198 ABSTRACT Jurassic rocks in the Hefei Basin were deposited by braided-fluvial and alluvial- fan systems, characterized by a general coarsening-upward sequence. Multiproxy provenance analyses demonstrate that the sediment source areas for the Hefei Basin are composed of a variety of rocks, including ultrahigh-pressure (UHP) and high-pressure (HP) metamorphic rocks and Yangtze basement rocks of the axial Dabie Shan metamorphic complex, the Luzhenguan complex granite, low- and medium-grade metamorphic rocks, and the Yangshan Group sandstone in the North Huaiyang fold and thrust belt. A Middle Jurassic section in the western part of the basin is characterized by relatively high ε Nd values (at 176 Ma), ranging from –13.8 to –11.3, whereas a section in the middle part of the basin has higher 147 Sm/ 144 Nd ratios, from 0.1168 to 0.1266 and somewhat lower ε Nd values (at 176 Ma), from –15.0 to –14.5. Sediments in a section in the eastern part of the basin have the lowest ε Nd values (at 176 Ma), ranging from –22.0 to –14.6, the highest T DM values, from 1.8 to 2.4 Ga, and low 147 Sm/ 144 Nd ratios, from 0.0937 to 0.1067. Provenance analyses of detrital composi- tions and Nd isotopic compositions of the sediments in the Hefei Basin clearly demon- strate that the depth of exhumation in the Dabie Shan orogen increases from the west to the east; the unroofing ages of the UHP and HP metamorphic rocks change from Early Jurassic to Late Jurassic westward. The exhumation rate during the Late Trias- sic and Jurassic is inferred to have increased eastward from ~1.4 mm/a to ~2.5 mm/a on average. The sediments in the basin record the episodic thrusting events and periodic unroofing in the orogen. INTRODUCTION The Dabie Shan orogen was formed by colli- sion of the Yangtze block, or the Qinling-Dabie Shan microcontinent, with the North China block along a north-directed subduction zone during the Paleozoic and the early Mesozoic (e.g., Mattauer et al., 1985; Zhang et al., 2001; Liu et al., 2003; Ratschbacher et al., 2006). The timing of the final collision has been constrained at 244–236 Ma (e.g., Li et al., 1993; Hacker et al., 1998) and ~230–220 Ma (e.g., Ames et al., 1993; Li et al., 2000; Liu et al., 2004) by several radiometric studies (e.g., Hacker et al., 2006; Liu et al., 2006). The two groups of ages may indicate a succession from the onset of high-pressure (HP)–ultrahigh-pressure (UHP) metamorphism to peak ultrahigh pressure con- ditions, respectively, during ultradeep subduc- tion (Hacker et al., 2006; Liu et al., 2006). The occurrence of coesite- and diamond-bearing, ultrahigh-pressure metamorphic rocks (e.g., Okay et al., 1989; Wang et al., 1989; Xu et al., 1992; Zhang et al., 1996; Liou et al., 1997) indi- cates that crustal rocks of the Yangtze block were subducted to depths of >100 km under the North China block (Liu et al., 2006; Hacker et al., 2006). The exhumation of the ultrahigh- pressure metamorphic complex has been pri- marily constrained by petrologic, structural, and 40 Ar/ 39 Ar thermochronology studies of the com- plex (e.g., Cong et al., 1995; Hacker et al., 1995, 1998; Li et al., 1999b; Li et al., 2000; Hacker et al., 2000; Ratschbacher et al., 2000). How- ever, this work focused mostly on the study of local ultrahigh-pressure metamorphic rocks and thus needs to be extended in order to reconstruct the exposed bedrock and structural framework of the Dabie Shan orogen during its various stages of exhumation, especially into the Juras- sic. Postorogenic erosion has removed many of the supracrustal units, limiting structural recon- structions of this area. However, sedimentation into adjacent foreland basins provides another constraint on the late stages of collisional defor- mation of the Dabie Shan orogen. Such informa- tion adds to our understanding of the evolution of this major continental suture zone. The exhumation of the UHP metamorphic complex in the Dabie Shan orogen is among the most interesting topics in geosciences because of its relationship with the dynamics of conti- nental lithosphere. Various lines of investiga- tion, including evidence from the provenance of Triassic and Jurassic sedimentary rocks in the Hefei Basin (e.g., Liu et al., 2001a, 2001b; Wang et al., 2002b; Li et al., 2005), the eastern and southern foreland of the Dabie Shan orogen (e.g., Grimmer et al., 2003), and the Songpan- Ganzi Basin (e.g., Enkelmann et al., 2007) have suggested similar ages (Late Triassic or Jurassic) for the exhumation of the UHP rocks. Using the presence of eclogite pebbles in basin sediments, sensitive high-resolution ion micro- probe (SHRIMP) dating of zircons, mineral chemistry of white K-micas and garnets, and dating of granite clasts, Wang et al. (2002b) and Li et al. (2005) were able to identify the time at which ultrahigh-pressure metamorphic rocks in the Dabie Shan orogen were first exposed. The abundance of Neoproterozoic zircons in Middle Jurassic sediments suggests that the sediment was mainly derived from the exhumed Yangtze block, which is located within the Dabie Shan orogen. The co-occurrence of detrital high-Si phengites, analyzed using electron microprobe, and Triassic detrital zircons, dated by SHRIMP, provides stratigraphic evidence that the first ex- posure of the UHP rocks at the Earth’s surface in the Dabie Shan orogen occurred in the Early For permission to copy, contact [email protected] © 2009 Geological Society of America GSA Bulletin; January/February 2010; v. 122; no. 1/2; p. 198–218; doi: 10.1130/B26524.1; 10 figures; 1 table; Data Repository item 2009185. † E-mail: [email protected] Tracing exhumation of the Dabie Shan ultrahigh-pressure metamorphic complex using the sedimentary record in the Hefei Basin, China Shaofeng Liu 1,† , Guowei Zhang 2 , Bradley D. Ritts 3 , Huiping Zhang 1 , Mingxing Gao 1 , and Cunchao Qian 2 1 State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China; College of Geosciences and Resources, China University of Geosciences, Beijing 100083, China; and Key Laboratory of Lithosphere Tectonics and Lithoprobing Technology of Ministry of Education, China University of Geosciences, Beijing 100083, China 2 State Key Laboratory of Continental Dynamics, Northwest University, Xi’an, Shanxi 710069, China 3 Chevron Energy Technology Company, San Ramon, California 94583, USA
Welcome message from author
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
198
ABSTRACT
Jurassic rocks in the Hefei Basin were deposited by braided-fl uvial and alluvial-fan systems, characterized by a general coarsening-upward sequence. Multiproxy provenance analyses demonstrate that the sediment source areas for the Hefei Basin are composed of a variety of rocks, including ultrahigh-pressure (UHP) and high-pressure (HP) metamorphic rocks and Yangtze basement rocks of the axial Dabie Shan metamorphic complex, the Luzhenguan complex granite, low- and medium-grade metamorphic rocks, and the Yangshan Group sandstone in the North Huaiyang fold and thrust belt. A Middle Jurassic section in the western part of the basin is characterized by relatively high εNd values (at 176 Ma), ranging from –13.8 to –11.3, whereas a section in the middle part of the basin has higher 147Sm/144Nd ratios, from 0.1168 to 0.1266 and somewhat lower εNd values (at 176 Ma), from –15.0 to –14.5. Sediments in a section in the eastern part of the basin have the lowest εNd values (at 176 Ma), ranging from –22.0 to –14.6, the highest TDM values, from 1.8 to 2.4 Ga, and low 147Sm/144Nd ratios, from 0.0937 to 0.1067.
Provenance analyses of detrital composi-tions and Nd isotopic compositions of the sediments in the Hefei Basin clearly demon-strate that the depth of exhumation in the Dabie Shan orogen increases from the west to the east; the unroofi ng ages of the UHP and HP metamorphic rocks change from Early Jurassic to Late Jurassic westward. The exhumation rate during the Late Trias-sic and Jurassic is inferred to have increased eastward from ~1.4 mm/a to ~2.5 mm/a on
average. The sediments in the basin record the episodic thrusting events and periodic unroofi ng in the orogen.
INTRODUCTION
The Dabie Shan orogen was formed by colli-sion of the Yangtze block, or the Qinling-Dabie Shan microcontinent, with the North China block along a north-directed subduction zone during the Paleozoic and the early Mesozoic (e.g., Mattauer et al., 1985; Zhang et al., 2001; Liu et al., 2003; Ratschbacher et al., 2006). The timing of the fi nal collision has been constrained at 244–236 Ma (e.g., Li et al., 1993; Hacker et al., 1998) and ~230–220 Ma (e.g., Ames et al., 1993; Li et al., 2000; Liu et al., 2004) by several radiometric studies (e.g., Hacker et al., 2006; Liu et al., 2006). The two groups of ages may indicate a succession from the onset of high-pressure (HP)–ultrahigh-pressure (UHP) metamorphism to peak ultrahigh pressure con-ditions, respectively, during ultradeep subduc-tion (Hacker et al., 2006; Liu et al., 2006). The occurrence of coesite- and diamond-bearing, ultrahigh-pressure metamorphic rocks (e.g., Okay et al., 1989; Wang et al., 1989; Xu et al., 1992; Zhang et al., 1996; Liou et al., 1997) indi-cates that crustal rocks of the Yangtze block were subducted to depths of >100 km under the North China block (Liu et al., 2006; Hacker et al., 2006). The exhumation of the ultrahigh-pressure metamorphic complex has been pri-marily constrained by petrologic, structural, and 40Ar/39Ar thermochronology studies of the com-plex (e.g., Cong et al., 1995; Hacker et al., 1995, 1998; Li et al., 1999b; Li et al., 2000; Hacker et al., 2000; Ratschbacher et al., 2000). How-ever, this work focused mostly on the study of local ultrahigh-pressure metamorphic rocks and thus needs to be extended in order to reconstruct the exposed bedrock and structural framework
of the Dabie Shan orogen during its various stages of exhumation, especially into the Juras-sic. Postorogenic erosion has removed many of the supracrustal units, limiting structural recon-structions of this area. However, sedimentation into adjacent foreland basins provides another constraint on the late stages of collisional defor-mation of the Dabie Shan orogen. Such informa-tion adds to our understanding of the evolution of this major continental suture zone.
The exhumation of the UHP metamorphic complex in the Dabie Shan orogen is among the most interesting topics in geosciences because of its relationship with the dynamics of conti-nental lithosphere. Various lines of investiga-tion, including evidence from the provenance of Triassic and Jurassic sedimentary rocks in the Hefei Basin (e.g., Liu et al., 2001a, 2001b; Wang et al., 2002b; Li et al., 2005), the eastern and southern foreland of the Dabie Shan orogen (e.g., Grimmer et al., 2003), and the Songpan-Ganzi Basin (e.g., Enkelmann et al., 2007) have suggested similar ages (Late Triassic or Jurassic ) for the exhumation of the UHP rocks. Using the presence of eclogite pebbles in basin sediments, sensitive high-resolution ion micro-probe (SHRIMP) dating of zircons, mineral chemistry of white K-micas and garnets, and dating of granite clasts, Wang et al. (2002b) and Li et al. (2005) were able to identify the time at which ultrahigh-pressure metamorphic rocks in the Dabie Shan orogen were fi rst exposed. The abundance of Neoproterozoic zircons in Middle Jurassic sediments suggests that the sediment was mainly derived from the exhumed Yangtze block, which is located within the Dabie Shan orogen. The co-occurrence of detrital high-Si phengites, analyzed using electron microprobe, and Triassic detrital zircons, dated by SHRIMP, provides stratigraphic evidence that the fi rst ex-posure of the UHP rocks at the Earth’s surface in the Dabie Shan orogen occurred in the Early
For permission to copy, contact [email protected]© 2009 Geological Society of America
GSA Bulletin; January/February 2010; v. 122; no. 1/2; p. 198–218; doi: 10.1130/B26524.1; 10 fi gures; 1 table; Data Repository item 2009185.
†E-mail: [email protected]
Tracing exhumation of the Dabie Shan ultrahigh-pressure metamorphic complex using the sedimentary record in the Hefei Basin, China
Shaofeng Liu1,†, Guowei Zhang2, Bradley D. Ritts3, Huiping Zhang1, Mingxing Gao1, and Cunchao Qian2
1State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China; College of Geosciences and Resources, China University of Geosciences, Beijing 100083, China; and Key Laboratory of Lithosphere Tectonics and Lithoprobing Technology of Ministry of Education, China University of Geosciences, Beijing 100083, China2State Key Laboratory of Continental Dynamics, Northwest University, Xi’an, Shanxi 710069, China3Chevron Energy Technology Company, San Ramon, California 94583, USA
Tracing exhumation of the Dabie Shan ultrahigh-pressure metamorphic complex
Geological Society of America Bulletin, January/February 2010 199
Jurassic during deposition of the Fanghushan Formation (Li et al., 2005). Li et al. (2005) also briefl y describe spatial variations in the compo-sition of detrital micas and garnets and in the U-Pb ages of detrital zircons. However, little is known about the spatial and temporal patterns of exhumation of the different rock units in the Dabie Shan orogen and their contribution to basin sediments. This paper documents a study of Jurassic sedimentary rocks in the Hefei Basin as they relate to postcollisional erosion of the orogen. Specifi cally, space-time changes within sedimentary sequences are correlated with exhumation events of the UHP and HP meta-morphic complex based on detrital provenance analysis, including Nd isotopic and geochemi-cal studies. This has allowed us to identify the distribution and contribution of source terrane lithologies to the Jurassic basin sediments in order to make inferences about exhumation of the Dabie Shan orogen.
GEOLOGICAL FRAMEWORK AND LITHOLOGICAL COMPOSITION OF THE DABIE SHAN OROGEN
The Dabie Shan orogen is linked with the East Qinling orogen across the Nanyang Basin (Fig. 1A) (Zhang et al., 1997). The Shangdan suture belt is exposed in the northern Dabie Shan orogen, between Nanyang and Xinyang, along the Xinyang-Shucheng fault (Fig. 1B) (Zhang et al., 2001), which was defi ned to be the Erlangping arc–Qinling unit suture by Ratschbacher et al. (2006). Another suture belt, the Qinling microcontinent–Yangtze craton suture belt, is located along the northern mar-gin of the UHP metamorphic complex in the Dabie Shan orogen (Ratschbacher et al., 2006). The Xiaotian-Mozitan fault zone reactivated the suture by Cretaceous sinistral strike-slip and normal faulting (Fig. 1B) (Ratschbacher et al., 2000). The Dabie Shan orogen can be divided into three generalized structural units: the northern Yangtze fold and thrust belt along the southern margin; the North Huaiyang fold and thrust belt along the northern margin; and the axial Dabie Shan metamorphic complex (Liu et al., 2003), which can be subdivided into the North Dabie Shan core zone (NDC), the ultrahigh-pressure metamorphic (UHP) and the high-pressure metamorphic (HP) zone on the basis of distinct lithologic and metamorphic facies (e.g., Hacker et al., 1998; Wang et al., 1998) (Fig. 1B). The Hefei and Middle Yangtze foreland basins are developed along the north-ern and southern margins of the orogen, respec-tively (Liu et al., 2003) (Figs. 1 and 2).
The North Huaiyang zone is located on the northern fl ank of the Dabie Shan orogen and is
bounded by the sinistral strike-slip and normal Xiaotian-Mozitan fault that places the North Huaiyang adjacent to the North Dabie Shan core zone to the south. The northern bound-ary of the North Huaiyang zone is covered by Ceno zoic rocks of the Hefei Basin but is pre-sumably the Xinyang-Shucheng fault. So the Hefei Basin is an episutural basin. Crosscutting relationships and age of basin-fi lling deposits demonstrate that the North Huaiyang zone was a long-lived (Triassic–Jurassic) thrust deforma-tion belt but was superposed by several periods of supracrustal extension during the Cretaceous to Tertiary (Liu et al., 2003) (Figs. 2A–2C). In-terpretation of seismic data (Zhao et al., 2000) shows that pervasive reverse faults cut an un-conformity between Jurassic and Proterozoic rocks and formed a small-scale imbricate thrust zone in the lowest parts of the Jurassic depos-its in the basin. In addition, the basin shows a simple asymmetric geometry consistent with fl exural subsidence, and a basin-bounding fault, the Xinyang-Shucheng fault, shows an older-over-younger relationship indicating re-verse motion, although this structure may not have been active during deposition (Fig. 2C). The eastern part of the North Huaiyang zone is composed of the Late Protero zoic Luzhenguan complex and Devonian Foziling Group, which have undergone greenschist- to amphibolite-facies metamorphism (e.g., Hacker et al., 1998; Anhui Geological Survey, 1999). Rock com-plexes in the western part of the North Huai-yang are represented by the Luzhenguan complex, the early Paleozoic Dingyuan For-mation, the lower part of the early Paleozoic Guishan Formation, and the upper part of the Devonian Nanwan Formation of the Xinyang Group, as well as the Carboniferous Yangshan Group. The Luzhen guan complex is com-posed mostly of red meta granite (or granite), granitic gneiss, mica-quartz schist, quartz-ite, and phyllite (Chen et al., 2003a), whereas the Dingyuan and Guishan Formations are composed of garnet-bearing mica schist, chlo-rite schist, and quartzite (Anhui Geological Survey, 1999), which represent a mélange accu-mulation that was formed along the Shangdan suture. The Nanwan Formation and Foziling Group as forearc basin sediments, deposited on the Guishan-Dingyuan-Luzhenguan complexes in front of the North China active margin (Li et al., 2001; Ratschbacher et al., 2006), are a turbiditic sequence with rhythmic layers of slate, phyllite, mica-quartz schist, and quartzite , as well as greenschists (Xu and Hao, 1988; Chen and Sang, 1995). The Yangshan Group is a shallow marine deposit, consisting mostly of sandstones, siltstone, and mudstone (Anhui Geological Survey, 1999).
The North Dabie Shan core zone includes the Luotian and Yuexi domes that lie in the footwall of the Dabie Shan orogen (Wang et al., 1998; Liu et al., 2004). The domes have similar lithologic features; they are composed of amphibolite- and granulite-facies felsic gneisses (mainly tonalitic gneisses) (75% of the total area), a supracrustal sequence (24%), and metamafi c-ultramafi c rocks (<1%) (Sang et al., 1997). The supracrustal sequence is made up of metavolcanic amphibole-biotite gneiss, amphibolite, and a small propor-tion of marble and magnetite-bearing quartzite (You et al., 1996). The radial dips of foliation and predominant NW and SE plunges of min-eral lineations outline the structural character of both domes (Wang et al., 1998). Xu et al. (2000) and Liu et al. (2000, 2001) found several fresh eclogite outcrops in the North Dabie Shan core zone and inferred that they were subjected to ultrahigh-pressure metamorphism in the north-ern Dabie Shan orogen.
The UHP/HP zone in the Dabie Shan orogen can be further differentiated into two subzones with different pressure-temperature (P-T) re-gimes: a coesite- and diamond-free HP unit in the south, and an UHP unit containing coesite- and diamond-bearing eclogites in the north (Fig. 1) (Okay, 1993; Carswell et al., 1997; Wang et al., 1998). The UHP/HP eclogite-bearing zone consists chiefl y of amphibolite-facies felsic gneisses (mainly granitic gneisses) with minor amphibolite, garnet-bearing peri-dotite, jadeite quartzite and marble, low-grade metamorphic basement-cover sequences (in-cluding metasediments and metavolcanics), and eclogites (Dong et al., 1996; Liou et al., 1997; Schmid et al., 2003). Ma et al. (2000) showed that most of the felsic gneisses in the UHP/HP zone have geochemical signatures of Neoproterozoic granites, and many refl ect magmatism in a rift environment. The protolith associations of the basement-cover sequences are compatible with deposition in a rift set-ting or along a passive continental margin during Neoproterozoic age. Eclogite-facies parageneses in the gneisses and the basement-cover units, along with P-T data demonstrate regional UHP metamorphism in the UHP/HP zone (Schmid et al., 2003).
JURASSIC SEDIMENTARY ROCKS OF THE HEFEI BASIN
Jurassic rocks in the Hefei Basin crop out along the northern fl ank of the Dabie Shan orogen. These rocks are stratigraphically sub-divided into the Fanghushan (J
1), Yuantong-
shan (J2), and Zhougongshan (J
3) Formations
in Feixi County, Anhui Province; the San-jianpu (J
2) and Fenghuangtai Formations (J
3) in
Liu et al.
200 Geological Society of America Bulletin, January/February 2010
LD
WM
ZF
FX
B
B′
C
C′
Wu
mia
o
Sh
an
gch
en
g
Fe
ng
hu
an
gta
i
Liu
an
’
Fa
ng
hu
sha
n
He
fei
Jin
zha
iD
ush
an
Ma
ota
nch
an
g
Hu
osh
an
Lu
oti
an
Yin
gsh
an
Taih
uYu
exi
DS
Fe
ixi
Xin
xia
n
31
00
FZ
L-2
(D)
92
F-1
(D
)
7ch
13
2D
-(
)
LJW
1E
-(
)
97
L0
03
F ()
96
110
F-
()
10
91
F-
()
97
L0
01
F()
D1
23
F()
94
30
3-2(
C′)
Bh
12
E-
()
Bh-
9b; 7
(E
)B
J93-
31; 3
2 (E
)92
H-2
4 (F
)9
2H
T-1
; 3; 8
(F
)
TH
12
E-
()
96
58
F-
()
01
HS
5C
-(
)0
1H
S7
C-
()
01
HS
8C
-(
)
01
HS
15
C-
()
01
HS
16
C-
()
PJL
1D
-(
)
ZF
A1
D-
()
6ch
10
7D
-(
)
01
HS
9D
-(
)
01H
S-1
1; 1
2 (D
)
2ch
53
B-
()
3ch
42
B3
ch4
42
B-
();
--
()
5ch
96
A-
()
4ch
76
A-
()
10
ch8
3B
-(
)
3ch
25
B-
()
95
HN
S1
3; 4
; 5; 6
(B
)-
-;
10
La
teJu
rass
icvo
lca
nic
rock
s
Jura
ssic
fore
lan
d d
ep
osi
tsY
an
gsh
an
Gro
up
Fo
zilin
gG
rou
p
Xin
yan
g
Din
gyu
an
Gro
up
s,
Lu
zhe
ng
ua
nG
rou
p
hig
h-p
ress
ure
me
tam
orp
hic
com
ple
xH
P(
)
ult
rah
igh
-pre
ssu
rem
eta
mo
rph
icco
mp
lex
(UH
P)
No
rth
Da
bie
Sh
an
core
me
tam
orp
hic
com
ple
x (N
DC
)
Hu
oq
iuA
rch
ea
nm
eta
mo
rph
icco
mp
lex
10
02
03
0 km
E11
5°0
0E
115°
30
E11
6°0
0E
116°
30
E11
7°0
0
N31°40 N31°20 N31°00 N30°40 N30°20 N30°00
E11
5°0
0E
115°
30
E11
6°0
0E
116°
30
E11
7°0
0
Qic
hu
n
2;3;
4;5;
6;7;
8;9
1;
Un
its
in N
ort
h H
ua
iya
ng
fold
an
d th
rust
be
lt:
Un
it in
No
rth
Ch
ina
blo
ck:
Un
its
in a
xia
l Da
bie
Sh
an
me
tam
orp
hic
co
mp
lex:
97
L0
03
F()
Gra
nit
e
Jura
ssic
sect
ion
s in
He
fei
ba
sin
an
d st
ruct
ura
l se
ctio
ns
Lo
cati
on
s n
um
be
rs &
en
dm
em
be
rso
fNd
iso
top
esa
mp
les
inso
urc
ea
rea
,-
Fa
ult
Qu
ate
rna
ry
Cre
tace
ou
s
NC
BX
inya
ng
Sh
uch
en
gfa
ult
(Sh
an
gd
an
Su
ture
)
-
Tanlufault
02
55
0 k
m
UH
P
HP
ND
C
UH
P
HP
E11
0E
115°
N3
1°2
0
N3
1°4
0
N3
0°
N3
0°4
0
E11
0°E
115°
Shang-Mafault
tluafnatizo
M-naitoaiX
tle
btsur
htd
na
dlof
gn
ayia
uH
htro
N tluafijgnauG-nafgnaiXNorth
ernYangtz
e fold
and thru
st belt
He
fei b
asi
n
N32°00
N3
2°00
YB
Ea
stQ
inlin
go
rog
en
QD
Xia
n’
NC
B
01
00
20
0 k
m
N3
0°2
0
N3
0°0
0
He
fei b
asi
n
Mid
dle
Ya
ng
tze
ba
sin
Stu
dy
are
a
Na
nya
ng
YB
N3
0°
N3
2°N
32°
Xin
yan
gH
efe
i
A
A′
B
B′ CC′
Na
nya
ng
ba
sin
A B
Da
bie
Sh
an
oro
ge
n
Fig
ure
1. G
eolo
gica
l sk
etch
map
sho
win
g m
ain
stra
tigr
aphi
c an
d st
ruct
ural
uni
ts o
f D
abie
Sha
n or
ogen
and
its
adj
acen
t re
gion
s, l
ocat
ions
of
Nd
isot
ope
sam
ples
in
Tabl
e D
R3
(see
foo
tnot
e 1)
in
sour
ce a
reas
, m
easu
red
Jura
ssic
str
atig
raph
ic s
ecti
ons
in H
efei
Bas
in a
nd s
truc
tura
l se
ctio
ns (
A–A
′, B
–B′,
and
C–C
′). J
uras
sic
stra
tigr
aphi
c se
c-ti
ons:
FX
—F
eixi
sec
tion
; Z
F—
Zhu
jiac
itan
g-F
engh
uang
tai s
ecti
on;
DS—
Dus
han
sect
ion;
LD
—L
iush
udia
n-D
ingb
acho
ng s
ecti
on; W
M—
Wum
iao
sect
ion.
Dat
a of
Nd
isot
ope
sam
ples
are
cit
ed fr
om: 1
—G
e et
al.,
200
1; 2
—X
ie e
t al.,
199
6; 3
—M
a et
al.,
200
0; 4
—L
i et a
l., 2
001;
5—
Lio
u et
al.,
199
7; 6
—D
ong
et a
l., 1
996;
7—
Che
n et
al.,
200
3a; 8
—X
u et
al.,
200
5; 9
—C
hen
et a
l., 2
003b
. Nd
isot
ope
sam
ples
in s
ourc
e ar
eas
are
divi
ded
into
six
end
mem
bers
—A
, B, C
(C
′), D
, E, a
nd F
—w
hich
are
sho
wn
wit
hin
pare
nthe
ses
at
ends
of
sam
ple
num
bers
. Ins
et m
aps
(A)
and
(B)
show
maj
or f
ault
s an
d st
ruct
ural
uni
ts in
Eas
t Q
inlin
g–D
abie
Sha
n or
ogen
and
Dab
ie S
han
orog
en, r
espe
ctiv
ely.
Abb
revi
a-ti
ons:
NC
B—
Nor
th C
hina
pla
te;
YB
—Y
angt
ze p
late
; Q
D—
Qin
ling-
Dab
ie S
han
plat
e; N
DC
—N
orth
Dab
ie S
han
core
zon
e; U
HP
—ul
trah
igh-
pres
sure
met
amor
phic
zon
e;
HP
—hi
gh-p
ress
ure
met
amor
phic
zon
e.
Tracing exhumation of the Dabie Shan ultrahigh-pressure metamorphic complex
Geological Society of America Bulletin, January/February 2010 201
the Maotanchang-Dushan-Jinzhai area, Anhui Province; and the Zhuji (J
2) and Duanji For-
mations (J3) in the Wumiao-Shangcheng area,
Henan Province (Anhui Geological Survey, 1999) (Fig. 3).
Fanghushan Formation (J1)
The Fanghushan Formation (J1) is only ex-
posed in the Feixi section (FX section in Fig. 3), where it unconformably overlies Archean rocks of the Huoqiu Group of the North China base-ment. The Fanghushan Formation is composed mainly of fl uvial deposits, including conglom-erate at its base and isolated channel-belt sand bodies distributed among fl ood-plain siltstones in its upper part. The entire Fanghushan Forma-tion is at most 400 m thick.
Zhuji, Sanjianpu, and Yuantongshan Formations (J2)
The westernmost section of the Zhuji For-mation that we measured is located at Wumiao (WM section) (Figs. 3 and 4). The Zhuji Forma-tion is ~1100 m thick and is composed of fi ne- to coarse-grained sandstone and pebbly sandstone with some conglomerate and mudstone inter-vals. This succession consists of a series of basal-scoured, upward-fi ning units, each of which begins with 5–15 m of massively bedded, pebbly or coarse-grained sandstone overlain by fi ne- to medium-grained sandstone with poorly devel-oped cross-stratifi cation or planar lamination. Paleocurrent indicators, from cross-bedding ori-entations, indicate fl ow toward the north (Fig. 3). Between sand bodies there are intervals of thin-bedded mudstone or very fi ne-grained sand-stone. This succession is interpreted to record a braided, channel-plain depositional system with some fi ne-grained overbank deposits.
The Sanjianpu Formation, measured along the Liushudian-Dingbachong (LD) section (Figs. 3 and 5), is ~1850 m thick and consists mostly of debris fl ow and braided stream con-glomerates. The lower part of the unit is com-posed of massive pebble conglomerate with internal scour surfaces. The middle part of the Sanjianpu Formation consists of coarse- to medium-grained sandstone with local con-glomerate lenses that are composed of tabu-lar, large-scale, planar cross beds. The upper part of the Sanjianpu Formation is fi ning up-ward, and is composed of cobble conglomer-ate at its base that fi nes to granule-rich pebble conglomerate toward the top. Paleocurrent indicators from imbricated gravel and cross-stratifi cation show fl ow that is directed toward the present-day north-northeast (Fig. 3). To the east of the Liushudian-Dingbachong section,
++
++
+++ + ++
++
++++
+
+
++++
+
++
+P
t2
J
AnC
-C
AnC
-CA
nC-C
J3-K
1C
-P
C-P
Nor
ther
nY
angt
ze fo
ld-t
hrus
t bel
t and
fore
land
bas
in
Yan
gtze
Blo
ck (
YB
)
XG
F (
Mia
nlue
sut
ure)
Qin
ling-
Dab
ie S
han
Blo
ck (
QD
)N
orth
Hua
iyan
g fo
ld-t
hrus
t bel
t and
fore
land
bas
inN
orth
Chi
na B
lock
(N
CB
)
Nor
ther
n bo
unda
ry fa
ult
of Q
inlin
g R
ange
Sha
ngda
n su
ture
–40
0
–10
–20
–30
–40
kmkm
0 –10
–20
–30
N-Q D-T
2Z
-SZ
-S
S
Z-S
D-T
N-Q
K2
C-O
C-O
J-K3
1E
C-P
-T
N-Q
C-P
-T
E
C-P
-T
E
C-P
-T
Pt 2
D
Pt
-Pz
31
Pt 1
Ar-
Pt 1
Pt3
K2
Pt 2
K2
Z-D
Pt2
010
20 k
m
Pt 3
E
K1
K2
Z-S
K2
Z-S
P-T
2
C-S
A
T-J3
2
?
A′
AQ
uate
rnar
y de
posi
ts
Rift
bas
in d
epos
its
Pyr
ocla
stic
roc
ks fi
lling
rift b
asin
For
elan
d ba
sin
depo
sits
Met
amor
phic
roc
k w
ithin
intr
usiv
e bo
dy0
1020
km
Xinyang-Shuchengfault
Shu
chen
g Liu'
anfa
ult
Fei
xi
Feiz
hong
faul
t Nor
ther
nbo
unda
ryfa
ult o
f Qin
ling
rang
e
J2-3
Ar 2
Pz 2
Z-P
z 1Q
n
Ar 2K
-E 2 Ar2
Pt 3
Z-P
z1P
z2Z
-Pz 1
Pz 2
J 1Z
-Pz1
J 2J 1
Z-P
z 1
K-E 2
K-E 2
J 2J2 J 1
Z-Pz
1P
z2
J1J 2-3
KE
2-P
t1-2
Pt1
-2
0 4 8 12 km
Hef
ei fo
rela
nd b
asin
CC
′
C
Pt3
Pt3
T-J 3
2K
1K
2 D-T
2
K2
T-J3
2D
-T2
K2
D-T
2P
t2
Pt 2
Pt2
Intr
usio
n
25°
Fuz
iling
Gro
up
Qin
gsha
n
Luzh
engu
an c
ompl
ex
04
km
J 2-3
Shi
shuy
uan
Youd
ian
Fuz
iling
Gro
upJ
-K 31
Dab
ie S
han
Met
amor
phic
rock
s
Xia
otia
n-M
ozita
n fa
ult
Nor
th H
uaiy
ang
fold
-thr
ust b
elt
BB
′
B
J-K 3
1P
z2P
t3P
t3
Fig
ure
2. S
truc
tura
l cro
ss s
ecti
ons
from
the
Dab
ie S
han
orog
en. L
ocat
ion
of s
ecti
ons
A, B
, and
C a
re s
how
n in
Fig
ure
1. S
ecti
ons
show
(A
) th
e en
tire
Dab
ie S
han
orog
en;
(B)
the
Nor
th H
uaiy
ang
fold
and
thr
ust
belt
; (C
) th
e X
inya
ng-S
huch
eng
thru
st b
elt
and
Hef
ei B
asin
, m
odifi
ed f
rom
Liu
et
al. (
2003
). A
ges
of u
nits
sho
wn
are:
Ar 2—
mid
dle
Arc
hean
; P
t 1–2—
Ear
ly t
o M
iddl
e P
rote
rozo
ic;
Pt 3—
Lat
e P
rote
ro-
zoic
; Z
—Si
nian
(eq
uiva
lent
to
Ven
dian
); P
z 1—ea
rly
Pal
eozo
ic;
Pz 2—
late
Pal
eozo
ic;
C–—
Cam
bria
n; O
—O
rdov
icia
n; S
—Si
luri
an;
D—
Dev
onia
n; C
—C
arbo
nife
rous
; P
—P
erm
ian;
T—
Tri
assi
c; T
1—E
arly
Tri
assi
c; T
2—M
iddl
e T
rias
sic;
T3–
J 2—U
pper
Tri
assi
c to
Mid
dle
Jura
ssic
; J 1—
Low
er J
uras
sic;
J2–
3—M
iddl
e to
Upp
er J
uras
sic;
J3–
K1—
Upp
erm
ost
Jura
ssic
to
Low
er C
reta
ceou
s; K
2—U
pper
Cre
ta-
ceou
s; K
2–E
—U
pper
Cre
tace
ous
to T
erti
ary;
N–Q
—N
eoge
ne t
o Q
uate
rnar
y; Q
—Q
uate
rnar
y; X
GF
—X
iang
fan-
Gua
ngji
fau
lt.
Liu et al.
202 Geological Society of America Bulletin, January/February 2010
Alluvial Fan
Braided streamand flood plain
E115°00
2°00
3N
E115°00
E117°00
E117°00
2°00
3N
ShangchengHefei
LD
WM
ZF
FXJinzhai
DS
200 40 km
Hefei Basin
Liu’an
MaotanchangHuoshan
.m
Fijn
au
D.
mFij
uh
Z
N
n = 31
N
n = 32
N
n = 51N
n = 29
N
n = 46
N
n = 45
.m
Fiat
gn
au
hg
ne
F.
mF
up
naij
na
S
N
n = 18
N
n = 34
Not to top
.m
Fiat
gn
au
hg
ne
F.
mF
up
naij
na
S
Not to top
.m
Fn
ahs
gn
og
uo
hZ
.m
Fn
ah s
gn
otn
au
Y.
mF
na
h su
hg
na
F
N
n = 32
.m
Fu
pn
aijn
aS
.m
Fiat
gn
au
hg
ne
F
cissar
u Jr
ep
pU
cissar
u Jel
ddi
Mciss
aru J
re
wo
L
Wumiao (WM)Section
Liushudian-Dingbachong(LD) Section
Dushan (DS)Section
Zhujiacitang-Fenghuangtai(ZF) Section Feixi (FX)
Section
West
East
0
250 m
Conglomerate Sandstone
Volcaniclasticrock
Siltstone andmudstone
Eclogitegravel
Pre-Mesozoicstrata
Cross-bedding
Ripple beddingN
n = 29
Paleo-currentdirection
Covered partin section
Troughcross-bedding
Figure 3. Jurassic stratigraphic sections and their lateral correlation, paleocurrents in Hefei Basin. Inset map shows general facies changes and location of sections in the Hefei Basin.
Tracing exhumation of the Dabie Shan ultrahigh-pressure metamorphic complex
Geological Society of America Bulletin, January/February 2010 203
–14 –12 0.10 0.11 1.7 1.8 1.9 2
1
2
(V)
(VI)
(IV)(III)
(II-2)
(I-4)
PQm
Qm F
LtK
03-11-22208-1,2
03-11-20
208-3
208-5208-6208-7,8208-9208-11208-13
208-14208-16
208-1703-11-25
208-19
208-20
208-21
208-22
208-24
03-11-26
03-11-27
03-11-28
03-11-19
J3
0
50
100m
Not to bottom
Qm-F-Lt Lithic compositionfrom thin sections
noit
amr
oFij
uh
Z
Qm-P-K
20 40 60 80% 20 40 60 80%
Stratigraphiccolumn and samples
noit
amr
oF
Sm/ Nd147 144 T (Ga)DMNd
seic
afort
eP
eg
alb
me ss
a
)J(
cissar
uJel
ddi
M2
eg
A
20 40 60 80%C G P C BF MMud
3
Figure 4. Stratigraphic section, detrital and Nd isotope composition of Zhuji Formation in the Wumiao section (location WM on Fig. 1). Values of εNd were calculated for t = 176 Ma at which deposition began to take place in the early Middle Jurassic. Figure shows lithic composition from thin sections, indicated by solid circles next to the stratigraphic column for various locations. Sandstone composition: Qm—monocrystalline quartz; F—plagioclase and potassium feldspar; Lt—total lithic fragments; P—plagioclase; K—potassium. Lithic petrofacies: I-4—felsic fragments; II-2—red granite, metagranite, and granite gneiss; III—quartzites, quartz schists, chlorite schists, and schists; IV—phyllites, granulites, and slates; V—low-grade metamorphic argillites and sandstones (V-1) and reworked sandstone and mudstone clasts; VI—altered rocks and felsic to mafi c metavolcanic rocks. Grain sizes are as follows: F—fi ne sandstone; M—medium sandstone; C—coarse sandstone; G—granule; P—pebble; C—cobble; B—boulder.
Liu et al.
204 Geological Society of America Bulletin, January/February 2010
noit
amr
oF
up
naij
na
Sn
oita
mro
Fiat
gn
au
hg
ne
F
Qt-F-L Qm-P-K
20 40 60 80% 20 40 60 80%
Qt F
L
Qm P
K
Stratigraphiccolumn and samples
C G P C B
1000m
500
0
noit
amr
oF
03-11-1
03-11-3
No1
03-11-6
No.11
No.12
L-10
No.14
03-11-7
03-11-8
03-11-5
L-26
L-25
L-21L-20
L-19
L-18
L-17
L-16
L-15
No.10
L-13
L-9L-8
L-7L-6
L-3
L-1
Liu-1
Liu-2
Liu-3
Liu-4
L-12
0.1260.1180.122–14.8
Sm/ Nd147 144 T (Ga)DMNd
2.2 2.3–14.4
1
2
3
4
5
6
7
Lithic composition fromthin sections and gravelcounts
)J(
cissar
uJel
ddi
M2
)J(
cissar
uJr
ep
pU
3e
gA
(I-1&2)
(II)
(III)
(IV)
(V)
(VI)
20 40 60 80%F MMud
seic
afort
eP
eg
alb
mess
a
Figure 5. Stratigraphic section, detrital and Nd isotope composition of Sanjianpu and Fenghuangtai Formations in the Liushudian-Dingbachong section (location LD on Fig. 1) modifi ed from Liu et al. (2001a). Figure shows lithic composition from thin sections and gravel counts, respectively, indicated by solid circles and squares next to the stratigraphic column for various locations. Sandstone composition: Qt—total quartz; L—total lithic fragments except polycrystalline quartz (Qp). Lithic petrofacies: I-1—granitic gneiss; I-2—plagiogneiss; II—containing white metagranite (II-1), red granite, metagranite, and granite gneiss (II-2), and metadiorite and tonalite (II-3). See other explanation in Figure 4.
Tracing exhumation of the Dabie Shan ultrahigh-pressure metamorphic complex
Geological Society of America Bulletin, January/February 2010 205
the Sanjianpu Formation in the Dushan (DS) section (Fig. 3) is mostly composed of con-glomerates with roughly tabular bedding and subrounded to rounded clasts ranging in size from ~5 to 12 cm.
Farther east, the Sanjianpu Formation along the Zhujiacitang-Fenghuangtai section (ZF section) (Figs. 3 and 6) is composed mostly of fi ne- to coarse-grained sandstones with some gravel. Conglomerate with poorly developed horizontal bedding is present in the lowest part of the formation. The upper part of the unit is characterized by vertically stacked, coarsening-upward, fi ne- to coarse-grained sand bodies, in which massive bedding and large, gently in-clined cross strata are developed. Paleocurrent indicators (cross beds) suggest paleofl ow was to the north-northeast (Fig. 3). Clear scour surfaces are located at the base of each sand body, and thin, pebble conglomerates are locally present at the bottom of these bodies. The Sanjianpu For-mation is interpreted to have been deposited by braided channel systems.
The Yuantongshan Formation conformably overlies the Fanghushan Formation in the Feixi section (FX, Fig. 3) and is equivalent to the Sanjianpu and Zhuji Formations in other areas of the Hefei Basin. The Yuantongshan Forma-tion consists of purple-red siltstone, mudstone, and intercalated medium- to fi ne-grained sand-stone. The siltstone and mudstone are composed of rippled or horizontal beds, and contain root traces and paleosols at the top of the layers. Clear scour surfaces are located at the base of the sandstones, within which inclined cross-beds are well developed. These deposits are interpreted to represent a fl uvial plain environ-ment with some small lakes (Li et al., 2005).
Duanji, Fenghuangtai, and Zhougongshan Formations (J3)
The Upper Jurassic stratigraphy in the Hefei Basin mostly consists of conglomer-ate and coarse-grained sandstone. The Zhou-gongshan Formation in the Feixi area consists of conglomerate, sandstone, and siltstone deposited in a fl uvial setting (Li et al., 2005). Paleo current indicators, from imbricated gravel and cross-bedding orientations, are to-ward the northeast (Fig. 3). The Fenghuangtai Formation is relatively thick, 2400 m in the Liushudian-Dingbachong section in Jinzhai and >1000 m in the Zhujiacitang-Fenghuangtai section in Huoshan, and is composed domi-nantly of cobble to boulder conglomerate (Figs. 5 and 6). The conglomerate sequence in the Liushudian-Dingbachong section contains units consisting of grain-supported, imbricated clasts within thick beds as well as units of massive,
matrix-supported conglomerate. The conglom-erate in the Zhujiacitang-Fenghuangtai section is mostly composed of grain-supported, thick, roughly laminated beds with scour surfaces at their base that contain a few thin, lenticular sand bodies. Paleofl ow directions in the Liushudian-Dingbachong and Zhujiacitang-Fenghuangtai sections, determined from imbricated clasts, are toward the present-day northeast. The Duanji Formation in Wumiao (Fig. 3), located to the west of Jinzhai, and the Fenghuangtai Forma-tion from Huoshan to Liu’an, located to the west of Maotanchang, consist of fi ning-upward sequences divided by scour surfaces overlain with cobble to pebble conglomerates at the base, and gravel sandstones and medium- to coarse-grained sandstones upward. These units are interpreted to be the deposits of a braided chan-nel plain system. Three big conglomeratic allu-vial fans are centered in western Shangcheng, Jinzhai-Dushan, and Maotanchang, respectively (Fig. 3), but the eastern one, in Maotanchang, is mostly covered by Cretaceous strata.
The Hefei Basin sections are interpreted to have formed from alluvial-fan and braided-fl uvial plain sedimentation derived from the Dabie Shan orogen to the south. The entire sequence is up to 6 km thick at the southern edge of the basin. Toward the north the basin fi ll gradually decreases in thickness, maximum grain size, and abundance of mass fl ow deposits.
Depositional Age
The Fanghushan Formation contains plant fossils such as Podozamites lanceolatus (Lindley et Hutton) Braun, Cldophlebis sp., Pityophyllum nordenskiöldi Heer, Neocalamites cf. Hoerensis (Schimper) Halle, N. carrerei (Zeiller) Halle, Cycadocapidium erdmanni Natherst, and Am-drupia stenodonta Harris (Bureau of Geology and Mineral Resources of Anhui Province, 1987). Plant fossils such as Neocalmites car-cinoides, Equisetites lateralis, and Equisetites takahashi, similar to those for Middle Jurassic strata in northern and northwestern China, have been found in the Yuantongshan Formation (Li et al., 2002). Fossils have not been found in the Sanjianpu and Fenghuangtai Formations, but the depositional ages of these formations are constrained by their position unconformably beneath the Maotanchang Formation, which contains Late Jurassic fossils such as Fergano-concha lingyuanensis, Carbicula, Sphaerium jeholense, Probaicalia, and Eosestheria (Li and Jiang, 1997). Wang et al. (2002a) reported K-Ar ages of 138.2 ± 2.2 Ma, 144.8 ± 2.3 Ma, and 146.5 ± 2.3–146.8 ± 2.3 Ma for samples of trachyte from the top, middle, and lower parts of the Maotanchang Formation, respectively,
and K-Ar ages of 147.5 ± 2.3 Ma and 148.8 ± 2.5 Ma for andesite samples from the lower part. Thus, the entire interval shown on Figures 4, 5, and 6 extends from the Middle Jurassic to Late Jurassic in age (Liu et al., 2003).
DETRITAL COMPOSITION OF BASIN FILL
Facies changes and paleocurrent indicators consistently demonstrate that sediments for the southern marginal alluvial-fan and braided-stream depositional systems in the Hefei Basin were derived from the adjacent Dabie Shan orogen (Fig. 3). The unroofi ng history of the Dabie Shan orogen as recorded in conglomerate and sandstone compositions of the Hefei Basin provides a useful indicator of orogenic exhuma-tion and erosion. The Dabie Shan orogen is well suited for such analysis because it has a wide range of exposed rock types, and thus detrital grain compositions are good indicators of the sediment source. The detrital compositions in the Hefei Basin represent the rock types, lithic assemblages, and tectonic setting of the Dabie Shan orogen.
Methodology
The Hefei Basin is dominated by abundant conglomerates and some sandstone, thus differ-ent techniques were used to identify the compo-sitions of these different grain sizes. Sandstone compositions were determined by counting 500–560 framework grains per thin section. The point-count results, i.e., the modal composi-tion (GSA Data Repository Table DR11), using the Gazzi-Dickinson method (Ingersoll et al., 1984), were plotted on Qm-F-Lt and Qm-P-K ternary diagrams to provide insight into the tectonic setting of the source areas (Fig. 7) and were plotted along side of the measured sections to show the changes in detrital composition with time (Figs. 4–6). Because the Dabie Shan oro-gen has a wide range of exposed rock types, lithic grain compositions are considered to be the most useful indicator of source area. In or-der to link rock fragments to specifi c protoliths, we also determined sandstone compositions by traditional point counting (e.g., Indiana method) (Suttner et al., 1985), in which all grains are counted as their respective rock type, including coarsely crystalline (e.g., individual sand-size crystals within a rock fragment are counted as that rock fragment). We calculated the contents
1GSA Data Repository item 2009185, provenance analyses of sediments of Hefei Basin, is available at http://www.geosociety.org/pubs/ft2009.htm or by request to [email protected].
Liu et al.
206 Geological Society of America Bulletin, January/February 2010
–20 –15 0.10 0.11 1.8 2.2
(V)
1
3
5
Lithic composition fromthin sections and gravelcounts
(III)
(VI)
(III)
(IV)
(III)
(II) (VI)
(V-1)
Qm-F-Lt
20 40 60 80%
Qm F
Lt
Qm P
K
C G P C BF MMud
No.3
03-11-41
03-11-53
03-11-36
03-11-48
03-11-47
0
50
100m
03-11-35
03-11-52
F-10
03-11-51
03-11-50
03-11-49
03-11-46
03-11-45
03-11-44
F-3
03-11-42
No.2
No.5
No.4
Qm-P-K
20 40 60 80%
Stratigraphiccolumn and samples
noit
amr
oF
Sm/ Nd147 144 T (Ga)DMNd
eg
A
20 40 60 80%
noit
amr
oF
up
naij
na
Sn
oita
mro
Fiat
gn
au
hg
ne
F
)J (
cissar
uJel
ddi
M2
)J(
cissar
uJr
ep
pU
3
Not to top
(I-2)
(I-3)
(II-
1)
(II-2)
(I-4)
)IIV(
seic
a fort
eP
eg
alb
mess
a
2
No.120 40 60 80%
4
6
(II)
Figure 6. Stratigraphic section, detrital compositions, and Nd isotope data for the Sanjianpu and Fenghuangtai Formations in the Zhujiacitang-Fenghuangtai section (location ZF on Fig. 1). Lithic petrofacies: VII—chert clasts. See other explanations in Figures 4 and 5.
Tracing exhumation of the Dabie Shan ultrahigh-pressure metamorphic complex
Geological Society of America Bulletin, January/February 2010 207
of different fragments and show the results for rock detrital composition (Table DR1 [see foot-note 1]), referred to as model content, which typically represents more than 15% of all the grains counted (Figs. 4–6).
In areas where conglomerates are abundant, we collected both gravel clast data in the fi eld and sandstones within the conglomerate. Nearly every conglomerate clast in an area of ~2 m2 was identifi ed, typically 100–180 clasts or more, and sandstones were identifi ed as described above (Table DR1 [footnote 1]).
Ternary Plots for Sandstone Compositional Data
As a whole, the Jurassic sandstones from the Hefei Basin are lithic poor (most <5% Lt) and dominated by quartz and feldspar fragments based on the Gazzi-Dickinson method of point counting. The plots of Qm-F-Lt and Qm-P-K clearly show a continental block provenance for the Jurassic Hefei Basin (Fig. 7), suggest-ing a deeply eroded continental basement prov-enance. The modal data can be divided into three groups, which are found in spatially and stratigraphically disparate parts of the Hefei Basin , based primarily on monocrystalline quartz content. Group 1 samples are composed of 65%–80% quartz fragments and less than 5% lithic fragments in the plot of Qm-F-Lt, and
65%–80% quartz fragments and 2%–25% po-tassium feldspar fragments in the plot of Qm-P-K. This group most commonly represents samples from the Zhuji Formation in the west-ern part of the Hefei Basin (Wumiao section) and the Sanjianpu Formation in the middle part (Liushudian-Dingbachong section). Group 2 samples are composed of ~50%–65% quartz fragments and less than 5% lithic fragments in the plot of Qm-F-Lt. This group is representa-tive of samples from the Sanjianpu Formation located in the eastern part of the Hefei Basin (Zhujiacitang-Fenghuangtai section). Group 3 samples contain ~30%–50% quartz fragments and less than 10% lithic fragments in the plot of Qm-F-Lt, and 30%–50% quartz fragments and 35%–50% potassium feldspar fragments in the plot of Qm-P-K. This group represents sam-ples from the Fenghuangtai Formation (upper Zhujiacitang-Fenghuangtai section).
As an indicator of compositional maturity, the Qm-F-Lt diagram distinguishes Group 1 samples from those of Groups 2 and 3 that contain less stable minerals and more feldspar, mostly potassium feldspar, as shown in the plot of Qm-P-K. This indicates that Groups 2 and 3, especially Group 3, are derived from source ter-ranes with more granitic rocks. The Sanjianpu Formation in the Liushudian-Dingbachong sec-tion contains more potassium feldspar than the Zhuji Formation in the Wumiao section, suggest-
ing that the former was derived from a source with a higher percentage of granite. The modal contents of Groups 1, 2, and 3 (e.g., 2%–25%, 10%–50%, and 35%–50% potassium feldspar fragments in Groups 1, 2, and 3 in the plot of Qm-P-K, respectively) suggests stronger tec-tonism and more rapid exhumation of the Dabie Shan orogen from the west to the east and from the Middle Jurassic to Late Jurassic. Although almost all samples plot in the continental block provenance in the Dickinson model (Dickinson et al., 1983), that does not mean that the source area, the Dabie Shan orogen, was a stable craton.
Lithic Petrofacies and Source-Area Analyses
Field study shows that the sediments in the southern Hefei Basin are mostly alluvial fan and braided stream in origin, and their fl ow is directed toward the present-day north-northeast (Fig. 3). The Jurassic strata in the basin mostly unconformably overlie the Paleozoic strata and Archean–Proterozoic metamorphic basement at the southern margin of North China block (Fig. 2). The source area for the sediments in the southern basin is the northern Dabie Shan oro-gen. The area to the east of the Tanlu fault might supply part of the sediments for the basin, but neither paleocurrent data nor lithic composition support this possibility.
LtFMAGMATIC ARC PROVENANCES
KP
Zhuji Formationin WM Section
Sanjianpu Formationin ZF Section
Fenghuangtai Formationin ZF Section
Sanjianpu Formationin LD Section
Qm
Increasing ratio ofplutonic (P) tovolcanic (V) sources
CONTINENTAL BLOCKPROVENANCES
RECYCLED OROGENPROVENANCES
(MIXED)
INCREASING RATIO OFPLUTONIC/VOLCANIC COMPONENTSIN MAGMATIC ARC PROVENANCES
INCREASING MATURITYOR STABILITY FROMCONTINENTAL BLOCKPROVENANCE
Qm
CIRCUM-PACIFICVOLCANOPLUTONICSUITES
LIM
ITO
F DETR
ITAL
MO
DES
Group 1
Group 2
Group 3
Group 1
Group 2
Group 3
Figure 7. Ternary plots of sandstone composition tested by Gazzi-Dickinson method. See explanation in Figure 4. Abbreviations: WM—Wumiao; LD—Liushudian-Dingbachong; ZF—Zhujiacitang-Fenghuangtai.
Liu et al.
208 Geological Society of America Bulletin, January/February 2010
Because the source areas in the axial Dabie Shan metamorphic complex and its northern marginal North Huaiyang zone develop dif-ferent kinds of rocks and some typical litholo-gies, source-area analysis through detailed comparison between the source rocks and lithic fragments, especially lithic petrofacies, is an effec tive method, although there are some un-certainties due to lithological complexity (Heller and Ryberg, 1983; Hendrix, 2000; Liu et al., 2003). Percentages of lithic grains, normalized as percentages of the total lithic content, are shown for the Hefei Basin in Table DR1 (foot-note 1), and Figures 4, 5, and 6. The samples are expressed as lithic petrofacies (petrofacies in short) consisting of groups of lithic fragments with similar lithologies that are found together, defi ning distinctive provenance terranes. In some measured sections the lithic petrofacies are further classifi ed into some subtypes accord-ing to their specifi c lithic types.
The lithic petrofacies and their subtypes are designated with Roman numerals and Arabic numerals, respectively. Petrofacies I is derived from granitic gneiss (Petrofacies I-1), plagio-gneiss (Petrofacies I-2), and marble (Petrofacies I-3) source areas exposed along the North Dabie Shan core and UHP/HP zones, as well as from the Late Proterozoic Luzhenguan complex. In addition, felsic fragments are common in many slides. Although it is diffi cult to confi rm their exact origin, they likely derived from the gran-itoid rocks and gneisses. Here we defi ne them as Petrofacies I-4. Petrofacies II contains white metagranite (Petrofacies II-1), red granite, meta-granite, and granite gneiss (Petrofacies II-2), and metadiorite and tonalite (Petrofacies II-3) clasts, in which the red granite, metagranite, and granite gneiss came from the Luzhenguyan Group, and the white metagranite, metadiorite, and tonalite came from the axial Dabie Shan metamorphic complex. Petrofacies III contains quartzites, quartz schists, chlorite schists, and schists. Petrofacies IV contains phyllites, granu-lites, and slates. Sources to the north of the axial Dabie Shan complex for these two groups in-clude the Luzhenguan complex and Fuziling Group in the east, and the Luzhenguan complex, Guishan, Dingyuan, and Nanwan groups in the west. These two petrofacies also contain a few clasts derived from the North Dabie Shan core and UHP/HP zones. Petrofacies V includes low-grade metamorphic argillites and sandstones (V-1), and reworked sandstone and mudstone clasts, all of which were derived from the Yang-shan Group, and parts of the Nanwan Forma-tion and Foziling Group. Petrofacies VI includes altered rocks and felsic to mafi c metavolcanic rocks, probably derived from arc volcanics in the Dingyuan and Guishan Formations in the
North Huaiyang zone (Li et al., 1999a). Petro-facies VII only contains a small amount of chert clasts in Zhujiacitang-Fenghuangtai (Fig. 6) mainly derived from Fuziling Group. Various kinds of lithic petrofacies with distinctive lithic composition at the different parts of the sections (Figs. 4–6) are expressed as petrofacies assem-blages, which represent the exposed rock types and tectonic setting of the source area in the Dabie Shan orogen during the syndepositional stage. The measured sections in the Hefei Basin are divided into various petrofacies assemblages with boundaries of lithic composition change, which contain different categories or propor-tions of lithic petrofacies (Figs. 4–6).
Zhuji Formation in the Wumiao SectionThe modal data of sandstone samples gen-
erated by point counting from the Zhuji For-mation in the Wumiao section show that the lithic fragments are mainly granite and meta-granite (Petrofacies II-2), quartzite and schists (Petrofacies III), phyllite, granulite, and slate (Petrofacies IV), metasandstone (Petrofacies V), altered rock and rhyolite (Petrofacies VI), and felsic fragments (Petrofacies I-4) (Fig. 4). Felsic fragments, which make up only a small fraction of the total, are mostly composed of quartz and potassium feldspar. Petrofacies II-2, mostly composed of granite and metagranite, is a higher component in places. Clast abundances of Petrofacies III and IV are both low, but those of Petrofacies V and VI are high. Petrofacies I-4 and Petrofacies II-2 were likely derived from the Luzhenguan complex, which contains abundant red metagranites and potassium feldspar. Petro-facies III and IV were mostly derived from the Nanwan, Guishan, and Dingyuan Formations, part of the Luzhenguan complex. Petrofacies V was probably derived from the Yangshan Group and Nanwan Formation. Finally, Petrofacies VI was derived from the Guishan and Dingyuan Formations, which contain some volcanic rocks. This analysis of the lithic portions of the petrofacies shows that sediment in the Zhuji Formation was derived from the Luzhen guan complex, the Nanwan, Guishan, and Dingyuan Formations, and the Yangshan Group in North Huaiyang. The lithic petrofacies for Wumiao section constitute three petrofacies assem-blages. The basal rocks, Petrofacies Assem-blage 1, mostly contain reworked sedimentary rocks, Petrofacies V (76.4%–83.9%), derived from the Yangshan Group and the Nanwan Formation, the basement cover rocks in North Huaiyang zone. The middle part of the section, Petro facies Assem blage 2, shows an increase in abundance of granite and granitic gneiss (Petrofacies II, mostly 10%–22% or more) and volcanic rocks (Petrofacies VI, 17%–67% in
some parts) from the Luzhenguan complex, the basement rocks, and the Dingyuan and Guishan Formations of North Huaiyang zone. Petro-facies Assemblage 3, at the top of the section, shows an increase in content of medium- to low-grade metamorphic source (Petrofacies III and IV, 18.5%–35.9%) and reworked sedimen-tary rock source (Petrofacies V, 23.8%–59%) (Fig. 4). The fi rst two petrofacies assemblages approximately record an unroofi ng event from cover strata to Luzhenguan basement, and the third one records a beginning of another unroof-ing event from cover strata in North Huaiyang.
Sanjianpu and Fenghuangtai Formations in the Liushudian-Dingbachong Section
The modal data generated by point counting and clast counting from sedimentary strata in the Liushudian-Dingbachong section (Table DR1 [footnote 1], Fig. 5) show input from six petro-facies sources (Petrofacies I to VI). Liu et al. (2003) suggested that the lithic petro facies for the Liushudian-Dingbachong section record two distinct unroofi ng events, but here we revise the interpretation to include three unroofi ng events which were represented by seven petro facies assemblages with a clear difference of lithic composition and content. Petrofacies Assem-blages 1 and 2 correspond to the fi rst unroofi ng event. Petrofacies Assemblage 1 is composed mostly of medium- to low-grade quartz schist, quartzite, chlorite schist, phyllite, and slate rocks (Petro facies III, 15%–92.2%; IV, mostly 21.8%–27%; and V, 7.8%–63.2%) derived from the Foziling Group on the northern fl ank of the orogen. The overlying deposits, which comprise Petrofacies Assemblage 2, also contain medium- to low-grade metamorphic rocks (Petrofacies III, IV, and V) from the Foziling Group but show a marked increase in the abundance of moderately high-grade granitic gneiss and plagio gneiss rocks (Petrofacies I, 40%–61.1%), granite and metadiorite rocks (Petrofacies II, mostly 13%–38.8%) from the orogen core and the Luzhen-guan complex. The high-grade plagiogneiss and metadiorite rocks are mostly sourced from the axial Dabie Shan complex. The second un-roofi ng sequence is represented by Petrofacies Assemblages 3 and 4. Petrofacies Assemblage 3, located at the top of the Sanjianpu Forma-tion, has an increase in medium- to low-grade metamorphic source rocks (Petrofacies III, 7%–41.7%; VI, 1.7%–38%; V, 6.6%–70%) derived from the Foziling Group. Petro facies Assemblage 4, located at the base of the Fenghuangtai Formation, contains a mixture of compositions transported from all source areas (i.e., polymict) in which basic volcanic detritus (Petrofacies VI, 3.2%–10.2% in some samples) from North Huaiyang is combined with gneiss
Tracing exhumation of the Dabie Shan ultrahigh-pressure metamorphic complex
Geological Society of America Bulletin, January/February 2010 209
gravels (Petrofacies I, mostly 8%–42.1%) from a high-grade metamorphic source in the axial Dabie Shan complex. The overlying middle to upper parts of the Fenghuangtai Formation cor-respond to a third unroofi ng sequence. This part begins with dominantly reworked sandstone and mudstone clasts (Petrofacies V, 81.9%) and low-grade metamorphic phyllites, granulites, and slates (Petrofacies IV, 18.1%) sourced from the Yangshan and Foziling Groups (Petro facies Assemblage 5), and then shifts to a detrital com-position composed of abundant medium- to low-grade metamorphic rocks, e.g., quartzites, quartz schists, phyllites, slates, and metamor-phic argillites (Petrofacies III, 14%–73.7%; IV, 11.5%–45.3%; V, 10.5%–65.3%) (Petro-facies Assemblage 6), mostly from the Foziling Group. A further shift in detrital composition to an atypical polymict with a lower content of gneiss (Petrofacies I, 0%–10%) and granite (Petrofacies II, 0%–21.1%) and a metabasalt (Petrofacies VI, 0%–15%) (Petrofacies Assem-blage 7) transported from the North Huaiyang (or North Dabie Shan core and UHP/HP zones) was observed at the top of the sequence. These three unroofi ng sequences represent three dis-tinct episodes of exhumation, each beginning with unroofi ng of cover strata in the North Huai-yang and then basement rocks in the UHP/HP zone and the Luzhenguan complex, but the up-per one is mostly derived from the cover strata in the North Huaiyang.
The fi rst occurrence of relatively high-grade metamorphic rocks (Petrofacies Assemblage 2) as detrital grains in the Sanjianpu Formation in the Liushudian-Dingbachong section indicates that the UHP and HP metamorphic rocks of the Dabie Shan orogen were exhumed and exposed during Middle Jurassic time. This occurrence, along with detrital grains of UHP rocks derived from the Dabie Shan orogen appearing in the Fenghuangtai Formation of the Hefei Basin (Wang et al., 2002b), indicates that signifi cant unroofi ng of the deep core was complete by Late Jurassic time. The lithic petrofacies includ-ing plagiogneiss, metabasalt, and eclogite grav-els (UHP rocks) are well developed at the base of the Fenghuangtai Formation in the Dushan section (Fig. 3), clearly showing that the UHP and HP metamorphic rocks were exhumed at that time. In the upper part of the Fenghuang-tai Formation in the Dushan section, the lithic petrofacies abruptly change into quartzite and quartz schist mostly eroded from the Foziling Group, representing a similar lithic composition to that in the Liushudian-Dingbachong section.
The evidence for three unroofi ng cycles in the stratigraphic section suggests that there were at least three episodes of uplift and exhumation in the source area. The rapid transition between
source areas of different metamorphic grade may suggest that unroofi ng within each cycle was also related to individual structural events. The major change in depositional style of the basin fi ll in the Hefei Basin (Fig. 5), indicated by the change from the Sanjianpu Formation coarse sandstone deposits to conglomerates of the Fenghuangtai Formation, is accompanied by a signifi cant change in source-area compo-sition from Petrofacies Assemblage 3 to 4, and represents a prominent period of unroofi ng and deformation. Detailed geologic mapping found that the Fenghuangtai Formation unconform-ably overlies the Sanjianpu Formation (Liu et al., 2001a; Liu et al., 1995), providing further evidence that tectonic deformation occurred be-tween deposition of these formations.
Sanjianpu and Fenghuangtai Formations in the Zhujiacitang-Fenghuangtai Section
The modal data generated by point counting for the Sanjianpu Formation and clast count-ing for the Fenghuangtai Formation reveal the presence of all seven lithic petrofacies (Table DR1 [footnote 1], Fig. 6) derived from the North Dabie Shan core and UHP/HP zones, the Luzhen guan complex, and Foziling Group. The lithic petrofacies in the section are classi-fi ed into six petrofacies assemblages. Petro-facies Assemblage 1, in the lower part of the San jianpu Formation, has a low content of felsic fragments (I-4, 7.9%–17.6%), consisting of quartz, plagioclase, and sparse potash feldspar, granite and diorite (II, 13.9%–23.4%), and al-tered rock and rhyolite (VI, 6.1%–7.9%), and a higher abundance of low-grade metamorphic rocks (IV, 17.6%–46.8%; V-1, 23.5%–35.3%). Petrofacies Assemblage 2, in middle part of the Sanjianpu Formation, is characterized by a high percentage of granite and diorite (64.6% at most) or felsic fragments (58.8% at most), which probably represent more sedi-ments derived from the axial Dabie Shan and Luzhen guan complexes. There is an absence of quartzite and schist in Petrofacies Assemblages 1 and 2, which are usually sourced from the North Huaiyang. Petrofacies Assemblage 3, in the upper part of the Sanjianpu Formation, con-tains all the petrofacies present in Petrofacies Assemblages 1 and 2, as well as Petrofacies III (mostly 3.3%–38.2%), which is composed of quartzite and schist. Petrofacies Assem-blage 3 is characterized by a high percentage of low-grade metamorphic lithic rocks (IV, 10.2%–63.6%) and an upward increase of gra-nitic and diorite lithic fragments (II) from 0% to 43.8% and felsic fragments (I-4) from 10.8% to 24.9%, which represent increasing input from the axial Dabie Shan complex and the Luzhen-guan complex. Petrofacies Assemblage 4, at the
base of the Fenghuangtai Formation, contains a high percentage (16.7%–45%) of plagio-gneiss (I-2) and marble (I-3) probably derived from the axial Dabie Shan complex, a moder-ate percentage of red granitic gneiss and meta-granite (II-2, 25%–50%), quartzite and schists (III, 11.1%–27.8%), and volcanic rocks (VI, 5.6%–16.7%) sourced from North Huaiyang. Petrofacies Assemblage 5, at the middle part of the Fenghuangtai Formation, contains a larger percentage of red granite and granitic gneiss (II-2, 33%–50%) that is presumed to have been derived from the Luzhenguan complex, a moderate percentage of quartzite and quartz schist (III, 24.4%–43.8%), sourced from the Foziling Group or the axial Dabie Shan com-plex, a lower percentage of plagiogneiss (I-2, 0%–22.2%), white granite (II-1, 0%–11.1%), and marble (I-3, 0%–9%) derived mostly from the axial Dabie Shan complex, and a few clasts of basic volcanic rock (VI, 0%–16.7%) whose source is presumed to be North Huaiyang arc volcanics. Petrofacies Assemblage 6, at the top of Zhujiacitang-Fenghuangtai section, is char-acterized by an increase percentage of plagio-gneiss (I-2, 16.7%) and white granite clasts (II-1, 22.2%), which implies more exposure of the axial Dabie Shan complex. The lithic detri-tal composition indicates that the Luzhenguan complex and Foziling Group in North Huaiyang are the main sources of the detritus. Although the six petrofacies assemblages developed in the Zhujiacitang-Fenghuangtai section were de-rived from a mixed provenance in which various units in the Dabie Shan orogen were exposed, the unroofi ng cycles, which correspond to the six assemblages, still can be distinguished. The lower four assemblages in the Sanjianpu and the base of the Fenghuangtai Formation display two unroofi ng cycles of the basement rocks in the axial Dabie Shan complex and the Luzhen-guan complex. The upper two assemblages in the Fenghuangtai Formation, conglomerate de-posited in an environment clearly different from that of the lower Sanjianpu Formation, was mostly shed from the proximal North Huaiyang area, but was derived more from the axial Dabie Shan complex at the top of the Zhujiacitang-Fenghuangtai section, which probably repre-sents another unroofi ng cycle.
Nd ISOTOPIC CONSTRAINTS ON SOURCES OF BASIN FILL
Sm-Nd isotope studies have been shown to be a powerful tool for investigating source ter-rains of sedimentary rocks (Faure, 1986; Clauer and Chaudhuri, 1992; Clift et al., 2002). Nd isotopic composition of the detrital sediments is assumed to result from mechanical mixing
Liu et al.
210 Geological Society of America Bulletin, January/February 2010
between old eroded crust and more recent de-trital input (Henry et al., 1997; Clift et al., 2001; Clift et al., 2002). The goal of the present study is to use Nd isotopes, combined with detrital composition analysis, to verify the evolution of the sedimentary record of the Hefei Basin in or-der to identify and quantify erosion of the Dabie Shan orogen.
Nd Composition of the Basin Sediments
Details of analytical methods are given in Table DR2 (see footnote 1). The εNd values of the sediments and the source rocks were calcu-lated or recalculated by using published age data (t = 176 Ma) for the time at which deposition and erosion began, and TDM (Nd crust residence times) was calculated using present-day ratios (147Sm/144Nd)DM = 0.2137 and (143Nd/144Nd)DM = 0.51315. The Sm-Nd data are plotted in Figures 4, 5, and 6 as a function of depth, and listed in Table DR2 (footnote 1) for each section. The vertical evolution of the Nd composition rep-resents the changes of contribution of different source rocks to the basin sediments.
The Nd composition in the Zhuji Formation in the Wumiao section is characterized by the highest εNd values, ranging from –13.8 to –11.3, and the lowest 143Nd/144Nd values, ranging from 0.511829 to 0.511966, in the three sections (Fig. 4; Table DR2 [footnote 1]). These values are clearly distinguishable from those of other sections and they suggest the sediments were derived from different sources. The TDM values of this formation are relatively low, ranging from 1.7 Ga to 2.0 Ga. The Nd isotopic values do not exhibit any clear trends, but they can be conveniently divided into three parts (cor-responding to Petrofacies Assemblages 1–3). Petrofacies Assemblage 1, at the base of the formation, is characterized by an upward de-crease in TDM from 2.0 to 1.8 Ga and an increase in εNd values from –13.3 to –11.3. In Petro facies Assemblage 2, the 147Sm/144Nd ratios and TDM values vary between 0.1009 and 0.1167, and between 1.7 and 1.9 Ga, respectively, and have an upward increase trend. In Petrofacies Assem-blage 3, TDM values increase upward from 1.8 to 1.9 Ga (Fig. 4). The Nd isotopic values of these sediments indicate one unroofi ng cycle in the lower-middle part and a beginning of an-other unroofi ng cycle in the upper part in the source terranes, which are similar to the lithic fragment cycles demonstrated by detrital com-position analysis, and suggest derivation from reworked sedimentary cover rocks to medium- to low-grade metamorphic rocks with relatively high εNd values.
Because the lithic fragments in Liushudian-Dingbachong section show clear unroofi ng
cycles in source areas (Liu et al., 2003), here we only tested the Nd composition on three sand-stone samples. The samples from the Sanjianpu Formation (Fig. 5; Table DR2 [footnote 1]) have remarkable isotopic characteristics; they have the highest 147Sm/144Nd values, ranging from 0.1168 to 0.1266, of all of the basin sediments and lower εNd values, ranging from –15.0 to –14.5, than those in the Wumiao section (Fig. 4; Table DR2 [footnote 1]). This suggests that the sediments of the Sanjianpu Formation were de-rived from a mixed axial Dabie Shan complex and North Huaiyang origin, including eclog-ite from the UHP/HP zone whose 147Sm/144Nd value is high.
Samples from the Zhujiacitang-Fenghuangtai section are characterized by the lowest εNd val-ues in the Hefei Basin, which range from –22.0 to –14.6 (Fig. 6; Table DR2 [footnote 1]), indica tive of sediments largely sourced from the basement of the axial Dabie Shan complex. The Nd isotopic composition regularly changes vertically along this section. In the lower part of the Sanjianpu Formation, Petrofacies As-semblage 1, the εNd and 147Sm/144Nd values are relatively constant at –15.4 and 0.1033–0.1067, respectively. In Petrofacies Assemblage 2, the εNd and 147Sm/144Nd values of sample 03-11-53 are –14.6 and 0.1057, respectively, and show minor differences with those from Petrofacies Assemblage 1. The upper part of the Sanjianpu Formation, Petrofacies Assemblage 3, exhibits decreasing εNd values from –19.4 (sample 03-11-49) to –22.0 (sample 03-11-47), which suggest that a basement rock with a low εNd value was gradually unroofed. The Fenghuang-tai Formation, Petrofacies Assemblages 4, 5, and 6, has medium values of εNd, ranging from –17.5 to –16.0 with the lowest 147Sm/144Nd val-ues from 0.0937 to 0.1024. But it also displays some Nd composition changes, and at Petro-facies Assemblages 4 and 6, for example, values of εNd show more negative relative to the others . This suggests two unroofi ng events in the source area in the Sanjianpu and Fenghuangtai Formations. The lower four assemblages (from Petrofacies Assemblages 1–4) record the fi rst unroofi ng process from cover strata in North Huaiyang to the basement in the axial Dabie Shan complex, which is little different from the results of lithic fragment analyses. The second unroofi ng event, in Fenghuangtai Formation, mostly supplies sediment from the medium- to low-grade metamorphic rocks and Luzhenguan granites with relatively high εNd values in North Huaiyang. This event is also demonstrated by lithic fragment analyses. The source difference between these two unroofi ng events likely in-dicates occurrence of a structural event in the early Late Jurassic.
End Members of Source Rocks and Their Contribution to Basin Filling
The plot of εNd values as a function of 147Sm/144Nd ratios (Fig. 8A) shows the general trend of the Nd isotopic composition in the basin sediments and suggests that the different sections have diverse provenances. The diagram indicates that plotted points for each section are relatively centralized, and that the εNd values of the sediments for all sections generally decrease from the western Wumiao section to the eastern Zhujiacitang-Fenghuangtai section. The regular variations in the Nd isotopic composition sug-gest that sediments in each section were derived from a variety of source rocks. Here, we attempt to determine the end-member source rocks for each section (Table DR3 [footnote 1]) and cal-culate their contribution to the basin sediments.
Defi nition of End Members of Source Rocks for Each Section
(1) The Zhuji Formation in the Wumiao sec-tion is located in the western part of the Hefei Basin. Its possible source rocks in the southern North Huaiyang and axial Dabie Shan complex include the Yangshan Group, the Guishan and Nanwan Formations of the Xinyang Group, the Dingyuan Formation, the Luzhenguan complex, and basement rocks in North Dabie Shan core and the UHP/HP zones. The plot of εNd versus 147Sm/144Nd ratios (Fig. 8B) for the Zhuji For-mation and the presumed source rocks suggests that at least three end members are needed to explain the range of the data. The clast-count and point-count data suggest three end-member lithologies: (A) argillaceous sandstone of the Yangshan Group; (B) slate, phyllite, and chlo-rite schist of the Nanwan and Guishan Forma-tions and metavolcanic rocks of the Dingyuan Formation; and (C) granite of the Luzhenguan complex. These three end members have typical upper crustal 147Sm/144Nd ratios (0.09–0.12), and εNd values of about –18.8 to –6.5 (Fig. 8B; Table DR3 [footnote 1]). End-member A has an aver-age εNd value of –7.5 (weighted average εNd of i samples = Σ(εNdi × [Nd]i)/Σ[Nd]i, where [Nd] is Nd concentration) and an average 147Sm/144Nd ratio of 0.111 (Xu et al., 2005). These values defi ne the upper end member of the plot of the Zhuji Formation sandstones. End-member B has a wider range of εNd values (–12.6 to –8.4) and 147Sm/144Nd ratios (0.11–0.19, but mostly 0.11–0.15) because of complex rock composi-tions (Li et al., 2001; Xu et al., 2005). This end member has an average εNd value of –11.0 and an average 147Sm/144Nd ratio of 0.134, which defi nes the right-hand end member of the data from the Zhuji Formation in Figure 8B. The Luzhenguan complex has a variety of rock types, but granite
Tracing exhumation of the Dabie Shan ultrahigh-pressure metamorphic complex
Geological Society of America Bulletin, January/February 2010 211
0.0
70
.09
0.1
10
.13
0.1
50
.17
–6
–8
–1
0
–1
2
–1
4
–1
6
–1
8
0.0
90
.10
.11
0.1
20
.13
0.1
40
.15
–6
–8
–1
0
–1
2
–1
4
–1
6
–1
8
–2
0
–2
2
–2
4
–2
6
–2
8 0.0
60
.08
0.1
00
.12
0.1
40
.16
0.1
8–
9
–11
–1
3
–1
5
–1
7
–1
9
–2
3
0.0
70
.08
0.0
90
.10
.11
0.1
2–
10
–1
5
–2
0
–2
5
–3
0
–3
5
En
d-m
em
be
r D
En
d-m
em
be
r F
(B)
WM
Se
ctio
n
( )
LD
Se
ctio
n(D
) Z
F S
ect
ion
C(A) A
ll S
ect
ion
s
En
d-m
em
be
r C
S
m/ N
d147
144
S
m/ N
d147
144
S
m/ N
d147
144
S
m/ N
d147
144
Nd
0.1
9
Nd–
20
0.2
–2
1
Nd
0.1
3
Nd
Pre
sum
ed
ero
de
d ro
cks
(ave
rag
es)
En
d-m
em
be
r AE
nd
-me
mb
er
BE
nd
-me
mb
er
CE
nd
-me
mb
er
C′
En
d-m
em
be
r E
Se
dim
en
ts
Liu
shu
dia
n-D
ing
ba
cho
ng
(LD
) S
ect
ion
Zh
ujia
cita
ng
-Fe
ng
hu
an
gta
i (Z
F)
Se
ctio
n
Wu
mia
o (W
M)
Se
ctio
n
Sa
nd
sto
ne
in Y
an
gsh
an
Fm
. as
En
d-m
em
be
r AM
eta
cla
stic
s, s
late
, ph
yllit
e a
nd
chlo
rite
sch
ist i
n N
an
wa
n a
nd
Gu
ish
an
Fm
s. a
nd
me
tavo
lca
nic
s in
D
ing
yua
n F
m. a
s E
nd
-me
mb
er
BG
ran
ite
in L
uzh
en
gu
an
com
ple
x a
s E
nd
-me
mb
er
C
Fe
lsic
gn
eis
s a
nd
ecl
og
ite
in U
HP
/HP
zo
ne
as
En
d-m
em
be
r E
Qu
art
z sc
his
t an
d q
ua
rtzi
te in
Fo
zilin
g G
rou
p a
s E
nd
-me
mb
er
D
Gn
eis
s a
nd
gra
nu
lite
in
No
rth
Da
bie
Sh
an
core
zo
ne
an
d m
eta
pe
lite
in
UH
P/H
P z
on
e a
s E
nd
-me
mb
er
F
Gra
nit
e in
Lu
zhe
ng
ua
n co
mp
lex
an
d m
eta
-ig
ne
ou
s ro
cks
in U
HP
/HP
a
s E
nd
-me
mb
er
C′
En
d-m
em
be
r A
En
d-m
em
be
r B
En
d-m
em
be
r C
En
d-m
em
be
r E
En
d-m
em
be
r D
En
d-m
em
be
r C
′
En
d-m
em
be
r D
En
d-m
em
be
r F
Qu
art
z sc
his
t an
d q
ua
rtzi
te in
Fo
zilin
g G
rou
p a
s E
nd
-me
mb
er
D
Gra
nit
e in
Lu
zhe
ng
ua
n co
mp
lex
as
En
d-m
em
be
r C
Fig
ure
8. ε
Nd v
ersu
s 14
7 Sm
/144 N
d. T
his
diag
ram
illu
stra
tes
the
mix
ture
of
vari
ous
end
mem
bers
, wit
h th
e co
mpa
riso
n be
twee
n 14
7 Sm
/144 N
d ra
tios
and
εN
d val
ues
obta
ined
for
the
ba
sin
sedi
men
ts a
nd t
hose
for
end
mem
bers
tha
t w
ere
calc
ulat
ed b
y pu
blis
hed
data
in T
able
DR
3 (f
ootn
ote
1) (
see
text
for
dis
cuss
ion)
.
Liu et al.
212 Geological Society of America Bulletin, January/February 2010
and metagranite are the most abundant grains and clasts in the basin sediments and are taken as end-member C. End-member C is characterized by lower εNd values (–18.8 to –13.9) and lower 147Sm/144Nd ratios (0.076–0.1) than the other sources, and has an average εNd of –16.0 and an average 147Sm/144Nd ratio of 0.095 (Chen et al., 2003a). These three end members encompass all the data. Two samples (2003-11-19 and 208-14) lie near the boundary defi ned by the end-members B and C, suggesting the possible presence of another end member, which is located below the boundary in Figure 8B. This suggests that metamorphic rocks in the UHP/HP and North Dabie Shan core zone units, which have compo-sitions matching the above condition, might have contributed minor detritus to these sediments.
(2) Point-count and fi eld clast data for the Liushudian-Dingbachong section clearly in-dicate that the source rocks of the Sanjianpu Formation were high-grade metamorphic rocks from the axial Dabie Shan complex, medium- to low-grade metamorphic rocks from the Foziling Group, and granitic rocks from the Luzhenguan complex, all of which lie south of the section. Here, we test three samples from the middle and upper parts of the Sanjianpu Formation (corresponding to Petrofacies Assemblages 2 and 3), which are shown in the plot of εNd ver-sus 147Sm/144Nd ratios (Fig. 8C). At least three end members are needed to explain the range of the data. The medium- and low-grade meta-morphic rocks, quartz schist, and quartzite in the Foziling Group represent end-member D, which defi nes source rocks with large negative εNd values (–14.2 to –11.9) and low 147Sm/144Nd ratios (0.10–0.12) (Li et al., 2001; Xu et al., 2005). Thus, end-member D has an average εNd value of –12.5 and an average 147Sm/144Nd ratio of 0.114 from published data. The Luzhenguan granite, with lower Sm/Nd ratios and εNd values, is taken as the second end member (C).
To account for the high Sm/Nd ratios in Figure 8C, another end member is needed, one with a 147Sm/144Nd ratio of at least 0.13. The best candi-date for this source is high-grade eclogites in the UHP/HP zone, which have 147Sm/144Nd ratios of 0.14–0.19 and εNd values >–18. Gneisses in the UHP/HP zone can be divided into two categories based on their Nd composition: one group has relatively low TDM values of 1.5 Ga to 1.9 Ga, εNd values (at 176 Ma) of –11.0 to –7.0 and Sm/Nd ratios of 0.11–0.13; the second group has TDM values of 2.1 Ga to 3.3 Ga, εNd values (at 176 Ma) of –22.4 to –12.2 and Sm/Nd ratios of 0.13–0.15 (Li et al., 1993; Li et al., 2001; Xie et al., 1996; Chavagnac and Jahn, 1996; Chen and Jahn, 1998; Ma et al., 2000; Ames et al., 1996; Liou et al., 1997). The second group probably represents the Yangtze basement in the
UHP/HP source terrane (Ma et al., 2000). The Nd composition of the gneiss (Fig. 8C) clearly shows that the latter, but not the former, fi ts an end-member composition for the sediments in the Liushudian-Dingbachong section. The spread of data in Figure 8C suggests the pres-ence of a third end member (E) characterized by high Sm/Nd ratios and εNd values between –17 and –13. This agrees with the observation that the UHP/HP zone constituted a major sedi-ment source in the middle part of Hefei Basin. We propose an end member (E) with an average εNd of –14.3 and an average 147Sm/144Nd ratio of 0.147 based on the exposed UHP/HP units.
The three proposed end members (C, D, and E) can explain the isotopic character of the Middle Jurassic sediments in the central part of the middle Hefei Basin. However, the three sam-ples from this section lie near or on the boundary between end-members C and E, suggesting that the sediment was predominantly derived from the UHP/HP terrane and the Luzhenguan complex.
(3) The Sanjianpu and Fenghuangtai Forma-tions in the Zhujiacitang-Fenghuangtai section, located in the eastern part of the Hefei Basin, have source rocks that are quite different from those in the west. Although the lithic petro-facies are similar, the isotopic character of the sediments is distinctly different. A plot of εNd versus 147Sm/144Nd ratios (Fig. 8D) shows that sediments in this section have low Sm/Nd ratios of 0.093–0.107 and very low εNd values of –22.0 to –14.6. To account for the range of the data, at least three end members are needed. An abun-dance of granite and low- to medium-grade metamorphic lithic fragments in the sediments suggests that the Foziling Group and Luzhen-guan complex can be end members for this section. Some of the metaigneous rocks (Dong et al., 1996) in the UHP/HP zone have isotopic compositions similar to those of the Luzhen-guan complex granites, so this material is added to the granites to form a new end member (C′). The average εNd value and 147Sm/144Nd of this end member are –16.0 and 0.092, respectively.
Thus, end-members C′ and D defi ne the upper boundary of the data in the plot of εNd versus 147Sm/144Nd ratios.
Due to the very low εNd values of sedimen-tary rocks in this section, an end member (F) with a low εNd value (<–22) is needed. All rocks in the axial Dabie Shan complex and the North Huaiyang, except high-grade metamorphic rocks in the North Dabie Shan core zone (Xie et al., 1996; Ma et al., 2000; Ge et al., 2001) and low-grade metamorphic rocks in the UHP/HP zone, the basement-cover rocks (Li et al., 2001; Schmid et al., 2003) (Table DR3 [footnote 1]), have higher values. Thus, we propose a new end member (F) that is composed of gneiss and granulite from the North Dabie Shan core zone and low-grade metamorphic rocks, the metapelite, in the UHP/HP zone. This end-member F has an average εNd value of –26.7 and an average 147Sm/144Nd ratio of 0.097.
(4) Summarizing the above discussion, we recognize six end members in the source area for the three sections. The end-member compo-sition is taken as the average of the published data (Table DR3 [footnote 1]) and is shown in Table 1. The specifi c end members and their compositions may deviate somewhat from the ones we defi ned, but our selection encompasses the total character and trend of the provenance.
Contribution of End Members to the Basin Sediments
Having characterized the different end mem-bers, we can now quantify the erosion of the Dabie Shan orogen by calculating the relative contribution of each end member for different sections and different stratigraphic levels. For our calculations we used the following mixing equations (Faure, 1986):
Σ(fi × [Nd]i × Xi) = Xm × Σ(fi × [Nd]i),
where Xm is either εNd value or 147Sm/144Nd ratio of each sediment sample; Xi is either εNd value or 147Sm/144Nd ratio of end member; and fi is the
TABLE 1. END-MEMBER COMPOSITIONS FOR MIXING CALCULATIONSEnd member Unit
Sm(ppm)
Nd(ppm) 147Sm/144Nd εNd(t)
F Gneiss and granulite in NDC and metapelite in UHP/HP zone
6.54 41.2 0.097 –26.7
E Felsic gneiss and eclogite in UHP/HP zone 4.54 19.47 0.147 –14.3D Quartz schist and quartzite in Foziling Group 4.5 24.11 0.114 –12.5C′ Granite in Luzhenguan complex and
metamagmatite in UHP/HP zone 5.22 33.18 0.092 –16.0
C Granite in Luzhenguan complex 5.54 33.96 0.095 –16.0B Metaclastics, slate, phyllite, and chlorite schist in
Nanwan and Guishan formations and metavolcanics in Dingyuan Formation
5.45 26.03 0.134 –11.0
A Sandstone in Yangshan Formation 7.10 38.80 0.111 –7.5Note: See text for discussion. Abbreviations: NDC—North Dabie Shan core zone;
UHP/HP—ultrahigh-pressure–high-pressure.
Tracing exhumation of the Dabie Shan ultrahigh-pressure metamorphic complex
Geological Society of America Bulletin, January/February 2010 213
weight percentage of each end member i, [Nd]i being the mass of Nd in each end member i.
To a fi rst approximation, this equation as-sumes that the Sm-Nd compositions in the ba-sin sediments result from mechanical mixing between the end-members A, B, C (C′), D, E, and F. The abundance of rock fragments and coarse detrital minerals supports this model (Henry et al., 1997). Because the sum of fi is
equal to 1, we can determine the percentage contribution of three end members by using the 147Sm/144Nd ratios and εNd values, which are in-dependent variables, for almost all samples in the three sections. Weathering and reheating from magmatic activity in the Cretaceous al-tered some major, trace, and rare earth elements in the sediments, so we do not use these ele-ments to determine the percentages of the end
members, but the isotopic character of the sedi-ments should not have been affected.
Figure 9 shows the relative contribution of each source area, calculated for each sample in all sections. These results refl ect variations observed in the petrofacies assemblages, the 147Sm/144Nd ratios, and the TDM and εNd values of the sediments (Figs. 3–5), which allow the depositional series to be divided into different
03-11-19
03-11-28
03-11-27
03-11-26
208-22
208-21208-20208-19
208-17
208-14
208-11
208-8
03-11-22
03-11-20
0
50
100m 03-11-1
1000m
500
0
03-11-5
03-11-8
03-11-35
03-11-36
03-11-53
03-11-50
03-11-49
03-11-4803-11-47
03-11-46
03-11-45
03-11-44
03-11-42
03-11-41
0
100
m200
0% 20% 40% 60% 80% 100%0% 20% 40% 60% 80% 100% 0% 20% 40% 60% 80% 100%
(A) Wumiao (WM) Section (B) Liushudian-Dingbachong(LD) Section
(C) Zhujiacitang-Fenghuangtai(ZF) Section
2A
P1
AP
2A
P1
AP
Are
bm
em-
dn
E
Cr
eb
me
m-d
nE
Br
eb
me
m-d
nE
D
Dr
eb
me
m-d
nE
C′
re
bm
em-
dn
E
3A
P
Er
eb
me
m-d
nE
Cr
eb
me
m-d
nE
Fr
eb
me
m-d
nE
1A
P2
AP
3A
P
3A
P4
AP
5A
P6
ci ssa r
uJel
ddi
Mciss
aruJ
re
pp
U
Figure 9. Relative proportions of end-members A, B, C (C′), D, E, and F versus depth for each section. End-member A—sandstone in Yangshan Formation; end-member B—metaclastics, slate, phyllite, and chlorite schist in Nanwan and Guishan Formations and metavol-canics in Dingyuan Formation; end-member C—granite in Luzhenguan complex; end-member C′—granite in Luzhenguan complex and meta igneous rocks in the UHP/HP zone; end-member D—quartz schist and quartzite in Foziling Group; end-member E—felsic gneiss and eclogite in the UHP/HP zone; end-member F—gneiss and granulite in North Dabie Shan core zone and metapelite in the UHP/HP zone. See text for discussion. Abbreviation: PA—petrofacies assemblage.
Liu et al.
214 Geological Society of America Bulletin, January/February 2010
cycles, almost the same as Petrofacies Assem-blages, in each section. In addition, we can draw several conclusions regarding the erosion of the Dabie Shan orogen during the Middle and Late Jurassic.
(1) In the Zhuji Formation in the Wumiao section, the sources of the sediments were mostly end-members A, B, and C, which sug-gest that the Paleozoic sedimentary sequence and Late Proterozoic granite in the North Huai-yang were exhumed and eroded. According to variations in the relative proportions of the end members, three parts, which are the three Petro-facies Assemblages, are observed (Fig. 9A). In the Petrofacies Assemblage 1, the percentage of the end-member C decreases from 49 wt% to 30 wt%, while end-member B remains rela-tively constant (35.7 wt% to 47.0 wt%), and end-member A increases from 4 to 34 wt%. These variations suggest an increasing contribu-tion from the Yangshan Group, and a decreasing input from the Nanwan, Guishan, and Dingyuan Formations and the Luzhenguan granite, and the sediment sources were mostly from the Paleo-zoic sedimentary sequence. Petrofacies Assem-blage 2 is characterized by a high percentage of end-members C and B between 34 wt% and 60 wt% and 0 wt% and 49 wt%, respectively, and a relatively lower percentage of end-members A between 6 wt% and 43 wt%. This suggests an increase in sediment derived from the Luzhen-guan complex and the low- and medium-grade metamorphic clastic and volcanic rocks of the Nanwan, Guishan, and Dingyuan Formations and a decreased contribution from the Yangshan Formation. From Petrofacies Assemblage 1 to 2, an unroofi ng process from the Yangshan re-worked sedimentary rocks to the Luzhenguan complex basement is demonstrated by both lithic fragment and Nd composition analyses. Contribution of end-members A, B, and C for Petrofacies Assemblage 3 keeps a similar level to the Petrofacies Assemblage 1, which prob-ably represents the beginning of another unroof-ing event characterized by the sources mostly from the Paleozoic sedimentary sequence (end-members A and B).
(2) The sources for the Sanjianpu Forma-tion in the Liushudian-Dingbachong section are quite different from those of its equivalent, the Zhuji Formation in the Wumiao section. Sediments in the Sanjianpu Formation contain detritus derived from the Luzhenguan granite (end-member C), quartz schist and quartzite of the Foziling Group (end-member D), and felsic gneiss and eclogite of the UHP/HP zone (end-member E) (Fig. 9B). The contribution of end-member E decreases upward from 55 wt% to 37 wt%, and that of end-member C increases upward from 29 wt% to 50 wt%, whereas the
content of end-member D remains essentially constant at ~16 wt%. Variations of end-member contributions can be matched with variations of Qt-F-L and Qm-P-K, and lithic composition in Petrofacies Assemblages 2 and 3 (Fig. 5), sug-gesting that the end members can be positively correlated with the detrital composition.
(3) The sources of the sediments in the San-jianpu and Fenghuangtai Formations in the Zhujiacitang-Fenghuangtai section were mainly end-members C′, D, and F, suggesting intensive exhumation and erosion of the axial Dabie Shan complex and the North Huaiyang (Fig. 9C). The variations observed in clast content, εNd values, and TDM (Fig. 6) point to six depositional petro-facies assemblages. In Petrofacies Assemblage 1, the lower part of the Sanjianpu Formation, the relative high proportions (49 wt% to 63 wt%) of quartz schist and quartzite (end-member D) suggest that the main sediment source was the Foziling Group in the North Huaiyang. In Petro-facies Assemblages 2, 3, and 4 the contribution of end-member F (basement gneiss in North Dabie Shan core zone and metapelite in the UHP/HP zone) increases from 7 wt% to 59 wt% as the contribution from the Foziling Group (D) decreases from 61 wt% to 5 wt%. The granite and metaigneous rock source (end-member C′) has an average contribution of ~35 wt%. In Petrofacies Assemblages 5 and 6, the most part of Fenghuangtai Formation, the contribution from end-member C′ is very high, increasing from ~52 wt% to 90 wt% at Petrofacies Assem-blage 5 and then decreasing to 54 wt% at Petro-facies Assemblage 6, while end-members D and F decrease from 36 wt% to ~7 wt% and from 12 wt% to 3 wt% at Petrofacies Assemblage 5, then increase to 24 wt% and 22 wt% at Petro-facies Assemblage 6, respectively. Variations in the proportions of end-members D, C′, and F agree with those of lithic composition discussed above and Qm, F, and Lt to some extent. The Lt content is probably related to the proportion of end-member D, and Qm and F contents are mostly associated with end-members C′ and F. The lower four petrofacies assemblages in the section clearly demonstrate an unroofi ng event from the cover strata in the North Huaiyang to the basement in the axial Dabie Shan complex in the Middle Jurassic and earliest Late Jurassic, al-though the lithic fragment analyses represent the two, and the upper two petrofacies assemblages probably suggest another unroofi ng event in the Late Jurassic.
DISCUSSION
Investigations of the space-time variations of the sediment sources for the Hefei Basin are now developed enough to formulate a con-
strained and testable tectonic model for the ex-humation of the axial Dabie Shan complex and North Huaiyang region.
(1) Regional tectonic analysis indicates that the Jurassic contractional deformation in east China is most likely an intraplate response to the northwestward subduction of paleo-Pacifi c plate, the postcollisional shortening along the Qinling-Dabie suture zone, collision of the amal-gamated blocks of China with those of Siberia along the Mongol-Okhotsk suture zone, and accretion of crustal blocks in southern China along the Bangong suture (Yin and Nie, 1993; Yin and Nie, 1996; Ratschbacher et al., 2000; Davis et al., 2001; Liu et al., 2007). Subsequent shortening that continued from Triassic through Jurassic time after the fi nal collision between the North China and Yangtze blocks controlled the evolution of a mountain-basin system in Dabie Shan orogen. The foreland and hinterland of the Dabie Shan orogen thrust southward and north-ward, respectively, formed the northern Yangtze fold and thrust belt and the North Huaiyang fold and thrust belt, which controlled the formation of the middle Yangtze and the Hefei foreland basins at both sides of the Dabie Shan orogen (Liu et al., 2003) (Figs. 1 and 2). 40Ar/39Ar metamorphic K-feldspar ages of ca. 180 Ma in the axial Dabie Shan complex indicate that the present explosive level was widely at ≥250 °C (Hacker et al., 2000). The Middle-Late Jurassic exhumation record in the Hefei Basin started when the now exposed rocks of the axial Dabie Shan complex were already at mid-crustal levels (25 km) (because a subduction-type geothermal gradient of ~10 °C/km must have prevailed after the UHP orogenesis [Ratschbacher et al., 2000]), and stopped when the present exposure level was at 4–8 kbar depth (an average depth of 20 km) at 150–130 Ma (intrusion depth of most of the Cretaceous plutons) (Ratschbacher et al., 2000). Therefore the rocks in the Dabie Shan oro-gen were exhumed by ~5 km thickness during Middle-Late Jurassic (from ca. 180 Ma to 150–130 Ma). Because of Cretaceous transtensional deformation that followed the Jurassic exhu-mation considered here, the present area of the Dabie Shan orogen has increased (Ratschbacher et al., 2000). Assuming the area of the Dabie Shan orogen to the east of Xinyang city (Fig. 1A) prior to Cretaceous extension was equal to that of the Hefei Basin, 60% of ~5 km thickness rocks exhumed in the Dabie Shan orogen were transported into the Hefei Basin in which the average thickness of the Middle-Upper Jurassic sedimentary rocks is ~3000 m. Therefore, most of exhumation in the Dabie Shan orogen was driven by tectonic unroofi ng.
(2) The source rocks for the sediments in the Jurassic Hefei Basin in the Dabie Shan orogen
Tracing exhumation of the Dabie Shan ultrahigh-pressure metamorphic complex
Geological Society of America Bulletin, January/February 2010 215
mainly include the North Dabie Shan core UHP/HP zones, the Luzhenguan complex, the Dingyuan Formation, the Guishan Formation, the Foziling Group (Nanwan Formation), and the Yangshan Group. One sediment source for the eastern Zhujiacitang-Fenghuangtai section is end-member F. It has the lowest εNd value and highest TDM, and is composed of low-grade metamorphic rocks from the UHP unit (e.g., metapelite) and the components of gneiss and granulite from the North Dabie Shan core zone. A study by Schmid et al. (2003) showed that the apparently low-grade metamorphic rocks were affected by UHP metamorphism, and a U-Pb single zircon age of 761 ± 33 Ma for volcano-clastic rocks demonstrates a similar Neo protero-zoic age to zircon ages from the gneisses in the UHP unit (e.g., Hacker et al., 1998). The low-grade metamorphic rocks, including metapelite, arkosic sandstone, carbonate, and metaigneous rock, as basement-cove sequences, belong to the same UHP terrane as the basement gneiss. End-member F is quite different from the North Dabie Shan core zone metamorphic ter-rane that has εNd values of about –32.4 to –17.0 (t = 176 Ma), TDM values of 2.1–3.1 Ga, and Sm/Nd ratios of 0.082–0.114 (Xie et al., 1996; Ma et al., 2000; Zheng et al., 2000; Li et al., 2001; Ge et al., 2001), and was not unroofed until the end of the Late Jurassic (Hacker et al., 1998). The above geological evidence suggests
that there is a unit, end-member F, located be-tween the modern UHP and HP metamorphic rocks and the North Dabie Shan core zone high-grade metamorphic rocks in the Jurassic of the Dabie Shan orogen, which is a mix of high-grade and apparently low-grade metamor-phic basement-cover rocks. This end member supplied a signifi cant amount of sediment to the Sanjianpu Formation in the Zhujiacitang-Fenghuangtai section. The high-grade compo-nent of the end-member F probably represents the Yangtze basement, the roots of the Dabie Shan orogen (Ma et al., 2000), and was under-thrusted below the UHP/HP zone postcollision.
(3) Provenance analyses of the sediments in the Hefei Basin clearly demonstrate that the depth and rate of exhumation in the axial Dabie Shan complex and the North Huaiyang increases from the west to the east. The provenance for the Hefei Basin changes from the cover rocks in the North Huaiyang to the UHP/HP and the North Dabie Shan core zone metamorphic rocks (probably including the Yangtze basement) east-ward. The unroofi ng ages of the UHP and HP metamorphic rocks in the Dabie Shan orogen are progressively younger (from Early Jurassic to Late Jurassic) westward (Fig. 10).
Study of the Feixi section by Li et al. (2005) shows that the UHP and the HP metamorphic rocks in the eastern part of Dabie Shan orogen were exhumed during the Early Jurassic. Our
current study of the Zhujiacitang-Fenghuangtai section shows that the North Dabie Shan core zone metamorphic rocks were fi rst exhumed in the Middle Jurassic, which suggests that UHP and HP metamorphic rocks above the North Dabie Shan core zone in the source area were unroofed before that time. If we choose the deepest subduction age and unroofi ng age of the UHP and HP rocks as 240 Ma and 190 Ma, respectively, and a subduction depth of 125 km, the average rate of exhumation is ~2.5 mm/a.
Provenance analyses in the Jinzhai and Dushan , the middle part of the southern Hefei Basin, show that the UHP/HP eclogite and felsic gneiss have high TDM values, but the North Dabie Shan core zone metamorphic rocks, in the Dabie Shan orogen were exhumed and eroded to provide detritus deposited in the Liushudian-Dingbachong section during the early Middle Jurassic at ca. 170 Ma. The exhu-mation rate in the middle part of the Dabie Shan orogen is ~1.8 mm/a.
Because we do not know when the UHP and HP rocks in the Dabie Shan orogen were eroded and transported to the basin in the Wumiao sec-tion, we cannot constrain the exhumation rate for these rocks from this study. The Wumiao sec-tion is located at the western part of the north-ern Dabie Shan orogen. If we assume that the unroofi ng age of the UHP and HP rocks was ca. 150 Ma, based on a signifi cant change in
WM LD DS
ca. 176 Ma
ca. 161 Ma
E
ca. 160 Ma
XFFZ
Unroofing ofEnd-memberF
ca. 190 Ma
?
HP
UHP
NDC
?
Unroofing ofEnd-member C
ca. 176 Ma
ca. 176 Ma
ca. 146 Ma
ca. 150 Ma
Blanket stratigraphy
ca. 2.5 mm/a
ca. 1.8 mm/aca. 1.4 mm/a
Exhumation depth
at J /Jboundary
12
Exhumation
depth at late J3
ca. 170 Ma
Co-occurrenceof high-Siphengitesand Triassic zircons
Occurrenceof eclogitegravels insediments
Unroofing ofEnd-member E
0 10 20 km
20
40
0
km
Figure 10. Simplifi ed map showing the spatial distribution of the representative end member or eclogite gravels, high-Si phengites, and Triassic zircons contributed to Wumiao (WM), Liushudian-Dingbachong (LD), Dushan (DS), Zhujiacitang-Fenghuangtai (ZF), and Feixi (FX) sections in the Hefei Basin, to demonstrate the depth of exhumation increasing from west to east and eastward increasing exhumation rate in Dabie Shan orogen during Jurassic. End-member C—granite in Luzhenguan complex; end-member E—felsic gneiss and eclogite in ultrahigh pressure (UHP) and high pressure (HP) zone; end-member F—gneiss and granulite in North Dabie Shan core zone and metapelite in the UHP/HP zone. Arrows indicate exhumation rate. See text for discussion. Abbreviation: NDC—North Dabie Shan core zone.
Liu et al.
216 Geological Society of America Bulletin, January/February 2010
the composition of detrital micas in samples from the Zhuji and Duanji Formations (Li et al., 2005), the exhumation rate would be ~1.4 mm/a.
(4) The Jurassic stratigraphy in this foreland basin clearly represents a coarsening-upward sequence and a sharp boundary between the Sanjianpu (Zhuji, Yuantongshan) and Feng-huangtai (Zhougongshan, Duanji) Formations characterized by changes in grain-size, deposi-tional environment, and sediment provenance, in response to backthrusting tectonic activity in the northern part of Dabie Shan orogen (Figs. 2B and 2C). The composition of lithic petro-facies and the contribution of end members in petrofacies assemblages in the Hefei Basin exhibit both blended clast compositions and in-verted clast stratigraphy; the inverted clast stra-tigraphy represents unroofi ng sequences that we interpret to record individual thrust faulting events in the Dabie Shan orogen (Steidtmann and Schmitt, 1988).
During the middle and earliest Late Jurassic, the Zhujiacitang-Fenghuangtai area was fi lled with sediments mostly transported from sources within the Foziling Group and Luzhen guan com-plex and in the gneisses of the Yangtze basement and low-grade metamorphic basement-cover rocks of the UHP/HP zone. The proportion of the latter sediment increases upward from Petro-facies Assemblages 1–4 and demonstrates com-bination stratigraphy of inverted and blended clast compositions, suggesting that unroofi ng of the upper North Huaiyang preceded uplift of the axial Dabie Shan complex, and the North Dabie Shan core zone (the Yangtze basement) and UHP and HP rocks continuously uplifted and progres-sively unroofed as compressional thrusting along the northern margin of the Dabie Shan orogen. According to depositional systems developed in the Feixi and Zhujiacitang-Fenghuangtai sections in the eastern part of the Hefei Basin (sandy, braided channel plains), the exhuma-tion rate in the Dabie Shan orogen was low. Rapid unroofi ng is recorded only in the middle part of the Hefei Basin as seen in the Dushan and Liushudian-Dingbachong sections that are mostly composed of conglomerate. The unroof-ing process in the Liushudian-Dingbachong in middle-earliest Late Jurassic is characterized by two exhumation cycles from the North Huai-yang stratigraphy to the UHP and HP rocks, which exhibit typical inverted clast stratigraphy with blended clast compositions at the tops. The second cycle of unroofi ng continued to the early Late Jurassic, at which time the UHP and HP metamorphic rocks and North Huaiyang stratig-raphy driven by thrusting were widely exposed. The western part of the Hefei Basin in the Wu-miao section in Middle Jurassic was fi lled by sediments eroded from cover strata to Luzhen-
guan basement in the North Huaiyang and re-cording one unroofi ng event and a beginning of another one, implying that the UHP and HP metamorphic rocks were not widely exhumed at this time.
Late Jurassic sediments in the Hefei Basin are mostly boulder and cobble conglomerate deposited by alluvial-fan and braided channel systems. The source rocks for these sediments were mostly from the Luzhenguan complex in the North Huaiyang, although the Yangtze basement in the axial Dabie Shan complex supplied some material to the Zhujiacitang-Fenghuangtai section with upward increased proportion in the east (Fig. 9). The source rocks for the Liushudian-Dingbachong section were mostly from the Foziling and Yangshan groups in Petrofacies Assemblages 5 and 6, but high-grade metamorphic rocks were not re-unroofed until the end of the Late Jurassic, which dem-onstrates well-developed inverted clast stratig-raphy (Fig. 5). There is some indication that the UHP and HP metamorphic rocks in Wumiao section of the western Dabie Shan, the Xinxian area, reached the surface at this time (Li et al., 2005). Therefore, the North Huaiyang sedimen-tary rocks and Luzhenguan complex probably supplied most of the detritus for the basin dur-ing the Late Jurassic and the North Dabie Shan core zone and the UHP/HP metamorphic zone was reexhumed during the latest Late Jurassic. This suggests that another unroofi ng sequence from the blanket stratigraphy in the North Huai-yang to metamorphic rocks in the North Dabie Shan core and UHP/HP zones was developed along the southern margin of the Hefei Basin, responding to episodic backthrusting event in the hinterland of the orogen. The assumed Late Jurassic backthrust belt was much closer to the southern margin of the present Hefei Basin than the Middle Jurassic basin as the backthrust ex-panded northward and controlled the deposition of the coarsening-upward Jurassic stratigraphy.
CONCLUSIONS
(1) Jurassic sediments in the Hefei Basin were deposited by braided-fl uvial and alluvial-fan systems and are characterized by a coarsening-upward sequence. In the Middle Jurassic , alluvial-fan conglomerate deposits were centered in the middle parts of the Hefei Basin. By Late Jurassic time, alluvial-fan con-glomerates were distributed along the entire southern margin of the basin.
(2) Multiproxy provenance analyses dem-onstrate that the sources of sediment in the Hefei Basin included the mixed low-grade metamorphic rocks in the UHP/HP and Yangtze basement rocks (end-member F), the
UHP/HP metamorphic gneiss and eclogite (end-member E) from the axial Dabie Shan complex, the granite in the Luzhenguan complex (end-members C and C′), low- and medium-grade metamorphic rocks (end-members B and D), and the Yangshan Group (end-member A) from the North Huiyang area, located along the north-ern fl ank of the Dabie Shan orogen. The contri-butions of these end members to the sediments are variable in the section, and represent a mixed provenance between the axial and fl anking parts of the Dabie Shan orogen.
The Middle Jurassic Wumiao section in the western portion of the basin is characterized by relatively high εNd values ranging from –13.8 to –11.3, and was derived from end-members A, B, and C (the North Huaiyang area). In the Petro-facies Assemblage 1, the percentage of the end-member C decreases from 49 wt% to 30 wt%, while end-member A increases from 4 wt% to 34 wt%. Petrofacies Assemblage 2 is character-ized by a high percentage of end-members C be-tween 34 wt% and 60 wt%, and a relatively lower percentage of end-members A between 6 wt% and 43 wt%. Contribution of end-members A, B, and C for Petrofacies Assemblage 3 keeps a simi-lar level to the Petrofacies Assemblage 1.
The Liushudian-Dingbachong section in the middle part of the basin has the highest 147Sm/144Nd ratios (0.1168–0.1266) and slightly lower εNd values than those of the Wumiao section, indicating that the source rocks are composed of end-members C, D, and E. The contribution of end-member E decreases up-ward from 55 wt% to 37 wt%, and that of end- member C increases upward from 29 wt% to 50 wt%, whereas the content of end-member D remains approximately constant at ~16 wt%.
The sediments in the Zhujiacitang-Feng-huangtai section of the eastern part of the basin have low εNd values, ranging from –22.0 to –14.6, high TDM values from 1.8 and 2.4 Ga, and low 147Sm/144Nd ratios from 0.0937 and 0.1067, indicating derivation from end-members C′, D, and F. In Petrofacies Assemblages 1, 2, 3, and 4, the contribution of end-member F (base-ment gneiss in North Dabie Shan core zone and metapelite in the UHP/HP zone) increases from 7 wt% to 59 wt% as the contribution from the Foziling Group (D) decreases from 63 wt% to 5 wt%. In Petrofacies Assemblages 5 and 6, the contribution from end-member C′ is very high, ranging from ~52 wt% to 90 wt%, while end-members D and F are lower, ranging from ~7 wt% to 36 wt% and from 3 wt% to 12 wt% at Petrofacies Assemblage 5, then increase to 24 wt% and 22 wt% at Petrofacies Assem-blage 6, respectively.
(3) Sandstone compositions and Nd isotopic analysis of sediments in the Hefei Basin clearly
Tracing exhumation of the Dabie Shan ultrahigh-pressure metamorphic complex
Geological Society of America Bulletin, January/February 2010 217
demonstrate that the depth of exhumation in the axial Dabie Shan complex and the North Huaiyang increased from west to east, and the unroofi ng ages of the UHP and HP metamor-phic rocks in the Dabie Shan orogen vary from Early Jurassic to Late Jurassic westward. The exhumation rate during the Late Triassic and Jurassic increased eastward from ~1.4 mm/a to ~2.5 mm/a on average.
(4) Unroofi ng of the UHP and HP metamor-phic rocks fi rst occurred in the Early Jurassic at the eastern end of the Hefei Basin. During the Middle Jurassic the eastern part of the basin re-cords an unroofi ng process from the early north-ern marginal thrust to the later Dabie Shan core. The middle part of the Hefei Basin records rapid unroofi ng and several exhumation cycles in the North Huaiyang and the UHP and HP rocks. The western part of the Hefei Basin was fi lled by sediment removed from the blanket stratigraphy and the Luzhenguan complex, before the UHP and HP metamorphic rocks were exhumed. During the Late Jurassic, source rocks for the sediments were mostly composed of the blan-ket stratigraphy and the Luzhenguan complex in the North Huaiyang, but North Dabie Shan core zone and UHP and HP metamorphic rocks began to supply sediments at the latest Jurassic, which represents another unroofi ng episode.
ACKNOWLEDGMENTS
We thank Prof. Shuguan Li and Zongqing Zhang for helpful discussions, Dr. Chen Fukun for his help during our study, and Profs. Paul Robinson, Lothar Ratschbacher, Ulf Linnemann, Mary Leech, Laura Webb, and Jake Covault for their detailed review of the paper. Financial support was provided by the Na-tional Natural Science Foundation of China (grants 40272055, 40234041, and 40672135), the 111 Project (B07011), Sinopec project (09), Project of Key Laboratory of Marine Reservoir Evolution and Hydro carbon Accumulation Mechanism, Ministry of Education, China (EEL2008-2). Work by B.D.R. and a visiting professorship for the senior author at Indiana University was supported by U.S. National Science Foundation grant EAR-0604443.
REFERENCES CITED
Ames, L., Tilton, G.R., and Zhou, G., 1993, Timing of col-lision of the Sino-Korean and Yangtze cratons: U-Pb zircon dating of coesite-bearing eclogites: Geology, v. 21, p. 339–342, doi: 10.1130/0091-7613(1993)021<0339:TOCOTS>2.3.CO;2.
Ames, L., Zhou, G., and Xiong, B., 1996, Geochronology and isotopic character of ultrahigh pressure meta-morphism with implications for collision of the Sino-Korean and Yangtze cartons, central China: Tectonics, v. 15, p. 472–489, doi: 10.1029/95TC02552.
Anhui Geological Survey (AGS), 1999, Explanation of the 1:250,000 geological map of the Dabie Mountains within Anhui Province: Anhui Geological Survey of Hefei, p. 1–44 (in Chinese).
Bureau of Geology and Mineral Resources of Anhui Prov-ince, 1987, Regional Geology of Anhui Province: Bei-jing, Geological Publishing House, 721 p. (in Chinese with English abstract).
Carswell, D.A., O’Brien, P.J., Wilson, R.N., and Zhai, M., 1997, Thermobarometry of phengite-bearing eclogites
in the Dabie Mountains of central China: Journal of Metamorphic Geology, v. 15, p. 239–252, doi: 10.1111/j.1525-1314.1997.00014.x.
Chavagnac, V., and Jahn, B.M., 1996, Coesite-bearing eclogites from the Bixiling Complex, Dabie Moun-tains, China: Sm-Nd ages, geochemical characteristics and tectonic implications: Chemical Geology, v. 133, p. 29–51, doi: 10.1016/S0009-2541(96)00068-X.
Chen, F., Guo, J., Jiang, L., Siebel, W., Cong, B., and Satir, M., 2003a, Provenance of the Beihuaiyang lower-grade metamorphic zone of the Dabie ultrahigh-pressure col-lisional orogen, China: Evidence from zircon ages: Journal of Asian Earth Sciences, v. 22, p. 343–352, doi: 10.1016/S1367-9120(03)00068-3.
Chen, F., Siebel, W., Guo, J., Cong, B., and Satir, M., 2003b, Late Proterozoic magmatism and metamorphism re-corded in gneisses from the Dabie high-pressure meta-morphic zone, eastern China: Evidence from zircon U-Pb geochronology: Precambrian Research, v. 120, p. 131–148, doi: 10.1016/S0301-9268(02)00162-6.
Chen, J.F., and Jahn, B.M., 1998, Crustal evolution of southeastern China: Nd and Sr isotopic evidence: Tectonophysics, v. 284, p. 101–133, doi: 10.1016/S0040-1951(97)00186-8.
Chen, Y.Z., and Sang, B., 1995, Metamorphic petrology and metamorphism of the Foziling Group in Northern Huaiyang and its age: Regional Geology of China, no. 3, p. 280–288. (in Chinese).
Clauer, N., and Chaudhuri, S., 1992, Isotopic signatures and sedimentary records, in Clauer, N., and Chaudhuri, S., eds., Lecture Notes in Earth Sciences 43, 529 p.
Clift, P.D., Shimizu, N., Layne, G.D., Blusztajn, J.S., Gaedicke, C., Schluter, H.-U., Clark, M.K., and Amjad , S., 2001, Development of the Indus Fan and its signifi cance for the erosional history of the West-ern Himalaya and Karakoram: Geological Society of America Bulletin, v. 113, p. 1039–1051, doi: 10.1130/0016-7606(2001)113<1039:DOTIFA>2.0.CO;2.
Clift, P.D., Lee, J.I., Hildebrand, P., Shimizu, N., Layne, G.D., Blusztajn, J., Blum, J.D., Garzanti, E., and Khan, A.A., 2002, Nd and Pb isotope variability in the Indus River System: Implications for sediment provenance and crustal heterogeneity in the Western Himalaya: Earth and Planetary Science Letters, v. 200, p. 91–106, doi: 10.1016/S0012-821X(02)00620-9.
Cong, B., Zhai, M., Carswell, D.A., Wilson, R.N., Wang, Q., Zhao, Z., and Windley, B.F., 1995, Petrogenesis of ultrahigh-pressure rocks and their country rocks at Shuanghe in Dabieshan, Central China: Eta, Journal of Mineralogy, v. 7, p. 119–138.
Davis, G.A., Zheng, Y., Wang, C., Darby, B.J., Zhang, C., and Gehrels, G., 2001, Mesozoic tectonic evolution of the Yanshan fold and thrust belt, with emphasis on Hebei and Liaoning provinces, northern China, in Hendrix, M.S., and Davis, G.A., eds., Paleozoic and Mesozoic tectonic evolution of central Asia: From continental assembly to intracontinental deformation: Boulder, Colorado, Geological Society of America Memoir 194, p. 171–197.
Dickinson, W.R., Beard, L.S., Brackenridge, G.R., Erjavec, J.L., Ferguson, R.C., Inman, K.F., Knepp, R.A., Lind-berg, F.A., and Ryberg, P.T., 1983, Provenance of North American Phanerozoic sandstones in relation to tec-tonic setting: Geological Society of America Bulletin, v. 94, p. 222–235, doi: 10.1130/0016-7606(1983)94<222:PONAPS>2.0.CO;2.
Dong, S., Wang, X., and Huang, D., 1996, Discovery and signifi cance of low-grade metamorphic rock slices in ultrahigh-pressure metamorphic belt in Dabieshan: Chinese Science Bulletin, v. 41, p. 815–820.
Enkelmann, E., Weislogel, A., Ratschbacher, L., Eide, E., Renno, A., and Wooden, J., 2007, How was the Tri-assic Songpan-Ganzi basin fi lled?: Tectonics, v. 26, p. TC4007, doi: 10.1029/2006TC002078.
Faure, G., 1986, Principles of Isotope Geology, 2nd edition: New York, Wiley, 589 p.
Ge, N., Li, H., Qin, L., Hou, Z., and Bo, L., 2001, Sr, Nd and Pb isotope geochemistry of granulites and TTG gneisses from the North Dabie Mountains: Acta Geo-logica Sinica, v. 75, p. 379–384.
Grimmer, J.C., Ratschbacher, L., McWilliams, M.O., Franz, L., Gaitzsch, J., Tichomirowa, M., Hacker, B.R., and
Zhang, Y., 2003, When did the UHP rocks reach the surface?: A 207Pb/206Pb zircon, 40Ar/39Ar white mica, Si-in-white mica single-grain provenance study of Dabie Shan synorogenic foreland sediments: Chemical Geology, v. 141, p. 1–34.
Hacker, B.R., Ratschbacher, L., Webb, L., and Dong, S., 1995, What brought them up?: Exhumation of the Dabie Shan ultrahigh-pressure rocks: Geology, v. 23, p. 743–746, doi: 10.1130/0091-7613(1995)023<0743:WBTUEO>2.3.CO;2.
Hacker, B.R., Ratschbacher, L., Webb, L., Ireland, T., Walker, D., and Dong, S., 1998, U/Pb zircon ages constrain the architecture of the ultrahigh-pressure Qinling-Dabie Orogen, China: Earth and Planetary Science Letters, v. 161, p. 215–230, doi: 10.1016/S0012-821X(98)00152-6.
Hacker, B.R., Ratschbacher, L., Webb, L., McWilliams, M.O., Ireland, T., Calvert, A., Dong, S., Wenk, H.R., and Chateigner, D., 2000, Exhumation of ultrahigh-pressure continental crust in east central China: Late Triassic–Early Jurassic tectonic unroofi ng: Journal of Geophysical Research, v. 105, B6, p. 13,339–13,364, doi: 10.1029/2000JB900039.
Hacker, B.R., Simon, R.W., Lothar, R., Marty, G., and George, G., 2006, High-temperature geochronology constraints on the tectonic history and architecture of the ultrahigh-pressure Dabie-Sulu Orogen: Tectonics, v. 25, p. TC5006, doi: 10.1029/2005TC001937.
Heller, P.L., and Ryberg, P.T., 1983, Sedimentary record of subduction to forearc transition in the rotated Eocene basin of western Oregon: Geology, v. 11, p. 380–383, doi: 10.1130/0091-7613(1983)11<380:SROSTF>2.0.CO;2.
Hendrix, M.S., 2000, Evolution of Mesozoic sandstone compositions, southern Junggar, northern Tarim, and western Turpan basins, northwest China: A de-trital record of the ancestral Tian Shan: Journal of Sedimentary Research, v. 70, p. 520–532, doi: 10.1306/2DC40924-0E47-11D7-8643000102C1865D.
Henry, P., Deloule, E., and Michard, A., 1997, The erosion of he Alps: Nd isotopic and geochemical constraints on the sources of the peri-Alpine molasse sediments: Earth and Planetary Science Letters, v. 146, p. 627–644, doi: 10.1016/S0012-821X(96)00252-X.
Ingersoll, R.V., Bullard, T.F., Ford, R.L., Grimm, J.P., Pickle, J.D., and Sares, S.W., 1984, The effect of grain size on detrital modes: A test of the Gazzi-Dickinson point-counting method: Journal of Sedimentary Petrol-ogy, v. 54, p. 103–116.
Li, R., Wan, Y., Cheng, Z., Zhou, J., Li, S., Jin, F., Meng, Q., Li, Z., and Jiang, M., 2005, Provenance of Juras-sic sedi ments in the Hefei Basin, east-central China and the contribution of high-pressure and ultrahigh-pressure metamorphic rocks from the Dabie Shan: Earth and Planetary Science Letters, v. 231, p. 279–294, doi: 10.1016/j.epsl.2004.12.021.
Li, S., Xiao, Y., Liou, D., Chen, Y., Ge, N., Zhang, Z., Sun, S., Cong, B., Zhang, R., Hart, S.R., and Wang, S., 1993, Collision of the North China and Yangtze blocks and formation of coesite-bearing eclogites: Timing and processes: Chemical Geology, v. 109, p. 89–111, doi: 10.1016/0009-2541(93)90063-O.
Li, S., Yue, S., and Wang, D., 2002, On the framework of Mesozoic strata on the northern margin of Dabie Oro-genic belt: Journal of Stratigraphy, v. 3, p. 176–220.
Li, S.G., Hong, J., Li, H., and Jiang, L., 1999a, U-Pb zir-con ages of the pyroxenite-gabbro intrusions in Dabie Mountains and their geological implications: Geological Journal of China Universities, v. 5, no. 3, p. 351–355.
Li, S.G., Jagoutz, E., Lo, C.H., Chen, Y., Li, Q., and Xiao, Y., 1999b, Sm/Nd, Rb/Sr and 40Ar/39Ar isotopic system-atic of the ultrahigh-pressure metamorphic rocks in the Dabie-Sulu belt, central China: A retrospective view: International Geology Review, v. 41, p. 1114–1124.
Li, S.G., Jagoutz, E., Chen, Y., and Li, Q., 2000, Sm-Nd and Rb-Sr isotopic chronology and cooling history of ultra-high pressure metamorphic rocks and their country rocks at Shuanghe in the Dabie orogen, central China: Geochimica et Cosmochimica Acta, v. 64, p. 1077–1093, doi: 10.1016/S0016-7037(99)00319-1.
Li, S.G., Huang, F., Nie, Y.H., Han, W.L., Long, G., Li, H.M., Zhang, S.Q., and Zhang, Z.H., 2001, Geochemical
Liu et al.
218 Geological Society of America Bulletin, January/February 2010
and geochronological constraints on the suture loca-tion between the North and South China Blocks in the Dabie Orogen, central China: Physics and Chemistry of the Earth, v. 26, no. 9–10, p. 655–672, doi: 10.1016/S1464-1895(01)00117-X.
Li, Y., and Jiang, L., 1997, Stratigraphy (Lithostratic) of Anhui Province: Wuhan, China, China University of Geo-sciences Press, 271 p. (in Chinese).
Liou, J.G., Zhang, R.Y., and Jahn, B.M., 1997, Petrology, geochemistry and isotope data on a ultrahigh-pressure jadeite quartzite from Shuanghe, Dabie Mountains, east-central China: Lithos, v. 41, p. 59–78, doi: 10.1016/S0024-4937(97)82005-1.
Liu, D., Jian, P., Kröner, A., and Xu, S., 2006, Dating of prograde metamorphic events deciphered from epi-sodic zircon growth in rocks of the Dabie-Sulu UHP complex, China: Earth and Planetary Science Letters, v. 250, p. 650–666, doi: 10.1016/j.epsl.2006.07.043.
Liu, S., Liu, W., Dai, S., Huang, S., and Lu, W., 2001a, Thrust and exhumation processes of bounding moun-tain belt: Constrained from sediment provenance analysis of Hefei Basin, China: Acta Geologica Sinica, v. 75, no. 2, p. 144–150.
Liu, S., Zhang, G., Zhang, Z., and Su, S., 2001b, Isotope chronological trace of granite gravel in Hefei Basin: Chinese Science Bulletin, v. 46, no. 20, p. 1716–1721, doi: 10.1007/BF02900659.
Liu, S., Heller, P.L., and Zhang, G., 2003, Mesozoic basin development and tectonic evolution of the Dabieshan orogenic belt, central China: Tectonics, v. 22, no. 4, p. 1038, doi: 10.1029/2002TC001390.
Liu, S., Zhang, J., Hong, S., and Ritts, B.D., 2007, Early Mesozoic basin development and its response to thrust-ing in the Yanshan fold-and-thrust belt, China: Inter-national Geology Review, v. 49, p. 1025–1049, doi: 10.2747/0020-6814.49.11.1025.
Liu, W., Ma, W., and Tan, Y., 1995, Regional geological investigations report (Suxianshi and Jinzhai County regions) (1:50,000): Hefei, China, Bureau of Geol-ogy and Mineral Resources of Anhui Province Press, p. 1–194 (in Chinese).
Liu, X.C., Jahn, B.M., Liu, D.Y., Dong, S.W., and Li, S.Z., 2004, SHRIMP U-Pb zircon dating of a metagabbro and eclogites from western Dabieshan (Hong’an Block), China, and its tectonic implications: Tectonophysics, v. 394, p. 171–192, doi: 10.1016/j.tecto.2004.08.004.
Liu, Y.C., Li, S.G., Xu, S.T., Li, H.M., Jiang, L.L., Chen, G.B., Wu, W.P., and Su, W., 2000, U-Pb zircon ages of the eclogite and Tonalitic gneiss from the Northern Dabie orogen, China and multi-overgrowths of meta-morphic zircons: Geological Journal of China Univer-sity. v. 6, no. 3, p. 417–423.
Liu, Y.C., Li, S.G., Xu, S.T., Jahn, B.M., Zheng, Y.F., Jiang, L.L., Chen, G.B., Wu, W.P., and Su, W., 2001, Sm-Nd isotopic age of eclogite from the Northern Dabie zone and its geological implications: Geochimica, v. 30, no. l, p. 79–87.
Ma, C., Ehlers, C., Xu, C., Li, Z., and Yang, K., 2000, The roots of the Dabieshan ultrahigh-pressure metamor-phic terrane: Constraints from geochemistry and Nd-Sr isotope systematics: Precambrian Research, v. 102, p. 279–301, doi: 10.1016/S0301-9268(00)00069-3.
Mattauer, M., Matte, Ph., Malavieille, J., Tapponnier, P., Maluski, H., Xu, Z., Lu, Y., and Tang, Y., 1985, Tectonics of the Qinling belt: Build-up and evolu-
tion of eastern Asia: Nature, v. 317, p. 496–500, doi: 10.1038/317496a0.
Okay, A.I., 1993, Petrology of a diamond and coesite-bearing metamorphic terrain: Dabie Shan, China: Euro-pean Journal of Mineralogy, v. 5, p. 659–675.
Okay, A.I., Xu, S., and Sengor, A.M.C., 1989, Coesite from the Dabie Shan eclogites, central China: European Journal of Mineralogy, v. 1, p. 595–598.
Ratschbacher, L., Hacker, B.R., Webb, L.E., McWilliams, M., Ireland, T., Dong, S., Calvert, A., Chateigner, D., and Wenk, H.R., 2000, Exhumation of the ultrahigh-pressure continental crust in east-central China: Cre-taceous and Cenozoic unroofi ng and the Tan-Lu fault: Journal of Geophysical Research, v. 105, p. 13303–13338, doi: 10.1029/2000JB900040.
Ratschbacher, L., Franz, L., Enkelmann, E., Jonckheere, R., Pörschke, A., Hacker, B.R., Dong, S., and Zhang, Y., 2006, The Sino-Korean–Yangtze suture, the Huwan detachment, and the Paleozoic–Tertiary exhumation of (ultra)high-pressure rocks along the Tongbai- Xinxian-Dabie Mountains, in Hacker, B.R., McClelland, W.C., and Liou, J.G., eds., Ultrahigh-pressure metamor-phism: Deep continental subduction: Geological So-ciety of America Special Paper 403, p. 45–75, doi: 10.1130/2006.2403(03).
Sang, L., Wang, R., He, B., Xie, D., Zhang, B., He, Y., and Wang, Q., 1997, Report of Regional Geological Sur-vey on Jiuzihe (H50E 006007) and Zhangjiazui (H50E 006008) Regions: Wuhan, China University of Geosci-ences, p. 1–162 (in Chinese).
Schmid, R., Romer, R.L., Franz, L., Oberhänsli, R., and Martinotti, G., 2003, Basement-Cover Sequences within the UHP unit of the Dabie Shan: Journal of Metamorphic Geology, v. 21, p. 531–538.
Steidtmann, J.R., and Schmitt, J.G., 1988, Provenance and dispersal of tectogenic sediments in thin-skinned, thrusted terrains, in Kleinspehn, K.L., and Paola, C., eds., New Perspectives in Basin Analysis: Springer-Verlag, p. 353–366.
Suttner, L.J., Basu, A., Ingersoll, R.V., Bullard, T.F., Ford, R.L., and Pickle, J.D., 1985, The effect of grain size on detrital modes: A test of the Gazzi-Dickinson point-counting method; discussion and reply: Journal of Sedimentary Petrology, v. 55, p. 616–618.
Wang, D.X., Liu, Y., Li, S., and Jin, F., 2002a, Lower time limit on the UHPM rock exhumation: Discovery of eclogite pebbles in the Late Jurassic conglomerates from the northern foot of the Dabie Mountains, eastern China: Chinese Science Bulletin, v. 47, no. 3, p. 231–235, doi: 10.1360/02tb9055.
Wang, X.D., Neubauer, F., Genser, J., and Yang, W., 1998, The Dabie UHP unit, central China: A Creta-ceous extensional allochthon superposed on a Tri-assic orogen: Terra Nova, v. 10, p. 260–267, doi: 10.1046/j.1365-3121.1998.00200.x.
Wang, X.M., Liou, J.G., and Mao, H.K., 1989, Coesite-bearing eclogites from the Dabie Mountains in central China: Geology, v. 17, p. 1085–1088, doi: 10.1130/0091-7613(1989)017<1085:CBEFTD>2.3.CO;2.
Wang, Y., Fan, W., and Feng, G., 2002b, K-Ar dating of late Mesozoic volcanism and geochemistry of vol canic gravels in the North Huaiyang Belt, Dabie orogen: Constraints on the stratigraphic framework and exhu-mation of the northern Dabie orthogneiss complex: Chinese Science Bulletin, v. 47, p. 1528–1534.
Xie, Z., Chen, J., Zhou, T., and Zhang, X., 1996, Nd isotopic compositions of metamorphic and granitic rocks from Dabie orogen and their geological signifi cance: Acta Petrolei Sinica, v. 12, p. 401–408.
Xu, B., Grove, M., Wang, Ch., Zhang, L., and Liu, S., 2000, 40Ar/39Ar thermochronology from the north-western Dabie Shan: Constraints on the evolution of Qinling-Dabie orogenic belt, east-central China: Tectonophysics, v. 322, p. 279–301, doi: 10.1016/S0040-1951(00)00092-5.
Xu, G.Z., and Hao, J., 1988, The characteristics of Foziling Group and the geotectonic environment of its forma-tion in the northern foot of the Dabie orogen: Scientia Geologica Sinica, no. 2, p. 95–109 (in Chinese).
Xu, Q., Ou Yang, J., and Zhang, B., 2005, Geochemical records of sedimentary source and tectonic evolution of Paleozoic strata along the northern margin in the Dabie Orogen, central China: Acta Geologica Sinica, v. 79, no. 3, p. 402–413.
Xu, S., Okay, A.I., Ji, S., Sengor, A.M.C., Su, W., Liu, Y., and Jiang, L., 1992, Diamond from the Dabie Shan metamorphic rocks and its implication for tectonic setting: Science, v. 256, p. 80–82, doi: 10.1126/science.256.5053.80.
Yin, A., and Nie, S., 1993, An indentation model for the North China and South China collision and the devel-opment of the Tan-Lu and Honam fault systems, East-ern Asia: Tectonics, v. 12, p. 801–813, doi: 10.1029/93TC00313.
Yin, A., and Nie, S., 1996, Phanerozoic palinspastic re-construction of China and its neighboring regions, in Yin, A., and Harrison, T.M., eds., The tectonic evolu-tion of Asia: Cambridge, Cambridge University Press, p. 442–485.
You, Z., Han, Y., Yang, W., Zhang, Z., Wei, B., Liu, B., and Liu, R., 1996, The high-pressure and ultra-high-pressure metamorphic belt in the East Qinling and Dabie Mountains, China: Wuhan, China University of Geosciences Press, p. 1–150.
Zhang, G., Meng, Q., Liu, S., and Yao, A., 1997, A giant intracontinental subduction belt in the southern Huabei block and present 3-D lithospherical structure of Qinling orogenic belt: Geological Journal China Uni-versity, v. 3, p. 129–143.
Zhang, G., Zhang, B., Yuan, X., and Xiao, Q., 2001, Qinling Orogenic Belt and Continental Geodynamics: Beijing, Chinese Science, p. 1–855 (in Chinese).
Zhang, R.Y., Liou, J.G., and Tsai, C.H., 1996, Petrogenesis of a high-temperature metamorphic terrane: A new tectonic interpretation for the North Dabieshan, cen-tral China: Journal of Metamorphic Geology, v. 14, p. 319–333, doi: 10.1111/j.1525-1314.1996.00319.x.
Zhao, Z., Yang, S., Chen, H., Zhu, G., and Lou, Z., 2000, Sedimentary environment of Carboniferous system in Shangcheng-Gushi area, Henan Province and its tec-tonic implications: Geology Review, v. 46, p. 407–416.
Zheng, X., Jin, C., Zhai, M., and Shi, Y., 2000, Approach to the source of the gray gneisses in North Dabie terrain: Sm-Nd isochron age and isotope composition: Acta Petrolei Sinica, v. 16, no. 2, p. 194–198.
MANUSCRIPT RECEIVED 26 AUGUST 2008REVISED MANUSCRIPT RECEIVED 4 APRIL 2009MANUSCRIPT ACCEPTED 6 APRIL 2009
Printed in the USA
Related Documents
