Geochemistry and isotopic constraints on the origin of the mesoproterozoic Rio Branco ‘anorogenic’ plutonic suite, SW of Amazonian craton, Brazil: high heat flow and crustal extension behind the Santa Helena arc? Mauro C. Geraldes a, * , Jorge S. Bettencourt b , Wilson Teixeira b , Joa ˜o B. Matos c a TEKTOS–Faculdade de Geologia, Universidade do Estado do Rio de Janeiro, Rua Sa ˜o Francisco Xavier 524, Rio de Janeiro, CEP 20550-013, Brazil b Instituto de Geocie ˆncias, Universidade de Sa ˜o Paulo, Rua do Lago 562, Sa ˜o Paulo-SP, CEP 05508-900, Brazil c Departamento de Geologia, Universidade Federal de Mato Grosso. Av. Fernando Correia da Costa s/n, Cuiaba ´-MT, Brazil Received 1 November 2002; accepted 1 May 2004 Abstract The Rio Branco plutonic suite (RBS) occurs in the southwestern Amazonian craton, crops out in an area of 1500 km 2 , and is emplaced into the ca. 1.79 Ga Alto Jauru terrane (Rio Negro/Juruena geochronological province). The RBS comprises basic (gabbro, diabase, and basalt) and felsic (porphyritic and rapakivi granite) rocks. Hybrid rocks (monzosyenite) with rapakivi-like textures indicate commingling and mixing among the basic and felsic magmas. Silica contents range 45–47% in the basic rocks (metaluminous) and 69–71% in the felsic rocks (slightly peraluminous–metaluminous). Lithogeochemical investigation also indicates higher contents of K 2 O, Rb, Zr, and Ba in felsic rocks, comparable with results reported elsewhere for rapakivi granites. Trace element discrimination diagrams indicate that the RBS felsic and basic rocks have within-plate signatures. In addition, the felsic rocks have strongly fractionated REE patterns that show marked negative Eu anomalies, probably due to plagioclase fractionation. The basic rocks are similarly LREE enriched but display flatter patterns, characteristic of weakly fractionated gabbros. Single-grain IDTIMS U–Pb analyses yield an upper intercept age of 1427G10 (MSWDZ1.7) for magmatic zircon from a granophyre of the RBS. This age contrasts significantly with an upper intercept age of 1471G8 Ma (with a concordant 207 Pb/ 206 Pb age of 1471G18 Ma) obtained for zircon from a sample of the basic group. The latter rocks show positive 3 Nd(1420) ranging from C1.2 to C1.9 (T DM Z1.86K1.82 Ga), which indicates mantle-derivation, whereas the felsic ones yield 3 Nd(1420) values from C0.2 to K1.0 (T DM Z1.80K1.73 Ga), indicating some older crust in their source. The RBS is interpreted to have formed at 1.47–1.42 Ga from a mixture of mantle source and crustal-derived magma. We propose high heat flow and an extensional environment for the origin of the RBS as a response to the inboard Santa Helena arc (ca. 1.45–1.42 Ga) that developed at the southwestern margin of the Amazonian craton at approximately the same time. q 2004 Elsevier Ltd. All rights reserved. Keywords: Amazonian craton; Mesoproterozoic plutonism; Rapakivi granite; Synorogenic magmatism Resumo A Suite Intrusiva Rio Branco (SIRB) esta ´ localizada no SW do craton Amazo ˆnico (Provı ´ncia Rio Negro/Juruena), aflorando em a ´rea de 1500 kM 2 e encaixada por rochas do terreno Alto Jauru de idade ca. 1.79 Ga. A suı ´te e ´ composta por um grupo de rochas ba ´sicas (gabros, diaba ´sios e basaltos) e fe ´lsicas (grano ´firos e granitos rapakivi). Rochas hı ´bridas monzosienı ´ticas com textura rapakivi indicam processos de mistura entre magmas ba ´sicos e fe ´lsicos. 0895-9811/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsames.2004.05.010 Journal of South American Earth Sciences 17 (2004) 195–208 www.elsevier.com/locate/jsames * Corresponding author. Tel.: C55-21-2587-7704; fax: C55-21-2254-6675. E-mail address: [email protected] (M.C. Geraldes).
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
Geochemistry and isotopic constraints on the origin
of the mesoproterozoic Rio Branco ‘anorogenic’ plutonic suite,
SW of Amazonian craton, Brazil: high heat flow and
crustal extension behind the Santa Helena arc?
Mauro C. Geraldesa,*, Jorge S. Bettencourtb, Wilson Teixeirab, Joao B. Matosc
aTEKTOS–Faculdade de Geologia, Universidade do Estado do Rio de Janeiro, Rua Sao Francisco Xavier 524, Rio de Janeiro, CEP 20550-013, BrazilbInstituto de Geociencias, Universidade de Sao Paulo, Rua do Lago 562, Sao Paulo-SP, CEP 05508-900, Brazil
cDepartamento de Geologia, Universidade Federal de Mato Grosso. Av. Fernando Correia da Costa s/n, Cuiaba-MT, Brazil
Received 1 November 2002; accepted 1 May 2004
Abstract
The Rio Branco plutonic suite (RBS) occurs in the southwestern Amazonian craton, crops out in an area of 1500 km2, and is emplaced into
the ca. 1.79 Ga Alto Jauru terrane (Rio Negro/Juruena geochronological province). The RBS comprises basic (gabbro, diabase, and basalt)
and felsic (porphyritic and rapakivi granite) rocks. Hybrid rocks (monzosyenite) with rapakivi-like textures indicate commingling and
mixing among the basic and felsic magmas.
Silica contents range 45–47% in the basic rocks (metaluminous) and 69–71% in the felsic rocks (slightly peraluminous–metaluminous).
Lithogeochemical investigation also indicates higher contents of K2O, Rb, Zr, and Ba in felsic rocks, comparable with results reported
elsewhere for rapakivi granites. Trace element discrimination diagrams indicate that the RBS felsic and basic rocks have within-plate
signatures. In addition, the felsic rocks have strongly fractionated REE patterns that show marked negative Eu anomalies, probably due to
plagioclase fractionation. The basic rocks are similarly LREE enriched but display flatter patterns, characteristic of weakly fractionated
gabbros.
Single-grain IDTIMS U–Pb analyses yield an upper intercept age of 1427G10 (MSWDZ1.7) for magmatic zircon from a granophyre
of the RBS. This age contrasts significantly with an upper intercept age of 1471G8 Ma (with a concordant 207Pb/206Pb age of 1471G18 Ma)
obtained for zircon from a sample of the basic group. The latter rocks show positive 3Nd(1420) ranging from C1.2 to C1.9
(TDMZ1.86K1.82 Ga), which indicates mantle-derivation, whereas the felsic ones yield 3Nd(1420) values from C0.2 to K1.0
(TDMZ1.80K1.73 Ga), indicating some older crust in their source.
The RBS is interpreted to have formed at 1.47–1.42 Ga from a mixture of mantle source and crustal-derived magma. We propose high heat
flow and an extensional environment for the origin of the RBS as a response to the inboard Santa Helena arc (ca. 1.45–1.42 Ga) that
developed at the southwestern margin of the Amazonian craton at approximately the same time.
taneously involving plagioclase, pyroxene, and magnetite
increases the total amount of REE in basaltic melts but does
not cause any significant LREE–HREE fractionation
(Fig. 8). Nevertheless, the absence of Eu-positive anomalies
in the basic rocks may be interpreted as a lack of
consanguinity between the magmas that formed the RBS
basic and felsic rocks.
6. Isotope data
U–Pb (single-grain) zircon geochronology was under-
taken on the felsic sample Rb-10 (20 kg) and the basic
sample Rb-04 (30 kg). Sample Rb-10 was processed to
concentrate heavy minerals, and a homogenous collection of
zircon grains was obtained. This collection consists of clear,
slightly caramel-colored grains, 50% of which have
biphasic (one gas and one liquid at room temperature)
fluid inclusions. Four single-zircon, fluid inclusion-free
grains were abraded and analyzed. The results yield an
upper intercept of 1423G10 Ma (Fig. 9), which we interpret
as the crystallization age of the felsic magma.
Fig. 3. Harker variation diagrams for oxides (K2O, Ca, MgO, and Fe2O3) and minor elements (Rb, Zr, Ba, and Cr) of the RBS mafic, hybrid, and felsic rocks.
Fig. 4. Alumina index of the RBS. The felsic rocks vary from slightly
peraluminous to metaluminous, and the basic rocks are metaluminous.
Fig. 5. Tectonic setting discrimination diagram (Pearce et al., 1984) for
RBS felsic rocks.
M.C. Geraldes et al. / Journal of South American Earth Sciences 17 (2004) 195–208202
Fig. 6. Tectonic setting discrimination, diagram (Pearce and Norry, 1979)
for RBS basic rocks.Fig. 8. REE patterns for RBS basic rocks.
Fig. 9. Plot of zircon data for sample Rb-10. The upper intercept yields a
crystallization age of 1423G02 Ma. Uncertainty at 2-s.
M.C. Geraldes et al. / Journal of South American Earth Sciences 17 (2004) 195–208 203
Five zircon grains obtained from sample Rb-04 were
analyzed, and the plotted results in the concordia diagram
(four fractions) yield an upper intercept age of
1471G18 Ma (Fig. 10). The grains are milky, and neither
the 001 faces nor the pyramidal ends are well defined. The
high MSDW (84) gives little confidence for this age, but a
concordant analysis (M(5) E in Table 2) indicates a207Pb/206 Pb age of 1471G18 Ma, which may be interpreted
as the crystallization age of the basic magma. Consequently,
zircon crystallization of basic magma took place 30–50
million years before felsic magma crystallization.
The U–Pb zircon ages are w300 Ma older than the
Rb–Sr whole-rock reference mixed line reported by Barros
et al. (1982) of 1130G72 Ma (87Sr/86SrinitialZ0.708) for the
RBS rocks. The Rb–Sr study is unreliable because the basic
and felsic rocks may have different sources. Recalculations
using only the felsic samples give an age of 1221G32,
200 Ma younger than the U–Pb ages, which indicates partial
resetting of the Rb–Sr ages. This resetting may be due to
younger events observed in the region, such as deformation
that produced the Aguapeı thrust (ca. 1000 Ma, Geraldes
et al., 1997).
Aliquots from the same powders used for whole-rock
major and trace element geochemistry were used for
Sm–Nd isotopic analyses (Table 3). The fractionation
factor (f) between Sm and Nd in the basic rocks varies
Fig. 7. REE patterns for RBS felsic rocks.
Fig. 10. Plot of zircon data for sample Rb-04. The upper intercept yields a
crystallization age of 1471G31 Ma. Uncertainty at 2-s.
Tab
le2
U/P
bre
sult
sfo
rsa
mp
les
Rb
-04
and
Rb-1
0
Fra
ctio
n*
Wei
gh
t(m
g)
20
7*/2
06
*
U(p
pm
)P
b(p
pm
)O
bse
rved
#R
atio
G2
SE
(%)†
Cal
cula
ted
ages
G2
SE
(Ma)
‡
206P
b/2
04P
b207*
Pb
/235U
206*
Pb/2
38U
207*
Pb
/206*
Pb
207*
Pb
/235U
206*
Pb
/238U
20
7*/2
06
*
Rb
-10
Gra
nop
hyr
eR
ioB
ranco
NM
(0)[
1]
0.0
02
13
33
85
20G
3.0
20
76
1.2
0G
0.2
43
57
1.1
7G
0.0
89
94
76
0.2
4G
0.9
80
14
13G
17
14
05G
16
14
24G
4.5
M(0
)[1
]0
.002
29
39
01
00
4G
3.6
03
36
0.7
9G
0.2
65
20
0.7
7G
0.0
98
54
52
0.1
8G
0.9
74
15
50G
12
15
16G
12
15
97G
3.3
M(1
)[1
]0
.003
29
97
81
52
5G
2.9
43
96
0.7
0G
0.2
37
79
0.6
8G
0.0
89
79
24
0.1
7G
0.9
69
13
93G
10
13
75G
09
14
21G
3.3
M(2
)[1
]0
.002
28
07
76
86G
2.9
68
79
0.8
2G
0.2
39
51
0.8
0G
0.0
89
90
09
0.1
8G
0.9
76
14
00G
11
13
84G
11
91
42
3G
3.4
Rb
-04
Ga
bb
roR
ioB
ran
co
M(5
)[2
]0
.005
44
09
28
86G
2.5
43
60
1.2
1G
0.2
00
64
51
.14G
0.0
91
94
31
0.4
0G
0.9
43
12
85G
16
11
79G
13
14
66G
7.6
M(5
)[1
]0
.003
49
71
37
61
6G
2.4
88
92
0.6
4G
0.2
04
26
80
.62G
0.0
88
37
10
.15
G0
.970
12
69G
08
11
98G
07
13
91G
03
M(5
)[1
]0
.002
51
71
08
56
9G
1.7
56
69
0.7
5G
0.1
54
67
80
.73G
0.0
82
36
96
0.1
8G
0.9
69
10
30G
07
92
7G
06
12
54G
3.6
M(5
)0
.004
78
82
07
99
8G
2.2
04
94
0.5
3G
0.1
87
30
80
.52G
0.0
85
37
66
0.1
1G
0.9
80
11
83G
06
11
07G
06
13
24G
2.1
M(5
)0
.002
19
35
94
94G
3.2
46
95
1.6
4G
0.2
55
52
21
.28G
0.0
92
16
08
0.9
7G
0.8
05
14
68G
23
14
67G
18
14
71G
18
NM
Zn
on
mag
net
ic;
MZ
mag
net
ic;
nu
mb
erin
par
enth
eses
ind
icat
esi
de
tilt
on
Fra
nz
sep
erat
or
at1
.5A
po
wer
;[1
]Zn
um
ber
of
gra
ins;
*d
eno
tes
radio
gen
icP
b;
†P
bco
rrec
ted
for
bla
nk
and
no
n-r
adio
gen
icP
b;
‡A
ges
giv
enin
Ma
usi
ng
dec
ayco
nst
ants
reco
mm
ended
by
Ste
iger
and
Jag
er(1
97
7);
ince
rtai
ns
inag
esar
e2s
.
M.C. Geraldes et al. / Journal of South American Earth Sciences 17 (2004) 195–208204
from K0.31 to K0.32 (with the exception of Rb-04, with
fZK0.61); in the felsic rocks, f varies from K0.37 to
K0.41. The anomalous f value for sample Rb-04 probably
results from its high concentration of accessory minerals
with high concentrations of REE. In addition, the
concentration values of Nd and Sm for sample Rb-07,
obtained from neutron activation (15.5 and 55.8 ppm,
respectively; Table 1) and isotope dilution (11.2 and
89.82 ppm, respectively; Table 2), are anomalously high
(37.88% for Nd and 37.29% for Sm). Due to its
anomalously f value, chemical and Nd isotope data for
Rb-04 are not considered further.
3Nd(1420) values for basic rocks range from C1.2 toC1.9,
suggesting mantle source crustal rock contributions.
3Nd(1420) values for felsic rocks range from C0.2 to K1.0,
which suggest that they contain an important older crust
component. TDM ages of the basic rocks vary from
1.73–1.80 Ga, and TDM ages of felsic rocks are slightly
older, 1.81–1.89 Ga. 3Nd(0) values of the basic rocks
range from K8.3 to K10.4; of the felsic rocks, from
K13.1 to K15.2, thus indicating that basic and felsic rocks
have different Nd isotopic evolutions compared with the
depleted mantle over time (Fig. 11).
7. Discussion
Isotopic ages usually confirm a close temporal associ-
ation of basic rocks with rapakivi granites (Haapala and
Ramo, 1999). Even gabbroic and anorthosite rocks intruded
by granitic rocks (AMCG suites) yield isotopic ages usually
undistinguishable within experimental error. For example,
Haapala and Ramo (1999) report an U–Pb age gap of 5 Ma
between gabbroic and quartz–feldspar porphyry dykes in the
Ahvenisto complex. Similarly, there is a difference of
approximately 20 Ma (U–Pb ages) in the gabbroic rocks and
granites from the Salmi batholith in Russian Karelia
(Neymark et al., 1994).
The reported U–Pb geochronological data obtained in
single-zircon grains yields an age of 1471G18 Ma for the
basic rocks and 1427G10 Ma for the felsic rocks, both of
which may be interpreted as crystallization ages. There are
two suggestions to explain the U–Pb age gap of 50–30 Ma.
First, the zircon grains analyzed from the basic sample may
be xenocrysts. This hypothesis is not consistent with the
zircon shape, because the studied grains are characteristic of
basic rocks. Second, basic and felsic magmas may have
crystallized at different temperatures. In this case, the basic
magma solidified 50–30 Ma before the felsic magma, which
corroborates the commingling textures that correlate with
the incomplete mixing of the basic and felsic magmas due to
brittle conditions.
The TDM ages of the basic (1.86–1.82 Ga; 3Nd(1420)C1.2
to C1.9) and felsic (1.80–1.73 Ga; 3Nd(1420)C0.2 to K1.0)
rocks are similar to Sm–Nd data from the Alto Jauru terrane
and Cachoeirinha suite rocks (3NdC0.5 to K0.8, TDM ages
Table 3
Sm/Nd isotopic properties of rocks from RBS
Sample Rock Nd (ppm) Sm (ppm) 147Sm/144Nd 143Nd/144Nd E(Nd) tZ0 E(Nd)