-
JOURNAL OF PETROLOGY VOLUME 38 NUMBER 6 PAGES 727755 1997
Magmatic Evolution and Tectonic Setting ofthe Iberian Pyrite
Belt Volcanism
J. MITJAVILA, J. MARTI AND C. SORIANOINSTITUTE OF EARTH SCIENCES
JAUME ALMERA, CSIC, LLUIS SOLE I SABARIS S/N, 08028 BARCELONA,
SPAIN
RECEIVED ON JUNE 1, 1996 REVISED TYPESCRIPT ACCEPTED FEBRUARY 4,
1997
result of oblique continental collision that followed the
subductionThe Iberian Pyrite Belt, which extends from Portugal to
Spain inof the South Portuguese plate beneath the Ossa Morena
plate. Thissouthwest Iberia, constitutes one of the worlds largest
reservoirstectonically driven magmatism does not have a modern
analogue,of massive sulphide deposits. Volcanic-hosted massive
sulphidebut it is not inconsistent with the proposed geodynamic
evolution ofmineralization occurs at several stratigraphic horizons
within anthe studied area. This model gives insights into the
petrology andEarly Carboniferous volcano-sedimentary package formed
of tur-geochemistry of strike-slip settings in the continental part
of sub-biditic siliciclastic deposits and basaltic, intermediate
and silicicducting plates, a region usually poorly constrained from
a petrologicalvolcanic rocks. Volcanic rocks do not show
significant temporal orpoint of view.spatial variations in the
stratigraphic sequence of the Iberian Pyrite
Belt and mainly occur as shallow intrusions into wet
marinesediments with some minor lavas, hydroclastic rocks and
volcanogenicsediments. A geochemical study, including major, trace
and rareearth elements, and Sr and Nd isotopes, of the least
altered volcanic
KEY WORDS: calc-alkaline volcanism; isotope geochemistry;
strike-sliprocks has been carried out to determine the primary
magmatic affinitytectonics; Iberian Pyrite Beltand tectonic setting
of the Iberian Pyrite Belt volcanism. Most of
the basaltic rocks are continental tholeiites, but a few samples
showan alkaline affinity. The origin of the basaltic rocks and
theirdiversity of compositions are explained by a single mixing
modelbetween E- and N-MORB (mid-ocean ridge basalt) and as-
INTRODUCTIONsimilation of crustal material. Calc-alkaline
intermediate and silicic Volcanic-hosted massive sulphide deposits
are mainlyrocks include basaltic andesites, andesites, dacites and
rhyolites. associated world-wide with calc-alkaline submarine
vol-Volumetrically, dacites and rhyolites are the most abundant.
Inter- canism. Petrological and geochemical data, together
withmediate and silicic rocks are not related by fractional
crystallization, stratigraphic and structural studies, have been of
majornor is there a relationship between the basaltic and
calc-alkaline importance in constraining geological models of the
vol-rocks by the same process. We suggest that in the Iberian
Pyrite canism associated with massive sulphide deposits. In-Belt
silicic calc-alkaline magmas were generated on a large scale
tegrated studies in areas such as the Kuroko province inby the
invasion of continental crust by mafic magmas generated in Japan
(Ohmoto, 1983), the Mount Read Volcanics inthe underlying upper
mantle. The diversity of compositions shown Tasmania (Crawford et
al., 1992), and the Mount Windsorby dacites and rhyolites can
mainly be explained either by differences Volcanics in northwestern
Australia (Stolz, 1995), havein the composition of the source rocks
or by different degrees of revealed that this volcanism may be
developed duringpartial melting of upper-crust rocks. Andesites,
however, formed by different stages of the subduction process,
always beingmixing between basaltic magmas and upper-crust
material. The new located on the overriding plate.geochemical data
agree with previously published tectonostratigraphic This paper
documents the petrology and geochemistrydata which suggest that the
Iberian Pyrite Belt volcanism formed of the volcanism of the
Iberian Pyrite Belt, an Earlyon the South Portuguese plate owing to
strike-slip tectonics. This Carboniferous metallogenic province
that extends from
Portugal to Spain in southwest Iberia and constituteslocal
extensional tectonic setting was related to transtension as a
Corresponding author. Telephone: 34-3-330 27 16. Fax: 34-3-411
0012. e-mail: [email protected] Oxford University Press
1997
-
JOURNAL OF PETROLOGY VOLUME 38 NUMBER 6 JUNE 1997
one of the worlds largest reservoirs of massive sulphide thrust
that emplaces the Ossa Morena Zone structurallyabove the South
Portuguese Zone (Fig. 1) and is linkeddeposits. Despite the
economic significance of this area,
the volcanic rocks have not been studied extensively and to a
major geological boundary of similar characteristicsin the southern
British Isles (Crespo-Blanc & Orozco,most of the existing
geodynamic models proposed to
explain the origin of volcanism and associated massive 1991).
Thus, the South Portuguese Zone was accretedto the rest of the
Iberian Massif during the Variscansulphide deposits are not based
upon petrological and
geochemical data. Thus, in spite of the existence of Orogeny
(Ribeiro et al. 1990) and represents the con-tinental crust of a
tectonic plate whose oceanic crust wasseveral studies aimed at
characterizing the geochemistry
of volcanic rocks (Rambaud, 1969; Strauss, 1970; Hamet totally
subducted under the continental crust of the Ossa& Delcey,
1971; Lecolle, 1977; Routhier et al., 1977; Morena Zone (Quesada et
al., 1994).Priem et al., 1978; Soler, 1980; Strauss et al., 1981;
The Iberian Pyrite Belt is one of the four VariscanMunha, 1983;
Moller et al., 1983; Schutz et al., 1988), structural units
distinguished by Quesada (1991) in theand some studies focused on
the stratigraphy and tectonic South Portuguese Zone (Figs 1 and 2).
It is bounded tosetting of the Iberian Pyrite Belt (Schermerhorn,
1971; the north by the Pulo do Lobo oceanic terrane and toStrauss
& Madel, 1974; Ribeiro et al., 1983; Oliveira, the south by the
Baixo Alentejo Flysch Group. The main1990; Silva et al., 1990;
Quesada et al., 1994; Giese et al., feature of the Iberian Pyrite
Belt is the occurrence of1994), the nature and evolution of the
volcanism are polymetallic sulphide deposits associated with basic
andstill poorly known. Geochemical data (elementary and silicic
volcanic rocks which are interbedded with Earlyisotopic) are mainly
concerned with the Portuguese sector Carboniferous turbiditic
siliciclastic deposits. Sedi-of the Iberian Pyrite Belt. In
contrast, petrological and mentation of these deposits is
continuous through thegeochemical data relating to the volcanic
rocks from the stratigraphic sequence of the Iberian Pyrite Belt
and noSpanish sector, which includes the eastern part of the
internal unconformities have been observed. DepositionIberian
Pyrite Belt, are relatively scarce. of these rocks took place in a
submarine continental
In this paper, we describe and interpret the magmatic platform
environment (Oliveira, 1983, 1990) from bottomevolution and
tectonic setting of the Iberian Pyrite Belt and turbidity
currents.volcanism. The volcanic rocks comprise basalts, andesites,
Volcanic rocks interbedded with marine sediments aredacites and
rhyolites which mainly appear as shallow concentrated in the
central part of the Iberian Pyriteintrusions into wet marine
sediments with some minor Belt stratigraphic sequence, whereas they
are lacking in itslavas, hydroclastic rocks and volcanogenic
sediments. upper and lower parts. Strauss (1970) and
SchermerhornThe detailed stratigraphy established by Soriano (1997)
(1971) distinguished three lithostratigraphic units ac-shows a lack
of significant temporal and spatial variations cording to the
presence or absence of volcanic rocks andin the distribution of
volcanic rocks. We report new volcanism-related hydrothermal
alteration. From the basemajor, trace and rare earth element (REE),
and SrNd to the top, these stratigraphic units are as
follows:isotope data from the Spanish sector of the Iberian (1) The
Phyllite and Quartzite unit is composed ofPyrite Belt which have
been integrated with previous phyllites, quartzites and
conglomerates deposited on apetrological and geochemical data,
mainly from the Por- continental platform (Oliveira, 1990). Rare
limestonetuguese sector. A comprehensive geodynamic model, lenses
bearing conodont fauna are located at the top ofwhich differs from
those deduced from other volcanic- the unit and indicate a Lower to
Upper Famennian agehosted massive sulphide deposit areas, is
proposed to (Fig. 3). Its base never crops out throughout the
Iberianexplain the origin and evolution of the Iberian Pyrite
Pyrite Belt.Belt volcanism. We also discuss the genetic
relationship (2) The Volcano-Sedimentary Complex is the strati-of
the different rock types found in this volcanism. graphic unit
which contains the ore deposits. Most of
the volcanic rocks are shallow intrusive bodies whichshow
irregular shapes and contacts with the host rocks.Therefore, the
lower and upper boundaries and the
GEOLOGICAL SETTING thickness of the Volcano-Sedimentary Complex
are vari-able. The sediments interbedded with the volcanic rocksThe
sector of the European Variscan Orogen that crops
out in the western Iberian Peninsula is known as the are mainly
turbiditic platform deposits with oceanic faunasuch as radiolaria.
Rare limestone lenses bearing con-Iberian Massif (Lotze, 1945),
where five major Variscan
geological units were distinguished by Julivert et al. (1974)
odont fauna are located at the top of the unit and indicatea Lower
to Upper Visean age (Fig. 3).(Fig. 1). Recent studies on the
boundary between the
South Portuguese Zone and the Ossa Morena Zone have (3) The Culm
Group is the uppermost stratigraphicunit in the Iberian Pyrite
Belt. Its top has been erodedinterpreted it as a major suture of
the European Variscan
Orogen (Munha et al., 1986; Crespo-Blanc & Orozco, and
cannot be observed. Shales and sandstones depositedfrom turbidity
currents are the main lithologies. Facies1988; Quesada, 1991). This
suture is interpreted as a
728
-
MITJAVILA et al. MAGMATIC EVOLUTION AND TECTONIC SETTING OF THE
IBERIAN PYRITE BELT
Fig. 1. Tectonostratigraphic terrane map of the South Portuguese
Zone and the southern part of the Ossa Morena Zone. Adapted
fromQuesada (1991). Inset: Variscan geological units of the Iberian
Massif ( Julivert et al., 1974). CZ, Cantabrian Zone; ALZ,
AsturianLeonese
Zone; CIZ, Central Iberian Zone; OMZ, Ossa Morena Zone; SPZ,
South Portuguese Zone. Adapted from Julivert et al. (1974).
distributions strongly suggest flysch sedimentation (Olive-
north to south polarity (Dallmeyer et al., 1993; De LaRosa et al.,
1993).ira, 1990). Goniatite fauna found at the base of the unit
Variscan tectonics in the Iberian Pyrite Belt is char-indicate
an Upper Visean age (Fig. 3).acteristic of a fold and thrust belt
with a southwardHydrothermal alteration took place during and
shortlyvergence. Thrust sequences followed a piggy-back pro-after
the volcanism of the Volcano-Sedimentary Complexpagation mode, and
related folds developed an axial plane(Munha, 1990). Oxygen,
hydrogen and sulphur isotopiccleavage sometimes transecting the
fold axes (Ribeiro etcompositions (Munha & Kerrich, 1980;
Barriga & Ker-al., 1983). An increase in the cleavage intensity
is seenrich, 1984) demonstrate that these hydrothermal pro-from
south to north across the entire South Portuguesecesses involved
the interaction of magmatic fluids withZone (Silva et al., 1990),
and is probably related to aseawater.thermal control on the
development of ductile structuresAll of the above-mentioned rocks
of the Iberian Pyriteby large intrusives in the northeastern part
of the SouthBelt were metamorphosed and deformed during
thePortuguese Zone. The detachment level of the thrusts isVariscan
Orogeny. A regional low-grade metamorphismfound at the base of the
Palaeozoic sequence of the Southof zeolite to greenschist facies
increases from south toPortuguese Zone (Ribeiro et al., 1983).north
across the Iberian Pyrite Belt and the South Por-
tuguese Zone, and towards the base of the stratigraphicsequence
(Munha, 1990). Large intrusive bodies of gran-
STRATIGRAPHY OF THE VOLCANO-itic, tonalitic and dioritic
composition, which appear atSEDIMENTARY COMPLEXthe northeastern
part of the South Portuguese Zone and
the southern part of the Ossa Morena Zone, are thought The
Volcano-Sedimentary Complex is characterized bythe presence of a
monotonous sequence (500800 mto be responsible for this regional
metamorphism and its
729
-
JOURNAL OF PETROLOGY VOLUME 38 NUMBER 6 JUNE 1997
Fig. 2. Geological map of the Spanish sector of the Iberian
Pyrite Belt showing the location of the stratigraphic sections in
Fig. 4 [modifiedafter Instituto Geologico y Minero de Espana
(1982)].
thick) of Late DevonianEarly Carboniferous sandstones the
Iberian Pyrite Belt silicic rocks are mostly found inthe lower part
of the Volcano-Sedimentary Complexand shales with shallow
sub-horizontal intrusions of ba-sequence, whereas in the north and
northeastern partsaltic, intermediate and silicic compositions, and
minorvery shallow silicic intrusives are located at its top
(Fig.interbedded lavas, hydroclastic rocks and volcaniclastic4).
Despite this trend, silicic intrusives also appear at thesediments
(Fig. 4). A detailed revision of the stratigraphytop of the
Phyllite and Quartzite unit in the north andof the
Volcano-Sedimentary Complex (Soriano, 1997)east of the Iberian
Pyrite Belt.has revealed that the emplacement of these
intrusives
Andesitic rocks do not crop out in the southwesternmosttook
place at different levels in the stratigraphic sequencepart of the
studied area. They mainly appear as shallowand that they are
interfingered with the host sediments.intrusives above and below
felsic volcanics in the centralThis has caused some
misinterpretations of the volcanicpart of the Iberian Pyrite Belt,
and can also intrude eitherevents represented in the
Volcano-Sedimentary Com-the top of the Phyllite and Quartzite Unit
or the highestplex, such as the distinction of several
well-separatedlevels of the Volcano-Sedimentary Complex. In
thevolcanic episodes made by Instituto Geologico y
Mineronortheasternmost part of the Iberian Pyrite Belt andesiticde
Espana (1982), which we will discuss in a later section.rocks are
always below silicic volcanics in the stratigraphicPrevious studies
of the Iberian Pyrite Belt volcanismrecord (Fig. 4). On the other
hand, basaltic rocks aresuggested the occurrence of primary
pyroclastic depositswidely distributed and do not show any specific
strati-(Schermerhorn, 1976; Lecolle, 1977). However, most ofgraphic
or palaeogeographic position.the volcaniclastic deposits found in
the studied area
are not of pyroclastic origin. They were formed byhydroclastic
fragmentation and erosion of submarinelavas and domes (Soriano,
1997).
PETROGRAPHY OF VOLCANICThe palaeogeographic and stratigraphic
distribution ofROCKSvolcanic rocks in the Volcano-Sedimentary
Complex is
relatively haphazard (Fig. 4), but some trends can be Felsic
rocks range in composition from dacite to rhyolite.inferred from
stratigraphic sections throughout the Iber- Most of them appear as
shallow intrusives which show
peperitic textures developed at the contacts betweenian Pyrite
Belt. In the south and southwestern part of
730
-
MITJAVILA et al. MAGMATIC EVOLUTION AND TECTONIC SETTING OF THE
IBERIAN PYRITE BELT
Fig
.3.
Age
sof
geol
ogic
alev
ents
inth
eSo
uth
Port
ugue
seZ
one.
The
met
hod
used
inda
ting
any
part
icul
arev
ent
issh
own
inth
elo
wer
row
.T
henu
mbe
rsin
the
Van
den
Boo
gaar
d&
Sche
rmer
horn
colu
mn
refe
rto
the
follo
win
gpa
pers
:1
Van
den
Boo
gaar
d&
Sche
rmer
horn
,198
1;2
Van
den
Boo
gaar
d&
Sche
rmer
horn
,197
5a;3
V
ande
nB
ooga
ard
&Sc
herm
erho
rn,
1980
;4
Van
den
Boo
gaar
d&
Sche
rmer
horn
,19
75b.
731
-
JOURNAL OF PETROLOGY VOLUME 38 NUMBER 6 JUNE 1997
Fig. 4. Stratigraphic sections of the Volcano-Sedimentary
Complex in the Iberian Pyrite Belt (see Fig. 2 for location).
intrusions and wet sediments (Boulter, 1993a, b). Oc-
hydroclastic deposits. Other field structures such as flow-banding
foliation, flow autobrecciation and soft-sedimentcasionally, felsic
rocks reached the oceanic floor as ex-
trusive domes giving rise to short-length lava flows and
intrusions through columnar jointing of volcanic rocks
732
-
MITJAVILA et al. MAGMATIC EVOLUTION AND TECTONIC SETTING OF THE
IBERIAN PYRITE BELT
also support a shallow intrusive emplacement for most procedures
used were inductively coupled plasma massspectrometry (ICP-MS)
after fusion for major elementsof the silicic volcanic rocks.
Rhyolitic rocks are mainly composed of albite and analysis plus
Ba, Sr, Y and Zr; ICP-MS after HF digestionfor trace metals (Cu,
Pb, Zn, Ag, Ni, Cd, Bi, V and Be);quartz phenocrysts and minor
biotite phenocrysts (par-
tially or totally replaced by chlorite). Occasionally, K- X-ray
fluorescence (XRF) pressed pellet for the elementsGa, Sn, S, Nb and
Rb; and instrumental neutron ac-feldspar may also be present. A
felsitic groundmass is
characteristic of rhyolites, which also show perlitic and
tivation analysis (INAA) for Au, As, Co, Cr, Cs, Hf, Sb,Sc, Ta, Th,
U and REE. All the analyses were performedspherulitic textures.
Plagioclase phenocrysts and those
parts of the groundmass enriched with plagioclase mi- at ACTLABS
(Canada) following standard proceduresfor each method. The
detection limits are indicated incrolites are often altered to
calcite, muscovite and sericite.
Accessory minerals in dacites and rhyolites include ap- Table 1.
The standards used by ACTLABS were:CCRMP SY-2, MRG-1, SY-3, USGS
G-2, W-2 andatite. Dacites usually show porphyritic and
glomero-
porphyritic to massive coherent textures. Embayed and AGV-1.
Some of the standards have been run in duplicateand the results
obtained do not differ from the certifiedcurviplanar quartz
phenocrysts, subhedral albitized pla-
gioclase, and rare biotite phenocrysts and clino- values. The
selection of the samples was done with theaim of covering all the
Spanish sector of the Iberianpyroxene microphenocrysts are set in a
micro-
crystalline quartzfeldspar groundmass. Pyrite Belt and all the
volcanic rock types present. Therocks show varying degrees of
alteration, therefore careIntermediate volcanic rocks are
porphyritic to glo-
meroporphyritic andesites with subhedral prismatic pla- was
taken in selecting the less altered rocks after
detailedpetrographic study.gioclase, occasionally albitized,
clinopyroxene
phenocrysts, Fe-oxides, and rare quartz and biotite Twelve of
these samples were selected for analysis ofRbSr and SmNd isotope
geochemistry (Table 2). These(mostly chloritized), embedded in a
microcrystalline
groundmass of microlitic plagioclase and microcrystalline 12
samples are the least altered and cover all the rocktypes
recognized in the Iberian Pyrite Belt. The samplesquartz. They
usually have amygdales filled with chlorite,
calcite, epidote and quartz. Plagioclase phenocrysts are were
ground to fine powder and acid washed for 45 minat 50C to remove
alteration products. Sample dissolutionoften altered to calcite,
muscovite and epidote, and the
groundmass can be altered to chlorite. Field exposures and
chemical separation methods for Rb, Sr, Sm andNd followed standard
procedures. Blank levels averagedof andesites can show highly
irregular, sometimes chilled,
contacts with the host sediments, and frequently have 01 ng for
Nd and 02 ng for Sr. Samples were analysedusing a Finnigan MAT 262
mass spectrometer at thecolumnar and spheroidal jointing. Two of
the studied
samples contained olivine phenocrysts, together with University
of Oslo (Norway). Sm and Nd were loadedonto Re filaments of a
double Re filament assembly andclinopyroxene and a Ca-rich
plagioclase. This suggests
the presence of restricted basaltic andesites in the vol- run as
metal ions. Sr was run in a single Ta filament.Nd isotopes were
measured using a single-jump triple-canism of the Iberian Pyrite
Belt.
Basaltic rocks show massive, intergranular, equi- collector
dynamic routine. Sr was measured in staticmulti-collection mode.
Analyses of inter-laboratory stand-granular and minor porphyritic
textures. Subhedral clino-
pyroxene and plagioclase (mostly albitized) frequently ards gave
the values of 143Nd/144Nd=051109210 forthe Johnson & Matthey
JMC321 Nd standard and ofshow intergrowing of crystals. In most of
the rocks the
clinopyroxene is a pale-coloured augite. However, a
87Sr/86Sr=071017712 for the NBS 987 Sr standard.few basaltic
samples contain a titaniferous salite, whichsuggests an alkaline
affinity. In the porphyritic varietiesthe groundmass contains
plagioclase microlites and mi- Alteration of volcanic
rockscrocystalline augite. Minor subhedral crystals of olivine The
volcanic rocks of the Iberian Pyrite Belt were affectedand FeTi
oxides are also present in all basaltic rocks. by several
alteration and modification processes, whichChlorite, epidote and
calcite usually fill original vesicles include low-temperature
hydration of glass (probablybut also appear as secondary minerals
replacing original caused by seawater percolation), hydrothermal
alteration,components of the rock. Field exposures occasionally and
regional low-grade metamorphism (greenschist fa-show minor pillows
and chilled margins developed at the cies). All these processes
have changed the primary chem-contacts with host sediments. istry
of the rocks, with significant gains and losses of
several elements and oxidation of Fe.Mobility of chemical
components of volcanic rocks
affected by hydrothermal alteration and low-grade
meta-GEOCHEMISTRYMethods morphism has been extensively documented
(Wood et
al., 1976; Floyd & Winchester, 1978; MacLean &
Kran-Fifty-nine new samples have been analysed for majorand trace
elements, and REE (Table 1). The analytical idiostis, 1987; MacLean
& Barret, 1993). To better
733
-
JOURNAL OF PETROLOGY VOLUME 38 NUMBER 6 JUNE 1997
Tabl
e1:
Maj
or,tra
cean
dra
reea
rthele
men
tana
lyse
sof
volca
nic
rock
san
don
ese
dim
ent(
FP-5
2)fro
mth
eSp
anish
secto
rof
the
Iber
ian
Pyrit
eB
elt;t
hetra
ceele
men
tdete
ction
limits
are
3r
Met
ho
dD
etec
.lim
its
FP-1
09FP
-80
FP-9
1FP
-34
FP-9
1AFP
-20
FP-1
18FP
-114
FP-7
FP-4
0FP
-105
FP-3
1FP
-117
FP-1
5FP
-16
FP-5
7FP
-119
FP-1
02FP
-116
FP-1
50
Fusi
on
ICP
SiO
20
0544
06
450
446
23
462
846
44
467
847
12
474
148
17
481
748
19
486
649
04
518
651
96
522
653
38
547
355
32
560
9
TiO
20
051
450
641
641
661
621
831
621
401
091
362
061
421
651
310
971
551
470
911
200
67
Al 2
O3
002
141
818
32
152
615
06
155
013
20
156
815
43
165
615
76
145
915
83
158
416
04
136
211
97
160
516
99
164
717
32
Fe2O
30
002
107
98
3310
40
128
010
22
110
19
8810
24
919
807
108
59
9310
83
852
109
09
348
928
617
755
26
Mn
O0
020
130
130
160
180
160
170
170
120
140
120
200
170
140
090
150
160
170
150
140
07
Mg
O0
036
1011
11
461
643
448
428
662
107
48
195
616
637
687
195
876
066
615
844
724
212
03
CaO
002
789
102
717
25
923
172
18
859
115
618
0813
45
103
49
677
868
109
828
774
815
694
827
52
Na 2
O0
022
571
380
464
010
243
013
884
232
592
262
862
902
963
913
373
655
022
894
510
95
K2O
000
50
320
380
05
-
MITJAVILA et al. MAGMATIC EVOLUTION AND TECTONIC SETTING OF THE
IBERIAN PYRITE BELT
Tota
ld
iges
tio
nIC
PC
u1
p.p
.m.
1220
3643
3920
3028
3553
2024
3425
3634
188
2914
Pb
5p
.p.m
.
-
JOURNAL OF PETROLOGY VOLUME 38 NUMBER 6 JUNE 1997
Tabl
e1:
cont
inue
d
Met
ho
dD
etec
.lim
its
FP-8
4FP
-144
FP-8
3FP
-101
FP-1
00FP
-97
FP-1
54FP
-95
FP-9
4FP
-93
FP-1
9FP
-134
FP-2
7FP
-17
FP-5
FP-4
9FP
-137
FP-2
2FP
-133
FP-7
9
Rec
alcu
late
d10
0%
wat
erfr
ee
SiO
260
99
613
561
83
633
163
70
649
265
51
638
164
63
653
568
33
691
369
36
704
371
64
758
076
10
755
574
96
747
1
TiO
20
850
890
780
650
820
860
650
840
810
770
470
340
540
360
700
120
330
300
190
20
Al 2
O3
168
616
79
171
715
69
176
014
84
159
813
93
154
115
78
169
415
57
133
315
49
115
714
17
126
713
01
122
213
29
Fe2O
36
568
286
466
076
626
933
576
846
415
223
134
436
283
614
911
642
561
801
962
69
Mn
O0
050
030
090
090
170
080
040
110
100
070
020
050
090
030
080
020
030
020
030
03
Mg
O4
532
094
533
122
003
070
804
312
601
761
711
590
901
981
283
092
830
380
400
78
CaO
363
319
453
785
197
269
676
385
422
611
199
235
370
115
160
028
017
094
025
181
Na 2
O5
596
523
942
706
465
266
455
254
064
543
134
753
893
754
831
941
255
090
923
93
K2O
080
067
058
043
050
124
012
093
163
025
420
167
177
313
323
290
399
283
904
251
P2O
50
140
190
110
090
160
120
120
130
130
140
100
120
130
080
140
030
060
080
040
06
Tota
l99
99
100
0010
002
100
0110
000
100
0010
000
100
0010
000
100
0010
001
100
0010
000
100
0199
99
100
0010
000
100
0110
002
100
00
Ba
3p
.p.m
.14
612
519
694
8917
337
201
351
6446
737
834
739
040
862
724
642
041
366
3
Sr
1p
.p.m
.18
222
028
818
015
310
267
581
200
126
119
153
158
7618
535
3012
528
148
Y1
p.p
.m.
2716
2627
2925
2228
3530
2946
8230
7249
2535
7442
Zr
3p
.p.m
.12
591
114
115
167
133
100
115
148
154
114
172
195
7317
158
150
7027
316
9
XR
Fp
ress
edp
elle
tG
a5
p.p
.m.
1910
2517
1620
1514
2022
2821
2625
2123
1613
2128
Sn
5p
.p.m
.
-
MITJAVILA et al. MAGMATIC EVOLUTION AND TECTONIC SETTING OF THE
IBERIAN PYRITE BELT
INA
AA
u(p
.p.b
.)5
p.p
.b.
13
-
JOURNAL OF PETROLOGY VOLUME 38 NUMBER 6 JUNE 1997
Tabl
e1:
cont
inue
d
Met
ho
dD
etec
.lim
its
FP-7
8FP
-128
FP-2
9FP
-130
FP-1
11FP
-122
FP-1
31FP
-121
FP-4
6FP
-127
FP-1
12FP
-113
FP-4
5FP
-129
FP-2
5FP
-107
FP-1
10FP
-28
FP-5
2
Rec
alcu
late
d10
0%
wat
erfr
ee
SiO
276
50
772
976
87
776
477
23
771
477
37
770
177
58
786
978
51
781
878
94
790
778
98
798
882
29
836
868
57
TiO
20
190
140
390
180
060
140
160
120
260
140
050
060
260
130
140
120
100
100
40
Al 2
O3
127
112
02
106
313
23
117
814
24
116
412
57
122
811
31
121
612
44
120
610
68
114
811
12
124
69
3616
12
Fe2O
33
523
032
122
021
180
932
191
361
711
230
410
541
461
641
090
370
501
024
35
Mn
O0
020
030
020
010
010
010
010
040
030
010
010
010
020
030
010
010
010
010
03
Mg
O2
923
212
252
431
520
500
100
240
340
230
600
370
310
250
050
220
740
074
33
CaO
032
056
071
196
008
016
042
020
197
009
006
009
034
035
001
022
020
042
088
Na 2
O1
021
222
241
411
034
653
764
334
621
891
122
224
431
194
353
170
265
000
60
K2O
275
241
469
103
711
220
431
409
113
636
706
607
212
660
387
486
342
032
467
P2O
50
060
080
060
090
020
040
040
040
070
060
030
030
060
070
020
030
030
020
05
Tota
l10
000
100
0099
99
100
0010
002
100
0110
000
100
0110
000
100
0110
001
100
0199
99
100
0210
000
100
0110
001
100
0110
000
Ba
3p
.p.m
.26
554
941
415
742
067
303
412
158
605
431
442
205
385
486
329
134
144
424
Sr
1p
.p.m
.42
3875
141
2323
5527
157
5427
2339
4537
7411
8613
Y1
p.p
.m.
3064
3165
5158
6539
3565
5650
3757
5028
3762
77
Zr
3p
.p.m
.16
413
384
176
7811
026
196
6988
8083
7116
012
013
588
4820
2
XR
Fp
ress
edp
elle
tG
a5
p.p
.m.
1923
1326
2220
2420
1722
2215
1523
208
2210
23
Sn
5p
.p.m
.9
88
14
-
MITJAVILA et al. MAGMATIC EVOLUTION AND TECTONIC SETTING OF THE
IBERIAN PYRITE BELT
INA
AA
u(p
.p.b
.)5
p.p
.b.
-
JOURNAL OF PETROLOGY VOLUME 38 NUMBER 6 JUNE 1997
Tabl
e2:
Isot
opic
com
posit
ions
ofSr
and
Nd
for
the
selec
ted12
sam
ples
Sam
ple
Rb
(p.p
.m.)
Sr
(p.p
.m.)
87S
r/86
Sr (0
)2r
87R
b/8
6 Sr
e Sr(
0)f(
Rb
/Sr)
87S
r/86
Sr (3
68M
a)e S
r(36
8)
FP-9
1B
asal
t0
2741
278
070
3884
180
0019
0
874
2
097
700
7038
74
274
FP-4
0B
asal
t16
94
591
930
7037
7518
008
275
10
30
0006
070
3342
10
30
FP-2
0B
asal
t69
71
864
070
7045
2618
023
333
037
181
8214
070
3304
10
82
FP-1
5B
asal
t4
7438
691
070
4491
160
0354
6
012
2
057
120
7043
053
39
FP-1
02A
nd
esit
e47
81
342
790
7066
8716
040
347
310
473
8787
070
4573
721
FP-1
54A
nd
esit
e1
6475
937
070
4237
160
0062
4
373
2
092
450
7042
041
95
FP-8
3A
nd
esit
e19
17
358
460
7054
802
015
468
139
070
8704
070
4670
856
FP-5
Dac
ite
113
9918
955
071
3245
161
7407
712
413
200
492
070
4125
091
FP-2
7D
acit
e46
60
173
270
7083
5018
077
818
546
438
4097
070
4273
296
FP-4
6R
hyo
lite
463
615
051
071
0941
160
8914
591
428
977
930
7062
7031
33
FP-1
30R
hyo
lite
344
615
79
070
9951
180
6316
377
375
663
760
7066
4236
58
FP-7
9R
hyo
lite
345
015
727
070
8287
160
6347
453
752
667
520
7049
6112
73
Sam
ple
Sm
(p.p
.m.)
Nd
(p.p
.m.)
143 N
d/1
44N
d(0
)2r
145 N
d/1
44N
d2r
147 N
d/1
44N
de N
d(0
)f(
Sm
/Nd
)14
3 Nd
/144
Nd
(368
Ma)
e Nd
(368
)
FP-9
1B
asal
t3
7713
27
051
2789
130
3483
931
017
182
938
0
1263
051
2375
410
FP-4
0B
asal
t1
906
030
5128
097
034
8441
30
1904
333
7
003
150
5123
503
63
FP-2
0B
asal
t0
922
050
5127
1311
034
8527
50
2713
145
90
3797
051
2059
2
05
FP-1
5B
asal
t2
258
680
5126
8628
034
8454
30
1569
093
8
020
190
5123
082
80
FP-1
02A
nd
esit
e4
7020
66
051
2461
90
3484
495
013
76
345
4
030
010
5121
29
068
FP-1
54A
nd
esit
e2
2610
86
051
2511
220
3484
366
012
57
247
6
036
060
5122
080
85
FP-8
3A
nd
esit
e4
1418
68
051
2425
10
3484
3213
013
39
415
6
031
890
5121
02
121
FP-5
Dac
ite
510
189
00
5124
971
034
8391
60
1632
2
75
017
010
5121
04
118
FP-2
7D
acit
e9
8939
12
051
2649
80
3484
213
015
280
207
0
2226
051
2281
226
FP-4
6R
hyo
lite
279
139
40
5123
798
034
8416
30
1209
5
048
0
3852
051
2088
1
49
FP-1
30R
hyo
lite
426
164
10
5124
626
034
8426
30
1571
3
433
0
2008
051
2083
1
58
FP-7
9R
hyo
lite
449
205
50
5124
539
034
8418
30
1320
3
601
0
3285
051
2135
0
57
740
-
MITJAVILA et al. MAGMATIC EVOLUTION AND TECTONIC SETTING OF THE
IBERIAN PYRITE BELT
constrain the changes caused by the alteration processes that
these basalts do not show calc-alkaline fractionationon the primary
geochemistry of the Iberian Pyrite Belt trends. Munha (1983) found
the same characteristics involcanic rocks, and to evaluate the
relative mobility the basalts from the Portuguese sector of the
Iberianof chemical elements, factor analysis of the principal
Pyrite Belt. The rest of the major elements show a highcomponents
has been used. This method has been largely dispersion of values,
mainly those which are more mobileused to reveal the elemental
association that, in this during secondary alteration (Na, Ca, Al,
etc.), whereascase, could be an indication of remobilization owing
to K and P values are less varied. Some of the observedsecondary
processes (Howarth & Sinding-Larsen, 1983). variations may be
primary, such as the contents in TiThe factors obtained give some
clues that confirm the and Mg, which may reflect the fractionation
of somemobility of some elements, already deduced by pet-
ferromagnesian minerals. These variations in Ti and Mgrography and
geochemistry. Thus, Na, Pb, Co, Si, Al, are accompanied by strong
variations in Ni and Cr, andK, Ba and Cu, and to a smaller degree
Rb, Mg, Ni, Sr, to a lesser degree Co, which agree with
ferromagnesianCr and Th, should be considered as mobile elements,
mineral fractionation.i.e. elements redistributed during secondary
alteration in Basaltic andesites and andesites can be distinguished
bythe Iberian Pyrite Belt. The mobility of some of these their
major element contents along with the petrographicelements differs
depending on the rock type. A similar analysis. Only one sample
analysed in this study is asituation is found in other
palaeovolcanic areas [see, e.g. basaltic andesite: FP-119. It shows
the highest Ti, Fe andThorpe et al. (1993)]. Mn contents of the
andesitic group and the lowest K
content, whereas it shows intermediate concentrations ofAl, Ca
and Na. The rest of the andesites show a cleargeneral decreasing
trend for Ti, Fe and Mg, whereas theGeneral classification and
description ofsilica content increases. Two groups of andesites can
begeochemical dataconsidered depending on their MgO content, which
show
The chemical compositions of the studied volcanic rocks two
divergent correlations with Ti, Fe and V contents.are shown in
Table 1. Major element analyses have been Also, the Fe2O3t/MgO
ratios vs Fe2O3t, Ti, Ti/V, etc.recalculated on a 100% water free
basis to minimize the show the same divergent correlations. Munha
(1983)effects of alteration by removing variations owing to found
similar subdivisions for the Portuguese andesites.different loss on
ignition values. The Nb/Y vs Zr/TiO2 These two groups of andesites
do not show any pref-variation diagram (Fig. 5; Winchester &
Floyd, 1977) has erential stratigraphic or geographic position.been
used to establish a broad classification of the studied The most
evolved rocks in the Iberian Pyrite Beltvolcanics. This diagram
shows the existence of different volcanism, which are
volumetrically dominant, are rep-types of rocks, which have already
been deduced in the resented by dacites and rhyolites. A gradual
geochemicalfield and from the petrographical studies. transition
exists between the two groups of rocks. Despite
the fact that secondary alteration has caused a significantMajor
elements change in the original composition of these rocks,
with
a high perturbation of the Al, K, Na and Si contents,Major
element compositions of basaltic rocks from theSpanish sector of
the Iberian Pyrite Belt (see Table 1) several general tendencies
can be established. Ti, Fe, Caindicate that most of them are
sub-alkaline basalts and and P show an increasing depletion towards
the mostthat a few samples show a slightly alkaline affinity. This
evolved rocks, whereas no tendency is apparent for Nais suggested
by the relatively high TiO2 (>12) and P2O5 and K. Mg is higher
for the Si-poor rhyolites than for(~02) contents (see Table 1), the
low contents in Zr and the dacites or the Si-rich rhyolites. These
differences inthe low (
-
JOURNAL OF PETROLOGY VOLUME 38 NUMBER 6 JUNE 1997
Fig. 5. Classification of the studied rocks on a Winchester
& Floyd (1977) diagram.
crust normalized values. They also have the highestcontents in
Ni, Cr and Mg, suggesting that these samplesrepresent the most
primitive basalts in the studied area,and that they formed by high
degrees of partial meltingof a peridotitic mantle source. Less
primitive basalts (FP-15, FP-16, FP-1097 and FP-20) have similar
con-centrations of compatible elements to the continentalcrust but
show higher normalized concentrations of theincompatible elements
than the average crust, withoutany possible correlation. Their low
contents of compatibleelements suggest that they originated by
lower degreesof partial melting, or that they suffered
fractionation ofprimitive minerals during their ascent. The
incongruent
Fig. 6. TiO2 vs Y/Nb diagram. Two samples fall in the alkaline
field, enrichment in incompatible elements suggests that
thesewhereas the rest fall on the continental tholeiites and
mid-ocean ridge basalts could also have assimilated continental
crust. In
basalt (MORB) fields. Fields are after Floyd & Winchester
(1975).addition, secondary alteration may have had a
moderateinfluence on the content of some incompatible elements.
Mg, Co, Fe) show significant variations and positive For
instance, all the basaltic rocks show a negativecorrelations among
the different samples. On the other anomaly in Cu, which is clearly
associated with a sec-hand, the variations observed in the
normalized con- ondary origin related to the ore deposition
process.centration of the incompatible elements (Rb, Pb, U, Th,
Trace element contents of andesitic rocks do not showBa, K) do not
show any correlation among the different typical intermediate
values between basalts and the moresamples of this group of rocks.
The variation observed evolved rocks. On the contrary, their trends
appear toin the compatible elements suggests differences in the be
independent of the rest of the volcanic rocks from theprimary
source of these basalts (differences in the source studied area. In
some cases, trace element contents ofor different degrees of
partial melting), or fractionation andesites are similar to the
contents either of the basalticof ferromagnesian minerals which
affects the contents of or the dacitic rocks. This suggests that
andesites are notNi, Mg, Cr, etc. Thus, in Fig. 7, FP-80 and FP-114
plot related either to basalts or to dacites or rhyolites by
fractional crystallization.between the primitive mantle values
and the continental
742
-
MITJAVILA et al. MAGMATIC EVOLUTION AND TECTONIC SETTING OF THE
IBERIAN PYRITE BELT
Fig
.7.
743
-
JOURNAL OF PETROLOGY VOLUME 38 NUMBER 6 JUNE 1997
Fig. 7. Elements from all the studied rocks normalized to
primitive mantle in order of incompatibility in the continental
crust (Hofmann, 1988).Average continental crust after Taylor &
MacLennan (1985) has also been plotted for comparison as shaded
circles. The basalts and rhyolites
have been plotted in separate diagrams to increase clarity.
Andesites are characterized by an irregular distribution silica
content variation, i.e. samples with the same silicacontent have
large differences (hundreds of p.p.m.) ofof highly incompatible
elements (Fig. 7), absence of K
and Na anomalies (except for sample FP-150, which has Ba, Zr,
Th, U, Hf, etc., in both rock types. Dacites arethe volcanic rocks
from the Iberian Pyrite Belt whicha positive anomaly in K and a
negative one in Na), lower
Mg and Ni contents than the basalts, and a strong positive show
normalized concentrations (Fig. 7) of trace elementscloser to the
average continental crust (see Taylor &anomaly of Sn and a
negative anomaly of Cu. As with the
basaltic rocks, the behaviour of the highly incompatible
McLennan, 1985). However, many dacites are slightlyenriched in some
incompatible elements with respect toelements may indicate
different source characteristics.
However, the original contents of Pb, U, Th, Rb and the
continental crust. Samples FP-154, FP-93 and FP-17, however, show
extremely low values of Pb, Ba, K,Nb are clearly masked by
secondary alteration. The
absence of Na and K anomalies implies that these ele- Nb, Sn and
Sr. Most of the dacites show negativeanomalies in Ti and Cu, and a
positive anomaly in Sn.ments were not highly mobilized. The
anomalies in Sn
and Cu, which are found in nearly all rock types, are All the
rhyolites show a very similar trend, with increasingnegative
anomalies in Sr, Eu, Ti, Ca and Fe, and positiveclearly related to
the ore-forming event.
Incompatible element contents of dacites and rhyolites anomalies
in K and Sn as silica content increases (Fig.7).are extremely
variable and they do not depend on the
744
-
MITJAVILA et al. MAGMATIC EVOLUTION AND TECTONIC SETTING OF THE
IBERIAN PYRITE BELT
the primary character of these rocks. The parallelismREEshown by
most of the normalized patterns of the dacitesIn the REE normalized
to chondrites (Nakamura, 1974)and rhyolites indicates the existence
of different degreesdiagram (Fig. 8) it is possible to observe that
most of theof partial melting in the formation of these two
groupsnormalized concentrations of the basaltic rocks show aof
rocks, and precludes mixing or assimilation betweenrelatively flat
pattern and are limited by two extremethem. Only the previously
mentioned anomalous rhy-samples (FP-20 and FP-80). Sample FP-20
shows theolites may have undergone such processes, as is
reflectedhighest fractionation between light REE (LREE) andby the
REE normalized pattern.heavy REE (HREE) [(La/Lu)n=578], i.e. it has
the
maximum La content and the minimum Lu content ofall the group.
Sample FP-80, however, shows an almostflat pattern with a positive
Tb anomaly and a fractionation Isotope geochemistry and age of
thevalue between Lan and Lun of 137. As has been indicated Iberian
Pyrite Belt volcanismbefore, FP-80 represents the most primitive
basalt of the Sr and Nd isotopic compositions have been obtained
instudied area. The rest of the basalts show (La/Lu)n values 12
selected samples comprising 4 basalts, 2 andesites, 3ranging from
168 to 495. This, together with the Eu/ dacites and 3 rhyolites.
The results are shown in TableSm values (026042), and the relative
flat trends of the 2. Initial ratios, as well as e values, have
been calculatedREE normalized to chondrite, suggests that most of
the for the age of 368 Ma, as we will discuss below. Thebasaltic
rocks from the Spanish sector of the Iberian constants used for all
the calculations are: 87Sr/86SrURPyrite Belt have the
characteristics of tholeiites from (0)=07045, 87Rb/86SrUR
(0)=00827, kRb=1421011/continental settings [see Cullers &
Graff (1984) and year for the Rb/Sr method, and 143Nd/144NdCHUR
(0)=references therein]. Samples which show high LREE/ 0511847,
143Sm/144NdCHUR (0)=01967, kSm=HREE ratios are those with a
slightly alkaline affinity. 6541012/year for the Sm/Nd couple.
Basaltic andesites and andesites show, in general, To constrain
the age of the Iberian Pyrite Belt vol-homogeneous patterns with
low fractionation between canism, several methods have been applied
by differentLREE and HREE. (La/Lu)n is 342 for the basaltic workers
(see Fig. 3 for details). The difficulty of datingandesite, and is
slightly higher (391483) for the andes- these rocks by isotopic
means is that they have undergoneites (Fig. 8). The main difference
among the REE patterns a low-grade metamorphism. Some of these
radiometricof these rocks is the Eu anomaly. The basaltic andesite
ages are those of the metamorphism, and others (see Fig.and one
andesite show no Eu anomaly (Fig. 8), whereas 3 for references)
correspond to the age of the plutonicthe rest of the andesites show
an increasing Eu anomaly rocks responsible for such metamorphism.
Hamet ¬ related to any other geochemical feature, thus in-
Delcey (1971) obtained an Rb/Sr age of 376 Ma for adicating the
existence of different original plagioclase group of six rhyolites
from Portugal. This age has alreadycompositions in the andesitic
group, or variations on f (O2) been recalculated with the new decay
constant (kRb=during crystallization. These changes in the
plagioclase 1421011/year) and is statistically
indistinguishablecomposition are not reflected in the other REE.
Sample from our result (see below). Priem et al. (1978) dated
theFP-144 shows the same fractionation between LREE and
metamorphism which affects the Volcano-SedimentaryHREE but with
very low values and with a straight Complex in Portugal using the
Rb/Sr method and ob-pattern (except for Lu). This feature may also
indicate tained an age of 30810 Ma, after recalculating it
withdifferences in the source, but factor analysis suggests that
the new decay constant. Dallmeyer et al. (1993), with thethis
sample has been significantly altered. use of the 40Ar/39Ar method,
have obtained a cooling
REE normalized diagrams of dacites and rhyolites (Fig. age of
335342 Ma for the intrusive bodies and the8) show parallel patterns
except for three samples of associated low-grade metamorphism in
the Ossa Morenarhyolite, which have irregular patterns. The
fractionation Zone. Finally, De la Rosa et al. (1993) have obtained
anbetween LREE and HREE is (La/Lu)n=314693 for errorchrone for
pluton emplacement at the north of thethe dacites, and
(La/Lu)n=248526 for the rhyolites. South Portuguese Zone by
including in their data theTwo extreme rhyolitic samples have a
value of 073 age obtained by Dallmeyer et al. (1993) in adjacent
units.(sample FP-113) and 703 (sample FP-122). The Eu In this
study, whole-rock KAr dating has been per-anomaly is clearly
defined and reaches a maximum value formed on some samples to check
the possibility of usingin the intermediate members of the group.
One dacitic this method with the less altered samples or at least
tosample shows low normalized contents in Yb and Lu date the
metamorphic phase. All the ages obtained are(Fig. 8) with respect
to the rest. These differences in the younger than the age of the
metamorphism itself. ThisYb and Lu normalized contents have no
equivalent in suggests that Ar has been removed (and probably K
hasthe other elements, at least for the same samples, sug- been
added) also after the metamorphic phase, as a
continuous process. Thus KAr ages in this case havegesting that
secondary alteration has probably masked
745
-
JOURNAL OF PETROLOGY VOLUME 38 NUMBER 6 JUNE 1997
Fig
.8.
RE
Eno
rmal
ized
toch
ondr
ites
afte
rN
akam
ura
(197
4)fo
rth
est
udie
dro
cks.
746
-
MITJAVILA et al. MAGMATIC EVOLUTION AND TECTONIC SETTING OF THE
IBERIAN PYRITE BELT
no geological meaning and no further discussion of these below
the sea-floor up to 1000 m (Soriano, 1997). Thus,results will be
presented here. their apparent position in the stratigraphic
sequence does
Using the isochron method with the Sr isotopic com- not
necessarily indicate a time relation with other volcanicposition on
whole-rock samples, divided into basaltic and rocks in the same
section. Also, many of these shallowcalc-alkaline rocks, we have
obtained an age of 36864 intrusions show interfingering with the
host sediments(1r error) Ma for the calc-alkaline suite (andesites,
dacites and give the appearance, in the same stratigraphic
profile,and rhyolites). This isochron age was calculated using of
different intrusive events. In addition, some of thethe Model 3
solution of the Yorkfit calculation, which former volcanic episodes
were defined based on theassumes that scatter is due to analytical
error plus nor- general assumption that some of them were
representedmally distributed error in initial 87Sr/86Sr, i.e. it
takes by primary pyroclastic rocks. However, a detailed studyinto
account not only the analytical error, but also the of the field
relationships and textures of the volcanicgeological error. The
basaltic rocks alone, however, give rocks has revealed that nearly
all the volcanic rocks fromages inconsistent with the stratigraphy
and with very the Spanish sector of the Iberian Pyrite Belt are
intrusivehigh 1r errors. with some minor extrusive silicic domes
which have
Using the Nd isotopic composition, the isochron age produced
small lavas and associated hydroclastic vol-is 369150 Ma. This
calculation assumes Model 3, as canogenic deposits (Soriano,
1997).before, and is the isochron for all the samples studied Field
characteristics of the calc-alkaline and basalticindependently of
their composition. If the age is calculated intrusives indicate
that they were emplaced nearly alwaysby groups of rocks
(calc-alkaline and basalts separately) at very shallow depths into
wet sediments (Boulter, 1993a,the ages obtained are meaningless and
the 1r errors are 1993b; Soriano, 1997). Volcanic intrusions
reached rel-larger than the age itself. In contrast to the use of
two ative higher positions in the stratigraphic succession
asdifferent isochrons calculated with the Rb/Sr method,
sedimentation progressed and, consequently, as the thick-the use of
a single Sm/Nd age for all samples can be ness of sediments
increased (Soriano, 1997). This fact,accepted, because the decay of
Rb into Sr is twice as together with the presence of some lavas and
volcanogenicfast as the decay of Sm into Nd. Thus, for the age
that
sediments interbedded at different levels of the Volcano-we are
studying (368 Ma) and for the Sm/Nd method,Sedimentary Complex,
suggests that volcanism was moreit is possible to consider all the
samples as members ofor less continuous during the entire
deposition of thethe same group.volcano-sedimentary package, which
covers a time spanThe age obtained, 36862 Ma, is statistically in
agree-of ~30 m.y. (see Fig. 3).ment with the ages obtained by other
workers (Fig. 3)
The calc-alkaline rocks show a preferential distributionusing
radiometric and palaeontological methods. Thein the stratigraphic
sequence depending on their pa-high 1r error on the final age is
probably related to thelaeogeographic position. They are
preferentially con-high dispersion of isotopic values owing to the
use ofcentrated towards the top of the stratigraphic series
indifferent rock types from different stratigraphic units.the north
and northeast of the Iberian Pyrite Belt, whereasintermediate and
silicic volcanics appear mainly at thebase of the stratigraphic
sequence in the south and
DISCUSSION OF RESULTS southwest of the Spanish sector of the
Iberian Pyrite Belt.The different positions in the stratigraphic
sequence ofTemporal and spatial evolution ofcalc-alkaline rocks do
not necessarily indicate the ex-volcanismistence of different
volcanic episodes affecting the wholeAccording to Instituto
Geologico y Minero de EspanaIberian Pyrite Belt. More probably,
these differences(1982) and Oliveira (1990), the volcanism of the
Iberianrepresent a migration of the focus of calc-alkaline
vol-Pyrite Belt has been divided into five volcanic episodes:canism
to the northeast, as suggested by Monteiro &(1) initial acid
volcanism; (2) basic volcanism; (3) middleCarvalho (1987), probably
related to the asynchronicityacid volcanism; (4) upper acid
volcanism; (5) intrusivein the opening of the main fractures that
controlleddiabases. This division is based on the stratigraphicthe
subsidence of the sedimentary basins. This is alsoposition of
volcanic rocks, their petrographic texture, andindicated by the
irregular structure of the basin and bythe occurrence of sediments
between volcanic packages.the migration of the depocentres
(Soriano, 1997).Thus, two volcanic rocks separated by a
sedimentary
The basaltic rocks, which are most commonly intrusive,interval
in any stratigraphic section have, until now, beendo not show any
stratigraphic or palaeogeographic con-interpreted as two different
volcanic events in time,trol and appear throughout the
stratigraphic sequencethe highest stratigraphic position
corresponding to theof the Volcano-Sedimentary Complex. In the
Portugueseyoungest volcanic event.sector, Munha (1983) found a
stratigraphic control inMost of the volcanics in the Iberian Pyrite
Belt are
shallow intrusives, ranging in depth from tens of metres the
distribution of tholeiitic and alkaline basalts, which
747
-
JOURNAL OF PETROLOGY VOLUME 38 NUMBER 6 JUNE 1997
were concentrated at the base and the top of the strati-
Petrological and geochemical data presented in pre-graphic
sequence, respectively. This relationship, how- vious sections
demonstrate that most of the basaltic rocksever, has not been
observed in the Spanish sector, where from the Iberian Pyrite Belt
have a tholeiitic affinity andbasaltic rocks with both affinities
appear as intrusives in that a few samples show a slightly alkaline
affinity. Thisany part of the stratigraphic sequence. difference
between the two types of basalts is indicated
Therefore, the previous division of the Iberian Pyrite by the
Y/Nb values, by their TiO2 and P2O5 contentsBelt volcanism into
several volcanic episodes should be (see Table 1), by the LREEHREE
fractionation, theabandoned, at least for the Spanish sector, as it
has no lack of mantle xenoliths or cumulates, and the
absencemeaning in terms of magmatic and volcanic evolution. of
normative Ne. The presence of both types of basaltsBased on
palaeogeographic distribution, stratigraphic po- is common in
different geodynamic settings and thesition, petrography and field
textures (Soriano, 1997), we distinctive chemical features of
tholeiites and alkalineconsider the volcanism of the Iberian Pyrite
Belt as a basalts are related to different degree of partial
meltingcontinuous, long-lived, volcanic episode which resulted from
the same source region (Moore et al., 1995).from a magmatic event
coeval with the tectonic processes The mixing model between E- and
N-MORB used tothat controlled the formation of basins where the
Iberian explain the origin of the most primitive basaltic
rocksPyrite Belt sedimentary succession was deposited. Differ- does
not discriminate between the origin of both tholeiiticent pulses
may logically be distinguished in this volcanism, and alkaline
affinities in the same basaltic volcanism.suggesting the existence
of alternating active and less However, the existence of tholeiitic
and alkaline affinitiesactive periods. However, this does not
indicate any may be explained by different degrees of partial
meltingchange in the tectonic conditions that caused this vol- of a
peridotitic mantle (see Carmichael et al., 1974; Hall,canism or in
the nature of its products. The geodynamic 1987; Wilson, 1989), as
is suggested by the REE patternsframework of this volcanism and the
origin of the as- shown by the basaltic rocks studied here. The
highersociated sedimentary basins are discussed in a later sec-
degree of partial melting is reflected in the lower LREE/tion. HREE
ratio of tholeiites with respect to that of the basalts
with a slightly alkaline affinity and by the flat patternsof the
normalized REE.
Origin of basaltsThe trace elements and REE compositions and the
high
Origin of intermediate and silicic rocksSr/Nd values (>20)
shown by some of the basaltic rocksfrom the Iberian Pyrite Belt
suggest that they originated Trace elements and REE compositions of
intermediatein the asthenospheric mantle (see Zindler et al.,
1981). and silicic rocks from the Iberian Pyrite Belt
demonstrateThese basalts are the most primitive of the studied area
that they are not related by fractional crystallizationand derive
from different mantle sources that can be processes. Munha (1983),
using classical geochemicalinternally related to the E- and N-MORBs
by a single data, also concluded that no fractional crystallization
ismixing model calculation (Figs 9 and 10). The rest of the
involved in the formation of the evolved rocks. Thatbasaltic rocks
may be explained by assimilationfractional researcher suggested
that andesites directly derived fromcrystallization (AFC) processes
or more likely by mixing of partial melting of the mantle, but
their high Sr contentthe mantle-derived basaltic magmas with
crustal materials precludes that possibility. Isotopic compositions
reveal(Figs 9 and 10). They have lower Sr/Nd values, which that
andesites (samples FP-83 and FP-102) fall in a moreindicate that
they fractionated plagioclase, assimilated evolved position than
the dacites (Figs 12 and 13), havingcrustal material or assimilated
magmas derived from very similar Nd isotope ratios to rhyolites but
different
Sr isotope ratios. This suggests that the andesites maycrustal
melts.The influence of continental crust on the primary have formed
by direct mixing of mantle-derived basalts,
which follow the mantle array, with young upper-crustmagmas of
some of the basaltic rocks from the IberianPyrite Belt is also
shown on the Ce/NbTh/Nb diagram material. Also, the Sr isotopic and
absolute compositions,
and the Zr/Nb, La/Sm and Ce/Th (Figs 9 and 10), andand by the
ratio Th/Nb (Figs 10 and 11). The Ce/NbTh/Nb diagram (Fig. 10)
shows how almost all the the low Sm/Nd ratios, confirm this point,
as andesites
are numerically between FP-40 (or even FP-91) andbasaltic
samples plot inside an area that represents mixingbetween E-MORB,
N-MORB and continental crust. upper-crust average. The fact that
andesites have higher
eSr and lower eNd than dacites implies that there is noIsotopic
compositions confirm mixing as the most plaus-ible mechanism to
explain the diversity of compositions relation by fractional
crystallization between andesites
and rhyolites through the dacites.shown by the basaltic rocks
from the Iberian Pyrite Beltand their relationship with the other
volcanic rocks (Figs The high variation in eNd, Sr (p.p.m.), Rb
(p.p.m.) and
Nd (p.p.m.), together with the low variation in eSr shown12 and
13).
748
-
MITJAVILA et al. MAGMATIC EVOLUTION AND TECTONIC SETTING OF THE
IBERIAN PYRITE BELT
Fig. 9. La/Sm normalized to chondrites vs Zr/Nb. The hyperbola
represents a single mixing model with the end-members in sample
FP-20and N-MORB. The mixing line passes through E-MORB, and samples
FP-40, FP-34, FP-31 and FP-80. Sample FP-20 is the most
LREEHREEfractionated basalt (Fig. 12), with clear tholeiitic
characteristics, whereas FP-80 is the more primitive of the studied
rocks and has a flat REEpattern (Fig. 12). From FP-80 and FP-31,
two mixing or AFC lines depart towards sample FP-7 (one clearly
contaminated basalt which also
falls in the field occupied by the rest of the samples, the
continental crust, and the analysed sediment).
by the dacites, and the high variation on eSr with almost and
REE (mainly Sr), and the high variation of the eNd,demonstrates
that not all of the dacitic samples supportno variation of eNd, Sr
(p.p.m.) and Nd (p.p.m.) shown
by the rhyolites (Fig. 13), suggests that they are derived
fractional crystallization as their genetic process. Thus,most of
the dacites, like the rhyolites, can each beby different degrees of
partial melting of different crustal
rocks. Moreover, despite the fact that sample FP-5 (dacite)
considered as an individual case of partial melting ofcrustal
material of different type (variations in Sr andhas coincident Nd
(p.p.m.) and eNd with FP-83 (andesite),
dacite samples appear not to be related by fractional Nd
concentrations) and age (differences in eSr and eNd).Only samples
FP-5 and FP-27 could be considered to becrystallization to any of
the andesites. This last point is
demonstrated by the lower values in eSr of the dacites generated
by fractional crystallization processes from amixture between
basalts and rhyolites.with respect to the andesites and [for two
dacites (FP-
27 and FP-154)] by the higher values of eNd with respect The
presence of high-silica content rhyolites is char-acteristic of
both sectors of the Iberian Pyrite Belt (Munha,to the andesites
(Fig. 13). Furthermore, there is no relation
between dacites and rhyolites by fractional crystallization
1983; this work). Despite the fact that this high silicacontent may
be due to a secondary silicification [most(see Fig. 12). In
contrast, the eSr of the analysed dacites
(Fig. 12) is nearly the same. This could suggest that of these
rocks have been classified as quartz keratophyresby previous
workers (Soler, 1980)], it may also be adacites are genetically
related to them by fractional
crystallization and, as shown in Fig. 12, they can be primary
feature. This is also suggested by the fact thatthe high-silica
content rhyolites show the highest negativederived by the same
process from a mixing product
between basalts and rhyolites. However, as we have seen, Eu
anomaly. Silica-rich rhyolites are typical of areaswith bimodal
volcanism rather than being generatedthe differences in
concentration of some trace elements
749
-
JOURNAL OF PETROLOGY VOLUME 38 NUMBER 6 JUNE 1997
Fig. 11. Nb vs. Th plot for the studied basalts. There is a
clear positiveratio between Th and Nb that relates the two
end-members FP-80 andFP-20 with other samples which plot on the
same line. It seems atypical fractional crystallization (FC) line,
but it passes over the lower-crust composition, suggesting that it
may be coincident with a mixingor contamination line. There are
three other trends: the first showsincreasing Nb and constant Th
and relates samples FP-31, FP-34 andFP-40 by partial melting
processes (these samples occupy intermediateFig. 10. Ce/Nb vs
Th/Nb. This diagram demonstrates the influencepositions on the
hyperbola in Fig. 16); the second, which is not as clearof
continental crust on the evolution of some basaltic rocks from
theas the others, relates some of the basalts to the basaltic
andesites andIberian Pyrite Belt. Most of the basaltic samples plot
inside a mixingto the sediment sample; the third line shows a high
increase in Th andtriangle with the three end-members E-MORB,
N-MORB and averageconstant Nb and connects samples FP-16, FP-15 and
FP-7. These threecontinental crust (Taylor & MacLennan, 1985).
Three samples (FP-samples always follow the same pattern, which
suggests mixing with16, FP-15 and FP-7) fall out of the triangle
towards an external end-source materials that are rich in Th but
poor in Nb (see also Fig. 10).member, which for samples FP-15 and
FP-16 could be the analysedAn increase in Th with no modification
in Nb may be explained bysediment, and for sample FP-7 could be an
artefact owing to its
secondary alteration or sediment assimilation.secondary
modification.
by fractional crystallization processes from
calc-alkalineandesites (Christiansen & Lipman, 1972). We
suggestthat the high silica content that characterizes some ofthe
Iberian Pyrite Belt rhyolites is a primary characterprobably owing
to differences in the source, consistentwith an origin by partial
melting of upper crust drivenby basaltic magmas.
Therefore, geochemical and isotopic data presented inthis paper
negate any relationship by pure fractionalcrystallization between
basalts, intermediate rocks andsilicic rocks. This fact, together
with the volumetricpredominance of silicic rocks and the widespread
oc-currence of basaltic volcanism, suggests that in the
IberianPyrite Belt calc-alkaline silicic magmas were generatedon a
large scale by the invasion of continental crust bymafic magmas
generated in the underlying upper mantle.
Fig. 12. Diagram of 87Sr/86Sr vs SiO2. A three end-member
mixingMantle-derived magmas can provide large quantities of (trend
1) can be established taking as end-members FP-91, FP-20 andheat
for partial melting and assimilation of lower- and the rhyolite
FP-130 (taken as an isotopic equivalent of the continental
crust in this area). It is also feasible to consider the
rhyolite FP-79 asupper-crustal rocks (Huppert & Sparks, 1988;
Kaczor etthe evolved end-member (trend 2). From these mixing fields
a regularal., 1988; Grunder, 1995). The diversity of compositions
FC pattern can be observed for the dacites, but it is not
consistent with
shown by dacites and rhyolites can be explained either Nd
isotope data nor with trace element contents. Andesites can
beconsidered as mixing products.by differences in the composition
of the source rocks or
by different degrees of partial melting of upper-crustrocks. In
contrast, andesites formed by direct mixing ofmantle-derived
basalts with young upper-crust material, the intrusion of
mantle-derived magmas or by direct
partial melting of the upper mantle.rather than by partial
melting of lower crust induced by
750
-
MITJAVILA et al. MAGMATIC EVOLUTION AND TECTONIC SETTING OF THE
IBERIAN PYRITE BELT
(4) The Baixo Alentejo Flysch Group, which is com-posed of a
thick sequence of syntectonic turbiditic depositsof Upper Visean to
Namurian age (Oliveira, 1990) andis bounded to the south by the
Iberian Pyrite Belt terranewith a thrust plane that dips to the
north (Quesada,1991). At the north of the Baixo Alentejo Flysch
Groupthe southern part of the Ossa Morena Zone is consideredto
represent a continental terrane with calc-alkaline arc-related
volcanism associated with the subduction of theBejaAcebuches
ophiolite (Santos et al., 1987).
The geological characteristics and boundaries of
thesetectonostratigraphic domains and the occurrence of
awell-defined suture between the South Portuguese andthe Ossa
Morena Zones (Munha et al., 1986; Silva et al.,1990; Crespo-Blanc
& Orozco, 1991; Dallmeyer et al.,
Fig. 13. eNd isotope composition vs. eSr isotope composition
diagram. 1993; Giese et al., 1994; Quesada et al., 1994) have
ledSample FP-40 falls on the mantle array and is very close to
E-MORB.to a general consensus on the tectonic evolution of
theSample FP-91 is slightly deviated towards the pattern defined by
the
other recent continental volcanism [see the review by Worner et
al. area. From Upper Devonian (Frasnian and Famennian)(1986)] which
is followed by FP-15 (a basalt with high influence of the through
the entire Carboniferous Period the South Por-upper crust or
sediment). Sample FP-20 falls between HIMU and EMI.
tuguese and the Ossa Morena plates were continuouslyThis sample
coincides with the zone equivalent to the North Atlanticconverging.
This plate convergence evolved with timeScottish Tertiary volcanic
provinces (Carter et al., 1978). These authors
explain this field as being produced by contamination of
granulites from a subduction of South Portuguese oceanic lith-from
the lower crust. With this distribution of basalts on the eNd vs.
eSr osphere beneath continental Ossa Morena crust to aplot, one can
deduce mixing and evolution lines towards the rhyolitic
continental collision of both plates (Monteiro &
Carvalho,end-members to explain the origin of all the studied
rocks.1987; Silva et al. 1990; Giese et al. 1994; Quesada et
al.,1994). In such a moving plates scenario the Iberian PyriteBelt
volcanism took place. The tectonic style of theGeodynamic setting
and origin of theIberian Pyrite Belt terrane (Soriano, 1996, 1997),
as withIberian Pyrite Belt volcanismthe rest of the South
Portuguese Zone terranes (Ribeiro
Tectonostratigraphic terrane analysis of the South Por- et al.,
1983), also agrees with such a geodynamic evolution,tuguese Zone
and the southern part of the Ossa Morena as most of the structures
clearly show an eastwest trendZone (Oliveira, 1990; Eden, 1991;
Quesada, 1991) has and south vergence.revealed the occurrence of
several well-differentiated Such a consensus, however, does not
surround thegeological domains which have a specific significance
in interpretation of the tectonic setting of the Iberian
Pyriteterms of Variscan plate tectonics evolution (Fig. 1). Thus,
Belt, which has been considered to have formed in afrom north to
south the tectonostratigraphic terranes of variety of settings.
These include: (1) a convergent platethe South Portuguese Zone are:
boundary, in an island arc generated over a subduction
(1) The BejaAcebuches ophiolitic complex, which has zone (Schutz
et al., 1988); (2) extensional tectonics as-recently been
interpreted as an Early Devonian oceanic sociated with back-arc
spreading developed on the Southcomplex (Munha et al., 1986;
Quesada et al., 1994) and Portuguese continental plate and
involving a subductionwhich represents an oceanic crust of the
South Portuguese of the Ossa Morena Zone (Soler, 1973); (3)
extensionalplate subducted beneath the continental crust of the
Ossa tectonics associated with the initial stages of a
back-arcMorena plate in Upper Famennian to Lower Car- spreading
developed on a continental plate (Munha,boniferous times (Silva et
al., 1990). 1983); (4) a forearc basin developed on the
continental
(2) The Pulo do Lobo oceanic accretionary prism, crust of an
overriding plate, the Ossa Morena Zonewhich is an oceanic terrane
composed of siliciclastic (Monteiro & Carvalho, 1987).sediments
of Lower to Upper Devonian age (Oliveira et These different
interpretations of the tectonic settingal., 1986) deposited on an
oceanic lithosphere. of the Iberian Pyrite Belt are in part due to
the lack of
(3) The intracontinental Iberian Pyrite Belt volcanic a precise
interpretation of the nature of the associatedterrane, which is
composed of marine turbiditic deposits volcanism. Most of the
previous studies concerning theand shallow calc-alkaline and
basaltic sill-complexes that tectonic setting of the Iberian Pyrite
Belt have con-intruded over a continental well-differentiated crust
centrated on the interpretation of its stratigraphical and(Munha,
1983). Fauna of turbiditic deposits and isotopic structural
features, but have not considered in detaildating of shallow
intrusions indicate a Late Devonian to whether the characteristics
of volcanism were compatible
with the various proposed tectonic settings. To determineEarly
Carboniferous age (Fig. 3).
751
-
JOURNAL OF PETROLOGY VOLUME 38 NUMBER 6 JUNE 1997
the validity of the previous interpretations of the Iberian
Quesada et al., 1994) preclude its location in a
fore-arcsetting.Pyrite Belt tectonic setting, we compare them with
the
Recently, Silva et al. (1990), Giese et al. (1994)
andpetrological and geochemical data presented in this
study.Quesada et al. (1994) have proposed a new
interpretationMineralogical and geochemical data of the Iberianfor
the Iberian Pyrite Belt volcanism based on terranePyrite Belt
volcanics (Routhier et al., 1977; Soler, 1980;analysis. They
suggest that local extensional tectonics,Munha, 1983; this paper)
indicate a clear continentalrelated to transtension owing to
oblique collision, de-crust affinity for the calc-alkaline rocks.
This precludesveloped in the continental crust of the South
Portuguesea certain number of tectonic settings for the
Iberianplate during a continental collision of the South Por-Pyrite
Belt volcanism. On the basis of this evidence, andtuguese and the
Ossa Morena plates. This strike-slipin accordance with previous
workers (Schermerhorn,tectonics caused the opening of pull-apart
basins, where1975; Munha, 1983), an island arc setting must
besubmarine terrigenous sedimentation occurred, as wellrejected, as
the volume of rhyolites and dacites in theas the bimodal volcanism
which characterizes the IberianIberian Pyrite Belt is too high
compared with modernPyrite Belt. The location of the Iberian Pyrite
Belt as-analogues of island arc environments (Carmichael et
al.,sumed by this model is consistent with the geodynamic1974;
Baker, 1982; Gill, 1981; Wilson, 1989). A well-framework described
above, and also with the palaeo-developed lower and upper
continental crust is necessarygeographic reconstruction of the area
(Soriano, 1997).to explain the geochemistry of the Iberian Pyrite
BeltHowever, there is not a clear modern analogue that
canvolcanism. In addition, basaltic rocks do not show anybe used to
define the main geochemical guidelines ofsubduction-related
affinity. They have higher TiO2 andsuch a volcanism. Previous
models of collision-zone mag-Fe2O3t contents, and lower K, Sr and
Zr contents than matism do not consider this possibility (see
Harris et al.,typical subduction-derived basaltic magmas
(Carmichael1986) and no indications of the geochemical signatureet
al., 1974; Wilson, 1989), and they do not have hydrousof this
magmatism exist. Despite this fact, we considerminerals. In
addition, andesitic rocks also differ min-that the geochemical and
isotopic data presented in thiseralogically from typical island-arc
andesites, which nor-study are compatible with this tectonic
setting.mally have orthopyroxene.
The ultimate source for the Iberian Pyrite Belt vol-Basalt and
rhyolitic associations such as those observedcanism is the
asthenospheric mantle, where large volumesin the Iberian Pyrite
Belt are commonly found in ex-of tholeiitic, and occasionally
alkaline, basaltic magmastensional tectonic setting such as
continental rifts ororiginated. Extensive melting of the
asthenospheric
continental back-arcs. However, mineralogical and geo- mantle
was probably induced by rapid decompressionchemical compositions of
the Iberian Pyrite Belt volcanics caused by the effect of regional
strike-slip tectonics whichdo not show any evidence of a subducting
slab de- affected the entire continental crust and probably
thehydration such as invariably occurs in back-arc volcanism
lithospheric mantle, thus causing a restricted lithospheric(Tarney
et al., 1977; Weaver et al., 1979; Wilson, 1989). thinning. The
large LREE depletion and the absence ofCalc-alkaline basalts are
also a common feature in mod- Eu anomaly observed in most of the
studied basalticern back-arcs (Weaver et al., 1979; Wilson, 1989),
but rocks are compatible with a continental breakup settingnone of
the basalts in the Iberian Pyrite Belt have (see Cullers &
Graf, 1984). It is important to note,this calc-alkaline pattern.
Moreover, the sequence of however, that the Iberian Pyrite Belt did
not form in atectonostratigraphic domains described above is
clearly post-collisional extensional setting. The strike-slip
tec-inconsistent with a back-arc setting for the Iberian Pyrite
tonics responsible for the opening of the basin and theBelt
volcanism. The geochemical features of this vol- associated
volcanism developed as a direct response tocanism also differ from
the distinctive characteristics of the oblique continental
collision between the Ossa Mo-modern and ancient intracontinental
rift zones [alkaline rena and the South Portuguese plates, which
lastednature, enrichment in large ion lithophile elements from
Middle Devonian until Late Carboniferous, and(LILE)] (see Bailey,
1983; Wilson, 1989). culminated with the Variscan orogeny in that
area. Thus,
A fore-arc location should be also rejected in the light in such
a scenario the existence of significant lithosphericof several
features: some influence of a subducting slab stretching caused by
the relaxation of compressionalshould be observed in the
geochemistry of the Iberian stresses is unlikely as the compression
had not ceased.Pyrite Belt volcanics if they had formed in a
fore-arc Basaltic magmas invaded a relatively thick continentalzone
developed above the continental crust that overrides crust causing
both assimilation of crustal materials anda subducting oceanic
crust. On the other hand, the nature extensive melting of
upper-crust rocks. This gave rise to(an accretionary prism) and
actual position of the Pulo the formation of andesites and silicic
rocks, respectively.do Lobo oceanic terrane and the Baixo Alentejo
Flysch The magmatic process that gave rise to the IberianGroup with
respect to the Iberian Pyrite Belt (Monteiro Pyrite Belt volcanism
was, thus, intimately related to
the tectonic regime which controlled the opening and&
Carvalho, 1987; Giese et al., 1988; Silva et al., 1990;
752
-
MITJAVILA et al. MAGMATIC EVOLUTION AND TECTONIC SETTING OF THE
IBERIAN PYRITE BELT
evolution of the sedimentary basin. This process was comments by
Cecilio Quesada and an anonymous re-viewer are also gratefully
acknowledged. This researchpulsating rather than continuous, as is
indicated by
changes in the structure of the basin with time and in was (in
part) supported by the research project entitled:Proyecto de
investigacion paleogeografica y vol-the position of the depocentres
(Soriano, 1997). These
pulses caused different intensities of decompression in
canologica en la Faja Pirtica del SW de Espana (ITGEFundacio Bosc i
Gimpera). We thank the Institutothe mantle and this would explain
the existence of
differences in the degree of partial melting of the as-
Tecnologico y Geo Minero for giving us permission topublish part of
the results from that project.thenospheric mantle, which would
explain the existence
of tholeiitic and alkaline affinities for the basaltic
magmas.However, in the Spanish sector there is no evidence
tojustify a sequence of events that could explain the tem-
REFERENCESporal evolution from tholeiites to alkaline basalts
thatBailey, D. K., 1983. The chemical and thermal evolution of
rifts.Munha (1983) suggested for the Portuguese sector.
Tectonophysics 94, 585597.Baker, P. E., 1982. Evolution and
classifications of orogenic volcanic
rocks. In: Thorpe, R. S. (ed.) Andesites: Orogenic Andesites and
RelatedCONCLUSIONS Rocks. New York: John Wiley, pp. 1123.
Barriga, F. J. A. S. & Kerrich, R., 1984. Extreme
18O-enrichedThe Iberian Pyrite Belt volcanism is mainly
representedvolcanics and 18O-evolved marine water, Aljustrel,
Iberian Pyrite Belt:by shallow intrusives emplaced into wet
turbiditictransition from high to low Rayleigh number convective
regimes.siliciclastic deposits of early Carboniferous
age.Geochimica et Cosmochimica Acta 48, 10211031.
Volumetrically, this volcanism can be considered as bi- Boulter,
C. A., 1993a. High level peperitic sills at Ro Tinto, Spain:modal,
with a predominance of basaltic rocks of tholeiitic implications
for stratigraphy and mineralization. Transactions, In-affinity and
calc-alkaline silicic rocks, despite the presence stitution of
Mining and Metallurgy, Section B: Applied Earth Sciences 102,
B30B38.of some basalts with alkaline affinity and
intermediateBoulter, C. A., 1993b. Comparison of Ro Tinto, Spain,
and Guaymascalc-alkaline rocks. Differences in composition shown
by
Basin, Gulf of California: an explanation of a supergiant
massivethe basaltic rocks can be explained by a single
mixingsulfide deposit in an ancient sillsediment complex. Geology
21,model involving three end-members: E-MORB, N- 801804.
MORB and continental crust. Silicic calc-alkaline rocks
Carmichael, I. S. E., Turner, F. J. & Verhoogen, J., 1974.
Igneousoriginated by large-scale partial melting of upper con-
Petrology. New York: McGrawHill.tinental crust induced by the
intrusion of basaltic magmas Carter, S. R., Evensen, N. M.,
Hamilton, P. J. & ONions, R. K.,
1978. Neodymium and strontium isotope evidence for crustal
con-generated in the underlying upper mantle. In
contrast,taminations of continental volcanics. Science 202,
743747.andesites originated by direct mixing of mantle-derived
Christiansen, R. L. & Lipman, P. W., 1972. Cenozoic
volcanism andbasalts with young upper-crustal material. The
volcanismplate-tectonic evolution of the western United States.
IILate
covers a long time span (15 m.y.) and does not show any
Cenozoic. Philosophical Transactions of the Royal Society of London
A271,significant temporal or spatial variation. The existence
249284.of tholeiitic and alkaline affinities in the basaltic rocks
is Crawford, A. J., Corbett, K. D. & Everard, J. L., 1992.
Geochemistry
of the Cambrian volcanic-hosted massive sulfide-rich Mount
Readinterpreted to be due to different degrees of partialVolcanics,
Tasmania, and some tectonic implications. Economic Geo-melting in
the asthenospheric mantle caused by differentlogy 87,
597619.intensities of tectonically driven decompression. The
new
Crespo-Blanc, A. & Orozco, M., 1988. The Southern Iberian
Sheargeochemical and isotopic data are consistent with theZone: a
major boundary in the Hercynian folded belt. Tectonophysics
stratigraphic and structural data, which suggest that 148,
221227.the Iberian Pyrite Belt developed in a complex setting,
Crespo-Blanc, A. & Orozco, M., 1991. The boundary between
theinvolving local extensional tectonics within the South
OssaMorena and South Portugues