Geochemistry of Upper Palaeozoic-Lower Triassic granitoids of the

Juurnd of South Amrricon Earth Sciences, Vol. Y. Nm 112. pp. 141-151, IYY6
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Geochemistry of Upper Palaeozoic-Lower Triassic granitoids of the
Central Frontal Cordillera (33”10-33’49, Argentina
DANIEL A. GREGORI, JOSE L. FERNANDEZ-TURIEL, ANGEL LOPEZ-SOLER and NICK PETFORD
Departamento de Geologia, Universidad National del SW-CONICET, San Juan 670, 8000 Bahia
Blanca, Argentina
Instituto de Ciencias de la Tierra “Jaume Almera”, CSIC, Marti i Franq s/n. 08028 Barcelona, Spain
School of Geological Sciences, Kingston University, Penrhyn Road, Kingston upon Thames, Surrey,
KTl 2EE, UK
Abstract - New petrographic and chemical data from Upper Palaeozoic-Triassic granitoids from the Frontal Cordillera of
Argentina (33”lO to 33”45) are presented.Five stocks with ages from Early Carboniferous to Late Permian were sampled. The
rocks are all talc alkaline with the mineralogy of each stock comprising essential plagioclase, alkali-feldspar and quartz with
monor biotite and hornblende. Chemically, the rocks are similar to granitoids from the-Frontal Cordillera of Chile, although the
Argentinian stocks have generally higher WRb ratios. REE chondrite-normalized patterns and overall abundances,along with
other petrological and geochemical similarities, suggesta common mode of origin for the granitoid stocks.The absenceof systematic and wide variations in abundanceof compatible trace elements suggestthat the stocks represent magmas largely unaffected by extensive high-level fractional crystallization, and that their compositionsare controlled primarily at source. Their age,
tectonic setting and geochemical trends are consistent with magma pulses generated during crustal extension. The magma
source region is considered to be relatively matic lower crust. Copyright 0 1996 Elsevier Science Ltd & Earth Sciences &
Resources Institute
Resumen - Se presentannuevosdatos petrogdficos y analisis quimicos de 10sgranitoidesde1 Paleozoic0 superior-TriGco de1
sector central de la Cordillera Frontal de Argentina (33”lO to 33”45’S). Cinco cuerpos intrusivos cuyas edades van desde el
Carbonifero temprano al Trihico tardo frueron muestreadosTodos 10s cuerpos presentan una mineralogia simple, constituida
fundamentalmentepor plagioclasa, feldsepato alcalino y cuarzo. La biotita y la homblenda aparecen en escasaabundancia. La
similitud en la abundancia de las tierras rams y 10s disethosde sus curvas normalizadas a condritos asi coma la similitud en
las caracteristicas petrogrtificasy geoquimicas indica un modo comun de origen. La ausencia de variaciones sistematicasy
amplias en la abundancia de muchos de sus elementos trams indica que estos intrusivos representanmagmas en 10sque no se
han llevado a cave procesosextensivos de cristalizacion fraccionada durante el emplazamiento y ascensode 10s mismos.
Los datos geocronologicosy algunas diferencias en sus tendenciasgeoquimicasindicanque cada intrusivo representaun pulse
separado de magma, probablementederivado por fusion partial de un magma padre de composition b sica, sometido a condiciones de metamortismo de la facies de amphibolitas en un ambiente extensional.
INTRODUCTION
Upper Palaeozoic plutonic and volcanic rocks of the
Frontal Cordillera are interpreted as a remnant magmatic
arc related to the subduction of ProtoPacific oceanic crust
(Ramos et al. 1986; Llambias et al. 1987; Mpodozis and
Kay, 1990). More recently Mpodozis and Kay (1990;
1992) have suggested that the Frontal Cordillera rocks
record a tectonic cycle that started with the emplacement
of granitoids produced by partial melting of subduction
complexes during the Late Carboniferous coincident with
the oblique collision of the Equis terrane, followed by the
emplacement of post-collisional units during the Triassic.
However, these interpretations are based in field relations,
age determinations and petrology of Late Carboniferous
to Triassic plutons north to 31”s. Although field studies
have been carried out on batholith rocks south of 31”s in
the Frontal Cordillera of Mendoza, relatively few petrological and geochemical data are currently available for
these rocks (see Caminos, 1979; Llambias and Caminos,
The geological configuration of the Andean Cordillera
documents the geological evolution of the south-western
Gondwana margin from Precambrian to Quaternary
times. In this geological context, the Frontal Cordillera of
Argentina and Chile is an important although as yet
poorly understood region which records a complex history of subduction, collision accretion and extension
processes. Composed predominantly of Palaeozoic rocks,
the Frontal Cordillera forms the eastern part of the Andes
Cordillera in the provinces of Mendoza and San Juan in
Argentina and in the Coquimbo and Atacama regions of
Chile, between 27”s and 345.
The Early Palaeozoic metamorphic rocks of the complex are currently interpreted as part of a microplate that
amalgamated with the south-western border of Gondwana
during Devonian times (Ramos, 1988). In contrast, the
Address all correspondence and reprint request to D.A. Gregori. Departamento de Geologia, Universidad National del
Sur-CONICET, San Juan 670, 8000 Bahia Blanca, Argentina.Telephone [54] 91 25196 ext. 360; Fax [54] 91 553933.
E-mail: [email protected]
UES9:W-J
141
D.A. GREGORI
142
1987). Radiometric ages (K-Ar, Rb-Sr) from the Frontal
Cordillera Composite Batholith record a long magmatic
event that extends from the Early Carboniferous
to the
Triassic, and extrapolation of current tectonic models to
other regions to the Frontal Cordillera of Mendoza may
not be applicable without first undertaking detailed field
and chemical studies of these rocks.
In this paper we present new petrological and geochemical data on granitoid stocks exposed from 33”lO to
33”45 in the Frontal Cordillera of Argentina, and attempt
to determine their origin and possible tectonic setting.
Preliminary
comparisons
are also made with rocks
exposed in the Frontal Cordillera of Chile and Argentina
north of 31”s.
GEOLOGICAL
SETTING
Upper Palaeozoic-Triassic
intrusive rocks of the Frontal Cordillera of northern Mendoza province that com-
et al.
prise the Frontal Cordillera Composite Batholith were
first described by Polanski (1958). The batholith has a
regional N-S elongation, extending for 130 km southward
to Mendoza River, varying in width from 20 km at the
Cordon de1 Plata to 40 km in the Cordon del Portillo
(Fig. 1). Several plutons and stocks were mapped and
described by Caminos (1967) and Polanski (1972) with
K-Ar and Rb-Sr radiometric ages reported by Dessanti
and Caminos (1967) and Caminos et al. (1979).
The La Quebrada de Guevara, La Carrera, La Cuchilla
de las Minas and Santa Clara Stocks form the eastern side
of the Cordon de1 Plata, whereas stocks of the Cuchilla de
Guarguaraz area (the Los Tabanos Stocks) were mapped
by Polanski (1972) and studied by Bjerg et al. (1990). In
the Cordon del Portillo area, the Las Cuevas, Las Delicias, Cerro Punta Negra, Quebrada Portillo and Cerro
Punta Blanca Stocks have also been recognized. In the
Cuchilla de Guarguarazand Cordon de1 Portillo the stocks
.____
_I 1 I I 1:‘:‘;:’
m
U
\
\-
Carboniferous 1
Sedimentites 1
Basaltic rocks
etamorphi cl
%.
Punta Blanca Stock
7Punta NegraStock
Portillo
YY”“““”
”
Y”
Y””
Y””
““V
“V”
Y”
“V
“Y”
>
Fig. 1. Simplified geological map of the Frontal Cordillera between 33”lO’ S to 33”45’ S showing location of major granitoid stocks.
Geochemistry of Upper Palaeozoic-Lower Triassic granitoids of the Central Frontal Cordillera
intrude metamorphic basement comprised of phyllites,
schists, marbles and quartzites, and greeschist facies, serpentinized ultrabasic rocks. Several phases of deformation and thermal effects relating to granitic intrusion have
been recognized (Bjerg et al. 1990). Radiometric age
determinations (Caminos et al. 1979) suggest an episode
of regional metamorphism with a minimum age of 500 ?
50 Ma. To the eastern side of the Cordon de1 Plata is the
Early to Late Carboniferous El Plata Formation (Caminos, 1965; Polansky, 1972), comprised of conglomerates,
sandstones and pelites deposited in a shallow to deep
water marine environment. These rocks were intruded by
the La Quebrada de Guevara and La Carrera Stocks, and
subsequently overlain by Permian to Triassic andesitic
and rhyolitic lavas of the Choiyoi Group.
During field mapping, Polanski (1964; 1972) divided
the igneous rocks of the Frontal Cordillera of Mendoza
province on field relationships into two plutonic-volcanic
associations, the Mesovariscian Plutonic-Volcanic Association and the Tardiovariscian Plutonic-Volcanic Association. In the former, Polanski (1972) described a general
intrusive trend from gabbros and tonalites to granodiorites, and a volcanic trend from andesites and dacites to
rhyodacites and rhyolites. The Tardiovariscian PlutonicVolcanic association comprise granitic and granodioritic
rocks as well as porphyritic granitic and rhyolitic rocks.
However, systematic radiometric dating (Caminos et al.
1979) has demonstrated at least three intrusive events in
the area. The first, assigned to the Somuncur phase by
Caminos et al. (1988) is represented by the Cerro Punta
Blanca Stock with radiometric ages of 348 f 35 Ma (RbSr) and 337 f 15 Ma (K-Ar). The second event, assigned
to the San Rafael phase, is represented by the Quebrada
Portillo Stock (Rb-Sr 264 + 8 Ma; K-Ar 291 + 10 Ma).
The final event includes the Quebrada de Guevara (K-Ar
23 1 + 1lMa, 202 f 10 Ma) and the Punta Negra (K-Ar
234210 Ma) Stocks. This magmatic event was assigned to
the Huarpes phase by Caminos et al. (1988).
From a geotectonic standpoint it is generally accepted
that the Frontal Cordillera igneous rocks represent a magmatic belt located along the Pacific margin of Gondwana
(Frutos and Tobar, 1975; Herve et al. 1981; Ramos et al.
1984; Caminos et al. 1988). The Devonian to Lower Carboniferous tonalites and granodiorites are interpreted as a
magmatic arc which shifted compositionally towards
granites and granodiorites during Permian and Triassic
times (Ramos et al. 1984; Herve et al. 1987). Detailed
work on the magmatic units in the northern segment of
the Frontal Cordillera of Chile have revealed a distinct
tectonic setting for these rocks. Thus, Mpodozis and Kay,
(1990, 1992) suggest that the source of the oldest units of
the Late Carboniferous-Early Permian Elqui Complex
represent a magmatic arc associated with Late Carboniferous subduction, while the youngest units are syncollisional granites related to uplift, crustal shortening and
thickening during the San Rafael deformational event.
This Late Permian to Triassic plutonism is considered
post-collisional, related to underplating of basalt magmas
during lithospheric thinning and crustal relaxation. The
143
Late Permian Colangeil Batholith the Frontal Cordillera
of Argentina is considered by Llambias and Sato (1990)
to have formed in a post-subduction extensional regime
during melting of thickened crust by underplating basaltic magmas.
Chemical Procedure
Samples of the Gabbroic Complex cropping out in
Arroyo Casas, Guevara and Los Tabanos Stocks and Cordon de1 Portillo bodies (Cerro Punta Blanca, Cerro Punta
Negra, Quebrada Portillo), collected mainly through eastwest traverses along arroyos, were selected for analysis of
major and trace elements. Major and trace elements were
analysed by X-ray fluorescence using common USGS and
French geostandards at “Jaume Almera” Institute, Barcelona, Spain. REE and trace elements were determined by
ICP-MS techniques at Royal Holloway and Bedford New
College, University of London, UK. Techniques and
standards are given in Jarvis (1992). Results are shown in
Table 1.
PETROGRAPHICAL AND CHEMICAL
CHARACTERISTICS OF THE FRONTAL
CORDILLERA ROCKS (33”10-33”45S)
In the following discussion, the igneous rocks of the
Frontal Cordillera of Mendoza are described in order of
younging. The basic rocks in the Arroyo Casas are considered Early Carboniferous in age, due to their close
occurrance with marine Carboniferous rocks. The Cerro
Punta Blanca Stock has an Early Carboniferous age, and
we assign the same age to Los Tabanos Stock on the basis
of field relationships, petroghapy, chemical composition
and tectonic setting. Both thus belong to the Somuncur
Phase. Radiometric dating implies the Quebrada Portillo
Stock is Late Carboniferous to Early Permian in age. The
youngest strocks, Guevara and Cerro Punta Negra, have
Middle to Late Triassic radiometric ages.
The Basic Rocks of the Arroyo Casas
The basic rocks from the Arroyo Casas from several
small (up to hundred of meters long and up to 20 m wide)
units elongated NE-SW interbedded with the El Plata Formation (Lower Carboniferous) in the eastern side of the
Cordon de1 Plata. They were mapped by Polanski (1972)
as the Gabbroic Complex, the lowest unit of the Mesovariscian Plutonic-Volcanic
Association. They are
medium to fine grained rocks, dark grey in colour. Rocks
from the margins of the basic units contain acicular radiating and curved actinolite, possibly after hornblende, set
in a fine groundmass of epidote, quartz and magnetite. No
olivine exists, but serpentine minerals are present.
Towards their centres the rocks are composed of hornblende, actinolite, diopside, plagioclase and interstitial
quartz showing a microcrystalline texture. No age determinations have been made on these rocks, but a Lower
Carboniferous age is assumed due to their held relationships with the El Plata Formation.
D.A. GREGORI
144
et al.
Table 1
Arroyo Casas Basalts
Sample Name
AL-3AC
AL-QAC
AL-5AC
Guevara Stock
AL-GAC
AL-IG
AI-7G
AL-2G
SiO2
45.00
46.08
44.42
63.36
70.60
Ti02
1.71
2.18
1.23
2.30
0.59
0.41
0.50
Al203
16.08
13.51
12.58
15.09
14.73
14.85
FeO'
15.18
1421
12.91
1481
13.87
3.65
2.68
3.00
MnO
0.16
0.08
0.70
0.09
0.05
0.07
M@
CaO
7.36
7.28
7.15
7.25
1.55
1.06
1.30
9.75
11.26
11.80
4.59
5.52
4.77
4.60
1.05
3.80
4.06
3.77
3.85
0.13
45.90
67 30
1.21
101
13.99
1.93
0.18
0.41
0.43
P205
0.11
0.10
0.20
2.21
0.67
1.12
0.22
0.70
0.15
LOI
0.18
1.15
0.59
0.15
0.32
0.40
98.95
97.40
97.72
98.15
99.42
99.59
99.87
57
69
230
50
13
60
10
11
240
53
50
7
Na20
KZO
total
co
V
117
249
Cl0
1 89
CU
Pb
nd
nd
nd
nd
nd
nd
nd
nd
16
14
15
Zn
nd
nd
nd
nd
57
27
32
307
11
92
87
94
70
60
686
434
55
411
534
371
402
18
1.96
18
1.40
S
310
310
309
Rb
8
10
18
69
122
243
Ba
Sr
Ga
Ta
Nb
Hf
Zr
Y
L
13
103
111
290
16
320
0.47
2.81
19
1.31
6.7
0.90
8.4
6.1
11.2
11.0
3.05
2.48
6.7
4.17
11.1
3.04
3.95
3.06
3.10
83
121
35
76
86
136
28
0.93
20
127
26
130
23
11.25
10.97
11.01
0.39
2.40
3.86
3.40
18.00
54.10
201
19
0.58
19
Th
32
1.14
1.33
24
0.81
U
0.44
0.61
0.40
La
Ce
Pr
0
17
9.14
5.47
4.51
5.84
19.72
17.69
22.95
26.95
20.35
22.50
56.75
53.10
2.38
3.61
1.80
2.33
6.20
5.65
6.10
13.10
6.09
9.89
4.34
12.50
22.15
Sm
11.41
4.87
5.35
5.76
23.15
5.18
22.95
5.40
Eu
1.76
1.98
Gd
Tb
4.22
1.69
5.41
1.63
3.92
1.68
4.56
0.85
5.43
0.82
6.54
Nd
DY
Ho
Er
Tm
Yb
Lu
Cl
1.14
2.48
0.49
3.64
1.04
1.55
3.24
0.88
4.00
3.88
0.72
4.13
4.89
0.86
2.28
0.43
0.83
3.16
0.48
0.47
0.71
2.96
0.41
62.89
84.20
207.70
2.28
1.76
0.73
4.29
0.88
4.36
0.76
3.55
4.60
3.87
0.79
2.58
0.84
0.80
2.61
3.26
0.52
1.61
2.71
0.70
3.42
0.32
100.00
0.31
153.70
0.53
0.32
133.24
143.00
The basic bodies range from 44 to 47 wt% Si02. They
are metaluminous
(AS1 = 0.43-0.88)
low-K tholeiites
(Table l), with relatively high MgO and CaO, typically
7.2 and 10 wt% respectively (Fig. 2). From their distribution on mantle trace elements discrimination
diagrams
(e.g. Pearce, 1982; Shervais, 1982; Myers and Breitkopf,
1989, and Fig. 3), we interpret the basalts at Arroyo Casas
to be derived from an oceanic mantle source. An
E-MORB signature is indicated by their REE patterns,
which although essentially flat at 10-30 times chondrite
show some LREE enrichment (Fig. 5). Given the close
0.60
2.54
asscoiation of these rocks with Carboniferous
marine
sediments it is probable that they represent basic material
erupted in an extensional or marginal basin setting.
Cerro Puntn Blanca Stock
The Cerro Punta Blanca is an irregular stock 9 km
wide and 7 km long, intruding metamorphic rocks in the
eastern side of Cordon de1 Portillo. It has a medium to
coarse grained equigranular to porphyritic texture, with
locally developed pegmatitic veins. Rhyolitic dykes and
145
Geochemistry of Upper Palaeozoic-Lower Triassic granitoids of the Central Frontal Cordillera
Table 1 kontdl
Los Tgbanos
SampleName
0106T
Stock
0406T
Quebrada
0706T
0906T
1106T
0222QP
Portillo Stock
0422QP
0223QP
0523QP
Si02
64.11
61.21
il.31
64.60
0.41
0.82
0.01
0.57
68.36
0.48
65.27
0.56
59.31
0.58
61.88
0.54
76.11
TiO2
Al203
FeO*
15.21
15.58
10.01
16.75
15.10
13.98
15.46
3.27
4.33
4.09
3.55
15.29
0.29
2.85
2.94
MnO
0.04
0.08
0.05
0.04
0.01
3.22
0.04
15.80
2.92
0.05
0.08
0.21
MgO
CaO
Na20
KZO
P205
LOI
total
2.05
1.21
1.45
1.79
0.12
1.03
1.47
1.30
I.22
3.75
5.19
4.08
2.37
4.79
0.74
3.05
3.20
4.77
6.10
5.21
5.39
1.70
5.59
2.79
5.54
3.91
3.50
3.61
4.44
3.23
3.50
4.49
4.30
4.42
4.49
4.99
0.31
0.18
0.15
0.25
0.01
0.21
0.24
0.25
0.13
1.30
I.74
0.66
0.91
1.29
0.60
2.04
5.22
99.42
99.36
99.35
99.39
1.96
100.73
99.00
100.78
99.50
98.57
3
5
7
4
I
43
59
54
55
13
16
109
co
V
cu
Pb
Zn
S
Rb
Ba
Sr
Ga
Ta
Nb
Hf
Zr
Y
Th
U
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
DY
Ho
Er
Tm
Yb
Lu
Cl
11
8
7
70
50
1
104
1
6
71
714
38
13
48
12
64
63
48
54
78
171
13
0.50
83
2
10
59
22
30
34
51
56
94
123
39
Ill
73
83
388
67
35
54
86
I8
39
524
438
1209
82
799
224
1040
549
52
247
105
581
915
546
592
99
947
612
581
20
24
21
132
18
0.14
0.64
23
0.75
11.8
458
21
0.86
21
225
I3
0.79
1.09
22
0.63
II.?
7.7
12.4
10.6
4.59
6.56
6.34
2.68
2.7
3.50
185
297
139
33
24
131
14
26
12.23
7.01
2.24
2.53
12.07
1.63
15
7.04
I.34
0.68
1.52
8.8
10.1
6.35
4.09
193
140
12
15
5.06
1.26
11.09
2.60
9.20
12
15.62
2.06
4.51
27.26
63.10
5.63
9.5
4.10
283
191
4
14
0.95
0.41
24.47
6.29
14.14
44.12
0.20
40.32
59.60
26.10
40.90
3.95
34.00
12.55
84.65
5.76
2.60
2.99
0.53
9.14
6.57
23.30
5.25
1.47
10.53
3.19
1.42
19.00
4.52
13.85
2.98
2.66
25.70
1.06
0.87
1.97
0.48
57.60
6.20
3.75
2.31
0.49
3.90
0.68
2.66
0.38
1.04
0.18
2.73
0.49
3.70
1.10
0.20
2.63
1.50
2.47
2.22
0.62
I.66
0.38
2.48
0.28
2.22
0.40
0.48
4.06
0.46
2.10
0.37
0.21
102.83
132.94
292.08
312.07
0.74
3.53
0.83
0.45
161.75
0.30
faults (striking N160”) cut the stock. Xenoliths of metamorphic rocks are found near the margin. The Cerro
Punta Blanca Stock, which ranges in composition from
quartz monzonite to monzogranite, gives radiometric ages
of 348 f 35 Ma (Rb-Sr) and 337 f 15 Ma (K-Ar). Quartz,
feldspar and plagioclase are the main minerals present.
Biotite is common but hornblende is scarce. Muscovite is
present in one sample infilling late-stage veins. Quartz
shows undulose extinction, and zoned plagioclase with
altered cores are common.
The Cerro Punta Blanca Stock has a relatively narrow
0.67
0.74
0.11
1.16
2.17
5.32
I.45
52.29
15.29
68.15
6.89
32.75
3.28
28.30
4.47
11.20
I.82
0.80
3.91
0.60
3.89
0.54
4.13
0.44
3.01
0.30
2.00
0.52
2.53
0.53
2.55
1.74
0.51
3.87
1.55
0.34
1.93
0.21
117.63
I.45
0.52
I.49
0.23
2.40
0.35
1.89
0.22
125.62
0.25
2.30
0.42
140.87
113.68
1.12
0.29
chemical composition, with SiO, ranging from 66 to 68
% and Al,O, from 13 to 15% (Table 1 and Fig. 2). The
stock is metaluminous and talc alkaline, with molecular
Al,O&aO+Na,O+K,O)
(A/CNK) values ranging from
0.63 to 0.84. Sample 0525 displays anomalous values of
SiO,, A/CNK, Sn, Rb, Sr, Ba, Ta, and other trace and rare
earth elements, consistent with post-magmatic hydrothermal alteration. Minor and trace elements in the Punta
Blanca Stock have similar contents, which do not change
greatly relative to SiO, .
D.A. GREGORI
146
et al.
Table 1 (contd)
Cerro Punta Negra Stock
Sample Name
SiO2
Ti02
A1203
0522PN
62.64
0522bPN
63.22
0 53
0.50
15.69
15.67
0624PN
0326PN
7410
0.08
1464
FeO’
3.87
3.51
0.58
MnO
0.10
0.00
M90
CaO
1.37
0.10
1.19
0.18
5.12
4.85
0.09
Na20
4 55
5 66
K20
413
3.87
P205
0.29
LOI
total
112
99.41
0823QP
0325 PB
0425PB
0625PB
0725PB
53 10
6729
66.14
66.66
68.05
0.94
057
0.52
0.54
67.12
0.60
14 96
9.34
1437
061
14.61
13.81
13.12
1479
3.69
3.88
3.31
3.39
3.87
0.06
6.17
0.13
0.08
0.07
0.08
0.09
1.23
1.68
142
1 52
146
3 76
415
3.00
2.61
4 75
a 98
2.54
1.98
3.70
4.37
3.92
4.59
4.74
498
2.14
4.31
4.44
4.75
417
0.28
0.61
0.00
0.14
0.21
1.50
1.00
0.16
1 97
465
0.21
0.46
0.25
0.53
0.25
0.84
99.46
100.90
99.37
99.46
99.46
99.45
100.85
25
11
12
10
11
13
1
182
65
72
52
11
57
69
CO
11
10
v
44
50
6
11
Pb
I6
19
20
Zn
S
63
a4
69
74
cu
Cerro Punta Blanca Stock
110
100.60
241
24
14
0
20
50
19
21
19
21
30
124
258
67
43
308
a3
416
22
67
100
143
194
69
Rb
117
134
194
28
69
108
98
894
926
170
325
686
156
710
103
Ba
1009
979
770
436
17
Sr
635
555
986
413
378
353
20
585
19
61
Ga
21
20
17
18
19
18
Ta
0.44
0.99
10.2
1.34
114
1.31
0.36
0.84
Nb
Hf
15.5
0.66
10.4
4.38
6.48
6.27
Zr
Y
125
15
101
5.88
a3
8.9
6.62
1.52
13.7
5.02
5.3
3.10
256
163
13
15
119
17
12
Th
9.41
7.20
9.49
3.13
293
18
5.04
U
2.08
2.14
2.50
0.95
5.46
1.31
22
19.63
5.25
1.39
La
Ce
18.14
18.71
7 11
18.20
41.39
48.15
26.25
39.65
83.50
51.03
83.15
2.72
8.74
5.94
6.64
8.31
29.55
24.25
28.75
5.73
a.3
7.69
1.04
11.1
4.86
324
17
179
7.34
1.97
15.01
24.35
37 15
55.80
71.20
35.12
77.50
20
2.61
Pr
4.97
46.60
4.34
Nd
18.80
4.39
10.65
4.26
16.05
8.88
31.30
Sm
19.95
5.76
3.82
3.82
7.23
5.75
21.60
4.57
Eu
1.55
1.42
3.45
2.30
4.55
2.11
4.43
1.78
3.16
1.97
3.30
0.53
2.53
1.83
3.01
163
Gd
Tb
0.75
3.14
4.36
0.50
2.86
0.54
0.53
2.61
0.74
3.57
0.60
3.49
0.51
3.29
4 55
0.60
0.63
165
0.62
1.70
0.60
0.71
0.79
0.65
0.71
0.98
2.29
2.06
i a5
0.35
0.28
1.64
0.28
1.43
0.26
2.37
0.37
0.38
2.10
2.34
0.37
0.23
233.21
0.53
198.80
DY
Ho
Er
Tm
Yb
Lu
Cl
0.52
1.49
0.20
1.57
0.31
273.03
0.48
2.71
0.43
1.32
0.31
1.75
0.28
2.15
0.31
235.84
49.41
3.63
024
101.00
Los Tabanos Stock
The Los Tabanos Stock is exposed as small and dispersed outcrops between the Rio de las Tunas in the north
and Arroyo de1 Arenal in the south, in a zone 10 km long
and 4 km wide. The stocks intrude low-grade garnetbearing greenschists and serpentinized
ultrabasic rocks
(Bjerg et al. 1990). A transition between intrusive and
volcanic rocks can be recognized in the northern area of
this stock. The rocks comprise phenocrysts of plagioclase
with altered cores and corroded quartz, set in an equigranular groundmass. Hornblende and biotite are com-
1.85
0.27
20468
4.89
029
300.90
3.36
2.40
0.37
292.81
mon minerals, both strongly altered to chlorite. These
mafic minerals represent an early stage of crystallization,
with later stage crystallisation characterized by unaltered
euhedral grains of plagioclase, perthitic alkali-feldspar
and a granophyric
intergrowth
of alkali-feldspar
and
quartz.
The Los Tabanos Stock has a large variation in chemical composition, with SiO, ranging from 61 to 76 wt%
(Fig. 2). The rocks have compositions
typical of calcalkaline magmatic suites (Fig. 4) ranging from metaluminous quartz monzonite
and quartz monzodiorite
to
Geochemistry of Upper Palaeozoic-Lower Triassic granitoids of the Central Frontal Cordillera
0 AC
9 T
0
2-
A QP
o
0
Tio2
147
0
l-
*
*
PN
X
PB
*
G
K20
.
+-.
,g(_
0
40
60
50
70
a0
a0
Na20
a0
20-9
a0
60
n
18-
,
.
16-
l
o
.mI.
*yr
*
Al203
‘4-
0
OO
0
.
l
;
X
CaO 10
Y8
*
x*
12103
40
60
50
I
a
0.30.2-
o
0
0.0
1
40
a0
60
.
O.l-
*
a0
70
0.4-
P205
0’
*
8
,
A
I
*
6-
Ir:
. A4
'es
.
L*
MgO 4-
l
* 1
,__
2-
.
70
50
o”o
80
40
50
s;;2
70
a0
::2
Fig. 2. Harker plots for the granitoid stocks and basaltic rocks of the Frontal Cordillera of Argentina.AC, Arroyo Casas basalts, T,
Los Tabanos, QP, Quebrada Portillo, PN, Cerro Punta Negra, PB, Cerro Punta Blanca, G, Guevara.
granodiorite and monzogranite. Harker variation diagrams
shows that decreasing trends exist for nearly all the major
and trace elements, except for K,O, Na,O and FeO*,
where some scatter occurs (Fig. 2). Differences in major
elements such as Ba, Th and REE contents may be due to
post-magmatic processes such as albitization and hydrothermal alteration. Comparison of average concentration
of trace elements (Rb 52 ppm, Ba: 566 ppm, Sr 425 ppm,
total REE: 95 ppm) of the Los Tabanos Stock with data
for the Cerro Punta Blanca Stock (average total REE 145
ppm) suggest that the former is the less evolved of the
two bodies. REE patterns are similar to those of the Cerro
Punta Blanca Stock, but overall enrichment in REE is
lower Table 1 and Fig. 5).
Quebrada Portillo Stock
The Quebrada Portillo Stock forms part of the second
magmatic cycle defined by Caminos et al. (1979) in the
Frontal Cordillera. Radiometric ages (Rb-Sr 264 * 8 Ma,
K-Ar 291 + 10 Ma) implying a Late Carboniferous-Early
Permian age. The stock has a very irregular shape,
D.A. GREGORI
148
~A14
l Al3
lot
4
et al.
present in small quantities. Chemically,
metaluminous and talc alkaline, with SiO,
62-74 wt% (Figs. 2 and 4). REE contents
lar to the Los Tabanos and Guevara stocks
the rocks are
ranging from
are very simi(Fig. 5).
DISCUSSION
Regional Comparisons
.,I
SrKFlbB~ThTaNbCeZrHfSmTi
Y Yb
Fig. 3. MORB- normalised Trace element spidergrams (after
Pearce, 1983) for basaltic rocks of Arroyo Casas.
extending 3 km N-S and 6 km E-W. Near Portillo Argentino the stock is cut by aplities, whereas near Manatiales
Brook the stock is cut by basic dykes. Samples from the
Quebrada Portillo Stock are coarse grained, with alkali
feldspar, plagioclase and interstitial quartz as the main
minerals. Biotite is common (up to 10%) whereas hornblende is scarce. Alkali feldspars show Carlsbard and
microcline twinning. Plagioclase has an albitic composition with a well developed zonation. Both alkali feldspar
and plagioclase are altered to sericite and occasionally
epidote. Accessory minerals include zircon and titanite.
The rocks are classified as monzonites, quartz monzonites, granodiorites and monzogranites, ranging in SiO,
from 59 to 68 wt%. Concentrations
of major element
oxides vary as a function of SiO, , with Al,O,, MgO and
CaO slightly decreasing with increasing SiO,. Na,O has
no trend and K,O increases (Fig. 2). The rocks are metaluminous, with A/CNK values range from 0.77 to 0.97.
Trace elements
vary systematically
with Si02, with
decreasing Sr, Ba and Eu consistent with minor feldspar
fractionation. Total REE contents (136-211 ppm) show a
range wider than in the Punta Blanca Stock. Total REE
contents decrease with increasing SiO*. Ce/Yb has no
obvious trend and average total REE content is 33.5 ppm.
The REE patterns shown slight enrichment in LREE, but
there are no Eu anomalies. These chemical trends are
very similar to those of the Punta Blanca Stock (Fig. 5).
Cerro Punta Negra Stock
The Ceno Punta Negra Stock lies in the eastern side of
the Cordon de1 Portillo. (Fig. 1). The stock is very well
exposed and contains a metamorphic roof-pendant. Caminos et al. (1979) report a K-Ar age for the stock of 234 +
10 Ma. The rocks are porphyritic to equigranular in texture, and locally pegmatitic. The mineralogy consists of
alkali feldspar, plagioclase and biotite. Quartz is interstitial between alkali feldspar and plagioclase.
Multiple
twinning and zonation are common in plagioclase. The
alkali feldspars show microcline twinning. Hornblende is
Granitoid rocks that date from Lower Carboniferous to
Upper Triassic occur on both sides of the Andes. In the
Frontal Cordillera of Chile at 28 to 31”S, Nasi et al.
(1985)
and Mpodozis
and Kay (1992)
describe
subduction-related
metaluminous
to peraluminous
calcalkaline granitoids formed from melting of subduction
complexes and later thickened crust following the collision of the Equis terrane. According to these authors, the
more basic units of the Elqui Complex have chemical and
petrological characteristics similar to magamatic arc granitic rocks, and are thought to relate to Late Carboniferous subduction. In the Argentinean
Frontal Cordillera,
both portions (29” to 31”s and 33”lO to 33”45’ S) are
made up of batholithic bodies elongated in a north-south
direction intruding Carboniferous basins (- 7 km thick)
oriented parallel to the present day coast and trench. In
the Frontal Cordillera of San Juan province (29” to 31”S),
Llambias et al. (1987) and Llambias and Sato (1990)
have proposed that the Colanguil Batholith was derived
from melting of basic crustal material during regional
extension.
We have compared our new data from the Frontal Cordillera batholith in Chile south of 3 1. Figure 6 shows a
plot of Na,O/K,O v K/Rb for the stocks and batholith
rocks. The genera1 fields defined by plutonic rocks from
Peru and the Aleutians are also shown. The rocks with the
highest
Na,O/K,O
are the Arroyo
Casas basalts.
Although many of the rocks from the Frontal Cordillera
of Argentina lie inside the field defined by Peru, samples
from Quebrada Portillo, Punta Negra and Guevara have
K/Rb ratios similar to rocks from the Aleutians. Perhaps
significantly, all of the samples from Los Tabanos stock
have high K/Rb ratios (up to 800), characteristic of Island
Arc plutons. For comparison, rocks from the Frontal Cordillera of Chile described by Mpodozis and Kay (1992)
lie mostly within the Peru field, with K/Rb ratios generally < 350. On a plot of La/Sm v LaiYb (Fig. 7) the REE
show a greater overlap with rocks from Chile. Thus, rocks
from Cerro Punta Negra, Quebrada Portillo and Los Tabanos stocks show relatively high La/Yb (ca. 20-35) at constant La/Sm (- 6), consistent with residual garnet in the
source region. Similar trends are seen in the Los Carricitos and Cochiguas plutons of the Ingaguas and Elqui
complexes in Chile (Mpodozis and Kay, 1992). The Cerro
Punta Blanca rocks show a trend on increasing LREE
enrichment (increasing La/Sm) with increasing LaNb,
consistent with garnet playing an important role in controlling magma composition at source. A similar trend is
seen in the Montosa rocks of the Elqui Complex in Chile.
In summary, there are apparent similarities in compositions of rocks from both the Chilean and Argentinian
Geochemistry of Upper Palaeozoic-Lower Triassic granitoids of the Central Frontal Cordillera
149
Calc-Alkaline
Na20
l
K20
MO
Fig. 4. AFM plot of the rocks of the Frontal Cordillera. The Arroyo Casas basalts are tholeiitic. The granitoid stocks show a typical
talc alkaline trend. Symbols as in Fig. 2.
Fig. 5. REE patterns of selected Frontal Cordillera granitoids and basalts from Arroyo Casas. Note the similarity between individual
stocks and with the Arroyo Casas basal& and lack of negative Eu anomalies. Symbols as in Fig. 2.
Frontal Cordilleras. However, while many of the Argentinian stocks are similar to the older, Elqui complex in
Chile (itself similar to rocks from Peru), samples from the
Frontal Cordillera of Argentina have higher K/Rb ratios
that may be due to magma derivation from a more mafic,
Island Arc source. Other Andean batholiths have recently
been shown to have close genetic links with basic (marginal basin) source material. For example, the Coastal
Batholith of Peru is now thought to have formed from
partial melts of basic-intermediate volcaniclastic material during a major episode of Albian continental rifting
(Atherton, 1990; Atherton and Petford, 1993). As a
150
D.A. GREGORI
8
0
held of Chdean Fronti
. . .:. ::.,
,q C$rddlera Bathohths
‘L:
0
200
400
600
800
1000
K/Rb
Fig. 6. Plot of Na,O/K,O v K/Rb for the rocks of the Frontal
Cordillera (Argentina). The field defined by granitoid rocks
from the Frontal Cordillera of Chile (hatched) is shown for
comparison (Mpodozis and Kay, 1992). While there is some
overlap with the Chile rocks (K/Rb similar to Peru), rocks from
Quebrada Portillo, Punta Negra Guevara and Los Tabanos
stocks have K/Rb ratios similar to Island Arc plutons charactrerised by the Aleutians. Symbols as in Fig. 2.
working model, we tentatively propose a similar origin
involving
Late Palaeozoic
crustal extension
for the
Frontal Cordillera Batholith magmas of Mendoza Province (Petford and Gregori, 1994).
et al.
larities in age, petrological and geochemical characteristics for the Guevara, Los T banos and the Cordon de1
Portillo Stocks suggest the stock magmas share a common source, with each stock possibly representing a separate pulse of magma.
The Arroyo
Casas basalts,
interpreted by Polanski (1972) as the first magmatic pulse
from which the later rocks of the Frontal Cordillera Composite batholith of Mendoza are derived is at odds with
our data and interpretation, which on the basis of their
chondrite-normalized
REE patterns, suggests they originated in a back arc or marginal basin setting. There are
general chemical similarities between the rocks of the
Frontal Cordillera of Chile and those analysed in this
present study. However, some Argentinian stocks (notably
Los Tabanos) have WRb ratios up to 800, characteristic
of Island Arc (e.g. Aleutian) plutons. Similar high K/Rb
rocks are apparently absent from the Frontal Cordillera in
Chile. Preliminary analysis suggests that the Frontal Cordillera granitoids may have originated by partial melting
of relatively mafic, high-K talc alkaline lower crust during a period of Late Paleozoic extension. These ideas are
currently under investigation.
Acknowledgements
-
We would like to thank to M. Cabane and R.
Bartroli for helping during XRF
determinations at
Barcelona, and K.
Jarvis, J. Williams and J. Wills for help with ICP-MS analysis. D. Green
and M. Holdtman provided invaluable assistance during fusion procedures at Kingston University. D.A.G. is particularly grateful to A. Lopez
Soler and 1. Jarvis for their guidance during his EC fellowship at Jaunt:
Almera Institute. Barcelona, Spain and Kingston University,
UK. Our
CONCLUSIONS
thanks to S. M. Kay and an anonymous referee for their constructive
New geochemical and mineralogical data of granitoid
rocks from the central area of the Frontal Cordillera presented in this paper show that the intrusive bodies have
chemical and mineralogical
characteristics
(relatively
high Na,O contents, A/CNK ratio < 1.1, normative diopside, modal hornblende
and sphene) similar to I-type
granitoids (Chappell and White, 1974). The overall simi-
thanks to the Directorate General XII of the Commission of the Euro-
comments, which improved the early version of this paper. Special
pean Communities for the
“Marie Curie” to
D.A.G.
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