UNIVER SIDAD DE CONCEPCIÓN
DEPARTAMENTO DE CIENCIAS DE LA TIERRA
10° CONGRESO GEOLÓGICO CHILENO 2003
THE EARLY JURASSIC SUBCORDILLERAN PLUTONIC BELT OF
PATAGONIA (42–44º S): PROTO-PACIFIC SUBDUCTION COEVAL
WITH THE KAROO MANTLE PLUME.
RAPELA, C.W.1, R.J. PANKHURST, R.J.2, FANNING, C.M.3
1
Centro de Investigaciones Geológicas, Calle 1 No. 644, 1900 La Plata, Argentina. [email protected].
Visiting Research Associate, British Geological Survey, Keyworth, Nottingham NG12 5GG, U.K.
[email protected].
3
PRISE, Research School of Earth Sciences, The Australian National University, Canberra ACT 0200, Australia.
[email protected].
2
Late Triassic to Jurassic magmatism is widely represented in Patagonia by the Chon Aike
volcanic province, the Batholith of Central Patagonia, and the Subcordilleran Plutonic Belt, the
principle subject of this work (Fig. 1). The very extensive distribution of this magmatism
contrasts with that of the main Cordillera and coastal belts in the central Andes between 23º and
38ºS.
The Subcordilleran Plutonic Belt (SCPB) of Río Negro and Chubut provinces is a discontinuous
NNW-SSE trending belt of small batholiths and plutons, extending for more than 300 km
immediately to the east of North Patagonian cordillera (Fig. 1). This was originally called the
‘Subcordilleran Patagonian Batholith’ by Gordon & Ort (1993), but we propose the this name
should be abandoned in favour of ‘Sub-Cordilleran Plutonic Belt’ (Haller et al., 1999). The SCPB
is mostly composed of I-type, calc-alkaline zoned plutons dominated by Bt-Hbl granodiorites and
quartz-monzodiorites, with Bt monzogranites, minor diorites and leucogranites (Spikermann et
al., 1988, 1989; Gordon & Ort, 1993; Busteros et al., 1993; Haller et al., 1999). The northernmost
plutons, in the Rio Alto Chubut and Leleque areas, have modal compositions indistinguishable
from those of granites in the Patagonian batholith (Rapela, 2001). Earlier radiometric ages
(mostly K–Ar) are rather variable but, in the most reliable cases, suggest an early Jurassic age.
Rb-Sr isochron ages of 200 ± 24 Ma and 183 ± 13 Ma were reported for the Río Alto Chubut area
(Gordon & Ort, 1993), with initial 87Sr/86Sr ratios of 0.7047 and 0.7059. The published data for
other individual plutons are considered below in comparison with the new results.
The first purpose of this paper is to accurately constraint the age of the SCPB by means of U-Pb
SHRIMP dating. Secondly, these crystallization ages are used, together with geochemical and SrNd isotopic data, to investigate the spatial distribution of the SCPB and its possible relationships
with the Chon Aike province and the Patagonian batholith. Finally, the probable tectonic setting
of the Late Triassic-Jurassic plutonic belts of Patagonia is considered in the light of the new
results.
Todas las contribuciones fueron proporcionados directamente por los autores y su contenido es de su exclusiva responsabilidad.
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Figure 1: Distribution of the Triassic and Jurassic magmatism of Patagonia (modified from Rapela, 2001). Location
of the main magmatic units of the Subcordilleran Plutonic Belt: (1) Alto Río Chubut; (2) Leleque; (3) Aleusco
batholith; (4) José de San Martín. Faults associated with subcordilleran plutonism are from Gordon & Ort (1993).
Isotopic ages of the Chon Aike province from Pankhurst et al. (2000).
GRANITES OF THE SUBCORDILLERAN PLUTONIC BELT
Samples from the Leleque, Aleusco and José de San Martín areas were collected for chemical,
isotopic and U-Pb dating. The Leleque area was studied by Lizuaín (1983) and Gordon & Ort
(1993), who recognized two units: a small gabbro pluton, intruded by a more voluminous
granodioritic to monzogranitic suite. Sample LEL-052 is a hornblende-biotite granodiorite from
the dominant suite, which carries microgranular mafic enclaves of 6–15 cm. A chemical analysis
of this sample indicates a typical metaluminous rock. It has an Alumina Saturation Index of 0.96
(ASI= molecular Al2O3/CaO+Na2O+K2O), with 69.66% SiO2, 7.21% Na2O+K2O, 3.58%
FeOt+MgO, normal levels of HFS (high field-strength) and LIL (large-ion lithophile) elements,
e.g. Y = 18 ppm, Nb = 8.7 ppm, Ta = 0.8 ppm, Rb = 109 ppm, Sr = 262 ppm, Ba = 651 ppm, and
a REE pattern with [La/Yb]cn = 10.5 and a weak negative Eu anomaly (Eu/Eu* = 0.78). A K-Ar
age of 141 ± 5 Ma has been reported for the Leleque unit (Lizuaín, 1983). Isotopic analysis of
the Leleque sample yields an initial 87Sr/86Sr ratio of 0.7054 ± 0.0001 and an initial εNd value of
–1.6 using the mean age of 185 Ma defined below.
The Aleusco pluton, emplaced in Liassic sediments in the Precordillera of Chubut, was first
described by Turner (1982) and Spikermann et al. (1988, 1989). This is a composite epizonal
pluton consisting of a main facies of hornblende-biotite granodiorite and subordinate diorite with
abundant mafic enclaves, and late plutonic leucogranite and aplite (Spikermann et al., 1989;
Haller et al., 1999). Sample ALE-055 is a hornblende-biotite granodiorite, with ASI = 0.95,
67.49% SiO2, 7.28% Na2O+K2O, 5.05% FeOt+MgO, Y = 26 ppm, Nb = 7.4 ppm, Ta = 0.69 ppm,
Rb = 152 ppm, Sr = 203 ppm, Ba = 573 ppm, and a REE pattern with [La/Yb]cn = 7.8 and
Eu/Eu* = 0.67. In almost all parameters the composition falls within the range of four samples
analysed by Haller et al. (1999). Previous age determinations by the K–Ar method have given
180 ± 10 Ma (Spikermann et al. (1988) and three amphibole ages of 177 ± 6 Ma, 179 ± 7 Ma and
184 ± 6 Ma (Haller et al., 1999). The initial 87Sr/86Sr ratio of ALE-055 at 185 Ma is 0.7062 ±
0.0001 and its εNdt value is –1.2.
Near the town of José de San Martin in Chubut province, a string of NE-SW granitic bodies,
mainly emplaced into Carboniferous sediments of the Tepuel basin, has been related to the SCPB
(Gordon & Ort, 1993). The petrographical and chemical characteristics of these granitoids have
been described by Spikermann (1978), Franchi & Page (1980) and Busteros et al. (1993). The
predominant lithology is medium-K metaluminous granodiorite varying to monzodiorite
(Busteros et al., 1993). The largest of these exposures is a 10 km long, 2 km wide pluton with a
thermal aureole in Upper Palaeozoic sediments, 6 km to the east of the locality of José de San
Martín (Spikermann, 1978; Busteros et al., 1993). Sample JSM-058 is a Hbl-Bt quartz
monzodiorite, with ASI = 0.91, 61.46% SiO2, 6.01% Na2O+K2O, 8.25% FeOt+MgO, Y = 31
ppm, Nb = 7.0 ppm, Ta = 0.56 ppm, Rb = 93 ppm, Sr = 297 ppm, Ba = 472 ppm, and a REE
pattern with [La/Yb]cn = 4.7 and Eu/Eu* = 0.79. Abundant quartz monzodiorites and quartz
monzonites are typical of the so-called calc-alkaline monzonite series (Lameyre, 1987), which
also dominate the Early Jurassic suites of El Deseado Massif (Rapela & Pankhurst, 1996).
Franchi & Page (1980) reported K-Ar ages of 167 ± 30 Ma, 197 ± 10 Ma and 207 ± 10 Ma for
these rocks, which they referred to as the José de San Martín Formation. The initial 87Sr/86Sr
ratio of JSM-058 at 185 Ma is 0.7050 ± 0.0001. Its εNdt value is +1.5, slightly higher than that of
the other two samples.
U–PB GEOCHRONOLOGY
U–Pb dating was carried out using a sensitive high-resolution ion microprobe (SHRIMP RG) at
The Australian National University, Canberra, following the procedures of Williams (1998).
Cathodo-luminescence images were used to target the magmatic rims and tips of euhedral grains.
Analysis spots, mostly within the well-zoned ends of grains, were selected avoiding cracks and
inclusions. Data for the SHRIMP analyses were processed using SQUID (Ludwig 2001) and
Isoplot/Ex (Ludwig 1999), and ages were calculated from the 206Pb/238U ratios after correction
for the appropriate composition of common Pb based on the measured 207Pb.
Sample
Location
Locality
Rock Type
Age (Ma)*
LEL-052
42º21.38´S
71º11.18´W
Leleque
Hbl-Bi granodiorite 181 ± 3
ALE-055
43º06.24´S
70º28.66´W
Aleusco
Hbl-Bi granodiorite 185 ± 2
JSM-058
44º00.46´S
70º23.66´W
José de
San
Martín
Hbl-Bi quartz
monzodiorite
182 ± 2
* errors are 95% confidence limits, including uncertainty in the standard
The table (above) shows results for the analysed samples described above from the Leleque,
Aleusco and José the San Martín areas. Nineteen grains were analysed from the Leleque
granodiorite, of which one is clearly inherited (c. 330 Ma). Apart from this, the data spread
outside expected analytical error (MSWD=3.2) and the four youngest 206Pb/238U ages were
disregarded as probably suffering from slight Pb-loss. The remaining 14 define an Early Jurassic
age of 181 ± 3 Ma. From the 14 analysed grains from the Aleusco granodiorite, 13 grains define
an Early Jurassic age of 185 ± 2 Ma. Finally, 12 grains of a quartz monzodiorite from the José de
San Martín body also gave a Early Jurassic age of 182 ± 2 Ma, with Pb-loss in three grains.
DISCUSSION
The U-Pb SHRIMP crystallization ages of these typical samples of metaluminous granodiorites
and quartz monzodiorites are remarkably consistent. These new precise data are consistent with
the Rb-Sr age of 183 ± 13 Ma reported by Gordon & Ort (1993) and the K–Ar amphibole ages of
Haller et al. (1999). Together they indicate that the 300 km long SCPB is dominated by roughly
coeval I-type intruded in late Early Jurassic times. The interval 185–181 Ma falls within the
Toarcian stage of the ICS and GSA time-scales. The previous Rb-Sr and K-Ar ages of ~ 200 Ma
mentioned above could be taken as suggesting that some plutons were intruded slightly earlier, at
the beginning of Jurassic and the transition Triassic to Jurassic but U-Pb data are needed to
confirm this possibility.
No plutons of this age have been found south of 44º30'S, either in the subcordilleran or
cordilleran sectors of southern Patagonia. The subcordilleran sector in southern Chubut and Santa
Cruz is generally well-exposed, so it is most probable that Early Jurassic plutonism did not
continue to the south along the axis of the Andean Cordillera and Patagonian batholith. Gordon
& Ort (1993), based on the direction of major lineaments and reported K-Ar ages from drill cores
of granitic rocks at the basement of the San Jorge basin, suggested that south of 44º30´ the SCPB
trends towards the SE, re-appearing in erosional windows in the north-eastern sector of the
Deseado Massif. Geochemical and isotopic study of the 203 ± 2 Ma monzonitic series of the
Deseado Massif has shown that they were subduction-related, deep-seated melts emplaced in
distal zones of the orogen (Rapela & Pankhurst, 1996). The initial 87Sr/86Sr ratio and εNdt values
of the three granites analysed here are very similar to those of the Deseado massif quartz
monzonites, which show restricted ranges of 0.7048–0.7052 and -2.4 to -0.2, respectively.
Overall, the Triassic–Early Jurassic magmatism of the SCPB and Deseado Massif has been
interpreted in terms of a subduction regime along the southern edge of Gondwana (Franchi &
Page, 1980; Busteros et al., 1993; Godeas, 1993; Gordon & Ort, 1993; Pankhurst et al., 1993;
Márquez, 1994; Rapela et al., 1996; Haller et al., 1999). The age and isotopic similarities
between the Early Jurassic suites of the SCPB and the Deseado Massif are at least consistent with
a common tectonic scenario for these I-type plutonic granitoids, which are treated hereon as
single belt.
Another important conclusion derived from the new precise ages of the SCPB, is that I-type
cordilleran plutonic rocks, most probably associated with subduction, were coeval with the
extensive Early Jurassic rhyolites developed at the same latitude in the North Patagonian Massif.
Originally defined in the eastern sector of the North Patagonian Massif (Marifil Formation),
Early Jurassic rhyolites have also been recognised in the central part of the massif (Garamilla
Formation, 188 ± 1.5 Ma, U-Pb SHRIMP, Franzese et al., 2002). This Early Jurassic pulse of
188–178 Ma (also referred to as V1 by Pankhurst et al., 2000 and Riley et al., 2001) has been
related to rise and spreading of the Karoo mantle plume head from beneath South Africa and East
Antarctica. However, this scenario must now include a contemporaneous convergent protoPacific margin in western Patagonia.
The space-time relationships between the Late Triassic to Jurassic I-type magmatic belts and
coeval acid volcanism are shown in four main intervals in Fig. 2.
A. During the Late Triassic (224–207 Ma) the NW-SE trending Batholith of Central Patagonia
was emplaced, associated with the dextral Gastre Fault System (Fig. 2a) (Rapela et al., 1991,
1992; Rapela & Pankhurst, 1992). This has calc-alkaline ‘volcanic arc’ geochemical and isotopic
signatures (εNdt = -2.2 to -3.1, initial 87Sr/86Sr = 0.7055–0.7059), and has been related to NNEdirected subduction (Rapela, 2001). Late Triassic calc-alkaline rhyolitic ignimbrites to the north
of the BCP show more evolved isotopic signatures (e.g., the Los Menucos ignimbrites: εNd = 8.0 to –9.4; initial 87Sr/86Sr = 0.7078–0.7079; Rapela et al., 1996). Other scattered outcrops of
Late Triassic pyroclastic and andesitic rocks have been inferred either from fossil flora or from
K-Ar ages (Carrillo & Hurtado, 1985; Zavattieri et al., 1994). The discovery of Late Triassic
fossils in sediments of the accretionary prism of the Chonos Archipelago at 45ºS (Fang et al.,
1998) may be significant in linking this magmatism to a palaeo-subduction zone.
B. By the beginning of Jurassic times the locus of I-type cordilleran magmatism shifted
southwestwards to the SCPB, at the time when the rhyolites of the Marifil Formation were being
erupted over most of the eastern and central sectors of the North Patagonian Massif (Fig. 2b).
The volcanic rocks are typically bimodal and subalkaline, with 69–77 % SiO2 rhyolites
dominating over occasional basaltic andesite and basalt (Pankhurst & Rapela, 1995). Peralkaline
rhyolites with 425–650 ppm Zr occur in Península de Camarones at 45ºS on the Atlantic coast,
but such indications of intraplate affinities are not common (Rapela, 2001). The rhyolites have
geochemical characteristics compatible with lower crustal melting of an old hydrous, mafic
source (Riley et al., 2001). Pankhurst & Rapela (1995) proposed melting of a mafic granulitic
lower crust, sometimes contaminated by an upper crustal component on the basis of εNdt values
of -3.9 to -8.2 and initial 87Sr/86Sr ratios of 0.7055–0.7112, but with a prominent mode at 0.7067.
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Figure 2: Spatial variation with time of the Late Triassic to Late Jurassic I-type cordilleran belts and volcanic
sequences of Patagonia. See text for explanation.
In contrast, both the Late Triassic and the Early Jurassic I-type batholiths have lithological,
chemical and isotopic signatures suggesting a different origin (Rapela et al., 1996; Rapela, 2001).
The dominant rock types are hornblende-biotite granodiorites and quartz monzodiorites with 6369% SiO2; more basic rocks are usually cumulate-rich fractions, while monzogranites result from
fractionation of the dominant intermediate magmas (e.g., Rapela & Pankhurst, 1996). The Sr and
Nd isotopic signature of the SCPB and Deseado Massif plutonic suites (εNdt = +1.5 to –2.5;
initial 87Sr/86Sr = 0.7048–0.7064) is more primitive than that of the volcanic rocks and falls as a
fairly tight grouping within the wider range exhibited by the Patagonian batholith (Bruce et al.,
1991; Pankhurst et al., 1999). Are at least some of the Marifil rhyolites related to the Pacific
subduction represented by the SCPB? This is a difficult question, but at least some constraints
should arise from a systematic isotopic and geochemical study of the rhyolites in the central and
western sectors of the North Patagonian Massif, and the comparison with those at the eastern side
which include the type section of the Marifil Formation.
C. During the Middle Jurassic(Fig. 2c), magmatic activity shifted towards southern Patagonia and
the Antarctic Peninsula, developing the extensive rhyolites sequences of the Chon Aike
Formation (V2 event, Pankhurst et al., 2000). In Tierra del Fuego the peraluminous Darwin
granite suite is thought to be correlative of the V2 rhyolites (164.1 ± 1.7 Ma, Mukasa & Dalziel,
1996). This Middle Jurassic acid event (~172–160 Ma: εNdt = -1.8 to -3.9; initial 87Sr/86Sr =
0.7064–0.7169) is now the only one for which no I-type cordilleran or subcordilleran coeval
counterpart has yet been reported. Further systematic and precise geochronological work will be
necessary to confirm if this temporal hiatus in the subduction-related cordilleran magmatism
along the southern and austral Andes is real.
D. From Late Jurassic times onwards, the main locus of the magmatic activity was within and
subparallel to the modern Andean Cordillera (Fig. 2c) (Rapela & Kay, 1988). The I-type plutonic
component of this activity is represented by the Patagonian batholith which ranges in age from
~155 to 5 Ma. The oldest, 155-143 Ma Late Jurassic granites are restricted to the eastern side of
the batholith and as satellite plutons east of the Patagonian batholith (Ar-Ar and U-Pb ages at
46º30´ -50º 20´S: Bruce et al., 1991; Martin et al., 2001; Suarez & De la Cruz, 2001). Late
Jurassic K-Ar ages from the eastern side of the Patagonian batholith at 40º–42ºS (González Díaz,
1982) also suggest that the earliest plutonism is restricted to a narrow eastern belt of the batholith
along the proto-Pacific margin. However, Late Jurassic volcanic rocks occur all along the
Patagonian Andes, as well as in extra-Andean sectors of the North Patagonian and Deseado
massifs (Fig. 2c) (see references in Pankhurst et al., 1999, 2000; Suarez & De la Cruz, 2001).
Thick 154–138 Ma rhyolite sequences in the Andean sector of southern Patagonia are considered
active-margin volcanic products associated with the Late Jurassic granites (Pankhurst et al.,
2000; Pankhurst et al., this volume).
The spatial distribution of the I-type plutonic belts in Patagonia indicates a clock-wise shift of
belt axes during the Late Triassic-Late Jurassic interval (Fig. 2). However, such a westward
migration has not been observed in the main body of the Patagonian batholith, where a
symmetrical pattern defined by Cretaceous margins, a Cenozoic centre, and Neogene satellite
plutons to the east and west is the prevailing spatial arrangement (Pankhurst et al., 1999; Suarez
& de la Cruz, 2001 and references therein). To explain this regular age distribution, Bruce et al.
(1991) proposed a model of magmatic inflation, produced by the long-term persistence of the
magmatic arc in the same place (Early Cretaceous to Recent).
No Late Jurassic plutons have been identified along the western side of the Patagonian batholith,
so that the Late Jurassic plutons seem to be the lattermost I-type plutonic rocks associated with
the clock-wise rotation of the plutonic axis during the Late Triassic-Jurassic interval. Such a
strong differences between the spatial arrangements of the Late Triassic-Jurassic and the
Cretaceous-Neogene I-type plutonism in Patagonia, could be related to microplate displacements
in the early stages of the Gondwana rifting and break-up. Such displacements are seen in the
hypothetical dextral displacement of the South Patagonian block during the Late Triassic-Early
Jurassic times (Rapela & Pankhurst, 1992), associated in turn with displacement and rotation of
the Falkland/Malvinas and Ellsworth microplates (Mitchell et al., 1986; Taylor & Shaw, 1989;
Grunow et al., 1991; Rapela & Pankhurst, 1992; Marshall, 1994). Clockwise displacement of
intra-Patagonian blocks may have been consequent upon crustal extension and microplate
movement away from the Karoo mantle plume. This scenario came to and end with the opening
of the South Atlantic in Early Cretaceous times, as the symmetrical distribution of the Late
Jurassic to Miocene Patagonian batholith suggests a quasi-stationary locus of subduction relative
to the continental margin.
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