Eastern Sierras Pampeanas Research Articles

Evolution of exhumation and erosion in western West Gondwanaland as recorded by detrital zircons of late Neoproterozoic and Cambrian sedimentary rocks of NW and Central Argentina

The evolution of the provenance areas for Late Neoproterozoic, Cambrian and Early Ordovician sedimentary and meta-sedimentary rocks of north central and northwest Argentina is discussed using 123 maximum ages of detrital zircons from 42 samples from this and previously published studies. Most detrital zircon ages fall into two groups: 1,200–900 Ma and 670–545 Ma. These ages are essentially identical for the non- to very low grade metamorphic late Neoproterozoic to Early Cambrian Puncoviscana Formation and the low to high grade metamorphic rocks of Eastern Sierras Pampeanas. Hence, both units are related to similar provenance areas at the same time of sedimentation. The time span from zircon crystallization in the Earth’s crust to exhumation and erosion may be very long. This is important when determining maximum ages of sedimentary rocks. Variation of zircon maxima may also be influenced by concurrent sedimentary cover of proposed provenance areas. For the late Mesoproterozoic to early Neoproterozoic zircon age group, an active mountain range of the southwest Brazilian Sunsas orogen is the most probable provenance area. The younger, late Neoproterozoic zircons are related to the continuously developing mountains of the Brasiliano orogen of southwest and south central Brazil. Young zircons, up to 514 Ma, from fossil-bearing Puncoviscana and Suncho Formation outcrops are related to late Early Cambrian volcanism contemporaneous with sedimentation. This situation continues through the Late Cambrian to the Early Ordovician, but the Sunsas orogen provenance diminishes as possible Rio de la Plata craton origins become important.

Fault controlled Carboniferous A-type magmatism in the proto-Andean foreland (Sierras Pampeanas, Argentina): Geochemical constraints and petrogenesis

The intrusion of granitoids into the Eastern Sierras Pampeanas in the Early Carboniferous took place after a long period of mainly compressional deformation that included the Famatinian (Ordovician) and Achalian (Devonian) orogenies. These granitoids occur as small scattered plutons emplaced in a dominant extensional setting, within older metamorphic and igneous rocks, and many of them are arranged along a reactivated large shear zone. A set of 46 samples from different granitic rocks: Huaco granitic complex, San Blas pluton, and the La Chinchilla stock from the Sierra de Velasco, Zapata granitic complex from Sierra de Zapata, and the Los Árboles pluton from Sierra de Fiambalá, display high and restricted SiO 2 contents between 69.2 and 76.4 wt.%. On both FeO/(FeO + MgO) vs. SiO 2 and [(Na 2O + K 2O) − CaO] vs. SiO 2 plots the samples plot in the ferroan and alkaline-calcic to calco-alkaline fields (FeO/(FeO + MgO) = 0.88–1.0%;[(Na 2O + K 2O) − CaO] = 6.3–8.3%), thus showing an A-type granitoid signature. The high concentrations for the High Field Strength Elements (HSFE), such as Y, Nb, Ga, Ta, U, Th, etc. and flat REE patterns showing significant negative Eu anomalies are also typical features of A-type granites. Our petrogenetic model supports progressive fractional crystallization with dominant fractionation of feldspar and a source mineral assemblage enriched in plagioclase. Biotites have distinctive compositions with high FeO/MgO ratios (7.8–61.5), F (360–5610 ppm), and Cl (120–1050 ppm). The FeO/MgO ratios together with the F and Cl content of igneous biotites seem to reflect the nature of their parental host magmas and may be useful in identifying A-type granitoids. The isotopic data (Rb–Sr and Sm–Nd) confirm that the A-type granites represent variable mixtures of asthenospheric mantle and continental crust and different mixtures lead to different subtypes of A-type granite (illustrating the lack of consensus about A-type magma origin). We conclude that prominent shear zones play an important role in providing suitable conduits for ascending asthenospheric material and heat influx in the crust, a hypothesis that is in accord with other recent work on A-type granites.

Provenance of the late Proterozoic to early Cambrian metaclastic sediments of the Sierra de San Luis (Eastern Sierras Pampeanas) and Cordillera Oriental, Argentina

Provenance studies have been performed utilising major and trace elements, Nd systematics, whole rock Pb–Pb isotopes and zircon U/Pb SHRIMP data on metasedimentary rocks of the Sierra de San Luis (Nogolí Metamorphic Complex, Pringles Metamorphic Complex, Conlara Metamorphic Complex and San Luis Formation) and the Puncoviscana Formation of the Cordillera Oriental. The goal was the characterisation of the different domains in the study area and to give insights to the location of the source rocks. An active continental margin setting with typical composition of the upper continental crust is depicted for all the complexes using major and trace elements. The Pringles Metamorphic Complex shows indications for crustal recycling, pointing to a bimodal provenance. Major volcanic input has to be rejected due to Th/Sc, Y/Ni and Cr/V ratios for all units. The εNd (540 Ma) data is lower for the San Luis Formation and higher for the Conlara Metamorphic Complex, as compared to the other units, in which a good consistency is given. This is similar to the T DM ages, where the metapsammitic samples of the San Luis Formation are slightly older. The spread of data is largest for the Pringles Metamorphic Complex, again implying two different sources. The whole rock 207Pb/ 206Pb isotopic data lies in between the South American and African sources, excluding Laurentian provenances. The whole rock Pb–Pb data is almost indistinguishable in the different investigated domains. Only the PMC shows slightly elevated 208Pb/ 204Pb values. Possible source rocks for the different domains could be the Quebrada Choja in the Central Arequipa–Antofalla domain, the Southern domain of the Arequipa–Antofalla basement, the Brazilian shield or southern Africa. Zircon SHRIMP data point to a connection between the Puncoviscana Formation and the Conlara Metamorphic Complex. Two maxima around 600 Ma and around 1000 Ma have been determined. The Nogolí Metamorphic Complex and the Pringles Metamorphic Complex show one peak of detrital zircons around 550 Ma, and only a few grains are older than 700 Ma. The detrital zircon ages for the San Luis Formation show age ranges between 590 and 550 Ma. A common basin can be assumed for the Conlara Metamorphic Complex and the Puncoviscana Formation, but the available data support different sources for the rest of the Complexes of the Sierra de San Luis. These share the diminished importance or the lack of the Grenvillian detrital peak, a common feature for the late Cambrian–early Ordovician basins of the Eastern Sierras Pampeanas, in contrast to the Sierras de Córdoba, the PVF and the Conlara Metamorphic Complex.

Petrology and geochemistry of the banded iron formation in the Eastern Sierras Pampeanas of San Luis (Argentina): Implications for the evolution of the Nogolí Metamorphic Complex

The metamorphosed banded iron formation from the Nogolí Metamorphic Complex of western Sierra de San Luis, Eastern Sierras Pampeanas of Argentina (Nogolí area, 32°55’S–66°15’W) is classified as an oxide facies iron formation of Algoma Type, with a tectonic setting possibly associated with an island arc or back arc, on the basis of field mapping, mineral and textural arrangements and whole rock geochemical features. The origin of banded iron formation is mainly related to chemical precipitation of hydrogenous sediments from seawater in oceanic environments. The primary chemical precipitate is a result of solutions that represent mixtures of seawater and hydrothermal fluids, with significant dilution by mafic–ultramafic volcanic and siliciclastic materials. Multi-stage T DM model ages of 1670, 1854 and 1939 Ma and positive, mantle-like ξ Nd (1502) values of +3.8, +1.5 and +0.5 from the banded iron formation are around the range of those mafic to ultramafic meta-volcanic rocks of Nogolí Metamorphic Complex, which are between 1679 and 1765 Ma and +2.64 and +3.68, respectively. This Sm and Nd isotopic connection suggests a close genetic relationship between ferruginous and mafic–ultramafic meta-volcanic rocks, as part of the same island arc or back arc setting. A previous Sm–Nd whole rock isochron of ∼1.5 Ga performed on mafic–ultramafic meta-volcanic rocks led to the interpretation that chemical sedimentation as old as Mesoproterozoic is possible for the banded iron formation. A clockwise P–T path can be inferred for the regional metamorphic evolution of the banded iron formation, with three distinctive trajectories: (1) Relict prograde M 1–M 3 segment with gradual P and T increase from greenschist facies at M 1 to amphibolite facies at M 3. (2) Peak P–T conditions at high amphibolite-low granulite facies during M 4. (3) Retrograde counterpart of M 4, that returns from amphibolite facies and stabilizes at greenschist facies during M 5. Each trajectory may be regarded as produced by different tectonic events related to the Pampean? (1) and the Famatinian (2 and 3) orogenies, during the Early to Middle Paleozoic. The Nogolí Metamorphic Complex is interpreted as part of a greenstone belt within the large Meso- to Neoproterozoic Pampean Terrane of the Eastern Sierras Pampeanas of Argentina.

Magnetic petrology of the Devonian Achala Batholith, Argentina: titanohaematite as an indicator of highly oxidized magma during crystallization and cooling

SUMMARY Devonian magmatism in the Eastern Sierras Pampeanas (Cordoba, Argentina) is represented by elliptical, porphyritic, batholithic, late- to post-orogenic monzogranites emplaced in a metamorphic–plutonic basement. Two groups of Devonian granitoids are distinguished: (1) metaaluminous to weakly peraluminous, hornblende-biotite bearing granitoids, and (2) peraluminous, biotite-(muscovite) bearing granitoids. Groups 1 and 2 are characterized by accessory phases of sphene-magnetite with high susceptibility, and ilmenite-(monazite-uraninite) with low susceptibility, respectively. The magnetic assemblage of the Group 2 Achala Batholith, predominantly paramagnetic, is the subject of this paper. Forty-three sites were sampled in the Achala monzogranite. Overall, it is weakly magnetic, with a mean magnetic susceptibility of 1.5 × 10−4 SI. Twenty-one sites showed titanohaematite as the magnetic carrier of a stable remanence. The titanohaematite is an accessory mineral and contains exsolved disc-shaped intergrowths of ilmenite. The weak magnetism of Achala monzogranite facies contrasts with the magnetic character of other Devonian porphyritic, batholithic monzogranites in the region. The virtual absence of magnetite, together with the presence of exsolved titanohaematite and Mn-ilmenite indicate crystallization and cooling under more oxidizing conditions. Oxidizing conditions continued into the subsolidus, causing late-stage reactions of ilmenite host grains to ilmenite + rutile+ haematite, and titanohaematite to ilmenite-poorer haematite + rutile. These reactions resulted in low susceptibility values and a strong reduction of natural remanent magnetization (NRM) intensities. Thermal demagnetization, up to 620–640 °C, isolated steeply dipping, dual-polarity remanence directions. The coincidence of the Achala paleomagnetic pole with the 380–360 Myr segment of the Gondwana Apparent Polar Wander Path and the U-Pb zircon age implies that the remanence was acquired soon after emplacement.

Primary structure influence on compositional banding in psammites: Examples from the Puncoviscana Formation, north-central Argentina

We investigate the influence of sedimentary structures in development of foliations in banded psammites and pelites in the meta-turbiditic Pampean Belt, Eastern Sierras Pampeanas, northwest Argentina. These anchizone to upper greenschist facies rocks were exhumed from near surface to mid-crustal levels, and preserve sedimentary structures and gradual changes in tectonic fabrics and microstructures. Primary variations in clay content affected the degree of development of a compaction-related pressure-solution cleavage ( S P) resulting in enhancement of the original sedimentary banding. The resulting compositional banding was overprinted by a tectonic cleavage ( S 1) related to mid-Cambrian chevron folding. In banded sandstones, pressure-solution and mica growth that define S P and S 1 foliations are increasingly better developed in phyllosilicate-rich domains with increasing structural depth and metamorphic grade. Clay-rich laminae in the protolith acted as the locus for mica nucleation and growth, favoring dissolution of quartz (up to 42%) causing passive concentration of mica. Pressure-solution thus produced banded sandstones with a spaced, bedding-parallel S P cleavage in shallower sections and compositionally banded psammites and schists at depth. These observations demonstrate that formation of compositional banding does not require tectonic transposition of an older planar fabric. Similar processes may produce compositional banding in a wide range of meta-sedimentary rocks.

THE MANGANESE DEPOSITS OF THE PAMPEAN RANGES, ARGENTINA

The example of manganese mineralization studied in this paper makes up the major concentration of this element in Argentina. It comprises an area of 70 by 30 km in the eastern Sierras Pampeanas, located in Cordoba and Santiago del Estero provinces. It is quite unusual since it is hosted in granodioritic and rhyodacitic rocks. Geochemical, petrological and structural analyses demonstrate that these rocks are unrelated to the mineralization process. The geometry and distribution of the veins are controlled by a dextral shear system related to north–south lineaments. Textural and structural analyses of the veins indicate four different stages of mineralization. Ore minerals precipitated during the first three, whereas the gangue was mainly formed during the last stage. X-ray diffraction and electron-microprobe studies show that Mn 4+ oxides (hollandite, pyrolusite, ramsdellite, romanechite and cryptomelane) associated with Fe 3+ oxides (goethite and hematite) are the main ore minerals. Calcite, opal, barite, and lesser amounts of fluorite, quartz and “chalcedony” are the most common gangue minerals. Fluid-inclusion studies show that this mineralization was associated with aqueous solutions of low salinity and a temperature of about 125°C. Isotopic analyses based on several samples of calcite (δ 13 C, δ 18 O) and barite (δ 34 S, δ 18 O) have demonstrated the dominance of meteoric waters in the hydrothermal fluids. Finally, the chemical composition of manganese oxides supports the hypothesis of an epithermal system developed in continental environments.

The Basement of the Central Andes: The Arequipa and Related Terranes

The basement of the Central Andes provides insights for the dispersal of Rodinia, the reconstruction of Gondwana, and the dynamics of terrane accretion along the Pacific. The Paleoproterozoic Arequipa terrane was trapped during collision between Laurentia and Amazonia in the Mesoproterozoic. Ultrahigh-temperature metamorphism correlates with the collapse of the Sunsás-Grenville orogen after ∼1000 Ma and is related to slab break-off and dispersal of Rodinia. The Antofalla terrane separated in the Neoproterozoic, forming the Puncoviscana basin. Its closure was coeval with the collision of the eastern Sierras Pampeanas. The rift-drift transitions of the early Paleozoic clastic platform showed a gradual younging to the north, in agreement with counterclockwise rotation based on paleomagnetic data of Antofalla. North of Arequipa arc magmatism and high-grade metamorphism are linked to collision of the Paracas terrane in the Ordovician, during the Famatinian orogeny in the Sierras Pampeanas. The early Paleozoic history of the Arequipa massif is explained by a backarc, which further south changed to open oceanic conditions and subsequent collision. The Antofalla terrane reaccreted to the continental margin by the late Ordovician. These accretions and subsequent separations during the Mesoproterozoic, Neoproterozoic–early Cambrian, and late Cambrian–middle Ordovician are explained by changes in absolute motion of the Gondwana supercontinent during plate global reorganization.

Suya Taco and Sol de Mayo mafic complexes from eastern Sierras Pampeanas, Argentina: Evidence for the emplacement of primitive OIB-like magmas into deep crustal levels at a late stage of the Pampean orogeny

The age and petrogenetic features of the Suya Taco and Sol de Mayo mafic complexes reported here provide important new insights into the tectonic setting and evolution of the Pampean orogen. Specifically, the results of this study support models that are capable of explaining the introduction of non-arc mantle-derived mafic magmas into the developing orogen at or very near the time of its metamorphic peak. Although a continental collision model for the Pampean orogen cannot be categorically ruled out, an additional slab break-off or lower crustal delamination event immediately following collision would be required in order to account for the observations presented here. Alternatively, the recently proposed hypothesis of subduction of a mid-ocean ridge beneath an accretionary prism as the cause for Pampean orogeny can account for rise of non-arc, OIB-like mafic magmas through an already thickened supracrustal assemblage, as well as the close spatial and temporal association of this magmatism with the development of high-grade granulitic metamorphic and migmatitic assemblages in the Pampean supracrustal rocks.

Dating fault fabrics using modern techniques of40Ar/39Ar thermochronology: evidence for Paleozoic deformation in the Eastern Sierras Pampeanas, Argentina

Dating fault fabrics using modern techniques of⁴⁰Ar/³⁹Ar thermochronology: evidence for Paleozoic deformation in the Eastern Sierras Pampeanas, Argentina

Timing and Duration of the Calc‐Alkaline Arc of the Pampean Orogeny: Implications for the Late Neoproterozoic to Cambrian Evolution of Western Gondwana

The Pampean Orogen in the Eastern Sierras Pampeanas contains two paired magmatic belts, an eastern calc‐alkaline magmatic belt and a western peraluminous granite/high‐grade metasedimentary belt. The relationship between the two belts and their relative timing are constrained through new U‐Pb zircon ages on granodiorites, monzogranites, and associated volcanic rocks from Sierra Norte and the easternmost Sierras de Córdoba. These ages indicate that calc‐alkaline arc magmatism was active over at least a 30‐m.yr. period from 555 to 525 Ma, terminating at the same time that peraluminous magmatism and associated high‐grade metamorphism began in the adjoining metasedimentary belt (525–515 Ma). These temporal relationships and the metamorphic characteristics of the two belts appear to be in conflict with previously proposed models for the Pampean Orogeny as a continental‐collision event, but they are consistent with models that propose eastward‐dipping subduction of oceanic crust initiated at ca. 555 Ma, followed by ridge‐trench collision at ca. 525 Ma. Similar‐aged belts of arc‐related and peraluminous magmatism occur elsewhere along the paleo‐Pacific margin of Gondwana, suggesting that similar processes of subduction and noncollisional peraluminous magmatism occurred along much of the Gondwana margin in late Neoproterozoic to Cambrian time.

Field and textural evidence of two migmatization events in the Sierras de Córdoba, Argentina

In the metamorphic basement of the central eastern Sierra de Comechingones (Sierras Pampeanas of Córdoba, Argentina), two types of migmatites are found: Tala Cruz metatexites and Cañada del Sauce diatexites. Rafts of deformed metatexites in the diatexites are interpreted as dismembered and partially assimilated remnants of the Tala Cruz metatexites. Two migmatization events are recognized: the M2 metamorphic peak produced high-grade paragenesis and the first anatexis generating the metatexites; 810–840 °C and 8.5–9 kbar were calculated for M2. The M4 decompression event produced the diatexites by a second melting of the metatexites; 725–780 °C and 5.5–6 kbar were estimated for M4. A dynamic contractional M3 event in high-ductility and high-temperature conditions was recognized between the two migmatization events. A correlation could be established between the second migmatization event (M4) and the event dated as Cambrian (520–530 Ma) in other areas in the Sierras de Córdoba, in which case the first migmatization event (M2) would have been older, perhaps as old as Neoproterozoic. The tectonic evolution of the Sierra de Comechingones would imply the occurrence of extension–shortening cycles, interpreted as “pull–push” cycles mirroring the subduction and collision of Pampia terrane against the western Gondwana margin. These cycles would have controlled the evolution of the Eastern Sierras Pampeanas, during the Neoproterozoic?–Early Palaeozoic times.

Solid inclusions of magmatic halite and sylvite in felsic granitoids, Sierra Norte, Córdoba, Argentina

This study provides evidence for the existence of halite and sylvite solid inclusions in igneous quartz and feldspars, the first to be reported in intrusive rocks, and to partially constrain the physicochemical environment that lets halides crystallize under magmatic conditions. Halite and sylvite solid inclusions were found included in quartz and feldspars from a micrographic–granophyric assemblage in a miarolitic aplite and, rarer, in alkali-feldspar from a miarolitic monzogranite. Monzogranite and aplite represent I-type, K-enriched postcollisional rocks of the Late Cambrian–Early Ordovician Sierra Norte–Ambargasta batholith in the Eastern Sierras Pampeanas. Both granitoids fall among the most evolved felsic rocks of the batholith, with aplite approaching haplogranitic compositions. Halite is far more common than sylvite and the presence and distribution of one or both halides are erratic within the felsic intrusive bodies. Halides occur as small skeletal grains, commonly in cross-shaped aggregates of less than 50 μm. No K or Na was found at the detection limits of EDS in either halite or sylvite respectively. Textural relationships suggest that the alkali-chlorides separated from the melt near the minima along the quartz–feldspar cotectics of P H 2O > 160 < 200 MPa in a silica-, and potassium-rich magmatic system at approximately 750–700 °C, prior to the H 2O-vapor saturated miarole-forming stage. Computed ratios for the magmatic volatile phase (MVP) coexisting with melt at the early stage of aplite crystallization are: NaCl/HCl = 0.11–0.97 and KCl/HCl = 0.24–1.62, being the highest range of values (0.79–0.97 and 1.45–1.62, respectively) found in those alkali-chloride-bearing samples. Maximum HCl/ΣCl (MVP) (0.28 to 0.31) indicates higher total Cl concentration in the MVP of alkali-chloride-bearing aplites, which is much higher in the halite-free aplite samples (HCl/ΣCl (MVP) = 0.59 to 0.74). One miarolitic monzogranite sample, where halite solid inclusions are present, also yielded the highest ratios for NaCl/HCl (MVP) (0.91) and KCl/HCl (MVP) (1.46), and the HCl/ΣCl (MVP) is 0.30. A high HCl concentration in the fluid phase is suggested by the log f(HF)/ f(H 2O) = − 4.75 to − 4.95, log f(HCl)/ f(H 2O) = − 3.73 to − 3.86, and log f(HF)/ f(HCl) = − 0.88 to − 1.22, computed at 750 °C after biotite composition. The Cl concentrations at 800 °C, computed with a D v/l Cl = 0.84 + 26.6 P ( P at 200 MPa), yielded values within the range of ∼ 70 to 700 ppm Cl in the melt and ∼ 4000 to 40 000 ppm Cl in the coexisting MVP. The preferential partitioning of Cl in the vapor phase is controlled by the D v/l Cl; however, the low concentration of Cl in the melt suggests that high concentrations of Cl are not necessary to saturate the melt in NaCl or KCl. Cl-saturation of the melt and coexisting MVP might have been produced by a drop in Cl solubility due to the near-haplogranitic composition of the granitoids after extreme fractionation, probably enhanced by fluctuating reductions of the emplacement pressure in the brittle monzogranite host. Liquid immiscibility, based in the differential viscosity and density among alkali-chloride saturated hydrosaline melt, aluminosilicate felsic melt, and H 2O-rich volatiles is likely to have crystallized halite and sylvite from exsolved hydrosaline melt. High degrees of undercooling might have been important at the time of alkali-chloride exsolution. The effectiveness of alkali-chloride separation from the melt at magmatic temperatures is in line with the interpretation of “halite subtraction” as a necessary process to understand the origin of the “halite trend” in highly saline fluid inclusions from porphyry copper and other hydrothermal mineralizations, despite the absence of the latter in the Cerro Baritina aplites, where this process preceded the exsolution of halite-undersaturated fluids. Pervasive alteration of the monzogranite country rock as alkali-metasomatic mineral assemblages, the mineral chemistry of some species, and the association of weak molybdenite mineralization are compatible with the activity of alkaline hypersaline fluids, most likely exsolved during the earliest stages of aplite consolidation.

Crustal structure of the south-central Andes Cordillera and backarc region from regional waveform modelling

SUMMARY We investigate the crustal structure in the Andes Cordillera and its backarc region using regional broadband waveforms from crustal earthquakes. We consider seismic waveforms recorded at regional distances by the CHile-ARgentina Geophysical Experiment (CHARGE) during 2000‐2002 and utilize previous seismic moment tensor inversion results. For each single station-earthquake pair, we fixed the source parameters and performed forward waveform modelling using ray paths that sample the crust of the highest elevation Cordillera and the accreted terranes in the backarc region. Our investigation indicates that synthetic seismograms for our earthquake-station geometry are most sensitive to crustal parameters and less sensitive to mantle parameters. We performed a grid search around crustal thickness, P-wave seismic velocity (Vp) and P -t oS-wave seismic velocity ratio (Vp/Vs), fixing mantle parameters. We evaluated this waveform analysis by estimating an average correlation coefficient between observed and synthetic data over the three broadband components. We identified all acceptable crustal models that correspond to high correlation coefficients that provide best overall seismogram fits for the data and synthetic waveforms filtered mainly between 10 and 80 s. Our results indicate along strike variations in the crustal structure for the north‐south high Cordillera with higher P-wave velocity and thickness in the northern segment (north of 33 ◦ S), and persistently high Vp/Vs ratio (>1.85) in both segments. This is consistent with a colder mafic composition for the northern segment and a region of crustal thickening above the flat slab region. In contrast, the results for the current volcanic arc (south of 33 ◦ S) agree with a warmer crust consistent with partial melt related to Quaternary volcanism presumably of an intermediate to mafic composition. A distinctive feature in the backarc region is the marked contrast between the seismic properties of the Cuyania and Pampia terranes that correlates with their heterogeneous crustal composition. The Cuyania terrane, composed of mafic-ultramafic rocks, exhibits high Vp, high Vp/Vs and a thicker layered crust versus the thinner more quartzrich crust of the eastern Sierras Pampeanas associated with low Vp and low Vp/Vs. These differences may have some effect on the mechanism that unevenly generates crustal seismicity in the upper ∼30 km in this active compressional region. In particular, the seismic properties of the Cuyania terrane, which shows evidence for a high Vp and high Vp/Vs crust, may be reflecting the complex tectonic evolution history of this terrane including accretion-rifting and re-accretion processes since the Palaeozoic that promote a high level of crustal seismicity in the upper ∼30 km, enhanced by the flat slab subduction in the segment between 31 ◦ S and 32 ◦ S. Another possible mechanism could be directly related to the presence of a strong lower crust above the flat slab that efficiently transfer stresses from the slab to the upper crust generating higher seismicity in the Cuyania terrane between 30 ◦ S and 34 ◦ S.

Cenozoic provenance history of synorogenic conglomerates in western Argentina (Famatina belt): Implications for Central Andean foreland development

In Famatina, at the modern northern flat-slab segment of the Central Andean belt, synorogenicconglomeratefaciesprovideaunique opportunity to reconstruct paleogeography and tectonic setting. Stratigraphically constrained provenance analyses record a multi-event history of the Andean foreland and a complex pattern of exhumed uplands, which, in turn, controlled the depocenter history. Western and eastern sources can be differentiated. The western domain was composed of equigranular granitoids and lower and upper Paleozoic volcano-sedimentary and sedimentary units, whereas the eastern domain was mainly formed by porphyrytic granitoids and minor slightly metamorphosed lower Paleozoic clastic rocks. The surrounding regions expose medium- to high-grade metamorphic rocks, which are not recorded in any Cenozoic conglomerate of Famatina. Unroofing sequences can be deduced from the compositional trends. Conglomerate composition and paleocurrent data show input from intraba-sinal sources and important participation of basement since the beginning of the Neogene. We infer an early Miocene broken foreland during deposition of the Del Creston Formation.A significant source inversion, with clasts supplied from the eastern basement domain, is recorded ∼2000 m from the base, within the middle Miocene Del Abra Formation; reactivation of western granitic thrust sheets occurred by the late middle Miocene and late Miocene, as indicated by provenance studies of the Santo Domingo and El Durazno Formations. In contrast to simple asymmetric foreland models, provenance analysis documents early participation of crystalline rocks, suggesting early development of intrafore-land basement highs within the southern Central Andes. At a regional scale, basement thrusting and associated intermontane basin development were diachronous. This interpretation is supported by broad propagation of basement thrusting from the northern Sierras Pampeanas (and Famatina) toward the central and eastern Sierras Pampeanas between early-middle Miocene and late Miocene–Pliocene time. Early Miocene reconstructions of the Juan Fernandez Ridge position on the Central Andean margin suggest that during this interval, shallow subduction was likely unrelated to oceanic plateau subduction under the Sierras Pampeanas.

Counterclockwise tectonometamorphic evolution of the Pringles Metamorphic Complex, Sierras Pampeanas of San Luis (Argentina)

The crystalline basement of the Sierra de San Luis, which belongs to the Eastern Sierras Pampeanas in central Argentina, consists of three main units: (1) Conlara, (2) Pringles, and (3) Nogolí metamorphic complexes. In the Pringles Metamorphic Complex, mafic–ultramafic bodies occur as discontinuous lenses along a narrow central belt concordant with the general NNE–SSW structural trend. A metamorphic gradient from granulite to greenschist facies is apparent on both sides of the mafic–ultramafic bodies. This work focuses on the characteristics of the mylonitization overprinted on the mafic–ultramafic intrusives in the Pringles Metamorphic Complex and their gneissic–migmatitic surroundings, both previously metamorphosed within the granulite facies. Petrogenetic grid and geothermobarometry applied to the paragenesis equilibrated during the mylonitic event, together with mineral deformation mechanisms, indicate that mafic and adjacent basement mylonites developed under upper amphibolite transitional to granulite facies metamorphic conditions at intermediate pressures (668–764 °C, 6.3–6.9 kbar, 0.3 < XCO 2 < 0.7). However, the following mylonitic assemblages can be distinguished from the external limits of the Pringles Metamorphic Complex to its center: lower amphibolite facies ⇒ middle amphibolite facies ⇒ upper amphibolite transitional to granulite facies. Geothermobarometry applied to mylonitic assemblages indicate a temperature gradient from 555 °C to 764 °C and pressures of 6–7 kbar for the mylonitic event. This event is considered to have developed on a preexisting temperature gradient attributed to the intrusion of mafic–ultramafic bodies. The concentration of sulfides in mylonitic bands and textural relationships provide evidence of remobilization of primary magmatic sulfides of the mafic–ultramafic rocks (+PGM) during the mylonitic event. A lower-temperature final overprint produced brittle fracturing and localized retrogression on mafic–ultramafic minerals and ores by means of a water-rich fluid phase, which gave rise to a serpentine + magnetite ± actinolite association. Concordantly in the adjacent country rocks, fluids channeled along preexisting mylonitic foliation planes produced local obliteration of the mylonitic texture by a randomly oriented replacement of the mylonite mineralogy by a chlorite + sericite/muscovite + magnetite assemblage. Observed mineral reactions combined with structural data and geothermobarometry suggest a succession of tectonometamorphic events for the evolution of the Pringles Metamorphic Complex of Sierra de San Luis, developed in association with a counterclockwise P– T– d path. The most likely geological setting for this type of evolution is a backarc basin, associated with east-directed Famatinian subduction initiated in Mid-Cambrian times and closed during the collision of the allochthonous Precordillera terrane in Mid-Ordovician times.

Cambrian initiation of the Las Pirquitas thrust of the western Sierras Pampeanas, Argentina: Implications for the tectonic evolution of the proto-Andean margin of South America

The 40 Ar/ 39 Ar hornblende crystallization ages from the Las Pirquitas thrust of the Sierra de Pie de Palo, Argentina, indicate that imbrication of the proto-Andean margin had initiated by ca. 515 Ma. The ages do not permit the Pie de Palo block to represent basement of the adjacent Argentine Precordillera terrane, but rather require it to be a separate fragment not juxtaposed with the Precordillera prior to the Ordovician. The Pie de Palo block was likely autochthonous to the Famatina arc of the proto-Andean margin in the Early Cambrian. When combined with existing regional ages of magmatism, metamorphism, and deformation, the ages suggest that east-dipping subduction beneath the Famatina arc initiated by the Early to Middle Cambrian. The newly established convergent margin resulted from a westward shift of the plate boundary at the end of the Pampean orogeny in the eastern Sierras Pampeanas. The Famatina arc persisted as an Andean-type active margin until the collision and accretion of the Precordillera terrane in the Middle Ordovician.

Deformation of Gondwana margin turbidites during the Pampean orogeny, north-central Argentina

Basement uplifts in the Eastern Sierras Pampeanas of north-central Argentina expose sedimentary rocks that originated on the Neoproterozoic to early Cambrian margin of Gondwana for over 1000 km along strike. A series of transects through unmetamorphosed sections of the Vendian-aged Puncoviscana Formation and metasedimentary equivalent rocks from deeper structural levels demonstrates that an early stage compaction cleavage has been folded into chevron structures, with a concomitant axial planar tectonic cleavage, prior to erosion and deposition of the overlying Middle to Late Cambrian Meson Group. Changes in structural style are shown to be gradational; all mesoscopic ductile fold structures in the region can be explained with one major deformation episode. Late-stage Andean brittle folds and thrusts are more noticeable in the northern sections, but do not significantly alter the overall structural style or geometry of the basement rocks. A static metamorphic overprint in the Ordovician with associated granitic intrusions only affects fabrics and mesoscale structures adjacent to plutons. Published age constraints indicate an overlap between the timing of sedimentary deposition and that of structural and metamorphic events at depth. We develop a tectonic model for the Pampean orogeny that involves the buttressing of off-scraped sedimentary rocks above a subducting slab beneath the Gondwana margin. The collision of outboard terranes did not occur in this area until at least the mid-Ordovician Famatinian orogeny.

Genesis of aluminous and intermediate granulites: A case study in the eastern Sierras Pampeanas, Argentina

We report petrologic and geochemical features of anhydrous granulites from the eastern Sierras Pampeanas, Argentina. Aluminous Grt–Crd-bearing granulite is the most abundant rock type in the study region, and hosts intermediate Opx–Grt bearing granulites as interlayered lenses. Both granulite types crystallized under similar P– T conditions of T = 850–900 °C and P c. 7.5 kbar. The formation of Grt–Crd granulites by subsolidus dehydration conflicts with the absence of K-feldspar in them. Moreover, the aluminous granulites have an REE pattern that precludes postulating they derived by close-system metamorphism of an extensively weathered protoliths. Comparing mineral and rock chemistry with experimental results in simple systems (ACFM, AKF, and AFM) permits the mineralogical changes and the bulk composition evolution experienced by granulites during melting to be identified. This work demonstrates that orthopyroxene is a primary phase in Opx–Grt granulites, however never crystallized within aluminous granulites. The aluminous granulites have both mineral and chemical compositions matching those expected for a residuum derived from the partial melting of a greywacke-like sedimentary precursor. Compared with the composition of the average continental crust, aluminous granulites are depleted in most lithophile trace elements and LREE, have somewhat low K / Rb ratios, and high contents of elements having high partition coefficients for garnet and/or ilmenite (HREE—Y, Cr, Ni). However, aluminous granulites have initial Sr isotope ratios (< 0.708) significantly lower than those measured in the rest of the metasedimentary rocks of the region (> 0.713). We discuss the possibility that an intense but short-lived melting event led to isotopic disequilibrium during melting of biotite-bearing protoliths, leaving a granulitic residuum with lower initial Sr isotope ratios. In contrast, Opx–Grt granulites have trace element features (e.g., high Sr contents and REE patterns with marked positive Eu anomalies) that do not support the idea that they are simply residues left after biotite-dehydration melting and melt extraction. Rather, these rocks display several petrological, major and trace element and isotopic features that suggest they formed through the interaction of mafic magmas and aluminous granulites.

Crustal deformation in the south-central Andes backarc terranes as viewed from regional broad-band seismic waveform modelling

SUMMARY The convergence between the Nazca and South America tectonic plates generates a seismically active backarc region near 31 ◦ S. Earthquake locations define the subhorizontal subducted oceanic Nazca plate at depths of 90‐120 km. Another seismic region is located within the continental upper plate with events at depths 1.80, th = 45‐55 km) than those of the eastern Sierras Pampeanas (Vp = 6.0‐6.2 km s −1 , Vp/Vs < 1.70, th = 27‐35 km). In addition, we observed an apparent distribution of reverse crustal earthquakes along the suture that connects those terranes. Finally, we estimated average P and T axes over the CHARGE period. The entire region showed P- and T-axis orientations of 275 ◦ and 90 ◦ , plunging 6 ◦ and 84 ◦ , respectively.