Tectono-metamorphic evolution of the proto-Andean margin of Gondwana: Evidence of internal high-grade metamorphism along the northern portion of the Famatinian orogen, Sierra de Aconquija, Sierras Pampeanas Orientales, Argentina

Abstract

The present work gives a detailed analysis of the metamorphic and structural evolution of the back-arc portion of the Famatinian Orogen exposed in the southern Sierra de Aconquija (Cuesta de La Chilca segment) in the Sierras Pampeanas Orientales (Eastern Pampean Sierras). The Pampeanas Orientales include from north to south the Aconquija, Ambato and Ancasti mountains. They are mainly composed of middle to high grade metasedimentary units and magmatic rocks. At the south end of the Sierra de Aconquija, along an east to west segment extending over nearly 10 km (Cuesta de La Chilca), large volumes of metasedimentary rocks crop out. The eastern metasediments were defined as members of the El Portezuelo Metamorphic–Igneous Complex (EPMIC) or Eastern block and the western ones relate to the Quebrada del Molle Metamorphic Complex (QMMC) or Western block. The two blocks are divided by the La Chilca Shear Zone, which is reactivated as the Río Chañarito fault. The EPMIC, forming the hanging wall, is composed of schists, gneisses and rare amphibolites, calc– silicate schists, marbles and migmatites. The rocks underwent multiple episodes of deformation and a late high strain-rate episode with gradually increasing mylonitization to the west. Metamorphism progrades from a M1 phase to the peak M3, characterized by the reactions: Qtz + Pl + Bt ± Ms → Grt + Bt2 + Pl2 ± Sil ± Kfs, Qtz + Bt + Sil → Crd + Kfs and Qtz + Grt + Sil→ Crd. The M3 assemblage is coeval with the dominant foliation related to a third deformational phase (D3). The QMMC, forming the foot wall, is made up of fine-grained banded quartz – biotite schists with quartz veins and quartz-feldspar-rich pegmatites. To the east, schists are also overprinted by mylonitization. The M3 peak assemblage is quartz + biotite + plagioclase ± garnet ± sillimanite ± muscovite ± ilmenite ± magnetite ± apatite. The studied segment suffered multiphase deformation and metamorphism. Some of these phases can be correlated between both blocks. D1 is locally preserved in scarce outcrops in the EPMIC but is the dominant in the QMMC, where S1 is nearly parallel to S0. In the EPMIC, D2 is represented by the S2 foliation, related to the F2 folding that overprints S1, with dominant strike NNW – SSE and high angles dip to the E. D3 in the EPMIC have F3 folds with axis oblique to S2; the S3 foliation has striking NW – SE dipping steeply to the E or W and develops interference patterns. In the QMMC, S2 (D2) is a discontinuous cleavage oblique to S1 and transposed by S3 (D3), subparallel to S1. Such structures in the QMMC developed at subsolidus conditions and could be correlated to those of the EPMIC, which formed under higher P-T conditions. The penetrative deformation D2 in the EPMIC occurred during a prograde path with syntectonic growth of garnet reaching P-T conditions of 640°C and 0.54 GPa in the EPMIC. This stage was followed by a penetrative deformation D3 with syn-kinematic growth of garnet, cordierite and plagioclase. Peak P-T conditions calculated for M3 are 710°C and 0.60 GPa, preserved in the western part of the EPMIC, west of the unnamed fault. The schists from the QMMC suffered the early low grade M1 metamorphism with minimum PT conditions of ca 400°C and 0.35 GPa, comparable to the fine schists (M1) outcropping to the east. The D2 deformation is associated with the prograde M2 metamorphism. The penetrative D3 stage is related to a medium grade metamorphism M3, with peak conditions at ca 590°C and 0.55 GPa. The superimposed stages of deformation and metamorphism reaching high P-T conditions followed by isothermal decompression, defining a clockwise orogenic P-T path. During the Lower Paleozoic, folds were superimposed and recrystallization as well as partial melting at peak conditions occurred. Similar characteristics were described from the basement from other Famatinian-dominated locations of the Sierra de Aconquija and other ranges of the Sierras Pampeanas Orientales.