Structural evolution and metamorphic timing of the Famatinian orogeny in Eastern Sierras Pampeanas (NW Argentina)

LARROVERE, MARIANO A.; DE LOS HOYOS, CAMILO R. ; WILLNER, ARNE P.; VERDECCHIA, SEBASTIAN O.; BALDO, EDGARDO G. ; CASQUET, CESAR; BASEI, MIGUEL A.S.; HOLLANDA, MARIA H. ; MORENO, GABRIEL G. ; ROCHER, SEBASTIÁN; ALASINO, PABLO H.

San Miguel de Tucuman

Congreso; XX Congreso Geológico Argentino; 2017

Universidad Nacional de Tucumán

In NW Sierras Pampeanas at ca. 28° S latitude vast Ordovician metamorphic complexes outcrop that results from the Famatinian orogeny. They consist of medium to high grade metasedimentary rocks intruded by scattered granitoid plutons (González Bonorino 1950, 1951, Larrovere et al. 2011, 2015). Metamorphic rock can be grouped into two complexes on a nearly SE-NW transect from the Sierra de Ancasti to the Sierra de Ambato-Aconquija (AA-transect): the El Portezuelo Metamorphic-igneous Complex (EPMIC) and Quebrada del Molle Metamorphic Complex (QMMC), separated by La Chilca Shear Zone (LCSZ).In this contribution, we applied structural methods and several geochronological methods (U Th-Pb monazite EPMA dating, LA-MC-ICP-MS and SHRIMP U-Pb zircon and monazite geochronology, 40Ar/39Ar mica dating) in a set of field-controlled rock samples in order: (1) to constrain the timing of metamorphism and deformation; (2) to unravel the structural evolution of the region; (3) to assess the role of this region in the evolution of the Famatinian belt. The EPMIC is mainly composed of K-feldspar-sillimanite ± garnet ± cordierite migmatites. Migmatitic layering in metatexite defines a predominant S2 foliation with NNW-SSE/N-S strike and ENE-E dip. Anaxial planarsynmigmaticS3 foliation overprints the S2 foliation at high angles. Thermal peak conditions were 670-820 °C and 4.5-5.3 kbar, consistent with a high T/ medium-low P metamorphism at mid-crustal levels (~ 17-21 km depth). New monazite and zircon datingyielded two groups of ages: 503-496 Ma and 484-465 Ma. The QMMC is dominated by garnet-biotite-muscovite schists and cordierite-biotite-muscovite gneisses derived from pelitic and psammitic protoliths.The main metamorphic foliation (S1) is parallel to S0and shows an intermediate to low ENE-dipping with NNW strike. A crenulation foliation (S2) is superposed toS1.Mineral assemblages suggest upper greenschist to lower-middle amphibolite facies conditions (~500-650 °C). Cordierite indicates pressures below 5-6 kbar. Weighted mean ages obtained by EPMA monazite dating were 474 Ma, 440 Ma and 408 Ma.40Ar-39Ar mica dating yielded ages in the ranges: 417-404 Ma (muscovite) and 376-367 Ma (biotite). The LCSZ consists of mylonite, protomylonite and weakly deformed rocks. The main structure is an S-C fabric. The shear zone is foldedinto a kilometer-scale open fold. The sense of shearing is reverse, with top-to-the-WSW (250°) sense of displacement (Larrovere et al. 2008). 40Ar-39Ar dating of biotite yielded a cooling age of ca. 385 Ma. Two main deformational phases occurred during regional anatexis. S2 migmatitic layering formed with shallow to intermediate dips in low-medium grade metasedimentary rocks, conincident with the first anatectic stage in the EPMIC. S1 metamorphic foliation in the QMMC was contemporary with migmatitic S2 fabric in the EPMIC (474 Ma and 484-465 Ma, respectively). Afterward an axial planar foliation S3 developed in the EPMIC at a high angle to S2, suggesting that this phase −which resulted from close-to-horizontal contraction− begun under partial-melting conditions. Deformation continued under subsolidus conditions. This contractional phase is well recorded in the Cuesta de La Chilca profile as evidenced by: 1- development of the reverse ductile LCSZ; 2- folding and generation of S2 foliation in the QMMC.Reverse shearing at the LCSZ caused metamorphic inversion, stackingof hotter rocksof the EPMIC over cooler rocksof the QMMC. Protracted contraction led to further folding of the shear zone when temperature decreased. Sub-horizontal attitude of the older foliations during horizontal shortening favored folding. As a result of this same phase the QMMC underwent multi-scale folding of S1 and generation of an axial planar foliation S2 at high angles to S1. Microfolding of pre-S2 cordierite porphyroblasts suggests that ductile conditions still prevailed in the QMMC. We link this contractional phase with the age of 440 Ma obtained from monazite EPMA dating. The youngerages recorded from EPMA monazite dating in the QMMC are almost coincident with the40Ar-39Ar muscovite ages (408±9 Ma and 406-404±6 Ma respectively). The latter suggests that re-equilibrium took place during either retrogression under low-exhumation rates or during a final stage of shear zone reactivation.Crustal tectonic stability was attained in theLate Devonian as suggested by 40Ar-39Ar biotite ages of 385 Main the QMMC and the LCSZ. EPMA monazite dating has revealed a thermal episode older than the syn-S2 metamorphic peak in the EPMIC not recorded so far. A consistent group of ages from the EPMIC are evidence that metamorphism probably took pla - ce at ca. ~503-496 Ma. The inferred late Cambrian metamorphic episode (ca. 500 Ma) evolved into a second one under high T- low P granulite facies conditions that produced widespread partial melting in the EPMIC. This second episode took place between 484 to 465 Ma. Shortening increased under partial melting conditions (e.g. syn-anatectic S3 axial planar foliation) in the early- to middle Ordovician (e.g. Finch et al. 2017). After that, contraction was focused along reverse ductile shearing andsubsequent folding. This late deformationphase brought to an end the Famatinian oregonian the early Silurian (~440 Ma). After a period of moderate exhumation rates during arc construction (i.e. main magmatism and deformation) throughout the Ordovician, exhumation rates decreased (de los Hoyos et al. 2011) until crustal stability was attained in the late Devonian ? early Carboniferous.