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Abstract

Towards unravelling the geodynamic setting of the northern Sierras Pampeanas (NW Argentina) we describe the tectonometamorphic and geochronologic evolution of sub-greenschist to granulite facies metamorphic sediments, granitoid plutons, and pegmatites in the Ordovician Sierra de Quilmes metamorphic complex. The protoliths of the metasediments are represented by a sequence of turbidites and minor calcsilicate rocks of the Neoproterozoic to Cambrian Puncoviscana Formation. The metamorphic complex consists of four zones including the (1) chlorite, (2) biotite–muscovite, (3) garnet–cordierite–sillimanite, and (4) orthopyroxene zones. Zones (3) and (4) show an increasing degree of anatexis, reaching large-scale diatexis in the orthopyroxene zone at P–T conditions exceeding ~800 °C and 600 MPa. At, or shortly after the metamorphic peak, the granitic to tonalitic Cafayate pluton intruded approximately along the boundary between anatectic and non-anatectic rocks. Retrograde near-isobaric cooling of the middle crust was accompanied by non-penetrative ductile shearing at granulite to amphibolite facies P–T conditions. Evidence for significant prograde deformation is absent in the Sierra de Quilmes metamorphic complex. Monazite and titanite U–Pb isotopic data constrain the metamorphic peak in migmatites and calcsilicate rocks to be at or slightly prior to ~470 Ma. Retrograde amphibolite facies mineral reactions led to continuous formation of monazite and titanite during slow cooling between ~470 Ma and 455 Ma (U–Pb data). The composite Cafayate pluton intruded over a time interval of several million years between ~477 Ma (Sm–Nd isochron) and ~460 Ma (monazite and titanite U–Pb isochron), followed by pegmatites. A younger group of pegmatites was emplaced in the country rocks at the end of the Ordovician (~440 Ma, Rb–Sr mineral isochrons), postdating most of the retrograde shear zones. Resetting of the muscovite K–Ar and 40Ar–39Ar system in weakly deformed pegmatites and crystallisation of new muscovite in low-grade metamorphic sediments at ~400–416 Ma is attributed to minor late greenschist and sub-greenschist facies metamorphism and deformation. The massive Early Ordovician heat transfer, the absence of major prograde deformation, and subsequent, prolonged phases of non-penetrative deformation in the high-grade metamorphic zones during slow near-isobaric cooling contradict crustal thickening and continent collision models, as have been proposed for the southern Sierras Pampeanas. We suggest continuous or stepwise extensional tectonics in a back-arc or a mobile belt tectonic environment for the Ordovician Sierra de Quilmes, and the northern Sierras Pampeanas in general. An extensional setting of the northern Sierras Pampeanas in the Ordovician is in agreement with the coeval formation of marine extensional sediment basins in vicinity of the Sierras Pampeanas in northwestern Argentina and southern Bolivia.