The Early Palaeozoic high-grade metamorphism at the active continental margin of West Gondwana in the Andes (NW Argentina/N Chile)

Abstract

The evolution of the Early Palaeozoic orogen of West Gondwana in the Cambrian to Ordovician basement of the Andes between ~18° and 32° S is investigated for pressure and temperature conditions and age of metamorphism. It is characterized by mid-crust temperatures commonly above the wet granite solidus (~650°C). Widespread felsic migmatite and rare granulite formed at pressures of ca 0.5–0.7 GPa, locally 1.0 GPa. These rocks represent the deepest exhumed sections of the Early Palaeozoic crust. High pressure–low temperature rocks are absent. The crystallization ages, compiled from the literature in combination with new data, for near peak metamorphic conditions of these high-grade metamorphic rocks in NW Argentina and N Chile are ~530–500 Ma and ~470–420 Ma. Both age groups are spatially overlapping. Radiogenic isotope signatures (Sr, Nd, Pb) are used to characterize the Early Palaeozoic basement. The Pb and Sr isotope compositions of the Early Palaeozoic basement indicate mixing arrays between pre-Palaeozoic unradiogenic and radiogenic crust. Crustal residence ages (Sm–Nd TDM) indicate a prominent event of crust formation around ~2 Ga, which is known continent-wide. This material was recycled during Midproterozoic and Early Palaeozoic orogenies without prominent additions of new crust present in the isotope record, i.e. accretion of compositional exotic material is absent.