Subgreenschist facies metamorphism of metabasites from the Precordillera terrane of western Argentina; constraints on the later stages of accretion onto Gondwana

Abstract

The metamorphic character of metabasites of the Precordillera terrane of western Argentina provides constraints which help to distinguish between contrasting models for the tectonic settings during the late stages of the accretion of this terrane onto the margin of Gondwana in late Lower Palaeozoic times. Metamorphic conditions are constrained to a low-temperature/low-pressure setting at ca. 250-350°C and 2-3 kbar, with geothermal gradients of between 30-35°C km -1 . The metamorphic character of the rocks and the derived P-T conditions demonstrate that the metamorphism did not develop in an ocean-floor setting as has been previously proposed. Neither is the metamorphic character compatible with being derived from enhanced heat flow in an extensional setting at the western edge of the Precordillera in the mid-Ordovician following docking against Gondwana. The most compatible model is one in which the metamorphism developed as a result of collision between the Chilenia and Precordillera terranes in early Devonian times. In addition, the metamorphism constrains the subduction depth to < ca. 15 km had the leading western edge of the Precordillera been subducted westwards beneath Chilenia. However, the metamorphic character is perhaps better linked to subduction of the leading edge of Chilenia eastwards against the Precordillera.