Abstract

Foliated meta-rhyolites of the Late Jurassic Tobífera Formation, form two thrust slices in the Patagonian Andes fold and thrust belt. They display a penetrative mylonitic foliation which has been developed at a regional scale. Except for quartz porphyroclasts, most of the mineral assemblage present in the rocks is of metamorphic origin. The synkinematic minerals in the strain shadows are similar in composition to those in the matrix. The metamorphic mineralogy indicates a metamorphic evolution ranging between 3 kbar (ca. 250°C) and close to 7 kbar (ca.350°C), and the synkinematic growth of the minerals suggests that they formed during the Early to middle Cretaceous compressional phase in the area, which possibly involved west directed subduction of the quasi oceanic floor that underlies the Tobífera Formation.