Evidence for oceanic subduction at the NE Gondwana margin during Permo-Triassic times

Blueschist was recently recognized within the Lhasa terrane, which is one of the NE Gondwana blocks. In this rock, the Mn and Mg contents of garnet enclosing aegirine-rich clinopyroxene, rutile and quartz decrease and increase, respectively, from core to rim. Amphibole changes from glaucophane through Na-Ca amphibole to Ca amphibole. The Si contents of phengite are high in the centre but low along the rim. The P-T path, starting above 2.5 GPa-450 degrees C and showing subsequently first a temperature increase to 500 degrees C and then a pressure release via blueschist conditions to 0.6 GPa, was reconstructed using a P-T pseudosection calculated for the P-T range 0.4-2.8 GPa and 250-650 degrees C. This path points to deep subduction of a cold oceanic crust probably beneath the NE Gondwana margin during Permo-Triassic times. This finding contributes to a better understanding of the pre-Cenozoic history of major terranes of NE Gondwana.