High pressure garnet amphibolites in ophiolitic mélange from the Changning-Menglian suture zone, southeast Tibetan Plateau: P-T-t path and tectonic implication

Abstract

The garnet amphibolites from the newly identified Wanhe ophiolitic mélange in the Changning-Menglian suture zone (CMSZ) provide a probe to elucidate the evolution of the Triassic Palaeo-Tethys. An integrated petrologic, phase equilibria modeling and geochronological study of the garnet amphibolites, southeast Tibetan Plateau, shows that the garnet amphibolites have a peak mineral assemblage of garnet, glaucophane, lawsonite, chlorite, rutile, phengite and quartz, and a clockwise <i>P-T</i> path with a prograde segment from blueschist-facies to eclogite-facies with a peak-metamorphic <i>P-T</i> conditions of 2000–2100 MPa and 495–515°C, indicating a cold geothermal gradient of about 240–260°C/GPa. Theretrograde metamorphic <i>P-T</i> path is characterized by nearly isothermal decompression to lower amphibolite-facies and subsequent cooling to greenschist-facies. The metamorphic zircons have fractionated HREE patterns and significant negative Eu anomalies, and therefore the obtained zircon U-Pb age of 231 ± 1.5 Ma is interpreted to be the timing of the amphibolite facies metamorphism occurrence. The present study probably indicates that the garnet amphibolites in the Wanhe ophiolitic mélange was the retrograded high-pressure eclogite-facies blueschist, instead of the previously proposed eclogites, and the garnet amphibolites recorded the subduction and exhumation process of the Palaeo-Tethys Oceanic crust in the Triassic.