Triassic record of Paleo-Tethyan subduction: Evidence from prograde metamorphic P-T paths of the Baqing eclogite, eastern Qiangtang, central Tibet

The prograde metamorphic P-T conditions of eclogites correspond to the subduction processes of oceanic plates. The conditions may be recorded in mineral inclusions and the compositional zoning of garnets, and they play significant roles in revealing the dehydration depth, geothermal structure of the subduction plate and in understanding arc magmatism. The P-T conditions of the Baqing eclogite have been determined previously using traditional geothermobarometers and Ti-in-zircon geothermometers. To obtain more precise prograde metamorphic P-T conditions and details of the Paleo-Tethyan subduction process, we carried out phase equilibrium modeling using a newly collected Baqing eclogite sample. The sample is characterized by well-developed prograde zoning in metamorphic garnets with increasing Mg and decreasing Mn contents from cores to rims, a peak metamorphic mineral assemblage of garnet + omphacite + phengite + rutile + quartz, and a retrograde metamorphic mineral assemblage of amphibole + plagioclase that replaced omphacite. We calculated the peak metamorphic conditions of the Baqing eclogite using the Garnet-Omphacite-Phengite-Quartz thermobarometers to be 26.5 +/- 1 kbar and 730 +/- 60 degrees C, consistent with the results (23.5-27.5 kbar, 730-750 degrees C) calculated using a P-T pseudosection. The prograde metamorphic P-T conditions were constrained to similar to 21.4 kbar and similar to 580 degrees C using pyrope and grossular isopleths. These results are consistent with previous calculations for the Baqing eclogite. Eclogites of the Central Qiangtang metamorphic belt, the Baqing metamorphic complex, and Changning-Menglian metamorphic belt show parallel P-T paths with different geothermal gradients, that relate to their different protoliths but similar heating and burial processes during Triassic subduction of the Shuanghu Paleo-Tethyan oceanic plate.

Automated summary

This study used phase equilibrium modeling to calculate the peak metamorphic conditions of Baqing eclogite to be 26.5 +/- 1 kbar and 730 +/- 60 degrees C, which confirms the previous calculation results. The eclogites of Baqing, Central Qiangtang metamorphic belt, and Changning-Menglian metamorphic belt show similar P-T paths, indicating similar heating and burial processes during Triassic subduction of the Shuanghu Paleo-Tethyan oceanic plate.