P-T-t Path of Garnetites in South Altyn Tagh, West China: A Complete Record of the Ultradeep Subduction and Exhumation of Continental Crust

摘要

We report here a detailed investigation of mineral chemistry, phase modelling and geochronology for garnetites from the Bashiwake ultrahigh pressure (UHP) terrane, South Altyn Tagh, West China. Three generations of mineral assemblages including two stages of melting were identified. The first generation is represented by the zoning with decreasing grossular and increasing pyrope and multiphase inclusions in garnet core. It records an early-stage prograde evolution from amphibolite facies to eclogite facies (1.7-2.5 GPa/710-850 degrees C) where the first stage of melting occurs. The second generation is characterized by the zoning in garnet mantle with increasing grossular and nearly constant pyrope, defining a later-stage prograde evolution to UHP peak with the minimum condition of 6.5-7.0 GPa/990 degrees C. The pressure is consistent with that constrained from the pigeonite exsolution particularly along (401) planes in clinopyroxene. The third generation includes the presence of perthite, plagioclase along with amphibole, suggesting the decompression from eclogite facies to HP-ultrahigh-temperature (UHT) granulite facies (similar to 3.3-L2 GPa/960-1020 degrees C) coupled with the second stage of melting. It is for the first time to get two metamorphic ages about 500 Ma and 486 Ma using in situ SIMS zircon dating which represent the prograde eclogite facies and decompressional granulite facies stages, suggesting the rocks staying at mantle depths for similar to 14 million years. The garnetites have recorded a complete metamorphic P-T-t history of continental ultradeep subduction to mantle depths of similar to 200 km or more followed by diapiring exhumation to the crust bottom.