Newly recognized retrograde eclogites overprinted by high‐temperature metamorphism in the Amdo microcontinent, Central Tibet: Implications for subduction erosion during continental subduction

Abstract

The Amdo microcontinent located within the Bangong-Nujiang Suture Zone (BNSZ) holds one of the important keys to understand the tectonic evolution of Central Tibet before the Qiangtang-Lhasa terrane collision during the Mesozoic. Newly discovered eclogites were reported within the Amdo microcontinent in this study, and their metamorphic evolution is deciphered by petrographic observations, pseudosection modelling, geochemistry, and zircon U-Pb dating. The eclogites are characterized by peak metamorphic mineral assemblages of garnet, omphacite, rutile, and quartz, and underwent a four-stage metamorphic evolution, including eclogite facies stage (M 1 ) at 20-24 kbar and 580-620 C, followed by HP granulite facies decompression stage (M 2 ) at 13-15 kbar and 750-780 C, subsequent MP-HT granulite facies heating stage (M 3 ) at 8-10 kbar and >840 C, and final amphibolite facies retrogression (M 4 ) at 5.3-6.0 kbar and 560-580 C. The eclogites exhibit rare earth element (REE) distribution patterns and trace element abundance similar to those of N-MORB and arc-related volcanics, which are inferred to have formed in a back-arc Basin tectonic setting. Zircon U-Pb dating yields eclogite facies metamorphic age of 190 ± 1 Ma. The clockwise P-T-t paths and the oceanic protolith signature of retrograde eclogites suggest that part of the back-arc Basin was subducted to the depths of 80 km. The subsequent ultra-high temperature metamorphic overprinting may be related to the upwelling of the asthenosphere. Tectonic erosion associated with the subduction of the Amdo microcontinent beneath the Tethys Ocean accounts for the deep subduction of the back-arc Basin and the absence of arc magmatic rocks in the northern Amdo microcontinent.