The discovery of high-pressure granulite at Shelama in Dongco area along the Bangong Co-Nujiang River suture zone and its tectonic significance

A high pressure metamorphic (HP) and ultrahigh pressure metamorphic (UHP) belt is exposed in Baxoi, Amdo and Pengco areas along the middle-eastern Bangongco-Nujiang suture zone (BNSZ). However, the similar HP-UHP metamorphic rocks have not discovered in the western section of BNSZ. In this paper, the authors first report the high-pressure granulite (probably retrograde eclogite) at Dongco area in the western section of the BNSZ. Dongco high-pressure granulite occurs in lenticular, stratoid and massive forms in plagioclase amphibolites and metamorphic gabbros. Based on the intensive study of petrology and mineralogical geochemistry, the authors hold that the HP granulite was made up of the early mineral assemblage of garnet (Grt)+clinopyroxene (Cpx) + plagioclase (Pl) (mostly saussuritized) and the late massive retrograde mineral assemblages amphibole (Hb)+plagioclase (Pl). Garnets in high-pressure granulites are surrounded by plagioclase which forms the corona. LA-ICP-MS zircon U-Pb dating yielded the age of original rock from the high-pressure granulite of 254±2Ma, indicating that the opening time of Bangong Co-Nujiang River oceanic basin can be traced to the Late Permian. Geological temperature pressure indicator shows that the temperature-pressure conditions of high-pressure granulite phase are 780~900℃, 13~16kPa, while those of retrogressive metamorphic amphibolite phase are 430~480℃, 4.5~5.2kPa, respectively, suggesting that the high-pressure granulites were most likely the product of granulite facies (even eclogite facies) high pressure metamorphism of the Bangong Co-Nujiang River Tethyan oceanic crust. The discovery of Dongco high pressure granulites shows that a high-pressure metamorphic belt might exist within the BNSZ, and might have been formed by the deep subduction of the Bangong Co-Nujiang River Tethyan Ocean. Further study of the high-pressure granulites is of great significance for recognizing the deep subduction process, continent-continent collision orogeny and the tectonic evolution of BNSZ in the Tibetan Plateau.