Crustal Extension of the Bangong Lake Arc Zone, Western Tibetan Plateau, After the Closure of the Tethys Oceanic Basin

A series of NS-and EW-striking dykes occur in Bangong lake region, western segment of the Bangong lake-Nujiang suture in Tibetan plateau, including both granite porphyry and diorite porphyrite. Based on petrochemical analyses and zircon U-Pb LA-ICP-MS dating combined with field investigation, the present authors propose that these dykes represent a crustal extension event of the Bangong lake arc zone after the closure of Bangeng lake middle-Tethys oceanic basin. The granite porphyries occuring only in NS-strike and the diorite porphyrites occuring both in NS- and EW-strike respectively yield a weighted mean age of 79.59±0.32Ma (MSWD=1.08) with 13 zircons and (76.9±1.2)Ma (MSWD=2.8) with 6. These results indicate that the crustal extensional process of the Bangong lake arc zone occurring in Late Cretaceous epoch was initialed only in EW-trending and slightly later also in NS-trending. Petrochemically, the two types of the dykes appear arc magmatic features characterized by enrichment of large iron incompatible elements (Rb, U, Th, K, Pb) and depletion of high field strength elements (Nb and Ti) which were attributed to metasomatism of Indian-MORB mantle by subducted sediment melt. Furthermore, by analyzing geochemical characteristics of the dyke, we come to the conclusion that the granite porphyries were generated at a shallower depth under amphibolite facies conditions, and the diorite porphyrites were probably under eclogite facies or garnet amphibolite conditions. And the amount of sediment melt involved in the mantle metasomatism related to diorite porphyrites varies largely from 1% to 10% with a source partial melting degree ranging from 8% to 15%. Whereas in the granite porphyries, the amount varies from 10% to 15% and their source partial melting degree is up to 15%.