ORIGIN AND TECTONIC IMPLICATIONS OF THE JURASSIC HIGH-MG ANDESITES AND DIORITES ALONG THE MIDDLE-WESTERN SHIQUANHE OPHIOLITE BELT, TIBET

The Shiquanhe ophiolitic mélange belt is one of two major ophiolitic mélange belts along the Bangong-Nujiang suture zone (BNSZ) separating the Lhasa and Qiantang terranes of central Tibet. This study reports new geochronological, petrological and geochemical data for high-Mg dioritic dikes intruding the Shiquanhe ophiolitic peridotite. Based on LA-ICP-MS zircon U-Pb dating, dioritic dikes yield a Late Jurassic age of 159.7±1.2 Ma. These are high-Mg diorites (54.9%~56.8% SiO₂, 7.1%-8.8% MgO) with moderate TiO₂ (0.95%~1.01%), high K₂O+Na₂O (6.1%~8.0%), and high contents of Cr (150~182 ppm), Ni (91~145 ppm) and Th (2.8~3.3 ppm). These dioritic dikes are very similar to sanukites of the Setouchi volcanic rock belt (SW Japan). Shiquanhe diorite samples have high Sr/Y ratios (up to 41.3), distinct positive Pb anomalies, negative Nb anomalies and continental crust-like trace element patterns without negative Eu anomalies (average Eu/Eu^*=0.96). Interpretation of these geochemical characteristics indicates that Shiquanhe dioritic dikes were derived by partial melting of mantle peridotite as a result of interaction with hydrous melts of subucted sediments. Similar high-Mg magmas are commonly formed during early stages in the evolution of a subduction zone. Integrated with geologic observations in the field and incorporating results from other studies of Late Jurassic high-Mg andesites and diorites along the Shiquanhe ophiolite belt, we argue that the Shiquanhe high-Mg dioritic dikes formed at an early stage of northward subduction of the Bangong Tethyan ocean beneath the Qiangtang terrane. Therefore, these results provide critical constraints on the Jurassic tectonic evolution of the Shiquanhe ophiolitic mélange belt and are significant for understanding the demise of the Bangong Tethyan ocean.

Keywords: Tibet, igneous geochemistry, BNSZ, high-Mg diorite, subduction