Origin and tectonic implications of Late Jurassic high-Mg diorites along the Bangong-Nujiang suture zone, Tibet

The Shiquanhe-Num Co ophiolitic melange belt (SNOB) is one of two such 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 162.8 +/- 1.3 Ma. These are high-Mg diorites (53.0%-56.8% SiO2, 7.07%-8.82% MgO) with moderate TiO2 (0.95%-1.01%), high K2O+Na2O (6.02%-8.01%), and high contents of Cr (150-182 ppm), Ni (91-145 ppm) and Th (2.8-5.9 ppm). They are very similar to sanukites of the Setouchi volcanic rock belt (SW Japan). Shiquanhe diorites 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). These geochemical characteristics indicate that Shiquanhe diorities were derived by partial melting of mantle peridotite as a result of interaction with hydrous melts of subducted sediments. Similar high-Mg magmas are common in the early evolution of a new subduction zone. Integrated with field observations and results from other studies of Late Jurassic high-Mg andesites and diorites along the SNOB, we argue that the Shiquanhe high-Mg diorites formed at an early stage of northward subduction of the Bangong-Nujiang Tethys ocean (BNO) beneath the Qiangtang terrane. Therefore, these results provide critical constraints on the Jurassic tectonic evolution of the Shiquanhe-Num Co ophiolitic melange belt and are significant for understanding the evolution of the BNO.

Automated summary

The high-Mg diorites in the Shiquanhe-Num Co ophiolitic melange belt formed in the Late Jurassic period with geochemical characteristics similar to sanukites of the Setouchi volcanic rock belt in southwest Japan. These diorites were likely derived from partial melting of mantle peridotite due to interaction with hydrous melts of subducted sediments during the early evolution of the northward subduction of the Bangong-Nujiang Tethys ocean beneath the Qiangtang terrane.