Chi Yan

Magmatic activity is crucial for identification of the tectonic framework of the ancient oceanic crust. In this study, systematic investigation, including a field survey, zircon LA-ICP-MS U-Pb dating, and whole-rock geochemical analysis, has been carried out on the intrusive quartz- and granodiorites within the Meso-Tethyan Shiquanhe Ophiolitic Mélange (SQM), Tibet. Zircon U-Pb dating yields the weighted mean ages of 174.7 ± 1.4 Ma (quartz diorite) and 178.9 ± 1.2 Ma (granodiorite), respectively, demonstrating the Early Jurassic formation age. The quartz diorite samples are metaluminous (A/NKC = 0.77–0.95) (molar/Al₂O₃/(CaO + Na₂O + K₂O)), while the granodiorite samples are weakly peraluminous (A/NKC = 0.95–1.21), and both of them exhibit tholeiitic to calc-alkaline geochemical characteristics and can be classified as I-type granites. The right-dipping rare-earth element (REE) patterns, enrichment in large ion lithophile elements (LILEs: Rb, Ba, Th), and depletion in high-field-strength elements (HFSEs: Nb, Ta, Ti), as well as relatively high (La/Yb)_N ratios, are features compatible with an island arc setting. Combined with previous works, we suggest that the Shiquanhe ophiolitic mélange not only preserves records of mid-late Jurassic island arc magmatic activity but also contains evidence of island arc magmatism from the late Early Jurassic.

The magmatic activity is crucial for identify the tectonic framework of ancient ocean. In this study, a systematically work including field survey, zircon LA-ICP-MS U-Pb dating, and whole-rock geochemical analysis is carried on to investigated the intrusive magmatic rocks of quartz diorite and granodiorite within the Meso-Tethyan Shiquanhe Ophiolitic Mélange (SQM), Tibet. Zircon U-Pb dating yields the weighted mean ages of 174.7±1.4 Ma (quartz diorite) and 178.9±1.2 Ma (granodiorite) respectively, indicating the Early Jurassic formation age. The quartz diorite is metaluminous (A/NKC = 0.77-0.95), while the granodiorite is weakly peraluminous (A/NKC = 0.95-1.21), and both of them exhibit tholeiitic–calc-alkaline geochemical characteristics and classified as I-type granites. The right-dipping rare earth element (REE) patterns, enrichment in large ion lithophile elements (LILEs: Rb, Ba, Th), and depletion in high field strength elements (HFSEs: Nb, Ta, Ti), as well as relatively high (La/Yb)N ratios represent the is-land arc TTG-like magmatism origin. Combine with the previous works, we suggest the Shiquanhe opiolitic mélange represent an island arc and back-arc assemblage rather than an independent ocean basin. Meanwhile, this Early Jurassic island arc magmatism initiated a new era of the Mesozoic mineralization in Shiquanhe area.