Partial melting of subducted Southern Qiangtang crust in northern Tibet: evidence from the geochemistry and geochronology of the Riwanchaka granodiorite porphyry in Central Qiangtang

This article presents new zircon U–Pb geochronology, Hf isotopic, and whole-rock major- and trace-element geochemical data that provide insights into the petrogenesis and tectonic history of the Riwanchaka granodiorite porphyries of Central Qiangtang, Tibet. Zircon U–Pb ages of 236–230 Ma indicate an early Late Triassic age of emplacement of the porphyries, and zircon Hf isotopic data yield εHf(t) values of – 7.0 to – 1.5 and ancient zircon Hf crustal model ages (T_DM^C) of 1524–1220 Ma. The granodiorite porphyries are characterized by low K₂O contents, high Mg# values, and relatively high Cr and Ni contents. They are classified as I-type calc-alkaline granite and are considered to have formed through the anatexis of ancient mafic crustal rocks with contributions from mantle-derived components. The geochemistry and isotopic compositions of all samples are similar to those of magmatic rocks that originated in the South Qiangtang crust. However, field observations indicate that the pluton intrudes the North Qiangtang crust, and we propose that the granodiorite porphyries were derived by partial melting of subducted continental crust of the South Qiangtang terrane. These new data have been integrated with data from previous studies to construct a new model of slab rollback during northward subduction of the Southern Qiangtang continental crust at <i>ca</i>. 245–226 Ma, thereby improving our understanding of magmatic processes involved in continental subduction in collision settings.

本文报道了全新的锆石U-Pb年代学、铪同位素以及全岩主量与微量元素地球化学数据，这些数据为揭示西藏羌塘中部日万恰卡花岗闪长斑岩的岩石成因与构造演化历史提供了重要依据。锆石U-Pb年龄介于236~230 Ma之间，指示该斑岩岩体的侵位时代为晚三叠世早期；锆石铪同位素数据显示其εHf(t)值为-7.0至-1.5，锆石Hf同位素地壳模式年龄(T_DM^C)为1524~1220 Ma。该花岗闪长斑岩具有低氧化钾(K₂O)含量、高镁指数(Mg#)以及相对较高的铬(Cr)和镍(Ni)含量特征，被归为I型钙碱性花岗岩，其成因被认为是古老镁铁质地壳岩石发生深熔作用并伴随幔源组分的加入。所有样品的地球化学与同位素组成均与源自南羌塘地壳的岩浆岩相似，但野外地质观察显示该岩体侵位于北羌塘地壳之中，因此我们提出该花岗闪长斑岩起源于南羌塘地体俯冲大陆地壳的部分熔融。本文将这些新数据与前人研究数据相结合，构建了南羌塘大陆地壳在约245~226 Ma期间北向俯冲过程中的板片回卷新模型，从而加深了我们对碰撞构造环境下大陆俯冲相关岩浆作用过程的认识。