Age determination of granite from Himalaya

The Gangdese belt, Tibet, records the opening and closure of the Neo-Tethyan ocean and the resultant collision between the Indian and Eurasian plates. Mesozoic magmatic rocks generated through subduction of the Tethyan oceanic slab constitute the main component of the Gangdese belt, and play a crucial role in understanding the formation and evolution of the Neo-Tethyan tectonic realm. U-Pb and Lu-Hf isotopic data for tonalite and granodiorite from the Xietongmen-Nymo segment of the Gangdese belt indicate a significant pulse of Jurassic magmatism from 184 Ma to 168 Ma. The magmatic rocks belong to metaluminous medium-K calc-alkaline series, characterized by regular variation in major element compositions with SiO2 of 61.35%-73.59 wt%, low to moderate MgO (0.31%-2.59%) with Mg# of 37-45. These magmatic rocks are also characterized by LREE enrichment with concave upward trend in MREE on the chondrite-normalized REE patterns, and also LILE enrichment and depletion in Nb, Ta and Ti in the primitive mantle normalized spidergrams. These rocks have high zircon eHf(t) values of + 10.94 to + 15.91 and young two-stage depleted mantle model ages (TDM2) of 192 Ma to 670 Ma. The low MgO contents and relatively depleted Hf isotope compositions, suggest that the granitoid rocks were derived from the partial melting of the juvenile basaltic lower crust with minor mantle materials injected. In combined with the published data, it is suggested that northward subduction of the Neo-Tethyan slab beneath the Lhasa terrane began by the Late-Triassic, which formed a major belt of arc-related magmatism.

西藏冈底斯带（Gangdese Belt）记录了新特提斯洋（Neo-Tethyan Ocean）的开合过程以及印度板块（Indian Plate）与欧亚板块（Eurasian Plate）由此发生的碰撞事件。由特提斯洋洋壳板片俯冲形成的中生代岩浆岩是冈底斯带的核心组成部分，对于解析新特提斯构造域的形成与演化机制具有至关重要的作用。采自冈底斯带谢通门-尼玛段（Xietongmen-Nymo segment）的英云闪长岩（tonalite）与花岗闪长岩（granodiorite）的U-Pb与Lu-Hf同位素数据显示，该区域存在184 Ma至168 Ma的显著侏罗纪岩浆活动幕。该类岩浆岩属于准铝质中钾钙碱性岩系，主量元素组成随二氧化硅（SiO₂）含量（61.35%~73.59 wt%）呈现规律性变化，氧化镁（MgO）含量介于低至中等水平（0.31%~2.59%），镁指数（Mg#）为37~45。其还表现出轻稀土元素（LREE）富集特征，球粒陨石标准化稀土元素配分模式中中稀土元素（MREE）呈凹型上倾趋势；同时在原始地幔标准化微量元素蛛网图中呈现大离子亲石元素（LILE）富集，以及铌（Nb）、钽（Ta）与钛（Ti）亏损的特征。该类岩石的锆石εHf(t)值介于+10.94至+15.91之间，两阶段亏损地幔模式年龄（TDM₂）为192 Ma至670 Ma，相对年轻。低氧化镁含量与相对亏损的Hf同位素组成表明，该类花岗岩类岩石起源于新生基性下地壳的部分熔融，并有少量幔源物质注入其中。结合已发表的相关数据，研究认为新特提斯洋洋壳板片在拉萨地体（Lhasa terrane）之下的北向俯冲始于晚三叠世，并由此形成了大规模的弧相关岩浆岩带。