玉树地区让娘贡巴辉长岩锆石SHRIMP U-Pb测年和地球化学特征

青海玉树地区出露一套晚古生代—早中生代的蛇绿混杂岩。其中,让娘贡巴辉长岩的锆石SHRIMP U-Pb年龄为258.4±2.9 Ma,表明该岩石形成于晚二叠世。地球化学特征表明,岩石具有富碱,高TiO2、Fe2O3的主量元素特征,稀土元素总量相对较高,富集轻稀土元素,(La/Yb)N=5.09~5.52,与原始地幔相比,富集大离子亲石元素(Rb、Ba、Th、K)及高场强元素(Nb、Ta、Zr、Hf)。综合研究表明,让娘贡巴辉长岩的形成环境为洋岛,这足以表明,至少在晚二叠世时,玉树地区已经存在洋壳。

Zircon SHRIMP U-Pb dating and geochemistry of the Rangnianggongba gabbro in Yushu area, Qinghai Province

The Yushu ophiolite mélange, which has preserved important record concerning the evolution of the Palo-Tethys Ocean, is mainly composed of pillow basalts, diabases, gabrros and ultramafic rocks. The zircon SHRIMP U-Pb age of Rangnianggongba gabbro in this mélange is 258.4±2.9 Ma, suggesting that the emplacement age of the rock is Late Permian. Petrologic and geochemical studies suggest that the gabbro is alkaline and has characteristics of high Ti and Fe. All the rock samples have relatively high REE abundances, and are enriched in LREE . The rock is enriched in large ion lithophile elements (LILE: Rb,Ba,Th,K) and some high field strength elements (HFSE: Nb, Ta, Zr, Hf). These data indicate that the Rangnianggongba gabbro was derived mainly from the enriched mantle. According to geochemical characteristics, the authors have concluded that the gabbro was formed in an oceanic island setting. The age dating and geochemical data indicate that an oceanic crust was already existent at least in the Late Permian. Moreover, the existence of large-scale Paleozoic Plume activities on the southwest margin of Yangtze block has been extensively reported. It is considered that the emplacement of the Rangnianggongba gabbro was related probably to activities of the Plume.