青海东昆仑东段加当辉长岩LA-ICP-MS锆石U-Pb测年及其地质意义

本文对青海东昆仑东段加当辉长岩进行了岩石学、年代学和地球化学分析,有助于丰富东昆仑古特提斯构造岩浆演化的整体认识。利用LA-ICP-MS锆石U-Pb定年技术,获得加当辉长岩加权平均年龄为262.5±2.5Ma(n=38,MSWD=0.18),表明岩体形成时代为中二叠世晚期。岩石地球化学研究表明,加当辉长岩具有低TiO_2(0.79%~1.09%)、贫P_2O_5(0.06%~0.09%)、低碱(K_2O+Na_2O=1.99%~2.39%)及Na_2OK_2O的特征,属拉斑玄武岩系列。岩石微量元素总体上富集大离子亲石元素(LILE:Rb、Ba、K)和Pb,相对亏损高场强元素(HFSE:Nb、Ta、P),稀土元素总量(ΣREE)为32.08×10~(-6)~46.41×10~(-6),(La/Yb)N为1.59~2.00,平均1.82,δEu=0.95~1.32,具有弱的Eu正异常。地球化学特征表明辉长岩源区为岩石圈幔源岩浆,岩浆在演化过程中遭受了地壳物质的同化混染;构造环境分析表明加当辉长岩是形成于俯冲环境的岛弧岩浆岩。结合东昆仑区域构造演化,认为加当辉长岩是阿尼玛卿古特提斯洋向北俯冲的产物,这为东昆仑晚古生代-早中生代构造演化提供了新的年代学证据。

Zircon LA-ICP-MS U-Pb Dating and its Geological Significance of the Jiadang Gabbro in the Eastern Section of East Kunlun,Qinghai Province

This paper presents a study on petrography, geochronology and geochemistry of the Jiadang gabbro in the eastern section of East Kunlun, Qinghai Province, which will furtehr the understanding of Paleo-Tethys tectono-magmatic evolution in East Kunlun. LA-ICP-MS zircon U-Pb dat-ingindicatesthattheweightedmeanageoftheJiadanggabbrois262.5±2.5Ma(n=38,MSWD=0.18),inthelateMiddlePermian. Petrogeochemical studies show that the gabbro, which belongs to the tholeiitic series, is characterized by low content of TiO2 (0. 79% ~1. 09%), P2 O5 (0. 06% ~0.09%),alkali(K2O+Na2O=1.99% ~2.39%),andNa2O>K2O. ThissuiteofrocksareenrichedinLILE(suchasRb,Ba,andK)andPb,and relatively depleted in HFSE (such as Nb, Ta, and P). The rocks have low REE content, with slightly positive Eu anomalies (∑REE=32.08 ×10 -6 ~46.41 ×10 -6, (La/Yb)N =1.59~2.00,δEu=0.95~1.32). The geochemical characteristics show that the parental magma of the gabbro is derived probably from the mantle. The parental magma was subject to crustal assimilation contamination during the magma evolution. Tectonic setting analysis shows that the Jiadang gabbro is an island arc magmatic rock formed in a subduction environment. Combining with tectonic evolution of East Kunlun, we consider that the Jiadang gabbro was the product of the northward subduction of the Anyemaqen-Paleo-Tethys ocean, providing a piece of new chrono-logical evidence for Late Paleozoic-Early Mesozoic tectonic evolution in East Kunlun.