藏南哲古基性岩SHRIMP锆石U-Pb年龄及地质意义  

U-Pb SHRIMP zircon ages of the mafic-ultramafic rocks from Chigu Co of South Tibet and their geological significance

作　　者：任冲[1] 马飞宙[2] 朱振华[1] 张建国[1] 黄荣才[1] 张亚[1] 

机构地区：[1]中国人民武装警察部队黄金第十一支队,西藏拉萨850000 [2]中国地质调查局,北京100037

出　　处：《中国地质》2015年第4期881-890,共10页Geology in China

基　　金：中国地质调查局地质大调查项目(1212011220659)资助

摘　　要：藏南地区广泛发育近东西向的基性岩墙群，其中哲古地区辉石岩以夹层形式产于辉长岩中，本文对哲古基性岩进行SHRIMP锆石U-Pb定年，得出辉长岩和辉石岩加权平均年龄分别为（131．71±2．7）Ma（2σ，MSDW=2．8）和（135．0±2．4）Ma（2σ，MSDW=2．1），辉石岩的结晶时代比辉长岩早约4Ma。元素地球化学研究表明，辉长岩SiO：含量为50．85％，TiO2含量为1．45％，MgO含量为10．05％，全铁含量为8．57％，Mg#为71．33。辉石岩SiO2含量为48．28％，TiO2含量为2．02％，MgO含量为18．68％，全铁含量为13．08％，Mg#为78．76。辉长岩和辉石岩K2O含量较高，分别为0．68和0．81，Eu的亏损不明显，相对亏损Nb、Ta等元素，稀土分布型式与板内洋岛玄武岩（Om）类似。其形成与同时期的地幔热事件关系密切，并在上升过程中可能受到了地壳物质的混染。The nearly EW-trending mafic dyke swarms are widely distributed in South Tibet, and the layered pyroxenite occurs in the gabbro. Two weighted average ages of gabbro and pyroxenite were obtained by SHRIMP zircon U-Pb dating, i.e., （131.71±2.7） Ma （2σ, M SDW=2.8） and （135.0±2.4）Ma （2 σ, MSDW=2.1 ）respectively. The crystallization of the ultramafic rocks took place 4 Ma earlier than that of the mafic ones. Element geochemical study shows that, in gabbro, SiO2 is 50.85%, TiO2 is 1.45%, MgO is 10.05%, TFe203 is 8.57%, and Mg# is 71.33, whereas in pyroxenite, SiO2 is 48.28%, TiO2 is 2.02%, MgO is 18.68%, TFe2O3 is 13.08%, and Mg# is 78.76. Their K2O content is high （0.68 and 0.81）, Eu loss is not obvious, Nb, Ta and other elements are depleted, and the patterns of REE distribution are similar to those of OIB. The formation of the mafic- ultramafic rocks was related to the mantle thermal events occurring in the same period, and might have been contaminated by crustal materials in the rising process.

关 键 词：藏南 基性岩 SHRIMP锆石定年 地球化学 

分 类 号：P597.3[天文地球—地球化学] P588.124.5[天文地球—地质学]