摘要:
提要：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4 Ma。元素地球化学研究表明，辉长岩SiO2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（OIB）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。
Abstract:
Abstract: 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, MSDW=2.8) and (135.0±2.4)Ma (2σ, 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%, TFe2O3 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.
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