摘要:
通过近3年野外观测、测试分析及综合研究,在西藏泽当矿田鉴别出五期中酸性岩浆活动;应用LA-MC-ICPMS(激光探针)锆石 U-Pb 同位素测年方法分别测定各期岩浆形成时代,并分析不同时期岩浆形成的构造背景及资源效应。泽当西奥长花岗岩锆石U-Pb年龄为(156±1.0)~(152.5±1.3) Ma,为埃达克岩,其岩浆演化与特提斯古大洋板块北向俯冲存在成因联系,形成于洋内弧构造环境;泽当北部桑布加拉矽卡岩铜矿成矿母岩花岗闪长岩锆石U-Pb年龄(94.10±0.96) Ma,显示埃达克岩特征,源于俯冲板块熔融,并在上升过程中受到了地幔楔物质的混染,岩浆演化可能与桑日岛弧洋内俯冲相关;在泽当东侧发现古新世和始新世两期花岗闪长岩,锆石U-Pb年龄分别为(63.0±1.0) Ma和(50.2±1.3)~(45.93±0.21) Ma,均显示陆缘弧岩浆岩特征,与印度—亚洲大陆初始碰撞有关;在泽当东获得石英二长岩锆石 U-Pb 年龄为(31.64±0.47)~(29.88±0.39) Ma,其中冲木达岩体为埃达克质岩,而明则岩体显示高钾特征,前者可能与碰撞中晚期俯冲到拉萨地块之下的印度板块的熔融有关,但这两者很可能是外来岩体(浆),不能代表泽当矿田的构造演化特征。综合锆石U-Pb同位素年龄及岩石地球化学特征推断,泽当矿田前四期岩浆活动可能分别与多期新特提斯洋内俯冲、印度—亚洲大陆初始碰撞有关,并且存在受后期断层影响的外来渐新世岩体(浆)。
Abstract:
Five-phase acidic magmatism was identified though nearly three years’ field observation, test analysis and study in the Zetang orefield on the southern margin of the Gangdise belt. The LA-MC-ICPMS (laser probes) method was used to determine the zircon U-Pb isotope magma formation age and analyze the magma-forming tectonic background and the effects of natural resources. The zircon U-Pb isotope age of trondhjemites in the west of Zetang County is (156±1.0)~(152.5±1.3) Ma. The rocks are adakites, suggesting that the Paleo-Tethy ocean plate subducted northward and formed an intra-oceanic arc tectonic setting in Jurassic. The zircon U-Pb isotope age of host granodiorite rocks (adakites) of the Sangbujiala deposit located in the north of Zetang County is (94.1±0.96) Ma. The magma probably originated from melting of subduction block, and was contaminated by mantle wedge material. Magmatic evolution was closely related to the intra-oceanic subduction of Sangri Island Arc. There existed two phases of magmatic activities in the east of Zetang County, with the zircon isotope age of granodiorite being (63.0±1.0) Ma and (50.2±1.3)~(45.93±0.21) Ma respectively, both of them exhibited continental margin arc features, which indicates initial India-Asian collision. The zircon isotope age of adamellite in the east of Zetang Country is (31.64±0.47)~(29.88±0.39) Ma, and the petrogeochemistry of these two kinds of rocks shows different characteristics. The Chongmuda granites are adakitic rocks, while the Mingze granites are potassium-rich rocks. It is estimated that the adakitic magma came from the melting of India plate underneath the Lhasa Block. However, the two intrusive rocks are probably exotic blocks or are not original magma, whose geochemistry can not represent the tectonic and magmatic evolution of the Zetang polymetallic orefield. Based on the zircon isotope ages, petrology and geochemistry, the authors have reached the conclusion that the first four phases of magmatic activities had close dynamic relationship with the multistage ocean subduction and initial India-Asian continent collision, and the Late Oligocene alien rocks were affected by the faults in the Zetang orefield.
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