内蒙古凉城蛮汗山石榴石二长花岗岩LA-MC-ICP-MS锆石U-Pb年龄及成因讨论
LA-MC-ICP-MS zircon U-Pb dating of garnet monzonitic granite in the Manhan Mountain of Liangcheng, Inner Mongolia, and its petrogenesis
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摘要: 内蒙古凉城地区的石榴石二长花岗岩20世纪70年代初1:20万区域地质调查时归入太古代早期侵入岩(γ11(2)),岩石定名为中粗粒似斑状花岗岩,称蛮汗山岩体。1:25万区调将其划分为弱片麻状石榴二长花岗岩、弱片麻状变斑状含黑云石榴二长花岗岩等不同地质单元。为进一步研究该石榴石花岗岩的形成时代,本文对弱片麻状变斑状含黑云石榴二长花岗岩中锆石进行了LA-MC-ICP-MSU-Pb测年研究,获得其U-Pb年龄值(1933.3±9.8)Ma,为石榴石二长花岗岩原岩的形成年龄。岩石化学、地球化学分析表明该石榴石二长花岗岩为新太古界孔兹岩系部分熔融的产物,为强过铝S型花岗岩。结合已有的年龄数据推断,1900~2000 Ma(古元古代中期)华北克拉通曾发生过大规模的构造岩浆热事件。Abstract: The garnet monzonitic granite in the Liangcheng area of Inner Mongolia was classified as Early Archean intrusive rocks (γ11(2))by the 1:200000 regional geological survey in the early 1970s. The rock was named porphyroid medium-coarse granite and called Manhanshan pluton. 1:250000 regional geological survey divided it into weakly-gneissic garnet monzonitic granite, weaklygneissic porphyroblastic biotite-garnet monzonitic granite and other different geological units. To further study the formation age of the garnet-granite, the authors conducted zircon LA-MC-ICP-MS U-Pb dating of weakly-gneissic porphyritic biotite-garnet monzonitic granite, and the results yielded an age of (1933.3±9.8) Ma, which represents the magmatic crystallization ages of the garnet monzonitic granites. Geochemical analyses show that the garnet monzonitic granites resulted from partial melting of the Neoarchean khondalite series, with the features of strongly peraluminous S- type granites. Combined with existing age data, it is held that the North China Craton occurred in large- scale tectono- magmatic thermal events during 1 900- 2 000 Ma (Middle Paleoproterozoic).
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