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    湖南南岳岩体LA-ICP-MS锆石U-Pb年龄及其地球化学特征

    LA-ICP-MS zircon U-Pb age and geochemical characteristics of Nanyue granites in Hunan Province

    • 摘要: 提要:分布于湖南衡山的南岳岩体,侵位于新元古代冷家溪群中,为复式岩体。通过锆石U-Pb年龄测定,其锆石LA-ICP-MS U-Pb年龄为(215.5±1.5)Ma,主体形成于晚三叠世;其中部分小岩体形成于早白垩世,锆石LA-ICP-MS U-Pb年龄为(140.6±0.8)Ma。晚三叠世花岗岩由三个岩石单元组成,第一期细中粒斑状角闪石黑云母二长花岗岩、第二期中细粒黑云母二长花岗岩、第三期二云母二长花岗岩;早白垩世花岗岩为二云母二长花岗岩。晚三叠世花岗岩属过铝质高钾钙碱性系列;早白垩世花岗岩属强过铝质高钾钙碱性系列。岩石明显富集大离子亲石元素,亏损高场强元素;∑REE中等(92.88×10-6~296.56×10-6),Eu均为弱负异常(δEu=0.28~0.87),其中早白垩世花岗岩Eu亏损更大;ISr值较大(0.7093~0.7189);低εNd(-8.04~-11.38), 高T2DM(1.63 ~1.90 Ga)。综合研究表明,南岳花岗岩石为壳源含白云母过铝花岗岩类(MPG),为华南前寒武系基底重熔而成,晚三叠世花岗岩有少量下地壳或地幔物质加入;其形成的构造背景应为碰撞造山作用晚期或结束时期,是陆内碰撞造山作用增温减压体制下的产物。

       

      Abstract: Abstract: Located in Hengshan of Hunan Province, Nanyue complex pluton intruded in the Neoproterozoic Lengjiaxi Group. The LA-ICP MS zircon U-Pb ages indicate that the main pluton was formed in the Triassic with a weighted mean age of 215.5±1.5 Ma. However, some small granitic plutons were formed in the early Cretaceous with a 206Pb/238U age of 140.6±0.8 Ma. The late Triassic granites are composed of three individual rock units, being mainly fine- and medium-grained porphyritic hornblende biotite monzonitic granite and surbordinately two-mica monzonitic granite. The early Cretaceous granite is two-mica monzonitic granite. The major and trace element analyses show that the late Triassic granites are of the peraluminous high-K calc-alkaline series, while the early Cretaceous granite is of the high-peraluminous high-K calc-alkaline series. The rocks are enriched in large-ion lithophile elements and depleted in high field-strength elements, with the data ∑REE=92.88-296.56 and weak negative Eu anomaly (δEu=0.28-0.87). The Sr-Nd isotope analyses indicate high ISr values (0.7093-0.7189), low εNd values (-8.04--11.38) and high T2DM values (1.63 Ga-1.90 Ga). The results obtained by the authors show that the Nanyue muscovite-bearing peraluminous granitoids were derived from remelting of Precambrian basement, the magma source of late Triassic granites had the mixed mantle and lower crust materials, and the lithogenesis was related to the lithospheric extension after the orogenesis.

       

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