哈图正长花岗岩年代学和成因及对东昆仑东段稀土元素成矿地质背景的约束

Geochronology and petrogenesis of Hatu syenogranite and its constraint on the geological background of REE mineralization in the eastern part of East Kunlun

东昆仑东段哈图地区稀土元素富集与正长花岗岩关系密切。LA-ICP-MS锆石U-Pb同位素定年结果显示哈图正长花岗岩的结晶年龄为251.6±1.3 Ma。哈图正长花岗岩富硅(SiO_(2)=74.78%-75.78%)和碱(Na_(2)O+K_(2)O=7.68%-8.11%),低铝(Al_(2)O_(3)=12.58%-13.06%),铝饱和指数A/CNK介于0.96-1.03之间,主体为准铝—弱过铝质中钾钙碱性系列。哈图正长花岗岩富集轻稀土元素,亏损重稀土元素,具较明显的Eu负异常(δEu=0.67-0.91);微量元素富集Rb、Th、Ba、Cs等大离子亲石元素(LILE),亏损Nb、Ta、Ti等高场强元素(HFSE),具有较低的Nb/Ta比值和较高的Mg^(#)值。以上显示,哈图正长花岗岩具壳源特征,为镁铁质岩浆底侵下地壳,使其部分熔融形成的原生岩浆再经过分异形成。哈图正长花岗岩具弧岩浆岩地球化学特征,结合东昆仑造山带岩浆岩分布以及沉积地层特征显示,早三叠世东昆仑地区处于布青山-阿尼玛卿洋的俯冲阶段,大规模的岩浆事件引起东昆仑造山带东段稀土元素成矿作用。

The enrichment of rare earth elements is related to granites in the Hatu region in the eastern part of East Kunlun. Zircon U-Pb dating by LA-ICP-MS reveals that the crystallization age of Hatu syenogranite is 251.6±1.3 Ma. The Hatu syenogranite is high in SiO_(2)(74.78%-75.78%), Na_(2)O+K_(2)O(7.68%-8.11%), relatively low in Al_(2)O_(3)(12.58%-13.06%), with A/CNK ratios ranging from 0.96 to 1.03, and displays metaluminous to weak peraluminous middle-K calc-alkaline affinities. The rocks are enriched in the large lithophile elements such as Rb, Th, Ba, Cs and LREE, depleted in the high field strength elements like Nb, Ta, Ti and HREE, possess low Nb/Ta ratio, high Mg^(#) value and display obvious negative Eu anomalies(δEu=0.67-0.91). Based on the geochronological and geochemical data, it is demonstrated that the Hatu syenogranite has the characteristic of crust source and is the product of magma fractional crystallization, triggered by the underplating of mafic magma into lower crust. As the Hatu syenogranite has the characteristics of arc magmatic rocks, and in combination with the properties of magmatic and sedimentary rocks in the East Kunlun orogenic belt, it is inferred that the Early Triassic East Kunlun area is in the subduction stage of the Buqingshan-Anyêmaqên ocean. Large scale magmatic events resultedin the Early Triassic REE mineralization in the eastern part of East Kunlun.