渭河盆地南缘花岗岩中生氦元素(U、Th)赋存状态及制约因素研究——以华山复式岩体为例

The Occurrence State and Restraint Factors of Helium-produced Elements(U,Th)in the Granites from the Southern Margin of Weihe Basin:Evidences from Huashan Complex

花岗岩在氦气生成、储存和释放机理方面已经取得了诸多进展,但对其中U、Th生氦元素的赋存状态及相应的制约因素仍缺乏系统研究。渭河盆地南缘分布的巨量花岗质岩基被认为是盆地重要的氦源岩。笔者以渭河盆地南缘代表性的华山复式岩体为研究对象,对岩体样品进行了电子探针定性分析和副矿物组合定量分析。分析结果显示,样品中放射性U、Th元素主要赋存于锆石、榍石、磷灰石之中;燕山期侵入体中Th含量主要受控于锆石,U含量由锆石和褐帘石联合制约;而印支期侵入体中Th、U含量主要受控于锆石和榍石及少量钍石。对放射性U、Th含量和部分熔融模拟联合研究表明,基底太华群不均一的成分及其部分熔融程度可能是样品中放射性U、Th含量的根本制约因素。

Granites have been constantly concerned and much progress has been achieved on the production,storage and release of helium.However,there is lack of further study about the occurrence state,restraint factors and effective assessment of helium-produced elements(U,Th)in the granites when granites had been generated.A large number of granitic batholiths,located at the southern margin of Weihe basin,were considered to be the crucial source of helium.This study focused on the representative Huashan granitic batholith.The EMPA analyses and identifications for placer minerals showed that U and Th were mainly stored in zircons,titanites and apatites from Huashan granitic batholith.The Th content was mostly depended on the content of zircon and U content was controlled by zircon and allanite in the Yanshanian pluton,while Th and U content were dominantly related to zircon,titanite and minor thorite in the Indonian pluton.Meanwhile,the contents of U and Th in the samples mentioned above were determined and synthetical study suggested that the Taihua complex with various gneisses and the degree of parting melting were the dominant two factors for the divergence of U and Th in the Huashan granite.