云南牟定县戌街地区黄草坝岩体锆石微量元素地球化学特征及其地质意义

Geochemical characteristics and geological significance of trace elements in zircon from Huangcaoba pluton in Xujie Area,Mouding County,Yunnan Province,China

黄草坝岩体位于扬子陆块西南缘,是康定杂岩的重要组成部分,对其开展元素地球化学研究可以为扬子陆块西缘的构造岩浆演化提供新的依据。本文通过LA-ICP-MS方法对该岩体3期花岗岩中的锆石微量元素进行了系统研究,结果表明:黄草坝岩体的3期花岗岩中的锆石富集重稀土元素,亏损轻稀土元素,具有明显的Eu负异常(δEu介于0.01~0.17)及显著的Ce正异常(δCe介于1.09~199.27),整体呈左倾配分型,Th/U比值大于0.2,与岩浆锆石的特征一致。锆石的结晶温度大部分<750℃,推测岩体岩浆可能来源于水近饱和条件下古老地壳的部分熔融。相对氧逸度计算结果表明,黄草坝岩体与铌钽(中期二长花岗岩)、钨成矿作用有关的花岗岩(晚期黑云母花岗岩)相比岩浆氧逸度相对较高。锆石的微量元素地球化学特征显示,早中期花岗岩可能形成于裂谷环境,与地幔柱活动相关,而晚期花岗岩可能形成于大陆弧边缘的弧后伸展环境。

The Huangcaoba pluton is located in the southwestern margin of the Yangtze Block and is an important part of the Kangding Complex.Elemental geochemical studies on it can provide a new basis for the tectonic-magmatic evolution of the western margin of the Yangtze Block.In this paper,the LA-ICP-MS method was used to systematically study the trace elements of zircon in the third-stage granite of the pluton.The results show that the trace elements of zircon in the third-stage granite of the Huangcaoba pluton are enriched in heavy rare earth elements and depleted in light rare earth elements.The elements are characterized by obvious negative Eu anomalies(δEu between 0.01 to 0.17)and significant positive Ce anomalies(δCe between 1.09 to 199.27).The whole is a left-leaning partition type,and the Th/U ratio is greater than 0.2,which is similar to that of magmatic zircon.characteristics are consistent.The crystallization temperature of zircon is mostly less than 680℃or not greater than 750℃.It is speculated that the magma of the pluton may originate from the partial melting of the ancient crust under the water-saturated condition.The calculation of relative oxygen fugacity shows that the magmatic oxygen fugacity of Huangcaoba pluton related to niobium-tantalum(mid-stage monzonitic granite)and tungsten mineralization(late-stage biotite granite)is relatively high.The trace element geochemical characteristics of zircon show that the early and mid-stage granites may have been formed in rift environments,which are related to mantle plume activities,while the late-stage granites may have been formed in the back-arc extension environment of continental arc margins.