Zircon U-Pb geochronology,geochemistry and Sr-Nd-Hf isotopic compositions of the rhyolite porphyry from the Zhoujiaoshan deposit in Xiangshan uranium ore field,Jiangxi Province,SE China
Zircon U-Pb geochronology,geochemistry and Sr-Nd-Hf isotopic compositions of the rhyolite porphyry from the Zhoujiaoshan deposit in Xiangshan uranium ore field,Jiangxi Province,SE China
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作者：
Y. Shui
在相山铀矿田周家山铀存款中发现一条侵入相山火山杂岩碎斑熔岩中的流纹斑岩脉，经LA-ICP-MS锆石U-Pb定年，其形成时代为134.6±1.2Ma（MSDW=1.2），与相山火山杂岩早期流纹英安岩和晚期流纹英安斑岩的年龄相近，岩石地球化学特征为：流纹斑岩富Si、K，A/CNK为1.1，富Rb、Th、U、La、Ce、Nd和轻稀土元素，亏损Nb、Ta、Sr、P、Ti;具中等负Eu异常（Eu/Eu ~*=0.32~0.46），~（87）Sr/~（86）Sr初始比值范围为0.7097 ~ 0.7136，eNd（t）值范围为-8.49 ~-9.62，锆石eHf（t）值为-3.6 ~-9.4（大部分在-7和-9之间），这些地球化学和同位素数据表明，流纹斑岩的源区可能是地壳物质的部分熔融，根据流纹斑岩的源区与地幔源区的相似性，流纹斑岩的源区可能是地壳物质的部分熔融，流纹斑岩的源区可能是地壳物质的部分熔融流纹斑岩的Sr-Nd同位素、锆石Hf同位素及稀土元素配分模式与相山火山岩、次火山岩及基底变质岩的对比分析表明，相山火山杂岩的源区与变质岩有密切的关系;它们可能主要来自于类似基底变质岩的地壳岩石的部分熔融，结合前人的研究，认为象山流纹斑岩可能形成于与古太平洋板块俯冲有关的弧后拉张环境。
A rhyolite porphyry dyke was found in the Zhoujiashan uranium ore deposit from the Xiangshan uranium ore field,which intruded into the porphyroclastic lava in the Xiangshan volcanic complex.LA-ICP-MS zircon U-Pb dating shows that this rock was formed at 134.6±1.2Ma(MSDW=1.2).This age is similar to those of rhyodacite in the early stage and rhyodacitic porphyry in the late stage of the Xiangshan volcanic complex,which suggests that the volcanic-intrusive activity in the Xiangshan region began in Cretaceous and the activity is concentrative.Geochemically,the rhyolite porphyry shows high Si and K contents and peraluminous with A/CNK1.1.The rock is enriched in Rb,Th,U,La,Ce,Nd and LREE;depleted in Nb,Ta,Sr,P and Ti;and has moderate negative Eu anomaly(Eu/Eu*=0.32~0.46).The 87Sr/86Sr initial ratios range from 0.7097 to 0.7136,the eNd(t) values range from-8.49 to-9.62,and zircon eHf(t) values range from-3.6 to-9.4(mostly between-7 and-9),with Hf depleted mantle model ages mostly between 1.6Ga and 1.7Ga.These geochemical and isotopic data indicate that the sources of the rhyolite porphyry were likely derived from partial melting of crustal materials.Based on the similar patterns of Sr-Nd isotopes,zircon Hf isotopes and REE patterns of the rhyolite porphyry to those of the Xiangshan volcanic rocks,subvolcanic rocks and the metamorphic rocks of the basement,it is suggested that the sources of the Xiangshan volcanic complex have a close relationship with the metamorphic rocks;they may have been derived mainly from partial melting of crustal rocks resembling the basement metamorphic rocks.In combination with previous studies,we concluded that the Xiangshan rhyolite porphyry may have formed in a back-arc extensional setting which related to subduction of the Palaeo-Pacific plate.