CCSD HP-UHP变质岩中石英脉微量元素和稀土元素地球化学

利用等离子质谱(ICP-MS)测定了CCSD HP-UHP变质岩中石英脉及东海水晶的微量元素和稀土元素组成,结果显示其中微量元素总体含量很低,Bb/Sr、Nb/Ta和Zr/Hf变化很大,说明Rb和Sr、Nb和Ta、Zr和Hf之间发生了不同程度的分异,但变质岩未曾发生过广泛强烈的水岩反应.部分样品中Rb和Cs含量特别低,显示CCSD中部分石英脉可能曾经历过超高压变质过程.CCSD石英脉和东海水晶均具有LREE富集而HREE明显亏损的特征,显示其来源主要为壳源.CCSD石英脉中正负Ce异常同时出现,显示板块俯冲前,超高压变质岩的原岩曾在地表较氧化的环境下经历过强烈的风化作用.对比研究显示,CCSD不同围岩中石英脉、地表石英脉和东海水晶的微量元素N-MORB标准化模式和BEE球粒陨石标准化配分曲线,与其各自围岩具有一定的对应相似关系,表明大多数石英脉(水晶)来自于围岩,同时显示板块俯冲和折返所释放的流体活动范围较小,没有发生大规模的迁移.

Trace elements and REE geochemistry of quartz veins from CCSD HP-UHP metamorphic rocks

Trace element and rare earth element(REE)compositions of quartz veins in CCSD HP-UHP metamorphic rocks and crystals in Donghai County in Jiangsu Province were analysed by TCP-MS in this study.The results show that most of the quartz and crystal samples possess very low trace elements total contents and greatly varied Rb/Sr,Nb/Ta and Zr/Hf ratios,implying that their host metamorphic rocks were not suffered extensive and strong water-rock interactions.Rb and Cs contents in some quartz samples are very low,suggesting part of the CCSD quartz veins might have been suffered ultrahigh metamorphism.REE analyses show that all of the CCSD quartz veins and Donghai crystals have LREE enriched and HREE depleted characters,implying they were mostly derived from crust.In addition,concurrence of negative and positive Ce abnormities in CCSD quartz veins suggests that parts of protolithes of the CCSD UHP metamorphic rocks might have been weathered in an oxidative environment at the surface before plate subduction. Comparatively,N-MORB normalized trace elements patterns and chondrite normalized REE patterns of the CCSD quartz veins and Donghai erystals are quite similar to those of their respective host rocks,suggest that most of the quartz veins and the crystals derived from their host metamorphic rocks,and crust fluids released during plate subduction and exhumation might migrated at grain scale.