CCSD HP-UHP变质岩中石英脉和东海水晶流体包裹体惰性气体同位素组成及其成因指示意义  

Noble gases isotopic compositions of fluid inclusions in quartz veins and crystals collected from CCSD and Donghai HP-UHP metamorphic rocks.

作　　者：孙晓明[1] 徐莉[1] 翟伟[1] 汤倩[1] 梁业恒[1] 梁金龙[1] 张泽明[2] 沈昆 

机构地区：[1]中山大学地球科学系 [2]中国地质科学院地质研究所,北京100037 [3]山东省地质科学研究院,济南250013

出　　处：《岩石学报》2006年第7期1999-2008,共10页Acta Petrologica Sinica

基　　金：国家自然科学基金(编号:40399142);中国科学院同位素年代学和地球化学实验室合作基金(GIGIso-0504);国家重点基础研究发展规划项目(973项目)(编号:2003CB716501);中国地质大学地质过程与矿产资源国家重点实验室开放基金(编号:2003015)联合资助.

摘　　要：中国大陆科学钻探(CCSD)岩心中存在大量石英脉和长英质脉体,关于其成因目前存在板块折返过程中的减压和流体释放以及后期流体穿插等不同成因观点。本文利用高真空气相质谱系统分析了CCSD石英脉和东海水晶流体包裹体中惰性气体同位素组成,其He-Ar、Xe和Ne同位素体系均清楚显示它们主要由地壳变质流体和大气饱和水组成,而基本不含深源地幔流体。在CCSD 900—1500m深度的石英脉流体包裹体出现40Ar／36Ar和δ18O同步下降,且与前人对CCSD中HP-UHP岩石的矿物原位δ18O测定结果变化趋势相似,说明该段原岩在板块俯冲前曾与大气降水发生较为充分的水／岩反应,石英脉继承了围岩的同位素组成特征。流体包裹体惰性气体同位素组成显示CCSD中石英脉和东海水晶可能具有相似的成因,它们均主要形成于HP-UHP岩石在板块折返到地壳中的压力降低和流体释放过程。CCSD HP-UHP岩石及其石英脉中基本不含地幔流体的原因在于本区印支期快速的俯冲和折返过程,被俯冲地壳物质无法与地幔进行充分的同位素交换。此外,退变质作用及其伴生的地壳流体也进一步减弱了HP-UHP岩石中的地幔流体信息。A lot of quartz or microcline quartz veins are found in the CCSD HP-UHP rocks, and different genetic models of the veins have been proposed, among which the two most representative ones are plate exhumation and fluid releasing model and late stage fluid penetration model. In this study, noble gas isotopic compositions of fluid inclusions in the CCSD quartz veins and quartz crystals in the Donghai area were analysed by using a high vacuum gas mass spectrum, and their He-Ar, Xe and Ne isotopic compositions clearly show that the fluid inclusions are composed mainly of crustal metamorphic fluid and air-saturated water, and the mantle-derived fluid is nearly negligible. At depth of 900 - 1500m in the CCSD, ^40Ar/^36Ar of the quartz veins declined in step with δ^18O, and where marked decline of δ^18O in minerals of HP-UHP rocks were recognized by in-situ δ^18O analyses, suggesting that the protolithes of the HP-UHP at this depth might have been reacted with air-saturated water before being subducted, and the quartz veins inherit their host rocks＇s isotopic features. Similar noble isotopic compositions of fluid inclusions imply that the CCSD quartz veins and the Donghai quartz crystals were formed in similar geological environment, and both of them were formed during fluid releasing and pressure decline of the exhumating plate. The negligible content of mantle-derived fluids in the CCSD quartz veins and Donghai quartz crystals show that the Indosinian subduction and exhumation of the HP-UHP rocks in this area may be quite quick, so that the mantle-crust interaction and isotopic exchange might be very limited. Besides, the late stage retrograde metamorphism tends to weaken mantle-derived information of the CCSD HP-UHP rocks.

关 键 词：流体包裹体 惰性气体同位素组成 石英脉 东海水晶 中国大陆科学钻探(CCSD) 

分 类 号：P597.2[天文地球—地球化学] P578.494[天文地球—地质学]