大陆俯冲带流体活动:超高压变质岩矿物水含量和稳定同位素制约  

Fluid activity in continental subduction zones:Insights from stable isotopes and water contents in minerals from ultrahigh-pressure metamorphic rocks

作　　者：陈仁旭[1] 郑永飞[1] 龚冰[1] 

机构地区：[1]中国科学院壳幔物质与环境重点实验室中国科学技术大学地球和空间科学学院,合肥230026

出　　处：《岩石学报》2011年第2期451-468,共18页Acta Petrologica Sinica

基　　金：国家973计划项目(2009CB825004);国家自然科学基金项目(40903003;40921002);中国博士后科学基金(20090460724;201003319);中国科学院王宽诚博士后工作奖联合资助

摘　　要：对超高压变质岩中含水矿物和名义上无水矿物的地球化学研究,极大地深化了我们对大陆碰撞带地壳俯冲和折返过程中流体体制的认识。就流体体制和化学地球动力学来说,有关研究在大别-苏鲁造山带进行的最为详细,因此已经成为研究大陆俯冲带变质的典型地区。本文以大别-苏鲁造山带为对象,从矿物水含量的角度,结合稳定同位素论述了大陆俯冲带流体活动。超高压变质岩中名义上无水矿物含有大量的水,以结构羟基和分子水形式存在。名义上无水矿物中结构羟基和分子水出溶与含水矿物分解共同构成了折返过程中退变质流体的主要来源。名义上无水矿物所释放的水以富集轻的氢氧同位素为特征,而含水矿物分解则提供了富集D的流体来源。折返过程中,名义上无水矿物降压脱水存在亏损D的分子水的优先丢失和不同形式水之间的相互转化。不同岩性的水含量差异导致了它们在折返过程中不同的流体活动行为。大陆板块俯冲和折返过程中,在不同矿物、不同岩性以及板片不同部位之间存在水的再分配;板片的一部分作为富水流体的源,而另一部分可能作为汇。Much of our understanding of fluid regime during subduction and exhumation in continental deep-subduction zones has been recently deduced from various records in hydrous and nominally anhydrous minerals from ultrahigh-pressure（UHP）metamorphic rocks.The Dabie-Sulu orogenic belt has been a type locality for the study of not only fluid regime but also chemical geodynamics in continental subduction zones.Hence the UHP metamorphic rocks from this belt are the main focus of this review.Sources of metamorphic fluid can be deduced from study of hydrous minerals,structural hydroxyl and molecular water in nominally anhydrous minerals（NAMs）.The decomposition of hydrous minerals and the exsolution of molecular water and structural hydroxyl during exhumation are responsible for the origin of retrograde fluid during exhumation of the deeply subducted continental crust.Aqueous fluid released by the exsolution of molecular water and structural hydroxyl from UHP nominally anhydrous minerals is characterized by light hydrogen and oxygen isotope compositions,whereas the decomposition of hydrous minerals may provide a source of D-rich fluid.D-poor molecular water was preferential loss from the NAMs by diffusion relative to structural OH during exhumation.The difference of water contents between different lithologies dictates their different behaviors of fluid activity during exhumation.During plate subduction and exhumation,water was reallocated between different minerals,different lithologies and different parts of the subducted plate.One part of the UHP slab is a source of aqueous fluid,whereas another part is a sink.

关 键 词：大陆地壳 超高压变质 水含量 稳定同位素 流体活动 地球动力学 大别-苏鲁造山带 

分 类 号：P588.3[天文地球—岩石学]