喜马拉雅造山带的部分熔融与淡色花岗岩成因机制

Partial Melting of Himalayan Orogen and Formation Mechanism of Leucogranites

喜马拉雅造山带核部由高级变质岩和淡色花岗岩组成,是研究大陆碰撞造山带部分熔融与花岗岩成因的天然实验室.基于最新研究成果,探讨了喜马拉雅造山带核部变质作用的条件、类型以及P-T轨迹、部分熔融的方式与程度及熔体成分以及变质作用与部分熔融的时间和持续过程.相关证据表明,造山带核部经历了高压麻粒岩相至榴辉岩相变质作用,具有以增温增压进变质和近等温降压退变质为特征的顺时针型PT轨迹.这些高压变质岩石发生了长期持续的高温变质与部分熔融.在泥质岩石的进变质过程中白云母和黑云母脱水熔融可以形成不同成分的熔体.同时,总结了淡色花岗岩的形成时间、地球化学特征和源区熔融方式,结果表明碰撞造山过程中加厚下地壳的脱水熔融形成了喜马拉雅造山带的淡色花岗岩.

The core of the Himalayan orogen consists of high-grade metamorphic rocks and leucogranites,forming a natural laboratory for studying crustal anatexis and granite origin during the collisional orogeny.Based on recent achievements of the related studies,the condition,type and P-T path of metamorphism,and mechanism,degree and melt composition of anataxis as well as metamorphic and anatectic timing and duration of high-grade metamorphic rocks in the orogenic core are discussed in this paper.The obtained evidence shows that the orogenic core experienced high-pressure granulite-facies to eclogite-facies metamorphism,with a clockwise-type P-Tpath characterized by increasing temperature and pressure prograde and early retrogression of near-isothermal decompression,and that the high-pressure rocks record a prolonged high-temperature metamorphic and anatectic process.The muscovite-and biotite-dehydration melting of meta-pelitic rocks during the prograde metamorphism resulted in formation of melts with highly variable chemical compositions.In addition,the formation time and geochemical feature of the Himalayan leucogranites are also summarized.Finally,it is concluded that the leucogranites were derived from the dehydration melting of thickened lower crust during the collisional orogeny.