大陆俯冲带地壳深熔作用的记录:来自超高压变质岩中多相晶体包裹体的研究

Anatexis of Crustal Rocks During Continental Collision: Evidence from Multiphase Crystal Inclusions in UHP Metamorphic Rocks

大陆碰撞过程中会发生广泛的部分熔融现象,形成深熔熔体。深入认识深熔熔体的组成和演化是大陆俯冲带化学地球动力学的主要研究内容。在熔融过程中产生的熔体会被超高压岩石中的转熔矿物所捕获,最终以多相晶体包裹体(multiphase crystal inclusion,简称MCI)的形式保存下来。多相晶体包裹体通常具有典型的负晶形和爆裂纹,主要以硅酸盐和碳酸盐矿物为主含有少量的硫酸盐矿物。矿物学、岩石学和地球化学原位微区分析的研究结果表明,多相晶体包裹体是由岩石部分熔融产生的初始硅酸盐或/和碳酸盐初始熔体熔体结晶而成。在降压折返过程中,高压岩石中的含水矿物,如多硅白云母、钠云母和帘石等脱水引发部分熔融产生硅酸盐熔体,而碳酸盐熔体则由碳酸盐矿物部分熔融产生。富Na矿物如钠云母脱水熔融产生的包裹体具有相对较高的Na含量,而部分富K的包裹体主要由富K矿物如多硅白云母脱水熔融所产生。近年来随着微区原位技术的飞速发展,从矿物的形态结构到矿物地球化学组成的测定技术有了飞速发展,通过对超高压岩石中包裹的多相晶体的详细研究,可限定大陆碰撞造山过程中部分熔融的组成、时限和形成机制,对大陆深俯冲的构造热演化和折返机制有重要制约。

Anatexis of crustal rocks during continental collision has a great bearing on mineral reactions,isotope resetting,element mobility,and rheological behavior of subducted slabs in continental collision zones. Anatectic melts are often trapped as multiphase crystal inclusions(MC inclusions) enclosed by peritectic minerals in ultrahigh-pressure(UHP)metamorphic terranes during continental collision. These MC inclusions usually exhibit negative crystal shapes with wedgeshaped offshoot structure in hosted mineral. They are composed of a number of silicate,carbonate and sulfate minerals.Many studies suggest that the MC inclusions are the primary crystallization product of former silicate or/and carbonate melts. The silicate melts were derived from dehydration melting of hydrous minerals such as phengite and paragonite in UHP metamorphic rocks,and the carbonate melts were produced by partial melting of subducted carbonate minerals. Some MC inclusions with high Na contents come from dehydration melting of paragonite. In contrast,K-bearing MC inclusions are produced by dehydration melting of K-bearing hydrous minerals like phengite. With the rapid development of in-situ microanalytical techniques,various analytical methods have been used to characterize the morphology,texture,mineral chemistry and trace element composition of MC inclusions. The results provide new insights into the physicochemical properties of anatectic melts in continental subduction zones. This process has great bearing on the tectonothermal evolution of continental subduction zones and the exhumation mechanism of deeply subducted crustal slices.