大别超高压变质过程中流体作用的微量元素效应及其地球化学意义

Trace element effect of fluid-metasomatism during the HP-UHP metamorphism in the Dabie Mountains and its geochemical significance

对大别地区角闪岩相-麻粒岩相和榴辉岩相岩石进行了较系统的地球化学研究，探讨了超高压俯冲变质及折返过程中元素的再分配行为，并就其对变质过程中的部分熔融作用和流体交代作用的指示进行了分析。研究表明，大别榴辉岩原岩在其俯冲过程中发生了部分熔融作用，并在其早期脱水和后期退变质作用过程中与变质流体相之间产生了交换。脱水作用使榴辉岩 Nb/Ta比值升高，但后期的退变质作用使榴辉岩与其寄主围岩间的 Nb/Ta比值相对均一化。角闪岩相-麻粒岩相岩石以相对富集 LREE和 LILE为特征，并具有较大的 Nb/Ta比值变化范围，但其 Nb/Ta比值的变化规律与榴辉岩相岩石区别明显，反映了大别地区超高压变质岩类与其他变质相岩石具不同的变质流体交代过程和变质机制。

A comprehensive geochemical study is made on the meta-mafic rocks of metamorphic facies range of amphibolite, granulite to eclogite and their felsic host rocks from the Dabie Mountains, which aims to understand elemental redistributions of the Dabie rocks during the subduction and exhumation of the continental crust, and to evaluate their potential indications referring fluid-metasomatism and partial melting during the processes. It is evident that some partial melting occurred when the protolithes of eclogites subducted into the HP-UHP depths, and the interactions between the UP-UHP meta-rocks and fluids, which were originated from the dehydration and retrogress metamorphism during the subduction and exhumation separately, initiated geochemical changes of the HPM-UHPM rocks. The fluid-metasomatism from the dehydration of the subducted rocks caused an obviously increasing and decreasing in the Nb/Ta ratios of eclogites and their host rocks respectively, but these changes, however, were buffered by the fluids resulted from the retrogress metamorphism of eclogites during their exhumation. Rocks of amphibolite-granulite facies in the terrane are characterized by relative enrichments in LREE and LILE. Though highly variable Nb/Ta ratios of these rocks are also recognized, their distinct regulations of the diversification in Nb/Ta ratios from those of the HP-UHP metamorphic rocks imply that these relative lower-grade metamorphic rocks had experienced some quite dissimilar processes both in metamorphism and fluid-metasomatism.