高压麻粒岩相变质作用及深熔作用：以柴北缘都兰地区为例

Research on HP granulite-facies metamorphism and anatexis: A case study of Dulan area in the North Qaidam Mountains

在一些俯冲/碰撞造山带中,高压麻粒岩相变质作用通常伴随着广泛的深熔作用。本文以柴北缘超高压变质带都兰地区的基性高压麻粒岩和浅色体为研究对象,在详细的野外观察的基础上,结合岩相学和年代学等研究方法,探讨高压麻粒岩相变质作用与深熔作用的关系及形成机制。从野外关系来看,浅色体主要呈层状、似脉状、补丁状或网络状分布在暗色的基性高压麻粒岩(残留体,residuumormelanosome)中,或与基性高压麻粒岩在露头上互层产出,并显示出混合岩的特征。基性高压麻粒岩主要由石榴子石、单斜辉石、斜长石和石英等矿物组成,在不同样品中还可含有少量蓝晶石、角闪石、金红石、黝帘石/斜黝帘石、黑云母、方柱石、绿泥石;浅色体主要由斜长石、钾长石和石英等矿物组成,一些样品中也含有少量的石榴子石和蓝晶石,与典型的长英质高压麻粒岩的矿物组合特征较为相似。锆石成因年代学结果显示浅色体中既发育深熔锆石,也有变质锆石生长,但两种锆石给出的年龄结果基本一致,其加权平均年龄为434±2Ma(MSWD=1.1),与前人获得的高压麻粒岩相变质作用和深熔作用时代基本一致。因此,综合野外关系、岩相学、地球化学特征及年代学结果,我们推测高压麻粒岩相变质作用及深熔作用可能形成于同一动力学过程,即在俯冲带的上盘环境,(变)基性岩石中的含水矿物(如角闪石、帘石或云母类矿物等)脱水熔融形成高Sr/Y熔体,而基性高压麻粒岩为残留体。

In some typical subductional /collisional orogens,high-pressure granulite facies metamorphism often accompanies extensive anatexis. This article focuses on mafic HP granulite and leucosome in the Dulan area,the North Qaidam Mountains.0n the basis ofdetailed field observation,in combination with petrographic and geochronologicaldata,we will discuss the relationship between HP granulite-facies metamorphism and anatexis. In the field,the leucosome lithologies appear as veins,layers small leucocratic patches injected into the HP mafic granulite,and locally contain lenses,blocks or layers of mafic granulite,which show a migmatization and are considered as the principal evidence for partial melting. The mafic HP granulite primarily consists of garnet, clinopyroxene,plagioclase and quartz,withdistinct amounts of kyanite,amphibole,rutile,zoisite/clinozoisite,biotite,scapolite and chlorite. The leucosome is mainly composed of K-feldspar,plagioclase and quartz,with minor kyanite and garnet in some samples,which is analogous to typical felsic HP granulite. Cathoduluminescence （ CL） image of zircons in leucosome exhibits both anatectic and metamorphic origin,which yielded similar weighted mean236 Pb /238 U ages of434 ±2Ma （ MSWD = 1. 1） ,analogous to the existing geochronological data. Therefore,the combination of field relationship,petrography,geochemistry and geochronology defined identicaldynamic process for HP granulite-facies metamorphism and anatexis,suggesting that the high Sr/Y meltwas probably derived from partial melting of hydrous minerals （ e. g. amphibole,zoisite or mica） in the （ meta-） mafic rocks,leaving the mafic HP granulite as residuum.