CAJ | 학술논문

在柴北缘高压-超高压变质带的东端都兰地区,高压麻粒岩以透镜体的形式存在于石榴白云母片岩、花岗质片麻岩以及斜长角闪岩中.高压麻粒岩的主体为基性麻粒岩,并含少量中酸性麻粒岩.基性麻粒岩主要由石榴子石、单斜辉石、斜长石和石英等组成,而中酸性麻粒岩峰期矿物组合为:石榴子石+斜长石+钾长石+蓝晶石+石英±单斜辉石.根据显微构造和反应结构特征,主要识别出3期变质作用:①峰期高压麻粒岩相阶段(M_1);②退变质高角闪岩相阶段(M_2);③绿片岩相/低角闪岩相阶段(M_3).选取典型的中酸性麻粒岩样品进行了锆石LA-ICP-MS U-Pb 原位定年分析,获得加权平均年龄为446.9±6.5 Ma,且CL图像显示锆石内部发育石榴子石、单斜辉石、斜长石等矿物包体,反映锆石可能形成在峰期高压麻粒岩相变质条件下.岩石学和年代学结果显示都兰高压麻粒岩和邻近的榴辉岩同时形成于同一俯冲带的不同热构造环境,高压麻粒岩并非榴辉岩热松弛作用形成的,两者具有各自独立的变质演化历史.
재시북연고압-초고압변질대적동단도란지구,고압마립암이투경체적형식존재우석류백운모편암、화강질편마암이급사장각섬암중.고압마립암적주체위기성마립암,병함소량중산성마립암.기성마립암주요유석류자석、단사휘석、사장석화석영등조성,이중산성마립암봉기광물조합위:석류자석+사장석+갑장석+람정석+석영±단사휘석.근거현미구조화반응결구특정,주요식별출3기변질작용:①봉기고압마립암상계단(M_1);②퇴변질고각섬암상계단(M_2);③록편암상/저각섬암상계단(M_3).선취전형적중산성마립암양품진행료고석LA-ICP-MS U-Pb 원위정년분석,획득가권평균년령위446.9±6.5 Ma,차CL도상현시고석내부발육석류자석、단사휘석、사장석등광물포체,반영고석가능형성재봉기고압마립암상변질조건하.암석학화년대학결과현시도란고압마립암화린근적류휘암동시형성우동일부충대적불동열구조배경,고압마립암병비류휘암열송이작용형성적,량자구유각자독립적변질연화역사.
High-pressure granulite occurs as lenses in muscovite schist, granitic gneiss and amphibolite of Dulan area at the eastern end of North Qaidam HP-UHP metamorphism terrane, Northwest China. The high-pressure granulite could mainly be divided into two types: mafic granulite and acid-intermediate granulite. The dominant mafic granulite is composed of garnet, clinopyroxene, plagioclase and quartz, whereas the peak assemblage of intermediate-acid granulite is composed of garnet, plagioclase, K-feldspar, clinopyroxene, kyanite and quartz. Based on microstructure and reaction textures, the authors have recognized three main metamorphic stages from the high-pressure granulite: ① the estimated p-t conditions for the metamprphic peak stage of high-pressure granulite facies (M_1) were 1.4～1.85 GPa and 800～925℃;② retrogression began at the high amphibolite facies stage (M_2) with p-t conditions of 0.8～1.05 GPa and 580～695℃;③ subsequent retrogression occurred at the green-schist/high-amphibolite facies stage (M_3). Combined with cathodoluminescence patterns and mineral inclusions(garnet, clinopyroxene and plagioclase inclusions in zircon), U-Pb dating of zircons revealed that the peak metamorphism age of the high-pressure granulite-facies is 466.9±6.5 Ma. Petrological and chronological studies suggest that the granulite and adjacent eclogite were formed in different thermal environments of the same subduction zone, i.e., the ecolgite was formed in the subduction zone while the high-pressure granulite was formed at the root of the thickened crust above the seduction zone. The high-pressure granulite experienced its unique metamorphic history, and was not associated with the action of thermal relaxation after the eclogite-facies metamorphism recorded in adjacent eclogites.