钦杭结合带加里东期榴闪岩超高压变质作用研究∗

摘要

对钦杭结合带首次发现的加里东期榴闪岩进行了详细的年代学、矿物学和变质作用研究。结果表明,榴闪岩的变质时代为454±4Ma,经历了早期角闪岩相进变质、中期榴辉岩相峰期变质和晚期角闪岩相退变质三个阶段,具顺时针 P-T-t 轨迹。早期角闪岩相进变质作用阶段的矿物共生组合为角闪石+斜长石+石英,温压条件分别为719℃~795℃和7.56Kpa~8.30Kpa；中期榴辉岩相峰期变质作用阶段的矿物共生组合为石榴子石+绿辉石(后期退变为透辉石+钠长石)±石英,温压条件分别为668℃~821℃和26.42Kpa~33.46Kpa；晚期角闪岩相退变质作用阶段的矿物共生组合为石榴子石+角闪岩+斜长石±石英(即榴闪岩),相应的温压条件分别为611℃~854℃和4.76Kpa~9.30Kpa。结合前人资料推断,区域内加里东期可能经历了榴辉岩相超高压变质→高温麻粒岩相退变质→角闪岩相退变质的演化过程。钦杭结合带首次厘定出加里东期超高压变质作用,表明华夏和扬子两大陆块碰撞拼合发生在加里东期。%By studying on the chronology,mineralogy and metamorphism of Caledonian garnet amphib-olite first discovered in the Qinzhou-Hangzhou joint belt,it is concluded that the garnet amphibolite, which possesses typical clockwise P-T-t path including amphibolite-facies prograde metamorphism in early stage,eclogite-facies(UHP)peak metamorphism in middle stage and amphibolite-facies retrogressive met-amorphism in late stage,yields a metamorphic age of 454±4 Ma.The mineral assemblage of the amphibo-lite-facies prograde metamorphism in early stage is amphibole(Amp)+plagioclase(Pl)+quartz(Qtz)with temperature of 71 9℃~795℃ and pressure of 7.56Kpa~8.30Kpa,the eclogite-facies(UHP)peak meta-morphism in middle stage has the mineral assemblage of garnet(Gt)+ akmite (retrograde to diopside+al-bite)±Qtz with temperature of 668℃~821℃ and pressure of 26.42 Kpa~33.46 Kpa,the amphibolite-fa-cies retrogressive metamorphism in late stage is composed of Gt + Amp + Pl ± Qtz with temperature of 61 1℃~854℃ and pressure of 4.76 Kpa~9.30 Kpa.Combined with the previous studies,it is proposed that the Qinzhou-Hangzhou joint belt had undergone evolutionary process of eclogite-facies(UHP)meta-morphism,granulite-facies retrogressive metamorphism and amphibolite-facies retrogressive metamor-phism in Caledonian.The first discovery of the Caledonian UHP metamorphism in the Qinzhou-Hangzhou joint belt indicates that the collision between Cathaysia and Yangtzs blocks occurred in Caledonian.