敦煌造山带长山子地区变质演化及年代学研究

Metamorphic evolution and geochronology of the Changshanzi area,Dunhuang Orogenic Belt,Northwest China

长山子地区位于敦煌造山带东北部,瓜州南部约100km处。该区主要出露一套中-高级变质表壳岩,主要岩石类型有长英质片麻岩、变泥质麻粒岩、高压基性麻粒岩、斜长角闪片麻岩。高压基性麻粒岩岩块、斜长角闪片麻岩岩块以构造透镜体或布丁(长度为0. 5~15m)的形式,被夹持于长英质片麻岩、变泥质麻粒岩组成的基质之中,呈现典型"基质夹岩块"的混杂带特征。高压基性麻粒岩、斜长角闪片麻岩、泥质麻粒岩中,普遍保留了二至三个阶段的变质矿物组合。进变质阶段矿物组合(M1)为石榴子石变斑晶中的细小矿物包裹体,变质高峰期矿物组合(M2)为石榴子石变斑晶和基质矿物,退变质阶段矿物组合(M3)主要为围绕石榴子石变斑晶边部发育的"白眼圈"状后成合晶。本区各类变质岩石均记录了顺时针型变质作用P-T轨迹,系典型俯冲-碰撞造山带变质作用特征。退变质阶段P-T轨迹属于西阿尔卑斯型,说明变质岩折返速率较快。变质高峰期(M2)属于中压变质相系,P-T条件分别为790~870℃/1.29~1.37GPa(高压基性麻粒岩)、680~685℃/0.89~0.97GPa(斜长角闪片麻岩)、860~880℃/0.90~1.14GPa(变泥质麻粒岩),它们之间存在大的差异。这说明,它们是形成于同一俯冲隧道内不同深度的变质岩石,在构造折返阶段才混杂在一起形成构造混杂岩。二次离子质谱(SIMS)锆石U-Pb定年表明,长山子地区变质杂岩记录了早泥盆世的俯冲事件(419~417Ma)。

Medium-to high-grade metamorphic supracrustal rocks are exposed in the Changshanzi area,northeastern Paleozoic Dunhuang Orogenic Belt,ca. 100 km south of Guazhou City,Gansu Province,Northwest China. The Changshanzi metamorphic complex mainly consists of felsic gneiss,metapelitic granulite,amphibolite and high-pressure mafic granulite. Amphibolite and highpressure mafic granulite occur as lenses or puddings（ 0. 5 ～ 15 m in length） and are intercalated with the matrix（ felsic gneiss,metapelitic granulite）,and show typical block-in-matrix structure. Therefore,the Changshanzi area is in fact a tectonic mélange. Two to three stages of metamorphic mineral assemblages are ubiquitously found in the high-pressure mafic granulite,amphibolite and metapelitic granulite. The prograde assemblage（ M1） is the inclusion assemblage preserved in the garnet porphyroblast, the metamorphic peak（ M2） assemblage includes the garnet and the matrix minerals,and the retrograde assemblage（ M3） consists of＂white-eye socket＂symplectite surrounding the garnet porphyroblast which were formed during tectonic exhumation. Clockwise metamorphic P-T paths were retrieved from the high-pressure mafic granulite,amphibolite and metapelitic granulite,which are generally believed to have been formed in subduction,collision and later tectonic exhumation. The retrograde P-T paths indicate nearly isothermal decompression and thus are ascribed to the western Alpine type,suggesting fast uplift in the tectonic exhumation stage. The peak metamorphism（ M2） belongs to the medium P/T series but the peak P-T conditions show large gaps. For high-pressure mafic granulite,the M2 P-T conditions were determined to be 790 ～ 870℃/1. 29 ～ 1. 37 GPa. For amphibolite,they are 680 ～ 685℃/0. 89 ～0. 97 GPa. For metapelitic granulite,the M2 P-T conditions are estimated to be 860 ～ 880℃/0. 90 ～ 1. 14 GPa. The data suggest that these rocks were metamorphosed at different depths in the same one subduction channel,and later they amalgamated at shallower level in the uplift stage. SIMS U-Pb dating of metamorphic zircon suggests that the metamorphic event of the Changshanzi area possibly occurred at 419 ～ 417 Ma,i. e.,Early Devonian.