北阿尔金地区古元古代ca.2.0Ga岩浆-变质事件

Paleoproterozoic ca. 2.0Ga magmatic-metamorphic event in the northern Altyn Tagh area

中国有三个主要的克拉通,分别是华北、华南和塔里木,它们在显生宙经造山过程聚集到一起。塔里木克拉通位于中国的西北部,面积超过60万平方千米,其北侧为中亚造山带,南侧为西昆仑造山带和阿尔金造山带。塔里木克拉通的前寒武纪岩石主要出露在其南北两侧边缘,包括库鲁克塔格、敦煌、阿尔金、铁克里克和阿克苏地块,它们记录了塔里木克拉通早期的构造演化。北阿尔金地块的阿克塔什塔格地区位于塔里木克拉通的东南边缘。该地区最老的岩石被称为米兰群或阿克塔什塔格杂岩。主要岩石包括太古宙的TTG岩石和表壳岩,以及古元古代的片麻状花岗岩,另有少量变质基性岩呈包体状出露在强变形的长英质侵入体中。本文对该区闪长质片麻岩开展了锆石SHRIMP U-Pb定年,同时还对变质基性岩进行了锆石LA-ICP-MS U-Pb测年和地球化学分析,目的是要约束北阿尔金地区古元古代的岩浆-变质事件。闪长质片麻岩的结晶年龄为2.04～2.03Ga,它们形成于岛弧环境。地球化学分析表明,变质基性岩的原岩是拉斑玄武岩。它们有类似于E-MORB的平坦的稀土配分模式,Nb、Ta、Zr、Hf不亏损,说明它们形成于大洋板内环境。在变质基性岩中还识别出两期变质锆石,其中2.05～2.01Ga的早期锆石代表了麻粒岩相的变质作用,而1.98～1.96Ga的晚期锆石可能与角闪岩相的退变质作用有关。无论该区岩浆作用还是变质作用都与约2.0Ga发生的俯冲增生造山事件有关。

China consists of three major cratonic blocks： North China,South China and Tarim cratons,which were amalgamated during Phanerozoic orogenic processes. The Tarim Craton is located in northwestern China with an area of more than 600,000km-2 and is bounded by the Central Asian Orogenic Belt to the north and the Western Kunlun Orogenic Belt and the Altyn Orogenic Belt to the south. Precambrian rocks in the Tarim Craton mainly occur at its northern and southern margins,including the Kuruktag Block,the Dunhuang Block,the Altyn Tagh Block,the Tiekelike Block and the Aksu Block,which recorded its early tectonic evolution. The Aketashitage area in the northern Altyn Tagh Block is located at the southeastern margin of the Tarim Craton. The oldest rocks in this area are known as the Milian Group or the Aketashitage Complex. The dominant rocks are Neoarchean TTGs and supracrustal rocks and Paleoproterozoic gneissic granitoids,whereas meta-mafic rocks occur only as minor enclaves in the strongly deformed felsic plutonic rocks. In this study we present zircon SHRIMP U-Pb age studying on the dioritic gneisses and zircon LA-ICP-MS U-Pb age and wholerock elemental composition studing on the meta-mafic rocks in the Aketashitage area in order to constrain the Paleoproterozoic magmatic-metamorphic events of the northern Altyn Tagh Block. The dioritic gneisses have crystallization ages of 2. 04 - 2. 03 Ga,which were suggested to be formed in an arc setting. Geochemical analyses indicate that the protoliths of meta-mafic rocks are tholeiite.The meta-mafic rocks have nearly flat REE patterns that bear a remarkably close resemblance to E-MORB,and no distinct depletions in Nb,Ta,Zr and Hf,suggesting that they were formed within an oceanic plate. Two episodes of metamorphic zircons are recognized in the meta-mafic rocks. The older one has ages of 2. 05 - 2. 01 Ga,representing granulite-facies metamorphism,whereas the younger one with ages of 1. 98 - 1. 96 Ga may relate to retrograde amphibolite-facies metamorphic overgrowth. Both magmatism and metamorphism are considered to result from the ca. 2. 0Ga subducted-accretionary orogenic event.