柴北缘HP-UHP变质带多期构造热事件的再厘定:鱼卡地区高压变泥质岩锆石和独居石U-Pb定年

柴北缘高压-超高压变质带西段鱼卡地区变泥质岩中夹有榴辉岩透镜体,已有的研究显示变泥质岩的变质程度也达到了榴辉岩相,并与榴辉岩一起经历了高压-超高压变质作用,是柴北缘曾经历早古生代大陆深俯冲作用的直接证据,也是研究柴北缘大陆深俯冲过程重要的岩石"探针"。本文选择柴北缘西段鱼卡超高压变质单元中的3件蓝晶石榴白云母石英片岩HP变泥质岩样品分别进行了SHRIMP、LA-ICP-MS锆石和原位独居石U-Pb定年。样品Q06-1-2的锆石给出了920±18Ma(MSWD=1.3)的加权平均年龄,其CL图像特征和极低的Th/U比显示其为变质年龄,代表了与罗迪尼亚超大陆碰撞拼合相关的变质事件。样品A03-11-2.2的锆石给出了450±7Ma(MSWD=0.2)的年龄,认为其代表变泥质岩的榴辉岩相变质年龄。样品A03-14-11的薄片原位独居石定年给出了439±8Ma(MSWD=0.072)的加权平均年龄,结合岩相学观察,认为其可能为榴辉岩相峰期之后的早期退变质年龄。这些资料显示柴北缘鱼卡地区早古生代大陆深俯冲的时限为440~450Ma。结合已有研究资料,鱼卡高压变泥质岩记录了新元古代早期和早古生代两期变质事件,进一步证明了柴北缘地区经历了格林威尔期和早古生代两期造山事件

Redefinition of the polyphase tectonothermal events of the North Qaidaim HP/UHP metamorphic terrane: Evidence from zircon and monazite U-Pb geochronology of the Yuka HP metapelites

Eclogitic metapelites were recognized as country rocks of eclogites in Yuka area of the North Qaidam HP-UHP metamorphic belt. The metapelites enclose lenses of eclogites, and are locally intercalated with eclogites. The eclogites and eclogitic metapelites are the direct evidence that the North Qaidam underwent continental deep subduction process and are considered as the important lithoprobes to explore the earth''s interior process. In this contribution, we investigate three high pressure metapelites samples from the Yuka UHP metamorphic unit, which are suitable for U-Pb analyses of zircon and monazite. SHRIMP U-Pb dating of zircon from Sample Q06-1-2 provides a weighted mean age of 920±18Ma (MSWD=1.3), which was interpreted as metamorphic age associated with the Rodinia supercontinent assembly event. LA-ICP-MS dating of zircon from Sample A03-11-2.2 yields an age of 450±7Ma (MSWD=0.2) from CL-gray outer rims, which is interpreted as timing of eclogite-facies metamorphism. In situ monazite U-Pb dating from Sample A03-14-11 yields a weighted mean age of 439±8Ma (MSWD=0.072). Based on the petrographic characteristics, we postulated ca. 440Ma as early retrograde age, following peak eclogite facies metamorphism. These data indicate that the timing of continental deep subduction in Yuka area could be restricted to 440~450Ma. Combining all these data suggest that the North Qaidam Mountains experienced both Grenville-age orogeny and Caledonian stage orogeny, and the high pressure metapelites record a complex tectono-metamorphic evolution that involved Neoproterozoic and Early Paleozoic orogenies.