柴北缘都兰地区片麻岩的起源及形成时代:锆石U-Pb年代学、REE和Hf同位素的证据

Provenance and age of gneisses in the Dulan area,North Qaidam UHP metamorphic belt:Evidence from zircon U-Pb geochronology,REE and Hf isotopic analyses

在柴北缘HP-UHP变质带东端的都兰地区,大量正、副片麻岩中包裹有少量榴辉岩和超基性岩。这些片麻岩普遍显示出角闪岩相变质作用特征,但是野外关系和少量副片麻岩中的柯石英包体显示这些片麻岩与榴辉岩一起经历了超高压变质作用。本文通过两个片麻岩样品的锆石U-Pb年代学测定,结合稀土元素和Hf同位素分析,来确定这些片麻岩的原岩起源和变质作用时代。锆石的阴极发光图像分析、稀土元素特征、U-Pb定年结果显示锆石具有明显的核、幔、边结构;岩浆成因的锆石核部获得的不一致线上交点年龄为2322～2365Ma,反映了源区经历过一期古元古代早期的岩浆热事件;岩浆和变质成因的幔部给出的不一致上交点年龄为1830～1837Ma,反映了片麻岩在古元古代晚期经历的变质和深熔作用。锆石幔部获得的不一致线下交点年龄451～465Ma与边部3个近于谐和点的加权平均年龄466Ma在误差范围内基本一致,结合锆石增生的边部较为平坦的重稀土配分型式、低的Th/U值,代表片麻岩发生榴辉岩相变质作用的时代。古元古代早期和晚期年龄的锆石大部分具有负的εHf(t)值,锆石的二阶段Hf模式年龄主要分布于2725～2835Ma之间,说明片麻岩的继承性碎屑锆石源区存在太古代的地壳生长事件,并在古元古代经历过两期再循环和改造事件。都兰地区的部分片麻岩具有与其北侧欧龙布鲁克微陆块中变质基底相似的早期演化历史,但前者在早古生代与所夹的榴辉岩的原岩一起发生了深俯冲作用。

In the eastern part of the North Qaidam HP/UHP metamorphic belt,near Dulan,orthogneiss and paragneiss enclose minor eclogites and ultramafic rocks.Although showing a common amphibolite-facies metamorphism overprint,field relationship and coesite inclusion in gneiss indicates that both gneiss and eclogite experienced UHP metamorphism.In this study,we link U-Pb age,trace element analyses and Hf isotope composition to unravel the provenance and possible metamorphic age of two gneisses.The combination of cathodoluminescence(CL)image,trace element analyses and U-Pb age reveals that the zircon retains distinct core,mantle and rim.Zircon cores yield ages of 2322~2365Ma,suggesting an Early Paleoproterozoic magmatic event in the source area.Zircon mantle gives upper intercept ages of 1830~1837Ma,indicating that the gneisses experienced metamorphism and anatexis in Late Paleoproterozoic time.Zircon rim gives lower intercept ages of 451~465Ma consistent with Ordovician eclogite facies metamorphism previously determined from eclogite and paragneiss.The negative εHf(t)values of both Early Paleoproterozic detrital zircon cores and Late Paleoproterozic metamorphic/anatectic zircon mantles and corresponding Archean Hf model ages(2.73~2.84Ga)imply a crust growth at Neoarchean and experienced two episodes of reworking events during Paleoproterozoic.This is similar to the Archean-Paleoproterozoic history of crust evolution decribed in the Oulongbuluke microcontinent to the north of the Dulan area,but gneisses from Dulan area experienced deep subduction together with enclosed eclogite during Early Paleozoic era.