柴北缘托莫尔日特古元古代变质基底的发现——对早古生代缝合线位置的约束

Discovery of the Paleoproterozoic metamorphic basement in the Tuomoerrite area of North Qaidam:Constraint on the location of Early Paleozoic suture.

柴北缘构造带东部托莫尔日特地区发现了古元古代片麻岩和斜长角闪岩,其构造归属对认识区域构造格局具有重要意义。托莫尔日特地区发育大量早古生代火山-沉积岩、蛇绿岩和侵入岩,片麻岩和其中透镜状斜长角闪岩仅出露于填图区东北侧,其原岩分别为花岗闪长岩和基性岩。用LA-ICP-MS技术测得片麻岩和斜长角闪岩中锆石的U-Pb年龄分别为2413±28 Ma(上交点年龄,n=29,MSWD=2.3)、1966±46 Ma(207 Pb/206 Pb年龄加权平均值,n=4,MSWD=1.7)和1922±22 Ma(上交点年龄,n=29,MSWD=1.3)。结合锆石特征认为,2413±28 Ma可代表片麻岩原岩岩浆的结晶年龄,1966±46 Ma和1922±22 Ma可分别代表片麻岩和斜长角闪岩的变质年龄。片麻岩具有典型埃达克岩高Sr低Y的地球化学特征,MgO、Cr和Ni含量较低,锆石εHf(t)值介于-0.8~+4.0之间,二阶段模式年龄为2704~2965 Ma,全岩εNd(t)值介于+0.8~+1.6之间,显示其可能为加厚下地壳部分熔融成因。古元古代变质岩的岩石组合、地球化学特征、形成时代等均与北侧德令哈杂岩特征一致,表明其属于北侧欧龙布鲁克地块变质基底,而非柴北缘构造带。这一认识约束了柴北缘缝合线的位置,进一步将该地区缝合线限定在赛坝沟蛇绿岩和古元古代变质岩之间很窄的范围内。

The Paleoproterozoic gneiss and amphibolite were recognized in the Tuomoerrite area,eastern part of the North Qaidam ultra-high pressure metamorphic belt.Their tectonic affinities have significance for the understanding of regional tectonic framework.In the Tuomoerrite area,there are abundant early Paleozoic volcano-sedimentary rocks,ophiolitic rocks and intrusives,with minor gneiss and amphibolite lens discovered in the northeast part.The protoliths of gneiss and amphibolite are granidiorite and basic rock respectively.LA-ICP-MS U-Pb dating of zircon from gneiss and amphibolite yields ages of 2413±28 Ma(upper intercept age,n=29,MSWD=2.3),1966±46 Ma(weighted mean 207 Pb/206 Pb age,n=4,MSWD=1.7),and 1922±22 Ma(upper intercept age,n=29,MSWD=1.3)respectively.Combined with cathodoluminescence images of zircon,2413±28 Ma represents the magmatic crystallization age of the protolith of the gneiss.1966±46 Ma and 1922±22 Ma represent the metamorphic ages of the gneiss and amphibolite respectively.The gneiss shows typical adakitic geochemical feature,with high Sr,low Y,MgO,Cr,and Ni.The magmatic zircons haveεHf(t)values between-0.8 and+4.0 and yield two-stage Hf model ages ranging from 2704 Ma to 2965 Ma.They also have positiveεNd(t)ranging from+0.8 to+1.6.These geochemical and isotopic characteristics indicate that the gneisses were likely formed by partial melting of thickened mafic lower-crust.The rock assemblage,geochemical features and ages of these metamorphic rocks are consistent with those of the Delingha Complex to the north,indicating that they belong to the northern Oulongbuluke block rather than the North Qaidam belt.This understanding further constrains that the suture of the North Qaidam belt lies between the Saibagou ophiolite and the newly recognized Paleoproterozoic metamorphic rocks.