青海南山“金水口岩群”的时代与构造属性研究

Formation Age and Tectonic Attribute of Jinshuikou Group Complex in the Nanshan Area,Qinghai

在青海南山出露一套变质程度达角闪岩相的变沉积岩组合,被认为可与东昆仑造山带中的金水口岩群相对应,其形成时代和构造属性仍存争议。本文对该套组合中的矽线石云母石英片岩和花岗伟晶岩侵入体进行了LA-ICP-MS锆石U-Pb测年和Lu-Hf同位素分析,结果表明矽线石云母石英片岩碎屑锆石U-Pb年龄峰值为779Ma,明显年轻于东昆仑造山带金水口岩群物源区最小年龄峰值(>1.9(Ga);对应的ε_(Hf)(t)值主要为负值(-23.7^-11.8),T_(DM2)年龄为2436~3172Ma,来自于再循环的古元古代—太古代地壳物质,少量碎屑锆石年龄为1744~2819Ma。这些特征与南祁连构造带党河南山和化隆地区元古宙变沉积岩相一致,完全不同于东昆仑造山带中的金水口群。~779Ma碎屑锆石与中祁连和南祁连构造带中新元古代岩浆岩时代相近。花岗伟晶岩形成于439±2Ma,相对应的ε_(Hf)(t)和T_(DM2)分别为-14.2^-7.7Ma和1909~2316Ma,可能为形成于俯冲相关环境下古元古代地壳物质再循环形成的S型花岗岩;775Ma继承性锆石与矽线石云母石英片岩碎屑锆石年龄峰值一致,表明矽线石云母石英片岩为花岗伟晶岩的形成提供了部分物质来源。这些事实表明,青海南山所谓"金水口岩群"的形成时间应不早于779Ma,但不晚于439Ma,其沉积物源区应为~779Ma岩浆岩,在空间上与南祁连构造带党河南山地区以及拉脊山南侧化隆地区的同时期变沉积岩具有相似的沉积物源区和构造属性。

A suit of amphibolites facies meta-sedimentary rocks,outcropping in the Nanshan area of Qinghai,has been previously considered to be comparable with the Jinshuikou Group in the East Kunlun orogenic belt.But its formation age and tectonic attributes are still debated.This study carried out LA-ICP-MS zircon U-Pb dating and Lu-Hf composition analysis for sillimanite-mica-quartz schist and granitic pegmatite pluton.The results show that detrital zircons from sillimanite-mica-quartz schist have a U-Pb peak age of^779 Ma,distinctly younger than the lowest peak value（〉1.9 Ga） of the Jinshuikou Group in the East Kunlun orogenic belt.In addition,corresponding ε_（Hf）（t） values（- 23.7^-11.8） and TDM2 ages（2436 ~3172 Ma） indicate that they were sourced from recycled Paleoproterzoic and Archean crustal materials,with minor zircon ages being 1744~2819Ma.The characteristic is consistent with that of meta-sedimentary rocks in the Danghe-Nanshan and Hualong areas in the south Qilian tectonic belt and very different from the Jingshuikou Group in the east Kunlun tectonic belt.The detrital zircon age of ~779 Ma is similar to are identical to that of the Neoproterozoic magmatism in the central Qilian and south Qilian belts.The granitic pegmatite was formed at 439 ± 2 Ma,accordingly with ε_（Hf）（t） and TDM2 of-14.2^-7.7 and1909~2316Ma,likely indicating it should belong to S-type granite sourced from recycled Paleoprotertozoic crustal material in a subduction-related setting.Inherited zircon with an age of 775 Ma is identical with the peak age of detrital zircons from sillimanite-mica-quartz schist,indicating that the latter may provide partial material for formation of granitic pegmatite.Our study shows that the meta-sedimentary rocks in the Nanshan area was formed no earlier than 779 Ma,but later than 439 Ma,dominated by sediments from magmatic rocks with an ange of ~779 Ma.They possess similar sediment sources and tectonic attributes with contemporary meta-sedimentary rocks in the Danghe-Nanshan and Hualong areas of the South Qilian tectonic belt.