柴达木盆地东北部拉合根地区中生代火山岩的发现及其地质意义:来自地球化学与锆石年代学证据

Discovery of the Mesozoic Volcanic Rocks in Lahegen Area of the Eastern North Qaidam Tectonic Belt,and Its Geological Implications:Evidence from Geochemistry and Zircon Geochronology

柴达木盆地北缘东部地区哇洪山—温泉断裂与宗务隆山南缘断裂之间地带出露系列火山岩,由于其中一部分缺乏精确的地质年代学制约,导致其地层归属难以确定,因此该地区大地构造单元划分尚存争议。本文基于详细的野外调查,在柴达木盆地北缘东部拉合根地区新识别出一套中生代火山岩,岩石类型为流纹岩、中酸性角砾熔岩及英安岩。对代表性流纹岩样品进行LA-ICP-MS锆石U-Pb定年,获得^(206)Pb/^(238)U加权平均年龄为(248.3±1.2)Ma(MSWD=0.30,n=20),表明岩石形成于早三叠世。全岩元素组成分析结果显示,该火山岩为准铝质高钾钙碱性岩石,轻重稀土元素分馏明显,轻稀土元素富集、重稀土元素亏损,δEu平均值为0.63,显示为弱负异常;微量元素呈现Rb、Ba等大离子亲石元素和Th、U、Hf和Zr等高场强元素相对富集,Sr强烈亏损,Ta、Nb相对亏损的特征,具有典型岛弧火山岩地球化学特征。锆石εHf(t)值分布范围为-19.62~-3.07,对应的t_(DM2)为2421~1639 Ma,表明岩石来自古元古代地壳物质的部分熔融。综合岩石年代学和地球化学特征,认为该火山岩为早三叠世阿尼玛卿洋向北俯冲于东昆仑地块,导致地壳基底(如达肯大坂岩群)发生深熔作用的产物。通过区域对比,提出该火山岩与鄂拉山岩浆弧洪水川组火山岩的地球化学性质较为相似,因此将其厘定为下三叠统洪水川组火山岩。该火山岩的发现指示拉合根地区大地构造单元属于东昆仑造山带鄂拉山岩浆弧。研究成果为柴达木盆地北缘东部地区大地构造划分提供了新的数据资料支撑。

The stratigraphic attribution of volcanic rocks in the eastern area of the northern margin of the Qaidam Basin is difficult to determine due to a lack of geochronological data.These rocks are located between the Wa-hongshan-Wenquan Fault and the southern margin of the Zongwulong Mountain Fault.This also leads to signifi-cant disputes over the division of geotectonic units in this area,as no Mesozoic volcanic rocks have been found here before.In this study,a set of Mesozoic volcanic rocks consisting of rhyolite,intermediate-acid brecciate lava and dacite was newly confirmed in the Lahegen area.Zircon U-Pb dating was performed on the rhyolite samples using the LA-ICP-MS technique with weighted mean age of 248.3±1.2 Ma,indicating an Early Trias-sic age.The geochemical data show that the volcanic rock is a metaluminous high-potassium calc-alkaline rock with clear fractionation of light rare earth elements,enrichment of light rare earth elements and depletion of heavy rare earth elements.The meanδEu value is 0.63,indicating a weak negative anomaly.The rocks are relatively enriched in large ion lithophile elements(e.g.,Rb and Ba)and high field strength elements(e.g.,Th,U,Pb,and Zr),and are significantly depleted in Sr,Ta,and Nb.These are typical geochemical charac-teristics of continental island arc volcanic rocks.Zircon Hf isotope compositions showεHf(t)values of~-19.62 to-3.07 and t_(DM2)values of~1,639 Ma to 2,421 Ma,indicating that the volcanic rocks are derived from the partial melting of Paleoproterozoic crustal materials.Based on geochronology and geochemical charac-teristics,the volcanic rocks are attributed to the anatexis of crustal materials(the Darken Daban Group)caused by the northward subduction of the Anemaqen Ocean to the East Kunlun block in the Early Triassic.The vol-canic rocks of the Hongshuichuan Formation are similar to those in the magma arc of Olashan,according to the regional correlation.Therefore,these rocks are identified as Early Triassic Hongshuichuan Formation volcanic rocks.The confirmation of the studied volcanic rocks indicates that the Lahegen area has a close tectonic affini-ty with the Olashan magmatic arc of the East Kunlun orogenic belt.This study provides new evidence for the tectonic division of the eastern area of the northern margin of Qaidam Basin.