喜马拉雅东部三叠系涅如组地球化学和碎屑锆石U-Pb年代学特征及构造意义

GEOCHEMISTRY AND DETRITAL ZIRCON U-Pb GEOCHRONOLOGY OF THE TRIASSIC NIERU FORMATION IN THE EASTERN HIMALAYAS AND ITS TECTONIC IMPLICATIONS

喜马拉雅中—新生代地层沉积于印度被动大陆边缘,三叠系沉积物物源分析是冈瓦纳大陆裂解和古地理研究的重要载体。特提斯喜马拉雅东段上三叠统涅如组碎屑岩的物质来源和沉积背景具有较大的争议,本次研究采集隆子县斗玉乡晚三叠世粉砂质板岩进行碎屑锆石U-Pb年代学和全岩主微量元素地球化学分析。涅如组碎屑锆石年龄具有两个明显的峰值:206-224 Ma(峰值213 Ma)和462-567 Ma(峰值512 Ma),表明涅如组沉积时代为晚三叠世诺利期。涅如组w(SiO_(2))含量较低(平均值为57.7%),w(Al_(2)O_(3))值较高(17.2%),轻稀土元素富集,重稀土元素亏损,轻稀土/重稀土元素平均值为8.1。高场强元素中Nb,Ta,Ti亏损,Zr,Hf,Th,U富集;大离子亲石元素中Ba,Sr亏损,Rb和K富集。综合分析认为源区岩石以酸性岩浆岩为主。本次研究认为上三叠统涅如组浊流相沉积形成于裂谷环境,代表新特提斯洋的打开;标志着拉萨板块从冈瓦纳大陆北缘裂解,并开始向北漂移。

Mesozoic-Cenozoic strata occur on the margin of the passive Indian continent and the provenance analysis of the Triassic sediments is an important carrier for the study of the breakup and paleogeography of Gondwana Continent.The material source and sedimentary background of the clastic rocks of the Upper Triassic Nieru Formation in the eastern Tethyan Himalaya have been a controversial topic.In this study,the late Triassic silty slate from Douyu Township,Longzi County is collected for detrital zircon U-Pb geochronology analysis and geochemical analysis.Detrital zircon U-Pb geochronology of Nieru Formation yields two obvious peaks:206-224 Ma(peak 213 Ma)and 462-567 Ma(peak 512 Ma),indicating that the Nieru Formation was deposited during the Late Triassic Norian Period.The content of w(SiO_(2))in the Nieru Formation is low(57.7%on average),the value of w(Al_(2)O_(3))is high(17.2%).The LREE is enriched and the HREE is depleted with the average LREE/HREE of 8.1.For high field strength elements,Nb,Ta,Ti are depleted and Zr,Hf,Th,U are enriched.For large ion lithophile elements,Ba and Sr are depleted and Rb and K are enriched.Comprehensive analysis shows that the Nieru Formation source rocks are mainly acidic magmatic rocks.It is considered that the turbidite facies of the Upper Triassic Neru Formation was formed in a rift environment,indication of and corresponding to the opening of the New Tethys Ocean,which marks the breakup of the Lhasa plate from the northern margin of Gondwana and the initiation of northward drift of Lhasa plate.