青藏高原拉萨地体南部林芝岩群的物质来源与形成年代：岩石学与锆石U-Pb年代学
Provenance and formation age of the Nyingchi Group in the southern Lhasa terrane, Tibetan Plateau: Petrology and zircon U-Pb geochronology
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摘要: 本文对位于青藏高原拉萨地体东南部林芝岩群中的变质岩进行了岩石学和年代学研究。研究表明，林芝岩群由角闪岩相的变质沉积岩和正片麻岩组成。变质沉积岩主要为含石榴石白云斜长角闪片岩、含石榴石云母石英片岩、含石榴石黑云钾长片麻岩、大理岩和石英岩等，代表性矿物组合包括石榴石+斜长石+角闪石+石英+黑云母+白云母，或石榴石+斜长石+钾长石+石英+夕线石+黑云母+白云母。花岗质片麻岩（含二云母片麻岩）的矿物组合是石英+斜长石+钾长石+黑云母+白云母。锆石U-Pb年代学分析表明，变质沉积岩中的碎屑锆石主要为岩浆成因，获得了2708~63Ma的206Pb/238U年龄范围，在~1100Ma和~550Ma出现两个年代峰值。碎屑锆石的变质增生边给出了35Ma的变质年龄。正片麻岩获得了496Ma的锆石结晶年龄和1158Ma的继承年龄。基于上述研究结果、区域对比和相邻变质岩石中获得的多期变质年龄，我们认为林芝岩群的原岩很可能形成在早古生代，其沉积物质主要来源于印度陆块，与特提斯喜马拉雅早古生代的岩石一起同为印度大陆北缘的沉积盖层，在环冈瓦纳大陆周缘造山过程中被寒武纪花岗岩侵入。在新特提斯洋向北的俯冲过程中，林芝岩群经历了晚中生代的安第斯型造山作用，在印度与欧亚大陆的俯冲-碰撞过程中，林芝岩群部分地经历了新生代的变质和岩浆作用再造。本研究证明，林芝岩群并不是传统上认为的拉萨地体的前寒武纪变质基底，其角闪岩相至麻粒岩相变质作用发生在中、新生代。Abstract: Petrology and geochronology of the Nyingchi Group located in the southeastern Lhasa terrane, Tibetan Plateau, were reported in this paper. The Nyingchi Group consists of the metasedimentary rocks and orthogneiss which were metamorphosed under an amphibolite-facies condition. The metasedimentary rocks include garnet-muscovite-plagioclase-hornblende schist, garnet-mica-quartz schist, garnet-biotite-k-feldspar gneiss, marble and quartzite. The typical mineral assemblages are garnet+plagioclase+hornblende+quartz+biotite+muscovite and garnet+plagioclase+k-feldspar+quartz+silimanite+biotite+muscovite. The orthogneiss has a mineral assemblage of quartz+plagioclase+k-feldspar+biotite+muscovite. Detrital zircons from the metasedimentary rocks are typical for magmatic origin, and give widely variable 206Pb/238U ages ranging from 2708Ma to 63Ma, of which two major age populations are~1100Ma and~550Ma. The overgrowth rims of the detrital zircons give metamorphic ages around ~35Ma.Magmatic zircons from the orthogneiss yield a crystallization age around ~496Ma and an inherited age around ~1158Ma. Based on these results, together with the regional comparison and multi-stage metamorphic ages obtained from adjacent metamorphic rocks, it is concluded that the Nyingchi Group has been formed in the Early Paleozoic, and derived from the Indian provenance. This group together with the Tethyan-Himalaya presented as Early Paleozoic sedimentary cover located on the northern margin of the Indian continent and then intruded by Cambrian granites formed during the circum-Gondwana orogeny. The Nyingchi Group initially experienced an Andean-type orogenesis through the northward subduction of the Neo-Tethys oceanic lithosphere during Late Mesozoic time and then the collision between the Indian and Eurasia in the Cenozoic. This work demonstrates that the Nyingchi Group was metamorphosed at Mesozoic and Cenozoic, not Precambrian metamorphic basement as previously considered.
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Key words:
- Nyingchi Group /
- Sediment province /
- Metamorphic evolution /
- Geochronology /
- Lhasa terrane /
- Tibetan Plateau
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