Provenance and formation age of the Nyingchi Group in the southern Lhasa terrane, Tibetan Plateau: Petrology and zircon U-Pb geochronology

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 gamet-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/ 238 U ages ranging from 2708Ma to 63 Ma, 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.

本文對位于青藏高原拉薩地體東南部林芝巖群中的變質巖進行了巖石學和年代學研究。研究表明,林芝巖群由角閃巖相的變質沉積巖和正片麻巖組成。變質沉積巖主要為含石榴石白云斜長角閃片巖、含石榴石云母石英片巖、含石榴石黑云鉀長片麻巖、大理巖和石英巖等,代表性礦物組合包括石榴石+斜長石+角閃石+石英+黑云母+白云母,或石榴石+斜長石+鉀長石+石英+夕線石+黑云母+白云母。花崗質片麻巖（含二云母片麻巖）的礦物組合是石英+斜長石+鉀長石+黑云母+白云母。鋯石U-Ph年代學分析表明,變質沉積巖中的碎屑鋯石主要為巖漿成因,獲得了2708～63Ma的206Pb/238U 年齡范圍,在～1100Ma和～550Ma出現兩個年代峰值。碎屑鋯石的變質增生邊給出了35Ma的變質年齡。正片麻巖獲得了496Ma的鋯石結晶年齡和1158Ma的繼承年齡。基于上述研究結果、區域對比和相鄰變質巖石中獲得的多期變質年齡,我們認為林芝巖群的原巖很可能形成在早古生代,其沉積物質主要來源于印度陸塊,與特提斯喜馬拉雅早古生代的巖石一起同為印度大陸北緣的沉積蓋層,在環岡瓦納大陸周緣造山過程中被寒武紀花崗巖侵入。在新特提斯洋向北的俯沖過程中,林芝巖群經歷了晚中生代的安第斯型造山作用,在印度與歐亞大陸的俯沖-碰撞過程中,林芝巖群部分地經歷了新生代的變質和巖漿作用再造。本研究證明,林芝巖群并不是傳統上認為的拉薩地體的前寒武紀變質基底,其角閃巖相至麻粒巖相變質作用發生在中、新生代。