Reworked Precambrian metamorphic basement of the Lhasa terrane, southern Tibet: Zircon/titanite U–Pb geochronology, Hf isotope and geochemistry
Reworked Precambrian metamorphic basement of the Lhasa terrane, southern Tibet: Zircon/titanite U–Pb geochronology, Hf isotope and geochemistry
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DOI：
10.1016/j.precamres.2019.105496
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发表时间：
2020-01
影响因子：
3.8
通讯作者：
Xin Dong;Zeming Zhang;Y. Niu;Z. Tian;L. Zhang
中科院分区：
文献类型：
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作者：
Xin Dong;Zeming Zhang;Y. Niu;Z. Tian;L. Zhang
由于暴露物的缺乏，对拉萨地体前寒武纪基底的形成和演化仍然知之甚少。地球化学资料表明，黑云母-角闪岩片麻岩和黑云母片麻岩的原岩分别为花岗闪长岩和花岗岩。从这些正长岩中继承的岩浆锆石核给出了1520-1506 Ma的原岩结晶年龄，而过度生长的锆石边缘给出了605-590 Ma的变质年龄。中元古代花岗岩体岩εNd(t)值为−3.6 ~ +0.1，锆石芯εHf(t)值为−4.5 ~ +2.6，tdm2age分别为2.35 ~ 2.05 Ga和2.54 ~ 2.10 Ga，表明它们来源于古元古代地壳物质的部分熔融。花岗质岩石也是拉萨地体古生界中元古代碎屑锆石的局部物源。钛矿锆石u - pb年龄进一步表明，东九正长岩在~26 Ma经历了较近的变质作用。矿物组合和热压计算表明渐新世变质作用发生在中压角闪岩相条件下（5.4-7.2 kbar, 691-765 °C）。我们认为东九片麻岩代表拉萨地体的前寒武纪变质基底，但由于碰撞后印度-亚洲的持续辐合，在SLT中受到了变质作用的强烈改造。
Due to the paucity of exposure, the formation and evolution of the Precambrian basement of the Lhasa terrane remain poorly known. Here we report zircon and titanitein situU–Pb ages, bulk-rock geochemical and zircon Hf isotopic data on the orthogneisses from the Dongjiu area of the southern Lhasa subterrane (SLT), southern Tibet. Geochemical data suggest that the protoliths of the biotite-amphibole gneiss and biotite gneiss are granodiorite and granite, respectively. Inherited magmatic zircon cores from these orthogneisses give protolith crystalline ages of 1520–1506 Ma, whereas the overgrown zircon rims give metamorphic ages of 605–590 Ma. The Mesoproterozoic granitic rocks have bulk-rock εNd(t) values of −3.6 to +0.1 and zircon core εHf(t) values of −4.5 to +2.6, which give similar TDM2ages of 2.35–2.05 Ga and 2.54–2.10 Ga respectively, suggesting their derivation from partial melting of Paleoproterozoic crustal material. The granitic rocks are also local provenance for the Mesoproterozoic detrital zircons in the Paleozoic strata in the Lhasa terrane. Titanitein situU–Pb ages further indicate that the Dongjiu orthogneiss experienced more recent metamorphism at ~26 Ma. The mineral assemblage and thermobarometry calculations indicate that the Oligocene metamorphism occurred under medium-pressure (MP) amphibolite-facies conditions (5.4–7.2 kbar, 691–765 °C). We propose that the Dongjiu gneisses represent the Precambrian metamorphic basement of the Lhasa terrane, but have been intensively reworked by metamorphism in the SLT in response to the continued India-Asia convergence since the collision.