Zircon U–Pb and garnet Lu–Hf geochronology of eclogites from the Lhasa Block, Tibet
Zircon U–Pb and garnet Lu–Hf geochronology of eclogites from the Lhasa Block, Tibet
复制标题
DOI：
10.1016/j.lithos.2012.09.011
复制
发表时间：
2012-12
期刊：
影响因子：
3.5
通讯作者：
Hao Cheng;Chao Zhang;J. Vervoort;Honghua Lu;Chao Wang;D. Cao
中科院分区：
文献类型：
--
作者：
Hao Cheng;Chao Zhang;J. Vervoort;Honghua Lu;Chao Wang;D. Cao
青藏高原是一系列冈瓦纳大陆衍生地块（如，羌塘、拉萨和印度）与亚洲大陆的关系。高压榴辉岩的产出与古特提斯俯冲和板块俯冲过程有重要关系。对采自西藏北方拉萨地块吉朗县的石英岩及其寄主榴辉岩进行了岩石学、地球化学和Lu-Hf、U-Pb联合年代学研究。吉朗榴辉岩的成分与弧后盆地中的超俯冲带玄武岩相似（LREE亏损，Nb-Ta负异常，Pb正异常，εNd（t）=~+8）。榴辉岩中柯石英/柯石英假像的观察和榴辉岩的P-T条件估计为3.4-3.8GPa和753-790°C，表明吉朗榴辉岩经历了超高压变质作用。吉朗榴辉岩的Lu-Hf年龄为265.9±1.1Ma（2σ，14点，MSWD=1.8），很可能反映了石榴子石的生长过程，并被解释为反映榴辉岩相的变质作用，这是由于石榴子石中从核到边都存在绿辉石包裹体以及尽管石榴子石中保存完好的绿辉石分带仍存在球形几何效应。榴辉岩中锆石变质边的U-Pb分析表明，榴辉岩中锆石的生长在~ 261 Ma和~ 238 Ma发生过两次。石英岩和榴辉岩岩浆核的锆石~（206）Pb/~（238）U年龄分别为291± 4 Ma（2σ，MSWD=2.2，n=12）和290.6±6.2Ma（2σ，MSWD=1.1，n=11），表明石英岩中的碎屑锆石与榴辉岩中的原岩锆石具有相同的成因。拉萨地块弧后盆地的初始开启时间不晚于约2000年。290Ma;拉萨地块弧后盆地的最终闭合是由冈底斯岛弧与北方拉萨地块的弧-陆碰撞作用所导致的，其时间不早于约1000年。266Ma。
The Tibetan Plateau is the result of several collision events between a series of Gondwana-derived blocks (e.g., Qiangtang, Lhasa and India) and the Asian continent since the early Paleozoic. The occurrence of high-pressure eclogites has a significant bearing on the Paleo-Tethys subduction and plate suturing processes in this area. A quartzite, together with its host eclogite from the Jilang County in the Lhasa Block of northern Tibet, was investigated in terms of petrology, geochemistry, and combined Lu–Hf and U–Pb geochronology. The composition of the Jilang eclogite is similar to supra-subduction-zone basalts found in back-arc basins (LREE depleted, negative Nb–Ta anomalies, positive Pb anomalies, εNd(t)=~+8). The observations of coesite/coesite pseudomorph in the eclogites and the estimated P–T conditions of 3.4–3.8GPa and 753–790°C for the eclogites imply that the Jilang eclogites have undergone ultra-high-pressure metamorphism. The Lu–Hf age of 265.9±1.1Ma (2σ, 14 points, MSWD=1.8) of the Jilang eclogite most likely reflects garnet growth and is interpreted to reflect eclogite–facies metamorphism due to the occurrence of omphacite inclusions from core to rim in garnets and the spherical geometry effect despite the well-preserved prograde zoning in the garnets. U–Pb analyses of the metamorphic rims of zircons from eclogites indicate that two episodes of zircon growth occurred at ~261Ma and ~238Ma. The identical zircon206Pb/238U age of 291±4Ma (2σ, MSWD=2.2, n=12) for the quartzite and 290.6±6.2Ma (2σ, MSWD=1.1, n=11) for the magmatic cores of the eclogite suggest that the detrital zircon in quartzite has the same origin as the protolith zircon in the eclogite. The initial opening of the back-arc basin in the Lhasa Block was no later than ca. 290Ma; the final closure of the back-arc basin in the Lhasa Block, led by the arc–continent collision between the Gangdese island arc and the northern Lhasa Block, was no earlier than ca. 266Ma.