青藏高原拉萨地块中的大洋俯冲型榴辉岩:古特提斯洋盆的残留?

Oceanic subduction-type eclogite in the Lhasa block, Tibet, China:Remains of the Paleo-Tethys ocean basin?

在西藏拉萨地块中新发现的榴辉岩为厚层状、块状,岩石新鲜,带宽500～600m,呈构造岩片产在含石榴子石云母石英片岩中。榴辉岩岩石类型简单,榴辉岩相矿物为石榴子石+绿辉石+金红石+多硅白云母+(石英)。Grt-Omp-Phe矿物温压计估算出榴辉岩的形成温压条件为T=730℃,p=2.7GPa,接近于柯石英和石英的转变线。岩石地球化学和Sr-Nd同位素表明其原岩为典型的MORB玄武岩,来自亏损地幔。榴辉岩锆石的SHRIMPU-Pb年龄介于(242.4±15.2)～(291.9±12.8)Ma之间,15个测点的平均值为261.7Ma±5.3Ma,结合矿物包裹体的研究,认为代表榴辉岩的变质年龄,与Sm-Nd同位素等时线年龄(305Ma)可以对比。结合区域地质资料,推测榴辉岩的原岩时代为石炭纪—二叠纪。二叠纪MORB榴辉岩和区域上同时代岛弧火山岩的产出,表明拉萨地块中可能存在一条石炭纪—二叠纪的古缝合带,其中榴辉岩代表古特提斯洋壳的残留。

An eclogite belt about 500-600 m wide is found recently in the Lhasa block, Tibet. The rock is thick-bedded and fresh and occurs as tectonic slices in garnet-bearing mica-quartz schist. It consists chiefly of garnet ＋ omphacite ＋ ruitile ＋ phengite ＋ （quartz）. According to the Grt-Omp-Phe mineral geothermobarometry, the T-p conditions for the formation of the eclogite are estimated to be 730℃ and 2.7 GPa, close to the phase boundary between coesite and quartz. Petrochemistry and Sr-Nd isotope analysis suggest that its protolith was typical MOR33, derived from the depleted mantle. The SHRIMP U-Pb zircon dating of the eclogite yielded ages ranging from 242.4±15.2 to 291.9 ±12.8 Ma, with an average of 261.7±5.3 Ma for 15 spots. Mineral inclusion study shows the ages represent the metamorphic age of the rock, which may be correlated with the whole-rock Sm-Nd age （305.5±50 Ma）. According to the regional geological data, it is inferred that the eclogite protolith is Carboniferous-Permian in age. The occurrence of Permian MOR33 eclogite and regionally contemporaneous island arc volcanic rocks is believed to mark a Carboniferous - Permian suture zone in the Lhasa block, and the eclogite therein is interpreted as renmants of the Paleo-Tethys oceanic crust.