西藏日多地区古近纪双峰式脉岩年代学、地球化学及其揭示的伸展背景

青藏高原南部拉萨地块古近纪岩浆岩带的岩石成因和动力学对于揭示新特提斯洋演化及印度-亚洲大陆碰撞具有重要意义。对日多地区新发现的平行成群排列的辉绿玢岩脉和花岗斑岩脉进行了地球化学和锆石LA-ICP-MS U-Pb定年研究。结果表明:辉绿玢岩的定年结果为(58. 8±3. 6) Ma,花岗斑岩的定年结果为(55. 2±2. 5) Ma。野外特征、岩相学和地球化学分析认为辉绿玢岩与花岗斑岩存在明显的组分间断,具有双峰式侵入岩特征。辉绿玢岩Si O_2含量为49. 67%～55. 53%,具有右倾的稀土元素配分模式,无Eu异常,相对富集K、Rb、Th、U,亏损Nb(Ta);花岗斑岩样Si O_2含量为74. 06%～74. 25%,显示右倾的稀土元素配分模式,具弱Eu异常,相对富集Rb、K、Ba、Th、U,相对亏损Nb(Ta)、Sr、P、Ti。基性岩石Th/Hf-Ta/Hf图解落入大陆伸展带,结合前人研究成果,显示印度-亚洲大陆碰撞的大构造背景下,存在局部伸展的构造环境,暗示深部北向俯冲的新特提斯洋板片发生回转、断离。

Paleogene bimodal intrusions dike in Riduo,Tibet:geochemistry,geochronology and implications for extension

The petrogenesis and geodynamic of Paleogene magmatism from the southern Lhasa Terrane,Tibet,played very important role in discovering the evolution and processes of the Tethyan Ocean before the collision between India and Asia. In research of this paper,the LA-ICP-MS zircon U-Pb dating and geochemistry of the diabase and granite porphyry veins arranged in parallel in the Riduo have been conducted. The results showed that the zircon ²⁰⁷Pb/²⁰⁶Pb weighted average age is 58.8±3.6Ma for the diabase and is 55.2±2.5Ma for the granite porphyry. Field characteristics,petrology, and geochemical analysis suggest that the diabase and granite porphyry have obvious compositional discontinuities with bimodal intrusive features, The diabase SiO₂ content is 49.67-55.53%,has a right-dipping pattern of rare earth elements,no Eu anormalies, relatively rich K, Rb, Th, U, loss Nb (Ta). For the granite porphyry, SiO₂ content is 74.06-74.25%, their trace elements generally have feature of enrichment of Rb, K, Ba, Th and U,obvious depletion of Nb (Ta), Sr, P, Ti,right-oblique shape and obvious depletion of Eu.. In combination with the further basic rock Th/Hf-Ta/Hf diagram falling into the continental stretch,showing a large tectonic setting in the Indian-Asian continental collision,there is a local extension of the tectonic setting,suggesting the deep northward subduction of the new Tethyyan Ocean has occured slab roll-back,slab break-off.