青藏高原拉萨地块中部晚白垩世镁铁质岩脉的成因及其地质意义

Mafic dykes have important significances for tracing the deep mantle characteristics, paleotectonic stress field and tectonic evolution. In a bid to determine the Late Mesozoic geodynamic setting and mantle characteristics of Lhasa block in Tibetan plateau, in this proposal, systematical field investigation on geological characteristics (distribution and geometry) and sampling, and comprehensive studies of petrology, mineralogy, element geochemistry, Sr-Nd-Pb-Hf-O isotopes, and precise U-Pb dating of zircon, baddeleyite and titanite will be carried out for the Late Cretaceous (~ 88-83 Ma) mafic dykes in Tangra-Yumco area, central Lhasa. The integrated results will provide important constraints on the petrogenesis, tectonic stress field, mantle source and tectonic setting for these dykes. Taking into account the regional geological and contemporaneous magmatism data, the deep geodynamics beneath Lhasa block during the Late Cretaceous will be further constrained; Furthermore, these mafic dykes can be used to constrain the latest pre-collisional mantle characteristics of the central Lhasa sub-block and thus, the evolution of the Lhasa mantle before and after Lhasa-India collision and its relationship with Indian continental subduction can be well discussed by comparing the pre-collisional mafic dykes to the post-collisional Miocene ultrapotassic rocks. This will provide important basic data for the geological evolution of southern Tibet.

镁铁质岩脉对于探讨深部地幔性质、古构造应力场和构造演化等方面具有重要的科学意义。为揭示青藏高原拉萨地块晚中生代的动力学背景和地幔特征，本项目拟对新发现的拉萨地块中部当惹雍错地区晚白垩世（~ 88-83 Ma）镁铁质岩脉进行系统地野外地质（分布、几何形态）调查并采样和岩石学、矿物学、元素地球化学、Sr-Nd-Pb-Hf-O同位素地球化学研究以及锆石、斜锆石、榍石的U-Pb精确定年。综合的研究结果可以深入剖析这些岩脉的成因及其形成构造应力场，阐明其地幔源区和构造背景。结合区域地质和同期岩浆岩的资料，进一步综合限定拉萨地块晚白垩世岩浆作用的深部动力学机制；利用这些镁铁质岩脉对印度-拉萨板块碰撞前最晚期的中拉萨地体地幔属性进行制约，并与碰撞后中新世幔源超钾质岩对比，探讨碰撞前后拉萨地幔的改造及其与印度大陆俯冲的关系，为青藏高原南部的地质演化提供重要的背景资料。

镁铁质岩脉对于探讨深部地幔性质和构造演化等方面具有重要的科学意义。本项目主要以青藏高原拉萨地块中部当惹雍错裂谷地区的晚白垩世镁铁质岩脉为研究对象，进行了系统的年代学和地球化学研究。全岩Ar-Ar以及锆石和榍石U-Pb定年结果表明区域内镁铁质岩脉主要形成于92-84 Ma，主要岩石类型为辉绿玢岩和闪长玢岩。辉绿玢岩显示弧型微量元素分配型式，相对富集的Sr-Nd同位素组成[εNd(t)= -7.92~ -3.45; 87Sr/86Sr(i)= 0.708189~ 0.710799]；而闪长玢岩显示镁质埃达克岩的属性，如高的Sr/Y (44.6-53.4)和La/Yb (27.3-49.6)，低的Y (13.8-15.3 ppm)和Yb (1.1-1.3 ppm)含量，同时也具有富集的Sr-Nd同位素组成，可能来源于拆沉的古老下地壳部分熔融。结合文献资料，我们认为拉萨地块西北带晚白垩世岩浆作用形成于羌塘-拉萨碰撞后的造山伸展环境。而晚中生代和新生代地幔属性的显著差异有力地支持了新生代印度大陆俯冲对拉萨地幔的进一步交代改造。