西藏多巴地区片麻岩变质演化过程及其地质意义

Metamorphic Evolution Processes and Their Geological Implications of Gneisses from Duoba Area of Tibet

拉萨地体普遍出露一套角闪岩相、局部达麻粒岩相的变质岩系,以往的区域地质研究普遍认为这套变质岩为拉萨地体的前寒武纪变质基底,称之为念青唐古拉岩群。尽管现有的研究认为念青唐古拉岩群经历了前寒武纪变质作用,但部分地区仍缺少令人信服的岩石学及地质年代学证据。通过对拉萨地体北部多巴地区出露的片麻岩进行岩石学及矿物学研究,认为其为一套经历角闪岩相变质作用的中级变质岩系,主要岩石类型有云母片岩和含石榴石黑云母斜长片麻岩。矿物电子探针分析表明,组成岩石的石榴石、长石、黑云母等矿物均为变质成因矿物。石榴石无明显核边分带结构,成分相对均一。利用传统矿物对温压计进行了温压条件估算,得到640℃~660℃和0.60~0.67GPa的角闪岩相变质温压条件。首次利用LA-ICP-MS方法对该区花岗质片麻岩中的锆石进行年代学研究,得到（847.0±5.7）Ma的新元古代变质年龄,可与该区域念青唐古拉岩群的变质时代对比,为该片麻岩归入念青唐古拉岩群提供了切实可靠的年代学证据。

A suit of metamorphic rocks,which experience amphibolite and partly granulite facies metamorphism exposes on Lhasa block,are recognized as the basement of Lhasa block and named as Nyainqentanglha Group in the former literatures.Although the rocks are thought to be part of Nyainqentanglha Group,however,it is still lack of reliable geochronological data in some area.By means of the petrological and mineralogical investigations on the gneisses exposed in Duoba area of the north part of Lhasa block,it shows that the gneiss suit is composed of mica schist and garnet-bearing plagioclase gneiss,which has experienced amphibolite facies metamorphism.The garnet in the gneiss with homogeneous composition does not have core-rim structure.The traditional mineral-pair geothermobarometer shows that the metamorphic pressure-temperature conditions are 640℃-660 ℃ and 0.60-0.67 GPa,which belong to amphibolite facies metamorphism.LA-ICP-MS zircon U-Pb dating technique is used for the first time to date the metamorphic age of the gneiss in Duoba area and yield the Proterozoic metamorphic age of（847.0±5.7）Ma,which can be compared with Nyainqentanglha Group in the area,exhibiting strong evidence that the gneiss in Duoba area is also part of Nyainqentanglha Group.