The amphibolite-facies and granulite-facies metamorphic rocks from the Lhasa terrane, South Tibet, have been traditionally grouped as the Precambrian metamorphic basement, but related research is still in the initial stages, particularly the study on geochronological and tectonic attribute. This paper reports petrological and geochemical features and zircon U-Pb age of typical ortho-metamorphic rocks, including garnet-bearing amphibolite, plagioclase gneiss and felsic veins from Nyainqentanglha Group in Yongzhu area, northern part of Lhasa terrane. We report for the first time Late Neoproterozoic metamorphic and magmatic events from the Lhasa terrane and provide important data for study on origin and Paleozoic tectonic evolution of the Lhasa Terrane. Geochemical data indicate that the garnet-bearing amphibolite (Sample TQ18 and Sample Q11T15) , originated in both E-MORB and island-arc tectonic settings, and that the plagioclase gneiss ( Sample TQ19) are of similar geochemical feature to plagiogranites which are interpreted to have derived from anatexis of hydrated amphibolites by ductile shearing during transports of the oceanic crust. In situ zircon U-Pb LA-ICP-MS dating on the zircons possess residual magmatic cores yielded an protolith age of the 758. 3 ±6.1Ma, 730. 8 ± 6. 9Ma and 742. 1 ±8. 3Ma, representing the age of Neoproterozoic ocean basin formation, metamorphism and melting process during transports of the oceanic crust and island arc magmatism. Zircons from garnet-bearing amphibolite (Sample Q11T15) show typical features of metamorphic origin, and yield peak-metamorphic ages of 665.7 ± 6. 5Ma, probably representing the age of continent collision during the oceanclosing. Geochemical evidence indicated that felsic veins (Sample Q11T16) are similar to adakite. Zircon rim from felsic veins record ages of 660. 1 ± 4. 2Ma, slight later the peak-metamorphic ages of sample (Q11T15), which are preliminary explained as the age of anatexis during initial extensional collapse or exhumation processes of the high-pressure rocks.%青藏高原南部拉萨地块中分布的中高级变质岩一直被认为是前寒武纪变质基底,但并未获得可靠的年代学证据,对其岩石成因及构造属性也缺乏系统的研究工作,严重制约了对拉萨地块早期构造演化的进一步研究.本文以拉萨地块北部永珠地区念青唐古拉岩群中的正变质岩系及深熔作用成因的长英质脉体为研究对象,进行了系统的岩石学、地球化学及同位素年代学研究.地球化学研究结果表明,永珠地区念青唐古拉岩群中正变质岩系的原岩为一套E-MORB型蛇绿岩、剪切型大洋斜长花岗岩和岛弧岩浆岩组合,深熔脉体则具有典型埃达克岩的特征,暗示高压条件下镁铁质岩石部分熔融的成因；LA-ICP-MS定年结果进一步表明,洋壳的形成时代为758Ma,与Rodinia超大陆裂解时期相一致,可能是在这一期全球性裂解事件中新生的新元古代洋盆记录；在洋壳的运移和进一步的演化过程中形成了730Ma的剪切型大洋斜长花岗岩和742Ma的岛弧岩浆岩；变质锆石定年结果表明大洋可能最终在666Ma的碰撞造山作用中闭合,并在造山带垮塌初期或高压变质岩系折返过程中,由于减压熔融,形成一期660Ma深熔脉体.本文的研究证明了拉萨地块前寒武纪变质基底的存在,首次获得了精确的新元古代变质及深熔作用年龄,填补了拉萨地块早期构造演化的空白,为进一步探讨拉萨地块起源,恢复其在超大陆的汇聚及裂解事件中古地理位置提供了重要的资料.
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