Delineation of Middle Neoproterozoic ophiolite mélange in the northern Lhasa terrane, South Tibet and its significance
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摘要:
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。
Abstract: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.
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Key words:
- Lhasa terrane /
- Nyainqentanglha Group /
- Neoproterozioc /
- E-MORB /
- Island arc magmatic rocks /
- Tibetan Plateau
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图 1
拉萨地块高级变质岩分布图、研究区地质简图及实测剖面图(a，据Dong et al., 2011修改；b、c为项目实测)
Figure 1.
Sketch geological map of the Lhasa terrane, showing the distributions of the high-grade metamorphic rock (a, after Dong et al., 2011), geological sketch map of Yongzhu area (b) and geologic sections of Nyainqentanglha Group in Yongzhu area (c)
图 2
样品的野外及镜下主要特征
Figure 2.
Outcrop photograph and photomicrographs of typical ortho-metamorphic rocks from Nyainqentanglha Group in Yongzhu area
图 3
念青唐古拉岩群变质基性岩Th-Co (据Hastie et al., 2007)和Nb/Y-Zr/TiO2(据winchester and Floyed, 1977)分类图解
Figure 3.
Th-Co (after Hastie et al., 2007) and Nb/Y-Zr/TiO2 (after winchester and Floyed, 1977) discrimination diagrams for the metabasic rocks from Nyainqentanglha Group in Yongzhu area
图 4
永珠地区念青唐古拉岩群中代表性正变质岩系的稀土元素球粒陨石标准化图及微量元素原始地幔蛛网图(标准化值据Sun and McDonough, 1989)
Figure 4.
Chondrite-normalized REE and primitive mantle-normalized trace element patterns of typical ortho-metamorphic rocks from Nyainqentanglha Group in Yongzhu area (normalization values after Sun and McDonough, 1989)
图 5
念青唐古拉岩群斜长片麻岩及长英质脉体A/NK-A/CNK图解(据Maniar and Piccoli, 1989)
Figure 5.
Al2O3/(Na2O+K2O) molar vs. Al2O3/(CaO+Na2O+K2O) molar plot of the plagioclase gneiss and felsic veins from Nyainqentanglha Group in Yongzhu area (after Maniar and Piccoli, 1989)
图 6
念青唐古拉岩群斜长片麻岩及长英质脉体An-Ab-Or图解
Figure 6.
An-Ab-Or diagram of the plagioclase gneiss and felsic veins from Nyainqentanglha Group in Yongzhu area
图 7
念青唐古拉岩群中典型锆石的阴极荧光图像及其206Pb/238U年龄
Figure 7.
CL images and 206Pb/238U ages of typical zircons from Nyainqentanglha Group in Yongzhu area
图 8
石榴石斜长角闪片麻岩中锆石的稀土元素球粒陨石标准化配分模式及锆石U-Pb谐和图
Figure 8.
Chondrite-normalized REE patterns and U-Pb concordia diagram of zircons in garnet amphibolite
图 9
石榴石斜长角闪岩中锆石的稀土元素球粒陨石标准化配分模式及锆石U-Pb谐和图
Figure 9.
Chondrite-normalized REE patterns and U-Pb concordia diagram of zircons in garnet amphibolite
图 10
石榴石角闪斜长片麻岩中锆石的稀土元素球粒陨石标准化配分模式及锆石U-Pb谐和图
Figure 10.
Chondrite-normalized REE patterns and U-Pb concordia diagram of zircons in garnet plagioclase gneiss
图 11
长英质熔融脉体中锆石的稀土元素球粒陨石标准化配分模式及锆石U-Pb谐和图
Figure 11.
Chondrite-normalized REE patterns and U-Pb concordia diagram of zircons in felsic veins
图 12
变质基性岩的不活动元素构造环境判别图解
Figure 12.
distinguishing diagram of tectonic settings for the metabasic rocks from Nyainqentanglha Group in Yongzhu area
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