Origin of Serpentinite in Sumdo Area, Xizang and Its Constraint on Subduction of the Sumdo Paleo-Tethys Ocean
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摘要: 蛇纹岩中的锆石不仅可以提供年代学信息，还可以约束蛇纹岩的成因和区域构造演化过程.本文以唐加‒松多古特提斯缝合带中的龙崖松多蛇纹岩为研究对象，对其进行了全岩地球化学、锆石U-Pb年代学和Lu-Hf同位素研究.结果显示，龙崖松多蛇纹岩具较高的MgO和TFeO含量、高Mg#值以及较低的Al2O3和TiO2含量.样品稀土元素配分曲线呈较为宽缓的“U”型.微量元素蛛网图显示，样品富集U、Ta，亏损Th、Nb、Zr和Hf.龙崖松多蛇纹岩锆石U-Pb定年结果为（230.3±2.3）Ma，εHf（t）值在+13.4到+16.0之间.年代学和地球化学研究表明，龙崖松多蛇纹岩的原岩为地幔楔内尖晶石二辉橄榄岩部分熔融后的残留体.龙崖松多蛇纹岩受到松多古特提斯洋板片俯冲产生的超临界流体的交代形成了交代锆石，可能还受到富水流体的交代，导致流体活动性元素的富集.结合前人研究，本文认为松多古特提斯洋在晚三叠世可能尚处于北向俯冲阶段.Abstract: Zircons in serpentinite can not only provide chronological information, but also constrain the genesis of serpentinite and regional tectonic evolution process. Longyasongduo serpentinite in Tangjia-Sumdo Paleo-Tethys suture zone was studied by means of zircon U-Pb dating, whole-rock geochemistry, and zircon Hf isotope in this paper. The results show that Longyasongduo serpentinites have high MgO, TFeO and Mg# values but low Al2O3 and TiO2 contents. The chondrite-normalized REE patterns display a gentle U-type. The primitive mantle-normalized spider diagrams exhibit enrichment in U, Ta and depletion in Th, Nb, Zr and Hf. Zircon U-Pb dating of Longyasongduo serpentinite yielded ages of (230.3±2.3) Ma, with εHf(t) values of +13.4 to +16.0. Through chronological and geochemical studies, we argue that the protolith of Longyasongduo serpentinite is the residual of partially melted spinel lherzolite in the mantle wedge. Longyasongduo serpentinite was metasomatized by supercritical fluid generated by subduction of Sumdo Paleo-Tethys oceanic crust to form metasomatic zircon, which may also be metasomatized by aqueous solutions, resulting in enrichment of fluid-mobile elements. Based on previous studies, we infer that the Sumdo Paleo-Tethys Ocean was still in a northward subduction setting in the Late Triassic.
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Key words:
- QinghaiXizang Plateau /
- Sumdo Paleo-Tethys Ocean /
- serpentinite /
- metasomatic zircon /
- petrology /
- geochemistry
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图 1 青藏高原及其邻区大地构造单元划分简图(a；据Xu et al., 2015修改)，唐加‒松多地区地质简图(b；据Wang et al., 2021修改)，龙崖松多蛇纹岩采样位置(c)
1.下白垩统楚木龙组；2.上侏罗统‒下白垩统林布宗组；3.下侏罗统叶巴组；4.上三叠统雄来组；5.上石炭统‒下二叠统松多岩组；6.下石炭统诺错组；7.白垩纪岩浆岩；8.侏罗纪岩浆岩；9.三叠纪岩浆岩；10.大理岩；11.洋岛辉长岩；12.洋岛玄武岩；13.蛇绿岩；14.超基性岩；15.二叠纪闪长岩；16.二叠纪玄武岩；17.榴辉岩；18.蓝片岩；19.断层；20.本文采样点；21.洋壳残片年龄；22.榴辉岩年龄；23.岛弧岩浆岩年龄；24.松多岩组年龄
Fig. 1. Tectonic framework of the Qinghai-Xizang Plateau and adjacent area (a; modified from Xu et al., 2015), geological sketch of the Tangjia-Sumdo area (b; modified from Wang et al., 2021), sampling location map of Longyasongduo serpentinite (c)
图 2 西藏松多地区龙崖松多蛇纹岩野外照片和镜下照片
a. 龙崖松多蛇纹岩与松多岩组野外接触关系；b. 龙崖松多蛇纹岩宏观照片；c. 龙崖松多蛇纹岩近景照片；d. 龙崖松多蛇纹岩显微照片. Serp.蛇纹石；Phl.金云母；Mag.磁铁矿
Fig. 2. Field photos and photomicrographs of Longyasongduo serpentinite in Sumdo area, Xizang
图 4 龙崖松多蛇纹岩中锆石阴极发光照片、U-Pb年龄谐和图和稀土元素球粒陨石标准化配分曲线
球粒陨石数据引自McDonough and Sun（1995）
Fig. 4. Zircon cathodoluminescence, zircon U-Pb concordia diagram and chondrite-normalized zircon REE patterns for Longyasongduo serpentinite
图 5 Al2O3-CaO-MgO图解(a, b; Coleman, 1977); TFeO-Na2O+K2O-MgO图解(c; Irvine and Baragar, 1971); SiO2/MgO-Al2O3图解(d; Bodinier and Godard, 2014)
Fig. 5. Al2O3-CaO-MgO diagram (a, b; Coleman, 1977); TFeO-Na2O+K2O-MgO diagram (c; Irvine and Baragar, 1971); SiO2/MgO-Al2O3 diagram (d; Bodinier and Godard, 2014)
图 6 龙崖松多蛇纹岩Ti-Yb图解(a; Parkinson and Pearce, 1998) 和全岩稀土元素部分熔融模型(b; Niu, 2004)
Fig. 6. Ti versus Yb diagram (a; Parkinson and Pearce, 1998) and whole rock REE partial melting model (b; Niu, 2004) of Longyasongduo serpentinite
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