雅鲁藏布江蛇绿岩带西段达机翁地幔橄榄岩组成特征及其形成环境分析

Composition characteristics and tectonic setting of the Dajiweng peridotite in the western Yarlung-Zangbo ophiolitic belt

雅鲁藏布江蛇绿岩带自萨嘎以西分成南北两个亚带。对两个亚带蛇绿岩的各自特征及成因联系的研究,是探讨雅鲁藏布江西段的新特提斯洋构造演化的关键。北亚带蛇绿岩呈构造岩块产于冈底斯山前喀喇昆仑断裂带的南侧。其中,位于北亚带西北段的达机翁蛇绿岩,主要由地幔橄榄岩,玄武岩夹硅质岩组成,各单元间断层接触。对达机翁蛇绿岩的地幔橄榄岩开展的组成特征研究表明：（1）地幔橄榄岩主体为方辉橄榄岩,含少量的纯橄岩。方辉橄榄岩内产有豆荚状铬铁矿（呈豆状,块状以及浸染状）,铬铁矿有一层纯橄岩的外壳;（2）达机翁方辉橄榄岩单斜辉石含量低,组成矿物以及全岩的地球化学特征均指示了这些样品经历了相对高的部分熔融作用;（3）方辉橄榄岩具有U型的球粒陨石标准化的稀土元素分配模式,Nb相对亏损,Ta,Zr和Hf具有弱的正异常,同时Sr和U具有强烈的正异常,这些特征可能与残余地幔和俯冲带熔/流体之间相互作用导致的轻稀土元素和部分微量元素的选择性富集有关。定量计算表明,达机翁地幔岩中的方辉橄榄岩来源于一个尖晶石相地幔源区的部分熔融,部分熔融程度大于25%,高于深海地幔橄榄岩的部分熔融程度（10%～2%）。这些橄榄岩形成时的氧逸度条件位于FMQ和FMQ＋1之间,高于深海地幔橄榄岩（FMQ-1）,与俯冲带环境的氧逸度条件一致。因此,我们认为达机翁蛇绿岩中的地幔橄榄岩形成于大洋中脊的环境,随后发生了洋内俯冲作用,位于俯冲带上部的地幔橄榄岩经历了俯冲带流/熔体的交代作用。

The Yarlung Zangho suture zone （YZSZ）, the youngest and southernmost suture in Tibet, separates the Lhasa Block to the north from the Indian continent to the south. Ophiolitic massifs crop out discontinuously along this ~ 2000km-long suture and these are interpreted as relicts of Neo-Tethyan oceanic lithosphere that separated India from Eurasia during the Mesozoic. This suture zone is typically divided into eastern （Luobusa ophiolite ）, central （e. g. , Xigaze ophiolite ） and western （e. g. , Dongbo and Purang ophiolites） segments. In the central and eastern segments, the ophiolites lie along a single lineament, whereas in the western part, the suture is divided into a northern and southern branch. The northern branch appears to be a continuation of the suture, whereas the southern branch is marked by a tectonic m61ange composed mostly of Tethyan sedimentary rocks. Because the western segment of the suture is very remote and difficult to access, most workers have focused on the ophiolites exposed in the eastern and central segments. However, in recent years a coordinated study of the western segment by a team from the Chinese Academy of Geological Sciences has provided much new data on the age, petrology and structure of these remote ophiolites. The Dajiweng ophiolite in the northern sub-beh of the suture zone is composed mainly of mantle peridotite, with some basalt and siliceous rocks. The mantle peridotites consist dominantly of harzburgite, accompanied by minor dunite and podiform chromitite. Our study reveals that the Dajiweug harzburgite is strongly depleted, indicating a relatively high degree of partial melting and/or fluid/melt-rock reaction. Extensive fluid/melt-rock reaction in the harzburgite is indicated by its well-defined, U-shaped, chondrite-normalized REE patterns and variable contents of high field strength elements. We suggest that the Dajiweng harzburgite originated by greater than 25% partial melting of a spinel-facies mantle source and was later modified by melts and fluids in a suprasubduction zone mantle wedze.