【作者】 钟立峰；

【导师】 夏斌； 陈根文；

【作者基本信息】 中国科学院研究生院（广州地球化学研究所） ， 构造地质学， 2006， 博士

【摘要】 罗布莎蛇绿岩地处雅鲁藏布江蛇绿岩带的东段。该蛇绿岩岩石单元出露齐全,又有其自身的特点,如:以地幔橄榄岩为主,地幔橄榄岩构造变形强烈;在堆积杂岩中上部还产出有仰冲型斜长花岗岩小岩块;辉绿岩以脉状形式产出于地幔橄榄岩的方辉橄榄岩中;壳层岩石相对很薄,壳层熔岩比较复杂。 罗布莎蛇绿岩辉绿岩中的锆石SHRIMP U-Pb ²⁰⁶pb/²³⁸U表观年龄呈现复杂的分布格局,主体年龄在148.3～174.2Ma范围之内,获得高精度的锆石谐和年龄为162.9±2.8Ma,即罗布莎蛇绿岩的形成年龄。获得的另外两组年龄数据说明,罗布莎蛇绿岩形成过程中可能俘获了古老洋壳或深海沉积物中的锆石;同时进一步证明罗布莎蛇绿岩在晚白垩世已经开始侵位。 罗布莎蛇绿岩壳层熔岩属于亚碱性玄武岩与安山岩/玄武岩之间的过渡类型,辉绿岩样品均处于亚碱性玄武岩范围内。这些岩石的地球化学特征指示罗布莎蛇绿岩来源于一个高度亏损的地幔源区,具有N-MORB的特征,但在形成过程中可能受消减作用的影响,遭受了陆源物质的混染。说明罗布莎蛇绿岩属于俯冲带之上（SSZ型）的弧间盆地蛇绿岩。罗布莎蛇绿岩壳层熔岩和辉绿岩具有高的、类似于来自DMM地幔端员的MORB的e_Nd（t）值。Sr-Nd-Pb同位素特征表明,罗布莎蛇绿岩地幔源区为混染或混合了EMⅡ地幔端员组分的高亏损的地幔储源库;并且该地幔源区具有南半球特有的Dupal异常性质,这种地幔源区Dupal异常是DMM和EMⅡ这两种地幔端员混合的结果。 对比整个特提斯蛇绿岩带的同位素分析结果发现:①它们都具有印度洋型的Sr、Nd、Pb同位素组成特征,并显示出明显的Dupal同位素异常;②这种大范围的Dupal同位素异常可能来源于南半球的一个或几个地幔柱;③这个携带着Dupal物质组分的地幔柱至少已经存在了350Ma;④并且该地幔柱可能是导致冈瓦纳古陆裂解,老新特提斯洋先后形成的源动力。 罗布莎地区特提斯洋的形成演化过程可归纳为:在二叠纪晚期至三叠纪初,印度板块的北缘分裂出拉萨地块,并在拉萨地块和印度板块之间逐渐形成特提斯洋;至晚三叠世时,特提斯洋盆已经具有一定规模;随着洋中脊继续扩张和更多还原

【Abstract】 The Luobusa ophiolite exist in the eastern segment of Yalu Zangbu（Yarlung Zangbo） suture zone, whose rock units are complete and which has many characteristics in itself, such as the major rock unit is mantle peridotite and the crust rock is complicated;there exist obduction-type granitoid rocks in the upper level of cumulate;diabases as veins crop out in the mantle harzburgite.Zircons were picked out from diabases in The Luobusa ophiolite, whose ²⁰⁶Pb/²³⁸U apparent ages SHRIMP U-Pb dating show very complex, but a majority of which distribute between the 148.3Ma and 174.2Ma. A precise zircon SHRIMP U-Pb concordia age obtained was 162.9 ± 2.8Ma. This age was interpreted as formation time of The Luobusa ophiolite. In addition, two different age data had been obtained in the precedure of analysis, which demonstrate that it is possible during the formation of the neo-tethys, that there are some older zircons coming from an older oceanic crust or pelagic sediment which have been caught when the mantle magmas upwell beneath mid-oceanic ridges and prove further that The Luobusa ophiolite has emplaced in late Cretaceous.The crust lavas of The Luobusa ophiolite belong to transition type between alkaline basalt and andesite/basalt, but diabases fall into the range of subalkaline basalt. Geochemical characteristics of these rocks suggest The Luobusa ophiolite derived from a very depleted mantla source with the characteristic of N-MORB, but may undergo the contamination of crust-derived materials. All the geochemical characteristics indicate that The Luobusa ophiolite belongs to arc-type ophiolite （SSZ）. That is, its formation background is interarc basin of supra-subduction zone. The crust lavas and diabases in The Luobusa ophiolite have big eNd（t） which resemble N-MORB. It was found that the mantle source of The Luobusa ophiolite may be a depleted mantle reservoir contaminated or mixed by EM II from Sr、 Ndand Pb isotopic data. The mantle source has Dupal isotopic anomaly and the Dupal anomaly of mantle source results from the mix of DMM and EM II mantle end-member.All the results of the Sr-Nd-Pb isotope analysed of Tethyan ophiolites show that they have the isotopic characteristics similar to those of Indian Ocean and reveal the Dupal anomaly. The large-scale Dupal isotopic anomaly may derive from one or some plumes of south hemisphere. These plumes with Dupal composition have existed 350Ma and more, and may be the cause which results in the splitting of Gondwana continent and opening of paleo-Tethyan and neo-Tethyan ocean early and late.In late Permian to early Triassic, the Lhasa block separated form Indian plate. The Tethyan ocean began to form between the Indian plate and the Lhasa block. The Tethyan ocean had been large in the late Triassic. With the spread of mid-ocean ridge and excursion northward of Indian plate, a inter-mantle-type thrusted ductile shear zone had been developed in the Tethys which results in the subduction between oceanic crusts. In mid-Jurassic, the magmas in the mantle wedge of supra-subduction zone convected intensively. Upwelling of mantle magmas cause northern older oceanic crust spreading and forming new oceanic crust. Later on, Zedong island arc was produced in the north of new oceanic crust. At one time, the Tethyan oceanic crust to the south of Lhasa block began to downthrust toward the Lhasa block. The island-arc magmas spewed at the southern edge of the Lhasa block and gave birth to the Sangri arc in the late Jurassic to early Cretaceous. But later the Sangri arc took place disassembly due to the tectonic event. The downthrusting between ocean crusts in Tethys had been completed in late Cretaceous. Both old and new oceanic crust emplaced on the stratum of Langjiexue Formation. In Tertiary, all the downthrusting has been finished and the collision between Indian plate and Eurasian plate began.更多还原