青藏高原蛇绿岩中镁铁-超镁铁质地壳形成过程及壳幔成因联系研究

Increasingly published Re-Os isotopic data have revealed that ancient mantle domains (>1.0 Ga) are ubiquitous in Tibet ophiolites, such inherited mantle could document the early upwelling of asthenospheric mantle during continental rifting and participate in the formation of chromitite deposits. However, little is known about their contribution to the formation of Tethyan oceanic crust and crust-mantle relationship. Mafic-ultramafic plutonic rocks in the lower crust of ophiolites can reflect tectonic settings of ophiolites, furthermore, their Os-Hf isotopic compositions have the potential to evaluate the contribution of ancient mantle domains. The transfer mechanism and tectonic control for the contribution of inherited mantle in the formation of lower oceanic crust are unclear. Therefore, detailed studies on continuous sections from upper mantle to lower crust are critical to clarify crust-mantle relationships and magma evolution during melt transport. Based on field observation on crust-mantle boundary, petrological observation, mineral composition and geochemistry data as well as modeling by pMELTS software, this proposal will systematically analyze Nd-Hf-Os isotopic compositions and PGE contents of whole-rock and minerals from subduction-related Xigaze and Dongqiao ophiolites and possible mid-ocean ridge ophiolites from Yongzhu and Kaimeng, respectively, to determine the nature of parental magma and the formation process of mafic-ultramafic rocks, placing constraints on tectonic settings and their genetic link with ancient inherited mantle during formation processes of several typical ophiolites in Tibetan Plateau.

近年来Os同位素研究表明青藏高原蛇绿岩中普遍存在古老地幔组分（>10亿年），这种继承性地幔组分可以记录大陆裂解的信息，还会参与形成铬铁矿。然而，它们对特提斯洋壳形成有多少贡献，壳幔成因联系还缺乏研究。下地壳镁铁-超镁铁质岩可以准确指示蛇绿岩形成环境，其Os-Hf同位素研究有潜力识别出源区古老地幔的贡献。但是，继承性地幔参与洋壳形成的机制还不清楚且缺乏构造背景条件约束。开展上地幔-下地壳岩性剖面精细研究成为认识壳幔成因联系和岩浆迁移演化的关键。本申请以俯冲相关的日喀则和东巧蛇绿岩和大洋扩张环境的永珠和凯蒙蛇绿岩中镁铁-超镁铁质岩石为研究对象，详细观察壳幔边界岩石产出关系，在岩相学、矿物学和地球化学研究基础上，以全岩和单矿物Nd-Hf-Os同位素及PGE分析为主要研究手段，结合pMELTS软件模拟，界定地壳岩石形成机制和岩浆性质，约束壳幔成因联系和构造背景，探究青藏高原典型蛇绿岩的形成过程。

本项目选择西藏地区班公湖-怒江缝合带两侧的蛇绿岩中的镁铁-超镁铁质堆晶岩为研究对象，针对样品严重蚀变的问题，以矿物组成和Os同位素为主要分析手段，完成了下地壳岩石样品的成因、构造背景和年代学研究。主要取得进展如下：（1）永珠蛇绿岩堆晶岩底部以方辉橄榄岩为主，具有低Mg#（84）和较高的Os同位素（187Os/188Os :0.1283-0.1672），是地幔橄榄岩早古生代熔体交代的产物。上部堆晶岩为辉石岩和辉长岩是幔源岩浆结晶分异形成的，锆石年龄216 Ma显示他们是班公湖-怒江特提斯洋早期扩张的岩浆记录；辉石岩的微量元素组成，Nd同位素（εNd:-2.3至+4.3）和Os同位素组成(γOs在-3.0到-0.8)显示他们是软流圈和岩石圈地幔相互作用的产物。（2）凯蒙堆晶岩和地幔岩是新生大洋岩石圈，堆晶岩全岩化学和尖晶石组成（低Cr#:30-45）以及地幔单元中的纯橄岩通道的矿物组成，都指示着MORB型熔体(εNd在+6至+8, 187Os/188Os :0.126-0.128)从岩浆迁移到结晶分异的演化过程；凯蒙地幔橄榄岩具有与深海橄榄岩相似的Os同位素特征。（3）安多橄榄岩体是形成于俯冲带弧前环境，不同世代的橄榄石和单斜辉石是硅酸盐熔体和含碳酸盐流体改造形成的。东巧蛇绿岩中斜方辉石岩脉锆石年龄为 495 Ma，是含水玄武质熔体与地幔岩反应的产物，表明蛇绿岩地幔是继承的早古生代冈瓦纳大陆北缘的陆下地幔。（4）罗布莎蛇绿岩的堆晶岩矿物学和锆石年代学研究显示他们是84Ma MORB型岩浆分异的产物，这种含水软流圈熔体可能与罗布莎铬铁矿形成密切相关。结合我们对丁青地幔铝尖晶石脉的矿物学研究(Cr# <10)，母岩浆的物理条件改变而非熔体-地幔岩反应控制着尖晶石矿物的大量结晶。硅酸盐岩浆与尖晶石熔体不混溶可能是形成大型铬铁矿床的重要机制，假说将作为工作模型用于进一步探讨蛇绿岩铬铁矿成因。