【作者】 徐向珍；

【导师】 杨经绥； 巴登珠；

【作者基本信息】 中国地质科学院 ， 矿物学、岩石学、矿床学， 2009， 博士

【摘要】 西藏雅鲁藏布江蛇绿岩带是喜马拉雅特提斯洋壳和地幔的残余,被认为是中生代冈瓦纳板块裂解再拼合的一条缝合带。近年来,在西藏雅鲁藏布江缝合带中的罗布莎蛇绿岩型铬铁矿中,发现可能来自深部(＞300km深度)异常地幔矿物群。引起国内外高度关注,引发了许多新的问题,例如,含有金刚石等异常地幔矿物的铬铁矿的成因?与其伴生的地幔岩的成因?产有这些铬铁矿和地幔岩的蛇绿岩的成因等等?本论文拟开展罗布莎地幔橄榄岩体中的康金拉铬铁矿床及其围岩地幔橄榄岩的研究,查明该铬铁矿矿体,尤其矿体围岩的地幔橄榄岩中是否也存在金刚石等特殊地幔矿物;通过详细的矿物学和岩石学的研究,探讨铬铁矿和地幔岩的成因,以及两者之间的成因联系。通过详细的野外调查和室内岩石薄片的岩相学、矿物成分、岩石地球化学和锆石SHRIMP U-Pb同位素年代学,铬铁矿和地幔橄榄岩的人工重砂大样的矿物学研究,取得以下进展和认识:(1)认为康金拉地幔橄榄岩为部分熔融MOR地幔残余,叠加了SSZ流体的改造。地幔橄榄岩主要由纯橄岩、方辉橄榄岩和二辉橄榄岩组成,为部分熔融残余地幔橄榄岩,成分表现为高度亏损的特征:铬尖晶石成分变化区间很大,被认为是熔融程度存在巨大差别的反映,可能代表多次熔融事件的产物。低Cr#30-40方辉橄榄岩和二辉橄榄岩属于深海橄榄岩类型;而高Cr#40-77的方辉橄榄岩和纯橄岩归于岛弧环境橄榄岩类,认为高Cr#的岩石是深海橄榄岩在洋内俯冲环境被再次熔融叠加的结果:岩石地球化学表现出的特征同样也分出LREE亏损和LREE富集的类型,前者为MOR型地幔橄榄岩特征,后者反映SSZ流体作用的印记。(2)人工重砂发现铬铁矿和地幔橄榄岩中的金刚石等特殊地幔矿物。1116 Kg铬铁矿和384Kg地幔橄榄岩大样经人工重砂分选和双目显微镜下挑选矿物,取得如下成果:(a)首次在康金拉11号铬铁矿体中发现了上千粒金刚石,以及碳硅石等强还原环境的超高压矿物,数量远远超过在罗布莎铬铁矿矿区中发现的金刚石;(b)首次在康金拉11号铬铁矿矿体的近矿围岩中发现了大量金刚石和碳硅石等一批异常地幔矿物,为探讨铬铁矿的成因及与近矿围岩地幔橄榄岩的关系奠定了基础;(c)除挑选出金刚石和碳硅石等超高压矿物,在康金拉11号铬铁矿矿体与围岩地幔橄榄岩中还发现了自然铁球、金属互化物、氧化物、硫化物、硅酸盐等一批异常地幔矿物。康金拉铬铁矿中发现的金刚石等特殊矿物组合,与罗布莎铬铁矿中发现的矿物组合可以对比,尤其从铬铁矿的围岩地幔岩中也发现同样矿物组合,对探讨铬铁矿的成因,铬铁矿与地幔岩的关系,以及地幔岩的成因提供了关键证据。(3)认为康金拉豆荚状铬铁矿是深部成因。康金拉铬铁矿石与矿体围岩地幔橄榄岩的铬尖晶石、橄榄石和单斜辉石等成分方面明显不同,认为矿石铬尖晶石并非是由副矿物铬尖晶石富集而成,与地幔橄榄岩也不存在成因联系,认为豆荚状铬铁矿床对于近矿围岩地幔橄榄岩而言为外来体,但两者可能均是由地幔深部被地幔柱带到上部。康金拉矿区的地幔橄榄岩的地球化学特征反映了其形成过程的复杂性,表现为:洋底扩张期间,扩张脊下的MOR型地幔的经历了不同程度的部分熔融,形成了亏损地幔橄榄岩,该地幔橄榄岩中赋存了来自深部的豆荚状铬铁矿。在洋盆扩张期间,发生了洋内俯冲作用,俯冲板片释放的富含轻稀土和大离子亲石元素的含水流体向上运移交代上覆的残留地幔橄榄岩。(4)围岩橄榄岩中发现地壳成因的老锆石,认为是地幔不均匀和地壳物质再循环的重要证据。锆石SHRIMP U-Pb测年发现两个样品锆石形态特征、锆石的Th/U值及测年结果均显示出相似特征,锆石形态的多样性、Th/U比的高易变性和年龄值的极度分散显示了锆石形成背景的复杂性和多源性。纯橄岩样品四个测点得到加权平均值为130.0±2.8Ma,属早白垩世,代表了纯橄岩的结晶年龄;研究中发现许多地壳成因的古老锆石(最老可达太古代2770Ma),远远早于目前主流观点中蛇绿岩的形成时间中生代晚期,综合考虑认为所选锆石有大量俯冲地壳成分,在地幔中保存,后随地幔循环运移出露地表。在异常地幔矿物和地壳循环物质共存的基础上,探讨了罗布莎豆荚状铬铁矿成因与地幔柱(Mantle plume)之间的可能关联。更多还原

【Abstract】 The Yarlung Zangbo ophiolites in southern Tibet are remnants of the Neo-Tethys oceanic lithosphere and are considered as one of the suture zone representing the breakup and reconnection of Gondwana during Mesozoic. Recently, numerous unusual mantle minerals with the possible deep source (>300km) have been recovered from podiform chromities in the Luobusa ophiolite within the eastern Yarlung Zangbo suture zone, Tibet. This new finding has attracted high attention in geosciences field and arose many significant issues: does these deep mantle minerals also occur in the similar chromitite deposite in the Luobusa? Can we discover the similar mantle minerals from the mantle rocks associated with the chromitite? What is the origin of the chromitite and mantle rocks containing these minerals like diamond, together with the origin of the ophiolite? This study wants to perform researches on the Kangjinla chromitite and its host rock-mantle peridotite, to determine the existence of special mantle minerals, to discuss the origin of the chromtite and mantle peridotite together with the genetic relation between them. Basing on detailed field survey and laboratory researches on lithological features, mineralogical composition, geochemical and zircon SHRIMP U-Pb data, together with the mineralogical study of chromitite and mantle peridotite heavy mineral bulk sample, various proceedings and recognitions have been achieved:(1) The mantle peridotite is the remnants of partial melted MOR mantle altered by SSZ fluids. The evidences include: the mantle peridotite mainly consists of dunite, harzburgite and lherzolite; the highly depleted composition; significant variation of chrome spinel suggesting the huge difference in melting degree, which might indicate the production of various melting events; the harzburgite and lherzolite with low Cr#30-40 are abyssal peridotite while the harzburgite and dunite with high Cr#40-77 are island-arc peridotite, the latter is formed by the melting of the former during intra-ocean subduction; the geochemical characteristics can also separate the depleted and enriched LREE, the former is attribute to the MOR mantle peridotite and the latter is to the SSZ fluids action.(2) Special mantle minerals have been discovered from manual heavy minerals of chromitite and mantle peridotite. For a 1116Kg chromitite and a 384Kg mantle peridotite sample, by heavy mineral separation and picking up under a binocular microscope following productions are obtained: (a) the UHP minerals including moissanite and over 1000 diamond grains are discovered firstly from the Kangjinla chromitite orebody Cr-11, which is far more abundant than the Luobusa district; (b) a series of unusual mantle minerals like moissanite and a great deal of diamond grains are discovered firstly from the host rocks of chromitite orebody Cr-11, providing the foundation of discussing the chromitite origin and its relation with the host rocks; (c) besides the UHP minerals like moissanite and diamond, various special mantle minerals are revealed from the chromitite orebody Cr-11 and its host rocks including some native elements, alloys, oxides, sulphides, silicates, carbonates, and tungstates. The unusal minerals assembledge discovered from the Kangjinla district is similar to that from the Luobusa district. Especially, that the same mineral assembledge was found firstly from mantle peridotite, the host rocks of chromitite, which provided new key evidence for discussing the origin of the chromitite, the hosted mantle rocks and the relation between them.(3) The deep origin of Kangjinla podiform chromitite is suggested by: that for the Kangjinla chromitite and the mantle peridotite rocks, the compositions of the chrome spinel, olivine and clinopyroxene are rather different, which indicates that the chrome spinel in the chromitite is neither the simple concentration of that in the host rocks nor with certain genic relation with mantle peridotite. Though the chromitite might be exotic for host rocks, both of them were carried from deep mantle to the shallow place by mantle plum. According to the geochemical data, the formation of mantle peridotite might be: during the spreading of ocean floor, the MOR mantle underwent variable degrees of partial melting and formed the depleted mantle peridotite with podiform chromitite; during the spreading of ocean basin, intra-ocean subduction happened, REE and LILE rich fluids containing water derived from the subduction slab migrated upwards and metasomatism happened to peridotite relic.(4) The ancient crustal zircons discovered from the chromitite are key evidence of mantle inhomogenity and recycled crust materials. Zircon grains are separated from the mantle peridotite, the host rocks of the chromitite orebody Cr-11, and the SHRIMP U-Pb age reveals sophisticated background and multiple sources of zircons. The weighted mean value of four measuring points (130.0±2.8Ma) might be the crystallization age of dunite. Many crustal zircons have the age (up to 2770Ma) that is much earlier than the formation of ophiolite considered by previous researchers, indicated the abundant subduction crust components occur in the zircons separated, which was preserved in the mantle, and outcropped with the mantle cycle process. Considering the coexistence of unusual mantle minerals and possible cyclic crustal materials, the origin of the Luobusa chromitite might bear certain relationship with mantle plume.更多还原