【作者】 林广春；

【导师】 李献华；

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

【摘要】 扬子块体西缘的新元古代岩浆活动非常强烈,其成因研究对于认识Rodinia超大陆的演化具有重大的意义。目前在地学界对这些岩浆岩的成因和形成的构造背景存在很大争议,主要是地幔柱和岛弧成因之争。本文选取川西康滇裂谷中广泛分布的基性岩墙群和与其密切共生的花岗质岩石(瓦斯沟杂岩体和石棉花岗岩体)为研究对象,在野外地质观察的基础上,通过SHRIMP锆石U-Pb年代学、岩石学、地球化学和Nd-Hf同位素等综合研究探讨这些岩浆岩的形成时代、成因及其大地构造意义。 川西瓦斯沟杂岩体和石棉花岗岩体形成于新元古代,为Ⅰ型花岗岩,岩浆经历了斜长石、角闪石、磷灰石和Fe-Ti氧化物等的分离结晶作用。样品具有明显的Nb-Ta亏损等类似于岛弧岩浆的地球化学特征,但同时也表现出强烈的Sr、P、Eu负异常,并具有较高的微量元素含量,这与典型的形成于岛弧环境岩石不同,而与形成于板内环境的岩石相类似。本文的研究认为这些新元古代花岗质岩石是板内岩浆活动的产物,来源于初生岛弧地壳的部分熔融,并受到古老地壳物质的混染。它们的Nb-Ta亏损是继承了源岩(初生岛弧地壳)的地球化学特征,不代表其形成于岛弧构造环境。 在川西康定-泸定-石棉地区发育大量的基性岩墙群,与上述新元古代花岗质岩石在时-空上密切共生,尤其在瓦斯沟地区,基性岩墙与瓦斯沟花岗质杂岩呈现明显的岩浆混合现象。野外地质关系和SHRIMP锆石U-Pb年龄测定结果表明,这些岩墙群的侵位结晶发生在780-760 Ma。基性岩墙为拉斑玄武质系列岩石,岩更多还原

【Abstract】 Neoproterozoic magmatic rocks are widespread in western margin of the Yangtze block, and their origin and genesis have significant implications for understanding the evolution of the Rodinia supercontinent. At the present time, the genesis and tectonic setting of these magmatic rocks have been an issue of hot debate, i.e., mantle plume vs island arc in origin. On the basis of field investigations, mafic dyke swarms and associated granitoid rocks including the Wasigou complex and the Shimian granite from western Sichuan were selected for comprehensive studies on SHRIMP zircon U-Pb geochronology, petrology, geochemistry and Nd-Hf isotope in this paper, with the aim of revealing their petrogenesis and tectonic implications.Granitoid rocks within the Wasigou complex and Shimian granite from the Kangdian Rift of western Sichuan are of metaluminous I-type. They were emplaced in Neoproterozoic, and experienced the fractional crystallization of plagioclase, hornblende, apatite and Ti-Fe oxides. These granitoids display some geochemical similarities to rocks formed in the island-arc environments such as Nb-Ta depletion. However, they are also depleted in Sr, P and Eu, and high in most other trace elements, which differ from those of typical rocks formed in the island-arc environments, but resemble those of intra-plate granitoids. It is suggested that these granitoid were derived from partial melting of pre-existing, young island arc crust, with contamination of old crust materials during magma ascending and emplacement. Their arc-like geochemical features (such as Nb-Ta depletion) should have been inherited from theprotoliths, rather than inflection of their tectonic setting when the granitoids formed.There are abundant mafic dykes in the Kangding-Luding-Shimian region within the Kangdian Rift, which were spatially and temporally coeval with the Wasigou complex and Shimian granite. In the Wasigou region, mafic dykes show clear phenomena of magma mingling with granitoid wall rocks. Mafic dykes are tholeiite in composition. They expereinced fractional crystallization of olivine, apatite, etc. Field geologic observation and SHRIMP U-Pb zircon data indicate that these mafic dykes were formed at 780 ~ 760 Ma. They are characterized by two-fold geochemical characteristics of intra-plate and island arc basalts. The highest eNd(T) (~8) and eHf(T) (~17) values for the mafic dykes suggest that mafic magmas were derived from depleted asthenosphere mantle source. Modal calculation for the high MgO samples suggests that their melting temperature is about 1450—1470 °C, significantly higher than the potential temperature of 1280—1350 °C for normal asthenosphere mantle. In addition, these high MgO mafic dykes are also high in Fe/Mn ratios, suggesting an anomalously hot mantle plume for origin of these mafic dykes. The mafic magmas were contaminated by young island arc crust to variable degrees while ascending and emplacement, resulting in some "arc-like" geochemical features for some highly contaminated samples.These ca. 780—760 Ma mafic dykes in western Sichuan have a close genetic relationship in time and space to the plume-related ca. 780 Ma mafic magmatic rocks in western North America and ca. 755 Ma mafic dyke swarms in NW Australia. Our results support the reconstruction model of Rodinia in which the South China Block was located between Australia and Laurentia.更多还原