西藏日喀则蛇绿岩镁铁质岩石Re-Os同位素特征及意义

通过对日喀则蛇绿岩的镁铁质岩石进行岩石学和地球化学研究,探讨其岩石成因及源区性质,同时根据Re-Os同位素的地球化学约束来探索雅鲁藏布蛇绿岩的形成机制。研究表明,日喀则蛇绿岩镁铁质岩石微量元素的标准化配分型式与洋中脊玄武岩类似,又具有岛弧玄武岩的地球化学特征。结合微量元素和同位素特征,均表明岩石的形成与俯冲作用有关。日喀则蛇绿岩产出于远离大陆地壳的SSZ环境,其形成过程未受到陆壳的混染;同时,Re-Os同位素体系受蚀变作用的影响也不明显。日喀则蛇绿岩镁铁质岩石的Re、Os含量低,¹⁸⁷Os/¹⁸⁸Os同位素比值较高,主要是源区性质和俯冲作用影响的结果。特提斯洋早期发生的多次俯冲作用造成地幔源区不均一。新特提斯洋壳俯冲过程中,上述不均一地幔发生部分熔融产生的镁铁质岩浆上升,经过遭受了早期熔体/岩石作用的纯橄岩通道,发生强烈的Re-Os同位素分馏,使熔体与地幔残余Os同位素组成表现出明显的解耦现象,进而形成现今的日喀则蛇绿岩。

Re-Os Isotopic Characteristics and Geological Significance of Mafic Rocks from the Xigaze Ophiolite,Tibet

Petrology and elemental geochemistry of mafic rocks from the Xigaze ophiolite are studied so as to investigate the petrogenesis and source characteristics of the ophiolite. At the same time, Re-Os isotope geochemical constraints in formation mechanisms of the Yarlung Zangpo ophiolite is also explored. The result shows that the primitive mantle-normalized multi-element pattern of the mafic rocks is similar to the normal Mid-Ocean Ridge Basalt (N-MORB) with the characteristics of island arc. Combined with the characteristics of the trace elements and isotope, all of these are shown that the petrogenesis is related to subduction. The Xigaze ophiolite was formed in the SSZ environment far away from the continental crust, so it’s not contaminated by the continent crust during its development, also the alteration in Re-Os isotope systematics was not obvious. The reason of Re-Os isotopic characteristics with low Re, low Os and high ratios of ¹⁸⁷Os/¹⁸⁸Os for the mafic rocks is manifested by the source characteristics and the subduction. The multi-stage Tethyan subduction resulted in the mantle heterogeneity. During the subduction of Neo-Tethys, the mafic magma from the above-mentioned heterogeneous mantle went through the peridotite channel influenced by the early melts, leading to the Re-Os fractionation and amplifying the Os-isotope mismatch between the melt and residual mantle, that’s the petrogenesis of the Xigaze ophiolite.