IOCAS-IR
大洋闭合时洋壳熔融的物质证据与大陆地壳生长
陈硕
第一作者单位中国科学院海洋研究所 ; 海洋地质与环境重点实验室
学位类型博士
导师牛耀龄
2018-05-06
学位授予单位中国科学院大学
学位授予地点中国科学院海洋研究所
学位名称理学博士
学位专业海洋地质
关键词大陆地壳增长 洋壳熔融 花岗岩类 暗色包体 微量元素分析
摘要

经过几代人的努力,我们对陆壳化学组成的认识日趋完善,且熟知它是非常重要的地球化学储库,然而我们对陆壳成因和演化的认识还远远不够,如大陆地壳是如何生长以及生长速率是多少至今仍然悬而未决。在认识到大陆地壳增长经典“岛弧模型”的缺陷后,Niu及其合作者基于藏南地区林子宗同碰撞火山岩研究,提出了全新的假说,即“大陆碰撞带是大陆地壳生长的主要场所Niu et al.2013)。包括祁连山在内的青藏高原出露有大量的同碰撞花岗岩类(及火山岩),是研究同碰撞岩浆作用以及检验大陆碰撞带为陆壳增长主要场所假说最理想的天然实验室。本文选取北祁连造山带同碰撞岩体为研究对象,通过详细的野外调查、岩相学、矿物学、年代学以及地球化学的研究,主要取得了以下五点成果:1)揭示了造山带岩体中暗色包体是同源岩浆早期堆晶的产物,并不是流行观点认为的基性岩浆和酸性岩浆混合的产物;并通过模拟计算进一步建立了暗色包体成因模型。2)在全面评估现有的埃达克岩及埃达克质岩石成因模型基础上,本文提出了埃达克质岩石成因新认识,即研究区埃达克质花岗闪长岩是暗色包体结晶分异所致。3)北祁连老虎山两类暗色包体的发现,进一步证实了暗色包体堆晶成因。角闪石成分研究以及成果1和成果2 的重要启示下,本文确认了北祁连造山带同碰撞花岗岩(及暗色包体)是大陆碰撞时洋壳与沉积物部分熔融的物质证据。4)老虎山花岗岩与暗色包体“完美”互补及与大陆地壳的高度相似特征,为大陆碰撞带是大陆地壳生长的主要场所这一假说提供了新的证据。5)本文依托中国科学院海洋研究所“大洋岩石学与地幔动力学”实验室,通过一系列实验,建立了地质样品微量元素高效消解及ICP-MS高精度分析方法,并开发数据处理软件OxideCALTraceCAL。这一新方法,简单易行、高效经济,可被广泛用于ICP-MS精确、快速分析地质样品中的微量元素。

其他摘要

It has been well established for the bulk composition of continental crust after painstaking effort built on works of generations, yet our knowledge remains limited on its origin, its way and rate of growth. To overcome the shortcomings of the standard “island arc” model for continental crust growth, Niu and co-authors proposed a novel hypothesis that continental collision zones are primary sites of net continental crustal growth on the basis of their studies on the India-Asia syn-collisional~55 Maandesitic rocks in southern Tibet. The abundant syn-collisional granitoidsand equivalentproduced and preserved at northern margin of the Tibetan Plateau is considered to provide an exceptional window into syn-collisional felsic magmatism and continental crust growth. Here we present the results of a comprehensive study of the syn-collisional granitoids and their enclosed mafic magmatic enclavesMMEson plutons from North Qilian orogenic beltNQOB. Collectively, the main achievements of this study can be summarized as follows: 1The MMEs and their host granitoids share many characteristics in common, including identical crystallization age~430 Ma, same mineralogy, similar mineral chemistry and whole-rock isotopic compositions, indicating the MMEs being of cumulate origin formed at earlier stages of the same magmatic systems rather than representing mantle melt required by the popular and alleged magma mixing modelAccordingly, we established a cumulate model for the formation of orogenic MMEs. 2By accepting our model for the petrogenesis of the MMEs, we have proposed a new model for the origin of adakitic rocks in syn-collisonal sittings after thorough assessment of existing models, e.g., the QMS adakitic granodiorite resulted from fractional crystallization dominated by mineral assemblages represented by the MMEs. 3The two types of MMEs and the LHS granitoid host are coeval and have indistinguishable bulk-rock Sr-Nd-Hf isotopic compositions, futher suggesting that the MMEs consistent with being earlier crystallized cumulate of the same magmatic systemsThe syn-collsional granitoids in the NQOB presented here are material evidence of melting ocean crust and sediments4The remarkable compositional complementarity between the LHS granitoid plutonMMEs and the host granitoidand the model bulk continental crustin terms of major and trace elements, and some key incompatible trace element ratiosindicate that the syn-collisional magmatism in the NQOB makes net contribution to the mass of continental crust, and that the way of continental growth in the Phanerozoic through syn-collisional felsic magmatismproduction and preservationis straightforward and does not require any additional processes. 5Incomplete sample digestion, isobaric interferences and instrumental drift remain obvious problems that must be overcome in order to obtain precise and accurate analysis of trace elements in rocks using the ICP-MS technique. For this reason, we have done many experiments and developed a set of simple, cost-effective and practical methods widely applicable for precise and rapid determination of trace element abundances in geological materials using ICP-MS.

学科门类理学::海洋科学
语种中文
文献类型学位论文
条目标识符http://ir.qdio.ac.cn/handle/337002/154442
专题中国科学院海洋研究所
推荐引用方式
GB/T 7714
陈硕. 大洋闭合时洋壳熔融的物质证据与大陆地壳生长[D]. 中国科学院海洋研究所. 中国科学院大学,2018.
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