| 喜马拉雅淡色花岗岩 |
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| 引用本文： | 吴福元,刘志超,刘小驰,纪伟强.喜马拉雅淡色花岗岩[J].岩石学报,2015,31(1):1-36. |
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| 作者姓名： | 吴福元 刘志超 刘小驰 纪伟强 |
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| 作者单位： | 中国科学院地质与地球物理研究所岩石圈演化国家重点实验室, 北京 100029;中国科学院地质与地球物理研究所岩石圈演化国家重点实验室, 北京 100029;中国科学院地质与地球物理研究所岩石圈演化国家重点实验室, 北京 100029;中国科学院地质与地球物理研究所岩石圈演化国家重点实验室, 北京 100029 |
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| 基金项目： | 本文受国家自然科学基金重点项目(41130313)和中国科学院战略性先导科技专项项目(B类)(XDB03010200)联合资助. |
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| 摘 要： | 在青藏高原南部的喜马拉雅地区,分布有两条世界瞩目的淡色花岗岩带。南带主要沿高喜马拉雅和特提斯喜马拉雅之间的藏南拆离系(STDS)分布,俗称高喜马拉雅淡色花岗岩带,构成喜马拉雅山的主体。北带淡色花岗岩位于特提斯喜马拉雅单元内,又被称之为特提斯喜马拉雅淡色花岗岩带。这些花岗岩多以规模不等的岩席形式侵入到周边沉积-变质岩系之中,或者呈岩株状产出于变质穹窿的核部。岩体本身大多岩性均匀,变形程度不等,但岩体边缘可见较多的围岩捕虏体,并在部分情况下见及围岩的接触变质作用,反映它们的异地侵位特征。上述两带中的淡色花岗岩在矿物组成和岩石类型上表现为惊人的相似性,主要由不同比例的石英、钾长石、斜长石、黑云母(5%)、白云母、电气石和石榴石等构成二云母花岗岩、电气石花岗岩和石榴石花岗岩三大主要岩石类型。从不同地区的野外观察来看,二云母花岗岩为喜马拉雅淡色花岗岩的主体岩石类型,而电气石花岗岩和石榴石花岗岩主要以规模不等的脉体形式赋存于二云母花岗岩之中,反映前两者晚期侵位的特征。地球化学特征上,这些花岗岩具有高Si、Al、K,低Ca、Mg、Fe、Ti的特点,接近花岗岩的低共熔点组分。绝大多数淡色花岗岩具有较高的含铝指数,属于过铝花岗岩。微量元素表现为较大的变化范围,但总体上表现为富集大离子亲石元素K、Rb和放射性元素U,而不同程度亏损Ba、Th、Nb、Sr、Ti等元素。稀土元素总量总体上明显低于世界上酸性岩的平均丰度,且绝大部分表现为轻-中等程度的稀土元素分馏和不同程度的Eu负异常。传统认为,喜马拉雅淡色花岗岩是原地-近原地侵位的纯地壳来源的低熔花岗岩。但本文通过分析提出,该花岗岩可能是从一种高温的花岗岩浆演化而来,其岩浆源区的性质或成因类型目前还难以确定。该岩浆在上升侵位的过程中曾经历过大规模地壳物质的混染,并发生了高度分离结晶作用。因此,喜马拉雅淡色花岗岩首先是一种高分异型的花岗岩,是真正意义上的异地深成侵入体,而并不是原地或半原地的部分熔融体。这种以大规模地壳混染和结晶分异作用为特征的花岗岩系,在花岗岩的研究内容中还未被充分地讨论。以前根据相关信息认为这些岩石来自于沉积岩部分熔融的结论,只是较多地注意到了后期地壳混染和结晶分异作用的特征。即使这些岩石的原始岩浆将来被证明真的来源于沉积岩系的部分熔融,那以前的结论也只能说是"歪打正着"。根据形成年龄和地质-地球化学特征,本文将这些花岗岩划分为原喜马拉雅(44~26Ma)、新喜马拉雅(26~13Ma)和后喜马拉雅(13~7Ma)三大阶段。其中第一阶段对应印度-亚洲汇聚而导致的大陆碰撞造山作用,而后两个阶段同加厚的喜马拉雅-青藏高原碰撞造山带拆沉作用有关,对应青藏高原的全面隆升。根据这些淡色花岗岩的岩石与地球化学特征,我们还不能支持青藏高原存在广泛的中地壳流动的模型。相反,俯冲的高喜马拉雅岩系在深部的部分熔融及随该岩系折返而发生的分离结晶作用可很好地解释淡色花岗岩所具有的系列特征。
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| 关 键 词： | 地壳流动 高度结晶分异作用 淡色花岗岩 喜马拉雅 |
| 收稿时间： | 5/4/2014 12:00:00 AM |
| 修稿时间： | 2014/8/27 0:00:00 |
Himalayan leucogranite: Petrogenesis and implications to orogenesis and plateau uplift |
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| WU FuYuan,LIU ZhiChao,LIU XiaoChi and JI WeiQiang.Himalayan leucogranite: Petrogenesis and implications to orogenesis and plateau uplift[J].Acta Petrologica Sinica,2015,31(1):1-36. |
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| Authors: | WU FuYuan LIU ZhiChao LIU XiaoChi and JI WeiQiang |
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| Institution: | State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China |
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| Abstract: | The Himalayan orogenic belt is characterized by widespread of leucogranites which are mostly peraluminous with mafic mineral less than 5%. Two sub-parallel belts, i.e., Higher Himalayan and Tethyan Himalayan, have been recognized according to their distribution. Petrologically, this kind of granitic rocks can be classified as three sub-groups of muscovite-, tourmaline- and garnet-bearing rocks. It was previously thought that the Himalayan leucogranite was formed during India-Asia continental collision by in situ partial melting of the Higher Himalayan paragneisses at amphibolite- to granulite-facies metamorphic conditions, since it locates near the minimum eutectic point in the granitic Q-Ab-Or system. It was also proposed that the muscovite-granite was formed at higher temperature than those of the tourmaline- and garnet-bearing granites. However, the detailed investigations indicate that this granite might be in fact a kind of high temperature magma, which had undergone an intensive crystal fractionation, and could be considered as an end-member of fractionated granites on our Earth. It is almost impossible to constrain its genetic type due to later fractional crystallization although it is mostly accepted that it is an S-type granite of purely crustal derived. The available geochronological data indicate that this granite can be subdivided into Eohimalayan, Neohimalayan and Posthimalayan with a duration of ~40Ma. The Eohimalayan(44~26Ma)leucogranite was formed during the India-Asia collision and subsequent slab breakoff, and the Neohimalayan(26~13Ma), as the main part of the Himalayan leucogranites, is closely related to delamination of the thickened Himalaya-Tibetan Plateau and induced exhumation of the subducted Higher Himalayan material. The Posthimalayan(13~7Ma)was probably resulted from the east-west extension during the ongoing convergence. Whatever, the Himalayan leucogranite cannot be considered as a representive of syn-collisional granitic magmatism in the continental-continental collisional zones. In addition, no documentation is found from these leucogranites to support model of the crustal channel flow beneath the Himalaya-Tibetan Plateau. In contrast, the partial melting of the subducted Higher Himalaya in depth, and the subsequent fractional crystallization associated with the exhumation of the partially molten rocks is more reasonably to be used to explain the development of the Himalayan leucogranites and their source characters. |
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| Keywords: | Crustal channel flow Highly fractional crystallization Leucogranite Himalaya |
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