Zircon U-Pb geochronological constraints on rapid exhumation of the mantle peridotite of the Xigaze ophiolite, southern Tibet
Zircon U-Pb geochronological constraints on rapid exhumation of the mantle peridotite of the Xigaze ophiolite, southern Tibet
复制标题
藏南日喀则蛇绿岩地幔橄榄岩快速折返的锆石U-Pb年代学约束
DOI：
10.1016/j.chemgeo.2016.09.015
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发表时间：
2016-12
期刊：
影响因子：
3.9
通讯作者：
Xu, Yang
中科院分区：
文献类型：
--
作者：
Liu, Chuan-Zhou;Ji, Wen-Bin;Zhang, Chang;Xu, Yang
日喀则蛇绿岩出露于藏南雅鲁藏布江缝合带中段。它的特点是大量的超镁铁岩单元和少量的镁铁岩。基性岩由辉长岩、辉长岩和玄武岩组成。日喀则蛇绿岩中的辉长岩呈层状体或各向同性脉岩侵入地幔剖面。大竹渠、白岗、吉顶等地均保存有六角形辉长岩体。但是，它们的形成时间和生成机制尚未系统地提出或很好地理解。本文对日喀则蛇绿岩中的9个镁铁质岩石样品进行了原位锆石U-Pb和Hf同位素分析，其中包括7个辉长岩和2个由辉长岩蚀变而来的钠长岩。地球化学特征表明，这些岩石是由正常洋中脊（N-MORB）型熔体侵入形成的。U-Pb测年结果一致，为124-129 Ma，但不确定。锆石εHf（t）值为正值，表明样品为亏损地幔源区。结合前人对基性岩脉、斜长角闪岩块体和放射虫硅质岩的研究，可以认为雅鲁藏布江蛇绿岩形成于119 ~ 132 Ma的较短时间内。因此，日喀则蛇绿岩中的地幔橄榄岩和辉长岩可能发生了快速折返，从而侵入了闪长岩脉和岩床。它排除了长期存在的古老岩浆房或透镜的存在。辉长岩更可能是一系列深成侵入体，位于化石缓慢扩张的山脊之下，而不是岩浆房的产物。因此，雅鲁藏布江蛇绿岩的形成可以应用“查普曼分离模型”。
The Xigaze ophiolite outcrops in the central segment of the Yarlung Zangbo suture zone, southern Tibet. It is characterized by large amounts of ultramafic units with minor mafic rocks. The mafic rocks consist of gabbros, diabases and basalts. The gabbroic rocks of the Xigaze ophiolite occur as layered bodies or isotropic dikes intruding into mantle sections. Hectometric-sized gabbroic bodies are well-preserved in Dazhuqu, Baigang and Jiding. However, their formation time and generation mechanism are not systematically proposed or well understood. In this study, nine samples of mafic rocks from the Xigaze ophiolite, including seven gabbros and two rodingites (altered from diabases), were selected for in situ zircon U-Pb and Hf isotopic analyses. The geochemical feature suggested that these rocks formed by the intrusion of melts of normal mid-ocean ridge (N-MORB) type. The U-Pb data yielded identical ages of 124–129 Ma within uncertainties. Positive zirconεHf(t) values indicated that these samples had an origin of depleted mantle source. Combined with previous studies on mafic dikes, amphibolite blocks and radiolarian cherts, it can be concluded that the Yarlung Zangbo ophiolites formed over a short period of time from 119 to 132 Ma. Hence, a rapid exhumation of the mantle peridotites and gabbroic rocks of the Xigaze ophiolite may have occurred to get intrusion of the diabase dikes and sills. It excludes the existence of a long-term ancient magma chamber or lens. It is more likely that the gabbroic rocks are a series of plutonic intrusions beneath a fossil slow-spreading ridge, rather than products of magma chambers. Therefore, the “Chapman detachment model” may be applied to the generation of the Yarlung Zangbo ophiolites.
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