西藏冈底斯带侏罗纪岩浆作用的时空分布及构造环境

    Spatial-temporal distribution and tectonic setting of Jurassic magmatism in the Gangdise belt, Tibet, China

    • 摘要: 在新近完成的1∶25万区域地质调查资料和相关研究成果的基础上,初步研究了西藏冈底斯带侏罗纪岩浆作用的分布特点及其年代学,并利用已有的地球化学数据重点分析了早期关注程度较低的侏罗纪花岗岩类岩浆作用的性质。目前在冈底斯弧背断隆带未发现侏罗纪火山岩;在冈底斯东部地区,早侏罗世岩浆活动几乎同时发生于南冈底斯(叶巴组火山岩和乌郁、尼木花岗岩类)、冈底斯弧背断隆带(宁中、金达、布久花岗岩类)和北冈底斯(聂荣花岗岩类),中晚侏罗世接奴群和拉贡塘组火山岩断续分布于北冈底斯,晚侏罗世岩浆活动零星分布于沙莫勒-麦拉-洛巴堆-米拉山断裂以北。将冈底斯侏罗纪岩浆活动置于时空框架内分析发现,南冈底斯和北冈底斯在侏罗纪时主要受俯冲作用的影响,而冈底斯弧背断隆带和中冈底斯自早侏罗世以来除了受到俯冲作用的影响外,还受到自东向西逐步扩展的碰撞作用的影响。结合古地磁重建资料和其他新发现,认为冈底斯带侏罗纪这种岩浆活动的特点可用班公湖-怒江洋壳向南、新特提斯洋壳向北的双向剪刀式(剪刀口向西张开)俯冲模式来解释。

       

      Abstract: The distribution and age-constrained by stratigraphy, paleontology and isotopic age dates of Jurassic igneous rocks, and the nature of Jurassic granitoids that are poorly discussed previously in the Gangdese Belt are preliminarily summarized on the basis of regional geological survey(1∶250000) and related works. At present no Jurassic volcanic rocks have been found in the Gangdise back-arc fault uplift belt; Early Jurassic magmatism in the eastern Gangdise took place almost synchronously in the southern Gangdise (e.g. the Yeba Formation volcanic rocks and Oiyug and Niêmo granitoids), Gangdise back-arc fault uplift belt (e.g. Nyingzhong, Gyinda and Puqu granitoids) and northern Gangdise (Nyainrong granitoids). Mid-Upper Jurassic Jienu Group and Lagongtang Formation volcanic rocks are discontinuously distributed in the northern Gangdise and Late Jurassic magmatism is only sporadically observed to the north of the Shamolei-Maila-Luobadui-Milashan fault. Geochemical analysis combined with spatial and temporal constraints on the Jurassic magmatism indicates that the southern Gangdise and northern Gangdise were primarily affected by subduction during Jurassic time, while the Gangdise back-arc fault uplift belt and central Gangdise were not only affected by subduction but also by westward-propagating collision since Early Jurassic time. Considering paleomagnetic reconstruction and other related new findings, it may be considered that the features of such Jurassic magmatism in the Gangdise may be explained using the bidirectional scissors-style (opening westward) subduction model involving the southward subduction of the Bangong Co-Nujiang Tethyan oceanic crust and northward subduction of the Neo-Tethyan oceanic crust.

       

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