摘要
班公湖—怒江缝合带作为青藏高原的主缝合带之一,其研究对班公湖—怒江特提斯洋的演化和造山带的研究具有重要意义。本文首次对该缝合带西段昂龙岗日地区沙木罗组火山岩进行系统的野外地质调查,并结合全岩地球化学分析,探讨火山岩的成因、构造环境及地球动力学背景。沙木罗组火山岩从底部至顶部共划分出3个火山喷发亚旋回,岩性为安山岩和安山质火山碎屑岩。地球化学分析表明3个亚旋回的火山岩均为钙碱性系列,LREE和不相容元素相对富集,而相对亏损HREE和Nb、P、Ti等高场强元素,具岛弧火山岩的地球化学特征。第一喷发亚旋回火山岩具典型的高镁安山岩特征,暗示是大洋板块俯冲作用的产物,从第一喷发亚旋回至第三喷发亚旋回构造环境由大洋岛弧过渡为大陆边缘弧,火山岩形成于大洋地壳向大陆地壳之下俯冲的背景,且洋壳初始俯冲的时间不晚于晚侏罗世末。岩浆起源于受俯冲板片流体交代的亏损地幔—弱富集地幔,来自俯冲板片的流体上升,引起上覆地幔楔物质发生部分熔融,形成幔源岩浆,在密度差的作用下幔源岩浆向上运移,底侵下地壳,引发下地壳物质发生部分熔融,并与之发生混合作用,形成了研究区火山岩的母岩浆,在岩浆源区或岩浆上升过程中遭受了地壳物质的混染作用,且岩浆演化以部分熔融作用为主,结合区域大地构造背景认为沙木罗组火山岩的形成与班公湖—怒江特提斯洋盆向北向羌塘地块之下俯冲关系密切,沙木罗组角度不整合覆盖在木嘎岗日岩群之上不能作为班公湖—怒江特提斯洋盘闭合的证据。
Objectives: Bangong Lake-Nujiang River Suture Zone is one of the main suture zones in Qinghai-Xizang( Tibet) Plateau,and researches of it have great significance on the researches of the evolutional process of the Tethyan ocean and orogenic belt.Methods: This paper,for the first time,investigates the causes,tectonic settings,and instructions on regional tectonic evolution of volcanic rock by using bulk geochemistry analysis based on systematic field geological investigation of the Shamuluo Formation volcanic rocks in Anglonggangri area located in the west section of the Bangong Lake-Nujiang River Suture Zone.Results: Three volcanic eruption cycles are divided from the top to the bottom with the lithology of andesite and andesitic volcanoclastic rock. Geochemical analysis shows that volcanic rocks in these three cycles belong to calc-alkaline series featured by island arc volcanic rock with relative enrichment of LREE and incompatible element and relative depletion of HREE and high field strength elements like Nb,P,and Ti. Volcanic rocks in the first cycle are featured by classical high-Mg andesite,which implies that plate subduction is likely to start. Tectonic setting from the first eruption cycle to the third one is a transition to continental marginal arc from oceanic island arc. Volcanic rocks in Anglonggangri area are formed during oceanic plate subduction from oceanic crust to the blow of it,and the initial time of subduction is no later than late Jurassic. Source region is depleted-weak enriched mantle influenced by fluid metasomatism of subduction slab. Mantle magma is formed during the partial melting of overriding mantle wedge resulted from ascending fluid of subduction slab,and it migrates upwards as a result of density difference,then intrudes into the lower crust and triggers the partial melting of mantle wedge in the lower crust. Moreover, mantle magma blends with those in the lower crust, thus mother magma of andesite in Anglonggangri area comes into being. Magma source may be contaminated with crustal materials during its ascent process. In addition,magmatic evolution is dominated by partial melting process.Conclusions: Combined with regional tectonic setting,it is considered that the formation of the Shamuluo volcanic rocks is closely correlated with northward subduction of Bangong Lake-Nujiang River’s Tethyan oceanic crust beneath the Qiangtang blocks,and the Shamuluo Formation rests with angular unconformity on the Muggar Kangri Group--complex in the area cannot be the evidence for the close of Tethys ocean of Bangong Lake-Nujiang River Suture Zone.
出处
《地质论评》
CAS
CSCD
北大核心
2019年第3期573-588,共16页
Geological Review
基金
国家重点研发计划--深地专项(编号:2018YFC0604101
2018YFC0604106)
中国地质调查局项目资助(编号:DD20190167)联合资助的成果~~
