西藏拉萨地块北部白垩纪火山岩及其对冈底斯岛弧构造演化的制约
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国家重点基础研究发展规划(973)资助项目(2011CB403100)


Cretaceous volcanic rocks in northern Lhasa Block: constraints on the tectonic evolution of the Gangdise Arc
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    摘要:

    拉萨地块北部广泛分布着白垩纪火山岩。本文及已有年代学研究表明,这些火山岩主要形成于早白垩世中期 (140~110 Ma)和晚白垩世早期 (100~80 Ma) 两个阶段。早白垩世中期火山岩主要为则弄群和多尼组地层中的火山岩夹层,岩性组合包括玄武岩、玄武安山岩、安山岩、英安岩和流纹岩,主体为高钾钙碱和钾玄质系列,具有初始87Sr86Sr比值(0.7068~0.7102) 较高、εNd(t) (-9.3~-1.5) 较低和δ18OV_SMOW (7.2‰~9.8‰) 较高等同位素组成特征,源区为受消减沉积物和/或蚀变玄武质洋壳的含水流体/熔体交代的富集岩石圈地幔楔。晚白垩世早期火山岩零星分布在拉萨地块北部和羌塘地块南部,岩石类型以玄武岩、玄武安山岩、安山岩为主,有少量的酸性火山岩(英安岩), 主体为典型的钙碱性系列组合(低钾拉斑+中钾钙碱),具有较高的Mg(Mg#可高达59)和相容元素Cr(162×10-6)、V(216×10-6)和Ni(80×10-6)含量以及更为原始的Sr-Nd同位素组成,(87Sr/86Sr) t = 0.7041~0.7048,εNd(t)=0.9~2.2,指示源区可能为受到上涌软流圈地幔熔体二次交代的交代富集岩石圈地幔,本文将其命名为尼玛火山岩。综合以上白垩纪火山岩和拉萨地块南部叶巴组、桑日群和林子宗群火山岩的时空分布特征,认为中生代至早第三纪冈底斯岛弧的演化发生了两次大的空间迁移:中生代早侏罗世以来由南向北变新,从最南部的叶巴组(早侏罗世,193~174 Ma)和桑日群火山岩(J3—K1),到北部的则弄群和多尼组火山岩(早白垩世中期,140~110 Ma),最后到最北部的尼玛火山岩(晚白垩世早期,100~80 Ma);早第三纪由北向南跃迁回南部,从北部的尼玛火山岩迁移到南部的林子宗群火山岩(70~40 Ma)。冈底斯岛弧火山岩的时空分布特征和成分演化规律,揭示了新特提斯洋板块的俯冲历史,即早期新特提斯洋壳由南向北低角度俯冲,然后再高角度反向旋转,直至最后发生拆沉。

    Abstract:

    Before the continental collision between India and Asia, there existed widespread Early Jurassic to Eocene arc magmatism in the Lhasa Block of southern Tibet, which resulted from the northward subduction of the Tethyan oceanic lithosphere beneath Asia. This paper has reported geochronological (40Ar/39Ar dating), major and trace elements and Sr-Nd-O isotopic data of the Cretaceous volcanic rocks widely exposed in the north of the Lhasa Block and tried to investigate the tectonic evolution of the Gangdise Arc. 40Ar/39Ar geochronolo gical data indicate that these Cretaceous volcanic rocks could be divided into two groups, namely Early Cretaceous (140~110 Ma) and Late Cretaceous (100~80 Ma). Early Cretaceous volcanic rocks refer to the interbeds in Zenong Group and Duoni Formation, which are mainly distributed in such central-northern areas of the Lhasa Block as Taruoco, Coqên, Dangruoyongco and Xainza and consist of basalt, basaltic andesite, andesite, dacite and rhyolite (mostly divided into high-K and shoshonitic series and subordinately into medium-K series). The authors focused their study on the basalt and basaltic andesite of Zenong Group (SiO2=47.76%~56.25%,high-K calc-alkaline and shoshonitic series in SiO2-K2O diagram), which are characterized by relative enrichment of light rare earth elements and large-ion lithophile elements, negative Eu anomaly (δEu=0.77~0.95), negative Nb, Ta, Ti anomalies, and relatively high initial (87Sr/86Sr)t (0.7063~0.7072), low 143Nd/144Nd (0.5124~0.5126) ratios and εNd(t) (-5.4~-1.5), and high δ18OV_SMOW (7.2‰~9.8‰), indicating that the magma source was the lithospheric mantle wedge metasomatized by the fluid/melts from the oceanic sediments or/and the oceanic crust. Late Cretaceous volcanic rocks mainly consist of intermediate-basic volcanic rocks in calc-alkaline (low-K tholeiitic and medium-K calc-alkaline, SiO2=50.91%~70.45%) series, sparsely exposed inthe north of the Lhasa Block and south of the Qiangtang Block. Compared with Zenong Group volcanic rocks, these volcanic rocks are characterized by younger ages, high Mg (Mg#=59) and compatible elements (Cr=162×10-6, V=216×10-6, and Ni=80×10-6) content, relatively primitive Sr-Nd isotopic composition (87Sr/86Sr)t=0.7041~0.7049; 143Nd/144Nd=0.5125~0.5128; εNd(t)=-3.5~2.2), suggesting that the magma source might have been the metasomatized lithospheric mantle wedge subsequently metasomatized by the melts from asthenospheric mantle. Such volcanic rocks are named Nima Group volcanic rocks in this study. The temporal-spatial distribution of Mesozoic-Paleogene volcanic rocks of the Lhasa Block implies that Gangdise Arc volcanic rocks have undergone two times of spatial migration. One is the northward migration, i.e., from Yeba Formation (193~174 Ma) and Sangri Group (J3—K1) in the southernmost area through Zenong Group/Duoni Formation in the north (140~110 Ma) to Nima Group in the northernmost area (100~80 Ma). The other is the migration from north (Nima Group volcanic rocks, 100~80 Ma) to south (Linzizong Group volcanic rocks, 70~40 Ma). The temporal-spatial distribution and geochemical characteristics of volcanic rocks reveal the north-dipping subduction history of the Neo-Tethys oceanic lithosphere, i.e., the evolution from low-angle and north-dipping subduction in the early stage through high-angle roll-back to final detachment.

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马国林,岳雅慧, 2010. 西藏拉萨地块北部白垩纪火山岩及其对冈底斯岛弧构造演化的制约[J]. 岩石矿物学杂志, 29(5):525~538.
MA Guo_lin, YUE Ya_hui, 2010. Cretaceous volcanic rocks in northern Lhasa Block: constraints on the tectonic evolution of the Gangdise Arc[J]. Acta Petrologica et Mineralogica, 29(5): 525~538.

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