蛇绿混杂岩的基质组成及其构造内涵:以东古特提斯为例

Matrix of the ophiolitic mélange zone and its tectonic implications:insights of the eastern Paleo-Tethys

蛇绿岩是恢复大洋演化最直接的证据,是识别板块汇聚边界的一级地质学标志。相比于蛇绿岩,蛇绿混杂岩的基质记录了更多关于大洋板块演化的信息。为了深入理解蛇绿混杂岩基质组成的构造内涵,我们选择了位于越南北部—中国滇东南地区的古特提斯哀牢山-Song Ma(马江)-Song Chay(斋江)缝合带开展研究。本文综合了前人有关该带蛇绿混杂岩基质碎屑锆石U-Pb定年和Hf同位素分析的结果,认为哀牢山-Song Ma-Song Chay蛇绿混杂岩基质具有强烈的横向不均一性,可以划分为M1、M2、M3和Song Chay单元。其中,M1位于哀牢山-Song Ma蛇绿混杂岩中部,构成了哀牢山蛇绿混杂岩的主体,具有440 Ma和960 Ma的特征碎屑锆石年龄峰值,结合其Hf同位素特征,我们认为M1的物源为印支板块。M2位于哀牢山-Song Ma蛇绿混杂岩的NW部,显示出单一的260 Ma左右的碎屑锆石年龄峰值,结合其Hf同位素特征,我们认为M2主要来自于华南板块上的峨眉山大火成岩省。M3位于哀牢山-Song Ma蛇绿混杂岩的SE部,碎屑锆石年龄结果显示出250 Ma的主要峰值以及370 Ma和780 Ma的次要峰值,结合其Hf同位素特征,我们认为M3的主要物源为印支板块的弧岩浆岩,少量来自华南板块。Song Chay蛇绿混杂岩基质中的碎屑锆石主要是来自作为被动陆缘的华南板块,只有很少一部分来自上覆的印支板块。上述蛇绿混杂岩组成的横向不均一性,指示了洋盆关闭过程中物源的变化。更重要的是,基质中鲜明的俯冲板块(华南)被动陆缘的信息,使我们推断古特提斯东部在某一阶段或某一部位表现为有限洋盆,其对我们理解东古特提斯板块拼合过程具有重要意义。

The ophiolite is the direct evidence to restore the oceanic evolution,and it is used to identify the convergence boundary of the plates.Compared with ophiolite,the matrix of ophiolite records more information about the evolution of oceanic plates.In order to understand the tectonic implications for matrix of ophiolitic mélange,the Mesozoic Paleo-Tethys Ailaoshan-Song Ma-Song Chay suture zone located in the North Vietnam-Southeast Yunnan region acts as an ideal study area.On the basis of the structural geology,we reviewed previous zircon U-Pb dating and Lu-Hf isotopic analyses on the detrital zircon from the Ailaoshan-Song Ma ophiolitic mélange.Accordingly,we subdivide the matrix of the Ailaoshan-Song Ma ophiolitic mélange into three parts(M1,M2,and M3).M1 is mainly located in the middle segment of the Ailaoshan-Song Ma belt.It shows the age peaks of 440 Ma and 960 Ma withεHf(t)values of-19.6~+10.3,indicating the affinity of the Indochina block.M2 is mainly located in the NW segment of the Ailaoshan-Song Ma belt,showing a dominant age peak of~260 Ma.Particularly,it hasεHf(t)values of-18.9~+8.1,likely sourced from the ELIP on the South China block.M3 is mainly located in the SE segment of the Ailaoshan-Song Ma belt,showing the peaks at~250 Ma,440 Ma,and 960 Ma withεHf(t)values of-11.9~+10.1;its main provenance is the magmatic arc on the Indochina block.The detrital zircons of the Song Chay ophiolitic mélange matrix are mainly from the subducting South China plate,with only a small part from the overlying Indosinian plate.The above-mentioned heterogeneity of the ophiolitic mélange indicates the change of detrital zircon provenances during the closure of the ocean basin.Importantly,we argue that the detrital material is mainly from the passive continental margin of the subducted SCB plate,while does not invariably come from the overriding Indochina plate.It likely implies that the eastern Paleo-Tethys was a limited oceanic basin instead of a vast ocean.