川西归宁地区西康群物源和构造背景:来自碎屑岩地球化学的约束

Tectonic setting and provenance of the Xikang Group in the Guining area,West Sichuan Province:Constraints from geochemistry of clastic sedimentary rocks

川西归宁地区紧邻江浪穹隆和长枪穹隆,大面积出露三叠系西康群,主体表现为一套浅变质的浊积岩复理石建造。对该套地层的碎屑岩进行岩石学、地球化学特征研究,并对物源和其亲缘关系进行了探讨。结果显示,砂岩主量元素SiO_(2)、P_(2)O_(5)与上地壳相近,CaO、MgO较高,而Al_(2)O_(3)、Fe_(2)O_(3)、K_(2)O、TiO_(2)含量偏低;微量元素蛛网图呈"M"型向右缓倾,富Th、U、La、Nd、Zr、Sm,贫K、Nb、Sr、P、Hf元素;样品轻、重稀土元素分馏明显,属轻稀土元素富集型,具中等负Eu异常;化学蚀变指数及成分成熟度均偏低,反映出物源区化学风化程度弱,沉积物具近源快速堆积的特征,是活动构造带的首次沉积;砂岩-泥岩套的物源区指纹判别图解及微量元素值显示该区西康群为上地壳长英质物源区。根据地球化学构造判别图解,并结合野外调查的古水流和古地理重建的证据,该套碎屑岩沉积于活动大陆边缘及大陆岛弧的构造背景,其物源区有可能来自于南东方向邻近的康滇古陆,与扬子地块西缘具有较强的亲缘关系。

The Xikang Group,distributed in the Guining area between the Changqiang Dome and Jianglang Dome in West Sichuan Province,is one set of low metamorphosed turbidite flysch.Based on the study of their petrological and geochemical characteristics of the clastic rocks,the provenance and affiliation were analyzed.The results show that the major elements of the sandstone,SiO_(2) and P_(2)O_(5),are similar to the upper crust,with relatively high CaO and MgO,while the contents of Al_(2)O_(3),Fe_(2)O_(3),K_(2)O and TiO_(2) are relatively low.The Chondrite-normalized trace element spider diagram of clastic sedimentary rocks is "M"-shaped and inclined to the right,rich in Th,U,La,Nd,Zr,Sm and poor in K,Nb,Sr,P,Hf.The rare earth elements of the samples display obvious fractionation in light and heavy REE,strong enrichment in light REEs with medium negative Eu anomaly.The chemical alteration index(CIA) and composition variation index value(ICV) are both low,reflecting the weak weathering degree of the source area and the characteristics of rapid accumulation of sediments near source,which represents the first deposition of the active structural belt.The fingerprint discrimination diagram and trace element value show that the source of the Xikang Group in this area was felsic in upper crust.According to geochemical tectonic setting discrimination diagrams,the structural background of the clastic rocks is active continental margin and continental island arc.Based on the paleocurrent evidence of field investigation and the reconstruction of paleogeography,it is inferred that the provenance of the Xikang Group might have come from the Kangdian ancient land in the southeast direction,and have an affinity to the Yangtze Block.