Provenance of the Laga Formation at the Nixiong Area of the Western Lhasa Terrane: constraints from geochemistry of clastic rocks
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摘要: 拉萨地块西段缺失晚古生代岩浆岩，造成不能从岩浆岩角度探索其晚古生代地质演化，因此沉积岩的对比研究对理解拉萨地块晚古生代的差异演化过程至关重要。本文通过对尼雄地区上石炭统—下二叠统拉嘎组碎屑岩地球化学特征分析，试图揭示其物源特征及源区构造背景，提供拉萨地块晚古生代沉积岩对比分析的基础信息。尼雄地区拉嘎组一段和二段主量和微量元素特征略有差异，二段较一段CaO、Na2O含量更高，V、Cr、Ni、Co及轻稀土含量也更高，反映二者沉积源区有一定差异。拉嘎组物源区的构造背景兼具被动大陆边缘与活动大陆边缘特征，并且二段较一段显示更强的活动大陆边缘亲缘性。结合区域地质资料，认为拉嘎组碎屑岩主要来自成熟的大陆源区，为冈瓦纳大陆北缘基底岩石及早古生代沉积岩的再旋回沉积，二段源区的少量酸性及基性物质组分对应于冈瓦纳大陆北缘泛非期及寒武纪岩浆活动。Abstract: Owing to the lack of Late Paleozoic magmatic rocks in the western segment of Lhasa terrane, it is difficult to understand its Late Paleozoic tectonic evolution in terms of magmatism. Here we report the geochemical characteristics of the clastic rocks from Laga Formation in Nixiong area, in order to reveal its tectonic background and provenance. The contents of main and trace elementcs of the First and Second Members of Laga Formation in Nixiong areas are slightly different. The Second Member of Laga Formation show higher CaO, Na2O and V, Cr, Ni, Co contents, and light rare earth elements than the Second Member of Laga Formation, indicating their different sedimentary sources. The tectonic settings of the Laga Formation are characterized by passive continental margin and active continental margin, and the Second Member is with more features of active continental margin. The clastic rocks of the Laga Formation were mainly derived from the matural continental source areas, and the source materials were from recycling deposition of the basement rocks and the early Paleozoic sedimentary rocks in the northern margin of Gondwana land. The source rocks of the Second Member of Laga Formation comprised a few acid and basic components, which equated with magmatic activities during Pan-African period and Cambrian in the northern margin of Gondwanaland.
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Keywords:
- Lhasa terrane /
- Nixiong area /
- the Laga Formation /
- geochemisty /
- provenance
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