浙黔桂地区寒武纪硅质岩的地球化学特征及其形成背景

Geochemistry and tectonic setting of the Cambrian siliceous rocks in the Zhejiang-Guangxi-Guizhou zone

关键词:  硅质岩  寒武纪  常量元素  稀土元素

Key words:  siliceous rock  Cambrian  major element  REE

摘要:

本文采用常量和稀土元素地球化学方法对浙黔桂地区寒武纪硅质岩的形成背景进行了研究。研究表明,硅质岩的Al₂O₃/(Al₂O₃+Fe₂O₃)为0.56~0.87;Ce/Ce^*为0.74±0.19;La_n/Ce_n为0.90~1.58;La_n/Yb_n平均为6.38,轻稀土明显富集,这些特征表明研究区硅质岩形成于大陆边缘环境或大陆边缘与深海过渡带。形成于裂陷盆地初期的硅质岩,比古华南残留洋盆和扬子陆块边缘的硅质岩轻稀土元素更加富集。构造拉张期形成的硅质岩为热液成因硅质岩,其稀土配分模式显示出轻稀土相对亏损、Eu正异常特征;构造稳定期形成的硅质岩为沉积成因硅质岩,稀土配分模式显示出轻稀土相对富集、Eu弱负异常特征。由硅质岩稀土配分模式的差异性反映出寒武纪时期的拉张为幕式拉张。位于扬子地台边缘的硅质岩所具有的Ce负异常,暗示着江南造山带寒武纪后曾发生过由东向西的大规模逆冲推覆。

Abstract:

The tectonic setting of the Cambrian siliceous rocks in the Zhejiang-Guangxi-Guizhou zone are discussed with the aid of major and rare earth element geochemistry. In the siliceous rocks, Al₂O₃/(Al₂O₃ + Fe₂O₃) ratios range from 0.56 to 0.87. Ce/Ce^* ratios equal to 0.74±0.19. La_n/Ce_n ratios range from 0.90 to 1.58. Average values of La_n/Yb_n ratios equal to 6.38. All these results show that the Cambrian siliceous rocks in the Zhejiang-Guangxi-Guizhou zone were formed in the continental-margin environment or transitional zone from the continental margin to deep sea. LREEs are more enriched in the siliceous rocks from early rift basins than those from the ancient Huanan residual oceanic basin and Yangtze block. The siliceous rocks formed during tectonic extension tend to have hydrothermal origin, and display the REE distribution patterns of being depleted in LREE and positive Eu anomaly. The siliceous rocks formed during stable tectonism tend to be sedimentary in origin, and display the REE distribution patterns of being relative enriched in LREE and slight negative Eu anomaly. The differences in the REE distribution patterns cited above indicate the episodic extension during the Cambrian. The negative Ce anomaly in the siliceous rocks on the margins of the Yangtze platform implies that the large-scale EW-trending overthrusting once occurred in the Jiangnan orogenic zone after the Cambrian.