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

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.