Geochemistry Characters And Background Of Paleozoic Cherts In Jiangnan Orogenic Belt

The research on siliceous rocks bears very important theoretical significance and practical value. The previous study show that the geochemical characteristics of siliceous rocks has very important significance on discussing its formation causes, sedimentary environment, geological background and revealing the evolutionary history of the regional geological structureJiangnan orogenic belt is a complex structural belt that experiences multi-stage evolution. There are a large number of stratified or layered-like dark siliceous rocks (radiolarian siliceous rocks) that developed in the orogenic belt. The thesis uses major-elemental micro-elements and rare-earth elemental(REE) geochemical methods to research the origin of the Paleozoic siliceous rocks within the orogenic belt and the geotectonic background of its formation in detail. Studies show that the Paleozoic siliceous rocks within the Jiangnan orogenic belt are muddy siliceous rocks with higher terrigenous muds, and the coexistence of hydrothermal and sedimentary origin indicates it's not the previous stated pure hydrothermal The Early Paleozoic siliceous rocks form in the continental margin or the transition zone of the continental margin and the ocean basins. The Late Paleozoic siliceous rocks form in continental margin, ocean basin, the oceanic ridges and other different structural positions.The thesis summarizes the geochemical characteristics differences between the hydrothermal and non-hydrothermal sedimentary siliceous rocks through the study on the Paleozoic siliceous rocks within the research region, and set up REE distribution and recognition patterns of hydrothermal and non-hydrothermal sedimentary siliceous rocks based on the above research. The REE patterns of hydrothermal siliceous rocks show the characteristics of light rare earth element (LREE) is relative defected, with positive anomaly in Eu; And the REE patterns of non-hydrothermal show the LREE is relative enriched, with weak negative anomaly in Eu.The thesis systematically summarizes REE geochemical characteristics of siliceous rocks under different tectonic settings: (1) The REE of siliceous rocks changes significantly from the continental margins to the deep ocean ridges. The siliceous rocks near the continental margin generally don't show positive anomaly in Ce element because of sufficient land-sourced supply. With the deepening of water depth, the siliceous rocks formed in the deep sea environment show significant negative anomaly in Ce element due to the strong transformation of seawater. (2) The LREE is much more enriched in siliceous rocks formed in continental margin environment than that formed in ocean basins, and show strong right inclination of their REE patterns (3) The LREE is much more enriched in siliceous rocks formed in the early stage of rift basin than that formed in remnant ocean basin of the ancient South China and the edge of the Yangtze Block.Research shows that siliceous rocks has obvious geochemical response to the tectonic evolution.The Early Paleozoic siliceous rocks indicate the collision 10Ga ago between Cathaysia block and Yangtze Block is oblique collision; Geochemistry characteristics of siliceous rocks support the fact that there exists residual basin between Cathaysia block and Yangtze Block. The late Paleozoic siliceous rocks formed in the complex background of the coexistence of continental margin, ocean basin and ridge, indicating the intense tectonic extension in this period, and forms small ocean basin and the oceanic crust in the strong extension part . The siliceous rocks located in the southeastern margin of Yangtze Block is formed in the transition zone of continental margin or continental margin and ocean basin, the source area location indicated by them is not coordinated with the current structure location ,and this indicate the Xuefeng archicontinent in the Late Cambrian had Napped in large-scale from east to west.There exists coupling relationship between the causes of siliceous rocks and tectonic activity. The episodic tensions cause episodic fluid release - tectonic activity period, and form hydrothermal sedimentary siliceous rocks; the Interval - stable structure period of the episodic tensions form normal sedimentary siliceous rocks...