Geochemical Characteristics Of The Devonian Cherts In Changning-Fengqing, Yunnan And Its Implications For The Tectonic Evolution

Changning-Fengqing County is in Yunnan province and its tectonic activity is relatively stable in Devonian periods. There are a large number of stratified or layered-like dark siliceous rocks(including some radiolarian siliceous rocks)with a thick layer of quartz sandstone and thin siltstone, shale combination formed a set of clastic rocks-siliceous rock built that developed in this area. The paper ues petrology, petrography and constant, trace elements and REE geochemical methods to study the origin and tectonic background of Devonian cherts. This studies show that the origin of Devonian siliceous rocks is both a normal biological sedimentary genesis and hydrothermal sedimentary genesis,and its mainly formed in continental margin or continental margin and ocean Marginal Basin wide transition zone.Through the study on the devonian siliceous rocks in this area, geochemical characteristics of rare earth elements of the siliceous rocks under different genesis and tectonic background is systematically summarized: From the continental margin to the deep ocean ridge, rare earth elements of the siliceous rocks has a obviously change. The siliceous rocks formed by normal biological sedimentary near the continental margin because of adequatly terrigenous supply, Ce elements generally is abnormal or no abnormalities, rare earth distribution patterns show that the relative enrichment characteristics of light rare earth; Along with the depth deepening, the hydrothermal fluid sedimentary siliceous rocks that formed in the deep-water environment are well developed due to the strong reformation of seawater showed obvious Ce negative anomaly, The distribution patterns of rare earth shows the relative loss of light rare earth and Ce negative anomaly characteristics. The siliceous rocks formed by normal biological sedimentary relative to by the hydrothermal fluid sedimentary have a weak enrichment of light rare earth, characterized by the trend of rare earth distribution patterns of gently to the right.The genesis of siliceous rocks and tectonic activity have a close relationship. Continental margin environment, stable structure, adequatly source supply, a large number of microbial life, form the normal biological sedimentary origin of the siliceous rocks; Deep sea basin environment, the structure is not stable, source supply, the absence of microorganisms, the frequently seafloor hydrothermal activity, a hot-water sedimentary siliceous rocks is formed.According to the response of siliceous rocks and its rock assemblage from the study area to basin evolution,it can be discriminated that the plate extensional basin was formed in the early Devonian period or earlier tectonic activity in this area. And in the Devonian period, tectonic activity was stable, submarine exhalative- hydrothermal activity were weak.Therefore,Pangaea began to break up form the end Cambrian to early Paleozoic.In the Paleozoic Period, the Zhenkang-Baoshan block is in a state of passive continental margin for a long time with the development of the proto Tethys and Paleo Tethys,and it was also affected by the Changning-Menglian Paleo Tethys ocean basin inherited expansion.And then,favorable tectonic and paleogeographic environment for the formation of the silica rocks were formed.