The Origin Of Ordivian Cherts In Qinling Area And It's Significance To The Study Of Proto Tethys Ocean Evolution

The Ordovician strata contains a large number of siliceous rocks in Qinling area.We chose siliceous rocks from Xichuan in Henan,Zhashui in Shanxi outcrops as the research object,studied and analyzed its'characteristics of sedimentary petrology,geochemistry in order to figure out it's distribution and origin,by trying to the make statistic about the same period siliceous rocks through the large area and discussing about tectonic background.We made the following recognition:The exposed forms of siliceous rocks from Xichuan and Zhashui are mainly nodular and banded,which was contained in limestone,dolomitic limestone.Cherts distributed along the layer significantly in Xichuan.Cherts from Xichuan has oolitic structure,and siliceous minerals distributed around the oolitics firmly?caused by metasomatism of siliceous hydrothermal?.Cherts from Zhashui has hidden crystal structure,is full of siliceous minerals as microcrystalline quartz,chalcedony and cryptocrystalline quartz form.Combining with the characteristics of Ordivician strata in Qinling area,north China block and Yangtze block,we try to find out the distribution features of the Ordivician cherts,in order to get some information about the episodic siliceous sedimentary events.There are two perionds of the event in the north China block,one of them is on the south of the block,the other one is on the north-east area of the block.Nodular and banded cherts are symbiosis with carbonate rocks.There are four main periods of the event in the yangtze block,first of them mainly occurs in the north area of Yangtze block,?mainly nodular cherts?,the other three of them sustained in the lower Yangtze area?mainly banded and stratiform cherts?,which are corresponding to Honghuayuan Formation,Miaopo Formation,Wufeng Formation.Cherts from Xichuan and Zhashui are high in silicon and calcium,has great geochemical characteristics.The Al-Fe-Mn triangle diagram and Fe-Mn-?Cu+Ni+Co?*10 diagram mainly fall in the typical hydrothermal silicates area.The ratio of Al/?Al+Fe+Mn??Fe/Ti??Fe+Mn?/Ti and Ba/Sr of siliceous rocks manifest the close relationships with hydrothermal sedimentation.Weak negative abnormality in Ce,Less enrichment in HREE,left-dipping REE distribution curves after NASC-normalized indicate the characteristics of hydrothermal sedimentary siliceous rocks.There is a negative correlation between the ratio of Si/Al and Al₂O₃ of the Xichuan siliceous rocks.All the chert samples from this two outcrops are hydrothermal sediments and their siliceous sources is the deep water.The sedimentary environment of Xichuan chert and Zhashui chert is likely to be the continental margin and deep sea basin,continental margin environment,respectively.The changes of Xichuan siliceous rocks'geochemical parameters reflect the enhance-ment in hydrothermal activity and the conversion of sedimentary environment from continental margin to the ocean basin environment,which was caused by ocean expansion.The sustained expansion of the Proto Tethys ocean,made it possible to the deep water circulation system to get Silicon from the submarine volcano or magmatic activities.Siliceous-rich deep water was transfered from the deep sea area to the shallow sea area,and deposited in the stable of shallow sea environment.