The Origin Of Siliceous Rocks Of The Taiyuan Formation In Jiaozuo Area

This paper selects the Taiyuan Formation cross-section in northwestern Jiaozuo as the study object, and analyzes the typical siliceous rocks and limestone samples to reveal the causes of the local siliceous rock. This research has highlighted the shape characteristic, mineral composition, and geochemical feature of siliceous rock, thus discussed its cause. By studying this issues, we conclude the following knowledge.(1)The lump limestone and banded siliceous rock in Taiyuan Formation distribute as layers.It was transformed from the original laminated siliceous rocks, which are the cap of liquefied limestone, under the trigger of a certain strength seism. The inter bed between the unconsolidated limestone and siliceous rock provide a good environment for liquefaction.(2) The siliceous rock in Taiyuan Formation mainly consists of opal,chalcedony, microcrystalline quartz and calcite. Genetic mineralogy and macro- and micro- fabric analysis indicate that the minerals in siliceous rock are metasomatic from calcite.(3) Abundant of pyrophylite mineral were found in the bottom laminated siliceous rocks of Taiyuan Formation. The original rock of these siliceous rocks is marl,which is rich in volcanic ash. Within mineral system, hydrolysis of volcanic ash generates Al2O3 colloid. Combining with the H4SiO4 colloid form outside system,pyrophylite could be formed in micro fractures, which is result of impact load.(4) The common element analysis of the siliceous rocks shows that there is a well negative correlation between SiO2 and CaO, which supports the metasomatic origin of the studied siliceous rock. Lump and banded siliceous rocks inherited the trace element characteristics of limestone. Due to the difficult transition element stuck in the mineral system after the hydrolysis of volcanic ash, the contents of trace elements in the laminated siliceous rock are higher than those in volcanic ash. The REE distribution pattern, δEu and δCe of the lump and banded siliceous rocks inherited the features of limestone. However, these features of the laminated siliceous rock could be a reflection of volcanic ash and its hydrolysis.