Abstract:

The Middle Permian Qixia Formation in Chaohu area, Anhui Province, was mainly composed of fine-grained clastic rocks and limestones which were deposited in the coastal marsh and carbonate platform. Limestones were characterized by nodular and banded cherts. Most of cherts were axiolitic and moniliform in outcrops. Among some cherts and limestones developed transitional belts with 0.5 cm width around cherts. The limestones were composed of packstone and wackestone which consisted mainly of fragments of fossils and microcrystalline calcite through polarizing microscope observation. The transitional belts were mainly composed of micrites and the aragonite-shell fragments which were replaced or half replaced by quartz prior to the micrites. Meanwhile the amorphous silica was found to be precipitated into the pores due to the micrites’dissolution under SEM observation in the transition belts. The chert consists of microcrystalline or cryptocrystalline quartz together with the fossil fragments. The categories of the organic matter and fossil fragments in the cherts were similar to those found in limestones, however, most of them were replaced by quartz completely. The petrology analysis of the cherts and limstones indicates that the organic acid which was produced by degradation of organic matters, would greatly increase the saturation of CaCO₃, promote the dissolution of aragonitic fossils and marls in the sediment. As a result, the calcium precipitation was inhibited yet gelatinous silica precipitated as well. The geochemistry of nodular cherts revealed that the average ratio of Al/(Al+Fe+Mn) was 0.63, which was higher than the maximum value (0.35) of hydrothermal deposition. The average ratio of Fe/Ti was 9.5 also lower than the minimum value (20) of hydrothermal deposition. The Al-Fe-Mn data of the cherts fell into in the non-hydrothermal field in the Al-Fe-Mn ternary diagram. The Al₂O₃ value (average 0.20%) was far beyond the other contents such as MgO,Na₂O,K₂O in the cherts and similar to the terrigenous silicon. Such geochemistry features indicated that the silica of the cherts in limestones was probably sourced from the continental materials in research area.

Key words: nodular chert, transitional belt, limestone, source of silica, Middle Permian Qixia Formation, Chaohu, Anhui Province