Spherical quartz sandstone of Mesoarchean Yuanjiang Group in central Yunnan and its petrogenesis

The Yuanjiang Group is a new lithostratigraphic unit in the central Yunnan region. It is the first time to identify the Mesoarchean shallow metamorphic strata in China, and it records a series of important geological events in the early stage of the earth's development and evolution. It was found that a typical quartz sandstone - spherical quartz sandstone was deposited in the Manlin Formation and Chahe Formation of the Lower Yuanjiang Group, associated with massive quartzite of volcanic, volcano-clastic and chemical sedimentary origin. On the macro level, the spherical quartz sandstone could be formed by graded bedding with tuff argillaceous slate and argillaceous slate, or it might be developed separately in medium-thick bedding with tuff, volcanic rock and quartzite, indicating that it was formed in an intensely active tectonic setting. The spherical quartz grains exhibit equal crystals, likely spherical crystal, showing no or slightly blue glass in the CL images, and could be formed by plastic growth. The results of hydrothermal inclusions test show that the quartz particles were formed from the SiO₂-saturated water in the hydrothermal sedimentary environment of 60~100 ℃ under weak hydrodynamic conditions. They belong to authigenic minerals in the basin. Scanning the surface of quartz particles, it is found that the surface of quartz particles is smooth and has no scratches, indicating that these quartz particles were not transported over a long distance, and should belong to the autochthonous to semi-autochthonous sediments in the basin. The Manlin Formation in the Lower Yuanjiang Group contains a large number of tuffaceous detritus, and its contemporaneous different facies Yinachang Formation commonly contains magnetite slate, tuff, and massive copper-bearing dolomite intercalation, which indicates a similar sedimentary environment. It shows that the volcanic activity was more frequent, and the water temperature in the basin was generally higher and changed rapidly. Such a water environment provided the first microorganisms with the energy they needed to survive, seting the basis for a blue-green algae bloom at the end of the Middle Archean.