Mineralogical and geochemical characteristics of lithophysa structure of the Cretaceous Qingshan Group in Sulu orogenic belt : Implications for genesis of lithophysa rhyolite

Lithophysae is a special primary structure in acid volcanic rocks, commonly found in rhyolite. A study of genesis of lithophysa structure is significant for understanding the properties and eruptive environment of rhyolitic magma. However, research on the magmatic properties, formation mechanism and influencing factors of lithophysae formation are still limited. In this paper, considering the lithophysae of Cretaceous Qingshan Group rhyolite of Sulu orogenic belt as the research object, the development law of the lithophysae, the mineral composition and element changes of the lithophysae and the cementation slurry between the lithophysae have been systematically studied through mineralogy, petrology, geochemistry and other methods. The results indicate that the lithophysa rhyolite mainly develops in the upper subfacies of the overflow facies, and lithophysae can be divided into two types: solid type and hollow type. The geochemical characteristics show that lithophysa rhyolite and fluidal rhyolite belong to homologous magma, and lithophysa rhyolite has experienced a higher degree of differentiation. The water content in the cementation slurry of lithophysa rhyolite is higher than that of the glassy in the lower fluidal rhyolite, resulting in a relatively obvious trend of escape of volatiles in the upper subphase of the overflow phase. Due to the high viscosity of silicon-rich magma, volatiles fail to escape smoothly, and a sudden cooling occurs at the escape point to form the lithophysa wall. Affected by instantaneous stress and degree of undercooling, various cavities are formed. As the crystallization temperature decreases, the residual magma inside the cavity condenses and crystallizes successively to form chalcedony and quartz. The formation of the lithophysae means the addition of water in the process of magmatism or eruption environment, which is of great significance for the study of the evolution of acid magma and the mechanism of volcanic eruption.