Diffusion Coronas Around Quartz Xenocrysts in Basaltic Andesite from Capus (Cionca Hill, Gilau Mountains - Romania)

Diffusion coronas consisting of Al – poor pyroxene (augite), glass and silica polymorphs are developed around the quartz xenocrysts in the olivine basaltic andesite from Cionca Hill (Gilău Mountains). The quartz of xenolitic origin was magmatically corroded and sometimes reacted with the basaltic melt. According to the time of coexistence between quartz and the basaltic melt, two different spatial disposition of the pyroxene of reaction can be noticed. For a shorter reaction time the Al – poor pyroxene are commonly tangentially to the quartz rim while for the crystals which presumably had longer reaction time the pyroxene are radially from points on the outer corona margins. The glass of the coronas has a high content of SiO2 (73.994 – 78.515 wt %) and alkalis (Na2O + K2O = 5.617 – 7.046 wt%). Pyroxene of reaction from different parts of the corona were analysed. Analyses performed on the crystals of pyroxene point out a very high chemical homogeneity. According the chemical composition and using the diagram of Morimoto et al. (1988), all the analysed crystals are represented by augite