Quantitative Classification of Permian Volcanic Facies in Western Sichuan Basin Based on Mineral Composition and Unit Cell Parameter Characteristics

There are significant lithological differences among different Permian volcanic facies in the western Sichuan Basin, and there is uncertainty in dividing volcanic facies based on petrological properties when samples are insufficient. In this paper, techniques such as X-ray diffraction (XRD) Rietveld method, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analysis, and polarizing microscope identification were employed to quantitatively characterize indicators of volcanic rocks including mineral composition, rock crystallinity, and plagioclase unit cell parameters, as well as to analyze their intrinsic relationships. The results show that among the different Permian volcanic facies in western Sichuan, characteristic minerals exhibit differential enrichment patterns, rock crystallinity has different distribution ranges, and plagioclase unit cell parameters are significantly different; among them, chlorite, quartz, labradorite, and calcite are the characteristic minerals enriched in the explosive facies, effusive facies, subvolcanic facies, and volcano-sedimentary facies, respectively (with average contents of 43.14%, 10.86%, 38.99%, and 28.42% respectively); the rock crystallinity of each volcanic facies is 84.79%–100%, 76.22%–91.06%, 73.90%–97.28%, and 81.02%–100% respectively; in addition, the a-b axis parameters and c-axis parameter of high-temperature albite in the explosive facies show a strong positive correlation and a negative correlation with the unit cell volume V, respectively; the a-b axis parameters and c-axis parameter of plagioclase in the effusive facies have a weak positive correlation; the a-b-c axis parameters of labradorite in the subvolcanic facies all show a positive correlation with the unit cell volume V; and the a-b axis parameters of low-temperature albite in the volcano-sedimentary facies are negatively correlated, while the b-c axis parameters show a strong negative correlation. Based on these findings, a quantitative volcanic facies division method was established, which is based on the logical chain of “mineral content–mineral interrelationships–rock crystallinity–plagioclase crystal structure–volcanic facies type”, providing a new insight for the accurate division of volcanic facies.