Quantitative Classification of Permian Volcanic Facies in the 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 work, 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, to divide the types of volcanic facies of the Permian period in different regions, 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. Specifically, chlorite, quartz, labradorite, and calcite are the characteristic minerals enriched in the explosive, effusive, subvolcanic, and volcano-sedimentary facies, respectively (average contents: 43.14%, 10.86%, 38.99%, and 28.42%), with corresponding rock crystallinity ranges of 84.79%–100%, 76.22%–91.06%, 73.90%–97.28%, and 81.02%–100%. In the explosive facies, the a-b axis parameters and c-axis parameter of high-temperature albite show a strong positive correlation and a negative correlation with the unit cell volume V, respectively; in the effusive facies, the a-b axis parameters and c-axis parameter of plagioclase exhibit a weak positive correlation; in the subvolcanic facies, the a-b-c axis parameters of labradorite all show a positive correlation with the unit cell volume V; and in the volcano-sedimentary facies, the a-b axis parameters of low-temperature albite 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.