Reprint of Very low-grade metamorphism of the upper Permian Linxi Formation from the southern Da Xing'an Mountains, NE China: Constraints from clay mineral genesis

Abstract

The metamorphic pressure–temperature conditions and the timing of metamorphism were determined for the upper Permian Linxi Formation, southern Da Xing'an Mountains, NE China, based on the genetic characteristics of clay minerals and K–Ar dating of illite from pelitic rocks. Detrital minerals are dominated by quartz and plagioclase, and clay minerals consist mainly of illite, chlorite, and mixed-layer chlorite/smectite. The XRD analytical results indicate that illite crystallinities are in the range of 0.25–0.57 (average value, 0.35), that illite polytypism is dominated by 2M1 and minor 2M1+1Md and 1Md types, and the illite b dimension is in the range of 8.9982–9.0312Å (average, 9.0174Å). These characteristics indicate that the strata containing the illite experienced very low-grade metamorphism. The temperatures estimated by vitrinite reflectance thermometry are in the range of 202°C–293°C (average, 245°C), indicating that the pelitic rocks of the upper Permian Linxi Formation experienced anchizone-grade metamorphism. The illite grains used for dating are <0.15μm in size, which is similar to that of autogenic illite grains. The K–Ar age of the illite is 127.64±2.12Ma, which is interpreted to represent the timing of the very low-grade metamorphism.