Clay mineralogy and geochemistry of pelitic rocks in the middle Permian Fanjiatun formation, East Changchun, China: implications for metamorphism and provenance

Abstract

Metamorphic pressure–temperature conditions and sedimentary provenance were determined for the middle Permian Fanjiatun Formation in east Changchun, China, based on clay mineralogy, vitrinite reflectance and geochemical data. The pelitic rocks in the Fanjiatun Formation contain illite + kaolinite + chlorite ± mixed-layer chlorite/smectite and detrital quartz + plagioclase. Illite crystallinity in the formation varies from 0.38 to 0.49 (average = 0.44), and consists dominantly of the 2M1 and mixed 2M1 and 1Md polytypes. The illite b0 dimension is 8.9860–9.0130 A (average = 9.0010 A), indicating the Fanjiatun Formation experienced very-low-grade metamorphism at <1.5 kbar. The metamorphic temperatures obtained based on vitrinite reflectance are 191–308 °C, indicating the formation experienced anchizone-zone facies metamorphism. Al2O3/TiO2 ratios vary from 23.93 to 24.22 (average = 24.11), and the chemical index of alteration is 64–69 (average = 67). Rare earth element contents are high and exhibit enrichment in light rare earth elements and negative Eu anomalies. Trace element characteristics indicate that the source of the pelitic rocks was moderately weathered silicic rocks. The sediments were deposited in a reducing continental setting.