Geochemical characteristics and origins of the diagenetic fluids of the Permian Changxing Formation calcites in the Southeastern Sichuan Basin: Evidence from petrography, inclusions and Sr, C and O isotopes

Abstract

Abstract The Upper Permian Changxing Formation in the Southeastern Sichuan Basin exhibited complex fluid activities, and the petrographic features, fluid inclusions and stable isotopes (C, O, Sr isotopes) of calcites and host rocks from this formation were determined. The aim of the study is to determine the crystallization sequence, fluid trapping time and origins of diagenetic fluids of calcites in the Changxing Formation. The results show that four generations of calcite cements were formed in the Changxing Formation, and the corresponding four groups of primary inclusions were captured by these calcites. Group Ⅰ inclusions are characterized by high salinity (16.15–23.18 wt%, average 19.5 wt% NaCl equivalent) and low temperature (60–97 °C), and they were trapped in fine-grained calcite during from the Early Triassic to the early Middle Triassic (T1-T2). Inclusions of the other three groups, which were captured by medium-grained to ultra-coarse-grained calcites, are characterized by moderate salinity (average