Diagenetic conditions and geodynamic setting of the middle Permian hydrothermal dolomites from southwest Sichuan Basin, SW China: Insights from in situ U–Pb carbonate geochronology and isotope geochemistry

The presence of hydrothermal dolomites in the middle Permian carbonates from the southwest Sichuan Basin has been suggested by petrographic features including zebra structure and extensive saddle dolomite cementation. Two major tectono-thermal events, i.e., the intensive volcanism related to the Emeishan large igneous province (ELIP) during the latest middle Permian and the tectonic evolution of the Longmenshan Orogenic Belt since the Late Triassic, might have contributed to the regional fluid flow for hydrothermal dolomitization. Here, we apply LA-ICP-MS U–Pb geochronology, petrography and isotopic geochemistry to constrain the diagenetic and geodynamic conditions of these hydrothermal dolomites. These dolomites resulted from injection of hot fluids up to 95 °C higher than the ambient temperatures. U–Pb ages (from 216.4 ± 7.7 Ma to 206 ± 15 Ma), combined with the highly radiogenic 87Sr/86Sr compositions (up to 0.712151), suggest that the previously proposed ELIP-related dolomitization model may not apply to this site. Instead, our new geochronological data suggest a syn-orogenic flow of basinal brines probably driven by thrust-related compression and/or tectonic valving. Moreover, the indistinguishable U–Pb ages and carbon, oxygen and strontium isotopic compositions among the replacement and cement carbonate phases in these hydrothermal dolomites further suggest that the paragenetic sequence of “replacement dolomite→saddle dolomite→blocky calcite” was originated from a common fluid in evolution. This study emphasizes the merits of LA-ICP-MS U–Pb dating in addressing the discrimination of, as well as diagenetic and geodynamic conditions for, hydrothermal dolomites.