Reflux dolomitization and subsequent hydrothermal dolomitization induced by the Alkaline Igneous Province in the Middle Devonian Nowshera Formation (Peshawar Basin, NW Pakistan)

Abstract

Dolomitized strata are potential exploration targets because they host economic mineral and hydrocarbon deposits around the globe. Establishing a petrogenetic history for dolomite is thus very vital. In this study, dolomitic bodies present in the Devonian carbonates of Nowshera Formation in Peshawar Basin, North-West Lesser Himalayas in north Pakistan are investigated through field observations, petrographic studies, and geochemical analysis. The carbonates of Nowshera Formation show evidence for multistage dolomitization and a complex diagenetic history. In a first stage the succession was completely dolomitized probably by the reflux of penecontemporaneous mesohaline seawater. This process resulted in both fabric-retentive and fabric-destructive dolomite types under near surface to shallow burial realms. In the subsequent stage saddle dolomites (matrix & cement) were formed under intermediate and/or deep burial realm. This later saddle dolomite phase likely resulted from the circulation of exotic fluids at comparatively high temperature. This local hydrothermal event, evidenced from the fluid inclusion homogenization temperatures, and stable isotopic signatures, also resulted in vuggy, fracture, and dissolution enhanced porosity. Microthermometric analysis of primary fluid inclusions in saddle dolomite confirm the presence of hot (125–178 °C) and highly saline brines (17–25 mass% NaCl equivalent). The δ18Owater (+2 to +9.2‰V-SMOW) calculated from fluid-inclusion homogenization temperatures in conjunction with the comparatively high salinity values is compatible with a magmatic origin of the hydrothermal fluid. It is suggested that the investigated hydrothermal dolomites in the Nowshera Formation formed in Carboniferous-Permian time. During this time, thermal convection heated by the Peshawar Plain Alkaline Igneous Province (PPAIP) may have provided the high temperature and sufficient magnesium (Mg) flux for several millions of years (~40 M.y.), ultimately leading to the formation of hydrothermal dolomite bodies. The pore spaces resulted from the leaching of these hydrothermal fluids were subsequently occluded by meteoric calcite related to the Eocene to Middle Miocene Himalayan exhumation of the studied area. This study provides important implications for potential source of Mg in understanding the genesis and timing of dolomitization.