Late Neogene hydrothermalism in the Central Oman Mountains

The Jabal Akhdar Dome of the Central Oman Mountains provides insights into the autochthonous Neoproterozoic to Cretaceous Arabian rocks. A superbly exposed outcrop of the dark carbonate Ediacaran Hajir (or Khufai) Formation displays NW/SE-trending, non-plunging tight folds and SW thrusts. As the Permo-Mesozoic rocks lack this deformation pattern, the folds/faults are inferred to have formed during the Infracambrian Cadomian Orogeny. White calcite veins in fold hinges and within faults/calcmylonites provide U-Pb ages of 8.95 ±0.92, 7.71 ±1.44, 5.91 ±2.92, 4.89 ±6.1, 2.41 ±1.46 and 2.03 ±0.78 Ma (all 2 SE). These ages do not match the field observation, indicating that pre-existing Cadomian(?) calcite veins were overprinted at a later stage. Geochemically, almost all carbonates from the veins display strongly positive Eu values in shale normalization ([Eu/(0.5*Sm+0.5*Gd]MUQ=1-8.4) indicative of precipitation from hydrothermal fluids at temperatures >200-250°C under non-oxidizing conditions. The δ13C and δ18O values of the limestone host rocks range between +3.93‰ to +5.93‰ and between +19.8‰ to +23.3‰, respectively. In contrast, vein calcites show considerably higher δ13C values, varying between +5.85‰ to +7.01‰ and slightly lower δ18O values from 16.5‰ to 20.7‰. Fluid inclusion homogenization temperatures of vein calcite suggest two phases of precipitation, with values of 125-150°C and 160-260°C. Based on fluid inclusion data, two phases of fluid entrapment can be discerned, consistent with two different calcite generations or recrystallization events. Stable isotope data seem to be a product of one calcite precipitation or recrystallization event, rather than a mixture of two different generation, because δ18O values of vein calcites define a fairly narrow range, requiring precipitation from fluids at similar temperatures and oxygen isotope compositions, during a time of similar palaeohydrological conditions. Fluid inclusions from primary inclusions in calcite indicate relatively high temperatures (at ~225°C) and are possibly a product of the fluid-flow event in response to the Cadomian Orogeny. Since the U-Pb isotopic system of the calcites was reset, providing late Cenozoic ages (<10 Ma), δ18O, values of vein calcites are most likely the product of the younger calcite recrystallization event that must have occurred at relatively shallow depths at temperatures between 125 and 150°C. Our U-Pb vein calcite ages, combined with geochemical, stable O-C isotopes and fluid inclusion data reveal that former Cadomian(?) calcite veins formed at temperatures of ~225°C, which was subsequently overprinted by near-surface hydrothermal activity at <10 Ma. This study highlights and dates a hydrothermal event in the Jabal Akhdar Dome. The inferred hydrothermal event correlates with an undated thermal overprint of Mesozoic shelfal rocks from the Jabal Akhdar Dome (Pracejus et al., 2022) and a hydrothermal-fluid flow east of the dome, which is related to listwaenite formation and Arabia-India convergence.   Pracejus, B., Scharf, A. & Mattern, F. (2022) Thermal overprinting of Mesozoic shelfal limestones on Jabal Akhdar, Oman. European Geosciences Union (EGU), Vienna, Austria, vEGU 2022-3985.