A Vein-Hosted Carbonate Record of Late Miocene To Pleistocene Hydrothermal Activity in the Hajar Mountains (Jabal Akhdar Dome, Sultanate of Oman)

Abstract

The Jabal Akhdar Dome offers insights into the Neoproterozoic to Cretaceous geology of the Central Hajar Mountains in Oman. Ediacaran carbonates display Cadomian NW/SE-trending, non-plunging tight folds and SW-facing thrusts that are partly mylonitized. Calcite veins from the extended part of fold hinges of individual limestone layers and within faults/calc-mylonites provide six U–Pb ages ranging between 9 and 2 Ma. These ages are in conflict with field observations, indicating that pre-existing Cadomian(?) calcite veins were overprinted. Geochemically, vein carbonates display strongly positive Eu values in shale normalization ([Eu/(0.5*Sm + 0.5*Gd]MUQ = 1–8), indicative of precipitation from non-oxidizing hydrothermal fluids at temperatures >200–250 °C. The δ13C and δ18O values of the limestone host rocks range from +3.93 to +5.93 ‰ (V-PDB) and + 19.8 to +23.3 ‰ (V-SMOW), respectively. Vein calcites show slightly lower δ18O values between 16.5 and 20.7 ‰ and considerably higher δ13C values between +5.85 to +7.01 ‰. Homogenization temperatures from primary fluid inclusions in the calcite suggest precipitation at 125–160 °C (average of 142 °C). Stable isotope data are a product of a single calcite precipitation/recrystallization event, which required precipitation from fluids with similar temperatures and oxygen isotope compositions during fluid migration. Combined field geological, geochemical, isotopic, thermometric and petrographic observations suggest that original calcite within the Cadomian thrusts and fold hinges recrystallized at <10 Ma during a previously unknown hydrothermal event near the surface at temperatures of ∼142 °C. This proposed hydrothermal event in the Jabal Akhdar at <10 Ma coincides with Late Cenozoic convergence of Arabia with Eurasia and/or India. © 2025 Elsevier B.V.