Dolomite cementation and recrystallisation of sedimentary breccias along the Musandam Platform margin (United Arab Emirates)

Two main events of dolomite formation were identified along the southern border of the Late Jurassic Musandam carbonate platform of the UAE. The first dolomitisation phase (type 1) was restricted to specific stratigraphic layers in Jurassic platform limestones that were subsequently brecciated by mass flow and collapse processes on the platform margin. These dolomites are planar-s, have crystal sizes ranging from 5 to 25 µm and exhibit an orange to pink and sometimes zoned red-yellow luminescence. This dolomite phase was formed shortly after deposition by fluids of marine or slightly modified marine composition. The second dolomite phase (type 2) mainly affected dolomite type 1 breccias by recrystallisation, dolomite cementation and replacement. Type 2 dolomites are planar-e and non-planar-c, with crystal sizes between 20 and 70 µm. They have more elongated forms with purple luminescent to non-luminescent cores overgrown by pink to yellow luminescent rims. Stable isotope analyses show a covariant trend between δ18O and δ13C from marine (− 4.2 to − 1.8 and + 0.8 to + 2.1‰ VPDB respectively) towards depleted values (− 10.2 and − 8.9‰ VPDB respectively) . This depletion is explained by recrystallisation during type 2 dolomitisation and it is interpreted in terms of high temperatures during precipitation and the incorporation of light carbon as hydrocarbons matured. Dolomite type 2 formation is thought to be the result of tectonically induced fluid flow which supplied hot magnesium-rich fluids. Two possible time scenarios for this fluid flow event are proposed: 1) during thrust emplacement of the tectonic nappes on top of the Musandam Platform in the Late Cretaceous. Volcaniclastic and basaltic rocks of the Hawasina Complex and the Oman-UAE ophiolites are a possible magmatic source for magnesium in this case; or 2) coinciding with the migration of hot, post-evaporative brines along the Hagab thrust, which acted as a fluid conduit during the Cenozoic orogeny