Thermal overprinting of Mesozoic shelfal limestones on Jabal Akhdar, Oman

The Jabal Akhdar Dome of the Hajar Mountains (northern Oman) has long been considered to have had no significant thermal overprinting since the start of its doming (Eocene, ~40 to 30 Ma). Only the Semail Ophiolite, obducted during the Late Cretaceous, metamorphosed the overridden sedimentary rocks at its base. However, this is stratigraphically well above the positions of the rocks discussed here. Our findings describe the first evidence for an increased metamorphic alteration of Late Permian, Jurassic and Lower Cretaceous shelfal limestones. Two independent sites were identified, where calcite was either replaced by wollastonite or sulfides. 

 

The calc-silicates, which occur southeast of the Saiq Plateau (stratigraphically above the plateau), contain up to centimeter-sized wollastonite crystals. The conversion into marble has been interrupted, as indicated by relict fossils and ooliths of Jurassic and Lower Cretaceous limestones. So far, the outcrop has been mapped over a length of ~1.2 km. It is dissected by several NW-striking dextral faults in a difficult terrain and, thus, the occurrence may be significantly wider. Wollastonite concentrates in sub-horizontal to gently SE-dipping limestone layers, neighbouring strata may be almost void of it. In places, strong and coarse-grained dolomitisation coincides with decreased wollastonite content. The area is cross-cut by irregular quartz-wollastonite-rich veins.

 

Adjacent to the outcrops are younger quartz-siderite veins, which have almost completely replaced limestone layers (encased wollastonite-carrying limestone relicts). Distal to the mineralisations, the limestones contain decimeter-sized chert nodules. This entire silica-dominated system must have reached 450 ºC in order to form the well crystallised wollastonite. The mostly oxidising character of the environment during overprinting is reflected by fine euhedral hematite grains throughout the examined profile. However, slightly reducing settings promoted the formation of very rare and tiny crystals of erdite (NaFeS₂·2H₂O) in two places.

 

Sulfides in finely laminated Permian carbonates, which contain fine as well as very coarse-grained black carbonates, occur on the northwestern side of the Saiq Plateau in no longer accessible excavation materials. So far, the search for another outcrop failed, due to the sub-vertical wadi walls near-by. The strongly dominating pyrite is accompanied by trace amounts of sphalerite and less galena. Collectively, sulfides replaced carbonate laminae with fine crystalline impregnations and concentrated in up to decimeter-large lensoid concretionary shapes. Dark carbonaceous laminae and recrystallised coarse-grained materials contain finest graphite flakes. This again indicates temperatures of ~450 ºC, at which the graphite formation started during decarbonisation, also promoting a reducing regime (the sulfides show no signs of oxidation).

 

Our working hypothesis is that the thermal overprint (>450 ºC) coincided with the late Eocene to Oligocene doming event, leading to multiple mafic intrusions. Similar intrusions are known from the Muscat and Batain area and have the same age.