This study is based on field, petrographic and geochemical investigations of the Hajar Supergroup autochthonous rocks (Ruus A1 Jibal Group, Musandam Peninsula, and Akhdar Group, Jebel Akhdar, Oman). The petrographic investigations were carried out by using transmission light microscope, scanning electron microscope, and cathodoluminescence. They were supplemented by geochemical analysis, using microprobe, X-ray diffraction, ion chromatography, atomic absorption, stable isotopes, and quantitative analysis by the scanning electron microscope. Field and petrographic evidence indicates that the rocks were deposited in shallow marine environments, particularly tidal flat, lagoon, reef, back-reef, and shoal environments that were part of the Arabian Platform during Permian and Triassic times. However they are almost entirely dolomitized and the rocks show different petrographic texures ranging from perfect preservation of the original texture by mimetic dolomitization to complete obliteration and destruction of the original limestone texture giving rise to inequicrystalline and equicrystalline fabrics. Dolomites were categorised on the basis of textural variations; crystal size, shape and impurity or inclusion distribution within crystals, and whether occurring as rock forming (replacive) or as cements. The dolomite types display variations in stoichiometry, ordering, and trace element concentrations, indicating differences in dolomitizing fluid chemistry and recrystallization stages that prevailed through time. Although dolomitization is pervasive, dolomites are petrographically and chemically immature. The petrographic and geochemical evidence strongly suggests that dolomitization began soon after deposition and that fluids close in composition to seawater were involved. Rocks showing preservation of allochems as well as marine cements by mimetic dolomite crystals, suggest that dolomitization was early (at shallow depth) with very active water circulation and occurred in a relatively short time. In some cases, there is evidence of meteoric water diagenesis which is clearly visible within dolomitized bioclasts and ooids that underwent a dissolution event and cementation prior to dolomitization. Petrographic and geochemical evidence from crystalline dolomites indicates several crystallisation events at shallow burial depths, under marine waters modified by increased temperature and mixing probably with evaporitic brines. Oxygen isotopes indicate baroque dolomite samples to have formed under shallow-burial, which were marked by elevated geothermal gradient, rather than under deep burial conditions. A recently proposed technique for analysing dolomites by the ion chromatography (Staudt et al., 1993) was found to give results which are nearly consistent with the rest of the petrographic and geochemical evidence, but stressed the idea of non-evaporitic related dolomitization. The only fluid capable of early dolomitization in the case of the Oman Mountains dolomites was warm seawater from the Tethys Ocean which was circulating in the subsurface.