Fabric Preserving and Fabric Destroying Dolomitization: A case of Seawater Dolomitization

This study is based on field, petrographic and geochemical investigations of Hajar Supergroup autochthonous rocks: Ruus Al Jibal Group- Musandam Peninsula, and Akhdar Group- Jebel Akhdar, Oman and U.A.E., and para-autochthonous Maqam Formation-Sumeini Group-Jebel Sumeini-U.A.E.  Petrographic evidence indicates that the rocks were deposited in a shallow marine shelf environment, 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 features ranging from perfect preservation of original texture by mimetic dolomitization to complete obliteration and destruction of the original limestones giving rise to inequicrystalline and equicrystalline fabrics.  Dolomites analyzed by geochemical methods were categorized on the basis of textural variations; crystal size, shape and impurity or inclusion distribution within crystals, and whether these crystals are found as rock forming (replacive) or cements. The dolomites display variations in stoichiometry, ordering and trace element concentrations indicating differences in dolomitizing fluid chemistry and recrystallization stages that prevailed through time. It indicates also that although dolomitization is pervasive, dolomites are petrographically and chemically immature. All the petrographic and geochemical evidence strongly indicates seawater and/or mixing zone dolomitization which may have been initiated soon after deposition of the host sediments.  Rocks showing preservation of allochems as well as the marine cements by mimetic dolomite crystals, suggest that dolomitization was early (at shallow depths) with very active marine-water circulation and occurred in a relatively short time.  Evidence from crystalline dolomites indicates several crystallization events at shallow burial depths, under marine waters modified by increased temperature and mixing probably with evaporitic brines. 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. KEYWORDS:


Fabric Preserving and Fabric Destroying Dolomitization: A case of Seawater Dolomitization
Alham Jassim Al-Langawi Authority of Applied Education and Training, College of Education, Department of Science, Kuwait, Email: alhamaqeel@hotmail.com

Introduction
he Permian and Triassic rocks studied here include dolomites which were sampled from: Wadi Al Bih and Wadi Hajil-Musandam Peninsula, Jebel Sumeini, Wadi Bani Kharus, Wadi Hajir and Wadi Bani Harras-Jebel Akhdar (Figure 1 and 2).Dolomites are described according to grain size, crystal shape, presence or absence of inclusions and their distribution within the crystals, and degree of preservation of the depositional texture.Dolomites are divided into those which show good preservation of the depositional fabric of the original limestone (fabric-preserving or mimetic dolomitization) and those where there is little or no evidence of the original limestone fabrics (fabric-destroying dolomitization).In the latter case crystal textures are described using the terminology of Sibley and Gregg (1987).The rocks that underwent fabric-destroying dolomitization show equicrystalline and inequicrystalline textures which are planar (idiotopic), nonplanar (xenotopic), and hypidiotopic, with crystals showing differences in impurity distribution.

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Figure 1.A map showing simplified geology of the Oman Mountains and major fieldwork localities.

Methods used for petrographic and chemical investigation.
The petrographic investigations were carried out by using transmission light microscope, scanning electron microscope, and cathodoluminescence.After the petrographic investigation of all samples (220 samples were examined petrographically using thin sections) 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 Color Illustrated Guide to Carbonate Rock Constituents (Scholle, 1978) and the Atlas of Sedimentary Rocks Under the Microscope (Adams et al., 1984) were used as aids in determining the appropriate name for the dolomites exhibiting preserved host-limestone textures.In order to get an indication of the nature of the original limestone texture and structure in samples with little obvious fabric preservation a sheet of white paper was inserted between the thin section and the stage and the light intensity increased.This technique is referred to as the "white paper technique", also known as white card technique (Delgado, 1977;Zenger, 1979).Cathodoluminescence photomicrographs are used to show the differences in dolomite and calcite trace element composition and to determine the cross-cutting relationship between the different fabric components in the samples.The description of colours was based on what was observed while examining the polished thins under the beam.Although the beam voltage and current were kept constant, the quality and intensity of colours in the photomicrographs are different.This was due to either the quality of film used in the photography, or to the exposure times used to enhance differences.
The petrographic investigations were supplemented by geochemical analysis, using microprobe, X-ray diffraction, ion chromatography, atomic absorption, stable isotopes, and quantitative analysis by the scanning electron microscope.

Fabric-preserving dolomitization
The terms "fabric-preserving" or "mimetic dolomitization" are given to rocks showing selective replacement of original limestones by dolomite crystals without any destruction of the original relationships between the fabric components (Sibley, 1980;Sibley, 1982;Bullen and Sibley, 1984;Sibley, 1990;Purser et al., 1994a and b).The fabric-preserving texture in dolomites is controlled mainly by two major factors; the precursor mineralogy and grain/crystal size (Murray and Lucia, 1967;Sibley, 1982;1990).The previous authors suggested that high magnesium calcite tends to be dolomitized with retention of the primary fabrics, whereas low magnesium calcite is more resistant to dolomitization and replacement results in fabric-destroying textures.Tucker and Wright (1990) explained the retention of the host limestone fabrics after dolomitization as being a result of the balance between dissolution and dolomite precipitation.
The fabric-preserving dolomites studied here exhibit grain-supported and mud-supported textures that can be classified into; stromatolitic dolomites, intraformational breccias, dolomudstones, wackestones, packstones, and grainstones.The wackestones, packstones and grainstones are mainly bioclastic, oolitic and pisolitic.The preserved depositional features suggest that these rocks were deposited in shallow subtidal, intertidal and supratidal environments with variable wave energy levels.Two main fabric components can be distinguished in the rocks which underwent fabric-preserving dolomitization.These are microcrystalline dolomite and void filling dolomites.

Microcrystalline rock-forming dolomites
The rock forming microcrystalline dolomite crystals are generally ≤ 8 µm in size.These crystals either replaced the original allochems or matrix in granular host limestones, or the carbonate mud of stromatolites and mudstones.The microcrystalline dolomites tend to be cloudy and anhedral giving rise to very fine nonplanar textures (Figure 3).At some stratigraphic levels the microcrystalline dolomites which replaced pellets and geopetal sediments within fenestrae were affected by aggrading neomorphisim, which resulted in the production of larger sizes and the development of some planar boundaries.These aggraded crystals often show the same luminescence as void filling dolomite crystals in the same samples.Microcrystalline dolomites also replaced micrite envelopes and wall structure of some bioclasts, such as forams and algal grains (Figure 3).This replacement helped in preserving parts of the fabric components as relicts even at later stages of pervasive dolomitization which resulted in the development of crystalline texture.Cathodoluminescence of rocks exhibiting fabric-preserving textures showed that all the microcrystalline dolomites have homogeneous dull brownish-red to orange-red luminescence, or with dark-grey luminescence (Figure 4).Some original oolitic grainstones and packstones show good textural and structural preservation of the rock components.The ooids are formed of alternating concentric layers of microcrystalline dolomites and slightly coarser, nonplanar cloudy and clear dolomite crystals.It is not certain whether these different structural preservation modes reflects variations in ooid composition, or is due to predolomitization diagenesis and fluctuations in dolomitizing environments from marine to meteoric or mixed water conditions.The cathodoluminesence patterns of the microcrystalline, and cloudy very fine and fine nonplanar dolomite crystals within ooids are always comparable to the luminescence in other replacive microcrystalline dolomites, whereas the luminescence of the clear fine nonplanar or planar dolomite crystals within the ooids is comparable to the luminescence of void filling sparry dolomite crystals (Figure 5).These variations in the luminescent patterns and interrelationships between the fabrics indicate that all the microcrystalline and very fine-fine nonplanar cloudy dolomite crystals formed under the same fluid conditions which predate dissolution and compaction features in the rocks.The slightly coarser planar clear dolomite crystals formed after meteoric water diagenesis which induced dissolution of the aragonite and/or high Mg-calcite, and after compaction which resulted in grain fracturing.

Void filling dolomites
Void filling dolomites are the crystals found infilling the intergranular, intragranular, fenestral, and fracture porosity within rocks that underwent fabric-preserving dolomitization.They are divided into; isopachous cements and void filling sparry mosaics.

Isopachous cement
Most of the granular dolomites were subjected to early cementation giving rise to isopachous rims around the outer surfaces of allochems.Some samples show evidence for phreatic meteoric water diagenesis, which produced isopachous cement around both the outer surface of allochems as well as the inner boundaries of the mouldic cavities of bioclasts.This indicates a dissolution stage by meteoric-water and probably cementation by isopachous calcite prior to dolomitization.Whether the isopachous dolomite is of replacement or cement origin is of great importance to this study.Staining with Alizarin Red-S and backscattered images revealed that the isopachous crystals are composed of dolomite.
The isopachous crystals are mainly prismatic, and to a lesser extent rhombic, and are generally ≤ 20 µm long.The prismatic isopachous dolomites are cloudy and clear, while the rhombic isopachous dolomite crystals are clear or show alternating clear and cloudy zones.Cathodoluminescence study indicates that the cloudy prismatic isopachous and the cores of rhombic isopachous dolomites in some samples, develop under the same conditions as the microcrystalline dolomites which replaced the allochems and micrite envelopes (Figs. 4 and 5).The clear isopachous prismatic dolomite crystals show dark-brown and dark-grey luminescence, and black nonluminescence that, in some samples, are capped by thin yellow, orange-red and light green luminescent zones.Cathodoluminescence indicates that the rhombic isopachous dolomite crystals are strongly zoned (Figure 5).

Void filling sparry mosaics
The sparry dolomites mainly have a planar texture, composed of subhedral and euhedral rhombic crystals, which are either clear or cloudy, or have cloudy centres and clear rims.In some sparry dolomites impurities are arranged in zones in any given crystal producing alternating clear and cloudy zones.Backscattered images reveal that some of the cloudy cores of sparry dolomites have calcite inclusions, and that most of the void filling dolomite crystals are lined with minute intracrystalline void spaces.Equant equicrystalline mosaics are present, especially within the intergranular pore spaces of intraformational breccias and some biomouldic porosity (Figure 3).The crystals comprising the equant equicrystalline mosaics are nonplanar, cloudy and are always fine-crystalline.Investigations with the polarizing microscope and the white paper technique reveal that the void filling sparry dolomites are zoned, especially the medium and coarse crystalline dolomites.Crystals with alternating clear and cloudy zones as seen under transmitted light, produce bright orange-red and yellow luminescence colours at locations with manganese oxides inclusion, and black nonluminescence at places where iron oxide inclusions exist (Figure 5).Calcite inclusions within some dolomite crystals produce speckled orange patterns under cathodoluminescence.
A few samples experienced cementation by coarse sparry dolomite crystals exhibiting curved crystal boundaries, curved cleavages, and sweeping extinction indicative of saddle dolomite.Clear baroque (saddle) dolomite crystals show homogeneous black nonluminescence patterns, and cloudy crystals and cloudy parts of crystals show speckled patterns of black nonluminescent backgrounds with bright red spots.The latter indicate a replacive origin for baroque dolomite.

Fabric-destroying dolomitization.
Rocks displaying fabric-destroying dolomitization are crystalline dolomites which exhibit a wide range of shapes and sizes.The crystals comprising these rocks are referred to here as rock forming dolomites.Petrographic studies show the distribution pattern of inclusions, and that most of the rocks lack any relicts.A few coiled forams and echinoderms are the only relicts found.The use of white paper gave better images of the original host limestone texture and structure, although these rocks are pervasively dolomitized (Figure 6).In most crystalline dolomites, especially the ones with wide range of impurity distribution, it is the cloudier crystals that replaced the original allochems, while clearer crystals replaced or filled the intergranular and intragranular porosities of the host rocks.The dolomites that replaced the cement or filled the porosity of the precursor rocks always display planar boundaries, and are clear and coarsely crystalline, whereas rock forming crystals which replaced the original allochems are mainly nonplanar, and are cloudy and fine crystalline.The white paper technique revealed that all fracture and void filling sparry dolomites are clear and have a planar texture, and that boundaries of most of the rock forming dolomites are lined with organic matter or iron oxides.
Backscattered images revealed no calcite inclusions within the rock-forming dolomites.This may indicate that the impurities are minute fluid inclusions which developed during the growth of the dolomite crystals.Clear and cloudy parts of the rock-forming dolomites show different luminescent patterns (Figure 7).Clear rockforming crystals and clear crystal rims show luminescence identical to the void filling dolomite in the same samples (Figure 7).Dolomite crystals filling intercrystalline pores, vugs, and fractures are mainly zoned, producing patterns composed of alternating thick zones and thin sub-zones of bright red, dull red, yellow and green luminescence, and black nonluminescence.

Stoichiometry and ordering.
The samples analysed by X-ray diffraction are of two types; either whole rock samples or separate baroque and normal dolomites.The microcrystalline and fine-crystalline dolomite samples and stromatolitic dolomites show either the lowest degree of ordering (0.43), or poor stoichiometry (53.9 Mole% CaCO 3 ) (Table 1, and Figure 8).The data collected on Ca and Mg show near-stoichiometric composition for dolomites displaying medium crystalline textures and that most of the microcrystalline, fine and coarse crystalline dolomites are nonstoichiometric (Table 1).Dolomites comprising the rocks which underwent fabric-preserving dolomitization are mainly poorly stoichiometric, and most of the dolomite crystals are Ca-rich.Dolomites comprising the rocks which underwent fabric destroying dolomitization are mainly near stoichiometric (Table 1).

Trace-element concentration.
Whole rock analysis provided important results, especially for the distribution of Si, Al and SO 4 in the dolomites (Table 2).Rocks which are of stromatolitic and peloidal origin and crystalline rocks that are composed of cloudy rock-forming dolomites display high Al and Si values (1800 and 2304 ppm respectively).The lowest Al and Si values are found in rocks that underwent fabric-destroying dolomitization and which are composed mainly of clear dolomite crystals or crystals with cloudy centres and clear or less cloudy rims (Tables 1 and 2).The SO 4 values, and the Na and Cl values from Table 2, reflect the actual concentrations in the dolomite lattice.The concentration of SO 4 in the samples ranges up to 630 ppm, and tends to be concentrated more in rocks that are of stromatolitic and mudstone origin.
Plotting δ 18 O values against Fe and Mn ppm concentrations for the same samples indicates a general antipathetic relationship between 18 O depletion and Mn concentrations, and direct relationship between 18 O depletion and the decrease in Fe concentration (Figure 9). Figure 9a shows that as crystal size increases Fe concentrations decrease and crystals tend to become more depleted in 18 O.Variations in Fe and Mn concentrations were also evident when the thin sections were examined by cathodoluminescence.The different colour zones were analysed by the microprobe to gain the actual Fe and Mn concentrations.This showed that crystals and zones displaying dark-grey luminescence are relatively Fe-rich (60-9460 ppm) and Mn-poor (0-540 ppm), the crystals and zones that are nonluminescent have Fe and Mn values which are below the detection limits, and that all other luminescent patterns are generally relatively Mn-rich (10-6510 ppm) and with Fe concentrations which are mainly below the detection limit ranges up to 930 ppm).Table 2 shows that Na and Cl tend to be low in the dolomites regardless of texture and composition.Na mean values range from 42-358 ppm, and Cl mean values range from 76-379 ppm.Mn in the whole rock samples is very low, ranging from 10-133 ppm.Excess Na and Cl were plotted against oxygen isotope values ( Figure 10b ) , with the following results: (i) Excluding sample BIH50 (354 ppm excess Na), all the microcrystalline dolomite samples show a general decrease in Cl values and increase in Na concentrations with 18 O depletion.Therefore, as the temperature of dolomitizing waters increases (i.e. more depletion in 18 O) Na increases and Cl decreases.This suggests that the microcrystalline dolomites with higher Cl concentrations would have formed from initially marine fluids, but as these waters became modified by increasing in temperature (and evaporation?) the crystals became enriched in Na. (ii) Na and Cl concentrations is variable in fine to medium dolomite crystals.Three samples show a general decrease in Cl and increase in Na which is equivalent to what is recognized in the microcrystalline dolomites.(iii) Coarse to very coarse crystalline dolomites become more depleted in 18 O they become enriched in Na.The δ 18 O values for these crystals (-5.954 to -6.987 ‰ PDB) suggest a higher temperature of formation which may be related to an increase in burial or in geothermal gradient.4) Baroque dolomite sample (BIH29-2) is Napoor in contrast to all other dolomite samples.Oxygen and carbon isotope analysis was carried out on samples with a range of textural and compositional variations from different locations throughout the Oman Mountains.Table 3 lists all the isotopic results from the Permian-Triassic dolomites of the various formations.Plotting δ 18 O against δ 13 C for all the samples (Figure 11) revealed that there are two distinct areas which can be differentiated and which relate to crystal size.The rocks displaying microcrystalline, fine and medium crystal sizes have nearly constant δ 18 O and δ 13 C values which are different from those of the coarse to very coarse crystal sizes.Figure 11 also shows that dolomite samples which are composed of coarse to very coarse crystals are depleted in δ 18 O compared to rocks which are composed of finer sizes.
Table 3 and Figure 12, show that δ 13 C values are higher in rocks of Permian age than those of Triassic age.A limestone sample from the Milaha Formation (WBM6) was tested for oxygen and carbon isotopes (Table 3).The isotopic values do not plot within any of the two areas which were differentiated according to crystal size (Figure 11a), but show a similar δ 13 C to the Triassic dolomites (Figure 11b).All the coarse and very coarse dolomites and baroque dolomite crystals plot together in a relatively small field ranging between -5.954 to -8.470 δ 18 O ‰ PDB and +1.619 to +3.296 δ 13 C ‰ PDB.The microcrystalline to medium dolomite crystals plot in a wide field, scattered between -5.184 to +0.095 δ 18 O ‰ PDB and -0.707 to +5.848 δ 13 C ‰ PDB (Figure 12). Figure 12 suggests that microcrystalline to medium crystalline dolomite samples in this study could have developed due to replacement of host rocks in the shallow burial environments by modified seawater of a formation temperature of ≤ 60°C, and dolomitized early by normal marine waters.On the other hand, the coarse and very coarse crystalline dolomites and baroque dolomites may have developed under shallow burial conditions marked by increase in temperature.None of the Permian and Triassic dolomites falls within the composition interpreted as of mixed-water origin by Choquette and Steinen (1980) and sabkha origin presented by Perkins et al (1994) (Figure 12).Few microcrystalline and fine-medium crystalline samples fall within the field of mixed-water dolomites presented by Dunham and Olson (1980) (Figure 10).

Ion chromatography.
The ion chromatography technique of Staudt et al. (1993) apparently provides a way of differentiating environments of dolomitization.Plotting the results gained for Cl, SO 4 and Na against the graphs presented by Staudt et al. (1993) indicated some contradictions (Figures 13 and 14).Table 3. Showing the distribution of oxygen and carbon isotopes within the Ruus Al Jibal and Akhdar Groups, Sumeini Group-Maqam Formation Member B, and Milaha Formation.FP=Fabric-Preserving Dolomitization, FD=Fabric-Destroying Dolomitization, Ruus al Jibal Group: (WBB, WBH, WBG, BIH, HAGIL), Milaha Formation:( WBM), Akhdar Group: Saiq Fm.(WHS, KHARUS, HARRAS), Mahil Fm. (WKM), Sumeini Group; Maqam Fm.Member B (SMB).While most of the values plot within the overlap field (Figure 13), but when plotting Na against Cl, analyses fall within the seawater/ mixing zone field (Figure 14).Baroque dolomite samples were also analyzed and they plot within the overlap and seawater/mixing zones of Figures 13 and 14.Furthermore, some data points did not plot within any of the designated fields, and only four samples plot in the seawater/mixing zone fields of both figures and two samples in the overlap fields of both figures.However, these figures clearly suggest a non-evaporitic related dolomitization.
The above mentioned correlation between the isotopes and SO 4 concentrations indicates that the range produced by Staudt et al. (1993) for SO 4 in dolomites does not necessarily indicate a definite environment of dolomitization, especially in the overlap fields, which range between 40-570 ppm.As mentioned earlier, most of the Oman Mountain dolomite samples from all textural categories lie within this range, which made it difficult to interpret their environment of dolomitization, whether burial or seawater/ mixing zone dolomitization according to Staudt et al. (1993).Moreover, comparing the results on the relationship between oxygen isotopes, Cl and Na with the results from the method of Staudt et al. (1993) indicates that enrichment in Cl does not necessarily indicate burial conditions.These results also show that the microcrystalline dolomites from the Oman Mountains with oxygen isotope values close to equilibrium with near normal marine waters possess high Cl concentrations.Therefore, the isotope analysis was beneficial to verify the environment of deposition for the Oman Mountain dolomites while the method of Staudt et al. (1993) failed to elucidate or explain the relationship between the values gained and the environments of dolomitization.

Mechanisms of dolomitization and the possible source of domitizing fluids.
The dolomitization of a carbonate sediment composed of 100% high Mg-calcite using the connate water will produce ~ 30% dolomite (Tucker and Wright, 1990), assuming the existence of a reasonably high original porosity.Therefore, even if all the Mg from the allochems is totally consumed in producing the dolomites, there was insufficient Mg in the host rock to account for dolomitization.Consequently, this must be compensated by other Mg-sources, such as marine water, formation waters and migrating brines, in order to produce massive dolomite sequences.The extent of change will depend upon the initial porosity and permeability relationships of the host sediment/rock.The fact that nearly the entire thickness of the examined rock groups in this study was totally dolomitized indicates that these petrographical properties were very favourable.However, a viable mechanism must operate to affect the mass transfer of fluids required.To cause the pervasive dolomitization of a thick sequence a major requirement is the vast through-put of fluids needed.The envisaged system must satisfy the mass transfer requirement of Mg with time.
The perfect preservation of host rock structures and textures indicates very early dolomitization, probably prior to any early diagenetic alteration, and in a relatively short time interval, before the end of the Triassic.Fast nucleation will produce very fine crystals which preserve original allochem structure and host rock textures.The only fluid capable of such dolomitization and in a short time interval is warm seawater circulating in the shallow subsurface.The rocks that underwent fabric-destroying dolomitization, and especially those displaying coarse crystalline textures, reflect several stages of dissolution-reprecipitation and neomorphism of dolomite in the shallow subsurface by marine and/or mixed waters, as indicated by the isotopic values and trace elements.
The palaeogeography indicates that the Oman region was situated at the edge of the Arabian Craton, away from the recharge area in the west and thus affected by seawater supplied from the vast Tethys Ocean to the east with the potential for dolomitization (Figure 15).The geothermal gradient increased in Early to Late Permian when the rifting of the Arabian Craton started (Blendinger et al 1990;Robertson and Searle, 1990).Rifting and sea floor spreading during the Permian and Triassic increased the circulation of hot subsurface marine waters towards the edge of the Arabian platform (Oman Mountains) (Glennie et al., 1974: Bechennec et al., 1990;Robertson and Searle, 1990), allowing greater influx of warm waters capable of dolomitization in short time interval (Figure 15).Dolomitization may have occurred during the times when sea level was at its lowest levels which led to the migrating of the mixing zones toward the east (seaward).
Figure 15.Permian and Triassic sea floor spreading and its effect on the geothermal gradient and movement of warm subsurface waters.Modified after Blendinger et al. (1990).
The mixing between marine fluids from the east and evaporitic brines from the vast evaporitic-shelf on the Arabian Platform during Permian and Triassic times would also have been a possible source of fluids for dolomitization.Because the Permian and Triassic dolomites from the Oman Mountains show a wide range of textural preservations and chemical variations it is hard to attribute dolomitization to one single process.All the results indicate the interplay of different dolomitizing conditions through time, mainly early dolomitization by normal seawater and modified marine waters by increased temperature, and relatively warm marine water conditions in the shallow subsurface.

Conclusion
The Permian and Triassic carbonates from the Oman Mountains display a great variety of textures embracing most, if not all, sedimentological dolomite variations known in the literature.They show a range from fabric-preservation to complete obliteration of host limestones textures.There is also fracture cementation as well as replacement of whole rocks by baroque dolomite.This petrographic study presented evidence on the systematic distribution of carbonate types between the north and the south in Oman Mountains.The samples from the Musandam peninsula are mainly rocks deposited in a supratidal and lagoonal environments, whereas most of the samples from the southern parts of Oman Mountains are rocks deposited in high energy environments such as shoals, reefs and back reefs.
The most important factor controlling the type of dolomite preservation (fabric-preserving or fabricdestroying) is the diagenetic stages that host rocks underwent prior to dolomitization.Early mimetic replacement by dolomite, prior to any exposure to meteoric waters and/or mixed waters, is important in producing dolomites which retain host rock textures.The combination of petrographic features and geochemistry in these dolomites suggests two main dolomitization stages.The first started soon after deposition in a marine setting either after or prior to cementation by high Mg-calcite and/or aragonite.This stage shows the following features: (i) Mimetic replacement of the micritic parts of allochems, micrite envelopes and matrix by cloudy nonplanar microcrystalline dolomites, (ii) Replacement of the bioclasts by fine cloudy nonplanar dolomites, (iii) Mimetic replacement of the first stage cement (high Mg-calcite or aragonite), where present, by cloudy dolomite crystals, and (iv) Development of clear-rhombic isopachous dolomite cement on the outer surface of allochems.The second stage dolomite is a cement which shows competitive growth in to cavities.Variations in the composition of this cement are reflected in the zoned patterns seen under cathodoluminescence, and confirmed by microprobe analyses obtained from sequential points in single crystals.
The poor ordering, slightly Ca-rich composition to near stoichiometry, textural preservation features, trace element composition, especially high Al, Si and Na, and enrichment to slight depletion in 18 O values are features compatible with other dolomites inferred to have developed early under normal and near normal marine fluids.On the other hand, well ordered rock-forming dolomites, near to stoichiometric compositions, with a crystalline texture, low Na, Fe and depleted 18 O values, are features compatible with dolomites inferred to have formed under oxidising mixed and marine waters in the shallow subsurface under temperatures not higher than 60°C.Moreover, stable isotopes in various types of dolomites suggest two main dolomitizing fluids.The isotopic data are compatible with seawater and mixed water dolomitization.The marine fluids could be of near-normal seawater composition and/or slightly modified marine fluids by heat and evaporation.
Therefore, in the absence of any evidence of hypersaline brines and the relative sparsity of evidence for dolomitization during burial, the Permian and Triassic carbonates of the Oman Mountains must have been dolomitized early in diagenesis by marine-derived fluids perhaps mixed with varying amounts of meteoric water.The only fluid capable of early dolomitization, which is marked by host-rock/sediment preservation in a short time interval, is warm seawater from the vast Tethys Ocean.Seafloor spreading which was initiated during the Late Permian and Triassic was an important factor in elevating the geothermal gradient in the shallow subsurface during dolomite development.Mixing zone dolomitization may have developed during several sea level

Figure 3 .b
Figure 3. Photomicrographs taken with plane polarized light of: (a) dolomite with mud-supported textures from the Saiq Formation, Kharus15; the internal structure of the algal fragments [A], and the matrix are replaced by microcrystalline cloudy dolomite and the intragranular, mouldic and fracture porosities are filled by planar sparry dolomite mosaics [SD] and nonplanar fine dolomite cement [ND].(b) a peloidal-bioclastic packstone which is composed of molluscan [M] and algal fragments [A], from Hagil Formation, Bih31.(c) a bioclastic wackestone, the bioclasts are algal [A] remains and ostracods shells [O], this rock show the replacement of original carbonate mud by microcrystalline dolomite, and the filling of the fenestral, mouldic, intergranular and fracture porosity by clear dolomite mosaics.Field of view= 8.75 mm.

Figure 5 .
Figure 5. (a) A photomicrograph taken with plane polarized light and (b) Cathodoluminescence photomicrograph of a part of an oolitic grainstone; the microcrystalline dolomite [MD], very fine nonplanar cloudy dolomite spars [ND], and the isopachous dolomite crystals [IP] display dark-grey luminescence.The cloudy isopachous dolomite crystals are capped by a clear rim showing red luminescence and dark-grey terminations (arrow).The clear rhombic [RSD] and subhedral [SD] void filling dolomite crystals display homogeneous red luminescence.The rock was affected by compaction which post dated the precipitation of the isopachous cement and resulted in fracturing the ooids and the cement rim, top centre of the photomicrographs.Saiq Formation-WHS17, field of view = 1 mm.

Figure 6 .Figure 7 .Figure 8 .
Figure 6.(a) A photograph taken with plane polarized light, (b) the same view but with a sheet of white paper inserted between the thin section and the stage.Although the rock is completely crystalline with no textural preservation of the host rock, there exists a ghost texture of a granular host.The host rock may have been a peloidal and/or oolitic bioclastic grainstone or packstone.The allochems show the effect of meteoric water dissolution which produced intragranular and mouldic porosity that were later cemented by clear crystals [Cl] or with crystals having cloudy centres and clear rims [Cd].The micritic parts are replaced by cloudy dolomite crystals and palaeo-voids are filled by clear dolomite crystals.Bih Formation-Hagil10, field of view = 3.5 mm.

Figure 9 .
Figure 9. Relationship between crystal size and Mn, Fe and oxygen isotopes concentrations in dolomites.

Figure 10 .
Figure 10.Relationship between crystal size and SO 4 , excess Na and Cl, and oxygen isotopes concentrations in dolomites.

Figure 11 .
Figure 11.Distribution of oxygen and carbon isotopes within Permian-Triassic rocks from Oman Mountains: (a) according to crystal size, (b) according to age.

Figure 12 .
Figure 12.Correlating the isotopic data of the Oman Mountain dolomites with others inferred dolomitizing mechanisms.(PS)= Samples from the present study.

Figure 13 .
Figure 13.Environment of dolomitization for Oman Mountains Dolomites according to Staudt et al (1993) plot of excess Na and Cl against SO 4 concentrations in ppm.

Figure 14 .
Figure 14.Environment of dolomitization for the Oman Mountain dolomites according to Staudt et al (1993) plot of Cl against Na concentrations in dolomites: (a) Samples from the Ruus Al Jibal Group and Milaha Formation.(b) Samples from the Akhdar Group and Sumeini Group-Maqam Formation Member B. .

Table 2 .
Trace-elements concentration and stoichiometry in the dolomite samples.Cl and SO 4 were analyzed by ion chromatography, all other elements by atomic absorption.