The Carbonatite of Oman
S. Hanna, S. Hajeri, S. Nasir
Carbonatites are very rare igneous rocks. While most of the world’s
igneous rocks contain from 40-80% SiO2, carbonatites rarely have
more than 10%, and often contain only traces. They are distinguished by having
50% carbonate minerals by volume. The dominant carbonate mineral in most
of the world’s carbonatites is calcite, CaCO3, but dolomite, CaMg(CO3)2,
ankerite, Ca(Fe,Mg,Mn)(CO3)2, or another carbonate can
dominate. (Woolley and Kempe, 1989) There are extremely rare cases of
natro-carbonatites which are dominated by Na20 and contain sodium
carbonatites; Oldoinyo Lengai, the only known active carbonatite volcano
in the world, is a natrocarbonatite volcano (see below) (Bell and Keller, 1995). There are only 330 known carbonatite
localities on Earth. Most are found in cratons, and almost half are
associated with rifting or extensional stress regimes. (Woolley, 1989) They
occur as tephras, lava flows, dikes and sills, and virtually never as large
plutonic bodies. They are often spatially associated with highly alkalic
rocks such as nephelinites and phonolites. (Barker, 1989). Carbonatites
are derived from the Earth's mantle, but there is mounting evidence that the
carbon that they contain came from the crust, and had been recycled deep into
the mantle during subduction. Carbonatite magmas differ from silicate magmas in
many ways, but one of the most important is that an essential ingredient,
carbonate, is not everywhere present in the mantle. The process by which
recycled carbon may eventually be incorporated into carbonatite magma and
returned to the crust is a circuitous one involving transport in a variety of
phases. These phases include: the carbonate minerals; graphite, diamond, silicon
carbide, and amorphous material in intergranular films; silicate and oxide
minerals in which carbon atoms substitute in tetrahedral sites; fluids
containing CO2. CO, CH4 components; and magmas containing
the CO32- component. As it forms and migrates through the
mantle, carbonate-rich liquid efficiently concentrates some elements,
especially Sr, Nb, Ba, the light rare-earth elements, Pb, Th, and U, while
strongly rejecting Si, Al, Mg, Cr, and Ni. However, calculated trace-element
enrichments, assuming 1% carbonatite liquid equilibrated with
"average" upper mantle in a single melting episode, are much smaller
than the enrichments observed in carbonatites. There must be multiple episodes
of progressive trace-element enrichment in the mantle, before the observed
trace-element compositions of carbonatites are achieved. The genetic
significance of extrusive carbonatites has been highlighted by Gittins and Extrusive carbonatites are
believed to represent magmatic carbonate liquids which escaped CO2
exsolution plus decarbonation processes and remained above the solidus until
they reached the volcanic P-T regime (Gittins and Jago, 1991; Bailey, 1993). Carbonatite magma can be stopped in its ascent by
reaction with olivine and orthopyroxene, to produce new silicates and a CO2
rich fluid. Ultramafic xenoliths from the upper mantle, as found in many
localities, show the effects of reaction with repeatedly invading
carbonate-rich liquids or fluids. Apparently carbonate-rich liquid can only
survive its upward passage if the rock through which it flows has already lost
its capacity to react with the liquid. The rarity of carbonate-rich magma in
the upper crust is probably caused by difficulty in its ascent, rather than by
a shortage of raw material. The carbonate-bearing igneous rocks (carbonatites)
and associated alkaline rocks are exceptionally important petrological systems
for trying to understand and elucidate the geological processes that pertain in
the Earth's crust and upper mantle . The origin of carbonatites has been
the subject of much debate. Some scientists believe that the parental
magmas for most or all or the world’s carbonatites were natrocarbonatites, like
those at Oldoinyo Lengai. Sodium carbonate dissolves quickly in rain
water, so it is not surprising that there is virtually no record of ancient
natrocarbonatites; the absence of sodium carbonate in intrusive
carbonatites is explained by it’s removal by hydrothermal fluids during cooling
and solidification. The fluids transfer the alkalis into to the country
rocks in a process called fenitization. (Bell and Keller,1995).
The second prevailing theory is that the calcite carbonatites are primary, crystallizing from magmas produced by fractional melting or by liquid immiscibility (separation of multiple magmas from one, analogous to the separation of oil and water.) In this case, natrocarbonatites are an extreme result of these processes, in which the alkalis are concentrated in the melt to a greater extent than in most carbonatite magmas. (Bell and Keller, 1995) Some of the evidence supporting this theory is presented below.
Massive carbonatite blocks have an average mode of 53% Sr-Ba-rich calcite, 23% Fe-monticellite, 9% Th-perovskite plus Ti-magnetite, 6% Cr-phlogopite, 6% forsteritic olivine, about 2% Zr-schorlomite and ca.1% Si-CO-OH apatite. Perovskite, schorlomite, and apatite form cognate phases, whereas olivine and phlogopite, often replaced by monticellite, occur as nodules and as discrete grains with compositions and deformation features typical of mantle xenocrysts found in alkali basalts and ultramafic rocks. High modal content of Ca-carbonate, high Sr, Ba and LREE contents of calcite, the presence of rare minerals peculiar to carbonatitic rocks.
The rock represents a carbonatitic melt strongly contaminated by mantle crystal debris. It displays unusual geochemical features having trace elements closer to those of the regional-associated kamafugitic rocks rather than to those of common carbonatites.
Economic importance of carbonatites
The rare earth elements (REE), niobium (Nb) and tantalum (Ta) are a suite of elements that form coherent mineral assemblages predominantly occurring in carbonatites and associated rocks. Only small tonnages of these elements are produced world-wide, yet they are essential components in many high-technology industries. They are also a source, in some cases a unique source, of a range of economically important elements and commodities. The most common accessory minerals found in carbonatites are phlogopite, KMg3Si3AlO10(OH,F), apatite group minerals, Ca5(PO4)3(F,Cl,OH), calcic and alkalic (Na and K) amphiboles and pyrochlore, (Na,Ca)2Nb2O6(OH,F). Carbonatites are strongly enriched in rare earth elements (REEs), and contain higher average percentages of REEs than any other igneous rock type. The relative abundance pyrochlore and other niobium-bearing minerals make carbonatites a commonly mined source of Nb. Carbonatite mines also produce phosphates, fluorite and rare earth minerals. (Mariano, 1989) .
The Carbonatites and Carbonate bodies of Oman
Large occurrences of extrusive carbonatites were found to occur within the metamorphic rocks beneath the Semail Ophiolite Complex in the Uyaynah area near Dibba, and at the Hatta area, northern Oman mountains (Alleman nd Peters, 1972; Ziegler et al. 1991; Woolley et al., 1991; Nasir and Klemd, 1998). They are associated with pillow lavas, meta-volcanics and radiolarian cherts. Petrographic and isotopic data confirm a mantle source for the Uyaynah carbonatites, which were probably formed on volcanic islands in a transition zone of continental to oceanic crust (Woolley et al., 1991; Ziegler et al., 1991; Nasir and Klemd, 1998). Since carbonatites are well known for their valuable mineral resources on a world-wide scale, the report merited further investigation for new occurrences. Accordingly, a programme of prospecting work was drawn up and executed in 1999 to 2000.
The results of the new finding of carbonatites in Hatta area were encouraging in all respects and so a decision was taken to extend it into adjacent regions with similar geological characteristics to those of Hatta and Dibba. Many of the geologic features of Dibba and Hatta are present in the area that was prospected for carbonatites. However, the mantle sequence of the Oman Mountains is bounded by an upper cumulate zone (mainly gabbro) and an imbricate basal thrust zone. Both zones have permitted fluid circulation and intense water-rock interaction resulting in total serpentinization, carbonation and /or silisification. 'Amqat' is retained as a convenient term for silicified serpenitinte lithology. It was first used by Glenie et al. (1974) for silicified serpentine with striking high relief features which stand proud of the normal serpentine topography. Amqat forms hard resistany outcrops within the basal serpentinite. In outcrops it is very similar to carbonatite where the color ranges from bright orange-brown to red brown. 'Birbirite' is the same lithological term for amqat. It was first used by Glennie et al. (1974) and Allemann and peters (1972 ) for silicified serpentine in the northern extinsion of the Oman Mounatains. The Amqat and Birbirite ourcrop pattern follows fault zone which have developed in the ophiolite nappe due to Palaeogene uplift. Glennie et al. (1974) proposed that the silicified serpentinite was produced by " selective leaching of magnesium under tropical conditions". Alternatively Stanger (1985) concludes that silicification was a low temperature chemical replacemet feature and not a weathering phenomenon.
The geology of the Oman mountains has been widely investigated and several reports and papers have been published (Glennie et al., 1974; Coelman, 1986; Robertson and Searle, 1990). The Arabian continent formed a passive continental margin to the southern Tethys ocean at the end of the Paleozoic. Thick sediments and volcanic were deposited along the margin. Seamounts cored by the Haybi complex were found in the ocean basin. A northwestwards-dipping subduction zone developed from about 100 Ma onwards. This zone consumed the crust lying between it and the continental margin. As the subduction zone approaced the continental margin, oceanic crust was obducted onto it.
Tectonic Setting of Carbonatites in Oman
The tectonic setting of the carbonatite in Dibba zone is interpreted as a transform or transentional passive margin (Robertson et al., 1990a). In the geologically similar Hatta transform fault zone, 70 km to the south of Dibba Zone, interpreted by Robertson et al. (1990b) as a transform passive margin- the present study confirms the existence of carbonatites within the metamorphic rocks underlying the Semail Ophilite in Rawda area which are relatively similar to those in Hatta (Nasir and Klemd , 1998).
The Hatta zone, is an approx. 50 km long left-lateral offset of the nortern Oman passive margin that was generated by right-lateral transform faulting during the spreading of Neo-Thyses ocean (Watts, 1990; Cooper, 1988; Robertson et al., 1990b). It forms a WNS-ESW trending elongate window thorugh the Semail ophiolite . Rock units in this zone, structurally below the Semail ophiolite, include platform slope carbonate (Sumeini Group), base slope redeposited sedimentary rocks and more distal deep-sea sedimentary rocks belonging to the Hamrat Dura Group (including the Haliw Formation. Cenomanian volcanic rocks, metamoprhic rocks (green schist, calc-silicate and amphibolite) and peridotites and gabbros of the Semail ophiolites. The newly observed carbonatites from Rawda area either occur mainly as dikes, few occur as intercalations, boudins and lenses within the Hamrat Duru Group, which consists mainly of thin bedded purple, red and green turbiditic radiolarian cherts, calciturbidites, shales and belongs to Late Triassic (Robertson et al. 1990b; Bechennec et al., 1988). Associated Triassic alkali basalts include pillow lavas, pillow breccias and hyaloclastites and rocks described in the field as amphibolite and chlorite schist (Searle et al., 1980). The associated volcanics show abundant textural evidence of submarine origin (pillow lava, hyaloclastites). The approximately EW-trending carbonatite dykes and lenses show sharp, sheared contacts with the assocaited pillow lavas and cherts. They consist of numereous subparalle intercalations which range between 20 cm and 2 m in thickness and occur in strongly deformed less comptent country-rocks, although the carbonatites themself lack internal deformation. The carbonatite lenses range in thickness between 2 and 20 m. They are easily recognizable in the field due to the light yellow to dark brown colour (iron oxides) resulting from weathering and frequently show late calcite and quartz veining. The entire sequence was metamorphosed to green schist facies conditions (Robertson et al.1990b).
The Hawasina nappe is a tectonic megaunit thrust over the Arabian Platform during the obduction of the Semail Ophiolite and comprising Permian-Cretaceous sedimentary and volcanic rocks of the southern passive continental margin of the Tethyan Ocean.
The history of the Hawasina Basin began in the Late Permian with the formation of a vast intracontinental basin (the Hamrat Duru Basin) on the northeastern edge of the Gondwana (Be'chennec et al., 1988). The Hamrat Duru Basin became a passive continental margin during the Middle-Late Triassic (Be'chennec et al., 1988). Sedimentation in the Hawasina Basin began mainly in the Triassic and continued up to the Late Turonian-Early Senonian. Sedimentation ended when the basin closed and overthrusting during obduction of the Semail Ophiolite occurred. These main stages in the evolution of the Hawaina Basin are marked by extinseive development of alkaline magmatism. Glennie et al. (1974) assumed Permian to Late Triassic-Early Jurassic rifting and simple in sequence thrusting of the Hawasina Basin. Graham (1980) drew a close comparison between the Hawasina Basin and the Mesozoic rift history of the North Atlantic. The Oman exotics were seen as seamounts along the continent-ocean boundary. Cooper (1990) suggests the presence of two depositional sub-basins within the Hawasina basin. A shale-rich northerly sub-basin was pornded by an ocean floor-ridge at its southern end, and relatively small dimension sediment bodies were fed into this from numerous point sources along the Oman margin. Searle et al. (1980) and Robertson and Searle (1990) suggested that the Oman exotics were oceanic seamounts, while the Hawaisna and the Haybi complexxxes are interpreted as oceanic units that were incorporated into a subduction-accretion complex.
Tectonic setting of the carbonate bodies in the Semail Nappe
The primary minerals of the mantle sequence (olivine, pyroxene, Cr-Spinel + plagioclase and ampbibole) have, to varying degrees, been altered to secondary assemblages of lizardire, chrysotile, iron oxids, cholrite and carbonates. Only Cr-spinel is inert during alteration. Extinsive alterations of olivine will produce a "chicken-wire" or mesh texture, in which a network of serpentine will enclose a tesselated array of polygonal cells with a core of primary olivine. The carbonates, magnesite, dolomite and calcite, occur in veins and shear zones throughout the mantle sequence. Magnesite is by far the most common carbonate in the peridotites (Stanger, 1985). The majority of the mantle sequence rocks of the Semail Nappe are between 50% to 80 % altered. The basal unit of Searle (1980) at the base of the Nappe is intensely sheared and generally 100% altered. The altered rocks appear darker at outcrop than the mantle sequence proper. Campanian-Maastrichtian lateritic alterations of the Semail ophiolites under a tropical climate were described by Alsharhan and Nasir (1996). Locally high degrees of alteration occur along imbricate or high-angle fracture zones or lineaments that cut through the mantle sequence and throughout most of the leading edge of the Semail Nappe. Neal and Stanger (1984) and Stanger (1985) suggest that there are two types of alteration in the Semail Nappe; (1) high temperature alteration serpentinization and (2) low temperature (30-45oC) precipetation serpentinization. The first type is the most intense (100%) along major thrust and fracture zones. It is relatively uniform in its effects and it is possible that it took place either whilst the ophiolite was an in situ part of the oceanic lithosphere or during its detachment. The second type of alteration is related to the movement of present day meteoric waters through the rocks. Rothery (1984) used Landsat multispectral scanner (MSS) to mappe this alteration unit. He argues that the correlation between alteration and emplacement-related structures to be due either to syn- or post-emplacement processes.
The carbonate bodies forms hard resistant outcrops that occur sporadically in the highly altered rocks along the basal thrust of the semail nappe and along a few fault zones through the mantle sequence.
Three dikes were identified as carbonatites during fieldwork in 2000 in Rawda area. The dikes are one to three meters wide; some thin to less and most are vertical. All have sharp contacts made clearly visible by the contrast of the yellow ochre color of the carbonatite dikes with the grey or dark red color of the host radiolarian cherts of the Hamrat Duru unit within the Hawasina group. The Carbonatites occur as hard, tough, unfoliated rocks forming craggy ridges and upstanding outcrops. Weathered surfaces are extremely rough. They are typically coated in black and/or brown secondary iron oxides. Dike 1 and 2 are fine-grained and contains numerous veins and patches of white calcite and grey quartz. Several sills (10-20 cm thick) penetrates the radiolarian cherts and alternate with the thin chert layers. Thin dikes (1-3 cm thick) cross cut the associated pilow lava. Dike 3 is medium-grianed and is dark grey on fresh surfaces. Most samples are characterised by the presence of perfectly spherical lapilli, which vary from 10 to 80 vol.% of the whole rock. The lapilli are up to 1 cm in size and usually consist of carbonate, Mg-chlorite and magnetite. Most calcites within the lapilli are fine-grained, while euhedral calcite rhombs in veins are medium to coarse-grained. Few dolomite microcrystals occur as lamela enclosed by calcite. Apart from calcite (up to 70 vol.%) further principle minerals are apatite (often carbonatized), and brown spinel. Most apatite occurs as isolated, sub-prismatic, sometimes broken crystals up to 5 mm long. Some apatite occurs as cluster of equigranular crystals 2-3 mm across. Bailey (1989) interpreted the occurrence of Cr-spinel (Brown spinel) as a direct evidence for the mantle origin of some carbonatites in Zambia. Chlorite, sphene, epidote, allanite and barite occur as a minor minerals. Besides hematite other secondary minerals include quartz, and Fe-chlorite. The carbonatites show magmatic textures such as shard textures and perefectly spherical lapilli. These textures are clearly of pyroclastic origin (cf. Keller, 1989) and therefore indicate subaerial and subaquatic extrusion as already suggested for the meta-carbonatites from the Dibba Zone (Woolley et al., 1991; Ziegler et al., 1991) and Hatta Zone (Nasir and Klemd, 1998).
The carbonate bodies
The carbonate-rich bodies, somtimese the basal thrust of the serpentinte, but more commonly forming discontinuous planar features sub-parallel to the basal thrust. It forms hard resistant outcrops. From a distance the colour ranges from ornage-brown to red-brown. They occur as lenses and dykes with blue-grey and/or brown to orange colors. The rocks are comapcted and hard. Most of them contain chromite grains and display a mesh texture outlined by goethite exsolution. In most cases carbonation is complete. The calcite has crystallized as ehuderal open-space precipitates within the cavities. Some calcite crystals are tabular in shape. Many crystals show twinning and epitaxial growth. Samples from dike 1 and 2 contain microcrystals of dolomite forming parallel lamellae within calcite. Samples fro the Tawa and Bowa area are clast-supported breccia. The diagenetic calcite rims all of the clast and fills the fracture. The clasts consists of serpentine and chromite. The clast shape is angular to subangular, suggesting ver short transport and rapid deposition. The grains are arenaceous in size with pseudoporphyric features. Samples from the Wadi Jizi and Wadi Bani Omar contains remnant of plagioclase crystals. X-ray data of the carbonate show that the samples consist of calcite and dolomite as major minerals and ankerite, serpentine and quartz as minor minerals.
The chemical composition of matrix carbonate in the three dikes is low Mg calcite . MnO is higher than FeO and SrO contents is significant as is tyical for calcite in carbonatites. Exsollution lamellae in the calcite prove to be dolomite, which contains higher Fe and Mn, but lower Sr than the enclosing calcite. Microprobe analysis of the calcite from the carbonate indicated a normal, low-Mg calcite with a variable amount of Mn.
The apatite from dike 1 and 2 is a relatively low La-Ce-F apatite, the others are relativley high La-Ce-F apatite . Apatite from dike 3 contains the highest SrO.
Magnetite from the carbonatites are rich with TiO2 in comaprison to magnetites
from the carbonates .
Spinels are rimmed by iron oxides in the carbonate but occurs as single large grains (0.1-0.2 mm) in the carbonatites. All spinels are chromain. Spinels from the carbonatites is Al2O3-rich and Cr2O3-poor in comparison to spinels from the carbonate .
Analysed mica from the carbonates are proved to be Cr-rich fuchsite. Secondary chlorite is associated with fuchsite and is also Cr-rich .
Geochemical data are tabulated in Table 6 and presented diagramatically in Figs 1 and 2. In Fig. 1 the cconcentrations of trace elements have been normalized to hypothetical primordial mantle composition (Wood et al., 1979). General features of the geochemistry of carbonatites are:
the typically high abundance of Ba, Th, LREE, Sr, variable abundances of Nb, Ta, P and low Cs, Rb, K, Ti and HREE abundances of all samples, similar to carbonatites world wide (Woolley and Kempe, 1989)
High SiO2 contents (12-23 wt.%) in the samples of dike 1 and 2, similar to the South African Goudini and South Australian carbonatites (Nelson et al., 1988).
All samples posses large negative U and Ti anomalies as well as small negative Sr and Zr anomalies. . The average of the Rawda carbonatites fall within the envelope of compositional variation noted in world ferro-carbonatite listed by Woolley and Kempe (1989), which is consistent with their classification as carbonatites. However, Nb, Ba and Sr concentrations are lower than the established golbal range. P2O5 is higher than the golbal average. However, when compared to the carbonatites of Uyaynah and Hatta, the Rawda carbonatites show lower REE, Zr, Nb, Th, U, TiO2 contents and higher SiO2, Ba, Sr, Y and MnO contents. The variation in the chemistry of the three carbonatites might be due to variable fractionation of apatite.
Chondrite-normalized REE profiles generally have the LREE-enriched, 'steep' patterns typical of world carbonatites (Wooley and Kempe, 1989). The chondrite-normalized REE distribution pattern (Fig. ) displays a strong light REE enrichment and low HREE abundance in the Rawda carbonatite, which is closely comparable with the ferrocarbonatite pattern of Woolley and Kempe (1989). The Rawda carbonatites show higher Ti, Al, Fe, Mn, Na and P and lower Ca and K contents than normal carboante rocks and display much higher values of REE and trace elements. The variation in the chemistry of the three carbonatite is consistant with variable fractionation of apatite.
The carbonate bodies
The grey-black carbonates (samples H-4, H-7, J-1, J-5, SH-1, SH2, WBN-1 , WBN-2) are characterized by their high MnO- (0.44 - 1.85 wt.%), high CaO- (> 40 wt. %) and low Fe2O3- (2.1-5.91 wt. %) and low MgO contents (<4.2 wt.%). Black pigmentation in calcite is generally caused by manganese oxide and/or graphite inclusions (e.g., Hanold and Weber, 1982). In comparison, the brown colored carbonates (samples H1, H2, T-4, T-5, SB-3 and SB-4) have lower MnO (<0.23 wt%), lower CaO (<37 wt.%) and higher Fe2O3 (5.18-11.5 wt. %) and higher MgO (8.2 - 16.4 wt. %). The brown coloration is mainly due to iron oxides. The highest SiO2 contents is observed in the silicified serpentinite sample SB-2 (69.3 wt.%). High Cr (1170-2640 ppm) and Ni (160-1280 ppm) contents were observed in most samples. However, two samples (SB-3 and SB-4) have low Cr (40-100 ppm) and high Ni (360-440 ppm). These two samples show also the highest Sr and Ba contents (1135-1470 ppm Sr and 60-225 ppm Ba). Sr content in the other samples varies between 130 and 1160 ppm and Ba vaires between zero and 70 ppm. All samples have very low contents of REE, Pb, Zr, Y, Nb, U, Th, F, Cl and S. Average rare-earth element concentrations normalized to chondritic values are plotted in Fig. . The rare-earth element patterns for secondary vein carbonate of sedimentary origin (Nelson et al., 1988) and for peridotites from the Semail ophiolite (Pallister and Knight, 1981) are also shown for comparison . All carbonates have very low REE concentrations and are characterized by a relatively flat REE pattern. These characteristics are very similar to the pattern of the secondary vein carbonate as well as to normal carboante. Small negative Eu anomalies are evident in most samples, except those of the The Jizzi and the Wadi Bani Omar areas which possesse a positive Eu anomalies. In comparisons, the peridotites of the Semail ophiolite have lower REE concentrations with V-shaped pattern and a negative Eu anomalies. These patterns are typical for depleted peridotite from ophiolites and oceanic basement (Pallister and Knight, 1981). The carbonates from the Wadi Jizi area show the lowest HREE which are similar to HREE valuse in the peridotites. The silicified serpentinite (sample SB2) show the lowest overall REE content which is similar to REE contents in the peridotites.
Carbonatites are normally closely associated with alkaline volcanic rocks and intracontinental rifting (e.g., Woolley, 1989). However, the carbonatites of the Canary and Cape Verde islands are associated with oceanic fracture zones (Woolley, 1989). The Uyaynah and the Hatta carbonatite occurrences along the Dibba and the Hata fault Zones are interpreted to be of primary mantle origin and have formed in an oceanic island setting similar to the tectonic setting of the Canary and Cape Verde islands (Ziegler et al., 1991; Woolley et al., 1991; Nasir and Klemd, 1998).
Detailed investigations in a number of carbonatite occurrences related to the East African Rift indicate a wide range of relationships between several factors such as seismically reactivated fracture zones or rifts with elongate negative Bouger anomalies, asthenospheric swells and crustal domes and the generation of carbonatite-nephelinite ring complexes. These factors are independent and resurgent suggesting a cyclic nature of such tectonic and magmatic activity. Carbonatite-nephelinite plugs, veins, dykes and flows mark the closure of the cyclic stemming from plate movement and updoming. The Rawda carbonatites are asocciated with large volumes of alkaline volcanic rocks, mainly ankaramites but with local trachytes and nephelinites, belonging to the Haybi Complex, which were partly interpreted on the basis of geochronological and geochemical data as Triassic and as within-plate magmas that erupted on volcanic islands or seamounts (Searle et al., 1980). Therefore, the Rawda carbonatites are probably related to the initial Triassic rifting phase of the Neo-Tethys along the eastern margin of the Arabian platform (Searel et al., 1983; Be'chennec et al., 1988). The Hatta zone including Rawda area shows lithological, stratigraphical and structural similarities with the Dibba zone and is interpreted as a right-lateral transform fault along the continetal margin (Robertson et al., 1990b). The Hatta and Rawda carbonatites occur in a stratigraphic interval rich in a variety of Triassic volcanogenic rocks, radiolarian cherts, calciturbidites and shales, all now metamorphosed in the greenschist facies. This association is characteristic of an oceanic environemnt of deposition, clearly suggesting that the carbonatites and related alkaline volcanic rocks were deposited in an oceanic island during a period of more general alkaline volcanic activity. The occurrences of carbonatites at the Rawda area within the Hamrat Duru sedimenys may indicates a genetic relationship between the carbonatites of Hatta, Rawda and Dibba. However, the three carbonatite occurrences have different chemical characteristics. These differences can be ascribed to primary igneous fractionation or different element mobilization during regional low-grade metamorphism. The high modal Sr-Ba-REE- rich calcite, the typical mineralogy, and the high amount of Sr-group elements identify these rock as a carbonatite. Very high Mg#, mantle debris in the range of mantle values indicate a near-primary character which is distinctive of a restricted group of extrusive carbonatites only found in continental rift areas. A strong correlation exists between carbonatite occurrences in Oman and mid-continental rifts or highly fractured linear zones.
The carbonatites appears to pre-date gensis of the Semail ophiolite. They may represent magmas related to the volcanism associated with the Triassic rifting as suggested for the Hatta and the Dibba carbonatites (Woolley et al., 1991; Ziegler et al., 1991; Nasir and Klemd, 1998), or they may have resulted from regional crustal extension that preceded or accompanied genesis of the ophiolite. The extension may occurred when Triassic Neo-Tethys oceanic crust began to be consumed along a newely activated subduction zone. Pre-existing fracture zone acted as zones of weakness and seamounts and volcanic islands were constructed along zones of transform lineaments such as Hatta and Dibba
Recrystallization of magmatic calcite, through subsolidus plastic flow, deutric alteration, and solution-precipitation, leads to an evolutionary convergence of carbonatite textures toward those that are normally associated with hydrothermal carbonate rocks. However, the discrimination between these hydrothermal carbonate products from those in which juvenile carbon is an essenital component is not an easy job. Recognition of a magmatic heritage for carbonatites must be based on the mineralogy, trace element composition and isotopic ratios of the carbonate-rich rock. Kapustin (1982) and Barker (1989) emphasized that carbonatites are usually enriched in REEs, P, Sr, F and Ba in amounts very much larger than those found in other carbonate rocks of hydrothermal or sedimentary origin. Table 8 shows that most of the carbonate-rich samples are rich in Cr, Ni and poor in other trace elements in comparison to carbonatites . Their chemistry is similar to those carbonate rocks of sedimentary origin. The presence of Cr-rich mica and the high Cr and Ni contents in these rocks argues for a hydrothermal origin for the carbonate- rich bodies in the Umar Group. These rocks can be interpreted as hydrothermal alteration products of mafic dykes and sills. The basal thrust of the Semail nappe displays substantial mineralogical variety. The earliest variation originates from the time of emplacement, in the form of greenschsit-faceies metamorphism. Subsequent uplift has given rise to silicification (Stanger, 1985) and alteration to hastingsite. The most recent efects are aqueous precipitates from bicarbonate-rich groundwaters (Neal and Stanger, 1984;1985, Stanger, 1985, Stanger and Neal. 1994). Glennie et al., (1974) suggested that the silicified seerpentine of the Semail nappe was due to selective leaching of magensium under tropical weathering conditions.
All samples show geochemical and petrofabric evidences pointing to a serpentinte protolith. The immobile elements Cr, Ni and REEs concentrations are similar to those of serpentinite. Constant volume replacement is shwon by the preservation of the serpentinte mesh texture. Elsewhere minor occurrences of carbonated serpentine have been found in the northern part of the Semail nappe ( Glennie et al. (1974). The outcrop pattern of the carbonated serpentine follows fault zones which have developed in the ophiolite nappe due to Palaeogene uplift. The alteration of olivine and pyroxene under high alkaline condition (pH> 11, Neal and Stanger , 1983) created zones of high porosity. Slow mixing of near-surface bicarbonate type water with deeper hydroxide groundwaters favour the precipitation of calcite (e.g., Stanger et al., 1988). The silisification and carbonation of the serpentine in the Semail ophiolite seems to be of regional extents and extend for mor than 600 km from the southern to the northern parts of the nappe.
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