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.
Geological
Setting
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.
Rawda
Carbonatites
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.
Mineralogy
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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