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MINERALOGY AND GEOCHEMISTRY OF MARBLES IN THE PAN-AFRICAN

BASEMENT ROCKS: AN EXAMPLE FROM ABU SWAYEL AREA, SOUTH
EASTERN DESERT, EGYPT

Article · January 1999

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BULL.FAC.ASSIUT UNIV., 28
(2-f), P-P. 77-98(1999) .

MINERALOGY AND GEOCHEMISTRY OF MARBLES IN THE

PAN-AFRICAN BASEMENT ROCKS: AN EXAMPLE FROM
ABU SWAYEL AREA, SOUTH EASTERN DESERT, EGYPT

Gala! H. El Habaak
Geology Department, Faculty of Science, Assiut University
Assiut, Egypt

Received: 27/11/1999
The marbles in the studied area were subjected to low-pressure
regional metamorphism: The marbles suffered later from
retrograde metamorphism along shear zones. Four
metamorphic assemblages would be recognized in the prograde
stage, The textures and mineral paragenesis of each assemblage
are studied. Also the P-T-Xco2 conditions at which these two
metamorphic phases were stable are also discussed. Major and
trace elements data of the marbles are presented in the present
study. Petrologic investigations of these marbles provide
important constraints on the P-T-Xcoz conditions and fluid flow
during metamorphism. It is shown that the major element
geochemistry of the studied marbles is controlled mainly by the
amounts of silicate minerals admixed with carbonate minerals
during deposition and later diagenesis. Reducing condition was
prevailing during the deposition of the protolith.
78 Galal H. El Habaak

INTRODUCTION
The Abu Swayel area is located in the basement complex in the
southwestern part of the Eastern Desert of Egypt between latitude 22° 40'
and 2T 50yN and longitudes 33° 33' and 33° 407E (Fig 1). Several studies
were carried out on the marbles of the Abu Swayel area including El
Shazly et al. (1965. 1975), El Ramly et al. (19BOX Hunting (1967),
ZagWoul et al (1984), El Gaby et a!. (1990) and Ghazsly (1996). El
Shazly et al. (1965) and Hunting (1967) considered that thr marbles
belong to shelf sediments represented by mature to sen-imature
assemblage deposited in a shallow water approaching neritic conditions. El
Gaby et al. (1900) agree with Hunting (1967) that the marbles represent
shelf sediments which were later overlain by a thick pile of molasse type
sediments comparable with those of the Hammainat sediments. Church
(1982) considered that the schists of Abu Swayel represent shelf sediments
intercalated with rnafic-felsic volcanics. On the other hand, Takla el al.
(1994) considered that the marbles encountered in Gabel r'elat area, south
Eastern Desert, ?.re allochthonous fragments incorporated in an ophioiitic
melange.
The present work focuses on the petrography, mineral cheiiMstry
and geochemistry of the marbles to declare the origin of their protolith and
the metamorphic conditions that affected these marbles.
FIELD RELATIONS
Five rock units have been differentiated in the study area arranged
from the younger to the older:

Younger
5- Granites (tonalites, pink biotite granites and red muscovite granites).
4- A Younger mafic-ultramafic intrusion affected by low-pressure
metamorphism and now represented by orthoamphibolites enclosing
Cu-Ni sulphides.
3- Metamorphosed Dokhan volcanics and Molasse sediments.
2-Ophiolitic melange including serpentinites, metagabbros and island arc
metasediments.
1-Shelf sediments represented by marbles and mica schists.
Older
80 Galal H. El Habaak

The marbles are widely distributed in the area as numerous lenses

and layers of variable thicknesses concentrated in a belt around the eastern
border of Um Shilman granite pluton. The marbles are structurally
overlain by ophiolitic melange rocks and pelitic sediments.
Metamorphosed clastic sediments present in the area are differentiated
into two types: (1) island arc metasediments, and (2) molasse sediments of
active continental margins (El Habaak, 1999). The marbles and the
enclosing country rocks were subjected to low-pressure regional
metamorphism, up to the amphibolite facies, and intruded by tonalite,
white biotite granites, pink biotite granites and red muscovite granites.

PETROGRAPHY
The marble is fine to medium-grained and massive, but sometimes
shows colour banding due to variation in graphite and mafic minerals
content. It shows equigranular, granoblastic texture and varies
compositionaUy from calcitic to dolomitic. The dolomite marbles are
distinguished by the relative abundance of brucite, graphite and forsterite.
The brucite marbles have very restricted occurrence and occur only near
the contact with the granite intrusion. Sheared and mylonitized marbles are
also detected along shear zones.
Table (1) summarizes the modal composition of the studied
marbles. The mineral parageneses are characteristic of contact
metamorphism. Staining with alizarin red solution (Warne, 1962) was used
to differentiate between calcite and dolomite in thin sections.

MICROSTRUCTURES AND TEXTURES

The micro structures of the studied marbles allow an assessment of
the role of deformation and recrystallization. The marbles exhibit various
colours ranging from milky white, yellow to black. They may contain
isolated quartz grains and/or veinlets of quartz. They contain, besides
quartz and carbonates, talc, tremolite, diopside and wollastonite. Mineral
relations suggest two phases of mineral growth. The first generation
includes the paragenesis calcite/dolomite, quartz, graphite and tremolite.
They occur as randomly oriented mineral clusters without dimensional
preferred orientation (DPO); weak colour banding defines the bedding in
hand specimens. Grain boundaries are generally serrated, bulbous and
 Mineralogy and Geochemistry of Marbles in The Pan-African

interpenetrating. The bulbous intergrowths suggest that some

grain-boundary migration has occurred during the deformation of the
protolith. Calcite/dolomite grains are dominated commonly by tapering
narrow twin lamellae suggesting a mechanical origin (plastic
deformation). The second phase of mineral growth comprises tremolite,
serpentine and talc.

Table (1): Summary of modal analysis for the studied marbles

Calcite marbles Dolomite marbles
(n=8) (n=5)
Minerals Average Range Average | Range
Calcite 71 92-55 35 28-41
Dolomite 6 0.55-7 40.95 32-53
Quartz 11 7.2-21 8.3 9.2- 19.8
Graphite 3.52 0.18-8.16 5.52 0.25- 14.22
Plagioclase 0.2 0-0.32 nd nd
Epidote 1.2 0.16-2.41 1.11 0.11 -2.81
Wollastonite 1.52 0-3.88 1.44 0-2.14
Forsterite 0.20 0-3.0 2.41 0.-3.11
Diopside 2.3 0-3.12 1.82 0 - 2.02
Hornblende 0.1 0-0.11 nd nd
Tremolite 1.2 0.81-3.12 1.86 0-2,07
Biotite tr tr nd nd
Sphene 0.2 0-0.90 nd nd
Rutile tr tr nd nd
Brucite 0.82 0-2.11 1.02 0 - 2.22
Apatite tr tr o.n 0-0.35
Magnetite 0.21 0 - 0.82 0.53 0.11 -0.89
Ilmenite nd nd tr tr
Pyrite 0.13 0.08-0.51 0.14 0 - 1 .44
tr: traces, nd: not detected

Some of the studied marbles underwent recrystallization especially

along grain boundaries and this grades into porphyroclasts in the
protomylonite stage. The protomylonites are characterized by calcite
porphyroclasts exhibiting undulatory extinction embedded in a matrix of
equent and smaller calcite grains characterized by uniform extinction.
Undulatory extinction of the porphyroclasts suggests that dislocation
82 Galal H. El Habaak

creep was the dominant deformation mechanism. Secondary overgrowths

on many grains suggest that solution mass transfer was also active (Busch
and Pluijm, 1995). Grain boundaries of these porphyroclasts are usually
irregular with curved segments while the grain boundaries of the
groundmass are still straight. At this stage weak lineation and foliation
could be noticed. Generally the studied mylonitic marbles consist of
recrystallized dynamically deformed equant calcite grains, and this fabric
symmetry has been attributed to the relative contributions of coaxial strain
(e.g. Erskine et al., 1993). The transition from massive rock to mylonite is
considered as a progressive mylonitization of the coarse- grained marbles
to fine-grained dark colourd well foliated and lineated mylonite along
shear zones. These mylonitized marbles are usually dissected by a network
of fractures filled with secondary calcite, quartz and epidote. These
features are probably related to fluid-flow occurred after the peak of
metamorphism.

MINERAL CHEMISTRY
Analyses of selected minerals by both electron microprobe
(Max-Plank Institut, Heidelberg, Germany) and EDX (scanning electron
x-ray analyzer of Assiut University) are listed in Table (2). The studied
samples contain both calcite and dolomite in variable amounts, together
with small amounts of silicate minerals. Calcite grains in all marble
types are chemically unzoned with regards to major and minor
constituents. Trace amounts of SiOs, NazO and K2 O are incorporated in
the analyzed calcite. Ca content (per formula unit) varies in the studied
calcite from 1.66 to 1.54 with an average of 1.627. The calcite contains
variable amounts of (Fe + Mn) atoms per formula unit (0,001-0.02) and
is relatively rich in MgO content (MgO = 0.72 - 0.54). The ratio
(Mg+Fe24+Mn)/Ca is 3.53 on the average indicating limited substitutions
of Ca by the other divalent cations and consequently limited metasomatic
process accompanying the metamorphism. Fractures filled with calcite
dissecting the marble bands contain higher amounts of Mg and Fe than the
calcite of the host marble. In addition to calcite (cc), diopside (di),
forsterite (fo), wollastonite (wl) quartz (q), serpentine (spr) and talc (ta)
are also analyzed (Table 2). Most of the studied samples contain secondary
minerals, for example periclase is hydrated to brucite, forsterite and
diopside are partially altered to serpentines, as well as secondary
dolomite was produced as a by product of alteration of periclase or by
exsolution from calcite.
 Calal H. El Habaak

PRESSURE AND TEMPERATURE OF MINERAL GROWTH

Mineral paragnesis and textural relations suggest the PT conditions
under which the marble was formed. Petrographic studies reveal the
presence of metamorphic zones around the emplaced the Urn Shilman
granite intrusions. The appearance of talc, tremolite, and forsterite and
diopside coexisting with dolomite, calcite and quartz in the studied marbles
would suggest increasing temperature as the granite approached (prograde
stage). The estimation of PT conditions for the various mineral
assemblages detected in the studied marbles is based mainly on the
CMSCH (CaO-MgO-SiO 2 -CO 2 -H 2 0) assemblages (Baker and Mathews,
1994) (Fig. 2). Based on the inferred metamorphic minerals, four
metamorphic assemblages could be differentiated within the prograde
stage. The talc-tremolite-dolomite-quartz-calcite assemblage is stable at
temperatures of 470 - 490°C. The
diopside-forsterite-tremolite-calcite-dolomite-quartz assemblage is stable
at temperatures of 585-665°C. Temperatures based on the composition of
calcite coexisting with dolomite in the presence of quartz range from 475
to 600° C. In some cases, relatively high temperatures were determined
for marbles containing an isobarical univariant assemblage consisting of
calcite + dolomite ' wollastonite

P Ikbars)

10 •

:O2 stability of P-T- X C02 stal d ilityi of

emolitc-calciie- op si d e-tf e m olife -calcite-

360 4 •' s 560 50 TIC)

Fig. 2: P-T diagram appropriate for constraining the conditions of growth of the firsi
generation of minerals observed in the studied marbles. Bold lines show univarianl
reactions along which talc (la), tremolite (tr), dolomite (del), quartz (q). diopside (di).
calcite (cc) and fluid (f) are stable and contoured for X< : o2 of the coexisting fluid phase
(see Baker ct al., 1991 and Baker & Matthews, 1994). Horizontal lines give the
pressure range determined for mineral growth (Buick & Holland, 1989). Dashed lines
show the range of Ihc conditions at which the observed ta-(r-dol-cc-q and di-tr-dol cc q
assemblages could be stable.
 Mineralogy and Geochemistry of Marbles in The Pan-African
8
5

The peak prograde mineral growth took place at a pressure of 6+2

kbars (Buick and Holland, 1989; Baker and Mathews, 1994). Fluids
coexisting with talc-tremolite-calcite-dolomite-quartz assemblage are
relatively water-rich, with Xco2 ^ 0.22 for pressures in the range 4 - 8
kbars. The high grade assemblage containing
diopside-tremolite-calcite-dolomite-quartz coexisted with CO 2 -rich fluids
with 0.74 Xeo2^ 0.88 for pressure of 6 kbars (Fig. 3).

ROLE OF FLUIDS IN MINERAL GROWTH

Fluids are potentially important agents of metamorphism. The
presence of a significant volume of fluids during metamorphism may (1)
control the stability of mineral assemblages, (2) cause metasomatism and
deposition of economic mineralization, (3) transfer heat and/or (4) affect
the rheology of the crust and influence tectonic processes (Cartwright et
al., 1997). It has been shown that two mineral generations are detected in
the studied marbles. The first generation was developed during prograde
metamorphism while the second developed during retrograde
metamorphism. The presence of wollastonite in the studied thermally
metamorphosed marbles indicate that these rocks have been infiltrated by
hot water-rich fluids (Tracy and Frost, 1991). Prograde mineral
assemblages of the studied marbles provide an important information about
the composition of the fluids during prograde metamorphisms and
alteration. The assemblages stable within the siliceous-carbonate lithology
are believed to comprise calcite ± dolomite ± quartz during low-grade
metamorphism. The growth of new, high temperature minerals observed in
the siliceous-carbonate assemblages requires the presence of a fluid, which
allows the following reaction to take place, and enables talc and tremoiite
to grow (Baker and Matthews, 1994). 3CaMg (CO 3 ) 2 + 4Si0 2 + H2 O -
Mg 3 SiO, 0 (OH) 2 + CaCO3 + 3CO2
Dolomite + quartz + fluid = talc + calcite + in the fluid

Figure (4) is a schematic T-X C o2 section showing selected reactions

between some minerals and COjAHaO fluids.
5CaMg(CO ? ) 2 + 8Si0 2 + H 2 O = Ca 2 Mg 5 Si 8 O 22 (OH) 2 +3CaCO 3 +7CO 2
dolomite + quartz + fluids = tremoiite + calcite +
fluids
Periclase + HaO = Brucite
MgO +H 2 O
=Mg(OH) 2
Galal ff. El Habaak

retrograde T-XC02
path of the

Fig. 3: T - Xco2 diagram appropriate for constraining the conditions of the growth of the prograde
minerals in the studied marbles. Dashed fields show T - Xco2 ranges for which prograde
diopside- and tremolite-bearing assemblages and prograde pristine dolomite are stable. Arrows
show changes in temperature and XCTO of the coexisting fluid required to grow the overprinting
tremolite-calcite and talc-calcite retrograde assemblages (see Baker and Matthews, 1994).

700

0 0,2 0.4 0.6 0.8

Fig. 4: T-Xcoa diagram depicting selected phase equilibria among periclase (Per), brucite
(Brc), forsterite (Fo), serpentine (Spr), calcite (CC), dolomite (Dol), and CO2-H 2 O fluid at
1000 bars. Inset shows chemographic relationships in the vicinity of the Fo-Spr-Brc-CC-Dol
isobaric invariant point. Dashed curves illustrate the T-Xco2 evolution of the fluid-rock system
during retrograde mineral-fluid reaction in the studied marbles (see Ferry and Rumble DI,
1997).
 Mineralogy and Geochemistry of Marbles in The Pan-African
8
7

At a higher grade of metamorphism tremolite is unstable and if SiO 2 is still

available after the above reaction, tremolite reacts with calcite to form
diopside (Deer, et al. 1992):
Ca 2 Mg g Si 8 O22(OH) 2 + 3CaCO 3 + 2 SiO 2 - 5CaMgSi 2 O 6 + 3CO 2 + H 2 O
The temperature for the above reactions depends upon H 2 O and CO 2
concentrations. All the above reactions demonstrate that the fluids
associated with the retrograde assemblages (and alteration) were more
water-rich than those present during the peak of prograde metamorphism
(Fig- 3).
RETROGRADE METAMORPHISM AND ALTERATION
The retrograde metamorphism is particularly recognized along
shear zones and areas of intense veining. Identification of retrograde
mineral reactions is based on texture relations in thin sections. The zones
rich in quartz, epidote and calcite veins indicate H 2 O-rich fluid infiltration.
In the present study, the growth of amphibole + epidote + talc ± chlorite ±
serpentine after silicate minerals (e.g. pyroxene and plagioclase) and
brucite after periclase is referred to as retrograde. This mineral assemblage
is certainly related to the most recent equilibration experienced by rocks
and mineral paragenesis indicating that this mineral generation has grown
at a temperature lower than that of the prograde minerals in the
metamorphic complex. In this stage calcite also occurs as coarse grains, as
well as thin films overgrowning the quartz grains or as thin venlites
dissecting the marble bands. Retrograde epidote and tremolite are
commonly replace diopside and plagioclase crystals and occur in a
congruent fashion. Quartz is abundant in this retrograde assemblage and is
essentially an end-member SiO? (sample No 27 in Table 2, for example).
Chernosky et al., (1988) concluded that antigorite is the equilibrium
serpentine mineral during alteration of forsterite. When tremolite and
serpentine are both present, serpentine appears to be texturally later.
Serpentines rim or cross-cut forsterite and diopside.
CRYSTALLINE CARBONACEOUS MATTER
The studied marbles contain variable amounts of poorly crystalline
carbonaceous particles in a dispersive form. These particles are
concentrated along mineral boundaries, cleavage planes and in some
sedimentary laminae. The studied carbonaceous matter are differentiated
according to morphology, grain size and crystallinity, using TEM., into
88 GalalH. El Habaak

flakes and globular/ovoid forms. The globular forms are characterized by

their uniformity in shape and size. They occur as separately rounded to
semi-rounded bodies, 12 ^ in diameter, or concentrated and interconnected
in patches. These particles appear less transparent to the electron beam
along their outer margin suggesting a hollow shell structure in three
dimensions (Large et al. 1994). The flaky forms include composite and
simple particles. The composite variety is characterized by irregular
outlines and shapes and exhibits internal structure. In the low-grade
marbles, the carbonaceous particles occasionally display considerable
structural variation, probably reflecting that their organic precursors were
heterogeneous in both chemistry and molecular structures. With increasing
grade of metamorphism, these particles become more homogenous and
gradually display limited structural organization due to increasing
crystalline graphite content (Fig. 5).
Graphite occurs either dispersed throughout the rock or
concentrated in parallel laminae. The graphite forms 1 mm thick hexagonal
flacks or unevenly distributed shapeless bodies mostly arranged in a pattern
subparallel to the bedding planes. They also form elongated streaks of
fragmented crystals. In the mylonites, cataclasis produced extremely small
graphite particles. The occurrence of globular panicles with crystalline
graphite indicates that they are non-graphitising, which is not unexpected
as their small size and structure would inhibit the formation of long range
crystalline order. A change in crystallinity can be recognized by a polygonal
appearance of some globular carbonaceous particles coexisting with
graphite in the high grade marbles. It is suggested that these carbonaceous
particles probably evolved from metamorphosed kerogen of microporous
type graphite (Large et al., 1994); analyses of S13 C should be done.
Graphitization of these kerogen in metasediments depends on pressure and
shear stress causing collapse and flattening of pore walls and pore
coalescence (Bonijoly et al., 1982)
CONTROLS ON GRAPHITIZATION
It is believed that carbon in the graphite in the studied marbles is of
syngenetic biogenic origin. During diagnesis and metamorphism, the
organic materials associated with the carbonate protolithes were converted
into carbon accompanied with loss of volatile constituents like oxygen,
nitrogen and hydrogen. During metamorphism, some of the remaining
carbon was partly recrystallized into graphite. The transition from poorly
90 Galal H. El Habaak

crystallized carbon to graphite is termed graphitization. Pressure,

temperature, duration of the thermal event, shear stress, original kerogen
type and fluid composition are the main parameters which influence
graphitization. Syngenetic graphite formation in metamorphic terrains was
previously considered a progressive, temperature dependent transition of
amorphous kerogen to crystalline graphite (Landis, 1971), Recent studies
reveal a far more complex discontinuous process depending on pressure
and shear stress (Deurbergue et al., 1987). Stress was found to reduce
greatly the activity energy for graphitization and allowing graphite to form
at lower temperatures and confining pressures (Ross and Bustin, 1990).
Low oxygen kerogens are more prone to graphitization where pore sizes
decrease as the oxygen content of the kerogen increases (Bonijoly et al.,
1982). Metamorphic fluid composition may also influence the
graphitization process. Wintsch et al. (1981) noted that interstitial,
non-aromatic hydrocarbons (CtrU) in porous carbonaceous matter from
greenschist grade metasedirnents inhibit the formation of a well ordered
graphite.
CHEMICAL ANALYSES OF THE MARBLES
Chemical analyses and Niggli values for sixteen representative
marble samples are presented in Table (3); some average values of
unmetamorphosed carbonate rocks are given in Table (4) for comparison.
The chemical analyses were performed at the Nuclear Material
Corporation, Cairo and the Geology Department, Assiut University. Major
elements were analyzed by the wet method and trace elements were
analyzed by X-ray fluorescence and atomic absorption. The average major
element compositions of the investigated dolomite marble are similar to the
unmetamorphosed dolostones and stoichiometrically similar to the impure
dolomite but differ in being relatively rich in CaO and A12O3. On the other
hand, the calcite marbles are richer than the dolomite marble in almost all
the major elements except MgO and K 2 O. The average chemical
composition of the studied calcite marbles is close to the average of
unmetamorphosed impure limestone (Table 3&4). Also the high SiO 2
contents of the calcite marbles are reflected in the high modal quartz and
silicate minerals in these rocks.
A series of inter-element plots is represented in Fig. (6 & 7). It is
noted that elements with almost equal ionic potential correlate positively
with each other. This is shown for example between CaO with Sr, K 2O
with Rb and Ba. On the other hand, CaO shows positive correlation with
 nd: not determined
P20 5 . KiO is also positively correlated with A^Os and this could be
attributed to adsorption phenomenon. The major element geochemistry of
the studied marbles is controlled mainly by the original variations in the
amounts of silicate minerals admixed with carbonate minerals during
deposition or added during diagenessis. The major elements are plotted
92Galal H. El Habaak

against A^CK (Fig. 7) because A1 2 O 3 shows substantial variations among

the marble samples, and because it commonly behaves in immobile fashion
during metamorphism (Munyanyiwa and Hanson, 1988). It is found that
there is a positive correlation between A^Os and Na 2 O, TiO 2 , Zr and V
which were brought into the basin of deposition as components of clastic
ingredients.

Fig. 6: Variation diagrams of A1203 vs K2O & Zr, K20 v.s Rb & Ba and CaO vs Sr &
P2O5 in the studied marbles,
 Mineralogy and Geochemistry of Marbles in The Pan-African

Fig. 7: Varaition diagrams of major oxides (wt %) vs A1 2 O 3 and CaO vs al-alk in the
studied marbles.

All the above features suggest that the studied marbles contain
substantial amounts of detrital particles. The positive correlation of some
of the major and trace elements with A^Os indicates that an
alumina-bearing phase made up a major fraction of the clastic material
admixed with the carbonate minerals. Clay, detrital micas and feldspars are
the most
 Galal H. El Habaak

important carriers of A1 2 O 3 in sediments. Positive correlations of FeO,

MgO, TiO 2 , Zn and Ni with A1 2 O 3 are inconsistent with feldspar being the
major carrier of AlaOs This clearly indicates that the clastic sediments
brought into the basin were essentially mature, very poor or devoid of
feldspars. Hence the linear compositional trends are most probably
controlled by admixture of phyllosilicates and carbonates. This is also
confirmed by the Niggli parameters al-alk [(A1 2 O?) - (Na ? 0 + K 2 O)], where
al-alk ratios of the studied marble samples have an average of 0.68
indicating that the studied marbles are free of either albite or K-feldspars.
Chalcophyle elements such as Cu, Zn and Ni are correlated with S
indicating that they are restricted to the sulphide phase. The marbles rich in
graphite are also rich in Cr, V and Zn which indicates reducing condition
during the deposition of the protolith and preservation of organic matter
containing Cr, V and Zn. It is concluded from the chemistry of the marbles
that the caldte marbles of Abu Swayel area were derived from impure
limestone and dolomite marbles were derived from impure dolostones.

Table (4): Chemical analyses of dolostones, pure

and impure limestones (after, Pettijohn,
1975)
Dolostones Pure limestones Impure limestones
Si02 3 98 1.88 10.61
A120. 0.78 0.83 4.3
FeO 0.64 0.26 2.97
MnO 0.02 0.01 0.22
MgO 20.26 2.75 2.01
CaO 29.06 50.89 41.9
Na2O 0.14 0.06 1.38
K2O 0.04 0.01 0.56
TiO2 0.04 0.01 0.07
P205 0.32 001 0.21

NATURE OF THE PROTOLITHS

The geochemical features of the studied marbles reveal that the
calcite marbles were derived from impure limestones while the dolomite
marbles were derived from impure dolostones. The major and trace
element geochemistry of the studied marbles is controlled mainly by the
variation in the degree of contamination of precipitating carbonate minerals
and clastic silicates and oxides during deposition. The restricted
occurrence
 Mineralogy and Geochemistry of Marbles in The Pan-African
9
5

of dolomite marbles in the present area could reflect that the prevailing
physicochemical conditions favorable for the deposition of calcite with
restricted conditions for dolomite deposition. The clastic admixtures are
represented mainly by clay minerals and micas, together with minor
amounts of sphene, rutile, ilmenite and apatite as shown in Table (1). The
variation diagram (Fig. 6 & 7), depict that the chemical characteristics of
the protoliths were largely preserved through the different degrees of
metamorphism. This appears to be a common feature of regionally
metamorphosed impure calcareous rocks. Senior and Leake (1978)
concluded that the chemical trends shown by amphibolite-facies
calc-i.:hcate rocks and marbles in the Dalradian of Connemara, Ireland,
are mainly primary and reflect original variation in source rocks. Such
preservation of the original composition suggests that major metasomatic
modification did not play extensive role during metamorphism.
CONCLUSION
The studied marbles represent shelf sediments and demonstrate the
development of two distinct generations of mineral growths. The first is
related to the prograde metamorphism and the other was developed after
the peak metamorphism (retrograde along shear zor;es). The talc
-tremolite -dolomite - quartz - calcite assemblage is stable at the
temperature range 470 -490° C. The diopside - forsterite - tremolite
-calcite - dolomite - quartz assemblage is stable at temperatures of 585
-665° C. Temperature calculations, based on the composition of calcite
coexisting with dolomite in the presence of quartz, range from 475 - 600°
C. It is shown that the increasing crystallinity of carbonaceous matter in the
studied marbles is roughly proportional to metamorphic grade. Major
element geochemistry of the studied marbles is controlled mainly by the
amounts of silicate minerals admixed with carbonate minerals during
deposition. Reducing condition was prevailing during the deposition of the
protolith.

REFERENCES

Baker J. and Mattews A. (1994): Textural and isotopic development of marble

assemblages during the Barrovian-style M2 metamorphic event, Naxos,
Greece. Contrib. Mineral. Petrol., 166, 130:144. Baker J. 5 Holland T.J.
and Powell R (1991): Internal buffering in systems
without solid solution: principles and examples. Contrib. Mineraf. Petrol.,
106, 170:182.
96 GalalH. El Habaak

Bonijoly M., Oberlin M. and Oberlin A. (1982): A possible mechanism for natural
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