Journal of African Earth Sciences 38 (2004) 255–268
www.elsevier.com/locate/jafrearsci
Pressure, temperature and oxygen fugacity conditions of calc-alkaline
granitoids, Eastern Desert of Egypt, and tectonic implications
H.M. Helmy *, A.F. Ahmed, M.M. El Mahallawi, S.M. Ali
Department of Geology, Minia University, Minia 61111, Egypt
Received 26 June 2003; accepted 21 January 2004
Abstract
Five calc-alkaline plutons; Um Tagher, Abu Zawil, Um Gidri, Um Anab and El Ghuzah, in the northern Eastern Desert of
Egypt were subjected to petrographic and mineralogical investigations. They are composed of varying proportions of
quartz + plagioclase + potash feldspar + biotite + hornblende ± epidote ± calcite + titanite + magnetite + apatite and zircon.
Electron microprobe analyses of coexisting hornblende and plagioclase (hornblende-plagioclase thermometry), Al content in
hornblende (aluminum-in-hornblende barometry) and the assemblage titanite–magnetite–quartz were used to constrain the P , T and
f O2 during the crystallization of the parent magmas in the different plutons. The plutons crystallized under varying pressures (5.4–
2.1 kbar) and wide range of temperature (785–588 °C) from highly oxidized magmas (log f O2 )21 to )13).
The pressure data discriminate three categories of granitoid emplaced at different crustal levels: (a) upper crust granitoids (e.g., El
Ghuzah, and Abu Zawil) emplaced at depths <9 km; (b) intermediate crust granitoids (e.g., Um Gidri and Um Anab) emplaced at
depths <13 km; and (c) lower crust granitoids (e.g., Um Tagher) emplaced at depths <21 km. The depths of emplacement seem to
increase from northwest to southeast.
It is likely that the magmas forming these plutons were generated at different depths; they were similar in composition but varied
substantially in their water and volatile contents. High water and volatile contents allowed the magma of some plutons to reach
shallower crustal levels without complete solidification. Although these complexes were crystallized at different depths, they were
later uplifted to the same level by upward faulting.
Ó 2004 Elsevier Ltd. All rights reserved.
Keywords: Mineral chemistry; Physico-chemical parameters; Calc-alkaline granites; Eastern Desert; Egypt
1. Introduction
The mineral assemblages and compositions in igneous rocks are closely related to the compositions and
evolving conditions of the melt during crystallization
(Abbott and Clarke, 1979; Abbott, 1985). This fact was
the basis for the use of the composition of certain
minerals and coexisting mineral phases in deciphering
the physicochemical parameters (pressure and temperature of crystallization, and oxygen fugacity), which
prevailed during crystallization of the magma. Several
thermodynamic equations are employed for the computation of pressure, temperature and oxygen fugacity
from the compositions of minerals, especially horn-
*
Corresponding author. Fax: +2-086-342601.
E-mail addresses: [email protected], hassan64_1999@yahoo.
com (H.M. Helmy).
0899-5362/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jafrearsci.2004.01.002
blende, feldspars, micas, titanite and Fe–Ti oxides. The
compositions of hornblende, feldspars and micas are
pressure and temperature indicators. For example, total
Al in hornblende is considered as a geobarometer
(Hammarstrom and Zen, 1986), Ti in hornblende
(Raase, 1974), and the mineral pair hornblende-plagioclase (Holland and Blundy, 1994) as geothermometers,
while titanite and Fe–Ti oxides are important for oxygen
fugacity calculations (Wones, 1989) although the composition of titanite is also P –T dependent (Enami et al.,
1993).
Calc-alkaline granitoids commonly contain the magmatic assemblage quartz + plagioclase + alkali feldspar + biotite + hornblende + titanite + magnetite (and/
or ilmenite), i.e., all mineral phases of petrologic significance. These granitoids were worldwide the objects
of petrologic studies using the compositions of the rockforming minerals (e.g., Ague, 1997; Speer, 1987; Vyhnal
et al., 1991).
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H.M. Helmy et al. / Journal of African Earth Sciences 38 (2004) 255–268
In Egypt, granitoids cover 40,000 km2 of the area
covered by basement rocks, among which calc-alkaline
granitoids represent 60%. The wide distribution of
calc-alkaline granitoids attracted their study. Nevertheless, only few papers dealt with the mineralogical aspects
of these granitoids.
This contribution focuses on five granitoid plutons
(Um Tagher, Abu Zawil, Um Gidri, Um Anab and El
Ghuzah) exposed in the northern Eastern Desert, between latitudes 26° 000 and 27° 000 N and longitudes 33°
300 and 34° 150 E. These plutons are mainly calc-alkaline, one of them (El Ghuzah) being highly fractionated
(Ahmed et al., in press). The aims of this paper are: (1)
to describe the mineral compositions, (2) to decipher the
physico-chemical conditions (pressure and temperature
of crystallization and oxygen fugacity) prevailing during
crystallization of magmas, and (3) discuss the implications of the data on the geotectonic interpretation of the
evolution of the northern Eastern Desert.
2. Geologic setting
Granitoids constitute >40% of the basement complex
of the Eastern Desert (ED) and Sinai. Various classifications of these granitoids were used by many authors
(El Ramly and Akaad, 1960; El Shazly, 1964; El Sukkary, 1970; El Gaby, 1975; Akaad et al., 1979; El Shazly
et al., 1982; Hussein et al., 1982, Noweir et al., 1990).
The granitoids were intruded in a variety of geologic
settings and into different rock units. In the South
Eastern Desert (SED), granitoids intrude high-grade
metasedimentary rocks, ophiolitic melange and metavolcanic rocks. In the Central Eastern Desert (CED),
they intrude gneisses, island arc metavolcanic rocks, and
ophiolitic melange. In the North Eastern Desert (NED),
granitoids are associated with Dokhan volcanic rocks
and molasse-type sedimentary rocks. The scarcity of
supracrustal cover in the NED led El Gaby (1983) to
suggest that granitoids were intruded along a prominent
shear zone (50 km-wide) forming the Qena-Safaga line
at the boundary between NED and CED. The contact
between the CED and SED is a major shear zone
marked by the Aswan-Ras Banas line. Stern and Hedge
(1985) suggested that NED and CED are two distinct
terranes.
The calc-alkaline granitoids from Egypt are classified
into older gray granites and phase-I younger granites.
Both are widely accepted to be the result of subduction
at active-continental margin (El Mahallawi, 1989; El
Mahallawi et al., 1996).
Granitic rocks from the NED were the subject of
extensive geological and geochemical studies (e.g., Sabet
et al., 1972; El Gaby, 1975; El Gaby et al., 1990). These
rocks were classified according to their presumed relation to orogeny into, synorogenic older granitoids and
late to post-orogenic younger granites. The former
group comprises intrusive bodies of tonalites and granodiorites. The late-to post-orogenic younger granitoids
are porphyritic pink and red granites. They range from
calc-alkaline granitoids to alkali-granite and the last
phases may be peralkaline. Sabet et al. (1972) considered
the studied five plutons as quartz diorite (Um Tagher),
tonalite–granodiorite (El Ghuzah and Abu Zawil) and
late-orogenic calc-alkaline granites (Um Anab and Um
Gidri).
The studied calc-alkaline bodies are located along, or
to the north, of the Qena-Safaga line and intrude
metavolcanic rocks, molasse-type sedimentary rocks,
metapyroclastic and small metagabbro-diorite complexes and were in turn intruded by alkali granite bodies
(Fig. 1).
(1) The Um Tagher granitoid pluton is an elongate
body divided by the Qena-Safaga road. It intrudes
the metavolcanic rocks and metagabbros and was
intruded with sharp contacts by alkali granite. Within the granitoid body, small schlieren of mafic minerals and lenses of pegmatite are common.
(2) The Abu Zawil pluton is a circular body, 7 km in
diameter. It is divided into two masses by a NE-SWtrending fault. This pluton intruded metavolcanic
rocks and is dissected by dyke swarms, including felsic, intermediate and mafic varieties (Sabet et al.,
1972).
(3) The Um Gidri and
(4) Um Anab plutons are part of one big circular granitic body, 10 km in diameter. The latter intrudes
metavolcanic rocks, and was later cut by alkali granites. The two plutons are separated by a major NW
fault. Sharp intrusive contacts occur between the
Um Gidri and Um Anab plutons and the alkali
granites. Mafic xenoliths of different sizes are common in these granites, with sharp contacts against
the host rocks.
(5) The El-Ghuzah pluton forms an elongate body, 14
km-long stretched in a northwesterly direction. It
is affected by faults and intrudes molasse-type sedimentary rocks, metavolcanic rocks and alkali granite. Almost vertical, mafic dykes dissect the El
Ghuzah granite with sharp contacts.
3. Analytical methods
Mineral analyses were performed at the Institute of
Mineralogy and Petrology, Karl-Franzens University,
Graz, Austria using a Jeol JSM-6310 Scanning Electron
microscope with attached energy dispersive system
(EDX) and microspec wavelength dispersive system
(WDX). The accelerating voltage was 15 keV and a
probe current of 5 nA. Silicate standards were adularia
H.M. Helmy et al. / Journal of African Earth Sciences 38 (2004) 255–268
257
Fig. 1. Geological map of the area north of Qena-Safaga road (after Sabet et al., 1972). Legend: 1. Wadi deposits; 2. Granites; 3. Volcanics;
4. Adamellites; 5. Tonalite–granodiorite; 6. Metavolcanic rocks; 7. Molasse sediments; 8. Metagabbro-diorite; 9. Metasedimentary rocks including
migmatites; 10. Faults, 11. Boundary between NED and CED.
for Si, K and Al, garnet for Fe, Mg and Mn, titanite for
Ca and Ti, chromite for Cr and jadeite for Na.
Chemical compositions and structural formulae of
feldspars, biotite, hornblende, and other accessory
minerals are listed in Tables 1–4. The structural formula
of amphiboles is calculated by the computer program
AMPH-IMA97 (Mogessie et al., 2001).
Q
El Ghuzah
Abu Zawil
Um Tagher
Um Anab
Um G idri
(1) The Um Tagher pluton is a medium to coarsegrained granodiorite, with hypidiomorphic equigranular texture. It is composed of quartz, plagioclase,
alkali feldspar, biotite and hornblende together with
apatite, titanite, zircon and epidote as accessories.
Epidote crystallized after hornblende and shows a
reactive relation to it suggesting a magmatic origin
Qz-monzonite
A
s
alite
Ton
Although the five granitoid plutons are distributed
over a large area, they show very similar mineralogic
and textural features. The color indices are usually in
the range 5–15. Hornblende and biotite tend to occur
as interstitial clots between quartz and feldspars.
The assemblage quartz + plagioclase + alkali feldspar +
biotite + hornblende + titanite + magnetite (or ilmenite)
is common, but the minerals (alkali feldspars, plagioclase and quartz) vary in proportion considerably
(Fig. 2).
Granites
tes
iori
nod
Gra
4. Petrography
Monzodiorite
P
Fig. 2. QAP modal composition of the studied plutons, fields from
Streckeisen (1976).
(Zen and Hammarstrom, 1984). Iron oxides are represented by magnetite, which may contain ilmenite
exsolutions.
(2) The Abu Zawil granitoids are mainly granodiorites
with minor granites. The rocks are medium to
coarse-grained equigranular and are composed of
plagioclase, quartz, alkali feldspar, biotite and hornblende.
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H.M. Helmy et al. / Journal of African Earth Sciences 38 (2004) 255–268
(3) The Um Gidri granitoids are not uniform; the
margins of the pluton are dominated by granodiorite–monzonite, which grade inward to granite. Granodiorites and monzonites are coarse-grained and
grayish white to pinkish gray, while the granite is
medium-grained and pale pink.
(4) The Umm Anab pluton comprises three petrographic varieties; namely granodiorite, quartz-monzodiorite, and monzogranite. The former two
varieties are leucocratic, porphyritic to equigranular
with hypidiomorphic to allotriomorphic granular
texture and contain amphibolitic xenoliths. They
are foliated and display cataclastic features. The
rocks are mainly composed of albite grading to
perthitic microcline, oligoclase and quartz. Dark
minerals are biotite and green hornblende, whereas
titanite, magnetite and epidote are the common
accessory constituents.
(5) The El Ghuzah granitoids are medium to coarsegrained and grayish white to pinkish white.
They are classified on the QAP modal diagram
(Fig. 2), as monzogranite which rarely grades to
granodiorite. Plagioclase, quartz, alkali feldspar,
biotite and hornblende are the essential minerals
while the accessories are the same as in Um Tagher
pluton.
5. Textural equilibrium of the critical mineral assemblage
As stated above, the assemblage quartz + plagioclase +
hornblende + biotite + titanite + magnetite (±ilmenite)
was observed in all the plutons (Figs. 3 and 4).
Quartz varies considerably in size (3–7 mm across),
and mostly exhibits polygonal outlines. It occurs in
association with feldspars and as inclusions in the mafic
minerals. Inclusions of hornblende and biotite are
common in quartz.
Plagioclase forms anhedral to subhedral twinned
grains. The larger plagioclase grains commonly contain
small inclusions of hornblende and biotite. Plagioclase
exhibits weak oscillatory zoning where the rim compositions are slightly more sodic.
Alkali feldspars are microcline or microcline microperthite, sometimes with cloudy appearance due to
alteration. Inclusions of biotite and quartz are common
in alkali feldspars.
Mafic minerals tend to form clots between felsic
minerals (Fig. 3a). In all granite intrusions, biotite (up to
8 vol%) is the most abundant mafic mineral. It forms
large flakes (Fig. 3a and b), several millimeters in length.
It occurs also as inclusions in the feldspars, in the
interstices between feldspars and quartz and is intergrown with hornblende (Fig. 3a). Apatite, hornblende,
Fig. 3. Back scattered electron images showing the mineral assemblage of the studied plutons. (a) A clot of mafic minerals; biotite (Bt), hornblende
(Hbl) together with titanite (Ttn) in quartz (Qtz) and feldspar (Pl) matrix, Um Tagher sample 303. (b) Large biotite (Bt) and hornblende (Hbl)
crystals, biotite contains inclusions of hydrogarnet (Grt), titanite, and apatite (Apt) and altered to chlorite (Chl), Um Gidri, sample 451. (c)
Hornblende inclusions (Hbl) in quartz (Qtz) which is intergrown with biotite (Bt) and plagioclase (Pl), Um Anab, sample 136. (d) Euhedral titanite
crystals (Ttn) developed between large hornblende crystal (Hbl), biotite (Bt) and quartz (Qtz), Um Tagher, sample 305. Mineral abbreviations
according to Kretz (1983).
H.M. Helmy et al. / Journal of African Earth Sciences 38 (2004) 255–268
259
Fig. 4. Back scattered electron images showing textures of accessory minerals. (a) Titanite inclusions (Ttn) in biotite (Bt) which is partially altered to
chlorite (Chl) and intergrown with plagioclase (Pl) and magnetite (Mgt), Um Tagher, sample 305. (b) Secondary titanite and chlorite after biotite,
Um Anab, sample 139. (c) Inclusion of ilmenite (Ilm) with magnetite (white) lamellae in titanite (Ttn), El Ghuzah, sample 257. (d) Subhedral
magnetite (Mgt) containing lamellae of ilmenite (dark gray) in quartz matrix (Qtz), Um Tagher, sample 312.
quartz, feldspars, titanite, zircon and opaque minerals,
and rarely hydrogarnet (as in Um Gidri) are common
inclusions in biotite (Fig. 3b). Biotite is frequently altered to chlorite (Fig. 4a and b) and secondary titanite as
a by-product.
Hornblende occurs as euhedral to subhedral prismatic crystals, up to 0.5 cm long. It develops in equilibrium with other minerals (Fig. 3a–d). Reaction rims
of epidote are sometimes observed between hornblende
and titanite. Twinned hornblende crystals are recorded
but uncommon. Hornblende usually encloses biotite and
also the minerals present as inclusions in biotite. Intergrowth of hornblende and biotite is also common (Fig.
3a). Hornblende is locally transformed into actinolite
along its margins in the Um Gidri granitoid.
Epidote has been observed in Um Tagher pluton as
small euhedral inclusions in plagioclase. It forms also
thin reaction rims around hornblende along the contacts
with titanite. These textural positions of epidote in Um
Tagher suggest magmatic origin (Zen and Hammarstrom, 1984).
Titanite occurs as euhedral to subhedral grains, up to
a few millimeters long included in plagioclase, along
the margins of large hornblende crystals (Fig. 3d) or
as discrete grains in the matrix. It forms also thin
rims around magnetite and biotite. This mineral is
commonly zoned and contains inclusions of magnetite.
This variety is of magmatic origin. Tiny inclusions of
titanite were observed in biotite altered to chlorite
(Fig. 4b) which is considered of secondary origin
resulting from the decomposition of biotite (TiO2 ¼ 0.7–
4.5 wt.%).
The oxide mineral assemblage comprises magnetite,
titanomagnetite and ilmenite. Magnetite forms small
(<1 mm) euhedral grains in the matrix and as fine
lamellae in ilmenite (Fig. 4c). It commonly contains, in
Um Tagher granite, fine ilmenite lamellae (Fig. 4d) as
well as inclusions of ilmenite grains. The magnetite is
sometimes surrounded by wide ilmenite rims. These
ilmenite grains are thought to represent ‘‘oxyexsolutions’’ of primary titanomagnetite. Magnetites from El
Ghuzah and Um Anab do not contain exsolved phases.
Regarding the paragenetic sequence, the textural
relations suggest that magnetite, plagioclase, and alkali
feldspars crystallized early and were followed shortly by
biotite, hornblende, quartz and titanite. This sequence is
common in coarse-grained granitoids (Speer et al.,
1986). The paragenetic sequence is important from the
petrologic point of view, as it ensures that hornblende
crystallized while all mineral phases of the assemblage,
were present in association with pore magma.
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H.M. Helmy et al. / Journal of African Earth Sciences 38 (2004) 255–268
biotite (AlT /Ca + Na + K, ASI) is significantly low (1.1–
1.5) and reflects decreased alumina activity in the crystallizing magma (Zen, 1988) of each area.
6. Mineral chemistry
6.1. Feldspars
Representative analyses of plagioclase and their calculated formulae are given in Table 1. The plagioclase
composition ranges from Ab54 An43 Or3 to Ab88 An11 Or1 .
Plagioclases from Um Tagher granite have the highest
anorthite content (An2831 ). At Um Gidri, plagioclase is
oscillatory zoned with cores richer in anorthite relative
to rims (An43:9 and An10:9 , respectively). A narrow range
of plagioclase composition is noted in each area, which
is necessary for the Al content in hornblende to be solely
a function of pressure (Hollister et al., 1987).
6.2. Biotite
Microprobe analyses of biotite from the different plutons (Table 2) indicate compositions lying approximately
midway between phlogopite and annite (Fig. 5) with an
intermediate Al content. The most important feature of
biotites is that they are Mg-rich. The range of molar Fe/
(Fe + Mg) is high (32–50) and reaches its maximum value
in Um Tagher samples. The alumina saturation index of
1.0
6.3. Amphiboles
Amphibole compositions of each pluton were determined and representative analyses together with their
chemical formulae are given in Table 3. The amphiboles
show some variation both within single and between
different grains. In the IMA-approved nomenclature
(Leake, 1997; Mogessie et al., 2001), these amphiboles
are magnesio-hornblendes. Amphiboles from the different intrusions have a wide compositional range with a
Mg/(Mg + Fe) ranging from 0.90 to 0.54 and a Si content of 6.55 to 7.59 atom per formula unit (afu).
Fig. 6 illustrates the relationship between AlIV and
T
Al in the amphiboles from the studied granitoids,
where an excellent positive correlation is observed as
also reported by Hammarstrom and Zen (1986). This
strong positive correlation shows the systematic difference between the Al contents of the amphiboles in different areas despite the variations within each area.
Two types of hornblende zoning are observed, viz.,
weak zoning with low Al content in the rim as in El
Ghuzah and Um Tagher samples (Fig. 7a) and strong
zoning (Fig. 7b) with actinolitic hornblende rims as in
El Ghuzah
Abu Zawil
Um Tagher
Biotite
1.6
Um Anab
0.6
1.2
Um Gidri
AlIV
Fe/Fe+Mg (a.f.u.)
0.8
0.4
0.8
El Ghuzah
Abu Zawil
Um Tagher
Um Anab
Um Gidri
0.4
0.2
Phlogopite
0.4
2.0
2.5
3.0
3.5
0.8
4.0
1.2
1.6
2.0
AlT (a.f.u)
Al (a.f.u.)
Fig. 6. Relationship between AlIV and AlT in the amphiboles of the
studied granitoids.
Fig. 5. Nomenclature of biotites of the studied plutons.
2.0
(a)
AlT
1.6
(b)
1.2
Xmg
Xmg
0.8
AlT
0.4
0.0
50
100
150
Distance in microns
200
50
100
150
Distance in microns
Fig. 7. Types of hornblende zoning in the studied plutons.
200
H.M. Helmy et al. / Journal of African Earth Sciences 38 (2004) 255–268
261
Table 1
Representative electron microprobe analyses of plagioclase from the studied calc-alkaline granitoids, Egypt
Area
Um Tagher
Sample no.
Analysis
description
312
Rim
SiO2
Al2 O3
FeO
CaO
Na2 O
K2 O
Total
62.60 62.55 63.01 63.61 58.37 65.76 65.19
23.08 23.54 23.53 22.99 26.45 21.71 21.90
0.25
0.25
6.88
5.92
6.25
4.81
8.96
2.31 2.19
7.25
8.21
7.79
8.76
6.16 10.19 9.83
0.21
0.15
0.12
0.29
0.27
0.16 0.13
100.27 100.37 100.70 100.46 100.21 100.38 99.24
O
Si
Al
Fe2þ
Ca
Na
K
Cation sum
Xan
Xab
Xor
8.000
2.771
1.204
0.009
0.326
0.622
0.012
4.944
33.96
64.79
1.25
307
Rim
Um Gidri
305
Core
451-3
Rim
Um Anab
451-2
Core
8.000
2.762
1.225
8.000
2.771
1.219
8.000
2.801
1.193
8.000
2.605
1.391
0.280
0.707
0.008
4.982
0.294
0.664
0.007
4.955
0.227
0.748
0.016
4.985
0.428
0.533
0.015
4.972
28.14
71.06
0.80
30.47
68.81
0.73
22.91
75.48
1.61
43.85
54.61
1.54
453
Rim
8.000
2.882
1.121
0.009
0.108
0.866
0.009
4.995
G136-2
Core
8.000
2.881
1.140
0.104
0.842
0.007
4.974
10.99 10.91
88.10 88.35
0.92 0.73
Abu Zawil
G136–3
Rim
515
Core
64.34
22.67
0.22
4.71
8.54
0.45
100.93
64.34 64.91 64.88
22.14 22.35 22.12
0.23
0.33
3.88
3.49
2.86
9.28
9.48 10.11
0.16
0.23
0.22
100.03 100.79 100.19
8.000
2.820
1.171
0.008
0.221
0.726
0.025
4.971
22.74
74.69
2.57
8.000
2.840
1.152
0.008
0.183
0.794
0.009
4.986
18.56
80.53
0.91
251-5
Core
8.000
2.843
1.153
0.012
0.164
0.805
0.013
4.990
16.70
81.98
1.32
251-11
Rim
El Ghuzah
251-6
Rim
65.13
23.05
257-3
Rim
64.98
22.83
257-8
Rim
64.93
22.53
257-11
Rim
65.43
22.85
3.17
3.44
3.21
2.99
9.39
9.48
9.44
9.13
0.19
0.23
0.39
0.36
100.93 100.96 100.50 100.76
8.000
2.854
1.147
8.000
2.837
1.183
8.000
2.835
1.174
8.000
2.846
1.164
8.000
2.849
1.176
0.135
0.862
0.012
5.010
0.148
0.793
0.013
4.974
0.161
0.802
0.013
4.985
0.151
0.802
0.022
4.985
0.140
0.773
0.020
4.958
13.38
85.43
1.19
15.51
83.12
1.36
16.50
82.17
1.33
15.49
82.26
2.26
15.01
82.85
2.14
Table 2
Representative electron microprobe analyses of biotite from the studied calc-alkaline granitoids
Area
Um Tagher
Um Gidri
Um Anab
Sample no.
301
302
314-1
314-2
511
512
513
514
SiO2
TiO2
Al2 O3
FeO
MnO
MgO
CaO
Na2 O
K2 O
F
Total
37.92
2.43
15.74
19.99
0.25
11.32
0.00
0.00
9.41
0.26
97.32
38.30
2.52
15.92
20.46
0.23
11.41
0.00
0.00
9.58
0.15
98.57
37.28
4.42
13.21
17.87
0.34
12.53
0.14
0.11
8.65
na
94.55
37.55
3.98
13.44
18.07
0.28
12.44
0.14
0.08
9.09
na
95.07
37.04
4.50
13.40
15.96
0.50
14.51
2.10
0.07
9.50
1.13
98.64
37.60
2.20
13.90
16.40
0.56
14.40
2.11
0.00
8.90
0.00
96.07
37.40
3.40
14.50
17.40
0.70
13.50
0.03
0.00
10.30
0.00
97.23
O
Si
Al
Al
Ti
Fe
Mn
Mg
Ca
Na
K
F
Total
22.00
6.19
1.81
1.22
0.30
2.73
0.03
2.75
0.00
0.00
1.96
0.00
16.98
22.00
6.17
1.83
1.19
0.31
2.76
0.03
2.74
0.00
0.00
1.97
0.00
17
22.00
6.20
1.80
0.79
0.55
2.49
0.05
3.11
0.02
0.04
1.84
0.00
16.88
22.00
6.23
1.77
0.86
0.50
2.51
0.04
3.07
0.02
0.03
1.92
0.00
16.94
22.00
6.23
1.77
0.69
0.53
2.08
0.07
3.36
0.35
0.02
1.88
0.56
17.53
22.00
5.99
2.01
0.60
0.26
2.18
0.08
3.42
0.36
0.00
1.81
0.00
16.96
0.5
1.55
0.5
1.53
0.44
1.36
0.45
1.34
0.38
1.09
0.39
1.20
Fe/Fe + Mg
ASI
Abu Zawil
El Ghuzah
515
251-1
251-2
257-1
257-2
257-3
257-7
37.97
1.60
14.70
16.50
0.60
16.20
0.11
0.09
8.90
0.00
96.58
37.50
2.13
14.30
16.30
0.64
15.96
0.04
0.13
9.80
0.00
96.67
38.71
2.33
14.71
17.45
0.33
13.52
0.10
0.03
9.03
0.00
96.18
39.60
2.23
14.66
17.30
0.28
13.55
0.08
0.02
10.01
0.00
97.71
36.30
0.70
11.70
20.50
0.10
15.70
0.11
0.00
8.50
0.33
93.94
37.20
2.70
13.70
14.90
0.70
17.60
0.10
0.10
8.30
2.29
97.49
37.60
2.70
13.60
14.20
0.60
16.20
0.10
0.00
9.02
1.52
95.54
40.60
3.20
13.10
14.80
0.10
16.30
0.10
0.10
8.86
1.84
98.90
22.00
6.24
1.76
0.95
0.41
2.31
0.09
3.19
0.01
0.00
2.08
0.00
17.04
22.00
6.28
1.72
1.00
0.19
2.17
0.08
3.80
0.02
0.03
1.78
0.00
17.24
22.00
6.32
1.68
0.95
0.25
2.13
0.08
3.71
0.01
0.04
1.95
0.00
17.28
22.00
6.36
1.64
1.14
0.28
2.34
0.04
3.23
0.02
0.01
1.84
0.00
16.9
22.00
6.34
1.66
1.11
0.27
2.32
0.04
3.23
0.01
0.01
2.05
0.00
17.03
22.00
6.20
1.80
0.56
0.09
2.93
0.01
4.00
0.02
0.00
1.85
0.18
17.64
22.00
6.35
1.65
0.84
0.31
1.92
0.09
4.04
0.02
0.03
1.63
1.12
18.02
22.00
6.69
1.31
1.10
0.30
1.78
0.08
3.62
0.02
0.00
1.73
0.72
17.36
22.00
6.41
1.59
0.85
0.38
1.95
0.01
3.84
0.02
0.03
1.78
0.92
17.8
0.42
1.30
0.36
1.49
0.36
1.32
0.42
1.49
0.42
1.34
0.42
1.26
0.32
1.48
0.33
1.38
0.34
1.33
262
H.M. Helmy et al. / Journal of African Earth Sciences 38 (2004) 255–268
Table 3
Representative electron microprobe analyses of amphiboles from the studied calc-alkaline granitoids
Area
Um Tagher
Sample no.
Analysis
description
300
Rim
306
Core
306
Rim
454
Rim
459
Rim
SiO2
TiO2
Al2 O3
FeO
MnO
MgO
CaO
Na2 O
K2 O
Total
44.87
0.93
10.02
18.97
0.35
10.24
11.43
0.96
0.94
98.78
44.77
1.01
9.97
19.29
0.45
10.07
11.29
1.14
0.98
99.11
44.95
1.08
10.28
18.35
0.47
10.28
11.52
1.27
0.88
99.01
46.53
1.9
7.55
16.97
0.21
12.32
11.05
1.36
1.2
98.92
O
Si
TiVI
AlIV
AlVI
Fe3þ
Fe2þ
Mg
Mn
Ca
Na
K
Cation sum
23.000
6.539
0.104
1.461
0.271
0.926
1.401
2.24
0.043
1.797
0.273
0.176
15.231
23.000
6.554
0.111
1.446
0.275
0.896
1.466
2.196
0.056
1.77
0.325
0.184
15.279
23.000
6.565
0.119
1.435
0.334
0.734
1.507
2.237
0.058
1.802
0.361
0.163
15.315
0.62
0.60
1.732
1.721
Xmg (Mg/
Mg + Fe2þ )
Al total
Um Gidri
Um Anab
Abu Zawil
El Ghuzah
314-1
Core
136-1
Rim
136-2
Rim
251-1
Rim
251-2
Core
251-3
Rim
257-1
Rim
257-2
Rim
257-3
Rim
44.38
1.05
8.99
21.2
0.33
9.09
11.48
1.19
1.07
98.64
46.49
0.47
7.75
20.29
0.22
9.86
11.56
1.08
0.68
98.42
45.98
1.1
6.38
12.99
0.55
13.08
10.92
1.1
0.55
92.65
49.9
0.83
6.25
12.98
0.33
12.67
11.36
0.83
0.47
95.62
48.95
1.3
6.5
14.4
0.55
14.07
11.7
1.09
0.56
99.12
48.9
1.19
6.49
14.2
0.67
13.97
11.69
0.88
0.54
98.53
49. 16
1.24
6.43
13.49
0.51
14.58
11.23
1.52
0.53
98.69
49.09
0.87
6.09
13
1.2
15.25
11.01
1.15
0.62
98.28
47.88
0.77
5.79
13.31
1.24
15.56
10.97
1.49
0.55
97.56
47.05
0.97
6.69
14.37
1.12
14.81
10.53
1.66
0.69
97.89
23.000
6.721
0.209
1.279
0.046
0.688
1.534
2.497
0.026
1.788
0.35
0.204
15.342
23.000
6.881
0.052
1.119
0.233
0.674
1.838
2.175
0.028
1.833
0.31
0.128
15.271
23.000
6.751
0.208
1.249
0.042
0.752
1.307
2.665
0.026
1.719
0.302
0.221
15.323
23.000
7.006
0.125
0.869
0.277
0.639
1.108
2.97
0.071
1.783
0.325
0.107
15.28
23.000
7.332
0.092
0.576
0.507
0.329
1.269
2.774
0.041
1.788
0.236
0.088
15.032
23.000
6.986
0.14
0.874
0.27
0.625
1.122
2.992
0.066
1.789
0.302
0.102
15.218
23.000
6.992
0.128
0.88
0.209
0.689
1.064
2.977
0.081
1.791
0.244
0.049
15.104
23.000
6.934
0.132
0.934
0.135
1.22
0.357
3.065
0.061
1.697
0.416
0.093
15.044
23.000
6.98
0.093
0.927
0.094
1.041
0.491
3.232
0.145
1.672
0.317
0.112
15.104
23.000
6.849
0.083
0.976
0
1.23
0.413
3.137
0.15
1.682
0.413
0.1
15.033
23.000
6.709
0.085
1.125
0
1.435
0.389
3.147
0.135
1.609
0.459
0.126
15.219
0.60
0.62
0.54
0.67
0.73
0.69
0.73
0.74
0.90
0.87
0.88
0.89
1.769
1.325
1.352
1.291
1.146
1.083
1.146
1.044
1.069
1.021
0.976
1.125
Table 4
Representative electron microprobe analyses of titanite from the studied calc-alkaline granitoids
Area
Um Tagher
Sample no.
300-2
300-3
300-4
SiO2
TiO2
Al2 O3
FeO
MnO
MgO
CaO
Na2 O
F
Total
29.55
36.80
1.41
0.96
0.00
0.00
26.21
0.00
0.27
95.20
29.77
37.69
1.06
1.18
0.00
0.00
26.29
0.00
0.29
96.28
O
Si
Al
Ti
Fe
Mn
Mg
Ca
Na
F
Cation sum
20.00
4.01
0.21
3.82
0.11
0.00
0.00
3.80
0.00
0.11
12.06
20.00
4.02
0.17
3.83
0.13
0.00
0.00
3.80
0.00
0.12
12.07
Um Gidri
Um Anab
Abu Zawil
El Ghuzah
300-4
51-1
51-2
45-2
251-1
251-2
251-3
251-12
257-1
257-3
257-6
30.79
38.75
1.32
1.04
0.14
0.00
27.69
0.00
0.59
100.32
30.04
37.92
1.14
1.17
0.00
0.00
26.23
0.00
0.49
96.99
31.39
36.89
1.55
2.42
0.14
0.00
27.56
0.00
0.45
100.40
31.16
36.83
1.83
1.91
0.12
0.02
27.64
0.15
0.52
100.18
31.05
35.00
2.35
1.31
0.03
0.24
28.89
0.04
1.04
99.95
31.28
36.58
1.42
1.61
0.00
0.00
27.28
0.00
0.00
98.17
31.04
37.92
1.01
1.17
0.00
0.00
27.12
0.00
0.00
98.26
31.28
38.00
1.14
1.48
0.00
0.00
27.54
0.00
0.00
99.44
30.58
37.36
1.18
1.66
0.00
0.00
27.03
0.00
0.00
97.81
30.97
35.93
1.50
2.42
0.19
0.01
26.74
0.06
1.23
99.05
30.46
36.75
1.06
1.89
0.00
0.00
25.92
0.00
0.77
96.85
30.11
35.91
1.30
1.96
0.00
0.00
25.23
0.00
0.53
95.04
20.00
4.02
0.2
3.8
0.11
0.00
0.00
3.87
0.00
0.39
12.39
20.00
4.03
0.18
3.81
0.13
0.00
0.00
3.80
0.00
0.22
12.17
20.00
4.04
0.26
3.6
0.25
0.00
0.00
3.85
0.00
0.20
12.20
20.00
4.05
0.28
3.6
0.21
0.00
0.00
3.85
0.01
0.21
12.21
20.00
4.09
0.37
3.48
0.14
0.00
0.01
4.09
0.00
0.43
12.61
20.00
4.09
0.21
3.38
0.17
0.00
0.00
3.59
0.00
0.00
11.44
20.00
4.09
0.16
3.78
0.13
0.00
0.00
3.85
0.00
0.00
12.01
20.00
4.1
0.18
3.75
0.16
0.00
0.00
3.87
0.00
0.00
12.06
20.00
4.08
0.19
3.75
0.19
0.00
0.00
3.87
0.00
0.00
12.08
20.00
4.03
0.23
3.52
0.26
0.00
0.00
3.73
0.00
0.50
12.27
20.00
4.05
0.17
3.67
0.00
0.00
0.00
3.69
0.00
0.33
11.91
20.00
4.09
0.21
3.66
0.22
0.00
0.00
3.67
0.00
0.23
12.08
H.M. Helmy et al. / Journal of African Earth Sciences 38 (2004) 255–268
Um Gidri samples. Actinolitic hornblende probably
crystallized as a subsolidus phase since it falls outside
the limit of igneous hornblende crystallization (Leake,
1971).
263
(up to 2.1 wt.%). Ilmenite is commonly rich in MnO (up
to 13.3 wt.%).
7. Discussion
6.4. Accessory minerals
7.1. Pressure estimates
Titanites from the different plutons are Al-poor
(<0.28 afu, Table 4), typical of magmatic titanite in
granitoids (Enami et al., 1993). Variable F contents
(0.23–1.23 wt.%) are recorded in titanite from the different plutons. The highest F contents (average 0.9
wt.%) are recorded in El Ghuzah samples while the
lowest contents are from Um Tagher granitoids (average
0.4 wt.%).
A single electron microprobe analysis of epidote from
Um Tagher granite (Table 5) shows Ps (Al/Al + Fe3þ )
value of 0.25 which confirms its magmatic origin (Zen
and Hammarstrom, 1984) and suggests high pressure of
crystallization (>5 kbar).
Apatite contains relatively high contents of F (2.6–5
wt.%, Table 5). The highest values are recorded in El
Ghuzah samples.
Microprobe analyses of magnetites from Um Anab
and El Ghuzah samples show them to be nearly endmember compositions. High contents of TiO2 (up to
11.9 wt.%) in magnetite have been documented in Um
Tagher, Abu Zawil and Um Gidri granitoids. The TiO2
contents in magnetites correlate positively with MnO
Understanding the evolution of a granitoid pluton
requires knowledge of the depth at which the various
minerals crystallized and the amount of post-crystallization upward movement. The pressure of emplacement
of a granite pluton can be constrained by geologic and
petrologic criteria.
Several studies (e.g., Hammarstrom and Zen, 1986;
Hollister et al., 1987) revealed that Al content of hornblende in calc-alkaline granitoids varies linearly with
pressure of crystallization, thereby providing means of
determining the depth of pluton emplacement. Estimation of the pressure of solidification of a calc-alkaline
granitoid body from the Al content of hornblende assumes that: (a) the equilibrium pressure of the hornblende barometer and the adjoining country rocks are
the same and; (b) the equilibrium pressure of the hornblende is the same as the pressure of emplacement of the
pluton. These assumptions were questioned by many
authors (e.g., Hollister et al., 1987; Ghent et al., 1991).
The computed pressure may be affected by ion substitutions in hornblende, oxygen fugacity, volatiles and
Table 5
Representative electron microprobe analyses of oxides, apatite and epidote from the studied calc-alkaline granitoids
Area
Um Tagher
Mineral
Sample no.
Mgt
300-5
Ilm
300-7
Ap
300-1
Ep
300-6
Mgt
45-3
Ilm
45-1
Ap
51-2
Mgt
251-10
Ilm
251-11
Mgt
257-3
Ilm
257-5
Ap
257-1
SiO2
TiO2
Al2 O3
FeO
MnO
MgO
CaO
P2 O5
F
Total
0.52
0.31
0.36
93.5
nd
nd
nd
nd
nd
94.69
0.26
50.44
0.23
43.92
7.12
nd
0.31
nd
nd
102.3
0.42
nd
nd
nd
nd
nd
54.95
42.68
nd
98.05
38.59
nd
22.65
13.05
nd
nd
22.7
nd
nd
96.99
0.51
0.88
0.23
87.99
nd
0.17
0.17
nd
nd
89.96
0.28
50.44
0.14
47.63
2.31
0.03
0.05
0.04
nd
100.92
0.23
0.02
nd
0.17
nd
0.06
55.6
43.5
2.58
102.10
0.32
6.44
0.59
82.4
0.33
0.42
0.21
nd
nd
90.71
0.28
52.11
nd
35.79
13.25
nd
0.32
nd
nd
101.75
0.34
0.63
0.2
91.37
0.13
0.08
0.25
nd
nd
93.05
0.29
47.47
0.15
36.36
13.78
0.23
0.48
nd
nd
98.80
0.57
nd
0.13
0.3
nd
0.13
53.32
43.2
4.93
102.62
8.00
0.05
0.04
0.02
7.79
0.00
0.00
0.00
0.00
0.00
7.90
8.00
0.02
0.02
2.53
2.45
0.40
0.00
0.02
0.00
0.00
5.45
26.00
0.07
0.00
0.00
0.00
0.00
0.00
10.20
6.26
0.00
16.54
12.00
3.04
2.10
0.00
0.86
0.00
0.00
1.91
0.00
0.00
7.91
8.00
0.05
0.03
0.07
7.67
0.03
0.02
0.00
7.87
0.00
8.00
8.00
0.02
0.01
2.56
2.69
0.13
0.00
0.00
5.41
0.00
8.00
26.00
0.04
0.00
0.00
0.02
0.00
0.02
10.20
6.28
1.39
17.93
8.00
0.03
0.07
0.48
6.77
0.03
0.06
0.02
0.00
0.00
7.46
8.00
0.02
0.00
2.60
1.99
0.75
0.02
0.00
0.00
0.00
5.38
8.00
0.03
0.02
0.05
7.74
0.01
0.01
0.03
0.00
0.00
7.89
8.00
0.02
0.01
2.48
2.11
0.81
0.02
0.04
0.00
0.00
5.50
26.00
0.10
0.03
0.00
0.04
0.00
0.03
9.87
6.32
2.70
19.11
O
Si
Al
Ti
Fe
Mn
Mg
Ca
P
F
Cation sum
*
Um Anab
Mgt ¼ magnetite, Ilm ¼ ilmenite, Ap ¼ apatite, Ep ¼ epidote.
Abu Zawil
El Ghuzah
264
H.M. Helmy et al. / Journal of African Earth Sciences 38 (2004) 255–268
magma composition. Also, the computed pressure may
reflect the level at which the hornblende crystallizes rather than the pressure at which the granite consolidates
(upward movement may continue after hornblende
crystallization, Ghent et al., 1991). The fact that Al
content in hornblende geobarometer is only applicable
in the presence of quartz and plagioclase; alkali feldspars, biotite, hornblende, titanite and magnetite or
ilmenite clearly limits compositional influences (Leake
and Said, 1994). Pressures estimated by the Al-inhornblende barometer for many granitoid plutons were
found to be consistent with pressures suggested by
geologic features and estimated from the syn-intrusive
metamorphic assemblage (Ague, 1997). Thus, the reliability of the Al-in-hornblende geobarometer has been
proved.
In the analyzed thin sections, quartz + plagioclase + alkali feldspar + hornblende + biotite + titanite +
magnetite or ilmenite represent a magmatic, equilibrium
assemblage, which makes the aluminum-in-hornblende
barometry to be applicable. The narrow compositional
range of feldspars in the studied plutons provides the
additional restriction necessary for Al content in hornblende to be solely a function of pressure (Hollister
et al., 1987). The pressure of crystallization of the
magma was calculated according to the following
equation (Schmidt, 1992):
P ð0:6 kbarÞ ¼ 3:01 þ 4:76 Altot hbl
r2 ¼ 0:99
Table 6 summarizes the results of crystallization
pressures calculated from hornblende composition. As
hornblende from the studied plutons is weakly zoned,
small difference (within the reported uncertainty of the
calibration) in the calculated pressure is estimated from
core and rim compositions. However, analyses of
amphibole rims within 20 lm of grain boundaries are
commonly used in the calculations. The compositions of
hornblende showing alteration into actinolite along rims
have been excluded as they yield zero or negative values.
These actinolitic hornblende rims probably crystallized
as a subsolidus phase (Leake, 1971) and thus, give
unrealistic pressures. Due to the fact that the crystallization of magma must be well advanced to produce the
entire mineral assemblage required for aluminum-inhornblende barometry, such plutons are likely to be
largely solidified and the calculated pressures probably
represent the conditions during final emplacement.
7.2. Temperature estimates
Temperature estimates are based on the hornblendeplagioclase geothermometer of Blundy and Holland
(1990). This calibration has a reported uncertainty of
±75 °C. We have used the microprobe analyses of
coexisting amphibole-plagioclase pairs from many
samples representing each pluton to calculate the temperature (Table 6). In most cases we have selected
hornblende (with no actinolite rims) and plagioclase
sharing a common grain boundary.
The bulk of the calculated crystallization temperatures for the different granitoid plutons lies between 819
and 571 °C (Table 6) but usually >650 °C; the highest
temperatures are recorded from El Ghuzah samples,
while the lowest temperatures are from Um Anab.
Inferences on temperature of crystallization were also
made from the titanite composition. Enami et al. (1993)
discussed the dependence of titanite composition on
pressure and temperature. They showed that high-T
Table 6
Summary of petrological data of the studied calc-alkaline granitoids, Egypt
Area
Um Tagher
Um Gidri
Um Anab
Abu Zawil
El Ghuzah
Plagioclase
Xab ¼ (Na/Ca + Na + K)
0.68–0.72
0.55–0.87
0.80–0.88
0.80–0.85
0.82–0.84
Biotite
Xmg ¼ (Mg/Mg + Fe)
ASI ¼ (AlT /Ca + Na + K)
F content (wt.%)
0.5
1.53–1.55
0.15–1.2
0.44–0.45
1.34–1.36
nd
0.58–0.64
1.09–1.49
0.0–1.13
0.58
1.34–1.49
nd
0.58–0.76
1.26–1.48
0.33–2.29
Hornblende
Xmg ¼ (Mg/Mg + Fe2þ )
Total Al (afu)
0.60–0.62
1.72–1.77
0.52–0.62
1.33–1.58
0.69–0.73
1.08–1.143
0.73–0.90
1.04–1.10
0.87–0.89
0.976–1.13
Titanite
Al + Fe3þ
F (wt.%)
0.24–0.38
0.22–0.59
0.49–0.51
0.45–0.52
0.51
1.04
0.29–0.38
nd
0.17–0.49
0.53–1.23
Pressure (kbar)
Temperature (°C)
Oxygen activity (f O2 )
Depth of emplacement (km)
5.2–5.4
724–800
)13.1 to )15.0
20–20.8
3.3–4.5
670–738
)15.1 to )17.3
12.1–13.2
2.1–2.2
571–656
)17.8 to )21
8.3–9.4
2.0–2.2
681–712
)16.2 to )17
7.5–9.2
1.6–2.3
755–819
)13 to )14.7
6.3–9.02
nd ¼ not detected.
H.M. Helmy et al. / Journal of African Earth Sciences 38 (2004) 255–268
titanite contains Al + Fe3þ (afu) contents less than 1.4
(calculated on the basis of 20 oxygens, 12 cations). All
titanites from the studied plutons (Table 4) have
Al + Fe3þ (afu) contents less than 0.8, similar to high
temperature (>700 °C) titanites of Enami et al. (1993).
Calculations of the temperature (Stormer, 1983) from
microprobe point analyses of exsolved oxides from the
studied granitoids (magnetite-ilmenite thermometry
yields temperatures between 350 and 500 °C for the
studied plutons) shows subsolidus re-equilibration consistent with the observed textures (see mineralogy section).
7.3. Oxygen fugacity estimates
The intrinsic oxygen fugacity of a magma is related to
its source material, which in turn depends on tectonic
setting. Sedimentary-derived granitic magmas are usually reduced, while I-type granites are relatively oxidized. More highly oxidized magmas are commonly
associated with convergent plate boundaries (Ewart,
1979) while felsic magmas formed by fractionation from
mantle-derived magmas in rift zones are reduced (Loiselle and Wones, 1979).
It is difficult to establish the original oxygen fugacities
of primary magmas from the study of granitiods, as
magnetite usually becomes Ti free during slow cooling
and ilmenite undergoes one or more stages of oxidation
and exsolution (Haggerty, 1976). However, some inferences on the oxidation state of the magma can be made
using the rock mineral assemblage and mineral chemistry. Mg-rich amphiboles suggest relatively oxidized
magmas. The occurrence of euhedral titanite and magnetite as early-crystallizing phases in felsic rocks indicates that the magma was relatively oxidized (Enami
et al., 1993).
Wones (1989) suggests that the assemblage titanite + magnetite + quartz in granitic rocks permits an
estimate of relative oxygen fugacity. He made quantitative estimation of oxygen fugacity based on the equilibrium expression
log f O2 ¼ 30930=T þ 14:98 þ 0:142ðP 1Þ=T
where T is temperature (in Kelvins) and P is pressure (in
bars). We used this equilibrium expression to estimate
the prevailing oxygen fugacity in the different plutons
(Table 6). Temperatures and pressures estimated from
hornblende-plagioclase thermometry and aluminum-inhornblende barometer were used in these calculations.
As can be seen from the data presented in Table 6, all
plutons crystallized from relatively oxidized magmas,
although differences in the oxidation degree do occur.
The Um Anab pluton shows the minimum oxygen
fugacity with values ranging from )21 to )19, while El
Ghuzah pluton was formed from the most oxidized
magma (from )16 to )13).
265
8. Tectonic implications
Ahmed et al. (in press) studied the geochemistry of
the Um Tagher, Abu Zawil, El Ghuzah, Um Gidri and
Um Anab granitoids, and used the variation in Rb/Sr
ratio and total alkalis to suggest various degrees of
partial melting and different depths of melt generation.
The authors suggest that the parent magmas of Um
Tagher and Abu Zawil granitoids were generated by
higher degrees of partial melting of the lower crust,
while the El Ghuzah magma was generated by lowdegrees of partial melting from the upper crust, i.e.,
shallower depth. Um Gidri and Um Anab parent melts
were formed by intermediate degrees of partial melting.
Vertical crustal displacements, after complete solidification of the magma, during tectonic events are difficult to quantify because pressure and age estimates may
be absent or they may record different thermal events.
Contact metamorphic assemblages may provide thermobarometric data, but informative assemblages may
be absent or overprinted, as in the case in north Eastern
Desert. In such areas, the hornblende barometer is
useful in understanding vertical crustal motions on a
regional scale (Vyhnal and McSween, 1990; Stein and
Dietl, 2001). Magmas crystallizing hornblende used for
pressure calculations must have been nearly solidified
(as all the minerals for the concerned assemblage required for the application of the aluminum-in-hornblende barometer were already formed). Therefore, the
calculated pressures probably represent conditions during final emplacement. In the following discussion, the
calculated pressures from the total aluminum-in-hornblende are used to understand vertical motion on a regional scale.
The emplacement pressures were converted to depths
(Table 6 and Fig. 8) by using an average crustal density
of 2.65 g/cm3 and taking in consideration the limits,
which these data impose on vertical crustal movements.
The depths were calculated from the average of all calculated pressures.
Although different samples within the same pluton
give different depths of emplacement, it is easy to distinguish three different depths of emplacement in the
studied plutons: (a) El Ghuzah, and Abu Zawil plutons––depth 9 km, (b) Um Gidri and Um Anab plutons depth––13 km, and (c) Um Tagher pluton––depth
20 km. These results (Table 6) enable us to realize that:
1. Depths of emplacement of Um Anab (9 km) and Um
Gidri (13 km), which are separated by a fault plane,
indicate that as much as 4 km of displacement may
have occurred on this fault.
2. The Um Tagher pluton (20 km depth) which is only
20 km away from the Abu Zawil pluton (9 km depth)
must have been uplifted as much as 11 km relative
to the latter pluton.
266
H.M. Helmy et al. / Journal of African Earth Sciences 38 (2004) 255–268
N
Um Gidri
Um Anab
P = 2.1 - 4.2 kbar
-f O2 = 16.5 - 21
T = 571 - 686 ˚C
D = 13 km
P = 3.3 - 4.5 kbar
T = 670 - 738 ˚C
-f O2 = 15.1 - 17.3
D = 13 km
El Ghuzah
P = 1.6 - 2.3 kbar
-f O2 = 13 - 14.7
T = 755 - 819 ˚C
D = 9 km
Abu Zawil
P = 2 - 2.2 kbar
-f O2 = 16.2 - 17
T = 681 - 712 ˚C
D = 9 km
Um Tagher
P = 5.2 - 5.4 kbar
T = 739 - 800 ˚C
-f O2 = 13.1 - 15
D = 20 km
Fig. 8. Location map showing the estimated pressures (P ), temperatures (T ) and oxygen fugacity (f O2 ) and calculated depth, the bold arrow shows
the direction of increase in depth of emplacement. The dashed bold line is the boundary (Qena-Safaga line) between CED and NED.
3. A weak SE trend of increasing depth of emplacement
is tentatively recognized (Fig. 8).
A considerable debate exists among geologists about
the border between the CED and NED. Whether this
border is formed by a shear zone (El Gaby et al., 1990)
or reflects different terranes for CED and NED (Stern
and Hedge, 1985) is a problem, which has not been
resolved. The very high depth of emplacement of Um
Tagher pluton (20 km), which is located along the
boundary between north and central Eastern Desert
(Fig. 1), may support the suggestion of El Gaby et al.
(1990). However, the 11 km vertical uplift of Um Tagher
pluton was confined to the boundary line and do not
affect other plutons far north.
The depth of emplacement of any granitic body depends upon internal and external factors. Internal factors comprise magma composition, temperature and
water pressure, while the external factors are mainly the
tectonic regime. Concerning magma composition, the
water and volatile contents are among the most important factors. Wyllie (1977) and Kenah and Hollister
(1983) showed that for hydrous tonalitic to granodioritic
magmas, the melting intervals lie between 700 and 900
°C and are nearly independent of pressure. Moreover,
Johnson and Rutherford (1989) showed that the pressures calculated from hornblende composition are
indirectly a function of fluid composition. The water
and volatile content of magma are reflected in the existence and modal abundance of certain mineral phases.
Mafic minerals (micas, hornblende) are the only major
silicate minerals that accommodate water (and some
volatiles, e.g., fluorine and chlorine) in their structure.
Accessory minerals, like titanite and apatite, are the
main carriers of the volatile components (F and Cl). The
modal compositions of the rocks reflect changes in the
bulk compositions of the parent magma. High modal
percentage of mafic hydrous minerals, titanite and
apatite reflect high water (and volatile) content in the
magma. High volatile (F, Cl) contents in the magma are
easily indicated by analyses of the mafic minerals, apatite and titanite.
To help answer the preceding question in the light of
the above discussion, we summarize here the important
points gathered from the mineralogical investigations
of the studied plutons.
1. Different modal contents of mafic minerals is observed, the highest values are recorded at El Ghuzah,
Abu Zawil and Um Anab plutons.
2. High F contents are recorded in biotite (up to 2.3
wt.%) and the highest contents are recorded in biotites from El Ghuzah and Um Anab plutons.
3. High F contents are recorded in apatite (up to 4.9
wt.%) and titanite (up to 1.2 wt.%). The highest values are recorded in apatites and titanites from El
Ghuzah samples, while the lowest values are recorded
in Um Tagher samples.
4. The highest temperatures of emplacement were estimated for W. El Ghuzah pluton.
5. The highest values of oxygen activity (reflecting high
water content of the magma) were reported from El
Ghuzah pluton, while the lowest values are from
the Um Anab pluton.
Interesting to note is that the El Ghuzah pluton was
emplaced at a shallower depth relative to Um Tagher.
The stated points 1–5 above support that the El Ghuzah
parent magma was richer in water and volatiles, relative
to the other plutons. This pluton was emplaced at the
shallowest estimated depth. It is likely that the high
water and volatile content of the magma together with
high temperature allowed the magma to reach shallow
crustal levels without complete solidification.
H.M. Helmy et al. / Journal of African Earth Sciences 38 (2004) 255–268
9. Conclusions
Coexisting mineral phases and their compositions
from five calc-alkaline granitoid plutons (El Ghuzah,
Abu Zawil, Um Anab, Um Gidri and Um Tagher) in the
north Eastern Desert were used to estimate the physicochemical parameters of their crystallizing parent
magmas. The aluminum-in-hornblende barometer, hornblende-plagioclase thermometer and the assemblage
quartz-magnetite and titanite were used to calculate
pressure, temperature and oxygen activity, respectively.
These plutons were emplaced at different depths ranging
from 9 to 20 km at a temperature between 650 and 819
°C, from oxidized magmas (log f O2 from )21 to )13).
High water and volatile contents allowed the magma
of some plutons to reach shallower emplacement levels
relative to the others. Aluminum-in-hornblende barometry was used to quantify the post-Pan-African vertical crustal displacements. These granitoid plutons were
emplaced at depths of 20, 13 and 9 km. The Um Tagher
pluton along the Qena-Safaga line has undergone as
much as 11 km of vertical displacement relative to the
other plutons farther north. Faults controlled the significant vertical displacements recorded, as documented
in the case of Um Gidri and Um Anab plutons.
Acknowledgements
We are greatly indebted to G. Hoinkes for putting the
analytical facilities of the Institute of Mineralogy and
Petrology, Karl Franzens University of Graz, Austria at
our disposal. S. El Gaby (Assiut University) is thanked
for critically reading the first version of this manuscript.
Discussions with M. Ghoneim (Tanta University) are
greatly acknowledged. This manuscript has benefited
from comments by A. Mogessie. Editorial handling and
helpful suggestions by A.B. Kampunzu are gratefully
acknowledged.
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