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Pergamon Journal of African Earth Sciences, Vol. 32, NO. 1, pp. 61-66. 2001
0 2001 Elsevier Science Ltd
Pll:S0899-8382(00)80033-8 All rights reserved. Printed in Great Britain 0899.5362/01 S- see front matter
Pan-African granite emplacement mechanisms in the Eastern Desert, Egypt
T.J. FOWLER’*2 ‘Geology Department, School of Management Technology and Environment,
LaTrobe University, Bendigo. PO Box 199, Bendigo, Victoria 3550, Australia.
2Present address: Geology Department, University of the United Arab Emirates, PO Box 17551 Al-Ain, Abu Dhabi, United Arab Emirates
ABSTRACT-The late Pan-African Fawakhir, Urn Had and Urn Effein Granite Plutons in the Egyptian Central Eastern Desert are small, elongate to circular bodies of pink monzogranite and syenogranite, and grey monzodiorite. Structural investigation of the intrusive contacts of these plutons reveals marginal sub-horizontal intrusive sheets as magma injection along pre- existing flat-lying structures including mineral foliations and thrust faults. Space for emplacement .of granitic plutons was accommodated by uplift of the country rocks along steep marginal faults with rotation of the wall rocks whose outer contacts are in the style of laccoliths. Overall, the Urn Had Granite Pluton has a phacolithic shape controlled by a south-plunging domed mylonitic shear zone, which separates gneissic rocks (preserved as a pluton core) from low grade overthrust units. The structural features of these plutons are consistent with upper crustal level emplacements at depths where o3 is vertical. Emplacement is likely to have occurred after northeast-southwest transpression, rather than in an extensional tectonic environment. @ 2001 Elsevier Science Limited. All rights reserved.
RESUME-Les plutons granitiques pan-africains tardifs de Fawakhir, Urn Had et Urn Effein dans le centre du Desert Oriental Bgyptien sont des petits corps allonges a circulaires form&s de syeno- et monzogranites roses et de monzodiorites grises. Les observations structurales aux contacts intrusifs de ces plutons indiquent qu’il s’agit de lames intrusives marginales subhorizontales, correspondant a des injections de magmas le long de structures planes preexistantes incluant foliations minerales et failles de charriage. L’espace necessaire a la mise en place de ces plutons granitiques a 6te accommode par le bombement des roches environnantes encaissantes dont les contacts exterieurs sont dans le style des laccolites. D’une man&e g&Wale, le pluton granitique d’Um Had possede une forme phacolitique contr6lee par un couloir cisaillant mylonitique bombe et a pente sud, qui separe les roches gneissiques (preservees comme coeur du pluton) des unites charriees peu metamorphiques. Les caracteristiques structurales de ces plutons sont en accord avec une mise en place dans la croute la plus superieure, a une profondeur ou o3 est vertical. Cette mise en place s’est probablement produite apres une transpression NE-SO plutot que dans un environnement. Q 2001 Elsevier Science Limited. All rights reserved.
(Received 12/l l/98: accepted 6/4/00)
INTRODUCTION The Egyptian Eastern Desert has been described as African alkali feldspar granite to monzogranite plutons one of the most intensively dyke-intruded and granitoid of the Eastern Desert range in Rb-Sr age from 620 to pluton-pierced segments of the continental crust (Vail, 570 Ma (Hassan and Hashad, 1990). These plutons 1968; Bentor, 1985). The small epizonal late Pan- are typically round to elliptical or teardrop-shaped.
Journal of African Earth Sciences 6 1
T.J. FOWLER
They have been described previously as late to post- tectonic, but there are widely differing interpretations
with respect to the tectonic setting and stress state of the continental lithosphere during their intrusion.
Ragab et a/. (1989) and Ragab (I 991) proposed that
these late Pan-African monzogranites to syeno- granites were intruded during active thrusting, i.e. in
a compressional terrain, while Abdel-Rahman (I 995) pictured them in an Andean setting with their intrusion
occurring at the time of stress state change from compression to extension. Greenberg (I 9811, Bentor
(19851, Stern et a/. (1984, 1988) and Stern and Gottfried (1986) have suggested an extensional
setting accompanied by dyking for the late Pan-
African granites of the North Eastern Desert. Late
Pan-African granite intrusion into active Najd Shear
Zones has also been reported (Davies, 1982; Stern, 1985). Fritz et al. (I 996) has described a trans- pressional tectonic model for the development of the
Meatiq Dome to the east of the Urn Had area, although this setting is relevant to events which pre-date the
intrusion of the late Pan-African monzogranites to
syenogranites. Hassan and Hashad (1990) propose
more than one tectonic setting for these granites
(including both compressional and extensional) to
account for trace element pattern differences. Therefore, it is important to establish (for each granite
individually) the tectonic stress conditions accom- panying intrusion in order that the radiometric dates
on these granites can be used to properly delimit the Pan-African tectonic events in the Eastern Desert.
The stress conditions accompanying intrusion may
clarify how intrusions at high crustal levels in brittle
rocks attain circular to elliptical shapes Wigneresse, 1995a; Vigneresse et a/., 1999).
This paper investigates the tectonic conditions
and mechanisms of intrusion for three late Pan- African granite plutons: the Fawakhir, Urn Had and
Urn Effein Granites from the western part of the
Central Eastern Desert (CED) (Fig. 1). These plutons
were chosen partly for their ready accessibility, and
partly because they are considered to be typical of
the late Pan-African granites in their dimensions and
their chemical signatures (El-Ramly and Akaad, 1960).
They also show a range of shapes in mapped outline including elongate, elliptical and circular.
After an introduction to the regional geology of the Urn Had area, geochemical data on the plutons are
presented for the purpose of accurately naming the
various granite phases and for use in determining the tectonic stage at which the granites were emplaced.
This is followed by the structural details of pluton
margins and wall rocks. The latter structural data
are used to determine:
i) the 3-D shape of the plutons; ii) the mechanism(s) of emplacement, including the
role of pre-existing wall rock structures during in-
trusion; and ii.!. the crustal stress state during intrusion.
Geological setting of the Urn Had area The three late Pan-African granite plutons are located
in the western part of the CED, referred to here as
the Urn Had area (Fig. I). Details of the structure of
the Urn Had area are summarised in Fowler and Osman (1998). A brief account only is presented here.
Gneiss-cored dome The Urn Had area (Fig. 1) contains a large elliptical
structure trending northwest outlining a core of medium- to high-grade partly gneissic rocks enclosed
by a domed thick mylonitic shear zone. The northern
tip of the dome plunges 2O’to the northwest, while
the southern rounded closure of the dome plunges
gently southeast, although its structural details are
obscured by the later Urn Had Granite. The dome core
consists of folded sheared slices of garnetiferous amphibolites, mica-schists, paragneisses and minor
migmatites. No age data on these rocks are yet available. They have been interpreted variously as
high-grade metamorphosed Pan-African formations
or pre-Pan-African basement (El-Gaby et al., 1984;
Krliner et a/. , 1988; Neumayr et al., 1996). For the
latter, a metamorphic core complex model has been
suggested for nearby gneissic complexes (Meatiq Complex: Sturchio et a/. , 1983) as it has for other
gneissic domal structures in Pan-African fold belts (e.g. the Damaran Orogen: Oliver, 1994).
North-northwestward thrust-relatedstructures in the
dome
The dome core lithologies are cut by moderately
southeast-dipping retrograde shear zones, and are
pervaded by a retrograde foliation of the same orien-
tation as the shear zones. Retrograde metamorphism has reduced the high-grade gneisses to schists.
Both retrograde foliations and shear zones curve
into and merge with the sheared dome margin. The domed mylonite is the roof of an antiformal duplex
structure, which formed during north-northwest- ward regional thrusting and was later folded about
upright northwest-southeast-trending folds. Fowler
Figure 1. Geology of the Urn Had area, Eastern Desert, Egypt. Locations for Figs 4, 6 and 9 maps are shown as insets. The Nubia Sandstone occupies the unornamented areas along Wadi Muweih.
62 Journal of African Earth Sciences
Pan-African granite emplacement mechanisms in the Eastern Desert, Egypt
012345 \i
G:.. \ Fig. 6
Gneisses,, migmatites and metasediments
Arc metavolcanics lmafic)
Primary layering
trends
Mylonitic and schistore
shear zones
Ophiolitic melange fundifferentiatedl
Hammamat Group (greywackes)
Hammamat Group (conglomerates)
Hammamat Group [siltstones)
Dokhan Volcanics
Post-Hammamat Felsites (intrusive)
Younger Granites (undifferentiated)
Journal of African Earth Sciences 63
T.J. FOWLER
and Osman (1998) have argued against a metamor- phic core complex interpretation of these mylonite sheathed and shear dissected gneissic rocks, the main evidence being that the shears within, and branching
from, the mylonite sheath show consistent thrusting kinematics. The north-northwestward thrusting event
post-dates the deposition of late Pan-African Hammamat Group sediments (as cover sequences
above the mylonitic shears and affected by them), which are reported to be about 590 Ma, from Rb-Sr
data (Willis and Stern, 1988). The thrusting event
must also predate the Urn Had area granites with Rb-
Sr ages ranging from 570-590 Ma, which intrude
these thrusts and show only very minor brittle
deformation.
low-grade metamorphic cover units thrust north-
north westward over the dome
Surrounding the dome are low-grade metavolcanics
and metasedimentary cover rocks, which were em-
placed over gneissic dome core rocks during the north-
northwestward thrusting event described above. These Pan-African units are represented by serpen-
tinites, metabasalts and metagabbros of ophiolitic melange origin: conglomerates, greywackes and
pelites (Hammamat Group), and silicic to basic calc-
alkaline volcanics (Dokhan Volcanics).
The large ophiolitic mass surrounding the
Fawakhir Pluton (Figs 1 and 4) consists dominantly of serpentinite melange, which has a gently dip-
ping foliation. The Hammamat Group conglomer-
ates have crude anastomosing southeast-dipping
cleavages with southeast-trending stretched pebble lineations. The Dokhan Volcanics show localised
northwest-striking mylonite foliations. The cleav-
ages in the Hammamat Group and some of the
mylonites in the Dokhan Volcanics are kinematically
related to the north-northwestward thrusting event.
Other mylonitic foliations in the latter unit are
apparently related to Najd faulting as mentioned
below.
Northeast-dipping and south west-dipping later thrusts
and related folding
Northeast-southwest-trending folds in the domed
mylonite sheath and retrograde foliations occurred as a result of a later-apparent northeast-southwest compressional tectonic event. This event also produced re-thrusting of the overlying low-grade cover
rocks. Northeast-dipping thrusts in the cover rocks
are well exposed along Wadi Atalla, whereas southwest-dipping thrusts dominate along Wadi Muweih. Thrust-related lineations on the northeast-/ southwest-dipping thrusts are variable but typically steeply pitching. The thrusts appear to precede Najd
64 Journal of African Eatth Sciences
strike-slip faulting (since they are dissected by the latter faults); however, the presence of some oblique pitching lineations on the thrust planes may be explained either by transpressional regional stresses
or simply by reactivation of thrusts during strike-slip
faulting. There is no evidence in this area of a ‘flower structure’ relationship between the thrusts and the
Najd Faults. The timing of the activity of the northeast- /southwest-dipping thrust structures relative to the
intrusion of the Urn Had area granitoids is discussed later in this paper.
Najd faulting and related structures
A sinistral transcurrent deformation regime, identified
as Najd fault-related, has mainly affected the low-
grade metavolcanics and metasediments east of the dome in Wadi Atalla. The Najd event produced steeply
dipping, ductile, sinistral transcurrent shears of notth-
northwest- to northwest-strike. The Urn Had Granite
appears to post-date the Najd Faults, as marginal apophyses from the granite cut across Najd Shears
without evincing deformation.
Late Pan-African granitoid intrusions
These include the Urn Had area granitoids and their
equivalents scattered throughout the Egyptian Eastern
Desert and emplaced during Pan-African times
throughout the African Craton and Arabian Peninsula.
The widespread occurrence of these late- to post- erogenic talc-alkaline to transitional A-type granitoids
was interpreted by Black and Liegeois (1993) as being
due to delamination of the continental lithospheric
mantle following the tectonic collisions leading to the assembly of Gondwana. These granitoids are
characterised by unfoliated, small, nearly circular to
elongate epizonal plutons with sharp intrusive
contacts. The reported forms of these plutons include
lopoliths, thick sills, ‘tack-shaped’ bodies and steep-
sided plutons (Noweir et al,, 1990).
GEOCHEMISTRY OF THE UM HAD AREA GFtANlTOlDS
The multicationic scheme of de la Roche et a/. (1980)
is used in Fig. 2a to represent the major elements
from chemical analyses of the three Urn Had area granitoids. The R, and R, parameters for the Urn Had Granitoid span the range from granodiorite to alkali
granite but lie mainly in the monzogranite and syenogranite fields. The Urn Effein Granitoid has a
similar compositional range but includes one sample in the quartz monzonite field. These two granitoids are typically pink in-hand specimen and have field characteristics described below. The Fawakhir Gra- nite includes an early grey-coloured mafic phase that
2000
1000
; -. / - -L / /
,, --___--7
,’ ALKALI ,I’ /’ GABBRO - a
/‘GABBRO /’
4 ‘I NORITE
, OLIVINE ’ I \
‘\ ,’ I’ \
GABBRO ‘I \, ./’
7: I’ I _M
/’ I *., *
I ‘. C- / I
/ . . 4’ .r
‘. /’ , ---
I -/- -- -- _/, -
/
‘. ,‘sYENOGABBRO,‘“ONZO ; /’
GABERO ./
/- ESSEXITE /*.
_-’ I I SYENODIORITE
. /’ r/C
/’ c’
-y: .’
, -. 0 .Jr
I’ I--__ / ,-
, / /’ --_
, NEPHELINE I’ , TONALITE
SYENITE /’ .* GRANODIORITE
I * I’ /
I SYENITE I ,I I’
I I //
I SYENITE
,I II
,I I’ ALKALI GRANITE I ,’
I _____-_----- -- _-‘______-----~---------_________
----_-_____-____
I I I 1 , Rl 1000
o Fawakhir Ganite
s Urn Had Granite
2000 3000
1000
Fawakhir Fawakhir
Urn Had Urn Had
Urn Effein
A Urn Effein Granite
@ (Rogers and Greenberg, 1983)
0 (Noweir ef a/., 1990)
+ (El-Gaby, 1975) + (Noweir et al., 1990)
\ \ Mantle
\ Fractionates
\
\
\
\ \
\
b
Late . \
.
erogenic - V . \
@@
---_ ---_ --
Anorogenic I)
Post-erogenic
R,
Figure 2. (al De la Roche et al. 119801 multicationic plot for geochemical data for the Fawakhir, Urn Had and Urn Effein Granites. R, = 4Si- 11 INa f KI-2(Fe + Til; R, = 6Ca + 2Mg +Al. lb) Same data as in (al plotted on a de la Roche et al. (1980) multicationic diagram (modified by Batchelor and Bowden, 19851 by superposing tectonomagmatic field boundaries.
T.J. FOWLER
is intruded by a pink granite phase similar in
appearance to the Urn Had and Urn Effein Granites.
The earlier mafic phase analyses occupy the field of
monzodiorite with one analysis in the syenodiorite
field. The later pink-coloured phase of the Fawakhir
varies from quartz monzonite and granodiorite to
monzogranite. With the exception of the monzodiorite
of the Fawakhir, the remaining chemical data from
the three granitoids forms a chemical continuum with
significant compositional overlaps for samples from
each pluton.
In order to characterise the probable tectonic stage
of evolution of the Urn Had area granitoids from their
geochemical signatures, the same granitoid chemical
data was plotted on a multicationic diagram modified
by Batchelor and Bowden (1985) (Fig. 2b), and on Nb
versus Y and Rb versus Nb +Y diagrams (Fig. 3). The
modified multicationic diagram in Fig. 2b shows that
the chemical data from the Urn Had area granitoids
lie dominantly in the late erogenic and syn-collision
granitoid associations identified as group 4 (sub-
alkaline monzonitic) and group 6 (anatectic two-mica
leucogranites) by Lameyre and Bowden (I 982). The
earlier monzodiorite phase of the Fawakhir Granite
lies in the post-collision uplift field or group 3, a high-
potassic talc-alkaline group.
Batchelor and Bowden (1985) explained the
compositional trends on these multicationic diagrams
in terms of a model where a source magma (of high
Al basaltic composition) progressively fractionates
to produce Na, K-richer derivative melts. The pro-
gressive stages of fractionation correlate with tec-
tonic stages. At each tectonic stage, the fractionated
source magma mixes with melts from felsic crustal
material producing hybrids of intermediate com-
position. The hybrids then experienced in situ fractionation to produce the observed series com-
positions. The series converge on the restricted area
of anatectic granite compositions. On the basis of
this model, the Urn Had and Urn Effein Granitoids,
and to a lesser degree the Fawakhir later phase,
represent rather strongly fractionated late-collision
to post-collision granitoids, whereas the earlier dioritic
phase of the Fawakhir would belong to the previous
tectonic stage of the cycle with relatively little
fractionation having occurred.
The range of Nb and Y values for the three
granitoids is shown in Fig. 3a. The data for each of
the granitoids extends into both fields of syn-collision/
volcanic-arc granites (SYN-COL/VAG) and within-
plate granites (WPG). None of the CED granitoid data
lies in the oceanic ridge granite (ORG) field. The Rb
versus Nb +Y diagram in Fig. 3b shows data from all
of the granitoids straddling the boundary between the
VAG and WPG fields. The data lies very close to the
66 Journal of African Earth Sciences
meeting point of these latter two fields and the field
of syn-collision granites. Again, no data falls in the
ORG field. Pearce et al. (1984) noted that post-
collision granites are not well distinguished on these
diagrams, apart from plotting in all fields except ORG,
and lying near the top of the VAG field. Variable
mixtures of mantle- and crust-derived magmas in post-
collisional settings were suggested to explain the
chemical variability of this group. Since each of the
Urn Had area granitoids crosses the VAG-WPG field
boundary, it is preferred to interpret this Nb, Y, Rb
data as suggesting a post-collisional setting for the
CED granitoids.
THE FAWAKHIR GRANITOID
As noted above, the Fawakhir Granitoid (Fig. 4) is
composed of two compositionally contrasting granitic
phases: an earlier grey monzodiorite phase (including
minor meladiorites and hornblendites) intruded with
sharp contact by a larger pink mainly monzogranite
phase. The petrography and structure of these phases
are briefly described, followed by the structure of
their margins and wall rocks. Reported radiometric
ages for the Fawakhir intrusion are unclear about
which phase was dated. Age determinations include
the following whole rock Rb-Sr ages: 574 Ma (Rogers
and Greenberg, 1983); 586 *9 Ma (Fullagar and
Greenberg, 1978); and 565-590 Ma (El-Bouseily et
a/., 1986).
The Fawakhir monzdiorite
This phase is mainly represented by grey coarse- to
medium-grained hornblende + biotite, quartz monzo-
diorite, which locally becomes plagiophyric and fine-
grained near wall rock contacts. This early phase is
lighter in colour and richer in quartz at northeastern and,
particularly, southwestern exposures, e.g. around the
El-Sid Gold Mine where it encloses numerous fine-grained
mafic enclaves. The monzodiorite contains numerous
fracture zones filled and altered by secondary epidote,
chlorite, carbonate and quartz. In the eastern exposures,
minor outcrops of hornblende mdadiorite and homblendite
are also found, which show signs of intense brecciation ’ and minor ductile deformation. The meladiorite/horn-
blendite is locally incorporated as breccia fragments
within monzodiorite, and on the basis of this evidence it
represents a distinct intrusive phase. Hornblende
meladiorite is also found locally along the western
contact of the pluton, e.g. near the El-Fawakhir Qah-
wa in Wadi Hammamat (Fig. 4).
The Fawakhir syenogranite to monzogranite This is typically a pink medium-grained biotite
syenogranite to monzogranite with occasional large
Pan-African granite emplacement mechanisms in the Eastern Desert, Egypt
a
Nb
10 -
&VAG .
- Fawakhir Granite
--- Urn Had Granite
----- Urn Effein Granite
Fawakhir 0 (Rogers and Greenberg,
Urn Had l (El-Gaby, 1975)
1000 r
100
10
SYN-COLG
ORG
1 10 100
Nb+Y
1000
b
19t
Figure 3. Pearce et al. ‘s (1984J granitoid tectonic environmental discrimination diagrams.
/al Nb versus Y. (bl Nb + Y versus Rb.
orthoclase phenocrysts. These phases contain typi- quartz veins, particularly in the vicinity of the El-Sid tally c 1% by total surface area of ovoid fine-grained and El-Fawakhir Gold Mines. The geological relations porphyritic mafic igneous enclaves of biotite and petrology of these veins and fractures have been microdiorite composition (Fig. 5e), some appearing analysed and reported by El-Bouseily et al. (I 987) partly assimilated. There is generally intense jointing and Loizenbauer and Neumayr (I 996). Also numerous with silicified or sericitised joint surfaces parallel to minor faults, cross-cutting the Fawakhir Granitoid,
Journalof African Eatth Sciences 67
T.J. FOWLER
+ +: A FAWAKEIR GRANITE oPliIoLITIc haLANGE
serpendnite
:phase
DoKE4N VOLCANICS HAMMAMAT GROUP
meta-almites. conglomerates
Felsite (intrusive)
/ Shear Zone
Siicic dgke
primary contacts &&tins. inmsive r/
high-angle fault
rn hnmhlmdifc dvke sillcic dvke Q (dip,dipdirection)
_, gz%IiPI J&l
7 (strike &dip)
foliatiott (ii ophiolites, 133
strike & dip) mike & dip)
synform, atttuorm +++++++ +++++++ +++++++ + El-Fmakbirqahwa i
$ ++++++++
++++++++++_, (roadside caf6)
-?TT++++++. r++++++++++, * El-Fawakhir Resthouse
a_++++++++++ (J%Sid Gold Mine site) ++++++++++ ++++++++++
vet&d cross- SWtiOllS
0 1 2 3 4
Figure 4. Map of the Fawakhir Granite (for location reference see Fig. IJ. The monzodiorite intrusive contacts dip outwards gently, while the syenogranite intrusive contacts are gently outward dipping at the northern and southern ends of the pluton and elsewhere moderate to steeply dipping. The sheared conglomerates along the eastern edge of the map have been included with the Hammamat Group.
68 Journal of African Earth Sciences
Pan-African granite emplacement mechanisms in the Eastern Desert, Egypt
n”#,l I , ,,,,,,,,, IF,, ..,, ,.... -.m,- I,,,,,. ,a, I,,_ r”,“,ll I,,, .I,_ I,,, c --a-- -. “_ .,.... _I_. I.“_” “““.,“..” “. . ..” I_._,..,,.
_.I ,,.. “.
contain carbonate or quartz fibres indicating variable directions of oblique slip. A red graphic granite, which forms minor sub-horizontal veins in the pluton, appears elsewhere to be gradational with the main monzo- granite. Aplite forms gently dipping dykes but is uncom- mon, and pegmatite is virtually absent. Thick white micro-granite sills apparently preceded the emplace- ment of the Fawakhir Syenogranite but post-date its monzodiorite variant and intruded the serpentinite wall rocks in the northern part of the pluton.
contacts between tha Fawakhir monzodiie and wall rocks Monzodiorite has intruded both serpentinites and metabasalt thrust sheets. In most exposures, the contact of the monzodiorite with these lithologies is sub-horizontal to gently dipping (usually 30°0r less) away from the pluton (Figs 4 and 5d). These contacts are usually concordant to the local gently dipping ser- pentinite foliation (Fig. 5b) and discrete low angle shear zones (Fig. 5~). However, where the serpentinite folia- tion dips steeply, the monzodiorite intrudes discor- dantly as a gently dipping sheet, e.g. along the south- em margin of the Fawakhir Pluton. Along the western side of the Fawakhir Pluton, the monzodiorite shows
steeper tabular form controlled and affected by thrust faults (Figs 4, sections A-A’ and B-B’, and 5a).
The 3-D form of the monzodiorite is well repre- sented in the northeast part of the Fawakhir Pluton where this phase forms extensive thick intrusive ton- gues and sheets with undulating tops (Fig. 4, section A-A’). In the latter area, the serpentinite and meta- basalt form roof inliers (Fig. 5d), which the monzo- diorite also intrudes by accessible faults in the roof rocks. The monzodiorite along these faults is itself brittly disrupted and intensely carbonate-veined. In this area, it appears that the monzodiorite has par- ticularly favoured intrusion along the gently northwest- to southwest-dipping active thrust contact between the serpentinites and metabasalts. The undulating dip of the roof of the intrusive tongues may be primary, or be a result of gentle folding, or represents rotated roof blocks. Some of these gently dipping tabular units have incorporated roof blocks of sheared serpentinite within them.
The meladiorites and hornblendites, which ap- parently preceded diorite emplacement, also adopted shear zone concordant shapes and are always in- tensely sheared and brecciated. The shear zones are southwest-dipping thrusts belonging to the second
Journal of African Earth Sciences 69
T.J. FOWLER
70 Journal of African Earth Sciences
Pan-African granite emplacement mechanisms in the Eastern Desert, Egypt
thrusting event in the region. In Wadi Hammamat near Wadi Atalla, irregular veinlets of hornblende mela-
diorite intrude the metabasalts. At the El-Fawakhir
Qahwa (Fig. 4, section A-A’), a hornblende meladioriie, which has intruded along a southwest-dipping thrust in the serpentinite, incorporates blocks of sheared ser- pentinite and has been sheared to biotite schist along the same thrust (Fig. 5a). The evidence above indicates intrusion of the monzodiorite actively during, or before the end of, the second thrusting event in the region.
The commonality of style and similarity of orientation
and structural level of the eastern and western ex-
posures of the monzodiorite suggests that it was
continuous between these exposures before inter- ruption by the later Fawakhir Syenogranite. If so, the intrusive form of monzodiorite was a flat-lying sheet,
or set of sheets, with local contacts taking advantage of sub-horizontal foliations and shear zones.
Contact between the Fawakhir syenogranite and wall rocks The syenogranite describes a southward tapering mass
intruded through the monzodiorite. There is no obvious hornfelsed zone or reaction zone between the syeno-
granite and the serpentinite or metabasalt wall rocks.
There are very few examples of wall rock xenoliths in the syenogranite. Near the contact, however, there are
frequent examples of dark ellipsoidal igneous enclaves
reaching a metre in length. These are most common along the western margin of the pluton (Fig. 5e). At the
northern contact, the syenogranite dips at about 20-
40” northwards beneath the serpentinites. The contact
is locally steep (70-80”) in the northeastern contacts
against the metabasalts but in the same locality is seen
to be stepped and composed of alternating steep and
gently dipping segments. Along the western contact,
the granite margin is again stepped with alternating
steep and gently dipping fault-related segments (Fig. 5f).
The western contact is typically moderately to steeply
dipping (Fig. 4, sections B-B’ and C-C’), although granite
dykes extending from the contact also show a step- like geometry related to faulting.
Contact between the Fawakhir syenogranite and monzodiorite The contact between the Fawakhir syenogranite and
monzodiorite is sharp, and the size (up to 0.75 m
long) and abundance of dark ovoid fine-grained igneous
enclaves increases towards these contacts. There
is no chilling of the syenogranite against the monzo-
diorite. The syenogranite usually displays 60” or steeper contacts against the monzodiorite and may form dykes of gentle to steep dip (20-80”) within the
monzodiorite. Syenogranitic dykes, which transect faults in the monzodiorite, show little evidence of deformation. At the southern contact along the Qift-
Quseir Road, the syenogranite has incorporated sub- metre scale blocks of monzodiorite and serpentinite.
Syenogranite has apparently intruded completely into
crystallised monzodiorite.
THE UM HAD GRANITOID
The Urn Had Granitoid (Fig. 6) adopts an approximately circular outline measuring 10 km in diameter. The
boundary of the pluton is sharply defined at its
western, southern and eastern margins. The western
and southern margins lie against Hammamat Group
metasediments, which show a spotted hornfels zone
reaching hornblende-facies grade extending 1.5 km
radially outwards from the contact. Hydrothermal al-
teration and veins containing epidote and quartz along
joints and faults in the hornfels are common. The
eastern contact is shared with highly sheared Dokhan
Volcanics, which show more subtle contact meta-
morphic effects. The central and northern parts of the pluton are
occupied by weakly gneissic quartzofeldspathic
metasediments, with numerous small intrusions of
syenogranite penetrating them (Fig. 6). These meta-
sediments are continuous with similar units north of
the Urn Had Pluton (El-Gaby et a/., 1988a). The
metasediments have been referred to as a ‘roof’ of
the pluton (e.g. Kamal El-Din eta/., 1996). However,
the mass of the Urn Had Pluton surrounding these metasediments lies structurally above these units,
having intruded between the metasediments and the
overthrust Hammamat Group, as described below.
The metasediments, therefore, constitute a wall rock core of the pluton, which is intensively intruded by
smaller Urn Had Granitoid bodies. The northern limit
of these subsidiary intrusions lies approximately along
Wadi Urn Sheqila (Fig. 6, sections A-A‘ and B-B’),
where they have intruded along a major south-dipping
Figure 5. Outcrop features for the Fawakhir Granite. la) Monzodiorite diorite dyke within serpentinite wall rock, from the western margin of the p&on, showing transection by a wall rock shear. Ibl Monzodiorite, from the western margin of the pluton, intruded concordantly along westerly dipping foliation in serpentinite ISI. /c/ Monzodiorite diorite dykes, from the southeastern margin of the pluton, intruded concordantly along shears in metabasalt. The coarse-grained unit beneath the hammer is diorite. fdl Serpentinite residual above flat-topped thick monzodiorite tabular body at the northeastern margin of the pluton. lel Microdiorite enclaves in syenogranite granite at the northwestern margin of the pluton. (fl Stepped contact of the syenogranite granite against serpentinite at the western margin of the pluton.
Journal of African Earth Sciences 7 1
T.J. FOWLER
Figure 6. Map of the Urn Had Granite ffor location reference see Fig. I). The arcuate broken line is the approximate location, before
Urn Had Granite intrusion, of the sheared boundary between the core metasedimentary gneissic rocks and the over-thrusted
Hammamat Group. The southwestern, southern and southeastern outer intrusive contacts dip outwards generally at moderate
angles. The western and eastern contacts are steep and associated with syn-intrusion faulting. The northern edge of the pluton is
defined by the northernmost significant dykes cropping out along Wedi Sheqila.
72 Journal of African Earth Sciences
Pan-African granite emplacement mechanisms in the Eastern Desert, Egypt
Bt
wadi Urn Had
HAMMAMAT GROUP
Figure 6. continued. Cross-sections and legend of the Urn Had Granite.
shear zone (El-Gaby et a/. , 1988a). This wadi is taken
to mark the northern limit of the Urn Had Pluton.
A subsurface extension of the Urn Had Pluton
northwards towards the Urn Effein Granitoid has been
suggested in several articles on the metamorphic
history of this area (Noweir and El-Sharkawi, 1988;
Kamal El-Din et a/., 1996). The subsurface extension
of the Urn Had Granitoid was invoked to explain the
broader belt of metamorphism north of the pluton than
south of it where the pattern of metamorphic zones
is independent of the shape of the pluton. In addition,
this northern metamorphic aureole has significantly
higher pressure assemblages, including garnet and
staurolite, and shows migmatisation. Apart from
staurolite, these features are also found in the core
metasediments of the pluton. Fowler and Osman
(1998) have pointed out that the garnet, sillimanite
and cordierite porphyroblasts in the Wadi Urn Sheqila
retrograde shear zone have been deformed by the
retrograde foliation and that the Urn Had Granitic
dykes have intruded post-kinematically along this
foliation. There is, therefore, no doubt that the garnet,
cordierite and sillimanite phases, and the migma-
tisation relate to an earlier regional metamorphism
rather than having been caused by contact meta-
morphism from the intrusion of the Urn Had
Granitoid. This was stated by El-Gaby (I 994) and
El-Gaby et al. (1988b) following El-Gaby et al.
(1988a) in their distinction between the Urn Had
contact aureole and the earlier Urn Sheqila thermal
aureole, the latter including the garnetiferous core
rocks of the pluton. There are few clearly discernible
Journal of African Earth Sciences 73
T.J. FOWLER
Urn Had Granitoid contact effects in these rocks apart from retrogression of the earlier porphyro- blasts.
As with the Fawakhir Granitoid, the Urn Had Granitoid consists of two intrusive phases: an ear- lier grey phase consisting of diorite to tonalite in composition, intruded by a more voluminous mon- zogranite to syenogranite. Age determinations on this pluton are few but include a reported whole rock Rb-Sr age of 590 f 11 Ma (Ries et al., 1983).
The Urn Had monzodiorite The diorite of Urn Had is quartz-poor and rich in biotite and hornblende, showing a wide variation in grain size. It is volumetrically a minor phase re- stricted to the northeast part of the pluton (Fig. 61, where it occurs as low-lying sheets and as minor intrusions semi-concordant with the core meta- sediment gneissosity (Fig. 6, section C-C’). In both cases, it is intruded by pink syenogranite to monzo- granite.
The Urn Had syenogranite to monzogranite The younger pink granitic phase of the Urn Had Granitoid is a coarse-grained potash feldspar (and more rarely quartz) porphyritic biotite syenogranite to monzogranite, which becomes typically medium- grained within a distinct 30 m thick chilled zone at the contact. Unlike the Fawakhir Granitoid, the Urn Had syenogranite to monzogranite is virtually devoid of mafic igneous enclaves. Wall rock xenoliths are rare at the contact but are common in the minor in- trusions in the core gneisses and especially in zones of intensive intrusion along core metasediment gneissosity (Fig. 7d) (El-Kalioubi, 1988). A red graphic granite is also present as dykes. Pegmatites are very common near the intrusive contacts and are present as minor dyke-like or sill-like intrusions in the core gneisses but are absent within the pluton itself.
The details of the intrusive contacts for the Urn Had Granitoid are described below, where they are divided into those related to the smoothly arcuate outer contact against the Hammamat Group, Dokhan Volcanics and felsite, and those related to the inner contact shared with the gneissic core metasediments of the pluton.
UM HAD PLUTON AND ITS OUTER CONTACTS WITH WALL ROCKS
In detail, the shape of the outer intrusive contact of the Urn Had Pluton departs from being smoothly circular. There is an outward bulge in the southern contact described by a roughly circular arc of smaller radius than for the pluton (Fig. 6). The eastern end of the bulge is marked by a re-entrant cusp in the contact zone, where the contact meets curved major thrust faults in the aureole. The intrusive contacts of the southern bulge also depart from a smooth arc by describing undulations, which correspond to variations in contact dip from typically 20-45’outwards (Fig. 7f). There are generally smooth gradients between these dip values; however, local sudden steepening of the contact to 65-80° (either towards or away from the pluton) is found and is associated with pre- or syn-intrusion faults along which the granite has penetrated. The evidence for the existence of these faults during intrusion includes pegmatitic sheets parallel to the fault plane and tongues of granite cutting across the fault plane.
Beds in the Hammamat Group, which dip towards the contact, adopt gentler dip values progressively nearer the contact (Fig. 6, section A-A’). Beds dipping away from the contact have progressively steeper dip values nearer the contact. These systematic bedding dip changes are best explained by an initially gently dipping intrusive contact being locally rotated to steeper outward dips, presumably to accommodate magma emplacement.
The eastern and western margins of the pluton are steepest, typically vertical to 65” outwards or in- wards. In these parts of the contact, the pluton has intruded along and across steeply dipping wall rock foliations and has included metre-scale blocks of foliated wall rock in rare xenolith zones, confined to within 20 m of the contact. The northeastern outer edge of the granite dips about 65’ outwards, roughly parallel to wall rock foliations (Fig. 7b), although gently dipping segments corresponding to marginal tongues persist (Fig. 6, section C-C’). The northern part of the contact is best included with the inner contact details below.
Beyond the outer contact, there are typically nu- merous granitoid dykes, some of which can be traced directly into the Urn Had Pluton, representing
Figure 7. Outcrop features for the Urn Had Granite. (a) Sub-horizontal syenogranite dykes extending from the southern margin of the pluton into Hammamat Group sediments discordantly to the steep foliation. IbJ Steep pink syenogranite dyke from the northeastern margin of the pluton intruded along the wall rock foliation and incorporating wall rock xenoliths. Ic) Massive pink syenogranite sill from the northern edge of the pluton. (d) Gneissic metasedimentary xenoliths from the zone of intensive intrusion of pink syenogranite along gneissosity at the northern edge of the pluton. leJ Film of contaminated syenogranite surrounding gneissic xenoliths at the northern edge of the pluton. If) Moderate outward dipping contact of syenogranite against Dokhan Volcanics at the southeastern margin of the pluton.
74 Journal of African Earth Sciences
Pan-African granite emplacement mechanisms in the Eastern Desert, Egypt
Journal of African Earth Sciences 75
T.J. FOWLER
marginal apophyses. The marginal dykes include those which are sub-parallel to the dip of the granite contact, e.g. the thick north-trending dyke on the eastern side of the pluton, as well as others which strike at a large angle to the contact and dip gently to moderately, independently of foliations in the wall rocks (Fig. 7a). Both dykes and the pluton contain thin sheets of pegmatite within a metre of the contact (upper contact in dykes) and parallel to it.
Inner contact with the core The weakly gneissic core metasediments are rarely garnet-bearing and are not generally spotted like the hornfelsed Hammamat Group. The core metasedi- ments show south- or southeast-dipping retrograde micaceous foliation, which are related to retrogression during north-northwest directed thrusting. Where this foliation is well developed, granitoid and rarer pegmatite sheets have intruded approximately along the foliation plane (Figs 6, sections A-A’and B-B’, and 8). The orientation of the retrograde foliation in the core is fairly constant over large areas (apart from mild rotations associated with folding) suggesting that there has been no significant disruption of these rocks during intrusion. Granitoid dykes have commonly intruded along the retrograde foliation forming low- lying tabular bodies. Locally, two or more tabular bodies, one above the other, are linked by dyke-like segments, which cut across the foliation. Both foliation concordant and discordant dykes contain wall rock xenoliths, which appear to have behaved plas- tically during intrusion (Fig. 8).
Both the retrograde foliation and the schistosity are feeble in the inner parts of the metasedimentary core. In these parts, the style of granite intrusion changes to that of large flat-lying sack-like bodies, kilometre- scale horizontally and several tens of metres thick, which were emplaced discordant to the gneissosity.
Along the northeastern inner contact, the core rock gneissosity is well developed and dips about 70’ outwards. Here the pink granite intrudes in two styles. The first is as minor injections along the gneissosity, in which case it incorporates gneissic fragments as xenoliths (Fig. 7d). Discrete tabular bodies of pink syenogranite to monzogranite along the gneissosity are found to pass upwards into the examples of the second style of intrusion, namely flat-lying discordant sack-like bodies, which are similar to those noted above.
In a similar manner in the northeastern parts of the Urn Had Pluton, diorite has intruded as both schistosity discordant sub-horizontal tabular bodies and along the gneissic foliation. Diorite intrusion along the schistosity also incorporates gneissic xenoliths, and these appear to have contaminated the magma,
76 Journal of African Earth Sciences
producing mafic compositional flow bands enveloping the xenoliths (Fig. 7e).
THE UM EFFEIN GRANITOID PLUTON
The Urn Effein Granitoid Pluton (Fig. 9) is a roughly elliptical intrusion measuring 10.0 km by 5.5 km. This pluton was emplaced into the northern closure of the domed shear zone that separates gneissic rocks and high grade schists from low grade meta- volcanics and Hammamat metasediments. The long axis of the intrusion lies parallel to the dome axial plane but is displaced a little to the east of it. Its eastern margin intrudes sheared and mylonitised felsite, and at its northern end it cuts across the hinge of the dome. The western margin lies against silicic meta-volcanic schists, while the southern margin lies against amphibolites.
Syenogranite to monzogranite and other phases of the Urn Effein Pluton The Urn Effein Granite is a generally coarse-grained biotite-poor pink syenogranite to monzogranite with abundant orthoclase and quartz phenocrysts. The granite is remarkably homogeneous, although it has a narrow chilled contact. A common feature is the presence of variably trending sub-horizontally flow- aligned potash feldspar phenocrysts. There are two phases at the southern margin: a coarse-grained pink porphyritic main syenogranite phase and a sheet of pale grey microgranite with phenocrysts of potash feldspar, hornblende and quartz. At this location there are rare examples of fine-grained porphyritic mafic igneous enclaves in the syeno- granite. Veinlets of pink syenogranite in the grey microgranite indicate that the grey microgranite is the older of the two phases. Quartz veining and silicification along tensional fractures and faults in the syenogranite indicate widespread and intense hydrothermal activity. Aplite veins are steep and strike approximately east-west. Minor pegmatite bands are found at the contact towards thesouth- eastern end of the pluton.
Urn Effein wall rock contacts As with the Urn Had and Fawakhir Plutons, the Urn Effein intrusive contact dips gently to moderately outwards beneath the country rocks and has the lowest dips at the northern and southern ends of the pluton where ZOO dips are typical (Fig. 9). The entire eastern margin of the pluton lies against felsite, and the contact is concordant with typically 30-40 (and locally up to 65”) easterly-dipping mylonitic foliation in the marginal felsite (Figs 9, section A-A’, 1 Ob and 11). For several kilometres along the eastern margin,
Pan-African granite emplacement mechanisms in the Eastern Desert, Egypt
____----
__--- ---___ -_d_-_
___ ---- a
b
Figure 8. Examples of syanogranita dyke and sill intrusions in the core gneisses of the pluton. Both la) and lb) are approximately vertical sections viewed towards the northeast. la) Discordant dyke passes into concordant sill with numerous detached sill roof blocks. Ibl Discordant dyke with several retrograde foliation concordant apophyses and engulfed wall rock blocks. The broken lines represent orientation of retrograde shear foliations in the gneisses.
Journal of African Earth Sciences 77
A
Figure 9. Map, legend and cross-section of the Urn Effein Granite (for location reference see Fig. I!. Intrusive
contacts dip outwards generally at low to moderate angles.
Pan-African granite emplacement mechanisms in the Eastern Desert, Egypt
the location and orientation of the Urn Effein Granite contact is controlled by a single massive tabular felsite unit, which has intruded sheared felsite along the mylonitic foliation. The northern tip of the pluton narrows and its margins lie at a low angle or parallel to the felsite foliation. Overall, concordance of the contact with the felsite foliation continues along the northwestern contact. The western margin of the pluton lies against steep shear foliations in the silicic metavolcanics. Here, there is a strong element of intrusion along this foliation and incorporation of wall rock blocks (Figs 1 Oc and 11). On this western side of the pluton, stepped low-lying sheets and steep marginal dykes are common (Fig. 11). The southwest- ern contact has dips as low as IO” and is discordant to the weak retrograde foliation in the amphibolites (Fig. 1 Oa).
SUMMARY COMPARISON OF THE CENTRAL EASTERN DESERT PLUTONS
Although the shapes and dimensions of the plutons and the nature of their wall rocks vary, there are several shared petrological and structural features between the Fawakhir, Urn Had and Urn Effein Plutons.
Petrological features The following petrological features are common to
the three plutons: i) an early mafic dioritic to monzodioritic (grey)
phase, which is much less voluminous than a later pink syenogranite to monzogranite, although the Fawakhir Granitoid includes a significantly large body of monzodiorite;
B all syenogranites to monzogranites have enclaves of similar composition to those found in monzodiorite, although the percentage is significantly higher in the Fawakhir than in the Urn Had and Urn Effein Plutons. Monzodiorites have much fewer enclaves, and then only in the more silicic representatives. The enclaves are much more likely to be found at pluton margins.
iiil chilled contacts are present in all except the Fawakhir Granitoid; and
iv) all the plutons have low pegmatite content (also noted by Rogers et al., 1978 for northeast African post-tectonic pink granitoids). The Urn Had Pluton is unique amongst the three plutons in showing evidence of wall rock contami- nation of magma.
STRUCTURAL FEATURES
The following structural features are common to the plutons:
$ the two elongate plutons (Fawakhir and Urn Effein Granitoids) have their long axes parallel to the main tectonic trend;
It) no syenogranite shows any evidence of tectonic foliation. Foliations related to shearing along intruded faults are found only in the earliest Fawakhir mela- diorite and hornblendite;
i@) apart from faulting, modest rotation of wall rocks outwards from the plutons and minor block engulf- ment, there is minimal evidence for wall rock dis- turbance as a result of intrusion;
iv) all of the plutons emphasise gentle outward- dipping contacts against their wall rocks (also noted by El-Gaby et al., 1988a);
v) all have sub-horizontal marginal dykes, or like the monzodiorite of the Fawakhir, are sub-horizontal tabu- lar themselves in form;
u3 all show modification of the sub-horizontal contact orientations to take advantage of available sub-hori- zontal wall rock structures, e.g. foliations and faults;
~7) when the wall rock foliation is steep, all adopt either or both foliation discordant sub-horizontal form, and to a lesser degree, foliation concordant shape; and
v@) all show very low wall rock xenolith content. On the basis of these emplacement features, it ap- pears that the intrusions were accommodated with minimal forceful intrusion. El-Bouseily et a/. (1986) commented that the Fawakhir Granitoid was pas- sively emplaced. A late tectonic timing of intrusion of the monzodiorite of the Fawakhir Granitoid is sug- gested by its association with active thrusts. Akaad and Noweir (I 969) regarded the Urn Had Granitoid as late erogenic. Kamal El-Din et a/. (I 996) described the Fawakhir and Urn Had Granitoids as late tectonic. Loizenbauer and Neumayr (1996) interpret the Najd strike-slip fault activity as affecting the Fawakhir Granitoid.
DISCUSSION Emplacement mechanism for the Fawakhir, Urn Had and Urn Effein Granitokis Access of magma to the level of emplacement for these plutons was probably via moderately dipping faults and shear zones. The Fawakhir monzodiirite (Fig. 12a) was emplaced during the late stages of southwest-dipping thrust faulting, with congealed early magma pulses being brittly disrupted and incorporated as fragments in the following pulse. The later Fawakhir syenogranite and monzonite is interpreted to have intruded as a flat-lying sheet, which accommodated further magma by lifting its roof along marginal faults (Fig, 12b-c). The magma filling the Urn Had (Fig. 12d-f) and Urn Effein (Fig. 12g- i) Plutons apparently took advantage of the domed myionite zone, which separates gneissic metasediments
Journal of African Earth Sciences 79
T. J. FOWCER
Figure 10. Outcrop features for the
Urn Effein Granite. (al Gently out-
ward dipping contact of the syeno-
granite against amphibolites from the
southern margin of the pluton. IbJ
Northeasterly-dipping felsite dykes
intruded along m ylonitised felsite
from the eastern margin of the pluton.
Syenogranite contact is concordant
to the mylonitic foliation. IcJ Large
block of silicic metavolcanics en-
gulfed b y s yenogranite at the western
margin of the pluton.
80 Journalof African Earth Sciences
Pan-African granite emplacement mechanisms in the Eastern Desert, Egypt
-
Figure 7 1. Detailed structure at selected locations along the margin of the Urn Effein Granite. The block diagrams show the orientation of the wall rock foliation (ruled lines) and the relationship to these foliations of the Urn Effein intrusive contact and marginal dykes (shaded areas). The front face of each block is vertical, east-west; while the side face is vertical, north-south.
from younger lower grade units. The Urn Had Granitoid has adopted a phacolith shape controlled by the curvature of the domed mylonite zone. The Urn Effein Granitoid shows strong control of its eastern margin by felsite mylonitic foliations, which are parallel to the dome- enveloping mylonites. The magma for the Urn Effein is interpreted to have ascended along this sheared boundary (Fig. 129).
Lateral growth of the plutons was achieved via low angle magma tongues penetrating the wall rocks and prying apart any available low angle structures, otherwise by cutting across them in order to maintain a low-dipping tabular shape (Fig. 12a, e, h).
Controls on the emplacement of flat-lying tabular intrusive bodies
Unfortunately, the gently dipping tabular form perse, which represents a link between the intrusion styles
of the three studied granites, is equivocal in the matter of tectonic environment. Mudge 11968) argued that sills and laccoliths intrude into anorogenic settings, while Anderson (195 I) and Gretener (1969) considered that these intrusions must indicate horizontal compression. Hutton and Ingram (I 992) described sill emplacement in an active thrust zone. More recently, thin flat-lying tabular granitic plutons have been described from extensional terrains (Vigneresse, 1995a, b; Ameglio et al., 1997; Vigneresse et al., 1999).
Apart from regional stresses, pre-existing partings may have assisted the intrusion of these CED granitoid intrusions, e.g. the domed shear zone intruded by the Urn Had Granitoid and the thrusts intruded by the Fawakhir monzodiorite. The ascent of the magmas may also have been controlled by neutral buoyancy mechanisms, since this would explain the similar emplacement levels of the three plutons. However,
Journal of African Earth Sciences 8 1
T.J. FOWLER
a b
Figure 12. Schematics showing the interpreted mechanisms of intrusion of the Urn Had area plutons. Details are discussed in the text. Fine stipple represents the monzodiorite of the Fa wakhir Granite. Coarser stipple represents the syenogranite of all the plutons. The approximate present-day level of erosion is shown in (c), (f) and (i). (a-c) Fawakhir Granite; (cl is based on Fig. 4, cross-section B-B’. S: Serpentinite; B: metabasalt. (d-f) Urn Had Granite; If) is based on Fig. 6, cross-section A- A’. G: Gneissic metasediments; H: Hammamat Group. (g-iJ Urn Effein Granite; (il is based on the Fig. 9, cross-section A- A’. A: Amphibolite; V: silicic metavolcanics; F: felsite.
the Fawakhir Granitoid lies at the same crustal level
as the Urn Effein, despite the former lying within
serpentinites and the latter within felsites.
Stress controls on the intrusion of the Urn Had area
plutons The only mechanism, which seems prospective in
explaining the shape and level of emplacement of the Urn Had area plutons, is stress controls. Price and
Cosgrove (1990) argued that at the level where a rising dyke spreads out to form a sill in a flat-lying
anisotropy, the difference in vertical and horizontal
stress (ov-o,) is less than the difference between tensile strengths tested parallel and normal to the
anisotropy (TjI -T-L). They also concluded that TII -TI
is unlikely to exceed 100 bars (IO MPa) and is probably
much less in intact rock. Under normal lithostatic (anorogenic) conditions o” = pgz and oh = oJv/l -v) where p: bulk rock density; g: gravity constant; z:
depth; v: Poisson’s constant. Using p = 2.65-2.75 g
cm-3 and v =0.15-0.28 as typical values for rock
types of the Urn Had area (Lama and Vutukuri, 19781, it may be seen that under lithostatic (anorogenic)
conditions the maximum estimated depth of intrusion
of the three granites would be about 650 m (Fig. 13a).
If the rocks, already broken by faults or joints, are
being referred to, then (TII -T-l_) could be higher, allowing sill intrusion to depths of about 1.25 km.
The Fawakhir, Urn Had and Urn Effein Plutons are
indeed epizonal on the basis of their intrusion into
anchizonal metamorphosed Hammamat Group
(Osman et al., 1993; Warr et al., 1996) and sup- ported by the fact that they have narrow metamorphic
aureoles, common hypersolvus and occasional
miarolytic textures (Greenberg, 1981). However, the depths calculated above for anorogenic intrusion of
the Urn Had area plutons seem too shallow since the intrusions already range over a depth of several
hundred metres and PHZO estimates by Rogers and
Greenberg (1983) for plutons such as the Urn Had
82 Journal of African Earth Sciences
Pan-African granite emplacement mechanisms in the Eastern Desert, Egypt
a Uz - uf-~ (MPa)
Figure 73. (al Calculated variation in the stress difference (oz-o,) with depth, using density and Poisson’s constants (14 appropriate for the lithologies of the Urn Had area for lithostatic (anorogenicl conditions (or: vertical stress; a,: horizontal stress). Rock densities of 2.65 and 2.75 g cm- 3 were used. For each v value, the linear relationship between depth and stress difference is represented as a shaded bar. The upper boundary of the bar pertains to a rock density of 2.65 g cm-3, while the lower boundary represents calculations using a rock density of 2.75 g cmm3. The maximum depth at which oz-oh is 110 MPa is 650 m. (bl Calculated variation in o,, 9. and oh_ with depth using density and v appropriate for the lithologies of the Urn Had area for compressive tectonic setting. a,,.: Maximum compressive horizontal stress; o,,_: minimum compressive horizontal stress. Applied tectonic stresses parallel to oh, and a,,_ are 50 and 125 MPa, respectively. For 9, the upper boundary of the shaded bar relates to a rock density of 2.75 g cm-? while the lower boundary represents calculations using a rock density of 2.65 g cm-3. The differences in calculated values of oh, and oh_ for densities from 2.65 to 2.75 g cm-3 are small enough to be neglected in this figure. The maximum depth, where q is the minimum stress, is about 3.3 km.
area granitoids vary from 1.5 to 2.5 kbars at the time of crystallisation.
The condition of 0,-o,, being small may also occur in a compression al terrane. Brisbin (I 986) considered how the shape of pegmatitic intrusions changes with increasing depth under directed stress conditions. If only one horizontal stress were tectonically loaded, it may exceed ov, although the other horizontal stress will fall short of o, and vertical dyking will be favoured except in the top 100 m or so of the surface. If tec- tonic compressional stresses are applied to both hori- zontal principle stresses, o,,, and o,,_ (where o,,, > o,J, then it is possible that both of these stresses will exceed sv over considerable depths, allowing flat-lying intrusive sheets to develop. Brisbin (1986) chose a value of 0.75 kbars (75 MPa) (o,,, = 1.25 kbars, o,. =0.5 kbars) and these values have been used in Fig. 13b to show the expected variation of o,, o,,. and oh. with depth for the Urn Had area. Above a ‘critical’ depth of about 3.3 km, oy is the minimum stress 03, and flat-lying tabular intrusions are favoured. Below this critical depth, vertical dyking normal to o,,” is expected. The maximum depth for flat-lying tabular intrusions of about 3-4 km seems realistic for the Urn Had area plutons.
Vigneresse eta/. (I 999) have explored the influence of the regional tectonic stress field on the shape and orientation of granitoid plutons. One category in their pluton shape classification is flat-floored tabular plutons, intruded at high crustal levels in brittle crust. They concluded that flat-floored plutons are fed by vertical dykes (therefore, regional o3 should be initially horizontal). Their model assumes that the rate of supply of magma in the dykes may be sufficient to allow magma pressure to increase the horizontal stresses. If the two horizontal principal stresses are initially o2 and o3 (with o, therefore vertical), the arrival of magma in the vertical dykes allows switching of o2 and o3 and an increase in their magnitude until they exceed the vertical principal stress, which then switches to 03. At this stage, a horizontal tabular intrusion forms. During the horizontal principal stress switching stage, the form of the dyke conduit becomes modified to a vertical cylinder. Vigneresse et a/. (I 999) suggest that a cylindrical conduit below a flat-floored tabular intrusion is an indicator of initial o, vertical, indicating an extensional or transtensional environment of intrusion. However, o, vertical also occurs in the situation of lithostatic loading (no applied tectonic stresses), which would be expected under post-tectonic conditions. Secondly, if no vertical cylindrical feeder is evident, and only the flat-lying tabular element of the intrusion is seen, (as is the case for the Urn Had area plutons), it is not clear from Vigneresse et a/.‘~ (I 999) analysis whether the
Journal of African Earth Sciences 83
T. J. FOWLER
plan shape of this tabular part should be a regional stress-state indicator. In fact, this seems unlikely since other factors, e.g. the geometry of sub-horizontal wall rock structures, control the shapes of flat-lying tabular intrusions.
There have been reports of sub-horizontal granitoid sheets intruding into active extensional low angle shear zones (Hutton, 1988; McCarthy and Thompson, 1988; Hutton et al., 1990; Antonellini and Cambray, 1992; Scaillet et a/. , 1995). However, the possibility of an extensional tectonic setting for the three late Pan-African plutons of this study is unlikely because of the following:
i) the granites do not show extensional shear deformation; and
iij they are intruded in the brittle crust not at the level of development of active extensional ductile shear zones.
The above discussion suggests that the Urn Had area plutons were not emplaced under conditions of tectonic extension or transtension and are likely to be anorogenic intrusives. Flat-lying tabular late Pan-African granite intrusions in the Eastern Desert and Sinai have been reported a number of times to have dyke-like feeders, not cylindrical ones (e.g. Stern et a/. , 1984). Another possibility is that o, was horizontal. However, the absence of significant deformation in the Urn Had and Urn Effein Plutons suggests that, if so, it would be remnant, i.e. follow active deformation but precede relaxation of the stress. The (earlier) monzodiorite phase of the Fawakhir Granite was intruded along and sheared by southwest-dipping thrust faults, which relate to an apparent northeast-southwest compressional event (Fowler and Osman, 1998). o,. (o,), in this interpretation, would trend approximately north- east-southwest during intrusion of this phase.
ACKNOWLEDGEMENTS
The author would like to extend his special thanks to Mr A.M. Badawi at Cairo University and Mr S.M. Mansour at the Fawakhir Resthouse for their invalu- able assistance in the commission of this project. This work was partly financed by a LaTrobe Uni- versity Central Starter Research Grant No. 8816 and derives from field work completed in co- operation with the Geology Department of Ain Shams University, Cairo, with the assistance of Prof. B. El-Kaliouby, Dr A.F. Osman and Dr. H. Dowidar.
The penetrating critical comments by two anony- mous reviewers on earlier versions of this paper were much appreciated. Editorial handling - G. J. H. Oliver and P. Bo wden
84 Journal of African Earth Sciences
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