Download Pan-African granite emplacement mechanisms in the Eastern Desert

Journal
Pergamon
of African
Pll:S0899-8382(00)80033-8
Earth Sciences,
Vol. 32, NO. 1, pp. 61-66.
2001
2001 Elsevier Science Ltd
All rights reserved. Printed in Great Britain
0899.5362/01
S- see front matter
0
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 preexisting 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
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 Panare typically round to elliptical or teardrop-shaped.
The Egyptian Eastern Desert has been described as
Journal of African Earth Sciences 6 1
T.J. FOWLER
They have been described previously as late to posttectonic, but there are widely differing interpretations
This is followed
with respect to the tectonic setting and stress state
of the continental lithosphere during their intrusion.
are used to determine:
Ragab et a/. (1989)
and Ragab (I 991) proposed that
these late Pan-African
monzogranites
to syenogranites 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
margins
by the structural
details of pluton
and wall rocks. The latter structural
data
i) the 3-D shape of the plutons;
ii) the mechanism(s)
role of pre-existing
of emplacement,
including the
wall rock structures
during in-
trusion; and
ii.!. the crustal stress state during intrusion.
occurring at the time of stress state change from
compression to extension. Greenberg (I 9811, Bentor
Geological setting of the Urn Had area
The three late Pan-African granite plutons are located
(19851, Stern et a/. (1984,
1988)
Gottfried (1986)
have suggested
the Urn Had area (Fig. I). Details of the structure of
setting
accompanied
African
granites
Pan-African
by dyking
and Stern and
an extensional
for the late Pan-
of the North Eastern Desert.
Late
in the western
part of the CED, referred to here as
the Urn Had area are summarised in Fowler and Osman
(1998). A brief account only is presented here.
granite intrusion into active Najd Shear
Zones has also been reported (Davies, 1982; Stern,
1985).
Fritz et al. (I 996) has described a transpressional 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
syenogranites.
monzogranites
Hassan and Hashad (1990)
more than one tectonic
setting
for these
to
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
propose
the southern
granites
gently southeast,
the northwest,
while
rounded closure of the dome plunges
although
its structural
details are
to
obscured by the later Urn Had Granite. The dome core
account
for trace element
pattern differences.
Therefore, it is important to establish (for each granite
consists of folded sheared slices of garnetiferous
amphibolites, mica-schists, paragneisses and minor
individually) the tectonic stress conditions accompanying intrusion in order that the radiometric dates
migmatites.
No age data on these rocks are yet
available. They have been interpreted variously as
on these granites can be used to properly delimit the
Pan-African tectonic events in the Eastern Desert.
or pre-Pan-African
(including
both compressional
The stress conditions
and extensional)
accompanying
intrusion may
high-grade
metamorphosed
Pan-African
basement
Krliner et a/. , 1988;
formations
(El-Gaby et al., 1984;
Neumayr
et al., 1996).
For the
clarify how intrusions at high crustal levels in brittle
latter, a metamorphic
rocks attain circular to elliptical shapes Wigneresse,
1995a; Vigneresse et a/., 1999).
suggested for nearby gneissic complexes (Meatiq
Complex: Sturchio et a/. , 1983) as it has for other
This paper investigates
the tectonic
conditions
and mechanisms
of intrusion for three late PanAfrican granite plutons: the Fawakhir, Urn Had and
Urn Effein
Granites
from the western
part of the
core complex model has been
gneissic domal structures in Pan-African
(e.g. the Damaran Orogen: Oliver, 1994).
North-northwestward
thrust-relatedstructures
Central Eastern Desert (CED) (Fig. 1). These plutons
dome
were chosen partly for their ready accessibility,
The dome core lithologies
partly because they are considered
the late Pan-African
and
to be typical of
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.
southeast-dipping
fold belts
in the
are cut by moderately
retrograde
shear zones, and are
pervaded by a retrograde foliation of the same orientation as the shear zones. Retrograde metamorphism
has reduced the high-grade gneisses to schists.
Both retrograde
foliations
and shear zones curve
After an introduction to the regional geology of the
Urn Had area, geochemical data on the plutons are
into and merge with the sheared dome margin. The
domed mylonite is the roof of an antiformal duplex
presented
structure, which formed during north-northwestward regional thrusting and was later folded about
for the purpose of accurately
naming the
various granite phases and for use in determining the
tectonic stage at which the granites were emplaced.
Figure 1. Geology of the Urn Had area,
Nubia
Sandstone
occupies
upright northwest-southeast-trending
Eastern Desert, Egypt. Locations
the unornamented
areas along Wadi Muweih.
62 Journal of African Earth Sciences
for Figs 4, 6 and 9 maps
are shown
folds. Fowler
as insets.
The
Pan-African
granite emplacement
mechanisms
in the Eastern Desert, Egypt
G:..
\
Fig. 6
012345
\i
Gneisses,,
migmatites
Arc metavolcanics
lmafic)
Primary layering
trends
Mylonitic
and metasediments
and schistore
Ophiolitic
melange
fundifferentiatedl
Hammamat
Group (greywackes)
Hammamat
Group (conglomerates)
Hammamat
Group [siltstones)
Dokhan
Volcanics
Post-Hammamat
Felsites
(intrusive)
shear zones
Younger
Granites
(undifferentiated)
Journal of African Earth Sciences 63
T.J. FOWLER
and Osman (1998)
phic core complex
sheathed and shear
evidence being that
have argued against a metamorinterpretation of these mylonite
dissected gneissic rocks, the main
the shears within, and branching
from, the mylonite sheath show consistent thrusting
kinematics. The north-northwestward
thrusting event
post-dates
Hammamat
the deposition
of late Pan-African
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
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.
must also predate the Urn Had area granites with RbSr ages ranging from 570-590
Ma, which
these
very
thrusts
and show
only
intrude
minor
brittle
Najd faulting and related structures
A sinistral transcurrent deformation
regime, identified
deformation.
as Najd fault-related,
low-grade
grade metavolcanics and metasediments east of the
dome in Wadi Atalla. The Najd event produced steeply
metamorphic
north westward
cover units thrust north-
over the dome
the low-
dipping, ductile, sinistral transcurrent shears of notth-
Surrounding the dome are low-grade
and metasedimentary
has mainly affected
metavolcanics
cover rocks, which were em-
placed over gneissic dome core rocks during the north-
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
northwestward
thrusting event described above.
These Pan-African units are represented by serpen-
without evincing deformation.
tinites, metabasalts
and metagabbros
of ophiolitic
melange origin: conglomerates,
greywackes
and
Late Pan-African
pelites (Hammamat
equivalents scattered throughout the Egyptian Eastern
Group), and silicic to basic calc-
alkaline volcanics
The
large
(Dokhan
ophiolitic
Volcanics).
mass
Desert
surrounding
granitoid intrusions
These include the Urn Had area granitoids and their
and emplaced
during
Pan-African
times
the
throughout the African Craton and Arabian Peninsula.
Fawakhir Pluton (Figs 1 and 4) consists dominantly
of serpentinite
melange, which has a gently dip-
The widespread occurrence of these late- to posterogenic talc-alkaline to transitional A-type granitoids
ping foliation.
Group conglomer-
was interpreted by Black and Liegeois (1993)
southeast-dipping
due to delamination
The Hammamat
ates have crude anastomosing
cleavages with southeast-trending
stretched pebble
lineations. The Dokhan Volcanics show localised
northwest-striking
mylonite
ages in the Hammamat
foliations.
Group
mylonites in the Dokhan Volcanics
related to the north-northwestward
Other
mylonitic
apparently
foliations
related
The cleav-
and some
in the
to Najd faulting
of the
are kinematically
thrusting event.
latter
unit are
of the continental
as being
lithospheric
mantle following the tectonic collisions leading to the
assembly
of Gondwana.
These granitoids
are
characterised
elongate
by unfoliated,
epizonal
small, nearly circular to
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).
as mentioned
below.
OF THE UM HAD
AREA GFtANlTOlDS
GEOCHEMISTRY
Northeast-dipping
and south west-dipping later thrusts
and related folding
The multicationic scheme of de la Roche et a/. (1980)
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
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
syenogranite fields. The Urn Effein Granitoid
and
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 Granite includes an early grey-coloured mafic phase that
2000
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_-‘______-----~---------_________
GRANITE
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----_-_____-____
I
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1000
o
,
1
I
Rl
3000
2000
Fawakhir Ganite
s Urn Had Granite
A
Fawakhir
Fawakhir
Urn Effein Granite
@ (Rogers
and Greenberg,
b
\
1983)
\
0 (Noweir ef a/., 1990)
Mantle
\
Fractionates
\
Urn Had
Urn Had
+ (El-Gaby,
1975)
\
+ (Noweir et al., 1990)
\
\
Urn Effein
\
\
1000
.
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
appearance
by a pink granite
phase similar
in
to the Urn Had and Urn Effein Granites.
meeting point of these latter two fields and the field
of syn-collision
granites. Again, no data falls in the
occupy the field of
in the syenodiorite
ORG field. Pearce et al. (1984) noted that postcollision granites are not well distinguished
on these
field. The later pink-coloured
phase of the Fawakhir
varies from quartz monzonite
and granodiorite
to
monzogranite.
With the exception of the monzodiorite
diagrams, apart from plotting in all fields except ORG,
and lying near the top of the VAG field. Variable
The earlier mafic phase analyses
monzodiorite
with one analysis
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
mixtures of mantle- and crust-derived magmas in postcollisional
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
data as suggesting
a post-collisional
CED granitoids.
this Nb, Y, Rb
setting for the
geochemical
signatures, the same granitoid chemical
data was plotted on a multicationic
diagram modified
by Batchelor
and Bowden
(1985)
THE FAWAKHIR
(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
As noted
lie dominantly
minor meladiorites
in the late erogenic
and syn-collision
above,
sharp contact
leucogranites)
are briefly
and Bowden
(I 982).
The
earlier monzodiorite
phase of the Fawakhir
Granite
lies in the post-collision
uplift field or group 3, a highpotassic
talc-alkaline
Batchelor
and
group.
Bowden
(1985)
explained
the
Granitoid
and hornblendites)
phase. The petrography
described,
their margins
and structure
followed
and wall rocks.
gressive
stages
of fractionation
fractionates
melts.
correlate
the following
whole
rock Rb-Sr ages: 574 Ma (Rogers
producing
position.
The
hybrids
hybrids
then
with
tec-
of intermediate
com-
experienced
in situ
fractionation
to produce the observed
series compositions. The series converge on the restricted area
of anatectic
this model,
granite
compositions.
On the basis of
the Urn Had and Urn Effein
and to a lesser
represent rather
degree the Fawakhir
strongly fractionated
Ma (Fullagar and
Ma (El-Bouseily et
The pro-
tonic stages. At each tectonic stage, the fractionated
source magma mixes with melts from felsic crustal
material
of
radiometric
ages for the Fawakhir
intrusion
are unclear about
which phase was dated. Age determinations
include
a/., 1986).
progressively
derivative
of these phases
Reported
Al basaltic
Na, K-richer
with
monzogranite
by the structure
and Greenberg,
1983);
586 *9
Greenberg,
1978); and 565-590
composition)
intruded
by a larger pink mainly
compositional
trends on these multicationic
diagrams
in terms of a model where a source magma (of high
to produce
(Fig. 4) is
composed of two compositionally
contrasting granitic
phases: an earlier grey monzodiorite
phase (including
granitoid
associations
identified
as group 4 (subalkaline monzonitic)
and group 6 (anatectic two-mica
by Lameyre
GRANITOID
the Fawakhir
Granitoids,
later phase,
late-collision
The Fawakhir monzdiorite
This phase is mainly
represented
by grey coarse-
to
medium-grained
hornblende + biotite, quartz monzodiorite, which locally becomes plagiophyric
and finegrained
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,
to post-collision
granitoids, whereas the earlier dioritic
phase of the Fawakhir would belong to the previous
minor outcrops of hornblende mdadiorite and homblendite
are also found, which show signs of intense brecciation
tectonic
and minor ductile
deformation.
The meladiorite/horn-
blendite
incorporated
as breccia
stage
fractionation
of the
having
cycle
with
relatively
little
occurred.
The range of Nb and Y values for the three
granitoids
is shown in Fig. 3a. The data for each of
is locally
within monzodiorite,
the granitoids
extends
into both fields of syn-collision/
represents
meladiorite
volcanic-arc
granites
(SYN-COL/VAG)
contact
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
’
fragments
and on the basis of this evidence
it
a distinct
intrusive
phase. Hornblende
is also found locally along the western
of the pluton,
wa in Wadi Hammamat
e.g. near the El-Fawakhir
Qah-
(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
---
Urn Had Granite
-----
Urn Effein
Fawakhir
0
(Rogers
Urn Had
l
(El-Gaby,
Granite
Granite
19t
and Greenberg,
1975)
1000 r
SYN-COLG
b
100
10
ORG
1
100
10
1000
Nb+Y
Figure 3. Pearce et al. ‘s (1984J
granitoid
/al Nb versus Y. (bl Nb + Y versus Rb.
tectonic
environmental
discrimination
diagrams.
orthoclase phenocrysts. These phases contain typitally c 1% by total surface area of ovoid fine-grained
quartz veins, particularly in the vicinity of the El-Sid
and El-Fawakhir Gold Mines. The geological relations
porphyritic
microdiorite
and petrology of these veins and fractures have been
analysed and reported by El-Bouseily et al. (I 987)
mafic igneous
enclaves
of biotite
composition (Fig. 5e), some appearing
partly assimilated. There is generally intense jointing
with silicified or sericitised joint surfaces parallel to
and Loizenbauer and Neumayr (I 996). Also numerous
minor faults,
cross-cutting
the Fawakhir
Journalof
Granitoid,
African Eatth Sciences
67
T.J. FOWLER
+
+:
A
FAWAKEIR
GRANITE
oPliIoLITIc haLANGE
serpendnite
:phase
DoKE4NVOLCANICS
HAMMAMATGROUP
meta-almites.
conglomerates
Felsite (intrusive)
Shear Zone
Siicic dgke
/
high-angle
primary contacts
&&tins.
inmsive
rn hnmhlmdifc
dvke
Q (dip,dipdirection)
_,
gz%IiPI
fault
r/
sillcic dvke
7
(strike
J&l
foliatiott (ii
133
ophiolites,
strike& dip)
i
&dip)
mike & dip)
synform,
atttuorm
+++++++
+++++++
+++++++
++++++++
+
$ ++++++++++_,
-?TT++++++.
El-Fmakbirqahwa
(roadsidecaf6)
* El-Fawakhir Resthouse
(J%SidGold Mine site)
r++++++++++,
a_++++++++++
++++++++++
++++++++++
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
--a--
I
-.
,
“_
,,,,,,,,,
IF,,
.,....
_I_.
..,,
I.“_”
,....
-.m,“““.,“..”
I,,,,,.
,a,
“.
I,,_
.
r”,“,ll
..”
I,,,
.I,_
_.I
I,,,
,,..
c
“.
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 monzogranite. Aplite forms gently dipping dykes but is uncommon, and pegmatite is virtually absent. Thick white
micro-granite sills apparently preceded the emplacement 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 serpentinite foliation (Fig. 5b) and discrete low angle shear
zones (Fig. 5~). However, where the serpentinite foliation dips steeply, the monzodiorite intrudes discordantly as a gently dipping sheet, e.g. along the southem 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 represented in the northeast part of the Fawakhir Pluton
where this phase forms extensive thick intrusive tongues and sheets with undulating tops (Fig. 4, section
A-A’). In the latter area, the serpentinite and metabasalt form roof inliers (Fig. 5d), which the monzodiorite 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 particularly favoured intrusion along the gently northwestto 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 apparently preceded diorite emplacement, also adopted
shear zone concordant shapes and are always intensely 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
long) and abundance of dark ovoid fine-grained igneous
thrusting event in the region. In Wadi Hammamat near
Wadi Atalla, irregular veinlets of hornblende mela-
enclaves
diorite intrude the metabasalts.
is no chilling of the syenogranite
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 serpentinite 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
posures
level of the eastern
of the monzodiorite
and western
suggests
ex-
that it was
increases
towards
these contacts.
There
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 QiftQuseir Road, the syenogranite has incorporated submetre scale blocks of monzodiorite and serpentinite.
Syenogranite
has apparently intruded completely into
crystallised monzodiorite.
continuous between these exposures before interruption by the later Fawakhir Syenogranite. If so, the
intrusive form of monzodiorite was a flat-lying sheet,
THE UM HAD GRANITOID
or set of sheets, with local contacts taking advantage
of sub-horizontal foliations and shear zones.
The Urn Had Granitoid (Fig. 6) adopts an approximately
circular outline measuring 10 km in diameter. The
Contact between the Fawakhir syenogranite and wall
rocks
western, southern and eastern margins. The western
The syenogranite describes a southward tapering mass
metasediments,
intruded through the monzodiorite. There is no obvious
hornfelsed zone or reaction zone between the syeno-
reaching hornblende-facies
granite and the serpentinite or metabasalt
teration and veins containing epidote and quartz along
boundary
wall rocks.
of the pluton
is sharply
radially outwards
grade extending
from the contact.
frequent examples of dark ellipsoidal igneous enclaves
Volcanics,
reaching a metre in length. These are most common
along the western margin of the pluton (Fig. 5e). At the
morphic effects.
The central and northern
northern contact, the syenogranite
occupied
by weakly
is locally steep (70-80”)
syenogranite
penetrating
against the metabasalts but in the same locality is seen
sediments
the Urn Had Pluton
gently dipping segments.
metasediments
with alternating
steep and gently dipping fault-related segments (Fig. 5f).
alThe
meta-
parts of the pluton are
gneissic
quartzofeldspathic
with numerous
to be stepped and composed of alternating steep and
the granite margin is again stepped
are common.
which show more subtle contact
metasediments,
Along the western contact,
1.5 km
Hydrothermal
eastern contact is shared with highly sheared Dokhan
40” northwards beneath the serpentinites. The contact
contacts
Group
which show a spotted hornfels zone
joints and faults in the hornfels
in the northeastern
at its
and southern margins lie against Hammamat
There are very few examples of wall rock xenoliths in
the syenogranite. Near the contact, however, there are
dips at about 20-
defined
small intrusions of
them (Fig. 6). These meta-
are continuous
with similar units north of
(El-Gaby
et a/.,
1988a).
The
have been referred to as a ‘roof’ of
the pluton (e.g. Kamal El-Din eta/.,
1996).
However,
The western contact is typically moderately to steeply
the mass of the Urn Had Pluton surrounding these
metasediments
lies structurally above these units,
dipping (Fig. 4, sections B-B’ and C-C’), although granite
having intruded between the metasediments
and the
dykes extending from the contact
like geometry related to faulting.
overthrust
below.
also show a step-
Hammamat
Group, as described
The metasediments, therefore, constitute a wall rock
core of the pluton, which is intensively intruded by
Contact between the Fawakhir syenogranite and
monzodiorite
smaller Urn Had Granitoid bodies. The northern limit
The contact between
Wadi Urn Sheqila (Fig. 6, sections A-A‘
monzodiorite
the Fawakhir syenogranite
is sharp, and the size (up to 0.75
and
m
of these subsidiary intrusions lies approximately
along
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.
Journalof African Earth Sciences 7 1
T.J. FOWLER
Figure 6. Map of the Urn Had Granite
Urn Had
Granite
intrusion,
of the
ffor location reference see Fig. I). The arcuate broken
sheared
boundary
between
the core metasedimentary
line is the approximate
location, before
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).
to mark the northern
A subsurface
northwards
suggested
history
This wadi is taken
extension
of the
Urn Had Pluton
towards the Urn Effein Granitoid has been
in several articles on the metamorphic
of this area (Noweir
Kamal El-Din et a/., 1996).
of the Urn Had Granitoid
and El-Sharkawi,
The subsurface
was invoked
1988;
extension
to explain
(1998)
have pointed
and cordierite
limit of the Urn Had Pluton.
the
shear zone have been deformed
retrograde
foliation
dykes
have
foliation.
cordierite
tisation
south of it where
of metamorphic
morphism
higher
pressure
staurolite,
staurolite,
assemblages,
and shows
these
metasediments
features
including
migmatisation.
are also found
of the pluton.
Fowler
garnet
Apart
and
(1988a)
from
in their
aureole
in the core
aureole,
the latter
the
by El-Gaby
including
meta-
Urn
(I 994)
El-Gaby
between
and the earlier
rocks of the pluton.
of
following
distinction
contact
metamorphism
by contact
intrusion
et al. (1988b)
this
and the migma-
regional
This was stated
from
and Osman
phases,
been caused
the
along
no doubt that the garnet,
to an earlier
having
by the
the Urn Had Granitic
post-kinematically
and sillimanite
than
Granitoid.
El-Gaby
intruded
relate
rather
is independent
of the shape of the pluton. In addition,
this northern metamorphic
aureole has significantly
and that
There is, therefore,
north of the pluton than
zones
sillimanite
in the Wadi Urn Sheqila
retrograde
broader belt of metamorphism
the pattern
out that the garnet,
porphyroblasts
Had
and
et al.
the Urn Had
Urn Sheqila
the garnetiferous
thermal
core
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 porphyroblasts.
As with the Fawakhir Granitoid, the Urn Had
Granitoid consists of two intrusive phases: an earlier grey phase consisting of diorite to tonalite in
composition, intruded by a more voluminous monzogranite 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 restricted 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 metasediment gneissosity (Fig. 6, section C-C’). In both
cases, it is intruded by pink syenogranite to monzogranite.
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 mediumgrained 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 intrusions 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 preor syn-intrusion faults along which the granite has
penetrated. The evidence for the existence of these
faults during intrusion includes pegmatitic sheets
parallelto 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 inwards. 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
parallelto 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 numerous 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 metasediments 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 lowlying 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 plastically 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, kilometrescale 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 metavolcanics 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 flowaligned 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 syenogranite. 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 thesoutheastern 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 ZOOdips 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’, 1Ob 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.
Journalof African EarthSciences 77
A
Figure 9. Map,
contacts
legend
dip outwards
and
cross-section
generally
at low
of the
Urn Effein
to moderate
angles.
Granite
(for location
reference
see
Fig.
I!.
Intrusive
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 1Oc and 11). On this western side
of the pluton, stepped low-lying sheets and steep
marginal dykes are common (Fig. 11). The southwestern contact has dips as low as IO” and is discordant
to the weak retrograde foliation in the amphibolites
(Fig. 1Oa).
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 contamination 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 meladiorite and hornblendite;
i@)apart from faulting, modest rotation of wall rocks
outwards from the plutons and minor block engulfment, there is minimal evidence for wall rock disturbance as a result of intrusion;
iv) all of the plutons emphasise gentle outwarddipping 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 tabular themselves in form;
u3 all show modification of the sub-horizontal contact
orientations to take advantage of available sub-horizontal 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 appears that the intrusions were accommodated with
minimal forceful intrusion. El-Bouseily et a/. (1986)
commented that the Fawakhir Granitoid was passively emplaced. A late tectonic timing of intrusion of
the monzodiorite of the Fawakhir Granitoid is suggested 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 congealedearly 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. 12gi) 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
Urn Effein
Granite.
ward
dipping
granite
southern
against
features
(al
contact
out-
of the syeno-
amphibolites
margin
for the
Gently
of the
from the
pluton.
Northeasterly-dipping
felsite
intruded
along m ylonitised
IbJ
dykes
felsite
from the eastern margin of the pluton.
Syenogranite
contact is concordant
to the mylonitic
foliation.
IcJ Large
block of silicic
metavolcanics
engulfed b y s yenogranite at the western
margin
80 Journalof African Earth Sciences
of the pluton.
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 phacolithshape controlledby 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 domeenveloping 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
intrusive bodies
of flat-lying
tabular
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,
Journalof African EarthSciences8 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 AA’. G: Gneissic metasediments; H: Hammamat Group. (g-iJ Urn Effein Granite; (il is based on the Fig. 9, cross-section AA’. A: Amphibolite; V: silicic metavolcanics; F: felsite.
the Fawakhir
Granitoid lies at the same crustal level
as the Urn Effein,
serpentinites
despite
the former
lying within
and the latter within felsites.
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
Stress controls on the intrusion of the Urn Had area
of the three granites would be about 650 m (Fig. 13a).
plutons
The only mechanism,
If the rocks, already broken by faults or joints, are
which seems prospective
in
explaining the shape and level of emplacement of the
Urn Had area plutons, is stress controls. Price and
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
Cosgrove (1990) argued that at the level where a
rising dyke spreads out to form a sill in a flat-lying
indeed epizonal
anisotropy,
(Osman et al., 1993; Warr et al., 1996) and supported by the fact that they have narrow metamorphic
the difference
in vertical and horizontal
stress (ov-o,) is less than the difference between
tensile strengths tested parallel and normal to the
anchizonal
aureoles,
on the basis of their intrusion into
metamorphosed
common
Hammamat
hypersolvus
Group
and occasional
is unlikely to exceed 100 bars (IO MPa) and is probably
miarolytic textures (Greenberg, 1981). However, the
depths calculated above for anorogenic intrusion of
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:
the Urn Had area plutons seem too shallow since the
intrusions already range over a depth of several
hundred metres and PHZOestimates by Rogers and
depth; v: Poisson’s constant.
Greenberg
anisotropy (TjI -T-L). They also concluded that TII -TI
82 Journal of African Earth Sciences
Using p = 2.65-2.75
g
(1983)
for plutons such as the Urn Had
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 cm3 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 tectonic compressional stresses are applied to both horizontal 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
Journalof African EarthSciences83
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 northeast-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 invaluable assistance in the commission of this project.
This work was partly financed by a LaTrobe University Central Starter Research Grant No. 8816
and derives from field work completed in cooperation 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 anonymous 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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