pan-african granite emplacement mechanisms in the eastern ...rjstern/egypt/pdfs... · or...

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 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

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 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 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 accompanying 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-northwestward 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


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 metamorphic 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

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‘. /’ , ---

I -/- -- -- _/, -

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‘. ,‘sYENOGABBRO,‘“ONZO ; /’

GABERO ./

/- ESSEXITE /*.

_-’ I I SYENODIORITE

. /’ r/C

/’ c’

-y: .’

, -. 0 .Jr

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, 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


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


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.



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 monzogranite. Aplite forms gently dipping dykes but is uncom- mon, 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 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 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 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 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


T.J. FOWLER



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 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 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.


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.



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


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 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 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 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 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.


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 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,


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 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 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 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,


____----

__--- ---___ -_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.


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.


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 southwestern 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 meladiorite 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 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 pas- sively 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 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


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


-

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,


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



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 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


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 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


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