Geochemistry of an island-arc plutonic suite: Wadi...

Pergamon Journal of African Earth Sciences, Vol. 24, No. 4, pp. 473-496. 1997

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Geochemistry of an island-arc plutonic suite: Wadi Dabr intrusive complex, Eastern Desert, Egypt

FAWZY F. ABU EL-ELA Geology Department, Faculty of Science, Assiut University, Assiut, Egypt

Abstract--The Wadi Dabr intrusive complex, west of Mersa-Alam, Eastern Desert, Egypt ranges in composition from gabbro to diorite, quartz diorite and tonalite. The gabbroic rocks include pyroxene-hornblende gabbro, hornblende gabbro, quartz-hornblende gabbro, metagabbro and amphibolite. Mineral chemistry data for the gabbroic rocks indicate that the composition of clinopyroxenes ranges from diopside to augite and the corresponding magma is equivalent to a volcanic-arc basalt. Plagioclase cores range from AnTs to An34 for the gabbroic varieties, except for the metagabbro which has Anl~.l 8. The brown amphiboles are primary phases and classified as calcic amphiboles, which range from tschermakitic hornblende to magnesiohornblende. Green hornblende and actinolite are secondary phases. Hornblende barometry and hornblende-plagioclase themometry for the gabbroic rocks estimate crystallisation conditions of 2-5 kb and 885-716°C. The intrusive rocks cover an extensive silica range (47.86-72.54 wt%) and do not exhibit simple straight-line variation on Harker diagrams for many elements (e.g. TiO 2, AI203, FeO*, MgO, CaO, P205, Cr, Ni, V, Sr, Zr and Y). Most of these elements exhibit two geochemical trends suggesting two magma sources. The gabbroic rocks are relatively enriched in large ion lithophile elements (K, Rb, Sr and Ba) and depleted in high field strength elements (Nb, Zr, Ti and Y) which suggest subduction-related magma. Rare earth element (REE) data demonstrate that the gabbroic rocks have a slight enrichment of light REE [(La/Yb)N= 2.67-3.91] and depletion of heavy REE [(Tb/Yb) N = 1.42-1.47], which suggest the parent magma was of relatively primitive mantle source. The diorites and tonalites are clearly calc-alkaline and have negative anomalies of Nb, Zr, and Y which also suggest subduction-related magma. They are related to continental trondhjemites in terms of Rb-Sr, K-Na-Ca, and to volcanic-arc granites in terms of Rb-(Y + Nb) and Nb-Y. The Wadi Dabr intrusive complex is analogous to intrusions emplaced in immature ensimatic island-arcs and represents a mixture of mantle (gabbroic rocks) and crustal fusion products (diorites and tonalites) modified by fractional processes.

R6sum6--Le complexe intrusif de Wadi Dabr, & I'ouest de Mersa-Alam, Ddsert Oriental, Egypte, est compos0 de gabbro, diorite, diorite quartzique et tonalite. Lee roches gabbrdfques comprennent un gabbro ~ pyrox~ne-hornblende, un gabbro hornblende, un gabbro quartzique ~ hornblende, un m(~tagabbro et une amphibolite. Les donn6es chimiques des min6raux des roches gabbrdiques montrent que les clinopyrox~nes varient de diopside ~ augite et que le magma correspondent est dquivalent & un basalte d'arc volcanique. Les coeurs de plagioclase 6voluent de An75

An34 pour les varidt6s gabbrdl'ques, sauf le mdtagabbro qui contient An~11 s. L'amphibole brune est une phase primaire calcique variant de hornblende tschermakitique ~ magndsio-hornblende. L'amphibole verte et I'actinote sont des phases secondaires. Le barom~,tre hornblende et le thermom(~tre hornblende- plagioclase sur les roches gabbro'iques donnent des conditions de cristallisation estimdes & 200-500 MPa et 885-716°C. Les roches intrusives montrent un grand intervalle de teneurs en silice (47.86-72.54% en poids d'oxydes) et ne montrent pas de simples variations lin6aires dane lee

Journal of African Earth Sciences 473

F. F. A B U EL-ELA

diagrammes de Harker (par exemple, TiO 2, AI203, FeO*, MgO, CaO, P205, Cr, Ni, V, Sr, Zr et Y). La plupart de ces 616ments montrent deux tendances g6ochimiques sugg6rant deux sources magmatiques. Les roches gabbro'='ques sont relativement enrichis en LILE (K, Rb, Sr et Ba) et appauvris en HFSE (Nb, Zr, Ti et Y) ce qui sugg~re un magma de subduction. Les donn6es de terres rares montrent que les roches gabro'iques ont un faible enrichissement en terres rares 16g~res [(La/Yb)N=2.67-3.91] et un appauvrissement en terres rares Iourdes [(Tb/Yb) N = 1.42-1.47], ce qui sugg6re un magma parent issu d'une source mantellique relativement primitive. Les diorites et les tonalites sont clairement calco-alcalines et pr~sentent des anomalies n6gatives en Nb, Zr, Y, ce qui sugg6re aussi un magma de subduction. Elles sont reli6es aux throndhj6mites continentales en termes de Rb-Sr, K-Na-Ca et aux granites d'arc volcaniques en termes de Rb-(Y + Nb) et Nb-Y. Le complexe intrusif de Wadi Dabr est analogue aux intrusions mises en place dans des arcs insulaires ensimatiques immatures et repr6sente un m61ange des produits de fusion de roches mantelliques (roches gabbro'iques) et crustales (diorites et tonalites), modif6s par des processus de fractionnement.

(Received 14 May 1996: revised version received 8 February 1997)

INTRODUCTION Although there are many reconnaissance studies of island-arc intrusive rocks (e.g. Kesler et al., 1977; Mason and McDonald, 1978; Hine and Mason, 1978), detailed studies of individual intrusions (e.g. Chivas, 1978; Perfit et al., 1980; Kay et al., 1983; Whalen, 1985) are relatively uncommon. An understanding of these rocks has implicat ions for the genesis of calc-alkaline volcanic rocks, which may represent extrusive equivalents. As is the case for calc-alkaline volcanic suites, knowledge of the characteristics of island-arc intrusive rocks has great ramifications for tectonic interpretations in older orogenic belts (e.g. Whalen and Currie, 1982).

Island-arc volcano-sedimentary rocks, which are composed of weakly metamorphosed calc-alkaline intermediate volcanics (mainly of andesites, dacites, volcaniclastics of comparable composition and subordinate basalts and rhyodacites), are already established and identified in the Egyptian basement (e.g. Stern, 1981; Abu EI-Ela, 1990, 1992; Abu EI- Ela and Hassan, 1992). However, the plutonism of the island-arc in the Egyptian basement is much less studied (e.g. AbdeI-Rahman, 1990; Abu EI-Ela, 1996). This paper addresses the question of how plutonic rocks in the Egyptian basement are related to the island-arc association using geology, mineral and whole-rock chemistry.

GEOLOGY The investigated Wadi Dabr intrusive complex is a gabbro-diorite-tonalite suite (Fig. 1) which was mapped as an epidiorite-complex (El Ramly and Akaad, 1960), as metagabbros and diorites (Akaad and Essawy, 1964; El Ramly, 1972) and

as a metagabbro-diorite complex (Hassan and Essawy, 1977). The gabbro-diorite rocks have been a f f ec ted by th ree p rocesses , v iz . hybridisation, post-magmatic (deuteric) alteration and regional metamorphism and were closely fo l l owed by emplacement of synorogen ic granites (Hassan and Essawy, op. cir.) .

The present study reveals that the gabbroic rocks include pyroxene-hornblende gabbro, hornblende gabbro, quartz-hornblende gabbro, metagabbro and amphiboli te. They are dark green, show marked variation in grain size, from fine to coarse grained, and vary in the ratio of mafic to felsic minerals. They are massive and devoid of any trace of igneous layering. The gabbro ic rocks in some places are h igh ly tectonised wi th developed minor folds and stretching lineations.

The dior i te and quartz dior i te are local ly foliated, cataclased and have a gneissic texture along Wadi Dabr, Wadi Um Markha and Wadi Thamili El Zarka. They contain relics of pegmatite and quartz veins which occur as pinch and swell textures. The coarse grained diorite, quartz diorite and appinitic diorite are intruded into the gabbro ic rocks. Xeno l i t hs of gabbro and amphibolite are enclosed within the dioritic rocks. The diorite and quartz diorite are dissected by veins and dyke-like bodies of muscovite granites (post-orogenic granites).

The tonal i tes form small masses and veins that invaded both gabbroic and diori t ic rocks. They show agmati te s t ructure and enclose d i f ferent shapes and sizes of f ine gabbro, coarse grained gabbro and quartz doler i te xenol i ths. Some tonal i te veins are cataclased and display augen texture.

474 Journal of African Earth Sciences

Geochemistry of an island-arc plutonic suite

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The ophiol i t ic melange, along Wadi Dabr (central and southwestern parts of the map area) and along Wadi Thamili EI-Hamra (western part of the map area), is composed of foliated slates and pebbly slates to pebbly metamudstones, with fragments of different sizes of ultramafic schists, serpentinite lenses, fine metagabbros, metabasalts, quartzites and metagreywackes. The contact between the ophiolitic melanges and the Wadi Dabr intrusive complex is intrusive, where the latter intruded along the foliation planes or the trace of bedding of the matrix of the ophiolitic melange, with injection of veins and veinlets of gabbroic and dioritic rocks. Along the contacts both the ophiolitic melange and the Wadi Dabr intrusive complex are folded into isoclinal minor folds.

The c o n t a c t b e t w e e n the is land-arc metavolcanics of Gabal Igla El Iswid and the Wadi Dabr intrusive complex is also intrusive, where the latter encloses xenoliths of the former (Abu EI-Ela, 1992).

The Wadi Dabr intrusive complex in turn is invaded by young gabbros (pos t - tec ton ic intrusive phases) along Wadi Dabr. The young gabbros compr ise ol iv ine gabbro, nor i te, gabbro and anorthosit ic gabbro. They caused a thermal metamorphic effect up to hornblende hornfels-facies.

In addition, the Wadi Dabr intrusive complex, young gabbros and ophiolitic melange are also invaded by late to post-orogenic granites. The contacts are sharp with no interaction with these rocks, which probably indicates that these granitic masses were emplaced at shallow depths.

PETROGRAPHY A brief petrographic description of Wadi Dabr intrusive complex is summarised under the following headings.

Gabbroic rocks Pyroxene-hornblende gabbros These rocks are fine to coarse grained, dark green in co lour and composed mainly of p lag ioc lase , c l i n o p y r o x e n e and b r o w n hornblende. Actinolite, chlorite, epidote, zoisite and sericite are secondary components. Ilmenite and apat i te are c o m m o n accesso r ies . Uralitisation and saussuritisation are common. Poikiolitic texture is characteristic.

Hornblende gabbros These are medium to coarse grained, dark grey- green in colour and composed mainly of

p lag ioc lase and b r o w n to b r o w n - g r e e n hornblende. Chlorite, epidote, zoisite, sericite, calcite and quartz are secondary minerals. Apat i te, sphene and i lmenite are common accessories. Clinopyroxene is lacking. Poikiolitic and equigranular textures are characteristic.

Quartz-hornblende gabbros These are similar to hornblende gabbros, except for the presence of subordinate amounts of quartz, and they are coarse to very coarse grained.

Metagabbros These rocks represent the product of low-grade regional metamorph ism of the previously mentioned rock types and are represented by the new mineral assemblage of albite, epidote, actinolite/chlorite, quartz and sphene. The fine grained metagabbros are schistose and the original texture can not be traced, whereas the coarse grained metagabbros show traces of the original poikilitic texture.

Amphibolites The amphibolites occur either as xenoliths within diorites and quartz diorites or as a local outcrop along Wadi Thamili EI-Hamra. Two main varieties are recorded, viz. hornblende amphibolites and bioti te-hornblende amphibolites. Hornblende amphibolites are fine grained, schistose and composed mainly of hornblende and plagioclase (An34). Chlor i te, ser ic i te and epidote are secondary minerals. Sphene apatite and Fe oxides are common accessories. Hornblende- biotite amRhibolites are similar to the previously mentioned amphibolites except for the presence of minor amounts of biotite and quartz.

Dioritic rocks Coarse-grained diorites They are composed mainly of plagioclase and hornblende. Locally, chlorite partially replaces hornblende and quartz is a minor constituent. Hypidiomorphic texture is characteristic, Apatite, zircon and Fe oxides are common accessories.

Gneissose diorites They are coarse grained and composed mainly of plagioclase, hornblende and minor biotite. Gneissic texture is characteristic. Apatite, zircon and iron oxides are common accessories.

Quartz diorites Texturally, three varieties of quartz diorites are distinguished, viz. hypidiomorphic quartz diorite, cataclased quartz diorite and gneissose quartz


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diorite. They are coarse grained and composed mainly of variable amounts of plagioclase and hornblende with subordinate biotite and quartz. Apat i te, zircon, sphene and Fe oxides are common accessories.

Tonalitic rocks Cataclased microtonalites These rocks occur as quartzo-feldspathic veins cross-cutting the gabbroic and dioritic rocks. They are fine grained and composed mainly of plagioclase and quartz with minor amounts of hornblende, chlorite and biotite. Zircon and apati te are accessor ies. Augen texture is characteristic of the plagioclase porphyroclasts.

Coarse grained tonalites The coarse gra ined tona l i tes are main ly composed of plagioclase and quartz w i th subordinate amounts of hornblende. Chlorite, biotite and microcline are minor constituents. Hypidiomorphic texture is common. Locally, some of these rocks are foliated. Zircon and apatite are accessories.

MINERALOGY AND MINERAL CHEMISTRY Analytical methods Microprobe analyses of the studied minerals were performed at the Institute of Earth Science, Utrecht University, The Netherlands, with a Jeol JXA 8600 superprobe and Tractor 5500 ED, using wavelength dispersive technique for Na, Cr, Mn and Fe, and energy d ispers ive spectrometer for Mg, AI, Si, K, Ca and Ti. Operating conditions were 20 kV acceleration voltage and 10 nA sample current. Matrix corrections were applied using ZAF program.

Plagioclase is an important phase in the gabbroic rocks of the Wadi Dabr intrusive complex. Chemical compositions and structural formulae of plagioclase are l isted in Table 1. Most plagioclase shows no significant optical zoning; for this reason only cores of plagioclase were analysed. The core composition is presumably representative of the original crystallisation and should provide the best indication of magma fractionation. Plagioclase core compositions in the gabbroic rocks range from An~s.9 [100 Ca/ (Ca + Na + K)] to An5o for pyroxene-hornblende gabbro, from An~. 4 to An384 for hornblende gabbro and from An380 to An34.s for quartz-hornblende gabbro. Metagabbros have An content ranging from An18.3 to Anlo.9.

Clinopyroxenes occur as relict cores within the calcic amphiboles, or as subhedral plates that

enclose poikiolitically subhedral prismatic crystals of plagioclase, and have their peripheries altered to actinolite. Chemical compositions and structural formulae of relict cores of clinopyroxene are listed in Table 2. The clinopyroxenes are classified as diopside and augite (Wo41.o: En438:Fs73 to Wo47 s: En4~ s: Fs12.8) according to the classification scheme of Poldervaart and Hess (1951 ). The conventional Si02-AI203 (Fig. 2) method of Le Bas (1962) is likely to be useful for slowly cooled igneous rocks (Coish and Taylor, 1979). The clinopyroxenes are located in the subalkaine field of Le Bas (1962). In the Ti + Cr versus Ti diagram (not shown), most of the clinopyroxenes plot in the volcanic arc basalt field of Leterrier et al. (1982). In addition, most of the ctinopyroxenes occupy the calc-alkali basalt field (Fig. 3) on the Ti-AlCt ~ discrimination diagram of Leterrier et al. (op. cit.).

Amphiboles are the dominant ferromagnesian minerals in the gabbroic rocks. They generally display wel l - formed crystal margins, good cleavages and uniform brown and brownish green colour. The brown hornblende occurs as subhedral plates that poikiol i t ically enclose plagioclase crystals, or as subhedral crystals containing relic cores of clinopyroxene. The brown hornblende is partially altered to green hornblende and chlorite.

The chemical composit ions and structural formulae of amphiboles are listed in Table 3. The amphiboles, according to the classification of Leake (1978) and Hammarstrom and Zen (1986), are calcic amphiboles (Fig. 4) belonging to the tschermak i te -ac t ino l i te series. The brown hornblende in the pyroxene-hornblende gabbros ranges from tschermak i t i c hornblende to magnes iohornb lende, whereas the brown hornblende in hornblende and quartz-hornblende gabbros is classified as magnesiohornblende. The gabbroic rocks contain subsolidus fibres of pale green actinolite replacing earlier clinopyroxene. Both subsolidus actinolite and metamorphic actinolite, associated with the metagabbros, fall in the actinolite field of the amphibole classification diagram (Fig. 4).

According to Leake's (1965) Si-Ti diagram, to differentiate between igneous and metamorphic amphiboles (Fig. 5), the brown hornblende in the gabbroic rocks falls within the igneous amphiboles f ield, whereas the green var iet ies (green hornblende and actinolite) fall within the field of metamorphic amphiboles. In addition, the Si-Ti diagram (Fig. 5) shows that the calcic hornblendes display decreasing Ti with increasing Si, which may suggest continual chemical change with magmatic differentiation.


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Figure 2. Si02-AI203 diagram o f LeBas (1962) for clinopyroxene relict cores of pyroxene-hornblende.gabbros.

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The hornblende barometry is based on the pressure sens i t iv i ty of total AI content in hornblende, where AI content increases with pressure. Hornblende crystallisation pressures for the gabbroic rocks have been calculated using the calibration of Hammarstrom and Zen (1986), Johnson and Rutherford (1989) and Schmidt (1992). The pressure estimates for the gabbroic rocks range f rom 2-5 kb (Table 4). A geothermometer has been devised (Blundy and Holland, 1990) based on the Al(iv) content of hornblende coexisting with plagioclase. Only those hornblendes that satisfied the igneous

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criteria were utilised for P-Tdeterminations (see Fig. 5). Eight hornblende-plagioclase pairs yield crystallisation temperatures ranging from 885- 716°C (Table 4).

Ilmenite is commonly found in hornblende gabbro and quartz-hornblende gabbro. Ilmenite analyses are listed in Table 5.

Chlorite is mostly after brown hornblende. It is characterised by a relatively high Fe/(Fe + Mg) ratio which ranges from 0.28 to 0.36 per formula unit and low Si which ranges from 5.571 to 5.425 per formula unit (Table 5) and is classified as ripidolite (Hey, 1954).

All epidote-group minerals are optically negative and defined as epidote. Compositionally, epidote is AI-rich (Table 5) and tool.% of pistacite (ps)= Fe/ (Fe+AI) is 0.12 which is characterist ic of metamorphic epidote. By comparison, Fe epidote


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Table 5. Representative microprobe analyses of ilmenite, chlorite, epidote and sphene from the gabbroic rocks of the Wadi Dabr intrusive complex

Rock types Sample No. SiO2 A!203 TiO2 Cr203 FeO MnO Mgo CaO Na20 K20 Total

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99.190 99.970 per 3 0

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0.000 0.000 0.000 0.000 1.011 1.013 0.001 0.000 0.919 0.917 0.059 0.057 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 1.989 1.987

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0.280 0.300 0.340 37.040 0.080 0.100 0.000 0.000

16.480 16.720 6.870 0.930 0.260 0.240 0.000 0.030

22.480 22.390 0.210 0.000 0.000 0.000 2 2 . 4 5 0 27.970 0.040 0.050 0.000 0.000 0.000 0.000 0.000 0.000;

88.050 89.050 97.480 99.170 i

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of primary magmatic textural features has been reported with composit ion between PSo21 to PSo.33 (Zen and Hammarstrom, 1984; Evans and Vance, 1987; Barth, 1990; Vyhnal et al., 1991 ).

Sphene is a widespread accessory mineral throughout the gabbroic rocks. It occurs as dusty aggregates, intimately associated with opaque phases, and as subhedral grains enclosed within actinolite and plagioclase. Sphene analyses (Table 5) are c o n s i s t e n t w i t h those of greenschist-facies rocks in terms of SiO~, AI203, TiO 2, FeO and CaO (Ernest, 1976).

WHOLE ROCK GEOCHEMISTRY Thirty nine representative samples of the Wadi Dabr gabbro-diorite-tonalite intrusive complex were analysed for their major elements on an ARL-34000 ICP emission spectrometer. Trace element analyses were carried out using the XRF automated Philips PW 14OO spectrometer at the Institute of Earth Sciences, State University of Utrecht, Holland. SiO 2 and LOI were determined using the wet-chemical methods of Shapiro

(1975) . Rare earth e lements (REE) were determined by INAA at IRI, Delft, using the established techniques (De Bruin, 1983).

Whole.rock compositions of typical rock types of the Wadi Dabr intrusive complex are shown in Table 6. The intrusive rocks cover an extensive silica range from 47.86 to 72.54 wt%. The main compositional trend of the studied intrusive rocks are shown on Harker variation diagrams (Fig. 6). Figure 6 shows that many elements do not have straight line variations (e.g. TiO 2, AI203, FeO*, P2Os, Cr, Ni, V, Zr, Y and St). This suggests two different compositional trends; one for the gabbroic rocks and the other for diorite- tonalite association. Therefore, two magma sources may be suggested. The compostional trend of the gabbroic rocks is characterised by an increase of TiO 2, AI203, Na20 + K20, P2Os, Zr, Y, Sr and Ba with increasing SiO 2, whereas FeO*, MgO, CaO, Cr, Ni and V decrease with increasing SiO 2. This trend is close to the calc- alkaline series except for TiO 2, AI203 and PzOs. The compositional trend of diorite and tonalite is characterised by a decrease of TiO 2, AI203,


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• AI 2 03 (wt %) O O

• o,,,,.~, % • °So"~Ip"°~ • ~ .., g O B "

A

I I I I I

2

0

8

6

4

2

0

I 1 I

O o O o

o • •

• t

I !

CaO(wt %)

/%/k

I I I I I

5

0

5

0

5

0 45

I I I I I

a TiO2(w t %)

X

X

o * * .t • • ~ ~

% I I I I I

55 65 45

Si 02(w t %)

0.4

0.3

0.2

0.1

0.0 75

! i ! I I

P205 (wt %)

O o • O

I I I I I

55 65

S i 02 (wt %)

75

Figure 6. (a) Plots o f major e lement contents versus SiO 2 for the Wadi Dabr intrusive complex.

FeO*, MgO, Ca•, P2Os, Cr, Ni, V and Sr with increasing SiO 2, whereas Na20+K20 and Ba increase with increasing SiO 2 (Fig. 6). This trend is identical to the calc-alkaline series. The decrease of Zr and Y contents with incresing SiO= in the diorites and tonalites (Fig. 6) suggest that these

rocks may be derived from an island-arc protolith rather than from an evolved continental crust, as discussed later.

A plot of the Wadi Dabr intrusive rocks on a Na20 + K20 versus SiO 2 diagram (Fig. 7) of Irvine and Baragar (1971) show that they fall within


Geochemistry o f an island-arc p lutonic suite

3 O0 ' ' ' , , 2501 ' ' ' ' '

x V ( p p m ) • , d k z~ A

2 0 0 ==' °o ~ z~ z~ o" 1501 ox

• " oOe ° • GI) • O

100 • • o ~o~oW * • a~ az~z~ z~ 501 e l , [b Z r ( p p m )

0 l l J i i 01 l t i , i

3 0 0 - , , , , , , , , , ,

N i ( p p m ) 4 0 15 x. Y ( p p m )

20 C 3 0 • '( _ o

• 20 =o%°1O8o • • A

10C • • ~ z~ • o ° • ~

• 10 • • ° " 4 ,.,,.x • " ~

0 o, ~ i ( , / %,,, , , , 0 , , , , ,

5 0 0 ' ' ' ' ' 5 0 0 ' ' ' ' '

• • C r ( p p m ) 4 0 0 O x o % • • z~

• "~i~ • 3 0 0 • 3 0 0 o • • ,%% • x Z~Z~ Z~ o| •

• 200 A • O 0 x O 0

I 0 0 % %o° • AA Z~ 100 S r ( p p m )

0 0 , , , , ,

45 55 65 75 6 0 0 ' ' , , ,

5 0 0

4 0 0

3 0 0

200

100

0

45

0 0

e • I n

55

Z~ Z~

Z~

• A • • A

B a ( p p m )

I I I

65 SiO 2 (wt %)

SiO2(wt%)

• Pyroxene-hornblende gobbros

o Hornblende gobbros

13 Quor tz -hornb lende gobbros

* Me~ogobbros

x Amphibol i tes

• D io r i tes

Z~ Tonal i t es

75

Figure 6. (b) Plots o f trace element contents versus SiO 2 for the Wadi Dabr intrusive complex.


F. F. ABU EL-ELA

8

o

0

J s

. t J

s

A l k a l i n e • •

I /

, , " S u b a l k a l i ne tJ I I I I I I

35 /.5 55 65 75

Z~

S i O 2 °/,,

Figure 7. N a 2 0 + K 2 0 versus SiO 2 plots of Irvine and Baragar (1971) for the Wadi Dabr intrusive complex. Symbols as for Fig. 6.

F

A v M

Figure 8. AFM diagram of Irivine and Baragar (1971) for the Wadi Dabr intrusive complex. Symbols as for Fig. 6.

the subalkaline field. On the AFM diagram (Fig. 8) of Irvine and Baragar (op. cit.), most of the rock units are calc-alkaline rather than tholeiitic and show only limited Fe enrichment.

Figure 9a and b shows hygromagmatophile element (HYGE) abundances for the Wadi Dabr intrusive rocks normalised to the primitive mantle abundances given by McDonough et aL (1985). These illustrate the characteristic enrichment of large ion lithophile elements (LILE: Sr, K, Rb, Ba) and depletion of high field strength elements

(HFSE: Nb, P, Zr, Ti, Y) in the gabbroic rocks. LILE and HFSE abundances show a steady increase from the least evolved pyroxene- hornblende gabbro to the most evolved quatrz- hornblende gabbro (Fig. 9b). Figure 9c and d show trace elements of the dioritic and tonalitic rocks normalised to ocean ridge granites abundances given by Pearce et al. (1984). These rocks are enriched in K, Rb and Ba and depleted in Nb, Zr and Y with respect to oceanic ridge granites (ORG). The patterns are very similar to those of

4 8 8 J o u r n a l o f A f r i c a n Ear th Sc iences


c- E}

4.n

E

E U3

100 -

a

10

• Pyroxene-hornblende gabbros

( 523A, 515A,503C, 522, 50z0 B, 2A, 504A, 525,

13A, 532,534A, 523C, 540,538 )

I I I I I I I I I

Bo Rb K Nb $r P Zr Ti Y

Figure 9. (a) Spidergram of trace element concentrations for pyroxene-hornblende gabbros normalised to primitive mantle after McDonough et ai. (1985). (b) Spidergram of trace element concentrations for hornblende gabbros and quartz-hornblende gabbros normalised to primitive mantle after McDonough et al. (1985). (c) Spidergram of trace element concentrations for diorites normalised to ocean ridge granite (ORG) after Pearce et al. (1984). (d) Spidergram of trace element concentrations fo( tonalites normalised to ocean ridge granite (ORG) after Pearce et al. (1984).

volcanic arc granites (Fig. 1 b of Pearce et al., op. cit.). In general, the gabbro-diorite-tonalite complex is characterised by the depletion of HFSEs and a marked negative Nb anomaly. HFSEs are depleted in subduction-related magmas derived at least in part from the mantle-wedge (Perfit et al., 1980). Also, the low concentration of Nb in association with low Ti is a feature of arc basalts (Saunders et a l . , 1980 ; Pearce, 1982 ; Ellam and Hawkesworth, 1988). The HYGE abundances of the gabbroic rocks (Fig. 9a) are similar to those of island-arc calc-alkaline basalt from the Taupo Volcanic Zone (TVZ), New Zealand (Graham and Hackett, 1987). The TVZ forms part of the Taupo- Hikurangi subduction system (Cole and Lewis, 1981).

The discriminant K-Na-Ca diagram (Fig. 10) after Barker and Arth (1976) clearly shows

the trondhjemit ic aff inity of the diorites and tonalites of the Wadi Dabr intrusive complex. A plot of Rb versus Sr (Fig. 11) indicates that the diorites and tonalites of the Wadi Dabr i n t r u s i v e c o m p l e x p lo t in the f ie ld of continental trondhjemite and quartz diorite of Coleman and Peterman (1975) and do not belong to the oceanic plagiogranite group. The latter are evolved rocks, formed in association with ophioiite complexes, and are distinguished by extremely low Rb contents.

The chondr i te -normal ised REE pat terns, est imated after Waki ta et al. (1 971) for the gabbroic rocks of the Wadi Dabr intrusive complex are reported in Fig. 12. They are relat ively enr iched in l ight REE (LREE) [(La/ Y b ) , = 2 . 6 7 - 3 . 9 1 ; Tab le 7 ] , less f r a c t i o n a t e d , s h o w no s i g n i f i c a n t Eu


F. F. ABU EL-ELA

m e"

:E

o.

cL E ¢}

u3

100 -

b

10

1

o H o r n b l e n d e gabbros ( 3A, 527A, 506A, 539, 506C, 502A)

[] Q u a r t z - h o r n b l e n d e gabb ros

( 14A, 1 4 B )

I I I I I I I I I

Bo Rb K Nb Sr P Zr Ti Y

100

C

10

0

t~

1

R a b r

100

d

1C

1

0.1 K20 Rb Bo Nb Zr Y

Figure 9. continued.

4 9 0 Journal o f Af r ican Earth Sciences


K

No Ca

Figure 10. Ternary K-Na-Ca projection for diorites and tonalites of the Wadi Dabr intrusive complex after Baker and Arth (1976). Ca" calc-alkaline trend; Tr: trondhjemitic trend. Symbols as for Fig. 6.

s I I

r

s

f ~

~. jJ'

//

10 1 O0 1 0 0 0

5r ppm

Figure 11. Rb-Sr variation diagram for diorites and tonalites of the Wadi Dabr intrusive complex. Symbols as for Fig. 6.


F. F. ABU EL-ELA

0

_=

=o x I s l a n d - a r c c a l c - a l k a l i n e basal t of TVZ,

New Zealand (Cole et al., 1983)

I I I I I

Le Ce Sm Eu Tb Yb Lu

Figure 12. Chondrite-normalised REE patterns for some gabbroic rocks of the Wad/Dabr intrusive complex. Symbols as for Fig. 6.

Table 7. REE (in ppm) in some gabbroic rocks of the Wadi Dabr intrusive complex

Sample No. La Ce Sm Eu Tb Yb Lu (L=/Yb )W (La/Sm)N (Tb/Yb)N Eu / Eu*

523.A 506A 14B 4.630 9.110 12.680 9.870 19.260 26.170 1.690 3.730 3.964 0.723 1.290 1.30 0.341 0.636 0.659 1.119 2.017 2.214 0.167 0.380 0.410 2.670 2.92 3.91 1.540 1.40 1.83 1.420 1.47 1.39 1.240 1.08 1.04

1000

100 O. Q.

.Q

z 10

.s f

WPG . . - " J

s y n - C O L G " v ~ / / / /

10 1 O0 1000 Y(ppm)

Figure 13. Discrimination diagram Nb versus Y after Pearce et al. (1984) for diorites and tonalites of the Wadi Dabr intrusive complex. ORG: ocean- ridge basalt; syn-COLG + VAG: syn-collision granites + volcanic-arc granites; WPG: within-plate granites. Symbols as for Fig. 6.

anomaly [ ( E u / E u * ) = 1 . 0 4 - 1 . 2 4 ] and are mildly depleted in heavy REE (HREE) [(Tb/ Yb)N= 1.42-1.47]. Depleted HREE indicates the presence of garnet in the source (Weaver and Tarney, 1981). Total REE abundances (13.54-47.79 ppm) increase from the least evolved gabbro (pyroxene- hornblende gabbro) to the most evolved gabbro (quartz-hornblende gabbro) and this can be explained in terms of fract ional crystallisation. Minor Ce anomalies (Fig. 12) have been widely described as typical of

volcanic-arc rocks. Negative Ce anomalies have been explained by either the presence of small amounts of subducted sediments in the source (Gill, 1981; Hole eta/., 1984), by fractionation of fluids originating from dehydration of the subducted slab (White and Patchett, 1984), or by hydrothermal alteration (Brouxel et al., 1987). The REE profile of the gabbroic rocks (Fig. 12) is similar to those of the island-arc calc- alkaline basalt of the Taupo Volcanic Zone (TVZ), New Zealand (Cole eta/ . , 1983).

4 9 2 J o u r n a l o f A f r i c a n E a r t h S c i e n c e s


1000

100

10

syn-COLG

J

Y wPG

VAG ORG

| I I I I I I I I I I I I I I l l I ~ I I I I I 1 |

10 100 1000

Y,Nb(ppm)

Figure 14. Discrimination diagram of Rb versus Y + Nb after Pearce et al. (1984) for diorites and tonalites of the Wadi Dabr intrusive complex, syn- COLG: syn-collision granites; VAG: volcanic-arc granites; WPG: within- plate granites. Symbols as for Fig. 6.

TECTONIC SETTING The REE profile of the gabbroic rocks resembles that of island arc basalts (lABs) and is distinctive from that of N-type mid-ocean ridge basalts (MORB). The similarity in REE profile between the gabbroic rocks and lAB suggests an island- arc tectonic setting.

A plot of d ior i tes and tona l i tes on the discrimination diagram (Fig. 13) of Nb versus Y depicts that they fall in the field of VAG + syn- COLG (volcanic-arc granites and syn-collision granites) of Pearce e t al. (1984). However, plotting on the discrimination diagram (Fig. 14) of Rb versus Y + Nb shows that they fall within the VAG (volcanic-arc granites) of Pearce et al.

(op. c i t . ) .

SUMMARY AND CONCLUSION The Wadi Dabr intrusive complex is composed of a gabbro-diorite-tonalite suite that shows a bimodal silica range (48.39-54.79 and 57.31- 72.54). The gabbroic rocks (48.39-54.79 SiO 2 %) display a subalkaline character (Fig. 7) and calc-alkaline trend (Fig. 8) on the basis of their major element chemistry. This is corroborated by the behaviour of major and trace elements on variation diagrams (Fig. 6). The spidergram (Fig. 9a, b) suggests that the gabbroic rocks are similar to island-arc calc-alkaline basalt from the Taupo Volcanic Zone, New Zealand (Graham and Hackett, 1987), where the gabbroic rocks are enriched in LILE (Ba, Rb, K, Sr) and depleted in

HFSE (Nb, P, Zr, Ti, Y). Both LILE and HFSE increase steadily from the least evolved gabbro (pyroxene-hornblende gabbro) to the most evolved gabbro (quartz-hornblende gabbro), which may be due to magmatic differentiation. The REE pattern for the gabbroic rocks shows slight enrichment of LREE and depletion in HREE, which is similar to island-arc calc-alkaline basalt from the Taupo Volcanic Zone, New Zealand (Cole et al., 1983). This suggests that the parent magma was derived from relatively primitive mantle source. The clinopyroxene chemistry depicts that the parent gabbroic magma is comparable to volcanic-arc basalt. The gabbroic rocks were crystallised, under conditions of 5-2 kb pressure and 865-716 °C temperature (Table 4), passing from the least evolved gabbro to the most evolved gabbro. The decrease in P-T conditions, from the least evolved to the most evolved gabbros, may be due to concomitant ascent, cooling and geochemical differentiation in the magma chamber. The foregoing discussion suggests the parent magma of the gabbroic rocks of the Wadi Dabr intrusive complex to have been a subduction-related magma.

The diorites and tonalites of the Wadi Dabr intrusive complex show a clear calc-alkaline trend on an AFM diagram (Fig. 8). The calc-alkaline trend is supported by major and trace element variation diagrams (Fig. 6), except for the variations of both Zr and Y versus SiO 2 (Fig. 6). Zirconium and Y decrease with increasing SiO 2, which suggests that the diorites and tonalites


F. Fo ABU EL-ELA

Calc-alkaline and tholeiiti¢ lavas

Diorite - tonal~ te / t rondhj em

Fusion of amoh'~bolitized

thole'l ' lt i c

Oceanic lithosphere

Hontle fusion

M a n t l e

~onatlon of basaltic IO

l ie f lux

Figure 15. Magma-forming processes and magma types formed during the evolution of an oceanic island-arc, based mainly on Ringwood (1974), Greene (1982) and Brown (1982) for the Wadi Dabr intrusive complex.

may have been derived f rom an island-arc protol i th rather than from evolved cont inental crust. In support of this v iew, both diorite and t o n a l i t e have t he c h e m i s t r y o f the trondhjemit ic trend (Fig. 10) of Baker and Arth (1976) , the con t inen ta l t r ondh jem i te and quartz diorite field (Fig. 11) of Coleman and Peterman (1 975) and volcanic-arc granite (Figs 9c, d, 13 and 14) of Pearce et al. (1984). Therefore, the parent magma of diorites and tonal i tes of the Wadi Dabr intrusive complex has similar features to a subduct ion-related magma.

A proposed model for the plutonism within an island-arc, which is control led by magmatic processes related to subduct ion (Ringwood, 1974; Brown, 1982), is suggested for the Wadi Dabr intrusive complex (Fig. 15). The model of ensimatic island-arc development occurs when the crust is sufficiently thick to retard the upward movemen t of tho le i i t i c basa l t ic magmas. Crystallisation of such magmas near to the base of the crust will involve amphibole, pyroxene, magnetite and sphene (Greene, 1982). Crystal- liquid fractionation will curtail the Fe enrichment trend and enhance the calc-alkaline magmatic trend. The emplacement of magma in the lower crust also causes melting of the amphibolitised tholeiitic protolith. The first felsic melts produced in this way have diorite-tonalite or trondhjemitic aff init ies. In this way, the protol i th of the gabbroic rocks of the Wadi Dabr intrusive complex is of mantle source and the diorites and tonalites are of tholeiitic crustal source.

Marzouki et al. (1982) and Jackson (1986) have also demonstrated that the 900-700 Ma diorite-tonalite complexes in the Arabian Shield were produced by mantle-derived magmas that were emplaced in an island-arc setting.

ACKNOWLEDGEMENTS A c k n o w l e d g e m e n t is made to the Dutch Government for the donation of my scholarship and to the Inst i tute of Earth Sciences, State Universi ty of Utrecht, for the support of this research.

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F. F. ABU EL-ELA

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Geological map A new geological map for the basement rocks in the

Eastern and Southwestern Desert of Egypt; 1:1 000 000. 1972. Compiled by El Ramly, M. F. Annals Geological Survey Egypt 2, 1 - 18.


Geochemistry of an island-arc plutonic suite: Wadi...

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