geocheinistry and petrogenesis of pan-african i-type ...rjstern/egypt/pdfs...the precambrian rocks...

larmal OfAfriccm Earth Scienm. Vol. 22, No. 1. pp. 29-42 19% COppightOl!MEkVkSChCC~

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Geocheinistry and petrogenesis of Pan-African I-type granitoids at Gabal Igla Ahmar, Eastern Desert, Egypt

MOHAMED A. HASSANEN, SAID A. EGNISR and FATHY H. MOHAMED

Geology Department, Faculty of Science, AIexandria University, Egypt

(Received 7 July 1994: revised version received 20 September 1995)

Abstract - Yotmger granites (post-kctonic) are common throughout the Precambrian igneous/metamorphic terrain of Egypt and they played a signiflcsnt role in the evolution of the Pan-African crust. The Gabal Igla Ahmar pluton comprises two magmatic suites: a calcalkalme diorite/quartz dioritc+granodiorite suite and an aluminous monzogranitf+gmnophyre suite. The calc-alkalme rocks have low KS, relatively low LFZE and display fractionated HREE (Tb/Ylbn=1.3-2.2). They appear to repmsent a suite of-andean-type intrusives emplaced in an active continental margin. The monzogranites are metal~ous to slightly perahuninous, highly diffemntlated I-type granitoids apparently representing a post-collision phase of intrusion.

Three distinct petrogenetic models for magma genesis are suggested to explain the petrological, major, trace and REE element varlations in these magmatic suites:

i) The caLdkahe quartz diorlte was derived by partial melting of garnet amphiboltte leaving a homblende- rich residue.

ii) The monzogmnites evolved by 7345% crystal fractionation of the quark dioribz melt. The crystallization took place at depth from a water saturated magma of minimum melt composition. After a further interv& the granitic melt was emplaced at shallow crustal levels at pressures of l-3 kbar.

iii) Simple mixing of quark dioritic and granitic melts as two end-member components could explain the origin of the granodiorite.

This model is consistent with the field, petrographical and chemical characteristics of the granodiorite. At a late stage of monzogranlte crystallization, the water contents in residual, intercrystaUine melt became sufficiently high to promote the development of eutectoid intergrowths of quark and feldspar to form the granophyre.

Resume - Les younger granites (post-tectoniques) sont abondants darts tous les terrains ignes et metamor- phiques d’Egypte et ont joue un role important dans l’@volution de la crotne pan-afrlcaine. L.e pluton de Gabal Igla Ahmar comprend deux suites magmatiques: une suite calco-alcaline diorite/diorite quartzique/granodiorite et une suite ahunineuse monzogranite/granophyre. L.es m&es calco-alcalines ont des teneurs faibles en KzO, rela- tivement faibles en terms rams, dont les termes lourds sont fraction&s (Tb/ybn=1.3-2.2). Elles semblent repr&enter une suite d’intrusions de type andin mises en place dans une marge continentale active. L.es monzogranites sont m&al~eux a faiblement hyperalw, fortement differencies et de type I. Ils repr&entent une phase dWrusions postcollisionnelles.

Trois mod&s p&rog&&iques distincts ont et6 sugg&& pour la gen&se de ces magmas afin d’expliquer les variations observ&es en &menk majeurs et en traces (y compris les terms rams):

i) les diorites quarkiques cako-akalines derivent par fusion partielle dune amphibolite a grenat, laissant un r6sidu riche en hornblende.

ii) Les monzogranites se fonnent par,aistallisation fraction&e (F=75-85%) a partir du magma dioritique. La cristallisation se serait d&o&e en profondeur a partir dun magma sat& en eau et de composition eutecticale. Le magma gmnitique se serait ensuite mis en place a faible profondeur (pression de 1 a 3 kbar).

iii) Un melange simple entre les magmas dioritique et granitique comme termes extremes expliquerait lorigine des granodiorites.

Ce modele est en accord avec les caracteristiques de terrain, petrographique et chimiques des granodiorites. Lors dune &ape tardive de la cristalbsation du monzogranite, la teneur en eau du magma residue1 intergranulaire devient suffimmment elev6e oue oour nermettre le developpement en condition eutectical d’intercroissances de quartz et de feldspath pour fo&t&les g&nophyres.

INTRODUCIION

Granitoid rocks constitute a major component of the Nubian Shield of Egypt (covering cu 60,000 km2). Two major and distinct Late Proterozoic to Early Pa- Iaeozoic granitoid suites have been recognized (El- RamIy, 1972; El-Gaby, 1975; Hashad, 1980; El-Gaby et al., 1987; Hassan and Hashad, 1990). The oldest of

these includes syn-tectonic, cak-aIkaIine I-type granitoids known as the older granites (Hassan and Hashad, 1990). These range in age from 1000-850 Ma and comprise diorite, quartz diorites, tonahtes, granodiorites and rarely granites. The second suite is made up of late- to post-tectonic granites known as the younger granites, which range in age from 620- 530 Ma (Hassan and Hashad, op. cit.). They are high-K,

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Geochemisty and petrogenesis of Pan-African I-type granitoids at Gabal Igla Ahmar 31

Quartz

Alkali Feldspar Plagioclase

Figure 2 Modal analysis (18 samples) of the GIA granitoids plotted on the classification diagram of Streckeisen (1976). The fields of A-, I- and Stypes granites are shown for comparison.

LILE-enriched talc-alkaline/alkaline to peralkaline granites which were emplaced during the main peak of Pan-African igneous activity (650-550 Ma, Dixon, 1981). The genetic relationship between the rocks of these two suites is still problematic.

The Gabal Igla Ahmar (GIA) granitoid complex (25”06’N, 34W’E) lies 10 km northwest of Marsa Alam in the central Eastern Desert (Fig. la). The GIA has been dated at 621 Ma by the RbSr whole-rock method (Hashad et al., 1’972). This suggests that the GIA belongs to the early magmatism of the younger granite suite of the Egyptian Nubian Shield. This paper presents the results af a detailed .mvestigation involving petrography and major, trace and rare earth elements (REE) analysis of the intrusive rocks of GIA. The main aim is to integrate the geochemical data using geochemical madelling to assess the plausible petrogenetic process(es) associated with silicic magmatism in the Nubian Shield.

GEOLOGICAL SE’ITING

The Precambrian rocks exposed in the studied area (Fig. lb) include lithological units which constitute members of the tectono-stratigraphical sequence of the Egyptian Basement complex (Akaad and Noweir, 1980). These units include me&&tone, metagabbro-diorite, basic metavolcanics, Hammamat molasse type sediments and granitoids.

The GIA forms an irregular granitoid body exposed over an area of about 30 krnr. A major dextral wrench fault (along the Idfu-Mama Alam asphaltic road; Ber- nau et al., 1987) occurs 10 km south of the GIA pluton. Like many other yormger granites (Greenberg, 1981; Rogers and Greenberg, 1!983) the GIA pluton is a post- tectonic, unfoliated body that was emplaced at a shallow crustal level. The eastern margin and the internal granitic structures dip east-southeast. It rarely exhibits any metamorphic effect on its host rocks.

Two main lithological varieties of granite can be recognized. The first variety is a coarse-grained

1 2 3 4 5 AR

Figure 3. A1203+(KzocNatO)/~~~-(K,ocNa~O) (Akalinity ratio: AR) verrms SiO,. Symbolsz triangles=quark diorite, crosseqrano- diorite, circles=monz.ogmnite, asterisks=granophyre.

pink to red monzogranite with high colour index, which constitutes the bulk of the GIA pluton. The second variety is an extremely leucocratic granophyre, which forms a central mass about a kilometre across. The location of this granophyre at the structural top of the granitoid intrusion is similar to that in other plutons, e.g. the Notch Peak granite (Nabelek, 1986). The contact between the granophyre and the monzogranite is gradational. At the extreme eastern margin of the pluton, the rocks change to dark grey in colour and form a discon- tinuous zone of granodioritic composition (a hybrid zone?; Fig. lb) with no chilling at the contact. Angu- lar to subrounded cognate xenoliths of dioritic composition are quite common.

Diorite and quartz diorite with a fine- to medium- grained texture and a dark green colour is encoun- tered beyond the hybrid zone along the eastern margin of the GIA. Irregular patches of coarse-grained appinite occur locally in the quartz diorite, particu- larly close to its contact with monzogranite. Cross- cutting relationships indicate that the diorite and quartz diorite are the earliest intrusive phases of the suite, followed by monzogranite. The granophyre represents the last crystallizing unit in the suite.

The region is intersected by two major sets of faults, one north-northwest-south-southeast (the Red ‘Sea trend) and a second east-northeast-west- southwest. The granitoid body is also cut by intermediate dykes and aplite veins typically less than 50 cm wide. These follow the same trends as the frac- tures and faults in the area.

PETROGRAPHY

Petrographical descriptions of some of the rock units from the studied area were given by Mafouz et al. (1979). Additional samples collected for the present investigation were point counted and the modal data is shown in Fig. 2.

32 M. A. HASSANEN et al.

Table 1. Representative major, trace and rare earth element analyses of GIA granitoids rocks, Eastern Desert, Egypt

Serial No 1 2 3 4 5 6 7 8 9 10 11 12 13

SamDIe 528 517 512 516 507 508 525 526 509 534 535 536 506*

SiOz 56.72 59.34 66.65 67.64 71.39 71.03 73.56 72.66 73.09 74.27 74.4 73.29 74.19 Ti02 1.08 1.07 0.49 0.55 0.17 0.21 0.25 0.19 0.21 0.22 0.22 0.22 0.20

A1203 17.16 17.3 15.21 16.31 14.49 14.12 14.09 13.55 13.95 14.11 12.53 13.42 13.23

Fe@3 1.93 1.36 0.59 0.42 1.79 1.44 2.28 2.72 1.85 1.6 1.63 0.95 1.94

Fe0 4.86 4.65 3.51 2.17 0.89 1.23 0.78 1.09 0.89 0.94 1.16 1.95 0.65

MllO 0.21 0.11 0.03 0.09 0.00 0.01 0.02 0.01 0.01 0.02 0.03 0.02 0.00

MgO 5.01 5.62 2.02 1.99 0.08 0.08 0.09 0.08 0.09 0.08 0.22 0.14 0.14

CaO 4.89 4.49 3.47 3.52 1.15 1.17 1.27 1.20 1.21 1.16 1.19. 1.06 0.13 NazO 2.58 3.51 4.12 3.09 4.85 4.82 4.87 4.83 4.56 4.67 5.01 4.49 4.54

K20 1.09 1.10 2.98 2.34 3.35 3.95 3.14 2.93 3.25 3.99 3.31 3.78 4.89

p2os 0.3 1 0.39 0.29 0.19 0.09 0.10 0.14 0.16 0.30 0.08 0.11 0.07 0.08

LoI** 3.98 1.56 0.81 1.59 0.67 1.29 0.53 0.52 0.74 0.48 0.78 0.84 0.71

Total 99.82 100.50 100.17 99.90 98.92 99.45 101.02 99.94 100.15 101.62 100.59 100.23 100.70 Trace elements (ppm) Ba 350 180 300 250

Sr 420 440 265 340

Zr 92 86 120 90

Y 9 10 12 11

Nb 8 12 15 14

Cr 66 85 85 70

Ni 25 23 50 34

SC 3 2 4 7

V 140 180 85 46

Ta 0.9 1.2 Bd 0.4

Hf 0.31 2.8 3.4 3.9

Th 12.4 14.2 16.3 7.5

La 24 22 34 39

Ce 50 43 67 79

Nd 20.4 15.1 22.7 31.5

Sm 3.92 2.68 4.84 5.74

EU 2.08 1.88 1.31 1.54

Tb 0.58 0.38 0.47 0.62

Yb 1.56 1.25 0.95 1.2

15

120

170

21 16

10

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Bd

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8.4

12.2

78

128

60.5

10.08

1.65

1.75

4.2

17 30 25 105 22 25 20 25

120 120 190 165 130 185 190 60

195 145 165 170 180 280 245 50

19 21 20 25 29 25 24 19

13 18 15 24 30 19 17 27

50 50 15 12 34 6 11 15 4 2 8 7 3 8 20 2

7 6 4 4 5 9 8 8 Bd Bd Bd Bd Bd Bd Bd Bd

0.7 0.62 0.52 0.89 1.2 2.5 1.4 0.3

8.9 9.2 7.3 8.9 9.4 21 18 6.6

10.4 12.5 9.3 16.5 18 9.9 15 8.4

73 74 76 85 95 70 72 61

133 158 122 195 202 148 135 118

69.25 85.3 64.8 96.5 100 83.1 101.5 50.41

12.07 11.62 11.52 11.13 18.9 12.13 11.47 9.14

1.46 1.81 1.48 1.95 2.53 3.76 2.46 1.75

1.8 1.4 1.12 1.82 2 1.47 2.3 0.66

4.5 4.3 4.3 4.6 5.4 4.8 6.5 1.4

Lu Bd Bd Bd Bd 0.46 0.49 0.71 Bd 0.68 Bd 0.61 0.73 0.19 *: Samples 18~ 2 Quark diorite; 3 L 4 Granodiorite; 5 - 12 Monzogranite; 13 Gmnophyre. Bd: Below detection limit. **: Loss on ignition.

The diorites and quartz diorites are fine- to medium-grained with nearly equal proportions of mafic and felsic constituents. The dominant mafic mineral is actinolitic hornblende, which rarely has a remnant clinopyroxene core. The actinolitic hornblende often shows an intercumulus texture and is partly to com- pletely altered to aggregates of chlorite, calcite and epidote. The plagioclase feldspar (labradorite- andesine) occurs as subhedral aligned laths, which form a cumulus texture in the more mafic diorite. The crystals are strongly zoned with highly saussuritized cores. Interstitial K-feldspar and quartz are minor

constituents occurring most commonly in the quartz diorites. The most common accessory minerals are magnetite, titan& and apatite.

The granodiorites in the hybrid zone have medium- to coarse-grained, granular to seriate textures. Pla- gioclase (40%) and quartz (30%) are the dominant felsic minerals with interstitial K-feldspar (1520%). Mafic phases (10-15%) are dominated by green hornblende and less common biotite. The plagioclase composition varies from andesine to oligoclase and becomes more sodic toward the monzogranite. The crystals show oscillatory zoning and are selectively

Geochemist-q and petrogenesis of Pan-African I-type granitoids at GabaI Igla Ahmar

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Figure 4. Major element oxides versus silica variation diagrams. Symbols as in Fig. 3.

altered to sericite, chlorite and calcite. Apatite is a fre- quently abundant accesstory mineral with zircon and titanite. Microxenoliths of dioritic composition consist- ing of finegmined equant crystals of labradorite and hornblende are common.

The monzogranite typically consists of micro- perthite (31.9%), quartz (32.5%), plagioclase (22.6%), biotite (58%), hornblende (l-2%) and accessory titanite, zircon, apatite and opaque oxides. The rocks are generally coarse- to medium-grained with hypidiomorphic textures. Rapakivi, micrographic and myrmekitic textures are not uncommon. A micropor- phyritic texture was also found in some samples col-

lected near the outer margin of the pluton. Plagioclase and quartz crystals are frequent as microphenocrysts set in a fine-grained groundmass of quartz, plagio- &se and potash feldspars. The plagioclase normally zoned forms subhedral crystals of oligoclase composition (An15-Arm).

The granophyre has a similar felsic mineralogy to the monzogranite, but with different mineral proportions, and is characterized by a granophyric texture. Riebeckite and bluish green hornblende are minor but represent the only mafic phases in the rocks. Plagio- clase feldspar (Am-Anu) and quartz form subrounded microphenocrysts set in a granophyric groundmass.


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Si 02 (wt %)

Figure 5. Variation of selected trace elements and ratios with silica. Symbols as in Fig. 3.

Unlike the monzogranite, accessory minerals in the granophyre are rare and include only apatite and zircon.

GEOCHEMISTRY

Analytical techniques

All of the major elements, as well as Sr and Ba, were analysed by classical wet chemistry, flamephotometry and atomic absorption spectrophotometry (AAS). Fer- rous Fe was determined using I-IF dissolution and the

potassium dichromate titration technique (Goldich, 1984). Trace elements ( Ni, Cr, V, Ba, Sr, Zr, Y, Nb and Sc) were analysed by an optical emission spec- troscopy (Q24). The accuracy and precision of the analytical results were monitored using USGS standards (G-l, AGV-1, BCR-1) and found to be in the range 2-596 for major elements and lo-15% for trace elements. The REE, along with I-If, Ta and Th, were determined by instrumental neutron activation analysis (INAA) at the Mineralogical - Geological Museum in Oslo, Norway. International rock standards (GZ,

Geochemistry and petrogenesis of Pan-African I-type granitoids at Gabal Igla Alumu 35

-5 s z4 0 N3 0

=2

7 01 234567

K20 ( wt ‘10)

Figure 6. K20 versus NazO plot of GIA samples. Dashed line based on the criteria of ChappelI and White (1974). Stippled area represents the compositional field of the younger granites of Egypt (based on data collected from references therein). Symbols as in Fig. 3.

A

yA S-type A x 0

e-w_---._ -_ - _ -- - _ -a-

I-type

a I I Si 02 ‘10

50 680 70 !502 (wt%)

Figure 7. A1203/(CaO+Na20+K20) (molar proportions) versus silica. Symbols as in Fig. 3.

BCR-1, BHVO-1 and GSP-1) were used for calculating the precision of the INAA, which was found to be 5- 10% for the Hf, Ta, Th and REE.

Major and trace element variations

Chemical analyses of representative samples from all the rock types are presented in Table 1. The granitoid rocks at GIA are metaluminous to weakly peraluminous [mole AlQ/(CaO+NazO+KzO) in the range 0.89 to 1.401. The major element chemistry and mineralogical composition (Table 1 and Fig. 3) allows two groups of granitoids to be distinguished, a calc- alkaline diorite, quartz diorite and granodiorite group which is least evolved (5567% SioZ) and a second group including the highly evolved monzogranite and granophyre (72-76% SQ). Figures 4 and 5

100

Nb PPm

10

1

VAG +

_ Syn- COLG

I I I I I,

10 20 30 50

v PPm

Figure 8. Nb versus Y plot of the GIA samples. WPG=within-plate granites; VAG=volcanic arc granites; Syn-COLG=syn~ollision granites (Pearce et al., 1984). Symbols as in Fig. 3.

show that the major and some trace elements define rather regular trends when plotted against SiOz, although there is a distinct compositional gap between 60-65% SiOz. Generally, with increasing SiOz there is an increase in Na20, KzO, Zr, Ta, REE, I-If, Sc and Nb and a corresponding decrease in TiOz, ti203, FeO, MgO, CaO, E205, Ba and Sr. Most of the elements display curved trends on the variation diagrams (Figs 4 and 5), which suggests fractional crystallization processes. However, linear trends such as those of K20, TiOz, MgO and Zr, might be the result of either hybridization or fractionation.

The chemical and petrographical data on GIA granites can be compared to the specific granite types defined and described by Chappell and White (1974), White (1979) and Colhns et al. (1982). The relatively high NarO/K20 ratio (~1, Fig. 6), the broad compositional range, the occurrence of magnetite and titan& and the slightly peraluminous character of the granites are all consistent with their being I-type g-rani- toids (Fig. 7). Also, when the GIA granites are plotted on a Nb versus Y diagram using the compositional discriminant fields of Pearce et al. (1984) (Fig. S), they fall in the volcanic arc granite field except for one sample, which lies in the WPG field.

Rare earth elements (REE)

REE data are given in Table 2 and the chondrite- normalized patterns are shown in Figs 9 and 10). The REE’s generally increase in abundance with increasing SiO2 from quartz diorite to granite. The quartz diorites have low REE abundances and low

36 M. A. HASSANEiN et al.

Table 2. Distribution coefficients (Kds) used in petrogenetic modelling of GIA granites

Ce Nd Sm Eu Gd Yb Sr Ba Zr

Plagioclase Biotite 0.20 (b, c) 0.04 (f ) 0.11 (a, b) 0.06 (f ) 0.21 (a, b) 0.06 (f )

2.25 2.24 0.09 (g) 0.28

0.07 0.18 (f) 3.20 (b, c) 0.12 (f)

3.30 (e) 6.40 (f ) 0.03 2.00

Hornblende 0.09 (a)

0.34 0.34 (a)

0.36 5.00

0.46 (a) 0.23 (e)

0.30 1.20

Apatite Magnetite 35 (f, g) 2.0 (d) 63 (f, s) 0.25 (d) 63 (f, g) 0.30 (d) 31 (f, g) ---mm 56 (f, g) 0.30 (d) 15 (f, g) 0.25 (d)

_--- mm___ ---- ___-_ ____ 0.1

Ti 0.06 3.00 4.10 ____ 12 a: Schnetzlcr and Philpotts (1970); b: Philpotts and Schnetzler (1970); c: Ewart and Taylor (1969); d: Reid (1983);

e: Gill (1978); E Arth (1976); g: Hanson (1980).

??Quartz Diorite

*. Gronodiorite

21 ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ 1 La Ce Nd Sm Eu Gd Tb Yb

Figure 9. Chondrite-xwrmaked REE patterns of the GIA quartz diorite and granodiorite (nommlizing values from !&WI, 1982). Stippled area delineates the field of continentahar@n intemwdi- ate volcanic rocks (Thorpe et al., 1976; Dosti et nl., 1977).

to moderately fractionated chondrite normalized patterns &a/Y&=10.3-11.8; Fig. 9). The small positive ELI anomalies (Eu/Eu*=WO-2.5) in the quartz diorite probably result from apatite or hornblende fractionation, or crystal accumulation of plagioclase (Taylor et al., 1981; Gromet and Silver, 1983; Fowler and Doig, 1983). Continental margin, intermediate talc-alkaline rocks often show ‘HREE fractionation (i.e. Tb/Yb,,>l), which is rare or absent in the island arc counterparts, and posses an overall higher abundance of REE (Thorpe et al., 1976; Do&al et al., 1977). The fractionated REE patterns of the less evolved samples (quartz diorik) with Tb/Yb,,=1.34-2.19 are closely comparable to those of some andesites from North Chile (Tb/Yb,,=1.3-2.00 at Sio2=55.6-64.6 and IGO=l.O-3.0; Thorpe et al., 1976).

The granodiorites have LREE-enrichment and moderate HREE fractionation (Tb/Yb,,=2.09-2.19) and no Eu anomaly. Otherwise they have similar REE patterns to the quartz diorites (Fig. 9). They all fall in the field of talc-alkaline intermediate volcanic rocks of the continental margin (Thorpe et al., 1976; Dostal et al., 1977).

The monzogranites display moderately fractionated REE patterns @a/%=7.32-12.41) with moderate to strong negative Eu anomalies (Eu/Eu*=0.26-0.66; Fig. 10). These patterns are comparable to those of many Late Proterozoic younger granites in the north East- em Desert of Egypt (Stem and Gottfried, 1986; Fig. 10) and also to the younger granites from the Red Sea Hills in Sudan (Klemenic and Poole, 1988). The relatively unfractionated HREE patterns (Tb/Yb,,= 1.10-1.76) of the monzogranites resemble those of post-erogenic granites (Stern and Gottfried, 1986). However, the inverse relationship of Eu between the monzogranites and quartz diorite and their similar (La/Yb), ratios suggests that these rocks are genetically related and evolved from the same parental magma.

The one granophyre sample (no. 506) which was analysed for REE shows a similar LREE pattern as the monzogranite but has marked HREE depletion (Tb/&=2.0; (Fig. 10). Zircon and hornblende have high Kd (mineral/melt distribution coefficients) for HREE and Y. Fractionation of these minerals could have depleted these elements in the granophyre. Al- ternatively, volatile loss is a possible cause of HREE and some HFS element depletion. This might be in- voked to explain the rapid crystallization of the fine- grained gmnophee after a sudden release of volatiles (Nabelek, 1986).

PETROGENESIS

The petrogenetic model for the talc-alkaline gmnitoids from GIA must take the following

Geochemistry and petrogenesis of Pan-African I-type granitoids at Gabal Igla Ahmar 37

??Monzogrcinite

*Gronophyre

10 -

4 1 1 ’ 1 ’ ’ ’ ’ ’ ’ ’ ’ ’ La Ce Nd SmEu Gd Tb Yb

I

Figure 10. The chondrite-normaked RF% patterns of monzogranites and granophyxe. The stippled area delineates the Egyptian younger granites (Stem and Go&fried, 1986).

constraints into account: i) predominance of diorite/quartz diorite and

limited abundance of granodiorite; J”’ ’ ’ 1

1

t -Yb

k?l -Y

J”’ ’ ’ 1 4

ii) enrichment and/or depletion of the trace and rare’ earth elements in granodiorite with respect to quartz diorite;

0.1

iii) the similar behaviour and abundances of Ni, Cr, V and REE’s (except E!u anomalies). A model of simple fractional crystallization from a mafic parent magma is inconsistent with the gee- chemical data. For example, the quartz diorites have relatively low contents of Cr (83 ppm) and Ni (43 ppm), which are comparable to those in the granodiorite ( Cr and Ni are 77 and 42 ppm, respectively). Most of the published Sr isotope work on diorite and quartz diorite rocks in the the Eastern Desert yield a low and restricted range of initial 87Sr/%r ratios (e.g. 0.7024, Dixon, 1981; 0.7042, Abdel-Rahman and Doig, 1987; 0.7034, El-Sayed, 1994). Assuming a similar low %r/%r ratio for the diorite and quartz diorite suites in the Eastern Desert, the low KzO contents and less fractionated HREE of GIA quartz diorites suggest the formation of the diorite and quartz diorite from a mantle source (Dixon, ‘1981). Partial melting of ec- logite or garnet amphibolite at mantle depths, leaving a residue of garnet, clinopyroxene and plagioclase, could produce melts delpleted in HREE with positive Eu anomalies and mildly fractionated REE patterns (Arth and Hanson, 1975). Similar geochemical features are found in the GlA rocks (Table 1, Fig. 9).

Figure 11. Graphical analysis of the potential mixing of monzogranite (534) and two potential hybrids (samples 512 and 516) normalized against quartz diorite (517) (after the proportional mixing diagram of McGarvie, 1985).

the quartz diorites. However, trace and REE modelling fails to substantiate the evolution of the granodiorite from the quartz diorites via fractional crystallization or partial melting. Field and petrographical observations (such as the oscillatory zoning in plagioclase, the presence of microgram&r xenoliths of dioritic composition and transitional contacts between the lithologies) point to a mixing process to explain the relationship between them. A proportional mixing diagram (M&&vie, 1985) has been used as a first step in evaluating the hybridization hypothesis for the GIA pluton. In Fig. 11 a highly evolved granite (such as sample 534) as one potential end-member is normalized against a possible quartz dioritic end-member (sample 517). All element values for hybrids derived from mixing the two end- members should plot between these compositions (between the central vertical line and that defined by sample 534 in Fig. 11). With some exceptions (Ta, Th, Yb and Y), the granodiorites (samples 512 and 516) do lie between the two end-members.

The granodiorites of GIA exhibit field and petro- Quantitative evaluation of the mixing model can be graphical features, major and trace element geo- made using the binary scatter diagram L.a/Sm vs La chemical characteristics and REE patterns (Fig. 9) (Fig. 12) (see Allegre and Minster, 1978; Hofmann and which substantiate their close genetic relationship to Feigenson, 1983). This shows that the granodiorites

Rock/ Quartz-dioriteW7)

Potential hybrides

kWranodiorite (512)

&Granodiorite (516)

mAcii.end-member granite (534)

c(

.

P ,’

y:

‘P Sr

/ P

! Al

- si

-To

1 _

-Th

Zr

-Nb

-Hf

-te


could indeed be the products of mixing of two end- members magmas. Although, a partial melting trend cannot be distinguished from a mixing line on this diagram, petrographical and geochemical evidence suggest that partial melting relationships are unlikely. Using La as a reference element and the mixing relationship given by Faure (1986), the amount of La and La/Sm in the mixture (La, and La/Sm,, respectively) can be calculated by:

(1)

(La/Sm),=(La/Sm),La~~/Sm),rLa~cl~~+~B(l-n. La4f (2)

where f represents the proportion of mixing and La, and La, are the contents of La in the two end- members A and B, respectively. The two end-member components in Fig. 12 used to construct the mixing hyperbola were a quartz diorite (sample 517) and a monzogranite (sample 534). The proportion of mixing is given on the hyperbola. Figure 12 indicates that the granodiorite samples could represent a mixing of about 80% quartz diorite magma with a monzogranite magma. The relationship between potential dioritic and monzogranitic magmas in the evolution of the GIA pluton is more difficult to explain. The mineralogical and geochemical characteristics of the monzogranites in the GIA pluton are consistent with its being a mildly alkaline, highly differentiated I- type granite. These are usually thought to be derived from a crustal mafic igneous source (White and Chappell, 1977; Chappell, 1984). In the experimental system Q-Ah-Or-HZ0 (Fig. 13), the GIA monzogranite samples lie close to the low-temperature minima in the water saturated granite system (Luth and Tuttle, 1%9). The plots are clustered between 0.5 and 3 kbar PHzo suggesting emplacement at depth of about 1.5 to 9 km.

The petrogenetic relationship between the monzogranites, which might be simple partial melts, and the more mafic portions of the pluton will now be in- vestigated in terms of these models:

i) batch partial melting of mafic or intermediate crustal material;

ii) fractional crystallization from mafic or intermediate melts;

iii) mixing fractional crystallization (MFC).

Batch partial melting

Batch melting processes can be modelled using the equation of Schilhng and Winchester (1967):

C,/Co=V P(l-F)+Fl (3)

where C, is the concentration of an element in a melt

generated by melting, C, is the weight of that element in the source, D is the bulk distribution coefficient of the residue in equilibrium with the melt and F is the degree of partial melting. Stern and Hedge (1985) suggested that amphibolitic crustal material might be the source of the felsic magmas in the central Eastern Desert of Egypt. They also added that about 15-25% batch melting of that source could be one of the alternative explanations for the chemical variation and evolution of the felsic magmatism. During melting of garnet or amphibole-bearing sources, Ce and Yb should be compatible elements (D c<l); and Ce should be more compatible than Yb (Dcc~cDyb). Consequently, relative to the parent rock, melts should have higher REE abundances, be LREE enriched with a steep REE pattern (Ce/Yb,,=19-25) and perhaps have no or weak negative Eu anomalies as a result of amphibole melting, which is unstable under mantle conditions. These features are partly inconsistent with those of the GIA monzogranite (Fig. lo), thus suggesting that partial melting of a mafic (amphibolitic) source is an unlikely model for the monzogranite in the GIA pluton. Anderson (1983) argued that the high K,O and FeO*/MgO of granites is consistent with lo-30% f’u- sion of talc-alkaline crust of quartz dioritic material. Similarly, Compston and Chappell (1979) indicated that more mafic I-type granites commonly represent about 25% melting of intermediate lower crust. Fig- ure 14 presents REE modelling of quartz diorite melting as a possible source for the monzogranite. The composition of quartz diorite (sample 517) is taken to be the source. The recalculated mineralogy (in wt.%; based on calculated average modal mineralogy of the quartz diorite) of the source is: plagioclase (42%), hornblende (40%), biotite (8.7%), quartz (6%), magnetite (3%) and apatite (0.3%). The Kd values in Table 2 are used in modelling. REE patterns of melts generated by l-5% melting of quartz diorite (Fig. 14) are similar to those of the monzogranite.

Fractional crystallization

The major and trace elements in the GIA magmatic suite display curved trends on most of the variation diagrams (Figs 4 and 5). Such trends are commonly attributed to melt fractionation of early crystallizing minerals such as plagioclase, amphibole and biotite. Depletion of Sr and Eu are commonly explained by fractionation of plagioclase and K- feldspar (Ewart et al., 1985; London, 1987). The REE also increase in abundance in the GIA from quartz diorite through to monzogranitic composition (Table 1). With increasing SiO, from quartz diorite through granodiorite to monzogranite, the abundances of the trivalent REE increase systematically and the Eu anomalies change from positive to negative. These


LalSm

20 40 60 80 100

La (ppm)

Figure 12. Plot of La/Sm versuls La @pm) for the granitic rocks of GIA. The dashed curve is the mixing hyperbola between components A (quartz diorite sample 517) and B (monzogranite sample 534). The mixing proportions Ifl are annotated on the mixing hyperbola (see text for details). The arrow FC represents the fractionation trend calculated using the Rayleigh fractionation law. The amount of residual melt is also shown on the fractionation line (see text for further details). Symbols as in Fig. 3.

features are compatible with fractionation involving plagioclase, amphibole (and biotite, which enrich residual liquids in the trivalent REE and deplete their Eu content.

To evaluate fractional crystallization processes applied to the GIA pluton, trace element and REE modelling has been done using the Rayleigh fractionation equation as applied by Neumann et al. (1954).

c)JKo=P’

where F is the fraction of liquid, D is the bulk distribution coefficient and CL and C, are the weight con- centrations of trace elements in the derived and parent melts, respectively. Using the appropriate published mineral/melt distribution coefficients (Table 2), estimated fractionated components are as follows: quark=7%, plagioclase=50%, hornblende=35%, biotite=5%, apatite=O.2?6 and magnetite=2% and the calculated bulk D valaes are Ds,=O.43, D,=1.63, DZr=0.54, Dc,=0.204. Using the trace element composition of quartz diorite sample 517 as an assumed source melt (CO) and monzogranite sample 534 as a daughter melt (CL), the degree of fractional crystallization of sample 517 necessary to produce sample 534 ranges from 7&85%. The resulting calculated F for each individual element is &=0X, Fs~O.15 and Ffie0.22.

The result obtained from REE modelling (Fig. 15) are consistent with those from the trace elements which point to the possibility of amphibole fractionation during the evolution of the GIA monzogranites and their alverall evolution by progres- sive fractionation from a hydrous quartz diorite magma.

Mixing fractional crystallization (MFC)

Although a simple fractional crystallization model could explain much of the geochemical data and the element variations in the monzogranite of the GIA pluton, other petrological and chemical constraints remain. The linear trends of some elements (Zr, TiO,, .KrO and MgO) on the variation diagrams (Figs 4 and 5) and some petrographical features (such as oscillatory, patchy and normal zoning in feldspar) suggest that simple two component mixing processes could also explain some of these features. Unlike the situa- tion in the volcanic rocks, the chemical signature of mixing processes in the intrusive rocks are often overshadowed by other processes such as crystal fractionation, assimilation, volatile transfer and zone refining. A simplified model for the mixing of intermediate, mantle derived, talc-alkaline magma of quartz dioritic composition, with the most evolved silicic monzogranitic melt at depth in the crust followed by fractional crystallization, is i&&rated in Figure 12. The binary scatter diagram (La versus La/Sm) is used in the mixing model for the petrogenesis of the granodiorite. The monzogranite data clearly fall away from the mixing hyperbola. This may be attributed to the inadequacy of the mixing model or alternatively may indicate that the chemical composition of the hybrid melt was modified after mixing by other processes such as fractional crystallization.

Using the hypothetical mixing composition representing about 60% monzogranite and 40% quartz diorite (point M), a fractional crystallization trend (FC) has been calculated using the Rayleigh fractionation equation (4). The proposed fractionated phases are quartz (19%), plagioclase (40%), K- feldspar (18%), hornblende (15%), biotite (7.5%) and apatite (0.5%). The distribution coefficients are given in Table 2. Figure 12 shows that the monzogranites could be derived by W-30% fractionation from a hybrid melt of composition (M). This model apparently provides a feasible explanation for the petrographical data and the varying behaviour of some major and trace elements observed in the GIA pluton (Figs 4 and 5). Nevertheless, the sharp intrusive contacts of the monzogranite, as well as some of the other binary variation diagrams (Fig. 4), still put constraints on this model. Moreover, the low degree of fractionation (1030%) is not sufficient to explain the enrichment of LIL elements (K, La and Ce), high K/Sr and Zr/Hf and the depletion of Cr, Ni, V and Sc in the monzogranite.

The petrogenesis of the granophyre

Trace elements and REE abundances in the GIA granophyres provide some constraints on their origin. In the field, these rocks have a restricted occurrence, in the centre of the GIA pluton, and have gradational

M. A. HASSANEN et al.

Ab Or

Figum 13. CIPW normative composition of the GIA granitoid plotted in the system Quartz-Albite-0rthoclase+H20+Anorthite+F. Mimimum melt compositions are from Winkler (1979) and JZbadi and Johannes (1991). Symbols as in Fig. 3.

500 I Notch Peak, Utah; Nabelek, 1986).

10 ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ La cc Nd Sm Eu Cd lb Vb Lu

Figure 14. Batch melting model for the generation of the REE patterns in the monzogranite from a quartz diorite 8oufce. Stippled field indicates the range of REE in the GIA monzogranite. Heavy lines refer to the modelled RF22 with annotated values (1% and 5%) equal to the fraction of the melt generated.

contacts. They have large trace element variations, although their range of major element compositional variations are small and they overlap with those of the host monzogranite. The small volume of the granophyre (~1% of the mass of the GIA pluton) and its chemical characteristics suggest that it is genetically related to the monzogranite. The granophre might be a product of crystal fractionation, or more probably from crystallization of a residual aqueous fluid. All the granophyres are markedly depleted in many elements, especially Na,O, CaO, MgO, Zr, Y, Hf, Ta and Th. The granophyres are also characterized by low contents of HREE and the low Tb/Yb ratio is important to note. These geochemical features are typical of many granophyres relative to their host granites (e.g.

IO ’ ’ ’ ’ ’ ’ ’ ’ ’ 1 ’ ’ 1 ’ Lace Nd Sm Eu Gd Tb Vb Lu

Figure 15. REE fractional aysta.Uization model of monzogranite gener&d from a quarkdiorite. The heavy lines represent the calculated melt composition at 80% and 35% fractionaticm (see text for details). The stippled field outlines the range of the REE pattern of the GIA monzogranite.

In the haplogranite system Qz-Ah-Or (Fig. 13), the granophyre data are shifted to the right of the ternary minimum, arguing against the role of crystal/melt fractionation. Moreover, the low concentration of CaO, MgO and III, elements and the absence of negative Eu anomalies in the g-ranophyre are all better explained by crystallization from an aqueous fluid rather than from a true magmatic melt. The occurrence of irregular appinite patches in the quartz diorite near the monzogranite contact and the presence of pegmatitic pods and veinlets in the latter also em- phasizes the incremental build up of a late magmatic volatile-rich fluid phase in the magma. At a late stage in the crystallization of the monzogranitic magma, the water and volatile contents in the residual inter- crystalline fluid probably became sufficiently high to segregate in the upper portion of the magma cham- ber. Rapid loss of these volatile components resulted in fine-grained eutectoid intergrowths of quark and feldspar to form granophyre.

CONCLUSION

The granitoid rocks in the GIA are petrographically and chemical@ class&d into two magmatic intrusions:

i) the talc-alkaline diorite/quartz diorite- granodiorite; and

ii) the monzogranite and granophyre. Thecalc-alWnero&areoflowKrOandhavefrac- tionated REE patterns &a/*1&24). The monzogm&e whichconstitute9themainmassoftheGIAishighlydif- ferentiated I-type granite with metahuninous to mildy peraluminous chamckr. It repxesenk postdbioxd emplacementalongacontinemalmargin

Geochemical modelling suggests the formation of


talc-alkaline quartz diorite by partial melting of garnet amphibolite. The monzogranite evolved through 7585% crystal/melt fractionation from quartz diorite melt. The crystallization took place at depth from water saturated (hydrous) magma of minimum melt composition. An alternative explanation is the formation of the monzogranite by the mixing of quartz dioritic magma and a highly silicic magma followed by fractional crystallization (mixing fractional crystallization). Granodiorite is formed by a simple mixing of about 80% quartz diorite and 20% differentiated granite as the two end-member components. The origin of the granophyres, according to their chemical characteristics and in the light of experimental work,’ is better explained by crystallization from fluid that was building up during the late stage of granite magma crystallization.

Acknowledgements

The authors are grateful to Prof. A. Bajorlykke, Institute of Geology, Oslo University, Norway and H. Z. Harraz for the REE analyses. This paper benefited from the discussion, comments and revision of Prof. Y. Anwar and Prof. S. El-Gaby. Two anonymous re- viewers are greatly acknowledged for their valuable comments and critical reviews of the manuscript. Special thanks are due to Prof. N. Eby for suggesting numerous improvements to the manuscript.

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geocheinistry and petrogenesis of pan-african i-type ...rjstern/egypt/pdfs...the precambrian rocks...

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