geochemical evolution of arc-related mafic plutonism in ...rjstern/egypt/pdfs/ce desert... · au...

OS99-5362196 Si5.00 + O.a,

PII: SOtW-5362(%)00018-8

Geochemical evolution of arc-related mafic plutonism district, Eastern Desert of Egypt

F. H. MOHAMED and M. A. HASSANEN

in the Umm Naggat

Alexandria University, Faculty of Science, Geology Department, Egypt

(Received 23 May 1995: revised version received 8 January 1996)

Abstract - The Umm Naggat metagabbro-diorite suite (UNGD) forms a part of the mafic intrusive province in the Arabian-Nubian Shield. The rocks range in composition from gabbro through diorite to quartz diorite. The suite forms a compositional continuum with a wide major element variation, particularly in terms of SiO, (49-61%), CaO (5-11%) and MgO (3-10%) content. Geochemically, the suite has a tholeiitic/calc+lkaline affinity typical of subduction- related rocks. The compatible behaviour of Mg, Ca, Fe, Cr, Ni and Sc, together with the incompatibility of Na, K, Ba, Rb, Zr and Y, suggest that the evolution of the suite is largely controlled by crystal melt fractionation.

In the Egyptian Shield, three chemically different groups of the erogenic-related mafic magmatism a_re discriminated: the I-type gabbro (group I), to which the UNGD complex belongs, is do minantly a talc-alkaline, arc-related suite characterized by a rather more fractionated trend with evolution towards more silica-rich compositions; the O-type gabbro (group II) includes the ophiolitic-related metagabbros with typical tholeiitic affinity and closely resembling the mid-ocean ridge basalk (MORB); the Y-type gabbro (group III) encompasses talc-alkaline/ tholeiitic fresh gabbros that have a more primitive and less fractionated character compared to group I.

The UNGD suite is characterized by low abundances of HFS elements such as Nb, Ti, Zr and Y, which reflects a depleted (relative to MORB) mantle wedge overlying the subducted slab. The addition of a slab-derived fluid to such a depleted source is responsible for the relatively high abundances of LIL elements, particularly K, Rb, Ba, Th and Sr. Subsequently, the primary magma was differentiated largely by fractional aystallization of plagioclase, clinopyroxene, amphibole and magnetite to generate the more evolved rocks in the UNGD suite.

RCsumC - IX complexe m&agabbroique-dioritique d’Umm Naggat (UNGD) appartient B la province intrusive mafique du Bouclier Arabs-Nubien. La composition des roches varie des gabbros aux diorites quartziques, en passant par des diorites. Le complexe est caract&i& par une continuite dans la composition chimique, avec grande variabilitk des teneurs en &me&s majeurs, en particulier SiO, (49-61%), CaO (S-11%) et MgO (3-10%). Du point de vue g6ochimique, la suite de roches presente une affinite thol&ique/calco-alcaline typique d’un contexte de subduction. Le comportement compatible du Mg, Ca, Fe, Cr, Ni et Sc ainsi que l’incompatibilite du Na, K, Ba, Rb, Zr et Y sugg&ent que l’&olution du complexe ait et6 largement contr~l6e par une cristallisation fraction&e B pa&r d’un liquide.

Au sein du Bouclier Egypt& trois groupes de magmatisme mafique orogtique, chimiquement diff&enk, sont distingu&: le gabbro de type I (groupe I), auquel appartient le complexe UNGD, est a p&dominance calco-alcaline, de type arc insulaire et caract- par une tendance prononcee au hactionnement ainsi que par une &olution vers des termes enrichis en silice; le gabbro de type 0 (groupe II) comporte les m&agabbros ophiolitiques a affinite tholtitique typique et est tres semblable aux basaltes des rides m#dio-oc&miques (MORB); le gabbro de type Y (groupe III) a trait B des gabbros non alMr& calco-alcalins/tholeiitiques, ayant par rapport au groupe I un caract&re davantage primitif et done fraction&.

L.e complexe UNGD est caract&+& par de faibles teneurs en elements HFS tels que le Nb, Ti, Zr et Y, indicateurs de la pr&ence d’un biseau de mar&au appauvri (par rapport au MORB) coiffant l’&ailIe subduct&. La contamination de ce genre de source appauvrie par des fluides provenant de l’&aille est h l’origine des abondances relativement elev&s des grands elements lithophiles, en particulier le K, Rb, Ba, Th et Sr. Ult&ieurement, la diff&nciation du magma primaire s’est op&& par la cristallisation fractionnee de plagioclase, clinopyrox&ne, amphibole et magn&ite, engendrant ainsi les roches plus &volu&s du complexe UNGD.

INTRODUCTION

The Arabian-Nubian Shield (ANS) was evolved and cratonized during the Late Precambrian ‘Pan-African’ event (ca 1100-450 Ma; e.g. Gass, 1977). Numerous o&iolit&earing suture zones had been identified and interpreted in terms of the microplate arc accretion model for the formation of the Afro-Arabian crust

(Stoeser and Camp, 1985; Vail, 1985; Sultan et al., 1992). Strontium, Nd and Pb isotope data further confirms such a model (Harris et al., 1990; Sultan et al., 1992). The stratigraphical units in the ANS comprise volcanosedimentary successions, dismembered ophiolite complexes, gabbro-diorite-tonal complexes and unmetamorphosed volcanic and pyroclastic sequences that are extensively intruded, particularly in

269

F. H. MOHAMED and M. A. HASSANEN 270

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Figure 1. Distribution of major mafic plutonites in the Egyptian sector of the Arabian-Nubian Shield. 1: Gabbro Akarem; 2: El Ginena El Garbiya-Ras Banas; 3: Wadi Ghadir; 4: Gabal Atud; 5: Wadi Dubr; 6: Wadi El Irma; 7: Umm El Rus; 8: Fawakhir; 9: Ras Gharib. Modified after El Ramly (1972), Hashad (1980) and Shirnron (1984).

the northern Egyptian shield, by batholithic granodiorite-granite complexes (e.g. El Ramly, 1972; Vail, 1985).

Mafic plutonites in the Egyptian basement were generated in different tectonic environments and occur in several outcrops throughout the Eastern Desert (Fig. 1). Certain gabbros are known to form an integral part of abducted ophiolitic sequences, whereas others constitute members of subduction-related, talc-alkaline metagabbro-diorite-tonalite complexes, namely the older metagabbros. A variety of intrusive gabbros, known as the younger gabbros, differs from the older ones as being mostly fresh, unmetamorphosed and post- erogenic (El Sharkawy and El Bayoumi, 1979; Takla et al., 1981; El Gaby et al., 1990).

Because island-arc metagabbros and ophiolitic metagabbros are regionally metamorphosed up to the lower amphibolite facies and are petrographically identical (e.g: El Gaby er al., 1990), they are difficult to discriminate and the two types were grouped as older gabbros (e.g. El Ramly, 1972; Takla er al., 1981). The ophiolitic and island-arc metagabbros originated in different tectonic regimes and should therefore have

different geochemical characteristics. This study focuses on the geochemistry and petrogenesis of the metagabbro- diorite complex located at the Umm Naggat district, central Eastern Desert of Egypt. Further, a compilation of geochemical data from earlier and current investigations on different gabbroic rocks of the Egyptian basement is presented in an attempt to assign distinct geochemical characteristics for each type of gabbro.

REGIONAL GEOLOGY AND PETROGRAPHY

The metagabbro-diorite complex (UNGD) forms low-lying terrains with discontinuous exposures that surround the main younger granite pluton of Gabal Umm Naggat (Fig. 2). The complex is represented by heterogeneous basic and intermediate rocks showing varying degrees of colour index, granularity and mineral proportions. The rocks are usually dark green, fine- to coarse-grained, massive and consist essentially of amphibole and plagioclase. The complex intrudes a metavolcano-sedimentary sequence and is itself intruded by voluminous Gabal Umm Naggat granitic magmatism. Contacts with the country rocks are irregular, sharp and of an intrusive nature. Pegmatitic and appinitic gabbro patches are locally present, particularly along the contact with the granite. The metagabbro-diorite complexes, together with the regionally metamorphosed, talc-alkaline volcanics and volcanoclastics sequence, represent the earliest manifestations of island-arc activity in the Arabian- Nubian Shield (Abdel-Rahman, 1990; El Gaby et al.,

1990). Rb-Sr whole-rock dating of the metagabbro- diorite complexes yield early Pan-African ages ranging from -980 to 870 Ma (Hashad, 1980; Abdel-Rahman and Doig, 1987).

The metagabbro is medium- to coarse-grained with a hypidiomorphic texture. It consists of plagioclase, amphibole and relict pyroxene with accessory magnetite and apatite. The plagioclase (An,,,) crystals are generally fresh although some are completely sericitized and saussuritized (epidote and zoisite). Primary amphiboles are represented by brown-green subhedral hornblende with ophitic inclusions of plagioclase laths and abundant magnetite inclusions. Actinolite and uralite (after amphibole and pyroxene) represent the common secondary amphiboles, forming aggregates with chlorite. Chlorite and magnetite locally mantle or completely replace hornblende and pyroxene.

The diorites are medium- to coarse-grained hypidiomorphic rocks, composed essentially of plagioclase, hornblende, biotite and minor quartz; actinolite, epidote and sericite are secondary minerals. Accessory minerals include apatite and sphene with some magnetite, closely associated with the mafic minerals. The plagioclase, dominantly andesine (An&, is sericitized and saussuritized. Hornblende occurs as primary subhedral prismatic green-brown pleochroic crystals, which are commonly altered and

Geochemical evolution of arc-related mafic plutonism in the Unim Naggat district 271

Figure 2. Geological map of the Gabal Umm Naggat area (modified after Refaat, 1970).

LEGEND -- Wadi Deposits

Trachyte Necks and Dykes Serpentinites

Volcanic Breccia El Metavolcanics

Granites and Granodiorites El Metasediments

Metagabbro-Diorite Complex / Faults

replaced by actinolite and/or chlorite. Ragged shreds of brown-yellow biotite, usually overgrown along crystal boundaries of hornblende, are abundant.

GEOCHEMISTRY

The major and trace elements were determined by XRF at the Technical University of Berlin, Germany. The precision of the analytical data wasmonitored by using international rock standards and was found to be better than 3% for the major elements and within 5-10% for the trace elements. Representative analyses of the metagabbro-diorite complex from the Umm Naggat district are given in Table 1.

Major element variations

The UNGD suite is compositionally broad in terms of SiO, (49-61%), CaO (5-11%) and MgO (3-10%) content. The total alkalis consistently increases with SiO, and the rocks range in composition from gabbro to diorite according to the classification scheme of le Maitre (1989; Fig. 3). &O contents increase from 0.17-2.30%

with increasing silica such that the mafic samples could be classified as a low-K series, while the most evolved rocks belong to the medium-K series, according to the scheme of Gill (1981; Fig. 4). The other major elements exhibit sublinear to curvilinear coherent variation trends (Fig. 5). The concentration of MgO and Fe,O, show a sharp inflection (at SiO, -52%) in their variation trends, which may be attributed to the early fractionation of Mg-Fe-rich phases such as olivine and pyroxene from the gabbroic magma. The asymmetrical bell shape-trend of A40,, with a decrease of CaO and increase of Na,O in the late differentiates, suggests an incoming role for plagioclase as the dominant separated phase from the magma. The plots of Fe, Ti, Al and Mn (Fig. 5) show a continuous magmatic evolution, increasing in the gabbro and defining a tholeiitic trend, then sharply declining in the diorite following a talc-alkaline trend. P20y Na,O and KrO show a considerable scatter in the gabbros and the diorites. P205 content decreases slightly and both Na,O and KrO increase with increasing SiO,

On the AFM diagram (Fig. 6), the UNGD rocks plot dominantly within the talc-alkaline field, although the gabbros.plot closer to the tholeiitic field. The continuous

272 F. H. MOHAMED and M. A. HASSANEN

Table 1. Representative major and trace element analyses of the LJmm Naggat metagabbro-diorite (UNGD) complex

and associated basic dykes, Eastern Desert, Egypt

Rock Type

Sample No.

SiOr

TiOr

A1203

Fe203 MIIO

MgG CaO

NarO

K20

p205

LOI”

Total

Cr

Ni

co

Sc

V

cu

Pb

Zn

Rb

Ba

Sr

Ga

Nb

Zr

Y

Th

U

La

Ce

Pr

Nd

Sm

Mg# Zr/Y

Zr/Ti

Metagabbro

948 949 951 970

52.52 50.21 51.29 49.81

0.30 0.40 0.31 0.60

17.88 16.63 19.01 17.42

6.24 6.57 5.58 9.04

0.13 0.12 0.11 0.14

8.78 10.28 9.84 6.76

10.35 10.76 11.12 10.01

2.25 1.42 1.92 1.99

0.17 1.18 0.32 0.82

0.02 0.03 0.01 0.11

1.31 2.11 0.92 2.21

99.95 99.71 100.43 98.91

238 959 512 144

112 153 119 88 67 55 76 54 35 14

47 43 24 77 36 20 49 24 49 10

34 37 26 40 29 22 24 21 15 14

110 118 88 280 263 237 297 223 155 168

55 100 187 173 178 131 43 145 85 27

0 4 0 7 4 7 3 8 11 6

43 51 41 73 75 70 78 62 61 111

Bd 46 7 14 11 27 10 37 28 17

165 129 98 209 294 406 374 583 774 264

318 318 394 531 558 463 490 418 388 430

13 13 14 19 17 15 19 16 16 22

2 1 2 1 2 3 5 2 3 29

14 24 15 36 44 55 61 63 92 282

8 14 10 12 15 16 21 17 21 47

3 7 3 1 3 5 2 6 12 11

Bd 6 3 Bd 1 3 Bd 2 Bd 2

Bd Bd Bd Bd Bd Bd 5 Bd Bd Bd

Bd 5 Bd 13 12 7 Bd 16 68 63

Bd Bd Bd Bd Bd Bd Bd 1 Bd Bd

Bd 8 Bd 12 9 11 8 12 27 30

2 3 2 3 3 3 2 4 6 6

73.59 75.60 77.74 59.69 58.79 59.15 57.88 57.14 54.80 50.20 45.79

1.75 1.71 1.50 3.00 2.93 3.44 2.90 3.71 4.38 6.00 5.62

0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.02 0.03 0.02 0.02

l Major and trace elements were determined by XRF at the ‘I mica1 University of Berlin, Germany.

Diorite 952 966 967 969 972

53.61 56.62 54.77 57.73 61.28

0.66 0.68 0.80 0.65 0.57

17.23 17.10 16.49 16.21 15.62

8.15 7.40 8.36 6.79 5.70

0.14 0.12 0.14 0.11 0.12

5.87 5.41 5.80 4.57 3.49

8.49 7.14 8.16 6.61 5.29

2.31 2.57 2.69 2.24 2.67

0.77 1.41 0.79 1.98 2.30

0.17 0.19 0.19 0.16 0.14

2.35 1.32 1.67 1.85 2.35

99.75 99.% 9986 98.90 99.53

125 103 156 111 81

Basic Dykes

943 944

51.90 49.18

1.99 1.93

17.49 15.52

9.51 8.91

0.14 0.16

4.84 8.80

7.57 9.27

3.25 3.16

1.03 1.06

0.41 0.40

1.78 1.59

99.91 99.98

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magmatic evolution of the UNGD rocks from primitive tholeiites to the more mature talc-alkaline series is also evident in a Miyashiro plot (Fig. 7).

Trace elements

The trace elements also show coherent enrichment/ depletion curvilinear trends with SiO, (Fig. 8). Compatible elements (Cr, Ni and Sr) consistently decrease with increasing SiO,, although they exhibit a sharper decrease in gabbros relative to diorites. Incompatible high field strength elements (Y and Zr) and large ion lithophile elements (Ba and Rb) are positively correlated with SiO,. Such curvilinear trends

defined by major and trace element variations are typical of rock series evolved by crystal-melt fractionation, where the changes in element trends record modifications in the composition of the separating mineral assemblages. The compatible behaviour of Mg, Ca, Cr, Ni and Sr are broadly consistent with subtraction of the observed mineral assemblage (clinopyroxene, plagioclase, anphibole and magnetite) in the most mafic member of the UNGD suite.

Some ratios of incompatible elements (e.g. Zr/Ti, Zr/Y and Nb/Y) can be used as petrogenetic indicators because they are not usually transported in aqueous fluids and tend to remain unaffected during alteration and/or low-grade metamorphism (Floyd and

Geochemical evolution of arc-related mafic plutonism in the Unim Naggat district 273

“k--- Si& wt% --

Figure 3. Total alkahs versus silica chemical classification diagram (le Maitre, 1989) for the Umm Naggat metagabbro-diorite complex (UNGD). Symbols are as follows: A=metagabbro; l =diorite; *alkaline basic dykes. Shown for comparison are the fields for other mafic plutonites from the Egyptian Shield. The stippled, line-shaded and dashed areas encompass the fields of intrusive metagabbros (I- type), ophiohtic metagabbros (O-type), and younger gabbros (Y- type), respectively. Data sources of the fields: younger gabbros: Gabbro Akarem area (Takla et al. 1981), Urn Rus area and Gabal Atud (Ghonehn, 1989) and Wadi Dubr (Hassanen, unpubl.); intrusive metagabbros: metagabbrodiorite suite of El Ginena El Garbiya- Ras Banas district (Takla et al., 1981), Ras Gharib complex (Abdel- Rahman, 1990), and Wadi El-Imra complex (El Sayed,. 1994); ophiolttic metagabbros: Wadi Ghadir ophiohtes (Elbayoumi, 1980) and Fawakhir ophiolites (Hassanen, 1985).

Winchester, 1978; Pearce and Norry, 1979). Since Zr is a more incompatible element than Y and Ti in mantle phases, the Zr/Y and Zr/Ti ratios will increase as the magma evolves by fractional crystallization from basic to acidic compositions. A plot of Zr as a fractionation index (Pearce and Norry, 1979) versus Zr/Y and Zr/Ti ratios (Fig. 9) shows that both ratios consistently increase from gabbros to diorites. The vectors of single- phase and multi-phase assemblages are calculated using Rayleigh fractionation to show the change in melt composition in a closed system fractionation process. The appropriate mineral-melt distribution coefficients for Zr, Y and Ti of basic and intermediate compositions are shown in Table 2. On the Zr/Y vs Zr diagram (Fig. 9a), the gabbros of the UNGD suite show a nearly horizontal trend that is broadly consistent with a combination of plagioclase-clinopyroxene-olivine as crystallizing phases (equivalent to modelled trend 1). Thereafter, the observed trend for the diorite samples shows a remarkable increase in Zr/Y ratio with a concomitant change of the slope to approximate unity. This reflects the onset of amphibole fractionation, which led to a decrease in Y, although Zr will still remain enriched in the liquid (between modelled trends 2 and 4). It is worth noting that the gabbro trend is similar to that of the tholeiitic arc lavas where crystallization of olivine, clinopyroxene and plagioclase are the dominant fractionating phases. This gives rise to a residual melt with relatively constant Zr/Y ratio (Gorton, 1977). The diorite shows a trend typical of volcanic arc talc-alkaline lavas (Brown et al., 1977) with a higher Zr/Y ratio as a result of amphibole crystallization. The mineral vectors

55 SiO, wt% 65

75

Figure 4. K,O versus SiO, discrimination diagram for the UNGD complex. The high-, medium- and low-K fields are from Gill (1981). Symbols and shaded fields as in Fig. 3.

show that amphibole and magnetite can cause an increase of the Zr/Ti ratio due to a decrease of Ti (Fig. 9b). Again, the observed trend of gabbro samples from the UNGD suite is horizontal, consistent with a crystallizing assemblage of clinopyroxene-plagioclase- olivine (modelled trend 1). The observed diorite trend shows a subsequent increase in the Zr/Ti ratio that can be attributed to incoming amphibole and magnetite as crystallizing phases (between modelled trends 2 and 4).

TECTONIC ENVIRONMENT

The abundance of talc-alkaline diorites in a calc- alkaline/ tholeiitic series suggests an island-arc tectonic setting for the UNGD suite. On the Zr-Y-Nb ternary diagram (Fig. 10; Meschede, 1987), the UNGD rocks plot in the field of volcanic arc lavas, whereas the basalt dykes fall in the within-plate field. The Y-Cr discriminant diagram (Pearce, 1982) further confirms the island-arc setting of the suite (Fig. 11) and the basic dyke rocks again plot in the within-plate basalt field.

A striking chemical signature of the volcanic arc basalts is their depletion in HFS elements (e.g. Nb, Zr and Y) relative to LIL elements (e.g. K, Rb, Ba and Sr; Pearce, 1982; McCulloch and Gamble, 1991). A more detailed insight analysis to arc lavas can be verified on MORB-normalized multi-element diagrams (Fig. 12a, b). In addition, the average composition of both tholeiitic and talc-alkaline arc lavas (Pearce, 1982) are plotted to compare them with the UNGD rocks.

MOREnormalized multi-element diagrams (plotted in the order of Pearce, 1982) of gabbros (Fig. 12a) from the UNGD suite closely match island-arc tholeiites, expressed by a selective enrichment of LIL elements (Sr, K and Ba) and a corresponding depletion of incompatible elements (Nb and Y) relative to MORB (Pearce, 1982). Moreover, the diorite patterns (Fig. 12b) exhibit characteristics of talc-alkaline volcanic arc basalk (e.g. Pearce, 1982). They are enriched in LIL elements together with Th, Ce and P. However, the other incompatible elements (Nb, Zr, Ti and Y) remain depleted relative to


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the MORB source. Except for Sr, the diorites exhibit higher levels of both LIL elements and incompatible elements relative to gabbros. The normalized patterns of basic dykes (Fig. 12~) are similar to alkaline basalts of within- plate affinity (Pearce, 1982) and show enrichment in all elements from Sr to Ti relative to MORB.

Geochemical grouping of erogenic-related mafic plutonites in Egypt

The mafic plutonic rocks of the Egyptian shield (mostly of gabbroic composition) were earlier classified by Akaad and El Ramly (1960) into two major groups: older gabbros and post-erogenic younger gabbros.

Noweir and Takla (1975) gave an explanation of some of the petrochemical characteristics of each group. In view of the published work on the Egyptian gabbroic rocks and the present geochemical study, the mafic plutonites in the Egyptian basement complex are subdivided into three distinct groups (cfi Hassan and Hashad, 1990). The first group constitutes a member of the metagabbro-diorite-tonalite association, which is considered to be a syn-erogenic suite. The second group forms an integral part of the ophiolitic sequences (ophiolitic metagabbros) in the Nubian Shield. The third group consists of unmetamorphosed post-erogenic younger gabbros. A compilation of published data (some 150 whole-rock analyses) is represented for the

Geochemical evolution of arc-related mafic plutonism in the Umm Naggat district 275

FeO*

A

Calc-Alkaline

Figure 6. A-F-M diagram for the UNGD complex. The field boundary between the tholeiitic and talc-alkaline fields is after Irvine and Baragar (1971). Symbols and shaded fields as in Fig. 3.

300 , I

h E 100 AA Q II 1 A ..*

A 0-0 f 0 0

1000 1

75

45

0 I &go 3 L

FeO*/

Figure 7. SiO;! versus FeO*/MgO diagram for the UNGD complex. The tholeiitic and talc-alkaline boundary is after Miyashiro (1974). Symbols and shaded fields as in Fig. 3.

100 I I I I 46 50 62

1) 46 50 50 62

Figure 8. Variation diagrams of trace elements versus silica. Symbola as in Fig. 3.


0.001 L 10

Figure 9. Observed and modelled fractionation trends on Zr/Y versus Zr (a) and Zr/Ti versus Zr (b) diagrams. Symbols and shaded fields as in Fig. 3. Multi-phase fractionation vectors are calculated using K, values for both basic (B) and intermediate (I) melt compositions. Modelled fractionation vectors are: 1: p&,cpx,,o&,, (B);

2: pLcpx,oLmt, (1); 3: pLam,mf, (1); 4: plssan-b2cpx0,mL (I). Mineral vectors are calculated using K, values for an intermediate composition. The K, values used are given in Table 2. Tick marks mdicate 50% crystallization (F=0.5).

three different types of gabbroic rocks. Group I: metagabbro-diorite suite of the El Ginena El Garbiya- Ras Banas district (Takla et al., 1981), the Ras Gharib complex (Abdel-Rahman, 1990) and the Wadi El-Imra complex (El Sayed, 1994); Group II: ophiolitic metagabbros collected from the Wadi Ghadir (Elbayoumi, 1980) and Fawakhir area (Hassanen, 1985); Group III: younger gabbros of the Gabbro Akarem area (Takla et al., 1981), the Urn El-Rus area and Gabal Atud (Ghoneim, 1989) and of the Wadi Dubr (Hassanen, pen. comm., 1995).

Both the ophiolitic and younger gabbroic suites have restricted compositions and are commonly dominated by a gabbroic composition. The non-ophiolitic (intrusive) metagabbro suites exhibit wide compositional variations and display more extreme fractionation trends with evolution towards tonalitic compositions (Figs 3 and 4). The majority of the UNGD samples cluster within the field delineated for the non- ophiolitic metagabbro suites (Figs 3,4,6,7 and 9). The ophiolitic gabbros exhibit a typical tholeiitic affinity, while the non-ophiolitic metagabbros and younger gabbro groups have a tholeiitic/calc-alkaline character (Figs 6 and 7). Moreover, the non-ophiolitic metagabbros are dominated by more evolved calc- alkaline compositions when compared with the younger gabbros. The latter shows primitive tholeiitic

2Nb

Zr14 Y

Figure 10. Zr/4-Y-2Nb digram for the UNGD complex. Fields are after Meschede (1987). Symbols and shaded fields as in Fig. 3. WPA: within-plate alkali basalts; WlT within-plate tholeiites; MORB: mid- ocean ridge basalt; VAB: volcanic arc basalt.

i I il

Figure 11. Cr-Y discrimina lion diagram (Pearce, 1982) for the UNGD complex. Symbols as in Fig. 3.

compositions (Figs 6 and 7). The more primitive nature of the younger gabbro suites relative to the non- ophiolitic metagabbro is again confirmed on the binary Zr versus Zr/Y and Zr/Ti diagrams (Fig. 9). The younger gabbros follow the tholeiitic fractionation trend (Gorton, 1977) dominated by the crystallization of olivine, clinopyroxene and plagioclase (trend 1, Fig. 9). This is consistent with the mineralogy of the younger gabbros (Takla et al., 1981). It should be noted that the non-ophiolitic metagabbro suites display a more fractionated trend, which characterizes the talc-alkaline volcanics (Brown et al., 1977) where amphibole is a common precipitating phase during fractionation (between modelled trends 2 and 4, Fig. 9). The ophiolitic metagabbros are characterized by lower Zr/Y and Zr/ Ti ratios than the non-ophiolitic varieties. These ratios


Table 2. Recommended mineral-melt partition coefficients’

Basic

Intermediate

Ti Zr Y

Ti Zr Y

01 Pl cpx opx am mt 0.02 0.04 0.30 0.10 1.50 7.50 0.01 0.01 0.10 0.03 0.50 0.10

0.01 0.01 0.50 0.20 1.00 0.20 0.03 0.05 0.40 0.25 3.00 9.00 0.01 0.03 0.25 0.08 1.40 0.20 0.01 0.06 1.50 0.45 2.50 0.50

’ After Pearce and Nony (1979).

show a regular increase with the fractionation index (Zr). The mineral vectors reveal that both ratios are significantly controlled by the progressive separation of clinopyroxene and magnetite. The latter two minerals, together with olivine and plagioclase, seem to control the evolutional trend of the ophiolitic metagabbros (trend 2, Fig. 9). On the Zr-Nb-Y diagram of Meschede (1987), the ophiolitic metagabbros are comparable with the tholeiitic MORB volcanics, whereas both the younger and older non-ophiolitic gabbros plot entirely in the volcanic arc basalt field (Fig. 10). MORB-normalized geochemical patterns for the younger gabbros and non-ophiolitic metagabbros (Fig. 13b, c) show a close resemblance to the volcanic arc basalts (enrichment in LIL elements Sr, K and Ba and depletion in I-IFS elements Zr, Y and Ti relative to MORB).

The non-ophiolitic metagabbros (Fig. 13a) exhibit relatively higher Sr, K and Ba abundances than the younger gabbros. The enrichment of the LIL elements is attributed to their transport by aqueous fluids derived from the dehydrated subducted slab to the overlying mantle wedge (Saunders and Tarney, 1979; McCulloch and Gamble, 1991; Davidson et al., 1993). Therefore, this argues the derivation of the non- ophiolitic metagabbros from a hydrated mantle source more enriched in a subduction-zone component relative to the younger gabbros. The ophiolitic metagabbros have geochemical patterns significantly different from the other two groups. They exhibit a flat pattern compared to the tholeiitic MORB and are closely similar to the basaltic tholeiites of Geotime lavas (Pearce, 1982).

The terms I-type and O-type gabbros are used here to correspond to the non-ophiolitic ‘intrusive’ island- arc metagabbro (group I) and the ophiolitic metagabbro (group II), respectively. The Y-type gabbro (group III) encompasses all the fresh younger gabbros with island- arc characteristics. The petrological features and the major chemical pecularities of the three gabbroic groups are summarized in Tables 3 and 4. As noted above, the I-type gabbroic rocks display a more fractionated nature, while the other two groups (0- and Y-types) still have a low and restricted range of fractionation. Table 3 shows that no single chemical parameter effectively discriminates between the I-type and the

other two types of gabbro. Nevertheless, the average of some element abundances and elemental ratios (e.g. Ni ~100 ppm, Co ~40 ppm, SiO, >50 wt.%, Mg# <6O,Zr/Y >3 and Zr/Ti 20.02) are collectively suggested to be useful discriminating parameters for the I-type relative to the other two types.

PETROGENESIS

The UNGD exhibits a continuous magmatic evolution from primitive arc tholeiites to more mature talc-alkaline magmatism, a feature characteristic of many island arc suites such as St. Martin Island, Northern Lesser Antilles (Davidson et al., 1993), the Ariab Island arc, the Red Sea Hills, Sudan (Schandelmeier et al., 1994) and the Shirahama Group, Japan (Tamura, 1994). The geochemical characteristics described above suggest that the UNGD suite has evolved from a mantle source region in which the subduction zone components enhanced the LIL elements in the magmas, particularly during the later stages of evolution, to generate the dioritic member of the complex.

The role of source heterogeneity

Compositional heterogeneity in the source of arc magmatism is believed to result from an addition to the mantle wedge of incompatible elements by fluids or melts released from the undergoing slab (e.g. Gill, 1981; Peccerillo and Wu, 1992). This mechanism of mantle metasomatism generates a marked increase in the abundances of incompatible elements, a feature which is likely to be inherited by magmas generated by the melting of such a source. These chemical aspects are a common feature in many island-arc volcanics, where an initial period of primitive tholeiitic volcanism is followed by a shift to increasing silica compositions of talc-alkaline affinity. This implies a change in magma source composition as a result of the incorporation of a subduction zone component.

Tamura (1994) provided a model in which tholeiitic and talc-alkaline arc magma associations can be evolved from two primary magmas, each generated independently from the dry and wet regions in the same


Sr K Ba Th Nb Ce P Zr Ti y

Figure 12. Mid-ocean ridge basalt-normalized incompatible element Figure 13. Range of mid-ocean ridge basalt-normalized incompat- patterns of the UNGD complex. (a) metagabbros; (b) diorites; (c) ible element patterns of mafic plutonites, Egypt (stippled pattern). alkaline basic dykes. Data for comparison are: the average composi- (a) ophiolitic metagabbros (O-type); (b) intrusive metagabbros (I- tions of tholeiitic and talc-alkaline volcanic arc basalts and within type); (c) younger gabbros (Y-type). Data sources for the plate basalts (Pearce, 1982). Normalizing values are from Pearce compositional ranges are as in Fig. 3. Data for the different types of (1982). basalts are as in Fig. 12.

mantle diapir. A hydrated wet mantle diapir was first generated above the cool subducting slab. It further ascends and is heated as it penetrates the overlying hot mantle wedge. Subsequently, the heated diapir has a dry and hot rind while its interior still wet and relatively cool. Temperature and HrO gradients established in the diapir result in simultaneous generation of basaltic magma from its drier parts and boninitic magma from its wetter regions.

The role of fractional crystallization

The curvilinear trends defined by some element variations in the rocks of the UNDG suite, together with the rapid depletion of Ni and Cr (Figs 5 and 8), suggest that a fractional crystallization process is potentially significant during the evolution of the complex. To constrain the fractional crystallization hypothesis quantitatively, the least-squares approximation program of Wright and Doherty (1970) was applied. Since the major element variation trends show a marked changeover of the entire fractionation trend from the less differentiated tholeiitic magma (gabbros) to the talc-alkaline diorites, two fractionation stages are therefore postulated.

-+- Calc-AlkalineVAB

f Tholeiitic “AB

A D Op_hioht’c Mrtogobbros

(0 type)

“. -Y- Tholcnitjc MORB

t I o-1 1 I I I I I 1 ._I

Sr K Ba Th Nb Ce P Zr Ti y

In the first stage, the parent magma is represented here by the least differentiated mafic sample with the highest Mg number (no. 951), while the most evolved gabbroic sample (no. 970) is used as the daughter (residual liquid) magma. In a similar way, the least evolved diorite (no. 952) and the most evolved quartz diorite (no. 972) were selected as respective parent and daughter compositions (second stage). The composition of the fractionated minerals (plagioclase, augite and amphibole) was taken from the metagabbro-diorite complex of the Gharib block, Egypt (Abdel-Rahman, 1990), whereas the composition of olivine and magnetite is from Tamura (1994). The results of calculations are presented in Table 5. The program also calculates the sum of the squares of the residuals; R* (a low value indicates a good fit). As shown in Table 5, the evolution of the gabbroic rocks is controlled by the fractionation of augite, plagioclase and olivine in the respective ratios; 38.81:33.17:28.02, with a residual liquid (no. 970) of 28.46%. Although R2 is not low, it gives a reasonable fit. The results of the geochemical modelling are consistent with the petrographic observations, where both plagioclase and clinopyroxene are the most abundant mineral phases in the gabbros. The model also


Table 3. Average major-oxide and trace element compositions of mafic plutonites and of Umm Naggat

metagabbro-diorite suite from the Egyptian Shield

TYPE

SiO2 51.84 4.17 49 TiOz 0.84 0.56 49 A1203 16.80 2.22 49 Fez03 8.98 3.59 49 MnO 0.17 0.04 49

MgO 6.10 2.61 49 CaO 9.63 2.46 49 Na20 2.71 0.86 49 K20 0.76 0.51 49 F205 0.14 0.08 49

Mg# 56.18 10.93 49 Cr 318 464 49 Ni 90 88 49 co 40 15 49 Sc 41 18 38 V 251 137 43 Rb 13 8 49 Ba 206 145 49 Sr 364 194 49 Nb 4 3 38 Zr 59 34 49 Y 17 8 49 Th nd nd nd Ce 11 9 26 Zr/Y 4.17 2.55 49 Zr/Ti 0.02 0.01 49

I-type X S N

o-type X S N

49.45 3.54 30 1.61 1.25 30

15.06 2.18 30 11.52 3.26 30 0.13 0.12 30 9.52 4.08 30 8.78 1.73 30 2.71 0.89 30 0.53 0.41 30 0.08 0.09 30

59.75 14.14 30 508 1077 -19 108 92 19 84 42 19 nd nd nd nd nd nd 28 16 19

264 140 19 355 207 19 12 4 19 86 58 19 29 11 19 nd nd nd nd nd nd 2.8 1.25 19

0.01 0.01 19

y- type X S N

49.44 3.15 18 0.76 0.40 18

15.88 3.49 18 9.42 1.05 18 0.19 0.04 18

10.69 3.17 18 9.96 1.50 18 1.99 0.69 18 0.30 0.24 18 0.10 0.12 18

68.12 6.75 18 654 689 18 128 61 28 63 16 12 36 14 12

200 43 12 6 8 18

65 32 18 323 204 18 16 19 18 49 23 18 19 7 18 3 1 12

16 15 12 2.88 0.99 18 0.01 0.01 18

Umm Naggat X S N

53.81 3.20 16 0.82 0.48 16

16.93 0.66 16 7.50 1.14 16 0.13 0.01 16 6.56 2.25 16 8.34 1.87 16 2.45 0.47 16 1.09 0.57 16 0.17 0.12 16

61.93 8.91 16 215 226 16 71 39 16 37 17 16 26 8 16

193 62 16 22 11 16

297 194 16 417 78 16

2 1 16 57 36 16 16 5 16 4 2 16 13 21 16

3.53 1.47 16 0.02 0.01 16

N: number of analyses; X: mean; S: standard deviation; nd: no data available.

Source of mafic plutonites as in the text.

closely matches the estimated mineral assemblage (trend l), which is taken to represent the fractionation trend for gabbros using trace element data (Fig. 9).

The least-squares fractionation model for diorite shows a dominant role for plagioclase, amphibole and magnetite crystallization in the following ratios; 50.19:46.59:3.22, with the residual liquid (no. 972) of 30 44% .W bnl tiofl? shows a good fit. Again the model is consistent with the mineral assemblage trends, particularly trend 3, which is suggested to represent the evolutionary fractionation trend of the diorites (Fig. 9). The coherent smooth variation (Figs 5 and 8), combined with the results of geochemical modelling indicates that a fractional crystallization process played a vital role during the evolution of the UNGD suite. However, the fractionation of two distinct mineral assemblages is required in order to explain the divergence of chemical attributes within the complex from the tholeiitic mafic compositions ti the intermediate calc+&aline compositions.

SUMMARY AND CONCLUSION

The Umm Naggat metagabbro-diorite (UNGD) suite forms a heterogeneous assemblage of gabbroic and dioritic rocks that are exposed as low hills surrounding the main younger granite pluton of Gabal Umm Naggat. The suite constitutes a compositional continuum with a wide range of major element composition. The major oxides exhibit smooth coherent and continuous variations in the UNGD suite from primitive arc tholeiites to more mature talc-alkaline rocks. The modelled fractionation trends for some incompatible elemental ratios (e.g. Zr/Y and Zr/Ti) indicate that the gabbroic rocks of the UNGD suite are dominated by olivine, clinopyroxene and plagioclase fractionation. The diorites exhibit a trend typical of volcanic arc calc- alkaline lavas with amphibole and magnetite as the dominant separated phases. On the MOREnormalized, multi-element plots, the UNGD suite rocks exhibit many


Table 4. Field, petrological and geochemical characteristics of mafic plutonites in the Egyptian Shield

r GROUP IYPE Field Relation

Mineralogy

Chemistry

%# jiO2 (wt%)

vi (ppm) zo (PPm) Zr/Y

Group I Group II Group III I-type O-type Y-type

Occurs as large masses as an Large masses, genetically Small plutons of limited early differentiate of the associated with ophiolitic extension (~20 km2) with gabbro-diorite-tonalite- serpentinite and tholeiitic intrusive contacts; associated granodiorite series basic volcanics. They have volcanics are of talc-alkaline

restricted petrological affinity variation

Hornblende + plagioclase f Hornblende + plagioclase f plagioclase f clinopyroxene f clinopyroxene + opaque clinopyroxene f olivine + olivine f opaque oxides and (ilmenite > magnetite) f opaques (magnetite > sulphides (pyrite, quartz f secondary minerals ilmenite). Secondary minerals chalcopyrite, pentlandite and including chlorite, actinolite, are chlorite and actinolite, pyrrohotite) epidote and calcite often metasomatized locally to

rodingite (sea floor metamorphism)

Wide variations in major and Moderate variations in major Restricted variations in major ltrace element compositions and trace element and trace element (see Table 3) 32-75 (56*)

compositions 29-79 (60)

compositions 49-79 (68)

43-64 (52) 40-57 (49) 46-59 (49) 15-456 (90) 13-302 (108) 52-248 (128) 17-93 (40) 51-202 (84) 34-81 (63) 1.27-11.55 (4.17) 1.13-5.57 (2.80) 0.65-4.03 (2.88)

Magma Type Mildy tholeiitic to evolved Tholeiitic Tholeiitic to mildy talc-alkaline talc-alkaline

Tectonic Setting Island-arc (VAB) Oceanic tholeiites chemically Island-arc (VAB) close to the MORB composition.

Metamorphism Greenschist up to amphibolite Greenschist up to epidote- Unmetamorphosed facies metamorphism amphibolite facies, often

overprinted by sea floor metamorphism

vhneralization No mineralization is Rarely contains ore minerals, Ilmenite, Cu-Ni sulphides or recorded but chromite lenses occur in associated Au mineralization

the associated serpentinites sxample Ras Gharib, W. Dubr, Umm Ghadir, Fawakhir Umm Rus, Atud, Gabbro

Naggat, W. El-Imra Akarem, Abu Ghalaga * Parentheses indicate average values.

geochemical features of island-arc lavas, such as the enrichment in the LIL elements (Sr, K and Ba) with a corresponding depletion in the incompatible elements (Nb, Zr, Ti and Y).

A comparison between the UNGD suite with other mafic plutonites in the Egyptian Shield suggests three chemically different groups. The first group constitutes a member of the metagabbro-diorite-tonalite series and represents an intrusive ‘non-ophiolitic’ type (I-type). The second group forms an integral part of the ophiolitic sequences (O-type), while the third group represents the unmetamorphosed younger gabbros (Y-type). Although some petrological similarities do exist between the three groups, each has distinct geochemical attributes. Compared with the Y-type, the I-type is

dominated by the more evolved, talc-alkaline compositions of arc lavas. The Y-type exhibits the primitive compositions of arc tholeiites. The O-type has characteristics of the tholeiitic MORB volcanics. The whole-rock chemistry of the mafic plutonites shows that no single chemical parameter could distinguish the more fractionated I-type metagabbros from the relatively primitive O-type metagabbros and the Y-type fresh gabbros. However, averages of certain element concentrations and ratios do point to a discrimination of the I-type metagabbros.

The geochemical characteristics of the UNGD suite suggest its evolution from a mantle source region enriched in a subduction zone component. The results of geochemical modelling confirm the role of crystal

Tab

le 5

. R

esul

ts o

f cry

stal

fra

ctio

nati

on l

east

-squ

ares

mod

elli

ng f

or U

mm

Nag

gat

met

agab

bro-

dior

ite

com

plex

(U

NG

D)

(Wri

ght

and

Doh

erty

, 197

0)

Obs

erve

d O

bser

ved

Cal

cula

ted

daug

hter

pa

rent

pa

rent

N

o. 9

70

No.

951

So

luti

on

Obs

erve

d da

ught

er

No.

972

Obs

erve

d C

alcu

late

d pa

rent

pa

rent

N

o. 9

52

Solu

tion

SioZ

T

ia

Al2

03

FeO

* M

I0

*gO

C

aO

Na2

0

52.3

1 0.

6 17

.42

8.13

0.

14

6.76

10

.01

1.99

0.

82

Sta

ge

-

49.7

9 0.

31

18.5

1 5.

02

0.11

9.

84

11.1

2 1.

92

0.32

1 St

age

- 2

52.0

4 P

I 33

.17

61.2

8 53

.61

55.1

3 Pl

50

.19

0.89

au

g 38

.81

0.57

0.

66

0.79

am

46

.59

18.8

7 01

28

.02

15.6

2 17

.23

17.6

1 m

t 3.

22

5.42

R

r =

3.19

7 5.

13

7.33

7.

53

R2

= 1.

07-l

0.

1 F

= 0.

285

0.12

0.

14

0.14

F

= 0.

304

9.87

3.

49

5.87

6.

05

11.4

8 5.

29

8.49

8.

75

2.81

2.

67

2.31

3.

06

0.29

2.

3 0.

77

0.93

R

2: S

um o

f th

e sq

uare

s of

the

res

idua

ls;

F: F

ract

ion

of l

iqui

d re

mai

ning

.

Min

eral

s da

ta:

pl (

pla

gioc

lase

); am

(a

mph

ibol

e);

aug

(aug

ite)

; 01

(ol

ivin

e);

mt

(mag

neti

te).

The

com

posi

tion

s of

pla

gioc

lase

, am

phib

ole

and

augi

te a

re f

rom

the

min

eral

ana

lyse

s of

the

met

agab

bro-

dior

ite

com

plex

fr

om R

as G

hari

b ar

ea,

Egyp

t (A

bdel

-Rah

man

, l!B

O),

whe

reas

the

cbm

posi

tion

of

olk

ne

and

mag

neti

te

are

from

Sh

irah

ama

volc

anic

gr

oup,

Ja

pan

(Tam

ura,

19

94).


melt fractionation in a mantle-derived magma. However, a separation of two different mineral assemblages is required to explain the changeover of chemical attributes within the suite from a tholeiitic mafic composition to an intermediate talc-alkaline composition.

Acknowledgements

The authors are deeply grateful to Dr G. Matheis for the XRF facilities at TU Berlin, Germany. Thanks to Dr Y. Anwar of Alexandria University for his fruitful discussions and reviews of an early draft. Dr Bogoch and an anonymous reviewer are also greatly acknowledged for their valuable comments and critical reviews of the manuscript.

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