the petrology of the abu zawal gabbroic intrusion, …rjstern/egypt/pdfs/general/... · the...

lovmal of.&ican Emh Sciencw Vol. 22. No. 2, pp. 147-157. 19% c%pyright819%ElseviascicnaLtd

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The petrology of the Abu Zawal gabbroic intrusion, Eastern Desert, Egypt: an example of an island-arc setting

F. F. ABU EL-ELA

Geology Department, Assiut University, Assiut, Egypt

(Received 19 January 1994: revised version received 8 December 1995)

Abstract - The Abu Zawal gabbroic intrusion consists of three gabbroic zones. Each of these zones has a distinc- tive mineralogical composition. Plagioclase and altered clinopyroxene are abundant in the lower zone gabbro (umlitized gabbro). Hornblende and Fe-Ti oxides occur in the middle and upper zone gabbros (hornblende and feno-gabbros, respectively). Fe-Ti oxides are more abundant in the upper zone gabbro. The composition of the plagioclase cores ranges from An.ss (Tower zone) to Arus (upper zone). The primary clinopyroxene and cakzic amphibole are augite and magnesio-hornblende, respectively. Application of the hornblende geobarometer indicates a pressure of crystallization ranging from 2.9 to 3.5 kbar. in addition application of the amphibole-plagioclase geothermometer yieY1d.s crystalhzation temperatures of about 1050-11CQ“C. Major oxide, trace element and RRE data are suggestive of an island-arc evolved high alumina basalt as the parent for these gabbros. The differentiation of the gabbmic zones can be accounted for by low pressme, &sed-system in situ aystaUization under wet conditions.

Resume - L’intmsion gabbrolque de Abu Zawal est compo&e de trois ensembles gabbroiques, presentant chacun une composition min&&gique distincte. Dans 1’ ensemble inf&ieur abondent le plagioclase et le clinopyrox&ne ah&n? (gabbro ourahtise), tandis que darts les ensembles moyen et sup6rieur la hornblende et les oxydes de Fe-Ti sont pr&ents (respectivement gabbros a hornblende et ferrogabbros). C’est dans l’ ensemble gabbrdque superieur que les oxydes de Fe-Ti sont davantage abondants. La composition des noyaux de plagioclases zon& varie de AXIS (ensem ble inferieur) a An10 ( ensemble sup&ieur). Le clinopyrox&ne primaire et Famphibole calcique correspondent respectivement 21 de l’augite et B de la hornblende magnesienne. L’apphcation du g&obarom&e hornblende indique une pression de cristallisation comprise entre 2,9 et 3,5 kbar. Par atlleurs, l’utilisation du g&othermometre amphibol~plagioclase donne des temperatures de cristalhsation d’environ lOt30-1100°C. Les &ments majeurs et en trace ainsi que les Terms Rams sugg&ent pour ces gabbros un magma parental de type ba- salte d’art insulaire, &volt16 et hyper&mineux. La d&%nciation en ensembles gabbroiques distincts peut s’ex- phquer par un syst&ne de cristallisation fern& in situ, A base pression et sous conditions hydratees.

INTRODUCIION

The Late Precambrian Pan-African gabbroic rocks in the Eastern Desert of Egypt occur in two main groups. The first group was mapped as an epidiorite complex (El-Ramly and Akaad, 1960) or as metagabbros and diorites (Akaad and Essawy, 1964; El- Ramly, 1972) or as older metagabbros (Takla et al., 1981). This group is nowadays considered to represent a member of an ophiolite sequence (El-Sharkawy and El-Bayoumy, 1979,: Abu El-Ela, 1990,199l). The second group was mapped as younger gabbros (Takla, 197l; El-Ramly, 1972; Basta and Takla, 1974a, b). These gabbros are post Hammamat (molasse-type sediments) intrusions, presumably just older than the post tectonic younger granites.

Although the concept of Precambrian plate- tectonics is generally accepted, the plutonic equiva- lents of island-arc andesites and dacites, as well as mantle derived island-arc gabbros in the Eastern

Desert of Egypt, are less well-known (El-Gaby et al., 1988). The mantle derived island-arc gabbro may have been mapped by some workers as first group gabbros.

The present paper deals with the Abu Zawal gabbroic intrusion from the point of view of mineral chemistry, major and trace element bulk analyses and REE. These are used to unravel the magmatic evolution of the gabbroic intrusion and to infer some constraints on the origin of the parent magma.

GEOLOGY

The Abu Zawal gabbroic intrusion has been mapped as amphibolite (El-Tahir, 1978) or as part of an island-arc association (Sharara et uZ., 1998). It forms an elongate body (25 km?) trending northeast- southwest (Fig. 1). The gabbroic rocks are intruded by syn-kinematic granodiorites, comparable to the Gl granites of Hussein et al. (1982), and post-kinematic

147

148 F. F. ABU ELELA

* , ,I

26U 5 3;2ioii

.................. .................. .................. . ................... ................... . .................... .................... .................... ................... ................... .................. .................. .................. ...... ............ ................ ................ . . ... . . ....... .

1 Wadi deposlls L-l + 1 Post-kinematic granites II- ( Youngest) u = 1 . . . .

El :I:: Syn-kinemat i c granites

c

c

Figure 1. Geological map of the Abu Zawal gabbroic intrusion.

granites, comparable to the G2 and G3 granites of Hussein et al. (op. tit). The contacts are sharp and irregular. Swarms of gabbroic xenoliths are enclosed with@ the granitic intrusions. The xenoliths are angu- lar, possess sharp boundaries and are highly dis- sected by granitic veinlets. The Abu Zawal gabbroic rocks are intruded into metavolcanics of island-arc affinity (Charara et al., op. tit) outside of the south- western part of the map area. The contacts are mostly

sharp, although the gabbroic intrusion sent tongues into the metavokanics.

Three zones of gabbroic rocks are disthguished in the Abu Zawal gabbroic intrusion, a lower zork (LX), middle zone (MZ) and upper zone (UZ), passing from the western to the eastern edge of the intrusion (Fig. 1). The division into zones is based on the distribution of rock types and textures. The LZ is represented by fine- to medium-grained uralitized gabbro,

The petrology of the Abu Zawal gabbroic intrusion 149

Table 1. Selected plagioclase analyses from the Abu Zawal gabbroic intrusion

TiOz 0.05 0.05 0.03 A1203 28.06 27.60 27.03 Fee* 0.06 0.07 MlIO 0.01 0.01 0.01 CaO 11.89 9.74 9.41 Na20 4.71 6.06 6.31

K20 0.10 0.10 0.19 Total 99.92 99.18 99.32

0.02 25.89

8.39 6.74 0.20

98.74 FeO”=totdinmasFeO.

Table 2 Selected clinopyroxene analyses from the Abu Zawal gabbroic intrusion.

Sample NC Lower Zone Middle Zone Upper Zone

SiO2 TiOz

Al203

FeO* MIIO

MgO CaO Na20 Total

(218C) -210 52.44 52.71

0.17 0.22 0.12 0.12 0.84 11.25 0.95 0.65 8.66 9.44 9.39 9.82 0.34 0.38 0.40 0.55

14.51 14.09 13.35 12.99 22.08 21.82 23.02 22.70 0.18 0.20 0.22 0.23

99.47 100.15 99.89 99.77 F~=totalimmasFeO

which is composed mainly of plagioclase and augite. Augite is partly to completely altered to actinolite. Hornblende was not found. Primary (igneous) Fe-Ti oxides are lacking. An adcumulus texture is characteristic of this zone. The average modal composition (vol.%) is: plagioclase 66.8X, augite and actinolite 31.9% and fine opaques 1.3%. The MZ is represented by medium-gra:med hornblende gabbro. It is composed essentially of plagioclase, pale-brown hornblende and altered augite. Small quantities of primary ibnenite and magnetite occur in the rocks of this zone. The presence of a considerable amount of pale-brown hornblende and the presence of some primary Fe-Ti oxides characterize the MZ gabbro compared with the LZ gabbro, in which both hom- blende and primary Fe-Ti oxides are absent. The average modal composition of hornblende gabbro is: plagioclase 68.2%, pale-brown hornblende 22.5%, altered augite 4.4%, Fe-Ti oxides 3.6%, quartz 0.7% and apatite 0.6%. An adcumulus texture is also characteristic of this zone. The LJZ is represented by coarse to pegmatoidal ferro-gabbros. It is composed mainly of plagioclase, brown hornblende and Fe-Ti oxides. Al- tered augite and apatite are minor phases. A great abundance of Fe-Ti o)ddes is characteristic of this zone and results from the high TiOr content of these

54

1

Si02

?§j

46- --

42 W3 I I

Figure 2. Clhopyroxene dkaimhnt diagram of le EJas (l%Z) for the Abu Zawal gabbroic intrusion. O=LZ gabbro; ??=h-lZ gabbro; ??=UZ gabbro.

rocks. The Fe-Ti oxides are enclosed in brown hom- blende and augite relicts and they also form interstitial grains between hornblende and plagioclase. Green hornblende occurs in these rocks and appears to have been formed by the recrystallization and replacement of brown hornblende. A mesocumuhis texture is characteristic of this zone. The average modal composition of the ferro-gabbro is: plagioclase 56.2%, brown hornblende 22.0%, Fe-Ti oxides l&8%, altered augite 2.1% and apatite 0.9%.

The relation between the LZ gabbro (uralitized gab bro) and the MZ gabbro (hornblende gabbxo) is transi- tional, whereas that between the middle and upper zone gabbros (ferrogabbro) is sharp. No primary igneous layering was seen in any of the zones. The contacts with the cormtry rocks are generally sharp with no chilled margins and no development of magma&es.

MINERALOGY

Compositions of the analysed minerals were determined in polished thin sections with a Jeol Jxa 8600 superprobe and Tracer 5500 ED, using wavelength dispersive techniques for Na, Cr, Mn and Fe and en- ergy dispersive spectrometry for Mg, Al, Si, K, Ca and Ti. Operating conditions were 20 kV accelerating voltage and 10 nA sample current. Matrix corrections were applied using a ZAF program. The analyses were carried out at the Department of Geochemistry, Utrecht University, the Netherlands. Each mineral analysis represents an average of four points.

Plagioclase is the most important mineral phase in the Abu Zawal gabbroic intrusion. The average core compositions of plagioclase for the LZ is Anss. In the MZ, the average ranges from An474 and in the UZ it is An40 (Table 1).

150 F. F. ABU ELELA

Table 3. Selected calcic amphiboles from the Abu Zawal gabbroic intrusion.

Sample N#

SiO2 TiQ

i

Fe0 MnO

Ml@ CaO NazO KZO Total

Middle Zone Upper Zon Reaction Amphibole!

(n8C) -210 (202A) (218C) Mg-Hb Mg-Hb Act. Act. -Hb

45.44 45.64 46.66 52.48 50.92 1.78 1.74 0.7l 0.10 0.88 8.43 8.18 7.73 3.59 4.98 15.03 15.34 15.09 10.80 12.73 0.25 0.32 0.29 0.22 0.26 12.21 12.39 12.92 18.12 15.82 12.31 11.79 12.17 11.92 12.12 0.95 0.93 1.08 0.23 0.39 0.82 0.83 0.60 0.05 0.18 97.22 97.16 97.25 97.00 98.28

7.50 7.25 Si 6.50 6.25 5.75 6 100

P A A

.

250 * 0 c w 00 D E

P 8

O< F G H I J

Figure 3. Composition of cakic amphiboles (after Lake, 1978) in the Abu ZawaI gabbmic intrusion. Whomblende of the MZ; W=hornblende of the UZ; A=reaction amphibole of the LZ; A=reaction amphibole of the LZ. A=actinoIite; B=actinolitic hornblende; C=Mg-hornblende; D=tschermakitic hornblende; E=tschermakite; F=fernxxtinoIite; G=ferro-actinoIitic hornblende; H=ferro-hornblende; I=ferro-tschermakitic hornblende; J-ferro- tschermakite.

The clinopyroxene is classified as augite according to the scheme of Poldervaart and Hess (1951). It occurs as irregular crystals and as relicts in uralitic amphiboles. Its low A1203 content (0.65-1.25 wt.%; Table 2) is suggestive of crystallization at low pressure (Green and Ringwood, 1968). The clinopyroxene plots in the subalkaline field (Fig. 2) of le Bas (1962).

Calcic amphiboles form pale-green fibrous rims around the clinopyroxene, but they also occur as sub hedral pale-brown to brown crystals interstitial between the palgioclase crystals, especially in the hornblende gabbro and ferro-gabbro. The amphiboles forming rims around clinopyroxene occupy the actinolite and the actinolitic hornblende fields (Fig. 4) on the classification diagram of Leake (1978), pointing to an origin by replacement of clinopyroxene (Nakajima and Ribbe, 1981). The subhedral amphibole crystals in the hornblende gabbro and ferro-gabbro occupy the magnesio-hornblende field on the same classification diagram (Table 3; Fig. 3).

Application of the hornblende geobarometer of Hammarstrom and Zen (1986) and Hollister et al. (1987) suggests crystallization at pressures of about 2.9 to 3.5 kbar. This range of crystallization pressure can be reasonably extrapolated to all of the gabbroic zones, as is also corroborated by the low AhO3

Table 4. Selected oxide analyses from the Abu Zawal gabbroic intrusion.

FeO* = toti ironasFe0

Magnetite Ilmenite (218C,MZ) (210,UZ) (218C,MZ) (210,UZj

0.30 0.20 0.48 0.16 0.03 0.21 45.42 46.48 0.07 - 0.08 0.18 0.11 0.12 -

91.51 91.51 50.96 49.55 2.01 2.30

0.07 0.08 0.24 0.18 0.09 0.20 0.28 0.02

92.18 92.32 99.47 98.87

content of clinopyroxenes. In addition, application of the amphibole-plagioclase geothermometer (Bhmdy and Holland, 1990) yields crystallization temperatures of about 1080 to 1lOO’C for the amphibole- plagioclase pairs in the middle and upper zones.

Among the opaque oxides, ilmenites from the middle and upper zones are Al, Mg and Cr poor but Mn rich (~2.0%; Table 4) and the magnetites are Ti, Al, Mg, Cr and Mn poor (Table 4).

GEOCHEMISTRY

Representative samples of the Abu Zawal gab broic intrusion have been analysed for major and trace elements (Table 5). The major elements have been determined by ICP methods using an ARG 34000 emission spectrometer. SQ, Fe0 and LO1 (Loss on Ignition) were determined using the wet- chemical methods of Shapiro (1975). Trace elements were determined by an automated Philips 1400 XRF spectrometer. REE were determined by instrumental neutron activation analysis using the methods of de Bruin (1983). All analyses were carried out at the Geochemistry Department, Utrecht University and at IRI, Delft, the Netherlands.

Whole rock chemistry

Table 5 shows chemical analyses of representative samples from the Abu Zawal gabbroic intrusion ar- ranged in order of increasing FeO*/MgO and hence decreasing Mg number ~g#=lOO molar MgO/ (MgO+FeO)]. The uralitized gabbro (LZ) has FeO*/ MgO ranging from 0.56 to 0.85 and Mg# from 77.5 to 69.7. The hornblende gabbro (MZ) has FeO*/MgO ranging from 1.16 to 1.38 and Mg# from 63.4 to 58.4 and the ferro-gabbro (LIZ) has Fo/MgO ranging from 1.97 to 2.90 and Mg# from 51.1 to 42.2. Therefore, the three gabbroic zones may represent three stages of fractional crystallization in which the lower, middle and upper zone gabbros represent the early, middle

Tabl

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Maj

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wt%

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f th

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awal

gab

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P205

L.

O.I.

To

tal

Mg#

Tr

ace

elem

R

b Sr

Ba

Zn

C

ll co

N

i V

C

r Y zr

N

b n.d.

= n

ot de

202A

51

3R

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..

0.36

19

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1.50

3.

46

0.10

8.

53

11.7

0 2.

81

0.20

0.

05

Low

er Z

one

(ura

litiz

ed g

abbr

os)

Mid

dle

Zone

(ho

rnbl

ende

gab

bros

) U

pper

Zon

e (f

erro

-gab

bros

) 21

2 21

5 20

98

206C

22

1 20

3 21

9 23

0B

218

208

250

220

210

X.4?

52

.82

51.4

2 57

17

i-__

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6&i

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.19

55.9

4 53

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45.9

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0.35

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0.29

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0.63

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0.

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1.48

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3.77

18

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17.1

9 18

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18.6

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17.4

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14.8

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14.5

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1.48

1.

52

1.32

1.

59

2.19

2.

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3.14

2.

43

2.37

6.

94

6.04

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00

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6 3.

62

4.06

4.

62

4.06

3.

44

4.07

4.

28

3.70

5.

75

6.80

7.

72

6.27

7.

40

0.10

0.

11

0.12

0.

10

0.10

0.

11

0.13

0.

15

0.14

0.

13

0.25

0.

17

0.26

8.

11

8.79

8.

80

7.96

6.

54

7.03

6.

12

4.74

5.

73

6.61

5.

7l

5.23

5.

74

11.3

9 9.

82

11.7

9 10

.00

8.91

10

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9.65

7.

11

8.84

10

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7.83

9.

64

9.41

2.

%

3.15

2.

68

3.51

3.

52

3.54

3.

68

5.12

3.

74

2.70

3.

59

3.16

2.

79

0.77

1.

16

0.49

0.

75

1.33

0.

23

1.25

1.

29

0.90

0.

81

1.43

0.

52

0.70

0.

02

0.02

0.

02

0.02

0.

08

0.07

0.

15

0.15

0.

18

0.32

0.

37

0.45

0.

47

0.73

0.

90

1.16

0.

92

0.96

1.

30

0.60

1.

47

1.57

0.

89

i.15

1.37

0.

41

0.65

99

.96

100.

44

100.

10

100.

60

100.

04

100.

06

100.

62

99.1

8 10

0.49

10

0.14

10

0.10

99

.47

100.

34

100.

55

77.4

8 76

.00

75.7

0 74

.18

73.6

5 70

.64

69.7

3 63

.41

61.6

4 58

.44

51.1

3 46

.74

43.6

0 42

.23

its (i

n pp

m)

2 13

28

6

16

25

1 35

21

16

17

31

7

16

735

655

830

7l4

684

779

816

781

522

771

591

473

723

695

84

132

137

93

122

252

124

174

234

407

634

288

508

571

44

41

52

46

45

55

52

79

81

75

62

174

90

105

104

44

59

30

5!

75

92

43

6 53

16

5 74

27

12

4 31

36

35

33

36

31

35

34

23

38

69

48

64

64

20

4 21

0 17

8 17

6 83

86

10

9 58

49

55

10

9 64

13

47

84

11

3 10

0 91

10

2 12

0 11

2 15

6 15

8 21

4 63

6 64

5 46

9 61

6 16

7 72

67

54

51

14

24

25

37

14

11

7

5 5

7 7

7 6

7 12

9

19

18

14

14

19

18

15

10

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4 17

8

22

24

22

26

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n.d.

n.

d.

n.d.

n.

d.

n.d

3 n.

d n.

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3 3

2 2

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&t&

.

F. F. ABU EL-ELA 152

0.4

0.2

0.0 16

12

0

4

0 20

% 16

12

6 4

2

0 56

52

46

44

40

0.5

0.3

8

6 10

0

6

4

.rn ????

No70

0.5 1 2 3 FeO*/ MgO

Figure 4. Variation diagram for major elements illustrating the main trends exhibted by rock samples of the Abu Zawal gabbroic intrusion. Symbols as for Fig. 2.

and late-stages of crystallization, respectively. Variation diagrams for the major and trace ele-

menk plotted versus FeO*/MgO as a differentiation index are shown in Figs 4 and 5. Five significant geochemical points are demonstrated by these variation diagrams:

i) Ti@ increases with increasing FeO*/MgO. This behaviour is also followed by FeO*, MnO, Co and V;

ii) strontium decreases gently with FeO*/MgO. This trend is followed by Al203 and CaO;

iii) yttrium displays a bell-shaped trend against FeO*/MgO, where some of the UZ gabbros have lower values than those of the MZ gabbros. This behaviour is also followed by SiOr, NazO and Zr;

iv) chromium decreases with increasing FeO*/MgO for all gabbroic zones. This trend is also followed by MgO and Ni; and

v) barium increases with increasing FeO*/MgO for all gabbros. This behaviour is also followed by PzOs. The noted decrease in Cr and Ni contents from the LZ gabbro (167 ppm Cr, 204 ppm Ni) to the UZ gabbro (5 ppm Cr, 13 ppm Ni) is consistent with the fractionation of spine1 and clinopyroxene.

Titanium and V abundances correlate with the modal abundances of Fe-Ti oxides. Strontium con- tenk reflect the modal abundance of plagioclase.

The average of the hygromagmatophile element abundances in the three gabbroic zones have been

normalized to N-type MORB concentrations (Pearce, 1984) and plotted in Fig. .6. This figure demonstrates that there is a relative enrichment in large ion litho- phiIe (LIL) elemenk (Rb, Sr, Ba and K) over the other incompatible elements (Nb, P, Zr, Ti and Y) in all gabbroic zones. These hyg-romagmatophile element patterns (Fig. 6) are very distinct from those of mod- em alkali basalk (which are enriched in Nb) and mid oceanic ridge basalk (Wood et al., 1981; Tamey et al., 1980), but are comparable to those of talc-alkaline basalt (Wood et aZ., 1981). Depletion in Nb and other high field strength elements (HFSE) (P, Zr, Ti and Y), especially in the LZ gabbros (uralitized gabbro) relative to LIL elements, is a characteristic feature of all subduction-related magma (Saunders et d., 1980). This has been attributed to:

i) partitioning of HFSE into residual Ti phases (e.g. ihnenite and sphene) which are stabilized during hydrous partial melting conditions; and

ii) the transportation of the LIL elements into the source regions of the talc-alkaline magmas as a result of dehydration of the downgoing slab (Saunders et al., 1980). In addition, the low Zr/Y (0.57-1.86) observed among the gabbroic zones support an oceanic-arc setting rather than a continental-arc setting (Pearce, 1984).

Chondrite-Rormalized REE patterns for representative gabbroic samples from the lower, middle and


OLower zone 0 Middle zone

0 1 2 3 0 1 2 3

FeO*/ MgO FeO*/ big0

20 - rfh ?? ????0

10 - so Zr 0

25 -

41 ??15 - 0 w

ppm _ 5_ SO” Y

0 ??

200 0 -

- A# 0

0 Ba

0

900 -

- O8 ??o 700 - 3 4

Figume 5. Variation diagram for trace elements illustrating the main trends exhibted by rock samples of the Abu Zawal gabbroic intrusim.

upper zone gabbros (Table 6) are plotted in Fig. 7. REE become steadily enriched from the LZ to the UZ gabbros. The gabbros have moderately frac- tionated REE patterns with (La/Yb)N from 2.27 to 5.75 and (Ce/Yb)N from 2.14 to 4.00, passing from the LZ gabbro (uralitized gabbro) to the UZ gabbro (ferro-gabbro). This is due to marked light rare earth elements (LREE) enrichment with increasing differentiation (i.e. with increasing FeO*/ MgO from 0.56 to 2.90). The heavy rare earth elements (HREE) show a smooth and flat trend with (Tb/Yb)N from 1.20 to 1.95. This indicates that the generation of magma was not accompanied by significant HREE fractionation and that the parent magma was generated in the spine1 stability field rather than the garnet stability field (cJ Weaver and Tarney, 1981; Gill, 1974). The positive Eu anomaly in the LZ gablbro is due to preferential Eu incorporation by the first accumulating plagioclase. The variations in the scale of the Eu anomaly in the middle and upper zone gabbros are ascribed to a combination of the degree of Eu fractionation in the magma and the amount of cumulus plagioclase present in each sample.

ESTIMATION OF THE PARENT MAGMA COMPOSITION

A common problem in studying mafic layered intrusions is the estimation of the parent magma composition in as much as the bulk chemical composition of rock samples is unlikely to match that of the parental magma because of the occurrence of cumulus processes (Irvine, 1979).

The chilled margin method used to obtain the parent magma composition (Wager and Brown, 1968) cannot be applied to the Abu Zawal gabbroic intrusion because extensive interaction processes have been operative along all contacts. However, the parental magma composition can be estimated by the weight summation method (Ragland and Butler, 1972; Klewin, 1990; To mmasini and Poli, 1992). This method has been applied to the Abu Zawal gabbroic intrusion summing the average chemical composition of each gabbroic zone, weighted according to its outcrop surface. The calculated major element composition of the parent magma is reported in Table 7. This composition is similar to the average high alumina basalt in island-arc settings (see Table 7).

154 F. F. ABU EL-ELA

1

10

0.1

0 Lower zone

0 Middle zone

m Upper zone

Sr K Rb Ba Nb P Zr Ti Y

Figure 6. Spidergrams of the averages of the Abu Zawal gabbroic zones. Nommlization data after Pearce (1984).

DIFFERENTIATION OF THE GABBROIC ZONES

A model of low-pressure, closed-system in situ crystallization is proposed for the differentiation of the Abu Zawal gabbroic intrusion. The mafic magma was emplaced probably in a single, relatively rapid injection and crystallization commenced throughout.

The following order of crystallization is proposed for the gabbroic zones on the basis of petro- graphical and mineral chemistry data. Plagioclase (Ar@+clinopyroxene nucleated at the begining of crystallization. Then, plagioclase grading from Anss to &+calcic amphibole (brown hornblende)+Fe-Ti oxides followed in the crystallization sequence and were successively joined by the crystallization of quartz and apatite.

The hydrous phases crystallized in response to an increase in Hz0 activity due to the early crystallization of anhydrous phases, as has been docu- mented in experimental studies (Baker and Eggler, 1983). On the basis of microscopic and mineral chemistry studies, magnesio-hornblende crystallized directly from the evolving liquid, whereas actinolite and actinolitic hornblende formed by reaction between the liquid and clinopyroxene. The

Table 6. REE abundances (ppm) for representative samples from the Abu Zawal gabbroic intrusion.

Sample No Lower Zone Middle Zone Upper Zone (202A) (218C) (210)

La 2.46 9.31 11.22 Ce Sm- Eu L Tb Yb Lll

6.02 18.29 20.37 1.15 2.98 3.812

0.576 1.22 1.365 0.198 0.395 0.583 0.73 1.555 1.318

0.129 0.219 0.211 -I

estimation of the crystallization temperature on the amphibole - plagioclase pairs in the middle and the upper zone gabbros ranges from 1080 to 1100°C and indicates a water content in the magma of about 4-5 wt.% and a liquidus temperature of about llOO- 1150°C (Baker and Eggler, 1983, Fig. 3). The water content in the magma is close to the water-saturated curve for basalts at 3-4 kbar (Holloway and Bum- ham, 1972; Hughes, 1982). Therefore, the crystallization of the gabbroic zones took place under wet conditions.

DISCUSSION AND CONCLUSION

The Abu Zawal gabbroic intrusion shows features which in part reveal the emplacement mechanism: the absence of a chilled margin and grain-sized graded layering (Irvine, 1982) indicate that the gabbro was probably not emplaced in a completely mol- ten state. The absence of magmatitic structures indicates that the gabbro crystallized in situ and was not emplaced as a crystal mush nor were significant por- tions being crystallized as new magma was still being injected. Thus, the Abu Zawal cumulates probably began to crystallize in the crust at pressures which were not significantly higher than those where final solidification took place (cfi Sutcliffe et al., 1989). This conclusion is confirmed by the hornblende geoba-

,rometer, which gives a crystallization pressure of about 2.9-3.5 kbar, and the low A1203 content of the clinopyroxene suggests crystallization at relatively low pressure.

The Abu Zawal gabbroic intrusion consists of three gabbroic zones (LZ, MZ and UZ). These three zones may represent three stages of fractional crystallization which can be demonstrated by the evolu- tional geochemistry. For example, the PROS content is as low as 0.02-0.08 wt.% in the LZ gabbro (early-stage gabbro) but during fractional crystallization PZOS probably became highly concentrated in the residual liquid. The very low P205 content in the LZ gabbro suggests that this intercumulus liquid was driven out


La Cc Sm Eu Tb Yb Lu

Figure 7. Chondrite-mmnaked (after Evensen et al., 1978) REE patterns of the Abu Zawal gabbroic intrusion, Symbols as for Fig. 6.

Table 7. Estimated parental magma composition according to the weight summation method and comparsion of the estimated parent magma with high-alumina basahs.

1.z MZ uz N 7 3 4 P 56.:30% 32.50% 11.20% Estimated

Ti02 0.38 1.13 3.66 0.99 0.92 0.73 1.01 0.71 AI203 18.33 17.18 14.67 17.54 18.98 17.30 18.10 19.70 Fez03 1.68 2.65 8.06 2.71 - 3.40 - - Fe0 3.90 4.58 7.05 4.48 9.79* 5.54 9.45* 7.75 MnO 0.10 0.14 0.20 0.13 0.19 0.22 0.21 0.13 MgO 7.97 5.53 5.82 6.94 5.77 5.50 4.47 5.66 CaO 10.61 8.53 9.24 9.77 10.69 8.94 8.93 10.00 Na20 3.17 4.24 3.06 3.50 3.36 3.10 3.49 2.51 IGO 0.70 1.15 0.87 0.86 0.99 0.90 0.75 0.52 E205 0.04 0.16 0.40 0.11 0.22 0.20 0.23 0.97 L.O.I. 0.94 1.31 0.90 1.06 -

N=number of samples; P=percentage of the outcrop surface of each zone, A=average composition; ??=total iron

as FeO; l=average of Aleutian high-ahunina basalt (Marsh, 1976; Brophy, 1984); 2=New Georgia (Solomon Is.)

high-alumina basalt (Brown and khairer, 1967); 3,4=high-ahnnb~a basalt (cf. Crawford et al., 1987, Table 1,

sample No. 1,2).

by post-accumulation crystal growth (i.e. this gabbro may have adcumulus properties as defined by Wager and Brown, 1968). Then, during the middle- and late- stage fractionation (middle and upper zone gabbros), the E205 content of the gabbros was increased by being fixed in crystaking apatite. The upper zone gab bros have between 0.32 and 0.47% I’&,. In addition,

the TiO2 content of the LZ gabbro is low, ranging from 0.29 to 0.63 wt.% (Table 5), which suggests that during this early stage of fractionation TiOz was also highly concentrated in the liquid. Then, during the crystallization of the MZ gabbro, the TiOr content of the gabbros increased due to the crystallization of Fe- Ti oxides. In the UZ gabbro, TiOz is 4.00 wt.%. This

156 F. F. ABU ELELA

probably means that most of the TiOz remained in the liquid in the early-stage of fractionation and most of it entered cumulates in the late-stage of fractionation (UZ gabbro).

That the Abu Zawal gabbroic intrusion may have crystallized from an island-arc high alumina basaltic magma, which was derived from a mantle source, is suggested by the following observations:

i) The gabbroic rocks have very low abundances of incompatible elements (K, Rb, Ba, Nb, I’, Zr, Ti and Y) and the abundances of these elements increase with increasing differentiation. LIL elements (Sr, K, Rb and Ba) have higher abundances relative to high field strength (I-IFS) elements (Ti, P, Zr and Nb). In addition, the low concentration of Cr and Ni are characteristic of an island-arc basalt parentage.

ii) LILE/LREE enrichment, in combination with negative Nb anomalies, are characteristic features of basaltic rocks from recent destructive plate margins (Pearce, 1984; Hohn, 1985).

iii) The estimated parent magma composition for the Abu Zawal gabbroic intrusion is equivalent to an evolved high alumina basalt (Table 7).

iv) The crystallization sequence in the Abu Zawal gabbroic intrusion is similar to that in experimental high alumina basaltic systems crystallized under wet conditions (Green and Ringwood, 1968; Brophy and Marsh, 1986).

Acknowledgements

A scholarship from the Dutch Government and the Institute of Earth Sciences, State University of Utrecht, is gratefully acknowledged. Discussion with Dr. J. P. P. Huysmans was very helpful. Prof. Dr. El-Gaby is thanked for his reading and criticism of the original draft. Comments by Dr. D. Hughes and another re- viewer have greatly improved the manuscript.

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