Medium- to high-pressure garnet-amphibolites from Gebel ...rjstern/egypt/PDFs/SE...Une etude a la microsonde &ctronique mdique que le grenat, l’ amphibole et le plagioclase du Wadi

Pergamon Jwmd OfAfricmr firrh Sciences. Vol. 21. No. 3, 443-457. 1995 pp.

Cap&h 0 1995 5evier Science Ltd

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Medium- to high-pressure garnet-amphibolites from Gebel Zabara and Wadi Sikaic south Eastern Desert, Egypt

ADEL A. SUROUR

Geology Department, Faculty of Science, Cairo University, Giza, Egypt

(Received 21 February 1995: revised version received 18 July 1995)

Abstract - Garnet-amphibohtes from Gebel Zabara and Wadi Sikait in the southern Eastern Desert of Egypt occur as highly flattened metamorphosed basic volcanic bands enclosed within gametiferous metasediments. Sam- ples from both localities have almost the same metamorphic assemblage of gamet-amphibole-plagioclase-ilmenite- rutile. An electron microprobe study indicates that garnet, amphibole and plagioclase are cryptically zoned lonly in samples from Wadi Sikait. The composition of amphiboles (tschermakitic hornblende to tschermakite) reflects a temperature range equivalent to that of the staurolite-kyanite zone of the metapelitic sequences. Geothermometric calculations of the pairs garnet-amphibole and amphibole-plagiadase indicate average temperatures of 55O“C for samples from Wadi Sikait and Gebel Zabara, respectively. Pressures of about 6.8 kbar and 7.7 kbar are obtained using some mineral equilibria of both silicates and opaque phases. The garnet-amphibolites are considered as a part of the infrast~ctural suite in the Rastern Desert. A comparison with the Pan-African amphibolites from the Eastern Desert and Sinai is presented.

R&urn6 - Les amphibolites a grenat du Gebel Zabara et du Wadi Sikait darts le sud du Desert Oriental d:Egypte se presentent en rubans volcaniques basiques m&amorphos& et fortement &i&s au sein de m&as4diments a grenat. Les echantilkms de deux regions &udi&es sent form& quasiment par le m&ne assemblage metamotphique grenat-amphibole-pla~~~~~~~~e. Une etude a la microsonde &ctronique mdique que le grenat, l’amphibole et le plagioclase du Wadi Sikait possedent un zonage cryptique. La composition des amphiboles (hornblende tschermakitique B tschermakite) permet d’avancer une gamme de temp6rature comparable a la zone a staurolitecyanite des sequences m&apelitiques. L.es calculs g&obarom&riques des paires grenat-amphibole et amphibole-plagioclase, pour les 6chantillon.s du Wadi Sikait et du Gebel Zabara respectivement, mdiquent tme temperature moyenne de 550°C. Des pressions de -6.8 kbar et 7.7 kbar ont et& obtenues en se basant sur des e@ibres mineraux aussi bien sur phases &cat&s qu’opaques. Les amphibolites a grenat sont consider&s comme faisant partie dune suite infracrustale du Desert Oriental. Une comparaison entre les amphibolites pan-africaines du Desert Oriental et du S&ii est present&s.

INTRODUCTION

Amphibolites are widely distributed in the Pre- cambrian (Neoproterozoic) supracrustal sequence of the Arabo-Nubian shield in Egypt and some neigh- bouring countries. In the Egyptian Eastern Desert, amphibolites are associated with more than one rock unit, namely, from oldest to youngest, gneisses, metasediments, metavolcanics, serpent&rites and metagabbros. The present work classifies the amphibolites in Egypt into two categories:

il high-grade garnetiferous amphibolites associating the infrastructural metasediments of pm-Pan- African age; and

ii) low-grade garnet-free amphibolites restricted to the Pan-African suprastructural metasediment- metavolcanic associations.

During the last decade, many studies have suggested that some arnphibolites occur as enclaves in the old continental or infrastructural gneisses, for ex-

ample at Wadi Ghadir in the Eastern Desert (Takla et al., 1992), and in the area around Taba in northeast Sinai. On the other hand, the low-grade amphibolites are of younger age and represent a part of the ophiolitic and non-ophiolitic Pan-African metavolcan- its. Shenouda (1977) recorded many greenschist fa- ties amphibolites from many localities in the Eastern Desert. Also, El-Gaby and Habib (1980) mentioned the occurrence of Abu Kalb amphibohtes in the metavolcanics of the Abu Marawat Formation in the area southwest of Port Safaga in the East&n Desert. Many other occurrences of Pan-African amphibolites in the Eastern Desert are also known. In Sinai, El-Tokhi (1992) described some Pan-African ~garnet-free amphibolites from Wadi Feiran, western Sinai. These amphibolites form bands, irregular lenses and elon- gated bodies interbedded with gneisses and mig- matites. It was concluded that thejse amphibolites are of igneous origin and have been derived from a tholeiitic rather than alkaline magma. Pan-African

443

444 A. A. SUROUR

(a)

&wan “:---, _ ___

34%. _ _ -.\A __.. -&

--_ .s, _

f%?mmamt mlasse + Ddchan volcanics

InfIas~ gne.isses intruded by gxanitiids

Metosediments - mefovo/conics

$i;;;;::: Garnet - mica ??._.,__. ;.: :.::::. schists :::.

cl xXxX Younger gronifes x x f white granite)

cl vvv Mcfagabbro-dio’rite vvv

cl i:f;iiiiii Gornef -mica ~!~::I::~: schisfs

cl \w Strike and dip of foliation

cl t Thrusts

Figure 1. Location of the two studied localities shown on a simplified geological map of the basement rocks in the Eastern Desert and Sinai (from El-Gaby, 1983), in addition to their detailed geology. (a) Gebel Zabara; (b) Wadi Sikait.

Medium- to high-pressure garnet-amphibolites from Gebel Zabara and Wadi Sikait 445

ultramafic-related amphibolites in the Eastern Desert minor horses. The present work considers that the de- are also recorded (e.g. Akaad and Noweir, 1980) and formational style of the garnet-amphibolites is that of termed as ‘metaultramafites’ by Basta et al. (1981). The the third order duplex. The Sikait duplex is a foreland last authors also distinguished between the Egyptian dipping duplex and the geometry of the thrust unit para- and ortho-amphibolites (of sedimentary and indicates transport from east-northeast to west- igneous origins, respectively) on the basis of opaque southwest towards the cratonic foreland (Greiling et mineralogy. al., 1987).

The garnet-amphibolites from Sikait-Zabara district associate high-grade gametiferous pelitic and psammopelitic schists. The present work gives details of a mineralogical investigation of some of these garnet-amphibolites, with emphasis on their geothermo- barometry as a new contribution to the Egyptian amphibolites, to deduce pressure-temperature paths of their metamorphic history. Implications on the geology of the country rocks and the tectonic position are also discussed.

GEOLOGICAL SETTING

Geochronologically, a 1770 Ma Uj(Pb age of zircons from the gneissose granite masse of Wadi Sikait was given by Abdel-Monem and Hurely (1979), but they considered the mass to be composed of para- gneisses (psammitic gneisses). They ~mterpreted the given age as the age of the source rock that supplied the detrius. They also considered the ages around 700 Ma as the re-setting ages during the Pan-African orogeny. The so-called ‘psammitic gneisses’ of Sikait were accepted as gneissose granites lby most of the workers (e.g. Basta and Zaki, 1961; Hegazy, 1984; El- Gaby et al., 1987; Surour, 1990; El-Maghraby, 1995).

Garnet-amphiboltes in both areas of Gebel Zabara and Wadi Sikait (Fig. la, b) are common. They always occur as highly flatteneed dykes or sills enclosed within the gametiferous metasediments (mainly garnet-mica schists and garnet-quartzites). The amphibolitic bands vary in thickness from 0.5 to 1.7 metres with an average length of 4.5 metres. The rock is always fine-grained, melanocratic, with schistose appearance and contains megascopic garnet porphyro- blasts 3-7 mm wide. A few bodies laking garnet are also present. Basta and Zaki (1961), Hegazy (1984) and others described the amphibolites associating the metasediments of Wadi Sikait as amphibole schists that could be of volcanic origin. Surour (1990) also described the garnet-amphibolites of Wadi Sikait and pointed out that their field relations, opaque mineralogy and gecochemistry indicate igneous protoliths. Generally, the present work considers these amphibolites as a part of the old metavolcanic within the infrastructural metasediments.

ANALYTICAL PROCEDURES

Some amphibolitic bands, as well as the enclosing schists, are folded, for example in a spot to the northwest of the old Roman Temple of Sikait. Tight folding is more conspicuous in the occurrence of Ge- be1 Zabara. From the detailed structural point of view, Greiling et al. (1987) agreed with Ries et al. (1983) that the Sikait antiform represents a second order duplex since it forms a horse of a major first order one (Hafafit duplex) to the northwest of the Si- kait area. These workers believed that the lowermost exposed horse of the Wadi Sikait duplex is built up of a gravitative dome of granitic rocks that are assumed to have overprinted the horse shape and formed a northwest-southeast structural culmination in the Si- kait duplex to the west of the wadi track. Ries et al. (1983) concluded that the Sikait metasediments them- selves represent a third order duplex composed of

Mineral chemistry analyses of well-selected mineral assemblages were conducted on ~a Cameca SX50 electron microprobe housed at the Institut fur Min- eralogie und Petrographie, ETH-Zurich, Switzerland. The qualitative analyses were obtained using five spectrometers operated at an acceleration potential of 15 kV and a sample current of 20 nA. The counting time of 15-20 seconds for each element was automa- tized and the standards used were either natural or synthetic minerals. Absorption and fIuorescence cor- rections were made using the procedure of Bence and Albee (1968). Computer programs in routine use at the ETH-Zurich labratories were used to calculate the cation proportions of minerals. For garnet, plagiocalse and ilmenite, Fe0 and Fe203 were calculated as- suming stoichiometry and charge balance. In the amphibole structure, Fez03 was recalculated using the formula given by Hawthorne (1981) on the basis of 23 oxygens and 13 cations+K+Na+Ca. Total Fe as ferric oxide was recalculated for epidotes on the basis of 12 oxygens and one OH-group.’ Only minerals of the prograde metamorphic assemblages: were used for the microprobe analyses. The crystals were as fresh as possible and with mutual boundaries so that the compositional data obtained would represent nearly true equilibrium composition of the co-existing minerals.

MINERAL ASSEMBLAGES AND TEXTURES

Microscopic investigation of 6everal garnet- amphibolite samples from the two studied localities revealed their almost identical metamorphic assemblages. They mainly consist of garnet, amphibole, epidote, plagioclase, ilmenite, sphene, rutile and little

446 A. A. SUROUR

Figure 2. Petrography of garnet-amphibolites. (a) Poikilobastic granet with coarse quartz inclusions, plane polarized light, G. Zabara. (b) Retrograded chlorite in the form of fine veinlets at the rim of a garnet porphyroblast, cross-nicols, W. Sikait. (c) Big amphibole sieved by quartz, plane polarized light, G. Zabara. (d) Granular clinozoisite (czI) and euhedral clinozoisite with rhombic outline (czII), plane polarized light, G. Zabara. (e) Reaction rim of sphene (dark) after ilmenite (light), reflected light, W. Sikait. (f) Fe-poor rutile (light) and Fe-bearing rutile (dark) replacing ilmenite, plane polarized light, G. Zabara.

quartz. Very fine needles of apatite are also present as inclusions in both garnet and amphibole. Epidotes are much more common in samples from Gebel Zabara, whereas they are less frequent in samples from Wadi Sikait. In the latter case, epidotes occur as inclusions in garnet. On a textural basis, the investigated samples are of visibly schistose character. Amphibole, plagioclase and ilmenite constitute the rock foliation. On the other hand, garnet occurs as large porphyro- blasts overprinting the foliation. Garnet is a syn- kinematic mineral and no remarks of post-kinematic origin are evident. Growth of the mineral might have continued till the latest stages of prograde metamorphism as most of the other minerals are swept inside it or partly enclosed by it. On this ground, early nu- cleation of garnet is not negligable. This is also supported by the significant cryptic zoning in Mg and Mn, as will be given later, in addition to the nature of the inclusions. Most of the garnets are colourless to netural, euhedral to subhedral and contain many inclusions of other minerals (Fig. 2a). Garnet porphy- roblasts from Wadi Sikait are characterized by Y- shaped cracking and they are nearly devoid of inclusions except for a few epidote, quartz and amphibole fine crystals, especially at the rims (Fig. 2b). Ilmenite

is sometimes found at the cores of garnets. In a few cases, garnet from Wadi Sikait is slightly altered to chlorite in the form of veinlets close to the rims (Fig. 2a) as a feature of limited retrograde metamorphism. Garnets from Gebel Zabara are of typical poikiloblastic nature with common inclusions of quartz and, to a lesser extent, plagioclase and amphibole laths (Fig. 2a).

Amphibole is the most abundant mineral in all samples, amounting to about 70-92%. It is usually of dark bluish-green colour, pleochroic and sometimes includes a few inclusions of quartz (Fig. 2c) and plagioclase, especially in the cores. Plagioclases are commonly recrystallized, but a very few relict mag- matic crystals (labradorite to bytwonite, on the basis of their optical properties) are also present. Twinning lamellae are much more predominant in the latter. From microprobe analyses, the An-content of the recrystallized metamorphic plagioclases from Gebel Zabara amphibolites is 19-21 mole%, whereas the range is wider (15.5-29 mole%) in the case of plagioclases in the Wadi Sikait samples. Garnet-free Pan- African amphibolites of volcanic origin from other localities in the Eastern Desert contain recrystallized plagioclases with an An-content of 6-18 mole% (e.g.


Samples from W. Sihoit

Somples from G. Zobara

Amphibole

Garnet t pyroxene t epidofe

Plogioclase + quartz t orthoclose

Figure 3. Modal plot of garnet-amphibolites on the diagrams of amphibole bearing metamorphites of Tonika (in Suk, 1983; pp168- 169). (1) Homblendite; (2) melanocratic amphibolite; (3) amphibolite; (4) eclogitic amphibolite and hornblende eclogite; (5) eclogite; (6) pyroxene gram&e, gneiss and quartzite.

Shenouda, 1977). Generally, plagioclases from the present garnetiferous samples from Gebel Zabara are commonly replaced by anhedral granular epidote (clinozoisite of generation I). A second generation of clinozoisite is much coarser, occurring as 30- 70 mm long, euhedral to subhedral crystals with parting and rhombic cross-sectional cut (Fig. 2d). The latter generation cuts the amphiboles and encloses some of them as inclusions and also partly encloses generation I clinozoisite. As will be discussed later, the appearance of the euhedral generation II clinozoisite is attributed to some thermal effect by the intrusive post-erogenic white and pink granites.

Detailed opaque mineralogical investigation classifies the studied garnet-amphibolites as ortho- ampibolites (Shenouda, 1977; Basta et al., 1981). The opaque content (5-9%) is mainly ilmenite occurring as fine skeletal prismatic crystals as a part of the rock foliation. This ilmenite is subhedral to anhedral with length up to 110 pm. In some other instances, extensive rim replacement of ilmenite by sphene is observed (Fig. 2e). The sphene reaction rim is either continuous or discontinuous. Fine streaks of sphene are also sometimes found along cracks and cleavage planes of the ilmenite. It is believed that the Ca*+ nececssary for the formation of sphene is obtained from the breakdown of calcic plagioclase by saus- suritization, starting from the greenschist facies, and then added with silica to the ilmenite. Ilmenite in

Table 1. Mineral assemblages of the host metasediments

the garnet-amphibolites of Sikait-Zabara district also shows variable degrees of alteration to rutile. The latter occurs either in the from of orange (Fe- free) crystals or as dark reddish brown (Fe-rich) crystals (Fig. 2f). Haggerty (1981) concluded that rutile in the metabasites represents a high- temperature alteration product of ilmenite in the amphibolite facies. He restricted alteration of ilmenite to anatase to very low-temperature conditions. In some samples, ilmenite i$ recrystallized and no alteration products are observed. This again supports the thermal effect in some ~samples by the intrusive granites which also resulted in the formation of euhedral clinozoisite II.

Following the classification of amphibole-bearing metamorphites of Ton&a (in Suk, 1983), the gamet- amphibolites fall within the field of melanocratic amphibolites (Fig. 3). Based on the co ‘position of pla-

? gioclases, the present garnet-amphi olites belong to the oligoclase-amphibolites (Wenk and Keller, 1969).

Table 1 gives the average mineralogical composi- tions of the common metasediments hosting the garnet-amphibolites. Petrographically, two major schist series are distinguished, namely the metapelite series (mainaly garnet-mica schists) and the psammopelite series (mainly garnet-quartzites).

MINERAL CHEMISTRY

Garnets

Electron microprobe analyses of amets are pre- $e sented in Table 2. Garnets from Ge 1 Zabara have

relatively higher pyrope content than those from Wadi Sikait (11-12 and 7-10 mole%, respectively). Contents of spessartine, grossular d almandine in

a!m garnets from both localities are a ost the same, whereas the range of almandine in garnets from Gene1 Zabara is narrower than that~ in granets from Wadi Sikait (56-58 and 55-60 mole%, respectively). Cryptic zoning in Wadi Sikait gam’ ts

1 is evident, in

which pyope at the cores (7-7 mole% is lower than at the rims (8.6-9.9 mole%). On the 0th r hand, spessartine content at the core (9.1-9.2 mole f ) is higher than at the rim (4-4.6 mole%). These gar*ts are similar in composition to many garnets in many worldwide garnet-amphibolites, such as those ~ from Chile and Vermont, USA (Kohn and Spear, 199b).

Mineral Rock type garnet biotite muscovite staurolite quartz

Garnet-mica schists Garnet auartzites

xxx xxx xx X xx

xxx xx xxx xxx: very common; xx: common; x: scarce; -: absent.

Tabl

e 2.

Ele

ctro

n m

icro

prob

e an

alys

es

of g

arne

ts

Sam

ple

1 si

k75

sik7

5 Si

k76

Sik7

7 Si

k78

SiO

2 ri

m

36.5

4 ri

m

37.0

3 ri

m

36.8

1 co

re

36.9

7 co

re

36.6

9 Ti

02

0.12

0.

17

0.10

0.

15

0.13

C

r203

0.

00

0.03

0.

00

0.03

0.

02

Alfi

21

.14

21.4

2 21

.40

21.2

7 21

.25

Fe20

3 2.

78

2.19

2.

28

2.26

2.

79

Fe0

24.6

9 25

.61

26.4

9 24

.85

24.2

8 M

nO

2.75

2.

59

1.73

4.

03

3.94

M

gO

2.09

2.

24

2.43

1.

74

1.84

C

aO

9.42

9.

15

8.85

9.

54

9.59

N

a20

0.09

0.

09

0.04

0.

00

0.03

K

ZO

0.01

0.

03

0.00

0.

00

0.00

To

tal

99.7

0 10

0.55

10

0.14

10

0.92

10

0.58

C

atio

ns a

ssum

ine

stoi

chio

met

rv

and

char

ge b

alan

ce

Si

2.92

11

2.93

22

2.92

63

2.92

64

2.91

53

2.97

26

2.95

98

2.97

50

2.98

43

2.96

44

2.97

80

2.97

44

Ti

0.00

74

0.01

00

0.00

59

0.00

88

0.00

78

0.00

58

0.00

46

0.00

51

0.00

58

0.00

49

0.00

52

0.00

26

Cr

0.00

00

0.00

16

0.00

00

0.00

21

0.00

10

0.00

00

0.00

00

0.00

04

0.00

00

0.00

06

0.00

01

0.00

14

Al

1.99

21

1.99

90

2.00

53

1.98

71

1.99

03

1.99

67

1.98

03

1.98

50

1.97

77

1.98

65

1.99

09

1.98

71

,Fe3

+ 0.

1670

0.

1307

0.

1365

0.

1346

0.

1668

0.

0529

0.

0998

0.

0612

0.

0473

0.

0770

, 0.

0462

0.

0623

IF

d’

1.65

09

1.69

62

1.76

09

1.64

72

1.61

37

1.70

40

1.69

25

1.72

70

1.70

40

1.67

83

1.69

44

1.73

23

Mn

~Mg

0.18

62

0.17

37

0.11

66

0.27

08

0.26

50

0.20

70

0.23

03

0.24

98

0.23

41

0.23

94

0.21

71

0.21

54

0.24

88

0.26

44

0.28

82

0.20

56

0.21

78

0.33

70

0.34

59

0.36

32

0.33

78

0.34

45

0.35

61

0.36

89

Ca

0.80

69

0.77

58

0.75

41

0.81

00

0.81

68

0.71

75

0.68

80

0.62

65

0.70

34

0.69

96

0.70

86

0.65

11

Na

0.01

46

0.01

30

0.00

60

0.00

02

0.00

43

0.00

52

0.00

76

0.00

68

0.00

52

0.00

28

0.00

25

0.00

34

‘K

0.00

14

0.00

27

0.00

00

0.00

00

0.00

00

0.00

13

0.00

12

0.00

00

0.00

00

0.00

00

0.00

10

0.00

12

IEnd

-mem

bers

Zab8

2 Za

b83

Zab8

3 Za

b83

Zab8

4 Za

b85

Zab8

5 ri

m

37.4

4 ri

m

37.6

2 co

re

37.4

3 co

re

37.6

8 co

re

37.4

9 ri

m

37.5

7 ri

m

37.5

4 0.

10

0.08

0.

09

0.10

0.

08

0.09

0.

04

0.00

0.

00

0.01

0.

00

0.01

0.

00

0.02

21

.34

21.3

6 21

.19

21.1

9 21

.32

21.3

1 21

.28

0.89

1.

69

1.02

0.

79

1.29

0.

78

1.04

25

.66

25.5

7 25

.98

25.7

3 25

.38

25.5

6 26

.14

3.08

3.

46

3.71

3.

49

3.57

3.

23

3.21

2.

85

2.95

3.

07

2.86

2.

92

3.01

3.

12

8.43

8.

16

7.36

8.

29

8.26

8.

34

7.67

0.

03

0.05

0.

04

0.03

0.

02

0.02

0.

02

0.01

0.

01

0.00

0.

00

0.00

0.

01

0.01

99

.82

100.

95

99.9

0 10

0.17

10

0.38

99

.91

100.

11

Gro

ssul

ar

0.18

7 0.

191

0.18

4 0.

202

0.18

9 0.

211

0.17

9 0.

176

0.20

8 0.

194

0.21

1 0.

185

Ahn

andi

ne

0.56

9 0.

581

0.60

2 0.

561

0.55

3 0.

574

0.57

0 0.

581

0.57

1 0.

566

0.56

9 0.

583

Pyro

pe

0.08

6 0.

091

0.09

9 0.

070

0.07

5 0.

113

0.11

7 0.

122

0.11

3 0.

116

0.12

0 0.

124

Spes

sart

ite

0.06

4 0.

059

0.04

0 0.

092

0.09

1 0.

070

0.07

8 0.

084

0.07

8 0.

081

0.07

3 0.

073

And

radi

te

1 0.

086

0.06

7 0.

070

0.06

9 0.

086

1 0.

027

0.05

1 0.

031

0.02

4 0.

039

0.02

3 0.

031

Tabl

e 3.

Ele

ctro

n m

icro

prob

e an

alys

es o

f am

phib

oles

Sam

ple

SiO

2 Ti

Oz

Cr2

03

A12

03

Fe20

3 Fe

0 M

IIO

MgO

C

aO

Naz

O

K20

H

20

Tota

l C

atio

ns

Si

Ti

Cr

Al

Fe*

Fe2’

M

n

Mg

Ca

Na

K

OH

Avv

A

V)

Na(

M4)

N

a(A

)

- si

k75

sik7

5 S&

75

sik7

5 Si

k76

Sik7

6 Si

k78

T

Zab8

2 Za

b82

Zab8

3 Za

b83

Zab8

4 Za

b85

rim

core

co

re

rim

ri

m

rim

ri

m

41.8

1 41

.63

41.7

9 42

.16

41.9

0 42

.49

42.3

2 0.

63

0.67

0.

58

0.61

0.

65

0.72

0.

63

0.00

0.

05

0.04

0.

01

0.04

0.

05

0.08

14

.74

15.2

3 15

.20

14.8

9 15

.37

14.4

0 15

.50

12.7

2 13

.45

13.6

8 13

.60

13.3

2 13

.44

11.9

1 4.

71

4.20

4.

58

4.26

4.

86

4.28

5.

16

0.14

0.

09

0.17

16

0.

10

0.10

0.

13

9.20

9.

25

9.09

9.

44

9.12

9.

49

8.99

11

.02

10.5

6 10

.47

11.1

8 10

.73

11.6

8 11

.01

1.65

1.

82

1.84

1.

67

1.77

1.

62

1.75

0.

40

0.39

0.

39

0.38

0.

50

0.40

0.

41

rim

42

.73

core

42

.49

rim

42

.72

core

42

.88

rim

ri

m

42.7

1 43

.00

2.04

2.

05

2.05

2.

07

2.06

2.

07

2.06

99

.09

99.4

7 99

.88

100.

43

100.

45

100.

74

99.9

9 ku

late

d on

the

bas

is o

f 23

0 a

nd 1

3 ca

tions

+K+N

a+C

a 6.

1474

6.

0906

6.

0995

6.

1163

6.

0865

6.

1502

6.

1551

0.46

0.

48

0.46

0.

38

0.43

0.

47

0.00

0.

03

0.00

0.

03

0.01

0.

02

16.6

4 16

.56

16.1

1 15

.53

16.2

0 16

.08

4.10

4.

59

4.09

3.

00

5.03

3.

24

10.3

8 10

.49

10.7

7 12

.76

9.78

11

.40

0.21

0.

16

0.19

0.

18

0.26

0.

16

9.32

9.

46

9.56

9.

08

9.70

9.

43

10.6

4 11

.04

11.2

0 11

.92

10.8

0 11

.16

2.17

2.

05

1.92

1.

38

2.15

1.

94

0.28

0.

31

0.24

0.

37

0.25

0.

26

2.04

2.

05

2.04

2.

03

2.05

2.

04

98.9

9 99

.72

99.3

1 99

.58

99.3

8 99

.21

ca

-t-

6.27

31

6.21

54

0.06

95

0.07

39

0.06

36

0.06

70

0.07

11

0.07

79

0.06

91

0.00

00

0.00

52

0.00

44

0.00

08

0.00

44

0.00

57

0.00

92

2.55

37

2.62

61

2.61

44

2.54

65

2.63

10

2.45

66

2.65

62

1.40

70

1.48

13

1.50

30

1.48

47

1.45

58

1.46

37

1.30

32

0.57

96

0.51

43

0.55

97

0.51

64

0.59

01

0.51

81

0.62

77

0.01

81

0.01

14

0.02

13

0.01

94

0.01

18

0.01

29

0.01

59

2.01

54

2.01

72

1.97

72

2.04

11

1.97

54

2.04

83

1.94

95

1.73

60

1.65

62

1.63

66

1.73

84

1.67

05

1.81

14

1.71

57

0.47

01

0.51

52

0.52

05

0.46

95

0.49

83

0.45

52

0.49

38

0.07

54

0.07

35

0.07

26

0.07

09

0.09

17

0.07

29

0.07

66

2.00

00

2.00

00

2.00

00

2.00

00

2.00

00

2.00

00

2.00

00

_ .

0.05

12

0.05

32

0.00

00

0.00

30

2.88

01

2.85

44

0.45

34

0.50

56

1.27

51

1.28

38

0.02

65

0.01

92

2.04

07

2.06

3 1.

6739

1.

7301

0.

6186

0.

5826

0.

0515

0.

0569

2.

0000

2.

0000

6.27

15

6.32

47

0.05

08

0.04

16

0.00

00

0.00

30

2.78

78

2.70

02

0.45

23

0.33

28

1.32

23

1.57

40

0.02

30

0.02

26

2.09

23

1.99

64

1.76

16

1.88

34

0.54

68

0.39

47

0.04

53

0.06

98

2.00

00

2.00

00

6.25

39

0.04

68

0.00

15

2.79

63

0.55

44

1.19

81

0.03

29

2.11

61

1.69

46

0.60

95

0.04

76

2.00

00

6.31

75

0.05

24

0.00

23

2.78

38

0.35

77

1.40

13

0.01

94

2.06

50

1.75

73

0.55

32

0.04

87

2.00

00

-rat

ins_

1.

909

1.85

3 1.

913

1.85

0 1.

845

1.72

7 1.

785

1.72

9 1.

675

1.74

6 1.

683

0.70

1 0.

717

0.71

4 0.

663

0.71

8 0.

607

0.81

1 1.

153

1.07

0 1.

059

1.02

5 1.

050

1.10

1 0.

264

0.34

4 0.

363

0.26

2 0.

329

0.18

9 0.

284

0.32

6 0.

270

0.23

8 0.

117

0.30

5 0.

243

0.20

6 0.

171

0.15

7 0.

207

0.16

9 0.

266

0.21

0 0.

292

0.31

3 0.

308

0.27

8 0.

304

0.31

1

450 A. A. SUROUR

Amphiboles

Some representative and average microprobe analyses of amphiboles from the garnet-amphibolites are given in Table 3. For normalization, Fe3+/Fe2+ of amphiboles were estimated using the procedure of Laird (1978). According to the classification of amphiboles given by Leake (1978), modified by Haw- thorne (1981), all amphiboles from Wadi Sikait samples are tschermakite, whereas those from Gebel Zabara are tschermakitic hornblende (except for analysis No. 83 which represents tschermakite). The Al(IV)-CNa+K diagram (Fig. 4a) supports this nomenclature, and also illustrates that the amphiboles under investigation have a higher XNa+K than those from the other Egyptian metabasites in the Eastern Desert, as given by Ghoneim (1988). On the other hand, the present amphiboles have almost the same zNa+K range (0.45-0.80) as the amphiboles from the fresh unmetaorphosed younger Egyptian gabbros, but the latter have lower Al(IV). Ghoneim (1988) classified the amphiboles from the metabasites as magnesio to actinolitic hornblende and those from the fresh gabbros as edenitic hornblende. The mineral chemistry of amphiboles from the Feiran amphibolites of Sinai indicates a hornblende composition. Figure 4a shows the linear trend of substitution of the amphiboles from the present garnet-amphibolites towards the tschermakite-pargasite end-members.

Table 3 indicates that some amphiboles from the garnet-amphibolites from Wadi Sikait are slightly zoned (analyses Sik 75 to Sik 78). The cores are more enriched in Na (0.51-0.52) than the rims (0.47-0.49). The Na(M4) or crossite content is higher at the cores than at the rims (OX-O.36 and 0.19-0.28, respectively). Unzoned amphiboles from Gebel Zabara samples have 0.177-0.326 Na in the (M4)-sites. The amount of Ca (1.67-1.88) is always inversely propor- tional to the amount of Na(M4), which is characteristic of amphiboles from carbonate-free assemblages (Spear, 1982).

Laird and Albee (1981) demonstrated that the composition of amphiboles in metamorphosed basic rocks is very sensitive to temperature. They corre- lated the composition of amphiboles with the metamorphic grades of the Barrovian sequence of biotite, garnet and staurolite-kyanite zones in the enclosing pelitic schists. Figure 5 shows that amphiboles from all analyzed samples are enriched in total Al (2.6-2.8), AI (1.67-1.91) and CAl(VI)+Fe3+ +Cr+Ti. Thus, they reflect a temperature range equivalent to that of the staurolite-kyanite zone in the metapelites. This conclusion seems reasonable, since the studied garnet-amphibolites are enclosed in schists rich in garnet and bear some staurolite. Analyses of zoned amphiboles from Wadi Sikait samples suggest higher temperatures for the cores.

Also, the present data are in agreement with Liou et al. (1974) that Fez+/Mg in amphiboles increases with prograde metamorphism.

The Al-Ti diagram of Hynes (1982) shows that Ti in amphiboles is low (0.041-0.078). Accordingly, they plot in the field of medium- to high-pressure amphiboles (Fig. 6a). The TiOz of the studied amphiboles (0.35-0.72 wt%) is lower than those of the low- pressure Feiran amphibolites (averaging 1.12 wt%; El-Tokhi, 1992). The AI(Na(M4) diagram (Fig. 6b) of Brown (1977) also supports the idea of a high- pressure origin for the amphiboles from the investigated garnet-amphibolites. These amphiboles plot at the extention of the high-pressure amphibole trend, which is characteristic of 6-7 kbar metamorphic pressure. In comparison to other famous world occurrences of metabasites (Fig. 6b, c), the amphiboles from the Egyptian garnet-amphibolites seem similar to those from medium- to high-pressure metamorphic terrains.

Plagioclases and epidotes

Electron microprobe analyses of plagioclases and epidotes are given in Table 4. Plagioclases from Wadi Sikait are zoned with more Ca at the cores. Generally, the range of An-content in the Gebel Zabara plagioclases is narrow (19-21 mole%). The lack of both visi- ble and cryptic zoning in the latter is very characteristic The orthoclase component in the latter is very low (about 0.005 mole%). This is most probably related to plagioclase alteration to sericite during metamorphism. Miyashiro (1973) made a major dis- tinction between low-pressure and medium- to high- pressure metabasites in terms of the co-existence of sodic plagioclase and tschermak-rich amphiboles in the latter. This is the case of the Egyptian gamet- amphibolites. Also, An-content (up to 19 mole%) supports regional metamorphism in the staurolite zone. Data in Table 4 show that generation I clinozoisite have higher Fe3+ and Ti (0.29-0.35 and 0.003- 0.007, respectively) than in generation II clinozoisite (0.10-0.15 and 0.001-0.002, respectively). Also, contents of Ca and Al [AI in particular] are relatively higher in generation II.

Ilmenites

Ihnenites in garnet-amphibolites from both occurrences (Table 5) are typically normal ihnentes of almost constant formula and free of Fez03 or hematite. A small amount of Mn-ilmenite in solid solution is notable (1.6-1.9 mole%). Based only on ore-microscopic investigation, Shenouda (1977) and Basta et al. (1981) identified ihnenite in the Egyptian Pan-African ortho-amphibolites of volcanic origin as ferri- ilmenite.

Medium- to high-pressure garnet-amphibolites from Gebel Zabara and Wadi Shit 451

“I I I I I 1

0 014 0!8 1.2

ENa+ K

Field of amphiboles from Egypfian metagabbros

(offer Ghoneim, 1988)

Field of amphiboles from Egypfion younger gabbros (after Ghoneim, 1988)

Amp hiboles from G. Zabara

Amphiboles from W.Sihaif

field of high- pressure

amphiboles f Mengel and

Rivers, 1991 J

Al +-Fe t2TiS

lb)

AllV

Figure 4. (a) Compositional trend of amphibole. @) Substitution trend in amphiboles to that of high-pressure amphiboles.

GEOTHERMOMETRIC CALCULATIONS

Qualitative estimation of pressure-temperature conditions have been already discussed in the section on mineral chemistry. The advantage of the quantitative estimation is the evaluation and measuring of the actual ionic exchange between the coexisting phases in equilibrium rather than in a single crystal phase. The semi-quantitative rough estimation of the metamorphic conditions of Plyushina (1982) suggest a high-pressure of around 8 kbar and a temperature range of about 515°C (in the case of low An-content at the rims of plagioclase) to about 545°C (for higher An-content at the cores).

Two geothermometers and three geobarometers have been applied to the investigated garnet- amphibolites (Table 6). A temperature range of 502°C to 570°C is obtained based on the garnet-amphibole equilibria of Graham and Powell (1984). A relatively lower temperature range (495°C to 560°C) is obtained from the plagioclase-amphibole geothermometer of Bhmdy and Holland (1990) who based their equa- tions on the basis of Al(IV) in amphiboles co-existing with plagioclase in silica saturated rocks. In the studied district, some sort of metamorphic zonation in the country rocks hosting the amphibolites is present. Sometimes, the garnetiferous metapelites contain staurolite but its abundance is very low causing a notable decrease in the calculated temperatures of the mineral equilibrium. Many details on the geother-

mometry of the metapelitic country rock are still in progress (Surour, in prep.). Metapelite$ with abundant staurolite in the garnet-mica schists~ in absence of chloritoid and primary chlorite, are characterized by metamorphic tempertures above 550°C (e.g. Powell and Holland, 1990). Only in very few ~ samples of garnet-amphibolites would the obtained temperatures be slightly lower than expected as some averages of analyses are for cryptically zoned garnets and plagioclases. In most instances, the Mn-rich cores of garnet give the lowest temperatures. The GRIPS (garnet- rutile-ilmenite-plagiocalse-silica) geobarometer of BoNen and Liotta (1986), as well ‘as the garnet- amphibole-plagioclasequartz geobarometer of Kohn and Spear (1989), modified by the g~et-amphibole- plagiocalse geobarometer of Kohn and Spear (1990), suggest pressures of 6.8 and 7.7 kbar for granet- amphibolites from Wadi Sikait and C&be1 Zabara, respectively. The obtained data and thb suggested P-T path in comparison with those of the Pan-African amphibolites are shown on a metamorphic facies diagram (Fig. 7).

DISCUSSION AND CONCLUSIONS

Garnet-amphibolites from Gebel Zabara and Wadi Sikait are characterized by the ass mblage: gamet-

K amphibole-epidote-plagioclase-ilme ‘te-rutile. Most of the amphiboles are dark bluish-green, ranging in composition from kchermakitic ~ hornblende to

452 A. A. SUROLJR

IO0 Al

SiiAI /- \ VS 1.0 ’ / %

Al 0.5 .0*3- (1 XXX

0 6.5 1.0 1.5 2.0

IV Al

o.3J

0.2-

_I_._ Biotlte zone - _ -- Garnet zone - Sfourolite -

kymite tome

K / -\ 0.I - /

*,.@*+ X

./ *- ./ _kM

*f 0 &, L.,,d

(cl 0 .H ‘, I I I

0 0.5 1.0 1.5 2.0

IV A/

Figure 5. Composition of amphiboles as an indicator of the metamorphic grade equivalent to the Barrovian zones in pelitic rocks (after Larid ahd Albee, 1981). Symbols as in Fig. 4.

tschermakite. Garnet porphyroblask are of common poikiloblastic nature, enclosing almost all the other phases as inclusions. Early neucleation of garnet is favourable because of the notable zoning and ar- rangement of inclusions. The appearance of garnet (Ahns7 Grossz0.s Pyriz Spess7.5 An&) from Gebel Za- bara and (Alm5s.s Gross19 Pyrs.5 Spess7 And7) from Wadi Sikait in such a lithology requires relatively high-perssure and depends also on the original composition of the basic volcanics (Glassely and Sorensen, 1980). Even in epidote-rich samples it is believed that the temperature exceeded that of the epidote-amphibolite facies metamorphism since epidotes could remain stable in amphibolites up to stauraolite-kyanite and sillimanite zones (Laird and Albee, 1981). Petrographic investigation indicates that

the generation I clinozoisite of granular shape is formed by the breakdown of more calcic plagioclase produced during prograde regional metamorphism. On the other hand, much coarser and euhedral clinozoisite (generation II) is of metasomatic origin.

The mineral chemistry of the amphiboles indicates regional metamorphism in the staurolite-kyanite zone, which is consistent with the appearance of staurolite in the enclosing garnetiferous pelitic schists. The amphiboles have a remarkably high crossite or Na in (M4)-sites, which is consistent with the presence of Fe-Ti oxides in the mineral assemblage at the time that the primary plagioclase is un- stable, altering to epidote or becomes more sodic (Grapes, 1975; Brown, 1977; Spear, 1980, 1982). Tschermak’s substitution of Al(IV) for Si in amphi-


Ti

Maximum Ti of low-pressure

amphibole

Low-pressure

2 .O _ Shuksan, USA

No

0 0.‘5 I.‘C l.!S

rv Al

No

(M4) l.O{F

- Sonbugwa -t Franciscan

(high- pressure 1

we - Dulrodian (medium-pressure]

.- .- Abukuma (low- pressure j

(cl 0 0.5 1. 0

No (” -I K

Figure 6. Composition of amphiboles as an indicator of the metamorphic pressure. (a) after Hynes (1982); (b) after Brown (1977); (c) after Laird and Albee (1981). Symbols as in Fig. 4.

boles from Wadi Sikait is higher than those from Ge- be1 Zabara, indicating relatively higher metamorphic temperatures for the former occurrence. A decrease of Al(IV) from amphibole cores towards the rims only in a few crystals suggests a limited retrograde metamorphism in samples from Wadi Sikait. This conclusion is also supported by the growth of some fine chlorite in the form of thin veinlets close to the rims of some garnet prophyroblasts.

On the basis of the available thermodynamic

equilibria for the mineral assemblages in the garnet- amphibolites, it is suggested that the metamorphic temperatures for samples from Wadi Sikait are relatively higher than those from Gebell Zabara (545 to 571°C and 484 to 515”C, respectively). Nevertheless, temperature in both occurrences was suitable for the transformation of ilmenite to rutile. I/n some samples from Gebel Zabara, clear orange rut$ (Fe-poor) with perfect internal reflection is present. For both occurrences, the estimated temperatures are in harmony

Tabl

e 4.

Ele

ctro

n m

icro

prob

e an

alys

es o

f pl

agio

clas

e,

epid

ote

and

iIm

enite

Min

eral

Sa

mpl

e Si

O2

Tio2

C

r203

A

1203

Fe

203

Fe0

I&O

Mgo

N

iO

CaO

N

a20

KZO

To

tal

Cat

ions

Si

Ti

C

r A

I Fe

3+

Fe2’

M

n

Mg

Ni

Ca

Na

K

Alb

ite

Ano

rtbi

te

Ort

bocl

ase

. . .

Plag

iocI

ase*

E

pido

te**

II

men

ite*

Zab8

2 Za

b83

sik7

5 SW

7 SW

7 Za

b82

Zab8

4 Za

b84

Zab8

5 Za

b84

Sik7

6 62

.50

62.6

1 62

..16

62.3

3 58

.59

38.3

7 38

.50

38.8

8 39

.12

0.01

0.

05

0.03

0.

00

0.02

0.

01

0.01

0.

06

0.13

0.

02

0.04

53

.50

53.0

5 0.

06

0.00

0.

00

0.00

0.

00

0.07

0.

00

0.03

0.

03

0.06

0.

01

23.0

2 22

.63

22.6

2 22

.63

25.1

3 29

.20

28.7

5 31

.49

31.0

1 0.

08

0.07

0.

12

0.05

0.

22

0.19

0.

29

5.89

6.

12

2.61

2.

87

0.00

0.

00

45.6

8 44

.86

0.82

0.

85

0.04

0.

03

0.00

0.

04

4.51

4.

26

3.77

3.

46

6.41

24

.01

23.8

0 24

.67

24.5

7 0.

04

0.07

9.

24

9.27

10

.15

10.3

8 8.

72

0.02

0.

00

0.02

0.

00

0.00

0.

00

0.07

0.

10

0.08

0.

08

0.05

0.

00

0.00

0.

00

0.03

0.

00

0.01

99

.61

98.9

6 99

.00

99.1

4 99

.24

97.7

4 97

.47

97.8

7 97

.67

100.

24

99.0

3

2.77

35

2.79

47

2.75

52

0.00

11

0.00

11

0.00

00

0.00

20

0.00

00

0.00

00

1.20

41

1.19

07

1.18

19

0.00

41

0.00

17

0.00

74

2.75

47

2.66

05

2.97

78

2.99

57

2.98

08

2.99

27

0.00

04

0.00

12

0.00

05

0.00

03

0.00

36

0.07

70

0.00

13

0.00

25

1.01

35

0.00

00

0.00

01

0.00

43

0.00

02

0.00

18

0.00

15

0.00

11

1.17

87

1.31

87

2.67

04

2.63

66

2.84

47

2.88

61

0.00

24

0.00

63

0.00

95

0.34

37

0.35

82

0.15

08

0.15

79

0.00

00

0.96

24

0.01

76

0.00

14

0.00

00

0.16

38

0.30

60

1.99

63

1.98

38

2.02

59

2.01

39

0.00

11

0.88

94

0.75

30

0.00

35

0.00

08

0.00

33

0.00

00

0.00

00

0.00

45

0.00

28

0.00

00

0.00

04

0.00

00

0.00

28

0.00

00

1.01

73

0.00

02

0.00

20

0.00

00

0.95

65

0.01

83

0.00

11

0.00

08

0.00

20

0.00

02

0.21

43

0.20

37

0.17

90

0.79

53

0.80

23

0.87

20

0.00

38

0.00

58

0.00

42

core

78

.5

21.1

0.

4

rim

79

.3

20.1

0.

6

0.00

04

rim

ri

m

core

G

ener

atio

n (I

) G

ener

atio

n (I

I)

82.6

84

.1

70.9

17

.0

15.5

28

.8

0.4

0.4

0.3

._.

_ .

_ _

?

“: ca

tions

are

cal

cula

ted

assu

mm

g st

oich

iom

etry

an

d ch

arge

ba

lanc

e.

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Table 5. Calculated temperatures and pressures for the garnet-amphibolites

Sample Zab82 Temperature in “C

G. and P. 502 B. and H. 495

Pressure in kbars 8. and L. 7.8 K. and S. 7.7

Zab83 Zab84

508 515 505 509

8.1 7.7 7.9 8.0

Zab85 sik75 Sik76 sik77 Sik78

509 555 570 556 550 510 545 560 550 545

7.5 6.8 6.5 6.9 6.5 7.9 7.0 6.5 6.7 6.4

Temperature CC I

Figure 7. Thermobarometric results (in “C and kbar) for the garnet-amphibolites (ruled area) in comparison with the Pan-African amphibolites (dotted area, based on data cited in El-Tokhi, 1992 and Jarrar, 1994). Arrowed line represents the suggested P-T tra- jectory for the studied granet-amphibolites. The AhSiOs phase diagram is after Berman (1988).

with the absence of clinopyroxenes in the studied assemblages, which requires metamorphic temperatures in the sillimanite zone (Ghent et al., 1983 and others). Amphibole chemistry and the other mineral equilibration together prove that there were higher pressure conditions for the Gebel Zabara samples (averging 7.7 kbar) in comparison with those from Wadi Sikait (averging 6.8 kbar).

Finally, the present work contributes to the presence of medium temperature and medium- to high- pressure amphibolite meatmorphism in the southern portion of the Easten Desert of Egypt. On a regional scale, it is suggested that the grade of metamorphism of the garnetiferous metapelite-metabasite pile increases southwards. Some field and laboratory obser- vations by the author in the Abu Swayel area (further south of the Sikait-Zabara district) support such a conclusion and the garnet-amphibolites there are much coarser in grain size with very distinct megatextural characteristics. El-Tokhi (1992) presents some microprobe analysis (for amphiboles only) from the Pan-African volcanic amphibolites from Wadi Feiran, western Sinai. Although he did not make any P-T calculations and based only on the absence of clinopyroxene from his assemblages (mainly amphibole, plagioclase and no garnet) he assumed a temperature range of 550-740°C and 2-4 kbar pressure.

These pressure values seem reasonablk, but the given temperatures are somewhat high as his hornblendes plot in the garnet zone on the diagram of Laird and Albee (1981) in addition to the absence of clinopyroxenes. Jarrar (1994) gave the same low-pressure estimation of 2-4 kbar for some volcanic amphibolites from southwest Jordan. This led him to the conclusions that low-pressure regional metamorphism is very characteristic of the evolution of the Pan-African rocks in the Arabo-Nubian shield. On ithe order hand, it is concluded here that the garnet-amphibolites from the Sikait-Zabara district and Abu Swayel area in the southern Eastern Desert are infrastructural rocks and they are not equivalent to the low-pressure suprastructural Pan-African ones. The enclosing garnetiferous metasediments must have been metamorphosed in the same conditions as the garnet- amphibolites. These metasediments are accepted as infrastructural rocks (Hegazy, 1984; El-Gaby, 1983; El Gaby et al., 1987). As a final statement, metamorphic conditions in the Arabo-Nubian shield are predicta- bly not uniform vertically and horizontally and it is therefore recommended to investigate the metamorphic history of every horizon (especially in the presence of major and/or local thrusts) prior to correla- tion on a regional scale. A detailed structural and mineralogical study is recommended to elucidate the roles of subduction-obducation and lcrustal thicken- ing in the appearance of high-pressure assemblages in the Egyptian Eastern Desert.

Acknowledgements

The author is greatly indebted to Prof. V. Trommsdorff for providing the facilities for electron microprobe analyses. Deep thanks are due to Prof. M. Takla and Prof. A. Hafez for their support, construc- tive discussion and review of the manuscript. Valu- able recommendation and discussions of the Journal’s reviewers are also acknowledged. Their opinions helped a lot in the preparation of the final version of this paper.

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Medium- to high-pressure garnet-amphibolites from Gebel ...rjstern/egypt/PDFs/SE...Une etude a la microsonde &ctronique mdique que le grenat, l’ amphibole et le plagioclase du Wadi

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