A-type granite and the Red Sea opening

Tectormphysics, 204 Elsevier

Science

(1992)27-40

Publishers

27

B.V., Amsterdam

A-type granite and the Red Sea opening Robert G. Coleman ‘, Susan DeBari a and Zell Peterman UGeology Department,Stanfor~ Lini~~ersity,Stanford, CA 94305, USA

’

’ isotope Branch, U.S. Geological Suwey, Denrer, CO 80226, USA (Received

March

6, 1989; revised version

accepted

June 10. 1990)

ABSTRACT Coleman. R.G., DeBari, S. and Peterman, Z., 1992. A-type granite and the Red Sea opening. Afro-Arabian Rift System. Tecto~ophysics, 204 (spec. sect.): 27-40.

In: R. Altherr

(Editor),

The

Mi~~cene-Oligocene A-type granite intrudes the eastern side of the Red Sea margin within the zone of extension from Jiddah. Saudi Arabia south to Yemen. The intrusions developed in the early stages of continental extension as Arabia began to move slowly away from Africa (around 30-20 Ma). Within the narrow zone of extension silicic magmas formed dikes, sills, small plutons and extrusive equivalents. In the Jabal Tirf area of Saudi Arabia these rocks occur in an elongate zone consisting of late Precambrian basement to the east, which is gradually invaded by mafic dikes. The number of dikes increases westward until an igneous complex is produced parallel to the present Red Sea axis. The Jabal Tirf igneous complex consists of diabase and rhyolite-granophyre sills (20-24 Ma). Although these are intrusine intrusive rocks their textures indicate shallow depths of intrusion (i 1 km). To the south, in the Yemen, contemporaneous with alkali basaltic eruptions (26-30 Ma) and later silicic eruptions, small plutons, dikes, and stocks of alkali granite invaded thick (1500 m) volcanic series, at various levels and times. Erosion within the uplifted margin of Yemen suggests that the maximum depth of intrusion was less than 1-2 km. Granophyric intrusions (20-30 Ma) within mafic dike swarms similar to the Jabal Tirf complex are present along the western edge of the Yemen volcanic plateau, marking a north-south zone of continental extension. The alkali granites of Yemen consist primarily of perthitic feldspar and quartz with some minor alkali amphiboles and acmite. These granites represent water-poor. hypersolvus magmas generated from parent alkali basalt magmas. The granophyric, two-feldspar granites associated with the mafic dike swarms and layered gabbros formed by fractional crystallization from tholeiitic basalt parent developed in the early stages of extension. Initial “‘Sr/‘“Sr ratios of these rocks and their bulk chemistry indicate that production of peralkaline and metaluminous granitic magmas involved both fractionation and partial melting as they ascended through the late Precambrian crust of the Arabian plate.

Introduction The occurrence of Tertiary A-type granites has been known for some time in the Yemen Arab Repubiic and southwestern Saudi Arabia near Jizan (Lamare, 1923; Roman. 1925; Huzayin, 1936; Shukri and Basta, 195.5; Karenberg, 1959; Geukens, 1966; Greenwood and Bleakly, 1967). None of these early reports attempted to relate the occurrence of these granites to the regional geology. Recent studies by Coleman et al. (19771,

Currespondeace partment

to: R.G.

of Geology,

0040-1951/92/$05.00

Coleman,

Stanford,

Stanford

University,

CA 94305, USA.

0 1992 - Elsevier

Science

Publishers

De-

Grolier and Overstreet (1978), McGuire and Coleman (19861, Capaldi et al. (1987) and Coleman and McGuire (1988) have shown that these rocks are Miocene in age. They occupy a structural setting that is related to the early volcanic activity which resulted from the crustal extension that finally led to the formation of the present Red Sea. This study will attempt to characterize these rocks petrologically and to demonstrate that A-type granites have developed in various ways. A-type granites, first introduced by Loiselle and Wones (1979), are thought to form most commonly in anorogenic situations, such as rifted continental crust, where previous partial melting has taken place. Alkali feldspar is the predomi-

B.V. All rights reserved

K.G. COLEMAN

28

ET AL.

nant mineral with albite-orthoclase intergrowths (perthites) often developing micrographic intergrowths with quartz. Minor amounts of acmite and arfvedsonite with annite-rich biotites are characteristic (Collins et al., 1982; Clemens et al., 1986). The presence of these A-type granites within the well studied extensional zones of the Red Sea provide a special opportunity for understanding their genesis. Geologic setting Tertiary A-type granites occupy a narrow structural zone, less than 50 km wide, which parallels the Red Sea axis and extends from the southern tip of the Arabian peninsula, at the Straits of Bab el Mandeb 600 km northward to the vicinity of Jizan (Fig. 1). This elongate zone is separated from the Red Sea shoreline by a narrow coastal plain cover of Quaternary and Tertiary sedimentary rocks. Many of the better exposures coincide with a steep escarpment that rises above the Red Sea coast. This study is centered around three occurrences of these granites: (1) Jabal Tirf east of Jizan, previously described by Coleman (19841, and McGuire and Coleman (1986). (2) Jabal Hufash approximately 100 km west of Sana along the Al Hudaydah road. (3) Jabal Sabir a few kiIometers south of Taizz (Fig. 1). The narrow extensional zone containing the granites varies from north to south, with the abundance of the granites increasing southward. In the Jizan segment, the granites are contained in a narrow zone (Tihama Asir Complex) that strikes parallel to the Red Sea axis (Fig. 1). This narrow zone comprises Precambrian metamorphic rocks (exposed on the east border) which are invaded by diabase and rhyolite dikes, Iayered gabbros, and granophyric intrusions. The crosscutting intrusive relations suggest that the mafic and leucocratic magmas are essentially contemporaneous (McGuire and Coleman, 1986). Further south, near Jabal Hufash, larger plutons of granite appear in what is essentially the same structural situation as Jabal Tirf, except that these plutons are further away from the

14i

, Fig. 1. Geologic of Tertiary nophyre

map of Felix Arabia

volcanic

associated

(Samples granite

JT-I, pluton

and

intrusive

JT-3,

invading

Yemen

MJG-76.72D.

Jabal

granite

(Samples

alkali

75-OT-27,

gabbros

JT-5);

72A, MJG-76”72B, Sabir

showing rocks.

with layered

JT-2,

i_...._J

i

75-OT-28). and Overstreet

and

2 = Jabal

volcanics 75%17.lA,

pluton

the distribution

I = Jabal

cutting

Map

Tirf gra-

sheeted

dikes

Hufash

alkali

(Samples

MJG-76-

758-17-101, Yemen

modified

after

3=

volcanics Grolier

(1978).

coastal plain (Fig. 1). A comparison of our map (Fig. 1) with that of Capaldi et al. (1987) shows that they consider the area1 extent of the granite to be much more restricted than is portrayed on the map of Grolier and Overstreet (1978). In the vicinity of Al Hirsch, Capaldi et al. (1987) describe a zone of granophyric and basaltic dikes similar to Jabal Tirf, which suggests that this extensional style of magmatic emplacement continues nearly as far south as Bab el Mandeb. The

A- I’YPt

GKANI’I‘k

AND

29

‘I‘HE RED SEA OPENING

present level of erosion along the scarp from Yemen to Jizan has removed approximately 2 km of overburden and the intrusion of these granites into the Yemen volcanics (basalts, trachytes, and comendites) indicates development of shallow magma chambers (McGuire and Coleman, 1986). At the Jabal Sabir locality, nearly 90 km from the Red Sea coast, the granite appears to occupy a position near intersecting faults (Grolier and Overstreet, 1978) (Fig. 1). At this locality, the granite intrusion cuts the Yemen volcanics, which here consist of a sequence of interiayered rhyolitic, comenditic, and trachytic tuffs and flows, accompanied by minor alkali basalt flows, Mafic dikes and layered gabbros of tholeiitic affinities are not found here. In general, the granites (granophyres) to the north of Jabal Tirf are directly intruded into zones of active extension where the main co-magmatic rocks are mafic magmas of tholeiitic parentage (Coleman and McGuire, 1988). Further south, in the same structural setting, the alkali granite intrusives are larger and are associated with contemporaneous eruptions of rhyolites, comendites, and trachytes punctuated by periods of alkali basalt eruption. Evidence for crustal extension is not apparent because normal faults, if present, have been covered by continuing volcanic activity. Age relationships Radiometric data on the granophyres of the Jabal Tirf area yield K/Ar ages of 20-23 Ma, which overlap the gabbro and hornfels ages of 20-24 Ma (Coleman et al., 1977). Fossils from the Jizan group into which the granophyres intrude imply Oligocene ages (Schmidt et al., 1982). A single K/Ar age on the arfvedsonite from the alkali granite of Jabal Sabir has an age of 22 Ma (Grolier and Overstreet, 1978). These alkali granites are intrusive into the Yemen voicanics and continental sediments of the Tawilan Group and Medjzir Series, which Grolier and Overstreet (1978) consider to be no younger than Early Miocene. A continuous set of northwest-trending dikes within the narrow coastal zone extend northward to the Gulf of Aqaba. At the Gulf of

Aqaba, these dikes are offset by the Dead Sea fault along the southern part of the Sinai Peninsula and have the same age as the granophyres in the Jabal Tirf area (Bartov et al., 1980). The radiometric and geologic ages fix the Early Miocene as a period of widespread extension and igneous activity related to the opening of the p‘roto Red Sea. One of the most significant aspects of this igneous activity is the development of A-type granitic intrusions in the southern sector on the Arabian side of the Red Sea. Similar granites of the same age and composition also occur in the Afar depression (Barberi and Varet, 1975). The rapid thermal uplift and subsequent erosion has exposed these hypabyssal, dike-like intrusions in the southern segment and in the Afar, however it is possible that there are extensive tracts of A-type granites underplating extended crust all along the coastal plain (Pallister, 1987). Petrology Mineralagy and mineral chemistry

The granophyre at Jabal Tirf is a light tan, fine-grained quartz feldspar rock. Euhedral plagioclase crystals (lo-30 modal percent) are surrounded by granophyric intergrowths of alkali feldspar and quartz, and interstitial plagiociase. Granophyric areas make up 5570% of the rock, and the composition of the feldspar ranges from orthoclase-rich to albite-rich alkali feldto Or,Ab,,,An,) to oligoclase spar (Or,, Ab ,4 (Or,Ab,,An,,). Plagioclase is zoned, sometimes with fine oscillatory zoning near the rim. Compositions range from An,, to An,,, averaging about An,, in the cores, with margins varying from An,, to An,,. Subhedral clinopyroxene (Wo,, En,, Fs,,) is similar in composition to the late pyroxene reported in the Skaergaard intrusion (McGuire and Coleman, 1986). The Yemen granites from Jabal Hufash are coarse-grained quartz feldspar rocks with hypidomorphic-granular texture. The distinctly perthitic potassium feldspar makes up 60-70 modal percent of the rock, with interstitial quartz making up 25-30 modal percent. Arfvedsonite and

30

K.G. COLEMAN

Or,,Ab,,. The mafic minerals consist of arfvedsonite to magnesio-katophorite, aegerine-augite to augite, and minor biotite (Fig. 2). Accessory zircon is present in all of the specimens studied. Capaldi et al. (1987) reported similar petrographic findings to ours using a much more extensive rock collection.

ferro-cckermannite (2-5 modal percent) from anhedral grains with minor acmite (Fig. 2). Graphic intergrowths of quartz and feldspar are rare in these specimens. Plagioclase and orthoclase are found only as exsolved phases in the perthite. Accessory zircons are present in all specimens. Sub-solidus alteration has clouded the feldspars and altered the dark minerals to opaques and secondary phyllosilicate mineraIs. Capaldi et al. (1987) describe the Jabal Hufash as an alkali amphibole granite. The Jabal Sabir granite in Yemen is coarse graincd, hypidiomorphic-granular with graphic interstitial intergrowths of perthitic feldspar and quartz. These intergrowths are interstitial to, and form the margins of, the larger feldspar crystals. the perthitic feldspar makes up to 65-70 modal percent of sample 75-OT-28 and more than 90 modal percent of sample 75-OT-27. The exsolved orthoclase is 0r,,Ab3 and exsolved plagioclase is Ab,,Or, to Ab,,Or,An,. Microprobe analyses of cryptocrystalline perthite or incompletely exsolved feldspar yield values of Or,,Ab,, to

N4h

Major element chemistry

The rocks from Jabal Sabir and Jabal Hufash in Yemen are silica-saturated peralkaline granites (Table 1). Except for sample, 75OT-27, collected from the margin of Jabal Sabir, these granites contain normative acmite (AC) and sodium metasilicate (Ns). Sodium metasilicate, a rare normative mineral, is present in the norm when there is a molar excess of Na,O over AI,O,. The mineralogicai expression of the normative Ns is manifested in these rocks by the presence of arfvedsonite. In contrast, the granites from Jabal Tirf complex in Saudi Arabia are silica saturated metaluALKALI AMPHIBOLES

AMPHIBOLES fNa+K&<0.50

I

fNa+K),LO.SO

1.0

0.0 m’/

I m

M@

0.5

0.5 MAGNESIQ-

FERRO-BARROISITE I

75

ECKERMANNITE I

I

ARFVEDSONITE

I

t

l.OI

.

0.5

0.0

si p.f.u.

-

t

1

1.0

Fe3+I(Fe3++V’A1)

-

(Nri+ Kf,< 0.50

(Na+K&r0.50 t.01

IF Lkc 2: 2.5)

Mg+F@+

FERROWHCHrrE

0.c 8.0

Imz”mE

ECKERMANNlTE

-

Mg+Fi$+

I

ET AL.

0.0

, 1

I MAGNESIO-

MAGNESIO-

KATO PHORITE

TARAMITE

I

WESECKfTE

m --

0.5,

CRCXSSITE

-

Mg+ FerC MAGNESIGTARAMITE

KATO PHORITE

GLALJCOPHANE RiEBEWTE

R#>CTERlTE I

0.0

1

a.0

I

I

I

7.5

a.5

a.0

Fig. 2. Composition

of amphiboles

I

1.0

’ 0.0

si p.f.u. from the Yemen

alkali granites

several grains).

determined

Classification

by microprobe

after Leake (19781.

0.3 Fe3+/(F$++

0.7

1.0

“‘Al) -

(each spot represents

the average

of

A-TYPE

CiRANITE

AND

THE

RED

SEA

31

OPENING

granophyres range from 66.9 to 74% SiO, (Table I) and overlap the marginal rock from Jabal Sabir but are, in general, less alkaline than the Yemen granites. The Yemen alkali granites and the granophyres overlap in Na,O + K,O contents, with a tendency for the alkali granites to have a higher total alkali content (Fig. 3). Dikes related to the granophyre reveal a trend which, in general, sug-

minous granites with rare arfvedsonite but without normative acmite. The marginal rock (75OT27) from Jabal Sabir is not peralkaline and is silica-poor, having 66.9% SiO, as compared to the 73.5-75.4% SiO, of the other Yemen peralkaline granites. In addition, the amphiboles in this sample are not of the alkali class (i.e., magnesio-katophorite/richterite) (Fig. 2) The Jabal Tirf TABLE

1

Chemical analyses of A-type granites in Felix Arabia Component

Sample Jabal Hufash 3

2

1

Jabal Sabir

Jabal Hufash

4

6

5

7

Ma’alata

Jabal Tirf

8

10

9

11

12

Major oxides (wt.%) SiO

74.0

74.5

75.4

66.9

74.0

73.5

73.9

75.4

66.9 0.86

74.0 0.19

73.5 0.15

73.9

TiO:

0.39

0.37

0.11

0.68

0.19

0.15

0.45

0.11

AW,

11.86

10.85

10.74

15.79

12.06

11.73

11.76

11.56

12.8

12.7

13.2

13.2

0.45

Fe,& Fe0

0.34

0.33

0.22

0.41

0.26

0.21

0.38

0.12

3.7

3.8

3.5

5.9

2.44

2.38

1.60

2.95

1.91

1.54

2.75

0.91

2.0

1.6

2.5

4.5

MnO

0.21

0.08

0.20

0.22

0.15

0.09

0.19

0.07

0.14

0.13

0.16

0.25

MgO CaO

0.15

0.16

0.08

0.81

0.19

0.13

0.31

0.28

0.47

0.40

0.85

0.85

0.24

0.63

0.25

1.23

0.33

0.37

0.28

0.36

1.7

1.7

3.0

3.8

Na,O

4.64

3.75

4.24

6.31

4.76

4.66

4.95

4.75

4.5

4.6

4.0

4.2

K2O

4.80

4.48

4.90

4.61

4.83

4.76

4.81

4.80

2.80

3.0

2.3

1.8

pzos

0.03

0.02

0.01

0.21

0.03

0.02

0.04

-

0.13

0.12

0.27

0.37

99.09

97.56

97.75

100.30

98.72

97.16

99.82

98.51

98.67

98.22

98.30

97.73

Q

31.57

36.66

38.04

9.00

31.11

32.69

30.66

35.10

32.84

31.71

32.35

25.70

Or

26.90

25.32

27.60

25.76

27.11

27.08

26.77

26.76

15.87

17.09

13.07

10.47

Ab

34.51 _

31.34

28.29

53.58

35.43 _

34.57

33.70

32.78

38.76

39.83

34.53

37.13

10.48

11.46

0.14

0.11

0.09

0.11

0.09

0.16

0.49

6.20 _

4.96

Ac NS

0.97

1.08

1.25

1.73

1.77

_

_

Di

0.94

2.87

1.12

3.39

1.37

1.64

1.06

1.69

1.08

2.42

1.78

4.52

HY Mt

4.25 _

2.99

2.85

5.01

3.42

2.39

5.11

1.65

1.02

0.12

2.45

2.80

0.06

3.09

2.41

2.93

5.06

II

0.64

0.62

0.18

1.42

0.31

0.25

0.74

0.21

0.89

0.82

1.51

1.89

AP

0.06

0.04

0.02

0.42

0.06

0.04

0.08

-

0.27

0.25

0.55

0.77

Total Normative minerals

Ai?

1.08 1.82

0.34

Minor elements (ppm) Nb Y

158

99

51

49

122

96

132

-

30

30

20

30

86

63

27

38

70

56

81

-

150

150

150

150 179

Sr

30

32

27

211

18

34

17

-

116

93

146

Rb

I54

137

202

85

218

223

141

-

83

85

86

58

Zr

1249

702

286

279

644

426

441

-

700

700

700

5110

Ba

92

84

91

1890

124

158

89

-

470

490

500

410

Rb/Sr

5.51

4.71

7.35

0.4

IR (Sr)

0.7064

0.7061

0.7072

0.7058

12.1 0.7057

6.6

8.3

-

0.7060

0.7060 -

0.72

0.92

0.59

0.32

0.7046

0.7057

-

0.7041

sample No. 1: (MJG-76.72A), Sample 2: (MJG-76-72B), Sample 3: (MJG-76-72D), Sample 4: (76-OT-27), Sample 5: (75-OT-281, sample 6: (75B-17-lA), Sample 7: (75B-17-lB, Sample 8: (Afar), Sample 9: (JT-l), Sample 10: (JT-2), Sample 11: (JT-31, Sample 12: (JT-5).

K.G. COLEMAN

32

ET AL.

14I3Alkali

I2I

Gramte

I-

I o9s

6-

5

7-

0 s z +

5-

0 h?

4

6 Grahophyre

3_ “$Red 2-

Sea basalt

I -

III

III

40

Fig. 3. Harker tholeiitic

diagram

dikes; circled

and I-type granites

with

SiO,

Ill1

44

48

versus

crosses = Jabal

(data from Whalen

Na,O

Abyad

1

52

56

+ K,O.

sequence;

et al. (1987)). Average

other data of the alkali granites

from Capaldi

I

PERALKALk

’

I

’

I

I

I

I

1.4 1.3 1.2 I. I 8 -s

1.0 0.9

0 y”

wt %

= granophyres;

= published

-0.8 +0 0 0” $0.7 z 0.6 0.5 0.4

1

68

72

11 76

circles = alkali

average

values

Red Sea and dike compositions

et al. (1987) and Jabal Abyad

gests the possibility that fractionation of tholeiitic magmas could produce granophyric compositions. A sequence of extrusive flows from Jabal 1.5

11

11 64

SiO 2 Triangles squares

I

60

sequence

I 60

granites;

for A-type

crosses = Jabal

granites,

from Coleman

S-type

and McGuire

Tirf

granites (1988),

from Baker et al. (1973).

Abyad near Khaybar, Saudi Arabia, which represent a fractionation series of Tertiary alkali basalts to hawaiites, mugearites, trachytes, phonolites, and finally comendites is shown for comparison (Fig. 3). Even though the volume of these differentiated rocks is small, their alkali-silica relationship illustrates that the Yemen alkali granites may have been generated by crystal fractionation of similar magmas (Fig. 3). The higher silica content of the alkali granites, however, does suggest some crustal contamination or more extreme modification of the primary magma. The Yemen alkali granites have agpaitic indexes that extend to 1.25 but a large number also show metaluminous and peraluminous compositions (Fig. 4). The granophyres are exclusively either metaluminous or peraluminous and are similar to the differentiated tholeiitic dike rocks (Fig. 4).

0.3

Trace elements

0.2 0.5

.6

.7

.8

.9

No,0 Fig.

4. (Na,O+K,O)/Al,O,

+CaO)

(molecular Symbols

1.0 I.1

1.3 1.4 1.5 1.6

A’2O 3 + KzO + Co0 (mo’) versus

%o) for A-type and sources

1.2

AI,O,/(Na,O+K,O

granites

of Felix Arabia.

the same as Fig. 3.

A-type granites are typically enriched in minor elements such as Ga, Nb, Y, Zr, and rare earth elements (REE) (Whalen et al., 1987). The available data on Y, Zr, and Nb for the Yemen alkali granites and granophyres are plotted against SiO, (Fig. 5). The alkali granites are strongly enriched

A-TYPE

c

GRANITE

Y+Zr+Nb

AND

THE

RED

33

SEA OPENING

pprn

IO

100

50

I

55

I 65

I

60 so2

I 70

I

75

I

80

wt %

Fig. 5. The sum of Y +Zr+Nb (ppm) versus SiO, (wt.%) for A-type granites of Felix Arabia. Symbols and sources the same as Fig. 3.

in these three elements, as are the granophyres at SiO, values between 65 and 75%. Enrichment of Y, Zr, and Nb in the Jabal Abyad series is clearly tied to differentiation and demonstrates that these same trace elements in the granites and granophyres are probably enriched by the same process. The general diminution of Y, Zr, and Nb as silica increases has no obvious explanation. Yemen granites show high Rb/Sr ratios and relatively low Ba concentrations, whereas the Jabal Tirf granites have lower RB/Sr ratios and higher Ba concentrations (Fig. 6, Table 1). Sample 75 OT-28 has a Rb/Sr ratio of less than 1, somewhat lower Zr, and a Ba concentration greater than the other Yemen granites. The marginal

IO

100

9

wm

Fig. 6. Rubidium-strontium plot for Felix Arabia A-type granites and associated mafic igneous rocks. Symbols and sources the same as Fig. 3.

position of the Jabil Sabir intrusion of 75OT-27 suggests that its trace etement variations is due to either magma contamination or that it is less fractionated. Preliminary REE data on the Yemen alkali granites (Capaldi et al., 1987) and the granophyres (Coleman et al., 1977) show that there is considerable enrichment of all REE except for a pronounced Eu depletion probably related to plagioclase fractionation. It is interesting to note that in two samples of alkali granite Capaldi et al. (1987) found no Eu depletion,

TABLE 2 Strontium isotope and Y, Zr, Nb, Rb, and Sr analyses Sample

75OT-28 76OT-27 MJG-76-72A MJG-7672B MJG-76-72D 75817-1A 758-17-18

Y

Zr

Nb

Rb

Sr

(ppm)

(ppm)

(ppm)

(ppm)

(ppm)

63 32 78 56 27 55 71

683 286 1497 818 289 443 492

169 86 238 152 69 138 201

217.8 85.2 153.6 137.30 202.10 222.90 140.80

17.65 211.20 29.75 31.91 27.09 34.39 17.15

“Rb/“Sr

s7Sr/?3r

IR23Ma

35.78 1.17 14.96 12.46 21.62 18.78 23.80

0.71732 0.70619 0.71125 0.71011 0.71420 0.71209 0.71382

0.70566 0.70581 0.70637 0.70~5 0.70715 0.70597 0.70606

*

Rb and Sr were determined by isotope dilution; Y, Zr, and Nb by X-ray fluoresence analysis. * Initial X7Sr/86Sr ratios for each sample calculated on the basis of an age of 23 Ma. ** Initial Sr isotope ratio calculated on the basis of T = 23 k 5 Ma, U.S. Geological Survey, Isotope Lab., Denver.

&SMa ** 0.00250 0.00008 0.00105 0.~87 O.OOlSi 0.00131 0.00167

KG.

34

COLEMAN

ET AL.

0.720

I

--’ 0.716

I

/

I

I

.

.

23 t 3.7 Ma 0.712

I

‘-:_:I::I~

0

5

IO

15

Fig. 7. X7Sr/XhSrversus s’Rb/s’Sr

suggesting partial lower crust.

melting

25

20 87

Rb I 86

30

35

40

Sr

isochron plot for alkali granites from Yemen, See Table 2 for analytical data.

of plagioclase-poor

Selected Yemen granites and Jabal Tirf granophyres were analyzed separately for Y, Zr, Nb, Rb, Sr, and a7Sr/ah Sr (Table 2). Samples of the Jabal Tirf granophyres had been analyzed earlier in the same laboratory by C.E. Hedge (Coleman et al., 1977). A regression of data using Isoplot (Ludwig, 1987) for all of the Yemen granite samples gives an isochron corresponding to an age of 23 rt 4 Ma and an initial 87Sr/86Sr ratio (hereafter called IR (Sr)), of 0.7062 zt 0.0011 (Fig. 7). A two-point isochron, defined by the Jabal Sabir samples, corresponds to an age of 22.6 + 0.3 Ma with an IR (Sr) of 0.70581 rfr0.0004. The Jabal Hufash data (5 samples) produce an isochron of 25 I): 13 Ma with an IR (Sr) of 0.7058 + 0.0011. These ages are consistent with a K/Ar age of 22.7 + 0.9 Ma on a separate sample of arfvedsonite from the Jabal Sabir granite (Grolier and Overstreet, 1978). Coleman et al. (1977) reports K/Ar ages of 20.0-24.3 Ma on gabbro, hornfels, and granophyre from Jabal Tirf. Capaldi et al. 0987) and Civetta et al. (1987) report K/Ar whole rock ages on the Yemen granites ranging from 20.4 k 0.7 to 26.1 + 0.8 Ma and have divided the rocks into two separate rock groups:

(1) an older group at 26 Ma, similar in age to the Yemen lower trap series; (2) a younger group at 20-22 Ma matching the age of the upper Yemen trap series and that of the Jabal Tirf granophyre. IR (Sr) values for each sample are calculated on the basis of an age of 23 Ma (Table 2). Uncertainties shown for the IR (Sr) values are based on arbitrary age uncertainty of rfr5 Ma. These calculated IR (St-1 values range from 0.75057 to 0.70720, a span which probably represents primary variation rather than post-crystalhzation disturbance. Most of the IR (Sr) of the Yemen granites are higher than those of the Jabal Tirf granophyres. Other significant differences include higher Rb, lower Sr, and much higher Rb/Sr ratios in the Yemen granites. Both the Jabal Tirf granophyres and the Yemen granites have IR (Sr) values significantly higher than the mafic igneous rocks of the region (Coleman et al., 1977) (Fig. 81 suggesting some degree of crustal involvement in their formation. The Iack of significant correlations between IR (Sr) and Rb or Sr contents and the general coherence of the data to a 23 Ma age suggests that this contamination occurred before final petrogenic diversification of the magmas. It seems probable that the parental magmas for the Tertiary granites were contaminated by the Proterozoic rocks of the underlying Arabian craton. Tonalitic rocks

A-TYPk

GKANITE

0.7oe

--

AND

35

THE RED SEA OPENING I

1

’

r

0 Alkali d + x

gronlfa

Granophyra Silicic

volcaoic

Dikes

Fig. 8. X7Sr/HhSr versus Sr concentrations for A-type granites and associated igneous rocks. Symbols and sources the same as Fig. 3, black circles = alkali basal%; additional data from DeMange et al. (1983).

with low Rb/Sr ratios are present within the Arabian craton in and around the Jabal Tirf area and could, perhaps, represent a source of this excess radiogenic Sr (Fleck and Hadley, 1982). The late Precambrian Pan-African event (500600 Ma) on the Saudi Arabian shield, developed during the final cratonization of the Arabian plate, produced A-type granites which are chemically and isotopically nearly identical to the Tertiary A-type granites in Yemen, IR (Sr) values of the western Saudi Arabian late Precambrian island-arc rocks average about 0.704, with Rb/Sr ratios of about 0.2. Early Paleozoic, post-erogenic peralkaline granites within the eastern part of the Saudi Arabia craton have IR (Sr) values as high as 0.706, with Rb/Sr ratios all greater than 5.0 (Fleck and Hadley, 1982; Stacey and Hedge, 1984). Metasediments associated with these peralkaline granites probably increase the IR (Sr) to above 0.710, however the Proterozoic crust of Saudi Arabia contains no older Archean rocks. This means that contamination by older radiogenie Sr of the ascending alkali granites or partial melts of this crust would not be as great as

usually found in Archean crust. The sediments associated with the Saudi Arabian Precambrian rocks are generally derived from talc-alkaline igneous arc materials and, therefore, would not introduce excess alumina into the melts or contain enriched Sr. However, mineralogic variations and melting reactions could produce metaluminous granitic melts. The initial “‘Sr/%r for these rocks varies from 0.703 to 0.701 with extremely low Sr. The nature of these A-type granites gives an indication that the Saudi Arabian crust from which these granites formed contains no large amounts of older Precambrian continental crust and explains the relatively high Rb/Sr for the Tertiary A-type granites of Yemen and Saudi Arabia. ~0ns~~ai~I~son magma generation The occurrence of plagioclase within perthites of the Yemen granites (hypersolvus) rather than as separate zoned plagioclase grains (subsolvus) is good evidence for crystallization of the Yemen granites from a magma low in volatiles and hot (around 9OO”C), as suggested by Whalen et al. (1987) and supported by the Clemens et al. (1986) experimental study. The presence of separate plagioclase and orthoclase grains in the Jabal Tirf granites characterizes them as subsolvus granites, which may relate to higher water pressures combined with shallow depths of crystallization. Luth et al. (1964) found that, in general, alumina undersaturation (peralkalinity) is characteristic of hypersolvus granites. In contrast, corundum is common as a normative mineral in subsoivus granites (Whalen et al., 1987). This chemical aspect of granites is reflected in their respective modes: a~fedsonite-riebeckitc and acmite are characteristic of the hypersolvus granites and micas are predominant in the subsolvus granites. Melting of sedimentary material could also produce peraluminous and water-rich magmas, sometimes referred to as S-type (White and Chappell, 1977). Luth et al. (1964) show that alumina undersaturated rocks, such as the Yemen peralkaline granites, represent a direct line of descent from basic magmas with low volatile content. Furthermore, these same authors suggest

36

that melting of sedimentary rocks would not produce a peralkaline magma. Experimental work on alumina-undersaturated systems has been carried out by Carmichael and Mackenzie (1963). They showed that addition of Acmite (AC) and sodium metasilicate (Ns) to the Ab-Or-Qz system at PHzO = 1000 kg/cm31 caused the feldspar minima to shift towards the orthoclase-quartz join, producing the opposite effect to increasing PHzO. The Yemen granites cluster between the thermal valleys (depression running from the alkali feldspar minimum to the quartz-feldspar co-tectic) for 0% AC + Ns and for 4.5% AC + Ns, whereas the Jabal Tirf granites plot distinctly below the minima and are transverse to its trend (not shown). Such a configuration suggests a fractionation trend comparable to that seen in Thingmuli (Carmichael, 1964). The alkali granites of Yemen have silica and alkali contents similar to the highly fractionated comendites of the Jabal Abyad sequence, as well as the Erta’Ale series of the Afar Depression (Barberi et al., 1975), whereas the granophyres of the Jabal Tirf are generally lower in silica and can be tied to the fractionated tholeiitic dikes of Jabal Tirf (Fig. 3). This same relationship can be seen in Fig. 4 where it is apparent that fractionation of the Jabal Abyad sequence leads to peralkaline volcanics, whereas the differentiated tholeiitic dikes lead to metaluminous or peraluminous granitic compositions. There is no obvious explanation for the trend of the Yemen alkali granites from peralkaline to peraluminous. Discussion

The development of the peralkaline and alkali granites within the zone of extension of the Red Sea can be explained partly on the basis of fractionating basalt magmas. Peralkaline magmas can develop from either tholeiitic or alkali basaltic parents. Their development is generally attributed to the “plagioclase effect” of Bowen (19451, where pure albite cannot fractionate from liquid containing Ca. Thus precipitation of Cabearing plagioclase will continue until the magma is almost depleted in calcium. High degrees of plagioclase fractionation, relative to olivine and

R.G. COLEMAN

ET AL.

clinopyroxene, could rapidly deplete aluminum relative to sodium and potassium and promote formation of peralkaline residual liquids enriched in Zr and Rb, and strongly depleted in Ba, Sr, and Eu. Carmichael and MacKenzie (1963) demonstrated that the “plagioclase effect” reaches a limiting control on magma composition when normative sodium metasilicate (Ns) reaches significant levels in the liquids. Here the remaining calcium combines with the ferro-magnesian constituents of the liquid to form a Ca-bearing pyroxene. Therefore the early fractionating magmas will preferentially incorporate the calcium into feldspar. However, once an excess of soda over alumina develops, alkali feldspar and Ca-bearing pyroxenes will crystallize. In Afar, Barberi et al. (1975, 1974) determined that pantellerites and comendites, considered here to be the younger extrusive equivalents of the Tertiary Arabian granites, are differentiates of transitional olivine basalts and that the “plagioclase effect” has played an important role. High-silica comendites from Erta’Ale, in the Afar depression of Ethiopia, are very similar in composition to the Yemen granites and are the first peralkaline liquids in the Erta’Ale series. From the varying chemical trends observed in Bonina, Afar, and Pantelleria, Barberi et al. (1975) concluded that the more alkalic the starting basalt, the more peralkaline are the final members of the fractionation series. Erta’Ale yields mildly peralkaline silica-rich comendites associated with the less alkalic “transitional” basalt. The Miocene to Pliocene Aden volcanics of south Arabia have a chemistry intermediate to Yemen and Afar, whose parent is a mildly alkaline olivine basalt yielding trachytes and rhyolites (Cox et al., 1970). A similar trend is found at Jabal Abyad, near Khaybar, Saudi Arabia (Baker et al., 1973). These rhyolites are comenditic and the composition is close to the Yemen alkali granites. These various illustrations show that compositions similar to the alkali granites are repeated in volcanic centers throughout Arabia and Afar associated with the Tertiary opening of the Red Sea. Ma’alata, another locality in Afar studies by Barberi et al. (1974), contains peralkaline comen-

A-TYPE

GKANITE

AND

THE

RtD

StA

37

OPENING

dites as well as peralkaline granites of almost the same composition as the Yemen granites (Table 1). These rocks have high initial 87Sr/86 Sr ratios up to 0.708 and it is obvious that their magmas have undergone some crustal contamination. Barberi et al. (1975) contend that partial melts of any continental crust, with excess silica (quartz normative) would produce comendites but they would plot along the quartz-feldspar minima following a path of increasing peralkalinity. The fact that the Ma’alata peralkaline rocks plot on such a transverse path relative to the quartz-feldspar co-tectic indicates that a fractionation origin is more probable than anatexis. Even though the bulk compositions of the Ma’alata series suggest magmatic differentiation, crustal contamination by partial melting of low Sr tonalites, similar to those found in the late Precambrian rocks of Saudi Arabia, could also produce the observed increase in X7Sr/XhSr. However, both the alkali granites and the granophyres seem to be related to fractionation trends of alkali and tholeiitic basalt magmas, and their enrichment in the high field strength elements (Zr, Y, and Nb) as well as the REE elements, further support a genesis mainly by fractionation. From the discussion above, it is possible that the Yemen and Jabal Tirf A-type granites could have formed by differentiation of parent magmas ranging in composition from alkali basalt to tholeiites. However, the large volume of these granites and their extrusive equivalents when compared to the exposed volumes of mafic magmas, suggests that crustal assimilation could have been extensive. As pointed out earlier, the mafic-felsic ratio in Jabal Tirf is 3 : 1 and for Yemen it appears to be greater than 1 : 1; however, more chemical and isotopic data is required to evaluate this idea further. It is also possible that deep crustal melting by ascending mafic magmas underplating the base of the extended crust produced more partial melting, due to the higher temperatures and confining pressures. Such a dry liquid would be confined to deep levels by its density until feldspar fractionation or partial melting of the deep crustal tonalites drove the magmas towards less dense peralkaline composition. Continued extension would allow

these light fractionated magmas to rise and produce shallow magma chambers where continued fractionation would enhance the peralkaline trend by the “plagioclase effect”. A more recent idea advanced by Whalen et al. (1987) and Collins et al. (1982) is that partial melting at high temperatures of lower crustal tonalites containing F- and Cl-rich amphibole or biotite would produce water-poor magmas. The Clemens et al. (1986) experiments on A-type melts show that they form at temperatures above 900°C and have low viscosity with moderate H,O and fluorine contents. These magmas, rich in Fl and Cl, could have distorted aluminosilicate frameworks in the melt, allowing stable complexing of compounds consisting of the highly charged metal ions of Ta, Nb, MO, W, and U associated with the zirconium fluoride melt complexes, preventing early crystallization of zircon (Collins et al., 19821. Thus, some A-type magmas could be products of basaltic magma underplating and partial melting of tonalites (I-type granites) at the base of the extending late Precambrian crust. The Jabal Tirf granites are associated with layered gabbros and dikes of intermediate to mafic composition. However, these mafic bodies are so small that it is quite impossible for them to have fractionated the large volumes of graniterhyolite seen at these same levels. It is much more likely that ponded magmas at the base of the extending crust either fractionated and/or partially melted the lower crust to form granophyric magma at deeper levels. Using the volume relationships of the exposed mafic and silicic rocks may, therefore, be misleading. Conclusions

The A-type granites of Felix Arabia exhibit nearly the same characteristics as most alkaline granites from other parts of the world (Hyndman, 1985; Black et al., 1985). Tectonically, they are aligned along flanks of a major rift zone and intrude through Proterozoic continental crust. They formed in the early stages of continental extension as a part of the first magmatic activity around 22 Ma and represent H,O-deficient magmas that form shallow intrusions. The Yemen

R.G. COLEMAN

38

granites are hypersol~s and the Jabal Tirf granophyres show subsolvus trends. The Jabal Tirf granophyres appear to be cogenetic with fractionated tholeiitic magmas, whose source can be related to shallow mantle melting in the early stages of continental extension. The Yemen alkali granites are intruded into and associated with rhyolite, comendite, pantellerite, trachyte, and alkali basahs. The alkali basalts of Yemen indicate a deeper mantle source for the primary magmas compared to the Jabal Tirf tholeiites. The volume of the Yemen peralkaline granite is many orders of magnitude greater than that of the Jabal Tirf granophyre or the Afar peralkaline granite (Fig. 1). The volume relationships between the mafic and silicic Red Sea rift zone rocks are distinctly different from the occur-

l

Map

Alkali

modified

rence of mafic rocks formed in an oceanic extensional regime and reflect the influence of the continental basement. The initial *‘Sr/%r ratio -JR &)--for the Yemen alkali granites is above 0.7057-0.7072, and those of the Jabal Tirf granophyres are about 0.7041-0.7057. The IR (Sr) of the gabbros and dikes associated with the granophyres average 0.7035, nearly the same as the nearby As Sirat alkali basalts (0.7035). Younger alkali basalts from Saudi Arabia have IR (Sr) values in the range 0.7035-0.7030 (Coleman et al., 1977; De Mange et al., 1983). On the basis of the IR (Sr) values it appears that the precursor alkaline or tholeiitic basalts may have assimilated some older continental crust during fractionation, perhaps accompanied by partial melting of the crust. The high Zr, Y, and Nb of these peralka-

Granites

after

Fig. 9. Distribution

Black et 01 (19851

of A-type

granites

in Africa

E-f‘ AL.

and Arabia

modified

after Black et al. (1985).

A-TYPE

GRANITE

AND

THE

RED

SEA

3Y

OPENINCI

line granites and granophyres indicates that both partial melting of continental crust and fractionation must have modified the original magmas. Crystal fractionation of a parent magma and partial melting of the late Precambrian tonalitic crust may be important factors in the development of the peralkaline A-type granites of Felix Arabia. Some of the contemporaneous basalt magmas, however, erupted on the surface without much change in their original parent magma compositions. Thus a bi-modal sequence formed; in the thicker, late Precambrian crust of Yemen the volume of exposed mafic versus silicic rocks is nearly 1: 1, whereas in Jabal Tirf, where the tholeiitic magmas encountered a thinner extended continental crust, a 3 : 1 (by volume) relatio~ship was produced. The effect of the continental crust in the evolution of these granites and their extrusive equivalents is evident, as can be seen by their volume relationships and isotopic ratios. In extensional environments, it is possible to produce peralkaline and metaluminous granites from both alkali and tholeiitic magmas as they partially melt the continental crust and fractionate. Late Precambrian A-type granites from the Arabian shield have similar chemical and isotopic compositions within the adjacent Saudi Arabian shield (intruded during the final accretion of the African plate). A-type granite formation in this region is not confined to the specific setting of either extension or compression. It can be proposed that, where there is lower crust of tonalitic composition and a source of magma hot enough to produce partial melts, it is possible to form A-type granites, as originally suggested by Wahlen et al. (1987). The distribution of A-type granites in Africa and Arabia is shown in Figure 9, where it can be seen that such granites are present within mobile zones and have ages ranging from 20 to 650 Ma (Black et al., 1985). The formation of A-type granite within mobile belts is not restricted to extensional regimes: many of the PanAfrican A-type granites are related to the final consolidation of Gondwanaland in a compressional mode similar to those late Paleozoic A-type granites produced during the final consolidation of Central Asia (Coleman, 1989).

Acknowledgements We are indebted to Carl Hedge, USGS, Denver, Colorado for part of the Sr isotopic values reported in this paper. The samples of Jabal Hufash and Jabal Sabir were collected by Maurice J. Grolier and WifIiam C. Overstreet of the U.S. Geological Survey as part of an investigation carried out for the Yemen Arab Republic (Grolier and Overstreet, 1978). The material from Jabal Tirf represents specimens collected during an investigation by R.G. Coleman supported by the Saudi Arabian Directorate General for Mineral Resources. During the preparation of this paper Capaldi et al. (1987) published a paper on the Yemen granites and we have inco~orated their new analytical data into our discussion. References Baker,

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