Contribution to the Geochemistry and Tectonic Setting of the ...kau.edu.sa/Files/320/Researches/60258_31067.pdfThe Oligo-Miocene A-type granites of Yemen Republic were emplaced in

JAKU: Earth Sci., Vol. 22, No. 2, pp: 37-68 (2011 A.D. / 1432 A.H.)

DOI: 10.4197 / Ear. 22-2.3

37

Contribution to the Geochemistry and Tectonic Setting of

the Oligo-Miocene A-Type Granites, South West of the

Arabian Shield, Yemen Republic

Rasmy I. EL-Gharbawy

Geology Department, Faculty of Science,

Ain Shams University, Cairo, Egypt

[email protected]

Received: 28/9/2009 Accepted: 15/6/2010

Abstract. The Oligo-Miocene A-type granites of Yemen Republic

were emplaced in the early stages of rifting and Red Sea floor

spreading, as Arabian Plate began to move slowly away toward ENE

direction. These granites intrude the eastern margin of the Red Sea

coast, within a nearly north-south trending extensional zone. Within

this zone, alkali magma formed isolated intrusive masses, as well as

various volcanic rocks spread allover this part of the Shield. These

Tertiary intrusive bodies are represented essentially by alkali granites

and syenites. The porphyritic texture of the studied granites indicates

shallow depths of intrusion (1-2 km). They consist of perthitic

feldspar and quartz, with alkali amphiboles. Geochemically, these

granites belong to the alkaline or peralkaline suite of A-type granite,

characterized by high FeOt/MgO and Ga/Al values. They have high

alkalies (8.29 – 10.29 %), high-field strength elements (HFSE, such as

Nb, Ta, Hf, Th, U and Y) and Zn contents, along with low Sr and Ba

contents. They are enriched in REE. The granites are specially

characterized by large amounts of independent Nb-Y-Th minerals

which appear in the late stages of continental extension. The low

Nb/Y and high Nb/Y granites, as well as major and trace elements

data revealed two geochemical processes; both assimilation and

fractional crystallization origin, with little opportunity of crustal

contamination. The textural features of these granites indicate that

they were originated from either water-poor hypersolvus or subsolvus

magma, generated from alkali basaltic magma. They formed in within

plate environment under an extensional tectonic setting pertaining to

rift related anorogenic granites. This agrees well with the extensional

environment accompanying the Red Sea opening.

38 R.I. EL-Gharbawy

Keywords: A-type granite; Anorogenic; Crust; Laccolith; Red Sea

Rifting.

Introduction

The Arabian Nubian Shield (ANS), which occupies large area in north

east Africa and Arabia is formed by the accretion of intra-oceanic island

arcs during the Neoproterozoic (Kusky et al., 2003). The majority of the

granitic magma were emplaced in this shield during the culmination of

the Pan-African magmatic activity (Roger et al., 1978). With the end of

the Pan-African orogeny, calc-alkaline magmatism was followed by post-

orogenic granitic magmatism of alkaline nature (Abdel Rahman 1995;

Jarrar et al., 2003; and Moussa et al., 2008).

The post-orogenic alkaline granites were commonly formed at

shallow depths, marking the beginning of the crustal stabilization of the

Arabian Nubian Shield (Roger et al., 1978). These intrusive bodies are

nonfoliated and usually have sharp contacts with their neighboring

country rocks. The majority of them are confined to structurally weak

planes or extensional environment possessing within plate characteristics

(Moghazi et al., 1999). The post-orogenic granites have been considered

to be formed either by the fractional crystallization of calc - alkaline

magma (Roger and Greenberg, 1990; and Azer, 2007) or represent the

initial stage of within plate A-type magmatism, (Beyth et al., 1994). The

alkaline granites are distinctively enriched in alkalis, HFSE and REE and

therefore, they have the geochemical characteristics of the anorogenic A-

type granites (Abdel-Rahman and Martin, 1990; Moufti et al., 2002; and

Mohamed and El-Sayed, 2008).

Loiselle and Wones (1979) studied the A-type granites and

suggested that they formed in anorogenic region or rift related

environment. Alkali feldspars are the most predominant mineral

constituents of these rocks.

Capaldi et al. (1987) studied the Tertiary anorogenic granites of

Yemen and stated that they belong to alkaline to peralkaline A-type

granite. They are either hypersolvus or subsolvus, originated from

alkaline parental basic magma by crystal liquid fractionation at shallow

depth. In Gabal Al Hirsh area, these authors described a zone of

granophyric and basaltic dikes similar to that of Gabal Tirf, east of Jizan

in Saudi Arabia. These intrusive masses vary in abundance and volume

Contribution to the Geochemistry and Tectonic Setting… 39

from north to south. The extensional zone containing these intrusive

bodies, comprises also metamorphic rocks, intruded by basic and acidic

dykes as well as granophyric and gabbroic intrusions. These magmatic

emplacement continues as far as Bab Al Mandab, at the entrance of the

Red Sea.

McGuire and Coleman (1986) and Coleman and McGuire (1988)

stated that the A-type granite of Yemen has Miocene age, related to the

volcanic activities resulted from the crustal extension associating the Red

Sea opening. Coleman et al. (1992) on their studies on the Oligo-

Miocene A-type granites of the eastern margin of the Red Sea, reported

that these plutonic rocks represent water-poor hypersolvus melt,

generated from alkali basaltic magma and developed in the early stages

of continental extension accompanying the Red Sea opening.

El Tokhi (1998) studied the riebeckite granites of Gabal Mousa in

the Precambrian rocks of southern Sinai, as an example of the alkali

granites of the Nubian Shield. He stated that they are A-type granite,

formed by partial melting and fractional crystallization of alkaline

magma. He reported also that these granites crystallized under low

temperature at a relatively shallow depth (3-4 km), possessing within

plate environments.

Qhadi (2002) studied Um Al Birak granite pluton in the western

part of the Arabian Shield in Saudi Arabia and stated that, it is alkaline to

peralkaline in nature comparable to many other peralkaline granites of

the shield and pertain to A-type granite. He stated that this granitic body

belongs to the anorogenic granitic magmatism and formed by partial

melting of juvenile crust. He reported also that Um Al Birak granite

pluton developed in within plate environments, following the field

delineated for Nigerian alkaline-peralkaline younger granites emplaced in

continental crust.

Moufti et al. (2002) studied the Rawda peralkaline suite in Al Hijaz

Terrain, west of the Arabian Shield. They reported that this plutonic body

represents an example of A-type granite consisting of aegirine-riebeckite

granite, and developed in anorogenic magmatism tectonically possess

within plate environment.

Kepede and Koeberl (2003) in their study on the A-type granites

from Wallagga area, western Ethiopia reported that they have chemical

40 R.I. EL-Gharbawy

and mineralogical characteristics of within-plate granite and were

generated and emplaced along an extensional tectonic environment.

Major and trace element modeling shows that the Ganjii monzogranite of

the investigated area was formed by fractional crystallization of largely

hornblende, plagioclase, and biotite from a monzodioritic parental

magma, enriched in incompatible elements, such as LILE, HFSE, and

REE. The authors contributed also that the data from western Ethiopian

within-plate granitoids are consistent with other granitoids elsewhere in

the Arabian Nubian Shield, indicating considerable new crust formation

during the Neoproterozoic.

EL-Bialy and El-Omla (2007) studied the A-Type granites of

Sharm El-Shiekh in Sinai and concluded that they represent the last

major magmatic activity and the terminal granitic plutonism in Sinai

massive. They suggested also that these granites are anorogenic evolved

in within plate environment and derived from continental crust through

island arc magmatism (post-orogenic).

Yemen Republic occupies the southwestern corner of the Arabian

Peninsula between latitudes 12° 40′ and 17° 26′ N and longitudes 43º and

53º E. Yemen can be subdivided geographically into four major

provinces; 1) The coastal plains of the Red Sea and Gulf of Aden, 2)

Yemen plateaus and highlands, (~3666 m), 3) Hadramawt - Mahra

uplands well dissected plateaus and highlands, and 4) Rub Al Khali and

Ramlat Al Sabatayn desert and sand dunes, (Fig. 1). This part of the

Arabian Shield comprises various rock units beginning with the

Precambrian, passing through the Phanerozoic, till recent representing

nearly most of the geologic column.

The Tertiary intrusive bodies, which formed essentially of alkali

granites and syenites occur in different locations, especially in the

western part of Yemen, nearly parallel to the general trend of the Red

Sea. The study of these alkaline intrusive bodies helps to understand the

evolution of the Oligo-Miocene igneous activity in Yemen, and also to

assess the relationships between magmatism and the tectonic

environments affecting this segment of the Arabian Shield, as well as

those related to the crustal extension accompanying the opening of the

Red Sea.

The Tertiary anorogenic granites cover nearly 3500 square

kilometers of the total area of Yemen. They are closely related to the


Tertiary volcanics. The available radiometric data for these intrusive

rocks suggest two phases of igneous activity, at 20 Ma and 26 Ma,

occurred in the Early Miocene, which represents a period of widespread

extension and igneous activity accompanying the Red Sea rifting (Civetta

et al., 1978 ; and Capaldi et al., 1987).

The largest intrusive masses lie along the coastal side of the main

Red Sea trend, (Fig, 1). The alkaline silicic bodies consist of plutons,

stocks and possibly some laccoliths, while the majority of other intrusive

bodies form mostly large stocks and plutons, having nearly N-S or

NNW-SSE trend.

The geologic features of these intrusive rocks and also their

porphyritic textures indicate shallow depths of intrusion (~1-2 km). The

alkali basaltic eruptions (26-30 Ma) (Capaldi et al., 1987) 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 2 km. Granophyric intrusions (20-30 Ma) are present along

the western edge of Yemen volcanic plateau, marking a north-south zone

of continental extension, (Fig. 1). One of the most significant aspects of

this igneous activity is the development of A-type granite in the southern

sector of the Arabian Shield, within a north-south extensional zone nearly

parallel to the Red Sea trend.

The present work sheds the light upon the differentiation and the

tectonic setting of the Oligo-Miocene A-type granite of Yemen, which is

represented by isolated magmatic bodies parallel to the direction of the

Red Sea coast. These intrusive rocks occur in the following locations

from the north to the south; Gabal Hufash, Gabal Bura, Gabal Raymah,

Gabal Marabit, Gabal Dubas, Gabal Ras, Gabal Al-Hirsh and Gabal

Sabir, (Fig. 1).

Geologic Setting

The southwestern part of the Arabian Shield was affected, in the

Oligocene-Miocene time by intense igneous activity, recorded by both

alkaline magmatism and volcanic activities (Capaldi et al., 1987a).

The Tertiary A-type granites of Yemen occupy a narrow zone, less

than 45 km wide parallel to the Red Sea trend and extend from the

42 R.I. EL-Gharbawy

southern tip of the Arabian Peninsula, to Jizan area in the north. This

elongate zone is separated from the Red Sea shoreline by a narrow coast

covered by Quaternary and Tertiary sedimentary rocks, (Fig. 1).

The granites are mostly light gray to pinkish gray in colour and

medium to coarse grained, formed essentially of perthitic alkali feldspars,

quartz as well as alkali femic components. The porphyritic varieties are

also present. They are commonly jointed and enclose xenoliths of older

rocks (Fig. 2). Intersecting joints, as well as cuboidal weathering, are the

most common characteristic features of these intrusive rocks.

The western side of the majority of the granitic masses coincides

with the escarpment of the Arabian Shield that rises above the Red Sea

coast. This is tectonically controlled with the general structural pattern of

the area, related to the crustal extension accompanying the Red Sea

opening.

The extensional zone containing granites varies in width and also

the abundance and extension of the granitic bodies from north to south.

The abundance of granites southwards is controlled by the general

structural setting of this sector of the Arabian Shield, which is related to

the opening of both Gulf of Aden and the Red Sea, due to the intersection

of both directions in this southern corner of the Shield (Coleman et al.,

1992).

Further south of Gabal Hufash, which represents one of the largest

granitic bodies in the investigated area, another mass of granite (Gabal

Bura) appears. It lies about 50 km east of Al Hudayadah city at the Red

Sea coast. Capaldi et al. (1987a) described granophyric and basaltic dikes

in Gabal Al Hirsh area.

The erosion along the escarpment of the Arabian Shield in the

study area removed nearly 1.5-2 km of the overburden. This indicates

that these alkali granites were formed at a shallower depth, not exceed 2

km. Also, the contact metamorphism, which was observed only in a

limited aureole in the country rocks around Gabal Raymah, (another

great intrusive body of the study area) indicates that these granitic bodies

developed at a relatively low temperature and shallow depth.


Fig

. 1.

Geo

logic

Map

of

Yem

en,

mod

ifie

d a

fter

Rob

erts

ton

Gro

up

, 1992.

Up

per

case

let

ters

ref

er t

o s

am

pli

ng l

oca

liti

es o

f th

e Y

em

en

gra

nit

es a

nd

gra

nop

hyre

s. A

: Jab

al

Hu

fash

gra

nit

e; B

: Jab

al

Bu

ra g

ran

ite;

C:

Ja

ba

l R

ay

ma

h g

ran

ite

an

d g

ran

op

hy

re;

D:

Ja

ba

l

Mara

bit

gra

nop

hyre

; E

: Jab

al

Du

bas

gra

nit

e an

d g

ran

op

hyre

; F

: Jab

al

Ras

gra

nit

e; G

: Jab

al

Al

Hir

sh g

ran

op

hyre

; H

: Jab

al

Sab

ir g

ran

ite.

44 R.I. EL-Gharbawy

Gabal Sabir granitic mass occupies a position near intersecting

faults (Grolier and Overstreet, 1978). This granitic intrusion cuts the

Yemen Volcanics (Fig. 3), which consist of rhyolites, trachytes as well as

alkali basalt (El-Gharbawy, in preparation).

Fig. 2. Basic xenoliths and intersecting joints in a granitic mass.

Fig. 3. A laccolith mass of the A-type granite intruding Yemen Volcanics.


Petrography

The granites of the study area are coarse to medium grained with

hypidiomorphic inequigranular, or possessing the commonly granular

porphyritic texture (Fig. 4). They consist essentially of quartz, perthitic

feldspar, plagioclase, alkali amphiboles (arfvedsonite, riebeckite), as well

as accessory minerals.

The quartz (~30% visual estimate) forms irregular subhederal or

anhederal grains, as being the last mineral to crystallize. It sometimes

shows undulose extinction forming interstitial mineral between other

early formed minerals. The quartz constitutes with the alkali feldspar the

intergrowth texture or skeletal quartz (Fig. 8c).

Orthoclase forms usually subhedral partially altered crystals.

Perthitic alkali feldspar constitutes about 65-70% of the rock (visual

estimate). The origin of perthite is commonly due to the exsolution

process and occasionally to the replacement action of ionic exchange at

low temperature. The micrographic intergrowths are usually observed

(Fig. 8b).

Zoned plagioclase crystals are also observed (Fig. 5), due to the

changes in the composition of both calcium and sodium contents of the

crystallized magma during the decrease of temperature of the silicate

melt. They form subhedral tabular crystals. Alteration of feldspars are

uncommon in which sericitization of orthoclase and saussuritization of

plagioclase giving rise to muscovite, sericite, saussurite and epidote as

secondary minerals.

Arfvedsonite and riebeckite are the most commonly alkali

amphibole observed in some thin sections of the investigated granites

(Fig.6). Fine grained riebeckite crystals scatter between other mineral

constituents of the rock and may be present as interstitial component

(Fig.6). This mineral is strongly pleochroic with x: green yellow, y: dark

gray and z: dark blue. Arfvedsonite forms large euhderal crystal, as being

the first alkali amphibole to crystallize, while riebeckite (Fig. 8b) formed

later. In some samples, prismatic hornblende crystals as well as biotite

flakes are enclosed within the felsic minerals. They are not associated

with the alkali amphiboles. The biotite occurs as flakes, partially altered

to chlorite. It shows moderate pleochroism and sometimes corroded and

replaced by quartz. String perthite (Fig.7 & 8d) as well as poikilitic

46 R.I. EL-Gharbawy

texture (Fig. 7), are commonly observed in some samples, in which

plagioclase and alkali amphiboles are enclosed in large crystals of k-

feldspar. Zircon, apatite, sphene and opaques (Fig. 8a), represent the

most abundant accessory phases.

Fig. 4. A photomicrograph showing

porphyritic texture, Gabal

Dubas, (CN, X25).

Fig. 5. A Photomicrograph showing

zoned plagioclase (Pl), A-type

granite, Gabal Hufash, (CN, X40).

Fig. 6. Scatter crystals of an alkaline

amphibole, riebeckite (Ri), A-

type granite, Gabal Hufash,

(CN, X25).

Ri


Fig. 7. A Photomicrograph shows large

alkali feldspar crystal (Kf) with

poikilitic texture in the A-type

granite of Gabal Bura, (CN,

X25).

Fig. 8a. Rod-like crystals of apatite

inside magnetite, as accessory

minerals, Gabal Bubas A-type

granite, (CN, X25).

Fig. 8b. Photomicrgraph showing

micrographic (Mi) and string

perthite texture (S) in Gabal

Dubas A-type granite, (CN,

X25).

Fig. 8c. Photomicrgraph showing

skeletal quartz (Sk) in Gabal

Bura A-type granite, (CN,

X25).

Kf

Qz

Mi

S

Sk

48 R.I. EL-Gharbawy

Fig. 8d. Photomicrograph showing

string perthite (St), alkali

amphibole (riebeckite, Ri) with

magnetite inclusions, (CN,

X25).

Geochemical Characteristics

Twenty seven samples representing the investigated granites in

various localities in Yemen Republic (Fig.1) were analyzed (Table 1).

Major and minor oxides as well as trace elements were determined at the

Technical University of Budapest, using Philips PW 1400 sequential

XRF spectrometer. Analytical precision, as determined from replicate

analyses, is generally better than 2%, except for MgO, Na2O and Nb,

which are better than 5% and Th which is better than 10%. Loss on

ignition (L.O.I.) was determined by treating each sample for about 1.5

hours at 1000˚C in an electric furnace. The rare earth elements were

determined by neutron activation analysis with at least two standard

reference materials.

Discriminations and correlation diagrams were applied in order to

shed the light upon the geochemical and the petrogenetic characteristics

of the investigated rock types.

Figure 9 shows the behaviour of major oxides relative to silica

variance. It is observed that TiO2, FeOt, MgO, and Na2O show negative

correlation with silica, while CaO shows slightly negative one, P2O5

gives scatter plot, while K2O and A2O3 data cluster in a small restricted

area on the diagram.

The diagram of Middlemost (1985) (Fig. 10) shows that the

investigated granitic samples occupy the two fields; alkali feldspar

granite (field 3) and the granite (field 6). It is observed that the samples

of Jabal Raymah and Jabal Bura lie together in a restricted area in the

granite field, indicating that they may have originated from the same

magmatic chamber; as will be discussed later.

Ri

St


Table 1. Chemical Analyses of A-Type Granite of Yemen.

SiO2

TiO2

Al2O3

Fe2O3

FeO

FeOt

Fe2O3t

MnO

MgO

CaO

Na2O

K2O

P2O5

0 0 0 0 0 0 0 0

50 R.I. EL-Gharbawy

Table 1. Cont.

SiO2

TiO2

Al2O3

Fe2O3

FeO

FeOt

Fe2O3t

MnO

MgO

CaO

Na2O

K2O

P2O5


Table 1. Cont.

Note: J R = Jabal Ras and J Ray = Jabal Raymah.

The values of Cl & F equal zero in all samples in the above table.

SiO2 TiO2

Al2O3

Fe2O3

FeO

FeOt

Fe2O3t

MnO

MgO

CaO

Na2O

K2O

P2O5

0 0 0 0 0 0 0 0

52 R.I. EL-Gharbawy

Fig. 9. Variation diagrams for silica vs. major oxides for A-type granites from Yemen.

60 70 80

60 70 80

60 70 80

60 70 80 60 70 80

60 70 80

60 70 80

60 70 80


Fig. 10. Variation diagram of SiO2 vs. (Na2O+K2O) (After Middlemost, 1985).

1. Alkali feldspar syenite, 2. Alkali feldspar quartz syenite, 3. Alkali feldspar granite, 4. Syenite, 5.

Quartz syenite, 6. Granite, 7. Monzonite, 8. Quartz monzonite, 9. Monzodiorite, 10. Quartz

monzodiorite, 11. Granodiorite, 12. Diorite and gabbro, 13. Quartz diorite, 14. Tonalite.

Fig. 11. Variation diagram of total alkalis vs. silica of the studied granites.

54 R.I. EL-Gharbawy

Fig. 12. AFM diagram of the studied granites (fields after Irvine and Barager, 1971).

The An-Ab-Or ternary diagram of Strickeisen (1976 b) (Fig. 13),

shows that all samples fall very close to middle part of the Ab-Or side

line, indicating that the studied samples fall within the granite field. This

ternary diagram shows equal amounts of both Na-plagioclase and alkali-

feldspars, it is clear also from the values of Na2O and K2O.

Fig. 13. An-Ab-Or ternary diagram of the studied granites (Strieckeisen 1976 b)

A = Tonalite; B = Granodiorite; C = Adamallite; D = Trondhjemite; E = Granite.


K2O-TiO2-P2O5 ternary diagram (Fig. 14), shows that the

investigated granites fall within the continental field, very close to K2O

apex.

Fig. 14. TiO2-K2O-P2O5 ternary diagram of A-type granites from Yemen.

Some selected trace elements were plotted against silica (Fig. 15),

in order to shed the light upon their evolution during the fractional

crystallization of their parent magma. It is noticed that some elements

like V, Zr, Sr, Ba and Rb show slightly marked trends, while others like

Li, Hf and Nb show scatter plot.

By using the Rb-Ba-Sr ternary diagram of El Bouseily and El

Sokkary (1975) (Fig. 16), it is noticed that all samples occupy a zone

parallel to the Ba-Rb side line. The samples of Gabal Raymah fall within

the normal granite field (A), while the rest of samples lies along the trend

of the strongly differentiated granite field (B).

Figure 17 shows the variations of some selected trace elements

within the low Nb/Y and high Nb/Y granites. The diagram revealed that

the samples of Gabal Raymah and Gabal Bura lie in the area of low Nb/Y

granites, while other samples of Gabal Hufash, Gabal Dubas and Gabal

Ras spread allover the area between low and high Nb/Y granites. This

bivariant correlation may shed the light upon the variation of these

selected trace elements with magma crystallization.

56 R.I. EL-Gharbawy

Fig. 15. Variation diagrams for silica vs. trace elements for A-type granites from Yemen.


Anomalous granite

Granodiorite

Qz diorite

Fig. 16. Rb-Ba-Sr diagram for the investigated A-type granites from Yemen (El Bouseily

and El Sokkary, 1975). A:normal granite, B:strongly differentiated granite.

Fig. 17. Trace element variation diagram within low Nb/Y and high Nb/Y granites.

58 R.I. EL-Gharbawy

The two diagrams of Pearce et al. (1984) (Fig. 18 & 19) revealed

that all the investigated granitic samples fall inside the within plate

granite field, supporting the extensional environment. This tectonic

setting fit well with the above mentioned geologic information.

Fig. 18. Diagram showing the tectonic environment of A-type granites from Yemen (after

Pearce et al., 1984).

ORG = Ocean Ridge Granites; VAG = Volcanic Arc Granites; syn-

COLG = syn-Collision Granites; WPG = Within Plate Granites.

Fig. 19. Diagram showing the tectonic environment of A-type granites from Yemen (after

Pearce et al., 1984).

ORG = Ocean Ridge Granites; VAG = Volcanic Arc Granites; syn-

COLG = syn-Collision Granites; WPG = Within Plate Granites.


By applying the discrimination diagrams (Fig. 20 A & B), for A-

type granites from Whalen et al. (1987) and Eby (1990), it is observed

that the studied granites fall within the A-type granite field.

Fig. 20. Discrimination diagrams for A-type granites from Whalen et al., (1987) and Eby (1990).

OGT: field for I-S-, and M-type granitoids, FG: field for fractionated I-type granites.

Ce/Nb and Yb/Ta vs. Y/Nb discrimination diagrams (Fig. 21 A, B)

are used to estimate possible genetic links of the A-type granites and

granophyres with crustal sources or mantle derived magmas.

Fig. 21. Ce/Nb (A) and Yb/Ta (B) vs. Y/Nb discrimination diagrams to estimate possible

genetic links of the Yemen granites and granophyres with crustal sources or mantle

derived magma.

IAB: island arc basalts, CG: collision granite, VAG: volcanic arc granites, OIB:

oceanic island basalts (fields from Eby, 1990). YTS: basalts fields for the Yemen

Trap Series basalts, from Data by Chiesa et al., (1989),

60 R.I. EL-Gharbawy

The chondrite-normalized REE patterns of the studied alkali

granites are shown in Fig. 22. Two slightly different types of REE

patterns could be distinguished. The first pattern, which is represented by

the plots of Gabal Ras and Gabal Dubas, shows LREE enrichment with

very slightly negative Eu anomaly. The second pattern, which is

represented by the plots of the other three localities; Gabal Hufash, Gabal

Raymah and Gabal Burra is characterized by LREE enrichment and

strong negative Eu values. The REE patterns form a tight bundle plot,

except for the pattern of Gabal Dubas. This observation indicates that, the

rare earth elements have been significantly unaffected either by tectonics

or metamorphism. It should be noticed also that, the REE patterns of the

investigated granites, reflect a unique origin of these rocks. Some

accessory phases retain the HREE and could produce a considerable

effect of fractionation. Although, the abundance of REE and lack of

significance of HREE depletion, suggest a source enriched in the REE,

originated probably by the melting of basic crustal blocks.

Fig. 22. Chondrite normalized REE patterns for the A-type granites from Yemen (chondrite

values after Anders and Ebihara, 1982).


Discussion

The granitoids are subdivided into several types according to their

mineral assemblages, their geologic setting, petrographical features, and

their geochemical characteristics. This typology complements most of the

recent classifications, because it is not based solely on one category. In

most cases, it thus has the advantage to distinguish the various granitoid

rocks in the field. Both types of peraluminous granitoids are of crustal

origin; the alkaline and peralkaline granitoids are of mantle origin; and

both types of calc-alkaline granitoids are of mixed origin and involve

both crustal and mantle materials. Each granitoid rock is generated and

emplaced in a very specific tectonic setting.

The post-collisional alkali granites, as a subdivision of A-type

granites could be generated from I-type magma (Stoeser and Elliott,

1980; Sylvester, 1989; and Hassanen, 1997). Wahlen et al. (1987)

considered the possibility that A-type granites may be originated from

highly fractionated I-type magma. If so, the observed geochemical

characteristics of the alkali granite would be a function of the degree of

fractional crystallization. The feldspars represent the most important

mineral in any fractional crystallization scheme in the granitic rocks. The

decrease of Ca, Sr and Ba in the alkali granites together with the

pronounced negative Eu anomalies reflect the important role of feldspar

separation during their genesis.

The diagrams of major elements geochemistry revealed that the

sample plots of Gabal Bura and Gabal Raymah occupy a definite field.

This field can be easily delineated to form a definite area (Fig. 9 & 12 &

20 & 21) and the rest of the samples of the other localities can be

grouped together in another field. The geologic setting and also the

geologic map of the study area (Fig. 1) show that Gabal Bura and Gabal

Raymah granitic bodies are closely related, and can be grouped to form

one body, while the granitic bodies of the other localities occupy a zone

surrounding them from the north and the south. Two generations of one

basic magmatic chamber can be proposed for the origin of these granitic

bodies; the later or younger phase was represented by Gabal Bura-

Raymah granite body and the other granitic bodies represent the earlier

one. This observation can be proved from the sample plots of both groups

in the geochemical diagrams (Gabal Bura-Raymah granite as one group

and the other localities of granite represent another group).

62 R.I. EL-Gharbawy

Also, the contact metamorphism, which was observed only in a

limited aureole in the country rocks around Gabal Raymah indicates that

these granitic bodies developed at a relatively low temperature and

shallow depth. This observation may support the idea that Gabal Bura-

Raymah represents a relatively later shallower phase of intrusion than

those of the other localities.

Also, the data collected by applying some trace elements diagrams

(Fig. 15-21) revealed that Gabal Bura-Raymah samples can be easily

grouped in a separate area suggesting two generations of magmatic

bodies. These two generations of magmatic bodies originated in within

plate environment during the Red Sea rifting, as confirmed by applying

some geotectonic diagrams. The precisely-dated granitoids can then

complement the structural approaches and indicate a definite geotectonic

environment. With reference to some case-studies allover the Arabian-

Nubian Shield, the use of granitoid rocks as tracers of the geodynamic

evolution is also of great significance.

Conclusions

The A-type granites have long been recognized as a distinct group

of granites occur in geodynamic contexts ranging from within-plate

settings to plate boundaries, locations and times of emplacement are not

random. They are fairly common at shallower depths, especially at the

subvolcanic level. Characteristic features include hypersolvus to

transsolvus to subsolvus alkali feldspar textures, iron-rich mafic

mineralogy, bulk-rock compositions yielding alkali-calcic to alkaline

affinities. The high LILE+HFSE abundances, and the pronounced

anomalies most probably due to the high degrees of mineral

fractionation. As it occurs in association with mafic igneous rocks in

continents as well as on the ocean floor, the A-type granite is likely to

come from mantle-derived transitional to alkaline mafic magmas.

The alkali granites of Yemen consist essentially of perthitic

feldspar and quartz with minor alkali amphiboles. These granites were

originated from water-poor hypersolvus magma generated from parent

alkali basaltic magmas.

These alkali intrusive bodies are closely associated with the

Tertiary volcanics and are connected with the rifting and the Red Sea


floor spreading. The appearance of the acidic intrusive bodies in the form

of laccolith, indicates that these bodies were formed and developed at

relatively shallow depths. Also, the intrusion of the granitic rocks is

significant because, it indicates that the rifting and crustal extension

occurred simultaneously and coincided with the normal faulting and early

rifting that led to the opening of the Red Sea.

The A-type granites are formed in the early stages of continental

extension in the first magmatic activity and represent H2O-deficient

magmas. The data obtained from the geochemistry supports the

extensional environments of the Yemen granites. They are intruded into

and associated with the Tertiary rhyolite, comendite, trachyte, and alkali

basalts.

The high Zr, Y and Nb of these granites indicates a fractional

crystallization which modified the original mafic magmas. The fractional

crystallization of this mafic parent magma was concluded to represent an

important factor in the development of the A-type granites of the Arabian

Shield. Some of the contemporaneous basalt magmas, however, erupted

on the surface without much change in the compositions of the original

parent magma.

It can be proposed that, where is lower crust of tonalitic

composition and a source of magma hot enough to produce partial melts,

it is possible to form A-type granite, as suggested by Wahlen et al.

(1987).

The geochemical parameters of some trace elements, revealed that

the investigated alkali granites developed in within plate environment.

This conclusion agrees well with the previous data obtained from other

localities in the Arabian Nubian Shield.

It is clearly observed that the chondrite-normalized REE patterns of

the studied granitic rocks, form a tight bundle plot, except for the pattern

of Gabal Dubas granite. This observation indicates that, the REE have

been significantly unaffected either by the tectonic or metamorphism.

The chondrite-normalized REE patterns show enriched light REE

moderate to strong negative Eu anomalies and more or less flat heavy

REE patterns. Also, the negative anomaly of the REE pattern suggests

that a partial melting have undergone an early fractional crystallization. It

64 R.I. EL-Gharbawy

should be noticed also that, the similarity in the REE patterns of the

investigated granitiods, reflects a unique origin.

Acknowledgements

I would like to express my appreciation and sincere thanks to Prof.

Imbarak S. Hassan, Suez Canal University for his kind help to carry out

the chemical analyses of the investigated granites and also for fruitful

discussion during the progress of this research work. The author also is

greatly indebted to the reviewers and editors of the Journal of KAU:

Earth Sciences and their comments which actually improved this article.

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Contribution to the Geochemistry and Tectonic Setting of the ...kau.edu.sa/Files/320/Researches/60258_31067.pdfThe Oligo-Miocene A-type granites of Yemen Republic were emplaced in