Table Of ContentKINGDOM OF SAUDI ARABIA
KING ABDULAZIZ UNIVERSITY
Abdullah O. Nasseef
President
FACULTY OF EARTH SCIENCES
Ahmed N. BaSahel
Dean
KING KHALID
I. A. G. BULLETIN No. 3
E V O L U T I ON A ND M I N E R A L I Z A T I ON
OF T HE
A R A B I A N - N U B I AN S H I E LD
Volume 4
Published for the
FACULTY OF EARTH SCIENCES, KING ABDULAZIZ UNIVERSITY
JEDDAH, KINGDOM OF SAUDI ARABIA
by
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Copyright © 1980 Faculty of Earth Sciences, King Abdulaziz University, Jeddah
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First edition A.D. 1980-A.H. 1400
British Library Cataloguing in Publication Data
Evolution and mineralization of the Arabian-Nubian
Shield. - (King Abdul Aziz University. Institute
of Applied Geology. IAG bulletins; no. 3).
Vol. 4
1. Geology - Near East - Congresses
I. Al-Shanti, Ahmed II. Series
555.6 QE319.N4 80-40043
ISBN 0-08-024481 5
Printed in Great Britain by Hillman Printers (Frome) Ltd., Somerset
Bound by A. Wheaton & Co. Ltd., Exeter
SYMPOSIUM PERSONNEL
Convenor: A. M. S. AL-SHANTI
Organizers: M. L. ABDEL-KHALEK, A. B. AL-
KHATIB, A. M. S. AL-SHANTI, A. N. BA-SAHEL,
P. L. BINDA, O. R. EL-MAHDY, A. A. A. HUSSEIN,
N. J. JACKSON, W. J. MACLEAN, J. G. MOLTZER,
F. I. OSMAN, C. R. RAMSAY, M. J. ROOBOL, S. A.
TAHOUN, and M. A. TOFIQ.
Field Excursion Leaders: A. M. AL-SARI, P'. L.
BINDA, O. R. EL-MAHDY, N. J. JACKSON,
M. E. KUKANDY, C. R. RAMSAY, M. J. ROOBOL,
J. H. SCHELLEKENS, M. I. THEKAIR, and M. O.
ZUBEIR.
Editorial Board: A. M. S. AL-SHANTI, P. L. BINDA,
C. R. RAMSAY, and S. A. TAHOUN.
Referees: M. L. ABDEL-KHALEK, A. A. ABDEL-
MONEM, M. A. AL-GAZZAZ, A. N. BA-SAHEL
Ε. Z. BASTA, P. L. BINDA, H. R. BLANK,
C. B. DUBOIS, O. R. EL-MAHDY, H. A. HAKIM,
A. A. A. HUSSEIN, N. J. JACKSON, M. M. KHUDA,
W. J. MACLEAN, J. G. MOLTZER, C. R. RAMSAY,
M. J. ROOBOL, J. WAKEFIELD, and A. J. WARDEN
Arabic Abstracts : I. A. M. FARAG
Associate Editors: P. L. BINDA, A. A. A. HUSSEIN,
N. J. JACKSON, M. M. KHUDA, C. R. RAMSAY,
and M. J. ROOBOL.
Editors of Volume 4: P. G. COORAY and S. A.
TAHOUN.
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POST-OROGflNIC PERALKALINE AND CALC-ALKALINE GRANITES
AND ASSOCIATED MINERALIZATION OF THE ARABIAN SHIELD,
KINGDOM OF SAUDI ARABIA
D. B. STOESER and J. E. ELLIOTT
U.S. Geological Survey, Jeddah, Saudi Arabia
Abstract—Following a long period of island arc-like orogenic activity, post-orogenic granites of Pan-African age
(670-550 Ma) were emplaced throughout the Arabian Shield. Most of these rocks are typical subsolvus calc-
alkaline biotite monzogranite. The early Pan-African granites are associated with widespread small tholeiitic
gabbro intrusions, many of which are layered. The calc-alkaline granites are not evenly distributed and their area
1
2
D. Β. STOESER and J. E. ELLIOTT
of exposure relative to other rock types increases from S.W. to north and N.E. In the north-eastern part of the
shield, they are associated spatially with rhyolites of similar composition and with molasse derived from granitic
terrane.
The occurrence of 45 late alkali granites, most of which are peralkaline, is described and their chemistry
discussed. The bulk of these rocks are medium- to coarse-grained hypersolvus alkali granites, but phenocrystal
and micrographie varieties are also common. The mafic silicate minerals of the alkali granites are arfvedsonite,
aegirine, aegirine-augite, riebeckite, and sparse biotite, aenigmatite, and barkevikite. The alkali granites do not
occur in the south-western third of the Arabian Shield nor in the south-eastern portion of the adjacent Nubian
Shield of Africa. A syenite-shonkinite province located in the south-western part of the Arabian Shield is
identified.
No economic mineral deposits are known to be associated with the Pan-African granites. Numerous mineral
occurrences and geochemical anomalies have been found that contain W, Mo, Sn, Be, Zr, Nb, Th, U, rare earths,
and fiuorite. The most important of these occurrences are tabulated and their distribution shown.
INTRODUCTION for a granite that contains more alkali metals than
is considered average for the group of rocks to
Among the projects of the Saudi Arabian Direc-
which it belongs. An alkali granite is one containing
torate General of Mineral Resources (DGMR)
soda pyroxene and/or amphibole (Sorensen, 1974).
five-year plan is a study of the petrogenesis and
A peralkaline granite is one in which molecular
mineral potential of the granitic rocks of the
A1 0 < Na 0 + K 0 (Carmichael et al, 1974). A
Arabian Shield. The Acid Plutonic Rock Project 2 3 2 2
metaluminous granite, however, has A1 0 <
involves two DGMR geologists and the two 2 3
CaO + Na 0 + K 0 but A1 0 > Na 0 + K 0 (Car-
authors, members of the U.S. Geological Survey 2 2 2 3 2 2
michael et al, 1974). Common granites are
Saudi Arabian Mission. Stoeser's primary concern
metaluminous. As used in this paper, a shonkinite
is the petrology of the rocks and Elliott's their
is a mafic syenite whose essential mineralogy is
economic geology. This paper is our first con-
potassium feldspar plus augite and that is com-
tribution to the geology of the Arabian Shield
posed of more than half mafic silicates (pyroxene,
granites.
amphibole, olivine, and biotite).
The Arabian Shield is the eastern half of the
Arabian-Nubian Shield, which was exposed by
early to middle Tertiary uplift and ensuing erosion ANALYTICAL METHODS
and split into two segments by the Red Sea rift Nineteen chemical analyses are presented in this
(Coleman et al, 1977). The Arabian portion of the paper, of which ten are new and nine are taken
shield occupies an area of approximately from the literature. The new analyses were per-
770,000 km2 and is composed of approximately formed by Skyline Labs, Inc. of Wheat Ridge,
40% granitoid plutonic rocks (quartz diorite and Colorado, USA. The following were determined by
tonalité to granite) and most of the remainder is atomic absorption: Si0 , A1 0 , MgO, CaO, Na 0,
2 2 3 2
volcanic and sedimentary rocks, many of which K 0, and Li. Ti0 and P 0 were determined by
2 2 2 5
have been metamorphosed to the greenschist colorimetry (Ti0 using Tiron and P 0 by the
2 2 5
facies (Brown and Jackson, 1960; Greenwood and vanadate-molybdate procedure). P 0 was below
2 5
Brown, 1973; Greenwood et al, 1976). The rocks the limit of detection (0.2%) for all samples and
of the Arabian Shield appear to be late Proterozoic therefore no values for this oxide appear in Table
in age (Fleck et al, 1976, 1979; Baubron et al, 2. Sr and Rb were determined by flame emission
1976). Their history can be divided into two main spectroscopy, and fluorine by the specific ion elec-
periods, an early island arc-like orogenic phase trode method using an ammonium citrate solution
(1165-680 Ma) succeeded by a post-orogenic after fusion in sodium hydroxide. Total volatiles
period (Pan-African, 670-550 Ma) during which were detected by loss on ignition (L.O.I.) at 1000°C
massive amounts of granite were emplaced for two hours.
throughout most of the shield (Brown and Jackson,
1960; Greenwood et al, 1976; Delfour, 1977). It is
the purpose of this paper to briefly describe the EVOLUTION OF THE SHIELD
general geologic setting, distribution, characteris- The evolution of the southern shield has been
tics, and economic geology of the post-orogenic documented by Greenwood et al (1976) and by
granites, with emphasis on the peralkaline rocks. Fleck et al (1976, 1978, 1979) and the northern part
The primary method of plutonic rock of the shield has been described by Baubron et al
classification used is the IUGS system (1976) and Delfour (1977). The geologic history of
(Streckeisen, 1975). In addition, the following the Arabian Shield can be divided into two main
modifying terms for granites are used: alkaline, periods. The first and longer period (1165-680 Ma)
alkali, peralkaline, and metaluminous. Alkaline (or is dominantly orogenic and was characterized by
alkalic) granite is used as a general chemical term calc-alkaline intermediate volcanic and plutonic
Post-Orogenic Peralkaline and Calc-Alkaline Granites and Associated Mineralization of the Arabian Shield 3
rocks and associated clastic sediments. The second plutons, although some were ring structured
period (680-550 Ma) is dominantly non-orogenic (Dodge, 1979). In the following 80 to 100 Ma there
and characterized by silicic volcanics, granites, and was a general shift from calc-alkaline to alkaline
arkosic molasse deposits. The volcanic, sedimen- granite. In the northern shield these granitic mag-
tary, and plutonic rocks of the first period appear mas also appear to be represented by the Murdama
to be typical of those found in subduction-related and Shammar Group rhyolitic intrusives (Delfour,
late Phanerozoic island arcs and, therefore, a 1977; Schmidt et al, 1979). The Murdama Group,
number of subduction models for the evolution of although containing acidic flows, is dominantly
the Arabian-Nubian Shield were put forth (Jack- composed of clastic sediments that apparently are
man, 1972; Greenwood et al, 1976; Bakor et al, a molasse derived from this granitic terrane
1976; Garson and Shalaby, 1976; Neary et al, (Schmidt et al, 1979). In the southern shield the
1976; Frisch and Al-Shanti, 1977). Murdama and Shammar are lacking, possibly due to
Studies of the southern shield indicate that the a greater depth of erosion.
oldest known rocks are the Baish meta-basalt One outstanding question presently being
greenstones and overlying Bahah meta-graywackes, debated is whether the late Proterozoic rocks of
which are cut by 900 Ma plutons and may be as old the Arabian Shield are underlain by an older
as 1165 Ma (Greenwood et al, 1976; Fleck et al, evolved crystalline basement. At present there is
1979). To the east and intercalated(?) with the no good evidence to support this idea. For the
Baish-Bahah belt is a broad belt of andesitic and southern shield the lack of older dates, low initial
dacitic metavolcanic rocks and lesser amounts of 87Sr/86Sr ratios of 0.702 to 0.704 (Fleck et al, 1979)
metasedimentary rocks, the Jiddah Group, which is and lack of old granites suggest no contamination
about the same age or somewhat younger by, or partial melting of, an underlying older
(Greenwood et al, 1976). evolved crust, thus indicating that no such base-
The layered rocks of the Baish, Bahah, and ment exists. For the northern part of the shield,
Jiddah Groups have been intruded by a belt of however, the question of an older basement is
tonalitic calc-alkaline batholiths that stretches more debatable. Although no dates are available
along the western side of the shield from the Yemen that would provide direct evidence for an older
border to north of Jiddah. Greenwood et al (1976) evolved crust, six granites dated by Baubron et al
report that these rocks were emplaced during two (1976) have initial 87Sr/86Sr ratios in the range 0.704
main episodes. Their "first dioritic series" (960 Ma) to 0.7122. The higher of these ratios clearly suggests
had an early phase of gabbro, diorite, and quartz that evolved crustal material was involved in the
diorite that was followed by more voluminous and formation of these granites. Fundamentally, the
more leucocratic biotite-hornblende tonalité (Fig. question awaits more and definitive isotopic
1). This first episode of tonalitic plutonism was studies.
followed by orogeny and a "second episode of
tonalitic plutonism" (800-680 Ma) which included
PETROGRAPHY
tonalité and trondjemite. The tonalités of the
Calc-alkaline Granites
second series are similar to those of the first series,
but appear to be spread over most of the Arabian The bulk of the calc-alkaline granites of the
Shield (Greenwood et al, 1976; Baubron et al, 1976; Arabian Shield are pink to gray subsolvus, equi-
Schmidt et al, 1979). During the later part of the granular to porphyritic, medium-grained mon-
second episode, granodiorite was emplaced largely zogranites and granodiorites that contain 1-8%
as syn-tectonic gneiss domes throughout at least the mafic silicates, mostly biotite with or without
southern half of the shield (Nebert, 1970; Fleck et al, hornblende. They occur mainly as oval to some-
1978; Schmidt et al, 1979; Cooper et al, 1979). In the what irregularly shaped plutons, and more rarely
northern half of the shield during this time massive as ring dikes having calc-alkaline granite or coun-
amounts of intermediate volcanics and associated try rock cores (Dodge, 1979). No attempt will be
clastic sediments, the Halaban (Hulayfah) Group made here to synthesize the considerable data
were deposited (Greenwood et al, 1976; Baubron et already available on this group of rocks; for further
al, 1976; Delfour, 1977). information the reader is referred to Greenwood
After the end of the long orogenic episode at and Brown (1973); Nasseef and Gass (1977); Delfour
about 700-680 Ma, plutonism changed from inter- (1977); and Kanaan (1979).
mediate composition to acidic so that massive Less common members of the calc-alkaline
amounts of calc-alkaline monzogranite accom- group are rapakivi granites, alaskites with or
panied by minor gabbroic magma were emplaced without garnet, and 'big-feldspar' porphyroblastic
throughout the shield (Fleck et al, 1976, 1979; monzogranites. The garnet-bearing alaskites are
Baubron et al, 1976). These granites were volumetrically insignificant but appear to be dis-
generally emplaced as simple circular or ovoid tributed throughout at least the central and southern
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Post-Orogenic Peralkaline and Calc-Alkaline Granites and Associated Mineralization of the Arabian Shield 5
shield and are reported from Egypt as well feldspar of the red granites is typically turbid and
(Schurmann, 1966). They are typically medium- clouded by very fine, dark inclusions (hematite?).
grained, have xenomorphic inequigranular texture, In the north-eastern part of the shield, the alkali
and contain less than 5% total garnet, biotite, and granites are commonly associated with pink to red
muscovite. What are interpreted to be potassium- granophyric granites. These granophyric granites
metasomatized monzogranites, containing por- typically consist of 10-50% phenocrysts of alkali
phyroblasts as large as 5-10 cm, are concentrated feldspar and lesser quartz in a micrographie
mainly in the southern shield. Insufficient data are groundmass. Mafic minerals are usually exten-
presently available on the Arabian rapakivi sively altered or completely destroyed. The alkali
granites to describe them accurately. granites and granophyres are often closely asso-
In addition to the calc-alkaline granites, post- ciated with rhyolitic volcanics of the Shammar
orogenic gabbros occur throughout most of the Group, which typically are also porphyritic and
Arabian Shield but are distinctly concentrated in have phenocrysts of alkali feldspar and quartz
the south-central shield (Coleman et α/., 1972). The (Delfour, 1977).
best criterion for field recognition is the presence
of olivine and pyroxene, both of which have been
destroyed in the older gabbros through metamor-
DISTRIBUTION OF GRANITES
phism. Many of these late gabbros are layered, and
Arabian Shield
the layering appears to be due to repeated in-
fusions of magma rather than to crystal settling On the basis of the distribution of the calc-
(Al-Shanti, 1974; Coleman et α/., 1977). The com- alkaline and alkali granites, the Arabian Shield is
position of the parental magma of these gabbros here divided into three regions (Fig. 2). From the
appears to have been olivine tholeiitic. Coleman et S.W. to the N.E. these are the Asir, Hijaz-Najd,
al. (1972, 1977) give Κ-Ar dates on these gabbros and Hail-ad Dawadimi regions. The post-orogenic
that fall in the range 415-769 Ma, many of them granites as a whole are not homogeneously dis-
around 630 Ma. In the southern shield, we have tributed over the shield. They are least abundant in
found these gabbros in intrusive complexes with the Asir region and increase in areal extent to the
the older calc-alkaline granites but never with the N.E. Small-scale maps and LANDSAT imagery
younger alkaline granites. In general it seems un- covering the north-eastern portion of the Arabian
likely that these gabbros could be any younger Shield reveal that granites are the dominant
than about 500 Ma since neither they nor the rock type.
granites have been observed to intrude the basal The first two are separated by the alkali-granite
Cambro-Ordovician cover rocks. line which is defined by the distribution of alkali
granites in the Arabian Shield. A literature search
combined with the authors' own work produced
Alkali Granites
the location of 45 Saudi Arabian alkali granites
The alkali granites of the Arabian Shield are (Fig. 2). One noticeable aspect of their distribution
mainly gray, pink, or red, medium- to coarse- is that an alkaligranite line can be drawn through the
grained, hypersolvus alkali granites. In the field, shield, south and west of which no such granites
they can often be distinguished by their dark red to have been reported.
almost black weathered surface and their tendency The boundary between the Hijaz-Najd and
to weather to very large, smooth, rounded blocks. Hail-ad Dawadimi regions, is based on the relative
Seen in thin section, they consist of alkali feldspar amounts of granite in the two regions. The increase
that has exsolved to a patch perthite, 20-30% in area of exposed granite from the central shield
quartz and 5-15% late interstitial, sodic mafic sili- to the N.E. is not gradational but is sharply
cates, which include aegirine, aegirine-augite, arf- demarcated. This demarcation is the dividing line
vedsonite, and less commonly aenigmatite, and between our Hijaz-Najd and Hail-ad Dawadimi
barkevikite. A reddish-brown biotite is also present regions.
in some samples. Deep blue, very pleochroic rie- Calc-alkaline granites are least abundant in the
beckite (?) is present in many samples, usually as a Asir region, and in south-western Asir they are
secondary amphibole replacing the primary sodic very sparse. Interestingly, it is here that recent
pyroxene or amphibole. Most of the peralkaline mapping has revealed a linear shonkinite-alkali
granites are more mafic than the calc-alkaline syenite province (Fig. 2). Coleman (1973b) and
granites. Accessory minerals are magnetite, Ratte (1974) reported a pluton, a plug, and several
allanite, zircon, fluorite, and in several samples dikes of shonkinite in the Abha area in the south-
brick-red elpidite (Na ZrSi Oi 5Ή2Ο). Plagioclase is ern part of the Asir region. Farther to the N.W. is
2 6
usually present only as secondary albite, partially the Lakathah ring complex which consists of a
replacing the primary alkali feldspar. The alkali core of layered mafic and ultramafic rocks and
