Precambrian Research, 16 (1981) 195--230
Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
ENSIMATIC VOLCANIC ROCKS, CENTRAL EASTERN DESERT OF
AND TECTONIC SETTING OF LATE PRECAMBRIAN
ROBERT JAMES STERN
Department of Terrestrial Magnetism, 5241 Broad Branch Road N. W., Washington, DC
(Received January 28, 1980; revision accepted August 5, 1981)
Stern, R.J., 1981. Petrogenesis and tectonic setting of Late Precambrian ensimatic vol-
canic rocks, central eastern desert of Egypt. Precambrian Res., 16: 195--230.
Early stages in the geologic evolution of the central eastern desert of Egypt (CED)
reflect an ilatense episode of ensimatic volcanic activity similar to modern magmatism of
the ocean floors and island arcs. This paper reports results from studies of the petrology
and petrogenesis, and interprets the significance of these Late Precambrian volcanic rocks.
A three-fold stratigraphy is preserved in the basement of the CED. A basal section of
oceanic crust includes ultramafics, gabbros and pillowed basalts. These older metavol-
canics (OMV) are conformably succeeded by dominantly volcanogenic metasediments,
which are in turn succeeded by a dominantly andesitic, calc-alkaline sequence of
younger metavolcanics (YMV). The OMV and YMV are largely restricted to the CED
in Egypt, but analogous terranes are found in northern Arabia.
(40--400 ppm) and Ni (30--260 ppm). They are poor in K20 (0.05---0.92%), Rb (0.3--5.0
ppm) and Ba (11--89 ppm). On Ti--Zr---Cr--V--Ni--P discriminant diagrams, the OMV
plot in the field of modern abyssal tholeiites. High K/Rb (450--1800) and light REE
depletions support this inference, although K/Ba (25--45) is lower than modern mid-
ocean ridge basalts (MORB). The sum of OMV geochemical characteristics requires that
these magmas were derived by the fractional fusion of the mantle. It is suggested that the
OMV were generated by 20--25% fractional melting of previously depleted mantle at
depths of less than 60 kin. Relatively little fractionation accompanied ascent to the sur-
face, where the OMV were erupted in a primitive crustal environment, either a small
oceanic rift or a back-arc basin.
Metamorphism of the YMV resulted in little elemental redistribution. These andesites
have sub-alkaline clinopyroxenes and major-element geochemical characteristics indis-
tinguishable from modern calc-alkaline andesites. YMV andesites in the central and wes-
tern CED have K/Rb = 400-600, K]Ba = 20-40 and are light REE-enriched and heavy
REE depleted. High concentrations of Cr (50--150 ppm) and Ni (20--100 ppm) and low
initial '~Sr/S+Sr ratios (0.7028--0.7030) indicate that these magmas were generated by
melting in the mantle. Modelling studies and consideration of experimental data indicate
that these andesites were formed by 2--10% fractional fusion of hydrous, undepleted,
garnet lherzolite at depths of 65 km or more in the mantle.
The data show that an intense episode of instability, convection, and widespread
melting occurred in the mantle beneath Afro-Arabia at the end of the Precambrian.
0301-9268/81/0000--0000/$02.50 © 1981 Elsevier Scientific Publishing Company
A growing body of evidence indicates that an episode of ensimatic crustal
generation and related sialic crustal disturbance took place in parts of North
Africa and Saudi Arabia at the end of the Precambrian. Such activity was
largely confined to the northern extensions of cratonic terranes rejuvenated
during the Pan-African orogeny (Fig. 1). Especially intense activity was con-
centrated around the modern Red Sea, in Saudi Arabia, Sudan and eastern
Egypt. Igneous and tectonic processes in this region resulted in the develop-
ment of primitive ensimatic crustal terranes which formed, collapsed, and
were engulfed by granites during the final 400 Ma of the Precambrian
(Engel et al., 1980).
The purpose of this paper is further to elucidate the nature and signifi-
Fig. I. Generalized tectonic map of Africa, showing the relationship of terranes affected
or generated during the Pan-African event to older and younger crustal blocks. Pan-
African crustal units studied in this paper are situated in NE Africa, west of the modern
cance of Late Precambrian igneous activity in North Africa. The volcanic
rocks which constitute the earliest manifestation of such activity are con-
sidered to be especially important. Thus, this study is focused on the petrol-
ogy and petrogenesis of the volcanic rocks of a portion of NE. Africa, the
central eastern desert of Egypt. Such data are important because very little
is known of Late Precambrian volcanism in NE Africa and Saudi Arabia.
Similar sequences in Saudi Arabia have been investigated by Bakor et al.
(1976); petrochemical studies of Egyptian volcanic sequences similar to those
described here are limited to that of Awadallah and Shaalan (1979).
REGIONAL GEOLOGY AND STRATIGRAPHY
Most of the Precambrian outcrops in Egypt are restricted to the area be-
tween the Nile and the Red Sea. Within this region, the prebatholithic vol-
canic and sedimentary succession is concentrated in the central eastern
desert (CED). This study concentrates on the volcanic succession of the
CED, especially between 25 ° 30' and 26 ° 30'N. The following areas were
studied in detail: Wadi Ambagi, Wadi E1 Dabbah, Wadi Kareim, Wadi Um
Seleimat and the region around Fawkhir; Wadi Arak, Wadi E1 Mahdaf, and
the region just east and north of the junction of Wadis Arak and Zeidun
(Arak/Zeidun). The location of these areas is shown on a geologic map
(Fig. 2) of a portion of the CED. This map represents the integration of the
author's geologic mapping in these areas with other regional investigations
(Noweir, 1968; Akaad and Shazly, 1972; E1 Ghawaby, 1973).
Despite more than 150 years of study, the basement stratigraphy of this
area is incompletely understood. The volcanic units discussed here would
be included in the Shadli Middle Schists of Hume (1934). Akaad and E1
Ramly (1960) suggested that these metavolcanics and associated ultramafics
and immature sediments reflected activity in a developing geosyncline. E1
Shazly (1964) divided this episode into 'early- and late-geosynclinal struc-
tural stages'. The latter stage was characterized by the intrusion of 'synoro-
genic plutonites' (tonalites and granodiorites) and the eruption of the Dokhan
Volcanics. The volcanic units discussed herein would be included in the early
geosynclinal structural stage. E1 Shazly further subdivided the volcanic rocks
of this stage into: (1) a 'basic--ultrabasic association'; and (b) an 'inter-
mediate association'. The first was known as the 'Baramia Association';
E1 Shazly noted that it "exhibits ophiolite characteristics". In contrast, the
'intermediate association' was dominated by andesites. The author's field-
work substantiates the most important aspects of E1 Shazly's (1964) stratig-
The results of the present stratigraphic investigation are schematically
summarized in Fig. 3, condensed from the more extended discussion of
Stern (1979a). The volcanics shown in Fig. 3 are informally subdivided on
the basis of relative stratigraphic position into 'older metavolcanics' (OMV)
and 'younger metavolcanics' (YMV).
33":~U" 33°60 , 33*50' 34*00 ....
SCALE IN KM
a ~1 ~
Fig. 2. Geologic map of a portion of the central eastern desert (CED) of Egypt.
The OMV are characterized by a thick, monotonous succession of aphyric
pillowed metabasalts. Sedimentary interbeds are rare. At Arak/Zeidun, the
OMV are associated with 10--30 m thick metagabbroic sills. These meta-
gabbros appear to be comagmatic, and have been included with the volcanic
rocks for the purposes of petrologic study. As a result of the homogeneous
nature of the OMV, it is difficult to ascertain the importance of tectonic
H" + +
0 0 U
!I '~ "PINK" "z~ vo LCANOGE NIC ~z~B R E CClASAZ~ ~
:~'' e'l~ ~I i 574GRANITES-594
Z x 10 6 y,s ~ ~,0
~O + + + Z
+ + + p I LLO W E D~f~')/,~ )Z~
4- + ÷
I, ,, , i i i, I
+ + + [ID'A ASE/iI' II
'1 i I I
+ + + X X X X
X x x
+ + X X X X X
+ ÷ + ~
, ....... .,
Fig. 3. Generalized statigraphy for [,ate Ptecambrian basement units of the central
eastern desert (CED), Egypt. Range of ages reported for the younger metavolcanlcs
(YMV) are from the work of Stern (1979a); ages for the younger of 'pink' granites are
from the work of Fullagar an(] Greenberg (1978) recalculated for X = 1.42 X 10-~y -t.
repetition or deletion within the succession; hence, thickness determinations
are equivocal. At Arak/Zeidun, the OMV is folded, but appears to be ca. 1 km
thick. East of Fawkhir, the OMV appears to extend ca. 1 km beneath the
metasediments before it passes downward into diabase dikes (?) and coarse-
Although widespread, the succession of pillowed basalt--gabbro--ultra-
mafics is rarely preserved in its original sequence. Fawkhir-Um Seleimat is
the only area where gabbros and ultramafics have been seen (by the author)
to unequivocally underlie pillowed basalt. The top of this sequence consists
of pillowed basalts with the local development of sedimentary pillow breccia.
Down-section, the pillows are succeeded by aphyric, more massive meta-
basalt which may include diabase dikes. These, in turn, are underlain by
metamorphosed anorthositic gabbros around Bir E1 Sid. Along the Qena-
Quesir road, the gabbroic succession is ca. 500 m thick. This grades down-
section into 500 m of pyroxenite. The pyroxenites overlie more altered
serpentinites, derived from dunites and harzburgites. The serpentinites
form the base of an allochthon thrust to the west over the Hammamat For-
The Fawkhir-Um Seleimat succession shows characteristics similar to
ophiolites. Ophiolites have been interpreted as fragments of oceanic (Moores
and Vine, 1971; Church, 1972), marginal basin (Pearce, 1975; Bakor et al.,
1976; Hawkins, 1977), or island-arc crust (Ewart and Bryan, 1972; Miyashiro,
1973). The volcanologic and stratigraphic characteristics of the OMV and its
gabbroic--ultramafic substrate are consistent with such paleo-environmental
The OMV or similar ophiolitic units are of regional extent. Similar suc-
cessions are widespread in northern Saudi Arabia (Bakor et al., 1976; Delfour,
1977; Shanti and Roobol, 1979).
The OMV are conformably succeeded by a series of dominantly vol-
canogenic sediments, especially tuffaceous sandstone and breccias. These
immature metasediments include distinctive beds of hematite-rich Fe-for-
mations, jaspers, rare marls, and distinctive, poorly-sorted conglomerates
(Stern, 1979a). These conglomerates contain cobbles of granite, arkose
and quartzite. Pb-isotope ages on zircons from granite cobbles in the
range of 1.1--2.3 Ga (Dixon, 1981). These conglomerate beds attest to
the proximity of a continental mass during the generation of the volcanic
rocks; they may be glaciogenic.
The YMV are a dominantly andesitic volcanic sequence which overlies
and interfingers with the immature metasediments. In the field, the YMV
are distinguished from the OMV by the abundance of porphyritic and more
felsic lithologies in the former succession. The abundance of volcaniclastic
sediments, and the paucity of large pillows further distinguishes the YMV
from the OMV. The absence of serpentinites in the YMV contrasts with
their more general association with the OMV.
Interpretation of the thickness of the YMV is complicated by its deforma-
tion. West of Wadi E1 Dabbah, the section is at least 1.3 km thick. In Wadi
Arak, the YMV is at least 5 km thick, and is overlain by 500--1000 m of
rhyodacite. In Wadi E1 Mahdaf, the YMV is 1.2 km thick before it is ter-
minated by a fault. Thus, the YMV is everywhere over 1 km thick.
GEOCHEMISTRY OF THE METAVOLCANIC ROCKS
Three metamorphic facies are recognized in both the OMV and YMV:
(1) prehnite--pumpellyite; (2) greenschist; and (3) epidote--amphibolite.
Metamorphic conditions have been discussed in greater detail elsewhere
Ba is relatively more enriched in the melt than predicted. Such a discrepancy
might be the result of the fractionation and preferential transport of Ba,
along with K and Rb, in a hydrous or CO2-rich fluid medium (Best, 1975;
Green, 1979). Er and Yb are significantly higher than that predicted from the
model. This discrepancy can be reconciled if a slightly higher proportion of
garnet to clinopyroxene enters the melt. Finally, the transition elements Ni
and Cr are lower in the meta-andesites than predicted in the model. The dis-
crepancy can be resolved if a small amount of refractory spinel (Dc r = 50--
570; Dni = 5--10; Irving, 1978) in the parent peridotite or the further pre-
cipitation of olivine and pyroxene or separation of an immiscible sulfide
phase is allowed during ascent.
A minimum depth for the generation of the YMV meta-andesites can be
estimated from the requirement that garnet be present in the residuum after
melting. Garnet is not stable in the mantle at pressures of less than 20--30
kbar, corresponding to depths in the order of 65--100 km (Ito and Kennedy,
1967; Kushiro et al., 1972; Wyllie, 1971). The REE data thus indicate that
the depth of mantle fusion was greater than the 65--100 km required to
stabilize garnet peridotite.
Although relatively little of the research on the petrology of Pan-African
volcanics has been reported, it is worthwhile comparing the conclusions of
those investigations that have been completed. Late Proterozoic volcanic
rocks from Algeria and Morocco are very similar to YMV andesites. In the
Tassendjanet Area, Algeria, Chikhaoui et al. (1978, 1980) report andesites
with similar major-element compositions to YMV andesites which are also
enriched in Cr and Ni relative to modern andesites. These investigators
argue that Tassendjanet andesites were generated by melting of hydrous
peridotite. Modeling of andesites was not reported, but model results of
calc-alkaline basalts indicated that these were generated by 10--20%
equilibrium partial melting of garnet lherzolite. McCurry and Wright (1977)
studied Late Precambrian andesites, dacites, and rhyolites from NW. Nigeria
which have (Ce/Yb)s of 9--35. These investigators conclude that the an-
desites were generated at depths great enough to permit garnet fractionation.
Thus all available data on Late Proterozoic calc-alkaline volcanic rocks sup-
port an origin from melting of garnet peridotite. This suggests that Pan-
African calc-alkaline igneous activity was rooted at depths of greater than
60 km in the mantle.
The following are the most important conclusions of this study:
(1) The end of the Precambrian was a period of instability and melting
in the mantle which lead to the formation of the entire crust of the central
eastern desert of Egypt.
(2) The earliest stage was the generation of MORB-like tholeiitic basalts
(the 'older metavolcanics') in a restricted back-arc basin or incipient Red
Sea-like rift. The OMV represent about 20% melts of depleted upper mantle,
at depths of 60 km or less.
(3) The next stage was generation of a calc-alkaline sequence of minor
basalt, andesite and rhyodacite in an arc setting. The 'younger metavolcanics'
represent lesser degrees (< 10%) of partial melting of a less-depleted mantle
than the OMV, at depths greater than 65--100 kin.
Thus, the volcanic rocks of the central eastern desert of Egypt record an
episode of primitive crustal growth that is distinct from other terranes of
similar age elsewhere in Africa which indicate that large-scale remobilization
of older sial was common during the Late Precambrian.
This paper reflects work undertaken as part of a Ph.D. Thesis at the
Scripps Institution of Oceanography and would not have been possible
without the advice and assistance of many friends and associates. Field
studies in Egypt would not have been possible without the help of Dr.
W.H. Kanes of the University of South Carolina, Dr. E.M. E1 Shazly,
Hafez Aziz, Ahmed Abdullah Abdel-Meguid and others in Egypt. The REE
analyses were possible with the assistance of Dr. Carl Hedge and Kiyoto
Futa, U.S.G.S., Denver, and Dr. Rodey Batiza, Washington, University, St.
Louis. The rest of the analytical work was performed at the Scripps Institu-
tion of Oceanography, San Diego, CA, and was greatly aided by the efforts
of Ron LaBorde and Dr. C. Engel. Dr. T.J. Chow made possible some of the
Rb and Sr and all of the Ba analyses. I would especially like to thank Dr.
A.E.J. Engel, who suggested my studies in the Precambrian of Egypt and
who provided encouragement, advice, and financial support throughout
these investigations, and T.H, Dixon, for many helpful discussions. This
study was financed by the National Science Foundation through grant
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