Petrogenesis of the Wadi Dib alkaline ring complex, Eastern Desert of EgyptPetrogenese des ...

27 Pages·2015·1.62 MB·English

Mineralogy and Petrology (1999) 65:249-275 ygolareniM
dna ygolorteP
© Springer-Verlag 9991
Printed in Austria
Petrogenesis of the Wadi Dib alkaline
ring complex, Eastern Desert of Egypt
W. Frisch 1 and A. M. Abdel-Rahman 2
~Institut ffir Geologie und Pal~iontologie, University of Tfibingen, Federal Republic of
Germany
2 Department of Geology, American University of Beirut, Beirut, Lebanon
With 31 Figures
Received January 7, 1998;
revised version accepted February 9, 1999
Summary
The Wadi Dib magmatic complex is the oldest known alkaline ring complex in the
Egyptian part of the Pan-African orogenic belt. Rb-Sr isotope data for seven samples
suggest a Vendian age of 578=k16 Ma, and a rS6S/rS78 initial ratio of 0.7048±0.0010.
The igneous complex has a diameter of 2 km and was emplaced within granodioritic
Pan-African host rocks at the intersection of two faults. It shows distinct concentric
compositional zoning with several syenitic outer ring sheets, a mainly trachytic
intermediate ring sheet, and a quartz syenite inner ring sheet with a granitic core;
relative ages decrease from margin to core. The mineralogical and chemical features are
characteristic of within-plate (A-type) magmatic complexes. Major and trace element
patterns underline the co-magmatic origin of the suite but indicate three stages of
evolution with several pulses of emplacement. A common feature of element
distribution patterns is the small systematic change in the early lithologies, but a
distinct evolution trend in the late quartz-bearing rocks.
We propose that an alkali-basaltic parent magma was emplaced within deep or
middle levels of the juvenile Pan-African crust. Differentiation mainly occurred by
fractional crystallization of olivine, clinopyro×ene, plagioclase, and apatite. During the
late stages of evolution, limited assimilation of island-arc magmatic rocks may have
occurred. Emplacement took place along ring fractures at a subvolcanic level and was
probably related with formation of a caldera during emplacement of the trachytic
lithologies. The anorogenic character of the magmatic suite indicates consolidation of
the Pan-African crust of NE Africa at the time of emplacement of the alkaline body.
250 .W Frisch and A. M. Abdel-Rahman
gnussafnemmasuZ
esenegorteP sed alkalischen Wadi Dib Ringkomplexes, Ostliche Wiiste snetpyg
Der Wadi Dib-Komplex ist die filteste bekannte Ringstruktur im figyptischen Teil des
Panafrikanischen Orogengfirtels. Rb-Sr Isotopendaten yon sieben Proben ergeben ein
vendisches Alter von 578±16 Ma und ein initiales 87Sr/86Sr-Verhfiltnis yon
0,7048±0,0010. Der magmatische Komplex besitzt einen Durchmesser von 2 km und
hat am Schnittpunkt zweier St6rungen innerhalb panafrikanischer Granodiorite Platz
genommen. Er weist eine konzentrische Zonierung mit mehreren syenitischen/iul3eren
Ringen, einem vorwiegend trachytischen mittleren Ring und einem quarzsyenitischen
inneren Ring mit einem granitischen Kern auf; die relativen Alter der Gesteine nehmen
vom Rand zum Kern hin ab. Mineralogische und chemische Charakteristika sind die
von Intraplatten- (A-Typ-) Komplexen. Haupt- und Spurenelementmuster weisen auf
eine ko-magmatische Entstehung hin, zeigen aber eine Entwicklung in drei Stadien mit
mehreren magmatischen Pulsen auf. Charakteristika der Elementverteilungen sind
wenig systematische Anderung in den ~ilteren Lithologien, aber ein gerichteter
Entwicklungstrend in den spfiten, quarzfiihrenden Lithologien.
Wir schliegen, dab ein alkali-basaltisches Magma in ein tiefes oder mittleres Niveau
der jungen panafrikanischen Kruste intrudierte. Differentiation erfolgte im wesent-
lichen durch fraktionierte Kristallisation yon Olivin, Klinopyroxen, Plagioklas und
Apatit. Wfihrend spfiter Entwicklungsstadien gab es vermutlich begrenzte Assimilation
yon Inselbogen-ga-uste. Die Platznahme erfolgte entlang von Ringbrfichen in einem
subvulkanischen Stockwerk und war vermutlich mit der Bildung einer Caldera w~ihrend
der Platznahme der trachytischen Lithologien verbunden. Der anorogene Charakter der
magmatischen Folge zeigt an, dab die panafrikanische Kruste Nordost-Afrikas zur Zeit
der Platznahme der alkalischen Intrusion bereits konsolidiert war.
Introduction
Several hundred, mostly circular alkaline complexes (e.g., El-Ramly and Hussein,
1985; Harris, 1985; Vail, 1985a) were emplaced in an intracontinental setting
within the Arabian-Nubian Shield, which had been consolidated during the Pan-
African orogeny. Emplacement of the post-orogenic or anorogenic complexes in
NE Africa took place during an extended period of time spanning almost the entire
Paleozoic and Mesozoic eras (Vail, 1990). The location of many of the alkaline ring
complexes is controlled by faults and shear zones.
The occurrence and tectonic environments of the Egyptian alkaline ring com-
plexes have been reviewed by several workers (e.g., deGruyter and Vogel, 1981;
Serencsits et al., 1981; E1-Ramly and Hussein, 1985). However, only a few detailed
petrological studies have been carried out on individual complexes. These include
the Mount Gharib A-type complex of NE Egypt (Abdel-Rahman and Martin,
1990a) and the Abu Khruq ring complex of SE Egypt (Landoll et al., 1994).
Since the introduction of the term "A-type granite" (Loiselle and Wones, 1979;
Collins et al., 1982) and "within-plate granite" (Pearce et al., 1984), the sources of
such felsic magmas have been studied by several authors (e.g., Bonin and Giret,
1984; Bonin, 1990; Skjerlie and Johnston, 1992; Landoll et al., 1994; Nedelec et al.,
1995). Kerr and Fryer (1993) concluded that the formation of A-type magmas
commonly involves a complex interplay of crustal and mantle components. Eby
Petrogenesis of the Wadi Dib alkaline ring complex, Eastern Desert of Egypt 152
(1992) subdivided the A-type granitoids into two groups: the 1A group represents
differentiates of mantle-derived alkaline basalt emplaced in an intraplate or rift
zone setting, and the A2 group represents crust-derived magmas of post-orogenic
settings.
The aim of this study is to describe the mineralogical and chemical
characteristics of the Wadi Dib ring complex and to evaluate the magma source,
the role of magma differentiation processes, and the tectonic setting of the
complex. The paper is based on field work carried out by the senior author, which
included detailed mapping on the basis of an aerial photograph.
General geology
The Wadi Dib ring complex (Fig. ,)1 located in the northern part of the Eastern
Desert of Egypt (latitude 27035 ', longitude 32°56'), is emplaced within Pan-
African granodioritic rocks. These host rocks are calc-alkaline and exhibit geo-
chemical and mineralogical traits of volcanic-arc granitoids. They were interpreted
to have been formed in an active continental margin setting during the late stages
of the Late Proterozoic (Vendian) Pan-African orogeny (e.g., Gass, 1982; Hussein
et al., 1982; Abdel-Rahman and Martin, 1987). The Pan-African crust of the
Arabian-Nubian Shield is considered to have developed by the welding of a series
of oceanic island arcs and, finally, a continental margin (e.g., Frisch and Al-Shanti,
1977; Gass, 1977; Engel et al., 1980; Vail, 1985b).
Francis (1972) was the first to note the occurrence of the Wadi Dib ring
complex, and described it as an alkaline syenitic-granosyenitic body. He
considered this complex to be of similar age (Late Mesozoic) as the Abu Khruq
ring complex of southeastern Egypt. Sabet et al. (1978) proposed that the Wadi Dib
complex evolved during several Late Proterozoic and Early Paleozoic stages of
igneous activity. The first radiometric age came from Serencsits et al. (1981), who
analysed biotite from quartz syenite and umptekite (syenite) and obtained ages of
549-t-11, 5534-11, and 5584-11 Ma. Rb/Sr isotope data indicate an emplacement
age of 5784-16 Ma (Frisch, 1982; see below).
The Wadi Dib ring complex is a circular structure about 2 km in diameter,
consisting of several ring sheets (Fig. .)1 These comprise syenite and pegmatitic
syenite outer rings, a trachytic intermediate ring (including early porphyritic
syenite), which is incomplete, and a quartz syenite inner ring, which encloses an
off-centered, roughly elliptical granitic core with a long axis measuring 0.8 km. A
number of generally NNW-trending, post-intrusive, mafic to felsic dikes cut across
the ring structure and its Pan-African host rocks (Fig. .)1 This dike swarm is
parallel to the Red Sea rift. Therefore, its emplacement is probably related to
crustal extension in the course of the Tertiary rifting process.
The Wadi Dib complex was emplaced at the intersection of an ENE-trending
fault and a fault belonging to the ESE-trending, late- to post-Pan-African Najd fault
system. The contacts of the outermost rings of the intrusive body dip away from the
centre with intermediate angles. The contacts between the individual ring sheets
within the complex steepen from the margin towards the trachytic unit, where they
are almost vertical. The inner part of the complex shows contacts which steeply dip
towards the centre.
252 .W Frisch and A. M. Abdel-Rahman
Dikes
Granite
A x x Red quartz syenite
Grey quartz syenite
o Younger trachyte
Q
Older trachyte
Q
Porphyritic syenite
r~ ° °" Pegmatitic syenite
• ::-- Syenite.
Shield rocks
Granodiorite (Pan-African)
: o 1000 rr
Fig. .1 Geological map of the Wadi Dib ring complex. Numbers indicate samples used for
Rb-Sr age determination (Fig. 4). Symbols on left side of legend are same as used in the
diagrams of Fig. 2 to .31 Inset map shows location of the complex
Lithologic units and their sequence of emplacement
The oldest rock of the Wadi Dib complex is syenite intruded by pegmatitic syenite,
which forms several ring sheets in the outer zone of the complex (Fig. 1). These
lithologies are followed by a porphyritic syenite, dikes of which radiate into the
syenitic outer ring sheets. The porphyritic syenite has a fine-grained matrix and is
Petrogenesis of the Wadi Dib alkaline ring complex, Eastern Desert of Egypt 352
texturally transitional to the trachyte. Only small bodies of the porphyritic syenite
are present; larger parts probably have been replaced by the trachyte. The trachyte
forms an incomplete ring sheet and contains small pegmatitic syenite and map-
sized porphyritic syenite inclusions. It occurs in several textural varieties including
rocks with trachytic texture ("older trachyte") and originally probably glassy rocks
("younger trachyte"; see below). This plutonic-volcanic association is suggestive
of a very shallow, subvolcanic level of emplacement.
Quartz syenite followed the trachyte in the sequence of emplacement,
asymmetrically replacing the trachyte. It occurs as two varieties. A grey quartz
syenite variety embraces large, mappable blocks of trachyte (Fig. )1 and forms a
ring sheet, which displays transitional boundaries towards a younger, red quartz
syenite variety, which takes a more internal position within the ring complex. The
red quartz syenite lacks direct trachyte inclusions but encloses xenoliths of the grey
quartz syenite which, in turn, carry trachyte inclusions (Fig. .)1 The content of
quartz gradually increases from the grey towards the red quartz syenite. The
youngest rock encountered is a dark red, fine-grained alkali-feldspar granite (>20
vol-% modal quartz) forming the core of the ring complex. The contacts between
the quartz syenite and the granite are transitional. Rare granite dikes radiate out
into the red quartz syenite. Within the granite mass, the highest quartz content
(about 30 vol-%) occurs in its centre.
The sequence of emplacement from the lithologies at the margins of the ring
complex towards those in its centre is reflected by the concentric compositional
zonation and consistent crosscutting relationships.
lacitylanA seuqinhcet
For radiometric age determinations, ca. 200 mg of whole rock samples were spiked
with a highly enriched S7Rb and 84Sr spike. The overall blank of the whole
chemical treatment is less than 2 ng .rS The determination of the isotope ratios was
carried out using a V.G. "Micromass M 30" mass spectrometer at University of
Vienna. 10-11 × 1.42 a-1 was used as decay constant. The rS68/rS78 isotope ratios of
the samples are relative to rS68/rS78 = 0.71014 in NBS987. Data are normalized to
86Sr/SSSr= 0.1194. The estimated analytical error (2o-) for individual analysis of
the 87Rb/s6sr ratio is 1%, and for the rS6S/rS78 ratio 0.1%. The regression line was
calculated using the program of Faure (1977).
Mineral analyses were carried out on the electron microprobe at University of
Ttibingen. Major and trace element (Rb, Sr and partly Zr and Y) concentrations
were obtained on fused lithium-metaborate discs and pressed pellets by X-ray
spectrometry at University of Ttibingen. Loss on ignition (LOI) was determined by
heating sample powders for 50 minutes at 1000 °, and Fe203 was determined by the
titration method. Concentrations of the rare earth elements as well as Ba, ,Y Nb, Zr,
Hf, Ta and Th were determined by the ICP-MS sintering techniques at Memorial
University (Newfoundland). The advantage of the sintering technique is that it
practically ensures complete digestion of resistant REE-bearing accessory phases
(e.g., zircon, fluorite), which may not dissolve during an acid digestion (for full
details of the procedure, see Longerich et al., 1990). The chondrite values used for
normalization are those of Taylor and McLennan (1985).
254 W. Frisch and A. M. Abdel-Rahman
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Petrogenesis of the Wadi Dib alkaline ring complex, Eastern Desert of Egypt 552
Petrography and mineral chemistry
Most lithologies of the Wadi Dib ring complex show a single feldspar phase
(mesoperthite) and are therefore hypersolvus. Rare, early-formed oligoclase
crystals are found only in a limited number of samples from the oldest, least
evolved syenite or the trachyte. The plutonic members of the complex consist of
variable amounts of alkali feldspar (60-93 vol-%), quartz (0-30 vol-%), amphibole
(5-20 vol-%), biotite (2-10 vol-%), clinopyroxene (0-4 vol-%), and ubiquitous
opaque Fe-Ti oxides.
Mesoperthite shows Carlsbad and Baveno twinning with some chessboard
albite. Patch and string perthite are most common. Graphic intergl-owth of quartz
and alkali feldspar is observed in some of the granitic samples. The compositions of
representative alkali feldspars and one plagioclase (oligoclase) are given in Table .1
The pyroxenes (for nomenclature, see ,otomiroM 1989) are calcic (Na <0.25
cations per formula unit). They vary in composition from augite to hedenbergite
and contain an acmitic component, especially in the pegmatitic syenite sample 73
(Table ,1 Fig. 2). Amphibole is represented mostly by bluish- to brownish-green
varieties. The composition of the amphibole ranges from hastingsite (Si < 6.50
cations p.f.u.) in syenite and trachyte, to ferro-edenite (Si > 6.50 cations p.f.u.) in
porphyritic syenite, quartz syenite and granite (Table ,1 Fig. 3). Most amphiboles
are titanian and/or potassic amphiboles, which underlines the alkaline character of
the rocks.
Biotite forms large, mostly reddish-brown crystals (5-8 mm long), and contains
inclusions of mesoperthite, zircon, and apatite. It ranges from biotite with Mg/
(Mg+Fe) --- 0.44 to annitic biotite with Mg/(Mg+Fe) ---- 0.16 (Table ,)1 which is
typical of anorogenic alkaline suites ,namhaR-ledbA( 1994). Zircon, apatite,
astrophyllite and fluorite are the main accessory phases, among those zircon is the
o2=sgMoc 6 S)~oer~oON(aC ~
C a .oaN( 5 Fe;i 5) Si206
gM z Oz/S
l
,II
MaC ~ 0 CaFeSi206
\,
Mg2Si206 Fe2Si206
Fig. .2 Tetrahedron showing pyroxene compositions. Symbols show different rock types
(see Figs. 1 and .)5 For results from microprobe analysis, see Table 1
256 .W Frisch and .A .M Abdel-Rahman
6.00 6.50 7.00
0.5
Mg
0.4- Mg + Fe • ferro-edenite
l -
0.3- • AA Q
II
0.2- II
II
0.1- hastingsite
nlr
si
O0
6.00 ~50 ~00
Fig. .3 Si Mg/(Mg+Fe) diagram showing amphibole compositions after (1997).
vs. Leake
Diagram parameters: BaC >_ 1.50, (Na÷K)A >_ 0.50, iT < 0.50 per formula unit. Symbols
show different rock types (see Figs. 1 and .)5 For results from microprobe analysis, see
Table 1
most common. It occurs as minute inclusions in mesoperthite and ferromagnesian
minerals, as large zoned crystals, and occasionally as hopper-shaped grains.
The early syenites (syenite und syenite pegmatite; Fig. )1 exhibit medium to
coarse-grained and pegmatitic textures. Bluish-green hastingsite is the main
ferromagnesian phase and is much more abundant than biotite or clinopyroxene.
Clinopyroxene is commonly rimmed by amphibole. Anhydrous conditions during
the early phases of crystallization (pyroxene, alkali feldspar) were replaced by
hydrous conditions during late stages of crystallization (amphibole, biotite). This is
also suggested by the pegmatitic rocks, which intrude the non-pegmatitic syenites.
Oligoclase (< 5 vol-%) occurs as inclusions in large alkali feldspars, thus indicating
its early crystallization. Small (0.1 ram) grains of nepheline are only present
(< 1 vol-%) in sample 56, forming inclusions in hastingsitic amphibole crystals.
The general absence of both quartz and nepheline indicates that these rocks are
silica-saturated in the strict sense.
The porphyritic syenite is a precursor of the trachytic lithologies. With its fine-
grained groundmass, it is texturally transitional between the early syenites and the
trachytes. The phenocrysts are alkali feldspar, abundant bluish-green ferro-edenite,
and minor hedenbergitic pyroxene.
The trachyte shows several textural varieties. Older, porphyritic trachyte shows
phenocrysts, often with a rounded shape, embedded in a microcrystalline
groundmass of aligned feldspar laths representing a trachytic flow texture.
Younger, partly aphyric trachyte contains a groundmass made up of a fine-grained,
equigranular mosaic of alkali feldspar and some amphibole with no or only minor
microphenocrysts. Layered textures may suggest formation of part of these rocks
as tufts. Among these younger trachytes is a quartz trachyte (sample 57), which
consists of a mosaic of minute alkali feldspar and quartz grains. We interpret these
textures to have resulted from devitrification. Trachyte breccia contains fragments
of the porphyritic and fine-grained trachyte within devitrified groundmass material.
The quartz syenite is texturally similar to the early syenite but differs from it by
the high degree of transformation of its mafic minerals (mostly amphibole) to
Petrogenesis of the Wadi Dib alkaline ring complex, Eastern Desert of Egypt 257
Table 2. Rb-Sr analytical data for whole rock samples of Wadi Dib complex. rS68/rS78
values era normalized ot the NBS standard 987 value of .O .41017 For sample localities, see
.giF 1
sample 87Rb/86Sr rS68/rS78 Rb (ppm) Sr(ppm)
54 33.1 0.7155 7.88 264
56 0.728 0.7106 9.88 482
76 0.788 0.7104 5.09 453
73 3.23 1559.0 721 9.51
59 54.1 0.7169 221 133
57 2.94 0.7308 277 373
70 9.17 0.7805 971 77.6
17 30.7 0.7619 951 7.98
opaque iron oxides. In the alkali-feldspar granite, the amount of the reddish-brown
biotite (annite) is about the same as the ferro-edenitic amphibole. Astrophyllite
commonly replaces amphibole and may reach 4 vol-% of the rock. The granite is
leucocratic and contains only accessory or no ferromagnesian minerals. Graphic
intergrowth of quartz and alkali feldspar as well as fluorite as an accessory phase
are observed in some samples in the central part of the intrusion.
Radiometric age
Rb-Sr isotopes (Table 2) have been determined for eight samples selected from the
various lithologies of the Wadi Dib ring complex (syenite, pegmatitic syenite,
trachyte, and granite; for locations of the samples, see Fig. .)1 Sample 73, which is
a syenite pegmatite, was collected near the margin of the ring complex, and
displays evidence of weathering. It is characterized by obvious loss of radiogenic
Sr and plots far below the data regression line. It was therefore not used for the age
calculation. The regression line for the seven remaining samples (Fig. 4) yields an
age of 5784-16 (2a) Ma (Vendian age according to the time table of Bowring and
Erwin, 1998), and a S7Sr/S6Sr initial ratio of 0.70484-0.0010, which is consistent
with a mantle origin. The regression line has an MSWD of 2.6, therefore it is not an
isochron in the strict sense.
Major and trace element geochemistry
Major and trace element abundances and the calculated CIPW norms are given in
Table 3 and in Fig. 5. The rocks classify as syenites and alkali granites in the
alkalis vs. silica diagram after Cox et al. (1979) (Fig. 6). All rock types plot into the
alkaline field according to the subdivision of Miyashiro (1978; Fig. 6). The granite,
however, shows less pronounced alkalinity, due to its lower total-alkali content at
higher SiO2 values as compared to the other lithologies. The SiO2 vs. Zr/TiO2
systematics (Floyd and Winchester, 1978) lead to the same result (Fig. 7). In this
diagram, most granites and the (quartz-bearing) trachyte sample 57 plot outside the
fields of alkaline suites, whereas all the other lithologies plot well within the
alkaline field.
852 .W Frisch and A. M. Abdel-Rahman
.790 /
87Sr / 8~Sr
.7BO -
077.
,760
.750
.740 -
57~ (8'Sr / o)rS68 = 0.7048 + 0.0010
.730 -
,720
.710
87Rb / 86Sr
65
.700
r i i i t I i
1 2 3 4 5 6 7 8 9 01
Fig. 4. Whole-rock Rb-Sr isotope data regression line for the Wadi Dib complex
(MSWD -- 2.6). Lithologies are syenites (samples ,45 56, 76), trachytes (57, 59), and
granites (70, 71). Locations of samples are shown in Fig. .1 See also Table 2
The concentration patterns of the different elements through time, i.e., from the
older, marginal to the younger, central lithologies show various trends of evolution
(Fig. 5). Three groups of rocks can be distinguished, which are separated by sharp
boundaries in the field and indicate distinct stages of magma influx and/or
differentiation in the magma chamber. These groups, in turn, are subdivided into
sub-units representing separate magmatic pulses of emplacement.
The first group is represented by the syenite and shows an evolution trend
towards higher silica, alumina, and differentiation index values and towards lower
contents in mafic minerals (shown as normative colour index in Fig. 5), MgO and
FeO. Most of the syenite samples are slightly nepheline normative. The syenite
pegmatite (sample 73), according to the field relations a late phase of the early
syenite but emplaced in a separate pulse, shows deviations from the syenite trend,
which may be due to hydrothermal alteration. We already stated that sample 73
shows strong deviation in the Rb-Sr isotope data.
The second group contains the porphyritic syenite and the trachytes. The
element concentrations of these rocks scatter, but the colour index generally
decreases, coupled with a decrease in total iron (Fig. 5). This rock group is silica
saturated and practically shows no normative nepheline or quartz. The silica
content is generally between 58 and 61 wt-%. Only sample 57 (quartz trachyte)
shows an abnormally high silica content but also deviating concentrations of other
elements (Table 3, Fig. 5), which may indicate contamination by wall rocks during
ascent of the magma. This rock contains quartz in its microcrystalline groundmass
and, according to its chemical characteristics, shows similarities with the group
three rocks.