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A new look on Imperial Porphyry: a famous ancient dimension stone from the Eastern Desert of Egypt—petrogenesis and cultural relevance

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Abstract

Imperial Porphyry, a famous dimension stone of spectacular purple color, was quarried in the Mons Porphyrites area north of Jabal Dokhan in the Eastern Desert of Egypt, from the beginning of the first until the middle of the fifth century AD. During this period, the valuable material was processed as decorative stone and was used for objects of art, reserved exclusively for the Imperial court of the Roman Empire. Later on, only antique spoils of smaller or bigger size have been re-used for these purposes. The Imperial Porphyry is a porphyritic rock of trachyandesitic to dacitic composition that occurs in the uppermost levels of shallow subvolcanic sill-like intrusions, forming a member of the Dokhan Volcanic Suite. Its purple color is mainly due to dispersed flakes of hematite, resulting from hydrothermal alteration of a dark green Common Porphyry of similar composition, underlying the Imperial Porphyry. Both, the Common Porphyry and the purple Imperial Porphyry’, are extensively exposed in the Roman quarries. Contacts between Common and Imperial Porphyry are irregular and gradational. In both rock types, intrusive breccias are frequent, indicating a complex intrusion history. U–Th–Pb zircon geochronology on two samples of Imperial Porphyry and one sample of the Common Porphyry yielded an age range of 609–600 Ma, thus confirming earlier results of radiometric dating. Geochemical evidence indicates that both the Imperial and the Common Porphyry are of medium- to high-K calc-alkaline affinity. The magmas have formed by partial melting of a subduction-modified upper mantle. The subsequent intrusion took place within a highly extended terrane (HET).
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International Journal of Earth Sciences
https://doi.org/10.1007/s00531-018-1604-z
ORIGINAL PAPER
A new look onImperial Porphyry: afamous ancient dimension
stone fromtheEastern Desert ofEgypt—petrogenesis andcultural
relevance
MahrousM.AbuEl‑Enen1· JoachimLorenz2· KamalA.Ali3· VolkervonSeckendor4· MartinOkrusch4·
UlrichSchüssler4· HeleneBrätz5· Ralf‑ThomasSchmitt6
Received: 6 November 2017 / Accepted: 8 March 2018
© Springer-Verlag GmbH Germany, part of Springer Nature 2018
Abstract
Imperial Porphyry, a famous dimension stone of spectacular purple color, was quarried in the Mons Porphyrites area north of
Jabal Dokhan in the Eastern Desert of Egypt, from the beginning of the first until the middle of the fifth century AD. During
this period, the valuable material was processed as decorative stone and was used for objects of art, reserved exclusively for
the Imperial court of the Roman Empire. Later on, only antique spoils of smaller or bigger size have been re-used for these
purposes. The Imperial Porphyry is a porphyritic rock of trachyandesitic to dacitic composition that occurs in the uppermost
levels of shallow subvolcanic sill-like intrusions, forming a member of the Dokhan Volcanic Suite. Its purple color is mainly
due to dispersed flakes of hematite, resulting from hydrothermal alteration of a dark green Common Porphyry of similar com-
position, underlying the Imperial Porphyry. Both, the Common Porphyry and the purple Imperial Porphyry’, are extensively
exposed in the Roman quarries. Contacts between Common and Imperial Porphyry are irregular and gradational. In both
rock types, intrusive breccias are frequent, indicating a complex intrusion history. U–Th–Pb zircon geochronology on two
samples of Imperial Porphyry and one sample of the Common Porphyry yielded an age range of 609–600Ma, thus confirm-
ing earlier results of radiometric dating. Geochemical evidence indicates that both the Imperial and the Common Porphyry
are of medium- to high-K calc-alkaline affinity. The magmas have formed by partial melting of a subduction-modified upper
mantle. The subsequent intrusion took place within a highly extended terrane (HET).
Keywords Eastern Desert· Dokhan Volcanics· Imperial Porphyry· Petrogenesis· Cultural relevance· U–Pb zircon age
dating
Introduction
During the period of the Roman emperor’s Imperial Por-
phyry, a porphyritic igneous rock of trachyandesitic to
dacitic composition with spectacular purple color (Fig.1),
was extensively quarried in the Mons Porphyrites area north
of Jabal Dokhan in the Eastern Desert of Egypt (Fig. S1
Electronic supplementary material The online version of this
article (https ://doi.org/10.1007/s0053 1-018-1604-z) contains
supplementary material, which is available to authorized users.
* Mahrous M. Abu El-Enen
mahrousabuelenen@gmail.com
1 Geology Department, Faculty ofSciences, Mansoura
University, Mansoura35516, Egypt
2 Graslitzer Str. 5, 63791KarlsteinamMain, Germany
3 Department ofMineral Resources andRocks, Faculty
ofEarth Sciences, King Abdulaziz University,
P.O. Box80206, Jeddah21589, SaudiArabia
4 Lehrstuhl für Geodynamik und Geomaterialforschung,
Institut für Geographie und Geologie, Am Hubland,
97074Würzburg, Germany
5 Geozentrum Nordbayern, Universität Erlangen,
Schlossgarten 5a, 91054Erlangen, Germany
6 Museum für Naturkunde, Leibniz-Institut für Evolutions- und
Biodiversitätsforschung, Invalidenstr. 43, 10115Berlin,
Germany
International Journal of Earth Sciences
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in the electronic supplement). This was the only known
source of the Imperial Porphyry in antiquity. Judging from
local inscriptions dated at the reign of Emperor Tiberius
(14–37AD), the Mons Porphyrites site was discovered by
the Roman explorer Caius Cominius Leugas who erected a
black porphyry stele, which he dedicated to Pan-Min, the
God of the Eastern Desert and the goddess Sarapis, indicat-
ing that he was the finder of the Imperial Porphyry and other
rock types (van Rengen 1995, 2001). The text, written in
Greek language, reads in English translation (V.v.S.): “Gaios
Kominios Leugas, the finder of the quarries of the Porphy-
rites and Knekites and the black Porphyrites and various
stones, has cut a prayer to Pan and Sarapis, the greatest gods,
for the wealth of his children. Regnal year 4 of Tiberius, the
Holy Kaisar, Epeiph 29”. This date corresponds to July 23,
18AD. Thus, quarrying likely commenced before the year
18AD, but came to a standstill in the middle of the fifth
century AD (Klein 1988; Brown and Harrell 1995; Maxfield
and Peacock 2001; Klemm and Klemm 2008). During this
time c. 10,000 tons of dimension stone were extracted from
numerous quarries (Klemm and Klemm 2001a). At Mons
Claudianus, about 30km south of Mons Porphyrites, light-
colored quartz diorite was extensively quarried during the
first to fourth century AD (Fig.2 inset, S1) (Klemm and
Klemm 2008; Harrell and Storemyr 2009).
The Roman quarries are situated in high topographic
positions between c. 800 and 1200m on the hills forming the
eastern and western slopes of Wadi Abu Ma’amel (Fig.2)
(e.g., Dardir and Abu Zeid 1972; Maxfield and Peacock
2001). In the nineteenth century, the quarries were detected
again by the British explorers James Burton and John Gard-
ner Wilkinson (1822/1823), the Italian explorer Giovanni
Battista Brocchi (1823) and the French petrographer Achille
Delesse (1852). In 1875, Georg Schweinfurth, a Baltic Ger-
man explorer, investigated and named three of the different
quarry areas (Schweinfurth 1887), i.e., the Rammius and
Lykabettus quarries at the west side and the Lepsius quarries
on the east side of the valley (Fig.2). In the NW part of the
area, Meredith and Tregenza (1950) detected the Northwest
quarries and the Bigfoot quarries. Finally, Nick Bradford
recovered the Northeast (Bradford) quarries (Maxfield and
Peacock 2001) at the eastern side of Wadi Abu Ma’amel.
The Roman quarries were hardly visited since the end of
the fifth century AD and, therefore, provide ample, undis-
turbed evidence for the production and dressing techniques
of Roman dimension stone. Unfinished porphyry blocks
show characteristic traces of different working stages
(Figs. S2, S3). Downhill transport of the porphyry blocks
was achieved by a system of stone chutes that connected
the quarries with the main loading platform at the opening
of the Wadi Umm Sidri towards the east. The quarry areas
were accompanied by villages containing housing estates for
workers, working areas for blacksmiths and stonemasons as
well as relics of a large fort, a public bath and of two tem-
ples dedicated to Isis and Serapis (Fig. S2d) (Maxfield and
Peacock 2001). The extremely remote, hyperarid mountain-
ous mining area was connected to the ancient port of Καινή
(now Qena) at the Nile River by a Roman trade route (Fig.
S1), via porphyrites, which was first described by Strabo
(63BC–23AD) and shown on a map by Claudius Ptolemy
(c. 100–170AD; Werner 1998). Porphyry is defined as “A
general term for any igneous rock that contains phenocrysts
in a finer-grained groundmass” (LeMaitre etal. 2002), i.e.,
displaying a porphyritic texture, as developed in many (sub-)
volcanic rocks (see also Bates and Jackson 1997). To the
best of our knowledge, the term πορφυρίτης (purple dye
or purple stone) appeared first in the Greek inscription, on
an Imperial Porphyry stele of Caius Cominius Leugas (van
Rengen 1995, 2001).
A concise account on the geological setting, mineralogy,
petrology and geochemistry of Imperial Porphyry is given
by Williams-Thorpe etal. (2001). A much more extensive
study was presented by Makovicky etal. (2016a, b), but
is essentially focused on samples from the Lepsius quar-
ries and some boulders from Wadi Abu Ma’amel. Our new
investigation covers samples from Wadi Umm Sidri, Wadi
Abu Ma’amel and all quarries. Therefore, new fieldwork
and extensive sampling was carried out in the framework of
an interdisciplinary Porphyry Project (Lorenz 2012), pro-
viding new insights into the mutual contact relationships
between the important rock types and their petrographic
Fig. 1 a Roman column of brecciated Imperial Porphyry, mined in
the Lykabettus quarries, 3m in length, with a sculpture of two male
figures, interpreted as Roman co-emperors of the tetrarchy period
(293–313AD; Delbrueck 1932), Vatican Library, Rome. b Close-up
of the brecciated rock consisting of fragments with porphyritic tex-
ture, rich in white colored plagioclase phenocrysts, in a fine-grained
matrix of dark purple color (from Lorenz 2012)
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Fig. 2 Detailed geological map of the Wadi Um Sidri-Wadi Abu Ma’amel area (based on our own field work and Landsat image processing).
Inset map: Geological map of the Eastern Desert of Egypt (modified after Eliwa etal. 2010; Ali etal. 2012)
International Journal of Earth Sciences
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characteristics. Moreover, results of geochronological and
geochemical analyses of the samples collected will be dis-
cussed together with the crucial problem of the spectacular
purple coloring of the Imperial Porphyry. Our results may
contribute to a better understanding of the petrogenesis of
this exceptional dimension stone and, at best, provide evi-
dence for the exact provenance of individual works of art.
In addition, an archeological sample WS-E9 from a medi-
eval portable altar found in the castle “Schlössel” close to
Klingenmünster in Germany was added to this study (Barz
etal. 2012).
Cultural relevance ofImperial Porphyry
Imperial Porphyry was used for the production of architec-
tural elements and works of art, exclusively reserved for the
imperial court of the Roman Empire. High appreciation was
mainly due to its characteristic purple color, traditionally
regarded as an imperial status symbol (e.g., Klemm and
Klemm 2001a). Careful polishing of the rock achieved a
highly reflective finish and a sparkling appearance of the
feldspar phenocrysts. Remarkably, brecciated varieties of
Imperial Porphyry were regarded as the most attractive
and thus were frequently used for fine works of art (Fig.1
and Figs. S4, S5, S6). Under the Roman emperors Trajan
(98–117AD) and Hadrian (117–138AD), the period of por-
phyries’ fashion reached a first maximum, while a second
one occurred under the reign of Diocletian (284–305AD),
Constantine the Great (306–337AD) and his followers on
the throne of the Byzantine Empire.
The different steps of extraction and transport of Imperial
Porphyry involved a wealth of masterly technical perfor-
mances, all the more as a widespaced joint pattern allowed
the production of large, cleavage- and fracture-free blocks,
suitable for manufacturing of monolithic or sectioned col-
umns, mantle pieces, pedestals, sarcophagi, sculptures, cer-
emonial bathtubs, magnificent giant bowls and other mas-
terworks of Roman craftsmanship. For instance, individual
segments of the Column of Constantine in Constantinople
(330AD) attain a weight of up to 45 tons (Fig. S6a). Moreo-
ver, it must be assumed that the Roman engineers had dis-
posed of a fundamental knowledge on geological survey.
Otherwise, they would have hardly been able to detect the
occurrences of Imperial Porphyry, suitable for quarrying.
Works of art designed of Imperial Porphyry in Pharaonic
Egypt are rare. Only small sculptures and plates have been
shaped from loose boulders (Klein 1988). According to Del
Bufalo (2013), the oldest example so far known is a man’s
head from dynasty XXVI (664–525BC). In contrast, many
architectural elements and artworks from Roman antiquity
are still preserved on-site in historic monument areas of
the eastern and central Mediterranean, or are exhibited in
museums.
Throughout the Middle Ages and, especially, in the
Renaissance and Baroque eras, the demand for this rock
type, called Porfido rosso antico by Italian sculptors,
remained high and, after the closure of the ancient quarries,
was only met by the use of antique spoils. These spoils,
derived from destroyed Roman buildings, were either used
as raw materials to design sculptures or floor and wall cov-
ers, or were incorporated, in their original form, in new
churches or palaces. A compilation of the most important
architectural elements and art objects made of Imperial
Porphyry, which can be visited in Europe, is given in the
electronic supplement.
General geological setting
The crystalline basement of the southern Sinai Peninsula
and the Eastern Desert of Egypt was formed during the
Pan-African Orogeny, due to collision of East and West
Gondwana and the related closure of the Mozambique
ocean in Neoproterozoic times (e.g., Shackleton 1986; Stern
1994). By this orogenic event, Neoproterozoic sedimentary
sequences with intercalated (sub-) volcanic and plutonic
rocks were deformed and metamorphosed under elevated
pressures and temperatures, and subsequently intruded by
Older I-type and Younger A-type granitoid magmas. (e.g.,
Abu El-Enen and Whitehouse 2013 and references therein;
see Fig.2 inset). The basement complex contains three dif-
ferent groups of Neoproterozoic volcanic rocks of mafic to
intermediate, subordinately of felsic composition (Eliwa
etal. 2006, 2014), i.e., the older metavolcanics, the younger
metavolcanics (Stern 1981) and the Dokhan Volcanics. The
later ones are represented by volcano-sedimentary succes-
sions containing varicolored lava flows, pyroclastic rocks
and ignimbrites differing in thickness from a few tens of
meters to about 1200m (Wilde and Youssef 2000; Eliwa
etal. 2006, 2010). Their composition ranges from basaltic
andesite, via andesite, trachyandesite, and dacite to rhyo-
lite. Moreover, subvolcanic rocks with porphyritic texture
intruded into shallow crustal levels. Most of the Dokhan
Volcanics are un-metamorphosed or, in some occurrences,
underwent low-pressure metamorphism (Eliwa etal. 2006,
2014 and references therein).
The Dokhan Volcanics contain enclaves of the Older
I-type granitoids, but on the other hand, are more or less of
the same age as the late- to post-orogenic Younger A-type
Granitoids (e.g., Stern and Gottfried 1986; Eliwa etal.
2006; see Fig.2). Rb–Sr whole rock dating yielded wide
age ranges of 610–560Ma for the Dokhan Volcanics (Res-
setar and Monard 1983; Ries etal. 1983) and of 610–550Ma
for the Younger Granites (e.g., Stern and Hedge 1985). A
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broadly conforming age range of 630–590Ma was obtained
by SHRIMP dating on single zircons from the Dokhan Vol-
canics (Wilde and Youssef 2000; Breitkreuz etal. 2010; and
own data, see below). Simultaneously, the molasse-type
Hammamat sediments, present outside the area of Fig.2,
were deposited forming interlayers with the volcanic rocks.
The volcano-sedimentary sequence and the granites were
intruded by contemporaneous, or slightly younger, swarms
of NE–SW to E–W trending, bi-modal or composite dikes of
andesite and rhyolite composition. These were emplaced in
a highly extensional tectonic regime similar to the Basin and
Range Province (Stern etal. 1988, and references therein).
Field relations
The ancient quarry sites at Mons Porphyrites are situ-
ated north of Jabal Dokhan in the central Eastern Desert
about 50 km west of the seaside resort Hurghada at
27°14.2–16.1N, 33°16.5–18.5E (Fig. S1; compilation by
Brown and Harrell 1995 and description by; Maxfield and
Peacock 2001). Although evidence of subaerial volcanism
exists in the Jabal Dokhan area, it became clear during our
recent field campaigns that most of the “volcanic rocks”
exposed in Wadi Um Sidri and Wadi Abu Ma’amel do not
represent subaerial lava flows, but in fact are formed by
shallow intrusion of magma. Presumably, this took place
in at least two different batches, as indicated by structural
evidence (see below). Consequently, we will avoid the term
“lava” in the following description. Samples of different
types of subvolcanic porphyries were collected from out-
crops in Wadi Umm Sidri, from the northern entrance of
Wadi Abu Ma’amel and from the Roman quarries. Sample
localities are shown in Fig.2.
Outcrop situation andstate ofrock preservation
Due to the present, hyperarid climatic conditions, blocks
of dimension stone quarried by the Romans more than
1500years ago are still absolutely fresh, as shown by traces
of stone working (Figs. S2, S3). In contrast, older fractured
rock surfaces are covered by a dark patina (Fig.3d), the
desert varnish. Accordingly, the structural features, char-
acteristic of different rock types, are much better seen in
Fig. 3 Field relations exposed
in Wadi Umm Sidri and Wadi
Abu Ma’amel, Jabal Dokhan
area: a Branching aplite dike
in dark-colored aphyric rock;
Wadi Umm Sidri; N27°1815.6
E33°1911.7. b Purple veinlets
rich in Mn-epidote in deep red-
dish brown porphyritic Imperial
Porphyry with white plagioclase
phenocrysts. Northwest quarries
(coin diameter = 2.5cm). c
Polished sample of subvolcanic
rocks of Imperial Porphyry
characterized by plagioclase
phenocrysts altered to pink
epidote/piemontite (Width of
view is 8cm). d Transitional
contact between dark greenish
gray Common Porphyry (left)
and purple colored Imperial
Porphyry (right) within a few
centimeters. Lepsius quar-
ries (coin diameter = 2.5cm).
e Greenish gray intrusive
breccia of Common Porphyry,
consisting of fragments up to c.
50cm in diameter in a matrix
of slightly darker color; Lepsius
quarries; length of the hammer
40cm. f Deep purple intrusive
breccia of Imperial Porphyry.
The fragments are purple and
richer in phenocrysts of white
feldspar than the matrix. Lyka-
bettus quarries
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objects of art, rather than in outcrop. Moreover, weathering
under more humid conditions during the Pleistocene has
produced a light to dark brown weathering crust, 1–5mm
thick, on surfaces of joints. Rock faces along the wadi slopes
are covered by extensive drift sheets. These consist of sand,
grit, weathered rock fragments or quarried freestone blocks
up to 500kg in weight, thus documenting the force of ero-
sion produced by occasional rain storms, the last of which
happened in 1999 and 2014.
Rock types andfield relationships
The great majority of the igneous rocks in the Wadi Umm
Sidri and Wadi Abu Ma’amel area is represented by mas-
sive subvolcanic rocks, in part with brecciated structure
(Figs.1, 3e, f and Figs. S4b, d, S5c, e). The predominant
Common Porphyry is a dark green colored porphyritic rock
that contains phenocrysts of white, rarely pale pink, plagio-
clase and dark amphibole in a greenish gray to dark greenish
gray matrix. Rarely, the larger crystals are aligned to form
flow banding. Some rock portions contain amygdales, filled
with secondary minerals, especially pistachio-green epidote
and calcite. Rocks with aphyric texture are rare. They are
exposed in northern Wadi Abu Ma’amel and Wadi Um Sidri,
where they are intruded by a branching aplite dike (Fig.3a).
The Roman quarries are the only ancient source of Impe-
rial Porphyry. This conspicuous variety is distinguished,
from the Common Porphyry, by its magnificent purple color
of its matrix, whereas plagioclase phenocrysts are white or
pale pink (Fig.3b, c). Both rock types adjoin each other with
irregular and diffuse contacts. The width of the gradational
transition zone differs from a few centimeters (Fig.3d) to
several meters (Fig. S7a). Judging from evidence in outcrop
and polished faces on works of art and architecture, most
of the Imperial Porphyry displays a brecciated structure,
showing different types of angular or rounded clasts in a
darker and finer-grained matrix. Poorly defined angular to
slightly rounded clasts, up to 50cm in size, rich in feldspar
phenocrysts, are set with relatively sharp contact in a matrix
that is poorer in feldspar phenocrysts (Fig.3e, f). Moreover,
the Imperial Porphyry is intruded by hydrothermal veinlets
of deep purple color, up to 1cm wide (Fig.3b). These facts
suggest that the matrix of the Imperial Porphyry got its char-
acteristic purple color by a later, pervasive overprint, per-
formed by hydrothermal solutions (see also Makovicky etal.
2016a, b). At the climax of this process, even the plagioclase
phenocrysts attain a pale pink color.
Although the Imperial Porphyry is by far best exposed
in the Roman quarries in the upper parts of the subvolcanic
intrusions, it also occurs down to the lower western faces of
Wadi Abu Ma’amel, resting on top of a granitoid intrusion
(Fig. S7b; see below). However, in these porphyry occur-
rences, a narrow-spaced joint system prevented quarrying
of big blocks. In total, the subvolcanic sequence must have
attained a minimum thickness of 600m, presumably up to
900m.
The Dokhan subvolcanics are intruded, with sharp con-
tact, by coarse-grained, light colored quartz-bearing syen-
ite, characterized by spectacular orthogonal jointing (Fig.
S7c). Xenoliths of dark subvolcanic rock (Fig. S7d) clearly
indicate that the syenite intrusion postdates the subvol-
canic activity. In the southern part of the study area, the
bulk syenite is coarse-grained, due to an intrusion depth
of 600–900m. A porphyritic structure has been attained
only close to the contact with the subvolcanic country
rock, where the syenite is more rapidly cooled. Moreover,
a porphyritic structure is observed also in Wadi Um Sidri
in the northern part of the study area, where the syenite has
intruded a porphyry of common type, obviously at a shal-
lower level. No additional alteration zones are visible in the
immediately adjacent country rocks of Common or Imperial
Porphyry type.
In the study area, the syenite and the Common Porphyry
are dissected, with sharp contacts, by steeply inclined,
approximately N–S trending mafic dikes, 0.7–1.5m, rarely
up to 5m in thickness (Fig.2). These fine-grained dikes,
best exposed near the northeastern water well in Wadi Abu
Ma’amel, are reddish brown and rich in needle-shaped
amphibole. Swarms of NE- to ENE-trending aplitic dikes are
more common in the areas of Imperial Porphyry exposures
(Fig.2). Whereas, due to the hydrothermal activity, the plu-
tonic and subvolcanic rocks described have been subjected,
along fissures, to alteration in deep reddish brown colors, the
younger basic and acidic dike rocks were not affected by this
process. A more detailed description of individual quarries
is given in the electronic supplement.
Microscopic characteristics
The fine grain size of the matrix and the high degree of sec-
ondary alteration does not allow reliable modal analyses of
the rock samples. Consequently, their classification is based
on the bulk rock analyses. The mineral content for selected
samples, based on microscopic observations and X-ray pow-
der diffractometry, is listed in TableS1. Results of repre-
sentative microprobe analyses are summarized in TableS2.
Purple Imperial Porphyry type
These rocks exhibit a well-developed porphyritic texture
with phenocrysts of plagioclase occurring as single crys-
tals or aggregates (Fig.4a, c), but heavily altered into
fine-grained aggregates of sericite + albite + quartz + hem-
atite ± calcite ± pink Mn-bearing epidote ± pink pie-
montite. In addition, phenocrysts of amphibole occur
International Journal of Earth Sciences
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in glomerophyric aggregates (Fig.4b). They are largely
altered to fine-grained, hematite-rich opacite (Fig.4a),
also containing pink Mn-bearing epidote, pink piemon-
tite (Fig.4c), dark mica, chlorite, tremolite and (?) talc.
Sporadically, relics of the original brown amphibole are
observed (Fig.4c). The matrix of the Imperial Porphyry
is extremely fine-grained and consists predominantly of
albite + K-feldspar + quartz + “sericite” ± Mn-bear ing
pink epidote ± pink piemontite ± titanite ± apatite (Fig.4a,
b). Besides ilmenite, hematite is the clearly predominant
ore mineral and occurs as tiny tablets visible in polished
sections and also confirmed by X-ray powder diffraction.
Fig. 4 Photomicrographs of Imperial and Common Porphyry: a Phe-
nocrysts of plagioclase, highly altered to form pale pink Mn-epidote
and albite, and amphibole partially to totally altered to opacite, set
in a fine-grained matrix consisting of quartz, feldspars, platelets of
hematite, and accessory tourmaline (upper right corner); sample D12-
22, Northwest quarries; 1 polar. b Phenocrysts of twinned amphibole
and sericitized plagioclase in a fine-grained matrix of quartz, feldspar
and hematite; sample D12-43, Lykabettus quarries; crossed polars. c
Phenocrysts of brown hornblende, partly altered to opacite and Mn-
epidote in association with those of highly altered plagioclase set in
fine-grained quartzo-feldspathic, hematite-bearing matrix; sample
D12-42, Lykabettus quarries; 1 polar. d Newly grown, distinctly pleo-
chroic crystals of piemontite in fine-grained matrix; sample D12-24,
Northwest quarries; 1 polar. e Porphyritic subvolcanic rock of Com-
mon Porphyry showing phenocrysts of altered twinned plagioclase
and vugs filled with epidote set in fine-grained matrix of plagioclase,
quartz, biotite, epidote and Fe oxides; sample D12-12, crossed polars.
f Common Porphyry with brecciated texture, containing porphyritic
sub-rounded clasts in a matrix consisting of albite, epidote, sericite,
biotite, chlorite, and opaques; sample D12-31, Northeast (Bradford)
quarries; crossed polars
International Journal of Earth Sciences
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Dark green colored common porphyry type
In the common porphyry, phenocrysts of plagioclase
are white to pale green in color and are largely altered to
aggregates of colorless, pale pink or yellowish green epi-
dote + sericite ± calcite + opaques. Phenocrysts of amphi-
bole occur in few samples only and are largely altered to
chlorite + phlogopite ± talc ± bluish-green or colorless
amphibole ± colorless or pale pink epidote; only few relics
of initial brown amphibole are preserved. The matrix con-
sists of very fine-grained aggregates of albite, quartz and
epidote. The distinctly predominant ore mineral is ilmenite,
commonly with minor hematite lamellae, whereas (titano-)
magnetite is rare. At the contact zones between Imperial
Porphyry and Common Porphyry, a transitional type with
reddish brown matrix occurs, but with the same petrographic
composition as the Common Porphyry.
Bulk rock composition
Geochemical analyses of major oxides were carried out by
X-ray Fluorescence (XRF) on 35 representative samples, 16
of the Common Porphyry (including brecciated samples)
and 19 of the Imperial Porphyry (including the archeo-
logical sample WS-E9). From these, 21 selected samples
were analyzed for trace elements and REE by laser abla-
tion inductively coupled plasma-mass spectrometry (LA-
ICP-MS) (TableS3). For analytical methods, see electronic
supplement.
Samples collected in Wadi Umm Sidri revealed signifi-
cant variations in the concentrations of both major and trace
elements, whereas in samples from the other localities, con-
tents of trace elements are nearly identical, and only some
of the major elements show certain variability (TableS3).
In the TAS diagram (Na2O + K2O) vs. SiO2 (Cox etal.
1979), all data points form a relatively wide cluster overlap-
ping the fields of trachyandesite, andesite and dacite (Fig.5a).
In detail, most data points of the Common Porphyry plot into
the dacite field, whereas most points of the Imperial Porphyry
plot into the trachyandesite field or straddle the border to
the dacite field. However, an andesite composition is indi-
cated for nearly all samples using the Al–(Fe + Ti)–Mg cation
plot of Jensen (1976) or discriminations based on relatively
immobile trace elements such as the (Zr/TiO2) vs. (Nb/Y)
diagram of Winchester and Floyd (1976), (Fig.5b). All sam-
ples are characterized by Ni contents of 16–49ppm and Mg#
(100MgO/(MgO + FeOtot)) ratios of 28–37, distinctly lower
than the composition of “primitive” rocks with Ni contents
between 235 and 400ppm (Sato 1977) and Mg# between 63
and 73 (Green 1971). This composition is far from that of
primary mantle-derived magmas indicating substantial frac-
tionation of olivine and/or pyroxenes from the initial magma
composition.
Within the TAS diagram (Fig.5a), but also in dia-
grams using more immobile elements such as Zr, Ti, Nb,
Y (+ Si) (Winchester and Floyd 1976, 1977), the sam-
ples display subalkaline affinity. In the diagram K2O vs.
SiO2 of Peccerillo and Taylor (1976) (Fig. S8a), the data
points scatter within the fields of the calc-alkaline series,
clustering around the boundary between medium-K and
Fig. 5 Geochemical classification of subvolcanic rocks from Wadi
Umm Sidri—Wadi Abu Ma’amel area, Eastern Desert of Egypt: a
Total alkalis–silica diagram (Cox etal. 1979); discrimination between
alkaline and subalkaline after Irvine and Baragar (1971); b selected
enlarged part of Nb/Y–Zr/TiO2 diagram (Winchester and Floyd 1976)
International Journal of Earth Sciences
1 3
high-K series (Rickwood 1989). Calc-alkaline affinity
is clearly supported by further discriminations, e.g., the
diagram FeOtot/MgO vs. SiO2 (Fig. S8b) after Miyashiro
(1974), the Jensen cation plot (Jensen 1976), and the
Th–Hf–Ta–Zr–Nb triangles of Wood (1980). An affiliation
to low-Fe series is verified by the discrimination of Arcu-
lus (2003) in the FeOtot/MgO vs. SiO2 diagram (Fig. S8b).
The chondrite-normalized REE patterns show similar
slopes that decrease from La to Ho and are almost flat from
Er to Lu. There are no conspicuous differences between
the Common Porphyry and the Imperial Porphyry (Fig.6a,
b). Although sample D12-05 shows the same trend, the
enrichment factors are distinctly lower than for all other
samples. In general, the REE patterns may indicate minor
amphibole fractionation. In contrast to the majority of
samples, which do not show a notable Eu anomaly, small
negative anomalies with EuN/EuN* of 0.83 and 0.76 in
samples D12-08 and D12-29 point to subordinate plagio-
clase fractionation (Fig.6a).
The MORB-normalized multi-element variation diagrams
(Pearce and Parkinson 1993) are characterized by positive
anomalies of the LIL elements Rb, Ba and K, the HFS ele-
ments Th, U and Pb as well as Al and Ga, whereas the HFS
elements Ta, Nb, Ti, but also V display negative anomalies
(Fig.6c, d). Particularly the Ta–Nb anomaly underscores the
calc-alkaline character of the samples.
U–Pb zircon dating
Two Imperial Porphyry samples (D12-24 and D12-36) and
one Common Porphyry sample (D12-30) have been dated
using SIMS (Secondary Ion Mass Spectrometry). Zircons
were separated after crushing of the samples, using heavy
liquids and a magnetic separator, then were handpicked
under a binocular microscope to obtain the most transpar-
ent and inclusion-free grains. Microphotography shows
euhedral crystals of zircon with sizes varying between 100
and 200µm. Their internal zoning (Fig.7) was detected
Fig. 6 Cl chondrite-normalized REE patterns for a the Common Por-
phyry type and b the Imperial Porphyry type using the normalization
values of Sun and McDonough (1989). MORB-normalized trace ele-
ment patterns for c the Common Porphyry and d the Imperial Por-
phyry using the normalization values of Sun and McDonough (1989)
and Pearce and Parkinson (1993). The interpolated and extrapolated
line of lowest enrichment factor (elements Nb, Zr, Ti, Y, Yb) is used
to estimate the concentrations of other elements in the mantle wedge
before modification during subduction (Pearce and Parkinson 1993)
International Journal of Earth Sciences
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by cathodoluminescence (CL) using a scanning electron
microscope. Ion microprobe U–Th–Pb analyses were per-
formed using a CAMECA IMS-1280 high resolution ion-
microprobe at the Swedish Museum of Natural History in
Stockholm (Nordsim facility). Details of instrument param-
eters and basic analytical techniques and data reduction have
been given by Whitehouse etal. (1999) and Whitehouse and
Kamber (2005). The zircon U–Pb isotope data are presented
in TableS4. The analytical data are plotted as two-sigma
error ellipses on Concordia diagrams (Tera and Wasserburg
1972), and the calculations were done using the Isoplot rou-
tines of Ludwig (2001).
Fig. 7 Representative CL images with 206Pb/238U ages (in Ma) of zircon grains from the analyzed Imperial and Common Porphyry samples
International Journal of Earth Sciences
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Sample D12-24 (27°1458.68N, 33°1628.71E) zircons
are subhedral to euhedral and yellow to pale brown, and
exhibit well-preserved oscillatory growth zoning (Fig.7).
A total of 15 measurements were carried out on 15 zir-
cons (TableS4; Fig.8a). The U content varies from 19 to
241ppm, Th from 11 to 175ppm, and the Th/U ratios are
high (0.40–1.34), as expected for magmatic zircons (Corfu
etal. 2003). In general, out of 15 zircon grains analyzed,
10 yielded a concordia age of 609 ± 4Ma [2σ] (Fig.8a)
based on a group of 10 concordant and equivalent analy-
ses (MSWD = 1.7). The rejected analyses were excluded
from age calculation because three analyses are discord-
ant (zr-4, zr-6 and zr-10), one analysis (zr-3) has a low U
content (19ppm) and shows a large uncertainty (± 55Ma)
[2σ] (TableS4). One spot (zr-14) is a concordant grain and
yields an older 206Pb/238U age (713 ± 5Ma) [2σ] than the
other analyses, which is interpreted to represent an inherited
zircon grain (TableS4).
Sample D12-36 (27°1451.81N, 33°1825.89E) zir-
cons are euhedral (100–200µm) and yellow to pale brown.
Cathodoluminescence (CL) images (Fig.7) show well-
developed zoning as expected for magmatic zircons. One
measurement was carried out on each of the 15 grains
(TableS4). Their U contents vary from 56 to 167ppm, Th
contents from 22 to 137ppm and Th/U ratios from 0.16 to
0.97 (TableS4). 12 analyses cluster tightly, defining a con-
cordia age of 600.4 ± 2.4Ma (2σ; MSWD = 0.96; Fig.8b).
Two spots (zr-7 and zr-11) show reverse discordance and
one spot (zr-1) is a concordant grain and yields an older
206Pb/238U age (623 ± 4Ma), perhaps reflecting a xenocryst
derived from older material (Fig.8b).
Sample D12-30 (27°162.57N, 33°185.04E) zircons
are euhedral (100–200µm) and yellow to pale brown. One
measurement was carried out on each of the seven grains
(TableS4). U contents vary from 101 to 212ppm, Th con-
tents from 26 to 117ppm and Th/U ratios from 0.24 to 0.74.
Four analyses cluster tightly, defining a concordia age of
608 ± 6Ma (95% confidence, MSWD = 1.9; Fig.8c).One
spot (zr-4) is discordant and distinctly younger than the
other six analyses. Two spots (zr-1 and zr-2) are concord-
ant grains and yield older 206Pb/238U ages of 786 ± 4 [2σ]
and 828 ± 4 Ma [2σ], respectively, perhaps representing
xenocrysts derived from older material (Fig.8c).
Discussion
Geotectonic setting andpetrogenesis
All samples show similar rare earth and trace element pat-
terns (Fig.6), thus indicating that they are derived from
a similar source and/or have crystallized from a common
parental magma. Based on their subduction-related trace
element signature and the negative anomalies of certain
HFS elements such as Nb, Ta and Ti, we conclude that the
porphyritic andesite to dacite, both of Common and Impe-
rial Porphyry types, were produced by partial melting of a
prior subduction-modified upper mantle, either within an
Fig. 8 Inverse (Tera–Wasserburg) Concordia diagrams for SIMS zir-
con U–Pb data from Imperial and Common Porphyry samples
International Journal of Earth Sciences
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active subduction zone or a later transtensional tectonic
regime (e.g., Johnson etal. 2011). Such transtensional tec-
tonic regime is confirmed from the Th/Yb vs. Ta/Yb ratios
plot (Fig. S9), where the Common and Imperial porphyries
straddle the within plate and active continental margin geo-
tectonic fields (Gorton and Schandl 2000). Lower NbN/LaN
ratios (0.31–0.40, average 0.35) and Zr/Ti ratios of the sub-
volcanic samples (0.02–0.05, average 0.03) further confirm
the arc settings (Pearce 1982; Pearce and Peate 1995; John
etal. 2004; Pearce and Stern 2006). Higher Th/Yb ratios of
the subvolcanic samples investigated (2.1–6.6, average 3.1)
point to continental arc setting, where Condie and Kröner
(2013) used a ratio of 0.5 for basalts to discriminate between
oceanic and continental arcs. Positive Pb and Ba anomalies
may be attributed to a Pb- and Ba-rich crustal component in
the mantle source, such as subducted pelagic (meta-)sedi-
ments or interaction with granitic magmas. This conforms
to the model, evaluated for the formation of the Dokhan
magmas by Stern etal. (1988) and Moghazi (2003).
Field evidence, such as the regional distribution of the
Hammamat sediments and the Dokhan volcanic suite in
broad shallow basins (e.g., Eliwa etal. 2010; Bühler etal.
2014), as well as the ENE–WSW trending dike swarms,
indicates that the magmas were emplaced into a N-to-NW-
directed extensional crustal regime, at least between c. 600
and 575Ma (Stern etal. 1988, and references therein). Such
a geotectonic position testifies to a highly extended terrane
(HET) as originally characterized by Olsen and Morgan
(1995). This is underscored by the calc-alkaline, subduction-
related geochemical signature of the igneous rocks in the
Eastern Desert of Egypt, which is similar to the “type area”
for HET, the Basin and Range Province (Zoback etal. 1981,
and references therein, cf.; Wang and Shu 2012). Occasion-
ally, HET syn-subduction or post-subduction extensional
regime was misleadingly interpreted to be a rift or having
a rifting phase (e.g., Burke 1978), a term also applied to
the Dokhan volcanic suite by Moghazi (2003). However,
the term rift is currently used synonymously to continental
rift (Olsen and Morgan 1995), a tectonic process associated
with magmatism of SiO2-undersaturated basalt to trachyte
as well as nephelinite–carbonatite composition (e.g., Wil-
son 1989). The proposed geotectonic model is similar to the
model assuming an extensional regime after crustal thicken-
ing by Stern etal. (1984, 1988), Stern and Gottfried (1986);
Mohamed etal. (2000), and it is distinct from the compres-
sional subduction regime model of Abdel-Rahman (1996)
and the model of El Gaby etal. (1989) for the formation of
the Dokhan magmas.
Intrusion mechanism
Based on their erosion forms with morphological steps
as typically found in lava plateaus, the Common and
Imperial Porphyry were interpreted as extrusive lava flows
and pyroclastic deposits during former studies (Mohamed
etal. 2000; Wilde and Youssef 2000; Moghazi 2003; Eliwa
etal. 2006). However, Horseman etal. (2005, 2010), Gud-
mundsson (2012), and Menand (2011) present and illus-
trate a type of repeated magma emplacement in subvol-
canic magma chambers, in which the individual magma
pulses form sheets of nearly constant thickness within
multiple sills. In our new interpretation, the occurrences
of Common and Imperial Porphyry in the Wadi Umm-
Sidri and Wadi Abu Ma’amel area most likely were formed
by such a subvolcanic intrusion mechanism. The parental
magma was derived from a mantle source and stored in a
magma chamber in the lower crust, where it underwent
minor lower crustal contamination and minor fractional
crystallization of plagioclase and amphiboles. During an
age interval between 609 and 600Ma, this magma was
then emplaced as subvolcanic magma pulses in a tran-
stensional tectonic regime to form multiple sills beneath
earlier erupted parts of the Dokhan Volcanic suite. The lat-
ter are dated to a volcanic period between 628 and 615Ma
(Breitkreuz etal. 2010) and occur just to the north of the
study area.
Interpretation oftheU–Pb zircon data
U–Pb SIMS dating on single zircons yielded ages of
600 ± 2 and 609 ± 4Ma for two samples of Imperial Por-
phyry and 608 ± 6Ma for a sample of Common Porphyry,
all interpreted as crystallization ages. The age difference
between the two Imperial Porphyry samples implies that
these may have derived from two different sills of some-
what older and younger age, respectively.
Our age data are consistent with the SHRIMP U–Pb
zircon data of 602 ± 9 and 593 ± 13Ma of the Imperial
Porphyry (Wilde and Youssef 2000). However, U–Pb
SHRIMP ages obtained for zircons from other localities
of the Dokhan volcanics cover a much wider range of
630–580Ma (Wilde and Youssef 2000; Breitkreuz etal.
2010), broadly overlapping with the ranges of the Rb–Sr
whole rock ages for volcanic rocks (610–560Ma) and
granites (610–550Ma). There is no doubt that igneous
activity took place, towards the end of the Pan-African
Orogeny, in a dynamic setting around isolated volcanic
centers and basin systems with different structural con-
trols and different ages (Breitkreuz etal. 2010; Johnson
etal. 2011). Contemporaneous to the intrusion of the post-
collisional, calc-alkaline I- to A-type Younger Granites is
the last phase of regional metamorphism of the crystal-
line basement in the Sinai Peninsula (Abu El-Enen and
Whitehouse 2013) and the Eastern Desert (Abu El-Enen
etal. 2016).
International Journal of Earth Sciences
1 3
The coloring oftheImperial Porphyry
One of the most interesting features of the Imperial Por-
phyry is the presence of pink piemontite (Fig.4d) and pale
pink, Mn-bearing epidote. Both minerals either replace
plagioclase phenocrysts as granular aggregates, giving the
plagioclase a patchy color distribution from pure white to
pale pink (Fig.3b, c), or they form coarser grained aggre-
gates, which occur as isolated patches, irregularly distrib-
uted in the fine-grained matrix (Fig.4d). Some authors
assume that the presence of piemontite is responsible for
the impressive purple color of the Imperial Porphyry as a
whole (Ghobrial and Lofti 1967; Basta etal. 1978; Klemm
and Klemm 2001b). The main reason for the purple color
of the Imperial Porphyry, however, is a dust of tiny hema-
tite tablets, confirmed by X-ray diffraction that are widely
and regularly distributed in the matrix of the rock. Con-
versely, the dark grayish-green Common Porphyry rather
contains fine-grained magnetite/ilmenite, whereas hema-
tite is subordinate. Such an interpretation was already pre-
sented by Dardir and Abu Zeid (1972) who have stressed
that Mn-epidote is found in the common, green varieties
of the Dokhan volcanics as well. On the other hand, Mak-
ovicky etal. (2016a) assigned the red coloration of the
Imperial Porphyry to both the presence of piemontite and
hematite dusting.
The transformation of the Common Porphyry to the
Imperial Porphyry type can be explained by a pervasive
hydrothermal-metasomatic overprint, leading to a wide-
spread formation of hematite in the rock matrix. Such
a process is also indicated by veins filled with purple-
colored piemontite (width up to 1cm, Fig.3b), spreading
out from fissures into the adjacent rock and thus forming a
metasomatic front. The replacement of plagioclase crystals
by Mn-epidote and piemontite may testify to an intensified
metasomatic process.
Gradational contacts between the Common and Imperial
Porphyries and the distribution of the Imperial Porphyry
exposures along ENE- and NNW-trends (Fig.2), parallel to
the nearby Younger Granites indicate that the hydrothermal
alteration around Mons Pophyrites (Fig.2) can be related
to the intrusion of the Younger Granites (e.g., Basta etal.
1978). These could have acted as a heat source for trans-
forming meteoric water into hydrothermal fluid, which pref-
erably moved along joints and faults. An interesting model
has been proposed by Makovicky etal. (2016a,b) who relate
the coloring to the increasing oxidation potential of the
Earth’s atmosphere during the Second Great Oxidation event
in late Neoproterozoic times (e.g., Frei etal. 2009; Campbell
and Squire 2010). However, the gradual transition from the
subvolcanic rocks of the predominant Common Porphyry
type into the Imperial Porphyry type clearly testifies to a
process of local rather than of global scale.
Conclusions
Based on field evidence, petrographic and geochemical
characteristics, as well as on the U–Pb zircon SIMS ages,
we propose the following model for the evolution of the
Dokhan subvolcanic rocks of both, Common and Imperial
Porphyry type:
Stage 1 An andesitic magma was formed by partial
melting of a subduction-modified upper mantle.
Stage 2 The andesitic magma underwent differentia-
tion towards dacite to trachyandesite composition by
fractionation of minor amphibole and, if any, of minor
plagioclase and, in addition, was modified by minor
contamination with subducted sediments and/or admix-
ture of granitic magmas.
Stage 3 At about 610–600Ma, these compositionally
modified magmas intruded into a highly extended ter-
rane (HET) within a transtensional tectonic regime,
thus forming multiple sills of nearly constant thickness.
The presence of repeated magma pulses is indicated
by the brecciated structure, visible in the field and on
many objects of art and architecture.
Stage 4 Epidote, formed by propylitic hydrothermal
alteration, produced the dark gray to greenish gray
color of the matrix in subvolcanic rocks of Common
Porphyry type.
Stage 5 Due to pervasive hydrothermal alteration under
oxidizing conditions, fine-grained flakes of hematite
were formed in the amphibole phenocrysts and the
matrix of the Imperial Porphyry, thus producing its
spectacular purple color. Between the two rock types,
gradational transitions exist.
Stage 6 Ongoing pervasive hydrothermal alteration,
combined with emplacement of hydrothermal veins,
locally overprinted the Imperial Porphyry that was
formed during stage 5. A more pronounced Mn-mobi-
lization yielded Mn-epidote and piemontite that fill fis-
sures and the cores of plagioclase (Fig.4c).
The metasomatic alteration of stages 4–6 was caused
by hydrothermal fluids, presumably formed by heating of
meteoric water, whereby the contemporaneous Younger
Granite sills may have acted as a heat source.
Acknowledgements The authors thank Hesham Sallam and Hassan
Eliwa for their field assistance. Sample DKK from Mons Porphyrites
was collected and kindly provided to this study by Rosemarie and
Dietrich D. Klemm (Dießen, Germany). Thanks to Martin Whitehouse,
Kerstin Lindén and Lev Ilyinsky, Swedish Museum of Natural History
Stockholm, for their help with CL-images and zircon isotope analy-
ses. Field work was partially supported by the Mansoura University,
Egypt, which is gratefully acknowledged. We highly appreciate the
International Journal of Earth Sciences
1 3
useful suggestions of the editor and two reviewers Peter Johnson and
Ghaleb Jarrar.
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... One of the most striking features of the ANS is the abundance of post-collisional plutons and associated volcano-sedimentary sequences, whereas older rocks, now comprising parts of metamorphic complexes, ophiolites and island arc assemblages, are less well represented. Rocks of the post-collisional phase have U-Pb zircon ages ranging from ca. 635 to 580 Ma (Moussa et al. 2008;Breitkreuz et al. 2008;Ali et al. 2009a;Azer et al. 2010;Be'eri-Shlevin 2009aAbu El-Enen et al. 2018). The magmatic history of the post-accretionary stage of the ANS includes two major magmatic episodes: the calc-alkaline phase (630-610 Ma) and the alkaline phase (610-580 Ma) (Beyth et al. 1994;Bentor and Eyal 1987;Jarrar et al. 2003;Ali et al. 2009a;Be'eri-Shlevin et al. 2009aEyal et al. 2010). ...
... Old geochronological methods applied to the post-accretionary volcanic rocks in the ANS, such as Rb-Sr whole rock, gave inaccurate and scattered ages (e.g., El-Ramly 1962;El-Shazly et al. 1973); therefore, we exclude most of these data from Table 20.1. On the other hand, modern geochronology techniques using zircon U-Pb have been applied by many authors and give more precise ages Youssef 2000, 2002;Kennedy et al. 2004Kennedy et al. , 2005Be'eri-Shlevin et al. 2009bMoghazi et al. 2012;Andresen et al. 2014;Abu El-Enen et al. 2018;Abd El-Rahman et al. 2019). The modern techniques applied to the post-accretionary volcanic rocks in the ANS are based mainly on SIMS (SHRIMP and CAMECA) secondary ion mass spectrometry and LA-ICP-MS (laser ablation inductively coupled plasma mass spectrometry). ...
... It represents the first igneous rock identified from the subduction stage of the ANS that was unquestionably derived from old continental crust and confirms the presence of an old continental crust substrate beneath the ANS. This diagram is based on published geochronology, geochemical and Nd isotope data (Kröner et al. 1992;Ali et al. 2009b;Moussa et al. 2008;Wilde and Youssef 2000;Ali et al. 2010;Abu El-Enen et al. 2018). ...
Chapter
The Arabian–Nubian Shield (ANS), formed by collision between East and West Gondwana during the Pan-African orogeny, serves as an excellent example of the product of major crustal accretion processes. One of the most striking features of the ANS is the widespread abundance of post-collisional plutons and associated volcano-sedimentary sequences. The magmatic history of the post-accretionary stage of the ANS includes two major episodes, both Ediacaran in age: the earlier calc-alkaline phase (635–610 Ma) and the later alkaline/peralkaline phase (610–580 Ma). Each of these magmatic episodes includes plutonic rocks and their volcanic equivalents. Early calc-alkaline volcanism was emplaced during two cycles of eruption of medium- to high-K calc-alkaline volcanic rocks, each accompanied by deposition of immature clastic sediments that remain undeformed and unmetamorphosed. The volcanic rocks of the early episode include intermediate to felsic subaerial lava flows, tuffs and ignimbrites, as well as subvolcanic bodies associated with minor basalt. The mostly high-K calc-alkaline character and other traits previously interpreted to indicate arc magmatism may simply reflect remelting of earlier arc-related material from the pre-accretionary stage (850–740 Ma). Some of the more evolved early episode calc-alkaline volcanic sequences are transitional to alkaline A-type, but this is interpreted to reflect extensive fractionation of I-type parental magmas rather than a switch in the tectonic regime. In addition, some of the early calc-alkaline volcanic rocks are adakitic in character, perhaps recording early manifestations of lithospheric delamination, including melting of the mafic lower crust. The late alkaline/peralkaline volcanic units were emplaced after the termination of the Pan-African Orogeny. They are represented by alkali rhyolite, comendite and pantellerite flows, with abundant ignimbrites and pyroclastic deposits. They were emplaced during a non-orogenic period associated with tensile stresses, block faulting and differential uplift. During the closing stages of this volcanic phase, large-scale caldera subsidence occurred and ring dykes were injected into the bordering fractures. There is some overlap in the emplacement of calc-alkaline and alkaline rocks between 610 and 590 Ma, and peralkaline magmatism also extended into the early Cambrian (~530 Ma). The overlap period of coeval calc-alkaline and alkaline volcanic series implies that, on a regional scale, two magma sources coexisted during a complex transitional phase in the evolution of the ANS.
... In the case of lithologies used as ornamental stone, they are valued for their aesthetics and their attractiveness and are generally used as a sign of distinction and just sometimes power, such as Roman porphyry (Abu El-Enen et al. 2018). The international interest in certain lithologies is well known, as is the case of Carrara marble (Primavori 2015) (Pereira and Cárdenes Van den Eynde 2019). ...
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Construction and ornamental stones are important elements of cultural heritage and geoheritage. The quarries, where these materials are extracted, are a type of site that combines these two types of heritage. Both the ornamental character of the rock and its place of origin can be deeply rooted in the local society. Red Ereño is a red micritic limestone (Lower Cretaceous) with abundant white rudist fossil shells. This stone has been exploited since Roman times in the north of the Iberian Peninsula (Basque Country, Spain) and exported internationally. The main quarry related to the extraction of Red Ereño , Cantera Gorria , is currently a cultural and geoheritage site. This emblematic site brings together numerous geologic (palaeontological, petrological, geomorphological and tectonic) and mining features that make it a reference point for both research and teaching activities. The link between geoheritage and cultural heritage that exists in Cantera Gorria is evident and makes this place an essential point for dissemination of geology as well as for tourism. The quarry is currently abandoned, and because of this, there is an urgent need for its protection and development, and in order to increase awareness of its importance and potential use.
... The marble used for the floor and ceiling was quarried from Anatolia (present-day eastern Turkey) and Syria, while bricks (used in the walls and parts of the floor) are from different construction phases with different clays composition [126]. The interior of Hagia Sophia is lined with enormous stone slabs sourced from multilple places: Green marble of Karystos (Greece), rose-colored marble from Phrygia (Turkey) [127], red Imperial porphyry from Egypt [128], Green porphyry from Sparta (Greece), buff lassikos from Caria (Turkey), white-yellowish marble from Lydia (Turkey), gold-colored marble from Libya, chunky black and white Celticum breccia (so-called marmum celticum) from France, honey-colored onyx from Pamukkale (Turkey), green Verde Antique serpentinite breccia from Thessaly (Greece) [129,130], white marble from Proconnesos (Turkey) [131] and the grey-colored marble from Vosporos (Greece) [132]. Hagia Sophia's columns were brought from the temple of Artemis in Ephesus (Turkey), from Egypt, and from other locations of the Byzantine Empire. ...
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Human activity has required, since its origins, stones as raw material for carving, construction and rock art. The study, exploration, use and maintenance of building stones is a global phenomenon that has evolved from the first shelters, manufacture of lithic tools, to the construction of houses, infrastructures and monuments. Druids, philosophers, clergymen, quarrymen, master builders, naturalists, travelers, architects, archaeologists, physicists, chemists, curators, restorers, museologists, engineers and geologists, among other professionals, have worked with stones and they have produced the current knowledge in heritage stones. They are stones that have special significance in human culture. In this way, the connotation of heritage in stones has been acquired over the time. That is, the stones at the time of their historical use were simply stones used for a certain purpose. Therefore, the concept of heritage stone is broad, with cultural, historic, artistic, architectural, and scientific implications. A historical synthesis is presented of the main events that marked the use of stones from prehistory, through ancient history, medieval times, and to the modern period. In addition, the main authors who have written about stones are surveyed from Ancient Roman times to the middle of the twentieth century. Subtle properties of stones have been discovered and exploited by artists and artisans long before rigorous science took notice of them and explained them.
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For centuries, Roman emperors ruled a vast empire. Yet, at least officially, the emperor did not exist. No one knew exactly what titles he possessed, how he could be portrayed, what exactly he had to do, or how the succession was organised. Everyone knew, however, that the emperor held ultimate power over the empire. There were also expectations about what he should do and be, although these varied throughout the empire and also evolved over time. How did these expectations develop and change? To what degree could an emperor deviate from prevailing norms? And what role did major developments in Roman society – such as the rise of Christianity or the choice of Constantinople as the new capital – play in the ways in which emperors could exercise their rule? This ambitious and engaging book describes the surprising stability of the Roman Empire over more than six centuries of history.
Chapter
In the previous chapters of this book, the different types of mineral deposits and occurrences have been reviewed in the countries covered by the Arabian–Nubian Shield (ANS): Saudi Arabia, Yemen, Eastern Desert of Egypt, Sudan, Eritrea, and Ethiopia. The ANS represents one of the Earth’s largest blocks of juvenile Neoproterozoic crust, which is also one of the largest repositories of Neoproterozoic metallic minerals.
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The gabbro-diorite complex of the Um Balad prospect hosts lode gold mineralization. The complex is dated at 723 ± 4 Ma using the LA-ICP-MS U-Pb zircon method and is correlated with the late Tonian-early Cryogenian subduction-related magmatic stage during the evolution of the Arabian-Nubian Shield. The gabbro-diorite complex evolved through the crystallization of a calc-alkaline magma and the subduction signature of this magma is verified by primitive mantle-normalized trace element patterns that show enrichment in large ion lithophile elements, U, and Th relative to high field strength elements as well as negative Nb and Ti anomalies. The high La/Ybcn (3.1–9.4) and Ta/Yb (0.12–0.41) ratios are consistent with a continental arc rather than an oceanic arc system. Amphibole chemistry indicates that this complex might have crystallized under a moderately oxidizing condition from a hydrous magma (>6 wt% water content) at temperature and pressure estimates of about 800°C and 3 kbar, respectively. Gold mineralization in the Um Balad prospect is confined to structurally controlled massive quartz±carbonate veins and surrounding alteration halos. Alteration in the prospect is represented by localized sericitization and carbonation as well as pervasive chlorite-sericite alteration. The alteration halos are characterized by enrichment in K and Rb and depletion in Ca and Sr compared to their host rocks. The veins of the prospect are related to lower order extensional fractures associated with the regional first order transpressional Najd Fault System. The high Fe contents of the gabbro-diorite complex represent a suitable chemical trap for gold through sulfidation of the host rocks. Supergene alteration resulted in the formation of goethite in association with atacamite and chrysocolla. Free mill gold is associated with these supergene phases, which were deposited in near neutral to slightly alkaline conditions.
Article
ABSTRACT. In the northernmost segment of the Arabian–Nubian Shield, a postcollisional high-K calc-alkaline volcanic sequence is exposed along Wadi Abu Ma’amel, Eastern Desert of the Nubian Shield. It comprises a series of intermediate to silicic volcanics and associated pyroclastics that include the Imperial Porphyry and calc-alkaline volcanics typical of the Dokhan Volcanics. The Imperial Porphyry occurs as subvolcanic sill-like intrusions forming the young member of the Dokhan Volcanics. The volcanic sequence extruded through synorogenic granite and was intruded by post-collisional granite, which also caused thermal contact metamorphism. The red and purple colors of the Imperial Porphyry reflect hydrothermal alterations, which resulted in the formation of dispersed flakes of hematite, epidote, and piemontite. The entire high-K calc-alkaline volcanic sequence, ranging from andesite through dacite and rhyodacite, exhibits post-collisional geochemical characteristics. Most samples of the Imperial Porphyry and some of the typical Dokhan Volcanics have characteristics of adakitic rocks, including high Sr (694–889 ppm), low Y (10.6–18.8 ppm), high Sr/Y (41.1–83.8), (La/Yb)n (8.6–15.6), and low (Yb)n (5.4–9.0). The mostly calc-alkaline character and other traits of the studied volcanics that were previously interpreted to indicate arc magmatism reflect, instead, remelting of earlier (pre-collisional) arc-related material. The formation of Wadi Abu Ma'amel volcanics resulted from upwelling of hot asthenospheric material during thinning of the previously thickened lithosphere as a consequence of lithospheric delamination. The parental magma was generated by partial melting of mafic lower crust that mixed with upper-crust-derived magma. It evolved mostly through fractionation of clinopyroxene and plagioclase, accompanied by apatite and Fe–Ti oxides in the more-evolved dacitic and rhyodacitic rocks.
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A thorough characterisation of porphyry from China (Province of Fujian) was made, regarding three chromatic variants: red, brown and grey. The properties object of study are: petrographic, chemical and mineralogical analysis, real and apparent density as well as open and total porosity, water absorption at atmospheric pressure, resistance to salt crystallization, rupture energy, compressive strength, flexural strength, abrasion resistance and slip resistance. The achieved results remain into the expected ones for this kind of stone. Nevertheless, small differences were found according to the colour of the sample. Finally, those properties which are covered in the CE marking were compared with the representative values of commercial samples from countries as Italy, Argentina and Mexico.
Chapter
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ABSTRACT Volcanic arc basalts are all characterized by a selective enrichment in incompatible elements of low ionic potential, a feature thought to be due to the input of aqueous fluids from subducted oceanic crust into their mantle source regions. Island are basalts are additionally characterized by low abundances [for a given degree of fractional crystallization) of incompatible elements of high ionic potential, as feature for which high degrees ot'melting, stability of rninor residual oxide phases, and remelting of depleted mantle are all possible explanations. Calc-alkaline basalts and shoshonites are additionally characterised by enrichment of Th, P and the light REE in addition to elements of low ionic potential, a feature for which one popular explanation is th contamination of their mantle source regions by a melt derived from subducted sediments. By careful selection of variables, discrimination diagrams can be drawn which highlight these various characteristics and therefore enable volcanic arc basalts to he recognized in cases where geological evidence is ambiguous. Plots of Y against Cr, K[Yb, Ce/Yb, or Th/Yb against Ta/Yb, and Ce/Sr against Cr are all particularly successful and can be modelled in terms of vectors representing different petrogenctic processes. An additional plot of Ti/Y against Nb/Y is useful for identifying 'anomalous' volcanic arc settings such as Grenada and parts of the Aleutian arc. Intermediate and acid rocks from volcanic are settings can also be recognized using a simple plot of Ti against Zr. The lavas from the Oman ophiolite complex provide a good test of the application of these techniques. The results indicate that the complex was made up of back-arc oceanic crust intruded by the products of volcanic arc magmatism.
Book
Mons Porphyrites, in the heart of the Red Sea mountains which dominate the Eastern Desert of Egypt, was the only source of imperial porphyry known to the ancient world. The quarries seem to have been worked from the Tiberian period until the early fifth century AD. A five-year programme of investigation of the quarries was undertaken between 1994 and 1998 and the first volume on the topography of the area appeared in 2001 (EES Excavation Memoir 67 by V A Maxfield and D S Peacock). This second volume includes reports of the excavations and provides a review of the overall development of the quarry complex. 450p, b/w illus (Excavation Memoirs 82, Egypt Exploration Society 2007)
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Rifts occur at all stages of the Wilson Cycle of opening and closing of oceans, but those most likely to be preserved form either at continental breakup or continental collision. All preserved rifts are “failed rifts” because they have not “succeeded” in developing into oceans. Hundreds of ancient rifts ranging up to 2.6 Ga in age have been recognized. Axial mafic dike systems have been located in both ancient and active rifts and activation of the basalt-eclogite transition in these systems may be an effective way of promoting the repeated subsidence that characterizes many rift complexes, such as those of the North Sea. Ancient rifts striking into fold belts are known as aulacogens (after Shatski). Recognition (after Wilson) that many fold belts mark places where oceans have closed has made it possible to interpret numerous aulacogens as rifts formed at continental rupture and to interpret their history in plate-tectonic terms. In many ways the best known of rift systems are those formed at the opening of the Atlantic Ocean because they have been subjected to intense petroleum exploration. Eleven classes of rifts with diverse histories can be distinguished among the rifts presently facing Atlantic-type continental margins and aulacogen histories are even more diverse.
Article
The prestigious red Imperial Porphyry was quarried from Mons Porphyrites in the Red Sea Mountains of Egypt. The porphyry, reserved for imperial use in Rome and Constantinople, was widely reused in Romanesque and Renaissance times, and in the Ottoman Empire. At the locality, the rocks vary from dark grey to red and are characterized by abundant, weakly aligned white to pink feldspar phenocrysts. The magmatic phenocrysts - plagioclase, hornblende, pyroxene, opaque components and apatite - are always altered. The red colour of the porphyry stems from alteration of phenocrysts and groundmass which generated Mn-epidote or piemontite (both rich in Mn3+) and a hematite dusting (dominated by Fe3+). Plagioclase relics consist of plagioclase (An2-47) and microcline (< 5-6 % Ab); they recrystallized during alteration processes into a mixture of sodium feldspar with segregations of epidote group minerals, K-feldspar, and minor anorthite and calcite. Rare pyroxene retains its primary morphology but is completely altered to epidote group minerals. Primary hornblende lies along the magnesiohastingsite-edenite join but recrystallized to low-Na, low-AlIV magnesiohornblende, and tremolite-actinolite. Primary oxide grains occur as exsolved ilmenite-titanomagnetite; they recrystallized to hematite or hematite-magnetite mixtures. The rocks are not pervasively recrystallized and retain a spectrum of magmatic textures. Alteration produced epidote-group minerals, notably Mn3+-containing epidote and more rarely piemontite. Both are pleochroic from pink to yellow; the depth of pleochroism increases in the reddest porphyries. Other metamorphic-grade minerals include tremolite-actinolite, aluminian titanite, phlogopite, muscovite, chlorite and chalcedony. These phases indicate essentially isochemi-cal greenschist facies conditions which took place under relatively high oxygen fugacity. Mineralogical observations, rock colour and texture, and particularly the pleochroic piemontite, should allow archaeologists to reliably assign pieces of Imperial Porphyry to their Egyptian source. Elemental and isotope geochemistry of the Imperial Porphyry is described in Part II. © 2015 E. Schweizerbart'sche Verlagsbuchhandlung, Stuttgart, Germany.