The following report was published in: Newsletter - "Pan-African Crustal Evolution in the Arabian-Nubian Shield", International Geological Correlation Programme Project 164, Number 3, March 1980.
GEOLOGY OF THE JABAL IDSAS - JABAL TAYS - JABAL ZRIBA
AREAS IN THE EASTERN ARABIAN SHIELD
The report is based upon a five day visit (Feb. 20th - March 1st, 1980) to the Jabal Idsas region of the Eastern Arabian Shield organized by the Faculty of Earth Sciences of King Abdulaziz University, Jeddah, as part of IGCP Project 164. (N. Jackson and J. Roobol - Notes on the Geology of the Jabal - Idsas - Jabal Tays areas in the Eastern Arabian Shield, for an IGCP Project 164 Field Excursion between Feb 26th and March 2nd 1980, Generalized map of the Eastern Arabian Shield based on a Riofinex report : A preliminary review of mineral opportunites in Saudi Arabia, 1977.)
All localities mentioned in the following text, along with the Riofinex map, have been indexed in the Google Earth file 'Saudi Arabia.kml' (+ associated image file) available at: http://instruct.uwo.ca/earth-sci/fieldlog/Google_Earth/ . [Click the file and save it to your c: drive, e.g. saudi.zip; unzip the file into e.g. c:\saudi, and load the .kml file into Google Earth.] The Riofinex map has been registered to Google with a reasonable degree of accuracy. A Google overlay based on the maps of Kahr et al. 1972, USGS Saudi Arabian Reports 124 and 126, and of later workers Al Shanti and Mitchel, 1976, Nawab, 1979, and the 1980 IGCP field reconnaissance, is in preparation.
Within the general Al Amar - Idsas area 14 tectono-lithostratigraphic units were
1) Paragneiss (Wadi Jifr)
2) Volcanic sequence I (east of the main ophiolite belt)
3) Volcanic sequence II (west of the main ophiolite belt)
4) Conglomerate unit I (Jabal Bitran)
5) Conglomerate unit II (N.N.W. of Fawara; = Idsas conglo-merate)
6) Conglomerate unit III (Jabal Zriba)
7) Mudstone-sandstone unit (main belt of the Abt Formation)
8) Mudstone-pebbly mudstone unit I (Jabal Tays)
9) Mudstone-pebbly mudstone unit II (east of main ophiolite belt; S.E. of Fawara village )
10) Coarse sandstone unit I (Jabal Zriba)
11) Coarse sandstone unit II (east of main ophiolite belt; N.W. of Fawara village)
12) Ophiolite unit I (Jabal Tays)
13) Ophiolite unit II (west of Jabal Bitran)
14) Ophiolite unit III (Fawara to Jabal Rugan)
Paragneiss occurs as a large pendant within granodiorite north of Wadi Jifr at
lat. 23° 27' long. 45° 21'.
Centimetre-scale layering in the gneisses reflects the presence of varying proportions of quartz, feldspar and biotite (samples WJ. 80. 5, 6). The existence of calc-silicate layers (sample W.J. 80. 1: quartz-plagioclase amphibole-garnet-calcite-imenite; mineral analyses below) within the gneiss suggests they are of sedimentary origin, whereas the abundance of K-feldspar in some of the layers indicates a continental source.
The gneisses, although in places tightly folded (sample W.J. 80. 4), are annealed, micas in the rocks exhibiting no strong preferred orientation. However, near their western contact with the surrounding granodiorite, the gneisses are strongly schistose, as is the granodiorite. Further study of the relative deformation history of the gneisses and the granodiorite is clearly warranted.
The gnesisses are transected by a younger set of mafic dikes: further study of these rocks would probably also prove fruitful.
At least two volcanic lithostratigraphic units other than those associated, with
the ophiolite complexes can be recognised in the general area examined between
Jabal Bitran and Jabal Rugaan.
Volcanic sequence I, which constitutes the main area of volcanic rocks located east of the Al Amar - Idsas line, is mostly composed of mafic to intermediate lavas and intrusive sheets, with lesser poorly defined units of conglomerate (conglomerate unit I) and cherts. The sequence is steeply dipping and transected by shear zones which impart a low grade schistosity to the rocks. Volcanic sequence II lies to the west of the main ophiolite belt, extending between Fawara and Jabal Rugaan. The sequence appears to be dominantly silicic in character, and relatively unmetamorphosed and undeformed. No observations were made concerning the age and structural relationship of sequences I and II, either to each other, to the ophiolite complexes, or to the Abt Formation.
The main belt of sediments in the Al Amar-Idsas region is represented by the Abt Formation, a sedimentary unit composed largely of mudstones and sandstones. Although folded and cleaved, rocks of the Abt Formation appear little metamorphosed (sericite - chlorite grade )
and the sandstones in particular show only signs of incipient recrystallization.
Sedimentary structures such as graded bedding and other parts of the Bouma
sequence, and flame and load structures, are discernable in the more sandy
facies of the Abt formation outcropping along the eastern margin of the main
belt of Abt rocks. To the west the Abt Formation seems to be largely composed of
hemipelagic deposits with thin interbeds of sandstone. However, in the vicinity
of Jabal Tays, associated with the mudstones are conglomeratic channel deposits
which grade into pebbly mudstones (Mudstone-pebbly mudstone unit I). Clasts
include plutonic rocks and marble. Similar conglomeratic mudstones are present
east of the Fawara-Jabal Rugaan ophiolite belt (mudstone-pebbly mudstone unit
II). The mudstones are steeply inclined and bordered to the east by a zone of
mylonites intruded by granodiorite. To the north the mudstones appear to be
located in the core of an antiformal belt. Coarse sandstone - mudstone rocks
facing west also occur east of the Fawra - Jabal Rugaan ophiolite belt just to
the north of the village of Fawara. The sandstones are poorly graded, and
contain cross-laminated ripples, and flame and load structures. As a sedimentary
facies they most closely resemble the sandstones of the lower part of the Abt
Formation. Although not penetratively cleaved, the Fawara sandstones are folded
by a set of S-shaped folds with vertically plunging axes. To the north this unit
appears to extend to at least as far as latitude 23 19'; to the south it is cut
off bv a fault and an intrusive body of granodiorite. Other than grains of
quartz, plagioclase, "floating mica", epidosite, felsic volcanic rock,
quartzite, and graphic-granite, the sandstones also contain
garnet and chromite. The: presence of chromite might indicate that the
sandstones are younger than the rocks of the adjacent ophiolite belt. The clast
content of the sandstones is similar to that found in sandstone blocks
associated with the ophiolitic melanges of the Eastern Desert of Egypt and the exogeosynclinal (foreland basin) flysch of the
Immature sandstones overlying conglomerates (conglomerate unit I) form an important unit within the Jabal Zriba syncline in the Bir Al Badriyah quadrangle. The detrital content of the sandstones is similar to that in the Fawara sandstone, and includes garnet, abundant mica, ilmenite and rutile, as well as clasts of volcanic glass, epidosite, graphic granite, and quartz-plagioclase plutonic material (sample J.Z.80.1) Sandstones form thick massive beds exhibiting large scale grading. The underlying conglomerate is also thickly bedded, some beds containing metre-size boulders of carbonate. The relationship of the conglomerate to the volcanic rocks beneath them was not observed.
Boulder conglomerates interlayed with coarse cross bedded sandstones form a unit
between Volcanic sequence I and coarse sandstone unit II north of Fawara village
and in the vicinity of latitude 23° 19' near Idsas. Bedding in the conglomerate
is vertical but the rocks are undeformed . The sequence seems to represent a
fluviatile facies, and in this respect are quite unlike the mudstone - pebbly
mudstone units associated with the Abt Formation. The relationship of the Idsas
conglomerate to adjacent units was not observed.
In the vcinity of latitude 23° 08', longitude 45° 15', a sheeted diabase unit with
screens of metagabbro was found within the ultramafic-mafic succession of the
Fawara - Jabal Rugaan ophiolite belt. The sheeted diabases are associated with
gabbros, serpentinites, and a possible unit of chert and shale. The unit appears
to face westwards, and to be capped by a carbonate unit. The ultramafic-mafic
succession east of Jabla Bitran seems to be disposed in the form of a southerly
plunging syncline. Sedimentary carbonates (low nickel rock bodies) also occur
within the succession at this locality. Near the village of Fawara, gabbroic and
fine grained mafic rocks are associated with plagiogranite (sample Faw 80.4;
It now seems likely that the that the Al amar - Idsas ophiolitic rocks form a vertically disposed belt, possibly synclinal in nature, located between two sedimentary units with similar flysch-like characteristics. The possibility arises therefore that the ophiolitic rocks form part of an allochthonous sheet thrust over the sediments of the Abt Formation, both units being subsequently interfolded and tectonically dislocated during a later phase of east-west lateral shortening and perhaps retrocharriage. The presence of chromite in the west facing sandstones east of the ophiolite belt raises the possibility that they represent exogeosynclinal (foreland basin) flyschi deposits laid down during obduction of the ophiolitic complexes from the northeast (present coordinates). It is equally feasible that at least part of the succession of rocks east of the Al Amar - Idsas line is allochthonous, perhaps representing the continental margin bordering the northeast edge of the ocean supposedly represented by the ophiolitic succession. However the relationship of the Abt Formation and the ophiolite tectonic unit to the volcanic sequence east of the Al amar - Idsas line is totally unknown, as is the relationship of the latter to the paragneisses of Wadi Jifr.
Further progress in the elucidation of the significance of the Al Amar - Idsas
line now requires detailed field mapping and petrographic studies of certain key
areas, along with a regional facies analysis of all the mud stone-sand stone
sequences in the Jabal Batran and Bir Al Badriyah quadrangles. With regard to
the former it is particularly recommended that attempts be made to 1) establish
the internal stratigraphy and structural state of the volcanic sequence east of
the Al Amar Idsas line, paying special attention to the distribution of high
strain zones; 2) determine the nature and age of the Wadi Jifr paragneiss
terrain, 5) establish the internal stratigraphy and structure of the ophiolite
slice west of Jabal Bitran and south of Fawara; 4) date the ophiolites using
zircons from the plagiogranite associated with the gabbro near Fawara village;
5) resolve the relationship between the ophiolite sequence, the pebbly mudstone
unit, and the volcanic unit, east of the ophiolite southeast of Fawara, and 6)
ascertain the stratigraphic relationship of the Idsas conglomerate to the
adjacent volcanic rocks at Idsas and other potential localities in the Bir Al
Badriyah quadrangle. It is also suggested that reconnaisance studies based on
observations currently made in the Jabal Bitran quadrangle be extended into the
south-east part of the Bir Al Badriyah quadrangle.
Department of Geology, University of Western Ontario,
London, Ontario, Canada N6A 5B7
ADDENDUM - chemical composition of metamorphic minerals in calc-silicate sample WJ80 1 (Wadi Jifr calc silicate band in quartzo-feldspathic gneiss) and detrital minerals in sample Faw80 3 (Abt greywacke, north of the village of Fawara, east of the ophiolite belt):
Si02 47.45 41.24 48.81 36.11 1.63
Ti02 0.06 0.15
A1203 30.73 27.80 2.56 5.16 0.20
FeO 0.23 4.16 18.70 24.65 92.02
MnO 0.46 0.69
MgO 0.02 0.06 6.06 0.10 0.05
CaO 16.42 23.08 22.53 29.98 0.13
Na20 2.24 0.33
TOTAL 97.11 96.34 99.60 96.69 94.7
Sample WJ80 1. 1-Plagioclase; 2-Epidote; 3-clinopyroxene; 4-garnet; 5-magnetite.
Si02 0.86 1.02 1.92 39.27
Ti02 94.42 0.06 0.09
A1203 2.25 21.02 7.09 21.99
FeO 1.54 18.84 20.28 27.65
MnO 0.89 1.00 0.41
MgO 0.06 12.03 9.12 9.74
CaO 0.06 0.05 0.05 0.77
Sample Faw80 3. 1-Anatase/rutile; 2-Chromite; 3-Chromite; 4-Almandine garnet.
N. Jackson and J. Roobol - Notes on the Geology of the Jabal - Idsas - Jabal Tays areas in the Eastern Arabian Shield, for an IGCP Project 164 Field Excursion between Feb 26th and March 2nd 1980.
"Two geotectonic models have been proposed for this region. Al-Shanti and Mitchell (1976) proposed that the Al-Amar—Idsas fault zone represents the site of an eastward dipping subduction zone at the site of collision between an island arc volcano-sedimentary-plutonic rock complex (Ar-Rayn Province) with a passive continental margin (western Ad-Dawadimi Province). In this interpretation the three formations of the Urd Group are interpreted as stable shelf sediments (Ar-Ridaniyah fm.), ocean floor ophiolites (Ophiolitic Complex) and ocean floor sediments (Abt fm.).
Nawab (1979) modified this interpretation to suggest that the Ad-Dawadimi Province represents a small "back arc" oceanic basin developed as a result of rifting of continental crust, while the Ar-Rayn plutonic-volcanic-sedimentary complex represents a "continental" volcanic arc generated above a westward dipping subduction zone.
An alternative hypothesis is that the eastern and western plutonic complexes formed a continuous "continental" substrate beneath the Urd and Halaban Groups. These complexes were intruded by mafic-ultramafic rocks ("ophiolite complexes"), possibly as a result of crustal rifting, but an oceanic substrate did not form. The Ar-Radaniyah and Abt Formations would therefore, represent intercontinental marine deposits, while the Halaban volcanics may be related to subduction or continental rifting processes?"
BIBLIOGRAPHY (Jackson and Roobol, arranged by date)
Vincent G. 1966. The magmatite occurrance of field zones. in
Eijkelboom G. The mineral occurrances and geology of the Idsas—Wadi Jifr region. B.R.G.M. SG-JED-66 A-15 App 5.
Eijkelboom G. 1968. Preliminary report on the structure of the AI-Amar area. B.R.G.M. 68-JED-3D.
Bouladon, J. 1969. Mineral exploration in zones 1,11 + 111; J. Sayid, Nuqrah» As Safar, Umm Ash Shalahib. B.R.G.M. 64-JED-41.
Bois, J. and Shanti, M. 1970. Mineral resources and geology of the As Sakhen Quadrangle. B.R.G.M. Report 70-JED-6.
Eijkelboom. G., Gendi. M., Henry. B.. Leca. X., Shanti. M.. Delanye, P. and P Flaum, J. 1970. Geology and mineral resources of the Al-Amar—Ar Rayn Quadrangle. Mineral Maps. MI-18.
Kahr, V.P.. Overstreet. W.C. 1972. Reconnaissance geology of the Jabal Bitran Quadrangle. U.S.G.S. Report SA(IR)-124.
Khukandy. M.E. 1974. Geology of the mafic, ultramafic and related rocks of Jabal Bitran area. Unpob. M.Sc. thesis, I.A.G., Jeddah.
Letalenet, J. 1975. Geology and mineral exploration of the As Sakhin and Sabkhat Muraysis Quadrangles, 22/45 A+B, Revision, B.R.G.M. Report 78-JED-2.
Al-Sari, A.M. 1976. Comparative studies of the mineralization at Al-Amar and Umm Ash Shalahib areas. Unpub. M.Sc. thesis I.A.G. Jeddah.
Al-Shanti, A.M.S. & Mitchell, A.H.G. 1976. Late Precambrian subduction and collision in the Al-Amar—Idsas region, Arabian Shield, Kingdom of Saudi Arabia. Tectonophysics 30, T41 - T47.
Moore, J.McM. 1976. A major lineament in the Arabian Shield and its relationship to mineralization. Mineral Deposits (Bert.) 11, 323-328.
Thekair, M.E. 1976. Carbonate rocks in A1-Amar—Asihailiya district of Saudi Arabia. I.A.G. Research Series No. 1.
Abdulaziz, M.I. 1977. Geological, mineralogical and geological studies in Jabal Idsas iron ore and its origin. I.A.G. Research Series No.3.
Gass, I.G. 1977. The evolution of the Pan African crystalline basement in NE Africa and Arabia. Jl.. Geol. Soc. 134, 129-138.
Riofinex, 1977. A preliminary review of mineral opportunities in Saudi Arabia. Report RF-1977-1.
Nawab, Z.A. 1979. Geology of the Al-Amar—Idsas region. I.A.G. Bulletin 3 Vol. 2. 29-40.
Ramsay, C.R., Jackson, N.J. and Roobol, M.J. 1979. Structural/Lithologica1 provinces in Saudi Arabian Shield geotravers. Proc. Cont. "Evolution and Mineralization of the Arabian-Nubian Shield". I.A.G. Bull. No. 3, Vol. 1, 63-84.
NOTES ON SOME STRATIGRAPHIC RELATIONSHIPS IN THE JABAL IDSAS AREA N.J. Jackson, A. Kroner, W.R. Church, and A.H. Hashad, published in: Newsletter - "Pan-African Crustal Evolution in the Arabian-Nubian Shield", International Geological Correlation Programme Project 164, Number 3, March 1980.
The objectives of this field group were to determine the nature of the Abt formation and its relationship to the mafic-ultramafic rocks of the Urd Group, to the Jabal Hammer metavolcanics, and to the Idsas conglomerate.
Abt Formation. This unit comprises an assemblage of meta-arenites and. conglomerates, phyllites, slates, schists and marble derived from arkosic, quartzitic, micaceous, calcareous and lithic sandstones and silts-tones, polymictic conglomerate, and limestone. Primary structures such as planar-trough-, and cross-bedding, slump bedding, ripples, cut-and-fill structure: and normal and reverse grading are preserved. Lithofacies variations in the small area studied suggest an east-wards coarsening succession. Near the eastern boundary polymictic conglomerates containing unfoliated granite boulders and cobbles probably represent channel-fill deposits, and these are associated with thick, bedded sandstone-siltstone successions indicate of a fluviatile-deltaic regime. Finer grained sandstone and silt stone become increasingly dominant to the -west and there is an increase in carbonate content until thin bedded carbonate-wackes predominate. These facies variations are tentatively interpreted as indicating derivation of terrigenous material from tvo sources, namely, an eastern, predominantly granitic terrain, and a western, 'carbonate-rich terrain.
An east-west- traverse across about 20 km of the Abt formation established two phases of pervasive deformation. The earlier phase (F1 ) produced tight, steeply plunging (80°) isoclinal Folds trending 170°. The strong axial plane cleavage of these folds was refolded to produce tight to open F2 folds with fold axes trending 200 degrees and plunging 17-20 south. The fjrst phase of folding may have been generated by N-S strike-slip faulting; the second phase is compatibl.e with east.-west oriented compressive stress. The second phase of deformation was concommitant with the emplacement of quartz porphyry and rhyolite dyken which were emplaced into the S l axial plane cleavage and deformed with it.
Ophiolite complex - Urd Group Irregular shaped bodies of mafic-ultramafic rock are emplaced into the Abt metasediments. Such complexes consist of combinations of serpentinized ultramafics, gaibbro, dolerite and basalt. (Contacts with the metasediments are always tectonic and sheared (usually high-angle reverse faults), and they were deformed along with the Abt during the F2 phase of folding. It therefore seems likely that they were emplaced between the F1 and F2 phases of deformation, although their original statigraphic position may have been below, contemporaneous with, or above the Abt sediment. On balance it seems more feasible that they represent a pre-Abt substrate.
Jabal Hammer volcanics They occupy an open synformal structure on the east side of the Abt basin (see P. Baker, this Newsletter). The volcanics have suffered only one phase of deformation which produced the open (20 degrees inward dipping) syncline, and are virtually unmetamorphosed. The junction with the Abt is obscured, but in view of their lesser deformation and metamorphism, and of the complete absence of metamorphic debris in the Abt , the volcanics are considered to be younger. An important point to be established is whether these volcanics are part of the major N-S trending belt of volcanics which parallel the Al Amar - Idsas fault.
Idsas Conglomerate This unit is located in a NW 0 trending (? Najd) fault zone. The conglomerate - fault breccia complex contains blocks of magnetite derived from the Jabal Idsas magnetite deposits emplaced in volcanics which are probably equivalent to the Jabal Hammer volcanics. If this assumption is correct, then this tectonic sedimentary deposit is probably the youngest Precambrian(?) deposit in the district. The cartoon diagram below summarizes our interpretations of the stratigraphic relationships in the area.
Comments on paper by Le Metour, J., Johan, V., and Tegyey, M., Origin and structural setting of the basic to ultrabasic complexes in the Al Amar and Bi'r 'Umq areas (Precambrian Shield of Saudi Arabia) Bureau de Recherches Geologiques et Minieres, Service Geologique National - B.P. 6009, 45060 Orleans Cedex, France
"Al Amar-Idsas region
1) Many of the rock stratigraphic units referred to by the authors have already been named by Kahr et al. (1972) in their description of the Jabal Bitran Quadrangle.
2) It is not clear why the volcanic rocks at Bir Maltabiga
are placed in the Abt Formation, there being no clear description of the contact
between the volcanics and the sandstone unit of the Abt. Further south
near Fawara the Abt sandstones transect the boundary between gabbro and volcanic
rocks of the 'Fawara', and the boundary could here be tectonic. If it is certain
that the volcanic rocks are intercalated with the Abt sandstones, given that the
volcanic rocks clearly constitute a mappable unit they should be given
formational status and the Abt raised to Group or Supergroup status. The
volcanic rocks could even be given group status since they contain two mappable
Sandstones collected from the Abt (Abt 80 1) west of Fawara village, although folded, are not penetratively cleaved and it is not possible to differentiate them from sandstones collected NE of the village (Faw80 1, 2, 3) where they are east of the serpentinite belt and west of a fault sliver of the Idsas conglomerate of Khar et al (1972) (northern facies of the Wadi Sderr of the authors). Both sample Faw80 1 and Abt80 1 contains detrital grains of chromite, whereas sample Faw80 1 also contains almandine garnet (Church 1980). All samples contain abundant 'floating mica'. Further south the Zriba Fm is composed of conglomerates which passes up into thick graded sandstones (with detrital Garnet, Church, 1980), units attributed to the Abt by Overstreet et al (1972). The Zriba overlies a volcanic unit but there is no record of interlayering. The Zriba contains large boulders of carbonate, and in this respect it should be remembered that extensive outcrops of marble occur further south at Jabal al Badr al Hambra as well as to the west (Ar Ridaniyah).
3) Rocks referred to in the paper as Al Amar - Idsas were named the Um Mushraha Fm by Kahr et al.
4) The rock unit near Idsas referred to as Wadi Sderr was named the Idsas Fm by Kahr et al. whereas the conglomeratic units to the south (Selib area ) were referred to as the Abu Sawarir. Furthermore, whereas the unconformable nature of the Idsas Conglomerate is indicated by the presence of fragments of magnetite derived from the adjacent magnetite deposits of the Al Amar - Idsas (Um Mushraha), the authors state that the basal conglomerate of the Selib area is intercalated with volcanic rocks of the Al Amar - Idsas (Um Mushraha). Could the intercalations be tectonic? It should also be noted that the volcanic unit west of the ultramafic belt ( Fawara volcanics of the authors) was named the Basdriyah Fm by Kahr et al. Since these rocks are not in physical continuity with the Al Amar - Idsas it is proper that they should be named differently from the latter. Is it impossible that the Fawara volcanic unit (Badriyah of Kahr et al. ) is equivalent to the volcanic unit of the Bir Maltabiga area?
5) As a facies the conglomeratic mudstones north of Selib (Abu
Sawarir ) of Kahr el al.) resemble the conglomeratic mudstones of the Abt Fm in
the Jabal Taif area. The mudstones there I would interpret as distal
flysch with conglomeratic channel fill deposits. The Idsas conglomerate
seems to be alluvial and therefore quite distinct from the Selib conglomerates
(??). It would be of interest therefore to know if the conglomerate with
volcanogenic and eruptive pebbles is clast or matrix supported. If clast
supported could there be a tectonic break between the latter and the marble
conglomerates which are matrix supported. The relatively rapid facies
change described in Wadi Sderr could be interpreted in terms of the presence of
a large channel, with the limestone also representing a turbidite deposit rather
than a platform deposit. It is also conceivable that the volcanic and
putonic pebble conglomerate is a locally derived island-fringing fault scarp
deposit within the Abt flysch basin.
It would seem that the placing of the Abt as older than the Al Amar-Idsas ( Um. Mushraha ) is based on the correlation of the Idsas conglomerate of Kahr et al with the conglomerates (Abu Sawarir of Kahr et al. ) of the Selim region. If this correlation is incorrect there would be no reason to correlate the Fawarah Fm (Badriyah Fm of Kahr et al.) with the Al Amar-Idsas (Um Mushraha Fm of Kahr et al) rather than with the volcanics of Bir Maltabiga. On the other hand all the volcanics other than those of Jabal Ahremer could be equivalent and older than the Abt including the Wadi Sderr.
6) It would be advisable to separate the Jabal Ahrmer volcanic rocks from the Wadi Sderr (Abu Sawarir Fm) since their stratigraphic relationship is not well established at this time. If the correlation is based on the assumed equivalence of the Jabal Ahrmer volcanics with those east of the conglomerate units of Wadi Sderr then it is doubtful that the correlation is proper.
6) The light hornfelsing of the Abt at the contact with serpentinite could result from sepentinization reactions associated with the latter being exothermic.
7) The metamorphic rocks mentioned could be derived from a dynamothermal aureole associated with an ophiolite. Similar rocks, but including garnet bearing units, occur in association with the Ballantrae ophiolite of Scotland and the Bay of Islands ophiolite of Newfoundland (Church, 1973, 1979)
8) Near Selib there is an undoubted occurrence of sheeted diabase with gabbro screens (Church, 1980)."
Publications concerning the geology of the Al Amar - Idsas region, since 1980
Stacey J.S.; Stoeser D.B.; Greenwood W.R.; Fischer L.B. 1984. U-Pb zircon geochronology and geological evolution of the Halaban-Al Amar region of the Eastern Arabian Shield, Kingdom of Saudi Arabia Journal of the Geological Society, v. 141, 6,, p. 1043-1055(13)
Abstract: U-Pb zircon model ages for eleven major units from the Halaban-Al Amar region of the eastern Arabian Shield indicate three stages of evolution: (1) plate convergence, (2) plate collision, and (3) post-orogenic intracratonic activity.
Convergence occurred between the western Afif and eastern Ar Rayn plates that were separated by oceanic crust. Remnants of oceanic crust now comprise the ophiolitic complexes of the Urd group. The oldest plutonic unit in the study is from one of these complexes and gave an age of 694±8 Ma. Detrital zircons from the sedimentary Abt formation of the Urd group, which is intercalated with the ophiolitic rocks, were derived from source rocks with a mean age of 710 Ma. The Abt formation may be an accretionary wedge on the western margin of the Ar Rayn plate. Plate convergence was terminated by collision of the Afif and Ar Rayn plates during the Al Amar orogeny which began about 670 Ma. During collision, the Urd group rocks were deformed and in part obducted on to one or both plates. Synorogenic leucogranitoid rocks were intruded from 670 to 640 Ma. From about 640 to 630 Ma, widespread unfoliated dioritic plutons were emplaced in the Ar Rayn block, and represent the end of orogenesis related to collision. There is no definitive evidence for a significantly older basement beneath the study region.
Agar R.A.1985. Stratigraphy and palaeogeography of the Siham group: direct evidence for a late Proterozoic continental microplate and active continental margin in the Saudi Arabian Shield Journal of the Geological Society, v. 142, 6, p. 1205-1220.
AL-SALEH A.M.; BOYLE A.P.; MUSSETT A.E 1998. Metamorphism and 40Ar/39Ar dating of the Halaban Ophiolite and associated units: evidence for two-stage orogenesis in the eastern Arabian Shield. Journal of the Geological Society, Volume 155, Number 1, p. 165-175 (11)Al-Saleh, A. M. 1998. Terrane amalgamation and the Late Proterozoic growth of the Eastern Arabian Shield. Arab Gulf Jour. Sci. Res., 16, 2, p. 283-295. ISSN 1015-4442
Abstract: The Arabian Shield has long been recognized as a region where plate-tectonic processes have been in action during most of the Late Proterozoic resulting in the amalgamation of the shield’s five constituent terranes along four major suture zones. Studies thus far carried out on these collisional belts have concentrated on the origin and dating of intermediate to acidic igneous rocks, and very little emphasis has been placed on the understanding of metamorphic processes operative during plate convergence. The Al-Amar Suture separates the Afif microplate from the Ar Rayn Block in the easternmost section of the Shield. Among several ophiolite occurrences in this suture, the Halaban Belt is by far the largest and preserves within and around it a record of a number of thermal/structural events related to the opening and eventual closure of a supra-subduction zone back-arc basin that existed in the period 695–680 Ma. The bimodal nature of 40Ar/39Ar dates from the Halaban Ophiolite and associated units is indicative of a two-stage orogenesis climaxing at 680 and 600 Ma, instead of the previously proposed model of a single orogenic episode between 670 and 630 Ma. The first stage (680 Ma) is believed to be related to basin closure and ophiolite emplacement as the Ar Rayn island arc collided with the Afif microcontinent. The second episode (600 Ma) was the outcome of a major collisional event between the Arabian craton and a large continental mass east of the Ar Rayn Block.
Comment: Al-Saleh says that the Hail terrane in comparison to the 'Nabitah Suture' possesses a much longer history of almost continuous volcanomagmatic activity from 740-565 Ma. However, the map of Pallister et al. (1988, Fig. 17) shows three bodies in the 'Nabitah Suture' terrane with ages of 729, 732, 782 Ma, respectively. Furthermore, Al-Saleh also shows the Afif crust thrust over the southern rim of the Hail terrane whereas Pallister et al (1988) clearly show the boundary as the Hail thrust over the Afif . He also says that late successions such as the Zarghat and Hadn formations do not exist with the Nabitah suture. Given the existence of the Jabilah basin with the Afif terrane it is not obvious why this is a relevant point of difference.
Al-Saleh, A.M. and Boyle, A.P. 2001. Structural rejuvenation of the eastern Arabian Shield during continental collison: 40Ar/39Ar evidence from the Ar Ridayniyah ophiolitic melange. Jour. African Earth Sci., 33, 1, p. 135-141. Fig 1 - simplified geologic map of the Al-Amar suture
The Ar Ridayniyah ophiolitic melange is one of a number of such complexes found within or at the peripheries of the Neoproterozoic Al-Amar Suture of the eastern Arabian Shield. This suture is sandwiched between the Ar Rayn island-arc terrane on the east and the much larger Afif continental block to the west, and is thought to represent the site of a 695-680 Ma back-arc basin that separated the two terranes. A thick and monotonous unit of metagraywacke (Abt Schist) underlies most of the suture along with scattered outcrops of metavolcanics and ophiolitic melange. One of these bodies is the Ar Ridayniyah melange, which occurs as a longitudinal belt of sheared ultramafic schists enclosing abundant blocks of oceanic serpentinites, as well as subordinate gabbros and basalts. The western boundary of this melange is defined by the Ar Ridayniyah thrust fault. The 610-600 Ma ages obtained from the metagabbros of this complex are considered to record the reactivation of the Ar Ridayniyah Fault during continental collision, 60 Ma after ophiolite emplacement.
Comment: This paper presents the results of Ar-Ar dating of amphibole in gabbro blocks within the Ar Ridayniyah ophiolitic melange. The time range of 610-600 Ma of the amphiboles records the last time the gabbros were cooled below the blocking temperature of hornblende, and this age marks the exhumation of the melange along the Ar Ridayniyah fault, itself a manifestation of regional strike-slip faulting of the Najd system during collision of the Arabian Craton with a large continental mass east of the Ar Rayn arc.
The author argues that collision of the Afif and Ar Rayn terranes along the Al-Amar suture delineated by the outcrop of the Abt formation was coeval with the emplacement of the Urd ophiolite onto the eastern border of the Afif 690-680 Ma ago, and that the period between 680 Ma and 620 Ma was a period of tectonic quiescence. Tectonic activity then resumed at c. 620-610 as the result of collision of the accreted Arabian Craton and a large continental mass east of the Ar Rayn arc, leading to thrust stacking and folding in the Al-Amar zone, high grade metamorphism of the Hillit (and Anjal Groups?), and extensive granite magmatism between 620 and 575 Ma. However, to the contrary it would seem that development of major transpressional fault systems and associated magmatism in the Saudi Shield (Junayah and Nabitah shear zones) had already begun been initiated by 654 Ma following the formation of the Murdama (including volcanics) and Bani Ghayy basins marginal to the thrust front demarcated by the Urd ophiolite belt, and that the magmatism (Musayrah, Abss, Junayah, Tathlith, etc, etc) continued through to 580 Ma and later. The formation of the Jurdhawiyaj and Hibshi basins between 640 Ma and 620 Ma, and the Jibalah basin at 580-570 Ma mark the surface manifestation of the faulting. It would seem therefore that the data presented in this paper is of only local significance, and unfortunately provides no basis for any major reorganisation of thought concerning the development of the Arabian Shield.
SHUJOON, AbdulRahman A. and PRIDE, Douglas E., 2001. Paper No. 173-0 EVALUATION OF THE MINERAL POTENTIAL OF THE SAUDI ARABIAN SHIELD: A GIS APPROACH. GSA Annual Meeting, November 5-8, Department of Geological Sciences, The Ohio State Univ, 125 South Oval Mall, Columbus, OH 43210-1398, email@example.com
Abstract: The Precambrian Shield of Saudi Arabia covers more than 680,000 km² of the western Arabian Peninsula, encompassing a third of Saudi Arabia. Rocks there are principally upper Proterozoic volcanics and volcanoclastics intruded by igneous plutons and dykes. More that 2,100 metallic mineral occurrences are known throughout the Shield, 138 of which have been shown to contain significant resources. This research extends the potential of known occurrences, and predicts where similar deposits may lie concealed.
Data layers covering the Shield include topography, geology, lineaments, lithotectonic terranes, gravity, magnetics, and rock ages. Geographic boundaries, and roads and cities provide additional coverages. Occurrences of 12 metals were utilized as "skeletons" upon which to examine spatial relationships between mineral locations and the data in the GIS layers. These relationships in turn were used to produce four exploration models: (1) Fuzzy Logic Model with Gamma Operation (FLM-GO), (2) Boolean Logic Model (BLM), (3) Index Overlay Model with Binary Evidence Maps (IOM-BEM), and (4) Index Overlay Model with Multi-Class Maps (IOM-MCM).
The FLM-GO identified 838 km² of high silver potential in the Ad Dawadimi sub-terrane of the Afif terrane, and 14,146 km² of high copper potential mainly in the Zalm sub-terrane of the Afif terrane. The Boolean Logic Model highlighted 14 km² of silver potential in the Ad Dawadimi, and 4,153 km² of copper potential in the Zalm. The IOM-BEM highlighted 44 km² and 8,397 km² respectively for silver and copper in the sub-terranes, and the IOM-MCM identified 412 km² and 9,633 km² in the two sub-terranes. All of the models highlight known occurrences of silver and copper, plus significant areas where there are no known deposits - the latter are particularly intriguing, especially those identified by IOM-MCM modeling. Results vary but are equally intriguing for gold and the other metals in the data base.
Al-Saleh, A. 2003. Metamorphic history of gabbroic blocks within the Ar Ridayniyah ophiolitic melange, Eastern Arabian Shield, Saudi Arabia. Jour. King Saud Univ., Science, v. 15, 2, p. 71-80.
Comment: This paper reports the observation that gabbros of the Ar Ridayniyah serpentinite melange have suffered both an early retrograde metamorphism and a late-stage epidote-amphibolite metamorphism. Based on this observation, Dr. Al-Saleh proposes that the serpentinites and gabbros, depicted as isolated oceanic diapirs injected into back-arc basin sediments (Abt Schist), were buried to a depth of more than 10 km during westward thrusting c. 610 Ma ago, and then rapidly exhumed by faulting, represented by the Ridayniyah Fault, during "the final collisional orogeny between the Ar Rayn and Afif terranes" Fig. 3 - successive stages in the metamorphic history of the Ridayniyah melange
The main problem with these conclusions concerns Dr Al-Saleh's supposition that the Abt sediments were layed down on the Urd - Al-Amar ocean floor. Given the presence of both continental (including garnet) and ultramafic (chromite) detritus in the Abt turbidites and associated debris flows of the Al-Amar - Idsas region, the Abt sediments are unlikely to be 'primary' oceanic abyssal deposits. The Abt sediments must be entirely younger than any ophiolite obduction event, but older than the culminating Afif/Ar Rayn collisional event, and they could have been laid down in a successor basin developed above the obducted ophiolite foreland sequence following the initial phase of accretion of the Ar Rayn to the Afif. In this case the Abt would be located stratigraphically above the ophiolitic sequences, and the Ridayniyah Fault would be located along the western margin of the melange.
Alternatively, the Abt turbidites may have been deposited in a foreland basin during the obduction process, in which case the eastern Abt Formation would initially be structurally located beneath the obducted oceanic material. As in the case of the complex structure of the ophiolite and the Abt and Abu Sawarir sedimentary formations along the line of the Idsas - J. Rugain (Selib) structural front, one can envisage that the Ar Ridayniyah ophiolitic melange may be bounded to the west by a primary obduction thrust, and to the east by a relatively young collisional, out-of-sequence thrust represented by the Ridayniyah fault.
` One other difficulty concerns that fact that the Abt Schist is invaded by numerous granite bodies, some which are thought to be derived from the melting of the Abt sediments. It is therefore possible that this thermal event may be responsible for the localized growth of prograde amphibolite assemblages in the melange gabbros.
It should also be noted that amphibole may also overgrow low grade actinolite in ophiolitic metagabbros during the development of inverted thermal aureoles related to the subduction/obduction process, and the Ar-Ar age of 610 Ma may therefore not represent the actual crystallization age of the amphiboles, but rather the age of exhumation of the melange along the fault.
http://baby.indstate.edu/gga/pmag/egon.pdf - A synopsis of events related to the assembly of Eastern Gondwana by J Meert