Central Arabian Shield Research Articles

Structural mapping of the west central Arabian Shield (Saudi Arabia) using downward continued magnetic data

In this study, we have applied a stable downward continuation technique to magnetic anomalies from the global geomagnetic field model MAG2V3 to improve the resolution of magnetic data of the west central Arabian Shield, Saudi Arabia. Then, advanced interpretation methods such as the tilt angle of the horizontal gradient (TAHG), horizontal gradient of a modified tilt angle (HGSTDR), and enhanced horizontal gradient (EHGA) have been applied to downward continued magnetic dataset to highlight the main structural features in the region. The observed geologic structures from the magnetic analysis revealed NW-SE, NE-SW, and ENE-WSW orientations in the west central Arabian Shield. The depths of these structures were computed by the analytic signal (AS) technique and the obtained result shows depths varying from 1.2 to 9.2 km. This study provided a new map of the subsurface geological structures for a better understanding of the tectonic evolution of the west central Arabian Shield.

Petrogenesis and Tectonic Implications of the Jabal Hadb Ad Dayheen Granitic Complex, Central Arabian Shield

Petrogenesis and Tectonic Implications of the Jabal Hadb Ad Dayheen Granitic Complex, Central Arabian Shield

Geochemical and geochronological constraints on the gold-sulfide mineralization and host granitoid rocks in the Bulghah and Al-Maham prospects, central Arabian Shield, Saudi Arabia

Among several gold-sulfide deposits and prospects scattered in the Afif terrane of the Arabian Shield, the Bulghah and Al-Maham prospects are investigated to assess the spatial and temporal relationships between gold mineralization and the host lithologies and regional structures. Both prospects are hosted by the sulfide-bearing Hulayfah Group metavolcano-sedimentary successions, and are confined to cross-cutting granitoid intrusions. Tonalite-granodiorite and monzogranite from the two areas have geochemical characteristics of metaluminous to weakly peraluminous calc-alkaline I-type granitoids. Younger alkali feldspar granite in the Bulghah area exhibits geochemical features of A-type granite, likely formed in a post-collisional or transitional setting. ID-TIMS dating of zircons from two granodiorite samples from the Al-Maham prospect yielded 206Pb/238U ages of ∼662 and 651 Ma. A tonalite sample from the Bulghah mine area gave a concordia age of 674.8 ± 0.8 Ma (2σ; MSWD = 0.42), whereas the alkali feldspar granite sample has an age of ∼ 631 ± 1 Ma. The newly defined ages can be considered as the upper age limit of the auriferous quartz veins, evidently attendant to the Nabitah orogeny (∼680–640 Ma). Gold-bearing quartz veins in the two prospects are identical in terms of ore mineralogy, textures and paragenesis. Gold-bearing quartz veins are associated with intersecting veinlets and stockwork of arsenopyrite, pyrite, and less common pyrrhotite, chalcopyrite, and sphalerite. Recrystallized pyrite and arsenopyrite are ubiquitous in quartz veins, whereas pyrrhotite-chalcopyrite intergrowths in-fills or replacements prevail in the deformed host metavolcanic rocks. Free gold typically occurs as fine inclusions in quartz veins, commonly in deformed pyrite and arsenopyrite crystals. The sulfides of Al-Maham prospect have significantly higher Re-Os concentrations (ppb level), compared with those in the Bulghah prospect, and define an isochron with a Model-1 age of 360 ± 2 Ma and an initial 187Os/188Os ratio of 1.962 ± 0.032. The obtained Re-Os age and the crustal initial Os isotope ratio do not necessarily define the primary age of gold mineralization since ore textures indicate that pyrrhotite was a paragenetically late sulfide phase in both prospects. Alternatively, this age may be interpreted as the time of remobilization and recrystallization of older sulfides in response to Hercynian uplift and deformation (370–290 Ma), manifested as intermittently rejuvenated reverse faults elsewhere in the Arabian Shield.

Mineral paragenesis and stratigraphic setting of the Neoproterozoic volcano-sedimentary succession of Samran Group, Jabal Farasan Area, West central Arabian Shield, Saudi Arabia

Jabal Farasan is located in the west central part of the Arabian Shield. The present study aims to reveal the stratigraphic setting and mineral paragenesis of the exposed volcano-sedimentary succession of the Samran Group in Jabal Farasan area. The succession of Samran Group is composed of three successive facies: the lower interbedded andesitic tuffs and tuffaceous andesites and trachyte’s (F1), the middle-altered quartz diorite/andesite (F2) and, an upper interbedded andesitic tuff, tuffaceous andesites and trachytes (F3). The vertical variations in the petrographic lithotypes depend mainly on the position of the depositional sites relative to the volcanic centers (proximal and distal). Both the volcanic rocks and the associated units are subjected to syn-and post-depositional diagenetic/metamorphic processes. These processes include the formation of the Fe2+-silicates: (i.e., chlorite instead of the volcanic ashes and the formation of plagioclase minerals). Post diagenetic recrystallization of the tuffaceous mudstones and the formation of microcrystalline quartz. The diagenetic oxidation of the Fe2+-silicates led to the formation of volcaniclastic red beds in certain stratigraphic horizons.

Stratigraphic setting, facies types, and mineral paragenesis of the carbonate-bearing succession of Jabal Farasan Area, West Central Arabian Shield, Saudi Arabia

Stratigraphic setting, facies types, and mineral paragenesis of the carbonate-bearing succession of Jabal Farasan Area, West Central Arabian Shield, Saudi Arabia

Petrogenesis of the post-collisional rare-metal-bearing Ad-Dayheen granite intrusion, Central Arabian Shield

At Hadhb't Ad-Dayheen, in the central Arabian Shield, a post-collisional igneous complex called the Ad-Dayheen intrusion is exposed. It was emplaced in the Early Ediacaran (613–625 Ma), during the final tectono-magmatic stage of Arabian Shield development. Despite limited and discontinuous ring-shaped outcrops due to alluvial cover and later faulting, three pulses of intrusion can be recognized in the field: an early pulse of monzogranite; a second pulse of syenogranite and alkali feldspar granite; and a final pulse of alkaline and peralkaline granite, mineralized microgranite, and pegmatite. Samples show distinctively low contents of CaO, MgO, and Sr in contrast to elevated concentrations of alkalis, Rb, Nb, Y, Ta, Hf, Ga, Zr and rare-earth elements (REE); these are common characteristics of post-collisional rare-metal-bearing A-type granites. The suite displays positive Nb–Ta anomalies and pronounced negative Eu anomalies (Eu/Eu* = 0.11–0.35). The alkaline/peralkaline granites and microgranite of the Ad-Dayheen intrusion feature disseminated mineralization, whereas mineralization is localized in the pegmatite. The primary magma feeding the Ad-Dayheen intrusion was mostly generated by partial melting of the juvenile crust of the Arabian Shield, with a minor mantle contribution. We argue that an episode of lithospheric delamination led to crustal uplift, erosional decompression, and generation of mantle melts that supplied heat to drive crustal melting. The anatectic deep crustal melts assimilated a F-bearing component that also added rare metals to the magma. Each pulse can be described by a fractional crystallization model, but the parental liquid of each subsequent pulse was first modified by further addition of fluorine and rare metals and loss of CaO, Sr, Ba, and Eu due to fluorite fractionation. Texture and morphology of the ore minerals indicate that mineralization (U, Th, Zr, Nb, Ta, Y, Hf and REE) took place in two stages: a magmatic stage coinciding with emplacement of the intrusion, followed by a hydrothermal stage. The magmatic process enriched the residual melt in high field strength elements (HFSE) and REE. The later hydrothermal stage further localized these elements and increased their concentrations to economic grades. The pegmatite is highly mineralized and contains high concentrations of U (81–179 μg/g), Th (244–600 μg/g), Zr (2397–14,927 μg/g), Nb (1352–2047 μg/g), Ta (96–156 μg/g), Y (828–2238 μg/g), Hf (131–377 μg/g) and ∑REE (1969–4761 μg/g).

Landforms Evolution of Wadi Qudaid Area, West Central Arabian Shield, Saudi Arabia: An Example of the Role of the Geological Factors in the Urban Extensions

Wadi Qudaid is located in the west central part of Saudi Arabia. It about 135 km to the northeast of Jeddah city along Al Haramein highway and it represents the upstream of the very large alluvial plain along the Red Sea coast. It runs in NE direction parallel to many wadis of the west central part of Saudi Arabia i.e. Wadi Fatima, wadi Sitarah. The wadi floor is filled by Quaternary deposits which represent good groundwater aquifer. Geologically, the present-day residual landforms of Wadi Qudaid are composed mainly of Precambrian Arabia shield rocks overlained by Tertiary sedimentary rocks and finally harrat (Tertiary volcanic). The Precambrian rocks are represented by 1) a lower layered basic and intermediate volcanic and the intercalated volcaniclastics. This unit is correlatable with Samran Group, 2) an upper layered acidic volcanic and the intercalated acidic volcaniclastics. The Arabian shield rocks are intensively folded and dragged along the major NE faults. They are directly overlain by Tertiary basic volcanic (harrat) and the related volcanoclastic red beds). The main geomorphologic elements of the study area include plateau, scarps, and the wadi floor. The plateau is represented by the black basaltic sheet that contains some semi-rounded depressions filled with Quaternary eolian sands. The scarps of the main wadi and its tributaries are nearly steep and contain many asphaltic roads with some isolated cone hills detached from the scarps. Geomorphological, Wadi Qudaid represents the incomplete erosion cycle that begins with the formation of deep galleries and very steep and narrow wadies formed along the major NE faults and related fractures and folds. The progress of the erosion processes led to the formation of narrow interfluves as a result of pedimentation and sculpturing of the wadies sides by scarp retreat. The peniplanation stages of the erosion cycles are reached in the southwestern and the central part of the wadi where fast peniplained areas were formed. The results of this study revealed the role of the different geological processes (lithology, structural elements and climatic conditions) in the distribution of present-day human populations in urban extensions. Qudaid, Dhubaya-Jumah, Dabyah, Al Khamrah, Almansa and Al Massamah are the main villages of Wadi Qudaid area.

Hydrogeology of Wadi Qudaid Area, Northeast Jeddah, West Central Arabian Shield, Saudi Arabia

The study area is a part of the Arabian Shield rocks of west central part of Saudi Arabia (150 km to the northeast of Jeddah). Geologically, the study area comprises five main geologic units i.e. 1) Layered basic volcanics and related volcaniclastics which are composed mainly from intercalated basalts and andesites and the related volcaniclastic derivatives, 2) Acidic volcanics and related volcaniclastics which are composed from layered and laminated dark and light acidic to intermediate igneous rocks, quartz and chert and marbles, 3) The Tertiary sedimentary succession which of volcaniclastic red beds and the intercalated clays; 5) Tertiary volcanics of Harrart, and 4) The Quaternary wadi fill deposits which are composed from friable pebble supported conglomerates, sandstones and clays. Hydrogeologically, the groundwater aquifer of Wadi Qudaid is present mainly in two main horizons i.e. i) unconfined shallow aquifer (13 - 37 m) within the well porous and permeable conglomerates of the Quaternary Wadi deposits, ii) The deep confined aquifer of the bedded tuffaceous sandstones and mudstone of the Tertiary sedimentary succession of Ash Shumaysi Formation. The water samples are analyzed for major elements i.e. Ca, Mg, Na, Cl, SO4, HCO3 and the results show the normal content of these elements. The study related the addition and depletion of many elements during the running trip of the groundwater from the northeast (recharge area) to the southwest (downstream) area.

Petrographic Characterization of the Different Types of Basalts of Harrat Al Fatih, Ablah Area, West Central Arabian Shield, Saudi Arabia

Harrat Al Fatih is located southwest of Ablah area, Assir terrain, southwestern Arabian Shield. It is present as black laterally extended horizontal basaltic sheet just overlying folded strata of the volcano-sedimentary succession of Ablah Formation (green mudstones, dolostones and green and red volcaniclastic red beds). These basalts are rift-related and represent part of the Oligo-Miocene basic volcanics of the western part of Saudi Arabia. The detailed field and microscopic description of these basalts enable the author to classify it into two main types: 1) Grey tuffaceous glassy basalts that present overlying the Ablah Formation. This basalt type is generally soft, bedded and intercalated with grey, green and red tuffaceous mudstones. Microscopically it composed of minute lath-like plagioclase and pyroxene crystals embedded in glassy groundmass. 2) Black olivine-rich basalts which are present in the topmost part of harrat Al Fatih are generally black, hard and contain remarkable reddish brown oxidized olivine spots and some calcite and amorphous quartz domains. Under the microscope, this basalt type is microcrystalline and composed mainly of lamellar twined Ca-plagioclase and colored olivines and pyroxenes. The olivines show different stages of oxidation and formation of amorphous blood red iron-oxyhydroxides and black hematite. The present study revealed the formation of the grey basalts at the initial stages of the volcanic eruption in ephemeral lakes and the second type of basalt was during the consolidation of proper basic magmas at the final stages of the volcanic eruption.

Origin and geochemistry of the Late Proterozoic intra-arc rift-related volcaniclastic red and green beds of Tayibit El Esm area, Ablah district, south central Arabian shield, Saudi Arabia

The volcaniclastic red beds of the study area are present overlying the rhyolite-dolostone succession of wadi Girshah area, Ablah district. It is composed of seven shallowing-upward cycles. The lower part of this sequence is dominated by gray tuffaceous mudstone which grades upward into mudstone/siltstone. The middle part is dominated by red hematitic volcaniclastic siltstone-sandstone while the upper part is dominated by thinly to thickly bedded silicified stromatolitic dolostone. This vertical distribution reflects the gradation from deeper conditions of high volcaniclastic input into shallower conditions of very low volcaniclastic input into more shallower and highly silicified restricted depositional environments dominated during the deposition of the uppermost silicicfied dolostones of the upper parts of the succession. The field, mega, microscopic, and geochemical results conclude the formation of the present volcaniclastic red beds during the following stages, these are: (1) deposition of the basic and intermediate volcanic ashes in slightly deeper back-arc setting, (2) the degradation of the deposited volcanic ash and the diagenetic authigenesis of green celadonitic clays either along the sediment/water interface or beneath the sea floor by the interaction between Fe2+, Mg2+, Si4+, and Al3+, (3) the diagenetic hematitization of the formed green celadonitic clays of the cycles of the middle and upper parts of the succession and the formation of the iron-oxyhydroxide mineral, i.e., goethite and hematite as a result of the change in the pore water sediments from reducing to oxidizing conditions, and (4) finally, the direct hematitization of the original tuffaceous materials and formation of iron minerals especially in the upper parts of the volcaniclastic red bed succession.

The role of aeromagnetic data analysis (using 3D Euler deconvolution) in delineating active subsurface structures in the west central Arabian shield and the central Red Sea, Saudi Arabia

In this paper, we present a case study of structural mapping by applying the 3D Euler method to the high-resolution aeromagnetic data that was collected in the west central Arabian Shield region and the coastal region of the central Red Sea in Saudi Arabia. We show the 3D Euler deconvolution algorithm and apply it to magnetic potential field data from the west Central Arabian Shield and the Central Red Sea. The solution obtained with 3D Euler deconvolution gives better-focused depth estimates, which are closer to the real position of sources; the results presented here can be used to constrain depth to active crustal structures (volcanisms) for the study area. The results indicated that the area was affected by sets of fault systems, which primarily trended in the NNW–SSE, NW–SE, EW, and NE–SW directions. Moreover, estimated Euler solution map from aeromagnetic data delineated also the boundaries of shallow, small, and confined magnetic bodies on the offshore section of the study area. These nearly exposed basement intrusions are most likely related to the Red Sea Rift and may be associated with structures higher up in the sedimentary section. These volcanic bodies are extended to the continental part (onshore) of the west central Arabian Shield, particularly beneath both sides of the Ad Damm fault zone. This extension verifies that the fault was largely contemporaneous with a major period during the extension of the Red Sea Basin. Moreover, according to the distribution of circular magmatic-source bodies (circular-shaped ring dikes) that resulted from this study, we can state that the clustering of most earthquakes along this fault may most likely be attributed to the active mantle upwelling (volcanic earthquakes), which are ultimately related to volcanic processes. Furthermore, the oceanic crustal structures near and in the Red Sea offshore regions were also estimated and discussed according to the ophiolite occurrences and further opening of the Red Sea. Our results are largely comparable with studies of previous crustal sections, which were performed along the Red Sea Rift and the Arabian Shield. As a result, the areas above these anomalies are highly recommended for further geothermal study. This example illustrates that high-resolution aeromagnetic surveys can greatly help delineating the subsurface active structures in the west central Arabian Shield and the middle coastal region of the Red Sea of Saudi Arabia.

Mechanism of formation of Haddat Ash Sham ironstones (Oligo-Miocene), Makkah Al Mokaramah District, West Central Arabian Shield, Saudi Arabia

The present study aims to shed light on the mechanism of formation of the Oligo-Miocene oolitic ironstones of Haddat Ash Sham area, Saudi Arabia. These ironstones are enclosed within the middle part of the Oligo-Miocene siliciclastic succession of the western part of the Arabian Shield, western Saudi Arabia. The ironstone beds were formed during marine incursion and creation of short-lived starved time periods of high organic matter activities, ferrous iron, and low clastic input. The depositional and diagenetic processes involved in the formation of Haddat Ash Sham ironstones are summarized here as follows: (1) the deposition of detrital components (i.e., amorphous iron-bearing clays admixed with silt and sand-sized quartz grains) and their distribution by the waves and current actions in areas of different water depths (bars and inter-bar areas); (2) the deposition of the iron-bearing clays in slightly reducing transgressive conditions (dysaerobic zone) led to the authigenesis of green marine chamositic clays of variable mineralogical and chemical compositions according to the predominated depositional environments; and (3) in the upper parts of the depositional cycles, the iron-bearing clays become admixed with detrital quartz grains which resulted in the formation of silty and sandy ironstones of low iron content. The diagenetic processes led to the oxidation of the green chamositic clays and formation of amorphous Fe-oxyhydroxides, ferrihydrites, goethite, and hematite. These iron mineral phases are related to each other and show progressive steps of transformation during the diagenetic processes. The iron ooids represent in situ formed irregular domains formed during the diagenetic crystallization and dehydration of the amorphous iron oxyhdroxides resulted from the diagenetic oxidation of green chamositic clays. This is supported by the absence of detrital cores of the iron ooids, the gradational contact between the iron ooids and the enclosing matrix and also by the presence of many ooids of unclear and ill-defined internal structure.

Facies Analysis and Depositional Environments of Ash Shumaysi Formation (Oligocene–Miocene), Makkah Quadrangle, Wadi Ash Shumaysi, West Central Arabian Shield, Saudi Arabia

Ash Shumaysi Formation represents Oligo-Miocene fluvio-lacustrine succession consisting mainly of conglomerates, sandstones, siltstones, muddy, sandy and glauconitic ironstones, fresh water carbonates, tuffaceous mudstone and basalts in descending order. It shows the transition from proximal to distal parts of the depositional environments of braided streams into floodplain and finally into lacustrine facies. This succession comprises four main facies associations arranged according to their predominance from the lower to the upper parts of the succession of Ash Shumaysi Formation as follows: (1) proximal (conglomerates facies association I) braided streams which includes four facies of sandstones and conglomerates (F1, F2, F3 and F4), (2) distal (sandstones facies association II) braided streams which comprises three facies (F5, F6 and F7), (3) meandering streams (facies association III) which include the thinly bedded muddy, glauconitic and sandy ironstones–ferruginous sandstone facies (F8), (4) lacustrine delta (facies association IV) which includes three facies (F9, 10 and 11). The Oligo-Miocene siliciclastic sections of Ash Shumaysi represent deposition in mid to distal parts of alluvial fans formed as a result of interplay between climatic conditions, tectonic and sedimentation. In the extreme distal parts of these alluvial fans, river lakes were filled rapidly with conglomerates, sandstones, mudstones and finally fresh water carbonated were formed during arid periods of low siliciclastic input.

Major elements hydrochemistry and groundwater quality of Wadi Fatimah, West Central Arabian Shield, Saudi Arabia

Wadi Fatimah is the largest NE-SW trending low topographic area in the west central Arabian Shield, Saudi Arabia. It is bounded by in the NW side by the faulted and folded volcano-sedimentary succession of Fatima Formation and in the SE side by the metamorphic rocks of Jeddah Group. Some massive gabbros and granites and foliated amphibolites are also present along both sides and in the upstream of Wadi Fatimah. The groundwater of Wadi Fatimah is present in the shallow Quaternary alluvial deposits and also in the underlying fractured Precambrian bedrocks. The description of the chemical composition of the groundwater of Wadi Fatimah includes the variations of the groundwater salinity, the major and trace constituents as well as the groundwater character. According to the chemical analysis and the trilinear and durov diagrams, the prevalent chemical character is calcium chloride followed by sodium chloride water types. All the sub-basins of Wadi Fatimah are characterized by a relatively low to medium saline water. Highly mineralized water zone marked the lower part of the wadi, e.g., Al Jumum-Bahrah area, where the groundwater salinity reached up to 26,500 μS/cm with an average of about 9,500 μS/cm. Major and trace elements analyses of the groundwater samples from the different tributaries of Wadi Fatimah revealed its suitability for irrigation purposes.

Geochemistry of the Late Neoproterozoic Hadb adh Dayheen ring complex, Central Arabian Shield: Implications for the origin of rare-metal-bearing post-orogenic A-type granites

The Hadb adh Dayheen ring complex (HDRC), Central Arabian Shield, is an alkaline to peralkaline A-type granite complex. It consists of an inner core of monzogranite followed outward by porphyritic alkali feldspar granite (hornblende biotite granite and aegirine riebeckite granite). Field and textural observations indicate that the different granite types were separated from magma reservoir, at different stages, and emplaced at higher levels along pre-existing fractures. The geochemical characteristics indicate that their magma was most plausibly originated by partial melting of juvenile lower crust following collision between East and West Gondwana at the final stage of the Arabian Shield evolution. The alkali feldspar granites have high abundances of albite and fluorite and wide variation of HFSE and REE that indicate interaction with hydrothermal F-rich fluids. Although there are many geochemical, mineralogical and textural evidence of secondary metasomatic alteration superimposed on the granitic rocks, they show textural features such as the arrangement of albite inclusions along growth planes in quartz (snowball texture) and aegirine that indicate early magmatic crystallization of albite. Also, the strong linear positive correlation of Ta vs. Nb and Zr vs. Hf emphasize that the behavior and enrichment of Ta and Nb are largely controlled by magmatic processes. The events that can explain the evolution of these rocks are: (1) during magmatic evolution, F dissolved in the magma and lowered the crystallization temperature causing REE and HFSE to form complexes and thus behave as incompatible elements, (2) prolonged crystallization of the major mineral phases (quartz and feldspar) formed a late-stage magmatic fluid enriched in volatiles (H 2O, F) and trace elements, (3) accessory minerals crystallized from such a phase in the interstices between the major mineral phases, and (4) post-magmatic re-equilibration and formation of secondary albite (Na-metasomatism) has probably led to the redistribution of some major elements and limited redistribution of HFSE and REE.

Late Ediacaran to early Cambrian (Infracambrian) Jibalah Group of Saudi Arabia

ABSTRACT This paper is one of a series that document the Neoproterozoic – Cambrian rock units in the Middle East Geologic Time Scale. It is focused on the oldest sedimentary succession in Saudi Arabia, the late Ediacaran – early Cambrian (Infracambrian) Jibalah Group (ca. 585 to 530–520 Ma). The group crops out in disconnected, pull-apart basins (ca. 10–100 km long and up to 20 km wide) along the NW-trending, strike-slip Najd Fault System in the Arabian Shield. It was described and mapped in the 1960s to 1980s, and several formations were defined and named in two areas separated by ca. 400 km. The stratigraphic successions in these two areas have not been correlated, nor has their relationship to the subsurface been resolved. This paper reviews the nomenclature, type sections, lithologies and ages of the formations and members (sometimes units and/or facies) of the Jibalah Group. The Jibalah Group unconformably overlies the Ediacaran Shammar Group (ca. 620–585 Ma, consisting mainly of rhyolite or granitic plutons), or older Proterozoic rocks. The age of the intervening Sub-Jibalah Unconformity is here estimated at ca. 585 Ma based on radiometric data and regional correlations. The lower part of the Jibalah Group is defined in the northern Arabian Shield in the Mashhad area, where it consists of three formations, in ascending order: (1) undated Rubtayn Formation, divided informally into the “Volcanic Conglomerate Member” (up to ca. 700 m thick), “Polymictic Conglomerate Member” (up to ca. 1,500 m thick) and “Sandstone Member” (up to ca. 1,000 m thick); (2) poorly dated Badayi Formation consisting of andesite-basalt flows (ca. 150 m thick); (3) undated Muraykhah Formation (330–370 m thick) consisting of the informal “Cherty Limestone Member” (ca. 135 m thick), “Siltstone and Mudstone Member” (ca. 20 m thick) and “Dolomitic Limestone Member” (ca. 135–175 m thick). The Rubtayn, Badayi and Muraykhah formations in the northern Arabian Shield, by stratigraphic position and lithology, correspond to the Umm Al ‘Aisah Formation in the Najd pull-apart basins of the central Arabian Shield. In particular, the Cherty Limestone unit (300–500 m thick) of the Umm Al ‘Aisah Formation is correlated to the Muraykhah Formation, which represents a marine flooding event. Above the Muraykhah Formation, the uppermost part of the group is defined in the central Arabian Shield by the undated Jifn Formation (up to ca. 2,500 m thick). The Jibalah Group is unconformably overlain by the lower Cambrian Siq Sandstone Formation (Asfar Sequence), and the intervening Sub-Siq Unconformity (Angudan Unconformity) has an estimated age between ca. 530–520 Ma.

Structural mapping from high resolution aeromagnetic data in west central Arabian Shield, Saudi Arabia using normalized derivatives

Aeromagnetic data covering an area of about 40,000 km2 at the west central Arabian Shield, Saudi Arabia has been collected and interpreted to provide structural map of the area. A number of normalized derivatives were used to help interpret the signature of magnetic data so that weak and small amplitude anomalies can be amplified relative to the stronger and larger amplitude anomalies. The interpretations obtained from these geophysical techniques of the field data demonstrated a strong correlation between magnetic anomalies and mapped subsurface geology. Based upon the variation in magnetic lineaments, shape amplitude, and trend structural map of the west central Arabian Shield on Saudi Arabia were obtained.

Geochemistry and Depositional Environments of the Tertiary Clays in Makkah and Rabigh Quadrangles, West Central Arabian Shield, Saudi Arabia

The present work deals with a detailed chemical study on the clay mineral assemblages characterizing some clay deposits, shales and mudstones, which differ in their texture, colour, lithology, and depositional environments. Samples selected from seven Tertiary formations located in Makkah and Rabigh quadrangles are chemically analyzed; some are used as environmental discriminators. The distribution of major and some trace elements in 77 samples has been investigated and the results obtained were tabulated and graphically illustrated. A wide range of variation is shown in the results of both major and trace constituents. This is due to their variable content of clay mineral constituents and the abundance or paucity of other admixtures, e.g. quartz, feldspar and minor heavy mineral grains. The local occurrence of minor gypsum and carbonate veinlets in some formations and ferrugination in others are also effective. A positive correlation exists in some formations between Fe2O3 and each of MnO, Cr, Ni, Cu & Zn; between each of Sr & Cu versus CaO; and in most cases between Ga and Al2O3. Whereas SiO2 shows a negative correlation with each of Al2O3 and L.O.I . Most of trace elements entered the depositional basin in close association with clay minerals and Ca-Fe-bearing detritus. The examined clay minerals and their trace elements are mainly source controlled (e.g. granitic and some pelitic rocks) and, an environmental illustration using Ga, B & Rb in a ternary diagram would 98 R.J. Taj et al. indicate transitional to marine depositional environments for the studied clay deposits.

Proterozoic ophiolites of the Arabian Shield and their significance in Precambrian tectonics

Abstract Neoproterozoic ophiolites, ranging in age from c. 870 Ma to c. 627 Ma, occur in several discrete suture and/or fault zones within the Arabian Shield and display a record of riftdrift, sea-floor spreading and collision tectonics during the evolution of the East African Orogen. The ophiolites within the Yanbu and Bir Umq suture zones in the west are among the oldest (870–740 Ma) in the Shield, locally show a Penrose-tyep complete pseudostratigraphy, and have chemical compositions typical of modern forearc oceanic crust. They are spatially associated with coeval and younger volcanic arc assemblages and were incorporated into the Arabian Shield during a series of collisional events that amalgamated these ensimatic arc terranes. The ophiolites of the Hulayfah-Ruwah suture zone in the central Arabian Shield are coeval with and/or slightly younger ( c. 843–821 Ma) than the ophiolites in the west and probably developed in a rifted ensimatic arc system that evolved as a volcanic archipelago near the Afif continental plate. Younger ophiolites ( c. 694 Ma) of the Halaban and Al Amar suture zones in the eastern Arabian Shield were incorporated into a subduction-accretion complex that evolved at the Andean-type active margin along the eastern edge (in present coordinate system) of the Afif continental plate. The Halaban suture zone ophiolites represent forearc oceanic crust, whereas the Al Amar suture zone ophiolites are scraped-off fragments of Mozambique ocean floor, seamounts and/or ocean island(s); the Abt Schist between them corresponds to a Franciscan-type accretionary prism of the ‘Halaban’ subduction zone. The incorporation of these ophiolites and the continental plates (Afif and Ar Rayn) into the Arabian Shield during 640–620 Ma marks a major shift in the direction of convergence (from northerly to westerly) during the assembly of the Shield and distinct episodes of continental collisions during closure of the Mozambique Ocean. The ophiolites of the Nabitah-Hamdah fault zone within the Asir terrane are the youngest ( c. 627 Ma) in the Shield, post-collisional in origin, display mid-ocean ridge basalt chemical affinity, and represent Ligurian-type oceanic crust developed in an intracontinental pararift zone. The ophiolite tectonics of the Arabian Shield indicates an eastward progression of continental growth through time as the East African Orogen was built during the late Neoproterozoic, following the breakup of Rodinia.

Mineralogy of the Tertiary Clay Deposits in Makkah and Rabigh Quadrangles, West Central Arabian Shield, Saudi Arabia

The mineralogy of the Tertiary clay deposits in Makkah and Rabigh quadrangles was thoroughly investigated by X-ray diffraction and differential thermal and thermogravimetric analyses in addition to other techniques. Results show that the investigated samples are predominantly composed of montmorillonite (Ca++ and/or Mg++ rich variety) and kaolinite, associated with subordinate illite and minor chlorite. Mixed layer montmorillonite-illite is recorded only in two samples. The relative abundance of these minerals by X-ray diffraction analysis showed that the studied clay deposits are of three types. The first, which is the most common, is highly montmorillonitic, the second is made up of a mixture of montmorillonite followed by kaolinite and illite and the third is highly kaolinitic with some montmorillonite. Generally, kaolinite shows a southward increase in Makkah quadrangle while chlorite, as a minor component, shows a northward increase in Rabigh quadrangle.