Geological and geophysical exploration of the groundwater aquifers of As Suqah area, Makkah district, Western Arabian Shield, Saudi Arabia

As Suqah area is a NW–SE trending wadi present in the west central part of the Arabian Shield. It comprises Precambrian–Cambrian basement rocks, Cretaceous–Tertiary sedimentary succession, Tertiary–Quaternary basaltic lava flows, and Quaternary–Recent alluvial deposits. The magnetic anomalies indicated the presence of many recent local buried faults. These affected the distribution of the clastic sedimentary succession and seem to have controlled the deep groundwater aquifers. Groundwater movement is towards the west and northwest, following in general the surface drainage system. Hydraulic gradient varies greatly from one point to another depending on the pumping rates and cross-sectional area of the aquifer in addition to its transmissivity. The detailed results of the resistivity and seismic measurements were integrated with those obtained from test holes drilled in the study area. Groundwater occurs mainly in two water-bearing horizons, the alluvial deposits and within the clastic sedimentary rocks of Haddat Ash Sham and Ash Shumaysi formations. The shallow zone is characterized with a saturated thickness of 3–20 m and water is found under confined to semi-confined conditions. Water levels were encountered at depths varying from 3 to 16 m in the alluvial wadi deposits and from 18 to 62 m in the sedimentary succession. The combinations of vertical electrical sounding, horizontal electrical profiling, and drilling led to the identification of groundwater resources in the study area. Resistivity soundings clearly identified the nature of the lithological depth and proved useful at identifying water-bearing zones. Significantly, the majority of the groundwater was found within the deep confined aquifer gravelly sandstone, rather than in the shallow unconfined aquifer.     

Microfacies, diagenesis, and depositional environments of the Tertiary carbonates of Usfan Formation in Haddat Ash Sham area, Western Arabian Shield, Saudi Arabia

The studied carbonates are present within the middle part of the Tertiary siliciclastic dominated succession in Haddat Ash Sham area. This succession consists of conglomerates, sandstones, siltstones, mudstones, and ironstones arranged in upward coarsening and fining cycles of intermittent depositional environments ranging from fluvial coastal plain to shallow marine. The carbonates represent deposition in slightly restricted depositional environments during periods of very low clastic input. The studied two carbonate sections show a pronounced lateral and vertical changes from predominantly dolostones in the west to mixed dolostones and limestones in the east. Both sections began by egg yellow dolomitic lime mudstones and fine crystalline dolomite which indicate the formation of dolomite by late depositional to early diagenetic recrystallization of precursor Fe and Mg lime mud. During this time period, the western part of the study area continues in the deposition of Fe and Mg lime mud in slightly restricted depositional environments and formation of interbedded fine and coarse crystalline dolostones in coarsening-upward cycles. In the eastern part, the depositional environments become deeper and more circulated with short-lived periods of fluctuation in sea level and hence the deposition of foraminiferal wackestone, bioclastic packstones, and bioclastic rudstone in coarsening and shallowing-upward cycles. Both the western and eastern parts of the study area are terminated by characteristic yellow coarse crystalline phosphatic dolomite which indicate ultimate stages of shoaling and increasing the level and concentration of phosphorous in the restricted water and hence the formation of in situ and reworked phosphatic components. The diagenetic processes affected the dolostones and limestones microfacies associations include: dolomitization of Fe and Mg-rich lime mud, hematitization, calcitization, gypsification, and phosphatization of dolomite. Neophormation and recrystallization of micritic matrix and micritic intraclasts as well as shell fragments are the main diagenetic processes affected the limestones.     

Using lithologic modeling techniques for aquifer characterization and groundwater flow modeling of the Sohag area, Egypt

Three dimensional lithologic modeling techniques have been used for detailed characterization and groundwater flow modeling of the Quaternary aquifer system of the Sohag area, Egypt. Well log data were used for building the lithologic model using RockWorks. A groundwater flow model, facilitated by MODFLOW 2000, was built using results of the lithologic model. The obtained lithologic model honored the well log data and revealed a complex sedimentary system, which is mainly composed of six lithologic categories: clay, clay and sand, fine sand, coarse sand, sand and gravel, and gravel. Inter-fingering and presence of lenses are the main characteristics of the sedimentary basin in the study area. A wide range of hydraulic conductivities is present, which vary spatially and control the groundwater flow. Heterogeneity of the aquifer system is spatially represented where different hydraulic conductivities are found in the different directions. Sandy layers tend to be connected. Hydraulic continuity is represented by inter-fingering and connection of sandy materials within the aquifer system.     

Interaction of aqueous fluids with calcareous metasediments during high-T, low-P regional metamorphism in the Qadda area, southern Arabian Shield

The S.W. Nabitah Mobile Belt, Saudi Arabia, contains a Proterozoic island-arc complex. In the Qadda area, the metavolcanic-dominated supracrustal sequence records amphibolite facies regional metamorphism of high-T , low-P type. Calcsilicate rocks and aluminous dolomitic marbles within the supracrustal sequence have been studied in detail to refine estimates of peak metamorphic P–T conditions and assess the role of fluids during prograde and retrograde metamorphism. Fluid-independent thermobarometers (including the calcite–dolomite thermometer and P-sensitive equilibria involving grossular, wollastonite, anorthite, meionite, quartz and calcite) yield peak P–T conditions of c. 650–660 °C, 4 kbar, both higher than previous estimates, giving a revised average thermal gradient of c. 45 °C km^–1. The close match between the peak temperatures implied by calcite–dolomite thermometry and those recorded by univariant devolatilization equilibria suggests that the calcareous rocks were fluid-bearing during late-prograde and peak metamorphic stages. These fluids were essentially binary H₂O–CO₂ mixtures with low NaCl and HF concentrations. Most were H₂O-rich, with X_CO2 between 0.02 and 0.2, but values of c. 0.6 are recorded by two samples. High modal abundances of the solid products of decarbonation reactions (e.g. c. 10–50% wollastonite) in many of the rocks that record low-X_CO2 equilibrium fluids implies infiltration of significant quantities of externally derived aqueous fluid during late-prograde metamorphism, but not enough to exhaust the buffering capacity of the rocks. Calculated minimum time-integrated fluid-to-rock ratios of five wollastonite-bearing calcsilicate rocks range from 0.7±0.22 to 1.39±0.46 (1σ); those of six marbles range from c. 0 to 4±1.4. The latter variation occurs on a metre-scale, implying focusing of fluid flow. Diopside-rich rocks record fluid-to-rock ratios of up to 88±48. Penetrative wollastonite lineations indicate a temporal link between infiltration and distributed ductile deformation. Infiltrating fluids were probably derived both from the prograde dehydration of adjacent metabasalts and metatuffs and from crystallization of voluminous pretectonic granitoid intrusions. In general, fluid-to-rock ratios deduced for the metavolcanic-dominated Qadda area are similar to those recorded by rocks in the metasediment-dominated terrane of N. New England. The occurrence of post-tectonic retrograde hydration textures in both carbonate-bearing and carbonate-free rocks otherwise lacking hydrous minerals testifies to infiltration of aqueous fluids during retrograde metamorphism in the absence of penetrative deformation. Minimum fluid-to-rock ratios calculated for secondary grossular reaction rims in some calcsilicates are c. 0.04. Later patchy hydration of scapolite probably utilized static, pore-filling fluids remaining after the early retrograde infiltration.     

Caldera-related volcanic rocks in the Shammar Group, northern Arabian Shield

Volcanic formations of the ca 630-620 Ma old Shammar Group in the Tuluhah area in the northern Arabian Shield occupy an oval area some 8×12 km. They overlie sedimentary rift-fill of the Kuara Formation and are interpreted as related to the formation of a caldera, here named the Awad Caldera. The earliest of the volcanic formations, the Dabsah Tuff, is more than 450 m thick in the south and wedges out in the north. It is composed of silicic, medial to proximal pyroclastic flow rocks that record an eruption during which an initial caldera is interpreted to have formed by probably trapdoor-style collapse. The Nijab Basalt, more than 200 m thick and present as flows overlying the Kuara Formation to the north of the caldera, is presumed to have originated outside the study area during an interval between periods of silicic volcanic activity, and to have flowed onto the Dabsah Tuff in the first-stage caldera. The succeeding Mindassa Megabreccia contains large rafts of the older Shammar rocks, mainly Nijab Basalt, in a tuff matrix, and is regarded as probably a caldera collapse and fallback megabreccia formed during a silicic eruption that led to the second stage of caldera development. The megabreccia is overlain by the post-collapse Sutayih Tuff, more than 450 m thick, composed of proximal pyroclastic flow units.     

Infracambrian superimposed tectonics in the Late Proterozoic units of Mount Ablah area, southern Asir Terrane, Arabian Shield, Saudi Arabia

Mount Ablah is a mining prospect, hosted by a dioritic igneous body that is bound to the east by greenschist grade metamorphic rocks and to the west by Ablah group volcano–sedimentary rocks. Rock units of Mount Ablah area were remapped through field investigations, petrological studies, and analysis of enhanced TM Landsat data. Ablah group rocks were divided into lower tuffaceous and upper epiclastic units. The epiclastic unit was divided into three subunits. During remapping, a sliver of serpentinite was discovered, which occurs between the dioritic igneous body and Ablah group rocks. The greenschist grade metamorphic rocks were intruded by Late Proterozoic quartz diorite prior to deposition of Ablah group rocks. The epiclastic unit is an infracambrian molassic unit that filled a graben, known as the Ablah graben. The Mount Ablah area was intruded by post-tectonic granitic rocks and affected by two superimposed F1-F2 folding events, associated with thrust and dextral faults, respectively. The first folding event involved N–S folding and thrusting. Simultaneously, stress partitioning at E–W accommodations zones produced E–W minor folds (F2) and associated E–W dextral faults. The F1–F2 folding events are contemporaneous with the Pan African deformation event, also known as the East African Orogeny (EAO). The EAO is infracambrian in age and culminated in development of the Najd sinistral fault system. The E–W dextral faults were probably reactivated during Cenozoic Red Sea rifting. The Ablah graben's infracambrian sedimentary rocks, such as siltstone, sandstone, and limestone that are mainly bound within the Ablah graben were not deformed prior to F1–F2 folding. Thus, the upper epiclastic unit of the Ablah group rocks is rheologically different from the surrounding greenschist rocks, responded to the late E–W compression in a more ductile manner than the surrounding greenschist rocks. Therefore, the Ablah graben was inverted, refolded, and crosscut by E–W dextral faults during the infracambrian EAO event, prior to development of Najd sinistral fault system, which are evident in Asir Terrane and crosscut Ablah graben.     

PGE exploration in the Wadi Amarah complex,southwestern Arabian Shield

A stream sediment survey targeting PGE and their pathfinders was carried out at a gabbro/pyroxenite complex in Wadi Amarah in the SW Arabian Shield. Twenty-nine stream sediment samples were collected from wadi channels and analyzed for PGE and base metals. None of the samples contained detectable PGE except for WAS-3 and WAS-27 which returned values above 70 ppb of combined Pt and Pd; these two locations are also anomalous for Au. A follow-up survey was conducted to further investigate the anomalies at these locations. A total of 48 soil and rock samples were collected from the two anomalous localities. Unlike the initial survey, most samples contained detectable PGE and Au albeit not as high as the original anomalies. Factor analysis of the results from the initial survey returned five main factors, with the first four reflecting the effects of mineral detritus as well as adsorption onto Fe–Mn oxyhydroxides; however, the last factor is loaded only for Cu and Ni and is therefore considered an ore factor indicating the presence of Cu–Ni sulphides. Another five factors were obtained from the follow-up survey, and as was the case in the initial survey, the first four factors are detrital/adsorption-related, while factor 5 has high loadings for only Pd and Pt and is therefore interpreted as a PGE ore factor. These findings reveal different modes of dispersion of PGE and base metals in the secondary environment and, more importantly, demonstrate the effectiveness of factor analysis in detecting even faint anomalies from disseminated mineralization.     

Seismic identification of geothermal prospecting in Harrat Rahat,Northern Arabian Shield

Influenced by mining activities in adjacent coal seams, stresses on rocks surrounding roadway were redistributed, and the roadways in lower coal seam were subjected to the asymmetrical roof falling and roof sagging. Considering stresses effect on the plastic zone around the roadway, numerical models were carried out by FLAC to investigate plastic zone with respect to stress ratio and direction of stresses. The relationship between the properties of surrounding rock and plastic zone boundary was also investigated by another numerical model and analytical study, whereby the tailgate stability of panel 30,501 in Tashan coal mine was implemented. It is shown that the rocks surrounding a roadway in the lower coal seam were subjected to unequal stresses, and the principal stress direction was deflected from the original direction. High stresses and big stress ratio can produce butterfly-shaped or X-shaped plastic zone. The direction of stresses was deflected, causing the plastic zone around the roadway to be transferred from the shoulder to the roof of the roadway. Consequently, asymmetrical stresses produce asymmetrical plastic zone. On this basis, the tailgate should be assigned conditions of the stresses and stress ratio at a low level. In this way, the tailgate was arranged at the position where the horizontal distance from the roadway in the lower seam to the centre line of the coal pillar in the upper seam (x) is 52.5 m, and was stable relatively.     

Structural analysis for metavolcanics and their metapyroclastics at gold deposit of the Mahd Ad Dahab area, Arabian Shield, Saudi Arabia

The present study focuses on the gold mining in Mahd Ad Dahab region of Saudi Arabia. The study aims to assess the spatial relationship between tectonic contacts in Mahd Ad Dahab area and to provide a meaningful hypothesis relating gold metallogeny to the evolution of the Arabian Shield. Distribution and localization of gold occurrences in the study area was envisaged based on the different styles of microstructures and the major deformation phases affecting the area. The detailed petrographical and mineralogical investigations indicate that the metavolcanic rocks at the Mahd Ad Dahab gold mine area can be classified into metabasalt, metaandesite, and the felsic varieties (metadacite, metarhyodacite and metarhyolite) associating their metapyroclastics (conglomerate and tuffs). Furthermore, quartz forms allotriomorphic crystals which exhibit wavy extinction, deformational lamina and foliation due to subsequent deformations. Furthermore, we conclude that finite strain in the deformed rocks is of the same order of magnitude for all units of metavolcano-sedimentary rocks. Furthermore, contacts formed during intrusion of plutons with some faults in the Mahd Ad Dahab area under brittle to semi-ductile deformation conditions. In this case, finite strain accumulated during superimposed deformation on the already assembled nappe structure. It indicates that the contacts formed during the accumulation of finite strain.     

Entropy in Markov chains and discrimination of cyclic patterns in lithologic successions

The concept of Markov chains, applied to stratigraphic sections, is reliable in analyzing cyclic patterns in lithologic successions. Randomness in the occurrence of lithologies repeating in a succession is evaluated generally in terms of entropies which can be calculated for the Markov matrix equated with the succession. Two types of entropies pertain to every state; one is relevant to the Markov matrix expressing the upward transitions, and the other, relevant to the matrix expressing the downward transitions. The latter and the former with respect to a certain state, making an entropy set, correspond to the degree of randomness in its linking with the others which occur as the precursor and the successor, respectively. It is obvious that the entropy sets which are calculated for all state variables serve as a reliable criterion in the discrimination of cyclic pattern of the succession. We are able based on the entropy sets to classify the various patterns into asymmetric, symmetric, and random cycles, which are exhibited also in actual lithologic successions. The entropy sets are calculated for Markov matrices which have been reported from a number of areas in the world, and compared with the cyclic patterns supposed there. Entropy for the whole system of sedimentation also is introduced to discuss variability of the condition in the depositional processes.     

Tectonic effects of sea-floor spreading on the Arabian Shield

Paleogeographic evidence shows that the series of broad E-W anticlines and synclines on the Arabian Shield (Southern Hadramawt Arch, Wadi Hadramawt Syncline, Northern Hadramawt Arch, Rub Al Khali Syncline, Tuwaiq Homocline, Nafud Basin) are not old, inherited structures, but were formed in late Eocene and Oligocene times, as indicated by the warping of Middle Eocene sediments. The fold axes of these structures trend parallel to the Gulf of Aden, and their separation increases from S to N, i. e., with increasing distance from the Gulf of Aden. The most pronounced orogenic phase of the Toros mountain belt and the folding of the foreland belt (Lebanon, Antilebanon, Palmyra Arch, Jebel Sinjar, etc.) took place simultaneously with the warping of the shield. Furthermore, the early Tertiary Trap Volcanism occurs only in the neighborhood of the Gulf of Aden (Yemen, W-Aden Protectorate, Eritrea, Ethiopia, Somaliland). Geophysical-oceanographic research in the Gulf of Aden suggests that emplacement of basic magmatic material forms a quasi-oceanic crust (sea-floor spreading) in that rift trough. This apparently causes the displacement of the continental blocks. The close connections in time as well as in directional trends of epirogenic, orogenic and volcanic activities on the Arabian Shield to the sea-floor spreading in the Gulf of Aden indicates tectonic interrelations.This impression is still emphasized, if one considers the younger tectonic development on the shield. The young Tertiary Aden-Volcanics Belt (Miocene-Recent) extends from the East African Rift system over the West Arabian Shield all the way up to Turkey, that is to say its trend is more or less parallel to the Red Sea. Warping effects (Ras en Naqb Uplift, Jafr Depression, Bayir Uplift, Wadi Sirhan Depression, Rutba Dome and the Mesopotanian Basin) on the northern part of the Arabian Shield, where the earlier developed (Aden Gulf-related) structures die out, can be related in time and direction to the rifting in the Red Sea. Faulting along the Aqaba-Dead Sea System is of the same age and cuts the foreland belt. Finally the folding of the Zagros mountain belt is of Miocene age too.The Arabian Shield, bounded by still-active rifting structures of different direction and age, provides a classical example of the effect of sea-floor spreading on a shield area itself, and on its surrounding instable belt. The correct interpretation of these tectonic connections eventually may allow far reaching, basic conclusions.

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Zusammenfassung Die känozoische tektonische und vulkanologische Entwicklung auf und um den Arabischen Schild ist relativ jung und im Vergleich zu anderen Gegenden in ihrer Gesamtheit noch verhältnismäßig gut überschaubar. Sie bietet daher ein Beispiel, dessen Verständnis möglicherweise von grundlegender Bedeutung für die Interpretation gebirgsbildender Vorgänge werden kann.Der Arabische Schild ist im Süden und Südwesten umrahmt von den Rift-Systemen des Golfes von Aden und Roten Meeres, deren zentrale Teile durch quasi-ozeanische Kruste gekennzeichnet sind. Die Einschübe basischen magmatischen Materials (sea-floor spreading) in die Rifttröge verursachten offenbar eine Verdrängung der kontinentalen Blöcke (Bewegungssinn senkrecht zum Streichen der Zonen des aktiven sea-floor spreading). Jedenfalls läßt sich ein solcher Beanspruchungsplan von den verschiedenen tektonischen Teilvorgängen auf dem Arabischen Schild ableiten.Die Entwicklung des Golfes von Aden ist älter als die des Roten Meeres, und das gilt dementsprechend für die Eo- bis Oligozänen vulkanischen, epirogenen und orogenen Vorgänge, die räumlich und zeitlich Beziehungen zum Geschehen im Golf von Aden aufweisen (Trap-Vulkanismus, Verbiegungen des Süd- und Zentral-Arabischen Schildes und Auffaltung des Taurusgebirges und Palmyra-bogens). Alle Ereignisse, die räumliche Beziehungen zur Rotcn-Meer-Streichrichtung zeigen (Aden-Vulkanismus, epirogene Verbiegung des Nord-Arabischen Schildes und Auffaltung des Zagrosgebirges) sind jünger, d. h. seit dem Miozän besonders aktiv.

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Résumé L'évidence paléogéographique montre que, comme l'indique le gauchissement des sédiments de l'Eocéne Moyen, les séries des grands géanticlinaux et synclinaux, orientés est-ouest, sur le craton arabique (S. Hadramawt Arc, Wadi Hadramawt syncline, N. Hadramawt géanticline, Rub Al Khali syncline, Tuwaiq homocline, Nafud bassin) ne sont pas des vieilles structures antérieures, mais ont été formés durant l'Eocéne et l'Oligocène. Les axes de plissement de ces structures ont tendance à être parallèles au Golfe d'Aden. Leur séparation augmente du sud vers le nord, c'est à dire de la même manière que leur distance du Golfe d'Aden augmente. La phase orogénique la plus prononcée de la ceinture montagneuse de Toros et les prémontagnes de l'Arabie septentrionale (Lebanon, Antilebanon, Palmyra Arc, Jebel Sinjar) eurent lieu au même moment que le gauchissement du craton arabique. Bien plus, le «Trap»-volcanisme du Tertiaire Inférieur n'a lieu que dans le voisinage du Golfe d'Aden (Yemen, W-Aden Protectorat, Éritrea, Ethiopia, Somali). Des recherches géophysiques et océanographiques dans le Golfe d'Aden et la Mer Rouge suggèrent que des emplacements de matériaux d'origine magmatique forment une croûte quasi-océanique («Sea-floor Spreading») dans le fossé d'effondrement. Ceci est apparement la cause du déplacement des blocs continentaux. Les proches coïncidences aussi bien en époque qu'en direction des activités épirogéniques, orogéniques et volcaniques entre le craton arabique et le «Sea-floor Spreading» du Golfe d'Aden indiquent des correspendances tectoniques.Cette impression est encore plus renforcée, si l'on considère les développements plus récents du craton arabique. La jeune ceinture Tertiaire d'«Aden»-Volcanisme (Miocène-Recent) s'étend depuid le rift d'Afrique de l'est jusqu'en Turquie au travers du craton arabique de l'ouest. Elle se trouve de ce fait être plus ou moins parallèle à la Mer Rouge. Les gauchissements (Ras En Naqb, Jafr dépression, Bayir hautes plaines, Wadi Sirhan bassin, Rutba dome, Bassin Mésopotanien) de la partie septentrionale du craton arabique, où disparaissent des structures (identiques à celles du Golfe d'Aden) dévelopées au paravent, peuvent être associées en temps et direction au « rifting » de la Mer Rouge. Les failles le long du système Aquaba-Mer Morte sont du même âge et coupent les prémontagnes de l'Arabie septentrionale. Finalement le plissement de la ceinture montagneuse de Zagros appartient aussi au Miocène.Le craton arabique délimité par des structures d'âge et de directions différentes et toujours en cours de séparation, est un example classique de l'effet de la «Sea-floor Spreading» sur un craton et sur sa ceinture instable environnante. L'interprétation correcte de ces effets tectoniques resultera, le cas échéant, en des conclusions fondamentales très importantes.

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Formerly Southwest Center for Advanced Studied P. O. Box 30 365 Dallas Texas 75 230(Contribution No.81).     

Volcanogenic mineralization and a rhyolite dome in the Arabian Shield

The district of Mahawiyah in the Proterozoic shield of Arabia contains a group of Zn-Cu-Au-Ag-Ba mineral prospects in folded meta-sedimentary, volcanoclastic and volcanic rocks, ranging in composition from basalt to rhyolite. The mineralization occurs in veins and as strata-bound, disseminated orebodies associated with intense argillic alteration of adjacent rocks. An intrusive rhyolite dome or laccolith is situated at the centre of an eight square kilometre area of slight but pervasive alteration whose outline can be traced from aerial photographs and within which many of the ore mineral occurrences lie. A model is proposed to explain the pattern of alteration in the volcano-sedimentary pile and formation of the volcanogenic mineralization, based on a concept of the dome acting as a heat source to drive a geothermal "cell". Circulating connate-hydrothermal fluids could have caused alteration and redistribution of trace metals within the volcanics and sediments which mantle the sub-volcanic, rhyolite intrusion. The ore genetic model implies that clusters of veins, disseminated strata-bound and stratiform massive sulphide orebodies occur in distinct areas of the shield, marked by tracts of pervasive alteration which can be identified in aerial photographs and satellite images.     

A lead isotope study of mineralization in the Saudi Arabian Shield

New lead isotope data are presented for some late Precambrian and early Paleozoic vein and massive sulfide deposits in the Arabian Shield. Using the Stacey Kramers (1975) model for lead isotope evolution, isochron model ages range between 720 m.y. and 420 m.y. Most of the massive sulfide deposits in the region formed before 680 m.y. ago, during evolution of the shield. Vein type mineralization of higher lead content occurred during the Pan African event about 550 m.y. ago and continued through the Najd period of extensive faulting in the shield that ended about 530 m.y. ago. Late post-tectonic metamorphism may have been responsible for vein deposits that have model ages less than 500 m.y. Alternatively some of these younger model ages may be too low due to the mineralizing fluids acquiring radiogenic lead from appreciably older local crustal rocks at the time of ore formation.The low²⁰⁷Pb/²⁰⁴Pb ratios found for the deposits in the main part of the shield and for those in north-eastern Egypt, indicate that the Arabian craton was formed in an oceanic crustal environment during the late Precambrian. Involvement of older, upper-crustal material in the formation of the ore deposits in this part of the shield is precluded by their low²⁰⁷Pb/²⁰⁴Pb and²⁰⁸Pb/²⁰⁴Pb characteristics.In the eastern part of the shield, east of longitude 44°20E towards the Al Amar-Idsas fault region, lead data are quite different. They exhibit a linear²⁰⁷Pb/²⁰⁴Pb-²⁰⁶Pb/²⁰⁴Pb relationship together with distinctly higher²⁰⁸Pb/²⁰⁴Pb characteristics. These data imply the existence of lower crustal rocks of early Proterozoic age that apparently have underthrust the shield rocks from the east. If most of the samples we have analyzed from this easterly region were mineralized 530 m.y. ago, then the age of the older continental rocks is 2,100±300 m.y. (2).The presence of upper crustal rocks, possibly also of early Proterozoic age, is indicated by galena data from Hailan in South Yemen and also from near Muscat in Oman. These data are the first to indicate such old continental material in these regions.     

柴达木盆地东坪地区侏罗系分布及基岩岩性预测

柴达木盆地东坪地区东坪1井天然气的发现揭开了柴达木盆地基岩天然气藏勘探的序幕。勘探和研究认为,东坪地区基岩气藏以侏罗系烃源岩为气源,纵向上通过断裂、裂缝系统迁移,横向上通过不整合面、优质砂体运移,在花岗岩和片麻岩基岩中聚集成藏。因此,侏罗系分布和基底岩性预测是该区基岩气藏勘探的关键问题。通过CEMP勘探结合重磁资料研究,明确了侏罗系的分布,预测了东坪-冷湖地区基岩分布及特征,与钻井吻合良好,对东坪-冷湖地区的基岩气藏勘探具有指导意义。     

Late Neoproterozoic adakitic magmatism of the eastern Arabian Nubian Shield

Late Neoproterozoic adakitic magmatism within the Eastern Arabian Nubian Shield has been dated at633.2±9.0 Ma(2σ).These magnas intrude the forearc Ad Dawadimi Basin,which is composed of metapelitic schists and greywacke along with ophiolitic melanges of boninitic affinity which underwent inversion and deformation by~620 Ma.This adakitic magmatism,while intruding parts of the Ad Dawadimi Basin,predates this deformation,but is possibly coincident with basin closure.As adakitic magmatism requires melting of an amphibolite or eclogitic source,empirical and experimental constraints require anomalously hot supra-subduction zone mantle.Considering that this magmatism immediately predates basin inversion,these magmas possibly pinpoint the timing of the slab breaking,marking the terminal stages of arc magmatism,terrane accretion and the influx of hot oceanic asthenospheric mantle.This influx of hot asthenospheric mantle may also be responsible for postcolltsional A-type magmatism.