Transpressive Structures in the Ghadir Shear Belt, Eastern Desert, Egypt: Evidence for Partitioning of Oblique Convergence in the Arabian-Nubian Shield during Gondwana Agglutination

Transpressional deformation has played an important role in the late Neoproterozoic evolution of the ArabianNubian Shield including the Central Eastern Desert of Egypt.The Ghadir Shear Belt is a 35 km-long,NW-oriented brittleductile shear zone that underwent overall sinistral transpression during the Late Neoproterozoic.Within this shear belt,strain is highly partitioned into shortening,oblique,extensional and strike-slip structures at multiple scales.Moreover,strain partitioning is heterogeneous along-strike giving rise to three distinct structural domains.In the East Ghadir and Ambaut shear belts,the strain is pure-shear dominated whereas the narrow sectors parallel to the shear walls in the West Ghadir Shear Zone are simple-shear dominated.These domains are comparable to splay-dominated and thrust-dominated strike-slip shear zones.The kinematic transition along the Ghadir shear belt is consistent with separate strike-slip and thrustsense shear zones.The earlier fabric(S1),is locally recognized in low strain areas and SW-ward thrusts.S2 is associated with a shallowly plunging stretching lineation(L2),and defines^NW-SE major upright macroscopic folds in the East Ghadir shear belt.F2 folds are superimposed by^NNW–SSE tight-minor and major F3 folds that are kinematically compatible with sinistral transpressional deformation along the West Ghadir Shear Zone and may represent strain partitioning during deformation.F2 and F3 folds are superimposed by ENE–WSW gentle F4 folds in the Ambaut shear belt.The sub-parallelism of F3 and F4 fold axes with the shear zones may have resulted from strain partitioning associated with simple shear deformation along narrow mylonite zones and pure shear-dominant deformation in fold zones.Dextral ENEstriking shear zones were subsequently active at ca.595 Ma,coeval with sinistral shearing along NW-to NNW-striking shear zones.The occurrence of upright folds and folds with vertical axes suggests that transpression plays a significant role in the tectonic evolution of the Ghadir shear belt.Oblique convergence may have been provoked by the buckling of the Hafafit gneiss-cored domes and relative rotations between its segments.Upright folds,fold with vertical axes and sinistral strike-slip shear zones developed in response to strain partitioning.The West Ghadir Shear Zone contains thrusts and strikeslip shear zones that resulted from lateral escape tectonics associated with lateral imbrication and transpression in response to oblique squeezing of the Arabian-Nubian Shield during agglutination of East and West Gondwana.

Geochemistry and Petrogenesis of Late Ediacaran Rare-metal Albite Granites of the Arabian-Nubian Shield

The Abu Dabbab albite granite(ADAG),in the central Eastern Desert of Egypt,hosts the most significant rare metal ore deposit in the northern part of the Neoproterozoic Arabian-Nubian Shield.Here,we report detailed field,petrographic,mineralogical and geochemical investigation of the ADAG,an isolated stock-like granitic body with sharp intrusive contacts against metamorphic country rocks,probably emplaced at about 600 Ma.The fine-grained porphyritic upper unit is a preserved remnant of the shallowly-emplaced apex of the magma chamber,whereas the medium-grained lower unit crystallized at deeper levels under subvolcanic conditions.The peraluminous leucocratic ADAG shares common geochemical characteristics with post-collisional intraplate A-type magmas.In addition to the conspicuous enrichment in Na2O,the ADAG is remarkable for its anomalous concentrations of Ta,Nb,Li,Hf,Ga,Sn,Zn and heavy rare-earth elements.Nb-Ta minerals in the ADAG are mixed with Fe-Mn oxides,forming black patches that increase in abundance toward of the base of the intrusion.Columbite-tantalite,cassiterite and wolframite are the most important ore minerals.Pronounced negative Eu anomalies(Eu/Eu*=0.10–0.24)reflect extreme magmatic fractionation and perhaps the effects of late fluid-rock interaction.The ADAG was most likely generated by partial melting of the juvenile middle crust of the ANS as the geotherm was elevated by erosional uplift following lithospheric delamination and it was emplaced at the intersection of lineations of structural weakness.Although formation of the ADAG and its primary enrichment in rare metals are essentially due to magmatic processes,late-stage metasomatism caused limited redistribution of rare metals.Fluid-driven subsolidus modification was limited to the apex of the magma chamber and drove development of greisen,amazonite,and quartz veins along fracture systems.

The possible source of uranium mineralization in felsic volcanic rocks,Eastern Desert,Egypt of the Arabian-Nubian Shield:Constraints from whole-rock geochemistry and spectrometric prospection

The present work deals with the detailed geology,mineralogy,geochemistry,and spectrometric prospection of the felsic volcanic rocks at the Eastern Desert,Egypt of the Arabian-Nubian Shield.Felsic volcanic rocks are an essential source for rare earth elements(REEs)and uranium occurrences in this area.They are compositionally uniform with tholeiitic to calc-alkaline affinities,peraluminous and belong to the series of rhyolite with high-K melt.They exhibit more enrichment in high field strength elements(HFSE,e.g.Zr,Ta,Nd,Th,and U)and large-ion lithophile elements(LILE,e.g.Pb and Rb)compared to the country rocks of the studied area,with REE ranging from 188.20 to 442.70 ppm and strong depletion in Ti,Sr,P with deep negative Eu oddities.The felsic volcanic rocks were mostly generated from the partial melting of quartz-amphibolite facies accreted during the Neoproterozoic.Positive oddities of Zr-U-Th for the felsic volcanic rocks determine the involvement of crustal materials.Felsic volcanic rocks are found in A-type suites of magma and represent highly fractionated rocks derived from rhyolitic magma,with insignificant interaction with continental crust in the low-pressure environment and during fractional crystallization.Felsic volcanic rocks have higher values of radioactivity in which eU range from 0.5 to 121 ppm and eTh from 1.0 to 415.10 ppm.The high values of eU and eTh can be ascribed to the mineralization of uranium and the presence of accessory minerals of radiogenic nature such as uranophane,uranothorite,zircon,and monazite.Uranophane is considered as the mineral with most enriched uranium contents in the studied felsic volcanic rocks in which(UO2=87.30 wt%).Also,they are enriched with REE-bearing accessory minerals comprising allanite,titanite,and apatite.The geological investigations of the felsic volcanic rocks in the studied areas are inappropriate to clear the feasible economic potentialities of rare earth elements and U occurrences;itemized and invaluable explorational work is as yet needed.Whilst,the environmental impact of mineralization,owing to U and Th and their radiogenic daughter products,is observed and must be elaborated minutely.

On the cratonization of the Arabian-Nubian Shield: Constraints from gneissic granitoids in south Eastern Desert, Egypt

The Shaitian granite complex(SGC)spans more than 80 Ma of crustal growth in the Arabian–Nubian Shield in southeast Egypt.It is a voluminous composite intrusion(60 km2)comprising a host tonalite massif intruded by subordinate dyke-like masses of trondhjemite,granodiorite and monzogranite.The host tonalite,in turn,encloses several,fine-grained amphibolite enclaves.U-Pb zircon dating indicates a wide range of crystallization ages within the SGC(800±18Ma for tonalites;754±3.9 Ma for trondhjemite;738±3.8 Ma for granodiorite;and 717±3.2 Ma for monzogranite),suggesting crystallization of independent magma pulses.The high positiveεNdi(+6–+8)indicate that the melting sources were dominated by juvenile material without any significant input from older crust.Application of zircon saturation geothermometry indicates increasing temperatures during the generation of melts from 745±31℃ for tonalite to 810±25℃ for trondhjemite;840±10℃ for granodiorite;and 868±10℃ for monzogranite.The pressure of partial melting is loosely constrained to be below the stability of residual garnet(<10 kbar)as inferred fromthe almost flat HREE pattern((Gd/Lu)N=0.9–1.1),but>3 kbar for the stability of residual amphibole as inferred from the significantly lower NbN and TaN compared with LREEN and the sub-chondrite Nb/Ta ratios exhibited by the granitic phases.The inverse relation between the generation temperatures and the ages estimates of the granitoid lithologies argue against a significant role of fractional crystallization.The major and trace element contents indicate the emplacement of the SGC within a subduction zone setting.It lacks distinctive features for melt derived from a subducted slab(e.g.high Sr/Y and high(La/Yb)N ratios),and the relatively low MgO and Ni contents in all granite phases within the SGC suggest melting within the lower crust of an island arc overlying a mantlewedge.Comparisonwith melts produced during melting experiments indicates an amphibolite of basaltic composition is the best candidate as source for the tonalite,trondhjemite and granodiorite magmas whereas the monzogranite magma is most consistent with fusion of a tonalite protolith.Given the overlapping Sm-Nd isotope ratios as well as several trace element ratios between monzogranite and tonalite samples,it is reasonable to suggest that the renewed basaltic underplating may have caused partialmelting of tonalite and the emplacement ofmonzogranite melt within the SGC.The emplacement of potassic granite(monzogranite)melts subsequent to the emplacement of Na-rich granites(tonalitetrondhjemite-granodiorite)most likely suggests major crustal thickening prior arc collision and amalgamation into the over thickened proto-crust of the Arabian-Nubian shield.Eventually,after complete consolidation,the whole SGC was subjected to regional deformation,most probably during accretion to the Saharan Metacraton(arc–continent collisions)in the late Cryogenian-Ediacaran times(650–542 Ma).

Constraints of Mantle and Crustal Sources Interaction during Orogenesis of Pre-and Post-collision Granitoids from the Northern Arabian-Nubian Shield:A Case Study from Wadi El-Akhder Granitoids,Southern Sinai,Egypt

The Egyptian older and younger granitic rocks emplaced during pre-and post-collision stages of Neoproterozoic Pan-African orogeny,respectively,are widely distributed in the southern Sinai Peninsula,constituting 70% of the basement outcrops.The Wadi El-Akhder,southwestern Sinai,is a mountainous terrain exposing two granitoid suites,namely the Wadi El-Akhder Older Granites(AOG)and the Homra Younger Granites(HYG).The AOG(granodiorites with subordinate tonalite compositions)have geochemical characteristics of medium-K calc-alkaline,metaluminous to mildly peraluminous granitoids formed in an island-arc environment,which are conformable with well-known Egyptian older granitoids rocks,whereas the HYG display calc-alkaline to slightly alkaline nature,peraluminous syeno-,monzogranites and alkali feldspar granites matching well those of the Egyptian younger granites.With respect to the AOG granitoids,the HYG granites contain lower Al_(2)O_(3),FeO^(*),MgO,MnO,CaO,TiO_(2),Sr,Ba,and V,but higher Na_(2)O,K_(2)O,Nb,Zr,Th,and Rb.The AOG are generally characterized by enrichment in LILE and LREE and depletion in HFSE relative to N-MORB values(e.g.,negative Nb and Ta anomalies).The geochemical features of the AOG follow assimilation-fractional crystallization(AFC)trends indicative of extensive crustal contamination of magma derived from a mantle source.The chemical characteristics of the AOG are remarkably similar to those of subduction-related granitoids from the ArabianNubian Shield(ANS).The compositional variations from monzogranites through syenogranites to alkali feldspar granite within HYG could not be explained by fractional crystallization solely.Correlating the whole-rock composition of the HYG to melts generated by experimental dehydration melting of meta-sedimentary and magmatic rocks reveals that they appear to be derived by extended melting of psammitic and pelitic metasediments,which is similar to the most of younger granitic suites in the ANS.

Integrated interpretation of dual frequency induced polarization measurement based on wavelet analysis and metal factor methods

In mineral exploration, the apparent resistivity and apparent frequency （or apparent polarizability） parameters of induced polarization method are commonly utilized to describe the induced polarization anomaly. When the target geology structure is significantly complicated, these parameters would fail to reflect the nature of the anomaly source, and wrong conclusions may be obtained. A wavelet approach and a metal factor method were used to comprehensively interpret the induced polarization anomaly of complex geologic bodies in the Adi Bladia mine. Db5 wavelet basis was used to conduct two-scale decomposition and reconstruction, which effectively suppress the noise interference of greenschist facies regional metamorphism and magma intrusion, making energy concentrated and boundary problem unobservable. On the basis of that, the ore-induced anomaly was effectively extracted by the metal factor method.

非洲东北部阿拉伯-努比亚地盾(ANS)构造演化与金成矿作用

阿拉伯-努比亚地盾(Arabian Nubian Shield,简称ANS)是900~550Ma期间冈瓦纳超大陆汇聚过程中形成的增生造山带,这一造山过程也被称为是泛非造山运动。它记录了一个长期的造山演化历史,经历了从大洋俯冲、岛弧形成及弧后的岩浆作用到大陆板块碰撞地体的拼合,再到新生地壳的逃逸构造、走滑剪切、张性断裂一系列的构造演化过程。这个演化可以分为四个阶段:(1)洋盆形成阶段(870~800 Ma);(2)洋壳俯冲阶段(800~670 Ma);(3)造山阶段(750~550 Ma);(4)后造山阶段(550 Ma~三叠纪),其中后三个阶段都有金的富集成矿作用。洋壳俯冲阶段的金矿化主要赋存在Algoma型含铁建造层(BIF)、凝灰质变质碎屑岩,以及火山成因的块状硫化物矿床内。造山阶段的主要金矿化类型为含金石英-碳酸盐脉状金矿化、与斑岩铜矿化有关的金矿化,以及与辉长岩类岩体有关的含金石英脉状矿化。与后造山阶段有关的金矿化以少量浸染状、网脉状并伴有Sn-W-Ta-Nb矿化的石英脉为特征。目前在ANS中发现了大量金矿床或矿点,它们具有各种不同的成因类型。根据构造背景及赋矿围岩,ANS原生金矿化可以划分为三类:(1)与火山沉积序列有关的金矿化,包括VMS型、浅成热液型;(2)空间分布上与碳酸盐化蛇绿岩带相关的金矿化;(3)与后造山或造山晚期闪长岩-花岗岩岩体或次火山岩有关的金矿化。

Serpentinized Peridotites at the North Part of the Wadi Allaqi District(Egypt): Implications for the Tectono-Magmatic Evolution of Fore-arc Crust

The Neoproterozoic Allaqi-Heiani suture （800-700 Ma） in the south Eastern Desert of Egypt is the northernmost linear ophiolitic belt that defines an arc-arc suture in the Arabian- Nubian shield （ANS）. The Neoproterozoic serpentinized peridotites represent a distinct lithology of dismembered ophiolites along the Allaqi-Heiani suture zone. The alteration of peridotites varies, some contain relicts of primary minerals （Cr-spinel and olivine） and others are extremely altered, especially along thrusts and shear zones, with development of talc, talc-carbonate and quartz-carbonate. The fresh cores of the chromian spinels are rimmed by ferritchromite and Cr- magnetite. The fresh chromian spinels have high Cr# （0.62 to 0.79）, while Mg# shows wider variation （0.35-0.59）. High Cr# in the relict chromian spinels and Fo content in the primary olivines indicate that they are residual peridotites after extensive partial melting. The studied ophiolitic upper mantle peridotites are highly depleted and most probably underwent high degrees of partial melting at a supra-subduction zone setting. They can be produced by up to -20%-22% dynamic melting of a primitive mantle source. The mineralogical and geochemical features of the studied rocks reflect that the mantle peridotites of the north part of the Wadi Allaqi district are similar to the fore-arc peridotites of a supra-subduction zone.

Geochemical Constraints of the Ediacaran Volcano-Sedimentary Succession of the Sa'al Metamorphic Complex at Wadi Zaghra, South Sinai, Egypt

The volcano-sedimentary succession around Wadi Zaghra in Sinai, at the northernmost segment of the Arabian Nubian Shield, comprises volcanic rocks interbedded with rather immature sediments. The succession is dominated by intermediate to silicic volcanics of medium-to high-K calc-alkaline affinity. It is divided into two units, the lower unit includes intermediate rocks and dacites interbedded with graywackes, semi-pelites and pelites and topped by polymict conglomerates. This unit is subjected to folding and regional metamorphism(up to garnet zone) and is intruded by quartz diorite-granodiorite inducing, locally, low-pressure contact thermal metamorphism. The unmetamorphosed upper unit encompasses acid volcanics intercalated with litharenite, sublitharenite and minor arenite. The rhyolites of this unit pertain to the highly fractionated granites and are characterized by an agpaitic index(NK/A) ranging from 0.87 to 0.96. They may reflect either extensive interaction of subduction-related magmas with the continental crust or a change in the tectonic regime. The present lithological and geochemical characteristics of the studied sediments together with available zircon ages indicate rather distal provenance of their detritus. This detritus comprises fluvial-alluvial sediments accumulated in the intermontane basins, which are half-grabens or tilted fault blocks. The tectonic setting of the depositional basins is active continental margin and continental island arcs. Geochemical patterns of the Zaghra volcano-sedimentary succession indicate their correlation with the Dokhan Volcanics-Hammamat Clastics sequence of the Eastern Desert of Egypt. Also, the Zaghra volcanics display geochemical similarities with those exposed in Sinai, at the Rutig, Ferani and Iqna Shar'a areas. The Zaghra succession is dated as Ediacaran but is not related either to the ensimatic island arc assemblage or to the rift-related assemblage formed during the early stages of the break-up of Rodinia as previously thought.

Genesis and Tectonic Implications of the Kabr El-Bonaya Ultramafic Rocks,Sinai Peninsula,Egypt:Constraints from Mineralogical and Geochemical Characteristics

The Kabr El-Bonaya mafic–ultramafic intrusion is exposed along the southeastern border of the Sinai Peninsula and the northernmost segment of the Arabian-Nubian Shield(ANS).It occurs as an elliptical intrusive body that is located along the major NE–SW trending fracture zones that prevail in the Kid metamorphic complex.The ultramafic rocks in the complex comprise ultramafic cumulates of peridotites(dunite,harzburgite and wehrlite)and pyroxenite.These rocks are generally unmetamorphosed and have intrusive contacts with the country rock.Mineral chemistry and whole-rock chemical compositions of these ultramafic rocks are mostly consistent with those of residual mantle peridotites from refractory suprasubduction tectonic settings.Based on the variations of the major elements,the studied ultramafic rocks are consistent with those of a supra-subduction zone mantle,as it seems to have melted at 1–2 GPa and 1300–1450℃.Linear variations of Al2O3,Ca O,V and Ni with Mg O,coupled with incompatible and rare-earth-element depletion and mineral compositions,suggest prior events of partial melting in both wehrlites and harzburgites.The LREE enrichment in the harzburgite,as well as the development of Cr-rich spinel,is consistent with a history of melt–peridotite interaction.The calculated(Sm/Yb)N variations for the studied peridotites indicate a general increase in the addition of fluids with an increasing degree of melting from the wehrlite(~13–15 wt%of fluid)in the source,after initial spinel peridotite melting to the harzburgite(~20–25 wt%of fluid)in the same source,which is contrary to normal abyssal peridotites.The estimated equilibration temperature ranges from 1214 to 1321℃for the studied wehrlites and from 1297 to 1374℃for harzburgites.The Mg-rich nature of the analysed olivines from the studied ultramafic rocks(Fo=81.41 to 91.77)reflect their primary composition and are similar to olivines in Alaskan-type ultramafic rocks.The Fo content of the analyzed olivines decrease slightly from the dunite to the harzburgite to the wehrlite and to pyroxenite,reflecting a fractional crystallization trend.The high Cr#and very low TiO_(2)contents(0.03–0.12 wt%)of the Cr-spinels from the studied peridotites are mostly consistent with modern highly refractory fore-arc peridotites,indicating that these peridotites developed in a supra-subduction zone environment.

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.

Petrogenesis of granitic rocks of the Jabal Sabir area,South Taiz City,Yemen Republic

The Tertiary granitic intrusive body(～21 Ma) of the Jabal Sabir area was emplaced during the early stages of the Red Sea opening.This intrusive body occupies the southern sector of Taiz City.It is triangular in shape,affected by two major faults,one of which is in parallel to the Gulf of Aden,and the other is in parallel to the eastern margin of the Red Sea coast.The petrogenesis of such a type of intrusion provides additional information on the origin of the Oligo-Miocene magmatic activity in relation to the rifting tectonics and evolution of this part of the Arabian Shield.The granitic body of Jabal Sabir belongs to the alkaline or peralkaline suite of A-type granites.It is enriched in the REE.The tight bundle plot of its REE pattern reflects neither tectonism nor metamorphism.This granite body is characterized by high alkali(8.7%-10.13%),high-field strength elements(HFSE),but low Sr and Ba and high Zn contents.The abundance of xenoliths from the neighboring country rocks and prophyritic texture of the Jabal Sabir granite body indicate shallow depths of intrusion.The major and trace elements data revealed a fractional crystallization origin,probably with small amounts of crustal contamination.It is interpreted that the Jabal Sabir intrusion represents an anorogenic granite pertaining to the A-type,formed in a within-plate environment under an extensional tectonic setting pertaining to rift-related granites.