petrogenesis and tectonic setting of late precambrian ensimatic volcanic rocks central eastern desert of egypt abstract early stages in the geologic evolution of the central eastern desert of egypt ced reflect an intense episode of ensimatic volcanic activity similar to modern magmatism of the ocean floors and island arcs this paper reports results from studies of the petrology and petrogenesis and interprets the significance of these late precambrian volcanic rocks a three fold stratigraphy is preserved in the basement of the ced a basal section of oceanic crust includes ultramafics gabbros and pillowed basalts these older metavolcanics omv are conformably succeeded by dominantly volcanogenic metasediments which are in turn succeeded by a dominantly andesitic calc alkaline sequence of younger metavolcanics ymv the omv and ymv are largely restricted to the ced in egypt but analogous terranes are found in northern arabia 40 400 ppm and ni 30 260 ppm they are poor in k 2 o 0 05 0 92 rb 0 3 5 0 ppm and ba 11 89 ppm on ti zr cr v ni p discriminant diagrams the omv plot in the field of modern abyssal tholeiites high k rb 450 1800 and light ree depletions support this inference although k ba 25 45 is lower than modern mid ocean ridge basalts morb the sum of omv geochemical characteristics requires that these magmas were derived by the fractional fusion of the mantle it is suggested that the omv were generated by 20 25 fractional melting of previously depleted mantle at depths of less than 60 km relatively little fractionation accompanied ascent to the surface where the omv were erupted in a primitive crustal environment either a small oceanic rift or a back arc basin metamorphism of the ymv resulted in little elemental redistribution these andesites have sub alkaline clinopyroxenes and major element geochemical characteristics indistinguishable from modern calc alkaline andesites ymv andesites in the central and western ced have k rb 400 600 k ba 20 40 and are light ree enriched and heavy ree depleted high concentrations of cr 50 150 ppm and ni 20 100 ppm and low initial 87 sr 86 sr ratios 0 7028 0 7030 indicate that these magmas were generated by melting in the mantle modelling studies and consideration of experimental data indicate that these andesites were formed by 2 10 fractional fusion of hydrous undepleted garnet therzolite at depths of 65 km or more in the mantle the data show that an intense episode of instability convection and widespread melting occurred in the mantle beneath afro arabia at the end of the precambrian

role of fluid mixing and wallrock sulfidation in gold mineralization at the semna mine area central eastern desert of egypt evidence from hydrothermal alteration fluid inclusions and stable isotope data abstract the semna gold deposit is one of several vein type gold occurrences in the central eastern desert of egypt where gold bearing quartz veins are confined to shear zones close to the boundaries of small granitoid stocks the semna gold deposit is related to a series of sub parallel quartz veins along steeply dipping wnw trending shear zones which cut through tectonized metagabbro and granodiorite rocks the orebodies exhibit a complex structure of massive and brecciated quartz consistent with a change of the paleostress field from tensional to simple shear regimes along the pre existing fault segments textural structural and mineralogical evidence including open space structures quartz stockwork and alteration assemblages constrain on vein development during an active fault system the ore mineral assemblage includes pyrite chalcopyrite subordinate arsenopyrite galena sphalerite and gold hydrothermal chlorite carbonate pyrite chalcopyrite and kaolinite are dominant in the altered metaggabro whereas quartz sericite pyrite kaolinite and alunite characterize the granodiorite rocks in the alteration zones mixtures of alunite vuggy silica and disseminated sulfides occupy the interstitial open spaces common at fracture intersections partial recrystallization has rendered the brecciation and open space textures suggesting that the auriferous quartz veins were formed at moderately shallow depths in the transition zone between mesothermal and epithermal veins petrographic and microthermometric studies aided recognition of co 2 rich h 2 o rich and mixed h 2 o co 2 fluid inclusions in the gold bearing quartz veins the h 2 o co 2 inclusions are dominant over the other two types and are characterized by variable vapor liquid ratios these inclusions are interpreted as products of partial mixing of two immiscible carbonic and aqueous fluids the generally light 34 s of pyrite and chalcopyrite may suggest a magmatic source of sulfur spread in the final homogenization temperatures and bulk inclusion densities are likely due to trapping under pressure fluctuation through repeated fracture opening and sealing conditions of gold deposition are estimated on basis of the fluid inclusions and sulfur isotope data as 226 267 c and 350 1100 bar under conditions transitional between mesothermal and epithermal systems the semna gold deposit can be attributed to interplay of protracted volcanic activity dokhan volcanics fluid mixing wallrock sulfidation and a structural setting favoring gold deposition gold was transported as au bisulfide complexes under weak acid conditions concomitant with quartz sericite pyrite alteration and precipitated through a decrease in gold solubility due to fluid cooling mixing with meteoric waters and variations in ph and f o 2

age determinations in the precambrian basement of the wadi araba area southwest jordan abstract the precambrian basement of jordan belongs to the northern margin of the arabian nubian shield age determinations by u pb isotopic analyses on sized and magnetic zircon fractions a monazite and an apatite sample and by rb sr isotopic studies on whole rocks and minerals of metasedimentary rocks granodiorites granites and dykes have elucidated the following events 1 a major regional high grade metamorphism accompanied by migmatization and synkinematic plutonism occurred at about 800 ma according to u pb zircon ages of metasediments and granites 2 during a postkinematic plutonic event between 615 and 600 ma extensive masses of granodioritic to granitic composition and dykes were emplaced the u pb data of zircons of the rocks yielded upper intercept ages with the concordia consistent with rb sr biotite ages the rb sr mineral ages of the older metasedimentary rocks document the resetting of the rb sr system due to the thermal pulse at this time 3 a younger plutonic event produced diorites and dykes at about 570 ma the plutonic events are related to the pan african orogenic phase the low initial87sr 86sr ratios of the plutonic rocks 0 7032 0 7046 correspond to values reported from equivalent rocks throughout the arabian nubian shield and suggest that no significant portions of ancient sialic crustal material contributed to the generation of the granitic to granodioritic magmas

pan african volcanism petrology and geochemistry of the dokhan volcanic suite in the northern nubian shield the late proterozoic dokhan volcanic suite 620 ma of the northern nubian shield is the product of late pan african volcanism the suite covers the entire spectrum from basalt to high silica rhyolite and occurs as two units a dark coloured unit containing basalt andesite dacite and a light coloured unit encompassing dacite rhyodacite rhyolite the latter unit is made up largely of ash flow tuffs and ign imbrites that are locally interstratified with basalt and andesite lava flows the suite forms a continuum in composition with a wide range of si0 2 48 77 wt cao 0 1 8 9 wt sr 81 906 ppm zr 85 340 ppm as well as most other elements and is moderately enriched in incompatible elements including rare earth elements ree the suite exhibits fractionated subparallel ree patterns that are similar overall to andean andesites and ignimbrites well defined major and trace element trends and fractionated ree profiles are consistent with a fractionated basalt to rhyolite calc alkaline magma series it is a typical calc alka line orogenic complex and exhibits mineralogical geochemical traits of arc related volcanism the suite neither resembles products of extensional nor transitional tectonic regimes as previously thought but was produced in a subduction related tectonic environment the mafic nature of the least evolved rocks of the suite along with its relatively low initial 87 sr 86 sr ratio 0 7039 are considered to indicate a mantle source a mantle derived basaltic magma fractionated with amphibole and plagioclase dominating the fractionating assemblage to produce the more felsic varieties as suggested by major and trace element fractionation modelling

petrology of the motaghairat mafic ultramafic complex eastern desert egypt a high mg post collisional extension related layered intrusion abstract the geodynamic settings of the precambrian mafic ultramafic complexes in the eastern desert of egypt have important bearing on understanding the geotectonic evolution of the arabian nubian shield we present a detailed petrological study on a layered mafic ultramafic intrusion that is located at the contact between the precambrian continental crust and the miocene red sea oceanic crust the motaghairat layered intrusion consists of basal lherzolite orthopyroxenite troctolite olivine gabbro and anorthosite on the top variations in modal mineralogy and mineral chemistry along with the chemical composition of these units suggest their derivation from a common high mg tholeiitic parent melt through fractional crystallization processes the parental magma was derived from a metasomatised mantle source the primitive mantle normalized patterns of the calculated melts exhibit enrichment in u relative to th and ba relative lree which indicate that the enriched lithospheric mantle source was metasomatised by fluids derived from a subducted oceanic crust rather than by a sediment melt geological and petrological evidences suggest that the layered motaghairat intrusion was emplaced during post orogenic extension following subduction break off and lithospheric delamination after the collision between the amalgamated island arc terranes and the saharan metacraton the heat source required to melt the metasomatised lithospheric mantle was derived from the upwelling of hot asthenosphere after the subduction break off

thermo hydro mechanical response of granite to temperature changes this paper examines the thermo hydro mechanical behaviour of a granitic rock the cylindrical sample of granite contained a sealed fluid filled cavity which was subjected to a cycle of temperature changes on its outer surface the temperature and fluid pressure changes in the fluid filled cavity were measured since the measurement of fluid pressure and temperature in the sealed cavity is experimentally feasible the technique is a common feature in thermo hydro mechanical experiments however it was observed that regardless of how precisely the cavity was filled with de aired water air bubbles can still exist in the cavity and influence the fluid pressure changes a novel technique is suggested for taking into account the influence of the volume of trapped air and eliminating its effect in the estimation of permeability the experiment was computationally modelled using the finite element code comsol multiphysics and the experimental results were compared with the computational estimates

gebel dahanib egypt a late precambrian layered sill of komatiitic composition gebel dahanib is a well preserved late precambrian layered mafic ultramafic sill about 1 5 km thick forming a prominent peak in the southeastern desert of egypt dunites harzburgites and thin chromite layers at the base grade upward through pyroxenites to layered gabbros and rare anorthosite at the top peridotitic dykes from the main body unsheared margins and ubiquitous cumulate textures all suggest magmatic rather than tectonic emplacement

age and geochemistry of late precambrian sediments of the hammamat series from the northeastern desert of egypt willis k m stern r j and clauer n 1988 age and geochemistry of late precambrian sediments of the hammamat series from the northeastern desert of egypt precambrian res 42 173 187 late precambrian crustal evolution in the northeastern desert of egypt culminated in a major episode of strong extension accompanied by bimodal igneous activity and deposition of clastic sediments in restricted terrigenous basins these sediments known as the hammamat series record the uncovering of the evolving crust of the region we have undertaken petrographic geochemical and rb sr and k ar geochronologic studies of hammamat samples collected along a single traverse the lower half of a 400 m thick section consists of coarse detritus shed from nearby ensimatic terranes while the upper half was derived from rapid reworking of bimodal igneous rocks similar to those of the dokhan volcanics and pink granites units that are preserved within the region rb sr whole rock analyses give an 8 point isochron age of 585 15 ma with an initial stsr sr of 0 70323 0 00013 interpreted to approximate closely the time of sedimentation this is very similar to the k ar ages of the coarser clay fractions 2 0 5 m 588 567 ma this result is consistent with stratigraphic considerations and other geochronological studies indicating that hammamat deposition occurred at 590 ma rb sr analyses of leached and unleached size fractions indicate thermal resetting at 524 17 ma accompanied or followed by fluxing of a radiogenic sr rich pore fluid stsr s sr 0 7124 this is further indicated by k ar ages of the 0 2 0 5 ttm size fraction of 542 532 ma

geochemistry of pan african volcanic arc sequences in southeastern sinai peninsula and plate tectonic implications abstract major and trace elements of lavas dykes and plutonic rocks of the late proterozoic orogenic sequence in se sinai the kid group have been analysed the dykes and lavas of the southermost sequences the tarr complex and heib formation are calc alkaline whereas the lavas of the malhak formation and the sharira gabbro to the north of the heib formation show both calc alkaline and tholeiitic trends the trace element characteristics of the tarr complex and the heib and malhak formations despite between sequence variations are all comparable with ensialic island arc magmatism whereas the sharira gabbro shows some morb characteristics in addition to the island arc imprints the sharira gabbro and the lavas of the malhak formation possibly formed in a developing back arc basin behind a continental marginal ensialic island arc the tarr complex and heib formation during the pan african orogeny the constituent units of the kid group were mutually juxtaposed along major ductile shear zones of thrust fault character this plate convergence regime involved initial magmatic arc development following northward subduction and subsequent collision between the arc complex and the proterozoic continental margin

reappraisal of strain estimations and measurement methods in the hammamat group sediments comparison of primary and secondary grain fabrics with new data from wadi zeidun and wadi arak basins ced egypt abstract in this study pebble and sand grain shape and orientation data from undeformed and deformed conglomerates and sandstones in the egyptian central eastern desert ced wadi zeidun and wadi arak basins were collected with the aim of providing more accurate tectonic strain estimations for deformed hammamat sediments by comparing their primary and secondary sedimentary fabrics common assumptions about primary pebble fabrics and pebble deformation behaviour in previous strain studies of the region are shown to be invalid this raises doubts about the accuracy of earlier reported strain estimates for the hammamat that were based on such assumptions for low strains in the hammamat sediments the best strain measurement method is normalized fry nfry analysis of sand grain centre to centre distance data an alternative is conventional fry analysis of pebble ellipse centre to centre distance data provided that large numbers of pebbles 300 are used and the pebbles form closely packed aggregates with high degrees of anticlustering this study also provides systematic guidelines for pebble axial length and orientation measurements in the field the progressive changes in the mechanical behaviour of pebbles with increasing bulk strain are discussed

age of feiran basement rocks sinai implications for late precambrian crustal evolution in the northern arabian nubian shield basement exposed on the perimeter of the red sea was created during the pan african event at the end of the precambrian pre pan african crust in the northern part of this region has not yet been identified this paper reports the results of rb sr whole rock and u pb zircon dating of gneisses and related basement units from the wadi feiran area in the sinai peninsula where the existence of such older basement has previously been suggested a post tectonic extensional dyke gives a rb sr age of 591 9 ma with an initial 87 sr 86 sr ratio of 0 7034 0 0002 rb sr whole rock and thin slab dating of paragneisses gives ages of c 610 ma with an initial 87 sr 86 sr ratio of 0 7035 a u pb zircon age of 632 3 ma is interpreted as either the time of formation of these gneisses or the age of the crust sampled by protolith sediments granodiorite to the east gives a u pb zircon age of 782 7 ma and is interpreted as representing the westernmost extent of a 780 50 ma terrane that extends across sinai into jordan uplift and erosion of the 780 50 ma terrane supplied detritus to flanking terranes in n and se sinai this region thus acted as a foreland to the younger accretionary and extensional units to the south and west that were active later in the pan african event there is still no evidence for pre pan african basement in the precambrian units around the northern red sea east of the nile

experimental modelling of the near field thermal regime in a nuclear fuel waste disposal vault abstract experiments have been performed to study the thermal performance of a backfilled disposal vault in which the waste containers are emplaced in boreholes and surrounded by a bentonitesand buffer the near field thermal regime in a buffer backfill disposal vault in which the nuclear fuel waste is emplaced in canisters is studied by one eight scale experimental models the thermal performance of the buffer and backfill are examined in a thermal drying environment prior to the ground water recharge of the disposal vault the formation of shrinkage cracks in the buffer due to drying and the response of these shrinkage cracks to the rewetting condition and their influence on the near field thermal regime are assessed

pre alpine evolution of a segment of the north gondwanan margin geochronological and geochemical evidence from the central serbo macedonian massif abstract the serbo macedonian massif smm represents a composite crystalline belt within the eastern european alpine orogen outcropping from the pannonian basin in the north to the aegean sea in the south the central parts of the massif i e southeastern serbia southwestern bulgaria eastern macedonia consist of the medium to high grade lower complex and the low grade vlasina unit new results of u pb la icp ms analyses coupled with geochemical analyses of hf isotopes on magmatic and detrital zircons and main and trace element concentrations in whole rock samples suggest that the central smm and the basement of the adjacent units i e eastern veles series and struma unit originated in the central parts of the northern margin of gondwana these data provided a basis for a revised tectonic model of the evolution of the smm from the late ediacaran to the early triassic the earliest magmatism in the lower complex vlasina unit and the basement of struma unit is related to the activity along the late cadomian magmatic arc 562 522 ma subsequent stage of early palaeozoic igneous activity is associated with the reactivation of subduction below the lower complex and the eastern veles series during the early ordovician 490 478 ma emplacement of mafic dykes in the lower complex due to aborted rifting in the middle ordovician 472 456 ma and felsic within plate magmatism in the early silurian 439 2 ma the third magmatic stage is represented by carboniferous late to post collisional granites 328 304 ma these granites intrude the gneisses of the lower complex in which the youngest deformed igneous rocks are of early silurian age thus constraining the high strain deformation and peak metamorphism to the variscan orogeny the permian triassic 255 253 ma stage of late to post collisional and within plate felsic magmatism is related to the opening of the mesozoic tethys

geothermal measurements in the northern red sea implications for lithospheric thermal structure and mode of extension during continental rifting the northern red sea is a continental rift in the process of transition from continental to oceanic rifting we present 191 new heat flow measurements from the northern red sea forming three traverses across the water covered portion of the rift the heat flow across the rift systematically increases from values of about 125 mw m2 seaward of the coasts to average values greater than 250 mw m2 in the axial depression the heat flow measurements are evaluated for environmental disturbances these are found to be generally small the largest estimated disturbance results from the relief of the seafloor and of the top of a subbottom evaporite layer the relief on these surfaces can account for the 20 point to point scatter typically observed in the heat flow measurements limits are placed on systematic disturbances to the heat flow pattern across the rift the estimated largest systematic disturbance results from sediment blanketing which may cause a reduction in the heat flow on the order of 10 the heat flow variation across the rift is then utilized to examine the lithospheric thermal structure and the geometry and mechanism of extension we employ a two dimensional time dependent numerical technique to follow the advection and diffusion of heat for a simple shear model and various pure shear models of lithospheric extension the model of simple shear lithospheric extension along a planar shallow dip detachment produces significantly lower than observed heat flow results in relatively small amounts of thermal thinning of the lithosphere compared to pure shear extension and does not result in conditions likely to generate melt the pure shear models with widening or constant width zones of extension also do not match the observed heat flow nor generate partial melt the modeling study indicates that the actively extending region must become narrower through time in order to match the heat flow high in the center of the rift generate some degree of partial melt and produce crustal subsidence within the observed width of the rift a narrowing of the zone of active extension is also consistent with the pattern of tectonic activity inferred from sedimentary structures observed on seismic reflection profiles which indicate an abatement of activity in the marginal areas and intense present day activity in the axial depression in all of the models an additional component of lithospheric heating other than that produced by extension appears necessary to match heat flow values in the marginal and near coastal areas as well as explain the observed magnitude of rift flank uplift several scenarios in which the zone of extension eventually becomes narrow are consistent with the heat flow data alone including those resulting in seafloor spreading for the past several million years however additional information provided by gravity magnetic and seismic reflection and refraction data appear to be most consistent with a history in which a period of relatively uniform extension across the rift is followed by a concentration of extension toward the center of the rift such a model retains a small thickness of continental crust in the center of the rift and does not necessitate active seafloor spreading in the northern red sea the three heat flow traverses indicate that maximum lithospheric thinning is occurring within the axial depression in the northern red sea which is very close to initiating seafloor spreading there is evidence that upper crustal extension is locally concentrated in specific areas along the strike of the axial depression forming deeps associated with large recent intrusions these deeps may constitute a precursory stage to seafloor spreading and appear to be nucleating at discrete and regularly spaced sites along the axis of the rift the deeps are located midway between regularly spaced cross trending fault zones or accommodation zones which in the marginal areas separate sets of fault blocks along strike crustal extension appears to be taken up more diffusely in the accommodation zones than in the intervening fault block domains which are characterized by fewer larger faults as a result initial crustal rupturing and emplacement of large intrusive bodies occurs halfway between the accommodation zones where more focused crustal extension forms the deeps comparison with other rifts suggests that the pattern of fault block domains separated by cross trending fault zones is a common feature that it originates early in the development of a rift and that it may continue to influence the development of the rift up to the establishment of a continuous seafloor spreading axis

on the relationship between auriferous talc deposits hosted in volcanic rocks and massive sulphide deposits in egypt abstract the volcanic hosted massive sulphide vhms deposits in the eastern desert of egypt e g um saki deposit are associated with precambrian coarse acid pyroclastic rocks the upper contacts of the massive sulphide body are sharp and well defined while the keel zone to the mineralization is always associated with pervasive alteration characterized by the presence of septechlorite and talc associated with variable amounts of carbonate and tremolite on the other hand the economic talc deposits in egypt are hosted intensively altered volcanic rocks besides talc chlorite carbonates and tremolite that occur in variable amounts in these deposits anomalously high concentrations of gold are also present the present study showed that alterations in the talc deposits of darhib el atshan abu gurdi egat um selimat and nikhira are similar to those occurring in the keel zone underlying the vhms of um samuki and that the chemical modifications due to alteration processes additions of mg fe mn and ca coupled with depletions in silica alkalies alumina and titanium are comparable even the host rocks are different thus reflecting a genetic relationship it is suggested that the examined localities of talc deposits are hosted in the intensively altered volcanics in the keel zones of volcanogenic massive sulphide deposits recently detailed geophysical prospecting program including electric resistivity self potential and induced polarization electromagnetic and magnetic methods was carried out at darhib abu gurdi and um selimat talc deposits the quantitative interpretation of these geophysical measurements revealed the presence of subsurface bodies of sulphides the present distribution of talc and allied minerals in darhib el atshan abu gurdi egat um selimat and nikhira could be explained by a tectonic process in which the coarse acid pyroclastic rocks with massive sulphides have tilted in such way that the footwall rock alterations talc and allied minerals are exposed on the present day surface at these localities structural studies are currently under way in an attempt to explain the deformation regime that led to the present situation of talc deposits two distinct spatial and mineralogical associations of gold mineralization could be identified in the volcanogenic massive sulphide deposits and their footwall alterations the keel zone in the eastern desert of egypt these are 1 gold silver zinc association and 2 gold copper association in the former gold grades are very low and silver is anomalous this association occurs typically in the upper levels of the vhms deposit where low temperature sulphides are abundant gold was deposited because of the mixing between the ascending hot solutions and the sulphate rich seawater the upper levels of um samuki sulphide body represent this association gold copper association on the other hand typically occurs in the footwall altered rocks the keel zone and the lowest parts of the massive sulphide body gold grades reach up to 5 54 ppm but the average is 1 ppm silver is very low usually in the range of 4 10 ppm lead usually but not always accompanies gold in this association deposition of gold probably took place due to decreasing of temperature and or increasing ph of the ascending hot brines the keel zones at darhib abu gurdi el atshan um selimat nikhira and egat talc mines better represent this association

coarse grained submarine fan deposits of magmatic arc affinity in the late precambrian aaron formation north carolina u s a abstract coarse grained submarine fan deposits in the late precambrian aaron formation in the carolina slate belt are derived from a volcanic arc as indicated in the detrital grain population lithic volcanic clasts lv are the major component followed by plagioclase p and subordinate monocrystalline quartz qm recrystallisation at lower greenschist facies has not been sufficient to destroy relict grains or mesoscopic sedimentary structures in these units seven sedimentary facies are recognizable in the aaron formation 1 massive framework supported conglomerate 2 massive to stratified pebbly arenite 3 trough crossbedded arenite 4 horizontally stratified arenite 5 siltstone 6 argillite and 7 vitric tuff these facies are found in 4 facies associations a through d which are stacked in a sequence displaying an upward fining and thinning of coarse beds the overall upward fining of the sequence is analogous to a coarse grained retrogradational submarine fan model associations a and b are equivalent to braided inner fan to mid fan channels whereas association c may represent mid fan or suprafan depositional lobes association d records the transition to basin plain sediments in which outer fan cycles are absent the sedimentary sequence of the aaron formation is analogous to a poorly efficient submarine fan system in which rapid erosion in the source terrane provides abundant coarse grained material to the depositional basin the aaron formation may be related to other late precambrian volcanic sedimentary sequences especially those of the pan african orogens the distinct similarity of constituents in the aaron to those of younger magmatic arc terranes suggests that active plate margin sedimentation and volcanism was prevalent at this locality in the late precambrian

origin and evolution of the um egat and dungash orogenic gold deposits egyptian eastern desert evidence from fluid inclusions in quartz shear zone related mesothermal gold deposits of um egat and dungash in the egyptian eastern desert are hosted by greenschist facies metavolcanic and or metasedimentary rocks of pan african age both deposits comprise boudinaged quartz veins that show evidence of incipient recrystallization and are similar in alteration style structural control and mineralogy the ore mineralogy includes pyrite arsenopyrite pyrrhotite chalcopyrite galena gold occurs both in the veins usually included in arsenopyrite pyrite or pyrrhotite next to fragments of altered country rocks or disseminated in the alteration haloes arsenic in arsenopyrite and aliv in chlorite geothermometers indicate that wall rock alteration and ore mineral precipitation occurred at temperatures between 400 and 250 c fluid inclusions in vein quartz occur in clusters or along trails three types of fluid inclusions were identified based on petrography and laser micro raman spectroscopy 1 two phase carbonic inclusions with co2 ch4 n2 h2o 2 two phase aqueous inclusions and 3 three phase aqueous carbonic inclusions with ch4 final melting of ice tmf for most inclusions occurs at temperatures between 4 and 0 c indicating a low salinity 300 c but cluster into two distinct groups for each type of inclusion inclusions from the same trail or cluster are commonly characterized by different degrees of fill or different th values field petrographic and microthermometric data suggest that low salinity aqueous carbonic fluids interacted with graphite bearing metasedimentary rocks to form ch4 at t 400 c and p 3 kbars these reduced fluids leached gold as they circulated through the metavolcanic rocks carrying it in the form of bisulfide complexes interaction of these aqueous carbonic fluids with the country rocks caused hydrothermal alteration and precipitated gold bearing sulfides in the alteration zones a drop of pressure during the migration of these fluids to shallower depths along the shear zones led to phase separation at t 300 c and p 2 3 kbars quartz crystallizing over a range of lower temperatures and pressures trapped carbonic and aqueous fluids as separate inclusions in clusters along pseudosecondary and secondary trails postdepositional deformation caused decrepitation of some inclusions and the stretching and leakage of others increasing th to 250 c deformation also remobilized the gold depositing it as globules of higher fineness in secondary sites

trace elements and isotope data of the um garayat gold deposit wadi allaqi district egypt trace element composition of sulfides and o c sr and s isotopic data are assessed to constrain the evolution and potential fluid and metal sources of the um garayat gold deposit ore microscopy and bse investigations of quartz veins show blocky arsenopyrite and pyrite replaced in part by pyrrhotite chalcopyrite sphalerite galena and gersdorffite free milling gold occurs commonly in close association with the late sulfides and along fractures in pyrite on the other hand recrystallized pyrite is disseminated in host metavolcaniclastic metasedimentary rocks that commonly contain carbonaceous material in situ la icp ms analysis of sulfides shows the recrystallized pyrite enriched in most trace elements while blocky pyrite contains only some traces of arsenic detected concentrations of gold up to 17 ppm were only reported in arsenopyrite disseminated in quartz veins the 34s values of blocky pyrite and pyrrhotite in quartz veins define a narrow range 1 6 to 3 7 suggesting a homogenous sulfur source which is consistent with the dominantly mafic host rocks the recrystallized pyrite has a distinctive sulfur isotope composition 34s 9 3 to 10 6 which is rather comparable to diagenetic sulfides hydrothermal carbonate in quartz veins and wallrock have nearly constant values of 18o 10 5 to 11 9 and 13c 4 2 to 5 5 based on constraints from mineral assemblages and chlorite thermometry data of six samples indicate that carbonate precipitation occurred at 280 c from a homogenous hydrothermal fluid with 18oh2o 4 4 0 7 and 13c 3 7 0 8 strontium isotope values of two samples 87sr 86sr 0 7024 and 0 7025 are similar to the initial 87sr 86sr ratios of island arc metabasalts 710 ma in the south eastern desert the generally homogenous sulfur c o sr isotope data are suggestive of metamorphogenic fluids likely produced from dominantly mafic volcanic rocks at the greenschist amphibolite facies transition

review of late proterozoic sutures ophiolitic melanges and tectonics of eastern egypt and north east sudan the distribution structural relations and ages of ophiolites and ophiolitic melanges in the eastern deserts of egypt and north east sudan and their relation to arc and post arc magmatism are used to infer the probable positions of sutures and the directions of obduction and subduction the ages of the successive tectonic stages in the terranes decrease northwards by about 100 ma in the region discussed obduction was north westwards the obducted sheets of ophiolite and ophiolitic melange extending about 500 km north from the onib sol flamed suture may all represent a single back arc basin the direction of subduction is still not clearly proved more geochemical evidence of arc magmatic polarity is needed the late tectonic north westward thrusting the nw se stretching lineation the transpressional north south hamisama shear zone and the nw se sinistral najd faults are all attributed to north westward movement present coordinates of terranes towards the older craton west of the nile the najd faults are interpreted as intracontinental transform faults

remote sensing guided stratigraphic dissection of an ediacaran terrestrial molasse basin kareim basin egypt with implications for sedimentary evolution abstract the kareim basin in the central eastern desert is one of the largest ediacaran post amalgamation hammamat basins in egypt it formed at a time of extensional tectonism 600 20 ma in the northern arabian nubian shield the kareim basin preserves a 6500 m fill of rapidly deposited alluvial fan and lake sediments due to monotonous lithology and complex facies variations the stratigraphic architecture of this and other large hammamat basins is poorly known in this study landsat 8 oli data for the kareim was processed to provide best band ratio and pca images that allow tracing of sandstone dominant and conglomerate dominant packages revealing a configuration of eight new stratigraphic units units 1 8 units 1 3 are basinwide while units 4 8 are localized and partly bounded by intraformational angular unconformities that formed during syn sedimentary tectonism fan palaeoslopes and palaeocurrents were w directed in units 1 3 then switched to sw and s directed in later units implying that the provenance areas lay east and north of the basin not south as in previous basin evolutionary models this study provides the first rs guided stratigraphic dissection of a hammamat basin and provides a sensitive tool for detecting intraformational unconformities in basins filled with monotonous molasse sequences it paves the way for the sequence stratigraphic analysis of the egyptian hammamat basins

geochronology geochemistry and tectonics of the ne bayuda desert n sudan implications for the western margin of the late proterozoic fold belt of ne africa abstract five suites of rocks collected from the precambrian basement in the ne bayuda desert of central northern sudan give late proterozoic whole rock rb sr isochron ages the abu harik complex thought by some previous workers to be an older basement gives an age of 898 51 ma upper amphibolite facies metasediments give a metamorphic age of 761 22 ma the supposedly younger greenschist facies el koro volcanic series were erupted 800 83 ma ago these are chemically similar to the volcanics which unconformably overlie the sol hamed ophiolite in the red sea hills of ne sudan and to some modern island are volcanics the metasediments were intruded 678 43 ma ago by the diefallab granite which is itself deformed the younger weakly deformed amaki series with a basal conglomerate containing basement clasts overlain by purple grits is probably equivalent to the molasse type hammamat group of the eastern desert of egypt which was deposited between 616 and 596 ma ago finally the post tectonic shallal granite with within plate geochemistry was intruded 549 12 ma ago geochemical data suggest that the abu harik complex the el koro volcanic series and the diefallab granite are arc related magmatic rocks they were intruded into or thrust onto shallow water shelf sediments during subduction and then collision between c 900 and 550 ma the data presented here give no support to previous views that the high grade metasediments were metamorphosed prior to the late proterozoic events that they unconformably overlie a still older perhaps archaean basement or that they are unconformably overlain by younger late proterozoic low grade volcanics the precambrian rocks along the e side of the bayuda desert must now all be assigned to the late proterozoic and the boundary between this late proterozoic fold belt and an older craton known to crop out at jebel uweinat must lie farther to the w

the role of fracture zones during early red sea rifting structural analysis using space shuttle radar and landsat imagery the structural pattern of the red sea region an area where continental lithosphere is cut by a young oceanic rift was studied by means of satellite and space shuttle radar imagery the space shuttle mission 2 made two passes over the red sea bracketing a region transitional between the southern red sea where true seafloor spreading started about 5 ma bp and the northern red sea where crust is probably continental though thinned and stretched structures on these passes define bands of shearing in the african and arabian margins parallel to the dead sea fracture zone 000 020 combinations of these data with landsat information reveal four shear bands along the margins of the red sea and one oblique to the gulf of suez we have named the northern three red sea shear zones detected on the sir a data the brothers dead sea fracture zone the zabargad fracture zone and the shagara fracture zone two sets of normal faults were also detected trending 330 340 parallel to the northern red sea coastline and 310 320 parallel to the axial deeps in the red sea a map of the 310 320 faults bordering the red sea resembles a v shaped wedge narrowing to the north near the gulf of suez these faults may have formed as a result of the most recent push of seafloor spreading if this is so then there may be a propagating tectonic wedge moving from south to north which thins the continental crust up to distances of 300 km from the axis of spreading following continental deformation oceanic crust may be emplaced in 9hot points9 along the rift axis these 9punctiform9 oceanic spreading centres should propagate in the same direction as continental rifting during this episode palaeo shear zones probably act as barriers 9locked zones9 to the axial propagation one such case is at the zabargad fracture zone where the oblique impact of the propagating rift against the palaeo shear may have placed the red sea9s western side under compression and uplift while the eastern side may have been subject to oblique extension with the formation of pull apart basins the space shuttle imagery allows integration of some of the northern red sea morphotectonic features with structures on the continent

possible flexural accommodation on the eastern edge of the altiplano in relation to focussed erosion in the rio la paz drainage system it has been proposed that erosional unloading by focused incision can be accommodated by flexural rebound which in turn potentially exerts a positive feedback on erosion this paper presents morphometric data from the la paz drainage basin in the bolivian andes to illustrate the effects of such a feedback mechanism between erosion and crustal bending the rio la paz is a prominent system with the headwaters on the altiplano and a trunk stream segment that cuts across the cordillera real thereby connecting large portions of the altiplano with the amazon basin the flexural feedback model explains why all drainages beyond the watershed disperse their waters to the altiplano it also provides an explanation for the presence of the highest peaks just next to the location where the la paz river cuts into the bedrock across the cordillera real

pan african magmatism in the wadi el imra district central eastern desert egypt geochemistry and tectonic environment the late proterozoic rocks of the wadi el imra district of the central eastern desert egypt comprise a metagabbro diorite complex oldest and older and younger granitoid complexes trace element characteristics show that the basic and granitoid rocks are of calc alkaline affinity but some of the younger granitoids grade into alkaline granites the o nd values of the the basic and granitoid rocks range from 8 3 to 4 9 and nd model ages are younger or rather similar to crystallization ages these data suggest a predominant depleted mantle derivation indicating formation by subduction in an oceanic setting ree patterns of the granitoid rocks suggest different modes of petrogenesis this coupled with the occurrence of amphibolite xenoliths in the older granitoids may indicate that some of these rocks represent partial melts from basic rocks that have been amphibolitized at deep crustal levels of the oceanic arc

the significance of continental derivation of the proterozoic mahanid formation southeastern arabian shield the mahanid formation is a quartzo feldspathic meta sedimentary unit exposed in the southeastern part of the arabian shield detritus was derived from a continental landmass located to the southeast erosional products were deposited in an adjacent marginal basin which existed from middle to late proterozoic time the mahanid formation is correlated with other formations in the eastern shield area but differs from meta sedimentary units in the western part of the arabian shield

geochemistry and geotectonic interpretation of granitoids from central iforas mali w africa abstract late proterozoic granitoids from central iforas mali w africa show a large range of chemical variations e g sio 2 52 8 75 6 and include diorites monzotonalites tonalites granodiorites and granites the major and trace element composition of these rocks is comparable to modern calc alkaline rocks of continental margins which were emplaced far from the trench in an area with thick continental crust the chemical variations were produced by crustal contamination and by fractional crystallization dominated by hornblende and plagioclase the granitoids are also similar in composition to pan african orogenic volcanic rocks of the hoggar geodynamic interpretations of the adrar des iforas belt are discussed in the light of these data

heat induced moisture movement in a clay barrier i experimental modelling of borehole emplacement abstract the paper presents the results of a series laboratory experiments which were conducted to study the heat induced moisture movement in a bentonitic clay buffer region in the experimental investigations hygro thermal phenomena are induced by a cylindrical heater which is placed within the compacted buffer material located in a borehole centrally situated in a granite block the paper summarizes the experimental methodologies and presents results for the time dependent distribution of temperatures within the granite block and the residual moisture distribution within the buffer at the termination of the experiment

origin of late precambrian intrusive carbonates eastern desert of egypt and sudan c o and sr isotopic evidence abstract the problem of the origin of northeast african basement carbonates is approached using a regional study of c o and sr isotopic compositions in whole rock samples of late precambrian carbonate rocks of the egyptian and sudanese shields ess from the eastern desert of egypt and sudan the isotopic data indicate that three distinct reservoirs were available for generation of ess intrusive carbonates 1 sedimentary carbonates with moderately high 87sr 86sr and heavy c and o 2 depleted mantle with low 87sr 86sr and light c and o and 3 enriched mantle or lower crust with high 87sr 86sr and light c and o isotopic data indicate that the intrusive carbonates of the north eastern desert were derived from reservoir 2 and a sample from the interior of sudan was derived from reservoir 3 the origin of the remaining intrusive carbonates of the central eastern desert and sudan is best explained as mixing between remobilized sedimentary carbonates and mantle fluids i e reservoirs 1 and 2 the source of the sedimentary carbonates may have been carbonate bank sediments deposited during pan african rifting and evolution of a passive continental margin on the north flank of the south eastern desert now structurally buried under the central eastern desert melange

age and chemical characteristics of some pre pan african rocks in the egyptian shield abstract u pb ages on zircons from several types of sialic rocks together with a variety of geochemical data on one sample of early plutonic activity within the egyptian shield are presented the earliest autochthonous granitic rocks in the egyptian shield are probably quartz diorites tonalites emplaced just prior to the main peak of pan african igneous activity 550 650 ma the studied example has an age of 711 ma 7 ma and is characterized by very low initial sr ratio 87sr 86sr 0 7026 and very low abundances of k2o and related large ion lithophile trace elements ree contents are low less than 20 chondritic abundances abundance patterns show only moderate light rare earth enrichment la yb 4 5 these characteristics are inconsistent with any models requiring fractional fusion of pre existing continental crustal material granitic and arkosic cobbles occur in rare conglomerate beds within volcanoclastic and greywacke sedimentary sequences the cobbles show a wide range in ages 1 1 2 3 ga and have no obvious source within the egyptian shield it is suggested that they are not indicative of any sialic plutons in the egyptian shield which might predate deposition of the enclosing immature sediments more probably they were derived from adjacent continental areas such as the uweinat region to the west and were deposited in an evolving arc ocean basin complex

provenance tectonic setting and geochemistry of greywackes and siltstones of the late precambrian hammamat group egypt abstract greywackes and siltstones from the neoproterozoic hammamat group in the wadi hammamat and wadi bali areas eastern desert of egypt were analyzed for major and trace elements the sedimentary rocks are characterized by na 2 o k 2 o 1 a wide range of both al 2 o 3 na 2 o 2 5 7 and al 2 o 3 tio 2 13 33 plus a chemical index of alteration between 60 and 44 these data suggest that the hammamat sedimentary rocks are chemically immature and that there was minimal chemical weathering involved in their erosion and deposition the sedimentary units show two broad compositional groupings in the two studied areas the wadi bali sedimentary rocks are characterized by low mgo cr ni v ba nb y and zr and high na2o cao and sr relative to the wadi hammamat sedimentary rocks these geochemical differences suggest contrasting provenance for the sedimentary rocks in the two areas the wadi hammamat sedimentary rocks were derived from 30 mafic rocks 25 granodiorite 25 intermediate volcanics and 20 felsic volcanics whereas those of the wadi bali area were derived from a relatively homogeneous source of 90 intermediate volcanics and 10 felsic volcanics the wadi hammamat sedimentary rocks were deposited in an intra arc basin that developed as a result of the creation of considerable topographic and structural relief after development of a continental arc the source rocks were continental arc volcanics dokhan volcanics and the uplifted older oceanic tholeiites and island arc assemblages the wadi bali sedimentary rocks were formed by the same scenario as the wadi hammamat sedimentary rocks but the absence of rock fragments older than the dokhan volcanics indicates a closed intra arc basin where the dokhan volcanics were the main source of clasts

geochemistry and sr and nd isotopic study on rare metal bearing granitic rocks central eastern desert egypt abstract three post orogenic granitic plutons exposed in the egyptian basement complex namely homr akarem homrit mukbid and nuweibi were selected for geochemical and nd sr isotopic studies based on petrographic and geochemical considerations the granitic rocks are divided into medium grained pink granites and albite rich granites both types exhibit peraluminous character with many chemical similarities to the high y nb a subtype granites owing to the extreme fractionated nature and complex magma source of these granitic rocks in addition to the high volatile activity their compositions resemble within plate magma originated in attenuated crustal lithosphere whole rock rb sr isochron ages of the medium grained pink granites from homr akarem and homrit mukbid plutons are 541 4 ma and 527 26 ma respectively the nuweibi albite granite nag represents the youngest granite phase and yields a rb sr age of 490 27 ma the medium grained pink granites show a moderate and narrow range of initial 87 sr 86 sr ratio 0 7064 0 7081 and negative t nd values 1 31 to 6 68 the nag has an initial 87 sr 86 sr 0 7071 and t nd 3 26 to 4 72 comparable to those of the medium grained pink granites the sm nd data yield model ages t dm of 0 96 to 1 70 ga similar nd model ages 1 2 1 8 ga have been reported for other post orogenic granites from southwest egypt and northern sudan the nd and sr isotopic compositions of the studied mineralized granites suggest the involvement of old lree enriched continental crust of pre pan african age and lower crust or mantle derived magmas the nd and sr isotopic variations can be explained by a mixing hypothesis

carbonatite crystallization and alteration in the tarr carbonatite albitite complex sinai peninsula egypt carbonate dykes occurring in the arabian nubian shield ans are clearly intrusive in origin and carbonatites according to the iugs classification yet previous investigations refer to them as int

creep behaviour of a buffer material for nuclear fuel waste vault in the canadian nuclear fuel waste disposal concept currently under study one of the prime candidate procedures is the borehole emplacement technique each fuel waste container will be placed in a

diversity of platinum group minerals in podiform chromitites of the late proterozoic ophiolite eastern desert egypt genetic implications abstract podiform chromitites are frequently distributed as lensoidal pods in the central and southern parts of the eastern desert egypt they are in most cases hosted by fully serpentinized peridotite which is a part of dismembered ophiolite complexes of the pan african belt of late precambrian age serpentinites are the predominant components in the ophiolitic melange either as a matrix or as variably sized blocks and are derived from harzburgite and subordinate dunite the central eastern desert ced chromitites have a wide compositional range from high cr to high al varieties whilst those of the southern eastern desert sed have a very restricted compositional range the cr ratio of spinel ranges from 0 5 up to 0 8 in the former while it is around 0 8 in the latter platinum group element pge mineralization has been recently reported in podiform chromitites from the late proterozoic pan african ophiolite of the eastern desert of egypt the populations of platinum group minerals pgm in the studied ced and sed chromitites are quite distinguishable they are mainly sulfides os rich laurite in the former and os ir alloy in the latter sulfarsenides and arsenides are also found in subordinate amounts from both chromitites the most abundant base metal sulfides bms in the eastern desert chromitites of egypt are millerite heazlewoodite pentlandite chalcopyrite and pyrite the sulfur fugacity and temperature conditions are the main controlling factors of pge mineralogy in the host chromitite at the initial stage within the upper mantle os rich laurite is stable at high sulfur fugacity and low temperature conditions whereas os ir alloy is stable at lower sulfur fugacity and higher temperature conditions the diversity of pge mineralogy combined with the differences in petrological characteristics of chromian spinels from ced chromitites to sed ones suggests different degrees of partial melting of the mantle rocks of this ophiolite which in turn may be attributed to different tectonic settings during the post magmatic processes i e serpentinization the primary pgm e g os rich laurite and os ir alloy can be modified at low temperatures to secondary pgm

a new classification of the gold deposits of egypt abstract gold deposits and occurrences located in the nubian shield have been known in egypt since predynastic times despite the fact that these deposits were long under exploitation and investigated many times they are still insufficiently classified in harmony with the crustal evolution models suggested for the evolution of the nubian shield several plate tectonic models were proposed for the development of the nubian shield and the present classification relies heavily on the model that implies collision of arc inferred continent through subduction and obduction of oceanic lithosphere a three fold classification of gold deposits of egypt is offered here in harmony with this evolutionary model these are stratabound deposits and non stratabound deposits hosted in igneous and metamorphic rocks as well as placer gold deposits the stratabound deposits are hosted in island arc volcanic and volcaniclastic rocks of comparable composition formed in ensimatic island arcs they are thought to have formed by exhalative hydrothermal processes during the waning phases of sub marine volcanic activity stratabound deposits are sub divided into three main types gold bearing algoma type banded iron formation gold bearing tuffaceous sediments and gold bearing volcanogenic massive sulphide deposits non stratabound deposits occur in a wide range of igneous and metamorphic rocks they were formed during orogenic and post cratonization periods by mineralizing fluids of different sources non stratabound deposits are divided into vein type mineralization which constituted the main target for gold in egypt since pharaonic times and disseminated type mineralization hosted in hydrothermally altered rocks alteration zones which are taken recently into consideration as a new target for gold in egypt placer gold deposits are divided into modern placers and lithified placers the former are sub divided into alluvial placers and beach placers conglomerates occurring on or near ancient eroded surfaces represent lithified placers

petrological evolution of the crust and mantle in its simplest form crust mantle evolution can be thought of as a study of the processes by which material is lost from the mantle and forms new crust the overall goal is achievement of mass balance between the average composition of material known to have been added to the crust and the estimated average composition of the entire crust attainment of this goal for the earth has been elusive primarily because the materials available for study have generally been involved in several stages of evolution including fractionation within both the crust and mantle and recycling of material from crust to mantle thereby complicating the interpretation of available data although the overall composition of the continental crust generally has been modelled as andesitic taylor and mclennan 1981a the present upper continental crust is granodioritic and requires a more mafic lower crust it has been proposed taylor and mclennan 1981a that originally the archean crust had an andesitic composition throughout its volume and that partial melting in the lower crust developed more felsic melts that rose to enrich the upper crust leaving a more mafic residuum of lower crust this concept has been challenged by weaver and tarney 1981a who argue that the granulites of presumed lower crustal origin do not have the correct compositions for this model if one considers more recent additions to the crust from the mantle over the past several hundred million years it appears that these latest additions are basaltic with very little if any granitic additions in fact barker 1981 in his introduction to a special issue of jgr on granites and rhyolites states to date no one has demonstrated that such magmas may form in the mantle indeed much experimental and geochemical work indicates that such siliceous liquids cannot originate there therefore such magmas must be generated in the crust in island arc environments where the average composition of added material normally is thought to be andesitic a anderson 1982 has shown that the parent melt is basaltic not andesitic in addition kay 1980 has shown that the k content of the newly added material is less than the average k content of the previously existing crust in the oceanic crust the additions from the mantle are basaltic and according to o hara 1982 probably are derived from very mafic picritic melts as parents continental flood basalts which have also added material from mantle to crust also are clearly derived from basaltic parental melts basaltic volcanism study project 1981 thus the evidence for the nature of material subtracted from the mantle and added to the crust for the past several hundred million or perhaps a billion years points to very mafic melts this contrasts significantly with the overall andesitic composition of continental crust it implies furthermore that if the overall archean crustal composition was andesitic as estimated by taylor and mclennan 1981a then additions to the crust have become more mafic with time a concept in direct conflict with most models in which secular trends are from mafic to felsic with time veizer and jansen 1979 and mclennan et al 198o a question which is beginning to appear either explicitly or implicitly is whether crustal volcanic rocks or the crust as a whole really represents the parent melt that formed in the mantle a anderson 1982 o hara 1982 and nisbet and walker 1982

metamorphism and tectonics of a pan african terrain in southeastern sinai a reply abstract the precambrian crystalline basement of sinai represents a low pressure metamorphic terrain intruded by large volumes of granitic rock based on detailed fieldwork a general assessment of the metamorphic and tectonic history of the wadi kid area southeastern sinai is presented three lithostratigraphic units can be traced over the whole area the umm zariq formation arkoses greywackes pelites the tarr formation dolomitic calcareous rocks and unconformably overlying the previous two units the heib formation flows pyroclastics conglomerates d 1 deformation of this 3 5 km thick sequence resulted in upright folds with changing strike of the axial planes from ne to nw across the area low grade conditions prevailed during this phase d 2 produced recumbent folds and a subhorizontal cleavage leading to transposition of d 1 structures in the higher grade parts of the area metamorphism reached its peak conditions around d 2 pressures are estimated at 2 5 3 5 kb whereas temperatures vary from 450 660 c in the central wadi kid area garnet staurolite cordierite and andalusite occur in metapelitic rocks highest grade rocks are syn d 2 andalusite k feldspar gneiss diapirs metamorphic zones are shallow dippin and form a domed pattern most of the metavolcanics and the syntectonic and late tectonic plutonic rocks belong to the calc alkaline suite the kid group sediments and volcanics were deposited in a shallow basin and subaerially respectively probably on older sialic basement this basement is at present not exposed because post orogenic uplift directly after the pan african event was relatively small 3 6 km metamorphism and the d 2 formation phase can both be related to a rising mafic diapir the sinai peninsula may have been a continental margin or a cratonized mature island arc in late proterozoic times

influence of pressurized water influx on the hygro thermal behaviour of an engineered clay barrier in a waste emplacement borehole this paper presents the results of a series of experiments conducted to examine the hygro thermal behaviour of a compacted internally heated engineered clay barrier consisting of a mixture of bentonitic clay and sand compacted in a laboratory scale model simulating a waste emplacement borehole located in a granitic rock the experiments considered the supply of water at a prescribed pressure at the outer cylindrical and plane boundaries of the compacted engineered clay barrier the experiments examined the manner in which the water supply conditions influenced the extent of the moisture uptake within the compacted clay barrier the experimental results presented in this paper include contours of the time dependent variation of temperature within the granite block the time dependent variations of temperature at the midsection level of the internally placed heater and along the axis of the experimental facility and the residual volumetric moisture content distribution determined at the termination of the experiments

lead isotope systematics of some igneous rocks from the egyptian shield lead isotope data on late precambrian igneous rocks from the eastern desert of egypt are presented previous work has indicated that this igneous suite is characterized by uniformly low initial ratios of 37sr86sr 0 7035 the pb data define three groups loosely corresponding to age an older tonalite to granodiorite otg suite with ages in the range 610 710 ma has pb isotope characteristics similar to modern mantle derived oceanic mafic rocks the age corrected initial pb isotope ratios of the otg group lie near the ocean regression line in correlation diagrams of 206pb204pb vs 207pb204pb and 203pb204pb the isotope data imply an origin for the otg group analogous to calc alkalic igneous rocks in modern intra oceanic island arcs a younger suite of k rich plutonic rocks 570 595 ma has similar 206pb204pb and 208pb204pb initial ratios relative to the otg suite but have higher 207pb204pb initial ratios when calculated from measured whole rock u and pb contents however k feldspar and galena associated with this suite indicate initial pb isotope ratios close to the otg suite we interpret this to indicate that the younger granites could have originated by anatexis of otg type material but they probably experienced an episode of metasomatism after emplacement a suite of volcanic rocks the dokhan volcanics from the central eastern desert are intermediate between the younger granites and otg in both age 610 ma and pb isotope characteristics limited data on initial 208pb204pb ratios suggest that all the egyptian samples originated from a source with anomalously low thpb ratios although the relative abundance of granitic rocks in the egyptian shield decreases to the south no evidence of north south heterogeneity in the pb isotope composition of the respective source regions is apparent in the data however the westernmost sample studied from the aswan area contains distinctly more radiogenic pb relative to the mean this location may mark the boundary between the late precambrian ensimatic orogen now outcropping in the egyptian and saudi arabian shields and an older sialic craton to the west

cenozoic behind arc volcanism in the bolivian andes south america implications for mantle melt generation and lithospheric structure we develop a model of arc and behind arc volcanism as well as constraining the evolution of the lithospheric mantle structure beneath the bolivian andes during the last 25 ma using new geochronological and geochemical data on behind arc mafic volcanic rocks together with a regional isotopic study of geothermal helium emissions and a comparison with seismic tomographic images of the same region helium isotopes measured in natural gas emissions in geothermal and mineral water springs suggest recent mantle melting in a wide zone extending at c 20 s from a few tens of kilometres west of the volcanic arc to over 300 km behind the arc where the subducted slab is at depths increasing from c 100 to c 250 km during the neogene there has been behind arc mafic felsic magmatism right across the high andes between 17 and 22 s in the altiplano and western margin of the eastern cordillera modelling of plio pleistocene basalts and basaltic andesites c 15 depleted mid ocean ridge basalt morb source at depths between c 75 km and c 110 km for mafic volcanic rocks that crop out c 65 km and c 90 km for those that crop out 100 km behind the arc ree inversions for oligo miocene shoshonites and basalts c 25 21 ma suggest that they originated at significantly shallower depths c 45 km to c 100 km from a slightly enriched morb source eruption of shoshonitic lavas also occurred in the altiplano between 13 and 11 ma and widespread behind arc intermediate felsic volcanism has been more or less continuous since the early miocene all this suggests that since c 25 ma there has been a thin c 25 ma by the opening up of a mantle wedge and inflow of hot asthenosphere as a consequence of the steepening of the subducted slab and detachment of hydrated and weakened mantle lithosphere and possibly mafic lower crust

late cenozoic magmatism of the bolivian altiplano small basalt to dacite volcanic centers are distributed sparsely over the bolivian altiplano behind the andean volcanic front most are pliocene to recent in age and are characterized by textural and mineralogical disequilibrium with abundant xenoliths and xenocrysts true phenocrysts are rare in the more mafic samples compared with recent volcanic rocks from andean stratovolcanoes the bolivian centers overlap in major element trends incompatible element contents tend to be higher particularly in the eastern altiplano the ranges of isotopic compositions reflect ubiquitous crustal contamination pb isotope compositions are dominated by pb from isotopically heterogeneous basement resulting in a wide scatter of data lying between inferred crustal compositions and showing little overlap with possible mantle sources in the region rocks sampled from the bolivian altiplano were probably derived from asthenospheric mantle and subjected to extensive open system differentiation during ascent through the 70 km thick crust of the region major element trends are largely controlled by the fractionating phase assemblage olivine clinopyroxene and amphibole trace element and isotope systematics however defy realistic attempts at modeling due to the geographic scatter of samples the uniformity of compositions at a given center and the heterogeneity of the contaminant nevertheless there are first order systematic trace element variations that appear to relate to the geometry of the subduction zone in particular life hfse exemplified by ba nb and zr nb ratios decrease from the arc front eastward into the altiplano these variations are not easily reconciled with control by crustal contamination alone a model is proposed in which the asthenospheric source is fluxed by high ba nb slab derived fluid to induce melting beneath the arc high fluid flux increases the ba nb ratio of the asthenosphre and leads to high degrees of partial melting high zr nb to the east lower or no fluid flux leads to low ba nb and low degrees of partial melting low zr nb melting in the back arc region of the altiplano may be facilitated by lithospheric delamination that leads to decompression melting of counter flowing asthenosphere there is no unequivocal evidence that requires a significant role for the lithospheric mantle

pan african structures ophiolites and melange in the eastem desert of egypt a traverse at 26 n a detailed structural traverse across the basement rocks of the eastern desert of egypt shows that they consist apart from intrusions of four broadly recumbent tectonic units the lowest of arkosic metasediments of continental shelf facies is exposed in a dome this unit is overlain by an allochthonous ophiolitic melange containing complete and dismembered ophiolitic masses in a matrix of deep oceanic graphitic pelites and turbidites a near horizontal schistose to mylonitic fabric most intense near and below the base of the melange indicates extensive lateral tectonic transport of the ophiolitic melange over the shelf metasediments but the melange originated as an olistostrome before being tectonically transported the melange extends over an area of at least 10 000 km 2 it is locally overlain by calc alkaline volcanics in which deformation is less intense but increases and schistosity flattens downwards indicating some translation over the melange unconformable on the melange and calc alkaline volcanics is a molasse facies series itself also locally strongly deformed late tectonic granites preceded and locally post date the molasse facies sediments still later diapiric peralkaline riebeckite granites locally up domed the recumbent structures the tectonic evolution is related to a late proterozoic subduction zone to the se of the region

distribution of oceanic and continental leads in the arabian nubian shield new common lead data for feldspar whole rock and galena samples from the arabian nubian shield together with data from previous work can be divided into two main groups group i leads have oceanic mantle characteristics whereas group ii leads have incorporated a continental crustal component of at least early proterozoic age the group i leads are found in rocks from the red sea hills of egypt and the western and southern parts of the arabian shield group ii leads are found in rocks from the northeastern and eastern parts of the arabian shield as well as from the southeastern shield near najran they are also found in rocks to the south in yemen to the east in oman and to the west at aswan egypt this distribution of data suggests that the arabian nubian shield has an oceanic core flanked by rocks that have developed at least in part from older continental material two mechanisms are suggested by which this older lead component could have been incorporated into the late proterozoic rocks and each may have operated in different parts of the shield the older lead component either was derived directly from an underlying early proterozoic basement or was incorporated from subducted pelagic sediments or sediments derived from an adjacent continent

cenozoic uplift of the central andes in northern chile and bolivia reconciling paleoaltimetry with the geological evolution the cenozoic geological evolution of the central andes along two transects between 17 5 s and 21 s is compared with paleo topography determined from published paleo altimetry studies surface and rock uplift are quantified using simple 2 d models of crustal shortening and thickening together with estimates of sedimentation erosion and magmatic addition prior to 25 ma during a phase of amagmatic flat slab subduction thick skinned crustal shortening and thickening nominal age of initiation 40 ma was focused in the eastern and western cordilleras separated by a broad basin up to 300 km wide and close to sea level which today comprises the high altiplano surface topography at this time in the altiplano and the western margin of the eastern cordillera appears to be 1 km lower than anticipated from crustal thickening which may be due to the pull down effect of the subducted slab coupled to the overlying lithosphere by a cold mantle wedge oligocene steepening of the subducted slab is indicat

crust magma interactions and the evolution of arc magmas the san pedro pellado volcanic complex southern chilean andes field petrographic and trace element data indicate that volcanic rocks of the quaternary san pedro pellado complex lat 36 s chile have incorporated a silicic component that may be derived from the crust isotope ratios of sr nd and pb vary little among the volcanic rocks this is due primarily to the limited isotopic contrast between the mesozoic tertiary crust and the young magmas isotope and element compositions of san pedro pellado rocks are similar to those reported from other localities in the southern volcanic zone of the chilean andes we suggest that crustal assimilation is a significant process in the genesis of differentiated volcanic rocks in the southern volcanic zone the san pedro pellado example illustrates the importance of considering all types of evidence not just isotopic data in the evaluation of crustal contributions to arc magmas

late precambrian rifting and crustal evolution in the northeastern desert of egypt basement exposures along the northwestern flanks of the gulf of suez record the rapid formation of continental crust during the interval 670 550 ma a variety of field petrographic petrologic and isotopic considerations indicate that this episode of crust formation took place in an extensional tectonic setting analogous to that of the late paleozoic oslo rift of norway crustal evolution in this region thus contrasts with other regions of the afro arabian shield where the crust appears to have been formed by convergent margin accretionary processes and collisional tectonics this imposes new constraints on our understanding of late precambrian crustal evolution

the world s oldest surviving geological map the 1150 b c turin papyrus from egypt a papyrus map discovered around 1820 and now in the egyptian museum in turin italy illustrates the topography and geology of wadi hammamat in the mountains of the central eastern desert of egypt it accurately depicts the areal distribution of sedimentary and igneous metamorphic rocks which are shown as black and pink hills respectively the ancient map also shows the gold working settlement at bir umm fawakhir the gold bearing quartz veins on the adjacent mountain the famous bekhen stone quarry the lithologically diverse wadi gravels and various cultural features hieratic texts on the map comment on the occurrence of gold in the area and the quarrying of bekhen stone grayish green chloritic sandstones and siltstones this papyrus is the oldest surviving geological map in the world and it predates by 29 centuries the next oldest known geological map the map was drawn during the reign of ramesses iv 1151 1145 b c as an aid to or a record of one of this king s bekhen stone quarrying expedi

crustal structure of the altiplano from broadband regional waveform modeling implications for the composition of thick continental crust we have modeled the full waveforms from six intermediate depth and two shallow earthquakes recorded at regional distances by the banjo broadband andean joint experiment banjo and seismic exploration of the deep altiplano seda portable seismic networks in the central andes in this study we utilize data from those banjo and seda stations located within the altiplano and eastern cordillera we used reflectivity synthetic seismograms and a grid search to constrain four parameters of the altiplano eastern cordillera lithosphere crustal thickness average crustal velocity vp and crustal and upper mantle poisson s ratios crust and mantle using our grid search we investigated the crustal and upper mantle structure along 36 individual event station paths and applied forward modeling to 56 event station paths robust models for the altiplano that provide the best overall fit between the data and synthetic seismograms are characterized by an average vp of 5 75 6 25 km s crustal thicknesses of 60 65 km crust 0 25 and mantle 0 27 0 29 we find a north south variation in the structure of the altiplano with the crust south of the banjo transect characterized by either lower than average crustal p wave velocities or a slightly higher crust relative to crust north of the banjo transect these results are consistent with a model of crustal thickening caused predominantly by tectonic shortening of felsic crust rather than by underplating or magmatic intrusion from the mantle

geochemical characterization of pan african dyke swarms in southern sinai from continental margin to intraplate magmatism abstract the chemical and mineralogical features of three pan african dyke swarms occurring in the wadi feiran area southern sinai have been examined two swarms striking approx 35 exhibit calcalkaline chemical properties the oldest s1 can be related to calcalkaline rocks from active continental margins by means of incompatible element plots displaying negative nb ta anomalies they dykes were emplaced synchronously with post orogenic alkaline granites to which they cannot be assigned chemically a second dyke generation l1 with the same strike shows apparent similarities with continental flood basalts e g columbia river basalts indicating a growing contribution of continental components to melt chemistry subduction related features are displayed by negative nb ta anomalies the youngest dyke swarm s2 striking approx 135 and crosscutting s1 displays transitional to mildly alkaline properties without negative nb ta anomalies its chemistry is fully consistent with intraplate basalts of continental areas the source regions of the three swarms cannot be related via assimilation of crustal material because decreasing lile percentages coupled with increasing amounts of nb and ta cannot be explained by this means the overlap of m values of rocks to be found with the three swarms indicates the unlikeliness of a development of the three magmas via fractional crystallization the emplacement of two totally different types of dykes in the wadi el sheikh area requires a source region which is either chemically inhomogeneous in a more or less vertical scale or changing its composition and melting conditions with time although the dykes were emplaced in an extensional tectonic setting the subcrustal lithosphere still produced calcalkaline melts documenting its subduction related magmatic history

origin of the high plateau in the central andes bolivia south america the bolivian altiplano in the central andes of south america is part of the second largest high plateau on earth it is an elongate region of subdued relief 1 2 105 km2 and 4 km above sea level bounded by the eastern cordillera and volcanic arc western cordillera here the crust is up to 75 km thick we describe the cenozoic geological evolution of this region using a revised chronostratigraphy and an analysis of the crustal and lithospheric structure crustal shortening and magmatic addition and locally sedimentation are the main mechanisms of cenozoic crustal thickening leading to nearly 4 km of surface uplift since the paleocene addition of mafic melts appears to be a first order mechanism of cenozoic crustal growth contributing 40 of the crustal thickening beneath the volcanic arc removal of the basal part of the lithosphere may have caused two episodes of widespread arc and behind arc mafic volcanism at 23 ma and 0 5 ma contributing to the surface uplift the altiplano originated as a sedimentary basin several hundred kilometers wide between the proto western cordillera and a narrow zone of uplift proto eastern cordillera farther east the latter zone formed by inversion of the center of a wide lacustrine or marine cretaceous paleocene basin close to sea level at 45 ma a thickness of 2 4 km of paleogene continental elastics accumulated in the proto altiplano basin subsequently in the oligocene we estimate that this region and the western margin of the eastern cordillera were technically shortened 22 65 km resulting in 9 km of average crustal thickening the altiplano basin was rejuvenated at 25 ma and subsequently flooded with up to 8 km thickness of detritus eroded from the uplifting eastern and western cordilleras between 25 and 5 ma folding and thrusting in the western margin of the eastern cordillera migrated westward into the center of the altiplano basin essentially terminating deposition except in local subbasins and accommodating 13 30 km of shortening and an estimated 7 km of average crustal thickening subsequently there has been strike slip deformation and limited local thrusting 5 km of shortening geomorphological and geochronological evidence for 1 5 2 km of surface uplift of this region since the late miocene suggests 14 km of lower crustal thickening beneath an essentially rigid lid and can be explained by 100 150 km of underthrusting of the brazilian shield and adjacent regions beneath the eastern margin of the central andes the present subdued relief in the altiplano may be a result of ductile flow in the lower crust and sedimentation and erosion in an internal drainage basin

wadi el hudi migmatites east of aswan egypt a geological study and some geotectonic implications for the eastern desert of egypt abstract wadi el hudi migmatites are represented by successive homogenization stages from stromatic and augen migmatites metatexites and anatexites to veined anatexites diatexites the migmatites are surrounded by biotite gneisses and form a sheath around an intrusive adamellite core the chemistry of the biotites reveals that they are the product of medium pressure high temperature metamorphism that reached the biotite melting isograde garnetiferous leucogranite sheets and pegmatite mobilizates are associated with the migmatites the geological map of egypt shows the occurrence of the studied granite migmatite gneiss complex along ophiolite belts and large outcrops of similar migmatite gneiss complexes recent workers have suggested that this belt represents a suture zone formed by the closure of a marginal basin field petrographical and petrochemical studies indicate two processes of crustal melting acting at varying depth levels one process in the closed system migmatite sheath involving selective anatexis anatectic differentiation and mobilization of leucogranite sheets and pegmatites associating the migmatites the other process involving widespread anatexis at greater depths which produced intrusive biotite hornblende adamellites and probably left a granulitic residuum it is proposed that the el hudi granite migmatite gneiss complex represents an outer arc magmatic belt formed during the initial stages of arc arc collision a plate tectonic model is suggested to illustrate the heat sources for crustal melting in the geotectonic environment of the outer arc magmatic belt as well as its mode of formation two implications for the eastern desert of egypt are suggested 1 the migmatite gneiss domes in the eastern desert of egypt which occur along three ophiolitic belts may represent syn collision crustal anatexis at plate margins during the initial stages of arc arc collision and 2 the mineralized garnetiferous muscovite granites are similar in petrography and petrochemistry to the leucogranite sheets in the migmatite front their barite and galena mineralization is probably related to the breakdown and melting of biotite and the formation of a mobilized hydrous melt on a regional scale in the eastern desert of egypt it is observed that the barite deposits occur along the gneiss belts whereas the fluorite deposits which are related to intra arc post collision leucogranites occur mainly in the central eastern desert between suture zones

proterozoic deformation of the east saharan craton in southeast libya south egypt and north sudan abstract the basement areas in southeast libya south egypt and north sudan west of the nile between gebel uweinat and the bayuda desert are part of an approximately 1000 km wide complexly folded polymetamorphic zone with a regional n nne ne ene trend of foliation and fold axis since this belt extends southwestward into the area of zalingei in the southern darfur block west sudan it is named the northern zalingei fold zone sr and nd isotopic studies suggest that this zone is older than pan african and further indicate that apart from archean rocks in the gebel uweinat area this belt is of early middle proterozoic age an early middle proterozoic three stage deformational and anatectic event established the present day fold and fault geometry in the western parts of this zone in the gebel uweinat gebel kamil area the pan african tectono thermal episode was most effective in the eastern part of the belt near the boundary with the nubian shield volcano sedimentary ophiolite granitoid assemblages it caused migmatization granite emplacement mylonitization and large scale wrench faulting which was related to late proterozoic accretionary and collisional events of the arabian nubian shield with the margin of the east saharan craton

late proterozoic evolution of the northern part of the hamisana zone northeast sudan constraints on pan african accretionary tectonics new structural data from the red sea hills northeast sudan add constraints to accretionary tectonic processes that acted in the arabian nubian shield during the late proterozoic geometric constraints provided by field data are used together with landsat thematic mapper imagery to interpret the regional distribution and structural relations of spectrally and lithologically distinctive rock assemblages in contrast to interpretations reported elsewhere these studies indicate that the hamisana zone a prominent structural feature in the northern red sea hills post dated terrane accretion it is characterized by east west crustal shortening and by steep folds and thrust faults calc alkaline magmatism was contemporaneous with and outlasted main phase deformation thus deformation occurred in an intra arc setting thrust faults were reactivated after east west contraction compromising their usefulness in delineating 9sutures9 or as piercing points across older faults pan african accretionary processes may have been analogous to phanerozoic ophiolite and island arc accretion in the western north american cordillera where penetrative deformation occurred in response to periodic intra plate shortening events rather than an ultimate collision of unrelated crustal fragments such an analogy implies a mechanism for crustal thickening by structural as well as igneous processes during evolution of an accretionary belt in a long lived convergent margin setting consistent with the long standing observation that pan african magmatism became more evolved with time this view emphasizes that an interplay of deformation and magmatism resulted in protracted accretion and crustal thickening in contrast to models where crustal shortening results directly from collision of a volcanic arc microcontinent or other buoyant features

geochemistry petrogenesis and tectonic setting of late neoproterozoic dokhan type volcanic rocks in the fatira area eastern egypt the neoproterozoic dokhan volcanics of the fatira area in eastern egypt comprise two main rock suites a an intermediate volcanic suite consisting of basaltic andesite andesite dacite and their associated pyroclastic rocks and b a felsic volcanic suite composed of rhyolite and rhyolitic tuffs the two suites display well defined major and trace element trends and a continuum in composition with wide ranges in sio2 54 76 cao 8 19 0 14 mgo 6 96 0 04 sr 983 7 ppm zr 328 95 ppm cr 297 1 ppm and ni 72 1 ppm they are enriched in liles rb ba k th ce relative to high field strength elements nb zr p ti and show strong affinity to calc alkaline subduction related rocks however their undeformed character their emplacement temporally and spatially with post orogenic a type granite and their high zr y values suggest that their emplacement follow the cessation of subduction in eastern egypt in an extensional related within plate setting major and trace element variations in the intermediate volcanics are consistent with their formation via partial melting of an enriched subcontinental lithospheric mantle source followed by a limited low pressure fractional crystallization of olivine and pyroxene before emplacement the lile enrichment relative to hfse is attributed to the inheritance of a subduction component from mantle material which constituted the mantle wedge during previous subduction events in eastern egypt the evolution of the whole volcanic spectrum was governed mainly by crystal melt fractionation of amphibole plagioclase titanomagnetite and apatite in the intermediate varieties and plagioclase amphibole biotite fe ti oxides apatite and zircon in the felsic varieties at each stage of evolution crystal fractionation was accompanied by variable degrees of crustal contamination

3he evidence for a wide zone of active mantle melting beneath the central andes we report results of a regional survey of helium isotopes measured in water and gas samples in volcanic sulfataras and geothermal springs from the central andes of northern chile and bolivia between the latitudes 15 s and 23 s the highest 3he4he ratios reported as rra ratios r sample 3he4he ra air 3he4he are associated with the active volcanic arc of the western cordillera 0 92 rra 5 52 and approach ratios found at other convergent margins in the circum pacific region a significant 3he component is also present in fluid and gas samples from the high altiplano plateau 0 48 rra 3 56 and the eastern cordillera 0 03 rra 1 2 up to 300 km east of the active arc and more than 300 km above the subducting slab this wide zone of 3he anomalies is delineated both to the east and the west by regions with low 3he4he ratios 0 2ra typical of radiogenic helium production in the crust studies of the regional groundwater regime suggest that the wide zone of elevated 3he4he values away from the active volcanic arc is unlikely to be caused by lateral and shallow transport of magmatic helium and there is no evidence for significant crustal sources of 3he the high 3he4he ratios are interpreted as reflecting degassing of volatiles from mantle derived magmas emplaced over an area 400 km wide beneath and into crust up to 75 km thick the subducting slab is at depths of 100 350 km in this region in the west underneath the active volcanic arc mantle melting is probably largely controlled by mantle hydration and dehydration and the helium isotope data can be used to delineate the extent of the asthenospheric mantle wedge at depth in contrast mantle melting behind the arc beneath the altiplano and eastern cordillera may be a result of convective removal of the base of the lithosphere the sharp cut off in the mantle helium signal in the east is interpreted as marking the western edge of thick and relatively cold lithosphere devoid of mantle melts which could transport mantle volatiles towards the surface this may coincide with the limit of underthrusting of the brazilian shield beneath the eastern margin of the central andes

gneiss cored interference dome associated with two phases of late pan african thrusting in the central eastern desert egypt abstract structural analysis of the wadi um had area in the central eastern desert ced of egypt reveals the presence of another nw trending elliptical gneissic and migmatitic domal structure west of gabal meatiq outlined by a thick schistose and mylonitized carapace the sheared carapace formed during a nw nnw ward thrusting event which emplaced thrust slices of ophiolitic melange dokhan volcanics and hammamat group sediments over the higher grade gneisses the gneisses are subdivided by retrograde shear zones of this event the sheared carapace is interpreted as the roof thrust of an imbricate stacked antiformal duplex a second thrusting and folding event associated with ne sw shortening deformed the earlier structures producing the dome and re thrusting low grade units against it as sw and ne dipping thrusts the features of this gneiss dome are inconsistent with it being a metamorphic core complex caution must be exercised in the recognition of transcurrent shears in the eastern desert since folded low angle thrust related mylonite zones may be steepened by folding producing steep shear zones with low pitching slip lineations extension bi directional thrusting and najd faulting in the ced may be explained in an escape tectonic deformation regime involving extrusion of material away from the pivotal collision zone in the mozambique belt to the south

age and tectonic setting of granitoid gneisses in the eastern desert of egypt and south west sinai strongly deformed and locally migmatized gneisses occur at several places in the southern eastern desert of egypt and in sinai and have variously been interpreted as a basement to pan african 900 to 600 ma supracrustal and intrusive assemblages a suite of grabbroic to granitic gneisses was investigated in the hafafit area which constitutes an i type calc alkaline intrusive assemblage whose chemistry suggests emplacement along an active continental margin and whose granitoid members can be correlated with the so called older granites of egypt 207pb 206pb single zircon evaporation from three samples of the hafafit gneisses yielded protolith emplacement ages between 677 9 and 700 12 ma and document granitoid activity over a period of about 23 ma a migmatitic granitic gneiss from wadi bitan south west of ras banas has a zircon age of 704 8 ma and its protolith was apparently generated during the same intrusive event as the granitoids at hafafit single zircons from a dioritic gneiss from wadi feiran in south west sinai suggest emplacement of the protolith at 796 6 ma and this is comparable with ages for granitoids in north east sinai and southern israel none of the above gneisses is derived from remelting of older continental crust but they are interpreted as reflecting subduction related calc alkaline magmatism during early pan african magmatic arc formation

geological exploration history of the eastern sahara the major contributions to our knowledge of the geological history of the east saharan countries are described much work has been published on the history and progress of geographical socioeconomic and archaeological exploration in north africa little attention however has been paid to the question of how our knowledge of the geology of this vast area was developed thus even though progress in geology is vital to the development of saharan countries little has been recorded of this progress without an understanding of regional geology and without a good knowledge of groundwater mineral and petroleum potentials these countries would be in very different situations today the author has over the last 35 years been involved in scientific research and exploration of petroleum mineral and groundwater resources in libya egypt sudan and chad

the najd fault system saudi arabia and egypt a late precambrian rift related transform system the najd fault system is a complex set of left lateral strike slip faults and ductile shear zones that strike nw se across the precambrian of arabia and egypt this system was developed during the interval 540 620 ma it is up to 400 km wide with an exposed length of 1100 km inferred buried extensions of the najd give it a total length of 2000 km it is the best exposed and may be the largest pre mesozoic zone of transcurrent faulting on earth previous models for the najd fault system suggest it formed as a result of a major late precambrian continent continent collision this model is not preferred here because 1 the lack of evidence for a pre late precambrian continent to the east of the najd fault system 2 the difference between the orientation of the najd fault system and that predicted by slip line theory 3 the younger age of najd movements compared with that of collisional sutures in the arabian shield and 4 lack of evidence for wide spread crustal uplift that would be expected to accompany collision a new model for the origin of the najd fault system accounts for each of these objections the najd fault system formed in response to a broad zone of nw se directed crustal extension that accompanied juvenile continental crustal formation in northernmost afro arabia this model also accounts for the following observations 1 strands of the najd parallel the direction of extension in the north eastern desert of egypt and sinai 2 the timing of the principal rifting movements ca 575 600 ma overlap with those of the najd ca 560 620 ma 3 in spite of observation 2 the najd fault system is not recognized in northernmost afro arabia instead the najd deformation becomes increasingly ductile and these zones are more commonly intruded by sheared and foliated granites as the principal zone of extension is approached the najd fault system thus represents a set of continental transforms developed in response to a major episode of late precambrian extensional continental crust formation in northernmost afro arabia

post collision a type granites of homrit waggat complex egypt petrological and geochemical constraints on its origin abstract the homrit waggat complex hwc is a ring complex which belongs to the younger granite province of the nubian shield it comprises a central biotite granite encircled by an intermediate ring of mylonitized pink granite both of which have been emplaced by cauldron subsidence in a granodiorite body the mylonitized pink granite is locally cut across by hypersolvus coarse grained red granite the mylonitized pink granite is characterized by fully developed mylonitic fabrics and suffered inflation and flattening due to a ballooning effect which suggests that it has experienced stronger ductile deformation than the granodiorite and biotite granite the hw granites are mildly peraluminous highly fractionated and enriched in rb nb y ree except eu and f with corresponding depletion in mg fe ti ca sr and ba ree fractionation patterns and eu eu values decrease from granodiorite la yb n 6 24 6 69 eu eu 0 95 1 56 to biotite granite la yb n 2 48 4 68 eu eu 0 25 0 75 the mylonitized pink and red granites display flat patterns and large negative eu anomalies la yb n 0 33 1 09 eu eu 0 01 0 07 respectively although accessory minerals e g apatite allanite and fluorite exerted some control on ree concentrations volatile transfer largely controls ree abundances particularly hree heavy rare earth element geochemical data indicate that the granites of hwc are post collision a types the less fractionated granodiorite has an i type and volcanic arc geochemical signature least squares analyses of major elements and modelling of rees indicate that the hw a type granites were formed through two stages of differentiation a first stage crystalmelt fractionation was dominated by the removal of plagioclase biotite quartz and apatite from the granodiorite magma to yield the biotite granite further crystal fractionation of the biotite granite liquid with k feldspar as a dominant separating phase in addition to plagioclase biotite and quartz gave rise to the mylonitized pink granite second stage volatile phase transfer occurred during the second stage of fractionation in the magma chamber the red granite represents the last crystallizing granite type from a residual volatile rich fluids which accumulated and emplaced as ring dykes in the solidified part of the mylonitized pink granite

geochemical and sr nd pb isotopic data bearing on the origin of pan africangranitoids in the kid area southeast sinai egypt the kid plutonic suite in southeast sinai egypt is composed of 1 a gabbro diorite complex 2 quartz monzodiorite 3 granodiorite monzogranite association 4 syenogranite major and traceelement analyses and radiogenic isotope data rb sr sm nd and pb pb are presented for these rocks rb sr isochrons indicate ages of 581 11 ma 576 11 ma and 570 4 ma for the quartz monzodiorite granodiorite monzogranite association and syenogranite respectively the gabbro diorite complex is a layered intrusion varying in sio2 from 42 to 59 wt it shows a calc alkaline trend and the trace element characteristics of arc lavas subsequently this magma fractionated to give quartz monzodiorite andsyenogranite magmas the quartz monzodiorite has low initial 87 sr 86 sr ratios 0 703 and high nd values 4 4 0 3 with trace element characteristics of volcanic arc granites the granodiorite monzograniteassociation has higher initial sr ratio 0 7044 lower nd 1 5 2 7 and relatively high 207 pb 204 pb and 208 pb 204 pb significantly different from those of the older rocks isotopic data and ree modelling suggestthat it formed by partial melting of an early pan african amphibolitic lower crust the syenogranite isa typical highly fractionated post orogenic granite with a deep negative eu anomaly hfse and reeenrichment a 87 sr 86 sr initial ratio of 0 7033 nd of 4 5 0 5 and distinct variations in the 206 pb 204 pbratio the isotopic compositions of sr nd and pb indicate that the syenogranite and the quartzmonzodiorite are comagmatic increasing fluorine concentration from the quartz monzodiorite to thesyenogranite suggests that fluid fractionation involving fluorine complexing played an important role inthe genesis of the syenogranite this process led to the enrichment of hfse and hree to extensivefractional crystallization to changes in the u pb ratios and a distinct variation in 206 pb 204 pb

evidence from the northern red sea on the transition from continental to oceanic rifting abstract the northern red sea is an active rift in the last stages of continental rifting which is beginning to undergo the transition to oceanic seafloor spreading the rift can be divided into two distinct regions the marginal areas consist of a series of bathymetric terraces and as indicated by geophysical data are underlain by a series of rotated fault blocks the axial depression which forms the axis of deep water is a continuous region 10 to 25 km wide near the center of the sea which is characterized by high heat flow a large gravity minimum and very deformed sediments both regions are divided along strike into segments roughly 60 km long by cross trending transfer zones the presently active tectonics are characterized by the concentration of extension and deformation in the axial depression and by the emplacement of a number of large intrusions the intrusions occur at distinct sites located at the intersection of transfer zones with the faults bounding the rotated blocks in the marginal areas and at deeps located in the axial depression halfway between transfer zones although extension is becoming concentrated in the axial depression geophysical data imply that at least the upper few kilometers of the crust in that region consists of non magnetic continental type material and that this ca apace has been ruptured by large intrusions only at the deeps a wide range of crustal seismic velocities has been observed in the northern red sea including velocities normally associated with oceanic crust the velocity variations appear spatially related to corresponding large variations in basement rock type observed on shore in the eastern desert of egypt the northern red sea is in an intermediate stage between a continental rift valley and a young ocean basin and delineation of its structure and tectonics allows a model to be developed for the evolution of a rifted continental margin rifting starts in a series of linked half grabens 50 60 km long and formed over detachment surfaces which probably penetrate no deeper than the mid or lower crust this style of rifting can accommodate at least the amcunt of extension represented by the gulf of suez 35 km at some greater amount of extension the rift acquires a more symmetric shape probably through the development of antithetically faulted blocks in the hanging wall extension in this stage is still distributed throughout the rift and is accommodated in the upper crust by rotation of fault blocks at the stage represented by the northern red sea extension becomes concentrated in an axial depression and begins to be partially accommodated by large intrusions which form deeps the position of the deeps is controlled by the transfer zones established in the earliest stages of rifting with further extension the deeps serve as nucleation points that develop into small discrete cells of seafloor spreading these cells grow and coalesce to form a continuous mid ocean ridge spreading center such as is found in the southern red sea

a late precambrian 710 ma high volcanicity rift in the southern eastern desert of egypt the late precambrian shadli metavolcanics of se egypt constitute a slightly metamorphosed bimodal sequence that has been previously interpreted as manifesting volcanic activity at an island arc we report the first rb sr geochronologic trace element including ree and nd isotopic data for these rocks two types of basalt are recognized the stratigraphically lower suite having compositions like n morb ferrobasalt while the overlying basalt is similar to slightly fractionated e morb the two basalt types were derived from melting of a strongly depleted source most likely within the upper 60 75 km of the upper mantle the origin of the felsic melts is problematic and these could either have fractionated from a mafic melt or resulted from melting of juvenile crust the mafic and felsic lavas yield a rb sr isochron age of 712 24 ma that probably represents the time of volcanic eruption the trace element characteristics of both mafic and felsic members of the shadli metavolcanics show few of the hallmarks of subduction related melts and we reject the hypothesis that these formed at an island arc instead the field and geochemical data are most consistent with the hypothesis that these rocks originated in a magmatic rift where the eruption of large volumes of lava accompanied large scale lithospheric extension this inference suggests that the tectonic setting of the important 700 715 ma crust forming event in ne africa and arabia needs to be critically reexamined

heat induced moisture movement in a clay barrier ii computational modelling and comparison with experimental results the paper presents the application of a galerkin finite element technique for the numerical solution of the differential equations governing coupled heat flow and moisture movement in a clay buffer attention is focussed on the axisymmetric modelling of the hygro thermal processes encountered in the single borehole emplacement configuration developed for laboratory simulation the numerical results derived for the time dependent temperature distributions within the granite block and the residual moisture distribution within the buffer are compared with the respective experimental results

lithospheric evolution of the andean fold thrust belt bolivia and the origin of the central andean plateau we combine geological and geophysical data to develop a generalized model for the lithospheric evolution of the central andean plateau between 188 and 208 s from late cretaceous to present by integrating geophysical results of upper mantle structure crustal thickness and composition with recently published structural stratigraphic and thermochronologic data we emphasize the importance of both the crust and upper mantle in the evolution of the central andean plateau four key steps in the evolution of the andean plateau are as follows 1 initiation of mountain building by 70 ma suggested by the associated foreland basin depositional history 2 eastward jump of a narrow early fold thrust belt at 40 ma through the eastward propagation of a 200 400 km long basement thrust sheet 3 continued shortening within the eastern cordillera from 40 to 15 ma which thickened the crust and mantle and established the eastern boundary of the modern central andean plateau removal of excess mantle through lithospheric delamination at the eastern cordillera altiplano boundary during the early miocene appears necessary to accommodate underthrusting of the brazilian shield replacement of mantle lithosphere by hot asthenosphere may have provided the heat source for a pulse of mafic volcanism in the eastern cordillera and altiplano at 24 23 ma and further volcanism recorded by 12 7 ma crustal ignimbrites 4 after 20 ma deformation waned in the eastern cordillera and interandean zone and began to be transferred into the subandean zone long term rates of shortening in the fold thrust belt indicate that the average shortening rate has remained fairly constant 8 10 mm year through time with possible slowing 5 7 mm year in the last 15 20 myr we suggest that cenozoic deformation within the mantle lithosphere has been focused at the eastern cordillera altiplano boundary where the mantle most likely continues to be removed through piecemeal delamination d 2005 elsevier b v all rights reserved

shrimp u pb dating of detrital zircons from the hammamat group at gebel umm tawat north eastern desert egypt

geology and origin of meatiq dome egypt a precambrian metamorphic core complex meatiq dome a metamorphic complex in the precambrian basement of the eastern desert of egypt exhibits many of the essential features of cordilleran metamorphic core complexes it is an antiformal structure with low dipping foliation and unidirectional mineral slickenside lineation the core consists of granite gneiss and is conformably overlain by a heterogeneous isoclinally folded mylonitic carapace the carapace grades upward into a nonmylonitic cover of low grade ophiolitic rocks the mylonitic rocks are interpreted to have formed in a low angle ductile shear zone during a compressional tectonic event accompanied by syntectonic intrusion of tonalite doming postdated mylonitization the absence of a brittle detachment surface at meatiq dome is its primary distinction from the otherwise similar cordilleran complexes

samarium neodymium data on two late proterozoic ophiolites of saudi arabia and implications for crustal and mantle evolution whole rock and mineral samples from the jabal al wask and jabal ess ophiolites northwestern saudi arabia yield sm nd isochron ages of 743 24 ma and 782 38 ma respectively these formation ages which provide maximum limits for possible obduction ages are in broad but not precise agreement with the previously known geologic history of the arabian shield they indicate that the ophiolitic rocks are roughly coeval with nearby volcanic and plutonic rocks supporting a back arc origin for the two ophiolites we suggest that the jabal al wask and jabal ess ophiolites were parts of the same northeast southwest trending ophiolite belt now offset along the najd fault system initial nd values range from 6 6 to 7 6 indicating derivation from a mantle source that has been lil depleted for at least 2 ga reported nd values from the arabian shield that are lower than this suggest the presence of older reworked continental crust

extension of the najd shear system from saudi arabia to the central eastern desert of egypt based on integrated field and landsat observations the najd shear system in saudi arabia extends over 1200 km in a nw se direction and has a width of approximately 300 km a digital color mosaic compiled from seven landsat thematic mapper scenes was used to delineate characteristic structural features of the najd system in the midyan region of saudi arabia and to search for similar features in the egyptian eastern desert the digital mosaic was generated using ratios of landsat thematic mapper bands bands 5 4 3 4 5 1 5 7 that are sensitive to the rock content of fe bearing aluminosilicates spectrally opaque phases and hydroxyl bearing or carbonate minerals respectively the mosaic covers approximately 130 000 km of late proterozoic exposures of the arabian nubian shield and has the eastern desert and the midyan region placed in their approximate pre red sea locations the ajjaj shear zone ajz marks the termination of the najd system against the eastern margin of the red sea in the midyan region the ajz aligns with the central eastern desert based on analysis of pre red sea locations analyses of landsat data and field observations show that the ajjaj shear zone and the central eastern desert exhibit the following features in common 1 outcrops that are generally elongate in a nw se direction as a result of folding with fine scale lithologic heterogeneity at the outcrop scale related to deformation associated with faulting 2 nw trending left lateral faults and ductile shear zones 3 subhorizontal nw trending mineral lineations and variably dipping nw trending foliations with local changes in attitude around large competent e g granitic bodies and 4 lithologic contacts that are generally tectonic in nature and related to faulting these features are less common to the north and south of both the ajjaj shear zone and the central eastern desert results are consistent with the najd shear system extending into the eastern desert and dominating the structural patterns within the central part of the eastern desert

geochronology of the late precambrian hamisana shear zone red sea hills sudan and egypt the hamisana shear zone hsz is a broad zone of deformation approximately 50 km wide and at least 300 km long making it one of the largest basement structures in ne africa it has been interpreted as a precambrian suture as a zone of strike slip displacement or as a zone of crustal shortening the results of new rb sr and u pb zircon geochronological studies indicate that the northern hsz was thermally active during the pan african event until c 550 mg ago initiation of the structure may have begun 40 110 ma earlier all units have low initial 87 sr 86 sr ratios indicating juvenile derivation the timing of activity in the hsz is 50 150 ma younger than collisional suturing and terrane assembly in the arabian nubian shield but is similar to the 655 540 ma najd fault system of egypt and arabia deformation and metamorphism along the hsz clearly post date terrane accretion and probably are closely related to the najd tectonic cycle the most important deformation of the hsz is unrelated to suturing and at least one late precambrian suture must extend west from arabia into the interior of n africa

end member models for andean plateau uplift abstract diverse techniques have been applied over the past decade to quantify the uplift history of the central andean plateau ap in this study opposing models for surface uplift are evaluated including a rapid rise of 2 5 km 10 6 ma and a slow and steady rise since 40 ma these end member models are evaluated by synthesizing observations of the ap lithosphere and the history of deformation sedimentation exhumation magmatism uplift and fluvial incision structural and geophysical studies estimate variable shortening magnitudes 530 150 km involving cover to basement rocks an isostatically compensated thick crust 80 65 km high heat flow and zones of variable velocity and attenuation in the crust and mantle these observations have invoked interpretations such as a hot weak lithosphere partial melt crustal flow and perhaps current localized delamination but do not provide strong support for massive delamination required by the rapid uplift model deformation and associated exhumation began 60 40 ma and generally migrated eastward with consistent long term average shortening rates 12 8 mm yr in bolivia favoring the slow uplift model volcanic and helium isotope evidence show an ap wide zone of shallow mantle melting and thin lithosphere that has existed since 25 ma which is inconsistent with the rapid rise model that suggests lithospheric thinning occurred 10 6 ma paleoaltimetry data suggest a rapid 2 5 km elevation gain 10 to 6 ma but are equally consistent within error with a linear rise since 25 ma widespread fluvial incision 2 5 1 km occurred along the western flank since 11 8 ma and may be associated with surface uplift as proposed by the rapid rise model however the paleoaltimetry and incision data can also be explained by regional climate change associated with plateau uplift implications of these results for reconstructions of ap evolution are that 1 substantial deformation of a weak lithosphere is essential 2 ap growth has taken significantly longer 40 myr and was more uniform along strike 1500 km than previously appreciated and 3 the slow and steady uplift model is most consistent with available constraints we conclude that the rapid uplift model may be an overestimate and that a more protracted cenozoic uplift history is tenable