upper mantle anisotropy beneath the african iris and geoscope stations summary upper mantle anisotropy beneath the african iris and geoscope stations is investigated through the measurements of splitting of teleseismic shear waves such as sks skks and pks phases seismic anisotropy data are interesting on their own as a measure of upper mantle active or frozen deformation beneath a given station but each station is of potential interest since it can be used to retrieve source side seismic anisotropy at remote sites if one is able to perform station side anisotropy correction we performed systematic investigations of teleseismic shear wave splitting at 15 stations from the iris and geoscope global seismic networks which are located on both the oceanic and the continental parts of the african plate anisotropy is generally well observed at continental stations the patterns we present generally show much more complexity than the results previously published from smaller data sets despite this complexity the splitting parameters generally appear in several places to contain a signature of the regional geodynamic setting rift structures archaean craton panafrican belt although a deeper source of anisotropy asthenospheric may be present at the oceanic stations anisotropy measurements are much more difficult to perform because the signal is generally of poor quality msey in the seychelles indian ocean is the exception and displays a clear correlation of the azimuth of the fast split shear wave with the trend of the absolute plate motion as defined by hotspot tracks

a field based classification scheme for gravel beaches a tripartite classification of gravel beaches based upon morphodynamic properties is proposed and demonstrated for 42 new zealand beaches the main advantage of this scheme is that it is based on simple visual classification that can be applied globally in the field and is underpinned by morphodynamic differences between the beach types the three types identified from the results are 1 pure gravel beach 2 mixed sand and gravel beach 3 composite gravel beach pure gravel beaches have steep slopes tan 0 08 0 24 and gravels extending from the storm berm to below mean low water spring tide level mixed sand and gravel beaches have moderate slopes tan 0 04 0 13 with sand and gravel entirely mixed both cross shore and at depth composite gravel beaches have a steep gravel berm fronted by a low angle intertidal terrace with overall beach slopes of tan 0 05 0 14 on composite beaches there is distinct hydrodynamic cross shore sorting of the sand and gravel component these broad types are tested using discriminant analysis and the three classes are shown to be highly significant f 16 24 df 8 p 0 00005 the key discriminating variables are iribarren number beach width average grain size and storm berm height

micro drilling id tims u pb dating of single monazites a new method to unravel complex poly metamorphic evolutions application to the uht granulites of andriamena north central madagascar in terranes subject to complex metamorphic evolution zircon and monazite u pb system can record successive growth and or resetting episodes conven tional isotope dilution thermal ionisation mass spec trometry id tims dating of fractions or single grains from conventionally separated zircon and monazite populations may produce inaccurate ages a new tech nique combining textural analysis and id tims dating on single monazite crystals is proposed this method is applied directly on thin sections with three successive steps comprising 1 a characterisation of textural rela tionships using electron microprobe analyses and ima ges 2 extraction by micro drilling of selected monazites and 3 u pb dating of each extracted grain by id tims the potential for this in situ dating technique are tested on ultra high temperature uht granulites from north central madagascar four dis tinct events on a single thin section were distinguished the uht metamorphism was dated at 2 5 ga while two retrogression events were dated at 790 and 500 ma finally a single crystal shielded in quartz seems to preserve a pre metamorphic age around 2 7 ga

ree patterns nd sm and u pb ages of the metamorphic rocks of the diagorou darbani greenstone belt liptako sw niger implication for birimian palaeoproterozoic crustal genesis

origin and age of paleoproterozoic conglomerates and sandstones of the tarkwaian group in burkina faso west africa abstract the tarkwaian rocks of burkina faso overlie the birimian supergroup which is considered to be part of a juvenile crust formed during the eburnean orogen they comprise fluviatile pebbles quartz rhyolite chert and schist embedded in an acid volcaniclastic matrix during a sinistral transpressive shearing they underwent a low grade metamorphism characterized by a paragonitic muscovite assemblage formed during isoclinal folding fold axes are nearly horizontal parallel to a stretching lineation and oriented 035 215 a typologic study of zircons in the volcaniclastics coupled with radiometric datings enables the volcanism to be characterized and its age to be determined the typologic study has shown that 1 zircons from the rhyolitic pebbles the matrix of the conglomerates and from the sandstone are basically the same 2 the zircons morphology characterizes a calc alkaline to alkaline evolutionary trend implying three main episodes of volcanism 3 the morphology of this zircon population is very similar to those usually observed in post orogenic metasediments or sediments the tectonic setting is one of intracontinental extension associated with rifting and crustal thinning in the course of a major period of crustal growth around 2 1 ga the continental volcaniclastics accumulated in the rift at the same time as the sediments the radiometric study has shown that there are no reworked archean rocks the ages cover the period 2170 to 2124 ma three main volcanic episodes may be defined 2170 2150 and 2124 ma the last one was probably contemporaneous with the sedimentation deformation of tarkwaian rocks took place between 2124 9 ma the age of the youngest inherited zircon and 1991 12 ma the age of a post tarkwaian granite this period 2170 to 2124 ma fits well with the main birimian crustal growth and the ages reported for the tarkwaian rocks of ghana the same period of accretion is also recognized in south america the tarkwaian conglomeratic rocks of burkina faso do not have gold concentrations similar to those in ghana and guyana with respect to the tarkwaian rocks of ghana where gold is concentrated in the quartz pebbles they also differ because 1 there is a volcaniclastic contribution 2 the zircons from the tarkwaian rocks in burkina faso are euhedral indicating that transport distances were short and 3 they have been deformed by shearing with respect to guyana the rocks are very similar and probably have the same origin the short transport distance may not allow a secondary concentration of gold these differences at least may help explain the lack of economic gold concentration in the tarkwaian metasediments

petrological and geochronological constraints on lower crust exhumation during paleoproterozoic eburnean orogeny nw ghana west african craton we present new petrological and geochronological data on high grade ortho and paragneisses from northwestern ghana forming part of the paleoproterozoic 2 25 2 00 ga west african craton the study area is located in the interference zone between n s and ne sw trending craton scale shear zones formed during the eburnean orogeny 2 15 2 00 ga high grade metamorphic domains are separated from low grade greenstone belts by high strain zones including early thrusts extensional detachments and late stage strike slip shear zones paragneisses sporadically preserve high pressure low temperature hp lt relics formed at the transition between the blueschist facies and the epidote amphibolite sub facies 10 0 14 0 kbar 520 600 c and represent a low 15 c km 1 apparent geothermal gradient migmatites record metamorphic conditions at the amphibolite granulite facies transition they reveal a clockwise pressure temperature time p t t path characterised by melting at pressures over 10 0 kbar followed by decompression and heating to peak temperatures of 750 c at 5 0 8 0 kbar which fit a 30 c km 1 apparent geotherm a regional amphibolite facies metamorphic overprint is recorded by rocks that followed a clockwise p t t path characterised by peak metamorphic conditions of 7 0 10 0 kbar at 550 680 c which match a 20 25 c km 1 apparent geotherm these p t conditions were reached after prograde burial and heating for some rock units and after decompression and heating for others the

re evaluation of leonian and liberian events in the geodynamical evolution of the man leo shield west african craton abstract the leonian 3400 3100 ma and liberian 2850 2700 ma events are important magmatic and tectono metamorphic events in the archean domain of the man leo shield in the southern west africa craton the older leonian event was associated with crust formation which was variably reworked during the younger liberian event in the man domain of western cote d ivoire zircon in situ dating by la icpms of granulitic grey gneisses gggs the mangouin charnockite and the lagoulale augen orthogneiss allows for the re evaluation of magmatic and metamorphic activity related to the leonian and liberian events we report here our findings that the gggs represent the oldest rocks in the study area and formed during the leonian event the ages recorded in the gggs in the man area are 3156 24 3098 19 3093 49 and 3019 53 ma the ages of 3156 24 and 3098 19 ma represent leonian juvenile magmatism while those of 3093 49 and 3019 53 ma represent leonian metamorphism the ages of the mangouin charnockite and the lagoulale augen orthogneiss are 2798 8 ma and 2795 9 ma respectively and are interpreted to record peak metamorphic conditions during the liberian event zircon from the augen orthogneiss includes an inherited core with an age of 3121 37 ma which corresponds to leonian ggg ages this inheritance indicates that the mangouin charnockite and the lagoulale augen orthogneiss were derived either partially or completely from leonian formations a previously constrained sm nd model age of 3250 ma for the charnockite and augen orthogneiss agrees with such a process the majority of th u is higher indicating a magmatic origin for most zircons the leonian and liberian events are proposed as two distinct events in man leo shield

age constraints on tarkwaian palaeoplacer and lode gold formation in the tarkwa damang district sw ghana two major epigenetic gold forming events are recorded in the world class gold province of southwest ghana a pre tarkwaian event was the source of the world class tarkwa palaeoplacers whereas post birimian and tarkwaian deformation which was related to the eburnean orogeny gave rise to the world class e g prestea to giant e g obuasi orogenic gold deposits which have made the region famous for more than 2 500 years a maximum age of 2133 4 ma for tarkwaian sedimentation is provided by 71 of 111 concordant shrimp ii u pb dates from detrital zircons in tarkwaian clastic rocks from damang and bippo bin northeast of tarkwa the overall data distribution broadly overlaps the relatively poorly constrained ages of birimian volcanism and associated dixcove type granitoid emplacement indicating syntectonic development of the tarkwaian sedimentary basin these zircon ages argue against derivation of the palaeoplacer gold from an orogenic gold source related to the compressional phase of an orogeny significantly older than the eburnean orogeny instead they suggest that the gold source was either orogenic gold lodes related to an earlier compressional phase of a diachronous eburnean orogeny or ca 2200 2100 ma intrusion related gold lode the co2 rich fluid inclusions in associated vein quartz pebbles are permissive of either source at the damang deposit an epigenetic orogenic lode gold system clearly overprinted and sulphidised low grade palaeoplacer hematite magnetite gold occurrences in the banket series conglomerate within the tarkwaian sedimentary sequence gold mineralisation is demonstrably post peak metamorphism as gold related alteration assemblages overprint metamorphic assemblages in host rocks in alteration zones surrounding the dominant subhorizontal auriferous quartz veins there are rare occurrences of hydrothermal xenotime which give a shrimp u pb age of 2063 9 ma for gold mineralisation the similar structural timing of epigenetic gold mineralisation in tarkwaian host rocks at damang to that in mainly birimian host rocks elsewhere in southwest ghana particularly at obuasi suggests that 2063 9 ma is the best available age estimate for widespread orogenic gold mineralisation in the region argon argon ages of 2029 4 and 2034 4 ma for hydrothermal biotite from auriferous quartz veins appear to represent uplift and cooling of the region below about 300 c as estimates of the temperature of gold mineralisation are higher at around 400 c if peak metamorphism with temperatures of about 550 c is assumed to have occurred at about 2100 ma the biotite ages in combination with the xenotime age suggest a broadly constant uplift rate for the region of about 1 km per 10 million years from about 2100 to 2025 ma

petrology major and trace element geochemistry geochronology and isotopic composition of granitic intrusions from the vicinity of the bosumtwi impact crater ghana abstract the bosumtwi crater is 10 5 km in diameter 1 07 ma old well preserved impact structure located in ghana centered at 06 30 n 01 25 w it was excavated in rocks of the early proterozoic birimian supergroup part of the west african craton here we present a full and detailed characterization of the three granitoid complexes and one mafic dike in the vicinity of the bosumtwi crater in terms of petrology major and trace element geochemistry geochronology and isotopic composition this allows us to characterize magmatic evolution of the west african craton in this area and better understand the geological framework and target rocks of the impact this study shows that the similar composition strongly peraluminous muscovite granites and granodiorites and age between 2092 6 ma and 2098 6 ma of all granitic intrusions in the proximity of the bosumtwi crater suggest that they are co genetic granitoids were probably formed as a result of anatexis of ttgs or rocks derived from them at relatively low pressure and temperature we propose that the intrusions from the bosumtwi area are genetically related to the banso granite occurring to the east of the crater and can be classified as basin type late stage granitoids also a mafic dike located to the ne of the bosumtwi crater seems to be genetically related to those felsic intrusions based on those findings a revised version of the geological map of the bosumtwi crater area is proposed

zircon hf and o isotope constraints on the evolution of the paleoproterozoic baoule mossi domain of the southern west african craton abstract the use of zircon isotopic systems such as lu hf and o has proven to be an invaluable tool for unraveling the evolution of the early earth in order to achieve a comprehensive understanding of the evolution of the southern west african craton and in particular the paleoproterozoic baoule mossi domain zircon grains from 75 felsic intrusions were analyzed for hf isotopes 22 of those samples were analyzed for o isotopes covering an area that covers more than 1000 km2 from southwest burkina faso across southern mali and into eastern guinea the results of the study reveal that the baoule mossi domain comprises two magmatic regions that display broadly juvenile isotopic character but with significant localized evidence of crustal contamination the hf model age data show that the crustal contaminant is potentially as old as ca 2800 ma and indicate that the two domains may have evolved slightly differently in fact the westernmost region presents a slightly older or less radiogenic hf isotope signature when compared to the easternmost region furthermore the o isotopic data support the notion that the felsic magmas were subject to supra crustal contamination as 18o values range between 6 and 12 the data presented in this study provide robust evidence that the baoule mossi domain of the southern west african craton evolved by accretionary processes between ca 2250 2050 ma

reos and upb geochronological constraints on the eclogite tonalite connection in the archean man shield west africa abstract mantle derived eclogite xenoliths and tonalite trondhjemite granodiorites ttg that occur in the man shield west africa sample different levels of archean lithosphere chemical and oxygen isotope systematics indicate that low mgo eclogites from the koidu kimberlite are ancient remnants of subducted oceanic crust that may have been involved in regional archean crust formation re os whole rock isotopic data for these eclogites scatter about a line with slope corresponding to an archean age of 3 44 0 76 ga 2 with re os model ages ranging from 1 4 to 3 9 ga this wide range of model ages overlaps with the age range for crust formation and metamorphism in the man shield in situ u pb ages of zircons from crustal rocks have been measured by laser ablation icp ms a tonalitic gneiss has discordant zircons with rare old cores 3 6 ga and an upper concordia intercept at 2890 9 ma 2 zircons from a mafic lower crustal granulite xenolith are concordant at 2686 32 ma our results together with previously published ages for man shield rocks indicate an early archean crust formation event followed by major crustal growth at 2 9 3 0 ga and a last major metamorphic event at 2 7 ga these data show that the eclogites and the continental crust of the west african craton overlap in time of formation but only at the very broad age uncertainty provided by the eclogite re os results they are permissive of archean crustal growth by melting of the protoliths of the materials now sampled as the koidu eclogite xenoliths if so this suggests that archean crustal growth in the man shield occurred in a convergent margin setting

new rb sr age determinations on the archaean basement of eastern sierra leone abstract the archean basement of sierra leone is a typical example of granite greenstone terrains found in ancient continental nucleii reconnaissance field mapping showed that the area can be subdivided into old gneiss which predates the greenstone belts and young granite which is later than the greenstone belts new rb sr whole rock age determinations on two suites of old tonalitic gneiss yield ages of 2786 49 ma and 2770 137 ma which either reflect the time of formation of the original tonalites or their metamorphism three new rb sr whole rock age determination on young granites yield ages of 2786 143 ma 2780 79 ma and 2770 50 ma which are interpreted as the time of emplacement the widespread occurrence of similar young granites throughout the archaean of west africa suggests that these results date a major event in the evolution of this segment of the crust a published pb pb age of the old gneiss and the new ages of the young granite bracket the age of the greenstone belts to 3000 2770 ma however if the rb sr ages of the old gneiss reported in this paper reflect the time of their formation the age of the greenstone belts is tightly bracketed to ca 2770 ma there is no isotopic evidence for rocks substantially older than 3000 ma in the west african archaean

fluid pressure dominated orogenic gold mineralization under low differential stress case of the yaoure gold camp cote d ivoire west africa the 1 5 moz yaoure gold deposit which is located within the birimian greenstone belts of central cote d ivoire west africa is associated with two stages of economic gold mineralization both ore forming stages occurred during protracted e w directed shortening associated with the eburnean orogeny the first mineralization stage is hosted by fault fill veins along conjugate ene striking dextral and wnw striking sinistral strike slip fault arrays this ore stage is associated with a polymetallic assemblage including mo bi pb w te ag and au and cryptic alteration in wall rocks the second and main gold mineralization stage is associated with a low displacement fault fracture vein mesh formed during a reverse faulting episode this second gold phase is associated with a polymetallic assemblage including w and au pb and is characterized by pronounced albite ankerite pyrite hydrothermal alteration in wall rocks shear veins associated with this second mineralization episode formed during episodic fluid pressure variations under low differential stress within pre existing low angle reverse fault planes permeability destruction by albite ankerite pyrite hydrothermal sealing of these low angle reverse faults resulted in fluid pressure build up and ultimately in the development of extension veins under low differential stress in the late stages of the structural evolution of the deposit this study presents the structural evolution of a polyphase orogenic gold deposit and highlights that the interplay between fault zone activity hydrothermal alteration and fluid pressure fluctuations at low differential and deviatoric stresses were key in controlling fluid flow and the formation of ore within optimally oriented faults

the importance of along margin terrane transport in northern gondwana insights from detrital zircon parentage in neoproterozoic rocks from iberia and brittany abstract detrital zircons from late neoproterozoic rocks of the peri gondwanan cadomian belt of sw iberia and north armorican domain of brittany record neoproterozoic ca 860 550 ma palaeoproterozoic ca 2300 1800 and archaean ca 3300 2600 ma u pb ages the absence of mesoproterozoic zircons suggests that these terranes evolved in a peri w african realm this is in contrast to other western european terranes that preserve mesoproterozoic zircons and are likely to have evolved in a peri amazonian realm such a contrast in detrital zircon populations coupled with the presence of mesoproterozoic zircons in the ordovician armorican quartzite deposited in a peri african platform is interpreted to record along margin terrane transport the change in provenance suggests that subduction was replaced by transform faults that juxtaposed amazonia derived terranes against w africa derived terranes to form the avalonia and armorica microcontinents subsequent extension along the margin resulted in the birth of the rheic ocean and the outboard drift of avalonia

controls on coastal dune morphology shoreline erosion and barrier island response to extreme storms abstract the response of a barrier island to an extreme storm depends in part on the surge elevation relative to the height and extent of the foredunes which can exhibit considerable variability alongshore while it is recognized that alongshore variations in dune height and width direct barrier island response to storm surge the underlying causes of the alongshore variation remain poorly understood this study examines the alongshore variation in dune morphology along a 11 km stretch of santa rosa island in northwest florida and relates the variation in morphology to the response of the island during hurricane ivan and historic and storm related rates of shoreline erosion the morphology of the foredune and backbarrier dunes was characterized before and after hurricane ivan using empirical orthogonal function eof analysis and related through canonical correlation analysis cca the height and extent of the foredune and the presence and relative location of the backbarrier dunes varied alongshore at discrete length scales of 750 1450 and 4550 m that are statistically significant at the 95 confidence level cospectral analysis suggests that the variation in dune morphology is correlated with transverse ridges on the inner shelf the backbarrier cuspate headlands and the historical and storm related trends in shoreline change sections of the coast with little to no dune development before hurricane ivan were observed in the narrowest portions of the island between headlands west of the transverse ridges overwash penetration tended to be larger in these areas and island breaching was common leaving the surface close to the watertable and covered by a lag of shell and gravel in contrast large foredunes and the backbarrier dunes were observed at the widest sections of the island the cuspate headlands and at crest of the transverse ridges due to the large dunes and the presence of the backbarrier dunes these areas experienced less overwash penetration and most of the sediment from the beachface and dunes was deposited within the upper shoreface it is argued that this sediment is returned to the beachface through nearshore bar migration following the storm and that the areas with larger foredunes and backbarrier dunes have smaller rates of historical shoreline erosion compared to areas with smaller dunes and greater transfer of sediment to the washover terrace since the recovery of the dunes will vary depending on the availability of sediment from the washover and beachface it is further argued that the alongshore pattern of dune morphology and the response of the island to the next extreme storm is forced by the transverse ridges and island width through alongshore variations in storm surge and overwash gradients respectively these findings may be particularly important for coastal managers involved in the repair and rebuilding of coastal infrastructure that was damaged or destroyed during hurricane ivan

geochemical and nd isotopic compositions of juvenile type paleoproterozoic birimian sedimentary rocks from southeastern west african craton ghana constraints on provenance and tectonic setting abstract the paleoproterozic birimian supergroup consists of sedimentary basins composed predominantly of wackes and argillites and volcanic belts composed predominantly of basaltic rocks whole rock geochemical and nd isotopic studies were carried out on fine grained sedimentary rocks shales and fine grained sandstones from one of the birimian sedimentary basins the kumasi basin the sedimentary rocks are immature in nature with very little recycled materials relative to average early proterozoic upper continental crust the birimian sedimentary rocks are depleted in the alkalis and alkaline earth elements and the high field strength elements and enriched in transition metals chondrite normalized rare earth element ree plots for both the shales and sandstones show fractionated lree patterns average la n sm n 3 45 with slight negative eu anomalies average eu eu 0 79 and a slightly fractionated hree patterns average gd n yb n 1 84 low to moderate degree of chemical weathering in the sediment source area is indicated by chemical index of alteration values of between 52 and 80 elemental ratios such as la co th co th cr and eu eu that are critical to provenance suggests that the sediments were derived from rocks of mixed felsic and mafic compositions mixing calculations based on the rees indicate that the shale is best modeled with a mixture having 62 ttg 16 granite and 22 basalt whereas the sandstone is almost 100 ttg the e nd values for the analyzed samples calculated at the age of deposition i e 2 1 ga range from 1 69 to 1 07 suggesting derivation from an undifferentiated source the nd isotopic compositions together with eu eu and th u values and other trace element characteristics suggest young undifferentiated arc provenance and this is in agreement with previous studies that show that the birmian crust is juvenile and little amount of archean crust is present in the sedimentary rocks the geochemical and nd isotopic systematics further suggest that the bulk of detritus was supplied by adjacent paleoproterozoic birimian greenstone belts with only 10 or less contribution from the archean crust trace element characteristics further suggest that deposition took place in an island arc setting

precise and accurate determination of 147sm 144nd and 143nd 144nd in monazite using laser ablation mc icpms abstract the 147sm 143nd radioactive decay system has been widely used in the earth sciences as a geochronometer and isotopic tracer recent studies have shown that laser ablation multiple collector inductively coupled plasma mass spectrometry la mc icpms is capable of precise and rapid 143nd 144nd measurements with high spatial resolution but its ability to measure 147sm 144nd accurately is still poorly constrained a critical analytical issue is the fractionation effects between 147sm and 144nd in la mc icpms which must be carefully corrected to determine robust initial 143nd 144nd particularly for ancient geological samples here we show that la mc icpms can produce substantial 5 147sm 144nd fractionation during analysis of monazite and that the fractionation can be corrected with uncertainty of 1 by using an independently defined 147sm 143nd isochron for an archean monazite standard in addition we describe a method for improving the precision and accuracy of la mc icpms isotopic measurements by calibration of faraday amplifier response and isotopic ratio normalization using a synthetic standard this method has been applied to one neogene and two archean monazites which were also analyzed by isotope dilution thermal ionization mass spectrometry id tims the la mc icpms data were acquired from 16 37 m single spots the la mc icpms determinations of the 145nd 144nd for all samples and 143nd 144nd for the neogene monazite compare well with the id tims results indicating the robustness of our method to measure nd isotopic ratios for the archean monazites the method yields initial 143nd 144nd values identical to the id tims values with reproducibilities 2 s d of 39 and 58 ppm analytical precisions of individual spot analyses for initial 143nd 144nd were calculated by taking into account the uncertainty in i the nd isotopic normalization to the synthetic monazite ii the correction factor for 147sm 144nd fractionation and iii the age which results in 2 of 45 87 ppm these results clearly demonstrate the capabilities of the la mc icpms technique applied to monazite sm nd isotope systematics for early crustal evolution studies

a 3 5 ga granite gneiss basement in guinea further evidence for early archean accretion within the west african craton abstract a granite gneiss formation guelemata orthogneiss was mapped on the northern and western slopes of the mount nimba hill range in guinea the original rocks were high al low yb medium to high k granites similar to most archean ttg analyses of u and pb isotopes in zircons using an ion microprobe shrimp gave ages of 3542 and 3535 ma respectively for the granite gneiss and a neighbouring granulitic metagabbro these are the oldest ages so far reported from the archean kenema man domain of west africa partial melting of an unfractionated basalt under eclogite facies conditions appears to be a suitable model for the origin of the granite gneiss

swash zone sediment transport step dynamics and morphological response on a gravel beach abstract state of the art morphodynamic field data were collected from slapton sands uk a fine gravel beach d50 2 10 mm over a 10 day period under a range of wave tide conditions comprising a mixture of southerly swell and easterly wind waves hs 0 5 1 m ts 4 8 s tide range 3 4 m over the field period the upper beach experienced accretion and berm formation during tides with low steepness swell dominated wave conditions h l 0 01 surprisingly the intertidal beach showed an opposite trend with sediment losses under swell conditions and an increase in sediment volume due to wind waves and suggests that upper beach morphological change can be decoupled from the overall sediment budget a submerged accretionary feature the beach step grew and migrated onshore on the rising tide typically reaching a maximum relief of around 0 5 m then gradually decreased in prominence while migrating offshore as the tide fell resulting in little net morphological change the step was better developed during the falling tide than the rising tide and through its control on the swash hydro and sediment dynamics the step had a significant effect on the development of the berm swash flow velocities under the conditions measured were often in excess of 2 m s 1 for both the uprush and the backwash onshore mean flows occurred in the upper swash region 40 inundation was characterised by offshore mean flows swash infiltration losses in the swash zone increased from 40 in the lower swash region to 100 in the upper swash region the most energetic swash flows and the strongest offshore mean flows occurred at the transition between swash zone and surf zone 90 100 inundation

a review of the mineral potential of liberia abstract the republic of liberia in west africa is underlain mostly by precambrian rocks of archaean liberian age in the west and of proterozoic eburnean age in the east by analogy with similar terranes elsewhere in the world and in west africa in particular the geology of liberia is favourable for the occurrence of deposits of a wide range of metals and industrial minerals including gold iron ore diamonds base metals bauxite manganese fluorspar kyanite and phosphate known gold deposits mostly orogenic in style occur widely and are commonly associated with north east trending regional shear zones gold mining commenced at the new liberty deposit in western liberia in 2015 while significant gold resources have also been identified at several other sites in both archaean and proterozoic terranes liberia has large resources of itabirite type iron ores most of which are located in the liberian terrane and was the largest producer in africa prior to the onset of civil war in 1989 production of iron ore is currently restricted to a single mine yekepa in the nimba range other important deposits some of them previously mined include bong the western cluster putu and goe fantro there is a long history of alluvial diamond production in western and central liberia together with more than 160 known occurrences of kimberlite most of the known kimberlites occur in three clusters of small pipes and abundant dykes located at kumgbor mano godua and weasua close to the border with sierra leone many of these are considered to be part of a single province that includes jurassic age diamondiferous kimberlites in sierra leone and guinea deposits and occurrences of a wide range of other metals and industrial minerals are also known several of these have been worked on a small scale in the past mainly by artisanal miners but most are poorly known in detail with sub surface information available at only a few localities by comparison with most other countries in west africa the geology of liberia is poorly known and there has been very little systematic exploration carried out for most commodities other than gold iron ore and diamonds since the 1960s and 1970s further detailed field and laboratory investigations using modern techniques are required to properly evaluate the potential for the occurrence of economic deposits of many minerals and metals in a variety of geological settings digital geological geochemical geophysical and mineral occurrence datasets including new national airborne geophysical survey data provide a sound basis for the identification of new exploration targets but in almost every part of the country there is a need for new and more detailed geological surveys to underpin mineral exploration

reassessment of proterozoic granitoid ages in ghana on the basis of u pb zircon and monazite dating abstract the west african craton in ghana is of early proterozoic age and forms alternating volcanic belts and sedimentary basins made up of rocks of the birimian supergroup as well as younger conglomerates and quartzites of the tarkwaian group and extensive granitoid plutons no consensus exists on absolute and relative ages of plutons within belts and basins u pb geochronology of four different granitoid complexes indicates that contrary to long held views belt granitoid plutons ashanti belt 2172 2 ma sefwi belt 2179 2 ma are 60 to 90 ma older than basin plutons kumasi basin 2116 1 ma belt granitoid are approximately coeval with volcanic rocks of the birimian supergroup while basin plutons are late kinematic and postdate tarkwaian sedimentation the first high precision geochronological data for rocks from ghana i e the eastern part of the birimian domain on the west african craton presented here show that magmatism within this region occurred over a time span of at least 90 ma and provide further evidence that the period between 2 1 and 2 2 ga was one of important magmatism in west africa

an improved u th pb age calculation for electron microprobe dating of monazite abstract determination of u th pb ages using the electron probe microanalyser epma is an inexpensive alternative method for dating monazite the method is rapid and reliable both for simple monogenenetic monazite and for complex polygenetic monazite having undergone metamorphic events involving fluid interaction and recrystallization the main limitation of the method is its rather poor precision i e 45 to 120 ma for ages ranging from 300 to 3000 ma calculated on each spot the precision is limited by pb content and by counting statistics which cannot give precision better than 2 for individual determinations on u th and pb even at high levels a procedure that uses the new th pb vs u pb diagram to improve the calculated precision on u th pb ages gives results to within 5 to 15 ma for ages ranging from 300 to 3000 ma with complex polygenetic monazite in which either the points show large scattering indicated by a large mswd or the regression line exhibits a slope very different from neighbour theoretical isochrons the procedure must be applied separately on homogeneous domains only this makes it possible to distinguish between events separated by a gap of 20 to 60 ma according to the range of ages concerned i e 300 to 3000 ma several examples are given to illustrate these systematics

mechanisms to account for the longshore spacing of overwash throats on a coarse clastic barrier in southeast ireland abstract the longshore spacing of overwash throats on two contrasting sections of a coarse clastic reflective barrier beach in southeast ireland is examined for evidence of spatial periodicity a fixed barrier section carnsore with an irregular barrier top dune field exhibits random longshore throat positions related to pre existing irregularities in the seaward dune boundary a free barrier section tacumshin without a barrier top dune field shows regular longshore throat positions with a peak throat to throat periodicity of between 30 40 m similar longshore periodicity is found in persistent high level beach cusps above mhws though not always spatially in phase there is no consistent near shore rhythmic morphology that can account for this spacing the storm event which generated the tacumshin overwash appears to have dominant longer incident wave periods than normal in celtic sea storms on weight of circumstantial evidence it is reasoned that this dominant wave period range 9 11 s can be related to a zero mode sub harmonic edge wave whose longshore wavelength matches the peak periodicity of overwash spacing overwash sites on this reflective barrier may be determined by an edge wave controlled extension of run up which generates high level cusps and thereby creates washover routeways the presence or absence of a barrier top dune field is a secondary control on whether washover is successful in reaching the back barrier usually only where surge penetration and pre existing dune irregularities coincide does washover occur

variability over time in the sources of south portuguese zone turbidites evidence of denudation of different crustal blocks during the assembly of pangaea this study combines geochemical and geochronological data in order to decipher the provenance of carboniferous turbidites from the south portuguese zone sw iberia major and trace elements of 25 samples of graywackes and mudstones from the mertola visean mira serpukhovian and brejeira moscovian formations were analyzed and 363 u pb ages were obtained on detrital zircons from five samples of graywackes from the mira and brejeira formations using la icpms the results indicate that turbiditic sedimentation during the carboniferous was marked by variability in the sources involving the denudation of different crustal blocks and a break in synorogenic volcanism the visean is characterized by the accumulation of immature turbidites mertola formation and the base of the mira formation inherited from a terrane with intermediate to mafic source rocks these source rocks were probably formed in relation to devonian magmatic arcs poorly influenced by sedimentary recycling as indicated by the almost total absence of pre devonian zircons typical of the gondwana and or laurussia basements the presence of carboniferous grains in visean turbidites indicates that volcanism was active at this time later serpukhovian to moscovian turbiditic sedimentation mira and brejeira formations included sedimentary detritus derived from felsic mature source rocks situated far from active magmatism the abundance of precambrian and paleozoic zircons reveals strong recycling of the gondwana and or laurussia basements a peri gondwanan provenance is indicated by zircon populations with neoproterozoic cadomian avalonian and pan african zircon forming events paleoproterozoic and archean ages the presence of late ordovician and silurian detrital zircons in brejeira turbidites which have no correspondence in the gondwana basement of sw iberia indicates laurussia as their most probable source

archaean crustal evolution in west africa a new synthesis of the archaean geology in sierra leone liberia guinea and ivory coast abstract a new synthesis of the geology and geochronology of the little known archaean rocks in sierra leone liberia guinea and ivory coast is presented in order to better understand the processes of archaean crustal evolution in this region and to attempt to interpret these data in the light of our current understanding of archaean crustal evolution in addition this study seeks to identify those aspects of archaean crustal evolution which are currently not known in this area and which need to become the subject of future studies given the economic importance of this region in terms of the mineral deposits hosted in the archaean rocks these include greenstone belt hosted iron ore lode gold chromite and columbite tantalite and younger diamondiferous kimberlites intrusive into archaean felsic gneisses the new results show that this cratonic nucleus comprises of four main geological units 1 the oldest crust is made up of 3 5 3 6 ga ttg tonalite trondhjemite granodiorite gneisses these only outcrop in the east of the craton in guinea but their presence is indicated elsewhere in the central part of the craton though xenocrystic zircon cores in younger rocks 2 the major rock type found throughout the craton is 3 26 2 85 ga ttg gneiss in detail these magmas are thought to have formed in two episodes one between 3 05 3 26 ga and the other between 2 85 2 96 ga the presence of inherited zircons in the younger suite indicate that this event represents the partial reworking of the older gneisses 3 4 ga eclogite xenoliths in kimberlite derived from the sub continental lithospheric mantle are thought to be the restite after the partial melting of a basaltic protolith in the production of the ttg magmas 3 supracrustal rocks form linear belts infolded into the ttg gneisses and metamorphosed to amphibolite and granulite grade they are of different sizes contain a variety of lithological sequences and may be of several different ages the larger supracrustal belts in sierra leone contain a thick basalt komatiite sequence derived by the partial melting of two different mantle sources unconformably overlain by a sedimentary formation they are seen as an important resource for gold iron ore chromite and columbite tantalite 4 a suite of late archaean granitoids formed by the partial melting of the ttg gneisses in a craton wide deformation metamorphic partial melting event at 2800 20 ma this thermal event is thought to be responsible for the stabilisation of the craton this new synthesis highlights major geological and geochronological similarities between the archaean rocks of sierra leone liberia guinea and ivory coast and those in the reguibat shield in the northern part of the west african craton suggesting that the two regions were once more closely related

single mineral pbpb dating a new stepwise leaching procedure applied to ca bearing silicates enables the selective recovery of radiogenic and common pb from a mineral and thus makes single phase pbpb dating possible this permits independent dating of rock forming minerals within a paragenetic sequence and the direct combination of age data with petrological results furthermore the stepwise leaching technique has the advantage over multi phase pbpb dating in that it does not require initial pb isotope equilibrium among the phases but may be used to test it the presence of microscopic solid inclusions which complicate dating of single phases if they are not in equilibrium with the host can be traced via the behaviour of the 208pb 204pb ratio through the various leaching steps this constitutes a major advantage over conventional bulk phase pbpb dating in addition the precision of an isochron age can be improved relative to that of the bulk mineral approach since the 207pb 204pb and 206pb 204pb data spread is significantly enhanced therefore the technique may both be used to date younger phases and or minerals with less favourable u pb ratios and to extend the applicability of self consistent p t t modelling

episodic collisional orogenesis and lower crust exhumation during the palaeoproterozoic eburnean orogeny evidence from the sefwi greenstone belt west african craton abstract new constraints on the architecture deformation sequence and tectonic evolution of the sparsely exposed palaeoproterozoic sefwi greenstone belt ca 2200 2070 ma in sw ghana are derived from the integration of field observations structural mapping and interpretation of regional airborne geophysical data our findings provide new insights into the tectonic evolution of the west african craton during the palaeoproterozoic eburnean orogeny ca 2150 2070 ma domains of contrasting metamorphic grade comprising high grade paragneisses and meta volcanic rocks are tectonically juxtaposed against low grade volcano sedimentary packages the earliest deformation event is characterised by a bedding parallel high grade metamorphic foliation s1 that is axial planar to isoclinal folds formed during nnw sse shortening and late d1 partial melting this crustal thickening event generated biotite muscovite granites emplaced parallel to the nw margin of the sefwi belt between ca 2092 and 2081 ma providing a minimum age for d1 d2 deformation is characterised by a moderate to steeply dipping ene wsw to ne sw striking penetrative s2 mineral foliation within the sefwi belt coaxial f2 fold axes and l2 stretching lineations pitch shallowly ene wsw to ne sw oblique to the major shear zones indicative of ene wsw transtension and associated constrictional deformation differential exhumation of middle and lower crustal segments along nne striking d2 extensional detachments was coincident with sinistral oblique reactivation of ne sw regional scale shear zones the timing of exhumation is constrained by metamorphic monazite u pb ages at ca 2073 ma subsequent e w shortening is associated with dextral reactivation of regional ne sw striking shear zones and refolding of earlier structures away from the major shear zones we propose that the nw margin of the sefwi belt represents the collision d1 of southern ghana and central and nw ghana ivory coast which record different tectonic histories furthermore we suggest the amalgamation of the west african craton is the product of episodic collisional orogenesis providing new insights into the evolution of orogenic processes during the palaeoproterozoic

concepts in gravel beach dynamics abstract the dominant processes in gravel beach dynamics are reviewed highlighting some common themes which unify the various components of the gravel beach system the repercussions of which impart on how gravel beach dynamics might be understood conceptually in particular gravel beach dynamics are thought to be highly dependent on the temporal and spatial variation in grain size and the continual adjustments made by an active beach step both of which act not only as the expression of changing morphodynamic conditions but also as a controlling influence morphodynamics the notion that the exchanges on beaches between the hydrodynamics sediment transport and morphological change takes the form of reciprocal relationships which are mediated through feedback mechanisms in such a way that they cannot be thought of or studied independently is not a new one yet it appears that for the gravel beach morphodynamics must be re defined to describe conditions where variations in sediment size are thought to deserve parity rather than as merely a sequent entity or boundary condition morpho sedimentary dynamics is a phrase coined to intuit such cause and effect detailing the co evolution of morphology hydro hydraulics and sediment properties whilst acknowledging causative pluralism feedbacks and multiplier effects this is the recommended conceptual framework within which to crystallise thought and organise further research for the gravel beach essentially it increases the minimum number of parameters needed to describe the state of the gravel beach as a physical system therefore it is advised that simplicity will be most expedient in our future modelling efforts if complexity is to be adequately encapsulated

measured and modeled wave overtopping on a natural beach abstract the rate of wave overtopping of a barrier beach is measured and modeled unique rate of wave overtopping field data are obtained from the measure of the carmel river california lagoon filling during a time when the lagoon is closed off with no river inflow volume changes are based on measured lagoon height changes applied to a measured hypsometric curve wave heights and periods are obtained from directional wave spectra data in 15 m fronting the beach beach morphology was measured by gps walking surveys three empirical overtopping models by van der meer and janssen 1995 hedges and reis 1998 and pullen et al 2007 with differing parameterizations on wave height period and beach slope and calibrated using extensive laboratory data obtained over plane impermeable beaches are applied in a quasi 2d manner and compared with the field observations three overtopping events are considered when morphology data were available less than 2 weeks prior to the event the models are tuned to fit the data using a reduction factor to account for beach permeability berm characteristics non normal wave incidence and surface roughness influence in addition the run up model by stockdon et al 2006 based on field data is examined and found to underestimate run up as the calculated values were too small to predict any of the observed overtopping the three overtopping models performed similarly well with values of 0 72 0 87 for the two narrow banded wave cases with an average reduction factor of 0 78 the european model pullen et al 2007 performed best overall and in particular for the case of the broad banded double peaked wave spectrum

age of younger tonalitic magmatism and granulitic metamorphism in the south indian transition zone krishnagiri area comparison with older peninsular gneisses from the gorur hassan area abstract a major episode of continental crust formation associated with granulite facies metamorphism occurred at 2 55 2 51 ga and was related to accretional processes of juvenile crust dating of tonalitic trondhjemitic granitic gneisses and charnockites from the krishnagiri area of south india indicates that magmatic protoliths are 2550 2530 5 ma as shown by both u pb and 207pb 206pb single zircon methods monazite ages indicate high temperatures of cooling corresponding to conditions close to granulite facies metamorphism at 2510 10 ma these data provide precise time constraints and sr nd isotopes confirm the existence of late tonalitic granodioritic juvenile gneisses at 2550 ma pb single zircon ages from the older peninsular gneisses gorur hassan area are in agreement with some previous sr ages and range between 3200 20 and 3328 10 ma these gneisses were derived from a 3 3 3 5 ga mantle source as indicated from nd isotopes they did not participate significantly in the genesis of the 2 55 ga juvenile magmas all these data together with previous work suggest that the 2 51 ga granulite facies metamorphism occurred near the contact of the ancient peninsular gneisses and the 2 55 2 52 ga juvenile tonalitic trondhjemitic terranes during synaccretional processes subduction mantle plume rb sr biotite ages between 2060 and 2340 ma indicate late cooling probably related to the dextral major east west shearing which displaced the 2 5 ga juvenile terranes toward the west

constraints from fluid inclusion compositions on the origin of mississippi valley type mineralization in the illinois kentucky district fluid inclusions hosted by mississippi valley type mvt ores in the illinois kentucky district were analyzed using microthermometry laser ablation inductively coupled plasma mass spectrometry la icp ms and raman spectroscopy to characterize the composition and temperature of mineralizing fluids in order to seek insights into the process of ore formation results from the present study support the interpretation of previous microthermometry studies that relate mineralization to mixing of two higher temperature brines and a cooler dilute meteoric fluid fluid inclusions in the illinois kentucky district have overall higher atomic ratios of ba na ca mg k na sr na and lower atomic ratios of mg na compared to inclusions in the ozark mvt districts and are thus compositionally distinct these differences may indicate that the illinois kentucky and ozark mvt ore fluids were derived largely from different source basins or may reflect input from igneous rocks in the illinois kentucky district low ca mg ratios of illinois kentucky fluid inclusions indicate that mineralizing fluids were dolomitizing and therefore could not have traveled far through the limestone ore host formations but instead traveled through deeper nonlimestone units before rising along faults and fractures in the district some fluid inclusions in illinois kentucky mvt mineralization yielded strong la icp ms responses for ore metals a single sphalerite hosted fluid inclusion out of 18 analyzed contained detectable pb corresponding to a concentration of 420 ppm whereas numerous fluorite hosted fluid inclusions contained detectable pb zn and cu corresponding to maximum concentrations on the order of 1 000s of ppm the high and similar concentrations of cu pb and zn in some fluorite hosted fluid inclusions in the illinois kentucky district are problematic because cu sulfide mineralization should be much more abundant than pb and zn sulfide mineralization owing to the lower solubility of cu relative to pb and zn in the presence of reduced sulfur which is not observed in the field this raises the possibility that ore metals may exist as solid phases in suspension or adhering to fluid inclusion walls in some inclusions rather than as dissolved species in aqueous solution raman microprobe analyses of fluid inclusions hosted by minerals occurring throughout the paragenetic sequence detected methane concentrations ranging from about 40 to 1 200 ppm with most on the order of 100s ppm even for the lowest methane concentration determined redox conditions would have been reducing at a log of 55 4 at a temperature of 100 c at such reducing conditions sulfide would predominate over sulfate this is supported by high concentrations of ba in the fluid inclusions the overall reducing conditions throughout the paragenetic sequence and the possible presence of both metal poor and rich fluid inclusions would suggest that mixing between a metal rich sulfide poor fluid and a metal poor sulfide rich fluid were an important mechanism for sulfide mineral precipitation minimum depths of mineralization estimated from methane concentrations in fluid inclusions range to about 1 500 m and possibly 2 500 m which would not have been reached until late pennsylvanian to permian time i e during the alleghanian orogeny

compilation of west african mineral deposits spatial distribution and mineral endowment abstract the west african craton is highly endowed in minerals and their spatial and temporal distribution varies from single to multi phase mineralization events they are broadly related to three major tectono metallogenic elements and formed during distinct mineral epochs 1 in both archean shields kenema man and reguibat and paleoproterozoic domains baoule mossi eglab these are characterized by giant iron ore deposits that formed between ca 2 5 2 3 ga nearly all gold porphyry copper lead zinc and sedimentary manganese ore that developed between 2 2 and 2 1 ga and primary diamonds that formed between two intervals at ca 2 2 2 0 ga and in the mesozoic 2 across pan african and variscan belts these are distinguished by major precambrian iocg s copper gold that formed at ca 2 1 ga and approximately 680 ma and neoproterozoic sedimentary iron ore and phosphate deposits 3 within intracratonic and coastal basins these include the development of cenozoic lateritic bauxites over mesozoic dolerites tertiary quaternary mineral sands deposits oolitic iron ore and sedimentary phosphate deposits geological spatial and temporal correlations using the multi commodity west african mineral deposit database highlight that gold and non gold commodities formed in multiple phases this commenced in the liberian orogeny 2 9 2 8 ga with the enrichment of iron ore nickel sulphides diamonds and gold in the earth s crust the pre eburnean or tangaean eoeburnean eburnean i event yielded gold and the major eburnean orogeny yielded gold iron ore manganese diamonds magmatic nickel sulphides copper gold lead zinc and ree minerals throughout the pan african event sedimentary manganese deposits lead zinc ree minerals sedimentary phosphates and again gold were formed primary diamonds and magmatic nickel sulphides are related to the break up of gondwana followed by an intense lateritic weathering period that formed bauxite deposits along the craton margin

geochronological evidence for reworking of archean terrains during the early proterozoic 2 1 ga in the western coˆte d ivoire man rise west african craton abstract a period of accretion of continental crust is recognized during the archean at ca 3 2 3 3 ga on the basis of wholerock nd model ages and pb ages on detrital zircons from various gneisses of the man area in co te d ivoire there is no significant evidence of any older event such as the 3 5 3 6 ga accretion episode recognized in the archean of the reguibat rise in mauritania the earliest magmatic events recorded in the northern domain of the man area correspond to tonalitic grey gneisses of a minimum age ca 3 05 ga leonian event a subsequent major phase of igneous and metamorphic activity around 2 8 ga liberian event is recorded in the entire archean terrain without any evidence of new accretion this second event began at 2830 7 ma its climax is indicated by charnockitic intrusions followed by granodioritic magmas at ca 2800 10 pb pb and u pb ages on monazite and zircon cooling after the liberian event occurred ca 2 75 ga ago k feldspar whole rock garnet sm nd ages paleoproterozoic reworking is mainly found in the southern part of the archean terrain of the man area zircon monazite sphene and garnet geochronometers all indicate an important thermal effect during the birimian event i e 2 1 ga the earliest influence of this event is recorded in a range of 2250 40 70 ma with a major event ca 2 1 ga partial to total resetting of u pb and sm nd geochronometers indicate that high temperature conditions were reached during the birimian event this metamorphic overprint and the diapiric emplacement of coeval paleoproterozoic juvenile magmas at 2 1 ga in the southern domain toule pleu massif suggest that the 2 1 ga overprint could be related to major underplating processes under the archean basement the thermal flow related to the differentiation and the accretion of these juvenile birimian magmas may account for the reworking of the surrounding archean domain which had been possibly thinned during underplating these results suggest close proximity between the archean continents and an accretion zone associated with juvenile magmatism 2 1 ga ago

geochemistry of upper birimian sediments major and trace elements and nd sr isotopes and implications for weathering and tectonic setting of the late paleoproterozoic crust abstract the upper birimian paleoproterozoic formations consist mainly of a detrital sedimentary pile intercalated with calc alkaline volcanites that were accreted during the eburnean orogeny 2 1 ga the sediments of the upper birimian group were analyzed for their major and trace elements concentrations and their nd sr isotopic compositions the shales are enriched in al 2 o 3 and k 2 o relative to paas and depleted in the other mobile major elements and lile whereas the fe shales show enrichments in cao and mno and depletions in al 2 o 3 and k 2 o relative to paas but have similar depletion in lile together with the subarkose samples the shales have undergone significant post depositional k metasomatism as indicated in the a cn k diagram the calculated pre metasomatism cia values 83 92 indicates a high degree of source weathering compatible with steady state weathering under tropical to subtropical conditions the fe shales have lower cia values 46 57 reflecting less intense weathering conditions low eu anomalies and th sc ratios combined with high cr th ratios for certain samples indicate a significant input of basic detritus for the analyzed sediments provenance modeling indicates that the shales are best modeled with a mixture having 75 basalt 75b and 25 granite 25g the fe shales have a similar provenance with their compositions being modeled with a 80b 20g mixture the nd values of selected shales calculated at 2 1 ga the age of the deposition vary from 2 8 to 8 6 which indicate that the sediment received 60 100 of its detritus from a juvenile birimian volcanic arc therefore the birimian sediments deposited within an active continental margin setting mostly supplied by the adjacent volcanic arc

extreme events and the morphodynamics of gravel dominated coastal barriers strengthening uncertain ground abstract the long term morphodynamic ordering of gravel dominated coastal systems gdcs many of which serve as coastal defences in northwest europe is dominated by extreme events that generate barrier crest overflow an understanding of this morphodynamic ordering is fraught with several unresolved difficulties these are related to the twin problems of the inadequacy of pertinent morphodynamic parameterisation and of obtaining data from modern shores enabling such parameterisation major uncertainties concern the timing of over crest flow in terms of return period of extreme elevation the intensity and structure of the overflow field antecedent beachface characteristics in response to storms the rate of relative sea level change tidal stage control and barrier resistance to forcing itself determined by a number of unknowns including barrier form and size sediment size and mosaics and barrier resilience while generalised extreme value modelling may provide a means of characterising overwashing return period and its variability exceptional tsunami events are outside the scope of such modelling the characterisation of gdcs morphodynamics in terms of the forcing extreme events will necessitate integrating some or all of these parameters into a single model

chemical and sr nd isotopic compositions and zircon u pb ages of the birimian granitoids from ne burkina faso west african craton implications on the geodynamic setting and crustal evolution abstract the west african craton was the site of massive juvenile crustal addition in the early proterozoic during the eburnean orogeny 2 1 ga in this study we determined chemical and isotopic compositions as well as zircon u pb ages of granitoid samples from ne burkina faso these data were then used to constrain the chemical characteristics emplacement age and tectonic environment of these rocks in the context of the west african craton evolution birimian paleoproterozoic granitoids of ne burkina faso are associated with volcanic and volcano sedimentary sequences of the greenstone belts analyses indicate that the na rich birimian granitoids of tonalite trondhjemite granodiorite ttg composition were generated by partial melting of a garnet bearing amphibolite crust granite intrusions derived from partial re melting of the deep basement of felsic calc alkaline nature with contribution of metasediments zircon u pb isotopic analyses yielded ages between 2122 15 ma and 2181 7 ma for the birimian na rich granitoids and 2151 10 ma for a biotite granite ij10 sr nd isotopic data show very low initial sr composition i sr t 0 7007 positive epsilon neodymium values nd t 0 7 to 2 5 and sm nd model ages t dm of 2363 2558 ma these data are similar to those of mafic rocks of the pissila greenstone belt i sr t 0 7003 0 7015 nd t 3 2 to 4 1 t dm 2336 2552 ma and further suggest that the na rich granitoids are of juvenile nature the present study together with published data on the birimian volcanics and meta sedimentary rocks of the west african craton preclude contamination of the juvenile birimian rocks of ne burkina faso by reworked archean materials the birimian granitoids of ne burkina faso were mostly emplaced during the early stage 2 19 2 15 ga of a large scale crustal growth event 2 2 2 0 ga where large amount of juvenile materials were added to the crust from the depleted mantle in a tectonic environment comparable to modern volcanic arcs

a weathered skarn type mineralization in ivory coast the ity gold deposit abstract although the ity deposit is the oldest operating mine in ivory coast very few studies exist on this deposit it is situated in western ivory coast some 700 km northwest of abidjan the mine is presently operated and mined by smi societe des mines d ity society of ity mines and has produced one million ounces of gold since its opening in 1991 all mined ore bodies are hosted in laterite and clay formations the gold deposits in the ity area form a cluster of mining operations of various sizes flotouo zia mont ity hosted in a birimian volcano sedimentary belt the mineralization is mostly located at the contact between a granodiorite tonalite intrusive body and sediments primary gold mineralization occurs in the form of veinlets and mostly skarn at the contact between the granitic intrusion and limestone calc silicate minerals such as diopside and andradite dominate the skarn however the economic gold mineralization at ity is associated with the saprolitic zone 50 120 m thick and consists of several mineralized lenses of clayey and ferruginous material terres noires and of sedimentary breccia layers a thick laterite horizon a minimum of 10 m overlies the saprolite and covers the land throughout the region it is mineralized in the deposit area this contribution provides a summary of the geological and mineralogical relationships at the ity deposit presents new data on skarn type gold mineralization and summarizes its history from the first findings through exploration and mining assessment

geochronology of polygenetic monazites constrained by in situ electron microprobe th u total lead determination implications for lead behaviour in monazite abstract monazite grains are generally concordant and with an electron microprobe give th u total pb isochron ages in agreement with conventional u pb data monazites from four samples of post archean migmatite from the ivory coast record two main ages an archean age at 2 80 ga partly overprinted by a late migmatization event at 2 03 ga the in situ electron microprobe determination has also shown a third event that systematically appears at 2 72 ga for all the monazites from two of the four studied rocks the three events at 2 80 2 72 and 2 03 ga recorded by the th u total pb system in monazite were each obtained on independent homogeneous parts of a single grain with no sign of pb diffusion thus the oldest monazite remained closed during a high grade resetting migmatization at a temperature estimated at around 700 c and also remained closed during the growth of secondary monazite conventional u pb data isotope dilution on monazite from the same rocks did not allow precise age determinations because the representative points were highly up to 83 discordant and did not fit well along a chord in the concordia diagram the u pb isotope dilution method gave a lower intercept age of 2029 25 ma in agreement with the electron microprobe age although with an unusually high uncertainty due to a poor fit of the data the pb evaporation method gave an old archean age at 2830 7 ma and proterozoic ages ranging from 2417 10 to 2074 7 ma thus although the oldest archean age obtained with the pb evaporation and electron microprobe methods is similar within the analytical error the proterozoic ages obtained by the pb evaporation method are significantly older than the th u total pb proterozoic age obtained with the electron microprobe this discrepancy is presumed to be due to the influence in the pb evaporation method of radiogenic lead contained within archean zones of the monazite grains in other words it means that migmatization took place later than 2074 7 ma and probably around 2030 ma the second archean event at 2 72 ga revealed by high spatial resolution of the electron microprobe could not be determined by other methods the th u total pb method appears to be an inexpensive alternative method for dating simple monazites with a precision close to 20 ma even if the u pb isotopic dilution technique remains the reference method moreover owing to the very high spatial resolution 1 m of the electron microprobe it appears to have no competitor for dating complex polygenetic monazites especially when more than two events are recorded homogeneous zones of different ages coexist in a single grain with no major indication of lead diffusion a procedure for age and error calculations is defined

whole grain evaporation for 207 pb 206 pb age investigations on single zircons using a double filament thermal ion source a technique has been developed and tested to analyse 207pb 206pb apparent ages by thermal evaporation of radiogenic lead directly from untreated whole zircon grains 0 3 mm the evaporation analyses are performed in the double filament arrangement of a thermal ion mass spectrometer thims the method is a powerful tool to distinguish between different lead components occurring in the same grain because differing activation energies of the competing lead components cause their sequential evaporation from the zircons the evaporation of test samples results in 207pb 206pb apparent ages in good agreement with u pb ages known from literature single zircons from a granite of the marble mountains california yield an age of crystallization of 1 410 30 ma ceylon zircons from heavy mineral bearing gravels yield 560 40 ma as age of crystallization of the pegmatitic gravel sources individuals from a heterogeneous zircon population of a diatexite from the southern schwarzwald sw germany indicate metamorphic zircon formation around 500 ma and the existence of middle proterozoic relics 1 95 0 05 ga

lithostratigraphic evolution of the bandamian volcanic cycle in central cote d ivoire insights into the late eburnean magmatic resurgence and its geodynamic implications late to post collisional volcanism in the southern portion of the west african craton swac has been described along the paleo craton margin between the paleoproterozoic baoule mossi domain and archean kenema man nucleus new lithostratigraphic and isotopic data for central cote d ivoire as well as revised regional stratigraphic correlations across the western swac indicate that this volcanic resurgence is not only restricted to the paleo craton margin but extends to most greenstone belts of the western half of the swac this late volcanic event historically referred to as the ca 2117 2080 ma bandamian volcanic cycle has received little attention since its identification in 1996 detailed geological investigation of the bouafle granite greenstone belt in cote d ivoire located 150 km east of paleo craton margin along with newly acquired u pb geochronological data indicate a peak in igneous activity showing evidence for bimodal magmatism affiliated to the bandamian volcanic cycle in central cote d ivoire between 2108 5 ma and 2105 4 ma whole rock geochemistry and lu hf isotopic measurements on magmatic zircons from the bouafle region show that these bandamian rocks are marked by a juvenile signature with little degree of crustal contamination collectively the data collected in this study show that the bandamian volcanic cycle represents an important thermal event that affected the swac during the late stages of the eburnean orogeny spanning the incremental change from folding and thrusting to dominantly transcurrent tectonics along major faults and shear zones this paper discusses the possible geodynamic settings that may account for such a peak in thermal activity during the late stages of the eburnean orogeny

two phases of granulite facies metamorphism during the neoarchean and paleoproterozoic in the east hebei north china craton records from mafic granulites abstract archean cratons are commonly reworked during the late paleoproterozoic orogeny which may result in multi phase granulite facies metamorphism in many domains the east hebei terrane of the north china craton is developed with two phases of granulite facies metamorphism in the neoarchean and paleoproterozoic the neoarchean granulite facies metamorphism is recognized mainly in the rafts or slivers of supracrustal rocks from the taipingzhai ovate structural domain characteristic of anticlockwise p t paths the mafic granulites at this domain contain two pyroxene granulite assemblages which are commonly overprinted by a late phase of granulite facies marked by tiny grained vermicular orthopyroxene bearing assemblages they only record the neoarchean metamorphic ages at 2 53 2 52 ga and 2 51 2 48 ga as indicated by zircon u pb data para simultaneously with the emplacement of ttg rocks in contrast the paleoproterozoic granulite facies metamorphism can be well recognized in the saheqiao linear structural belt which is characterized by high pressure granulite assemblages in mafic rocks with clockwise p t paths in this belt the archean two pyroxene granulite assemblages are locally preserved showing red eye socket overprinting textures consisting of garnet coronas or fine grained garnet and clinopyroxene bearing assemblages surrounding the early stage pyroxenes zircon u pb data indicate that mafic granulites from the saheqiao belt record not only the neoarchean metamorphic ages of 2 50 ga but also some paleoproterozoic ages of 1 97 1 83 ga the first application of lu hf garnet whole rock isochron method to garnet bearing mafic granulites from the saheqiao belt yields ages of 1766 6 7 0 and 1787 22 ma confirming that the overprinting of garnet bearing assemblages occurred in the late paleoproterozoic thus the saheqiao linear structural belt may represent a strongly reworked domain during the paleoproterozoic orogeny rather than an archean high grade greenstone belt as previously considered

petrogenesis of the 2090 ma zanhuang ring and sill complexes in north china a bimodal magmatism related to intra continental process abstract it is widely debated as to whether the 2100 ma bimodal magmatic events in the eastern north china craton represent intra continental rifting or marginal processes the genetic relationship between the two components of bimodal suites is the key solution to this issue and this is investigated on a group of newly identified ring complexes as well as the apparently associated sills in zanhuang southern taihang mts these ring complexes comprise varied components pyroxenite gabbro anorthosite k rich a type granite and minor na rich a type granite u pb analyses on baddeleyite and zircon grains liberated from a gabbro sample both yield a 2090 ma age which represents crystallization timing and is contemporaneous with a type granites of the ring complexes as well as the sills in the gantaohe group the mafic rocks of the ring complexes and sills show similar chemical features they were characterized by enriched light rare earth elements compared to middle and heavy ones la ybn 1 4 2 8 enriched large lithosphile elements but depleted high field strength elements and enriched sr nd isotopes endt t 2090 ma 1 9 0 6 nd tdm 2 7 ga they were most likely originated from the same magma source resided in the sub continental lithospheric mantle which formed in the late archean the compositional variation of the mafic sills may be resulted from the fractionation of olivine and plagioclase the k rich a type granite as well as the dacitic sills could be originated from this same source with firstly olivine plagioclase dominated fractionation in the lower magma chamber to more felsic compositions and then with significant removal of clinopyroxene plagioclase in the upper magma chamber which at the same time produced pyroxenite gabbro and minor anorthosite cumulates in the ring complexes na rich a type granite was likely formed locally under relatively high water fugacity with fractionation of k feldspar plagioclase clinopyroxene from a parental magma similar to the k rich a type granite this bimodal zanhuang ring and sill complexes and other comparable bimodal suites widely distributed in the eastern north china craton were likely intra continental representing a craton scale extension at 2100 ma

conventional u pb dating versus single grain pb evaporation dating of complex zircons from a pegmatite in the high grade gneisses of southwestern sweden u pb multi grain dating of zircons from a pegmatite dyke in southwestern sweden yield analyses which define a discordia with upper and lower intercept ages of 1510 32 ma and 631 64 ma respectively dating by the single zircon evaporation method on prismatic zircons from the same dyke indicates a primary crystallization age of 1409 20 ma in addition four oval zircon grains yield a 207pb206pb age of 969 27 ma the older single zircon evaporation age is interpreted as representative of pegmatite emplacement whereas the younger age is suggested to correspond to isotopic disturbance and new zircon growth during a late sveconorwegian metamorphic event to explain the false intercept ages in the u pb concordia diagram this study suggests a three stage model including 1 crystallization of zircons during pegmatite intrusion 2 episodic lead loss and new zircon growth during high grade metamorphism and 3 a recent lead loss event as the dykes are strongly deformed the protolith age of the pegmatite provides an upper age limit for the gneiss forming event the single zircon age of 969 27 ma is interpreted to be associated with this event which took place during high grade metamorphic conditions

metallogenic portfolio of the west africa craton abstract the west african craton hosts major resources of gold iron ore aluminium ore diamonds phosphates and manganese this portfolio of ore deposits is linked to the formation of archean paleoproterozoic greenstone belts jurassic rifting and extended periods of mesozoic to cenozoic weathering and erosion we give a brief overview of the temporal and spatial distribution patterns of west african ore deposits with emphasis on the main commodity types the oldest ore forming processes generated major resources in iron ore and gold in the kenema man and reguibat shields during the neo archean the majority of gold porphyry copper lead zinc and sedimentary manganese deposits formed during the paleoproterozoic dominantly within the baoule mossi domain at the same time diamond bearing kimberlites developed in ghana another distinct diamond event has been recognized in the mesozoic of the kenema man shield isolated occurrences of iocg s as well as copper gold and gold formed in pan african variscan belts during the neoproterozoic the majority of mineralization consists of sedimentary iron ore and phosphate deposits located within intracratonic basins during the phanerozoic aluminium ore phosphates and mineral sands concentrated along the margins of the coastal and intracratonic basins

late eburnean granitization and tectonics along the western and northwestern margin of the archean kenema man domain guinea west african craton abstract recent mapping of the eastern part of guinea has revealed a vast plutonic belt that rims the margin of the archean kenema man craton and that is made up of a variety of granitic rocks granodiorite biotite granite monzogranite two mica granite granodiorite with common clinopyroxene being the most abundant single zircon dating by stepwise pb evaporation carried out on characteristic rock types granodiorite monzogranite two mica granite reveals their age to be late eburnean c 2090 2070 ma and indicates that their emplacement occurred within a relatively short time interval 20 ma the granodiorite biotite granite and monzogranite of the plutonic belt all present a calc alkaline and highly potassic nature and show numerous analogies with the neogene calc alkaline rocks of the central andes there is evidence to suggest strong contamination from the archean crust these analogies make it possible to envisage the emplacement of the plutonic belt directly above a subduction zone the occurrence of peraluminous two mica granite at the northern extension of the plutonic belt in the siguiri basin may suggest that the convergence here was associated with the local melting of metasedimentary rocks in the deep part of the basin contemporaneously with its emplacement the late eburnean plutonic belt accommodated regional convergence by major wnw ese sinistral movements along the northwestern margin of the archean block horizontal shortening towards the wsw is envisaged at the scale of eastern guinea in relation with these kinematics the late eburnean plutonic belt of eastern guinea is a key element of the archean proterozoic transition zone in the southern part of west africa evidence along the western and northwestern border of the archean kenema man domain leads us to interpret this transition zone as an active margin along which strike slip tectonism accommodated regional convergence towards the end of the eburnean cycle