geodynamic evolution of the mauritanide bassaride and rokelide orogens west africa abstract the mauritanide bassaride and rokelide orogens occur along the western edge of the west african craton these record a polyphase tectonothermal evolution including pan african i c 650 ma and pan african ii c 550 ma events together with local hercynian late paleozoic overprinting pan african i activity is most penetratively recorded in the bassarides and resulted from late proterozoic collision of a western continental structural block pan african ii orogenesis increases in intensity from the southern mauritanides through the bassarides and dominates the rokelides this tectonothermal activity appears to reflect collision of the west african and guyanean cratons during assembly of gondwana hercynian activity is concentrated along the margin of a western continental block which underwent relative eastward translation during collision of gondwana and laurentia this resulted in extensive thrusting of intracontinental foreland sequences external nappes and more ductile imbrications of pre deformed and metamorphosed late proterozoic rift sequences and western calc alkaline igneous successions internal nappes

the skaergaard layered series part ii magmatic flow and dynamic layering re examination of textural and structural features of the skaergaard intrusion has clarified the process responsible for various forms of layering two general types are distinguished one produced by magmatic flow and another by non dynamic processes such as compaction and varied nucleation and growth of crystals our concern here is mainly with layering of the first type magmatic flow results in contrasting layers and strong foliation accompanied by a linear orientation of elongated minerals it is associated with slumping and cross bedding in a zone near the margins of the intrusion rocks in the interior of the layered series are foliated and commonly layered but have little or no lineation their layering cannot be due solely to deposition from currents the principal structural elements produced by dynamic processes in the magmatic state foliation lineation shear zones folds and current structures define parallel belts mainly around the western and northern margins their distribution suggests that the strongest component of flow was for the western side and that its strength decreased toward the east shear flow trajectories deduced from lineations and kinematic indicators confirm this asymmetry and point to mass movement toward the southeastern part of the floor

proto avalonia a 1 2 1 0 ga tectonothermal event and constraints for the evolution of rodinia the neoproterozoic evolution of avalonia is thought to have been geodynamically linked to the amalgamation and dispersal of rodinia similar sm nd isotopic signatures for different periods of arc activity suggest that avalonian basement or proto avalonia was generated in a series of primitive oceanic island arcs between 1 2 and 1 0 ga because this interval coincides with the amalgamation of rodinia proto avalonia is inferred to have been located in a panthalassa like peri rodinian ocean an early 760 660 ma phase of avalonian arc activity is attributed to renewed subduction in the peri rodinian ocean following the breakup of rodinia which caused the accretion of avalonian terranes to the gondwanan margin by ca 650 ma further subduction along the margin occurred outboard of these terranes and resulted in the onset of main phase avalonian volcanism at 630 ma the diachronous cessation of arc magmatism is attributed to ridge trench collision and the generation of a continental transform the geodynamic link between avalonia and rodinia is analogous to that between the mesozoic dispersal of pangea and the tectonothermal evolution of western north america this event also resulted in the accretion of outboard terranes and in arc related magmatism that is currently being terminated in a diachronous manner by ridge collision and the generation of the san andreas transform the model implies that the neoproterozoic evolution of avalonia and other peri gondwanan terranes provide important constraints on the tectonic history of a large portion of the rodinian continental margin

history of the west african neoproterozoic ocean key to the geotectonic history of circum atlantic peri gondwana adrar souttouf massif moroccan sahara abstract bordered by the archaean basement of the reguibat shield to the e and s the adrar souttouf massif is located in the southern regions of the moroccan sahara and represents the northern part of the mauritanide belt the central areas of this massif comprise the dayet lawda and sebkha matallah units that are mainly composed of mafic rocks ten samples taken from these rocks yielded 531 zircon grains that were analysed with respect to their morphology u th pb and lu hf isotope composition additionally 155 apatite grains from six samples were also dated by the u th pb method mostly well rounded zircon grains commonly showed bright cathodoluminescent cl overgrowth domains and leaching zones indicating metamorphic overprint around unaffected oscillatory zoned core areas all samples but one yielded two significant zircon age populations 605 ma and 634 ma with some inherited grains in the range of 650 740 ma and very scarcely up to 1190 ma for six samples the hf isotope composition suggests a major contribution of neoproterozoic juvenile magmas from depleted mantle source the zircon hf data for the remaining samples points to a predominant recycling of older archaean to palaeoproterozoic crust based on the geochemical composition of the rocks an island arc setting is assumed at the periphery of the west african craton close to the cryogenian ediacaran boundary subsequent metamorphism during accretion and partial obduction onto the basement rocks took place at about 605 ma a minor variscan overprint could be demonstrated for only one sample this case study exemplifies the great potential of the widely occurring metamorphosed mafic and ultramafic rocks along the western margin of the west african craton for palaeogeographic and geodynamic reconstructions including the peri gondwanan terranes during the late neoproterozoic

lithostratigraphie et datation du proterozoique de la boutonniere d ifni anti atlas maroc abstract in the ifni window in the southwestern part of the moroccan anti atlas unfolded volcanic rocks and sediments rest on a lower proterozoic basement their lithology and relations with dated magmatic rocks lead to a separation into three upper proterozoic groups the absence of the middle proterozoic could also be demonstrated as well as the progressive transition from uppermost proterozoic to lower cambrian the originality of the radiometric ages obtained in the bou azzer region is emphasized as well as the evolution of the pan african tectonics throughout the western anti atlas the intensity of which progressively decreases southwestward

colour banding in a latest neoproterozoic cambrian microbially variegated sabkha of the taoudeni basin adrar of mauritania abstract the taoudeni basin is a large intracratonic trough located in the west african craton with more than 2 km of neoproterozoic lower palaeozoic sedimentary thickness the occurrence of late ediacaran cambrian sabkha deposits in which playa lake evaporites are bounded by proximal braided fluvial and distal shoal complexes is easily distinguishable in the field by their variegated colour banding the diagenetic processes recorded in the sabkha substrates were largely facies controlled associated with microbial development and mat decay mineralization due to anaerobic decay of mat microorganisms chemical conditions beneath mats and biofilms tended to be strongly reducing as a result precipitation of authigenic indicators of anoxic conditions such as pyritic crusts led to variegated colours parallel to stratification reworking of microbial mats and structures by ephemeral streams also generated isolated reduction spots around their microbial clasts leading to the formation of pyritic haloes the subsequent alteration of pyrite to amorphous reddish hematite and subsequent recrystallization to brownish to yellow goethite contributed to the variegated aspect of the unit final reddening of the sediment took place irregularly as a result of dehydration of ferric hydroxides a process relatively common in desert environments

the geology of the madina kouta basin guinea senegal and its significance for the geodynamic evolution of the western part of the west african craton during the upper proterozoic period abstract the madina kouta basin is a southward extension of the upper proterozoic taoudeni basin this sedimentary basin formed in a sw ne tectonic fault zone in the leo shield at 1000 800 ma the deposits indicate a quiet and slightly subsiding area an important uncorformity allows us to distinguish two lithostratigraphic groups the segou group below and the madina kouta group above sedimentation in this area ended with the late proterozoic glaciation 650 620 ma the correlations with the taoudeni basin show a close relationship between the foreland area and the mauritanides bassarides fold belt during the pan african i cycle 800 650 ma in particular we suppose that the lower part of the taoudeni basin deposits which correlate with the madina kouta group group of souroukouto is on the passive margin of the pan african i rift and the post madina kouta deposits in the taoudeni basin group of bakoye correspond to the molassic stage of the pan african i fold belt rift closure stage

the tasiast deposit mauritania abstract the tasiast gold deposits are hosted within mesoarchean rocks of the aoueouat greenstone belt mauritania the tasiast mine consists of two deposits hosted within distinctly different rock types both situated within the hanging wall of the west vergent tasiast thrust the piment deposits are hosted within metasedimentary rocks including metaturbidites and banded iron formation where the main mineral association consists of magnetite quartz pyrrhotite actinolite garnet biotite gold is associated with silica flooding and sulphide replacement of magnetite in the turbidites and in the banded iron formation units the west branch deposit is hosted within meta igneous rocks mainly diorites and quartz diorites that lie stratigraphically below host rocks of the piment deposits most of the gold mineralisation at west branch is hosted by quartz carbonate veins within the sheared and hydrothermally altered meta diorites that constitute the greenschist zone at tasiast gold mineralisation has been defined over a strike length 10 km and to vertical depths of 740 m all of the significant mineralised bodies defined to date dip moderately to steeply 45 to 70 to the east and have a south southeasterly plunge gold deposits on the tasiast trend are associated with second order shear zones that are splays cutting the hanging wall block of the tasiast thrust an age of 2839 36 ma obtained from the hydrothermal overgrowth on zircons from a quartz vein is interpreted to represent the age of mineralisation

late proterozoic to early palaeozoic supracrustal succession of sierra leone an aulacogen at the western margin of the west african craton three parallel zones of upper proterozoic to lower palaeozoic sedimentary volcanic cover rocks have been recognized in western sierra leone the kasila group m the sw ist metamorphosed to the granulite facies and grades into the marampa schist group and this in turn grades into the unmetamorphosed rokel river group in the ne prior to this investigation these rocks were thought to belong to different ages the most highly metamorphosed was thought to be the oldest and the unmetamorphosed the youngest this view is unrealistic since field mapping has failed to reveal any sharp contacts between them and all available age data indicate their involvement in the pan african ca 550 ma orogeny a model is proposed involving an intracontinental graben aulacogen extending from western sierra leone into the adjacent territories developed about 1000 ma ago during the pan african orogeny a portion of the aulacogen fill overlying the remobilized basement was subjected to a regional metamorphism and deformation the grade of metamorphism and the degree of deformation decreased steadily from the centre of the pan african domain to the contact with the west african craton the unmetamorphosed and mildly deformed to undeformed equivalent rests on the craton

crystal mush compaction in the cohassett flood basalt flow hanford washington abstract the 40 m thick central part of the cohassett flow which was probably emplaced by inflation of an earlier more ti rich flow exhibits textural evidence of crystal mush compaction during solidification although most of the crystallization started in the roof zone as water circulated through fractures in the crust plumes of dense crystal mush must have dripped from the roof to the floor of the sheet thus displacing upward the colonnade entablature boundary the mush initially consisted of loosely stacked plagioclase crystals and liquid surrounding rapidly grown pyroxene oikocrysts containing radial chadacrysts of plagioclase upon compaction in the floor zone the liquid between the loosely stacked plagioclase crystals was expelled as the crystals packed tightly together to form sub parallel grains wrapped around the pyroxene oikocrysts which recrystallized into granular aggregates of augite and pigeonite stacks of sinking plagioclase crystals and rising liquid accumulated on the upper and lower sides respectively of horizontal plagioclase phenocrysts this asymmetric texture clearly indicates the relative motion of solids and liquid the degree of compaction as determined from variations in the amount of interstitial liquid and quantitative measures of textural anisotropy accounts for the chemical variation through the central part of the flow with compatible elements being concentrated in the lower part of the flow and incompatible elements concentrated in the upper part modeling of the compaction process using the iridium melt infiltration and reaction program of boudreau a 2003 iridium a program to model reaction of silicate liquid infiltrating a porous solid assemblage computers and geosciences 29 423 429 shows that the observed chemical and compaction dilation profiles are to be expected in a 40 m thick sheet of basaltic magma solidifying on the earth s surface compaction of crystal mush should be even more likely in slowly cooled intrusive bodies

fault seal analysis south marsh island 36 field gulf of mexico smi 36 is a predominantly gas field on the gulf of mexico shelf the principal trap is updip fault closure in the hanging wall of a large down to the north normal fault a second normal fault the intrareservoir fault separates the reservoir into two compartments juxtaposition allan diagrams of the stratigraphy projected onto this intrareservoir fault surface show several intervals of sand on sand juxtaposition in the lowermost part of the section the ma sand in the footwall is juxtaposed against the ln and lp sands in the hanging wall differential fluid contacts and pressures in these sands across this fault suggest that the fault is sealing we hypothesize shale gouge as the mechanism for fault seal estimates of shale percent along the fault plane between the sand intervals range between 40 and 80 mud weights and limited reservoir pressure data show that the deep section is overpressured with a water pressure differential as much as 600 psi 4 13 106 pa across the fault the cross fault differences in the aquifer pressure partially seal the hydrocarbons in the hanging wall the hydrocarbons in the footwall reservoir support a capillary pressure as much as 80 psi 5 51 105 pa for a shale gouge of 45 production pressure profiles record a cross fault pressure difference of 3000 psi 2 06 107 pa across the fault before pressure support between the reservoirs suggesting seal breakdown

recognition of late precambrian glaciogenic sediments in liberia late precambrian glaciation in west africa is now suggested to have extended as far south as gibi mountain in west central liberia 200 km farther south than previously recognized glacial deposits in central sierra leone the gibi mountain formation includes a basal diamictite interpreted as a probable tillite and overlying shallow marine laminites containing isolated ice rafted dropstones and dropgrains these rocks rest on late archean age gneisses and are overlain by late archean age quartzite klippen emplaced during the pan african orogeny ca 550 ma

40 ar 39 ar polyorogenic mineral age record within the southern mauritanide orogen m bout bakel region west africa the southern mauritanide orogen exposed between m bout and bakel is characterized by several internally imbricated polydeformed and variably metamorphosed infrastructural allochthons sup 40 ar sup 39 ar incremental release ages recorded by hornblende within undeformed granodiorite of the guidimakha complex suggest post magmatic cooling through appropriate argon closure temperatures at approx 670 ma sup 40 ar sup 39 ar ages recorded by muscovite within lithologic elements of both the guidimakha complex and the diala bouanze series suggest initial regional metamorphism associated with pan african i orogenesis was following by cooling through muscovite argon closure temperatures between approx 600 and 620 ma slight rejuvenation of muscovite argon systems occurred locally between approx 325 and 350 ma muscovite and whole rock slate phyllite argon systems within metavolcanic and metavolcaniclastic components of the infrastructural calc alkaline igneous complex easternmost sectors of the m bout series record sup 40 ar sup 39 ar plateau age of approx 300 to 320 ma muscovite and whole rock slate phyllite argon systems within westernmost portions of the study area western portions of the m bout series record sup 40 ar sup 39 ar plateau ages of approx 267 to 312 ma all foreland units within the m bout bakel area were affected by post emsian folding effects of this tectonic activity are widespread throughout the parautochthon and western metamorphic sequences these effects includemore emplacement of suprastructural allochthons and local reactivation of older thrust faults within infrastructural units less

first record of early cambrian shelly microfossils from west africa two incomplete specimens of aldanella attleborensis were recovered from calcareous dolomite of the walidiala group taoudeni basin northern guinea this is the first record of cambrian shelly microfossils from west africa and the first record of aldanella from a western gondwanan continent these fossils the regional stratigraphy and radiometric dating place the dolomite previously considered to be late proterozoic in age in the early cambrian proterozoic and lower paleozoic strata in the taoudeni basin are generally unfossiliferous hence a attleborensis and associated microfossils are biostratigraphic markers of geologic importance in west africa the lower cambrian dolomite near walidiala is separated from precambrian tillite by a previously unrecognized disconformity within the walidiala group that represents a hiatus of uncertain duration

40ar 39ar polyorogenic mineral age record within the central mauritanide orogen west africa the central mauritanide orogen in the vicinity of magta lahjar is characterized by several internally imbricated polydeformed and variably metamorphosed infrastructural allochthons which include 1 a calc alkaline igneous complex kelbe intrusive and el hneik t extrusive sequences 2 portions of an ophiolitic sequence el aoueija and oued amour units and 3 continental margin rift sequences fark ka association which include amphibolite low ti continental tholeiite dike protoliths mylonitic quartzite and kyanite schist several variably meta morphosed quartzitic supracrustal nappes g oua and sangarafa allochthons structurally overlie the infrastructural units 40 ar 39 ar incremental release ages have been determined for 10 hornblende and 22 muscovite concentrates prepared from various units of the infrastructural and supra structural allochthons hornblende from fark ka amphibolite is characterized by internally discordant spectra reflecting wide spread contamination with extraneous argon 36 ar 40 ar versus 39 ar 40 ar isotopic correlations are well defined for several concentrates and the resultant ages suggest that post metamorphic cooling occurred following distinct late proterozoic tectonothermal events at ca 700 720 ma and ca 640 650 ma muscovite within fark ka lithologies initially cooled through argon closure temperatures between ca 570 and 595 ma muscovite records similar post metamorphic cooling ages in the other infrastructural allochthons and in the g oua suprastructural allochthon these muscovite ages are interpreted to reflect cooling associated with widespread post orogenic uplift and are consistent with deposition of upper proterozoic lower paleozoic flysch in the proximal foreland tichilit el beida group the tichilit el beida group and correlative parautochthonous sequences djon ba group were penetratively deformed prior to the early ordovician 40 ar 39 ar incremental release analyses of 6 very fine grained white mica concentrates from mylonitic quartzite along thrust faults within the parautochthon indicate that this deformation occurred prior to ca 540 550 ma there is no clear thermal record of this orogenic activity in either infrastructural or suprastructural allochthons suggesting that it must have occurred at relatively high structural levels and was probably largely of intracontinental character all foreland units were affected by post emsian folding effects of this tectonic activity are widespread throughout the parautochthon and western metamorphic sequences these effects include emplacement of the suprastructural allochthons into their present structural positions at ca 300 325 ma with an associated partial rejuvenation of muscovite argon systems proximal to basal thrust faults local reactivation of older thrust faults within the parautochthon and infrastructural allochthons is also indicated by variable 300 325 ma rejuvenation of muscovite argon systems within some fault zones muscovite within westernmost exposures of the infra structural allochthons was totally rejuvenated during the early carboniferous and records 300 m y plateau ages the early tectonic history recorded within the central mauritanide orogen appears to have been related to restricted rifting of a western crustal block from the west african craton and formation of a limited intervening oceanic domain subsequent closure resulted in compression and imbrication of continental crust with resultant development of an ensialic volcanic arc calc alkaline sequences within the infrastructural allochthons together with widespread metamorphism and deformation between ca 700 and 650 ma later orogenesis at ca 550 ma could have reflected distal effects of collisional events which dominate the rokelide orogen in sierra leon and guinea late paleozoic tectonotherma activity within the central mauritanides is interpreted to have developed as a result of a renewed translation between the western continental block and the west african craton during final collision of laurentia arc gondwana

compaction of igneous cumulates when initially deposited mafic igneous cumulates consist of a solid matrix of mineral phases plus 50 60 interstitial liquid due to its greater density the matrix compacts squeezing the interstitial liquid upward a numerical fluid mechanical model of this process indicates that if the cumulate deposition rate is constant compaction occurs in one of two distinct models for rapid deposition the system is permeability controlled with compaction occurring only at the base of the cumulate pile for slowly deposited cumulates an additional compaction zone that is partially viscosity controlled develops at the top of the pile if the extent of compaction and thickness of this zone are known for a real system the model can be used to estimate the permeability and effective viscosity of the compacting cumulates such calculations yield a permeability of 1 5 times 10 3 a 2 f 3 1 f 2 for olivine cumulates in the muskox intrusion reliable viscosity estimates await an empirical law relating ma

differentiation and compaction in the skaergaard intrusion igneous differentiation processes are constrained from bulk compositions densities and mineral modes of 116 cumulate gabbro samples in a new reference profile through the layered series of the skaergaard intrusion east greenland the stratigraphic distribution of p u and rb in cumulates and residual magma modeled by rayleigh fractionation constrains the final porosities or trapped liquid contents to 30 52 in lza troctolites decreasing to 4 12 at the top of lzb olivine gabbros and remaining low 1 13 4 middle dot 6 on average in the oxide gabbros of lzc mz and uz local variations in trapped liquid content are associated with modal layering leucocratic low density rocks have higher proportions of trapped melt than adjacent melanocratic high density rocks these observations are explained by varying degrees of compaction compaction was most important after the onset of fe ti oxide crystallization because of the high densities of the crystal matrix here computed rates of compaction exceed the rate of crystal accumulation in layers that are metres to a few tens of metres thick in the basal section lza the crystal pile was too thin and the density of the crystal matrix too low to drive the rate of compaction above the high rate of crystal accumulation promoted by initial cooling through the intrusion floor in the overlying section lzb the efficiency of compaction gradually increased as a result of thickening of the crystal pile and lowering of the rate of crystal accumulation the modeling constrains the p2o5 content of the residual magma to 1 middle dot 7 wt at the level of apatite in suggesting that the magma contained 49 wt sio2 and followed a trend of iron enrichment compaction of the uppermost metres to tens of metres of crystal mush at the top of the cumulate pile was an efficient means of differentiation and resulted in layers with variable final porosities and trace element contents depending on the mineralogy and density of the crystal matrix

crustal and upper mantle electrical conductivity structures in west africa geodynamic implications abstract measurements of the electrical and magnetic fields at the surface of the earth have been carried out in west africa in the period range 10 10 000 s in this area the method of magnetotelluric mt sounding finds its application in a variety of geological problems such as the deep shape of sedimentary basins the geometry of orogenic belts and craton deep structures the present article is an attempt to correlate the conductivity model with large scale tectonics mt surveys have revealed the presence of several areas where electrical resistivity is abnormally low at crustal and upper mantle depths and the existence of lateral electrical conductivity inhomogeneities within the lithosphere subsurface electrical conductivity distribution is in good relation with the geological features and its variation associated with lithological and structural changes a major discontinuity found at a depth of several hundred kilometers in the lithosphere separates two structural blocks the eastern one with higher resistivities being the west african craton the lithosphere derived from this work has a maximum thickness of 300 km under the senegalo mauritanian basin and more than 400 km beneath the craton the electrical asthenosphere is certainly not well developed under the craton a sketch of electric structures across west africa is presented and a geodynamic interpretation is proposed to explain the present structural pattern of the southern segment of the mauritanides belt

geology and stable isotope o h c s constraints on the genesis of the cachoeira gold deposit gurupi belt northern brazil the cachoeira gold deposit is one of several large n20 tonnes au shear zone hosted hydrothermal gold deposits of the proterozoic gurupi belt in northern brazil o h c and s isotope compositions of carbonate silicate and sulfide minerals and from graphite and inclusion fluids from this deposit have been measured in order to infer the composition of ore forming fluids the data show small variation at the deposit scale irrespective of host rock calculated y 18 o values of h2o in the fluid in equilibrium with quartz hydrous silicates and carbonates are respectively 6 2 to 12 4x 5 0 to 9 0x and 5 9 to 11 0x smow the ydh2o of inclusion fluids is 22x whereas values for chlorite and white mica are 16x and 20 to 28x smow respectively these values are compatible with a metamorphic origin which is in agreement with the postmetamorphic timing of the mineralization the calculated y 13 c value of co2 of the fluid in equilibrium with the carbonates is from 9 5x to 12 7x pdb whereas the graphite bearing host rocks have y 13 c value of graphite carbon of 29 7x the calculated y 34 so f h2s of the fluid varies mainly between 0 7x and 5 5x cdt and is interpreted as reflecting a magmatic or average crustal source for sulfur the restricted isotopic compositions indicate a relatively homogeneous fluid and deposition under restricted temperature conditions nonetheless a relatively wide temperature range of 272 383 8c is estimated for the ore bearing fluid using the quartz dolomite oxygen isotope thermometer collectively the data indicate that gold was transported as a reduced sulfur complex by a metamorphic fluid having a near neutral ph at relatively low co2 furthermore the data suggest that gold was deposited during the structural evolution of the associated shear zone probably as a result of sulfidation and carbonatization reactions with the fe and c rich host rocks which resulted in changes of the ph and in the redox state of the ore fluid d 2005 elsevier b v all rights reserved

chemical evolution of intercumulus liquid as recorded in plagioclase overgrowth rims from the skaergaard intrusion the intercumulus liquid of a crystal mush fills pore spaces and typically solidifies to form overgrowths on cumulus grains and poikilitic post cumulus minerals if the liquid is immobile solidification produces zoned intercumulus minerals as a result of progressive fractionation of the residual liquid convection within the mush results in buffering of the liquid composition and thus limits mineral zonation for fully solidified cumulates fossil changes in liquid composition or porosity are difficult to identify however detailed study of immobile minor components of plagioclase overgrowth rims can provide information about the progressive solidification of intercumulus material ti contents of plagioclase overgrowths in samples from the lowermost parts of the skaergaard intrusion show strong variations with anorthite content with decreasing xan ti concentrations first rise and then fall consistent with changing tio2 contents of the intercumulus liquid during solidification tio2 in plagioclase decreases sharply at an55 reflecting local saturation of fe ti oxides ti in clinopyroxene oikocrysts also falls rimward but zoning in faster diffusing species fe mg is limited other than slight reverse zones that may occur on the plagioclase margins xan falls continuously during crystallization the reverse zoning is interpreted as the result of compaction driven dissolution and reprecipitation of plagioclase the continual decrease in xan is exploited together with back scattered electron images of the cumulates to produce calibrated images showing regions of progressive crystallization this allows the regions crystallizing at each stage of solidification to be visualized these images show that the final remnants of interstitial melt were present in triangular pockets and as thin grain boundary melt films this approach can provide information about the progressive reduction of porosity during cumulate solidification

40ar 39ar polyorogenic mineral age record in the northern mauritanide orogen west africa abstract northern segments of the mauritanide orogen in the akjoujt region are dominated by suprastructural nappe complexes these structurally overlie other tectonic elements including 1 autochthonous parautochthonous foreland units composed of late proterozoic late paleozoic frasnian sedimentary cover sequences and unconformably underlying crystalline basement late archean middle proterozoic of the west african shield 2 parautochthonous successions largely represented by low grade metasedimentary rocks and 3 allochthonous infrastructural complexes with characteristics similar to those of late proterozoic early paleozoic pan african tectonic elements exposed in central and southern parts of the mauritanide orogen amphibole from various structural levels of the ijibiten and akjoujt suprastructural nappe complexes generally display internally discordant saddle shaped 40 ar 39 ar release spectra suggestive of extraneous excess argon contamination one sample records a well defined intermediate temperature plateau of 2420 ma muscovite within lower and intermediate structural levels of the complexes displays variably discordant 40 ar 39 ar age spectra which suggest that variable thermal rejuvenation of older up to 2600 ma mineral systems occurred during a ca 310 300 ma metamorphism uppermost structural levels of the complexes are represented in part by penetratively mylonitic metagranite muscovite within these rocks displays internally concordant 40 ar 39 ar age spectra which define plateau ages of 300 310 ma there is no indication in either amphibole or muscovite results of any late proterozoic early paleozoic pan african thermal record together with field relationships the 40 ar 39 ar results suggest that structural units of the suprastructural nappe complexes are in part composed of variably overprinted basement rocks these structurally overlie allochthonous pan african sequences and therefore must have been derived west of and now largely conceal the pan african orogen the complexes were transported eastward to higher structural levels during late paleozoic orogenesis the extent of late paleozoic thrusting within the akjoujt region contrasts with more limited transport distances documented southward in the orogen this contrast probably reflects a kinematic influence of the reguibat promentory of the west african shield which during the late paleozoic collision of gondwana and laurentia effected extensive compression in proximal sequences

petrologic evolution of partially molten cumulate the atok section of the bushveld complex the postcumulus evolution of a portion of the bushveld complex that includes the merensky reef is inferred from the study of a continuous 56 m drill core the core penetrated the basal orthopyroxenites of the merensky and bastard units the massive anorthosites overlying the two pyroxenites and about 10 m of norite underlying the merensky pyroxenite detailed profiles of major minor and rare earth element ree contents of clinopyroxene and orthopyroxene were determined by electron and ion probe good correlations exist between textural and lithological variations and the ree contents of the pyroxenes specifically enrichments in pyroxene ree abundances are observed in the basal pyroxenites of the merensky and bastard units relative to the underlying and overlying rocks in the pyroxenites the nd content of clinopyroxene is typically over 12 ppm and reaches nearly 40 ppm 90 chondrite and nd yb ratio is in the range 8 to 25 these extreme enrichments in ree are not accompanied by large variations in major element contents computations of the compaction parameters relevant to the conditions of crystallization of the bushveld complex combined with a consideration of cooling history confirm the importance of compaction as a post cumulus process this analysis indicates that the geochemical variability is a result of redistribution of interstitial melt driven by compaction and cannot reflect variations in the initial porosity of the accumulating crystal pile a model for the development of the atok section is developed the merensky anorthosite is interpreted to have served as a barrier to the upward porous flow of late stage hydrous and incompatible element enriched melt which was thus trapped in the underlying merensky pyroxenite the flow was driven by compaction of the 350 meter thick section of predominantly norite beneath the anorthosite the introduction and accumulation of this melt in the pyroxenite and subsequent cooling resulted in partial dissolution recrystallization and ree enrichment of the rock forming minerals and in the formation of the main lithologic features of the merensky pyroxenite further upward percolation of the interstitial melt through the merensky anorthosite was restricted to channels due to the relatively impermeable nature of the cemented anorthosite this melt accumulated in and metasomatized the bastard pyroxenite in the same manner as the merensky pyroxenite having again been trapped by the overlying bastard anorthosite

emplacement age of post tectonic granites in southern guinea west africa and the peninsular florida subsurface implications for origins of southern appalachian exotic terranes the coyah granite of guinea intruded western portions of the rokelide orogen following a tectonothermal event at ca 550 ma it records a biotite muscovite plus whole rock rb sr isochron age of 533 7 m y mswd 1 6 which is interpreted to closely date crystallization the osceola granite represents a major lithotectonic element in the pre cretaceous crystalline basement of peninsular florida five biotite concentrates from cuttings recovered from four wells penetrating the pluton record 40ar 39ar plateau ages ranging between 527 4 1 9 m y and 534 6 2 0 m y petrographic characteristics of the four samples suggest relatively rapid post magmatic cooling and the plateau ages are interpreted to closely date emplacement of the pluton similarities in age and petrographic characteristics combine to suggest that the coyah and osceola granites were initially part of a sequence of plutons emplaced along the northwestern margin of the gondwana continent portions of this terrane amalgamated with laurentia during late paleozoic assembly of pangea various exotic lithotectonic terranes which had previously accreted or were at least proximal to the north american craton were transported to their present structural position in the southern central appalachian orogen during this collision

liquidus temperatures of the skaergaard magma the liquidus temperatures of the skaergaard intrusion can be estimated for the layered series from plagioclase compositions plagioclase saturation in one atmosphere melting experiments on evolved north atlantic basalts is a function of an content mol and can be described by an empirical linear relationship t c 899 3 6 an 1 20 c this relationship predicts a total crystallization interval for the intrusion of 150 c plagioclase crystallized in the hidden zone an 71 at 1155 c at the base of the lower zone an 66 at 1137 c and finally in the sandwich horizon an 30 at 1007 c these temperatures are in good agreement with previous estimates based on melting studies of suitable chilled margin rocks and gabbros from the skaergaard intrusion and liquid line of descent modeling our temperature estimates however are markedly lower by up to 60 c than recent temperature estimates based on an extrapolation of high pressure experimental data for the kiglapait intrusion to skaergaard emplacement conditions proposed variations in magmatic pressure during skaergaard evolution and reasonable estimates of magmatic water contents do not alter these conclusions

age of the late paleozoic tectonothermal activity in northcentral mauritanide west africa abstract late proterozoic sedimentary successions unconformably overlie each other on the crystalline basement in the northcentral mauritanide foreland clay sized m fractions from allochthonous metasedimentary samples collected close to the basal thrust are characteristic of an upper anchizone very low grade lower epizone low grade thermal development these fractions record conventional k ar ages ranging between 301 8 and 312 7 ma which are interpreted to date structural emplacement of the allochthon clay fractions separated from the underlying autochthonous sedimentary sequence record a diagenetic to low middle anchizonal very low grade thermal development these fractions yield k ar values between 337 8 and 472 12 ma which probably reflect variable diffusional rejuvenation processes of detrital and or diagenetic precursors 40 ar 39 ar incremental release analyses of hornblende and biotite concentrates from crystal line basement beneath the sedimentary sequence suggest that the respective closure temperatures last occurred at 2400 and 1850 ma ago as no record of any other thermal overprint was observed the results suggest that emplacement of the allochthon occurred in the late carboniferous and was not associated with a regional metamorphism of the north central mauritanide foreland

crystallization of apatite in natural magmas under high pressure hydrous conditions with particular reference to orogenic rock series two separate series of hydrous experiments involving 1 imposing apatite saturation on a series of igneous rock compositions from basanite to rhyolite and 2 crystallizing similar natural rock compositions progressively until apatite appears demonstrate a close dependence between apatite saturation and silica content of the magma and determine p2o5 levels at a given silica value and temperature at which that composition may be expected to crystallize apatite the effect of pressure on apatite solubility is not great and is most significant for silicic compositions

40ar 39ar mineral age record of polyphase tectonothermal evolution in the southern mauritanide orogen southeastern senegal 40 ar 39 ar incremental release mineral ages have been determined for several tectonostratigraphic units in southernmost portions of the maiuritanide orogen in senegal here middle proterozoic crystalline basement rocks of the west african craton are unconformably overlain by an upper proterozoic ca 1100 ma continental sedimentary succession madina kouta group transitional into volcanic and intercalated deep marine sediments tennesse group western portions of the volcano sedimentary unit were variably deformed and metamorphosed prior to deposition of an unconformably overlying upper precambrian lower cambrian sedimentary succession mali group muscovite within cleavage domains in the low grade metamorphic terrane records 40 ar 39 ar plateau dates of ca 655 665 ma and are interpreted to closely date the pan african i phase of orogenic activity the mali group itself was locally folded during a second phase of pan african tectonothermal activity which also affected a western calc alkaline volcanic plutonic sequence niokolo koba group igneous hornblende from several granites within this sequence displays internally discordant 40 ar 39 ar release spectra which record variably developed plateaus of ca 640 660 ma these are interpreted to approximately date initial postmagmatic cooling through 500 c and are consistent with a previously published rb sr whole rock iso chron crystallization age of 683 18 ma for one of the plutons associated calc alkaline volcanic and volcaniclastic rocks are more penetratively deformed muscovite that is aligned along a regional cleavage records 40 ar 39 ar plateau ages of ca 550 560 ma which are interpreted to closely date the pan african ii orogenesis widespread deposition of a molassic sequence youkoukoun formation occurred in the middle cambrian early ordovician this was disconformably overlain by upper ordovician upper devonian sedimentary rocks bove basin succession which are locally folded more significant late paleozoic tectonothermal activity is recorded in north westernmost portions of the study area where the calc alkaline upper proterozoic granites locally display penetrative mylonitic fabrics development of these fabrics is interpreted to be closely dated by the 40 ar 39 ar 270 to 280 ma plateau ages recorded by dynamically recrystallized muscovite developed along fluxion foliations these results allow demarcation of a late paleozoic tectonic front and permit correlations with genetically related events in southeastern portions of the appalachian orogen

correlation of proterozoic sediments of western and central africa and south america based upon radiochronological and palaeontological data abstract nearly 70 new rb sr isochron ages and many k ar conventional ages have been determined between 1975 and 1980 on proterozoic sedimentary or metasedimentary sequences in western and central africa and south america some stratigraphic results have been established 1 1 five formations have been dated of the lower proterozoic 2 2 a long sedimentation gap occurs mainly in western african and in some regions of central africa and south america between nearly 1600 and 1100 ma 3 3 the upper riphean assemblages of stromatolites have been dated and compared to those of the eurasian craton 4 4 two main glacial events have been dated the first one placed at ca 950 ma the second one during the vendian at ca 650 620 ma 5 5 it can be stated that when applied to precambrian sequences all stratigraphic methods must be used together

etude regionale magneto tellurique des structures de la conductivite electrique sur la bordure occidentale du craton ouest africain en republique du senegal magnetotelluric soundings were carried out in the senegal republic on the western border of the west african craton by a team of geophysicists from mbour s observatory along an approximately east

the geology and mineralogy of the loulo mining district mali west africa evidence for two distinct styles of orogenic gold mineralization loulo is a world class orogenic gold mining district in the birimian terrane of western mali orebodies are located along second or higher order shears associated with the senegal mali shear zone with gold mineralization largely linked to a transtensional event the mine camp is divided into two distinct styles of gold deposit on the basis of differing geologic characteristics one group is typified by the gara deposit whereas the other by the yalea deposit both deposit styles are hosted by similar rock types calcareous graywackes and calciticdolomitic marbles gara style orebodies occur as sulfide disseminations or ankerite rich shear vein stockworks hosted in folded tourmalinized sandstones and breccias mainly within 2 km of the senegal mali shear zone these deposits are characterized by intense multi stage albitization and tourmalinization pre syn and postmineralization gold lodes are fe rich dominated by nickeloan pyrite contain cu ni co minor and trace sulfides e g chalcopyrite gersdorffite pentlandite cobaltite millerite and show consistently high levels of p ree w bearing phases apatite monazite xenotime and scheelite base metal concentrations show a marked increase in marble host rocks with the formation of nickeloan pyrite cobaltite clausthalite ores in contrast yalea style deposits are associated with quartz ankerite vein lodes and disseminated sulfide stringer zones mineralization occurs along highly altered tourmaline absent brittle ductile shears up to 8 km away from the senegal mali shear zone wall rock alteration is characterized by addition of k 2 o cao co 2 h 2 o and sio 2 with mineral assemblages consisting of chlorite sericite carbonate quartz albite ore paragenesis is enriched in as mainly as multistage growth of arsenopyrite and arsenian pyrite base metal sulfides scheelite and ree phosphates are extremely rare the diversity in the ore paragenesis is controlled by a dynamic hydrothermal system that sourced fluids and metals from different reservoirs within the region the as rich yalea style deposits have characteristics typical of birimian gold mineralization in ghana and auriferous fluids are likely derived from the dewatering of sedimentary rocks during regional metamorphism on the other hand the polymetallic fe b rich gara style orebodies show atypical features for birimian gold mineralization instead field relations and the mineralogy and geochemistry of the gara style lodes indicate a strong hydrothermal influence from surrounding intrusive stocks with possible links between gold mineralization and iron oxide skarn development in the region the data collected at loulo highlight the diverse nature of orogenic gold deposits especially in west africa this style of ore deposit can form from a variety of fluid sources both metamorphic and magmatic

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

tectonometamorphic evolution of ghana togo and benin in the light of the pan african brasiliano orogeny abstract the south pan african belt of west africa ghana togo and benin province can be divided into three domains 1 1 the external nappes thrust over the west african craton derived from passive margin sedimentary deposits middle to late proterozoic and displaying middle to high pressure metamorphism such as the kante and atacora nappes 2 2 the intermediate nappes including the suture zone rocks with eclogitic assemblages protoliths ca 800 ma and mainly monocyclic metasedimentary rock and orthogneiss that are found in central togo and northwest benin 3 3 the internal nappes composed of high grade commonly anatectic gneiss anatexis ca 600 ma including orthogneiss eburnian plutons elongated granulite belts reworked archean basement and an intrusive granite and charnockite association ca 600 ma which are exposed in benin and nigeria the p t t d paths calculated from rocks in these three domains show a prograde retrograde evolution 1 1 an initial clockwise evolution related to an early burial stage hp ht in the intermediate nappes followed by oblique thrusting in a sw direction that caused nappe stacking and subsequent uplift 2 2 a second anti clockwise evolution with late temperature and pressure increases only recorded in the more internal units this temperature increase was a consequence of the previous crustal thickening with subsequent doming extension and anatexis the pressure increase was related to movement along the kandi fault working as a dextral strike slip fault with a transpressional component under late e w shortening the retrograde trend corresponds to the final uplift the evolution of the belt can thus be divided into three successive stages 1 1 a first stage was characterized by an oblique collision inducing sw oblique thrusting with associated syn foliation reverse metamorphism 2 2 during a second stage anatectic doming took place followed by 3 3 a third stage at still high temperature with dextral wrench faulting and associated n s post foliation folding under late e w shortening comparison between the west african north brasiliano and central african late proterozoic belts shows very strong similarities between their lithology kinematics and metamorphism and indicates that the pan african brasiliano chain resulted from collision between three major continental domains the west african and sao francisco congo cratons and a late proterozoic mobile domain

new information on the probable isotopic age of the late proterozoic glaciation in west africa abstract new rb sr age determinations on smectitic clay fractions from a formation of the late proterozoic glaciation in west africa constrain the occurrence of that major event to between 630 and 595 ma

a tectonic linkage between the rodelide orogen sierra leone and the st lucie metamorphic complex in the florida subsurface the st lucie metamorphic complex comprises part of the non laurentian paleozoic suwannee terrane beneath the gulf and atlantic coastal plains of the southeastern united states hornblende concentrates prepared from cuttings from two deep test wells penetrating the complex display internally concordant 40 ar 39 ar incremental release spectra defining plateau ages of 510 8 pm 1 1 ma and 513 1 pm 1 8 ma which are interpreted to date post metamorphic cooling through temperatures required for intracrystalline argon retention the kasila group constitutes the western segment of the rokelide orogen in sierra leone four hornblende concentrates prepared from amphibolite within the kasila group yield 40 ar 39 ar ar plateau ages of 505 0 pm 5 2 508 2 pm 2 1 510 5 pm 2 6 and 546 1 pm 6 8 ma they are interpreted to date post metamorphic cooling through appropriate argon closure temperatures following a ca 550 560 ma pan african ii phase of tectonothermal activity

postrift history of the eastern central atlantic passive margin insights from the saharan region of south morocco the passive margin of south morocco is a low elevated passive margin it constitutes one of the oldest margins of the atlantic ocean with an early jurassic breakup and little geological data are available concerning its postrift reactivation so far we investigated the postrift thermal history of the onshore part of the margin with low temperature thermochronology on apatite crystals fission track and u th sm he ages we obtained are significantly younger than the breakup 190 ma fission track ages range from 107 8 to 175 16 ma with mean track lengths from 10 7 0 3 to 12 5 0 2 m u th sm he ages range from 14 1 to 185 15 ma using inverse modeling of low temperature thermochronological data we demonstrate that the south moroccan continental margin underwent a complex postrift history with at least two burial and exhumation phases the first exhumation event occurred during late jurassic early cretaceous and we attribute this to mantle dynamics rather than to intrinsic rifting related processes such as flexural rebound the second event from late cretaceous to early paleogene might record the onset of africa europe convergence we show a remarkably common behavior of the whole moroccan passive margin during its early postrift evolution the present day differences result from a segmentation of the margin domains due to the africa europe convergence finally we propose that varying retained strengths during rifting and also the specific crustal lithospheric geometry of stretching explain the difference between the topographical expressions on the continental african margin compared to its american counterpart

compositional convection in a reactive crystalline mush and melt differentiation fractional crystallization and partial melting involve relative motion of liquid and solid phases and chemical and thermal interactions between them to elucidate some physical principles of thermosolutal convection in a reactive porous medium we describe experiments on the directional solidification of aqueous ammonium chloride solutions the addition of small amounts of a polymerizing agent permits variation of the solution viscosity independently of thermal conditions the phase diagram and permeability solutions were cooled from below and crystallization developed at the base of the tank generating at first a field of thin plumes of light residual fluid released from the boundary layer at the top of the mush the interstitial fluid within the mush also became unstable eventually the onset of convection occurring when the porous medium rayleigh number of the mush reached a critical value this threshold value was found to be 25 at low initial superheat and to decrease with increasing superheat local thermodynamic equilibrium between crystals and liquid within the mush coupled the evolution of temperature composition and porosity convective motions locally caused dissolution and precipitation and hence fluctuations of porosity developed dissolution occurred preferentially in the central parts of upwellings and the upflow gradually focused being ultimately channelized into narrow chimneys devoid of crystals all else being equal the areal density of chimneys was greater the lower the viscosity chimney diameter increased with increasing solution viscosity in the liquid above the growing mush the convective plumes were very similar to salt fingers depending on solution viscosity and temperature gradient they exhibited a phenomenon of collective instability such that vertical motion was disrupted by wave instabilities the base plate temperature chosen was above the eutectic and hence the total amount of crystals at the end of experiments with the same initial composition and the same final temperature was a constant fixed by the phase diagram the spatial distribution of crystals and the final porosity of the mush were however determined by the strength of compositional convection as measured by the porous medium rayleigh number when no convection occurred mush thickness finally became equal to the initial layer thickness and the system was homogeneous when compositional convection occurred the overlying reservoir underwent chemical evolution and mush growth slowed dramatically in experiments with progressively lower solution viscosity and hence more vigorous convection final mush thickness was progressively less and final porosity lower final mush thickness was found to scale with solution viscosity to the power 0 33 during fractional crystallization of magma the effects of compositional convection can be recorded in the chemical and mineralogical features of cumulate rocks we speculate that fossil chimney structures can be found in the lower zone of the bushveld ultramafic complex in the form of iron rich platinum bearing dunite pipes this could explain the adcumulate nature of the lower zone rocks and the chemical and mineralogical similarities between the pipes and the overlying merensky reef the merensky reef would be interpreted as a hiatus in mush growth the results may also have applications to the flow structure in regions of partial melting and in the earth s core

sadiola hill a world class carbonate hosted gold deposit in mali west africa the 8 moz sadiola hill gold deposit is located in the kedougou kenieba inlier a window of deformed ca 2200 to 2050 ma rocks that crop out in eastern senegal and western mali the geology of the inlier differs from other paleoproterozoic granite greenstone belts and sedimentary basins by the abundance of carbonate rocks the sadiola hill gold deposit occurs within 3 km of the senegal mali shear zone and country rocks in the region have undergone polycyclic deformation the lithostratigraphy of the open pit consists of impure limestones overlain by younger sequences of detrital sedimentary rocks which include arenite wacke and siltstone the sedimentary rocks have been subjected to regional greenschist facies metamorphism and have been intruded by multiple generations of synkinematic calc alkaline stocks and dikes the bulk of the ore is hosted within a n s to nnw trending 10 to 50 m wide brittle ductile dilational shear zone defined as the sadiola shear zone gold lodes also occur along an array of steep nne trending shear zones geometric and kinematic analyses indicate that the ore hosting structures were undergoing sinistral displacement at the time of mineralization locally defined as the d3s nnw sse shortening event hydrothermal alteration is polyphase and includes an early high temperature calc silicate phase i e porphyroblastic growth of actinolite tremolite followed by a potassic phase i e biotite calcite quartz k feldspar tourmaline actinolite that was synchronous with ore mineral deposition paragenetic studies reveal a multistage ore development that includes an as rich sulfide stage followed by an au sb stage the ore is associated with a metal enrichment suite of au as sb cu fe w mo ag bi zn pb te the sadiola hill deposit shares a similar relative timing and structural setting to that of other world class orogenic gold systems in the west african craton e g ashanti loulo with gold deposition occurring during a period of transcurrent tectonics after the cessation of region wide compressional deformation the high temperature paragenesis at sadiola hill is atypical of gold mineralization in the craton and indicates that the late eburnean tectono magmatic activity between ca 2080 and 2060 ma played an important role in the dynamics of hydrothermal fluid circulation along the senegal mali shear zone

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

proterozoic microbial reef complexes and associated hydrothermal mineralizations in the banfora cliffs burkina faso abstract the proterozoic guena souroukoundinga formation of the mopti arm gourma aulacogen southerm taoudeni basin consists of a shale dominated succession up to 200 m thick with scattered microbial reef complexes quarry exposures of the tiara reef complex allow reconstruction of a transect across back reef peritidal laminites reef margin and peri reef ooidal shoals and fore reef slope strata microbial carbonate productivity nucleated on isolated palaeohighs during transgression whereas its end was controlled by two tectonically induced drowning pulses that led to the successive record of onlapping kerogenous limestones and pelagic shales reef carbonates are crosscut by fractures and fissures occluded by hydrothermal mineralizations which are related to the rifting activity of the gourma aulacogen the tiara reef complex is similar to other proterozoic reefs in being composed nearly entirely of stromatolites although calcimicrobial filamentous and thromboid textures are locally abundant which contrast with their scarcity or absence in coeval stable platform microbial reefs of the northern taoudeni basin

timing and duration of volcanism in the north atlantic igneous province implications for geodynamics and links to the iceland hotspot abstract we combine new and published 40 ar 39 ar age determinations from incremental heating experiments on whole rocks and mineral separates to assess the timing duration and distribution of volcanic activity during construction of the north atlantic igneous province we use these ages together with volume estimates of erupted magmas and their cumulates to calculate melt production rates for the early tertiary flood basalts of east greenland and the faeroes islands the lavas lie at opposite ends of the greenland iceland faeroes ridge the postulated iceland hotspot track and record volcanic activity leading up to during and following continental breakup between greenland and europe dominantly basaltic but also alkalic and picritic magmas were erupted from beneath thick continental lithosphere simultaneously at 61 ma from baffin island to the western and eastern margins of greenland to the faeroe islands and the western british isles a roughly circular area 2000 km in diameter volcanic activity was increasingly intermittent by 57 56 ma but at 56 1 0 5 ma the average melt production rate increased by more than an order of magnitude over previous levels from 3000 km 3 km of rift my coinciding with continental rifting and shallow decompressional melting of the mantle that fed subaerial volcanic activity along most of the eastern margin of greenland and its complement on the northwestern european margin offshore abnormally thick oceanic crust was produced along the line of plate separation the upper part of this crust comprises seismically imaged seaward dipping subaerially erupted lavas by 50 ma eruption rates had diminished drastically and volcanic activity had narrowed to a much restricted portion of the east greenland margin at the western end of the greenland iceland ridge that subsequently connected in an age progressive trend to present hotspot activity in southeastern iceland the startup mantle plume head and tail model with moderate excess temperature t 100 c and active upwelling best explains the time space and compositional aspects of volcanism in this province

geochronology of sedimentary and metasedimentary precambrian rocks of the west african craton abstract this contribution summarizes current knowledge of the geochronology of the upper proterozoic sedimentary rocks covering the west african craton this was done by using direct dating methods correlations between the northern edge of the tindouf basin and the northern and southern part of the taoudeni basin as well as the volta basin are proposed an important depositional lack over almost the entire craton is confirmed between 1 7 ga which represents the time of the latest eburnean activities or cooling effects and about 1 0 ga which is the start of the general upper proterozoic sedimentation this lack is remarkable as the other cratons from gondwana continent are covered by middle proterozoic sediments the upper proterozoic sedimentation on the west african craton may be divided into a basal mainly sandstone period ca 1000 900 ma a middle mainly limestone period ca 900 750 ma and an upper mainly siltstone period ca 750 650 ma tectonic volcanic and thermal activities in connection with the pan african orogeny are recorded only around the craton they induced either sedimentation lacks in morocco or sedimentation excesses in hoggar unsolved problems such as the precise stratigraphic position of the uppermost proterozoic tillitic episode and the correlation within the moroccan anti atlas are also raised

the archaean kasila group of western sierra leone geology and relations with adjacent granite greenstone terrane abstract the kasila group an archaean highly metamorphosed supracrustal succession composed of basaltic igneous clastic and chemical sedimentary rocks outlines the western border of the granite greenstone terrane of the west african craton in sierra leone the contact between the two tectonic units is characterised by intense overthrust ductile shear deformation operating during the late archaean at high metamorphic grade followed by increasingly brittle shear motions of the same sense indicating prolonged eastwards directed overthrusting subsequent to the formation of the adjacent granite greenstone terrane to the east the kasila group poor in granitic intrusives is unlike archaean granite greenstone and high grade gneiss terranes and may represent the deep seated remains of a telescoped supracrustal succession perhaps resulting from the collision and suturing of archaean terranes now represented by the west african craton and the guyana shield

postmagmatic sulphur loss in the skaergaard intrusion implications for the formation of the platinova reef abstract the stratabound platinova au pd reef in the skaergaard intrusion is intimately linked to cu fe sulphide minerals and occurs at a level that shows changes in the sulphur concentration and the cu s ratio these features suggest that the reef is related to silicate sulphide liquid immiscibility in the skaergaard magma however although the evidence is strong present day sulphur concentrations are too low to support such a model the detailed knowledge of the shape volume and compositional relations of the skaergaard lithologies allows for numerical modeling of sulphur and copper concentrations during fractionation of the magma these data can be evaluated with the current models for sulphur saturation in iron rich mafic magmas to assess the original sulphur concentrations the extent of fractionation the composition of sulphide liquids and the extent of postmagmatic sulphur loss mass balance modeling yields initial copper and sulphur concentrations of 250 and 894 ppm for the parental magma to the lower zone a prior to sulphur saturation cu behaves as an incompatible element with a bulk partition coefficient in the cumulus material dcumuluscu of 0 13 indicating some incorporation of cu into mafic silicates probably augite sulphur can be assumed to be perfectly incompatible in the cumulus minerals although it will be included as a component of the interstitial liquid these values yield sulphur saturation at around 635 ppm cu and 2607 ppm s assuming the sulphur to originally be present as a mixture of monosulphide mss and intermediate solid solution sulphide iss the primary sulphides in the cumulates would amount to 0 07 modal through lower and middle zones and 0 53 modal on average through upper zone the expected weight proportion of primary chalcopyrite pyrrhotite evolves from 30 70 at the base of lza through 26 74 at the level of the platinova reef to 18 82 at the sandwich horizon the present day bornite magnetite mineral assemblage is likely to have been derived from a primary pyrrhotite chalcopyrite assemblage through reequilibration and postmagmatic oxidation significant sulphur appears to have been lost from the lower and middle parts of the layered series lower zone a to upper zone b whereas the sulphides of the upper zone c can be explained in the context of a sulphur gain local scale reequilibration of cu from mafic silicates into sulphides may have contributed to the sulphide mineralogy of the lower and middle zones the structure of the platinova reef probably formed from an original single stratabound reef sulphur loss through oxidation in upward migrating deuteric or hydrothermal fluids could have caused dissolution and reprecipitation of pd and au in a cumulate with several stratiform redox barriers this process can thus explain the successive pd peaks of the reef as well as the variable stratigraphic offset of pd and au across the intrusion