calcic amphibole equilibria and a new amphibole plagioclase geothermometer there is currently a dearth of reliable thermobarometers for many hornblende and plagioclase bearing rocks such as granitoids and amphibolites a semi empirical thermodynamic evaluation of the available experimental data on amphibole plagioclase assemblages leads to a new thermometer based on the aliv content of amphibole coexisting with plagioclase in silica saturated rocks the principal exchange vector in amphiboles as a function of temperature in both the natural and experimental studies is left na square 1 right a left alsi 1 right t1 we have analysed the data using 3 different amphibole activity models to calibrate the thermometer reactions 1 edenite 4 quartz tremolite albite 1 2 pargasite 4 quartz hornblende albite 2 the equilibrium relation for both 1 and 2 leads to the proposed new thermometer t frac 0 677p 48 98 y 0 0429 0 008314 ln k and k left frac si 4 8 si right x ab plag where si is the number of atoms per formula unit in amphiboles with p in kbar and t in k the term y represents plagioclase non ideality rtln ab from darken s quadratic formalism dqf with y 0 for xab 0 5 and y 8 06 25 5 1 xab 2 for xab 0 5 the best fits to the data were obtained by assuming complete coupling between al on the t1 site and na in the a site of amphibole and the standard deviation of residuals in the fit is 38 c the thermometer is robust to ferric iron recalculation procedures from electron probe data and should yield temperatures of equilibration for hornblende plagioclase assemblages with uncertainties of around 75 c for rocks equilibrated at temperatures in the range 500 1100 c the thermometer should only be used in this temperature range and for assemblages with plagioclase less calcic than an92 and with amphiboles containing less than 7 8 si atoms pfu good results have been attained on natural examples from greenschist to granulite facies metamorphic rocks as well as from a variety of mafic to acid intrusive and extrusive igneous rocks our analysis shows that the pressure dependence is poorly constrained and the equilibria are not suitable for barometry

depletion of light rare earth elements in felsic magmas contrary to simple generalizations about their behavior light rare earth elements lree do not act as incompatible elements in very felsic magmas in fact lree concentrations typically decrease often drastically during differentiation of such magmas the simplest explanation for this depletion involves the separation of minute easily overlooked quantities of an extremely lree rich accessory mineral either monazite or allanite our data indicate that felsic liquids with 50 ppm lree may be saturated in either of these accessories and that the concentration required for saturation decreases in increasingly felsic liquids this accounts for incompatible behavior of lree even at high concentrations in mafic magmas in contrast to compatible behavior at low concentrations in felsic magmas partitioning of lree into solid rather than liquid has important implications for trace element and nd isotope modeling of crustal anatexis as well as for magma differentiation

kinematics of large syn orogenic intrusions example of the lower proterozoic saraya batholith eastern senegal the large saraya batholith intruded the lower proterozoic sequences of eastern senegal structural analysis indicates that this large granitic batholith corresponds to a composite body consisting of several coalescent plutons and interfering diapirs interpretation of the structural fabric patterns in these plutons and in the host rocks constitutes the basis of a hypothetical model of initiation ascent and emplacement as follows 1 development of a deep n s granitic ridge controled by a major crustal fracture generated by an earlier extensional phase re activated by a strike slip fault which probably would facilitate segregation collection and ascent of magmas 2 intrusion and splitting up of the ridge into several diapirs which ascended simultaneously and separately in a setting of a n s sinistral transcurrent deformation inducing a transverse nw se shortening and forceful emplacement of plutons and 3 final emplacement characterized by interference between regional deformation and pluton ballooning the latter leading to the coalescence of intrusions and to the formation of a larger apparently homogeneous body

major and trace element geochemistry of the archaean ntem plutonic complex south cameroon petrogenesis and crustal evolution the ntem complex crops out in south cameroon and belongs to the congo craton it corresponds to a 2 8 ga basement which in the northern part is made up of plutonic rocks 1 charnockites and 2 soo tonalites both have similar compositions and define a typical tonalite trondhjemite and granodiorite ttg suite the study of isotopes isr major elements and ree allows us to propose a petrogenetic model that may summarized as follows 1 1 partial melting of the mantle gave rise to tholeiites 2 2 tholeiites transformed into garnet bearing amphibolites underwent partial fusion thus generating the parental magma of the ttg suite 3 3 fractional crystallization mainly of hornblende and plagioclase produced the differentiated ttg suite this scheme is typical of archaean ttg suites and is supposed to have occurred in a subduction like geodynamic environment where because of the high archaean geothermal gradients the subducted oceanic crust underwent fusion before dehydration operated it is also possible to show that the ttg suite was not emplaced during the granulite facies metamorphism but that this occurred later affecting only some parts of the basement consequently the ntem complex appears to be an archaean high grade terrane where the granulitic assemblages may be early proterozoic in age characteristic that seems to be more widespread among archaean high grade terranes

very low frequency earthquakes along the ryukyu subduction zone 1 a total of 1314 very low frequency earthquakes vlfes were identified along the ryukyu trench from seismograms recorded at broadband networks in japan f net and taiwan bats in 2007 the spectra of typical vlfes have peak frequencies between 0 02 to 0 1 hz among those waveforms from 120 vlfes were inverted to obtain their centoroid moment tensor cmt solutions and locations using an examination grid to minimize a residual between the observed and synthetic waveforms within an area of 11 14 in latitude and longitude and at depths of 0 to 60 km most of the vlfes occur on shallow thrust faults that are distributed along the ryukyu trench which are similar to those earthquakes found in honshu and hokkaido japan the locations and mechanisms of vlfes may be indicative of coupled regions within the accretionary prism or at the plate interface this study highlights the need for further investigation of the ryukyu trench to identify coupled regions within it

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

structural and gold mineralizing evolution of the world class orogenic mana district burkina faso multiple mineralizing events over 150 million years abstract the mana district located in the northern part of the birimian hounde greenstone belt in western burkina faso is a world class paleoproterozoic orogenic gold district 8 moz including five gold deposits fofina nyafe siou wona kona and yaho these deposits are located in specific lithostratigraphic domains and gold is controlled by various structural features deposit and regional scale mapping intrusion age and geochemistry as well as airborne aeromagnetic and electrical resistivity geophysical data were used to decipher the tectonic evolution of each gold deposit and the district five deformational and four gold mineralizing events were recognized the first deformation event d1md e w oriented shortening affected the metamorphosed volcanic and sedimentary rocks of the lower birimian group this early deformation episode was correlated with the formation of gently n plunging folds f1md and n s striking thrusts faults coeval with emplacement of the pre to synkinematic wona kona and siou plutons dated at 2172 ma under greenschist facies metamorphism the quartz carbonate veins v1md at fofina and siou formed during d1md at eoeburnean time manifesting the first gold event at approximately 2172 ma the following deformation event d2md e w oriented extension is associated with the deposition of the upper birimian group mana basin overlying the lower birimian group the geometry of the mana basin is controlled by the mana and maoula shear zones the tarkwaian type rock formation overlying the upper birimian group controlled by the wona kona and siou shear zones is constrained at the end of d2md or at the beginning of the d3md event with a maximum deposition age at 2113 ma the third deformation event d3md e w to wnw ese transpression affected the entire supracrustal rock such event is correlated with the formation of map scale f3md folds and dextral shear zones during the eburnean orogeny 2113 2090 ma a second gold mineralizing event occurred during d3md and is manifested by quartz carbonate veins v3md and disseminated sulfides at the yaho fofina and nyafe and possibly wona kona deposits the fourth deformation event d4md nnw sse transpression is correlated with sinistral shearing along the major transcurrent faults and the development of asymmetric nne striking folds f4md associated with vertical fold axes syn d4md mineralization is characterized mainly by a strong silicification si4md with disseminated pyrite and arsenopyrite along the wona kona shear zone and by tiny quartz carbonate veinlets v4md this event is considered the main gold bearing event in the western margin of the mana district the fifth and last deformation event d5md is brittle in character and was responsible for the formation of e w subvertical crenulation cleavages and reverse faults under overall n s shortening this late deformation event is tentatively associated with a last gold event recorded as free gold associated with muscovite in brittle fractures developed in competent orebodies at the wona kona and siou deposits this event could be as young as 2022 ma the age obtained from ar ar datation of muscovite schists at the wona kona deposit our main contribution is that we decipher multiple gold mineralizing events at the district scale based on deposit and regional scale mapping it is interpreted that gold was introduced as early as 2172 ma and possibly as late as 2022 ma during at least 3 or even 4 shortening tectonic events in a timeframe not yet recognized at the district scale for all the birimian belts

experimental deformation of partially molten westerly granite under fluid absent conditions with implications for the extraction of granitic magmas the mechanical behavior of partially molten westerly granite was investigated in the temperature range 800 1100 c 250 mpa confining pressure by means of constant strain rate creep and stress relaxation tests the only water in the samples came from the breakdown of hydrous phases biotite minor chlorite and muscovite and alteration products of feldspars thus the amount of melt was controlled by the test temperature and ranged from 3 at 800 c to 50 at 1100 c over that temperature range strength decreased from 500 mpa to less than 1 mpa and a preliminary constitutive flow law for the partially molten rock was obtained to allow extrapolation to low strain rates the comparative viscosity of the melt alone was estimated at 950 and 1000 c from the distance it could be made to penetrate into a porous sand under a known pressure gradient under all conditions deformation of the matrix of solid grains was by brittle fracture only samples containing up to 10 vol melt failed with the formation of a shear fault zone at higher melt fractions melt filled pores collapsed by shear enhanced compaction squeezing the melt into axial cracks above 40 vol melt unfractured solid grains were carried about passively in the flowing liquid there was no evidence of a rheologically critical melt percentage in this system by analogy with the uniaxial compaction of water saturated soils a simple model is erected to describe a two stage process for the extraction of granitic melts from their protoliths with the aid of nonhydrostatic stress shear enhanced compaction is inferred to drive melt into a network of melt filled veins whereupon porous flow through the high permeability vein network allows rapid drainage of melt to higher crustal levels

origin of high potassium talc alkaline i type granitoids many i type granitoid magmas are generated through partial melting of older metaigneous rocks and the compositions of such melts are broadly calc alkaline and metaluminous these melts are granitic to tonalitic and result from thermal extremes in their lower crustal source regions data on the experimental partial melting of common crustal rocks suggest that high k i type granitoid magmas can be derived only from the partial melting of hydrous calc alkaline to high k calc alkaline mafic to intermediate metamorphic rocks in the crust because of their low k2o contents metabasaltic rocks of all kinds are unsuitable as sources and models that propose mixing of mantle derived basaltic magmas and crustal melts are also inadequate there is no requirement that i type calc alkaline magmatism be related in any way to subduction processes

an experimental deformation study of partially molten amphibolite application to low melt fraction segregation an amphibolite has been experimentally deformed under subsolidus and partially molten conditions to evaluate the influence of melt on the mechanical behavior of a natural mafic rock and to assess low melt fraction segregation processes experiments were performed under fluid absent conditions at 1 8 gpa between temperatures of 650 c and 1000 c these conditions are similar to those of thickened lower continental crust or intermediate depths in subducting oceanic lithosphere at 850 c melt is granitoid in composition and its viscosity is that of wet granite 103 105 pa s the results can be summarized as follows 1 under subsolidus conditions tests at 650 c and 750 c the amphibolite is macroscopically ductile and deformation is homogeneously distributed throughout the sample 2 at near solidus conditions 800 c 0 5 vol melt fractures 1 10 m in width displace hornblende and plagioclase grains and melt formed in situ is found in some of these cracks the formation of a ductile shear zone in one sample is attributed to the presence of very fine grained reaction products from combined dehydration hydration reactions that involve plagioclase the dehydration reaction products have apparently changed the deformation mechanism and even overrode the melt embrittlement process trapping melt in pockets of lower strain with higher melt fractions 10 15 vol broad melt bearing shear zones form and grains within these zones are brittlely deformed 3 at 20 vol melt 1000 c additional weakening occurs but fractures are not observed and the overall deformation is by viscous flow the results show that at low melt fractions 15 vol fluid absent melting reactions can induce fracture in previously ductile rocks this suggests that the percentage at which melt may escape by fracture is lower than the theoretical critical melt fraction or cmf of 26 40 vol in general estimating the fraction of melt at the onset of segregation cannot be predicted by the cmf melt segregation models have to be adapted to variations in such major factors as pressure temperature type of melting reaction rate of melting and strain rate such models will greatly assist our understanding of continental growth and evolution during orogenesis

intrusion related affinity and orogenic gold overprint at the paleoproterozoic bonikro au mo deposit cote d ivoire west african craton the bonikro gold deposit and satellite hire open pits are located at the southern tip of the oume fettekro granite greenstone belt in cote d ivoire country rocks in the region have undergone polyphase deformation and prolonged arc magmatism during the paleoproterozoic eburnean orogeny intrusive host rocks at hire and bonikro have been dated at 2180 6 and 2086 4 ma u pb on zircon respectively these plutonic bodies acted as favourable sites for fluid flow due to their brittle rheological characteristics gold mineralisation at bonikro is hosted by i a sheeted quartz vein array characterised by an au w bi te ag metal association in the cupola of a porphyritic granodiorite and ii an overprinting swarm of fault fill smoky quartz molybdenite veins gold mineralisation at hire is shear hosted and shares a similar relative timing with the latter set of auriferous veins this second phase of gold mineralisation occurred during a period of transcurrent tectonics late in the eburnean tectono magmatic history soon after the cessation of compressional deformation it is responsible for the bulk of gold resources in the region e g bonikro hire agbahou and has been dated at 2074 16 ma re os on molybdenite the integration of structural paragenetic relationships with high precision dating of magmatic and hydrothermal events highlights the superposition of late eburnean orogenic gold mineralisation on an earlier intrusion related gold system linked to the local emplacement of a porphyritic granodiorite at bonikro and illustrates the genetic diversity of paleoproterozoic granitoid hosted gold mineralisation in the west african craton it also further supports that gold mineralisations in the west african craton occurred diachronously throughout the eburnean orogeny through a variety of deposit types including au skarn intrusion related au orogenic au and porphyry cu au

melt segregation in the continental crust before melt will segregate from its restite and source there must be both a driving force and space into which the melt can move the continental crust is anisotropic and in most cases is subject to deviatoric stresses during melting under these conditions melt segregation begins when an interconnected network of melt forms and the source becomes permeable melt is driven by pressure gradients from its restite toward dilatant sites set up in the heterogeneously deforming source the source is continually drained of melt and so need never contain more melt than that required to maintain permeability 26 where the solid matrix breaks down and diapirism can occur

petrogenesis of juvenile type birimian paleoproterozoic granitoids in central cote d ivoire west africa geochemistry and geochronology abstract the birimian paleoproterozoic granitoids of central cote d ivoire are associated either with greenstone belts or with basin sedimentary rocks field data indicate two stages of granitoid emplacement which according to singlezircon and monazite 207 pb 206 pb age determinations occurred within a short period of time the first generation of granitoids 2123 2108 ma intruded and metamorphosed the greenstone belts whereas the second generation 2108 2097 ma intruded and metamorphosed the sedimentary pile and subordinate andesitic volcanic rocks of the bandama basin unconformably overlying the greenstone belts geochemical and isotopic data show that the various granitoids belong to four magmatic groups 1 sodic calc alkaline nacg 2 potassic calc alkaline kcg 3 peraluminous alg and 4 alkaline alkg all the granitoids show juvenile features with mantle like low initial sr isotopic compositions 0 700 0 702 and positive epsilon nd values 1 5 2 7 these data indicate that a the nacg may have formed by partial melting of a garnet amphibolite crust b the kcg and alkg derived from partial melting of crustal mafic to intermediate metaigneous rocks and c the alg epsilon nd 1 5 represent infracrustal melting of young sialic igneous materials during a regional crustal shortening in a transcurrent tectonic context sm nd model ages 2 25 2 4 ga preclude the existence of an archean crust reworked in the birimian magmas an underplating model is discussed

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

partial melting of apatite bearing charnockite granulite and diorite melt compositions restite mineralogy and petrologic implications melting experiments p 6 9 kbar t 850 950 c nno 02 hm were done on mafic to felsic charnockites a dioritic gneiss and a felsic garnet granulite all common rock types in the grenville basement of eastern north america a graphite bearing granulite gneiss did not melt water h2o 0 60 to 2 0 wt is bound in low grade retrograde metamorphic minerals and is consumed during the earliest stages of melting most melts are water undersaturated melt compositions range from metaluminous silicic granodiorite diorite starting composition to peraluminous or weakly metaluminous granites all others in general liquids become more feldspathic less silicic and less peraluminous and are enriched in feo mgo and tio2 with increasing temperature residual feldspar mineralogy controls the cao k2o and na2o contents of the partial melts and the behavior of these elements can be used particularly if the degree of source melting can be ascertained to infer some aspects of the feldspar mineralogy of the source k feldspar a common restite phase in the charnockite and granulite but not the diorite should control the behavior of ba and possibly eu in these systems and yield signatures of these elements that can distinguish source regions and in some cases bulk versus melt assimilation apatite a common restite phase is enriched in rare earth elements ree especially middle ree retention of apatite in the restite will result in steep light ree enriched patterns for melts derived from the diorite and charnockites

manaslu leucogranite a collision signature of the himalaya a model for its genesis and emplacement the manaslu pluton is one of 10 leucogranites that formed in the overthrusted higher himalaya after the indo eurasian collision field and analytical data indicate that the underlying migmatites of the tibetan slab may be where the leucogranitic melts were generated the himalayan crustal thrusting of a hot slab over a rather cold volcano sedimentary pile le fort 1975a provides the necessary release of fluids these fluids cross the main central thrust mct induce the partial anatexis of the overheated tibetan slab and produce a leucogranitic magma the emplacement of the magma at first is located along the main disharmonic plane above the mct between the infrastructure and the superstructure there it generates a convective hydrothermal system extending very far laterally according to the stratification of permeabilities progressive emplacement of the granite proceeds as the convected fluids including the fluids released by the saturated magma dissolve the mainly calcareous host rocks of tibetan sedimentaries a frozen image of this caving out progression is given by the extensive network of granitic dikes outside the pluton the two quite independant fluid cycles of the generation and of the emplacement were triggered or guided by the tectonics due to collision this two fold model dominated by fluid activity may be of importance for other leucogranites and granites

the palaeoproterozoic birimian of haute comoe in the west african craton ivory coast a transtensional back arc basin abstract deformation in the palaeoproterozoic of haute comoe ivory coast 2150 ma to 1800 ma birimian or approximately orosirian period according to the new precambrian time scale of plumb 1991 was controlled by ductile faults trending n s n 25 e and n 130 e of which at least the first and third sets were essentially transcurrent faults the volcanosedimentary and subordinate mainly tholeftic to calc alkaline volcanic rocks of the tehini succession serie de tehini that characterise the haute comoe belt overlie detrital sedimentary rocks and are overlain by the thick comoe succession of sedimentary rocks serie de la comoe geochemically the tehini succession has affinities with ensialic rift rocks in the south and with rocks of back arc basins in the north this model differs from the oceanic plateau setting proposed by abouchami et al 1990 for the birimian of west africa tehini and comoe successions have been affected throughout only by transcurrent type eburnean deformations d2 and d3 of milesi et al 1989 the sediments are moderately folded weakly metamorphosed and strongly marked by ductile strike slip deformation the sequence could represent a rift type or transtensional back arc basin established after 2200 ma and within the burkinian crust that had previously been deformed in an early palaeoproterozoic orogenic cycle at 2400 to 2150 ma lemoine et al 1985 tempier 1986 burkinian crust specifically dabakalian basement granitoids of rhyacian to siderian age of plumb 1991 was the source of the detrital sediments which were deposited at the base of the birimian and has been almost entirely consumed during the massive intrusion of calc alkaline plutons the s1 s2 and s3 cleavages are marked by greenschist facies mineral assemblages this generally low grade metamorphism is related to the emplacement of early granodiorites the deformation particularly s1 cleavage which has grown in conformity with the stratification is closely related to granitic plutons we regard this s1 cleavage as unrepresentative of a regional horizontal shortening phase if the basin was oriented obliquely to the direction of closure the presence of oceanic material and the lack of evidence for thrusting in a closed marginal basin can be explained by transcurrent shearing it is probable that the basin controlled by major intracontinental faults was deformed at 2075 ma resulting in the eburnean tectonometamorphic phases d2 and d3 by nw se compression and strike slip shearing a terminal phase of ne sw compression superimposed n 120 e faulting and crenulation cleavage s4 and ssw thrusting upon the earlier deformation fabrics

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

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

mechanisms of pluton emplacement and structural evolution of a 2 1 ga juvenile continental crust the birimian of southwestern niger abstract the 2 1 ga birimian terranes of southwestern niger consist of a granite greenstone association the structures of the greenstone belts result from the interference between pluton emplacement and regional transcurrent deformation sinistral n s strike slip faults ne sw trending schistosity the regional cleavage trend is observed to change in the vicinity of elliptical plutons cleavage trajectories bend around their contacts resulting in triple junctions at the pluton extremities and narrow aureoles of intense penetrative schistosity and high grade amphibolitefacies metamorphism are developed around them structural features of plutons suggest syntectonic emplacement and age data indicate that all plutons were emplaced between 2 16 and 2 10 ga the plutons are assembled in two distinct types of batholith elongated either parallel or perpendicular to the inferred regional shortening direction a batholith parallel to the inferred regional shortening direction dolbel batholith consists of km sized subcircular alkali granite plutons except for the southernmost one which is of a larger size and exhibits an elliptical shape with a nw se short axis parallel to the shortening direction crosscutting relationships indicate a progressively northward emplacement trend for those plutons internal fabrics generally correspond to layering and magmatic flow fabrics and accessorily to schistosity and shear zones which represent late stage plastic deformation restricted to the pluton margin batholiths which are perpendicular to the inferred regional shortening direction are composed of large elliptical plutons 500 to 5000 km 2 in area consisting mainly of granodiorite and covering 70 of the region the plutons never show crosscutting relationships but are moulded against each other their internal fabric corresponds to a solid state foliation it either dips towards the centre of the pluton funnel or teardrop shaped intrusions or is subhorizontal in the centre and vertical at the margin dome shaped intrusions the foliation and the geometry of the largest pluton tera pluton were acquired during pluton ascent and inflation whereas the geometry of later shear zones indicates that deformation in the pluton was then passively acquired at the final emplacement level due to regional deformation the contrast in both the orientation and structural features of the batholiths reflects distinct emplacement mechanisms the small alkali granite plutons resulted from magma injection within nw se fractures parallel to the inferred regional shortening direction whereas the granodiorite plutons are assumed to be emplaced as diapirs forceful intrusions with long axes perpendicular to the shortening direction these two intrusion mechanisms for plutons emplaced during the same deformation event and at the same crustal level are considered to reflect a local change in the rheological behaviour of the crustal segment studied in connection with the thermal effect induced by the large granodioritic plutons this vertical tectonic regime appears to be typical of palaeoproterozoic juvenile continental crust areas and related to an interplay between emplacement of tremendous volumes of granodioritic magma and a regional transcurrent deformation

crustal structure of southern burkina faso inferred from magnetotelluric gravity and magnetic data abstract understanding the architecture of the west african craton at depth is essential to be able to reconstruct its evolution our study focuses on the crustal imaging of structures and geometries characterizing the crust of the leo man shield with broadband and long period magnetotelluric data collected in southern burkina faso and covering a 220 km long profile the resulting 3d resistivity crustal model highlights the distribution of the granite greenstone assemblages with depth showing excellent correlation with mapped surficial lithologies the whole crust of southern burkina faso is resistive with lateral as opposed to vertical major resistivity contrasts reflecting the location of major scale shear zones characterizing this part of the baoule mossi domain ground gravity data acquired along the same line as the mt data were also modeled and show relatively good correspondence with the resistivity model the new resistivity and gravity models compared with results from joint inversion of gravity and aeromagnetic data highlight significant changes between the greenstone belts and granitoid domains along the profile the comparison of the geophysics with the geology enables us to define new depth constraints of the main tectonic features in the area the observed large scale dipping shear zones favour the model of crustal building through major parallel thrust faults

ree geochemistry and isotopic data of archean silicic volcanics and granitoids from the pilbara block western australia implications for the early crustal evolution eighteen silicic volcanic rocks of the warrawoona group and ten associated plutonic rocks from the pilbara block western australia have been chosen for geochemical and isotopic studies silicic volcanics of the unsb upper member of north star basalt are dated at 3 56 3 57 by both the rb sr and the sm nd methods the respective 1 initial isotopic composition values are 0 7005 5 sr and 0 50810 39 nd this age is consistent with the stratigraphic interpretation that the talgatalga subgroup in which the north star basalt occupies the lowermost position is overlain by the duffer formation whose age was earlier established at 3 45 by the zircon u pb method the new rb sr data on six silicic lava samples from the duffer formation yield an isochron of 3 23 0 28 2v though imprecise this age agrees with the zircon age within error limits rb sr ages of 2 3 2 4 obtained for the panorama rocks and the wyman formation do not correspond to their initial eruption ages chemical arguments suggest that these ages represent the time of metasomatism associated with the widespread thermal event in this region about 2 3 2 4 ago geochemically most of these analyzed rocks volcanic and plutonic are of tonalite trondhjemitegranodiorite ttg composition a typical feature found in many other archean terrains they generally show fractionated ree patterns except the panorama formation rocks furthermore the wyman formation rhyolites and the post tectonic adamellites show significant negative eu anomalies suggesting a similar mode of magma generation and a probable genetic link theoretical considerations suggest that most of these ttg rocks could have been generated by partial melting of amphibolitic or basaltic sources followed by fractional crystallization although the archean granitic gneisses often possess mantle like isr values the trace element data indicate that they could not have been derived by direct melting of upper mantle materials the immediate tectonic implication is that in any archean terrain the formation of na rich continental crust of ttg suite must be preceded by the presence of basaltic crust the occurrence of this basaltic crust is a matter of controversy such crust might have been totally destroyed by repeated melting processes or its remnants are now represented by some of the mafic ultramafic enclaves within the tonalite trondhjemite batholiths

40ar 39ar thermochronology of paleoproterozoic granitoids of northeast burkina faso west african craton implications for regional tectonics abstract the greenstone belts of northeast burkina faso in the west african craton wac comprise birimian meta sedimentary and mafic ultramafic complexes that are intruded by granitoids and late dolerite dykes the granitoid greenstone belt assemblages underwent at least two phases of deformation that have resulted in their principal nw and ne trends we performed a detailed 40ar 39ar dating together with zircon u pb ages in order to unravel the thermo chronological history of the studied granitoids our previous zircon u pb data indicated that the granitoids were emplaced between 2138 7 and 2181 7 ma this period is well known for crustal growth in the wac the emplacement of granitoids was concomitant with d1 or the tangaean event 2130 2170 ma amphibole and micas yield two 40ar 39ar age groups the first group 2046 12 2132 13 ma constrained the d2 deformation event and the second group 1918 24 to 1984 12 ma constrained the d3 or wabo tempelse event considering closure temperatures of the isotopic systems the u pb ages and the present 40ar 39ar dating results suggest that after emplacement the granitoids cooled to 315 c at ca 2020 ma with a mean cooling rate of 6 5 c ma corresponding to a mean uplift rate of 0 65 km ma the additional k feldspar 40ar 39ar data indicate a later slow cooling stage with a rate of 0 54 c ma corresponding to a mean uplift rate of 0 054 km ma subsequent to the end of the major crustal growth in the wac at 2000 ma

constraints on melting and magma production in the crust major intrusions of granitic rocks are found in several tectonic settings and in all cases crustal melts may contribute to the volumes of granitic magma high grade metamorphism and partial melting of the crust take place predominantly under fluid absent conditions we present a model for calculating the amounts of melt that may be formed by fluid absent breakdown of micas and amphiboles in common crustal rock types pelitic quartzofeldspathic intermediate and mafic melt proportions depend mainly on the kind of source rock the pressure at which melting takes place the temperature and the hydrous mineral h2o content of the source as a consequence of the pressure dependence of water solubility in silicate melts any given source rock will produce more melt by a given fluid absent reaction at lower pressure at a given pressure higher temperature reactions can produce more melt from a given source rock based on a survey of the compositions of common rock types we show that the amounts of melt can vary from 50vol thus crustal rocks vary widely in their fertility as magma sources depending on the types and amounts of hydrous phases they contain in general muscovite breakdown in pelites will yield only small quantities of melt and lead to migmatite formation biotite breakdown in pelites occurs at higher temperature and because most high grade pelites below granulite grade are biotite rich can yield up to about 50 vol of granitoid melt rocks of intermediate composition and hornblende rich mafic rocks are potentially highly fertile magma sources also provided that the high temperatures necessary for biotite and hornblende breakdown are realized pyroxene rich mafic rocks and quartzofeldspathic rocks are much less potentially fertile data suggest that mechanisms exist for the efficient segregation of melt and restite in systems with 30and probably 20vol melt the pressure temperature history of a region can greatly influence crustal source fertility through its control over the occurrence of subsolidus dehydration and melting equilibria

geochemical comparison between himalayan and hercynian leucogranites abstract isotopic sr pb nd o ree and trace element data from three himalayan nepal and six hercynian brittany france leucogranites are compared for the himalayan granites makulu mustang and especially manaslu the 87sr 86sr versus 87rb 86sr scatter diagrams and hence variability of the initial 87sr 86sr ratios reflect heterogeneity of the source materials these ratios are very high 0 730 0 770 for manaslu the very high 207pb 204pb ratios 15 8 at the corresponding 206pb 204pb ratio of 18 8 the very low 143nd 144nd ratios i nd 13 to 17 and the very high 18o 16o ratios 12 confirm the crustal origin with the paragneisses of the tibetan slab formation i as the probable parent source for the six hercynian granites with an individual rb sr isochron established for many of them the initial 87sr 86sr ratios range from 0 706 to 0 717 the lead neodymium and oxygen isotope compositions confirm their crustal source of presumably palaeozoic age but not specifically identified the rare earth patterns and thorium contents are comparable to those for himalayan granites the similarities of the geochemical signatures isotopes and trace elements of the himalayan and hercynian leucogranites suggest that the petrogenetic processes were comparable and that the himalayan continental collision model can be applied to the hercynian the depletion in ree and th implies either interaction of the granitic liquids with fluid or more probably early precipitation of monazite the increase in sm nd in the granites compared to their parent materials implies that the neodymium model ages of primary mantle extraction are maximum rather than minimum ages in contrast to the hercynian leucogranites the extremely radiogenic sr of the himalayan leucogranites reflects the very long crustal residence time before the melting event the apparent uniformity of the initial 87sr 86sr ratios of the individual hercynian granites may be related to an aging effect where the straight line fitting improves with increase in age of the pluton

geochemical and isotopic disequilibrium in crustal melting an insight from the anatectic granitoids from toledo spain in the hercynian anatectic complex of toledo act the anatectic granitoids include leucogranitic leucosomes leucogranitic massifs and restite rich granites they show a broad range of initial sr and nd isotope ratios from 0 711 to 0 720 and 0 51164 to 0 51203 respectively which clearly indicate the absence of isotopic homogenization in the melts broadly the ranges reflect the isotopic variation of the metapelitic protoliths if crustal melting occurs under water undersaturated conditions as is the case of the act the generated melts do not isotopically and chemically equilibrate with the granulitic residuum the preservation of heterogeneities could arise through a number of processes 1 duration of the process in which the presence of melts with disequilibrium features and the high solid content of several of the granites in the act point to a very short lived magmatic system 2 limited diffusion in which poorly segregated and volatile undersaturated granitoids have a very restricted opportunity for isotopic and chemical homogenization and 3 magma dynamics in which the low melting rates the high restite and low water contents and the silica rich composition of these granites together with the small volume and high viscosity values of the melts seriously restrict the physical processes leading to homogenization

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

origin and early growth rate of continents abstract data suggest that early continents formed over mantle sinks and grew rapidly between 2 7 and 3 0 ga ago nd isotopic results indicate that archean tonalites which are largely responsible for early continental growth are derived chiefly from ancient depleted mantle sources trace element and oxygen isotope data however indicate enriched sources for such tonalites these constraints can be explained if the mantle is fractionated into depleted dmr and enriched emr reservoirs during early degassing of the earth 4 4 ga blocks of emr are recycled back into the mantle and the emr dmr mixtures serve as sources for archean basalts these basalts are enriched in incompatible elements by deep sea alteration metasomatism in sinks or by other mantle processes and then undergo wet partial melting to produce tonalite magmas enrichment must occur not more than 150 ma prior to tonalite production basalt plateaus accumulate at sinks and as cooling continues geotherms at sinks pass into the eclogite stability field dense eclogite roots form on the basalt plateaus which pulls them into the mantle between 2 7 and 3 0 ga ago resulting in rapid tonalite production and thus in rapid continental growth

melt segregation in migmatites anatectic stromatic migmatites have a symmetrical layered structure with a low ratio of thickness to length and a periodicity features that have not been explained satisfactorily but which are related to physical processes of melt segregation we evaluate the compaction model for segregation as it applies to migmatites and develop models for melt segregation based upon convection driven by volume change and upon advection down pressure gradients that result from applied differential stress acting on an anisotropic multilayer protolith compaction by gravity driven two phase flow results in asymmetric separation yields calculated segregation times that are slow or geologically unreasonable and yields relative volumes that are not consistent with the abundance of leucosome seen in natural migmatites although calculated segregation times for very wet granite might allow segregation by compaction some driving force in addition to gravity is needed to cause widespread melt segregation in the crust for segregation at low volume fraction of melt we develop a two dimensional two phase matrix and fluid viscous flow cell model length much greater than thickness with phase changes which predicts either expansion or contraction convection depending on whether volume change vr is positive or negative convection leads to melt segregation to form a stromatic structure in a geologically reasonable timescale at moderate volume fraction of melt segregation may occur by filter pressing in compositionally layered rocks in response to applied differential stress melt migration is by porous medium flow driven by differences in mean normal stress between the layers as a consequence of differences in rheology and shear enhanced matrix collapse calculated segregation rates are fast and the model yields adequate volume of leucosome the positive vr for volatile phase absent melting reactions under crustal pressures promotes melt escape unless extensional deformation facilitates melt accumulation if the rate of melt production exceeds the rate of melt escape then the increase in melt pressure may lead to melt enhanced embrittlement and fracture and melt may migrate out of the system in contrast water rich volatile phase present melt producing reactions have a negative vr which promotes melt retention after segregation unless deformation generates melt escape pathways evolution of stromatic structure through anatectic erosion of mesosome by increasing the melt fraction in situ may lead to breakdown of the solid matrix and will lead to instability due to buoyancy and magma may become mobile and entrain residual material restite

isotopic study of the manaslu granite himalaya nepal inferences on the age and source of himalayan leucogranites a detailed isotopic study of the manaslu leucogranite was carried out a u pb age of 25 ma and a whole rock rb sr age isochron of 18 ma were obtained suggesting that the magmatic activity lasted at least 7 ma initial sr isotopic ratios are very high 0 740 to 0 760 and initial nd isotopic ratios are low nd in 13 to 16 and they show the existence of large isotopic variations even at the metre scale these are not the result of perturbations by fluids but rather they reflect the initial isotopic heterogeneity of the source material which has not been obliterated by magmatic processes e g fusion mixing by convection these results also support the crustal origin of this leucogranite the tibetan slab paragneisses whose sr and nd isotopic ratios are very similar to those of the granite at an age of 20 ma are the most probable parental material nd model ages for both the leucogranite and the gneisses are in the range 1 5 2 ga a model of formation of the manaslu granite by coalescence of different batches of magma is in agreement with the present data

geologic evolution of the serrinha nucleus granite greenstone terrane ne bahia brazil constrained by u pb single zircon geochronology abstract u pb single zircon crystallization ages were determined using tims and sensitive high resolution ion microprobe shrimp on samples of granitoid rocks exposed in the serrinha nucleus granite greenstone terrane in ne brazil our data show that the granitoid plutons can be divided into three distinct groups group 1 consists of mesoarchaean 3 2 2 9 ga gneisses and n s elongated ttg tonalite trondhjemite granodiorite plutons with gneissic borders group 2 is represented by ca 2 15 ga pretectonic calc alkaline plutons that are less deformed than group 1 group 3 is ca 2 11 2 07 ga late to post tectonic plutons shoshonite syenite k rich granite and lamprophyre groups 2 and 3 are associated with the transamazonian orogeny xenocryst ages of 3 6 ga the oldest zircon yet recorded within the sao francisco craton are found in the group 3 euclides shoshonite within the uaua complex and in the group 2 quijingue trondhjemite indicating the presence of paleoarchaean sialic basement group 1 gneiss migmatitic rocks ca 3200 ma of the uaua complex constitute the oldest known unit shortly afterwards partial melting of mafic material produced a medium k calc alkaline melt the younger santa luz complex ca 3100 ma to the south subsequent ttg melts intruded in different phases now exposed as n s elongated plutons such as ambrosio 3162 26 ma araci 3072 2 ma requeijao 2989 11 ma and others which together form a major part of the archaean nucleus some of these plutons have what appear to be intrusive but are probably remobilized contacts with the transamazonian itapicuru greenstone belt the older gneissic rocks occur as enclaves within younger archaean plutons thus serial additions of juvenile material over a period of several hundred m y led to the formation of a stable micro continent by 2 9 ga evidence for neoarchaean activity is found in the inheritance pattern of only one sample the group 2 euclides pluton group 2 granitoid plutons were emplaced at 2 16 2 13 ga in a continental arc environment floored by mesoarchaean crust these plutons were subsequently deformed and intruded by late to post tectonic group 3 alkaline plutons this period of transamazonian orogeny can be explained as a consequence of ocean closure followed by collision and slab break off the only subsequent magmatism was kimberlitic probably emplaced during the neoproterozoic braziliano event which sampled older zircon from the basement

a new metamorphic constraint for the eburnean orogeny from paleoproterozoic formations of the man shield aribinda and tampelga countries burkina faso abstract progressive metamorphic zones developed in pelites around the aribinda granite burkina faso indicate contact metamorphism at 5 6 kbar biotite chloritoid staurolite kyanite zones are distributed concentrically around the aribinda pluton on its northern margin but are displaced by later thrust faults in the south of the aribinda area and in the nearly tampelga area these metamorphic and structural characteristics provide new constraints for the geodynamical evolution of the eburnean orogeny in the north part of the baoule mossi domain they show the leading part of the granitoid emplacement in the thermic structuration of the greenstone belts and the post metamorphic character of the tangential tectonic in burkina faso the eburnean orogeny underwent 1 initial greenschist facies metamorphism 2 emplacement of granitoids at mid deep crustal levels leading to unusual medium pressure metamorphism 3 later thrusting

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

geochemical investigations of the origin of the manaslu leucogranite himalaya nepal the himalayan orogeny is characterized by an intraplate subduction zone the thrusting of the tibetan slab over the foreland is probably responsible for the origin of the manaslu type leucogranites this geochemical study tries to examine the crustal origin of the granite to verify whether the paragneisses of the tibetan slab are the most probable parent material and to determine the subsequent petrogenetic processes the rb sr systematics suggest that 1 1 the scatter of data points in the 87sr86srvs 87rb86sr diagram may be due to the heterogeneity of the initial 87sr86sr ratios 2 2 the initial isotopic ratios are very high 0 730 0 760 and are in agreement with those of the gneisses of the tibetan slab at the time of the melting pb isotopic ratios are homogeneous they are also consistent with a crustal prehistory the ree and th contents are unexpectedly low in comparison with the much higher contents of the parent material monazite left in the residue of melting or more probably extracted in an early stage of the magmatic history may explain the observed ree and th abundances alternatively field observations show that the emplacement of the granite has been accompanied by a very strong hydrothermal activity in consequence the low ree and th contents of the granite could also be due to the loss by solution during prolonged fluid circulation throughout the granitic melt and the sr and pb isotope distribution to the effect of late to post magmatic fluids the isotopic character of which was determined by the underlying tibetan slab

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

phase relationships of s type granite with h2o to 35 kbar muscovite granite from harney peak south dakota muscovite granite 13 8 muscovite 4 8 normative corundum was reacted with varying percentages of h 2o in cold seal vessels at 2 kbar and in piston cylinder apparatus between 10 and 35 kbar the diagrams illustrating melting crystallization relationships are p t sections with both excess h 2o and with no h 2o added 0 66 h 2o in rock t x h 2o sections at 15 kbar and 25 kbar showing h 2o undersaturated conditions the h 2o undersaturated surface for the crystallization of quartz coesite small amounts of aluminosilicate minerals persist to higher temperatures glass compositions measured by electron microprobe from samples with 5 h 2o at 15 kbar confirm that liquids are syenitic through at least 100 c above the solidus as predicted from the effect of pressure on the residua system results are explained successfully by phase relationships involving muscovite quartz and orthoclase in k 2o al 2o 3 sio 2 h 2o with reactions depicted in a p total p e h 2o t model with special reference to the divariant surfaces in the region p e h 2o p total with reduced p e h 2o or a h 2o produced either by small amounts of h 2o and h 2o undersaturation or by co 2 h 2o mixtures subsolidus dehydration reaction temperatures decrease vapor present solidus temperatures increase and muscovite stability in presence of liquid increases in general muscovite biotite and amphibole can be precipitated from magmas containing only a few tenths per cent h 2o although the h 2o undersaturated liquids coexisting with crystals may contain 3 or more dissolved h 2o this particular granite cannot be a primary magma from mantle or subducted oceanic crust it is a possible product of partial fusion of pelitic rocks between about 20 km and 40 km depth given sufficient h2o and xenocrystal muscovite or sillimanite from the source rocks the phase relationships are consistent with the idea of s type granites but not sufficient to prove the origin of this rock additional tests require phase relationships of other associated granitic rocks and details of geochemistry geophysics and field relationships

coupled two dimensional modeling of bed evolution and bank erosion in the upper jingjiang reach of middle yangtze river abstract at the initial stage of dam operation in an alluvial river significant vertical bed incision and transverse bank erosion processes always occur in the river reach downstream of the dam with the latter being usually an integrated product of several interacting processes a coupled two dimensional 2d model for the processes of bed evolution and bank erosion has been proposed in this study which integrates a 2d morphodynamic module with a bank erosion module and a groundwater flow module in addition to the action of fluvial erosion the effect of groundwater flow on bank erosion was considered with the changes in pore water pressure and matrix suction being calculated the proposed model was calibrated and validated through simulating channel evolution processes in a 30 54 km sub reach of the upper jingjiang reach of the middle yangtze river with the effects of infiltration recharge on bank erosion and incoming flow on the central bar evolution being investigated the results show that i the proposed model had a good performance in reproducing flow and sediment factors in terms of water depth depth averaged velocity and suspended sediment concentration daily averaged discharge and river stage and flow and sediment diversion ratios in local anabranching subreaches ii the proposed model correctly calculated the bank erosion processes in the study reach with the calculated bank retreat width being close to the measured value however it was not enough to fully reproduce the evolution of central bars indicating that a specialized and improved module may be necessary for central bar evolution and its interaction with bank erosion iii the groundwater level change lagged behind the river stage variation and it showed a more obvious phase lag and a higher value at the same river stage corresponding to a higher pore water pressure when encountering with a more rapidly changing river stage and iv the rate of infiltration recharge caused by rainfall process would generally increase the groundwater level and thus the bank erosion degree in the study reach in addition the shifting of incoming main flow might take an obvious impact on the calculation of nearby central bar evolution

genesis of the two main types of peraluminous granitoids two mica monzogranites to leucogranites and biotite rich cordierite bearing tonalites to monzogranites form two distinct groups of peraluminous granitoids they can be distinguished by mineral and rock associations and by the variation of their peraluminosity during differentiation except for the rare muscovite bearing granitoids produced by extreme fractionation or local contamination of metaluminous magmas the majority of peraluminous granitoids are produced by partial melting of crustal rocks production of either muscovite bearing granites or biotite rich cordierite bearing granitoids does not depend only on the nature of the sources but is also controlled by the physical parameters of partial melting and consequently by the way anatexis of a thickened crust is enhanced biotite rich cordierite bearing granitoids are generated where mantle derived magmas are injected into or have underplated crustal rocks two mica granites are generated where thickened crust is affected by major crustal shears or thrusts correlations between differentiation and peraluminosity indicate the dominant role of either restite unmixing or fractional crystallization in the production and evolution of the various types of peraluminous granitoids

geology geodynamics and orogenic gold prospectivity modelling of the paleoproterozoic kumasi basin ghana west africa abstract this paper describes the geology and tectonics of the paleoproterozoic kumasi basin ghana west africa as applied to predictive mapping of prospectivity for orogenic gold mineral systems within the basin the main objective of the study was to identify the most prospective ground for orogenic gold deposits within the paleoproterozoic kumasi basin a knowledge driven two stage fuzzy inference system fis was used for prospectivity modelling the spatial proxies that served as input to the fis were derived based on a conceptual model of gold mineral systems in the kumasi basin as a first step key components of the mineral system were predictively modelled using a mamdani type fis the second step involved combining the individual fis outputs using a conjunction product operator to produce a continuous scale prospectivity map using a cumulative area fuzzy favourability caff curve approach this map was reclassified into a ternary prospectivity map divided into high prospectivity moderate prospectivity and low prospectivity areas respectively the spatial distribution of the known gold deposits within the study area relative to that of the prospective and non prospective areas served as a means for evaluating the capture efficiency of our model approximately 99 of the known gold deposits and occurrences fall within high and moderate prospectivity areas that occupy 31 of the total study area the high and moderate prospectivity areas illustrated by the prospectivity map are elongate features that are spatially coincident with areas of structural complexity along and reactivation during d4 of ne sw striking d2 thrust faults and subsidiary structures implying a strong structural control on gold mineralization in the kumasi basin in conclusion our fis approach to mapping gold prospectivity which was based entirely on the conceptual reasoning of expert geologists and ignored the spatial distribution of known gold deposits for prospectivity estimation effectively captured the main mineralized trends as such this study also demonstrates the effectiveness of fis in capturing the linguistic reasoning of expert knowledge by exploration geologists in spite of using a large number of variables the curse of dimensionality was precluded because no training data are required for parameter estimation

early proterozoic crustal evolution in the birimian of ghana constraints from geochronology and isotope geochemistry abstract representative suites of ghanaian granitoids metavolcanics and metasediments were selected from birimian rock units for rb sr pb pb and sm nd analyses isotopic data on the igneous rocks provide evidence for a major early proterozoic crust forming episode which took place over a maximum time interval from 2 3 to 2 0 ga by a differention from a depleted mantle source there is little evidence for the involvement of significantly older e g archaean crust in the genesis of the igneous rock units the only exception so far found is the winneba granitoid which appears to contain a magmatic contribution from presumably underlying archaean basement sm nd model ages indicates that birimian sediments were derived from adjacent penecontemporaneous volcanic belts the birimian of ghana forms part of a major proterozoic eburnean episode of juvenile crustal accretion which has been recognised in the surrounding areas of mauretania senegal ivory coast burkina faso mali and niger where it has been dated at 2 1 2 2 ga abouchami et al the birimian terranes bridge a major gap in mantle activity and associated crustal evolution for a period considered as quiescent on other continents

the alamoutala carbonate hosted gold deposit kedougou kenieba inlier west africa the alamoutala gold deposit is located in the kedougou kenieba inlier a window of paleoproterozoic rocks that crop out in eastern senegal and western mali the deposit is part of the 3 moz au yatela district and produced 308 400 oz au between 2002 and 2012 country rocks in the alamoutala open pit consist of carbonate rocks arenites wackes and siltstones the sedimentary rocks have been subjected to polycyclic deformation and regional greenschist facies metamorphism synkinematic calc alkaline intermediate quartz feldspar porphyry stocks intruded the metasedimentary rocks at 2083 7 ma u pb zircon amphibolite facies contact metamorphism of carbonate rocks resulted in the formation of a high temperature low pressure magnetite bearing skarn field relationships and microtextural data however indicate that economic gold mineralization is shear hosted and occurred during retrograde contact metamorphism geometric and kinematic analyses indicate that the ore hosting structures were undergoing sinistral reverse displacement at the time of mineralization ore minerals deposited synchronous with a potassic hydrothermal alteration assemblage defined as biotite calcite pyrite k feldspar actinolite tremolite quartz muscovite and tourmaline petrographic studies reveal that ore related sulfides largely consist of pyrite with lesser pyrrhotite and chalcopyrite the ore also contains accessory ag sb as hg bi te mo w zn pb ni co u bearing mineral species the alamoutala deposit represents two distinct hydrothermal systems in which a locally developed magnetite bearing skarn was overprinted by a region wide orogenic gold event in the late eburnean

petrogenesis of archaean trondhjemites tonalites and granodiorites from eastern finland major and trace element geochemistry

petrogenesis of hercynian leucogranites from the southern armorican massif contribution of ree and isotopic sr nd pb and o geochemical data to the study of source rock characteristics and ages abstract ree geochemistry and isotopic pb sr nd o compositions have been studied in up to nine separate leucogranite massifs from three structurally adjacent zones in south brittany ages decrease from 340 ma in the north to 300 ma in the south whereas initial 87 sr 86 sr ratios increase from 0 706 to 0 710 0 717 ree abundances are relatively low la n 20 80 chondrites with a strong enrichment of lree relative to hree la n lu n 13 4 26 6 pb isotope ratios are homogeneous for a given massif but heterogeneous among massifs and plot very close to a normal terrestrial lead growth curve at 300 ma ago 18 o 16 o ratios are high and relatively uniform at 10 7 to 12 8 mean 11 9 u pb studies on zircon indicate the presence of an inherited radiogenic pb component and that the sediment dominated source region had been metamorphosed during the siluro devonian ligerian negativee nd t values imply that the source region had been enriched in lree prior to the melting episode based on the combined data large scale partial melting of crustal material dominantly of sedimentary origin is proposed the model age of the source rocks is not very old 600 700 to 1200 ma the pb and sr isotopic variations observed among different massifs is probably the result of age differences in the source materials of the different leucogranites