late archaean to early proterozoic granitoid magmatism and high grade metamorphism in the central limpopo belt south africa the central zone of the limpopo belt in southern africa has previously been interpreted as a segment of archaean crust which experienced its main deformation and metamorphism around 2 7 ga ago we report new single zircon u pb and pb pb ages for granitoid gneisses supracrustal rocks and anatectically derived granitic melt patches in the area around messina south africa the sand river gneiss is a composite suite of tonalitic to trondhjemitic rocks with protolith ages between 2 6 and 3 2 ga the singelele gneiss a heterogeneous granodioritic to quartz monzonitic rock has protolith ages between 2 55 and 2 58 ga since both the sand river and singelele gneisses experienced polyphase high strain ductile deformation this must have occurred later than 2 55 ga ago granulite facies pelitic gneisses of the beit bridge complex contain abundant spherical multifacetted zircons which reflect new zircon growth near or at the peak of metamorphism these zircons provide ages with a mean at 2026 5 6 3 ma which is interpreted as reflecting a high p t event 10 kbar 825 25 c granitic melt patches in the metapelites as well as in the sand river gneiss and anatectic granites are probably related to rapid near isothermal decompression to below 3 5 kbar and 600 750 c these rocks contain new magmatic zircons which yielded a mean age of 2005 6 4 4 ma and probably reflect a crustal melting event resulting from rehydration of the granulitic assemblage our zircon data support previous suggestions for only one single granulite facies event in the central zone and we suggest that this event occurred c 2027 ma ago since most of the deformation seen in the gneisses of the messina area must have occurred later than 2 55 ga ago it is likely that the limpopo orogeny at least in the central zone is not an archaean event but took place in the early proterozoic

carbonate silicate inclusions in garnet as evidence for a carbonate bearing source for fluids in leucocratic granitoids associated with granulites of the southern marginal zone limpopo complex south africa abstract we present a study of carbonate bearing polyphase inclusions in garnets from leucocratic granitoids intruding metapelitic granulites of the southern marginal zone smz of the neoarchean limpopo high grade complex south africa during the post peak stage 2710 2650 ma u pb ages for zircons and monazites ternary feldspar thermometry suggests that the granitoid magma cooled from temperatures 800 900 c at a pressure of ca 6 5 kbar abundant co2 fluid inclusions in quartz and t xco2 phase equilibria modeling via perple x imply action of an essentially carbonic fluid in the granitoids cores of almandine rich garnet grains from the granitoids contain polyphase carbonate bearing inclusions with a distinct negative crystal shape the major carbonate in the inclusions is a strongly zoned magnesite siderite variety whereas pyrophyllite is the predominant silicate phase raman spectra of unexposed inclusions revealed a presence of co2 as well as ch4 and h2o the carbonate bearing inclusions coexist with larger polyphase inclusions composed of biotite quartz k feldspar plagioclase sillimanite which are interpreted as relics of granitic melts modeling the mineral assemblage preserved within the carbonate bearing inclusions shows that their present mineral and chemical compositions are a product of interaction of the trapped aqueous carbonic fluid with host garnet during cooling below 400 c despite strong modifications the inclusions bear evidence for initial saturation of the fluid with mg carbonate this is taken as an evidence for an origin of the fluids by devolatilization of the mg rich carbonate bearing ultrabasic greenstone rocks of the kaapvaal craton that were buried under the smz being generated at temperatures between 650 and 700 c the fluid subsequently participated in anatexis and coexisted with the granite magma during exhumation and interaction of the smz granulites with cratonic rocks

structural and p t evolution of a major cross fold in the central zone of the limpopo high grade terrain south africa the central zone cz of the limpopo complex of southern africa is characterized by a complex deformational pattern dominated by two types of fold geometries large sheath folds and cross folds the sheath folds are steeply ssw plunging closed structures whereas the cross folds are north south oriented with near horizontal fold axes in the area south of messina this complexly folded terrain grades continuously towards the south into a crustal scale ene wsw trending ductile shear zone with moderate dip towards the wsw all sheath folds document consistent top to the ne thrust movement of high grade material the timing of this shear deformational event d and thus of the gneissic fabric s is constrained at 2 6 ga by the syntectonic emplacement throughout the cz of precursors to quartzo feldspathic gneisses singelele type gneisses cross folds deform the s fabric and are characterized by a near vertical axial planar cleavage s recent single phase pb pb dating of garnet from a metapelitic gneiss with an s fabric from one of the largest cross folds in the cz constrains the timing of the deformational d and metamorphic m event at 2 0 ga mineral chemistry for metapelites from this cross fold shows a single peak on an n mg histogram for garnet reflecting a single phase of mineral growth metapelites from this cross fold also preserve evidence for only one well developed reaction texture grt sil qtz crd this reaction is accompanied by the simultaneously operating reaction grt fsp h o bt sil qtz both these divariant reactions belong to the univariant kfmash equilibrium crd grt fsp h o bt sil qtz the progress of the two divariant retrograde reactions leads to the consumption of grt and fsp k feldspar or never occurs with both cordierite and garnet microprobe profiling coupled with calculated isopleths for bt grt and crd in divariant equilibria define a decompression cooling p t path that reflects a single m high grade metamorphic event during the evolution of the cross fold this decompression cooling p t path traverses from 780 c 5 7 kbar to 600 c 3 3 kbar

the role of radiogenic heat production in the thermal evolution of a proterozoic granulite facies orogenic belt eastern ghats indian shield abstract the eastern ghats belt egb is a deeply eroded proterozoic orogenic belt juxtaposed against the archaean dharwar bastar singhbhum cratons in the indian shield the tectono metamorphic history of this belt is broadly similar to the grenvillian orogenic belts the egb exposes dominantly the granulite facies rocks such as charnockites khondalites and migmatitic gneisses with less abundant intermediate to mafic granulites the granulites are intruded by small plutons of granites syenites and anorthosites the northern and southern parts of the belt negb and segb have distinct crustal histories but 1600 1000 550 ma old tectono metamorphic events are common to both the belts in this research we attempt to understand the present day and past thermal state of this orogen on the basis of our radioelement k u and th measurements at 562 sites using in situ gamma ray spectrometry with other geological and geophysical constraints the dominant rock types of the negb and segb have similar radioelement abundances 3 k 3 ppm u and 30 ppm th and heat production heat production of charnockites gneisses khondalites and intermediate granulites of the negb is 2 9 2 8 2 9 and 1 1 w m 3 respectively in the segb they are 2 7 2 5 2 8 and 0 6 w m 3 respectively and the mafic granulites are the lowest in heat production 0 3 w m 3 on the basis of the heat production data and crustal petrologic models we show that the crustal contribution of the negb and segb is 54 and 45 mw m 2 respectively which are broadly in good agreement with the available surface heat flow data thermal models of the present day egb crust indicate that the temperature at the moho is 550 c radioelements and heat production of the eastern ghats granulites are higher than the other granulite belts in the indian shield the crustal radiogenic heat contribution and moho temperatures of the egb are higher than the adjoining archaean cratons this could also be responsible for the development of inverted metamorphic isograds in the tectonic boundary between the egb and the adjoining cratons during a collisional orogenic event at about 550 ma ago

plutonic rock types series discrimination of various granitoid series and related rocks abstract the streckeisen qap diagram discriminates four characteristic type series among the large variety of granitoids associated in intrusions three of them integrate varied compositions from basic to acid terms 1 the calcalkaline series with its low k or trondhiemite medium k or granodioritic high k or monzonitic variants 2 the alkaline series with its characteristic syenitic members 3 the tholeiitic series deprived of k feldspars the fourth 4 corresponds to mobilizates associated with migmatites of varied composition but always quartz rich the generalization in the qapf system allowing the plotting of undersaturated alkaline series provides an excellent visual display of the major associations and trends for the plutonic rocks

meso archaean and palaeo proterozoic sedimentary sequence stratigraphy of the kaapvaal craton abstract the kaapvaal craton hosts a number of precambrian sedimentary successions which were deposited between 3105 ma dominion group and 1700 ma waterberg group although younger precambrian sedimentary sequences outcrop within southern africa they are restricted either to the margins of the kaapvaal craton or are underlain by orogenic belts off the edge of the craton the basins considered in this work are those which host the witwatersrand and pongola ventersdorp transvaal and waterberg strata many of these basins can be considered to have formed as a response to reactivation along lineaments which had initially formed by accretion processes during the amalgamation of the craton during the mid archaean faulting along these lineaments controlled sedimentation either directly by controlling the basin margins or indirectly by controlling the sediment source areas other basins are likely to be more controlled by thermal affects associated with mantle plumes accommodation in all these basins may have been generated primarily by flexural tectonics in the case of the witwatersrand or by a combination of extensional and thermal subsidence in the case of the ventersdorp transvaal and waterberg wheeler diagrams are constructed to demonstrate stratigraphic relationships within these basins at the first and second order levels of cyclicity and can be used to demonstrate the development of accommodation space on the craton through the precambrian

the composition and evolution of the continental crust rare earth element evidence from sedimentary rocks the composition of the present day upper crust inferred from the uniformity of sedimentary rock r e e rare earth element patterns is close to that of granodiorite a revised andesite model is used to obtain total crustal composition the lower crust is the composition remaining assuming that the upper crust one third of the total is derived from intracrustal partial melting the upper crustal r e e pattern has pronounced eu depletion eu eu 0 64 the lower crustal pattern has eu enrichment eu eu 1 17 and the total crust has no eu anomaly relative to chondritic abundances the eu depletion in the upper crust is attributed to retention of eu in plagioclase in the lower crust because plagioclase is not stable below 40 km 10 kbar the anomaly is intracrustal in origin the archaean upper crust has a different r e e pattern to that of the present day upper crust being lower in total r e e and la yb ratios and lacking an eu anomaly these data are used to infer the archaean upper crustal composition which resembles that of the present day total crust except that ni and cr contents are higher the archaean crustal composition can be modelled by a mixture of tholeiites and tonalite trondhjemites the latter have steep light r e e enriched heavy r e e depleted patterns consistent with equilibration with garnet and hence probable mantle derivation there is little reason to suppose that the archaean lower crust was different in composition from the upper crust except locally where partial melting episodes occurred the r e e evidence is consistent with isotopic and geological evidence for a low continental growth rate in the early archaean a massive increase to about 70 of the total crust between about 3000 and 2500 ma b p and a slow increase until the present day the change from archaean to post archaean r e e patterns in the upper crust is not isochronous but is reflected in the sedimentary rock r e e patterns at differing times in different continents on the basis of a model composition for the mantle 36 of the potassium 30 of uranium 15 of lanthanum and 3 of ytterbium are concentrated in the present continental crust this enrichment is related to ionic size and valency differences from common mantle cations e g mg fe pre 3 9 ga b p crusts were obliterated by meteorite bombardment no geochemical evidence exists for primordial anorthositic sialic or mafic crusts

the structural framework of the southern margin of the limpopo belt south africa abstract the southern marginal zone is characterized by two prominent regional structural features i massive crustal wedges 20 km 50 km containing large oval shaped closed fold structures which are related to an early d 1 phase of deformation and which are bounded by ii major ductile shear zones that strike in a general e w direction the hout river shear zone forms the southern boundary of the southern marginal zone where granulites are juxtaposed against lower grade granite greenstones of the kaapvaal craton this fault system also acted as the sole thrust for the other high grade shear zones in the southern marginal zone and played a major role in the development of the zone of retrogression and the establishment of the orthoamphibole isograd in the southern marginal zone the hout river shear zone and related faults within the southern marginal zone clearly post date d 1 and a period of peak metamorphism m 1 they are composite structures showing dominantly reverse sense displacements and are responsible for the emplacement of the granulite terrane over and onto the kaapvaal craton in contrast to these older structures are the reactivated shear zones these structures clearly cross cut and displace the high grade shear zones in the southern marginal zone as well as the retrograde orthoamphibole isograd the fact that the hout river and related shear zones are in metamorphic equilibrium with the surrounding wall rocks suggests their age is that of the metamorphism i e 2670 ma barton and van reenen 1992

composition and source of fluids in high temperature graphite bearing granitoids associated with granulites examples from the southern marginal zone limpopo complex south africa abstract p t conditions fluid regime and carbon isotope composition of graphite and fluid inclusions from garnet sillimanite bearing leucocratic tonalites trondhjemites and granites associated with orthopyroxene bearing granulite metapelites in the southern marginal zone smz of the limpopo complex south africa are presented in the paper re integrated compositions of perthitic alkali feldspars and antiperthitic plagioclase as well as p t and t xco2 phase equilibria modeling using perple x software indicate that the granitoids began to crystallize at temperatures of 900 940 c and pressures of 7 9 kbar and were equilibrated with a fluid phase with xco2 0 5 0 6 as is recorded in dense fluid inclusions in quartz a small fraction of a saline fluid accumulated during cooling only average 13cpdb values for graphite 6 52 to 8 65 and fluid inclusions 2 50 to 5 58 from the granitoids differ substantially from the values which have been obtained in previous studies for graphite from the surrounding smz metapelites isotope data thus indicate that fluids associated with the granitoid magmas of the smz originated from a source unrelated to the host metapelites in terms of the heavy isotope signatures of carbon the granitoids might carry fluids which have been produced during devolatilization of mafic rocks amphibolites interlayered with hydrothermal carbonate veins in the adjacent granite greenstone successions of the kaapvaal craton that have been buried underneath the smz granulites during their exhumation the cratonic rock could serve as a source for the trondhjemite magmas intruding the smz granulites in this scenario the studied granitoids crystallized from these magmas variously contaminated by metapelitic material graphite precipitated via reduction of co2 rich fluid as the result of dissolution of pyrrhotite rich metapelites in the magmas

isotopic and geochemical evidence for crust mantle interaction during late archaean crustal growth abstract the charnockites and enderbites of the north marginal zone of the limpopo belt in southern zimbabwe represent a suite of dry magmas of tonalite trondhjemite granodiorite ttg affinity which crystallised directly in the lower crust they were emplaced between 2 6 and 2 7 ga and were derived from a source which showed remarkable lead isotope homogeneity unlike many other lower crustal rocks they have low k rb and low k u ratios and have not experienced depletion of their heat producing elements th u k and rb and thus represent an unusual suite of granulites in this study we have combined the constraints of trace element and isotope geochemistry to characterise the nature of the protolith to the nmz enderbites and charnockites we use a combination of high values calculated lead and strontium isotope initial ratios and old nd model ages to show that the enderbites and chamockites were derived from a protolith which was in part formed more than 500 ma before magma genesis we argue from the very high values that this protolith constituted two separate components an old high low sm nd crustal component and a younger low high sm nd mantle derived component both of which were homogenised by extensive mixing prior to 2 7 ga the presence of an older crustal component contrasts with other archaean ttg suites worldwide which show little evidence for a crustal prehistory the process of mantle crust interaction during late archaean crustal growth in southern zimbabwe invites comparison with modern convergent margins such as the andes

3 07 2 97 ga greenstone belt formation in the northeastern kaapvaal craton implications for the origin of the witwatersrand basin

mesoarchean ttg magmatism from the northeastern margin of the kaapvaal craton southern africa arguments for an exotic terrane remnant of pilbara craton abstract high grade terranes along archean cratonic margins either represent remobilized equivalents of the adjacent low grade domain or an exotic entity which accreted to the developing craton this study addresses this debate for the southern marginal zone high grade terrane along the northeastern margin of the kaapvaal craton the hout river shear zone separates the high grade terrane from the adjacent low grade pietersburg block of the kaapvaal craton tonalite trondhjemites of the baviaanskloof gneiss unit constitute the dominant rock in the southern marginal zone both pyroxene bearing and pyroxene absent variants occur locally the granitoids are affected by anatectic and metasomatic overprints magmatic textures are better preserved in the massive granitoids u pb zircon analyses of two orthopyroxene bearing granitoids gave mesoarchean crystallization ages 3088 11 ma massive 3121 9 ma banded however overprint ages differ with 2706 50 34 ma from the massive variety and 1925 19 ma 1976 11 ma from the banded variety the syn tectonic 2 71 ga is interpreted as metamorphic overprint age while the post tectonic 2 ga represent age of local overprint available geochemical data on the baviaanskloof granitoids are similar to archean ttgs a possible spatial temporal transition from 3 13 ga low pressure through 3 12 2 98 ga medium pressure to 2 92 ga high pressure ttg is delineated from the northwest towards the southeastern southern marginal zone this trend is compatible with the southward directed polarity of accretion events recognised in the adjacent low grade pietersburg block we argue that the northeastern margin of the kaapvaal craton north of the hout river shear zone is an exotic terrane because 1 the baviaanskloof granitoid event 3 1 2 9 ga with peak at 3 1 ga is older than the accretion episodes 2 97 2 92 ga with peak at 2 95 ga and 2 87 2 84 ga with peak at 2 86 ga in the adjacent pietersburg block 2 the younger 2 86 ga event occurs closer to the hout river shear zone while the older 2 95 ga event was away from it and 3 contemporaneous 3 1 3 0 ga detrital zircons are absent in greenstone belts of the pietersburg block that are proximal to the hout river shear zone possible connection to rocks of similar age from the pilbara craton as part of the vaalbara supercraton is explored

involvement of fluids in the metamorphic processes within different zones of the southern marginal zone of the limpopo complex south africa an oxygen isotope perspective abstract we present oxygen isotope data obtained by laser fluorination for three outcrops from the south marginal zone smz of the limpopo granulite complex south africa these outcrops are located within the granulite zone bandelierkop quarry on the retrograde anthophyllite in isograd manamead locality and within the zone of rehydrated granulite south of the retrograde isograd klipbank locality studied samples also include felsic granitoid rocks that occur as intrusive bodies veins and veinlets in the host tonalite gneisses or the metapelite the tonalite gneiss tg and the k feldspar rich segregations kfs g at klipbank have marginally different 18 whole rock wr values 7 9 and 7 3 correspondingly the kfs g may have formed by partial melting of the tg in the presence of external saline h 2 o co 2 fluid metapelites at manamead locality have 18 o values 9 5 10 3 lower than those at bandelierkop quarry 10 0 11 4 the two mineral oxygen isotope temperature estimates for metapelite minerals from manamead cluster around two values 620 30 and 730 40 c oxygen isotope temperature estimates for the metapelites from bandelierkop quarry define a narrow temperature range of 700 740 the oxygen isotope temperatures are in reasonable agreement with the results of classical geothermobarometry the 18 values of the whole rocks and individual constituent minerals of metapelites strongly negatively correlate with the rocks volatile content loi values this suggests that a fluid phase was involved in the hydration of metapelites in the vicinity of the regional anthophylite in isograd of the southern marginal zone the fluid may have been released during crystallization of leucogranite melt oxygen isotope and geochemical features of the leucogranite indicate genetic relation to the tonalitic baviaanskloof gneisses but do not support the formation of leucogranite via in situ partial melting of metapelites

geochemistry of archean shales from the pilbara supergroup western australia archean clastic sedimentary rocks are well exposed in the pilbara block of western australia shales from turbidites in the gorge creek group ca 3 4 ae and shales from the whim creek group ca 2 7 ae have been examined the gorge creek group samples characterized by muscovite quartzchlorite mineralogy are enriched in incompatible elements k th u lree by factors of about two when compared to younger archean shales from the yilgarn block alkali and alkaline earth elements are depleted in a systematic fashion according to size when compared with an estimate of archean upper crust abundances this depletion is less notable in the whim creek group such a pattern indicates the source of these rocks underwent a rather severe episode of weathering the gorge creek group also has fairly high b content 85 29 ppm which may indicate normal marine conditions during deposition rare earth element ree patterns for the pilbara samples are characterized by light ree enrichment lanybn 7 5 and no or very slight eu depletion eueu 0 82 0 99 a source comprised of about 80 felsic igneous rocks without large negative eu anomalies felsic volcanics tonalites trondhjemites and 20 mafic ultramafic volcanics is indicated by the trace element data very high abundances of cr and ni cannot be explained by any reasonable provenance model and a secondary enrichment process is called for

gulf stream dynamics pad ii eddy energetics at 73 w abstract pointwise energy balances in the gulf stream system at 73 w downstream of cape hatteras are examined five current meter moorings obtained verlocity and temperature data at four different depths for approximately one year these data were used to estimate eddy kinetic and potential energy flux divergences and the rates of energy conversion between the eddy and mean fields in both the gulf stream and the deep western boundary current based on our results and those of others we argue that i the gulf stream is on average baroclinically unstable in this region although energy release from the horizontal shear dominates near the surface ii eddy pressure work is an important component of gulf stream eddy energetics and acts as a sink of eddy energy at 73 w iii the deep western boundary current appears to be baroclinically unstable iv gulf stream eddies are affected by the far field and v the eddy field at 73 w is significantly different in its effects on the gulf stream than that u

fluid inclusions in hydrated granulite facies rocks southern marginal zone of the limpopo belt south africa abstract the archean granulite terrane of the southern marginal zone of the limpopo belt south africa was rapidly uplifted and thrust over a low grade granite greenstone terrane van reenen et al 1987 during cooling the granulites were rehydrated in a zone parallel to the contact with the low grade terrane a sharply defined retrograde orthoamphibole isograd at conditions of 6 kbar 625 650 c x co 2 0 8 van reenen 1986 was established we made microthermometric analyses of over 1000 fluid inclusions in four samples representing a traverse across the isograd with the goal of characterizing the hydrating fluid we found patterns of composition and density of inclusions to be similar to those reported from other granulite facies terranes a few apparently pure co 2 inclusions with densities appropriate for the p t conditions many co 2 inclusions with lower densities and aqueous inclusions of variable salinity and containing no detectable co 2 the results suggest that the hydrating fluid may be represented by secondary co 2 rich fluid inclusions because of their small size most between 5 and 10 m and equant shapes h 2 o contents of the co 2 rich inclusions of up to 0 3 mole fraction may not be detectable a model for hydration of the granulites includes infiltration of a fluid into the partially cooled overthrust granulites with a temperature gradient decreasing towards the underthrust greenstone terrane the fluid was probably generated by devolatilization of the greenstones as they were heated by the overthrust granulites the high co 2 content of the calculated fluid and of the fluid inclusions suggests the hydrating fluid was co 2 rich at the p t conditions of the retrograde isograd

the development of continental crust through geological time the south african case nd isotopic compositions and147sm144nd ratios were measured in fifty eight south african shales and greywackes with depositional ages ranging from 0 2 to 3 3 b y elements such as the rare earths which are poorly soluble in water and not fractionated during exogeneous processes preserve the signature of the original crustal source the147sm144nd ratios appear to be approximately constant throughout the time interval sampled we calculated nd model ages of crustal differentiation knowing that the shales represent a true blend of different continental areas we consider these model ages representative of the mean ages of their primitive continental sources then using the inverse technique developed by alle gre and rousseau in 1984 we computed a growth curve for the continental crust in south africa two periods of important crustal genesis archaean and around 1 5 b y can be compared with the observed geology and with other continental crust growth curves obtained in previous studies in southern africa and in australia the observation of large variations in the mgo content and ni cr u and th concentrations between archaean south african shales and post archaean samples compared to the constancy of the147sm144nd ratios leads us to propose that the archaean crust was composed of both granite 70 5 and a mafic component 29 5 which could have been komatiite the small dispersion of 147sm 144nd ratios suggests that erosion and sedimentation processes yielded homogeneous archaean shales the present day continental crust is much more heterogeneous because it has undergone several episodes of recycling thus recent shales are characterized by more variable 147sm 144nd ratios

shear wave velocity structure of the lower crust in southern africa evidence for compositional heterogeneity within archaean and proterozoic terrains 1 the nature of the lower crust across the southern african shield has been investigated by jointly inverting receiver functions and rayleigh wave group velocities for 89 broadband seismic stations located in botswana south africa and zimbabwe for large parts of both archaean and proterozoic terrains the velocity models obtained from the inversions show shear wave velocities 4 0 km s below 20 30 km depth indicating a predominantly mafic lower crust however for much of the kimberley terrain and adjacent parts of the kheis province and witwatersrand terrain in south africa as well as for the western part of the tokwe terrain in zimbabwe shear wave velocities of 3 9 km s are found below 20 30 km depth indicating an intermediate to felsic lower crust the areas of intermediate to felsic lower crust in south africa coincide with regions where ventersdorp rocks have been preserved suggesting that the more evolved composition of the lower crust may have resulted from crustal reworking and extension during the ventersdorp tectonomagmatic event at c 2 7 ga citation kgaswane e m a a nyblade j julia p h g m dirks r j durrheim and m e pasyanos 2009 shear wave velocity structure of the lower crust in southern africa evidence for compositional heterogeneity within archaean and proterozoic terrains j geophys res 114 b12304 doi 10 1029 2008jb006217

migmatization by metamorphic segregation at subsolidus conditions implications for nd pb isotope exchange abstract we have carried out major and trace element analyses as well as nd and pb isotope measurements on a stromatic migmatite in the phikwe complex botswana in order to assess the effect of high grade metamorphic event leading to migmatization on apparent nd model ages the phikwe complex considered as an entity of the central zone of the limpopo belt underwent a high grade metamorphic event at 2 0 ga leading to migmatite formation migmatites in this area present a very good layering of migmatitic components leucocratic layers were segregated from mafic layers dominated either by hornblende or biotite on a centimeter to millimeter scale accessory minerals such as monazite apatite and zircon were not identified in hornblende rich and quartzo feldspathic layers the combination of petrography and trace element concentrations in all layers indicates that the migmatite may have been formed either by partial melting according to biotite and or hornblende dehydration reactions or by metamorphic segregation at subsolidus conditions ree modeling applying batch fractional and disequilibrium melting equations allows to discriminate between these two processes the modeled ree results of the quartzo feldspathic layers match those observed via metamorphic segregation at subsolidus conditions the end values at 2 0 ga range from 0 3 to 8 9 with lower values in the ree rich migmatitic layers thus the distribution of major and trace elements as well as the scatter nd isotope data are interpreted as reflecting the proportional amount of each mineral in the different migmatitic components the positive end value of hornblende rich layers suggests hornblende crystallization from a mantle derived source and sm nd fractionation at protolith formation age apparent nd model ages ranging from 2 3 to 3 1 ga do not reflect the crustal evolution in southern africa in archean to proterozoic time based on the geochemical features and the pb pb wr isochron at 2 0 ga mass conservation during migmatization can be assumed therefore chemical and nd isotopic compositions of the protolith prior to migmatization can be calculated the results indicate a si and k poor diorite as protolith moreover calculated protolith compositions yield a two point sm nd isochron indicating an age of 2 7 ga and very similar nd model ages of 2 73 2 75 ga those ages constrain the timing of an archean crust formation episode in the phikwe complex

mean state of the troposphere over south east asia and the east indies december 1978 the mean state of the troposphere over south east asia from 8 30 december 1978 is described in the lower troposphere a jet was observed over the south china sea extending south westward from the philippines evidence suggests that this north easterly jet was supported by downstream blocking induced by the borneo land mass at the equator the jet had the appearance of being channelled between the highlands of borneo and sumatra on its left flank near the west coast of borneo was a semi permanent and quasi stationary vortex being warm core this vortex was probably maintained in part by latent heat release in cumulus convection by partitioning the data into means at 00 and 12 gmt the intensity of the rising branch of the hadley circulation over borneo was shown to increase by a factor of two from 100 10 5 to 200 10 5mb s 1 from 00 to 12 gmt

geochronology of the hout river shear zone and the metamorphism in the southern marginal zone of the limpopo belt southern africa abstract in this paper monazite u pb and zircon evaporation dates stepleaching pb pb results on garnet staurolite and kyanite and hornblende ar ar data are presented which constrain the timing of granulite facies metamorphism in the southern marginal zone of the limpopo belt and its thrusting onto the kaapvaal craton the southern marginal zone of the limpopo belt is considered to be a lower crustal equivalent of the northern kaapvaal craton granulite exhumation is associated with southward thrusting along the hout river shear zone which is a set of thrust and strike slip shear zones zircon ages for the matok intrusive complex which was emplaced within the zone during this thrusting charno enderbites 2671 4 ma granodiorites between 2667 and 2664 ma have previously been interpreted as evidence for rapid exhumation of the southern marginal zone within only 7 ma we have obtained a u pb date of 2691 7 ma for monazite from the bandelierkop quarry in the southern marginal zone interpreted as the age of high grade metamorphism a single zircon evaporation pb pb date of 2643 1 ma from a leucosome band at the same locality may indicate longer lasting metamorphism or decompression melting during exhumation anatexis of metapelitic xenoliths within the matok intrusive complex was dated at 2663 4 ma by u pb on monazite indistinguishable from existing zircon ages for this complex pb pb step leaching dates obtained on synkinematically grown garnet 2691 20 ma staurolite 2712 37 ma and kyanite 2672 51 ma from the khavagari hills in the giyani greenstone belt in the immediate footwall of the hout river shear zone indicate that early thrusting was contemporaneous with peak metamorphism in the southern marginal zone ar ar dating and geochemistry on syntectonic hornblende separates from the same shear zone system yielded disturbed spectra and indicated multiple populations probably reflecting repeated or continuous tectonic activity of the hout river shear zone up to about 2600 ma

neoarchean crustal recycling and mantle metasomatism hf nd pb o isotope evidence from sanukitoids of the fennoscandian shield abstract the growing knowledge of the geological record reveals evidence of crustal recycling in the archean eon in order to assess the role of recycling of crust into the mantle we have analyzed radiogenic hf in zircon as well as whole rock nd and pb isotope compositions of mantle derived granitoids of the sanukitoid series in the neoarchean karelian province fennoscandian shield homogeneous initial hf and nd values suggest that sanukitoids are derived from well mixed source s limited scatter in the initial pb isotope data within intrusions as well as in other radiogenic isotopes point to effective mixing processes in the mantle o isotope data for zircon from karelian sanukitoids indicate a dominant mantle source 18 o 5 5 6 1 with minor contributions from sources with elevated 18 o 7 0 to 8 5 such as sediments and altered upper oceanic crust our results suggest that the parental melts of sanukitoids are mixtures of depleted mantle and recycled crustal components a slab breakoff at the end stage of subduction is a plausible trigger mechanism for sanukitoid magmatism and it explains variable contributions from different 18 o sources

hydration of cordierite and hypersthene and a description of the retrograde orthoamphibole isograd in the limpopo belt south africa

exhumation of limpopo central zone granulites and dextral continent scale transcurrent movement at 2 0 ga along the palala shear zone northern province south africa abstract the palala shear zone is part of a major tectonic lineament in southern africa which extends over 1000 km from central botswana to the soutpansberg in south africa the lineament is situated between the central zone of the limpopo belt and the kaapvaal craton at its main exposure in the koedoesrand window 50 km e of ellisras northern province the palala shear zone comprises a c 15 km wide mylonite zone which can be subdivided into a northern central and southern domain this study addresses the tectono metamorphic history of the palala shear zone in the koedoesrand window and its role in the tectonics of the limpopo belt in order to do this structural and petrographical work was combined with geochronology age data from relic mineral parageneses show that metapelites from the central domain underwent granulite facies metamorphism around 2 6 ga however because of an intense proterozoic overprint field relationships of the archean event are largely disrupted during a second high grade event at c 2 0 ga granulite facies metamorphism was reached in the northern domain and the adjacent limpopo central zone charnockites were emplaced in the northern domain after 2 1 ga and were affected by retrograde amphibolite and greenschist facies metamorphism around 1 97 ga post orogenic transcurrent faulting under sub greenschist facies conditions characterizes the central and southern domains field observations and microstructural investigations indicate a dextral shear sense for the retrograde mylonitisation these findings document that the palala shear zone played an important role during the exhumation of the proterozoic granulites in the limpopo central zone associated with a transpressive orogeny between 2 05 and 1 95 ga low grade post orogenic transcurrent faulting in the palala shear zone at c 1 9 ga may be associated with the transtensional opening of the soutpansberg and palapye grabens on the basis of the tectono metamorphic history in the koedoesrand window and regional geophysical geochemical and sequence stratigraphic data the palala shear zone is interpreted as a proterozoic suture zone along which the archean kaapvaal and zimbabwe provinces were juxtaposed

role of fluids in the exhumation of the southern marginal zone of the limpopo complex south africa abstract the limpopo complex lc of southern africa displays structural metamorphic and geochronological evidence for a complex exhumation controlled evolutionary history that followed after burial and attainment of peak metamorphic conditions p 9 kbar t 900 c in the root zone central zone cz of the orogen before 2 72 ga the southern marginal zone smz of the lc in addition provides evidence for thrust controlled emplacement of a hot granulite facies nappe onto the adjacent low grade granite greenstone terrane of the northern kaapvaal craton nkvc in the interval 2 69 2 62 ga a tectono metamorphic scenario in which initial near vertical channeling exhumation at 2 69 ga was transferred into horizontal channeling thrusting at 2 67 2 64 ga is interpreted to have commenced with in situ partial melting of crustal rocks in the root zone of the orogen cz this resulted in formation of melt weakened crust migmatized gneisses that with continuous shortening led to vertical transfer of lower crustal material accompanied by fluid fluxed decompression melting that resulted in production of large volumes of leucocratic granitic material this material detached to form buoyant diapirs that accompanied the emplacement at 2 65 ga of mega sheath folds at the middle crustal level throughout the entire cz of the lc this early stage of near vertical exhumation is reflected i by the fact that linear elements of sheath folds and shear zones plunge consistently at steep angles to the sw throughout the entire cz and ii by decompression cooling dc segments of p t paths constructed for metapelitic rocks dc paths in the far northern part of the smz that are associated with the steep north dipping portion of the hout river shear zone hrsz traversed from p 8 5 kbar t 850 c to p 6 5 kbar t 720 c early near vertical exhumation transferred into horizontal channel flow that controlled emplacement at the middle crustal level of the granulite facies nappe smz horizontal channeling thrusting is recorded in metapelitic granulite by near isobaric cooling p 6 kbar t 700 550 c segments of earlier dc p t paths that are associated with the shallow north dipping portion of the hrsz devolitilization of underthrusted greenschist due to thrusting hot granulite over cool greenschist released large volumes of high temperature aqueous fluids from the underthrusted rocks that constantly infiltrated the overriding granulite during southwards migration of the nappe immiscible co 2 rich and brine fluids thus derived persistently infiltrated into and reacted with cooling granulite and triggered the main pulse of anatexis at p 7 5 kbar t 750 c similar fluids established a regional retrograde isograd and associated zone of 4500 km 2 retrograde hydrated granulite at p 6 kbar t 600 630 c located in the hanging wall of the shallow north dipping hrsz that bounds the smz in the south shear zone hosted metasomatic alteration including formation of lode gold deposits occurred at t 600 630 c p 6 kbar in the zone of rehydrated granulite and at higher p t conditions in the granulite zone the granulite facies smz of the lc thus preserves clear evidence for a direct link between persistent fluid flux and fluid rock interaction as an integral component of a conveyor like tectonic mechanism driven by vertical tectonics that operated from the core region of the orogen cz this mechanism explains the important link between the thrust controlled emplacement of the smz onto the nkvc and deep crustal fluid flow

unraveling the record of successive high grade events in the central zone of the limpopo belt using pb single phase dating of metamorphic minerals abstract dating of relic metamorphic assemblages can provide important information about the timing and character metamorphic grade and or pt evolution of early high grade episodes in polymetamorphic provinces using pb stepwise leaching of metamorphic silicates we have dated multiple granulite facies metamorphic episodes in the central zone cz of the limpopo belt ages of 2 52 ga were obtained from sillimanite and cogenetic garnet and ages of about 2 01 ga from titanite garnet and clinopyroxene together with new and published conventional age data from accessory phases and in the context of combined petrological and structural data these results lead us to a reinterpretation of the tectono metamorphic history of the cz three distinct high grade events at about 3 2 3 1 ga 2 65 2 52 ga and 2 0 0 05 ga are recognized each of these is suggested to correspond to a tectonic episode of distinct character a for the 3 2 ga event magmatic activity can mainly be identified best represented for example by the sand river gneisses or the messina layered intrusion the field relationships concerning the tectonometamorphic history of this early archean event are largely erased by at least two high grade metamorphic overprints b late archean 2 6 2 52 ga low pressure granulite facies metamorphism was associated with voluminous granitic and charnockitic plutonism the anticlockwise p t evolution of these granulites probably reflects deep crustal processes associated with magmatic underplating or in plating contemporaneous with vertical crustal growth of the zimbabwe craton around 2 6 ga c during the proterozoic event 2 05 1 95 ga tectonic thickening was caused by the collision of the kaapvaal and zimbabwe cratons the cz was squeezed between these two cratons and as a consequence underwent high pressure granulite facies metamorphism with a clockwise p t evolution the structural metamorphic and geochronological data can be best explained with a tectonic model that describes this final event as a dextral transpressive orogeny

comment on ultrahigh temperature granulites and magnesian charnockites evidence for the neoarchean accretion along the northern margin of the kaapvaal craton by rajesh et al

the behaviour of nd and pb isotopes during 2 0 ga migmatization in paragneisses of the central zone of the limpopo belt south africa and botswana we have carried out major and trace element analyses and nd pb isotope measurements on different components of two migmatite outcrops in the limpopo belt lose quarry botswana and palala south africa in order to assess the effect of migmatization during a high grade metamorphic event on sm nd systematics and nd model ages the outcrops studied are located in the mahalapye tshipise straightening zone at the south western part of the limpopo central zone close to its contact with the kaapvaal craton leucosomes of peraluminous granitic composition were formed by incongruent biotite melting in an fe rich metagreywacke lose quarry and in a metapelite palala respectively monazite u pb and garnet sm nd ages yield close to 2 0 ga for the high grade event confirming other recent work in the central zone the chemical compositions of the main mineral phases feldspar biotite and garnet indicate minimum melt formation at t 750 c and low pressure isocon plots and pb isotope data indicate that the partial melts are locally derived however lil trace element distribution between the leucosomes and paleosomes indicate the incomplete equilibration of the nascent melts with the residual feldspars and biotite in the paleosomes sm nd results on leucosomes and paleosomes of the metagreywacke migmatite show considerable scatter when calculated back to 2 0 ga the paleosomes fall in a narrow range of nd 4 and give tdm model ages between 2 45 and 2 66 ga the nd values of leucosomes are on average four units lower causing them to have apparent tdm ages of 2 7 2 92 ga on average 250 ma older than those of the paleosomes the ree th p2o5 and zr budgets of paleosomes and leucosomes indicate dominance of the lree by monazite and the hree by apatite incorporation of proportionally more monazite than apatite into the melt is inferred as the cause for the nd isotope discrepancy and hence the uncertainty of the average protolith mantle derivation age in contrast pb pb results define an apparent age of 2025 29 ma reflecting nearly full pb isotope exchange in the case of the palala metapelitic migmatite the nd values at 2 0 ga of all but two migmatitic constituents cluster between 7 and 9 5 pointing out extensive nd isotope equilibration during migmatization whereas the pb isotopes yield an errorchron date of 2311 65 ma indicating incomplete pb isotope exchange here garnet dominates the hree distribution whereas the lree are controlled by feldspar and monazite considering that nd isotope equilibration was nearly reached during migmatization nd model ages on the whole rocks represent a good approximation of those of the original protolith 3 0 3 2 ga in the regional context the pb isotope data from the lose quarry locality combined with the nd model age uncertain but within the range of 2 5 2 9 are reminiscent of the northern marginal zone of the limpopo belt and the southern zimbabwe craton whereas the nd and pb isotope data from the palala locality are typical for the limpopo southern marginal zone and the northern kaapvaal craton

deep crystal response to continental collision the limpopo belt of southern africa the limpopo belt is an example of an archean 2700 ma continental collisional zone in which the thickened crust 70 km has virtually attained equilibrium with the adjacent non thickened plates by a process of upward movement and lateral spreading of the underthrust high grade metamorphosed plates

reply to comments on p t record of two high grade metamorphic events in the central zone of the limpopo complex south africa by a zeh and r klemd

the triangle shearzone zimbabwe revisited new data document an important event at 2 0 ga in the limpopo belt abstract the limpopo belt in southern africa has been used to demonstrate that modern style continent continent collision operated during the late archaean 2 6 2 7 ga we have studied the age and pt conditions of strike slip tectonism along the important right lateral triangle shearzone our results substantiate existing proterozoic metamorphic mineral age data of prior uncertain significance using the pb pb and sm nd garnet chronometers and the ar ar step heating technique for amphibole we have dated pre and syn tectonic metamorphic minerals at 2 2 and 2 0 ga thus the triangle shearzone can now be regarded as an important proterozoic suture examination of corresponding high grade pt conditions reaching 800 c at 9 kbar indicates a clockwise metamorphic evolution with pronounced isothermal uplift although the evidence that thrusting of the marginal zones of the limpopo belt over the adjoining cratons occurred during the late archaean clearly remains it is now very uncertain to which event the various pt paths obtained in the limpopo belt may be assigned therefore the question of whether the 2 6 2 7 ga tectonism fits on its own a modern style continental collision model remains open and has to be reassessed

greenstone belts at the northernmost edge of the kaapvaal craton timing of tectonic events and a possible crustal fluid source abstract the hout river shear zone a complex thrust system marks the boundary between the low to medium grade metamorphic granitoid greenstone northern kaapvaal craton province and the high grade southern marginal zone of the limpopo complex to the north of it the giyani greenstone belt located on the northern kaapvaal craton immediately adjacent to the hout river shear zone is in a key position for studying the tectonics of this shear zone we have mapped two central transects through the greenstone belt dated metamorphic minerals by 207 pb 206 pb stepwise leaching and 40 ar 36 ar and compared our results with data from the pietersburg polokwane and rhenosterkoppies greenstone belts which are close to the hout river shear zone further west the se domain of the ne sw striking giyani greenstone belt appears to constitute a nw vergent nappe movement along the hout river shear zone is consistently sw vergent varying in character between frontal and lateral ramps and nappe the nw domain of the giyani greenstone belt is dominated by ultramafic schists with mafic and banded iron formation bif intercalations it is considered a separate tectonic unit from the se domain in which mafic intrusive and pillow volcanic rocks are prevalent with intercalated clastic sediments correlations between this se domain and the lower mafic unit of the pietersburg greenstone belt and with the mafic bif association of the rhenosterkoppies greenstone belt are possible ages for peak metamorphism in the immediate footwall of the hout river shear zone from this and previous work yield a weighted mean age of 2717 28 ma 95 confidence documenting a hot iron metamorphism a garnet age of 2833 38 ma in the se domain of the giyani greenstone belt overlaps with ages from 2854 to 2870 ma for youngest zircons in syntectonic intrusions within the giyani and the pietersburg greenstone belts as well as youngest detrital zircons in the probably syntectonic clastic sedimentary uitkyk formation of the latter these ages constrain a tectonometamorphic event in both belts termed here the pgb lwaji orogeny amphibole 40 ar 39 ar dates from the hout river shear zone from this and previous work and shear zones within the giyani greenstone belt yield disturbed age spectra with step ages mainly in the 2550 2670 ma range the amphiboles are chemically heterogeneous and the ages can be shown to reflect mixtures of older and younger amphibole generations due to reactivation of the shear zones and fluid activity after the main event in the hout river shear zone thermal modelling shows that in spite of the very low content of radioactive heat producing elements in the southern marginal zone its lower crust after moderate crustal thickening could be heated to granulite facies conditions in 30 60 ma a much shorter time than the 120 ma age difference between the pgb lwaji orogeny and the hout river shear event a causal link between the two tectonic events is thus precluded on the other hand previously low grade supracrustal rocks from the kaapvaal craton buried at lower crustal levels during hout river shear event could provide a source for fluids up to about 80 ma after the event

melting carbonic fluids and water recycling in the deep crust an example from the limpopo belt south africa the proposed retrograde orthoamphibole isograd in the southern marginal zone of the limpopo belt separates hydrated amphibolite grade metapelites from their granulite grade precursors and provides an intriguing geological dilemma widespread rehydration of metapelitic granulites under conditions of 660 600 c and 0 6 gpa and co2 dominated fluid inclusion populations appear to suggest thorough flushing of the high grade crust with an externally derived carbonic fluid however past studies of the carbon and oxygen isotope geochemistry of the hydrated rocks have not demonstrated the involvement of any voluminous out of equilibrium fluid in the evolution of the rocks this contribution proposes a model wherein the hydrating fluids are derived from crystallizing anatectic leucosomes generated by in situ fluid absent biotite melting along the prograde path model equilibrium fluid compositions suggest that reaction between this melt derived h2o and biogenic graphite produced co2 rich fluid compositions and potentially high fluid rock ratios at the wet granite solidus declining temperature resulted in fluid compositions shifting to higher xh2o with the precipitation of graphite essentially at the sites of initial fluid generation thereby preserving original pre metamorphic isotopic heterogeneities the hydration pattern of the southern marginal zone appears to be a function of melt migration in the hydrated zone leucosomes generally approximate minimum melt compositions and in this zone h2o was effectively recycled between the prograde and retrograde assemblages in contrast leucosomes in the granulite grade portion of the terrane have lost a k2o and h2o rich melt fraction and although some hydration has occurred in this zone orthopyroxene is generally preserved in metapelites in a general context in situ crystallization of graphitic partially melted source rocks has the potential to produce high fluid rock ratios at temperatures close to the wet granite solidus this single process holds the potential for widespread retrogression of formerly high grade assemblages at a variety of ah2o values without external fluid input

the johannesburg dome south africa new single zircon u pb isotopic evidence for early archaean granite greenstone development within the central kaapvaal craton abstract the johannesburg dome located in the central part of the kaapvaal craton constitutes one of the key areas to better understand the archaean crustal evolution of this part of the craton the dome comprises a variety of archaean granitic rocks intruded into mafic ultramafic greenstone remnants this study presents new precise u pb single zircon dating for seven different granitoid samples and an amphibolite dyke collected from the johannesburg dome a trondhjemitic gneiss sampled on the northwestern part of the dome yielded an age of 3340 3 ma and represents the oldest granitoid phase recognized so far this result has important implications with regard to the age of the mafic and ultramafic greenstone remnants scattered throughout the dome as it implies that the greenstone remnants are older than c 3 34 ga this initial magmatic episode involving early greenstone development and the intrusion of trondhjemitic and tonalitic granitoids on the northern half of the dome was followed by the emplacement of a 3201 5 ma hornblende biotite tonalite gneiss in the south following the trondhjemite tonalite gneiss emplacement a further period of magmatism took place on the dome which resulted in the intrusion of mafic dykes that are manifest as hornblende amphibolites the age of these dykes has yet to be determined quantitatively but they fall within the time constraints imposed by the age of the trondhjemitic gneisses 3340 3200 ma and later crosscutting potassic granitoids these rocks consisting dominantly of granodiorites constitute the third magmatic event and occupy an area of batholithic dimensions extending across most of the southern portion of the dome the southern and southeastern parts of the batholith consist mainly of medium grained homogeneous grey granodiorites dated at 3121 5 ma their porphyritic granodiorite equivalents in the southwestern part of the dome yielded an age of 3114 2 3 ma an age of 3117 12 ma from zircons extracted from one of the mafic dykes possessing granitic microveins provided confirmation of the timing of this third magmatic event lastly pegmatites that crosscut all these earlier granitoid events are younger than 3114 ma and might be at least 3 0 billion years old these new data provide confirmation of the conclusion that the witwatersrand basin was deposited after c 3074 ma on an archaean basement as young as c 3120 ma the data combined with that from other parts of the kaapvaal craton further supports the view that the evolution of the craton was long lived and episodic and that it grew by accretionary processes becoming generally younger to the north and west of the c 3 5 ga barberton swaziland granite greenstone terrane situated in the southeastern part of the craton

paleoproterozoic amphibolite facies retrogression and exhumation of archean metapelitic granulites in the southern marginal zone of the limpopo belt south africa abstract granulite facies terrains that have undergone extensive retrogression provide insights into the recycling of volatiles in the crust during orogenesis the timing of retrogression is key to understanding the geodynamic circumstances of such events this study has investigated amphibolite facies retrogression of melt depleted granulite facies metapelites from the south marginal zone smz of the limpopo belt the granulites formed via incongruent partial melting reactions that consumed biotite and produced garnet and rutile as prominent peritectic products at 2 7 ga under conditions of 11 kbar and 875 c peak metamorphic garnet is replaced by cordierite orthopyroxene symplectites with cordierite subsequently partially replaced by fine grained intergrowths of amphibolite facies assemblages including rutile zr in rutile thermometry yielded a temperature range of 731 846 c for the granulite facies rutiles and 629 690 c for rutile in the retrograde assemblage all rutile generations yielded a youngest u pb age of 2 1 ga some granulite facies rutiles record older dates 2 55 2 25 ga which are non systematic with regard to crystal size and represent non reset relicts of metamorphic recrystallization during a potentially complex history of amphibolite facies metamorphism between 2 7 and 2 2 ga the youngest 2 1 ga rutile u pb age represents the time of final exhumation related cooling of the rocks following a discrete paleoproterozoic amphibolite facies retrograde event coexisting orthoamphiboles in the peak metamorphic assemblage indicate a maximum peak temperature of 625 c the lack of age size correlation and intragrain age zoning in rutile suggest fast cooling following the amphibolite facies retrograde event which is inferred to be not much older u pb dating of zircon from the same rocks confirms that the metapelites underwent granulite facies metamorphism at 2695 35 ma thus the metapelites of the smz record both an archean granulite facies event and a polymetamorphic paleoproterozoic amphibolite facies history that culminated in a mid amphibolite facies peak inducing partial retrogression of the granulites the paleoproterozoic amphibolite facies event is tentatively correlated with mantle melting documented between 2 3 and 2 1 ga to produce mafic and intermediate volcanic rocks in the pretoria group on the kaapvaal craton

hafnium isotopic composition of the bushveld complex requires mantle melt upper crust mixing new evidence from zirconology of mafic felsic and metasedimentary rocks

an isotopic and geochemical study of the northern kaapvaal craton and the southern marginal zone of the limpopo belt are they juxtaposed terranes abstract to test existing lateral accretion models for the archean kaapvaal craton in south africa metapelites and leucocratic granitoids from two different provinces the granulite facies southern marginal zone of the limpopo belt and the northern part of the adjacent greenschist to amphibolite facies kaapvaal craton have been studied both provinces resemble each other in terms of isotope and trace element characteristics they show a general depletion of incompatible elements independent from the grade of metamorphism this observation conflicts with theories involving mobilisation during granulite facies metamorphism a further common fingerprint is the unusually high cr and ni concentrations in the metapelites in addition they define a very narrow range of nd model ages 2 9 3 05 ga constraining the average crustal age of their cratonic source region the granitoid gneisses show somewhat larger variations of nd model ages indicating some age heterogenity within the crust however the data from both provinces completely overlap the southern marginal zone and the kaapvaal craton also show uniform low 207 pb 204 pb relative to their 206 pb 204 pb ratios that differ clearly from other southern african provinces these results support the view that the southern marginal zone represents a part of the kaapvaal craton exposed at a deeper crustal level it rules out a hypothesis that both provinces are separate amalgamated terranes furthermore there is very little difference between granitoids and metapelites which indicates that the latter largely derived from the former the exposure of the granulite facies metapelites of the southern marginal zone at the same level as the lower grade metamorphic rocks of the kaapvaal craton indicates uplift along a sharp tectonic boundary

archaean to proterozoic crustal evolution in the central zone of the limpopo belt south africa botswana constraints from combined u pb and lu hf isotope analyses of zircon a combined set of u pb and lu hf in situ laser ablation icp mc ms zircon analyses were obtained from orthogneisses and granitoids in the central zone of the limpopo belt which comprises the beit bridge and mahalapye complexes the results indicate that by combining the two isotope systems primary magmatic zircon domains can be distinguished from those formed during later metamorphic events even if the distinct zircon domains underwent multiple pb loss and the texture age relationships as obtained by cathodoluminescence images and u pb analyses are ambiguous furthermore the applied technique allows distinction of zircon grains formed in juvenile magmas from those generated by melting of older continental crust or affected by substantial crustal contamination the combined u pb and lu hf data reveal that the sand river gneiss suite of the beit bridge complex was emplaced at 3283 8ma and formed from melting of an older archaean crust which was derived from a depleted mantle source at around 3 65 ga the hafnium model age tdm is significantly older than those obtained from zircons from numerous neoarchaean granitoids of the beit bridge complex comprising the singelele gneiss 2647 12ma the bulai granite 2612 7ma the regina gneiss 2649 9ma and two samples of the zanzibar gneiss 2613 6ma these granitoids show initial ehf t values between 0 5 and 7 1 which correspond to initialtdm between 3 46 and 3 01ga these variable tdminitial and ehf t initial values are interpreted to be the result of different mixtures of reworked 3 65 ga palaeoarchaean crust with juvenile magmas extracted from the depleted mantle during the neoarchaean at 2 65 ga this conclusion is supported by results obtained from the mahalapye complex which was affected by migmatization and granite intrusions during the palaeoproterozoic at 2 02 2 06 ga the mokgware granite 2019 9ma contains zircon xenocrysts with pb pb ages of 2 52 2 65 ga and 2 93 ga and hafnium model ages of 3 0 3 4 ga indicating that this granite is derived from remelting of archaean crust in contrast uniformtdm hf initial ages of 2 61 2 67 ga obtained from a diorite gneiss 2061 6ma of the mahalapye complex indicate that its protolith may have been formed from remelting of a neoarchaean juvenile crust variable ehf t initial values from 3 7 to 6 3 of zircon cores 2711 11ma in an adjacent leucosome also support a model of mixing of juvenile mantle derived matter with older crust in the neoarchaean

the timing of successive fluid events in the southern marginal zone of the limpopo complex south africa constraints from 40ar 39ar geochronology abstract we have carried out a geochronological study in the high grade metamorphic southern marginal zone of the limpopo complex south africa aimed at providing time constraints on episodes of fluid activity this work chiefly comprises 40 ar 39 ar dating with accompanying petrography of amphibole and mica and further incudes zircon u pb geochronology on a post peak metamorphic trondhjemite intrusion the so called zone of rehydration within 30 km n of the hout river shear zone where the southern marginal zone is thrust over the kaapvaal craton was studied in particular but sampling also included areas of patchy metasomatism in the granulite zone further north where no post peak metamorphic rehydration on a regional scale occurred amphibole in a mafic granulite and an older generation of zircons in the petronella trondhjemite yielded dates in the 2700 2730 ma range corresponding to the published age of peak granulite metamorphism in the southern marginal zone a cluster of 40 ar 39 ar dates on amphiboles from the zone of rehydration indicate an age between 2660 and 2670 ma for this event in accord with results from previous work on the hout river shear zone the main population of u pb zircon dates from the trondhjemite dating its intrusion also falls in this age range as does the published age of the matok pluton these data suggest that rehydration at 2660 2670 ma was a discrete episode a further 40 ar 39 ar age cluster defined by dates on both amphiboles and micas is seen at 2010 2040 ma which is the age of a major tectonometamorphic event in the central zone of the limpopo complex and previously reported from the southern marginal zone as rb sr biotite dates events we found in the 2010 2040 ma bracket are strike slip shearing dated by fabric forming amphibole as well as localized potassic metasomatism dated by both amphiboles and micas the latter was not confined to the zone of rehydration and appears in one case to be associated with gold mineralization a number of 40 ar 39 ar dates intermediate between 2600 and 2000 ma were obtained but these are diverse and can be accounted for by mixing incomplete resetting and or excess 40 ar they do not indicate events in this intermediate age bracket

sulfur geochemistry across a terrestrial permian triassic boundary section in the karoo basin south africa concentrations of sulfur and carbon and isotopic compositions of sulfur were determined in sedimentary rocks from a section across the terrestrial permian triassic p tr boundary in the northern karoo basin south africa high concentrations of sulfide produced by sulfate reducing bacteria were found in the sedimentary rocks at and just below the perceived p tr boundary the ratios of organic carbon to sulfide of the sedimentary rocks with high concentrations of sulfide are relatively constant and similar to those of marine environments this means that the enhanced accumulation of sulfide is a result of the enrichment of sulfate in the water as the sulfide concentrations do not correlate with concentrations of any major elements indicative of weathering intensity the supply of sulfate was independent of environmental factors such as temperature and co2 concentrations in the atmosphere which control weathering intensity therefore it is reasonable to consider that the sulfate might have been supplied to the freshwater directly as acid rain by an event such as a bolide impact or a volcanic eruption the absence of evidence for an impact event in the geological record though favors volcanic input as the cause of the acid rain

shear zones of the limpopo belt and adjacent granitoid greenstone terranes implications for late archaean collision tectonics in southern africa abstract granulite metamorphism in the 2 65 ga limpopo belt of southern africa is related to crustal thickening due to late archaean collision tectonics the central zone of the belt is a major thrust sheet of distinctive lithological structural and isotopic character separated from adjacent terranes by crustal scale shear zones and the northern and southern marginal zones are up thrust deep level granulite facies terranes of the adjacent cratons geometrically the marginal zones are mirror images and the limpopo belt is essentially symmetrical in n s section conventionally regarded as a n s collisional orogeny the gross structural and lithological differences between the central zone and the marginal zones kinematic data from shear zones and geophysical data showing the three dimensional geometry of the central zone are here used to suggest that the direction of emplacement of the central zone was also the collision direction this was from northeast to southwest with the upthrust outwardly directed marginal zones developing in mechanical response to this collision

pb isotopes a multi function tool for assessing tectonothermal events and crust mantle recycling at late archaean convergent margins abstract lead isotopes are a powerful tool in studying archaean granitoids and continental growth because 207pb is a sensitive indicator of involvement of lead from old archaean crustal sources this paper aims to demonstrate the significance and promote the use of lead isotopes by reviewing the theory concepts and traditions for interpretations of pb isotope data as well as by introducing a global model based on crust mantle recycling for discriminating late archaean convergent margin settings of granitoids case plots of k feldspar and whole rock data of ca 2 7 ga granitoids from several archaean cratons show that lead isotope systematics when interpreted in the correct geological context can be used for resolving many questions on crustal evolution from magmatic and tectonothermal history of individual intrusions to large scale tectonics and supercontinent correlations

geochronological problems related to polymetamorphism in the limpopo complex south africa abstract the integration of new and published geochronologic data with structural magmatic anatectic and pressure temperature p t process information allow the recognition of high grade polymetamorphic granulites and associated high grade shear zones in the central zone cz of the limpopo high grade terrain in south africa together these two important features reflect a major high grade d 3 m 3 event at 2 02 ga that overprinted the 2 63 ga high grade neoarchaean d 2 m 2 event characterized by sw plunging sheath folds these major d 2 m 2 folds developed before 2 63 ga based on u pb zircon age data for precursors to leucocratic anatectic gneisses that cut the high grade gneissic fabric the d 3 m 3 shear event is accurately dated by u pb monazite 2017 1 2 8 ma and pbsl garnet 2023 11 ma age data obtained from syntectonic anatectic material and from sheared metapelitic gneisses that were completely reworked during the high grade shear event the shear event was preceded by isobaric heating p 6 kbar and t 670 780 c which resulted in the widespread formation of polymetamorphic granulites many efforts to date high grade gneisses from the cz using pbsl garnet dating resulted in a large spread of ages 2 0 2 6 ga that reflect the polymetamorphic nature of these complexly deformed high grade rocks

formation of an archaean continent about 30 of the earth is covered by continents but only about 10 small kernels of these continentsae known as archaean cratonsae are continental fragments formed before 2 5 gyr ago the kaapvaal craton of south africa which formed and stabilized between 3 7 and 2 7 gyr ago is one of the oldest reasonably sized examples of these continental fragments it consists of a mosaic of subdomains that have been welded together by processes similar to those of modern day plate tectonics the earliest subdomains may have owed their origin to the onset of efficient recycling from the earth s hydrosphere into the mantle

crustal heat production and style of metamorphism a comparison between two archean high grade provinces in the limpopo belt southern africa abstract different characteristics of late archean metamorphism in the northern and southern marginal zones of the limpopo belt in southern africa are examined in the light of differences between these provinces in contents of heat generating elements in the northern marginal zone intrusion of charnoenderbitic magmas lasted from 2 75 to 2 58 ga accompanied by repeated low to medium pressure granulite facies metamorphism with some anticlockwise p t paths in contrast the medium pressure granulites of the southern marginal zone experienced peak metamorphism at 2 69 ga without much evidence of plutonism at that time followed by near isobaric cooling at approximately 6 5 kbar as a basis for our approach we used steady state geotherms calculated for the deeply eroded marginal zones for late archean times on the basis of their average k th and u concentrations and assuming low basal heat flow values of 20 mw m 2 the southern marginal zone and the adjacent northern kaapvaal craton are both characterised by low average th and u concentrations th 1 1 and 1 5 ppm u 0 3 and 0 65 ppm which are not a consequence of high grade metamorphism the average th and u concentrations in the zimbabwe craton 8 8 and 2 2 ppm are much higher than those in the kaapvaal craton and those found in the northern marginal zone are even higher 13 8 and 2 6 ppm in the southern marginal zone peak metamorphic temperatures are much above reasonable steady state geotherms and peak conditions could only have been attained through transient very high basal heat flux which in the context of a subduction zone might have resulted from slab breakoff or slab windows the northern marginal zone in contrast has such a large internal heat generation that high grade metamorphic conditions plot close to steady state geotherms even with basal heat flux values of 20 mw m 2 and minor crustal thickening or basal heat flux increase would have sufficed to exceed fluid absent tonalite solidus temperatures in the lower crust the repeated high grade metamorphism and charnoenderbite intrusions over a period of 1 7 108 years in the province can be explained by this observation both marginal zones of the limpopo belt can be understood as archean active continental margins of the kaapvaal and zimbabwe craton respectively while the problems of explaining the metamorphism in the southern marginal zone i e the source of the required heat are similar to those faced in many present day margins or orogens the northern marginal zone is non uniformitarian in the sense that its extreme heat generation rate could not be a property of any geological province today

tectonic model for the evolution of the limpopo belt abstract constraints on which any model for the tectonic evolution of the limpopo belt must be based include the following data crustal thickening to at least 65 km between 2700 and 2650 ma ago was responsible for the formation of the granulite terrane exposed in the limpopo belt today this crustal thickening probably resulted from the thrusting of the kaapvaal craton over the zimbabwe craton along the south dipping triangle tuli sabi shear zone other northwardverging thrusts related to this event occur in the kaapvaal craton south of the limpopo belt in the central and southern marginal zones and the kaapvaal craton this shortening is directed to the southwest while in the northern marginal zone it was directed to the north northwest peak metamorphism was superimposed on the thickened crust and then nearly isothermal decompression of several kbars occurred throughout the belt during this decompression rocks moved upward and spread outward onto the adjacent cratons from the zone of thickened crust along several inward directed shear zones creating a regional pop up high grade rocks were thrust over low grade rocks on the cratons producing along the terrane boundary retrograde metamorphism in the limpopo belt and prograde metamorphism on the cratons

initial pb of the amitsoq gneiss revisited implications for the timing of early archaean crustal evolution in west greenland reply abstract the rapid change in 207 pb 206 pb ratio during early earth history provides a powerful constraint for temporal resolution of archaean crustal evolution new and published data for amitsoq gneiss feldspars and whole rocks define the earth s least radiogenic silicate pb known until now the most important feature of the data is that independent of the exact slope of the regression in a common pb diagram a very well defined intercept with plausible terrestrial pb evolution lines is obtained between 3 63 and 3 66 ga identical to a combined pb pb regression age of 3654 73 ma mswd 17 6 we conclude that the magmatic precursors of the amitsoq orthogneisses separated from a depleted mantle like source at ca 3 65 ga this contrasts sharply with claims based on ion probe u pb zircon dating that the amitsoq gneisses had a prolonged episodic magmatic evolution extending from ca 3 87 to 3 60 ga pb isotopes offer no evidence that any amitsoq gneiss analysed until now began its evolution in the crust as long ago as 3 85 ga we propose that zircons significantly older than 3 65 ga were inherited from chemically evolved rocks as old as ca 3 85 ga geochemical arguments are presented to support this proposal new feldspar and whole rock pb isotope data are also reported for sheets of quartz dioritic gneiss on akilia island that yield the oldest zircon u pb age clusters in the range 3 84 3 87 ga these sheets are discordant to a metasedimentary enclave containing accessory apatite with graphite inclusions whose c isotope ratios are regarded as biogenic in origin as above pb isotopes in the sheets are consistent with a magmatic age of 3 65 ga which we regard as the true minimum age for these waterlain sediments and their possibly biogenic markers additional pb isotope data as well as limited published sm nd and u pb zircon data on the akilia enclaves themselves suggest an age between ca 3 65 and 3 70 ga for their original deposition some 150 200 ma younger than proposed previously thus the question of overlap of earliest life with a bolide impact scenario terminating 3 80 ga as on the moon becomes irrelevant

archean magmatic granulites diapirism and proterozoic reworking in the northern marginal zone of the limpopo belt the northern marginal zone nmz of the limpopo belt southern africa is a high grade gneiss belt dominated by magmatic granulites of the charnoenderbite suite which intruded minor mafic ultramafic and metasedimentary rocks between 2 74 and 2 57 ga the intrusive rocks have crustal and mantle components and occur as elliptical bodies interpreted as diapirs peak metamorphism p 800 mpa t 800 850 c occurred at ca 2 59 ga the highly radiogenic nature of the rocks in the nmz supplemented by heat from mantle melts led to heating and diapirism culminating in the intrusion of distinctive porphyritic charnockites and granites horizontal shortening and steep extrusion of the nmz during which crustal thickening was limited by high geothermal gradients contrast with overthickening and gravitational collapse observed particularly in more recent orogens the granulites were exhumed by the end of the archean the pervasive late archean shortening over the whole of the nmz contrasts with limited deformation on the zimbabwe craton possibly owing to the strengthening effect of early crust in the craton in the southeast of the nmz strike slip kinematic indicators occur within the transition zone and the triangle shear zone where dextral shearing reworked the archean crust at ca 1 97 ga

isotopic and ree characteristics of the intrusive charnoenderbite and enderbite geographically associated with the matok pluton limpopo belt southern africa abstract intrusive charnoenderbite and enderbite in the southern marginal zone of the limpopo belt are genetically linked with peak metamorphism m1 during the 2700 ma to 2650 ma limpopo orogeny they occur geographically associated with and are intruded by the granodiorite and granite of the matok pluton which were emplaced between the response phase m2 and the rehydration phase m3 of that orogeny rb sr pb ree and sm nd elemental and isotopic data for whole rock samples and u pb data for populations of zircon suggest that the charnoenderbite and enderbite were intruded 2671 ma ago and were derived from a crustal precursor the granodiorite and granite of the matok pluton were intruded at approximately the same time between 2667 ma and 2664 ma ago they also were derived from a crustal precursor nd model ages and ree patterns for the charnoenderbite and enderbite and the granodiorite are distinct this relationship may imply that they are not cogenetic alternatively these rock types could be cogenetic if the magma from which they were derived was assimilating older crustal material as it evolved nd isotopic data indicate that the magma giving rise to the charnoenderbite and enderbite formed by partial melting of rocks compositionally similar to some portions of the baviaanskloof gneiss data for the granodiorite are consistent with the possibility that this unit formed in a similar manner from rocks compositionally similar to other portions of the baviaanskloof gneiss the close temporal relationship between the emplacement of the charnoenderbite and enderbite and the granitic rocks of the matok pluton indicates that the retrograde portion of the limpopo orogeny beginning 2670 ma ago occurred rapidly perhaps over 7 ma

norite from the mahalapye granitoid complex northern edge of kaapvaal craton implications for the extent of the paleoproterozoic bushveld lip abstract the paleoproterozoic centred at 2 054 ga bushveld lip is one of the largest intracratonic lips in the world its components are primarily distributed within the archean kaapvaal craton this study reports dyke like and sill like norite intrusions from lose quarry within the mahalapye complex a paleoproterozoic granitoid migmatitic gneiss terrane along the northern edge of kaapvaal craton field relations indicate the norite postdate the 2 061 ga diorite gneiss and predate the 2 039 ga granodiorite significantly it exhibit structures indicative of comingling with 2 053 ga leucosome leucogranite a product of partial melting of diorite gneiss the cross cutting relations provide unequivocal evidence that the norite and the leucogranite were liquid at the same time and should be of similar age fe rich orthopyroxene xmg 0 43 0 46 constitutes the dominant mafic mineral in the norite geochemical characteristics of the mahalapye complex norites are similar to those of marginal facies norites from the bushveld complex b1 sills and molopo farms complex layered intrusions two of the prominent constituents of the bushveld lip this is the first solid evidence that the original extent of the bushveld lip covered an area larger than the current known limits of the archean kaapvaal craton into the youngest and westernmost terrane of the high grade limpopo complex the potential of high grade terranes along archean cratonic margins as locales to look for lip fragments is explored

shear zones bounding the central zone of the limpopo mobile belt southern africa abstract contrary to previously suggested north directed thrust emplacement of the central zone of the limpopo mobile belt we present evidence indicating west directed emplacement the central zone differs from the marginal zones in rock types structural style and isotopic signature and is an allochthonous thrust sheet it is bounded in the north by the dextral tuli sabi shear zone and in the south by the sinistral palala shear zone which are crustal scale lateral ramps published gravity data suggest that the lateral ramps are linked at depth and they probably link at the surface in a convex westward frontal ramp in the vicinity of longitude 26 30 e in eastern botswana two phases of movement the first between 2 7 and 2 6 ga and the second between 2 0 and 1 8 ga occurred on both the tuli sabi and the palala shear zones

adakites the key to understanding lile depletion in granulites abstract for more than 20 years it has been argued that granulites of the lower continental crust are depleted in fluid mobile elements such as cs rb u and th either because they were removed by melting of the lower crust or through dehydration we argue that there is little evidence for a simple relationship between granulite facies metamorphism and element depletion and propose that for felsic orthogneisses the depletion may be a primary feature of crust generation processes modern adakites show a range of fluid mobile element ratios and adakites from the austral volcanic zone avz chile show fluid mobile element ratios very similar to those found in lower crustal felsic granulites this suite of adakites has a strong slab derived component we propose that modern adakites of the type found in the austral volcanic zone are analogous in their genesis to lower crustal felsic granulites and that their mafic precursors were already depleted in fluid mobile trace elements prior to melting an incremental melting model in which fluid mobile elements are released from an eclogitic slab prior to partial melting can explain the fluid mobile element behaviour of both modern adakites and lower crustal felsic granulites

discrete metamorphic events in the limpopo belt southern africa implications for the application of p t paths in complex metamorphic terrains essentially all models for the tectonic evolution of the high grade limpopo belt of southern africa invoke a single orogenic event at 2 7 ga involving the collision of the zimbabwe and kaapvaal cratons with a distinct central zone by implication rocks within the belt must have followed parallel pressure temperature time p t t paths the main reasons for this situation are the belief that syn to postdeformational granitic magmatism in the belt was coeval and the fact that p t paths are similar new geochronological data indicate however that this magmatism occurred at distinct times in different parts of the belt over a span of at least 70 m y in addition the data indicate that high grade metamorphism and associated tectonism in the central and southern marginal zones occurred at about 2 0 and 3 15 ga respectively therefore p t t paths of these zones were independent until their juxtaposition more recently than 2 0 ga

tectonic model of the limpopo belt constraints from magnetotelluric data a b s t r a c t despite many years of work a convincing evolutionary model for the limpopo belt and its geometri cal relation to the surrounding cratons is still elusive this is partly due to the complex nature of the crust and upper mantle structure the significance of anatectic events and multiple high grade meta morphic overprints we use deep probing magnetotelluric data acquired along three profiles crossing the kaapvaal craton and the limpopo belt to investigate the crust and upper mantle lithospheric struc ture between these two tectonic blocks the 20 30 km wide composite sunnyside palala tshipise shear zone is imaged in depth for the first time as a sub vertical conductive structure that marks a fundamental tectonic divide interpreted here to represent a collisional suture between the kaapvaal and zimbabwe cratons the upper crust in the kaapvaal craton and the south marginal zone comprises resistive granit oids and granite greenstone lithologies integrating the magnetotelluric seismic and metamorphic data we propose a new tectonic model that involves the collision of the kaapvaal and zimbabwe cratons ca 2 6 ga resulting in high grade granulite limpopo lithologies this evolutionary path does not require a separate terrane status for each of the limpopo zones as has been previously suggested

development of a protocol to obtain the composition of terrigenous detritus in marine sediments a pilot study from international ocean discovery program expedition 361 abstract the geochemical and isotopic composition of terrigenous clays from marine sediments can provide important information on the sources and pathways of sediments international ocean discovery program expedition 361 drilled sites along the eastern margin of southern africa that potentially provide archives of rainfall on the continent as well as dispersal in the agulhas current we used standard methods to remove carbonate and ferromanganese oxides and stokes settling to isolate the clay fractions in comparison to most previous studies that aimed to extract the detrital signal from marine sediments we additionally applied a cation exchange wash using cscl as a final step in the sample preparation the motivation behind the extra step not frequently applied is to remove ions that are gained on the clay surface due to adsorption of authigenic trace metals in the ocean or during the leaching procedure either would alter the composition of the detrital fraction if no cation exchange was applied moreover using cscl will provide an additional measure of the cation exchange capacity cec of the samples however no study so far has evaluated the potential and the limitations of such a targeted protocol for marine sediments here we explore the effects of removing and replacing adsorbed cations on the clay surfaces with cs conducting measurements of the chemical compositions and radiogenic isotopes on a set of eight clay sample pairs both sets of samples underwent the same full leaching procedure except that one batch was treated with a final cscl wash step in this study organic matter was not leached because sediments at iodp site u1478 have relatively low organic content however in general we recommend including that step in the leaching procedure as expected significant portions of elements with high concentrations in seawater were replaced by cs 2sd 2 8 from the wash including 75 of the sodium and approximately 25 of the calcium 10 of the magnesium and 8 of the potassium trace metals such as sr and nd whose isotopes are used for provenance studies are also found to be in lower concentrations in the samples after the exchange wash the exchange wash affected the radiogenic isotope compositions of the samples neodymium isotope ratios are slightly less radiogenic in all the washed samples strontium and pb isotopes showed significant deviations to either more or less radiogenic values in different samples the radiogenic isotopes from the cscl treated fractions gave more consistent correlations with each other and we suggest this treatment offers a superior measure of provenance although we observed changes in the isotope ratios the general trend in the data and hence the overall provenance interpretations remained the same however the chemical compositions are significantly different we conclude that a leaching protocol including a cation exchange wash e g cscl is useful for revealing the terrestrial fingerprint cec could with further calibration efforts be useful as a terrestrial chemical weathering proxy

p t conditions mechanism and timing of the localized melting of metapelites from the petronella shear zone and relationships with granite intrusions in the southern marginal zone of the limpopo belt south africa

lewisian gneiss geochemistry and archaean crustal development models abstract the geochemistry of lewisian amphibolite facies gneisses from northwest scotland is described with particular reference to the rare earth elements ree and compared with the geochemistry of lewisian granulite facies gneisses the results show that there are no significant differences between laxfordian amphibolite facies and scourian granulite facies gneisses in terms of ree and other immobile trace elements at equivalent silica levels although the mobile radioactive heat producing elements k rb th u are significantly lower in the granulites in both types the basic gneisses have moderately fractionated ree patterns while the intermediate and acid gneisses have strongly fractionated ree patterns with low heavy ree abundances and decreasing levels of total ree with increasing sio 2 the most silicic gneisses develop large positive europium anomalies these gross chemical similarities between gneisses from intermediate amphibolite facies and lower granulite facies crustal levels constrain models for the evolution of the archaean crust the depletion of k rb cs th and u in granulites but not other incompatible trace elements cannot be explained by magmatic processes the positive eu anomaly in the more siliceous gneisses of both facies is a function of the primary processes of crustal generation and not secondary processes such as intracrustal melting or fractional crystallisation fractionation of radioactive heat producing elements from other trace elements is a result of granulite facies metamorphism with these elements being removed by an active fluid phase the apparent lack of partial melting in lower crustal granulites suggests a model for archaean crustal growth largely through underplating by primary tonalitic magmas