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

water magma interaction some theory and experiments on peperite formation experiments using molten thermite as a magma analog produce peperite when the melt interacts with wet sand these experiments also show explosive behavior developing strombolian and surtseyan like bursts the results demonstrate that the application of fuel coolant interaction fci theory is appropriate for interpretation of peperites the theory described includes discussion of the importance of mass interaction ratios of wet sediment and magma rs which determine thermal equilibrium temperature limits and contact interface dynamics the dynamics of the interface between magma and wet sediments involves heat transfer over a wide range of rates from passive quenching to explosive fragmentation a vapor film layer develops at the interface and acts both as an insulating barrier promoting passive quenching as well as a potential energy reservoir that can cause magma fragmentation mingling of the magma with wet sediments and explosive quenching when the vapor film becomes unstable an important parameter in determining the behavior of the vapor film is the value of rs which controls whether heat can be convectively removed from the layer as more is being added from its contact with magma if rs 1 for fully saturated sediments there is enough water in the sediments to make convective heat flow effective in quenching the magma but below that value there is the potential that the vapor film will be unstable producing highly dynamic phenomena including explosive fragmentation at values of rs 0 1 there is insufficient water to allow the escalation of explosive fragmentation

rootless tephra stratigraphy and emplacement processes volcanic rootless cones are the products of thermohydraulic explosions involving rapid heat transfer from active lava fuel to external sources of water coolant rootless eruptions are attributed to molten fuel coolant interactions mfcis but previous studies have not performed systematic investigations of rootless tephrostratigraphy and grain size distributions to establish a baseline for evaluating relationships between environmental factors mfci efficiency fragmentation and patterns of tephra dispersal this study examines a 13 55 m thick vertical section through an archetypal rootless tephra sequence which includes a rhythmic succession of 28 bed pairs each bed pair is interpreted to be the result of a discrete explosion cycle with fine grained basal material emplaced dominantly as tephra fall during an energetic opening phase followed by the deposition of coarser grained material mainly as ballistic ejecta during a weaker coda phase nine additional layers are interleaved throughout the stratigraphy and are interpreted to be dilute pyroclastic density current pdc deposits overall the stratigraphy divides into four units unit 1 contains the largest number of sediment rich pdc deposits units 2 and 3 are dominated by a rhythmic succession of bed pairs and unit 4 includes welded layers this pattern is consistent with a general decrease in mfci efficiency due to the depletion of locally available coolant i e groundwater or wet sediments changing conduit vent geometries mixing conditions coolant and melt temperatures and or coolant impurities may also have affected mfci efficiency but the rhythmic nature of the bed pairs implies a periodic explosion process which can be explained by temporary increases in the water to lava mass ratio during cycles of groundwater recharge

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

seismic crustal structure of the limpopo mobile belt zimbabwe abstract a 145 km n s seismic traverse was deployed to determine the crustal structure of the limpopo mobile belt in southern zimbabwe and the nature of its northern boundary with the zimbabwean craton rockbursts from south african gold mines to the south and regional seismicity from the kariba south zambia belt to the north were used as seismic sources p wave relative teleseismic residuals were also measured to assess whether any velocity contrast between the craton and the mobile belt extended into the upper mantle interpretation of reduced travel times from the local buchwa iron ore mine blasts which were broadside to the traverse revealed an upper crustal interface in the limpopo mobile belt at a depth of 5 8 0 6 km dividing material with a velocity of about 5 8 km s from that of about 6 4 km s on the craton arrivals from the same source showed a 4 4 0 5 km thick 5 5 km s layer overlying crust of about velocity 6 5 km s p wave arrivals from the regional seismicity were used to construct a crustal cross section absolute crustal thickness was tentatively estimated from the identification of a moho reflection on the mine blast recordings to the south of rutenga the crust thins from around 34 km to 29 km in association with a positive gravity anomaly centred over the late karoo nuanetsi igneous province and karoo tuli syncline north of rutenga to the boundary with the zimbabwean craton the crust is about 34 km thick the craton boundary was found to be a steeply southerly dipping zone associated with high velocity material which could either be deep seated greenstones or mafic material associated with the margin in the region studied this zone divides cratonic crust which was found to be about 40 km thick from that typical of the mobile belt and implies a step in the moho of around 6 km analysis of relative teleseismic residuals showed that the velocity contrasts are not confined to the crust but extend into the uppermost upper mantle with the cratonic lithosphere being about 4 faster than that of the limpopo mobile belt the resolution of the technique is such that it is difficult to ascertain whether these differences are features of precambrian evolution or are due to reactivation of the upper mantle during karoo igneous and tectonic activity

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

single zircon dating constraining the maximum age of the barberton greenstone belt southern africa one of the central issues in early precambrian crustal evolution is the age and genetic relationship between greenstone belt supracrustal assemblages and nearby high grade gneiss terranes the crucial question in most cases is which of the two is older we report zircon ages using the new single grain evaporation technique from a metaquartzite and a felsic volcanic flow of the onverwacht group in the barberton greenstone belt bgb south africa and swaziland that constrain the age of this basal part of the greenstone sequence between 3451 15 and 3438 6 ma this is almost 100 ma younger than previous age estimates assigned to the onverwacht and agrees well with recent sm nd pb pb and zircon ion microprobe dating tonalitic orthogneisses of the ancient gneiss complex agc adjacent to the bgb have ages up to 3644 4 ma and are thus demonstrably older than the greenstones also they cannot be derived from them by partial melting as has been proposed previously this age relationship as well as the presence of metaquartzites and thrust slices of pregreenstone tonalite in the onverwacht tectonic sequence rule out an entirely oceanic evolution for the early history of the bgb and suggest that continental crust was either below or nearby

tectonic evolution of the bindura shamva greenstone belt northern zimbabwe progressive deformation around diapiric batholiths abstract the bindura shamva greenstone belt northern zimbabwe and adjacent batholiths are regarded as one of the type areas for granite greenstone tectonics structural observations and three dimensional strain analyses have been used to propose a model for the tectonic evolution of this late archaean greenstone belt the data indicate that pluton emplacement triggered the deformation of the greenstone sequences and that the deformation history is single phase progressive rather than polyphase they do not however lead to an unambiguous interpretation of the emplacement mechanism structural features are best explained by a combination of pluton diapirism and ballooning plutonism criteria used as evidence for pluton related deformation include the pattern of foliation and stretching lineation trajectories the variation in strain type and strain intensity fold types sense of shear at the batholith greenstone interface metamorphic zoning and timing of porphyroblast growth specific structural and strain patterns within cleavage triple points interdomal synclinoria saddle culminations and batholith margins demonstrate interference between strain fields developed around each batholith

the sierra de mil cumbres michoacan mexico transitional volcanism between the sierra madre occidental and the trans mexican volcanic belt abstract the sierra de mil cumbres is a miocene volcanic range located in central mexico in the north eastern part of the state of michoacan near the city of morelia structurally it is a ene trending horst that covers an area of 1022 km2 approximately 20 km wide 60 km long and contains exposures of chemically bimodal volcanism in the form of ignimbrites lava domes lava flows cinder cones and related deposits the main volcanic manifestations of this range are the la escalera caldera 16 3 23 ma the garnica volcanic complex 18 3 17 9 ma the atecuaro caldera 16 3 19 4 ma and the indaparapeo volcanic complex 14 1 17 5 ma the sierra de mil cumbres stands in space and time at the intersection between the miocene recent trans mexican volcanic belt and the late cretaceous early miocene sierra madre occidental and so provides new insights into the geological evolution of central mexico arc volcanism in the sierra de mil cumbres was initiated by a massive nnw sse extension probably during the counterclockwise rotation of the sierra madre occidental new geological mapping stratigraphic analysis detailed geochemistry and 40ar 39ar geochronology demonstrates that this intra plate volcanism was emplaced between 14 and 23 ma

quantitative experiments on phreatomagmatic explosions abstract an experimental set up for controlled generation of thermal explosions fuel coolant interactions has been built by an interdisciplinary research group of volcanologists physicists and engineering scientists the tee haus thermal explosion experiment during the last 3 years more than 500 experimental runs were performed aim of the project was not to build a mini volcano but to produce igniteable mixtures of water and hot melt on a small scale in order to investigate basic physical aspects such small scale mixtures however most probably also occur during phreatomagmatic events in nature mixtures of water and magma on a large scale may be modelled by combination of small elements earlier experimental studies of thermal explosions were carried out with the use of metal melts nearly exclusively therefore a carbonate melt approx 0 3 kg of 1 1 mass ratio na2co3 k2co3 was used for initial experimental series because of easy handling and more magma similarity by injection of water into this melt explosive mixtures were produced and ignited self triggering explosive interactions occurred in a wide melt temperature range 720 to 1040 c at melt temperatures exceeding 1000 c co2 increasingly degassed presence of non condensible gas bubbles in the melt strongly reduced the explosivity the intensity of explosions only slightly increased with higher melt temperatures but strongly increased with higher water injection velocities because of more intensive mixing a minimum injection velocity was found to be approx 0 7 m s 1 experiments with silicate melts derived from diverse remelted volcanic rocks were carried out with a slightly modified set up explosions were ignited by an additional trigger a shock wave of approx 8 j in volcanic systems shock waves of this magnitude are abundant volcanic tremor etc strong explosions with silicate melts were produced at melt temperatures between 1350 and 1750 c the produced explosions exerted repulsion forces of up to 24 000 n with a typical pulse duration of 1 ms carbonate melt resp 1 5 ms silicate melts the ejection velocity was found to be in a range between 200 and 400 m s 1 explosion energy values up to 500 j were calculated the maximum explosion pressure was calculated to be in the range between 10 and 100 mpa only part of the melt so called interactive melt reacts explosively with injected water most of the melt is passively ejected by and after the thermal explosion different fragmentation processes result in specific grain size and grain shape populations that can be distinguished from each other particles resulting from the fine fragmentation process that precedes and causes the thermal explosion are characterized by angular to subrounded shapes and grain sizes ranging from 20 m to 180 m the amount of interactive melt is proportional to the intensity of the explosion the maximum amount of water vapourized at the explosive interaction so called interactive water was estimated thus the water melt mass ratio for explosive interactions could be calculated the values range between 1 25 and 1 35 carbonate melt resp between 1 6 and 1 25 silicate melts

aeromagnetic interpretation in the south central zimbabwe craton reappraisal of crustal structure and tectonic implications regional aeromagnetic data from the south central zimbabwe craton have been digitally processed and enhanced for geological and structural mapping and tectonic interpretation integrated with gravity data to constrain previous interpretations based on tentative geologic maps and provide new information to link these structural features to known tectonic events the derived maps show excellent correlation between magnetic anomalies and the known geology and extend lithological and structural mapping to the shallow near subsurface in particular they reveal the presence of discrete crustal domains and several previously unrecognised dykes faults and ultramafic intrusions as well as extensions to others five regional structural directions ene nne nnw nw and wnw are identified and associated with trends of geological units and cross cutting structures the magnetic lineament patterns cut across the 2 7 ga greenstone belts which are shown by gravity data to be restricted to the uppermost 10 km of the crust therefore the greenstone belts were an integral part of the lithosphere before much of the upper crustal brittle deformation occurred significantly the observed magnetic trends have representatives craton wide implying that our interpretation and inferences can be applied to the rest of the craton with confidence geological tectonic correlation suggests that the interpreted regional trends are mainly 2 5 ga great dyke age and younger and relate to tectonic events including the reactivation of the limpopo belt at 2 0 ga and the major regional igneous dyking events at 1 8 2 0 ga mashonaland 1 1 ga umkondo and 180 ma karoo thus their origin is here inferred to be inter and intra cratonic collisions and block movements involving the zimbabwe and kaapvaal cratons and the limpopo belt and later lithospheric heating and extension associated with the break up of gondwana the movements produced structures or reactivated older fractures that were exploited by late archaean and proterozoic mafic intrusions there was interplay between vertical and horizontal tectonics as seen in similar terrains worldwide

the limpopo mobile belt a result of continental collision the 600 km long limpopo mobile belt is discussed within the frame of a proterozoic supercontinent model piper 1976 evidence is presented that the rhodesian and kaapvaal cratons may have been separated by distances of more than 1000 km of oceanic crust from about 3350 ma ago the kaapvaal craton appears to have been driven intermittently n nw and then wnw against the rhodesian craton forming the ne sw trending collision zone the limpopo mobile belt and all the major fold and fracture patterns found this movement would be similar to the oblique movement of the pacific plate into the aleutian trench when collision ceased around 2500 ma ago it is likely that the great dyke and other complexes intruded along release fractures formed at right angles to the compression

subkilometer martian volcanoes properties and possible terrestrial analogs kilometer to subkilometer cones with large central craters are the most numerous volcanic structures in the northern plains of mars based on examination of more than 12 000 high resolution viking orbiter images most of these very small cones are located in parts of eastern acidalia and utopia isidis elysium planitia measurements of base and pit diameter for some 2000 well resolved features show small variations in the histogram of base diameters between and within regions but remarkable similarity in the distribution of crater cone diameter ratios this suggests that the small cones had a similar origin with only minor variations in local conditions the acidalia cones are generally larger more widely distributed and occur on a greater variety of terrains than those in utopia isidis elysium based on morphology distribution and the crater cone ratio histograms the smallest martian cones may be similar to icelandic pseudocraters somewhat larger steeper cones with smaller crater cone ratios which also occur in eastern acidalia may be analogs of terrestrial cinder cones

multiple zircon growth within early archaean tonalitic gneiss from the ancient gneiss complex swaziland abstract u pb age determinations by ion microprobe reveal multiple episodes of zircon growth and recrystallization within a single sample of tonalitic gneiss from the ancient gneiss complex the oldest episode at3644 4ma 2 produced the dominant type of zircon characteristically purplish brown and massive in texture this probably constitutes unaltered zircon precipitated from the original magma recrystallization accompanied and obscured by early pb loss took place within the oldest grains at3504 6ma and3433 8ma whole new grains grew at these times also we interpret the post 3644 ma growth as due to episodic deformational and metamorphic events that transformed the tonalite pluton into foliated banded gneiss in addition many grains are visibly overgrown by two layers of younger zircon of different colour and texture dated at2986 20ma and2867 30ma euhedral finely zoned whole grains having the 2986 ma age are present also evidently contributed by very thin felsic veins associated with the nearby lochiel granite the age of3644 4ma combined with precise zircon u pb dating of volcanics from the onverwacht group reported elsewhere demonstrates that at least part of the ancient gneiss complex is older than the barberton greenstone belt

single zircon ages for granitoid gneisses in the central zone of the limpopo belt southern africa and geodynamic significance abstract ion microprobe evaporation and vapour digestion single zircon emplacement ages for granitoid rocks throughout the central zone cz of the limpopo mobile belt south africa define three broad age groups namely at 3188 3314 2637 2734 and ca 2500 ma respectively all these granitoids including the youngest group display polydeformational features suggesting that much of the tectonic history of the cz occurred in the late archaean and early proterozoic the oldest group of predominantly tonalitic trondhjemitic granodioritic gneisses was found in relatively small domains from the extreme w to the e of the cz and probably represents remnants of a once more widespread early archaean terrain the most abundant granitoid gneisses belong to the 2 6 2 7 ga age group and were emplaced into already ductilely deformed gneisses of the oldest group several of these rocks were derived from intracrustal melting rare metamorphic zircons with ages of 2560 2575 ma reflect a high grade metamorphic event in the cz that has also been postulated by other workers a strongly deformed orthogneiss with a protolith emplacement age of 2510 ma proves beyond doubt that at least part of the structural history in the cz occurred in the early proterozoic an anatectic granite in the extreme w of the cz intruded at 2022 ma and is probably related to the peak of granulite facies metamorphism previously dated at 2027 6 ma in the messina area the second period of granitoid magmatism is broadly contemporaneous with charnockite and enderbite emplacement and granulite facies metamorphism in the northern and southern marginal zones of the limpopo belt broad contemporaneity of extensive granitoid magmatism in all three zones raises doubts about the exotic nature of the cz and suggests a revised evolutionary model for the limpopo belt

ice volcano interaction of the 1996 gjalp subglacial eruption vatnajokull iceland volcanic eruptions under glaciers can cause dangerous floods and lahars1 2 3 and create hyaloclastite fragmented glassy rock mountains4 5 6 7 8 but processes such as the rate of heat transfer between ice and magma edifice formation and the response of the surrounding glacier are poorly understood because of the lack of data here we present observations from the fissure eruption at vatnajokull ice cap iceland in october 1996 in the 13 days of the eruption 3 km3 of ice were melted and the erupted magma fragmented into glass forming a hyaloclastite ridge 6 7 km long and 200 300 m high under 500 750 m of ice meltwater of temperatures of 15 20 c flowed along a narrow channel at the glacier bed into the grimsvotn subglacial lake for five weeks before draining in a sudden flood or jokulhlaup subsidence and crevassing of the ice cap occurred over the eruptive fissure and the meltwater path whereas elsewhere the glacier surface remained intact suggesting that subglacial eruptions do not trigger widespread basal sliding in warm based glaciers

predictive mapping of prospectivity for orogenic gold giyani greenstone belt south africa we present a mineral systems approach to predictive mapping of orogenic gold prospectivity in the giyani greenstone belt ggb by using layers of spatial evidence representing district scale processes that are critical to orogenic gold mineralization namely a source of metals fluids b active pathways c drivers of fluid flow and d metal deposition to demonstrate that the quality of a predictive map of mineral prospectivity is a function of the quality of the maps used as sources of spatial evidence we created two sets of prospectivity maps one using an old lithologic map and another using an updated lithological map as two separate sources of spatial evidence for source of metals fluids drivers of fluid flow and metal deposition we also demonstrate the importance of using spatially coherent or geologically consistent deposit occurrences in data driven predictive mapping of mineral prospectivity the best predictive orogenic gold prospectivity map obtained in this study is the one that made use of spatial evidence from the updated lithological map and spatially coherent orogenic gold occurrences this map predicts 20 of the ggb to be prospective for orogenic gold with 89 goodness of fit between spatially coherent inactive orogenic gold mines and individual layers of spatial evidence and 89 prediction rate against spatially coherent orogenic gold prospects in comparison the predictive gold prospectivity map obtained by using spatial evidence from the old lithological map and all gold occurrences has 80 goodness of fit but only 63 prediction rate these results mean that the prospectivity map based on spatially coherent gold occurrences and spatial evidence from the updated lithological map predicts exploration targets better i e 28 smaller prospective areas with 9 stronger fit to training gold mines and 26 higher prediction rate with respect to validation gold prospects than the prospectivity map based on all known gold occurrences and spatial evidence from the old lithological map

when was the limpopo orogeny abstract the term limpopo orogeny refers to the common high grade metamorphic and tectonic event that affected the archaean rocks of the central and southern marginal zones of the limpopo belt and its influence on the adjacent portions of the kaapvaal craton available geochronological data from muscovite kyanite prograde metamorphic assemblages zircon in igneous charnoenderbite and enderbite muscovite in syntectonic pegmatite bodies formed in shear zones partial melts and syn and post tectonic plutons of granodiorite and granite indicate that this orogeny occurred during the time interval from 2700 ma ago to 2650 ma ago the retrograde phase of this orogeny from the onset of the response phase m 2 until the rocks were subjected to rehydration m 3 was very rapid probably lasting 7 ma from 2671 ma to 2664 ma ago similar but limited data from the northern marginal zone and adjacent portions of the zimbabwe craton suggest that rocks there were subjected to a more recent interval of high grade metamorphism and tectonism extending from 2600 ma until 2550 ma

moho depth and crustal composition in southern africa abstract we present new results on structure thickness and composition of the crust in southern africa based on 6300 seismic receiver functions at 85 stations application of hk stacking to the entire sase dataset and use of multi frequency bands improve resolution substantially we observe a highly heterogeneous crustal structure with short wavelength variations in thickness h vp vs ratio composition and moho sharpness which defines 20 blocks that do not everywhere coincide with surface tectonic features in the zimbabwe craton the tokwe block has h 35 38 km and vp vs 1 74 1 79 whereas the thicker crust in the tati block h 47 51 km may be related to deformation of the archean crust along the cratonic margin two distinct crustal blocks with similar crustal thickness 42 46 km but significantly different vp vs ratios are recognized in the limpopo belt extreme values of 1 90 1 94 at the dyke swarms in eastern limpopo and 1 84 at the olifants river dyke swarm and easternmost bushveld intrusion complex bic indicate voluminous magmatic intrusions in the whole crust we find no evidence for magmatic intrusions in the central inferred part of bic where the crust is thick 45 50 km and vp vs is low 1 68 1 70 this thick crustal root may have deflected rising magmas to form the two bic lobes most of central kaapvaal has thin 35 40 km crust and vp vs 1 74 these characteristics are similar to the tokwe block in zimbabwe craton and may indicate delamination of pre existing lower crust which is further supported by a very sharp moho transition the exposed cross section in the vredefort impact crater is non representative of cratonic crust due to shallow moho 34 km and high vp vs 1 80 attributed to shock metamorphism high vp vs 1 76 is typical of the witwatersrand basin and anomalously low vp vs 1 66 1 67 marks the kaapvaal kheis namaqua transition highly heterogeneous crust both in thickness and vp vs ratio is typical of the namaqua natal and cape fold belts

the motloutse complex and the zimbabwe craton limpopo belt transition in botswana abstract the archaean rocks of northeastern botswana are divided into named lithostratigraphic complexes each a different assemblage of stratiform lithologies granitoids and gneisses and each with differences in metamorphic grade structural style and metallic mineralisation the motloutse complex lies between four other complexes it includes granitoids and orthogneisses typical of the zimbabwe craton to the east detached segments of the quartzite carbonate pelite association of the anomalous matsitama schist belt to the west and migmatites similar to those of the limpopo belt to the south but is itself a distinctive lithological assemblage it shows a higher grade of metamorphism than the adjacent cratonic terranes but lacks the granulites of the limpopo belt it is delineated by relatively abrupt changes in structural style and fabric orientation of which most if not all coincide with shear zones or faults it is partly bounded against the limpopo belt by the magogaphate shear zone but also flanks the western boundary of the limpopo belt central zone west of 27 35 e the motloutse complex was formed in the late archaean between 2 7 ga and 2 5 ga ago in a zone of plutonism which separated a crustal fragment bearing the depositional basin of the matsitama shelf type sequences from the developing greenstone belts to the east during regional deformation elements of the matsitama belt and associated paragneisses were thrust from the southwest over the active magmatic zone and were then infolded with and underplated by migmatites orthogneisses and granitoids of the developing motloutse complex the terranes which were thus accreted to the southwestern margin of the zimbabwe craton during this episode of deformation and crustal thickening d1 d3 were then cut across by dextral movement on the magogaphate shear zone during d4 so juxtaposing the limpopo belt with the motloutse complex the area of the motloutse complex was previously regarded as a transitional boundary between the craton and the mobile belt but is here seen instead as an intracratonic accretionary assemblage on which the ene wsw limpopo trend was partly superimposed during d4 shearing delineation of lithostratigraphic complexes elsewhere in the zimbabwe craton and the limpopo belt may be possible and can be expected to clarify the pattern to which these provinces were assembled during the archaean prior to d4 deformation

the magmatic metamorphic and tectonic evolution of the northern marginal zone of the limpopo belt in zimbabwe the northern marginal zone of the limpopo belt in southern africa comprises a plutonic assemblage of granitoids including a distinctive suite of porphyroclastic granites and a much less abundant supracrustal assemblage of metabasites and iron formations these rocks are at granulite facies above a normal thickness of continental crust most of the plutonic assemblage are intrusive rocks that crystallized from dry melts from 2800 to 2600 ma with a relatively simple thermal history they may have been derived from partial melting of a mafic source some supracrustal rocks have experienced two thermal events at granulite facies a reverse sense shear zone forms the boundary of the northern marginal zone with the zimbabwe craton the southern boundary is the triangle shear zone which is proven as a continuous structure along a much greater strike length than previously documented a widespread sub vertical foliation in the northern marginal zone and the reverse shear zone formed during progressive nnw sse shortening crustal thickening occurred both magmatically and tectonically in the late archaean and was accompanied by synchronous uplift protracted magmatism provides a mechanism to incorporate supracrustal rocks into the lower crust and can explain the occurrence of more than a single thermal event

regional geological setting of the limpopo belt abstract the 2 7 ga limpopo belt granulite terrane is situated between two lower grade granite greenstone cratons the kaapvaal craton in the south and the zimbabwe craton in the north this granulite facies terrane is subdivided into three zones each with a distinctive geological signature and separated from each other and from the adjacent cratons by well defined shear zones 1 the southern marginal zone composed of granite greenstone material very similar to that of the kaapvaal craton has retrogressed granulites on its southern margin in the hanging wall of the northward dipping terrane boundary hout river shear zone separating the granulite terrane from the adjacent kaapvaal craton 2 the northern marginal zone a granite greenstone terrane at granulite grade which is separated from the zimbabwe craton by a southward dipping terrane boundary and 3 the central zone a unique crustal element with distinctive carbonate platform lithologies and a metagabbroic layered complex which intruded before or at 3250 ma this zone cannot be related to the kaapvaal craton or to the zimbabwe craton it is separated from the two marginal zones by mylonitic shear zones with horizontal stretching lineations the entire limpopo belt is characterized by a distinctive deformational and metamorphic style which contrasts markedly from that of the adjacent granite greenstone terranes of the kaapvaal and zimbabwe cratons this distinctive pattern developed during the limpopo orogeny probably as a consequence of continental collision during the late archaean

reaction history of garnet sapphirine granulites and conditions of archaean high pressure granulite facies metamorphism in the central limpopo mobile belt zimbabwe sequential reaction textures in archaean garnet corundum sapphirine granulites from the central zone of the limpopo belt document a progression from early coarse grained high pressure p 9 5 kbar granulite facies assemblages m1 to late low pressure p 6 kbar granulite facies sub assemblages m2 the stable m1 assemblage was garnet 57 pyrope mg mg fe 62 sapphirine corundum gedrite phlogopite rutile late m1 boron free kornerupine grew at the expense of garnet and corundum and coexisted with garnet sapphirine and gedrite partial or complete breakdown of coarse garnet and kornerupine during m2 resulted in the development of pseudomorphs and coronas consisting of fine grained symplectic intergrowths of cordierite gedrite and sapphirine later spinel the majority of reaction textures can be explained in terms of a stable reaction sequence and a model time sequence of mineral facies can be constructed when compared with a qualitative petrogenetic grid of fe mg discontinuous reactions in the fmash multisystem sapphirine garnet corundum spinel cordierite gedrite kornerupine the facies sequence indicates decompression at essentially constant t assuming constant a h2o exhumation of m1 corundum inclusions during m2 breakdown of kornerupine resulted in production of metastable spinel by a disequilibrium reaction with gedrite a second disequilibrium reaction of the spinel with cordierite produced sapphirine the operation of such reaction while pressure was decreasing the opposite dp from that implied by the texture if assumed to be the product of an equilibrium reaction has serious implications for the use of reaction textures in the construction of p t vectors garnet biotite thermometry on garnet interiors and phlogopite inclusions in corundum yields temperatures of ca 850 c for the m1 stage a minimum late m1 pressure of ca 7 kbar is indicated by the former association of kornerupine and corundum relict m1 kyanites reported by other workers indicate a minumum early m1 pressure of 9 5 kbar implying metamorphism at depths of at least 33 km probably 38km the high pressure granulite facies metamorphism was followed by an almost isothermal pressure decrease of 5 kbar indicative of rapid uplift the p t path is interpreted as the product of a single metamorphic cycle which probably took place in response to tectonic thickening of the crust such a process contrasts with the extensional origin recently proposed for isobarically cooled granulite facies terranes

the significance of 3000 ma granulite facies mafic dikes in the central zone of the limpopo belt southern africa abstract structural petrographic and chemical relationships indicate that a suite of mafic dikes possibly 3000 ma old were emplaced at high crustal levels deformed and metamorphosed epicontinental supracrustal rocks of the central zone of the limpopo belt these dikes were subsequently metamorphosed at granulite grade conditions probably during the limpopo orogeny about 2700 ma ago the earlier metamorphism was under conditions of at least amphibolite grade and isotopic data suggest that it may have occurred between about 3200 ma and 3100 ma ago these relationships indicate that the rocks comprising the central zone did not follow a single p t time loop but instead were involved in two distinct orogenies the orogenies were separated by a period of crustal stability during which the supracrustal rocks presently and originally exposed at the surface were again moved near to the surface this tectonic history also implies that the supracrustal rocks of the central zone and the sand river gneisses are of great antiquity if the sand river gneisses with their unique suite of older mafic dikes form a previously deformed basement to the supracrustal rocks then the sand river gneisses have been involved in three orogenies pb isotopic data indicate that the mafic dikes were derived from a mantle source with a high average ratio at some stage in its evolution prior to 3200 ma all archean lithologic units within the central zone of the limpopo belt are characterized by having been derived from rocks that had a large mu ratio at some time in their history and this relationship suggests that a link existed between the crustal rocks of the central zone and this mantle source from at least 3200 ma ago until it was destroyed by or after the limpopo orogeny the tectonic history of the central zone is different from those of the lithologies forming the adjacent terranes and suggests that it evolved separately from these terranes until they were juxtaposed during the limpopo orogeny

physical controls and resulting morphological forms of quaternary ice contact volcanoes in western canada abstract volcanism in association with large quantities of ice manifests itself in a variety of morphological forms developed under differing physical conditions these physical conditions include the location amount thickness and type of confining ice the location and quantity of trapped water and the surrounding topography ice in the form of thick continental ice sheets thinner alpine glaciers or even perennial accumulations of snow can influence the resulting morphological form of the volcano the chemical composition of the erupting magma effusion rate and total erupted volume will also affect the shape of the resulting edifice quaternary volcanoes influenced by many of these factors are found in western canada the morphological forms range from tuyas to subglacial mounds to stratovolcanoes this paper provides a review of ice contact volcanism and volcanic products formed in these environments using canadian examples for illustration

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

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

early basic magmatism in the evolution of the northern marginal zone of the archean limpopo belt abstract basaltic rocks enclosed in quartzo feldspathic gneisses and metamorphosed to granulite grade at 2 9 ga from the north marginal zone of the limpopo belt are subdivided into three types group a basalts are tholeiitic and have flat rare earth element ree patterns field and chemical evidence suggest that these metabasic rocks may have originated as basaltic lavas prior to the 2 9 ga metamorphism group b basalts are enriched in trace elements relative to group a particularly in the lree this group merges into a smaller group of basalts group c which are highly enriched in trace elements group b and possibly group c basalts are tentatively identified as dykes which cut the quartzo feldspathic gneisses but predate the metamorphism the cause of the trace element enrichment in groups b and c cannot be uniquely identified for whilst some evidence suggests crustal contamination it is impossible to unambiguously choose between crustal contamination a small percentage of melting and source region heterogeneity trace element patterns for the group b basalts show a striking similarity to those for phanerozoic basalts from this region and which are thought to have been derived from an enriched mantle source

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

mars a review and synthesis of general environments and geological settings of magma h2o interactions the advent of a global cryosphere likely occurred very early in the history of mars and much of the available water and related volatiles co 2 clathrates etc were sequestered within and below the cryosphere this means that magmatism plutonism and volcanism as a geological process throughout the history of mars cannot be fully understood without accounting for the interaction of magma and water and related species in both solid and liquid form we review and outline the probable configuration of water and ice deposits in the history of mars describe environments and modes of magma h 2o interaction and provide specific examples from the geological record of mars magma and water ice interactions have been interpreted to have formed 1 massive pyroclastic deposits 2 large scale ground collapse and chaotic terrain 3 major outflow channels 4 mega lahars dwarfing terrestrial examples 5 sub ice sheet eruptions and edifices 6 pseudocraters 7 landslides on volcanic edifice flanks and 8 hydrothermal sites the global nature of the cryosphere its longevity and the diversity of environments means that mars is an excellent laboratory for the study of magma h 2o interactions and the role of related volatile species

himalayan tibetan analogies for the evolution of the zimbabwe craton and limpopo belt abstract the limpopo orogeny which lasted from 2700 to 2600 ma was characterised by nnw sse directed shortening this resulted in both crustal thickening accommodated by folding and nnw directed thrusting and associated crustal extrusion whereby a number of crustal blocks including the central zone of the limpopo belt was displaced to the west southwest along wsw ene trending shear zones this was part of a regionally distributed phase of orogenic deformation which affected the adjoining kaapvaal and zimbabwe cratons from the witwatersrand basin in the south to the harare greenstone belt in the north this deformation which migrated northwards with time postdated an earlier phase of crustal accretion during which the archaean crustal unit now marked by the kaapvaal and zimbabwe cratons and the limpopo belt grew towards the north northwest shortening of this unit during the limpopo orogeny may be regarded as the last stage of the crustal accretion process this tectonic process of continental growth by accretion followed by crustal shortening with associated crustal extrusion has many features in common with tibetan tectonics from the mesozoic to the present

metamorphic p t profiles from the zimbabwe craton to the limpopo belt zimbabwe abstract metamorphic p t profiles in the eastern and western parts of the limpopo belt zimbabwe were examined on the basis of mineralogic and petrologic data from areas which include the southernmost zimbabwe craton the northern marginal zone and the central zone the zimbabwe craton consists of a granite greenstone terrane of 2900 ma to 3600 ma old the northern marginal zone represents a high grade equivalent of the granite greenstone lithologies the central zone has suffered granulite facies metamorphism during the late archean metamorphic temperature of the zimbabwe craton adjacent to the northern boundary of the limpopo belt is calculated to be 650 680 c for amphibolite and 710 720 c for quartzofeldspathic gneiss in the eastern part these temperatures are slightly lower than that calculated from the same assemblages in the northern marginal zone and central zone peak metamorphic conditions in the eastern part of the northern marginal zone are estimated at about 740 790 c and 7 5 7 9 kbar from the assemblage garnet core orthopyroxene plagioclase quartz in pelitic gneiss these conditions were followed by a retrograde event of about 630 660 c and 3 8 4 5 kbar which were calculated from the assemblage garnet rim cordierite sillimanite quartz in the same rock the p t path is very similar to the retrograde path from the southern marginal zone it implies that both the northern marginal zone and southern marginal zone may have shared a similar metamorphic evolution garnet bearing mafic gneiss in the central zone indicates somewhat higher p t conditions than those in the northern marginal zone central zone metamorphic conditions are estimated at 790 890 c and 11 0 13 3 kbar for garnet clinopyroxene plagioclase quartz assemblages and 750 870 c and 7 6 8 8 kbar for garnet hornblende plagioclase quartz assemblages the difference of p t conditions between the northern marginal zone and central zone may be related to movement along the pre triangle shear zone the northern marginal zone appears to have been displaced along the shear zone relative to the central zone during uplift carrying the deeper crust to the surface

zircon ion microprobe studies bearing on the age and evolution of the witwatersrand triad abstract the ventersdorp and witwatersrand supergroups and the dominion group collectively referred to as the witwatersrand triad represent some of the oldest and economically most important volcano sedimentary basins within the kaapvaal craton of southern africa precise age constraints which are vital to models aimed at understanding the formation mineralisation and evolution of these basins have been hampered by analytical and geological problems these problems have been overcome by this ion microprobe study on zircons from volcanic rocks within the three basins felsic and mafic volcanics from the ventersdorp supergroup yield ages of 2709 4 ma and 2714 8 ma respectively constraining the timing of the closure of the witwatersrand basin a maximum age for the witwatersrand basin is indicated by the age of 3074 6 ma for volcanics from the underlying dominion group thus a period of c 350 ma was available for the accumulation of the full witwatersrand basin succession unfortunately dating within the witwatersrand basin itself is not complete and future work could show that extended time breaks may exist within the basin these new data show that previous age determinations on the ventersdorp witwatersrand and dominion basins were inaccurate and that the economically important witwatersrand basin must now be recognised as archaean

a terrane interpretation of the archaean limpopo belt the limpopo belt is a zone of thickened archaean crust whose origin is currently explained by a late archaean continent continent collision between the kaapvaal and zimbabwe cratons this review shows that the two cratons have fundamentally different geological histories and that the zimbabwe craton was unlikely to have behaved as a stable cratonic block at the time of the limpopo belt collision the geological histories of the zimbabwe craton the north marginal central and south marginal zones of the limpopo belt and the kaapvaal craton are shown to be sufficiently different from one another to warrant their consideration as discrete terranes the boundaries between the five units outlined above are all major shear zones further supporting a terrane model for the limpopo belt the five units were all intruded by late to syn tectonic granites c 2 6 ga constraining the accretion event to c 2 6 ga

the crustal architecture of the kaapvaal crustal block south africa between 3 5 and 2 0 ga following terrane amalgamation of early oceanic lithosphere the southern and central parts of the kaapvaal craton were a coherent unit by 3 1 ga juxta position of the northern and western granitoid greenstone terranes including the murchison island arc was the result of terrane accretion that started at 3 1 ga the culmination of these events was the collision of the kaapvaal craton the pre cratonic zimbabwe block and the central zone to generate the limpopo granulite gneiss terrane coeval with these orogenic events the central kaapvaal craton underwent extension to accommodate the development of the dominion witwatersrand pongola and ventersdorp basins the craton scale thabazimbi murchison lineament development during the 3 1 ga accretion event and continued to influence the tectonic evolution of the kaapvaal block throughout the period under review as indicated by the syn sedimentary tectonics of the 2 64 ga wolkberg group overlying black reef formation and the transvaal sequence the transvaal and griqualand west basins developed in the late archaean 2 55 ga with basin dynamics influenced by far field stresses related to the limpopo orogeny during this period the thabazimbi murchison lineament lay close to the northern margin of the depository reactivation of the lineament between 2 4 and 2 2 ga resulted in inversion of the transvaal basin and formation of the northward verging mhlapitsi fold and thrust belt the half graben setting envisaged for the deposition of the pretoria group was influenced by the thabazimbi murchison lineament as was the emplacement and subsequent deformation of the bushveld complex

athabasca valles mars a lava draped channel system athabasca valles is a young outflow channel system on mars that may have been carved by catastrophic water floods however images acquired by the high resolution imaging science experiment camera onboard the mars reconnaissance orbiter spacecraft reveal that athabasca valles is now entirely draped by a thin layer of solidified lava the remnant of a once swollen river of molten rock the lava erupted from a fissure inundated the channels and drained downstream in geologically recent times purported ice features in athabasca valles and its distal basin cerberus palus are actually composed of this lava similar volcanic processes may have operated in other ostensibly fluvial channels which could explain in part why the landers sent to investigate sites of ancient flooding on mars have predominantly found lava at the surface instead

oxygen boron chromium and niobium enrichment in native au and ag grains a case study from the linglong gold deposit jiaodong eastern china quartz vein type gold mineralization in the linglong deposit constitutes one of the important sources of gold in china here we report the first finding of anomalous enrichment of boron b oxygen o chromium cr and niobium nb in native gold and ag te grains from the linglong deposit the oxygen content of gold and silver grains ranges up to 75 83 at boron up to 97 85 at chromium up to 26 23 at and tellurium up to 33 4 at the au precipitation is mainly linked to pyrite precipitation under reducing conditions the hfse enrichment including chromium cr and niobium nb in the gold and silver grains indicate that the ore forming fluids were derived from high temperature magmas sourced from depth the enrichment of low temperature te in the native gold and silver grains combined with the typical features of fluid boiling as inferred from fluid inclusion studies indicate that the ore fluids derived from depth upwelled rapidly and metal precipitation occurred through decompression at shallower levels our results suggest good prospect for gold and silver at depth in linglong

topographic and stochastic influences on pahoehoe lava lobe emplacement a detailed understanding of p hoehoe emplacement is necessary for developing accurate models of flow field development assessing hazards and interpreting the significance of lava morphology on earth and other planetary surfaces active p hoehoe lobes on k lauea volcano hawai i were examined on 21 26 february 2006 using oblique time series stereo photogrammetry and differential global positioning system measurements during this time the local discharge rate for peripheral lava lobes was generally constant at 0 0061 0 0019 m3 s but the areal coverage rate of the lobes exhibited a periodic increase every 4 13 0 64 min this periodicity is attributed to the time required for the pressure within the liquid lava core to exceed the cooling induced strength of its margins the p hoehoe flow advanced through a series of down slope and cross slope breakouts which began as 0 2 m thick units i e toes that coalesced and inflated to become approximately meter thick lobes the lobes were thickest above the lowest points of the initial topography and above shallow to reverse facing slopes defined relative to the local flow direction the flow path was typically controlled by high standing topography with the zone directly adjacent to the final lobe margin having an average relief that was a few centimeters higher than the lava inundated region this suggests that toe scale topography can at least temporarily exert strong controls on p hoehoe flow paths by impeding the lateral spreading of the lobe observed cycles of enhanced areal spreading and inflated lobe morphology are also explored using a model that considers the statistical likelihood of sequential breakouts from active flow margins and the effects of topographic barriers

can archean granulites be direct crystallization products from a sialic magma layer archean granulite terrains are generally dominated by charnockite enderbite suites these rocks are geochemically similar to the tonalite trondhjemite granodiorite ttg gneisses of archean cratons following an open system fractional crystallization model for formation of ttg crust through solidification from a shallow level tonalitic magma ocean we propose that such granulites could have formed by crystallization at depth at the base of the same magma layer which is undergoing convection increasing solubilities with depth of fluids in a magma mean that the base of a thick convecting magma layer will be undersaturated with respect to h 2 o and co 2 this allows the formation of charnockitic assemblages crystallization at depth can involve the reverse of dehydration melting reactions e g producing biotite and quartz at the expense of k feldspar orthopyroxene and liquid such reactions buffer the activity of water but not of co 2 during crystallization hence fluids exsolved during progressive solidification are likely to be co 2 rich

sapphirine kornenipine bearing rocks and crustal uplift history of the limpopo belt southern africa sapphirine kornerupine bearing rocks occur within the anorthosites of the messina layered intrusion in the limpopo mobile belt of zimbabwe the xmg range of the major minerals is as follows cordierite 0 98 0 93 enstatite 0 97 0 86 chlorite 0 98 0 92 phlogopite 0 98 0 90 sapphirine 0 98 0 86 kornerupine 0 94 0 88 gedrite 0 96 0 85 spinel 0 92 0 78 there are four rock types the constituent minerals of which have different x m g values which decrease in the above mineral order other minerals are corundum sillimanite and relict kyanite we recognise twenty reactions without phlogopite and nine reactions involving phlogopite the textural relations and the plots of the microprobe data of coexisting minerals in the mgo al2o3 sio2 h2o system are consistent with the following sequence of main reactions 1 enstatite corundum cordierite sapphirine 4 sapphirine sillimanite cordierite corundum 8 kornerupine corundum cordierite sapphirine 13 kornerupine cordierite sapphirine enstatite 15 enstatite spinel chlorite sapphirine 18 cordierite sapphirine chlorite corundum 20 sapphirine chlorite corundum spinel the early reactions are shown by coarse grained reaction intergrowths kornerupine and gedrite breakdown is shown by finer grained symplectites and the latest reactions by very fine grained products in micro fractures these selected reactions illustrate a remarkably steep trajectory from thept peak close to 10 kbar and 800 c to the minimum observable at 3 5 4 5 kbar and 700 c as indicated by the pure mash system very rapid uplift took place under nearly isothermal conditions the protolith of this material was possibly sedimentary derived from altered volcanic rocks the bulk composition is close to the composition of kornerupine or to a mixture of alunite chlorite and pyrophyllite these texturally and mineralogically complex rocks contain a wealth of relevant data for documenting crustal uplift history

the significance of cordierite hypersthene assemblages from the beitbridge region of the central limpopo belt evidence for rapid decompression in the archean

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

rootless cone eruption processes informed by dissected tephra deposits and conduits rootless cones result from the explosive interaction between lava flows and underlying water saturated sediment or volcaniclastic deposits rootless explosions can represent a significant far field hazard during basaltic eruptions but there are few detailed studies of their deposits a rootless cone field in the 8 5 ma ice harbor flow field of the columbia river basalt province nw usa is revealed by sections through rootless conduit and cone structures the ice harbor lava flow hosting the rootless cones was emplaced across a floodplain or lacustrine environment that had recently been mantled by a layer of silicic volcanic ash from a major explosive eruption our observations indicate a two stage growth model for the rootless cones 1 initial explosions generated sediment rich tephra emplaced by fallout and pyroclastic density currents and 2 later weaker explosions that generated spatter rich fountains variable explosive activity resulted in a wide range of pyroclast morphologies and vesicularities cross sections through funnel shaped conduits also show how the conduits were constructed and stabilised the growth model is consistent with decreasing water availability with time as inferred for rootless cones described in iceland the ice harbor rootless cones provide further lithological data to help distinguish between rootless cone derived tephra and tephra generated above an erupting dyke

magma water interactions in subaqueous and emergent basaltic in the subaqueous growth and emergence of a basaltic volcano clasts are formed by one or a combination of 1 explosive release of magmatic volatiles 2 explosive expansion and collapse of steam formed at magma water contact surfaces 3 explosive expansion of steam following enclosure of water in magma or entrapment of water close to magma and 4 cooling contraction these processes named respectivelymagmatic explosivity contact surface steam explosivity bulk interaction steam explosivity andcooling contraction granulation can be enhanced by mutual interaction and feedback the first three explosive processes are limited at certain water depths hydrostatic pressures and become increasingly vigorous at shallower levels the depth of onset of magmatic explosivity depends largely on juvenile volatile content it is up to 200 m for tholeiitic magmas and up to 1 km for alkalic magmas at the depth where formation of clastic deposits becomes predominant over effusion of lavas magmatic explosivity is subordinate to steam explosivity as a clast forming process the upward transition to accumulation of dominantly clastic deposits is not simply related to the onset of substantial exsolution of magmatic volatiles and can occur without it contact surface explosivity commonly requires initiation by a vigorous impact between magma and water and although no certain depth limit is known likelihood of such explosivity decreases rapidly with depth clast generation by bulk interaction explosivity appears to be restricted to depths much shallower than that of the critical pressure of water which in sea water is at about 3 km cooling contraction granulation can occur in any depth of water but at shallow levels may be replaced by contact surface explosivity during continuous eruption under water tephra can be ejected and deposited within a cupola of steam such that rapid quenching does not occur emergent volcanoes are characterized by distinctive steam explosive activity that results primarily from a bulk interaction between rapidly ascending magma and a highly mobile slurry of clastic material water and steam the water gets into the vent by flooding across or through the top of the tephra pile and violent explosions cease when this access is sealed the eruptions during emergence of surtsey and capelinhos typify the distinctive explosive activity the style and controls of which are different from those of maar volcanoes

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

rootless cones on mars indicating the presence of shallow equatorial ground ice in recent times high resolution mars orbiter camera moc images have revealed the existence of clusters of small cones in the cerberus plains marte valles and amazonis planitia mars these cones are similar in both morphology and planar dimensions to the larger of icelandic rootless cones which form due to explosive interactions between surficial lavas and near surface groundwater impact crater size frequency relationships indicate that surfaces upon which the cones sit are no older than 10 ma if martian cones form in the same manner as terrestrial rootless cones then equatorial ground ice or ground water must have been present near the surface in geologically recent times

structure and origin by injection of lava under surface crust of tumuli lava rises lava rise pits and lava inflation clefts in hawaii tumuli are positive topographic features that are common on hawaiian pahoehoe lava flow fields particularly on shallow slopes and 75 measured examples are presented here to document the size range tumuli form by up tilting of crustal plates without any crustal shortening and are thus distinguished from pressure ridges which are up buckled by laterally directed pressure the axial or star like systems of deep clefts that characterize tumuli are defined here as lava inflation clefts their tips advanced into red hot lava and they widened as uplift proceeded and while the lava crust was thickening flat surfaced uplifts formed like tumuli by injection of lava under a surface crust were previously called pressure plateaus but lava rise is proposed instead the pits that abound among lava rises previously attributed to collapse or subsidence are generally formed because the lava around them rose and the name lava rise pit is proposed unique examples of tumuli and lava rises from which lava drained out under a surface crust 1 5 to 2 5 m thick are described from kilauea caldera these examples show that in tumuli and lava rises the crust floats on considerable bodies of fluid lava and is able to do so because of its higher vesicle content the fluid lava loses many of its gas bubbles during residence beneath the crust the bulk densities of samples from tumuli show a general downward increase the form of the density profile is consistent with the relationship that for any given crustal thickness the density of fluid lava closely matched the average density of that crust suggesting that the lava was stably density stratified it is inferred that stable stratification was regulated by out flows of the more vesicular lava fractions loss of bubbles through the lava inflation clefts and entry of injected lava at its level of neutral buoyancy below the uppermost meter the downward decrease in vesicularity closely conforms with that expected by compression of a uniform mass of gas per unit mass of lava

the deep structure of the limpopo belt from geophysical studies abstract geophysical studies in the limpopo belt and adjoining cratons contributed to the fundamental understanding of the deep structure in these tectonic provinces the seismic and geoelectrical data sets prove that the terrane boundaries of the limpopo belt dip in towards the belt to give it the appearance of a pop up structure the gravity data show that high grade metamorphic rock with densities that are high compared to the normal low grade upper crustal granitoids form the upper crust in the northern and southern marginal zones of the limpopo belt these rocks also show a low magnetic susceptibility the geophysical data clearly defined the position of the southern boundary of the limpopo belt the northern boundary of the central zone can be related to a southward dipping zone of seismic reflectors while the southern boundary of this terrane as formed by the palala shear zone seems to be near vertical the greenstone belts of the craton have a limited depth extent and are allochtonous the present position of these structures could be related to thin skinned tectonics that affected the upper crust on the kaapvaal craton and which were revealed by seismic reflection data

on the geotectonic nature of the fen wei rift system abstract the fen wei rift system 1200 km long and 10 100 km wide is situated in the northern part of east china following a zigzag path trending nne ssw it cuts the shanxi shaanxi plateau average altitude 2 km reaching depths of 5 6 km and is filled with cenozoic sediments thickness 3 4 km accompanying the mainly vertical displacement of the marginal fault zones the rift system extends laterally up to 7 km greatest in the southern part earthquakes with foci 10 30 km deep are frequent m 8 eruptions mainly of alkaline basalts took place 24 times in the cenozoic heat flow appears to be rather high as marked by 17 hot springs negative bouguer anomalies are dominant reaching a maximum of 200 mgal the crust is 38 km thick on average 2 4 km thinner than that of the marginal uplift zones possibly a mantle arched belt exists below in the upper mantle v p 7 95 km s originally the rift system was part of the broad ancient china platform in the early jurassic the latter underwent activation and became the north china diwa geodepression region in the cenozoic the fen wei rift system formed as a result of contraction of the crust in this region combined with creeping of asthenospheric material it may be referred to the crevice type of post platform continental rift zones in e e milanovsky s classification

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

explosive lava water interactions in elysium planitia mars geologic and thermodynamic constraints on the formation of the tartarus colles cone groups 1 volcanic rootless constructs vrcs are the products of explosive lava water interactions vrcs are significant because they imply the presence of active lava and an underlying aqueous phase e g groundwater or ice at the time of their formation combined mapping of vrc locations age dating of their host lava surfaces and thermodynamic modeling of lava substrate interactions can therefore constrain where and when water has been present in volcanic regions this information is valuable for identifying fossil hydrothermal systems and determining relationships between climate near surface water abundance and the potential development of habitable niches on mars we examined the western tartarus colles region 25 27 n 170 171 e in northeastern elysium planitia mars and identified 167 vrc groups with a total area of 2000 km2 these vrcs preferentially occur where lava is 60 m thick crater size frequency relationships suggest the vrcs formed during the late to middle amazonian modeling results suggest that at the time of vrc formation near surface substrate was partially desiccated but that the depth to the midlatitude ice table was 42 m this ground ice stability zone is consistent with climate models that predict intermediate obliquity 35 between 75 and 250 ma with obliquity excursions descending to 25 32 for lava thicknesses ranging from 30 to 60 m and ground ice fractions ranging from 0 1 to 0 3 an ice volume of 4 23 km3 could have been melted and or vaporized by the time the lava solidified and the associated hydrothermal systems could have retained temperatures 273 k for up to 1300 years

explosive lava water interactions ii self organization processes among volcanic rootless eruption sites in the 1783 1784 laki lava flow iceland we have applied quantitative geospatial analyses to rootless eruption sites in the hnuta and hrossatungur groups of the 1783 1784 laki lava flow to establish how patterns of spatial distribution can be used to obtain information about rootless cone emplacement processes and paleo environments this study utilizes sample size dependent nearest neighbor nn statistics and voronoi tessellations to quantify the spatial distribution of rootless eruption sites and validate the use of statistical nn analysis as a remote sensing tool our results show that rootless eruption sites cluster in environments with abundant lava and water resources but competition for limited groundwater in these clusters can cause rootless eruption sites to develop repelled distributions this pattern of self organization can be interpreted within the context of resource availability and depletion topography tends to concentrate lava fuel and water coolant within topographic lows thereby promoting explosive lava water interactions in these regions given an excess supply of lava within broad sheet lobes rootless eruption sites withdraw groundwater from their surroundings until there is insufficient water to maintain analogs to explosive molten fuel coolant interactions rootless eruption sites may be modeled as a network of water extraction wells that draw down the water table in their vicinity rootless eruptions at locations with insufficient groundwater may either fail to initiate or terminate before explosive activity has ceased at nearby locations with a greater supply of water thus imparting a repelled distribution to observed rootless eruption sites

evolution of continental crust in southern africa abstract nd isotopic data from the zimbabwe and kaapvaal cratons and the limpopo kalahari namaqualand and damara mobile belts imply that over 50 of present day continental crust in this region had separated from the mantle by the end of the archaean and that

discrete blasts in granular material yield two stage process of cavitation and granular fountaining a discrete blast within granular material such as a single subterranean explosion within a debris filled diatreme structure is typically considered to produce a single uprush of material our experiments demonstrate that apparent debris jet deposits can be formed by a two stage process of cavitation and subsequent granular fountaining bench scale experiments reported here demonstrate that for a range of overpressures and depths individual discrete buried gas blasts open space and expel particles from the blast site in two largely decoupled stages expanding gas initially pierces material nearest the blast source to open a cavity above it then a fountain of grains rises from the source into the cavity this staged motion dynamically segregates source grains from host material grains and the rates of cavity opening versus fountain rise show a power law decay relationship with initial pressure our experimental analysis has implications for maar diatreme systems field scale detonation experiments and underground nuclear testing

syn and posteruptive hazards of maar diatreme volcanoes abstract maar diatreme volcanoes represent the second most common volcano type on continents and islands this study presents a first review of syn and posteruptive volcanic and related hazards and intends to stimulate future research in this field maar diatreme volcanoes are phreatomagmatic monogenetic volcanoes they may erupt explosively for days to 15 years above the preeruptive surface a relatively flat tephra ring forms below the preeruptive surface the maar crater is incised because of formation and downward penetration of a cone shaped diatreme and its root zone during activity both the maar crater and the diatreme grow in depth and diameter inside the diatreme which may penetrate downwards for up to 2 5 km fragmented country rocks and juvenile pyroclasts accumulate in primary pyroclastic deposits but to a large extent also as reworked deposits ejection of large volumes of country rocks results in a mass deficiency in the root zone of the diatreme and causes the diatreme fill to subside thus the diatreme represents a kind of growing sinkhole due to the subsidence of the diatreme underneath the maar crater is a subsidence crater and also grows in depth and diameter with ongoing activity as long as phreatomagmatic eruptions continue the tephra ring grows in thickness and outer slope angle syneruptive hazards of maar diatreme volcanoes are earthquakes eruption clouds tephra fall base surges ballistic blocks and bombs lahars volcanic gases cutting of the growing maar crater into the preeruptive ground formation of a tephra ring fragmentation of country rocks thus destruction of area and ground changes in groundwater table and potential renewal of eruptions the main hazards mostly affect an area 3 to possibly 5 km in radius distal effects are comparable to those of small eruption clouds from polygenetic volcanoes syneruptive effects on infrastructure people animals vegetation agricultural land and drainage are pointed out posteruptive hazards concern erosion and formation of lahars inside the crater a lake usually forms and diverse types of sediments accumulate in the crater volcanic gases may be released in the crater compaction and other diagenetic processes within the diatreme fill result in its subsidence this posteruptive subsidence of the diatreme fill and thus crater floor is relatively large initially but will decrease with time it may last millions of years various studies and monitoring are suggested for syn and posteruptive activities of maar diatreme volcanoes erupting in the future the recently formed maar diatreme volcanoes should be investigated repeatedly to understand more about their syneruptive behaviour and hazards and also their posteruptive topographic limnic and biologic evolution and potential posteruptive hazards for future maar diatreme eruptions a hazard map with four principal hazard zones is suggested with the two innermost ones having a joint radius of up to 5 km areas that are potentially endangered by maar diatreme eruptions in the future are pointed out

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

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

lava and ice interaction at stratovolcanoes use of characteristic features to determine past glacial extents and future volcanic hazards structures resulting from lava and ice interaction are common at glaciated stratovolcanoes during summit eruptions at stratovolcanoes meltwater is produced and travels freely down steep slopes and thin permeable valley glaciers eroding the ice and enlarging preexisting glacial drainages as a result eruptions in this environment have produced few catastrophic floods lava flowing into the open channels and voids in the glaciers becomes confined and grows thicker filling the available space and producing steep sided bodies with smooth bulbous contact surfaces quenching of lava against ice or by water forms small scale features such as tensional fractures and glass as the amount of meltwater in contact with the lava increases the type and abundance of smaller scale features become similar to those produced during subglacial eruptions into meltwater lakes identification of large and small scale lava ice contact features in the field can be used to reconstruct paleoglacial extent and combined with geochronology of lavas to determine past paleoclimate an understanding of lava ice interaction allows us to better assess the hazards posed by future eruptions at glaciated volcanoes

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

structural analysis and implicit 3d modelling of high grade host rocks to the venetia kimberlite diatremes central zone limpopo belt south africa abstract the beit bridge complex of the central zone cz of the limpopo belt hosts the 519 6 ma venetia kimberlite diatremes deformed shelf or platform type supracrustal sequences include the mount dowe malala drift and gumbu groups comprising quartzofeldspathic units biotite bearing gneiss quartzite metapelite metacalcsilicate and ortho and para amphibolite previous studies define tectonometamorphic events at 3 3 3 1 ga 2 7 2 5 ga and 2 04 ga detailed structural mapping over 10 years highlights four deformation events at venetia rules based implicit 3d modelling in leapfrog geo provides an unprecedented insight into cz ductile deformation and sheath folding d 1 juxtaposed gneisses against metasediments d 2 produced a pervasive axial planar foliation s 2 to isoclinal f 2 folds sheared lithological contacts and s 2 were refolded into regional open predominantly southward verging e w trending f 3 folds intrusion of a hornblendite protolith occurred at high angles to incipient s 2 constrictional prolate d 4 shows moderately ne plunging azimuths defined by elongated hornblendite lenses andalusite crystals in metapelite crenulations in fuchsitic quartzite and sheath folding d 4 overlaps with a 1 2 03 2 01 ga regional m 3 metamorphic overprint b transpressional deformation at 2 2 1 9 ga and c 2 03 ga transpressional dextral shearing and thrusting around the cz and d formation of the avoca bellavue and baklykraal sheath folds and parallel lineations

steep extrusion of late archaean granulites in the northern marginal zone zimbabwe evidence for secular change in orogenic style the northern marginal zone nmz of the limpopo belt in southern zimbabwe is an essentially plutonic terrain now consisting of granulite gneisses that was assembled by extensive magmatic under and intraplating between ca 2 7 and 2 6 ga during late archaean crustal shortening in this study we present the first evidence for regional scale normal shear in the nmz normal shear has been identified along an up to 500m wide steeply dipping belt of mylonites the mtilikwe shear zone geochronologic and mineralogical textural data indicate that normal shear occurred coeval with crustal shortening that was accommodated along gently dipping thrust zones the bulk strain pattern consisted of horizontal shortening vertical stretching and sub vertical extrusion of a crustal segment horizontal shortening persisted throughout the orogenic evolution in the nmz and no features typical of gravitational collapse in modern orogens are seen this could imply that there was never major overthickening of the crust by thrusting which may be a characteristic feature of archaean orogeny

geochemistry and origin of the archaean beit bridge complex limpopo belt south africa the beit bridge complex bbc in the central zone of the limpopo belt in southern africa includes metasedimentary rocks of archaean age chiefly metapelites quartzites and carbonates amphibolites chiefly mafic dykes and sills and granitoids which comprise 50 of the complex the amphibolites are tholeiites with relatively unfractionated ree patterns and pronounced subduction zone geochemical signatures in terms of relatively immobile incompatible elements tonalites and granites which also exhibit such a signature can be formed by batch melting of a mafic source with residual garnet or amphibole most granites have negative eu anomalies and appear to represent residual liquids bbc metapelites have negative eu anomalies and other immobile element distributions similar to phanerozoic shales one group of metapelites that shows significant depletion in ca na and sr appears to have been derived from intensely weathered sources the negative eu anomalies in bbc metapelites suggest intracrustal melting of their sources and thus the presence of evolved continental crust in this part of southern africa by about 3000 ma bbc quartzites are quartz rich sandstones that are depleted in hfs elements ba and sr relative to phanerozoic counterparts a feature that probably reflects less recycling during the early archaean bbc banded iron formation bif is similar to other archaean bif and shows large positive eu anomalies bbc carbonates are dolostones depleted in ree hfs elements u th ba sr ni sc v and y and enriched in fe mn na rb and cs relative to phanerozoic marine carbonates

rootless cones on mars a consequence of lava ground ice interaction fields of small cratered cones on mars are interpreted to have formed by rootless eruptions due to explosive interaction of lava with ground ice contained within the regolith beneath the flow melting and vaporization of the ice and subsequent explosive expansion of the vapour act to excavate the lava and construct a rootless cone around the explosion site similar features are found in iceland where flowing lava encountered water saturated substrates the martian cones have basal diameters of c 30 1000 m and are located predominantly in the northern volcanic plains high resolution mars orbiter camera images offer significant improvements over viking data for interpretation of cone origins a new model of the dynamics of cone formation indicates that very modest amounts of water ice are required to initiate and sustain the explosive interactions that produced the observed features this is consistent with the likely low availability of water ice in the martian regolith the scarcity of impact craters on many of the host lava flows indicates very young ages suggesting that ground ice was present as recently as less than 10 l00 ma and may persist today rootless cones therefore act as a spatial and temporal probe of the distribution of ground ice on mars which is of key significance in understanding the evolution of the martian climate the location of water in liquid or solid form is of great importance to future robotic and human exploration strategies and to the search for extraterrestrial life

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

the geology and tectonic evolution of the pietersburg greenstone belt south africa abstract the pietersburg greenstone belt occurs near the northern margin of the low grade granite greenstone terrain of the kaapvaal craton to the north lies the high grade granite greenstone terrain which constitutes the southern marginal zone of the limpopo belt the original relationship between the two terrains has been overprinted by the effects of the limpopo orogeny and are therefore not known a better understanding of the tectonic evolution of the pietersburg greenstone belt may help to clarify this relationship the rocks of the southwestern part of the pietersburg greenstone belt have been mapped in considerable detail the rocks can be subdivided into two sequences the first comprises a sequence of massive to pillowed metabasalts metagabbros and metaperidotites as well as bif fine grained mafic tuffs and sediments the sequence was subjected to extensive hydrothermal metamorphism at low pressures and variable temperatures in the greenschist to amphibolite facies this suggests an oceanic like environment the second sequence known as the uitkyk formation comprises a sequence of terrestrial clastics which overlie the first sequence with a profound unconformity sedimentology indicates that the uitkyk formation formed in a prograding alluvial fan or braided alluvial plain the uitkyk sediments were deformed and infiltrated by fluids that precipitated quartz tourmaline gold and siderite soon after deposition age relationships bedding cleavage relationships and younging directions indicate that both the sediments and simatic rocks were involved in a phase of thrust related deformation d2 that resulted in south over north stratigraphic repetition under generally low grade conditions of metamorphism post d2 granites date this thrusting at pre 2 69 ga d2 was probably diachronous and at surface the deformation was accompanied by syntectonic terrestrial sedimentation and placer gold deposition the tectonic environment was probably one of an intermontane accretionary or terrestrial foreland basin within this tectonic environment the basement and cover became tectonically stacked across evolving shear zones combined field and microstructural relationships within a major fault zone have revealed a complex history of progressive d2 deformation metamorphism mineralisation and associated sedimentary processes the zone represents a deformed syn sedimentary duplex that was carried piggy back above an allochthonous thrust sheet of simatic rocks at the deepest levels this fault and other shear zones were intruded by syntectonic granitoids this late archaean northward verging d2 thrust stack model for the greenstone belt with the syntectonic granites intruding the thrusts from beneath the greenstone belt is in good accord with the recent geophysical observations the age of cover sedimentation is older than 2 69 ga but younger than 2 98 ga in age this supports suggestions that these deposits are age equivalent of the upper gold bearing sediments of the witwatersrand basin and that the northwards thrusting in both basins may be of the same age and related to the same regional tectonic forces the simatic rocks of the pietersburg greenstone belt may have been part of a more extensive allochthonous simatic terrain which includes the similar greenstone remnants of the southern marginal zone of the limpopo belt such a terrain must have been disrupted and metamorphosed during the northward directed imbrication and tectonic burial recorded in the pietersburg belt prior to the climax of the limpopo orogeny the latter subsequently severely disrupted the pietersburg greenstone belt during ventersdorp times 2 6 2 7 ga

constraints on the form of the p t loop in the southern marginal zone of the limpopo belt south africa abstract a generalised clockwise p t path consisting of a prograde portion a high grade isothermal decompression portion and an essentially isobaric cooling portion has been suggested for the metamorphic evolution of the southern marginal zone previous studies have concluded that the high grade decompression path started in the kyanite stability field peak metamorphic mineral compositions have been reset during the equilibrium decompression event and hydration during the cooling dominated isobaric portion occurred in the kyanite stability field the present petrographic study from the bandelierkop quarry a locality in the granulite zone of the southern marginal zone identified a series of sequential anatectic reactions which record a period of prograde heating from approximately 700 c to 850 c during this period pressure is constrained to within the sillimanite stability field by the production and preservation of prograde generations of sillimanite the maximum temperature melting event is closely followed by two divariant reactions ga qz sil cord and ga qz cord hy which occur in response to a period of general isothermal decompression a subsequent cooling dominated rehydration period is indicated by a crystallisation hydration reaction cord ksp melt bi ky qz which occurred at a minimum temperature of approximately 640 c and pressure greater than 6 5 kbar the present data confirms the clockwise form of the loop and also supports the previously suggested flat slope of the retrograde cooling portion additional mineral chemistry data relating to the second decompression reaction indicate that the zonation profiles of cordierite and garnet as well as the growth of small quantities of garnet after the reaction are in strong support of the suggested p t path the present mineral compositions in the southern marginal zone reflect a near total re equilibration during the high grade cooling dominated portion of the p t path and as a result reflect p t conditions considerably lower than those of the peak metamorphism

evidence of crustal thinning beneath the limpopo belt and lebombo monocline of southern africa based on regional gravity studies and implications for the reconstruction of gondwana abstract the 600 km long by 250 km wide limpopo belt forms a wedge shaped structure between the zimbabwe and kaapvaal cratons in southern africa the results of a gravity survey which included the part of the limpopo belt within zimbabwe are incorporated with other data and presented as a regional gravity map three gravity profiles crossing the limpopo belt and one within mozambique have been interpreted in terms of lateral changes in crustal structure these interpretations are controlled by density measurements and seismic refraction data across the northern and central parts of the belt the zimbabwe craton appears from gravity modelling to be characterised by a horizontal moho at 34 km depth the limpopo belt is parallel to the ene trending gravity contours and in the east has a southwest northeast line of gravity highs coincident with the nuanetsi sabi volcanic rocks this study has shown that the regional gravity high associated with these features is consistent with crustal thinning probably caused by the breakup of gondwana the crust thins in a south southeast direction to about 30 km in the central limpopo belt with a dip on the moho which varies from about 4 to 7 the short wavelength positive gravity anomalies occurring on the nuanetsi sabi regional gravity high are due to late karoo igneous intrusions and volcanics a north south similar line of gravity highs exists along the lebombo monocline on the mozambique south africa border beyond the lebombo monocline to the east the gravity anomaly suggests a thickness of 6 km or more of sediments that could represent a paleo deltaic environment prograding over the continental margin the lebombo nuanetsi sabi gravity anomaly that dominates the gravity response in the eastern part of the study area is likely to be a failed arm triple junction and could imply a much tighter fit of gondwana than has been previously suggested as such the geometry of such a margin may provide a major constraint on the geometric fit of east antarctica with africa

the morphology and formation of flow lobe tumuli on icelandic shield volcanoes three types of tumuli are recognized on monogenetic shield volcanoes in iceland namely 1 lava coated tumuli 2 upper slope tumuli and 3 flow lobe tumuli flow lobe tumuli grade into lava rises which are larger inflation structures but both structures have a similar mode of emplacement the areal distribution of flow lobe tumuli in the flow field differs from that of lava coated tumuli and upper slope tumuli flow lobe tumuli are frequent in the medial and distal parts of pahoehoe flow fields whereas the other two tumulus types are more frequent in the proximal parts of the flow fields the flow lobe tumuli also differ from the two other tumulus types as regards their morphology they are significantly larger have shallower flanks and do not have extensive outflows from the cracks the vesicle distribution within the flow lobe tumuli is similar to that of p type flow lobes a fairly dense zone is situated between the upper and lower vesicular zones each flow lobe tumulus is an individual lobe that inflates and forms tension cracks in the lava crust major lava tubes are mostly absent from the areas where flow lobe tumuli form and therefore the emplacement of flow lobe tumuli must be related to secondary short lived pathways of lava the flow lobe tumuli inside the flow fields of icelandic shield volcanoes are morphologically identical to those flow lobe tumuli that are found in the margins of flow fields of other icelandic volcanoes these observations allow us to model the flow lobe tumuli as morphological variants of p type flow lobes forming along terminating secondary lava paths flow lobe tumuli form at low lava supply rates of the order of 10 2 10 5 m3 s and are generated by inflation of the lava crust as a result of magmatic overpressure in the associated lava core we model the tumulus as a circular elastic plate that becomes uplifted inflated and fractured by a uniform magmatic overpressure at its bottom we conclude that 1 the effective thickness of the lava crust that is subject to bending is 0 2 0 4 m and remains essentially constant during the formation of the tumulus and 2 for flow lobe tumuli that are supplied with magma from tubes that originate in overflow from the lava lake of a shield volcano the elevation difference between the tumuli and the surface of the source lava lake is commonly around 100 m this elevation difference can give rise to a static magmatic overpressure of 2 5 mpa in the tumulus whereas the overpressure needed to form a typical flow lobe tumulus is only 0 2 1 mpa

tectonic evolution of a greenstone sequence in northern zimbabwe sequential early stacking and pluton diapirism structural metamorphic relationships in the 2 71 2 64 ga harare shamva greenstone belt in northern zimbabwe demonstrate that the archean stratigraphy of the zimbabwe craton can not be regarded as a coherent and nonrepetitive autochthonous succession of ultra mafic and felsic greenstones the greenstone belt shows an early set of amphibolite grade d1 m1 structures and associated synkinematic metamorphic assemblages that were formed during an episode of layer parallel shearing at about 2 64 ga this resulted in sequential west directed imbricate stacking and recumbent folding of distinct tectonostratigraphic greenstone sequences and includes the development of a syntectonic sedimentary succession d2 m2 fabrics strain patterns and contact aureoles overprint d1 geometries and are related to the diapiric emplacement of the chinamora batholith the structural data indicate that granite gneiss terrains can be subdivided into two types 1 structurally emplaced terrains the madziwa gneiss terrain and 2 diapiric domes the chinamora batholith the history of the harare shamva greenstone belt indicates that the late archean 2 7 2 6 ga evolution of the zimbabwe craton must allow for accretion of diverse crustal fragments including oceanic and back arc mafic crust volcanic arc felsic crust continental crust and related sedimentary sequences the process of accretion was probably diachronous across the craton and may have involved concomitant diapirism strike slip faulting magma intrusion and sedimentation

explosive lava water interactions i architecture and emplacement chronology of volcanic rootless cone groups in the 1783 1784 laki lava flow iceland to determine the relationships between rootless cone emplacement mechanisms morphology and spatial distribution we mapped the hnuta and hrossatungur groups of the 1783 1784 laki lava flow in iceland we based our facies maps on differential global positioning system dgps measurements photogeological interpretations and supporting field observations the study area covers 2 77 km2 and includes 2216 explosion sites to establish the timing of rootless cone formation we incorporated tephrochronological constraints from eighty eight stratigraphic sections and determined that the hnuta and hrossatungur groups are composite structures formed by the emplacement of six geographically and chronologically discrete domains rootless eruptions initiated in domain 1 on the first day of the laki eruption june 8 1783 and lasted 1 2 days the second episode of rootless activity began in domain 2 on june 11 and lasted 1 3 days the four domains of the hrossatungur group dominantly formed after june 14 and exhibit a complex emplacement sequence that reflects interactions between the laki lava contemporaneously emplaced rootless cones and an existing topographic ridge in the study area we identify three distinct rootless cone archetypes i e recurring morphological forms that are related to tube channel and broad sheet lobe fed eruptions we assert that emplacement of lava above compressible substrates e g unconsolidated sediments may trigger rootless eruptions by causing subsidence induced flexure and failure of the basal crust thereby allowing molten lava fuel to come into direct contact with groundwater coolant and initiating analogs to explosive molten fuel coolant interactions mfcis

ultrahigh temperature granulites and magnesian charnockites evidence for neoarchean accretion along the northern margin of the kaapvaal craton abstract shrimp u pb analysis of zircon grains from an ultrahigh temperature mg al granulite from the northern margin of the kaapvaal craton southern marginal zone shows ages of 2 72 ga for the peak metamorphism geochemical characteristics of syn tectonic charnockites in the area are similar to magnesian charnockites typically formed in subduction settings in contrast to the prevailing models on peak metamorphism in the context of neoarchean continent collision between archean zimbabwe and kaapvaal cratons resulting in the formation of the limpopo complex we relate the neoarchean events to the accretion of this marginal block to the rest of the kaapvaal craton in this model the subduction event is marked by the formation of magnesian charnockites and the ultrahigh temperature metamorphism dates the collision event

the paleoproterozoic waterberg group south africa provenance and its relation to the timing of the limpopo orogeny abstract four paleoproterozoic formations of the waterberg group in south africa are composed of coarse clastic detritus derived from erosion of the limpopo belt timing of the limpopo orogeny an event involving the collision of the kaapvaal and zimbabwe cratons has long been a contentious issue the results of point counting major and trace element geochemistry and u pb detrital zircon geochronology indicate that the waterberg sedimentary formations in the study area were primarily sourced by siliceous rifted margin sedimentary and minor mafic volcanic rocks of the archean beit bridge complex limpopo central zone the volumetrically predominant beige brown sandstones in the four studied formations are quartz rich with average qfr ratios of 80 7 13 blouberg 70 19 11 setlaole 88 5 7 makgabeng and 89 3 8 mogalakwena chert and arenite account for 90 of lithic fragments in all formations with minor siliceous gneiss fragments although all formations are silica enriched the makgabeng dune samples produce extremely high sio2 abundances 92 99 wt which are attributed to the presence of silica cement and quartz within rock fragments geochemically the stratigraphically highest mogalakwena formation is unique with elevated ti and zr values and intra formational differences in ree patterns the latter feature is consistent with a mixed provenance volumetrically minor green and purple sandstones in the waterberg formations contain the greatest cr ni ti and v abundances which supports localized derivation from a mafic or ultramafic source chemical index of alteration cia values range from 57 to 89 which could indicate significant chemical weathering of the source rocks but a plot of th sc versus zr sc illustrates that the sandstones have undergone recycling which was probably responsible for enrichments in al2o3 relative to na2o3 cao and k2o combinations of well rounded and subangular quartz grains support a recycled origin detrital zircons in the lowermost blouberg formation produced a wide array of ages ranging from ca 3379 to 2043 ma the youngest peak at 2046 ma is also the largest and represents the maximum age of deposition for the formation additional peaks at 3281 ma 3330 ma and 3379 ma are consistent with ages previously determined from the beit bridge complex whereas peaks at 2578 ma and 2649 ma coincide with ages determined from gneisses of the limpopo central zone derivation of detritus from the beit bridge complex is directly indicated by sedimentary and siliceous gneiss fragments in the sandstones subrounded quartz grains suggestive of relatively short transport distances green and purple sandstone drapes derived from mafic volcanic units and paleocurrent patterns consistent with south to southwest flow directions therefore the timing of deposition of the blouberg formation 2046 ma equates to the end stages of the limpopo orogeny this negates previous suggestions that the limpopo orogeny occurred only during the neoarchean