sequence stratigraphy of the neoproterozoic infra krol formation and krol group lesser himalaya india a bstract a sequence stratigraphic study of terrigenous and carbonate rocks of the infra krol formation and krol group in the lesser himalaya fold and thrust belt of northern india was undertaken as part of a broader investigation of the significance of carbon isotope data in neoproterozoic successions eight regional stratigraphic discontinuities were traced over a distance of nearly 300 km and interpretations were anchored in a series of local studies involving the mapping of key beds and the measurement of closely spaced sections three of the regional surfaces are interpreted as sequence boundaries on the basis of 1 locally developed incised valleys 60 m deep 2 paleokarstic depressions with 50 m of mappable relief 3 subaerial dissolution and weathering products breccias and calcrete filling vertical fissures dikes cavities and shallow de pressions in underlying carbonate rocks and 4 small scale evidence for sub aerial exposure at an erosion surface the remaining five discontinuities are regional flooding surfaces identified on the basis of either facies changes with an abrupt upward deepening across the surface or transitions in facies stacking patterns typically from forestepping to backstepping a glacio eustatic origin is permitted although not required for the three sequence boundaries but no evidence has been found for marked lowering of sea level at other horizons a mismatch between the stratigraphic location of sequence boundaries and car bon isotope minima suggests that local diagenetic alteration or oceanographic phenomena unrelated to glaciation may be in part responsible for observed isotopic variation and that small ice sheets may have existed during apparently nonglacial times without producing either cap carbonates or negative carbon isotope excursions

zircon shrimp dating confirms a palaeoarchaean supracrustal terrain in the southeastern kaapvaal craton southern africa abstract we report shrimp zircon ages for felsic volcanic rocks of the early archaean nondweni greenstone belt ngb located in northern kwazulu natal province south africa the ngb is part of a chain of greenstone belt remnants that occurs south of the well known barberton greenstone belt bgb two samples of felsic volcanic rocks of the basal toggekry formation yielded zircon ages of 3 53 ga whereas significantly younger ages of 3 41 ga were obtained for two samples of felsic rocks from near the top of the witkop formation the latter date now constrains the age of stromatolite like structures in cherts associated with the felsic rocks which probably represent some of the oldest preserved evidence of life on earth the zircon ages also indicate that the ngb correlates well chronologically with the bgb and is one of the oldest volcano sedimentary successions in the southeastern kaapvaal craton geochemical data for the felsic ngb rocks suggest formation by partial melting or crystal fractionation of a mafic source that was contaminated by older crust possibly in a back arc environment initial e nd values for ngb rocks range from 1 2 to 0 3 and suggest involvement of pre greenstone continental crust that was possibly extensive in the southeastern part of the kaapvaal craton during palaeoarchaean times

the early archaean nondweni greenstone belt southern kaapvaal craton south africa part i stratigraphy sedimentology mineralization and depositional environment abstract the nondweni greenstone belt is an exceptionally well preserved 3 4 ga succession of predominantly volcanic rocks near the southern margin of the kaapvaal craton nondweni group rocks show both similarities and differences to the barberton sequence of similar age 300 km to the north in contrast with the latter none of the nondweni lavas exceed 23 mgo and spinifex textures are exclusively of pyroxene type the characteristics of the lavas and magma genesis are discussed in part ii of this series of papers three formations are recognized in the nondweni group the lowermost mogongolozi formation comprises basaltic andesites komatiitic andesites and basalts with intercalated coarser grained pyroxenites and gabbros the toggekry formation is a felsic unit of quartz sericite schists with small cu zn massive sulphide ore bodies of volcanogenic origin the uppermost witkop formation comprises mainly basalts komatiites and komatiitic basalts intercalated with chert layers and capped by a clastic sedimentary succession the total exposed thickness of the nondweni group is 16 500 m is the result of duplication of stratigraphy and tectonic thickening during early d1 thrusting in the bedding plane of much thinner lithological units tuffaceous sediments comprise a relatively minor component of the stratigraphy but are indicative of depositional environment banded chert layers are largely derived from accumulations of accretionary lapilli tuffs although a diverse origin is indicated for some varieties at the base of the witkop formation a number of undeformed banded coarse grained graphitic quartzite bodies entirely devoid of sedimentary structures are considered to have originated as hydrothermal deposits resulting from fumarolic activity selective brecciation of layers resulted from explosive release of fluids and gases the textures of these bodies are in marked contrast to the highly rounded pseudo conglomerates from the clastic sedimentary succession at the top of the witkop formation this 500 m thick succession passes upwards through reworked tuffs and alluvial fan deposits into a zone of evaporites and stromatolites swallow tail twin crystals of barite and an evaluation of crystal interfacial angles are indicative f replacement of primary evaporative gypsum most primary features of the nondweni greenstone belt suggest a shallow water or subaerial depositional environment and together with characteristics of the magma composition indicate a tectonic setting in a subduction related continental margin setting

a review of the superior province of the canadian shield a product of archean accretion abstract the superior province consists of northern and southern high grade gneiss subprovinces and a central region of alternating granite greenstone and metasedimentary belts subprovinces are commonly fault bounded and display contrasting lithological assemblages metamorphic and structural styles geophysical characteristics and ages they are comparable to phanerozoic suspect terranes the middle and late archean rocks of the superior province are of mantle and juvenile crustal origin with little evidence of inheritance of components older than ca 3 1 ga high grade gneiss subprovinces are mainly deeper level equivalents of the supracrustal rich belts but one minnesota river valley is part of an older ca 3 6 ga terrane that was juxtaposed late in the tectonic history granite greenstone terranes are characterized by low grade volcano sedimentary sequences tholeiitic komatiitic submarine lava plain tholeiitic calc alkalic submarine to subaerial central complexes early platform type quartz arenite stromatolitic carbonate late timiskaming type shoshonitic alkalic volcanic fluvial sediment that range in age from ca 3 0 ga to 2 7 ga the volcanics represent oceanic island arc and continental arc volcanism the quartz arenite carbonate sequences deposition on relatively stable early formed ca 3 0 ga crust and the timiskaming type sequences deposition in pull apart basins developed along strike slip faults late archean ca 2 75 2 70 ga greenstone sequences occur throughout the superior province middle archean ca 3 0 2 8 ga sequences are common in northern and central superior province rare or absent in the south volcanism was accompanied by mafic to trondhjemitic plutonism the metasedimentary belts consist of variably metamorphosed volcanogenic turbidites with detrital zircons ranging in age from ca 3 0 ga to 2 7 ga the metasediments were deposited after major volcanism and probably represent accretionary prisms late archean orogenesis accompanied and followed by calc alkalic plutonism youngs southward from about 2 73 2 72 ga to 2 70 2 68 ga it involved large scale north south compression and dextral transpression and displays thin to thick skinned transitions the contemporaneity of events along the lengths of the belts and their southward younging coupled with the evidence of major compression are consistent with subduction driven oblique accretion of oceanic and continental volcanic arcs accretionary sedimentary wedges older microcontinental fragments etc in a convergent margin setting analogous to parts of the pacific rim differences eg abundance of komatiites and tonalitic plutonic suites are attributable to a hotter archean mantle

archaean flow top alteration zones formed initially in a low temperature sulphate rich environment the 3 500 3 400 myr onverwacht group in the southern part of the barberton greenstone belt south africa includes up to 12 km of komatiitic and tholeiitic lavas containing minor sedimentary and felsic volcanic units1 komatiitic lavas showing spinifex textures chrome spinels and high ni and cr contents are common throughout the sequence2 the rocks have been subject to polyphase archaean deformation and low grade metasomatism2 4 at several positions within the onverwacht group fig 1 the tops of komatiitic lavas underlying chert beds are marked regionally by 1 50 m thick zones of silicification carbonatization and k2o enrichment fig 2 because these zones have been variously interpreted to represent the felsic tops of mafic to felsic volcanic cycles weathering horizons and shear zones their origin is germane to any interpretation of the stratigraphy structure and evolution of the barberton belt here we present evidence that they formed inlarge part by low temperature near surface alteration of komatiitic lavas during intervals of volcanic quiescence they do not mark fault zones and cannot be used to subdivide the greenstone belt sequence into a series of fault bounded structural slices

archaean stromatolites from the ngesi group belingwe greenstone belt zimbabwe preservation and stable isotopes preliminary results abstract the petrology and stable isotope chemistry of cyanobacterial stromatolites of archaean age 2 7 ga from the cheshire and manjeri formations of the belingwe greenstone belt in zimbabwe have been examined palaeomagnetic data suggest that the stromatolites formed in tropical to subtropical latitudes the cheshire formation shows little evidence of either anion or cation exchange during metamorphism and the stable carbon and oxygen isotope ratios suggest a formation at temperatures perhaps considerably below 80 c the manjeri formation only slightly older but overlain by a thick volcanic sequence shows a low grade of metamorphism and isotope ratios that are consistent with a metamorphic temperature of around 200 c

cherts of the barberton greenstone belt interpreted as products of submarine exhalative activity cherts studied from the southern barberton greenstone belt are identified as replacement products of various pillow lavas pyroclastic deposits and sedimentary deposits silica filled fracture systems within the cherts have been found to be associated with this silicification process these systems include 1 silica filled fractures 2 silica vein stockworks 3 silica cemented breccias and 4 pseudoconglomerate bodies some of the silica filled fracture systems can be shown to have formed soon after sedimentation silica vein stockworks can be seen to develop laterally into silica cemented breccias the fragments of which have sometimes become rounded within the fracture systems to form pseudoconglomerate the cherts are interpreted as silicified hemi pelagic sediments that overlie pillow lavas some of which are now also cherts cells of convective sea water interacted with oceanic crust to produce silica rich fluids these fluids silicified lava and sediments on re entering the near surface env

geochemical characteristics of archean cherts and other sedimentary rocks in the pilbara block western australia evidence for archean seawater enriched in hydrothermally derived iron and silica abstract geochemical and mineralogical characteristics of archean sedimentary rocks such as cherts banded iron formations bifs shales and sandstones collected from the pilbara block western australia were studied lithologic features of various types of chert occurring within the pilbara supergroup are closely related to their chemical compositions red and brown bands of banded cherts generally show a higher fe 2 o 3 total iron as fe 2 o 3 concentration than do other cherts whereas grey greenish grey and pale yellowish grey cherts are characteristic of relatively higher concentrations of al and ti white cherts are generally composed almost exclusively of sio 2 geochemical characteristics of the cherts suggest that they were precipitated from hydrothermal solutions evidence for the hydrothermal origin includes 1 low mno fe 2 o 3 values 2 low concentrations of heavy metals 3 positive euanomalies 4 low co zn and ni zn values and 5 high concentrations of mn and fe in carbonates positive correlations between fe and heavy metals such as ni and zn imply the intermittent precipitation of fe oxides hydroxides along with these heavy metals in the ancient ocean cherts except for grey greenish grey and pale yellowish grey cherts are highly depleted in detrital materials this may have been caused by rapid precipitation of hydrothermally derived fe and si the archean pilbara ocean is believed to have been enriched in fe and si derived from hydrothermal activity

archaean plate tectonics revisited 1 heat flow spreading rate and the age of subducting oceanic lithosphere and their effects on the origin and evolution of continents a simple model which relates the rate of seafloor creation and the age of the oceanic lithosphere at subduction to the rate of continental accretion can successfully explain the apparent differences between archaean and phanerozoic terrains in terms of plate tectonics the model has been derived using the following parameters 1 the spreading rate at mid ocean ridges 2 the age of the oceanic lithosphere at the time of subduction 3 the area age distribution of the seafloor 4 the continental surface area as a fraction of the total surface area of the earth and 5 the erosion rate of continents as a function of continental surface area and the total number of continental masses observations in phanerozoic terranes suggest that there are profound differences in the nature and volume of subduction zone igneous activity depending upon the age of the oceanic lithosphere being subducted and the nature of the overriding plate that is either continental or oceanic the subduction of young oceanic lithosphere less than 50 m y old which is thermally buoyant appears to result in a reduced volume of igneous activity most of the igneous activity caused by subduction of young oceanic lithosphere is either siliceous plutonism or bimodal tholeiitic rhyolitic volcanism when very young lithosphere is being subducted 50 m y old appears to result in greater volumes of igneous activity including the eruption of andesitic magmas thus andesites could only begin to be abundant in the rock record when older oceanic lithosphere began to be subducted our model predicts that as the earth aged and as heat flow from the interior of the earth diminished the proportion of old oceanic lithosphere being subducted increased fundamentally changing the nature of subduction zone igneous activity and the rate of continental accretion if the subduction of old oceanic lithosphere results in an 8 10 times greater volume of subduction zone magmatism our model predicts or explains all of the following observed features of earth history 1 archaean terranes appear to record two periods of rapid continental accretion between 3 8 and 3 5 b y ago and between 3 1 and 2 6 b y ago 2 there are very few differences and many marked similarities between rocks from archaean terranes and equivalent rocks from phanerozoic terranes 3 the total continental area appears to have remained essentially constant for the past 2 b y 4 archaean andesites are comparatively rare and the relative abundances of mafic and siliceous rocks appear to change during the archaean and the proterozoic with siliceous volcanics becoming proportionately more abundant in the geologic record with time 5 plutonic tonalites and trondhjemites appear to have been relatively much more abundant during the archaean plate tectonics is thus shown to have evolved over time due to a gradually decreasing rate of creation of oceanic lithosphere meaning that archaean tectonics and phanerozoic tectonics are but two points on an evolutionary continuum

the umsoli chert turbidite testament for a major phreatoplinian event at the onverwacht fig tree transition swaziland supergroup archaean south africa abstract the volcanogenic origin of accretionary grains in the oolitic umsoli chert is proved by the unequivocal identification of constituent vesicular and pumiceous ash particles by the discovery of authigenic alkali feldspars and by the k meta bentonite parentage of associated illites celadonites ash particle morphology and ash grain size distribution suggest a phreato magmatic process of eruption sedimentary structures indicate that the felsic ash precursor of this ancient chert was deposited by turbidity currents the sedimentologic model invokes the transition of parts of pyroclastic falls and base surges into turbidites accretionary lapilli forming a major constituent of this silicified tuff oolite may be indicators of subaerial eruption but they can probably not be used as evidence for subaerial deposition

gliding and overthrust nappe tectonics in the barberton greenstone belt abstract regional recumbent folds inverted stratigraphy nappes and olistostromes are described from the southern part of the 3 3 3 5 ga barberton greenstone belt overthrusting of thin rigid silicified slabs with minimum dimensions of 25 km2 and up to 500 m in thickness occurred over minimum distances of 86 km more ductile and coherent units were overfolded up to at least 2 km during their emplacement the glide planes on which these nappes travelled were zones of high fluid pressures related to hydrothermal fluid circulation patterns driven by heat sources from igneous intrusions the upwelling areas of the geothermal convection cells were sites of mud pools and hydrothermal vents which may mark the trailing edges pull aparts of the overthrust units progressive silica and carbonate precipitation due to decreasing temperatures within the zones of fluid migration distant from the areas of high heat flow probably acted as built in braking systems below the travelling slabs active sedimentation and metasomatism during this tectonism indicates a protracted history for the evolution of the greenstone belt the recognition of nappe and overthrust tectonics in the barberton belt processes which may have been commonplace in archaean terrains necessitates a re evaluation of the stratigraphy of this belt

do graded units of accretionary spheroids in the barberton greenstone belt indicate archaean deep water environment we report here a re analysis of the sedimentology of the msauli chert oolite which shows that environmental conditions during the early depositional phase of the fig tree group in the barberton greenstone belt were of a deep water nature the unit is compared with quaternary sediments deposited in the pacific ocean at bathyal to abyssal depths

a smnd and pb isotope study of archaean greenstone belts in the southern kaapvaal craton south africa abstract a sm nd and pb study on a wide variety of lithologies in archaean greenstone belt fragments in the southern kaapvaal craton reveals a complex petrogenetic history the fragments are important because they represent a 350 km transect through the craton south of barberton to its southern margin the commondale greenstone belt yields a precise sm nd age of 3334 18ma on an exceptionally well preserved peridotite suite of komatiitic affinity the wide range of sm nd from 0 6 to 1 0 is attributed to the unusual occurrence of orthopyroxene in the spinifex bearing rocks a considerably younger age of about 3 2 ga is suggested for the nondweni greenstone belt close to the southern margin of the craton on the basis of separate sm nd isochrons on individual lithologies ranging from komatiite through komatiitic basalt and basalt to felsic volcanic rocks the most precise sm nd age for the greenstone belt of 3208 87ma is based on a combined mineral whole rock isochron for komatiitic basalts but the possibility cannot be excluded that this represents a later metamorphic age on the basis of the present study the greenstone belts appear to have been emplaced at progressively younger ages towards the southern margin of the craton of greater significance is that irrespective of age the individual lithologies of the nondweni sequence have isotopically distinct initial ratios combining isotopic data from these lithologies invariably yields apparent older ages the isotopic characteristics are consistent with a model of progressive contamination by older sialic crust thought to be the basement to the nondweni sequence

sea floor massive sulfide deposits from 21 degrees n east pacific rise juan de fuca ridge and galapagos rift bulk chemical composition and economic implications

a procedure for partitioning bulk sediments into distinct grain size fractions for geochemical analysis a method to separate sediments into discrete size fractions for geochemical analysis has been tested the procedures were chosen to minimize the destruction or formation of aggregates and involved gentle sieving and settling of wet samples freeze drying and sonication pretreatments known to influence aggregates were used for comparison freeze drying was found to increase the silt clay ratio by an average of 180 percent compared to analysis of a wet sample that had been wet sieved only sonication of a wet sample decreased the silt clay ratio by 51 percent the concentrations of metals and organic carbon in the separated fractions changed depending on the pretreatment procedures in a manner consistent with the hypothesis that aggregates consist of fine grained organic and metal rich particles the coarse silt fraction of a freeze dried sample contained 20 44 percent higher concentrations of zn cu and organic carbon than the coarse silt fraction of the wet sample sonication resulted in concentrations of these analytes that were 18 33 percent lower in the coarse silt fraction than found in the wet sample sonication increased the concentration of lead in the clay fraction by an average of 40 percent compared to an unsonicated sample understanding the magnitude of change caused by different analysis protocols is an aid in designing future studies that seek to interpret the spatial distribution of contaminated sediments and their transport mechanisms

hydrothermal chert and associated siliceous rocks from the northern pacific their geological significance as indication od ocean ridge activity adachi m yamamoto k and sugisaki r 1986 hydrothermal chert and associated siliceous rocks from the northern pacific their geological significance as indication of ocean ridge activity sediment geol 47 i25 148 analyses of 42 cretaceous rock samples from deep sea drilling project dsdp leg 32 in tile northern pacific show that chert and porcellanite are enriched in fe mn p cu ba mo ni and pb relative to both triassic biogenic cherts of central japan and cretaceous dsdp cherts of leg 62 the iron enrichment up to 6 68ve fe20 as total iron is most prominent in chert immediately overlying basal basah which is the only chert free of radiolarians and veined by megaquartz containing many fluid inclusions chemical features characterized by a low ratio for a1 ai fe mn and ai z tio 2 and a high ratio for total fe2oj tio 2 mno tio 2 and mo tio 2 as well as petrological evidence indicatc that the cherts formed in an intense submarine hydrothermal environment on or near the cretaceous japanese spreading center which separated the kula plate from the pacific plate the spreading rate of the cretaceous japanese spreading center estimated from the chemical composition of the cherts was about 3 7 cm r i the rate is comparable to that inferred from the geomagnetic data

sedimentology of the middle marker 3 4 ga onverwacht group transvaal south africa abstract the middle marker is a thin 3 6 m sedimentary unit at the base of the hooggenoeg formation in the 3 4 ga old onverwacht group barberton mountain land south africa the original sediments consisted largely of current deposited volcaniclastic detritus now represented by green to buff colored silicified volcaniclastic rock and fine grained gray chert black chert possibly formed by the silicification of a non volcaniclastic precursor makes up a significant part of the unit the middle marker is underlain and overlain by mafic and commonly pillowed volcanic flowrock although the original sediment has been replaced by and or recrystallized to a microquartz chlorite sericite carbonate and iron oxide mosaic under lower greenschist grade metamorphism sedimentary textures and structures are remarkably well preserved textural pseudomorphs indicate the primary volcaniclastic sediment consisted of a mixture of crystal vitric and lithic debris middle marker sediments were deposited as a prograding cone flanking volcaniclastic sedimentary platform in a relatively shallow and locally current wave influenced subaqueous sedimentary environment available paleocurrent data indicate a largely bimodal orthogonal distribution pattern which is quite similar to both ancient and modern shallow marine shelf systems diagnostic evidence for tidal activity is lacking as felsic volcanic activity waned an extensive airfall blanket of fine grained volcanic ash and dust was deposited in a low energy subaqueous environment the sedimentary cycle was terminated with a renewal of submarine mafic volcanism middle marker volcaniclastic sediments accumulated in an anorogenic basin removed or isolated from the influence of continental igneous and metamorphic terranes although compositionally dominated by a volcanic source middle marker sediments owe their final texture and sedimentary structures to subaqueous sedimentary rather than volcanogenic processes

comparative sedimentology of the principal volcanic sequences of archean greenstone belts in south africa western australia and canada implications for crustal evolution abstract significantly different assemblages of sedimentary rocks are present in the volcanic sequences of archean greenstone belts in the barberton mountain land pilbara block and canadian shield thin sedimentary layers in the 3 55 3 3 ga old onverwacht and warrawoona groups in the barberton mountain land and eastern pilbara respectively are made up of silicified volcaniclastic sediments banded cherts carbonaceous cherts silicified evaporites and locally stromatolites deposited under shallow water anorogenic conditions the volcanic sequences in these belts essentially lack terrigenous debris derived by weathering and erosion algoma type iron formation volcanogenic massive sulfide deposits hyaloclastic breccias and evidence for the existence during their accumulation of older sialic basement sedimentary rocks in the volcanic sequences of the 2 8 2 6 ga old superior and slave provinces canada and perhaps in other late archean belts in the rhodesian craton and yilgarn block include generally unsilicified volcaniclastic units mafic hyaloclastic breccias terrigenous clastic layers abundant algoma type iron formation and massive sulfide deposits sedimentation took place under relatively deep water conditions local tectonic activity was widespread and older sialic basement was present these younger volcanic sequences contain few shallow water deposits such as carbonaceous cherts evaporites and stromatolites the volcanic sequences of both generations of greenstone belts seem to have formed platforms that stood topographically above the surrounding sea floor differences between older and younger greenstone belt volcanic sequences appear to reflect the increasing abundance and influence of sialic crust from early to late archean time and an accompanying increase in ocean water depth over the volcanic platforms evolution of the sialic crust can be traced through at least five archean and early proterozoic stages 1 1 an early period of ensimatic shallow water greenstone belt formation 3 55 3 3 ga ago 2 2 an early episode of intrusive and metamorphic activity from ca 3 3 2 9 ga ago during which some of the older greenstone belts were cratonized 3 3 a late period of greenstone belt formation 2 8 2 6 ga ago during which volcanism occurred under relatively deep water conditions upon and or adjacent to areas of older sialic basement 4 4 a late archean interval of continental crust formation during which the younger greenstone belts were cratonized 2 7 2 5 ga ago 5 5 a latest archean and early proterozoic period dominated by stable cratons and relatively minor greenstone belt activity

stromatolites from the 3 300 3 500 myr swaziland supergroup barberton mountain land south africa a morphologically variable assemblage of stromatolites has been discovered in thin chert layers within the fig tree group of the swaziland supergroup south africa they are commonly low relief nearly stratiform laterally linked domes rarer forms include pseudocolumns and crinkly stratiform stromatolites the stromatolites grew on a substrate of altered komatiitic lava and sediments deposited on the lava surface and in most places are covered by later komatiitic flows abundant fine grained tourmaline included within the stromatolite laminae suggests that stromatolites formed in an environment dominated by boron rich hot spring emissions and evaporitic brines

formation of massive sulfide deposits on oceanic ridge crests incremental reaction models for mixing between hydrothermal solutions and seawater equilibrium path calculations have been used to model mixing between hot 350 c hydrothermal solutions and ambient seawater in an attempt to simulate mineral precipitation at seafloor vents these calculations predict temperatures of precipitation paragenetic sequence of minerals and chemical composition of chimney deposits associated with vents on the seafloor at 21 n epr assuming sulfate sulfide disequilibrium during mixing the paragenetic sequence revealed is chalcopyrite anhydrite pyrrhotite pyrite sphalerite graphite and barite when sulfate sulfide equilibria is permitted during mixing however reduction of small amounts of sulfate results in early precipitation of pyrite and a sequence of cu rich sulfide minerals chalcopyrite bornite chalcocite covellite this sequence is analogous to that observed in thin chimney walls the calculations indicate that sulfide mineral precipitation occurs in response to both cooling and change in composition of the hydrothermal solutions as a result of mixing varying the amount of mixing with respect to temperature simulating conductive heating of seawater prior to mixing results in only minor variations in the sequence and abundance of precipitated phases anhydrite precipitation during mixing occurs early which is consistent with formation of an anhydrite leading edge of chimney structures similarly extrapolation of warm spring data from galapagos to zero so4 concentration suggests anhydrite formation due to mixing with seawater beneath the seafloor most likely below the level of reactive calcareous sediments subsequent interaction of the mixed hydrothermal solution with those sediments results in elevated and variable ca concentrations estimated for end member solutions from the galapagos precipitation of mg hydroxide sulfate hydrate in the walls of the vent chimneys at 21 n epr occurs as a result of conductive heating of ambient seawater with only very minor amounts of mixing in contrast precipitation of amorphous silica in the vents must be due to conductive cooling of the hydrothermal solutions thus incremental reaction calculations demonstrate that reactions occurring in and associated with venting ridge crest hydrothermal solutions can be effectively modeled using the thermodynamic data and reaction modeling codes available today departures from equilibrium required to accurately model the mixing process are easily accommodated and consistent with data from the vents and vent forming materials

restricted shallow water sedimentation of early archean stromatolitic and evaporitic strata of the strelley pool chert pilbara block western australia abstract the 3 4 ga old strelley pool chert is a 25 m thick sedimentary unit near the top of the predominantly volcanic warrawoona group in greenstone belts of the eastern pilbara block western australia it is here subdivided into 5 members containing 13 lithofacies the basal member i is composed of quartzose sandstone deposited in a high energy wave or tide dominated shallow water system overlying this are members ii and iii which make up the bulk of the formation and were deposited in a low energy partially restricted hypersaline basin they record a predominantly regressive succesion of deposits including subaqueous laminite stromatolite and evaporite stromatolite carbonaceous laminite black and white banded chert evaporite and intraformational detrital units deposited under intermittently to predominantly exposed conditions and subaerially deposited windblown sand evaporite and evaporite solution layers members iv and v record the progradation of a volcaniclastic alluvial fringe the strelley pool chert represents an association of sedimentary environments directly comparable to that observed in modern low energy shallow marine carbonate evaporite systems such as along the trucial coast of the persian gulf and abundantly developed in phanerozoic carbonate platform deposits there is no evidence however that uniquely identifies the environment as having been marine deposition may have taken place in either a large hypersaline lake or a restricted marine basin evidence of predominantly low energy depositional conditions and a paucity of terrigenous detritus indicate that sedimentation was dominated by orthochemical and biological processes silicified evaporites including coarsely crystalline layers resembling messinian selenite are widespread and similar to younger evaporite deposits they clearly indicate that evaporites were common within shallow water archean sequences the presence of an assemblage of biogenic deposits including organic laminite stromatolites encrusting carbonaceous mats carbonaceous granules and oncolites deposited under conditions ranging from fully subaqueous to nearly subaerial and locally evaporitic points to the existence of an ecologically and probably biologically diverse microbial community 3 4 ga ago

the early archaean nondweni greenstone belt southern kaapvaal craton south africa part ii characteristics of the volcanic rocks and constraints on magma genesis the early archaean nondweni greenstone belt is dominated by mafic volcanic rocks which have a wide range in compositions primary volcanic structures and compositional characteristics point to this succession being unique in archaean geology and an important indicator of controls of magma formation and environment of deposition compositionally the nondweni lavas are broadly similar to those of the barberton sequence high values for cao al2o3 ti y and low al2o3 tio2 they are also distinctly different in that sio2 content is markedly higher and incompatible trace element ratios are also different the high silica content has promoted the subdivision of komatiitic and basaltic rocks into the following categories komatiite komatiitic andesite komatiitic basalt basalt and basaltic andesite the recognition of the distinct suite of komatiitic andesites is aided by the lower zr ti ratios than those lithologies of equivalent mgo content reported from other greenstone belts and sio2 contents as high as 58 these characteristics suggest similarity to recent high mg andesites none of the lava compositions exceed 23 mgo anhydrous and pyroxene rather than olivine dominates the crystallization assemblage olivine spinifex textures are entirely absent in the nondweni greenstone belt and instead pyroxene or amphibole after pyroxene spinifex is abundantly developed and is characterised by spectacular radial conical structures which have their apices towards the tops of the flow units some of the spinifex cone structures attain heights of up to 6 m branching pyroxene crystals and feathery textured plagioclase typify the spinifex textures of the komatiitic andesites the common occurrence of coned and branching spinifex textures in a wide range of magma compositions indicates that these were controlled by thermal characteristics and depositional environment for the flow units rather than solely by composition this may have been facilitated by emplacement in shallow water rather than in deep water as envisaged for the onverwacht group of the barberton sequence shallow water depositional environments in the nondweni group are supported by sedimentary lithofacies well preserved pillow structures also show a wide range in textural forms sizes of pillows degree of welding of pillow margins development of hyaloclastite and formation of shelf structures formed by repeated filling of lava tubes are related to both compositional type as well as physical controls such as extrusion rate and nature of the substrate komatiites seldom form pillows and very large pillow formation is restricted to the komatiitic andesite lava type komatiite flow successions are typified by spinifex textured units as well as by massive and highly brecciated aphyric flow units unaltered pyroxene and plagioclase characterise some of the massive flow units and pillowed basalts of the witkop formation the present paper is the second part of a series on the nondweni greenstone belt part i wilson and versfeld 1994 describes the stratigraphy sedimentology mineralization and depositional environment there are strong of kaapvaal craton may have been different to that of the barberton greenstone belt a present day analogue to the nondweni tectonic setting may be an ensialic back arc basin adjacent to a continental margin

stratigraphic and sedimentological evidence bearing on structural repetition in early archean rocks of the barberton greenstone belt south africa abstract the 3 5 3 3 ga old swaziland supergroup in the barberton greenstone belt south africa includes a lower predominantly volcanic unit the onverwacht group and an upper largely sedimentary section the fig tree and moodies groups although viljoen and viljoen concluded that the onverwacht group exceeds 15 km in stratigraphic thicknesses more recent workers have suggested that it represents a much thinner section of rocks repeated by folding and faulting our mapping indicates that the upper half of the group includes at least 8 km of unrepeated stratigraphic section inferences of structural repetition have arisen in part because a major shear zone is present in the upper half of the onverwacht group on the west limb of the onverwacht anticline shearing and the overturning of large blocks of the igneous sequence have combined with the later intrusion of felsic magma to produce a generally stratiform highly complex zone of deformation up to 2 km wide the displacement across the zone is unknown but the presence of unfaulted sections of this part of the stratigraphic sequence indicates that the gross stratigraphic arrangement of units within the group has not been significantly altered structures previously mapped as major fold hinges within the upper part of the onverwacht group can be shown to have formed by local deformation and reflect neither large scale folding nor structural repetition of the rocks several kilometers higher in the section rocks of the uppermost onverwacht and fig tree groups are stacked and repeated in a series of thrust nappes individual nappes extend for up to 15 km along strike and include as much as 900 m of allochthonous volcanic and sedimentary rocks they were apparently emplaced by faulting or gravity sliding at or near the earth s surface their formation involved the movement of sheets made largely up of sedimentary rocks along shear surfaces within underlying much weaker serpentinized ultramafic komatiites locally the sheets were deformed to form fold nappes the lower limbs of the fold nappes commonly show extreme attenuation and brittle deformation whereas the upper limbs are largely intact the thrust faults are the loci of small hypabyssal igneous bodies that appear to be co magmatic with a suite of upper fig tree volcanic rocks this style of faulting has not affected rocks of the overlying moodies group although estimates of the timing of deformation are as yet imprecise these features suggest that it occurred during a late fig tree phase of tectonism and volcanism

the proterozoic ophiolite problem continental emergence and the venus connection to account for the lack of preservation of ophiolites fragments of oceanic crust and mantle in old orogenic belts age 1000 to 2500 million years a hypothesis proposes that the magmatic oceanic crust formed during sea floor spreading was thicker during the cited time interval this thickening led to reduced contrast between the elevation of continental and oceanic regions and to greater average flooding of the continents the resultant distribution of elevation may have resembled modern venus more than modern earth

pseudoconglomerate and a re examination of some paleoenvironmental controversies pseudoconglomerate an unusual fragmental rock that contains pebblelike clasts is described from the barberton greenstone belt south africa it occurs in concordant and discordant bodies that are closely associated with silicification carbonation and in situ brecciation and veining pseudoconglomerate is interpreted as a product of submarine exhalative activity in a hemipelagic environment it is thought to develop in subsurface breccia bodies that form by hydraulic disaggregation of the host rock breccia clasts are jostled during continued fluid flow and become rounded as the result of attrition between adjacent fragments the previous interpretation of this rock as a normal marine sediment at barberton and possibly in other archean and paleozoic sequences may have resulted in paleoenvironmental misinterpretation