early diagenesis in deep sea turbidites the imprint of paleo oxidation zones sediment cores up to 35 m long have been recovered from turbidite sequences in the madiera abyssal plain and the southern nares abyssal plain detailed geochemical analyses of these sediments si al fe mn ca organic and inorganic carbon and associate pore waters nitrate ammonia silicate fe2 mn2 and alkalinity provide representative profiles of both steady state and nonsteady state diagenetic reactions initiated by oxidation and reduction processes models demonstrate that chemical transport in the pore water system is diffusion dominated over periods of a few thousands of years to several hundreds of thousands of years mn and fe mobilized in the subsurface mildly reducing sediments are precipitated as easily reducible oxyhydroxides in the near surface oxidized sediments nonsteady state profiles show the influence of episodic and rapid turbidite deposition and the imprint of oxidation zones that developed as the result of oxygen diffusion into the upper portions of turbidites during intervals of slow pelagic deposition the time required for development of oxidation zones between turbidites ranges from 2 0 to 15 ka in the madeira abyssal plain and from 0 2 to 2 4 ka in the southern nares abyssal plain paleo oxidation zones can be recognized in old turbidites by distinct changes in color between lower and upper portions of the deposit in the basal unoxidized portion of the turbidite the sediments are characteristically dark green gray or brown in the paleo oxidized zone the sediments are pale green blue or yellowish brown these zones in both the madeira and southern nares abyssal plains contain distinctly less organic c and carbonate in the paleo oxidized zone as compared with the unoxidized sediments depletion of carbonate content in the oxidized zones in the southern nares abyssal plain is particularly significant in that as much as 80 of the initial carbonates have been lost also the relative amounts of weak acid soluble fe and mn are generally less in the paleo oxidized zones due to the initial oxidation of metal carbonate and metal hydroxide forms to easily reducible oxide forms which are then subsequently solubilized under the mildly reducing conditions at depth in the sediment column

quaternary sediments and environments on the continental shelf off northern norway abstract based on lithologic palaeontologic and chronostratigraphic investigations of close to 200 gravity cores from troughs deep banks and shallow banks the following late quaternary environment can be outlined in the weichselian deposition of basal tills was followed by deposition of laminated clay in a sea ice environment later a pebbly pelite was deposited in the troughs at the same time as the banks were iceberg ploughed then 13 000 yrs b p a period with incipient winnowing occurred on the deep banks and deposition of sandy pelite took place in the troughs the holocene commenced with a marked environmental change due to intrusion of atlantic water the fauna changed from arctic to boreal high energy winnowing forming a lag deposit took place on the banks and high accumulation rates in the troughs occurred due to the winnowing and sediment influx from the downwasting continental icesheet during the later part of the holocene the winnowing diminished on the deeper banks on the shallow banks and on the shelf break it still prevails and in the troughs calcareous sandy mud is being deposited the surface sediments comprise three main facies bouldery and pebbly sand sand and sandy mud whose distribution mainly depends on the prevailing bottom current regime the composition of the older quaternary sediments is demonstrated by some selected seismic profiles

the magnetic stratification of layered mafic intrusions natural examples and numerical models abstract mafic ultramafic intrusions commonly display a prominent petrographic and geochemical layering attributed to differentiation in a magma chamber this layering occurs at scales from several hundreds of meters in some large intrusions down to scales of a few millimeters layering forming processes are numerous and include both internal processes such as crystal settling double diffusive convection flow segregation and compaction and external processes such as magma recharge extraction or mixing mafic magma chambers may operate as a closed system or as an open system several recent investigations have been conducted on borehole cores through the insizwa sill south africa the great dyke zimbabwe and the bushveld complex south africa we present an overview of magnetic results on these intrusions the magnetic properties of mafic rocks are dominated by the presence of magnetite in most layers pyrrhotite the only ferrimagnetic sulfide is generally restricted to sulfide rich reefs merensky j m bohmke or to massive sulfide deposits at the base of the intrusion all intrusions display distinct variations of their magnetic properties anisotropy of magnetic susceptibility magnetic remanence magnetic saturation high field magnetic susceptibility across the layering the significance of these magnetic zones varies depending on the nature of variations from zone to zone for example a drop in magnetic susceptibility does not necessarily correspond to a new magma batch unless it is also associated with a change in magnetic anisotropy parameters in the studied cases all open systems the magnetic susceptibility and the total iron content estimated from high field measurements increase towards the top of these intrusions suggesting a general pattern of iron enrichment a numerical petrologic model for a tholeiitic magma predicts magnetic susceptibility variations across a statically differentiated closed system intrusion no convection the effects of sequential magma recharge and discrete magma extraction are investigated from the magnetic properties point of view conversely these models can be used to interpret the magnetic stratification pattern in natural examples and test for its possible causes

the geology of the great dyke zimbabwe crystallization layering and cumulate formation in the p1 pyroxenite of cyclic unit 1 of the darwendale subchamber

archaean granulites of the limpopo belt zimbabwe one slow exhumation or two rapid events the peak of high temperature low pressure anticlockwise granulite facies metamorphism in the northernmost part of the northern marginal zone of the limpopo belt has been dated at 2591 4 ma using conventional u pb zircon geochronology on a syntectonic charnockite intrusion the thermal peak of 800 c at 0 5 gpa reflects a strong perturbation of the geotherm and was reached during compressional deformation and coincides with vast granite intrusion into the overlying craton the coincidence of the temperature t maximum and compression is thought to be a response to lithospheric mantle thinning that resulted in granite emplacement and a consequent change in upper crustal rheology after initial exhumation and cooling these high temperature low pressure rocks resided at a comparatively shallow depth 10 km for 620 ma field microstructural and geochronological investigation of a low temperature overprint establishes that reworking and final exhumation occurred at 1 97 ga along discrete greenschist facies thrusts the 40ar 39ar laser ablation of hornblende and biotite and pb pb step leaching of sphene are shown to be useful geochronometers in dating the low grade assemblages our results indicate that early proterozoic deformation lasted 10 to 20 ma however a number of ages intermediate between the events at 2 6 and 2 0 ga have been reported for the northern marginal zone these include pb pb garnet ar ar step heating amphibole and u pb zircon ages such ages suggest that these minerals were partially rejuvenated below their commonly believed closure temperature reinterpreting these intermediate ages as partial overprint or inheritance was possible in the northern marginal zone due both to the large time gap between the two tectonometamorphic events and the use of a multimethod multichronometer approach these observations call for great caution when extrapolating mineral ages into cooling histories especially so when the resulting cooling is slower than thermally and or mechanically possible and suggests that the geochronometry in other regions that underwent multiple tectono metamorphic events may need to be reassessed

interlaminated ice proximal glacimarine sediments in muir inlet alaska muir inlet in glacier bay alaska is a glacial fjord receiving a tremendous volume of sediment annually the rate of sediment accumulation is greatest proximal to muir glacier about 9 m yr 1 and decreases away from the glacier the primary sediment sources are meltwater streams discharging at subglacial and ice marginal positions to form overflows interflows and underflows continuous turbidity currents overflows and interflows interact with diurnal tidal currents and their volume and sediment concentration varies diurnally and annually with meltwater discharge these effects produce cyclic deposits of a thin fine grained sand or silt lamina that grades normally to a thicker poorly to very poorly sorted mud lamina this lamina couplet is termed a cyclopel underflows are suggested to occur in this glacimarine environment because of conditions unique to subglacial fluvial systems underflow deposits occur only in proximal positions 0 5 km from glacier face and are coarse grained reverse to normal graded and exhibit an increase in sorting and sand content up layer ice rafted debris identified as particles 177 m is ubiquitous though low 5 by weight and occurs as isolated particles frozen pellets or as lenses that in cores may have a lamina appearance proximally ice rafted debris is difficult to identify because proximal sediment is often as coarse grained deposited sediment may be reworked by tidal currents and sediment gravity flows depositional processes operating in muir inlet produce interlaminated sand silt clay that characterizes sediment proximal to a glacier and fines seaward to mud sediment is classified into one of three sediment types 1 1 type i sediment is very fine grained mean 8 65 7 17 o low in sand 0 1 11 2 and very poorly to poorly sorted it is the dominant sediment type in muir inlet and is transported by plumes and deposited by suspension settling 2 2 type ii sediment is fine to coarse grained mean 6 70 3 12 o low to high in sand 5 1 86 6 and very poorly to moderately sorted it represents reworked sediment proximal plume deposits or coarse grained laminae of cyclopels 3 3 type iii sediment is coarse grained mean 3 89 2 38 o high in sand 58 0 100 0 and poorly to well sorted it is deposited by sediment gravity flows or underflows

holocene paleoclimatology of the queen elizabeth islands canadian high arctic abstract a wide variety of evidence reflecting in different ways the changing climate of the queen elizabeth islands during the holocene is reviewed all proxies pertain to summer conditions many sources of information provide weak or equivocal paleoclimatic signals but a general pattern of events can be discerned temperatures in the early to mid holocene were highest comparable with or higher than temperatures prevailing for much of this century although much evidence points to a mid holocene thermal maximum there is also considerable evidence that conditions were warmer in the early holocene possibly related to orbitally induced radiation anomalies the apparent mid holocene thermal maximum may reflect lags in the response of the environment and of some proxies in recording paleoclimatic conditions more direct indices point to warmest conditions in the early holocene before 7500 bp temperatures declined from 3000 bp culminating in exceptionally low temperatures from 100 400 bp this may have been the coldest period of the entire holocene resulting in glacier advances to post glacial maximum positions the period since 1925 has witnessed a pronounced increase in temperature leading to negative mass balances on glaciers and ice sheets throughout the archipelago this period is the warmest for at least 1000 years and perhaps for several thousand years modern climate in the queen elizabeth islands may thus be characteristic of the early to mid holocene and atypical of conditions which prevailed for much of the last few thousand years

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

high closure temperatures of the u pb system in large apatites from the tin mountain pegmatite black hills south dakota usa abstract the cores of several large 1 2 cm apatites from the early proterozoic tin mountain pegmatite black hills south dakota have concordant u pb ages the age uncertainties of these samples are less than 5 ma the lead isotopic compositions of muscovite and k feldspar coexisting with the apatites were used for initial lead isotopic corrections the resulting u pb concordia ages agree well with u pb and pb pb isochron ages based solely on multiple fragments from single apatites this agreement justifies the use of coexisting muscovite and k feldspar for initial lead corrections the precision of these u pb ages is comparable to those of the u pb systems of more commonly analyzed minerals such as zircon monazite and sphene the estimated u pb closure temperature t c of these apatites is about 620 c this t c is in close agreement with the t c calculated for 0 5 cm diffusion radii and published data for diffusivity and activation energy of lead diffusion in apatite apatite can therefore yield high precision u pb ages that are close to its crystallization age in igneous rocks

field evidence of eros scale asteroids and impact forcing of precambrian geodynamic episodes kaapvaal south africa and pilbara western australia cratons abstract the role of asteroid and comet impacts as triggers of mantle crust processes poses one of the fundamental questions in earth science i present direct field evidence for close associations between impact ejecta fallout units major unconformities and lithostratigraphic boundaries in archaean and early proterozoic terrains including abrupt changes in the composition of volcanic and sedimentary assemblages across stratigraphic impact boundaries with implications for the nature and composition of their provenance terrains as originally observed by d r lowe and g r byerly in the barberton greenstone belt eastern kaapvaal craton south africa 3 26 3 24 ga asteroid mega impact units are closely associated with the abrupt break between an underlying simatic mafic ultramafic volcanic crust and an overlying association of turbidites banded iron formations felsic tuff and conglomerates of continental affinities contemporaneous stratigraphic relationships are identified in the pilbara craton western australia evidence for enrichment of seawater in ferrous iron in the wake of major asteroid impacts reflects emergence of new source terrains likely dominated by mafic compositions attributed to impact triggered oceanic volcanic activity relationships between impact and volcanic activity are supported by the onset of major mafic dyke systems associated with 2 48 ga and possibly the 2 56 ga mega impact events

thrust related accretion of an archaean greenstone belt in the midlands of zimbabwe abstract detailed structural data from the midlands greenstone belt show that in contrast to pre existing models the evolution of the greenstone belt involved an early episode of thin skinned thrusting affecting separate lithological domains these display different sedimentary and structural metamorphic histories prior to their juxtaposition across steep west directed thrust zones the domains include 3 5 ga old gneiss 2 67 2 88 ga mafic felsic volcanic units and early syn tectonic clastic sedimentary sequences concomitant with thrusting along domain boundaries upright folding and minor shear accommodated strain within domains the early thin skinned and later steeper thrusting events can be interpreted as progressive stages in an accretionary and crustal thickening sequence possibly associated with under plating or low angle subduction the clastic sedimentary sequences contain intraformational clasts and show coarsening upward cycles below major early thrust horizons and may have formed on alluvial fans developing in front of west moving nappes strike slip motion on the main shear zones in the midlands greenstone belt only occurred late in the tectonic history and there is no evidence that this resulted in large displacements a shift of 1 from e w to n s during the late stages of archaean deformation of the greenstone belt can be attributed to extensional collapse following e w shortening and crustal thickening of the zimbabwe craton

millennial scale climatic cycles in the early pliocene pollen record of ptolemais northern greece early pliocene lacustrine sediments from the ptolemais basin northern greece exhibit a climatically induced cyclicity which can be correlated with the orbital cycle of precession superimposed on precession controlled cycles palynological time series data reveal a higher order climatic cyclicity with periodicities of 10 2 5 and 1 5 ka these millennial scale vegetation changes are likely to reflect changes of the soil moisture gradient on mountain slopes caused by fluctuations in orographic winter precipitation such fluctuations corroborate the concept of a nao like north atlantic climatic teleconnection during early pliocene times the periodicities are similar to those of climate oscillations inferred from quaternary records the occurrence of millennial scale cyclicity in a time interval when the northern hemisphere was essentially ice free implies an ultimate forcing mechanism that operated independently of changes in the thermohaline circulation in the atlantic ocean the cycles are likely to be related to long period variations in solar activity

foraminiferal stratigraphy and environment of late weichselian deposits on the continental shelf off troms northern norway abstract based on the analysis of 8 selected gravity cores from the upper sediment layers of andfjorden and malangsdjupet off northern norway six late weichselian foraminiferal assemblage zones are defined the two oldest assemblage zones 8 and 7 are found in basal tills and contain mixed faunas reworked and deposited by glaciers which reached the shelf edge during the early late weichselian during the earliest phase of the deglaciation of the continental shelf a sparse fauna characterized by buccella frigida zone 6 was deposited zone 5 is dominated by cassidulina laevigata and a fairly high content of planktic foraminifera indicating periods of ice free normal marine surface waters during a glacial advance between 14 000 and 13 000 b p the fauna zone 4 was dominated by elphidium excavatum this fauna reflects near glacial conditions characterized by unstable bottom water conditions the last c 500 years of this period suffered from oxygen depleted bottom waters zone 3 formed between 13 000 and 10 000 b p comprises several species e g cassidulina reniforme nonion barleeanum n labradoricum and cibicides lobatulus during the first 2000 years of this period the fauna reflects a climatic amelioration e g an increase in temperature salinity and current activity probably due to intrusion of atlantic water during the younger dryas zone 3 reflects a climatic deterioration

pb and nd isotope systematics of stromatolitic limestones from the 2 7 ga ngezi group of the belingwe greenstone belt constraints on timing of deposition and provenance abstract pb pb isochrons have been obtained for stromatolitic limestones from the late archaean belingwe greenstone belt of zimbabwe providing direct age constraints on the deposition of these shallow water marine sediments samples from the manjeri formation and stratigraphically higher cheshire formation yield age estimates of 2706 49 ma mswd 11 5 and 2601 49 ma mswd 0 93 respectively these data are in agreement with published u pb zircon and pb pb whole rock ages of associated volcanics and we therefore interpret our pb pb ages as representing the timing of early diagenesis thus providing a minimum age for carbonate precipitation a 2543 70 ma age mswd 5 1 for one sample from the cheshire formation is considered to reflect either late stage diagenesis a craton wide thermal chemical disturbance or tectonic activity along the crustal scale mtshingwe fault calculated model 1 values for manjeri and cheshire limestones are 8 40 0 02 and 9 02 0 01 similar to values for approximately 3 5 and 2 9 ga archaean basement units adjacent to the belingwe belt negative end tdeposition and fsm nd suggest derivation of the ree from old lree enriched continental crust two stage nd model ages for the carbonates indicate that precursor rocks were extracted from the mantle at around 3 5 ga cheshire formation and 3 3 ga manjeri formation in good agreement with mantle extraction ages for local basement units model tdm 3 5 3 3 ga

sedimentary processes of canadian arctic fjords abstract conceptual models of circulation and sedimentation in arctic fjords are presented to illustrate the current understanding of these environments circulation is a vigorous process in these fjords and is enhanced by the loss of the freshwater cap during the long winter period of near zero inflow the process of brine rejection during the formation of sea ice and the role of glaciers and icebergs injecting freshwater at depth on the other hand the presence of sea ice blocks reduces the mixing action of wind which may otherwise represent the largest input of energy to fjord circulation it also provides a freshwater layer on melting and may in some circumstances damp the amplitude of tides during summer large loads of glacial sediment entering arctic fjords are separated into streams of overflowing suspended load and bed load in the presence of salt water as the concentrations of suspended load in the freshwater are normally much less than required to overcome the density of the salt water deposition occurs throughout the fjord by rapid settling of flocculated particles from the overflowing brackish water and from grain flows in the proximal environment created as the fluvial traction carpet moves over the delta foreset slope after sediment has been deposited it may be remobilized by slides which in turn may develop into fully turbulent flows as they move down the fjord floor a range of deposits from slump debris to minor turbidites are postulated to occur with the flocculated muds although not all have yet been examined in the arctic fjord stratigraphic record sea ice is an important sedimentological agent in arctic fjords it passively carries fluvial aeolian and colluvial material which is subsequently released from the ice during the stage of puddle draining at break up sea ice also actively incorporates fine and coarse sediments up to large boulders during freezing along the shores in winter and by wave and tidal current washing controlled by the presence of the ice during break up

composition of the ultramafic mafic contact interval of the great dyke of zimbabwe at ngezi mine comparisons to the bushveld complex and implications for the origin of the pge reefs the great dyke contains the world s second largest platinum resource after the bushveld complex isotopic and trace element data from the interval straddling the contact between the ultramafic and mafic sequences of the great dyke indicate a less enriched composition than in the bushveld complex great dyke sri 0 7024 0 7028 end mostly 1 to 1 ce sm 2 6 bushveld sri 705 0 709 end 5 to 7 ce sm 5 15 these data are interpreted to reflect relatively moderate amounts of contamination of the great dyke parent magma all analyzed isotopes show little variation across the main sulfide zone and the ultramafic mafic contact this corroborates earlier work by other researchers that the great dyke crystallized from a single magma type mixing of compositionally distinct magmas proposed to have caused sulfide melt saturation in the bushveld complex seemingly played little or no role in the formation of the pge mineralization in the main sulfide zone and neither did enhanced crustal contamination of specific magma batches instead sulfide melt saturation of the magma was likely triggered by silicate fractionation the mechanism of concentration of the sulfide melt remains uncertain but theoretical considerations suggest that phase sorting in response to slumping of crystal mushes possibly caused by chamber subsidence played an important role this model is consistent with the highly irregular undulating nature of the contact between the mafic and ultramafic zones of the intrusion in the hanging wall of the main sulfide zone

a short term cooling event 4 205 million years ago in the ptolemais basin northern greece abstract a distinct clay rich layer in the otherwise regular succession of alternating lignites and marls of the early pliocene ptolemais formation reveals an equally distinct palynomorph assemblage only in this layer fresh water dinoflagellate cysts spiniferites cruciformis and gonyaulax apiculata are encountered these co occurring dinoflagellate species may be taken to indicate that surface water temperatures must have been remarkably low for early pliocene mid latitudes contrasting other indications that overall warm humid climates prevailed at this time a multi component study of this so called dinolayer with an astrochronologically derived age of 4 205 0 01 ma was undertaken to further investigate the apparently contrasting climatic signals integrated sedimentological micropaleontological palynological and geochemical data indicate that the dinolayer was deposited in a shallow fresh water lake the pollen record points to slightly cooler conditions during deposition of the dinolayer in comparison with conditions just below and above an anomalously low winter temperature is proposed as a possible cause for the recorded phenomena

two step deglaciation of the southeastern barents sea marine geologic evidence from the western barents sea shelf leaves little doubt that theareawascoveredbygroundediceduringthelastglaciation butthepatternandtiming of the subsequent deglaciation were not well determined here we reconstruct the timing and mechanism of ice sheet retreat as constrained by seismic stratigraphy and lithostratigraphy and accelerator mass spectrometer 14 c dating of foraminifera and mollusc shellsinsedimentboringsfromthesoutheasternbarentssea 1000kmfromthewestern shelf edge and former ice sheet margin the deposition of ice proximal glaciomarine sediments upon till began at or shortly before 12 7 ka indicating that the ice sheet retreat whichmostlikelycommencedalongthewesternmarginofthebarentsseashelfat15 14 5 ka reached the southeastern part of the shelf in 2 ka the subsequent accumulation of glaciomarinesedimentstookplaceintwodistinctpulses 12 7 12 1kaand 10 5 9 4ka separatedbyanintervalofnondepositionlasting 1 5ka thetwopulsesofsedimentation were coeval with periods of increasing surface water and air temperatures in the northern atlanticregionandacceleratedeustaticsea levelrise suggestingthattheremainingretreat of the barents shelf ice sheet was paced by these factors

archean layered mafic ultramafic intrusions in the west pilbara craton western australia a synthesis of some of the oldest orthomagmatic mineralizing systems in the world considerable exploration interest has been generated by the platinum group element pge and ni cu potential of the archean layered mafic ultramafic intrusions in the west pilbara craton western australia the munni munni intrusion contains the largest resource of pge associated with a layered intrusion in australia and the radio hill and mount sholl intrusions host significant resources of ni cu co sulfides titaniferous magnetite layers remobilized sulfides and structurally controlled hydrothermal polymetallic deposits have also been a focus for exploration in recent years the ca 2 9 ga munni munni andover radio hill mount sholl and sherlock layered intrusions are a cogenetic suite of high level 5 kbars bodies that represent some of the oldest mineralizing systems of their type in the world although they display similar field relationships and mineralogical geochemical and isotopic features their contrasting chalcophile metal distribution patterns show that the timing and mechanism of the s saturation event were critical for the development of pge enriched sulfide bearing layers and basal segregations of base metal sulfides the intrusions form thick 5 5 km dikelike bodies or relatively thin 0 5 2 km sheets and sills emplaced at different levels along major lithological discontinuities in the upper crust rhythmically layered ultramafic components are generally thinner than and occur along the northern sides of more massive overlying mafic components the ultramafic zones consist of dunite lherzolite wehrlite olivine websterite clinopyroxenite and websterite inverted pigeonite gabbronorite magnetite gabbro olivine gabbro anorthositic gabbro and anorthosite comprise the mafic sequences olivine and clinopyroxene were generally the first minerals to crystallize except in the andover intrusion where orthopyroxene preceded clinopyroxene and possibly reflects greater contamination of the parent magma by felsic crustal material the crystallization of chromite was inhibited in the ultramafic zones by the partitioning of cr into early crystallizing clinopyroxene thus downgrading the potential for pge chromite associations the pge mineralization in the munni munni intrusion occurs in the upper levels of a porphyritic plagioclase websterite orthocumulate layer directly below the ultramafic gabbroic zone contact mineral compositional trends and nd isotope data indicate that a pd pt au enriched s undersaturated magnesian basaltic magma was frequently injected into a small magma chamber during formation of the ultramafic zone a major influx of more fractionated s saturated tholeiitic gabbroic magma related to the resident magnesian magma rapidly inflated the chamber and induced turbulent magma mixing that resulted in the formation of the pge bearing porphyritic websterite layer in contrast the parent magmas that formed the mount sholl radio hill andover and maitland intrusions were saturated in s before they were emplaced into the magma chambers in these intrusions gravitational and structural controls were important for the concentration of pge poor 5 400 ppb pt pd au massive ni cu co sulfides in depressions and structural embayments along the basal contacts beneath the thickest sequences of mafic ultramafic cumulates the parent magmas to the west pilbara intrusions were siliceous high magnesian basalts of al depleted komatiitic affinity barberton type with 9 to 12 percent mgo 15 to 25 ppm sc 12 to 18 ppm y low al2o3 tio2 ca 11 or half chondrite ratios and light rare earth enrichment chondrite normalized la sm 2 7 la lu 9 0 they were generated with garnet in the residual asthenospheric mantle with probable involvement of a pre 3 0 ga subduction modified lithospheric mantle isotopic and geochemical modeling suggests that the magmas were contaminated by ca 3 0 to 3 3 ga archean tonalitic to granodioritic crust before being emplaced into high level magma chambers

compositional and lithological controls on the pge bearing sulphide zones in the selukwe subchamber great dyke a combined equilibrium rayleigh fractionation model the platinum group element pge bearing main and lower introduction sulphide zones of the selukwe subchamber of the great dyke are an increasing number of mafic layered intrusions are made up of a series of subzones within which the ratios pd pt are reported to have distributions of platinum group elements effectively constant whereas these ratios vary significantly between pge au and base metals characterized by vertically the subzones fractionation of pd with respect to pt varies by a offset profiles offset profiles involve the vertical sepfactor of 10 and cannot be modelled using a sulphide collector aration of metal peaks and were first reported in the phase and constant partition coefficients the link with sulphide is great dyke prendergast keays 1989 wilson et al indisputable and the control is likely to have been the degree of 1989 naldrett wilson 1990 wilson tredoux oversaturation of pge micro nuggets in the magma the apparent 1990 the phenomenon has since been recognized in partition coefficients for pt and pd between silicate and sulphide the munni munni intrusion of western australia barnes liquid are dependent on the degree of oversaturation and thereby et al 1990 the skaergaard intrusion bird et al 1991 exhibit spurious correlation with the pge content of the sulphide and the rincon del tigre complex of eastern bolivia modelling replicates the pt and pd distribution and ratios only by prendergast 2000 pge are also postulated to be transdramatically changing the effective partition coefficients pyroxene ported in fluids emanating from expulsion of intercumulus compositions including tio2 are shown to be strongly dependent liquid from the crystal pile and there is strong evidence on the incompatible element content of the whole rock and specific to support such processes in several gabbroic sections linear arrays relating these variables can be related to the pge of layered intrusions such as the skaergaard intrusion subzones the overall control is rayleigh fractionation but constancy andersen et al 1998 and in the stillwater complex of the ratio pd pt and the initial pyroxene composition before re boudreau mccullum 1986 1992 in contrast the equilibration with trapped liquid within the subzones is indicative mineralized zones in the great dyke and munni munni of equilibrium crystallization this layered structure may have been intrusion are located in pyroxenite and primary magderived from liquid layers in the magma chamber matic processes have been invoked to explain the min

understanding cratonic flood basalts the origin of continental flood basalts has been debated for decades these eruptions often produce millions of cubic kilometers of basalt on timescales of only a million years although flood basalts are found in a variety of settings no locale is more puzzling than cratonic areas such as southern africa or the siberian craton where strong thick lithosphere is breached by these large basaltic outpourings conventionally flood basalts have been interpreted as melting events produced by one of two processes 1 elevated temperatures associated with mantle plumes and or 2 adiabatic decompression melting associated with lithospheric thinning in southern africa however there are severe problems with both of these mechanisms first the rifting circumstances of several wellknown basaltic outpourings clearly reflect lithospheric control rather than the influence of a deep seated plume specifically rift timing and magmatism are correlated with stress perturbations to the lithosphere associated with the formation of collisional rifts second the substantial lithospheric thinning required for adiabatic decompression melting is inconsistent with xenolith evidence for the continued survival of thick lithosphere beneath flood basalt domains as an alternative to these models we propose a new two stage model that interprets cratonic flood basalts not as melting events but as short duration drainage events that tap previously created sublithospheric reservoirs of molten basalt formed over a longer time scale reservoir creation existence stage i requires long term e g 1 ma supersolidus conditions in the sublithospheric mantle that could be maintained by an elevated equilibrium geotherm appropriate for the archean a slow thermal perturbation e g thermal blanketing or large scale mantle upwelling or a subduction related increase in volatile content the drainage event stage ii occurs in response to an abrupt stress change in the lithosphere which leads to the initiation and propagation of lithospheric dikes such a model accounts for the short eruption time of flood basalts the evidence for lithospheric control of the eruptions and the continued survival of cratonic lithosphere following these magmatic events the most notable consequence of this model is that it implies the existence of large reservoirs of magma comparable to the eruption volume of flood basalts that have been and are still likely present beneath stable continental lithopshere

timing of thermal stabilization of the zimbabwe craton deduced from high precision rb sr chronology great dyke abstract dating low temperature events such as magmatic cooling or hydro thermal surges in archean and proterozoic terranes is crucial in defining cratonal thermal stabilization after episodic continental growth during the archean and early proterozoic rubidium sr chronology is potentially a powerful tool in this regard because of its low closure temperature i e

analysis of the railway heave induced by soil swelling at a site in southern france in order to better understand the heave observed on the railway roadbed of the french high speed train tgv at chabrillan in southern france the swelling behaviour of the involved expansive clayey marl taken from the site by coring was investigated the aim the study is to analyse the part of heave induced by the soil swelling first the swell potential was determined by flooding the soil specimen in an oedometer under its in situ overburden stress on the other hand in order to assess the swell induced by the excavation undertaken during the construction of the railway a second method was applied the soil was first loaded to its in situ overburden stress existing before the excavation it was then flooded and unloaded to its current overburden stress after the excavation the swell induced by this unloading was considered finally the experimental results obtained were analyzed together with the results from other laboratory tests performed previously and the data collected from the field monitoring this study allowed estimating the heave induced by soil swelling subsequently the part of the heave due to landslide could be estimated which corresponds to the difference between the monitored heave and the swelling heave

the chromite deposit of the ipueira medrado sill sao francisco craton bahia state brazil the ipueira medrado mafic ultramafic sill hosts the largest chromite deposit in brazil the sill is a 7 km long and 300 m thick elongated body consisting mainly of interlayered dunite and harzburgite but also containing minor mafic rocks the most distinctive feature of the ipueira medrado sill is a continuous 5 to 8 m thick massive chromitite layer that is currently being mined the anomalous thickness of this chromitite layer provides important constraints on current genetic models for the origin of massive chromitites the sill is divided into three zones marginal ultramafic and mafic the marginal zone 5 20 m thick consists of sheared gabbro and orthopyroxene rich harzburgite the ultramafic zone up to 250 m thick is subdivided into the lower ultramafic unit the main chromitite layer and the upper ultramafic unit the lower ultramafic unit consists of dunite with minor harzburgite and chain textured chromitite the main chromitite layer is composed of chain textured and massive chromitites and the upper ultramafic unit consists mainly of harzburgite but also minor chain textured chromitite and dunite pyroxene abundance increases toward the top in this zone and grades to an orthopyroxenite at the top the presence of magmatic intercumulus amphibole is significant up to 20 vol in this unit the mafic zone up to 40 m thick consists of leuconorites to melanorites that are partially metamorphosed under amphibolite facies conditions the variation of olivine and orthopyroxene compositions throughout the sill layers reveals two intervals of distinct magmatic evolution the interval located below the main chromitite layer is characterized by gradual upward evolution of mineral compositions toward more mg rich compositions fo93 and en94 the interval located above the main chromitite layer is characterized by rapid evolution toward more fe rich compositions fo85 and en84 with increasing stratigraphic height the interval below the main chromitite layer suggests crystallization in a dynamic magma chamber undergoing frequent replenishment with primitive magma the interval above the main chromitite layer suggests crystallization in a mainly closed fractionating magma chamber mass balance considerations suggest that an enormous amount of magma 10 000 m thick was associated with the formation of the main chromitite layer at the ipueira medrado sill such a volume of magma is obviously not represented in the stratigraphy of the ipueira medrado sill mineral chemistry data indicate that the composition of the resident magma was most primitive at the stratigraphic interval immediately below the main chromitite layer we suggest that the sill acted as a conduit through which a large volume of magma flowed under these conditions contamination of the parental magma with crustal material and fluids is a likely mechanism for explaining the origin of the chromitite abundant intercumulus amphibole in the upper ultramafic unit and high fe3 in chromites intercumulus orthopyroxene in main chromitite layer chromitites and sm nd and re os isotope data for silicate rocks and chromitites respectively indicate that the ipueira medrado sill formed from magmas contaminated by crustal material the host rocks of the ipueira medrado sill consist mainly of high grade gneiss and marbles that would have been appropriate source of fluids and sio2 and could have caused the increase in f o2

benthic and planktic community changes at the north siberian margin in response to atlantic water mass variability since last deglacial times the eastward penetration of atlantic derived water adw into the eurasian basin of the arctic ocean was investigated at the western laptev sea continental margin for the time since c 17 6 ka using a high resolution investigation of the lithology geochemistry planktic and benthic foraminifers and ostracods on a sediment core from 270 m water depth major steps in the environmental evolution of the region are recognized in general adw was continuously present in the study area between 17 6 and 15 4 ka adw manifested itself through open water polynyas and associated upwelling events comparison between the laptev sea and northern svalbard shelf using cassidulina neoteretis allows assuming an unmodified subsurface inflow of adw within its northern branch between 15 4 and 13 2 which was strongest after 14 7 ka and in line with the overall climate amelioration a local freshwater event at 13 ka followed by shelf flooding and the establishment of a freshened shelf water mass resulted in an off shelf displacement of adw from the studied site as suggested by the disappearance of c neoteretis between 12 and 7 ka as evidenced by an abundance peak in nonion labradoricum the sea ice marginal zone was located at the site around 12 11 ka but then shifted northward during the early holocene warming enhanced adw inflow since 7 ka correlated with climate cooling and southward retreat of the seasonal drift ice margin the inflow of adw during mid late holocene differed from deglacial times because of the combined influence of northern and eastern adw branches

zircon geochronology of archaean felsic sequences in the zimbabwe craton a revision of greenstone stratigraphy and a model for crustal growth u pb ion microprobe shrimp work on zircon populations from 13 zimbabwean archaean felsic rocks are presented and interpreted samples were extracted from felsic volcanic sequences from most of the major greenstone belts and represent the first zircon geochronological data from within the greenstone belts themselves the data demonstrate a late archaean volcanicity spanning 250 ma which began at least 2900 ma ago and ended at 2650 ma the intrusion of extensive granitoid sills of the chilimanzi suite at c 2 6 ga marks the widespread stabilization of the craton based on the new zircon data and a re evaluation of published mapping a new stratigraphic subdivision is presented for the late 3 3 ga fragment of archaean crust in zimbabwe these data suggest that the early archaean crust is restricted to the tokwe segment in the south of the country since even the oldest of the felsic volcanics 2 90 ga lower belingwean have zircons which are 50 ma older it is suggested that remnants of earlier cyclic greenstone granitoid events must underlie the present craton and that all of the currently exposed greenstone belts of zimbabwe were developed on sialic crust

reprint of benthic and planktic community changes at the north siberian margin in response to atlantic water mass variability since last deglacial times abstract the eastward penetration of atlantic derived water adw into the eurasian basin of the arctic ocean was investigated at the western laptev sea continental margin for the time since c 17 6 ka using a high resolution investigation of the lithology geochemistry planktic and benthic foraminifers and ostracods on a sediment core from 270 m water depth major steps in the environmental evolution of the region are recognized in general adw was continuously present in the study area between 17 6 and 15 4 ka adw manifested itself through open water polynyas and associated upwelling events comparison between the laptev sea and northern svalbard shelf using cassidulina neoteretis allows assuming an unmodified subsurface inflow of adw within its northern branch between 15 4 and 13 2 which was strongest after 14 7 ka and in line with the overall climate amelioration a local freshwater event at 13 ka followed by shelf flooding and the establishment of a freshened shelf water mass resulted in an off shelf displacement of adw from the studied site as suggested by the disappearance of c neoteretis between 12 and 7 ka as evidenced by an abundance peak in nonion labradoricum the sea ice marginal zone was located at the site around 12 11 ka but then shifted northward during the early holocene warming enhanced adw inflow since 7 ka correlated with climate cooling and southward retreat of the seasonal drift ice margin the inflow of adw during mid late holocene differed from deglacial times because of the combined influence of northern and eastern adw branches

a re appraisal of the epoch nickel sulphide deposit filabusi greenstone belt zimbabwe a hydrothermal nickel mineral system abstract the neoarchaean epoch nickel sulphide nis deposit discovered in 1970 and subsequently mined from 1972 to 1998 has been previously interpreted to be a magmatic ore system however on the basis of field petrographic and analytical data obtained during the initial exploration and evaluation stages integrated with limited new analytical data sem and ore microscopy we propose that the epoch nis may be of hydrothermal origin key features that have prompted a re appraisal of the genesis of the epoch mineralisation include a ore lenses are irregularly distributed along foliation fabrics of the host talc carbonate rocks and as such no relationship to basal ultramafic mafic units b the thickness of the ore lenses are highly variable from tens of centimetres to several metres c ore mineralogy is characterised by dominant millerite pyrite chalcopyrite assemblages and d high pd pt ratios although our hydrothermal model is somewhat speculative we suggest that on present evidence there is little doubt that the epoch nis lacks the features of typical magmatic sulphide deposits more geochemical and isotopic data are needed to test our model

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

a multielement geochronologic study of the great dyke zimbabwe significance of the robust and reset ages abstract new sm nd u pb and pb pb age determinations indicate that the great dyke of zimbabwe an elongate intrusion of mafic and ultramafic rocks some 550 km long and between 3 and 10 km wide is over 100 ma older than previously believed based on rb sr ages the intrusion was emplaced as a series of subchambers with similar stratigraphy comprising a lower ultramafic sequence with cyclic layering of dunite or harzburgite grading upwards into bronzitite the top sections of which include pt enriched sulfide zones and an upper mafic sequence of pyroxenites capped by olivine gabbro and gabbronorite the sm nd method has yielded a combined mineral whole rock isochron of 2586 16 ma and e nd t of 1 1 for samples from the darwendale sebakwe and wedza subchambers as well as the satellite east dyke this isochron age is in excellent agreement with the u pb age for three concordant rutile fractions extracted from a feldspathic pyroxenite of the selukwe subchamber with an error weighted mean at 2587 8 ma two zircon fractions from the same feldspathic pyroxenite sample as the rutile are discordant and although not well constrained suggest pb loss from the zircons at ca 830 ma this may be related to the onset of the widespread and diachronous pan african tectonothermal event in southern africa whole rock samples and clinopyroxene and plagioclase separates from a darwendale subchamber drill core yielded a 207 pb 204 pb vs 206 pb 204 pb isochron age of 2596 14 ma which is in agreement with the sm nd isochron and the rutile u pb crystallization age this new age information shows that emplacement of the great dyke and its satellite dikes closely followed the amalgamation of the kaapvaal and zimbabwe cratons and was contemporaneous with emplacement of the youngest of the trondhjemite tonalite granodiorite granitoid suite in the zimbabwe craton assuming that amalgamation of the kaapvaal and zimbabwe cratons was largely by nnw directed convergence it follows that the source of the great dyke was asthenospheric mantle hydrated and enriched in incompatible elements by subduction processes isochrons of 206 pb 204 pb vs 238 u 204 pb and 207 pb 204 pb vs 235 u 204 pb yield ages with large errors but well constrained initial pb ratios 206 pb 204 pb 14 15 0 30 and 207 pb 204 pb 15 04 0 06 assuming a two stage model for common lead evolution this result yields a value of 9 5 along with the calculated initial sr and nd isotopic compositions these data are consistent with derivation of the great dyke magmas by large volume melting of a mantle that has been hydrated and enriched by subduction while a small amount of crustal contamination of magma derived from depleted mantle could produce the composition of the great dyke the uniformity of initial ratios between subchambers supports the notion of enrichment in incompatible elements being an intrinsic characteristic of the mantle source

nickel sulfide deposits in australia characteristics resources and potential abstract australia s nickel sulfide industry has had a fluctuating history since the discovery in 1966 of massive sulfides at kambalda in the eastern goldfields of western australia periods of buoyant nickel prices and high demand speculative exploration and frenetic investment the nickel boom years have been interspersed by protracted periods of relatively depressed metal prices exploration inactivity and low discovery rates despite this unpredictable evolution the industry has had a significant impact on the world nickel scene with australia having a global resource of nickel metal from sulfide ores of 12 9 mt five world class deposits 1 mt contained ni and a production status of number three after russia and canada more than 90 of the nation s known global resources of nickel metal from sulfide sources were discovered during the relative short period of 1966 to 1973 australia s nickel sulfide deposits are associated with ultramafic and or mafic igneous rocks in three major geotectonic settings 1 archean komatiites emplaced in rift zones of granite greenstone belts 2 precambrian tholeiitic mafic ultramafic intrusions emplaced in rift zones of archean cratons and proterozoic orogens and 3 hydrothermal remobilized deposits of various ages and settings the komatiitic association is economically by far the most important accounting for more than 95 of the nation s identified nickel sulfide resources the ages of australian komatiitic and tholeiitic hosted deposits generally correlate with three major global scale nickel metallogenic events at 3000 ma 2700 ma and 1900 ma these events are interpreted to correspond to periods of juvenile crustal growth and the development of large volumes of primitive komatiitic and tholeiitic magmas caused by large scale mantle overturn and mantle plume activities there is considerable potential for the further discovery of komatiite hosted deposits in archean granite greenstone terranes including both large and smaller high grade 5 to 9 ni deposits that may be enriched in pges 2 to 5 g t especially where the host ultramafic sequences are poorly exposed analysis of the major komatiite provinces of the world reveals that fertile komatiitic sequences are generally of late archean 2700 ma or paleoproterozoic 1900 ma age have dominantly al undepleted al 2 o 3 tio 2 15 to 25 chemical affinities and often occur with sulfur bearing country rocks in dynamic high magma flux environments such as compound sheet flows with internal pathways facies kambalda type or dunitic compound sheet flow facies mt keith type most precambrian provinces in australia particularly the proterozoic orogenic belts contain an abundance of sulfur saturated tholeiitic mafic ultramafic intrusions that have not been fully investigated for their potential to host basal ni cu sulfides voisey s bay type mineralization the major exploration challenges for finding these deposits are to determine the pre deformational geometries and younging directions of the intrusions and to locate structural depressions in the basal contacts and feeder conduits under cover stratabound pge ni cu cr deposits hosted by large archean proterozoic layered mafic ultramafic intrusions munni munni panton of tholeiitic affinity have comparable global nickel resources to many komatiite deposits but low grades

glacial history and paleoceanography of the southern yermak plateau since 132 ka bp abstract the southern yermak plateau yp is situated at the entrance to the arctic ocean in the narrow marginal ice zone miz between the polar and arctic fronts north west of svalbard a gravity core jm10 02gc has been analysed in order to reconstruct paleoceanographic conditions and the movement of the sea ice margin as well as the glacier ice conditions of the svalbard barents sea ice sheet sbis during the last interglacial glacial cycle the distribution of planktic and benthic foraminifera planktic and benthic oxygen and carbon isotopes and variations in ice rafted debris ird has been investigated the sediment core covers the time interval from the marine isotope stage mis 6 5e transition termination ii c 132 ka bp to the early holocene during termination ii tii the sbis retreated and the sea ice margin was in distal position whereas during mis 5 to mis 4 the sea ice margin was close to the core site several core intervals interpreted as representing mis 5e mis 5c mis 5a mis 3 and mis 1 were barren of calcareous microfossils whereas the intervals representing mis 4 and mis 2 were characterised by high productivity hp of planktic and benthic foraminifera these glacial hp zones were associated with the open water conditions resulting from the advection of atlantic water aw and retreat of the sea ice margin the barren zones during mis 5 mis 3 and mis 1 resulted from the proximity of the sea ice margin whereas during mis 2 the likely cause was an advance of the sbis

the distribution of living benthic foraminifera on the continental slope and rise off southwest norway abstract surface sediment samples taken by corer from 45 stations on the norwegian continental margin and in the norway basin have been investigated for their benthic foraminiferal content unlike previous studies the living benthic foraminiferal fauna was differentiated from empty tests comprising the foraminiferal death assemblage factor analysis of both the living and dead faunal data reveals six living species assemblages and five corresponding dead assemblages the additional living assemblage is characterized by the arenaceous speciescribrostomoides subglobosum that dominates between 1400 and 2000 m water depth but is rare in the dead faunal data trifarina angulosa and to a lesser extent cibicides lobatulus characterize the shallowest foraminiferal assemblage from 200 to 600 m water depth where it is associated with strong bottom currents and warm saline atlantic water of the north atlantic drift on the slope between 600 and 1200 m water depth themelonis zaandami species assemblage dominates particularly in areas characterized today by rapid sedimentation of terrigeneous material between 1000 and 1400 m depth where the slope is covered by fine grained organic rich terrigeneous mud the living foraminiferal assemblage is characterized bycassidulina teretis andpullenia bulloides below 1400 m three foraminiferal assemblages are found c subglobosum is found from 1400 to 2000 m cibicidoides wuellerstorfi andepistominella exigua predominantly live from 2000 to 3000 m water depth and below 3000 m oridorsalis umbonatus andtriloculina frigida dominate the fauna all of theelphidium excavatum tests found in this study and thecassidulina reniforme tests found above 500 m water depth were found to be reworked analysis of the sediment grain size distribution and the organic carbon content in surface samples from the deepest stations suggest that the abundance ofc wuellerstorfi ande exigua is positively correlated to relatively coarse caused by planktic foraminifera and organic rich sediments whereas high frequencies ofo umbonatus andt frigida coincide with low organic carbon content we suggest thatc wuellerstorfi is adapted to deep sea environments with relatively high food supply tolerating relatively low interstitial water oxygen content whereaso umbonatus may tolerate lower food supply prefering well oxygenated interstitial waters

magmatic nickel sulfide mineralization in zimbabwe review of deposits and development of exploration criteria for prospectivity analysis significant magmatic nickel sulfide deposits in zimbabwe occur in archean greenstone belts and archean to proterozoic mafic ultramafic intrusions such as madziwa and the great dyke here we review exploration criteria for magmatic nickel sulfide deposits taking into account nickel sulfide mineral systems in general zimbabwe s cratonic evolution and the geological setting of the deposits we propose that the essential ingredients for nickel sulfide mineral systems in zimbabwe are i proximity to greenstone belts craton margins and orogenic belts ii large mafic ultramafic intrusions and their feeders iii crustal contamination of basic ultrabasic magma with sulfidic crustal lithologies and iv the vicinity to major fault zones that allowed fertile magma to ascend into the upper crust we suggest that there is potential for new nickel sulfide discoveries both within and outside the greenstone belts including the karoo flood basalt province the mashonaland dolerites and the limpopo magondi and zambezi orogenic belts

the origin of massive diamicton facies by iceberg rafting and scouring scoresby sund east greenland almost 90 of 39 m of core material recovered from scoresby sund and the adjacent east greenland shelf is massive diamicton interpreted to be formed predominantly by the release of iceberg rafted debris and reworking by iceberg scouring there is also likely to be a contribution from suspension settling of fines derived from glaciofluvial sources model calculations suggest that the 14c derived holocene sedimentation rate of 0 1 0 3 m 1000 yr 1 in scoresby sund can be accounted for mainly by iceberg rafting of debris a further 4 of core material is of gravel or coarse sand lenses interpreted to reflect iceberg dumping of debris intensive iceberg scouring which reworks sea floor sediments is observed on acoustic records from over 30 000 km2 of the scoresby sund fiord system and the adjacent east greenland shelf 69 72 n and 75 n the rate of iceberg production from greenland ice sheet outlet glaciers and iceberg drift tracks on the shelf suggests that iceberg rafting and scouring may be important over a significant proportion of the 500 000 km2 area above the shelf break the relatively extensive modern occurrence of massive diamicton formed by iceberg rafting and scouring together with suspension settling of fines suggests that it may also be a significant facies in the glacier influenced geological record the recognition in the geological record of the massive diamicton facies described above may also indicate the former presence of fast flowing ice sheet outlet glaciers

abundance of smectite and the location of a plate boundary fault barbados accretionary prism isolating the respective roles of factors responsible for the initiation and localization of fault zones remains one of the more important goals of research in neotectonics the frontal decollement zone of the northern barbados accretionary prism provides several important clues as to how the interwoven variables of clay mineralogy fluid flow chemical interactions and sediment physical properties affect strain localization this plate boundary fault is centered at a lithologic contact between miocene claystone and fine grained oligocene turbidites the fault zone is nearly 40 m thick at ocean drilling program site 948 and its upper part passes through smectite rich deposits a sharp minimum in percent smectite and a maximum in percent illite mark the base of the decollement there is a consistent increase in percent smectite with distance above the base of the decollement but the top of the decollement is poorly defined by clay mineralogy the intrinsic mechanical weakness of strata with abundant smectite group clays probably influences where the fault tip propagates into the undeformed stratigraphy of the atlantic abyssal plain a second inherited parameter is the local abundance of radiolarians which contribute to higher than normal porosities sediment shear strength also decreases because pore pressure within the fault zone is significantly greater than hydrostatic the principal cause of excess pore pressure seems to be updip fluid advection in theory however decreases in pore fluid salinity and porosity collapse should increase the amount of physicochemical stress generated by expandable clay minerals the imported fluid is unusually low in salinity because it has migrated from zones of deeper seated dehydration reactions if fresher pore water migrates to the propagating tip of the decollement its arrival should increase smectite swelling and reduce the shear strength of the mudstone even more the location and evolution of the decollement therefore are controlled by a complicated interplay of static factors inherited from the abyssal atlantic stratigraphy and dynamic factors associated with episodic fluid flow and changing fluid chemistry

horizontal tectonic deformation geometries in a late archaean sedimentary sequence belingwe greenstone belt zimbabwe in the belingwe greenstone belt of zimbabwe structural evidence from the circa 2 65 ga old sedimentary cheshire formation which overlies and is imbricated with a mafic volcanic unit is consistent with thrusting of the greenstone sequence the cheshire formation consists of a karstified carbonate ramp sequence overlain by siliciclastic turbidite deposits that formed in a southeast deepening basin the earliest deformational structures formed during a syndepositional to postdepositional thin skinned thrusting event d1 that affected poorly consolidated sediments and is recorded in bedding parallel ductile shear zones boudins folds and block in matrix structures d1 shear zones separate the volcanic sequence and the overlying sediments and occur between the carbonate and siliciclastic units syntectonic sulphide mineralization and silicification of mainly fine grained sediments along the main thrust faults gave rise to the formation of rocks similar in appearance to banded iron formations stratigraphic units were locally duplicated along the d1 thrust faults including a tectonic slice of mafic volcanics that was emplaced onto carbonates along a chaotic unit similar to a tectonic melange the elongation of inclusions in the melange zone and lineations in ironstones together with kinematic indicators suggest that stratigraphic duplication resulted from northwestward directed tectonic transport deposition of the cheshire formation took place in an asymmetric foreland type basin contemporaneously with thrusting soon after deposition the formation was incorporated into the thrust stack subsequent deformation events include tight upright folding gentle cross folding and dextral strike slip faulting of the greenstone succession evidence for early thrusting suggests that horizontal tectonic processes played an important role in the evolution of the belingwe greenstone belt

changes in the flow of atlantic water into the arctic ocean since the last deglaciation evidence from the northern svalbard continental margin 80 n 1 the svalbard archipelago is located in the high arctic 76 80 n within the northernmost reach of the west spitsbergen current which is the continuation of the north atlantic current in this specific setting close to the polar front even small variations in the current system are expected to give large and distinct signals in paleoceanographic parameters thus the svalbard area is ideal for monitoring the past history of the inflow of atlantic water to the arctic ocean we have studied sediment core np94 51 taken outside the mouth of the hinlopen strait on the northern svalbard shelf the paleoceanographic development of the last deglaciation and the holocene has been reconstructed using benthic and planktonic foraminifera oxygen isotopes and ice rafted detritus the results show that the first strong subsurface inflow of atlantic water to the arctic ocean after the last glacial maximum commenced at 12 6 14c kyr b p circa 15 000 calendar cal years b p during the bolling allerod interstadial during the younger dryas polar conditions prevailed at the surface with extensive sea ice cover and the inflow of atlantic derived water was generally diminished the inflow of more saline but still cold atlantic water was relatively strong during the early holocene and caused intensified seasonal biological productivity the atlantic water inflow declined gradually during the mid holocene between 4500 and 1100 cal years b p the inflow was very weak and the bottom waters over the northern svalbard margin were cold and of lower salinity during the last 1000 years climatic conditions improved such that the subsurface inflow of atlantic derived water increased however the surface waters were still cold

multivariate analysis and quantitative paleoecology of benthic foraminifera in surface and late quaternary shelf sediments northern canada abstract benthic foraminifera in surface sediments from northeastern canada and the beaufort shelf were studied to determine the quantitative relationship between recent assemblage distributions and modern water depth temperature and salinity factor analysis shows that 7 factor assemblages account for 92 of the variance between samples multiple linear regression shows that different factors are significantly related to water depth august bottom water temperature or salinity multiple correlation coefficients for depth temperature and salinity are 0 94 0 72 and 0 92 respectively labrador fjords contain arenaceous faunas dominated by eithersaccammina atlantica andreophax fusiformis or byeggerella advena regression analysis shows that thesaccammina reophax factor is most strongly related to salinity theeggerella factor is related to depth and temperature a fauna dominated bytrochammina nana andbuliminella elegantissima is correlated with warm deep water in davis strait calcareous assemblages dominated byelphidium excavatum orcassidulina reniforme are found in arctic bays and shelf areas these factor assemblages are strongly related to temperature and depth cassidulina is also related to salinity anislandiella helenae fauna on the inner labrador shelf is strongly correlated with depth and salinity anonionellina labradorica fauna in deep shelf basins is closely related to salinity the multiple regression model for the sea bed data estimates water depth august bottom water temperature and salinity within confidence limits of 67 m 1 3 c and 1 0 paleoecological transfer functions were used to quantify environmental changes recorded by the foraminifera in two piston cores on a transect across the labrador shelf paleo depth estimates obtained from the core data produce relative sea level curves which are similar in shape to theoretical curves for this region paleo salinity and temperature estimates for offshore core 12 show that the change from a postglacialnonionellina fauna to a late glaciale excavatum fauna is accompanied by a decrease of about 2 in salinity and 4 c in temperature paleoecological estimates for fjord core 111 show that the change from a modernsaccammina reophax fauna to a mid holocenei helenae fauna corresponds to a salinity increase of 3 5 and temperature increase of 1 2 c

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

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

the shabogamo intrusive suite labrador sr and nd isotopic evidence for contaminated mafic magmas in the proterozoic abstract the shabogamo intrusive suite comprises numerous bodies of variably metamorphosed gabbro which intrude archean and proterozoic sequences at the junction of the superior churchill and grenville structural provinces in western labrador combined sm nd and rb sr systematics in two bodies ranging from unmetamorphosed to lightly metamorphosed document a crystallization age of about 1375 m y and suggest that both bodies crystallized from magmas with similar nd and sr isotopic compositions this age is in accordance with the existence of a regional magmatic event in the churchill province at approximately 1400 m y rb sr systematics in two bodies of amphibolite grade gabbro suggest a regional metamorphic event at about 950 m y corresponding to the waning stages of grenville activity sm nd systematics in these high grade bodies are affected to a much lesser degree than rb sr initial ratios for143nd 144nd and87sr 86sr are lower and higher respectively than bulk earth values at 1375 m y both these displacements are in the direction of older crustal material at 1375 m y and a model is proposed to produce the shabogamo magma by mixing a mantle derived magma with a partial melt of crustal rocks approximately 4 1 by volume young volcanic rocks with anomalous nd and sr isotopic ratios which have previously been taken as evidence for enriched mantle may be interpreted similarly

sediment accumulation in low sedimentation wave dominated glaciated inlets abstract many fjords situated in lowland areas of resistant bedrock are characterized by a comparatively low fluvial sediment flux and are often dominated by wave processes several such fjords in eastern canada are used as examples to develop a generalized sediment distribution model physiographically the fjords can be subdivided into nearshore platforms basin slopes and basin floors where there has been marine transgression the principal sediment source is shoreline erosion of glacial till in areas of crustal rebound late glacial and early holocene muds deposited on highs are eroded as they come within the zone of wave erosion with the falling sea level storm waves resuspend fine grained sediment in water depths of many tens of metres and produce near bottom turbid water zones within basins fine sand and silt settle out rapidly leading to the deposition of graded beds on the basin floors residual sand and gravel accumulate on the near shore platforms while silt and mud prograde over or are advected beyond basin slopes burying older basin fill sediment

meaning measurement and field application of swelling pressure of clay shales the meaning and laboratory measurement of swelling pressure are examined by swelling and swelling pressure tests on undisturbed specimens of taylor shale from san antonio texas swelling pressure response of a shale element is defined in terms of a void ratio against swelling pressure relationship rather than a single value of swelling pressure and is interpreted by means of constant volume free swell and controlled load or controlled deformation unloading tests a concept of unsaturated fissured shale is introduced and is used to explain the magnitude and time rate of swelling pressures measured in laboratory tests and mobilized in the field the magnitude of swelling pressure that a shale surface exposed in an excavation mobilizes against a stiff support is expressed in terms of an expansion of the shale mass adjacent to the excavation the expansion of shale is mainly through the opening of joints and fissures during excavation and before installation of the lining la signification et la mesure

constraints on pb closure temperature in titanite based on rocks from the ungava orogen canada implications for u pb geochronology and p t t path determinations on the basis of multiequilibrium metamorphic pressure temperature p t estimates we have determined that 660 700 c is a reasonable minimum estimate of the closure t of pb diffusion in titanite a temperature significantly higher than previously suggested activity corrected reactions that produce titanite define narrow t stability fields 40 c that correspond closely to metamorphic p t determinations derived for coronitic garnet clinopyroxene hornblende samples growth of titanite occurring between 1789 and 1814 ma in the underthrust archean basement of the paleoproterozoic ungava orogen northern quebec is associated with retrograde metamorphism during underthrusting of the archean basement approx 1 b y after formation of the original granulitic assemblages

tabular intrusion and folding of the late archaean murehwa granite zimbabwe during regional shortening the murehwa granite belongs to the late archaean chilimanzi suite of granites that occupy more than 50 of the zimbabwe craton the granite has compositional layers on metre to tens of metre scales with generally shallow dips and dip directions towards the centre of the granite near its margins the layering is systematically folded about sub horizontal east west axes microcline phenocrysts are generally strongly aligned parallel to the layering but locally crosscut the layering because they are axial planar to the folds crystals are not significantly deformed fabrics are thus magmatic the granite was intruded in a tabular sheet that may have been only about 6 km thick and fed by dykes contrary to previous concepts of diapirism ballooning for some archaean granites of the zimbabwe craton the systematic orientation of magmatic folds and axial planar fabrics demonstrate that the granite was intruded during regional north south shortening this shortening is consistent with the orientation of cratonic scale fractures that were intruded by the great dyke within 25 ma of the intrusion of the murehwa granite the granite may have intruded in a relatively short interval of less than 1 26 ma within the longer duration of the entire chilimanzi granite event

paleoceanographic evolution of the sw svalbard margin 76 n since 20 000 14c yr bp abstract two cores from the southwestern shelf and slope of storfjorden svalbard taken at 389 m and 1485 m water depth have been analyzed for benthic and planktic foraminifera oxygen isotopes and ice rafted debris the results show that over the last 20 000 yr atlantic water has been continuously present on the southwestern svalbard shelf however from 15 000 to 10 000 14c yr bp comprising the heinrich event h1 interval the bolling allerod interstades and the younger dryas stade it flowed as a subsurface water mass below a layer of polar surface water in the benthic environment the shift to interglacial conditions occurred at 10 000 14c yr bp due to the presence of a thin upper layer of polar water surface conditions remained cold until ca 9000 14c yr bp when the warm atlantic water finally appeared at the surface neither extensive sea ice cover nor large inputs of meltwater stopped the inflow of atlantic water its warm core was merely submerged below the cold polar surface water

lead diffusion in apatite and zircon using ion implantation and rutherford backscattering techniques abstract ion implantation was used to introduce pb into the minerals zircon and apatite diffusion profiles were obtained using rutherford backscattering and the results were fitted with a model to determine the diffusion coefficients this approach is both simple and useful in studying diffusion over a temperature range of geologic interest without inordinate annealing times results for apatite over the temperature range 600 900 c are in good agreement with earlier results obtained for higher temperatures watson et al 1985 and are described by the following arrhenius law d 1 27 10 4 cm 2 s 1 exp 54 6 1 7 kcl mol 1 rt this suggests that radiation damage induced by ion implantation has little effect on pb diffusion in this case due to rapid annealing of induced damage at low temperatures the activation energy stated above is somewhat smaller than that previously determined 70 kcal mol 1 reflecting the larger range in 1 t sampled by the present results closure temperatures calculated with these diffusion parameters are in good agreement with those inferred from geochronologic data diffusion coefficients for pb in zircon determined in this study are greater than those obtained from a single experimental measurement and estimates of pb diffusion based on geochronologic data and u pb zoning in natural zircons the differing results for zircon and apatite obtained in this study appear to be related to each mineral s ability to anneal the radiation damage induced by ion implantation

paleoceanographic development in the sw barents sea during the late weichselian early holocene transition abstract the late weichselian early holocene variability of the north atlantic current has been studied with focus on the zonal component of this meridional transport during the transition from glacial to interglacial conditions the investigated sediment core is from 409 m water depth in the sw barents sea eight accelerator mass spectrometry ams 14c dates show that the core covers the last 20 000 cal yr b p with a centennial scale resolution during late weichselian early holocene planktic foraminiferal assemblages were analyzed using the 100 m size fraction and foraminiferal planktic and benthic 13c and 18o isotopes were measured furthermore a range of physical and chemical analyses has been carried out on the bulk sediment samples four time periods have been identified which represent the varying oceanographic conditions in ingoydjupet a glacial trough located off the north coast of norway in the sw barents sea 1 the late glacial before ca 15 000 cal yr b p influenced by the nearby ice sheets with high amounts of sea ice or iceberg transported detritus 2 the late oldest dryas stadial and the bolling allerod interstadial ca 15 000 12 700 cal yr b p with cold surface water conditions influenced by the collapse of the nearby ice sheets high amounts of sea ice or iceberg transported detritus and melt water and weak subsurface inflow of atlantic water 3 the younger dryas cold stadial 12 700 11 650 cal yr b p with low primary productivity and extensive sea ice cover and 4 the preboreal and early holocene 11 650 6800 cal yr b p cal yr b p with strong influx of atlantic water into the area near absence of ice rafted debris and generally ameliorated conditions in both surface and bottom water masses as seen from a high flux of foraminifera and increased marine primary production

constraints on archaean crustal evolution of the zimbabwe craton a u pb zircon sm nd and pb pb whole rock isotope study the u pb ages of zircons from seven felsic volcanic and plutonic rocks from northern zimbabwe combined with field data and pb pb and sm nd whole rock isotope data constrain the timespan of development of the harare shamva granite greenstone terrain and establish the relative involvement of juvenile mantle derived and reworked crustal material basement cover field relationships and isotope and geochemical data demonstrate that the greenstones were deposited onto 3 2 2 8 ga basement gneisses in ensialic continental basins geodynamic models for the generation of the areally extensive bimodal magmatic products and growth of the pre existing crustal nucleus consistent with our interpretations are rift related 1 intracontinental rifting related to mantle plume activity or 2 rifting in a back arc environment related to a marginal volcanic arc the data in conjunction with field evidence do not indicate the presence and accretion of an older ca 2 70 ga and a younger ca 2 65 ga greenstone sequence in the harare part of the greenstone belt as was recently postulated on the basis of shrimp zircon ages zircon ages for basal felsic volcanics 2715 15 ma and a subvolcanic porphyry 2672 12 ma constrain the initiation and termination of deposition of the greenstone sequence the timespan of deposition of the upper bulawayan part of the greenstone sequence corresponds well with radiometric ages for upper bulawayan greenstones in the central and southern part of the craton and supports the concept of craton wide lithostratigraphic correlations for the late archaean in zimbabwe zircon ages for a syn tectonic gneiss 2667 4 ma and a late syn tectonic intrusive granodiorite 2664 15 ma pinpoint the age of deformation of the greenstone sequence and compare well with a pb pb age of shear zone related gold mineralization 2659 13 ma associated with the latter intrusive phase the intimate timing relation of greenstone deformation and granitoid emplacement but also the metamorphic evidence for a thermal effect of the batholiths on the surrounding greenstone belts and the structural and strain patterns in the greenstone sequence around and adjacent to the batholiths imply that the intrusion of the granitoids had a significant influence on the tectono thermal evolution of the greenstone belt prolonged magmatic activity is indicated by the zircon ages of small post tectonic plutons intrusive into the greenstone belt with a mineralized granodiorite dated at 2649 6 ma and an unmineralized tonalite at 2618 6 ma the 2601 14 ma crystallization age of an external chilimanzi type granite agrees well with existing radiometric ages for similar granites within the southern part of the craton demonstrating a craton wide event and heralding cratonization the similarity between u pb zircon ages and tdm model ages 2 65 2 62 ga and the positive ndt values 3 to 2 for the late syn tectonic and post tectonic intrusive plutons within the greenstone belt indicate magmatism was derived directly from the mantle or by anatexis of lower crustal sources with very short crustal residence times and minor contamination with older crust the rather high model 1 values 8 2 8 6 are unlikely to indicate the involvement of significant amounts of older crust and may be inherited from a high u pb mantle source as was suggested by previous workers for the archaean mantle beneath southern africa the older tdm ages for the felsic volcanics 3 0 2 8 ga and the porphyries 2 8 2 7 ga suggest that these felsic magmas were derived by partial melting of a source that was extracted from the mantle ca 200 ma prior to volcanism or may indicate interaction between depleted mantle derived melts and older crustal material

definition of archaean comment and a proposal on the recommendations of the international subcommission on precambrian stratigraphy abstract the proposed definition of the archaean proterozoic boundary at 2500 ma sims 1980 is unsatisfactory as it is difficult to use contravenes the international stratigraphic guide and does not allow for revisions of isotopic decay constants instead it is proposed that the boundary be set at the cooling age of the great dyke of zimbabwe 2 5 ga which provides an equivalent definition in the rock the division of the archaean at 2 9 ga and 3 5 ga sims 1980 is also unsatisfactory and instead reference successions are proposed in the rhodesian craton the pilbara block in australia and in greenland to serve as possible future stratotypes for archaean eras

a multi proxy reconstruction of oceanographic conditions around the younger dryas holocene transition in placentia bay newfoundland abstract a marine sediment core from placentia bay off the south coast of newfoundland was studied to reconstruct past oceanographic conditions based on a multiproxy approach including diatoms benthic foraminifera grain size analysis ip25 x ray fluorescence calcium carbonate and total organic carbon content based on 6 radiocarbon dates the 513 5 cm long core spans the age interval from ca 13 to 10 cal kyrs bp and thus captures the transition from the younger dryas stadial into the holocene at sub decadal resolution the younger dryas was characterized by cold arctic water taxa of diatoms and benthic foraminifera and a stratified water column with high sea ice cover and low productivity due to strong influence of the labrador current during the second half of the younger dryas after ca 12 2 cal kyrs bp the influence of the labrador current gradually decreased and sea ice conditions became more variable the transition towards the holocene is characterized by a sequence of events starting with turnovers in the biogenic proxies followed by an abrupt retreat of sea ice and a rise in productivity these events are believed to be related to a northward migration of the gulf stream labrador current oceanic front the younger dryas termination is directly followed by a detrital carbonate layer associated with the final phase of the heinrich 0 event linked to laurentide ice sheet dynamics during the early holocene productivity remained high with minimal sea ice cover reduced stratification and stronger influence of warmer gulf stream derived waters