u pb evidence for abitibi gold mineralization postdating greenstone magmatism and metamorphism two widely held theories for the origin of archaean lode gold deposits the magmatic1 and metamorphic replacement2 models predict that gold emplacement should have occurred near the end of late syn kinematic greenstone magmatism here we report that at the camflo mine in the abitibi subprovince of canada u pb ages for rutile and titanite in auriferous alteration 2 625 7 myr are much younger than the age of the host quartz syenite 2 680 4 myr and ages for regional magmatism 2 670 myr 3 and metamorphism 2 680 myr 4 mineral associations and pb pb systematics indicate that the hydrothermal minerals record crystallization ages that are probably coeval with the gold mineralization these ages and others from the abitibi subprovince4 6 rule out a direct connection between gold and magmatism or metamorphism associated with regional folding of the greenstone belt they are broadly contemporaneous with metamorphism at deep crustal levels7 and may record late devolatilization of the lower crust

large scale intertidal polygonal features of the abu dhabi coastline this study provides the first quantitative description of modern giant polygons these large scale up to 166 m diameter sedimentological features are described here in detail from the intertidal zone of the arabian gulf near abu dhabi these features form by the displacive carbonate cementation of carbonate sand below a 3 to 5 cm thick cover of unconsolidated sediment the oxygen isotopic composition of cements is consistent with precipitation from evaporated sea water cementation proceeds via a poorly consolidated layer that grades downward into an 8 to 14 cm thick well cemented grainstone the margins of this expanding hardground form the overlapping borders of the polygons tepee structures at the borders of polygons are only occasionally preserved as supporting sediment is removed by strong tidal currents and high energy events such as storms that erode the unsupported tepees these observations have clear implications for the use of tepees in the interpretation of ancient polygons and their associated environments when preserved in the sedimentary record tepee structures are interpreted to indicate lower energy depositional environments than that observed in this study

environmental implications of salt facies in the dead sea thick sequences of salt halite have been recovered in a 456 m long core drilled at the deepest floor of the dead sea by the dead sea deep drilling project and extending 200 k y back in time the salt sequences were precipitated in the ancient lake that occupied the dead sea basin during the last three interglacials during intervals of extreme aridity in the lake s watershed the salt layers alternate with mud layers that indicate wetter periods in the watershed when floods transported fine detritus matter to the lake the salt sources include brine discharge and freshwater runoff that dissolved halite units dissolved salts accumulated in the lake during glacials and relatively wet periods when the lake expanded and precipitated during interglacials when the lake levels dropped this study establishes for the first time the evaporite facies and sedimentological features of the deep dead sea brine during interglacial periods by using the modern precipitation of halite in the dead sea as an analogue for past halite depositional periods as recorded in the drill core the halite intervals provide a record of facies characterizing a deep water evaporitic environment the halite layers consist mainly of two types of crystals small cumulate crystals containing halite rafts which indicate precipitation from the surface brine of the lake epilimnion and bottom growth usually large halite crystals that precipitated on the lake floor hypolimnion the layers of small halite crystals formed during drier periods as compared to the bottom growth crystals the bottom growth halite crystals contain variable quantities of detritus and show mild dissolution structures at the contact between the mud and the halite crystals these two main types of halite in combination with muds and gypsum comprise seven categories of salt facies that reflect the hydrological conditions dry to wet and that display a cyclic decadal to millennial pattern along the sampled core intervals frequent alternation of these two salt crystal types suggests seasonal changes whereby the small cumulate crystals were formed during the summer and the bottom growth crystals were formed during the winter when the surface temperatures of the lake were low and the surface water was less saline and less likely to be saturated with respect to halite comparison of the last interglacial halite with the modern halite facies together with the absence of significant dissolution features within the halite and the cyclic nature of the facies indicates that the lake was continuously deep 100 m during the last 200 k y

high precision u pb ages for granulite metamorphism and deformation in the archean kapuskasing structural zone ontario implications for structure and development of the lower crust abstract precise u pb isotopic ages for two subhorizontal granulite gneisses in the kapuskasing structural zone indicate that formation and emplacement occurred more than 50 m y after cessation of volcanism in the steeply folded nearby greenstone belts by successive stages of ductile underplating across a zone of crust mantle delamination volcanism and batholith formation had ceased by 2700 and 2680 ma respectively while granulite formation occurred between 2660 and 2640 ma and ductile deformation continued until 2585 ma emplacement of the oldest granulite caused the development of shear planes and melting in the older overlying tonalite while emplacement of the younger granulite caused rupturing and rehydration of the overlying older granulite widespread tectonic underplating is implied since all superior province granulites have similar young ages anomalous ar ages that post date accretion by about 150 m y and scrambled pb pb and rb sr whole rock systems also suggest that late low temperature alteration accompanied this event temporal relationships and the presence of post volcanic 2670 ma conglomerate cobbles at granulite grade support a model of ductile underplating of a post accretion thermally softened back arc fore arc complex with components welded onto an overlying volcanic plutonic complex at successively deeper levels multistage deformation in active fault zones and hydrothermal emplacement of major gold deposits in greenstone belts are coeval with ductile underplating with ages between 2670 and 2585 ma anomalously abundant gold production from archean rocks may indicate that shallow underplating granulitization and devolatization was a widespread phenomenon at this time post accretion ductile underplating must be considered as a primary cause of seismic reflectors in archean terrains

precambrian u pb zircon ages in eclogites and garnet pyroxenites from south brittany france an old oceanic crust in the west european hercynian belt abstract u pb zircon ages have been determined for two eclogites from the vendee and for two garnet pyroxenites from the baie d audierne in an episodic pb loss model the two discordia would give upper intercept ages around 1300 1250 ma and lower intercepts ages of 436 384 ma two interpretations are proposed 1 the 1250 1300 ma ages may reflect an unspecified upper mantle event or process the paleozoic ages correspond to the tectonic emplacement of an eclogitic mantle fragment into the continental crust 2 the protolith may have been extracted from the upper mantle 1250 1300 ma ago and stored in a crustal environment until it was metamorphosed under high pressure conditions around 400 ma ago this latter model is favoured by available geologic and isotopic data consequently we propose that a 1300 ma old oceanic crust was tectonicly incorporated into a sialic basement during the proterozoic this mixture was subsequently subducted during the paleozoic

the record of temperature wind velocity and air humidity in the δd and δ18o of water inclusions in synthetic and messinian halites abstract deuterium and oxygen isotope fractionations between liquid and vapor water were experimentally determined during evaporation of a nacl solution 35 g l 1 as a function of water temperature and wind velocity in the case of a null wind velocity slopes of d 18o trajectories of residual waters hyperbolically decrease with increasing water temperatures in the range 23 47 c for wind velocities ranging from 0 8 to 2 2 m s 1 slopes of the d 18o trajectories linearly increase with increasing wind velocity at a given water temperature these experimental results can be modeled by using rayleigh distillation equations taking into account wind related kinetics effects deuterium and oxygen isotope compositions of water inclusions trapped by the precipitated halite crystals were determined by micro equilibration techniques these isotopic compositions accurately reflect those of the surrounding residual waters during halite growth isotopic compositions of water inclusions in twenty natural halites from the messinian realmonte mine in sicily suggest precipitation temperatures of 34 4 10 c that match the homogenization temperatures obtained by microthermometry median 34 5 c the similarity between the measured and experimental slopes of the d 18o evaporation trajectories suggests that the effect of wind was negligible during the genesis of these halite deposits hydrogen and oxygen isotope compositions of water inclusions from realmonte halite also define a linear trend whose extrapolation until intersection with the mediterranean meteoric water line allows the characterization of the water source with d and 18o values of 70 10 and 11 5 1 5 respectively these results reveal that the huge amounts of salts deposited in sicily result from the evaporation of seawater mixed with a dominant fraction 50 of meteoric waters most likely deriving from alpine fluvial discharge

heat flow gravity and structure of the abitibi belt superior province canada implications for mantle heat flow the results of new heat flow measurements are presented for eleven sites located in the abitibi greenstone belt of the archean superior province these new and previously published heat flow data are used with gravity and seismic data to constrain the crustal structure and composition of the abitibi subprovince and to determine the mantle heat flow beneath the canadian shield in order to analyze the long wavelength variations in heat flow and bouguer gravity anomaly the crust was divided into three layers with distinctive composition and physical properties average density heat production and thickness for each layer were varied within a range compatible with measurements and yielded crustal models consistent with gravity and heat flow data the dominant contribution to crustal heat production is that of a tonalitic type layer which has the lowest density the models require mantle heat flow to range between 10 and 14 mw m 2 the westward increase in heat flow requires the tonalitic layer to thicken from the grenville front toward the kapuskasing uplift at the expense of the greenstone supracrustals long wavelength gravity data are more affected by changes in crustal thickness than by composition and the low bouguer anomaly near the eastern grenville front is caused by 5 6 km of crustal thickening in greenstone terranes the long wavelength variations in heat flow are more useful for determining average crustal composition and the thickness of supracrustals than the gravity data

40ar 39ar geochronological evidence for multiple postmetamorphic hydrothermal events focused along faults in the southern abitibi greenstone belt thermal histories in the abitibi and pontiac subprovinces vary for three general domains 1 zones of impermeable rock with cooling rates of 8 c ma 2 zones of impermeable rock that are spatially associated with monzogranite plutons with apparent cooling rates of 1 2 cima and 3 permeable high strain zones with apparent cooling rates of 2594 ma from the volcanogenic massive sulphide related sericite alteration with the coarser mica yielding the younger date based on radiometric dating impermeable rocks were affected weakly by postmetamorphic hydrothermal events minerals in zones of higher permeability were reset during the intrusion of monzogranite plutons highly permeable zones such as the larder lake cadillac and porcupine destor faults were overprinted repeatedly this produces a drawn out series of dates from minerals with varying closure temperatures radiometric ages from fault zones or ore deposits cannot be used to interpret the cooling history and evolution of the abitibi belt as a whole

decoding the mediterranean salinity crisis this historical narrative traces the steps to unravel over a span of 40 years an extraordinary event in which 5 of the dissolved salt of the oceans of the world was extracted in a fraction of a million years to form a deposit more than 1 million km 3 in volume a buried abyssal salt layer was identified with reflection profiling and sampled during the deep sea drilling project leg 13 the dolomite gypsum anhydrite and halite in the drill cores paint a surprising picture of a mediterranean desert lying more than 3 km below the atlantic ocean with brine pools that shrank and expanded by the evaporative power of the sun the desert drowned suddenly when the gibraltar barrier gave way the explanation of deep basin shallow water desiccation and the notion of a catastrophic zanclean flood had a mixed reception however the hypothesis became broadly accepted following subsequent drilling expeditions nevertheless as experts examined the evaporate facies and sequences of equivalent age in the terrestrial outcrops weaknesses appeared in the concept of repeated flooding and drying to account for the magnitude of the deposits the rosetta stone used to decipher conflicting interpretations turns out not to be the deposits but the erosion surfaces that enclose them these surfaces and their detritus formed in response to the drop in base level during evaporative drawdown extend to the basin floor evaporative drawdown began halfway through the salinity crisis when influx from the atlantic no longer kept up with evaporation prior to that time a million years passed as a sea of brine concentrated towards halite precipitation during the later part of this interval more than 14 cyclic beds of gypsum accumulated along shallow margins modulated by orbital forcing the thick salt on the deep seabed precipitated in just the next few cycles when drawdown commenced and the brine volume shrank upon closure of the atlantic spillway the remnants of the briny sea transformed into salt pans and endorheic lakes fed from watersheds of eurasia and africa the revised model of evaporative concentration now has shallow margin shallow water and deep basin deep water precursors to desiccation

high frequency cyclicity in the mediterranean messinian evaporites evidence for solar lunar climate forcing abstract the deposition of varved sedimentary sequences is usually controlled by climate conditions the study of two late miocene evaporite successions one halite and the other gypsum consisting of annual varves has been carried out to reconstruct the paleoclimatic and paleoenvironmental conditions existing during the acme of the messinian salinity crisis 6 ma when thick evaporite deposits accumulated on the floor of the mediterranean basin spectral analyses of these varved evaporitic successions reveal significant periodicity peaks at around 3 5 9 11 13 20 27 and 50 100 yr a comparison with modern precipitation data in the western mediterranean shows that during the acme of the messinian salinity crisis the climate was not in a permanent evaporitic stage but in a dynamic situation where evaporite deposition was controlled by quasi periodic climate oscillations with similarity to modern analogs including quasi biennial oscillation el nino southern oscillation and decadal to secular lunar and solar induced cycles particularly we found a significant quasi decadal oscillation with a prominent 9 year peak that is commonly also found in modern temperature records and is present in the contemporary atlantic multidecadal oscillation amo index and pacific decadal oscillation pdo index these cyclicities are common to both ancient and modern climate records because they can be associated with solar and solar lunar tidal cycles during the messinian the mediterranean basin as well as the global ocean were characterized by different configurations than at present in terms of continent distribution ocean size geography hydrological connections and ice sheet volumes the recognition of modern style climate oscillations during the messinian suggests that although local geographic factors acted as pre conditioning factors turning the mediterranean sea into a giant brine pool external climate forcings regulated by solar lunar cycles and largely independent from local geographic factors modulated the deposition of the evaporites

high resolution strontium isotope stratigraphy of the messinian deep mediterranean basins implications for marginal to central basins correlation new 87sr 86sr data from dsdp odp messinian cores from deep mediterranean basins suggest that the usually envisaged correlation of offshore upper evaporites with onshore upper gypsum deposits of sicily cyprus and crete recording the stage 3 5 53 5 33 ma of the messinian salinity crisis may be not entirely correct high resolution stratigraphic calibration of sr isotope data indicates that only a very thin unit commonly 50 m in the uppermost part of the seismic upper evaporites is characterized by the typically lower values for sr isotopes with respect to the global ocean which characterize stage 3 onshore successions lago mare event these deposits mainly consist of interbedded clastic or cumulate gypsum and marls halite recovered from cores in the upper evaporites unit is actually characterized by sr isotope values consistent with stage 2 deposits of onshore successions according to these results the messinian trilogy of the western mediterranean basin could be as a whole correlated with the halite unit of the eastern basin suggesting that different hydrologic conditions characterized the two deep areas during the peak of the salinity crisis

dynamic cause of marginal lithospheric thinning and implications for craton destruction a comparison of the north china superior and yilgarn cratons we present a comparative tectonic analysis of the north china craton ncc which has lost parts of its root with the yilgarn and superior cratons which preserve their roots we compare the geophysical structure and tectonic histories of these cratons to search for reasons why some cratons lose their roots while others retain them based on the comparison and analysis of geological geophysical and geochemical data it is clear that the lithospheric thinning beneath craton margins is a common phenomenon which may be caused by convergence between plates however craton destruction is not always accompanied by lithospheric thinning except for cratons that suffered subduction and collision from multiple sides the western block also known as the ordos block of the ncc yilgarn and superior cratons have not experienced craton destruction the common ground among them is that they are surrounded by weak zones e g mobile belts or orogens that sheltered the cratons from deformation which contribute

meganodular anhydritization a new mechanism of gypsum to anhydrite conversion palaeogene neogene ebro basin north east spain a number of palaeogene to early neogene gypsum units are located along the southern margins of the ebro basin north east spain these marginal units of eocene to lower miocene age formed and accumulated deposits of ca sulphates gypsum and anhydrite in small shallow saline lakes of low ionic concentration the lakes were fed mainly by ground water from deep regional aquifers whose recharge areas were located in the mountain chains bounding the basin and these aquifers recycled and delivered ca sulphate and na chloride from mesozoic evaporites triassic and lower jurassic in outcrop the marginal sulphate units are largely secondary gypsum after anhydrite and exhibit meganodules from 0 5 to 5 m across and large irregular masses in the sub surface these meganodules and masses are mostly made of anhydrite which replaced the original primary gypsum the isotopic composition 11 1 to 17 4 for 18ovsmow 10 7 to 15 3 for 34svcdt of secondary gypsum in this meganodular facies indicates that the precursor anhydrite derived from in situ replacement of an initial primary gypsum as a result of ascending circulation of deep regional fluid flows through the gypsum units near the basin margins the gypsum was partly altered to anhydrite within burial conditions from shallow to moderate depths from some metres to a few hundred metres at such depths the temperatures and solute contents of these regional flows exceeded those of the ground water today these palaeoflows became anhydritizing solutions and partly altered the subsiding gypsum units before they became totally transformed by deep burial anhydritization the characteristics of the meganodular anhydritization for example size and geometry of the meganodules and irregular masses spatial arrangement relations with the associated lithologies and the depositional cycles presence of an enterolithic vein complex and palaeogeographic distribution are compared with those of the anhydritization generated both in a sabkha setting or under deep burial conditions and a number of fundamental differences are highlighted

the behaviour of zirconium hafnium and rare earth elements during the crystallisation of halite and other salt minerals abstract halite crystals from messinian and tortonian evaporites from sicily and spain and current precipitated halite crystals and the relative parent brines active evaporation systems were investigated in order to evaluate the zr hf and rare earth element ree behaviour halite crystallisation from evaporating brines fractionates zr hf and ree through a two step process during the first step dissolved complexes of studied elements are scavenged onto the surfaces of crystallising halite during the second step elements are co precipitated into the crystal lattice as it grows the first step mechanism is determined by the dissolved ree speciation in saltworks where carbonate ree complexes occur surface complexation of ree onto halite crystals does not occur on the contrary surface ree complexes onto halite crystals are forming in the dead sea water where aqueous ree speciation is dominated by chloride complexes under the latter conditions halite crystallises with cubic and cubic octahedral composite habitus octahedral planes involve the formation of strong coulombic interactions mainly with hf h 2 o 3 oh 5 rather than with zr h 2 o 4 oh 4 0 complexes as a consequence newly formed halite in the dead sea shows strong subchondritic zr hf ratios based on these indications analyses carried out on salt minerals from messinian and tortonian evaporites in sicily and spain show that their overall ree content can be considered a discriminating parameter between authigenic minerals and diagenetic modified materials however features of shale normalised ree patterns are driven by the mineralogical composition of evaporites rather than their authigenic or secondary nature on the contrary the zr hf signature of salt minerals is influenced by their origin indeed subchondritic zr hf values are found in primary salt minerals whereas larger zr hf values are recognised in those diagenetically modified calculated distribution coefficients of zr hf and ree are employed for modelling the ree distribution in halite equilibrated with the deep sea brines from typo medee and thetis basins eastern mediterranean the obtained indications allow us to discriminate brines formed by dissolution of evaporites relative to those representing relics of fossil evaporated seawater

the messinian salinity crisis in the adriatic foredeep evolution of the largest evaporitic marginal basin in the mediterranean abstract the recent release of a large number of subsurface geological data by the italian minister of economic development including boreholes and seismic profiles provided the occasion for a new assessment of the deposits associated with the messinian salinity crisis msc in the adriatic foreland basin system and a new integration with the outcropping successions of the apennines in particular the study of the messinian evaporites allowed to reconstruct a new detailed palaeogeographic and palaeobathymetric framework for all the stages of the crisis we identified the largest evaporitic marginal basin ever described for the mediterranean hosting the precipitation of the primary shallow water gypsum deposits plg primary lower gypsum during the first stage of the crisis during the second and third stages of the crisis the plg basin underwent uplift and erosion and the evaporite accumulation moved to the deeper part of the basin and was characterized by the deposition of the resedimented lower gypsum unit including clastic evaporites recycling the plg ones primary halite and terrigenous deposits the distribution of the different evaporitic facies was the basis for an improved reconstruction of the upper miocene tectonic evolution of the apennines thrust belt our results show a clear separation between shallower depocenters located in the wedge top and in the adriatic foreland basins and characterized by msc stage 1 plg deposition and deeper water ones located in the adriatic foredeep and close to the calabrian arc where msc stage 2 terrigenous and gypsum bearing clastic deposits and primary halite accumulated

diagenetic evolution of aptian evaporites in the namibe basin south west angola the widespread and dissected nature of the angolan gypsiferous salt residuals offers a uniquely detailed view of the lateral and vertical relations inherent to secondary evaporite textures which typify exhumed salt masses worldwide such secondary textures are sometimes misinterpreted as primary evaporite textures thin metre scale and patchy dome like gypsum accumulations are well exposed within strongly incised present day river valleys along the eastern margin of the namibe and benguela basins south west angola these sections are time equivalent to the main basinward subsurface evaporites aptian loeme formation which mostly consist of halite the gypsum here called the bambata formation is interpreted to represent the final residual product of fractional dissolution and recrystallization of the halite mass that occurred during late cretaceous margin uplift and continues today this halite underwent multiple episodes of diagenetic alteration between its deposition and its final exhumation leading to the formation of various secondary gypsum fabrics and solution related karst and breccia textures that typify the current evaporite outcrop four different diagenetic gypsum fabrics are defined thinly bedded alabastrine nodular alabastrine displacive selenite rosettes and fibrous satin spar gypsum current arid conditions are responsible for a thin weathered crust developed at the top of the outcropping gypsum but the fabrics in the main core of the current at surface evaporite unit mostly formed during the telogenetic stage of uplift prior to complete subaerial exposure alteration occurred as various dissolving and rehydrating saline minerals encountered shallow aquifers in the active phreatic and vadose zones geomorphological and petrographic analyses mostly based on the cross cutting relations and crystallographic patterns in the outcrop are used to propose a sequence of formation of these different fabrics

synsedimentary dissolution pipes and the isolation of ancient bacteria and cellulose coarse clear halite from synsedimentary dissolution pipes in the permian salado formation in southeastern new mexico usa yielded viable halotolerant bacteria and well preserved cellulose believed to be permian in age here we show that geologic and hydrologic conditions have isolated these rocks since permian time pipes were dissolved most likely along cracks created by thermal contraction or desiccation and the boundaries of salt polygons saucers on exposed salado formation salt pan surfaces down to the level of the water brine table macropores developed at the brine level in some horizons as the water level rose coarse halite cemented the open space fluid inclusions millimeter scale trapped bacteria which were probably in a spore state as well as cellulose inclusion water from pipes in some cycles may have isotopic values reflecting permian meteoric water while other cycles may show evaporated permian seawater exposure surfaces are more prominent in the upper part of salado formation depositional cycles due to basin desiccation as indicated by cracks and dish shaped laminae clays were concentrated on many exposure surfaces by floods wind and dissolution dissolution pipes formed from these surfaces and a synsedimentary age is confirmed by undisturbed overlying halite beds salado formation halite has very low permeability 10 22 m 2 effectively preventing significant fluid flow and passage of fluids through the formation either to recrystallize salt or to introduce modern bacteria stratigraphic relationships halite textures fluid inclusion chemistry and hydraulic properties are all consistent with a synsedimentary origin of the dissolution pipes their crystals and the bacteria and cellulose recovered from the salado formation

salt deposition and brine evolution in the granada basin late tortonian se spain abstract a group of sedimentary basins in the betic chain were formed during the middle late miocene as a result of the closure of the tethys during the alpine orogeny in the late miocene tortonian messinian the connections between the atlantic ocean and mediterranean sea were interrupted and those basins hosted major evaporites the granada basin an inner basin located far from the mediterranean contains a thick rock salt deposited during the latest tortonian in the transition from marine to non marine conditions in the centre of the basin three halite bearing units overlie a basal anhydrite bed the lower halite unit the intermediate sandstone unit and the upper halite unit fluid inclusion compositions and bromine concentrations in halite together with stable isotopes 34 s sulfate 18 o sulfate and 87 sr 86 sr indicate a mixture of different inflow waters in the granada basin beginning with a marine lagoon that evolved into a salt pan strongly isolated from the sea saline waters evolved from sulfate rich marine derived to sulfate depleted non marine brines influenced by the addition of cacl 2 rich inputs these cacl 2 rich waters were probably linked to thermal fluids associated with a major crustal fracture system crevillente or cadiz alicante fault system that cuts through the granada basin

the major ion composition of cenozoic seawater the past 36 million years from fluid inclusions in marine halite fluid inclusions from ten cenozoic eocene miocene marine halites are used to quantify the major ion composition mg2 ca2 k na so42 and cl of seawater over the past 36 my criteria used to determine a seawater origin of the halites include 1 stratigraphic sedimentologic and paleontologic observations 2 br in halite 3 34s of sulfate minerals 4 87sr 86sr of carbonates and sulfates and 5 fluid inclusion brine compositions and evaporation paths which must overlap from geographically separated basins of the same age to confirm a global seawater chemical signal changes in the major ion chemistry of cenozoic seawater record the end of a systematic long term 150 my shift from the ca2 rich mg2 and so42 poor seawater of the mesozoic cacl2 seas to the mgso4 seas with higher mg2 and so42 ca2 of the cenozoic the major ion composition of cenozoic seawater is calculated for the eocene oligocene 36 34 ma serravallian tortonian 13 5 11 8 ma and the messinian 6 5 ma assuming chlorinity 565 mmolal salinity and the k concentration 11 mmolal are constant and the same as in modern seawater fluid inclusions from cenozoic marine halites show that the concentrations of mg2 and so42 have increased in seawater over the past 36 my and the concentration of ca2 has decreased mg2 concentrations increased from 36 mmolal in eocene oligocene seawater 36 34 ma to 55 mmolal in modern seawater the mg2 ca2 ratio of seawater has risen from 2 3 at the end of the eocene to 3 4 and 4 0 respectively at 13 5 to 11 8 ma and 6 to 5 ma and to 5 in modern seawater eocene oligocene seawater 36 34 ma has estimated ranges of so42 14 23 mmolal and ca2 11 20 mmolal if the ca2 so42 product is assumed to be the same as in modern seawater 300 mmolal2 eocene oligocene seawater had ca2 16 mmolal and so42 19 mmolal the same estimates of ca2 and so42 for serravallian tortonian seawater 13 5 11 8 ma are so42 19 27 mmolal and ca2 8 16 mmolal and so42 24 mmolal and ca2 13 mmolal if the ca2 so42 product is equal to that in modern seawater messinian seawater has an estimated range of so42 21 29 mmolal and ca2 7 15 mmolal with so42 26 mmolal and ca2 12 mmolal assuming the ca2 so42 product is equal to that in modern seawater regardless of the estimation procedure so42 shows progressively increasing concentrations from 36 ma to the present values which are the highest of the cenozoic

revealing hidden structures the application of cathodoluminescence and back scattered electron imaging to dating zircons from lower crustal xenoliths abstract the use of cathodoluminescence cl and or back scattered electron bse imaging techniques with in situ ion probe analyses of zircons can help unravel complex crustal histories of metamorphic rocks that otherwise might remain elusive using these techniques we have imaged zircons from three lower crustal xenolith suites that have previously been dated by shrimp sensitive high resolution ion microprobe in all three cases the zircons are featureless in transmitted light but cl and bse reveal internal structures that correlate with distinct growth events generally cl and bse images reveal similar structures with cl showing finer detail neither imaging technique is capable of delineating all growth features in every sample and the best results are obtained using a combination of the two techniques igneous cores in zircons commonly emit a different color cl emission in many cases zoning in the cores is truncated indicating that the zircons either spent time in the supracrustal environment after their initial crystallization and prior to the granulite facies event s or that part of the core zircon was resorbed during the subsequent metamorphic event metamorphic rims when present are commonly 10 to 30 m thick and are nearly always unzoned and featureless igneous cores and metamorphic overgrowths commonly have distinctive cl emission spectra and trace element concentrations however the cl spectra can only be used to qualify chemical differences as a linear relationship has not been shown to exist between cl intensity and trace element concentration in natural zircons in many cases the hf y p u and the heavy rare earth elements hrees concentrations can be correlated to igneous and metamorphic growth using a combination of cl and bse imaging techniques and in situ trace element analyses with either the electron microprobe or pixe particle induced x ray emission

zircon u pb strain chronometry reveals deep impact triggered flow abstract large 100 km meteorite impact cratering events play important roles in surface and biosphere evolution however their potential for widespread ductile modification of the lithosphere has been difficult to assess due partly to our inability to isotopically age correlate deep mineral fabrics with surface records we have integrated benchmark u pb zircon dating methods id tims shrimp with new microstructural techniques ebsd xrd to demonstrate that crystal plastic deformation can cause rapid out diffusion of radiogenic pb and accompanying trace element alteration in crystalline zircon we have used this phenomenon to directly date fabric in archean zircons and xenoliths of the lower crust of south africa at 2023 15 million years coeval with the 2020 3 million year old vredefort cratering event at surface with extent 20 000 km 2 our findings indicate that regional exogenic fabrics similar to high temperature tectonic fabrics exist in ancient crust moreover our results establish that crystal plastic deformation in the lithosphere can now be directly dated and linked to planetary evolution by zircon u pb strain chronometry

evolution of the late miocene mediterranean atlantic gateways and their impact on regional and global environmental change marine gateways play a critical role in the exchange of water heat salt and nutrients between oceans and seas as a result changes in gateway geometry can significantly alter both the pattern of global ocean circulation and associated heat transport and climate as well as having a profound impact on local environmental conditions mediterranean atlantic marine corridors that pre date the modern gibraltar strait closed during the late miocene and are now exposed on land in northern morocco and southern spain the restriction and closure of these miocene connections resulted in extreme salinity fluctuations in the mediterranean leading to the precipitation of thick evaporites this event is known as the messinian salinity crisis msc the evolution and closure of the mediterranean atlantic gateways are a critical control on the msc but at present the location geometry and age of these gateways are still highly controversial as is the impact of changing mediterranean outflow on northern hemisphere circulation here we present a comprehensive overview of the evolution of the late miocene gateways and the nature of mediterranean atlantic exchange as deduced from published studies focussed both on the sediments preserved within the fossil corridors and inferences that can be derived from data in the adjacent basins we also consider the possible impact of evolving exchange on both the mediterranean and global climate and highlight the main enduring challenges for reconstructing past mediterranean atlantic exchange

trace element and age characteristics of zircons in granulite xenoliths from the udachnaya kimberlite pipe siberia trace element and shrimp u pb geochronological data are presented for zircons from granulite xenoliths from the devonian udachnaya kimberlite pipe situated in the central part of the siberian craton 66 05 n 110 45 e the xenoliths include three two pyroxene garnet granulites one garnet granulite lacking orthopyroxene three felsic garnet granulites one kinzigite and one amphibolized garnet granulite most of them are clearly distinct from the local upper crustal lithologies and have pressure and temperature estimates indicative of derivation from the lower crust zircons from these granulites show a record of several magmatic and metamorphic events with different domains revealed by cl imaging and variations in ree content and th u ratios the first event is shown by magmatic zircon in the cores of some grains that yield discordant archaean to proterozoic ages all other domains of the studied zircons have textures typical for metamorphic zircons they have proterozoic ages and were formed by subsolidus growth or as a result of solid state recrystallization of pre existing zircon they give a range of concordant ages from 1 94 to 1 81 ga in which several distinct episodes can be distinguished that relate to the collision of terranes during the accretion of the siberian craton accompanied by formation of granite melts during these events the xenolith protoliths were metamorphosed under granulite facies conditions at pressures corresponding to lower crustal depths except for the kinzigite that is probably derived from shallower depths

the messinian salinity crisis past and future of a great challenge for marine sciences forty years after the image of the mediterranean transformed into a giant salty lake was first conceived the fascinating history of the messinian salinity crisis msc still arouses great interest across a large and diverse scientific community early outcrop studies which identified severe palaeoenvironmental changes affecting the circum mediterranean at the end of the miocene were followed by investigations of the marine geology during the 1950s to 1970s these were fundamental to understanding the true scale and importance of the messinian event now after a long period of debate over several entrenched but largely untested hypotheses a unifying stratigraphic framework of msc events has been constructed this scenario is derived mainly from onshore data and observations but incorporates different perspectives for the offshore and provides hypotheses that can be tested by drilling the deep mediterranean basins the msc was an ecological crisis induced by a powerful combination of geodynamic and climatic drivers which had a great impact on the subsequent geological history of the mediterranean area and on the salinity of the global oceans these changed the mediterranean s connections with both the atlantic ocean and the freshwater paratethyan basins causing high amplitude fluctuations in the hydrology of the mediterranean the msc developed in three main stages each of them characterized by different palaeoenvironmental conditions during the first stage evaporites precipitated in shallow sub basins the msc peaked in the second stage when evaporite precipitation shifted to the deepest depocentres and the third stage was characterized by large scale environmental fluctuations in a mediterranean transformed into a brackish water lake the very high resolution timescale available for some late miocene intervals in the mediterranean makes it possible to consider environmental variability on extremely short time scales including in some places annual changes despite this fundamental questions remain some of which could be answered through new cores from the deepest mediterranean basins improvements in seismic imaging and drilling techniques over the last few decades make it possible to plan to core the entire basinal messinian succession for the first time the resulting data would allow us to decipher the causes of this extreme environmental change and its global scale consequences

tepee shaped agglutinated microbialites an example from a famennian carbonate platform on the lennard shelf northern canning basin western australia distinctive metre scale antiformal structures are well developed in a famennian carbonate platform in the chedda cliffs area of the lennard shelf reef complexes the structures are distinguished by chevron shaped crests and thickened cores and contain abundant non skeletal allochems ooids pisoids peloids and intraclasts of silt to pebble size and variably developed laminations and fenestrae the internal morphology and pervasive occurrence of fenestral clotted and wavy laminated fabrics suggest that these structures are microbial mounds composed of agglutinated stromatolites and thrombolites microbial fabrics most probably originated through sediment trapping and binding by microbial mats with early lithification involving microbial calcification and cementation of trapped sediment the facies and stratigraphic context of the mounds support a shallow subtidal transitional backreef to reef flat setting however alone these mounds do not provide unequivocal environmental information other large antiformal structures in famennian platforms on the lennard shelf previously described as tepee structures show morphological similarities to the chedda cliffs mounds which suggests that these other structures may also be microbial mounds the presence of microbial mounds in platform successions further highlights the importance of microbial communities in the lennard shelf reef complexes

depositional environments of the mediterranean lower evaporites of the messinian salinity crisis constraints from quantitative analyses abstract we use simple quantitative analyses to evaluate controversial water level scenarios for the mediterranean lower evaporites of the messinian salinity crisis our results indicate that a shallow water scenario for the lower gypsum units with mediterranean water level lower than the sill at gibraltar would imply unrealistic salt thicknesses on the order of 3 km some outflow to the open ocean must have persisted implying that the mediterranean was a deep water basin during lower gypsum formation since glacio eustatic fluctuations do not seem to have had a major influence on lower gypsum deposits mediterranean water level was even substantially higher than the gibraltar sill our analyses furthermore show that precessional changes in the freshwater budget may explain the observed cyclic lithological changes of gypsum and non evaporitic sediments potential precipitation of gypsum in the deep mediterranean basins would have critically depended on the availability of oxygen and thus on the stratification of the water column finally our results indicate that the deep mediterranean halite units could have been deposited under shallow conditions assuming that they correspond to the 70 kyr time interval between glacials tg12 and tg14 when mediterranean outflow to the atlantic was blocked

time dependent uplift of the kenai peninsula and adjacent regions of south central alaska since the 1964 prince william sound earthquake leveling global positioning system very long baseline interferometry and tide gauge observations indicate that time dependent uplift has occurred on the kenai peninsula and adjacent regions of southern alaska that subsided coseismically in the 1964 prince william sound earthquake these observations have been critically assessed and used along with seismological and geological information as constraints in various aseismic slip and viscoelastic flow models of crustal deformation the postseismic uplift rate in most of the region is significantly greater than the expected interseismic average some but not all of the data indicate that the uplift rate was quite rapid more than several centimeters per year in the years immediately following the earthquake and has slowed to a few centimeters per year or less since then the modeling shows that the uplift can be explained by aseismic slip at depth but not by viscoelastic flow the slip at depth since 1964 must have a transient as well as a steady component since the deduced slip is considerably greater than the roughly 1 5 m expected from north american pacific plate convergence since the 1964 event the minimum amount of slip required to fit the observations comes from an elastic model and is about 2 75 m viscoelastic models require more slip since the dominant effect of viscoelastic flow is to produce subsidence in the region that is uplifted a simple parameterization of the transient component using an amplitude and decay time is adequate to explain the observations while the parameters deduced from the viscoelastic models have some ambiguity the viscosity of models that best fit the observations cannot be much less than several times 1019 pa s when the thickness of the north american plate is taken to be about 90 km

the demise of a salt giant driven by uplift and thermal dissolution abstract the development of giant salt basins and eventual cessation of rapid salt deposition is founded on a delicate balance of salinity and heat fluxes within the water body governed by tectonic climatic and eustatic change the onset of salt deposition in such basins is widely accepted to be initiated by basin restriction however the processes that lead to the termination of salt deposition are comparatively unclear here we use an array of 2d and 3d seismic surveys to reveal that the truncation surface at the top of a thick salt sequence in the eastern mediterranean is far more extensive than previously thought we show that uplift of the salt driven by deformation and thermal dissolution initiated the demise of the salt giant even prior to the final dilution and emplacement of brackish lago mare and fluvial deposits progressive uplift of the salt through the thermocline and into the under saturated epilimnion led to dissolution we argue that dissolved salt was recycled and re precipitation from the hypolimnion in the deepest sections of the basin contemporaneous with dissolution of halite from the shallower epilimnion these findings explain how rapid basinwide salt deposition was brought to an end in the eastern mediterranean and present a novel process for sculpting the final architecture of a salt giant

localized granulite and eclogite facies metamorphism at flatraket and krakeneset western gneiss region u pb data and tectonic implications

how dry was the mediterranean during the messinian salinity crisis the messinian salinity crisis msc 5 97 5 33 ma is an enigmatic episode of paleoceanographic change when kilometers thick evaporite units were deposited in the mediterranean basin it is generally accepted that during the msc interval there was a dry climate in the mediterranean region it is difficult to assess how dry the climate was during the msc because a modern analogue in size and duration is absent here we reconstruct hydrological changes in the mediterranean basin during the three main msc stages using excellently preserved biomarkers we used the hydrogen isotopic composition of the long chain n alkanes dn alkanes to reconstruct the hydrological changes on the land adjacent to the mediterranean sea additionally the d of long chain alkenones dalkenones is used to observe changes in the mediterranean sea water source the dn alkanes recorded during the deposition of primary lower gypsum stage 1 in monte tondo indicate a d of the precipitation comparable to the present day mediterranean implying a similar hydrologic regime indicated by experiments modelling the miocene pliocene transition elevated dalkenones values from halite unit stage 2 of the realmonte mine are associated with kainite and giant polygons consistent with presumably high evaporative conditions during halite deposition the dn alkanes recorded during the deposition of upper gypsum stage 3 in eraclea minoa indicate a dprecipitation typical for much drier settings similar to the red sea region the relative contribution of the different alkenones from eraclea minoa is similar to the one observed in present day marine settings suggesting that during stage 3 connections to the open ocean were likely maintained however the dalkenones records during deposition of the evaporites in eraclea minoa are similar to those synchronously registered in the black sea implying that a similar hydrologic regime characterized by extended drought covered large areas of southeastern europe based on the dalkenones similarity and the paratethys type of lago mare fauna in the mediterranean we speculate that the surface water during stage 3 was at times derived from the black sea

a summary of electromagnetic studies on the abitibi grenville transect electromagnetic surveys on the abitibi grenville lithoprobe transect have elucidated a number of conductivity signatures that can be genetically linked to precambrian tectonic processes some major fault zones are moderately conductive possibly signalling graphite deposition from a mantle co2 flux along crust penetrating fault systems however conductive graphitic metasedimentary rocks characteristic of foreland basins are apparently absent from the transect area a weak inverse correlation between metamorphic grade and electrical conductivity was observed by following rock units across the grenville front into high grade equivalents within the parautochthonous belt a uniformly conductive mid crustal layer extends across the grenville front apparently without change in character the existence of this ubiquitous mid crustal conductor has been interpreted to mean that electrical conductivity is controlled by the present day pressure temperature and fluid saturation of the lower crust independent o

an x ray spectroscopic perspective on messinian evaporite from sicily sedimentary fabrics element distributions and chemical environments of s and mg the messinian salinity crisis is a dramatic hydrological and biological crisis that occurred in the mediterranean basin at 5 97 5 33 ma the interpretation of the facies and stratigraphic associations of the messinian salt deposits is still the object of active research because of the absence of modern depositional analogues of comparable scale in this study the spatial distributions of na mg s o si and al in a potassic magnesian salt and a halite layers of messinian evaporites from the realmonte mine on sicily were determined using synchrotron based micro x ray fluorescence the dominant molecular host site of mg and s obtained by x ray absorption near edge structure xanes is applied to specify the hydrochemistry of hypersaline brines and the presence of diagenetic minerals thus shedding light on evaporative concentration processes in the caltanissetta basin of sicily mg and s k edge xanes spectra revealed the presence of highly soluble mg bearing sulfates the massive halite layer unit c contains less soluble minerals thus did not exceed the stage of halite crystallization we infer that as evaporative concentration increased the density of the brine at the shallow margin of the basin increased as salinity increased to concentrations over 70 times the starting values creating brines that were oversaturated with mg sulfate density stratification of the deep basin caused heavy brines to sink to the bottom and become overlain by more dilute brines we propose lateral advection of dense mg sulfate brines that certainly affected marine biota

proterozoic zircon growth in archean lower crustal xenoliths southern superior craton a consequence of matachewan ocean opening granulite grade anorthositic and mafic xenoliths recovered from a jurassic kimberlite pipe near kirkland lake ontario are fragments of the lower crust that underlies the ca 2 7 ga abitibi greenstone belt of the superior craton cathodoluminescence imaging and or backscatter electron microscopy of zircon from four individual xenoliths reveals a complex crystallization history characterized by two main stages of zircon growth the age of the two stages has been constrained by combining imaging results with isotope dilution u pb dating of grain fragments and single grains minimum ages for the first crystallization stage in individual xeno liths are 2584 7 ma 2629 8 ma 2633 3 ma whereas an approximate crystallization age for a fourth sample is 2788 57 ma the second main stage of growth consists of chemically and isotopically distinct metamorphic zircon overgrowths times of solid state zircon growth are most broadly constrained in three samples to the interval between 2 52 ga to 2 40 ga and most precisely dated in a meta anorthosite at 2416 30 ma these complex zircons are intergrown with garnet and clinopyroxene of the host granulite facies assemblage and thus the paleoproterozoic ages of the metamorphic overgrowths are interpreted to reflect an interval of isobaric granulite grade metamorphism of the lower crust beneath the greenstone belt approximately 150 million years after craton formation this interval of metamorphism is broadly coeval with the intrusion of the matachewan dyke swarm across the southern superior craton and with mafic magmatism and deposition of huronian rift margin sediments 200 km to the south during the opening of the matachewan ocean it is proposed that a significant volume of magma intruded the crust mantle interface during rifting promoting isobaric metamorphism and zircon growth in the deep levels of the superior craton subsequent major rifting events along this margin apparently failed to produce a similar lower crustal response the results have important implications for the structure of lithosphere beneath archean continental crust

lithoprobe s impact on the canadian diamond exploration industrythis article is one of a series of papers published in this special issue on the theme lithoprobe parameters processes and the evolution of a continent earth sciences sector contribution 20070261 lithoprobe contribution 1479 lithoprobe aided and will continue to guide diamond exploration in canada via a number of diverse ways and at scales from cratons to individual ore bodies specific transects outlined the domainal nature geometry and age relations of crust subjacent mantle and diamond bearing roots in unusually rich detail for cratonic settings such as those of the superior and slave cratons as well as pericratonic shield settings such as those of the sask craton and buffalo hills terrane the superior and slave examples of craton construction by multiple underthrust layers as inferred from structural geometries observed in lithoprobe seismic and magnetotelluric surveys remain the clearest resolved to date the first definition of the sask craton occurred 10 15 years ago and helped fuel the diamond industry s ongoing revision of clifford s rule for exploration so as to include targets in proterozoic shield settings main lithoprobe transect results and supporting geoscience studies have helped to demonstrate the

chronology with a pinch of salt integrated stratigraphy of messinian evaporites in the deep eastern mediterranean reveals long lasting halite deposition during atlantic connectivity abstract the messinian salinity crisis msc 5 97 5 33 ma is considered an extreme environmental event driven by changes in climate and tectonics which affected global ocean salinity and shaped the biogeochemical composition of the mediterranean sea yet after more than 50 years of research msc stratigraphy remains controversial recent studies agree that the transition from the underlying pre evaporite sediments to thick halite deposits is conformal in the deep eastern mediterranean basin however the age of the base and the duration of halite deposition are still unclear also disputed is the nature of the intermediate and upper msc units which are characterized as periods of increased clastic deposition into the eastern mediterranean based on marginal outcrops and seismic data we provide a multidisciplinary study of sedimentary geochemical and geophysical data from industrial offshore wells in the levant basin which recovered a sedimentary record of deep basin mediterranean evaporites deposited during the msc in combination with previous observations of the msc throughout the mediterranean basin our results promote the need for a new chronological model remarkably the one kilometer thick lower part of the evaporitic unit is composed of essentially pure halite except for a thin transitional anhydrite layer at its base the halite is undisturbed and homogeneous lacking diverse features apparent in more proximal sections indicating a deep sea depositional environment we find that distinct meters thick non evaporitic intervals interbedded with the halite previously thought to be clastic layers are diatomites while xrd analysis confirms an increase in clastic components in these sediments they are composed primarily of well preserved marine and freshwater planktonic diatoms the occurrence of marine planktonic diatoms in these intervals indicates the input of atlantic waters into the mediterranean basin during the deposition of the massive halite unit seismic stratigraphy and well log cyclostratigraphy further support deep basin halite deposition which started about 300 kyr earlier than widely assumed 5 97 ma we propose that halite deposition in the deep mediterranean took place during stage 1 of the msc rather than being limited to the short 50 kyr msc acme when sea level was presumably at its lowest thus brine formation salt precipitation and faunal extinction occurred at least in part in a deep non desiccated basin with a restricted yet open mediterranean atlantic connection that allowed inflow of oceanic water we observe an increase in heavy minerals and reworked fauna within the clastic evaporitic interbedded evaporites of the basinal msc section and argue that these settings correspond in the deep basins with a significant sea level drawdown during stage 2 of the msc as observed in the marginal sections this correlation is corroborated by astrochronology and chemostratigraphic markers such as the distribution of n alkanes and biomarker based thermal maturity indices the levant deposits indicate that high sea level and partial connectivity with global oceans promoted the deposition of deep basin deep water halite while sea level drawdown promoted deposition of reworked and transported material from the margins into deep mediterranean basins this study modifies the current understanding of the mechanisms governing salt deposition throughout the msc with implications for other evaporitic events in the geologic record

petrogenesis of the early proterozoic matachewan dyke swarm canada and implications for magma emplacement and subsequent deformation major minor and rare earth element analyses of fine grained marginal material from 249 dykes of the matachewan giant radiating swarm show that the range in compositions cannot be produced by any simple fractionation model instead a two stage process is required whereby melts were modified by both fractionation and assimilation of lower crustal material before rising and ponding at shallower crustal levels 15 20 km maximum where a combination of fractional crystallization plagioclase phenocryst formation and periodic melt replenishment took place these shallow crustal chambers acted as feeders for the numerous dykes one hypothesis compatible with these observations involves an extensive rising plume that would provide heterogeneous melts over a large area accompanied by stresses that could produce a radial pattern of dykes in a search for indications of lateral magma flow a comparison of longitudinal variations in composition along four sample traverses across the western subswarm showed no t

geochemical indicators in western mediterranean messinian evaporites implications for the salinity crisis abstract the messinian salinity crisis msc led to deposition of one of the youngest saline giant on earth the increasing restriction of the connections between the mediterranean the atlantic ocean and the freshwater paratethyan basins resulted in the deposition of massive amounts of evaporites gypsum anhydrite halite and potash salts in shallow marginal basins as well as in deep mediterranean basins here we show that each gypsum unit in the circum mediterranean marginal basins in sicily and spain is characterized by a narrow range of sulfate isotopic values 34s 23 and 18o 14 in the lower gypsum 34s 23 and 18o 17 in the upper gypsum sulfate isotope compositions found in msc evaporites from a variety of circum mediterranean basins are homogenously high relative to expected late miocene marine evaporites 34s 22 and 18o 12 this points to a stratified mediterranean sea with a high salinity dense and anoxic bottom water mass an intermediate depth gypsum saturated brine flooded marginal basins from which selenite deposits formed during the msc stage 1 primary lower gypsum and msc stage 3 upper gypsum messinian brines were gradually affected by biogenic redox processes and isotopically differentiated from global seawater values the homogeneity of isotopic signatures between distant synchronous gypsum deposits further supports the deep basin deep water model for the mediterranean during the entire msc event

1 45 ga granulites in the southwestern grenville province geologic setting p t conditions and u pb geochronology in the southwestern grenville province the parautochthonous britt domain includes a variety of pre grenvillian metamorphic and plutonic rocks that were reworked at upper amphibolite facies during the grenvillian orogeny near pointe au baril ontario a crustal block containing pre grenvillian granulite facies mineral assemblages and pre grenvillian to early grenvillian tectonic fabrics has been identified the block is bounded on the northwest and southeast by extensional shear zones that may have isolated it from regional late gren villian deformation multiequilibria pressure temperature p t calculations for orthopyroxene bearing mafic rocks suggest conditions of 625 700 c and 7 2 8 4 kbar for the pre grenvillian metamorphism the granulite facies assemblages were locally overprinted during higher pressure grenvillian metamorphism which peaked at 720 775 c and 10 8 11 5 kbar u pb zircon data from migmatitic mafic supracrustal gneiss indicate metamorphism and leucosome development at ca 1450 1430 ma in agreement with other pre grenvillian metamorphic ages for the central gneiss belt and grenville front tectonic zone an expanding data base on pre grenvillian events in the southwestern grenville province indicates that high grade metamorphism at ca 1450 1430 ma affected a large region of crust and was coeval with widespread felsic to intermediate plutonism

giant polygons in the realmonte mine agrigento sicily evidence for the desiccation of a messinian halite basin abstract the messinian realmonte salt deposit consists of a 400 600 m thick halite succession that can be divided into four main lith ologic units base to top a cumulates of plates settled out from a stratified water column b plate cumulates in a shallowing upward sequence containing six kainite layers near the top of the unit c cu mulates of skeletal hoppers with chevron overgrowths precipitated from a nonstratified water body and d skeletal halite and anhydrite spectacular vertical fissures cut through the upper part of unit b at its boundary with salt unit c these fissures are spaced at intervals of up to 5 m apart extend down to a depth of 6 m and are filled by red mud the salt beds affected by the fissures are commonly upturned buckled and are truncated and onlapped by the succeeding flat lying halite beds of unit c additionally the upturned layers are cut by vertical dissolution pipes that extend down to as much as a depth of 4 m below the fissured surface to explain the observed superimposed structures we propose two mechanisms both induced by the desiccation of the evaporite basin 1 when the salt surface was exposed the halite layers were buck led broke into polygonal crusts and formed tepee structures the buckling was caused by a net volume increase due to thermal expan sion and what is more important precipitation of new halite induced by evaporative pumping of brines moving up from the groundwater table the buckled salt layers were then affected by meteoric dissolu tion which created vertical pipes 2 the tepee structures and the associated zone of dissolution pipes are further crosscut by a new network of large polygons these new polygonal sets are outlined by deep fissures in which red silt collected blown by the wind or carried by episodes of surface flooding this type of deep contraction crack is known to be produced by volumetric changes induced by annual temperature fluctuations the documentation of contraction polygons and other exposure fea tures represents the first report of desiccation events during the de position of the sicilian salt no geological data are yet available to settle the questions if the salt of sicily is correlatable with the halite present beneath the floor of the mediterranean and if the desiccation of the realmonte salt basin could have been induced by a local tectonic causes uplift of the basin floor by thrust activity b simple evaporitic drawdown c basinwide drop of the mediterranean sea level or d complex interaction among these factors

u pb zircon and monazite ages from the kapuskasing uplift age constraints on deformation within the ivanhoe lake fault zone a decade of u pb dating of zircon and monazite from high grade metamorphic rocks in the kapuskasing uplift has identified a series of magmatic and metamorphic events between 2700 and 2585 ma and indicates that the onset of regional granulite metamorphism took place at mid crustal levels of the southern superior craton ca 2660 ma new u pb ages for zircon and monazite have been used to constrain the age of ductile deformation fabrics at two sites in the ivanhoe lake fault zone the structure along which the granulite facies kapuskasing structural zone was uplifted these results suggest that the fault zone was probably active in the late archean as young as 2630 ma and again at approximately 2500 ma

crustal structure of the kapuskasing uplift from lithoprobe near vertical wide angle seismic reflection data reprocessing of one lithoprobe kapuskasing deep seismic reflection profile discloses significant new information on the structure of the kapuskasing uplift ku the shallow structure of the ivanhoe lake fault zone ilfz along which the high grade granulites of the ku were thrust to the surface is conspicuously imaged on a seismic section as a series of prominent northwest dipping reflections with listric geometry the new images show that the ilfz is a steep fault 50 near the surface which quickly flattens out at shallow depths these reflections coincide with bright positive aeromagnetic anomalies over the fault zone direct correlation with geological observations indicates that the high reflectivity and high magnetism associated with the fault zone likely originate from mylonites the reprocessing also reveals a pronounced midcrustal reflector within the abitibi greenstone belt agb dipping northwest and plunging under the ku the existence of such a reflector is independently confirmed by wide angle reflection data acquired from a cross profile this reflector is apparently also detected on two other reflection profiles crossing the ilfz about 80 km to the southwest its concave down shape and broad lateral extent suggest that it represents underthrusting of the agb beneath the ku with these new results a more complete structural cross section can be constructed it shows that on the northwest side the ku was overthrust along the northwest dipping ilfz and on the southeast side the agb was underthrust northwesterly it seems evident that the ku is a product of intraplate collision during the early proterozoic and the underthrusting of the agb is primarily responsible for the emplacement of the ku

u pb zircon geochronology of the kapuskasing structural zone and vicinity in the chapleau foleyet area ontario geological development of the archean abitibi and wawa greenstone granite subprovinces of the superior province occurred in three main stages 1 a volcanic sedimentary depositional stage 2750 2696 ma ago 2 a metamorphic tectonic event 2700 2685 ma ago and 3 a late plutonic event at 2685 2668 ma the area between wawa in the west and ivanhoe lake in the east represents an oblique section through some 20 km of the crust where geochronological relationships can be examined at different structural levels much of the eastern wawa subprovince is characterized by tonalite granodiorite gneiss containing abundant mafic xenoliths and represents a crustal level intermediate between high level metavolcanic rocks near lake superior and the deep seated gneisses of the kapuskasing structural zone a tonalitic phase of the complex gneiss of the wawa subprovince has a minimum u pb zircon date of 2707 ma but the fine zircons are partly reset possibly in response to lit par lit injections of gneissic to foliat

large and rapid climate variability during the messinian salinity crisis evidence from deuterium concentrations of individual biomarkers during the messinian 6 m y ago massive sea level fall and widespread deposition of evaporites occurred in the mediterranean sea when it became isolated from the world oceans here we present the first hydrogen isotope data from individual sedimentary biomarkers n alkanes and isoprenoids that tracked climatically driven hydrographic changes in response to extreme evaporation during the messinian salinity crisis the stable hydrogen and carbon isotope compositions of these biomarkers show a range of 160 in d values and 14 in 13 c values and roughly covary this indicates that the source waters of the biomarkers were therefore in some cases extremely enriched in deuterium having average d as great as 66 vsmow vienna standard mean ocean water such values are only known from desert climates today because the offset between the d values of n alkanes and isoprenoids preserved in the miocene sedimentary rocks is similar to the offset found in modern biological samples we conclude that diagenesis did not significantly affect the primary deuterium concentrations

paleoclimate reconstruction during the messinian evaporative drawdown of the mediterranean basin insights from microthermometry on halite fluid inclusions 1 data constraining the paleoenvironmental conditions during the messinian evaporative drawdown of the mediterranean basin are still conflicting here we present a comprehensive paleoclimatic reconstruction during messinian halite deposition from several italian sites we performed fluid inclusion analyses to define better both the composition of the mediterranean water body and the paleotemperatures of the messinian brine during halite crystallization we measured homogenization temperatures on 218 primary all liquid fluid inclusions in messinian halite from the volterra crotone and caltanissetta evaporite basins these measurements provided mean homogenization temperatures of 17 18 c with a range between 10 11 c and 28 29 c which should be close to the sst of the mediterranean sea during halite deposition the occurrence of major elements such as cl na mg s ca and k within the halite fluid inclusions together with the presence of minerals such as pentahydrite polyhalite and ca k and mg sulfates indicates that these salt bodies originated from mainly marine water taking into account both the present day annual sst of the mediterranean sea around the sampling sites which ranges between 18 and 20 c and the lower latitude of the mediterranean basin during the messinian salinity crisis our homogenization temperatures point to a colder climate during the messinian halite deposition compared to the present interglacial climate stage this conclusion is consistent with halite deposition during a late miocene glacial interval tg12

the deep record of the messinian salinity crisis evidence of a non desiccated mediterranean sea abstract this research is focused on a complete reexamination of the evaporite facies present in all the cores that cut through the topmost deposits of the messinian salinity crisis lying below the floor of the mediterranean sea dsdp legs 13 and 42a odp legs 107 and 161 this review suggests that the uppermost evaporite units in both western and eastern deep mediterranean basins consist mainly of clastic gypsrudite gypsarenite and gypsiltite and fully subaqueous deposits laminar gypsum selenite and cumulate halite that are partially affected by burial anhydritization and tectonic induced recrystallization no unequivocal evidence of shallow water or even supratidal sabkha deposition is in evidence suggesting that at the very last phase of the salinity crisis the mediterranean sea did not experience desiccation but that deposition took place under permanent subaqueous conditions

stratigraphic variations control deformation patterns in evaporite basins messinian examples onshore and offshore sicily italy three dimensional seismic data are increasingly resolving original compositional heterogeneity and structural complexity in evaporitic successions within sedimentary basins the relationship between basin structure evaporite composition and its influence on subsequent deformation are investigated here using messinian examples from the maghrebian thrust system of sicily and applied to the adjacent ionian sea bed by integrating outcrop and onshore subsurface data we demonstrate variations in evaporite stratigraphies deposited across thrust top basins and how these variations have controlled subsequent deformation of these basins gypsum and carbonate units develop broad single layer buckle fold trains with wavelengths reflecting layer thickness the development of deformation appears limited by bending resistance at fold hinges which can be overcome by syntectonic erosion in contrast the thick halite and k salt accumulations in growth synclines have deformed with short wavelength folds and distributed strain these structures can display rapid lateral variations on a scale of hundreds of metres similar structural styles with buckle fold trains passing laterally into more homogeneously shortened short wavelength folding are evident on seismic data from the buried messinian interval beneath the ionian sea using the sicilian outcrop as analogues the structural styles for the ionian may be used to infer evaporite type in these subsurface examples

estimation of the physical petrophysical and mineralogical properties of messinian salt rocks sicily implications for multidisciplinary applications abstract the messinian salinity crisis msc was a short term 5 96 5 33ma and dramatic paleoenvironmental event which led to the deposition of a thick evaporitic layer throughout mediterranean area however several studies have demonstrated that messinian evaporites have different characteristics and properties from east to west in the mediterranean area and the caltanissetta basin sicily west mediterranean provides an excellent messinian onshore record for investigating evaporitic succession given the scarcity of representative halite samples and at the same time widespread research and scientific drilling in the mediterranean area new physical mechanical petrophysical mineralogical and thermal characterization of messinian halite could provide useful information for several disciplines and applications hence 50 samples of messinian halite were obtained from a sicilian mine italkali analyzed and tested using different instruments ultrasound velocities measured under hydrostatic confining pressure up to 120 mpa ranged from 4210 to 4780 m s and 2510 2650 m s respectively for p and s waves the results further showed that the halite samples had low porosity 2 5 8 5 and negligible permeability demonstrating the weak presence of pores and the total lack of interconnection among them which is typical of rock salt units densities of the halite determined with the hydrostatic weighing method and gas porosimeter were low and ranged from 2 14 to 2 23 g cm3 while high values were obtained for thermal conductivity 17 0 and 32 0 c it is significant that the mineralogical heterogeneity of the halite was due to the presence of clay bearing veins although this work was performed mainly using onshore data and observations the results have wider applications for offshore studies of the msc and its deposits furthermore the study provides important insights for future mediterranean drilling projects geological modelling hydrocarbon exploration and or fluids storage and the understanding of salt rheology under different conditions of pressure and temperature

modeling deformation and salt tectonics in the eastern mediterranean ridge accretionary wedge this study addresses some aspects of the kinematics and structural setting of the mediterranean ridge accretionary complex particularly those aspects related to the occurrence and distribution of messinian evaporites the approach is by 1 identification of the geometry of deformation through reprocessing of existing multichannel seismic reflection data from the levantine basin aimed at the definition of the velocity field and depth imaging 2 physical modeling in the laboratory aimed at the study of the kinematics of shortening of sedimentary sequences detaching on viscous decollements and 3 comparison between the results of the two approaches we conclude that the thick messinian sequence of the levantine basin is composed primarily of salt so that the entire evaporitic sequence can be expected to behave as a viscous material it follows that the main detachment of the post messinian wedge is located at the base of the messinian salt layer a viscous salt decollement would produce thrusting trending normal to the shortening direction but boundary conditions affect structural trends even more than stress and or movement direction both strain partitioning and the formation of major strike slip faults within the post messinian wedge are prevented by the high angle of convergence between the african and aegean plates as well as by the low intraplate friction in addition we show that curved and anastomosing thrust fronts are reflected in the cobblestone topography of the mediterranean ridge we finally postulate that extension may occur in the central mediterranean ridge as a result of the geometry of the plate boundaries this extension is considered as a possible cause of mud volcanism and mud diapirism

models of convergent boundary tectonics implications for the interpretation of lithoprobe data physical models of convergent boundary processes can provide insights into compressional orogens such as those studied by lithoprobe we summarize the qualitative tectonic style of a series of mode

deposition and early alteration of evaporites summary the depositional settings for primary and early diagenetic evaporite deposits generally fall into three categories marginal mixed shallow subaqueous and subaerial shallow and deep subaqueous these three environmental groupings hold for both marine and nonmarine settings although the details of continental evaporites may be far more complex than in most marine fed water bodies the primary evaporite morphologies from many continental playa hypersaline marine and marine marginal depositional settings are reasonably well understood because of the numerous detailed studies of recent holocene and cenozoic deposits that serve as models for sedimentary interpretation the sedimentological features that develop in deeper water settings are inferred based on examination of unaltered cenozoic deposits each environmental setting develops characteristic depositional features and patterns and one facies grades into the next because there may be significant physiochemical changes in water composition during deposition as well as sudden change s in both water depth and basinal circulation description and interpretation of evaporative rocks should not be based on mineralogy alone but on the distinct sedimentary characteristics of each part of a deposit in cases where sediments still reflect their primary mineralogy and morphology most of the environments can now be recognized however geochemical studies are commonly required to determine the source s of the original water the determinative geochemical techniques that presently serve as a support to sedimentologic study include studies of fluid inclusions bromine content of chloride salts and the stable isotopes of strontium sulphur carbon and oxygen only after these and perhaps other chemical analyses are considered can the full depositional history of a region be reasonably unravelled

age and evolution of the lower crust beneath the western churchill province u pb zircon geochronology of kimberlite hosted granulite xenoliths nunavut canada abstract zircon from granulite facies xenoliths can preserve unique geodynamic records of deep and mostly inaccessible levels of the continental crust within archean cratons we present the first such records from beneath two archean crustal blocks that were affected by multiple episodes of regional scale proterozoic metamorphism and are located within the western churchill province nunavut canada zircon u pb geochronology and trace element analysis were carried out on samples that originated in the deep crust at the time of kimberlite eruption as confirmed with raman spectroscopy zircon was recovered from four metabasites grt cpx plg from the rankin inlet area of the chesterfield block and five metabasites grt cpx plg hbl opx and three metatonalite anorthosites plg cpx plg hbl opx from the repulse bay area of the rae craton sem cl imaging reveals several types of growth zoning diagnostic of igneous and metamorphic zircon crystallization the latter are related to local high temperature recrystallization and micro fracturing and are spatially associated with zones of apparent pb loss and or intracrystalline pb variability evidence for the formation of paleoarchean mesoarchean to neoarchean crust is preserved in oscillatory zoned igneous cores having 207pb 206pb ages of ca 3 5 ga and ca 3 0 2 6 ga two periods of paleoproterozoic metamorphism and zircon growth followed with the oldest episode at ca 2 0 1 9 ga in the chesterfield block metamorphic zircon rims from all twelve xenoliths have an age range of ca 1 75 to 1 70 ga the trace element compositions of the metamorphic rims differ from those of older igneous and metamorphic cores with the rims characterized by lower hree contents and low lu gd n suggesting that garnet crystallization occurred syn to pre rim growth at ca 1 75 ga the archean and paleoproterozoic ages correspond broadly to the upper crustal histories of the western churchill province however the prevalence of ca 1 75 1 70 ga metamorphic rims indicates regional scale metamorphic reworking of the lower crust across the region during a period of intracratonic extension and associated magmatism regional magmatic underplating and or remobilization of the lithosphere at ca 1 75 1 70 ga to an extent not widely appreciated represents a terminal stage in western churchill cratonization including stabilization of reworked archean crust relative to underlying diamondiferous mantle