u pb dating of monazite and its application to geological problems monazite is an underutilized mineral in u pb geochronological studies of crustal rocks it occurs as an accessory mineral in a wide variety of rocks including granite pegmatite felsic volcanic a

distribution of high field strength elements y zr ree hf ta th u in adjacent magnetite and chert bands and in reference standards fer 3 and fer 4 from the temagami iron formation canada and the redox level of the neoarchean ocean abstract adjacent magnetite and chert bands and reference standards fer 3 and fer 4 from the neoarchean temagami iron formation if show shale normalized rare earth and yttrium rey patterns with low nd yb ratios and positive eu gd and y anomalies indicating that they formed as marine chemical sediments in contrast to previous claims none of the samples shows any ce anomaly indicating the absence of oxidative ce rey decoupling and arguing against oxic conditions in the wider vicinity of the neoarchean temagami seabasin the distribution of zr hf and ta yields zr hf and hf ta ratios that differ from those of chondrites average upper continental crust and local shales suggesting that the temagami if is the only case observed so far in which a significant fraction of these elements is non detrital but sourced from seawater if neoarchean seawater was characterized by zr hf and hf ta ratios similar to those of modern seawater these ratios point towards preferential scavenging of hf over zr and ta as is typical of the modern ocean similar to the 2 9 ga old mozaan if in the pongola supergroup south africa the temagami if shows low th u ratios that differ from those of the respective local shales and from that of average upper continental crust decoupling of u and th results from u4 oxidation in the earth s surface system and fractionated th u ratios in these marine chemical sediments are therefore at odds with the lack of ce anomalies this suggests a different redox sensitivity of the two paleo redox proxies th u and ce rey and demonstrates that the temagami if and the mozaan if warrant further study of other paleo redox proxies

an electron microprobe study of the u th pb systematics of metamorphosed monazite the role of pb diffusion versus overgrowth and recrystallization abstract an electron microprobe emp was used to obtain u th pb chemical ages from monazite that underwent an amphibolite facies metamorphic overprint in the monashee complex southern canadian cordillera u pb isotope dilution dating of single grains yielded discordant ages that plot in linear arrays on a concordia diagram upper intercepts of the arrays indicate primary growth in the proterozoic and cretaceous and lower intercepts are coeval with a tertiary metamorphic event the microprobe s high spatial resolution 2 m and ability to quickly differentiate between domains with greatly contrasting u th and pb concentrations made it the ideal tool for investigating the cause of the u pb isotopic discordance the microprobe dating shows that proterozoic monazite lies directly adjacent to generally small

a laser 40ar 39ar study of minerals across the grenville front investigation of reproducible excess ar patterns 40ar 39ar ages have been determined on single grains of biotite and amphibole from rocks on a 40 km traverse across the grenville front gf in the temagami area ontario minerals in the vicinity of the gf contain excess ar in varying amounts characterized by both anomalously high integrated and plateau ages the excess ar in biotite varies with distance from the gf such that the overall age distribution along the gf forms an asymmetric wave like pattern an argonami with two age maxima one on either side of the gf the integrated age maxima are 2 34 ga for biotite and 3 21 ga for amphibole located 6 and 0 2 km north of the gf respectively whereas maxima are 1 33 ga for biotite and 1 50 ga for amphibole 7 km south of the gf this wave like pattern of apparent ages shows a remarkable similarity with that seen in an earlier k ar study across the gf at chibougamau quebec and also with a small scale pattern of excess ar associated with the emplacement of a dyke variations in the magnitude of excess a

estimation of entrainment rate in simple models of convective clouds a method for estimating lateral entrainment rate in clouds is suggested linking its magnitude to buoyant production of vertical kinetic energy within a cloud updraught in single column model studies the formulation captures the high values of entrainment seen in shallow convection and the values an order of magnitude lower in cases of deep convection as suggested by observational studies and large eddy model simulations together with a realistic vertical variation comparison is made with other recent attempts to parametrize entrainment

review of archean supracrustal assemblages of the southern abitibi greenstone belt in ontario canada products of microplate interaction within a large scale plate tectonic setting abstract previously formulated plate convergent models account for many aspects of the large scale regularity and parallelism between different subprovinces of the archean superior province such models do not however appear to adequately explain the within subprovince evolution of 2 75 2 68 ga supracrustal assemblages of the late archean southern abitibi greenstone belt principal supracrustal assemblage classes include l assemblages dominated by tholeiitic metabasalt and metakomatiite with varying amounts of tholeiitic intermediate to felsic metavolcanic rocks these assemblages probably formed in extensional settings the ratio of basalt to komatiite and the ratio of basalt komatiite to intermediate felsic metavolcanic rocks may be a function of mantle temperature and crustal thickness at the site of extension respectively 2 assemblages dominated by intermediate to felsic fragmental and or effusive rocks are generally of calcalkalic affinity and likely formed in island arc settings 3 assemblages containing a substantial component of iron formation may represent older supracrustal assemblages that form structural and or stratigraphic basement to some of the younger supracrustal assemblages 4 assemblages consisting almost wholly of turbidite facies metasedimentary rocks appear to postdate volcanic activity related to the previous assemblages deposition of these sediments broadly coincides with the onset of regional deformation these assemblages may represent distal deposits related to an emerging collisional orogen 5 assemblages consisting of alluvial fluvial metasedimentary rocks with or without spatially associated alkalic metavolcanic rocks are associated with crustal scale shear zones and are the youngest assemblages within the greenstone belt alluvial fluvial assemblages likely represent the final emergence of the consolidating orogen geochronological data and assemblage distribution suggest that the supracrustal assemblages of the southern abitibi greenstone belt may represent the amalgamation of products of diverse and intricate microplate tectonic settings repeated in time and space prior to the impingement of the large scale plate tectonic regime that dictated the final large scale configuration of the archean superior province

neoarchean trondhjemitic and tonalitic orthogneisses identified within the northern grenville province in ontario by precise upb dating and petrologic studies abstract integrated field petrologic and geochronological studies reveal the presence of heretofore unrecognized neoarchean trondhjemitic and tonalitic orthogneisses north and northwest of lake nipissing in the northern grenville province of ontario the high al type trondhjemitic orthogneiss termed warren trondhjemite forms several intrusions with a total exposed area of 300 km 2 the tonalitic orthogneiss including the tilden tonalite forms a discontinuous zone of intrusions about 20 km long and up to 1 6 km wide both trondhjemitic and tonalitic orthogneisses are geochemically distinct from paragneiss that forms the country rocks precise u pb zircon and titanite dating of the warren trondhjemite at six locations and of the tilden tonalite at one location indicates that the former was emplaced at 2679 2 ma whereas the latter was intruded at 2669 4 ma a 996 to 975 ma metamorphism shown by the lower intercept ages of concordia plots of all samples caused partial pb loss from primary neoarchean zircons and titanites and also induced new growth of zircons in a local pegmatitic patch and of titanite in orthogneisses intrusive relationships between the orthogneiss and the country rock paragneiss indicate that the paragneiss is also archean in age the orthogneiss and the paragneiss together constitute an archean crustal sequence southeast of the grenville front boundary fault within the ontario segment of the grenville province this archean sequence resembles the pontiac subprovince of the superior province in western quebec in several important ways including the dominance of compositionally similar metasediments comparable depositional ages of the metasediments and contemporaneous granitoid plutonism at about 2680 to 2670 ma differences also exist between the two for example the major granitoids in the pontiac subprovince are generally more k rich

tungsten in archean banded iron formation of the former sherman and adams mines ontario possible mineralization processes anomalous tungsten occurs in archean banded iron formation from the previous sherman and adams mines ontario in sherman samples from three open pits and one drill core site with one exception anomalous tungsten occurred only in the sherman north pit adams mine samples from three pits carried anomalous tungsten in samples of chlorite rich silicate facies statistical evaluation showed that 10 ppm tungsten was anomalous with respect to the entire data set the implications of restricted tungsten distribution as seen in sherman occurrences were evaluated with respect to tungsten mineralization processes scenarios considered involve syngenetic metamorphic and magmatic hydrothermal processes thermal constraints provided by iron bearing silicate assemblages and oxygen isotopes attest to contact metamorphism as a potential mechanism for tungsten redistribution within iron formation however tungsten of magmatic hydrothermal origin emplaced as an epigenetic component is considered the essential miner

sulfur and carbon isotopes in archean banded iron formations implications for sulfur sources abstract sulfur isotope ratios of pyrite from two late archean banded iron formations were used to constrain possible sources of sulfur the iron formations studied were the temagami iron formation which hosts the sherman mine near temagami and the boston iron formation including the adams mine south of kirkland lake both in ontario in both cases algomantype oxide facies magnetite chert ores predominate the sulfur studies were supplemented by limited carbon isotopic data for the sherman mine iron formation in the sherman mine occurs on both limbs of a regional synformal structure each iron formation unit caps a thick calc alkaline andesite dominated volcanic sequence with one band hosted in sedimentary rocks of turbidite origin the adams mine includes four major iron formation bands interbedded with mafic and ultramafic volcanic rocks and greywacke of turbidite origin the sulfur isotope statistics for averages of pyrites from these three units are sherman mine volcanic hosted suite 34 s 1 7 4 6 n 14 sherman mine sediment hosted suite 34 s 1 4 1 6 n 43 and adams mine 34 s 1 8 2 9 n 17 these averages differ by from juvenile sulfur and together with their limited dispersions suggest a predominantly unfractionated source of sulfur it is argued that this sulfur has been derived from the associated volcanic piles most of the pyrite occurs as disseminated grains in oxide facies iron formation and interbedded clastic sediments a distinctive nodular pyrite in the sherman mine sediment hosted suite occurs in cherty graphitic sulfide facies iron formation while the sulfur isotopic properties of this subset 34 s 0 0 0 8 are not suggestive of biogenic sulfur the geological setting and carbon isotopic compositions of associated graphite 13 c 34 imply that anaerobic bacterial processes were active in this reducing environment the apparent lack of biogenic sulfide is rationalized by models suggesting that archean marine basins differed from phanerozoic analogues in various parameters which influence sulfur isotopic characteristics it is argued that biogenic sulfide is probably a significant component of the sulfur in sherman mine nodular pyrite

age and timing of igneous activity in the temagami greenstone belt ontario a preliminary report the southernmost remnants of archean supracrustal and intrusive rocks in eastern ontario are exposed through a window in the early proterozoic huronian supergroup near the town of temagami u pb zircon ages from this area indicate the presence of some of the oldest felsic magmatism so far discovered in this portion of the superior province the iceland lake pluton 2736 2 ma and a nearby rhyolite flow are contemporaneous which establishes that at least some of the intrusive rocks in the region are synvolcanic and coeval with the oldest volcanic cycle the youngest plutonic activity is the emplacement of a late rhyolite porphyry dike at 2687 2 ma an age that is bracketed by the 2675 2700 ma emplacement ages of late internal plutons found throughout the abitibi subprovince the 2736 ma dates however are older than the nearest portion of the exposed abitibi some 120 km to the north near kirkland lake

effects of vertical wind shear on the cumulus transport of momentum observations and parameterization abstract dynamical effects of organized cumulus convection on its environment with vertical wind shear are studied analyses of the wind field and momentum budget residual for mesoscale convective systems observed during sesame and pre storm reveal systematic differences in the vertical transport of horizontal momentum between mesoscale convective complex mcc and squall line cases in the mcc a distinct minimum in wind speed appears over the area of intense convection and the momentum budget residual acts to decelerate the environmental wind and to reduce the upper level vertical shear therefore the inferred vertical transport of momentum in the mcc is downgradient in the upper layer on the other hand in the squall line there is no wind speed minimum and the upper level vertical shear of the line normall component of the environmental wind increases as convection develops thus the vertical transport of momentum normal to the squall line is upgradient in the upper layer although the transport of

the tectonic evolution of the abitibi greenstone belt of canada based on structural geochemical sedimentological and geochronological studies we have formulated a model for the evolution of the late archaean abitibi greenstone belt of the superior province of canada the southern volcanic zone svz of the belt is dominated by komatiitic to tholeiitic volcanic plateaux and large bimodal mafic felsic volcanic centres these volcanic rocks were erupted between approximately 2710 ma and 2700 ma in a series of rift basins formed as a result of wrench fault tectonics the svz superimposes an older volcanic terrane which is characterized in the northern volcanic zone nvz of the abitibi belt and is approximately 2720 ma or older the nvz comprises basaltic to andesitic and dacitic subaqueous massive volcanics which are cored by comagmatic sill complexes and layered mafic anorthositic plutonic complexes these volcanics are overlain by felsic pyroclastic rocks that were comagmatic with the emplacement of tonalitic plutons at 2717 2 ma the tectonic model envisages the svz to have formed in a series of rift basins which dissected an earlier formed volcanic arc the nvz analogous rift environments have been postulated for the hokuroko basin of japan the taupo volcanic zone of new zealand and the sumatra and nicaragua arcs the difference between rift related submergent volcanism in the svz and emergent volcanism in the nvz resulted in the contrasting metallogenic styles the former being characterized by syngenetic massive sulphide deposits whilst the latter was dominated by epigenetic porphyry type cu au deposits

an improved u th pb age calculation for electron microprobe dating of monazite abstract determination of u th pb ages using the electron probe microanalyser epma is an inexpensive alternative method for dating monazite the method is rapid and reliable both for simple monogenenetic monazite and for complex polygenetic monazite having undergone metamorphic events involving fluid interaction and recrystallization the main limitation of the method is its rather poor precision i e 45 to 120 ma for ages ranging from 300 to 3000 ma calculated on each spot the precision is limited by pb content and by counting statistics which cannot give precision better than 2 for individual determinations on u th and pb even at high levels a procedure that uses the new th pb vs u pb diagram to improve the calculated precision on u th pb ages gives results to within 5 to 15 ma for ages ranging from 300 to 3000 ma with complex polygenetic monazite in which either the points show large scattering indicated by a large mswd or the regression line exhibits a slope very different from neighbour theoretical isochrons the procedure must be applied separately on homogeneous domains only this makes it possible to distinguish between events separated by a gap of 20 to 60 ma according to the range of ages concerned i e 300 to 3000 ma several examples are given to illustrate these systematics

electron microprobe dating of monazites from high grade gneisses and pegmatites of the kerala khondalite belt southern india monazites of five samples one leptynitic garnet biotite gneiss one khondalite one augen gneiss and two pegmatites from the central and northern part ponmudi unit of the kerala khondalite belt kkb in southern india were analyzed with the electron microprobe dating technique monazites in the augen gneiss and the pegmatites yield grain sizes between 200 800 m th abundances are rather low 10 wt and the distribution of th u and pb within single grains is fairly homogeneous contrasting to this monazites in the leptynitic gneiss and the khondalite are small 150 m they often display very complex th u pb patterns and contain high th concentrations up to 20 wt the statistical treatment of individual ages from the investigated samples revealed three populations of lower proterozoic 1 9 ga upper proterozoic 580 ma and ordovician age 470 ma as well as mid proterozoic ages between 0 8 1 7 ga which are not regarded to be of geological significance lower proterozoic ages are preserved in the cores of monazites from leptynitic gneisses and khondalites they fairly agree with sm nd model ages for similar rocks of the kkb and give a minimum age for first monazite growth or complete homogenization the prominent pan african population with mean values between 540 and 580 ma is present in the leptynitic gneiss the khondalite and the augen gneiss and in line with published isotope ages for the kkb the ordovician population finally marks the emplacement of granitic pegmatites subsequent to the pan african high grade metamorphic event there is an obvious discrepancy between khondalites and leptynitic gneisses on the one hand and augen gneisses on the other concerning the presence of lower proterozoic ages while these are abundant in the former often rimmed by upper proterozoic ages they are completely absent in the latter it appears unlikely that lower proterozoic ages were completely reset during a pan african event exclusively in the augen gneisses while they were preserved in leptynitic gneisses and khondalites it is further concluded that the augen gneisses are of magmatic origin and were derived from porphyritic granites thus the upper proterozoic age of 605 37 ma calculated for the investigated sample approximates the time of magma emplacement which slightly precedes the peak stage of pan african high grade metamorphism in the kkb and of monazite crystallization from the granitic melt a characteristic feature of the investigated monazites is the resetting of lower proterozoic and pan african ages to significantly younger values due to partial pb loss monazites not shielded by other minerals e g garnet suffered selective mobilization of pb along fractures or at their rims while th and u concentrations remained almost unchanged the results presented in this study indicate that this was mainly due to fluid rock interaction it is concluded that thermal diffusion of pb even at the suggested temperatures of 900 c only had minor influence on the th u pb composition in monazite and that the closure temperature for this system must be significantly higher than previously assumed 750 c

the effect of horizontal pressure gradients on the momentum transport in tropical convective lines part i the results of the convective parameterization abstract measuremenits of the momentum transport in tropical convective lines suggest that horizontal momentum can be generated by the pressure low located near the center of the convective part of the line a simple convective parameterization is used to evaluate this effect the parameterization is a version of the fritsch and chappell scheme modified in order to evaluate the influence of the horizontal pressure gradients on momentum transport the results suggest that in modeling of convective particularly slow moving lines with 20 km resolution subgrid horizontal pressure gradients should be taken into account sensitivity studies show that the magnitude of the calculated momentum flux strongly depends on the average vertical velocity and the vertical velocity in clouds

a three dimensional method for calculating independent chemical u pb and th pb ages of accessory minerals abstract previous theoretical considerations on the chemical u th total pb dating method failed to distinguish between thorogenic and uranogenic lead however it can be shown that the data points are located on a plane in the three dimensional tho2 pbo uo2 space the calculation of the best fit plane yields a slope in the tho2 pbo and uo2 pbo coordinate projections and an initial pbo value from the two slopes th pb and u pb ages can be calculated independently the method described in this paper is applied to monazites from the hercynian g4 granite of the fichtelgebirge germany th pb and u pb ages were calculated at 323 20 ma and 304 15 ma respectively the intercept value close to zero indicates that no significant amounts of common lead are present in the monazites studied

geochronological constraints on the timing of magmatism deformation and gold mineralization in the red lake greenstone belt northwestern ontario the paper presents u pb ages on zircon baddeleyite titanite rutile and monazite from volcanic and plutonic rocks of the red lake greenstone belt in the northern superior province the purpose of the study was to refine a previously outlined time frame for the volcanic evolution of the belt define the time of major plutonism and deformation and constrain the age of formation of gold deposits volcanism spanned at least 270 ma between 3000 and 2730 ma new ages of 2989 3 and 2894 2 ma for units of a lower volcanic sequence and and 2744 1 ma for units of an upper volcanic sequence corroborate and refine the previously established relationships local gabbroic and felsic intrusions cutting through the lower sequence yield ages of 2870 15 and respectively volcanism was succeeded by major plutonism deformation alteration and metamorphism between about 2730 and 2700 ma these processes were interrelated and progressed as a relatively continuous sequence of events accentuated by major activity d

parameterization of convective momentum transport in highly baroclinic conditions the organization of convection by vertical shear strongly affects the transport of horizontal momentum yet the concept of organization has received little attention in convective parameterization here the focus is on open cellular convection in cold air outbreaks under strongly baroclinic conditions that occur to the rear of midlatitude cyclones the principles derived herein are envisaged to apply to baroclinic advection at large in similar shear flows open cellular convection forced by surface fluxes of sensible and latent heat and organized by unidirectional shear is simulated using a three dimensional cloud resolving numerical model the effects of the in cloud pressure field organization and shear on momentum transport are quantified for the simulated clouds a simple analytic model of blocked overturning provides dynamical insight into downgradient momentum transport by the simulated three dimensional cumulonimbus ensemble the overturning circulation having vorticity of the same sign as the ambient shear maintains the far field flow while the blocking effect decelerates in cloud momentum the combined effect is to maintain the mean flow somewhat below its undisturbed value the numerical results and analytic predictions are used to evaluate and interpret two parameterization schemes for convective momentum transport used in operational global weather prediction and climate models at the u k meteorological office and the european centre for medium range weather forecasts finally the authors comment on how the downgradient momentum transport by three dimensional cumulus convection contrasts with momentum mixing concepts as well as countergradient momentum transport by two dimensional squall lines

open ocean convection observations theory and models we review what is known about the convective process in the open ocean in which the properties of large volumes of water are changed by intermittent deep reaching convection triggered by winter storms observational laboratory and modeling studies reveal a fascinating and complex interplay of convective and geostrophic scales the large scale circulation of the ocean and the prevailing meteorology two aspects make ocean convection interesting from a theoretical point of view first the timescales of the convective process in the ocean are sufficiently long that it may be modified by the earth s rotation second the convective process is localized in space so that vertical buoyancy transfer by upright convection can give way to slantwise transfer by baroclinic instability moreover the convective and geostrophic scales are not very disparate from one another detailed observations of the process in the labrador greenland and mediterranean seas are described which were made possible by new observing technology when interpreted in terms of underlying dynamics and theory and the context provided by laboratory and numerical experiments of rotating convection great progress in our description and understanding of the processes at work is being made

convective momentum transport and perturbation pressure field from a cloud resolving model simulation this study uses a 2d cloud resolving model to investigate the vertical transport of horizontal momentum and to understand the role of a convection generated perturbation pressure field in the momentum transport by convective systems during part of the tropical ocean and global atmosphere coupled ocean atmosphere response experiment toga coare intensive observation period it shows that convective updrafts transport a significant amount of momentum vertically this transport is downgradient in the easterly wind regime but upgradient during a westerly wind burst the differences in convective momentum transport between easterly and westerly wind regimes are examined the perturbation pressure gradient accounts for an important part of the apparent momentum source in general it is opposite in sign to the product of cloud mass flux and the vertical wind shear with smaller magnitude examination of the dynamic forcing to the pressure field demonstrates that the linear forcing representing the interaction between the convective updrafts and the large scale wind shear is the dominant term while the nonlinear forcing is of secondary importance thus parameterization schemes taking into account the linear interaction between the convective updrafts and the large scale wind shear can capture the essential features of the perturbation pressure field the parameterization scheme for momentum transport by zhang and cho is evaluated using the model simulation data the parameterized pressure gradient force using the scheme is in excellent agreement with the simulated one the parameterized apparent momentum source is also in good agreement with the model simulation other parameterization methods for the pressure gradient are also discussed

electron microprobe observations of pb diffusion in metamorphosed detrital monazites electron microprobe analyses have been made on monazite grains from paragneiss samples in the andalusite sillimanite transition 620 15 c and sillimanite orthoclase 680 15 c zones of the cretaceous ryoke metamorphic belt southwest japan monazites from pelitic gneisses are of metamorphic origin euhedral to subhedral and chronologically homogeneous giving chemical th u total pb isochron chime ages of 98 8 3 3 98 0 3 2 ma two psammitic gneisses of individual metamorphic grade contain both metamorphic monazite grains and detrital ones as old as ca 1700 ma most detrital monazite grains are heterogeneous in the tho2 and uo2 concentrations and have multiple or single rims as young as ca 100 ma several detrital monazite grains are well rounded in form exhibit homogeneous th and u distributions and show a pb diffusion profile in the margin the width of the diffusion zones is approximately constant throughout grains from each psammitic gneiss 18 22 m for 620 c and 48 58 m for 680 c assuming the isothermal diffusion of pb from homogeneous monazite spheres during a 5 ma duration of peak metamorphism we obtain diffusion coefficients of 1 9 0 3 10 21 and 1 5 0 3 10 20 cm2 s at 620 c and 680 c respectively these data derive an activation energy of 2 44 2 85 1 26 105 j mol and a frequency factor of 3 4 10 7 8 5 10 12 2 2 107 cm2 s taking account of uncertainties of 15 c in the temperatures and of 20 in the diffusion coefficients the diffusion parameters obtained from natural samples in this study provide a reliable insight into the closure temperature for pb in monazite that has been poorly understood so far

parametrization of momentum transport by convection ii tests in single column and general circulation models diagnostics derived from cloud resolving model simulations in part i of this study relating to the vertical transport of horizontal momentum by convection are used to develop a parametrization of convective momentum transports for deep convection based upon the mass flux convection scheme discussed by gregory and rowntree the importance of cloud pressure gradients to the flow within the cloud is emphasised and a simple method of representing their effect is suggested the scheme is able to reproduce the fluxes derived from the cloud resolving model studies where cloud organization by the flow is unimportant inclusion of the parametrization in a version of the meteorological office unified model demonstrates that convective momentum transports play a large role in the momentum balance of the atmosphere generally simulation of the mean atmospheric circulation by the unified model is improved by the inclusion of such transports

geochronological evidence for reworking of archean terrains during the early proterozoic 2 1 ga in the western coˆte d ivoire man rise west african craton abstract a period of accretion of continental crust is recognized during the archean at ca 3 2 3 3 ga on the basis of wholerock nd model ages and pb ages on detrital zircons from various gneisses of the man area in co te d ivoire there is no significant evidence of any older event such as the 3 5 3 6 ga accretion episode recognized in the archean of the reguibat rise in mauritania the earliest magmatic events recorded in the northern domain of the man area correspond to tonalitic grey gneisses of a minimum age ca 3 05 ga leonian event a subsequent major phase of igneous and metamorphic activity around 2 8 ga liberian event is recorded in the entire archean terrain without any evidence of new accretion this second event began at 2830 7 ma its climax is indicated by charnockitic intrusions followed by granodioritic magmas at ca 2800 10 pb pb and u pb ages on monazite and zircon cooling after the liberian event occurred ca 2 75 ga ago k feldspar whole rock garnet sm nd ages paleoproterozoic reworking is mainly found in the southern part of the archean terrain of the man area zircon monazite sphene and garnet geochronometers all indicate an important thermal effect during the birimian event i e 2 1 ga the earliest influence of this event is recorded in a range of 2250 40 70 ma with a major event ca 2 1 ga partial to total resetting of u pb and sm nd geochronometers indicate that high temperature conditions were reached during the birimian event this metamorphic overprint and the diapiric emplacement of coeval paleoproterozoic juvenile magmas at 2 1 ga in the southern domain toule pleu massif suggest that the 2 1 ga overprint could be related to major underplating processes under the archean basement the thermal flow related to the differentiation and the accretion of these juvenile birimian magmas may account for the reworking of the surrounding archean domain which had been possibly thinned during underplating these results suggest close proximity between the archean continents and an accretion zone associated with juvenile magmatism 2 1 ga ago

denudation history of the high t p ryoke metamorphic belt southwest japan constraints from chime monazite ages of gneisses and granitoids chime chemical th u total pb isochron method monazite ages were determined for gneisses and granitoids from the eastern and western parts of the ryoke belt separated by about 500 km the monazite ages for the gneisses are concentrated between 102 and 98 ma and are interpreted as the time of monazite formation under lower amphibolite facies conditions the peak metamorphism seems to be contemporaneous with the emplacement of the geologically oldest plutons that are dated at c 95 ma in both the eastern and western parts in the eastern part plutonism continued from c 95 ma to c 68 ma at intervals of 2 10 ma whereas in the western part it ceased at c 85 ma the chime monazite ages agree well with the relative age of granitoids derived from intrusive relationships of granitoids in both parts these lines of evidence are incompatible with a current view that the plutonometamorphism in the ryoke belt becomes younger towards the east the chime monazite ages coupled with available data on the depth at which the ryoke metamorphism took place and the emplacement of individual plutons show that the western part was eroded more rapidly about 1 5 mm year 1 than the eastern part about 0 8 mm year 1 over the time span from 91 to 85 ma the denudation rates agree well with those in active orogenic belts like the alps and himalayas

middle precambrian detrital monazite and zircon from the hida gneiss on oki dogo island japan their origin and implications for the correlation of basement gneiss of southwest japan and korea abstract monazite and zircon from seven paragneiss samples of the upper amphibolite facies grade from oki dogo island in the hida terrane western japan were analyzed for th u and pb on an electron microprobe most monazite grains are chronologically unzoned and give unequivocal chime chemical th u total pb isochron ages of 250 ma but some grains show a distinct core overgrowth relationship the cores retain evidence of pre 250 ma ages and are interpreted to be of detrital origin suggesting that monazite can be more immune to diagenetic and low grade metamorphic decompositions than had been thought previously zircon grains rounded to nearly euhedral give chime ages ranging from 3000 to 350 ma of particular interest are several rounded 3000 ma recycled zircons which perhaps originated from an archean terrain in the sino korean craton the youngest age of detrital zircons constrains that the gneiss protolith deposited after the 350 ma event the chime monazite and zircon ages for the paragneisses from oki dogo island and from the main part of the hida terrane reveal that the gneisses formed from permo carboniferous clastic sediments during a single metamorphism at ca 250 ma the results are in marked contrast to the current view that 1 the hida gneiss is a polymetamorphosed precambrian complex 2 the central part hida gneiss region and the eastern marginal part unazuki schist region of the hida terrane are correlative with the precambrian kyonggi massif and the paleozoic okchon zone in the korean peninsula respectively instead we consider that the okchon zone alone can be correlatable with the paleozoic hida terrane

decoupled hf nd isotopes in neoarchean seawater reveal weathering of emerged continents marine chemical sediments from the temagami banded iron formation bif in canada exhibit nonchondritic zr hf and y ho ratios and seawater like rare earth element patterns indicating that their hf and nd are not detrital but derived from seawater this is confirmed by sm nd and lu hf isochron ages of 2605 140 ma initial end 0 03 4 1 and 2760 120 ma initial ehf 7 2 5 3 respectively that overlap within error the 2 7 ga u pb age of associated igneous rocks the temagami bif is therefore an excellent archive of the nd hf isotopic composition of neoarchean seawater whereas graphic 1 values cluster around 1 graphic 2 values range from 6 7 to 24 1 substantially more radiogenic than those of ambient neoarchean mantle and continental crust such an ehf end distribution is typical of modern seawater plotting above the terrestrial array as defined by igneous and clastic sedimentary rocks the only mechanism known to produce natural waters with decoupled nd and hf isotope compositions is the incongruent mobilization of hf from continental crustal material therefore input of such highly radiogenic hf into seawater requires substantial amounts of evolved neoarchean continental crust that was exposed above sea level and available to erosion and terrestrial weathering 1 embed inline graphic 1 gif 2 embed inline graphic 2 gif

geochronology of polygenetic monazites constrained by in situ electron microprobe th u total lead determination implications for lead behaviour in monazite abstract monazite grains are generally concordant and with an electron microprobe give th u total pb isochron ages in agreement with conventional u pb data monazites from four samples of post archean migmatite from the ivory coast record two main ages an archean age at 2 80 ga partly overprinted by a late migmatization event at 2 03 ga the in situ electron microprobe determination has also shown a third event that systematically appears at 2 72 ga for all the monazites from two of the four studied rocks the three events at 2 80 2 72 and 2 03 ga recorded by the th u total pb system in monazite were each obtained on independent homogeneous parts of a single grain with no sign of pb diffusion thus the oldest monazite remained closed during a high grade resetting migmatization at a temperature estimated at around 700 c and also remained closed during the growth of secondary monazite conventional u pb data isotope dilution on monazite from the same rocks did not allow precise age determinations because the representative points were highly up to 83 discordant and did not fit well along a chord in the concordia diagram the u pb isotope dilution method gave a lower intercept age of 2029 25 ma in agreement with the electron microprobe age although with an unusually high uncertainty due to a poor fit of the data the pb evaporation method gave an old archean age at 2830 7 ma and proterozoic ages ranging from 2417 10 to 2074 7 ma thus although the oldest archean age obtained with the pb evaporation and electron microprobe methods is similar within the analytical error the proterozoic ages obtained by the pb evaporation method are significantly older than the th u total pb proterozoic age obtained with the electron microprobe this discrepancy is presumed to be due to the influence in the pb evaporation method of radiogenic lead contained within archean zones of the monazite grains in other words it means that migmatization took place later than 2074 7 ma and probably around 2030 ma the second archean event at 2 72 ga revealed by high spatial resolution of the electron microprobe could not be determined by other methods the th u total pb method appears to be an inexpensive alternative method for dating simple monazites with a precision close to 20 ma even if the u pb isotopic dilution technique remains the reference method moreover owing to the very high spatial resolution 1 m of the electron microprobe it appears to have no competitor for dating complex polygenetic monazites especially when more than two events are recorded homogeneous zones of different ages coexist in a single grain with no major indication of lead diffusion a procedure for age and error calculations is defined

timing of magmatism and deformation in the kamiskotia kidd creek area western abitibi subprovince canada abstract the kamiskotia kidd creek area comprises well preserved granitoid greenstone belt lithologies of the western most abitibi subprovince similar in composition and structural style to other late archean granitoid greenstone terranes e g kalgoorlie norseman western australia belingwe zimbabwe slave province canada it contains the kidd creek volcanogenic massive sulfide deposit and lesser deposits of the kamiskotia district and lode gold deposits associated with the destor porcupine fault zone near the prolific porcupine gold district the magmatic and tectonic history for the area is provided through detailed structural mapping and high precision u pb geochronology field observations support the following sequence of magmatic and structural events in the kamiskotia area 1 formation of older mafic and intermediate volcanic rocks followed by the emplacement of the kamiskotia gabbroic complex kgc and cogenetic massive sulfide bearing bimodal volcanic rocks kv also accompanied by coeval intrusion of granitoid a into the crystallizing kgc 2 regional bulk rotation about a sub horizontal axis of this stratigraphy to form a ne facing monocline 3 emplacement of three large granitoid intrusions imparting their respective contact strain aureoles on the stratigraphy 4 regional sub horizontal bulk north south shortening that formed a non penetrative east west flattening fabric with an element of 5 late dextral transpression that formed high strain zones related to western destor porcupine fault zone dpfz within less competent volcanic units and 6 emplacement of alkalic dikes associated with the late stages of transpressive deformation u pb ages help substantiate this chronology and aid in the correlation of kamiskotia lithologies across the southern abitibi subprovince zircon ages for the kgc and a kamiskotia rhyolite are 2707 2 ma and 2705 2 ma respectively these ages are coeval with the reid rhyolite with a zircon age of 2706 2 ma but are significantly younger than the kidd creek rhyolite and the prosser rhyolite with zircon ages of 2717 2 ma data refined by the addition of an abraded concordant analysis and 2716 6 3 ma respectively two of the granitoid intrusions have zircon ages of 2696 1 5 and 2694 4 ma the older granitoid intrusion has a titanite population with a 207 pb 206 pb minimum age of 2692 5 ma which may represent a protracted cooling history or a metamorphic event after the setting of the zircon u pb system two zircon fractions from the younger granitoid intrusion exhibit inheritance with 207 pb 206 pb minimum ages up to 2926 ma indicating the presence of significantly older crust in this region timing of north south compression and late transpression may be constrained by a u pb garnet titanite age on a late to post tectonic alkalic dike at 2687 3 ma

textural geochemical and isotopic data from silicified rocks and associated chemical sedimentary rocks in the 2 7 ga abitibi greenstone belt canada insight into the role of silicification abstract silica rich precambrian rocks often preserve geochemical information and microfossil remnants from the early biosphere and could play a critical role in the formation of early crust because these rocks are important geochemical and paleontological archives we need to better constrain their geochemical and isotopic attributes and generate a refined picture of the evolving archean silica cycle here we investigate a series of sub to greenschist facies si rich archean rocks from the 2 7 ga abitibi greenstone belt canada that represent chemical sedimentary rocks and rocks formed via silica addition through the process of silicification we report data for major and trace element geochemistry multi crystal silicon and oxygen isotopes of quartz using isotope ratio mass spectrometry and texture specific silicon isotope values measured using secondary ion mass spectrometry on neoarchean chemical sedimentary rocks their silicified equivalents and associated silicified volcanic rocks we find that in such a well preserved terrane we can utilize petrographic textures and geochemical attributes to establish rock origin distinguishing siliceous rocks that form via chemical sedimentation from those that form via silicification chemical sedimentary rocks display a wide range of 30si depleted silicon isotopes values that vary with stratigraphy similar to other archean iron formation silicified volcanic rocks possess 30si enriched values similar to archean silicified basalts we conclude that because silicon isotope values of iron formation shift toward 30si enriched values up stratigraphy basinal changes in the composition of the silicon isotope reservoir may be preserved silicon isotope values of silicified volcanic rocks by contrast likely represent precipitation from an isotopically heavy silicon reservoir influenced by downward percolating seawater and upward moving convecting fluids interacting with host volcanic rock basalt or andesite overall we confirm that neoarchean silicified rocks are 30si enriched like their paleoarchean counterparts

decoupling of neoarchean sulfur sources recorded in algoma type banded iron formation abstract neoarchean algoma type banded iron formations bifs are widely viewed as direct chemical precipitates from proximal volcanic hydrothermal vents however a systematic multiple sulfur isotope study of oxide facies bif from a type locality in the ca 2 74 ga temagami greenstone belt reveals mainly bacterial turnover of atmospheric elemental sulfur in the host basin rather than deposition of hydrothermally cycled seawater sulfate or sulfur from direct volcanic input trace amounts of chromium reducible sulfur that were extracted for quadruple sulfur isotope 32s 33s 34s 36s analysis record the previously known mass independent fractionation of volcanic so2 in the archean atmosphere s mif and biological sulfur cycling but only minor contributions from juvenile sulfur despite the proximity of volcanic sources we show that the dominant bacterial metabolisms were iron reduction and sulfur disproportionation and not sulfate reduction consistent with limited availability of organic matter and the abundant ferric iron deposited as fe oh 3 that sulfur contained in the bif was not a direct volcanic hydrothermal input as expected changes the view of an important archive of the neoarchean sulfur cycle in which the available sulfur pools were strongly decoupled and only species produced photochemically under anoxic atmospheric conditions were deposited in the bif forming environment

parametrization of momentum transport by convection i theory and cloud modelling results summary the theory of the mass flux approach to parametrization of convective momentum transport is presented a cloud resolving model is used to simulate momentum transport by ensembles of deep convective clouds in two very different regimes a mid latitude cold air outbreak and tropical convection forced by convergence idealized unidirectional wind profiles are used to simplify interpretation of the results diagnostics relevant to the parametrization problem are presented and it is shown that for both regimes the approximations inherent in the parametrization equations are reasonable the pressure gradients inside clouds play an important role in determining horizontal velocities in the clouds for the cold air outbreak with linear shear the results suggest that these pressure gradients are proportional to the shear and to the up downdraught mass fluxes sensitivity studies suggest that the results are not very sensitive to resolution or parametrization of subgrid scale processes giving some confidence that the results are reasonably accurate for the tropical case with a low level jet the pressure gradients change sign with the wind shear in part i1 of this paper these results will be used to develop and validate a mass flux parametrization of convective momentum transport which is tested in single column and global versions of the meteorological office unified model

a metamorphic history from micron scale 207pb 206pb chronometry of archean monazite ion microprobe measurements of pb isotope ratios in monazites have been obtained in situ from thin sections using the cambridge isolab 120 molecular interferences are sufficiently resolved at an rp of 6500 to allow 2 7pb z 6pb dating of monazite with precisions as low as 4 5 ma 2 r the results presented here provide important information on the chronological history of the late archean metamorphism of the wind river range wyoming usa matrix monazites and monazite inclusions in garnets from a metapelite from the northern wind river range have been analysed by sims in a previous study peak metamorphic conditions t 800 c p 8 1 kb were estimated using inclusion assemblages in garnets from this same sample isolated monazite inclusions in garnet yield 2 7pb z 6pb age estimates of 2781 6 to 2809 10 ma those along fractures yield lower ages 2603 2687 ma which are similar to tims and sims ages of matrix monazites a single large 500 xm monazite grain locally preserves growth zoning but has a recrystallised core and a resorbed recrystallised rim age estimates for these three regions are 2788 9 ma 2663 4 and 2523 6 ma respectively thus the inclusion assemblages of sharp and essene may record peak metamorphic conditions at ca 2 8 ga and indicate a phase of metamorphism that predates by over 100 ma the emplacement of the bridget batholith the major lithologic component of the northern wind river range the analysed monazite grains appear to preserve ca 300 ma history even within a single grain monazite inclusions in garnet that are fully armoured may provide estimates for the time of garnet growth even in high grade terranes where most chronometers are reset the age pattern preserved by the large monazite grain cannot be simply related to diffusion controlled closure instead a chronology is preserved which can be related to the petrographic setting of individual grains through in situ analysis

evolution of the south central part of the archean abitibi belt quebec part i stratigraphy and paleogeographic model the region comprises parts of the southern zone of the abitibi greenstone belt and of the bellecombe gneiss belt and the belleterre angliers greenstone belt the stratigraphic sequence of the abitibi belt consists of 1 a lower ultramafic division 2 a middle tholeiitic division and 3 an upper diverse division these divisions lap onto an older volcanic sequence to the north and west in the bellecombe belt sediments overlie the lower ultramafic division these sediments coarsen toward the abitibi belt and toward the stratigraphic top the volcanic rocks of the belleterre angliers belt overlie the sediments of the bellecombe belt five paleogeographic phases have been recognized during phase i a deep marine ultramafic lava plain covered the south part of the abitibi belt and the bellecombe belt faulting took place at the duparquet destor break during phase ii tholeiitic basalts formed a deep marine lava plain in the west of the abitibi belt whereas flows erupted mainly from central volcanic c

a comprehensive mass flux scheme for cumulus parameterization in large scale models abstract observational studies indicate that a mass flux approach may provide a realistic framework for cumulus parameterization in large scale models but this approach through the introduction of a spectral cloud ensemble leads normally to rather complex schemes in this paper the question is addressed whether much simpler schemes can already provide realistic values of the thermal forcing by convection under various synoptic conditions this is done through verifying such a scheme first on data from field experiments for periods of tropical penetrative convection gate marshall islands tradewind cumuli atex bomex and extratropical organized convection sesame 79 and then in a nwp model the scheme considers a population of clouds where the cloud ensemble is described by a one dimensional bulk model as earlier applied by yanai et al in a diagnostic study of tropical convection cumulus scale downdrafts are included various types of convection are represented i e penetrative convection in c

the role of momentum transport in the motion of a quasi idealized mesoscale convective system abstract momentum transport is examined in a simulated midlatitude mesoscale convective system mcs to investigate its contribution to mcs motion momentum budgets are computed using model output to quantify the role of specific processes in determining the low level wind field in the system s surface based cold pool results show that toward the leading convective line of the mcs and near the leading edge of the cold pool the momentum field is most strongly determined by the vertical advection of the storm induced perturbation wind across the middle rear of the system the wind field is largely a product of the pressure gradient acceleration and to a lesser extent the vertical advection of the background environmental i e base state wind the relative magnitudes of the vertical advection terms in an eulerian momentum budget suggest that for gust front driven systems downward momentum transport by the mcs is a significant driver of mcs motion and potentially severe surface winds results furthe

u pb zircon geochronology in the southwestern abitibi greenstone belt superior province the abitibi belt is the largest continuous greenstone belt in the superior province of the canadian shield it comprises several composite komatiitic tholeiitic calc alkalic and sedimentary sequences that are folded transected by major faults and intruded by various generations of plutonic rocks precise u pb geochronology has been carried out in the belt for the past decade to solve chronostratigraphic and metallogenetic problems this paper presents new zircon ages and reassesses previously published ones now refined by the addition of abraded and concordant zircon analyses volcanic and subvolcanic units of the timmins area yield the following ages 2727 1 5 ma for a tuff of the upper deloro formation 2703 1 5 ma for a tuff of the upper tisdale formation and 2698 4 ma for the krist fragmental assigned to the top of the tisdale formation the age of a dunite intrusive into the upper deloro formation is revised at 2707 3 ma whereas the aquarius diorite east of timmins yields a poorly define

momentum transport by a line of cumulonimbus abstract the vertical transport of horizontal momentum normal to a line of cumulonimbus observed during gate on 14 september 1974 is against the vertical momentum gradient contrary to the predictions of mixing length theory data from repeated aircraft passes normal to the line s axis at heights from 0 15 to 5 5 km are used to document the flux and determine its source the flux is concentrated in roughly a 25 km wide active zone just behind the leading edge of the line in kilometer scale convective updrafts accelerated upward by buoyancy and toward the rear of the line by mesoscale pressure forces the fall in mesoscale pressure from the leading edge to the rear of the active zone is mainly hydrostatic resulting from relatively high virtual temperatures and the 60 degree tilt of the leading edge from the vertical with the clouds at the surface well ahead of those aloft evaluation of the terms in the momentum flux generation equation confirms that the above process reflected by the velocity buoyan