evidence for a widespread mafic cover sequence and its implications for continental growth in the northeastern superior province abstract archean greenstone belts of the northeastern superior province nesp were emplaced over a 160 ma period 2 88 2 72 ga spanning a major episode of crustal reworking in which early tonalite trondhjemite plutonism evolved to dominant granite granodiorite and pyroxene bearing felsic plutonism the numerous greenstone belts that crop up across the craton contain lavas belonging to three mafic volcanic suites 1 mg tholeiites that have chemical compositions typical of many archean basalts with 4 10 wt mgo 9 15 wt fe2o3t 0 4 1 2 wt tio2 and la sm ratios between 1 and 3 2 fe tholeiites that have similar mgo contents and la sm ratios but markedly higher fe2o3t 11 20 wt tio2 1 0 2 6 wt and gd yb ratios that may reflect derivation from a distinct garnet bearing mantle source 3 light rare earth element lree enriched rocks that have the chemical characteristics typical of calc alkaline mafic magmas with higher sio2 al2o3 and la sm ratios but lower fe2o3t and tio2 than the tholeiitic suites at a given mgo content the presence of unconformities within single greenstone belts numerous inheritance ages in volcanic rocks replicating those of older volcanic plutonic assemblages and the recognition of widespread geographically separate yet coeval 2 78 ga extrusive assemblages containing the distinctive fe tholeiites do not support a plate tectonic model for a large portion of the nesp the correlations in composition and ages of greenstone belts are more consistent with a model in which they represent the remnants of an extensive autochthonous mafic cover sequence nd isotopic evidence for the presence of evolved crust in some syn volcanic felsic plutons and volcaniclastic rocks imply that this mafic cover sequence was erupted onto an older granitoid crust the amalgamation of two isotopically distinct terranes at ca 2 76 2 74 ga created a proto cratonic mass that consisted of a composite tonalite trondhjemite greenstone crust underplating by mafic mantle melts and widespread insulation after 2 75 ga increased temperatures sufficiently to produce extensive melting of this crust to generate pyroxene bearing felsic rocks and granodiorite to granite partial melts the isotopically enriched character of mantle derived magmas after 2 75 ga may reflect more extensive contamination by a felsic crust affected by widespread partial melting furthermore a decrease of nb th ratios in the lree enriched mafic rocks younger than 2 75 ga reflects a change in the nature of the crustal contaminant from tonalite trondhjemite prior to 2 75 ga to granite granodiorite afterwards

protracted fluid induced melting during barrovian metamorphism in the central alps the timing and dynamics of fluid induced melting in the typical barrovian sequence of the central alps has been investigated using zircon chronology and trace element composition multiple zircon domains in leucosomes and country rocks yield u pb ages spanning from 32 to 22 ma the zircon formed during alpine melting can be distinguished from the inherited and detrital cores on the basis of their age th u 0 1 and trace element composition ti in zircon thermometry indicates crystallization temperatures around 620 700 c their composition allows discriminating between 1 zircon formation in the presence of early garnet 2 zircon in equilibrium with abundant l mree rich accessory phases allanite titanite and apatite typical of metatonalites and 3 zircon formed during melting of metasediments in feldspar dominated assemblages the distribution of zircon overgrowths and ages indicate that repeated melting events occurred within a single barrovian metamorphic cycle at roughly constant temperature that in the country rocks zircon formation was limited to the initial stages of melting whereas further melting concentrated in the segregated leucosomes that melting occurred at different times in samples a few meters apart because of the local rock composition and localized influx of the fluids and that leucosomes were repeatedly melted when fluids became available the geochronological data force a revision of the temperature time path of the migmatite belt in the central alps protracted melting over 10 my followed the fast exhumation of alpine eclogites contained within the same region and preceded fast cooling in the order of 100 c ma to upper crustal levels

microprobe monazite geochronology understanding geologic processes by integrating composition and chronology abstractmonazite is a light rare earth element lree bearing phosphate mineral that is present in a wide variety of rock types has an extremely variable composition reflecting host rock conditions and is a robust geochronometer that can preserve crystallization ages through a long history of geological events monazite crystals typically contain distinct compositional domains that represent successive generations of monazite which in turn can provide a detailed record of the geologic history of its host rocks the electron microprobe can be used to characterize the geometry of compositional domains analyze the composition of each domain and when carefully configured determine the u th total pb age for domains as small as 5 m in width these data allow the monazite to be linked with and place timing constraints on silicate processes in the host rocks current applications span a broad range of geologic processes in igneous metamorphic hydrothermal and sedimentary rocks

isotopic and chemical alteration of zircon by metamorphic fluids u pb age depth profiling of zircon crystals from barrow s garnet zone northeast scotland u pb isotopic analyses were done using ion microprobe depth profiling on zircon crystals from dalradian rocks in barrow s garnet zone northeast scotland to determine the timing of metamorphic fluid infiltration and investigate fluid zircon interactions zircon crystals collected from altered metasedimentary rock adjacent to a cross cutting quartz vein and from relatively unaltered continuously equivalent layers distal to the vein indicate that paleozoic fluid influx caused isotopic and chemical alteration along zircon crystal surfaces and internal fractures ion drilling into natural zircon crystal faces yielded u pb depth profiles with 50 100 nm spatial resolution that permitted u pb isotopic age analysis of thin grain rims affected by fluid mediated growth recrystallization at 500 550 c we interpret the measurements to indicate that fluid influx at 462 8 8 ma caused the growth recrystallization of new zircon along pitted crystal surfaces and fractures in archean zircon grains our age estimate overlaps the accepted age range ca 467 464 ma for peak barrovian garnet growth confirming that metamorphism and fluid influx were contemporaneous the new zircon was enriched in u th and common pb relative to host grains suggesting that u th and pb were actively transported by upper greenschist facies metamorphic fluids we present the first field based evidence that common pb in zircon can serve as an indicator of metamorphic fluid infiltration archean ages for some detrital zircons suggest contributions from a lewisian scourian provenance

synchronous vertical and horizontal tectonism in the neoarchean kinematic evidence from a synclinal keel in the northwestern superior craton canada the long and narrow carrot river greenstone belt in the northwestern superior craton is a synclinal keel between two broad granitoid domes its unique structural geometry and kinematics place significant constraints on the timing relationship between vertical and horizontal tectonism a high strain zone spatially coincident with the belt records evidence for synchronous greenstone down granitoid up dip slip and dextral strike slip components kinematic and regional considerations lead to the conclusion that the dip slip component was a result of vertical tectonism and the strike slip component was a result of horizontal tectonism and therefore that the vertical and horizontal tectonism were synchronous it is emphasized that vertical and horizontal tectonism do not have to act exclusively of each other it is not necessary to downplay the significance of vertical tectonism to validate horizontal tectonism and vice versa evidence for synchronous vertical and horizontal tectonism reported here makes it necessary that both processes as opposed to either one or the other and their potential interactions are considered in interpretations of archean terrains it is suggested that synchronous vertical and horizontal tectonism was a common process in the neoarchean and might represent a transition from dominant vertical tectonism in the mesoarchean and paleoarchean to dominant horizontal tectonism in the proterozoic and phanerozoic it might be an important process for gold mineralization at the late stages of archean cratonization 2005 elsevier b v all rights reserved

characterization of transpressive deformation in shear zones of the archean north caribou greenstone belt nw superior province and the relationship with regional metamorphism abstract the 2 7 3 0 ga north caribou greenstone belt ncgb host to the musselwhite bif hosted gold deposit possesses abundant shear zones on its northern margins which appear to have formed under amphibolite facies conditions protracted deformation and regional metamorphism are coeval with widespread magmatism and accretion events during crustal amalgamation of the western superior province and are responsible for folding the ore hosting bif and channeling fluids the importance of shear zones in behaving as conduits for fluids during the tectonic evolution of the ncgb is not well known and their relationship with metamorphism is equivocal yet higher grade syn to post tectonic metamorphic minerals seem to correlate with loci of higher strain structural analyses support oblique transpressive collision that produced steeply dipping planar and shallowly plunging linear fabrics with dominant dextral kinematics that trend broadly parallel to the doubly arcuate shape of the belt electron backscatter diffraction analyses were conducted on strategic samples across one shear zone in order to characterize crustal conditions during transpressive deformation the dinnick lake shear zone cuts through mafic metavolcanics and at its core is an l tectonite granite composed of recrystallized quartz whole rock geochemistry shows little variation in ca na mg and k often used as indicators of hydrothermal alteration from surrounding less deformed units suggesting deformation in a dry environment microstructural analysis indicates subgrain rotation recrystallization and deformation by prism a and c slip in quartz as well as aligned hornblende that suggest deformation temperatures above 500 c quartz in mafic rocks along the margins of the shear zone also exhibits a basal a slip component indicating a slight decrease in strain or temperature although the ncgb exhibits some first order evidence of vertical tectonism dome and keel geometries the dominant strain record within shear zones is that of horizontal oblique transpressive displacement this is in agreement with other greenstone belts in the western superior province where vertical tectonism and horizontal tectonism were coeval

the 10a phase a high pressure expandable sheet silicate stable during subduction of hydrated lithosphere h2o storage and release in deep subducting lithosphere is controlled by complex reaction suites involving a variety of hydrous phases as a result of its relatively large thermal stability and intermediate composition the 10a o phase mg3si4o10 oh 2wnh2o has been regarded as a relevant h2o reservoir in a wide range of rock compositions and mineral assemblages high pressure syntheses of the 10a o phase were carried out at 6 7 gpa and 650 c under fluidsaturated conditions in a walker type multi anvil apparatus from 5 min to 430 h x ray powder diffraction of large platy hexagonal crystals of the 10a o phase up to 100 wm were indexed on the basis of a trioctahedral type structure long term run products s 110 h reveal sensitivity of the 10a o phase to treatment with acetone leading to the appearance of diffractions at greater d spacings 10 2 11 6 a o with respect to the basal peak of the 10a o phase 9 64 10 07 a o this swelling behavior is strongly related to synthesis run duration the raman spectrum of the 10a o phase at frequencies less than 800 cm 31 shows a strong similarity to talc in the si o stretching region 800 1100 cm 31 the 10a o phase exhibits three modes 909 992 and 1058 cm 31 as compared to two in talc the bending mode of water x2 i s found at 1593 cm 31 in the oh stretching region peaks at 3593 3622 and 3668 cm 31 were observed the acetone treated sample shows a c h stretching mode at 2923 cm 31 while the double bond cno signal is absent the swelling behavior of the 10a o phase is interpreted as due to intercalation of acetone with pre existing interlayer water the efficiency of this process is dependent on the amount of the interlayer water which in turn depends on run duration the relation between the response to acetone treatment and run duration is therefore interpreted as a time dependent hydration of the 10a o phase the fractions transformed from non expandable to expandable fractions was fitted to the avrami empirical law which suggests that kinetics are mainly controlled by diffusion rather than phase boundary reactions the ability to accommodate variable amounts of h2o makes the 10a o phase a major h2o sink whenever a hydrous phase such as chlorite and serpentine breaks down during prograde transformations in the subducted lithosphere under h2o saturated conditions a fully hydrated 10a o phase occurs when h2o undersaturated conditions prevail a h2o deficient 10a o phase incorporates the volatile component available the exchange capacity of interlayer molecules in the 10a o phase structure opens new scenarios on the control of fluid compositions escaping from subducted

influence of averaging interval on shear velocity estimates for aeolian transport modeling abstract recent investigations of aeolian transport have focused on increasingly short time scales because of growing recognition that wind unsteadiness is a major factor in the dynamics of sediment transport however the statistical reliability of shear velocity u estimates becomes increasingly uncertain as averaging interval is decreased this study provides an empirical assessment of the influence of averaging interval on the reliability of u estimates the data consist of 15 min wind speed profiles 1 hz sampling collected at four coastal sites each profile was subdivided into progressively shorter fixed length time intervals and estimates of u and the 95 confidence interval for u were determined for each time block using standard statistical techniques the logarithmic model accurately represents the measured wind speed profiles even with relatively brief averaging intervals mean r 2 values remain robust down to block lengths as short as 10 20 s typically retaining better than 98 of the r 2 value found for the full length data sets fewer than 2 of the individual 10 s blocks had r 2 values less than 0 9 however mean confidence intervals typically expanded by 70 80 of the full record value as block length decreased from 900 to 10 s for highly log linear profiles this amounted to an absolute increase from about 8 to only 14 of u so that the additional information gained through the use of shorter averaging intervals may outweigh the increase in statistical uncertainty nevertheless given that rates of aeolian transport are generally modeled as a function of u 3 this increase in uncertainty may be significant for transport modeling thus very short averaging intervals should be used with caution when predicting aeolian sediment flux it is proposed that transport modeling should incorporate the shear velocity confidence interval as an indicator of the potential error associated with this source of uncertainty

the pikwitonei granulite domain manitoba a giant neoarchean high grade terrane in the northwest superior provincethis article is one of a series of papers published in this special issue on the theme of geochronology in honour of tom krogh n machado t e krogh and w weber are deceased the pikwitonei granulite domain located at the northwestern margin of the superior province is one of the largest neoarchean high grade terranes in the world with well preserved granulite metamorphic assemblages preserved in a variety of lithologies including enderbite opdalite charnockite and mafic granulite u pb geochronology has been attempted to unravel the protolith ages and metamorphic history of numerous lithologies at three main localities natawahunan lake sipiwesk lake and cauchon lake the u pb age results indicate that some of the layered enderbite gneisses are mesoarchean 3 4 3 0 ga and the more massive enderbites are neoarchean the high grade metamorphic history of the pikwitonei granulite domain is complex and multistage with at least four episodes of metamorphic zircon growth identified 1 2716 1 3 8 ma 2 2694 6 0 6 ma 3 2679 6 0 9 ma and 4 2642 5 0 9 ma metamorphic zircon growth during episodes 2 and 3 are interpreted to be regional in extent corresponding

leaching and differential recrystallization of metamict zircon under experimental hydrothermal conditions we report results of hydrothermal experiments on four alluvial zircons from sri lanka which cover a wide range of radiation damage at 450 c and 1 3 kbar for 744 h with 2 m cacl2 solution as reactive fluid after the hydrothermal treatment the most metamict samples show micrometer thick reaction rims which surround apparently unreacted zircon as revealed by cathodoluminescence cl and nomarski differential interference contrast ndic images these rims have sharp curved and transgressive boundaries with unreacted zircon and are in some cases spread out along cracks the thickness of reaction rims increases with increasing cumulated dosage of the starting materials the reaction rims are strongly enriched in ca up to 7000 ppm and a water species and depleted in radiogenic pb zr and si as revealed by electron microprobe analyses a significant th loss from the reaction rims was detected in the case of the most metamict sample whereas u remained in the structure ft infrared spectrometry and x ray diffraction measurements revealed that the bulk run products were recrystallized using micro raman spectrometry we were able to demonstrate that differential recrystallization took place the reaction rims are strongly recrystallized whereas the unreacted grain interiors underwent only minor recrystallization recrystallization of the rims is accompanied by an enhancement of the integral cl intensity it is suggested that recrystallization in the reaction rims was catalyzed by water infiltration and ion exchange and prevented significant congruent zircon dissolution under the given experimental conditions previous zircon studies have shown that 1 a transgressive morphology 2 a reduced th u ratio and 3 an enhanced cl emission are also characteristics of rims in zircons from high grade metamorphic rocks based on these similarities between natural and experimentally produced rims it is suggested that leaching catalyzed recrystallization is an important alteration process in zircon under wet geological conditions and can account for many complex core rim structures found in natural zircons furthermore the strong enrichment of ca in the reaction rims supports previous assumptions that high ca concentrations in natural zircons are of secondary origin it is suggested that lower u pb concordia intercept ages obtained from single phase zircons with high ca contents date a leaching event

establishment of a 3 83 ga magmatic age for the akilia tonalite southern west greenland a recurring challenge to ion microprobe u pb zircon geochronology has been to discriminate between preservation of original igneous zircon populations and inherited grains this has proved particularly problematic in studying the polyphase metamorphic rocks that dominate early archean gneissic terranes for example differing interpretations exist for the origin of complex zircon populations investigated by u pb ion microprobe zircon geochronology from a granitoid orthogneiss body previously used to establish a minimum age for a supracrustal enclave on akilia island in southern west greenland we describe a method whereby the geochemistry of the akilia orthogneiss coupled with a u th pb vs age depth profile in a zircon from the same rock permits direct assessment of zircon inheritance results reveal evidence for three phases of concentric zircon growth at 3 83 ga v3 6 ga and 2 7 2 5 ga zircon growth at both v3 6 ga and 2 7 2 5 ga is consistent with precipitation from a metamorphic fluid depth profile u th pb data demonstrate that only the s 3 8 ga zircon core could have crystallized from the host rock we conclude that the magmatic age of this rock is 3 83 7 0 01 ga not v3 65 ga as has been previously proposed the u th pb zircon depth profile technique has wider applications to resolving other geochronological debates in highgrade metamorphic terranes where zircons populations are diverse and individual grains record complex overgrowth and dissolution re precipitation features 2002 elsevier science b v all rights reserved

relative oxidation states of magmas inferred from ce iv ce iii in zircon application to porphyry copper deposits of northern chile major and trace element compositions of zircons and whole rocks from 14 barren and seven ore bearing calc alkaline intrusions from the chuquicamata el abra porphyry copper belt of northern chile have been measured in situ by excimer laser ablation ela icp ms these data permit the ce iv ce iii ratio within zircon to be calculated using a lattice strain model for mineral melt partitioning of ce iv and ce iii zircon ce iv ce iii and eun eun ratios and by inference magmatic oxidation states generally increase from older mafic to younger felsic units within this sequence porphyry copper mineralization is directly associated only with intrusions with zircon ce iv ce iii 300 and eun eun 0 4 such trends can be understood in terms of interdependent relations between oxygen fugacity sulfur speciation and solubility and chalcophile element partitioning in silicate magmas because zircon occurs in most calc alkaline intrusions and is resistant to subsolidus alteration zircon ce iv ce iii ratios provide a useful tool for evaluating the economic potential of such rocks for magmatic hydrothermal cu au mineralization the approach is general and may provide a means to infer relative oxidation state in a wide range of intermediate to felsic igneous rocks

geochronology of the north caribou greenstone belt superior province canada implications for tectonic history and gold mineralization at the musselwhite mine abstract the 3 ga north caribou greenstone belt comprises a package of volcanic and sedimentary rocks metamorphosed from greenschist to upper amphibolite grade surrounded by 2 7 3 0 ga granitoids and gneisses and is host to the musselwhite gold mine a large orogenic gold deposit new u pb zircon monazite ar ar biotite and sm nd garnet ages for volcanic sedimentary and intrusive rocks from the musselwhite mine and surrounding plutons have been obtained felsic volcanic rocks yielded ages of 2972 4 1 6 and 2982 4 0 8 ma broadly consistent with existing ages for volcanic rocks and plutonism an age of 2857 3 1 4 ma for the schade lake gneiss is consistent with published plutonic and volcanic ages a younger and previously unrecognized plutonic event is represented by three granites dated at 2723 1 0 8 2727 7 1 0 and 2729 4 7 1 ma an sm nd age of 2690 9 ma for garnets associated with gold mineralization is bracketed by peraluminous granites with ages of 2668 8 1 1 and 2715 8 1 6 ma ar ar ages range from 2420 11 97 to 2658 13 46 ma broadly synchronous with the youngest intrusive age and are consistent with progressive resetting due to metamorphism the new geochronological data for the ncgb show that volcanism in the opapimiskan lake area spanned 10 million years from 2982 4 to 2972 4 ma allow for new stratigraphic interpretations of the belt and are broadly correlative with other greenstone belts in the north caribou terrane the new data for the ncgb and associated plutons are consistent with 3 ga plume related activity forming a proto craton that was the site of successive sedimentary basins over the next 150 million years subsequent plutonism is consistent with supra subduction zone processes at the margins of this proto continent by 2 87 ga and is best interpreted as the result of horizontal tectonic processes consistent with those operating in the phanerozoic with the plutons emplaced behind continental margin arcs on the edges of the craton sm nd ages of garnets that are coeval with mineralization suggest that the orogenic gold mineralization at the musselwhite mine was broadly synchronous with the 2700 ma orogeny caused by the collision of the oxford stull domain with the north caribou terrane 75 km northeast of the musselwhite mine and furthermore argues that s type granites may play a greater role in the formation of some orogenic gold deposits than has been previously recognized

phyllosilicates in hydrothermally altered basalts from dsdp hole 504b leg 83 a tem and aem study phyllosilicates occurring as replacements of olivine clinopyroxene and interstitial materials and as veins or fracture fillings in hydrothermally altered basalts from dsdp hole 504b leg 83 have been studied using transmission and analytical electron microscopy the parageneses of phyllosilicates generally change systematically with depth and with the degree of alteration which in turn is related to permeability of basalts saponite and some mixed layer chlorite smectite are the dominant phyllosilicates at the top of the transition zone chlorite corrensite and mixed layer chlorite corrensite occur mainly in the lower transition zone and upper levels of the sheeted dike zone chlorite talc and mixed layer talc chlorite are the major phyllosilicates in the sheeted dike zone although replacement of talc or ohvine by saponite is observed the phyllosilicates consist of parallel or subparallel discrete packets of coherent layers with packet thicknesses generally ranging from 100 a to a few hundred a the packets of saponite layers are much smaller or less well defined than those of chlorite corrensite and talc indicating poorer crystal linity of saponite by contrast chlorite and talc from the lower transition zone and the sheeted dike zone occur in packets up to thousands of a thick the si si al ratio of these trioctahedral phyllosilicates increases and fe fe mg decreases in the order chlorite corrensite saponite and talc these relations reflect optimal solid solution consistent with minimum misfit of articulated octahedral and tetrahedral sheets variations in composition of hydrothermal fluids and precursor minerals especially in si si al and fe fe mg ratios are thus important factors in controlling the parageneses of phyllosilicates the phyllosilicates are generally well crystallized discrete phases rather than mixed layered phases where they have been affected by relatively high fluid rock ratios as in high permeability basalts in veins or areas adjacent to veins intense alteration in basalts with high permeability indicating high fluid rock ratios is characterized by pervasive albitization and zeolitization minimal alteration in the basalts without significant albitization and zeolitization is characterized by the occurrence of saponite mixed layer chlorite smectite in the low temperature alteration zone and mixed layer chlorite corrensite or mixed layer talc chlorite in the high temperature alteration zone textural non equilibrium for phyllosilicates is represented by mixed layering and poorly defined packets of partially incoherent layers the approach to textural equilibrium was controlled largely by the availability of fluid or permeability

the hydrothermal stability of zircon preliminary experimental and isotopic studies experimental investigations of the stability of the u pb isotopic system in nonmetamict zircons show that appreciable losses of pb and u can be induced at amphibolite grade conditions 400 c to 600 c 4 to 6 kb in 2 m nacl and 2 hno sub 3 solutions the severity of u loss and to a lesser extent pb loss varies with solution composition in this case the 2 m nacl solution induced more pb and u loss than the 2 hno sub 3 solution at the same p t conditions scanning electron microscopy of the run products also revealed a range of corrosion related surface features which suggests that some of the observed trends in pb and u loss must be attributed to zircon dissolution backscattered electron bse imaging of the run products further suggests that partial homogenization of chemical zoning patterns occurred during the experiments microprobe analyses of treated and untreated grain show that both populations have a similar range of hf contents thus the apparent loss of sharp well defined zoning features is most likely due to small scale smearing out of formerly sharp chemical gradients and is perhaps related to the annealing of lattice defects caused by alpha recoil damage thus experimentally induced u pb isotopicmore discordance in zircon is a complex function of zircon stability and annealing effects less

the world class musselwhite bif hosted gold deposit superior province canada new high precision u pb geochronology and implications for the geological setting of the deposit and gold exploration abstract new high precision u pb ages from the host succession of the musselwhite banded iron formation bif hosted epigenetic gold deposit 5 7 moz au in the north caribou greenstone belt of the superior province provide timing constraints on the setting of the deposit with implications for geological and exploration models the deposit is mainly hosted by a folded garnet grunerite chert silicate facies of the northern bif part of the opapimiskan markop assemblage dating of a volcaniclastic sedimentary rock a garnet biotite staurolite schist and a cross cutting foliated felsic dike indicate that the age of the northern bif is between 2967 and 2909 ma a felsic tuff from the south rim volcanic assemblage structurally overlies the ore bearing northern bif but yields an older age of 2979 ma reappraisal of stratigraphic and polyphase structural relationships in conjunction with the u pb ages indicate that the mine stratigraphy is inverted as it is located on the overturned limb of a kilometre scale refolded f1 syncline greatly influencing the distribution and geometry of the bif units hosting the bulk of the gold felsic ash tuff and a fine grained siliciclastic sedimentary rock from the lower sediments unit zeemel eyapamikama assemblage have much younger ages of

multi stage zircon and monazite growth revealed by depth profiling and in situ u pb geochronology resolving the paleoproterozoic tectonics of the trans hudson orogen on southeastern baffin island canada abstract understanding the growth and recrystallization of zircon and monazite during regional metamorphism is important for identifying the nature and timescales of tectonic processes in continental crust we present extensive in situ u pb monazite geochronology and u pb zircon depth profiling analyses from the mid crust exposed on southeastern baffin island canada in the internal zone of the paleoproterozoic trans hudson orogen in situ la icp ms u pb monazite dating on ten metasedimentary rocks totalling 300 analyses from different paleodepths reveals a dominant ca 1860 1820 ma monazite population that records regional mid amphibolite to granulite facies metamorphism with coeval fabric development subordinate monazite with ages between ca 1800 1750 ma are interpreted to have formed through dissolution reprecipitation the sims u pb depth profiling technique allowed dating of ca 1861 1833 ma zircon rims and less abundant ca 1800 1764 ma zircon rims that would be unresolvable using conventional spot dating trace element depth profiling via la icp ms shows that zircon rims typically have lower th u values higher hf contents and different ree compositions than zircon interiors zircon rim compositions are highly variable with no apparent link to u pb age and are interpreted as the combined result of metamorphic zircon growth and dissolution reprecipitation our data demonstrate that the timing of peak metamorphism is recorded by both monazite and zircon and that they are both sensitive to u pb age resetting at lower temperatures during late orogenic tectonism regional mid amphibolite to granulite facies metamorphism and east directed crustal shortening are bracketed between ca 1860 1820 ma these events are interpreted to result from the accretion of cratonic blocks in the internal zone of the trans hudson orogen the integrated geochronological approach on regionally distributed samples enabled us to document ca 1800 1750 ma monazite domains and zircon rims which we interpret to result from the terminal collision with the lower plate superior craton followed by late to post orogenic tectonothermal activity that continued for at least 70 myr after thermal peak this prolonged history is characteristic of the northeastern trans hudson orogen and implies that orogenic collapse may not be a hallmark of all hot thickened orogens

the effect of turbulent flow structures on saltation sand transport in the atmospheric boundary layer the effect of turbulent flow structures on saltation sand transport was studied during two convective storms in niger west africa continuous synchronous measurements of saltation fluxes and turbulent velocity fluctuations were made with a sampling frequency of 1 hz the shear stress production was determined from the vertical and streamwise velocity fluctuations the greatest stress bearing events were classified as turbulent structures with sweep ejection inward interaction and outward interaction described according to the quadrant technique the classified turbulent structures accounted for 63 5 per cent of the average shear stress during the first storm and 56 0 per cent during the second storm the percentage of active time was only 20 6 per cent and 15 8 per cent respectively high saltation fluxes were associated with sweeps and outward interactions these two structures contribute positively sweeps and negatively outward interactions to the shear stress but have in common that the streamwise velocity component is higher than average therefore the horizontal drag force seems primarily responsible for saltation sand transport and not the shear stress this was also reflected by the low correlation coefficients r between shear stress and saltation flux 0 12 and 0 14 respectively while the correlation coefficients between the streamwise velocity component and saltation flux were much higher 0 65 and 0 57 respectively

mg ca variation in planktonic foraminifera tests implications for reconstructing palaeo seawater temperature and habitat migration the nature of compositional variability within the tiny calcitic shells tests that are precipitated by planktonic foraminifera has been investigated using laser ablation inductively coupled plasma mass spectrometry la icp ms systematic large and correlated variation of mg ca mn ca ba ca and zn ca but relatively uniform sr ca are observed through the test walls of analysed species globigerinoides sacculifer globigerinoides ruber neogloboquadrina pachyderma and neogloboquadrina dutertrei distinct chamber and chamber wall layer compositions can be resolved within individual tests and mg ca compositional differences observed in sequentially precipitated test components of the different species analysed are consistent with seawater temperature changes occurring with habitat migration during their adult life cycle stages estimated test calcification temperatures are in keeping with available seawater temperature constraints indicating the potential for accurate seawater temperature reconstruction using la icp ms mg rich 1 6 mol mg surface veneers that are also enriched in mn ba and zn have been found on all species and all fossil tests as well as on live sampled tests of g ruber with the latter suggesting a possible biogenic origin these mg rich surfaces bias bulk test compositions toward higher mg ca values by between 5 and 35

zircon reaction and stability of the u pb isotope system during interaction with carbonate fluid experimental hydrothermal study the interpretation of metamorphically induced u pb isotopic discordance requires a thorough understanding of zircon fluid interactions with this aim we have studied the behaviour of metamict and crystalline zircon phases and their u pb systems by cathodoluminescence after treatment by 2m na2co3 solution at t 200 800 c and p 1 5 kbar for 3 14 days x ray diffraction microprobe and isotope dilution analysis the data indicate that zircon transformation under hydrothermal conditions depends on the experimental conditions and the degree of structural damage reconstitution of defective and impurity enriched zones of metamict zircon homogenization of impure element concentrations and increase of crystallinity was observed at 400 c and p 1 kbar considerable lead and uranium loss occurred under these conditions as a result of zircon dissolution newly formed baddeleyite accommodating u from 2m na2co3 solution and zr na silicate were recognized this process intensified with increasing pressure study of crystalline zircon indicates that migration of u and pb took place only during dissolution of zircon at t above 650 c in the presence of carbonate ions essential u and pb amounts are lost from metamict zircon at a lower p t than is typical for greenschist facies metamorphism

u pb isotopic behaviour of zircon during upper amphibolite facies fluid infiltration in the napier complex east antarctica understanding the factors that contribute to u pb discordance in zircon is essential for interpreting isotopic data and for assessing the validity of concordia intercept ages modification caused by interaction with metamorphic fluids is often cited as a primary means by which significant or even complete isotopic resetting of u pb systematics in zircon might be achieved under subsolidus conditions we present a field example from the napier complex east antarctica in which a palaeoproterozoic 2450 70 ma zircon population interacted locally with an early palaeozoic 498 1 7 ma aqueous fluid at upper amphibolite facies conditions conventional ion microprobe analysis of sectioned and polished grain surfaces indicates that fluid interaction resulted in minor disturbance of u and pb in zircons both normal and reverse discordance with limited displacement along a chord with a lower intercept that coincides with the timing of fluid infiltration in contrast ion probe drilling or depth profiling into unpolished natural zircon crystal surfaces revealed extensive disturbance of u pb systematics to depths of 0 2 m with near surface ages consistent with the timing of fluid influx at 498 ma although zircon underwent some radiogenic pb redistribution during fluid interaction infiltrating fluids resulted in minimal grain scale isotopic modification of zircon based on ion probe depth profiling results we propose that limited normal discordance observed in the conventional ion microprobe zircon analyses in this case is controlled by an analytical mixture of reset and or recrystallised zircon along penetrative micro fracture networks with that of adjacent unaffected zircon we also suggest that the observed reverse discordance is genuine resulting from localised intra grain net accumulations of radiogenic pb we conclude that the isotopic response of zircon in this case is controlled by the interaction of an aqueous metamorphic fluid of low to moderate salinity resulting in sub micrometre depth scale isotopic modification at natural crystal faces and along penetrative micro fracture networks and that grain scale isotopic modification was negligible therefore we urge caution when considering regional chronological interpretations that appeal to significant zircon isotopic resetting caused exclusively by metamorphic fluid interaction at upper amphibolite facies conditions

cambrian granitoids in pre alpine basement of crete greece evidence from u pb dating of zircon the pre alpine basement of eastern crete consists of at least three sub complexes which show differences in age and grade of pre alpine metamorphism u pb analyses of zircons of two orthogneisses yielded upper intercepts in the concordia diagram at 511 16 ma chamezi crystalline complex and 514 14 ma myrsini crystalline complex these values are interpreted as protolith ages of the gneisses suggesting that parts of the basement underwent cadomian pan african imprints at the northern margin of gondwana the lower intercept ages are attributed to alpine subduction which led to high pressure low temperature metamorphism t ca 300 c brittle ductile deformation and hydrothermal fluid activity due to pervasive alpine fluid migration the zircons of the chamezi crystalline complex display disturbed zonation and solution resulting in 87 radiogenic pb loss ca and al gain and zr depletion

unscrambling the stratigraphy of an archean greenstone belt a upb geochronological study of the favourable lake belt northwestern ontario canada abstract new zircon and baddeleyite u pb ages show that the major units of the archean favourable lake greenstone belt in the northern superior province were formed by episodic magmatism spanning more than 250 ma and demonstrate that the original stratigraphy is disrupted by thrusts that juxtaposed older supracrustal rocks on top of younger ones the oldest rocks of the area are a 2950 ma gneissic tonalite and a 3000 2960 ma granodiorite clast from a ca 2725 ma conglomerate five distinct volcanic and sedimentary groups formed during presumably short lived episodes at 2925 ma i 2870 ma ii 2858 ma iii 2734 ma iv and 2725 ma v the youngest group contains the thickest sedimentary unit a turbiditic and alluvial fluvial sequence compression caused thrusting that placed groups i and v on top of iv iii on v and ii on iii the thrust complex was subsequently isoclinally folded compression was accompanied by major plutonism that emplaced the bulk of the bounding batholiths between 2732 and 2711 ma ago the late tectonic mo mineralized setting net lake stock in the centre of the belt has an age of about 2708 ma and was overprinted by younger hydrothermal events that produced monazite 2706 ma titanite ca 2695 2690 ma and rutile the youngest rutile at 2657 ma similar late hydrothermal pulses are recorded by secondary titanite elsewhere in the belt and within the batholiths the protracted magmatic evolution of the belt is typical of that observed in a number of greenstone belts of the northern superior province but is uncommon farther to the south in contrast the structural complications and put of sequence stratigraphy appear to be a quite common tectonic characteristic of greenstone belts of the whole superior province

formation mechanisms of illite chlorite and mixed layer illite chlorite in triassic volcanogenic sediments from the southland syncline new zealand clay minerals from the three principal kinds of zeolitic sediments from the type area for zeolite facies alteration the triassic murihiku supergroup southland new zealand have been studied by tem bentonitic tuff consists largely of smectite and heulandite with minor illite they occur as replacements of glass shards and are inferred to be direct alteration products of tuff alteration both analcime and laumontite rich tuffs contain chlorite illite and mixed layer illite chlorite including 1 1 mixed layer sequences subhedral to euhedral phyllosilicate crystal shapes and other textural features imply that phyllosilicates crystallized from solution derived in part by dissolution of precursor smectite intralayer transitions involving illite and chlorite are inferred to be products of crystallization rather than direct alteration and replacement petrographically similar bentonitic and analcimized tuffs overlap each other in the stratigraphic section supporting earlier observations that there is no systematic change in smectite relative to the illite plus chlorite derived from smectite in sections up to 8 5 km thick the data imply that smectite may be metastable relative to illite plus chlorite permeability and fluid chemistry are inferred to be as significant as temperature in promoting reactions in clay minerals as well as zeolites during burial metamorphism

archaean quartz arenites in the canadian shield examples from the superior and churchill provinces abstract units of remarkably pure archaean quartz arenite occur in the northwestern part of the superior province and in the northern terrane of the western churchill province rae province of the canadian shield in the superior province silica cemented quartz arenites of archaean age are well preserved in several greenstone belts the example from the keeyask lake sedimentary assemblage displays tabular planar and trough cross beds ripple marks reactivation surfaces and pebble lag deposits in spite of penetrative deformation and greenschist grade metamorphism primary textures are extremely well preserved showing framework grains to be very well rounded and sorted the succession of keeyask lake quartz arenite beds is overlain by siltstones containing small scale stratiform domal and columnar stromatolites a shallow marine environment of deposition is inferred detrital heavy minerals include pyrite magnetite zircon tourmaline apatite sphene and topaz in the northern part of the western churchill province rae province archaean quartz arenites occur in northeasterly trending belts where intense structural deformation has in most places obscured or obliterated primary textures and structures this has led to speculation that some of these units are metachert or recrystallized vein quartz but local preservation of primary textures and structures provides clear evidence of epiclastic origin in the example described herein quartz arenites of the woodburn lake group display sparse occurrences of trough and tabular planar cross beds channels ripple marks and pebble lag deposits probable environments of deposition for these quartz arenites include fluvial systems and shallow marine shelf settings the occurrence of unequivocal quartz arenite clasts in beds of intercalated conglomerate provides direct evidence of at least two episodes of accumulation of almost pure quartz sand thin sections and polished slabs reveal frameworks of clastic quartz grains with little to no matrix now mainly muscovite and rare detrital grains of accessory heavy minerals predominantly zircon and opaque iron oxides pyrite and other sulphides have been introduced along fractures but some intergranular sulphide grains may be of detrital origin the principal source for the quartz arenites in both areas must have been quartz rich granitoid rocks conditions of intense chemical weathering are indicated the widespread occurrence of extremely mature quartz arenites throughout archaean terranes of the canadian shield and in other shields of the world are suggestive of crustal stability during early earth history the association of quartz arenites and ultramafic rocks uncharacteristic of younger terranes is now recognized in many archaean greenstone belts of the canadian shield

nature of mixed layer clays and mechanisms of their formation and alteration abstract mixed layer clay minerals are intermediate products of reactions involving pure end member clays they come from natural environments ranging from surface to low grade metamorphic and hydrothermal conditions most often mixed layering is essentially two component but more complicated interstratifications have also been documented variable tendency to form regular 1 1 interstratifications has been observed and explanations of this phenomenon have been proposed mixed layer clays are either di or trioctahedral di trioctahedral interstratifications are rare most mixed layer clays contain smectite or vermiculte as a swelling component exceptions are all trioctahedral serpentine chlorite in low temperature environments and mica chlorite and talc chlorite at high temperatures solid state transformation and dissolution crystallization are the two mechanisms responsible for the formation of different mixed layer clays in general the weathering reactions that produce mixed layering are revers

transport of uranium thorium and lead in metamict zircon under low temperature hydrothermal conditions abstract understanding the stability of radiation damaged zircon under low temperature hydrothermal conditions is crucial to the application of zircon for u pb geochronology and as a host phase for the disposal of plutonium waste we report the results of an investigation of the stability of partially metamict zircon by leaching experiments at 175 c with a 2 m alcl 3 and a 1 m hcl cacl 2 solution as hydrothermal fluids for 1340 h cathodoluminescence cl and backscattered electron bse images show that the zircon grains have developed a reaction rim several micrometers thick or deeply penetrating reticulated alteration zones with sharp boundaries to unaltered metamict zircon these zones have experienced severe loss of si u th and pb and gain of al or ca and a water species as revealed by electron microprobe sensitive high resolution ion microprobe shrimp analyses and infrared spectroscopy micro raman and infrared measurements on the altered areas show that disordered crystalline remnants of the partially metamict zircon structure were partially recovered whereas recrystallization of the embedding amorphous phase was not observed no detectable structural or chemical changes were detected inside the unaltered areas intensive fracturing which was most intense in the hcl cacl 2 experiment occurred inside the altered areas due to the volume reduction associated with the recovery of the disordered crystalline material and probably with the leaching reactions we explain the formation of deep penetrating alteration zones by a percolation type diffusion model which is based on the assumption that percolating interfaces or areas of low atomic density between crystalline and amorphous regions as well as between amorphous domains exist along which fast chemical transport is possible the idea of the existence of fast diffusion interfaces is supported by the sharp chemical gradients at the margin to unreacted zircon the model was used to estimate for the first time diffusion coefficients for u th and pb diffusion in amorphous zircon at 175 c by assuming that volume diffusion inside the amorphous domains is the loss rate limiting process these estimates show that volume diffusion in metamict zircon can cause significant loss of u th and especially loss of radiogenic pb over geological time scales even at temperatures as low as 175 c our results show that recent pb loss discordias can be generated 1 by predominate pb loss from metamict zircon through volume diffusion at low temperatures where thermal healing of the structure is insignificant and 2 by leaching of pb and u and th from metamict zircon through an external fluid

zircon u pb and trace element zoning characteristics in an anatectic granulite domain insights from lass icp ms depth profiling abstract understanding the geochemical characteristics of metamorphic zircon and how they may be modified by recrystallization processes is fundamental to defining the timescales of tectonic processes affecting continental lithosphere we utilize laser ablation split stream lass icp ms depth profiling analysis to obtain a continuous rim to core record of the u pb ages and trace element composition preserved within variably recrystallized zircon from different rock types within a well studied granulite domain in the western grenville province canada detailed analysis of the depth resolved signal enables definition of chemically distinct homogeneous internal domains and heterogeneous intervening zones that can generally be correlated with textural features observed in cl three age populations have been distinguished within the 35 m deep profiles that correlate well with the established timing of protolith formation granulite facies metamorphism and amphibolite facies shearing respectively the u pb isotopic system and th u ratios in much of the crystal interiors have undergone considerable modification as evidenced by a linear correlation between 207pb 206pb age and th u ratio interior and rim domains commonly contain blurred or faded oscillatory zoning patterns suggesting that solid state recrystallization is at least partially responsible for the modified u th pb composition a number of systematic trends in trace element composition are also observed between interior domains and recrystallized rims including 1 decreased th u to 0 1 2 tighter clustering of hf concentrations 3 decreased total ree 4 unchanged eu anomalies and 5 a widened spread of hree enrichment values ybn gdn both ybn gdn vs th u and u ce vs th plots show increasing degree of compositional differentiation from protolith zircon as a function of metamorphic reworking processes i e sample type the transition zones between interior and rim domains exhibit textural features consistent with solid state recrystallization and migration of a transgressive recrystallization front the main compositional characteristics of the transition zones are 1 spikes in u and ta 2 steady to abrupt increases in hf toward maximum values in the rims 3 steady to abrupt decreases in th toward minimum values in the rims and 4 variable ree patterns that include spikes troughs and montonic decreases most of these characteristics are consistent with the purging of larger ions from the crystal lattice during recrystallization with local bands of high concentration associated with recrystallization fronts

experimental hydrothermal alteration of partially metamict zircon we present the results of a series of hydrothermal experiments on grains from two partially metamict zircon samples from sri lanka in the temperature range 350 to 650 c and with different solutions 2 m alcl 3 2 m cacl 2 pure h 2 o and a multi cation solution under these conditions sharply bounded reaction fronts penetrated into the zircon grains and developed complex lobate and rim structures that resemble structures found in natural zircon systems the reaction zones are characterized by a marked increase in the cathodoluminescence intensity a decrease of the back scattered electron emission and an increased degree of structural order as revealed by micro raman and infrared spectroscopy sensitive high resolution ion microprobe and electron microprobe measurements revealed that the altered areas gained solvent cations e g ca 2 ba 2 mg 2 al 3 from the solution and lost variable amounts of zr si hf the ree u th as well as radiogenic pb a comparison between dry and hydrothermal annealing trends shows that the kinetics of structural recovery including recrystallization of the amorphous phase in metamict zircon is strongly enhanced under hydrothermal conditions this finding suggests that water catalyzes structural recovery processes in metamict zircon we found that the structure of the reacted areas does not resemble that of crystalline zircon i e is still characterized by a temperature dependent degree of disorder which would not be expected if the reaction is controlled by a coupled dissolution and re precipitation process instead the alteration process can be described best by a diffusion reaction recrystallization model in this model it is postulated that the diffusion of water into the metamict structure is the driving force for moving recrystallization fronts we found that the rate and the extent of solid state recrystallization of the amorphous phase is an important factor in determining the mobility of trace elements this interpretation is indicated by the observation that trace elements including u and th were preferentially lost during the reaction with a fluid at low temperatures where recrystallization of the amorphous material was slow or not activated at all the observed chemical alteration patterns are believed to reflect a competition between the kinetics of long range diffusion and ion exchange and the kinetics of the short range diffusion necessary for the recrystallization process

linking growth episodes of zircon and metamorphic textures to zircon chemistry an example from the ultrahigh temperature granulites of rogaland sw norway in situ u th pb analyses by ion microprobe on zircon in intact textural relationships are combined with backscatter and cathodoluminescence imaging and trace element analyses to provide evidence for growth episodes of zircon this approach helps a to unravel the polymetamorphic history of aluminous migmatitic and granitoid gneisses of the regional contact aureole around the rogaland anorthosite norite intrusive complex and b to constrain the age of m 2 ultrahigh temperature uht metamorphism and the subsequent retrograde m 3 event all samples yield magmatic inherited zircon of c 1035 ma some an additional group at c 1050 ma this suggests that loss of pb by volume diffusion in non metamict zircon is not an important factor even under extreme crustal conditions furthermore the identical inheritance patterns in aluminous garnet cordierite osumilite bearing migmatites and orthogneisses indicate a metasomatic igneous instead of a sedimentary protolith for the migmatite results for the m 1 metamorphic event at c 1000 ma bp are consistent in all samples including those from outside the orthopyroxene in isograd the latter do not show evidence for zircon growth during the m 2 metamorphic episode zircon intergrown with or included within m 2 metamorphic minerals magnetite spinel orthopyroxene give an age of 927 7 ma 2 n 20 the youngest observed results are found in zircon outside m 2 minerals some overgrown by m 3 mineral assemblages late garnet coronas garnet quartz and orthopyroxene garnet symplectites and yield a slightly younger pooled age of 908 9 ma 2 n 6 these textures are relative time markers for the crystallization of zircon overgrowths during discrete stages of the uht event these youngest age groups are consistent with the emplacement age of the rogaland intrusive complex and the last magmatic activity tellnes dyke intrusion respectively this is direct and conclusive evidence for uht metamorphism in the regional aureole being caused by the intrusions and corrects earlier notions that the events are not linked trace element behaviour of zircon tb u and y content has been tracked through time in the samples and shows variations both within and between samples this heterogeneous behaviour at all scales appears to be common in metamorphic rocks and precludes the use of rules of thumb in the interpretation of zircon chemistry but chemical tracers are useful for recognition of zircon growth or recrystallization during metamorphism

a simulation model of blowout development blowouts are the main feature of wind erosion in the dunes along the dutch coast a number of characteristics such as their growth against the prevailing wind and the spontaneous stabilization when certain dimensions are reached are believed to be caused by a prominent quality of the southwest wind viz its gustiness this hypothesis is tested in a two dimensional computer model which simulates the effect of wind gusts on a sandy surface along the main axis of the blowout a number of constraints based on field observations are built into the model the results closely resemble the longitudinal section of the blowout

aeolian sand transport an optimized model consideration of the available equations for prediction of aeolian sand transport led to the development of a model which relies on optimization techniques for the production of improved performance the model has been applied to the characterization of dunesand movement in coastal environments in eastern australia

u pb zircon ages for magmatism in the red lake greenstone belt northwestern ontario u pb dating was carried out on nine volcanic rocks and two felsic intrusions from the red lake greenstone belt in order to establish an absolute time framework for the magmatic evolution of the area and yield first indications on the time of deformation and gold mineralization the data indicate a protracted period of igneous activity spanning at least 270 ma felsic volcanic rocks near the top of the tholeiitic to komatiitic sequence in the eastern part of the belt yield ages of and a third unit dated at contains inherited zircons older than 2982 ma which casts some uncertainty on the validity of the inferred intercept age rocks in the western part of the belt previously believed to form a relatively young calc alkalic sequence but now known to be dominantly tholeiitic are shown to be relatively old with ages of and these two dates also bracket the age of stromatolites occurring in chemical sediments that are under and overlain by the dated units another volcanic horizon in the centre of the b

monazite th pb age depth profiling the significant capabilities of the ion microprobe for thermochronometric investigations of geologic materials remain largely unexploited whereas sup 208 pb sup 232 th spot analysis allows approximately 10 mm scale imaging of pb loss profiles or overgrowths in sectioned monazite grains the spatial resolution offered by depth profiling into the surface region of natural crystals is more than two orders of magnitude higher the authors document here the ability of the high resolution ion microprobe to detect sup 208 pb sup 232 th age differences of 1 m y with better than 0 05 micro m depth resolution in the outer micron of tertiary monazites from the hanging wall of the himalayan main central thrust age gradients on this scale are inaccessible to ion microprobe spot analysis or conventional thermal ionization mass spectrometry interpretation of the near surface sup 208 pb distributions with available monazite pb diffusion data illustrates the potential of the approach for recovering continuous high temperature thermal history information not previously available

kulkeite a new metamorphic phyllosilicate mineral ordered 1 1 chlorite talc mixed layer kulkeite occurs as platy colorless porphyroblastic single crystals up to 2 mm in size in a low grade dolomite rock associated with a triassic meta evaporite series at derrag tell atlas algeria it is associated with sodian aluminian talc unusual chlorite polytypes and both k and na phlogopite kulkeite is optically biaxial negative nx 1 552 ny 1 5605 nz 1 5610 2vz 24 obs based on microprobe analysis the empirical formula is na0 38k0 01ca0 01 mg8 02al0 99 al1 43si6 57o20 oh 10 with some variation in na si and tetrahedral al the crystals are monoclinic with a 5 319 1 b 9 195 2 c 23 897 10 a 97 1 3 z 2 the calculated density is 2 70 g cm 3 the four strongest lines in the x ray powder pattern are d i hkl 7 90 100 003 1 533 100 060 7 42 80 002 3 38 80 007 the 001 reflection with 23 7 a has intensity 10

fluid channelling during ductile shearing transformation of granodiorite into aluminous schist in the tauern window eastern alps ductile shearing in the core of the tauern window austria transformed metagranodiorite into si undersaturated garnet chlorite staurolite schist at a depth of c 35 40 km during the alpine orogeny four distinct zones have been recognized extending from the wallrock into the centre of the shear zone zone i unaltered metagranodiorite with subordinate amphibolite zone ii biotite white mica garnet schist zone iii biotite phengite schist zone iv quartz absent garnet chlorite staurolite schist with garnets up to 10 cm across whole rock analyses show a dramatic decrease in sio2 from 65 wt in zone i to 35 wt in zone iv ca na and sr also decrease across the shear zone whereas al ti fe mg p cr ni zn and rb all increase towards zone iv mass balance calculations indicate that shearing was accompanied by up to 60 volume loss near the centre of the shear zone comparison of the tauern window samples with other shear zones in granitic hosts indicates that silica loss accompanied by gains in mg fe and ti is typical for volume loss shear zones but is distinctly different from the element behaviour exhibited in shear zones that are thought to represent approximately isovolumetric behaviour in the samples studied here volume loss appears to have resulted from channellized fluid flow during shearing producing time integrated fluid fluxes of 108 cm3 cm 2 in zone iv this large volume of fluid may have originated in part from dehydration of flysch carried beneath the metagranodiorites during eocene movement on the north penninic subduction zone development of an inverted thermal gradient during subduction would have allowed the fluid to scavenge large amounts of silica from the shear zone during ascent and heating

timing of gold mineralization at red lake northwestern ontario canada new constraints from u pb geochronology at the goldcorp high grade zone red lake mine and the madsen mine the goldcorp high grade zone at the red lake mine has reserves proven and probable established at 1 775 million tons mt at an average grade of 80 6 g t au new u pb geochronologic data combined with detailed mapping and crosscutting relationships provide timing constraints and new insights into the formation of the exceptionally rich goldcorp high grade zone the results show that the main stage of the high grade mineralization in the high grade zone formed before 2712 ma and that a second stage of gold mineralization much smaller in terms of total gold content but spectacular in terms of grade formed after 2702 ma this second stage is attributed to gold remobilization caused by the enhanced thermal gradient and deformation associated with the emplacement of the cat island pluton 2699 ma approximately 7 km east of the deposit or by the ca 2 63 to 2 66 ga postorogenic regional thermal event indicated by hornblende muscovite and biotite ar ar cooling ages from the uchi subprovince it is proposed that the main stage of high grade mineralization formed between ca 2723 to 2712 ma possibly synchronous with emplacement of the dome and mckenzie island stocks the abino granodiorite and hammell lake batholith as well as with penetrative d2 main stage regional deformation lamprophyre dikes spatially associated with the deposit postdate main stage mineralization by at least 10 m y a postore granodiorite dike from the madsen mine the second largest deposit in the district was dated at 2698 1 ma this age is similar to a postalteration and late deformation diorite dike at the creek zone showing near the former starratt olsen mine which was dated at 2696 2 ma the crosscutting relationships confirm that the main stage of mineralization at the madsen mine is pre 2698 ma a folded and metamorphosed polymictic conglomerate located stratigraphically above the campbell red lake deposit and deposited after ca 2747 but before 2712 ma is correlated with the newly recognized huston assemblage crosscutting and overprinting relationships between the conglomerate and the alteration indicate protracted multistage aluminous and iron carbonate quartz veining and or alteration event s before and after deposition of the conglomerate the age of the conglomerate confirms that the campbell red lake deposit is proximal to a folded regional unconformity consistent with the empirical relationship between large gold deposits and unconformities elsewhere in archean greenstone belts this paleosurface represents a key first order exploration target and it is proposed that areas of high potential for gold exploration in red lake occur in the balmer assemblage rocks within 500 m to 1 km of the unconformity

a continuously weighing high frequency sand trap wind tunnel and field evaluations abstract a new continuously weighing high frequency sand trap cwhf has been designed its sampling efficiency is evaluated in a wind tunnel and the potential of the new trap has been demonstrated in field trials the newly designed sand trap allows fully automated and high frequency measurement of sediment fluxes over extensive periods we show that it can capture the variations and structures of wind driven sand transport processes and horizontal sediment flux and reveal the relationships between sand transport and meteorological parameters its maximum sampling frequency can reach 10 hz wind tunnel tests indicated that the sampling efficiency of the cwhf sand trap varies between 39 2 to 64 3 with an average of 52 5 it achieved a maximum sampling efficiency of 64 3 at a wind speed of 10 m s 1 this is largely achieved by the inclusion of a vent hole which leads to a higher sampling efficiency than that of a step like sand trap at high wind speeds in field experiments we show a good agreement between the mass of sediment from the cwhf sand trap the wind speed at 2 m and the number of saltating particles at 5 cm above the ground surface according to analysis of the horizontal sediment flux at four heights from the cwhf sand trap 25 35 50 and 100 cm the vertical distribution of the horizontal sediment flux up to a height of 100 cm above the sand surface follows an exponential function our field experiments show that the new instrument can capture more detailed information on sediment transport with much reduced labor requirement therefore it has great potential for application in wind blown sand monitoring and process studies

eocene tectonometamorphism on serifos western cyclades deduced from zircon depth profiling geochronology and mica thermochronology abstract phengite bearing metasedimentary sequences preserving relict glaucophane from serifos western aegean are indicative of high pressure conditions during cycladic blueschist metamorphism depth profiling and conventional sims u pb geochronology and o isotope analysis on zircon from highly strained orthogneisses at the base of the sequence reveal xenocrystic zircon with varying degrees of recrystallization one end member population has needle like or prismatic morphologies is internally oscillatory zoned with little to no rim material created during recrystallization these zircons yield triassic to carboniferous ages elevated th u and 18 o mostly ranging between 4 and 11 in marked contrast a separate zircon population has a spongy structure created by complete recrystallization of the pre existing crystal these zircons possess low th u and flat hree patterns and yield eocene ages with 18 o 7 chondrite normalized ree profiles are extremely depleted within this spectrum other zircons show variable states of recrystallization and mixed ages regression of u pb data from three mylonitic orthogneiss samples that exhibit near complete recrystallization and similar age depth profiles yields c 40 ma ages these new data are the first geochronological constraints that successfully demonstrate an eocene metamorphic event in the western cyclades and new 40 ar 39 ar phengitic mica ages of 38 32 ma presented here suggest the metamorphic wedge was exhumed into the shallow crust shortly thereafter

the effect of wind gusts moisture content and fetch length on sand transport on a beach this paper reports on an analysis of instantaneous sediment transport in relation to wind gusts moisture content and fetch length from a field study carried out at skallingen denmark wind speeds were measured with cup anemometers and instantaneous sediment transport with vertical traps coupled to electronic balances moisture content was measured gravimetrically using samples taken along a beach profile at intervals of 1 to 2 h sediment transport and wind speeds were sampled at 1 hz and a 5 s running mean was used to smooth the data from the trap because of limitations of the balance resolution three measures of the threshold of sediment motion the intermittency threshold minimum threshold and maximum threshold were explored using data from five runs carried out on october 26 2000 the intermittency threshold and maximum threshold were largely insensitive to wind speed but varied spatially along the fetch distance the minimum threshold increased significantly with increasing mean wind speed as a result of reduced time for drying of the surface layer examination of time series for instantaneous wind speed and sediment transport for a number of runs on october 26 and november 6 2000 showed close agreement between fluctuations in wind speed and fluctuations in sediment transport cross spectral analysis of instantaneous wind speed and sediment transport for one run on each day where there was continuous transport showed high coherence between the two variables over a range of frequencies from 0 2 hz to 0 01 hz plots of all non zero instantaneous sediment transport values versus wind speed show considerable scatter in general transport rates are highest for traps located further from the upwind non erodible boundary i e along the fetch best fit power relationships show r2 values generally ranging from 0 4 to 0 6 and exponents ranging from 4 5 to 6 94 i e much higher than the traditional value of 3 found in most transport equations traps located close to the upwind boundary had lower r2 values and exponents 2 5

neoarchean disaggregation and reassembly of the superior craton the southern and western superior craton of canada swsc is widely considered to be a tectonic collage accreted from north to south by multiple coeval subduction zones we propose an alternative non plate tectonic scenario where swsc continental fragments are not exotic but derived by partial disaggregation of a heterogeneous older superior i craton in response to a mantle overturn event that started at ca 2780 ma during overturn radial mantle outflow stretched and disaggregated the lithosphere to create the concentric neoarchean fabric of the superior craton the southern part of which swsc broke up into ribbon continents separated by oceanic tracts neoarchean calc alkaline magmas record interaction between plume related magma and older crust a change in the mantle flow field at ca 2720 ma caused southward drift of the northern superior cratonic block as a result of mantle traction on its lithospheric keel and swsc terranes accreted to its leading edge

corrensite and mixed layer chlorite corrensite in metabasalt from northern taiwan tem aem empa xrd and optical studies many chloritic minerals in low grade metamorphic or hydrothermally altered mafic rocks exhibit abnormal optical properties expand slightly upon glycolation expandable chlorite and or have excess alvi relative to aliv as well as significant ca k and na contents chloritic minerals with these properties fill vesicles and interstitial void space in low grade metabasalt from northern taiwan and have been studied with a combination of tem aem empa xrd and optical microscopy the chloritic minerals include corrensite which is an ordered 1 1 mixed layer chlorite smectite and expandable chlorite which is shown to be a mixed layer chlorite corrensite corrensite and some mixed layer chlorite corrensite occur as rims of vesicles and other cavities while later formed mixed layer chlorite corrensite occupies the vesicle cores the tem observations show that the mixed layer chlorite corrensite has ca 20 and the corrensite has ca 50 expandable smectite like layers consistent with xrd observations and with their abnormal optical properties the aem analyses show that high si and ca contents high alvi aliv and low fevi fe mg vi ratios of chlorites are correlated with interstratification of corrensite or smectite like layers in chlorite the aem analyses obtained from 200 500 a thick packets of nearly pure corrensite or chlorite layers always show that corrensite has low aliv siiv and low fevi fe mg vi while chlorite has high aliv siiv and high fevi fe mg vi this implies that the trioctahedral smectite like component of corrensite has significantly lower aliv siiv and fevi fe mg vi the ratios of fevi fe mg vi and aliv siiv thus decrease in the order chlorite corrensite smectite the proportions of corrensite or smectite like layers relative to chlorite layers in low grade rocks are inferred to be controlled principally by fe mg ratio in the fluid or the bulk rock and by temperature compositional variations of chlorites in low grade rocks which appear to correlate with temperature or metamorphic grade more likely reflect variable proportions of mixed layered components the assemblages of trioctahedral phyllosilicates tend to occur as intergrown discrete phases such as chlorite corrensite corrensite smectite or chlorite corrensite smectite a model for the corrensite crystal structure suggests that corrensite should be treated as a unique phase rather than as a 1 1 ordered mixed layer chlorite smectite

formation and behavior of aeolian streamers received 5 december 2004 revised 31 march 2005 accepted 9 june 2005 published 7 september 2005 1 field experiments were conducted to determine the characteristic spatial and temporal dimensions and behavior of aeolian streamers and to identify the processes responsible for their formation an instrument array that included anemometer towers piezo electric transport sensors safires and hot film probes was deployed to measure streamers and airflow dynamics on both a coastal dune and a desert sand mound in california aeolian transport occurs predominantly in the form of families of intertwining and bifurcating streamers while under intense wind forcing more complex patterns of nested streamers and clouds with embedded streamers develop the streamers display a characteristic width of approximately 0 2 m and an average lateral spacing of about 1 m these dimensions appear to be independent of mean airflow characteristics the observations and measurements of streamers under different environmental conditions suggest that bed surface control in the form of differentiation in moisture content grain size or microtopography is not a necessary condition for the formation of streamers the results show little correlation between possible streamwise vortices or burst sweep events and streamers it is proposed that streamers are generated by near surface gusts that originate from large eddies propagating to the ground from higher regions of the boundary layer these elongated and stretched eddies scrape across the surface and initiate saltation along their path this concept accounts for the characteristic size and spacing of streamers their rapid propagation and the fast response of saltation to wind speed fluctuations

u pb zircon ages in supracrustal and plutonic rocks north spirit lake area northwestern ontario the north spirit lake greenstone belt in the sachigo subprovince of the superior province comprises parts of three sequences of volcanic and sedimentary rocks the main youngest is separated from the older sequences by an unconformity to disconformity the belt is bounded by large granitoid batholiths and was metamorphosed under greenschist to locally hornblende cordierite facies u pb zircon dating was performed on volcanic sedimentary and plutonic rocks in order to establish an absolute chronology for the evolution of the area a tuff breccia in the lowermost supracrustal sequence is dated at 3023 2 ma zircons from a tuff in the middle sequence show complex u pb relationships although they do not allow a precise age determination the data suggest that the tuff formed sometime between 2950 and 2800 ma ago zircons from a quartz arenite also in the middle sequence yield a simple data pattern and define an age of suggesting derivation of the sediment from a uniform source of this age a zirco

zircons from syros cyclades greece recrystallization and mobilization of zircon during high pressure metamorphism zircons were studied from high pressure low temperature metamorphosed meta igneous lithologies from syros these rocks carry several zircon generations related to each other by dissolution reprecipitation processes one generation is pristine zircon that shows growth zoning relatively elevated contents of trivalent cations and high th u ratios the other end member is a skeletal zircon generation with negligible trivalent cation contents and low th u ratios 0 1 texturally between these two there is a range of zircon crystals with complex inclusion populations of y hree th phases and fluid inclusions showing variable progress of replacement recrystallization both end members yield distinct sensitive high resolution ion microprobe shrimp u pb ages the pristine generation has an age of 80 2 1 6ma from a metagabbro and 76 4 2 1ma from a meta plagiogranite dyke the skeletal low th u zircon generation yields an age of 52 4 0 8ma the older late cretaceous zircons are interpreted to date emplacement of the magmatic protoliths in a small segment of oceanic crust the younger eocene age however dates a zircon recrystallization event which possibly coincides with high solubility and mobility of high field strength elements in a high pressure aqueous fluid phase intergrowth relations between zircon and peak metamorphic garnet and excellent agreement of the u pb ages with white mica ar ar ages for the same samples support the conclusion that eocene is the true age of high pressure metamorphism on syros

grain scale variations in trace element composition of fluid altered zircon acasta gneiss complex northwestern canada sem back scattered electron images of zircon separates from a 2 9 ga granitic dyke from the acasta gneiss complex slave province reveal primary igneous oscillatory zoning which is transgressively overprinted by irregular alteration domains electron microprobe analyses show that altered zircon domains are depleted in zr and si relative to unaltered zircon while retaining a constant zr si ratio sims trace element analyses indicate that lree are preferentially enriched in the altered domains altered zircon contains elevated concentrations of ba up to 580 ppm ca up to 2 weight of the element and common pb 50 ppb the presence of lree ca ba and common pb in the zircons is interpreted to be the result of secondary alteration by an aqueous fluid and not by primary incorporation during crystallization we propose that the alteration related elements are located in amorphous zircon domains

insights into low to moderate temperature recrystallization of zircon unpolished crystal depth profile techniques and geochemical mapping abstract depth profiling techniques for u pb geochronology and ree geochemistry were applied to unpolished metasomatized archean zircon from metasedimentary rock samples within the north caribou greenstone belt western superior province canada samples were selected in order to investigate the isotopic and chemical effects of fluid interaction on zircon at temperatures between 300 and 700 c zircon rims are 207 pb 206 pb ages between ca 2788 2667 ma with two thirds of rim ages ranging between 2754 and 2735 ma which is 100 myr younger than primary crystallization interior ages these dates correspond to regional tectonothermal episodes including potassic alteration suggesting that metamorphism and regional hydrothermal fluid flow were synchronous similarly ree geochemical analyses on the outer margins of the same unpolished zircon reveal that rims are characterized by variable lree concentrations high hf and low th u ratios indicating that elements were mobile during fluid zircon interaction unpolished zircon crystal faces were also mapped with a la icp ms technique to expose their 2 dimensional trace element variability the patterns reveal a dendritic network or a patchy mosaic form which is perhaps promoted by the presence of fractures adjacent mineralogy or variations in the crystallinity of the zircon the integration of these microanalytical techniques into the study of zircon can provide insight into the timing of low to moderate temperature history of rocks that do not contain other common chronometers but have witnessed significant tectonothermal events

the significance of titanite and apatite u pb ages constraints for the post magmatic thermal hydrothermal evolution of a batholithic complex berens river area northwestern superior province canada abstract titanite apatite and in one case rutile from a suite of twenty seven samples representing the major phases of an extensive and well dated archean plutonic domain of the northwestern superior province have been dated by u pb to determine the temporal progress of thermal and hydrothermal events accompanying and following major plutonism and to assess the isotopic behaviour of these geochronometers most samples contain 1 dark brown titanite with 350 20 ppm u showing shapes and textural relationships indicative of a primary magmatic origin and yielding ages corresponding either to the time of magmatic crystallization or to that of secondary metamorphic or magmatic events known in the area 2 pale titanite with 30 2 ppm u showing textural evidence for a secondary origin by breakdown of biotite hornblende oxide or older titanite and yielding ages that in general post date the major plutonic and metamorphic events in the area 3 apatite containing 30 5 ppm u occasionally up to 150 ppm u showing igneous shapes and textural relationships but yielding post magmatic ages similar to those of coexisting pale titanite 4 rutile in one sample postdating the ages of coexisting titanite and apatite the common coexistence of two noncoeval titanite generations demonstrates that the often invoked mechanism of pb diffusion during slow cooling in this case has no general validity the distribution of primary dark titanite ages however could be explained by late closure to diffusion or alternatively as the result of resetting by the thermal or hydrothermal effects caused by younger plutonic activity the ages of the secondary pale titanites probably reflect crystallization during discrete retrogressive events promoted by moderate heating or enhanced fluid activity the correspondence between pale titanite and apatite ages suggests that the same processes also caused the isotopic resetting of apatite on a regional scale the data pattern permits the identification of domains characterized by differences in the length and intensity of the thermal and hydrothermal processes the most protracted activity is recorded in northern domains of the berens river area whereas the gold mineralized red lake area in the south underwent a very rapid decline of the thermal hydrothermal activity these data also highlight important differences between the red lake and other gold camps of the superior province such as hemlo and val d or where hydrothermal activity locally associated with mineralization occurred as much as 30 80 m y after the major orogenic developments

new high precision u pb ages for the island lake greenstone belt northwestern superior province implications for regional stratigraphy and the extent of the north caribou terrane a combined u pb and field mapping study of the island lake greenstone belt has led to the recognition of three distinct supracrustal assemblages these assemblages record magmatic episodes at 2897 2852 and 2744 ma voluminous plutonic rocks within the belt range in age from 2894 to 2730 ma with a concentration at 2744 ma u pb data also show that a regional fault that transects the belt the savage island shear zone is not a terrane bounding structure the youngest sedimentary group in the belt the island lake group has an unconformable relationship with older plutons sedimentation in this group is bracketed between 2712 and 2699 ma this group and others similar to it in the northwestern superior province is akin to timiskaming type sedimentary groups found throughout the superior province and in other archean cratons these data confirm that this belt experienced a complex geological history that spanned at least 200 million years which is typical of greenstone belts in this area age correlations between the island lake belt and other belts in the northwest superior province suggest the existence of a volcanic megasequence this evidence in combination with nd isotopic data indicates that the oxford stull domain and the munro lake island lake and north caribou terranes may have been part of a much larger reworked mesoarchean crustal block the north caribou superterrane it appears that the superior province was assembled by accretion of such large independent crustal blocks whose individual histories involved extended periods of autochthonous development

a field experiment on the role of small scale wind gustiness in aeolian sand transport estimates of aeolian sand transport generally use an average wind speed to assess the wind energy to determine if variation in small scale wind gustiness on the order of several seconds duration has a significant effect on sand movement detailed field measurements were made of wind speed and transport rate average wind speed and several gust parameters were derived from the wind data the results indicate that small scale gust information does not improve prediction of transport rates

single shot laser ablation split stream ss lass petrochronology deciphers multiple short duration metamorphic events single shot laser ablation split stream ss lass petrochronology offers spatial resolution of 1 m per surface analysis this enables the technique to interrogate metamorphic zircon overgrowths that are very thin or that preserve a composite age structure to demonstrate the advantages of ss lass the technique was applied to metamorphic zircon overgrowths in five rocks from the cordillera de la costa venezuela in these rocks ss lass was able to decipher discrete short duration 106 yr zircon growth events at c 33 0 c 28 3 c 23 0 and c 18 2 ma comparison with existing geo thermochronology suggests that the ss lass dates represent hydro thermal events that mark distinct episodes of tectonism affecting the northern margin of south america with an external error of 5 incorporated into isotope ratios as required to achieve satisfactory analytical precision ss lass dates for reference materials were accurate to within 1 5 of published values the ss lass technique is thus in favorable cases capable of providing 106 yr resolution for cenozoic rocks methods in geospeedometry which employ forward models to reproduce observed diffusion length scales have obtained anomalously short time scales of 104 106 yr for metamorphism the high spatial and temporal resolution of ss lass offers tremendous promise for investigating the veracity of these claims

spatial grain size sorting in eolian ripples and estimation of wind conditions on planetary surfaces application to meridiani planum mars the landscape seen by the mars exploration rover mer opportunity at meridiani planum is dominated by eolian wind blown ripples with concentrated surface lags of hematitic spherules and fragments these ripples exhibit profound spatial grain size sorting with well sorted coarse grained crests and poorly sorted generally finer grained troughs these ripples were the most common bed form encountered by opportunity in its traverse from eagle crater to endurance crater field measurements from white sands national monument new mexico show that such coarse grained ripples form by the different transport modes of coarse and fine grain fractions on the basis of our field study and simple theoretical and experimental considerations we show how surface deposits of coarse grained ripples can be used to place tight constraints on formative wind conditions on planetary surfaces activation of meridiani planum coarse grained ripples requires a wind velocity of 70 m s at a reference elevation of 1 m above the bed from images by the mars orbiter camera moc of reversing dust streaks we estimate that modern surface winds reach a velocity of at least 40 m s and hence may occasionally activate these ripples the presence of hematite at meridiani planum is ultimately related to formation of concretions during aqueous diagenesis in groundwater environments however the eolian concentration of these durable particles may have led to the recognition from orbit of this environmentally significant landing site

u pb geochronology and trace element composition of zircon from the horseshoe lake greenstone belt superior province canada implications for the tectonic and metamorphic history1 depth profile techniques for u pb geochronology and rare earth element ree geochemistry were conducted on unpolished archean zircon with metasomatic rims from metasedimentary rocks within the horseshoe lake greenstone belt western superior province canada these zircon crystals are shown to have isotopically distinct rims typically 100 million years younger than the cores the 207pb 206pb rim ages can be temporally correlated with regional magmatism and with a later potentially au bearing hydrothermal event synchronous with greenschist facies metamorphism and regional deformation notably the zircons do not record evidence of local ca 2741 2715 ma magmatism manifested by the emplacement of a quartz feldspar porphyry dike swarm laser ablation inductively coupled plasma mass

design and field test of a continuously weighing tipping bucket assembly for aeolian sand traps a new tipping bucket assembly t bass for aeolian sand traps has been designed and field tested with encouraging results it facilitates high frequency monitoring of sediment flux over extensive time periods and therefore offers improved performance over other continuously weighing mechanisms that are generally limited to either a small total load capacity or poor resolving ability the t bass is a modified version of the tipping bucket meteorological rain gauge mounted on a cantilever and pulley system linked to an electronic load cell as trapped sand accumulates in one of the buckets the increasing mass exerts a downward force on the cantilever arm which translates into a slight deflection of the thin beam element of the load cell the resulting voltage output is proportional to the load and the analogue signal may be monitored by a data acquisition system eventually the bucket fills to capacity and tips the sediment load is emptied into a reservoir container the other bucket of the bucket pair is positioned beneath the funnel and the system is automatically reset to zero load for continued measurement in this way a high frequency record of sediment accumulation is obtained field testing of five prototypes demonstrated that the t bass can be configured to yield i linear calibrations for conversion of voltage to gram weight with r2 values exceeding 0 99 ii weight resolution of 0 99 ii weight resolution of 0 1 g or better depending on load cell specifications and bucket capacity and iii detailed temporal information order of 1 s on sediment flux allowing investigation of its relation to attributes of the wind field suggested modifications may produce improved performance in future versions copyright 1998 john wiley sons ltd