u pb evidence for polymetamorphic history of huronian rocks within the grenville front tectonic zone east of sudbury ontario canada abstract this paper presents u pb geochronological data for zircon titanite monazite apatite allanite and rutile from street township east of sudbury that help unravel two main facets of the geology of the grenville front tectonic zone gftz they document the presence of extensive huronian supergroup magmatic ca 2480 2460 ma and sedimentary rocks inside the gftz and highlight the significance of bimodal magmatism in the genesis of the huronian supergroup the data also provide further insights into the complex orogenic evolution of the gftz showing in particular that amphibolite facies metamorphism migmatization and deformation during grenvillian compression at 995 980 ma overprinted but did not totally erase the memory of earlier metamorphic events at ca 1720 and 1470 ma from the point of view of u pb systematics the results emphasize the nearly ubiquitous state of isotopic disequilibrium in these rocks most of the minerals investigated comprise two or more generations and are generally in a state of partial resetting because of the superimposed effects of younger events the preservation of pre grenvillian ages in titanite and allanite demonstrates that processes related to final grenvillian collision and exhumation occurred at great speed only apatite and to some degree rutile yield ages that are clearly post tectonic thereby reflecting the final stages of exhumation and cooling of the region

cross cutting moraines reveal evidence for north atlantic influence on glaciers in the tropical andes surface exposure dating of boulders on an exceptionally well preserved sequence of moraines in the peruvian andes reveals the most detailed record of glaciation heretofore recognised in the region the high degree of moraine preservation resulted from dramatic changes in the flow path of piedmont palaeoglaciers at the southern end of the cordillera blanca 10 00 s 77 16 w which in turn generated a series of cross cutting moraines sixty 10be surface exposure ages indicate at least four episodes of palaeoglacier stabilisation 65 ca 65 ca 32 and ca 18 15 ka and several minor advances or stillstands on the western side of the nevado jeulla rajo massif the absence of ages close to the global last glacial maximum ca 21 ka suggests that if an advance culminated at that time any resulting moraines were subsequently overridden the timing of expanded ice cover in the central peruvian andes correlates broadly with the timing of massive iceberg discharge heinrich events in the north atlantic ocean suggesting a possible causal connection between southward migration of the intertropical convergence zone during heinrich events and a resultant increase in precipitation in the tropical andes copyright 2010 john wiley sons ltd

devonian extension in northwestern newfoundland 40ar 39ar and u pb data from the ming s bight area baie verte peninsula abstract the ming s bight group of northwestern newfoundland an outlier of humber zone continental margin rocks is entirely surrounded by ophiolitic rocks of the dunnage zone structures in the ming s bight group and adjacent units record three main phases of deformation the earliest structures relate to silurian sinistral transpression previously documented in the region two later phases of extensional deformation produced a series of dextral oblique normal shear zones and faults that now separate the ming s bight group in the footwall from ophiolitic and granitoid rocks in the hangingwall 40ar 39ar and u pb data constrain the times of oblique normal shear and cooling metagabbro in the point rousse ophiolite complex which lies in the hangingwall preserves disturbed ordovician hornblende 40ar 39ar ages whereas adjacent shear zones record devonian ages hornblendes in pacquet harbour group amphibolites within extensional shear zones mainly record 40ar 39ar ages of 390 380 ma synkinematic titanite

magmatism and metamorphism in a grenvillian fragment u pb and 40ar 39ar ages from the blair river complex northern cape breton island nova scotia canada the blair river complex in northern cape breton island nova scotia is interpreted to be an exposure of grenvillian basement which formed the southeastern tip of a promontory on the proto atlantic continental margin of north america u pb zircon data reported here confirm grenvillian ages for the major components in the blair river complex the locally granulite facies sailor brook gneiss has a minimum protolith age of 1217 ma and was metamorphosed at 1035 12 10 ma the lowland brook syenite has an igneous crystallization age of 1080 5 3 ma and the otter brook granitic orthogneiss yielded an igneous crystallization age of 978 6 5 ma an igneous crystallization age could not be obtained for anorthositic rocks in the blair river complex but the red river anorthosite suite was metamorphosed at 996 6 5 ma silurian thermal activity is recorded in the blair river complex by granitic magmatism dated at 435 7 3 ma and a widespread amphibolite facies metamorphic overprint of similar age these events are corroborated by 206pb 238u ages of ca 423 ma for titanite from seven widely distributed samples from both silurian and proterozoic units interpreted to represent cooling from igneous or peak metamorphic conditions through temperatures of ca 550 600 c rutile in one sample gave a u pb age of 410 2 ma which may represent postmetamorphic cooling through 405 c the u pb cooling ages are supported by similar 40ar 39ar ages of ca 420 ma for phlogopite and muscovite from rare calc silicate lenses in the blair river complex and for hornblende from a silurian diorite intrusion the widespread silurian metamorphism of this inlier is similar in age to that in western newfoundland and indicates involvement of the blair river complex in silurian orogenesis our data support the interpretation that the blair river complex represents laurentian grenvillian basement analogous to basement exposures in the humber zone of western newfoundland

determining accurate temperature time paths from u pb thermochronology an example from the kaapvaal craton southern africa thermochronology has revolutionized our understanding of the establishment and evolution of lithospheric thermal structure however many potential benefits provided by the application of diffusion theory to thermochronology have yet to be fully exploited this study uses apatite tc 450 550 c and titanite tc 550 650 c u pb id tims thermochronology at the single to sub grain scale to separate the variable effects of volume diffusion of pb from metamorphic over growth above and below the tc of a mineral data are presented from two ca 3227 ma tonalite samples from north and south of the barberton greenstone belt bgb southern africa two distinct populations of apatite from a sample north of the bgb record fast cooling followed by metamorphic growth 10 myr later both apatite and titanite dates from south of the bgb show a strong correlation with the grain size and record 100 myr of post emplacement cooling complex core rim zoning observed in cathodoluminescence images of apatite is interpreted to reflect metamorphic overgrowth above the tc the age and topology of grain size versus date curves from titanite and apatite are used in combination with a finite difference numerical model to show that slow non linear cooling and not thermal resetting is responsible for the observed distribution the thermal histories from either side of the bgb are very different and provide unique insight into the bgb s tectonic evolution a 70 myr period of apparent stability after ca 3 2 ga terrane assembly was followed by fast exhumation south of the bgb that led to lower crustal melting and intrusion of granitic batholiths ca 3 14 3 10 ga

origin and consequences of cyclic ice rafting in the northeast atlantic ocean during the past 130 000 years deep sea sediment cores recovered from the northeast atlantic ocean were examined in order to elucidate the influence of the earth s orbital parameters on major ice rafting analyses of coarse grained ice rafted debris and planktonic foraminifers revealed a strong reaction to the precession signal since 130 000 yr b p dropstone layers have been deposited each half period of a precessional cycle 11 000 1000 yr ice rafting occurs during times of winter minimum summer maximum insolation and summer minimum winter maximum insolation in the first case high summer insolation forces meltwater discharge from the ice sheets into the polar seas which subsequently enhances formation of sea ice during the winter in the second case growth of continental ice enhances iceberg production which also leads to a salinity reduction of surface seawater both situations result in a southward penetration of polar water thus the marine record of dropstones documents ice rafting not only during weichselian stades but also during cold events within interstades the regularity of ice rafting yields a useful framework to calibrate and elucidate climatic changes not only in the region of the north atlantic ocean but also in remote areas such as the pacific ocean and the antarctic

a new mg in plagioclase geospeedometer for the determination of cooling rates of mafic rocks abstract we present a new mg in plagioclase geospeedometer based on the diffusive exchange of mg between plagioclase and clinopyroxene that allows cooling rates of a wide range of mafic rocks to be determined the major element composition of plagioclase is shown to play an important role in driving the flux of mg as well as the partitioning of mg between plagioclase and clinopyroxene therefore commonly used analytical solutions to the diffusion equation are not applicable in this system and an alternate approach is developed practical aspects of the method such as the role of anorthite zoning patterns and influence of shapes of cooling paths and grain geometries in controlling the mg concentration profile shapes are discussed propagation of uncertainties in the knowledge of diffusion and partition coefficients shows that the method is capable of resolving small differences in cooling rates e g log d t d t c yr 1 3 0 3 in one example the approach is illustrated by application to two samples from the lower oceanic crust

high latitude forcing of the south american summer monsoon during the last glacial the climate of the last glacial period 10 000 to 110 000 years ago was characterized by rapid millennial scale climate fluctuations termed dansgaard oeschger d o and heinrich events we present results from a speleothem derived proxy of the south american summer monsoon sasm from 16 000 to 50 000 years ago that demonstrate the occurrence of d o cycles and heinrich events this tropical southern hemisphere monsoon reconstruction illustrates an antiphase relationship to northern hemisphere monsoon intensity at the millennial scale our results also show an influence of antarctic millennial scale climate fluctuations on the sasm this high resolution precisely dated tropical precipitation record can be used to establish the timing of climate events in the high latitudes of the northern and southern hemispheres

interpreting zirconium in rutile thermometric results at first sight experimental results and observations on rocks suggest that the zr content in rutile where equilibrated with quartz and zircon should be a useful thermometer for metamorphic rocks however diffusion data for zr in rutile imply that thermometry should not for plausible rates of cooling give the high temperatures commonly observed in high grade metamorphic rocks it is suggested here that such observations can be accounted for by high t diffusive closure of si in rutile causing the interior of rutile grains to become insensitive to the thermometer equilibrium well above the temperature of zr diffusive closure paired with comparatively slow grain boundary diffusion and problematic zircon nucleation this allows for cases of zr retention in rutile through temperatures where zr is still diffusively mobile within rutile grains other observations that may be accounted for in this context are large inter grain ranges of rutile zr contents uncorrelated with rutile grain size and flat zr profiles across individual rutile grains counter to what would be expected from diffusive closure a consequence is that it is unlikely that zr in rutile thermometry will be useful for estimating rock cooling rates

methods for determining deformation history for chocolate tablet boudinage with fibrous crystals abstract chocolate tablet boudinage with fibrous crystal growths between the boudinaged plates from two localities were studied in one from leytron valais switzerland the deformation history was found to be a succession of plane strain increments with the shortening direction perpendicular to the boudinaged sheet and the extension direction showing a progressive change in orientation within the sheet the incremental and finite strains were evaluated the other specimen from parys mountain anglesey great britain was found to have a more complex history with diachronous break up of the competent layer and flattening strain increments it was found that under these circumstances the direct graphical methods of determining finite and incremental strains gave inconsistent results a numerical model was developed which allowed the simulation of chocolate tablet structure with a complex deformation history the model was applied to the anglesey specimen and three possible strain histories for this structure were tried

variscan tectonics in the iberian pyrite belt south portuguese zone this paper aims to discuss the structural evolution of the iberian pyrite belt during the variscan orogeny it provides new structural data maps and cross sections from the eastern part of the iberian pyrite belt regional geology of the south portuguese zone and lithostratigraphy of the iberian pyrite belt are first briefly summarised three roughly homoaxial deformation phases are distinguished and are mainly characterised by south verging multi order folds axial planar cleavages and thrusts three structural units are distinguished the la puebla de guzman and valverde del camino antiforms are rooted units related to the propagation of southward directed thrust systems that may branch onto the lower decollement level of the south portuguese zone el cerro de andevalo is a structurally higher unit mainly composed of allochthonous d1 thrust nappes no evidence of sinistral transpression has been found in the transected cleavage and the strike of s3 with respect to s2 better evidence of transpression is the moderately to steeply westerly plunging folds that show s type asymmetry in down plunge view variscan deformation in the iberian pyrite belt is defined as the combination of a dominant southwards shear and a sinistral e shear caused by oblique continental collision between the south portuguese plate and the iberian massif

zr and hf diffusion in rutile abstract chemical diffusion of zr and hf under anhydrous conditions has been measured in synthetic and natural rutile the sources of diffusant used were hafnon or zircon powders or a hafnon rutile mixture experiments were run in crimped pt capsules in air or in sealed silica glass capsules with solid buffers to buffer at nno or qfm rutherford backscattering spectrometry rbs was used to measure zr and hf diffusion profiles from these measurements the following arrhenius relations were obtained for zr diffusion parallel to c over the temperature range 750 1100 c d zr c 9 8 10 15 exp 170 30 kj mol 1 rt m 2 s 1 for hf diffusion parallel to c over the temperature range 800 1000 c d hf c 9 1 10 15 exp 169 36 kj mol 1 rt m 2 s 1 for hf diffusion normal to c over the temperature range 750 1050 c d hf c 2 5 10 12 exp 227 62 kj mol 1 rt m 2 s 1 diffusivities for experiments buffered at qfm and nno are similar to those run in air diffusivities in synthetic and natural rutile are likewise similar indicating that these findings can be applied directly in determining zr diffusivities in rutile in natural systems these data indicate that rutile should be moderately retentive of zr chemical signatures with zr diffusivities within an order of magnitude of those for pb in rutile over most geologic conditions when applied in evaluation of the relative robustness of the recently developed zr in rutile geothermometer t zack r moraes a kronz temperature dependence of zr in rutile empirical calibration of a rutile thermometer contributions to mineralogy and petrology 148 2004 471 488 e b watson d a wark j b thomas crystallization thermometers for zircon and rutile contributions to mineralogy and petrology 151 2006 413 433 these findings suggest that zr concentrations in rutile will be somewhat more likely to be affected by later thermal disturbance than the geothermometer based on zr concentrations in titanite l hayden e b watson d a wark a thermobarometer for sphene abstract 16th v m goldschmidt conference 2006 and much less resistant to diffusional alteration subsequent to crystallization than the ti in zircon geothermometer e b watson d a wark j b thomas crystallization thermometers for zircon and rutile contributions to mineralogy and petrology 151 2006 413 433 e b watson t m harrison zircon thermometer reveals minimum melting conditions on earliest earth science 308 2005 841 844

coeval folding and boudinage in four dimensions scaled analogue experiments have been carried out incrementally to demonstrate the growth of constrictional folds and boudins through space and time pure constriction acted on a single stiff layer that was embedded in a weak matrix with the layer trending parallel to the x axis of the constrictional strain ellipsoid the viscosity ratio between non linear viscous layer and matrix was set at ca 13 three dimensional images of the incrementally deformed layer have been obtained using computer tomography they yielded clear evidence that d1 folds and d1 boudins show striking interactions when growing simultaneously at low to moderate finite strain the axes of d1 folds are subparallel and the axes of d1 boudins are subperpendicular to the x axis of the constrictional strain ellipsoid the normalized wavelengths of d1 folds and d1 boudins and the amplitudes of d1 folds are fixed already at low finite strain 5 and do not significantly change as deformation proceeds the normalized wavelength of both structures is not so in line as suggested by theoretical studies at higher degrees of finite strain d1 boudins are affected by d2 folds and rotated limbs of the latter show d2 boudinage resulting in complex deformation patterns that are difficult to identify in the field the model results are important for the analysis and interpretation of deformation structures in rheologically stratified rocks undergoing dislocation creep under bulk constriction tectonic settings where constrictional folds and boudins may develop simultaneously are stems of salt diapirs subduction zones or thermal plumes

the boudinage question in the high ardenne slate belt belgium a combined structural and fluid inclusion approach abstract the terms boudin and boudinage 1 were first introduced as descriptive terms for particular geometric features associated with quartz veins in competent metaquartzitic layers in the high ardenne slate belt belgium the occurrence of the boudins has subsequently been used to draw kinematic conclusions on a pre variscan extensional event in the rhenohercynian belt in this paper a combined structural and fluid inclusion approach allows proposing a new and more representative kinematic model for the development of the veins and associated structures moreover the kinematic significance of interboudin veins is emphasized in this regard microthermometry of primary co 2 n 2 fluid inclusions in the quartz of the interboudin veins suggests that the origin of vein development is hydraulic fracturing in an already compressional setting this suggestion implies the presence of fluid and near lithostatic fluid pressures to permit fracturing at great depths where the rocks otherwise would be either stable or in the realm of ductile behaviour the vein filling fluid had a local origin and was most likely related to the formation of bedding parallel dissolution seams within the host rock veining is followed by layer parallel shortening resulting in the development of double sided mullions at both interfaces of metaquartzitic layers pinned by pre existing veins subsequently these polyphase structures behave passively in the variscan deformation

an exhumation history of continents over billion year time scales the continental lithosphere contains the oldest and most stable structures on earth where fragments of ancient material have eluded destruction by tectonic and surface processes operating over billions of years although present day erosion of these remnants is slow a record of how they have uplifted eroded and cooled over earth s history can provide insight into the physical properties of the continents and the forces operating to exhume them over geologic time we constructed a continuous record of ancient lithosphere cooling with the use of uranium lead u pb thermochronology on volcanically exhumed lower crustal fragments combining these measurements with thermal and pb diffusion models constrains the range of possible erosion histories measured u pb data are consistent with extremely low erosion rates persisting over time scales approaching the age of the continents themselves

a new geodynamic interpretation for the south portuguese zone sw iberia and the iberian pyrite belt genesis 1 the south portuguese zone spz constitutes the southernmost segment of the variscan iberian massif it is bounded to the north by the beja acebuches ophiolitic complex and related accretionary wedge to the south lie the iberian pyrite belt ipb and flysch deposits forming the southern extent of the zone structural analysis within the spanish side of the spz supports continuous south propagating deformation evolving from early synmetamorphic thrusting in the internal zone to thin skinned tectonics in the southern external domain the accretion of the spz to the ossa morena zone is also witnessed by the presence of various melanges observed throughout the investigated area part of the melanges observed in the ipb are related to the volcanics and mineralizations setting a key point to understand the ipb mineralizations genesis is to constrain the volcanogenic model one underestimated feature is the large amount of submarine calc alkaline ignimbritic facies implying the presence of caldera structures within the province such correlation between caldera environment and ore deposits strongly suggests that the ipb developed in a continental arc our geodynamic model proposes an early north directed subduction associated with the obduction of the oceanic crust toward the south southward this episode is immediately followed by the development of the accretionary prism while farther south a second subduction zone responsible for the arc setting of the ipb initiates subsequent visean continental collision is associated with the deposit of the south propagating flysch and the present geometry of the spz

multistage exhumation and juxtaposition of lower continental crust in the western canadian shield linking high resolution u pb and 40ar 39ar thermochronometry with pressure temperature deformation paths 1 new u pb titanite apatite rutile and 40ar 39ar hornblende muscovite biotite data are linked with existing pressure temperature deformation paths to impose detailed temporal constraints on the juxtaposition and unroofing of domains in an extensive 20 000 km2 region of exhumed lower crust in the east lake athabasca region of the western canadian shield in the 200 m y between circa 1 9 ga high pressure granulite facies metamorphism and circa 1 7 ga unconformable deposition of athabasca basin sediments on the exhumed rocks our analysis reveals at least three distinct phases of unroofing at different rates specifically we distinguish 1 an early phase of extensional unroofing in the chipman domain at rates of 1 5 2 0 km m y from 1 0 gpa to 0 8 gpa associated with mafic magmatism and metamorphism 2 an episode of regional contractional uplift and associated unroofing along the legs lake shear zone at circa 1850 ma from 0 7 0 8 gpa to 0 4 0 5 gpa and 3 a final period of extensional unroofing at rates of 0 2 0 3 km m y from 0 4 to 0 5 gpa that culminated in transport of current exposures to near surface conditions the cooling patterns and retrograde assemblages are consistent with pauses between exhumational phases the apparent convergence of disparate higher temperature histories in several deep crustal domains at circa 1 89 1 88 ga implies their juxtaposition at 0 7 0 8 gpa conditions regional east directed thrusting of the deep crustal domains as a coherent unit at 1 85 ga juxtaposed the granulites with middle crustal hearne domain rocks at 0 4 0 5 gpa conditions the detailed exhumational history allows correlation with changing regional tectonic regimes associated with the amalgamation of laurentia the temporal and spatial heterogeneity of exhumation patterns in the east lake athabasca region may be a common feature of the unroofing histories of lower crustal rocks

zirconium in rutile speedometry new constraints on lower crustal cooling rates and residence temperatures abstract the incorporation of zirconium into the mineral rutile tio2 has been both empirically and experimentally calibrated as a measure of rutile crystallization temperatures watson et al 2006 this temperature sensitive system has been employed as a geothermometer with applications to a number of different geologic settings and rock types experimentally measured kinetics for zr diffusion in rutile cherniak et al 2007 indicate that zr can be lost to temperature dependent diffusion warranting further investigation of the geologic significance of calculated temperatures coupling diffusion kinetics with numerical solutions to the diffusion equation provides a means to forward model the time and temperature dependency of the system modeled results indicate a strong dependency of zr concentration in rutile on both 1 initial cooling rate following high temperature metamorphism crystallization and 2 temperature and duration of long term geologic residence zr concentrations measured in rutile from lower crustal xenoliths that resided at 25 45 km depths for 2000 my reveal zr concentrations in the approximate grain center that are consistent with temperatures measured by independent thermometers forward models for zr diffusion show that preserving a zr record of these initial temperatures in the center of a rutile crystal with a 50 m radius requires rapid cooling 300 c ma from magmatic metamorphic temperatures followed by a long term residence 2000 my at temperatures

paleoproterozoic amphibolite facies retrogression and exhumation of archean metapelitic granulites in the southern marginal zone of the limpopo belt south africa abstract granulite facies terrains that have undergone extensive retrogression provide insights into the recycling of volatiles in the crust during orogenesis the timing of retrogression is key to understanding the geodynamic circumstances of such events this study has investigated amphibolite facies retrogression of melt depleted granulite facies metapelites from the south marginal zone smz of the limpopo belt the granulites formed via incongruent partial melting reactions that consumed biotite and produced garnet and rutile as prominent peritectic products at 2 7 ga under conditions of 11 kbar and 875 c peak metamorphic garnet is replaced by cordierite orthopyroxene symplectites with cordierite subsequently partially replaced by fine grained intergrowths of amphibolite facies assemblages including rutile zr in rutile thermometry yielded a temperature range of 731 846 c for the granulite facies rutiles and 629 690 c for rutile in the retrograde assemblage all rutile generations yielded a youngest u pb age of 2 1 ga some granulite facies rutiles record older dates 2 55 2 25 ga which are non systematic with regard to crystal size and represent non reset relicts of metamorphic recrystallization during a potentially complex history of amphibolite facies metamorphism between 2 7 and 2 2 ga the youngest 2 1 ga rutile u pb age represents the time of final exhumation related cooling of the rocks following a discrete paleoproterozoic amphibolite facies retrograde event coexisting orthoamphiboles in the peak metamorphic assemblage indicate a maximum peak temperature of 625 c the lack of age size correlation and intragrain age zoning in rutile suggest fast cooling following the amphibolite facies retrograde event which is inferred to be not much older u pb dating of zircon from the same rocks confirms that the metapelites underwent granulite facies metamorphism at 2695 35 ma thus the metapelites of the smz record both an archean granulite facies event and a polymetamorphic paleoproterozoic amphibolite facies history that culminated in a mid amphibolite facies peak inducing partial retrogression of the granulites the paleoproterozoic amphibolite facies event is tentatively correlated with mantle melting documented between 2 3 and 2 1 ga to produce mafic and intermediate volcanic rocks in the pretoria group on the kaapvaal craton

fluid assisted particulate flow of turbidites at very low temperature a key to tight folding in a submarine variscan foreland basin of sw europe 1 the problem addressed in this article is how sedimentary formations like turbidites in a foreland basin which include layers with apparently great competence contrast can be tightly folded in a regular manner under very low temperature and pressure this raises two major issues the rheological behavior of the rocks at the time of folding and the role played by fluids in order to understand very low temperature folding and the structural evolution of a submarine foreland basin we carried out detailed structural work in turbidites with alternating sandstone and shale for which estimated peak temperature conditions were top diagenetic to very low grade metamorphism folds are tight to isoclinal with local collapsed hinges which implies that the incompetent shale was mobile enough to flow away from strongly flattened areas we did not find evidence for cataclastic flow or crystal plasticity at mesoscopic and microscopic scales other structures mostly boudins foliations conjugate brittle faults and quartz veins associated with folds denote anisotropic compaction by fluid extraction during regional shortening this is possible if the folded rocks were unconsolidated fluid saturated sediments the estimated low peak temperature is consistent with the shale being unlithified poorly cemented grains are free to slide past one another under shallow burial or high pore pressure conditions following this line of thought we consider independent particulate flow assisted by fluids under very low confining pressure bean bag analogy as the rock deformation mechanism active during the described intense folding similar deformation is likely occurring and has occurred in other submarine accretionary wedges

trace element systematics in granulite facies rutile implications for zr geothermometry and provenance studies metamorphic rutile from granulite facies metapelitic rocks of the archean pikwitonei granulite domain pgd manitoba canada provides constraints on the systematics of trace elements in rutile during high temperature conditions and subsequent slow cooling compositional profiles and maps of the zr concentrations in rutile grains 120 600 m from three metapelitic gneisses were acquired by electron probe micro analysis using a spatial resolution of down to 2 m simultaneously profiles were analysed for nb cr and v which have significantly different diffusion characteristics in rutile the profiles of all elements show relatively homogeneous concentrations within most grains but significant inter grain differences even within a single thin section some rutile grains display a slight concentration decrease from a neighbouring garnet towards the matrix for all measured elements the lack of diffusion profiles for all analysed elements shows that these are highly immobile in rutile and that distributions of these elements are primary and preserve prograde information the nb and cr concentrations overlap with ranges that are ascribed to different provenances indicating that source discrimination based on these elements is not possible in all cases high retentiveness for zr implies that the zr in rutile geothermometer is highly robust to diffusive re equilibration even during very slow cooling 2 c ma 1 from granulite facies conditions most grains have high zr contents 3000 4600 ppm differences between high zr contents suggest that during growth under vapour absent conditions there may not be saturation of zr in rutile even if zircon is present therefore several rutile grains need to be analysed in a sample to obtain a useful minimum peak temperature the highest zr concentrations correspond to 900 c this is significantly higher than previous peak temperature estimates of 820 c based on two feldspar thermometry on a regional scale this implies that part of the pgd was affected by ultra high temperature uht metamorphism it also implies that rutile is able to preserve primary compositions even to uht conditions this study shows that if combined with textural information zr in rutile has the potential to be a very useful tool for estimating rutile crystallization temperatures and peak metamorphic conditions for granulite facies rocks zr in rutile yields more reliable peak metamorphic temperatures than most other exchange geothermometers which tend to partially re equilibrate by diffusion during cooling

indian ocean climate and an absolute chronology over dansgaard oeschger events 9 to 13 oxygen isotope ratios of a stalagmite from socotra island in the indian ocean provide a record of changes in monsoon precipitation and climate for the time period from 42 to 55 thousand years before the present the pattern of precipitation bears a striking resemblance to the oxygen isotope record from greenland ice cores with increased tropical precipitation associated with warm periods in the high northern latitudes the largest change at the onset of interstadial 12 occurred very rapidly in about 25 years the chronology of the events found in our record requires a reevaluation of previously published time scales for climate events during this period

prograde and near peak zircon growth in a migmatitic pelitic schist of the southeastern canadian cordillera abstract u pb dates and trace elements acquired simultaneously on zircon from a migmatitic pelitic schist from the southeastern canadian cordillera are linked with results of phase equilibria modeling the first of three phases of zircon growth occurred between 77 and 72 ma along a prograde path in the kyanite melt field between the wet melting and muscovite dehydration melting reactions 675 775 c this protracted phase of prograde growth is characterized by a shallow positive slope in heavy rare earth elements hree the second phase occurred at c 69 ma after significant garnet growth by biotite dehydration melting at 775 875 c which produced zircon depleted in hree the last phase occurred at c 63 ma on the retrograde path as garnet and rutile were consumed at 780 c as indicated by enrichments in hree and nb but prior to feldspar crystallization in the melt as suggested by the moderate eu anomaly this path is consistent with rutile and monazite petrogenesis documented herein and in a previous study respectively in contrast with the high temperatures at which zircon grew 675 875 c ti in zircon thermometry yields unrealistically low estimates 470 739 c with 80 below the wet solidus there are no uncertainties in the buffering assemblage as rutile and quartz were present throughout and no effect of pressure because a pressure dependent calibration was used this study demonstrates that zircon growth along a prograde path is possible and warrants further investigation on factors controlling the ti content of zircon thermometry in metamorphic rocks

c axis fabrics and microstructures in a recrystallized quartz vein deformed under fluid rich greenschist conditions the quartz v axis orientations and microstructures have been analyzed in an originally single crystal buckled quartz vein of greenschist metamorphic grade from the sambagawa metamorphic belt southwest japan the basal 0001 plane was inclined by ca 10 20 to the quartz vein surface before deformation and the flexure essentially kink which produced the fold was accomplished by basal 0001 slip alone in the hinges i e flexural slip folding in addition to crystal plasticity dissolution microstructures are ubiquitous in the quartz vein dynamic recrystallization resulted from both subgrain rotation and grain boundary migration rotation recrystallization associated with basal 0001 slip resulted in host controlled v axis orientation distribution along a great circle whereas migration recrystallization perhaps assisted by intergranular fluid resulted in a penetrative development of a weak x elongation maximum v axis fabric nowhere rotated by the folding the migration recrystallization is evidenced by large v axis misorientations across the grain boundaries as well as irregular grain shape based on the weak v axis fabric development and lack of grain shape fabric solution precipitation creep may have dominated in the fluid assisted grain boundary migration regime and hence it is inferred that the transition from rotation to migration recrystallization was caused by a local decrease of differential stress in the buckled quartz vein

eclogites of the snowbird tectonic zone petrological and u pb geochronological evidence for paleoproterozoic high pressure metamorphism in the western canadian shield eclogite occurs within the southern domain of the east athabasca mylonite triangle in northern saskatchewan situated at the boundary between the archean rae and hearne provinces of the western canadian shield the east athabasca mylonite triangle is a fundamental exposure of the 3 000 km long snowbird tectonic zone the eclogite occurs in association with a variety of lower crustal high pressure granulites that record a complex metamorphic history from 2 6 to 1 9 ga temperatures of the eclogite facies metamorphism are constrained by garnet clinopyroxene exchange thermometry at 920 1 000 c minimum pressure conditions are recorded by the jadeite quartz albite geobarometer at 1 8 2 0 gpa a near isothermal decompression path to granulite facies conditions is inferred from retrograde reaction textures involving the formation of granulite facies assemblages such as orthopyroxene plagioclase and pargasite plagioclase u pb idtims zircon geochronology of the eclogite yields a weighted mean 207pb 206pb date of 1 904 0 0 3 ma which we interpret as the time of peak eclogite facies metamorphism shrimp in situ analyses of metamorphic zircons included within omphacitic clinopyroxene support this interpretation with a weighted mean 207pb 206pb date of 1 905 19 ma inclusion suites of high pressure phases and the petrographic setting of zircon are a direct link between zircon growth and eclogite facies metamorphism zircon from one eclogite sample has older cores that are 2 54 ga which is a minimum age for the emplacement or earliest metamorphism of the gabbroic protolith u pb rutile data indicate slow cooling at 1 c ma below 500 c from 1 88 to 1 85 ga the formation and exhumation of the eclogites at ca 1 9 ga has important implications for the tectonic significance of the snowbird tectonic zone during the paleoproterozoic the eclogites described here are consistent with transport of continental crust to mantle depths during the paleoproterozoic followed by rapid buoyancy driven exhumation to normal lower crustal depths

the tectonometamorphic evolution of high pressure low temperature metamorphic rocks of eastern crete greece constraints from microfabrics strain illite crystallinity and paleodifferential stress abstract analysis of strain paleodifferential stress illite crystallinity and microfabrics of quartz and calcite suggest that multiply alternating late oligocene early miocene n s convergence and n s extension caused significant vertical discontinuities in the grade of high pressure low temperature metamorphism of the phyllite quartzite unit pqu of eastern crete constrictional fabrics and parallelism of d2 stretching lineations and fold axes are attributed to enhanced slab pull forces at greater depth of subduction a major d3 extensional detachment is present in the upper part of the pqu where slates with lowest metamorphic grade diagenesis anchizone transition are resting on top of anchimetamorphic rocks the highest metamorphic grade anchi epizone transition has been determined for skythian rocks on top of pre alpine basement revived n s convergence d4 led to e w trending map scale folds and top to the s directed brittle thrusts there is a clear correlation between d2 related metamorphic grade finite strain differential stress and deformation mechanisms at the anchi epizone transition solution precipitation and dislocation creep of quartz and calcite caused maximum finite strains rxz up to 7 at high strain rates and moderate differential stresses 80 200 mpa at the diagenesis anchizone transition deformation was accommodated by cataclasis dislocation glide and solution precipitation creep resulting in relatively low finite strains rxz

marching in near lock step over the past 100 000 years intervals of gradual climate cooling have repeatedly been followed by abrupt warming events collectively known as dansgaard oeschger d o cycles 1 these 1500 year long cycles are recorded in the oxygen isotope record of greenland ice cores and in layers of iceberg rafted sediments in the north atlantic ocean known as heinrich layers 2 that record collapses of the northern hemisphere ice sheets on page 570 of this issue kanner et al 3 report a stunning new archive of the oxygen isotopic composition of tropical precipitation recorded in the deposits of pacupahuain cave see the figure panel a located high on the eastern side of the peruvian andes that matches the north atlantic greenland records in striking detail the results help to elucidate how climate change both past and future in one region of the globe may drive climate changes in far flung regions

tropical climate changes at millennial and orbital timescales on the bolivian altiplano tropical south america is one of the three main centres of the global zonal overturning circulation of the equatorial atmosphere generally termed the walker circulation1 although this area plays a key role in global climate cycles little is known about south american climate history here we describe sediment cores and down hole logging results of deep drilling in the salar de uyuni on the bolivian altiplano located in the tropical andes we demonstrate that during the past 50 000 years the altiplano underwent important changes in effective moisture at both orbital 20 000 year and millennial timescales long duration wet periods such as the last glacial maximum marked in the drill core by continuous deposition of lacustrine sediments appear to have occurred in phase with summer insolation maxima produced by the earth s precessional cycle short duration millennial events correlate well with north atlantic cold events including heinrich events 1 and 2 as well as the younger dryas episode at both millennial and orbital timescales cold sea surface temperatures in the high latitude north atlantic were coeval with wet conditions in tropical south america suggesting a common forcing

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

a high resolution absolute dated late pleistocene monsoon record from hulu cave china oxygen isotope records of five stalagmites from hulu cave near nanjing bear a remarkable resemblance to oxygen isotope records from greenland ice cores suggesting that east asian monsoon intensity changed in concert with greenland temperature between 11 000 and 75 000 years before the present yr b p between 11 000 and 30 000 yr b p the timing of changes in the monsoon as established with 230th dates generally agrees with the timing of temperature changes from the greenland ice sheet project two gisp2 core which supports gisp2 s chronology in this interval our record links north atlantic climate with the meridional transport of heat and moisture from the warmest part of the ocean where the summer east asian monsoon originates

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

pb diffusion in rutile diffusion of pb was measured in natural and synthetic rutile under dry 1 atmosphere conditions using mixtures of pb titanate or pb sulfide and tio2 as the sources of diffusant pb depth profiles were then measured with rutherford backscattering spectrometry rbs over the temperature range 700 1100 c the following arrhenius relation was obtained for the synthetic rutile d 3 9 10 10exp 250 12 kj mol 1 rt m2s 1 results for diffusion in natural and synthetic rutile were quite similar despite significant differences in trace element compositions mean closure temperatures calculated from the diffusion parameters are around 600 c for rutile grains of 100 m size this is about 100 c higher than rutile closure temperature determinations from past field based studies suggesting that rutile is more resistant to pb loss through volume diffusion than previously thought

high pressure granulites formation recovery of peak conditions and implications for tectonics high pressure granulites are characterised by the key associations garnet clinopyroxene plagioclase quartz in basic rocks and kyanite k feldspar metapelites and felsic rocks and are typically orthopyroxene free in both basic and felsic bulk compositions in regional metamorphic areas two essential varieties exist a high to ultrahigh temperature group and a group representing overprinted eclogites the high to ultrahigh temperature type formerly contained high temperature ternary feldspar now mesoperthite coexisting with kyanite is associated with garnet peridotites and formed at conditions above 900 c and 1 5 gpa clinopyroxene in subordinate basic rocks is al rich and textural evidence points to a high pressure high temperature melting history the second variety contains symplectite like or poikilitic clinopyroxene plagioclase intergrowths indicating former plagioclase free i e eclogite facies assemblages this type of rock formed at conditions straddling the high pressure amphibolite high pressure granulite field at around 700 850 c 1 0 1 4 gpa importantly in the majority of high pressure granulites orthopyroxene is secondary and is a product of reactions at pressures lower than the peak recorded pressure in contrast to low and medium pressure granulites which form at conditions attainable in the mid to lower levels of normal continental crust high pressure granulites of nonxenolith origin mostly represent rocks formed as a result of short lived tectonic events that led to crustal thickening or subduction of the crust into the mantle short times at high temperature conditions are reflected in the preservation of prograde zoning in garnet and pyroxene high pressure granulites of both regional types although rare are known from both old and young metamorphic terranes e g c 45 ma namche barwa e himalaya 400 340 ma european variscides 1 8 ga hengshan china 1 9 ga snowbird saskatchewan and 2 5 ga jianping china this spread of ages supports proposals suggesting that thermal and tectonic processes in the lithosphere have not changed significantly since at least the end of the archean

timing of millennial scale climate change in antarctica and greenland during the last glacial period a precise relative chronology for greenland and west antarctic paleotemperature is extended to 90 000 years ago based on correlation of atmospheric methane records from the greenland ice sheet project 2 and byrd ice cores over this period the onset of seven major millennial scale warmings in antarctica preceded the onset of greenland warmings by 1500 to 3000 years in general antarctic temperatures increased gradually while greenland temperatures were decreasing or constant and the termination of antarctic warming was apparently coincident with the onset of rapid warming in greenland this pattern provides further evidence for the operation of a bipolar see saw in air temperatures and an oceanic teleconnection between the hemispheres on millennial time scales

formation of chocolate tablet boudins in a foreland fold and thrust belt a case study from the external variscides almograve portugal abstract chocolate tablet boudins of quartzite are restricted to steep limbs of d 1 folds along the sw coast of portugal the chocolate tablet geometry results from older vertical and younger horizontal quartz veins both sets of veins developed during similar conditions by extension fracture in pre existing necked domains 1 both veins show stretched crystal fibers 2 the boudin aspect ratio is the same in vertical 3 0 1 4 and in horizontal sections 2 9 1 2 3 temperatures obtained from fluid inclusions are similar 200 20 for vertical and 230 22 c for horizontal veins and are compatible with temperatures obtained from illite crystallinity ca 200 ca 250 c given thermal equilibrium between the host rock and the precipitating fluid the chocolate tablet boudins formed close to the metamorphic peak we interpret that the vertical veins developed after the limbs of the d 1 folds had attained their steep attitude and the orientation of the greatest and intermediate principal strain axes had interchanged subsequently the initial strain field was restored and opening of horizontal veins led to the chocolate tablet geometry the direction of the main shortening direction was constant from the initial buckling stage via isoclinal folding and during all boudinage stages

combined rutile zircon thermometry and u pb geochronology new constraints on early paleozoic hp uht granulite in the south altyn tagh north tibet china abstract hp granulite and garnet peridotite constitute a hp metamorphic unit in the south altyn tagh northern tibet previous data suggest that hp granulites experienced high pressure ultrahigh temperature metamorphism 870 1050 c 19 25 kbar followed by a medium pressure granulite facies overprint 780 820 c 9 5 12 kbar previous zircon u pb geochronology of hp granulites gave an age of ca 500 ma but it is unclear if this age reflects the metamorphic peak or the post peak overprint in this contribution new shrimp analyses indicate that the zircons associated with the peak metamorphic assemblage in hp felsic granulite are characterized by flat heavy ree patterns and 30 62 ppm ti yielding a temperature of 892 1002 c by ti in zircon thermometry suggesting that the growth of the zircons occurred under or close to peak ultrahigh temperature conditions rather than under post peak conditions these zircons gave a shrimp u pb age of 495 4 ma representing the age of peak metamorphism in contrast oscillatory zoned zircon cores yield magmatic ree patterns ti in zircon temperatures of 720 820 c and neoproterozoic u pb ages 936 22 ma this indicates that the protoliths of the felsic granulites were formed in the early neoproterozoic and were taken to extreme temperatures and pressures during the early paleozoic orogenic event in situ la icpms and emp analyses indicate that rutiles are characterized by broad scatter and bimodal distribution in both zr concentration and u pb age which are interpreted as a result of recrystallization or growth rather than volume diffusion the first rutile u pb age cluster is at ca 485 ma with a zr concentration of ca 3000 ppm yielding ca 950 c at 20 kbar implying that they grew at or near the peak metamorphic conditions the second rutile u pb age cluster is at ca 450 ma with a zr concentration of ca 1000 ppm corresponding to a temperature around 780 c at 10 kbar which is related to post peak reheating under medium pressure medium temperature granulite facies conditions these results demonstrate that hp granulites in the south altyn tagh experienced a peak ultrahigh temperature and high pressure metamorphism at ca 495 ma followed by medium pressure medium temperature granulite facies overprint before ca 450 ma

solving the production diffusion equation for finite diffusion domains of various shapes part i implications for low temperature u th he thermochronology abstract we propose an accurate fast and easy to use method to derive numerical solutions for production diffusion equations for finite diffusion domains of various shapes and arbitrary cooling histories previous studies provide solutions for spheres infinite cylinders and infinite sheets we extend this range and provide solutions for finite bodies i e finite cylinders and rectangular blocks of any aspect ratio this approach is important as recently it has become clear that for example the physical grain is the diffusion domain for he diffusion in apatite and titanite j geophys res 105 2000 2903 geochim cosmochim acta 63 1999 3845 we discuss the use of the new approach for forward modelling u th he production diffusion in apatite taking results with finite cylinders as a good approximation for apatite crystals it is found that approximating instead with spheres or infinite cylinders having the same radius yields differences in calculated ages that can easily be as large as 20 35 the relative differences are most pronounced in thermal histories that spend significant time at or near the closure temperature on the other hand reasonable agreement is found with spheres having the same surface to volume ratio