nitrogen isotopic composition of ammoniated phyllosilicates case studies from precambrian metamorphosed sedimentary rocks ammonium concentrations in phyllosilicates from archean and proterozoic schists and carbonates were evaluated by fourier transform infrared microspectrometry aftir the y 15 nair values of structural nitrogen in these phases were subsequently measured by laser mass spectrometry a rapid estimate of the concentration of nh4 in biotite by aftir allows the preselection of the samples to be analyzed for their isotopic composition micas from 1 90 2 09 ga metasediments from finland and from 1 025 1 500 ga biotite schists from the moine succession in scotland have high concentrations of structural ammonium between 176 and 1549 ppm suggestive of a biological origin for the nitrogen the heavy nitrogen isotopic signatures y 15 n 4 9x to 19 7x of structural nh4 in these micas are considered consistent with biological denitrification in operation at the time of sedimentation evidence for denitrification in the mesoproterozoic is in agreement with geological and geochemical data for abundant free oxygen in the atmosphere facilitating the stability of marine no3 sequences of early archean ca 3 8 ga garnet mica schists from the isua supracrustal belt isb in southern west greenland contain biotite rich in nh4 between 233 and 512 ppm and nitrogen isotopic signatures y 15 n 1 9x to 5 9x which possibly point to an early evolution of ammonium assimilation and or nitrogen fixation this result appears to be consistent with molecular phylogenetic studies in the bacterial and archaeal domains which suggest that nitrogen fixation and ammonium assimilation were metabolic features of the last common ancestor lca of all organisms d 2004 elsevier b v all rights reserved

geochemistry and isotopes of fluids from sulphur springs valles caldera new mexico abstract detailed geochemistry supported by geologic mapping has been used to investigate sulphur springs an acid sulfate hot spring system that issues from the western flank of the resurgent dome inside valles caldera the most intense activity occurs at the intersection of faults offsetting caldera fill deposits and post caldera rhyolites three geothermal wells in the area have encountered pressures 4 8000 mg l these conditions cause argillic alterations throughout a large area non condensible gases consist of roughly 99 co 2 with minor amounts of h 2 s h 2 and ch 4 empirical gas geothermometry suggests a deep reservoir temperature of 215 to 280 c comparison of 13 c and 18 o between caco 3 from well cuttings and co 2 from fumarole steam indicates a fractionation temperature between 200 and 300 c by decarbonation of hydrothermally altered paleozoic limestone and vein calcite in the reservoir rocks tritium concentrations obtained from steam condensed in a mudpot and deep reservoir fluids baca 13 278 c are 2 1 and 1 0 t u respectively suggesting the steam originates from a reservoir whose water is mostly 50 yrs old deuterium contents of fumarole steam deep reservoir fluid and local meteoric water are practically identical even though 18 o contents range through 4 thus precipitation on the resurgent dome of the caldera could recharge the hydrothermal system by slow percolation from analysis of d and 18 o values between fumarol steam and deep reservoir fluid steam reaches the surface either 1 by vaporizing relatively shallow groundwater at 200 c or 2 by means of a two stage boiling process through an intermediate level reservoir at roughly 200 c although many characteristics of known vapor dominated geothermal systems are found at sulphur springs fundamental differences exist in temperature and pressure of our postulated vapor zone we propose that the reservoir beneath sulphur springs is too small or too poorly confined to sustain a true vapor dominated system and that the sulphur springs system may be a dying vapor dominated system that has practically boiled itself dry

the structural geometry and geological history of archean rocks at kenora north western ontario a proposed type area for the kenoran orogeny the kenora area at the south margin of the english river subprovince consists of polydeformed gneisses intruded by granitoid plutons some of which are also deformed the gneisses are mainly tona

nitrogen in rock occurrences and biogeochemical implications 1 there is a growing interest in the role of bedrock in global nitrogen cycling and potential for increased ecosystem sensitivity to human impacts in terrains with elevated background nitrogen concentrations nitrogen bearing rocks are globally distributed and comprise a potentially large pool of nitrogen in nutrient cycling that is frequently neglected because of a lack of routine analytical methods for quantification nitrogen in rock originates as organically bound nitrogen associated with sediment or in thermal waters representing a mixture of sedimentary mantle and meteoric sources of nitrogen rock nitrogen concentrations range from trace levels 200 mg n kg 1 in granites to ecologically significant concentrations exceeding 1000 mg n kg 1 in some sedimentary and metasedimentary rocks nitrate deposits accumulated in arid and semi arid regions are also a large potential pool nitrogen in rock has a potentially significant impact on localized nitrogen cycles elevated nitrogen concentrations in water and soil have been attributed to weathering of bedrock nitrogen in some environments nitrogen released from bedrock may contribute to nitrogen saturation of terrestrial ecosystems more nitrogen available than required by biota nitrogen saturation results in leaching of nitrate to surface and groundwaters and where soils are formed from ammonium rich bedrock the oxidation of ammonium to nitrate may result in soil acidification inhibiting revegetation in certain ecosystems collectively studies presented in this article reveal that geologic nitrogen may be a large and reactive pool with potential for amplification of human impacts on nitrogen cycling in terrestrial and aquatic ecosystems

exhumation of the high pressure tsakkok lens swedish caledonides insights from the structural and white mica 40ar 39ar geochronological record

influence of deformation and fluids on ar retention in white mica dating the dover fault newfoundland appalachians abstract white mica 40ar 39ar analyses may provide useful constraints on the timing of tectonic processes but complex geological and thermal histories can perturb ar systematics in a variety of ways ductile shear zones represent excellent case studies for exploring the link s between dynamic re neo crystallization of white mica and coeval enhanced fluid flow and their effect on 40ar 39ar dates white mica 40ar 39ar dates were collected from compositionally similar granites that record different episodes of deformation with proximity to the dover fault a terrane bounding strike slip shear zone in the appalachian orogen newfoundland canada 40ar 39ar data were collected in situ by laser ablation and by step heating single crystals results were compared to each other and against complementary u pb zircon and monazite and k ar fault gouge analysis although step heat 40ar 39ar is a widely applied method in orogenic settings this dataset shows that relatively flat step heat 40ar 39ar spectra are in contradiction with wide spreads in in situ 40ar 39ar dates from the same samples and that plateau dates in some cases yielded mixed dates of equivocal geological significance this result indicates that the step wise release of ar from white mica likely homogenizes and obscures spatially controlled ar isotope reservoirs in white mica from sheared rocks in contrast in situ laser ablation 40ar 39ar analysis preserves the spatial resolution of 40ar reservoirs that have been variably reset by deformation and fluid interaction this study therefore suggests that laser ablation is the best method for dating the timing of deformation recorded by white mica final interpretation of results should be guided by microstructural analysis estimation of deformation temperature chemical characterization of white mica and complementary chronometers overall the dataset shows that granitic protoliths were emplaced between 430 and 422 ma u pb zircon high strain deformation along the wing pond shear zone occurred between ca 422 405 ma u pb monazite and 40ar 39ar subsequent patchy ar loss in white mica occurred locally during low t shear

dissolution and replacement creep a significant deformation mechanism in mid crustal rocks abstract zoning patterns and zoning truncations in metamorphic minerals in a granodioritic orthogneiss indicate that strain and s c fabrics in these rocks were produced by dissolution precipitation and replacement processes even at epidote amphibolite facies metamorphic conditions the metamorphic fabric is defined by alternating layers and folia dominated by quartz feldspars and biotite epidote zoning patterns in most metamorphic plagioclase orthoclase epidote and sphene are truncated at boundaries normal to the shortening direction suggesting dissolution interfaces of relict igneous orthoclase phenocrysts that face the shortening direction are embayed and replaced by biotite epidote and myrmekitic intergrowths of plagioclase and quartz metamorphic plagioclase grains are also replaced by epidote we interpret these microstructures to reflect strain enhanced dissolution the cores of many grains show asymmetric overgrowths with at least two generations of beards all oriented on the ends of grains that face the extension direction we interpret these textures to reflect precipitation of components dissolved by deformation enhanced dissolution while biotite and quartz probably deformed by dislocation creep the overall deformation was accommodated by dissolution perpendicular to the shortening direction and precipitation parallel to it these chemical processes must have been activated at lower stresses than the dislocation creep predicted from extrapolations of data from experiments in dry rocks thus wet crust is likely to be weaker than calculated from these experimental studies

40ar loss in experimentally deformed muscovite and biotite with implications for 40ar 39ar geochronology of naturally deformed rocks abstract the effects of deformation on radiogenic argon 40ar retentivity in mica are described from high pressure experiments performed on rock samples of peraluminous granite containing euhedral muscovite and biotite cylindrical cores 15 mm in length and 6 25 mm in diameter were drilled from granite collected from the south armorican massif in northwestern france loaded into gold capsules and weld sealed in the presence of excess water the samples were deformed at a pressure of 10 kb and a temperature of 600 c over a period 29 of hours within a solid medium assembly in a griggs type triaxial hydraulic deformation apparatus overall shortening in the experiments was approximately 10 transmitted light and secondary and backscattered electron imaging of the deformed granite samples reveals evidence of induced defects and for significant physical grain size reduction by kinking cracking and grain segmentation of the micas infrared ir laser co2 heating of individual 1 5 2 5 mm diameter grains of muscovite and biotite separated from the undeformed granite yield well defined 40ar 39ar plateau ages of 311 2 ma 2 identical experiments on single grains separated from the experimentally deformed granite yield results indicating 40ar loss of 0 35 in muscovite and 2 3 40ar loss in biotite intragrain in situ ultraviolet uv laser ablation 40ar 39ar ages 4 10 1 of deformed muscovites range from 309 13 to 264 7 ma consistent with 0 16 40ar loss relative to the undeformed muscovite the in situ uv laser ablation 40ar 39ar ages of deformed biotite vary from 301 to 217 ma consistent with up to 32 40ar loss no spatial correlation is observed between in situ 40ar 39ar age and position within individual grains using available argon diffusion data for muscovite the observed 40ar loss in the experimentally treated muscovite can be utilized to predict average 40ar diffusion dimensions maximum 40ar 39ar ages obtained by uv laser ablation overlap those of the undeformed muscovite indicating argon loss of

influence of tectonic overpressure onp tpaths of hp uhp rocks in continental collision zones thermomechanical modelling the principle of lithostatic pressure is habitually used in metamorphic geology to calculate burial exhumation depth from pressure given by geobarometry however pressure deviation from lithostatic i e tectonic overpressure underpressure due to deviatoric stress and deformation is an intrinsic property of flow and fracture in all materials including rocks under geological conditions in order to investigate the influences of tectonic overpressure on metamorphic p t paths 2d numerical simulations of continental subduction collision zones were conducted with variable brittle and ductile rheologies of the crust and mantle the experiments suggest that several regions of significant tectonic overpressure and underpressure may develop inside the slab in the subduction channel and within the overriding plate during continental collision the main overpressure region that may influence the p t paths of hp uhp rocks is located in the bottom corner of the wedge like confined channel with the characteristic magnitude of pressure deviation on the order of similar to 0 3 gpa and 10 20 from the lithostatic values the degree of confinement of the subduction channel is the key factor controlling this magnitude our models also suggest that subducted crustal rocks which may not necessarily be exhumed can be classified into three different groups i uhp rocks subjected to significant 0 3 gpa overpressure at intermediate subduction depth 50 70 km p 1 5 2 5 gpa then underpressured at depth 100 km p 3 gpa ii hp rocks subjected to 0 3 gpa overpressure at peak p t conditions reached at 50 70 km depth in the bottom corner of the wedge like confined subduction channel p 1 5 2 5 gpa iii lower pressure rocks formed at shallower depths 40 km depth p 1 gpa which are not subjected to significant overpressure and or underpressure

nitrogen and argon isotopic signatures in graphite from the 3 8 ga old isua supracrustal belt southern west greenland abstract the problems involved with the interpretation of carbon isotopes as indicators for early life in highly metamorphosed early archean rocks have prompted the search for additional chemical and isotopic biomarkers here we report an attempt to identify the origin of carbonaceous matter in the 3 8 ga old isua supracrustal belt in southern west greenland by measuring the concentration and isotopic composition of a trapped nitrogen component stepped combustion pyrolysis mass spectrometry of carbonaceous matter in several rock samples revealed three different reservoirs of trapped nitrogen 1 nitrogen associated with a very small amount of reactive carbonaceous material 2 nitrogen intercalated in graphite correlated with intercalated radiogenic argon 3 nitrogen strongly retained at defects or chemically bound in the graphite structure the 15n of nitrogen associated with reactive carbonaceous matter ca 6 overlaps with that of average phanerozoic sedimentary organic matter and is believed to be part of nonindigenous postmetamorphic biologic material in situ raman spectroscopy confirmed the high degree of crystallinity of the metamorphosed indigenous carbonaceous material and this material is further referred to as graphite graphite interpreted as epigenetic associated with mg mn siderite in metacarbonates contains a very small strongly retained nitrogen component with a low 15n ratio 3 to 1 this range overlaps with values that are typically found in archean kerogens but also those of a metamorphically emplaced inorganic basaltic source geological constraints suggest that this graphite incorporated nitrogen from surrounding metabasaltic rocks graphite interpreted as syngenetic and biogenic found in a turbidite deposit is relatively similar to this mg mn siderite derived graphite based on degree of graphite crystallinity amount of trapped radiogenic argon low nitrogen concentration and 15n signature we conclude that nitrogen concentration and its isotope ratio in graphite cannot be used conclusively as a biomarker in these rocks from the highly metamorphosed isua supracrustal belt

the effects of retrograde reactions and of diffusion on 40ar 39ar ages of micas the effects of metamorphic reactions occurring during decompression were explored to understand their influence on the ar ar ages of micas monometamorphic metasediments from the lepontine alps switzerland reached lower amphibolite facies during the barrovian metamorphism related to the collision between european and african adria continental plates mineral assemblages typically composed of garnet plagioclase biotite muscovite and paragonite or margarite were screened for petrological equilibrium to focus on samples that record a minimum degree of retrogression x ray diffraction data indicate that some mineral separates prepared for ar ar stepwise heating analysis are monomineralic whereas others are composed of two white micas muscovite with paragonite or margarite or biotite and chlorite in monomineralic samples ar ar and ar ar proportional to ca k and cl k ratios did not change and the resulting ages can be interpreted unambiguously in mineral separates containing two white micas ca k and cl k ratios were variable reflecting non simultaneous laboratory degassing of the two heterochemical ar reservoirs these ratios were used to identify each ar reservoir and to unravel the age in a chlorite margarite biotite calcschist equilibrated near 5608c and 0 65 gpa biotite margarite and muscovite all yield ages around 18ma at slightly higher grade 560 5808c 0 8 0 9 gpa the assemblage muscovite paragonite plagioclase is in equilibrium and remains stable during retrogression in this case muscovite and paragonite yield indistinguishable ages around 16 5ma above 5908c paragonite was mostly consumed to form plagioclase 45908c whereby the relict mica yields an age up to 5 6 ma younger than muscovite this partial or total resetting of the ar clock in paragonite is interpreted to reflect plagioclase growth during decompression where biotite is present within this same assemblage it systematically yields a younger age than muscovite by 0 5 2 ma however these biotites all show small amounts of retrograde chlorite formation we conclude that even very minor chloritization of biotite is apparently a more effective process than temperature in resetting the ar clock as is the formation of plagioclase from paragonite decomposition multi equilibrium thermobarometry is an excellent means to ensure that equilibrium in investigated samples is preserved and this helps to obtain geologically meaningful metamorphic ages however even samples passing such equilibrium tests may still show retrograde effects that affect the ar retention of micas a more robust interpretation of such ar ar results may require use of a second geochronometer such as u pb on monazite

distribution of ammonium in minerals of metamorphic and granitic rocks abstract sixty nine analyses are given for nh4 in minerals of metamorphic and granitic rocks mostly from the ryoke belt japan the distribution of nh4 in coexisting minerals is quite systematic suggesting that nh4 is one of the stable geochemical components in high temperature processes biotite has the highest content of nh4 followed by muscovite k feldspar and plagioclase pure quartz is almost free from nh4 calcic plagioclase contains less nh4 than does sodic plagioclase the partition coefficients d pl bi d kf bi and d kf bi are on the average 0 11 0 38 and 0 43 respectively the fractionation of nh4 in these minerals is quite similar to that of rb but much smaller than that of cs distribution of nh4 as well as those of rb and cs appears to be explained by its ionic radius and the shortest cation o distances in alkali positions of minerals

paradigms new and old for ultrahigh pressure tectonism regional ultrahigh pressure uhp metamorphic terranes exhibit a spectrum of lithological structural and petrological characteristics that result from the geodynamic processes that formed and exhumed them at least six geodynamic processes can be envisioned to have carried continental rocks to mantle depths i continental margin subduction ii microcontinent subduction iii sediment subduction iv intracontinental subduction v subduction erosion and vi foundering of a crustal root most of these processes have been investigated through numerical or analog models and most have been invoked for one or more specific occurrences of uhp rocks at least six geodynamic processes can be envisioned to have exhumed uhp continental rocks i eduction ii microplate rotation iii crustal stacking iv slab rollback v channel flow and vi trans mantle diapirs most of these processes have also been investigated through numerical or analog models and all have been invoked to explain the exhumation of at least one uhp terrane more detailed and systematic field investigations are warranted to assess the predictions of numerical models and more sophisticated and realistic numerical models are required to replicate and explain the petrological structural and chronological data obtained from uhp terranes

sm nd dating of seve eclogites norrbotten sweden evidence for early caledonian 505 ma subduction the seve nappes consist of long transported thrust sheets belonging to the upper allochthon within the scandinavian caledonides eclogites from two different megalenses within the seve nappe complex in norrbotten northern sweden have been dated with the sm nd method the two eclogite bearing megalenses have been subjected to different pressure and temperature conditions during the high pressure metamorphism garnet omphacite whole rock from ts2 within the tsakkok lens give an age of 505 18 ma 2 i 0 512231 0 000024 mswd 0 10 garnet and whole rock from ts5 and whole rock ts3 also fall on the above isochron the composite isochron gives an age of 503 18 ma 2 these results are interpreted to date the peak of the high pressure metamorphism 500 630 c and 12 15 kbars for the tsakkok lens the eclogites in the vaimok lens at grapesvarre were subjected to higher pressure and temperature conditions and more extensive reequilibration during the early cooling stages retrograde breakdown reactions accompanied by retrograde zoning of relict garnet seems to be associated with disturbance of the sm nd isotopic systems in contrast a sample with unzoned garnet and well preserved high pressure mineralogy from grapesvarre gives a sm nd garnet omphacite age of 503 14 ma 2 i 0 512010 0 000038 2 the ages for the seve eclogites are significantly older than the sm nd eclogite dates from the western gneiss region of norway wgr suggesting the existence of at least two eclogite forming events in the caledonide orogen the younger event has been related to the main continent continent collision stage of the caledonian orogeny while the older event that led to the production of the norrbotten eclogites must have taken place several hundred kilometres to the north and in a different tectonic setting more oceanward to the wgr it appears that the older event ca 505 ma was restricted to the subduction of dyke intruded sedimentary cover rocks which are thought to represent the rifted edge of the baltic continent

correlation of seismotectonic variables and gps strain measurements in western turkey period extending from 1981 to 1998 are examined in the eastern mediterranean and western turkey area this analysis was conducted in three tectonic subdivisions corresponding to regions dominated by shear extensional and compressional deformation the region of shear coincides with the northern anatolian fault zone nafz the region of extension with the aegean back arc region and the region of compression to the aegean subduction zone between the african and anatolian plates the median b value in the region of extension is larger than those observed in the regions of shear and compression and suggests that large magnitude earthquakes are less probable in this region than along the nafz and aegean subduction zone a significant correlation is observed between d2 and d15 with dilatation over the 10 40 km range in the back arc region of extension a marginal correlation between gps derived strain and b value is observed only in the region of compression where it correlates negatively with shear and positively with dilatation a significant positive correlation is observed between d and b along the nafz a clear relationship between d and b does not exist in regions deformed by extension and compression the relationship between d and b along the nafz may also be an indicator of increased seismic hazard since the positive relationship between d and b is a recent characteristic of the nafz observed only in the 20 year period preceding the izmit earthquake index terms 7215 seismology earthquake parameters 7209 seismology earthquake dynamics and mechanics 7223 seismology seismic hazard assessment and prediction 7230 seismology seismicity and seismotectonics keywords gps strain fractal dimension b value

exhumation of high and ultrahigh pressure metamorphic rocks by slab extraction exhumation of high and ultrahigh pressure metamorphic rocks in collisional orogens may be explained by upward extrusion of these rocks erosion of their overburden or extensional thinning of the overburden some high pressure terranes such as the adula nappe in the central alps fit none of these scenarios we propose an additional way in which part of the overburden may be removed it may sink off into the deeper mantle slab extraction structural and metamorphic relationships in and around the adula nappe indicate that the emplacement of this alpine high to ultrahigh pressure nappe to 3 2 gpa in a pile of lower pressure nappes resulted from the interaction of two subduction zones that accommodated the closure of two ocean basins ultimately leading to the extraction of the intervening slab in terms of mechanics the cause of the exhumation is in this case not the buoyancy of the high pressure rocks but the negative buoyancy of the extracted slab

15 n enriched gondwana lamproites eastern india crustal n in the mantle source abstract a comagmatic suite of lamproites ultramafic lamprophyres and minettes was intruded at 100 ma into thick siliciclastic sequences deposited in five intracratonic basins which developed during gondwana rifting in eastern india lamproites are dominated by microphenocrysts of ti rich phlogopite and olivine pseudomorphed by serpentine and talc with variable modal abundances of aegirine amphibole apatite carbonates cr spinel fluorite perovskite priderite and titanomagnetite compositionally lamproites are characterized by extremely high contents of tio2 4 2 10 wt k2o and p2o5 over a range of mg from 80 to 75 in keeping with lamproites worldwide they have high abundances of ba th u and ree 1019 5400 ppm with pronounced fractionation of ree where la smcn 3 5 5 6 and gd ybcn 7 3 18 consistent with residual garnet in the mantle source nitrogen isotope compositions range from 1 6 to 8 7 with an average of 3 8 and n contents average 107 ppm mantle 15n averages 5 with a n content of 2 ppm whereas continental crust varies from 2 to 6 with 30 90 ppm n atmospheric n2 is sequestered by n fixing microorganisms and stored as kerogen with an average 15n of 3 in sedimentary rocks some of the organic n is converted to nh4 which substitutes for k in crustal k silicates the 15n enriched lamproites are interpreted to have formed from partial melting of enriched harzburgitic mantle lithosphere by decompressional melting accompanying extension enrichment resulted from subduction erosion of continental crust or subduction of sediments low degree partial melting hybridization of the melts with mantle lithosphere and incubation prior to decompressional melting consequently some crustal n is recycled to the mantle

regional variations and correlations of gutenberg richter parameters and fractal dimension for the different seismogenic zones in western anatolia we investigated the regional variations of gutenberg richter g r parameters a and b and fractal correlation dimension dc and relations among these parameters for the different regions in western anatolia wa the whole examined area 26 33 e 33 40 5 n is divided into 15 different seismogenic regions based on their tectonic and seismotectonic regimes we used database including 69 182 earthquakes for the instrumental period from 1900 to 2011 we calculated b value which is the slope of the frequency magnitude gutenberg richter relationship from maximum likelihood method ml and dc value which is the slope of log10c r versus log10r from correlation integral using the least squares ls method computed values for 15 different seismogenic regions are mapped using different color scale for different range of values of b and dc regional distributions of these parameters reveal information about regional variation of stress level and geological complexity we concluded aegean arc and aegean islands aliaga fault and buyuk menderes graben the most vulnerable regions for occurrence of the large earthquakes in wa considering the computed lowest b values and the highest dc values in these regions since dc b values are the highest in these regions this ratio may be used as an indicator of earthquake hazard levels of different seismogenic zones in a studied region an effort is made to find relationships between the g r parameters and fractal dimension we observed negative correlation between dc and b values and positive correlation between dc and a b values for different regions of wa we observed that the relationship between a b and dc can be used for seismicity earthquake risk and hazard studies because of the computed high correlation coefficient and fewer scattering of these parameters

the behavior of nitrogen and nitrogen isotopes during metamorphism and mineralization evidence from the otago and alpine schists new zealand abstract metamorphism is a major mechanism for the re distribution of fluids and mass in the earth s crust with these processes most prominently highlighted by the occurrence of major gold resources within these terranes however although orogenic gold deposits have contributed over 20 of the global gold production their origins remain controversial the nitrogen concentration and isotopic composition of rocks and minerals are potentially powerful tracers of crustal metamorphism and mineralization but there have been few detailed applications of this approach to date although nitrogen isotopes have recently been used to elucidate the source of fluids in some neoarchean orogenic gold deposits and proterozoic to paleozoic mountain belts due to their age and geological complexity of these terranes major uncertainties as to the behavior of nitrogen remain the otago and alpine schists in the south island of new zealand comprise a large comparatively young the range of 15n values for mica and whole rock samples from the schists spans 0 2 to 7 0 and the nitrogen concentration from 23 to 3483 ppm sample provenance and rock type have minimal influence on the nitrogen concentration and isotopic value which appears to have been inherited from the original sedimentary kerogen there is no systematic variation between metamorphic temperature and 15n or n concentration in micas suggesting that there has been little discernible loss of 15n depleted fluids from silicates with increasing metamorphic temperature comparison of fluid mobile alkali element k rb and cs carbon and nitrogen whole rock concentrations indicates that for the otago and alpine schists in rocks up to upper greenschist facies significant nitrogen remains hosted in phases other than micas most likely poorly matured carbonaceous material samples from macraes flat a major gold producer have a similar range of 15n values to the host terrane but show distinctly higher nitrogen concentrations relative to unmineralized samples due to the incorporation of nitrogen from the mineralizing hydrothermal fluid this suggests that there is only a subtle metamorphic re distribution of nitrogen during mineralization albeit with minimal isotopic fractionation in the case of nitrogen at least the isotopic signatures of mineralized rocks support a metamorphic fluid source and are inconsistent with mantle or meteoric fluid reservoirs however due to the high relative abundance of nitrogen in sedimentary rocks compared to other potential reservoirs unless fluid fluxes are very large and well channeled nitrogen signatures are not sensitive recorders of fluid inputs from mantle magmatic or meteoric reservoirs conversely the absence of a sedimentary source nitrogen isotope signature similar to the host rock in an orogenic deposit would be a very strong indicator of an external exotic source for the mineralizing fluids

precambrian ammonium biogeochemistry a study of the moine metasediments scotland abstract the content and isotopic composition of ammoniacal nitrogen have been determined in a suite of amphibolite facies metasediments of precambrian age whole rock samples and mica separates from the moine succession proterozoic of scotland were analysed by a combination of capacitance manometry and static vacuum mass spectrometry prior to the analyses the presence of the ammonium ion was confirmed by obtaining infrared ir absorption spectra from the micas principally biotites whole rock ammonium contents are high for amphibolite facies metasediments 140 422 ppm the ammonium ion was not detected in the ir absorption spectra of biotites from one schist however this sample contains neither garnet nor white mica for most of the samples the 15n values are very positive between 12 4 and 16 6 with one sample having a lower value of 8 4 the major variations between samples can be interpreted in terms of original sedimentary features for the ammonium bearing samples muscovites have concentrations between 365 and 633 ppm biotites between 845 and 1739 ppm for individual samples nh4 mu nh4 bi varies between 0 31 and 0 43 a much narrower range than has been reported previously there are no significant or consistent differences in 15n between co existing biotite and white mica nor between the 15n values of the mineral separates and their respective whole rock values the latter suggests minimal selective partitioning of isotopes between micas and plagioclase feldspar the only other potential host for ammonium in the rocks assuming that the ammonium has been derived from organic matter originally present at the time of deposition and by comparison with previous studies it can be inferred that the original 15n value of the moine sediments was somewhere between 5 and 10 similar to recent sediments this suggests that the nitrogen cycle in the proterozoic shelf sea from which the moine sediments were deposited was broadly similar to the present day overall the study underlines how under developed the n system is compared to other stable isotope systems notably c and s and we use the opportunity to highlight some areas that could be developed in the future however one aspect is clear future ammonium studies of metamorphic rocks will need to be performed at high resolution visible scale of heterogeneity with strong mineralogical petrological and field controls

nitrogen and argon signatures in 3 8 to 2 8 ga metasediments clues on the chemical state of the archean ocean and the deep biosphere abstract n and ar elemental and isotopic analyses were conducted on archean metasediments of isukasia west greenland and pilbara craton western australia in order to investigate the n isotopic evolution during the first half of earth s history the selected samples are deep sea sediments and hydrothermal deposits having ages from 3 8 to 2 8 ga and affected by different degrees of metamorphism the release patterns of n and ar obtained by high resolution stepped combustion show the occurrence of at least two trapped components the first is released at 600 c and it is likely contained in fluid inclusions n is released together with primordial 36ar and shows a 15n value of 1 3 1 0 close to that of modern atmospheric n2 15n 0 this component is well preserved in hydrothermal vent silica deposits of north pole pilbara craton and nitrogen may represent ammonium salt dissolved in deep sea hydrothermal fluids the second n component released at temperatures higher than 1000 c is accompanied by radiogenic 40ar and shows a 15n value of 7 4 1 0 in a kerogen rich chert from north pole pilbara craton this n is likely biogenic and negative 15n values may reflect a metabolic isotopic fractionation induced by chemosynthetic bacteria using inorganic nh4 contained in hydrothermal fluids this 15n depleted biogenic component may occur in isukasia banded iron formation 15n 1 7 but further data are needed to confirm such a hypothesis in all other samples metamorphic induced rayleigh distillation has altered the pristine n isotopic signature

polyorogenic 40 ar 39 ar mineral age record in the seve and koli nappes of the gaddede area northwestern jamtland central scandinavian caledonides in the scandinavian caledonides eugeoclinal terranes represented in the koli nappes were thrust 500 km onto the baltoscandian platform during the early to middle paleozoic immediately underlying thrust sheets of the seve nappe complex are suspected to have been derived from the outer margin of continent baltica contrasting tectonothermal histories are recorded in seve and koli nappes exposed in the gad dede area northern jamtland county sweden eclogite bearing seve rocks underwent an initial amphibolite grade regional metamorphism smi with different structural units experiencing contrasting high to intermediate pressure conditions subsequent ductile imbrication sm2 of high and intermediate pressure structural units occurred prior to 460 470 ma cooling through temperatures required for intracrystalline retention of argon in hornblende a later thermal overprint sm3 everywhere rejuvenated seve biotite and muscovite and locally hornblende argon systems at 430 pm 5 ma 40 ar 39 ar plate

microfossil like objects from the archaean of greenland a cautionary note recent reports1 4 have described yeast like microfossils isuasphaera isua pflug2 in 3 800 myr old5 metaquartzites from the isua supracrustal belt of south west greenland a biogenic interpretation of these objects is inconsistent with the tectonic history of the isua region6 8 with the petrology of the metaquartzites and with the morphology of the microstructures themselves the putative microfossils are indistinguishable from limonite stained fluid inclusions microstructures which are demonstrably inorganic and post depositional in origin as such we contend here that these objects should not be regarded as evidence of early archaean life forms

pillow lavas as protoliths for eclogites evidence from a late precambrian cambrian continental margin seve nappes scandinavian caledonides pillow lavas containing an eclogitic mineral assemblage locally with glaucophane crossite are described from the tsakkok lens within the seve nappes north central scandinavian caledonides critical primary relationships to the host metasedimentary rocks which are dominated by marble and quartz garnet phengite schist are preserved it is argued that these clastic rocks and the ongoing igneous activity in the tsakkok lens are late precambrian to cambrian in age and that these rocks were deposited emplaced along the outermost part of a continental margin related to the continent baltica

exhumation of high pressure rocks a review of concepts and processes the exhumation of high pressure metamorphic rocks requires either the removal of the overburden that caused the high pressures or the transport of the metamorphic rocks through the overburden exhumation cannot be achieved simply by thrusting or strike slip faulting it may be caused by erosion of shortened and thickened crust but this is unlikely to be the only mechanism for exhuming rocks from depths greater than about 20 km one or more of the following additional mechanisms may be involved 1 corner flow of low viscosity material trapped between the upper and lower plates in a subduction zone can cause upward flow of deeply buried rock and may explain some occurrences of high pressure tectonic blocks in melange this process does not however appear to be adequate to explain the exhumation of regional high pressure terrains 2 buoyancy forces acting directly on metamorphic rock bodies may cause them to rise relative to more dense surroundings this is likely to be the most important mechanism of exhumation of crustal rocks subducted into the mantle but cannot explain the emplacement of coherent tracts of high density metamorphic rock into shallow crustal levels some high pressure blocks emplaced at shallow levels in accretionary terrains may have been entrained in diapiric intrusions of low density mud or serpentinite 3 extension driven by the forces associated with contrasts in surface elevation may explain the exhumation and structural setting of many high pressure terrains extension may occur in the upper part of an accretionary wedge thickened by underplating or it may affect the whole lithosphere in a region of intracontinental convergence if surface elevation has been increased by the removal of a lithospheric root in the second case extension may be accompanied by magmatism and an evolution towards higher temperature during decompression of the metamorphic terrain

extension parallel folding in the scandinavian caledonides implications for late orogenic processes abstract structural analyses of the devonian basins and basement rocks in the scandinavian caledonides show the existence of large scale e w oriented folds which affect the para autochthonous western gneiss region of the baltic craton these folds mostly occur in areas of widespread and intense late orogenic extension chronology based on structural relationships both within the devonian sediments and caledonian rocks shows that folding has been active from the first stages of the ductile extension and ended after devonian basin sedimentation folding evolved through time towards crustal scale transcurrent left lateral faulting in the zones of maximum deformation such an evolution is explained by progressive unloading of the western gneiss region due to extension which involved a permutation of the maximum stress from a vertical position at the onset of extension to a horizontal position in the subsequent stages folding parallel to late orogenic extension as well as left lateral translation on east to northeast trending crustal scale faults suggest the occurrence of a regional direction of compression perpendicular to the direction of extension this in turn argues for an external force triggering the late orogenic extensional process in the scandinavian caledonides

microdiamond discovered in the seve nappe scandinavian caledonides and its exhumation by the vacuum cleaner mechanism when a continent collides with an island arc or other continent continental crust of the subducted continent may be buried to depths exceeding 100 km and exposed to pressures that can cause formation of coesite and diamond this process leads to substantial density increase in sio2 rich rocks and in turn to a reduction of the buoyancy of the subducted material which should inhibit exhumation nevertheless coesite and diamond bearing continental crustal rocks are known from several occurrences worldwide we report on the discovery of microdiamond in kyanite garnet gneiss from allochthonous metasediments of the seve nappe complex in the scandinavian caledonides our discovery calls for general reconsideration of existing exhumation models of deeply subducted continental crust we propose that the diamond bearing rocks were subducted in an arc continent collision setting and their exhumation was facilitated by local pressure reduction resulting from extraction of the forearc lithospheric block

subduction ultrahigh pressure metamorphism and regurgitation of buoyant crustal slices implications for arcs and continental growth abstract intracontinental collisional belts that retain mineralogic relics of phanerozoic ultrahigh pressure uhp metamorphism are increasingly being recognized adjacent regions generally lack evidence of coeval calcalkaline volcanism plutonism following consumption of intervening oceanic lithosphere each orogen marks the site of a deeply subducted microcontinental promontory fragments of which later returned to midcrustal levels mafic and ultramafic rocks are volumetrically minor pressures in uhp complexes approached or exceeded 2 8 gpa at 600 900 c subduction zones involve low t prograde gradients and constitute the only plate tectonic realm where such conditions exist although internal portions of descending plates may have even lower geothermal gradients crustal upper margins are typified by p t trajectories of 5 10 c km attending underflow clinoamphibole rich metabasaltic rocks dehydrate at pressures 2 2 2 5 gpa this accounts for the kinetically favorable solid state production of mafic eclogites and partial fusion of morbs to form hydrous andesitic melts devolatilization of partly serpentinized mantle lithosphere also introduces h 2 o to the magmagenic zone as large masses of continental crust enter deeper portions of subduction zones mineralogic transformations slow or cease because micas the major oh bearing phases in such rocks retain volatile contents to pressures 4 gpa persistence of hydroxyl bearing layer silicates may explain the metastable preservation of low density assemblages in continental crust subjected to uhp conditions and the dearth of calcalkaline magmatism in most collision belts exhumed uhp terranes are intensely deformed and disaggregated all consist dominantly of quartzofeldspathic compositions mafic eclogites garnet peridotites constitute only a few volume percent eventual decoupling of the relatively low density subducted crust promotes buoyancy propelled transport to shallow depths along the subduction channel with the hanging wall lithosphere acting as a stress guide extensional collapse and erosional unroofing of rising masses result in the continued ascent of deeply subducted but buoyant crustal material although equidimensional massifs must return surfaceward large volume to surface areas render the contained uhp phase assemblages virtually unquenchable surviving uhp terranes thus consist of relatively thin slabs of continental crust allochthons that retain uhp relics apparently lost heat by conduction across both upper normal fault and lower reverse fault contacts based on present exposures estimated thicknesses of uhp sheets are qinling dabie sulu belt 5 km kokchetav complex 1 3 km dora maira massif 1 2 km western gneiss region 1 km slices evidently rose to midcrustal levels at exhumation rates approaching 10 mm per year back reaction attending decompression in all cases was nearly complete where uhp mineral inclusions persisted retrogression apparently was limited by the relative impermeability of the rocks to catalytic aqueous fluids and by declining temperatures the coarse grain size of host phases also disfavored complete retrogression

the timing of stabilisation and the exhumation rate for ultra high pressure rocks in the western gneiss region of norway new petrographic evidence and a review of the latest radiometric age data are taken to indicate that formation of the ultra high pressure uhp eclogites within the western gneiss region of norway probably occurred within the 400 410 ma time frame thus this event took place significantly later than the previous widely accepted age of c 425 ma for the timing of the high pressure metamorphism in this part of the scandinavian caledonides garnet growth under uhp coesite stable conditions is recognised as a discrete younger event following on from earlier garnet formed under firstly amphibolite facies then quartz stable eclogite facies conditions currently the best constrained and most precise age specifically for uhp mineral growth is the 402 2 ma u pb age for metamorphic zircon some of which retain coesite inclusions from the hareidland eclogite exhumation must have followed shortly thereafter and based on synoptic pressure temperature and depth time curves must have been very fast our data and those of others indicate an initial fast exhumation to about 35 km depth by about 395 ma at a mean rate of about 10 mm a 1 this rapid exhumation rate may have been driven by the appreciable residual buoyancy of the deeply subducted continental crustal slab due to incomplete eclogitization of the dominant proterozoic orthogneisses during the short lived uhp event subsequent exhumation to 8 10 km depth by about 375 ma occurred at a much slower mean rate of about 1 3 mm a 1 with the late stage extensional collapse of the caledonian orogen playing an increasingly important role especially in the final unroofing of the western gneiss region with some remarkably preserved uhp rocks

an 40ar 39ar thermochronology of the ofoten troms region implications for terrane amalgamation and extensional collapse of the northern scandinavian caledonides fifteen 40ar 39ar cooling ages are reported for metamorphic hornblende and muscovite from far traveled terranes constituting the ofoten nappe stack of northern norway eight cooling ages on hornblende range from 425 to 394 ma and seven muscovite ages from the same or nearby outcrops as the hornblendes range from 400 to 373 ma these data are compared with 40ar 39ar ages from over a large part of the northern caledonides to evaluate regional mineral cooling patterns results indicate that 1 scandian silurian devonian metamorphism was predominant 2 most of the nappes investigated contain some vestige of pre scandian tectonism and or metamorphism 3 hornblende and muscovite cooling ages are progressively younger to the west and south which suggests a hinged to the east mineral cooling pattern and 4 a late out of sequence thrust is the only disruption of this cooling pattern synmetamorphic amalgamation of the nappes resulted from scandian a type subduction the hinged to the east mineral cooling pattern implies isostatic adjustment and exhumation of the footwall of a west dipping crustal scale extensional fault located somewhere west of the present norwegian coast during late synorogenic gravitational collapse the late out of sequence fault formed contemporaneously with uplift in the hinterland implying a kinematic and temporal connection with east directed contractional faulting in the foreland

high temperature deformation during continental margin subduction exhumation the ultrahigh pressure western gneiss region of norway abstract a new dataset for the high pressure to ultrahigh pressure western gneiss region allows the definition of distinct structural and petrological domains much of the study area is an e dipping homocline with e plunging lineations that exposes progressively deeper more strongly deformed more eclogite rich structural levels westward although eclogites crop out across the wgr scandian deformation is weak and earlier structures are well preserved in the southeastern half of the study area the scandian reworking increases westward culminating in strong scandian fabrics with only isolated pockets of older structures the dominant scandian deformation was coaxial e w stretching the sinistrally sheared more trondelag fault complex and nordfjord mylonitic shear zone bound these rocks to the north and south there was moderate top e amphibolite facies deformation associated with translation of the allochthons over the basement along its eastern edge and the nordfjord sogn detachment zone underwent strong lower amphibolite facies to greenschist facies top w shearing a northwestward increase in exhumation related melting is indicated by leucosomes with hornblende plagioclase and scandian sphene in the western 2 3 of the study area exhumation related amphibolite facies symplectite formation in quartzofeldspathic gneiss postdated most scandian deformation further deformation was restricted to slip along biotite rich foliation planes and minor local folding that the western gneiss region quartzofeldspathic gneiss exhibits a strong gradient in degree of deformation implies that continental crust in general need not undergo pervasive deformation during subduction

mixing properties of phengitic micas and revised garnet phengite thermobarometers mixing properties for muscovite celadonite ferroceladonite solid solutions are derived from combining available experimental phase equilibrium data with tabulated thermodynamic data for mineral end members when a partially ordered solution model is assumed the enthalpy of mixing among the end members muscovite celadonite ferroceladonite is nearly ideal although the gibbs energies of muscovite celadonite and muscovite ferroceladonite solutions are asymmetric due to an asymmetry in the entropy of mixing thermodynamic consistency is achieved for data on phengite compositions inassemblages with a pyrope kyanite quartz coesite b almandine kyanite quartz coesite c talc kyanite quartz coesite and d garnet phengite pairs equilibrated both experimentally at high temperatures and natural pairs from low grade schists the muscovite paragonite solvus has been reanalysed using the asymmetric van laar model and the effects of the phengite substitution into muscovite have been quantitatively addressed in order to complete the simple thermodynamic mixing model for the solid solution among the mica end members results are applied to a natural pyrope coesite phengite talc rock from the western alps and to investigate the conditions under which biotite bearing mica schists transform to whiteschist like biotite absent assemblages for average pelite bulk compositions

subduction and eduction of continental crust major mechanisms during continent continent collision and orogenic extensional collapse a model based on the south norwegian caledonides during continental collision in the middle silurian the thickness of the lithosphere under the caledonides of s norway was doubled by subduction of the western margin of baltica including the western gneiss region under laurentia crustal rocks of the baltic plate reached sub moho depths of near 100 km or more as inferred from the presence of coesite in eclogites isostatic calculations indicate an average elevation of the mountain chain of about 3 km at this stage the subducted lithosphere experienced vertical constrictional strains as a result of slab pull by its heavy and cold root eduction of the deeply buried crustal material was initiated by decoupling of the thermal boundary layer in the subducted lithosphere isostatic rebound resulted in very rapid uplift 1 2 mm yr 1 and the deep crust was exhumed mainly by tectonic extensional stripping over a period of 30 40 myr the eduction was probably related to a rolling hinge footwall uplift mechanism and the early high pressure coaxial fabrics were overprinted by extensional simple shear as the deep crust reached middle and upper crustal levels the model explains the present day normal crustal thickness under the exhumed deep rocks without necessarily invoking large scale lateral flow of material in the lower crust or igneous underplating

monazite geochronology of uhp and hp metamorphism deformation and exhumation nordoyane western gneiss region norway u th pb monazite geochronology is combined with previous structural analysis and quantitative estimates of metamorphic conditions to date the thermotectonic evolution of uhp and hp plates 820 c 39 kbar 780 c 18 kbar metamorphosed during the late silurian early devonian collision between baltica and laurentia the upper plate contains a microdiamond bearing kyanite garnet graphite gneiss and associated kyanite eclogites independently indicating probable diamond forming conditions in situ dating of monazite in the microdiamond sample using the shrimp ii at the geological survey of canada yielded ages of 415 6 8 ma for those included in garnet and 398 6 ma for those in the matrix these ages compare to 408 0 5 6 and 397 5 4 4 ma determined using the electron microprobe at the university of massachusetts both methods also identified complexly zoned detrital cores up to 150 micrometers in diameter with ages of 1100 950 ma and scattered grains with ages of 900 500 ma but no ages of 1680 1650 ma equivalent to the local baltica basement were found agreement between the two techniques allowed evaluation of monazite age domains 198 analyses from the microdiamond rock and a kyanite garnet sillimanite mylonite produced from it using a combination of high resolution element imaging and trace element analysis of u th pb and y this comparison yielded three mean ages of 407 0 2 1 ma 394 8 2 3 ma and 374 6 2 7 ma combining this geochronology with previous p t estimates we propose that the uhp unit reached its maximum depth of 125 km at a maximum age of 407 ma when monazite was included in garnet and experienced 65 km of exhumation at an average rate of 10 9 mm year during top southeast thrusting that brought it into contact with the hp unit following these events both units were exhumed together at an average rate of 3 8 mm year until reaching a depth of 37 km at 395 ma where these rocks experienced extensive re equilibration and top west and left lateral shearing after 395 ma these units continued to be exhumed at an average rate of 0 8 to 1 4 mm year until 375 ma the time of last equilibration of asymmetric monazite porphyroclasts in the mylonite the exhumation histories of these units record a change in mechanism from syncollisional exhumation through late to post orogenic collapse that was a consequence of plate reorganization

ammonium contents of biotites from precambrian rocks in finland the significance of nh4 as a possible chemical fossil abstract the ammonium contents of biotites in precambrian rocks from finland have been determined to examine the possibility that ammonium in biotites represents a chemical fossil if biologic activity existed in precambrian sedimentary environments biotites formed from the sediments should contain distinctly larger quantities of nh 4 than biotites from igneous rocks biotites from svecokarelidic metasediments which were originally products of sedimentation about 2400 1900 ma ago have high nh 4 contents hundreds of ppm while biotites from svecokarelidic plutonic rocks of 1800 1900 ma old and postsvecokarelian rapakivi granites of 1650 1700 ma old have low nh 4 contents tens of ppm the results may indicate that biologic activity was plentiful during sedimentation of the svecokarelian strata this agrees with evidence for ancient life found in dolomites and greywacke slates of the svecokarelides biotites from presvecokarelidic schists of more than 2600 2800 ma old have only tens of ppm of nh 4 this value is at the same level as those of biotites from the rapakivi granites and the svecokarelidic plutonic rocks considering the rarity of fossils in presvecokarelidic rocks the low nh 4 content of biotite from a presvecokarelidic pelitic schist may indicate less biologic activity during the sedimentation of the original sediments these data emphasize that ammonium in biotites of precambrian metasediments may be a useful chemical fossil for identifying the geologic sites of ancient life

plate rotation during continental collision and its relationship with the exhumation of uhp metamorphic terranes application to the norwegian caledonides lateral variation and asynchronous onset of collision during the convergence of continents can significantly affect the burial and exhumation of subducted continental crust here we use 3 d numerical models for continental collision to discuss how deep burial and exhumation of high and ultrahigh pressure metamorphic hp uhp rocks are enhanced by diachronous collision and the resulting rotation of the colliding plates rotation during collision locally favors eduction the inversion of the subduction and may explain the discontinuous distribution of ultra high pressure uhp terranes along collision zones for example the terminal scandian collision of baltica and laurentia which formed the scandinavian caledonides resulted in the exhumation of only one large hp uhp terrane the western gneiss complex wgc near the southern end of the collision zone rotation of the subducting baltica plate during collision may provide an explanation for this distribution we explore this hypothesis by comparing orthogonal and diachronous collision models and conclude that a diachronous collision can transport continental material up to 60 km deeper and heat material up to 300 c hotter than an orthogonal collision our diachronous collision model predicts that subducted continental margin material returns to the surface only in the region where collision initiated the diachronous collision model is consistent with petrological and geochonological observations from the wgc and makes predictions for the general evolution of the scandinavian caledonides we propose the collision between laurentia and baltica started at the southern end of the collisional zone and propagated northward this asymmetric geometry resulted in the counter clockwise rotation of baltica with respect to laurentia consistent with paleomagnetic data from other studies our model may have applications to other orogens with regional uhp terranes such as the dabie shan and papua new guinea cases where block rotation during exhumation has also been recorded

continental plate collision p t t z conditions and unstable vs stable plate dynamics insights from thermo mechanical modelling there was much debate recently on the mechanisms of continental convergence and related pressure temperature p t conditions both in modeling and petrologic community depending on the mechanisms of convergence subduction collision folding or rt instability one can argue about the possibility of large scale deviations of pressure and temperature in the accretion prism and below it from reference i e lithostatic conditions commonly used for petrologic reconstructions of evolution of exhumed metamorphic rocks these deviations can be caused for example by tectonic overpressure that as suggested in some studies may be responsible for formation of uhp ultra high pressure rocks however overpressure in exhumation zones can be built only in specific collision scenarios associated with plate coupling pure shear folding in this study we analyze conditions that define various mechanisms of convergence and consequently of exhumation these mechanisms can be represented as a superposition of 1 simple shear subduction 2 pure shear collision 3 folding and 4 rayleigh taylor instability we study these scenarios using a thermo mechanical model that accounts for brittle elastic ductile rheology surface processes and metamorphic changes it appears that stable oceanic type subduction may occur in the case of cold lithospheres tmohob550 c and relatively high convergence rates n3 5 cm yr depending on the lower crustal rheology strong or weak either the whole upper and lower crust or only the lower crust can be involved in subduction in case of weak metamorphic rheologies phase changes improve chances for stable subduction pure shear becomes a dominant mechanism when tmohon550 c or convergence rates are lower than 3 cm yr large scale folding is favored in case of tmoho 500 650 c and is more effective in the case of mechanical coupling between crust and mantle e g strong lower crust gravitational rayleigh taylor instabilities overcome other mechanisms for very high values of tmoho n800 c and may lead to development of subvertical cold spots however it is reasonable to assume that in most cases continental collision is initiated at oceanic subduction rate which is rarely slower than 5 cm yr this rate is sufficient to drive continental subduction during the first several myr of collision in this case the subduction channel is characterized by nearly lithostatic pressure conditions large scale zones of tectonic overpressure may be built outside the channel but do not affect the exhumed rocks overpressure may be built inside the channel in the short moment of its closure we suggest that most continental orogenic belts could have started their formation from continental subduction this evokes small tectonic overpressures and thus deep origin of the uhp rocks that may be brought to the surface via the suggested multi level mechanism of exhumation

isotopic variations of the fixed nitrogen in metamorphic rocks the content and isotopic composition of fixed nitrogen nh 4 n in metasedimentary rocks phyllites mica schists and gneisses from prograde metamorphic zones in the erzgebirge saxon ore mountains g d r were measured and show that the nitrogen content decreases and 15n increases with increasing metamorphic grade the geochemical and isotopic behaviour of nitrogen during contact metamorphism was investigated in a mica schist profile through the contact aureole of a granitic pluton the nitrogen concentrations of the wall rocks decrease towards the contact whereas the 15n values become higher various model experiments were carried out in the laboratory to explain the changes observed in the rock samples the nitrogen isotope fractionation factor was determined for the isotopic exchange in the system ammonium sulphate ammonia at 250 c 1 0143 0 0012 and 350 c 1 0128 0 0006 in accordance with the model experiments the conclusion seems warranted that the change in nitrogen content and its isotopic composition in rocks during metamorphic processes depend mainly on the metamorphic temperature

distribution and significance of extraneous argon in uhp eclogite sulu terrain china insight from in situ 40ar 39ar uv laser ablation analysis in situ 40ar 39ar uv laser ablation analyses of garnet omphacite epidote phengite barroisite and symplectite in a sample of uhp eclogite from qinglongshan sulu terrain china reveal extremely heterogeneous argon isotopic ratios results from the 220 240 ma eclogite indicate a significant and variable extraneous argon signature that is restricted to the early formed phengite and epidote and later formed barroisite individual compositionally zoned phengite grains record a large variation in 40ar 39ar dates 300 1100 ma that are positively correlated with celadonite content these data are consistent with retrograde chemical exchange and argon isotopic repartitioning in response to decreasing partial pressures of argon during decompression consistent with the observation of atmospheric argon ratios in garnet omphacite and symplectite these data support an interpretation that the extraneous ar was incorporated early in the crystallization history of the eclogite and not from a late infiltrating fluid the ultimate source of the extraneous argon remains speculative however on the basis of supporting stable oxygen and hydrogen isotope data from this locality which support a closed isotopic system it is inferred to have been inherited from the protolith of the uhp eclogite

monazite allanite phase relations in metapelites abstract calculations of the transition from allanite to monazite bearing assemblages in typical pelitic bulk compositions have been made using thermodynamic data estimated from oxide sums and inferred from natural parageneses calculations in the cfashpce and mnnckfmashpce systems place the allanite to monazite transition in the middle amphibolite facies 525 600 c for a bulk composition similar to shaw s average pelite the temperature of the transition is pressure dependent and strongly dependent on the bulk rock cao content consistent with inferences from natural parageneses the transition is also a function of the bulk al 2 o 3 content although the calculated result is opposite to that inferred from natural samples comparison with published results in the la mg system suggest that the nature of the ree phosphate does not greatly influence the conditions of the transition

divergent plate motion drives rapid exhumation of ultra high pressure rocks abstract exhumation of ultra high pressure u hp rocks by upper plate divergent motion above an unbroken slab first proposed in the western alps has never been tested by numerical methods we present 2d thermo mechanical models incorporating subduction of a thinned continental margin beneath either a continental or oceanic upper plate followed by upper plate divergent motion away from the lower plate results demonstrate how divergent plate motion may trigger rapid exhumation of large volumes of u hp rocks directly to the earth s surface without the need for significant overburden removal by erosion model exhumation paths are fully consistent with natural examples for a wide range of upper plate divergence rates exhumation rates are systematically higher than the divergent rate imposed to the upper plate and the modeled size of exhumed u hp domes is invariant for different rates of upper plate divergence major variations are instead predicted at depth for differing model scenarios as larger amounts of divergent motion may allow mantle wedge exhumation to shallow depth under the exhuming domes the transient temperature increase due to ascent of mantle wedge material in the subduction channel has a limited effect on exhumed continental u hp rocks already at the surface we test two examples the cenozoic u hp terranes of the western alps continental upper plate and eastern papua new guinea oceanic upper plate the good fit between model predictions and the geologic record in these terranes encourages the application of these models globally to pre cenozoic u hp terranes where the geologic record of exhumation is only partly preserved

when can muscovite 40ar 39ar dating constrain the timing of metamorphic exhumation abstract metamorphic cooling rates determined using muscovite 40 ar 39 ar ages underpin many tectonic models but are based on several simplifying approximations we demonstrate that a number of these approximations especially those relating to the assignment of a single closure temperature might not hold for certain metamorphic scenarios and particularly for metamorphism during rapid orogenic cycles using numerical diffusion models that include a recently reported significant pressure dependence of ar diffusion in muscovite we systematically interrogate the approximations associated with linking 40 ar 39 ar dates to pressure temperature histories the results of the simulations are presented in a simple graphic form allowing evaluation of the pressure temperature pt regions in which ar diffusion is efficient and hence those in which the muscovite may yield true 40 ar 39 ar cooling ages we suggest that a robust method for determining whether metamorphic muscovite 40 ar 39 ar ages relate to the timing of cooling involves 1 the determination of the pt conditions and relative timing of muscovite crystallisation during the metamorphic cycle 2 the collection of high precision and high spatial resolution 40 ar 39 ar data from muscovite grains and 3 the comparison between analytical data and numerical diffusion models which test different post crystallisation pressure temperature histories

buoyancy driven rapid exhumation of ultrahigh pressure metamorphosed continental crust abstract preservation of ultrahigh pressure uhp minerals formed at depths of 90 125 km require unusual conditions our subduction model involves underflow of a salient 250 150 km wide 90 125 km long of continental crust embedded in cold largely oceanic crust capped lithosphere loss of leading portions of the high density oceanic lithosphere by slab break off as increasing volumes of microcontinental material enter the subduction zone buoyancy driven return toward midcrustal levels of a thin 2 15 km thick low density slice finally uplift backfolding normal faulting and exposure of the uhp terrane sustained over 20 million years rapid 5 mm year exhumation of the thin aspect ratio uhp sialic sheet caught between cooler hanging wall plate and refrigerating downgoing lithosphere allows withdrawal of heat along both its upper and lower surfaces the intracratonal position of most uhp complexes reflects consumption of an intervening ocean basin and introduction of a sialic promontory into the subduction zone uhp metamorphic terranes consist chiefly of transformed yet relatively low density continental crust compared with displaced mantle material otherwise such complexes could not return to shallow depths relatively rare metabasaltic metagabbroic and metacherty lithologies retain traces of phases characteristic of uhp conditions because they are massive virtually impervious to fluids and nearly anhydrous in contrast h2o rich quartzofeldspathic gneissose schistose more permeable metasedimentary and metagranitic units have backreacted thoroughly so coesite and other uhp silicates are exceedingly rare because of the initial presence of biogenic carbon and its especially sluggish transformation rate uhp paragneisses contain the most abundantly preserved crustal diamonds

ammonium loss and nitrogen isotopic fractionation in biotite as a function of metamorphic grade in metapelites from western maine usa abstract ammonium fixed in micas of metamorphic rocks is a sensitive indicator both of organic inorganic interactions during diagenesis as well as of the devolatilization history and fluid rock interaction during metamorphism in this study a collection of geochemically well characterized biotite separates from a series of graphite bearing paleozoic greenschist to upper amphibolite facies metapelites western maine usa were analyzed for ammonium nitrogen nh 4 n contents and isotopic composition 15nnh4 using the hf digestion distillation technique followed by the ea irms technique biotite separates sampled from 9 individual metamorphic zones contain 3000 to 100 ppm nh 4 n with a wide range in 15n from 1 6 to 9 1 average nh 4 n contents in biotite show a distinct decrease from about 2750 ppm for the lowest metamorphic grade 500 c down to 218 ppm for the highest metamorphic grade 685 c decreasing abundances in nh 4 are inversely correlated in a linear fashion with increasing k in biotite as a function of metamorphic grade and are interpreted as a devolatilization effect despite expected increasing 15nnh4 values in biotite with nitrogen loss a significant decrease from the garnet zones to the staurolite zones was found followed by an increase to the sillimanite zones this pattern for 15nnh4 values in biotite inversely correlates with mg mg fe ratios in biotite and is discussed in the framework of isotopic fractionation due to different exchange processes between nh 4 nh 3 or nh 4 n 2 reflecting devolatilization history and redox conditions during metamorphism

the tectonic significance of pre scandian 40ar 39ar phengite cooling ages in the caledonides of western norway pre silurian continental margin deposits in western norway non conformably overlying allochthonous continental orthogneisses retain ordovician 40 ar 39 ar cooling ages for phengites implying either rapid cooling immediately after a late ordovician orogenic event or less likely a slow cooling following an early ordovician or older orogeny the dalsfjord suite hoyvik group basement cover pair are probably a lateral equivalent to late proterozoic sandstones sparagmites covering the jotun nappe gneisses of the middle allochthon in central south norway the hoyvik group underwent polyphase deformation greenschist facies metamorphism t max 40 ar loss probably occurred during subsequent scandian deformation and late to post orogenic extension the hoyvik group rocks were unroofed before the wenlock time 423 428 ma and cooled through the temperature for argon retention in phengite at c 447 4 ma indicating a maximum cooling rate between 14 and 22c ma 1 through ashgill and llandovery times before being subjected to low grade metamorphism during the scandian orogeny rapid pre scandian cooling combined with peak metamorphic conditions of 450c or less may indicate that the dalsfjord hoyvik basement cover pair were avected by an orogenic event during the late ordovician caradoc time the data also suggest that the caledonian margin of baltica may have experienced a more protracted tectonism during the caledonian cycle than previously models focusing on early caledonian and tremadoc or older ophiolite obduction and the scandian continental collision between baltica and laurentia

eclogite facies polyphase deformation of the drosdal eclogite western gneiss complex norway and implications for exhumation abstract exhumation of the deep crust during orogenic extension is accepted as a geological phenomenon but structures formed during burial and the earliest stages of exhumation are often overprinted by deformation events occurring at shallower crustal levels the western gneiss complex wgc of norway comprises variably retrogressed high pressure hp and ultrahigh pressure uhp eclogite bodies enclosed in predominantly felsic amphibolite facies rocks the drosdal body located in the sunnfjord area is one of the largest and best preserved eclogites in the wgc and comprises over ca 3 km2 of exposed eclogite facies rocks it is an excellent example of an eclogite tectonite and displays a wealth of structures formed during deformation at a minimum depth of 50 km and peak temperatures of ca 800 c large volumes of mylonites with the eclogite facies assemblage garnet clinopyroxene quartz amphibole clinozoisite phengite kyanite rutile are preserved structures associated with the eclogite facies metamorphism include e w trending isoclinal folds boudinage hinge parallel lineations and meter scale kyanite dominated veins the drosdal body was tightly folded on the kilometer scale under eclogite facies conditions subsequently the shapes of the drosdal body and other mafic bodies in the sunnfjord area were modified during eclogite to amphibolite facies e w directed stretching to give boudin like lenses the timing of formation of a pervasive eclogite facies lineation eclogite facies folds and kyanite bearing veins overlapped substantially and this portion of the deformation history was dominated by a constrictional strain field partitioning of deformation occurred after a substantial amount of eclogite facies deformation had already taken place and resulted in the relative rotation of linear features in specific zones on the scale of 400 m the eclogite facies lineations and fold hinges within the drosdal body are subparallel to amphibolite and greenschist facies structures throughout the wgc although circumstantial this suggests that structures belonging to each of these three metamorphic facies formed during one progressive deformation event corresponding to extensional exhumation our observations are consistent with models involving inhomogeneous deformation of the lower crust during early orogenic extension we envisage that transtensional noncoaxial deformation dominated the extensional deformation of the wgc but that coaxial deformation must have occurred locally

thermochemistry of monazite la and dissakisite la implications for monazite and allanite stability in metapelites thermochemical properties have been either measured or estimated for synthetic monazite lapo4 and dissakisite calamgal2 sio4 3oh the mg equivalent of allanite a dissakisite formation enthalpy of 6 976 5 10 0 kj mol 1 was derived from high temperature drop solution measurements in lead borate at 975 k a third law entropy value of 104 9 1 6 j mol 1 k 1 was retrieved from low temperature heat capacity c p measured on synthetic lapo4 with an adiabatic calorimeter in the 30 300 k range the c p values of lanthanum phases were measured in the 143 723 k range by differential scanning calorimetry in this study la oh 3 appeared as suitable for drop solution in lead borate and represents an attractive alternative to la2o3 pseudo sections were calculated with the theriak domino software using the thermochemical data retrieved here for a simplified metapelitic composition la ree y and considering monazite and fe free epidotes along the dissakisite clinozoisite join as the only ree bearing minerals calculation shows a stability window for dissakisite clinozoisite epidotes t between 250 and 550 c and p between 1 and 16 kbar included in a wide monazite field the p t extension of this stability window depends on the bulk rock ca content assuming that synthetic lapo4 and dissakisite la are good analogues of natural monazite and allanite these results are consistent with the ree mineralogy sequence observed in metapelites where 1 monazite is found to be stable below 250 c 2 around 250 450 c depending on the pressure allanite forms at the expense of monazite and 3 towards amphibolite conditions monazite reappears at the expense of allanite

giant quartz vein systems in accretionary orogenic belts the evidence for a metamorphic fluid origin from δ 15 n and δ 13 c studies abstract nitrogen isotope compositions of hydrothermal micas in giant structurally hosted quartz vein systems place constraints on the origin of the hydrothermal fluids the vein systems are from neoarchean terranes in the superior province of canada and the norseman terrane western australia the four quartz vein systems studied in different terranes formed at metamorphic grades ranging from lower greenschist to lower amphibolite facies coevally with metamorphism of the host terrane nitrogen resides predominantly as nh 4 in hydrothermal mica for lower to upper greenschist facies quartz veins n contents and 15 n values of micas are between 40 and 200 ppm and 15 and 21 respectively whereas in quartz veins formed at the greenschist to amphibolite transition and lower amphibolite facies micas have n contents of 20 70 ppm and 15 n of 11 24 in contrast micas and k feldspars from granitoids in the neoarchean abitibi and red lake greenstone belts are characterized by systematically lower 15 n of 5 to 5 and generally lower n contents of 20 50 ppm comparable to other granitoids carbon and oxygen isotope compositions of hydrothermal ferroan dolomite and calcite show systematic depletions with increasing metamorphic grade the mean values range from 2 2 to 3 7 for 13 c and 13 8 to 12 3 for 18 o in veins formed at lower to upper greenschist facies but from 5 8 to 7 1 for 13 c and 11 4 to 9 8 for 18 o at higher metamorphic grades nitrogen isotope compositions rule out mantle 15 n mean 5 magmatic 15 n 5 to 15 n mean 4 4 2 0 for the quartz veins accordingly the results are consistent with fluids derived from metamorphic dehydration of oceanic crust and sediments within accretionary orogenic belts

calcite textures microstructures and rheological properties of marble mylonites in the bancroft shear zone ontario canada calcite textures in several microstructural domains from the bancroft shear zone reveal a sequence of transitions which indicate contributions from a variety of deformation mechanisms at greenschist facies conditions the coarse marble protolith has undergone low strains accommodated by twinning and grain boundary migration porphyroclastic mylonite has a well developed crystallographic preferred orientation and microstructures indicative of dislocation creep and rotation recrystallization mechanisms the textures are consistent with high temperature deformation experiments where r f and c slip systems were active complete dynamic recrystallization produced equant medium sized grains 50 m with a texture that is nearly random secondary calcite growth observed under cathodoluminescence and variation in 18o value with microstructurc indicate large fluid fluxes during mylonitization fluid assisted grain boundary sliding and a minor component of dislocation creep are the inferred deformation mechanisms finer grained s c mylonite has a well developed shape fabric and crystallographic preferred orientation with a point maximum and great circle girdle attributed to attainment of an easy slip orientation for most grains ultramylonites have homogeneous textures with finc elongate grains 20 30 m mantled by disseminated secondary phases textures in ultramylonites are characterized by single point maxima and great circle girdles the most evolved ultramylonite has a point maximum oblique to the shear plane with a symmetry indicating rotation of the maximum toward the extension direction texture microstructure relations in these mylonites indicate that the competition between deformation mechanisms was highly sensitive to grain size strain and secondary phases in addition to temperature strain rate and differential stress the varied deformation mechanisms and application of experimentally derived constitutive equations indicate significant differences in differential stress and strain rate among microstructural domains these differences reflect strain softening associated with dynamic recrystallization and work hardening associated with dissemination of secondary phases and the transition from grain boundary sliding to dislocation creep

interpreting high pressure phengite 40 ar 39 ar laserprobe ages an example from saih hatat ne oman new single grain fusion and core rim 40ar 39ar laserprobe phengite data from the saih hatat high pressure terrane in ne oman show that individual samples yield a range of apparent ages which is similar to that previously reported from across the entire terrane the majority of the determined ages are older than the previously reported u pb zircon peak metamorphic age core to rim age variations within individual grains range from no discernible difference across the grain to grains with older cores or rarely older rims some samples manifest all three patterns numerical diffusion modelling shows that due to the peak temperature of ca 550 c the measured apparent ages cannot be explained by simple cooling or by partial retention of crystallisation or detrital ages in an open system the age variability is better explained by spatially and temporally variable open or closed system behaviour at the mm cm scale coupled with pervasive and heterogeneously distributed excess argon anomalously old eclogite phengite 40ar 39ar ages are due either to internally derived 40ar inherited from a k bearing precursor or externally derived 40ar distributed by grain boundary fluids mica rich schists within the eclogite boudins yield younger phengite ages suggesting excess argon was absent or diluted pelites hosting the eclogite appear to have been affected by later fluid ingress during deformation and greenschist facies overprint and yield very variable ages commonly with apparently older rims on younger cores the grain and sample scale age variations measured in saih hatat indicate that the grain boundary network in eclogite pods was not an efficient transfer pathway for argon transport whereas the grain boundary network in the surrounding pelites acted as a more efficient pathway on the timescale of the metamorphic cycle

coesite and pure pyrope in high grade blueschists of the western alps a first record and some consequences a pyrope quartzite originally described by vialon 1966 from the dora maira massif was resampled and reinvestigated garnet up to 25 cm in size phengite kyanite talc and rutile are in textural equilibrium in an undeformed matrix of polygonal quartz the garnet is a pyrope almandine solid solution with 90 to 98 mol mg end member it contains inclusions of coesite which has partially inverted to quartz resulting in a typical radial cracking of the host garnet around the inclusions several lines of evidence show that coesite crystallised under nearly static pressure conditions and that the whole matrix has once been coesite

stratigraphic and geochemical evidence for the depositional environment of the early archaean isua supracrustal belt southern west greenland the highly deformed c 3800 ma isua supracrustal belt is a fragment of a more extensive early archaean sedimentary and volcanic succession intruded by and tectonically intercalated with tonalitic and granitic amftsoq gneisses in the period 3800 3600 ma the supracrustal rocks recrystallised under amphibolite facies conditions between 3800 and 3600 ma in the late archaean and locally at c 1800 ma layered sequences of rock of sedimentary and probable volcanic origin form over 50 of the belt bodies of high mg al basic rocks and ultramafic rocks were intruded into the layered sequences prior to isoclinal folding and intrusion of amitsoq gneisses the layered rocks which are 1 km thick are divided into two sequences that are in faulted contact with each other the way up of these sequences has been determined from facing directions of locally preserved graded layering in felsic metasediments at several localities the overall upwards change in sedimentary succession is interpreted as showing change from dominantly basic to dominantly felsic volcanism which provided the major clastic component of the sediments clastic sedimentation took place against a background of chemical sedimentation shown by interlayers of banded iron formation metachert and calc silicate rocks throughout the sequences the felsic rocks locally preserve graded bedding and possible conglomerate structures indicating deposition from turbidite flows and possibly as debris flows nodules in the felsic rocks contain structures interpreted as fiamme there is an irregular enrichment in k2o na2o in many of the felsic rocks at constant sio2 and al2o3 content interpreted as owing to alteration of original andesitic to dacitic volcanic rocks banded iron formations locally contain conglomeratic structures suggesting sedimentary reworking possibly under shallow water conditions lithological and geochemical characters of the clastic components of the supracrustal sequences are consistent with derivation from felsic and basic volcanic rocks and do not require a continental source

sm nd studies of archaean metasediments and metavolcanics from west greenland and their implications for the earth s early history metasedimentary and metavolcanic rocks from the archaean of west greenland have been examined for evidence of crustal components greater than 3 8 ga in age and for their compatibility with the presently adopted bulk earth sm nd parameters sm nd isotopic data have been obtained for the garbenschiefer metagabbro unit metasediments from the isua supracrustal belt gneisses interior to the isua belt and metasediments from the malene supracrustal belt using estimates of emplacement age t of between 3 77 and 3 67 ga for the parental volcanics to the garbenschiefer unit initial143nd 144nd ratios yield positiveendt values between 1 0 and 3 1 relative to the chur parameters for seven out of eight samples model sm nd ages for the isua gneisses and metasediments are only compatible with their estimated stratigraphic ages if their sources were ca 2end relative to chur at those times similarly model sm nd ages for the malene samples are only compatible with stratigraphic age constraints when based on a source evolution with positiveendt implications of these results for the early development of the earth s mantle are discussed

deformation and recrystallization of plagioclase along a temperature gradient an example from the bergell tonalite abstract syn to post mylonitic tilting of the bergell tonalite allows investigation of the deformation and recrystallization of plagioclase grains along a temperature gradient from the lower to the upper amphibolite facies at the lowest temperatures recrystallization occurs by nucleation and growth of new grains having a different composition from the old grains in contrast at the highest temperatures recrystallization occurs by subgrain rotation associated with grain boundary migration the temperature increase is inferred to induce this transition in two ways first at higher temperatures the anorthite content of the new grains is higher and thus the comositional difference between old and new grains is lower hence the chemical driving potential is small compared with recrystallization at lower temperatures second higher temperatures facilitate climb of dislocations and thus subgrain formation as well as grain boundary mobility the temperature increase is also associated with a transition in the dominant deformation mechanism intracrystalline plasticity dominates deformation at lower temperatures whereas at higher temperatures deformation occurs primarily by diffusion accommodated grain boundary sliding as indicated by the weakening of the indicatrix preferred orientation and the formation of a mixed plagioclase biotite matrix with increasing deformation therefore the strength of the mid to lower crust may be overestimated by dislocation creep flow laws for plagioclase lower crust rheology for polyphase rocks is better approximated by constitutive relationships involving diffusion accommodated grain boundary sliding

coesite in clinopyroxene in the caledonides and its implications for geodynamics coesite is reported for the first time from the caledonide orogen it occurs as inclusions in clinopyroxene in the dolomite eclogite at grytting norway and provides valuable new evidence to support the hypothesis of extremely high pressure 30 kbar in certain norwegian eclogites this finding has implications for the geodynamics and kinetics of the concepts of microscale intracrystalline tectonic overpressure and of macroscale local or regional tectonic overpressure and for the hypotheses of subduction and obduction involving a more than 100km vertical component of transport

postcollisional extension of the caledonide orogen in scandinavia structural expressions and tectonic significance the scandinavian caledonides are much more affected by postcontractional devonian extension than previously thought the extensional deformation is not restricted to the internal parts of the orogen but can be traced for more than 200 km from the west coast of norway toward the foreland it involved significant reactivation of the basal thrust zone as a low angle extensional detachment whic consequently contains abundant top to the north west sense of shear indicators the extensional deformation involved substantial backsliding of the orogenic wedge eduction of the baltic shield together with stretching of the basement by west dipping extensional shear zones after 30 km of backsliding the extensional detachment splayed to penetrate the overlying orogenic edifice in the coastal region in southern norway which led to inactivation of the main portion of the basal thrust zone as an extensional detachment

crystallization and very rapid exhumation of the youngest alpine eclogites tauern window eastern alps from rb sr mineral assemblage analysis multimineral rb sr internal isochrons from eclogite facies rocks of the eclogite zone tauern window eastern alps consistently yield an early oligocene age of 31 5 0 7 ma this age has been obtained both for late prograde dehydration related eclogitic veins and for rocks variably deformed and recrystallized under eclogite facies conditions 2 0 2 5 gpa 600 c initial sr isotopic equilibria among all phases indicate absence of significant post eclogitic isotope redistribution processes therefore the ages date eclogite facies assemblage crystallization equilibria also prove that no prolonged pre eclogite facies history is recorded in the rocks instead subduction prograde mineral reactions and eclogitization proceeded rapidly fast exhumation immediately after eclogitization with minimum rates 36 mm a is inferred from a 31 5 0 5 ma internal mineral isochron age of a post eclogitic greenschist facies vein assemblage such rates equal typical subduction rates late eocene to early oligocene subduction of the european continental margin with subsequent rapid exhumation of high pressure nappe complexes has previously been recognized only in the western alps the new data signify synchronous continental collision all along the alpine belt our results demonstrate the unique potential of rb sr assemblage system analysis for precise dating of both eclogite facies and post eclogitic events thus for precisely constraining exhumation rates of deep seated rocks and for straightforward linkage of petrologic evidence with isotopic ages

ammonium as a biomarker in precambrian metasediments abstract the following article assesses the potential of ammonium as a biomarker in precambrian metasediments a modern marine environment is considered to highlight the processes that contribute to the isotopic composition of organic nitrogen notably the roles played by nitrogen fixing organisms cyanobacteria and denitrifying bacteria the carbon and nitrogen biogeochemical cycles are compared and contrasted unlike carbon the major nitrogen isotope variations may be controlled by local environmental conditions such as season of the year and water depth however such local environmental factors may have been superimposed on more gradual changes related to the long term chemical evolution of the exterior environments of the earth the great potential of nitrogen as a biomarker is that the breakdown product of organic nitrogen ammonium behaves like a trace element and is stable in high temperature crustal processes thus ammonium may survive as a biomarker when all other evidence for former life has been destroyed and ammonium studies may be able to identify sedimentary pelitic components in high grade precambrian terrains previous results from precambrian rocks are reviewed to highlight that very little work has been completed to date and that only very general conclusions can be drawn much work needs to done in order to understand the basic biogeochemistry and geochemistry of crustal nitrogen and it is suggested that the best way to proceed is by numerous detailed case studies the interpretation of isotope ratios in ancient sediments usually assumes uniformitarianism and the justification of this approach is considered briefly overall this article highlights the under developed potential of precambrian ammonium biogeochemistry and suggests area for future research not only does the development of the subject have implications for the search for former life on mars but it may also open up new areas of precambrian research

mineral age record of a polyorogenic evolution within the seve and koli nappes trondelag norway abstract in the central scandinavian caledonides eugeoclinal terranes represented in the koli nappes were thrust 500 km onto the baltoscandian platform during the early to middle paleozoic immediately underlying thrust sheets of the seve nappe complex are suspected to have been derived from the outer margin of continent baltica hornblende within amphibolite at one locality within westernmost exposures of seve nappe units in trandelag norway displays an internally discordant 40 ar 39 ar age spectrum which suggests a polyphase thermal evolution including 1 1 initial post metamorphic cooling through argon retention temperatures at c 475 480 ma 2 2 partial rejuvenation during a silurian c 400 425 ma thermal overprint muscovite within the exposure was more extensively rejuvenated during the silurian overprint at twelve other trandelag sample locations hornblende muscovite and biotite within both the seve and koli nappe complexes record post metamorphic cooling ages between 420 and 435 ma and display no direct evidence of a pre silurian thermal record however the widespread occurrence of intracrystalline extraneous argon components and textural complexities clearly point toward a pre silurian tectonothermal history comparison with 40 ar 39 ar mineral ages reported elsewhere in the central scandinavian caledonides suggests a polyorogenic evolution consisting of 1 1 variable high pressure metamorphism of distal baltoscandian miogeoclinal sequences during entrainment in an accretionary wedge which developed in the late cambrian early ordovician over a west dipping subduction zone related arc volcanic activity occurred in more outboard western sectors 2 2 ductile imbrication of high and intermediate pressure sequences within the accretionary wedge diachronous cooling and or chronologically distinct tectonic episodes are indicated throughout the early and middle ordovician 3 3 ultimate accretion of a variety of eugeoclinal terranes along the baltica continental margin 4 4 imbrication and metamorphism of the eugeoclinal terranes and previously deformed portions of the miogeocline during eastward translation onto the baltoscandian platform in the silurian crystalline baltica basement and overlying structural cover units were depressed to significant crustal depths during eastward nappe transport with resultant formation of high temperature eclogues previously reported crystallization ages of c 425 ma suggest that the eclogites formed at approximately the same time as minerals within the superencumbent nappe units cooled through argon closure temperatures following eastward nappe transport the basement experienced rapid uplift and resultant adiabatic decompression this was followed by rapid cooling through argon retention temperatures at 390 400 ma

size and exhumation rate of ultrahigh pressure terranes linked to orogenic stage abstract a growing set of data indicates a stark contrast between the evolution of two types of ultrahigh pressure uhp terranes large terranes that evolved slowly over 10 30 myr and small terranes that formed and were exhumed on timescales of

recrystallization or cooling ages in situ uv laser 40ar 39ar geochronology of muscovite in mylonitic rocks the intra grain 40 ar 39 ar age distributions of muscovite from greenschist facies mylonite zones are shown to be sensors of metamorphic history textures compositions and in situ uv laser ablation 40 ar 39 ar data from two extensional shear zones demonstrate that cooling ages can be distinguished from neo or recrystallization ages deformed muscovite porphyroclasts and recrystallized shear band muscovite from the pogallo shear zone ivrea zone southern alps reveal variable intra grain 40 ar 39 ar ages with internal age variations of more than 65 ma the wide range of 40 ar 39 ar ages within compositionally homogeneous grains is consistent with diffusion dominated argon loss controlled by observable intra grain microstructures in contrast intra grain 40 ar 39 ar ages of muscovite fish from proterozoic mylonite of the porsgrunn kristiansand shear zone southern norway display only minor age dispersion and age variations are correlated with changes in muscovite composition the sympathetic age composition correlations in different muscovite microstructures directly relate to sequential neo and recrystallization of muscovite and ultimately provide insight into the protracted recrystallization history during extensional deformation along the porsgrunn kristiansand shear zone

exhumation of high pressure rocks driven by slab rollback rocks metamorphosed under high pressure hp and ultra high pressure uhp conditions in subduction zones come back to the surface relatively soon after their burial and at rates comparable to plate boundary velocities in the mediterranean realm their occurrence in several belts related to a single subduction event shows that the burial exhumation cycle is a recurrent transient process using the calabria apennine and aegean belts as examples we show that the exhumation of hp rocks is associated in time and space with the subduction of small continental lithosphere blocks that triggers slab rollback creating the necessary space for the exhumation of the buoyant continental crust that was deeply buried just before the buoyancy force of the subducted crust increases until this crust detaches from the downgoing slab it then exhumes at a rate that depends directly on the velocity of trench retreat to become part of the overriding plate heated from below by the asthenosphere that flows into the opening mantle wedge the exhumed crust weakens and undergoes core complex type extension responsible for a second stage of exhumation at a lower rate the full sequence of events that characterizes this model crust mantle delamination slab rollback and trench retreat hp rock exhumation asthenosphere heating and core complex formation arises entirely from the initial condition imposed by the subduction of a small continental block no specific condition is required regarding the rheology and erosion rate of hp rocks the burial exhumation cycle is transient and can recur every time a small continental block is subducted

understanding phengite argon closure using single grain fusion age distributions in the cycladic blueschist unit on syros greece abstract the preservation of 40ar 39ar ages of high pressure hp metamorphic white mica reflects an interplay of processes that mobilise 40ar either through mica recrystallisation or by diffusive 40ar loss the applicability of resulting ages for dating tectonic processes is critically dependent on whether either of these processes can be proven to be efficient and exclusively active in removing 40ar from mica if not preservation of an inherited or mixed age signal in a sample must be considered for interpretation the cycladic blueschist unit on syros has become a new focal area in the discussion of the geological significance of argon age results from multi grain step heating experiments while some argue that age results can directly be linked to deformation or metamorphic growth events others interpret age results to reflect the interplay of protracted recrystallisation and partial resetting preserving a mixed age signal here we demonstrate the potential of a new approach of multiple single grain fusion dating using the distribution of ages at the sample section and regional scale we show that in northern syros mica ages display systematic trends that can be understood as the result of three competing processes 1 crystallisation along the prograde to peak metamorphic path 2 a southward trend of increasing 40ar loss by diffusion and 3 localised and rock type dependent deformation or metamorphic reactions leading to an observed age spread typically limited to 10 myr at the section scale none of the sections yielded the anomalously old age results that would be diagnostic for significant excess 40ar the recorded trends in ages for each of the studied sections reflect a range of p t conditions and duration of metamorphism diffusion modelling shows that in a typical subduction metamorphic loop subtle variations in p t t history can explain that age contrasts occur on a regional scale but are limited on the outcrop scale our new approach provides a comprehensive inventory of the range of ages present in different rocks and at different scales which results in a more refined understanding of argon retention and isotopic closure of phengite and the geological significance of the ages we verify the added value of our new approach by comparison with multi grain step heating experiments on selected samples from the same sections

early to mid silurian extrusion wedge tectonics in the central scandinavian caledonides we present evidence for an extrusion wedge in the scandian fold thrust belt of the central scandinavian caledonides seve nappe complex rb sr multimineral geochronology in synkinematic assemblages indicates simultaneous movements at the normal sense roof shear zone and at the reverse sense floor shear zone between 434 ma and 429 ma a sm nd age of 462 ma from a mylonitic garnet mica schist documents prograde garnet growth and possible incipient subduction pressure temperature pseudosection calculations provide evidence for eclogite facies metamorphic conditions and nearly isothermal decompression at 670 50 c from 17 5 to 14 5 kbar in garnet kyanite mica schists during reverse sense shearing and from 15 to 11 kbar in garnet mica schists during normal sense shearing these data and the presence of decompression related pegmatites dated at 434 ma and 429 ma indicate that the seve nappes form a large scale extrusion wedge this wedge extends along strike for at least 150 km and marks an early stage of ultrahigh pressure metamorphism exhumation and orogenic wedge formation in this part of the scandinavian caledonides predating the major post 415 ma ultrahigh pressure exhumation processes in southwestern norway

evolution of caledonian deformation fabrics under eclogite and amphibolite facies at vardalsneset western gneiss region norway the vardalsneset eclogite situated in the western gneiss region sw norway is a well preserved tectonite giving information about the deformation regimes active in the lower crust during crustal thickening and subsequent exhumation the eclogite constitutes layers and lenses variably retrograded to amphibolite and is composed of garnet and omphacite with varying amounts of barroisite actinolite clinozoisite kyanite quartz paragonite phengite and rutile the rocks record a five stage evolution connected to caledonian burial and subsequent exhumation 1 a prograde evolution through amphibolite facies t 490 63 c is inferred from garnet cores with amphibole inclusions and bell shaped mn profile 2 formation of l s tectonite eclogite t 680 20 c p 16 2 kbar related to the subduction of conti nental crust during the caledonian orogeny lack of asymmetrical fabrics and orientation of eclogite facies extensional veins indicate that the deformation regime during formation of the l s fabric was coaxial 3 formation of sub horizontal eclogite facies foliation in which the finite stretching direction had changed by approximately 90 disruption of eclogite lenses and layers between symmetric shear zones characterizes the dominantly coaxial deformation regime of stage 3 locally occurring mylonitic eclogites t 690 20 c p 15 1 5 kbar with top w kinematics may indicate however that non coaxial deformation was also active at eclogite facies conditions 4 development of a widespread regional amphibolite facies foliation t 564 44 c p 10 3 8 1 kbar quartz veins and development of conju gate shear zones indicate that coaxial vertical shortening and sub horizontal stretching were active during exhumation from eclogite to amphibolite facies conditions 5 amphibolite facies mylonites mainly formed under non coaxial top w movement are related to large scale movement on the extensional detachments active during the late orogenic extension of the caledonides the structural and metamorphic evolution of the vardalsneset eclogite and related areas support the exhumation model including an extensional detachment in the upper crust and overall coaxial deformation in the lower crust

constraints from 40ar 39ar geochronology on the timing of alpine shear zones in the mont blanc aiguilles rouges region of the european alps the mont blanc and aiguilles rouges massifs which form part of the external crystalline massifs of the western european alps record a young and fast exhumation history as established by an extensive low temperature thermochronology and geochronology data set various kinematic and dynamic models for oligocene to neogene deformation and exhumation of the aiguilles rouges mont blanc system have been proposed however the timing of deformation along major shear zones in and around the massifs which is crucial to these models is still controversial our new 40ar 39ar data from key deformation zones in the mont blanc area show that nw directed thrusting lasted from oligocene to mid miocene times in the mont blanc massif before a nw ward jump of the deformation front to below the aiguilles rouges massif which led to updoming of the aiguilles rouges and mont blanc massifs this deactivated the main nw verging shear zones and caused back folding and back thrusting east of the mont blanc as well as upward extrusion of the aiguilles rouges massif subsequently there was a switch to more coaxial shortening between the aiguilles rouges and mont blanc massifs associated with strain partitioning and related dextral strike slip deformation on the eastern side of the mont blanc massif

metamorphic rocks seek meaningful cooling rate interpreting 40ar 39ar ages in an exhumed ultra high pressure terrane 40ar 39ar dating of metamorphic white mica is commonly used to elucidate the timing of cooling and or exhumation of metamorphic terranes in theory so long as the dated mineral originally crystallised above its closure temperature the yielded 40ar 39ar ages should not vary significantly within or between samples and should vary only due to grain size 3 ma variation depending on the estimated cooling rate infra red laser single grain fusion and ultra violet laser spot muscovite and or phengite data from mafic eclogite bodies and their host gneisses in the nordfjord region of the western gneiss complex wgc norway show that the ages yielded both within and between white micas from the same mafic eclogite sample vary by up to 284 ma ages yielded within the same pelitic gneiss sample vary by up to 28 ma overall the 40ar 39ar phengite dates from mafic eclogites range from 750 to 440 ma and are significantly older than the previously reported 405 400 ma u pb zircon age constraining peak metamorphism in the nordfjord wgc the felsic host gneiss samples which recrystalised during a post peak exhumation related amphibolite facies overprint yield single grain fusion and laser spot 40ar 39ar white mica dates spanning 420 to 385 ma encompassing previous estimates of the timing of the entire metamorphic cycle these data suggest that ar is heterogeneously distributed both within grains and between grains on the cm to sub mm scale in all samples and imply restricted ar transport and inefficient removal during the metamorphic cycle open system zero grain boundary ar concentration behaviour is an inherent assumption in the commonly applied closure temperature formulation but unless this can be demonstrated linking 40ar 39ar ages to temperature is no longer trivial the high spatial and chronological precision afforded by uv laser ablation mean that intra grain ar concentration distributions may be used to assess open vs closed system behaviour the data presented here suggest that open system behaviour cannot be assumed in metamorphic systems even those in which the pressure temperature and time conditions in theory allow for efficient diffusion during the metamorphic cycle high spatial resolution 40ar 39ar data and robust comparison with numerical models of open system behaviour provide the greatest future potential for unravelling the complexities of metamorphic exhumation timescales

thermal history of thrust sheets in an orogenic wedge 40ar 39ar data from the polymetamorphic seve nappe complex northern swedish caledonides the seve nappe complex in the scandinavian caledonides contains the fragmented late precambrian continent ocean transition between baltica and the iapetus ocean this passive margin was fragmented and thrust eastwards over the baltic shield during caledonian orogenesis the individual thrust sheets in the seve nappe complex went through different p t t evolutions resulting in dramatic metamorphic contrasts eclogite bearing nappes are juxtaposed with nappes showing no evidence of caledonian deformation or metamorphism in their interiors strain localization to the marginal parts of the thrust sheets left records of both pre orogenic rift and early orogenic subduction and subsequent uplift processes in the thrust sheets of the seve nappe complex even though it has been transported several hundred kilometres only the margins of the eastern part of the sarektjakka nappe are affected by penetrative caledonian deformation this part of the sarektjakka nappe is dominated by pristine tholeiitic dykes and cross bedded sandstones the dykes are 608 1 ma old and make up 70 80 of the nappe widely spaced thin shear zones of the ruopsok fault system represent the only caledonian penetrative deformation in the interior of the nappe previously published ar ar dates indicate cooling below the closure temperature of hornblende at c 470 ma but numerous ages have been recorded ar dating of biotite and muscovite from a cross laminated metapsammite in the sarektjakka nappe gave well defined ages of 428 5 3 6 and 432 4 3 8 ma respectively muscovite from a shear zone in the ruopsok fault system gave 428 2 4 0 ma whereas hornblende from the same locality did not yield interpretable data the results indicate that these rocks were completely degassed at some unknown time presumably at the emplacement of the dyke swarm no subsequent excess argon contamination can be detected a likely candidate for the degassing event is the emplacement of the dykes at 608 ma the interior of the nappe and thus the entire nappe complex cooled below 350 c at around 430 ma cooling from more than 500 c at c 470 ma to 350 c at c 430 ma suggests an average cooling rate of 4 c ma a prolonged period of slow cooling exhumation following the initial rapid uplift of the eclogite bearing nappes and early ordovician construction of the seve nappe complex is suggested

microdiamond on areskutan confirms regional uhp metamorphism in the seve nappe complex of the scandinavian caledonides metamorphic diamond in crustal rocks provides important information on the deep subduction of continental crust here we present a new occurrence of diamond within the seve nappe complex snc of the scandinavian caledonides on areskutan in jamtland county sweden microdiamond is found in situ as single and composite diamond carbonate inclusions within garnet in kyanite bearing paragneisses the rocks preserve the primary peak pressure assemblage of ca mg rich garnet phengite kyanite rutile with polycrystalline quartz surrounded by radial cracks indicating breakdown of coesite calculated p t conditions for this stage are 830 840 c and 4 1 4 2 gpa in the diamond stability field the ultrahigh pressure uhp assemblage has been variably overprinted under granulite facies conditions of 850 860 c and 1 0 1 1 gpa leading to formation of ca mg poor garnet biotite plagioclase k feldspar sillimanite ilmenite quartz this overprint was the result of nearly isothermal decompression which is corroborated by ti in quartz thermometry chemical th u pb dating of monazite yields ages between 445 and 435 ma which are interpreted to record post uhp exhumation of the diamond bearing rocks the new discovery of microdiamond on areskutan together with other evidence of ultrahigh pressure metamorphism uhpm within gneisses eclogites and peridotites elsewhere in the snc provide compelling arguments for regional at least 200 km along strike of the unit uhpm of substantial parts of this far travelled allochthon the occurrence of uhpm in both rheologically weak gneisses and strong lithologies eclogites peridotites speaks against the presence of large tectonic overpressure during metamorphism

optical microstructure of partly recrystallized calcite in some naturally deformed marbles abstract deformed marbles from the mt lofty ranges south australia have microstructures characterized by deformed relics of calcite interspersed with much finer grained recrystallized calcite and with minor quartz and diopside the deformed calcite relics contain numerous subgrains and show abundant evidence of strain induced migration of twin and grain boundaries new calcite grains have nucleated and grown preferentially along twins especially at twin intersections at grain and kink like boundaries and apparently randomly inside deformed relics optical microstructural evidence suggests that dynamic syntectonic recrystallization has taken place along with some static recrystallization probable mechanisms for the initiation of recrystallization include bulge nucleation and subgrain nucleation

absolute timing of caledonian orogenic wedge assembly central sweden constrained by rb sr multi mineral isochron data abstract multi mineral rb sr isochron ages for mylonites of the caledonian nappe pile jamtland sweden constrain the orogenic evolution of the central scandinavian caledonides an age of 437 8 3 9 ma from a metamorphic leucosome in the central nappe stack dates crystallization of migmatized gneiss thirteen isotopic ages derived from mylonites across all structural levels are between 434 6 6 9 ma and 426 3 3 1 ma these results complemented by a dense network of kinematic field data are interpreted to reflect the timing of protracted top to the ese general shear under decreasing amphibolite to upper greenschist facies conditions across all nappes three samples derived from the top middle and bottom of the nappe pile yield additional deformation ages between 416 0 3 6 ma and 410 1 3 0 ma these ages are interpreted to represent post assembly imbrication of the nappe stack under decreasing metamorphic conditions the new tectonochronologic data are consistent with a tectonic model that explains nappe stacking in jamtland by three stages of ese directed in and out of sequence thrusting in the first stage postdating earlier subduction exhumation of the high grade metamorphic seve nappe complex the latter was emplaced in sequence in the lower mid crust above nappes in its footwall foreland at c 430 ma this event is related to the collision of baltica with an island arc concurrently at c 430 ma determined by paleogeography and structurally unrelated to the first stage second stage out of sequence thrusting emplaced the koli nappe complex atop the developing nappe stack during final baltica laurentia collision eventually imbrication and exhumation of the composite jamtland nappe stack in the uppermost middle crust during the third stage at c 414 ma was caused by continuous underthrusting of baltica beneath laurentia this three stage thrust model reflects the complex interactions of baltica arc laurentia collisions that were accommodated by two merging subduction zones

zoning and recrystallization of phengitic micas implications for metamorphic equilibration white micas phengites in the metasediments of the scottish dalradian display a large range of compositions within single samples the variations in the composition of these phengites are strongly controlled by their structural age with early fabrics containing a paragonite poor celadonite rich phengite whereas in later fabrics the micas are generally paragonite rich and celadonite poor retrograde phengite growth identified using back scattered electron imaging occurs as celadonite rich rims on micas within all existing fabrics and appears to be preferentially developed along existing white mica plagioclase grain boundaries the presence of these chemically distinct phengite populations within single samples implies that chemical exchange between the individual micas was inefficient it is proposed that diffusion controlled exchange reactions in phengites have relatively high closure temperatures below which major element exchange is effectively impossible this closed system behaviour of micas questions the ease with which phengites may equilibrate with other phases during prograde greenschist and lower amphibolite facies metamorphism many of the chemical variations preserved in phengites from such metamorphic rocks may reflect deformation recrystallization controlled equilibria

high ammonium contents in the 3800 ma isua supracrustal rocks central west greenland abstract ammonium contents of whole rock and biotite samples from the metasedimentary sequence of the 3800 ma isua supracrustal belt central greenland have been determined and compared with the ammonium contents of volcanic sedimentary metamorphic and granitic rocks from the late archaean early proterozoic a sample of the amitsoq gneiss has an nh4 content whole rock 9 ppm biotite 29 ppm that is similar to many younger igneous rocks ammonium contents of the supracrustal rocks vary with chemistry and mineralogy of the rock and most likely were inherited from the original sediments a series of carbonate bearing and basic plagioclase bearing biotite rich schists from sequence a have varying nh4 contents whole rock 6 24 ppm that increase with an increasing chemical clastic component and decreasing volcanogenic component biotites in these rocks together with biotites in mica schists of the same sequence have high nh4 contents which vary over a small range of 54 95 ppm suggesting that the nh4 in these rocks was contained mostly in authigenic clays of the original sediments all these data imply that clay minerals were the major sink of nh4 or other nitrogen compounds on the earth s surface at 3800 ma two samples of graphite bearing garnet staurolite mica schist from sequence b have remarkably high nh4 contents whole rock 68 and 86 ppm biotite 234 and 253 ppm which are as high as or even higher than those of common late archaean early proterozoic argillites it appears almost certain that the original thick clay rich bottom sediments contained a large amount of nh4 or other nitrogen compounds the most likely major source of nitrogen in these nh4 enriched graphite bearing mica schists might be organic nitrogen

deformation progressive or multiphase abstract complexly deformed rocks that exhibit refolded folds and overprinting planar fabrics are often interpreted as the result of polyphase deformation and described in a scheme of deformation phases d 1 d 2 d n at the same time a number of models have been presented over the last decades that explain how complex and overprinting structures can form by single phase progressive deformation as strain accumulates strain generally accumulates heterogeneously in a deforming region depending on strain intensity and geometry flow parameters rheology and lithology the result is regions of different structural complexity fabrics and fabric orientations that can be sorted into different structural facies such structural pattern should be mapped and presented before defining a scheme of deformation phases and attempts should be made to explain them by a single structural kinematic model such as partitioned transpression only if this is not possible or if independent geochronologic data reveals a more punctuated history should we define an additional tectonic regime and deformation phase multiphase and progressive deformation histories are not mutually exclusive and it is important that both possibilities are considered when describing the deformation history of any complexly deformed region when the concept of deformation phases is employed the number of phases should be minimized and be related to major tectonic event of external character uncritically defining deformation phases can easily generate a complicated discrete deformation history with no link to tectonic reality

heterogeneous 40ar distributions in naturally deformed muscovite in situ uv laser ablation evidence for microstructurally controlled intragrain diffusion abstract micas are commonly used in 40 ar 39 ar thermochronological studies of variably deformed rocks yet the physical basis by which deformation may affect radiogenic argon retention in mica is poorly constrained this study examines the relationship between deformation and deformation induced microstructures on radiogenic argon retention in muscovite a combination of furnace step heating and high spatial resolution in situ uv laser ablation 40 ar 39 ar analyses are reported for deformed muscovites sampled from a granitic pegmatite vein within the siviez mischabel nappe western swiss alps penninic domain brianconnais unit the pegmatite forms part of the variscan 350 ma alpine basement and exhibits a prominent alpine s c fabric including numerous mica fish that developed under greenschist facies metamorphic conditions during the dominant tertiary alpine tectonic phase of nappe emplacement furnace step heating of milligram quantities of separated muscovite grains yields an 40 ar 39 ar age spectrum with two distinct staircase segments but without any statistical plateau consistent with a previous study from the same area a single 3 5 mm muscovite porphyroclast fish was investigated by in situ uv laser ablation a histogram plot of 170 individual 40 ar 39 ar uv laser ablation ages exhibit a range from 115 to 387 ma with modes at approximately 340 and 260 ma a variogram statistical treatment of the 40 ar 39 ar results reveals ages correlated with two directions a highly correlated direction at 310 and a lesser correlation at 0 relative to the sense of shearing using the highly correlated direction a statistically generated kriging method age contour map of the 40 ar 39 ar data reveals a series of elongated contours subparallel to the c surfaces which where formed during tertiary nappe emplacement similar data distributions and slightly younger apparent ages are recognized in a smaller mica fish the observed intragrain age variations are interpreted to reflect the partial loss of radiogenic argon during alpine 35 ma greenschist facies metamorphism one dimensional diffusion modelling results are consistent with the idea that the zones of youngest apparent age represent incipient shear band development within the mica porphyroclasts thus providing a network of fast diffusion pathways during alpine greenschist facies metamorphism the incipient shear bands enhanced the intragrain loss of radiogenic argon the structurally controlled intragrain age variations observed in this investigation imply that deformation has a direct control on the effective length scale for argon diffusion which is consistent with the heterogeneous nature of deformation

plate velocity exhumation of ultrahigh pressure eclogites in the pakistan himalaya u pb ages of zircon and allanite from the coesite bearing ultrahigh pressure uhp units in the kaghan valley northern pakistan demonstrate that peak uhp metamorphism along the northern margin of the indian plate occurred at 46 4 0 1 ma at peak pressure temperature conditions of 27 5 kbar 100 km and 720 770 c much lower pressure retrogressive growth of titanite took place between 46 4 and ca 44 ma indicating that the eclogites were exhumed to 35 km depth at or before 44 ma implying very rapid exhumation rates within the mantle of 30 80 mm yr or more comparable to rapid plate velocities once entrained in the base of the crust the eclogites followed a slower cooling history from 45 ma similar to the amphibolite facies gneisses of the pakistan himalaya

nitrogen isotope compositions of metasedimentary rocks in the catalina schist california implications for metamorphic devolatilization history abstract in the catalina schist subduction zone metamorphic complex california metasedimentary rocks show a decrease in n concentration and an increase in 15nair with increasing metamorphic grade lowest grade lawsonite albite rocks contain 632 185 ppm n with 15 n 1 9 0 6 whereas highgrade amphibolite equivalents contain 138 76 ppm n with 15 n 4 3 0 8 loss of n accompanied devolatilization reactions that evolved h2o rich c o h s n fluids through consumption of chlorite and phengitic white mica and production of mineral assemblages containing muscovite biotite garnet and kyanite whole rock n concentrations of up to 200 ppm in veins and pegmatites produced during high p t metamorphism reflect the redistribution of n during devolatilization and partial melting of the metasedimentary rocks bulk fluid rock n isotope fractionations 15n 15nfluid 15nrock of 1 5 1 were calculated with the rayleigh distillation equation taking into account variability in rock composition by comparison of samples with similar k2o concentrations these fractionations are similar to but slightly lower than published calculated fractionations for n2 nh4 exchange at the temperature range of 350 600 c over which most of the devolatilization occurred in the catalina schist approximately 3 4 to 2 25 the n systematics appear to be explained by n2 nh4 exchange and a devolatilization process intermediate in behavior to batch volatilization and rayleigh distillation the observed shifts in n concentration and 15n cannot be explained by nh3 nh4 exchange at these temperatures using the equilibrium models the distillation devolatilization process implicated in this study may govern the behavior of other trace elements partitioned into hydrous fluids during devolatilization e g b cs u similarity of the calculated fluid 15n 1 5 to 5 5 with compositions of natural gases inferred to be derived from metasedimentary sources indicates the possibility of using n as a tracer of large scale volatile transport

a mechanism for syn collisional rock exhumation and associated normal faulting results from physical modelling abstract properly scaled physical modelling of the subduction of continental lithosphere is performed with a three layer lithospheric model the model includes a strong upper crust a weak ductile lower crust and a strong mantle part the lithosphere is underlain by a low viscosity asthenosphere subduction is produced by a piston push force and the pull force from the mantle lithospheric layer which is slightly denser than the asthenosphere the results of the modelling are tested on the example of the himalayas in the experiments all lithospheric layers subduct into the mantle to a depth corresponding to 200 300 km in nature until the upper crustal layer fails in front of the subduction zone forming a first major thrust fault the mct in our interpretation of the himalayas underthrusting along this fault increases relief which upon reaching a height corresponding to several kilometres in nature is removed mechanically by a blade the blade is moved horizontally back and forth throughout the experiment this erosional unloading mechanism causes the previously subducted segment of the buoyant upper crust to slide upward over the weak lower crust at a rate comparable to the subduction rate the rise of the crustal slice accelerates displacement along the crustal thrust while producing a normal sense motion and exhumation of the material from great depth up to 40 50 km depending on the model parameters along the upper surface of the slice the rapid uplift of the crust then ceases and the orogenesis occurs further due to continuing underthrusting along the first major thrust at a certain stage the crust fails again producing a new major crustal thrust the mbt in the himalayas