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

eclogites and eclogites in the western gneiss region norwegian caledonides the western gneiss region wgr marks the outcrop of a composite terrane consisting of variably re worked proterozoic basement and parautochthonous or autochthonous cover units the wgr exhibits a gross structural petrographic and thermobarometric zonation from southeast to northwest reflecting an increasing intensity of scandian late palaeozoic

petrochronology and tims thermal ionization mass spectrometers or tims were developed by the pioneers of mass spectrometry in the mid 20th century and have since been workhorses for generating isotopic data for a wide range of elements later developed mass spectrometric techniques have many advantages over tims including higher spatial resolution with in situ techniques such as secondary ion mass spectrometry sims and laser ablation inductively coupled plasma mass spectrometry la icpms and greater versatility in terms the elements that can be easily and well measured the reason tims persists as an important method for geochronology is that for some key parent daughter systems e g u pb sm nd it can produce isotopic data and resultant dates with 10 100 times higher precision and more quantifiable accuracy than in situ techniques even when sample sizes are very small such as those that might result from single crystals or even small portions of zoned crystals for many questions in the geosciences the highest achievable precision and accuracy are required to resolve the timescales of processes and or correlate events globally as an example modern tims u pb geochronology is capable of producing dates with precision and accuracy better than 0 1 of the age for single crystals with only a few picograms pg of pb therefore it is possible to constrain the durations of single zircon crystal growth in magmatic systems over tens to hundreds of kyr in mesozoic and younger rocks if these dates and rates can be connected with other igneous processes such as magma transfer emplacement and crystallization then it becomes possible to calibrate thermal and mass budgets in magmatic systems and evaluate competing models for pluton assembly and subvolcanic magma storage as another example sm nd geochronology of garnet permits dates with precision better than 1 million years for garnets of any age including multiple concentric growth zones in single crystals such

investigating peatland stratigraphy and hydrogeology using integrated electrical geophysics hydrology has been suggested as the mechanism controlling vegetation and related surficial pore water chemistry in large peatlands peatland hydrology influences the carbon dynamics within these large carbon reservoirs and will influence their response to global warming a geophysical survey was completed in caribou bog a large peatland in maine to evaluate peatland stratigraphy and hydrology geophysical measurements were integrated with direct measurements of peat stratigraphy from probing fluid chemistry and vegetation patterns in the peatland consistent with previous field studies ground penetrating radar gpr was an excellent method for delineating peatland stratigraphy prominent reflectors from the peat lake sediment and lake sediment mineral soil contacts were precisely recorded up to 8 m deep two dimensional resistivity and induced polarization imaging were used to investigate stratigraphy beneath the mineral soil beyond the range of gpr we observe that the peat is chargeable and that ip imaging is an alternative method for defining peat thickness the chargeability of peat is attributed to the high surface charge density on partially decomposed organic matter the electrical conductivity imaging resolved glaciomarine sediment thickness a confining layer and its variability across the basin comparison of the bulk conductivity images with peatland vegetation revealed a correlation between confining layer thickness and dominant vegetation type suggesting that stratigraphy exerts a control on hydrogeology and vegetation distribution within this peatland terrain conductivity measured with a geonics em31 meter correlated with confining glaciomarine sediment thickness and was an effective method for estimating variability in glaciomarine sediment thickness over approximately 18 km 2 our understanding of the hydrogeology stratigraphy and controls on vegetation growth in this peatland was much enhanced from the geophysical study

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

a new method integrating high precision u pb geochronology with zircon trace element analysis u pb tims tea increased precision in isotope dilution thermal ionization mass spectrometry id tims u pb geochronology has revealed age complexities in zircon populations that require new tools for understanding how the growth of zircon is related to geologic processes u and pb are routinely separated from other elements in dated minerals by ion exchange separation prior to tims isotope measurement we develop a method in which trace elements in the exact same volume of zircon are redissolved and analyzed using solution nebulization inductively coupled plasma sector field mass spectrometry with matrix matched external liquid calibration using 0 5 ml solution resulting concentrations are between 1 ppt for elements such as ti nb and ta and tens of ppb for zr by analyzing a series of standard solutions zircons and procedural blanks we show that accurate measurements are performed on zr hf y sc and the hree while low concentration elements can be measured accurately to 5 ppt we performed combined u pb id tims geochronology with trace element analysis here called u pb tims tea on zircons from eight volcanic rocks comprising several volcanic systems and one metamorphic sample similar to previous in situ trace element analyses zircon geochemistry is distinct between different samples and records petrogenetic processes such as fractional crystallization assimilation and or magma mixing unique from in situ analysis u pb tims tea can trace geochemical evolution in accessory minerals with adequate age precision to resolve magmatic processes in rocks at least 200 million years old this provides a means to identify auto ante and xenocrystic zircon and lead to more robust age interpretations in id tims u pb geochronology one suite of cretaceous andesitic zircons shows correlations in geochemistry and absolute time that record evolution of a magmatic system over 250 ka prior to eruption future work will combine u pb tims tea with solution isotopic analysis of nd sr and hf and will be applied to a host of datable minerals such as monazite sphene apatite rutile xenotime and baddeleyite these combined tools will provide access to an improved understanding of a wide range of igneous and metamorphic processes as a function of time

experimental zircon melt and zircon garnet trace element partitioning and implications for the geochronology of crustal rocks garnet is the most commonly used mineral in thermobarometry whereas zircon is the most robust chronometer to date high grade metamorphic rocks to provide a basis for correlation of zircon and garnet growth we determined experimentally the trace element partitioningbetweenzircon ahydrousgraniticmeltandgarnetat20kbarand800 1000 cforp y rareearthelements ree zr hf th andu inrespecttomelt zirconpreferentiallyincorporatesallinvestigatedelementsapartfromreewithatomicnumberlowerthansm at this pressure and in the chosen composition the distribution coefficient between zircon and melt d zrn melt for ree increases with increasingatomicnumberofthereeandwithdecreasingtemperature dyb is 20at1000 c butmorethananorderofmagnitude higher at800 c the solubilityofzrinhydrousgraniticmelts buffered by zirconisabouta factoroftwo lower at20kbar than what has been previously established for mid crustal pressures large garnet produced in the experiments allowed determination of garnet melt trace element partitioning d grt melt at temperatures of 800 1000 c conditions relevant for partial melting of crustal rocks there is a systematic increase indree with decreasing temperature zircon contains significantly more heavy ree than garnet at temperatures of 800 850 c zircon garnetpartitioncoefficientsofheavy reedecreasewithincreasingtemperaturefromdlu of12at800 cto 1 4at1000 c middle reepartitioningisclosetounityforthewholeinvestigatedtemperaturerange differentempiricallydetermined zircon garnet partition coefficients from granulites and ultra high temperature granulites can potentially be explained by the experimentallydeterminedchangeofpartitioningasafunctionoftemperature thesedatacanassistinestablishingequilibriumbetween garnetandzirconzonesinnaturalrocks andintheconstructionofdetailedpressure temperature timepathsinhigh grademetamorphic rocks

u pb zircon geochronology of eclogites from the scandian orogen northern western gneiss region norway 14 20 million years between eclogite crystallization and return to amphibolite facies conditionsthis article is one of a series of papers published in this special issue on the theme of geochronology in honour of tom krogh reconstructing tectonic histories involving continental collision subduction and exhumation at plate tectonic rates of 1 cm year requires precise u pb zircon geochronology the western gneiss region has exceptional exposures of high pressure hp and ultra high pressure uhp rocks the strategy adopted here involved sampling eclogite and associated late unstrained pegmatites to acquire the time of eclogite crystallization and subsequent exhumation respectively the oldest eclogite sampled is 415 1 ma from layered probably uhp eclogite at tevik averoya also with a garnet hornblende assemblage at 410 1 ma the flem gabbro eclogite margin with implied uhp conditions is 410 2 ma hornblende eclogite at seth lepsoya never at uhp is 412 2 ma these compare to devonian ages of 401 1 ma for overgrowths on proterozoic baddeleyite in selnes gabbro 402 2 ma for coesite eclogite at hareidlandet 405 400 ma for coesite eclogite at flatraket and 405 2 ma for near uhp eclogite at hjelmeland

zircon geochronology and ca 400 ma exhumation of norwegian ultrahigh pressure rocks an ion microprobe and chemical abrasion study understanding the formation and exhumation of the remarkable ultrahigh pressure uhp rocks of the western gneiss region norway hinges on precise determination of the time of eclogite recrystallization we conducted detailed thermal ionization mass spectrometry chemical abrasion analysis and sensitive high resolution ion microprobe analysis of zircons from four ultrahigh and high pressure hp rocks ion microprobe analyses from the flatraket eclogite yielded a broad range of apparently concordant caledonian ages suggesting long term growth in contrast higher precision thermal ionization mass spectrometry analysis of zircon subject to combined thermal annealing and multi step chemical abrasion yielded moderate pb loss from the first lowest temperature abrasion step possible minor pb loss or minor growth at 400 ma from the second step and a 407 404 ma cluster of slightly discordant 206pb 238u ages most likely free from pb loss from the remaining abrasion steps we interpret the latter to reflect zircon crystallization at 405 400 ma with minor discordance from inherited cores zircon crystallization occurred at eclogite facies possibly post peak conditions based on compositions of garnet inclusions in zircon as well as nearly flat hree profiles and lack of eu anomalies in zircon fractions subjected to chemical abrasion these ages are significantly younger than the 425 ma age often cited for western norway eclogite recrystallization implying faster rates of exhumation 2 5 8 5 km myr and coeval formation of eclogites across the uhp portion of the western gneiss region

insights into u hp metamorphism of the western gneiss region norway a high spatial resolution and high precision zircon study abstract combining high spatial resolution and high precision geochronology and geochemistry of zircon provides constraints on the timing and duration of ultrahigh pressure uhp metamorphism resulting from the collision of baltica avalonia and laurentia during the scandian orogeny in the western gneiss region of norway zircons were extracted from a layered eclogite in the saltaneset region southern uhp domain and from an eclogite in the ulsteinvik region central uhp domain zircons were first analyzed for u pb and trace element compositions by laser ablation split stream lass inductively coupled plasma mass spectrometry icp ms followed by analysis of those same zircons that yielded scandian dates by integrated u pb isotope dilution thermal ionization mass spectrometry and trace element analysis tims tea lass results from a garnet quartz layer within the saltaneset eclogite give scandian dates of ca 413 397 ma with subsequent id tims analyses ranging from 408 9 0 4 ma to 401 4 0 2 ma 2 an omphacite rich layer from the same eclogite yields lass dates of ca 414 398 ma and a single id tims date of 396 7 1 4 ma in comparison the ulsteinvik eclogite lass results give dates spanning ca 413 397 ma whereas id tims analyses range from 409 6 0 6 ma to 401 3 0 4 ma id tims zircon data from the eclogites reveals two age populations 1 ca 409 407 ma and 2 ca 402 ma both in situ and solution trace element data show a distinct pattern for scandian zircons with strongly depleted hree and weakly negative eu anomalies eu eu whereas inherited zircon ree patterns are distinguished by steep hree slopes and marked negative eu eu when coupled with partition coefficients calculated for zircon and garnet these ree patterns indicate that zircon re crystallized during eclogite facies metamorphism at ca 409 407 ma and ca 402 ma at two widely separated uhp localities

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

smnd isotopic systematics of a gabbro eclogite transition abstract sm nd isotopic systems have been studied in 1 a sequence of coronitic olivine gabbros showing different degrees of transition to eclogite 2 coronitic leuco gabbro norite and 3 country rock eclogite all from the nordoyane brattvag area western norway isochrons defined by combinations of whole rock samples and relict igneous phases give ages of 1198 56 and 1289 48 ma olivine gabbro and 926 70 ma leuco gabbro norite and have been interpreted in terms of two different episodes of igneous intrusion in gabbro with advanced corona formation relict augite tends to retain its original nd isotopic composition while plagioclase may show selective disturbance related to recrystallization and partial replacement by garnet in completely eclogitized samples there is a correlation between the degree of isotopic equilibrium and microstructural equilibrium whole rock eclogites occurring within gabbro are characterized by pseudomorphs after igneous phases and preserve original igneous nd isotopic composition results for eclogite minerals suggest caledonian ages but these phases have not achieved complete isotopic equilibrium in contrast nd isotopic systems in texturally equilibrated and strongly deformed eclogite within adjacent gneisses have been disturbed on a whole rock scale minerals from an external eclogite define a sm nd isochron with an age of 400 16 ma which is interpreted to date synkinematic eclogite equilibration at high p and t conditions

pool patterning in a northern peatland geophysical evidence for the role of postglacial landforms a hydrogeophysical study conducted in a 2200 ha peatland in central maine shows evidence that a laterally continuous esker deposit extends below the peat soil following the direction of glacier retreat and coinciding with the presence of a pool system ground penetrating radar gpr electrical resistivity er imaging and terrain conductivity measurements were constrained with limited hydrological measurements direct sampling of inorganic sediments and radiocarbon dating of peat soils to develop a conceptual model for pool formation that could explain the unique pool patterning observed in other northern peatlands gpr profiles revealed three dimensional images of two esker crests below the easternmost edge of the pool area gpr data was supported by er imaging and terrain conductivity data and allowed for facies differentiation to infer processes related to esker formation during the glacier front retreat hydrologic data showed preferential flow of groundwater towards the esker suggesting that pools may persist as a result of enhanced peat decomposition and carbon loss due to oxygenation and or upward supply of alternative electron acceptors from the mineral soil along narrow permeable inorganic esker crests variations in pool depths and radiocarbon dating of peat samples from the pool bottom support a model whereby pools originate at a focus centered along the two esker crests and subsequently spread laterally outwards

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

new evidence for coesite in eclogite and gneisses defining an ultrahigh pressure province in the western gneiss region of norway widespread evidence of ultrahigh pressure metamorphism is reported from the nordfjord and stadtlandet area of the western gneiss region in norway outlining a distinct ultrahigh pressure province coesite has been found as inclusions in garnet omphacite or kyanite in eclogite from 5 new localities and quartz pseudomorphs after coesite occur in eclogite minerals at an additional 12 localities quartz pseudomorphs after coesite within garnet and clinozoisite are reported for the first time from associated gneisses which is evidence for in situ ultrahigh pressure metamorphism at least at a local scale the boundary to an ultrahigh pressure province is defined by a metamorphic break from lower pressure 2 8 gpa within a 10 km wide mixed zone in the nordfjord area this zone may mark the disrupted tectonic boundary to a small ultrahigh pressure unit and casts doubt on the assumption that the western gneiss region behaved as a coherent terrane during burial and exhumation

timing of the permian triassic biotic crisis implications from new zircon u pb age data and their limitations abstract the most profound biotic crisis in the earth s history causing the near extinction of both terrestrial and marine life occurred at the end of the permian period about 253 myr ago and marks the paleozoic mesozoic era boundary the cause of this event is still a matter of vigorous debate with both brief and catastrophic as well as gradual mechanisms having been proposed similar to a recent landmark study this study uses the u pb method on zircons from the uppermost permian lowermost triassic ash fall deposits at meishan zhejiang province se china in order to examine time and rate constraints for these events the results of both this study and previous work show that for these ash layers the effects of pb loss are combined with varying amounts and sources of inheritance resulting in an age scatter which prohibits the extraction of a statistically robust age in many cases though the effects of pb loss on the zircons analyzed in this study were reduced by leaching the grains in hydrofluoric acid as opposed to commonly applied air abrasion prior to analysis the presence within a single ash layer of multiple generations of older xenocrysts in many cases only slightly older than the depositional age has made quantitative interpretation even more difficult when these combined phenomena bias individual zircon ages by less than a percent they are extremely difficult to deconvolute and if multi grain analyses are used can become impossible to recognize because of the resulting age averaging monte carlo simulations using actual measurements of individual zircon crystals show that age excursions due to pb loss and xenocrystic contamination for the meishan bentonites are easily homogenized to the point of undetectability when replicate analyses of multi grain zircon samples are compared thus this study uses only high precision analyses of single crystals whether from our work or that of previous studies three main conclusions have emerged first our data require a significant increase in the age of the permian triassic boundary by more than 2 myr compared to the previous study which shifts the age to a value older than 253 ma second neither our data nor those from previous work can confirm or negate the possibility of a very abrupt biotic crisis third even large suites of very high quality single zircon u pb analyses for these tuffs cannot in most cases yield objective reliable and robust dates with accuracies at the sub myr level though the temptation to perform arbitrary selection of subsets of the analyses for that purpose is almost irresistible the last conclusion is not an indictment of zircon u pb dating in general other rocks and other zircon populations can and do behave very differently and further technical advances will likely improve our ability to prepare grains or sub grains of adequately enhanced quality for analysis consequently the results of the present study strongly suggest that for problems requiring time scale accuracy inferences from zircon u pb dating must be based on sufficiently large suites of single crystal or crystal domain high precision analyses

estimating carbon accumulation rates of undrained mires in finland application to boreal and subarctic regions equations based on empirical relationships of peat physical properties were used to estimate the average long term apparent rate of carbon accumulation lorca in finnish mire vegetation regions the results were generalized to the boreal and subarctic regions analyses of 1302 dated peat cores were used to infer carbon accumulation for each mire vegetation region of finland the area weighted lorca for finnish undrained mire areas was 18 5 g m 2 yr 1 and the total carbon sink 0 79 tg yr 1 1 tg 1012g the total carbon pool of finnish undrained mires was estimated as 2257 tg the aapa mire region included 80 of the total net accumulation rate of carbon and 85 of the total carbon reservoirs of finnish undrained mires lorca was signi cantly higher in the raised bog region 26 1 g m 2 yr 1 compared with the aapa mire region 17 3 g m 2 yr 1 and bogs generally had a higher lorca 20 8 g m 2 yr 1 than fens 16 9 g m 2 yr 1 the total c sink for boreal and subarctic mires was estimated at 66 tg yr 1 whi

an algorithm for u pb isotope dilution data reduction and uncertainty propagation this is the publisher s version also available electronically from http onlinelibrary wiley com

campaign style titanite u pb dating by laser ablation icp implications for crustal flow phase transformations and titanite closure abstract u pb dates of titanite from 150 samples of chiefly quartzofeldspathic gneiss and leucosomes were measured across the western gneiss region of norway to understand deformation and metamorphism of typical crustal rocks during ultrahigh pressure uhp subduction and exhumation titanite is unstable at these high temperatures and pressures and indeed most of the titanite yielded post uhp dates a modest number of titanites sampled across large areas however have pre uhp u pb dates indicating that they survived their excursion to and return from mantle depths metastably this has three important implications 1 titanite grains can remain closed to complete pb loss during regional metamorphism at temperatures as high as 750 c and pressures as high as 3 gpa 2 phase transformations in quartzofeldspathic rocks can be inhibited at the same conditions 3 quartz bearing rocks can remain undeformed even at high temperature and pressure both of the latter were previously recognized the present study simply presents a new method for evaluating both using titanite u pb dates

coesite micro inclusions and the u pb age of zircons from the hareidland eclogite in the western gneiss region of norway abstract we have identified by laser micro raman spectroscopy that inclusions of coesite occur together with other eclogite facies mineral phases within metamorphic zircons separated from the large eclogite body at ulsteinvik dimnoy on hareidland this is the first identification of coesite from this portion of the northwestern western gneiss region wgr and supports continuity of ultrahigh pressure uhp metamorphism between the documented coesite occurrences on stadlandet to the south and the microdiamond and coesite pseudomorph localities on fjortoft in the nordoyene to the north the zircons first analysed by u pb tims in 1973 have been re analysed and have yielded a much more precise age of 401 6 1 6 ma that overlaps with the previously determined age our discovery of coesite and the indication of a close to 402 ma formation age add to a growing number of mid late early devonian ages that signal that the uhp metamorphism in this part of west norway occurred relatively late in the caledonian orogenic cycle these observations should be incorporated in geodynamic models for the exhumation of these rocks and for the metastable preservation of eclogite facies mineralogies

the proterozoic hustad igneous complex a low strain enclave with a key to the history of the western gneiss region of norway the hustad igneous complex hic is a concentrically zoned high k calc alkaline intrusive complex with a core of quartz monzonite and granite surrounded successively by monzodiorite and pyroxenite preserved in a 10 km2 lens within the strongly caledonised western gneiss region wgr of norway upper intercept u pb zircon ages of 1654 1 and 1653 2 ma obtained from the granite and the monzodiorite respectively overlap with a rb sr whole rock age of 1656 49 ma i r 0 70193 0 0009 mswd 6 6 obtained from the felsic members of the complex and a sm nd whole rock age of 1555 180 ma i r 0 51067 13 mswd 3 3 the u pb age of 1654 1 ma is interpreted to date the time of intrusion a 50 m thick dolerite dyke that can be followed for almost 4 km across the hic yields a u pb zircon and baddeleyite age of 1251 3 ma the dyke also contains a secondary polycrystalline zircon generation which together with baddeleyite defines a lower intercept age of 384 9 ma the caledonian events were also marked by the emplacement of a swarm of pegmatites which crosscut all the other units and provide a u pb zircon age of 401 1 ma a rb sr whole rock age for the pegmatites is older at 452 26 ma i r 0 7105 mswd 4 3 indicating some initial isotopic heterogenity and formation of the pegmatites from material with a long crustal residence time a concordant u pb monazite age of 390 2 ma from the pegmatite and the above 384 9 ma lower intercept age date metamorphism caused by percolating fluids post pegmatite deformation transformed the complex along shear zones into gneisses indiscernible from the surrounding gneisses the hic is characterised by high mg numbers 0 39 0 80 high ni and cr values and depletion of nb and ta relative to lil elements phenocrysts of ortho and clinopyroxene have high concentrations of cr supporting the cr rich nature of the complex the suite is lree enriched has a fractionated and u shaped ree pattern cen 90 143 ybn 12 40 end values ranging from 0 0 to 2 7 and the felsic members display weak positive eu anomalies while the pyroxenite has a negative eu anomaly the complex bears a geochemical resemblance to the late tectonic high ba sr granite complexes of west scotland and particularly to their volcanic counterparts the mg rich old red sandstone andesites overall the data suggest generation of the suite in a subduction environment from material derived by melting of a recently enriched but previously long term lree depleted mantle the hic is one of the few well preserved remnants of the igneous protoliths of the 1686 1640 ma gneisses that make up the wgr in fact the pyroxenite is compositionally identical to orthopyroxene eclogite found as lenses in the surrounding gneisses and forms a likely protolith to this enigmatic rock the metamorphism had little influence on the rb sr and sm nd systems of the felsic rocks but it appears to have affected more strongly those in the mafic rocks probably as a result of the more intense mineralogical transformations and metasomatic loss of rb and lree during high grade metamorphism

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

fabric development during exhumation from ultrahigh pressure in an eclogite bearing shear zone western gneiss region norway abstract petrofabrics and trace element thermobarometry of deformed quartzofeldspathic gneiss and associated coesite bearing eclogite in the salt mylonite zone western gneiss region norway document a pressure temperature deformation path from ultrahigh pressure to amphibolite facies conditions the salt mylonite zone is dominated by quartzofeldspathic gneiss with a strong foliation and lineation coesite bearing eclogite within the shear zone contains a foliation and lineation defined by elongate omphacite consistent with that of the host gneiss suggesting that gneiss and eclogite were deformed in the same kinematic framework in eclogite omphacite preserves ls to l type crystallographic preferred orientation and quartz preserves prism fabrics that developed in quartz near coesite quartz transition conditions the quartzofeldspathic gneiss in the mylonite zone records prism and rhomb slip in quartz and reverse zoning in plagioclase higher ca rims consistent with re equilibration during decompression the ti concentration in quartz in gneiss is higher than that in quartz in eclogite suggesting that quartz recrystallized at a lower pressure in the gneiss ti in quartz thermobarometry of rutile bearing eclogite and titanite bearing gneiss indicates equilibration at t 750 c and t

metamorphic zircon formation at the transition from gabbro to eclogite in trollheimen surnadalen norwegian caledonides a transition from gabbro to eclogite has been investigated at vinddoldalen in south central norway with the aim to link reaction textures to metamorphic zircon growth and to obtain a direct u pb zircon age of the metamorphic process in the different rocks of the transition zone zircon occurs as i igneous prismatic grains ii metamorphic polycrystalline rims and pseudomorphs after baddeleyite and iii as tiny 10 mu m bead like zircon grains textural relations suggest that type ii zircon formed by breakdown of baddeleyite in the presence of silica whereas fe ti oxides were the main zr source for the type iii zircon subsolidus liberation of zr and formation of bead zircon took place by oxyexsolution of titanomagnetite during fluid assisted metamorphism and by resorption of fe ti oxide in rock domains that were completely recrystallized to eclogite sims secondary ion mass spectrometry and tims thermal ionization mass spectrometry dating provides comparable u pb ages of magmatic zircon and baddeleyite baddeleyite tims yielded an age of 1457 11 ma for the gabbro emplacement bead type metamorphic zircon from eclogite gave 425 10 ma tims dating the metamorphic transition from gabbro to eclogite in the upper basement of the lower allochthon in the south central scandinavian caledonides less

microdiamond in high grade metamorphic rocks of the western gneiss region norway three grains of microdiamond were recovered from high grade gneiss exposed in the western gneiss region norway identification and characterization of the diamond grains by raman and infrared spectroscopy indicate the presence of substitutional impurities of h and n primary fluid inclusions in garnet and quartz in the diamond bearing rock demonstrate the evolution of metamorphic volatile fluids from reduced n 2 co 2 compositions during the peak phase of metamorphism to n 2 ch 4 h 2 o bearing compositions during retrograde metamorphism compatible geologic petrologic and fluid composition data imply a metamorphic origin for the microdiamonds if so the metamorphic and fluid conditions recorded by the microdiamonds and gneissic host may be applicable to microdiamond investigations in other high pressure regionally metamorphosed orogens

geochronology and geochemistry of zircon from the northern western gneiss region insights into the caledonian tectonic history of western norway abstract the western gneiss region wgr of norway is divided by the more trondelag shear zone mtsz into a southern region that contains domains of caledonian ultrahigh pressure uhp metamorphic rocks 2 5 gpa and a northern area of similar caledonian aged rocks that record a maximum pressure reported thus far of 1 5 gpa although both regions contain similar lithologies primarily migmatitic quartzofeldspathic gneiss containing mafic lenses and structural relationship of basement rocks to infolded nappes this difference in maximum pressure implies a difference in tectonic history continental subduction south of the shear zone none to the north and raises questions about the role of the mtsz in the metamorphic history including exhumation of the wgr previous geochronology results indicated a difference in timing of peak metamorphism older in north younger in south in order to better understand the tectonic history of the northern wgr and the mtsz and in particular the late to post caledonian tectonic history u pb zircon geochronology and trace element abundances were obtained using the split stream laser ablation icpms technique from metabasaltic lenses and migmatitic quartzofeldspathic host rocks from the structurally lowest exposed region of the northern wgr roan peninsula basement as well as leucosomes from an intercalated portion of the seve nappe complex and a pegmatite in the mtsz zircon from roan gneiss and metabasite yield metamorphic ages of ca 410 406 ma and zircon from a variety of migmatite samples foliation parallel leucosome to dikes indicate melt crystallization at ca 410 to 405 ma the seve nappe leucosomes yield only early caledonian dates that cluster at ca 437 ma and ca 465 ma suggesting that the allochthons in this region did not experience or record the same scandian tectonic history as the basement rocks zircon from a weakly deformed pegmatite dike within the mtsz crystallized at ca 404 ma indicating that this shear zone was active during the end stages of high grade metamorphism in both the southern and northern wgr domains results of this study show that the northern and southern wgr experienced a coeval scandian metamorphic magmatic and deformation history despite a possible difference in maximum p t conditions

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

esker characteristics in terms of glacier physics katahdin esker system maine the characteristics of large subglacially formed eskers such as the katahdin system are closely related to two special peculiarities of water filled tunnels along the beds of ice sheets 1 the water pressure approximates the weight of the overlying ice and 2 in tunnels that descend and those that ascend less steeply than 1 7 times the ice surface gradient the walls melt producing a sharply arched tunnel cross section whereas in those that ascend more steeply they freeze producing a wide low one the first peculiarity primarily governs the paths of these eskers it causes the tunnels to follow the paths ordinary rivers would follow were the land tipped downglacier 11 times the local ice surface gradient the paths therefore trend in the general direction of the former ice flow but tend to deviate so as to follow valleys and to cross divides at the lowest passes as observed ice surface gradients calculated from path deviations at two localities on the katahdin esker system indicate relatively thin sluggish ice the surface of which lay 200 m below the summit of mount katahdin in agreement with independent geologic evidence the second peculiarity primarily governs the form composition and structure of these eskers strong melting causes a large inflow of basal ice and entrained debris to the tunnel and produces sharp crested eskers of poorly sorted poorly bedded sand gravel and boulders with lithologies like the adjacent till whereas weaker melting produces multiple crested ones of similar composition freezing precludes inflow and produces broad crested eskers of fairly well sorted well bedded more water worn coarse sand with few large clasts ice surface gradients calculated from transitions from the multiple crested type to small areas of broad crested type on the katahdin system agree closely with those computed from the paths at nearby localities an anomalously low gradient calculated from a transition to an area of broad crested type approximately twice as wide and long as the probable ice depth apparently confirms that as expected the basal ice was supported by water pressure over most if not all of the width of the esker

the impact of zircon garnet ree distribution data on the interpretation of zircon u pb ages in complex high grade terrains an example from the rauer islands east antarctica abstract the responsiveness of zircon to granulite metamorphism and post peak fluid infiltration in polymetamorphic terranes has been investigated utilising a case study from the rauer islands of east antarctica imaging bse cl textural analysis in situ ion microprobe trace element analysis of zircon garnet orthopyroxene plagioclase and apatite and ion microprobe oxygen isotope analysis of zircon have been integrated to establish the event significance of c 510 ma zircon age domains that occur within and on c 2840 ma protolith magmatic zircons initial magmatic zircon is characterised by moderate th u 0 4 0 6 and elevated steep hree yb n gd n 20 c 510 ma zircon domains feature lobate cuspate and planar bse and cl zones that have lower th u to 0 05 and either decreased mree or decreased mree and hree metamorphic garnet initially characterised by flat hree shows extensive replacement by orthopyroxene and plagioclase accompanied by near rim increases in hree to values that are consistent with equilibrium between garnet rims and product orthopyroxene zircon garnet mree hree distribution coefficients d ree calculated using compositions of modified zircon domains coupled with both initial metamorphic garnet cores and rims are far removed from presently accepted equilibrium values with the zircon mree being strongly depleted relative to garnet these d ree indicate that the observed modification of zircon did not occur in response to the major high grade metamorphic events recorded in the studied rock mineralogy but instead occurred after the high grade metamorphism the zircon was blind to these metamorphic events but instead was responsive to a later post peak fluid infiltration episode that shifted zircon 18 o values downwards by some 3 the likelihood of blind or unresponsive behaviour of zircon to high grade metamorphism must be considered when reconstructing crustal histories and terrane scenarios based on u pb age data and tested for using an integrated approach that includes in situ ree and other trace element analysis of zircon and its potentially coexisting minerals

temperature and bulk composition control on the growth of monazite and zircon during low pressure anatexis mount stafford central australia the formation age and trace element composition of zircon and monazite were investigated across the prograde low pressure metamorphic sequence at mount stafford central australia three pairs of inter layered metapelites and metapsammites were sampled in migmatites from amphibolite facies t 600 c to granulite facies conditions t 800 c sensitive high resolution ion microprobe u pb dating on metamorphic zircon rims and on monazite indicates that granulite facies metamorphism occurred between 1795 and 1805ma the intrusion of an associated granite was coeval with metamorphism at 1802 3ma and is unlikely to be the heat source for the prograde metamorphism metamorphic growth of zircon started at t 750 c well above the pelite solidus zircon is more abundant in the metapelites which experienced higher degrees of partial melting compared with the associated metapsammites in contrast monazite growth initiated under sub solidus prograde conditions at granulite facies conditions two distinct metamorphic domains were observed in monazite textural observations petrology and the trace element composition of monazite and garnet provide evidence that the first metamorphic monazite domain grew prior to garnet during prograde conditions and the second in equilibrium with garnet and zircon close to the metamorphic peak ages from sub solidus prograde and peak metamorphic monazite and zircon are not distinguishable within error indicating that heating took place in less than 20 myr

experimental determination of ree partition coefficients between zircon garnet and melt a key to understanding high t crustal processes the partitioning of rare earth elements ree between zircon garnet and silicate melt was determined using synthetic compositions designed to represent partial melts formed in the lower crust during anatexis the experiments performed using internally heated gas pressure vessels at 7 kbar and 900 1000 c represent equilibrium partitioning of the middle to heavy ree between zircon and garnet during high grade metamorphism in the mid to lower crust the dree zircon garnet values show a clear partitioning signature close to unity from gd to lu because the light ree have low concentrations in both minerals values are calculated from strain modelling of the middle to heavy ree experimental data these results show that zircon is favoured over garnet by up to two orders of magnitude the resulting general concave up shape to the partitioning pattern across the ree reflects the preferential incorporation of middle ree into garnet with dgd zircon garnet ranging from 0 7 to 1 1 dho zircon garnet from 0 4 to 0 7 and dlu zircon garnet from 0 6 to 1 3 there is no significant temperature dependence in the zircon garnet ree partitioning at 7 kbar and 900 1000 c suggesting that these values can be applied to the interpretation of zircon garnet equilibrium and timing relationships in the ultrahigh t metamorphism of low ca pelitic and aluminous granulites

protolith ages and exhumation histories of ultra high pressure rocks across the western gneiss region norway the timing of protolith formation ultrahigh pressure uhp subduction and subsequent exhumation for the ultrahigh pressure to high pressure units across the eastern part of the western gneiss region norway were assessed using u pb zircon th pb monazite and 40 ar 39 ar white mica ages u pb zircon ages from eclogites demonstrate that oceanic and continental allochthons were emplaced onto the baltica basement before the entire mass was subducted to ultra high pressure eclogites within the allochthons across the entire western gneiss region are caledonian and show a degree of zircon re crystallization that increases with peak pressure permitting the interpretation that the entire region underwent synchronous subduction 40 ar 39 ar white mica ages of 399 ma indicate that the eastern part of the western gneiss region had been exhumed to shallow crustal levels while uhp metamorphism was ongoing farther west indicating a westward dip to the slab the 40 ar 39 ar white mica ages also show a clear east to west gradient across the entire western gneiss region indicating that the western gneiss region rose diachronously to crustal levels from east to west between 399 and 390 ma

simulating vertical flow in large peatlands ground water flow simulations are used to evaluate the importance of three parameters on vertical flow in peatlands regional slope permeability of the mineral soil underlying the peat and peatland topography our results indicate that the extent of vertical ground water flow in peatlands is primarily controlled by mineral soil permeability local ground water flow cells that form under small water table mounds at bog domes can drive peat pore water into permeable mineral soil however when peat forms over low permeability mineral soil vertical movement of peat pore water becomes negligible and lateral flow of water in the upper portion of the peat column dominates the peatland hydrology this effect is not due to the low permeability of humified peat layers as is commonly assumed in many peatland studies field data from the hudson bay lowland canada and the glacial lake agassiz peatlands of northern minnesota confirm the validity of these models these results are relevant in settings other than peatlands where relief is low and small topographic mounds exist

u pb zircon ages constrain the architecture of the ultrahigh pressure qinling dabie orogen china new shrimp and tims zircon ages 40ar 39ar ages and eclogite locations contribute significantly to our understanding of the ultrahigh pressure dabie shan 1 the geographic extent of the yangtze craton that was subducted to ultrahigh pressure extends to the northern edge of the dabie shan 2 the northern half of the dabie shan is a magmatic complex intruded over a 10 myr interval between 137 and 126 ma that accommodated 100 n s stretching of the pre existing collisional architecture 3 granitic orthogneisses and enclosing ultrahigh pressure paragneisses have indistinguishable zircon populations the population of triassic zircon ages ranges from 219 to 245 ma leading us to question the prevailing assumption that 219 ma zircons formed at ultrahigh pressure and to propose instead that they reflect late retrogression at crustal pressures following the bulk of exhumation

high resolution imaging of the pyrenees and massif central from the data of the pyrope and iberarray portable array deployments the lithospheric structures beneath the pyrenees which holds the key to settle long standing controversies regarding the opening of the bay of biscay and the formation of the pyrenees are still poorly known the temporary pyrope and iberarray experiments have recently filled a strong deficit of seismological stations in this part of western europe offering a new and unique opportunity to image crustal and mantle structures with unprecedented resolution here we report the results of the first tomographic study of the pyrenees relying on this rich data set the important aspects of our tomographic study are the precision of both absolute and relative traveltime measurements obtained by a nonlinear simulated annealing waveform fit and the detailed crustal model that has been constructed to compute accurate crustal corrections beneath the massif central the most prominent feature is a widespread slow anomaly that reflects a strong thermal anomaly resulting from the thinning of the lithosphere and upwelling of the asthenosphere our tomographic images clearly exclude scenarios involving subduction of oceanic lithosphere beneath the pyrenees in contrast they reveal the segmentation of lithospheric structures mainly by two major lithospheric faults the toulouse fault in the central pyrenees and the pamplona fault in the western pyrenees these inherited hercynian faults were reactivated during the cretaceous rifting of the aquitaine and iberian margins and during the cenozoic alpine convergence therefore the pyrenees can be seen as resulting from the tectonic inversion of a segmented continental rift that was buried by subduction beneath the european plate

geophysical evidence for peat basin morphology and stratigraphic controls on vegetation observed in a northern peatland vegetation patterns in peatlands suggest autogenic peatland development and hydrological processes are controlling factors in peatland evolution these processes regulate vegetation ordination by influencing ph and nutrient availability at the peat surface geophysical studies in caribou bog a 2200 hectare peatland in central maine indicate a close correlation between underlying geologic deposits and dominant vegetation type the location of pools within the bog also appears to be related to subsurface geologic materials electrical resistivity imaging along a 1 km transect across the central unit of caribou bog resolved underlying lake sediment and glacio marine clay thickness as well as variability in depth to glacial till ground penetrating radar precisely defined the organic rich lake sediment the glacio marine interface and elevated till surfaces where peat thickness is less than 10 m direct verification of peat and lake sediment thickness in addition to samples of the glacio marine and till deposits constrained the geophysical interpretation wooded heath wh interspersed with occasional sphagnum lawn occurs where lake sediment and glacio marine clay accumulation is thickest abrupt thinning of both layers such that peat rests directly on till in places correlates with a sharp transition to shrub heath dominated vegetation a major open pool area located within the wh coincides with a localized sandy mineral soil lake sediment contact at the base of the bog the geophysical data suggest the underlying double basin system influences peatland development and dictates vegetation patterning limited groundwater elevation data suggest that sub organic sediment stratigraphy mapped with geophysics regulates water levels at the peat surface exerting a control on vegetation patterning the major pool area located between the basins appears to reflect isolation of stagnant water at a point where peat development is incomplete and thus suggests that pools exist concurrent with peatland formation

surface hydrology of low relief landscapes assessing surface water flow impedance using lidar derived digital elevation models abstract conventional hydrologic analyses of digital elevation models dems perform well in areas of high topographic relief where surface water flow is typically unidirectional convergent spatially static and directed toward a single discharge point at the edge of a catchment such analyses do not perform well on landscapes with low topographic relief e g floodplains river deltas coastal wetlands and estuaries where surface water flow is influenced by subtle topographic depressions and may be bidirectional divergent and spatially dynamic in response to hydrologic forcing such as tides or variation in river discharge we developed a framework for hydrologic analysis of low relief landscapes using a high resolution 1 m dem derived from light detection and ranging lidar data collected over a 8 8 km 2 section of the umatilla river floodplain oregon usa our approach assessed the pattern and characteristics of hydrologic facets landscape patches that have high internal surface water connectivity and therefore function as a single hydrologic unit where facet boundaries were defined by subtle topographic divides across the floodplain we initially identified nearly 6000 small fine scale hydrologic facets using standard gis processing algorithms we located the divide between each pair of adjacent facets and determined hydrologic impedance i e the maximum change in river stage necessary to inundate the divide for each divide n 17 000 using hydrologic impedance values we analyzed patterns of surface water connectivity among the fine scale facets and aggregated groups of adjacent facets that had high connectivity this process yielded a reduced number of larger facets useful for hydrologic analysis at coarser spatial scales we compared results derived using several alternate rule sets for aggregating facets with appropriate aggregation rules the results are useful for generating optimal link and node flow networks to support hydrologic modeling of surface water flux across low relief landscapes

u pb dates and trace element geochemistry of zircon from migmatite western gneiss region norway significance for history of partial melting in continental subduction article i nfo thewesterngneissregion wgr norway isdominatedbymigmatiticgneissthatcontainsinclusionsofeclogite some of which contain evidence for ultrahigh pressure metamorphism to evaluate geochemical and age relation ships between host migmatite and eclogite we obtained la icp ms u pb dates and trace element analyses for zircon from a variety of textural types of leucosome from layer parallel to crosscutting zircon textures euhedral oscillatory and sector zone grains indicate a likely magmatic origin of the leucosomes caledonian u pb zircon dates from zircon rim and near rim regions are as old as 410 406 ma coeval with previously determined ages of high and ultrahigh pressure metamorphismof wgr eclogite trace elementanalysesobtained simultaneously with u pb dates indicate crystallization of zircon under garnet present conditions in the majority of leucosomes other zircons including those from crosscutting pegmatite yield younger ages as young as 385 ma coinciding with dates determined for amphibolite facies retrogression of eclogite trace element analyses suggest that these zircons grew under plagioclase present garnet absent conditions combined age and trace element data for leucosome zircons record the transition from high pressure garnet present plagioclase absent crys tallization to lower pressure plagioclase present crystallization if the euhedral zircons that yield ages coeval with peak or near peak uhp metamorphism represent crystallization from anatectic leucosomes these results combined with field and petrographic observations of eclogite migmatite relationships are consistent with the presence of partially molten crust in at least part of the wgr during continental subduction the decreased viscosity and increased buoyancy and strain weakening associated with partial melting may have assisted the rapid ascent of rocks from mantle to crustal depths

metamorphic evolution of norwegian country rock eclogites as deduced from mineral inclusions and compositional zoning in garnets abstract detailed studies of pre early syn and post eclogite mineral associations in some west norwegian country rock eclogites have made it possible to reconstruct their metamorphic evolution pre eclogite stages are represented by various amphibolite facies associations found in the central parts of prograde zoned garnets early and syn eclogite stages are represented by various eclogitic assemblages containing low and high jd omphacites respectively the former located in the intermediate zones and the latter in the outermost rims of prograde zoned garnets and in the matrix post eclogite stages include granulite amphibolite and greenschist facies associations representing replacement phases after higher grade assemblages p t estimates indicate a prograde evolution along an extremely steep p t gradient 7 9 kbar over 100 200 c while the initial post eclogite evolution involved a nearly adiabatic decompression on the order of 10 kbar

trace element signature and u pb geochronology of eclogite facies zircon bergen arcs caledonides of w norway secondary ion mass spectrometry sims u pb and trace element data are reported for zircon to address the controversial geochronology of eclogite facies metamorphism in the lindas nappe bergen arcs caledonides of w norway caledonian eclogite facies overprint in the nappe was controlled by fracturing and introduction of fluid in the proterozoic sveconorwegian granulite facies meta anorthosite norite protolith zircon grains in one massive eclogite display a core rim structure sveconorwegian cores have trace element signatures identical with those of zircon in the granulite protolith i e 0 31 th u 0 89 heavy rare earth element hree enrichment and negative eu anomaly weakly zoned to euhedral oscillatory zoned caledonian rims are characterized by th u 0 13 low lree content minimum normalized abundance for pr or nd variable enrichment in hree and no eu anomaly a decrease of ree towards the outermost rim especially hree is documented this signature reflects co precipitation of zircon with garnet and clinozoisite in a feldspar absent assemblage and consequently links zircon to the eclogite facies overprint the rims provide a mean 206pb 238u crystallization age of 423 4 ma this age reflects eclogite forming reactions and fluid rock interaction this age indicates that eclogite facies overprint in the lindas nappe took place at the onset of the scandian silurian collision between laurentia and baltica

an integrated microtextural and chemical approach to zircon geochronology refining the archaean history of the napier complex east antarctica integrated textural and chemical characterisation of zircon is used to refine the u pb geochronology of the archaean ultra high temperature napier complex east antarctica scanning electron microscope characterisation of zircon and the rare earth element compositions of zircon garnet and orthopyroxene are integrated to place zircon growth in an assemblage context thereby providing tighter constraints on the timing of magmatic and metamorphic events data indicate that magmatism occurred in the central and northern napier complex at ca 2 990 ma a regional relatively low pressure metamorphic event occurred at ca 2 850 2 840 ma mineral ree data from garnet bearing orthogneiss indicate that ca 2 490 2 485 ma u pb zircon ages provide an absolute minimum age for the ultrahigh temperature uht foliation preserved in this rock internal zircon zoning relationships and estimated zircon garnet dree values from paragneiss suggest that an absolute minimum age of ultra high temperature metamorphism is ca 2 510 ma but that it is more likely to be older than ca 2 545 ma we suggest that the high proportion of published zircon u pb data with ages between ca 2 490 2 450 ma reflects late post peak zircon growth and does not date the timing of peak uht metamorphism

slow exhumation of uhp terranes titanite and rutile ages of the western gneiss region norway abstract u pb ages of titanite and rutile were obtained from the central western gneiss region norway to assess the style and timing of exhumation and cooling of the western gneiss uhp terrane approximately half of the titanite ages are concordant the majority of which yield a limited age range from 393 to 390 ma the other titanite data are discordant and define discordia arrays with upper intercept ages of either 938 ma or 1 6 ga and a lower intercept of 389 ma concordant rutile analyses range from 385 to 392 ma both titanite and rutile ages young wnw toward the core of the orogen and are 4 ma older than 40 ar 39 ar muscovite ages corresponding to a cooling rate of 90 c ma a well defined boundary between concordant and discordant titanite ages in combination with the wnw increasing p t gradient and the similarity between muscovite cooling ages in the east and eclogite ages in the west suggests that the wgr remained coherent throughout its exhumation history and was progressively unroofed from east to west a 390 2 0 8 ma titanite in the soroyane uhp domain indicates that exhumation occurred at a vertical rate of 7 mm yr for 12 ma these rates are slower than estimates from smaller uhp terranes but similar to other large uhp terranes suggesting that there may be fundamental differences in the mechanisms controlling the evolution of large uhp terranes that undergo protracted subduction and exhumation and smaller uhp terranes that undergo rapid subduction and exhumation

raman confirmation of microdiamond in the svartberget fe ti type garnet peridotite western gneiss region western norway introductionvestiges of ultra high pressure meta morphism uhpm havebeenfoundincontinental and oceanic rocks of dif ferentorogensworldwide smith 1988 coleman and wang 1995 carswelland compagnoni 2003 indirectuhpmevidencecomesfromtheappli cation of geothermobarometric tech niques lappin and smith 1978 ravna krogh and paquin 2003 ap plied to uhpm mineral assemblagesand directly from mineralogical indi cators such as coesite diamond andmajoritic garnet chopin 1984 smith 1984 sobolev and shatsky 1990 do brzhinetskaya et al 1995 van roer mund and drury 1998 nasdala andmassonne 2000 mposkos andkosto poulos 2001 van roermund et al 2002 godard et al 2003 neverthe less the mechanisms responsible forthe formation preservation and exhu mation of uhpm rocksin orogens arestill not fully understood the volume and spatial distributionof uhpm rocks within uhpm terr anes including the observed contrastin metamorphic grade between rocksthat contain uhpm minerals andadjacent country rocks that do notcontain them have led to di erentexplanations concerning the origin emplacement and preservation ofthese rocks to explain the occur rence volume and areal extent ofuhpm rocks in the western gneissregion wgr the following mecha nisms that assume a continental sub duction scenario have been proposed a the existence of major and orminor post uhpm tectonic contactssuch as nappes and or tectonic mel anges e g lappin and smith 1978 smith 1984 1988 wain 1997 cuth bert et al 2000 terry and robinson 2003 2004 or b the presence of uids and or deformation leading todi erential prograde or retrogradereactions and di erences in reaction kinetics between rock types austr heim et al 1997 cuthbert et al 2000 wain et al 2001 carswell andcuthbert 2003 cuthbert and car swell 2003 hypotheses that do notnecessarily involve continental sub duction may involve local regional tectonic overpressure in rocks smith 1984 mancktelow 1993 petrini and podladchikov 2000 it isconsidered here that the volume andareal extent of uhpm domains withina single metamorphic terrain are thusimportant parameters to determine asthey can modify or identify the loca tions of inferred or observed tectoniccontacts the relations between uidsand uhpm and or the scales atwhich continental crust subductionor tectonic overpressure might act here we report raman data thatcon rm the previously reported butnot proven presence of microdia monds at the svartberget uhpmlocality in the wgr of norway vrijmoed et al 2006 micro diamond con rmation is consistentwith the results of vrijmoed et al 2006 that the volume and arealextent of the northern uhpm domainof the wgr fig 1 are considerablyexpanded eastwards and followingthe reasoning in the paragraph above the new microdiamond nding willput important constraints on themodels described above ultra high pressure metamorphismin the western gneiss regionthe wgr represents a window inthe scandinavian caledonides wherescandian reworked high grade gneis sic rocks of proterozoic age presum ably belonging to the baltic plate are exposed in addition variableamounts of metadolerites amphi abstract

stratigraphic controls on pool formation in a domed bog inferred from ground penetrating radar gpr ground penetrating radar gpr and terrain conductivity measurements reveal a close correlation between the location of water filled pools and stratigraphic lithologic features in caribou bog a 2200 hectare peatland in central maine the geophysical profiles supported by coring define the general peatland stratigraphy as till underlain by glacio marine sediment overlain by organic rich lake sediment transitioning into terrestrial peat gpr measurements show that open pools coincide with abrupt thinning of lake sediment and glaciomarine clay such that elevated mineral soil surfaces are in direct contact with the base of terrestrial peat the width of these features the gpr reflection patterns the anomalously low terrain conductivity and the core samples obtained support the interpretation of these elevated mineral surfaces as the extension of buried eskers that outcrop on the periphery of caribou bog the geophysical results indicate that two parallel esker ridges are orientated n s and dip towards the south we suggest that the distribution of open pools is related to the distribution of depressions and beads associated with a beaded esker deposit the pools possibly being maintained by local groundwater flow cells associated with the esker gpr and em31 surveys across sphagnum pools defined here as sphagnum filled wet depressions suggest a continuous sequence of underlying lake sediment and glacio marine deposits devoid of elevated mineral surfaces we suggest that sphagnum pools developed after peatland formation secondary features due to changes in water table peat accumulation and or climatic controls q 2005 elsevier b v all rights reserved

the fate of subducted continental margins two stage exhumation of the high pressure to ultrahigh pressure western gneiss region norway thermobarometry suggests that ultrahigh pressure uhp to high pressure hp rocks across the western gneiss region ponded at the moho following as much as 100 km of exhumation through the mantle and before exhumation to the upper crust eclogite across the c 22 000 km 2 study area records minimum pressures of c 8 18 kbar and temperatures of c 650 780 c one orthopyroxene eclogite yields an uhp of c 28 5 kbar and evidence of former coesite has been found c 50 km farther east than previously known despite this widespread evidence of uhp to hp thermobarometry of metapelite and garnet amphibolite samples reveals a surprisingly uniform supra barrovian amphibolite facies overprint at c 11 kbar and c 650 750 c across the entire area chemical zoning analysis suggests that garnet in these samples grew during heating and decompression presumably during the amphibolite facies event these data indicate that the norwegian uhp hp province was exhumed from mantle depths of c 150 km to lower crustal depths where it stalled and underwent a profound high temperature overprint the ubiquity of late stage supra barrovian metamorphic overprints suggests that large scale collisional uhp terranes routinely stall at the continental moho where diminishing body forces are exceeded by boundary forces significant portions of the middle or lower crust worldwide may be formed from uhp terranes that were arrested at the moho and never underwent their final stage of exhumation

slow subduction of a thick ultrahigh pressure terrane 1 high precision lu hf and sm nd ages reported here cover a large portion of the western gneiss region wgr ultrahigh pressure uhp terrane norway and range from 413 9 3 7 to 397 1 4 8 ma collectively new and existing geochronologic data for eclogites demonstrate that eclogite facies metamorphism occurred over a large area 60 000 km2 for an unexpectedly long period of 20 ma a thermal model using slow subduction 2 4 mm a 1 of a relatively warm and thick slab can reproduce the observed p t t history of the wgr uhp terrane indicating that large uhp terranes with protracted peak or near peak histories probably reflect slow subduction of relatively thick crustal sections

relations between granitoid magmatism and migmatization u pb geochronological evidence from the western gneiss complex norway gneiss and migmatite samples from the southern part of the western gneiss complex of norway define a single u pb zircon discordia line with lower and upper intercepts at 1633 8 ma and 969 8 ma the upper intercept is interpreted as the magmatic age of the protoliths showing that the basement rocks were generated during the gothian orogeny 1750 1500 ma the migmatites yield titanite ages that are similar to the lower intercept zircon age and we interpret this to be the age of migmatization during the sveconorwegian orogeny u rich titanite cores yield an age of 973 3 ma whereas less u rich rims gave a slightly younger age of 961 4 ma a granitoid batholith in the area gave a u pb zircon age that is coeval with the migmatite age 966 2 ma the metaluminous granitic magma of the pluton formed by melting at relatively high temperatures in the lower crust in contrast the migmatites formed during upper amphibolite facies conditions at relative high crustal levels the close temporal and spatial relations between the migmatites and the granite batholith suggest that migmatite formation in the lower parts of the upper crust was related to the heat and mass flux linked to the rise of granitic magma from the lower crust

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

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

ground penetrating radar and spring formation in a groundwater dominated peat wetland summary ground penetrating radar gpr was used to identify the thickness and geometry of peat in a groundwater fed wetland in northern wisconsin the low electrical conductivity of the water saturated peat permitted high resolution imaging to a depth of 16 m the gpr survey results delineated a break in slope in the interface between peat and the underlying sand gravel springs and ponds are located above the break in slope suggesting that thinning of the sand and gravel may focus groundwater flow upward to the wetland surface with emergence as springs the results of the gpr surveys suggests a potential mechanism that controls the formation of large springs and pools above the break in slope