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

petrological constraints and pt path of devonian collapse tectonics within the scandian mountain belt western gneiss region norway in norway post caledonian extension controlled the formation and sedimentation of middle devonian sedimentary basins these collapse basins were deposited along a low angle extensional detachment fault on top of caledonian high grade metamorphic rocks of the western gneiss region this paper concerns the petrological analysis of eclogite and high pressure micaschist collected within the extensional shear zone study of gneisses and micaschists surrounding these high pressure rocks allows description of the complete pt path which characterized this area application of appropriate geothermometers and geobarometers confirms that the retrograde evolution following high pressure conditions ll 15kbar at 500 600 c was characterized by nearly isothermal decompression followed by amphibolite facies metamorphism the shape of the pt path is compared with theoretical models of thermal evolution of continental orogenic crust during uplift we can observe an increase in temperature during the first stages of decompression smaller than predicted by the models considering the evidence for extensional tectonic in the study area we propose to explain such a small increase in temperature during the decompression by the intervention of extension which accelerates the collapse process and reduces the late orogenic heating of the caledonian thickened crust

replacement of primary monazite by apatite allanite epidote coronas in an amphibolite facies granite gneiss from the eastern alps accessory monazite crystals in granites are commonly unstable during amphibolite facies regional metamorphism and typically become mantled by newly formed apatite allanite epidote coronas this distinct textural feature of altered monazite and its growth mechanism were studied in detail using backscattered electron imaging in a sample of metagranite from the tauern window in the eastern alps it appears that the outer rims of the former monazites were replaced directly by an apatite ring with tiny thorite intergrowths in connection with ca supply through metamorphic fluid around the apatite zone a proximal allanite ring and a distal epidote ring developed this concentric corona structure with the monazite core regularly preserved in the center shows that the reaction kinetics were diffusion controlled and relatively slow quantitative electron microprobe analyses suggest that the elements released from monazite breakdown p ree y th u were diluted and redistributed in the newly formed apatite allanite and epidote overgrowth rings and were unable to leave the corona this supports the common hypothesis that these trace elements are highly immobile during metamorphism furthermore microprobe data suggest that the preserved monazite cores lost little possibly none of their radiogenic lead during metamorphism thus metastable monazite grains from orthogneisses appear to be very useful for constraining u th pb protolith ages on the basis of these findings and a review of literature data it seems that monazite stability in amphibolite facies metamorphic rocks depends strongly on lithologic composition while breaking down in granitoids monazite may grow during prograde metamorphism in other rocks such as metapelites

experimental resetting of the u th pb systems in monazite abraded fragments 200 400 m of a large chemically homogeneous and non metamict brazilian monazite crystal characterised by a concordant u pb ages of 474 1 ma 208pb 206pb 19 5 were hydrothermally treated at varying temperatures with solutions of different compositions product monazites were analysed with scanning electron microscope sem electron microprobe emp secondary ion mass spectrometer sims and isotope dilution thermal ionisation mass spectrometer id tims experiments with pure water over a temperature range of 800 1200 c at 700 mpa and durations ranging from 5 to 60 days showed that even at 1200 c any dissolution and recrystallization of new monazite is confined to the outermost surface of the grain neither pb diffusion at the emp scale nor significant discordancy were observed we performed experiments at 800 and 1000 c for different durations using different fluid compositions at quartz saturation a 10 wt cacl2 fluid a 10 wt srcl2 fluid a 10 wt nacl fluid and a fluid containing nbs 982 pb standard 208pb 206pb 1 for all runs emp traverses revealed no pb diffusion profiles significant overgrowths of newly formed monazite are documented by sem analyses they occurred only in the 1000 c experiments when either cacl2 or pb bearing fluids were present in the cacl2 experiment two zones could be distinguished within the crystal a core possessing the initial monazite composition and a rim consisting of newly formed monazite produced by dissolution precipitation which was enriched in ca and pb free id tims dating of single grains treated with srcl2 and cacl2 solutions at 1000 c are significantly discordant experiments employing the nbs pb standard produced sub concordant monazite for which the 207pb 206pb apparent age has become older than prior to the experiment 544 ma at 800 c and 495 ma at 1000 c the newly grown monazite rim had obviously incorporated pb from the fluid none of our reaction products contained a detectable diffusion profile the only resetting mechanism we detected involved dissolution precipitation the extent of the dissolution precipitation process depends on fluid composition and is a more efficient mechanism than diffusion for controlling the resetting of monazite in natural rocks

pre caledonian granulite and gabbro enclaves in the western gneiss region norway indications of incomplete transition at high pressure the western gneiss region of norway is a continental terrane that experienced caledonian high pressure and ultrahigh pressure metamorphism most rocks in this terrane show either peak caledonian eclogite facies assemblages or are highly strained and equilibrated under late caledonian amphibolite facies conditions however three kilometre size rock bodies flatraket ulvesund and krakenes in outer nordfjord preserve pre caledonian igneous and granulite facies assemblages and structures where these assemblages are preserved the rocks are consistently unaffected by caledonian deformation the three bodies experienced high pressure conditions 20 23 kbar but show only very localized about 5 eclogitization in felsic and mafic rocks commonly related to shear zones the preservation of pre caledonian felsic and mafic igneous and granulite facies assemblages in these bodies therefore indicates widespread 95 metastability at pressures higher than other metastable domains in norway late caledonian amphibolite facies retrogression was limited the degree of reaction is related to the protolith composition and the interaction of fluid and deformation during the orogenic cycle whereby metastability is associated with a lack of deformation and lack of fluids either as a catalyst or as a component in hydration reactions the three bodies appear to have been far less reactive than the external gneisses in this region even though they followed a similar pressure temperature evolution the extent of metastable behaviour has implications for the protolith of the western gneiss region for the density evolution of high pressure terranes and hence for the geodynamic evolution of mountain belts

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

microfabrics of uhp metamorphic granites in the dora maira massif western alps no evidence of deformation at great depth in the dora maira massif western alps essentially undeformed ultrahigh pressure uhp metamorphic granites brossasco granite are embedded in and locally grade into granite gneisses or augengneisses and mylonites in this study the quartz microfabrics of the undeformed granites are compared against the augengneisses and mylonites in a representative number of samples from several locations in the undeformed granites the fine grained quartz aggregates that formed from coesite upon decompression are characterized by a foam structure and random crystallographic orientation in the deformed granites the quartz microstructures and the crystallographic preferred orientation cpo indicate deformation by dislocation creep most of the deformation of the granites if not all must have happened at a late stage during exhumation after transformation of coesite to quartz at greenschist facies conditions in the middle crust the deformed granites provide no evidence of deformation during subduction at u hp metamorphic conditions and in the earlier stages of exhumation the diameter of internally undeformed slices of continental crust subducted to and exhumed from about 100 km can exceed that of the presently exposed brossasco granite i e it can be on the kilometre scale

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

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

monazite paragenesis and u pb systematics in rocks of the eastern mojave desert california u s a implications for thermochronometry abstract studies of the paragenesis and u pb systematics of monazite in rocks from the eastern mojave desert california corroborate its potential usefulness as a prograde thermochronometer and in dating granite inheritance unmetamorphosed latham shale and its equivalents at grades ranging from greenschist to upper amphibolite facies are virtually identical in composition monazite is absent in the shale and low grade schists but it is abundant in schists at staurolite and higher grades lower grade schists instead include minute th and ce oxides and unidentified ce poor lree phosphates that apparently are lower temperature precursors to monazite thus monazite originates when the pelite passes through lower amphibolite facies conditions monazites from three upper cretaceous granites yield ages that are strongly discordant upper intercepts of 1 6 1 7 ga are similar to those defined by u pb data for coexisting zircons and coincide with a period of copious magmatism in the mojave crust as the host upper cretaceous granitic magmas were all above 700 c effective closure of the restitic monazites to pb loss must be well in excess of this temperature u pb compositions of monazite from proterozoic granitoids and schist also indicate high pb retentivity taken together these studies support the suggestion that monazite can be an effective prograde thermochronometer at least in pelites it is not usually retained as a detrital mineral but rather forms during moderate temperature metamorphism its u pb system should not be reset by subsequent higher grade metamorphism

three metamorphic monazite generations in a high pressure rock from the bohemian massif and the potentially important role of apatite in stimulating polyphase monazite growth along a pt loop abstract based on microstructural and compositional criteria three generations of monazite can be identified in a peraluminous garnet rich high pressure metamorphic rock from the bohemian massif the first monazite generation m1 formed probably on the prograde part of the clockwise pt loop a second m2 during the high pressure stage 26 3 kbar and 830 30 c and a third generation m3 during subsequent near isothermal decompression of the rock to 8 2 kbar the high pressure m2 monazites are volumetrically dominant and form 100 300 m grains they are characterised by an unusual sr brabantite component sro up to 2 wt and grew together with accessory sr rich fluorapatites sro up to 33 wt the sr contents of both minerals are indicative of high pressure conditions and result from the breakdown of plagioclase and the release of sr that occurred during this process m3 monazites occur exclusively as small grains 10 100 m within and at the rims of apatite microstructures show that they grew at the expense of the apatite extracting lrees from the latter under retrograde conditions these parasite monazites in apatite are characterised by very low th contents which is consistent with having been derived primarily from an apatite source m1 monazites are represented by resorbed irregular shaped cores which are occasionally preserved within the sr brabantite rich high pressure monazites m2 these m1 cores are generally sr poor indicating that they should have formed prior to the high pressure stage in the presence of plagioclase the dissolution of m1 monazite and the reprecipitation of a th u enriched m2 monazite during the high pressure stage implies that lrees were taken up by the high pressure sr rich fluorapatite to some extent and not completely processed for the formation of the m2 monazite this would also explain the widespread retrograde growth of m3 monazite within the apatite the ability of apatite to act as an interim host for lrees at peak pt conditions could be one of the reasons why monazite growth during regional metamorphism is often periodic

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

structure and 40ar 39ar thermochronology of an ultrahigh pressure transition in western norway abstract the nordfjord region of western norway hosts an archetypal subducted crustal section underpinned by ultrahigh pressure uhp eclogite overlain by devonian sediments and cored by a crustal scale extensional shear zone structural mapping reveals two distinct displacement zones that played different roles during the formation and exhumation of this section 1 the sandane shear zone is a nw dipping amphibolite facies high strain zone near the base of the eclogite bearing crust that separates allochthonous units from underlying crystalline basement it may have originated during early thrusting but was overprinted by top to the west extensional fabrics at lower crustal depths 2 structurally above this the nordfjord sogn detachment zone is a top to the west amphibolite to greenschist facies detachment shear zone within allochthonous units that defines the upper boundary of the eclogitized crust and was responsible for exhumation through at least mid crustal depths muscovite 40 ar 39 ar ages suggest that amphibolite facies deformation below the nordfjord sogn detachment was mostly finished by c 397 ma whereas muscovite ages from the deeper parts of the uhp domain indicate that it cooled after 390 ma during exhumation through the middle crust west directed stretching was accompanied by north south folding late sinistral transpressional faulting in the middle to upper crust truncated the earlier folds and shear zones supplementary material complete 40 ar 39 ar data and a summary geological map of the nordfjord region are available at http www geolsoc org uk sup18460

exhuming norwegian ultrahigh pressure rocks overprinting extensional structures and the role of the nordfjord sogn detachment zone 1 the nordfjord sogn detachment zone nsdz is widely cited as one of the primary structures responsible for the exhumation of norwegian ultra high pressure uhp rocks here we review data from the considerable volume of research describing this shear zone and compile a strikeparallel cross section along the nsdz from the solund basin in the south to the soroyane uhp domain in the north this cross section highlights several previously unrecognized patterns revealing a shear zone with top to the west asymmetric fabrics that 1 initiated at amphibolite facies 2 overprints metamorphic breaks and tectonostratigraphic contacts and 3 has a gradational continuum of muscovite cooling ages these patterns constrain the kinematic evolution of the nsdz and suggest a new three step model for the exhumation of norwegian u hp rocks the initial stages of exhumation were characterized by the rise of crustal rocks from u hp depths to the base of the crust by buoyancydriven mechanisms not specified in this paper mantle exhumation was followed by top to thewest normal sense displacement within a broad noncoaxial ductile shear zone near the base of the crust that overprinted tectonostratigraphic contacts formed previously during mantle exhumation in the final stages of crustal exhumation top w brittleductile detachments soled into and partially excised this ductile shear zone dropping the devonian basins into contact with rocks of varying tectonostratigraphic levels this new interpretation of the nsdz is significant as it accounts for the extreme crustal excision observed in western norway using three sequentially overprinting structures active at different stages of uhp rock exhumation citation johnston s m b r hacker and t b andersen 2007 exhuming norwegian ultrahigh pressure rocks overprinting extensional structures and the role of the nordfjord sogn detachment zone tectonics 26 tc5001

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

structural petrological and chemical analysis of syn kinematic migmatites insights from the western gneiss region norway evidence of melting is presented from the western gneiss region wgr in the core of the caledonian orogen western norway and the dynamic significance of melting for the evolution of orogens is evaluated multiphase inclusions in garnets that comprise plagioclase potassic feldspar and biotite are interpreted to be formed from melt trapped during garnet growth in the eclogite facies the multiphase inclusions are associated with rocks that preserve macroscopic evidence of melting such as segregations in mafic rocks leucosomes and pegmatites hosted in mafic rocks and in gneisses based on field studies these lithologies are found in three structural positions 1 as zoned segregations found in high pressure hp ultra mafic bodies 2 as leucosomes along amphibolite facies foliation and in a variety of discordant structures in gneiss and 3 as undeformed pegmatites cutting the main caledonian structures segregations post date the eclogite facies foliation and predate the amphibolite facies deformation whereas leucosomes are contemporaneous with the amphibolite facies deformation and undeformed pegmatites are post kinematic and were formed at the end of the deformation history geochemistry of the segregations leucosomes and pegmatites in the wgr defines two trends which correlate with the mafic or felsic nature of the host rocks the first trend with ca poor compositions represents leucosome and pegmatite hosted in felsic gneiss whereas the second group with k poor compositions corresponds to segregation hosted in ultra mafic rocks these trends suggest partial melting of two separate sources the felsic gneisses and also the included mafic eclogites the ree patterns of the samples allow distinction between melt compositions fractionated liquids and cumulates melting began at high pressure and affected most lithologies in the wgr before or during their retrogression in the amphibolite facies during this stage the presence of melt may have acted as a weakening mechanism that enabled decoupling of the exhuming crust around the peak pressure conditions triggering exhumation of the upward buoyant crust partial melting of both felsic and mafic sources at temperatures below 800 c implies the presence of an h2o rich fluid phase at great depth to facilitate h2o present partial melting

detection of inherited monazite in the manaslu leucogranite by 208pb232th ion microprobe dating crystallization age and tectonic implications abstract although leucogranites are among the least petrologically variable of all igneous rocks ironically they are among the most difficult to reliably date the high himalayan leucogranites have been the subject of numerous geochronological investigations because of their interrelationship with the most significant tectonic features of that mountain belt for a variety of reasons linked to the minimum melt composition of these leucogranites these dating studies have not been entirely successful we report results of a new ion microprobe dating method based on the decay of 232 th to 208 pb in monazite that has directly revealed the presence of inherited pb in monazite from the manalsu granite casting doubt on its previously accepted age monazite ages obtained from this leucogranite yield two distinct populations a large number of ages with a normal distribution and mean age of 22 4 0 5 ma 2 s e that we interpret to be the crystallization age and a smaller inherited fraction with an age of ca 600 ma because formation of the granite is thought to be related to slip on the main central thrust both this date and a second less precise result from a structurally similar pluton near mt everest indicate that the main central thrust was active in the interval 24 22 ma but do not constrain its initiation together with crosscutting relationships these data require that movement on the north himalayan fault occurred prior to 22 ma at both locations

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

middle precambrian provenance of jurassic sandstone in the mino terrane central japan th u total pb evidence from an electron microprobe monazite study precise electron microprobe analyses have been made on detrital monazites in jurassic sandstones of the mino terrane central japan and their th u pb relations are examined in terms of pbo and tho2 tho2 tho2 plus the equivalent of uo2 the pbo tho2 ratios of most detrital monazites 403 of 581 grains cluster in the range from 0 0763 to 0 0618 other grains show the ratio of about 0 0541 46 grains 0 0364 29 grains 0 0170 24 grains and 0 0116 79 grains the pbo tho2 ratio of 0 0763 0 0618 is nearly identical with that of monazites in a middle precambrian 1740 ma gneiss clast from the kamiaso conglomerate of the mino terrane this strongly suggests that the detrital monazites were derived from gneisses and or granitoids of middle precambrian age the ratio of 0 0116 is nearly the same as that of monazites in a permian 274 ma paragneiss of the hida terrane north to the mino terrane the pbo tho2 monazite data coupled with palaeocurrent analyses and petrological characteristics of sandstones show that middle precambrian high grade metamorphic rocks and granitoids were widespread both in the northern and southern source regions of the e w trending jurassic basin of the mino terrane the pbo tho2 ratio of detrial monazites is a useful new measure for the provenance analysis of clastic rocks

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

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

multi step tims and ca tims monazite u pb geochronology abstract multi step chemical abrasion thermal ionization mass spectrometry ca tims methods were developed for monazite using six samples that vary in composition and age amelia jefferson county burke elk mountain 554 and madagascar to evaluate whether the multi step ca tims approach reveals complexities in either age or composition that might be masked by single step analysis this study presents a side by side comparison of monazite samples dated via multi step tims not annealed and ca tims annealed and measurement of rare earth element ratios for each dissolution step the data demonstrate three important contributions first annealing reduces solubility after one partial dissolution step not annealed fractions were 3 3 4 1 times more digested than the annealed fractions the difference in solubility suggests that monazite does not fully self anneal at low temperatures second multi step tims analyses yielded high precision u pb plateau ages for the burke and amelia monazites ca tims analyses yielded high precision u pb plateau ages for madagascar and amelia monazites jefferson county elk mountain and 554 yielded more complex results and no u pb plateau ages third chemical analysis of partial dissolution steps reveals heterogeneous age and compositional data for annealed samples because not annealed samples yielded more consistent age and compositional data high temperature annealing is not recommended for monazite instead optimal tims results are provided by slow partial dissolution of monazite in weak acid

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

the correlation between 18o 16o ratios of meteoric water and surface temperature its use in investigating terrestrial climate change over geologic time abstract correlations between mean annual temperature mat and the weighted average oxygen isotope composition of yearly precipitation 18opt are well known but the utility of modern relations to make reliable estimates of temperature change over geological time is uncertain this question has been addressed by using seasonal subsets of the global data base of temperature and isotopic measurements to represent two different climate modes a comparison of middle to high latitude 18opt temperature relations for each climate mode reveals 1 a significant offset between them and 2 a difference in the strength of their correlations the offset in relations is due to differences in temperature and water vapor budget in the tropics and can lead to serious underestimates of temperature change differences in the strength of correlations arise from the influence of climate mode specific non temperature factors on 18opt the overall result is that no single relation can be used in all cases to make unambiguous temperature estimates using a temporal record of 18opt values one way to overcome these problems is to reconstruct 18opt temperature relations for the time periods being investigated if an appropriate proxy for 18opt is available it may also be possible to estimate temperature without relying on 18opt temperature relations a promising alternative to these options is to use records of 18opt to test predictions of global climate models an approach that may allow a reliable and more complete reconstruction to be made of climate change over geologic time

reply to comment by w p schellart on the thermal structure of subduction zone back arcs 1 currie and hyndman 2006 documented the evidence that the majority of continental back arcs are uniformly hot with thin lithospheres over regional extents of hundreds of kilometers even if where is no thermally significant extension using a number of independent observations we showed that eight such current subduction zones have similar thermal regimes with moho temperatures greater than 800 c 35 40 km depth and lithosphere thicknesses of 60 km the one exception the peru flat slab region of south america supports our model explanation we then presented the hypothesis that the elevated back arc temperatures have a common mechanism efficient transport of heat by vigorous thermal convection in hydrated low viscosity back arc upper mantle 2 the paper was structured such that the observations indicating regionally high back arc temperatures were separate and independent from our proposed model the evidence for high uniform temperatures seems secure the discussion is on the explanation schellart 2007 appears to agree that the back arcs included in our study are characterized by hot thin lithospheres but does not agree with our interpretation of the underlying cause his main criticisms are that 1 local tectonics in particular extension may account for the high temperatures in some areas and 2 three dimensional mantle flow and complex slab dynamics may produce elevated temperatures 3 we acknowledge as stated in the original paper that a number of site specific processes may affect the thermal regime at each back arc we also recognize that nearly all back arc regions have undergone at least some past extension as described by schellart 2007 notable exceptions are central alaska and the northern canadian cordillera the key question is whether or not this extension is the primary cause of the observed uniformly hot thermal regimes 4 schellart 2007 argues that high surface heat flow at some back arcs is due to recent extension including parts of the sunda borneo and ryukyu korea back arcs we note that our compilation used a number of independent observational constraints on lithosphere temperatures not just surface heat flow mantle temperatures determined using only surface heat flow data are highly uncertain due to the point sampling and measurement uncertainties and uncertainties in the thermal parameters used to calculate the geotherm other temperature constraints on upper mantle thermal structure include seismic velocities from refraction pn and tomography mantle xenolith thermobarometry effective elastic thickness te etc for each back arc the upper mantle temperatures from all constraints are in good agreement and show no significant lateral variation on a regional scale this is especially illustrated by the uniform low upper mantle seismic velocities for different back arcs currie and hyndman 2006 and references therein 5 extension affects the thermal regime by locally decreasing lithosphere thickness and by inducing upwelling of hot underlying mantle modeling studies have shown that such extension should only affect lithosphere temperatures and surface heat flow within 100 km of the extended region morgan 1983 alvarez et al 1984 and for continental crust extension may actually decrease surface heat flow due to thinning of the crustal radioactive heat generation waples 2001 in addition limited local extension should not significantly perturb the deep thermal region for the back arcs in our compilation there is no evident upper mantle temperature increase in more strongly extended regions suggesting that a more regional phenomenon is required to produce regionally elevated temperatures we also note that extension can only occur if the lithosphere is sufficiently hot that it is weak enough to fail under available extensional stresses a normal cool lithosphere is too strong to be deformed by plate tectonic forces e g hyndman et al 2005 we suggest that back arc extension is a consequence of high temperatures and associated weak lithosphere and not the primary cause 6 schellart 2007 also suggests a thermal role for a number of upper mantle dynamical processes including along margin mantle flow associated with a slab edge slab retreat advance and slab break off we agree that each of these factors may affect the back arc thermal regime in some locations however these are local phenomena if they are thermally important they should be evident in lateral variations in mantle seismic velocity and other deep temperature indicators lateral variations are not apparent in the current observational data 7 as stated in the original paper currie and hyndman 2006 paragraph 71 we do not rule out local factors in journal of geophysical research vol 112 b11408 doi 10 1029 2007jb005415 2007 click here for full article

exhumation of ultrahigh pressure rocks beneath the hornelen segment of the nordfjord sogn detachment zone western norway the nordfjord sogn detachment zone of western norway represents an archetype for crustal scale normal faults that are typically cited as one of the primary mechanisms responsible for the exhumation of ultrahighpressure uhp terranes in this paper we investigate the role of normal sense shear zones with respect to uhp exhumation using structural geology thermobarometry and geochronology of the hornelen segment of the nordfjord sogn detachment zone the hornelen segment of the zone is a 2 6 km thick top w shear zone primarily developed within amphibolite grade allochthonous rocks that juxtaposes the uhp rocks of the western gneiss complex in its footwall with lower grade allochthons and carboniferous devonian basins in its hanging wall new thermobarometry and sm nd garnet geochronology show that these top w fabrics were initiated at lower crustal depths of 30 40 km between 410 ma and 400 ma structural geology and quartz petrofabrics indicate that top w shear was initially relatively evenly distributed across the shear zone and then overprinted by discrete ductile brittle detachment faults at slower strain rates during progressive deformation and exhumation these results require a three stage model for uhp exhumation in which normal sense shear zones exhumed uhp rocks from the base of the crust along initially broad ductile shear zones that were progressively overprinted by discrete ductile brittle structures

uranium lead zircon and titanite ages from the northern portion of the western gneiss region south central norway abstract the geochronologic history of the northern portion of the western gneiss region south central norway has been investigated by u pb analyses of zircon and titanite from various basement units and one supracrustal gneiss a six point four zircon and two titanite discordia line from a migmatite melt pod and its host gneiss astfjord migmatite gneiss defines upper and lower intercept ages of 1659 1 1 8 ma and 393 6 3 6 ma respectively the upper intercept age is interpreted as a time of tonalite emplacement and migmatite formation in the region a seven point three zircon and four titanite discordia line from the ingdal granite gneiss has similar upper and lower intercept ages 1652 9 1 7 ma and 396 1 4 9 ma respectively and the upper intercept age is interpreted as the time of granite crystallization the lower intercept age from both theastfjord migmatite gneiss and the ingdal granite gneiss is interpreted as the time of regional metamorphic resetting and is evidence for caledonian influence in the region titanite from these two gneiss units as well as from other tonalite granite and supracrustal gneisses throughout the western gneiss region displays a remarkably uniform pattern of discordance titanite discordance is variable from 6 to 100 and in general the degree of discordance is proportional to the caledonian metamorphic grade of nearby supracrustal schists episodic diffusional lead loss from titanite during caledonian metamorphism probably caused the discordance pattern but a combined mechanism of mixing and diffusional lead loss cannot be ruled out in any case the seventeen point titanite and zircon discordia line clearly indicates that 1 the bulk of the granitoid terrane in this portion of the western gneiss region was emplaced migmatized and cooled in a short time interval about 1657 ma ago 2 a second short lived thermal event that exceeded the blocking temperature of titanite occurred about 395 ma ago and 3 titanite and zircon in the region were not isotopically disturbed by comparable geologic events in the period from 1657 to 395 ma or at any time after 395 ma

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

u pb monazite geochronology of granitic rocks from maine implications for late paleozoic tectonics in the northern appalachians crystallization ages of monazites from peraluminous granites and granitic pegmatites in southwestern maine constrain the timing of final ductile motion of a major shear zone to approximately 280 ma u pb monazite ages of granite at brunswick 278 pm 2 ma and evolved granitic pegmatites in topsham 275 269 ma provide evidence that synkinematic intrusion was followed by final emplacement at the end of ductile deformation these granitic rocks were evidently generated during the final high temperature event experienced by this portion of the appalachian orogen during this time the host rocks record initial rapid cooling c 16 c m y followed by slower cooling c 4 c m y into the mesozoic the similarity of ages of granite in brunswick and pegmatites in topsham permit the rocks to be related to the same magmatic system the spatially associated sebago batholith the largest exposed pluton in new england has a concordant u pb monazite age of 293 pm 2 ma thus the sebago batholith cannot be

crustal exhumation of the western gneiss region uhp terrane norway 40 ar 39 ar thermochronology and fault slip analysis article i nfo ar 39 ar muscovite thermochronology 40 ar 39 ar k feldspar thermochronology fault slip analysis new 40 ar 39 ar muscovite and k feldspar thermochronology combined with existing data reveal the timing and patterns of late stage exhumation across the western gneiss region u hp terrane muscovite age contours show that exhumation into the mid upper crust progressed westward over a 20 myr period 400 380 ma this exhumation was caused by i e w stretching and eastward tilting north of nordfjord where muscovite ages decrease from the foreland allochthons westward into the uhp domains and ii differential exhumation south of nordfjord where muscovite ages depict a ne sw dome like pattern and the western gneiss region is bounded by overlying units little affected by the scandian metamorphism exhumation of the uhp domains into the mid upper crust by late folding continued through 374 ma the smooth gradient of fairly flat muscovite age spectra demonstrates minimal influence of excess ar which is relatively unusual for a u hp terrane 40 ar 39 ar spectra and modeled cooling histories from k feldspar combined with brittle ductile

dislocation creep regimes in quartz aggregates abstract using optical and tem microscopy we have determined that three regimes of dislocation creep occur in experimentally deformed quartz aggregates depending on the relative rates of grain boundary migration dislocation climb and dislocation production within each regime a distinctive microstructure is produced due primarily to the operation of different mechanisms of dynamic recrystallization at lower temperatures and faster strain rates the rate of dislocation production is too great for diffusion controlled dislocation climb to be an effective recovery mechanism in this regime recovery is accommodated by strain induced grain boundary migration recrystallization with an increase in temperature or decrease in strain rate the rate of dislocation climb becomes sufficiently rapid to accommodate recovery in this regime dynamic recrystallization occurs by progressive subgrain rotation with a further increase in temperature or decrease in strain rate dislocation climb remains sufficiently rapid to accommodate recovery however in this regime grain boundary migration is rapid thus recrystallization occurs by both grain boundary migration and progressive subgrain rotation the identification of the three regimes of dislocation creep may have important implications for the determination of flow law parameters and the calibration of recrystallized grain size piezometers in addition the identification of a particular dislocation creep regime could be useful in helping to constrain the conditions at which a given natural deformation has occurred