slab detachment during continental collision influence of crustal rheology and interaction with lithospheric delamination abstract collision between continents can lead to the subduction of continental material if the crust remains coupled to the downgoing slab a large buoyancy force is generated this force slows down convergence and promotes slab detachment if the crust resists to subduction it may decouple from the downgoing slab and be subjected to buoyant extrusion we employ two dimensional thermo mechanical modelling to study the importance of crustal rheology on the evolution of subduction collision systems we propose simple quantifications of the mechanical decoupling between lithospheric levels and the potential for buoyant extrusion of the crust the modelling results indicate that a variable crustal rheological structure results in slab detachment delamination or the combination of both mechanisms a strong crust provides coupling at the moho low and remains coherent during subduction low it promotes deep subduction of the crust 180 km and slab detachment exhumation occurs in coherent manners via eduction and thrusting slab detachment triggers the development of topography 4 5 km close to the suture a contrasting style of collision occurs using a weak crustal rheology mechanical decoupling at the moho high promotes the extrusion of the crust high disabling slab detachment ongoing shortening leads to buckling of the crust and development of topography on the lower plate collisions involving rheologically layered crust allow decoupling at mid crustal depths this structure favours both the extrusion of upper crust and the subduction of the lower crust such collisions are successively affected by delamination and slab detachment topography develops together with the buoyant extrusion of crust onto the foreland and is further amplified by slab detachment our results suggest that the occurrence of both delamination apennines and slab detachment himalayas in orogens may indicate differences in the initial crustal structure of subducting continental plates in these regions

the sudetes as a palaeozoic orogen in central europe two metasedimentary complexes are exposed in the sudetes mountains of poland germany and the czech republic succession i comprises pelites and greywackes of neoproterozoic cambrian age deformed and metamorphosed prior to intrusion by s type porphyritic granites at 515 480 ma succession ii comprises a sandstone mudstone chert sequence and turbidite sequence of ordovician early carboniferous age accompanied by bimodal volcanogenic rocks both successions were intruded by late to post orogenic granitoid intrusions at 340 300 ma the sedimentary rocks of succession ii show increasing maturity until mid late devonian times inversion of the basins commencing in late devonian early carboniferous times was reflected in the emplacement of turbidites and olistostromes concurrent with the uplift of a metamorphic core complex of succession i rocks the original stratigraphic order of the successions was maintained thus crustal imbrication was not significant instead extensional faulting became important followed by transpression on almost orthogonal fault zones resulting in the presently observed juxtaposition of crustal blocks the palaeozoic sequences developed in mainly ensialic basins on cadomian and older basement parts of which became strongly reworked 2 6 to 0 54 ga zircon inheritance ages and incorporated into the palaeozoic structures the orogen developed either on the rifted margin of peri gondwana or on a rifted away fragment of pre baltica the sudetic section of the variscan orogen is of broadly alpine style with significant basement involvement but apparently without evidence for long lived subduction of wide oceans or the accretion of numerous exotic terranes

ultrahigh pressure metamorphism in the śnieznik mountains sudetes poland p t constraints and geological implications the petrology and p t conditions of eclogites and associated gneisses from the nieznik mountains sudetes sw poland were investigated to establish an exhumation path for these ultrahigh pressure rocks peak pressure conditions are represented by the assemblage garnet omphacite phengite kyanite coesite rutile post dating this assemblage but still in the eclogite facies garnet with lower grossular content and a first generation of amphibole are formed under lower pressure conditions giving rise to the assemblage garnet omphacite phengite kyanite quartz rutile calcic amphibole i continuing depressurization caused the successive destabilization of omphacite garnet kyanite and phengite in some places the primary eclogite assemblage was completely transformed into amphibolite facies mineral assemblages estimated peak pressures in the nieznik and miedzygorze units lie in the coesite stability field p 27 kbar at temperatures of about 700 800 c and are not significantly different to the zlote un

titanium mobilization by hydrocarbon fluids related to sill intrusion in a sedimentary sequence scotland abstract an occurrence of vein hosted bitumen in carboniferous rocks central scotland exhibits unusual enrichments in titanium zirconium and chromium detected by electron microprobe these elements occur as mineral inclusions of titanium and titanium iron oxides a close association between titanium rich tuffs and a quartz dolerite sill suggests that metals were leached from igneous rocks by a hydrocarbon rich fluid generated by the heat of the intrusion into organic rich shales this occurrence supports other evidence for widespread titanium mobility under low temperature conditions inclusions of nickel arsenide may reflect regionally important mineralization including former ore deposits related to sills and dykes as sills are extensively intruded into petroleum prospective basins offshore they may have a significant influence upon sediment geochemistry

constraints on the timing of multiple thermal events and re equilibration recorded by high u zircon and xenotime case study of pegmatite from pilawa gorna gory sowie block sw poland abstract the application of zircon and xenotime geochronometers requires knowledge of their potential and limitations related to possible disturbance of the age record the alteration of the intergrown zircon and xenotime in pegmatite from the gory sowie block sw poland was studied using the electron microprobe analysis x ray wds compositional mapping micro raman analysis and la icp ms u pb dating of zircon and xenotime as well as the u th total pb dating of uraninite these microanalytical techniques were applied to understand the formation mechanisms of the secondary textures related to post magmatic processes in the zircon and xenotime intergrowth and to constrain their timing textural and compositional features combined with u pb data indicate that the pegmatite related crystallization of the zircon and xenotime intergrowth occurred ca 2 09 ga 2086 35 ma for zircon and 2093 52 ma for xenotime followed by the re equilibration of zircon and xenotime ca 370 ma 373 18 ma and 368 6 ma respectively during the formation of the younger pegmatite the zircon and xenotime were most likely derived from precambrian basement rocks and emplaced in the pegmatite as a restite the zircon preserved textures related to diffusion reaction processes that affected its high u core up to ca 9 6 wt uo2 which underwent further metamictization and amorphization due to self radiation damage the zircon rim and xenotime were affected by coupled dissolution reprecipitation processes that resulted in patchy zoning age disturbance and sponge like textures xenotime was also partially replaced by fluorapatite or hingganite y and y enriched allanite ce the termination of the low temperature alteration was constrained by the u th total pb age of the uraninite inclusions that crystallized in zircon at 281 2 ma which is consistent with the age of 278 15 ma obtained from the youngest cluster of u pb ages in the re equilibrated high u zircon domains this study demonstrates the importance of the careful examination of compositional microtextural and geochronological data obtained using microanalytical techniques to reconstruct the complex thermal histories recorded by accessory minerals

u pb dating of serpentinization hydrothermal zircon from a metasomatic rodingite shell sudetic ophiolite sw poland abundant zircon grains were found within a metasomatic chlorite corrensite trioctahedral regularly interstratified chlorite smectite shell that surrounds rodingite from the jordanow gogolow jg serpentinite massif sudetic ophiolite bohemian massif the zircon grains bear two types of primary fluid inclusions containing either liquid co2 or aqueous solutions the inclusions were used to assign the p t conditions of the zircon formation 270 300 c ca 1 kbar using the crossed isochore method abraded single zircon grains give a u pb age of 400 4 3 ma which is interpreted as the age of zircon crystallization this zircon formed during rodingitization i e a metasomatic process directly connected with the serpentinization and thus provides the opportunity of direct dating of the early serpentinization process the zircon age from the jordanow gogolow massif is consistent with the crystallization age of metagabbro protoliths from the sudetic ophiolite as determined by oliver et al j geol soc lond 150 1993 355 a ce anomaly combined with the mineralogical record of greenschist facies metamorphism of the mafic rocks from sudetic ophiolite suggests that the sm nd whole rock isotopic data of pin et al lithos 21 1988 195 353 21 ma possibly characterize the age of later metamorphic event

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

vertical extrusion and horizontal channel flow of orogenic lower crust key exhumation mechanisms in large hot orogens a large database of structural geochronological and petrological data combined with a bouguer anomaly map is used to develop a two stage exhumation model of deep seated rocks in the eastern sector of the variscan belt an early sub vertical fabric developed in the orogenic lower and middle crust during intracrustal folding followed by the vertical extrusion of the lower crustal rocks these events were responsible for exhumation of the orogenic lower crust from depths equivalent to 18 20 kbar to depths equivalent to 8 10 kbar and for coeval burial of upper crustal rocks to depths equivalent to 8 9 kbar following the folding and vertical extrusion event sub horizontal fabrics developed at medium to low pressure in the orogenic lower and middle crust during vertical shortening fabrics that record the early vertical extrusion originated between 350 and 340 ma during building of an orogenic root in response to se directed saxothuringian continental subduction fabrics that record the later sub horizontal exhumation event relate to an eastern promontory of the brunia continent indenting into the rheologically weaker rocks of the orogenic root indentation initiated thrusting or flow of the orogenic crust over the brunia continent in a north directed sub horizontal channel this sub horizontal flow operated between 330 and 325 ma and was responsible for a heterogeneous mixing of blocks and boudins of lower and middle crustal rocks and for their progressive thermal re equilibration the erosion depth as well as the degree of reworking decreases from south to north pointing to an outflow of lower crustal material to the surface which was subsequently eroded and deposited in a foreland basin indentation by the brunia continental promontory was highly noncoaxial with respect to the se oriented saxothuringian continental subduction in the early visean suggesting a major switch of plate configuration during the middle to late visean

zircon geochronology and trace element characteristics of eclogites and granulites from the orlica śnieznik complex bohemian massif u pb zircon geochronology and trace element analysis was applied to eclogites and ultra high pressure granulites that occur as volumetrically subordinate rock bodies within orthogneisses of the orlica nieznik complex bohemian massif under favourable circumstances such data may help to unravel protolith ages and yet undetermined aspects of the metamorphic evolution for example the time span over which eclogite facies conditions were attained by means of ion probe and laser ablation techniques a comprehensive database was compiled for samples collected from prominent eclogite and granulite occurrences the 206 pb 238 u dates for zircons of all samples show a large variability and no single age can be calculated the protolith ages remain unresolved due to the lack of coherent age groups at the upper end of the zircon age spectra the spread in apparent ages is interpreted to be mainly caused by variable and possibly multi stage pb loss further complexities are added by metamorphic zircon growth and re equilibration processes the unknown relevance of inherited components and possible mixing of different aged domains during analysis a reliable interpretation of igneous crystallization ages is not yet possible previous studies and the new data document the importance of a carboniferous metamorphic event at c 340 ma the geological significance of this age group is controversial such ages have previously either been related to peak u hp conditions the waning stages of eclogite facies metamorphism or the amphibolite facies overprint this study provides new arguments for this discussion because in both rock types metamorphic zircon is characterized by very low total ree abundances flat hree patterns and the absence of an eu anomaly these features strongly suggest contemporaneous crystallization of zircon and garnet and strengthen interpretations proposing that the carboniferous ages document late stage eclogite facies metamorphism and not amphibolite facies overprinting

geochemical correlation in an exhumed submarine channel complex tabernas basin se spain comparison to sedimentological correlation at various length scales abstract whole rock inorganic geochemical data have previously been used to aid characterization and stratigraphic correlation of fossil poor clastic sedimentary deposits although this approach has not yet been calibrated by testing chemostratigraphic correlations against physical stratigraphic correlations from outcrop chemical data from a well exposed siliciclastic succession in a tortonian late miocene submarine slope channel system in the tabernas basin of southern spain were generated using x ray fluorescence supplemented by x ray diffraction sem analysis and light microscopy amongst the 60 elements measured nearly a quarter of them have proved to be useful or reliable for geochemically based correlation at the tested scales 10 m 50 m 100 m these elements are sio2 k2o al2o3 cao and tio2 as well as zr nb th rb cs as and the rare earth elements la nd and ce high resolution bed to bed geochemical correlation is feasible over short distances 10 m geochemical data from sections in beds that are traceable suggest that a given stratigraphic level seems to be geochemically homogeneous with respect to some key elements geochemical data also provide a useful aid for high resolution sedimentary correlation at distances of several tens of meters up to at least 100 m since each individual bed seems to be broadly homogeneous in composition at a larger scale it is possible to discern a provenance signal in geochemical data chemical stratigraphy thus seems to work even in stratigraphically complicated submarine channel complexes

exhumation of eclogitized continental basement during variscan lithospheric delamination and gravitational collapse sudety mountains poland a variscan deep crustal level eclogite facies continental basement massif in western poland the snieznik complex was tectonically exhumed crustal penetrating mylonite zones record three main kinematic events early top to the north directed thrusting right slip transpression tension and late top to the south and east normal faulting thrusting resulted in extreme crustal thickening and associated eclogite facies metamorphism right slip movements produced retrogressive crystal plastic simple shear zones normal faults flank carboniferous to early permian terrigenous sedimentary basins documenting tectonic and erosional denudation of the snieznik complex during lithospheric extension sm nd isotopic dates previously reported for the in situ eclogite facies metamorphic mineral assemblages are 341 337 and 329 ma brueckner et al 1991 40 ar 39 ar isotopic dates for metamorphic hornblende 338 333 and 332 ma muscovite 329 and 329 ma and biotite 328 ma reflect times of cooling through the 500 350 and 300 c isotherms respectively these nearly concordant mineral dates document rapid cooling from 850 c eclogue facies temperatures to 300 c rapid denudation of these deep crustal rocks 19 22 kbar pressures 70 km depth is attributed to processes similar to those of metamorphic core complexes that operated during lithospheric delamination and gravitational collapse the sequence of late paleozoic alleghanian crustal thickening followed by right slip transpression tension followed by normal faulting recognized in the u s appalachians implies that this tectonic pattern may exist throughout the afleghanian variscan belt

constraints on the devonian carboniferous closure of the rheic ocean from a multi method geochronology study of the stare město belt in the sudetes poland and the czech republic abstract this paper attempts to clarify the complex nature of how and when the rheic ocean closed in what is now central europe and with respect to the various terranes that were involved offer a regional chronology for the associated structural metamorphic and igneous processes that accompanied and followed this closure the variscan orogen in europe originated from the multiple collisions of gondwana derived terranes the armorican terrane assemblage with laurussia however many important structural details on the timing of these collision related events remain obscure in the sudetes the stare m sto belt represents a wnw dipping part of the rheic suture that developed from the continental collision of the eastern terranes of the armorican terrane assemblage now in the bohemian massif with the brunovistulian terrane a part of laurussia old red continent in this study the results of monazite th u total pb garnet lu hf and zircon u pb geochronology were integrated into a newly established d1 d3 tectonometamorphic sequence a th u total pb age of 368 ma from a monazite that grew concurrently with d2 metasedimentary garnet as well as lu hf ages of 361 ma and 355 ma obtained from d2 metasedimentary garnets implies that the regional contractional deformation and progressive metamorphism of d2 took place mainly during the famennian late devonian and extended into the visean middle mississippian of the early carboniferous the ion probe u pb zircon ages of 355 ma and 359 ma obtained from leucocratic neosomes in migmatized amphibolites confirmed a lag in the peak temperature that followed crustal thickening during d1 d2 metamorphic monazites dated at 340 ma provide a time for the subsequent d3 dextral transpressional regime the closure of the stare m sto belt segment of the rheic ocean probably resulted from a head on hard collision between the westerly subducting brunovistulian promontory of laurussia and the eastern members of the armorican terrane assemblage thus the rheic ocean closed during the late devonian at 370 360 ma and preceded the collision of the armorican terrane assemblage with east avalonia at the western margin of what is now the bohemian massif following ocean closure the rheic slab may have broken off resulting in the suture zone becoming dominated by lateral tectonic escape movements of the colliding terranes at 340 330 ma visean syntectonic d3 intrusions of granodiorite tonalite magma acted as a hot lubricant and stitched the suture zone together

temporal evolution of mineralization events in the bohemian massif inferred from the re os geochronology of molybdenite molybdenite is a common mineral accompanying sn w au and base metal mineralizations located in different geotectonic units of the bohemian massif but it is also widespread in granitoids and or related quartz veins pegmatites forming disseminated mo mineralization thirty re os ages were obtained for molybdenite samples from the bohemian massif to provide constraints on the timing and duration of mineralization event s within the framework of previously published geochronological data for the host and or associated rocks the obtained data for sn w li deposits in the erzgebirge metallogenetic province indicate the predominance of one and or multiple short time mineralization events taking place between 319 and 323 ma with the exception of the krupka deposit associated with the altenberg teplice caldera where the data may suggest prolonged activity until 315 ma the ages of the pb zn au mo h rky u rakovnika and fe cu as ob i d l mineralizations from the exocontacts of the cista pluton and krkonose jizera plutonic complex respectively provide evidence for synchronous emplacement of the ore and the associated granitic rocks in contrast the padr fe as mo mineralization postdates the age of the associated padr granite disseminated mo mineralization in cadomian and variscan granitoids and or related to quartz veins pegmatites provides re os ages that overlap with the previously published geochronological data for the host rocks suggesting coeval evolution molybdenite samples from the sazava suite granites of the central bohemian plutonic complex cbpc have resolvable younger ages than their host granites but similar to the age of spatially related au mineralization which is associated with the latest evolution of the cbpc

geochemical correlation in deltaic successions a reality check whole rock geochemical analysis can aid the stratigraphic correlation of faunally barren successions compared to petrographic techniques or heavy mineral separations such analysis is quick and objective yet rarely has the reliability of the method been tested against unequivocal correlations we have applied it to the kenilworth member of the upper cretaceous blackhawk formation a well exposed and easily correlated 100 m thick unit in the deltaic successions of the book cliffs utah united states this is the first evaluation of geochemical analysis as an aid to sequence stratigraphic interpretation at the parasequence scale from the geochemistry alone it is not possible with statistical confidence to separate individual parasequences or systems tracts nor the fluvial incised valley from the marine shoreface units the magnitude of geochemical variation in a single sandstone bed over a 30 m lateral traverse is as great as vertically through an entire parasequence y nb rb and ti abundances are inversely correlated with grain size thus geochemical signatures follow sedimentological trends and distal parts of a parasequence appear distinct from proximal parts y ratios compensate for this grain size effect but introduce confusing distortions for elements such as zr that are not correlated with grain size contrary to some predictions zr is as common in the finest grained units as the coarsest and sr is not correlated with ca in some sections sr is also inversely correlated to grain size and cannot be used as an indicator for cement abundance

high pressure granulites from the sudetes south west poland evidence of crustal subduction and collisional thickening in the variscan belt two high grade gneissic complexes of the western sudetes the gory sowie block and the nieznik area complex contain small predominantly felsic granulitic inliers with minor cpx bearing intercalations the p t conditions of the granulite facies events and of the subsequent re equilibration are estimated using the ternary feldspar thermometer and the geo calc computer program version twq jan 92 in the gory sowie granulites the peak granulitic event occurred at c 18 20 kbar and 900 c and the late decompressive re equilibration within a range of 4 10 kbar and temperatures decreasing to 600 700 c the latter event is thought to have coincided with the main metamorphic phase in the surrounding gneisses the p t estimates are more scattered in the nieznik granulites but the peak conditions for the granulitic event are estimated at pressure over 22 kbar possibly around 30 kbar and temperature exceeding 900 c the analysed samples from the nieznik area bear no significant evidence of lower pressure re equilibration integrating the thermobarometric data and some age constraints indicates that the gory sowie granulites belong to the early stage type i granulites of the variscan belt c 400 ma old which are interpreted as fragments of continental crustal materials subducted to mantle depths in the earliest stages of the variscan orogeny the nieznik granulites are more problematic they may belong to a younger high p suite c 350 ma old widespread in the southern and eastern parts of the bohemian massif and possibly related to the climax of the variscan continent continent collision

constraining long term denudation and faulting history in intraplate regions by multisystem thermochronology an example of the sudetic marginal fault bohemian massif central europe 1 the rychlebske hory mountain region in the sudetes ne bohemian massif provides a natural laboratory for studies of postorogenic landscape evolution this work reveals both the exhumation history of the region and the paleoactivity along the sudetic marginal fault smf using zircon u th he zhe apatite fission track aft and apatite u th he ahe dating of crystalline basement and postorogenic sedimentary samples most significantly and in direct contradiction of traditional paleogeographic reconstructions this work has found evidence of a large cretaceous sea and regional burial to 6 5 km of the carboniferous permian basement in the late cretaceous 95 80 ma during the burial by sediments of the bohemian cretaceous basin system the smf acted as a normal fault as documented by offset zhe ages across the fault at 85 70 ma the basin was inverted cretaceous strata eroded and basement blocks were exhumed to the near surface at a rate of 300 m ma as evidenced by late cretaceous paleocene aft ages and thermal modeling results there is no appreciable difference in aft and ahe ages across the fault suggesting that the smf acted as a reverse fault during exhumation in the late eocene oligocene the basement was locally heated to 70 c by magmatic activity related to opening of the eger rift system neogene or younger thermal activity was not recorded in the thermochronological data confirming that late cenozoic uplift and erosion of the basement blocks was limited to less than 1 5 km in the study area

u pb ages from sw poland evidence for a caledonian suture zone between baltica and gondwana faunal and palaeomagnetic evidence suggests the existence of a c 3000 km wide tornquist sea between gondwana and baltica in early ordovician times which narrowed to 50 2 ma an unmetamorphosed ophiolite gives a 420 20 2 ma age whilst in two other terranes amphibolite facies island arc lavas have fossiliferous lower ludlow 424 415 ma sequences all are unconformably overlain by unmetamorphosed upper devonian conglomerates a sinistral transpressive regime is observed in regionally extensive mylonite zones titanite and zircon ages 338 2 3 and 339 4 ma record variscan magmatism the data suggest considerable narrowing of the tornquist sea during the ordovician continuing ocean floor and island arc activity in the silurian and final sinistral transpressive closure by the mid devonian

pervasive horizontal fabric and rapid vertical extrusion lateral overturning and margin sub parallel flow of deep crustal migmatites northeastern bohemian massif abstract in the west sudetes northeastern bohemia massif geochronometry provides evidence for repeated episodes of rapid cooling that contrasts sharply with an absence of structural evidence for significant tectonic exhumation by crustal extension instead high grade assemblages of the orlica snieznik complex have a regional sub horizontal foliation and sub horizontal lineations that trend parallel to narrow sub vertical shear zones containing exhumed high pressure assemblages mesoscopic petrofabrics combined with anisotropy of magnetic susceptibility ams data from amphibolite facies to migmatitic meta sedimentary and meta igneous rocks reveal remarkably consistent average lineations that plunge shallowly to the ssw on both steep and sub horizontal nne trending planar fabrics the dominant ssw nne fabric orientation is parallel to the bohemia brunia suture which marks a major boundary along the eastern margin of the massif the shape of the ams ellipsoid is predominantly oblate revealing flattened fabrics with only local prolate ellipsoids we envisage that the continental brunian indentor operated as a rigid backstop and allowed the migmatized lower crustal orogenic root to be exhumed along the bohemian margin shortly following terminal arc collision sub vertical extrusion of the orogenic root was arrested in the mid crust where the lower ductile crust was laterally overturned at the base of rigid upper crustal blocks upon reaching the crustal high strength lid the exhumed ductile mass of continental material laterally spread sub parallel to the margin underwent subsequent supra barrovian metamorphism and quickly cooled the application of ams techniques to high grade metamorphic rocks in concert with macroscopic structural observations is a powerful approach for resolving the deformation history of a terrane where visible rock fabrics can be tenuous

exhumation and metamorphism of an ultrahigh grade terrane geochronometric investigations of the sudete mountains bohemia poland and czech republic the sudete mountains ne bohemian massif czech republic and poland preserve abundant eclogitic and granulitic centimetre to decimetre scale boudins enveloped in a predominantly migmatitic matrix published geochronometry and thermobarometry from the uhp and uht rocks broadly constrain the crystallization and initial stage 1 exhumation history for these units however the timing of stage 2 metamorphism and associated unroofing is less well constrained new in situ ion microprobe th pb monazite results together with complementary u pb zircon and electron microprobe analyser total pb monazite results on 11 amphibolite facies gneissic to migmatitic samples place important temporal constraints on the second stage of uhp and uht metamorphism exhumation the orlica snieznik dome records uhp metamorphism occurring at 375 ma and subsequent exhumation to midcrustal levels in supra barrovian conditions at c 345 330 ma in contrast the western gory sowie block preserves evidence of hp granulite conditions at c 400 ma and exhumation to mid crustal levels at 380 370 ma revealing a c 30 million years difference in exhumation events between the neighbouring terranes the eastern gory sowie block preserves ages similar to the orlica sneiznik dome suggesting that different preserved metamorphic cooling histories are juxtaposed across the sudetic marginal fault the bounding niemcza shear zone yields preliminary th pb dates that range from 380 8 ma to 283 2 ma preserving a protracted metamorphic record that spans the exhumation history of the region the distinct collapsed geochronologies of both terranes probably reflect rapid vertical transport of low viscosity crust under supra barrovian conditions near the mid crustal high strength lid during oblique transpressional convergence

geochronology and nd sr systematics of lusatian granitoids significance for the evolution of the variscan orogen in east central europe a variety of pre variscan granitoids and two variscan monzogranites occurring in the central and western parts of the lusatian granodiorite complex lgc saxonia were dated by the single zircon evaporation method complemented by whole rock nd isotopic data and rb sr whole rock and mineral ages the virtually undeformed pre variscan granitoids constitute a genetically related mostly peraluminous magmatic suite ranging in composition from two mica granodiorite muscovitebearing biotite quartz diorite tonalite and granodiorite to biotite granodiorite and monozogranite

heterogeneous extrusion and exhumation of deep crustal variscan assembly geochronology of the western tatra mountains northern slovakia abstract the nature and style of mid crustal assembly and exhumation during continental collision has been investigated in the tatra mountains of the western carpathians the pre alpine basement of the western carpathians represents the easternmost exposure of the variscan orogen in europe which marks the collision of laurasia with gondwanian affiliated terranes during the palaeozoic the tatric crystalline unit of the western tatra in northern slovakia displays an inverted metamorphic sequence where a high grade unit comprising migmatites with relicts of eclogite has been thrust over a lower grade mica schist unit new geochronological and thermochronological data together with published thermobarometry illuminate the metamorphic history of the western tatra the upper unit eclogites with occasionally preserved omphacite record near isothermal decompression from 1 6 gpa to 1 0 1 2 gpa at 750 800 c which lead to intensive re equilibration at high pressure granulite facies conditions comparable to the peak metamorphic conditions of the host migmatite both eclogite and migmatite shared a retrograde p t path following the insertion of the eclogite assemblage into the migmatites the metamorphic evolution of the lower unit mica schist is constrained to peak p t conditions of 0 6 0 8 gpa and 640 and 660 c followed by retrogression this suggests that different rock types of the western tatra metamorphic core shared only their exhumation path from mid crustal levels id tims sm nd dating of garnet from eclogite yields a whole rock garnet isochron age of 337 10 ma with an initial e nd isotopic composition of 8 3 in situ u pb dating of monazite from a migmatite surrounding the eclogite shows one age population of c 380 ma whereas monazite from a migmatite away from the eclogite preserves a robust 340 11 ma age which is indistinguishable from sm nd garnet age and u pb age of zircons in the anatectic leucosome of the migmatite 347 7 ma a younger monazite age population from the migmatite of 300 16 ma is consistent with 40 ar 39 ar mica ages of c 310 ma this argues for a contemporaneous and likely shared exhumation path of the assemblage pair in situ monazite total pb analyses from the lower unit mica schists yields xenocrystic and c 370 ma ages but no geochronologic evidence for peak variscan tectonism exhumation of the deep crustal root occurred most probably in a two stage process the timing of the high pressure eclogite facies metamorphism before the onset of exhumation into the mid crust was likely between c 380 ma and 360 ma subsequent exhumation into the middle crust was coeval with migmatite generation at c 340 ma and garnet diffusion modeling suggest 30 c ma cooling rates the exhumation was likely tectonically forced by the action of a rigid indentor which prompted the weak lower crust to be heterogeneously extruded to mid crustal levels at a time coeval with anatexis and subsequently extruded with mid crustal material to the upper crust

the timing of eclogite facies metamorphism and migmatization in the orlica śnieznik complex bohemian massif constraints from a multimethod geochronological study the orlica nieznik complex osc is a key geological element of the eastern variscides and mainly consists of amphibolite facies orthogneisses and metasedimentary rocks sporadic occurrences of eclogites and granulites record high pressure hp to ultrahigh pressure uhp metamorphic conditions a multimethod geochronological approach 40ar 39ar rb sr sm nd u pb has been used to gain further insights into the polymetamorphic evolution of eclogites and associated country rocks special attention was given to the unresolved significance of a 370 to 360 ma age group that was repeatedly described in previous studies efforts to verify the accuracy of c 370 ma k ar phengite and biotite dates reported for an eclogite and associated country rock gneiss from the location nowa wie suggest that these dates are meaningless due to contamination with extraneous ar extraneous ar is also considered to be responsible for a significantly older 40ar 39ar phengite date of c 455 ma for an eclogite from the location wojtowka attempts to further substantiate the importance of 370 360 ma zircon dates as an indicator for a melt forming high temperature ht episode did not provide evidence in support of anatectic processes at this time instead shrimp u pb zircon dating of leucosomes and leucocratic veins within both orthogneisses and u hp granulites revealed two age populations 490 450 and 345 330 ma respectively that correspond to protolith ages of the magmatic precursors and late variscan anatexis the results of this study further underline the importance of late carboniferous metamorphic processes for the evolution of the osc that comprise the waning stages of hp metamorphism and lower pressure ht overprinting with partial melting eclogites and their country rocks provided no chronometric evidence for an uhp and ultrahigh temperature episode at 387 360 ma as recently suggested for granulites from the osc based on lu hf garnet ages anczkiewicz et al 2007

a model for a continental accretionary wedge developed by oblique collision the ne bohemian massif structural and metamorphic investigations of the northeastern margin of the bohemian massif indicate three main sequential devonian carboniferous tectonic events 1 devonian rifting 2 early carboniferous oblique underthrusting and formation of a continental accretionary wedge 3 eduction of the wedge and late carboniferous transpression devonian rifting of the brunia microcontinent resulted in the formation of two crustal scale boudins associated with the development of two syn rift devonian basins this extensional template strongly influenced the nature of the ensuing variscan contractional deformation early carboniferous 350 330 ma progressive highly oblique underthrusting of the two crustal boudins beneath the lugian terrane to the west generated syn deformational barrovian metamorphism and the formation of a continental accretionary wedge the wedge was further compressed by continued underthrusting of brunia which resulted in the successive vertical extrusion eduction of an upper and lower allochthon derived from the more deeply underthrust crustal boudin the eduction was terminated by a late carboniferous 330 310 ma transpressional event resulting from continued plate convergence release of mantle derived magma during late stage eduction thermally softened the transpressional zones in the more external parts of the wedge the resultant differential displacements gave rise to extensional unroofing of the internal part of the wedge

new evidence of blueschist facies rocks and their geotectonic implication for variscan suture s in the bohemian massif blueschist facies rocks exposed within consolidated continental blocks provide some of the best evidence for the existence of previous suture s they usually occur as lenses or layers embedded within greenschist or amphibolite facies rocks and indicate reequilibration at medium to low pressure conditions in the bohemian massif a few occurrences of blueschists have been reported and here new evidence of high pressure hp metamorphism in various lithologies is presented that suggests a larger extent of blueschist facies rocks along the northern border of this massif an earlier blueschist facies metamorphism is documented by inclusions of glaucophane in garnet epidote and titanite from metabasites along with zoned white mica having a phengitic core and a muscovite rim in metapelites and orthogneisses the estimated p t conditions obtained using pseudosections and mineral isopleths correspond to blueschist and low temperature eclogite facies conditions 1 1 2 0 gpa at 350 550 c together with medium temperature eclogites from different units in the bohemian massif they indicate a geothermal gradient of 8 10 c km 1 which is typical for cool subduction radiometric dating on phengite from metapelites confirms an early palaeozoic cooling age of c 360 ma for this hp metamorphic event the presence of blueschist facies rocks their p t relations and age constraint together with those from eclogite facies rocks allows us to locate the variscan suture which straddles the se margin of saxothuringian zone from erzgebirge to sudetes and its possible continuation to the moldanubian zone where eclogite facies and uhpm rocks are abundant

post variscan thermal history of the moravo silesian lower carboniferous culm basin ne czech republic sw poland abstract apatite fission track analysis aft and zircon u th he thermochronology zhe have been carried out for a lower carboniferous greywacke succession of the moravo silesian culm basin in the nizk jesenik mountains the range of apparent zircon helium ages is 303 233 ma late carboniferous to early triassic in the eastern part of the basin whilst they are significantly younger in the western part ranging from 194 to 163 ma early middle jurassic apatite fission track central ages range from 152 latest jurassic to 44 ma eocene with the majority being grouped between 114 aptian and 57 ma paleocene all samples experienced substantial post depositional thermal reset both the aft ages and the zhe are considerably younger than the depositional ages the mean track length varies in the range between 12 5 and 15 4 m the unimodal track length distribution the relatively short mean track length in most samples and their rather low standard deviation values 1 2 to 2 1 m indicate that their thermal history was determined by variscan and post variscan heating event s followed by a prolonged residence in the apatite partial annealing zone in the mesozoic and finally by cooling in the paleogene geological evidence combined with thermal modeling based on aft and zhe data indicate that the lower carboniferous strata had already reached maximum palaeotemperatures in the late carboniferous however they were presumably later re heated during the permian triassic post variscan extensional tectonics events were responsible for high heat flow that together with carboniferous burial could account for the reset of both thermochronometers a major phase of cooling occurred in the late cretaceous finally exhumation was probably faster in the paleogene causing the present day exposure of the studied rocks

structural and anisotropy of magnetic susceptibility records of granitoid sheets emplacement during growth of a continental gneiss dome central sudetes european variscan belt 1 granitoid magmatic sheets emplaced syntectonically during growth of the orlica nieznik mantled gneiss dome central sudetes european variscan belt were examined by means of structural geology quartz and anisotropy of magnetic susceptibility ams fabric studies magmatic emplacement was localized along the eastern transpressive margin of the high metamorphic core and below the low grade detached mantle rimming the southern margin of the dome in the first area the magmatic sheets were emplaced parallel to the dilated subhorizontal foliation of an anisotropic pre orogenic block the resulting melt host rock multilayer localized the transpressive zone along which the bulk of magma was emplaced the ams study shows an across strike fabric zonation underlying a strongly transpressive solid state deformation in the hanging wall magmatic subhorizontal fabric in the foot wall and a transitional fabric in the center of the intrusion in contrast along the southern dome margin magmatic sheets intruded along steep foliations of weakly metamorphic mantle rocks and are affected by recumbent folding and subhorizontal shearing the bulk of the sheets present a shallow dipping magmatic foliation and an along strike magmatic lineation the variations of the quartz and magnetic fabrics are attributed to superimposition of pure shear dominated ductile thinning followed by simple shear dominated detachment onto the original steep fabric ams modeling confirms the role of variation of orientation of pre intrusive anisotropy during progressive deformation on the resulting fabric pattern and helps explaining observed variations in fabric orientations and symmetries this study highlights contrasting mechanical behavior of syntectonically emplaced magmas in different parts of a growing crustal scale mantled gneiss dome

new u pb monazite and zircon data from the sudetes mountains in sw poland evidence for a single cycle variscan orogeny new u pb monazite and zircon data combined with a study of their internal morphologies significantly clarifies the controversy surrounding the palaeozoic geodynamic evolution of the sudetes mountains in the northeastern bohemian massif variably thin rims of secondary magmatic overgrowth on zircons from an anatectic granite in the gory sowie block are dated by a u pb zircon lower intercept age of 353 61 ma by magmatic monazite growth at 378 2 ma and by xenotime crystallization at 383 370 ma in a late syntectonic pegmatite reflecting the anatectic event associated with a single cycle variscan orogeny in this area

marginal ice zone thickness and extent due to wave radiation stress abstractocean surface wave radiation stress represents the flux of momentum due to the waves when waves are dissipated or reflected by sea ice that momentum is absorbed or reflected resulting in

thermomechanical modeling of slab eduction 1 plate eduction is a geodynamic process characterized by normal sense coherent motion of previously subducted continental plate this mechanism may occur after slab detachment has separated the negatively buoyant oceanic plate from the positively buoyant orogenic root eduction may therefore be partly responsible for exhumation of high pressure rocks and late orogenic extension we used two dimensional thermomechanical modeling to investigate the main features of the plate eduction model the results show that eduction can lead to the quasi adiabatic decompression of the subducted crust 2 gpa in a timespan of 5 my large localized extensional strain in the former subduction channel flattening of the slab and a topographic uplift associated with extension of the orogen in order to further investigate the forces involved in the eduction process we ran systematic parametric simulations and compared them to analytic plate velocity estimations these experiments showed that eduction is a plausible mechanism as long as the viscosity of the asthenospheric mantle is lower than 1022 pa s while subduction channel viscosity does not exceed 1021 pa s we suggest that eduction can be a viable geodynamic mechanism and discuss its potential role during the orogenic evolution of the norwegian caledonides

thermochronology of the west sudetes bohemian massif rapid and repeated eduction in the eastern variscides poland and czech republic the sudete mountains of northeastern bohemian massif were amalgamated during the closure of the rheic ocean culminating with variscan orogenesis and contain occurrences of high pressure granulite and small relict ultrahigh pressure eclogite formed during subduction we performed 40 ar 39 ar thermochronometry on primarily amphibolite facies gneisses and schists from two crustal blocks within the sudetes gory sowie and orlica snieznik massifs hornblende and mica plateau ages from the mountainous portion of the gory sowie reflect relatively rapid cooling between 382 1 ma and 373 0 5 ma following peak conditions at ca 400 ma kyanite sillimanite and cordierite garnet mineral growth textures denote near isothermal decompression during eduction concordant hornblende and biotite cooling ages obtained from the eastern topographically flat region adjacent to the niemcza shear zone indicate markedly younger cooling at 337 0 8 ma 40 ar 39 ar results from the snieznik mountains 50 km to the south yield plateau cooling ages for white mica and biotite between 341 1 to 337 0 6 ma and 342 1 to 334 0 6 ma respectively and are remarkably similar in age to an amphibolite facies overprinting episode mica analyses from the orlica mountains also yield cooling ages between 338 0 9 to 335 0 5 ma thermochronometry illustrates temporally disparate cooling histories but similar metamorphic evolutions between the gory sowie block and the orlica snieznik complex in both terranes it took 35 m y from the time of peak high pressure conditions to residence in the upper crust and possibly as little as 10 m y between high pressure and mid crustal supra barrovian events these results in light of regional geochronology illustrate the rapid localized and repetitious nature of eduction exhumation of the sudetic orogenic root including the eclogite and hp granulite assemblages during protracted subduction

high resolution magnetic susceptibility and geochemistry for the cenomanian turonian boundary gssp with correlation to time equivalent core abstract the upper cretaceous cenomanian turonian c t boundary represents the most significant thermal climatic event in the mesozoic and it has been well studied here we present a relatively new approach for providing both independent climate based cyclostratigraphic analysis and correlation among c t sections we have collected samples for bulk initial low field magnetic susceptibility ms measurement and high resolution inorganic geochemical analyses from two c t sequences 1 the well defined c t global boundary stratotype section and point gssp located at lake pueblo colorado and deposited in the upper cretaceous western interior seaway of north america and 2 the well studied marlstone and limestone usgs 1 portland core drilled at portland colorado 40 km to the west of the c t gssp comparing the lithostratigraphy chemostratigraphy and magnetostratigraphic susceptibility developed from magnetic susceptibility ms data indicates that the usgs 1 portland core closely compares with the c t gssp sequence using any of the three measures demonstrating that intervals collected and sampled from these two sequences are highly correlated a preliminary magnetostratigraphic susceptibility zonation for the c t boundary is presented to which other ms work can be compared fourier transform ft analysis of the ms data from the gssp when compared with previous ft work using geochemical analysis of portland core samples shows close similarities to the gssp with milankovitch eccentricity obliquity and precession bands well defined in both data sets using this result we assign relative ages to the intervals sampled and evaluate changes in absolute sediment accumulation rates through the c t boundary interval we show that while sediment accumulation rate is relatively high in the upper cenomanian a change toward lower accumulation rates in the gssp section at the c t boundary is due to reduced productivity following the c t extinction events that resulted from global warming associated with oceanic anoxic event 2 oae2 that began in the uppermost cenomanian

crustal influx indentation ductile thinning and gravity redistribution in a continental wedge building a moldanubian mantled gneiss dome with underthrust saxothuringian material european variscan belt 1 the contribution of lateral forces vertical load gravity redistribution and erosion to the origin of mantled gneiss domes in internal zones of orogens remains debated in the orlica snieznik dome moldanubian zone european variscan belt the polyphase tectono metamorphic history is initially characterized by the development of subhorizontal fabrics associated with medium to high grade metamorphic conditions in different levels of the crust it reflects the eastward influx of a saxothuringian type passive margin sequence below a tepla barrandian upper plate the ongoing influx of continental crust creates a thick felsic orogenic root with hp rocks and migmatitic orthogneiss the orogenic wedge is subsequently indented by the eastern brunia microcontinent producing a multiscale folding of the orogenic infrastructure the resulting kilometre scale folding is associated with the variable burial of the middle crust in synforms and the exhumation of the lower crust in antiforms these localized vertical exchanges of material and heat are coeval with a larger crustal scale folding of the whole infrastructure generating a general uplift of the dome it is exemplified by increasing metamorphic conditions and younging of 40ar 39ar cooling ages toward the extruded migmatitic subdomes cored by hp rocks the vertical growth of the dome induces exhumation by pure shear dominated ductile thinning laterally evolving to non coaxial detachment faulting while erosion feeds the surrounding sedimentary basins modeling of the bouguer anomaly grid is compatible with crustal scale mass transfers between a dense superstructure and a lighter infrastructure the model implies that the moldanubian orlica snieznik mantled gneiss dome derives from polyphase recycling of saxothuringian material

pre variscan variscan and early alpine thermo tectonic history of the north eastern bohemian massif an 40 ar 39 ar study the orlica snieznik and jesenik mountains correspond to three main domes from west to east the snieznik keprnfk and desna domes they are composed of a basement of autochthonous gneisses a thick series of blastomylonites and a supposed para autochthonous or allochthonous metamorphic pre devonian to devonian cover their broad direction is nne ssw 40ar 39ar radiometric measurements allow three main groups of ages to be defined 1 300 310 ma represented in the keprnik and desna domes this age is interpretated following the constraints on the age of the metamorphism which is linked with the extensional process occurring during the westphalian 2 320 340 ma represented mainly in the snieznik dome but not in the keprnfk massif the nappe structure of orlik vysoka hole in the northern area of the desna dome also exhibits this age which is interpretated as reflecting the period of the major variscan barrowian metamorphism which accompanied the compressional process it is only represented in the zones where the extensional process was not strong enough to result in a complete overprinting 3 340 440 ma corresponding to a very strictly defined area in the eastern rim of the desna dome occupied by ultramylonites and mylonites these ages obtained on muscovites result from an incomplete resetting of the minerals developed during the cooling of a granitic protolith and mylonitized during the extensional process a laser probe analysis confirms the extreme inhomogeneity of the ages of the muscovites and their different resetting from one grain to another the late alpine overprinting is more discrete but can be deciphered through the low extraction temperatures with ages between 80 and 120 ma these ages can be compared with alpine ages in the close western carpathians

use of bulk chemical analyses in stratigraphic correlations of sandstones an example from the statfjord nord field norwegian continental shelf abstract sandstone cores from two wells in statfjord nord field have been studied to evaluate a local correlation problem in well 33 9 14 cores have been taken in sandstones of both the upper jurassic draupne formation and the middle jurassic brent group in well 33 9 13 a 67 m cored interval was originally identified as brent but was subsequently suggested to be draupne formation it has not been possible to confirm the correlation of this interval by biostratigraphic study or conventional heavy mineral analyses in order to determine the feasibility of identifying the sandstones in well 13 on the basis of their bulk chemical composition bulk chemical data from the known stratigraphic intervals in well 14 were examined using the statistical technique of discriminant analysis consistent c mpositional differences were found between the upper jurassic and middle jurassic sandstones and the sandstones from the disputed interval in well 13 were found to match the middle jurassic compositions the most significant compositional difference is higher phosphorous content in the upper jurassic sandstones this difference is consistent with the observation of glauconite pellets a feature commonly associated with phosphorous mineralization throughout the upper jurassic interval in well 14 and the complete absence of glauconite in both the brent in well 14 and the disputed interval in well 13 the approach used in this study including both the use of bulk chemical analyses as a correlation parameter and the use of discriminant analysis for differentiating between stratigraphic un ts has potential application to correlation problems in a wide variety of rock types

geochemistry and 40ar 39ar geochronology of the mafic metavolcanic rocks from the rýchory mountains complex west sudetes bohemian massif palaeotectonic significance the mafic metavolcanic rocks blueschists and greenschists of the r chory mountains crystalline complex west sudetes experienced sea floor hydrothermal alteration spilitization prior to regional metamorphism the metabasite geochemistry namely trace element and ree abundances indicates that the protolith was comparable in composition with 1 tholeiitic to transitional ocean floor basalts and 2 transitional and alkaline intra oceanic island basalts two main metamorphic events affected the r chory mountains metabasites in an earlier high pressure low temperature metamorphic event the rocks experienced blueschist facies metamorphism the results of 40 ar 39 ar geochronology studies on phengites from the mafic blueschists date the end of the earlier metamorphism to 360 ma the greenschist metamorphic overprint followed around 340 ma the elongated bodies of mafic metavolcanic rocks are situated within the prominent nne ssw leszczyniec shear zone following the trend of the r chory mountains and the rudawy janowickie mountains both the geochemical affinities and the blueschist facies metamorphism of the metabasites suggest that this shear zone evolved from the variscan suture dividing western and central terranes of the west sudetes according to the radiometric age for the end of the high pressure low temperature metamorphism the terranes accreted during the famennian a considerable time span between the formation of the metabasite protolith and the blueschist metamorphism may indicate long lasting subduction of a large oceanic plate between gondwana and laurussia possibly accompanied by terrane accretion prior to the variscan orogeny

continental subduction and a mechanism for exhumation of high pressure metamorphic rocks new modelling and field data from oman abstract the physical model presented reveals two principal regimes of continental subduction a highly compressional hc regime and a low compressional lc regime characterised by high and low pressure between the overriding and subducting plates respectively the pressure is inversely proportional to the pull force which depends on the difference between average density of the subducting lithosphere and density of the mantle the subducting continental crust reaches a maximum depth which is proportional to the strength of the crust inversely proportional to the interplate pressure and is ca 200 km on average the crust then fails forming a major crustal thrust the location of failure depends on the regime of subduction on the interplate pressure in the hc regime failure occurs near the front of the subduction collision zone whereas the crust fails at greater depth under the base of the overriding plate in the lc regime the failure is followed by buoyancy driven uplift of the subducted crustal slice while the lithospheric mantle keeps subducting the uplift causes a normal sense displacement formation of a normal fault along the upper surface of the crustal slice for the lc regime failure of the crust near the base of the overriding plate a rapid spontaneous crustal uplift intrusion into the interplate zone brings the deeply subducted crust to shallow depths under the pressure of this crust the frontal part of the overriding plate undergoes local extension and then fails forming a tectonic window the rising material the high pressure rocks is exhumed within this window in the hc regime uplift of the subducted crust after its failure in front of the overriding plate is possible only with erosion of the relief which provides an unloading effect allowing the subducted crustal slice to rise up the exhumation depth is generally smaller in this regime but the volume of the exhumed material is larger the hc subduction regime has been shown earlier to match the himalayan situation the lc regime fits the situation in the oman mountains considered in this paper

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

use of spectral gamma ray signature to interpret stratigraphic surfaces in carbonate strata an example from the finnmark carbonate platform carboniferous permian barents sea spectral gamma ray gr profiles were examined in well 7128 6 1 the stratigraphic reference section of the entirely subsurface finnmark carbonate platform detailed bulk chemical profiling of selected gr peaks shows that potassium k and thorium th are mutually correlated and are a direct index of siliciclastic aluminosilicate content whereas uranium u is uncorrelated with k th and all other chemical components measured uranium tends to be enriched in thin shale and argillaceous carbonate layers within otherwise carbonate dominated intervals uranium is thus associated with aluminosilicate minerals and is not particularly concentrated in dolomite two types of gr peaks are observed potassium thorium dominated peaks are suggested to indicate relatively major transgressions during which aluminosilicate detritus was derived from sources interior to the fennoscandian shield uranium dominated peaks correspond with relatively minor transgressions within intervals of cyclic shallow water carbonate deposits uranium enriched aluminosilicate detritus is suggested to be the product of extended subaerial exposure of the platform during which u was concentrated by groundwater movement these results can be useful as a basis for applying spectral gr signature as a tool for stratigraphic interpretation in uncored or incipiently understood carbonate sections

transient allanite replacement and prograde to retrograde monazite re crystallization in medium grade metasedimentary rocks from the orlica śnieznik dome czech republic poland textural and geochronological arguments abstract we use textural and geochronological arguments to constrain monazite petrogenesis and the age of metamorphic events in medium grade metasedimentary rocks from the european variscan belt orlica nieznik dome czech republic poland the relationships between monazite allanite and other major and accessory phases are characterized for staurolite sillimanite and kyanite grade garnet bearing mica schist samples garnet porphyroblasts with prograde compositional zoning include allanite in the core and monazite at the rim allanite is virtually absent from the matrix it shows replacement textures involving a cluster of th u y rich silicates or phosphates monazite with frequent micro cracks or trimmed grain boundaries abounds in the mica rich matrix it occurs as lone grains close to partially resorbed garnet apatite or staurolite and as subhedral and compositionally zoned aggregates associated with allanite and xenotime electron microprobe analyses of monazite define pb vs th isochrons ages 363 343 ma with positive intercepts pb 0 40 120 ppm which suggest the presence of initial pb and age overestimation isotopic analyses obtained by laser ablation coupled with mass spectrometry confirm the minor presence of initial pb which leads to variable discordance in the 206 pb 238 u 207 pb 235 u system conversely the 206 pb 238 u 208 pb 232 th system is not visibly affected analyses are concordant and 208 pb 232 th ages range from 358 to 299 ma 7 8 ma regardless of monazite texture or composition they define a main group at 330 310 ma with few older outliers all observations suggest that a first monazite generation mnzi formed via allanite breakdown at prograde to peak p t conditions 5 7 kbar 575 640 c a comparison with previous geochronological results reveals that the age of this prograde reaction is however preserved by only few domains yielding 360 340 ma instead our results emphasize the importance of retrograde processes which built the dominant age population of 330 310 ma the latter ages are ascribed to a combination of 1 transient monazite growth after allanite and controlled by p availability 2 variable recrystallization replacement of older monazite grains and 3 minor monazite neoformation due to the resorption of garnet or apatite the young monazite grains or domains mnzii mostly formed during fluid influx and chloritization they constrain the end of the orogenic history when mid crustal rocks were exhumed and re hydrated at temperatures below 300 c