rbsr and 40ar39ar laser probe dating of high pressure phengites from the sesia zone western alps underscoring of excess argon and new age constraints on the high pressure metamorphism abstract the combined use of the rb sr and 40ar 39ar laser probe methods allows confirmation of the existence of excess argon in phengites from eclogites from the sesia zone western alps despite systematic 40ar 39ar plateau ages 40ar 39ar phengite ages from two different areas show integrated age spreads between 65 4 0 3 ma and 109 5 0 4 ma the youngest 40ar 39ar plateau age 65 9 0 4 ma is concordant with two rb sr phengite whole rock isochron ages from the same outcrop which display a mean age at 64 2 2 5 ma and older than a rb sr biotite whole rock isochron age at 53 0 1 0 ma this age concordance being incompatible with the concept of isotopic closure temperature is probably fortuitous and probably results from an excess argon contamination however the concordance of the rb sr phengite isotopic closure temperature with the temperature reached during the eclogite facies event suggests that the age at 64 2 2 5 ma could be a crystallization age probably contemporaneous with the eclogite facies event in the sesia zone when compared to recent age data from the piemont zone and the dora maira massif the entire data set which needs to be confirmed by extra measurements support diachronous high pressure hp events in the western alps

salt rich aqueous fluids formed during eclogitization of metabasites in the alpine continental crust austroalpine mt emilius unit italian western alps abstract the metabasite bodies of the mt emilius continental unit western italian alps underwent a stage of alpine eclogite facies metamorphism 1 1 1 3 gpa and 450 550 c accompanied by polyphase deformation and recrystallization the metabasites consist of two main rock types 1 eclogites omphacite garnet glaucophane epidote phengite paragonite preserving no relics of their precursors 2 eclogitized granulites i e rocks whose incomplete eclogitic recrystallization clinopyroxene ii garnet ii epidote amphibole chlorite allowed survival of textural and mineralogical relics of pre alpine granulitic assemblages clinopyroxene i garnet i plagioclase amphibole in this latter case the pre alpine granulites were converted to eclogites as the result of infiltration of aqueous fluids during eclogitization in both eclogites and eclogitized granulites hydrated high pressure foliations are cut by eclogitic metamorphic veins the bulk rock chemistry of the metabasites influenced the compositions of both the vein and rock forming clinopyroxenes and the compositional correlation between the vein and rock forming clinopyroxenes indicates that the syn eclogitic fluids have re equilibrated with the metabasite hosts the predominant vein minerals omphacite epidote and garnet contain primary high salinity fluid inclusions the fluids consist of two phase liquid vapour and of multiphase liquid vapour salt additional quartz salty aqueous inclusions containing nacl cacl 2 and mgcl 2 as the main chloride species the vein inclusions show a salinity range from 17 to 45 wt salts in eclogites and from 20 to 50 wt salts in eclogitized granulites in contrast fluid inclusions in matrix minerals of the eclogitized granulites contain primary two phase inclusions displaying a salinity range between 10 and 25 wt salts the marked difference in fluid salinities recorded by the inclusions in the eclogitic veins and those in the partially re equilibrated eclogitized granulites is interpreted in terms of progressive hydration during eclogitization of granulite facies rocks which caused an increase in the salt content of the residual inclusion fluids

precise eclogitization ages deduced from rb sr mineral systematics the maksyutov complex southern urals russia abstract the maksyutov complex southern urals russia is a well preserved example of subduction related high pressure metamorphism one of its two litho tectonic units consists of rocks that experienced eclogite facies conditions published 40ar 39ar data on phengite u pb data on rutile and sm nd mineral data define a cluster of ages around 370 to 380 ma nevertheless no consensus exists as to the detailed interpretation of data and the exact age of eclogitization we present new high precision internal mineral rb sr isochrons for eclogite facies metabasites felsic eclogites and eclogite facies quartz veins nine isochrons mainly controlled by omphacite and white mica phases give concordant ages with an average value of 375 2 ma 2 microtextural features such as prograde growth zoning in eclogite facies phases suggest that the assemblages dated formed at a stage of prograde metamorphism sr isotopic equilibria among eclogite facies phases and among eclogite facies fluid veins and the host rocks indicate that our ages reflect crystallization ages related to the prograde metamorphic probably fluid mediated eclogitization reactions this interpretation is reinforced by data from fluid precipitated quartzitic eclogites whose modal composition together with intergrowth relationships conclusively imply closed system behavior after crystallization the possible occurrence of a pre 375 ma event of ultra high pressure metamorphism uhpm in the maksyutov complex is disproved by isotope systematics microtextures and mineral zoning patterns

metamorphic evolution of the sesia lanzo zone western alps time constraints from multi system geochronology the sesia lanzo zone is a polymetamorphic unit containing hercynian granulite relics overprinted by eclogite and greenschist facies metamorphism and deformation during the alpine orogeny different parts of the unit record different stages on the p t deformation evolution allowing multi system isotopic studies to unravel the precise timing of the metamorphic history new rb sr white mica and u pb sphene data constrain the age of eclogite facies metamorphism and deformation to 60 70 ma this substantially alters the common view of early to mid cretaceous eclogite facies metamorphism in this unit the new results are more consistent with the established geotectonic framework for the alpine orogeny since they do not require a prolonged period of depressed geothermal gradient at a time when the region was in extension it is also more concordant with recent studies of other units that demonstrate post cretaceous high pressure metamorphism step heated 40ar 39ar analysis of phengites yields good plateaux giving ages older than the corresponding rb sr age such anomalously high ages indicate the presence of radiogenic argon rich fluids in the grain boundary network under the fluid pressure conditions acting during this high pressure metamorphic event the u pb sphene ages are variable in polymetamorphic rocks and show inheritance of older pb or sphene crystals into the high pressure event two monometamorphic assemblages yield concordant ages at 66 1 ma reflecting crystallisation of the eclogite facies assemblage the gneiss minuti complex gmc lies structurally below the eclogitic micaschists and its pervasive greenschist facies fabric yields tightly clustered rb sr white mica ages at 38 39 ma this greenschist event did not affect the majority of the emc the 40ar 39ar ages of micas formed at this time were very disturbed whereas micas surviving from an earlier higher pressure assemblage had their 40ar 39ar system reset the greenschist event did not strongly affect u pb systematics in hercynian age sphenes suggesting that the gmc did not uniformly suffer an eclogite facies metamorphism during the alpine cycle but was juxtaposed against the emc later in the orogeny this model still requires that the locus of deformation and metamorphism and possibly fluid flux moved outboard with time leaving the sesia lanzo basement as a shear bounded unreactive block within the orogenic wedge

deep subduction of continental crust in accretionary orogen evidence from u pb dating on diamond bearing zircons from the qinling orogen central china abstract accretionary orogenic belts are considered the most important sites for the generation of continental crust however it is still not very clear whether in such settings the continental crust has been significantly transported back to the mantle accompanied with its lateral growth continental uhp rocks the best indicators of the recycling of the continental crust have rarely been discovered in accretionary orogenic belts which hinder the exploration of this essential process the possible occurrence of early paleozoic uhp metamorphism in the qinling orogen provides an excellent opportunity to address this question this article reports an integrated study of u pb age trace element and mineral inclusion of zircon from an amphibolite sample in the qinling orogen the zircon crystals show typical metamorphic growth zoning low th u ratios flat hree patterns and insignificant eu anomalies they give a weighted mean u pb age of 490 4 5 8 ma most importantly an in situ diamond inclusion was identified from one of the zircon crystals therefore the obtained 490 4 5 8 ma age was taken as registering the peak uhp metamorphism the discovery of in situ diamond inclusion provides important evidence for the uhp metamorphism of the qinling orogen and indicates that the north qinling microcontinent was subducted to mantle depths of 120 km when it collided with the erlangping arc attached to the southern margin of the north china block combining with previous results we suggested that the north qinling uhp terrane underwent fast exhumation from 120 km to ca 30 km within 10 myr intervals as a few examples of uhp metamorphism have been discovered in some accretionary orogens more attention needs to be paid to the occurrence of continental uhp metamorphism during accretionary orogenic process furthermore evident crustal growth has been documented in the north qinling orogen during the early paleozoic it is inferred that the accretionary orogenic belts may play important roles in recycling of continental crust into the mantle besides generation of continental crust

shallow stratigraphic drilling applied in hydrocarbon exploration of the nordkapp basin barents sea abstract the nordkapp basin is a late palaeozoic rift basin characterised by numerous salt diapirs the salt was deposited in the late carboniferous early permian and has been remobilised several times upper palaeozoic and mesozoic sediments filled in the basin and were uplifted by salt diapirism in the early middle triassic late jurassic late cretaceous and tertiary regional uplift and erosion in the early tertiary and extensive glacial erosion in the late pliocene pleistocene removed up to 1200 m of the mesozoic and cenozoic strata and left them heavily tilted and truncated around the salt diapirs only covered by a thin sheet of glacial sediments during much of the triassic and jurassic the nordkapp basin was a large marine embayment open to the ocean in the southwest this allowed deposition of potential hydrocarbon source rocks in anoxic basins during transgressive periods in the triassic and late jurassic coastal processes improved the reservoir quality of upper triassic to middle jurassic sands during regressive periods one exploration well within the basin and three wells surrounding the basin have produced oil shows but no commercial discoveries have been made so far during the review period of the present paper an exploration well encountered oil and gas in triassic sandstones in the southwestern segment of the basin this is the first direct indication of a mature oil producing source rock in the nordkapp basin and will be important for future exploration of the basin

the post collisional stabilization of thickened crust in the southern urals abstract the urals are characterized by a depression of the moho to a depth of 57 km this structure is interpreted as a relic orogenic root which has been conserved because no significant post collisional processes occurred however there is evidence that voluminous post collisional magmatism affected the lower crust in this paper we use thermal finite element models to quantify the influence of the post collisional magmatism on the stabilization of the root we show that at least 70 of the heat producing elements migrated in granitic melts from the lower crust to the upper crust as a result the crustal heat flow reduced and the lithosphere could stabilize at a thickness of 180 km furthermore we propose that a granulite metamorphic event during the thermal relaxation of the collision zone prevented the 57 km thick crust from delamination these results strongly indicate that post collisional processes were necessary for the stabilization of the uralian crust and lithosphere

40ar 39ar and rb sr geochronology of high pressure metamorphism and exhumation history of the tavsanli zone nw turkey geochronological investigations in high and ultra high pressure metamorphic rocks are problematic since firstly the low temperatures lead to fine grain size and disequilibrium assemblages and secondly the problem of excess argon affects 40ar 39ar systematics the most commonly used isotopic system the tavsanli zone is a belt of high pressure low temperature hp lt rocks spanning nw turkey and is one such region where previous geochronological studies have produced a range of estimates for the age of hp lt metamorphism raising the question of whether they are geologically significant this study presents new data from the tavsanli zone 40ar 39ar ages are in the range 60 ma to 175 ma whilst rb sr ages are restricted to 79 7 ma to 82 8 ma confirming the presence of excess argon detailed ultra violet laser ablation microprobe uvlamp studies have revealed younger 40ar 39ar ages in the cores of coarser white micas which in conjunction with 40ar 39ar ages from the finest grained lithologies and the rb sr white mica crystallisation ages constrain the post hp lt metamorphism exhumation rates of these rocks petrological and regional constraints suggest that syn subduction exhumation and cooling took place initially by synchronous subduction and exhumation by underplating this is followed by a phase of syn continent continent collision at a rate of approximately 1 5 mma 1 and exhumation to the surface via thrusting the 40ar 39ar hornblende data from a granodiorite intruding the hp lt rocks constrain the later parts of exhumation path this study highlights the importance of a multi system geochronological approach when attempting to determine the history of hp lt rocks

sr bearing zoisite and epidote in ultra high pressure uhp metamorphic rocks from the su lu province eastern china an important sr reservoir under uhp conditions sr bearing zoisite and epidote are common constituents of eclogites and associated paraschists throughout the su lu ultra high pressure uhp province eastern china the sro content of prograde zoisite and epidote reaches 3 2 wt in crystal cores and generally decreases toward crystal margins retrograde epidote is poorer in sro 0 1 wt preliminary rare earth element ree analyses of epidote give la2o3 up to 2 9 wt ce2o3 5 9 wt and nd2o3 3 0 wt ree contents of zoisite are distinctly lower la 2o3 up to 0 16 wt ce2o3 up to 0 26 wt and nd2o3 up to 0 16 wt than coexisting epidote apatite is always more depleted in sro 0 10 0 59 wt on average than coexisting zoisite and epidote and sr ca partition coefficients for zoisite and epidote and apatite sr ca zo ep ap range from 5 to 20 sro content of k white mica 0 012 0 044 wt is an order of magnitude lower than that of apatite an evaluation of the sro content in zoisite and epidote and their modal abundances in seven samples indicates that 70 of the whole rock sro is contained in these minerals apatite and k white mica are only minor reservoirs for sro in these rocks zoisite and epidote are thus regarded as the most important sr reservoirs at uhp conditions where calcic plagioclase and titanite are unstable

a crystal chemical basis for pb retention and fission track annealing systematics in u bearing minerals with implications for geochronology this study develops an empirical crystal chemical framework for systematizing the kinetics of pb loss and fission track annealing in u bearing minerals ionic porosity z the fraction of a mineral s unit cell volume not occupied by ions potentially accounts for kinetic behavior by monitoring mean metal oxygen bond length strength various tests of a general kinetics porosity relationship are presented based upon diverse mineral data including 1 pb diffusion parameters 2 measured closure temperatures tc for fission track annealing and 3 retentivities of both pb and fission tracks from apparent age data every kinetic parameter including tc and mineral age for both the u pb and fission track systems is inversely correlated with z within the sub assemblage zircon z 29 titanite 34 and apatite 38 assuming a diffusional closure model pb isotopic transport phenomena are described by a tc z scale calibrated with field based tc data for titanite 680 20 c and apatite 500 c extrapolation of this scale yields tc estimates for the following minerals staurolite tc 1060 c z 25 garnet 1010 c 26 5 zircon 900 c monazite xenotime and epidote 750 c 32 and ca clinopyroxene 670 30 c 34 1 depending on composition these empirical results imply that a u pb pb date for staurolite or garnet records the time of mineral growth not post growth isotopic closure as also concluded in recent field studies because z systematizes fission track annealing this recrystallization process like volume diffusion must also be rate limited by the strength of chemical bonds the extent to which other recrystallization processes are likewise rate limited is important to u pb geochronology because they potentially compete with diffusion as mechanisms for pb isotopic resetting in nature

rb sr isotopic and compositional retentivity of muscovite during deformation abstract permian metapegmatite muscovite from the upper austroalpine matsch unit in southern tyrol italy was investigated regarding its rb sr and compositional retentivity during cretaceous upper greenschist facies deformation the data imply that microstructurally relic permian magmatic muscovite largely maintained its major and trace element compositions during deformation whereas the rb sr geochronometer is strongly affected by a net loss of sr lower sr concentrations of muscovite correlate with higher 87rb 86sr and 87sr 86sr ratios in most samples the muscovite grain size and magnetic fractions with the lowest 87rb 86sr and 87sr 86sr ratios preserve a permo triassic muscovite whole rock rb sr apparent age interpreted as to reflect formation during or cooling after pegmatite emplacement contrastingly muscovite fractions with higher 87rb 86sr and 87sr 86sr ratios are arranged along a roughly linear array with a positive correlation of the 87rb 86sr and 87sr 86sr ratios in the 87rb 86sr vs 87sr 86sr space they yield successively lower muscovite whole rock rb sr apparent ages we explain the variations in the rb sr isotopic character of microstructurally relic muscovite by a presumably deformation related loss of sr during the cretaceous event contemporaneously only very limited amounts of isotopically different sr from the matrix reservoir might possibly have entered the muscovite consequently the rb sr of the relic muscovite is affected by a net loss of sr the results imply that at temperatures of

topographic control of oceanic flows in deep passages and straits saddle points between neighboring deep ocean basins are the sites of unidirectional flow from one basin to the next depending on the source of bottom water flow in these sites appears to be topographically controlled so the interface between the bottom water and the water above adjusts itself to permit bottom water flow from the basin that contains a source of bottom water into the next examples in the atlantic include flow in the romanche fracture zone the vema channel the ceara abyssal plain the anegada jungfern passage and the discovery gap but there are many more theoretical predictions of volume flux using a method that requires only conductivity temperature depth data archives and detailed knowledge of bathymetry near the saddle point are compared with volume flux estimates using current meters and or geostrophic estimates for seven cases the ratio of prediction to volume flux estimate ranges from 1 0 to 2 7 some ocean straits that separate adjacent seas are also found to critically control bidirectional flows between basins theory of the influence of rotation on such critical flows is reviewed predictions of volume flux in eight cases are compared with ocean estimates of volume flux from traditional methods

metamorphic evolution of eclogite and associated garnet mica schist in the high pressure metamorphic maksyutov complex ural russia metasedimentary garnet mica schists are interlayered with metabasic garnet omphacite schists and enclose eclogite boudins in the high pressure metamorphic maksyutov complex in the southern urals russia these three rock types were investigated in one outcrop and compared chemographically and thermobarometrically the fe mg distributions between garnet rim omphacite and garnet rim phengite pairs indicate different equilibration temperatures for the three samples with the lowest temperature 500 c 1 5 gpa for the eclogite boudin an intermediate temperature 630 c 1 7 gpa for the foliated eclogite and the highest temperature 650 c 1 7 gpa for the garnet mica schist the garnets in garnet mica schist enclose abundant chloritoid relics and the fe mg distribution between chloritoid and garnet records an earlier high temperature stage 650 c 2 0 gpa before the garnet rim phengite temperatures were reached together with some minimum and maximum pressure estimates three different prograde pressure temperature paths and a common retrograde metamorphic evolution are interpreted from the chemographic and thermobarometric data the different early metamorphic evolutions and conditions confirm the variability of protoliths which are also indicated by different u pb zircon and rutile ages

geology and geodynamic evolution of the high p low t maksyutov complex southern urals russia the high pressure low temperature maksyutov complex is situated in the southern urals between the silurian devonian magnitogorsk island arc and the east european platform the elongated n s trending complex is made up of two contrasting tectono metamorphic units unit 1 consists of a thick pile of proterozoic clastic sediments suggested to represent the passive margin of the east european platform the overlying unit 2 composed of paleozoic sediments volcanic rocks and a serpentinite melange with rodingites is interpreted as a remnant of the uralian paleo ocean devonian eastward subduction of oceanic crust beneath the magnitogorsk island arc resulted in an incipient blueschist facies metamorphism of unit 2 indicated by lawsonite pseudomorphs in the rodingites while unit 2 was accreted to the upper plate subduction of the continental passive margin caused the high pressure metamorphism of unit 1 buoyancy driven exhumation of unit 1 into the forearc region led to its juxtaposition with unit 2 along a retrograde top to the ene shear zone further exhumation of the maksyutov complex into its present tectonic position was accomplished by later shear zones that were active as normal faults and are exposed along the margins of the complex at the western margin a top to the west shear zone juxtaposed a low grade remnant of a paleozoic accretionary prism suvanyak complex above the maksyutov complex along the eastern margin a top to the east shear zone and the brittle main uralian normal fault emplaced the maksyutov complex against the magnitogorsk island arc in the hanging wall

excess argon in phengite from eclogite evidence from dating of eclogite minerals by smnd rbsr and 40ar39ar methods the existence of excess ar in phengites from coesite bearing ultrahigh pressure eclogites of the dabie mountains and su lu terrain china has been confirmed by dating of minerals using sm nd rb sr and 40ar39ar methods 40ar39ar ages of phengites for two samples studied are much older than the sm nd and rb sr mineral isochron ages sample ql 1 has rb sr age of 219 5 0 5 ma sm nd age of 226 3 4 5 ma and phengite 40ar39ar age of 877 5 15 9 ma the corresponding ages for sample zb 4 are 223 9 0 9 ma rb sr 228 4 6 0 ma sm nd and 943 3 12 4 ma 40ar39ar phengite an age of 225 ma based on the sm nd and rb sr mineral isochron ages is consistent with the time inferred from paleomagnetic and geological data for collision of the north china and yangtse blocks and formation of these eclogites we call for greater caution in 40ar39ar dating of phengites from other occurrences of ultrahigh pressure environments

thermotectonic evolution of the western fold and thrust belt southern uralides russia as revealed by apatite fission track data abstract the uralides a linear n s trending palaeozoic fold belt reveals an intact well preserved orogen with a deep crustal root within a stable continental interior in the western fold and thrust belt of the southern uralides devonian to carboniferous siliciclastic and carbonate rocks overlay mesoproterozoic to neoproterozoic sedimentary rocks deformation in the devonian carboniferous and permian caused thick skinned tectonic features in the western and central parts of the western fold and thrust belt a stack of several nappes characterizes the deformation in the eastern part along the e w transect ac ts 96 that crosses the western fold and thrust belt apatite fission track data record various stages of the geodynamic evolution of the uralide orogeny such as basin evolution during the palaeozoic synorogenic movements along major thrusts synorogenic to postorogenic exhumation and a change in the regional stress field during the upper jurassic and lower cretaceous the palaeozoic sedimentary cover and the neoproterozoic basement of the ala tau anticlinorium never exceed the upper limit of the paz since the devonian a temperature gradient similar to the recent one 20 c km would account for the ft data reactivation of the neoproterozoic zilmerdak thrust was time equivalent to the onset of the devonian and carboniferous collision related deformation in the east west directed movement along the tashli thrust occurred in the lower permian the devonian and carboniferous exhumation path of the neoproterozoic siliciclastic units of the tirlyan synclinorium mirrors the onset of the uralian orogeny the emplacement of the tirlyan nappe and the continuous west directed compression the five main tectonic segments inzer synclinorium beloretzk terrane ala tau anticlinorium yamantau anticlinorium and zilair synclinorium were exhumed one after another to a stable position in the crust between 290 and 230 ma each segment has its own t t path but the exhumation rate was nearly the same final denudation of the western fold and thrust belt and exhumation to the present surface probably began in late tertiary in jurassic and cretaceous south directed movements along w e trending normal faults indicate a change in the tectonic regime in the southern uralides

rb and sr diffusion in alkali feldspars with implications for cooling histories of rocks volume diffusion of sr and rb have been measured in orthoclase feldspar as well as sr diffusion in albite using isotope exchange hydrothermal solutions and analysis by ion microprobe for water pressures of 1000 bars the kinetics of diffusion parallel to the c axis are given by the arrhenius relations strontium transport in the or is isotropic to within a factor of four the measurements use a new procedure in which only the elements in question are present in the starting fluid cooling rates of igneous rocks can now be determined from closure temperatures material balance and the rb sr dating method based on the departures of the mineral data from the whole rock isochron for cooling rates less than approximately 100 c my

crustal scale structure and evolution of an arc continent collision zone in the southern urals russia the outcropping geology of the southern urals contains a well preserved accretionary complex related to the paleozoic collision that took place between the magnitogorsk arc and the former east european craton the crustal scale structure of the accretionary complex has been determined from outcropping field geology that is integrated with three reflection seismic profiles the reflection profiles show the accretionary complex to be highly reflective allowing direct comparison of many reflections with surface geological features we interpret the accretionary complex to be a thrust stack that is composed of shallowly subducted continental shelf and rise material syncollisional sediments derived from the arc deeply subducted high pressure gneisses that are intercalated with eclogites and blueschist and at the highest structural level ophiolite complexes it is bound at the base by a thrust and at the rear by a highly deformed zone the main uralian fault adjacent to the backstop the magnitogorsk arc deposition of the late devonian volcaniclastic sediments of the zilair formation appears to be related to collision uplift and erosion of the arc possibly following the arrival of the full thickness of the east european craton continental crust at the subduction zone with the arrival of the continental crust at the subduction zone offscraping and underplating of paleozoic slope and platform material took place at the base of the accretionary complex uplift of the arc was followed by its collapse and the unconformable deposition of lower carboniferous shallow water carbonates on top of it a time lag of 10 15 myr occurred between the high pressure metamorphism and the subsequent arrival of the east european craton at the subduction zone

40ar 39ar u pb and sm nd constraints on the timing of metamorphic events in the maksyutov complex southern ural mountains new 40 ar 39 ar u pb and sm nd data from 13 metamorphic samples constrain the formation and exhumation of the high pressure maksyutov complex in the southern uralide orogen the maksyutov complex records the highest pressure metamorphism in the southern uralides and is part of the thrust stack that composes the footwall beneath the main ural fault the new isotopic data from eclogite in the lower unit of the maksyutov complex include 40 ar 39 ar data from phengite u pb data from rutile and apatite and sm nd data from garnet clinopyroxene rutile and apatite these data indicate that eclogite facies metamorphism occurred in the mid devonian epoch c 380 ma corresponding with eastward subduction of the east european craton beneath the magnitogorsk island arc after partial exhumation the lower unit was juxtaposed c 360 ma based on 40 ar 39 ar white mica data with the middle and upper units at mid crustal levels during the development of a d 3 shear zone within the subduction zone by the late carboniferous epoch the maksyutov complex was exhumed to upper crustal levels compatible with sedimentologic evidence that at this time the foreland thrust and fold belt began to form and material was shed off it into the foreland basin

processing of crust in the root of the caledonian continental collision zone the role of eclogitization segments of the root to the caledonian collision zone are exposed along the west coast of norway allowing the study of processes and petrophysical conditions at the deepest crustal levels p t conditions prevailing in these crustal root zones correspond to eclogite facies eclogitization is associated with marked changes in petrophysical properties notably a density 10 15 and vp increase which may give a mantle signature to an eclogitized crust the observed ductility enhancement may be caused by transformation plasticity reduction in grain size formation of new mineral assemblages and presence of fluid in the western gneiss region and the bergen arcs eclogites form along shear zones interpreted as fluid pathways with sharp boundaries to their metastable protolith this makes eclogite shear zones potential deep crustal reflectors dry crust subducted into roots of continental collision zones will undergo eclogitization only if hydrous fluids are available timing of metamorphic reactions does not depend on crossing of equilibrium boundaries in the p t space but rather on the introduction of fluids into the system eclogites are not only passive recorders of p t t paths illustrating very deep subduction of crust 100 km rapid changes in petrophysical properties make fluid induced eclogitization a dynamic process that influences geodynamics stresses set up by the volume changes or by tectonic forces are released by ductile deformation along shear zones and by seismic faulting as evidenced by syn eclogitic pseudotachylytes in the metastable protolith deformation including seismicity enhances fluid infiltration and further eclogitization dense and rheologically weak eclogites may delaminate and sink into the mantle in the absence of fluids crustal doubling may occur without metamorphic re equilibration the evolution of a collision zone depends on the fluid regime which should be considered in geodynamic modelling in addition to p and t

the effects of ferromagnesian components on the paragonite muscovite solvus a semiquantitative analysis based on chemical data for natural paragonite muscovite pairs chemical data for 139 natural paragonite muscovite pg ms pairs illustrate the effects of ferromagnesian components on the p t x topology of the pg ms solvus the pairs were selected on the basis of reasonably accurate knowledge of the p t conditions of formation evidence for close approach to equilibrium at peak metamorphic conditions exclusion of pairs in which paragonite contains more than 5 mol margarite and exclusion of pairs from polymetamorphic rocks that contain more than one set of cogenetic pg ms pairs graphical analysis reveals considerable scatter in the data nevertheless it is evident that the muscovite limb of the solvus shifts markedly toward end member muscovite with increasing pressure from approximately 7 kbar to 21 kbar this shift is attributed to a pressure induced increase of the ferromagnesian content of muscovite which increases the size of the xii alkali site to the effect that k is more readily accommodated than na the data also suggest that the paragonite limb of the solvus migrates slightly toward end member paragonite with increasing pressure broadening of the pg ms solvus with increasing pressure reflects increasingly nonideal na k mixing as the phengite content of muscovite increases due to the wide scatter of data for pg phengitic ms pairs it is concluded that at the present time pg ms solvus thermometry is only viable for quasibinary pg ms pairs

main uralian thrust and main uralian normal fault non extensional palaeozoic high p rock exhumation oblique collision and normal faulting in the southern urals the crustal architecture of the southern urals is dominated by an orogenic wedge thrusted westward upon the subducted east european continental margin the n s trending wedge constitutes an antiformal stack composed mainly of the high p maksyutov complex the overlying suvanyak complex and the allochthonous synformal zilair flysch further west these tectono metamorphic units are separated by tectonic contacts and record discontinously decreasing metamorphic conditions from bottom to top in the east the e dipping main uralian normal fault cross cuts the metamorphic footwall and juxtaposes the non metamorphic magnitogorsk island arc this syncollisional normal fault compensated crustal thickening and exhumation of the high p rocks orogenic shortening was accommodated by the main uralian thrust a w vergent crustal scale shear zone at the base of the wedge geological investigations and reflection seismics urseis 95 argue in favour of a geodynamic evolution integrating subduction and basal accretion of high p rocks during sinistral oblique thrusting along the main uralian thrust and coeval normal faulting along the main uralian normal fault

petrotectonic evolution of the high to ultrahigh pressure maksyutov complex karayanova area south ural mountains structural and oxygen isotope constraints the maksyutov complex consists of three fault bounded lithologic units a quartzofeldspathic gneiss containing mafic eclogite boudins unit a1 a metasedimentary blueschist facies yumaguzinskaya unit and a meta ophiolitic melange unit a2 the geologic history of the high to ultrahigh pressure hp uhp assembly of the maksyutov complex is complicated by several stages of prolonged retrograde metamorphism and deformation the sakmara river exposes all three units near the former village of karayanova a structuralrpetrologic cross section through the area yields new quantitative data for the complex and regionally for the south urals analysis of the karayanova area has identified the major structures regional folding within the complex is parallel to the dominant foliation trending northeast southwest stereonet data show that during exhumation this large scale folding was refolded about axes trending southeast unit a1 and the yumaguzinskaya are tectonically and petrologically distinct units juxtaposed by west vergent thrusting and recrystallization within the same subduction zone a shear zone developed later between unit a2 and the unit a1q yumaguzinskaya tectonic package accompanying exhumation field relations and petrofabric demonstrate that blueschist facies recrystalliza tion overprinted an earlier eclogite facies metamorphism thermobarometric measurements yield p t values of 594 6378c 1 5 1 7 gpa for eclogite but these conditions may reflect annealing during the early stage exhumation at 375 ma cuboid graphite aggregates testify to precursor conditions for unit a1 within the diamond stability field if such textures are correctly interpreted measured 18 or 16 o partitioning between pairs of coexisting phases yield three main recrystallization temperature ranges 1 678 838c attending unit a1 eclogite facies metamorphism 2 453 178c during transitional blueschistrgreenschist facies metamorphism for the amalgamated unit a1q yumaguzinskayaq unit a2 assembly and 3 250 688c reflecting late stage hydrothermal alteration and exhumation oxygen isotope data for units a1 and a2 indicate that garnet blue amphibole and pyroxene crystallized in isotopic equilibrium validating previous thermobarometric calculations for a unit a1 retrograde metamorphic event variations in d 18 o values for phengites suggest the possibility of late metamorphic fluid infiltration retrograde recrystallization at high pressure in the presence of fluids and a calculated

a model of palaeozoic obduction and exhumation of high pressure low temperature rocks in the southern urals abstract a new evolutionary geodynamic model of devonian continental subduction and exhumation of high pressure low temperature metamorphic rocks in the southern urals is proposed based on the results of physical modelling geological and geophysical data and on the comparison of this belt with other orogens the model includes the following principal phases 1 closure of the western uralian ocean by eastward intraoceanic subduction associated with the magnitogorsk volcanic arc 2 subduction of the european continental margin causing failure of the overriding plate in the arc area along a fault dipping eastward under the arc 3 subduction into the mantle of the fore arc block together with the underlying continental crust and sedimentary layer the sedimentary cover as well as the crustal and mantle fragments of this block are scraped off and accreted in front of and under the arc the continental crust shielded from the hot mantle by the fore arc block subducts to depths of more than 150 km remaining at a relatively low temperature the crust then fails at depths of several tens of kilometres 4 the subducted crustal slice starts to rapidly rise and intrude the interplate zone scraping and pushing up the sediments and slivers of the oceanic fore arc block previously dragged down to different depths 5 exhumation of the rising high pressure low temperature rocks within the uraltau dome separating the subduction accretionary complex from the magnitogorsk arc the eastern border of the uraltau dome coincides with the main uralian fault this fault represents a complex suture zone which at the stage of deep crustal subduction was associated with a major thrust subduction fault and at the exhumation stage with a major normal fault corresponding to the upper surface of the rising crustal sedimentary body

rapid exhumation of the zermatt saas ophiolite deduced from high precision smnd and rbsr geochronology sm nd isotope data from garnets in ultrahigh pressure coesite bearing eclogite from the western alps were obtained to determine the age of peak metamorphism and exhumation rates of deeply buried oceanic crust in the zermatt saas ophiolite complex we report an exceptionally well constrained sm nd isochron age of 40 62 6 ma from the lago di cignana eclogite difficulties in dating alpine eclogites using this method were overcome by using an hf acid leaching procedure on garnet to remove mineral inclusions the zermatt saas rocks reached greenschist facies conditions around 38 2m a on the basis of rb sr whole rock phengite isochrons these data combined with existing estimates of the pressure and temperature conditions of the eclogite give an estimate for the initial exhumation rate of 10 to 26 km m y depending on the inferred pressure for the recrystallization of phengite followed by slow exhumation of 0 3 km m y from 34 to 14 ma the initial exhumation rates at lago di cignana are among the highest yet determined for high pressure terranes and are well constrained because the age has been determined directly on the high pressure assemblage the rapid exhumation rates presented here place critical constraints on collisional tectonic models 1999 elsevier science b v all rights reserved

a 40ar 39ar laser probe study of micas from the sesia zone italian alps implications for metamorphic and deformation histories two samples from the eclogite micaschist complex emc and the seconda zona diorito kinzigitica iidk of the sesia zone have been studied using a high spatial resolution laser probe 40ar 39ar technique with the aim of investigating the complexities of argon behaviour in metamorphic rocks and comparing their thermal histories data from a single large phengite grain from the emc show a range of ages from mid jurassic to upper cretaceous these apparent age variations are spatially related to both location within the grain and to intragrain microstructure modelling of the data shows that the profile formed by the diffusion of an excess argon component into the grain parallel to the mica cleavage profile asymmetry is explained by temporal variations in microstructural development enabling excess argon to enter the grain at different times in different places the temperatures of the initiation of deformation and the possible time scales for the deformation can be calculated as a function of cooling rate all estimates suggest deformation at greenschist facies in accord with the observed retrograde mineral assemblage absolute temperature estimates for deformation vary by less than 22 c for different cooling rates of 10 and 30 c ma 1 but vary by 80 c with different estimates of diffusion parameters the duration of deformation was for at least 2 ma at 10 c ma 1 or 0 7 ma at 30 c ma 1 biotites from the iidk sample record a permian to upper cretaceous age range that correlates with grain size the smallest grain sizes yielding the youngest ages this relationship is best explained by a partial resetting of biotites during an alpine thermal event initiated not more than 70 ma ago modelling of these data suggest that the sample never exceeded 300 c during the alpine the profoundly different thermal histories of the two units the emc recrystallized at 550 c whilst the iidk remained below 300 c suggests that they may not have been juxtaposed until much later than the eclogite facies metamorphism

geochemical window into subduction and accretion processes raspas metamorphic complex ecuador the high pressure low temperature p 1 3 2 gpa t is 600 c raspas metamorphic complex is an exhumed fragment of the partially accreted partially subducted amotape chaucha terrane in southwest ecuador comparative analysis of major and trace elements plus sr nd and pb isotopes in bulk lithologies and individual crystalline phases shows that the complex includes one to three layers of ordinary oceanic crust and underlying mantle lithosphere together with oceanic plateau fragments subduction and exhumation of oceanic lithosphere resulted in selective bulk trace element geochemical changes rb ba and sr have been lost in amounts from approximately 85 50 from the high p low t metamorphosed pelites and basalts whereas pb is enriched in mafic rocks during formation of the eclogite u pb and rare earth elements rees were immobile high p low t metamorphosed terranes form the basement of active ecuadorian arc volcanoes partial melting of this basement by mantle wedge derived basalt is a likely source of adakitic components

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

35 ma old ultrahigh pressure metamorphism and evidence for very rapid exhumation in the dora maira massif western alps abstract ion microprobe shrimp data on zircons from various rock types of an ultrahigh pressure uhp metamorphic whiteschist type pyrope quartzite lens of the dora maira massif dmm consistently show domains giving a late eocene age of 35 4 1 0 ma which is taken as the age of uhp metamorphism these domains partially replace the older oscillatory zoning pattern of the zircons formed during primary magmatic crystallization at about 275 ma zircons of a metagranitic country rock next to the uhp metamorphic lens have these primary features best preserved all zircons measured also yield intermediate ages between 275 and 35 ma with a statistical concentration between 260 and 210 ma thus the uniformity of the initial zircon population both in the lens and the country rock evidences a common protolith that is a granite intruded during the late herynian while the intermediate ages are at least partly due to incomplete resetting of the zircons during uhp metamorphism those in the 260 210 ma range may be related to rifting episodes in the permotriassic the mg rich chemistry of the whiteschist lenses is due to local metasomatic alterations of the granite perhaps by fluids derived from evaporitic sediments dating as early as the permotriassic as well the more pervasive resetting of zircon ages during uhp metamorphism in the pyrope quartzite lenses is explained by the ubiquity of fluids and or melts produced during subduction by a series of dehydration reactions that occurred only in the more hydrous mg rich protoliths and not in their drier granitic neighborhood fission track ages determined partly on the same zircon samples yielding 29 9 1 4 ma mark the time when the uhpm unit had reached about 290 c at a shallow location within the crust thus exhumation over a vertical distance of about 120 km must have occurred within about 5 6 ma indicating an average uplift rate of about 20 24 km ma and an average cooling rate of about 85 100 c ma the radiometric data obtained do not lend any support to an eo alpine cretaceous subduction event so that deep subduction and immediately following exhumation all took place during early to mid tertiary time this scenario seems to apply to large portions of the western and central alps as well calling for drastic geodynamic reinterpretations of these parts of the alps

diagenesis and incipient metamorphism in the western fold and thrust belt sw urals russia in the western fold and thrust belt of the southern urals the kubler and arkai indices determined on shales slates and phyllites record an increase from lower late diagenetic to epizonal grade from west to east the metamorphic grade varies strongly within the different tectonic segments which are separated by major thrusts the increase of diagenetic and incipient metamorphic grade from the footwall to the hanging wall of all major upper palaeozoic thrusts indicates a pre permo triassic origin west of the avzyan thrust zone the diagenetic to incipient metamorphic grade is related to the palaeozoic basin development and reached the final grades in late carboniferous to early permian times east of the first avzyan thrust in the yamantau anticlinorium the diagenetic to lower greenschist metamorphic grade is possibly of neoproterozoic origin and might be related to the development of the neoproterozoic basin at the eastern margin of the east european craton the eastern part of the yamantau anticlinorium was exhumed below 200 c in the late carboniferous or early permian the diagenetic grade of the autochthonous palaeozoic sedimentary units increases toward the stack of palaeozoic nappes and might partly be caused by the deformational process due to the emplacement of the palaeozoic nappes within the timirovo thrust sheet the decrease of metamorphic grade with stratigraphic age developed prior to the emplacement of the nappes the upper anchizonal metamorphic grade of the upper devonian slates of the zilair nappe results from the deformation process related to the lower carboniferous nappe emplacement

eo alpine eclogitisation of permian morb type gabbros in the koralpe eastern alps austria new geochronological geochemical and petrological data new petrological geochemical and isotopic data sm nd rb sr from meta gabbros eclogites and metapelites from the southern koralpe are presented pl cpx gabbros mostly olivine free that have preserved their magmatic minerals to varying extent occur as relics within eclogites transformation of gabbro into eclogite is documented in different stages ranging from macro to microscale based on textural characteristics the eclogites are classified into four groups types 1 4 typical eclogite parageneses are grt omp zo ky qtz amp phe rut the metapelitic eclogite host rocks are composed of qtz ms bt pl grt ky p t estimates for both metabasites and metapelites range at 1 8 2 0 gpa 600 650 c sm nd mineral isochrons pl cpx for the gabbroic eclogite precursors give ages of between 247 16 and 255 9 ma their initial nd and sr isotopic compositions are clearly within the range of modern depleted mantle values minerals from type 1 metagabbroic eclogites are in clear nd isotopic disequilibrium pervasively recrystallized type 3 eclogites yield grt wr or grt omp zo amp wr ages of 97 5 101 7 and 109 8 ma the metapelitic eclogite host rocks mica schists yield nd dm depleted mantle model ages of 1 44 1 63 ga grt wr isochron ages from these rocks are 86 1 87 4 and 89 3 ma the new data further support the permian early mesozoic rifting event for this part of the austroalpine basement which is related to initial oceanization at the western end of the alpine tethys during alpine plate convergence pre alpine continental basement and oceanic crustal fragments where subducted to depths of c 60 km the most likely time span for the high pressure peak is c 100 10 ma rapid exhumation in the upper cretaceous is inferred from mineral cooling ages between 80 and 60 ma