review of geochemical variation in lower palaeozoic metabasites from the ne bohemian massif intracratonic rifting and plume ridge interaction

geology and geochemistry of archaean granites in the kalgoorlie region of the eastern goldfields western australia a syn collisional tectonic setting abstract granites in the kalgoorlie region of the archaean yilgarn craton are predominantly biotite granodiorite and monzogranite with some hornblende bearing varieties granites are divided on the basis of field relationships petrography and chemical composition into five supersuites an older supersuite containing four suites pre dates regional folding and four supersuites post date regional folding the potassic composition of the mainly silica rich granites 68 sio2 suggest that the most appropriate source for the majority of supersuites is continental crust with a composition in the range tonalite to granodiorite most supersuites evolved by fractional crystallization mainly of biotite and plagioclase some suites contain synplutonic dykes and microgranite xenoliths that crystallized from enriched mantle derived magmas there was only limited and local chemical interaction between granite and mantle derived magmas geometric and temporal relationships between regional structures and granites indicate that the granites were emplaced during a major orogenic event however s type granites which are widespread in some modern syn collision settings are absent in the kalgoorlie region the predominance of high fo2 i type granites in the study area probably reflects the absence of large graphite bearing metasedimentary accumulations in the lower crustal source regions the distribution of suites and supersuites implies a common source region beneath the kalgoorlie and gindalbie granite greenstone terranes suggesting that they shared a common geological history dating back to late greenstone deposition

u pb zircon and sm nd model ages of high grade moldanubian metasediments bohemian massif czechoslovakia we report single grain u pb ion microprobe as well as conventional bulk size fraction ages for zircons from 3 metasediment samples of the moldanubian complex bohemian massif one of the largest crystalline complexes of the hercynian foldbelt in europe these are complemented by whole rock sm nd model ages the metasediments are of upper amphibolite to granulite grade and come from the bory massif in moravia nw of brno sample aa 1 and from the varied group aa 2 and monotonous group aa 3 in the ceske budejovice region of sw bohemia

continental subductions beneath tibet and the himalayan orogeny a review argand 1924 and later holmes 1965 proposed that the himalayan orogeny and tibet uplift were the result of large underthrusting of the indian continental crust beneath tibet since then plate motion studies have yielded new constraints on the relative displacements of india and asia and a large number of geological and geophysical investigations have helped to determine the structure of the himalayas and tibet the best natural laboratory for understanding mountain belts the aim of this paper is to review briefly the various models of himalayan mountain building in light of recent geological and geophysical data and to estimate the role of continental subductions

late cadomian crustal tilting and cambrian transtension in the tepla barrandian unit bohemian massif central european variscides the cadomian basement of the tepla barrandian unit is characterized by a classic barrovian type metamorphism the degree of which increases considerably towards the west reaching amphibolite facies conditions in the domaz lice crystalline complex dcc the number and volume of plutons also increases towards the west the emplacement ages of the te s ovice granite and the mrac nice trondhjemite have been determined at 521 7 2 ma and 523 4 5 ma respectively applying conventional u pb analyses of zircons pervasive high temperature prolate fabrics and north northwest dipping dextral oblique slip shear zones within the mrac nice trondhjemite suggest a synkinematic melt emplacement within a lower cambrian transtensional setting transtension is probably related to early stage rifting that introduced the separation of the tepla barrandian unit as part of armorica from gondwana structural and petrological data of the country rocks show that the barrovian type metamorphism and two deformation stages d1 with unknown kinematics and d2 top to the north shearing are older than the melt emplacement and thus can be attributed to the cadomian orogeny the intrusion depth of both plutons is nearly the same ca 7 km although the degree of barrovian type metamorphism differs significantly within the country rocks this suggests late cadomian eastward tilting of the metamorphic isograd planes the weak post plutonic lower greenschist to subgreenschist facies folding and thrusting result from variscan northwest southeast compression

composition of modern deep sea sands from arc related basins summary petrographic and chemical examination of modern deep sea sands shows a clear distinction between active and passive plate tectonic settings among active settings sands from forearc basins of island arcs and basins from strike slip continental margins can be distinguished but those from other arc related settings overlap considerably in composition in particular sands from continental margin subduction zones and those from the backarc of island arcs appear to be indistinguishable chemically arc related sands are very close in composition to ancient greywacke sandstones for instance sodium almost always exceeds potassium thus it is not necessary for sodium to be added to sands diagenetically to make greywackes as has been suggested the influence of tectonics on the composition of sandstones has long been a central feature of sandstone petrology the geosynclinal model of global tectonics stimulated much thought on this subject probably the best known ideas are those of krynine e g 1942 subsequently such research declined but the plate model for global tectonics has produced a new wave of interest e g crook 1974 schwab 1975 potter 1978 dickinson suczek 1979 much of this work has relied on data from ancient sediments where diagenetic changes and difficulty in de ciphering the tectonic setting limit the useful ness of the conclusions much more work is needed on recent sediments to establish a base line against which ancient sediments can be compared we have previously reported valloni maynard 1981 the results of our work on the petrography of a set of recent deep sea sands from piston cores in this paper we shall com pare this petrographic information with bulk chemistry and attempt to relate these to tecto nics in arc related settings

jurassic submarine arc apron deposits and associated magma wet sediment interaction northern sierra nevada california abstract jurassic metavolcanic rocks of the northern sierra terrane in northern california are part of an extensive triassic jurassic arc constructed along the western margin of north america in the english mountain area nevada and sierra counties a well exposed volcaniclastic sequence 3 6 km thick records jurassic island arc activity in a submarine environment in the lower part of the sequence thinly bedded andesitic volcanic sandstone turbidites and mudstones of the lower to middle jurassic sailor canyon formation were deposited below storm wave base in a long lived marine basin this unit is overlain conformably by the middle jurassic tuttle lake formation which records rapid accumulation of coarse grained calc alkaline volcanic debris in the proximal parts of a submarine apron around a major volcano that developed within the earlier sailor canyon basin the formation consists primarily of massively bedded matrix supported polymict basaltic to andesitic volcanic breccias deposited from submarine debris flows intercalated pillow hyaloclastite breccias formed during local extrusion of lavas on the seafloor numerous intrusions cut the sequence and are petrographically and chemically identical to clasts in the debris flow deposits peperite occurs along intrusion margins and as abundant isolated pockets indicating intrusion of magma into wet unconsolidated sediments elongate intrusive pods to 13 6 m in length are associated with smaller intrusive pillows and are inferred to represent a network of feeder tubes that extended from larger coherent intrusions and supplied magma to complex zones of magma wet sediment interaction at shallow levels beneath the seafloor syndepositional intrusive activity of this type including formation of abundant peperite probably is a characteristic feature of submarine volcaniclastic sequences developed in proximal island arc settings

magma pulses in the central variscan belt episodic melt generation and emplacement during lithospheric thinning magmatism in the central part of the variscan orogenic belt in europe has occurred in several distinct episodes each episode spanned a few million years and was characterized by a distinct geochemical signature and geodynamic setting i between 340 and 330 ma visean high k diorites and granitoids within a zone extending parallel the orogen axis possibly tracing a palaeosuture ii at 310 307 ma westphalian calc alkaline granitoids in the intra alpine domain iii at 304 295 ma stephanian subalkaline granitoids in intra alpine and pennine domains and finally iv at 280 270 ma permian granitoids in pennine austroalpine and south alpine domains the generation and emplacement of granitoid melts in the variscan crust are suggested to be short term pulses that are controlled by extensional processes in the lithosphere such as thermal erosion and relaxation of the lithospheric mantle and decompressional melting

sedimentary rocks and crustal evolution tectonic setting and secular trends a significant change in the composition of turbidites deposited at active tectonic settings is seen at the archean post archean transition in comparison to post archean active margin turbidites archean greenstone turbidites exhibit more uniform th sc ratios but with a similar average greater hree depletion gd n yb n 2 less eu depletion eu eu mostly 0 85 and an absence of low th u ratios th u 3 these data indicate that differing mantle sources and or conditions of mantle melting existed during the archean and that intracrustally differentiated rocks older cratons or young differentiated arcs were relatively unimportant as provenance components for archean turbidites in contrast some sedimentary rocks preserved in archean platformal sequences of high grade terranes display significant negative eu anomalies similar to the majority of post archean shales indicating that the general process of cratonization including intracrustal differentiation has taken place since at least 3

tectonics and plate tectonics model for the variscan belt of europe abstract a plate tectonics model is presented to explain the tectonometamorphic characteristics of the european variscides after the closing of two oceanic domains by two sided subduction 500 420 ma and obduction 420 380 ma collision of the european and african continental plates occurred we propose that the subsequent complex intracontinental deformation 380 290 ma is the result of a double subduction of the continental lithosphere accompanied by crust mantle decollement this mechanism explains the progressive crustal thickening and migration of the deformation through time from the sutures toward the external parts of the variscan belt accounting for this model and for the relationships between the european variscides and the other paleozoic peri atlantic belts caledonides appalachian mauritanides and morocco we infer the relative positions of africa america and europe between the silurian and the permian

the late variscan control on the location and asymmetry of the upper rhine graben the nne trending upper rhine graben urg of the european cenozoic rift system developed from c 47 ma onwards in response to changing lithospheric stresses in the northwestern foreland of the alps the composite graben structure consists of three segments each c 100 km long and 30 40 km wide but flares to c 60 km near its southern and to c 80 km near its northern termination normal faulting induced a total extension of 5 8 km of the 1 2 km thick mesozoic sedimentary franconian platform and underlying variscan basement rocks however distribution of an up to 3 5 km thick sedimentary graben fill and cumulative displacements near eastern and western main border fault systems suggest that subsidence of the graben floor and shoulder uplift created strong cross sectional asymmetries cumulative w down displacements 3 km along strongly segmented transfer faults in the east contrast with e down displacements 600 km long zone that delimits the trailing edge of a sw moving lithospheric block in the urg area ne sw oriented seismic anisotropy at sublithospheric depths of c 60 80 km suggest active mantle flow in this direction as a possible driving force for the reactivation of pre graben lithospheric shear zones

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

eclogites with a short lived granulite facies overprint in the moldanubian zone czech republic petrology geochemistry and diffusion modelling of garnet zoning the petrology and geochemistry of a newly discovered suite of high pressure garnet clinopyroxene bearing rocks from the monotonous series of the moldanubian zone of the bohemian massif southwest czech republic have been investigated three types common eclogites quartz kyanite clino zoisite eclogites and garnet hornblende clinopyroxenites are distinguished by petrography and geochemistry all underwent a significant degree of partial breakdown under granulite and amphibolite facies conditions during exhumation important features include the growth of orthopyroxene in breakdown domains after garnet and omphacite and anorthite spinel corundum exceedingly peraluminous sapphirine replacing kyanite garnet zoning and inclusion patterns support a prograde evolution from low pressures for at least some of the samples the post eclogite stage granulite facies overprint indicates that high temperatures prevailed during exhumation but preservation of zoning in some garnets and the results of diffusion modelling suggest that this overprint took place over a very short time scale the geochemical and petrological results allow characteristic differences to be recognized between these eclogites and metabasites found in other tectonic units of south bohemia and consequently the assigning of all high pressure rocks to a single now disrupted tectonic unit is a gross simplification that seriously misrepresents the tectono metamorphic history of the region

subsurface distribution and tectonic setting of the late variscan granites in the northwestern bohemian massif integrative gravity modelling reveals the 3d shape subsurface extent and basal morphology of the late variscan granites in the northwestern bohemian massif the combination of the results with structural and geochronological data enables the interpretation of the morphogenesis and the tectonic setting of the granite emplacement for the subsurface shaping two gravity methods were used the linsser filtering and the 2 5d density modelling the linsser filtering enables tomographic analyses of the gravity field in successive depth levels the 2 5d density models were computed by the gravmag program version 1 5 british geological survey which interprets gravity data vs geological cross sections at depth the granites show steep and flat sided wedge like plutons which are generally elongated in north northwest south southeast direction the maximum basal depths between 3 and 8 km occur along the root zones indicated by high gravity magnitudes the western and easternmost granites e g leuchtenberg bor are deeply eroded whereas the granites in the central part e g falkenberg rozvadov are greatly buried in the metamorphic host rocks in the northern oberpfalz the plutons show a west sided basal asymmmetry and in the western bohemia in contrast the plutons have an east sided basal asymmetry the steep pluton flanks parallel north northwest south southeast gravity gradients which result from the high density contrast of the granites and host rocks the density contrasts are caused by steep faults which enabled the emplacement of the granites the shape basal morphology depth and volume of the plutons presume a genetic link between magma intrusion and shear deformation however the orientation of the plutons 15 20 off the trend of the major faults and structural data suggest emplacement into opening fractures along initially transtensional faults owing to the lower to upper carboniferous extension in the northwestern bohemian massif

accretion of a rifted passive margin the late paleozoic rhenohercynian fold and thrust belt middle european variscides in the western rhenish massif the rhenohercynian fold and thrust belt of the middle european varsicides exposes a telescoped complete devonian to early carboniferous passive margin this permits the analysis of geometry and kinematic processes of passive margin accretion to an erogenic wedge during variscan collision 330 300 ma the sedimentary cover of the passive margin was shortened by some 50 or 180 km crustal scale balancing and restoration reveals a wide symmetric rift with a central graben a marginal plateau separated this failed lower devonian rift from an emsian middle devonian oceanic basin in the south remnants of which are preserved in the southernmost imbricates and the giessen harz nappes the seismically well imaged aachen midi detachment faille du midi acted as the basal decollement of this thin to thick skinned orogenic wedge it shows a ramp and flat geometry from the blind tip down to middle crustal levels owing to its position below the base of the basin fill the thick synrift sequence and structure controlled structural evolution during contraction by localizing thrust branch lines and by inversion of rift structures synthetic to the subduction direction with formation of basement footwall shortcuts moreover the three dimensional detachment geometry shows large scale oblique ramp flat features which control the architecture of the belt and the distribution of metamorphic grade rocks and fabrics from the detachment show that the latter was located at the transition into the ductile field at the fossil 300 400 c isotherm in the restored section the detachment trajectory displays a saucershaped geometry rising to the surface at the rear and at the front this suggests that the basal detachment propagated into the passive margin by ductile failure during lithospheric flexure under the load of an advancing upper plate the regional pattern of synkinematic metamorphic grade shows varying modes of margin accretion basal underplating into the orogenic wedge by the shelf slope and marginal plateau with formation of duplex stacks and elongate thrust sheets and frontal accretion of the remaining thicker failed rift in the north into the propagating wedge with formation of several imbricate fans cyclical changes in accretion history and or critical taper are suggested by spaced domains of out of sequence thickening these are also related to internal domainal kinematic partitioning of the wedge controlled by oblique collision

timing of magmatism foreland basin development metamorphism and inversion in the anglo brabant fold belt new data implying crustal activation of eastern avalonia along the anglo brabant fold belt are presented late ordovician subduction related magmatism in east anglia and the brabant massif coupled with accelerated subsidence in the anglia basin and in the brabant massif during silurian time indicate a foreland basin development final collision resulted in folding cleavage development and thrusting during the mid lochkovian to mid eifelian in the southeast of the anglo brabant fold belt acadian deformation produced basin inversion and the regional antiformal structure of the brabant massif the uplift inferred from the sedimentology petrography and reworked palynomorphs in the lower devonian of the dinant synclinorium is confirmed by illite crystallinity studies the tectonic model discussed implies the presence of two subduction zones in the eastern part of eastern avalonia one along the anglo brabant fold belt and another under the north sea in the prolongation of the north german polish caledonides

chronology of late paleozoic tectonothermal activity in the southeastern bohemian massif austria moldanubian and moravo silesian zones 40ar 39ar mineral age controls abstract new 40ar 39ar hornblende and muscovite ages from the moldanubian and moravian zones of the southeastern bohemian massif indicate rapid cooling associated with continental underplating during early carboniferous plate collision and closure of the moravo silesian foredeep 40ar 39ar ages have been determined for hornblende concentrates from 1 amphibolite from the moldanubian nappe complex including the dobra gneiss rehberg amphibolite and loosdorf complex all part of the variegated unit 2 micaschist structurally overlying the bittesch gneiss and 3 amphibolite within the bittesch gneiss moravian zone the five hornblende concentrates display variably discordant 40ar 39ar age spectra because of contamination with extraneous argon components however 36ar 40ar vs 39ar 40ar isotope correlations are well defined for the five hornblende concentrates mswd the hornblende and muscovite ages are interpreted to date post variscan metamorphic cooling through ca 500 c and ca 400 c respectively no record of pre variscan thermal events is preserved in either the hornblende or muscovite argon systems however the widespread occurrence of extraneous argon components suggest a pre variscan stage of tectonothermal development the near concordancy of the hornblende and muscovite cooling ages suggest relatively rapid cooling through the contrasting argon closure temperatures and indicate that relatively high crustal levels had likely been attained during the early carboniferous

provenance deposition and age of gneiss units from the ktb drill hole germany evidence from shrimp and conventional u pb zircon age determinations in order to investigate the provenance deposition age and metamorphism of metasedimentary gneisses from the pilot and the main holes of the german continental deep drilling program ktb zircons from volcano metasedimentary v unit and monotonous greywacke g unit gneisses were dated by ion microprobe shrimp and conventional u pb zircon methods the zircons were found to consist of anhedral detrital or subhedral cores surrounded by one or two metamorphic rims analysis of growth zones in 37 zircons indicated ages for the detrital cores between 2 25 and 2 05 ga in a v unit gneiss sample k52 and 1 8 2 35 ga in two g unit gneisses k747 and k798 neo proterozoic 650 545 ma subhedral igneous cores were identified in all investigated ktb gneiss samples and an additional two zircon growth episodes in the source area of the sediments at 712 15 and 525 11 ma were also recognised zircon overgrowths dated at 487 13 ma are interpreted to have formed during an early metamorphic episode following incorporation of the zircons into their host metasediments a second metamorphic overprint indicated by a date of 381 22 11 ma was determined for zircon overgrowth in the g unit gneisses the dates determined for zircon growth phases suggest that the ktb metasediments were derived from northern gondwana hypothetical provenance areas and a new model of terrane positions that is consistent with the geochronological geodynamic and palaeo geographic preconditions at the northern border of gondwana in late precambrian to early ordovician times are proposed

chapter 18 high k continental arc volcanism in the kettle rock sequence of the eastern mesozoic belt northern sierra nevada california implications for lower mesozoic cordilleran tectonics

high resolution observations of the small scale flow pattern associated with a poleward moving auroral form in the cusp 1 in this paper we present high resolution observations by the eiscat svalbard radar of the localized ionospheric flow response due to one single flux transfer event and the relative location of a poleward moving auroral form in a fast scan mode the radar was tracking a 50 60 km wide channel of westward flow in the cusp region this flow channel was surrounded by flow running in the opposite direction at the poleward edge of the narrow flow channel a poleward moving auroral form was situated consistent with the auroral form being the signature of an upward birkeland current filament our observations can be interpreted in terms of the southwood 1985 1987 flux transfer model and the fte twin cell flow pattern appears to be a ripple onto the larger scale background convection

rb sr and u pb isotope studies on migmatites from the schwarzwald germany constraints on isotopic resetting during variscan high temperature metamorphism mineral and isotope studies were undertaken on migmatites from the schwarzwald moldanubian zone of the variscan belt the aims of the study were to date the migmatite formation and to determine the processes involved in migmatization in order to evaluate their influence on isotopic resetting textural evidence and the comparison of mineral compositions from leucosomes and mesosomes of two centimetre scale migmatite profiles respectively suggest that migmatitic textures and mineral assemblages were formed by metamorphic segregation deformation enhanced mass transport rather than by partial melting anatexis the results of rb sr thin slab dating on these profiles indicate that sr isotopes were not completely reset during migmatization no true isochron ages but ages of approximate isotopic homogenization were obtained on the thin slabs by calculating 87sr 86sr ratios back to various stages in their evolution the coincidence of these rb sr data with u pb ages of monazites from migmatites and non migmatitic gneisses shows that gneisses and migmatites were formed during the same high temperature event in the carboniferous 330 335 ma the observation that high temperature metamorphism failed to equilibrate sr isotopes on the centimetre scale imposes limitations on the use of conventional whole rock isochron techniques in dating migmatites

hf radar signatures of the cusp and low latitude boundary layer continuous ground based observations of ionospheric and magnetospheric regions are critical to the geospace environment modeling gem program it is therefore important to establish clear intercalibrations between different ground based instruments and satellites in order to clearly place the ground based observations in context with the corresponding in situ satellite measurements hf radars operating at high latitudes are capable of observing very large spatial regions of the ionosphere on a nearly continuous basis in this paper we report on an intercalibration study made using the polar anglo american conjugate radar experiment radars located at goose bay labrador and halley station antarctica and the defense meteorological satellite program dmsp satellites the dmsp satellite data are used to provide clear identifications of the ionospheric cusp and the low latitude boundary layer llbl the radar data for eight cusp events and eight llbl events have been examined in order to determine a radar signature of these ionospheric regions this intercalibration indicates that the cusp is always characterized by wide complex doppler power spectra whereas the llbl is usually found to have spectra dominated by a single component the distribution of spectral widths in the cusp is of a generally gaussian form with a peak at about 220 m s the distribution of spectral widths in the llbl is more like an exponential distribution with the peak of the distribution occurring at about 50 m s there are a few cases in the llbl where the doppler power spectra are strikingly similar to those observed in the cusp

zircon ages and nd isotopic and chemical compositions of orthogneisses from the black forest germany evidence for a cambrian magmatic arc single zircon u pb dating combined with 207pb 206pb ages obtained by the evaporation method constrains the emplacement of tonalitic trondhjemitic and granodioritic orthogneisses of the moldanubian zone in the black forest between 500 and 510 ma two detrital zircon populations of 1 9 and 1 6 ga indicate early middle proterozoic material in the former setting of the basement the initial end values range from 0 1 to 3 4 and mean crustal residence ages of 1 0 1 4 ga are consistent with involvement of early middle proterozoic crust and a subordinate juvenile component probably originating from subduction related melting of the mantle the orthogneisses have fractionated ree patterns and slightly higher k2o na2o ratios than typical low k tonalite trondhjemite granite suites the chemical data are interpreted as evidence for melting of amphibolite and contributions from evolved crust the emplacement of the orthogneisses was superceded by a high temperature metamorphic event at 480 ma which we interpret as a result of lithospheric thinning in a marginal basin behind a cambrian magmatic arc

granitoid magmatism of the nw bohemian massif revealed gravity data composition age relations and phase concept the late variscan granitoids of the nw bohemian massif northeast bavaria west bohemia constitute four partly contiguous granitoid complexes fichtelgebirge northern oberpfalz waidhaus rozvadov and bor incorporating more than 20 intrusive units based on gravity data the granites can be modeled as steeply inclined slab and wedge like bodies with thicknesses between 2 and 8 km a rough estimate of the total volume of the granites is approximately 18 000 km3 within the four areas named above composition ranges from less evolved dioritic rocks known as the redwitzite suite to highly evolved granites the redwitzites comprise metaluminous rocks with dominant i type features these rocks yield aberrantly old rb sr ages 545 415 ma low initial sr ratios 0 706 0 708 and high and variable nd t values 1 to 4 sr nd isotopes of the redwitzites show contamination trends towards the granites suggesting mixing between mantle magma and crustal granitic melts an older plutonic association granites of bor leuchtenberg weissenstadt marktleuthen zainhammer is mildly peraluminous displaying features of both i and s type granitoids these granites are characterized by lower carboniferous ages rb sr k ar u pb low to intermediate initial sr ratios 0 707 0 708 and high nd t values 2 to 4 which overlap with those of paragneisses from the zone of erbendorf vohenstrauss zev and from the western part of the tepla barrandian it is postulated that the older granites were formed either by partial melting of zev or tepla barrandian crust or alternatively of preexisting mature crust contaminated by mantle material the younger granites are strongly peraluminous and of s type they yield upper carboniferous rb sr and k ar ages and exhibit a range towards high initial sr ratios 0 710 0 720 and low nd t values 4 to 8 similar values are found in moldanubian paragneisses and in saxothuringian metasediments both of which provide potential source rock lithologies for these granites

accretion and exhumation at a variscan active margin recorded in the saxothuringian flysch the saxothuringian flysch basin on the north flank of the central european variscides was fed and eventually overthrust by the northwestern active margin of the tepla barrandian terrane clast spectra mineral composition and isotopic ages of detrital mica and zircon have been analyzed in order to constrain accretion and exhumation of rocks in the orogenic wedge the earliest clastic sediments preserved are of early famennian age ca 370 ma they are exposed immediately to the nw of the suture and belong to the par autochthon of the foreland besides ultramafic ophiolite material these rocks contain clasts derived from early paleozoic continental slope sediments originally deposited at the nw margin of the saxothuringian basin these findings together with the paleogeographic position of the famennian clastics debris on the northwestern passive margin indicate that the saxothuringian narrow ocean had been closed by that time microprobe analyses of detrital hornblendes suggest derivation from the randamphibolit unit now present in the middle part of the saxothuringian allochthon munchberg nappes detrital zircons of metamorphic rocks formed a little earlier ca 380 ma indicate rapid recycling at the tectonic front the middle part of the flysch sequence ca early to middle visean both in the par autochthon and in the allochthon contains abundant clasts of paleozoic rocks derived from the northwestern slope and rise together with debris of cadomian basement 500 ma granitoids and 380 ma early variscan crystalline rocks all of these source rocks were still available in the youngest part of the flysch c middle to late visean but some clasts record in addition accretion of the northwestern shelf our findings permit deduction of minimum rates of tectonic shortening well in excess of 10 30 mm per year and rates of exhumation of ca 3 mm a and possibly more

variscan vs cadomian tectonothermal activity in northwestern sectors of the tepla barrandian zone czech republic constraints from 40ar 39ar ages five muscovite concentrates from high grade pelitic metasedimentary basement rocks exposed in northwestern sectors of the tepla barrandian zone czech republic record 40ar 39ar mineral plateau ages which range between ca 376 and 362 ma hornblende concentrates from metagabbro marianske lanz complex and fine grained basement amphibolite display plateaux which define 36ar 40ar vs 39ar 40ar isotope correlation ages of ca 370 and ca 375 ma the mineral ages are interpreted to date relatively rapid cooling through appropriate argon retention temperatures following early phases of variscan early devonian regional metamorphism a slate phyllite basement sample collected within lower grade metasedimentary rocks in southeastern portions of the tepla barrandian zone is characterized by an internally discordant 40ar 39ar whole rock age spectrum which suggests partial variscan rejuvenation of intracrystalline argon systems which had cooled through appropriate argon retention temperatures following an initial regional metamorphism at or prior to ca 500 ma cadomian hornblende from undeformed diorite of the kdyn complex records a well defined 40ar 39ar age plateau which corresponds to an isotope correlation age of ca 516 ma this is interpreted to date post magmatic cooling following emplacement

oblique crustal detachment in the variscan schwarzwald southwestern germany the schwarzwald is part of the central polymetamorphic crystalline belt of the variscan orogen moldanubian belt from north to south it consists of four terranes the metasedimentary zone of baden baden the polymetamorphic central schwarzwald gneiss complex the sedimentary metamorphic zone of badenweiler lenzkirch and the hotzenwald complex the largest of these terranes is the central schwarzwald gneiss complex csgc whose rocks record a history of protracted regional metamorphism and anatectic melt generation during variscan convergence between 350 and 325 ma the csgc became detached from a high temperature lower crustal substratum and was emplaced southeastward over paleozoic clastics volcanic rocks and crystalline slivers of the zone of badenweiler lenzkirch and the hotzenwald complex kinematic indicators suggest that these early convergent movements on retrograde shear zones and the concomitant crustal thickening were superseded by movements on divergent shear zones the ascent of voluminous granitic plutons from a mid crustal zone of melt generation into the upper crust was probably triggered by a change in the crustal kinematics from overall convergence to overall divergence at about 325 ma in detail this process was probably diachronous detachment of upper crust and large scale melt generation in the middle crust of the schwarzwald was probably facilitated by the tectonic stacking of water rich pelitic clastics and gneiss slivers with relatively even proportions of crystalline and pelitic materials

microstructural evolution and geologic significance of garnet pyriclasites in the hoher bogen shear zone bohemian massif germany ductile extensional movements along the steeply inclined hoher bogen shear zone caused the juxtaposition of tepla barrandian amphibolites granulites and metaperidotites against moldanubian mica schists and paragneisses garnet pyriclasites are well preserved within low strain domains of this shear zone their degree of metamorphism is significantly higher than that of the surrounding rocks microstructural and mineral chemical data suggest in situ formation of the garnet pyriclasite by dehydration of pyroxene amphibolite at t 750 840 c and p 10 13 kbar including recrystallization accommodated grain size reduction of plagioclase and clinopyroxene nucleation of garnet and breakdown of amphibole into garnet clinopyroxene rutile subsequent decompression and retrograde extensional shearing led to the formation of mylonitic epidote amphibolite the presence of lower crustal and mantle derived slices within the hoher bogen shear zone supports the view that a in upper devonian times the tepla barrandian unit was thrust over moldanubian rocks as a complete crustal unit and b that during the subsequent lower carboniferous orogenic collapse the garnet pyriclasite and metaperidotite were scraped off from the basal parts of the tepla barrandian unit being dragged into the hoher bogen shear zone due to dramatic and large scale elevator style movements

time calibration of a p t path from a variscan high temperature low pressure metamorphic complex bayerische wald germany and the detection of inherited monazite a temperature time path was constructed for high temperature low pressure ht lp migmatites of the bayerische wald internal zone of the variscan belt germany the migmatites are characterised by prograde biotite dehydration melting peak metamorphic conditions of approximately 850 c and 0 5 0 7 gpa and retrograde melt crystallisation at 800 c the time calibration of the pressure temperature path is based on u pb dating of single zircon and monazite grains and titanite separates on 40ar 39ar ages obtained by incremental heating experiments on hornblende separates single grains of biotite and k feldspar and on 40ar 39ar spot fusion ages of biotite determined in situ from sample sections additionally crude estimates of the duration of peak metamorphism were derived from garnet zoning patterns suggesting that peak temperatures of 850 c cannot have prevailed much longer than 2 5 ma the temperature time paths obtained for two areas approximately 30 km apart do not differ from each other considerably u pb zircon ages reflect crystallisation from melt at 850 800 c at 323 ma southeastern area and 326 ma northwestern area the u pb ages of monazite mainly coincide with those from zircon but are complicated by variable degrees of inheritance the preservation of inherited monazite and the presence of excess 206pb resulting from the incorporation of excess 230th in monazite formed during ht lp metamorphism suggest that monazite ages in the migmatites of the bayerische wald reflect crystallisation from melt at 850 800 c and persistence of older grains at these temperatures during a comparatively short thermal peak the u pb ages of titanite 321 ma and 40ar 39ar ages of hornblende 322 316 ma and biotite 313 309 ma reflect cooling through the respective closure temperatures of approximately 700 570 500 and 345 310 c published in the literature most of the feldspars ages 305 296 ma probably record cooling below 150 300 c while two grains most likely have higher closure temperatures the temperature time paths are characterised by a short thermal peak by moderate average cooling rates and by a decrease in cooling rates from 100 c my at temperatures between 850 800 and 700 c to 11 16 c my at temperatures down to 345 310 c further cooling to feldspar closure for ar was probably even slower the lack of decompressional features the moderate average cooling rates and the decline of cooling rates with time are not easily reconciled with a model of asthenospheric heating rapid uplift and extension due to lithospheric delamination as proposed elsewhere instead the high peak temperatures at comparatively shallow crustal levels along with the short thermal peak require external advective heating by hot mafic or ultramafic material

the response of the hf radar spectral width boundary to a switch in the imf by direction ionospheric consequences of transient dayside reconnection in the high latitude dayside ionosphere the movement of the hf radar spectral width boundary swb provides a good proxy for the movement of the open closed field line boundary around magnetic local noon by studying the dynamics of the spectral width boundary we can investigate features of the dayside ionospheric response to changes in the interplanetary magnetic field imf the high temporal and spatial resolution of the super dual auroral radar network superdarn hf radars make them good tools to study these features in this paper we use the halley hf radar in antarctica to study the equatorward motion of the swb which appears to occur in response to a large change in the direction of imf by the spectral width boundary initially moves equatorward in the form of a u shaped bulge close to magnetic local noon this bulge then expands longitudinally to earlier and later magnetic local times merged velocity vectors from two antarctic hf radars describe the flow velocity variation in the boundary region the flow equatorward of the boundary follows the contours of the boundary as it expands the flow poleward of the boundary is directed at more oblique angles to the boundary this study represents the first clear two dimensional observation of the formation of an equatorward bulge on the polar cap boundary which may be associated with changes in dayside reconnection and presents a unique observation of the variation of the ionospheric flow in the locality of the boundary we discuss the possible interpretations of this event and its consequences to our present understanding of the ionospheric response to changes in the imf

first in situ measurements of hf radar echoing targets 1 the sounding rocket investigation of cusp irregularities 2 ici 2 was launched into the cusp ionosphere over svalbard to investigate the production of decameter scale irregularities in the electron plasma associated with hf radar backscatter the main mission objective was to obtain high resolution measurements of decameter scale electron plasma irregularities and to quantify the growth rate for the gradient drift instability gdi at the 5 7 khz sampling rate of the absolute density measurements ici 2 has provided the first documentation in terms of absolute electron density measurements of how 10 m structures are located on km scale electron density gradients ici 2 traversed a cusp electron density structure created by ongoing soft precipitation 10 m scale irregularities were generated at km scale density gradients the estimated growth time for the gdi process was 10 50 seconds

u pb zircon and sm nd dating of moldanubian hp ht granulites from south ohemia czech republic conventional multigrain u pb systematics of zircon and single grain evaporation data of zircons in combination with nd model ages are presented for pyroxene free felsic garnetiferous granulites and orthopyroxene bearing granulites of felsic mafic and intermediate composition from the blansky les mountains granulite complex s bohemia zircons from mafic and intermediate granulites yield concordia intercept ages between 346 5 and 351 6 ma as well as a single grain evaporation age of 346 12 ma these ages define the time of variscan high grade metamorphism in s bohemia and are supported by an internal sm nd garnet zircon plagioclase whole rock isochron age of 343 21 ma euhedral zircons from felsic granulites of probable granitic derivation yield u pb ages of 366 5 ma and 373 12 ma interpreted as intrusion ages of the granitic precursor an inherited proterozoic crustal component is documented by poorly defined concordia upper intercept ages around 1700 ma and nd model ages between 1 3 and 2 1 ga

crustal scale balanced cross sections through the variscan fold belt germany the central egt segment abstract this paper presents crustal scale geometrically and isostatically balanced cross sections through the rhenohercynian and saxothuringian zones of the variscan fold belt of germany the cross sections are based on surface geological data and on results of the dekorp deep reflection profiles they were constructed by means of computer assisted iterative forward modelling the rhenohercynian zone corresponding to the externides of the variscan belt developed out of a some 300 km wide rifted continental margin sedimentary prism limited to the south by the narrow giessen oceanic basin during carboniferous imbrication this shelf sequence is stripped of its basement above one or more decollements and shortened by some 150 km closure of the giessen ocean and emplacement of the ophiolite bearing giessen nappe entailed a minimum additional 200 km of crustal shortening the structure of the model correlates well with the present outcrop pattern of rock units their metamorphic history the seismic and magnetotelluric architecture of the crust and its present regional distribution of thickness the deeper crust beneath the rhenish massif is probably devoid of extremely weak decollement horizons and pronounced rheological stratification in the southward adjacent saxothuringian zone nappe development involving the stacking of crustal slices during middle devonian to visean times accounts for a further 170 km of crustal shortening above an infracrustal partly blind thrust system with flat ramp geometry total crustal shortening in the analyzed profiles is estimated to be of the order of 520 km this figure is a conservative value prominent uncertainties include the unknown importance of crustal wrenching the state of strain within thrust units the distribution of strain within the lower crust and major changes in thrusting directions during the course of orogenic history

correlation of the rhenohercynian variscides recent structural and stratigraphical data from the rhenohercynian variscides of belgium germany and south cornwall indicate tectonic continuity within this zone prior to disruption along a late carboniferous dextral transcurrent fault this extends from the bristol channel to the vosges parallel to similar lineaments of variscan age in europe and north america consequently a tectonic model for south cornwall is extrapolated to europe and a plate tectonic setting for the evolution of the north european variscides by the closure of an oceanic basin is discussed

transformation of a magmatic arc and an orogenic root during oblique collision and it s consequences for the evolution of the european variscides mid german crystalline rise the architecture of the european variscides has been subdivided by kossmat 1927 into paleogeographically coherent units which are presently interpreted as former plate fragments the mid german crystalline rise mgcr separates two fragments rhenohercynian and saxothuringian belts at the site of an inferred plate boundary and reequilibrated orogenic root the commonly favoured model interprets the mgcr as the magmatic arc on saxothuringian crust above a south dipping subduction zone in upper devonian and carboniferous times data from the mgcr the kinematic evolution of the mid european variscides and first order volume balancing suggest a reinterpretation of this unit which challenges classical views on the mgcr as well as on the subdivision of variscan architecture the mgcr is composed of two rock groups with different tectonic identity a lower carboniferous low pressure high temperature magmatic arc association on lower paleozoic basement rests tectonically on a stack of medium pressuremedium temperature rocks of inferred rhenohercynian origin the latter were tectonically accreted to the base of the overriding plate by tectonic underplating the entire process was controlled by oblique convergence this led to regional partitioning of the plate kinematic vector into contractional domains lower rhenohercynian plate and back arc area of the upper saxothuringian plate bulk heterogeneous plate margin parallel extensional domains mgcr and plate margin parallel wrench domains mgcr boundaries during this process material was continually transferred from the lower plate to the upper plate uplifted and exhumed by net crustal extension the concomitant removal of parts of the former arc and the entire orogenic root necessitates a reappraisal of variscan architecture and evolution

the baltica gondwana suture in central europe evidence from k ar ages of detrital muscovites and biogeographical data

from cadomian subduction to early palaeozoic rifting the evolution of saxo thuringia at the margin of gondwana in the light of single zircon geochronology and basin development central european variscides germany

from jurassic shores to cretaceous plutons geochemical evidence for paleoalteration environments of metavolcanic rocks eastern california volcanic and plutonic rocks exposed in east central california record a long history of metasomatism and or metamorphism within the mesozoic cordilleran continental arc we use whole rock and mineral elemental compositions along with standard and cathodoluminescence petrography to characterize alteration histories of late triassic to middle jurassic metavolcanic rocks in the ritter range white inyo mountains and alabama hills although alkali metasomatism is widespread and pervasive ratios and abundances of ce th tb and ta suggest that mafic protoliths from the white inyo mountains were shoshonitic whereas those from the ritter range were calc alkaline alkali exchange apparently modified the compositions of many metavolcanic rocks much of this metasomatism may have occurred at low temperature t conditions and attended or shortly postdated deposition of the volcanic protoliths high 18 o values for k rich metatuffs from

silurian paleogeography of armorica new paleomagnetic data from central bohemia due to the lack of reliable paleomagnetic data from armorica for mid ordovician to late devonian times the drift history for this tectonic element is poorly understood in order to help rectify this problem a paleomagnetic study of upper silurian volcanic intrusive and sedimentary rocks from the barrandian basin of the central bohemian massif czech republic has been carried out this basin comprises a sequence of unmetamorphosed lower paleozoic rocks the folding of which has an uppermost age limit of early carboniferous stepwise thermal demagnetization of the samples collected 182 samples from 21 sites and principal component analysis of both great circle and stable endpoint data reveal a total of three directions of magnetization the first labelled a is in the direction of the present day local earth s magnetic field d i 360 67 and is thus interpreted as being of recent origin the other two directions termed b and c comprise the intermediate and high blocking temperature directions respectively the overall in situ mean direction obtained for b identified between 150 and 320 c is 195 8 d i k 18 5 95 13 2 eight sites it fails the fold test and yields an in situ paleo south pole position of 42 s 354 e this coincides with the late carboniferous segment of the apparent polar wander apw path for stable europe and is considered to be a secondary overprint of this age the third magnetization termed c is identified between 320 and 600 c gives an overall mean direction of 208 40 k 96 8 95 5 7 eight sites after bedding correction it passes fold and reversal tests on both local and regional levels and is interpreted as representative of the late silurian paleofield direction in the bohemian massif it yields a paleo pole position of 55 s 325 e assuming a normal polarity the inclination value obtained translates into paleolatitudes of 23 s for this part of armorica and implies a position adjacent to the southern margin of baltica this therefore indicates closure of the intervening ocean by late silurian times however adopting this polarity also implies large scale up to 140 anticlockwise rotations of the bohemian massif prior to deformation in the early carboniferous

thermobarometry diffusion modelling and cooling rates of crustal garnet peridotites two examples from the moldanubian zone of the bohemian massif abstract different origins for crustal garnet peridotites are illustrated by two variscan bodies in adjacent tectonically juxtaposed terranes in the moldanubicum of the bohemian massif eastern moravia the nove dvory garnet peridotite contains lenses of eclogite and is allochthonous and allofacial with respect to surrounding gfohl gneiss the mohelno spinel garnet peridotite is devoid of eclogite and has concordant contacts with surrounding felsic granulite the bulk of the mohelno body is spinel peridotite and garnet occurs only along its margins where it has formed at the expense of spinel thermobarometry of mineral cores from garnet bearing assemblages yields values for nove dvory from 1195 c 54 6 kbar to 970 c 41 8 kbar and for mohelno from 1265 c 27 9 kbar to 1070 c 23 4 kbar the compositional zoning profiles in garnets at both localities are matched closely by diffusion modelling in which bilinear cooling rates are utilized at nove dvory initial cooling rates of 50 200 c ma are followed by slower rates of 10 50 c ma in contrast at mohelno an extremely rapid initial cooling rate of 1 c year was followed by one of 500 c ma an idealized tectonic model is proposed for the emplacement of the mohelno peridotite to account for its cooling history the predicted garnet zoning profiles that result from conductive cooling agree closely with the observed zoning profiles both peridotites were derived from the upper mantle the nove dvory body was emplaced from a deep fault zone into the crust as garnet peridotite and the mohelno body was emplaced as a hot slab of spinel peridotite in which garnet formed only along contacts as the body cooled into the garnet stability field

faunal evidence for oceanic separations in the palaeozoic of britain the faunal criteria for the recognition of the relative separation of old continents are 1 the contrast between shallow not deep water faunas off opposite shorelines 2 the recognition of deeper water facies surrounding the edges of continents and 3 the recognition of the disposition of ancient climatic belts these criteria are defined and used to assess the relative position of the various parts of what is now britain in the palaeozoic the iapetus ocean is well characterized by faunal differences from the cambrian until the lower ordovician but from caradoc times onwards these differences dwindle until by the latest silurian only the distribution of the ostracods separates the two sides of the ocean on faunal grounds in the early ordovician s british faunas are comparable with those from bohemia france and elsewhere to the s indicating connection with gondwanaland and these faunas differ from those in the baltic area a distribution which we attribute to a true ocean tornquist9s sea between n and s europe tornquist9s sea probably closed in the upper ordovician and from ashgill times onwards the s british and baltic faunas remained similar during the middle and late silurian there is evidence of increasing faunal distinction between britain the baltic area and north america on the one hand and southern europe and gondwanaland on the other indicating an opening rheic ocean which appears to have persisted until late carboniferous times

crustal scale thrusting and extension in the hercynian schwarzwald and vosges central europe the hercynian outcrop areas in the schwarzwald of sw germany and vosges of easternmost france consist from north to south of three major lithotectonic complexes very low to medium grade metamorphics of the saxothuringian belt the polymetamorphic central gneiss complex and the tectonic melange of the southern complex juxtaposition of these three complexes occurred by oblique convergent motions in early carboniferous time the saxothuringian belt is part of a major se dipping accretionary wedge that consists of lower paleozoic meta sedimentary and metaigneous rocks and superimposed upper devonian to lower carboniferous synorogenic volcanics and elastics the central gneiss complex consists of polymetamorphic paragneisses orthogneisses metabasites and meta ultramafics dipping to the nw the southern complex consists of an imbricated melange of north to nw dipping synorogenic elastics volcanics and blocks of ultramafics and gneissic basement the central gneiss complex cgc experienced pervasive hercynian low pressure high temperature metamorphisrn before it was emplaced against and thrust over the sedimentary and volcanic fill of the adjacent synorogenic basins this convergence 340 330 ma produced mylonitic cataclastic shear zones in the cgc the most significant shear zones are the se verging todtnau fault zone between the cgc and the southern complex and the nw verging lubine fault zone between the central gneiss complex and the rocks of the saxothuringian belt these two fault zones enclose a bivergent pop up of the polymetamorphic basement the todtnau fault zone probably extends for more than 100 km along strike and can be traced to a depth of 10 km by deep seismic reflection studies detachment probably occurred along a mid crustal zone of anatexis expressed by a domain of low seismic reflectivity between about 10 and 15 km crustal thickening during early carboniferous convergence led to an extensional phase of crustal reordering between about 330 and 280 ma that was accompanied by the massive rise of posttectonic granitic plutons from middle to high crustal levels simultaneous creation of a structural lamination in the refractory lower crust neglected the main convergent tectonic boundaries it produced a basin and range type lower crust and mantle crust transition in early permian time and involved a major advective input of heat

simultaneous ground based optical and hf radar observations of the ionospheric footprint of the open closed field line boundary along the geomagnetic meridian previous studies have confirmed that the equatorward boundaries of oi 630 0 nm auroral emissions and broad doppler spectral widths in super dual auroral radar network superdarn data the so called spectral width boundary swb are good empirical proxies for the dayside open closed field line boundary ocb in the ionosphere however both observational techniques are associated with mapping errors superdarn uses a virtual height model for mapping but it is not well known how the mapping error responds to a changing background ionosphere or transient reconnection events optical instruments such as the meridian scanning photometers have high spatial resolution near zenith where the mapping error due to the assumed oi 630 0 nm auroral emission height becomes small by comparison in this work an adjusted method is introduced to identify the swb which does not require temporal smoothing across several scans the difference in latitude between the swb as identified using this method and the simultaneously observed oi 630 0 nm auroral emission boundary along a common line of sight is compared utilizing the oi 630 0 nm boundary as a reference location we present two case studies observed at different levels of solar activity in both instances the latitude offset of swb from the reference location is discussed in relation to the background ionospheric electron density the compared results indicate that the intake of high density solar extreme ultraviolet ionized plasma from subauroral latitudes causes a refraction of the hf radar beam path which results in an overestimation of range mapping the adjusted method would thus be a useful tool for identifying the ocb under changing ionospheric conditions in the cusp region

large scale imaging of high latitude convection with super dual auroral radar network hf radar observations the hf radars of the super dual auroral radar network superdarn provide measurements of the e b drift of ionospheric plasma over extended regions of the high latitude ionosphere with the recent augmentation of the northern hemisphere component to six radars a sizable fraction of the entire convection zone approximately one third can be imaged nearly instantaneously 2 min to date the two dimensional convection velocity has been mapped by combining line of sight velocity measurements obtained from pairs of radars within common volume areas we describe a new method of deriving large scale convection maps based on all the available velocity data the measurements are used to determine a solution for the distribution of electrostatic potential expressed as a series expansion in spherical harmonics the addition of data from a statistical model constrains the solution in regions of no data coverage for low order expansions the results provide a gross characterization of the global convection we discuss the processing of the radar velocity data the factors that condition the fitting and the reliability of the results we present examples of imaging that demonstrate the response of the global convection to variations in the interplanetary magnetic field imf in the case of a sudden polarity change from northward to southward imf the convection is seen to reconfigure globally on very short 6 min timescales

rb sr and k ar mineral data of the ktb and the surrounding area and their bearing on the tectonothermal evolution of the metamorphic basement rocks k ar and rb sr age determinations on muscovites and biotites and k ar age determinations on amphiboles are presented for leucocratic gneisses amphibolites and meta igneous rocks from the zone of erbendorf vohenstrauss zev and the ktb boreholes located in the northeastern zev and leucocratic gneisses from the erbendorf greenschist zone egz the investigations were carried out to contribute to the knowledge of the tectonothermal evolution of the area and to the response of the isotope systems to retrograde processes including recent elevated in situ temperatures in the boreholes a memory of an early ordovician igneous event is given by rb sr ages of 480 5 475 5 ma obtained on very coarse grained relic igneous muscovites of metapegmatites from the zev this memory is missing in micas from peg matoids of the drill site coarse grained muscovites of gneisses from zev and egz surface outcrops and of gneisses and pegmatoids from the boreholes yielded maximum k ar ages of 377 3 371 3 ma related to the end of the devonian regional metamorphism in both units consistent k ar and rb sr apparent ages of some muscovites from surface outcrops point to a fast cooling of the metamorphic rocks due to rapid tectonic uplift the lack of a depth related gradient in model ages of coarse grained ktb muscovites is explained by post variscan stacking processes for minerals from intermediate to greater depths of the ktb a strong age dependence on grain size and disturbances of the isotope systems due to various late to post variscan retrograde processes can be demonstrated a strong decrease in k ar model ages with increasing depth as obtained for biotites from the deepest section of the hb borehole probably indicates time integrated response of the isotope system to elevated in situ temperatures

k ar geochronology of different tectonic units at the northwestern margin of the bohemian massif abstract the polymetamorphic moldanubian mo of the northeasthern margin of the bohemian massif has been thrust to the north onto the mainly paleozoic sedimentary saxothuringian of the fichtelgebirge fg these two units have undergone polyphase deformation and the last regional event to affect both units was a low pressure metamorphism in which temperatures decreased towards the north in contrast the nappe units of the erbendorf vohenstrauss zone zev and the erbendorf greenschist zone egz which partly cover the border of the moldanubian and the saxothuringian and the munchberg nappe pile mm which lies on the saxothuringian were in parts subjected to a late medium pressure metamorphic event the zev the egz the mo and the fg are intruded by late carboniferous granites conventional k ar analyses mainly of hornblendes and muscovites from the autochthonous fg and mo the units beneath the nappes have yielded exclusively carboniferous dates the oldest dates point to a regional cooling of the rocks which outcrop at the present day surface at about 330 320 ma i e at the early late carboniferous boundary the late carboniferous cooling history was largely governed by the thermal influence of the post kinematic granites 320 295 ma especially in the fg and the northern mo the high grade metamorphic rocks in the western part of the zev and in the upper three nappes of the mm mostly yield dates around 380 ma i e early devonian the results show a relatively wide scatter moreover biotites frequently appear to be older than the coexisting muscovites both observations indicate that the rocks underwent a later thermal influence whether some groups of older dates e g 400 ma are due to excess argon or to inherited argon is still open to discussion slightly scattered muscovite dates around 366 ma were obtained for the prasinite phyllite series one of the lower nappes of the mm a single hornblende from the egz gave the same age these two nappes have therefore probably been affected by a late devonian thermal and or tectonic event the muscovite dates obtained from the paleozoic bavarian lithofacies the lowermost nappe of the mm and the hornblende dates from the eastern part of the zev are indistinguishable from those of the autochthonous units fg and mo

evidence for precollisional subduction erosion in ancient collisional belts the case of the mid european variscides analysis of the fore arc remnants and suture zone rocks in the european variscides using criteria established for the circum pacific margins geometry sedimentary evolution and style of deformation shows that precollisonal orogenic evolution can also be reconstructed for ancient collisional belts the collision zone of the lower plate thrust belt rhenohercynian zone and the upper plate arc and fore arc remnants saxothuringian zone and saar basin indicate subduction erosion of the upper plate by some 50 km length of upper plate lithosphere prior to collision removal started during early convergence in the famennian ca 365 ma and lasted until the late visean onset of collision ca 330 ma part of this material may have been underplated below the upper plate and has since resurfaced as high pressure rocks contributing to the metamorphic core of the orogen

migmatization and secondary granitic magmas effects of emplacement and crystallization of primary granitoids in southern bohemia austria the sauwald and muhl zones of the prebatholithic moldanubian middle crust in northern austria contain metapelites and metaluminous to weakly peraluminous metagreywackes respectively both zones were affected by low pressure high temperature metamorphism and anatexis the metapelites of the sauwald zone became in situ diatexites probably by fluid absent reactions involving the breakdown of muscovite and the partial breakdown of biotite the biotite plagioclase quartz gneisses of the muhl zone experienced only slight melting following this event and while the mid crust was still hot additional heat was locally advected into the muhl zone by the intrusion of the weinsberg granite this brought about fluid present partial melting of the biotite plagioclasequartz gneisses producing relatively large volumes of metaluminous to weakly peraluminous i type schlieren granite this cool wet restite rich magma remained close to its site of generation thus infracrustal i type granitoids may be formed anywhere in the crust and not always at high t under special circumstances the heat and fluids from granitic magmas can spawn secondary granites also relatively low initial 87sr 86sr values of around 0 707 in i type rocks do not necessarily indicate either lower crustal magma sources or mixing with mantle derived magma the weinsberg granite magma came from the lower crust p probably 700 mpa where widespread fluid absent breakdown of biotite plagioclase quartz assemblages occurred the necessary high heat flow was probably provided by newly underplated mafic magmas however these seem not to have mixed or mingled with the crustally derived weinsberg magmas deep equivalents of the muhl zone metagreywackes may have formed the weinsberg protolith fluid absent experiments show that the melting temperature probably exceeded 850 c and that a garnet bearing orthopyroxene rich residue should be present in the lower crust fluid present experiments demonstrate that the availability of free h2o can radically alter the characteristics of the partial melts from apparent s type mineralogy with fluid absent melting to i type mineralogy with wet melting

contraints on the evolution of the mid german crystalline rise a study of outcrops west of the river rhine several small outcrops along the western rhinegraben escarpment expose rocks which represent the western prolongation of the so called mid german crystalline rise this basement ridge separates the rhenohercynian and saxothuringian zones of the variscan belt of europe and thus marks the boundary between the external and the internal zones the variable rock association includes an orthogneissamphibolite complex weakly deformed low grade sediments devonian and visean and a number of different syn to post orogenic granodioritic to granitic intrusives all crosscut by late lower carboniferous undeformed lamprophyric dikes and unconformable overlain by permian sediments and volcanics

geochemistry and provenance of devono carboniferous volcano sedimentary sequences from the southern vosges basin and the geodynamic implications for the western moldanubian zone

precise upb chronometry of 345 340 ma old magmatism related to syn convergence extension in the southern vosges central variscan belt abstract the southern vosges host a large volcano sedimentary basin which contains clastic sediments of famennian late devonian ca 365 ma to late visean ca 340 335 ma age the basin is bordered by an exhumed high grade basement complex and intruded by several granitoid plutons a chronology for basin evolution and magmatism was established dating volcanic rocks of the basin as well as cross cutting granitoid rocks by the u pb technique applied to zircon and titanite the basin evolution started with olistolithic deposits and overlying marine shales that have a famennian fauna ca 365 ma and contain detrital zircons with an age of 386 ma the lowermost rhyolite flows interlayered with the basin sediments and tholeiitic basalts yielded an age of 345 2 ma following a tectonic event at the bottom of the upper visean the rates of sedimentation and volcanism were strongly enhanced leading to basin emersion and were concluded by the extrusion of the uppermost ignimbrite molkenrain rhyolite emplaced at 340 2 ma the whole volcano sedimentary association was subsequently intruded by melts of the ballons and cre tes granites as well as by small monzodioritic and dioritic satellite intrusions along the border of the ballons granite within 3 2 myr 342 1 to 339 5 2 5 ma the zircons of most of the volcanic and plutonic rocks investigated contain inherited lead of 0 6 and 2 1 ga age the differences in initial nd isotopic composition of the ballons and cre tes granites can be explained by differential uptake of crustal material during magma genesis the precise age data prove that magmatism and basin formation in the southern vosges were extremely short lived and coeval to the exhumation of adjacent high grade gneiss terrains revealing an episode of extension at the end of the lower carboniferous between 345 and 340 ma visean the tectonic regime in the internal part of the variscan orogen is that of extension and strike slip while convergent tectonics were still ongoing in the external part of the variscan orogen