the development of the central european continental crust since the early archaean based on conventional and ion microprobe dating of up to 3 84 b y old detrital zircons abstract archaean components up to 3 84 b y old have been detected by ion microprobe dating of zircon from various sediments and metasediments of the european hercynides moldanubian of northeastern bavaria and the massif central the early archaean age of 3 84 b y was obtained within the euhedral core of a detrital grain from a moldanubian paragneiss the same detrital zircon grain reveals at least three metamorphic overprints at 2 59 b y b p 1 94 b y b p and 460 m y b p and at least two cycles of mechanical abrasion during surface recycling in the early proterozoic and after the pan african cycle it is thus the crystal with the most complex geological history ever detected further archaean ages for the crystallization of primary magmatic zircons probably in the course of continental crust formation in the first cycle provenances of the two studied areas were found at 3 15 2 9 2 76 and 2 65 b y b p and in a few cases between 2 6 and 2 5 b y b p early proterozoic crust forming events between about 1 75 and 2 15 b y b p play an important role especially for the sediments and metasediments of the montagne noire at the southern rim of the massif central middle and late proterozoic ages cluster around 1 0 b y b p and 600 m y b p for all analysed samples therefore the deposition of the metasedimentary precursors took place after the pan african cycle which is in line with the rb sr whole rock systematics of central european metasediments the ages of metamorphism in the different source areas could be inferred in some cases by lower intercept ages obtained on single detrital grains the following sequence coinciding with times of magmatic events has been so far established 2 59 1 94 and 1 0 b y b p and 600 m y b p thus ion probe dating is also capable of defining secondary effects in the respective provenances which are typical for orogenic cycles the ubiquitous presence of pan african detrital zircons in european metasediments is a strong indication that the detrital material was derived from gondwana however from the presence of metamorphic units including eclogitefacies rocks which probably formed at different continental margins and at various times between about 500 m y and 330 m y ago it is probable that various gondwana derived microcontinental plates contributed to the different basin fillings the gondwana derived detritus is also supported by the observation of two periods of relative quiescence or non activity around 2 3 b y ago and especially around 1 5 b y ago which is a typical feature observed in gondwana cahen et al 1984 but not in laurasia the other possible source of the detritus the lack of zircons crystallized between 3 84 and 3 15 b y ago might have different causes thus the continental crust of the european hercynides probably developed since the early archaean and grew via many crustal accretion events to its present state

ion microprobe ages of zircons from early archaean granite pebbles and greywacke barberton greenstone belt southern africa abstract we report single grain and grain domain u pb zircon ages for three granitoid clasts from the moodies conglomerate and for detrital zircons from a fig tree greywacke from clastic sediments in the early archaean barberton greenstone belt of southern africa the crystallization ages obtained for the moodies clasts vary between 3570 6 ma and 3518 11 ma while the fig tree zircons show a surprisingly uniform source terrain age of 3453 9 ma that may date the stratigraphically underlying onverwacht group felsic metavolcanics the discordancy patterns are ascribed to a combination of variable pb loss at unspecified times in the past and new zircon growth and or recrystallization the shrimp data record details of isotopic variation even within single zircons that result in precise definition of upper concordia intercept ages our results suggest an age range between 3460 and 3570 ma for the source region of the barberton belt clastic metasediments presumably the ancient gneiss complex of swaziland and felsic volcanics of the overwacht group this source included geochemically evolved granitoids that predate the oldest greenstones by some 100 ma

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

provenance of upper precambrian lower cambrian shales in the central iberian zone spain evidence from a chemical and isotopic study geochemical and isotopic sr nd o data are presented for upper precambrian and lower cambrian shale sequences from the central iberian zone spain in order to define geochemical discriminants between them and to assess the nature of the source regions supplying these basins elemental data especially th sc ratios coupled with endt values of 2 5 suggest that the upper precambrian rocks contain a greater proportion of a much younger more mafic component than the lower cambrian sequence which has high th sc and endt of 7 3 coherent trends between elemental and o isotope data imply a homogeneous recycled source as the dominant provenance for the upper precambrian shales whereas a lack of such coherence for the lower cambrian sediments suggests a more heterogeneous provenance the sedimentological geochemical and isotopic data are consistent with the upper precambrian succession being derived ultimately from a polycyclic mixture of young pre cadomian mafic sources and older 2 ga crust in contrast the lower cambrian sequence was mainly derived from old more felsic crust which would have contributed increasingly greater proportions of material as the younger sources were progressively removed by erosion

single zircon dating constraining the maximum age of the barberton greenstone belt southern africa one of the central issues in early precambrian crustal evolution is the age and genetic relationship between greenstone belt supracrustal assemblages and nearby high grade gneiss terranes the crucial question in most cases is which of the two is older we report zircon ages using the new single grain evaporation technique from a metaquartzite and a felsic volcanic flow of the onverwacht group in the barberton greenstone belt bgb south africa and swaziland that constrain the age of this basal part of the greenstone sequence between 3451 15 and 3438 6 ma this is almost 100 ma younger than previous age estimates assigned to the onverwacht and agrees well with recent sm nd pb pb and zircon ion microprobe dating tonalitic orthogneisses of the ancient gneiss complex agc adjacent to the bgb have ages up to 3644 4 ma and are thus demonstrably older than the greenstones also they cannot be derived from them by partial melting as has been proposed previously this age relationship as well as the presence of metaquartzites and thrust slices of pregreenstone tonalite in the onverwacht tectonic sequence rule out an entirely oceanic evolution for the early history of the bgb and suggest that continental crust was either below or nearby

cadomian metasediments and ordovician sandstone from corsica detrital zircon u pb hf constrains on their provenance and paleogeography the island of corsica which belongs to the southern variscan realm was detached from southern france in the tertiary alongside alpine and variscan edifices it carries outliers of neoproterozoic metasediments and lower paleozoic siliciclastics including ordovician sandstone and conglomerates as well as hirnantian diamictites in its nw sector we investigated the u pb hf of detrital zircons of basement metasediments and overlying ordovician sandstone and channeling conglomerate to constrain their provenance the timing of their deposition and to deduce the late ediacaran to ordovician paleogeography the youngest detrital zircons in the metasediments are 0 55 0 53 ga indicating their maximum age of deposition is late ediacaran to early cambrian thus classifying the corsica basement metasediments as cadomian the u pb analyses revealed that a preponderance of the detrital zircons in the basement micaschist and quartzite portray neoproterozoic ages concentrating between 0 55 and 0 65 ga this is partly consistent with derivation from pan african terranes of north africa yet the presence of detrital zircons younger than 0 6 ga indicates significant input from cadomian magmatic arcs that resided within or at the margin of the cadomian basin in which the metasediments of corsica were deposited ehf values of the ediacaran zircons varies between samples indicating the provenance comprised both juvenile arcs and magmatic arcs that involved various degrees of mixing with old crustal components the hf tdm ages of many of the ediacaran aged zircons point to a plausible involvement of meso paleoproterozoic crust in the generation of these cadomian arcs the presence of small but fairly distinguished populations of mesoproterozoic aged 1 0 1 6 ga as well as 2 0 2 2 ga and 2 4 2 6 ga detrital zircons in the cadomian metasediments farther indicates the presence of such crust in the provenance although mesoproterozoic detrital zircons are usually considered the hallmark of avalonian terranes the presence of hirnantian glacial sediments at the corsican sequence indicates it resided in the vicinity of gondwana we therefore postulate that the pre neoproterozoic zircons have sourced from exotic crustal vestiges that were entrained and accreted within the cadomian realm itself the transition into the overlying ordovician sandstone and conglomerate marks a major change in the provenance possibly pointing to lateral motions along the strike of the peripheral cadomian domain the youngest concordant detrital zircon in the ordovician ciuttone sandstone yielded an age of 0 48 ga the detrital zircon ages define an overwhelming concentration at 0 55 ga indicating the source of the ordovician sandstone was cut off from the gondwana hinterland and that sand was exclusively derived from a latest ediacaran arc in view of the sharp detrital zircon age peak the corsica ordovician sandstone cannot be straightforwardly correlated with armorican sandstone because the detrital zircon spectra of the latter are generally broader indicating derivation from various sectors of the north gondwana crust ehf t values of the 0 55 ga zircons are mostly positive and the corresponding tdm ages at 0 7 1 2 ga indicating derivation from a juvenile island arc while tdm ages of this type are common for late ediacaran avalonian rocks the presence of hirnantian diamictite in corsica further substantiates that the aforementioned 0 55 ga island arc evolved in the peripheral cadomian realm as a whole the u pb hf zircon data from the corsica sequences reveal the presence of juvenile cadomian arcs alongside cadomian arcs that recycled ancient pre neoproterozoic crust along strike variations of this type are known from the japanese islands in line with the peripheral cadomian orogeny being an ancient analog of a western pacific type plate boundary

evidence of a paleoproterozoic basement in the moroccan variscan belt rehamna massif western meseta abstract for the first time an eburnian magmatic event has been identified in the rehamna massif moroccan variscan belt western meseta located north of the south meseta fault the best estimate of the crystallization age of rhyolitic porphyry is given by a weighted mean 207pb 206pb age of 2050 6 3 ma rhyacian orosirian the new u pb age obtained for rhyolitic porphyry from the rehamna massif is of great relevance for improving geological knowledge about the boundaries of the wac because i it overlaps the older eburnian magmatic event described in the anti atlas belt and the icartian magmatic event of the european variscan belt ii this suggests that exists paleoproterozoic basement in the western meseta a hundred kilometers further to the north of the south meseta fault as old continental crust slivers preserved in the cadomian and variscan belts and iii this means that the cambrian transgression in the western meseta probably took place based on a more complex structural architecture affecting the precambrian basement composed not only of ediacaran rocks as has been suggested in the literature but also with paleoproterozoic rocks ca 2 05 ga as discovered in this study

dating the onset of variscan crustal exhumation in the core of the bohemian massif new u pb single zircon ages from the high k calc alkaline granodiorites of the blatna suite central bohemian plutonic complex abstract the variscan central bohemian plutonic complex crops out between the upper crustal tepla barrandian and the high grade moldanubian units bohemian massif much of the complex is made up of calc alkaline plutonic rocks 1 the geochemically more primitive na rich 354 4 ma sazava suite which was emplaced syntectonically during regional shortening 2 the younger more evolved potassic blatna suite which records both the shortening along its nw contact and the onset of normal shearing related to exhumation of the moldanubian unit to the se new ion microprobe u pb zircon ages for the high k calc alkaline blatna suite pinpoint this major switch in the tectonic regime both blatna and kozarovice granodiorites 346 2 ma and 347 2 ma 2 were generated by melting of heterogeneous crust and were emplaced contemporaneously to form the blatna composite pluton from age spectra preserved in inherited zircons and whole rock sr nd isotope signatures the likely crustal sources for the magmas were immature greywackes rich in neoproterozoic 615 10 ma kozarovice or late cambrian early ordovician 492 4 ma blatna volcanogenic detritus an additional important petrogenetic process was variable mixing with enriched mantle derived monzonitic magmas which may also have supplied the extra heat for crustal anatexis supplementary material analytical techniques selected whole rock major and trace element geochemical analyses and u th pb data are available at http www geolsoc org uk sup18391

growth of early archaean crust in the ancient gneiss complex of swaziland as revealed by single zircon dating abstract we present new single grain zircon ages derived from ion microprobe and evaporation analysis for rocks of the ancient gneiss complex agc of swaziland southern africa that enable us to document a more extended history of early archaean crustal growth in the eastern kaapvaal craton than previously documented the oldest rocks appear to be strongly flattened tonalitic trondhjemitic orthogneisses dated as 3644 4 ma compston and kroner 1988 which are in faulted contact with rocks of the barberton greenstone belt bgb in an agc enclave in northwest swaziland and which also record subsequent thermal metamorphic plutonic events at 3580 3500 3430 3200 and 3000 ma these rocks are tectonically interbanded with 3200 ma old granodiorites and k rich leucogranites thereby providing evidence for a severe and important post 3 2 ga deformation event that is also inferred for 3500 to 3200 ma old tonalitic to trondhjemitic gneisses in the agc of northeast swaziland in central swaziland zircons from the oldest tonalitic gneiss of the agc have an age of 3560 ma whereas younger additions of granitoid which also intrude the dwalile greenstone enclave in the agc record ages of 3450 3460 ma intense ductile deformation has repeatedly obliterated original intrusive contacts so that preserved field relationships often cannot resolve the above chronology chemical and whole rock isotopic data are consistent with derivation of some agc gneisses from mantle sources and thus imply very short crustal residence times for their granitoid precursors but this is not generally true for the entire agc we suggest that at least some gneisses resulted from intracrustal melting perhaps during crustal thickening events as recorded in granulite parageneses within the agc documentation of 3 5 ga or older granitoid crust in all major regions of the agc now exposed suggests the presence of continental terrains of up to at least 100 km in diameter before the barberton greenstone succession was deposited the combined geochronologic and structural metamorphic data for the agc and bgb reveal a complex history of growth and deformation for the eastern kaapvaal craton that lasted for at least 600 ma from 3 64 to about 3 ga ago the magmatic and tectonic evolution of the agc was broadly similar to that of the bgb and suggests that early archaean greenstone and gneiss terrains were coeval but formed and evolved in different settings and at different crustal levels our age data and the structural metamorphic evolution of the eastern kaapvaal craton are compatible with models suggesting crustal growth through vertical magmatic underplating and early crustal thickening uplift and erosion as a result of tectonic interleaving due to repeated horizontal shortening

multiple zircon growth within early archaean tonalitic gneiss from the ancient gneiss complex swaziland abstract u pb age determinations by ion microprobe reveal multiple episodes of zircon growth and recrystallization within a single sample of tonalitic gneiss from the ancient gneiss complex the oldest episode at3644 4ma 2 produced the dominant type of zircon characteristically purplish brown and massive in texture this probably constitutes unaltered zircon precipitated from the original magma recrystallization accompanied and obscured by early pb loss took place within the oldest grains at3504 6ma and3433 8ma whole new grains grew at these times also we interpret the post 3644 ma growth as due to episodic deformational and metamorphic events that transformed the tonalite pluton into foliated banded gneiss in addition many grains are visibly overgrown by two layers of younger zircon of different colour and texture dated at2986 20ma and2867 30ma euhedral finely zoned whole grains having the 2986 ma age are present also evidently contributed by very thin felsic veins associated with the nearby lochiel granite the age of3644 4ma combined with precise zircon u pb dating of volcanics from the onverwacht group reported elsewhere demonstrates that at least part of the ancient gneiss complex is older than the barberton greenstone belt

sr nd isotopic constraints on the petrogenesis of the central bohemian pluton czech republic the sr nd isotopic data for selected granitoids of the central bohemian pluton show a broad negative correlation with the total range of 87sr 86sr 330 0 7051 0 7129 and nd330 0 2 to 8 9 the older intrusions have more depleted sr nd compositions and calc alkaline geochemistry sazava suite whereas the younger intrusions shift towards k rich calc alkaline blatna suite and shoshonitic rocks icany and certovo b emeno suites with more evolved isotopic signatures the distribution of the data is interpreted as reflecting a diversity of sources and processes rather than a single progressive crustal contamination trend the sazava suite could have originated by partial melting of metabasites or of a mantle source with an isotopic composition close to bulk earth or by hybridization of crustally derived tonalitic and mantle derived magmas variation within the blatna suite is modelled by mixing between a moderately enriched 87sr 86sr 330 0 708 nd330 3 mantle component with either an isotopically evolved metasedimentary component or with more evolved magmas of the suite the icany suite was most probably produced by partial melting of peraluminous lithologies possibly of the adjacent moldanubian unit the certovo b emeno suite evolved from strongly enriched mantle derived magmas 87sr 86sr 330 0 7128 end330 7 either through closed system fractional crystallization or interaction with magma corresponding to leucogranites of the central bohemian pluton

precadomian relicts in the armorican massif their age and role in the evolution of the western and central european cadomian hercynian belt abstract well dated precambrian is mostly developed in the north of the armorican massif in which area voluminous cadomian magmatism is dated at between 650 and 550 ma much older relicts occur at cap de la hague in guernsey and in the tregor and are also found in the northern continental margin of the iberian peninsula in all these occurrences whole rock systems have been opened so that the true ages cannot be determined by the rb sr whole rock isochron method four u pb zircon ages are between 1 8 and 2 ga in guernsey icart orthogneisses in tregor port beni trebeurden morguignen orthogneisses there is no evidence from strontium isotopes that these isolated and scattered relicts have a wide extension or that such ancient continental crust played an important role in magma genesis from 650 ma to 270 ma ago on the contrary the evolution of initial 87sr 86sr ratios with time shows that the observed mid and west european continental crust is probably not older than 700 ma the increase of the initial 87sr 86sr ratios of the magmas with time suggests that after its formation in cadomian times this segment of continental crust evolved virtually as a closed system and hercynian magmatism arose principally from re melting of relatively young sialic components

magnetotelluric study of the eastern margin of the bohemian massif relations between the cadomian variscan and alpine orogeny eastern margin of the bohemian massif is a geologically remarkable area where three different orogenic cycles are meeting the oldest cadomian building a basement for younger cycles a younger variscan and the youngest alpine covering the older units in the past this area was investigated by gravimetry and seismic methods recently we have supplemented broadband magnetotelluric measurements within the period range of 0 001 1000 s carried out on 29 stations distributed along a 140 km long west east regional profile the profile direction was based on local geology and then confirmed by a dimensionality and directionality analysis data showed moderate effects of cultural noise in the signals and could be successfully processed by robust methods we carried out a 2d inversion of the data using the rebocc approach the inversion results confirm the known near surface geology and reveal deeper structures on the west and in the central part of the profile units of the bohemian massif the moldanubian zone the brunovistulicum are interpreted in the east the western carpathians units are encountered short period data agree well with the known near surface geology of all inner smaller units and bring new knowledge in particular on their thickness in the shallow structure several conductive anomalies have been identified which are hypothesized to be related to graphitized layers in the moldanubian zone from long period data a new image of the moldanubian brunovistulicum contact and about the structure of the brunovistulicum especially the brunovistulian massifs was obtained

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

mapping seismic anisotropy of the lithospheric mantle beneath the northern and eastern bohemian massif central europe abstract we evaluate 3d body wave anisotropic parameters shear wave splitting fast s polarizations and delay times of the split slow shear waves and directional terms of relative p wave residuals from data of passive seismic experiment bohema ii 2004 2005 data from 34 temporary stations increased substantial density of observations in the bohemain massif bm and allowed us to scan in detail the upper mantle structure of its northern and eastern parts for the first time we delimit domains of the mantle lithosphere in this part of the bm and infer 3d self consistent models of their fabrics by a joint inversion of the body wave anisotropic parameters fabrics of the northern and north eastern parts of the bm are approximated best by peridotite aggregates with the a c foliations dipping approximately to the n and ne respectively whereas a model with the westerly dipping a lineation fits best the fabric of the south eastern domain the saxothuringian fabric nw of the eger rift extends to the east across the elbe fault zone and continues along this zone to the southeast beneath the cretaceous basin the south eastward continuation of the elbe fault zone seems to be related to the boundary between two different fabrics of the northern and southern brunovistulian domains below the moravo silesian zone this study shows an underthrusting of the brunovistulian micro plate beneath the eastern rim of the bm and indicates that its northern and southern fragments might have originally belonged to baltica and to gondwana respectively according to a zone of distinctly decreased anisotropic signals the brunovistulian micro plate extends at least about 100 km westward beneath the moldanubian with these new findings we update the domain like mantle structure of the bm and compare the results with inferences from the upper mantle velocity tomography and depth changes of the lithosphere asthenosphere boundary retrieved in a detailed model from relative residuals and from the receiver functions inferences on anisotropic structure beneath the permanent observatories included in different experiments are stable and are validated by the bohema ii experiment

is hydrology kinematic a wide range of phenomena natural as well as man made in physical chemical and biological hydrology exhibit characteristics similar to those of kinematic waves the question we ask is can these phenomena be described using the theory of kinematic waves since the range of phenomena is wide another question we ask is how prevalent are kinematic waves if they are widely pervasive does that mean hydrology is kinematic or close to it this paper addresses these issues which are perceived to be fundamental to advancing the state of the art of water science and engineering copyright 2002 john wiley sons ltd

hercynian thrust related shear zones and deformation of the varied group on the contact of granulites southern moldanubian bohemian massif the three deformation phases inferred from the detailed structural analysis of the cesky krumlov varied group record the hercynian development of the southern moldanubian of the bohemian massif the deformation is related to the nw se thrusting of the large crustal units including granulites the formation of the ns and nw se trending shear zones is connected with the thrust movement the structural development begins with f1 isoclinal fold formation that could originate in unmetamorphosed sediments in the final stage they were strongly flattened and b1 boudinage developed in the rocks aplite dykes and migmatitization of paragneisses occurred at the same time the rocks were metamorphosed the d2 deformational phase was produced by the simple shear deformation of the unit and folds of various styles around rigid inclusions and the strike slip shear zone near the boundary of the granulite and the varied group were formed the f1 and f2 folds are parallel with the stretching and mineral lineation indicating a ns to nw se direction of the thrusting the youngest deformation is characterized by spectacular boudinage and by folding of the vertically oriented planes

long lasting cadomian magmatic activity along an active northern gondwana margin u pb zircon and sr nd isotopic evidence from the brunovistulian domain eastern bohemian massif manuscript published version available in international journal of earth sciences 2016 doi 10 1007 s00531 016 1416 y

boundaries of mantle lithosphere domains in the bohemian massif as extinct exhumation channels for high pressure rocks abstract five domains microplates have been recognized by seismic anisotropy in the mantle lithosphere of the bohemian massif the mantle domains correspond to major crustal units and each of the domains bears a consistent fossil olivine fabric formed before their variscan assembly the present day mantle fabric indicates that this process consisted of at least three oceanic subductions each followed by an underthrusting of the continental lithosphere the seismic anisotropy does not detect remnants of the oceanic subductions but it can trace boundaries of the preserved continental domains subsequently underthrust along the paths of previous oceanic subductions the most robust continent continent collision was followed by westward underthrusting of the brunovistulian mantle lithosphere still detectable by seismic anisotropy more than 100 km beneath the moldanubian mantle lithosphere major occurrences of the high pressure ultra high pressure hp uhp rocks follow the ene and nne oriented sutures and boundaries of the mantle lithosphere domains mapped from three dimensional modeling of body wave anisotropy the hp uhp rocks are products of oceanic subductions and the following underthrusting of the continental crust and mantle lithosphere exhumed along the mantle boundaries the close relation of the mantle sutures and occurrences of the hp uhp rocks near the paleosubductions testifies for models interpreting the granulite garnet peridotite association by oceanic continental subduction underthrusting followed by the exhumation of deep seated rocks our findings support the bivergent subduction model of tectonic development of the central part of the bohemian massif the inferences from seismic anisotropy image the bohemian massif as a mosaic of microplates with a rigid mantle lithosphere preserving a fossil olivine fabric the collisional mantle boundaries blurred by tectonometamorphic processes in easily deformed overlying crust served as major exhumation channels of the hp uhp rocks

elevator tectonics and orogenic collapse of a tibetan style plateau in the european variscides the role of the bohemian shear zone variscan collision of peri gondwanan terranes led to a doubly vergent crustal wedge that was thicker than 55 km in the area of the bohemian massif this crustal thickness resulted in a highly elevated bohemian plateau with a topographic height 3 4 km the bohemian plateau was covered with unmetamorphic paleozoic strata all of which are today well preserved in the tepla barrandian unit because of crustal scale vertical slip along the bohemian shear zone bsz the bsz forms a subvertical ca 500 km long and up to 2 km wide belt of dip slip mylonites which show several 90 deflections in map view tepla barrandian down movements were active under retrograde metamorphic conditions starting with granulite and ceasing with greenschist facies conditions as slip along the bsz was largely vertical and led to a minimum throw of 10 km this type of crustal scale deformation is referred to as elevator tectonics the elevator style movements caused the juxtaposition of the supracrustal tepla barrandian lid the elevator against high grade rocks of the extruding orogenic root the bsz has further governed the foci of mantle derived plutonism new u pb zircon and monazite tims dating of six plutons suggest that emplacement of mantle derived melts along the bsz lasted for at least 20 m y starting with the emplacement of the klatovy granodiorite at 347 4 3 ma and ceasing with the emplacement of the drahotin pluton at 328 1 ma when taking into account the new ages of synkinematic plutons the simultaneous vertical slip along the individual segments of the bsz north west and central bohemian shear zone is bracketed to the period 343 337 ma elevator tectonics was probably controlled by delamination of thickened mantle lithosphere that caused a dramatic thermal turnover and heating up of the orogenic root the overheated lower crust was thermally softened by anatexis and diffusion creep resulting in channel flow vertical extrusion fast uplift and exhumation of the orogenic root

structure and stratigraphy of the tepla barrandian neoproterozoic bohemian massif a new plate tectonic reinterpretation abstract the tepla barrandian unit tbu of the bohemian massif exposes a section across the once extensive avalonian cadomian belt which bordered the northern active margin of gondwana during late neoproterozoic this paper synthesizes the state of the art knowledge on the cadomian basement of the tbu to redefine its principal component units to revise an outdated stratigraphic scheme and to interpret this scheme in terms of a recent plate tectonic model for the cadomian orogeny in the bohemian massif the main emphasis of this paper is on an area between two newly defined fronts of the variscan pervasive deformation to the nw and se of the barrandian lower paleozoic overlap successions this area has escaped the pervasive variscan late devonian to early carboniferous ductile reworking and a section through the cadomian orogen is here superbly preserved the nw segment of the tbu consists of three juxtaposed allochthonous belts of unknown stratigraphic relation the kralovice rakovnik radnice kralupy and zbiroh sarka belts differing in lithology complex internal strain patterns and containing sedimentary and tectonic melanges with blocks of diverse ocean floor meta basalts we summarize these three belts under a new term the blovice complex which we believe represents a part of an accretionary wedge of the cadomian orogen the se segment of the tbu exposes the narrow picin belt which is probably a continuation of the blovice complex from beneath the barrandian lower paleozoic and a volcanic arc sequence the davle group their stratigraphic relation is unknown flysch units the st chovice group and svrchnice formation overlay the arc volcanics and both units contain material derived from volcanic arc the former was also sourced from the nw segment whereas the latter contains an increased amount of passive margin continental material in contrast to the blovice complex the flysch experienced only weak cadomian deformation the new lithotectonic zonation fits the following tectonic scenario for the cadomian evolution of the tbu well the s to se directed cadomian subduction beneath the tbu led to the involvement of turbidites chaotic deposits and 605 39 ma ocean floor in the accretionary wedge represented by the blovice complex the accretionary wedge formation mostly overlapped temporally with the growth of the volcanic arc the davle group at 620 560 ma upon cessation of the arc igneous activity the rear of the wedge and some elevated portions of the arc were eroded to supply the deep water flysch sequences of the st chovice group whereas the comparable svrchnice formation 560 to

determining sedimentation ages of high grade metamorphic gneisses by their palynological record a case study in the northern schwarzwald variscan belt germany in the northern schwarzwald moldanubian zone of the variscan belt paragneisses drilled from approximately 2500 m depth at the buhl 1 drilling site have experienced amphibolite facies metamorphism and were later partly transformed to mylonites under greenschist facies conditions completely graphitised acritarchs and chitinozoans were recovered from these paragneisses and mylonitic gneisses whereas both groups of palynomorphs give evidence of a marine depositional environment only the chitinozoans are stratigraphically useful in this case and indicate a paleozoic age previous findings of acritarchs in a different high grade metamorphic gneiss unit within the schwarzwald indicate an upper proterozoic marine sedimentation age the spectrum of palynomorphs found thus far in the schwarzwald in combination with petrological data suggests that within the moldanubian domain of the armorican microplate rocks with different sedimentary records later experienced diverging prograde metamorphic histories in general the results reveal that palynological studies in combination with petrological data are a promising tool for reconstructing pre orogenic settings in collisional belts

ultrapotassic dykes in the moldanubian zone and their significance for understanding of the post collisional mantle dynamics during variscan orogeny in the bohemian massif abstract we report mineral textures geochemistry and age relations of two ultrapotassic dykes from a dyke swarm in the western part of the moldanubian zone at contact to the tepla barrandian block the dykes have orientation almost perpendicular to the nne ssw trending central bohemian plutonic complex and cross cut metamorphic foliation in basement gneisses and migmatites based on mineral compositions and geochemistry the dykes show close relations to mg k syenite plutons in the moldanubian zone the two dykes are vaugnerite and syenite in compositions and contain talc pseudomorphs after olivine within a fine grained matrix that consists of k feldspar phlogopite with small amounts of clinopyroxene and accessory quartz apatite titanite and sulphides of fe cu ni the syenite porphyry dyke cross cuts the vaugnerite it contains quartz phenocrysts and comparing to vaugnerite has lower modal content of talc pseudomorphs the vaugnerite and syenite porphyry have high k 2 o 6 to 7 wt and mg number 0 66 0 74 but low cao and na 2 o contents the vaugnerite is markedly rich in p 2 o 5 2 wt and comparing to syenite porphyry has higher amount of mantle incompatible elements e g rb cs ba pb th u v and cr id tims analyses on titanite in vaugnerite and on zircon in syenite porphyry yielded 338 59 0 68 and 337 87 0 21 ma respectively mineral and bulk rock chemistry of the dykes indicates that the source magma was formed by a low degree of partial melting of a highly anomalous domain in the upper mantle which subsequently fractionated and was contaminated with crustal material during its ascent formation of ultrapotassic magma is explained by transcurrent shear zones in the mantle lithosphere that occurred due to block rotation and post collisional mantle dynamics initiated by slab break off and asthenosphere upwelling into the moldanubian accretionary complex during the variscan orogeny

evidence from zircon dating for existence of approximately 2 1 ga old crystalline basement in southern bohemia czech republic zircon ages are reported for three moldanubian amphibolite grade orthogneisses from the southern bohemian massif obtained by conventional u pb analyses for two of these orthogneisses conventional u pb data are supported by ion microprobe single zircon ages or single grain evaporation data the amphibolite grade orthogneisses occurring in three small tectonic lenses within the varied group close to the south bohemian main thrust are of tonalitic granodioritic or quartz dioritic composition

chronological constraints on the pre orogenic history burial and exhumation of deep seated rocks along the eastern margin of the variscan orogen bohemian massif czech republic key lithological units of the high grade eastern margin of the bohemian massif were dated using the u pb and pb pb methods on zircons in order to establish a chronological framework for the geodynamic evolution of the variscan orogenic root the protolith ages for metagranitoids orthogneisses and granulites of thickened lower and middle crust reveal the existence of magmatic activity that occurred over a 100 million year time interval from cambro ordovician to early devonian times probably related to discontinuous intracontinental rifting of neoproterozoic crust our geochronological data suggest that the eastern part of the orogenic root represents thermally softened and rifted neoproterozoic crust preserved farther to the east as the brunia microcontinent zircon ages for felsic granulites high grade gneisses of the lower crust and of a syn convergence granodioritic intrusion in the upper crust indicate that thickening and exhumation of the crust occurred during a narrow time interval between 370 and 340 ma exhumation of the lower crust to mid crustal levels was a localized process that occurred at 340 ma and was associated with crustal scale folding in the internal part of the root as well as orogenic channel flow along the eastern collisional margin both types of exhumation mechanisms were driven by deep level wedging indentation of the easterly brunia continent followed by deposition of heavy minerals and pebbles derived from high pressure rocks in the adjacent foreland basin final orogenic development was characterized by ne sw dextral transpressive shearing parallel to the brunia margin as well as dextral trans tension associated with activity along the elbe lithospheric fault these processes affected the marginal parts of the orogenic root and were accompanied by 330 to 325 ma old syntectonic granitoid intrusions along reactivated lithotectonic boundaries rotation of the assembled orogenic belt accompanied by lithospheric faulting driven by westerly subduction roll back may be the most plausible model to explain late deformation of the orogenic root

early archaean zircon ages from orthogneisses and anorthosites at mount narryer western australia abstract u th pb isotopic analyses have been made with an ion microprobe of 20 30 m areas within zircons from gneisses adjacent to the mount narryer quartzite western australia in which detrital zircons up to 4200 ma old have been found the cores of the zircons yield protolith crystallization ages of 3678 6 ma 95 confidence limits for the meeberrie banded monzogranite gneiss and 3381 22 ma for the dugel syenogranite leucogneiss in agreement with observed intrusive relationships zircons from inclusions of deformed leucogabbro and meta anorthosite within the dugel gneiss formerly part of the manfred layered igneous complex are 3730 6 ma old the remnants of the manfred complex are therefore the oldest rocks currently known on the australian continent analyses of discrete post magmatic grains and of overgrowths on older grains indicate an episode of subsolidus zircon growth in the region at 3296 4 ma which is interpreted as a time of local metamorphism of the gneisses subsequently the zircons have been rounded and embayed and have lost radiogenic pb in both ancient and recent times all the zircon ages determined in this study are within the age range for detrital zircons in the mount narryer quartzite it is therefore possible that these felsic gneisses and anorthositic rocks were a local source for some of the sedimentary material however the lateral extent of this early archaean terrain and the source rocks for the rare 4100 4200 ma old zircons in the metasediments are yet to be determined

p t evolution of a variscan lower crustal segment a study of granulites from the schwarzwald germany pressure temperature time p t t paths of orogenic granulites introduction provide important information on the thermal and chemical structure general background of the lower continental crust through time and constraints on tectonic granulites are typical rocks of the earth s middle to processes we present the first detailed petrological investigation of lower crust under high temperature conditions they are granulites from the variscan schwarzwald pelitic granulites from the found as xenoliths in basaltic volcanic rocks mainly central schwarzwald gneiss complex csgc are characterized by within continental rifts but most granulites occur as the peak assemblage garnet rutile kyanite antiperthite complexes or terranes in orogenic settings orogenic quartz felsic to intermediate granulites from the southern granulites display a wide compositional range harley schwarzwald gneiss complex ssgc exhibit different peak as1989 1998 and are known from a variety of collisional semblages with clinopyroxene orthopyroxene ternary feldspar garnet belts that formed during different episodes since the quartz and sillimanite and manifold retrograde reaction textures archaean e g harley 1998 clarke et al 2000 moller peak p t conditions were calculated by two feldspar thermometry et al 2000 determination of bulk rock and mineral garnet orthopyroxene thermometry and various geobarometers minicompositions calculation of peak equilibration conmum estimates for peak conditions are 950 1010 c and 1 4 1 8 ditions and dating of orogenic granulites thus provide us gpa for the granulites of the csgc which followed a clockwise with important constraints on the thermal and chemical p t path the retrograde path is characterized by initial isothermal structure of the earth s continental crust at different decompression associated with partial melting followed by isobaric geological times cooling peak conditions for the ssgc are 1015 c and 1 5 gpa harley 1989 showed that equilibration conditions minimum temperature maximum pressure no prograde relics are deduced from natural orogenic granulites cover a wide preserved and isothermal decompression was less pronounced than range in particular pressures are highly variable and in the csgc other variscan hp ht granulites from central the granulite field may be divided into a low pressure europe show similar lithologies equilibration temperatures and lp a medium pressure mp and a high pressure hp ages 340 335 ma the heat for widespread high temperature facies according to green ringwood 1967 peak metamorphism in the variscan lower crust could have been supplied temperatures of many granulite terranes scatter around by repeated intrusion of subduction related basic magmas rapid 800 c bohlen 1987 but an increasing number of near isothermal decompression of the granulites may have been ultra high temperature uht granulite complexes facilitated by considerable volumes of partial melt and by orogenic extension 900 1100 c and 0 7 1 3 gpa harley 1998 are being recognized e g harley hensen 1990 dasgupta et al 1995 klemd brocker 1999 hokada 2001 regarding retrograde p t paths harley 1989 distinguished two

a new high precision u pb date from the oldest known rocks in southern britain a new high precision u pb zircon age of 710 8 1 5 ma for granophyric granitic rock from the stanner hanter complex of the welsh borderland lies just within error of an older rb sr isochron age nd values of 0 3 and 1 2 combined with t dm of 1394 ma and 1468 ma indicate that the magma incorporated an older crustal source component the nd data highlight differences with western avalonia the widely considered late neoproterozoic north american counterpart to southern britain and point toward a closer similarity with other peri gondwanan terranes that incorporate older cratonic source material

evolution of moldanubian rocks in austria review and synthesis the moldanubian zone in austria comprises three major lithological units despite general agreement that nappe tectonics contributed to its current structure the number and position of tectonic boundaries or continental pieces that were involved in its evolution as well as the age extent and position of oceanic sutures are disputed recent models ascribe the moldanubian tectonostratigraphic structure to its oblique n to ne directed collision with moravia only the rocks of the moldanubian bunte series and gfohl unit experienced a common intensive overprint in the range 700 800 c and 8 11 kbar textural evidence suggests that this overprint was attained during nearly isothermal decompression so the rocks experienced higher pressures prior to this overprint these conditions constrain a continent continent collision environment that contributed to the formation of the moldanubian granulites the estimated metamorphic temperatures are close to tmax during this hercynian high t overprint the minerals underwent extensive diffusion controlled homogenization of elements the early stages of retrogression of these units were characterized by isobaric cooling at c 6 kbar in the range 650 500 c that is related to the oblique collision of the moldanubian and moravian zones cooling to c 400 c is demonstrated by unstrained diasporized corundum inclusions in garnet of common moldanubian granulites the available age data including cooling ages from metamorphic rocks show a very wide variation between 490 and 280 ma that depends on sample characteristics and the dating method used they demonstrate clearly however that the metamorphic overprint is hercynian the possibility that the large variation in ages reflects homogenization resetting and closure of the isotopic systems attained at different sample and method specific times is discussed age data varying between c 370 and c 346 ma tentatively date different stages during the hercynian high t decompression the majority of zircon and monazite u pb ages as well as the hornblende and muscovite ar ar cooling ages cluster between c 345 and c 326 ma and date the effective closure conditions and the onset of rapid nearly isobaric cooling the continent continent collision that formed the granulites pre dates c 370 ma the intra moldanubian nappe stacking pre dates thrusting of the moldanubian zone over the moravian zone the range c 340 335 ma is the lower limit for completion of tectonic activity in the moldanubian zone the moldanubian series are post tectonically intruded by granitoids of the southern bohemian pluton recent age determinations and geochemical evidence suggest that the formation of the early granitoid types took place in the lower crust in connection with the hercynian high grade overprint the moldanubian monotone series in austria is separated from the other moldanubian units by a conspicuous tectonic horizon it also differs from them by its characteristic high t low p overprint which is best demonstrated by a widespread cordierite gneiss

early palaeozoic tectonothermal events in basement complexes of the eastern greywacke zone eastern alps evidence from u pb zircon data field observations and u pb zircon data yield evidence that the kaintaleck complex represents a distinct crystalline basement unit of the eastern greywacke zone and contrasts partly with other pre alpine basement complexes of the eastern alps the age data indicate possible late archean rock formation several magmatic and metamorphic events in the early palaeozoic ca 520 490 and ca 400 360 ma and low grade metamorphic overprint in the cretaceous zircons from a garnet gneiss layer in a plagioclase gneiss and amphibolite sequence yield an upper intercept age of ca 2 55 ga which is interpreted as a possible protolith age the lower intercept age at 514 ma represents either 1 a magmatic event or 2 a metamorphic overprint within amphibolite facies conditions magmatic zircons derived from granitic orthogneiss boulders of the kalwang conglomerate which covers the crystalline basement crystallised at ca 500 ma the lower discordia intercepts of the orthogneiss boulders mark the cretaceous low grade metamorphic overprint the lower intercept age of paragneiss zircons from another slice of the kaintaleck complex suggests a likely devonian metamorphic event at ca 390 400 ma the new data demonstrate that the kaintaleck complex experienced several stages of tectonothermal evolutions which are in contrast to the evolution of the main mass of the austroalpine basement the data constrain linkages of this part of the austroalpine basement to the cordillere ligerienne and cadomian block within west european variscides

pre hercynian mantle lead transfer to basement rocks as indicated by lead isotopes of the schwarzwald crystalline sw germany ii lead isotope evolution of the european hercynides

whole grain evaporation for 207 pb 206 pb age investigations on single zircons using a double filament thermal ion source a technique has been developed and tested to analyse 207pb 206pb apparent ages by thermal evaporation of radiogenic lead directly from untreated whole zircon grains 0 3 mm the evaporation analyses are performed in the double filament arrangement of a thermal ion mass spectrometer thims the method is a powerful tool to distinguish between different lead components occurring in the same grain because differing activation energies of the competing lead components cause their sequential evaporation from the zircons the evaporation of test samples results in 207pb 206pb apparent ages in good agreement with u pb ages known from literature single zircons from a granite of the marble mountains california yield an age of crystallization of 1 410 30 ma ceylon zircons from heavy mineral bearing gravels yield 560 40 ma as age of crystallization of the pegmatitic gravel sources individuals from a heterogeneous zircon population of a diatexite from the southern schwarzwald sw germany indicate metamorphic zircon formation around 500 ma and the existence of middle proterozoic relics 1 95 0 05 ga

unsupported radiogenic pb in zircon a cause of anomalously high pb pb u pb and th pb ages ion microprobe u th pb isotopic analyses of zircons from a granodioritic orthogneiss from the napier complex mount sones enderby land antarctica have identified an unambiguous example of unsupported radiogenic pb in a 3 950 ma old crystal at one 40 m spot on the crystal an unusually large heterogeneity in pb content was found the concentration of radiogenic pb ranging from 5 to 50 percent higher than could have been generated in 3 950 ma by radioactive decay of the co existing u and th this relative excess of radiogenic pb is attributed to pb gain rather than to u and th loss because first the pb content varied by more than the u or th contents and secondly changes in the pb u pb th and pb isotopic composition correlated directly with changes in the pb concentration the individual 207pb 206pb apparent ages ranged from 4 000 ma to 4 145 ma all greater than the inferred age of the crystal a correlation between 207pb 206pb and pb u shows that the pb excess has not resulted from recent pb movement the spot apparently gained radiogenic pb about 2 500 ma ago at the same time as the majority of the other zircons in the rock suffered substantial pb loss the pb movement occurred in response to a discrete geologic event reverse discordance is a phenomenon that must be considered when interpreting zircon u pb ages especially 207pb 206pb ages of single crystals or portions of crystals

chronology of early archaean granite greenstone evolution in the barberton mountain land south africa based on precise dating by single zircon evaporation abstract we report precise 207 pb 206 pb single zircon evaporation ages for low grade felsic metavolcanic rocks within the onverwacht and fig tree groups of the barberton greenstone belt bgb south africa and from granitoid plutons bordering the belt dacitic tuffs of the hooggenoeg formation in the upper part of the onverwacht group yield ages between 3445 3 and 3416 5 ma and contain older crustal components represented by a 3504 4 ma old zircon xenocryst fig tree dacitic tuffs and agglomerates have euhedral zircons between 3259 5 and 3225 3 ma in age which we interpret to reflect the time of crystallization a surprisingly complex xenocryst population in one sample documents ages from 3323 4 to 3522 4 ma we suspect that these xenocrysts were inherited during the passage of the felsic melts to the surface from various sources such as greenstones and granitoid rocks now exposed in the form of tonalite trondhjemite plutons along the southern and western margins of the bgb and units predating any of the exposed greenstone or intrusive rocks several of the granitoids along the southern margin of the belt have zircon populations with ages between 3490 and 3440 ma coeval with or slightly older than onverwacht felsic volcanism while the kaap valley pluton along the northwestern margin of the belt is coeval with fig tree dacitic volcanism these results emphasize the comagmatic relationships between greenstone felsic volcanic units and the surrounding plutonic suites some of the volcanic and plutonic units contain zircon xenocrysts older than any exposed rocks these indicate the existence of still older units possibly stratigraphically lower and older portions of the greenstone sequence itself older granitoid intrusive rocks or bodies of older unrelated crustal material our data show that the onverwacht and fig tree felsic units have distinctly different ages and therefore do not represent a single tectonically repeated unit as proposed by others unlike the late archaean abitibi greenstone belt in canada which formed over about 30 ma exposed rocks in the bgb formed over a period of at least 220 ma the complex zircon populations encountered in this study imply that conventional multigrain zircon dating may not accurately identify the time of felsic volcanic activity in ancient greenstones a surprising similarity in rock types tectonic evolution and ages of the bgb in the kaapvaal craton of southern africa and greenstones in the pilbara block of western australia suggests that these two terrains may have been part of a larger crustal unit in early archaean times

pre hercynian mantle lead transfer to basement rocks as indicated by lead isotopes of the schwarzwald crystalline sw germany lead isotopes of k feldspars from five granites of the se schwarzwald and from metamorphites are positively correlated in 207 204 as well as 208 204 vs 206 204 diagrams

model of syn convergent extrusion of orogenic lower crust in the core of the variscan belt implications for exhumation of high pressure rocks in large hot orogens reflection seismic section field structural analysis and gravimetric modelling of orogenic lower crust in the core of a carboniferous orogenic root reveal details of the polyphase process of exhumation subvertical amphibolite facies fabrics strike parallel to former plate margins that collided in the nw the fabrics are developed in both mid crustal and lower crustal high pressure granulite units as a result of intensive nw se intraroot horizontal shortening driven probably by the west directed collision in granulites the steep fabrics originated as a result of extrusion of orogenic lower crust in a 20 km wide vertical ascent channel from lower crustal depths at 350 340 ma the large granulite bodies preserve older granulite facies fabrics documenting a two stage evolution during the exhumation process surface exposures of granulites coincide with the absence of subhorizontal seismic reflectors at depth suggesting preservation of the 20 km wide subvertical tabular structure reaching moho depths horizontal seismic reflectors surrounding the vertical channel structure corroborate a dominant flat migmatitic fabric developed in all tectonic units this structural pattern is interpreted in terms of subhorizontal spreading of partially molten orogenic lower crust in mid crustal levels 765 c and 0 76 gpa at 342 337 ma large massifs of extruded and progressively dismembered felsic granulites disturbed mid crustal fabrics in the surrounding horizontally flowing partially molten crust the horizontal mid crustal flow resulted in collapse of the supra crustal tepla barrandian unit interpreted as the orogenic lid along a large scale crustal detachment above the extruded lower crustal dome the presence of felsic granulites at the bottom of the orogenic root is considered to be a key factor controlling the exhumation of orogenic lower crust in large hot orogens