highly fractionated i type granites in ne china ii isotopic geochemistry and implications for crustal growth in the phanerozoic ne china is the easternmost part of the central asian orogenic belt caob the area is distinguished by widespread occurrence of phanerozoic granitic rocks in the companion paper part i we established the jurassic ages 184 137 ma for three granitic plutons xinhuatun lamashan and yiershi we also used geochemical data to argue that these rocks are highly fractionated i type granites in this paper we present sr nd o isotope data of the three plutons and 32 additional samples to delineate the nature of their source to determine the proportion of mantle to crustal components in the generation of the voluminous granitoids and to discuss crustal growth in the phanerozoic despite their difference in emplacement age sr nd isotopic analyses reveal that these jurassic granites have common isotopic characteristics they all have low initial 87 sr 86 sr ratios 0 7045f0 0015 positive end t values 1 3 to 2 8 and young sm nd model ages 720 840 ma these characteristics are indicative of juvenile nature for these granites other late paleozoic to mesozoic granites in this region also show the same features sr nd and oxygen isotopic data suggest that the magmatic evolution of the granites can be explained in terms of two stage processes 1 formation of parental magmas by melting of a relatively juvenile crust which is probably a mixed lithology formed by pre existing lower crust intruded or underplated by mantle derived basaltic magma and 2 extensive magmatic differentiation of the parental magmas in a slow cooling environment the widespread distribution of juvenile granitoids in ne china indicates a massive transfer of mantle material to the crust in a post orogenic tectonic setting several recent studies have documented that juvenile granitoids of paleozoic to mesozoic ages are ubiquitous in the central asian orogenic belt hence suggesting a significant growth of the continental crust in the phanerozoic d 2003 elsevier science b v all rights reserved

the ktb apatite fission track profiles building on a firm foundation abstract deep boreholes serve as natural laboratories for testing thermochronometers under geological conditions the kontinentale tiefbohrung ktb is an interesting candidate because the geological evidence suggests that approximate isothermal holding since the last documented exhumation in the late cretaceous to palaeocene is a reasonable assumption for the thermal histories of the ktb samples we report 30 new apatite fission track ages and 50 new mean confined track lengths determined on cores from the 4 km deep pilot hole the and external detector ages are consistent with the population ages from earlier studies and together define a clear age profile the mean track lengths from this and earlier studies reveal the effects of experimental factors the measured age and length profiles are compared with the predictions of 24 annealing models for isothermal holding there are clear discrepancies between the measured and calculated profiles down to 1 5 km depth the measured mean track lengths are shorter than the predicted the balance of methodological evidence indicates that this is due to seasoning i e a shortening of the fossil confined tracks without attendant age reduction from 2 5 to 4 0 km depth the mean track lengths are longer than the predictions this suggests that the bias model that weights the probabilities of observing tracks of different length and which is based on experiments relating surface track densities to mean track lengths is not appropriate for confined tracks experimental and methodological factors are sometimes difficult to disentangle but present a sufficient margin for there to be no need to go against the independent geological evidence unknown geological events cannot be ruled out but their existence cannot be inferred from the fission track data alone much less can the nature or magnitude of such events be specified

chronology of deformation within the turbidite dominated lachlan orogen implications for the tectonic evolution of eastern australia and gondwana ar ar data from fabric forming white mica in slates syntectonic quartz veins and granitic mylonites constrain the timing of metamorphism deformation and exhumation in the lachlan orogen australia these data also help define the tectonic evolution of the tasmanides during paleozoic time the lachlan orogen formed by the progressive accretion of a thick turbidite fan sequence and volcanic terrains to gondwana during the closing of a small marginal ocean or back arc basin this tectonic setting was similar to the present western and southwestern pacific region accretion of the lachlan orogen to gondwana occurred by closing of the basin system by subduction accretion processes and some translation the process is typified in the western lachlan orogen by a major eastward migrating deformation involving chevron folding and faulting over an eastward propagating decollement melange zone and is recorded by ar ar mica ages ranging from 455 ma in the western part to 390 ma in the eastern part in the central lachlan orogen deformation migrated southwestward from 440 430 to 405 ma away from the high temperature wagga omeo metamorphic complex where deformation metamorphism occurred between 440 and 400 ma in the north 400 ma mica ages record deformation and inversion of structures in the cobar basin in the eastern lachlan orogen ar ar mica dates range from 450 to 340 ma ages of 455 445 ma are yielded by the narooma accretionary complex 405 390 ma ages are found along the major thrust faults bounding high grade metamorphic complexes and 360 340 ma cooling ages are found in the inverted extensional basins e g hill end and related structural zones the ar ar results also document periods of reactivation on early formed structures during later deformation elsewhere in the orogen

granitoids in the rozvadov pluton western bohemia and oberpfalz the rozvadov pluton is a complex of mainly variscan granitoid rocks situated near the bohemian bavarian border between barnau tachov rozvadov and waidhaus 25 km ese of the ktb site five mappable units can be distinguished which intruded as folows 1 slightly deformed leucocratic meta aplite metapegmatite dykes with garnet and tourmaline 2 a complex of cordierite bearing granitoids which have been divided into three facies a biotite granite with cordierite at the margin of the complex b biotite cordierite granite and c cordierite tonalite in the centre of the complex 3 fine grained biotite granite of the rozvadov type with associated pegmatite bodies 4 two mica barnau granite and 5 geochemically specialized albite zinnwaldite topaz granite kri ov kamen kreuzstein granite with indications of sn nb ta mineralization and associated phosphorus rich pegmatite cupolas rare earth element data suggest that meta aplite pegmatite dykes are the result of a batch partial melting process whereas the compositional variation of the other rock types was mainly controlled by fractional crystallization the genesis of the cordierite granitoid suite is best explained in terms of a batch melting of metapelitic source followed by crystallization of a cordierite rich cumulate and k feldspar enriched melt the leucocratic pluton constituents the meta aplites and the barnau and k i ov kamen granites are rich in phosphorus 0 5 0 8 the main carriers of phosphorus are alkali feldspars especially k feldspar up to 0 8 p2o5 the presence of p rich leucocratic granites is one of the features distinguishing the variscan granitoids within the moldanubian zone from the nearly contemporaneous granitoids in the saxothuringian zone

the moldanubian granitoid plutons of austria chemical and isotopic studies bearing on their environmental setting abstract the late palaeozoic moldanubian plutonic terrain of upper austria consists of granitoids that range from s type to i type in composition although many units show transitional compositional characteristics the austrian terrain displays some of the features of continent margin main arc batholiths the expanded compositional range monzodiorite to leucogranite of the rock types the high sr i types and the low to intermediate 87 sr 86 sr initials 0 705 0 708 of these i types however there are also important differences the austrian i types are low cao and high k2o units and transitional and s type granitoids are volumetrically dominant in the austrian terrain there is a scarcity of gabbros and an absence of rocks with positive nd initials the austrian terrain is dominated by granitoids of crustal or predominantly crustal origin we suggest that these formed in an inner continental arc setting rather than a main arc and can be correlated with those of the schwarzwald f r g together these are interpreted as components of a moldanubian inner continent plutonic belt

crustal evolution of the ktb drill site from oldest relics to the late hercynian granites the crustal unit penetrated during the german continental deep drilling program ktb the zone of erbendorf vohenstrauss zev comprises an association of metabasites and paragneisses and orthogneisses that underwent high and medium pressure metamorphic cycles during the early palaeozoic in this summary of the structural petrological geochemical and geochronological information from borehole and surface rocks we show that geological models proposed prior to drilling have had to be significantly modified features of the zev such as the dominant nw se trending structures devonian 370 ma medium pressure amphibolite facies metamorphism 0 6 0 8 gpa 720 c and earlier eclogite stage are directly comparable with those of the nearby bohemicum unit in western bohemia intervening units in contrast exhibit ne sw trending structures and carboniferous 315 325 ma low pressure metamorphism all units are cut by predominantly posttectonic granites mostly 320 ma earlier models to explain this contrast by means of a nappe emplacement of the zev in the narrow time interval between the younger regional metamorphism and granite intrusion could not be substantiated because contrary to predictions the same rock association was present in the whole of the drilled 9 1 km and therefore no nappe boundary was drilled instead it is interpreted that shallow crustal levels reached by the zev subsequent to the devonian event shielded most of it from the effects of the carboniferous metamorphism a lack of expected thermal and baric gradients within the drilled metamorphic pile is a result of repeated stacking of an original 3 km depth section in mesozoic times an important result of the geological investigations of the ktb project is the recognition that the present day geometry of the zev is a result of cold shallow level thin skinned tectonics that in the absence of the view into the third dimension offered by the drill hole could not have been accurately predicted from the fossil high grade assemblages and structures seen at the surface

depletion of light rare earth elements in felsic magmas contrary to simple generalizations about their behavior light rare earth elements lree do not act as incompatible elements in very felsic magmas in fact lree concentrations typically decrease often drastically during differentiation of such magmas the simplest explanation for this depletion involves the separation of minute easily overlooked quantities of an extremely lree rich accessory mineral either monazite or allanite our data indicate that felsic liquids with 50 ppm lree may be saturated in either of these accessories and that the concentration required for saturation decreases in increasingly felsic liquids this accounts for incompatible behavior of lree even at high concentrations in mafic magmas in contrast to compatible behavior at low concentrations in felsic magmas partitioning of lree into solid rather than liquid has important implications for trace element and nd isotope modeling of crustal anatexis as well as for magma differentiation

petrogenesis of the hercynian tichka plutonic complex western high atlas morocco trace element and rbzsr and smznd isotopic constraints the late tectonic hercynian tichka plutonic complex in morocco was emplaced in the lower cambrian volcanosedimentary sequence of the western high atlas and displays an exceptional acid basic association which consists of five well defined plutonic intrusions gabbros diorites granodiorites tonalites monzogranites and two types of leucogranites biotite amphibole leucogranites and two mica leucogranites with the exception of the late two mica leucogranites the plutonic suites present field relationships clearly indicating an almost synchronous magmatic emplacement the monzogranites and biotite amphibole leucogranites plot along the same rbzsr isochron and most of them also plot along the same smznd isochron yielding respectively ages of 291 5 ma and 283 24 ma with initial87sr 86sr and143nd 144nd ratios of 0 70348 4 and 0 51243 4 major and trace element data together with the rbzsr and smznd isotopic results rule out any genetic process involving a unique homogeneous source the smznd isotopic heterogeneity even at the scale of one unit implies the existence of contrasting source rocks and the combination of different mechanisms the gabbro and diorite magmas are assumed to be derived from the melting to different degrees of a depleted upper mantle source in a within plate geotectonic environment whereas a combined wallrock assimilation fractional crystallisation model is proposed for the evolution of the diorite suite in contrast the granitoids are mainly the result of the anatexis of a dominant basic to intermediate igneous heterogeneous continental crust

nd and sr isotope constraints on the origin of the cornubian batholith sw england combined nd and sr isotope data for the hercynian granites of sw england provide important new evidence for the origin of the cornubian batholith the nd isotope systematics are shown to be robust even in areas of extensive hydrothermal alteration e nd values for the main plutons range from 4 7 to 7 1 whereas values for the palaeozoic country rocks are significantly lower typically 8 to 11 depleted mantle nd model ages 1 3 1 8 ga indicate a heterogeneous source region for the granite magmas and imply that the age of the basement beneath southern england is significantly older than previously estimated calculated e nd values for the palaeozoic country rocks at the time of granite intrusion suggest that they were unlikely to have been the main source material for the magmas e nd e sr data for the granites do not define a simple binary mixing array between a mafic depleted mantle melt and proterozoic crust the best fit model is consistent with partial melting of a composite lower crustal source comprising immature metasedimentary and mafic meta volcanic rocks and mixing with a relatively minor contribution of basaltic magma extracted from a slightly enriched mantle source such hybrid magmas are thought to have been generated during an ivrea type underplating of the lower crust during continental collision their isotope signatures being determined by the relative proportions of crust and mantle material

age and emplacement of late variscan granites of the western bohemian massif with main focus on the hauzenberg granitoids european variscides germany abstract the variscan hauzenberg pluton consists of granite and granodiorite that intruded late to postkinematically into ht metamorphic rocks of the moldanubian unit at the southwestern margin of the bohemian massif passauer wald u pb dating of zircon single grains and monazite fractions separated from medium to coarse grained biotite muscovite granite hauzenberg granite ii yielded concordant ages of 320 3 and 329 7 ma interpreted as emplacement age zircons extracted from the younger hauzenberg granodiorite yielded a 207 pb 206 pb mean age of 318 6 4 1 ma the hauzenberg granite i has not been dated the pressure during solidification of the hauzenberg granite ii was estimated at 4 6 0 6 kbar using phengite barometry on magmatic muscovite corresponding to an emplacement depth of 16 18 km the new data are compatible with pre existing cooling ages of biotite and muscovite which indicate the hauzenberg pluton to have cooled below t 250 400 c in upper carboniferous times a compilation of age data from magmatic and metamorphic rocks of the western margin of the bohemian massif suggests a west to northwestward shift of magmatism and ht lp metamorphism with time both processes started at 325 ma within the south bohemian pluton and magmatism ceased at ca 310 ma in the bavarian oberpfalz the slight different timing of ht metamorphism in northern austria and the bavarian forest is interpreted as being the result of partial delamination of mantle lithosphere or removal of the thermal boundary layer

testing for the presence of a terrane boundary within neoproterozoic dalradian to cambrian siliceous turbidites at callander perthshire scotland the southern highland group dalradian and keltie water grit formation which includes the lower cambrian leny limestone form an inverted 1 4 km thick largely arenaceous sequence at callander the grits have the same detrital mineralogy throughout mainly quartz plagioclase an 1 3 muscovite and biotite chlorite formed from detrital biotite during low grade regional metamorphism t 40 ar 39 ar laser fusion dating of detrital muscovites yields an age spectrum with a peak at 1600 1800 ma in the dalradian rocks similar old ages occur in the keltie water grits but are diluted by ages of 507 886 ma we interpret these new data as showing that the rocks were most likely deposited as a single sequence possibly with a disconformity in neoproterozoic to early cambrian times before the onset of grampian orogenesis in the early palaeozoic no major structural or stratigraphical breaks have been identified and there is no direct evidence for the presence of two separate terranes

origin of late variscan granitoids from ne bavaria germany exemplified by ree and nd isotope systematics sm nd isotopic results are presented for late and post tectonic granitoids comprising peraluminous granites and subordinate metaluminous dioritic rocks redwitzites from the fichtelgebirge fg and northern oberpfalz nop ne bavaria the data combined with a number of earlier geochromological studies on these granitoids place severe constraints on the crustal evolution of this region during the carboniferous and pre carboniferous epoch redwitzites range in nd t from 4 to 0 inclusive data from p k holl and coworkers the nd t values of the granites s s are restricted to overlapping ranges of 8 to 3 fg and 8 to 2 nop in both domains the older granites 330 325 ma are characterized by nd t values of 4 whilst the younger granites 315 305 and 290 ma have nd t values of 4 the diversity observed in nd isotopic characteristic is interpreted in terms of different source material the nd isotopic compositions of the redwitzites exhibit source heterogeneity suggesting mixing between mantle magma and crust the nd isotopic features of the older granites are consistent with the magmatic precursors having been generated by partial melting of pre existing mature crust variably contaminated by mantle material or alternatively by melting of chemically less evolved crust resembling paragneisses of teh zev structural unit the origin of the younger granites can be confidently linked to anatexis of common moldanubian and saxothuringian metasediments two stage nd model ages tdm of the granitoids are in the range 1 1 1 7 ga these ages provide further support of substantial involvement of pre phanerozoic crust in the generation of the granitoids the leuchtenberg granite and the g2 and g3 granites yield late variscan sm nd isochron ages which are concordant with previous rb sr data on these granites by contrast the nd isotopic data for barnau and flossenburg define pseudo isochrons suggestive of mixing between contrasting felsic melts

extending a thickened crustal bulge toward a new geodynamic evolution model of the paleozoic nw bohemian massif german continental deep drilling site se germany abstract fault bounded tectonic metamorphic complexes assembling the nw bohemian massif around the german continental deep drilling ktb site are seen to be extremely heterogeneous in tectonic and metamorphic histories in current models the different complexes were supposed to reflect a puzzle of small pre devonian microplates and the related collision events supposedly lasted until the carboniferous opposed to these models it will be shown that all the boundaries among the complexes were formed by detachment late in a prolonged overall geodynamic history of a thickened crustal bulge during extensional tectonics and associated thermal events that outlasted the onset of collision in the silurian lower devonian by about 70 80 ma micro structures petrological and geochronological data of individual complexes predominantly preserve the late stages rather than the unbroken record of their tectonometamorphic histories such partial histories strongly different among individual complexes depict diverse snapshots taken at different places in the evolving thickened crustal bulge and at different instants in its overall evolution and do not define different precollisional microplates predominantly p t and deformation episodes after terrane juxtaposition are preserved this article presents an integrated view of the structural geology microscopic fabrics p t data and geochronology of such diverse metamorphic complexes this integrated view provides a new understanding of 1 the tectonic evolution during upper silurian devonian collision of the gondwana derived central european lithosphere with laurussia 2 the postaccretionary events that lasted through the upper carboniferous and 3 the earlier lower ordovician metamorphic and magmatic history which is only locally recorded metamorphic complexes occupying the structurally highest position upper tectonic complexes record devonian and earlier tectonometamorphic and magmatic events after the mid devonian such complexes did not experience any metamorphism the recorded devonian events consist of subduction and exhumation of hp rocks and their exhumation involved thrusting and extensional tectonics upper tectonic complexes comprise fragments of both the plate that was subducted in this period and the overriding plate in the footwall carboniferous extension has brought upper tectonic complexes against metamorphic complexes that essentially record lower and upper carboniferous tectonometamorphic and magmatic events lower tectonic complexes in the lower tectonic complexes such events are 1 consecutive extensional stages that created at least three sets of ductile normal detachment systems intersecting each other and various associated thermal pulses as well as 2 predetachment events that are only recorded as petrologic and structural relics transposed during extension inferred as predetachment events are crustal subduction as well as stacking that outlasted thrusting and exhumation of the upper tectonic complexes in the deeper portion of the thickened crustal bulge represented by the lower tectonic complexes spatial variations of p t t d histories during decompression occurred as a result of continued differential exhumation on the three sets of detachment systems and also from various thermal pulses that have affected different parts of this section during progressive shearing and exhumation to various degrees whereas the lower tectonic complexes of the erzgebirge gneiss dome preserve the record of a lower carboniferous history of hp metamorphism and crustal thickening followed by extension in those of the oberpfalz region ktb site a major upper carboniferous thermal pulse mostly erased the pre upper carboniferous strain fabrics and metamorphic record in the oberpfalz region ongoing extension emplaced mid crustal rocks that pervasively equilibrated in the upper carboniferous at high temperatures and low pressures low p t ratio against rocks not exposed to high temperatures at that time in summary a prolonged postaccretionary history concealed large scale structural relationships linked to crustal subduction and thickening associated with former juxtaposition of gondwana lithosphere to laurussia polyphase crustal extension and strike slip created the variety of different metamorphic tectonic complexes observed in the nw bohemian massif

brittle deformation events at the western border of the bohemian massif germany investigations of brittle deformation structures present within the crystalline rocks of the bavarian oberpfalz reveal a complex late to post variscan crustal evolution upper carboniferous mainly westphalian granites were emplaced into semibrittle to brittle rocks of the zev zone of erbendorf vohenstraus and the egz erbendorf greenschist unit respectively from both the alignment of the granites and the direction of granite related tension gashes a north east south west extension must be assumed for the period of magmatic activities apart from the granite intrusions rapid crustal uplift about 1 5 km my led to an increase in the geothermal gradient from 40 c km late variscan post granitic the increased geothermal gradient persisted during the succeeding reverse faulting which results from late carboniferous probably stephanian east west and northeast south west compression although not evidenced directly in the area considered strike slip faults seem to have played an important part during the late variscan crustal evolution particularly in the early permian the strike slip events indicate further crustal shortening and indentation under north south compression a similar indentation was present in cretaceous time after a weak phase of early cretaceous reverse faulting which results from north south compression strike slip faults formed under north west south east and north south compression all these faults in particular the strike slip faults seem to be related to the cretaceous and lowermost tertiary convergence of the alpine carpathian orogeny a late stage of crustal extension characterized by a radial stress tensor 2 3 is indicated through high angle normal faults which probably formed during the subsidence of the adjacent neogene eger graben

quartz and feldspar zoning in the eastern erzgebirge volcano plutonic complex germany czech republic evidence of multiple magma mixing abstract zoned quartz and feldspar phenocrysts of the upper carboniferous eastern erzgebirge volcano plutonic complex were studied by cathodoluminescence and minor and trace element profiling the results verify the suitability of quartz and feldspar phenocrysts as recorders of differentiation trends magma mixing and recharge events and suggest that much heterogeneity in plutonic systems may be overlooked on a whole rock scale multiple resorption surfaces and zones element concentration steps in zoned quartz ti and feldspar phenocrysts anorthite content ba sr and plagioclase mantled k feldspars etc indicate mixing of silicic magma with a more mafic magma for several magmatic phases of the eastern erzgebirge volcano plutonic complex generally feldspar appears to be sensitive to the physicochemical changes of the melt whereas quartz phenocrysts are more stable and can survive a longer period of evolution and final effusion of silicic magmas the regional distribution of mixing compatible textures suggests that magma mingling and mixing was a major process in the evolution of these late variscan granites and associated volcanic rocks quartz phenocrysts from 14 magmatic phases of the eastern erzgebirge volcano plutonic complex provide information on the relative timing of different mixing processes storage and recharge allowing a model for the distribution of magma reservoirs in space and time at least two levels of magma storage are envisioned deep reservoirs between 24 and 17 km the crystallisation level of quartz phenocrysts and subvolcanic reservoirs between 13 and 6 km deflation of the shallow reservoirs during the extrusion of the teplice rhyolites triggered the formation of the altenberg teplice caldera above the eastern erzgebirge volcano plutonic complex the deep magma reservoir of the teplice rhyolite also has a genetic relationship to the younger mineralised a type granites as indicated by quartz phenocryst populations the pre caldera biotite granites and the rhyodacitic schonfeld volcanic rocks represent temporally and spatially separate magma sources however the deep magma reservoir of both is assumed to have been at a depth of 24 17 km the drastic chemical contrast between the pre caldera schonfeld westfalian b c and the syn caldera teplice westfalian c d volcanic rocks is related to the change from late orogenic geotectonic environment to post orogenic faulting and is considered an important chronostratigraphic marker

detrital muscovite 40ar 39ar ages from carboniferous sandstones of the british isles provenance and implications for the uplift history of orogenic belts major progradations of clastic sediments are recorded in the sedimentary record of the famennian and the visean namurian of the united kingdom and surrounding waters we have determined 40ar 39ar ages of 162 detrital muscovites from 11 coarse sandstones which were deposited between 370 and 465 ma spanning both progradations detrital mica ages are dominated by a peak at 415 ma with minor peaks at 440 ma and 390 ma the 415 ma muscovites are derived from the unroofing of the scandian nappes during the compressional phase of the caledonian orogeny in scandinavia the 440 ma muscovites record pre scandian orogenic activity which is rarely preserved in the orogenic record thermochronological evidence suggests that episodic postorogenic uplift and exhumation events kept the scandian orogen a major topographic feature and likely a sediment source for over 100 million years after nappe emplacement and implicates tectonic rather than climatic control on the clastic sediment progradations the near total absence of detrital muscovites with ages 415 ma suggests that the scandian nappes had not been entirely eroded despite repeated uplift during the postorogenic extension the river s which supplied the sediments probably ran parallel to the strike of the major scandian thrusts along the length of the caledonian orogen in a manner analogous to the major river systems of contemporary orogenic highlands

physical modelling of deformation in the tasman orogenic zone abstract structural vergence within the western subprovince of the lachlan fold belt is towards the hinterland rather than the foreland in contrast to many well known orogenic belts high angle reverse faults and upright folds verge eastwards away from the australian craton towards the inferred centre of orogenic and magmatic activity we designed a series of analogue models to test the anomalous vergence in the western lachlan fold belt particularly the interaction of a stable australian craton with tasman line geometry interacting with weaker oceanic or transitional lithospheric material we found consistently that vergence direction in the models was towards the hinterland not the foreland as in the western lachlan fold belt irrespective of the way the model was deformed strength gradients between the oceanic and cratonic lithosphere control the deformation patterns an important result of the models is that they demonstrate that fold belts with different vergences can be generated without the requirement of subducting oceanic lithosphere

granitic magma transport by fracture propagation granitic magmas commonly ascend tens of kilometres from their source terranes to upper crustal emplacement levels or to the earth s surface apart from its obvious bearing on the interpretation of the geology and geochemistry of felsic igneous rocks the magma ascent mechanism critically affects any modelling of the metamorphic evolution of the upper lithosphere as well as its rheology we propose that in general granitic magmas ascend via propagating fractures as dykes in extremely short time periods long distance diapiric transport of granitoid magmas through crustal sections is not viable on thermal or mechanical grounds and there is an apparent total absence of field evidence for diapiric rise of such magmas even in supposed type localities neither the shapes nor internal or external characteristics of granitic plutons necessarily reveal anything about the transport of their precursor magmas these are purely arrival phenomena dictated by local structure kinematics and stress states based on existing numerical treatments of the problem we show that granitic magmas are apparently sufficiently inviscid to travel through fractures to high crustal levels without suffering thermal death by freezing for example a 2000 km3 granitic batholith can be inflated by a 1 km 3m 20 km deep dyke system in less than 900 years the model proposed has numerous far reaching petrological and rheological consequences some of which are outlined

experimental geochemical mineralogical and o isotope constraints on the late magmatic history of the fichtelgebirge granites germany mineralogical experimental geochemical and stable isotope data give evidence for an aqueous late stage fluid which develops increasingly from the less to the more highly evolved granites of the fichtelgebirge g1 g4 in less evolved granites particularly g1 only minor chloritization and sericitization minor hydration and corrosion of zircons low leachable fractions of sr and zr the nearly chondritic ratios of zr hf and y ho and rare earth element ree patterns lacking the tetrad effect provide evidence of a less differentiated granitic melt system poor in complexing agents and water with insignificant late magmatic fluid rock interaction the subsequent low temperature alteration which is suggested by oxygen isotope composition of quartz and biotite did not affect the whole rock chemical composition in the highly evolved granites particularly g4 albitization sericitization and fluoritization ubiquitous hydration and corrosion of zircons high leachable fractions of sr and zr zr hf and y ho ratios very different from the chondritic ratios and ree patterns showing a marked tetrad effect point to the presence of a highly specialized fluid system rich in water and complexing agents such as fluorine leading to a marked magmatic hydrothermal alteration the oxygen isotopes also indicate alteration by high temperature fluids which were likely exsolved during the final stage of crystallization

the origin of granite the role and source of water in the evolution of granitic magmas temperature and water content are the two most important parameters in the formation of granitic magmas evidence from volcanic and plutonic lithologies suggests that water contents of 2 to 4 wt are present in most silicic magmas calculations based on the stability of biotite yield water fugacities within the melt phase from about 500 to 2 000 bars although these calculations determine the log f h 2 o and the effective uncertainty of 0 5 log units yields a large absolute uncertainty comparison with crystallization experiments demonstrates that less than 2 water would require significant percentages of crystallization at temperatures above 900 c with liquidus temperatures about 1000 c water contents greatly in excess of 4 wt would mean that the magma would become vapor saturated at high pressures and would tend to crystallize during ascent to a fine grained granite before reaching shallow depths the main sources of water for magma generation are the dehydration of hydrous silicates within the crust and volatiles transported into the crust from subducted oceanic crust and upper mantle in the form of hydrous basalts and andesites dehydration reactions of muscovite biotite and horn blende are of particular significance anatectic granites may be partially classified in terms of the probable dehydration reaction responsible for their generation melts generated from muscovite dehydration are relatively cool peraluminous in composition high in k na ratio and generally high in initial 87 sr 86 sr and delta 18 o biotite generated melts tend to be higher in temperatures peraluminous to meta aluminous in composition moderately high in k na ratio and relatively high in strontium and oxygen ratios both types may contain metasedimentary enclaves granitoids generated by hornblende dehydration would be much higher in initial temperature peralka line to meta aluminous in composition and lower in k na ratio and in general would have lower strontium and oxygen ratios volatiles deposited in hydrothermally altered oceanic crust and upper mantle will be released during subduction in the form of hydrous basalts and andesites these melts are important energy transfer mechanisms to drive anatexis within the crust if these melts encounter a silicic magma chamber when intruding the crust they will be trapped below the lower density granitic melt while partially quenching against the cooler melt they may release volatiles and heat which will aid in the melting process granites generated in this way will vary in their geochemical properties depending on the relative importance of crustal versus mantle magma systems in general they will have moderate to high initial temperatures be meta aluminous in composition be variable but somewhat low in k na ratio and have lower initial strontium and delta 18 o ratios than will melts derived completely from the crust the fact that most terrestrial spreading centers are subaqueous thus may aid in the formation of a thick granitic continental crust

autometasomatic rb enrichments in highly evolved granites causing lowered rbsr isochron intercepts abstract whole rock samples of the younger members of a suite of the erzgebirge granites yield rb sr isochrons with remarkably low intercepts 0 689 0 702 whereas the older granites yield initial sr isotope ratios which are higher and rather uniform 0 70677 22 a model is proposed to explain this observation by a two stage evolution of the younger granites after a time span t 1 following the intrusion autometasomatic processes caused by residual magmatic solutions increase the rb contents of the younger granites by more or less the same amount this rb addition lowers the intercepts of the isochrons but it leaves their slopes undisturbed the rb amounts added during autometasomatosis can be derived by a petrogenetic model of fractional crystallization the time t 1 may be calculated approximately on the basis that the true initial ratio is at least as high as those of the older granite suite t 1 is in the order of 10 ma and agrees well with the difference in age between whole rock data and the rb sr mineral ages of metasomatically formed dark micas in the rocks 10 20 ma and with k ar data it is concluded that decreased granite isochron intercepts may be more common than hitherto expected

are strongly peraluminous magmas derived from pelitic sedimentary sources igneous compositions that require the presence of a phase more aluminous than biotite are here designated as strongly peraluminous ps such compositions are common but subordinate in felsic rocks and exceedingly rare in more mafic rocks the term s type has been applied to many igneous rocks based largely or even solely upon their ps compositions neglecting or placing less emphasis on other criteria for sedimentary parentage in chappell and white s i and s type classification system the s type designation was in fact initially restricted to granites which were thought to have been derived from a chemically mature strongly weathered pelitic shaly source application of chappell s and white s criteria together with others based upon isotopic and phase equilibria considerations leads to the following conclusions about ps igneous rocks worldwide 1 very few ps rocks are derived entirely from pelitic sources 2 a majority are derived largely from intermediate to felsic crustal sources including

diatom 63µm distribution offshore of eastern new zealand surface sediment record and temperature transfer function abstract diatom microfossils preserved in 38 core top samples from offshore of eastern new zealand are examined to develop the first offshore modern analogue dataset for diatoms in new zealand waters the sample set consists of seven approximately e w trending depth transects with samples from the continental shelf slope and abyssal plain between latitudes 33 and 54 s the identification of terrestrial and coastal species in open marine core top samples indicates that offshore transport of diatom microfossils occurs to an extent that could be useful for sediment provenance studies or at least to enable improved identification of transported versus in situ deposition on the continental shelf cluster analysis and ordination indicate that the chatham rise and or the overlying subtropical front are the most significant bathymetric oceanographic features in the dataset as this location shows up as a major barrier or transition zone in terms of diatom distribution the strongest environmental control on diatom distribution is the north south sea surface temperature gradient a statistically strong relationship r 2 0 81 rmsep 1 57 exists between diatom abundance and sea surface temperature making diatoms ideal for quantitative reconstruction of past sea surface temperatures in future paleoclimate studies

continental crust formation seen through the sr and nd isotope systematics of s type granites in the hercynian belt of western france the isotopic compositions of sr nd and pb in leucogranites which are intercorrelated bernard griffiths et al 1985 1 may be explained by the mixing of ancient basement 1800 ma with juvenile crust late precambrian or early palaeozoic this hypothesis does not involve the existence of mid proterozoic crust as apparently indicated by thetdm model ages of the leucogranites ranging between 1600 and 1100 ma the nd isotopes reveal the crustal reworking while sr isotopes mainly record juvenile crust formation this paradox is explained by the geochemical heterogeneity of the sources involved

aluminous subsolvus anorogenic granite genesis in the light of nd isotopic heterogeneity the problem of the genesis of anorogenic granites is far from being resolved and this may result from their petrogenetic variability several kinds of a type granites are indeed present in many anorogenic provinces a peralkaline and an aluminous trend commonly occur and the latter often comprises hypersolvus and subsolvus granites as an example of the aluminous subsolvus group we have studied major trace elements and nd sr isotopes of the tana granite sw corsica france along with the coeval mafic and intermediate rocks petrological and trace element constraints suggest that the associated mafic rocks are not representative of the magmatic precursor of the granite rather the intermediate rocks monzonites are more akin to the parental magma that yielded the granite through fractional crystallization the aluminous subsolvus anorogenic granite and related monzonite are characterized by variable nd isotopic compositions ndi from 4 7 to 1 3 and from 2 to 0 1 respectively while sr isotopes give good isochrons it is argued that the nd isotopic heterogeneity was not acquired at a late stage in the upper crust by contamination from partial crustal melts or f rich late to post magmatic fluids similarly the lack of correlation between trace element concentrations and nd isotopes in a single granitic body precludes a process of crustal contamination of mantle melts instead it is inferred that these low and very heterogeneous ndi s were probably inherited from the magma source region in the lower crust the preservation of this heterogeneity during magma ascent suggests that only limited mechanical and chemical mixing occurred in magma chambers it is more likely that separate batches of low viscosity a type magmas ascended individually mainly through fracture propagation in an extensional regime the possible derivation of granites from monzonites through fractional crystallization together with the examination of the corsican lower crust composition and already published dehydration melting experiments suggest that the aluminous subsolvus trend of a type granites may be generated through the partial melting of metamorphosed amphibole bearing and k2o rich mafic cumulates that have not undergone a previous partial melting event

isotopic and trace element arguments for the lower crustal origin of hercynian granitoids and pre hercynian orthogneisses massif central france abstract the cadomian hercynian basement of the massif central france contains both pre metamorphic orthogneisses 600 410 ma and post metamorphic undeformed granitoids 360 275 ma major trace element and sr nd isotopic analyses of these intrusions indicate slight differences in source material for orthogneisses and two types of underformed granitoid a granodiorite monzogranites and leucogranites orthogneisses have moderately enriched ree patterns la yb 5 3 13 3 variable hree contents ybn 4 2 25 and strong negative eu anomalies eu eu 0 23 0 36 their tnd values are low 5 8 to 1 7 indicating a strong crustal component in the source but tsr values are also generally low 25 to 100 and overlap the field of age corrected isotopic values for lower crustal granulite xenoliths from the massif central the source of the orthogneisses is therefore considered to be dominated by a lower crustal meta igneous component with additional input from a lower crustal metasedimentary source granodiorites monzogranites tend to show strong lree enrichment la yb 11 26 with small eu anomalies eu eu 0 54 0 75 and ybn values between 6 and 8 5 these granitoids form the least evolved end members of a series trending to highly evolved leucogranites with similar la yb ratios 11 36 but with generally larger eu anomalies eu eu 0 24 0 62 and lower ybn values 3 8 initial isotopic ratios of the leucogranites tnd 7 9 to 4 5 and tsr usually 100 mostly fall within the field of age corrected lower crustal metasedimentary granulite xenoliths from the region and it is proposed that these leucogranites were formed by partial melting of lower crustal metasediments granodiorites and monzogranites have tnd between 6 5 and 3 4 and tsr values

sources of hercynian granitoids from the french massif central inferences from nd isotopes and consequences for crustal evolution the french massif central is characterized by a great abundance and variety of granitoids emplaced during hercynian times 360 280 ma distinct nd i and 87sr86sr i are not associated with particular petrographic types however lower nd i values 8 5 occur among the younger leucogranites from the western part of the massif intermediate nd i 6 correlate with aluminous monzogranites and higher values 6 to 3 with calc alkaline granodiorites in the eastern massif central nd i are homogeneous at the regional scale in the western massif central the leucogranites and the monzogranite granodiorites display different trends of nd i with emplacement ages from these results extreme models involving a depleted mantle or an old archean or early proterozoic source may be ruled out the trend of increasing nd i with decreasing age observed in the monzogranite granodiorite group suggests either an increasing contribution of a mantle component or the progressive melting of more mafic more refractory crustal sources or both the reverse trend displayed by the leucogranites might be explained in terms of the upward migration of partial melting across an isotopically zoned metasedimentary pile the large amount of granites may be the result of favourable processes during the collision tectonics combined with the unusual abundance of fertile materials enriched in water and heat producing elements in the pre carboniferous crust

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

caledonian ages in variscan rocks rb sr and sm nd isotopic variations in dioritic intrusives from the northwestern bohemian massif west germany abstract dioritic intrusives and granites from the northwestern bohemian massif west germany were analyzed for the chemical and isotopic composition of rb sr and sm nd the marktredwitz intrusive suite and the tirschenreuth mahring sill intrusions yield apparent rb sr whole rock isochron ages of 468 9 ma and 470 33 ma respectively while the reuth erbendorf granodiorites display a scatter with an apparent age of 545 16 ma the rb sr whole rock ages conflict with geological evidence the diorites intruded metamorphic country rocks which experienced an early devonian metamorphism contact relationships between the diorites and porphyritic granites indicate nearly contemporary emplacement the intrusion ages of two porphyritic granites from this area have been dated at 319 ma and 311 ma the diorites display a variation in nd 320 ma ranging from 1 0 to 4 4 whereas the two granites yield homogeneous initial nd values of 3 7 and 6 6 the variations in elemental concentrations and initial isotopic ratios clearly show a mixing trend within the dioritic rock suites the mixing partners are a presumably mantle derived magma and crustal components of varying composition the isotope systematics of the diorites and the concordance of their rb sr whole rock age values with caledonian rock forming events in central europe point to a pre intrusive caledonian event inherited in these rocks we suggest that the diorites were formed by multistage anatectic processes

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

genesis of granitoid batholiths of peninsular malaysia and implications for models of crustal evolution evidence from a ndsr isotopic and upb zircon study nd sr and u pb isotopic data for the late triassic west coast province batholiths and permian to triassic east coast province batholiths of peninsular malaysia allow estimates of the ages of the crustal fragments comprising the peninsula to be made initial nd and sr values for granitoids from the west coast province range from 6 to 10 and 160 0 716 to 660 0 751 respectively nd model ages calculated based on a depleted mantle evolution model tdmnd range from 1300 ma to 1800 ma and are in general agreement with the mid proterozoic upper intersection ages of u pb zircon reverse discordia 1500 1700 ma initial nd and sr values for granitoids from the east coast province range from 0 8 to 6 and 10 0 705 to 130 0 714 respectively calculated tdmnd ages of 900 1400 ma for these granitoids are comparable to two u pb zircon reverse discordia intercepts that yield 800 ma and 1350 ma the general agreement of u pb zircon inheritance ages and tdmnd ages are interpreted to correspond to the proterozoic crust formation ages of the continental fragments represented by the west coast and east coast batholithic provinces mid proterozoic 1300 1900 ma crust formation ages are commonly shown by other phanerozoic continental margin plutonic and volcanic belts the ubiquitous mid proterozoic crust formation ages and the absence of archaean signatures suggest voluminous juvenile additions to the continental crust in the mid proterozoic such ages at continental margins would imply that many continental blocks had achieved very much their present day extent by the mid proterozoic

geochronology geochemistry and isotope compositions of piaochi s type granitic intrusion in the qinling orogen central china petrogenesis and tectonic significance abstract s type peraluminous granite can crop out in various tectonic settings the paleozoic piaochi granitic intrusion is a large s type peraluminous intrusion located in the north qinling orogen and has important implication for its tectonic evolution though intensively studied the age and tectonic setting of this intrusion are still controversial in this study we carried out an integrated study of zircon u pb age major and trace elements and sr nd hf o isotope compositions for the piaochi intrusion with aims to constrain its petrogenesis and tectonic significance sims and la icpms u pb zircon data obtained from magmatic zircons indicate that the piaochi intrusion was emplaced at 473 4 ma and contained ca 490 ma inherited zircons the granite samples from the piaochi intrusion show typical s type characteristics such as the relatively moderate aluminum saturation index 1 00 1 08 low zirconium saturation temperatures occurrences of muscovite and high zircon 18 o values 8 70 to 9 54 the relatively low sm nd 0 13 0 20 and feo t mgo ratios 2 21 3 9 suggest that the granites are unfractionated the geochemical compositions reveal that they were derived from partial melting of a clay poor but plagioclase and biotite rich psammitic source similar to metasedimentary rocks outcropping in the north qinling unit moreover the piaochi granites are moderately depleted in eu and heavy rare earth elements and have relatively high la yb n values 22 8 56 8 implying a high pressure plagioclase poor and garnet rich residual assemblage the piaochi granites have high and variable initial 87 sr 86 sr ratios 0 72167 0 72511 and low e nd t from 10 23 to 8 71 values with two stage nd model ages t dm2 of 1 91 2 03 ga which are comparable with those of supracrustal materials in the north qinling unit in addition our results show that the ca 470 ma magmatic zircons display a narrow e hf t array within those of the inherited cores these imply that the piaochi granites inherited their isotopic characteristics wholly from their sources and the hf isotopes might have been differently homogenized during the partial melting our new zircon u pb ages indicate that the piaochi granites were emplaced after the climax of the uhp metamorphism coeval with later retrograde metamorphism during exhumation therefore we suggest that the formation of the early paleozoic piaochi intrusion was triggered by the exhumation of the uhp rocks

late variscan magmatism revisited new implications from pb evaporation zircon ages on the emplacement of redwitzites and granites in ne bavaria pb evaporation zircon analyses coupled with a detailed cathodoluminescence cl study on the complete series of granitoids from the northern oberpfalz ne bavaria provide new evidence for the commencement and timing of late variscan magmatism all granitoids analysed in this study were dated before by rb sr and or k ar methods investigated samples comprise medium grained i type dioritic rocks redwitzites i s type granites leuchtenberg marktredwitz g1 zainhammer and s type granites falkenberg liebenstein mitterteich friedenfels steinwald flossenburg barnau the zircon evaporation technique reveals three groups of 207pb 206pb ages which are interpreted to represent magmatic crystallisation 1 ages of 324 321 ma are found in all analysed redwitzites and in almost all i s type granites 2 the granites of falkenberg and liebenstein yield ages of 315 ma 3 ages between 312 and 310 ma are recorded in the mitterteich friedenfels steinwald and flossenburg granites titanite crystals from different redwitzite bodies yield conventional u pb ages of 325 322 ma identical to the pb evaporation zircon data of these rocks the s type granites of groups 2 and 3 contain zircons with relict cores but only a small number of them yield older ages indicating that some of the cores must have lost their radiogenic pb from the geochronological data we infer that metamorphic conditions of the variscan crust produced different granite types at different times the data support a model involving an early period of mantle related magmatism which postdates the final convergence stage of the variscan orogen this magmatic activity was at the same time as the thermal peak of regional metamorphism and is followed by a late period of crustal anatexis which is probably related to post collisional extension of the thickened variscan crust

source of the lachlan fold belt flysch linked to convective removal of the lithospheric mantle and rapid exhumation of the delamerian ross fold belt during the cambrian ordovician the paleo pacific margin of gondwana underwent a rapid transition from a site of convergent deformation delamerian ross orogeny to one of sedimentation during which the thick turbiditic sequences of the adjacent basal lachlan fold belt were deposited laser probe 40 ar 39 ar data show that the synkinematic 523 486 ma mid crustal i and s type granites and metamorphic country rocks of the delamerian fold belt were rapidly cooled at 490 485 ma coincident with the intrusion of a suite of high level postkinematic a type granites and gabbros ca 497 481 ma 40 ar 39 ar data on detrital muscovites from the basal sections of the lachlan fold belt provide information on exhumation rather than cooling but yield an age pattern 507 480 ma identical to that of the delamerian fold belt demonstrating that this was the source of the sediments the combined data require that some 15 km of exhumation occurred very rapidly 5 15 mm yr 1 coincident as implied by overlapping ages with the cessation of convergent deformation and partial melting to form the postkinematic magmas this scenario implies a causative link which we infer to have been convective removal of lithospheric mantle following orogenic thickening the model is analogous to the tertiary uplift of the india asia orogen which provided the sediment source for the bengal fan our results suggest that similar processes were important in the evolution of mountain belts at least as far back as the early phanerozoic

geochemical and isotopic composition and inherited zircon ages as evidence for lower crustal origin of two variscan s type granites in the nw bohemian massif geochemical and nd sr isotopic compositions and u pb zircon ages of two variscan granites neunburg and oberviechtach from southern oberpfalz nw bohemian massif have been investigated in order to place constraints on their formation and on the crustal reworking both granites exhibit similar mineralogical chemical and isotopic characteristics they have peraluminous compositions a cnk ratios 1 2 1 3 and display high k2o na2o ratios of 2 2 2 3 consistent with typical s type granites in terms of trace elements they show an enrichment of lree and strong fractionation between lree and hree lan ybn ratios 46 to 60 compared with the primordial mantle distinct negative anomalies of several trace elements ba sr nb and ti are also observed in both granites they are further characterised by low initial end values of 6 2 to 8 2 and high initial 87sr 86sr ratios of 0 7114 to 0 7147 zircon u pb data indicate that the intrusion of both granites shortly post dates the ht lp metamorphism of the moldanubian basement and crystallised at about 320 ma the samples studied contain zircons mostly having xenocrystic cores with diverse morphologies these inherited zircons have early proterozoic to early palaeozoic ages this points to melting of sources comprising substantial sedimentary rocks the lan ybn and tbn ybn ratios of both granites are the highest so far reported from granitoids within this region melting of lower crustal rocks leaving garnet as a restite phase in the source provides a viable mechanism to reproduce the ree characteristics

structural model for the saxothuringian moldanubian suture in the variscan basement of the oberpfalz northeastern bavaria f r g interpreted from geophysical data abstract the moldanubian saxothuringian boundary marks a prominent subduction zone in the internal parts of the european variscides in the oberpfalz area deep reflection seismic profiles show a high density of various reflectors mainly in the upper crust indicating a complex polyphase deformation during the collision process seismic structures and related data from surface geology suggest that the moldanubian saxothuringian collision consisted of two main phases the first phase is marked by extensive nw directed overthrusting of moldanubian onto saxothuringian crust along se dipping master decollements some of which can be traced down to the moho during an advanced stage progressive overthrusting led to the development of wedge structures associated with se directed backthrusting and backfolding in the upper structural levels subsequently some of these wedges and their related decollements were folded to form the main ne striking anticlines the first phase of crustal imbrication correlates with a regional low pressure high temperature metamorphism at about 320 ma decreasing in grade northwestward the second phase is characterized by a reorientation of the main direction of crustal shortening from nw se to sw ne which is interpreted as the result of the n directed indentation of the vindelician terrane into the moldanubian this event is recorded by conjugate systems of overthrusting and subsequent strike slip faulting in the oberpfalz and the bavarian forest and the black forest vosges area in the area of the southern german block between these basement outcrops the indentation front is marked by the arc shaped axes of gravimetric and magnetic anomalies and permo carboniferous troughs

constraints on melting and magma production in the crust major intrusions of granitic rocks are found in several tectonic settings and in all cases crustal melts may contribute to the volumes of granitic magma high grade metamorphism and partial melting of the crust take place predominantly under fluid absent conditions we present a model for calculating the amounts of melt that may be formed by fluid absent breakdown of micas and amphiboles in common crustal rock types pelitic quartzofeldspathic intermediate and mafic melt proportions depend mainly on the kind of source rock the pressure at which melting takes place the temperature and the hydrous mineral h2o content of the source as a consequence of the pressure dependence of water solubility in silicate melts any given source rock will produce more melt by a given fluid absent reaction at lower pressure at a given pressure higher temperature reactions can produce more melt from a given source rock based on a survey of the compositions of common rock types we show that the amounts of melt can vary from 50vol thus crustal rocks vary widely in their fertility as magma sources depending on the types and amounts of hydrous phases they contain in general muscovite breakdown in pelites will yield only small quantities of melt and lead to migmatite formation biotite breakdown in pelites occurs at higher temperature and because most high grade pelites below granulite grade are biotite rich can yield up to about 50 vol of granitoid melt rocks of intermediate composition and hornblende rich mafic rocks are potentially highly fertile magma sources also provided that the high temperatures necessary for biotite and hornblende breakdown are realized pyroxene rich mafic rocks and quartzofeldspathic rocks are much less potentially fertile data suggest that mechanisms exist for the efficient segregation of melt and restite in systems with 30and probably 20vol melt the pressure temperature history of a region can greatly influence crustal source fertility through its control over the occurrence of subsolidus dehydration and melting equilibria

provenance of early palaeozoic sandstones southeastern australia part 1 vertical changes through the bengal fan type deposit abstract ordovician strata of the lachlan fold belt in southeastern australia are dominated by a turbidite succession that in its undeformed state was comparable in size to the modern day bengal fan the ancient fan was derived from the northern part of the cambrian ross delamerian orogenic belt that formed at a convergent boundary between plates containing gondwanaland and part of the palaeo pacific ocean respectively substrate to the submarine fan consisted of mafic volcanic rocks with an immature island arc affinity similar to the eocene izu mariana bonin system sandstone compositions determined from several stratigraphically constrained sections demonstrate a uniform framework composition dominated by monocrystalline quartz with less abundant feldspar polycrystalline quartz metamorphic rock fragments sedimentary rock fragments and mica these indicate derivation from a low grade metamorphic terrain up sequence variation is limited to decreasing feldspar and lithic fragments and increasing quartz indicating increased weathering and lowering of topography in the source terrain over time 25 of counted sandstones in the late ordovician interval contain a small component of mafic intermediate volcanic rock fragments that accompanied a change in palaeocurrent direction from eastwards to northwards in the early silurian the ordovician sequence was disrupted by the formation of several major deformation zones between these zones developed several deep marine sedimentary basins that contain a continuous succession from the ordovician into the early silurian early silurian sandstone is similar in composition to underlying early ordovician rocks except that the feldspar component is generally absent early silurian sandstone was derived from uplifted portions of the ordovician sequence and includes texturally inverted types with sandstone containing a bimodal mixture of coarse well rounded quartz and fine angular quartz

dioritic intrusions of the slavkovský les kaiserwald western bohemia their origin and significance in late variscan granitoid magmatism mafic and intermediate intrusions occur in the slavkovsk les as dykes sills and minor tabular bodies emplaced in metamorphic rocks or enclosed in late variscan granites near the sw contact of the western krusne hory erzgebirge granite pluton they are similar in composition and textures to the redwitzites defined in ne bavaria single zircon pb evaporation analyses constrain the age of a quartz monzodiorite at 323 4 4 4 ma and of a granodiorite at 326 1 5 6 ma the p t range of magma crystallization is estimated at 1 4 2 2 kbar and 730 870 c and it accords with a shallow intrusion level of late variscan granites but provides lower crystallization temperatures compared to the bavarian redwitzites we explain the heterogeneous composition of dioritic intrusions in the slavkovsk les by mixing between mafic and felsic magmas with a minor effect of fractional crystallization increased k ba rb sr and ree contents compared to tholeiitic basalts suggest that the parental mafic magma was probably produced by melting of a metasomatised mantle the melts being close to lamprophyre or alkali basalt composition diorites and granodiorites originated from mixed magmas derived by addition of about 25 35 and 50 vol respectively of the acid end member granite to lamprophyre or alkali basalt magma our data stress an important role of mafic magmas in the origin of late variscan granitoids in nw bohemian massif and emphasize the effect of mantle metasomatism on the origin of k rich mafic igneous rocks

two stage erosion and deposition in a continental margin setting an 40ar 39ar laserprobe study of offshore detrital white micas in the norwegian sea detrital white micas from a restricted region of the norwegian sea have been analysed by the 40ar 39ar laserprobe examining the ages of individual white mica grains from offshore triassic jurassic and cretaceous sandstones reveals a narrow range of detrital white mica ages 369 424 ma moreover there is an inverse relationship between sediment age and detrital white mica age with the triassic sands containing a larger proportion of younger white micas and the cretaceous sands containing the oldest white micas in addition residence times of the detritus which is the age of the sediment subtracted from the age of the detrital white mica range from c 150 to c 300 ma for the triassic and cretaceous sands respectively the inverse age relationships combined with the large residence times for these stratigraphic levels can best be explained by an erosion deposition model involving at least two stages taking into account previous provenance studies in the lofoten region of the norwegian sea these new data suggest that detritus shed initially from the exhuming caledonian nappes of nw norway in late silurian early devonian times was deposited in intramontane basins that were themselves subsequently unroofed and re eroded from permo triassic times this caused the recycling of the caledonian detritus from the devono carboniferous basins and led to the inverted detrital age distribution ovserved in the triassic cretaceous sediments examined in this study while sedimentary recycling is not a surprising feature for mesozoic basins in the north and norwegian seas this study documents a previously untapped method 40ar 39ar laserprobe provenance studies by which to identify its effect and extent

recycling processes in granite granodiorite complex genesis the querigut case studied by ndsr isotope systematics abstract using the isotopic rb sr and sm nd petrogenetic tracers we have studied the calc alkaline plutonic complex of querigut pyrenees france which is famous for its acid to mafic rock association and is a typical post tectonic hercynian granitoid its age of 280 m y is already known 2 23 24 and our purpose is to examine the relationship between its different petrographic units and the problem of their source composition the sm nd isotopic results rule out any genetic process involving a unique homogeneous source and imply the presence of old recycled continental materials in contrast the rb sr isotope system seems to have been homogenized by hot fluids maybe post magmatic this is also true for pb isotopes as shown previously by michard vitrac et al 18 on the basis of the isotopic results and the major element data a model of combined fractional crystallization assimilation of country rocks is proposed for the basic part of the complex in contrast acid parts are mainly the result of the continental anatexis this study also constrains the basement s age 1200 1900 m y of the area and suggests a scheme for the gological history of the eastern pyrenees

coeval potassic and sodic calc alkaline series in the post collisional hercynian tanncherfi intrusive complex northeastern morocco geochemical isotopic and geochronological evidence abstract the post collisional late hercynian tanncherfi intrusive complex tic is part of a widespread intrusive episode in the moroccan meseta the complex contains a wide range of rock types from monzogabbros to monzogranites two distinct magmatic series are recognized 1 a potassic shoshonitic series consisting of monzogabbros quartz monzonites and monzogranites and 2 a sodic granodioritic series represented by quartz monzodiorites and granodiorites all the tanncherfi plutonic rocks display similar spider diagram profiles with lile and lree enrichment and nb ta ti depletion which are typical of subduction related magmas combined major trace element compositions and sr nd isotopic results indicate that the two series have been derived from a lile and lree enriched continental lithospheric mantle source under different partial melting and or depth conditions intrusion of the tanncherfi rocks was not temporally related to subduction and the enrichment of their source is likely to be linked to preceding subduction events the two series evolved by fractional crystallization of clinopyroxene plagioclase hornblende biotite k feldspar and accessories fe ti oxide minerals titanite apatite and zircon for the potassic series while the sodic series combined fractional crystallization with assimilation of felsic magmas with lower sr isotopic ratio than the more mafic term of the series the quartz monzodiorite the intrusion of the potassic magmas 344 6 ma marks a major change in the tectonic regime of eastern meseta these magmas intruded during post thickening uplift and extension both probably favored by convective thinning of the lithosphere this model provides a reasonable mechanism for the genesis of other hercynian intrusive complexes in morocco

constraints on variscan granite emplacement in north east bavaria germany further clues from a petrogenetic study of the mitterteich granite petrochemical and rb sr k ar and sm nd isotopic data presented for the mitterteich granite provide information on whole rock and mineral compositional characteristics intrusion and cooling history and protolith nature and put further constraints on the variscan magmatic evolution in north east bavaria

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

the lanthanide tetrad effect and its correlation with k rb eu eu sr eu y ho and zr hf of evolving peraluminous granite suites lanthanide tetrad effects are often observed in ree patterns of more highly evolved variscan peraluminous granites of mid eastern germany central erzgebirge western erzgebirge fichtelgebirge and northern oberpfalz the degree of the tetrad effect te1 3 is estimated and plotted vs k rb sr eu eu eu y ho and zr hf the diagrams reveal that the tetrad effect develops parallel to granite evolution and significant tetrad effects are strictly confined to highly differentiated samples mineral fractionation as a cause for the tetrad effect is not supported by a calculated rayleigh fractionation which also could not explain the fractionation trends of sr eu and eu eu the strong decrease of eu concentrations in highly evolved rocks suggests that eu fractionates between the residual melt and a coexisting aqueous high temperature fluid mineral fractionation as a reason for the tetrad effect is even more unlikely as ree patterns of accessory minerals display similar tetrad effects as the respective host rocks the accessory minerals inherit the ree signature of the melt and do not contribute to the bulk rock tetrad effect via mineral fractionation these results point in summary to significant changes of element fractionation behavior in highly evolved granitic melts ionic radius and charge which commonly control the element distribution between mineral and melt are no longer the exclusive control the tetrad effect and the highly fractionated trace element ratios of y ho and zr hf indicate a trace element behavior that is similar to that in aqueous systems in which chemical complexation is of significant influence this distinct trace element behavior and the common features of magmatic hydrothermal alteration suggest the increasing importance of an aqueous like fluid system during the final stages of granite crystallization the positive correlation of te1 3 with bulk rock fluorine contents hints at the importance of ree fluorine complexation in generating the tetrad effect as the evolution of a ree pattern with tetrad effect m type implies the removal of a respective mirroring ree pattern w type the tetrad effect identifies open system conditions during granite crystallization

anticorrelated rb sr and k ar age discordances leuchtenberg granite ne bavaria germany the leuchtenberg granite oberpfalz ne bavaria displays a continuous differentiation trend ranging from mildy peraluminous coarse grained porphyritic biotite granites bg to strongly peraluminous medium to fine grained garnet bearing muscovite granites gmg the rb sr and k ar age determinations of whole rock and mineral samples from the granite and associated intermediate rocks redwitzites have revealed two divergent age gradients rb sr wholerock dates decrease and initial 87sr 86sr ratios increase for successively more evolved subsets of the granite all bg samples 87rb 86sr 2 16 yield a date of 326 2 ma with a low initial 87sr 86sr ratio of 0 70778 0 00013 1 while all gmg samples 87rb 86sr 70 to 1000 yield a younger date of 317 2 ma with an enhanced initial 87sr 86sr ratio of 0 7146 0 0039 the k ar measurements on biotites and muscovites give closely concordant dates for the gmg 326 323 ma and the southern lobe of the bg 324 320 ma the northern lobe of the bg including the redwitzites shows a well defined trend of decreasing k ar dates from 320 ma to 300 ma towards the northwest critical consideration of both isotope systems leads to the conclusion that the rb sr system of the gmg was disturbed by a later hydrothermal event the ca 326 ma whole rock rb sr date for the bg is not in conflict with any of the k ar mineral dates and is taken as approaching the crystallization age of the leuchtenberg granite the k ar age progression within the northern lobe of the bg indicates that this part either cooled down over a protracted period of some 20 ma or experienced reheating at ca 300 ma the study highlights the potential of combined rb sr and k ar dating in deciphering detailed chronology on the scale of a single igneous intrusion

new 40ar 39ar ages and geochemistry of late carboniferous early permian lamprophyres and related volcanic rocks in the saxothuringian zone of the variscan orogen germany

stable isotope composition of impact glasses from the nordlinger ries impact crater germany abstract the ries impact which today is represented by a 24 km diameter complex crater occurred at 15 ma it is estimated that a projectile of about 1 km diameter formed a transient crater of 6 to 7 km radius and 2 8 km depth impact melt glasses investigated are found within the suevite the breccia that forms the uppermost layers of the ejecta blanket around the crater the glasses are considered to represent quenched melts produced as a result of the impact the oxygen isotope compositions of several glasses sampled from widely spaced localities are very homogeneous with 18 o values in the range of 6 7 to 7 4 and 17 o of 3 3 to 3 7 with increasing devitrification and alteration the 18 o values increase up to 15 8 17 o up to 8 1 water content also increases with devitrification from fresh glasses with about 1 3 wt to 3 8 wt in devitrified glass d values decrease with increasing water content from about 87 to 127 whole rock oxygen isotope compositions of the sedimentary cover sequence range from 17 to 27 and crystalline basement rocks range from 8 8 to 13 5 for granites and gneisses whereas amphibolites have 18 o values of 5 2 and 6 1 models suggesting that the glass in the suevite represents a mixed melt derived from all the rocks present in the suevite in proportion of their occurrence engelhardt 1997 do not agree with the oxygen isotope composition of the glass the simplest explanation for the homogeneous chemical and oxygen isotope composition is that the glass represents quenched melts from a few closely spaced lithologies only high fe and ni contents in the glass cr ni and cr co ratios and relatively low 18 o values indicate involvement of amphibolites during melting the balance of the melt was made up of spatially associated granites or gneisses as indicated by the major element and rare earth element geochemistry the best agreement between major element and oxygen isotope composition of the glass and model melts is obtained by mixtures of amphibolite and granite in proportions similar to their average occurrence in the fallout suevite

early palaeozoic acid magmatism in the saxothuringian belt new insights from a geochemical and isotopic study of orthogneisses and metavolcanic rocks from the fichtelgebirge se germany rocks range from 316 to 298 ma for muscovites and from 306 to orthogneisses and acid metavolcanic rocks from the fichtelgebirge 280 ma for biotites demonstrating thermal influences from late ne bavaria germany are predominantly chemically evolved monvisean to late stephanian 325 290 ma granite intrusions the zo granites and rhyodacites dacites respectively the metavolcanic involvement of dominantly crustal derived melts is considered to rocks are variably tectonized and include samples with anomalously account for the peraluminous character a cnk 1 08 high low cao na2o and mno 80 initial sr sr 0 709 and negative end 500 ma 2 9 to 6 4 wt and k2o 6 wt concentrations implying selective of the intrusive and volcanic rocks the generation of the melts is element mobility during post eruptive events sm nd isotope data assumed to have taken place within an overall extensional geodynamic for samples from three main orthogneiss units wunsiedel selb setting the wider range in end 500 ma for the metavolcanic rocks waldershof yield a composite sm nd whole rock isochron cor 3 8 to 6 4 orthogneisses 2 9 to 4 0 suggests that differential responding to an age of 560 45 ma this age estimate is admixture of a pelagic sedimentary component during emplacement constrained however to be less than 560 ma by the presence of may have occurred nd model ages range from 1 5 to 1 7 ga and detrital cadomian zircons in the country rocks fifteen specimens are consistent with those of granitoids from the erzgebirge and the from the wunsiedel orthogneiss give an rb sr whole rock isochron lausitz but completely different from those of metavolcanic rocks of 480 4 ma with an initial sr sr ratio of 0 7095 from the thuringer wald in addition nd model ages of the early 0 0007 mswd 2 7 rb sr isotope data from the walpalaeozoic granitoids in the fichtelgebirge are consistent with those dershof orthogneiss and the metavolcanic rocks suggest however of the late carboniferous granitoids from the same area this that in general sr isotopic equilibrium if ever reached was suggests similar source material for the pre variscan and the late significantly modified during later events taking recent geovariscan fichtelgebirge granitoids chronological literature data into account it now appears that early palaeozoic acid magmatism in the fichtelgebirge commenced with the intrusion of the orthogneiss precursors during the early and mid ordovician and ended with the eruption of the volcanic

a jurassic garnet bearing granitic pluton from ne china showing tetrad ree patterns abstract ne china is characterized by the massive distribution of phanerozoic granitoids most of them are of i and a type granites whereas s type granites are rarely documented the present work deals with the dongqing pluton a small granitic body emplaced in the southern zhangguangcai range the pluton comprises a two mica garnet granite and a garnet bearing muscovite granite the latter occurs as veins in the former the pluton shows a gradational contact with the surrounding host granites rb sr and sm nd isotope analyses on whole rocks and minerals reveal that the two types of granites were emplaced synchronously at about 160 ma the pluton was emplaced coeval with the surrounding i type granitic pluton and had a rapid cooling history it is characterized by an initial sr isotopic ratio of 0 706 slightly negative end t values 0 5 to 1 9 and young depleted mantle model ages 970 1090 ma this suggests that the parent magma originated from partial melting of relatively juvenile crust which is largely compatible with the general scenario for much of the phanerozoic granitoids emplaced in the central asian orogenic belt geochemically the granites of the dongqing pluton are peraluminous with a shand index molar ratio a cnk of 1 0 1 1 for the two mica granites and 1 2 1 3 for the garnet bearing granites all the garnet bearing granites and some of the two mica granites show tetrad ree patterns tetrad group whereas most two mica granites show normal granitic ree patterns normal group the normal group granites exhibit depletion in nb ta p and ti in spidergrams and generally weak positive eu anomalies in ree patterns by contrast the tetrad group granites manifest depletion in ba nb ta sr p and ti and significant negative eu anomalies the trace element data constrain the parental magmas to having undergone extensive magmatic differentiation during their late stage magmatic evolution intense interaction of residual melts with aqueous hydrothermal fluids resulted in the non charac charge and radius controlled trace element behavior and the tetrad effect in ree distribution patterns this in turn leads to the invalidation of the commonly used tectonic discrimination criteria derived from trace element abundances of normal granites in view of this and previous studies we conclude that there were probably no s type granites produced in ne china during the phanerozoic consequently weathered sedimentary material did not play an important role in the genesis of the strongly peraluminous granites in the zhangguangcai range

neodymium and strontium isotope content of microdiorite enclaves points to mantle input to granitoid production microdiorite enclaves are arguably the most poorly understood of the more prominent features of granitoids surprisingly they have been invoked as support for completely different models for the origin of chemical zonation in granitoids depending on whether they are interpreted as restites syn plutonic blobs of magma remobilized cumulate or basified country rock xenoliths here we present the first detailed neodymium and strontium isotope study of suites of such enclaves from two plutons in both cases we find relatively high nd values most readily explained if enclaves represent syn plutonic injections of mafic magma into the host granitoid magma as such they are the tangible remains of a very important process of mantle derived heat and mass transfer operating in the generation of granitic magmas

the genesis of felsic mafic plutonic associations a sr and nd isotopic study of the hercynian braga granitoid massif northern portugal abstract the late tectonic braga plutonic complex in the centro iberian zone north portugal was emplaced during the hercynian orogeny within a silurian sedimentary sequence and displays an acid basic association which consists of three well defined intrusions the biotite dominant braga granite minor bodies of gabbro to granodiorite composition and the mildly peraluminous gonca leucogranite these three plutonic suites present field relationships indicating a synchronous magmatic emplacement for which a rb sr age of 310 10 ma is obtained the distribution of the three plutonic units along regular curves in major and trace element diagrams suggests that the different units can be genetically related however the rb sr and sm nd isotopic results do not indicate a unique homogeneous source and a simple fractional crystallization process the gabbros have chemical characters high k ba sr and light rare earth elements and low nb ti and zr contents and isotopic compositions sri 0 70497 and nd 2 5 which suggest an alkaline magmatic affinity of shoshonitic type they are probably derived from an enriched mantle source in contrast the peraluminous gonca leucogranite sri 0 70933 and nd 6 8 is more likely the result of crustal anatexis the braga granite and the evolved members of the basic series have an intermediate isotopic composition sri 0 70532 to 0 70733 and nd 3 0 to 6 2 which can be interpreted in terms of an hybridization process between the two previous end members the chemical and sr nd isotopic compositions of the braga plutonic series can be explained by an assimilation fractional crystallization afc model between a mantle derived magma equivalent to the gabbros and a crust derived magma the gonca granite

pyrite and pyrrhotite textures and composition in sediments laminated quartz veins and reefs at bendigo gold mine australia insights for ore genesis the various types textures and compositional zoning of pyrite in the gold bearing saddle reefs quartz veins and surrounding sedimentary rocks provide new information on the potential source and timing of gold and arsenic and related fluid processes responsible for mineralization at bendigo nodular diagenetic pyrite in the black shale tops to sandstone turbidites is enriched in invisible gold and arsenic with mean values of 0 61 ppm au 1 300 ppm as and au ag 1 based on la icpms analyses other elements enriched in the diagenetic pyrite within the organic rich shales are mn zn mo cu v ba ag cd tl co ni bi pb and te in contrast euhedral and growth zoned hydrothermal pyrite in the turbidites and bedding parallel laminated quartz veins contains lower contents of most trace elements but has higher contents of invisible au and as especially on the outermost rim of the pyrite the gold rich pyrite rims generally become thicker a few to hundreds of microns in proximity to the gold bearing saddle reefs pyrite in the reef commonly has the highest levels of invisible au and as and the lowest levels of other trace elements it is characterized by au ag 1 and au pb 0 01 in the deepest stratigraphic levels below known productive gold reefs diagenetic pyrite in the most carbonaceous shales has been replaced by pyrrhotite during metamorphism la icpms analyses reveal that the disseminated pyrrhotite contains similar levels of ni and co to the diagenetic pyrite but is strongly depleted in as and au the spatial relationships between organic rich shales folded bedding parallel laminated quartz veins and gold arsenic bearing saddle reefs combined with the consistent trends in the trace element composition of pyrite hosted by these three geological elements is interpreted to indicate that the black shales were an initial source of au and as and the laminated quartz veins acted as the initial pathways for hydrothermal fluid flow carrying au and as from the source shales to the saddle reefs maximum gold and arsenic input into the reefs principally as free gold plus arsenopyrite occurred late during deformation toward the end of the hydrothermal cycle and is expressed by the au as rich rims to hydrothermal pyrite in the sedimentary host rocks laminated quartz veins and reefs this corresponds with final fold lockup and the development of through going fault arrays linking adjacent anticlines the source of au and as for this final and most economically important fluid flow event is considered to be from carbonaceous shales deeper in the basin where original gold bearing diagenetic arsenian pyrite reacts with organic matter and is converted to pyrrhotite with release of au as and s to the metamorphic fluid

zircon hf isotope perspective on the origin of granitic rocks from eastern bavaria sw bohemian massif the petrogenetic potential of in situ laser ablation hf isotope data from melt precipitated zircons was explored through the analyses of about 700 individual crystals derived from about 20 different granitic intrusions covering the variscan basement segment of eastern bavaria se germany in combination with geochemical features four major suites of granitic rocks can be distinguished 1 ne bavarian redwitzites 52 57 wt sio2 intrusion ages around 323 ma have chondritic ehf t values 0 8 to 0 4 the redwitzites are hybrid rocks and the hf data are permissive of mixing of a mantle progenitor and crustal melts 2 various intermediate rock types dioritic dyke granodiorite palite 59 63 wt sio2 334 320 ma from the bavarian forest yield negative ehf t values between 3 4 and 5 1 these values which apparently contradict a mantle contribution fingerprint an enriched metasomatized mantle component that was mixed with crustal material 3 voluminous major crust forming granites sensu stricto 67 75 wt sio2 328 298 ma are characterized by a range in ehf t values from 0 5 to 5 6 different crustal sources and or modification of crustal melts by various input of juvenile material can explain this variation 4 post plutonic c 299 ma porphyritic dykes of dacitic composition 64 67 wt sio2 from the southern bavarian forest have chondritic ehf t values 0 6 to 1 1 and display large intergrain hf isotope variation the dykes form a separate petrogenetic group and the hf data suggest that the zircons crystallized when a pristine mantle derived parental melt was modified by infiltration of crustal material the zircon hf data form a largely coherent positive array with the whole rock nd data and both systems yield similar two stage depleted mantle model ages 1 1 1 7 ga

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

a reappraisal of the evolution of the palaeo pacific margin of gondwana from the pb and os isotope systematics of igneous rocks from the southern adelaide fold belt south australia abstract we have undertaken pb and os isotopic analyses of igneous rocks from the southern adelaide fold belt to further understanding of the neoproterozoic early palaeozoic evolution of the palaeo pacific margin of gondwana the new pb isotope data and a reappraisal of existing geochronological data suggest that layered mafic intrusions at black hill and cambrai may have intruded much earlier than previously thought intrusion may even have occurred prior to deposition of the normanville and kanmantoo group sediments and the onset of what is traditionally regarded as the delamerian orogeny we tentatively term this earlier event the hallett cove orogeny however equivalent ages are common in the formerly adjacent ross orogen when the pb isotope signatures of the suites of gabbros and i s and a type granites are compared at 487 ma they all span a very large range 206pb 204pbi 15 4 18 7 207pb 204pbi 15 4 15 7 208pb 204pbi 33 0 38 8 compared with other formerly adjacent terranes or the subsequent lachlan fold belt similarly 187os 188osi ratios span 0 122 1 17 but include one gabbro and one rhyolite that have subchondritic 187os 188osi ratios when taken together the combined data do not support models in which the observed range in initial isotope ratios reflects major and variable mid to upper crustal assimilation of magmas derived from the contemporary asthenospheric mantle rather it appears that magmas parental to the gabbros and a type rocks were derived from subduction modified lithospheric mantle that has a median metasomatic age of 1 4 ga large scale pb isotopic homogenization as proposed for the lachlan fold belt did not occur in the adelaide fold belt

depth of intrusion and thermal modeling of the falkenberg granite oberpfalz germany the late carboniferous falkenberg granite exposed 2 km to the east of the german continental deep drilling ktb drill site has solidified at a depth of approximately 9 12 km the initial temperature of the intrusion was 780 800 c the shape of the pluton is approximately that of a horizontal plate with an assumed original thickness of approximately 9 km 3 km of which having now been removed by erosion the results of simple one dimensional thermal modeling based on conductive heat transfer suggest cooling to 400 and 350 c over approximately 6 and 15 m y respectively with respect to the cooling ages of micas this suggests that the intrusion is somewhat older than previously assumed the lack of thermal influence in the nearby crustal section recovered by ktb compared with the width of the contact aureole inferred from the model is explained by considerable convergence between contact and drilling site the initial synemplacement distance between the granite wall rock contact and the ktb location was at least twice the recent value this is consistent with mesozoic crustal shortening which has resulted in the antiformal stack geometry of the supracrustal slices drilled by ktb

the early paleozoic xitieshan syn collisional granite in the north qaidam ultrahigh pressure metamorphic belt nw china petrogenesis and implications for continental crust growth abstract syn collisional magmatism produced by partial melting of subducted oceanic and continental crust during the continental collision plays an important role in understanding the orogenic evolution and crustal growth this contribution reports zircon la icpms u pb ages and hf isotopic compositions whole rock major and trace elements and sr nd isotopic compositions of syn collisional xitieshan intrusion within the north qaidam ultrahigh pressure metamorphic uhpm belt to study its petrogenesis and contribution to continental crust growth la icpms zircon u pb dating of the xitieshan granite yields magmatic crystallization ages of 441 2 ma and 442 2 ma which are consistent with the peak age of ultrahigh pressure eclogite facies metamorphism in the xiteishan terrane the temporal correlation between them confirms that the xitieshan granite was products of syn collisional magmatism during the continental collision between qaidam and qilian blocks these granites show high k calc alkaline and slightly peraluminous signature low zirconium saturation temperatures and high contents of k2o with the s type characteristics they have rare earth element and trace element patterns resembling those of bulk continental crust with strong fractionation of light and heavy rare earth elements la yb n 19 26 moderately eu negative anomalies eu 0 65 0 71 and the obviously nb ta p and ti negative anomalies the xitieshan granite also exhibits remarkable sr nd isotopic differences initial 87sr 86sr 0 70920 0 71080 and end t 4 54 4 11 from the contemporaneous granites within north qaidam uhpm belt combined with the positive ehf t 0 5 5 3 and ages of inherited zircons 475 518 ma the magmatism is best explained as resulting from melting of subducted oceanic and continental crust during continental collision isotopic mixing calculations suggest that ca 28 35 ocean crust and ca 65 72 continental materials contribute to the origin of the xitieshan granite thus the syn collisional felsic magmatism represents juvenile crust with input of oceanic crustal materials derived from the depleted mantle and the hypothesis continental collision zones are primary sites for net continental crust growth is applicable in the north qaidam

meta igneous origin of hercynian peraluminous granites in n w french massif central implications for crustal history reconstructions seventy samples of hercynian peraluminous granites gueret millevaches and saint sylvestre massifs and metamorphic units of the limousin area were analysed for rb sr and sm nd the source rocks of the peraluminous granites can be found in the metamorphic rocks of limousin among them meta igneous rocks were largely predominent over meta sedimentary rocks in the source of the three granites millevaches and gueret granites were generated by the partial melting of rocks comprising meta volcanics and meta sediments whereas the saint sylvestre granite was produced exclusively by the melting of late precambrian granites this leads to confusing t dm nd values the confusion being amplified by the segregation of monazite during the petrogenetic evolution of the peraluminous granites which leads to dramatic fractionation in sm nd ratios the data of the present study tend to demonstrate that peraluminous granites do not give a good representation of isotopic mean crustal estimates late precambrian time seems however to have been a period of extensive crustal generation in western europe

geochemical comparison between himalayan and hercynian leucogranites abstract isotopic sr pb nd o ree and trace element data from three himalayan nepal and six hercynian brittany france leucogranites are compared for the himalayan granites makulu mustang and especially manaslu the 87sr 86sr versus 87rb 86sr scatter diagrams and hence variability of the initial 87sr 86sr ratios reflect heterogeneity of the source materials these ratios are very high 0 730 0 770 for manaslu the very high 207pb 204pb ratios 15 8 at the corresponding 206pb 204pb ratio of 18 8 the very low 143nd 144nd ratios i nd 13 to 17 and the very high 18o 16o ratios 12 confirm the crustal origin with the paragneisses of the tibetan slab formation i as the probable parent source for the six hercynian granites with an individual rb sr isochron established for many of them the initial 87sr 86sr ratios range from 0 706 to 0 717 the lead neodymium and oxygen isotope compositions confirm their crustal source of presumably palaeozoic age but not specifically identified the rare earth patterns and thorium contents are comparable to those for himalayan granites the similarities of the geochemical signatures isotopes and trace elements of the himalayan and hercynian leucogranites suggest that the petrogenetic processes were comparable and that the himalayan continental collision model can be applied to the hercynian the depletion in ree and th implies either interaction of the granitic liquids with fluid or more probably early precipitation of monazite the increase in sm nd in the granites compared to their parent materials implies that the neodymium model ages of primary mantle extraction are maximum rather than minimum ages in contrast to the hercynian leucogranites the extremely radiogenic sr of the himalayan leucogranites reflects the very long crustal residence time before the melting event the apparent uniformity of the initial 87sr 86sr ratios of the individual hercynian granites may be related to an aging effect where the straight line fitting improves with increase in age of the pluton