plate tectonics and magma genesis the framework of plate tectonics with active boundaries and stable plates is the best yet devised for explaining the different types and styles of magmatic activity the dynamic mechanisms of plate tectonics transport rock masses across fusion boundaries in three distinct types of environment and source material associated with plate boundaries these are divergent boundaries where mantle peridotite is transported upwards melting to yield basalt convergent boundaries where oceanic crust is transported downwards melting to yield magma of intermediate sio 2 content and ocean continent convergent boundaries where the lower part of continental margins are melted to yield rhyolite magma hot spots beneath plates possibly generated by mantle plumes yield basaltic magma from mantle and rhyolite magma from overlying continental crust subducted h 2o is involved in the generation of andesites and batholiths and co 2 from uncertain sources is an influential component for the generation of kimberlite and other low sio 2 high alkali magmas below continental plates the chemical differentiation of the earth is accomplished through magmatic processes which are a direct manifestation of convection within the mantle igneous petrology is now a study of processes and products firmly rooted in geophysics and calibrated by laboratory experiments at high pressures and temperatures

the role of hybridization in the genesis of hercynian granitodis in the gredos massif spain inferences from sr nd isotopes the gredos massif is one the better exposed granitoid complexes of the iberian massif it is composed mainly of peraluminous granitoids with subordinate basic and ultrabasic complexes the massif also contains mega enclaves of migmatites with which the granitoids show transitional contacts two major magmatic associations have been distinguished in this study 1 one comprises the granitoids with microgranular enclaves the enclaves and basic rocks 2 the other is formed by leucogranites intrusive into the former series and free of microgranular enclaves field relationships and microstructures indicate that the rocks of the first series are related by a dominant hybridization process the sr nd isotopic study reveals that this process is complex relating different end members of mantle and crustal affinities and occurred around 295 ma ago late with respect to the main deformation phases of the hercynian orogeny the granitoids with microgranular enclaves gme are part of an overall mixing trend involving palaeozoic mantle derived magma and melts of older crustal material amphibole bearing gme in general contain greater proportions of the mantle derived component than the cordierite bearing gme the actual mixing processes took place on a variety of scales sometimes between melts which were themselves hybrids on a local scale this hybridization process can be modelled by simple binary mixing as documented in the case of a composite dyke the isotopic signatures of the basic rocks are probably to a large degree the result of interaction with crustal melts though additionally the presence of an enriched mantle source cannot be elmininated microgranular enclaves and their immediate hosts have differing initial sr and nd isotopic signatures indicating that isotopic equilibrium was not attained this suggests that the enclaves did not reside in their final granitic melt for long before cooling of the whole system the enclaves are considered to have been derived from basaltic melts which had fractionated and hybridised to varying degrees late stage peraluminous leucogranites have similar initial nd isotopic compositions to the evolved gme a crustal source with a radically different nd isotopic composition or age does not need to be invoked in their petrogenesis

formation and age of sphalerite mineralization in carbonate rocks of bajocian age in the swiss jura mountains evidence of mesozoic hydrothermal activity a combination of petrographic and geochemical techniques was applied to better constrain the origin and evolution of the fluid systems responsible for the formation of disseminated cd rich up to 0 6 wt sphalerite zns mineralization in the northeastern part of the jura mountains switzerland the rb sr ages of sphalerite samples indicate that a main phase of sphalerite formation occurred near the boundary between the late middle and early late jurassic at around 162 ma the negative 34s values 22 3 to 5 3 suggest that biogenic sulfide sulfur was involved in zns precipitation the strontium isotope composition is more radiogenic than that of contemporaneous seawater reflecting the interaction of mineralizing fluids with silicate rocks lead isotope signatures are very uniform 206pb 204pb 18 63 18 67 207pb 204pb 15 63 15 64 208pb 204pb 38 51 38 63 indicating an isotopically well homogenized fluid system the basement rocks underlying the jurassic strata are considered to be the main source of metals for the sphalerite mineralization the migration of deep sourced hydrothermal saline metal bearing fluids into the bajocian host carbonates containing sedimentary reduced sulfur resulted in the precipitation of sulfides the period of sphalerite formation near the middle late jurassic boundary is characterized by enhanced tectonic and hydrothermal activity in europe related to the opening of the central atlantic and tectonic thermal subsidence during spreading of the alpine tethys our study provides evidence that the bajocian carbonate rocks in the jura mountains area were affected by the circulation of deep sourced metal bearing hydrothermal fluids in response to these continent wide tectonothermal events the presence of sphalerite mineralization and associated geochemical anomalies in zn and cd contents in carbonate rocks may also be used to trace basement features

ore genesis of pb zn deposits in the nappe zone of northern tunisia constraints from pb s c o isotopic systems abstract the jalta and jebel ghozlane ore deposits are located in the extreme north of tunisia within the nappe zone the mineralization of jalta hosted in triassic dolostones and the overlying mio pliocene conglomerates consists of abundant galena barite and cerussite with accessory sphalerite pyrite and jordanite at jebel ghozlane large pb zn concentrations occur in the triassic dolostones and eocene limestones the mineral association consists of galena sphalerite barite and celestite and their oxidation products cerussite smithsonite and anglesite lead isotope ratios in galena from both districts are relatively homogeneous 206 pb 204 pb 18 702 18 823 207 pb 204 pb 15 665 15 677 208 pb 204 pb 38 725 38 875 the 34 s values for sulfates from both areas 12 2 to 16 2 at jalta and 14 3 to 19 4 at jebel ghozlane are compatible with a derivation of sulfur from marine sulfates possibly sourced from the triassic evaporites the 34 s values of the sulfides have a range between 10 and 12 5 at jalta and between 9 1 and 22 1 at jebel ghozlane the large range of values suggests reduction of the sulfate by bacterial and or thermochemical reduction of sulfate to sulfur the high 34 s values of sulfides require closed system reduction processes the isotopically light carbon in late calcites 6 3 to 2 5 and authigenic dolomite 17 6 suggests an organic source of at least some of the carbon in these samples whereas the similarity of the 18 o values between calcite 24 8 and the authigenic dolomite 24 7 of jalta and their respective host rocks reflects oxygen isotope buffering of the mineralizing fluids by the host rock carbonates the secondary calcite isotope compositions of jalta are compatible with a hydrothermal fluid circulation at approximately 100 to 200 c but temperatures as low as 50 c may be indicated by the late calcite of jebel ghozlane 18 o of 35 9 given the geological events related to the alpine orogeny in the nappe zone nappe emplacement bimodal volcanism and reactivation of major faults such as ghardimaou cap serrat and the neogene age of the host rocks in several localities a late miocene age is proposed for the pb zn ore deposits considered in this study remobilization of deep seated primary deposits in the paleozoic sequence is the most probable source for metals in both localities considered in this study and probably in the nappe zone as a whole

lead isotope and pb pb model age determinations of ores from central europe and their metallogenetic interpretation the paper presents lead isotope data from 211 samples from phanerozoic lead bearing ore occurrences in central europe particularly from the southern part of the former german democratic republic the data are interpreted in terms of amov s dynamic model of continuous lead isotope evolution the relationships between thoro genic and urano genic model ages and the source of lead in different regional units are discussed we observed differences in lead isotope evolution in the hercynian internides and externides within the moldanubian and saxothuringian zones we distinguish five main lead bearing ore associations 1 cambrian stratiform base metal hermsdorf waldsassen 206pb 204pb 17 50 17 70 2 devonian vein type sb bearing metamorphogene neumuhle hartmannsdorf 17 80 18 00 3 upper carboniferous permian polymetallic including tin vein type kutna hora freiberg altenberg 18 00 18 20 4 triassic jurassic pb ba vein type st ibro halsbrucke 18 20 18 60 5 cenozoic polymetallic vein type riftogene roztoky banska stiavnica 18 80 19 10 pb isotope characteristics from ores of the montagne noire and the brioude massiac district correspond to this subdivision ore associations from the rheno hercynian zone display higher 207pb 204pb ratios which can be explained by more evolved and less metamorphosed source rocks mineralizations of the eastern harz strasberg neudorf belong to the permian association those from the western harz clausthal bad grund to the triassic jurassic because of pb isotope agreement the stratabound rammelsberg and the vein bound ramsbeck i mineralization are presumed to be isogenetic pb isotope identity of distinct mineralizations in the basement zone halsbrucke bad grund and in the triassic sediments gorny slask mechernich bleiglanzbanke suggests a strong genetic coherence pb isotope conformity between the upper carboniferous permian triassic ore associations and hercynian postkinematic granitoids and lamprophyric rocks also favours a close relationship pb isotope and other data indicate crustal sources as the age of the ore associations decreases crustal influences generally increase apart from the roztoky mineralization

pb isotope evidence for the origin of lead in strata bound pb zn deposits in triassic carbonates of the eastern and southern alps a comparison of ore lead isotope ratios of pb zn deposits hosted in triassic carbonates of the eastern and southern alps with the isotopic composition of trace leads of their host rocks of triassic volcanics and of the underlying clastic sediments shows that these rocks could have supplied only part of the ore lead the isotopic signature of feldspar lead from crystalline basement rocks however reveals that they must have contributed a significant amount of metal to these deposits the presence of barite and of thallium is also indicative that feldspars were the main source of lead arsenic is one of the few important trace elements in these deposits and is known to occur in some areas of lower paleozoic metasediments in unusually high concentrations any model explaining the origin of these deposits must consider the extensive leaching of the basement and to some extent of the overlying clastic sediments as well how the metal bearing solutions entered the lagoonal back reef areas is still open to question as feeder channels have not yet been positively identified the results further demonstrate that the basement also acted as a metal source for galena bearing deposits hosted in permian sediments vein type deposits in permian volcanics and probably the barite fluorite polymetallic deposits along a devonian erosional surface in the carnic alps as well

u pb investigations on zircons from pre variscan gneisses i a study from the schwarzwald west germany abstract field observations in the schwarzwald indicate that pre variscan rocks were formed by the following events 1 1 intrusion of granitic to granodioritic magma into sediments 2 2 a sequence of two anatectic events with an intervening regional metamorphism and development of a gneissic schistosity the absolute chronology of these older gneisses and migmatites has not yet been satisfactorily resolved we collected several samples of a coarse grained homogeneous orthogneiss of granodioritic composition and one sample of a metasedimentary source rock related to this orthogneiss body in the einetwald forest north of steinach kinzig valley the metasedimentary rock corresponds to the biotitepoor paragneisses in the schwarzwald the zircons in the latter sample are detrital and only very small specimens 45 m are euhedral the zircons from the orthogneiss samples are euhedral some are zoned and some have dark inclusions analyses of the three coarser fractions of the paragneiss form a linear array on a concordia diagram with upper and lower intercepts of about 2000 myr and 524 20 myr respectively the 2000 myr value is interpreted as the average age of the sedimentary source rocks for this material the 524 myr intercept defines an event responsible for most of the lead loss in the zircons as well as new zircon growth analyses of the zircons from the orthogneiss samples do not show a linear array on the concordia diagram because of inclusions of older inherited zircons stepwise leaching of two zircon fractions shows newly grown u rich zircon rims around old cores a 207 pb 206 pb age of 517 15 myr was found for the first dissolution steps about 50 of the zircons dissolved in subsequent steps with progressive dissolution of the residue 207 pb 206 pb age increased to 950 myr the u pb zircon data for the paragneiss and the orthogneiss establish an age of 520 15 myr for the magma intrusion

lead isotopic composition of hercynian granitic k feldspars constrains continental genesis lead isotope compositions have been determined in many k feldspars from granitic rocks emplaced during the variscan hercynian orogeny sensu lato around 300 myr in various parts of europe and the eastern us the ratios show a regular geographical pattern with contours arched around southern france and parallel to the atlantic coast of the us this distribution parallels predriftpalaeogeographical zones during the palaeozoic the lead isotope pattern of variscan granitic rocks is similar to that of some orogenic volcanic suites but is better explained by crustal melting than by an hercynian mantle crust material mixing the continental crust from these areas must be older than 3 000 myr large scale fluid circulation in the crust during orogenic processes apparently explains the observed geographical patterns of lead isotopes

the pb isotopic systematics during crustal contamination of subcrustal magmas the hercynian magmatism in the serie dei laghi southern alps italy abstract the pb isotopic composition and pb u and th concentrations of a series of hercynian granite plutons and mafic intermediate stocks and dykes belonging to the serie dei laghi western southern alps have been analyzed the initial pb isotope ratios of both the granites and the dykes are typical of the lower crust and are spread over a significant range 206 pb 204 pb 280 17 883 18 530 207 pb 204 pb 280 15 597 15 710 and 208 pb 204 pb 280 38 077 38 708 four dykes all at the same locality display evidence of post emplacement fluid circulation with mobilization of u and pb in spite of the pb isotopic crustal signature of the studied rock series the 87 sr 86 sr ratios reach values as low as 0 704 in the dykes and as low as 0 707 in the granites suggesting a mantle contribution in the genesis of both lithologies indeed when compared with the metasediments from the same area the magmatic rocks have somewhat lower initial 207 pb 204 pb ratios which weakly trend towards the less radiogenic pb isotopic composition of the mantle the relationships between the pb isotopic composition and the pb content along with the pb and sr isotopic covariation suggest that the suite under examination was generated from a mantle derived primary magma that underwent extensive crustal contamination a primary magma similar to the neighbouring ivrea mafic complex has been inferred further assimilation of crustal material is particularly evident in the dykes while the granites appear to have been generated mainly by crystal fractionation processes from the intermediate magma already contaminated

the age of the kagenfels granite northern vosges and its bearing on the intrusion scheme of late variscan granitoids in the saxothuringian part of the vosges france a first series of variscan plutonic rocks diorites to granites has been intruded by several younger granites rocks of both the older generations have been cross cut by the late orogenic kagenfels granite the averages of the hitherto published mineral ages of the earlier rock generations are 331 and 334 ma respectively whereas rb sr and k ar dates around 290 ma have been reported for the kagenfels granite because of the unlikely large age hiatus a redetermination of the intrusion age of the kagenfels granite formation appeared to be irrevocable the newly obtained mineral ages on the kagenfels granite k ar and 40ar 39ar biotite ages as well as single zircon radiogenic 207pb 206pb data 331 5 ma are about 40 ma older than the previous results they are interpreted as giving the time of emplacement of the kagenfels granite during the latest viscan the mineral ages of the earlier plutonic rocks in this part of the variscan orogeny in all probability are not significantly different from their ages of intrusion therefore the age concordance of all three granitoid generations constrains a rather narrow time interval of orogenic magmatism close to the lower upper carboniferous boundary

reconnaissance isotopic investigations on rocks of an undeformed granite contact within the coesite bearing unit of the dora maira massif abstract recent research on ultrahigh pressure rocks in val gilba and the brossasco area has revealed the complete textural preservation of large volumes of a premetamorphic granite together with its former contact aureole that was subducted to depths of 100 km as indicated by coesite pseudomorphs petrographic studies indicate that although most minerals have been affected by metamorphism biotite and potassium feldspars in the rock are largely preserved except for thin peripheral rimming of biotite by phengite and garnet rb sr analyses on seven biotite samples fail to produce a valid isochron indicating either incomplete resetting or inheritance of radiogenic sr during metamorphism ar ar data from two biotites likewise yield very irregular patterns that are consistent with an episode of severe ar loss from a pre alpine parent rock followed later by addition of inherited radiogenic ar the biotite rb sr and k ar systems have therefore been extensively disturbed during alpine metamorphism in spite of the largely unaltered appearance of the crystals in thin sections attempts to measure u pb ages with apatite also failed although the geochronological studies yielded no reliable ages pb and sr ratios from four potassium feldspar samples are within the ranges found in known hercynian rocks providing new evidence that hercynian rocks played a prominent role as source materials for the dora maira rocks sr ratios from two apatite samples agree closely with the potassium feldspar values the ratios are also substantially lower than those found in the whiteschists in completely recrystallized rocks at parigi ca 3 km north of the present rocks these results indicate that the undeformed rocks in val gilba brossasco area are less disturbed by alpine ultrahigh pressure metamorphism thereby preserving the record of the protoliths somewhat better than do the completely recrystallized rocks at parigi to the north the pb and sr isotope data further show that the minerals of the undeformed rocks failed to reach isotopic equilibrium despite subduction to 100 km and heating to 700 c

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

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

coseismic gravity changes of the 2011 tohoku oki earthquake from satellite gravimetry 1 the massive tohoku oki earthquake of a moment magnitude mw of 9 0 occurred on 11 march 2011 off the pacific coast of the northeastern japan the mass redistribution in and around the focal region associated with this earthquake was studied using the gravity changes detected by gravity recovery and climate experiment grace satellite after the 2004 sumatra andaman and the 2010 central chile maule earthquakes the present study presents the third case of clear detection of coseismic gravity changes by grace the observed gravity changes were dominated by decrease over the back arc region of 7 gal or less this reflects to a large extent coseismic crustal dilatation of the landward plate they agree well with the changes calculated with the green s function for the realistic earth using fault parameters inferred from coseismic crustal movements the spatial patterns of the gravity changes of these earthquakes are very similar because they are all shallow angle reverse faulting at convergent plate boundaries we found linear relationship between gravity decreases and seismic moments

post collisional variscan lamprophyres black forest germany 40ar 39ar phlogopite dating nd pb sr isotope and trace element characteristics abstract subsequent to high pressure and temperature metamorphism of the axial zone of the variscan foldbelt in central europe at ca 340 to 330 ma formation of lamprophyre dikes during transtensional tectonics may be viewed as the beginning of the post collisional stage of the orogeny we report the results of 40 ar 39 ar mica dating major and trace element data and isotope compositions for lamprophyre and rhyodacite samples from the southern black forest the chemical compositions of these rocks shed light on the upper mantle and crust at the end of the variscan orogeny 40 ar 39 ar plateau ages for four phlogopite biotite separates from lamprophyres indicate emplacement at 332 to 314 ma this event coincides with melting of the crust as indicated by 40 ar 39 ar biotite plateau ages of ca 332 ma for rhyodacite dikes which are probably related to coeval undeformed granites incompatible trace element patterns of the lamprophyre samples reveal the characteristics of evolved continental crust and are interpreted as evidence for melting of sediment in a subduction modified mantle nd sr and pb isotope compositions indicate an enriched mantle source with end values of 1 5 to 6 8 which is similar to variscan crust significant contamination of the lamprophyric melts by variscan crust can be ruled out as mantle derived phlogopite phenocrysts have similar end values as in the whole rock samples we propose that the isotope compositions and incompatible trace element characteristics of the lamprophyres were predominantly inherited from melted sediment the isotope compositions of variscan lamprophyres from western europe suggest that enriched upper mantle was only partly delaminated when ascending hot mantle triggered melting of the lower crust as has been invoked for the origin of post collisional granites the isotope compositions of tertiary basalts and mantle xenoliths indicate a depleted upper mantle under western europe implying that the enriched variscan material was efficiently removed and mixed into the convecting mantle

metallogenic provinces and the remobilization process studied by lead isotopes lead zinc ore deposits from the southern massif central france the two main problems of ore genesis are the origin of metals and the process es of emplacement lead isotopes have proven to be a powerful tool for studying these questions we have studied pb zn mineralizations from montagne noire and cevennes southern massif central france by means of lead isotopes this study shows the existence of two populations of ore deposits defined by 17 76 206 pb 204 pb 17 94 and 18 25 206 pb 204 pb 18 63 a general model of lead evolution in these deposits is proposed after evolving in the earth s mantle until approximately 3 b y ago the lead became incorporated into a segment of continental crust then the less radiogenic population of ore deposits was emplaced contemporaneously with the formation of cambrian carbonate host rocks the second population is related to the emplacement of hercynian granitoids or in more general terms to fluid circulations associated with the hercynian orogeny it can also be shown that some of the deposits are remobilized from preexisting ore deposits

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

sr nd al36 1997 99 122 hercynian granodiorites and monzogranites massif central france variations in sr nd and pb initial isotope ratios of three large hercynian granitoid massifs of the french massif central are consistent with formation of the granitoids by partial melting of the lower crust the nature of the lower crust of the massif central is known from the granulite facies lower crustal xenoliths brought to the surface by tertiary alkaline volcanics studies of the xenoliths have revealed that during the hercynian orogeny 360 290 ma ago the pre existing metasedimentary and acid metaigneous lower crust was intruded by basic magmas causing anatexis the mantle derived basic magmas mixed with melts from the pre existing lower crust forming the hercynian granitoids the peraluminous gueret and margeride granitoids which are very similar in their major and trace element compositions have 87sr 86sri 0 7089 to 0 7121 and ndi 7 5 to 3 9 values which indicate that they have a high sedimentary component considered to have been derived from lower crustal metasedimentary rocks however the granitoids differ significantly in their initial pb isotope compositions with the gueret granodiorites being less radiogenic indicating the presence of a slightly different crustal source component for the gueret massif in contrast monzogranites from the millevaches massif are less peraluminous and have higher la ybn and lower 87sr 86sri values 0 7064 to 0 7109 these features together with the relatively radiogenic pb isotope compositions of the millevaches granitoids suggest derivation from the acid metaigneous lower crust

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

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

plumbotectonic evolution of the ossa morena zone iberian peninsula tracing the influence of mantle crust interaction in ore forming processes the ossa morena zone in the variscan belt of the iberian peninsula is characterized by a complex geotec tonic evolution it hosts a neoproterozoic paleozoic sequence that records the existence of two orogenic events cadomian and variscan separated by rift and stable platform stages middle cambrian to late devonian volcanogenic and sediment hosted massive sulfides skarns magmatic ni cu deposits synmetamorphic and intragranitic and perigranitic veins and replacement bodies are related to different magmatic hydrothermal systems ranging in age from 650 ma to about 250 ma the lead isotope signatures from 26 different mineral deposits show variable ratios of 206pb 204pb 17 49 18 44 207pb 204pb 15 47 15 67 and 208pb 204pb 37 37 38 78 that have low average values 9 7 but high values 38 3 as a whole the lead isotope data define an irregular cluster that plots around the stacey and kramers 1975 growth curve but clearly marks the existence of two main ore forming events during cadomian and variscan times the most radiogenic samples are from some minor zn pb stratiform sedimentary exhalative mineralization they are interpreted as having a dominantly crustal derivation 10 1 in which most of the lead was leached from a pre cadomian continental basement with a long crustal history this basement has strong geochemical affinities with the oldest rocks of the iberian terrane however most of the ore samples investigated plot below the stacey and kramers 1975 reference curve indicating polyphase but variable mixing of lead from upper crustal and mantle lead reservoirs at the terrane scale clear mixing lines between the two end member reser voirs are evident only in the oldest volcanic rock and sedimentary rock hosted massive sulfide deposits related to the cadomian late neoproterozoic early cambrian magmatic arc in the other deposit types lead isotopes have more restricted values the crustal lead is interpreted as having been derived from ancient crust 2 1 2 5 ga and the derived shales montemolin formation that predated the cadomian orogeny the mantle derived lead may have been derived from three different sources neoproterozoic tholeiitic orthoamphibolite syn cadomian calc alkaline andesite or calc alkaline metaluminous plutonic rocks intruded during the variscan orogeny the first two sources probably controlled the lead isotope signatures of the cadomian massive sulfide deposits and the variscan synmetamorphic veins the input of primitive lead is especially significant during variscan times 350 300 ma and is interpreted as having been related to the existence of a large primitive reservoir that controlled the variscan magmatic metallogenic and hydrothermal evolution of the ossa morena zone this primitive lead is recognized in most of the magmatic hydrothermal deposits including the typically crust derived w and pb bearing perigranitic hydrothermal systems likewise associated plutonic rocks have more primitive lead isotope signatures than their counterparts in other iberian terranes the lead isotope evolution of the region is consistent with recent geophysical data that suggest that a deep large mafic sill was intruded below the ossa morena zone during variscan times this sill was a likely key factor in the different pb isotope evolution of the ossa morena zone compared with the nearby iberian terranes and the entire western mediterranean province which are char acterized by lengthy crustal histories with very little input of primitive lead the extensive mantle crust inter action probably explains the wide variety and the unusual metallogenic features of the ossa morena zone

cadomian lower crustal contributions to variscan granite petrogenesis south bohemian pluton austria constraints from zircon typology and geochronology whole rock and feldspar pb sr isotope systematics zircons show a mean pb pb evaporation age of 355 a hybrid pyroxene bearing weinsberg type granitoid of the south 9 ma interpreted as the age of zircon growth during a carboniferous bohemian batholith austria consists of two independent mineral partial melting event in the lower crust granite emplacement at assemblages that were formed during two different magmatic events 345 5 ma is inferred from u pb analysis of the anhedral the older inherited assemblage forms unevenly distributed millimetrezircon population the comparably low radiogenic common pb sized multi grain patches of quartz mesoperthitic alkali feldspar isotope composition of megacrystic alkali feldspars suggests that at andesine bytownite clinopyroxene xmg 0 50 0 54 least some domains of these crystals are inherited from the older orthopyroxene xmg 0 35 0 42 ilmenite accessories pyroxene bearing mineral assemblage rb sr whole rock dating is it is interpreted to represent remnants of a mangeritic igneous thus severely jeopardized by the presence of the inherited alkali rock with a superimposed granulite facies re equilibration texture feldspar crystals leading to widely scattering data points and characterized by unzoned pyroxenes and plagioclase the enclosing errorchron ages of no geological significance younger assemblage with alkali feldspar oligoclase andesine quartz biotite accessories crystallized from a biotite bearing granitic melt with feldspars exhibiting typical magmatic zoning