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

metamorphic p t conditions of the kerala south india khondalite belt a granulite facies supracrustal terrain a discussion

tectonic implications of metamorphism and gravity field in the penokean orogen of northern michigan published geologic data together with new metamorphic pressure and temperature data and gravity data provide strong evidence for northward directed thrusting of lower proterozoic rocks in northern michigan and wisconsin metamorphic studies indicate pressures of 7 kbar at watersmeet node and 4 kbar at the peavy node the republic metamorphic node located 30 km to the north of peavy attained pressures of 2 3 kbar all three metamorphic highs reached temperatures in the range of 600 650 c and are correlated with areas of negative gravity anomalies relative to surrounding regions in the peavy high the center of the gravity anomaly lies on the sillimanite isograd and we suggest that the source of this negative anomaly may be remobilized underlying archean gneiss the highest grade rocks of the republic node lie well within a residual gravity anomaly which is interpreted to have a very shallow source and the watersmeet high lies above gneiss domes which show negative gravity anomalies a north south geologic cross section constructed from a two dimensional gravity model which extends northward from the dunbar dome in the magmatic arc terrane of wisconsin across a terrane boundary interpreted as a suture and marked by the niagara fault and across the peavy metamorphic node suggests that the dunbar dome was transported northward onto the continental foreland during the penokean orogenic event these north verging fold thrust sheets or nappes produced prominent tectonic thickening at the peavy and watersmeet as indicated by the relatively high metamorphic pressures there the thrust sheets evidently thinned to the north as suggested by the relatively low pressures at republic we suggest that the archean continental edge defined by the steep gradient in the regional northeast trending gravity high lies beneath the magmatic rocks of wisconsin and thus extends south of the niagara fault suture

the dunbar gneiss granitoid dome implications for early proterozoic tectonic evolution of northern wisconsin the dunbar dome in northeastern wisconsin is a critical structural feature in the early proterozoic penokean orogen it provides exposures of gneisses dunbar gneiss that structurally underlie the voluminous metavolcanic rocks of northeastern wisconsin and exposures of abundant granitoid rocks ranging from tonalite to granite the granitoid rocks cut both the gneisses in the core and the supracrustal cover metavolcanic rocks and were emplaced essentially along the core cover boundary the dunbar gneiss is calc alkaline and was derived from volcanic and intrusive rocks of intermediate composition the various intrusive rocks have calcic calc alkaline and alkali to alkali calcic compositions and they progress with time to more sio 2 and k 2 rich compositions u pb zircon ages indicate that accumulation of the layered rocks in the core and cover deformation and metamorphism and intrusion of the granitoid rocks spanned a relatively short time 1865 1835 ma we interpret the dome as being a large scale fold interference structure resulting from polydeformation modified by diapirism northeast oriented folds f 3 and a related mylonitic foliation s 3 nearly confined to the dome are superposed on northwest oriented folds f 2 that developed during regional deformation in the core cover boundary these structures are obliterated by a zone of intense deformation a mylonitic foliation and a steeply plunging stretching lineation as much as 500 m wide which we interpret as resulting from diapirism metamorphic zoning is concentric amphibolite facies in inner parts of the mantle and greenschist facies in the outer part of the mantle the wisconsin magmatic terrane as represented by the rocks in the dunbar dome differs from the epicratonic early proterozoic sedimentary volcanic sequence marquette range supergroup in michigan to the north in stratigraphy structure and volume and composition of igneous rocks whereas the basalts in northern michigan are compositionally similar to continental rift basalts the volcanic rocks in the dunbar dome have over all island arc compositional affinities the over all calc alkaline compositions of the intrusive rocks are similar to those in magmatic arcs formed at convergent plate margin settings accordingly we interpret the wisconsin magmatic terrane as an oceanic arc complex that was sutured to the north american continent during development of the penokean orogen similar interpretations based on broad regional observations have been proposed previously

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

the goodman swell a lithospheric flexure caused by crustal loading along the midcontinent rift system rb sr biotite ages of archean and early to middle proterozoic crystalline rocks in northern wisconsin and adjacent upper peninsula of michigan describe a regionally systematic pattern related to differential uplift an age low occurs in northern wisconsin where values range from 1070 1172 ma for rocks with crystallization ages of 1760 to 1865 ma these values overlap with the main episode of mafic igneous activity 1090 to 1120 ma along the midcontinent rift system mrs we interpret these low biotite ages as registering closure due to cooling below the 300 c isotherm as a consequence of uplift and rapid erosion of an area that we are informally naming the goodman swell the position of the goodman swell is independent of the archean and early proterozoic regional geologic framework but closely linked with the curvilinear concave to the south mrs we interpret the swell to be a forebulge imposed on an elastic crust by loading of mafic igneous rocks along and within the axis of the mrs the southward concavity of load axes resulted in flexural interaction that localized the swell because of rapid uplift the swell may have been a source area for clastic sediments in the rift sequence

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 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

the early proterozoic structural and tectonic history of the south central lake superior region the early proterozoic tectonic evolution of the south central lake superior region is complex owing to the presence of four tectonostratigraphic terranes which were affected by six phases of deformation the four terranes are the passive margin of the superior craton two paraautochthonous passive margin terranes crystal falls and florence niagara terranes and a southern magmatic arc complex which is probably allochthonous with respect to the other terranes four of the six deformational episodes accompanied subhorizontal shortening while two were caused by subvertical shortening the first and the most penetrative phase of deformation is marked by subhorizontal shortening in a nne ssw direction the second and fourth deformations were characterized by subvertical shortening and did not significantly modify the structural orientations from previous events in the study area the third fifth and sixth deformations mostly caused open folding and shortening directions were nw ne and w respectively because all the terranes in the south central lake superior region share parallel deformational histories it is suggested that the accretion of these terranes occurred during the first deformational episode after removal of younger deformational effects including open folding of the suture zone the tectonostratigraphic assemblages in this region show the following sequence from nne to ssw a platformal assemblage overlying sialic basement a basinal assemblage of tholeiitic volcanic rocks overlain by deep water turbidites an assemblage of basin floor deposits crystal falls terrane with apparently no demonstratably underlying crystalline basement a fault bounded terrane with highly strained passive margin strata florence niagara terrane and a calc alkaline magmatic arc assemblage such an arrangement of tectonostratigraphic assemblages is comparable with cross sections through phanerozoic accretionary continental margins and therefore supports an arc continent collision model

fold axes oblique to the regional plunge and proterozoic terrane accretion in the southern lake superior region abstract the niagara fault is believed to represent the suture between a magmatic arc terrane and a passive margin assemblage which were tectonically juxtaposed about 1 85 ga ago during the penokean orogeny accretion related deformation is concentrated within an 12 km wide belt that straddles the suture a distinctive structural feature of this belt is the rotation of early fold axes toward parallelism with the direction of bulk extension resulting in a great circle stereonet distribution of fold axes within a vertical axial plane rocks of both arc and passive margin assemblages crop out in this more highly strained belt but there has been no material transfer between them across the fault structural style and metamorphic assemblages demonstrate deep level beneath the brittle ductile transition erosion of the terrane accretion boundary in the southern lake superior region there is some evidence for pure shear during the accretion event but we are unable to evaluate the amount or sense of simple shear

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

florence niagara terrane an early proterozoic accretionary complex lake superior region u s a the florence niagara terrane is an elongate fault bounded block 10 km wide and 80 km long composed of miogeoclinal assemblage lithologies bounded on the north by a generally coherent miogeocline extending to the superior craton and on the south by a probably exotic magmatic arc terrane the florence niagara terrane is composed of a collage of eight fault bounded packets all fault packets are characterized by faults that trend west to west northwest with a dominant west northwest striking deeply dipping foliation elongation lineations plunge down the dip of foliations in all cases except in packet 1 where bulk extension locally parallels shallowly plunging fold axes a probable product of superimposed strains fold axes are subhorizontal or girdled in the mean foliation plane or plunge steeply packets containing girdled fold axes probably were subjected to greater strain than were packets with subhorizontal fold axes steeply plunging fold axes are not easily explained but they were perhaps formed by noncoaxial deformation one of the packets contains a younger generation of folds formed during east west shortening another packet contains folds associated with later strike slip faulting convergence between the magmatic terrane and the miogeocline probably resulted in accretion of miogeoclinal sediments in the forearc area of the magmatic terrane the florence niagara terrane may represent such a zone of forearc accretion formed prior to and during collision and thus it may be referred to as an accretionary complex

middle proterozoic uplift events in the dunbar dome of northeastern wisconsin usa isotopic ages of granitic and metamorphic rocks exposed in the dunbar structural dome of northeastern wisconsin identify a protracted series of tectonic and hydrothermal events that culminated in major regional uplift during middle proterozoic keweenawan ca 1 100 ma continental rifting and volcanism the major rock forming events and the structural development of the dome occurred during the interval 1 862 4 ma to 1 836 6 ma whole rock rb sr ages are partly reset in response to a widely recognized but cryptic event in wisconsin and michigan at about 1 630 ma the scale and systematic character of the whole rock resetting strongly suggests the presence of a fluid phase derived in situ from water dissolved in the silicates or externally from a subthrust plate of low grade metamorphic rocks the regional nature of the 1 630 ma disturbance possibly indicates that it is related to a major tectonic event such as an active plate margin far to the south rb sr biotite ages for the dunbar dome this study the southern complex of the marquette district van schmus and woolsey 1975 and the felch trough area aldrich and others 1965 provide a remarkably coherent pattern that reflects multiple episodes of differential uplift younger events superimposed on a regional 1 630 ma imprint are recorded at 1 330 ma and 1 140 ma the 1 330 ma disturbance could reflect stabilization following intrusion of the wolf river batholith at 1 485 ma the 1 140 ma uplift event occurred during keweenawan rifting and volcanism as a result of stresses imposed on a mosaic of fault bounded blocks with possible subcrustal influence the remarkably small variance in the 1 140 ma biotite age peak argues for rapid uplift and cooling and hence rapid erosion detritus from the uplift probably was being shed into nearby tectonic basins most of which did not survive subsequent uplift and erosion

continental thermal and tectonic regimes during the archaean metamorphic pressures estimated from mineral assemblages in archaean high grade terrains may be used to constrain geothermal regimes in the archaean continental crust in two ways first they may be combined with temperature estimates to yield bounds on the heat flux during the thickening events that produced the metamorphism the pressure temperature data do not require any greater supply of heat to the base of the continental crust in the archaean than is supplied at the present day second the pressures alone may be used to infer the height of archaean mountains these mountains appear to have had elevations comparable with those of present day mountain belts it seems probable that the elevation of present day belts is limited by the creep strength of the lithosphere and in view of the strong temperature dependence of creep in earth materials this second observation also implies that archaean continental thermal regimes were similar to present day ones these observations together with the common

metamorphic petrology of the negaunee iron formation marquette district northern michigan mineralogy metamorphic reactions and phase equilibria progressive regional metamorphism of the negaunee iron formation in the marquette district from chlorite to sillimanite grade has resulted in the development of three metamorphic zones and two subzones the diagnostic zone assemblages are in order of increasing grade 1 siderite quartz or ankerite or minnesotaite or stilpnomelane or magnetite 2 grunerite or quartz or ankerite or magnetite and grunerite or siderite or ankerite or magnetite 3a grunerite or garnet or quartz or magnetite and grunerite or ca amphibole or quartz or calcite or magnetite 3b grunerite or olivine or clinopyroxene or quartz or calcite or magnetite and grunerite or ca amphibole or olivine or magnetite the grunerite garnet and ca amphibole and olivine and clinopyroxene isograds define the boundaries between the respective zones and subzones temperature estimates are approximately 300 degrees c for zone 1 and approximately 600 degrees c for subzone 3b pressure is estimated at between 200 to 300 mpa microprobe analyses of coexisting minerals indicate general attainment of exchange equilibrium and a strong localized influence of bulk composition on mineral chemistry theoretical analysis of phase relations on isobaric t x sub co 2 diagrams leads to the following conclusions 1 throughout zone 1 minnesotaitc bearing assemblages are diagnostic of low x sub co 2 values stilpnomelane bearing assemblages represent low to medium values and chlorite bearing assemblages represent medium to high x sub co 2 values juxtaposition of these three assemblages at one locality indicates that fluid phase compositional gradients most likely existed 2 throughout zone 3 actinolite hornblende and garnet bearing assemblages reflect similar low to high x sub co 2 values the continued juxtaposition of these various assemblages suggests the continued presence of fluid phase compositional gradients into the highest grade metamorphic zones 3 metamorphic reactions caused substantial localized co 2 enrichment in the coexisting fluid phase a diversity of t x sub co 2 paths was followed typically differing on the scale of individual mesobands

geochemical comparison between minettes and kersantites from the western european hercynian orogen trace element and pbsrnd isotope constraints on their origin trace element and isotopic characteristics of late carboniferous to early permian minettes and kersantites have been determined these lamprophyres have been sampled throughout the western european hercynian orogen from brittany to the west to schwarzwald to the east in spite of sharp petrological differences reflected by mineralogy and major element geochemistry minettes and kersantites exhibit close identity with respect to trace element and isotopic features these features comprise enrichment in incompatible elements highcs rb and lowce pb ratios ta and ti relative depletion high abundance in transition elements and highni mg ratios pb isotope ratios are undistinguishable from those measured on hercynian continental crust initial143nd 144nd ratios are between0 5120 ei 5 and0 5122 ei 1 for minettes and kersantites whereas initial87sr 86sr ratios vary between 0 7055 0 710 for minettes and 0 707 0 708 for kersantites no simple mixing relations are visible on rbsr and smnd isochron diagrams the exceptional homogeneity of these geochemical characteristics along a 1000 km traverse does not allow for an hypothesis of enrichment through upper level assimilation and thus leads to propose that these rocks originated through melting of a mantle enriched by recycling of crustal material

metamorphic temperatures in the michigamme formation compared with the thermal effect of an intrusion northern michigan temperature and pressure conditions of metamorphism in the peavy node area in northern michigan have been estimated from the garnet biotite geothermometer and garnet plagioclase aluminosilicate quartz geobarometer sillimanite zone rocks yielded an average temperature 592 c in the staurolite zone temperatures ranged from 545 to 582 c and in the garnet zone temperature estimates were 485 to 557 c the pressure estimate of sillimanite bearing assemblages is 4 4 kb whilst that of andalusite bearing rocks is 2 9 kb at 534 c temperatures due to the peavy pond complex pvc intrusion as a heat source were calculated at various positions within the michigamme fm assuming that the magma was emplaced at 1000 c and taking into account the latent heat of solidification of magma and the heat consumed by metamorphic dehydration reactions if cooling proceeds by conduction a one dimensional 3 2 km thick sheet heat source reaches a maximum temperature of 355 c at 2 5 km from contact within the staurolite zone dur

review of nd isotopic data and xenocrystic and detrital zircon ages from the pre variscan basement in the eastern bohemian massif speculations on palinspastic reconstructions

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

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

two early proterozoic successions in central wisconsin and their tectonic significance geological mapping of 7 500 km 2 of previously virtually unmapped precambrian rocks in central wisconsin suggests the presence of two early proterozoic sedimentary volcanic plutonic successions evidence for the two successions includes structural stratigraphic metamorphic and lithologic relations that are consistent throughout the area the older succession unconformably overlies archean rocks it is characterized by amphibolite facies metamorphism folding about west to northwest plunging isoclinal f 1 fold axes with more open coaxial f 2 folding basal quartzite and pelitic sediments of the older succession are overlain by volcanic rocks and intruded by syntectonic tonalite granodiorite and trondhjemite plutons the younger succession consists primarily of a greenschist facies calc alkaline basalt rhyolite sequence that unconformably overlies the older amphibolite facies rocks structures in the greenschist sequence trend northeasterly with fold axes that plunge easterly the plutonic rocks are dominantly syntectonic to late tectonic granites and quartz monzonites with only minor tonalite in most areas the two successions have been juxtaposed along major faults the older amphibolitic sequence may correlate with the chocolay group farther north in the lake superior region and the younger green schist rocks with the menominee and or baraga groups if this correlation is correct the deformation metamorphism and intrusion of the green schist succession would have occurred during the penokean orogeny metamorphism to amphibole rank isoclinal folding and tonalite intrusion of the older succession would be a major pre penokean erogenic event within the early proterozoic that may correlate with the post chocolay pre menominee break in northern michigan wisconsin and minnesota we suggest that this earlier event has been unrecognized and that it may be a significant part of the early proterozoic history of the lake superior region

chaotic three dimensional distribution of ba rb and sr in feldspar megacrysts grown in an open magmatic system as has been demonstrated in recent years the heterogeneities of coeval magmas can be more successfully revealed by zoned megacrysts rather than by analysis of the whole rocks hosting them here the geochemical heterogeneities of feldspar megacrysts from the karkonosze granite poland are investigated by la icp ms the crystals are the product of migration and growth in regions of poorly mixed magmas 3d modeling of the ba sr and rb distributions emphasizes the importance of micro domain growth morphologies two models of element behavior a relative concentration model and a composition gradient model provide a potentially effective tool for tracking the mixing process on a microscale measured concentrations of elements of different mobilities do not agree with what might be expected from the mixing of two end member magmas if mixing was the only process occurring linear correlations between the concentrations of any two elements should be observed this however is not the case for combinations of any two of the three elements modeling reveals differing non linear correlations between concentrations the megacryst heterogeneities provide an insight into how mixing magmas are chaotically advected to growing crystals and the degree of inter magma element exchange between the magmas

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

minerals and potentially hazardous trace elements in the late triassic coals from the qiangtang basin china abstract the tumen coal mine has the largest coal resources in tibet which have been developed and utilized as an important energy source in the last decades it has raised some health problems during the coal exploration and utilization in tibet fifty late triassic coal or coaly samples were picked up from the qiangtang basin tumen mine woruoshan and hongshuihe areas to determine the minerals potentially hazardous elements and their mode of occurrence and possible sources coal samples from the qiangtang basin have medium and high ash yields 15 20 47 88 with low or medium high total sulfur content 0 04 4 86 minerals in qiangtang basin coal include clay minerals dolomite quartz pyrite siderite and hematite and trace amounts of halite feldspar anhydrite barite chromite and galena potentially hazardous trace elements in coal samples include as hg pb and se arsenic is controlled mainly by aluminosilicate minerals mercury occurs mainly as an organic bound form the organically bound pb is dominant selenium is controlled mainly by fe bearing probably pyrite a proportion of the elevated concentrations of as hg pb and se in qiangtang basin coal may be related to underlying shale bed these elements might have been eroded or leached from the shale bed which were subsequently transported and accumulated syngenetically in the coal forming peat swamps the enrichment of as hg pb and se in qiangtang basin coal however is also partly to be related to magmatic hydrothermal fluids

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

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

pressure temperature time paths of regional metamorphism i heat transfer during the evolution of regions of thickened continental crust the development of regional metamorphism in areas of thickened continental crust is investigated in terms of the major controls on regional scale thermal regimes these are the total radiogenic heat supply within the thickened crust the supply of heat from the mantle the thermal conductivity of the medium and the length and time scales of erosion of the continental crust the orogenic episode is regarded as consisting of a relatively rapid phase of crustal thickening during which little temperature change occurs in individual rocks followed by a lengthier phase of erosion at the end of which the crust is at its original thickness the principal features of pressure temperature time ptt paths followed by rocks in this environment are a period of thermal relaxation during which the temperature rises towards the higher geotherm that would be supported by the thickened crust followed by a period of cooling as the rock approaches the cold land surface the temperature increase that occurs is governed by the degree of thickening of the crust its conductivity and the time that elapses before the rock is exhumed sufficiently to be affected by the proximity of the cold upper boundary for much of the parameter range considered the heating phase encompasses a considerable portion of the exhumation decompression part of the ptt path in addition to the detailed calculation of ptt paths we present an idealized model of the thickening and exhumation process which may be used to make simple calculations of the amount of heating to be expected during a given thickening and exhumation episode and of the depth at which a rock will start to cool on its ascent path an important feature of these ptt paths is that most of them lie within 50 c of the maximum temperature attained for one third or more of the total duration of their burial and uplift and for a geologically plausible range of erosion rates the rocks do not begin to cool until they have completed 20 to 40 per cent of the total uplift they experience considerable melting of the continental crust is a likely consequence of thickening of crust with an average continental geotherm a companion paper discusses these results in the context of attempts to use metamorphic petrology data to give information on tectonic processes 1984 oxford university press

a technique for analyzing the thermal and uplift histories of eroding orogenic belts a scandinavian example geothermometric and geobarometric data from pelitic schists in the upper plate of the forsa thrust in the northern norwegian caledonides are interpreted as recording part of the uplift trajectory of this area during erosion and slow cooling a method developed in this paper for reconstructing the thermal history of orogenic belts from such data is based on a general mathematical description of thermal conduction in a lithospheric slab with an arbitrary initial temperature distribution and uplifted at a constant rate with certain restrictions the uplift trajectory tz path of any rock parcel can be calculated from only a few temperature depth points along that uplift trajectory by applying this method to geothermometry and geobarometry data from near the forsa thrust we can 1 extrapolate tz paths measured over a limited depth range upward to the surface 2 recover geotherms present at various times during the uplift and erosion history of the mountain belt 3 obtain tz paths for rock parcels above and below those for which pt data are available and 4 use pt data in combination with geochronological data to obtain accurate estimates of uplift rates our analysis suggests that shortly after nappe emplacement the nappe stack was hot near 800 c in the lower part of the nappe stack while the underlying autochthon was cold near thermal equilibrium prior to nappe emplacement the maximum burial depth of rocks near the forsa thrust was probably between 35 and 60 km