synorogenic melting of mafic lower crust constraints from geochronology petrology and sr nd pb and o isotope geochemistry of quartz diorites damara orogen namibia quartz diorites represent the earliest ca 540 ma and most primitive plutonic rocks in the pan african damara belt and they pre date the main phase of high t regional metamorphism two suites of synorogenic quartz diorites are unusual among damaran intrusive rocks in their elemental and isotopic features comparison of the diorite compositions with melts from amphibolite dehydration melting experiments points to a garnet bearing meta tholeiite probably enriched in k2o as a likely source rock partial melting processes generated mafic ca 50 wt sio2 quartz diorites in the deep crust at temperatures of between 1 000 and 1 100 c based on comparison with experimental results and similar temperature estimates based on p2o5 solubility in mafic rocks subsequently the quartz diorites evolved by multistage polybaric differentiation processes including fractional crystallization of mainly hornblende and plagioclase and assimilation of felsic basement gneisses although their chemical characteristics high lile low hfse resemble those of other quartz diorites with calc alkaline affinities they differ in their enriched sr initial 87sr 86sr 0 70943 0 71285 nd initial e nd 9 1 to 15 2 and o 18o 6 8 8 1 isotope compositions neodymium model ages tdm that range from 1 7 to 2 2 ga and large variation in 207pb 204pb relative to 206pb 204pb indicates involvement of ancient crustal material lead 206pb 204pb 17 08 17 23 207pb 204pb 15 53 15 62 208pb 204pb 37 71 38 16 isotope compositions are strongly retarded indicating that the source underwent a pre pan african u pb fractionation and u depletion it is proposed that the quartz diorites originated by synorogenic high temperature melting of mafic lower crust this contrasts with previous suggestions favouring an origin of these rocks by melting of an enriched mantle during pan african times with characteristics modified by subduction of oceanic crust and sedimentary rocks

evidence of isotopic equilibration between microgranitoid enclaves and host granodiorite warburton granodiorite lachlan fold belt australia abstract microgranitoid enclaves in the s type warburton granodiorite lachlan fold belt australia display a range in mineralogical and isotopic compositions many show obvious similarities to the host granodiorite they contain biotite as the sole ferromagnesian phase and have isotopic signatures similar to the host other enclaves contain cummingtonite actinolite and rare orthopyroxene mainly in the core of the enclaves these enclaves typically have higher nd i and lower sol 87 sr 86 sr i than the host rocks reversely zoned plagioclase megacrysts in some enclaves contain anhedral cores which are similar in composition to an poor plagioclase crystals in the host rock an45 these megacrysts are jacketed by an rich plagioclase an70 an80 the enclaves are interpreted to be globules of a more mafic magma which mingled with the host while it was still partially liquid megacrysts within the enclaves are thought to have been derived from the granodioritic magma the mafic enclave magma is likely to have been derived from a mantle source but equilibration between enclaves and host rock has changed the mineralogy and isotope geochemistry of all enclaves the equilibration process was incomplete for some enclaves resulting in mineralogical zoning and distinctions in isotopic signature between these enclaves and the host rock this equilibration process is likely to have been arrested as a result of rapid cooling of the pluton due to its small size and shallow emplacement level it is uncertain whether the compositions of the least equilibrated enclaves are truly representative of the original mafic enclave magma or whether their trace element signature has been affected by diffusional processes within a stratified magma chamber

dissolution kinetics of plagioclase in the melt of the system diopside albite anorthite and origin of dusty plagioclase in andesites the textures and kinetics of reaction between plagioclase and melts have been investigated experimentally and origin of dusty plagioclase in andesites has been discussed in the experiments plagioclase of different compositions an96 an61 an54 an23 and an22 surrounded by glasses of six different compositions in the system diopside albite anorthite was heated at temperatures ranging from 1 200 to 1 410 c for 30 min to 88 h textures were closely related to temperature and chemical compositions a crystal became smaller and rounded above the plagioclase liquidus temperature of the starting melt glass and remained its original euhedral shape below the liquidus whatever the temperature the crystal melt interface became rough and often more complicated sieve like texture composed of plagioclase melt mixture in the scale of a few m was developed from the surface of the crystal inward formation of mantled plagioclase if the crystal is less calcic than the plagioclase in equilibrium with the surrounding melt and the interface remained smooth if the crystal is more calcic than the equilibrium plagioclase from these results the following two types of dissolution have been recognized 1 a crystal simply dissolves in the melt which is undersaturated with respect to the phase simple dissolution and a crystal is partially dissolved to form mantled plagioclase by reaction between sodic plagioclase and calcic melt partial dissolution the amount of a crystal dissolved and reacted increased proportional to the square root of time this suggests that these processes are controlled by diffusion probably in the crystal mantled plagioclase produced in the experiments were very similar both texturally and chemically to some of the so called resorbed plagioclase in igneous rocks chemical compositions and textures of plagioclase phenocrysts in island arc andesites of magma mixing origin have been examined cores of clear and dusty plagioclase were clacic about an90 and sodic about an50 respectively this result indicates that dusty plagioclases were formed by the partial melting due to reaction between sodic plagioclase already precipitated in a dacitic magma and a melt of intermediate composition in a mixed magma during the magma mixing

microchemical and sr isotopic investigation of zoned k feldspar megacrysts insights into the petrogenesis of a granitic system and disequilibrium crystal growth k feldspar megacrysts kfm are used to investigate the magmatic evolution of the 7ma monte capanne mc monzogranite elba italy dissolution and regrowth of kfm during magma mixing or mingling events produce indented resorption surfaces associated with high ba contents diffusion calculations demonstrate that kfm chemical zoning is primary core to rim variations in ba rb sr li and p support magma mixing i e high ba and p and low rb sr at rims but more complex variations require other mechanisms in particular we show that disequilibrium growth related to variations in diffusion rates in the melt may have occurred as a result of thermal disturbance following influx of mafic magma in the magma chamber initial 87 sr 86 sr ratios isr obtained by microdrilling decrease from core to rim inner core analyses define a mixing trend extending towards a high isr rb sr melt component whereas the outer cores and rims display a more restricted range of isr but a larger range of rb sr lower isr at the rim of one megacryst suggests mixing with high k calc alkaline mantle derived volcanics of similar age on capraia trace element and isotopic profiles suggest 1 early megacryst growth in magmas contaminated by crust and refreshed by high isr silicic melts as seen in the inner cores and 2 later recharge with mafic magmas as seen in the outer cores followed by 3 crystal fractionation with possible interaction with hydrothermal fluids as seen in the rim the model is compatible with the field occurrence of mafic enclaves and xenoliths

trace element evidence for plagioclase recycling in calc alkaline magmas abstract trace element zoning in plutonic plagioclase feldspars from the val fredda complex southern adamello massif italy has been measured using secondary ion mass spectrometry sims the studied samples range from hornblende gabbro through diorite to granitoid with an overall range in plagioclase composition from an96 to an15 individual plagioclases display extensive zonation in anorthite through a considerable variety of zoning textures there is equally extensive zoning in trace elements notably sr and ba broadly similar trace element zoning patterns and concentrations are displayed by all plagioclases irrespective of host rock composition using a relationship betweenxan and the distribution coefficientsdsr anddba trace element concentrations in plagioclase are converted to those of the melt which precipitated each crystal zone the calculated variations in ba and sr are greater than can be attributed to changes in pressure or temperature alone indicating that compositional variation in the coexisting melt was the principal cause of zoning the ba sr melt trend for each crystal describes a two stage crystallisation history the calcic cores of all crystals show melt trends of increasing sr and ba which are attributed to mixing and fractionation in parental hydrous picrobasalt magmas in contrast the melt trends recorded by the rims vary from sample to sample but essentially show decreasing sr and ba consistent with fractionation of sodic plagioclase alkali feldspar and or biotite the plagioclase cores are thought to have crystallised in a common magma chamber at 6 10 kbar pressure while the rims crystallised at the emplacement level 1 5 kbar from interstitial melt whose composition is determined by that of the host magma the data suggest that early formed calcic plagioclase was recycled by the magma during fractionation vigorous convection is invoked to both retain relatively dense calcic plagioclase within the evolving magma and to periodically re entrain cumulus crystals into more evolved magmas the occurrence of calcic plagioclase cores in felsic derivative magmas indicates that fractionation from picrobasalt to tonalite occurred on a relatively short time scale

shrimp u pb zircon geochronology of the strzelin gneiss sw poland evidence for a neoproterozoic thermal event in the fore sudetic block central european variscides zircon ages recorded in gneissic rocks have recently been used as criteria to define and correlate various tectonic units and crustal blocks in the central european variscides a shrimp u pb zircon geochronological study of the strzelin gneiss in the fore sudetic block sw poland indicates the presence of 1 inherited zircon cores of palaeo to mesoproterozoic 206pb 238u ages between ca 2 000 and 1 240 ma and 2 zoned rims of neoproterozoic age with two distinct means of 600 7 and 568 7 ma the proterozoic age range of the cores suggests that different precambrian crustal elements were the source for the protolith of the gneiss a likely scenario is the erosion of various proterozoic granites and gneisses sedimentation after 1 240 ma and partial resistance of the original components to subsequent metamorphic dissolution and or anatectic resorption in neoproterozoic times the zoned zircon rims of both of the younger neoproterozoic ages are indistinguishable in the cathodoluminescence images the data are interpreted in terms of two different thermal events inducing zoned zircon overgrowth at ca 600 and 568 ma in general the new results confirm earlier assumptions of the proterozoic age of the gneiss protoliths and indicate their similarity to orthogneisses in the east sudetes tectonic domain e g the velke vrbno and desna gneisses the neoproterozoic dates are different from the age of 504 3 reported earlier for the go ciecice gneiss from a neighbouring locality in the strzelin massif the new data strongly indicate a moravo silesian bruno vistulian affinity for the strzelin gneiss and support the hypothesis that the strzelin massif lies within the tectonic boundary zone between the west and east sudetes domains which represents the northern continuation of the moldanubian thrust

paleozoic tectonic evolution of the northern xinjiang china geochemical and geochronological constraints from the ophiolites 1 the northern xinjiang of china is a composite orogenic belt in the south central part of the altaids formed by subduction accretion mainly during the paleozoic geochronologic and geochemical data from the paleozoic ophiolites and associated assemblages are reported here to place constrains on the tectonic evolution of this region and to reconstruct its history using a model of subduction accretion plus large scale map view strike slip faulting geologic and geochronologic review indicates that the paleozoic ophiolites and associated assemblages of the western and eastern junggar regions are tectonically related occurring as a pattern with the early paleozoic tangbale mayile hongguleleng aermantai complexes in the center the late paleozoic darbut karamaili and the northern tianshan complexes in the south and the late paleozoic kekesentao qiaoxiahala kuerti complexes in the north available geochemical data suggest that the paleozoic ophiolites of the northern xinjiang are mainly oceanic fragments of these three types mid ocean ridge basalt oceanic island basalts and island arc basalts the paleozoic subduction accretion complexes grew mainly along different segments of the kipchak arc and in associated with migration of magmatic arc the paleozoic ophiolite associated sedimentary sequences and subduction related volcanic rocks exhibit an evolution history from intraoceanic early paleozoic to marginal settings late paleozoic implying coeval strike slip stacking with subduction accretion processes and by which to transport the respective complexes together and further to amalgamate with neighboring units of the altaids subduction accretion processes terminated in the mid carboniferous in this region in the late permian early triassic a counterclockwise rotation of fault blocks in a giant sinistral strike slip and extension domain between altay and tianshan lastly generated present configuration of the paleozoic subduction accretion complexes in the northern xinjiang

crystal melt separation and the development of isotopic heterogeneities in hybrid magmas if a magma is a hybrid of two or more isotopically distinct endmembers at least one of which is partially crystalline separation of melt and crystals after hybridization will lead to the development of isotopic heterogeneities in the magma as long as some of the preexisting crystalline material antecrysts retains any of its original isotopic composition this holds true whether the hybridization event is magma mixing as traditionally construed bulk assimilation or melt assimilation once a magma scale isotopic heterogeneity is formed by crystal melt separation it is essentially permanent persisting regardless of subsequent crystallization mixing or equilibration events the magnitude of the isotopic variability resulting from crystal melt separation can be as large as that resulting from differential contamination multiple isotopically distinct sources or in situ isotopic evolution in one model a redistribution of onethird of the antecryst cargo yielded a crystal enriched sample with 87 sr 86 sr of 0 7058 whereas the complementary crystal poor sample has 87 sr 86 sr of 0 7068 in other models crystal rich samples are enriched in radiogenic sr isotopic heterogeneities can be either continuous controlled by the modal distribution of crystals and melt or discontinuous when there is complete separation of crystals and liquid the first case may be exemplified by some isotopically zoned large volume rhyolites formed by the eruptive inversion of a modally zoned magma chamber in the latter case the isotopic composition of any for example interstitial liquid will be distinct from the isotopic composition of the bulk crystal fraction the separation of such an interstitial liquid may explain the presence of isotopically distinct late stage aplites in plutons crystal melt separation provides an additional option for the interpretation of isotopically zoned or heterogeneous magmas this option is particularly attractive for systems whose chemical variation is otherwise explicable by fractionation dominated processes non isotopic chemical heterogeneities can also develop in this fashion

dehydration melting and water saturated melting of basaltic and andesitic greenstones and amphibolites at 1 3 and 6 9 kb

the magma mixing origin of mantled feldspars the key to mantled feldspar genesis is epitaxial nucleation of plagioclase on k feldspar or k feldspar on plagioclase once this nucleation takes place there is a relatively straightforward process of crystal growth yielding rapakivi and antirapikivi textures the most common mantling is plagioclase on k feldspar which occurs in both volcanic and plutonic environments in the volcanic environment the morphology of the plagioclase overgrowth typically is dendritic though in subvolcanic and shallow plutonic environments dendritic growth is followed by a more or less continuous non cellular shell of plagioclase in the plutonic environment early stages of plagioclase overgrowth also tend to be dendritic although with coarser grained characteristics dendritic morphology is thus a common denominator in rapakivi genesis since growth of dendritic plagioclase is clearly related to marked undercooling in silicate melt systems its occurrence in many volcanic rocks is to be expected equivalent quenching in the plutonic environment requires a cooling mechanism independent of conductive heat transfer to wallrock and also independent of effective cooling related to sudden loss of volatile phases that could only occur late in the crystallization of most magmas and therefore after much dendritic plagioclase had already formed internal quenching of portions of magma systems must occur if mafic magma is abruptly mixed with felsic magma such magma mixing yields a heterogeneous system at first one that is in a drastic state of disequilibrium and tending to force nucleation of one feldspar type on the surface of another resulting in epitaxial crystallization of dendritic plagioclase on k feldspar mantling of one feldspar type by another during magma mixing is paralleled by dendritic growth zones in coexisting plagioclase crystals

granite magma formation transport and emplacement in the earth s crust the origin of granites was once a question solely for petrologists and geochemists but in recent years a consensus has emerged that recognizes the essential role of deformation in the segregation transport and emplacement of silica rich melts in the continental crust accepted petrological models are being questioned either because they require unrealistic rheological behaviours of rocks and magmas or because they do not satisfactorily explain the available structural or geophysical data provided flow is continuous mechanical considerations suggest that far from being geologically sluggish granite magmatism is a rapid dynamic process operating at timescales of or 100 000 years irrespective of tectonic setting

fingerprinting feldspar phenocrysts using crystal isotopic composition stratigraphy implications for crystal transfer and magma mingling in s type granites microsampling of cm scale feldspar crystals within an s type granite from the lachlan fold belt of southeastern australia has revealed complex internal sr and nd isotopic variations the observed isotopic zonations are in part interpreted as recording feldspar crystallisation in a dynamically mixing magma system the isotopic composition of which was varying in response to the influx of more mafic and isotopically more mantle like magmas the latter stages of which are now represented in modified form by microgranular enclaves similar core to rim isotopic variations in feldspar megacrysts from a microgranular enclave and the adjacent host granite strongly suggest megacrysts in the enclave were transferred from the granitic magma during crystallisation feldspar rims have higher 87sr 86sri and lower nd i than adjacent whole rock analyses but match those of mineral separates from the surrounding enclave matrix this suggests that the final stages of megacryst growth occurred in the presence of a component that had previously interacted with a high 87sr 86sr low nd i component such as metasedimentary wall rocks isotopic heterogeneities are also presererved within different mineral phases in the enclave matrix suggesting that differing phases grew at differing stages of equilibration between the enclave magma and its host granitic magma our results reveal major isotopic heterogeneities on a single crystal and also inter mineral scale in a pluton which shows well constrained evidence for magma mingling these results indicate the suitability of feldspars as recorders of isotopic change in magmatic systems even those which have cooled slowly in the plutonic environment and suggest that much heterogeneity in plutonic systems may be overlooked on a whole rock scale

mineral scale sr isotope constraints on magma evolution and chamber dynamics in the rum layered intrusion scotland sr isotopic zoning within single plagioclase crystals from rocks from unit 9 of the rum layered intrusion is used to infer events during crystal growth in a magma undergoing contamination the 87sr 86sr diversity among minerals and between cores and rims of plagioclase crystals increases as the boundary between unit 9 and the overlying unit 10 peridotite is approached models of near solidus interaction of the cumulate with a fluid or melt or large scale textural re equilibration cannot easily account for the systematic differences in 87sr 86sr between small crystals and the rims of larger crystals

processes and sources during late variscan dioritic tonalitic magmatism insights from plagioclase chemistry gesiniec intrusion ne bohemian massif poland the ge siniec intrusion strzelin massif east sudetes 307 290 ma is composed predominantly of dioritic to tonalitic rocks with sr sr ratios ranging from 0 7069 to 0 7080 and end1 4 3 1 to 4 2 emplaced as post collisional magmas following thevariscan orogeny in situ sr isotope and trace element analyses of plagioclase were carried out on five diorite tonalite samples with variable whole rock sr sr compositions to constrain magma sources and elucidate magma chamber processes plagioclase is characterized by complex zoning patterns such as patchy zoning asymmetrical zoning and strong resorption between an rich cores and more albitic rims the range of sr isotopic compositions recorded in plagioclase is 0 7069 0 7091 greater than that of the whole rocks no change in isotopic composition is observed across resorbed core rim boundaries as would be expected if the resorption was caused by magma mixing two samples are close to sr isotopic equilibrium between plagioclase and the whole rock three samples are isotopically heterogeneous and exhibit the largest variations in sr isotope ratios within high an plagioclase cores associated with only slight resorption and almost no change in an content consequently we interpret the core resorption to be due to decompression during emplacement of phenocryst bearing magmas in the upper crust rather than to magma mixing whereas we interpret the isotopic heterogeneity in the high an plagioclase cores to be due to open system processes in the lower crust the trace element and sr isotopic compositions of the high an plagioclase cores provide an insight into the lower crustal processes that took place prior to decompression we show that different diorite tonalite types cannot be related to each other by assimilation fractional crystallization processes and consequently they probably evolved as separate magma batches processes recorded in plagioclase core compositions include contamination by high sr sr crustal material and interaction with water rich magma the studied samples represent magmas that probably formed either by continuous differentiation and crustal contamination of originally more mafic magmas or by remelting of 1 3 2 1 ga basaltic materials in the lower crust this study emphasizes the importance of integrating textural and in situ data to elucidate the processes that contribute to the formation of texturally and compositionally complex plagioclase crystals in plutonic rocks

late devonian palaeobiogeography of marine organic walled phytoplankton abstract acritarchs and prasinophytes have generally been considered as organic walled phytoplankton and their distribution patterns play a significant role in palaeogeographical and palaeoclimatical reconstructions in this paper the palaeobiogeography of late devonian phytoplankton mainly famennian is quantitatively analyzed based on a global database consisting of 95 genera from 15 geographical units the data are analyzed using cluster analysis nonmetric multidimensional scaling and minimum spanning tree analysis using the jaccard ochiai kulczynski and yule s y similarity coefficients the results show that there was provincialism in the late devonian and three phytoplankton palaeobiogeographical realms could be identified the west gondwana east gondwana and boreal realms there is a high degree of similarity between phytoplankton assemblages in east gondwana especially those in australia and iran and euramerica portugal of the iberian armorican block was situated to the north of algeria and probably acted as a stepping stone between euramerica and west gondwana the closed oceanic surface circulation pattern in the proto tethys ocean between euramerica and east gondwana might have hindered the exchange of phytoplankton between east and west gondwana the phytoplankton province that had been typically confined to the higher latitudes was still present while its characteristic genera gradually dispersed into the lower latitudes in the late devonian there is a high similarity between phytoplankton assemblages from the western junggar of xinjiang nw china and euramerica latitude and current influenced palaeotemperature and oceanic circulation patterns are considered to have been the major determinants of the geographical distribution and evolution of marine phytoplankton in the late devonian

geochemical investigations of microgranitoid enclaves in the s type cowra granodiorite lachlan fold belt se australia the cowra granodiorite is a relatively mafic enclave rich s type pluton in the lachlan fold belt which has been cited as a type example of a restitic origin for all varieties of enclaves in lachlan fold belt s type granites microgranitoid enclaves from the pluton are subordinate to metasedimentary varieties and can be subdivided into two groups according to their mafic mineral assemblage pyroxene microtonalites and biotite microgranites no geochemical or isotopic distinction can be made between the two varieties petrographic evidence acicular apatites xenocrysts from the host granite suggests an origin as mingled more mafic magmas which have been variably contaminated by the more felsic host magma this is supported by the fact that the microgranitoid enclaves have isotopic compositions 87sr 86sr i 0 7095 to 0 7144 end i 9 2 to 6 9 that are generally more primitive than or similar to those of the host granite 87sr 86sr i 0 7142 end i 8 8 the spread in isotopic compositions like their trace element compositions is considered to be the consequence of variable degrees of diffusive exchange between the felsic and more mafic magmas during slow cooling several studied metasedimentary enclaves are not in isotopic equilibrium with their host granite and therefore cannot represent pristine samples of the bulk source region of the granite instead they represent portions of a lithologically and compositionally diverse source terrane or accidental xenoliths entrapped during emplacement

magma mixing in the genesis of hercynian calc alkaline granitoids an integrated petrographic and geochemical study of the sazava intrusion central bohemian pluton czech republic the devonian early carboniferous 354 1 3 5 ma conventional zircon u pb age sazava intrusion biotite amphibole quartz diorite tonalite and granodiorite of the central bohemian pluton cbp associated with bodies of olivine pyroxene amphibole gabbro gabbrodiorite quartz diorite and rare hornblendite gives an opportunity for a comparative study of a rather shallow calc alkaline magma mixing zone at two levels separated by a vertical difference of approximately 1 km the deeper section p ibram displays the direct evidence for the existence of a long lived periodically tapped and replenished floored magma chamber masli the contacts between the subhorizontal sheet like basic bodies and the surrounding commonly cumulus rich sazava granitoid are arcuate and cut by a series of veins and ascending pipes shallow dipping swarms of strongly elongated and flow aligned mafic microgranular enclaves mme concordant with the contacts of the basic bodies are commonplace the higher level teletin section shows relatively independent basic intrusions some of them distinctly hybrid in character and mainly of quartz dioritic composition surrounded by relatively homogeneous nearly cumulus free sazava tonalite rich in texturally variable mme larger quartz microdiorite bodies and the mme both interpreted as hybrids contain varying proportions of highly heterogeneous plagioclase megacrysts with complex zoning particularly well shown by cathodoluminescence cl most often the megacrysts have cores of labradorite anorthite partly resorbed and overgrown by andesine rims but some are strongly brecciated and fragments have been annealed by rim growth also characteristic are long prisms of apatite oikocrysts of quartz and k feldspar and zoned amphibole the latter has brown pargasite and magnesiohastingsite cores resorbed and overgrown by magnesiohornblende compositionally similar to the amphibole in the sazava tonalite the brown cores are probably relics of a higher p t stage with temperature exceeding 900 c plots of major element analyses of the teletin quartz microdiorite and the mme fall close to the line joining the compositions of the evolved sazava granodiorite tonalite and the associated gabbros the estimated minimal proportions of the acidic end member range from ca 15 to 20 for the quartz microdiorite and ca 40 to 60 for the enclaves field relationships macroscopic and mineralogical features and whole rock geochemistry are consistent with a hybrid origin of the quartz microdiorite masses and enclaves unzoned calcic plagioclase cores are interpreted as having crystallized from a basic magma prior to its injection into a high level acidic magma chamber the sudden decrease in ca activity and introduction of water due to magma mixing with tonalitic melt led to their resorption and brecciation quartz ocelli that are abundant locally in some basic bodies are most likely xenocrysts introduced from the felsic magma in the basic melt they were resorbed and became a substrate for heterogeneous nucleation of the amphibole or pyroxene coronas strong undercooling of the basic magma resulted in the growth of long prismatic apatite together with dendritic boxy cellular plagioclase both types of plagioclase cores were subsequently overgrown by more sodic mantles at the same time small unzoned plagioclase crystals nucleated in the groundmass following thermal re adjustment crystallization of the hybrid magma ended with considerably slower growth of quartz and k feldspar oikocrysts

dissolution origin and syn entrapment compositional change of melt inclusion in plagioclase abstract in the system di ab an h 2 o melt inclusions in plagioclase were reproducibly formed in a series of partial dissolution experiments and the compositional differences from the surrounding matrix melt were evaluated at 200 mpa and 1050 1150 c a seed crystal of an 59 was partially dissolved into the matrix melt which was in equilibrium with more calcic plagioclase than the seed the reaction proceeded inward from the crystal surface by a dissolution and recrystallization process to form a reaction zone composed of melt channels and recrystallized calcic plagioclase the melt channels were choked necked off by the recrystallized plagioclase at the crystal surface to create melt inclusions the experimentally formed inclusions having square and zigzag shapes up to a few tens of micrometers in size are similar in texture to some common types of natural melt inclusion since the partial dissolution of phenocrysts can occur during various magma mixing processes it should be taken into consideration as one of the inclusion formation mechanisms when trying to interpret information from natural melt inclusions especially for those in hydrous arc magmas the chemical composition of the reproduced melt inclusions deviates from the matrix melt basically along the plagioclase liquidus isotherm the molar ratio of ca ca na and the mgo concentration in the synthesized melt inclusion are up to 17 and 61 relative lower than those in the matrix melt respectively this compositional difference is caused by a combination of factors 1 supersaturation with dissolved plagioclase component 2 insufficient diffusive homogenization in the melt channels due to rapid dissolution of plagioclase and 3 the choking off effect on the basis of the experimental results a model for diffusion in a growing melt channel in dissolving plagioclase was constructed which revealed that the concentration of incompatible elements with low diffusivities can be significantly lower in the melt inclusions than that in the matrix melt these results demonstrate the importance of petrography for interpretation of inclusion data and of more experimental studies on melt inclusion origin

the variscan post collisional volcanism in late carboniferous permian sequences of ligurian alps southern alps and sardinia italy a synthesis abstract in the considered wide sector of the west mediterranean southern europe the collisional phase of the variscan orogeny during late carboniferous and permian times was followed by magmatic intrusive and effusive activity and sedimentation into intracontinental alluvial to lacustrine basins originated by wrench to normal fault systems the first volcanic cycle generally late carboniferous early permian in age is represented by early calc alkaline andesites and rhyolites in variable amounts and by following large volume of rhyolites and by dacites both andesites and rhyolites show k normal and high k calc alkaline character the origin of the liquids of the first cycle is ascribed to partial melting processes at the mantle crust interface telescoped within a thickened crust the melting is considered as the consequence of thermal re equilibration following isostatic disequilibrium and the subsequent collapse of the orogenic belt the ascent of liquids occurred in a trans tensional regime the second magmatic cycle is represented by alkaline magmatism and exhibits typical anorogenic features consistent with a rifting regime this event was no more related with the collapse of the variscan belt but rather to the post variscan global re organization of plates that evolved during late triassic times to the neo tethyan rifting in both cycles important differences in timing areal distribution and outpoured volumes arise

post variscan evolution of the lithosphere in the rhine graben area constraints from subsidence modelling in the area of the cenozoic rhine rift system crustal and lithospheric thicknesses range between 24 and 35 km and 60 and 120 km respectively this rift system transects the deeply truncated variscan orogen and superimposed permo carboniferous wrench induced troughs and late permian and mesozoic thermal sag basins at the time of its westphlian consolidation the variscan orogen was probably characterized by 45 60 km deep crustal roots that were associated with its rheno hercynian saxo thuringian saxo thuringian bohemian and bohemian moldanubian sutures all of which are transected by the cenozoic rhine rift system during the stephanian early permian wrench induced disruption of the variscan orogen subducted lithospheric slabs were detached causing upwelling of hot mantle material during the resulting thermal surge partial delamination and or thermal thinning of the continental mantle lithosphere induced regional uplift at the same time the variscan orogenic roots were destroyed and crustal thicknesses reduced to 28 35 km in response to the combined effects of mantle derived melts interacting with the lower crust regional erosional unroofing of the crust and on a more local scale by its mechanical stretching towards the end of the early permian the potential temperature of the asthenosphere returned to ambient levels with this regional long term thermal subsidence of the lithosphere commenced controlling the development of a new system of late permian and mesozoic thermal sag basins however the evolution of these basins was repeatedly overprinted by minor short term subsidence accelerations that reflect the build up of far field stresses related to rifting in the tethyan and atlantic domains comparison of observed and modelled subsidence curves suggests that in the area of the rhine rift system the lithosphere had equilibrated with the asthenosphere at the end of the cretaceous at depths of 100 120 km before it became thermally destabilized again by cenozoic rifting and plume related magmatism modelled subsidence curves indicate that by the end of early permian times the thermal thickness of the remnant mantle lithosphere ranged between 10 and 50 km in areas that were later incorporated into mesozoic thermal sag basins this corresponds to mid permian thermal lithosphere thicknesses of 40 80 km the geological society of london 2004

modelling diverse processes in the petrogenesis of a composite batholith the central bohemian pluton central european hercynides the multiple intrusions making up the central bohemian pluton in the central european hercynides have petrographic and geochemical features consistent with the presence of four main granitoid suites major element trace element and sr nd isotopic compositions are used to model their petrogenesis partial melting of metabasic locks or of a chur like mantle source are interpreted to have produced melts parental to the most primitive calc alkaline sazava suite interaction of basic with more acidic magmas followed by extensive amphibole plagioclase dominated fractionation accounts for the production of trondhjemites alternatively the trondhjemites correspond to small degree melts of a metabasic source afc assimilation fractional crystallization modelling with a paragneiss as a contaminant and increasing d nd values simulates the characteristics of the blatna suite closed system fractionation of strongly enriched mantle derived magmas or their interaction with leucogranitic melts is deduced for the petrogenesis of the shoshonitic certovo bremeno suite partial melting of a metasedimentary source followed by k feldspar dominated fractionation accounts for the granites of rite ricany suite the progression from relatively primitive calc alkaline granitoids towards evolved k rich calc alkaline and shoshonitic rocks is interpreted to reflect the increasing enriched mantle input in the petrogenesis of the later suites the evidence for hercynian subduction is equivocal and the mantle enrichment could have been significantly older

interplay between geochemistry and magma dynamics during magma interaction an example from the sithonia plutonic complex ne greece orogenic granitoids often display mineralogical and geochemical features suggesting that open system magmatic processes played a key role in their evolution this is testified by the presence of enclaves of more mafic magmas dispersed into the granitoid mass the occurrence of strong disequilibrium textures in mineralogical phases and or extreme geochemical and isotopic variability in this contribution intrusive rocks constituting the sithonia plutonic complex northern greece are studied on the basis of mineral chemistry whole rock major trace element geochemistry and sr and nd isotopic composition sithonia rocks can be divided into a basic group bearing macroscopic mafic enclaves microscopic disequilibrium textures geochemical and isotopic evidence of magma interaction and an acid group in which most geochemical and isotopic features are consistent with a magma mixing process but macroscopic and microscopic features are lacking a two step mixing plus fractional crystallization mfc process is considered responsible for the evolution of the basic group the firststepexplainsthechemicalvariationinthemaficenclavegroup abasicmagma representedbytheleastevolvedenclaves interacted

crystallization and resorption in plutonic plagioclase implications on the evolution of granodiorite magma gesiniec granodiorite strzelin crystalline massif sw poland abstract granodiorite from the gesiniec intrusion strzelin crystalline massif sw poland contains complexly zoned plagioclases five chemically and structurally distinct zones can be correlated among crystals cores 25 35 an inner mantles 40 45 an outer mantles 40 25 an resorption zones 35 50 an and rims 35 30 an good structural and chemical major and trace elements correlation of zones between crystals indicates that zonation was produced by changes in conditions of crystallization on a magma chamber scale plagioclase being the liquidus phase records a time span from the beginning of crystallization to emplacement and rapid cooling of granodiorite as thin dykes crystallization began with the formation of inner mantles the paucity and different sizes of inner mantles suggests slow crystallization in high temperature magma normally zoned inner mantles were formed under increasing undercooling compositional trends in mantles suggest closed system crystallization the major resorption zones were caused by injection of less evolved magma as indicated by the strontium increase in plagioclase the injection triggered a rapid rise of magma and plagioclase crystals facilitating mixing but also inducing fast kinetically controlled growth of complex multiple oscillatory zonation within resorption zones the ascent of magma caused decompression melting of plagioclase and produced melt inclusions within inner mantles the cores the decompression range is estimated at a minimum of 2 kbar emplacement of granodiorite as thin dykes allow rapid cooling and preservation of magmatic zonation in plagioclases melt inclusions crystallized completely during post magmatic cooling the zonation styles of plutonic plagioclase differ markedly from volcanic ones suggesting different magma evolution zones in plutonic plagioclase are well correlated indicating crystallization in quiescent magma where crystals accumulation and compositional magma stratification may occur crystals probably did not travel between different regimes resorption occurred but as single albeit complex episodes good correlation of zones in plutonic plagioclases allows a distinction between the main processes controlling zonation and superimposed kinetic effects

episodic growth of the gondwana supercontinent from hafnium and oxygen isotopes in zircon it is thought that continental crust existed as early as 150 million years after planetary accretion but assessing the rates and processes of subsequent crustal growth requires linking the apparently contradictory information from the igneous and sedimentary rock records for example the striking global peaks in juvenile igneous activity 2 7 1 9 and 1 2 gyr ago imply rapid crustal generation in response to the emplacement of mantle super plumes rather than by the continuous process of subduction yet uncertainties persist over whether these age peaks are artefacts of selective preservation and over how to reconcile episodic crust formation with the smooth crustal evolution curves inferred from neodymium isotope variations of sedimentary rocks detrital zircons encapsulate a more representative record of igneous events than the exposed geology and their hafnium isotope ratios reflect the time since the source of the parental magmas separated from the mantle these model ages are only meaningful if the host magma lacked a mixed or sedimentary source component but the latter can be diagnosed by oxygen isotopes which are strongly fractionated by rock hydrosphere interactions here we report the first study that integrates hafnium and oxygen isotopes all measured in situ on the same precisely dated detrital zircon grains the data reveal that crust generation in part of gondwana was limited to major pulses at 1 9 and 3 3 gyr ago and that the zircons crystallized during repeated reworking of crust formed at these times the implication is that the mechanisms of crust formation differed from those of crustal differentiation in ancient orogenic belts

magnetic hysteresis of 0 6 110 μm magnetites across the verwey transition we report saturation magnetization ms saturation remanence mrs coercive force hc and remanence coercivity hcr as a function of grain size d and temperature t for 0 6 135 m magnetites

insights into magmatic evolution and recharge history in capraia volcano italy from chemical and isotopic zoning in plagioclase phenocrysts plagioclase phenocrysts in dacites from the high k calc alkaline capraia volcano were investigated for major trace element and sr isotope variations in order to gain better insight into the proposed open system behaviour of the volcano repeated dissolution zones in plagioclases from the early erupted monte castello mc and later monte rucitello mr dacitic units are associated with a sharp increase in an content and variation in isotopic compositions 87sr 86sr 0 70872 0 71004 which argue for repeated magma mixing in particular petrographic and mineralogical evidence suggest that recharge with hot basaltic magma occurred repeatedly during the lifetime of the capraia volcanic system we attempted to correlate shared crystallization and resorption histories using chemical mol an fe and sr isotopic zoning in plagioclase in this correlation framework the observed mismatch between chemical and isotopic data in some phenocrysts is interpreted to reflect either growth of phenocrysts in a separate magma chamber or compositional zoning within a single magma chamber a minor role of crustal contamination is inferred instead the intra crystal variations in isotopic composition in plagioclase reflect the existence of distinct components that may have thoroughly mixed at the end of the mc plagioclase growth history the dominant radiogenic component is similar to lamproites occurring in tuscany recharge with this component occurred throughout the genesis of the mr dacites but was restricted to the early growth history of the mc dacites the mr plagioclases are inferred to have crystallized from separate more differentiated batches of magma that were likely to have evolved at the top of a zoned magma reservoir decrease in the an content after most dissolution zones in all phenocrysts and low temperature estimates from zircon saturation thermometry 800 c also emphasize the importance of crystal fractionation during the evolution of the volcano in agreement with the occurrence of a significant amount of rhyolitic material thus it is inferred that a rather large magma chamber may have existed during the first cycle of eruptive activity

gabbro derived granulites from the northern apennines italy evidence for lower crustal emplacement of tholeiitic liquids in post variscan times apennines and the western alps the protoliths of the fe tithe mafic granulites from the northern apennines commonly contain oxide bearing mafic granulites can be related to the least evolved significant amounts of either olivine or fe ti oxide phases on the rocks through fractional crystallization and assimilation of crustal basis of mineralogy and whole rock major and trace element material the felsic granulites probably represent anatectic and compositions their protoliths are recognized as cumulus rocks derived migmatitic rocks resulting from multi stage melting of lower crustal from variably evolved tholeiitic liquids clinopyroxenes from the basement rocks olivine bearing rocks show peculiar trace element compositions e g low amounts of cr zr and lanthanides coupled with relatively high sr concentrations that record a process of granulite facies trace element redistribution controlled by a reaction between olivine and

intracontinental extensional magmatism with a subduction fingerprint the late carboniferous halle volcanic complex germany the halle volcanic complex hvc is part of the transtensional intracontinental saale basin which formed on the mid german crystalline rise located at the southern margin of the late carboniferous early permian volcanic province of central europe magmatic activity ranged from early trachybasalts trachyandesites and trachydacites followed by calc alkaline mildly peraluminous low si rhyolites the latter of which had intruded at a very shallow crustal level two groups of geochemically heterogeneous and isotopically distinct mafic intermediate rocks have to be distinguished which originated from enriched mantle lower crustal sources and experienced crustal contamination to various extents these rocks preceded the emplacement of rhyolites that are remarkably uniform in major and trace element chemistry as well as nd isotope composition distinctly negative end t 300 ma 6 7 to 7 0 of the rhyolites implies significant involvement of crustal material the pb isotopic composition of k feldspar and trace element content of the rhyolites are compatible with remelting of saxothuringian rather than rhenohercynian crustal domains of the variscan orogen slightly differing ree abundances in the rhyolites are attributed to an inhomogeneous distribution of accessory minerals in conflict with their generation in an extensional environment the trace element signature of the hvc rocks indicates a magmatic arc or collisional setting rather than an intracontinental within plate setting the composition of rhyolites from extensional settings at halle and the adjacent northeast german basin demonstrates that trace element composition and geodynamic environment may not be correlated furthermore the geochemistry of these rocks implies that the same type of magmatism may take entirely different chemical expressions in dependence of the structural and chemical composition of the underlying lithospheric block which might be used to map hidden destroyed terrain boundaries in ancient orogens

stratigraphy and facies development of the marine late devonian near the boulongour reservoir northwest xinjiang china late devonian to early carboniferous stratigraphic units within the zhulumute formation honggulelengformation stratotype hebukehe formation and the heishantou formation near the boulongourreservoir in northwestern xinjiang are fossil rich the hongguleleng and hebukehe formations are biostratigraphicallywell constrained by microfossils from the latest frasnian linguiformis to mid famenniantrachytera conodont biozones the hongguleleng formation 96 8 m is characterized by bioclastic argillaceouslimestones and marls the dominant facies intercalated with green spiculitic calcareous shales ityields abundant and highly diverse faunas of bryozoans brachiopods and crinoids with subordinatesolitary rugose corals ostracods trilobites conodonts and other fish teeth the succeeding hebukehe formation 95 7 m consists of siltstones mudstones arenites and intervals of bioclastic limestone e g blastoid hill and cherts with radiolarians a diverse ichnofauna phacopid trilobites echinoderms crinoids and blastoids together with brachiopods ostracods bryozoans and rare cephalopods have beencollected from this interval analysis of geochemical data microfacies and especially the distribution ofmarine organisms which are not described in detail here but used for facies analysis indicate a deepeningof the depositional environment at the boulongour reservoir section results presented here concernmainly the sedimentological and stratigraphical context of the investigated section additionally onelate devonian palaeo oceanic and biotic event the upper kellwasser event is recognized near the sectionbase