geochemistry and stratigraphic correlations application to the investigation of geothermal and mineral resources of tuscany italy contribution to the knowledge of the ore deposits of tuscany ii abstract major element geochemistry 364 analyses is used to obtain quantitative information on the initial mineralogical composition and the depositional environment of paleozoic more or less metamorphic rocks from tuscany and elba furthermore this method can be used to improve stratigraphic correlations by comparing the chemical compositions of the metamorphic rocks and their possible non metamorphic counterparts of known stratigraphic position the stratigraphic units were divided into two sets the first includes the units of known stratigraphic or tectonic position taken as reference groups permian red porphyries carboniferous rocks buti group l s porphyritic schists porphyroids lower phyllites and metabasites from the apuan alps and elba the second set comprises the units of uncertain position metapelites and metapsammites from outcrops mines and boreholes from the boccheggiano niccioleta area calamita schists and metabasites associated with these two units micaschists gneisses and associated amphibolites the main results are 1 1 the porphyroids and porphyritic schists differ strongly from the permian red porphyries thus confirming that they belong to two different volcanic episodes 2 2 all the reference units consist of shales sandstones s s or graywackes differing by their degree of maturity increasing from the lower phyllites to the buti group and to the carboniferous formations 3 3 the carboniferous rocks the buti group and the lower phyllites have distinct chemical compositions 4 4 the boccheggiano formation l s and the calamita schists are similar and include rocks chemically equivalent to the lower phyllites the buti group and the carboniferous formations 5 5 the micaschists and gneisses derive from shales and graywackes respectively and chemically recall the lower phyllites 6 6 the metabasites from the apuan alps and from niccioleta are within plate basalts whereas the amphibolites interlayered within the micaschists and gneisses seem ocean floor basalts

an isotopic mixing model for the origin of granitic rocks in southeastern australia a simple mixing process was a significant element in the genesis of many southeastern australian granitic rocks major and trace element abundances are near linearly correlated nd and sr initial isotopic compositions define a simple hyperbolic trend and many initial 87sr 86sr ratios are a regular function of rb sr ratios geological arguments and regression of geochemical variation diagrams place limits on the nature of the end members the high si end member is meta sedimentary derived magma comparable in composition to the cooma granodiorite the one pluton in the region surrounded by and clearly derived from regional metamorphic rocks the low si end member is basaltic material with similarities to rare gabbros also found in the area isotope dilution calculations based upon initial sr and nd isotopic compositions deduced for the end members yield realistic hypothetical sr and nd concentrations for the end members in a demonstration of the internal consistency of the model all of the granitic rocks from hornblende tonalites to cordierite granodiorites are a single broad family products of variable mixing between distinct batches of basaltic material and a uniform partial melt of the regional basement

partial melting of metagreywackes part ii compositions of minerals and melts a series of experiments on the fluid absent melting of a quartz rich aluminous metagreywacke has been carried out in this paper we report the chemical composition of the phases present in the experimental charges as determined by electron microprobe this analytical work includes biotite plagioclase orthopyroxene garnet cordierite hercynite staurolite gedrite oxide and glass over the range 100 1000 mpa 780 1025 c biotites are na and mg rich with ti contents increasing with temperature the compositions of plagioclase range from an17 to an35 with a significant orthoclase component and are always different from the starting minerals at high temperature plagioclase crystals correspond to ternary feldspars with or contents in the range 11 20 mol garnets are almandine pyrope grossular spessartine solid solutions with a regular and significant increase of the grossular content with pressure all glasses are silicic sio2 67 6 74 4 wt peraluminous and leucocratic feo mgo 0 9 2 9 wt with a bulk composition close to that of peraluminous leucogranites even for degrees of melting as high as 60 vol with increasing pressure sio2 contents decrease while k2o increases at any pressure the melt compositions are more potassic than the water saturated granitic minima the h2o contents estimated by mass balance are in the range 2 5 5 6 wt these values are higher than those predicted by thermodynamic models modal compositions were estimated by mass balance calculations and by image processing of the sem photographs the positions of the 20 to 70 isotects curves of equal proportion of melt have been located in the pressure temperature space between 100 mpa and 1000 mpa with increasing pressure the isotects shift toward lower temperature between 100 and 200 mpa then bend back toward higher temperature the melting interval increases with pressure the difference in temperature between the 20 and the 70 isotects is 40 c at 100 mpa and 150 c at 800 mpa the position of the isotects is interpreted in terms of both the solubility of water in the melt and the nature of the reactions involved in the melting process a comparison with other partial melting experiments suggests that pelites are the most fertile source rocks above 800 mpa the difference in fertility between pelites and greywackes decreases with decreasing pressure a review of the glass compositions obtained in experimental studies demonstrates that partial melting of fertile rock types in the crust greywackes pelites or orthogneisses produces only peraluminous leucogranites more mafic granitic compositions such as the various types of calk alkaline rocks or mafic s type rocks have never been obtained during partial melting experiments thus only peraluminous leucogranites may correspond to liquids directly formed by partial melting of metasediments other types of granites involve other components or processes such as restite unmixing from the source region and or interaction with mafic mantle derived materials

feeding and growth of a dyke laccolith system elba island italy from ams and mineral fabric data dykes feed laccoliths and sills however the link between feeder and intrusion is rarely observed the felsic san martino laccolith displays a clear feeder intrusion link allowing reconstruction of the influence of the size and location of feeder dykes on magma flow during formation of subhorizontal intrusions this work uses anisotropy of magnetic susceptibility ams combined with mineral shape preferred orientations of sanidine megacrysts to examine magma flow pathways through feeders into a laccolith strong correlation between ams and k feldspar datasets indicates that alteration affecting the paramagnetic mineralogy did not influence ams results the well established field relationships between feeder and laccolith provided a robust geo logical model for flow pathways that we have used as a framework to aid interpretation of ams data the position and size of the main feeder dyke helped to predict the flow paths in the overlying laccolith our results show that magma spread laterally from the feeding system and built the laccolith layers with propagating and inflating divergent flow where tabular particles became aligned perpendicular to the magma displacement direction the lack of internal discontinuities indicates that the magma was injected as a single pulse or a series of quickly coalescing pulses supplementary material ams methods ams data and detailed fabric maps are available at www geolsoc org uk sup18717

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

rapid incremental assembly of the monte capanne pluton elba island tuscany by downward stacking of magma sheets the late miocene monte capanne pluton elba island italy is characterized by the widespread occurrence of euhedral k feldspar megacrysts for which variations in size and abundance have been determined at 392 stations the variability of megacryst distribution defines three main facies characterized by low san piero facies high sant9andrea facies and intermediate san francesco facies megacryst abundance the three facies show minor yet systematic differences in whole rock major and trace element contents isotopic composition and biotite mineral chemistry with no detectable link between chemical variabilities and megacryst abundance these results along with the reconstruction of the crystallization sequence suggest that the facies formed at depth as distinct magma batches with their own peculiar geochemical features which were preserved after ascent and emplacement the new geological map based on k feldspar megacryst distribution thus reveals the composite structure of the pluton which was built up incrementally by downward stacking of three slightly different magma batches resulting in a sheeted pluton in the intermediate shallow crust the three magma batches were emplaced in a short time sequence in order to have internal magmatic contacts preserved thus giving a new perspective to the ongoing geochronological efforts to unravel age differences between internal plutonic facies

geochemistry of tuscan archipelago granitoids central italy the role of hybridization processes in their genesis this paper reports major trace and rare earth element data for selected samples from the elba giglio and monte cristo plutons which lie in the tuscan archipelago italy on the basis of field and petrologic constraints all the rocks can be divided into five facies and range in composition from granodioritic to alkali granitic with a strong predominance of monzogranites tuscan archipelago granitoids are clearly collision related rocks and they straddle the i and s types of chappell and white however mineralogical and chemical characteristics indicate that the more basic rocks are broadly i type while the high silica rocks are s type genetic models that relate these magmas through the segregation of their major and accessory mineral phases are inconsistent with major trace and ree element abundant data geological petrographical and geochemical constraints show that the microgranular mafic enclaves found within the rocks are blobs of mafic high temperature magma chilled within a cooler m

isotopic disequilibrium during melting of granite and implications for crustal contamination of magmas the assumption that the isotopic ratios of anatectic melts generated in response to crust magma interaction are equivalent to the bulk source rock from which they are derived was tested experimentally via dehydration melting of a proterozoic biotite granite heating of the coarse grained granite at 1250 c for durations of 1 to 48 h produced extreme disequilibrium among simultaneously generated mafic and felsic melts quenched to glass and whole rock 87sr 86sr ratios in situ microdrill sr isotopic analyses across gradational contacts between mafic high 87sr 86sr and felsic low 87sr 86sr glass register significant isotopic and chemical heterogeneity on the submillimetre scale our experiments suggest that during the early stages of wall rock melting when the rate of heating may exceed melting rates isotopic equilibrium may not be maintained thus producing contaminant liquids having isotopic compositions different from the bulk source rock if isotopic equilibrium is not attained during partial melting in the crust current geochemical models of open system magmatic processes may require modification

geochemical and petrological evidence of the subduction of delaminated adriatic continental lithosphere in the genesis of the neogene quaternary magmatism of central italy abstract serri g innocenti f and manetti p 1993 geochemical and petrological evidence of the subduction of delaminated adriatic continental lithosphere in the genesis of the neogene quaternary magmatism of central italy in m j r wortel u hansen and r sabadini editors relationships between mantle processes and geological processes at or near the earth s surface tectonophysics 223 117 147 the neogene quaternary magmatism of the northern apenninic arc took place in four phases separated in space and time which become progressively younger from west to east phase i 14 ma phase ii 7 3 6 0 ma phase iii 5 1 2 2 ma phase iv 1 3 0 1 ma this magmatism is the result of the activation of three physically separate sources 1 the adriatic continental crust extracted from the mantle in the late proterozoic 2 a strongly refractory recently k enriched harzburgitic mantle located in the mechanical boundary layer mbl of the lithosphere and 3 a recently metasomatized cpx rich mantle compositionally variable from iherzolite to wehrlite clinopyroxenite interpreted as an ephemerally k enriched asthenosphere the adriatic continental crust is the dominant source of the acid plutonic and volcanic rocks of the tuscan region the acid magmatism is mostly found inside an ellipsoidal area about 150 300 km centred on giglio island here defined as the tuscan crustal dome within this area mantle derived magmas unaffected by important crustal contamination processes and mixing with crustal anatectic melts have so far not been found pure crustal magmas are rare but are represented for example by some of the san vincenzo and roccastrada rhyolites virtually all the tuscan acid centres show evidence of mixing with potassic mantle derived magmas major and trace elements as well as 87 sr 86 sr and 143 nd 144 nd data on primitive rocks mg 65 reveal two groups of mantle derived magmas these define two distinct mantle enrichment trends both essentially due to the additions of k rich components which metasomatized separate compositionally diverse upper mantle sectors in both cases the most remarkable mineralogical effect of these enrichment processes is the production of variable amount of phlogopite through reaction between fluids and or melts with the mantle the rocks of group i ol hy and q normative lamproites ultrapotassic high mg latites ultrapotassic shoshonites and shoshonites saturated trend are considered to be derived by partial melting at low pressure 87 sr 86 sr 0 717 ce sr 0 3 and k 2 o na 2 o 6 7 and low ratios of 143 nd 144 nd 0 5121 0 5120 and ba la 87 sr 86 sr 0 712 and k 2 o na 2 o 8 9 values and low 143 nd 144 nd ba la 0 10 ratios these constraints do not allow to exclude a complete identity between the k rich components which metasomatized the mantle sources of the saturated and undersaturated trend magmas the geochemical and isotopic features of the components that metasomatized the mantle sources of the northern apenninic arc magmatism can be explained by a geodynamic process which causes a large amount of crustal materials to be incorporated within the upper mantle we propose that the delamination and subduction of the adriatic continental lithosphere related to the still ongoing northern apennine continental collision provide a viable mechanism to explain the genesis and eastward discontinuous migration of the magmatism in central italy the subduction of delaminated lithospheric mantle with lower crustal slivers would have exposed uppermost mantle adriatic mbl and crustal units previously imbricated in the apennine chain to the heating advected by the upwelling of a recently and ephemerally k enriched asthenospheric mantle wedge and by the underplating of magmas derived from it we consider that the diapiric uprising of a hot crustally contaminated asthenosphere occurs in the wake left above the sinking of the adriatic delaminated subducting continental lithosphere the delamination subduction process of the adriatic lithosphere has probably started in the early middle miocene but earlier than 15 14 ma ago as indicated by the age and petrologic characteristics of the first magmatic episode sisco lamproite of the northern apennine orogenesis

disequilibrium melting during crustal anatexis and implications for modeling open magmatic systems contamination of ascending mantle derived magmas by the continental crust was investigated and modeled for a suite of volcanic rocks and entrained crustal xenoliths from the central andes using bulk geochemical compositions for mantle derived and crustal end members as dictated by traditional approaches the assumption that the crustal contaminant in these open magmatic systems is a single composition was assessed through in situ analysis of quenched anatectic melt trapped within its crustal xenolith our results show for the first time significant chemical and sr isotopic disequilibrium between melt and source over submillimeter length scales in a natural system sampled glass is rhyolitic in nature enriched in large ion lithophile elements lile and depleted in heavy rare earth elements hree analysis of the melt for its 87sr 86sr composition revealed isotopic heterogeneity ranging from 0 7164 to 0 7276 the isotopic disequilibrium between melt and source is understood to reflect the melting of minerals with different rb sr and therefore 87sr 86sr more quickly than the isotopic composition can diffusively equilibrate between melt and minerals our results suggest that the mechanism of crustal anatexis produces contaminating melts which are geochemically heterogeneous both spatially and temporally furthermore time scales of sr diffusion and anatectic melt segregation promote the preservation of isotopic disequilibrium at the micro submillimeter and macro crustal scale this highlights the need for detailed microscopic investigations coupled with petrogenetic modeling in order to develop more robust characterization and quantification of contamination in open magmatic systems

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

using the sr isotope compositions of feldspars and glass to distinguish magma system components and dynamics the individual components of magmatic rocks are being increasingly targeted to gain insight into volcanic systems here we demonstrate the use of sr isotope microsampling to distinguish distinct populations of glass and plagioclase that represent the sources of components liquids and crystals within each erupted magma batch we analyzed glasses and plagioclase feldspar crystal cores to characterize crystal cargoes and host liquids from pumice fall units from the minoan cycle santorini greece using this approach we identify the magmatic components that interacted prior to eruption and formation of pumice clasts allowing us to interpret the magma dynamics associated with each eruption varying degrees of complexity can be identified from relatively simple cases where the crystals are in isotopic equilibrium with the glass to more complex such as the large caldera forming minoan eruption which is characterized by at least two liquid components and three different crystal provenances

pb isotopic zoning of k feldspar megacrysts determined by laser ablation multi collector icp ms insights into granite petrogenesis this study investigates pb isotopic zoning in magmatic k feldspar megacrysts from the monte capanne pluton elba italy using laser ablation multi collector icpms the studied crystals provide an ideal opportunity to use in situ techniques to assess the extent of open system processes and better characterize the components involved in the genesis of complex magma systems earlier investigations of the pluton identified the importance of magma mixing between mantle and crustal derived magmas the investigated k feldspar megacrysts exhibit strong zoning in 207pb 206pb and 208pb 206pb correlated with lead elemental variations we interpret these variations as reflecting growth zoning as opposed to secondary diffusive exchange despite a great variety of zoning patterns we were able to correlate different events of megacryst growth reflecting crystallization in a dynamic magma system our two step model includes 1 growth of a granitic magma chamber by addition of low 208pb 206pb magma to a high 208pb 206pb magma contaminated with crustal material i e the megacryst cores and 2 recharge by mantle derived magma i e the megacryst rims we interpret the thorogenic nature of the megacryst rims to reflect the mantle derived component involved in the mixing process taking account of other data from the tuscan magmatic province the mantle source is inferred to have been metasomatized by continental material during subduction tims sr isotopic data from microdrilled cores in one megacryst provides general support for the model but show that the two isotopic systems are decoupled

evidence from rb sr microsampling geochronology for the timing of alpine deformation in the sonnblick dome se tauern window austria

sr isotope disequilibrium during differentiation of the bandelier tuff constraints on the crystallization of a large rhyolitic magma chamber the 1 61 ma otowi member of the bandelier tuff jemez mountains volcanic field new mexico is a caldera forming high silica rhyolitic ignimbrite with a precursor fallout deposit the otowi member has high rb sr ratios the 87 rb 86 sr ratios from sanidines and glasses range from 14 to 570 most sanidines from glomerocrysts in the ignimbrite have 87 sr 86 sr i 0 7052 0 7056 whereas the ratios from glasses from glomerocrysts range from 0 7052 to 0 7079 quartz phenocrysts containing glass inclusions from both the initial fallout and the ignimbrite are markedly more radiogenic at 87 sr 86 sr i 0 7105 0 7113 despite having much lower rb sr ratios than glomerocryst glasses these relations require that the inclusion glasses are more contaminated with proterozoic country rock than are glomerocryst glasses textural isotopic and trace element data support a model in which crystals grow in a boundary layer with the most inclusion rich quartz grains closest to the magma country rock contact phenocrysts and glomerocrysts represent fragments from different zones of the chamber9s crystalline carapace disseminated throughout the magma prior to eruption an important implication of these results is that glass inclusions do not necessarily represent precursor magma compositions hence extrapolation of measured volatile contents of inclusion glasses to the entire volume of an erupted magma should be approached with caution

recurrent slope failure enhancing source rock burial depth and seal unit competence in the pearl river mouth basin offshore south china sea high quality 3 d seismic data are used to assess the significance of mass transport deposits mtds to the evolution of the pearl river mouth basin south china sea basal shear surfaces and lateral margins of seven recurrent mtds are mapped to reveal a general ne sw transport direction throughout the late miocene quaternary a key result of our analysis is the perceived relationship between the recurrence of slope instability in the study area and the dongsha tectonic event using borehole data to constrain the ages of interpreted mtds we show that tectonic uplift in the northern south china sea led to slope oversteepening in the late miocene between 10 5 ma and 5 5 ma preconditioning it to fail recurrently for more than 10 ma interpreted mtds are shown to enhance burial depths of source and reservoir units and improve seal competence in lower slope areas conversely upper slope regions record important erosion and reduced sealing capacity in late cenozoic strata as a result we postulate that the thickness variations imposed by mtds on late miocene quaternary strata have important implications to petroleum plays in the south china sea

isotope disequilibrium during anatexis a case study of contact melting sierra nevada california the geochemical consequences of contact melting were investigated in the sierra nevada batholith where trachyandesite magmas have intruded and melted the host granitic rocks partial melting of the granite varies from 10 20 at contact regions to 50 70 in a granite block within the trachyandesite thermal modelling suggests that the temperature of the granite block exceeded its solidus within 3 months of trachyandesite intrusion reached a maximum of 1000 c after ca 1 5 yr and remained above 500 c for ca 40 yr sr nd and pb isotope data of granitic melts record marked sr isotope and to a lesser extent pb isotope disequilibrium both within different glass fractions and between glasses and their source rb sr isochron calculations on residual mineral glass pairs fail to yield the age of melting obtained by40ar 39ar dating 11 9 ma sr and pb isotope data of glasses establish that major rock forming minerals of the granite had not attained isotope equilibrium before the onset of melting consequently the isotopic composition of the successive liquid fractions originating from granite melting depended on the relative contribution of each mineral to the elemental budget of the melts the extrapolation of these results to the petrogenesis of crustal melts and more generally to studies of crustal evolution suggests that isotope disequilibrium during anatexis and high grade metamorphism may be a common characteristic of many geologic settings if future studies substantiate these results some geochemical models of crustal melting and assimilation may need revision in addition marked sr isotope disequilibrium in the residual mineral phases within the partially melted granite raises concerns about the reliability of radiometric dating in metamorphic terranes

two stage growth of laccoliths at elba island italy at elba island italy nine shallow level late miocene granite porphyry layers connected by feeder dikes built up three nested christmas tree laccoliths detailed mapping and reconstruction of tectonic history led to restoration of the original 5 km thick sequence and determination of the dimensional parameters of each intrusive layer the laccolith layers were emplaced at depths between 1 9 and 3 7 km exploiting physical discontinuities that served as crustal magma traps inside a stack of nappes the intrusive layers are 50 700 m thick with diameters between 1 6 and 10 km length to thickness relationships for individual laccolith layers show a power law correlation that does not fit the known dimensional distribution for laccoliths but instead fits a line with a slope typical of the theoretical vertical inflation stage of laccolith development this is interpreted as the first reported natural example of the occurrence of a vertical inflation stage during laccolith growth the dimensional data for elba intrusive layers also suggest that laccoliths and plutons commonly form by amalgamation of smaller sheet like bodies while multilayer laccoliths form when coalescence fails possibly owing to the large availability of crustal magma traps

ree fractionation and nd isotope disequilibrium during crustal anatexis constraints from himalayan leucogranites abstract the processes controlling rare earth element ree behaviour during crustal anatexis have profound implications not only for ree abundances of crustally derived granites but also for their model nd ages for typical protoliths of anatectic granites such as are exposed in metapelites from the high himalayan crystalline series hhcs a high proportion 90 of bulk rock lree and mree are sited in the accessory phases apatite and monazite and of hree in garnet and apatite mass balance constraints indicate that observed ree abundances of himalayan leucogranites cannot be generated from ree deficient reactant phases in their metasedimentary protoliths muscovite biotite plagioclase and quartz but must result from the dissolution of accessory phases ree and th abundances in anatectic melts are modelled by combining empirical expressions for monazite apatite and zircon solubilities as a function of temperature melt composition and melt water content with analysed accessory phase ree abundances this approach which allows the contribution to the melt from each accessory phase to be identified successfully predicts himalayan leucogranite trace element abundances the lree and th concentrations of a metapelite derived melt are buffered by monazite stability although apatite dissolution contributes up to 10 of lree to the melt together with 50 of the mree and 90 of the hree zircon contributes a negligible proportion of ree to the melt during anatexis dissolution of monazite and apatite in the melt results in 1 increasing ree abundances and lree hree ratios with increasing temperature of melting 2 a negative eu anomaly in the melt 3 high sm nd ratios in the melt relative to its metasedimentary protolith and 4 nd isotope disequilibrium between the melt and its protolith all of these features are observed in the ree geochemistry of himalayan leucogranites the combined effects of sm nd fractionation and nd isotope disequilibrium during anatexis imply that model nd ages calculated for anatectic granites derived from apatite and monazite bearing protoliths should be treated with considerable caution

isotopic study of the manaslu granite himalaya nepal inferences on the age and source of himalayan leucogranites a detailed isotopic study of the manaslu leucogranite was carried out a u pb age of 25 ma and a whole rock rb sr age isochron of 18 ma were obtained suggesting that the magmatic activity lasted at least 7 ma initial sr isotopic ratios are very high 0 740 to 0 760 and initial nd isotopic ratios are low nd in 13 to 16 and they show the existence of large isotopic variations even at the metre scale these are not the result of perturbations by fluids but rather they reflect the initial isotopic heterogeneity of the source material which has not been obliterated by magmatic processes e g fusion mixing by convection these results also support the crustal origin of this leucogranite the tibetan slab paragneisses whose sr and nd isotopic ratios are very similar to those of the granite at an age of 20 ma are the most probable parental material nd model ages for both the leucogranite and the gneisses are in the range 1 5 2 ga a model of formation of the manaslu granite by coalescence of different batches of magma is in agreement with the present data

extreme mineral scale sr isotope heterogeneity in granites by disequilibrium melting of the crust abstract the broadest ranges of initial sr isotopic ratios 87 sr 86 sr i ever reported within a single igneous rock 2 10 2 are preserved within the late miocene laccolith pluton dyke felsic complex of elba island italy for these units the integration of textural and crystal scale isotope data allows tracing the evolution of the 87 sr 86 sr i of the melt from the emplacement level back to the earliest pre emplacement crystallization stage the rock matrix minerals record the 87 sr 86 sr i composition of the magma at the emplacement level 0 715 0 716 k feldspar megacrysts representing an earlier phase crystallized at depth record a rim to core increase of sr isotopic ratios from values similar to those of the matrix to significantly higher ones 0 719 remarkably biotites hosted within megacrysts representing the first crystallization stage have extreme and contrasting 87 sr 86 sr i values in the different intrusive units biotites within megacrysts from the laccolith record the lowest ratio in the intrusive complex 0 710 while those in the megacrysts from the pluton and associated felsic dyke have the highest 87 sr 86 sr i 0 732 this time transgressive record of isotopic variation in the magma reflects episodic recharge and mixing of magma batches formed by disequilibrium melting of crustal sources that produced melts through different reactions as temperature was increasing the progression from muscovite to biotite dominated fluid absent melting generates melts with increasing 87 sr 86 sr while at higher temperatures the progression from biotite to hornblende dominated melting reactions results in a decrease in the 87 sr 86 sr of the melt

coupling of anatectic reactions and dissolution of accessory phases and the sr and nd isotope systematics of anatectic melts from a metasedimentary source advances in field observations and experimental petrology on anatectic products have motivated us to investigate the geochemical consequences of accessory mineral dissolution and nonmodal partial melting processes incorporation of apatite and monazite dissolution into a muscovite dehydration melting model allows us to examine the coupling of the rb sr and sm nd isotope systems in anatectic melts from a muscovite bearing metasedimentary source modeling results show that 1 the sm nd ratios and nd isotopic compositions of the melts depend on the amount of apatite and monazite dissolved into the melt and 2 the relative proportion of micas muscovite and biotite and feldspars plagioclase and k feldspar that enter the melt is a key parameter determining the rb sr and 87 sr 86 sr ratios of the melt furthermore these two factors are not in practice independent in general nonmodal partial melting of a pelitic source results in melts following one of two paths in e nd 87 sr 86 sr ratio space a higher temperature fluid absent path path 1 represents those partial melting reactions in which muscovite biotite dehydration and apatite but not monazite dissolution play a significant role the melt will have elevated rb sr 87 sr 86 sr sm nd and e nd values in contrast a lower temperature fluid fluxed path path 2 represents those partial melting reactions in which muscovite biotite dehydration plays an insignificant role and apatite but not monazite stays in the residue the melt will have lower rb sr 87 sr 86 sr sm nd and e nd values than its source the master variables controlling both accessory phase dissolution and hence the sm nd system and melting reaction and hence the rb sr systematics are temperature and water content the complexity in sr nd isotope systematics in metasediment derived melts as suggested in this study will help us to better understand the petrogenesis for those granitic plutons that have a significant crustal source component

in situ zircon u pb oxygen and hafnium isotopic evidence for magma mixing and mantle metasomatism in the tuscan magmatic province italy in this study we have used in situ u pb hf and o isotopic analyses of zircon grains to gain insights into both magmatic processes and duration of magmatism in igneous rocks from the tuscan magmatic province 0 1 9 ma italy three plutonic centres have been investigated monte capanne and porto azzuro monzogranites in elba and the giglio monzogranite as well as capraia the only volcanic centre in the tuscan archipelago new ion microprobe zircon u pb data reveal a continuum of plutonic activity in elba over 2 ma 8 3 6 3 ma with coeval volcanic activity in capraia 7 1 7 6 ma and plutonic activity resuming in giglio 5 5 ma after a gap of 1 ma from these zircon data we also show that construction of the monte capanne pluton elba may have occurred over a period of c 0 5 ma a significant range of both 176hf 177hf determined by la mc icpms and 18o determined by ion microprobe in zircon 7 epsilon hf units and 5 respectively is present which together with zircon morphology and trace element data gagnevin et al 2010 emphasises the importance of mixing and replenishment involving magma batches with both metaluminous and peraluminous affinities inherited and xenocrystic zircons also occur but are scarce these have a wide range of 176hf 177hf and 18o values further emphasising that a variety of crustal components has contributed to the genesis of the tuscan magmas either as contaminants or magma sources while mixing undoubtedly occurred between mafic metaluminous and felsic peraluminous magmas the range of hf and o isotopic data suggests a diversity within the peraluminous component the unradiogenic hf composition ehf t 6 of the inferred mantle derived component represented by capraia volcanism and at least in part lamproitic in composition strongly supports the idea that the mantle source involved in tuscan magmatism was severely modified by subduction related crustal derived metasomatic fluids

source controlled 87sr 86sr isotope variability in granitic magmas the inevitable consequence of mineral scale isotopic disequilibrium in the protolith abstract the broad sr isotopic variability exhibited by granitoid rocks is commonly interpreted to reflect the mixing of magmas from different sources however evidence from granites and migmatites indicates that melting and magma extraction from crustal sources can occur sufficiently rapidly that trace element and isotopic equilibration between liquid and residual phases is commonly not achieved additionally evidence from unmelted high grade metamorphic rocks indicates that major reactant minerals in the fluid absent melting process principally biotite and plagioclase do not always attain equilibrium during regional metamorphism when these two circumstances occur in combination the melt does not inherit its radiogenic isotopic signature from the bulk source in a simple way in such situations the isotopic composition of the melt will be dependent on the isotopic compositions of the reactant phases and the stoichiometry of the melting reaction this study has used information from experimental studies of fluid absent partial melting in metapelites and metagreywackes to investigate the consequences of sr isotopic disequilibrium between the reactant minerals for magma composition the study demonstrates that a range of isotopically distinct magmas can arise from progressive melting of a single source that is able to undergo melting through different reactions as temperature increases when translated to the typically layered sources constituted by sedimentary and volcano sedimentary rocks this process will produce magmas characterized by sr isotope variability that reflects the differences in melting reaction stoichiometry within the different layers even with no bulk rock isotopic variability between layers this study demonstrates that the sr isotope variability commonly observed within granitic suites as well as at the grain and sub grain scale within individual magmatic bodies can be primary reflecting differences in composition between magma batches produced from the progressive melting of a single source

evolution of silicic magma through assimilation and subsequent recharge evidence from sr isotopes in sanidine phenocrysts taylor creek rhyolite nm

fluid absent melting behavior of a two mica metapelite experimental constraints on the origin of black hills granite we have studied experimentally the vapor absent melting behavior introduction of a proterozoic metapelite hp 60 1 from the black hills south peraluminous leucogranites such as the miocene granites dakota to assess whether the high do tourmaline bearing granites of the himalaya and tourmaline granites of the harney of the harney peak suite resulted from muscovite dominated depeak suite in south dakota are widely considered to be hydration melting at 10 kbar samples were sealed with no added the product of muscovite dehydration melting yet few h2o in gold capsules then run in a 0 5 inch 1 27 cm pistonexperimental studies of muscovite dehydration melting cylinder apparatus over the temperature interval 812 975 c reactions have been performed because the low temexperiments were conducted at 10 kbar to allow comparison with peratures of the reactions hinder the approach to equiother metapelite melting studies mass balance calculations imply librium in addition melt fractions generated are typically the melting reaction muscovite biotite plagioclase quartz low resulting in small melt pockets which are very difficult melt alkali feldspar aluminosilicate garnet until to analyze with the electron microprobe in this study muscovite is exhausted 812 c then the reaction biotite we assess the possible genetic relationship between the plagioclase quartz alkali feldspar aluminosilicate tourmaline bearing granites of the harney peak suite melt garnet the inferred muscovite reaction is completed below and the aluminous metasediments these granites intrude 812 c and produces 1 2 wt melt melt production of the although the attempt to investigate the muscovite decontinuous biotite reaction increases steadily with rising temperature hydration melting reaction was hindered by the surto 32 wt in our highest temperature run the low melt prisingly low melt productivity we were able to infer a productivity of the muscovite reaction at 10 kbar suggests that balanced reaction illuminating the dominantly peritectic melting at lower pressures may be necessary to generate mobile melt nature of fluid absent muscovite breakdown under the fractions by muscovite dominated reactions in our starting material conditions investigated in addition melts of hp 60 1 are considerably more mafic than isotopic and trace element studies indicate a genetic the tourmaline bearing granites with average combined tio2 relationship between s type granites and aluminous metamgo feo contents of 1 85 wt and 0 84 wt respectively sediments in the black hills of south dakota where the in particular melts of hp 60 1 are enriched in ti and would harney peak granite hpg intruded proterozoic and probably stabilize biotite rather than tourmaline upon cooling archean schists and metagreywackes undergoing regional arguing against hp 60 1 as a potential source rock for the metamorphism the timing and mode of emplacement of tourmaline granites the hpg and the deformation and regional metamorphic

mineral disequilibrium in lavas explained by convective self mixing in open magma chambers characteristic features of many porphyritic andesite and dacite lavas are that they are rich in crystals and display a range of disequilibrium features including reversely zoned crystals resorption surfaces wide ranges of mineral compositions and minerals which are not in equilibrium with the surrounding rock matrix these features are often interpreted as evidence of the mixing of magmas of contrasting composition temperature and origin1 2 here however we propose that such features can also be caused by convection within a magma body with a single composition that is heated from below and cooled from above we describe petrological observations of andesite lava erupted at the soufriere hills volcano montserrat which indicate a heating event and the intermingling of crystals that have very different thermal histories we present experimental data on a representative groundmass composition of this lava which indicate that it is difficult to explain the calcic compositions of plagioclase overgrowth rims and microphenocrysts unless parts of the magma were at temperatures much higher than the inferred average temperature the concept of convective self mixing allows us to explain the occurrence of compositions of minerals that apparently cannot coexist under equilibrium conditions