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

a search for elf vlf activity associated with earthquakes using isis satellite data for more than 10 years researchers using satellite observations have reported anomalous phenomena associated with seismic activity increases in elf vlf filter amplitudes have been associated with preshock and postshock events near satellites in a number of studies however an examination of the wideband elf vlf data from the low altitude isis satellites shows no significant changes in electromagnetic em activity associated with earthquakes in the analysis of wideband data from 164 satellite passes above 39 earthquakes no unique or familiar previously observed seismo active emission can be linked to the occurrence of a nearby earthquake the electromagnetic activity levels in elf vlf spectra observed by the isis 2 satellite before and after 37 shocks is examined and compared with a set of selected control orbits comparisons are made of em activity changes between earthquake passes and control passes over the epicenter location no increase in the activity for earthquake passes is found these results do not change when various constraints are placed on the data in an attempt to test for dependencies on different criteria no precursory coseismic or postseismic effects are found no significant evidence is found for differences between the data sets which would suggest any seismo active activity

weathering rates and 87sr 86sr ratios an isotopic approach the 87sr 86sr ratios of precipitation throughfall groundwater streamwater and mineral matter were analyzed precipitation strontium ratios are around 0 71 throughfall which is affected by strontium from the mineral soil has ratios of 0 72 0 73 and shallow groundwater in a drug well and runoff have ratios around 0 73 0 74 there is a tendency for this relationship to be reversed for discharge higher ratios are associated with lower discharges water from a fracture zone gave the highest value 0 78 approaching the value of the bedrock around 0 85 a calcium budget which reflects the mean conditions over a few years has been calculated this budget indicates that weathering should be of the order of 8 kg ca ha 1yr 1 to maintain the pool of exchangeable calcium at a constant level the major anion in the runoff is suphate which has entered the system in the form of acid rain sulphur deposition has been rather constant over the last two decades if acidification had an effect on weathering of silicates this would have shown up in a tendency of the strontium isotope ratios toward higher values in the soil pool and in runoff water thus its seems from the observed ratios that at least a major portion of the excess calcium export resulting from sulphur deposition is taken from the soil pool resulting in a gradual acidification of the soil

87sr 86sr ratios in hydrothermal waters and deposits from the east pacific rise at 21 n abstract 87 sr 86 sr ratios of three hydrothermal waters collected on the east pacific rise at 21 n define a mixing line between seawater and a hydrothermal end member at 0 7030 which is derived by seawater basalt interaction at ca 350 c and water rock ratio of about 1 5 sr concentrations are not affected in the process while mg uptake from seawater is almost complete up to2 3 of this hydrothermal component is involved in anhydrite precipitation while the sr isotopic ratio in sulfides chalcopyrite sphalerite cannot be distinguished from that of sulfate it is estimated that ca 1 10 10 moles of strontium are yearly cycled in the hydrothermal systems of mid oceanic ridges thereby affecting the 87 sr 86 sr budget of seawater mass balance between river runoff limestone precipitation and ridge basalt alteration suggests that the 87 sr 86 sr ratios of the river runoff are in the range 0 7097 0 7113 and are largely dominated by limestone alteration

the fate of basaltic enclaves during pyroclastic eruptions an origin of andesitic ignimbrites abstract igneous enclaves chilled bodies of magma with compositions contrasting with those of their hosts have long been recognized in felsic plutonic rocks similar enclaves occur in felsic pyroclastic rocks despite the apparent difficulty of their survival of the explosive eruption process without fragmentation the occurrence of andesitic ignimbrites with textural evidence of generation by mechanical mixing of felsic and mafic ash indicates that in some instances basaltic enclaves in felsic magmas that erupted explosively do indeed undergo fragmentation and homogenization with their host two exposures of rhyolitic ignimbrite that hosts basaltic enclaves and of andesitic ignimbrite in coastal maine demonstrate the set of conditions necessary for survival of basaltic enclaves during catastrophic explosive eruptions relatively lower viscosity of basaltic enclaves compared to the rhyolitic host magma permits vesicle networks to develop as volatiles exsolve from the melt and form bubbles the vesicle networks provide sufficient permeability for exsolving gases to escape the basaltic magma bodies hence sparing the basaltic enclaves from fragmentation if adequate permeability for volatile escape does not develop the expanding bubbles are trapped within the basaltic enclave and ultimately with depressurization during rise of the magma to the surface cause fragmentation of the basaltic magma in this case the basaltic ash and the host rhyolitic ash homogenize producing a hybrid ignimbrite while the surrounding viscous rhyolitic magma behaves typically with a small volume of the rhyolitic magma retaining its coherence as pumice bodies while most of the magma fragments shortly after vesiculation to become ash these observations suggest a distinction between the voluminous andesites associated with subduction zones for which attainment of intermediate composition occurred as a result of petrologic processes unique to subduction zones and hybrid andesitic ignimbrites which are spatially associated with bimodal magmatic systems in a variety of tectonic settings and are the result of mechanical mixing of ash during pyroclastic flow

crystallization and hybridism in microgranitoid enclave magmas microstructural evidence microstructural evidence indicates that many microgranitoid enclaves mafic inclusions in granitoids crystallize from globules of magma that have undergone previous magma mixing hybridism the evidence includes 1 rounded xenocrysts ocelli of quartz rimmed with fine grained aggregates of early formed minerals especially mafic minerals 2 k feldspar megacrysts commonly rimmed with plagioclase rapakivi structure and 3 corroded cores dendritic overgrowths and calcic spikes in plagioclase initial quenching of a magma globule in felsic magma involves rapid temperature equilibration which causes a high rate of nucleation of minerals precipitating from the enclave magma at the time followed by a period of slow cooling during which the nucleation and growth rates of later crystallizing minerals are similar to the much slower rates prevailing in the granitic melt this may result in markedly poikilitic microstructures involving inclusion of the earlier formed finer grained minerals in quartz and or k feldspar which are common in microgranitoid enclaves

interaction between coeval mafic and felsic melts in the st francois terrane of missouri usa abstract in the eastern st francois mountains of missouri a 1 48 ga subvolcanic ring pluton known as the silvermine granite hosts swarms of dark pillow shaped enclaves exhibiting classical features of mafic melt globules trapped in granitic magma the pillows formed by turbulent injection of near liquidus mafic melt into a resident visco plastic granitic magma exchange of crystals fluids and ions between the two melts produced a hybrid tonalite granite enclave series and modified host granite composition several lines of evidence suggest interaction between granitic melt that produced the granophyric cap of the pluton and basaltic melt that formed dikes in the area rapid cooling of the pillows re melted the host creating back veins in many enclaves mineral chemistry indicates the hybrid rocks have undergone extensive subsolidus re equilibration

sr isotope evolution of seawater the role of tectonics we use a high resolution seawater sr isotopic evolution curve for the last 100 m y in conjunction with modern riverine sr flux measurements and also geologic tectonic and geochronological data to make the case for a close relationship between seawater sr isotopic composition and the india asia continental collision using a simple seawater sr budget model we begin by showing that the sr flux associated with alteration of seafloor basalts is too small and does not have the right time evolution to account for much of the seawater sr isotopic curve of the last 100 m y the flux of dissolved sr carried by rivers originating in the himalaya tibet region on the other hand is presently a significant fraction of the global sr budget we calculate how this riverine flux would have had to change with time in order to match the observed seawater sr isotopic curve and find that the riverine flux remains relatively constant prior to the collision of india with asia but then increases very significantly after collision we note that the period of most rapid change in seawater sr isotopic ratio from 20 ma to 15 ma is also a period of exceptionally high erosion in parts of the himalayas and the tibetan plateau as further evidence that sr derived from the collision of india with asia plays a major role in the sr isotopic evolution of seawater we show that the total amount erosion of the himalaya tibetan plateau since collision which we calculate separately represents a total amount of sr that is very nearly the same as the cumulative amount required by the sr isotopic change of seawater since collision the relationship between erosion and riverine sr flux allows us to use the sr isotopic evolution of seawater to reconstruct a history of erosion since collision and we find that the erosion rate accelerates with time since collision with the present having the largest rate when we apply the sr budget model to the entire phanerozoic using a new compilation of deformed continental area versus time we find that we can account for the large scale structure of the seawater sr isotopic curve but fail to reproduce several local maxima and minima especially in the period 100 300 ma the present high s7sr s6sr of seawater and similar highs in the devonian and cambrian do correlate with extensive deformation on the continents

isotopic disequilibrium among commingled hybrid magmas evidence for a two stage magma mixing commingling process in the mt perkins pluton arizona this is the publisher s version also available electronically from http www jstor org stable 30070774 seq 1 page scan tab contents

strontium and neodymium isotopes in hot springs on the east pacific rise and guaymas basin we have determined the concentrations and isotopic compositions of sr and nd in hydrothermal fluids from 21 n east pacific rise and guaymas basin gulf of california the purest solutions analyzed from 21 n exhibit a small range in sr concentration between individual vents from 5 8 to 8 7 ppm close to normal seawater sr concentrations they exhibit a small range in 87 sr 86 sr from e sr 0 13 4 to 17 7 corresponding to e sr 0 18 2 in the pure hydrothermal end member these results indicate extensive but not complete isotopic exchange with sr in the depleted oceanic crust e sr 0 31 8 and suggest that sr concentrations in these solutions are buffered in contrast the concentration and isotopic composition of nd in solutions show large variations between vents the concentration of nd ranges from 20 to 336 pg g 6 100 times seawater nd concentrations the isotopic composition ranges from e nd 0 3 6 similar to pacific seawater to 7 9 many samples show substantial contributions from morb but all have e nd 0 well below morb at this locality e nd 0 9 7 in spite of very large enrichments in nd concentrations while complete isotopic exchange with water rock 2 or exchange with anomalous oceanic crust can explain the sr data the nd data require exchange with a reservoir having e nd 0 pacific seawater low temperature reactions with metalliferous sediments on the ridge flanks may provide such a source both sr and nd in the guaymas basin solution are very different from21 n e sr 0 11 0 and e nd 0 11 4 and are consistent with the fluid exchanging sr and nd with heated sediments having a substantial component of old continental detritus some irregularities in the nd isotopic data reported here indicate that there must be a problem of contamination for some ultra low level trace elements during sample collection and processing which requires further attention using a simple box model the estimates for hydrothermal nd fluxes are compared with fluxes which would be required to maintain the relatively radiogenic value of e nd 0 3 in the pacific against the influx of more negative antarctic waters e nd 0 9 it is shown that the hydrothermal flux of nd from mid ocean ridges falls far short of that necessary to maintain the isotopic balance this indicates that weathered material from volcanic terranes e nd 0 7 is the most reasonable major source of radiogenic nd in the pacific

strontium isotopes in trees as an indicator for calcium availability summary the increasing acidification and impoverishment of soil poses a great environmental problem today and in the future and is exemplified by the decreased availability of calcium in the soil pool unfortunately budgeting the inputs and outputs of calcium in the soil vegetation ecosystem is a very laborious task some of the fluxes are internal in the ecosystem and cannot be measured explicitely like weathering of primary minerals however since strontium and calcium exhibit similar geochemical behaviour it is possible to make an estimation of the calcium content in different media in the soil vegetation eco system using natural strontium isotopes preliminary analyses of water precipitation throughfall runoff soil water soil mineral matter and biological material trees mussel shells show that the use of the strontium isotope ratio 87 sr 86 sr is a powerful tool in estimating and understanding environmental changes

progressive hybridisation between late devonian mafic intermediate and felsic magmas in the meguma zone of nova scotia canada two felsic plutons of late devonian 385 370 ma age in the meguma zone of southwestern nova scotia contain three circa 376 ma synplutonic mafic intermediate intrusions that collectively record progressive stages of in situ hybridisation a 5 m wide spessartite dyke in the port mouton pluton probably underwent rapid cooling and crystallisation immediately after intrusion which heated and coarsened the adjacent tonalite an 85 m long sheet of pillowed kersantite also in tonalite of the port mouton pluton presumably contained residual magma after quenching and obtained k p ba rb more radiogenic sr zr nb and light ree from the tonalite during magma mingling the third synplutonic body a 100 m wide diorite sheet intrudes granodiorite of the adjacent shelburne pluton and has a circa 45 m wide gradational contact of metaluminous hornblende tonalite this tonalite dominantly records magma mixing by the transfer of ti mg fe ca and v in hornblende biotite plagioclase and at least in part apatite xenocrysts derived from dioritic pillows that were originally disaggregated in the granodiorite probably in response to convection scattered data points unusual concave down variation trends for al2o3 p2o5 and sr and non hyperbolic sr nd isotopic characteristics in the tonalite apparently reflect syn or post mixing fractionation and accumulation of xenocrysts from residual magma phosphorus may have assisted diffusion of sr zr nb and light ree and caused premature quenching of the hybrids at mcleods cove and birchtown during magma mingling and mixing

water transfer during magma mixing events insights into crystal mush rejuvenation and melt extraction processes many plutons preserve evidence of magma mixing between hydrous mafic magmas and resident felsic crystal rich mushes to investigate water transfer processes in such systems following thermal equilibration we conducted 24 h experiments to establish the petrological evolution of a water undersaturated 4 wt h 2 o in the interstitial melt quartz bearing dacite crystal mush 0 5 0 8 in crystal fraction intruded by a water saturated 6 wt h 2 o initially crystal free andesite magma at 950 c and 4 kbar 12 km depth our results show isothermal undercooling resulting from a change in liquidus temperatures of the interacting magmas due to their changing water content specifically mafic samples dramatically crystallize during water escape into the felsic end members and consequent increase in liquidus temperature conversely the addition of water to the felsic mush reduces the liquidus temperature leading to an increase in melt fraction the experiments provide insights into how volatiles contribute to crystal mush rejuvenation i e increase of melt fraction however h 2 o diffusion alone is not sufficient to promote melt extraction from short and long lived mushes in the earth s crust

strontium isotopes and rubidium in the ganga brahmaputra river system weathering in the himalaya fluxes to the bay of bengal and contributions to the evolution of oceanic87sr 86sr the concentrations of rb and sr and87sr 86sr isotopic ratios have been measured in the dissolved load of the ganga brahmaputra g b river system the ganga was sampled extensively from its source at gangotri in the higher himalaya to patna on the alluvial plains the brahmaputra was sampled in its stretch in assam in india the average sr concentration in the ganga at patna is 1 2 mol l and that in the brahmaputra at goalpara is 0 73 mol l the mean87sr 86sr ratios are 0 7239 and 0 7192 respectively the87sr 86sr in the ganga source waters the alaknanda the bhagirathi and their tributaries range between 0 7300 and 0 7986 considerably higher than the global average runoff value of 0 7119 the high87sr 86sr in the ganga source waters result from the intense weathering of precambrian granites and gneisses enriched in radiogenic sr the sr isotope systematics of the ganga waters is dominated by silicate weathering whereas carbonate weathering plays a significant role in their major ion chemistry the g b system transports about 910 million moles of dissolved sr annually to the bay of bengal with an average87sr 86sr of 0 7213 the sr transported by the g b system is about 2 7 of the global dissolved sr flux to the oceans via rivers model calculations reveal that the g b system has contributed significantly to the sr isotope evolution of seawater during the past 20 ma the flux of rb through the g b system is 24 million moles per year this is about 5 of the global river supply of rb to the oceans nearly twice the contribution of water via the g b system to the oceans our study suggests that the marine geochemistries of sr and rb and possibly u may have been influenced considerably by the himalayan orogeny

a global geochemical mass budget applied to the congo basin rivers erosion rates and continental crust composition rivers carry the products of continental denudation either in a dissolved form chemical erosion or in a solid form physical erosion we focus in this paper on the relationship between physical erosion and chemical erosion we establish the mass budget of the congo basin rivers using chemical complementarities between river suspended sediments sandy bedload and dissolved load of the congo basin rivers reported in a previous paper dupre et al 1995 a series of equations are presented assuming that the physical and chemical erosion processes are in a steady state during one year the total mass of river borne material dissolved and particulate transported in the river over a given period of time should balance the mass of upper continental crust eroded during this time we show that the local continental crust on each drainage basin can be estimated and solve our steady state weathering model using an inversion procedure the very good agreement between modelled and measured values of the river suspended sediment concentrations validates the steady state hypothesis in this wet tropical area consequently in this area the sediment yield provide a good estimate of the rates of mechanical denudation this result also validates the calculation of the chemical and isotopic composition of the local continental upper crust using the bulk river load erosion rates for the silicate upper crust and thus independent of the lithological variability silicates evaporites and carbonates of the drainage basins are calculated mechanical erosion rates and chemical erosion rates for the congo basin at brazzaville are 8 t km2 y and 5 t km2 y the corresponding consumption of atmospheric co2 by weathering process is estimated to 51 103 mol km2 an these weathering and consumption rates are low in spite of the severity of the weathering conditions of the high soil temperature and of the intensity of precipitations these conclusions indicate the limiting influence the dynamic equilibrium of soils for silicate weathering finally by estimating the local continental crust chemical composition before the onset of erosion processes especially for the most soluble elements we can test the model of taylor and mclennan

enclaves in the cadillac mountain granite coastal maine samples of hybrid magma from the base of the chamber the cadillac mountain intrusive complex is dominated by the introduction cadillac mountain granite and a 2 3 km thick section of interlayered enclaves occur in nearly all granites and have recently gabbroic dioritic and granitic rocks which occurs near the base of been the focus of many studies see reviews by vernon the granite the layered rocks record hundreds of injections of basaltic 1983 and didier barbarin 1991 although enclaves magma that ponded on the chamber floor and variably interacted may have originated in many different ways and from with the overlying silicic magma magmatic enclaves ranging in different sources there has been a growing consensus composition from 55 to 78 wt sio2 are abundant in granite that there are textural criteria to recognize a class of above the layered mafic rocks the most mafic enclaves are highly enclaves that formed from different magmas that were enriched in incompatible elements and depleted in compatible elemingled into the granitic magma while it was still mobile ments their compositions can be best explained by periodic vernon 1984 1990 these can appropriately be termed replenishment mixing and fractional crystallization of basaltic magmatic enclaves their compositions are commonly magma at the base of the chamber the intermediate to silicic intermediate but can vary from basaltic to highly silicic enclaves formed by hybridization between the evolved basaltic magma magmatic enclaves occur in all types of granite and are and resident silicic magma there is little evidence for significant probably the dominant type of enclave in most calcexchange between enclaves and the enclosing granite instead alkaline i type granites in spite of partial reequilibration hybridization apparently occurred between stratified mafic and silicic enclaves of this type commonly have nd isotopic commagmas at the base of the chamber enclaves in a restricted area positions that are distinct from the enclosing granite and commonly show distinctive compositional characteristics suggesting support their formation from mantle derived magmas they were derived from a discrete batch of hybrid magma enclaves holden et al 1987 were probably dispersed into a localized portion of the granitic the compositions of magmatic enclaves can vary magma when replenishment or eruption disrupted the intermediate widely even within a single pluton and they can only layer rarely be interpreted as simple mixtures between an original basaltic magma and the host granitic magma there is ample petrographic and chemical evidence in many enclaves for hybridization mixing of liquids and

genetic significance of multiple enclave types in a peraluminous ignimbrite suite lachlan fold belt australia the violet town volcanics lachlan fold belt australia are an s type ignimbrite suite containing microgranitoid enclaves basaltic andesite enclaves and enclaves of high silica rhyolite the microgranitoid enclaves are similar to those in peraluminous granites they typically have lower initial sr sr and higher e i than the host and represent globules of a mafic mantle derived magma ivhich was hybridized by mixing and diffusional exchange with the host magma the basaltic andesite enclaves were incorporated into the ignimbrite as xenoliths but their parental magma may have been similar to that of the microgranitoid enclaves they are isotopically less depleted than other mantle derived rocks from the lachlan fold belt reflecting contamination by crustal material or derivation from less depleted mantle sources the high silica rhyolite enclaves previously interpreted to be related to the ignimbrite by crystal fractionation have ejw values up to 3 units higher than their host and cannot be related to their host by crystal fractionation or assimilation fractional crystallization afc processes the coexistence of s type magmas and mantle derived magmas suggests that the latter may have played a role in the palaeozoic magmatism of the lachlan fold belt acting as a heat source for melting and perhaps also contributing chemical components to the crustally derived magmas

effects of magma mingling in the granites of mount desert island maine textures and compositional relationships associated with dark colored fine grained enclaves in the cadillac mountain and somesville granites mount desert island maine preserve abundant evidence for contamination of host granitic magmas by enclave liquids fine grained enclaves which apparently represent chilled magmatic droplets have affected the composition and texture of the host granites by three possible mechanisms 1 crystallization of feldspar quartz hornblende pegmatite pods from fluids of enclave origin in the granite surrounding enclaves and the disaggregation of the pods and dispersion of crystals into the granite 2 ionic exchange between enclaves and granitic magmas 3 the generation around enclaves of rinds consisting of an inner alkali feldspar quartz zone and an outer zone of hornblende enriched granite coarse grained pegmatite pods that occur outside of enclaves may result from migration of a hydrous residual fluid evolved during enclave crystallization out of the enclave

geochemistry and petrogenesis of mesoproterozoic 1 1 ga magmatic enclaves in granites of the eastern llano uplift central texas usa abstract mesoproterozoic 1 1 ga plutons of the eastern llano uplift central texas usa contain two types of magmatic enclaves type ii enclaves contained within the marble falls mf and lone grove lg plutons exhibit enrichments in y nb and zr relative to their respective host granites enrichments in these incompatible trace elements at low sio2 renders unlikely the possibility that the mf and lg type ii enclaves are the result of partial melting anatexis of mafic crustal rocks numerical modeling of fractional crystallization and simple mixing fails to explain the observed trace element trends because no coeval mafic to intermediate rocks are exposed in the uplift characteristics of type ii enclave source magma s is uncertain however assuming source magmas similar to primitive continental arc basaltic andesite trace element trends i e incompatible element enrichment and compatible element depletion can be adequately replicated by a replenishment fractional crystallization rfc model chemistry of the mf and lg type ii enclaves suggest repeated replenishment of primitive magmas with only limited interaction with the host granitic magmas the more primitive enclave magmas evolving in near chemical isolation by rfc processes however evidence from type ii enclaves in two other plutons in the llano uplift kingsland and enchanted rock suggest that the isolation was non ideal i e some limited mixing may have occurred rapid quenching likely limited the potential for physical and chemical exchange between type ii enclaves and their host granite magmas

microgranitoid enclaves in granites globules of hybrid magma quenched in a plutonic environment microgranitoid enclaves autoliths cognate xenoliths mafic inclusions are common in high level granitoid plutons they have relatively fine grained igneous microgranitoid microstructures1 and many show evidence of having flowed in a magmatic condition2 these features counter interpretations that the enclaves represent transformed solid fragments of diverse wall rocks3 5 or restite6 11 an alternative interpretation is that the enclaves represent globules of mafic magma that have mingled commingled and quenched in the granitoid host magma12 16 a detailed review of the literature occurrence morphology and composition of micro granitoid enclaves strongly supports this view hut indicates that the enclave magmas range in composition from mafic to felsic and that magma mixing may be involved in their formation17 i summarize here the main features of microgranitoid enclaves and briefly outline the possible processes leading to their incorporation as magma globules in the host granitoid

small scale disequilibrium in a magmatic inclusion and its more silicic host basaltic andesite inclusions and their host dacite from the purico chascon complex in northern chile are isotopically distinct textural characteristics of the inclusions are typical of those resulting from magma mingling serial sectioning across the interface of an inclusion and its host dacite complemented by microdrill sampling and detailed microprobe work has enabled us to examine the scale of mixing and chemical isotopic and trace element disequilibrium the results of this work show that 1 the composition of the inclusion is relatively homogeneous 2 the dacite host is generally higher in 87sr 86sr and lower in 143nd 144nd than the enclave but it is heterogeneous on a small scale and probably a hybrid 3 the isotopic composition in the marginal zone apparently on both host and inclusion sides of the weakly chilled interface actually shows the highest 87sr 86sr and lowest 143nd 144nd 4 large plagioclase crystals in the inclusions and host are xenocrystic with higher 87sr 86sr than any of the other samples these observations are reconciled with a model of magma evolution in a crustal magma chamber in such a scenario the mafic magma is overlain by a cap of rhyolite a partial melt of the dacitic ignimbrites which now underlie the purico chascon complex overturn of such a magma system gives rise to a hybrid dacite containing discrete mafic inclusions

sr isotopic evidence for ion exchange buffering in tropical laterites from the parana brazil sr isotopes have been measured in laterite sequences and in associated waters from the region of ribeirao preto parana basin brasil in order to characterize the water rock interactions corresponding to the successive stages of weathering of these continental flood basalts we analyzed detailed sections of concentric rims of alteration in ball shaped spheroids developed around cores of fresh basalts waters were also analyzed on the different types of facies in this region unaltered basalts thick weathered profiles of lateritic material and surrounding sandstones the results show that a major part of the initial sr is removed from the basalt during the earliest stages of weathering within the narrow grey rims surrounding the unweathered core radiogenic sr is also leached from hydrothermal minerals located in basalt fractures or disseminated in the basalt groundmass at this early stage very high water rock ratios of at least 100 000 are calculated for this process from the sr 87 sr 86 in the related waters a general trend of increasing sr 87 sr 86 ratios is observed in the increasingly weathered facies inner yellow cortexes red kaolinitic soils and matrices and in their related waters the very high sr 87 sr 86 ratios of the red kaolinitic horizons probably result from isotopic exchange with radiogenic rainwaters or sandstone waters and also from the presence of residual or neoformed radiogenic minerals budget calculations as well as experimental leaching of kaolinite samples suggest that significant amounts of radiogenic sr can be leached from these facies during weathering thus the kaolinitic facies of tropical laterite profiles are able first to trap strontium from surficial waters then to provide radiogenic sr to the waters that weather the fresh basalts at the weathering front therefore the thick weathered horizons must be considered as a natural cation exchange layer that play an active role in the weathering process

ocean crust vein mineral deposition rb sr ages u th pb geochemistry and duration of circulation at dsdp sites 261 462 and 516 abstract cation exchange experiments ammonium acetate and cation resin on celadonite smectite vein minerals from three dsdp holes demonstrate selective removal of common sr relative to rb and radiogenic sr this technique increases the rb sr ratio by factors of 2 3 to 22 without significantly altering the age of the minerals allowing easier and more precise dating of such vein minerals the ages determined by this technique site 261 121 4 1 6m y site 462a 105 1 2 8m y site 516f 69 9 2 4m y are 34 54 and 18 m y younger respectively than the age of crust formation at the site in the case of site 462a the young age is clearly related to off ridge emplacement of a massive sill flow complex at the other sites either the hydrothermal circulation systems persisted longer than for normal crust 10 15 m y or were reactivated by off ridge igneous activity celadonites show u and pb contents and pb isotopic compositions little changed from their basalt precursors while th contents are significantly lower celadonites thus have unusually high alkali u th ratios and low th u ratios if this celadonite alteration signature is significantly imprinted on oceanic crust as a whole it will lead to very distinctive pb isotope signatures for any hot spot magmas which contain a component of aged subducted recycled oceanic crust initial sr isotope ratios of ocean crust vein minerals smectite celadonite zeolite calcite are intermediate between primary basalt values and contemporary sea water values and indicate formation under seawaterdominated systems with effective water rock ratios of 20 200

enclaves from the a type granite of the megantic complex white mountain magma series clues to granite magmagenesis the subsolvus biotite hornblende granite at megantic contains mafic to leucocratic enclaves the groundmass of leucocratic enclaves is microgranitic that of mafic enclaves is meladioritic apatite is commonly acicular in the enclaves suggesting quenching of mafic magmas commingled with the granitic host mafic enclaves contain mafic clots hornblende cores biotite rims some with relic cores of vial cr rich diopsidic augite similar to phenocrysts in associated basaltic dykes the clots are interpreted to be pseudomorphs after mafic phenocrysts the enclaves also contain granitic microxenoliths xenocrysts of quartz with hornblende reaction rims partially resorbed oligoclase and perthite megacrysts and biotite xenocrysts the xenocrysts record hybridization between a mafic enclave magma and a granitic magma and or rock enclaves define mixing lines on many variation diagrams however rb sr ba zr nb y ree zn pb and f do not fit this pattern and appear to have been redistributed by residual granitic melts or exsolved volatiles the data suggest that the granite is anatectic heat being provided by crystallization of associated basalts mixing calculations suggest that as little as 5 hybridization with basalt or hawaiite can account for the sr isotopic heterogeneity of the granites

volcanic expression of bimodal magmatism the cranberry island cadillac mountain complex coastal maine the cranberry island series is a thick succession of silurian volcanic breccias ash flow tuffs lava flows and hypabyssal intrusions comprising the southern margin of mount desert island and the cranberry islands of southeastern maine the rocks of the series preserve remarkable textures indicative of the simultaneous eruption of mafic and felsic magmas during both explosive and effusive events the cadillac mountain intrusive complex is an approximately 20 km wide composite layered mafic to felsic plutonic body that dominates mount desert island immediately north of the outcrop of the cranberry island series the rocks of the cranberry island series and those of the cadillac mountain intrusive complex are correlative in age and similar in composition the age of the cranberry island series based on new u pb geochronology of zircon is 424 ma pm 1 ma the cadillac mountain granite the somesville granite and the southwest harbor granite all parts of the cadillac mountain intrusive complex are

chemical modification of enclave magma by post emplacement crystal fractionation diffusion and metasomatism hornblende bearing microgranitoid enclaves from the swifts creek pluton scp se australia display mineralogical and textural variations from their margins to their centers margins are fine grained and display quench textures and are enriched in amphibole biotite and in some cases magnetite relative to their coarser grained centers enclaves of this type and their adjacent granitoid host rocks have been sectioned into 0 3 to 1 cm thick slabs in order to determine the chemical variations associated with these mineralogical changes the fine grained margins are variably enriched in ti al mg fe mg k rb ba nb y sc v ni and the ree up to a factor of three relative to the enclave centers this enrichment is compensated by a depletion of si and zr elements like ca na and sr show less coherent variation from margin to center host rocks in immediate contact with these enclaves are depleted in mg na k rb and ba relative to host rocks that are not exposed to enclaves no one single process can account for all the variations instead we propose that the chemical variations are related to a combination of in situ crystal fractionation of isolated magma globules mass transfer by diffusion and metasomatic exchange

mafic inclusions from the glenbog and blue gum granite suites southeastern australia mafic inclusions from the i type granites of the glenbog and blue gum suites of the lachlan fold belt of southeastern australia are less than 1 m and mostly less than 10 cm in size and are generally in sharp contact with the surrounding host granites unless otherwise stated the term granite is used in this paper in the general sense to include granite senso stricto adamellite granodiorite tonalite etc although some inclusions are irregular in shape the majority have an elliptical or circular outline when viewed on exposed surfaces rounded and subrounded individuals outnumber those with an angular appearance abundances of inclusions in seven plutons of the glenbog and blue gum suites range from about 6 to less than 0 1 their abundance is negatively correlated with the sio2 content of the host rock mafic inclusions contain the same mineral species as the host granite but plagioclase and hornblende are more abundant and quartz and k feldspar are less abundant in inclusions mafic inclusions all contain more than 54 sio2 and are mostly intermediate in composition the abundances of many elements in the mafic inclusions follow those of that element in the host granites sr and nd isotopic compositions in both the granites and the inclusions are similar and relatively evolved various models for the origin of the inclusions are considered it is concluded that of those considered the only model capable of explaining all data is that the inclusions represent fragments of more refractory source material that were carried up with the magma it is suggested that this more refractory character resulted from a lesser degree of hydration of the source rocks of the magmas prior to partial melting

magmatism and continental break up in the south atlantic high precision40ar 39ar geochronology abstract a detailed ar ar study of the parana etendeka continental flood basalts cfb has been undertaken using laser spot analysis data provide information not only on the age of the samples but also on the variability of the non radiogenic ar component and state of alteration the results indicate the parana etendeka cfb were erupted over 10 million years between 137 and 127 ma providing a minimum overall eruption rate of 0 1 km3 yr 1 this is an order of magnitude less than that previously proposed for this the deccan and siberian cfb provinces but it is similar to estimates for hawaii iceland and the columbia river cfb in detail the new ar ar analyses indicate that magmatism within the parana etendeka province migrated from nw to se prior to and during the opening of the south atlantic providing an explanation for the asymmetry of the cfb lavas about the south atlantic moreover chemically defined magma types were erupted at different times in different places and so within the paranathey may not be used as reliable chronostratigraphic units rather such magmatic units may reflect the extent of compositionally distinct source regions in the uppermost mantle and indicate that partial melting took place over a wide area under the paranabasin the nw se migration of the onset of magmatism might be interpreted as a plume trace but the inferred rate of movement is 3 times faster than that inferred from subsequent magmatism on the rio grande rise it is argued that the onset of magmatism reflected extensional tectonics normal to the ponta grossa dyke swarm given that magmatism preceded the main phase of rifting and that extension across the ponta grossa was moderate current plume models have difficulty in predicting sufficient melting within the asthenosphere

silicic magma chambers as traps for basaltic magmas the cadillac mountain intrusive complex mount desert island maine the cadillac mountain intrusive complex located on mount desert island maine provides a superb record of episodic invasion of a floored silicic magma chamber by many pulses of basaltic magma the complex consists of three units the cadillac mountain granite cmg the somesville granite sg and gabbro diorite g d basaltic magmas were emplaced before during and after the emplacement and crystallization of the granitic plutons and the chambers of silicic magma acted as traps for basaltic magma basaltic magma emplaced into the surrounding country rocks crystallized as homogeneous diabase and gabbro while basaltic magma emplaced within the perimeter of the granite displays abundant evidence for commingling with silicic magma the g d unit about 1 5 km thick consists of interlayered gabbroic dioritic and granitic rocks although thin 1 m thick gabbroic layers in the g d unit are typically chilled on all margins thicker layers of gabbro commonly form the lower parts of macrorhythmic units

magmatic inclusions in rhyolites of the spor mountain formation western utah limitations on compositional inferences from inclusions in granitic rocks inclusions of quenched mafic magma occur in a 21 ma rhyolite lava and precursory non welded tuff that form the spor mountain formation in west central utah the mafic inclusions are not lithic inclusions no comparable volcanic unit was present at the surface when the spor mountain formation erupted mineral and bulk compositions preclude liquid immiscibility the mafic inclusions show clear morphologic and textural effects of magma mingling shortly before eruption of the rhyolite globular inclusions from both units are vesicular phenocryst poor plagioclase sanidine clinopyroxene orthopyroxene magnetite ilmenite latites and trachytes with quench temperatures of about 1000 c although overlapping in sio2 tio2 zr and hf concentrations inclusions from the underlying tuff lack negative eu anomalies and are enriched in p2o5 k2o al2o3 sr pb cr and ni whereas those hosted in the overlying lava have small negative eu anomalies and two fold enrichments in fe2o3 mno hree y ta th rb and cs the most reasonable explanation for the differences between the two sets of inclusions lies in selective chemical exchange between the rhyolite lava and the mafic inclusions after eruption limited mechanical mixing occurred after the inclusions solidified and became chemically modified the textures of the inclusions in the lavas and the elements selectively mobilized in the inclusions imply that vapor phase transport occurred in this low pressure volcanic environment if such substantial variations in inclusion compositions can arise during what must have been a short period of time before chemical reactions were halted by rapid cooling it seems unlikely that the compositions of mafic inclusions formed by magma mingling in slowly cooled granites preserve their original compositions mineralogies or information about their ultimate sources using the compositions of such chemically modified inclusions as end members for mixing calculations may lead to erroneous results regarding the significance of magma mixing in plutonic rocks

mafic enclaves in the wilson ridge pluton northwestern arizona implications for the generation of a calc alkaline intermediate pluton in an extensional environment the wilson ridge pluton is an epizonal calc alkaline pluton that formed about 13 5 ma during a period of mid miocene extension faulting and erosional dissection provide a cross section of the pluton the apex of the pluton in the boulder wash area nevada is composed of hypabyssal quartz monzonite and dacite the base of the pluton is 20 km to the south where quartz monzodiorite monzodiorite and diorite are in low angle intrusive contact with precambrian basement the pluton was separated from cogenetic volcanic rocks in the river mountains by movement along the saddle island detachment fault at about 13 4 ma the river mountains now lie 20 km to the west of the pluton the wilson ridge pluton is composed of the teakettle pass suite consisting of foliated monzodiorite and quartz monzodiorite and unfoliated quartz monzonite and the older horsethief canyon diorite rocks of the pluton contain 2 4 modal percent sphene intermediate rocks of the teakettle pass suite contain abundant basaltic and diorite enclaves basaltic enclaves are lensoidal and pillow like and commonly have crenulate and fine grained margins enclaves are chemically similar to mafic dikes of the wilson ridge pluton and to cogenetic alkali basalt flows in the river mountains they probably represent blobs of mafic liquid that commingled and mechanically mixed with felsic magma to produce the intermediate rocks of the pluton basaltic enclaves commonly occur as inclusion rich zones that represent synplutonic mafic dikes that were injected into a quartz monzonite host mafic magma was entrained and mechanically broken down by magmatic flow shear a continuum in shape exists from enclaves that are bulbous and ellipsoidal to those that are thin tabular mafic selvages and schlieren and ultimately to the mafic component in foliated quartz monzodiorite and monzodiorite diorite enclaves have angular contacts with host rocks and are interpreted as xenoliths field evidence and major and trace element models suggest that the intermediate rocks of the pluton were produced by the commingling of a large volume of mafic magma with a smaller volume of felsic magma a mafic felsic ratio of about 70 30 as well as fractional crystallization similar open system processes may be responsible for the production of calc alkaline intermediate rocks in other parts of the great basin

regional variations within the parana flood basalts southern brazil evidence for subcontinental mantle heterogeneity and crustal contamination abstract continental flood volcanism of the parana basin lower cretaceous is represented by two pyroxene tholeiitic basalts 90 vol the northern parana province npp is dominated by basalts high in tio2 and incompatible elements htib while the southern parana province spp is dominated by basalts low in tio2 and incompatible elements ltib npp basalts show relatively small variations of initial 120 ma 87 sr 86 sr r0 and 143 nd 144 nd nd ratios r0 0 7051 0 7062 and nd 0 5124 0 5125 respectively relative to those occurring in spp r0 0 7046 0 7120 and nd 0 5122 0 5128 respectively the latter basalts show significant positive correlations between r0 vs sio2 k2o rb and ba and negative correlations between r0 vs cr ni and mg value believed to be due to crustal granitic contamination in general the effects of contamination are pronounced in spp and tend to vanish towards npp 18o values range from 6 5 to 10 0 in the basalts from npp and spp and essentially reflect water magma interactions uncontaminated ltib and htib basalts from npp are isotopically distinct from the uncontaminated analogues from spp ltib r0 0 7055 vs 0 7046 nd 0 5124 vs 0 5128 respectively all chemical data consistently indicate distinct sources and a large scale mantle heterogeneity the npp mantle source is expected to have relatively high content of enriched components possibly related to small volume melts and metasomatic fluids tentatively the last stabilization age of the parana mantle heterogeneity is 0 5 1 0 ga old the existence of high and low tio2 basalt suites in both parana and karoo provinces in brazil and southern africa respectively indicates a large scale heterogeneity in the subcontinental mantle and suggests that basalt generation occurred in the lithospheric mantle

origin of mafic enclaves from the taihang mesozoic orogen north china craton abstract the origin of mafic enclaves in intermediate and felsic granitoids has long been debated in this paper petrological and geochemical analyses whole rock geochemistry nd sr and zircon in situ hf isotopic compositions are used to address this controversial issue mafic enclaves in the taihang felsic plutons north china craton are fine grained and show typical magmatic textures they occur as rounded to ellipsoidal globules and are sometimes highly elongated without solid state deformation due to stretching within the partially crystallized convective felsic magma such occurrence suggests that the mafic enclaves and host rocks represent two co existing compositionally distinct magmas the mafic enclaves contain abundant hydrous phases hornblende and biotite and show the reaction relationship of clinopyroxene being rimmed by hornblende they also contain plagioclase xenocrysts showing compositional and textural disequilibrium in terms of nd hf isotopes mafic enclaves have end t values of 10 to 15 and host rocks have lower values as a whole 13 to 18 zircons from a single enclave show a large variation in hf isotopic compositions ehf t 5 to 20 a model is proposed whereby an evolved basaltic magma first mixed with a granitic magma of crustal origin at depth and then the mixed magma broke up into discrete globules upon entering the felsic magma chamber forming enclaves by convective motion or forceful injection in the host felsic magma this was followed by a double mechanical transfer of plagioclase crystals at the contact of the two magmas and by the diffusion of mobile elements e g si alkalis p y yb pb from felsic magma to enclave forming magma