heterogeneity of initial 87sr 86sr ratios within a single pluton evidence from apatite strontium isotopic study abstract magmatic apatite in samples of the unmetamorphosed undeformed cretaceous pluton of the habu granodiorite southwestern japan was examined to determine its viability as an indicator of initial 87sr 86sr heterogeneity in magmatic bodies the habu granodiorite is a zoned pluton in the ryoke metamorphic belt of southwestern japan a published rb sr whole rock isochron age of 124 2 10 8 ma is significantly older than th u total pb monazite ages of ca 86 ma a rb sr mineral isochron of 90 0 2 5 ma for a sample of granodiorite from the margin of the pluton has an initial 87sr 86sr sri of 0 7056 0 0001 similar to a 87sr 86sr of 0 70539 for coexisting euhedral apatite similarly a rb sr mineral isochron of 91 4 2 5 ma for a sample of adamellite from the center of the pluton has an sri of 0 7068 0 0001 corresponding to a 87sr 86sr ratio of 0 70673 0 70684 for three fractions of coexisting euhedral apatite the uniformity of 87sr 86sr values of apatite in individual samples and accordance of 87sr 86sr values of apatite and sri from the mineral isochron suggest that magmatic apatite can be considered as a reliable indicator of sri in granitic rocks the sr isotopic compositions of euhedral apatite coupled with those determined through the mineral isochrons and whole rock data indicate that sri varies between samples from 0 7054 to 0 7074 this provides direct evidence of sri heterogeneity within a compositionally zoned pluton such heterogeneity can produce spurious whole rock age estimates in granitic plutons even where the true age of a pluton is uniform within measurable error assimilation cannot account for chemical differences between granodiorite and adamellite as well as sr isotopic heterogeneity it is likely that the heterogeneity of sri is caused by magma source materials

u pb evidence for abitibi gold mineralization postdating greenstone magmatism and metamorphism two widely held theories for the origin of archaean lode gold deposits the magmatic1 and metamorphic replacement2 models predict that gold emplacement should have occurred near the end of late syn kinematic greenstone magmatism here we report that at the camflo mine in the abitibi subprovince of canada u pb ages for rutile and titanite in auriferous alteration 2 625 7 myr are much younger than the age of the host quartz syenite 2 680 4 myr and ages for regional magmatism 2 670 myr 3 and metamorphism 2 680 myr 4 mineral associations and pb pb systematics indicate that the hydrothermal minerals record crystallization ages that are probably coeval with the gold mineralization these ages and others from the abitibi subprovince4 6 rule out a direct connection between gold and magmatism or metamorphism associated with regional folding of the greenstone belt they are broadly contemporaneous with metamorphism at deep crustal levels7 and may record late devolatilization of the lower crust

the trace element compositions of s type granites evidence for disequilibrium melting and accessory phase entrainment in the source within individual plutons the trace element concentrations in s type granites generally increase with maficity total iron and magnesium content and expressed as atomic fe mg in this study the degree of variability in trace element concentration also expands markedly with the same parameter the strongly peraluminous high level s type granites of the peninsular pluton cape granite suite south africa are the product of biotite incongruent melting of a metasedimentary source near the base of the crust leucogranites within the suite represent close to pure melts from the anatectic source and more mafic varieties represent mixtures of melt and peritectic garnet and ilmenite trace elements such as rb ba sr and eu that are concentrated in reactant minerals in the melting process show considerable scatter within the granites this is interpreted to reflect compositional variation in the source in contrast elements such as lree zr and hf which are concentrated within refractory accessory phases zircon and monazite show well defined negative correlations with increasing sio2 and increase linearly with increasing maficity this is interpreted to reflect coupled co entrainment of accessory minerals and peritectic phases to the melt leucocratic rocks cannot have evolved from the more mafic compositions in the suite by a process of fractional crystallisation because in this case they would have inherited the zircon saturated character of this hypothetical earlier magma trace element behaviour of granites from the peninsular pluton has been modelled via both equilibrium and disequilibrium trace element melting in the disequilibrium case melts are modelled as leaving the source with variable proportions of entrained peritectic phases and accessory minerals but before the melt has dis solved any accessory minerals thus the trace element signature of the melt is largely inherited from the reactants in the melting reaction with no contribution from zircon and monazite dissolution in the equilibrium case melt leaves the source with entrained crystals after reaching zircon and monazite saturation a significant proportion of the rocks of the peninsular pluton have trace element concentrations below those predicted by zircon and monazite saturation in the case of the most leucocratic rocks all compositions are zircon undersaturated whilst the majority of the most mafic compositions are zircon oversaturated however in both cases zircon is commonly xenocrystic thus the leucocratic rocks represent close to pure melts which escaped their sources rapidly enough that some very closely match the trace element disequilibrium melting model applied in this study zircon dissolution rates allow the residency time for the melt in the source to be conservatively estimated at less than 500 years

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

geochemistry of phosphorus and the behavior of apatite during crustal anatexis insights from melt inclusions and nanogranitoids

the thermal responses of rock art pigments implications for rock art weathering in southern africa the san rock art of southern africa is an international heritage subject to degradation and loss resulting from weathering the paintings occur within rock shelters where many are exposed to direct solar radiation for varying periods rather than occurring in dark caves as part of a study on the factors thought to be impacting weathering data were collected pertaining to rock and pigment temperatures as well as humidity within the rock shelters in addition xrd analyses were undertaken on pigment samples and the pigment to rock and pigment to pigment contacts were investigated by means of sem pigments were found to be composed of ferric oxide the ochre and a gypsum clay mix the white and to occur as a layer on top of rather than penetrating into the sandstone noncontact infrared sensors were used to monitor the temperatures of the actual pigments while micro thermocouples to monitor the surrounding nonpainted rock surfaces thermal data show that there are significant differences between the white and the ochre pigments both in terms of actual temperatures and short term thermal responses noticeably the white paint exhibits relatively large thermal fluctuations as compared to the ochre or the rock over the 20 s to 1 min timescale these thermal variations may induce pigment to pigment stresses within the painting the pigmented areas also exhibit different temperatures to the surrounding paint free rock suggesting that there may be both within painting and between painting and rock including the rock beneath the painting stresses that can lead to degradation humidity data were found to be inadequate for any meaningful evaluation of the moisture conditions

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

archean gold bearing quartz veins at the sigma mine abitibi greenstone belt quebec part i geologic relations and formation of the vein system the sigma deposit consists of an archean gold bearing vein system in metavolcanic rocks intruded by an irregular body of porphyritic diorite both affected by regional deformation and in turn cut by younger undeformed feldspar porphyry dikes the veins cut all rock types and are composed chiefly of quartz and tourmaline with minor amounts of carbonates pyrite chlorite scheelite and free gold they were emplaced during or after a greenschist facies metamorphism postdating the intrusion of feldspar porphyry dikes the veins were not significantly affected by any subsequent metamorphism or tectonism two principal types of veins occur in the mine subvertical veins and subhorizontal veins they are contemporaneous and cogenetic subvertical veins occur within steeply dipping east west trending ductile shear zones along which reverse subvertical displacements have taken place the veins occupy openings created by overriding of undulations and irregularities of foliation and slip planes during progressive shearing movements within the shears subhorizontal veins occupy extensional fractures which developed between ductile shear zones preferentially in the more competent host rocks these veins were formed by one or several episodes of open space filling geologic relations indicate that the mineralized veins were emplaced progressively in a dynamic tectonic environment during and shortly after the formation of ductile shear zones a structural analysis has shown that the ductile shear zones and the two vein types can be related to a single strain ellipsoid of the following orientation the z axis trending north south and horizontal the y axis plunging 10 degrees west and the x axis plunging 80 degrees east the deformation responsible for the generation of ductile shear zones and emplacement of the veins is ascribed to a late north south trending compression postdating regional deformation

experimental deformation of partially molten westerly granite under fluid absent conditions with implications for the extraction of granitic magmas the mechanical behavior of partially molten westerly granite was investigated in the temperature range 800 1100 c 250 mpa confining pressure by means of constant strain rate creep and stress relaxation tests the only water in the samples came from the breakdown of hydrous phases biotite minor chlorite and muscovite and alteration products of feldspars thus the amount of melt was controlled by the test temperature and ranged from 3 at 800 c to 50 at 1100 c over that temperature range strength decreased from 500 mpa to less than 1 mpa and a preliminary constitutive flow law for the partially molten rock was obtained to allow extrapolation to low strain rates the comparative viscosity of the melt alone was estimated at 950 and 1000 c from the distance it could be made to penetrate into a porous sand under a known pressure gradient under all conditions deformation of the matrix of solid grains was by brittle fracture only samples containing up to 10 vol melt failed with the formation of a shear fault zone at higher melt fractions melt filled pores collapsed by shear enhanced compaction squeezing the melt into axial cracks above 40 vol melt unfractured solid grains were carried about passively in the flowing liquid there was no evidence of a rheologically critical melt percentage in this system by analogy with the uniaxial compaction of water saturated soils a simple model is erected to describe a two stage process for the extraction of granitic melts from their protoliths with the aid of nonhydrostatic stress shear enhanced compaction is inferred to drive melt into a network of melt filled veins whereupon porous flow through the high permeability vein network allows rapid drainage of melt to higher crustal levels

from sediment to granite timescales of anatexis in the upper crust abstract granite formation is the culmination of a sequence of events initiated by prograde heating of the protolith and followed by formation of a grain boundary melt melt segregation into a vein network ascent of the melt through the network and finally crystallisation of the melt experimental constraints on the formation of a crustal melt from the incongruent melting of muscovite combined with geochemical studies of anatexis in the high himalaya allow the timescales required for each of these processes to be assessed discordant temperatures determined from monazite and zircon thermometry for himalayan anatectic granites indicate that at least for some intrusives the melt was undersaturated in lree implying that melts have probably been extracted in less than 10 ka experimental studies suggest that some himalayan melts are also undersaturated in zr implying segregation may have occurred within 100 years such short timescales confirm that deformation driven mechanisms are important in extracting these melts from their source the transport distances of himalayan granitic melts of 10 km may be achieved by the ascent of magma through dykes in about 1 day at such rates even the largest granite could theoretically be emplaced in 10 years crystallisation of himalayan melts involves much longer periods if emplaced as thin sheets 100 m wide a timescale of 500 years is required compared with 30 ka for single stage intrusion of the larger laccoliths for composite sheet complexes magma crystallisation rather than melt ascent comprises the rate determining step on the emplacement of the intrusion the overall timescales of melt segregation and emplacement for many orogenic granites are therefore less than 10 ka and possibly less than 1 ka in contrast the timescale required for prograde heating of the protolith is 1 ma since the melt production rate is determined by heat flow into the protolith and not by reaction kinetics for any geologically significant period we conclude that heat flow determined by both the mechanism of heating and the thermal diffusivities of crustal rocks provides the overall rate determining step of crustal anatexis

badrinath gangotri plutons garhwal india petrological and geochemical evidence for fractionation processes in a high himalayan leucogranite the gangotri leucogranite is the western end of the badrinath granite one of the largest bodies of the high himalayan leucogranite belt hhl it is a typical fine grained tourmaline muscovite biotite leucogranite the petrography shows a lack of restitic phases the inferred crystallization sequence is characterized by the early appearance of plagioclase quartz and biotite and by the late crystallization of the k feldspar this suggests that in spite of being of near minimum melt composition the granite probably had long crystallization or melting interval in agreement with previous experimental studies tourmaline and muscovite have a mainly magmatic origin even though the major element composition is homogeneous there are several geochemical trends when cao decreases there is an increase in na2o rb sn u b f and a decrease in k2o fe2o3 tio2 sr ba zr ree th which are best explained by a fractionation process with early crystallizing phases experimental solubility models for zircon and monazite in felsic melt support a magmatic origin for these two accessory phases as well rb sr isotope data show this granite to have like other hhl heterogeneous isotopic values for sr initial 87sr 86sr ratios calculated at 20 ma range between 0 765 and 0 785 therefore no mixing i e no convection occurred between the different batches of magma in contrast 18o data show little variation 13 04 0 25 implying a source with homogeneous 18o values differences in timing between fluid infiltration and the onset of melting related to differences in temperature of the source could explain why source homogenization occurred for the gangotri and not for the manaslu granite the use of experimental results for solubility and the position of the accessory minerals during melting predict a low viscosity for the melt during its extraction this in turn explains the lack of restitic phases major and accessory in the granite as well as some field features lensoid shape pronounced magmatic layering based on the petrographic and isotopic studies it is suggested that the mechanism of ascent was not diapiric but rather that the melt ascended along several fractures and the level of emplacement was partialy controlled by the density contrast between the melt and host rocks

melt segregation and magma interactions during crustal melting breaking out of the matrix abstract differentiation of the continental crust begins with its partial melting the products of crustal melting are silicic hydrous h2o undersaturated granitic liquids that are generated within matrices of residual crystals crustal differentiation requires that felsic magmas form and escape from these solid residua an important question is whether granitic magmas collect into large batches within or near their sources which then give rise to ascent or alternatively bleed out of the sources in smaller streams or pulses we demonstrate that the physical reality is closer to the second alternative and thus question the validity of the concepts of magma segregation and source fertility as they are sometimes visualised granitic plutons contain chemically distinct sub populations formed by source level entrainment of the peritectic assemblages into the melts rapid evacuation at source levels and high ascent rates protect the magmas from wall rock interactions during their tenure at source depths and during their ascent through cool upper crust the existence of different types of granites and hence of clearly defined chemically different magmas within plutons dictates that felsic magmas must separate efficiently from their anatectic sources and must ascend to the sites of their emplacement with minimal chemical interaction with crust through which the magma must move exposed deep crustal sections are typically lithologically diverse with the more fertile rocks commonly forming discrete layers surrounded by rocks with contrasting compositions if the melts were required to segregate and accumulate into large volumes within their hot sources especially by slow gravity driven melt percolation it is very likely that they would be substantially modified by reaction with diverse source rocks thus the processes mooted to occur in melting assimilation storage and homogenisation mash or deep crustal hot dch zones would act to erase original compositional heterogeneities and produce larger batches of more homogeneous magma perhaps carrying evidence of extensive magma mixing from various lines of evidence we conclude that mash and dch zones may not exist and even if they do they cannot form the sources of most granitic magmas the intrinsic heterogeneity in crustal source rocks the likely occurrence of peritectic assemblage entrainment and the inefficiency of magma mixing decree that granitic rocks will retain source inherited chemical and isotopic heterogeneities rapid semi continuous and disequilibrium withdrawal of small magma batches from source rocks means that elements of whole rock trace element chemistry and accessory mineral chemistry will be decoupled from major element variations this also means that a large source with low nominal fertility due to low hydrous mineral content for example could still produce a substantial granitic pluton by additions of individually small increments of magma

hafnium isotopic heterogeneity in zircons from granitic rocks geochemical evaluation and modeling of zircon effect in crustal anatexis abstract we carried out a geochemical evaluation and modeled the mechanism responsible for varied hf isotopic ratios in magmatic zircons of a single granitic rock specimen five representative granitic samples were selected from southern china based on preliminary hf isotopic data our new dataset of zircon hf isotopes confirmed significant hf isotope variations 5 9 epsilon units for each sample and these zircons show roughly positive th u versus t crystallizing temperature correlations while the lu hf variation is independent from t in addition some zircons show significantly higher ti concentrations in the rims compared to the interiors implying reverse thermal zonation based on the ti in zircon thermometry these geochemical features in zircons suggest open system processes which may have resulted from frequent replenishment we modeled zircon dissolution during crustal anatexis to reveal the hf isotopic evolution in the extracted melts the model suggests that the extracted melts may have extremely variable hf isotope compositions 20 epsilon units if the bulk zr concentration in the source is initially above 100 ppm and the melting is rapid 10 4 yr 1 the decoupled release of zircon hf and non zircon hf from a single crust derived magma source can lead to significant hf isotope variations in a solidified granitic body this work provides an alternative explanation for the hf isotopic heterogeneity in magmatic zircons of granitic rocks which is in contrast to the common interpretation by mixing with mantle derived magmas

weathering of sandstone by the combined action of frost and salt preliminary results from a continuing series of laboratory experiments designed to examine the combined effects of salt and frost weathering indicate that some salts greatly enhance the breakdown of rocks by frost samples of ardingly sandstone from southeast england were soaked in saturated solutions of sodium chloride or sodium sulphate and subjected to alternating cycles of freezing and thawing rapid disintegration of the sandstone occurred within twenty cycles in each of the salt solutions in contrast samples subjected to freezing and thawing in deionized water suffered very little damage unless they were saturated under vacuum when samples were subjected to twenty cycles of wetting and drying at temperatures above 0 c those soaked in deionised water or sodium chloride suffered no appreciable damage but those soaked in sodium sulphate rapidly disintegrated tentative explanations of these results are offered the enhancement of frost weathering by salt appears to have been overlooked by many writers yet it is likely to be an important geomorphological process in those areas of mid and high latitudes where rocks are saturated with sodium salts

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

thermal stress weathering and the spalling of antarctic rocks using in situ field measurements laboratory analyses and numerical modeling we test the potential efficacy of thermal stress weathering in the flaking of millimeter thick alteration rinds observed on cobbles and boulders of ferrar dolerite on mullins glacier mcmurdo dry valleys mdv in particular we examine whether low magnitude stresses arising from temperature variations over time result in thermal fatigue weathering yielding slow crack propagation along existing cracks and ultimate flake detachment our field results show that during summer months clasts of ferrar dolerite experience large temperature gradients across partially detached alteration rinds 4 7 c mm 1 and abrupt fluctuations in surface temperature up to 12 c min 1 the latter are likely due to the combined effects of changing solar irradiation and cooling from episodic winds the results of our thermal stress model coupled with subcritical crack growth theory suggest that thermal stresses induced at the base of thin alteration rinds 2 mm thick common on rocks exposed for 105 years may be sufficient to cause existing cracks to propagate under present day meteorological forcing eventually leading to rind detachment the increase in porosity observed within alteration rinds relative to unaltered rock interiors as well as predicted decreases in rind strength based on allied weathering studies likely facilitates thermal stress crack propagation through a reduction of fracture toughness we conclude that thermal stress weathering may be an active though undervalued weathering process in hyperarid terrestrial polar deserts such as the stable upland region of the mdv

laboratory simulation of rock weathering by salt crystallization and hydration processes in hot arid environments the results of a series of experiments are reported in this paper which were designed to differentiate between the forces of crystal growth and hydration in salt weathering using a single salt hydrate system sodium sulphate five contrasting rock types and several diurnal temperature relative humidity cycles which permitted or inhibited these processes and simulated ground surface climates in hot arid environments it was shown that hydration of sodium sulphate is an effective mechanism of rock disintegration but that it is significantly less destructive than crystal growth pressure crystallization of thenardite na2so4 is in turn more effective in rock weathering than the crystal growth of mirabilite na2so4 10h2o in general rates of disintegration were most rapid where the diurnal temperature range was extreme and relative humidity lowest

element concentrations in granitic magmas ghosts of textures past the geochemistry of granitic rocks appears to contain information about some aspects of the pre anatectic textures of the protoliths there are marked contrasts in the covariations between ba a proxy for melting proportion in the protolith and the p 2 o 5 tio 2 y zr and ce concentrations in i and s type granitic magmas rocks these contrasts are interpreted as features resulting from the interplay between differences in the locations of accessory mineral grains in the protoliths their solubilities in the granitic melts and the degree of foliation developed in the protoliths it is inferred that s type protoliths are more highly foliated with accessory minerals concentrated into the biotite rich layers that form the foci for partial melting i type protoliths appear to have less pronounced foliation and a more random distribution of both hydrous mafic silicates and accessory grains supplementary material a list of data sources and tables of analyses of granitic rocks are available at www geolsoc org uk sup18682

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

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

frost and salt weathering further experimental results a laboratory experiment has been conducted to examine the effects of frost and salt weathering i e physical breakdown by the freezing of salt solutions on a limestone results show that the presence of certain salts in solution can inhibit frost damage these findings are in direct conflict with those presented by goudie 1974 and more recently williams and robinson 1981 comparison of the experimental methods used in each of these three studies suggests that opposing results can be explained in terms of the different experimental procedures which were employed if salt supply is frequent and plentiful then it seems likely that rock breakdown will be enhanced this is the case represented by the experiment of williams and robinson conversely if the salt supply is limited and the amounts of salt remain more or less constant then rock breakdown will be inhibited the case of the present experimental study caution is therefore advocated when attempting to extrapolate laboratory derived results to infer on the behaviour of rocks under natural conditions several environmental situations in which frost and salt weathering may be a possibility are dsiscussed but it is concluded that further field data especially concerning temperature regimes and salt availability at and below rock surfaces in cold regions would be necessary before more definite statements could be made about the efficacy of this process

fluids and thrusting abstract recent studies of the structure of the continental crust by cocorp and birps have shown that major thrust structures in the lower crust are ubiquitous plate tectonic processes of continental crust destruction or construction at sites where subduction or collision occurs the himalayas or the andes involve thrusting of continental materials or ocean crust beneath continents and local overthrusting may accompany transcurrent motion in all such cases massive fluid flow must occur as first stressed by m k hubbert and w w rubey thrusting mechanisms require that fluid pressures are close to or greater than lithostatic pressures when thrusting occurs compaction of the underplate must occur rapidly expelling pore water crustal thickening leads to gradual heating of the underplate with prograde metamorphism near the top and even melting at the base if the overthrust plate is thick and hot at the base retrograde metamorphism will occur at the base of the overthrust plate and rising fluids will encounter inverted thermal gradients in such a region veins need not occur but leaching phenomena may dominate if an underthrust plate is hydrated and or contains sedimentary aquifers large fluid volumes are expelled through shear zones into the overthrust plate the scale of fluid release processes can be large thus in thin skinned tectonics where the overthrust plate is 10 15 km thick induced fluid release can easily reach 4 109 g m 2 of thrust surface the flow of such fluids will be controlled by the thrust surface and lithology both of which will influence hydraulic fracture mechanisms and spacing of fractures the chemistry of thrust derived fluids may be highly variable depending on lithology and the time constants of thrusting vertical thermal and redox environments will be similarly dependent in the subduction process impressive quantities of fluids must pass back up the thrust surface and such fluids have recently been directly observed during drilling dewatering of spilites serpentinites and sediment layers of the underthrust oceanic lithosphere must produce massive fluid flow back to the surface but some of these fluids may form hydrated minerals in the overlying mantle and at great depth flux mantle melting extreme metasomatism in blueschist belts must result from such fluids a case of particular interest involves the cessation of subduction when a high level slab reaches thermal equilibrium in general flow regimes in the thrust and fault zones associated with collisions follow a sequence from conditions of high t p with locally derived fluids at low water rock ratios during initiation of the structures to high fluxes of reduced metamorphic fluids along conduits as the structures propagate and intersect hydrothermal reservoirs later in the tectonic evolution and at shallower crustal levels there may be incursion of oxidizing near surface fluid reservoirs into the faults these fluids may have extremely low 18o values where mountain ranges form on rebound faults and high altitude depleted fluids penetrate down the rebound structure extensive mineralization may be associated with such thrust derived fluids examples which will be discussed include the mother lode au deposits of california u s a and the large u deposits of lagoa real brazil

sandstone weathering processes damaging prehistoric rock paintings at the albarracin cultural park ne spain the rock paintings in cliff foot caves of the albarracin cultural park are known as some of the most important evidences of the levantine prehistoric art of spain 8000 3000 bp the paintings are on sandstone buntsandstein facies of triasic age which may develop intense weathering the analysis of the variables controlling the weathering indicate that salt and wetting drying weathering are responsible for granular disintegration and flaking which lead to rock painting deterioration

decoupled ca and fe mg content of s type granites an investigation on the factors that control the ca budget of s type granites abstract the compositional variation of s type granites is thought to derive from source controlled processes whereby magmas acquire their composition as a result of entrainment of variable proportions of a peritectic rich residual mineral assemblage into the melt a corollary of this model is that the entrained phases keep the stoichiometric proportions they have in the partially melted source these proportions are controlled by the composition of the subsolidus minerals consequently it is expected that the ratio between maficity feot mgo and other compatible major elements in granites would be the same as those of the source this is true for ti but not for ca which shows systematically higher ca maficity ratios in order to explore the factors that cause this discrepancy thermodynamic calculations were used to study the partial melting of a representative metapelite and a metagreywacke under a range of crustal pressure and temperature conditions the modelling included segregation of melt or magma in pulses disequilibrium melting of plagioclase in which only a fraction of this phase participates in the melting system and produces two populations of plagioclase the unreacted cores with the original na rich composition and a second one with a ca rich composition and the entrainment of up to 40 wt minerals into the melt this entrained assemblage consists of two fractions one containing the minerals that are most likely to be surrounded by melt in the source and representing 80 wt of the total assemblage and another one consisting of the phases that are most likely to make up the solid framework with the minerals within each fraction keeping stoichiometric proportions the modelled magmas failed to reproduce the observed high ca maficity ratios this indicates that there are other processes that increase the ca content of granites several processes were evaluated including the presence of apatite enhanced availability of subsolidus plagioclase in the melting system disequilibrium ca rich melts and enhanced entrainment of ca rich plagioclase the last three processes were found to increase the ca content but only enhanced entrainment of ca rich plagioclase increases the ca maficity ratio to values similar to those of the granites a possible explanation for the preferential entrainment of ca rich plagioclase is that the density of this phase is different to that of garnet and ilmenite and similar to the density of the melt consequently a significant fraction of the plagioclase in granites may actually be of residual origin other relevant findings of this study are that metagreywackes seem to be the main source of s type granites as previous investigations suggested and that cordierite rich granitoids represent mineral rich magmas that are formed only from metapelites at pressures below 0 75 gpa this investigation emphasizes the importance of source controlled processes in the composition of s type granites and indicates that entrainment of a mineral assemblage into the melt is not a simple and straightforward process as previously thought

behaviour of apatite during partial melting of metapelites and consequences for prograde suprasolidus monazite growth abstract the suprasolidus behaviour of apatite and monazite is examined for an average metapelite composition using phase equilibria modelling coupled with solubility equations of these minerals both closed and open system scenarios are considered partial melting above the solidus requires apatite and monazite breakdown in order to saturate the anatectic melt in phosphorus and the light rare earth elements in general melt loss is predicted to increase the stability of apatite and monazite at high temperature most apatite is predicted to survive up to ultrahigh temperature conditions except for rocks with low bulk phosphorus concentrations by contrast most monazite is expected to be consumed by uht conditions thorium substitution in monazite is expected to increase the stability of monazite to higher temperatures the presence of lree rich apatite decreases the stability of monazite above the solidus but the breakdown of this apatite during anatexis may generate prograde monazite at the apatite melt interface in local pockets of melt oversaturation however prograde suprasolidus monazite along grain boundaries is expected to be consumed during further partial melting or during melt homogenization when an interconnected melt network develops anatectic melts are predicted to be saturated with respect to apatite except at uht conditions and for rocks with low initial p2o5 bulk concentrations

weathering geomorphology texts 2 cliff ollier 1984 longman group ltd essex england 2d ed 270 pp stg 11 50 paperback

strontium diffusion kinetics in plagioclase feldspars abstract a pronounced systematic variation by a factor of 104 in strontium tracer diffusion kinetics has been measured in four feldspar specimens that span the range of plagioclase compositions the experiments were carried out in air using isotopically enriched 86sr and analyzed with an ion microprobe the arrhenius relations for strontium diffusion are plagioclase ab d 0 m 2 s q kj g atom sr temperature range c albite 98 1 05e 279 550 1080 oligoclase 73 7 4e 7 261 750 1100 labradorite 36 1 1e 6 295 800 1300 anorthite 4 5 7e 9 267 900 1300 no difference was found within the measurement uncertainty between these results and those for albite under hydrothermal conditions at one kbar 100 mpa strontium diffusion rates parallel to and normal to the albite twin lamellae are the same there is no effect on the strontium diffusion rate in albite between an oxygen partial pressure of 0 2 bar air and 10 19 bar graphite buffer a relation is developed that permits calculation of the strontium diffusion arrhenius parameters for any normal plagioclase composition the large variation in diffusion rate with composition has a major effect on the ease of strontium isotope exchange at high temperatures between plagioclases and coexisting phases in rocks which may prove useful in determining thermal histories

a geochemical and experimental study of the role of k feldspar during water undersaturated melting of metapelites abstract the production of s type granites has been related to the extraction of melts during water undersaturated melting of metasediments in the lower continental crust if this mechanism is of widespread applicability a chemically distinctive granulitic restite must be left behind with a positive eu anomaly and depleted incompatible element concentrations however little geochemical evidence has been found in support of this hypothesis the rare earth element geochemistry of lower crustal granulites and associated melts will depend on the behaviour of the phases in which ree and trace elements are concentrated i e zircon monazite and k rich feldspar new experimental data on synthetic metapelites show that k feldspar can be a product or a reactant during water undersaturated melting of biotite depending on the h2o k2o of the melt relative to biotite past confusion over the role of k feldspar during melting is due to the reaction being close to degeneracy and changing its stoichiometry in response to only small changes in bulk composition coherent interpretation of the role of k feldspar in different experimental studies requires a better knowledge of phase water contents than has commonly been available the leucosomes formed in some antarctic granulite facies migmatites are likely to have had low h2o k2o ratios which suggests that k feldspar would have been a reactant during melting this could cause the positive eu anomalies observed in the leucosomes through the disequilibrium melting of k feldspar during water undersaturated melting the undersaturation of trace elements and ree in the leucosomes strongly suggests that disequilibrium melting took place caused by the incomplete dissolution of accessory phases monazite and zircon these phenomena give the melts a distinctive geochemical character which has also been seen in the other granulite facies leucosomes analysed to date this geochemical character contrasts with that of s type granites and therefore the production of granulites in the lower crust may not necessarily be associated with this type of magmatism if lower crustal granulite facies migmatites are the source region for peraluminous s type granites then the melts generated must undergo considerable modification during ascent

magma mixing recharge and eruption histories recorded in plagioclase phenocrysts from el chichon volcano mexico operative throughout the 200 ky history of the el chichon magma consistent core to rim decreases of sr sr ratios and coincident system increases in sr concentrations in plagioclase phenocrysts of varying size 1 cm to 2 mm are reported from samples of the 1982 and pre 1982 200 ka eruptions of el chichon volcano maximum sr sr ratios of 0 7054 significantly higher than the wholerock isotopic ratios 0 7040 0 7045 are found in the cores

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

accessory phase controls on the geochemistry of crustal melts and restites produced during water undersaturated partial melting the profound geochemical conseqences of accessory phase behaviour during partial melting of highgrade metapelites are demonstrated with reference to two geochemically distinct crustal melts produced by biotite dehydration melting reactions under granulite facies kbar 860 cc conditions these two leucogneiss suites from the brattstrand bluffs coastline eastern antarctica have similar field relations transport distances 10 100 s of metres and major element chemistry type 1 leucogneisses have low zr th and lree positive eu anomalies and zr zr and lreet lreet values less than 1 0 i e less than required to saturate the melt mass balance constraints suggest that these melts have been extracted before equilibration with host melanosomes the dry peraluminous nature of vapour undersaturated melts inhibits monazite and zircon solubility and results in concentration of these phases in the residue melts are consequently depleted in lree and hree melanosomes show complementary enrichment in lree while hree patterns are dominated by residual garnet type 2 leucogneisses in contrast have strongly enriched zr th and lree abundances negative eu anomalies and zr zr and lreet lreet 1 resulting from accessory phase entrainment vapour absent partial melting under moderate 6 8 kbar pressure granulite facies conditions of a pelitic source containing monazite is likely to give disequilibrium melts depleted in lree and hree as monazite and garnet are concentrated in the residue if temperatures are high enough 850 870 c to permit relatively large degrees of partial melting then the feldspar component of the source will be removed almost completely giving melts with large positive eu anomalies melts formed under vapour present conditions are unlikely to show such extreme lree and hree depletion or positive eu anomalies even at high degrees of partial melting disequilibrium melting coupled with source entrainment could fortuitously produce ree and trace element signatures similar to those typical of s type granites and usually ascribed to equilibrium melting conditions

isotopic equilibration during partial melting an experimental test of the behaviour of sr abstract experiments using isotopically enriched sr doped minerals designed to test for isotopic equilibrium between source and melt during partial melting reveal that 87 sr 86 sr ratios of the liquids are primarily determined by the proportions of minerals consumed and vary with the advancement of the melting reaction the experiments were performed at 1 atm on model crustal assemblages composed of pairs of natural plagioclase an 68 87 sr 86 sr 0 701 and synthetic fluorphlogopite doped with 90 ppm sr having 87 sr 86 sr 4 2 sims traverses showed that during the initial stages of the reaction liquids are isotopically zoned all the analyzed melts have 87 sr 86 sr markedly higher than that of the bulk starting assemblage i e the source because of the faster melting rate of fluorphlogopite at 1200 c and 1250 c melting occurs above a critical temperature wherein the dissolution rates of the crystals are controlled by diffusion of species in the melt and reactants and reaction products are out of isotopic equilibrium this is due to faster melt crystal boundary migration when compared to sr diffusion in the crystals equilibration is possible only if melting stops calculations show that total equilibration between melt and residue by sr tracer diffusion in the crystals takes 10 4 10 6 yr for temperature and grain size ranging respectively from 800 c to 1000 c and 0 1 to 1 cm when compared to the proposed residence time of crustal magmas at their sources this result strongly suggests that magmas that do not reflect the bulk isotopic characteristics of their source regions can be produced

rb and sr diffusion in alkali feldspars with implications for cooling histories of rocks volume diffusion of sr and rb have been measured in orthoclase feldspar as well as sr diffusion in albite using isotope exchange hydrothermal solutions and analysis by ion microprobe for water pressures of 1000 bars the kinetics of diffusion parallel to the c axis are given by the arrhenius relations strontium transport in the or is isotropic to within a factor of four the measurements use a new procedure in which only the elements in question are present in the starting fluid cooling rates of igneous rocks can now be determined from closure temperatures material balance and the rb sr dating method based on the departures of the mineral data from the whole rock isochron for cooling rates less than approximately 100 c my

nd o hf isotopic decoupling in s type granites implications for ridge subduction abstract the generation of s type granites in the continental crust generally requires partial melting of metasedimentary rocks at relatively high temperatures and is typically associated with syn or post collisional tectonics these melts are generally expected to inherit the geochemical characteristics of their metasedimentary protoliths and their nd hf and o isotopic compositions have been widely used to trace the origin of the magmas here we show that these isotopic systems are decoupled in s type granites from the chinese altai displaying a wide range of zircon hf t values 0 7 to 6 2 and a limited range of whole rock nd t values 6 1 to 2 2 and zircon 18o values 8 49 to 12 91 zircon xenocrysts are common in these plutons and show markedly negative hf t values from 30 7 to 18 and highly positive 18o values from 9 61 to 13 03 in addition these granites exhibit high lu hf 0 15 to 1 38 ratios and substantial depletions in zr and hf we develop a new model to explain the observed relationship between residual zircon content and nd hf o isotope decoupling we propose that these s type granites formed from partial melting of metasedimentary crustal rocks associated with a ridge subduction related heat source limited crustal heating due to mantle upwelling through the slab window on a small scale may have resulted in significant disequilibrium melting of the metasedimentary crust our findings provide new insights into melting processes in the deep continental crust

a spectrum of disequilibrium melting preserved in lava hosted partially melted crustal xenoliths from the wudalianchi volcanic field ne china abstract disequilibrium melting has been established as a common process occurring during crustal anatexis and thus demonstrates that crustal assimilation is not a closed system observations of extreme compositional heterogeneity within partial melts derived from crustal xenoliths have been documented in several recent examples however the retention or transfer of elements to and from residues and glasses and their relative contributions to potential crustal contaminants warrants further investigation recent eruptive products of the huoshaoshan volcano in the holocene wudalianchi volcanic field of northeast china contain crustal xenoliths which preserve a spectrum of partial melting both petrographically and geochemically thus providing an excellent natural example of crustal anatexis correlations exist between the volume of silicic glass preserved within the xenoliths and bulk rock sio2 70 83 wt al2o3 8 16 wt glass 87sr 86sr 0 715 0 908 abundances of elements common in feldspars and micas sr ba rb and elements common in accessory minerals y zr nb these correlations are likely associated with the consumption of feldspars and micas and the varying retention of accessory phases during partial melting the xenoliths which contain greater volumes of silicic glass and residual quartz interpreted as being the most melted were found within pahoehoe lava whilst the least melted xenoliths were found within scoria of the summit cone of huoshaoshan thus it is interpreted that the extent of melting is linked to the immersion time in the lava which we estimate to be on the order of days however the least partially melted samples display geochemical evidence in support of melt loss evidenced by the relatively depleted nature of their whole rock compositions and the larger difference between whole rock and glass yet the more melted samples appear to have experienced less melt loss thus the extent of melting cannot be linked linearly to the volume of melt loss which occurs small scale mm mingling and transfer of material from the enclosing lava to the xenolith are observed however modelling of potential contaminant compositions is inconsistent with crustal contamination during lava petrogenesis it is thus more likely that any crustal contamination in these eruptives is extremely localized within this open magmatic system and most likely occurs at the contact zone between the xenolith and host lava

isotopic disequilibrium in migmatitic hornfels of the gennargentu igneous complex sardinia italy records the formation of low 87sr 86sr melts from a mica rich source isotopic disequilibrium is increasingly recognized as a common feature of magmatic systems but the details of the mechanism s underpinning the development of isotopic disequilibrium during partial melting processes are not fully understood partial melting of mica rich lithologies may be predicted to generate melts enriched in radiogenic sr compared to the bulk protolith compositions due to the typically high rb sr ratio coupled with low melting temperature of mica in crustal rocks here we report a puzzling case study where the sr isotope composition of the melt fraction leucosome of partially molten metapelites migmatites is instead less radiogenic than the restitic component melanosome the examined migmatites fringe 50 m wide zone a low pressure 200 mpa high temperature 1050 c quartz dioritic intrusion which was emplaced in the gennargentu igneous complex sardinia italy at 306 26 ma bulk rock rb sr dating the migmatites derive from anatexis of the muscovite rich metapelitic wall rocks they include a quartzo feldspathic leucosome and a melanosome containing cordierite k feldspar plagioclase biotite fe ti oxide minerals and both corundum and hercynite the leucosome has a less radiogenic sr and more radiogenic nd isotope composition than the melanosome 87sr 86sr 306 ma 0 71068 and 0 71536 end 306 ma 6 4 and 9 2 respectively with bulk migmatite samples having intermediate compositions the significantly lower content of mica in the migmatites compared to the protolith indicates that muscovite and to a lesser extent biotite largely contributed to melt formation however the leucosome volume 50 estimated through mass balance calculations is considerably higher than the amount of melt 10 vol generally produced by mica dehydration melting in the crust suggesting that partial melting was enhanced by an external hydrous fluid the o isotope composition of the migmatites is lower than the typical metapelite values 10 but overlaps with the 18o range of the quartz diorites 8 8 9 9 suggesting that such a hydrous fluid was released from the quartz dioritic intrusion we put forward a model whereby the anatexis temperature conditions t 800 c favored the preservation of isotopic disequilibrium of micas and plagioclase in the protolith in this context the leucosome formed by the preferential melting of less radiogenic plagioclase rather than more radiogenic micas the melt was then efficiently separated from the melanosome containing restitic biotite and peritectic k feldspar and magnetite derived from mica breakdown during the anatectic process the quartz diorite provided not only the heat budget but also the fluid amount responsible for i the hydration of the pelitic country rock ii the increase of melt fraction and iii the higher mobility of anatectic magma

cryptic sr and nd isotopic variation across the leinster granite southeast ireland rb sr isotope whole rock data from the end caledonian leinster batholith define an errorchron age of 464 26 ma appreciably older than the accepted emplacement age of c 405 ma this anomalously old age is the consequence of a highly variable initial sr isotopic composition initial nd ratios and t dm model ages also show a wide range of values however these isotopic variations are neither randomly distributed nor related to petrographic changes but instead reflect in a simple way geographic position about the axis of the batholith the isotopic pattern revealed in the leinster batholith means that isochron ages obtained from s type granites may be largely dependent on the sampling scheme adopted the variable isotopic composition of the granite most likely resulted from either a heterogeneous metasedimentary source or hybridization of coeval dominantly crustal melts

synmetamorphic lode gold deposits in high grade archean settings lode gold deposits that formed under conditions of the middle to upper amphibolite and granulite facies have been identified in the yilgarn block of western australia these deposits which form a continuum with lower metamorphic grade mesothermal gold depos its show that at least some of the fluids responsible for mesothermal gold deposits originated in or below the middle crust studies of 18 o suggest that fluid compositions were controlled by the wall rocks at least for some elements these more proximal gold deposits provide more information about deep fluid sources than do investigations of higher level mineralization deposited from fluids strongly modified by wall rock interaction during the early part of their ascent

what controls chemical variation in granitic magmas abstract consideration of the models that have been applied to explain the chemical variations within granitic rock suites shows that most are inadequate to account for the main variations this stems from a variety of model deficiencies ranging from physical or energetic inadequacies to incompatibility with the chemical data or internal inconsistency between models based on for example isotope or trace element data and major element data we contend that any model that fails any of these tests of internal consistency cannot be considered further thus although we can point to examples in which many of the traditionally accepted mechanisms have played secondary roles in producing variation there presently remains but one viable choice for the primary mechanism by which most granitic magmas acquire compositions beyond the range defined by the compositions of crustal melts that primary mechanism is peritectic assemblage entrainment pae we infer that once a partial melt has formed in a crustal protolith it may segregate from its complementary solid residue carrying small crystals of the peritectic phase assemblage formed in the melting reaction and that the ratios of individual peritectic minerals in the entrained assemblage remains fixed in the ratio decreed by the stoichiometry of the melting reaction for those elements with low solubilities in granitic melts pae in varying degrees accompanied by co entrainment of accessory minerals is responsible for most of the primary elemental variation in granitic magmas in contrast the concentrations of elements with high solubilities in silicic melts reflect the protolith compositions in a simple and direct way the source is the primary control on granite magma chemistry it dictates what is available to dissolve in the melt and what will be formed as the entrainable peritectic assemblage the apparent complexity in granitic rock suites is largely a consequence of these processes in the source all other mechanisms contribute only as a secondary overlay

geochemical and isotopic disequilibrium in crustal melting an insight from the anatectic granitoids from toledo spain in the hercynian anatectic complex of toledo act the anatectic granitoids include leucogranitic leucosomes leucogranitic massifs and restite rich granites they show a broad range of initial sr and nd isotope ratios from 0 711 to 0 720 and 0 51164 to 0 51203 respectively which clearly indicate the absence of isotopic homogenization in the melts broadly the ranges reflect the isotopic variation of the metapelitic protoliths if crustal melting occurs under water undersaturated conditions as is the case of the act the generated melts do not isotopically and chemically equilibrate with the granulitic residuum the preservation of heterogeneities could arise through a number of processes 1 duration of the process in which the presence of melts with disequilibrium features and the high solid content of several of the granites in the act point to a very short lived magmatic system 2 limited diffusion in which poorly segregated and volatile undersaturated granitoids have a very restricted opportunity for isotopic and chemical homogenization and 3 magma dynamics in which the low melting rates the high restite and low water contents and the silica rich composition of these granites together with the small volume and high viscosity values of the melts seriously restrict the physical processes leading to homogenization

incongruent dissolution of ree and sr rich apatite in peraluminous granitic liquids differential apatite monazite and xenotime solubilities during anatexis the relative solubilities of monazite mnz xenotime xno and apatite ap ree and sr rich and ree and sr poor have been studied in peraluminous granitic liquids in month long experiments at 750 c and 200 mpa ph20 in contrast with the high solubility of the apatite 0 7 wtojop205 in strongly peraluminous liquids monazite and xenotime have much lower solubilities 0 05 wt jop205 in mildly to strongly peraluminous compositions p205 in the liquid is 0 03 0 04 wt joat xenotime saturation and 0 02 0 05 wt jo at monazite saturation in keeping with the low p in the glasses re203 contents are below emp detection thresholds 50 08 wt jo for all conditions of experiments saturation of liquid at equilibrium and local saturation around apatite apatite dissolves incongruently crystallizing ree rich monazite on its surface 1 4 m iong grains resulting in similar low ree contents in liquids monazite precipitation occurs along the margins of dissolving apatite crystals even though the bulk liquid is not monazite saturated the abundance of monazite microcrystals increases with the ree content of the apatite and the degree of apatite dissolution the reaction relationship ap liqi mnz liq2 stemming from differences in relative solubilities greater than an order of magnitude between apatite and monazite results in the dissolution of much smaller amounts of ree into peraluminous liquids than expected by simple evaluation of apatite ree contents the amount of ree contributed from apatite directly to peraluminous granitic liquid is related to the amount of apatite dissolved by simple mass balance only if the total ree content of the apatite is sufficiently low that monazite saturation in liquid 50 100 ppm re203 at 750 c is avoided during dissolution of sr rich apatite the sr partitions into the liquid and at 750 c and 200 mpa ph20 the diffusion coefficient of sr in liquid is 2 x 10 10 cm2js r2 0 659 the reaction relationships described above may have application to some textural features observed in natural igneous rocks for example clusters of monazite microcrystals might be indicators of dissolved apatite monazite morphology can be used to distinguish the source of its ree and the general petrological process rock anatexis or magma crystallization under which the monazite formed in addition monazite microcrystals could serve as nucleation sites for other minerals which might explain their common inclusion in biotite and amphibole within granitoids

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

the effect of accessory minerals on the redistribution of lead isotopes during crustal anatexis a model abstract the initial pb isotopic composition of anatectic granites is modelled as a mixture of pb derived from major minerals that have characteristically low 238 u 204 pb and 232 th 204 pb ratios i e feldspars biopyroboles etc with pb contributed from dissolved accessory minerals with characteristically high 238 u 204 pb and 232 th 204 pb ratios i e zircon monazite etc the initial pb isotopic composition of anatectic granites is a function of a the age type modal distribution and heterogeneity in the initial u and th content of the accessory minerals b variation in melt composition and temperature during anatexis c the fraction of the source melted and d the extent to which the melt was homogenized prior to crystallization the differential behavior of accessory minerals during anatexis can explain the common yet enigmatic decoupling of the pb isotopes from other chemical and isotopic systems in granite rocks granitic plutons derived by anatexis of the same source material are not required to have the same initial pb isotopic composition low temperature peraluminous granites are more likely to exhibit homogeneous initial pb isotopic compositions dominated by the pb contained within the major mineral assemblage of the source material in contrast the initial pb isotopic composition of higher temperature metaluminous or peralkaline anatectic granites is more likely to be heterogeneous unless the melts are homogenized during emplacement and will record a larger fraction of the radiogenic pb component contained in the accessory mineral assemblage of the source material heterogeneity in the initial pb isotopic composition of individual granites or granitic suites can preserve the weighted average age of the radiogenic pb present in the accessory mineral assemblage of the source region even if these minerals are entirely dissolved during anatexis the use of isotopic tracer studies in terrane analysis will be optimized by the isotopic characterization of granitic suites

vapour absent melting from 10 to 20 kbar of crustal rocks that contain multiple hydrous phases implications for anatexis in the deep to very deep continental crust and active continental margins

nd isotope disequilibrium during crustal anatexis a record from the goat ranch migmatite complex southern sierra nevada batholith california geological and geochemical studies of a pelitic migmatite complex within the isabella pendant of the southern sierra nevada batholith california suggest that the leucosomes represent the products of partial melting of the metapelite host driven by the emplacement of the adjacent goat ranch pluton ca 100 ma the leucosomes preserve a record of largemagnitude nd isotope disequilibrium with respect to their pelitic source the leucosomes have a wide range of nd 100 ma from 26 0 to 211 0 as compared to 28 7 to 211 3 for the source they can be subdivided into two groups based on their major elements and sr and nd isotope geochemistry group i leucosomes have higher p2o5 contents and nd 100 ma values than those of group ii the nd 100 ma values of group i leucosomes are significantly higher than those of metapelites and migmatites by two to four epsilon units suggesting that group i leucosomes are in nd isotope disequilibrium with their sources correlations among p2o5 contents nd 100 ma values and sm nd ratios in the leucosomes suggest that apatite or monazite has played a dominant role in fractionating sm from nd and generating nd isotope disequilibrium dissolution of apatite or monazite might play a critical role in regulating the behavior of the sm nd isotope systems and thus the nd isotope compositions of melts generated during crustal anatexis especially in metasedimentary protoliths

unravelling the complex interaction between mantle and crustal magmas encoded in the lavas of san vincenzo tuscany italy part ii geochemical overview and modelling abstract this work reports a geochemical overview and modelling of the lavas erupted 4 4 ma ago at san vincenzo tuscan magmatic province tmp although these lavas cover a relatively small area 10 km2 they show very large geochemical variations caused by the interaction of mantle derived and crustal anatectic magmas the lavas consist of peraluminous rhyolites 87sr 86sr i up to 0 726 hosting primarily variably sized magmatic enclaves with shoshonite latite compositions 87sr 86sr i down to 0 708 new whole rock data for a large shoshonite enclave show high concentrations of lree lile and tetravalent hfse coupled with pentavalent hfse depletions and enrichments in compatible elements such as cr and co the chondrite normalised ree pattern is strongly fractionated and characterised by a negative eu anomaly eu eu 0 79 hybridisation and afc models suggest that the shoshonite enclave is the result of 12 rhyolite contamination of a mantle derived magma akin to the potassic trachybasalt shoshonite lavas of capraia island 4 6 ma tmp following an 18 5 assimilation of late triassic metasediments 13 evaporite and 5 5 carbonate and 56 fractionation of clinopyroxene 39 plagioclase 10 and biotite 7 each rhyolite sample is characterised by mineral scale isotopic disequilibria e g 87sr 86sr i 0 711 0 726 glass inclusions with large k2o na2o variations 1 1 3 4 and a poli thermobarometric history of crustal melt production at eutectic conditions a multi parametric approach accounting for k2o na2o 1 3 2 2 87sr 86sr i 0 713 0 725 sr 104 311 ppm and rb 294 403 ppm whole rock variations allowed us to divide the anatectic a rhyolites into five groups a1 a2 1 a2 2 a2 3 a3 group a1 shows the highest 87sr 86sr i ratios and the lowest values of sr k2o na2o and rb it is related to a2 1 and a3 rhyolites by positive k2o na2o rb and k2o na2o feo correlations these three rhyolite groups crop out in the south of san vincenzo and are interpreted to derive from mixing and ascent of peraluminous magma batches locally extracted at progressively higher temperatures from the heterogeneous paleozoic metamorphic basement the a2 3 rhyolites show the highest rb concentrations up to 403 ppm and inverse sr rb and k2o feo correlations suggesting reactions involving the incongruent melting of a biotite bearing crustal source which was previously depleted in low temperature melting components such as muscovite and albite most of the a2 3 rhyolites crop out in the northern svvc area and appear spatially and geochemically related to the a2 2 rhyolites by similarly low 87sr 86sr i ratios however a2 2 rhyolites show lower rb contents 296 316 ppm and geochemical correlations e g 87sr 86sr i rb sr rb mgo feo clearly indicating hybridisation mixing with the shoshonite mantle derived magma rb 109 122 ppm finally we propose a four stage model for the magma plumbing system of san vincenzo produced by the uprising of a mantle derived magma in a normally faulted and doubled crustal basement characterised by two sections of paleozoic siliciclastic rocks separated by late triassic evaporite carbonate sequences

vapor absent melting at 10 kbar of a biotite and amphibole bearing tonalitic gneiss implications for the generation of a type granites vapor absent melting experiments on a biotite and amphibole bearing archean tonalitic gneiss agc150 at 10 kbar and 875 to 1050 c show that amphibole breaks down from 900 to 950 c producing garnet orthopyroxene and granitic melt biotite dehydration melting produces 10 wt melt up to 950 c via incongruent melting reactions that produce garnet orthopyroxene and titanomagnetite widespread biotite dehydration melting occurs between 950 and 975 c and produces orthopyroxene magnetite titanomagnetite and 20 wt fluorine rich melt up to 0 31 wt f minor f rich 2 7 wt biotite is present even at 1000 c our experiments show that under vapor absent conditions intrusion of hot mantle derived magmas into the lower crust is necessary to initiate widespread biotite dehydration melting in rocks with compositions like agc150 we propose that the high thermal stability of biotite in agc150 suggests that this rock is residual after a previous episode of partial dehydroxylation that left behind somewhat f enriched biotite we show that dehydration melting of such f enriched biotite produces f rich granitic liquids with compositions within the range of a type granite and leaves behind a granulitic residue consisting of orthopyroxene plagioclase quartz titanomagnetite and magnetite

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

melt production during granulite facies anatexis experimental data from primitive metasedimentary protoliths we conducted fluid absent partial melting experiments at 0 5 and 1 0 gpa in the temperature range 750 to 1000 c to investigate the influence of bulk rock mg 100mg mg fe and the effects of additional tio2 on the granulite grade anatectic evolution of relatively magnesian metapelites and metagreywackes in these experiments melting began between 780 and 830 c by the incongruent breakdown of biotite to produce quartz saturated granulite facies residual mineral assemblages in equilibrium with h2o undersaturated granitic melt the glass quenched melt compositions produced in this study vary little generally the glasses have compositions similar to those of many natural strongly peraluminous leucogranites the solidus temperatures in both rock types increase with increasing mg but are unaffected by the presence or absence of a tio2 component at 0 5 gpa the metapelites melted at temperatures up to 50 c lower than the equivalent metagreywackes but at 1 gpa there was no discernible difference this study suggests that the fluid absent solidus has a steep positive dp dt slope in metapelites and steep negative dp dt slope in metagreywackes the pattern of melt production with increasing temperature is strongly controlled by the upper limit of biotite stability in tio2 free compositions this was found to increase by 15 to 20 c in the metapelites and by 30 to 40 c in the metagreywackes as a function of increasing mg from 49 to 81 the presence of a tio2 component increases the upper limit of biotite stability by 50 c in the metapelites and by 80 c in the metagreywackes over that observed in the equivalent tio2 free compositions in consequence in the tio2 free samples large pulses of melt up to 35 wt are produced over narrow temperature ranges as little as 15 c in these experiments between 830 and 875 c in the tio2 bearing samples the major pulse of melt production occurs more gradually between 830 and 900 c

pb isotopes during crustal melting and magma mingling a cautionary tale from the miki fjord macrodike central east greenland abstract pb isotopic data are presented for hybrid rocks formed by mingling between mantle derived tholeiitic magma of the eocene miki fjord macrodike east greenland and melt derived from the adjacent precambrian basement bulk mixing and afc processes between end members readily identified in the field fail to model the pb isotope systematics selective contamination during diffusional exchange which can explain the complex sr and nd isotope compositions of the hybrid rocks blichert toft et al 1992 cannot fully account for the variability of the pb isotopic data using the identified crustal end members the crustal anatectic end member although similar in sr and nd isotope composition has a markedly different pb isotopic composition than its source gneiss the differences are consistent with preferential incorporation of radiogenic pb from accessory phases such as metamict zircon or loosely bound pb from grain boundaries during disequilibrium melting of the host rock by the mafic magma the crustal melt involved in magma interactions was therefore heterogeneous with respect to pb isotopes on a metre scale these results illustrate the difficulties inherent in interpreting isotopic variations in contaminated mafic magmas even when the end members are well constrained by field relations we show that the pb isotopic composition of the crustal contaminants and contamination trajectories for the miki fjord hybrid magmatic lithologies are markedly different from regional basement gneisses and contaminated lavas of the associated flood basalt province that were contaminated by characteristically unradiogenic pb and sr lithologies resembling lewisian basement not exposed locally

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

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

chemical composition and sr isotopes of apatite in the xiangshan a type volcanic intrusive complex southeast china new insight into petrogenesis abstract a series of late mesozoic a type granitoids including the xiangshan volcanic intrusive complex was recently identified along the gan hang volcanic belt which was close to the boundary of the yangtze and cathaysia blocks in south china however significant issues on the petrogenesis of the xiangshan complex especially how mantle derived magma contributes if any still remain controversial in this study we focused on the apatite geochemistry of the xiangshan complex to understand the characteristics and evolution of its primary magma in the xiangshan complex mainly including rhyolite porphyritic lava granite porphyry and quartz monzonite porphyry apatites occur mostly as inclusions in plagioclase and biotite and exhibit high srap sr content in apatite srhr sr content in host rock from 0 97 to 3 22 1 85 on average indicating these apatites crystalized before plagioclase in the melt therefore the negative eu anomalies from 0 06 to 0 83 in apatites of the xiangshan complex indicate that plagioclase is a remaining stable mineral phase in the source rocks during the partial melting in addition most of apatites in the xiangshan complex with varied whole rock asi aluminum saturation index values from 0 88 to 1 19 are characterized by high and relatively homogeneous f content from 3 14 to 3 55 therefore f rich mineral should be a primary phase in the source compared with apatites in three earlier rocks of the complex apatites from quartz monzonite porphyry have higher mn and cl contents stronger degree of negative eu anomalies and lower initial 87sr 86sr 0 70642 0 70714 suggesting that before apatites crystalized the magma to form quartz monzonite porphyry had been mixed with a cl rich magma derived from a mantle source that received addition of cl from subducting oceanic sediments of the paleo pacific plate the sr isotope ratios and halogens contents of apatite from quenched dark enclaves found in granitic porphyry imply that the enclaves were formed from this cl rich mantle derived magma through injecting part of this magma into the granitic magma and remaining as quenched enclaves without mechanic mixing and no significant chemical influence on the host granitic magma thorough mixing between this mantle derived magma and the felsic crustal magma did not occur until the latest stage of the xiangshan volcanism when quartz monzonite porphyry was forming

sandstone weathering a century of research and innovation abstract a review of sandstone weathering research particularly in the past 100 years reveals a trajectory of enquiry from early description and classification of features to development of process based explanations to decreasing scales of investigation and a disparity between understanding of process es and explanations of the genesis of sandstone weathering features developments in expositions on mesoscale weathering features on sandstone surfaces are discussed demonstrating a range of approaches to weathering phenomena field based and laboratory based that must be linked to provide an explanation of observed features on a landform scale throughout the twentieth century a thematic chronology highlights certain trends in research description of forms often in arid and semi arid environments single process form models an emphasis on experimentation difficulties in measuring weathering rates and a persistent emphasis on physical causes of breakdown a new research agenda is promoted in which biodeterioration and chemical processes gain parity a holistic approach based on conceptual modeling of weathering systems gains prominence and scale issues are addressed more rigorously

phosphorus in s type magmas the p2o5 content of feldspars from peraluminous granites pegmatites and rhyolites

the use of apatite as a record of initial 87sr 86sr ratios and indicator of magma processes in the inagawa pluton ryoke belt japan abstract sr isotopic analysis of apatite in the extensive inagawa pluton of southwestern japan reveals the spatial distribution of initial 87sr 86sr sri within this body with an unprecedented resolution while the sri values in older units show the largest variation 0 7093 0 7107 the younger ones have relatively homogeneous sri values around 0 7096 the older units also contain numerous mafic microgranular enclaves which are absent in the younger units combining these results with bulk rock analyses and field observations suggests the wide variation in petrology and bulk chemistry in the inagawa pluton is not solely due to fractional crystallization as previously thought mixing between mafic and felsic melts has also played a significant role the lack of enclaves and homogeneous sri values in the younger units suggest that mixing was complete by this time

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

classification of rock weathering at writing on stone provincial park alberta canada a study in applied geomorphology at writing on stone provincial park in southern alberta canada weathering is causing deterioration and loss of archaeologically important indian rock art a procedure devised for the use of park personnel identified four classes of weathering ranging from largely unweathered rock to severely weathered the technique employed simple visual qualitative assessment and photo interpretation of 50 sample sections of sandstone cliff face covering a total area of 354 m2 schmidt hammer tests indicated large variations in rock strength and provided a numerical basis for the visual assessment about 43 per cent of the cliffs are severely to completely weathered 41 per cent show moderate weathering

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

petrogenesis of granitoids from the lachlan fold belt southeastern australia the role of disequilibrium melting abstract s and i type granites from the lachlan fold belt southeastern australia have been investigated to assess the role of disequilibrium melting in their petrogenesis differences between the median initial ehf compositions of magmatic zircon populations and the host bulk rock ehfblk zrc range from 0 6 to 2 5 e units providing evidence for intra sample and hence inter phase hf isotopic heterogeneity linear variations on harker diagrams and o and hf isotope compositions of magmatic zircon preserved in many i and s type granites are inconsistent with assimilation or simple mixing hypotheses in contrast isotopic disequilibrium between the melt and a restite assemblage can explain the bulk rock versus zircon differences observed in these samples assuming that magmatic zircon records the melt composition differences between the bulk rock ehf and ehf of magmatic zircon ehfblk zrc values measured for i type granites 0 4 2 5 can largely be explained by disequilibrium amphibole dehydration melting of meta igneous protoliths that were either isotopically heterogenous at the time they were formed or perfectly homogeneous before being aged in the crust for 0 4 1 0 billion years prior to partial melting the currowong suite exhibits petrographic features and preserves geochemical and isotopic compositions that do not lend themselves to simple restite model or magma mixing explanations however these observations could be explained by the restite unmixing of magma batches generated from a single source rock if as modelling has suggested separate batches contain different melt compositions by investigating the application of disequilibrium melting to granite genesis this study demonstrates that isotopic heterogeneity at various sampling scales should actually be expected for the production of granites from a single source rather than necessitating the involvement of multiple sources and mixing processes as a result great care should be taken in the interpretation of isotope data from granitic bulk rocks or their zircons

rejuvenation of rbsr mica ages during shearing on the northwestern margin of the nanga parbat haramosh massif abstract the shear zone bordering the western margin of the nanga parbat haramosh massif nphm separates terranes with distinct but well documented cooling histories this study was undertaken in order to examine isotopic behaviour and if possible to date deformation or cooling within the contact zone itself rb sr muscovite and biotite ages and petrographic descriptions are presented for a range of metamorphic lithologies and granite sheets which have been affected to varying degrees by greenschist facies extensional crenulation this deformation has been kinematically linked to the differential uplift of the nphm ages for metamorphic lithologies and granite sheets are similar and are considered together determined muscovite ages mostly lie in the range 9 22 ma whilst the majority of biotite ages lie in the range 2 11 ma mica ages from the shear zone are significantly more variable than ages determined for unsheared lithologies from outside the shear zone even closely spaced samples yield variable mica ages and in some cases there is a significant difference between muscovite and biotite ages determined for the same sample muscovite ages yield a bimodal distribution with groups of ages in the range 17 21 ma and 8 12 ma the older group is thought to reflect the original purely thermostatic cooling of lithologies through the muscovite closure temperature prior to formation of the nphm structure in contrast the younger group of muscovite ages reflects variable degrees of isotopic resetting during shearing at temperatures below the muscovite closure temperature associated with the relative uplift of the nphm the majority of biotite ages also reflect partial to complete resetting during shearing however biotite samples from structurally competent amphibolite horizons preserve older 16 ma ages indicating that there was locally a strong lithological or fluid control on the degree of resetting the flanks of the nphm cooled through the muscovite closure temperature significantly earlier than the deeper structural levels presently exposed within the core of the nphm greenschist grade deformation was occurring within the shear zone at the same time that deeper levels of indian crust were passing through the muscovite closure temperature suggesting that the shearing reflects the relatively passive extensional movement of rocks sliding off the flank of the actively growing nanga parbat structure the results highlight the isotopic complexities of long lived shear zones present in orogenic belts but show that useful radiometric age information may be obtained if the regional tectonic history is well understood and if the samples for dating come from well constrained structural settings

scales of disequilibrium and rates of equilibration during metamorphism mounting evidence suggests that partial disequilibrium meaning disequilibrium for some elements but not for others may be a common but rarely detected phenomenon during metamorphic mineral growth even in ordinary prograde reactions that progress to completion detailed examination of compositional variations in garnet crystals over a range of metamorphic grade suggests distinctly different scales of equilibration for common elements with strong temperature dependence under lower greenschist facies conditions fe and mg may equilibrate at hand sample scale whereas mn and ca may not equilibrate even at millimeter scale although mn may achieve hand sample scale equilibration under upper greenschist facies conditions ca and many trivalent cations e g rees may not do so until temperatures exceed those of the middle amphibolite facies even in the lower granulite facies some elements e g y yb show indications of disequilibrium at sub centimeter scales during garnet growth analysis and numerical modeling of undisputed disequilibrium textures demonstrate that the most common impediment to equilibration during metamorphism is the sluggishness of intergranular diffusion the same mechanism known to govern porphyroblast crystallization in many metamorphic environments despite this importance to petrology few quantitative determinations exist of inter granular diffusion rates under metamorphic conditions using numerical simulations of coupled intergranular and intracrystalline diffusion processes in coronal textures around partially resorbed garnet crystals from the llano uplift u s a a very precise and relatively accurate estimate is obtained for the rate of intergranular diffusion of al in fluid undersaturated systems this result and an earlier estimate for fluid saturated systems provide bracketing values for al diffusivity during metamorphism

the crustal continuum model for late archaean lode gold deposits of the yilgarn block western australia most archaean gold ores belong to a coherent genetic group of structurally controlled lode deposits that are characteristically enriched in au with variable enrichments in ag as w sb bi te b and pb but rarely cu or zn and are surrounded by wallrock alteration haloes enriched in k lile and co2 with variable na and or ca addition evidence from the yilgarn block of western australia combined with similar evidence from canada and elsewhere indicates that such deposits represent a crustal continuum that formed under a variety of crustal regimes over at least a 15 km crustal profile at pt conditions ranging from 180 c at 1 kb to 700 c at 5 kb individual deposits separated by tens to hundreds of kilometres collectively show transitional variations in structural style of mineralisation vein textures and mineralogy of wallrock alteration that relate to the pt conditions of their formation at varying crustal depths specific transitions within the total spectrum may be shown also by deposits within gold camps although nowhere is the entire continuum of deposits recorded from a single gold camp or even greenstone belt