strontium distribution in the shell of the aragonite bivalve arctica islandica 1 the behavior of sr in the aragonite shells of the bivalve arctica islandica has been explored to determine its potential for high resolution environmental reconstruction the structural state of sr in the shell was investigated using x ray absorption fine structure xafs and sr ca variations across the prismatic layer in the umbo and outer shell were determined by secondary ionization mass spectrometry sims and laser ablation inductively coupled plasma mass spectrometry la icp ms respectively xafs data suggest that sr randomly substitutes for ca within the shell aragonite sr ca in both the umbo and the outer shell exhibits significant seasonal variation but does not correlate with seawater temperature sr ca varies significantly between parallel transects across the same annual bands sr concentrations are highest in transects along the maximum growth axis of the outer shell but lowest in these transects in the umbo scanning electron microscopy indicates changes in crystal morphology both within and between transects we do not however observe a clear relationship between shell growth rate and sr ca fluctuations in sr ca are most likely to be controlled by a complex interaction of biological and kinetic processes

a new method to quantify the real supply of mafic components to a hybrid andesite the eruption of soufriere hills volcano montserrat has been ongoing since 1995 the volcano is erupting a crystal rich hornblende plagioclase andesite with ubiquitous mafic inclusions indicating mixing with mafic magma this mafic magma is thought to be the driving force of the eruption supplying heat and volatiles to the andesite resident in the magma chamber as well as producing macroscopic mafic inclusions the magma mixing process involves incorporation of phenocrysts from the andesite into the mafic magma these inherited phenocrysts show clear disequilibrium textures e g sieved plagioclase rims and thermal breakdown rims on hornblende approximately 25 of all phenocrysts in the andesite show these textures indicating very extensive mass transfer between the two magma types fragments of mafic inclusions down to sub mm scale are found in the andesite together with mafic crystal clusters which are commonly found adhered to the rims of phenocrysts with disequilibrium features mineral chemistry also points to the transfer of microlites or microphenocrysts initially formed in the mafic inclusions into the andesite this combined evidence suggests that some of the mafic inclusions disaggregate during mingling and or ascent possibly due to shearing and raises the question what proportion of the andesite groundmass actually originated in the mafic inclusions and thus what is the true amount of mafic magma in the magmatic system we present a new method for quantifying the relative proportions of groundmass plagioclase derived from mafic and andesitic magma based on analysis of back scattered electron images of the groundmass preliminary results indicate that approximately 16 of all groundmass plagioclase belongs genetically to the mafic inclusions together with the crystal clusters disequilibrium phenocryst textures and mm scale inclusions there is a cryptic mafic component in the andesite of approximately 6 by volume this is significant compared with the proportion of macroscopic mafic inclusions typically 1 5 the new method has the potential to allow tracking of the mafic fraction through time and thus to yield further insights into magma hybridisation processes

plagioclase mantles on sanidine in silicic lavas clear lake california implications for the origin of rapakivi texture at clear lake california mixed dacite lavas contain sanidine mantled by oligoclase that closely resemble rapakivi feldspars in granites resorption and mantling of sanidine by plagioclase is one of many disequilibrium textures in these lavas resulting from maficfelsic magma interaction mantling of sanidine by plagioclase may occur directly in mixed magmas due to shifts in composition and temperature or indirectly in crystal rich silicic magmas influenced by thermal under plating or mafic felsic hybridization in adjacent regions of the magmatic system other silicic laves containing mantled sanidine also show evidence for mafic felsic magma interaction textural and compositional similarities between rapakivi texture in extrusive and intrusive rocks suggest that they have a common origin moreover modification of early formed textures due to protracted crystallization subsolidus reactions and metasomatic hydrothermal alteration in the plutonic environment accounts for most of the observed differences between extrusive and intrusive examples the association of coeval mafic rocks and evidence for mafic felsic magma interaction in many plutons containing rapakivi exture confirm that magma mixing plays a key role in its formation in some intrusive texamples evidence for a compositional control and multiple episodes of mantle formation are also compatible with repeated cycles of recharge and mixing regardless of the exact cause of k feldspar instability textural evidence from volcanic rocks and experiments indicates that development of rapakivi texture is controlled by cation diffusion in a dissolution boundary layer developed on k feldspar mantles appear to form in a two stage process that involves initial epitaxial nucleation of plagioclase on sanidine followed by simultaneous dissolution of sanidine and inward growth of plagioclase

behavior of alkalies during diffusive interaction of granitic xenoliths with basaltic magma a series of three experiments was made at 125 c and 10 kbar in which a 1 gram reservoir of molten oceanic tholeiite was maintained in contact with one end of a 6 5 mm diameter cylinder of partially molten fine grained granite detailed microprobe analysis 450 spots total of the quenched samples revealed progressive basalt granite interaction as run duration increased from 0 75 to 8 to 24 hours interdiffusion of melt components is limited for structure controlling species such as si 4 and al 3 and for divalent cations which all mix over a zone less than 0 5 mm wide in 24 hours at run conditions in contrast k and na are extremely mobile resulting in considerable uptake of k by the basalt melt and eventual loss of na from the basalt to the granite by uphill diffusion the chemical diffusivity of k in the basalt can be accurately estimated at 3 times 10 18 cm 2 sec for the specific experimental conditions used although this value is somewhat lower than previous estimates k o

a case for a downwasting mountain glacier during termination i vercenik valley northeastern turkey field evidence of palaeoglacial records in the vercenik valley in the eastern black sea mountains was examined and 19 samples for surface exposure dating with cosmogenic 10 be were collected with the aim of increasing knowledge on the amplitude and frequency of palaeoglacier advances in anatolia glacial erosional features were mapped and the flow directions of the palaeoglaciers were determined the vercenik palaeoglacier advanced before 26 1 k 1 2 k yr the last glacial maximum lgm advance continued until 18 8 k 1 0 k yr the vercenik palaeo glacier collapsed during termination i after 17 7 k 0 8 k yr there was no more ice in the main valley the vercenik palaeoglacier most probably then separated into five small glaciers that were restricted to the tributary valleys among these the hemsin palaeoglacier completed its recession around 15 7 k 0 8 k yr on the basis of glacial erosion features a lateglacial glacier advance can be identified evidence of the little ice age advance appears to be absent the results from this valley system seem to be consistent with the first results from the adjacent kavron valley and with the anatolian lgm palaeoclimate the sea surface temperature minima in the western mediterranean the deposition of red clay layers in the black sea and deposition of the heinrich 1 layer in the north atlantic copyright 2007 john wiley sons ltd

joint bounded crescentic scars formed by subglacial clast bed contact forces implications for bedrock failure beneath glaciers glaciers and ice sheets are important agents of bedrock erosion yet the precise processes of bedrock failure beneath glacier ice are incompletely known subglacially formed erosional crescentic markings crescentic gouges lunate fractures on bedrock surfaces occur locally in glaciated areas and comprise a conchoidal fracture dipping down ice and a steep fracture that faces up ice here we report morphologically distinct crescentic scars that are closely associated with preexisting joints termed here joint bounded crescentic scars these hitherto unreported features are ca 50 200 mm deep and involve considerably more rock removal than previously described crescentic markings the joint bounded crescentic scars were found on abraded rhyolite surfaces recently exposed 20 years beneath a retreating glacier in iceland as well as on glacially sculpted precambrian gneisses in nw scotland and various precambrian rocks in ontario glaciated during the late pleistocene we suggest a common formation mechanism for these contemporary and relict features whereby a boulder embedded in basal ice produces a continuously migrating clast bed contact force as it is dragged over the hard bedrock bed as the ice embedded boulder approaches a preexisting joint in the bedrock stress concentrations build up in the bed that exceed the intact rock strength resulting in conchoidal fracturing and detachment of a crescentic wedge shaped rock fragment subsequent removal of the rock fragment probably involves further fracturing or crushing comminution under high contact forces formation of joint bounded crescentic scars is favoured by large boulders at the base of the ice high basal melting rates and the presence of preexisting subvertical joints in the bedrock bed we infer that the relative scarcity of crescentic markings in general on deglaciated surfaces shows that fracturing of intact bedrock below ice is difficult but that preexisting weaknesses such as joints greatly facilitate rock failure this implies that models of glacial erosion need to take fracture patterns of bedrock into account

fracturing and diking in incompletely crystallized granitic plutons abstract dikes in granitic plutons commonly have mineralogic and textural characteristics indicating a genetic relation to their host the contact between the dike and the host is marked signifcantly by interlocking unfractured crystals rather than by truncations and overgrowths although megascopically the contacts between dike and host pluton may be exceedingly sharp and planar an explanation of these relationships by fracturing an incompletely crystallized host rather than by entirely post magmatic fracturing with subsequent diking is postulated to be consistent with experimental date and certain petrologic observations published experimental research indicates that fracturing of crystal melt systems may occur if there is at least 70 crystals in the system at which point the system is capable of transmitting compressive and shear stresses via a crystalline framework dilatancy pumping of melt into fracture zones and subsequent hydraulic fracturing is thought to be one possible way late stage fluids of granitic systems evolve into dikes in granitic plutons such as leucogranites aplites and pegmatites interlocking textures between dike and host may also develop if magma from some distant source is interjected along the plane of fracturing in crystal melt systems

distribution of magnesium in coral skeleton ion micro probe imaging of the aragonite skeleton of pavona clavus a massive reef building coral shows that magnesium and strontium are distributed very differently in contrast to strontium the distribution of magnesium is strongly correlated with the fine scale structure of the skeleton and corresponds to the layered organization of aragonite fibers surrounding the centers of calcification which have up to ten times higher magnesium concentration this indicates a strong biological control over the magnesium composition of all structural components within the skeleton magnesium may be used by the coral to actively control the growth of the different skeletal crystal components

magma mingling and melt modification between granitic pipes and host diorite guernsey channel islands within the northern igneous complex of guernsey cylindrical pipe intrusions 20 40 cm in diameter developed by buoyant emplacement of granite magma into overlying denser diorite magma the high viscosity of the partially crystallized magmas and lack of mechanical stirring inhibited physical mixing and mesoscopically sharp well defined interfaces developed the granitic pipes are surrounded by aureoles up to a metre in diameter which exhibit changes in bulk chemical composition and partial mineralogical re equilibration these formed in response to the modification of interstitial melt in the partially crystallized host resulting from residual melt migration and or melt interdiffusion across the pipe host interface such processes between partially crystallized magmas may in part provide an explanation for textural mineralogical and chemical compositional features commonly exhibited by mafic enclaves within granites

crystal fabrics and element impurities sr ca mg ca and ba ca in shells of arctica islandica implications for paleoclimate reconstructions sr ca mg ca and ba ca values are heterogeneously distributed in shells of arctica islandica these patterns are largely associated with crystal fabrics size habit and orientation of crystals or the processes controlling them the outer sublayer of the outer shell layer oosl homogenous and irregular simple prismatic crystal fabrics contained element ca values up to 62 higher than the inner sublayer iosl crossed acicular crossed lamellar fine crossed lamellar and irregular simple prismatic crystal fabrics a gradual decrease in sr ca and mg ca values was observed from the outer portions of the oosl toward the oosl iosl transition zone this chemical shift was accompanied by a gradual transition from homogenous crystal fabrics into crossed lamellar acicular crystal fabrics near annual growth lines irregular simple prisms i e during periods of slow growth sr and mg seemed to be deposited in equilibrium with the ambient environment because the sr ca and mg ca values approached values typical for seawater during the remainder of the growing season sr ca and mg ca remained far below values expected for thermodynamic equilibrium ba ca peaks however occurred erratically at different times of the year without any noticeable changes in crystal fabrics likely the environmental information contained in these peaks was less severely filtered by vital effects than in sr and mg the findings of the present study can help to develop new techniques with which extract environmental signals from the metal to calcium ratios of bivalve shells

dissolution of tonalitic enclaves in ascending hydrous granitic magmas an experimental study dissolution of natural mafic magmatic enclaves in a hydrous leucogranitic synthetic melt has been tested experimentally results suggest that the mechanism of enclave dissolution is a potential hybridization process in granitic systems experiments performed in decompression simulating ascending magmas show interesting results from 10 kbar to 4 kbar for a given temperature enhanced dissolution of the tonalitic enclave into the melt has been observed compared with experiments at constant pressure furthermore the composition of the melt changed to higher cao feo and mgo contents dissolution textures on relict crystals from the tonalitic enclave were monitored these results have implications for the generation of peraluminous monzogranites and granodiorites and an example for the cabeza de araya caceres spain from the serie mixta of the iberian massif is given the tonalitic enclaves that are partially dissolved may be early or coeval intrusions into the granite magma or even into the source migmatitic area many mineralogical geochemical and isotopic features of monzogranites e g reverse zoning in plagioclase low sr isotopic ratios are accounted for by this mechanism of enclave dissolution during ascent and emplacement

the ascent of felsic magmas in dykes abstract the results of a thermal and fluid dynamical analysis are presented that allow geologists to estimate the critical dyke or fracture width w c necessary for the transport of felsic magma through the crust without control by solidification the principal geological variables required to estimate w c are the magmatic viscosity the density and temperature contrasts and t between magma and country rock and the height of ascent h the results show that for typical values of and and for transport distances of 20 to 30 km the critical dyke width is constrained to lie between about 2 and 20 m

lateral plucking as a mechanism for elongate erosional glacial bedforms explaining megagrooves in britain and canada megagrooves are kilometre scale linear topographic lows carved in bedrock separated by ridges typically in areas of largely devoid of till they have been reported from several areas covered by pleistocene glaciations such as canadian nw territories michigan and nw scotland here we report two previously undocumented megagroove fields from ungava canada and northern england and present new analyses of the megagrooves from nw scotland this paper seeks to determine the nature of the lithological and structural controls on the occurrence and formation of megagrooves analysis of both geomorphological and bedrock properties shows that megagrooves are generally a confined to well stratified or layered bedrock such as meta sedimentary rocks with closely spaced joints and tend not to occur on massive rocks such as gneiss or granite or thick bedded sedimentary rocks b subparallel to palaeo ice flow and the strike of the strata and tend not to occur where palaeo ice flow is at high angles to the strike of strata c produced by significant glacial erosion by sustained unidirectional ice flow detailed analysis of megagrooves in nw scotland shows that neither glacio fluvial erosion nor differential abrasion was the dominant mechanism of formation a mechanism here termed lateral plucking is suggested that involves block plucking on rock steps parallel to ice flow removal of joint bounded blocks from such rock steps involves a component of rotation along a vertical axis block removal may be enhanced by a direct component of shear stress onto the vertical stoss sides the lateral plucking mechanism results in horizontal erosion at right angles to the ice flow and enhances the groove ridge topography megagrooves are potentially useful as palaeo ice flow indicators in areas devoid of till and can thus complement the palaeo ice stream datasets which are presently largely based on soft sediment landform studies

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

mixing and chemical interdiffusion of trachytic and latitic magma in a subvolcanic complex of the tertiary westerwald germany abstract a special kind of magma mixing is extraordinarily well exposed in the bittersberg subvolcanic complex in the tertiary volcanic field of the german westerwald a trachytic melt has been penetrated by a latitic dyke which has been dispersed within the host magma as small spherical enclaves globules whole rock analyses of the globules show a change in composition that cannot be explained by a simple mechanical mixing between the endmembers the most evolved globules have a phonolitic composition microprobe measurements in the microlithic matrix of the host rock and the guest indicate a diffusive motion of the alkalis from the host into the globules on the other hand an opposite trend can be observed for ca mg fe and ti which are impoverished in the globules the trace elements and the middle rare earth elements mree has also been involved in the diffusive exchange the ree pattern of the most evolved phonolitic globules shows a characteristic trough in the area of the mree which is almost identical to the ree pattern of many phonolites the phonolites and the alkali rich trachytes of the westerwald show similar globular textures as the bittersberg volcanics therefore generation of these rocks involving diffusive element exchange during mixing processes in a magma reservoir situated on a deeper crustal level may be possible

glacial erosion and bedrock properties in nw scotland abrasion and plucking hardness and joint spacing subglacial erosion beneath glaciers occurs predominantly by abrasion and plucking producing distinct erosional forms the controls on the relative importance of abrasion vs plucking are poorly understood on the one hand glacial conditions that favour or suppress cavity formation ice velocity ice thickness and water pressure are thought to favour plucking or abrasion respectively conversely bedrock properties are also known to control landforms but this has rarely been analysed quantitatively in this study we compare landforms and bedrock properties of sandstone and quartzite at the bed of a palaeo ice stream near ullapool in nw scotland the boundary between the rock types is at right angles to the westward palaeo ice flow and palaeoglacial conditions on both rock types were similar we report quantitative parameters for bedrock properties schmidt hammer hardness and joint spacing and use morphometric parameters to analyse the landforms torridon sandstone is soft but thick bedded and with a wide joint spacing erosional bedforms include roche moutonnees with smoothed tops and concave stoss sides whalebacks and elongate p forms indicating a high proportion of abrasion over plucking cambrian quartzite is hard but thin bedded with narrow joint spacing erosional landforms are angular to subangular with abundant plucked lee faces suggesting a high proportion of plucking over abrasion hardness and joint spacing thus exert a strong control on subglacial erosional landforms and the mechanisms that formed them thus glacial conditions ice velocity ice thickness can only be inferred from glacial erosional landforms if the effects of bedrock properties of the substrate are considered

petrogenesis and tectonic implications of a type granites in the dabie orogenic belt china geochronological and geochemical constraints the dabie orogenic belt is characterized by the presence of large volumes of intrusive and volcanic rocks that formed in late mesozoic times most of the intrusive bodies are i type granites but it is still unclear whether there are contemporary a type granites here we report the first unambiguous discovery of a type granite from baiyashan in the north dabie tectonic belt the crystallization age of the body has been fixed as 120 4 1 2 ma using u pb analysis of zircons by la icpms the baiyashan granite is enriched in si k na rb and ree has elevated feo tot feo tot mgo and ga al ratios and is depleted in mg ca mn ba sr p and ti the ree composition shows highly fractionated patterns with la yb n 6 95 16 68 and eu eu 0 33 0 59 its crystallization age field relationships petrographic and geochemical data show beyond doubt that the baiyashan granite is an early cretaceous a type granite sr nd isotope systematics are characterized by a high i sr of 0 708 0 714 and a low nd of 7 5 to 19 4 with t dm2 1 5 2 5 ga and these data indicate that the magmas were dominantly sourced from partial melting of middle to lower crustal intermediate felsic igneous rocks and mingling with mafic to intermediate magmas during rift related magmatism associated with subduction of the palaeo pacific plate beneath eastern china in early cretaceous times

the genesis of i and s type granitoid rocks of the early ordovician oledo pluton central iberian zone central portugal abstract the early ordovician oledo pluton consists of four distinct granodioritic to granitic phase g1 g4 which intruded a cambrian schist metagraywacke complex but were themselves intruded by a late carboniferous pluton id tims u pb ages for zircon and monazite from these granitic rocks indicate emplacements within a short period of time at 479 480 ma granodiorite g1 is the most deformed rock with shear zones and deformation at the border g1 and g3 contain fine grained biotite tonalite and biotite granodiorite microgranular enclaves which are darker and richer in mafic minerals than the host granodiorites the geological mineralogical geochemical and sr nd and o isotopic data show that tonalitic and granodioritic enclaves and host g1 are of i type and were related predominantly by a fractional crystallization process least square analysis of major elements and modelling of trace elements indicate that granodioritic enclaves and host g1 could be derived from the tonalitic enclave magma by fractional crystallization of plagioclase grunerite biotite and ilmenite granodiorite g2 is of hybrid origin most variation diagrams for granodioritic enclaves and host g3 granodiorite and their biotites show linear trends modelling of major and trace elements of granodioritic enclaves indicate that they result from mixing of relatively primitive granodiorite magma with magma derived from crustal melting tonalitic enclaves correspond to globules of a more mafic relatively primitive magma granite g4 has the most pronounced crustal signature and is of s type

an intensity ratio calibration method for the accurate determination of mg ca and sr ca of marine carbonates by icp aes an inductively coupled plasma atomic emission spectroscopy icp aes method for the accurate and precise simultaneous measurement of the mg ca and sr ca content of carbonates was established while a precision of 0 3 1 standard deviation sd is easily obtainable for both mg ca and sr ca analysis a ca matrix effect complicates achieving similar levels of accuracy with conventional calibration procedures an alternative ratio calibration procedure is proposed which overcomes the ca matrix effects and ensures the accuracy of the mg ca and sr ca analysis of marine carbonates to 0 3 almost an order of magnitude better than conventional calibration methods the longer term precision is 0 1 if the batch run average values are corrected for longer term drift the method is suitable for analysis of foraminiferal calcite and coral aragonite and can easily be adjusted for the analysis of other carbonates or microsamples

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

magma evolution and open system processes at shiveluch volcano insights from phenocryst zoning phenocryst zoning patterns are used to identify open system magmatic processes in the products of the 2001 eruption of shiveluch volcano kamchatka the lavas and pumices studied are hornblende plagioclase andesites with average pre eruptive temperatures of 840c and fo2 of 1 5 2 1 log units above nickel nickel oxide nno plagioclase zoning includes oscillatory and patchy zonation and sieve textures hornblendes are commonly unzoned but some show simple multiple or patchy zoning apatite microphenocrysts display normal and reverse zoning of sulphur the textural similarity of patchy hornblende and plagioclase together with ba sr systematics in patchy plagioclase indicate that the cores of these crystals were derived from cumulate material plagioclase liquid equilibria suggest that the patchy texture develops by resorption during h2o undersaturated decompression when h2o saturated crystallization recommences at lower pressure reduced ph2o results in lower xan in plagioclase causing more al rich hornblende to crystallize plagioclase cores with diffuse oscillatory zoning and unzoned hornblende crystals probably represent a population of crystals resident in the magma chamber for long periods of time in contrast oscillatory zoning in the rims of plagioclase phenocrysts may reflect eruption dynamics during decompression crystallization increasing fe al in oscillatory zoned rims suggests oxidation as a result of degassing of h2o during decompression a general lack of textural overlap between phenocryst types suggests that different phenocryst populations were spatially or temporally isolated during crystallization we present evidence that the host andesite has mixed with both more felsic and more mafic magmas olivine and orthopyroxene xenocrysts with reaction or overgrowth rims and strong normal zoning indicate mixing with basalt sieve textured plagioclase resulted from mixing of a more felsic magma with the host andesite the mineralogy and mineral compositions of a mafic andesite enclave are identical to those of the host magma which implies efficient thermal quenching and thus small volumes of intruding magma mixing of this magma with the host andesite results in phenocryst zoning because of differences in dissolved volatile contents we suggest that small magma pulses differentiated at depth and ascended intermittently into the growing magma chamber producing incremental variations in whole rock compositions

estimation of ancient and future shoreline positions in the vicinity of olkiluoto an island on the western coast of finland the difference between grid and tin based gis approaches abstract the rise of gis data and spatial modelling techniques has brought new insights into shore displacement and palaeogeographical studies during recent years the aim of this study was to compare the capabilities of a raster data and map algebra based technique with those of a vector data triangulated irregular network tin based technique in the reconstruction of ancient and future shoreline positions in the vicinity of olkiluoto an island on the rapidly uplifting western coast of finland the first two models were carried out using a raster subtraction technique with grid cell sizes of 200 m and 30 m and a third model utilised a tin intersection technique where the shoreline is derived from the intersection of the tin representing the modern topography and the tin representing the water plane of the reconstructed phase the results show that the raster based technique has limited capability in recognition of important landscape features common to glacially sculptured coastal areas subject to glacial isostatic adjustment gia whereas the tin intersection technique provides more detailed information about the shoreline features comparison of the results reveals that the advantage of the tin intersection technique is more obvious the finer the scale is novel spatial modelling techniques improve the reconstructions of palaeolandscapes and shoreline positions of different time periods

mg ca sr ca and stable isotope δ18o and δ13c ratio profiles from the fan mussel pinna nobilis seasonal records and temperature relationships 1 we present new annually resolved 18o 13c mg ca and sr ca ratio records for two shells of the fast growing mediterranean fan mussel pinna nobilis collected from proximal spanish coast sea grass meadows the relationship between the potential geochemical proxies and ontogenetic and environmental controlling factors is investigated specifically the use of shell mg ca and sr ca ratios as potential calcification temperature proxies the latter calculated from measured shell 18o values has been assessed the 18o cycles along the growth axis indicate that our p nobilis specimens are 10 5 and 4 5 years old shell sr ca ratios do not exhibit any consistent interannual cyclicity and are not correlated to temperature a subtle ontogenetic effect on shell mg ca ratios was observed during the first 4 5 years of recorded growth but was highly evident during the organism s later growth years in p nobilis shells different mechanisms influence ontogenetic variation in shell mg ca and 18o records shell mg ca ratios from the first 4 5 years of growth correlate significantly to temperature in a best fit relationship described by the equation mg ca 17 16 1 95 exp 0 022 0 004 t p nobilis shell mg ca records therefore are a valid temperature proxy only during an early growth phase for the same range of temperatures shell mg ca ratios in p nobilis are approximately 1 3 lower than those reported for inorganic calcite but 3 to 4 times higher than in another bivalve species mytilus trossulus and 4 to 16 times higher than in foraminifera we suggest these offsets are due to a higher degree of similarity between seawater and calcification fluid composition in p nobilis than in other bivalves and foraminifera the observed shell mg ca ratio change per c of 2 2 also is lower than that observed for inorganic and other biogenic calcites our findings strongly support taxon and species specific mg ca temperature relationships for bivalves and other calcifying organisms an appreciation of the physiology and calcification mechanisms of any biogenic carbonate archive therefore is paramount prior to the application of stable isotope and element ca ratio proxies for paleotemperature reconstructions

investigation of li ca variations in aragonitic shells of the ocean quahog arctica islandica northeast iceland interannual and intra annual variations in lithium to calcium ratio were investigated with high temporal resolution in the aragonitic outer shell layer of juvenile arctica islandica mollusca bivalvia collected alive in 2006 off northeast iceland li ca shell ranged between 7 00 and 11 12 mol mol 1 and presented well marked seasonal cycles with minimum values recorded at the annual growth lines a general pattern was a progressive increase in li ca shell from march to may followed by a plateau in june and a decrease down to minimum values in july august li ca shell was correlated with d 18 o shell derived temperature but the strength of this relationship was weak r 2 p shell in a islandica may most likely be explained 1 by calcification rate and or 2 by significant river inputs of li rich silicate particles flowing to the sea as soon as snow melts in the first case li ca shell may be a useful proxy for addressing seasonal variations of growth rate in bivalves that lack discernable microgrowth patterns abrupt decreases of li ca shell may in turn help identify growth retardations due to harsh environmental conditions alternatively if li ca shell variations are linked to particulate li inputs by rivers this could be a new proxy for the intensity of mechanical weathering of icelandic basalts with interesting perspectives for the reconstruction of frequency and intensity of past jokulhlaups subglacial outburst floods further works including experimental studies are needed to test these hypotheses

origin of sierra nevadan granite evidence from small scale composite dikes composite dikes at hell hole meadow in the central sierra nevada contain hybrids created by small scale mixing of andesitic and rhyolitic magmas early rhyolitic injections had partially solidified when subsequent andesitic magmas arrived and mixed with small increments of remnant rhyolitic magma in major element chemistry the hybrid rocks define linear mgo variation diagrams that closely resemble those for the half dome mt givens and eagle peak granodiorites the patterns suggest that mixing of mafic and felsic magmas has been important in the evolution of these plutons

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

granite magma generation ascent and emplacement within a transpressional orogen crustal thickening during transpressive orogenesis may produce anatectic granites which may then localize deformation leading to transcurrent movement granites may be transported from sites of generation through the mid crust in dyke like channelways within relatively narrow strike slip shear zones which link to major fault zones in the upper crust extensional jogs within fault systems provide developing sites for the assembly of plutons from magma arriving from below the model is based upon observations from the cadomian belt of nw france which exposes sections through middle and upper levels of the late precambrian crust within different elements of the orogen the mechanism provides a favourable alternative to diapirism and explains the common collocation of granites and shear zone fault systems within orogenic belts

rock jointing and abrasion forms on roches moutonnees sw finland the influence of bedrock structures especially joints on the formation of roches moutonnees and different glacial abrasion forms on roches moutonnees were studied in the southwest archipelago of finland the size and form of roches moutonnees are closely connected with the frequency and state of joints in the bedrock the fewer and smaller the joints are the larger are the roches moutonnees that can be formed vertical joints in the direction of the ice movement form the weakest zones and determine the width of the roches moutonnees joints dipping against glacial movement determine the dip of the eroded rock surface horizontal joints provide a suitable base for large and gently dipping roches moutonnees which are normally lower than others with less frequent horizontal joints striae friction cracks facets fracture lines and plucked surfaces and their positions on the surfaces of roches moutonnees were observed on the basis of these observations a stereographic model of the distribution of different abrasion features on the surfaces of roches moutonnees was produced

the mafic silicic layered intrusions of saint jean du doigt france and north guernsey channel islands armorican massif gabbro diorite layering and mafic cumulate pegmatoid association abstract the saint jean du doigt france and north guernsey channel islands intrusive complexes hereafter referred to as sjic and ngic respectively are examples of mafic silicic layered intrusions in the armorican massif both are characterized by the occurrence of 1 a basal peripheral gabbroic unit interlayered with sheets generally dioritic in composition occasionally gabbroic and crossed by leucocratic diapirs and pipes from monzodioritic to q monzonitic in composition 2 peripheral pegmatoids associated with mafic cumulates and 3 coeval granitoids beside these main similarities some contrasted features lead us to propose two distinct models of formation the variscan sjic includes tholeiitic mafic rocks monzogabbro that locally mingle and mix with leucocratic components monzonite or q monzonite the cadomian ngic is calc alkaline the sjic sheet bearing gabbro is homogeneous from a petrologic point of view whereas the ngic exhibits gabbroic macrorhythmic sequences with mineral layering the sr nd isotopic compositions of the sjic gabbros are significantly different from those of the associated dioritic layers this is not the case in the ngic where the magmas could be cogenetic we argue that the sjic gabbro was a liquid that crystallized in situ without significant crystal settling by contrast the rhythmic sequences of the ngic are consistent with crystal accumulation subsequently both can be seen as mafic reservoirs which were repeatedly invaded by magmas of intermediate composition we interpret the sheets in the sjic as the result of horizontal spreading of dioritic metastable magmas into a gabbroic reservoir crystallizing from below at levels of neutral buoyancy injections and convection in the central part of the reservoir possibly resulted in spectacular mixing mingling structures in the ngic the emplacement of the dioritic sheets was rather controlled by pre existing rhythmic cumulative structures in both intrusions late differentiated diapirs were extracted from the dioritic sheets associated peripheral pegmatoids are thought to result from the crystallization of liquids issued from a mafic intercumulus melt in the presence of a fluid phase this extraction might have been enhanced by the disruption of the peripheral cumulate stack perhaps following pressure drops

dynamics of the development of the isle au haut gabbro diorite layered complex quantitative implications for mafic silicic magma interactions the isle au haut igneous complex provides a unique opportunity to examine in detail the in situ physical and chemical interactions between contemporaneously emplaced mafic and silicic magmas the complex contains a 600 m thick sequence of 11 alternating layers of gabbro and diorite typically 15 40 m thick purely on the basis of density contrasts 2 65 g cm 3 gabbro vs 2 55 g cm 3 diorite the entire system should have undergone wholesale instability and mixing it is instead arrested in a grossly unstable state of interaction while molten chilled margins along the lower contacts of the gabbros and structural integrity of the diorite layers indicate that near liquidus gabbroic magma invaded partly crystalline cooler diorite mineral assemblages chemical analyses and phase equilibria calculations indicate initial temperatures during emplacement of 11808c gabbro and 10008c diorite conductive thermal models yield solidification timescales of 15 60 years for single gabbro layers and about a thousand years for the entire complex there is ample evidence for two phases of small scale interfacial rayleigh taylor type instabilities of dioritic melt into the gabbros phase i occurred immediately upon gabbro emplacement whereas evenly spaced slender more silicic pipes represent a much later stage phase ii pipe geometry and spacing estimated viscosities of the gabbroic magma and silicic melt and the sudden increase in silica near the upper contact of the diorite all indicate a thin 18 53 cm buoyant layer at the upper contact of the diorite as the source of the pipes compaction of the diorite produced this layer over a period of about 10 years simultaneous solidification along the lower contact of the overlying gabbro thickening inwards increased viscosity enough to arrest pipe ascent after a few meters crystal size distribution analyses of the gabbro layers yield crystal growth rates go1 4 2 4 10 10 cm s and nucleation rates jo1 410 10 6 cm 3 s indicative of conductive cooling coupled with some sluggish convective stirring owing to collapse of the roof ward gabbro solidification fronts were the complex larger with a much longer solidification time all this evidence would have been lost thus suggesting that in larger systems similar processes may commonly take place leaving little direct evidence of their operation apart from the ultimate final petrological product

streamlined hard beds formed by palaeo ice streams a review fast flowing ice streams occur within modern ice sheets and also operated in pleistocene ice sheets the reconstruction of palaeo ice streams normally relies on the mapping of mega scale glacial lineations msgls and drumlins composed of soft sediment mainly till analysis of new satellite imagery and digital terrain models demonstrates the presence of large fields of kilometre scale glacial lineations comprising rock drumlins megagrooves and megaridges in this paper we describe and analyse a number of such hard bed landform systems from the former laurentide and british irish ice sheets occurring in a variety of palaeo ice stream settings these are attributed to erosion of crystalline and sedimentary rock below fast flowing ice streams bedrock properties such as hardness fracture spacing and bedding and their orientation with respect to ice flow have a profound effect on the occurrence and character of elongate rock bedforms elongate streamlined forms on hard crystalline rock as on the canadian shield only form under special circumstances in contrast sedimentary strata are highly susceptible to form streamlined hard beds specifically if bedrock strike is parallel to ice flow large scale elongate rock bedforms are erosional in origin formed by preferentially focused abrasion or by lateral plucking depending on bedrock type many palaeo ice stream footprints previously mapped in the laurentide ice sheet on the basis of soft bed bedforms are shown to be significantly larger extending up ice across sedimentary strata and onto precambrian crystalline rocks hard bed streamlined forms further show that ice streaming does not necessitate a deformable bed but can equally occur on smooth hard beds

magma mixing in granitic rocks of the central sierra nevada california abstract the el capitan alaskite exposed in the north american wall yosemite national park was intruded by two sets of mafic dikes that interacted thermally and chemically with the host alaskite comparisons of petrographic and compositional data for these dikes and alaskite with published data for sierra nevada plutons lead us to suggest that mafic magmas were important in the generation of the sierra nevada batholith specifically we conclude that 1 intrusion of mafic magmas in the lower crust caused partial melting and generation of alaskite rhyolitic magmas 2 interaction between the mafic and felsic magmas lead to the observed linear variation diagrams for major elements 3 most mafic inclusions in sierra nevada plutons represent chilled pillows of mafic magmas related by fractional crystallization and granitoid assimilation that dissolve into their felsic host and contaminate it to intermediate granodioritic compositions 4 vesiculation of hydrous mafic magma upon chilling may allow buoyant mafic inclusions and their disaggregation products to collect beneath a pluton s domed ceiling causing the zoning mafic margins to felsic core that these plutons exhibit

magma mixing the marsco suite isle of skye scotland magma mixing has been proposed as a major mechanism for the origin of a wide variety of rock suites in mid ocean basalts there are few obstacles to mixing because of the similarities in chemical and physical properties of the proposed end members however in calc alkaline rocks the proposed end members have disparate properties and these present obstacles to mixing

the origin of small scale geochemical and mineralogic variations in a granite intrusion a crystallization and mixing model a post tectonic unzoned granite intrusion in the meatiq dome a late proterozoic metamorphic complex in the central eastern desert of egypt shows significant chemical and mineralogic heterogeneity on the scale of sampling 5 kg whole rock analyses of 21 samples indicate small variations in sio2 al2o3 na2o and k2o and larger systematic variations in the less abundant major elements such as feo tio2 cao mno and mgo and trace elements such as sc cr co rb sr zr la ce nd sm tb yb lu ta th and u these variations cannot be accounted for by processes such as marginal accretion assimilation alteration by late stage fluids or multiple intrusion instead we proposed a model involving 35 percent solidification of the granite magma followed by partial solid liquid segregation during emplacement resulting in rocks containing 7 71 volume percent early formed solids such randomly distributed local segregation could have been caused by filter pressing flow differentiation and possibly gravity segregation either singly or in combination thus each sample is interpreted as a mixture of two end members with nearly constant compositions an early formed solid assemblage of crystals and a complementary residual liquid early formed solids are enriched in tio2 feo cao p2o5 sc co cr sr la ce and depleted in sio2 rb yb lu ta th and u while the residual liquid has complementary enrichments and depletions this simple mixing model is consistent with field and petrographic observations experimental studies pertaining to the crystallization sequence in the system ab or qtz h2o at 1 kb and physical properties of silicic magmas furthermore it is quantitatively supported by trace element data for minerals computed endmember compositions at 35 crystallization using mineral analyses and reasonable kd values and internal consistency in the percent solid for each sample computed from each of 17 elements in the inferred end members we suggest that this model might also apply to other small epizonal granite intrusions that show small scale chemical heterogeneities

the significance of mafic microgranular enclaves in the petrogenesis of the dehno complex sanandaj sirjan belt iran abstract quartz monzodioritic and granodioritic enclaves occur in the upper cretaceous paleocene dehno complex nw of isfahan iran they are hosted by hornblende granodiorite and granodiorite monzogranite respectively the presence of xenocrystic plagioclase biotite quartz magnesio hornblende occasionally with relics of clinopyroxene in quartz monzodiorites minor k feldspar and the lack of peraluminous minerals dominate the mineralogy of these rocks and suggest a metaluminous source the plagioclase in the quartz monzodioritic enclaves has higher an content than that of the respective plagioclase of their host hornblende granodiorite indicating crystallization from more mafic magma the quartz monzodioritic enclaves generally contain higher values of fe 2 o 3 mgo cao mno na 2 o transition elements ni cr co v ree and lower values of sio 2 k 2 o p 2 o 5 and zr than their host which are in consistent with occurrence of abundant ferromagnesian minerals in these rocks compare to their host the granodioritic enclaves have higher amount of fe 2 o 3 mgo mno tio 2 p 2 o 5 rb zr y ni cr co and lower concentration of sio 2 k 2 o and ba aside from hornblende granodiorite and granodiorite monzogranite the dehno complex comprises leucogranodiorite and tourmaline leuco granites geochemically the rocks of the complex are metaluminous to strongly peraluminous and are enriched in lile e g rb k sr ba and depleted in hfse e g nb ta p ti zr hf with medium to high k calc alkaline nature these geochemical features are proposed for the volcanic arc granites emplaced in an active continental margin whole rock and quartz 18 o values for all rocks are high 9 7 14 4 and 10 1 14 9 respectively suggesting the interaction of magma with crustal derived components the strongly peraluminous composition can be interpreted in terms of this hypothesis all these mineralogical and geochemical features support the idea that the granitoid rocks of the dehno complex developed through variable degrees of mixing mingling between a basic magma and a supracrustal melt during subduction of the neotethys oceanic crust beneath central iranian microcontenent the quartz monzodioritic enclaves most likely represent the nearest composition to the original more mafic magma the granodioritic enclaves result from the mixing mingling of globules basic magma with host granitic magma

mantle and crustal sources in the genesis of late hercynian granitoids nw portugal geochemical and sr nd isotopic constraints abstract large volumes of granitoids were emplaced in the hercynian central iberian zone during the last ductile deformation phase d3 300 320 ma the biotite rich granitoids are the most abundant 1 syn d3 granodiorites monzogranites 313 319 ma with calc alkaline and aluminopotassic affinities 2 late d3 granodiorites monzogranites 306 311 ma related to subalkaline and aluminopotassic series these granitoids are associated with coeval gabbro norite to granodiorite bodies and or mafic microgranular enclaves both granitoids and basic intermediate rocks show petrological geochemical and isotopic evidence of interaction between felsic and mafic magmas the mantle derived melts represented by shoshonitic gabbro norites were probably derived from an enriched and isotopically homogeneous source sr i 0 7049 to 0 7053 e nd 2 1 to 2 5 in some syn and late d3 plutons there are evidences of essentially crustal granites represented by moderately peraluminous monzogranites of aluminopotassic affinity they have similar nd model ages 1 4 ga but different isotopic compositions sr i 0 7089 to 0 7106 e nd 5 6 to 6 8 revealing a heterogeneous crust potential protoliths are metasedimentary immature sediments and or felsic meta igneous lower crust materials large amounts of hybrid magmas were generated by the interaction of these coeval mantle and crust derived liquids giving rise to slightly peraluminous monzogranites granodiorites of calc alkaline and subalkaline affinities which display more depleted isotopic compositions than the crustal end members sr i 0 7064 to 0 7085 e nd 4 4 to 6 2 petrogenetic processes involving mingling and or mixing and fractional crystallization at variable degrees in multiple reservoirs are suggested a major crustal growth event occurred in late hercynian times 305 320 ma related to the input of juvenile mantle magmas and leading to the genesis of composite calc alkaline and subalkaline plutons largely represented in the central iberian zone

cryptic magma recharge associated with the most voluminous 20th century eruptions 1921 1948 and 1971 at villarrica volcano abstract villarrica volcano is one of the most active volcanoes of the andes rest cycles of 5 4 3 9 years and erupts products of basaltic to basaltic andesite compositions and in styles from hawaiian to vigorous strombolian we compare whole rock geochemistry and mineral abundances textures and compositions of the most voluminous basaltic lavas of the 20th century which were erupted in 1921 1948 and 1971 the analyzed lavas show a restricted range in whole rock compositions 51 7 52 4 wt sio2 but there is an increasing trend of mgo content from 1921 5 3 wt mgo to 1948 7 0 wt mgo and a decreasing trend from 1948 to 1971 6 0 wt mgo two groups of plagioclase compositions are observed in all lavas an poor an60 and an rich an80 an poor compositions are the most abundant in all lavas and were commonly formed in both the earliest cores and latest stages external rims and small and nearly unzoned phenocrysts of plagioclase crystallization whereas an rich plagioclase formed around an poor cores during intermediate stages additionally precipitation of an poor plagioclase in both rims and cores could simultaneously have occurred in highly dissolved an rich antecrysts equilibrium temperatures of plagioclase olivine clinopyroxene of about 1090 c were obtained in crystal clots of the three lavas thermometry calculations obtained from melts simulations of plagioclase formation indicate a heating thermal mixing of the reservoir at 0 5 kbar of up to 100 c with respect to the crystal clots temperatures incorporation of small amounts of volatile rich and hotter mafic magma of similar composition cryptic mixing into the villarrica reservoir could heat it probably in a more efficient way than heat conduction alone a longer interaction of the hot mafic magma within the reservoir could cause crystallization of an rich plagioclase dissolution of an poor plagioclase i e increase in an rich an poor modal content ratios and compositional modifications toward mgo rich and volatile rich compositions that ultimately result in higher intensity of eruption

laboratory investigations of viscous effects in replenished magma chambers abstract some laboratory experiments are described which investigate the dynamical effects of replenishment of a magma chamber containing high viscosity magma by hotter denser and much more fluid magma in the experiments a layer of hot kno3 solution is emplaced beneath cold glycerine which has a viscosity 3000 times greater less dense fluid is released immediately and continuously from the interface as a result of crystallization in the lower layer and rises as plumes through the overlying glycerine further crystallization occurs in the plumes and the crystals fall out but there is little mixing between the two fluids and a layer of depleted kno3 solution forms at the top the experiments demonstrate that interfacial processes begin to dominate where there are large viscosity differences between adjacent fluid layers as would be the case in a rhyolitic magma chamber replenished by basaltic magma

the significance of rock structure lithology and pre glacial deep weathering for the shape of intermediate scale glacial erosional landforms the main landforms within the glacially scoured precambrian rocks of the swedish west coast are closely connected to the principal structural pattern and have lately been explained as mainly a result of etch processes probably during the mesozoic and with a possible second period of etching during the neogene to explore the effect of multiple glacial erosion on the rock surfaces an island with two different lithologies and with striae from different directions was selected for a detailed study focusing on the shape of roches moutonnees air photo interpretation of bedrock lineaments and roches moutonnees combined with detailed field mapping and striae measurements are used to interpret the structural and lithological control on the resulting shape the study reveals a significant difference in shape between roches moutonnees in augen granite and orthogneiss low elongated and streamlined roches moutonnees occur in the gneiss area striated by a late weichselian ice flow from the ne this ice flow is subparallel with both the local dominant trend of topographically well expressed joints and the schistosity of the gneiss frequently there are no signs of quarrying on the lee sides of the gneiss roches moutonnees and hence they resemble the shape of whalebacks or ruwares as typically associated with the exposed basal weathering surface found in tropical areas the granite roches moutonnees were formed by an older ice flow from the ese which closely followed the etched wnw ese joint system of the granite late weichselian ice flow from the ne caused only minor changes of the landforms on the contrary marks of the early ese ice flow are poorly preserved in the gneiss area where it probably never had any large effect as the flow was perpendicular to both schistosity and structures and accordingly also to the pre glacial relief the study demonstrates that coincidence between ice flow direction and pre glacially etched structures is most likely to determine the effects of glacial erosion copyright 2002 john wiley sons ltd

extensional pluton emplacement models do they work for large plutonic complexes shallow crustal to mid crustal magma emplacement models commonly rely on local or regional extension to solve the room problem we argue that existing extensional models explain the emplacement of sheeted dike complexes or internally layered plutons but are not as easily reconciled with the emplacement of elliptically shaped plutons or batholiths for the following reasons 1 rock strengths and crustal stresses prevent large voids from existing at depth and therefore require wall rock extension to be balanced by magma influx 2 rates of extension are controlled by fault slip rates of millimetres to centimetres per year and thus require numerous small injections of magma and 3 because of their sheetlike shapes and small thicknesses these small magma injections will cool rapidly and form sheeted plutons or dike swarms extension may however facilitate emplacement of elliptically shaped plutons in concert with other mechanisms if certain kinematic constraints are satisfied

mixing between granitic and dioritic crystal mushes guernsey channel islands uk abstract the contact zone between the cobo granite and bordeaux diorite complex of guernsey channel islands uk displays numerous features which result from the interaction of these two penecontemporaneously emplaced intermediate to felsic magmas initial interaction resulted in the formation of chilled mafic enclaves in granite magma as thermal equilibrium was approached some physical mixing took place to produce a heterogeneous marginal facies continued interaction resulted in incorporation of previously mixed magma into intruding magma the early mixed material is locally preserved as enclaves but more commonly underwent disaggregation promoted by its incompletely crystallised nature the mineral components becoming distributed as xenocrysts and often as microenclaves or glomeroxenocrysts into surrounding magma further modification within the contact zone was brought about by the infiltration of melt through the interconnected pore space of the magma mushes on the scale of centimetres to hundreds of metres these processes produced geochemical profiles which do not exhibit perfect mixing trends the petrographic and geochemical features described not only demonstrate the efficacy of mixing between partially crystallised magma mushes of broadly similar composition but also provide criteria by which such interaction may be recognised elsewhere many features in particular mineral scale disequilibria and small scale modal heterogeneity bear striking similarities to those which occur widely in granite plutons where obvious evidence for magma mixing is absent as such it is possible that many granite bodies preserve a subtle record of hitherto overlooked mixing events

evidence for cold based glacial activity in the allan hills antarctica evidence is presented for cold based glacial erosion deposition and deformation from the allan hills south victoria land antarctica different erosional features such as scrapes striae and grooves depositional features including till isolated boulders and ice cored debris cones and three scales of glaciotectonism resulting from cold based glacial advance are described and conceptual models are presented based on these observations and those of advancing cold based glaciers elsewhere the models entail i ice block apron overriding and entrainment and ii ice bed separation leading to the formation of a cavity on the down glacier side of escarpments the models are most applicable to a horizontally stratified lithified sedimentary bedrock substrate but our criteria may assist in correctly interpreting features such as boulder trains modified bedrock tors and complex cosmogenic exposure histories which have been noted in several regions that may have experienced cold based glaciation during pleistocene glacial maxima

control of glacial quarrying by bedrock joints abstract a principal assumption of models that describe bedrock quarrying by glaciers is that cracks in the bed are small and isolated these cracks are assumed to grow slowly in response to glacial loading to eventually delimit the areas of quarried surfaces if this assumption is correct then quarried surface orientations will be controlled by orientations of principal stresses in the bed associated with sliding and resultant ice bed separation downglacier from rock ledges and roches moutonnees rather than by orientations of joints that predate glacial loading to test this hypothesis we compared orientations of quarried surfaces and preglacial joints in nine recently deglaciated forefields of glaciers in canada and switzerland with different bedrock lithologies and bedforms in all but one forefield quarried surfaces and major joint sets are coincident with orientations of quarried surfaces bearing no systematic relationship to sliding direction other than their obvious tendency to not face upglacier a difference in sliding direction of 64 between two areas of one glacier forefield had no effect on the orientation of quarried surfaces with the same joint set exploited in both areas these observations motivate a new characterization of subglacial bedrock that differs from that of existing quarrying models the bed is idealized as a group of rock blocks separated by preglacial joints slow crack growth necessary for quarrying would be confined to small bodies of rock bridges that connect the faces of joints this model of the bed does not allow it to be treated as an intact elastic solid with hydraulically isolated cracks as is usually assumed which would have a profound influence on characterization of quarrying mechanics

mafic microgranular enclave swarms in the chenar granitoid stock nw of kerman iran evidence for magma mingling abstract mafic microgranular enclaves mme are common in the early to middle miocene chenar granitoid stock northwest of kerman which is a part of central iranian eocene volcanic belt they occur individually and in homogeneous or heterogeneous swarms the mme form a number of two dimensional structural arrangements such as dykes small rafts vortices folded lens shapes and late swarms the enclaves are elongated rounded to non elongated and subrounded in shape and often show some size sorting parallel to direction of flow variation in the elongation of enclaves could reflect variations in the viscosity of the enclave the time available for enclave deformation and differential strain during flow of the host granitoid magma the most effective mechanism in the formation of enclave swarms in the chenar granitoid stock was velocity gradient related convection currents in the granitoid magma chamber gravitational sorting and the break up of heterogeneous dykes also form mme swarms the mme mainly diorite to diorite gabbro have igneous mineralogy and texture and are marked by sharp contacts next to their host granitoid rocks the contact is often marked by a chilled margin with no sign of solid state deformation evidence of disequilibrium is manifested in feldspars by oscillatory zoning resorbed rims mantling and punctuated growth together with overgrowth of clinopyroxene amphibole on quartz crystals the acicular habit of apatites and the development of fe ti oxides along clinopyroxene cleavages these observations suggest that the mmes are derived from a hybrid magma formed as a result of the intrusion of a mafic magma into the base of a felsic magma chamber the density contrast between hybrid magma and the overlying felsic magma was reduced by the release of dissolved fluids and the ascent of exsolved gas bubbles from the mafic magma into the hybrid zone further convection in the magma chamber dispersed the hybridized magma as globules in the upper parts of magma chamber where their entrainment and coalescence eventually formed swarms magma mingling between felsic and hybrid magmas led to the formation of mmes and swarms

mixing basaltic and dacitic magmas by forced convection although magma mixing is an old concept which has been applied to many magma types recent petrographic studies have revived interest in the theory by suggesting that it may account for the origin of calk alkaline magma1 6 and mid ocean ridge basalt7 9 there has been no previous experimental investigation into how two magmas with different properties can mix to form a homogeneous magma of andesitic composition or to produce textures such as observed in banded dacite we present here the results of experiments demonstrating that basaltic and dacitic magmas can be easily mixed by forced convection to form both banded dacite and homogeneous andesite in less than a few hours

re examining pluton emplacement processes abstract previous pluton emplacement studies have attempted to explain how space is made during pluton emplacement in fact the only means of making space during emplacement of mantle derived magmas in the crust are 1 lowering the moho or 2 outward displacement of the earth s surface other pluton emplacement mechanisms are material transfer processes mtps that do not increase the volume of the crust one frequently cited mtp for emplacement of concentrically zoned and ballooning plutons is ductile flow of wall rocks around the plutons the expected structures and total strains near such plutons are dependent on the three dimensional pluton shape the width of the deforming aureole whether the pluton is a piercing or non piercing diapir and in the latter case the number of body radii the pluton travels we have re examined a few such plutons and calculated the amount of material transfer caused by ductile flow in no case is more than 40 and in most cases only 15 35 of the required material transfer accommodated by wall rock flow the contact aureoles around these plutons are too narrow and or strains too low indicating that the plutons are predominantly discordant bodies with narrow 0 1 0 4 pluton radii concordant aureoles a comparison of strains in natural contact aureoles and those made in mechanical models of diapirs indicates that natural strains are at least an order of magnitude less than model strains we suggest that the similarities between models of diapiric ascent of spherical bodies and concentrically zoned plutons are superficial because the models are too simplistic instead we argue that the ascent and emplacement of magmas require multiple near field and far field mtps these mtps will show gradients with depth distance from the magma and with time rates of near field mtps must be rapid whereas far field rates are probably nearer to long term average rates of orogenic processes

microstructural evidence of orders of crystallization in granitoid rocks experimental melting studies of granitoid rocks have documented variations in the order of crystallization of minerals depending on the melt composition total pressure and the activity of water and other volatiles in the melt microstructural criteria for independent determination of the order of crystallization are needed to permit application of the experimental data to the evaluation of the conditions of crystallization unfortunately few criteria can be reliably applied to infer either the order of initial crystallization on the order in which minerals cease to crystallize in granitoid rocks most microstructures of granitoid rocks record simultaneous rather than sequential crystallization of minerals grain size criteria e g phenocrysts vs groundmass can be used to infer a partial order of crystallization in many volcanic rocks but the presence of extremely large phenocrysts of k feldspar that from experimental data must be the last mineral to crystallize highlights the dangers inherent in the use of grain size criteria providing it can be determined that included minerals did not form subsequently along cracks or from entrapped melt inclusions those inclusions that are demonstrably and consistently only near the centre of large grains must have ceased to crystallize before the host mineral where the host mineral occurs only as rims on the early formed mineral it is possible to infer both the order of cessation and initiation of crystallization common examples are provided by the rimming produced by a discontinuous reaction relationship most other examples of inclusion relationships could result from simultaneous crystallization microstructures involving moulding and impingement relationships are unreliable as the microstructure is produced only after the minerals have begun to crystallize in the melt or on the melt solid interface as most minerals continue to crystallize right to the solidus they cease to crystallize simultaneously new criteria perhaps involving detailed chemical zoning patterns need to be developed before the orders of crystallization can be reliably determined for granitoid rocks

development of joint controlled rock basins in bohus granite sw sweden the granite area of bohuslan along the swedish west coast is characterised by an abundance of rock basins of different sizes within a 163 km2 large part of the area over 400 basins occur ranging

quaternary evolution of glaciated gneiss terrains pre glacial weathering vs glacial erosion vast areas previously covered by pleistocene ice sheets consist of rugged bedrock dominated terrain of innumerable knolls and lake filled rock basins the cnoc and lochan landscape or landscape of areal scour these landscapes typically form on gneissose or granitic lithologies and are interpreted 1 either to be the result of strong and widespread glacial erosion over numerous glacial cycles or 2 formed by stripping of a saprolitic weathering mantle from an older deeply weathered landscape we analyse bedrock structure erosional landforms and weathering remnants and within the cnoc and lochan gneiss terrain of a rough peneplain in nw scotland and compare this with a geomorphologically similar gneiss terrain in a non glacial arid setting namaqualand south africa we find that the topography of the gneiss landscapes in nw scotland and namaqualand closely follows the old bedrock saprolite contact weathering front the roughness of the weathering front is caused by deep fracture zones providing a highly irregular surface area for weathering to proceed the weathering front represents a significant change in bedrock physical properties glacial erosion and aeolian erosion in namaqualand is an efficient way of stripping saprolite but is far less effective in eroding hard unweathered bedrock significant glacial erosion of hard gneiss probably only occurs beneath palaeo ice streams we conclude that the rough topography of glaciated cnoc and lochan gneiss terrains is formed by a multistage process 1 long term pre glacial chemical weathering forming deep saprolite with an irregular weathering front 2 stripping of weak saprolite by glacial erosion during the first glaciation s resulting in a rough land surface broadly conforming to the pre existing weathering front etch surface 3 further modification of exposed hard bedrock by glacial erosion in most areas glacial erosion is limited but can be significant beneath palaeo ice stream the roughness of glaciated gneiss terrains is crucial for modelling of the glacial dynamics of present day ice sheets this roughness is shown here to depend on the intensity of pre glacial weathering as well as glacial erosion during successive glaciations

potential effects of subglacial water pressure fluctuations on quarrying water pressure fluctuations beneath glaciers may accelerate rock fracture by redistributing stresses on subglacial bedrock and changing the pressure of water in bedrock cracks to study the potential influence of water pressure fluctuations on the fracture of subglacial bedrock ice flow over a small bedrock step with a water filled cavity in its lee is numerically modeled and stresses on the bedrock surface are calculated as a function of transient water pressures in the cavity stresses on the bed are then used to calculate principal stress differences within the step rapid reductions in cavity water pressure increase principal stress differences in the bed increasing the likelihood of crack growth in the step and the formation of predominantly vertical fractures relatively impermeable bedrock may be most susceptible to fracturing during water pressure reductions because high water pressure in cracks within the rock can be maintained as water pressure decreases in cavities these results when considered in conjunction with the strong likelihood that increases in water pressure accelerate the removal of rock fragments loosened from the bed suggest that in zones of ice bed separation where water pressure fluctuations typically are large rates of quarrying may be higher than along other parts of glacier beds

tropical marine climate during the late paleozoic ice age using trace element analyses of brachiopods abstract the late paleozoic ice age can be considered an important analogue to the modern ice age but comparisons between the two time intervals have been hampered by the difficulty of resolving climatic changes that occur over short e g seasonal time scales in the paleozoic record as a first step toward overcoming these limitations this study employed mg ca trace element ratios sampled across the growth bands of six specimens of the brachiopod composita to assess differences in tropical marine mean temperature and seasonality in visean and moscovian time these time intervals bracket the onset of the late paleozoic ice age which occurred in mid serpukhovian time and thus provide a snapshot of climate changes from a greenhouse to an icehouse world shells were screened for diagenetic alteration by cathode luminescence and by comparing the values of mn sr na and b to those reported for modern brachiopods in addition a 87sr 86sr value measured for one specimen agreed with a published strontium isotope curve for the late paleozoic era both the mean and range of mg ca values were larger in visean than moscovian time suggesting colder and less seasonal temperatures during the late paleozoic ice age the decline in seasonality was due to a greater proportional reduction of maximum values which presumably represent summer temperatures than minimum values these relative differences between the pre ice age and ice age tropical climate provide a first step toward understanding the broad patterns of seasonal scale changes in temperature during a critical time in earth s history