on the antarctic peninsula batholith the plutonic rocks of the antarctic peninsula magmatic arc form one of the major batholiths of the circum pacific rim the antarctic peninsula batholith is a 1350 km long by 210 km wide structure which was emplaced over the period 240 to 10 ma with a cretaceous peak of activity that started at 142 ma and waned during the late cretaceous early jurassic and late jurassic early cretaceous gaps in intrusive activity probably correspond to episodes of arc compression in a northern zone of the antarctic peninsula the batholith intrudes palaeozoic mesozoic low grade meta sedimentary rocks and in a central zone it intrudes schists and ortho and paragneisses which have late proterozoic nd model ages and were deformed during triassic to early jurassic compression in a southern zone the oldest exposed rocks are permian sedimentary rocks and deformed jurassic volcanic and sedimentary rocks all these pre batholith rocks formed a belt of relatively immature crust along the gondwana margin with few exceptions jurassic plutons crop out only within the central zone many are peraluminous having s like mineralogies and relatively high 87sr 86sri they are considered to consist largely of partial melts of upper crust schists and gneisses and components of mafic magmas that caused the partial fusion by contrast early cretaceous plutons crop out along the length of the batholith few magma compositions appear to have been affected by upper crust the bulk being compositionally independent of the type of country rock they intrude they are dominated by metaluminous calcic si oversaturated 1 type granitoid rocks with relatively low 87sr 86sri intermediate silicic compositions 5 mgo we interpret these to represent partial melts of basic to intermediate igneous locally garnet bearing lower crust contemporaneous mafic magmas e g syn plutonic dykes form a more alkaline si saturated series having higher 143nd 144nd at the same87sr 86sr than the intermediate silicic series to which they are not petrogenetically related the change from limited partial fusion of upper crust in jurassic times to widespread partial fusion of lower crust in early cretaceous times is considered to be a result of an increasing volume of basaltic intrusion into the crust with time

dehydration melting of amphibolite at 10 kbar the effects of temperature and time we have simulated the dehydration melting of a natural low k calcic amphibolite 67 4 hornblende 32 5 anorthite in piston cylinder experiments at 10 kbar and 750 1000 c for 1 9 days the solidus temperature is lower than 750 c garnet appears at 850 c the overall reaction is hb p l cpx al hb ca hb ga opx three stages of reaction are 1 melting dominated by the growth of clinopyroxene and garnet with little change in composition of liquid or garnet 2 a reversal of this reaction between 875 c and 900 c with decreases in the amounts of liquid and garnet and 3 a large increase in liquid along with the loss of hornblende and decrease of plagioclase while clinopyroxene and garnet increase garnet is enriched in pyrope and zoned from fe cores to mg edges range 3 mol pyrope liquid composition is enriched first in an to 950 c and then in ab the liquids are more calcic and aluminous than natural tonalites which is attributed to the plagioclase composition an90 the formation of peraluminous liquid from the metaluminous amphibolite is caused by anorthite not h2o saturated conditions the results are consistent with an amphibolite phase diagram with relatively high solidus temperatures in the garnet absent field 900 1000 c but with a solidus backbend at 7 9 kbar coincident with the garnet in boundary hornblende breakdown due to garnet formation in a closed system must make h2o available for h2o undersaturated melting right down to the h2o saturated solidus below 700 c which defines a large low temperature pt area where hydrous granitoid melts can be generated with residual garnet and hornblende

low melt fraction connectivity of granitic and tonalitic melts in a mafic crustal rock at 800 c and 1 gpa in the absence of an externally applied stress the segregation of small amounts of granitic or tonalitic melts from their residual mafic crystals is possible only if the melt forms an interconnected network phase accordingly this research focuses on melt connectivity at low melt fraction 4 wt or 5 vol connectivity of granitic and tonalitic melts in amphibole rich rock was assessed by performing two types of piston cylinder experiments at 1 gpa and 800 c the first involved annealing samples that consisted of either alternating layers or homogeneous mixtures of calcic amphibole and metaluminous obsidian powder the second type of experiment involved creating diffusion couples here an upper cylinder of amphibole saturated granitic or tonalitic melt was placed against a lower cylinder consisting of an amphibole rich rock containing zero or a small melt granitic or tonalitic fraction the upper part of the diffusion couple was doped with emitter 151sm or 14c and functioned as an infinite melt reservoir the lower part of the diffusion couple was considered to be the host rock the experiments approached textural equilibrium which allowed us to characterize the wetting behaviour of the calcic amphibole by the hydrous silicic melt granitic or tonalitic these particular experiments also provided information concerning diffusive transport because the emitter could diffuse through the connected melt liquid in the amphibole rich rock the dihedral angle measurements show that melt connectivity was achieved this conclusion is based on the fact that the dihedral angles consistently yielded median apparent values of 53 58 for an amphibole rich rock granitic melt system and 46 48 for an amphibole rich rock tonalitic melt system however the frequency distribution of angles is found to be relatively broad the results of the diffusion couple experiments assessed using the radiographic technique complement the dihedral wetting angle measurements by showing that melt connectivity is achieved at a melt fraction less than 4wt 5 vol

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

the growth of the zimbabwe craton during the late archaean an ion microprobe u pb zircon study abstract u pb zircon ages obtained by ion microprobe are presented for late archaean granitoids from the zimbabwe craton and the north marginal zone of the limpopo belt similar ages of crust formation in the felsic gneisses of the zimbabwe craton and the felsic granulites of the north marginal zone support the view previously argued on geological grounds that they represent the middle and lower crustal layers respectively of the zimbabwe craton zircons from the southern margin of the zimbabwe craton and the northern part of the north marginal zone contain old cores confirming the presence of an old c 3 5 ga nucleus to the zimbabwe craton and indicating that the areal extent of this old nucleus is greater than has been hitherto supposed thus late archaean crustal growth involved both the reworking of and the addition of new magmas to this old crustal nucleus at 2 7 ga crustal growth was contemporaneous with plume volcanism in the greenstone belts in contrast crustal growth at 2 74 2 67 2 64 and 2 62 ga was focused at the margins of the old craton9 and may be related to andean style continental margin magmatism lower crustal melting at 2 61 2 59 ga led to the emplacement of granitoids in the upper crust supplementary material a summary of previously published u pb zircon geochronology for the zimbabwe craton and the north marginal zone of the limpopo belt field relationships and sample localities for the samples used in this study and u th pb secondary ionization mass spectrometry data are available at www geolsoc org uk sup18466

adakite differentiation and emplacement in a subduction channel the late paleocene sabzevar magmatism ne iran this study describes the structural setting petrogenesis and geochronology of a suite of acidic magmatic rocks that are intruded in the metamorphic core of the tertiary ophiolitic suture zone of the sabzevar range ne central iran these intrusive bodies show tabular geometries with solid state fabrics documenting syntectonic emplacement during crustal shortening in the total alkalis silica tas diagram their compositions define a medium k calc alkaline suite spanning from basaltic andesite to the dacite and rhyolite fields they show characteristic low mgo 0 15 0 60 wt and ni 40 ar 39 ar white mica and amphibole geochronology constrains the sabzevar magmatism to the late paleocene at ca 58 ma genesis of the sabzevar magmatic suite is interpreted in terms of prograde wet amphibolite melting during oceanic subduction within a pressure temperature range between a plagioclase out and a hornblende out boundary magma differentiation and high pressure amphibole fractionation of pristine slab melts are proposed as the dominant factors that imparted the adakite signature in the sabzevar structural zone implications in terms of the regional tectonic scenario are discussed and framed within the advancing and retreating evolution of the neotethyan subduction during the mesozoic tertiary time span

towards a coupled physical and chemical model for tonalite trondhjemite granodiorite magma formation abstract tonalite trondhjemite granodiorite ttg magmatism provides a window into the composition of the earth s earliest crust and on rates of heat transfer during the archaean however the tectonic setting in which these magmas form and the physical mechanisms by which ttg melt segregates from its partially molten source rock are poorly understood here we present some simple models aimed at predicting the compositions of partial melts which segregate from thickened mafic amphibolitic or eclogitic lower crust our results suggest that buoyancy driven compaction with melt flow along grain edges can yield large volumes of segregated ttg melt over geologically realistic timescales 4000 years 10 my petrologic diversity is predicted even from a homogenous protolith the chemical composition of the segregated melt varies both spatially and temporally and is governed not only by the composition and mineralogy of the source rock the depth of melting and the melting reactions but also by the physical processes through which the melt migrates and segregates from its partially molten host this simple model represents the first step towards a properly coupled physical and chemical model of ttg formation which is closely constrained by geochemical field and laboratory data such models will compliment the existing geochemical work and help resolve some of the outstanding issues concerning ttg petrogenesis

trace element and isotopic effects arising from magma migration beneath mid ocean ridges abstract the trace element concentrations and isotopic ratios in the magma erupted on mid ocean ridges may differ from those in the source material due to physical effects such as porous flow dispersion exchange of trace elements between the fluid and solid phases during magma migration and convective mixing in magma chambers these differences are in addition to those produced by better known processes such as fractional cyrstallization and partial melting the effects of the three former processes are described it is predicted that magma typically reaches the sub ridge magma chambers with a spatial heterogeneity only slightly reduced from that of the source material but with a subdued variation in time convective mixing then further reduces the spatial heterogeneity application of the results for convective mixing to a recent fourier analysis of 87 sr 86 sr variations along the mid atlantic ridge suggests that the falloff in amplitude of variation observed with decreasing wavelength in the mid atlantic ridge data cannot be explained by convective mixing in magma chambers instead it is postulated that this falloff is due to the mechanics of the production and or the solid state convective mixing of chemical and isotopic heterogeneities in the solid mantle

a simple approach to define holocene sequence stratigraphy using borehole and cone penetration test data cone penetration testing has been widely used since the 1950s for determining the subsurface geotechnical conditions of unconsolidated sediments this paper highlights the potential value of cone penetration testing as an aid to define the stratigraphic structure of holocene sedimentary deposits by calibrating cone penetration test logs with adjacent borehole logs and by utilizing all the available information produced during geotechnical surveying stratigraphic models that accurately describe the vertical and lateral boundaries as well as the stacking pattern of late quaternary systems can be constructed the widespread application and technical simplicity of cone penetration testing combined with simple data interpretation via correlation with adjacent borehole logs yield a useful and inexpensive tool for sedimentological investigations this methodology is illustrated using data from 36 cone penetration tests and 11 boreholes on the holocene deltaic plain of the aliakmon river greece sedimentological and stratigraphic information from core log correlations the spatial distribution of cone penetration test parameters sediment grain size and per cent concentration of organic matter are utilized the results suggest that in sequence stratigraphic terms the delta is divided into a lowstand systems tract composed by fluvial gravels and sands u0 of late pleistocene age as well as from red oxidized clays and a transgressive systems tract represented by fluvial channel sands u1 overlain by a thin transgressive sand sheet of coastal origin u2 characterized by fining upward trends the highstand systems tract is constituted by a variety of stratigraphic units u3 to u7 and depositional environments characterized by coarsening upward sequences representing both aggradational and progradational facies and dominated by the presence of three prograding wedges detailed definition of the thickness vertical boundaries and stacking pattern of the resolved stratigraphic units presented as a two dimensional stratigraphic model demonstrates the applicability of the proposed method

granitic magma transport by fracture propagation granitic magmas commonly ascend tens of kilometres from their source terranes to upper crustal emplacement levels or to the earth s surface apart from its obvious bearing on the interpretation of the geology and geochemistry of felsic igneous rocks the magma ascent mechanism critically affects any modelling of the metamorphic evolution of the upper lithosphere as well as its rheology we propose that in general granitic magmas ascend via propagating fractures as dykes in extremely short time periods long distance diapiric transport of granitoid magmas through crustal sections is not viable on thermal or mechanical grounds and there is an apparent total absence of field evidence for diapiric rise of such magmas even in supposed type localities neither the shapes nor internal or external characteristics of granitic plutons necessarily reveal anything about the transport of their precursor magmas these are purely arrival phenomena dictated by local structure kinematics and stress states based on existing numerical treatments of the problem we show that granitic magmas are apparently sufficiently inviscid to travel through fractures to high crustal levels without suffering thermal death by freezing for example a 2000 km3 granitic batholith can be inflated by a 1 km 3m 20 km deep dyke system in less than 900 years the model proposed has numerous far reaching petrological and rheological consequences some of which are outlined

melt segregation in the continental crust before melt will segregate from its restite and source there must be both a driving force and space into which the melt can move the continental crust is anisotropic and in most cases is subject to deviatoric stresses during melting under these conditions melt segregation begins when an interconnected network of melt forms and the source becomes permeable melt is driven by pressure gradients from its restite toward dilatant sites set up in the heterogeneously deforming source the source is continually drained of melt and so need never contain more melt than that required to maintain permeability 26 where the solid matrix breaks down and diapirism can occur

the generation of sodic granite magmas western palmer land antarctic peninsula subduction related mesozoic to cainozoic granites s l in western palmer land antarctic peninsula have similar chemical compositions to archean tonalite trondhjemite granodiorite ttg suites phanerozoic slab melts adakites and to experimental partial melts of basaltic material in equilibrium with amphibole pyroxene garnet they are predominantly sodic metaluminous and most have al2o3 15 wt and y 36 km between triassic and tertiary times the initial nd t of the magmatism increased and sr t decreased suggesting that new continental crust was produced during this period underplating by mafic magma was an important crustal growth mechanism in the arc the generation of abnormally thick crust and its subse quent fusion is considered to be a consequence of ca 180 ma of subduction and associated magmatism in the region an implication of the model is that dense garnet amphibolite and eclogite residues from partial melting of the lower crust will accumulate in theory the setting was appropriate for such residues to detach from the base of the crust and to sink into the convecting mantle such a process would leave the rest of the crust enriched in large ion lithophile elements lree but depleted in hree y

evolution of major and trace element composition during melt migration through crystalline mush implications for chemical differentiation in the crust we present the first quantitative model of heat mass and both major and trace element transport in a mush undergoing compaction that accounts for component transport and chemical reaction during melt migration and which is applicable to crustal systems the model describes the phase behavior of binary systems both eutectic and solid solution with melt and solid compositions deter mined from phase diagrams using the local temperature and bulk composition trace element concentration is also determined the results demonstrate that component transport and chemical reaction generate compositional variation in both major and trace elements that is not captured by existing geochemical models in particular we find that even for the simplest case of a homogenous insulated column that is instantaneously melted then allowed to compact component transport and reaction leads to spatial variations in major element composition that in this case produces melt that is more enriched in incompatible elements than predicted by batch melting in deep crustal hot zones dchz created by the repeated intrusion of hot mantle derived magmas buoyant melt migrating upwards accumulates in high porosity layers but has a composition corresponding to only a small fraction of batch melting because it has locally equilibrated with mush at low temperature moreover melt migration and chemical reaction in a layered protolith may lead to the rapid formation of high porosity melt layers at the interface between different rock compositions in both of these cases the melt in the high porosity layer s is less enriched in incompatible trace elements than predicted if it is assumed that melt with the same major element composition was produced by batch melting this distinctive decoupling of major and trace element fractionation may be characteristic of magmas that originate in dchz application of the model to a number of crustal systems including the ivrea verbano zone the rum layered intrusion and the holyoke flood basalt suggests that composi tional heterogeneity can be explained by buoyancy driven melt migration and compo nent transport through a reactive crystalline mush

between beds and sequences stratigraphic organization at intermediate scales in the quaternary of the virginia coast usa stratigraphy presupposes a hierarchy of scales of spatial organization supplemented at the small scale end by sedimentological concepts beds bed sets and bed cosets and at larger spatial scales by sequence stratigraphic concepts systems tracts parasequences sequences between these two end members are intermediate sclae bodies described as lithofacies or simply facies a more restricted concept granulometric facies can be described in terms of horizontal grain size gradients facies change and cvclic vertical grain size gradients stratification assemblages of facies so defined also called depositional systems are not random loot occur in a limited suite of patterns such assemblages may be linked to two classes of bounding surfaces a source diastem the immediate source of the sediment and a surface of closure if preserved between which is sandwiched a transgressive or regressive basinward fining facies succession systems bounding surfaces are notably more continuous than internal gradational facies boundaries by thus restricting the definition of a facies assemblage depositional system it is possible to describe the quaternary of the virginia coast wish as few as 12 systems depositional systems in the quaternary of the virginia coast are allometric in that any system can be derived from any other by plastic expansion of one or more facies relative to another or by simple symmetry operations self similarity prevails across this intermediate scale of stratigraphic organization facies assemblages depositional systems consist of event beds which themselves have erosional basal boundaries and internal successions of microfacies at larger spatial so ales depositional systems are repeated either autocyclic repetitions forced by processes within the basin of deposition or allocyclic repetitions as parasequences and high frequency sequences in the virginia quaternary systems architecture is compatible with sequence architecture and nests conformably within its framework but analysis of systems architecture reveals rules beyond those governing sequence architecture

generation of sodium rich magmas from newly underplated basaltic crust sodium rich rocks of trondhjemite tonalite dacite ttd or granodiorite ttg suites form much of precambrian continental crust1 they are thought to have formed by partial melting of subducted oceanic crust2 3 a process that would have been much more widespread early in earth history than at present owing to the higher thermal gradients prevailing at that time4 phanerozoic ttd suites do exist however and seem also to relate to subduction zones5 defant and drummond6 proposed that these suites form where young 25 myr hot oceanic lithosphere is subducted and melts thus locally simulating the conditions that led to widespread crustal growth in the archaean here we describe plutonic and volcanic rocks from the cordillera blanca complex in peru which have characteristics of the high al ttd suite but which were produced above a subduction zone containing a 60 myr old slab we present evidence that the complex formed by partial melting of newly underplated basaltic crust and argue that this mechanism should be considered more generally as an additional way of generating sodium rich arc magmas

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

diffusion in silicate minerals and glasses a data digest and guide to the literature the rates of diffusion of cations oxygen and hydrogen in silicates are of fundamental importance to a wide range of petrological and geochronological problems in order that the earth scientist may readily apply the available experimentally determined diffusion rates to such problems a compilation of data for diffusion in silicate minerals and glasses is presented along with a working guide for its use published data and experimental conditions are listed in tables and these are preceded by an outline of diffusion mechanisms terminology equations and units diffusion regimes factors controlling diffusion rates measurement techniques and diffusional behaviour

rheology of granitic magmas during ascent and emplacement flow abstract considerable progress has been made over the past decade in under standing the static rheological properties of granitic magmas in the continental crust changes in h2o content co2 content and oxidation state of the interstitial melt phase have been identified as important compositional factors governing the rheodynamic behavior of the solid fluid mixture although the strengths of granitic magmas over the crystallization interval are still poorly constrained theoretical investigations suggest that during magma ascent yield strengths of the order of 9 kpa are required to com pletely retard the upward flow in meter wide conduits in low bagnold number magma suspensions with moderate crystal contents solidosities 0 1 0 3 viscous fluc tuations may lead to flow differentiation by shear enhanced diffusion ams and mi crostructural studies support the idea that granite plutons are intruded as crystal poor liquids 50 with fabric and foliation development restricted to the final stages of emplacement if so then these fabrics contain no information on the ascent verti cal transport history of the magma deformation of a magmatic mush during pluton emplacement can enhance significantly the pressure gradient in the melt resulting in a range of local macroscopic flow structures including layering crystal alignment and other mechanical instabilities such as shear zones as the suspension viscosity varies with stress rate it is not clear how the timing of proposed rheological transitions formulated from simple equations for static magma suspensions applies to mixtures undergoing shear new theories of magmas as multiphase flows are required if the full complexity of granitic magma rheology is to be resolved

dike transport of granitoid magmas thermal and fluid dynamical analyses suggest that for viscosities and density contrasts spanning the range considered typical for many calc alkalic granitoids dike ascent is a viable mechanism for the transport of large volumes of granitoid melt through the continental crust we present calculations showing that a granitoid melt with calculated viscosity of the order of 10 6 pa s and a density contrast between magma and crust of 200 kg m 3 can be transported 30 km through the crust in 1 month corresponding to a mean ascent velocity of 1 cm s using analysis modified from numerical studies of the flow of basaltic magmas in dikes we also present an expression that allows the calculation of the critical minimum dike or fault width required for granitic magma to ascend without freezing for all reasonable estimates of cordilleran granitoid viscosity and density contrast the critical dike width is determined to be between 2 and 7 m calculated peak batholith filling rates are orders of magnitude greater than mean cavity opening rates based on estimated fault slippage which is consistent with chemical evidence for intermittent supply of magma pulses

an experimentally based model for the origin of tonalitic and trondhjemitic melts abstract the results of melting experiments have been used to establish equations relating melt composition to temperature t pressure p and water content of the source y h2o for tonalitic and trondhjemitic melts produced by partial melting of average archean tholeiite aat a normative classification diagram for these rocks has been translated into a t p y h2o diagram showing the required conditions of partial melting tonalites form at high temperatures low pressures and high water contents while trondhjemites form at lower temperatures higher pressures and lower water contents as pressure increases the k content increases but the melts never achieve the composition of a granodiorite or granite a method for estimating the conditions for the formation of natural tonalites and trondhjemites is proposed the method has been successfully tested against other experimental studies and natural rocks where the temperature pressure conditions have been constrained by traditional geochemical methods

evolution of the hazelton arc near terrace british columbia stratigraphic geochronological and geochemical constraints on a late triassic early jurassic arc and cu au porphyry belt understanding the development of island arcs that accreted to the north american craton is critical to deciphering the complex geological history of the canadian cordillera in the case of the hazelton arc part of the stikine terrane or stikinia in northwestern british columbia understanding arc evolution also bears on the formation of spatially associated porphyry cu au epithermal and volcanogenic massive sulfide deposits the hazelton group is a regionally extensive long lived and exceptionally thick upper triassic to middle jurassic volcano sedimentary succession considered to record a successor arc that was built upon the paleozoic and triassic stikine and stuhini arcs in central stikinia near terrace british columbia the lower hazelton group telkwa formation comprises three volcanic intrusive complexes mt henderson mt o brien and kitselas that at their thickest constitute almost 16 km of volcanic stratigraphy basal telkwa formation conglomerates and volcanic rocks were deposite

observations on the origins and ascent mechanisms of granitic magmas this review presents a summary of most current beliefs about the origins of granitic magmas together with calculations of some important properties of granitic melts and magmas turning attention to magma ascent mechanisms diapirism and dyking represent two competing end member models the differences in processes and products between the two are fundamental and the implications of the differences are profound for the evolution of the continental crust the weight of evidence favours dyke ascent for the majority of granitic magmas and modelling shows that dyke ascent is both rapid and thermally effcient furthermore the structural and petrological features expected in crust through which granitic diapirs have passed have not been discovered in nature granites in the upper crust commonly have forms and structures indicating that diapirism could not have been the emplacement mechanism in contrast granitic dykes are common suggesting that it is reasonable to consider the ascent of granitic magmas through fracture systems many granitic batholiths are tabular and some have been fed by large dykes the rate limit in granitic plutonism is not magma ascent but either the rate of melting and segregation near the magma source or the rate of deformation at the emplacement site

melting kinetics of a plagioclase feldspar partial melting experiments on plagioclase feldspar have been carried out to investigate textures and kinetics of the melting a labradorite single crystal was heated at one atmosphere pressure and temperatures within its melting interval as a function of time so called honeycomb fingerprint or sieve textures were produced except for the runs just below the liquidus the melting was initiated by heterogeneous nucleation of melt at the surface and or interior cracks and possively dislocations of the crystal the pattern of the melt is dendritic with a few m arm spacing after the melt develops throughout the crystal the volumes of melt and residual crystal become larger and smaller respectively without changing the arm spacings the melt is homogeneous and has the approximate temperature dependent liquidus composition irrespective of the time there are compositional gradients in the residual crystal after short periods of melting the an content of the crystals increases with increasing time until it finally reaches equilibrium with the melt after several thousands minutes of heating it is concluded that the enlargement of the melt the main process of the melting is controlled by diffusion in the crystal the fact that partial melts have the composition of the equilibrium liquidus even from the first several minutes strongly suggests that the local equilibrium at the crystal liquid interface is satisfied during the melting some of the honeycomb fingerprint and sieve textures found in xenoliths and phenocrysts of sodic plagioclase in volcanic rocks would be caused by heating events such as magma mixing during which temperatures of magmas were temporarily higher than the solidus of some of the minerals

fluvial response to sea level changes since the latest pleistocene in the near coastal lowland central kanto plain japan abstract this paper addresses the influence of sea level changes on the stratigraphy and basal topography of latest pleistocene holocene incised valley fill of the arakawa and menuma lowlands in central japan by using sedimentary drill cores and columnar geologic sections in the study area four buried fluvial terrace surfaces i to iv formed in response to sea level fall during the last glacial period surface iii is covered with as yp tephra 15 16 5 ka and extends at least 65 km landward from the present mouth of the arakawa river a buried valley named surface v that extends from at least 85 km upstream from the present river mouth to about 120 m below present sea level on the continental slope was incised in response to the sea level lowstand of the last glacial maximum the incised valley fill can be divided into three units unit a gravelly river channel sediments including valley bottom gravels unit b fluvial sediments and unit c marine sediments which intertongues within unit b and pinches out up to 60 km upstream from the present river mouth unit c marks the landward limit of holocene transgression the lower part of unit b is a fining upward succession formed during rising sea level and indicates retrogressive aggradation of floodplain sediments during transgression whereas the upper part of unit b was formed during a sea level highstand the floodplain migrated more than 85 km upstream from the modern shoreline in response to sea level rise until the middle holocene and extends over several tens of kilometers beyond the landward limit of the holocene marine transgression

partial melting of subducting oceanic crust the conditions under which partial melting of subducting oceanic crust occurs can be determined by combining a partial melting model for basaltic compositions with two dimensional thermal models of subduction zones for porosities of 1 containing h20 the amount of partial melt generated at the wet basaltic solidus is limited to 100 mpa can be maintained by rocks close to or above their melting temperatures in the absence of high shear stresses substantial melting of the oceanic crust will only occur during subduction of very young 5 ma oceanic lithosphere partial melting of hydrated basalt amphibolites derived from the mid ocean ridge has been proposed e g 1 3 as being responsible for the generation of certain recent high al andesitic to dacitic volcanic rocks adakites three of these volcanic suites mount st helens southern chile and panama occur in volcanic arcs where oceanic crust 25 ma is being subducted at rates of 1 3 cm yr and the calculated thermal regime is several hundreds of degrees hotter than more typical subduction zone environments however oceanic lithosphere is not currently being subducted beneath baja and new guinea where recent adakites are also present suggesting that some adakite magmas may form by water undersaturated partial melting of underplated mafic lower crust or previously subducted oceanic crust further experimental work on compositions representative of oceanic crust is required to define the depth of possible adakite source regions more accurately

magmatic life at low reynolds number the extremely high viscosities of high silica liquids mean that intermediate and high silica magmas exist deep within the laminar range of fluid behavior many common interpretations of layering in plutonic rocks rely on analogies with sedimentation in water but for such high viscosity liquids including interstitial liquid in partially crystallized intermediate magmas these analogies cannot be correct particle reynolds numbers for high silica liquids are exceedingly small on the order of 10 6 or less in such a regime inertia is negligible and everyday experience is inapplicable layering and other pseudo sedimentary structures in granodiorites and granites likely arise from processes such as chemical diffusion self organization and crystal ripening

late quaternary depositional sequences and landforms in relation to sea level changes in the osaka intra arc basin japan a borehole database analysis abstract development of late quaternary depositional sequences in the osaka intra arc basin was reconstructed based on the analysis of sequence stratigraphy and new shazam stratigraphy using numerous and high density borehole data the latest pleistocene to holocene sequence i and the lower late pleistocene sequence ii underwent several significant phases 1 formation of alluvial fans and terraces during regression from marine isotope stage mis 5 to mis 2 2 formation of a boundary between the two sequences during rapid sea level fall of ca 30 20 ka 3 development of ravinement surfaces by wave and tidal erosion during the transgression from mis 2 to mis 1 4 development of barrier systems and coastal cliffs during the transgression to the maximum high stand of 6 5 ka and 5 progradation of alluvial fans deltas and strand plains during the high stand since 5 ka depositional facies and systems of the two sequences are different among the western study area around kobe the central area with the nishinomiya amagasaki lowland and the eastern area with the osaka plain due to differences in sediment flux wave tide shore current basement and landforms distribution such as plateau and hills development of the sequence ii could be also affected by tectonic differences which fault fold structure and flexure three regions with different landforms and depositional systems were identified within the osaka basin the systems of the three regions correspond to the three major types of the latest pleistocene to holocene transgressive regressive depositional sequences and alluvial plains recognized in japan

estuarine barrier island to strand plain sequence and related ravinement surface developed during the last interglacial in the paleo tokyo bay japan abstract recognition of sequence boundaries and transgressive surfaces i e ravinement surfaces rs is now known to be of great importance in stratigraphy the sedimentary features of deposits immediately above a transgressive surface are well exposed in the upper pleistocene kioroshi formation of the kanto plain in central japan the formation comprises mainly coastal and shallow marine deposits estuarine barrier island and the strand plain systems which accumulated along a wavedominated coast in the late pleistocene i e the last interglacial to last glacial period the kioroshi formation is bounded above and below by sequence boundaries that formed in the lowstand periods correlative to the glacial periods of oxygen isotope stages 4 and 6 respectively a significant transgressive surface that was formed by landward migration of barrier islands during the transgressive interval the ravinement surface rs is found within the deposits of the upper shelf environment this ravinement surface is characterized by the exotic nature of the overlying sediment veneer pebbles shells and scattered mud clasts which is poorly sorted the rs shows a very flattened erosional surface in the shore parallel sense and the gradient of the surface in shore normal sense is calculated as 0 0021 where the syndepositional tectonic movement is revised the rs commonly cuts through the lower sequence boundary however in the places where the river or tidal channel valleys incised the valley filling sediment shows a deepening upward sequence recognized as a transgressive systems tract and the rs can be clearly distinguished from the lower sequence boundary

sediment accumulation patterns in a tectonically subsiding incised valley insight from the echigo plain central japan abstract the echigo plain is on the japan sea coast of the island of honshu and is bounded to the west by the echigo plain western margin fault zone the shinano river incised valley lies east of the bounding fault zone and runs seaward approximately parallel to it the incised valley deepens to more than 160 m below mean sea level near the coast deeper than sea level fall during the last glacial maximum because of subsidence governed by the fault zone in this study we examined the sediment stacking patterns of the incised valley fill at 1000 year scale by analyzing sedimentary facies and radiocarbon ages from three sediment cores from the echigo plain the latest pleistocene to holocene shinano river incised valley fill sediments unconformably overlie a basement of late pleistocene flood plain sediments that contain aira tn at tephra dated at 25 120 220 bp ca 30 cal kyr bp the fill sequence comprises sediments of meandering river 12 cal kyr bp estuarine 13 9 cal kyr bp and barrier lagoon 9 0 cal kyr bp systems in ascending order thousand year isochrons based on radiocarbon ages show that the meandering river and estuarine systems were deposited aggradationally and retrogradationally and the barrier lagoon system was deposited progradationally and aggradationally these sediment stacking patterns resemble those of other incised valley fill sequences in east and southeast asia which indicates that the last deglacial eustasy was a major factor controlling sediment stacking patterns of incised valley fill in these regions however aggradational sedimentation in all three depositional systems is only observed in the echigo plain and suggests that long term subsidence at 2 7 3 8 mm yr is an important factor controlling the 1000 year scale sediment stacking pattern in this tectonically subsiding incised valley

quantitative modeling of granitic melt generation and segregation in the continental crust 1 we present a new quantitative model of granitic in a broad sense melt generation and segregation within the continental crust we assume that melt generation is caused by the intrusion of hot mantle derived basalt and that segregation occurs by buoyancy driven flow along grain edges coupled with compaction of the partially molten source rock we solve numerically the coupled equations governing heating melting and melt migration in the source rock and cooling and crystallization in the underlying heat source our results demonstrate that the spatial distribution and composition of the melt depends upon the relative upward transport rates of heat and melt if melt transport occurs more quickly than heat transport then melt accumulates near the top of the source region until the rock matrix disaggregates and a mobile magma forms as the melt migrates upward its composition changes to resemble a smaller degree of melting of the source rock because it thermodynamically equilibrates with rock at progressively lower temperatures we demonstrate that this process of buoyancy driven compaction coupled with local thermodynamic equilibration can yield large volumes of mobile granitic magma from basaltic and meta basaltic amphibolitic protoliths over timescales ranging from 4000 years to 10 myr the thickness of basaltic magma required as a heat source ranges from 40 m to 3 km which requires that the magma is emplaced over time as a series of sills concurrent with melt segregation these findings differ from those of previous studies which have suggested that compaction operates too slowly to yield large volumes of segregated granitic melt

melt segregation in migmatites anatectic stromatic migmatites have a symmetrical layered structure with a low ratio of thickness to length and a periodicity features that have not been explained satisfactorily but which are related to physical processes of melt segregation we evaluate the compaction model for segregation as it applies to migmatites and develop models for melt segregation based upon convection driven by volume change and upon advection down pressure gradients that result from applied differential stress acting on an anisotropic multilayer protolith compaction by gravity driven two phase flow results in asymmetric separation yields calculated segregation times that are slow or geologically unreasonable and yields relative volumes that are not consistent with the abundance of leucosome seen in natural migmatites although calculated segregation times for very wet granite might allow segregation by compaction some driving force in addition to gravity is needed to cause widespread melt segregation in the crust for segregation at low volume fraction of melt we develop a two dimensional two phase matrix and fluid viscous flow cell model length much greater than thickness with phase changes which predicts either expansion or contraction convection depending on whether volume change vr is positive or negative convection leads to melt segregation to form a stromatic structure in a geologically reasonable timescale at moderate volume fraction of melt segregation may occur by filter pressing in compositionally layered rocks in response to applied differential stress melt migration is by porous medium flow driven by differences in mean normal stress between the layers as a consequence of differences in rheology and shear enhanced matrix collapse calculated segregation rates are fast and the model yields adequate volume of leucosome the positive vr for volatile phase absent melting reactions under crustal pressures promotes melt escape unless extensional deformation facilitates melt accumulation if the rate of melt production exceeds the rate of melt escape then the increase in melt pressure may lead to melt enhanced embrittlement and fracture and melt may migrate out of the system in contrast water rich volatile phase present melt producing reactions have a negative vr which promotes melt retention after segregation unless deformation generates melt escape pathways evolution of stromatic structure through anatectic erosion of mesosome by increasing the melt fraction in situ may lead to breakdown of the solid matrix and will lead to instability due to buoyancy and magma may become mobile and entrain residual material restite

partial melting of two amphibolites contrasting experimental results under fluid absent conditions a large portion of the lower continental crust may be amphibolitic in composition and without a free fluid phase as a consequence h2o undersaturated or fluid absent melting of amphibolites may be responsible for the formation of some granites and migmatites produced during major orogenic events in an attempt to determine the systematics of melting under fluid absent conditions a series of piston cylinder experiments was conducted on two natural amphibolites one a meta alkali basalt aba with a total water content of wt contained in hornblende and the other a meta island arc tholeiite iat which has 1 1 3 wt water contained in hornblende cummingtonite and biotite the experimentally determined melting ranges of the two amphibolites showed that the solidus temperatures and sta temperature interval over which amphibole was stable were controlled by the amphibolites different bulk compositions and their resulting metamorphic assemblages the volume of melt produced by melting of the two amphibolites were compared with estimated amounts based on burnham s 1979 water melt solubility model and the fluid absent melting model presented by clemens and vielzeuf 1987 the observed melt volumes were greater than estimated as the water content of melt largely detemines the volume of melt produced independent measurements of the water content of the glass formed during partial melting in the aba were made by thermogravimetric analyses the water content of the aba glass is 2 wt which is less than the assumed melt water content water content of the melt used in previous modeling of fluidabsent anatexis in mafic lithologies as a consequence more melt can be expected during fluid absent partial melting of mafic lower crust as is observed in the experiments a modification of the clemens and vielzeuf 1987 fluid absent melting model for mafic compositions has been made using the experimental data available on melting in basaltic systems and is presented here for pressures of 5 8 and 10 kbar tectonic scenarios in which the crust is thickened i e by collision then undergoes extension or where a previously thinned crust is later rethickened provide enough heat so that amphibolite melting under fluid absent conditions can become importan and hence responsible for some melts produced during post collisional magmatism the results may also have applications to melting in hydrated oceanic crust in subduction zones and in island arc terains

vertically extensive and unstable magmatic systems a unified view of igneous processes volcanoes are an expression of their underlying magmatic systems over the past three decades the classical focus on upper crustal magma chambers has expanded to consider magmatic processes throughout the crust a transcrustal perspective must balance slow plate tectonic rates of melt generation and segregation in the lower crust with new evidence for rapid melt accumulation in the upper crust before many volcanic eruptions reconciling these observations is engendering active debate about the physical state spatial distribution and longevity of melt in the crust here we review evidence for transcrustal magmatic systems and highlight physical processes that might affect the growth and stability of melt rich layers focusing particularly on conditions that cause them to destabilize ascend and accumulate in voluminous but ephemeral shallow magma chambers

dehydration melting of a basaltic composition amphibolite at 1 5 and 2 0 gpa implications for the origin of adakites this study presents the results of dehydration melting experiments on a basaltic composition amphibolite under conditions appropriate to a hot slab geotherm 1 5 and 2 0 gpa and temperatures of 850 to 1150 c dehydration melting produces an omphacitic augite and garnet bearing residue coexisting with rhyolitic to andesitic composition melts at 1 5 gpa the amphibolite melts in two stages between 800 and 1025 c the 2 0 gpa data also define two melting stages at 2 0 gpa the first stage involves nearly modal melting of the original amphibolite minerals qtz pl amp to produce melt cpx grt during the second stage the eclogite restite melts non modally 0 86 cpx 0 14 grt 1 melt the experimental results were combined with data from the literature to generate a composite p t phase diagram for basaltic composition amphibolites over the 800 to 1100 c temperature range for pressures up to 2 0 gpa comparison of the major element compositions of the experimentally produced melts with compositions of presumed slab melts adakites shows that partial melting of amphibolite at conditions appropriate to a hot slab geotherm produces melts similar to andesitic and dacitic adakites except for significant mgo and cao depletions trace element modelling of amphibolite dehydration melting using the 2 0 gpa melting reactions produces ree abundances similar to those of adakites at 10 15 wt batch melting but the models do not reproduce the high sr y ratios characteristic of adakites taken together the major and trace element results are not consistent with the derivation of adakites by dehydration melting of the subducted slab with little or no interaction with the mantle wedge or crust if adakites are partial melts of the subducted slab they must undergo significant interaction with the mantle and or crust during which they acquire a number of their distinctive characteristics

episodic rapid sea level rises on the central mediterranean shelves after the last glacial maximum a review abstract submerged palaeo shorelines on the central mediterranean shelves identified from high resolution seismic profiles and bathymetric data mark distinct water depths at which sea level stationed for a period of time during the relative sea level rise that followed the last glacial maximum lgm the shorelines are commonly represented by palaeo coastal cliffs and barrier beaches that lie today at water depths between 100 m and 70 m and between 65 m and 40 m in most places irrespective of the different tectonic contexts even in the presence of significant vertical rates these morphological features are thought to have been drowned during melt water pulses 1a and 1b which occurred between 15 and 10 ka the evidence presented here confirms drowned shorelines documented elsewhere at similar water depths and shows that melt water pulses have punctuated the post glacial relative sea level rise with rates up to 60 mm yr for a few centuries the identification of morphological features related to melt water pulses in the central mediterranean sea has important implications to improve our knowledge on episodes of rapid glacio eustatic sea level rise this issue is critical to be able to forecast future sea level rises in the mediterranean which is characterized by densely populated coasts and important coastal infrastructures

strain localization and porphyroclast rotation it has been debated for decades whether rigid inclusions such as porphyroclasts and porphyroblasts do or do not rotate in a softer matrix during deformation experiments and numerical simulations with viscous matrix rheologies show ongoing rotation of circular inclusions whereas using mohr coulomb plasticity results in nonrotation because the rocks in which inclusions are found normally undergo deformation by dislocation creep we applied a full field crystal plasticity approach to investigate the rotation behavior of rigid circular inclusions we show that the inclusion9s rotation strongly depends on the anisotropy of the matrix minerals strongly anisotropic minerals will develop shear bands that reduce the rotation of inclusions inhibition of rotation can only occur after a significant amount of strain our results may help to explain why geologic rigid objects often show evidence for rotation but not necessarily in accordance with the viscous theory that is usually applied to these systems

liquid segregation parameters from amphibolite dehydration melting experiments we have experimentally obtained some of the parameters necessary for understanding the segregation of silicate liquid from amphibolite undergoing dehydration melting at 1 gpa and 750 1000 c the solidus for this calcic amphibolite 68 mg hb 32 pl an 90 is 750 c amphibolites begin to melt at relatively high temperatures in the garnet absent field but the solidus appears to backbend at 1 gpa coincident with the garnet in boundary hornblende breakdown due to garnet formation releases h 2o and causes melting thus in the garnet present field 1 gpa the amphibolite dehydration melting solidus may be coincident with the h 2o saturated solidus liquid interconnectivity may be achieved at 900 c and 5 vol liquid based on both physical and chemical data from solid rock runs mass balance calculations from powdered rock runs suggest that small amounts 5 15 vol of hydrous 4 wt h 2o low viscosity 10 3 10 4 pa s heavy rare earth element depleted felsic liquid may be segregated during amphibolite dehydration melting at 875 c the rapid breakdown of coarse grained hb cores may lead to the formation of transient h 2o saturated liquids with even lower viscosities 10 2 pa s although comprising only a small portion of the melting cycle these h 2o rich conditions may enhance the segregation of liquid by reduction of liquid viscosities and by mechanical effects on the restite e g increased deformation and liquid fracturing during anatexis of coarse grained natural rocks transient conditions may control the initial stages of liquid segregation

silicic magmas of protector shoal south sandwich arc indicators of generation of primitive continental crust in an island arc protector shoal the northernmost and most silicic volcano of the south sandwich arc erupted dacite rhyolite pumice in 1962 we report geochemical data for a new suite of samples dredged from the volcano geochemically the dredge and 1962 samples form four distinct magma groups that cannot have been related to each other and are unlikely to have been related to a single basaltic parent by fractional crystallization instead the silicic rocks are more likely to have been generated by partial melting of basaltic lower crust within the arc trace element and sr nd isotope data indicate that the silicic volcanics have compositions that are more similar to the volcanic arc than the oceanic basement formed at a back arc spreading centre and volcanic arc basalts are considered to be the likely source for the silicic magmas the south sandwich islands are one of several intra oceanic arcs tonga kermadec izu bonin that have 1 significant amounts of compositionally bimodal mafic silicic volcanic products and 2 6 0 6 5 km s 1 p wave velocity layers in their mid crusts that have been imaged by wide angle seismic surveys and interpreted as intermediate silicic plutons geochemical and volume considerations indicate that both the silicic volcanics and plutonic layers were generated by partial melting of basaltic arc crust representing an early stage in the fractionation of oceanic basalt to form continental crust

post glacial sea level change along the pacific coast of north america abstract sea level history since the last glacial maximum on the pacific margin of north america is complex and heterogeneous owing to regional differences in crustal deformation neotectonics changes in global ocean volumes eustasy and the depression and rebound of the earth s crust in response to ice sheets on land isostasy at the last glacial maximum the cordilleran ice sheet depressed the crust over which it formed and created a raised forebulge along peripheral areas offshore this combined with different tectonic settings along the coast resulted in divergent relative sea level responses during the holocene for example sea level was up to 200 m higher than present in the lower fraser valley region of southwest british columbia due largely to isostatic depression at the same time sea level was 150 m lower than present in haida gwaii on the northern coast of british columbia due to the combined effects of the forebulge raising the land and lower eustatic sea level a forebulge also developed in parts of southeast alaska resulting in post glacial sea levels at least 122 m lower than present and possibly as low as 165 m on the coasts of washington and oregon as well as south central alaska neotectonics and eustasy seem to have played larger roles than isostatic adjustments in controlling relative sea level changes

can slab melting be caused by flat subduction slab melting has been suggested as a likely source of adakitic arc magmas i e andesitic and dacitic magmas strongly depleted in y and heavy rare earth elements existing numerical and petrologic models however restrict partial melting to very young 5 ma oceanic crust typically at 60 80 km depth paradoxically most of the known pliocene quaternary adakite occurrences are related to subduction of 10 45 ma lithosphere which should not be able to melt under normal subduction zone thermal gradients we propose an unusual mode of subduction known as flat subduction occurring in 10 of the world s convergent margins that can produce the temperature and pressure conditions necessary for fusion of moderately old oceanic crust of the 10 known flat subduction regions worldwide eight are linked to present or recent 6 ma occurrences of adakitic magmas observations from chile ecuador and costa rica suggest a three stage evolution 1 steep subduction produces a narrow calc alkaline arc typically 300 km from the trench above the asthenospheric wedge 2 once flat subduction begins the lower plate travels several hundred kilometers at nearly the same depth thus remaining in a pressure temperature window allowing slab melting over this broad distance and 3 once flat subduction continues for several million years the asthenospheric wedge disappears and a volcanic gap results as in modern day central chile or peru the proposed hypothesis which reconciles thermal models with geochemical observations has broad implications for the study of arc magmatism and for the thermal evolution of convergent margins

geochemistry of amphibolitized eclogites and cross cutting tonalitic trondhjemitic dykes in the metamorphic kimi complex in east rhodope n e greece implications for partial melting at the base of a thickened crust in the ultra high pressure metamorphic kimi complex widespread tonalitic trondhjemitic dykes with an intrusion age ca 65 63 ma cross cut boudins and layers of amphibolitized eclogites geochemical investigation proclaims the tied genetic relationship of the amphibolitized eclogites and the associated tonalitic trondhjemitic dykes the major and trace element contents and rare earth element patterns of the amphibolitized eclogites indicate formation of their protoliths by fractional crystallization of tholeiitic magmas in a back arc environment the tonalites and trondhjemites are characterized by moderate to high sr contents 130 ppm and low y 8 2 ppm and heavy rare earth element contents yb content of 0 19 0 88 ppm the chemical composition of the tonalitic and trondhjemitic dykes are best explained by partial melting of a tholeiitic source like the amphibolitized eclogites with residual garnet and amphibole at the base of a thickened crust during early tertiary subduction accretion at the southern margins of the european continent

dubious case for slab melting in the northern volcanic zone of the andes it was first suggested by r w kay that adakites may represent melts of subducted slab since then the term adakite has become synonymous with slab melts based on their unusual geochemical signature this contribution using the northern volcanic zone of the andes as an example aims 1 to expose the weakness in simply associating a geochemical signature with a genetic mechanism and 2 to underline the importance of using several integrated lines of evidence in assessing the viability of slab melting we conclude that although slab melting probably occurs in some arcs regional geochemical trends in the northern volcanic zone and their relationship to the subduction zone architecture are not indicative of slab melting and can be accounted for by normal arc magmatic processes acting on wedge derived basaltic magmas

dynamic particle segregation and accumulation processes in time and space revealed in a modern river dominated delta a spatiotemporal record of the kiso river delta central japan abstract the holocene succession of the kiso river delta a sand rich river dominated delta in central japan was analyzed at high temporal and spatial resolution the results demonstrate how the spatiotemporal distribution of sedimentary facies gross sediment accumulation rates and accumulation rates of mud versus sand relate to the geomorphic development of the delta and the post glacial sea level history the centers of peak mud and sand accumulation have migrated seaward in parallel during a regressive stage from 6 5 ka to the present at the rate of 3 9 m y the mud and sand accumulation centers mark the bottom and top respectively of the delta front slope the sediments of the prograding delta show strong sorting and clear upward coarsening because of the steep slope of the delta front such internal topographic control redistributes and separates the coarse and fine fractions of the fluvial input during regression the topographic controlled sedimentary segregation constructs delta front topography in reverse therefore the proportion of sand to mud in the fluvial input controls the stability of the delta front morphology our modeling of the kiso river delta may have implications for prediction of coastal flooding adjacent to deltas in other parts of the world

petrology of the chilliwack batholith north cascades washington generation of calc alkaline granitoids by melting of mafic lower crust with variable water fugacity calc alkaline granitoid rocks of the oligocene pliocene chilliwack batholith north cascades range from quartz diorites to granites 57 78 sio2 and are coeval with small gabbroic stocks modeling of major element trace element and isotopic data for granitoid and mafic rocks suggests that 1 the granitoids were derived from amphibolitic lower crust having ree rare earth element and sr nd isotopic characteristics of the exposed gabbros 2 lithologic diversity among the granitoids is primarily the result of variable water fugacity during melting the main effect of fh 2 o variation is to change the relative proportions of plagioclase and amphibole in the residuum the ree data for intermediate granitoids quartz diorite granodiorite eu eu 0 84 0 50 are modeled by melting with fh 2 o 1 kbar leaving a plagioclase pyroxene residuum in contrast data for leucocratic granitoids leuco granodiorites and granites eu eu 1 0 0 54 require residual amphibole in the source and are modeled by melting with fh 2 o 2 3 kbar consistent with this model isotopic data for the granitoids show no systematic variation with rock type 87sr 86sri 0 7033 0 7043 end 0 3 3 to 5 5 and overlap significantly with data for the gabbroic rocks 87sr 86sri 0 7034 0 7040 end 0 3 3 to 6 9 the fh 2 o variations during melting may reflect additions of h2o to the lower crust from crystallizing basaltic magmas having a range of h2o contents chillwack gabbros document the existence of such basalts one dimensional conductive heat transfer calculations indicate that underplating of basaltic magmas can provide the heat required for large scale melting of amphibolitic lower crust provided that ambient wallrock temperatures exceed 800 c based on lithologic and geochemical similarities this model may be applicable to other cordilleran batholiths

a combined geochronological approach to investigating long lived granite magmatism the shap granite uk abstract with the advent of more precise dating methods it has become apparent that zircon dates from granite plutons frequently indicate older emplacement ages than other dating methods here we attempt to reconcile a number of dating methods from the c 5 km2 caledonian shap granite northern england the results reveal a more complex and protracted evolution than indicated by application of any single dating method zircon u pb dates give a weighted mean age of 415 6 1 4 2 ma a mafic enclave dated at 412 2 2 ma revised rb sr feldspar age from davidson et al 2005 contains resorbed k feldspar and zircon crystals scavenged from the host crystal mush these ages are at odds with field relations in the thermal aureole that suggest final emplacement at approximately 404 ma or later during acadian deformation previously reported re os ages on molybdenites associated with magmatic fluids have given ages of 405 2 1 8 2 ma selby et al 2008 and confirm the overlap of at least some magmatic activity with acadian deformation a similar emplacement age is supported by rb sr whole rock mineral and biotite k ar dates when adjusted for revised decay constants 402 3 ma and 401 7 ma respectively wadge et al 1978 the lower closure temperatures of these systems relative to the u pb system in zircon means that they are more likely to record the timing of final granite emplacement these data suggest that most zircons grew before final granite emplacement by about 10 ma on average we suggest that the majority of zircon crystals record pre emplacement magmatic activity within a deeper part of the system mafic enclaves and their scavenged cargo of crystals record the assembly of a mid crustal batholith where crystals remained at least locally mobile at 412 ma gravity data support the existence of an extensive 1500 km2 intrusive body originally at about 15 km depth beneath shap this batholith is likely to have remained below the granite solidus for much of its existence due to conductive heat loss but episodic influxes of silicic magma between c 412 and 405 ma are thought to have enabled periods of rejuvenation these influxes are recorded by complex compositional zoning patterns within k feldspar megacrysts the shap granite itself is likely to represent a rejuvenated crystal slurry emplaced as a cylindrical cupola above the main magma body during acadian transpression this study highlights the importance of integrating different dating techniques and that final emplacement of granites can only be indicated by the youngest zircon ages

chemical differentiation cold storage and remobilization of magma in the earth s crust the formation storage and chemical differentiation of magma in the earth s crust is of fundamental importance in igneous geology and volcanology recent data are challenging the high melt fraction magma chamber paradigm that has underpinned models of crustal magmatism for over a century suggesting instead that magma is normally stored in low melt fraction mush reservoirs 1 9 a mush reservoir comprises a porous and permeable framework of closely packed crystals with melt present in the pore space1 10 however many common features of crustal magmatism have not yet been explained by either the chamber or mush reservoir concepts1 11 here we show that reactive melt flow is a critical but hitherto neglected process in crustal mush reservoirs caused by buoyant melt percolating upwards through and reacting with the crystals10 reactive melt flow in mush reservoirs produces the low crystallinity chemically differentiated silicic magmas that ascend to form shallower intrusions or erupt to the surface11 13 these magmas can host much older crystals stored at low and even sub solidus temperatures consistent with crystal chemistry data6 9 changes in local bulk composition caused by reactive melt flow rather than large increases in temperature produce the rapid increase in melt fraction that remobilizes these cool or cold stored crystals reactive flow can also produce bimodality in magma compositions sourced from mid to lower crustal reservoirs14 15 trace element profiles generated by reactive flow are similar to those observed in a well studied reservoir now exposed at the surface16 we propose that magma storage and differentiation primarily occurs by reactive melt flow in long lived mush reservoirs rather than by the commonly invoked process of fractional crystallization in magma chambers14 magma storage and differentiation in the earth s crust mainly occurs by reactive melt flow in long lived mush reservoirs rather than by fractional crystallization in magma chambers as previously thought

self organisation and fracture connectivity in rapidly heated continental crust abstract volume expansion 1 5 volume strain with v melting positive and fluid absent partial melting in which v melting is positive of continental crust by intruding basaltic magma is a strongly irreversible process involving the dissipation of both thermal energy and matter partial melt using a simple random graph model we show by analogy how isolated fractures that form during rapid thermal perturbation in the source region can combine to form a single interconnected structure with high permeability once connected the fracture network may be thought of as a single structure or pattern that will remain stable so long as a strong temperature gradient is maintained in the source region estimates of fracture permeability that take into account changes in connectivity and fracture spacing range from approximately 10 10 to 10 5 m 2 many orders of magnitude greater than values considered typical during large scale crustal deformation and prograde regional metamorphism the ability of the isotropic fracture network to develop a top bottom directionality is crucial for buoyancy driven melt transport a physical model based on non linear evolution rules during thermal expansion is given that predicts the emergence of directionality vertical fracture alignment on a time scale of the order of 10 5 y the necessary ingredients are a deviatoric strain path a heterogeneous medium and a stiffness that evolves as a function of the local strain