microgranular enclaves as evidence of rapid cooling in granitoid rocks the case of the los pedroches granodiorite iberian massif spain microgranular enclaves from the los pedroches granodiorite lpg los pedroches batholith iberian massif spain have sr nd isotopic and mineral chemical compositions close to those of their host this similarity is not related to restite unmixing as indicated by the igneous textures of the enclaves a number of other geological and geochemical lines of evidence including the high ree and hfse contents of the microgranular enclaves relative to the host granitoid strongly suggest that this similarity cannot be explained by magma mixing alternatively a crystallization process by rapid cooling within the host granitoid magma could explain the geochemical and textural characteristics of the microgranular enclaves including shape grain size mineralogy texture chemistry and sr nd isotopic composition such a crystallization occurred at the walls of the magma conduits through which the granitic magmas were emplaced in the upper crust this process should be considered as an alternative hypothesis to magma mixing for the generation of some microgranular enclaves especially where no direct evidence exists for the presence of basic magmas coeval with granitoids and where there is a lack of isotopic contrast between hosts and enclaves as the process is favoured by feeder dyke related emplacement we suggest that abundance of microgranular enclaves can be related to the mechanism of emplacement of granitoid bodies

mineralogy and geochemistry of microgranular enclaves in palaeoproterozoic malanjkhand granitoids central india evidence of magma mixing mingling and chemical equilibration palaeoproterozoic ca 2 480 ma felsic magmatism of malanjkhand region of central indian precambrian shield referred to as malanjkhand granitoids mg contain xenoliths of country rocks and mesocratic to melanocratic fine grained porphyritic microgranular enclaves me the shape of me is spheroidal ellipsoidal discoidal elongated and lenticular varying in size from a few centimeters to about 2 m across the contact of me with the host mg is commonly sharp crenulate and occasionally diffuse which we attribute to the undercooling and disaggregation of me globules within the cooler host mg the me as well as mg show hypidiomorphic texture with common mineral hbl bt kfs pl qtz assemblage but differ in modal proportions the variation in minerals composition presence of apatite needles elongated biotites resorbed plagiclase ocellar quartz and other mafic felsic xenocrysts strongly oppose the restite and cognate origins of me compositions of plagioclases an3 an29 amphiboles mg mg fe2 0 55 0 69 and biotites mg mg fe2 0 46 0 60 of me are slightly distinct or similar to those of mg which suggest partial to complete equilibration during mafic felsic magma interactions al in amphibole estimates the mg pluton emplacement at ca 3 4 0 5 kbar and therefore magma mixing and mingling must have occurred at or below this level the hbox fe rightleftharpoons hbox mg substitution in biotites of me and mg largely suggests subduction related calc alkaline metaluminous i type nature of felsic melts most major and trace elements against sio2 produce near linear variation trends for me and mg probably generated by the mixing of mafic and felsic magmas in various proportions trace including rare earth elements patterns of me mg pairs however show partial to complete equilibration most likely governed by different degrees of elemental diffusion the available evidence supports the model of me origin that coeval mafic enclave and felsic mg magmas produced a hybrid me magma layer which injected into cooler partly crystalline mg and dispersed mingled and undercooled as me globules in a convectively dynamic magma chamber

ice stream d flow speed is strongly modulated by the tide beneath the ross ice shelf the flow velocity of ice stream d west antarctica has been measured to vary by a factor of three over the course of a day these fluctuations are measured at the grounding line as well as upstream of the grounding line in the ice plain of ice stream d the diurnal velocity fluctations appear to be driven by the tide beneath the ross ice shelf these results suggest that there is significant and heretofore poorly understood influence of the ocean tide and of the ice shelf on the dynamics of ice stream flow

sediment deformation beneath glaciers rheology and geological consequences experiments beneath breidamerkurjokull in iceland have led to development of flow laws for the subglacial till relating strain rate to shear stress and effective pressure and assuming either bingham fluid or nonlinearly viscous fluid behavior water pressures in the till are less than ice pressures and it is suggested that this may lead to infiltration of ice into the sediment which inhibits sliding at the ice sediment interface where water pressures are equal or near to ice pressures infiltration does not occur and sliding may result a one dimensional theory of subglacial deformation is developed in which the empirical flow law is coupled with a model of subglacial hydrology and consolidation this predicts stable states in which subglacial sediment either does not deform or a dilatant deforming horizon forms with positive effective pressures at the ice bed interface or unstable states where zero or negative effective pressures are predicted time dependent analyses show that response times following perturbations of the system may be of the order of 103 years and thus that unsteady behavior may be normal on glaciers flowing over unlithified sediment beds it is suggested that the natural variability of material properties in subglacial sediment beds leads to the development of drumlins on the glacier bed it is suggested that unstable deformation at zero or negative effective stress leads to piping in subglacial sediments at the glacier terminus and the growth of sediment floored subglacial tunnels their frequency is that which is sufficient to draw down subglacial water pressures so as to prevent unstable deformation where they discharge large water volumes subglacial sediments flow laterally toward them producing tunnel valleys this sediment is then removed by water flowing along the axial tunnel tunnel valleys can be regarded as the equivalent in soft sediment areas of eskers in bedrock areas

s type granitic magmas petrogenetic issues models and evidence abstract despite a perception that it represents a perverse divergence it is perfectly possible to believe in the existence of s and i type granites and the implications for the nature of their protoliths and to disbelieve in the applicability of the restite unmixing model for chemical variation in granitic magmas white and chappell erected the s i classification with impeccable validity the isotopic evidence demands contrasting source reservoirs for s and i type granitic magmas however the major advance was not the classification but the recognition that highly contrasting parental materials must be involved in the genesis of granitic magmas the restite unmixing model is commonly seen as a companion to the s i classification but it is really a separate issue this model implies that the compositions of granites image those of their source rocks in a simple way however there are other equally valid models that can explain the data and none of them represents a unique solution the most cogent explanation for the high grade metasedimentary enclaves in most s type granites is that they represent mid crustal xenoliths restitic enclaves are either rare or absent inherited zircons in s type rocks are certainly restitic however the occurrence of a substantial restitic zircon population does not imply an equally substantial restitic component in the rest of the rock zircon and zirconium behaviours are controlled by disequilibrium and kinetics and zr contents of granitic rocks can rarely be used to infer magma temperatures since the dominant ages among inherited zircons in lachlan fold belt lfb s type granites are ordovician and proterozoic it seems likely that crust of this age but geochemically different from the exposed rocks not only underlies much of the lfb but also forms a component in the granite magma sources the evidence is overwhelming that the dark microgranular enclaves that occur in both s and i type granites are igneous in origin they represent globules of quenched more mafic magma mingled and modified by exchange with the host granitic magma however magma mixing does not appear to be a significant process affecting the chemical evolution of the host magmas likewise the multicomponent mixing models erected for some granitic rock suites are mathematically nonunique and in some cases violate constraints from isotopic studies s and i type magmas commonly retain their distinct identities this suggests limited source mixing limited magma mixing and limited wall rock assimilation though intermediate types certainly exist they are probably relatively minor in volume crystal fractionation probably plays the major role in the differentiation of very many granitic magmas including most s types especially those emplaced at high crustal levels or in the volcanic environment minor mechanisms include magma mixing wall rock assimilation and restite unmixing isotopic variations within plutons and in granite suites could be caused by source heterogeneities magma mixing assimilation and even by isotopic disequilibrium however source heterogeneity coupled with the inefficiency of magma mixing is probably the major cause of observed heterogeneity normal geothermal gradients are seldom sufficient to provide the necessary heat for partial melting of the crust and crustal thickening likewise fails to provide sufficient heat generally the mantle must be the major heat source this might be provided through mantle upwelling and crustal thinning and possibly through the intra and underplating of mafic magmas upper crustal extension seems to have been common in regions undergoing granitic magmatism migmatites probably provide poor analogues of granite source regions because they are mostly formed by fluid present reactions granitic magmas are mostly formed by fluid absent processes where we do see rare evidence for arrested fluid absent partial melting the melt fraction is invariably concentrated into small shear zones veinlets and small dykes thus it seems likely that dyking is important in transporting granitic magma on a variety of scales and at many crustal levels however one major missing link in the chain is the mechanism by which melt fractions in small scale segregations occurring over a wide area can be gathered and focused to efficiently feed much wider spaced major magma conduits answers may lie in the geometry of the melting zones and in the tendency of younger propagating fractures to curve toward and merge with older ones self organization almost certainly plays a role

dynamics of laterally confined marine ice sheets we would like to thank dr m hallworth for valuable help with running the experiments and the technicians of the damtp g k batchelor laboratory for help with the set up of the experimental apparatus k n k is supported by a nerc phd studentship the experimental data are available as supplementary material

sedimentation beneath ice shelves the view from ice stream b abstract ice shelf development is favored by rapid flow of cold ice from outlet glaciers or ice streams into protected embayments with localized high spots basal melting of ice shelves is rapid near the ice front and may occur near the grounding line ice from outlet glaciers may contain significant englacial debris that is deposited as a dropstone diamicton in regions of basal melting englacial debris is sparse or absent in ice streams evidence from ice stream b draining into the ross ice shelf of west antarctica suggests that the rapid ice velocity arises from deformation of a several meter thick water saturated basal till layer that is eroding an unconformity on sediments beneath and that has deposited a till delta tens of meters thick and tens of kilometers long at the grounding line sea level fall would cause conveyor belt recycling of this till delta and grounding line advance across the ross sea to the edge of the continental shelf forming an ice sheet with a low ice stream profile resting on a several meter thick deforming till layer eroding an unconformity the modern ross sea is characterized by a regional unconformity overlain by a diamicton of probable latest pliocene pleistocene age measuring several meters to tens of meters thick we hypothesize that this diamicton is a deformed glacial till and that the ross sea sediments record one or more expansions of the till lubricated west antarctic ice sheet to the edge of the continental shelf

characteristics of the bed of the lower columbia glacier alaska an unplanned but unique experiment has given an in situ measurement of the strength of deforming subglacial till under the central region of a major valley glacier we report on both planned and unplanned borehole investigations of the subglacial shear zone of columbia glacier southeast alaska basal samples coring and down hole water samples show that the fiord filling lower reach of the glacier is underlain by a thin 7 cm veneer of rock debris fluidized debris intruded at least a meter up the borehole at a higher site 13 km from the terminus and above the fiord probing samples and the bending of a drill stem which was stuck in the basal zone for 5 days showed that the basal till layer was 65 cm thick horizontal velocity of the till decreased monotonically downward from the ice till interface till at the interface moved with the ice velocity plastic deformation of the drill stem gave an estimate of the strength of the basal till which is normally described as a viscoplastic material if the till is assumed to be either perfectly plastic or newtonian viscous then the strengths are as follows the plastic yield strength of the till was 5 5 10 3 pa 0 055 bar with an upper bound of 1 3 10 4 pa 0 13 bar while the nominal viscosity was of the order of 2 10 8 pa s 2 10 9 poise with an upper bound of 5 10 8 pa s in neither case is the till strength enough to supply the bulk basal shear stress to resist the glacier flow

the formation of grounding zone wedges theory and experiments we present a fluid mechanical explanation of the formation of sedimentary wedges deposited at ice stream grounding zones we model both ice and till as layers of viscous fluid spreading under gravity into an inviscid ocean to test the fundamentals of our theory we perform a series of laboratory experiments in which we find that a similar wedge of underlying less viscous fluid accumulates spontaneously around the grounding line we formulate a simple local condition relating wedge slopes which determines wedge geometry it expresses a balance of fluxes of till either side of the grounding line and involves upstream and downstream gradients of till thicknesses as well as the upper surface gradient of the ice it shows that a wedge will form that is the upstream till thickness gradients are positive when the flux of till driven by the glaciostatic pressure gradient of the overlying ice is greater than the flux of till ahead of the grounding zone this is related to the unloading of the till as the ice sheet crosses the grounding line

a model for monazite melt equilibrium and application to the generation of granitic magmas abstract in order to model the behaviour of zr hf ti p u th y and the rare earth elements ree in the continental crust the parameters which control the equilibria between accessory minerals and granitic melts must be determined this paper proposes equations for the equilibrium between monazite and ca poor felsic melts in a first step all the light ree are considered as a single component and an equation describing monazite solubility is deduced from the available experimental data in a second step the fractionation of ree by monazite is investigated on the basis of a natural example of coexisting monazite and obsidian from macusani peru the solubility of monazite can be used as a thermometer based on the total ree content of natural magmatic rocks application of ree thermometry to various peraluminous rock series suggests that it is a reliable tool for determining temperatures of magmatic processes the equation governing ree fractionation by monazite is used to discuss genetic relationships between a melt and a presumed parental rock using the composition of the monazite in the parental rocks and the ree pattern of the daughter rock this method has been applied successfully to high himalayan manaslu and tibetan slab granites it is suggested that detailed petrographical work on monazite including electron microprobe analyses and sem imaging of its internal structures combined with the model presented here can be a useful tool for deciphering the genesis and the differentiation of peraluminous granitic magmas in the continental crust

till beneath ice stream b 3 till deformation evidence and implications most of the velocity of ice stream b near the upstream b camp upb west antarctica appears to arise from deformation of a seismically detected subglacial till layer that averages 6 m thick available evidence indicates that the entire thickness of this till layer is deforming and is eroding subjacent bedrock into flutes parallel to ice flow and hundreds of meters across the resulting till flux beneath upb is equivalent to an average erosion rate of about 0 4 mm yr 1 in the catchment area and suggests that till deltas tens of kilometers long have been deposited at the grounding line during the holocene such deltas should be characterized by partial ice till decoupling across a water film and by a small ice air surface slope they may have been discovered by recent geophysical work

submarine glacial landforms and rates of ice stream collapse the rate of deglacial ice sheet retreat across polar continental shelves and possible ice stream collapse and sea level rise has been much debated high resolution imagery of seafloor morphology is available for many polar shelves and fjords the rapidity of ice retreat is inferred from diagnostic assemblages of submarine landforms produced at ice stream sedimentary beds these landforms exposed by ice retreat across high latitude shelves demonstrate that deglaciation occurs in three main ways rapidly by flotation and breakup episodically by still stands and or grounding events punctuating rapid retreat or by slower retreat of grounded ice submarine landform assemblages imply through the presence of grounding zone wedges overprinting mega scale glacial lineations on many polar shelves that ice stream retreat is more often episodic than catastrophic these observations provide a robust test of the ability of numerical models to predict the varied response of ice sheet basins to environmental changes

dynamics of viscous grounding lines we have used viscous fluids in simple laboratory experiments to explore the dynamics of grounding lines between marine ice sheets and the freely floating ice shelves into which they develop we model the ice sheets as shear dominated gravity currents and the ice shelves as extensional gravity currents having zero shear to leading order we consider the flow of viscous fluid down an inclined plane into a dense inviscid ocean a fixed flux of fluid is supplied at the top of the plane which is at sea level the fluid forms a gravity current flowing down and attached to the plane for some distance before detaching to form a freely floating extensional current we have derived a mathematical model of the flow that incorporates a new dynamic boundary condition for the position of the grounding line where the gravity current loses contact with the solid base the grounding line initially advances and eventually reaches a steady position good agreement between our theoretical predictions and experimental measurements and observations gives confidence in the fundamental assumptions of our model

the tynong pluton its mafic synplutonic sheets and igneous microgranular enclaves the nature of the mantle connection in i type granitic magmas in the lachlan orogen of south eastern australia the high level postorogenic 368 ma i type tynong pluton contains granitic to granodioritic rocks that crystallised from a variety of mainly crustally derived magmas emplaced in the shallow crust in an extensional regime the isotopic characteristics of the main plutonic rocks are relatively unevolved 87sr 86srt 0 705 0 706 and endt 0 4 to 0 6 suggesting source rocks not long separated from the mantle we infer that arc mafic to intermediate rocks and associated immature greywackes formed the main crustal source rocks and that these are located in the largely unexposed neoproterozoic cambrian selwyn block that forms the basement as exposed near its southern margin the pluton also contains minor pillowed sheet like intrusions of quartz dioritic rock that show mainly mingling structures with the enclosing granodiorites as well as some hybrid pods and fairly abundant igneous microgranular enclaves that we infer to have been derived from the quartz dioritic sheets despite this evidence of direct mantle input into the tynong magma system the main granodioritic series do not appear to have been formed by magma mixing processes of any i type granite in the region the tynong pluton has perhaps the most direct connection with mantle magmas nevertheless the main mantle connection here is probably in the mantle derived protolith for these crustal magmas and in the mantle thermal event that gave rise to melting of the deep crust in the selwyn block this degree of mantle connectedness seems typical for i type granitic rocks worldwide

the seismic architecture and geometry of grounding zone wedges formed at the marine margins of past ice sheets grounding zone wedges form where halts of decades to centuries during ice sheet retreat across polar shelves allow sediment buildup at the grounding zone through delivery of deforming basal debris from fast flowing ice streams thirty grounding zone wedges were identified on two dimensional seismic reflection profiles across the continental shelves of ne and nw greenland grounding zone wedges close to the seafloor are probably of late weichselian age others are on paleoshelves buried beneath thick prograding glacial sediments several greenland shelf grounding zone wedges occur on topographic highs that provide stabilizing pinning points in relatively shallow water grounding zone wedges are asymmetric in the ice flow direction with steeper ice distal sides typical grounding zone wedges are approximately 5 20 km long and 50 100 m thick with a lateral width of several tens of kilometers forty grounding zone wedges from the greenland norwegian and antarctica margins show considerable variability about these values grounding zone wedge dimensions are controlled by sediment flux the duration of halts in ice retreat subice cavity shape and ice stream width low angle ice shelf cavity roofs immediately beyond the grounding zone probably restrict vertical accommodation space preventing formation of high amplitude ridges grounding zone wedges are mainly transparent or chaotic on seismic profiles probably resulting from delivery of diamictic debris offlapping reflectors represent progradation into an ice roofed cavity reflector truncations of grounding zone wedge bases indicate erosion during initiation channels are present within some grounding zone wedges meltwater flow is under high pressure and v shaped incisions suggest high energy flow channels are not ubiquitous within greenland grounding zone wedges and may represent nonsteady meltwater release perhaps through lake drainage where ice sheet mass loss is dominated by meltwater runoff grounding zone wedges probably contain more sorted sediment than those from greenland simple models of grounding zone wedge architecture are presented grounding zone wedges indicate episodic rather than catastrophic ice sheet retreat and are a mechanism for ice shelf stabilization because wedge growth counteracts collapse induced by ice sheet thinning and sea level rise

geochemistry and origin of megacrystic granitoid rocks from eastern ghats granulite belt abstract the eastern ghats granulite belt eggb forms part of a continuous precambrian metamorphic terrain in gondwana it is characterised by widespread development of an archaean khondalite suite of metasedimentary rocks archaean to late proterozoic charnockites and late proterozoic anorthositic granitic and syenitic emplacements a 1900 ma megacrystic granitoid suite containing varying proportions of charnockites and granites forms an important and widely distributed litho unit in the central khondalite and eastern migmatite zones of the eggb it contains metasedimentary enclaves megacrystic k feldspar quartz plagioclase ovoids biotite garnet porphyroblasts and coronas apatite zircon ilmenite magnetite etc hypersthene is present in the charnockite phase monazite is present in some garnet free granites it is characterised by low na 2 o k 2 o ratios high alumina saturation index cao mgo and ree negative correlation of tio 2 al 2 o 3 fe 2 o 3 t mgo mno cao p 2 o 5 ba sr zr and v with sio 2 positive correlation of k 2 o ree th and rb with sio 2 fractionated lree relatively flat hree and negative eu anomalies the data suggest s type nature of the suite fractionation of the granitic magma and local variations in ph 2 o and fco 2 caused the formation of megacrystic charnockites formation of the corona garnet is related to the reworking of the suite during late proterozoic ca 1250 ma isothermal decompression associated with channelised co 2 rich fluid flux along narrow shear zones

lateral controls on grounding line dynamics we present a theoretical and experimental study of viscous gravity currents introducedat the surface of a denser inviscid uid layer of nite depth inside a vertical hele shaw cell initially the viscous uid oats on the inviscid uid forming a self similar buoyancy driven current resisted predominantly by the viscous stresses due to shearacross the width of the cell once the viscous current contacts the base of the cell the ow can be considered in two regions a grounded region in which the currentlies in full contact with the base and a oating region the subsequent advance ofthe grounding line separating these regions is shown to be controlled by the thickeningof the current associated with balancing the local shear stresses an understanding ofthe ow transitions is developed using asymptotic and numerical analysis of a modelbased on lubrication theory key words geophysical and geological ows hele shaw ows ice sheets

time vs composition trends of magmatism at sunlight volcano absaroka volcanic province wyoming sunlight volcano is a dissected but well preserved eruptive center in the eastern high k belt of the absaroka volcanic province on the basis of high precision 40ar 39ar ages magmatism at the volcano commenced prior to 49 6 ma and continued until ca 48 1 ma stratigraphic relationships along with geochemical and 40ar 39ar data distinguish three principal stages of effusive volcanism that are interpreted to reflect the inception climax and decay of a crustal magmatic system related to three major pulses of basalt and high eruptive rates during the first two stages shoshonitic and latitic lavas with small but distinct compositional ranges accumulated the third stage included eruption of picritic basaltic lavas and intrusion and solidification of magmas in the volcanic edifice as this stage produced magmas compositionally more evolved and diverse basalts to trachytes than in previous stages it likely represents the terminal pulse of basalt to the system followed by cooling protracted differentiation and cessation of eruptive activity petrographic geochemical and sr and nd isotope data for sunlight igneous rocks indicate that crystallization differentiation was the dominant magma evolution process although phenocryst accumulation and crustal contamination also occurred evidence for phenocryst accumulation is most pronounced in final stage rocks this relationship is interpreted to reflect recharge of late magmas into mature chambers laden with crystals deposited during earlier stages the importance of crystallization differentiation at sunlight volcano contrasts with recent petrogenetic models for calc alkaline magmas in the absaroka volcanic province which require a more important role for early magma mixing the results presented here indicate that across strike increases in k2o contents of rocks in the field derive from variations in the conditions of crustal differentiation rather than from mantle processes involving a subducting slab

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

fluid absent melting and the roles of fluids in the lithosphere a slanted summary abstract the presence or absence nature and compositions of lithospheric fluids are crucial factors in the evolution of igneous and metamorphic rocks despite this confusion still surrounds perceptions of the degrees of fluid involvement in many geological settings however experimental and field investigations constrain these roles and we are able to make some valid generalizations studies of prograde metamorphism have highlighted the spatial heterogeneity of fluid concentrations in many crustal environments this commonly indicates internal lithological control of volatile activities under fluid present conditions crustal fluids do not appear to mix readily fluid flow is generally structurally or lithologically controlled the transition from amphibolite to granulite facies is characterized by a decrease in ah2o as well as a temperature increase in many terranes these phenomena correlate with the appearance of migmatites some granulites are not migmatitic but have chemical and mineralogical characteristics of melt depleted restites this indicates efficient extraction of melts incongruent melting reactions involving the breakdown of crystalline hydrates in crustal rocks produce granitoid melts plus denser restites that are nearly or actually anhydrous experimental and theoretical treatments of fluid absent melting show that micas and amphiboles decompose at 700 to 900 c for mid to lower crustal pressures the melts resemble the compositions of s and i type granitoids pervasive aqueous fluids could not generally have been present during this melting since exposed granulite terranes are not characterized by high melt proportions and the magmas themselves are water undersaturated likewise the general presence of very h2o poor pervasive fluids is unlikely since this would allow only subsolidus dehydration at normally attainable crustal temperatures in this case the amphibolite granulite transition is possible at relatively low temperatures and the resultant rocks would not contain evidence of in situ melting the granulite granite connexion and effective crustal differentiation seem viable only during high temperature fluid absent metamorphism data on continental geotherms show that the lower crust is not entirely granulitic granulites are produced in regions of anomalously high heat flow the mantle connexion thus inferences on the nature of the lower crust should not be entirely based on studies of exposed granulite terranes mineral assemblages in eclogites also indicate widespread internal buffering of volatile activities and probable fluid absent conditions mantle derived basaltic magmas too are volatile bearing and it is plausible that volatile fixing phases also occur in the mantle volatiles may largely be introduced into the mantle through subduction processes and only released during the melting attending the breakdown of hydrous minerals as in the crust

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

the intrusion of fluid mechanics into geology during the last few years a number of fluid dynamicists often in collaboration with geologists have been investigating problems suggested by geology and creating the new field of geological fluid mechanics the research has uncovered many new fluid mechanical phenomena and the studies have ranged widely over theoretical contributions laboratory experiments and confirmatory field observations this paper presents an overview of part of the field with a strong emphasis on the novel fluid mechanical concepts the presentation includes discussion of some of the effects of cooling and crystallization and the resulting convective motions the flow of gravity currents at both high and low reynolds number and the erosion of solid retaining walls by hot turbulent flows

expansion and rapid retreat of the west antarctic ice sheet in eastern ross sea possible consequence of over extended ice streams abstract marine geological data from the eastern ross sea provide compelling evidence that the west antarctic ice sheet extended to the continental shelf break during the last glacial maximum swath bathymetry records show mega scale glacial lineations within each of the four large glacial troughs on the shelf these features extend virtually uninterrupted across the shelf and have sharp lateral boundaries sediment cores that sampled the lineations recovered diamicton that is interpreted as deformation till the combined data indicate streaming ice the great extent of the paleo ice streams up to 1000 km in length is attributed to the fact that they flowed across unconsolidated sedimentary strata geomorphic features that are known to form at the retreating grounding line such as recessional moraines and ridges are restricted to the southeastern portion of the shelf iceberg furrows that occur at depths of up to 650 m overprint mega scale lineations these deep furrows are interpreted as having been formed by icebergs calved from the grounding line of the ice sheet which indicates the absence of a fringing ice shelf sediment cores sampled a thin layer of glaciomarine sediments resting sharply on till sub ice shelf deposits similar to those sampled in the western ross sea were not recovered in the eastern ross sea these combined results imply rapid retreat of the ice sheet from the shelf crosscutting lineations with different orientations indicate changes in flow direction that probably occurred as the ice profile was lowered and the physiography of the sea floor across which the ice sheet was retreating changed grounding zone wedges vary in number and location within individual troughs which indicates diachronous retreat of ice streams due to a lack of carbonate material for radiocarbon dating all radiocarbon ages were derived from acid insoluble organic aio fractions these dates indicate early retreat of the ice sheet relative to the western ross sea and other areas of west antarctica however the older dates must be viewed as suspect due to a paucity of diatomaceous material and contamination from old carbon while we were not successful at constraining the timing of ice sheet retreat from the continental shelf the data do indicate that the ice sheet was grounded on the shelf during the lgm

ice sheet grounding zone wedges gzws on high latitude continental margins abstract grounding zone wedges gzws are asymmetric sedimentary depocentres which form through the rapid accumulation of glacigenic debris along a line source at the grounding zone of marine terminating ice sheets during still stands in ice sheet retreat gzws form largely through the delivery of deforming subglacial sediments the presence of gzws in the geological record indicates an episodic style of ice retreat punctuated by still stands in grounding zone position moraine ridges and ice proximal fans may also build up at the grounding zone during still stands of the ice margin but these require either considerable vertical accommodation space or sediment derived from point sourced subglacial meltwater streams by contrast gzws form mainly where floating ice shelves constrain vertical accommodation space immediately beyond the grounding zone an inventory of gzws is compiled from available studies of bathymetric and acoustic data from high latitude continental margins the locations and dimensions of gzws from the arctic and antarctic alongside a synthesis of their key architectural and geomorphic characteristics are presented gzws are only observed within cross shelf troughs and major fjord systems which are the former locations of ice streams and fast flowing outlet glaciers typical high latitude gzws are less than 15 km in along flow direction and 15 to 100 m thick gzws possess a transparent to chaotic acoustic character which reflects the delivery of diamictic subglacial debris many gzws contain seaward dipping reflections which indicate sediment progradation and wedge growth through continued delivery of basal sediments gzw formation is inferred to require high rates of sediment delivery to a fast flowing ice margin that is relatively stable for probably decades to centuries although the long term stability of the grounding zone is controlled by ice sheet mass balance the precise location of any still stands is influenced strongly by the geometry of the continental shelf the majority of high latitude gzws occur at vertical or lateral pinning points which encourage grounding zone stabilisation through increasing basal and lateral drag and reducing mass flow across the grounding zone

mechanical and hydraulic properties of till beneath bakaninbreen svalbard original article can be found at http www igsoc org journal copyright igs doi 10 3189 172756501781832304 full text of this article is not available in the uhra

till beneath ice stream b 1 properties derived from seismic travel times seismic experiments conducted on ice stream b part of the marine ice sheet of west antarctica show a meters thick layer immediately beneath the 1000 m thick ice a seismic experiment consisting of wide angle reflection profiling along a line parallel to ice stream flow was conducted to determine the properties of this layer inversion of seismic travel times yields a compressional wave speed of less than 1700 m s 1 and a shear wave speed less than 160 m s 1 for the layer these very low wave speeds imply that the material in the layer is highly porous and is saturated with water at a high pore pressure based on wave speeds in other saturated unconsolidated sediments we believe that a porosity substantially greater than 0 32 probably around 0 4 and an excess of overburden pressure over pore pressure of only 50 kpa 0 5 bar characterize the layer at this location

granitoid emplacement and deformation along a major crustal lineament the cordillera blanca peru abstract the cordillera blanca batholith and ignimbrites of the intermountain callejon de huaylas basin represent the last magmatic event in the andean cycle of central northern peru this most spectacular example of an extensional regime is characterised by acid magmatism spanning some 10 ma with both magma emplacement and subsequent deformation being fundamentally controlled by a ca 300 km lineament known as the cordillera blanca fault complex the western margin of the batholith has been strongly deformed by the cordillera blanca fault resulting in a series of ductile to brittle structures that increase with intensity away from the relatively undeformed core towards the margin the late stage brittle fabrics are coeval with extensional structures in the callejon de huaylas basin and relate to uplift and extension along the fault zone the more ductile fabrics as indicated by combined k ar and 40 ar 39 ar mineral dates formed earlier in the intrusion history and are consistent with emplacement into an active dextral strike slip shear zone the rapid uplift and high erosion rates seen from the late miocene onwards and the current state of stress in this region of thickened crust are due to gravitational instability body forces set up at high topographical levels and buoyancy forces resulting from the intrusion of young acid magmas at high crustal levels

magnetic properties of the ll5 ordinary chondrite chelyabinsk fall of february 15 2013 here we characterize the magnetic properties of the chelyabinsk chondrite ll5 s4 w0 and constrain the composition concentration grain size distribution and mineral fabric of the meteorite s magnetic mineral assemblage data were collected from 10 to 1073 k and include measurements of low field magnetic susceptibility v0 the anisotropy of v0 hysteresis loops first order reversal curves m spectroscopy and x ray microtomography the rem and rem 0 paleointensity protocols suggest that the only magnetizations recorded by the chondrite are components of the earth s magnetic field acquired during entry into our planet s atmosphere the chelyabinsk chondrite consists of light and dark lithologies fragments of the light lithology show logv0 4 57 0 09 s d n 135 while the dark lithology shows 4 65 0 09 n 39 where v0 is in 10 9 m 3 kg 1 thus chelyabinsk is three times more magnetic than the average ll5 fall but is similar to a subgroup of metal rich ll5 chondrites paragould aldsworth bawku richmond and l ll5 chondrites glanerbrug knyahinya the meteorite s room temperature magnetization is dominated by multidomain feni alloys taenite and kamacite no tetrataenite is present however below approximately 75 k remanence is dominated by chromite the metal contents of the light and dark lithologies are 3 7 and 4 1 wt respectively and are based on values of saturation magnetization

effect of sedimentation on ice sheet grounding line stability sedimentation filling space beneath ice shelves helps to stabilize ice sheets against grounding line retreat in response to a rise in relative sea level of at least several meters recent antarctic changes thus cannot be attributed to sea level rise strengthening earlier interpretations that warming has driven ice sheet mass loss large sea level rise such as the 100 meter rise at the end of the last ice age may overwhelm the stabilizing feedback from sedimentation but smaller sea level changes are unlikely to have synchronized the behavior of ice sheets in the past

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

discovery of till deposition at the grounding line of whillans ice stream we report on the discovery of a grounding line sedimentary wedge till delta deposited by whillans ice stream west antarctica our observation is that grounding line deposition serves to thicken the ice and stabilize the position of the grounding line the ice thickness at the grounding line is greater than that of floating ice in hydrostatic equilibrium thus the grounding line will tend to remain in the same location despite changes in sea level until sea level rises enough to overcome the excess thickness that is due to the wedge further our observation demonstrates the occurrence of rapid subglacial erosion sediment transport by distributed subglacial till deformation and grounding line sedimentation which have important implications for ice dynamics numerical modeling of ice flow and interpretation of the sedimentation record