structural history and granite emplacement in the rottenstone domain during closure of the trans hudson orogen davin lake northern saskatchewan the rottenstone domain at davin lake northern saskatchewan exhibits structural and granite emplacement evidence for crustal thickening and possible himalayan style extrusion overprinted by transpressional strain increasing toward the contact with the wathaman batholith three discrete rottenstone subdomains parallel the regional strike of the trans hudson orogen i the southeast rottenstone subdomain sersd with gently northwest dipping migmatitic straight gneiss s1 and white granitoid rocks with pinch and swell structures parallel to the straight gneissosity ii the central rottenstone subdomain crsd which partly preserves the same nw dipping fabric s1 but is overprinted at its margins by tight upright f2 folds and includes a stockwork of pink monzogranitic aplites and pegmatites and iii the northwest rottenstone subdomain nwrsd in which the f2 folds are generally tighter and penetrative and its network of narrow white granitoid rocks is deformed and transposed by the f2 folds but in

k feldspar megacrysts in granites phenocrysts not porphyroblasts abstract k feldspar megacrysts in granitoid plutons have been interpreted as either phenocrysts or porphyroblasts most of the microstructural mineralogical and chemical evidence e g shape alignment concentration ba content zoning inclusions and twinning favours a phenocryst origin the main features that have been used to support a porphyroblast origin are occurrence of megacrysts 1 across aplite vein boundaries 2 in country rocks and 3 in or across boundaries of microgranitoid enclaves mafic inclusions however these features can be explained by the phenocryst hypothesis in particular megacrysts in microgranitoid enclaves can be explained by growth or mixing in magma before a globule of that magma or a fragment of the resulting igneous rock was incorporated as an enclave all available evidence favours or is consistent with a phenocryst origin for k feldspar megacrysts in granitoid rocks and their enclaves the large size of the megacrysts is evidently due to nucleation difficulties for k feldspar in granitic melts though k feldspar is commonly the last mineral to begin crystallizing in granitic magmas abundant melt is still present at that stage allowing sufficient space for the megacrysts to grow the reason for the common lack of megacrysts in volcanic rocks may be that the phenocrysts do not grow large enough to be called megacrysts until the magma contains such a high proportion of crystals that it cannot erupt

study of plasmaspheric evolution using geosynchronous observations and global modeling we examine the dynamics of the outer plasmasphere during a 3 day interval in february 1992 by comparing observations from three widely spaced geosynchronous satellites with the predictions of the magnetospheric specification and forecast model msfm for this study the msfm was modified to include a cold plasmaspheric population subject to the effects of corotation convection and ionospheric refilling the model does a very good job of reproducing the observations in particular it clearly shows the formation and westward transport of duskside plasmaspheric plumes accounting for the progressively earlier local time observations of cold ions following periods of enhanced convection the msfm s ability to model dynamically the evolution of the outer plasmasphere under varying solar wind conditions improves upon previous plasmaspheric models and has aided our understanding of spatially and temporally separated observations of plasmaspheric ions at geosynchronous orbit

modelling granite migration by mesoscale pervasive flow abstract mesoscale pervasive magma migration leads to granite injection complexes common in hot crustal terranes pervasive migration is limited by magma freezing when intruding cold country rock here we explore numerically the feedback mechanism between magma intrusion and heating of the country rock which allows younger intrusive batches to reach increasingly shallower cooler levels this process relies on the higher solidus temperature of a rock compared to that of its melt once melt is segregated we define the free ride layer as the region above the melt source where magma may freely migrate because rock temperature is above melt solidus the top of the free ride layer which corresponds to the melt solidus t s isotherm is at the limiting depth z s after magma passes through the free ride layer the magma front is always at the limiting depth we modeled the thickening and heating of the crust above the source as melt at its liquidus t l intrudes it pervasively from below we found that a magma quickly warms crust below z s to about t l forming a step in temperature at z s b the front z s moves up through the crust as more magma is intruded c as magma is emplaced at the front a mingled layer of about half magma half crust forms below it so that the total rise of the front corresponds approximately to half of the thickness of magma added to the free ride layer d the rate of rise of the front depends on the temperature difference between crust and t l and slows down as the magma front rises e for most reasonable intrusion rates and volumes the crust above z s feels little influence of the intrusion because the diffusion time scale is much smaller than the rise rate of the front in summary pervasive migration is an efficient way of heating the lower to middle crust and can result in an injection complex several kilometers thick consisting of about half magma and half original crust

shisha pangma leucogranite south tibetan himalaya field relations geochemistry age origin and emplacement the shisha pangma pluton forming most of the xixabangma 8027 m massif in south tibet is one of the 20 larger leucogranite intrusives that mark the highest structural levels of the himalayan metamorphic core the pluton occurs immediately below the shisha pangma detachment a strand of the south tibetan detachment std system a low angle 30 north dipping normal fault placing paleozoic black slates atop sillimanite grade pelites and calc silicate rocks k feldspar augen gneisses containing fibrolite and sillimanite paragneisses along the footwall show strong internal s c fabrics indicative of down to the north extension the shisha pangma leucogranite is a heterogeneous polyphase intrusion with an earlier foliated biotite rich phase and a later tourmaline muscovite rich phase typically containing the assemblage kfs pl qtz ms tur gt bt sil ap the highly peraluminous granites have high 87sr 86sr ratios 0 738 0 750 typical of pelite derived anatectites nd depleted mantle model

metamorphic zonation migmatization and leucogranites along the everest transect of eastern nepal and tibet record of an exhumation history

overshielding event of 28 29 july 2000 1 on 29 july 2000 image euv observed a shoulder like bulge on the plasmapause goldstein et al 2002 showed that plasmaspheric shoulders may be caused by a pre dawn concentration of eastward overshielding electric e field triggered by a decrease in magnetospheric convection a simulation by the magnetospheric specification model msm captures the shoulder formation reproducing its approximate size and location rocsat 1 ion drift meter data are consistent with the strength time development and local time variation of the msm eastward penetration e field and associated radial flows during the shoulder formation on 28 july

the pangong injection complex indian karakoram a case of pervasive granite flowthrough hot viscous crust in the karakoram range of nw india the tangtse gorge cuts across deep crustal rock sexhumed by transpression along two strands of the dextral karakoram fault crustal leucogranitic magmas pervasively intruded amphibolites and migmatites in the form of sheets that locally coalesced and expanded to form kilometre scale plutons sheets resulted from slow seeping of granite into foliation planes magma wedging rather than from dyking driven by magma buoyancy and possibly syn intrusive tectonic deformation high temperature of the country rocks at the time of intrusion is inferred from similar crystallization ages of the intrusive leucogranite and in situ partial melt in the migmatites and reflected in the ductile structures developed whereas low country rock viscosity inhibited dyking high temperature freed the magma from the constraints of freezing and permitted pervasive intrusion the injection complex exposed along the tangtse gorge rather than representing the final emplacement structures represent transient structures of the granite pathways on their upward journey to build the karakoram batholith which crops out a few kilometres north at structurally shallower levels we suggest that pervasive magma flow is a transitional step between magma segregation at the source and later rise through cold crust and it may be one of several factors controlling whether dyking or diapirism becomes dominant during late ascent

himalayan leucogranites and migmatites nature timing and duration of anatexis widespread anatexis was a regional response to the evolution of the himalayan tibetan orogen that occurred some 30 ma after collision between asia and india this paper reviews the nature timing duration and conditions of anatexis and leucogranite formation in the greater himalayan sequence ghs and compares them to contemporaneous granites in the karakoram mountains himalayan leucogranites and associated migmatites generally share a number of features along the length of the mountain front such as similar timing and duration of magmatism common source rocks and clockwise p t paths despite commonalities most papers emphasize deviations from this general pattern indicating a fine tuned local response to the dominant evolution there are significant differences in p t xh2o conditions during anatexis and timing in relation to regional decompression further to that some regions underwent a second event recording melting at low pressures zircon and monazite ages of anatectic rocks range between c 25 and 15 ma suggesting prolonged crustal melting typically a single sample may have ages covering most of this 10 ma period suggesting recycling of accessory phases from metamorphic rocks and early formed magmas recent studies linking monazite and zircon ages with their composition have determined the timing of prograde melting and retrograde melt crystallization thus constraining the duration of the anatectic cycle in some areas this cycle becomes younger down section towards the leading front of the himalayas whereas the opposite is true in other areas the relationship between granites and movement on the south tibetan detachment std reveals that fault motion took place at different times and over different durations requiring complex internal strain distribution along the himalayas the nature and fate of magmas in the ghs contrast with those in the karakoram mountains ghs leucogranites have a strong crustal isotopic signature and migration is controlled by low angle foliation leading to diffuse injection complexes concentrated below the std in contrast the steep attitude of the karakoram shear zone focused magma transfer feeding the large karakoram baltoro batholith anatexis in the karakoram involved a cretaceous calcalkaline batholith that provided leucogranites with more juvenile isotopic signatures the impact of melting on the evolution of the himalayas has been widely debated melting has been used to explain subsequent decompression or conversely decompression has been used to explain melting weakening due to melting has also been used to support channel flow models for extrusion of the ghs or alternatively to suggest it triggered a change in its critical taper in view of the variable nature of anatexis and of motion on the std it is likely that anatexis had only a second order effect in modulating strain distribution with little effect on the general history of deformation thus despite all kinds of local differences strain distribution over time was such that it maintained the well defined arc that characterizes this orogen this was likely the result of a self organized forward motion of the arc controlled by the imposed convergence history and energy conservation balancing accumulation of potential energy and dissipation independent of the presence or absence of melt

effect of plasmaspheric drainage plumes on solar wind magnetosphere coupling evidence is uncovered that plasmaspheric drainage plumes flowing into the dayside reconnection site mass load the reconnection rate and thereby reduce the coupling of the solar wind to the earth s magnetosphere solar wind magnetosphere coupling is statistically analyzed with the 1963 2003 omni2 data set matched up with the ae au and pci geomagnetic indices times when plasmaspheric drainage plumes are flowing are discerned using multiple spacecraft with plasma detectors in geosynchronous orbit it is found that for a given value of vbz of the solar wind the geomagnetic indices ae au and pci are statistically lower when plasmaspheric drainage plumes are present than when plumes are not present this is taken as a measure of a weakened coupling of the solar wind to the earth s magnetosphere caused by the plumes an addendum examines the effects of polar cap saturation on the auroral electrojet index

crosscorrelogram migration of inverse vertical seismic profile data we present the theory of crosscorrelogram migration of ghost reflections also known as interferometric imaging to delineate reflector geometries from inverse vertical seismic profile data the theory includes the equations for forward modeling migration asymptotic inversion and model resolution of crosscorrelgrams rather than using primary reflections crosscorrelogram migration can use ghost reflections to reconstruct the reflector geometry other multiples can be used as well including pegleg multiples and higher order multiples its main advantages over conventional kirchhoff migration are 1 both source location e g drill bit position and source wavelet need not be known such as when using a drill bit as a source in a deviated well 2 it is insensitive to source related static errors and 3 it has wider subsurface illumination than conventional kirchhoff migration of primary reflections crosscorrelation migration can effectively widen the source receiver aperture by more than 50 co

simulation of van allen probes plasmapause encounters we use an e b driven plasmapause test particle ptp simulation to provide global contextual information for in situ measurements by the van allen probes radiation belt storm probes rbsp during 15 20 january 2013 during 120 h of simulation time beginning on 15 january geomagnetic activity produced three plumes the third and largest simulated plume formed during enhanced convection on 17 january and survived as a rotating wrapped residual plume for tens of hours to validate the simulation we compare its output with rbsp data virtual rbsp satellites recorded 28 virtual plasmapause encounters during 15 19 january for 26 of 28 92 virtual crossings there were corresponding actual rbsp encounters with plasmapause density gradients the mean difference in encounter time between model and data is 36 min the mean model data difference in radial location is 0 40 0 05 re the model data agreement is better for strong convection than for quiet or weakly disturbed conditions on 18 january both rbsp spacecraft crossed a tenuous detached plasma feature at approximately the same time and nightside location as a wrapped residual plume predicted by the model to have formed 32 h earlier on 17 january the agreement between simulation and data indicates that the model provided global information is adequate to correctly interpret the rbsp density observations

two mica and tourmaline leucogranites from the everest makalu region nepal tibet himalayan leucogranite genesis by isobaric heating in the higher himalaya of the region from cho oyu to the arun valley northeast of makalu the miocene leucogranites are not hosted only in the upper high himalayan crystallines hhc a network of dykes also cuts the lower hhc and the lesser himalayan crystallines lhc the plutons and dykes are mainly composed of two mica muscovite biotite tourmaline cordierite andalusite sillimanite leucogranite with tourmaline 2 6 and biotite 1 5 modal and tourmaline muscovite tourmaline biotite sillimanite garnet kyanite andalusite spinel corundum leucogranite with tourmaline 2 2 and biotite 1 5 modal both leucogranite types were produced by partial melting in the andalusite sillimanite facies series under lp ht conditions constrained by the occurrence of peritectic andalusite and cordierite the geochemical features of the leucogranites suggest that tourmaline leucogranite was produced by muscovite dehydration melting in muscovite rich metapelites at p 350 mpa and t 640 c whereas two mica leucogranite was produced by biotite dehydration melting in biotite rich metapelites at p 300 mpa and t 660 710 c melting in fertile muscovite rich metapelites of the top of both the hhc and lhc produced magmas which were emplaced at the same structural level in which they had been generated melting in the biotite rich gneiss of both the hhc and lhc produced hotter magmas which were transported upwards by dyking and eventually coalesced in the plutons of the upper hhc a similar process also produced a network of two mica granite at the top of the lhc in the ama drime nyonno ri range northeast of makalu the prograde character of leucogranite melt producing reactions in the everest makalu area suggests that here the generation of miocene leucogranites took place in a regime of nearly isobaric heating following nearly adiabatic decompression

ductile thrusting in the himalayas shear sense criteria and stretching lineations the most useful criteria used for the determination of the sense of shear can be found in the huge himalayan ductile shear zone known as m c t they allow the determination of the sense of shear in relation with an unequivocal southward directed intracontinental thrust the various criteria analyzed at different scales of observation show the permanence of the shear flow regime throughout the metamorphic evolution however most of the observed shear criteria are due to late nappe emplacement postdating the climax of the metamorphism the most striking feature is the stretching lineation which is roughly normal to the thrust trace all along the belt it parallels everywhere the shear direction and thus indicates the transport direction of the nappes the observed radial pattern of stretch trajectories implies that the local motion does not correspond to the average plate convergence

crustal generation of the himalayan leucogranites abstract detailed studies of the himalayan two mica leucogranites such as the manaslu pluton indicate that they have very uniform mineralogical petrological and structural characteristics one can relate their occurrence to the thickest zones of the underlying tibetan slab in these zones migmatization attains its greatest development and vertical extension the granite is emplaced at first along the main disharmonic plane above the main central thrust mct at the top of the tibetan slab infrastructure ductile deformation of the granite is variable the granite being syn to late kinematic with regard to the functioning of the mct major elements are very homogeneous except for na and k implying that p t conditions of melting were relatively uniform the melted material was of a similar composition over a vast volume and the percentage of melting was small 10 15 trace elements are highly variable some are characteristic for very evolved material ta rb cs u or show the link with the tibetan slab ba sr whilst others are problematic th ree ree and th abundances being much less in the granite than in the tibetan slab imply that they have been extracted during one of the main stages of formation possibly by monazite radiogenic pb sr nd and stable o isotopes are consistent with the origin of the granite from the tibetan slab however the heterogeneous sr isotopic ratios make age dating difficult and imply poor mixing or little fluid interaction during its evolution h isotope data indicate that magmatic compositions of the main body of the manaslu granite have been preserved late or post magmatic alterations are extremely local in the main pluton to the north another belt of two mica granites occurs whose characteristics are very similar to the high himalaya belt they were probably generated in a similar way during this recent intracontinental evolution

melt segregation in the continental crust distribution and movement of melt in anatectic rocks the grain and outcrop scale distribution of melt has been mapped in anatectic rocks from regional and contact metamorphic environments and used to infer melt movement paths at the grain scale anatectic melt is pervasively distributed in the grain boundaries and in small pools consequently most melt is located parallel to the principal fabric in the rock typically a foliation short branched arrays of linked melt bearing grain boundaries connect melt depleted parts of the matrix to diffuse zones of melt accumulation protoleucosomes where magmatic flow and alignment of euhedral crystals grown from the melt developed the distribution of melt leucosome and residual rocks normally melanocratic in outcrop provides different but complementary information the residual rocks show where the melt came from and the leucosomes preserve some of the channels through which the melt moved or sites where it pooled different stages of the melt segregation process are recorded in the leucosome melanosome arrays regions where melting and segregation had just begun when crystallization occurred are characterized by short arrays of thin branching leucosomes with little melanosome a more advanced stage of melting and segregation is marked by the development of residual rocks around extensive branched leucosome arrays generally oriented along the foliation or melting layer places where melting had stopped or slowed down before crystallization began are marked by a high ratio of melanosome to leucosome because most of the melt has drained away very few leucosomes remain to mark the melt escape path this is common in melt depleted granulite terranes many migmatites contain abundant leucosomes oriented parallel to the foliation mostly these represent places where foliation planes dilated and melt drained from the matrix via the branched grain boundary and larger branched melt channel leucosome arrays collected melt collected in the foliation planes was partially or fully expelled later when discordant leucosomes formed leucosomes or veins oriented at high angles to the foliation layering formed last and commonly lack melanocratic borders hence they were not involved in draining the matrix of the melting layer discordant leucosomes represent the channels through which melt flowed out of the melting layer

simultaneous miocene extension and shortening in the himalayan orogen the south tibetan detachment system separates the high grade metamorphic core of the himalayan orogen from its weakly metamorphosed suprastructure it is thought to have developed in response to differences in gravitational potential energy produced by crustal thickening across the mountain front geochronologic data from the rongbuk valley north of qomolangma mount everest in southern tibet demonstrate that at least one segment of the detachment system was active between 19 and 22 million years ago an interval characterized by large scale crustal thickening at lower structural levels these data suggest that decoupling between an extending upper crust and a converging lower crust was an important aspect of himalayan tectonics in miocene time

when did the roof collapse late miocene north south extension in the high himalaya revealed by th pb monazite dating of the khula kangri granite th pb ion microprobe measurements made on 12 monazite grains from the khula kangri granite tibet bhutan frontier are interpreted to indicate that crystallization occurred at 12 5 0 4 ma the leucogranite is cut by the gonto la detachment part of the southern tibet detachment system that has allowed upper level north directed extension of the himalayan orogen significant orogen normal extension in southern tibet appears to have continued 8 10 m y later than previously recognized this is the first reported crystallization age for a leucogranite east of the yadong cross structure an apparent 70 km offset of the high himalaya and southern tibet detachment west of the yadong cross structure reliable ages for high himalaya events major main central thrust slip granite generation and emplacement attainment of critical topography and major detachment extension group between ca 24 and 19 ma we interpret the west to east change across the yadong cross structure to be due to either 1 an abrupt 10 m y younging of principal high himalayan events or 2 a deeper thus younger exposed part of the footwall of the southern tibet detachment near khula kangri the southern tibet detachment is cut by the highly oblique yadong gulu rift a manifestation of tibet plateau east west extension integrated estimates of magnitude and rate of detachment displacement suggest that the observed postcrystallization north directed extension lasted for 1 3 m y after which time the yadong gulu rift formed this interpretation is consistent with initiation of east west extension of tibet at ca 8 ma

progressive adjustments of ascent and emplacement controls during incremental construction of the 3 1 ga heerenveen batholith south africa abstract the heerenveen batholith is part of a suite of areally extensive shallow crustal granitoid plutons intruded during the last regional phase of tectonism and nw se subhorizontal shortening recorded in the mesoarchean barberton granitoid greenstone terrain of south africa at 3 1 ga intrusive relationships allow at least four main successively emplaced intrusive stages to be distinguished each of these shows distinct geometries and intrusive styles that provide evidence for the progressive change of emplacement controls during the incremental construction of the heerenveen batholith the earliest sheet like granitoids intruded as foliation parallel sills along the shallowly dipping basement gneissosity emphasizing the role of favourably inclined pre existing wall rock anisotropies for granite emplacement during the early stages of pluton assembly continued sheeting and coalescence of sheets provided the thermal ground preparation that led to the formation of larger coherent magma bodies and the main phase of homogeneous commonly megacrystic granites these megacrystic granites form the central parts of the heerenveen batholith and are interpreted to represent steady state magma chambers the introduction of the rheologically weaker melt bodies into the shallow crust resulted in the nucleation of conjugate synmagmatic transpressive shear zones around the central granites the shear zones correspond to several km wide zones of shear zone parallel granite sheeting this stage marks a dramatic switch in emplacement styles while the initial stages of magma emplacement were largely determined by factors external to the magma most importantly the pre existing wall rock anisotropies subsequent stages are dominated by factors intrinsic to the magma namely strain localization and partitioning along melt bearing zones during syntectonic plutonism the associated melt transfer along these zones is independent of pre existing structures and mainly related to buoyancy and strain induced melt ascent the last granites of the heerenveen batholith are post tectonic they intrude as either plugs or stocks of seemingly random orientation but display a clear control by wall rock anisotropies where they are in contact with the country rocks on a regional scale the different phases of the heerenveen batholith describe an overall zonation of central homogeneous granites enveloped by composite sheeted and sheared margins this pattern is typical for most of the large 3 1 ga granite batholiths in the barberton granitoid greenstone terrain we suggest that the sequence of progressively changing emplacement controls and the formation of steady state magma chambers described here for the heerenveen batholith may be of wider application to other zoned and or incrementally assembled batholiths

rb sr geochronology of granites and gneisses from the mount everest region nepal himalaya nineteen samples of granites orthogneisses and paragneisses from the high himalaya basement nappe s of the mount everest region have been dated by the rb sr method the post metamorphic tourmaline leucogranites of the upper imja drangka nuptse lhotse glacier have high initial sr87 sr86 characteristic of an anatectic origin from crustal material a whole rock isochron age of 52 m y early eocene has been obtained for the samples from the granite body of lhotse glacier apparently sr isotopic homogenization was not reached throughout the much larger nuptse granite

volatile assisted intrusion and autometasomatism of leucogranites in the khumbu himalaya nepal abstract leucogranite crops out in the upper imja khola khumbu himalaya nepal where it makes up approximately 50 of the exposed bedrock leucogranites and their associated aplite pegmatites typically intruded as concordant sills millimeters to kilometers wide the exceptional nuptse granite is an ellipsoidal pluton of nearly 1 km radius which probably resulted from the ballooning of a sill and is wrapped in concordant country rock foliation small scale structures and mineral paragenesis indicate syntectonic intrusion into high temperature low pressure sillimanite muscovite cordierite bearing country rocks penetrative deformation of the country rock and sills is characterized by foliation striking n 60 e 40 nw and by north northwest plunging mineral lineation subparallel to fold axes of decimeter to meter scale isoclinal folds and centimeter to decimeter scale kink folds and crenulation a volatile rich phase played a fundamental role in the development of the imja khola leucogranites it

the gangotri granite garhwal himalaya laccolithic emplacement in an extending collisional belt the gangotri miocene leucogranite is composed of several laccoliths 6 7 km long 1 5 2 km thick which can be divided into two sets of lenses the southern lenses intruded in the lower part of the high himalaya sedimentary cover and the northern lenses intruded in an older porphyritic biotite bearing granite in both cases the magmatic fabric is commonly weak with no dominant stretching direction although a rough e w trend is present in the northern lenses the magnetic fabric is characterized by the fact that drastic changes in the direction in the magnitude of the magnetic parameters or in the fabric type planar or linear may take place over very short distances measurements of preferred orientations in thin sections indicate that the strain regime was largely dominated by a coaxial component for both sets of lenses in agreement with the large dispersion observed in both the field and magnetic lineations the comparison of the granite and host rock structures shows that the leucogranite emplacement dates the onset of the extensional tectonism in the high himalaya range and is not related to a southward directed thrust event associated with the main central thrust this is exemplified by the vertical attitude of the feeder dikes that intrude the metasedimentary rocks beneath die southern lenses the presence of these dikes indicates in turn that magma ascent occurred by fracture propagation the spatial disposition of the southern lenses could have resulted either from the disruption of a single laccolithic intrusion by crustal scale boudinage due to a northern gravity backslide of the top of the tibetan slab or from the intrusion of independent laccoliths the use of the elastic bending theory of pollard and johnson 1973 shows that in both hypotheses the current laccolith sizes are compatible with a laccolithic mode of magma emplacement however neither the density contrast between the magma and its enclosing rocks nor the lithological boundary between the tibetan slab and the overlying tibetan metasedimentary series controlled the level of magma emplacement rather flat lying collapse structures which intersected the upward propagating magma dikes are the most likely causes of magma arrest such a mechanism was favored by the schist rich lithology of the metasedimentary host rocks in addition field relationships indicate that the melt supply through the dike system was a continuous rather than pulsed process existing numerical treatments on the rates of magma transport through fractures show that in such a case the laccoliths could have been built in less than 100 years this short time of emplacement the small size of the laccoliths and their peripheral disposition relative to the badrinath granite suggest that the gangotri lenses may represent the initial stage of pluton accretion in the high himalaya which ultimately gave rise to a much larger massif such as the manaslu granite

global plasmasphere evolution 22 23 april 2001 1 the evolution of the plasmasphere during the moderate 22 23 april 2001 storm is studied by comparing observed plasmasphere behavior to the predictions of a simulation the plasmasphere observations include global images from image euv and density and flow measurements from lanl mpa the subjective uncertainty in the euv plasmapause was found to be 0 2l inward plasmapause motion was correlated with southward interplanetary magnetic field imf with a 27 min delay the electric e field at the plasmapause was approximately 13 of the solar wind e field the observations support the idea that plasmaspheric erosion begins with a partial plasmapause indentation which then gradually widens in magnetic local time mlt to encompass the entire nightside in situ measurements confirm a dayside plume of sunward flowing plasma and images show that the plume subsequently experienced phases of mlt narrowing and rotation wrapping to simulate the event we employed a test particle representation of the plasmapause using a parametric e field model that includes corotation and convection from two sources dayside magnetopause reconnection dmr and the subauroral polarization stream saps the model captures the phases of plume evolution and it reproduces the observed plasmapause with maximum mean error 0 7l 0 24l the model fails to reproduce important subglobal features e g a predawn plasmaspheric shoulder and grossly misplaces the inner edge of the rotating and wrapping plume saps contributes significantly to the duskside flows but the simulated flows are about 50 smaller than those seen by lanl mpa the simulated plasmapause e field is about 70 of the euv inferred value

deformation of leucogranites of the crystalline main central sheet in southern tibet china abstract towards the top of the main central sheet in southern tibet china leucogranites intrude metamorphic rocks related to rocks of the main central thrust in the himalayas our study shows that the leucogranites suffered a northward shear deformation the significance of this event is discussed in terms of gravity driven decollement of the 10 000 m thick tethyan sedimentary pile over its crystalline basement

origin of 2 5 ga potassic granite from the nellore schist belt se india textural cathodoluminescence and shrimp u pb data in a geochemical and geochronological investigation of archean and proterozoic magmatism in the nellore schist belt we conducted shrimp u pb analyses of zircons from two cospatial granitic bodies at guramkonda and vendodu the former is a ba and sr rich hornblende bearing tonalite whereas the latter is a rb zr pb th u and ree rich biotite bearing leucogranite the guramkonda tonalite displays a restitic texture with remnants of trapped granitic melt whereas the vendodu leucogranite contains residual partially melted plagioclase grains both rock types contain two generations of zircon tonalite contains a group of euhedral zoned zircons enclosed within plagioclase and a group of subhedral patchy zircons associated with trapped melt quartz feldspar matrix and leucogranite also contains a group of doubly terminated euhedral zircons included within orthoclase as well as a group of zircons with visible cores mantled by later rim growth cathodoluminescence images also clearly document two distinctly textured varieties of zircon the tonalite contains a population characterized by narrowly spaced uninterrupted oscillatory zoning and a second population lacking zoning but exhibiting a random distribution of dark u rich and light u poor regions the leucogranite contains u rich zoned zircons and u poor zircon cores mantled by u rich rims the ree chemistry of zircon cores from the vendodu leucogranite is very similar to the ree of zoned zircons from the guramkonda tonalite zircon ages from both plutons exhibit bimodal distributions in u pb concordia diagrams the tonalite defines an age of 2 521 ma 5 ma for zoned magmatic zircons and 2 485 ma 5 ma for unzoned newlyprecipitated zircons whereas the leucogranite has an age of 2 518 ma 5 ma for u poor zircon cores relics of the tonalite pluton and 2 483 ma 3 ma for u rich zoned magmatic zircons the trace element geochemistry of the 2 520 ma zircons is distinctly different from the 2 485 ma zircons irrespective of the host rock our textural cl image and shrimp u pb analyses document the origin of the leucogranite by partial melting of the tonalite high alkalis na2o k2o rb nb hree feot mgo and low ca al ba sr and large negative eu anomalies characterize the leucogranite as a thermal minimum melt whereas the very low k and rb of the tonalite attests to its residual nature we suggest that the leucogranite formed by high t 900 950 c moderate pressure 10 kbar dehydration partial melting of the tonalite under reducing conditions the calculated source compositions of the leucogranite melt and the tonalite residue show strong similarities to melts that are considered to have been produced in a subduction zone environment the leucogranite probably formed in a post collisional realm immediately after accretion of the tonalitic crust

long lived plasmaspheric drainage plumes where does the plasma come from long lived weeks plasmaspheric drainage plumes are explored the long lived plumes occur during long lived high speed stream driven storms spacecraft in geosynchronous orbit see the plumes as dense plasmaspheric plasma advecting sunward toward the dayside magnetopause the older plumes have the same densities and local time widths as younger plumes and like younger plumes they are lumpy in density and they reside in a spatial gap in the electron plasma sheet in sort of a drainage corridor magnetospheric convection simulations indicate that drainage from a filled outer plasmasphere can only supply a plume for 1 5 2 days the question arises for long lived plumes and for any plume older than about 2 days where is the plasma coming from three candidate sources appear promising 1 substorm disruption of the nightside plasmasphere which may transport plasmaspheric plasma outward onto open drift orbits 2 radial transport of plasmaspheric plasma in velocity shear driven instabilities near the duskside plasmapause and 3 an anomalously high upflux of cold ionospheric protons from the tongue of ionization in the dayside ionosphere which may directly supply ionospheric plasma into the plume in the first two cases the plume is drainage of plasma from the magnetosphere in the third case it is not where the plasma in long lived plumes is coming from is a quandary to fix this dilemma further work and probably full scale simulations are needed

mesoscale pervasive felsic magma migration alternatives to dyking abstract this paper reviews the literature on dyking as a mechanism of felsic magma extraction from a source and transport to shallower crustal levels and review the recent literature suggesting a range of alternative mechanisms of magma migration in hot crustal zones which produce mesoscale pervasive granite sheet intrusions recent papers have strongly favoured dyking as the main mechanism controlling magma migration however the initiation of dykes from a felsic magma source is fraught with difficulties even when magma is immediately available for transportation as in magma chambers within a partially molten source magma may reside in a range of structures with a wide range of shapes sizes and degrees of connectivity whereas the growth of individual dykes within a partially molten zone and the self propagation of large dykes into subsolidus crust have both been studied in some detail little attention has been given to the crucial intermediate step of the growth of a dyke network capable of producing wide crustal scale dykes the rarity of granite dyke swarms suggests that if dyking is the preferred mechanism of magma transport felsic magma sources produce only few major transporting dykes during their lifetime alternatively dyking is not an important mechanism the parameters controlling the volume of the catchment drained by one such dyke as well as other basic geometrical parameters controlling the structure of the dyke network within the source are unknown the ability of dyking to drain a partially molten source depends crucially on these variables and particularly on the horizontal permeability of the source the slow velocity of viscous felsic magmas traveling in rock pores implies that magma drained during dyking is mostly that previously extracted from the pores and resident in irregular magma bodies or dyke networks the observation that large volumes of buoyant magma are commonly present in migmatite zones and that dyking in these zones plays a secondary role suggest that dyking is inefficient and is able to extract only a fraction of the total melt available in the source in support of this conclusion recent detailed studies of exhumed hot crustal zones have revealed a range of alternative migration mechanisms characterized by mesoscale pervasive magma flow outcrop scale as opposed to porous flow pervasive migration gives rise to magma sheets preferentially emplaced parallel to high permeability zones such as foliation or bedding planes apart from local dyking three alternative mechanisms have been proposed to account for pervasive migration of magmas namely tectonic pumping magma wedging into low viscosity rocks and volatile driven intrusion because of their unfocused pervasive character these mechanisms are restricted to hot crust where magmas are not exposed to rapid freezing a model is proposed whereby heat advected with the magma pushes crustal geotherms upwards allowing pervasive magma migration to shallower depths

geochemistry of granitic melts produced during the incongruent melting of muscovite implications for the extraction of himalayan leucogranite magmas granites formed from the incongruent melting of muscovite preserve evidence of chemical disequilibrium both between restite and melt and within individual phases during crystallization miocene leucogranites from the himalayas exemplify melt formation from a pelitic source through muscovite breakdown trace elements rb sr and ba reside almost entirely in micas and feldspars in the protolith and therefore are appropriate for quantitative modeling equilibrium between melt and restite was established for sr under fluid phase absent conditions but some disequilibrium is suggested by rb abundances consistent with known diffusion rates for these elements abundances of zr and ce in the granites which provide essential structural constituents of the accessory phases zircon and monazite indicate that the himalayan granite melts were saturated in these elements if formed at temperatures of 670 700 c but slightly undersaturated if formed at somewhat higher temperatures ion microprobe traverses of y and heavy rare earth element abundances across single garnets from himalayan leucogranites provide evidence for disequilibrium between melt and phenocryst during garnet growth thus constraining the evolution of magmatic compositions during melt formation and crystallization in some cases progressive melting of the protolith and crystallization of garnet from the melt appear to have been concomitant indicating that minimum solidus temperatures were maintained throughout the melting event this suggests that thermal buffering rather than melt extraction provided the rate determining step in magma evolution

the contact between the higher himalaya crystallines and the tibetan sedimentary series miocene large scale dextral shearing space and time evolution of the synmetamorphic structures across the metamorphic pile have been studied in several areas along the himalayan belt east and central nepal garhwal zanskar from one area to the other the evolution is very similar 1 at the base of the pile in the main central thrust mct shear zone the stretching lineation penetrative and regularly oriented n0 e to n30 e indicates the mct transport direction very constant all along the belt from the eohimalayan main metamorphic development up to the late metamorphic movements 2 at the top of the pile at the contact between the crystalline unit and its sedimentary cover gravity driven structures are confirmed north vergent folds ductile normal faulting however there are numerous local indications of late miocene syn to late emplacement of the leucogranitic plutons dextral shearing 3 in between across the medium part of the pile the stretching lineation shows a conspicuous progressive regional clockwise rotation clearly indicated by the strain trajectories mapped in nepal and garhwal points 1 and 2 show that the crystalline sedimentary boundary despite its apparent structural and metamorphic continuity is not only a normal fault but also an important dextral shear zone which has acted since the upper miocene as the main southern limit of the eastward extruding tibetan block

in situ signatures of residual plasmaspheric plumes observations and simulation we compare in situ observations of the los alamos national laboratory lanl magnetospheric plasma analyzers with output of a dynamic plasmapause test particle ptp simulation for the moderately disturbed interval 18 20 january 2000 in the model weakly enhanced convection on 18 january creates a narrow drainage plume plume a that wraps completely around the main torus moderate convection on 19 january triggers significant plasmaspheric erosion forming a second plume b that coexists with the narrow wrapped residual plume a we fly three virtual lanl satellites through the simulation domain the observations are globally consistent with the ptp simulation lanl data contain several intervals of plume plasma in the model s predicted magnetic local time mlt sector the modeled durations of plume sector transits are in good agreement with the lanl data on a subglobal scale the mlt widths and timings of the simulated plumes do not precisely agree with observations however several observation intervals exhibit good morphological agreement with virtual spacecraft signatures of two distinct coexisting plumes a and b the fine scale structure in the ptp model arises from the merging of residual plume a with the newer plume b plume merging is one theoretical means of generating fine structure in the plasmasphere during multiple cycles of erosion and recovery successive layers of wrapped residual plumes can merge with newer plumes creating layers of filamentary density structure the model data comparisons suggest that the plasmaspheric density distribution may preserve some memory of prior epochs of erosion and recovery

on the thermal viability of dikes leaving magma chambers dikes beginning to propagate away from a magma source are thin and grow slowly and thus are susceptible to freezing a self similar solution is obtained for a dike propagating down a temperature gradient when the wallrock and magma temperatures are equal at the chamber wall the solution applies only to the special case of a single component magma intruded at the solidus temperature but provides insight into the case in which the magma temperature is above the solidus the dike propagation velocity relative to that in the absence of freezing is determined by a single dimensionless parameter for sufficiently large magma freezing chokes the dike faster than propagation can elastically widen it and the dike cannot grow if the magma intrudes at a temperature above the solidus there is some value of above which the dike will propagate a certain distance and freeze and below which it will propagate until depletion of the source or three dimensional elastic effects promote freezing for reasonable parameters this process can explain why basaltic dikes are found far from obvious source regions while granitic dikes are scarce except in the immediate vicinity of granitic plutons even if a rhyolite is 5 orders of magnitude more viscous than a basalt however this distinction can be offset if the rhyolite has an excess source pressure only 1 order of magnitude greater than that of the basalt

fluid enhanced melting during prograde metamorphism anatexis is a commonly recognized feature of high grade metamorphism but segregated melts are generally ascribed to anatexis during peak metamorphic conditions and little is known about melting along the prograde path a suite of small volume deformed two mica leucogranites has been recognized within the high himalayan crystalline series of the garhwal himalaya these granites are consistently more siliceous than minimum melt granite compositions and are characterized by low rb sr ratios high ba low abundances of hfs elements and positive eu anomalies such trace element characteristics contrast strongly with the geochemistry of the well studied early miocene leucogranites of the high himalaya derived from fluid absent melting sm nd garnet dating of one deformed granite indicates a crystallization age of 39 3 ma c 15 ma before the emplacement of the more voluminous high himalayan leucogranites whilst some entrainment of restitic phases cannot be excluded trace element signatures suggest a low temperature 2 o activity positive eu anomalies and unusually low rb sr ratios are indicative of rapid disequilibrium melting fluid enhanced melting may be a common feature of prograde upper amphibolite facies metamorphism of orogenic belts predating peak metamorphism by at least 15 ma these melts will only crystallize within this period if they segregate from their protoliths subsequent dating of long lived melts would indicate erroneously young ages for the prograde melting events however melts formed in this way may be recognized by their distinctive trace element chemistry the persistence of early formed melts within an orogen provides insights into the prograde heating path and may be critical in controlling the rheology of the middle crust and hence its deformational history

chemical and boron isotopic compositions of tourmaline from the nyalam leucogranites south tibetan himalaya implication for their formation from b rich melt to hydrothermal fluids abstract the miocene nyalam leucogranites from the south tibetan himalaya consist of two mica leucogranite and tourmaline leucogranite in which tourmaline is an important constituent in nodular tourmaline quartz segregations within the two mica leucogranite these nodules display two basic morphologies rounded 2mg ro type and radial or dendritic 2mg ra type the nodules in the tourmaline leucogranite share similar textural features with the 2mg ro type tourmaline nodules but without the leucocratic halo tourmaline crystals in the tourmaline leucogranite display overgrowth textures most commonly in the form of homogeneous cores tg i type surrounded by oscillatory zoned rims tg ii type based on the petrography chemical and boron isotopic compositions of the various types of tourmaline it is hypothesized that the 2mg ro type tourmaline nodules crystallized during the transition from magmatic to hydrothermal evolution of the granite while the 2mg ra type tourmaline nodules likely formed during syn magmatic crystallization from boron rich melts in this hypothesis the tg i type tourmalines also formed during the transitional stage from late magmatic to early hydrothermal process tg ii type tourmalines were obviously formed after the tg i type tourmaline and can be attributed to the mixing of juvenile fluids and volatiles coming from the wall rocks to the nodule in a post magmatic hydrothermal environment where tg i type tourmaline has already crystallized thus the origin of the tourmaline nodules is related to the different stages of syn magmatic crystallization from a b rich melt or the subsequent transition from magmatic to hydrothermal environment or crystallized in the post magmatic hydrothermal fluids the average 11b values of the 2mg ra type and 2mg ro type tourmaline are 15 1 and 14 4 respectively the tourmalines in tourmaline leucogranite have slightly higher 11b values with the tg i type tourmalines averaging 12 5 and the tg ii type tourmalines averaging 12 1 the boron isotopic composition of the tourmalines has implications for the origin of the leucogranites we suggest that the distinction of the boron isotopic compositions between the source rocks of the two types of leucogranite could account for this boron isotope distinction in tourmalines

magma driven hydraulic fracturing and infiltration of fluids into the damaged host rock an example from dronning maud land antarctica abstract excellent outcrops in dronning maud land antarctica provide unique insight into the mode and extent of fluid infiltration into metamorphic and plutonic rocks in the middle crust the fluids are liberated from pegmatitic veins and give rise to alteration halos in the alteration halos the conspicuous change in colour is correlated with 1 hydration mineral reactions and 2 high density of microcracks in quartz and feldspar exceeding that observed in the unaltered host rock by an order of magnitude the field relations indicate that the veins originated as melt driven hydraulic fractures sealed by pegmatite and aplite crystallising from volatile rich melts with the alteration halo being the wake of the process zone formed at the tip of the propagating fractures it is proposed that 1 the size of the alteration zone and the width of the vein are correlated resulting in higher values of both these quantities for cracks propagating at higher velocities and consequently higher crack propagation toughnesses 2 the damage zone is characterised by a transient state of high permeability which was short lived due to rapid healing and sealing of microcracks 3 the infiltration and retrogression of the high grade rocks can be considered as a quasi instantaneous process on geologic time scales with a duration of hours to weeks

emplacement of himalayan leucogranites by magma injection along giant sill complexes examples from the cho oyu gyachung kang and everest leucogranites nepal himalaya abstract the upper part of the high himalayan slab in north central nepal is comprised of a thick layer parallel sheet of biotite muscovite tourmaline garnet sillimanite cordierite leucogranite up to 3 4 km thick and dipping north at 5 20 these strongly peraluminous magmas were emplaced into high temperature low pressure sillimanite and cordierite bearing gneisses calc silicates and rare amphibolites which were metamorphosed at temperatures of 600 650 c some time during the oligocene early miocene parallel stringers of black xenolithic gneisses within the leucogranites suggest passive magmatic intrusion along fractures parallel to the schistosity in the country rocks in the mountains of cho oyu gyachung kang pumori lingtren and the base of the everest massif these leucogranites form part of a single structural horizon bounded at the top by the lhotse detachment the lower of two n dipping normal faults of the south tibetan detachment std system and below by the khumbu thrust kt an out of sequence fault which was partly responsible for the uplift erosion and exhumation of the leucogranites a model for the emplacement of these leucogranites is proposed where they represent viscous minimum melts produced by melting of a pelitic protolith similar to the underlying sillimanite grade gneisses through muscovite breakdown either during fluid absent melting at

crustal structure restoration and evolution of the greater himalaya in nepal south tibet implications for channel flow and ductile extrusion of the middle crust

analyzing electric field morphology through data model comparisons of the geospace environment modeling inner magnetosphere storm assessment challenge events 1 the storm time inner magnetospheric electric field morphology and dynamics are assessed by comparing numerical modeling results of the plasmasphere and ring current with many in situ and remote sensing data sets two magnetic storms are analyzed 22 april 2001 and 21 23 october 2001 which are the events selected for the geospace environment modeling gem inner magnetosphere storms im s assessment challenge imsac the imsac seeks to quantify the accuracy of inner magnetospheric models as well as synthesize our understanding of this region for each storm the ring current atmosphere interaction model ram and the dynamic global core plasma model dgcpm were run together with various settings for the large scale convection electric field and the nightside ionospheric conductance while keeping all other simulation settings the same dgcpm plasmaspheric parameters were compared with image euv plasmapause extractions and lanl mpa plume locations and velocities ram parameters were compared with dst lanl mpa fluxes and moments image mena images and image hena images both qualitative and quantitative comparisons were made to determine the electric field morphology that allows the model results to best fit the plasma data at various times during these events the simulations with self consistent electric fields were in general better than those with prescribed field choices this indicates that the time dependent modulation of the inner magnetospheric electric fields by the nightside ionosphere is quite significant for accurate determination of these fields and their effects it was determined that a shielded volland stern field description driven by the 3 hour kp index yields accurate results much of the time but can be quite inconsistent the modified mcilwain field description clearly lagged in overall accuracy compared to the other fields but matched some data sets like dst quite well the rankings between the simulations varied depending on the storm and the individual data sets indicating that each field description did well for some place time and energy range during the events as well as doing less well in other places times and energies several unresolved issues regarding the storm time inner magnetospheric electric field are discussed

progression from south directed extrusion to orogen parallel extension in the southern margin of the tibetan plateau mount everest region tibet abstract in the south tibetan himalaya major detachment systems exhumed midcrustal rocks from different structural levels that are exposed in the ama drime and mount everest massifs the south tibetan detachment system stds accommodated exhumation of the greater himalayan series ghs until the middle miocene field and laboratory based structural data presented here indicate that early melt present deformation in the footwall of the stds accommodated large scale flow of a low viscosity middle crust decompression related anatexis and emplacement of leucogranites in the structurally highest positions of the ghs 16 18 ma mark the final stages of south directed extrusion within a relatively narrow solid state mylonite zone that progressed into a brittle detachment estimates of mean kinematic vorticity and deformation temperatures record a progression in deformation conditions from ultra mylonitic leucogranite 400 500 c 51 67 pure shear to mylonitic marble 300 c 41 55 pure shear to mode

empirical high latitude electric field models electric field measurements from the dynamics explorer 2 satellite have been analyzed to extend the empirical models previously developed from dawn dusk ogo 6 measurements j p heppner 1977 the analysis embraces large quantities of data from polar crossings entering and exiting the high latitudes in all magnetic local time zones paralleling the previous analysis the modeling is based on the distinctly different polar cap and dayside convective patterns that occur as a function of the sign of the y component of the interplanetary magnetic field the objective which is to represent the typical distributions of convective electric fields with a minimum number of characteristic patterns is met by deriving one pattern model bc for the northern hemisphere with a y interplanetary magnetic field imf and southern hemisphere with a y imf and two patterns models a and de for the northern hemisphere with a y imf and southern hemisphere with a y imf the most significant large scale revisions of the ogo 6 models are 1 on the dayside where the latitudinal overlap of morning and evening convection cells reverses with the sign of the imf y component 2 on the nightside where a westward flow region poleward from the harang discontinuity appears under model bc conditions and 3 magnetic local time shifts in the positions of the convection cell foci the modeling above was followed by a detailed examination of cases where the imf z component was clearly positive northward neglecting the seasonally dependent cases where irregularities obscure pattern recognition the observations range from reasonable agreement with the new bc and de models to cases where different characteristics appeared primarily at dayside high latitudes the analysis of these differences for strong bz conditions using passes through different mlt zones led to finding that the deviations from both the bc and de models could be modeled by extending the evening cell foci toward and beyond noon with a deformation that roughly resembles a rotational twist of the extended evening cell the morning cell accommodation to the model bc deformation produces a narrow tongue of eastward convection near 75 invariant latitude on the dayside the model de deformation compresses the morning cell toward the night hours such that the sunward convection in the morning cell is diverted to polar latitudes in the 0400 to 0700 mlt sector in both cases the deformations of the two cell patterns lead to sunward convection in dayside polar regions while maintaining the integrity of the nightside convection pattern thus the nightside dilemmas that plague three and four cell models designed to explain sunward convection in polar regions under bz conditions do not appear

magnetospheric convection near a drainage plume 1 we report on equatorial convection associated with a plasmaspheric drainage plume using simultaneous observations from five satellites during the early recovery phase of the july 2000 bastille day magnetic storm the extreme ultraviolet sensor on the magnetopause to aurora global exploration satellite detected the plume near 16 00 17 00 magnetic local time extending outward to l 2 8 the plasmaspheric boundary was near l 2 at other local times we mapped simultaneously measured ionospheric plasma drifts from rocsat 1 and three defense meteorological satellite program dmsp spacecraft along magnetic field lines to infer equatorial convection velocities in the inner magnetosphere the zonal component of convection derived from rocsat 1 ion drift measurements had a sharp positive azimuthal gradient near the plume s boundaries reversing direction from westward to eastward the meridional profile of horizontal velocities deduced from dmsp measurements shows a large westward flowing subauroral polarization stream saps located outside the plasmapause the peak velocity of the saps centered at a radial distance of l 2 8 with a full width of 1 re in the inertial frame of reference equatorial plasmas flowed toward the plume from both its day and evening sides suggesting a negative gradient in the equatorial azimuthal velocity that was largest near the plume s outermost boundary these observations provide new evidence about diversion of saps plasma flows and distinctive azimuthal velocity patterns in the vicinity of plasmaspheric plumes