numerical modeling of colloid facilitated radionuclide decay chain transport in a coupled fracture matrix system a numerical model is developed to simulate the colloid facilitated transport of radionuclides in a single fracture host rock coupled system with planar geometry such a system may be encountered in case of leaching and transport of radionuclides from a high level radioactive waste repository in deep geological formations the presence of colloids in the system reduces the retardation effects on the radionuclide migration and can change the concentration distribution of radionuclides in the system significantly the processes such as advection dispersion surface sorption radioactive decay and diffusive loss to the host rock are considered for transport in the fracture whereas dispersion adsorption and radioactive decay are included for transport in the host rock since the radionuclides to be disposed into the high level waste repository may include a decay chain of progeny the performance assessment methodology must incorporate the ingrowth of progeny with time to carry out realistic estimates the inclusion of decay chain buildup and transport is essential to avoid underestimation with respect to the environmental impact assessment for the repository hence the numerical model based on implicit finite difference scheme is developed with the inclusion of decay chain transport the model is verified by comparing with well known analytical solutions the application of the model is demonstrated by applying it to a five member decay chain of 238u with increase in the concentration of colloids the concentrations of the radionuclides in the fracture water increase and also reach the steady state at an earlier time the concentrations of radionuclides almost become steady throughout the distance of 200 m in the fracture as the colloid concentration reaches 5 kg m 3 or more sensitivity analysis is carried out with respect to various model parameters such as dispersion coefficient in fracture dispersion coefficient in the porous host rock porosity of the host rock and distribution coefficient of radionuclide with respect to the mobile colloids the developed model can be a useful tool for the performance assessment of high level radioactive waste repositories

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

porphyroclast systems as kinematic indicators porphyroclasts of relatively strong minerals in mylonites commonly have an internal monoclinic shape symmetry defined by tails of dynamically recrystallized material the geometry of a porphyroclast and its tails called a porhyroclast system can serve as a valuable indicator of the sense of vorticity porphyroclast systems have been divided into and types on the basis of the geometry of the tails types have wedge shaped recrystallized tails whose median lines lie on opposite sides of a reference plane parallel to the tails and containing the symmetry axis for the system types are further subdivided into a a types in which the porphyroclast is isolated in a relatively homogeneous matrix and b types in which the porphyroclast system is associated with a shear band foliation in the matrix types typically have narrow recrystallized tails whose median lines cross the reference plane adjacent to the porphyroclast consequently embayments of matrix material occur adjacent to the porphyroclasts and the tails display characteristic bends a porphyroclast system in a mylonite develops when the relatively weak dynamically recrystallized grain aggregate in the porphyroclast mantle changes its shape due to non coaxial flow in the adjacent matrix this behaviour has been modelled in shear box experiments passive marker lines around rigid cylinders embedded in silicone putty were subjected to simple shear the experiments were modified to simulate a change in recrystallization rate r with respect to rate of deformation by decreasing the diameter of the rigid cylinder during deformation at variable rates the ratio r appears to be one of the most important factors in determining which porphyroclast system will develop at high r values flow of recrystallized material away from the porphyroclast is continuously appended by the production of new grains and wedge shaped a type tails develop at low r values relatively few new grains are added to the tails which become thinned and deflected by drag due to the spinning motion of the porphyroclast in addition most porphyroclast systems at low shear strains are of a type or lack monoclinic symmetry whereas types are only developed at high shear strain values complex porphyroclast systems characterized by two generations of tails are observed in many of the natural and model shear zones studied and may form due to fluctuating r conditions that allow isolated a and type porphyroclast systems to be used as sense of vorticity indicators are the systems should have a monoclinic shape symmetry matrix grain size should be small with respect to porphyroclast size matrix fabric should be homogeneous deformation history should be simple and observations should be made on sections normal to the inferred bulk vorticity vector for the mylonite

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

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

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

thermal and dynamical regimes of single and two phase magmatic flow in dikes finite element calculations of magma flow in dikelike channels with length to width ratios of 1000 1 or more have been used to investigate the coupling between thermal and dynamical regimes due to temperature dependent viscosity and dissipation steady state solutions with realistic thermal and dynamical parameter values have been obtained the models show that the onset of solidification on the boundaries of a basaltic or andesitic dike as predicted by idealized laminar flow models can be prevented or significantly delayed by a small amount of transverse flow induced by rising bubbles boundary roughness or turbulence this effect will reduce the critical initial widths of dikes estimated by bruce and huppert 1989 in the absence of transverse flow the bulk temperature of the magma may actually increase slightly with distance along the dike as a result of viscous dissipation even while solidification is occurring on the walls of the dike with converging or necking dike walls it is found that boundary temperatures fall to a minimum and then increase with distance along a dike even if viscous heating is neglected for viscous heating to offset significant rates of heat loss 2 kw m2 in a plane parallel 1 m wide basaltic dike an average flow velocity of 2 7 m s driven by a pressure gradient of 1 7 mpa km is required a 7 void fraction caused by exsolution of volatiles or chamber gas in the magma will produce this pressure gradient the ease of producing such a gradient by reducing the density of the magmatic column with addition of a gas phase makes it likely that flows of basaltic magma could be maintained in dikes tens of kilometers long furthermore a gas phase may be important for the propagation of the fracture prior to the initial injection of magma during dike emplacement rapid transport of more silicic magmas through dikes is inhibited by the requirement of large driving pressure gradients exceeding several hundred megapascals per kilometer however the pressure gradient can be substantially reduced if the magma is a heterogenous mixture of a predominantly silicic component with a more mafic component pipe flow experiments involving molten polymers that exhibit dynamical similarity to magmas strongly suggest that unmixing occurs when a two component magma mixture in which the components have different viscosities rises within a dike within a few dike widths of the inlet the less viscous mafic component encapsulates the more viscous silicic part effectively lubricating the passage of the more viscous component compositional variations in the obsidian dome volcano support the occurrence of this pressure reduction or self lubrication mechanism such a self lubrication process often may be necessary to permit very viscous magmas to reach earth s surface if so chemical and lithologic zoning would be anticipated as a common feature of near surface intrusions or lava flows that are characterized by a high silica and or high crystal content such as at deadman dome in long valley california

the wiley glacier complex antarctic peninsula pluton growth by pulsing of granitoid magmas early cretaceous gabbro quartz diorite granodiorite and tonalite granodiorite plutons of the wiley glacier complex in the antarctic peninsula magmatic arc were emplaced in a zone of syn magmatic extensional shearing the oldest component pluton is the creswick gap quartz diorite granodiorite pluton and this is cut by slightly younger hornblende gabbro of the moore point pluton these plutons form the wall rock to the burns bluff tonalite granodiorite pluton the youngest component of the complex the burns bluff pluton comprises tonalite granodiorite sheets which at the plutons margins are interleaved with screens of mylonite wall rock the moore point hornblende gabbro has nd141 of ca 4 and sr141 of ca 1 it is predominantly mantle in origin although its parent magma assimilated some continental crust during emplacement the creswick gap pluton has nd141 between ca 4 quartz diorite and ca 1 granodiorite and sr141 from ca 1 quartz diorite to ca 22 granodiorite this pluton contains both mantle and crustal components we suggest that the quartz diorite facies fractionated from a similar parent magma to that of the moore point gabbro whilst the granodiorite fractionated from a gabbro quartz diorite magma during continued crustal assimilation geochemical variations within the creswick gap pluton cannot be generated by afc models some of the granodiorite samples have chemical characteristics which typify partial melts of amphibole garnet bearing crust we conclude that the pluton comprises fractionates of basaltic magma and crustal partial melt the burns bluff tonalite granodiorite sheets have nd141 between ca 4 and ca 2 and sr141 from ca 5 to ca 31 their isotopic and chemical compositions suggest that some are fractionates of basalt gabbro magma others are predominantly partial melts of amphibole garnet bearing crust and some are mixtures of these two magma types the chemical and isotopic compositions of the burns bluff plutons constituent tonalite sheets and its dyke like internal structure suggest that it grew incrementally via the addition of melt batches from a variety of crustal and mantle sources the growth of this pluton via dyking may reflect magma pulsing associated with transtensional movement along the creswick gap shear zone moreover the burns bluff pluton may be a frozen conduit system for structurally higher plutons in the antarctic peninsula batholith

semibrittle and cataclastic deformation of hornblende quartz rocks in a ductile shear zone abstract hornblende quartz rock of the coniston shear zone i csz i at the grenville front canada was deformed dominantly by semibrittle flow of hornblende quartz aggregate and locally by cataclastic flow of hornblende aggregates fracturing and slip along fractures produced mesoscopic domino style deformation void formation associated with domino style deformation was prevented by leveling the corners of the slid segments of individual mesodominoes producing narrow shear zones the shear zones eventually coalesced to form the dominant mylonitic c foliation on the microscopic scale semibrittle flow was achieved by dynamic recrystallization of quartz concomitant with microcracking and microfaulting of hornblende into microdominoes and rotation shear and separation of the microdominoes plastic deformation of quartz accommodated the rotation and shear of the microdominoes thereby obviating the space problem local cataclastic flow occurred in the hornblende domains and produced r 1 riedel shear surfaces the observed processes are consistent with greenschist grade retrograde metamorphism accompanying post orogenic uplift along this zone

intracrustal derivation of na rich andesitic and dacitic magmas an example from volcan ollague andean central volcanic zone andesitic and dacitic lavas erupted from quaternary stratovolcanoes in the central volcanic zone cvz of the andes have major and trace element compositions similar to proposed melts of subducted mid ocean ridge basalt and na rich trondjhemites tonalites and granodiorites ttg in early archean high grade gneiss terrains suites of lavas from individual volcanoes have average la yb of 25 45 sr y of 30 65 yb contents of 1 5 1 1 ppm and na 2 o contents 3 8 wt with lavas erupted on the volcanic front displaying the most affinity to the early archean suites because slab melting is not involved in the derivation of the cvz magmas these data indicate that rocks with early archean ttg compositions can also be produced through intracrustal differentiation processes isotopic major element and ree data from volcan ollague are consistent with a model incorporating contamination of mantle derived basaltic magmas in garnet bearing deep continental crust to produce basaltic andesite magma followed

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

continental crust crustal underplating and low q upper mantle beneath an oceanic island arc a detailed structural model of the crust subducting slab and underlying upper mantle across the northern izu ogasawara bonin island arc system is derived from a marine seismic reflection and ocean bottom seismographic refraction survey and subsequent forward modeling combined with tomographic inversion the model indicates that the crust is thickest beneath the presently active rift zone and a granitic crust may have formed in the mid crust a highly attenuative mantle that is one with low quality q seems to be confined mainly beneath the presently active rift zone in contrast high p wave velocity persists in the lower crust between the forearc and eastern margin of the back arc basin suggesting a large scale magma input responsible for the arc formation

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

constraints on rates of granitic magma transport from epidote dissolution kinetics epidote is stable only at high pressure in granitic magmas but is found in magmas crystallized at shallow crustal levels this observation suggests qualitatively that magma transport is sufficiently rapid to prevent complete epidote resorption experimental measurements of epidote dissolution showed that the presence of epidote implies that magma transport from the deep crust in these systems was rapid and that the transport was by dikes rather than diapirs

geodynamic evolution of the antarctic peninsula during mesozoic times and its bearing on weddell sea history this review of the tectonic evolution of the antarctic peninsula during mesozoic times highlights four main events 1 late triassic late jurassic extension 2 late jurassic early cretaceous dextral transpression 3 early cretaceous extension and 4 mid cretaceous compression magmatism was virtually continuous during much of this period with the exception of possible breaks in the known record in early jurassic and late jurassic early cretaceous times the second of these breaks corresponded to the first compressional event there was no apparent hiatus in the magmatic record during the mid cretaceous compressional event although there was a significant change in the pattern of sedimentation in the larsen basin on the eastern margin of the weddell sea at about that time the tectonic evolution of the peninsula is compared to and puts some constraints on existing weddell sea models the late triassic late jurassic arc extension correlates with initial rifting in the weddell sea region during sinistral motion between east and west gondwana however there is no known record of large scale pre mid jurassic transcurrent deformation in the antarctic peninsula that would have been consistent with rotation of west antarctic crustal blocks in the initial rifting period the peninsula wide late jurassic early cretaceous compressional event may correlate with the major change in gondwana plate motions from e w to n s african reference frame separation of east and west gondwana this change probably resulted in formation of an ocean continent boundary along the northern margin of the weddell sea embayment and initial seafloor spreading geological data do not seem to support subduction of southwestern proto weddell sea oceanic lithosphere beneath the eastern margin of the peninsula at that time cretaceous arc extension was coeval with the initial seafloor spreading phase in the weddell sea mid cretaceous arc compression linked to a global increase in ocean floor spreading rates and a superplume event correlates with a change from ne sw to nw se spreading in the weddell sea

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

crustal growth of the antarctic peninsula by accretion magmatism and extension a subduction accretion model incorporating new geophysical data is presented to explain the geology of the antarctic peninsula from late palaeozoic to cenozoic time according to the model the peninsula consists of overlapping accretionary magmatic and extensional regimes that were diachronous across the peninsula and have built the crust to its present form the crust which contains a small proportion of sialic basement was mainly formed by accretionary and magmatic processes and modified to its present shape by extension the gondwanide orogeny for the antarctic peninsula is interpreted in terms of the accretionary processes

a review of criteria for the identification of magmatic and tectonic foliations in granitoids abstract foliations in granitoids can form by magmatic flow submagmatic flow high temperature solid state deformation and moderate to low temperature solid state deformation a review of previous work suggests that no single criterion can consistently distinguish foliations in granitoids formed by flow during ascent diapiric emplacement and expansion emplacement during regional deformation or regional deformation post dating emplacement however a magmatic origin is favoured for foliations defined by the alignment of igneous commonly euhedral minerals particularly where the foliation is parallel to internal or external pluton contacts foliations formed during expansion or ballooning of diapirs may be strictly magmatic in origin although some studies suggest that solid state deformation also may occur if so we would hope to find evidence of deformation of crystal melt systems and that the solid state deformation occurred at high temperatures the inference of syntectonic foliations is most convincing where magmatic and high temperature solid state foliations are subparallel these foliations are continuous with regionally developed foliations in the wall rocks synkinematic porphyroblasts are present in the wallrocks and igneous minerals have the same age as metamorphic minerals associated with the regional cleavage a strictly tectonic origin for foliations in granitoids is favoured when the foliation is defined by metamorphic minerals no alignment of igneous minerals occurs the foliation is locally at high angles to pluton wallrock contacts and the foliation is continuous with a regionally developed cleavage

the transfer of bomb radiocarbon and anthropogenic lead to the deep north atlantic ocean observed from a deep sea coral abstract deep ocean c 14 pb concentrations and pb isotopes were reconstructed from a deep sea coral enallopsammia rostrata from 1410 m depth off of bermuda our high resolution time series is created from closely spaced radial cross sections with samples taken from the center of concentric coral growth bands that we show to be the oldest portion of the section prebomb radiocarbon ages from the coral demonstrate that the vertical growth rate of the coral is linear and the age of the coral is estimated to be 560 630 yr old based on the growth rate using this age model to reconstruct c 14 in deep seawater we first detect bomb radiocarbon at the coral growth site around 1980 and show that c 14 increased from 80 1 average 1930 1979 to a plateau at 39 3 1999 2001 pb ca of the coral ranges between 1 1 4 5 nmol mol during the 16th and 17th centuries and pb isotope ratios 206pb 207pb 1 21 208pb 207pb 2 495 in this period agree with pre anthropogenic values found in the pelagic sediments of the north atlantic ocean basin coral pb ca is slightly elevated to 6 2 0 9 nmol mol between the 1740s and the 1850s and then increases to 25 1 0 2 nmol mol in the 1990s the increase in coral pb ca is accompanied by a decrease in coral 206pb 207pb and 208pb 207pb indicating that the increase was caused by the infiltration of anthropogenic pb to the coral growth site comparing our data to the surface coral c 14 and pb records from bermuda reveals a time scale of tracer transport from the surface ocean to the coral growth site some characteristic features e g the bomb derived c 14 increase appear in the deep ocean approximately 25 yr later than the surface but the overall increase of c 14 and pb in the deep ocean is smaller and slower than the surface showing the importance of mixing during the transport of these tracers

thermal requirements for extensional emplacement of granitoids numerical models of magma emplacement during crustal extension show that shallow 10 km rhyolitic to andesitic magma chambers can form and be maintained at typical rates of extension of continental crust chambers at depths of 8 km form within 90 ka after emplacement of a total width of 4 km of magma at half extension rates of 2 cm yr once even a small 1 km wide shallow chamber has formed by any process it can be maintained by extension at a half rate of 1 cm yr high temperatures are maintained at a half rate of 0 5 cm yr continental rifts and dilational jogs along strike slip faults in arcs provide areas where extension rates are most appropriate large granitoids and batholiths can form by this process over millions of years actual chambers at any one time may be considerably smaller than preserved granitoids

production of jurassic rhyolite by anatexis of the lower crust of patagonia abstract the mid jurassic marifil and chon aike volcanic rocks of eastern patagonia are part of one of the largest silicic igneous provinces known rb sr geochronology indicates eruptive ages of 175 190 ma for the marifil complex mostly toarcian aalenian the majority of the rocks are isotopically uniform with initial 87sr 86sr 0 7067 0 0003 and endt 4 2 primary magmas of andesitic composition were generated by partial melting of mafic grenvillian lower crust indentified by depleted mantle model ages of 1150 1600 ma lower crustal pyroxene granulite xenoliths can be modelled as residual jurassic source although they may alternatively be co genetic cumulates the dacite rhyolite suite formed by crystal liquid fractionation processes from the primary andesites involving multistage crystallization and re melting during magma ascent with the later stages of evolution being explicable by fractional crystallization of plagioclase amphibole and accessory minerals the province represents large scale lower crustal reworking associated with the unique tectonic and igneous environment of gondwana break up