incremental growth of an upper crustal a type pluton argentina evidence of a re used magma pathway abstract carboniferous igneous activity in the sierra de velasco nw argentina led to the emplacement of several magmas bodies at shallow levels t 5 8 and ehf t in zircon 2 9 to 4 5 the second group comprises the metaluminous to peraluminous granitic rocks of the youngest units san blas and hualco which were formed by a heterogeneous mixture between mantle and crustal sources end t 0 6 to 4 8 and ehf t in zircon 3 to 6 our results provide a comprehensive view of the evolution of an intrusive complex formed from multiple non consanguineous magma intrusions that utilized the same magmatic plumbing system during downward transfer of host materials as the plutonic system matures the ascent of magmas is governed by the visco elastic flow of host rock that for younger batches include older hot magma mush the latter results in ductile downward flow of older during rise of younger magma such complexes may reflect the plutonic portion of volcanic centers where chemically distinct magmas are erupted

late pleistocene granodiorite beneath crater lake caldera oregon dated by ion microprobe variably melted granodiorite blocks ejected during the holocene caldera forming eruption of mount mazama were plucked from the walls of the climactic magma chamber at 5 km depth ion microprobe u pb dating of zircons from two unmelted granodiorite blocks with shrimp rg sensitive high resolution ion microprobe reverse geometry gives a nominal 238 u 206 pb age of 100 78 80 ka or 174 89 115 ka when adjusted for an initial 230 th deficit shrimp rg u th measurements on a subset of the zircons yield a 230 th 238 u isochron age of 112 24 ka considered to be the best estimate of the time of solidification of the pluton these results suggest that the granodiorite is related to andesite and dacite of mount mazama and not to magmas of the climactic eruption the unexposed granodiorite has an area of at least 28 km 2 this young shallow pluton was emplaced in virtually the same location where a similarly large magma body accumulated and powered violent explosive eruptions 7700 yr ago resulting in collapse of crater lake caldera

discriminating assimilants and decoupling deep vs shallow level crystal records at mount adams using 238u 230th disequilibria and os isotopes article a suite of 23 basaltic to dacitic lavas erupted over the last 350 kyr from the mount adams volcanic field has been analyzed for u th isotope compositions to evaluate the roles of mantle versus crustal components during magma genesis all of the lavas have 230 th 238 u n1 and span a large range in 230 th 232 th ratios

magma flow paths and strain patterns in magma chambers growing by floor subsidence a model based on magnetic fabric study of shallow level plutons in the stiavnica volcano plutonic complex western carpathians the miocene stiavnica volcano plutonic complex western carpathians exposes two nearly coeval intra caldera plutons their roof basement of a stratovolcano and associated volcanic rocks the complex thus provides insights into mechanisms of magma chamber growth beneath large volcanoes as inferred from the anisotropy of magnetic susceptibility ams these plutons were emplaced through significantly different processes the diorite as a discordant stock with steep fabric and the granodiorite as a tabular bell jar pluton in detail we interpret that the latter was assembled in two stages first an upper layer intruded as a thin sill along a major subhorizontal basement cover detachment the subhorizontal magnetic fabric and strongly oblate ams ellipsoid in this layer record intrusive strain where the magma flow paths were subparallel to the pluton roof second in the lower layer of the pluton magnetic foliations dip moderately to the nw and wnw to vertical and are associated with down dip to subhorizontal lineations and prolate to weakly oblate shapes of the ams ellipsoids such a fabric pattern is compatible with piecemeal floor subsidence where magma flowed along multiple subsiding fault bounded blocks based on this case example we develop a conceptual model for magma flow paths and strain patterns for four main modes of floor subsidence 1 piston cauldron subsidence is characterized by convergent flow and radial principal stretching above the magma chamber floor 2 the piecemeal floor subsidence leads to steep to inclined magma flow paths in conduits along fault bounded blocks 3 asymmetric trapdoor subsidence produces first divergent flow paths near the conduit sides changing into convergent paths in the narrower space near the kinematic hinge and 4 symmetric cantilever funnel subsidence will lead to divergent flow from a central feeder and thus circumferential principal stretching of the magma if the growing pluton develops a two layer structure all the flow paths and associated strains are affected by the flat lying pluton roof and will convert into horizontal flattening as the roof is approached

depositional features and stratigraphic sections in granitic plutons implications for the emplacement and crystallization of granitic magma abstract many granitic plutons contain sheet like masses of dioritic to gabbroic rocks or swarms of mafic to intermediate enclaves which represent the input of higher temperature more mafic magma during crystallization of the granitic plutons small scale structures associated with these bodies e g load cast and compaction features silicic pipes extending from granitic layers into adjacent gabbroic sheets indicate that the sheets and enclave swarms were deposited on a floor of the magma chamber on granitic crystal mush and beneath crystal poor magma while the mafic magma was incompletely crystallized these structures indicate way up typically toward the interior of the intrusions and appear to indicate that packages of mafic sheets and enclave concentrations in these plutons are a record of sequential deposition hence these plutons preserve a stratigraphic history of events involved in the construction filling replenishment and crystallization of the magma chamber the distinctive features of these depositional portions of plutons allow them to be distinguished from sheeted intrusions which usually preserve mutual intrusive contacts and dike sill relations of different magma types the considerable thickness of material that can be interpreted as depositional and the evidence for replenishment suggest that magma chamber volumes at any one time were probably much less than the final size of the pluton thus magma chambers may be constructed much more slowly than presently envisaged the present steep attitudes of these structures in many plutons may have developed gradually as the floor of the chamber along with the underlying solidified granite and country rock sank during continuing episodes of magma chamber replenishment these internal magmatic structures support recent suggestions that the room problem for granites could be largely accommodated by downward movement of country rock beneath the magma chamber

late pleistocene granodiorite source for recycled zircon and phenocrysts in rhyodacite lava at crater lake oregon abstract rhyodacite tephra and three lavas erupted 27 ka interpreted to be early leaks from the climactic magma chamber of mount mazama contain ubiquitous resorbed crystals antecrysts that were recycled from young granodiorite and related plutonic rocks of the same magmatic system the shallow composite pluton is represented by blocks ejected in the 7 7 ka climactic eruption that formed crater lake caldera plagioclase crystals in both rhyodacite and granodiorites commonly have cores with crystallographically oriented fe oxide needles exsolved at subsolidus conditions at least 80 of plagioclase crystals in the rhyodacite are antecrysts derived from plutonic rocks other crystals in the rhyodacite notably zircon also were recycled sims 238 u 230 th dating indicates that zircons in 4 granodiorite blocks crystallized at various times between 20 ka and 300 ka with concentrations of analyses near 50 70 110 and 200 ka that correspond to periods of dacitic volcanism dated by k ar u th ages of zircon from a rhyodacite sample yield similar results no analyzed zircons from the granodiorite or rhyodacite are pre quaternary zircon minimum ages in blocks from different locations around the caldera reflect ages of nearby volcanic vents and may map the distribution of intrusions within a composite pluton survival of zircon in zircon undersaturated hydrous magma and of fe oxide needles in plagioclase suggests that little time elapsed from entrainment of antecrysts to the 27 ka eruption of the rhyodacite the 27 ka rhyodacite is an example of young silicic magma that preserved unstable antecrysts from a known source early during growth of a large high level magma chamber in contrast the voluminous 7 7 ka climactic rhyodacite pumice is virtually lacking in zircon indicating dissolution of any granodioritic debris in the intervening period mineralogical evidence of assimilation may be destroyed in hot vigorously growing silicic magma bodies such as ultimately produced the climactic eruption of mount mazama

klamath blue mountain lineament oregon regional gravity data clearly show a zone of southwest northeast lineations across oregon that defines a major crustal lineament its existence is supported by geologic and geophysical data its correlation with the northwestern boundaries of the klamath and blue mountain provinces suggests that these are continuous beneath the cascade volcanic arc the lineament may represent a pre tertiary strike slip continental margin which from paleomagnetic evidence later rotated clockwise into its present position

thermal mechanical modeling of the role of thermal stresses and stoping in magma contamination mantle derived magmas emplaced in the upper crust depth 10 km are in thermal disequilibrium with the surrounding country rock as a consequence of the large temperature contrasts not only will the magma cool quite rapidly but the surrounding country rock will be heated and affected by large thermal stresses using coupled time dependent thermal mechanical models we find that a a single stage 3 km radius magma body has an eruptive life of on the order of 104 years b near the magma wall rock boundary large thermal stresses develop over short time spans 10 years and c material stoped into the chamber as a result of these thermal stresses will be assimilated either in the upper reaches of the chamber or at its base leaving a relatively uncontaminated central core these results indicate that contamination of a primary magma by stoped crustal blocks is an efficient mechanism for producing hybrid lava sequences

local displacement of diapir contacts and its importance to pluton emplacement study abstract the extent to which mid crustal plutons have grown by diapirism cannot be deduced from the horizontal shortening of ductilely deformed host rocks and associated displacement of lateral intrusive contacts by contrast the vertical shortening above plutons and upward displacement of their roof contacts may provide a good measure of the diapiric component of pluton emplacement this is related to the vertical bulk translation of buoyant material in immature diapirs and associated escape flow of overlying rocks the length change of a vertical material line situated above the crest of an immature upright diapir corresponds to the upward displacement of the local contact the length change of horizonal material lines situated at lateral contacts of model diapirs however differs markedly from the contact displacement this is related to the absence of horizontal symmetry planes in the finite strain field about upright diapirs of any shape and the importance of vertical shear at the lateral contacts the host rock strain held of a hypothetical pluton grown in situ by spherical expansion ballooning has an infinite number of symmetry planes the strain held is characterized by large volume losses but the length change of all radial lines corresponds to the local magnitude of contact displacement this precludes the possibility of differentiating in ductile host rocks above plutons between vertical diapirism and spherical expansion unless gauges of volume change are available

shallow and deep crustal control on differentiation of calc alkaline and tholeiitic magma article u 230 th isotopes 87 sr 86 sr lower crust assimilation the role of changing crustal interaction and plumbing geometry in modulating calc alkaline vs tholeiitic magma affinity is well illustrated by the influence of 70 km thick crust beneath volcan parinacota changes in petrologic affinity correlate with periods of cone building sector collapse and rebuilding of the volcano over the last 52 ka and are well explained by changes in magma recharge regime with increasing recharge and magma output lavas transition from low fe strongly calc alkaline phenocryst rich silicic compositions to medium fe near tholeiitic mafic and aphanitic characteristics strontium isotope data show that the change in magma regime did not affect all parts of the system simultaneously these are characterized by distinctive 87 sr 86 sr ratios which suggest an initially compartmentalized system relatively high 230 th 232 th activity ratios of 0 72 in early erupted calc alkaline lavas are consistent with interaction with high u upper crust low 230 th 232 th activity ratios of 0 55 and up to 33 th excess in younger near tholeiitic lavas correlate with steep ree patterns indicating lower crustal interaction thorium excesses at the time of eruption approach the maximum that can be generated via small degree garnet residual melting in the lower crust or mantle and imply that transit time through the crustal column for the most tholeiitic magmas had to be short on the order of b2 10 4 yr in contrast lavas with greatest calc alkalinity are also at or near secular

a view from the roof magmatic stoping in the shallow crust chita pluton argentina nearly 2 km of vertical relief and exposure of continuous pluton roof wall corners around the chita pluton northwest argentina provides constraints on the 3 d host rock displacement field attending magma emplacement in the shallow crust the pluton is rectangular in crosssectional view and consists of weakly to non deformed granite to granodiorite structures in surrounding metasedimentary host rocks are truncated at a knife sharp contact and are only weakly deflected from their regional orientations final emplacement occurred by stoping together with minor doming of the earth s surface structural restoration of a cross section indicates that host rocks above the pluton may have been domed upwards during emplacement by as much as a few hundred meters accounting for up to 20 of the exposed pluton volume the remaining 80 of the space for pluton emplacement was made by downward removal of host rock from the present level of exposure although mechanisms such as floor subsidence are permissive late stoping has destroyed evidence for the early emplacement history however geometrical constraints provided by exposure of continuous wall to roof contacts indicate that early emplacement mechanisms at this level in the crust and immediately below were also dominated by the downward displacement of host rocks through a conduit that now is represented by the solidified pluton q 2002 elsevier science ltd all rights reserved

roof and walls of the red mountain creek pluton eastern sierra nevada california usa implications for process zones during pluton emplacement abstract the red mountain creek pluton has a sub circular outline in map section generally flat roof and steep sides the pluton roof sharply truncates host rock markers has a highly irregular geometry with rectangular steps and passes continuously into a steep wall the roof wall transition is not offset by faults and shows no evidence of extensive diking or synemplacement ductile strain which requires that the pluton roof and its wall remained attached to each other and were not faulted or ductilely deformed during emplacement of the pluton we propose that the exposed section through the red mountain creek pluton may represent the crestal portion of a vertically extensive piston shaped plutonic system the upper part of which was largely emplaced by magmatic stoping although other material transfer processes may have previously operated during emplacement characteristics of the pluton roof presented in this paper as well as in other papers on pluton roofs directly rule out some emplacement models commonly applied to plutons in the sierra nevada place limitations on other models and fit well the expected characteristics of visco elastic diapirs around which variable material transfer processes largely displace host rock downwards in the upper crustal plutons the most widespread preserved process of downward transport of host rock was magmatic stoping

osmium isotope constraints on lower crustal recycling and pluton preservation at lassen volcanic center ca osmium isotope compositions of intermediate to silicic composition calc alkaline volcanic rocks from the lassen volcanic region of the cascade arc are significantly more radiogenic qos 23 to 224 than typical mantle these evolved arc rocks in the lassen region have unradiogenic sr nd and pb isotope compositions which overlap with those of contemporaneous mafic lavas crystal fractionation of mafic to intermediate composition magmas produces re os ratios that are high enough to evolve to very radiogenic os isotope compositions in only a few million years providing a potential fingerprint for detecting the involvement of such young relatively mafic crust in magmatic systems however the sr nd and pb isotope compositions will remain constant over such short time intervals due to relatively low parent daughter enrichment during magmatic evolution the radiogenic os isotope compositions in typically evolved lassen rocks are interpreted to reflect significant interaction with lower crustal material that has radiogenic os isotope compositions beneath this section of the cascade arc large amounts of such high qos lower crust may have formed and been isolated from mash zone mixing and homogenization processes during the pliocene or late miocene the results from this study indicate that os isotopes may provide a unique glimpse into lower crustal processes such as recycling in primitive orogenic arcs 9 2002 elsevier science b v all rights reserved

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

late paleozoic to triassic evolution of the gondwana margin evidence from chilean frontal cordilleran batholiths 28 s to 31 s upper paleozoic to triassic chilean granitoids in the andean frontal cordillera between 28 s and 31 s record crustal and mantle conditions at the gondwana margin during the final assembly and initial breakup of the pangea supercontinent this period overlaps the end of paleozoic terrane accretion and precedes andean subduction integration of new trace element and isotopic data with other information on the granitoids and the regional geology leads to a tectonic model that has implications for other parts of the gondwana margin in the model the carboniferous to early permian is a period of oblique convergence associated elqui complex granitoids are diverse those in the guanta and montosa units are predominantly related to subduction processes whereas those in the cochiguas and el volcan units are dominated by melting of the subduction complex and older crust progressive oblique collision of the last pre pangea terrane equis along the margin resulted in crustal thickening associated with shortening deformation of foreland basinal sedimentary rocks and uplift of the elqui complex subsequent gravitational collapse of the inactive slab and lithospheric delamination resulted in the production of large amounts of basalt which intruded and melted the crust producing the post collisional ingaguas complex the los carricitos granitoids formed in thickened crust whereas the chollay el colorado and el leon units formed in thinner crust the ingaguas complex is part of the choiyoi granite rhyolite province whose formation similar to that of other gondwana silicic provinces was probably accentuated by anomalously hot upper mantle associated with the pangea supercontinent

deformation around a rising diapir modeled by creeping flow past a sphere a new experimental method for determining the pattern of flow and three dimensional progressive finite deformation of fluid elements in low reynolds number re viscous flow has been applied to creeping flow past a rigid sphere falling in a cylinder this is intended as a model analogue of natural rock deformation associated with magmatic and solid state diapirs rising through ductile crust the kinematics of finite strain in such analogue experiments may give useful information on the development of natural structures the experiment and a new method for quantifying the progressive finite deformation of fluid volumes in the transparent newtonian test fluid used sgm36 are discussed in detail the progressive finite deformation of fluid flowing along specific streamlines passing the sphere appears to be complex and largely a function of the initial distance of fluid volumes and the particular streamlines which they follow from the flow axis the progressive deformation history in the material surrounding a rising spherical diapir is characterized by progressive flattening and rotation of material above and progressive stretching below the sphere the results compare favorably with the external strain pattern associated with post tectonic mesozonal diapiric intrusions

simulations of crustal anatexis implications for the growth and differentiation of continental crust there is overwhelming and incontrovertible petrological and geophysical evidence for the significant role played by mantle derived mafic magma in the generation and growth of continental crust likewise intrusion of mafic magmas beneath or into continental crust is very likely the major source of enthalpy that drives intracrustal differentiation a simple dynamical model has been constructed to examine the critical factors that govern the evolution and time scale of crustal anatexic events when driven by basaltic magma underplating or injection into crust critical factors include the intensity of the enthalpy input i e the power dissipated and the rheological properties bulk composition and compositional structure of the source rocks undergoing partial fusion we have performed numerical simulations to evaluate these factors using phase equilibria and thermochemical and transport property data applicable to the binary eutectic system caal2si2o8 camgsi2o6 as a rough analog to study anatexis of mafic lower crust the role of enthalpy power is tested by varying the enthalpy or temperature along the base of the crustal block while maintaining a fixed temperature at the top as ratio tbot ttop increases modestly from 1 05 to 1 15 the average fraction of melt at steady state in the anatexic region increases from 50 to 75 time to attain steady state scales inversely with tbot ttop with an increase by a factor of 2 for a 10 decrease in tbot ttop typical anatexic timescales are in the range 103 to 105 years for length scales in range 102 to 103 m the consequences of different rheological models especially the importance of darcy percolative flow relative to en masse flow within the partial melt region was also investigated a value of the solid fraction called the critical value scrit is set such that for 0 0 5 relatively large volumes of nearly homogeneous melt are rapidly generated in distinction for scrit small more enthalpy is stored in the solid and less melt is produced however the compositional spectrum of liquids generated within the anatexic region is significantly larger for small scrit because of the sensitive dependence of solidi and liquidi on bulk composition melt productivity is related to bulk composition given a particular enthalpy power input and rheological model there is a factor of 2 difference in volume of melt generated when the bulk composition of the source is 10 modal percent less refractory compositionally zoned melts form by melting of either homogeneous or layered sources although composition frequency relations are sensitive to the initial subsolidus compositional structure the effects of anatexic events are to fundamentally reorganize the pattern of compositional structure within the crust intracrustal differentiation is a long term inevitable process associated with the underplating or injection of mafic magma into preexisting crust

crystallization of accessory phases in magmas by local saturation adjacent to phenocrysts abstract accessory minerals commonly occur attached to or included in the major crystalline phases of felsic and some intermediate igneous rocks apatite is particularly common as inclusions but fe ti oxides pyrrhotite zircon monazite chevkinite and xenotime are also known from silicic rocks accessories may nucleate near the host crystal liquid interface as a result of local saturation owing to formation of a differentiated chemical boundary layer in which accessory mineral solubility would be lower than in the surrounding liquid differentiation of this boundary layer would be greatest adjacent to ferromagnesian phenocrysts especially fe ti oxides it is with oxides that accessories are most commonly associated in rocks a boundary layer may develop if the crystal grows more rapidly than diffusion can transport incorporated and rejected elements to and from the phenocryst diffusion must dominate over convection as a mode of mass transfer near the advancing crystal liquid interface in order for a boundary layer to exist accumulation of essential structural constituent elements of accessory minerals owing to their slow diffusion in evolved silicate melt also may force local saturation but this is not a process that applies to all cases local saturation is an attractive mechanism for enhancing fractionation during crystallization differentiation if accessory minerals attached to or included in phenocrysts formed because of local saturation their host phenocrysts must have grown rapidly when accessories nucleated in comparison to lifetimes of magma reservoirs some inconsistencies remain in a local saturation origin for accessory phases that cannot be evaluated without additional information

coupled deformation and metamorphism in the roof of a tabular midcrustal igneous complex the 370 377 ma mooselookmeguntic igneous complex was emplaced at 14 km depth into steeply dipping metaturbidites that were folded and metamorphosed ca 400 405 ma during the acadian orog eny gravity and drill hole data isograd geometry thermal modeling and struc tural measurements all indicate that the eastern aureole of the complex represents an areally extensive roof above the gently east dipping tabular shaped intrusion this roof preserves a classic example of low pressure high temperature metamorphism caused by the underlying intrusion ther mal modeling indicates that the roof is no more than 1000 m thick over an area exceed ing 100 km2 unlike other roofs described in the literature the mooselookmeguntic igneous complex roof preserves a thick 600 m emplacement related strain gradi ent with a deformational fabric that evolved through all stages of crenulation cleavage to become an intense nondifferentiated folia tion approximately parallel to the intrusive contact and perpendicular to the steeply dip ping regional acadian age foliation fabric evolution through all stages of crenulation cleavage requires the vertical growth of the tabular intrusion to have been largely accommodated by dissolution precipitation creep which is a linear viscous deformation mechanism that can occur at lower differ ential stress than for example dislocation creep as the crenulation cleavage evolves a stage is reached where further deformation requires a change in deformation mecha nism which may lead to hardening of the rock development of crenulation cleavage requires a mica rich foliation at an initially high angle to the emplacement related fl at tening plane and this confi guration may be the primary reason why the strain aureole in the mooselookmeguntic igneous complex roof is so well developed compared to other published examples of midcrustal roofs

aureole deformation by flattening around a diapir during in situ ballooning the cannibal creek granite the cannibal creek granite a foliated pluton within a strongly deformed aureole shows several diagnostic features of a diapir the intensity of the aureole deformation diminished away from the contact the aureole foliation is continuous with that of the granite and forms closed elliptical trend lines and the contact metamorphic minerals are synkinematic with regard to the aureole foliation the pluton deformed its aureole in three stages involving initially brittle then ductile and finally brittle deformation styles initial intrusion of a low strength magma stoped kinked and flexed aside its overburden crystallization of the magma prevented further rise and it ballooned and deformed its aureole evidently entirely by ductile bulk shortening perpendicular to the contact this the major stage of aureole deformation records no evidence of vertical movement of the pluton or of progressive simple shear along its margin during this stage the magma may have been theologically a high strength bing

translation and the resolution of the pluton space problem abstract the late cretaceous mono creek granite has a pronounced nw se elongate shape 60 km long by 10 km wide characteristic of plutons from the eastern sierra nevada batholith an 8 km wide bulge exists on the ne side of this pluton which exhibits evidence of forceful emplacement or in situ ballooning such as deflection of metamorphic wallrock and igneous foliation and the orientation of fracture patterns three dimensional strain analysis indicates that wallrock strains do not provide enough volume to accommodate the emplacement of the bulge a recurring problem in studies of plutonic terranes we suggest that emplacement of the mono creek bulge was accommodated by all components of the three dimensional displacement field including translation rotation and pure strain shape change of the surrounding units classical strain analysis only addresses the rotation and pure strain components and is incapable of quantifying the translation component however our analysis suggests that translation plays the dominant role in the emplacement process a shell model of translation of the surrounding igneous and metamorphic units is proposed for the mono creek bulge which suggests that the translation component decreases dramatically away from the intrusion consistent with the observed geology and finite strain analysis we propose that translation is the solution to the recurring pluton space problem either through tectonically controlled passive or magmatically controlled active movement of the wallrocks translation is generally the neglected component of the displacement field but it may often be evaluated through judicious use of finite strain analysis and tectonic reconstruction

multistage emplacement of the mount givens pluton central sierra nevada batholith california the ca 90 ma mount givens pluton is one of the largest granodioritic to granitic intrusions in the sierra nevada batholith of california emplacement of the pluton occurred during a critical time in the tectonic evolution of the central sierra nevada magmatic arc marked by a transition from regional contraction to dextral transcurrent shear a model for the emplacement of the intrusion is developed based on detailed mapping of the pluton and its wall rocks and characterization of its internal structure by measurements of the anisotropy of magnetic susceptibility ams at 351 stations one of the key results of the study is documentation of a strong correlation between petrologic and structural fabrics in the pluton and determination that these fabrics reflect internal magma chamber dynamics more than regional tectonic strain the 80 km long 15 30 km wide pluton crystallized from a multiphase three segment magma chamber marked by a bulbous northern lobe and linear central and southern segments the pluton is interpreted to be tabular in shape with a thickness of 5 km most of the space for the pluton was created by piecemeal block downdrop of the magma chamber floor along three principal fracture sets the most important of which were steeply dipping northwest trending fractures formed parallel to the structural grain of the arc and vertical north trending extension fractures formed in response to a component of arc parallel dextral shear some of these fractures acted as magma conduits episodically filling the pluton as source rocks became depleted in melt an initial voluminous intrusive event stage 1 quickly filled the southern chamber with granodiorite magma but only partially filled the northern and central chambers stage 2 magmatism involved underplating of megacrystic granite in the northern chamber and lateral flow of a large batch of this magma from the northern to the central chamber the latter delineated by a 20 km long belt of megacrystic granite containing subhorizontal magnetic lineations that connects the pluton segments floor downdrop eventually ceased to be an effective space making process in the northern lobe and renewed magmatism stage 3 led to expansion and doming of the chamber as the northern lobe cooled a ring fault ruptured within the viscoelastic stage 1 2 carapace allowing ring dike intrusion stage 4 and sinking of a central flap of consolidated material the temporal and spatial variations in emplacement mechanisms demonstrated for the mount givens pluton i e fracture generation floor downdrop underplating inflation ring diking suggest that end member models e g fracture vs diapir are oversimplifications of the pluton assembly process

volcanic signature of basin and range extension on the shrinking cascade arc klamath falls keno area oregon 1 detailed geologic mapping of the klamath falls keno area revealed the complex relationship between subduction crustal extension and magmatic composition of the southern oregon cascade volcanic arc volcanism in the study area at 7 4 ma consisted of calc alkaline basaltic andesite and andesite lava flowing over a relatively flat landscape local angular unconformities are evidence that basin and range extension began at by at least 4 ma and continues today with fault blocks tilting at a long term rate of 2 ma to 3 ma minimum nw se extension is 1 5 km over 28 km 5 high alumina olivine tholeiite haot or low k low ti transitional high alumina olivine tholeiite lklt erupted within and adjacent to the back edge of the calc alkaline arc as the edge receded westward at a rate of 10 km ma at 2 7 0 45 ma the volcanic front migrated east much slower than the back arc migrated west 0 km ma for 6 0 4 ma calc alkaline rocks 0 7 km ma if 6 ma haot lklt is included and 1 km ma if highly differentiated 17 30 ma volcanic rocks of the early western cascades are included declining convergence probably decreased asthenospheric corner flow decreasing width of calc alkaline and haot lklt volcanism and the associated heat flow anomaly the margins of which focused on basin and range extension and leakage of haot lklt magma to the surface this declining corner flow combined with steepening slab dip shifted the back arc west compensation of extension by volcanic intrusion and extrusion allowed growth of imposing range front fault scarps only behind the trailing edge of the shrinking arc

diapiric ascent of magmas through power law crust and mantle there has never been a convincing explanation of the way in which diapirs of molten granite can effectively rise through mantle and crust we argue here that this is mainly because the country rocks have previously been assumed to be newtonian and we show that granitoid diapirs rising through thermally graded power law crust may indeed rise to shallow crustal levels while still molten the ascent velocity of diapirs is calculated through an equation with the form of the hadamard rybczynski equation for the rise of spheres through newtonian ambient fluids this well known equation is corrected by factors dependent on the power law exponent n of the ambient fluid and the viscosity contrast between the drop and the ambient fluid these correction factors were derived from results reported in the fluid mechanical and chemical engineering literature for the ascent of newtonian drops through power law fluids the equation allows calculation of the ascent rates of diapirs by direct application of rheological parameters of rocks the velocity equation is numerically integrated for the ascent of diapirs through a lithosphere in which the temperature increases with depth the depth of solidification of the diapir is systematically studied as a function of the geothermal gradient buoyancy of the body solidus temperature of the magma and rheological parameters of the wall rock the results show that when the wall rock behaves as a power law fluid the diapir s ascent rate increases without a similar increase in the rate of heat loss in this way diapirs rising at 10 to 102 m yr can ascend into the middle or upper crust before solidification strain rate softening rather than thermal softening is the mechanism that allows diapirism to occur at such rates the thermal energy of the diapir is used to soften the country rock only at late stages of ascent the transport of magmas through the lower crust and mantle as diapirs is shown to be as effective as magmatic ascent through fractures

magmatic stoping as an important emplacement mechanism of variscan plutons evidence from roof pendants in the central bohemian plutonic complex bohemian massif the presence of numerous roof pendants stoped blocks and discordant intrusive contacts suggests that magmatic stoping was a widespread large scale process during the final construction of the central bohemian plutonic complex bohemian massif the measured total length of the discordant contacts that cut off the regional cleavage and were presumably formed by stoping corresponds to about half of all contacts with the upper crustal host rocks in addition at least some of the straight cleavage parallel intrusive contacts may also have recorded complex intrusive histories ending with piecemeal stoping of thin cleavage bounded host rock blocks into the magma chamber based on the above we argue that the fast strain rates required for emplacement of large plutons of the central bohemian plutonic complex into brittle upper crustal host rocks over relatively short time span could not have been accommodated entirely by slow ductile flow or slip along faults instead the emplacement was largely accommodated by much faster thermal cracking and extensive stoping independent of regional tectonic deformation finally we emphasize that magmatic stoping may significantly modify the preserved structural patterns around plutons may operate as an important mechanism of final construction of upper crustal plutons and thus may contribute to vertical recycling and downward transport of crustal material within the magma plumbing systems in the crust

melting of a subducting oceanic crust from u th disequilibria in austral andean lavas understanding crustal genesis at convergent plate boundaries is important for determining mass transfer between different geochemical reservoirs in the earth s mantle and for deciphering the long term growth of the continental crust most arc magmas are thought to be generated from fluid induced melting of the mantle wedge above slabs of subducting oceanic crust1 such magmas frequently display 238u enrichments or radioactive equilibrium2 3 between 238u and its radiogenic product 230th but where a young and hot oceanic crust is being subducted it may itself partially melt and produce calc alkaline andesites and dacites termed adakites4 here we report a uniform excess of 230th over 238u but variable th isotope ratios in young adakites from the andean austral volcanic zone south of the triple junction where the chile ridge subducts beneath south america we show that these results are compatible with the adakites having been formed by approximately 20 equilibrium melting due to amphibole decomposition in a heterogeneous5 oceanic crust moreover both the degree of melting of the oceanic crust and its thermal structure appear to be uniform under most of the andean austral volcanic zone such partial melting of subducted oceanic slabs may have occurred throughout the earth s history where young oceanic plates were subducted

laser probe 40ar 39ar investigation of a pseudotachylyte and its host rock from the outer isles thrust scotland the formation of a friction melt or pseudotachylyte in the footwall of the outer isles thrust scotland during thrust movement in the early paleozoic caledonian orogeny caused argon loss from biotites in the host gneiss mean 40 ar 39 ar ages for biotite grains decrease from 1450 to 923 ma in a zone 730 c in the host gneiss these high temperatures may have caused catastrophic argon loss rather than loss by volume diffusion an 40 ar 39 ar age traverse across the pseudotachylyte vein revealed old ages adjacent to the margin reflecting the incorporation of partially outgassed host rock clasts apparent ages for the pseudotachylyte decrease unevenly toward the center of the vein a weighted mean age of 430 6 ma 2 obtained in the center corresponds closely to movement ages derived for the associated moine thrust zone

soluble impurities in the byrd station ice core antarctica their origin and sources polar ice cores contain long and detailed records of paleoatmospheric composition paleoclimate and their evolution through time by making a complete ionic balance in melted samples of ice it is possible to evaluate the origin identification of chemical compounds and sources continental marine volcanic extraterrestrial of soluble impurities contained in these ice cores once these are known an attempt can be made to try and explain the cause of any observed changes in this paper we present results of analyses of the major soluble impurities h na nh4 k so4 no3 cl made on bulk samples selected at approximately 50 m intervals down the length of an ice core from byrd station antarctica 80 01 s 119 31 w 1530 m most samples are in ionic balance and it is therefore possible to suggest with which compounds the ionic impurities are linked these compounds include an important contribution from both sea salts nacl and na2so4 and strong acids approximately equal amounts of hno3 and h2so4 the cl na mole ratio is found to be quite stable throughout the core and hovers about the expected bulk sea water ratio in general the marine derived components at byrd station decreased by a factor of 2 between the late glacial maximum and holocene while the gas derived acid components decreased by a factor of only 1 5 some possible causes of these variations are suggested

stoped blocks in plutons paleo plumb bobs viscometers or chronometers abstract stoped blocks potentially provide information about paleohorizontal during emplacement of plutons and about the timing of and viscosities and kinematics during magmatic fabric formation in the mount stuart batholith washington we have examined magmatic fabrics patterns around stoped blocks in tonalite located near the pluton roof and completed 1 1 scale three dimensional mapping of fabric patterns around stoped blocks in diorite located 1000 m below the roof in both settings no structural record of magma ascent early emplacement or settling of the blocks is preserved instead fabric formation largely post dates the settling and trapping of stoped blocks two dimensional finite difference thermal modeling and calculations of sinking rates support short

mechanism for middle to upper crustal contamination evidence from continental margin magmas lavas and feeder dikes of two quaternary volcanic complexes in southwest british columbia provide evidence of an upper crustal syntectic origin for many dacite magmas contamination initially involves solid liquid and liquid liquid interdiffusion between the magma and 1 cm wide partially melted 60 margins of wall rock xenoliths subsequent abrasion of mechanically unstable margins contributes both silicic melt and xenocrysts to the magma hybridization continues by elemental exchange between the magma and crustal liquid where mechanical mixing produces a meshlike distribution of 1 mm rhyolite strands in the lava matrix the magma may assimilate large quantities of crustal material through disaggregation of xenoliths within high level subvolcanic dikes

assimilation and crystal accumulation in a mid crustal magma chamber the sausfjellet pluton north central norway the sausfjellet pluton is made up of two intrusive units emplaced into high grade metamorphic rocks of the helgeland nappe complex of the uppermost allochthon in the norwegian caledonides the eastern part of the pluton intruded marble and less voluminous calc silicate and pelitic rocks the western half is hosted predominantly by semi pelitic migmatite with intercalated marble remelting of the migmatite during pluton emplacement occurred in a thermal aureole as much as 1000 m wide the early gabbroic unit forms the southeastern part of the body it consists of hornblende bearing to hornblende rich gabbro and diorite which is thought to have crystallized from an h2o rich andesitic parental magma the younger dioritic unit underlies the central and western parts of the pluton as well as a zone as much as 200 m wide that separates the rest of the pluton from its host rocks herein the annular zone the interior or central zone of the dioritic unit is pyroxene diorite that is locally interlayered with anorthosite the western and annular zones are by comparison mineralogically heterogeneous they range from diorite to quartz monzonite and from biotite bearing two and threepyroxene assemblages to biotite hornblende assemblages neither rock type nor mafic assemblage is correlated with position in the pluton or proximity to a contact stoped blocks of a distinctive coarse grained diorite as well as pyroxenerich calc silicates are present in the gabbroic unit and the central zone of the dioritic unit the few stoped blocks observed in the western zone of the dioritic unit are predominantly quartz rich gneiss chemical variation in the central zone of the dioritic unit is interpreted to result from accumulation of pyroxenes plagioclase from an h2o poor andesitic parent these rocks have approximately constant d 18 o of 6 6f0 2x and lack evidence of in situ assimilation heterogeneities in the western and annular zones of the dioritic unit are reflected in variable anomalously enriched incompatible element contents and in d 18 o which ranges from 6 7x to 8 6x petrologic models indicate that the magma parental to the central zone could also be parental to the western and annular zones if so evolution of the western and annular zone magma was by crystal accumulation and assimilation of metapelitic host rocks as much as 20 of the mass of the western and annular zones can be ascribed to assimilated material which apparently entered the magma by stoping therefore the asymmetrical

the extent of u series disequilibria produced during partial melting of the lower crust with implications for the formation of the mount st helens dacites the extent to which u series disequilibria can be produced during partial melting of mafic lower crust is quantified using a simple batch melting model and both experimental and theoretical partition coefficients for u th and ra we show that partial melting of mafic lower crust can only produce small disequilibria between 238u 230th and 226ra crystallisation of basalt and mixing between young basalt and crustal derived melts will have a similar or smaller effect consequently u series disequilibrium in arc andesites and dacites can generally only be an inherited feature derived from a mantle parent unless the timescales of silicic magma production within the crust are short compared to the half life of 226ra our results have profound implications for several recent models of silicic magma production by thermal incubation and partial melting of the lower crust we show that the 226ra excess observed in most arc andesites and dacites requires extremely rapid differentiation and or the involvement of mantle derived basalts less than a few thousand years old application to mount st helens suggests that crystallisation of young mantle derived magma is likely to be the dominant process in the formation of these dacites

garnet growth during amphibolite anatexis implications of a garnetiferous restite formation of a garnet clinopyroxenite restite may have important physicochemical consequences for deep crustal and mantle processes we have experimentally simulated the dehydration melting of amphibolite and report the modal and compositional evoltuion of the mafic granulitic mineral assemblage the restite with an emphasis on garnet a natural low k calcic amphibolite was powdered and run in sealed gold capsules in a piston cylinder apparatus at 10 kbar 750 1000 c 1 to 9 day runs and fo 2 ni nio garnet first appears in 8 day runs at 825 25 c above 850 c clinopyroxene and poikilitic garnet comprise most of the restite garnet peaks at 950 c at 32 vol of the material within the capsule or 41 vol of the restite assemblage and disappears between 975 1000 c there is no cross over of fe mg partitioning between coexisting liquids and garnets liquids remain mg poorer than garnets throughout the melting interval but do increase in mg with increasing temperature thus the hotter liquids may be less likely to react with mantle wedge peridotite the modes are consistent with ree patterns of archean tonalitic trondhjemitic rocks generated by the partial melting of garnet bearing mafic crust a garnet clinopyroxenite with 41 vol garnet has a density of 3 5g cm 3 which may induce lower crustal delamination in hot nonthickened amphibolitic crust this crust is seismically indistinguishable from lithosphere or mantle because garnet growth traps water bearing hornblende inclusions the restite may contain up to 0 3 wt water thus delivery of poikilitic garnet to the mantle possibly by delamination or by subduction may be an important mechanism for adding water to the mantle

exploding xenoliths and the absence of elephants graveyards in granite batholiths abstract biotite monzogranite near the eastern margin of the late devonian south mountain batholith at portuguese cove nova scotia surrounds a large 20 m 25 m anisotropic decimetre scale interbedded psammite and semipelite xenolith of the metamorphic regional greenschist thermal cordierite hornfels country rocks of the cambro ordovician meguma group the xenolith has two distinct types of surfaces against the monzogranite i one rounded diffuse and apparently old surface that had prolonged contact with the granite magma and ii two straight sharp and apparently new surfaces that had more recently come into contact with the granite magma more than 375 thin sheets width n 224 0 1 0 5 cm n 66 0 5 1 0 cm n 51 1 0 5 0 cm n 30 5 0 10 0 cm n 3 10 cm n 2 ranging in composition from monzogranite to pegmatite to coarse quartz sericite cut the xenolith parallel to bedding producing a cumulative volume expansion of 14 in the xenolith aspect ratios in some of the thin sheets exceed 1000 1 several sheets cut the xenolith perpendicular to bedding producing a volume expansion of

os and u th isotope signatures of arc magmatism near mount mazama crater lake oregon abstract interaction of mantle melts with the continental crust can have significant effects on the composition of the resulting melts as well as on the crust itself and tracing this interaction is key to our understanding of arc magmatism lava flows and pyroclastic deposits erupted from 50 to 7 7 ka at mt mazama crater lake oregon were analyzed for their re os and u th isotopic compositions mafic lavas from monogenetic vents around mt mazama that erupted during the buildup to its climactic eruption have lower 187 os 188 os ratios 0 1394 to 0 1956 and high 230 th excess 230 th 238 u 0 of 1 180 to 1 302 whereas dacites and rhyodacites tend to have higher 187 os 188 os ratios 0 2292 to 0 2788 and significant 238 u excess 230 th 238 u 0 of 0 975 to 0 989 the less radiogenic os isotope compositions of the mafic lavas can be modeled by assimilation of young 2 5 to 7 ma mafic lower crust that was modified during regional extension whereas the more radiogenic os isotope compositions of the dacites and rhyodacites can be attributed to assimilation of older 10 to 16 ma mid to upper crust that acquired its composition during an earlier period of cascade magmatism production of th excesses in the lower crust requires very young garnet formation accompanying dehydration melting in the lower crust at less than a few 100 ka by heat from recent basaltic magma injection the results from this study suggest that the combination of os and th isotopes may be used to provide insights into the timescales of evolution of the continental crust in arc settings as well as the influence of the crust on erupted magmas and suggest a link between the age and composition of the lower and upper crust to regional tectonic extension and or earlier cascade magmatism

trace element and isotopic evidence for two types of crustal melting beneath a high cascade volcanic center mt jefferson oregon mt jefferson is an andesite dacite composite volcano in the cascade range the locus of andesite and dacite dominated volcanism for at least 1 million years a large trace element data set for mt jefferson and its surrounding mafic volcanic platform effectively rules out any fractionation based model fc or afc for the generation of mt jefferson andesites several incompatible element zr nb y concentrations decrease in the range from basalt to andesite and then increase in the range from andesite to rhyodacite others ba rb la th remain constant or show a slight increase in the basalt to andesite range with modest increases from andesite to rhyodacite systematic variations in highly incompatible element ratios such as ba la and rb th suggest magma mixing dominates the trace element signatures rhyodacites are isotopically uniform 87sr 86sr 0 70325 0 70343 206pb 204pb 18 75 18 85 18o 6 3 0 3 whereas andesite and dacite are more variable 87sr 86sr 0 70291 0 70353 206pb 204pb 18 59 18 86 18o 6 0 0 6 typical basaltic andesite has 87sr 86sr 0 70326 0 70358 206pb 204pb 18 78 18 85 and 18o 5 9 0 4 sr rich 1 000 ppm basaltic andesite is more variable 87sr 86sr 0 70300 0 70360 206pb 204pb 18 70 18 89 18o 5 9 0 4 the data define mixing arrays with one end member at 87sr 86sr 0 7029 206pb 204pb 18 59 another at rhyodacite and a third at 87sr 86sr 0 7036 206pb 204pb 18 89 the first end member is defined by sr rich 800 1 200 ppm andesite with high al2o3 and low k2o ba and rb th the third one by k2o and very sr rich 2 000 ppm shoshonite isotopic data for basalts in northern oregon preclude any fractionation relationship between basalt and either rhyodacite or sr rich andesite e g the minimum 206pb 204pb ratio in basalt is 18 83 considered in light of geophysical models for the cascades these data suggest two types of crustal melting beneath the arc rhyodacite may be generated at 25 30 km depth by partial melting of arc basalt like amphibolite at 850 900 c sr rich andesite may be formed by partial melting of depleted morb like mafic granulite at 35 45 km depth at 1 000 1 100 c experimental and ree evidence supports these interpretations as does the restriction of sr rich andesite in the cascades to the area south of the 100 mw m2 heat flow contour between mt jefferson and mt hood thick crust and high heat flow are necessary to produce such andesite

on the emplacement of tabular granites granites in both crystalline terranes and continental magmatic arcs tend to be circular to elliptical in map view and vary in width from about 3 to 100 km available gravity and structural data suggests that many of these plutons are tabular in shape with an average thickness of about 3 km ductile structures observed around mesozonal granites indicate that space is created by a combination of lateral and vertical displacements of wall rocks whereas contact relationships of epizonal plutons imply that only vertical displacements are involved during emplacement in both settings magma arrives at the emplacement site via one or more vertical feeder zones and flows laterally with the exception of very high level epizonal plutons structural studies suggest that space for many tabular intrusions must be provided mainly by floor depression lopolith emplacement rather than roof lifting laccolith emplacement an emplacement model for this type of tabular granite is proposed which involves progressive depression of the floor of an initially horizontal chamber as it is filled by one or more vertical conduits a crustal scale balance in the rates of melt extraction magma ascent and pluton filling is required by the model and transfer of material from the source to the pluton is accommodated by broadly distributed deformation of low strain magnitude the process is evaluated with end member cantilever and piston sinking mechanisms the models predict that large 10 100 km wide tabular plutons 3 km thick can be emplaced quickly 100 a to 1 ma with floor depression and related wall rock strain rates similar those expected during tectonic deformation 10 10 to 10 15 s 1 bulk strains in the intervening crustal column rarely exceed a strain ratio of 1 5 which is likely to remain undetected in the geological record unless the required deformation is accommodated on discrete structures such as normal faults or shear zones at the base of the pluton

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

in defense of magmatic diapirs abstract diapirism as a crustal magma ascent mechanism has been recently criticized we contend that this reflects an overly simplistic view that diapirs must resemble modeled hot stokes diapirs and the perception that magma ascent in dikes is a more problem free mechanism for the construction of plutons we describe four cordilleran plutons that have characteristics much more compatible with diapirs than dike fed chambers these plutons were emplaced at depths ranging from 10 to 30 km and record different parts of diapiric ascent paths most ascended during complex visco elastic flow of host rock during regional deformation have narrow structural aureoles indicating power law behavior of host rock and were constructed of multiple batches of magma attributes enabling them to ascend greater distances than single hot stokes diapirs some features of these plutons are not typically attributed to diapirs and thus we introduce the term visco elastic diapir for bodies consisting of one or more batches of magma rising together with length to width ratios

new osmium isotope evidence for intracrustal recycling of crustal domains with discrete ages new 187os 188os ratios of quaternary mount adams volcanic rocks from the cascade arc in southern washington vary by 300 187 os 188 os 5 0 165 0 564 and fall into high 0 319 and low 0 166 to 0 281 groups of 187 os 188 os ratios that are sub stantially more radiogenic than mantle values these os isotope compositions and groupings are interpreted to reflect recycling of discrete intracrustal domains with high 187os 188os ratios but dif fering ages thus recording the process of crustal hybridization and homogenization os isotope compositions provide new constraints on amounts of intracrustal recycling in young subduction zone en vironments that reflect the magmatic history of the arc sr nd hf and pb isotope variations in this young mafic arc complex are too small to allow such constraints

direct evidence for the origin of low 18o silicic magmas quenched samples of a magma chamber s partially fused granitoid walls crater lake oregon abstract partially fused granitoid blocks were ejected in the climactic eruption of mount mazama which was accompanied by collapse of crater lake caldera quartz plagioclase and glass in the granitoids have much lower 18 o values 3 4 to 4 9 than any fresh lavas of mount mazama and the surrounding region 5 8 to 7 0 oxygen isotope fractionation between phases in granitoids is consistent with equilibrium at t 900 c following subsolidus exchange with hydrothermal fluids of meteoric origin assimilation of 10 20 of material similar to these granitoids can account for the o and sr isotopic compositions of lavas and juvenile pyroclasts derived from the climactic magma chamber many of which have 18 o values 0 5 or more lower than comparable lavas of mount mazama the o isotope data provide the only clear evidence for such assimilation because the mineralogy and chemical and radiogenic isotopic compositions of the granitoids dominantly granodiorite are similar to those of erupted juvenile magmas the granitoid blocks from crater lake serve as direct evidence for the origin of 18 o depletion in large shallow silicic magma bodies

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

evidence for significant granite space creation by the ballooning mechanism the example of the ardara pluton ireland the emplacement mechanism of the ardara granite a caledonian pluton located on the northwestern seaboard of ireland has been a widely cited and a key point of reference in the debate about the granite space problem for nearly half a century early models related its subcircular tailed shape concentric foliation pattern and concentrically deformed envelope to diapirism it was subsequently reinterpreted using strain data as a ballooning pluton that was created by 70 in situ expansion at the emplacement site and was later cut by a regional shear zone recently it was the leading example in a wide ranging review of forcefully emplaced plutons of this type and was reinterpreted as a series of nested diapirs associated with only modest amounts 30 of forceful space creation but a large component of inferred early block stoping we report the results of the complete remapping and extensive collection of new structural data from this 8 km diameter normally zoned three phase calc alkaline pluton we confirm the existence of the steeply inclined internal foliation pattern and that it was formed in the magmatic state although this and the outline of the pluton are modified by late stage high temperature solid state regional transcurrent shear zones along two of the pluton contacts structural mapping has failed to identify a macroscopic lineation in the magmatic foliation planes and this is confirmed by strain determinations using mafic enclave populations and magmatic crystal separations for the magmatic state deformation that show nearly pure flattening type strains k 0 strains measured by these techniques show an apparently smooth gradient increasing outward toward the pluton contacts from an inner low strain zone we argue that these features together with the simple normal petrographic zonation a lack of small scale structures that would indicate the pluton had moved up relative to the wall rocks and an exposure level that is not in the roof zone are inconsistent with diapirism either singly or multiply but are most easily explained by a ballooning emplacement mechanism synmagmatic shear sense data from both the pluton and the adjacent country rocks indicate however that within the overall context of radial ballooning there was a limited amount of late stage nonradial northward injection of magma an inversion of the deformed enclave and crystal spacing strain determinations and strain determinations within the country rocks suggest that more than 80 of the volume of the pluton was accommodated by the ballooning mechanism

growth rates of dike fed plutons are they compatible with observations in the middle and upper crust diapirism and dike fed expansion are two popular models for the ascent and emplacement of large batches of granitoid magma into the middle and upper crust many structural and petrologic relationships in and around plutons can be cited as permissive evidence for either model and so it has been difficult to differentiate between them on these grounds here we consider wall rock strain rates resulting from dike fed expansion at a constant volume pluton filling rate considering end member pluton shapes and dike flow rates and dikes 1 km in plan length the dike fed expansion model predicts wall rock strain rates over the entire pluton growth period that are up to six orders of magnitude faster than the fastest accepted regional tectonic strain rates 10 213 s 21 strain rates in the very early stages of growth can reach values several orders of magnitude faster our results suggest that evidence for or against very fast strain rates in deformation aureoles around plutons may form a key criterion for identifying dike fed expansion on the other hand our ability to extract strain rate information from pluton aureoles is currently extremely limited

midcrustal emplacement of the sausfjellet pluton central norway ductile flow stoping and in situ assimilation midcrustal 25 30 km emplacement of the dioritic sausfjellet pluton into rocks of the helgeland nappe complex central norway occurred in two stages stage 1 consists of two pyroxene hornblende gabbro and diorite stage 2 is asymmetrically zoned with a modally layered central zone of diorite and anorthosite and a western annular zone of quartz bearing monzodioritic rocks igneous layering was locally attenuated folded and boudinaged in the hyper solidus state the magmatic foliation trajectory pattern in the pluton defi nes a shallowly southwest plunging synform that crosscuts compositional zones mineral lineations plunge shallowly to moderately to the southwest the pluton intruded a major lithologic boundary within the nappe migmatitic pelitic gneisses are the main host rocks to the western part of the pluton whereas the eastern and central parts are hosted predominantly by metacarbonate rocks calc silicate marble quartzo feldspathic and dioritic xenoliths up to 200 m in length are present throughout the pluton they are most common in stage 1 and the central zone of stage 2 metapelitic xenoliths are conspicuously absent ductile fl ow during emplacement produced an 1 kmwide structural aureole in which host rock structures were defl ected into subparallelism with the steeply inward dipping margin of the pluton tight antiforms developed along the northeastern southeastern and southwestern margins amphibolite grade shear zones in the host rocks preserve plutonside up kinematic indicators in addition to the abundance of dioritic calc silicate and quartzo feldspathic gneiss xenoliths and geochemical evidence for assimilation in the western annular zone regional discordance of the pluton host rock contacts indicates that stoping was also an important emplacement process at midcrustal depths following magma emplacement foundering of the central portion of the chamber combined with possible ca 445 ma regional contraction produced the map scale synform defi ned by magmatic foliations and igneous layers this study demonstrates that stoping and assimilation may occur simultaneously with host rock ductile fl ow during magma chamber evolution at midcrustal levels and offers an explanation of why xenolith preservation may be compositionally dependent

controls on skarn mineralization and alteration at the cadia deposits new south wales australia the cadia fe cu au skarns in eastern new south wales australia are associated with the late ordovician shoshonitic cadia intrusive complex which hosts the largest mineralized intrusion related system in eastern australia both the big cadia and little cadia skarns are developed within a 40 m thick volcanic derived calcareous sandstone and adjacent units which represent the most important control on skarn formation the ore zones have undergone postore displacement along steeply dipping predominantly reverse faults without significant remobilization of ore chalcopyrite and minor native gold formed in intimate association with epidote chlorite quartz calcite and in the interstices of bladed hematite and magnetite aggregates the age of the au cu mineralizing event at cadia is constrained by 40ar 39ar dating of accompanying muscovite at 438 2 4 2 ma 1 gold to copper ratios within the skarns are generally lower than for porphyry au cu deposits at cadia particularly the high tonnage low grade cadia hill deposit ore and calc silicate gangue mineralogy are similar to other oxidized fe cu au skarns associated with porphyry deposits iron skarn at big cadia occurs several hundred meters distant from the margin of a mineralized intrusion of quartz monzonite porphyry magmatic dominated fluids reacted with carbonate in the wall rocks and produced classic skarn zonation over an 800 m interval from the intrusive contact located at cadia quarry toward more distal environments at big cadia skarn zonation comprises 1 proximal garnet pyroxene 2 intermediate garnet pyroxene scapolite and 3 distal fe au cu skarn alteration of noncalcareous metavolcanic rock units fine grained pyroxene phyric volcanic rocks and volcaniclastic rocks adjacent to the mineralizing quartz monzonite porphyry includes hydrothermal biotite k feldspar quartz hornfels and magnetite quartz biotite hornfels gold copper mineralization formed adjacent to garnet bearing veins peripheral to the main garnet rich zone indicating that garnet forming fluids carried the ore metals at little cadia and cadia east mineralization and skarn zonation similar to big cadia developed above additional intrusions of quartz monzonite porphyry retrograde hydrous alteration replaced much of the prograde garnet dominant mineralogy at cadia with the strongest overprint at little cadia the styles and distribution of alteration and mineralization suggest that the big cadia skarn formed from fluids that migrated laterally within calcareous units from strongly altered quartz monzonite phases of the cadia intrusive complex at cadia quarry at little cadia skarn formation was probably related to fluids derived from the cadia intrusive complex that migrated vertically and laterally within permeable calcareous units structural controls were important in focusing fluids and localizing the emplacement of late mineralizing phases of the cadia intrusive complex

eruptive history and geochronology of mount mazama and the crater lake region oregon geologic mapping k ar and 40 ar 39 ar age determinations supplemented by paleomagnetic measurements and geochemical data are used to quantify the quaternary volcanic history of the crater lake region in order to define processes and conditions that led to voluminous explosive eruptions the cascade arc volcano known as mount mazama collapsed during its climactic eruption of 50 km 3 of mainly rhyodacitic magma 7700 yr ago to form crater lake caldera the mazama edifice was constructed on a pleistocene silicic lava field amidst monogenetic and shield volcanoes ranging from basalt to andesite similar to parental magmas for mount mazama between 420 ka and 35 ka mazama produced medium k andesite and dacite in 2 1 proportion the edifice was built in many episodes some of the more voluminous occurred approximately coeval with volcanic pulses in the surrounding region and some were possibly related to deglaciation following marine oxygen isotope stages mis 12 10 8 6 5 2 and 2 magmas as evolved as dacite erupted many times commonly associated with or following voluminous andesite effusion establishment of the climactic magma chamber was under way when the first preclimactic rhyodacites vented ca 27 ka the silicic melt volume then grew incrementally at an average rate of 2 5 km 3 k y 1 for nearly 20 k y the climactic eruption exhausted the rhyodacitic magma and brought up crystal rich andesitic magma mafic cumulate mush and wall rock granodiorite postcaldera volcanism produced 4 km 3 of andesite during the first 200 500 yr after collapse followed at ca 4800 yr b p by 0 07 km 3 of rhyodacite the average eruption rate for all mazama products was 0 4 km 3 k y 1 but major edifice construction episodes had rates of 0 8 km 3 k y 1 the long term eruption rate for regional monogenetic and shield volcanoes was d 0 07 km 3 k y 1 but only 0 02 km 3 k y 1 when the two major shields are excluded plutonic xenoliths and evidence for crystallization differentiation imply that the amount of magma intruded beneath mount mazama is several times that which has been erupted the eruptive and intrusive history reflects competition between 1 crystallization driven by degassing and hydrothermal cooling and 2 thermal input from a regional magma flux focused at mazama before ca 30 ka relatively small volumes of nonerupted derivative magma crystallized to form a composite pluton because the upper crust had not been heated sufficiently to sustain voluminous convecting crystal poor melt subsequently and perhaps not coincidentally during mis 2 a large volume of eruptible silicic magma accumulated in the climactic chamber probably because of heating associated with mantle input to the roots of the system as suggested by eruption of unusually primitive magnesian basaltic andesite and tholeiite west of mazama

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