are granitic intrusions scale invariant published length l and thickness t data on 135 laccolith and 21 granite intrusions define power law relationships of the form l kt a typical of systems exhibiting scale invariant fractal behaviour both data sets are characterised by an exponent a 1 0 88 0 1 for laccoliths and 0 80 0 20 for plutons that reflects an inherent preference for scale invariant tabular sheet geometries these power law size relationships can be explained in mechanical terms by the need for an incoming magma sheet to travel laterally some distance before vertical thickening can occur sheet thickness is a function of available magma pressure which for an intrusion fed by a feeder dyke is proportional to the vertical magma transport distance

extreme alteration by hyperacidic brines at kawah ijen volcano east java indonesia i textural and mineralogical imprint article kawah ijen volcano located on the eastern tip of java and renowned for its large hyperacidic crater lake poses significant volcanic and environmental hazards to its immediate surroundings crater lake brines seep through the flanks of the volcano to form the banyu pahit river which is used in irrigation downstream resulting in extensive pollution sharply reduced crop yields and health problems the impact on the environment comes mainly from the high element load which is derived from leaching of rocks by the acid fluids and transported downstream our detailed study of water rock interaction in different parts of the kawah ijen system indicates that there are three settings for this alteration the crater lake and banyu pahit riverbed the hydrothermal system below the lake and the solfatara of the active rhyolite dome in all three settings the silicates are leached and altered to amorphous silica in the order olivine glassnan rich plagioclasenortho pyroxenenclino pyroxenenab rich plagioclase in contrast the alteration of titanomag netite is characterised by dissolution in the surficial setting replacement by pyrite and ti oxide in the hydrothermal system and pyritisation ti mobility in the fumarole conduits alteration progresses along crystallographically controlled planes in all phases and shows strong compositional control in plagioclase and titanomagnetite no secondary minerals develop except for minor barite cristobalite pyrite and jarosite this indicates that despite its high element load the waters are undersaturated with respect to most secondary minerals typically produced during alteration of these magmatic rocks by acid chloride sulphate brines and that water rock interaction at kawah ijen is not a sink of elements but rather contributes to the element load transported downstream

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

internal structure and emplacement of the papoose flat pluton an integrated structural petrographic and magnetic susceptibility study the papoose flat pluton of eastern california is commonly cited in the geologic literature as a classic example of a forcibly emplaced pluton although the relative importance attributed to magmatic versus tectonic processes in controlling the structural evolution of the pluton has been controversial reexamination of this late cretaceous 83 ma pluton using a combination of new structural petrographic microstructural and anisotropy of magnetic susceptibility ams data on the pluton wall rock system has shown that the pluton is an inclined and internally zoned tabular structure assembled by forcible intrusion of successive pulses of magma at a crustal depth of 12 15 km initial pluton formation involved magma ascent in a vertical west northwest striking feeder dike which was arrested at a stratigraphically controlled mechanical discontinuity in the overlying cambrian metasedimentary rocks leading to formation of a southwest dipping sill subsequent sill inflation accompanied by horizontal infilling from the feeder dike at the base of the sill resulted in deformation and vertical translation of previously emplaced magma pulses and local raising of the sill roof facilitated by thermal weakening as the wall rock temperatures progressively rose during emplacement of successive magma pulses cooling from the roof of the pluton downward resulted in cessation of vertical inflation on the west side of the pluton and promoted lateral expansion toward the northeast and floor depression below the eastern part of the pluton we have been unable to document any regional scale structures e g of equivalence to similar age synplutonic strike slip shear zones in the sierra nevada batholith to the west that may have controlled emplacement of the papoose flat pluton however this fact does not preclude the likelihood that the country rocks were subjected to a regional deviatoric stress field at this time indeed tectonic overpressuring was probably essential for producing sufficiently high magma pressures for laccolith formation at this midcrustal level simple thermal modeling using microstructural and thermobarometric data indicates that the total duration of emplacement of the pluton did not exceed 30 000 yr this rapid emplacement rate may explain why the pluton appears to be anorogenic even though it was emplaced during a period of regional scale deformation

granitic melt viscosity and silicic magma dynamics in contrasting tectonic settings models for granitic melt viscosities reveal the influences of h 2 o temperature and tectonics on melt segregation ascent and emplacement low t magmas are wetter than those formed at high t so log 10 melt viscosity pa s is 3 2 6 3 for leucogranite to tonalite only 10 1000 times more viscous than basalts there should be no correlation between critical dyke width to prevent freezing and tectonic setting granitic magma ascent rates in dykes vary by

magmatic history and geophysical signature of a post collisional intrusive center emplaced near a crustal scale shear zone the plechý granite pluton moldanubian batholith bohemian massif the plech pluton southwestern bohemian massif represents a late variscan complexly zoned intrusive center emplaced near the crustal scale pfahl shear zone the pluton thus provides an opportunity to examine the interplay among successive emplacement of large magma batches magmatic fabric acquisition and the late variscan stress field associated with strike slip shearing the magmatic history of the pluton started with the emplacement of the porphyritic plech and haidmuhler granites based on gravity and structural data we interpret that the plech and haidmuhler granites were emplaced as a deeply rooted ne sw elongated body its gross shape and internal fabric steep ne sw magmatic foliation may have been controlled by the late variscan stress field the steep magmatic foliation changes into flat lying foliation particularly recorded by ams presumably as a result of divergent flow magnetic lineations correspond to a sub horizontal ne sw finite stretch associated with the divergent flow subsequently the t istolicnik granite characterized by steep margin parallel magmatic foliation was emplaced as a crescent shaped body in the central part of the pluton the otherwise inward younging intrusive sequence was completed by the emplacement of the outermost and the most evolved garnet bearing granite the marginal granite along the southeastern margin of the pluton

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

pluton emplacement by sheeting and vertical ballooning in part of the southeast coast plutonic complex british columbia batholiths of the coast plutonic complex in the area of harrison lake british columbia are interpreted to have formed by horizontal sheeting and vertical inflation zoned ovoid batholiths exhibit sheeted margins as thick as 3 km shallow floors are indicated by moderately to gently inward dipping magmatic foliations the observed trace of floor contacts across topography and a published deep seismic reflection transect mineralogic aureoles under the floors preserve a paragenetic record of increasing pressure during aureole crystallization initial pressures of

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

pluton emplacement during transpression in brittle crust new views from analogue experiments analogue experiments were used to investigate pluton emplacement during transpression in brittle crust models consisted of silicone putty placed within slots in a rigid basement and an overburden of dry sand the models were transpressed causing emplacement of silicone intrusions into the overlying sand space was made for intrusions by upward displacements of overburden along oblique reverse shear zones in some cases accompanied by extensional shearing above the intrusions silicone putty was progressively accreted to the bases of uplifted blocks and growing intrusions rode up the shear zones leaving space for accretion of more silicone the resulting intrusion shapes depended largely on the geometries of shear zone systems which in turn depended on overburden thickness the intrusion profiles and the structural relationships between intrusions and host in the experiments are comparable to some natural examples a magma accretion model may thus account for pluton emplacement and batholith assembly within some transpressive orogens

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

deep crustal deformation textures along megathrusts from newfoundland and ontario implications for microstructural preservation strain rates and strength of the lithosphere lithospheric scale thrusts from the west newfoundland ophiolite belt white hills peridotite shear zone and the south western grenville province parry sound shear zone involve rocks of lower crustal and or upper mantle origin that exhibit intense crystal plastic deformation of plagioclase k feldspar orthopyroxene and clinopyroxene minerals that are commonly viewed as representative of low ductility phases the occurrence of this extreme deformation in shear zones that exhibit similar lower crustal syntectonic p t conditions suggests a phenomenological link between the megathrust environment and both the generation and subsequent preservation of the observed deformation microstructures an empirical homologous parameter is constructed in an attempt to characterize conditions for similar behaviour among different minerals and to explore the feasibility of refining a threshold recovery preservation condition within the megathrusts studied this parameter predicts at the estimated syntectonic temper

laccolith like emplacement model for the papoose flat pluton based on porphyroblast matrix analysis detailed porphyroblast matrix analysis within the concordant metasedimentary aureole rocks surrounding the papoose flat pluton of eastern california indicates that inclusion trails within porphyroblasts can be used as strain markers to restore the aureole rocks to their prepluton emplacement position using porphyroblast matrix relationships in combination with measurement of stratigraphic sections and whole rock geochemical analyses we have determined the kinematics of rotation the change in thickness and volume and the amount of translation of the metasedimentary formations within the aureole these data are consistent with initial emplacement of the magma as an inclined sill and subsequent inflation into a pluton or laccolith the combination of structural and porphyroblast matrix analysis leads to a three dimensional kinematic history of the wall rocks wherein vertical upward translation represents a significant part of the pluton emplacement related strain history

granite magma formation transport and emplacement in the earth s crust the origin of granites was once a question solely for petrologists and geochemists but in recent years a consensus has emerged that recognizes the essential role of deformation in the segregation transport and emplacement of silica rich melts in the continental crust accepted petrological models are being questioned either because they require unrealistic rheological behaviours of rocks and magmas or because they do not satisfactorily explain the available structural or geophysical data provided flow is continuous mechanical considerations suggest that far from being geologically sluggish granite magmatism is a rapid dynamic process operating at timescales of or 100 000 years irrespective of tectonic setting

getting granite dikes out of the source region whether a dike can propagate far from a magma reservoir depends upon the competition between the rate at which propagation widens the dike and the rate at which freezing constricts the aperture available for magma flow various formulations are developed for a viscous fluid at temperature t sub m intruding a growing crack in an elastic solid the initial solid temperature equals t sub m at the source and decreases linearly with distance from the source if t sub m is the unique freezing temperature of the fluid dike growth is initially self similar and an essentially exact solution is obtained if t sub m is above the solidus temperature the solution is approximate but is designed to overestimate the distance the dike may propagate the ability of a dike to survive thermally depends primarily upon a single parameter that is a measure of the ratio of the dike frozen margin thickness to elastic thickness perhaps more intuitively one may define a minimum distance from the essentially solid reservoir wall to the point at which the host rock temperature drops below the solidus necessary for dikes to propagate far into subsolidus rock it is concluded that for reasonable material properties and source conditions most basalt dikes will have little difficulty leaving the source region but most rhyolite dikes will be halted by freezing soon after the magma encounters rock at temperatures below the magma solidus while these results can explain why granitic dikes are common near granitic plutons but rare elsewhere the potentially large variation in magmatic systems makes it premature to rule out the possibility that most granites are transported through the crust in dikes nonetheless these results highlight difficulties with such proposals and suggest that it may also be premature to rule out the possibility that most granite plutons ascend as more equidimensional bodies

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

crustal scale en echelon p shear tensional bridges a possible solution to the batholithic room problem the apparent paradox of voluminous granitoid emplacement in an overall compressional magmatic are has been somewhat alleviated by the idea of pluton emplacement in tension cracks and dilational jogs within arc parallel strike slip fault systems as an alternative hypothesis we propose that an en echelon p shear array provides a continuous batholith scale zone of dilation along the trend of active magmatism and that it is favored by transpressional tectonics in the sierra nevada california the late cretaceous granitoids of the cathedral range intrusive epoch are consistent with emplacement into bridges between p shears because 1 they are elongated oblique to the magmatic trend consistent with p shear orientation within a dextral system 2 they were all emplaced within 5 10 m y along a 300 km trend resulting in a displacement rate of 1 cm yr across the arc and 3 syn to late magmatic dextral shear zones bound and crosscut the plutons as in a p shear tensional bridge

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

emplacement kinematics of a granite diapir the chindamora batholith zimbabwe the geometric features of a large 50 40 km granitic batholith and its surrounding greenstone belt envelope are described the batholith is made up of several intrusive components and an intrusion sequence based on contact relations and xenoliths has been established the igneous rocks show a local foliation s fabric and preferred orientation of minerals the intensity of this fabric accords with the finite strain determined from xenolith shape analysis when the strain variations are analysed in detail it appears that the batholith was built up by a succession of magmatic pulses each of which led to a stretching of the previously consolidated igneous material so called balloon tectonics evidence is presented that xenolith deformation commences only when the surrounding igneous material has crystallized and a method is suggested whereby the position of the surface separating crystal supported phase plastic behaviour from melt supported phase liquid behaviour at different stages in the plutonic history can be determined ductile shear zones are present and their geometry accords well with the shape changes necessary to allow the outer parts of the pluton to stretch

complex deformation during arc continent collision quantifying finite strain in the accreted alisitos arc peninsular ranges batholith baja california abstract the early cretaceous alisitos island arc located in the western part of the peninsular ranges batholith baja california accreted to north america during the mid cretaceous a syn to post collisional fold thrust belt dominated by sinistral transpression and orthogonal convergence developed along the northern and eastern edges of the arc respectively field observations across the fold thrust belt show a deformation gradient with stronger planar and linear fabrics fold tightening and greater finite strain towards the arc continent suture flattening strains dominate and finite strain intensity ranges from 0 08 to 2 71 and generally increases towards the suture in detail the fold thrust belt narrows southward from 12 km to 3 km furthermore finite strain is heterogeneous reflecting a heterogeneous fold thrust belt characterized by local high strain zones near faults folds and igneous intrusions finite strain data and field observations allow several conclusions 1 the colliding arc deformed significantly as a result of collision 2 strain contributes to bulk shortening and crustal thickening in the collision zone 3 geometry composition and tectonic setting of the continental margin prior to collision control along strike variations in the fold thrust belt and 4 narrowing of the fold thrust belt southward is offset by increased deformation in continental margin units

a tectonic model for the emplacement of the main donegal granite nw ireland the main donegal granite is shown to lie in a major late stage ne sw trending transcurrent sinistral shear zone in the caledonides of nw ireland the zone which is bent both laterally and upwards around the pluton has displacements which diminish sw along its length from a maximum of approximately 20 km at the ne end of the pluton the granite has also been deformed by this sinistral shear but it has a strain component of its own the nature and development of which indicate that the pluton was accommodated rather than forcefully emplaced in the shear zone strain variations between granite and wall rock and an early plagioclase fabric associated with the petrographic banding of the granite suggest that intrusion and deformation were more or less synchronous at around 400 ma it is suggested that unconsolidated granite in the shear zone locally created high strain rates and instability which caused the zone to bend split lengthwise and progressively create an internal low pressure zone into which the main granite was emplaced the wide variety of emplacement styles of the other radiometrically synchronous plutons of the batholith can be attributed to the progressive development of the shear zone thorr and fanad in tension between the zone and a complimentary structure to the n ardara and toories in local compression at the shear zone termination rosses in tension on the outer arc of the bending zone and trawenagh bay as displaced main granite magma in a similar tensional setting during the final stages of deformation

pluton accommodation at high strain rates in the upper continental crust the example of the central extremadura batholith spain abstract emplacement in the tensional bridge of a stepped dextral shear zone system is proposed for the central extremadura batholith spain the country rocks show a pervasive anisotropy that conditioned the style of the structures developed as a consequence of the transference of displacement from the stepped shear zones to the releasing area the kinematic evolution of the resulting megakink fold provided the volume increase necessary for the granite emplacement thermal and kinematic models suggest that the growth of individual plutons took place in periods of no more than several hundred to a few thousand years fast strain rates 10 10 10 11 s 1 must concentrate in local structures e g initiation of kink folds even in zones deforming as a whole under typical strain rates 10 14 1 s 1 granite plutons might be used as strain rate gauges for syn plutonic structures

structure and emplacement history of a multiple center cone sheet bearing ring complex the zarza intrusive complex baja california mexico the cretaceous zarza intrusive complex located in the peninsular ranges of baja california norte mexico is perhaps the bestpreserved multiple center cone sheet bearing ring complex documented in north america the 7 km 2 elliptical complex hosts three nested non concentric intrusive centers that are successively younger to the south the northern and central centers show the same evolutionary sequence of 1 intrusion of concentric gabbroic cone sheets 2 intrusion of massive core gabbros and 3 development of subvertical ductile ring faults ring fault kinematics indicate that both centers moved down relative to the surrounding country rocks suggesting collapse into an underlying magma chamber the southern center is composed of approximately equal proportions of gabbro and tonalite and lacks cone sheets aluminumin hornblende barometry on the tonalite indicates a maximum emplacement depth of 2 3 0 6 kbar the zarza intrusive complex is surrounded by a ductile deformation aureole and bedding is inward dipping and inward younging around the entire complex excellent preservation of the intrusive history allowed us to evaluate the origin of the aureole and the three most applicable models are 1 collapse of the complex into its underlying magma chamber 2 sinking of the complex and its chamber after solidification and 3 formation of the aureole prior to emplacement of the complex the preserved structural and intrusive relationships provide information on the dynamic evolution of subvolcanic magma chambers and suggest that the complex may have been overlain by a caldera

constraints on geological strain rates arguments from finite strain states of naturally deformed rocks a summary of known finite strain states is presented longitudinal strains 1 e as measured in many rocks often range from 1 to 40 and 1 to 0 025 the time span available to produce such measureable strains in young orogenic zones seems to be less than 10 m y possibly less than 1 m y which constrains conventional strain rates into the range of 10 sup 13 s sup 1 to 10 sup 15 s sup 1 for both pure and simple shear the most efficient way and much less efficient way to accumulate incremental strains respectively the ellipticity of the finite strain ellipse increases in a nonlinear manner finite strain variations in adjacent layers which give rise to features such as cleavage refraction arise with only slight differences in the strain rates within these layers

modeling the thermal evolution of fault controlled magma emplacement models implications for the solidification of granitoid plutons a two dimensional finite diaerence model is used to simulate the conductive thermal regime attend ing construction and maintenance of a continental magma chamber by intrusion of granite dikes into grano diorite host rocks displaced at various spreading rates final intrusion shapes include tabular square and vertical rectangular bodies emplaced in the shallow crust 5 15 km and tabular bodies emplaced in the middle crust 15 20 km fed by dikes with widths of between 20 and 100 m the formation of a steady state chamber is defined as the point at which the ambient temperatures surpass the intrusion solidus forestalling the solidifi cation of subsequently intruded material for spreading rates 10 mm year 1 construction of a steady state magma chamber in the shallow crust took 260 ka rectangular 360 ka square and 1 ma tabular whereas in the mid crust a steady state was reached in less than 30 ka tabular at faster spreading rates 25 and 50 mm year 1 ambient temperatures pass the solidus isotherm forming a steady state reservoir within 55 ka depending on intrusion depth and size for 10 25 mm year 1 spreading rates sheeted dikes make up from 10 to 100 of the intrusion the thermal modeling supports the following conclusions a episodic magma emplacement into a fault con trolled setting is a thermally viable means of constructing a steady state chamber at moderate to fast spreading rates only if the duration of faulting and intrusion are long enough to elevate ambient temperatures above the intrusion solidus b isotherms will migrate outward during successive intrusion before converging back on the center of the intrusion after chamber construction c the margins of most intrusions formed by this scen ario should contain sheeted dikes d the solidus isotherm and thus the solidification front that it tracks will become progressively curviplanar during the construction of the magma chamber and will not represent the in itial shape of the intrusions i e sheets e the steady state chamber will be smaller than the total intrusion dimensions and f magmatic fabrics will form diachronously and not always parallel to sheet margins as they track the migrating solidification front because it is unlikely that most large intrusions formed instan taneously the eaects of continued addition of heat on the migration of solidification fronts may have signifi cant implications for magmatic processes in many emplacement scenarios 1998 elsevier science ltd all rights reserved

the mortagne granite pluton france emplaced by pull apart along a shear zone structural and gravimetric arguments and regional implication the emplacement mechanism and kinematics and the 3 d geometry of the variscan granite pluton of mortagne are interpreted using systematic measurements of the granite structure allied with micro structural observations from a detailed gravimetric study it is proposed that magma infilled a pull apart structure which formed along a jog of a northwest southeast trending sinistral zone of shear a second episode of shearing under a dextral regime and independent of emplacement was responsible for perigranitic tectonites these successive transcurrent opposite movements already documented for variscan times in western france are discussed concerning the age of the mortagne pluton 313 15 m y

internal fabric and strike slip emplacement of the pan african granite of solli hills northern nigeria the solli hills pluton 250 km2 in northern nigeria is an example of a pan african granite emplaced circa 580 ma ago in the jos adamawa block its micro structures and internal fabrics have been studied in order to help constrain the kinematics of the pan african orogeny in this block we used field observations and measurements petrographic examinations and measurements of the magnetic susceptibility and anisotropy of magnetic susceptibility of 124 regularly spaced stations most samples are ferromagnetic i e belong to the magnetite series granites and present predominantly magmatic microstructures particularly remarkable are the asymmetrical zoning of the petrographic types the susceptibility magnitudes and the planar linear magnetic fabrics the magnetic foliations and lineations are very consistent in the pluton around n 20 e 60 w and n 205 e 10 respectively field and microscopic observations reveal that deformation occurred everywhere in the pluton and immediate country rock envelope during a dextral strike slip synplutonic episode it is suggested that the solli hills pluton was emplaced in a late pan african dextral tear fault system the striking fabric asymmetry of the pluton strongly suggests a shear zone termination emplacement model

structure emplacement and lateral expansion of the san jose tonalite pluton peninsular ranges batholith baja california mexico abstract the 108 km2 san jose pluton forms part of the jurassic to cretaceous peninsular ranges batholith of northern baja california mexico the pluton was formed by three nested southward migrating intrusive pulses and the internal contacts between them indicate juxtaposition while the adjoining pulses were magmas shrimp u pb zircon data indicate that the entire pluton was emplaced in less than 4 4 m y ages of the individual pulses cannot be separated at the 95 confidence interval owing primarily to low uranium content of zircon detailed structural data and geologic mapping are consistent with a component of asymmetrical lateral expansion at the site of emplacement the direction of maximum lateral expansion may have been controlled by thermal compositional and resulting rheological gradients in the surrounding wall rocks a carapace of predominantly solid state deformation marks the northern two thirds of the pluton and the early stages of this fabric may have formed in the presence of a small amount of melt lack of evidence for syn to post emplacement regional ductile deformation around the pluton suggests that this carapace was deformed during the lateral expansion of the pluton

modeling dislocation creep as a near field material transfer process during spherical pluton expansion implications for strain rates and their preservation in pluton aureoles abstract in this paper we model coupled spatial and temporal changes in stress and temperature to calculate dislocation creep strain rates in host rock associated with spherical magma chamber expansion with and without material removal by stoping and or assimilation given a constant magma chamber pressure of 100 mpa we show that stress and temperatures in the host rock range from 100 to 10 mpa and 800 to 300 c respectively using a flow law for dislocation creep of wet quartzite with recently calibrated parameters maximum dislocation creep strain rates on the order of ca 10 10 s 1 occur only close to the pluton margin despite the simple geometry relative to real plutons the models yield several thought provoking predictions if the outer portions of the pluton are allowed to cool a wide thermal and structural aureole forms maintaining the liquidus temperature in the entire pluton allows for rapid aureole deformation resulting in a narrow thermal and structural aureole because the deformation outpaces heat conduction dislocation creep strain rates in the inner portions of aureoles are extremely sensitive to pressure changes only potentially resulting in large transient variations in strain rate under isothermal conditions evidence for elevated strain rates in pluton aureoles may not be preserved in host rock aureoles if solidified pluton participates in the deformation or it may be completely removed

conditions of contact metamorphism papoose flat pluton eastern california usa implications for cooling and strain histories petrological study of highly strained carbonate and pelitic rocks within the contact aureole surrounding the western part of the papoose flat pluton yields thermal profiles plots of metamorphic temperature versus distance across the aureole that show temperature gradients which are relatively flat and narrow 100m the gradients occur close to the contact and indicate a slight decrease in temperature from 500 550 c at the pluton wall rock contact to 450 500 c at the outer margin of the aureole one thermal profile across low strain metasedimentary rocks located in the southern part of the aureole shows that thermal effects from emplacement extend no further than 600 m from the contact coexistence of andalusite and cordierite in pelitic rocks of the aureole constrain pressures to 4 kbar thermal modelling using an analytical solution of the conductive heat flow equation for a rectangular shaped pluton reproduces the observed thermal maxima and profile shape conductive rather than convective cooling also is supported by isotopic and field evidence for limited fluid flow along the strongly deformed margin of the pluton simple thermal models coupled with observed high temperature deformation features and a measured 90 attenuation of stratigraphic units in the plastically deformed western part of the pluton s aureole indicate that strain rates may have been of the order of 10 12s 1 evidence for episodic heating such as two distinct generations of andalusite growth in pelites from the aureole alternatively may indicate a longer heating event and therefore slower strain rates thermal models also indicate that parts of the pluton still may have been above the solidus during deformation of the pluton margin and aureole

sm nd and rb sr dating of amphibolite from the nellore khammam schist belt se india constraints on the collision of the eastern ghats terrane and dharwar bastar craton the nellore khammam schist belt se india is sandwiched in between the proterozoic eastern ghats terrane and the archaean dharwar bastar craton we report sm nd and rb sr mineral isochron ages of amphibolite from the schist belt the sm nd and rb sr ages are 824 43 ma and 481 16 ma respectively the sm nd age indicates the timing of peak metamorphism whereas the rb sr age indicates the pan african thermal overprint the peak metamorphism was related to collision of the eastern ghats terrane with the dharwar bastar craton which occurred during early neoproterozoic time

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

plate boundary reorganization at a large offset rapidly propagating rift the existence of rapidly spinning microplates along the southern east pacific rise has been documented by geophysical swath mapping surveys1 6 and their evolution has been successfully described by an edge driven kinematic model7 but the mechanism by which such microplates originate remains unknown proposed mechanisms1 10 have generally involved rift propagation11 possibly driven by hotspots or changes in direction of sea floor spreading here we present geophysical data collected over the earth s fastest spreading centre the pacific nazca ridge between the easter and juan fernandez microplates fig 1 which reveal a large offset propagating rift presently reorganizing the plate boundary geometry a recent episode of rapid duelling propagation of the historically failing spreading centre in this system has created a 120 120 km overlap zone between dual active spreading centres which may be the initial stage of formation of a new microplate

implications of rapid dike fed pluton growth for host rock strain rates and emplacement mechanisms dikes must maintain a critical width and flow velocity in order to propagate several kilometers from source to sink without freezing consequently they must support a high volumetric magma flow rate this in turn implies that plutons fed by dikes must fill rapidly in an effort to predict host rock strain rates required by dike fed growth we employ a three dimensional geometric model consisting of concentric shells to track instantaneous strain rates in a homogeneously deforming aureole around spheroidal plutons with a range of aspect ratios using published values for magma and host rock parameters appropriate for mid crustal levels we calculate for example a minimum instantaneous strain rate of approximately 10 210 s 21 in the deformation aureole of a 1 km radius spherical pluton aureole strain rates which range upward to greater than 10 24 s 21 are primarily a function of position in the aureole pluton size and filling rate pluton shape plays only a secondary role although preservation of evidence for high strain rates may be rare and although multiple mechanisms probably operate simultaneously we expect that deformation this rapid in the middle crust would be accommodated primarily by brittle mechanisms q 2003 elsevier ltd all rights reserved

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

thermomechanical evolution of a ductile duplex the thermomechanical evolution of a midcrustal ductile duplex in central australia has been reconstructed through space and time using 40ar 39ar thermochronology flow stress estimates cross sectional restoration of dislocation creep microstructures and microstructural and structural analysis a critical aspect of this analysis is the identification of populations of white micas in quartzite mylonites that have neocrystallized below their closure temperature and which record the time when ductile deformation ceased in dating these micas the mylonitic microstructures have effectively been dated the time temperature history of the duplex has been constrained through multidomain thermal modeling of k feldspar 40ar 39ar data the modeling demonstrates that a temperature gradient existed across the duplex during its formation the concept of microstructural continuity during ductile deformation has great potential for elucidating the kinematic evolution of ductile duplexes mapping of the deformation mechanisms and recrystallized grain sizes of quartzites deformed under greenschist facies conditions has been used to evaluate tectonic offsets that occurred after microstructural freezing this analysis shows that the duplex formed as a forward propagating thrust system accommodating 60 km of convergence between the upper and lower plates of the megathrust with a significant fraction of the displacement occurring after microstructural freezing finally using the data as input to published flow laws for quartz aggregates provides a strain rate history for the duplex although uncertainties are clearly large the timing of highest estimated strain rates during duplex evolution does indeed correlate with the highest rates of convergence between the upper and lower plates of the megathrust system according to regional cooling history studies and with coeval sedimentation in adjoining molasse basins

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

igneous emplacement in a shear zone termination the biotite granite at strontian scotland deformed xenolith strain data together with other structural information indicate that the biotite granite body 90 sq km of the strontian complex scotland was emplaced in the extensional termination of a dextral transcurrent shear zone this shear zone is a splay of the major great glen fault which lies along the southern boundary of the granite siting of the shear zone splay was probably controlled by a a slight releasing bend in the great glen fault in this area and b a large pre existing asymetric synform in proterozoic metasedimentary country rocks which intersects the great glen fault trace at a high angle a model is proposed in which moine thrust caledonian compression at ca 435 ma activated the great glen fault dextrally dextral movements around the releasing bend detached a flat segment from the inside fault wall and the biotite granite was emplaced side ways at depths of about 15 km as a sheet into this extensional listric fault bounded cavity

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

isotopic dating of strain fringe increments duration and rates of deformation in shear zones the time scales over which deformation in the earth9s crust remains localized in shear zones are poorly known as are the associated strain rates we have determined the longevity and rates of deformation using rubidium strontium rb sr microsampling dating of increments of fibrous strain fringes from a pyrenean shear zone the fibers grew quasi continuously through a protracted deformation history between 87 and 50 million years ago over a period comparable to that of an orogeny during a short interval between 66 and 62 million years a rise in strain rate from 1 1 10 15 to 7 7 10 15 seconds 1 occurred this acceleration correlates with an abrupt change in fiber growth direction and a stress field inversion from gravitational collapse to renewed horizontal crustal shortening

rates of processes in magmatic arcs implications for the timing and nature of pluton emplacement and wall rock deformation abstract the construction of arcs and in many cases the emplacement of plutons occur in tectonically active regions it is critical therefore to evaluate the rates of structural and magmatic processes when trying to understand the evolution of arcs and the associated pluton wall rock systems our best estimates of average rates or durations of processes in shallow to moderate level arc environments are the following 1 crystal growth rates in magma of 10 4 cm year 1 2 growth rates of metamorphic porphyroblasts between 10 5 and 10 2 cm year 1 3 long term magma supply rates of 10 1 km 3 year 1 and short term rates of up to 350 km 3 year 1 4 diapiric ascent rates for mafic plutons of 1 3 m year 1 5 cooling of plutons to ambient wall rock temperatures in 10 5 10 6 years 6 final crystallization of plutons in a small fraction of the time needed for complete cooling 7 fault displacements of 3 cm year 1 8 development of cleavages in fault zones in less than 10 6 years at strain rates of 10 13 s 1 or higher and 9 the development of regional cleavages in 10 6 years at strain rates of 10 14 s 1 these rates indicate that processes operating in magmatic arcs are relatively rapid pluton emplacement cleavage development etc occur over time spans of tens of thousands to no more than a few million years however the rate of ascent and crystallization of plutons at shallow levels is generally shorter than that needed to get large displacements on faults or widespread cleavages developed at deeper levels or in zones undergoing faster strain rates the time spans of the various processes approach one another thus plutons with otherwise similar characteristics emplaced in regions undergoing fast strain rates or at deeper crustal levels may appear quite different structurally from those emplaced at shallow levels or in regions undergoing slower strain rates comparison of these data also suggests that the rate at which wall rock deforms is the limiting factor controlling the rates of other structural processes during emplacement of plutons unless fast strain rates or multiple deformation mechanisms are considered thus emplacement mechanisms that rely on the transport of magma over 10 5 to 10 6 years are favored and need further consideration finally we argue that the structural and other characteristics of pluton wall rock systems will depend on rates of various processes involved and that these rates at the very least influence and sometimes invalidate the timing criteria previously published by us and others

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

measuring host rock volume changes during magma emplacement we use a technique to evaluate volume strains in contact aureoles during magma emplacement based on geochemical mass balance calculations dynamothermal contact metamorphism adjacent to the emigrant gap composite pluton sierra nevada california produced an 01 km wide thermal structural aureole consisting of an outer andalusite2cordierite zone and an inner potassium feldspar2sillimanite zone mass balance calculations using al as an immobile reference frame element in metapelites inside and outside of the aureole indicate that the percentage change in total rock mass between chlorite grade rocks outside of the thermal eaects of the pluton and the aureole is 11 1 21 4 mass balance calculations indicate that si was the major rock forming element depleted 0 16 from the aureole during contact metamorphism mass balance calculations and density measurements yield volume strains associated with mass transfer during contact metamorphism of 12 4 these data in conjunction with field relationships are interpreted to suggest that a element mobility during contact metamorphism is not restricted to volatiles and b volume losses within the contact aureole occur during magma emplacement and may contribute to the space making process during magma emplacement if intrusion spans the period necessary to engage hydrothermal circulation in the host rocks 1998 elsevier science ltd all rights reserved

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