high grade metamorphism and partial melting of basic and intermediate rocks rocks of basic and intermediate bulk composition occur in orogenic terranes from all geological time periods and are thought to represent significant petrological components of the middle and lower continental crust however the former lack of appropriate thermodynamic models for silicate melt amphibole and clinopyroxene that can be applied to such lithologies at high temperature has inhibited effective phase equilibrium modelling of their petrological evolution during amphibolite and granulite facies metamorphism in this work we present phase diagrams calculated in the na2o cao k2o feo mgo al2o3 sio2 h2o tio2 o2 nckfmashto compositional system for a range of natural basic and intermediate bulk compositions for conditions of 2 12 kbar and 600 1050 c using newly parameterized activity composition relationships detailed in a companion paper by green et al in this issue particular attention is given to mid ocean ridge basalt morb and diorite protolith bulk compositions calculated subsolidus mineral assemblages in all basic and intermediate rock types are modally dominated by hornblende and plagioclase with variable proportions of epidote clinopyroxene garnet biotite muscovite quartz titanite or ilmenite present at different pressures the h2o saturated wet solidus has a negative p t slope and occurs between 620 690 c at mid to lower crustal pressures of 5 10 kbar the lowest t melts generated close to the wet solidus are calculated to have granitic major element oxide compositions melting at higher temperature is attributed primarily to multivariate hydrate breakdown reactions involving biotite and or hornblende partial melt compositions calculated at 800 1050 c for morb show good correlation with analysed compositions of experimental glasses produced via hydrate breakdown melting of natural and synthetic basic protoliths with niggli norms indicating that they would crystallize to trondhjemite or tonalite diorite is shown to be significantly more fertile than morb and is calculated to produce high t melts 800 c of granodioritic composition subsolidus and suprasolidus mineral assemblages show no significant variation between different members of the basalt family although the p t conditions at which orthopyroxene stabilizes thus defining the prograde amphibolite granulite transition is strongly dependent on bulk rock oxidation state and water content the petrological effects of open and closed system processes on the mineral assemblages produced during prograde metamorphism and preserved during retrograde metamorphism are also examined via a case study analysis of a natural archean amphibolite from the lewisian complex northwest scotland

crystallisation and zircon saturation of calc alkaline tonalite from the adamello batholith at upper crustal conditions an experimental study the understanding of the geochemical and petrophysical evolution of magmas forming intermediate calc alkaline batholiths at shallow crustal levels critically depends on knowledge of the phase equilibria relations along the liquid line of descent here we present experimental results for a tonalitic system at a pressure of 200 mpa and under water saturated conditions melting experiments were performed at temperatures between 700 and 1000 c in externally heated hcm pressure vessels with oxygen fugacity controlled close to the ni nio buffer equilibrium nno employing an argon methane mixture as pressure medium and co pd redox sensors to verify fo2 conditions natural rock powder of a medium k tonalite from the adamello batholith in northern italy served as experimental starting material based on compositional data of stable phases in the run products and images of entire run charges mass balance calculations as well as image processing were performed to investigate the evolution of the crystal melt ratio with respect to temperature furthermore compositional trends of minerals as well as the liquid line of descent of residual melts were obtained orthopyroxene clinopyroxene and plagioclase were identified as near liquidus phases below 990 c at 900 c amphibole joins the solid phase assemblage at the expense of clinopyroxene indicating the existence of a peritectic relationship after an initial near linear decrease with temperature residual melt fractions exhibit a plateau of 45 55 vol between 750 and 850 c followed by a rapid decrease coinciding with quartz saturation at 725 c compositions of residual liquids evolve along a typical calc alkaline differentiation trend with decreasing temperature increasing sio2 and decreasing tio2 al2o3 cao mgo and feo contents and become peraluminous below 900 c intermediate to acidic rocks from the adamello follow the experimental liquid line of descent indicating that the observed compositional spread of the natural intermediate composition rock record can be explained by low pressure magma differentiation and liquid extraction experimentally determined zircon saturation levels are at low temperatures distinctly lower compared to existing and often used zr saturation models but fully consistent with observed and modelled zr evolution trends from the natural rock record of the southern adamello batholith inferring that zircon saturation in these intermediate to felsic plutonic rocks occurred at 800 830 c corresponding to a melt fraction of about 50 vol

the tectonically controlled emplacement of a vertically sheeted gabbro pyroxenite intrusion feeder zone of an ocean island volcano fuerteventura canary islands abstract the miocene px1 gabbro pyroxenite pluton fuerteventura canary islands is a 3 5 5 5 km shallow level intrusion 0 15 0 2 gpa and 1100 1120 c interpreted as the feeder zone to an ocean island volcano it displays a vertical magmatic banding expressed in five 50 to 100 metre wide nne ssw trending alkaline gabbro sequences alternating with pyroxenites this emplacement geometry was controlled by brittle to ductile shear zones generated by a regional e w extensional tectonic setting that affected fuerteventura during the miocene at a smaller scale the px1 gabbro and pyroxenite bands consist of metre thick differentiation units which suggest emplacement by periodic injection of magma pulses as vertical dykes that amalgamated similarly to a sub volcanic sheeted dyke complex individual dykes underwent internal differentiation following a solidification front parallel to the dyke edges this solidification front may have been favoured by a significant lateral horizontal thermal gradient expressed by the vertical banding in the gabbros the fractionation asymmetry within individual dykes and the migmatisation of the wall rocks pyroxenitic layers result from the fractionation and accumulation of clinopyroxene olivine plagioclase crystals from a mildly alkaline basaltic liquid they are interpreted as truncated differentiation sequences from which residual melts were extracted at various stages of their chemical evolution by subsequent dyke intrusions either next to or within the crystallising unit compaction and squeezing of the crystal mush is ascribed to the incoming and inflating magma pulses the expelled interstitial liquid was likely collected and erupted along with the magma flowing through the newly injected dykes clinopyroxene mineral orientation as evidenced by ebsd and micro x ray tomography investigations displays a marked pure shear component supporting the interpretation of the role of compaction in the generation of the pyroxenites conversely gabbro sequences underwent minor melt extraction and are believed to represent crystallised coalesced magma batches emplaced at lower rates at the end of eruptive cycles clinopyroxene orientations in gabbros record a simple shear component suggesting syn magmatic deformation parallel to observed nne ssw trending shear zones induced by the regional tensional stress field this emplacement model implies a crystallisation time of 1 to 5 years for individual dykes consistent with px1 emplacement over less than 0 5 my a minimum amount of approximately 150 km3 of magma is needed to generate the pluton part of it having been erupted through the central volcanic centre of fuerteventura if the regional extensional tectonic regime controls the px1 feeder zone initiation and overall geometry rates and volumes of magma depend on other source related factors high injection rates are likely to induce intrusion growth rates larger than could be accommodated by the regional extension in this case dyke intrusion by propagation of a weak tip combined with the inability of magma to circulate through previously emplaced and crystallised dykes could result in an increase of non lithostatic pressure on previously emplaced mushy dyke walls thus generating strong pure shear compaction within the pluton feeder zone and interstitial melt expulsion these compaction dominated processes are recorded by the cumulitic pyroxenite bands

rheological transitions in progressive melting of rock and their geological constraints from the fuhu metatexite diatexite profile in guangdong province se china abstract integration of the published experimental data of partially melted amphibolite orthogneiss granite and aplite reveals the existence of three rapid strength drops of the rock assemblage at melt fraction around 7 21 and 41 respectively the first and the last drops are equivalent to the well known melt connectivity transition mct and solid to liquid transition slt the drop at melt fraction around 21 is newly identified and termed as the framework melting transition fmt the fmt is defined as the transition from eutectic quartz feldspars melting to refractory biotite melting the three rheological transitions are well constrained by the geological data from the fuhu profile in guangdong province se china this profile is a 15 m high cliff rocks exposed on the profile are zoned as mottled stripped narrow banded and wide banded migmatites and diatexitic granite from the top of the profile downwards the leucosome proportions of the zones measured on the outcrops combined with the experimental data of the partially melted rocks have significant implications for understanding the role of rheological transitions during crustal melting the mct which corresponds to the boundary between the mottled and the narrow banded migmatites suggests the segregation of melt within the partially melted system along with the compaction of matrix thus the mct is deemed to be the transition of the partially melted system from matrix supported melt stored in pore space to framework supported melt stored in foliation bedding space of protolith the fmt corresponding to the boundary between the narrow and the wide banded migmatites denotes the inception of melting of the solid framework mechanically the fmt can be regarded as the transition of the framework from compaction to fusion and geochemically the transition from eutectic melting of quartz feldspars to that of refractory biotite the increasing melt due to framework melting after the fmt is basically limited within the solid framework until the melt fraction exceeding the slt the slt is geologically constrained by the in situ metatexite diatexite transition that is also termed as magma interface mi the slt mi represents the upper limit of a crustal magma layer because of the downward increasing temperature and melt fraction there should be no consecutive layer of felsic rock existed in the space between the slt mi and the mafic lower crust except the isolated rock blocks within the magma layer

tonalite granodiorite suites as cotectic systems a review of experimental studies with applications to granitoid petrogenesis abstract granodiorites and tonalites granotonalites in this review paper form large batholiths at active continental margins and collisional orogens although the relation with subduction is obvious the mechanisms of magma generation remain among the most debated topics in earth sciences today the most outstanding features of these calc alkaline batholiths are summarized in this review the problem has been addressed by laboratory experiments along decades dispersed data from experimental studies are compiled here in order to give phase equilibria constraints of general application it is apparent from chemical data that major element compositional trends follow cotectic patterns whose composition is largely dependent on intensive variables the comparison between experiments and batholith chemical trends produces an empirical thermodynamic framework useful to understand in a first approach the origin and evolution of magmas forming granite batholiths a new projection scheme of rock compositions onto the pseudoternary plane defined by the fe mg anorthite orthoclase is given this can be taken as first approach to phase equilibria model of batholiths it has been revealed useful to distinguish processes controlled by increasing melting or decreasing crystallization magma temperature from processes controlled by assimilation or magma mixing the available experimental data although scarce can be used to tentatively depict phase equilibria in the projected pseudoternary f an or diagram although important limitations are imposed by the projective procedure phase equilibria relations in equilibrium with liquids can be drafted in the diagram the stability fields of mineral in areas close to the liquid are defined according to reported phase assemblages in the experimental studies of reference the diagram is robust to identify cotectic and non cotectic relations phase equilibrium constraints and rock textures have revealed that one of the most important parameter controlling phase equilibria in granite magmas is the initial water content typically calc alkaline granitoids from qtz diorite to granotonalite and granite have crystallized from strongly undersaturated water magmas low initial water contents w i in the range 1 2 wt h 2 o may account for residual saturated melt fractions as low as 0 1 0 2 for plutons emplaced at 400 mpa the water content of these residual melt fractions is enough to stabilize hydrous minerals namely hbl and bt as predicted by experiments the initial water content is a primary feature of the magma and it is directly related to the melting reaction and more precisely to the temperature of magma generation the study of compositional trends supports magma fractionation by crystal liquid separation as a relevant role as a first approach the pressure composition relations in the melts can be seen as a response to the composition of the solid saturating assemblage and particularly related to the ratio grt pl in the solid residue

magnetic susceptibility anisotropy a new petrofabric tool in migmatites 1 the rheological behavior of the lower to middle continental crust during orogenic deformation is poorly understood migmatite common at this crustal level remains difficult to analyze from a structural viewpoint because of pervasive recrystallization we present new anisotropy of magnetic susceptibility ams data from 217 specimens of migmatites of the superior province in minnesota with the aim of investigating its potential as a petrofabric tool quarry exposures provide the ideal sampling playground for such a study the migmatites are coarse grained are granitic to tonalitic in composition and display a centimeter scale compositional layering the layering and the regional foliation are subhorizontal to shallowly dipping to the ne mineral lineations are difficult to observe due to recrystallization but trend around 70 n with gentle plunges magnetic properties are carried mainly by multidomain md primary magnetite grains ranging in size from 20 to 200 m the magnetic susceptibilities are generally high around 5 10 10 3 si the degree of magnetic anisotropy is large between 1 2 and 1 3 the principal axes of the ams ellipsoids coincide with those of the finite strain and the degree of anisotropy broadly correlates with the mineral fabric intensity this study is the first detailed investigation of magnetic fabrics in migmatites these preliminary results suggest that the ams technique is an easy and powerful tool for the petrofabric analysis of migmatites

archaean cratonization and deformation in the northern superior province canada an evaluation of plate tectonic versus vertical tectonic models abstract the archaean minto block northeastern superior province is dominated by tonalite trondhjemite enderbite pyroxene tonalite granodiorite and granite with subordinate mafic rocks and supracrustal belts the plutons have been interpreted as the batholithic roots of andean type plate margins and intra oceanic arcs existing horizontal tectonic models propose that penetrative recrystallization and transposition of older fabrics during terrane assembly at 2 77 and 2 69 ga produced a n nw tectonic grain in the douglas harbour domain northeastern minto block tonalite and trondhjemite dominate the faribault thury complex 2 87 2 73 ga and enderbite constitutes 50 100 km scale ovoid massifs troie and qimussinguat complexes 2 74 2 73 ga magmatic muscovite and epidote in tonalite trondhjemite have corroded edges against quartz plagioclase suggesting resorption during ascent of crystal charged magma foliation maps and air photo interpretation show the common development of 2 10 km scale ovoid structures throughout the douglas harbour domain outcrop and thin section scale structures imply that many plutons experienced a phase of syn magmatic deformation typically followed by high temperature sub magmatic overprints thermobarometric data for plutons indicate near solidus recrystallization at 4 6 kbar pressures the common preservation of syn magmatic fabrics in plutons of different ages seems incompatible with the origin of these fabrics through superimposed regional orogenesis the broad uniformity of intrusion ages and lithologies throughout the minto block and the rarity of shallowly dipping planar fabrics also seem inconsistent with accretion of disparate older terranes each of which should preserve distinct histories a possible alternative explanation for these features is provided by vertical tectonic models whereby buoyant felsic magmas ascended as crystal slurries while dense supracrustal rocks and solidified felsic intrusions emplaced into them subsided as cold fingers 10 20 km scale instabilities shear between upwelling and downwelling limbs would have concentrated in the weak intrusions generating steeply plunging syn magmatic fabrics and producing ductile overprints in solidified rocks

melt extraction from crystal mushes numerical model of texture evolution and calibration of crystallinity ordering relationships abstract mechanical crystal melt interactions in magmatic systems by separation or accumulation of crystals or by extraction of interstitial melt are expected to modify the spatial distribution of crystals observed as phenocrysts in igneous rocks textural analysis of porphyritic products can thus provide a quantitative means of interpreting the magnitude of crystal accumulation or melt loss and reconstructing the initial crystal percentage at which the process occurred we present a new three dimensional numerical model that evaluates the effects of crystal accumulation or interstitial melt removal on the spatial distribution of crystals both processes lead to increasing apparent crystallinity but also to increasing spatial ordering expressed by the clustering index r the trend of progressive crystal packing deviates from a random texture trend produced by static crystal nucleation and growth and it is universal for any texture with straight log linear crystal size distribution for sparse crystal suspensions 5 vol crystals r 1 03 up to 97 melt can be extracted corresponding to a new crystallinity of 65 vol and r 1 32 when the rheological threshold of crystal interlocking is reached for initially crystal rich suspensions the compaction path is shorter this is because the initial crystal population is more aggregated and it reaches the limit of interlocking sooner crystal suspensions with 35 vol crystals cannot be compacted without mechanical failure these results illustrate that the onset of the rheological threshold of magma immobility strongly depends on the spatial configuration of crystals in the mush the primary rigid percolation threshold 35 vol crystals corresponds to touching or interlocking crystal framework produced by in situ closed system crystallization whereas the secondary rigid percolation threshold 35 to 75 vol crystals can be reached by compaction which is particularly spatially efficient when acting on sparse crystal suspensions illustrative quantitative evaluation of the crystallinity clustering relationships to representative porphyritic granites from a single intrusive unit of the krkonose jizera pluton central europe reveals a single crystal accumulation path starting at low initial crystallinity 5 7 vol k feldspar phenocrysts with 24 84 melt extracted leading to the observed crystallinity of 9 26 vol by contrast a camptonite dyke from the ceske st edoho i volcanic province has experienced the onset of crystal accumulation later 18 vol amphibole crystals and lost 23 interstitial melt only the combination of modal and clustering analysis offers a sensitive tool for identifying differentiation processes in natural magma chambers and here it illustrates examples of mechanically dominated open system vs in situ nearly closed system crystallization from two contrasting magmatic settings

magnetic fabric of the barrington passage pluton meguma terrane nova scotia a two stage fabric history of syntectonic emplacement abstract structural analysis and the anisotropy of magnetic susceptibility ams are used to document the internal magmatic to high temperature subsolidus fabrics of the tonalitic late devonian barrington passage pluton the ams fabric orientations compare favorably with mesoscopic scale fabrics in general the k1 k2 principal plane of the ams is close to the observed mineral foliation defined by biotite and plagioclase and the k1 axis is aligned parallel to a mineral lineation defined by the same minerals and to a locally developed weak extension lineation defined by quartz measurement of the anisotropy of anhysteretic remanence aar and image analysis of biotite preferred orientations confirm that the orientation of the ams is principally controlled by biotite locally anomalous ams orientations are attributed to the influence of fine grained magnetite crystals the presence of which is revealed by measurements of hysteresis and coercivity properties a two stage fabric history is revealed first horizontal fabrics were formed foliation compositional banding during emplacement of a laccolith like pluton the second stage involved synmagmatic folding of the horizontal foliation and banding in response to regional contraction associated with the acadian orogeny with development of a lineation that records stretching parallel to the fold axis the results demonstrate that the barrington passage pluton was emplaced during the main phase of the acadian orogeny a model is proposed that depicts the barrington passage pluton as the folded base of a larger syntectonic pluton that is now largely eroded off

aluminous granulites from the pulur complex ne turkey a case of partial melting efficient melt extraction and crystallisation abstract in the pulur complex ne turkey a heterogeneous rock sequence ranging from quartz rich mesocratic gneisses to silica and alkali deficient fe mg and al rich melanocratic rocks is characterized by granulite facies assemblages involving garnet cordierite sillimanite ilmenite spinel plagioclase quartz biotite corundum rutile and monazite textural evidence for partial melting in the aluminous granulites particularly leucosomes is largely absent or strongly obliterated by a late stage hydrothermal overprint however inclusion relations high peak p t conditions the refractory modes bulk and biotite compositions of the melanocratic rocks strongly support a model of partial melting the melt was almost completely removed from the melanocratic rocks and crystallised within the adjacent mesocratic gneisses which are silica rich bear evidence of former feldspar and show a large range in major element concentrations as well as a negative correlation of most elements with sio2 peak conditions are estimated to be 800 c and 0 7 0 8 gpa subsequent near isothermal decompression to 0 4 0 5 gpa at 800 730 c is suggested by the formation of cordierite coronas and cordierite spinel symplectites around garnet and in the matrix sm nd rb sr and 40ar 39ar isotope data indicate peak conditions at 330 ma and cooling below 300 c at 310 ma

from sediment to granite timescales of anatexis in the upper crust abstract granite formation is the culmination of a sequence of events initiated by prograde heating of the protolith and followed by formation of a grain boundary melt melt segregation into a vein network ascent of the melt through the network and finally crystallisation of the melt experimental constraints on the formation of a crustal melt from the incongruent melting of muscovite combined with geochemical studies of anatexis in the high himalaya allow the timescales required for each of these processes to be assessed discordant temperatures determined from monazite and zircon thermometry for himalayan anatectic granites indicate that at least for some intrusives the melt was undersaturated in lree implying that melts have probably been extracted in less than 10 ka experimental studies suggest that some himalayan melts are also undersaturated in zr implying segregation may have occurred within 100 years such short timescales confirm that deformation driven mechanisms are important in extracting these melts from their source the transport distances of himalayan granitic melts of 10 km may be achieved by the ascent of magma through dykes in about 1 day at such rates even the largest granite could theoretically be emplaced in 10 years crystallisation of himalayan melts involves much longer periods if emplaced as thin sheets 100 m wide a timescale of 500 years is required compared with 30 ka for single stage intrusion of the larger laccoliths for composite sheet complexes magma crystallisation rather than melt ascent comprises the rate determining step on the emplacement of the intrusion the overall timescales of melt segregation and emplacement for many orogenic granites are therefore less than 10 ka and possibly less than 1 ka in contrast the timescale required for prograde heating of the protolith is 1 ma since the melt production rate is determined by heat flow into the protolith and not by reaction kinetics for any geologically significant period we conclude that heat flow determined by both the mechanism of heating and the thermal diffusivities of crustal rocks provides the overall rate determining step of crustal anatexis

melt segregation and magma interactions during crustal melting breaking out of the matrix abstract differentiation of the continental crust begins with its partial melting the products of crustal melting are silicic hydrous h2o undersaturated granitic liquids that are generated within matrices of residual crystals crustal differentiation requires that felsic magmas form and escape from these solid residua an important question is whether granitic magmas collect into large batches within or near their sources which then give rise to ascent or alternatively bleed out of the sources in smaller streams or pulses we demonstrate that the physical reality is closer to the second alternative and thus question the validity of the concepts of magma segregation and source fertility as they are sometimes visualised granitic plutons contain chemically distinct sub populations formed by source level entrainment of the peritectic assemblages into the melts rapid evacuation at source levels and high ascent rates protect the magmas from wall rock interactions during their tenure at source depths and during their ascent through cool upper crust the existence of different types of granites and hence of clearly defined chemically different magmas within plutons dictates that felsic magmas must separate efficiently from their anatectic sources and must ascend to the sites of their emplacement with minimal chemical interaction with crust through which the magma must move exposed deep crustal sections are typically lithologically diverse with the more fertile rocks commonly forming discrete layers surrounded by rocks with contrasting compositions if the melts were required to segregate and accumulate into large volumes within their hot sources especially by slow gravity driven melt percolation it is very likely that they would be substantially modified by reaction with diverse source rocks thus the processes mooted to occur in melting assimilation storage and homogenisation mash or deep crustal hot dch zones would act to erase original compositional heterogeneities and produce larger batches of more homogeneous magma perhaps carrying evidence of extensive magma mixing from various lines of evidence we conclude that mash and dch zones may not exist and even if they do they cannot form the sources of most granitic magmas the intrinsic heterogeneity in crustal source rocks the likely occurrence of peritectic assemblage entrainment and the inefficiency of magma mixing decree that granitic rocks will retain source inherited chemical and isotopic heterogeneities rapid semi continuous and disequilibrium withdrawal of small magma batches from source rocks means that elements of whole rock trace element chemistry and accessory mineral chemistry will be decoupled from major element variations this also means that a large source with low nominal fertility due to low hydrous mineral content for example could still produce a substantial granitic pluton by additions of individually small increments of magma

the relationship between length and width of plutons within the crustal scale cobequid shear zone northern appalachians canada the lengths and widths have been measured for 69 component bodies of composite plutons along the cobequid shear zone plutons on major fault strands those with mylonite zones 0 1 km wide exhibit evidence of multiple intrusion of magma batches small plutons along short faults in stepover zones appear related to rapid emplacement of magma in bodies 1 5 4 km long by 0 1 2 km wide such small plutons show low enrichment in incompatible elements in older component bodies but increasing amounts in younger bodies as a result of progressive magma expulsion from crystal mush during crystallization and shear enhanced compaction in fault zones wider plutons generally occur along longer fault strands accommodating more strain and penetrating deeper into the crust and show enrichment in incompatible elements the width of the mylonitic fault zone is about 15 of the width of these plutons the length to width ratio of component bodies and composite plutons varies between 2 and 11 the best fit line describing these data has a slope of 1 056 which implies scaling behavior between plutonism and tectonic processes scalar properties of plutonic bodies are similar to those of faults but scalar relationships observed in component bodies do not apply to composite plutons

effects of chaotic advection on the timescales of cooling and crystallization of magma bodies at mid crustal levels we numerically define the thermochemical evolution of a subduction related crystal bearing magmatic mass at mid crustal levels 0 7 gpa 20 25 km two main dynamic mechanisms are considered 1 a pure buoyancy driven system where the convective flow is induced by density changes during magma cooling 2 a buoyancy driven convective system governed by chaotic advection the non newtonian rheology of natural magmas is taken into account linking the herschel bulkley formulation with the results of fractional crystallization experiments of magmas with the same composition and at the same conditions of temperature and pressure of the studied system the latent heat of crystallization is also considered in order to address the thermal release in the system induced by the crystallization results indicate that the development of chaotic advection generates a complex thermochemical evolution of the system speeding up the crystallization process and the timing required to reach the jamming condition relative to the pure buoyancy driven convective system nearly 2 times faster our results have important implications for both the rheological history of the magmatic body and the refilling of shallower magmatic systems in particular 1 a time dependent composition ranging from basalt to andesite can be extracted from an initial basaltic magmatic batch 2 at the attainment of the maximum packing fraction i e just before the jamming condition homogeneous andesitic melts can be potentially extracted from the system and 3 the development of chaotic advection within the system allows for the extraction of andesitic melt in shorter times compared to a buoyancy dominated system

selective entrainment of peritectic garnet into s type granitic magmas evidence from archaean mid crustal anatectites entrainment of restite is commonly invoked to explain both the origin of relatively mafic granites and granodiorites as well as the chemical connection between granite magmas and their sources this concept has become linked to models for magma migration out of the source as restite entrainment is considered to take place when diatexitic sources mobilise en masse this is at odds with the common occurrence of relatively mafic granites as high level intrusions in the crust or their eruptive equivalents that must have formed from markedly water undersaturated magmas that ascended through narrow conduits we investigate pelitic migmatites from the mkhondo valley metamorphic suite mvms in swaziland where a mid crustal heating event produced metatexitic migmatites with minimal post anatectic recrystallisation in these rocks all the garnet is peritectic having arisen through biotite fluid absent melting which produced garnet poikiloblasts characterised by inclusions of melt quartz and biotite leucosomes that represent sites of melt transfer carry similar smaller typically 1 mm entrained garnet poikiloblasts which were capable of amalgamating to form larger composite grains in anatectic structures where melt was present for longer entrained garnet was extensively recrystallised via a dissolution precipitation process to adopt a more magmatic character the peritectic garnet in the pelitic source appears to have grown out of equilibrium with feldspar and hree rich accessory phases while the recrystallised garnet in the larger melt filled structures became progressively better equilibrated with these minerals thus peritectic garnet in the source grew sufficiently rapidly to prevent trace element equilibrium with the bulk rock composition and concurrent rapid magma segregation prevented the development of diatexitic source conditions the segregated magma consisted of melt the peritectic assemblage principally garnet and the accessory minerals monazite and zircon these rocks illustrate that mafic granites may arise purely as mixtures of melt and the peritectic assemblage produced by the incongruent melting reaction importantly under the circumstances which produced the mvms anatectites peritectic garnet is entrained as 1 mm poikiloblasts demonstrating how mafic granitic magmas can migrate out of the source without the source becoming diatexitic

observing silicic magma transport in dykes at depths of 8 19 km evidences from crustal xenoliths and numerical modelling abstract within the context of magma transport processes in a dyke this paper integrates petrography and thermodynamic modelling together with fluid dynamics to model the melting of the wall rock in three magma dykes at crustal depths from 8 to 19 km using this combination we explore the interaction between thermal and mechanical processes during silicic magma ascent and the implications for the relative abundance of different crustal xenoliths erupted at the surface we utilize a two dimensional thermal model of dacitic magma injection and flow in dykes of different thickness lengths and depths and associated partial melting of the wall rock and compare the results with field examples from the neogene volcanic province se spain the modelling results open the possibility to relate the range of observed xenoliths information microstructures size distribution at the surface p t evolution to their position in a transient thermal regime in the wall rock of a magma conduit and to the time spent immersed in magma in addition the effects of the modelled stress in the wall rock during magma ascent by the presence or absence of planar features in the xenoliths reveal that the stress patterns are independent of the dyke length and thickness

tectonics of the ninetyeast ridge derived from spreading records in adjacent oceanic basins and age constraints of the ridge 1 analysis of new and existing geophysical data for the central indian and wharton basins of the indian ocean were used to understand the formation and evolution of the ninetyeast ridge ner especially its relationship to the kerguelen hot spot and the wharton spreading ridge satellite gravity data and magnetic anomalies 34 through 19 define crustal isochrons and show fracture zones striking n5 e one of these at 89 e crosses the n10 e trending ner impacting the ner morphology from 77 to 43 ma the ner lengthened at a rate of 118 km myr twice that of the 48 58 km myr accretion rate of adjacent oceanic crust this difference can be explained by southward jumps of the wharton spreading ridge toward the hot spot which transferred portions of crust from the antarctic plate to the indian plate lengthening the ner magnetic anomalies document a small number of large spreading ridge jumps in the ocean crust immediately to the west of the ner especially two leaving observable 65 and 42 ma fossil spreading ridges in contrast complex magnetic anomaly progressions and morphology imply that smaller spreading ridge jumps occurred at more frequent intervals beneath the ner comparison of the ner dates and magnetic anomaly ages implies that the hot spot first emplaced ner volcanoes on the indian plate at a distance from the wharton ridge but as the northward drifting spreading ridge approached the hot spot the two interacted keeping later ner volcanism near the spreading ridge crest by spreading center jumps

influence of melt induced mechanical anisotropy on the magnetic fabrics and rheology of deforming migmatites central vosges france in the central vosges france we describe and model the structural relationships existing between orthogneiss domains and their surrounding viscous migmatites these domains show a structural and anisotropy of magnetic susceptibility ams zonation associated with an increasing amount of melt from the core to their margins a pre extension oblate to plane strain fabric is preserved in the core of the orthogneiss domains where the granite veins were emplaced along a dilated mechanical anisotropy the internal margin of the orthogneiss bodies exhibits development of a prolate fabric and active buckling of an orthogneiss granite multilayer the external margin of the orthogneiss bodies is characterized by oblate fabrics resulting from a layer perpendicular shortening of a melt orthogneiss multilayer pinch and swell structures and extensional kink bands along which granite veins have been injected the surrounding metasedimentary migmatites show a similar structural history but the degree of fabric resetting is significantly higher this systematic fabric and structural succession is due to contrasting rheological evolution between orthogneiss and metasedimentary migmatites during progressive melting and continuously evolving degree of mechanical anisotropy in the stronger orthogneiss numerical modeling confirms the role of the relationship between initial mechanical anisotropy and superimposed deformation overprints on finite ams fabrics in migmatites

development of igneous layering during growth of pluton the tarcouate laccolith morocco abstract we discuss the significance of igneous layering with respect to pluton growth processes the case study is the tarcouate laccolith morocco whose core consists of modally layered hornblende granodiorites with high amount of monzodioritic enclaves contrasting with peripheral non layered biotite granodiorites with low amount of enclaves rhythmic layering with modal grading cross stratification and trough layering is associated with monzodioritic layers and wraps around mafic enclaves its steep dips 45 result from tilting that occurred above solidus conditions as indicated by sub vertical and synmagmatic granite aplite and monzodiorite dykes cutting across the layering the systematic association of igneous layering with mafic enclaves in calc alkaline plutons suggests that layering originates from recurrent injection of mafic magma viscosity calculations suggest that the physicochemical properties of magma alone cannot account for the presence of layering in the central hornblende granodiorite and its coeval absence in the peripheral biotite granodiorite of the tarcouate laccolith intermittent pulses of hot mafic magma into crystallizing granodiorite likely produced thermal perturbations able to trigger local convection formation of mafic enclaves and development of igneous layering through protracted crystallization

mueilha rare metals granite eastern desert of egypt an example of a magmatic hydrothermal system in the arabian nubian shield abstract the mueilha granite pluton is one of the rare metals bearing peraluminous granitic plutons in the arabian nubian shield it represents the apical part of a highly evolved magma chamber emplaced at a shallow level subsequent to the post pan african orogeny the pluton can be seen as a highly leucocratic medium grained albite oligoclase framework infilled with quartz k feldspar and muscovite that are variably overgrown by k feldspar muscovite quartz and topaz megacrysts the increasing number and size of the k feldspar megacrysts at the expense of the whitened albite oligoclase framework imparts variably red color to the mueilha granite contacts between the milky white and red granites are usually gradational but may be locally sharp or may form narrow transition zones resulting from abrupt variations in texture and lithology textural relations indicate an initial stage of hydrothermal albitization of magmatic plagioclase and crystallization of topaz megacrysts resulting from infiltration of na rich fluorine bearing fluids a subsequent stage of metasomatic enrichment is characterized by extensive growth of large k feldspar quartz and muscovite megacrysts at the expense of the albite oligoclase crystals as a result of infiltration of k si rich hydrous fluids post magmatic infiltration of hydrous fluids along the fault planes is shown by the intense replacement of alkali feldspar megacrysts by quartz development of myrmekitic intergrowth pockets along the k feldspar megacrysts and sealing of the micro fractures by cryptocrystalline mixtures of clay minerals iron oxides sericite and chlorite compositionally the red granitic rocks have higher sio 2 fe 2 o 3total k 2 o na 2 o ree y th u zr and zn and lower al 2 o 3 ga ta nb and mo compared to the milky white granites lile and sn do not show clear variation trends throughout the mueilha granite pluton suggesting their immobility during hydrothermal alteration microthermometric measurements indicate that the interactions with the hydrothermal fluids started at a minimum temperature 400 c most likely during the late stage crystallization of the mueilha granite and continued after their complete solidification i e subsolidus conditions at a temperature as low as 120 c the high fertility of mueilha granite is most plausibly the result of partial melting within the undepleted juvenile crust of the arabian nubian shield that has formed during the pan african orogeny

rheology of granitic magmas during ascent and emplacement flow abstract considerable progress has been made over the past decade in under standing the static rheological properties of granitic magmas in the continental crust changes in h2o content co2 content and oxidation state of the interstitial melt phase have been identified as important compositional factors governing the rheodynamic behavior of the solid fluid mixture although the strengths of granitic magmas over the crystallization interval are still poorly constrained theoretical investigations suggest that during magma ascent yield strengths of the order of 9 kpa are required to com pletely retard the upward flow in meter wide conduits in low bagnold number magma suspensions with moderate crystal contents solidosities 0 1 0 3 viscous fluc tuations may lead to flow differentiation by shear enhanced diffusion ams and mi crostructural studies support the idea that granite plutons are intruded as crystal poor liquids 50 with fabric and foliation development restricted to the final stages of emplacement if so then these fabrics contain no information on the ascent verti cal transport history of the magma deformation of a magmatic mush during pluton emplacement can enhance significantly the pressure gradient in the melt resulting in a range of local macroscopic flow structures including layering crystal alignment and other mechanical instabilities such as shear zones as the suspension viscosity varies with stress rate it is not clear how the timing of proposed rheological transitions formulated from simple equations for static magma suspensions applies to mixtures undergoing shear new theories of magmas as multiphase flows are required if the full complexity of granitic magma rheology is to be resolved

hornblende dehydration melting in mafic rocks and the link between massif type charnockite and associated granulites eastern ghats granulite belt india a massif type intrusive charnockite body in the eastern ghats granulite belt india is associated with hornblende bearing mafic granulite two pyroxene granulite and enderbitic granulite the charnockite is characterised by pervasive gneissic foliation s1 this is axial planar to the folded layers of hornblende bearing mafic granulite f1 folds indicating that the granulite protoliths were present before the development of s1 two pyroxene granulite and enderbitic granulite occur as lenticular patches disposed along the foliation and hence could be syngenetic to s1 the tonalitic to granodioritic metaluminous to weakly peraluminous compositions and relatively high sr rb of the charnockite are consistent with its derivation by partial melting of a mafic protolith strong y depletion lack of sr depletion and strongly fractionated ree patterns with high la yb n ratio but relatively lower hree gd lu fractionation with marked positive eu anomalies suggest major residual hornblende as well as garnet but not plagioclase consistent with the hornblende dehydration melting in the source rocks such a residual mineralogy is broadly similar to those of some of the hornblende bearing mafic granulite inclusions which have compositional features indicative of a restitic nature quantitative modelling supports an origin for the charnockite melts by partial melting of a hornblende rich mafic granulite source although source heterogeneity is very likely given the rather variable trace element contents of the charnockite the whole rock and mineral compositions of the two pyroxene granulites and enderbitic granulites are consistent with them representing peritectic phase segregations of hornblende dehydration melting a clockwise p t path implies that melting could have occurred in thickened continental crust undergoing decompression

modeling ore generation in a magmatic context abstract magmatic ore deposit models are constructed from a set of measurables e g ore grade fluid composition structure from which important parameters temperature pressure chemistry can be estimated and possible genetic processes can be constrained such models include direct inverse or iterative problems direct problems separately consider an eulerian and a lagrangian formulation in the first case an analytical approach describes the bulk system from an external frame whereas the lagrangian approach provides a description from a discrete element attached to the system conversely inverse problems mostly rely on a system of equations or differential equations their solution requires a matrix inversion using statistical criteria to bracket errors subsequently an iterative approach is adopted commencing with an initial bulk model that is successively refined to fit the observations the direct problem always results in a unique solution though the formulation is highly overdetermined such unique solution highly depends on the input parameters and multiple solutions vary with the initial imposed conditions conversely the inverse problem is underdetermined by construction atlan 2001 accordingly it provides a set of solutions generally identified and separated by statistical tests as exemplified bythe least squares approximation both methods intrinsically ignore feedback loops iterative methods are weak in quantifying the results based on the insights gained from this review we developed a new integrative model for porphyry deposits that relies on the magmatic segregation of metals through a fluid sparging process a formulation under the direct problem basically considers the enrichment factor for groups of metals e g cu mo au a lagrangian approach using a lattice boltzmann model examines metal diffusion from the melt toward an immiscible phase commonly a salty aqueous fluid metals first diffuse in the melt the motion of which slows down when the mush development reduces the porosity thus tapping the mobile fluid phase gas bubbles turn to tubes they offer more mobility and allow the progression of the fluid phase leading to metal advection followed by metal precipitation the quantitative results are poorly constrained owing to the large uncertainties on the input parameters but the metal enrichment factor fits observations from crustal abundance to ore grade levels nevertheless the results aid in designing an inverse approach linking metal enrichment to metal partitioning diffusion and viscous melt motion from this approach a simple diagram can be deduced using a logarithmic scale to smooth the parametric uncertainties the diagram serves to illustrate the link between the above parameters and metal enrichment also explaining differences in ore grade for different metals magma pairs

structures and melt fractions as indicators of rheology in cordierite bearing migmatites of the bayerische wald variscan belt germany a melt but the degree of their shape preferred orientation increases the relation between melt volume fraction melt segregation and with increasing melt segregation and differential stress the mideformation mechanisms of minerals in migmatites and the concrostructures suggest that deformation mechanisms and hence the trolling effects of all these factors on the bulk mechanical behaviour bulk mechanical behaviour of migmatites change in space and time were investigated using rocks from the bayerische wald bohemian during partial melting the observed complex interplay of melt massif variscan belt germany biotite dehydration melting at volume fraction melt segregation bulk strength contrasts mechanical 800 850 c and 0 5 0 7 gpa was the migmatite forming process properties of different minerals and time cannot be described by a in this area four migmatite types were distinguished that alternate single flow law detailed mapping of migmatite areas along with on the scale of several decimetres to several tens of metres type microstructural observations deformation experiments considering mig1 is massive and undeformed types mig2 and mig3 are heterogeneous melt distribution and numerical models that integrate both stromatic leucosome mesosome interlayering but differ in the different flow laws for various stages of partial melting may serve degree of deformation type mig4 has an interlayering of meto derive quantitative models for the bulk mechanical behaviour of lanosome and leucosome and is strongly deformed melt volume crustal sections in the future fractions found by volume estimation of pure melt leucosomes on outcrop hand specimen and thin section scales are 20 40 vol and coincide fairly well with melt volume fractions produced in dehydration melting experiments with similar bulk compositions at the relevant pressure and temperature conditions 20 30 vol the degree of melt removal from mesosomes and melanosomes melt

on the mechanical effects of poroelastic crystal mush in classical magma chamber models author posting american geophysical union 2018 this article is posted here by permission of american geophysical union for personal use not for redistribution the definitive version was published in journal of geophysical research solid earth 123 11 2018 9376 9406 doi 10 1029 2018jb015985

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

fabric development as the key for forming ductile shear zones and enabling plate tectonics abstract lithospheric deformation on earth is localized under both brittle and ductile deformation conditions as high temperature ductile rheologies are fundamentally strain rate hardening the formation of localized ductile shear zones must involve a structural or rheological change or a change in deformation conditions such as an increase in temperature in this contribution i develop a localization potential that quantifies the weakening associated with these changes the localization potential corresponds to the increase in strain rate resulting from that change under constant stress conditions i provide analytical expressions for the localization potential associated with a temperature increase grain size reduction an increase in water fugacity melt content or the abundance of a weak mineral phase i show that these processes cannot localize deformation from a mantle convection scale 103 km to a ductile shear zone scale 1 km to achieve this is it necessary to invoke a structural transition whereby the weak phase in a rock forms interconnected layers this process is efficient only if one phase is much weaker than the others or if the weakest phase has a highly non linear rheology micas melt and fine grained aggregates unless dry rheologies are used have the necessary characteristics as none of these phases is expected to be present in the dry lithosphere of venus this concept can explain why venus unlike the earth does not display a global network of plate boundaries the diffuse plate boundary in the central indian ocean may be as yet non localized because serpentinization has not reached the ductile levels of the lithosphere

field and geochemical constraints on mafic felsic interactions and processes in high level arc magma chambers an example from the halfmoon pluton new zealand the 140 ma halfmoon pluton of stewart island new zealand provides direct evidence for a number of physico chemical processes that operate at depth within active arc settings it is characterized by a sequence of mingled mafic sheets and enclaves that are preserved within intermediate felsic host rocks way up structures and textures are consistent with a younging direction to the south and allow a south dipping magma chamber stratigraphy to be constructed the similarity in modal mineralogy geochemistry and isotopic signature between the various components and their proximity in space and time indicates that the system can be considered as a single composite pluton with discrete portions active at different times interpretation of mingling structures and textures in the field combined with detailed geochemical and geochronological analyses has allowed the identification of several physical and chemical processes operating within the chamber including 1 multiple mafic magma pulses that were extensively mingled and sometimes physically mixed at the exposed level and at depth 2 the development of mineral fabrics e g aligned plagioclase and hornblende crystals and alignment of mafic enclaves caused by processes of crystal accumulation magmatic flow and compaction 3 shortening of the chamber during crystallization as a result of magmatic loading 4 fractional crystallization of the intermediate felsic host magma that was interrupted periodically by mafic magma intrusion 5 incremental assembly of the pluton from the base up 6 limited chemical transfer between the mingled mafic sheets and enclaves and the surrounding intermediate felsic host rocks variations in morphology chemistry and texture of the mingled mafic rocks relative to their position within the magmatic stratigraphy indicate that processes operating within the chamber varied in both space and time mineral assemblages chemical characteristics primitive isotopic signatures and a lack of zircon inheritance indicate that the amphibole rich calc alkaline halfmoon pluton was emplaced into a juvenile arc setting and experienced no contamination with ancient crustal materials a model of pluton construction and evolution is presented that shows that the halfmoon pluton consisted of at least two adjacent magma pods that grew incrementally by episodic replenishments of mafic magma into a magma chamber of evolving intermediate felsic composition data are consistent with a model whereby hydrous amphibole rich basaltic magmas ponded at the crust mantle interface and episodically rose injected and mingled

the inception and growth of leucosomes microstructure at the start of melt segregation in migmatites the beginning stages of melt segregation and the formation of leucosomes are rarely preserved in migmatites most arrays of leucosomes record a more advanced stage where flow dominates over segregation however the early stages in the formation of leucosomes and the segregation of melt are preserved in a partially melted meta argillite from the metatexite zone 800 c of the contact aureole around the duluth complex minnesota the rock contains 2 4 modal leucosome in a matrix consisting of 40 5 in situ neosome and 57 1 cordierite plagioclase framework the domainal microstructure in the matrix is a pre anatectic feature resulting from the bulk composition terminal chlorite reactions produced a large volume of cordierite which with plagioclase formed a framework that enclosed patches of biotite quartz plagioclase k feldspar upon melting these fertile domains became patches of in situ neosome plagioclase in the neosome is less sodic than in the leucosome hence segregation of melt occurred during crystallization not melting segregation was delayed because the cordierite plagioclase framework was strong enough to resist dilatation and compaction until after crystallization started the leucosomes are small i e they are microleucosomes and display a systematic progression in morphology as length and aspect ratio increase from 1 to 19 mm and from 2 5 to 30 respectively small equant micropores form first and in places these coalesce into small 1 mm aspect ratio 2 5 isolated blunt ended elliptical microleucosomes in the next stage micropores develop ahead of and at 45 to the left and right of the blunt tip of a microleucosome one of these develops into an elliptical leucosome and an en echelon array of either a left or right stepping elliptical microleucosome forms each elliptical microleucosome in the en echelon arrays is separated by a bridge of matrix next microleucosomes of greater length 4 mm and aspect ratio 5 form when the bridges of cordierite plagioclase matrix rupture and the elliptical microleucosomes link together to form a zigzag shaped microleucosome finally still longer microleucosomes with greater aspect ratios 30 are formed by the joining of zigzag arrays such a progression is characteristic of the way ductile fractures grow the segregation of melt was driven by the pressure gradient between the dilatant fracture and an adjacent in situ neosome which drew melt to the growing fracture thereby creating a microleucosome the microleucosomes are filled arrays of ductile fractures melt was contiguous only between microleucosomes and adjacent patches of in situ neosome the length scale of segregation was 5 mm the size of a typical patch of in situ neosome and restricted by the surrounding impermeable cordierite plagioclase framework the melt in the microleucosome was the most fractionated and the last to crystallize all microleucosomes contain entrained minerals as a consequence of their mechanism of growth rupture of the bridges resulted in the entrainment of pre anatectic phases however microleucosomes that cross patches of in situ neosome are also contaminated with peritectic phases that were transported with the melt

metamorphism transient mid crustal rheology strain localization and the exhumation of high grade metamorphic rocks 1 we present a series of three dimensional numerical models investigating the effects of metamorphic strengthening and weakening on the geodynamic evolution of convergent orogens that are constrained by observations from an exposed mid crustal section in the new england appalachians the natural mid crustal section records evidence for spatially and temporally variable mid crustal strength as a function of metamorphic grade during prograde polymetamorphism our models address changes in strain rate partitioning and topographic uplift as a function of strengthening weakening in the middle crust as well as the resultant changes in deformation kinematics and potential exhumation patterns of high grade metamorphic rock results suggest that strengthening leads to strain rate partitioning around the zone and suppressed topographic uplift rates whereas weakening leads to strain rate partitioning into the zone and enhanced topographic uplift rates deformation kinematics recorded in the orogen are also affected by strengthening weakening with complete reversals in shear sense occurring as a function of strengthening weakening without changes in plate boundary kinematics

pervasive melt migration from migmatites to leucogranite in the shuswap metamorphic core complex canada control of regional deformation abstract the shuswap metamorphic core complex exhumed in the hinterland of the canadian cordillera displays a 15 km structural section from migmatites to leucogranite it comprises a lower unit dominated by anatectic migmatites exposed in the core of domes at higher structural level the transition from a middle amphibolite facies unit to an upper crustal unit is marked by a low angle detachment associated with the emplacement of laccoliths of leucogranite the amphibolite facies middle unit is permeated by a network of granitic veins feeding the laccoliths and structurally connected to the migmatites of the lower unit this paper describes the geometric textural and structural characteristics and the distribution of the granitic fraction at the outcrop scale in the amphibolite facies unit separating the migmatites from the leucogranite this analysis constrains the mechanisms of melt migration within a plurikilometer scale section across high grade rocks of the middle crust in fertile lithologies metapelites felsic amphibolites the granitic fraction in part generated in situ forms a diffuse network of concordant veins feeding discordant veins and structural sites such as the foliation shear zones and boudin necks discordant granitic veins are oriented perpendicular and to a lesser extent parallel to the mineral and stretching lineation in refractory lithologies the granitic fraction is dominantly intrusive and displays sharp contacts with the host rock however distribution of the granitic fraction shows the same characteristics as for fertile lithologies the melt fraction migrated through a network of fractures taking advantage of mechanical weaknesses such as the foliation plane or forming dikes oriented dominantly perpendicular to the regional stretching and mineral lineations these geometric and textural characteristics suggest that granite migration in these rocks is achieved dominantly by viscous flow of the melt solid through the solid matrix driven by its buoyancy and controlled by mechanical anisotropy of the rock and local dilation created by heterogeneous deformation the relationship between the regional fabric and the distribution of the granitic fraction indicates that regional deformation incremental and finite played a major role in providing pathways for melt migration the formation of laccoliths of leucogranite at higher structural levels suggests that upward melt migration led to accumulation of granitic magmas these observations are consistent with a model of pervasive melt migration through a network of interconnected concordant and discordant granitic veins feeding laccolithic plutons extracted from the dominantly solid matrix during vertical thinning and compaction at a regional scale the presence of both transposed and intact granitic networks indicate an overlap between deformation and granite migration as a continuum or in pulses the methodology and terminology proposed in this paper provide some insights on the behavior of the granitic fraction in the middle crust this approach should be applicable to other high grade terrains exhumed in the cores of orogenic belts

chemical heterogeneity in an enderbite hosted pseudotachylite eastern india evidence for syn deformation multi reaction melting in pseudotachylite the origin of chemical and mineralogical heterogeneity in tens of microns wide layers and domains in enderbite hosted couple of centimeters wide pseudotachylite vein is examined based on the results of bse and x ray element imaging and electron probe microanalyses of major elements of host rock minerals clasts microphenocrysts and pseudotachylite matrix the pseudotachylite layers and domains containing variable proportions of orthopyroxene and magnetite microphenocrysts continue as mantles around quartz k feldspar plagioclase and garnet clasts the clasts are chemically modified along margins and intra clast pseudotachylite injections the chemical modifications are extensive in smaller clasts 5 m diameter at least three chemically distinct layers and domains in the pseudotachylite and their fine grained matrices plot in sharply defined well segregated and non overlapping fields in feo mgo al2o3 sio2 feo cao mgo and cao na2o k2o and feo vs feo feo mgo diagrams the compositions of the layers and domains smeared between a feldspar quartz component and a ferromagnesian component of garnet fe ti oxides orthopyroxene possibly correspond to fractionated quenched melts or admixtures of microphenocrysts that cannot be resolved by the microbeam techniques employed the compositional variations are incompatible with deformation driven crystal fractionation in melt instead the layers and domains possibly are crystal melt mushes produced by syn deformation ultra high temperature 1 250 1 375 c melting reactions involving variable proportions of host rock minerals determined by time transient local phase aggregates experiencing strain the similar element variation trends in pseudotachylite examined here and those reported from anorthosite metapelite and charnockite elsewhere suggests local phase aggregate controlled multi reaction melting is a phenomenon commoner than hitherto realized in pseudotachylites

the compositional variation of i type granites constraints from geochemical analyses and phase equilibrium calculations for granites from the qinling orogen central china abstract there is a large compositional variation of i type granites although partial melting and fractional crystallization are the two primary mechanisms for the variation it is intriguing to know how they jointly work this issue is addressed by a combined study of geochemistry and phase equilibrium modelling for a triassic pluton from the qinling orogen the pluton consists of high k calc alkaline i type granites geochemical results suggest that the granites are likely sourced from triassic mafic intermediate igneous rocks compared with experimental melts it is inferred that the natural granites are not the crystallized product from pure magmatic melts this is confirmed by phase equilibrium calculations that have utilized the triassic mafic intermediate igneous rocks as the source yielding modelled melts that contain too low feot mgo and mg to match the target granites the addition of restitic mineral assemblages to melts can relieve the discrepancy but the modelled magmas have much higher cao than the granites replacing equilibrium melting by disequilibrium melting in terms of the plagioclase behavior can lead to a satisfactory match furthermore by taking the average of the target granites as a parental magma the modelled crystalline products can well reproduce the variation trends in the high k calc alkaline i type granites from the qinling orogen in this regard the crustal anatexis would produce the parental magma at first which experienced fractional crystallization to form the granitic pluton with the compositional variation as observed therefore these two processes would jointly lead to the compositional variation of i type granites in nature

old magma and a new intrusive trigger using diffusion chronometry to understand the rapid onset calbuco eruption april 2015 southern chile in april 2015 an unpredicted rapid onset eruption occurred at calbuco volcano southern andes of chile this event consisted of two sub plinian eruptions separated by a few hours by analysis of fe ti exchange between ilmenite and titanomagnetite crystals in samples of erupted material we determine timescales of pre eruptive heating experienced at the partially solidified chamber base and constrain the magma residence time for the bulk of the carrier magma analysis of the fe ti oxide pairs from a sample retrieved from a pyroclastic density current deposit cal 160 shows that it was affected by a significant heating event recording 70 220 c of heating while other collected samples did not record this late heating this sample is interpreted to represent a piece of crystal mush located at the bottom of a prolate ellipsoidal mush reservoir mobilised 4 days before the eruption by a triggering pulse of mafic magma considerably hotter than the typical magmatic temperature of the reservoir another two fall deposit samples lapillus cal 149tb and cal 155 of the eruption are interpreted to represent resident eruptible magmas that did not interact with any magma recharge immediately prior to or during the eruption we infer that these magmas had been at eruption temperature for some years based on their extensively equilibrated fe ti oxides

effect of finite deformation and deformation rate on partial melting and crystallization in metapelites 1 strain and strain rate partitioning in partially molten rocks are two of the important mechanisms that govern the process of coupling and or decoupling of the partially molten lithosphere consequently the proportion of partial melt and crystals and their network in partially molten rocks influence the degree of the partitioning along with the bulk rheology of the system this study explores the possible role of finite strain and strain rate on the rate and volume of partial melting and crystallization in a metapelitic system undergoing deformation cylinders of synthetic quartz muscovite aggregate 7 3 volume ratio were deformed in torsion at 750 c 300 mpa and constant shear strain rate 3 1 0 4 s 1 for finite shear strains g 1 15 the deformed samples were studied along the longitudinal tangential lt and axial la sections to obtain the data along a range of strain rates for a given finite strain and vice versa the results showed that deformation plays an important role on the kinetics of partial melting and crystallization with increasing strain rate amount and rate of crystallization comprise the volumetrically dominant process compared to partial melting at a given finite strain in contrast when the strain rate is constant partial melting is the dominant process over crystallization up to moderate strain g 5 the dominant process reverses at higher strain and the system shows more crystallization than partial melting application of the experimental data to geological systems implies that for metapelites a significant amount 20 of partial melt can generate at high strain rate and moderate strain g 7 but at high strain g 15 the system is melt depleted under such conditions decoupling should take place in brittle ductile mode on the other hand rocks undergoing deformation with low strain rates and strain g 3 contain more than 25 partial melt which can act as a major decoupling agent by localizing ductile shear zones

rheology of the plate interface dissolution precipitation creep in high pressure metamorphic rocks abstract subduction zone models invoke deformation to be concentrated along the plate interface in a region of particularly low temperature geophysical observations do not provide constraints on temperature stress and deformation patterns with desired resolution in contrast the record of high pressure metamorphic rocks exhumed from subduction zones provides details on p t history deformation mechanisms and stress state albeit not readily correlated with the former dynamic situation on larger scale here we review available information on dissolution precipitation creep dpc in high pressure metamorphic rocks which if representative for subduction zones in general can pose constraints on conditions rheology and flow patterns along the plate interface the key observations and conclusions are that 1 deformation is typically highly inhomogeneous and localized into shear zones 2 stresses are generally too low to drive crystal plastic deformation 3 microfabrics suggest dissolution precipitation creep to be the predominant deformation mechanism 4 an aqueous fluid at quasi lithostatic pressure is available throughout allowing for tensile fracturing and crack healing or sealing 5 low stress combined with high strain rates required for localized deformation at typical subduction rates implies low viscosity and 6 contribution of shear heating to the thermal budget of subduction zones should be moderate the dominant deformation mechanism dpc is reviewed in some detail including experimental and theoretical approaches various examples of dpc in high pressure metamorphic rocks are illustrated emphasizing the role of interphase boundaries as sites of dissolution rheology governed by dpc is proposed to control interplate coupling and development of a subduction channel with return flow being a likely candidate for rapid exhumation of high pressure metamorphic rocks

fabric evidence for granodiorite emplacement with extensional shear zones in the variscan gredos massif spanish central system abstract three granitoid bodies in the central part of the gredos massif spanish central system batholith are tabular around 1 km in thickness and intruded into a migmatitic middle crust during the d3 deformation phase of the variscan orogeny petrologically they are composed of bt granodiorite and crd monzogranite and they show varying abundance of large cm scale feldspar megacrysts a detailed study of the shape preferred orientation spo magmatic fabric defined by these megacrysts together with a kinematic analysis of the structures due to interactions between them and the measurement of quartz c axis fabrics in migmatites and granitoids suggests that granitic magma and country rocks were mechanically coupled during deformation the emplacement took place along large scale extensional shear zones active during the first stages of the d3 phase the shape of the spo ellipsoids varies from constrictional at the centre of the granitic bodies to flattening or even oblate at their external contacts with the migmatitic host rocks the favoured interpretation of this spatial fabric variation is the overprinting of the emplacement fabrics by a constrictional tectonic regime associated with the growth of tabular magma chambers along extensional detachments followed by shear zone development commonly at the top of the granitic bodies the entire structure was later folded during the last stages of the d3 phase

crustal melting by ponding of mafic magmas a numerical model abstract we examine the possibility that an abating period of mantle related volcanic activity at continental margins triggers the production of crustal derived melts a vertical zone permeated by rising mafic melts is established below the volcanic arc if long lasting mantle derived magmatism occurs this situation can be assumed to exist at continental arcs we propose that mafic magmas release heat and water into the crustal environment if subsequent magma ascent is stopped due to establishment of a density barrier this process enhances intracrustal partial melting using a finite element program we simulate in a 2d model area the time dependent change of the crustal thermal structure the spatial distribution of potential melting zones and their degree of partial melting within the crust is computed the amount of generated melt able to segregate is calculated and compared with volume estimates given by field investigation and geochemical constraints in a natural test case we chose the altiplano puna volcanic complex apvc which is part of the central andes as an example where the process may have happened and show from simple approximations that the model can explain the volumes of crustal melts represented by the felsic volcanics in this region although the model parameters and assumptions used are specific to this geologic context the proposed model is generally applicable to any arc setting

petrogenesis of the middle triassic erenhot granitoid batholith in central inner mongolia northern china with tectonic implication for the triassic mo mineralization in the eastern central asian orogenic belt abstract voluminous post orogenic magmatism holds a key to monitoring important geodynamic processes and crustal evolution within an extensional accretionary orogen the middle triassic erenhot granitoid batholith from central inner mongolia northern china is tectonically located in the eastern central asian orogenic belt this batholith consists of monzogranite syenogranite alkali feldspar granite and albite granite with minor mafic to felsic enclaves and dykes the formation of this composite batholith involved distinct mafic and felsic magma end members concomitant episodic mafic felsic interaction and intra felsic magma evolution mafic diabasic and high mg intermediate dioritic dykes show enrichment in large ion lithophile elements depletion in high field strength elements and heterogeneous isotopic compositions indicative of a metasomatized lithospheric mantle source inherited from prior oceanic slab subduction the monzogranites and aplites have arc like elemental features and variable isotopic compositions isr t 0 706132 end t 0 4 zircon ehf t 2 8 to 7 4 zircon 18o 5 32 to 6 74 clearly suggesting that the felsic magma end member of a dual heritage was derived from dominant newly underplated mafic crust and subordinate ancient lower crust hybridization processes between mantle derived mafic magma and crust derived felsic magma result in the formation of monzonitic enclaves isr t 0 704850 end t 0 1 with an isotopic resemblance to the host granites fractional crystallization of the monzogranitic magmas endows the differentiated syenogranites and alkali feldspar granites with highly evolved elemental characteristics we further deduce that the magmatic immiscibility between k rich and na rich residual melts in a deep reservoir produces albite granites with distinct deformation imprints variable k2o contents and wide range of crystallization temperatures combining with coeval regional igneous episodes as well as concomitant sedimentary and structural events in the northern china mongolia tract the triassic erenhot composite batholith can be considered as post orogenic magmatic products possibly within a coupled geodynamic scenario between lithospheric dripping and concurrent gravitational collapse therefore the documentation of this large scale triassic post orogenic magmatism could not only lead to the characterization of the eventual closure of the paleo asian ocean but also capture a snapshot into the tectonic background for the widespread porphyry type mo mineralization in the eastern central asian orogenic belt during early mesozoic

a review of mesoscopic magmatic structures and their potential for evaluating the hypersolidus evolution of intrusive complexes abstract the perception that plutonic bodies are structurally simple is disappearing with the recognition of an array of magmatic structures useful for constraining hypersolidus temporal histories evolving rheologies strain fields flow directions growth and cooling patterns tilting and syn emplacement tectonism these histories provide a powerful means of testing an array of growth emplacement chamber evolution and tectonic models important points include that 1 many structures form in hydrogranular or congested magma slurries during magma mush avalanching local convection and late hypersolidus strain challenging the notion that magmas must have 55 crystals to convect fractionate and form compositional diversity in upper crustal magma chambers 2 magmatic fabrics reflect transient strain in these slurries rather than flow directly the latter must be inferred by linking geometries with temporal and kinematic information 3 caution is needed when using anisotropy of magnetic susceptibility and similar quantitative tools that characterize through a single ellipsoid preferred orientations of mineral grains given the increasing recognition of multiple fabrics in plutons and 4 that future studies are particularly needed in plutons focusing on the distribution and styles of compositionally defined structures magmatic folding shear zones and faults multiple fabrics and of the physical chemical behaviors in hydrogranular slurries

consequences of progressive eclogitization on crustal exhumation a mechanical study summary the very low water content of the granulitic unit of holsnoy in the bergen arcs norway caused its partial metastable preservation throughout caledonian burial end exhumation leading to the observed mixture of completely eclogitized and uneclogitized rocks the eclogitization of the granulite facies protolith led to a density increase 10 but also to a large rheological weakening that resulted in the localization of all deep ductile deformation in the eclogite fraction we address the consequences on subduction dynamics of the concomitant evolution of large scale density and rheology during progressive eclogitization of crust by comparing the behaviour of buried crust with various properties in the channel flow model large scale buoyancy and density are integrated in a single adimensional parameter the exhumation number which describes the capacity of the crust to exhume for a crust whose eclogitization causes a very large viscosity decrease burial resistant end member goes through a maximum in the partially eclogitized zone that is there is a given proportion of eclogite for which the crust is strongly weakened but still significantly lighter than surrounding mantle for this partially eclogitized crust with max the maximum possible downward flow is very low this zone acts as a bottleneck and all incoming crust in excess is forced backwards and starts exhuming this return flow zone is bounded downwards in the partially eclogitized zone by max while its upper limit propagates upwards at a constant rate the curve eclogite fraction controls the maximum proportion of eclogite of the crust that can be exhumed we also demonstrate that large exhumation rates reached by certain uhp units are incompatible with any stationary flow regime whatever the nature of subducted crust the additional presence of a weak layer on top of the channel favours burial for narrow channels and exhumation for large ones but does not qualitatively change the dynamics of the subduction channel this study shows that deep crustal circulation in subduction zones and exhumation from large depths are controlled by the evolution of large scale crustal properties with progressive eclogitization

k feldspar megacryst accumulations formed by mechanical instabilities in magma chamber margins asha pluton nw argentina abstract the asha pluton the oldest unit of the san blas intrusive complex early carboniferous exhibits impressive examples of magmatic structures formed by accumulation of k feldspar megacrysts enclaves and schlieren almost all recognized structures are meter scale vertically elongate bodies of variable shapes defined as fingers trails drips and blobs they preferentially developed near the external margin of the asha pluton and generally are superimposed by chamber wide magmatic fabrics they mostly have circular or sub circular transverse sections with an internal fabric defined by margin parallel inward dipping concentric foliation and steeply plunging lineation at upper parts and flat foliation at lower parts the concentration of megacrysts usually grades from upper sections where they appear in a proportion similar to the host granite to highly packed accumulations of k feldspar along with grouped flattened enclaves at lower ends these features suggest an origin by downward localized multiphase magmatic flow narrowing and log jamming and gravitational sinking of grouped crystals and enclaves with compaction and filter pressing as main mechanisms of melt removal crystal size distribution analysis supports field observations arguing for a mechanical origin of accumulations the magmatic structures of the asha pluton represent mechanical instabilities generated by thermal and compositional convection probably owing to cooling and crystallization near the pluton margins during early stages of construction of the intrusive complex

lattice preferred orientations of late variscan granitoids derived from neutron diffraction data implications for magma emplacement mechanisms the lattice preferred orientation lpos of two late variscan granitoids the meissen monzonite and the podlesi dyke granite were determined from high resolution time of flight neutron diffraction patterns gained at the diffractometer skat in dubna russia the results demonstrate that the method is suitable for the lpo analysis of polyphase relatively coarse grained 0 1 6 mm rocks the meissen monzonite has a prominent shape preferred orientation spo of the non equidimensional minerals feldspar mica and amphibole whereas spo of the podlesi granite is unapparent at the hand specimen scale the neutron diffraction data revealed distinct lpos in both granitoids the lpo of the non equidimensional minerals feldspar mica and amphibole developed mainly during magmatic flow in the case of the meissen monzonite the magmatic flow was superimposed by regional shear tectonics which however had no significant effect on the lpos in both samples quartz shows a weak but distinct lpo which is atypical for plastic deformation and different in the syn kinematic meissen monzonite and the post kinematic podlesi granite we suggest that first of all the quartz lpo of the meissen monzonite is the result of oriented growth in an anisotropic stress field the quartz lpo of the podlesi granite which more or less resembles a deformational lpo in the flattening field of the local strain field developed during magmatic flow whereby the rhombohedral faces of the quartz crystals adhered to the 010 faces of aligned albite and to the 001 faces of zinnwaldite due to shape anisotropy of their attachments the quartz crystals were passively aligned by magmatic flow thus magmatic flow and oriented crystal growth are the major lpo forming processes in both granitoids for the meissen monzonite the solid state flow was too weak to cause significant crystallographic re orientation of the minerals aligned by magmatic flow finally the significance of our results for the evaluation of the regional tectonic environment during magma emplacement is discussed the discussion on the regional implications of the more methodologically oriented results provides the basis for future more regionally aimed studies in view of the fabric characteristics of such plutons and their developing mechanisms

non linear feedback loops in the rheology of cooling crystallizing felsic magma and heating melting felsic rock at least six major parameters control the rheology of partially molten systems melt content rate of melt production reaction to strain of the solid component reaction to strain of the molten component temperature and chemical composition of the source rocks we examine their interactions to understand the rheology of partially molten rocks and partly crystallised magmas this paper especially focuses on the rheology in the transitional domains between two pairs of thresholds that bracket a transitional regime between a solid state and a fluid behaviour during melting and crystallisation respectively we review related information and point out non linear effects that develop during heating of melting rocks and cooling of crystallising magmas owing to the non linear interactions positive or negative feedback loops accelerate or damp the system melt in migmatite experiences shear softening which along with strain partitioning facilitates melt segregation conversely the augmenting number of rigid crystals during cooling increases the suspension viscosity shear hardening which soon inhibits magma movement those effects reinforce the asymmetry between solid to melt and melt to solid transitions they severely contradict the concept of one rheological critical melt percentage valid for both melting and crystallisation transitions fabric acquisition competes with nucleation and crystal growth thus leading to hysteresis of the stress stain rate curves implications to field observations include horizontal magma segregation magma extraction and successive magma intrusions

schlieren formation in diatexite migmatite examples from the st malo migmatite terrane france schlieren are trains of platy or blocky minerals typically the ferromagnesian minerals and accessory phases that occur in granites and melt rich migmatites such as diatexites they have been considered as 1 unmelted residue from xenoliths or the source region 2 mineral accumulations formed during magma flow 3 compositional layering and 4 sites of melt loss in order to help identify schlieren forming processes in the diatexites at st malo differences in the size shape orientation distribution and composition of the biotite from schlieren and from their hosts have been investigated small biotite grains are much less abundant in the schlieren than in their hosts schlieren biotite grains are generally larger have greater aspect ratios and have except in hosts with low 10 biotite contents a much stronger shape preferred orientation than host biotite the compositional ranges of host and schlieren biotite are similar but schlieren biotite defines tighter sharper peaks on composition frequency plots hosts show magmatic textures such as imbricated tiled unstrained plagioclase some schlieren show only magmatic textures tiled biotite no crystal plastic strain features but many have textures indicating submagmatic and subsolidus deformation e g kinked grains and these schlieren show the most extensive evidence for recrystallization magmas at st malo initially contained a significant fraction of residual biotite and plagioclase crystals smaller biotite grains were separated from the larger plagioclase crystals during magma flow since plagioclase was also the major early crystallizing phase the plagioclase rich domains developed rapidly and reached the rigid percolation threshold first forcing further magma flow to be concentrated into narrowing melt rich zones where the biotite had accumulated hence increasing shear strain and the degree of shape preferred orientation in these domains schlieren formed in these domains as a result of grain contacts and tiling in the grain inertia regime final amalgamation of the biotite aggregates into schlieren involved volume loss as melt trapped between grains was expelled after the rigid percolation threshold was reached in the biotite rich layers

the sphene centered ocellar texture an effect of grain supported flow and melt migration in a hyperdense magma mush abstractthe sphene centered ocellar texture consists of leucocratic ocelli with sphene titanite crystals at the center enclosed in a biotite rich matrix this texture has been recognized worldwide in hybrid intermediate rocks on the basis of structural petrological and geochronological data from selected outcrops of the variscan ribadelago pluton nw iberian massif we propose that the ocelli were formed by migration and accumulation of a residual melt through a plagioclase and biotite dominated crystalline framework at the late stage of crystallization the magma acted as a hyperdense suspension and reacted to the pressure gradient caused by the regional stress field entering the domain of grain supported flow microstructures reveal that aligned crystal domains arose in the crystal framework from the shearing and compaction of the crystal mush and behaved as magmatic microshears relative displacement of adjacent crystal clusters along these microshears corresponded to the onset of reynolds di

rheological transitions and the progress of melting of crustal rocks abstract high grade igneous and metamorphic systems which possess a variable percentage of melt may undergo a transition between crystal supported and fully fluid behavior the melt fraction interval over which this transition occurs is not well constrained we have conducted six series of numerical simulations of crustal melting using melt fraction intervals between 0 05 to 0 15 for this mechanical transition the results of these simulations demonstrate the dependence of convection and progress of melting on the rheological transition between solid and melt convective heat flux varied by a factor of 8 over the duration of the simulations 160 yr and the mass of melt produced varied by nearly a factor of 4 the onset of convection in simulations with the most gradual rheological transition was suppressed convective heat flux and progress of melting were more sensitive to the melt fraction interval for the rheological transition than to the composition of a range of crustal protoliths the rheological transition between solid and melt may be one of the dominant factors controlling the thermal evolution of high grade crustal rocks

spatial distribution of melt bearing structures in anatectic rocks from southern brittany france implications for melt transfer at grain to orogen scale abstract in this study we present quantitative spatial information on the one and two dimensional distribution of inferred melt bearing structures in anatectic supracrustal rocks of the southern brittany migmatite belt south of the transcurrent south armorican shear zone sasz based on these data we infer the mechanism of melt extraction from partially molten crust former melt bearing structures include foliation parallel leucosomes and cross cutting granitic leucosomes that infill inter boudin partitions and extensional shear surfaces as well as discordant dykes of granite petrographic i e mineralogical and microstructural continuity of granite from structure to structure suggests that they once formed a continuous melt bearing network measurements along one dimensional line traverses perpendicular to layering of stromatic migmatite exposed in clean sub horizontal outcrop surfaces provide information about thickness and spacing distributions of foliation parallel leucosomes most leucosome thicknesses fall in the range of 1 10 mm with upper limits around 20 30 mm the number of thicker layers decreases abruptly with increasing thickness which is inconsistent with scale invariance this suggests that leucosome formation was controlled by short range melt movement along grain boundaries to form melt rich layers constrained by pre existing compositional layering spacing distributions also are not scale invariant however the large percentage of leucosomes 40 60 in these line traverses suggests that spacing distributions may be controlled in part by impingement of leucosomes making it difficult to derive genetic information from these data qualitative observation of inferred melt bearing structures in mutually perpendicular two dimensional exposures from the same outcrop reveals anisotropy of the leucosome network related to a well developed sub horizontal quartz feldspar lineation reflecting stretching associated with transcurrent movement along the sasz analysis of these two dimensional distributions using the box counting method corroborates the observed anisotropy but indicates that leucosome morphology and perhaps distribution is not scale invariant the applicability of the box counting method or of fractal analysis to understanding melt movement in migmatites is discussed in light of these results based on the anisotropy of melt bearing structures we infer that melt movement in structures now represented by layer parallel leucosomes was primarily sub horizontal these layers fed steeply dipping structures now represented by cross cutting leucosomes in particular those developed at inter boudin partitions and granite dykes the formation and orientation of these steeply dipping structures was in part controlled by far field stresses related to dextral displacement along the sasz melt extraction is inferred to have occurred along these steeply dipping structures extracted melt accumulated in plutons at higher crustal levels such as the quiberon sarzeau and guerande granites

contribution of ams measurements in understanding the migmatitic terrains of pointe geologie terre adelie east antarctica a detailed magnetic mapping using anisotropy of magnetic susceptibility ams technique was carried out in pointe geologie archipelago terre adelie east antarctica that represents a hot crust having experienced a long lived anatectic event during paleoproterozoic times 1 69 ga ago ams measurements allowed to better analyse the tectonic structure of the crystalline basement that is built up by rocks affected by various degrees of partial melting and then devoid of clear strain markers ams sampling was performed from main rocks types of pointe geologie migmatites including leucosomes and melanosomes coarse grained pink granites anatexites and mylonitic gneisses for melanosomes the magnetic foliation is dominantly in agreement with the observed field foliation i e dominantly n s sub vertical in shear zones and gently inclined in dome structures ams technique reveals a sub horizontal magnetic lineation in migmatites from shear zones and a gently plunging one in dome structures magnetic properties of leucosomes and of coarse pink granitic dykes contrast with melanosomes the bulk susceptibility and anisotropy degree are significantly lower in granitic magmas that in melanosomes in addition in well defined leucosomes granitic dykes and anatexites the magnetic ellipsoid is characterized by a higher plunge of the magnetic lineation which tends to be vertical this is associated to a rheological contrast between the solid state deformation suffered by oxide grains in the melanosomes and their reorientation in a viscous flow during the transfer of felsic melt to the granitic dykes magnetic structure of leucosomes granitic dykes and anatexites highlights the role of the gravity induced upwelling of a crust undergoing high degree of partial melting in a transpressional regime

timescales and mechanisms of batholith construction constraints from zircon oxygen isotopes and geochronology of the late variscan serre batholith calabria southern italy abstract the late variscan serre batholith in central calabria represents the middle portion c 13 km thick of a tilted crustal section continuously exposed from lower crustal granulites to upper crustal phyllites the batholith is zoned consisting of several granitoid types that were emplaced at depths ranging from c 23 to c 6 km deep strongly foliated quartz diorites and tonalites were emplaced into migmatitic metapelites the intermediate level granitoids are weakly foliated to unfoliated porphyritic granodiorites and monzogranites and the shallowest bodies are two mica granodiorites and granites grading upward to biotite amphibole granodiorites emplaced into paragneisses and phyllites five samples representative of the main granitoid types in terms of both composition and emplacement depth have been dated by shrimp zircon from a lower crustal quartz diorite gave an emplacement age of 297 3 3 1 ma two middle crustal strongly peraluminous k feldspar megacrystic granites were emplaced at 296 1 1 9 ma and 294 9 2 7 ma a middle to upper crustal two mica monzogranite was emplaced at 294 2 2 6 ma and finally an upper crustal weakly peraluminous granodiorite from the batholith roof was emplaced at 292 2 2 6 ma these results are consistent with the development of the batholith by incremental multipulse overaccretion each granitoid body being in a dominantly rigid state before intrusion of the next with little or no possibility of magma mixing the difference in age between the oldest and youngest granitoids from the batholith floor and roof respectively is 5 1 4 0 ma providing an upper limit of about 9 ma on the time taken for batholith construction the presence in the c 296 294 ma granitoids of c 305 302 ma anatectic zircon with varied th u and oxygen isotope compositions indicates a time interval of 8 9 ma between incipient partial melting in the lower crust and magma emplacement in the middle crust the emplacement of the first granitoid bodies into the top of the lower crust was controlled tectonically by the activation of a deep seated shear zone the shallowest granodiorites were emplaced during the waning stages of the shear zone activity producing late to post tectonic contact metamorphism in the upper crustal phyllites and mylonitic paragneisses

evolution of the ligurian tethys in the western alps sm nd and u pb geochronology and rare earth element geochemistry of the montgenevre ophiolite france abstract we provide geochemical and geochronological data for gabbro diorite and albitite samples from the montgenevre ophiolite in the western alps this well preserved remnant of the piemont ligurian oceanic basin shows evidence of intra oceanic deformation and metamorphism but has suffered minor ductile deformation and metamorphism during the alpine orogeny the gabbros have geochemical features and initial nd isotopic signatures similar to that of mid oceanic ridge gabbros indicating that they were extracted from a depleted mantle source with no evidence of continental contamination end t 8 cumulation played an important role in the genesis of these gabbros and their rare earth element ree patterns are controlled by the major cumulus phases modelling the ree data for the gabbros and diorites provides support for the hypothesis that the dioritic magmas have been derived from small degree 5 partial melting of the surrounding gabbros during shearing at high temperature high t zircons from a leucodioritic vein within sheared gabbros are poorly discordant and cluster the concordia at 156 3 ma while zircons from an albitite lens within the mantle rocks display concordant ages at 148 2 ma in both cases the zircons show no evidence of inheritance and these values are interpreted as crystallisation ages the 160 150 ma age bracket very likely records a late stage in the magmatic history of the montgenevre ophiolite since the emplacement of diorite clearly post dates the layered gabbros these radiometric ages correlate well with the late bathonian to early kimmeridgian stratigraphic age 160 140 ma of the earliest post ophiolitic radiolarian sediments the whole rock sm nd system of the gabbros yielded an isochron age of 198 22 ma end t of 8 8 this value is significantly older than the 165 ma age of western alps ophiolitic gabbros published elsewhere but is similar within error to the 180 ma age of eclogitised gabbros from the ligurides and corsica this early jurassic age is interpreted as the emplacement age of the gabbroic melts into the mantle lherzolites and could be associated with the early stage of rifting between the european and austro alpine continental margins these ages and their interpretation are consistent with the model of asymmetric mantle denudation by an oblique detachment fault proposed by lemoine et al 1987 this model implies that the newly formed lherzolite gabbro oceanic domain probably remained close to the spreading centre and therefore experienced slow cooling and low spreading rates this appears to be correlated by our radiometric results that suggest a life span of at least 30 ma for the formation of the oceanic crust in this part of the piemont ligurian ocean

magmatic structures and kinematics emplacement of the variscan granites from central portugal serra da estrela and castro daire areas abstract the late d3 variscan granites which represent the most important volume of granites of central portugal are studied here for their magmatic and solid state fabrics by means of the anisotropy of magnetic susceptibility ams technique in a 5000 km 2 area located between guarda and castro daire including the serra da estrela region two main directions n150e and n20e of stretching lineations are found indicating that the emplacement of these late kinematic granitoids was tectonically controlled a three stage model is proposed in which a openings along n150 striking dextral strike slip faults allow the first granitic magmas to be emplaced in the upper crust of palaeozoic metasediments and syn d3 granitoids b openings along n20 striking possibly sinistral strike slip faults conjugates to the prior strike slip faults and allow the enlargement of the granitic plutons and c the final emplacement of this huge volume of granitic rocks was mainly controlled by the magma pressure which explains the rarity of structures related to this stage of granites emplacement in the country rocks this structural study of granitic rocks yields new constraints on the tectonic evolution of this part of the variscan orogenic belt at c 300 ma

a physical model for metal extraction and transport in shallow magmatic systems 1 the highest concentrations of metals e g cu au ag mo in the earth s crust are found in porphyry type deposits the metals are ultimately sourced from magmas and appear to be concentrated hundred to thousand fold from typical magmatic contents ppm ppb in the exsolved volatile phase to better quantify the purging and transport of metals we develop a physical model of volatile evolution in an incrementally built upper crustal magma reservoirs that considers 1 partitioning of metals from the melt to an exsolved volatile phase and 2 advection of the buoyant volatile phase using a single dimensionless parameter the peclet number pe ratio of advection rate over diffusion rate we propose that metal extraction and segregation from magmas can occur in 3 stages with different pe 1 during exsolution of the magmatic volatile phase in shallow crystal poor magma bodies slow volatile advection pe 1 2 during the growth of volatile channels that develop in the reservoir as crystallinity increases pe 1 and 3 during advection in connected channels rapid volatile advection high pe 1 for each stage a metal enrichment factor can be calculated allowing insight into the optimal conditions to maximize metal mass flux into the overlying hydrothermal system the model predicts that the most efficient purging of metals occurs for magmas with intermediate volatile contents and is enhanced during late stage magmatic activity as the reservoirs reach high crystallinity and are not disturbed by volcanic venting in agreement with natural observations suggesting that ore formation post dates volcanic activity

crustal exhumation during ongoing compression in the variscan maures tanneron massif france geological and thermo mechanical aspects abstract the maures tanneron massif mtm together with corsica and sardinia represent the southeastern branch of the huge european variscan belt a continuous evolution from continental collision to exhumation is described from ca 350 ma to ca 320 ma based on an extended compilation of available geological and geochronological data this geological compilation is complemented with thermo mechanical modelling that tests the conditions under which deep seated hp units can melt and exhume massively during continued convergence we simulate in two dimensions the development of gravitational instabilities in a partially melting crust depending on rheological layering and heat sources and accounting for temperature dependent elasto visco plastic rheologies in order to reproduce pt exhumation patterns in the mtm over the appropriate time scale 20 my and spatial extent 100 km a best fit was obtained with a convergence rate of 0 5 cm yr and moderate diffusive surface processes the contribution of radiogenic heating in the pre stacked felsic crustal units is crucial a lateral alternation with mafic units is also required in order to prevent lateral spreading of the orogen a low viscosity partially molten crust 1020 pa s thus rather felsic accompanies crustal exhumation in a decoupled mode from the deeper mantle a shallow asthenosphere below the orogen lab at 70 km depth e g shallow slab break off produces too warm and sudden exhumation as opposed to a competent mantle lithosphere 120 km depth lab that restrains it the mtm witnesses the typical competition between far field plate convergence and internal body forces and our study pleads for a progressive evolution of transpression towards perpendicular extension from ca 320 ma

conditions for the growth of a long lived shallow crustal magma chamber below mount pelee volcano martinique lesser antilles arc 1 the compositional homogeneity of silicic andesites emitted during the 13 500 year long last eruptive cycle of mount pelee suggests that the physical state of the magma chamber was largely unmodified during this period experimental phase equilibria on mount pelee recent products indicate that pre eruptive magma temperatures and pressures were in the range of 875 900 c and 2 0 5 kbar respectively the estimated average eruption rate was 7 5 10 4 km3 a with an average volume of about 0 3 km3 per eruption an analytical model for a spherical magma chamber indicates that a magma flux of 4 5 10 4 km3 a can maintain a magma chamber of 0 3 km3 above 875 c below mount pelee however observations of plutons suggest that magma chambers may grow by addition of sheet like intrusions with numerical simulation we show that a minimum sheet accretion rate of a few centimeters per year is required to grow a persistently active magma chamber independently of the intruded volumetric flux this minimum injection rate is higher if the heat transfer is enhanced by convection processes the limited ability of an arc crust to extend and accommodate dikes suggests that magma injections are sills or if they are dikes that most of the volume injected in the magma chamber is eventually erupted in a conductively cooling igneous body formed by sills injected at a rate of a few centimeters per year most of the injected magma completely solidifies and only a small part of the body about 10 20 is above 875 c and able to feed eruptions

emplacement of the nain anorthosite diapiric versus conduit ascent estimation of settling velocities of large orthopyroxene megacrysts found within anorthosite intrusions are calculated and compared with ascent rates achieved by diapirism and conduit propagation calculations suggest that diapirism is far too slow to be an appropriate ascent mechanism for anorthositic crystal mush and favour conduit emplacement the intrusions of the nain plutonic suite nps are located along the abloviak shear zone which marks the boundary between the nain and churchill provinces and within the zone of juxtaposition of the saglek and hopedale blocks of the nain province these crustal weaknesses have probably controlled the emplacement and distribution of the intrusions contact relations between intrusions of anorthosite and their gneissic host rock provide evidence for two emplacement styles within the nps the first typified by strongly deformed and recrystallized rocks and the second by an outer border zone of mafic rocks it is proposed that these differences in intrusive style are due to differences in ductility contrast between the magma and its surrounding host rocks such that those intrusions emplaced into the thermally softened shear zone have deformed margins whereas those intruded into the cooler archaean crust have undeformed margins

magma migration folding and disaggregation of migmatites in the karakoram shear zone ladakh nw india effi cient extraction of granitic magma from crustal sources requires the development of an extensive permeable network of melt bearing channels during deformation we investigate rocks that have undergone deformation and melting within the karakoram shear zone of ladakh nw india in which leucosome distribution is inferred to record the permeable network for magma extraction delicate structures preserved in these rocks record the development of this permeable magma network and its subsequent destruction to form a mobile mass of melt and solids resulting from the interplay between folding and magma migration during folding magma migrated from rock pores into layer parallel and axial planar sheets forming a stromatic migmatite or metatexite with two communicating sets of sheets intersecting parallel to the fold axis once the network was developed folding and stretching was eased by magma migration and slip along axial planar magma sheets folding and magma migration led to layer disaggregation transposition and the formation of a diatexite where rock coherency and banding were destroyed a number of structures developed during this process such as cuspate fold hinges disharmonic folds truncated layering shear along axial planar leucosomes and fl ow drag and disruption of melanosomes in this system magma migration was an integral part of deformation and assisted the folding and stretching of metatexites while folding gave rise to a magma sheet network now preserved as leucosomes as well as the pressure gradients that drove magma migration and the breakup of the metatexite thus metatexite folding increased melt interconnectivity while magma mobility increased strain rate and released differential stresses

beryllium in silicic magmas and the origin of beryl bearing pegmatites this chapter addresses the portion of the geochemical cycle of beryllium that entails its transfer through the continental crust by the generation and migration of silicic magmas which will be referred to loosely as granitic but will include all of the textural variants of rocks derived from the granitic bulk compositions within the granite system we will examine the distribution of be between anatectic melts and residual crystals and we will track the accumulation of be in melt via crystal fractionation culminating in the saturation of pegmatite forming melts in the mineral beryl beryl is unquestionably the most common manifestation of be enrichment and the most abundant of the phases that contain essential beryllium see cern this volume to explain the presence of beryl in pegmatites we pose four questions 1 how much be does it take to saturate a granitic pegmatite forming melt in beryl 2 how much be is likely to be incorporated in silicic partial melts at their source 3 consequently how much fractionation of a typical granitic magma must occur to achieve beryl saturation in pegmatitic derivatives or is beryl saturation possible without extended magmatic fractionation 4 and should all granitic magmas be expected to achieve beryl saturation eventually or are there special attributes of the source material or crystalline phases in the magma that predetermine the likelihood of forming beryl in pegmatitic differentiates to address these questions we start with a review of beryl saturation in pegmatites we then examine the likely abundance of be derived by anatexis at the sources of granitic magmas with the two ends of the system bracketed we can then subject be fractionation to conventional modeling methods to ascertain how much fractionation must take place to bring granitic melts to beryl saturation as a review this chapter relies as much as possible on readily available publications

geochronology geochemistry mineralogy and metallogenic implications of the zhaojinggou nb ta deposit in the northern margin of the north china craton china abstract the large zhaojinggou nb ta deposit is located in the northern margin of the north china craton ncc inner mongolia china the deposit is mainly hosted in the mineralized dikes around the hutoushan pluton and the residual pegmatite shell at the top of the pluton samples from the pluton and the mineralized dikes are characterized by high contents of sio2 73 3 84 3 total alkalis 5 24 9 60 and al2o3 7 94 13 8 and moderate a cnk values with an average of 1 04 indicating a weakly peraluminous affinity the trace element patterns are characterized by rb nb ta zr and hf enrichment and markedly negative ba sr p eu and ti anomalies zircon u pb dating suggests that the emplacement of the pluton and the nb ta mineralization occurred coevally in the early cretaceous 126 1 ma and 127 2 ma respectively variations in both bulk rock geochemistry k rb zr hf nb ta and mineral composition mica and columbite consistently unveil a magmatic evolutionary trend from the hutoushan alkali feldspar granite through the hutoushan pegmatite shell to the mineralized dikes typical a type characteristics of the granite and ancient hf isotopic features ehf t ranging from 6 80 to 12 95 and crust model ages from 1 61 to 2 00 ga indicate that the parental magma was derived from the partial melting of palaeoproterozoic crust with insignificant mantle material involved considering the significantly higher bulk contents of nb ta and f in the pegmatite shell and the mineralized dikes than that in the alkali feldspar granite as well as the similar composition in the biotite from the pegmatite shell and the alkali feldspar granite we propose that a residual nb ta enriched brine melt phase segregated from the crystal mush at the late stage of crystallization the progressive concentration of f in the melt was conducive to the solubility of nb and ta in granitic melts and decreased the viscosity by depolymerizing the melt and thus contributed to the high magma differentiation and nb ta concentration the post magmatic fluid triggered the fluid rock interaction resulting in the extensive albitization m type tetrad effect extremely low nb ta ratio

time scales of pluton construction at differing crustal levels examples from the mount stuart and tenpeak intrusions north cascades washington deciphering the magmatic history of continental magmatic arcs in general and the growth history of individual intrusions in particular is key to understanding the complex history of magma generation segregation and transport that define the dynamics of crustal growth we utilize high precision u pb geochronology to resolve a detailed magmatic history from two composite intrusions the 2 4 kbar mount stuart batholith and the 7 10 kbar tenpeak pluton emplaced in the cretaceous north cascades arc this temporal framework provides a way to evaluate models of pluton growth that explain common features of intrusions such as concentric compositional zoning and internal magmatic contacts u pb zircon crystallization ages were obtained from 12 samples of the mount stuart batholith and 8 samples of the tenpeak intrusion representing the range of compositional diversity and geographical extent these dates indicate that the mount stuart batholith was constructed over a 5 5 m y time period that was punctuated by four intervals of high magma flux the durations of the high flux periods are short a few hundred thousand years relative to the duration of the batholith the consistent pattern of magmatic fabrics and the lack of distinct contacts in the batholith may be explained by the juxtaposition of melt rich and mush zones with subtle contacts between mineralogically and texturally similar tonalite and time transgressive magma fabrics in contrast the tenpeak intrusion was constructed over a 2 6 m y time period with magma influx distributed throughout the intrusive history and texturally distinct magma bodies the tenpeak intrusion lacks distinct age domains which suggests that any magma reservoir was smaller in size and potentially more ephemeral although the distinct age domains and discrete compositional and textural phases indicate that pluton growth occurred incrementally neither pluton bears resemblance to a purely end member incremental growth model whereby a pluton is constructed from hundreds to thousands of discrete magma pulses that have little if any interaction in particular ages from the youngest domain of the mount stuart batholith indicate that a melt rich magma reservoir of 520 km 3 existed over a 170 90 k y time span

microbiological processes in banded iron formation deposition banded iron formations have been studied for decades particularly regarding their potential as archives of the precambrian environment in spite of this effort the mechanism of their deposition and specifically the role that microbes played in the precipitation of banded iron formation minerals remains unresolved evidence of an anoxic earth with only localized oxic areas until the great oxidation event ca 2 45 to 2 32 ga makes the investigation of o2 independent mechanisms for banded iron formation deposition relevant recent studies have explored the long standing proposition that archean banded iron formations may have been formed and diagenetically modified by anaerobic microbial metabolisms these efforts encompass a wide array of approaches including isotope ecophysiological and phylogeny studies molecular and mineral marker analysis and sedimentological reconstructions herein the current theories of microbial processes in banded iron formation mineral deposition with particular regard to the mechanisms of chemical sedimentation and post depositional alteration are described the main findings of recent years are summarized and compared here and suggestions are made regarding cross disciplinary information still required to constrain the role of the biosphere in banded iron formation deposition

melt producing versus melt consuming reactions in pelitic xenoliths and migmatites abstract melt producing reactions have operated both in pelitic xenoliths and in pelitic migmatites but melt consuming reactions related to in situ melt crystallization are largely restricted to migmatites hence xenolith reaction textures may provide clues on the relative roles of both reaction types in migmatites a comparison of xenoliths and migmatites results in a catalogue of textural and chemical features some of which are argued to be diagnostic the results have implications for the identification of restite in igneous rocks and for the relative importance of prograde and retrograde processes in different domains of layered migmatites