granite ascent and emplacement during contractional deformation in convergent orogens based on a case study in the central maine belt of west central maine u s a it is proposed that crustal scale shear zone systems provide an eaective focussing mechanism for transfer of granite melt through the crust in convergent orogens during contractional deformation flow of melt in crustal materials at depths below the brittle plastic transition is coupled with plastic deformation of these materials the flow is driven by pressure gradients generated by buoyancy forces and tectonic stresses within the oblique reverse central maine belt shear zone system stromatic migmatite and concordant to weakly discordant irregular granite sheets occur in zones of higher strain which suggests percolative flow of melt to form the migmatite leuco somes and viscous flow of melt channelized in sheet like bodies possibly along fractures cyclic fluctuations of melt pressure may cause instantaneous changes in the eaective permeability of the flow network if self propa gating melt filled tensile and or dilatant shear fractures are produced due to melt enhanced embrittlement in homogeneous migmatite and schlieric granite occur in zones of lower strain which suggests migration of partially molten material through these zones en masse by granular flow and channelized flow of melt carrying entrained residue founded on the central maine belt case study we develop a model of melt extraction and ascent using the driving forces stress conditions and crustal rheologies in convergent especially transpressive orogens ascent of melt becomes inhibited with decreasing depth as the solidus is approached for intermedi ate a h2o muscovite dehydration melting the water saturated solidus occurs between 400 and 200 mpa near the brittle plastic transition during high t low p metamorphism where the balance of forces favors sub horizontal fracture propagation emplacement of melt may be accommodated by ductile flow and or stoping of wall rock and inflation may be accommodated by lifting of the roof at shallower crustal levels and or sink ing of the pluton floor the resultant plutons have sub horizontal tabular geometries with floors that slope down to the ascent conduits although these plutons may have locally discordant relations with country rock structures when viewed at the crustal scale granite ascent and emplacement in convergent orogens are syn tec tonic processes 1998 elsevier science ltd all rights reserved

preservation of chemical residue melt equilibria in natural anatexite the effects of deformation and rapid cooling a combined petrologic textural and chemical approach is used to interpret the relationship between chemical exchange and deformation during partial melting of a granodioritic gneiss in the contact aureole of the bergell pluton central alps in contrast to most regional metamorphic anatexites chemical equilibrium between residue and segregated melt was attained and preserved by several elements as shown by their present distribution equilibrium involved both the major and the trace elements hosted in the rock forming minerals and the light rare earth elements lree hosted primarily in allanite equilibrium with respect to lrees was achieved because allanite had a reactive behaviour during water present melting in contrast to the refractive behaviour of xenotime which mostly controls the distribution of the hree s the attainment and preservation of equilibrium between leucosome and residue requires peculiar boundary conditions we propose that the achievement of equilibrium between melt and residue was promoted by deformation operating via a mechanism of melt present granular flow i e dissolution accommodated grain boundary sliding active during partial melting microstructural observations indicate that melt present granular flow was the dominating deformation mechanism in these anatexites the preservation of residue melt equilibrium is inferred to result from rapid cooling under contact metamorphic conditions and from the lack of deformation below the solidus

the mechanism of ascent and emplacement of granite magma during transpression a syntectonic granite paradigm we propose a model for syntectonic ascent and emplacement of granite magma based on structural relations in part of the northern appalachians in the study area in western maine strain was distributed heterogeneously during devonian acadian transpression metasedimentary rocks migmatites at high grades record two contrasting types of finite strain in zones that alternate across strike rocks in both types of zones have a penetrative moderately to steeply ne plunging mineral elongation lineation defined by bladed muscovite fibrolite sillimanite at high grades in straight belts of enhanced deformation rocks have s l fabrics that record apparent flattening to plane strain apparent flattening zones afzs but rocks between these belts have l s fabrics that record apparent constriction apparent constriction zones aczs at metamorphic grades above the contemporary solidus rocks in afzs developed stromatic structure in migmatite which suggests that percolative flow of melt occurred along the evolving flattening fabric stromatic migmatites are intruded by concordant to weakly discordant m scale composite sheet like bodies of granite to suggest magma transport in planar conduits through the afz rocks inhomogeneous migmatite is found in the intervening aczs which suggests migration of partially molten material through these zones en masse probably by melt assisted granular flow inhomogeneous migmatites are intruded by irregular m scale bodies of granite that vary from elongate to sub circular in plan view and seem cylindrical in three dimensions these bodies apparently plunge to the northeast parallel to the regional mineral elongation lineation to suggest magma transport in pipe like conduits through the acz rocks we postulate that the form of magma ascent conduits was deformation controlled and was governed by the contemporaneous strain partitioning magma ascent in planar and pipe like conduits through migmatites is possible because oblique translation during contraction displaces isotherms upward in the orogenic crust to form a thermal antiform within this hot corridor it is the difference in temperature between melt producing reactions in the anatectic zone and the wet solidus for granite melt that enables magma to migrate pervasively up through the orogenic crust without congealing heat advected with the migrating melt promotes amplification of the thermal antiform in a feedback relation that extends the zone of plastic deformation and pervasive melt migration to shallower levels in the crust at the wet solidus we suggest melt flows obliquely toward axial culminations in the thermal antiform which are sites of melt accumulation and perturbations from which magma may escape to form plutons batches of melt that escape from these perturbations may be trapped by tectonic structures higher in the crust or ascent may become inhibited with decreasing depth by thermal arrest and solidification if the rate of arrival of subsequent melt batches exceeds the rate of crystallization at the site of pluton construction melt pressure ultimately may lead to sub horizontal magma fracture or viscous flow of wall rocks may allow lateral spreading the resultant plutons have sub horizontal tabular geometries with floors that slope down to the ascent conduit as the thermal antiform decays the height of sub solidus crust that separates the deepest part of the pluton from the anatectic zone increases consequently pluton inflation declines and solidification leads to infilling of the magma feeder channel to form a root zone that passes downward into migmatite which may explain the difficulty of determining precisely the floor to these deeper segments of large plutons using gravimetry

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

geochemical transition shown by cretaceous granitoids in southeastern china implications for continental crustal reworking and growth abstract zircon u pb ages and in situ trace elements and hf o isotope compositions together with whole rock geochemical and sr nd hf isotopic data are presented for cretaceous granitoids in southeastern se china in order to establish their origin and the evolution of the underlying lithosphere during the late mesozoic two stages of cretaceous magmatism with contrasting geochemical features have been identified an earlier adakite like biotite granite as represented by the shangying pluton and a later enclave bearing monzogranite as represented by the zaoshan pluton the shangying biotite granites have a zircon u pb age of 99 1 ma they have relatively low y and yb contents with high la yb and sr y ratios showing geochemical features of adakite their sr nd hf isotope compositions are similar to those of early cretaceous mafic rocks in the same area indicating that they were generated by partial melting of juvenile granulitic crust at a depth of about 40 km the source was formed by underplating of enriched lithosphere mantle derived magmas in contrast the zaoshan calc alkaline monzogranites their enclaves and associated dolerite dykes from the zaoshan pluton have an emplacement age of 88 1 ma the dolerites have high mgo contents relatively low sio 2 concentrations and low la yb ratios and depleted hf isotope compositions all these geochemical features suggest that they were derived from a depleted spinel iherzolite mantle source the enclaves have high sio 2 contents indicating that they were derived from a crustal source they have variable zircon hf and o isotope compositions suggesting that two components i e a high e hf t and a low 18 o component and a low e hf t and high 18 o component were involved in their origin the high zircon e hf t values and low 18 o values are similar to those of the dolerites indicating a common source thus we suggest that the enclaves were generated by partial melting of newly underplated depleted mantle derived materials the monzogranites have distinctly different zircon hf and o isotope compositions from the enclaves indicating that the parental magmas were mainly derived from ancient crust that interacted with underplated depleted mantle derived magmas the monzogranites have relatively high hree contents suggesting a garnet free source

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

origin of k poor leucosomes in a metasedimentary migmatite complex by ultrametamorphism syn metamorphic magmatism and subsolidus processes two types of stromatic leucosomes are identified in metasedimentary rocks from the skagit migmatite complex north cascades washington state u s a both types are trondhjemitic and appear similar in outcrop but although both contain low abundances of ree one type consists of leucosomes that are relatively ree enriched compared to the other and contains 1 small 700 c water rich fluid present inferences about the origin of the above listed mineralogical and fluid inclusion features and modeling of leucosome trace element abundances the second type of leucosome is interpreted to have formed entirely by subsolidus processes e g metamorphic differentiation because these leucosomes lack features consistent with an origin by partial melting k poor tonalitic trondhjemitic leucosomes associated with metasedimentary biotite bearing source rocks may form by water saturated partial melting or by subsolidus processes both general leucosome forming mechanisms may operate at different times during upper amphibolite facies regional metamorphism partial melting may be initiated by syn metamorphic magmatic activity if crystallizing plutons serve as external sources of the water rich fluid necessary for ultrametamorphism in the middle crust during orogenesis large scale migmatite complexes such as the skagit migmatites may form at least in part in response to contact effects of plutonism associated with high grade metamorphism so although migmatite complexes are a volumetrically substantial part of many orogenic belts they may not themselves represent a significant original source of magma for larger scale igneous bodies

magma accumulation and segregation during regional scale folding the holland s dome granite injection complex damara belt namibia abstract the regional scale upright fold of the holland s dome in the damara belt of central namibia contains a kilometre scale network of intrusive highly fractionated uraniferous leucogranites three broadly orthogonal and intersecting sets of leucogranite sheets that intruded parallel and at right angles to the axial plane of the first order fold can be distinguished the granites are internally sheeted and illustrate the growth of the injection complex through the successive addition of thousands of smaller magma batches spatial and timing relationships point to a stepwise evolution of the injection complex early dilatancy driven segregation and accumulation of granitic magmas in the core of the fold above a basal detachment was followed by compaction driven segregation of a melt phase during fold tightening the intersecting leucogranite sets provide a suitably organized permeability structure for melt segregation while the successive injection of magma batches ensures compatibility between regional strain rates during folding and the rates of magma segregation the three dimensional network of melt bearing structures further assisted regional shortening past the lock up of the fold the holland s dome injection complex illustrates the geometric complexity of magma transfer pathways and the significance of regional scale folding for the accumulation segregation and fractionation of granitic magmas in suprasolidus crust

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

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

viscoplastic flow in migmatites deduced from fabric anisotropy an example from the naxos dome greece 1 many migmatites represent crystallized partially molten crust and therefore record the mechanisms and pathways of orogenic crustal flow field and microstructural methods may be insufficient to characterize the planar and linear elements of rock fabric in migmatites due to obscured flow fabrics or protracted deformation in the naxos dome greece we test the anisotropy of magnetic susceptibility ams as a tool for recovering mineral fabric symmetry and the kinematic axes of flow in migmatites measurements of 155 migmatite samples yield dominantly low values 300 10 6 si of bulk magnetic susceptibility km consistent with biotite being the dominant carrier of the ams higher values of km thermomagnetic hysteresis and microstructural data however suggest a ferromagnetic contribution from magnetite in a subset of samples n 15 using electron backscatter diffraction ebsd analysis we establish the correspondence of the biotite subfabric with the ams and structural fabric of the naxos migmatites ebsd data from biotite suggests that magnetic lineation in these dominantly paramagnetic migmatites arises from a zone axis orientation of biotite crystals organized about the direction of viscoplastic flow over a range of spatial scales migmatitic foliation and magnetic foliation are well correlated the magnetic lineation recovered by ams displays a coherent organization despite the heterogeneous structure and composition of the naxos migmatites these data suggest that the apparent complexity of migmatites masks a simpler flow regime controlled by bulk viscoplastic flow furthermore our study demonstrates the utility of the ams method for studying the dynamics of partially molten orogenic crust

geochemical modeling of low melt fraction anatexis in a peraluminous system the pena negra complex central spain abstract a study was made of the chemical fractionation associated with four cases of anatectic segregation of low melt fraction cordieritic granites from migmatized meta greywackes the aims of the study were to 1 reveal the fractionation patterns of major and trace elements 2 compare the major element chemistry of leucogranites and the quantitative behavior of source minerals during anatexis inferred by mass balance adjustment with available experimental data for peraluminous systems and 3 discuss the behavior of trace elements in crustal melting by comparing the chemically determined composition of leucogranites with the results of three fractionation models two of these assume a perfect diffusive behavior of trace elements within residual solids but they use a different set of distribution coefficients the third assumes a perfect nondiffusive behavior in relation to their source rocks the leucogranites are strongly depleted in li transition elements and light rare earth elements but enriched in k2o sio2 and ba the k rb ratio in leucogranites is higher than in sources and reaches values typical of high grade metamorphism mass balance analysis using the anatexis mixing model shows that the chemistry of cordierite leucogranites is compatible with its having originated by closed system water undersaturated anatexis on previously migmatized meta greywackes leaving a residue enriched in cordierite plus biotite and exhausted in k feldspar biotite melts congruently unless important amounts of sillimanite were also present in the source compared with experimental melts obtained from sources with the same chemical composition but with a different femic mineralogy biotite sillimanite instead of cordierite biotite the pena negra leucogranites are richer in k2o and mgo with a lower fe fe mg ratio the differences in magnesium are believed to result from the changes in the mineral assemblage of the source rocks however the differences in potassium contents are still not well understood the behavior of trace elements during anatexis cannot be modeled by trace element fractionation equations because of the sluggish diffusion kinetics of trace elements at the temperature of anatexis making the melting rate of minerals greater than the diffusion rate of trace elements within minerals the behavior of trace elements essentially consists in following that of the minerals containing them

shear criteria in granite and migmatite deformed in the magmatic and solid states microstructural criteria for the determination of the sense of shear in rocks homogeneously deformed in the partially melted state are similar to those which apply to solid state deformation sense of shear determination is either direct deduced from the sense of rotation of markers or indirect involving the obliquity between the shear and foliation planes or between the successive foliations imprinted at different stages of progressive deformation this study is a by product of the detailed structural and microstructural investigation of a high grade metamorphic rock pile variscan vosges massif france which underwent subhorizontal shearing during partial melting and further solidification depending on the rock chemistry on the position in the pile and the relative timing of progressive deformation layered migmatites and homogeneous granites were variously deformed in the partially melted and solid states the sense of shear obtained from these rock types using the criteria presented here consistently gives a top to sw direction

microstructures of melt bearing regional metamorphic rocks the most reliable microstructural criterion for the former presence of felsic melt in regional migmatites is a three mineral quartz k feldspar and sodic plagioclase aggregate in veinlets several other criteria are potentially reliable namely 1 euhedral crystals of feldspar precipitated from liquid or peritectic minerals e g garnet cordierite orthopyroxene k feldspar lining felsic protoleucosomes 2 inclusionfree euhedral overgrowths of feldspar precipitated from liquid or peritectic minerals e g garnet cordierite orthopyroxene k feldspar on residual grains of the same minerals with abundant inclusions in the mesosome 3 aligned euhedral feldspar crystals 4 simple twinning in k feldspar 5 dihedral angles of 60 subtended where a grain of feldspar and or quartz inferred to have pseudomorphed former melt meets two grains of other minerals 6 cuspate volumes of quartz k feldspar or sodic plagioclase especially where surrounded by grains inferred to have been residual during melting 7 veinlets of inferred former melt now mineral pseudomorphs consisting of one of quartz k feldspar or sodic plagioclase preferably though less commonly involving two or three of these minerals along grain boundaries or along inferred former intragranular fractures 8 biotite pseudomorphed by feldspar 9 veinlets of plagioclase that is more sodic than plagioclase grains in the adjacent rock 10 plagioclase with oscillatory zoning 11 microgranophyric intergrowths of quartz and alkali feldspar in patches or veinlets between primary grains 12 symplectic replacement aggregates that can be explained by reactions between peritectic grains and cooling melt and 13 melanosome patches and layers from which leucosome has been extracted however all these criteria must be interpreted with care some other proposed criteria are questionable for example 1 random mineral distributions 2 grain size increase 3 interstitial grains 4 corroded relics of inferred reactant mineral grains surrounded by areas of quartz k feldspar or sodic plagioclase 5 projections into a mineral grain 6 lobes of myrmekite and 7 plagioclase rims with a constant sodic composition occurring on plagioclase cores that are more calcic and or of variable composition

basaltic phreatomagmatic fissure at 71 gulch part 1 sediment magma mingling and eruptive behavior 71 gulch volcano located in the western snake river plain southwestern idaho usa was formed by a basaltic fissure eruption into pliocene lake idaho deposits at and below the eruptive surface record the nature of explosive and non explosive sediment magma and water magma interactions the paleoenvironment and the volcanic plumbing system of 71 gulch eruption were reconstructed from a detailed study of the now exposed eruptive and subsurface deposits estimated water levels during the eruption range from 1 to 53 m across the entire volcano based on the distribution of pillow lavas billowed dikes and fluidal peperites are two types of intrusive features found 12 m below the syn eruptive surface that have similar continuous margins representing ductile behavior and emplacement into soft deformable sediment in comparison tabular dikes and blocky peperites occur throughout 41 m thick of exposed deposits below the syn eruptive surface a conical diatreme like vent structure dominates the subsurface exposure indicating excavation by phreatomagmatic explosions localized spatter within the main subsurface vent suggests temporary exclusion of water and thus a period of brief vent growth above the lake level evidence of sediment magma mingling through blocky and fluidal peperites shows how differing subsurface depths impact the resulting mingling textures

temporal and compositional differences between subsolidus and anatectic migmatite leucosomes from the quetico metasedimentary belt canada migmatites in the quetico metasedi mentary belt contain two types of leucosome 1 layer parallel leucosomes that grew during deformation and prograde metamorphism these are enriched in si02 sr and eu but depleted in ti02 fe203 mgo cs rb ree sc th zr and hf relative to the quetico metasediments 2 discordant leucosomes that formed after the regional folding events when metamorphic temperatures were at their peak these are enriched in rb ba sr and eu but display a wide range of lree th zr and hf contents relative to the quetico metasediments layer parallel leucosomes formed by a sub solidus process termed tectonic segregation this stress induced mass transfer pfocess began when the quetico sediments were deformed during burial and continued whilst the rocks were both stressed and heterogeneous sub solidus leucosome compositions are consistent with the mobilization of quartz and feldspar from the host rocks by pressure solution the discordant leucosomes formed by partial melt ing of the quetico metasediments possibly dur ing uplift of the belt the range of composition displayed by the anatectic leucosomes arises from crystal fractionation during leucosome emplacement some anatectic leucosomes pre serve primary melt compositions and have smooth ree patterns but those with negative eu anomalies represent fractionated melts and others with positive eu anomalies represent accumulations of feldspar plus trapped melt

manaslu leucogranite a collision signature of the himalaya a model for its genesis and emplacement the manaslu pluton is one of 10 leucogranites that formed in the overthrusted higher himalaya after the indo eurasian collision field and analytical data indicate that the underlying migmatites of the tibetan slab may be where the leucogranitic melts were generated the himalayan crustal thrusting of a hot slab over a rather cold volcano sedimentary pile le fort 1975a provides the necessary release of fluids these fluids cross the main central thrust mct induce the partial anatexis of the overheated tibetan slab and produce a leucogranitic magma the emplacement of the magma at first is located along the main disharmonic plane above the mct between the infrastructure and the superstructure there it generates a convective hydrothermal system extending very far laterally according to the stratification of permeabilities progressive emplacement of the granite proceeds as the convected fluids including the fluids released by the saturated magma dissolve the mainly calcareous host rocks of tibetan sedimentaries a frozen image of this caving out progression is given by the extensive network of granitic dikes outside the pluton the two quite independant fluid cycles of the generation and of the emplacement were triggered or guided by the tectonics due to collision this two fold model dominated by fluid activity may be of importance for other leucogranites and granites

on the pseudomorphing of melt filled pores during the crystallization of migmatites pseudomorphs of melt filled pores recognized by their generally cuspate shape are used as diagnostic for the former presence of partial melt they are commonly observed in migmatites from the mid to deep crust although they occur in the smaller pores in migmatites from shallower levels 1 2 kbar the pseudomorphing of meltfilled pores is controlled by the kinetics of nucleation and is a consequence of the greater supersaturation required for nucleation in a small pore compared with a larger one we examine three migmatites in detail a contact metamorphosed cherty band from an iron formation an archaean regional granulite from an accretionary prism and an amphibolite facies sample from the roots of an archaean mountain chain the greater undercooling required for nucleation in progressively smaller pores is recorded by the composition of plagioclase pseudomorphs a study of dihedral angles at the corners of pseudomorphed pores demonstrates that melt solid textural equilibrium was probably attained only in the contact aureole the regional granulite preserves an almost unmodified reaction controlled melt distribution with little evidence for either melt solid textural equilibration or solid solid re equilibration whereas the reactioncontrolled melt distribution in the regional amphibolite facies example has been modified by a partial approach to solid solid textural equilibrium it is not clear whether the differences in dihedral angle population are due to differences in uplift and exhumation rates or due to the presence of h2o on grain boundaries

a felsic mash zone of crustal magmas feedback between granite magma intrusion and in situ crustal anatexis abstract magma mixing and mingling are described from different tectonic environments and are key mechanisms in the evolution of granitoids the literature focuses on the interaction between mafic and felsic magmas with only limited research on the interaction between similar magmas here we investigate instead hybridization processes between felsic magmas formed during the 500 ma delamerian orogeny on the south coast of kangaroo island field relations suggest that a coarse megacrystic granite intruded and interacted with a fine grained diatexite that resulted from combined muscovite dehydration and water fluxed melting of kanmantoo group turbidites the two magmas hybridized during syn magmatic deformation explaining the complexity of relationships and variability of granitoids exposed we suggest that granite intrusion enhanced melting of the turbidites by bringing in heat and h 2 o with rising melt fraction intrusive magmas became increasingly unable to traverse the partially molten terrane creating a positive feedback between intrusion and anatexis this feedback loop generated the exposed mid crustal zone where magmas mixed and homogenized thus the outcrops on kangaroo island represent a crustal and felsic melting assimilation storage homogenization felsic mash zone where instead of having direct mantle magma involvement as originally proposed these processes developed in a purely crustal environment formed by felsic magmas

nature of the archean midcrust in the core of the vredefort dome central kaapvaal craton south africa extreme uplift associated with the formation of the 2 02 ga vredefort dome has exposed a substantial cross section through the crystalline early archean basement complex rocks of the kaapvaal craton the rocks comprise polydeformed high grade tonalite trondhjemite granodiorite ttg gneisses migmatites and late tectonic intrusive granitoids that straddle the upper amphibolite to granulite facies transition field petrographic and geochemical data indicate that compositional heterogeneity occurs on a local scale and reflects the migmatitic character of the rocks rather than crustal scale layering as has been previously proposed no evidence has been found to support exposure of either a melt depleted refractory lower crust or an upper crustal batholithic granite layer however the immense volume of granitic leucosome in the rocks suggests that the exposed section represents an intermediate level between these two zones granitic leucosomes in the upper amphibolite facies migmatites appear to be intrusive into the predominantly trondhjemitic host rocks rather than of in situ derivation leucosome compositions in the granulite facies migmatites are more variable ranging from granitic and charnockitic to enderbitic probably reflecting at least some local derivation leucosomes and small granitoid bodies show local scale geochemical variation that can be explained in terms of variable amounts of melt segregation and migration and fractionation of minerals such as k feldspar within the melts

metamorphozed hercynian granitoids in the alpine structures of the central rhodope bulgaria geotectonic position and geochemistry abstract orthogneisses of late hercynian protolith age crop out in the central rhodope high grade metamorphic complex which is part of the alpine orogen in south eastern europe they compose a tectonic unit bordered by late alpine extensional shear zones these rocks reflect eocene amphibolite facies migmatization

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

evidence for the granulite granite connection penecontemporaneous high grade metamorphism granitic magmatism and core complex development in the liscomb complex nova scotia canada abstract upper amphibolite granulite facies gneisses and granites of the liscomb complex nova scotia canada which are exposed in a core complex within the cambro ordovician meguma group of southern nova scotia yielded concordant u pb zircon monazite ages of 377 2 and 374 3 ma respectively geochronological and geochemical data suggest a single devonian high grade metamorphic event which generated the granitic magma by partial melting of the fertile liscomb gneisses at a depth of 30 km the melting was also synchronous with an extensional event during which the gneisses were uplifted in a core complex associated with the intrusion of granitoids to a depth of 10 km subsequently the gneisses and granites underwent rapid exhumation before the deposition of unconformably overlying late fammenian rocks at 364 ma these events took place during terminal stages of the acadian orogeny and the onset of extensional tectonics in atlantic canada during the middle late devonian the close temporal and spatial association of liscomb gneisses granulites and granites their major and trace element compositions and their overlapping isotopic characteristics confirm the hypothesis that high grade metamorphism and generation of granitic melt are complementary processes as the liscomb granites are of similar age mineralogy and chemistry to the voluminous granitoid plutons found throughout the meguma terrane a similar process is indicated for the rest of the terrane

the extent of equilibration between melt and residuum during regional anatexis and its implications for differentiation of the continental crust a study of partially melted metapelitic enclaves thisstudy presentsthefirst precise evaluation of the extent ofchem ical equilibrium between felsic melt and crystalline residuumduring the crustal anatexis of metasediments the high precision ofthese resultsstemsfrom thefactthat in thiscase the meltsare repre sented by pristine glasses that occur in the matrices of partiallymelted metapelitic enclaves and as abundant glassy melt inclusionstrapped by different minerals in the same enclaves in previous stu dies of anatexis the composition of the melt has been approximatedby that of leucosomes in migmatites which do not necessarily repre sent the composition of the melt very well the present study isbased on laser ablation analyses of 35 trace elements in the glassand principal minerals of enclaves included within neogene peralu minous dacites exposed at el hoyazo se spain the enclaves which contain the assemblage plagioclase biotite sillimanite garnet glass ilmenite graphite k feldspar cordieritequartz represent fragments of metapelitic continental crust thatwaspartiallymeltedatpressuresof5 7kbarduringaregionalmeta morphic event matrix melt and melt inclusions within the enclaveswere quenched to glass upon eruption of the dacite the glasses inthe melt inclusions are interpreted as the remains of melt producedby an initial melting reaction involving a muscovite rich assemblageat 700 7508c whereasthe matrixglassesare consistent with con tinuedmeltingand oralatermeltdominatedbytheincipientmeltingof biotite at 800 8508c the distribution of trace elements be tween residuum and melt duringanatexis of the enclaves isdifferentfrom that predicted by disequilibrium models that consider effectivedistribution coefficients close to unity instead the bulk melt andthe major minerals plagioclase biotite alkali feldspar cordierite were close to equilibrium except for the case of garnet duringthe generation of the melt inclusions and matrix glasses thequenchedmelts meltinclusionsandparticularlythematrixglasses were not in equilibrium with accessory zircon and monazite andare depleted in zirconium and the light rare earth elements recrystallization of mineralsand diffusion in the melt were import ant controls during the redistribution of trace elements betweenthe solids and the bulk melt local disequilibrium processes alsotook place at mineral melt interfaces during the crystallizationof peritectic minerals and the recrystallization of residual phases as shown by the depletion of compatible elements in the meltinclusions

nd isotope evolution of archaean plutonic rocks in southeastern superior province the opatica plutonic belt abitibi greenstone belt and pontiac subprovince represent a major proportion of the southeastern superior province which was formed and accreted rapidly between approximately 2 9 and 2 8 ga plutons in these belts are grouped into four types i trondhjemite tonalite granodiorite ttg suite 2 82 2 69 ga ii monzodiorite mzd suite 2 697 2 669 ga iii late alkaline granitoid alk suite 2 68 2 67 ga and iv anatectic granite and monzonite ana suite 2 69 2 64 ga the four suites are represented in all belts and show similar petrography and geochemistry in terms of nd isotope composition the ttg mzd and alk suites are typical of destructive plate margin magmatism and have 1 4 initial end 3 7 values which are very similar to that of the abitibi mantle end 2 5 the lower values for the ana suites end 0 1 to 2 4 result from recycling of crustal components in the opatica belt the ana granitoids fall on the nd isotope evolution curve defined b

water fluxed melting of the continental crust a review abstract water fluxed melting also known as fluid or water present melting is a fundamental process in the differentiation of continents but its importance has been underestimated in the past 20 years during which research efforts focused mostly on dehydration melting reactions involving hydrate phases in the absence of a separate aqueous phase the presence of a free aqueous phase in anatectic terranes influences all major physical and chemical aspects of the melting process from melt volumes viscosity and ability to segregate from rock pores to melt chemical and isotopic composition a review of the literature shows that melting due to the fluxing of aqueous fluids is a widespread process that can take place in diverse tectonic environments active tectono magmatic processes create conditions for the release of aqueous fluids and deformation driven transient high permeability channels capable of fluxing high temperature regions of the crust where they trigger voluminous melting water fluxed melting can be either congruent in regions at the water saturated solidus or incongruent at suprasolidus p t conditions incongruent melting reactions can give rise to peritectic hornblende or to nominally anhydrous minerals such as garnet sillimanite or orthopyroxene in this case the presence of an aqueous phase is indicated by a mismatch between the large melt fraction generated and the much smaller fractions predicted in its absence the relatively small volumes of aqueous fluids compared to that of rocks imply that melting reactions are generally rock buffered fluids tend to move upwards and down temperature however there are cases in which pressure gradients drive fluids up temperature potentially fluxing suprasolidus terranes crustal regions at conditions equivalent to the water saturated solidus represent a natural impediment to the up temperature migration of aqueous fluids because they are consumed in melting reactions in this case continued migration into supra solidus terranes take place through the migration of water rich melts thus melts become the transport agent of water into supra solidus terranes and responsible for water fluxed melting other processes such as the relatively rapid fluid migration through fractures also allow regional aqueous fluids to by pass the water saturated solidus fluid trap and trigger melting above solidus conditions when aqueous fluids or hydrous melts flux rocks at supra solidus conditions they equilibrate with the surroundings through further melting decreasing water activity and giving rise to undersaturated melts it is in these conditions that hornblende or anhydrous peritectic phases are stabilized unlike dehydration melting the melt fraction generated in this case is not limited by the water contained in hydrous minerals but by the volume of water added to the system unlike melting at the water saturated solidus these melts are capable of rising without freezing and do give rise to upper crustal granitic bodies

migmatites formed by water fluxed partial melting of a leucogranodiorite protolith microstructures in the residual rocks and source of the fluid abstract the opatica subprovince in the canadian shield is a late archaean 2761 2702 ma plutonic arc formed above a north dipping subduction zone anatexis 2690 2677 ma of leucogranodiorite and leucotonalite orthogneisses in the opatica generated migmatites in an area of north vergent back thrusts visible at the surface and in l ithoprobe seismic profile 48 schollen diatexite migmatites occur in the thrusts and metatexite migmatites between them the modal mineralogy microstructure and whole rock major trace and oxygen isotope compositions of the protolith and migmatites were investigated to 1 determine the melting reaction 2 find microstructural criteria for identifying residual rocks in leucocratic systems where there is no melanosome and 3 to determine the source of the fluid involved in anatexis partial melting of the protolith did not change the mineral assemblage but the abundance of quartz and microcline both declined and plagioclase and biotite increased in the residual rocks quartz plagioclase and microcline show evidence for dissolution and biotite does not thus water fluxed melting of quartz plagioclase microcline occurred a mass balance indicates 25 30 partial melting the melting reaction consumed the microcline and created essentially monomineralic domains of plagioclase extraction of 80 90 of the melt left a thin film of melt on the grain boundaries and crystallization of these in the plagioclase domains created diagnostic microstructures microcline fills the last remaining pore space and forms high aspect ratio crystals between plagioclases or triangular crystals at grain junctions quartz shows a range of morphologies from high aspect ratio films through the string of beads to isolated rounded grains as the microstructure progressively equilibrated after crystallisation most accessory phases including zircon remained in the residuum however almost all the schollen migmatites have high contents of th u nb ta and ree relative to the protolith due to contamination by accessory phases derived from mafic rocks disaggregation of the mafic rocks may have been facilitated by the high strain in the back thrusts where the schollen diatexites formed average whole rock 18 o for the protolith and migmatites are similar ca 8 2 and the small difference between melt rich 8 6 and residuum rich rocks 8 0 is consistent with fractionation thus the fluid that caused melting was probably of metamorphic origin with 18 o similar to the protolith the seismic profile shows several reflectors extending to a present depth of 20 km ca 40 km in the late archaean under the migmatites these are the paths along which the metamorphic fluid migrated and generated the migmatites now at the surface a new type of neosome reported in this study may have formed along fractures that the fluids migrated along however these are peripheral pathways in the metatexites adjacent to the back thrusts and schollen diatexites

crustal anatexis and melt migrations in the higo metamorphic terrane west central kyushu kumamoto japan abstract this paper deals with the anatectic migmatites and leucogranites developed in a high t low p metamorphic terrane kyushu japan the migmatites in this terrane are divided into metatexites and diatexites and they occur at and above the garnet cordierite grade layers lenses and pods of leucogranite a few centimeters up to 6 m in thickness are wide spread in the metamorphic terrane and are most abundant not in the highest grade zone but in intermediate grade zones in which migmatites do not develop the migmatite leucosomes are typically depleted in orthoclase component while many leucogranites contain abundant orthoclase it is suggested that the leucogranites are crystal cumulates precipitated from peraluminous melts that had been generated and ascended from deeper levels in the crust and that the zone of metatectic migmatites represents both the source region and passage region for the anatectic melts it is thus clear that the anatectic melts had segregated and ascended on the order of 5 km in the crust of this region diatexites on the other hand represent solid melt mixtures at more advanced stages of partial melting dominated by biotite breakdown implications of the observations made in the higo terrane are discussed in reference to an evolutionary scheme for crustal anatexis and granite magma genesis

u pb geochronology of the opatica tonalite gneiss belt and its relationship to the abitibi greenstone belt superior province quebec u pb ages for zircon monazite and titanite from samples collected from two transects across the opatica tonalite gneiss belt in the superior province of quebec indicate that the belt contains rocks that are significantly older than those in the adjacent low grade northern abitibi belt tonalite and tonalite gneiss which make up most of the belt formed over an interval of 100 ma from pre 2800 to 2702 ma five samples have ages of ca 2807 13 2773 23 and 2702 3 ma zircon growth at 2718 2721 ma in the two oldest samples may record an early period of high grade metamorphism in the belt hornblende diorite monzodiorite and tonalite plutons were intruded at 2693 2696 ma particularly along the southern boundary with the abitibi belt these include the canet pluton at ma the lac ouescapis pluton at 2693 2 ma and the barlow pluton at 2696 3 ma pink granite plutons and dykes are the youngest intrusive rocks in the belt three samples have yielded zircon ages of 2690 2 and 2686 4 ma

geochemical signatures of mid crustal melting processes and heat production in a hot orogen the kerala khondalite belt southern india abstract garnet bearing migmatites and associated leucogranites and leucosomes of the trivandrum block in the kerala khondalite belt were formed through granulite facies dehydration melting of metasedimentary protoliths significant trace element depletions in cs zr nb and the compatible transition metal elements v cr ni cu and zn relative to average post archaean shales and model middle and upper crust recorded in all samples require their sedimentary protoliths to have been impure sandstones and greywackes rather than shales leucogranites 70 75 wt and leucosomes sio2 68 70 wt which are uniformly peraluminous and classed as s type on the basis of their a nk and asi relations form a compositional array that shows strong correlated variations between tio2 and fe2o3 and tio2 or fe2o3 with co and y these reflect coupled variations in modal garnet and ilmenite and require the presence of up to 15 20 wt of entrained peritectic garnet in the higher y and hree leucosomes the leucosomes have ree patterns with normalised la sm of 10 negative eu anomalies eu yb collectively support petrological and field observations that the leucosomes are mixtures between former melt and entrained peritectic garnet and ilmenite the leucogranites are the products of melt extraction and migration on metres to several metres lengthscales leucogranite nb y ta yb eu and sr ba relationships demonstrate that their chemical diversity is principally controlled by variation in peritectic phase assemblage i e proportion of entrained garnet ilmenite and k feldspar with near situ feldspar dominated fractional crystallisation and accumulation coupled with the loss of fractionated melt being secondary controls age corrected heat productions of the best segregated leucogranites that most closely approach melt compositions are 2 7 0 8 wm 3 somewhat greater than the weighted average 1 7 0 5 wm 3 of preserved migmatite in situ leucosomes and leucogranites of all types correction of the 550 ma heat production to account for an estimated 30 of migration and loss of melt from this middle crustal migmatite complex yields a syn orogenic heat production of c 2 0 0 6 wm 3 which though elevated compared with model middle crust is unlikely to have been high enough to have facilitated the near uht to uht metamorphism in the trivandrum block without the addition of heat external to the orogenic crust itself

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

formation and evolution of granite magmas during crustal reworking the significance of diatexites taylor mclennan 1985 the physical processes ina continuous section through reworked archaean crust records the volved particularly how granite magma is formed remain generation of granitic magma and its subsequent development in the controversial miller et al 1988 white chappell opatica subprovince in the canadian shield there the transition 1990 a partially melted source can form a granitic from palaeosome to granite was a closed system process through magma only if large volumes of melt are separated from intermediate stages of patch migmatite and diatexite the average most of their residuum understanding this process is degree of partial melting was less than 30 but the melt crucial to determining how crustal recycling and differfraction was redistributed within individual diatexite layers during entiation occurs for example melt residuum separation deformation regions that lost melt became residual diatexites could be a nearly perfect single stage process at the enriched in tio2 feot mgo cao sc cr co sr rare earth site of partial melting as described for leucosomes in elements ree and high field strength elements hsfe melt metatexite migmatites wickham 1987a sawyer 1991 accumulated to create diatexite magmas enriched in large ion 1994 brown et al 1995 alternatively the separation lithophile elements lile but contaminated with residuum macould be an imperfect multi stage process that yields a terial such diatexite magmas are parental to granites found at magma with a large residual component as is observed higher crustal levels in the terrane flow of the diatexite magma in in diatexite migmatites bea 1991 greenfield et al response to deformation separated some of its residuum into schlieren 1996 sawyer 1996 the metatexite model necessarily parautochthonous plutons were created where ascending granitic produces leucocratic residuum free granites such as magma locally ponded below impermeable layers and structures those described by le fort 1981 and montel et al magma left the anatectic region in dykes and lost its remaining residuum as it crystallized consequently the allochthonous granite 1991 and maximizes the geochemical signature of magmas that rose through 20 km of crust to feed the highest level crustal differentiation in contrast the diatexite model plutons in this region are highly fractionated and essentially free of can produce residuum rich granites and so reduce the residuum geochemical effect of crustal differentiation as chappell 1996 noted how anatectic magmas formed in the deeper crust evolve to the granitic magmas emplaced in the upper crust is also poorly known because few crustal sections

orogen parallel and transverse shearing in the opatica belt quebec implications for the structure of the abitibi subprovince the late archean opatica granitoid gneiss belt is situated within the northern abitibi subprovince along the northern margin of the abitibi greenstone belt approximately 200 km of structural section was mapped along three traverses within the previously unstudied opatica belt the earliest preserved structures are penetrative foliations and stretching and mineral lineations recording regional ductile shearing d1 late d1 deformation was concentrated into kilometre scale ductile fault zones typically with l s tectonite fabrics two families of lineations are associated with d1 indicating shearing both parallel and transverse to the east northeast trend of the belt lineations trending east northeast or northwest southeast tend to be dominant within domains separated by major fault zones in light of the abundant evidence for early north south compression documented throughout southern superior province including the abitibi greenstone belt d1 is interpreted in terms of mid crustal thrusting prob

geochemical characteristics of crustal anatexis during the formation of migmatite at the southern sierra nevada california we provide data on the geochemical and isotopic consequences of nonmodal partial melting of a thick jurassic pelite unit at mid crustal levels that produced a migmatite complex in conjunction with the intrusion of part of the southern sierra nevada batholith at ca 100 ma field relations suggest that this pelitic migmatite formed and then abruptly solidified prior to substantial mobilization and escape of its melt products hence this area yields insights into potential mid crustal level contributions of crustal components into cordilleran type batholiths major and trace element analyses in addition to field and petrographic data demonstrate that leucosomes are products of partial melting of the pelitic protolith host compared with the metapelites leucosomes have higher sr and lower sm concentrations and lower rb sr ratios the initial 87sr 86sr ratios of leucosomes range from 0 7124 to 0 7247 similar to those of the metapelite protoliths 0 7125 0 7221 however the leucosomes have a much wider range of initial end values which range from 6 0 to 11 0 as compared to 8 7 to 11 3 for the metapelites sr and nd isotopic compositions of the leucosomes migmatites and metapelites suggest disequilibrium partial melting of the metapelite protolith based on their sr nd and other trace element characteristics two groups of leucosomes have been identified group a leucosomes have relatively high rb pb ba and k2o contents rb sr ratios 0 15 rb sr 1 0 and initial end values group b leucosomes have relatively low rb pb ba and k2o contents rb sr ratios 0 15 and initial end values the low rb concentrations and rb sr ratios of the group b leucosomes together suggest that partial melting was dominated by water saturated or h2o fluxed melting of quartz feldspar assemblage with minor involvement of muscovite breakdown of quartz and plagioclase with minor contributions from muscovite resulted in low rb sr ratios characterizing both group a and group b leucosomes in contrast group a leucosomes have greater contributions from k feldspar which is suggested by 1 their relatively high k concentrations 2 positive or slightly negative eu anomalies and 3 correlation of their pb and ba concentrations with k2o contents it is also shown that accessory minerals have played a critical role in regulating the partitioning of key trace elements such as sm nd nb and v between melt products and residues during migmatization the various degrees of parent daughter fractionations in the rb sr and sm nd isotopic systems as a consequence of nonmodal crustal anatexis would render melt products with distinct isotopic signatures which could profoundly influence the products of subsequent mixing events this is not only important for geochemical patterns of intracrustal differentiation but also a potentially important process in generating crustal scale as well as individual pluton scale isotopic heterogeneities

two new geobarometers for garnet amphibolites with applications to southeastern vermont two new geobarometers for the assemblage garnet hornblende plagioclase quartz have been calibrated on the basis of the equilibrium 6 anorthite 3 tremolite 2 grossular 1 pyrope 3 tschermakite 6 quartz and its fe end member equivalent data representing 37 natural samples that equilibrated at conditions of 2 5 to 13 kbar and 500 to 800 c were fitted to the general equation rt in keq a bt p i c by using a weighted least squares method multiple correlation coefficients are high 0 98 and 0 97 lines of constant keqhave extremely shallow slopes in p t space 2 to 8 bars per degree suggesting that pressures may be deduced precisely even where temperatures are only poorly constrained typical analytical errors and temperature imprecisions propagate to very small errors in pressure about 500 bars caution is advised in applying the barometers outside the range of calibrant mineral compositions application of the calibrations to samples from southeastern vermont near the strafford chester and athens domes documents relatively high pressure metamorphism 7 to 10 kbar for these structures

genesis of aluminous and intermediate granulites a case study in the eastern sierras pampeanas argentina abstract we report petrologic and geochemical features of anhydrous granulites from the eastern sierras pampeanas argentina aluminous grt crd bearing granulite is the most abundant rock type in the study region and hosts intermediate opx grt bearing granulites as interlayered lenses both granulite types crystallized under similar p t conditions of t 850 900 c and p c 7 5 kbar the formation of grt crd granulites by subsolidus dehydration conflicts with the absence of k feldspar in them moreover the aluminous granulites have an ree pattern that precludes postulating they derived by close system metamorphism of an extensively weathered protoliths comparing mineral and rock chemistry with experimental results in simple systems acfm akf and afm permits the mineralogical changes and the bulk composition evolution experienced by granulites during melting to be identified this work demonstrates that orthopyroxene is a primary phase in opx grt granulites however never crystallized within aluminous granulites the aluminous granulites have both mineral and chemical compositions matching those expected for a residuum derived from the partial melting of a greywacke like sedimentary precursor compared with the composition of the average continental crust aluminous granulites are depleted in most lithophile trace elements and lree have somewhat low k rb ratios and high contents of elements having high partition coefficients for garnet and or ilmenite hree y cr ni however aluminous granulites have initial sr isotope ratios 0 713 we discuss the possibility that an intense but short lived melting event led to isotopic disequilibrium during melting of biotite bearing protoliths leaving a granulitic residuum with lower initial sr isotope ratios in contrast opx grt granulites have trace element features e g high sr contents and ree patterns with marked positive eu anomalies that do not support the idea that they are simply residues left after biotite dehydration melting and melt extraction rather these rocks display several petrological major and trace element and isotopic features that suggest they formed through the interaction of mafic magmas and aluminous granulites

petrology of basement dominated terranes ii contrasting isotopic sr nd pb and o signatures of basement derived granites and constraints on the source region of granite damara orogen namibia abstract major and trace element and nd sr o and pb isotope data from granites of the high grade central part of the damara orogen khan and outlet gorge areas namibia indicate a dominantly lower crustal origin based on their appearance in field the granites can be divided into red granites white granites and grey granodiorites red granites and some of the white granites from the khan area are isotopically evolved initial e nd 12 5 to 18 9 and were likely derived from metaigneous sources with late archaean to early proterozoic crustal residence ages other white granites are less evolved initial e nd 6 5 to 8 8 and were likely derived from metasedimentary sources that are similar to the country rock metapelites grey granodiorites from the khan and outlet gorge area are also isotopically evolved initial e nd 9 9 to 13 1 but are derived from metaigneous sources with younger late proterozoic crustal residence ages major and trace element data do not support closed system fractional crystallization processes for all samples however some chemical features i e decreasing rare earth element ree abundances with increasing sio 2 underline the importance of crystal fractionation processes for each distinct magmatic pulse isotope data do not support mixing of different crust derived melts or assimilation of crustal rocks by a mafic magma instead highly evolved pb isotope compositions strongly negative e nd values and radiogenic sr isotope compositions argue for an undepleted basement as a potential source the most likely model involves mainly partial melting of different basement rocks of archaean to proterozoic age at different levels within the crust only some granites could be derived by melting of metasedimentary rocks of the pan african cover sequence the consistency of the chemical data with a crustal anatectic origin and the range in radiometric ages suggests that they intruded simultaneously with crustal thickening however some of them may have been emplaced during extensional tectonics c 40 50 ma later than the main period of crustal thickening the heating events that promoted melting of fertile deep crustal rocks might have been caused either by the inferred high heat productivity together with crustal thickening during the main periods of orogeny or by delamination of mantle lithosphere during the final extensional stages of the pan african orogeny

partial melting of aluminous metagreywackes in the northern sierra de comechingones central argentina we describe a suite of metamorphic and migmatitic rocks from the represent the source regions of anatectic magmas e g brown 1994 sawyer 1998 even though sawyer sierra de comechingones sierras pampeanas of central argentina that include unmelted gneisses migmatites and refractory granulites 1996 showed that diatexite migmatites have rheological and chemical properties that suggest that they are the gneisses are aluminous greywackes metamorphosed in the amphibolite grade and are likely to have been the protoliths for the parental to granite magmas it is also known that granulites depleted in incompatible major and trace higher grade migmatites and granulites mineralogical characteristics and major and trace element compositions show that metatexite elements are present in the lower continental crust e g lambert heier 1968 these rocks have been migmatites diatexite migmatites and granulites are all melt depleted rocks the migmatites both metatexites and diatexites have underinterpreted as refractory residues of partial melting complementary to granitic magmas e g clemens 1990 gone h2o fluxed melting and lost 20 melt whereas the granulites have lost as much as 60 melt formed chiefly by vielzeuf et al 1990 and there is ample theoretical and experimental evidence that supports this concept dehydration melting of biotite the granulites are rocks from which essentially all the granitic components have been extracted whereas e g brown fyfe 1970 thompson 1982 vielzeuf holloway 1988 patino douce johnston 1991 the migmatites cooled and solidified while still retaining a granitic fraction the only evidence for the presence of melt in the study patino douce beard 1995 it is thus possible that the source regions of some granitic magmas could area apart from migmatitic leucosomes are dikes and sills of leucogranite rich in cumulate k feldspar these leucogranite bodies consist of a wide spectrum of migmatites and meltdepleted granulites may represent the pathways along which melt was drained from the exposed rocks our study shows that in the sierra de comechingones in this paper we focus on a suite of metamorphic and migmatitic rocks from the sierras pampeanas of both migmatites and granulites represent source regions of anatectic magmas which were frozen in at different stages of development argentina the rocks in the study area include unmelted gneisses migmatites and refractory granulites we describe the fabrics mineral assemblages and field relations of the various rock types and present data

in situ migmatite and hybrid diatexite at mt stafford central australia metasedimentary gneisses show a rapid change in grade within a 10 km wide low p high t regional aureole at mt stafford arunta block central australia migmatites occur in all but the lowermost of five metamorphic zones which are characterized by 1 muscovite quartz schist 2 andalusite cordierite k feldspar granofels with small melt segregations 3 spinel sillimanite cordierite k feldspar migmatite 4 garnet orthopyroxene cordierite migmatite and minor diatexite and 5 biotite cordierite plagioclase diatexite that shows a transition to granite a subsolidus unit comprising interbedded sandstone and siltstone is equivalent to bedded migmatite the main rock type in zones 2 4 mesoscopic textures and migmatite classification of this unit vary with grade in zone 2 metatexite is developed in siltstone layers that are separated by quartz rich unmelted metapsammite layers melt segregation was less efficient in zones 3 and 4 where the dominant migmatite layering is a modified bedding high proportions of melt were present in zone 4 in which schlieren migmatite is transitional between bedded migmatite and metapelite sourced diatexite the preservation of sedimentary structures and coexistence of melt reactants and products in zone 4 metapelite imply that melting proceeded insitu without substantial migration of melt zone 5 biotite cordierite plagioclase diatexite carries rafts of bedded migmatite with strongly resorbed edges as well as large k feldspar and quartz augen this unit of comparatively ca rich migmatites is inferred to have been formed by the mixing of locally derived and injected granitic melt

early cretaceous lower crustal reworking in ne china insights from geochronology and geochemistry of felsic igneous rocks from the great xing an range this paper presents new zircon la icp ms u pb ages and whole rock geochemical data for two granitic plutons and rhyolites of the baiyingaolao formation in the western xing an range ne china the two syenogranite granitic plutons yield identical zircon u pb age of 142 1 ma and the baiyingaolao rhyolites yield zircon u pb age of 138 2 ma the granites contain some hornblendes and show low zr and zr nb ce y contents and low a cnk 0 98 1 11 mg 6 55 and feot mgo values rhyolite samples show similar geochemical characteristics with a cnk of 0 99 1 10 and mg of 14 21 in combination with the high k2o contents 4 43 5 61 wt and negative correlations between p2o5 and sio2 both the granites and rhyolites were classified as high k calc alkaline i type granitoids all samples give high zirconium saturation temperature of 794 964 c with few initially inherited zircons and belong to high temperature i type granitoids they were generated by dehydration melting of biotite muscovite from sub alkaline meta basalts in lower crust depth leaving garnet amphibole and plagioclase as the major residual minerals the syenogranites and rhyolites are likely formed in mongol okhotsk oceanic subduction setting incorporating other lower crust originated felsic rocks in erguna and xing an massifs and songliao basin it is argued that lower crustal reworking is pronounced in ne china during early cretaceous

an archean fold thrust belt in the northwestern abitibi greenstone belt structural and seismic evidence an integration of structural field data and lithoprobe seismic reflection line 28 in the northwestern abitibi greenstone belt agb reveals a crustal scale south to southwest vergent thrusting event that developed in sequence above a shallowly 15 north dipping sole thrust at a mid crustal level seismic reflector geometry above this decollement suggests a mid crust 6 20 km depth dominated by low angle thrusts with smooth trajectory ramps and culmination folds or antiformal stacks similar to the structural style of neighbouring high grade plutonic gneissic opatica and sedimentary pontiac subprovinces in contrast low to high angle east west trending thrusts at the upper crust greenstone belt level 6 9 km depth are interpreted to be listric they occur in two fault systems the chicobi and taibi that resemble imbricate fan systems the contrasting structural geometry of the upper and mid crust is interpreted as variations in level through the thrust stack and resembles paleozoic mountain

internal differentiation of the archean continental crust fluid controlled partial melting of granulites and ttg amphibolite associations in central finland fault bound blocks of granulite and enderbite occur within upper amphibolite facies migmatitic tonalitic trondhjemitic granodioritic ttg gneisses of the iisalmi block of central finland these units record reworking and partial melting of different levels of the archean crust during a major tectonothermal event at 2 6 2 7 ga anhydrous mineral assemblages and tonalitic melts in the granulites formed as a result of hydrous phase breakdown melting reactions involving amphibole at peak metamorphic conditions of 8 11 kbar and 750 900 c a nominally fluid absent melting regime in the granulites is supported by the presence of carbonic fluid inclusions the geochemical signature of light rare earth element lree depleted mafic granulites can be modelled by 10 30 wt partial melting of an amphibolite source rock leaving a garnet bearing residue the degree of melting in intermediate granulites is inferred to be less than 10 wt and was restricted by the availability of quartz pressure temperature estimates for the ttg gneisses are significantly lower than for the granulites at 660 770 c and 5 6 kbar based on the p t conditions melting of the ttg gneisses is inferred to have occurred at the wet solidus in the presence of an h 2 o rich fluid a hydrous mineralogy abundant aqueous fluid inclusions and the absence of carbonic inclusions in the gneisses are in accordance with a water fluxed melting regime low ree contents and strong positive eu anomalies in most leucosomes irrespective of the host rock composition suggest that the leucosomes are not melt compositions but represent plagioclase quartz assemblages that crystallized early from felsic melts furthermore similar plagioclase compositions in leucosomes and adjacent mesosomes are not a migmatite paradox as both record equilibration with the same melt phase percolating along grain boundaries

large volumes of anatectic melt retained in granulite facies migmatites an injection complex in northern quebec abstract the opinaca subprovince in the superior province of northern quebec is a metasedimentary belt characterized by metagreywacke rocks that record a late archean granulite facies metamorphism and contains abundant thin veins of leucogranite regional scale study of this subprovince provides insight to the net budget of anatectic melt loss versus accumulation in the lower middle continental crust during anatexis a petrological study indicates that the metagreywackes partially melted at 820 c and 7 kbar and generated 1 from the metamorphic peak at 2666 ma to the granite solidus at 2636 ma the large discrepancy about 50 between the volume of anatectic melt produced locally in situ and the observed volume of leucogranite in the terrain indicates that anatectic melt accumulated in the opinaca subprovince it is an injection complex this finding that granulite facies terranes can be enriched in melt contrasts with the general view that granulite terranes are melt depleted the presence of an injection complex in the deep crust has several broad implications for the continental crust 1 h 2 o released as the injected melt crystallized rehydrated the adjacent granulite facies rocks which is evident from orthopyroxene replaced by amphibole and biotite consequently these deep rocks would be more fertile in a subsequent anatectic event than non rehydrated residuum 2 the retention of voluminous leucogranite in the injection complex has thermal consequences for the crust first the bulk composition of the crust containing the injection complex is necessarily more felsic thus the budget of heat producing elements th u and k is locally enhanced and radiogenic heat production is higher secondly latent heat of crystallization is released at a deeper level where the leucogranite solidifies the combined effect of both maintains the lower crust several tens of degrees warmer for longer period of time

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

melt segregation in migmatites anatectic stromatic migmatites have a symmetrical layered structure with a low ratio of thickness to length and a periodicity features that have not been explained satisfactorily but which are related to physical processes of melt segregation we evaluate the compaction model for segregation as it applies to migmatites and develop models for melt segregation based upon convection driven by volume change and upon advection down pressure gradients that result from applied differential stress acting on an anisotropic multilayer protolith compaction by gravity driven two phase flow results in asymmetric separation yields calculated segregation times that are slow or geologically unreasonable and yields relative volumes that are not consistent with the abundance of leucosome seen in natural migmatites although calculated segregation times for very wet granite might allow segregation by compaction some driving force in addition to gravity is needed to cause widespread melt segregation in the crust for segregation at low volume fraction of melt we develop a two dimensional two phase matrix and fluid viscous flow cell model length much greater than thickness with phase changes which predicts either expansion or contraction convection depending on whether volume change vr is positive or negative convection leads to melt segregation to form a stromatic structure in a geologically reasonable timescale at moderate volume fraction of melt segregation may occur by filter pressing in compositionally layered rocks in response to applied differential stress melt migration is by porous medium flow driven by differences in mean normal stress between the layers as a consequence of differences in rheology and shear enhanced matrix collapse calculated segregation rates are fast and the model yields adequate volume of leucosome the positive vr for volatile phase absent melting reactions under crustal pressures promotes melt escape unless extensional deformation facilitates melt accumulation if the rate of melt production exceeds the rate of melt escape then the increase in melt pressure may lead to melt enhanced embrittlement and fracture and melt may migrate out of the system in contrast water rich volatile phase present melt producing reactions have a negative vr which promotes melt retention after segregation unless deformation generates melt escape pathways evolution of stromatic structure through anatectic erosion of mesosome by increasing the melt fraction in situ may lead to breakdown of the solid matrix and will lead to instability due to buoyancy and magma may become mobile and entrain residual material restite

neoarchean leucogranitoids of the kianta complex karelian province finland source characteristics and processes responsible for the observed heterogeneity abstract in this study we describe the geochemistry and discuss the genesis of neoarchean ca 2 7 ga leucogranitoids of the kianta complex within the karelian province of the fennoscandian shield the leucogranitoids occur extensively as dykes and intrusions of varying size and represent substantial recycling of older crust occurring as the last magmatic event before final cratonization of the karelian province geochemically the leucogranitoids resemble the leucosomes of migmatized ttgs both groups are compositionally heterogenous but are characterized by high silica contents mean sio2 73 4 wt and leucocratic appearance reflecting a paucity of mafic minerals mean mgo 0 37 wt mg 32 6 the majority of samples analyzed are weakly peraluminous mean a cnk 1 05 mineralogically the leucogranitoids form a series grading from trondhjemite to leucogranite which is observable as highly variable cao 0 07 3 84 wt mean 1 47 k2o 0 30 8 76 wt mean 3 53 and na2o 2 22 7 23 wt mean 4 49 we consider the local ttgs sensu lato which comprise both true ttgs k2o na2o 0 5 to be the predominant source material for the leucogranitoids however some samples overlap compositionally with local ttgs and could represent either partial melting of garnet amphibolites or alternatively products of nearly complete fusion of ttgs leucogranitoids and transitional ttgs can be differentiated to some extent on the basis of geochemistry but definitive discrimination requires demonstration of field relationships and or isotope dating the observed compositional scatter is interpreted to be a result of source heterogeneity retention of variable accessory minerals and accumulation processes melting is attributed to thickening of the crust in response to collisional tectonics and occurred dominantly under water fluxed conditions possible fluid sources are the sanukitoid and quartz diorite suites plus dehydration reactions and partial melting of the lower crust

the role of mantle delamination in widespread late cretaceous extension and magmatism in the cordilleran orogen western united states extension during plate convergence and mountain building is widely recognized yet the causes of synconvergent extension remain controversial here we propose that delamination of lithospheric mantle aided by decoupling of the crust from the mantle via a reduction in the viscosity of the lower crust through heating incursion of fl uids and partial melting explains many enigmatic yet prevalent aspects of the metamorphic magmatic and kinematic history of the sevier laramide orogen of the western united states during the late cretaceous extension heating anatexis magmatism and perhaps rock uplift were widespread during a restricted time interval in the late cretaceous 75 67 ma along the axis of maximum crustal thickening within the mojave sector of the sevier orogen and to a lesser extent within the interior of the idaho utah wyoming sector to the north similar processes may have been active in the peninsular range sierran western mojave and salinian segments of the mesozoic cordilleran arc these processes are viewed as predictable consequences of the thermal rheological and dynamic state of the overlying crust following delamination of mantle lithosphere beneath isostatically compensated mountain belts the proposed delamination would have occurred immediately prior to eastward propagation of lowangle subduction of the farallon plate during the inception of the laramide orogeny following delamination extension and anatexis of the north american crust were aided locally by egress of slab derived fl uids from the low angle farallon slab we suggest that lithospheric delamination may have aided in the shallowing of the slab to achieve lowangle subduction geometry delamination has been proposed to be common in areas of thickened continental lithosphere in the terminal phase or late in orogenesis the late cretaceous delamination event proposed here for the sevier laramide orogen occurred during protracted plate convergence and was synchronous with and followed by continued shortening in the external part of the orogen

the s type ladybird leucogranite suite of southeastern british columbia geochemical and isotopic evidence for a genetic link with migmatite formation in the north american basement gneisses of the monashee complex abstract the 62 52 ma ladybird granite lbg suite is a peraluminous leucocratic s type quartz monzonitic to granitic suite which occurs as batholiths stocks dikes sills and pegmatite veins predominantly in the high grade rocks of the shuswap complex in southeastern british columbia the emplacement of the lbg was synchronous with the production of abundant migmatites within thor odin dome of the monashee complex an exposure of north american basement exhumed from depths of ca 26 33 km by eocene extensional faults the lbg and the leucosome in migmatites from thor odin dome have similar major and trace element patterns and are both characterized by zircons which have inherited precambrian cores whole rock nd isotope compositions show a range of values for the lbg with end 55 ma values from 5 0 to 17 2 the end 55 ma for the leucosome samples range from 9 5 to 23 6 overlapping with those of the granitic suite these data support the interpretation of a genetic link between formation of the lbg suite and melting of north american basement rocks such as those exposed in the core of thor odin dome the leucosome samples have lower high field strength element hfse concentrations and positive eu anomalies whereas the lbg samples have higher hfse concentrations and negative eu anomalies the similar trace element characteristics suggest that the leucosome from the migmatites and the lbg are related whereby most of the leucosome samples are cumulates and the lbg samples represent evolved or residual melts the initial 87sr 86sr isotope values for both the lbg and leucosome samples have a large range however the initial sr isotopic ratios for the lbg suite are lower than those of the leucosome samples with 87sr 86sr 55 ma ranging from 0 70603 to 0 73688 and 0 74256 to 0 76593 respectively this isotopic discrepancy suggests either a isotopic disequilibrium during partial melting in the mid to lower crust where the leucosome formed b the distribution of sr during partial melting was controlled by different melt producing reactions and or c isotopic heterogeneity in the source rocks at least part of the lbg suite likely formed via melting of north american basement rocks that were dominantly of sedimentary origin melting of the proterozoic supracrustal metasedimentary rocks overlying north american basement may also have contributed to the formation of the different phases of the suite found at the regional scale however the abundant leucosomes in the basement rocks of thor odin dome may mark the paths along which anatectic melt migrated in the structurally overlying ladybird granites of the south fosthall pluton

archaean subduction inferred from seismic images of a mantle suture in the superior province plate tectonics provides the basis for the interpretation of most current terrestrial tectonic activity and is widely accepted as having been active over much of the earth s history1 yet the timing of initiation of this process is subject to debate2 9 so far the earliest seismic evidence for plate tectonics has come from a fossil mantle suture in the svecofennian orogen 1 89 gyr ago 10 and from inferred plate convergence subduction and accretion in the trans hudson orogen 1 91 1 79 gyr ago 11 as yet seismic data from archaean areas have been able to demonstrate only the importance of compression in the construction of the continental crust12 15 here we present seismic data from a collision zone in the superior province of canada involving the abitibi granite greenstone subprovince and the plutonic arc related opatica belt we interpret dipping seismic reflections that extend 30km into the mantle as representing a relict 2 69 gyr old suture associated with subduction although crustal structure lithospheric thicknesses and convergence rates may have differed from those seen today these seismic data provide direct evidence that plate tectonics was active in late archaean times 800 myr earlier than indicated by previous seismic reflection surveys

anatomy of garnets in a jurassic granite from the south eastern margin of the north china craton magma sources and tectonic implications abstract the late jurassic jingshan granite located at the south eastern margin of the north china craton contains abundant garnets which can be subdivided into three types based on texture and composition i euhedral garnet in mafic biotite and garnet rich enclave grt i ii coarse grained garnet grt ii in the host granite and iii small euhedral garnet in aplite grt iii in general grt i has higher feo cao and lower mno contents than grt ii grt iii has higher mn but lower ca contents than others grt i has lower mree and hree contents than grt ii grt iii has prominent and distinctly negative eu anomaly as well as higher mree composition compared to the others systematic variations in oxygen isotope compositions are observed among the three garnet types with 18 o values of 4 7 in grt iii some of the grt ii and grt iii display two distinct zonings with cores having similar major and trace element compositions to grt i cathodoluminescence cl images revealed that the zircons from different garnet bearing samples possess fine scale oscillatory zoned magmatic rims with inherited cores in situ zircon u pb dating and trace element analyses show that the dark luminescent magmatic rims all have jurassic concordia ages 160 ma and similar trace element patterns most of the inherited cores also display similar triassic ages of 210 236 ma which is similar to the ages of ultrahigh pressure uhp metamorphic rocks of the dabie sulu orogen 230 ma in addition jurassic concordia ages were also found in a zircon inclusion in grt i implying that the grt i was formed shortly before the main magmatic event the age data suggest that the three different garnet types may be genetically related and modified by cogenetic magmatic events based on the zircon u pb ages from different garnet bearing samples the major element trace element oxygen isotope and zoning textures of the three kinds of garnet we suggest that grt i may be peritectic garnet whereas grt ii and iii are probably the results of magmatic dissolution precipitation processes and re equilibration of garnets with changing magmatic conditions during melting differentiation crystallization and cooling within the granite we conclude from the oxygen isotopic character of the garnets and ages of the zircons that the source rocks for the jingshan granites are from dabie sulu orogen representing the south china craton

primary crustal melt compositions insights into the controls mechanisms and timing of generation from kinetics experiments and melt inclusions abstract we explore the controls mechanisms and timing of generation of primary melts and their compositions and show that the novel studies of melt inclusions in migmatites can provide important insights into the processes of crustal anatexis of a particular rock partial melting in the source region of granites is dependent on five main processes i supply of heat ii mineral melt interface reactions associated with the detachment and supply of mineral components to the melt iii diffusion in the melt iv diffusion in minerals and v recrystallization of minerals as the kinetics of these several processes vary over several orders of magnitude it is essential to evaluate in nature which of these processes control the rate of melting the composition of melts and the extent to which residue melt chemical equilibrium is attained under different circumstances to shed light on these issues we combine data from experimental and melt inclusion studies first data from an extensive experimental program on the kinetics of melting of crustal protoliths and diffusion in granite melt are used to set up the necessary framework that describes how primary melt compositions are established during crustal anatexis then we use this reference frame and compare compositional trends from experiments with the composition of melt inclusions analyzed in particular migmatites we show that for the case of el hoyazo anatectic enclaves in lavas the composition of glassy melt inclusions provides important information on the nature and mechanisms of anatexis during the prograde suprasolidus history of these rocks including melting temperatures and reactions and extent of melt interconnection melt homogenization and melt residue equilibrium compositional trends in several of the rehomogenized melt inclusions in garnet from migmatites granulites in anatectic terranes are consistent with diffusion in melt controlled melting though trace element compositions of melt inclusions and coexisting minerals are necessary to provide further clues on the nature of anatexis in these particular rocks

the role of hybridization and crystal fractionation in the evolution of the cairnsmore of carsphairn intrusion southern uplands of scotland the petrology of the cairnsmore of carsphairn granitic intrusion in the southern uplands of scotland was the subject of several classical studies by deer about 50 years ago the presence of concentric zonation from dioritic margins to granitic core with petrologically transitional contacts was explained in terms of successive episodes of intrusion and hybridization magmas of gabbroic granodioritic and finally granitic composition were involved producing extensive intermediate zones of hybrid diorite and hybrid granite these proposals are examined in the light of recent field observations and new major and trace element data using suitable end member compositions it is shown that hybridization or mixing alone could not have produced the intermediate rock types although a form of hybridization may locally account for transitional boundaries between the main rock types it is shown that geochemical variations through all units are consistent with crystal fractionation as the dominant mechanism of magma evolution but field observations of locally sharp contacts between different rock types and varying compositional gradients preclude a simple model of in situ fractional crystallization the preferred model for the pluton is that a large volume of dioritic magma was emplaced assimilated country rocks near the margins and crystallized inwards to form the diorite quartz diorite unit the enclosed melt fractionated towards a granodioritic composition and broke through the partially crystallized carapace to form the zone of granodiorite the melt continued to fractionate at depth and after a further interval a granitic melt was intruded through the largely crystallized granodiorite to form the inner zone of granite the first intrusion of granite formed the finer grained microgranite where more rapid cooling and or volatile loss occurred this was intruded by a second pulse of compositionally similar melt to form the coarse grained granite

origin of high sr y type granitic magmatism in the southwestern of the alxa block northwest china abstract the petrogenesis of high sr y type magmas is still open to debate usually such magmas could result from melting under high pressure settings 12 kbar in this paper we gave an example that some high sr y type magmas could originate from melting of crustal materials at pressure of 10 12 kbar we carried out a study of petrogenesis for devonian high sr y granites from the beidashan batholith 397 411 ma southwestern alxa block northwest china the beidashan granites have sio 2 69 21 74 60 wt and al 2 o 3 14 01 16 20 wt with a cnk ratios of 0 99 1 08 according to their trace element compositions and whole rock zirconium saturation temperatures t zr the beidashan granites can be divided into two groups group i dy yb n 1 2 3 0 eu eu 0 77 1 3 t zr 761 856 c resulted from fluid absent partial melting of mafic to intermediate crustal materials leaving garnet residuum at pressure of 12 kbar and group ii dy yb n 0 76 2 16 eu eu 1 7 5 3 t zr 651 785 c formed by fluid present melting of mafic to intermediate crustal materials with residual amphibole in the source at pressure of 10 kbar both group i and group ii show high sr y and la yb n features they show i sr 0 7134 0 7180 e nd t 6 61 to 9 71 t 2dm 1 7 1 9 ga e hf t 5 6 to 9 9 and t dm c 1 7 2 0 ga indicating that the beidashan high sr y granites were derived from melting of crustal basement materials our results suggest that some high sr y type granites formed under relatively lower pressure conditions 10 12 kbar and they could not be an indicative of partial melting of thickened crust

petrology of basement dominated terranes i regional metamorphic t t path from u pb monazite and sm nd garnet geochronology central damara orogen namibia abstract mineral chemistry combined with sm nd garnet whole rock and u pb monazite ages provides important constraints on the geological history of metapelites migmatites and granites from the high grade basement dominated part of the damara orogen namibia based on thermobarometric data the peak of regional metamorphism reached c 700 750 c and c 5 kbar u pb monazite and sm nd garnet mineral ages obtained from unmigmatized garnet and cordierite bearing metapelites indicate that these peak metamorphic conditions were reached between 525 2 and 504 3 ma some migmatites with restitic melanosomes record sm nd garnet ages of 518 3 ma indicating that high grade regional metamorphism also culminated in partial melting some other migmatites and granites give older sm nd garnet whole rock ages between 530 3 and 540 4 ma different initial nd isotope compositions of granites leucosomes and melanosomes indicate that in this case migmatization is intrusion related crust derived granites intruded at 515 2 512 3 and 509 5 ma and are probably related to the second peak of regional metamorphism the youngest granites intruded at 469 3 ma indicating that intrusion of melts is superimposed upon the regional metamorphic history age constraints for the period between c 540 and 510 ma provide substantial evidence for crustal thickening together with igneous activity whereas the high temperature event which occurred at c 470 ma must be related to extensional tectonics the concordance between the monazite and garnet ages suggest that monazite grew at high grade metamorphic conditions it is further demonstrated that combined monazite and garnet chronometry are useful in identifying the timing and nature of particular tectonometamorphic and intrusive events in polymetamorphic high grade terranes

on the dynamics of magma mixing by reintrusion implications for pluton assembly processes many plutons have formed by repeated intrusion with complex internal contacts characterized by sharp and diause kinematic and compositional domains we performed numerical experiments of mixing following magma chamber recharge that explicitly considers the fluid dynamics of multiphase mixtures in the limit of zero diausivity of intensive scalar quantities and low stokes number mingling by fluid instability is equivalent to deformation and persistent fluid structures are kinematic attractors three distinct regimes are exemplified and can be described by their multiphase reynolds or grashof number for a reynolds number greater than about 100 an internal intrusive contact will collapse by internal wave breaking and chaotic magma mingling and mixing yield a nearly chamber wide stratification this flow may scour mushy regions at the walls and widely distribute previously crystallized material for reynolds numbers from 10 to 100 the internal wave does not break and the stratification occupies less of the chamber leaving islands of unmixed material for reynolds numbers around 1 internal slumping and folding can occur and is the most likely to be preserved by a mineral fabric internal sub vertical contacts require a high absolute viscosity in the resident magma representing a time break between episodes of reintrusion the shape of perched mafic domains captures the progress of sinking from higher levels into regions of increasing strength and crystallinity 7 2000 elsevier science ltd all rights reserved

high pressure melting of metapelite and the formation of ca rich granitic melts in the namche barwa massif southern tibet abstract a detailed geochemical and geochronological study of anatectic migmatites from the namche barwa massif nbm southern tibet has been carried out to place important constraints on the thermal and tectonic evolution of the eastern himalayan syntaxis shrimp zircon u pb dating indicates that the granulite facies metapelite underwent metamorphism at 21 8 0 7 ma and 24 5 0 7 ma respectively the latter is similar to the timing of partial melting and the formation of ca rich leucosomes at 24 25 ma these leucosomes are characterized by 1 high cao na 2 o and na k ratios 2 radiogenic sr 87 sr 86 sr t 0 7407 0 7904 but unradiogenic nd e nd t 7 0 to 21 2 isotope compositions 3 depleted hfse and 3 variable but depleted hree relative to their host pelites some of the leucosomes show large degrees of nd isotopic disequilibrium up to 10 epsilon units with respect to their hosts these high cao and na 2 o leucosomes were derived from fluxing melting of metapelite at high pressures a similar process could have operated during the formation of the himalayan leucogranites and contributes to the heterogeneities in such granites

structure of the high grade opatica belt and adjacent low grade abitibi subprovince canada an archaean mountain front plutonic gneisses in the archaean opatica belt in canada record crustal scale wsw vergent i e longitudinal high temperature d1 ductile shearing similar structures occur in adjacent greenstone belts but only in narrow amphibolite facies zones near the base metavolcanics in the greenschist facies portions do not record penetrative d1 structures the greenstone belts are interpreted as allochthonous on the gneisses they form the uppermost d1 thrust sheets sse vergent i e approximately transverse d2 thrusting imbricated the opatica belt and formed a large antiform either a culmination or an antiformal stack in the southern opatica the imbricated opatica belt wedged under the northern part of the abitibi subprovince and created a tight syncline with nnw vergent d2 thrusts interpreted as backthrusts on its northern limb the opatica belt abitibi subprovince contact therefore resembles some phanerozic mountain fronts further southwards propagating imbrication of the crust beneath the abitibi subprovince created a s vergent foreland fold and thrust belt in the overlying greenstones high grade opatica gneisses and adjacent low grade abitibi greenstones contain complementary structural patterns and may therefore represent the internal and foreland parts of an archaean mountain belt similar in many respects to phanerozoic examples

ttg suite from the bundelkhand craton central india geochemistry petrogenesis and implications for archean crustal evolution abstract the bundelkhand craton is one of the important archean protocontinental nuclei in peninsular india for which not much information on tectonic setting evolutionary history and petrogenetic processes is available here we provide comprehensive geochemical data on the tonalite trondhjemite granodiorite ttg rocks which form the basement rocks for the craton compositionally these rocks are variably evolved between diorite and granites and display both high and low al ttg types with geochemical features typical of convergent margin tectonic setting the evolved granitoids are essentially low pressure type further grouped into sodic true ttg and potassic transitional ttg types the low pressure and moderate shallow depths of melting conditions and low mg ni and cr contents do not support a direct slab melting scenario alternately these features favor the derivation of the bngc granitoids through the melting of a sub arc basaltic source possibly at the lower part of thickened arc crust in a suprasubduction regime the occurrence of identical lithological assemblages in other cratons in peninsular india and similar petrogenetic conditions for the emplacement of ttgs suggest the involvement of subduction accretion processes in building the ancient cratonic nuclei of the indian shield

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

source parameters of the 11 june 1909 lambesc provence southeastern france earthquake a reappraisal based on macroseismic seismological and geodetic observations 1 destructive earthquakes are rare in france yet pose a sizable seismic hazard especially when critical infrastructures are concerned only a few destructive events have occurred within the instrumental period the most important being the 11 june 1909 lambesc provence earthquake with a magnitude estimated at 6 2 rothe 1942 the event was recorded by 30 observatories and produced intensity ix effects in the epicentral area 30 km north of marseille we collected 30 seismograms leveling data and earthquake intensities to assess the magnitude and possibly the focal mechanism of this event following this multidisciplinary approach we propose a source model where all relevant parameters are constrained by at least two of the input data sets our reappraisal of the seismological data yielded mw 5 7 6 1 6 0 preferred and ms 6 0 consistent with the magnitude from intensity data me 5 8 and with constraints derived from modeling of coseismic elevation changes hence we found the lambesc earthquake to have been somewhat smaller than previously reported our data sets also constrain the geometry and kinematics of faulting suggesting that the earthquake was generated by reverse right lateral slip on a wnw striking steeply north dipping fault beneath the western part of the trevaresse fold this result suggests that the fold located in front of the luberon thrust plays a significant role in the region s recent tectonic evolution the sense of slip obtained for the 1909 rupture also agrees with the regional stress field obtained from earthquake focal mechanisms and microtectonic data as well as recent gps data

petrogenesis of the puerto eden igneous and metamorphic complex magallanes chile late jurassic syn deformational anatexis of metapelites and granitoid magma genesis a suite of schists gneisses migmatites and biotite granitoids from the puerto eden igneous and metamorphic complex peimc and biotite hornblende granitoids of the south patagonian batholith southern chile has been studied for that purpose the chemistry of minerals and the bulk rock composition of major and trace elements including rb sr and sm nd isotopes were determined mineralogical observations and geothermobarometric calculations indicate high temperature and low pressure conditions ca 600 700 c and 3 to 4 5 kbar for an event of metamorphism and partial melting of metapelites in late jurassic times previously determined by shrimp u pb zircon ages structures in schists gneisses migmatites and mylonites indicate non coaxial deformation flow during and after peak metamorphic and anatectic conditions andalusite schists and sillimanite gneisses yield initial 87sr 86sr ratios of up to 0 7134 and end150 values as low as 7 6 contemporaneous biotite granitoids and a coarse grained orthogneiss have initial 87sr 86sr ratios between 0 7073 and 0 7089 and end150 values in the range 7 6 to 4 4 this indicates that metamorphic rocks do not represent the natural isotopic variation in the migmatite source thus a heterogeneous source with a least radiogenic component was involved in the production of the biotite granitoids the peimc is considered as a segment of an evolving kilometre sized and deep crustal shear zone in which partial melts were generated and segregated into a large reservoir of magmas forming composite plutons in late jurassic times a biotite hornblende granodiorite and a muscovite garnet leucogranite show initial 87sr 86sr ratios of 0 7048 and 0 7061 and end100 values of 2 6 and 1 8 respectively and are thus probably related to early cretaceous magmas not involved in the anatexis of the metasedimentary rocks

the petrogenesis of some migmatites and granites central damara orogen namibia evidence for disequilibrium melting wall rock contamination and crystal fractionation

petrogenesis of migmatites in maine usa possible source of peraluminous leucogranite in plutons high t low p facies series conditions during deformation within a devonian crustal scale shear zone system defined by kilometer scale straight belts of apparent flattening strain that anastomose around lozenges of apparent constrictional strain at upper amphibolite facies grade metapelites are partially melted the onset of which is introduction