the stabilization of self organised leucogranite networks implications for melt segregation and far field melt transfer in the continental crust abstract net structured leucocratic vein systems are ubiquitous in high grade and migmatite terrains and are interpreted to represent interconnected former magma bearing structures that accommodated the migration of granitic magmas out of their anatectic source regions net structured leucogranite vein systems in high grade gneisses from the damara belt in namibia describe a size based hierarchy and spatial distribution that indicates self organisation of the magma transfer system the internal geometry and connectivity of the net structured leucogranites are controlled by variations in melt pressure whereas regional strains control the orientation of the leucogranites foliation parallel stromatic and shear band hosted granite veins accommodate transport at relatively low melt volumes and account for the metre scale segregation of melt close to the anatectic source during times of higher melt supply the formation of extensional fractures that cross cut stromatic and shear band hosted melt sheets at high angles is promoted extensional fracturing and associated dilatancy create gradients in magmastatic head that cause melt to be drained from pervasive stromatic and shear band hosted vein networks into spaced dilatant fractures this has the potential to initiate buoyancy driven far field melt transfer along suitably orientated extensional fractures as vertical dykes or self propagating hydrofractures our results suggest that melt transfer systems are able to adjust to variations in melt supply by forming conduits that accommodate varying melt volumes and will tend to develop towards a state of self organisation

origin of fine scale sheeted granites by incremental injection of magma into active shear zones examples from the pilbara craton nw australia in the archaean pilbara craton of western australia three zones of heterogeneous centimetre to metre scale sheeted granites are interpreted to represent high level syn magmatic shear zones evidence for the syn magmatic nature of the shear zones include imbricated and asymmetrically rotated metre scale orthogneiss xenoliths that are enveloped by leucogranite sheets that show no significant internal strain at another locality granite sheets have a strong shape preferred alignment of k feldspar suggesting magmatic flow while the asymmetric recrystallisation of the grain boundaries indicates that non coaxial deformation continued acting upon the sheets under sub solidus conditions elsewhere randomly oriented centimetre wide leucogranite dykes are realigned at a shear zone boundary to form semi continuous layer parallel sheets within a magma dominated dextral shear zone it is proposed that the granite sheets formed by the incremental injection of magmas into active shear zones magma was sheared during laminar flow to produce the sheets that are aligned sub parallel to the shear zone boundary individual sheets are fed by individual dykes with up to 1000s of discrete injections in an individual shear zone the sheets often lack microstructural evidence for magmatic flow either because the crystal content of the magma was too low to record internal strain or because of later recrystallisation

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

mapping the distribution of melt during anatexis at the source area of crustal granites by synchrotron μ xrf proyecto fapa universidad de los andes bogota colombia centro nacional de pesquisa em energia e materiais brazilian synchrotron light laboratory lnls proposal n xafs1 17105 cariparo cassa di risparmio di padova e rovigo project makearth european union s horizon 2020 research and innovation programme under the marie sklodowska curie grant agreement no 654606

the influence of melting and melt drainage on crustal rheology during orogenesis partial melting significantly weakens crustal rocks by introducing a low viscosity liquid phase however near concomitant melt drainage can remove this weak phase potentially reversing the rheological effects such that the strength of a specific lithology depends on when the prograde pressure temperature path intersects a melting reaction how much melt is produced and how long this melt is retained before it is lost phase equilibria and mixed rheology modeling of typical metapelite and metagreywacke compositions indicate that these rocks undergo continuous but pulsed melt production during prograde metamorphism depending on whether melt removal is continuous or episodic and assuming geological strain rates the lithologies can retain a very low strength less than 1 mpa or transiently strengthen to 5 mpa following melt loss lithologies undergoing episodic melt loss can therefore cycle between being relatively weak and relatively strong components within a composite crustal section melt production retention and weakening in the middle to lower crust as a whole is more sustained during heating and melt production consistent with geodynamic inferences of weak melt bearing lower crust however the long term consequence of melting and melt loss is a 50 400 increase in the strength of residual lithologies the strengthening is more pronounced in metapelite than metagreywacke and is achieved through a combination of dehydration and the removal of the weak mica framework coupled to increased proportions of strong feldspars and garnet despite prolonged weakness melting and melt loss therefore ultimately result in a dry and elastic lower crust

reply to comment on interplay of deformation and magmatism in the pangong transpressional zone eastern ladakh india implications for remobilization of the trans himalayan magmatic arc and initiation of the karakoram fault abstract in the eastern part of ladakh the right lateral karakoram fault zone kfz bifurcates into two strands called the pangong strand and the tangtse strand these two strands bound a region called the pangong transpression zone ptz which consists of migmatitic dioritic gneiss calc silicates and the durbuk pluton a pluton of two mica leucogranite outcrop scale observations suggest pervasive migration of leucogranitic melt through the existing tectonic structures of the gneiss magnetic fabric from both the tectonized and undeformed parts of the durbuk pluton show parallelism with the tectonic fabric of the host gneiss which along with pervasive melt migration indicates syn kinematic relationship between deformation along the kfz leucogranite magmatism and emplacement of the durbuk pluton u pb geochronology of zircons from the dioritic gneiss yields a crystallization age of 63 6 1 5 ma and also shows younger zircon growth down to 13 ma which suggest arc magmatism at 65 ma followed by partial melting and leucogranite magmatism in the kfz till 13 ma one two mica leucogranite sample from the durbuk pluton gives a crystallization age of 22 7 0 5 ma as the durbuk pluton is syn tectonic with deformation along kfz it is inferred that the kfz initiated at least 23 ma ago

self organisation and fracture connectivity in rapidly heated continental crust abstract volume expansion 1 5 volume strain with v melting positive and fluid absent partial melting in which v melting is positive of continental crust by intruding basaltic magma is a strongly irreversible process involving the dissipation of both thermal energy and matter partial melt using a simple random graph model we show by analogy how isolated fractures that form during rapid thermal perturbation in the source region can combine to form a single interconnected structure with high permeability once connected the fracture network may be thought of as a single structure or pattern that will remain stable so long as a strong temperature gradient is maintained in the source region estimates of fracture permeability that take into account changes in connectivity and fracture spacing range from approximately 10 10 to 10 5 m 2 many orders of magnitude greater than values considered typical during large scale crustal deformation and prograde regional metamorphism the ability of the isotropic fracture network to develop a top bottom directionality is crucial for buoyancy driven melt transport a physical model based on non linear evolution rules during thermal expansion is given that predicts the emergence of directionality vertical fracture alignment on a time scale of the order of 10 5 y the necessary ingredients are a deviatoric strain path a heterogeneous medium and a stiffness that evolves as a function of the local strain

modelling granite migration by mesoscale pervasive flow abstract mesoscale pervasive magma migration leads to granite injection complexes common in hot crustal terranes pervasive migration is limited by magma freezing when intruding cold country rock here we explore numerically the feedback mechanism between magma intrusion and heating of the country rock which allows younger intrusive batches to reach increasingly shallower cooler levels this process relies on the higher solidus temperature of a rock compared to that of its melt once melt is segregated we define the free ride layer as the region above the melt source where magma may freely migrate because rock temperature is above melt solidus the top of the free ride layer which corresponds to the melt solidus t s isotherm is at the limiting depth z s after magma passes through the free ride layer the magma front is always at the limiting depth we modeled the thickening and heating of the crust above the source as melt at its liquidus t l intrudes it pervasively from below we found that a magma quickly warms crust below z s to about t l forming a step in temperature at z s b the front z s moves up through the crust as more magma is intruded c as magma is emplaced at the front a mingled layer of about half magma half crust forms below it so that the total rise of the front corresponds approximately to half of the thickness of magma added to the free ride layer d the rate of rise of the front depends on the temperature difference between crust and t l and slows down as the magma front rises e for most reasonable intrusion rates and volumes the crust above z s feels little influence of the intrusion because the diffusion time scale is much smaller than the rise rate of the front in summary pervasive migration is an efficient way of heating the lower to middle crust and can result in an injection complex several kilometers thick consisting of about half magma and half original crust

pervasive granitoid magma transfer through the lower middle crust during non coaxial compressional deformation granitic magmas migrated through early proterozoic middle lower crust at mt hay central australia via a diverse network of narrow structurally controlled channelways during a period of progressive w sw directed thrusting d1a d1d they utilized existing folds boudins and shear zones or created new channels by magmatic fracture either parallel to layering or rarely in irregular arrays the magmas rose obliquely parallel to the plunging 50 60 regional elongation direction which was defined by coaxial folds boudin necks and a strong mineral elongation lineation megacrystic charnockitic magmas migrated through metre scale conduits during d1a d1b but leucosomes were generally restricted to smaller centimetre scale structures that existed throughout the entire deformation history thus d1a d1b leucosomes were potential feeders of in situ partial melts to the adjacent larger conduits of charnockite magma thereby providing a pervasive interconnected network that allowed efficient migration of all magma types during the early stages of thrusting the upper middle crust of the anmatjira reynolds range area contains abundant megacrystic granitoid sheets that are of similar age and geochemistry to those at mt hay they are considered to have formed as syntectonic intrusions emplaced during w sw directed thrusting as at mt hay suggesting that granitic magmas formed near the base of the continental crust passed through the mid lower crustal level 25 30 km exposed at mt hay and accumulated in batholithic proportions at shallower crustal levels 12 20 km such as the anmatjira reynolds range area the observations imply that granitoid magmas in the deep crust are capable of pervasive migration through the crust during major compressive noncoaxial shear deformation localization of magmas by sequentially developed narrow compressive structures suggests that dilatancy followed successive foliation forming events a situation that can occur during steady state deformation if the effective confining pressures are low which would be a result of high and possibly variable rates of magma influx the inferred rapid melt segregation and migration during deformation suggest that large chambers do not form until magma reaches neutral buoyancy in the middle to upper continental crust

heterogeneous brittle ductile deformation at shallow crustal levels under high thermal conditions the case of a synkinematic contact aureole in the inner northern apennines southeastern elba island italy abstract we present an example of interaction between magmatism and tectonics at shallow crustal levels in the late miocene the metamorphic units of the eastern elba island northern apennines were intruded at very shallow crustal levels by a large pluton 60 km 2 with the development of an hectometre sized contact aureole defined by growth of low pressure high temperature mineral assemblages p max max 650 c structural data show that the contact aureole is associated with a km sized antiform of the foliation and by several metre to decametre thick high strain domains consisting of strongly foliated rocks containing synkinematic ht lp mineral assemblages and ductile shear zones of variable thickness these shear zones are characterized by a mylonitic foliation variably overprinted by cataclasis quartz microfabrics indicate that the dynamic crystallization processes progressively changed from grain boundary migration associated with the thermal peak of contact metamorphism to subgrain rotation and bulging recrystallization the latter mostly associated with the cataclastic overprint these transitions of recrystallization mechanisms in quartz are related to a progressive decrease of temperature during deformation deformation accompanied the development and cooling of the contact aureole which recorded the switch from high temperature ductile to low temperature brittle conditions the geometry of the studied deformation structures is consistent with the constraints of the regional tectonic evolution and its local interaction with the localized and transient thermal anomaly related to the coeval emplacement of igneous rocks

paleozoic to mesozoic deformation of eastern cathaysia a case study of the chencai complex zhejiang province eastern china and its tectonic implications

the pangong injection complex indian karakoram a case of pervasive granite flowthrough hot viscous crust in the karakoram range of nw india the tangtse gorge cuts across deep crustal rock sexhumed by transpression along two strands of the dextral karakoram fault crustal leucogranitic magmas pervasively intruded amphibolites and migmatites in the form of sheets that locally coalesced and expanded to form kilometre scale plutons sheets resulted from slow seeping of granite into foliation planes magma wedging rather than from dyking driven by magma buoyancy and possibly syn intrusive tectonic deformation high temperature of the country rocks at the time of intrusion is inferred from similar crystallization ages of the intrusive leucogranite and in situ partial melt in the migmatites and reflected in the ductile structures developed whereas low country rock viscosity inhibited dyking high temperature freed the magma from the constraints of freezing and permitted pervasive intrusion the injection complex exposed along the tangtse gorge rather than representing the final emplacement structures represent transient structures of the granite pathways on their upward journey to build the karakoram batholith which crops out a few kilometres north at structurally shallower levels we suggest that pervasive magma flow is a transitional step between magma segregation at the source and later rise through cold crust and it may be one of several factors controlling whether dyking or diapirism becomes dominant during late ascent

mafic magma intraplating anatexis and hybridization in arc crust bindal batholith norway distinct with nd and o in the migmatites from 7 6 to 9 6 the dioritic velfjord plutons 448 ma were emplaced into and from 10 9 to 13 5 and in the contact granites regional migmatitic metapelitic and metacarbonate rocks at midfrom 5 2 to 7 5 and from 9 6 to 12 3 respectively crustal levels corresponding to pressures of 700 mpa exthus the contact granites could have a source that is isotopically humation to 400 mpa began while the migmatites were in a distinct from but mineralogically similar to the diatexites or they partly molten state with increasing proximity to the plutons could result from mixing of magma similar to the high k leucosomes regional stromatic migmatites change to diatexite and diatexitic with dioritic magmas mass balance calculations are consistent dikes are common within 500 m of the contacts we interpret these with the latter interpretation with proportions of granitic to dioritic relationships to indicate that heat from the plutons resulted in contact magmas from 7 1 to 7 3 emplacement and solidification of the migmatization in a zone up to 1 km wide typical residual dioritic plutons provided zones of structural anisotropy along which mineralogy in the diatexites is plagioclase quartz biotite high k leucosome magmas and contact granite magmas collected garnet sillimanite k feldspar consistent with biotite these magmas were injected by additional dioritic magma and dehydration melting podand dike like leucosomes consist of two further hybridized because the solidi of the plutons were several types earlier high k granitic ones with mineral assemblages hundred degrees higher than that of the granitic magmas the pluton identical to the migmatites and later low k tonalitic ones in which walls acted as long lived hot rigid surfaces along which magmas sillimanite is sparse and garnet absent the high k leucosome collected and migrated magmas can be explained by biotite dehydration melting at 700 mpa within the aureole mafic magmas were locally injected into and hybridized with the diatexites and the high k leucosome

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

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

emplacement of the giant kunene amcg complex into a contractional ductile shear zone and implications for the mesoproterozoic tectonic evolution of sw angola massif type anorthosites are voluminous plagioclase dominated batholiths that occur worldwide but only during the proterozoic eon granitoids are closely associated with the anorthosites and they collectively may form an amcg suite i e anorthosite mangerite charnockite granite many fundamental questions regarding the origin of the amcg complexes remain unresolved including their tectonic setting and mode of emplacement here we present new data from reconnaissance structural mapping u pb la icp ms zircon dating and 40ar 39ar amphibole and mica dating from a 500 km2 area straddling the western margin of one of the world s largest 15 000 km2 but also least studied amcg suites the kunene complex kc in angola the oldest recognized pre kunene deformation event d1 produced a steep wnw ese striking s1 fabric in supracrustal rocks migmatite and paleoproterozoic 1800 1780 ma granodiorite before 1680 ma this was folded by the d2 event resulting in steep n s striking s2 leucosomes and gneissosity in the rocks surrounding the kc concordant magmatic layering and lamination and high to low temperature gneissic foliation in the marginal kunene anorthosite and coeval megacrystic granite are parallel with s2 fabrics in the paleoproterozoic country rocks mineral stretching lineations l2 in all rock types across intrusive contacts are steep and associated with west side up thrust shearing overlapping u pb zircon baddeleyite ages from the kunene megacrystic granite and kunene anorthosite with 40ar 39ar mica ages from the mantling paleoproterozoic country rocks attest to progressive contraction thrusting and exhumation of the paleoproterozoic country rocks onto the kunene anorthosite in the interval 1410 1380 ma we suggest that thermal softening of the country rocks caused by heat dissipation from the earliest phase 1500 1410 ma of ponding of hot anorthositic magma in the deep crust localized d2 deformation at the margin of the kc our data point to a tectonic model whereby anorthosite sheets were emplaced at a high angle to the shortening direction and along host rock s2 anisotropy deep in the crust and cooled during progressive ductile thrusting and exhumation of the country rocks these results question the current view that proterozoic amcg suites are emplaced in extensional environments as large diapiric bodies and favor formation in a continental arc setting

strain partitioning during partial melting and crystallizing felsic magmas abstract the rheology of partly molten felsic systems changes dramatically during the onset of melting and through crystallization solid to liquid and liquid to solid transitions are not rheologically identical contradicting the concept of a unique rheological critical melt percentage we use four thresholds separating the successive stages according to the melt fraction for the melting transition 1 the liquid percolation threshold connects the melt pockets at about 8 of melt thus allowing local magma displacement at the metric scale and 2 for greater melt percentages 20 25 a melt escape threshold corresponds to the onset of transport over large distances of the melt and part of the residual solid phase leading ultimately to granitic bodies for the solidification transition 3 the first formation of a rigid framework of crystals which can accumulate stress although the melt can still flow corresponds to the rigid percolation threshold 55 solids and 4 the system becomes totally locked at about 72 75 solid which corresponds to the particle locking threshold the present paper focuses on strain partitioning induced by the coexistence of two contrasting rheological phases between these thresholds on a variety of scales non coaxial deformation is preferentially accommodated by the lower viscosity melt which increases the vorticity in the melt phase and reinforces asymmetries in the flow pattern strain partitioning also increases the scale length of the weakest phase during melting and magma ascent whereas strain partitioning decreases the scale length during crystallizing strain partitioning also affects the migration of melt by raising the particle locking threshold and lowering the melt escape threshold for three dimensional deformations such as transpression the direction of melt segregation e g horizontal may differ from the direction of melt migration e g vertical

continuous vs discontinuous melt segregation in migmatites insights from a cellular automaton model a cellular automaton model is presented that simulates melt extraction from migmatites the varying parameters are the relative amount of melt thresholds for melt connection leading to movement and melt escape and type of deformation pure and or simple shear it is shown that when the melt percentage is about half that of the melt escape threshold the competition between production and deformation assisted extraction results in a discontinuous rate of melt extraction the process can be compared to stick slip motion during frictional experiments at higher melt fraction it is continuously extracted deformation assists significantly in segregating melt particularly when a pure shear compaction component is added to simple shear

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

cretaceous oblique extensional deformation and magma accumulation in the fosdick mountains migmatite cored gneiss dome west antarctica 1 the fosdick mountains west antarctica expose a 15 x 80 km migmatite cored gneiss dome consisting of migmatitic gneisses diatexite migmatite and subhorizontal leucogranite sheets the fosdick dome was emplaced and exhumed in the mid cretaceous due to oblique extension associated with the west antarctic rift system along the west antarctic new zealand segment of east gondwana the dome is bounded to the south by a dextral oblique detachment structure and to the north by an inferred dextral strike slip fault within the fosdick dome and in the detachment zone granite occupies leucosomes dikes sills and dilatant and shear structures the pattern of kilometer scale domains of migmatite and granite suggest that lithologic variations and heterogeneous deformation boudinage resulted in pressure gradients that enhanced melt flow and magma accumulation in the fosdick dome steep foliations are overprinted folded and transposed by subhorizontal fabrics the crosscutting relationship is interpreted as a transition from wrench deformation to oblique divergence steep structures in the dome host concordant subvertical leucosome and granite sheets yielding shrimp u pb zircon ages between ca 117 and 114 ma prevalent subhorizontal domains host large volumes of subhorizontal diatexite migmatite and granite sheets that yield u pb zircon ages between ca 109 and 102 ma these ages indicate a timescale for melt influx of approximately 15 ma and that the transition from wrench to oblique divergence may have occurred in as little as 5 ma granites with crystallization ages between ca 109 and 102 ma were also emplaced in the south fosdick detachment zone indicating that the detachment was active during oblique divergence shrimp u pb titanite ages between ca 102 and 97 ma for late to post tectonic diorite dikes are interpreted as emplacement ages and give a minimum age for gneissic foliation development during detachment faulting the fosdick mountains preserve a record of the middle to lower crustal response to a transition from wrench to oblique extensional deformation overprinting structural relationships show that a change in the angle of oblique extension can induce accumulation of subhorizontal magma sheets and lead to initiation of a detachment zone

melt segregation and far field melt transfer in the mid crust abstract migmatites in the damara belt in namibia preserve relationships between anatectic leucosomes and intrusive leucogranite sheets illustrating the consecutive stages of partial melting local melt segregation and far field mobilization of melts over hundreds of metres and out of the anatectic region initial melting and localized segregation of melts are controlled by gradients in fluid pressure created by pervasively developed shallowly dipping dilatant fractures subsequent melt transfer out of these initial sites of melting and melt storage occurs along subvertical disc shaped leucogranite sheets that intersect the leucosomes the leucogranite sheets propagate as isolated melt filled hydrofractures driven by the pressure differential along the subvertical fractures the collapse of wall rocks into former melt bearing fractures and the presence of residual peritectic phases of the melting reaction trapped in wall rocks testify to the efficient extraction of melt by the mobile hydrofractures during their ascent this process of melt drainage into and transport by hydrofractures leaves almost no trace of the ascent conduits this study also shows that melt transport and the stability of melt pathways in strongly layered mid crustal levels characterized by low deviatoric stresses are determined by the presence of pre existing anisotropies and the progressively evolving structure in deforming orogens

transcurrent shearing granite sheeting and the incremental construction of the tabular 3 1 ga mpuluzi batholith barberton granite greenstone terrane south africa structural petrographic and geochronological studies show that the tabular 3 1 ga mpuluzi batholith in the barberton granite gneiss terrane in south africa was emplaced via a combination of external and internal processes external structural controls are indicated by systematic variations in intrusive relationships and strain along the margins of the mpuluzi batholith and are consistent with an emplacement of the granite in a dilational jog within a ne ene trending system of dextral transcurrent synmagmatic shear zones internally the mpuluzi batholith is essentially made up of granite sheets the structurally higher parts of the granite are made up of shallowly dipping sheets that are underlain by an anastomosing network of steeply dipping variably deformed dykes and sheets these granite sheets at lower structural levels intruded either into the actively deforming shear zones or into extensional sectors between and along the bounding shear zones multiple intrusive relationships and geochronological evidence suggests that granite sheeting and the assembly of the pluton occurred over a period of 3 13 ma the spatial and temporal relationship between deformation and magma emplacement reflects episodes of incremental dilation related to deformation along the bounding shear zones and granite sheeting the transition to the mainly subhorizontal granite sheets at higher structural levels of the tabular mpuluzi batholith indicates the intrusion of the granites during subhorizontal regional shortening where the reorientation of the minimum normal stress to vertical attitudes at the shallow levels of emplacement allowed for vertical dilation and subhorizontal emplacement of the granite sheets

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

controls on the locus of intraplate deformation in central australia abstract the locus of intraplate deformation in central australia changed from the presently preserved southern margin of the amadeus basin during the late neoproterozoic to early phanerozoic petermann orogeny to the northern margin in the late phanerozoic alice springs orogeny immediately prior to each event the thickness of the amadeus basin sediments varied from as little as 1 5 km in the central parts of the basin to as much as 7 km in the vicinity of the presently preserved margins of the basin with the locus of deformation mimicking the locus of maximum sedimentary thicknesses at the onset of each orogenic event as such these orogenies represent extreme examples of basin inversion a process observed in many intracratonic basins worldwide we show that differential burial of the central australian proterozoic basement complexes beneath the amadeus basin is capable of producing variations in moho temperature of up to 30 c prior to the petermann orogeny and up to 110 c prior to the alice springs orogeny for a brace goetze model of lithospheric rheology the variations in moho temperature equate to variations in effective lithospheric strain rate of 1 4 orders of magnitude implying that variations in thickness of the sedimentary blanket may have played a primary role in localising phanerozoic intraplate deformation in central australia an appealing aspect of this model lies in its corollary that the removal of the sedimentary blanket during denudation of the orogen will be accompanied by dramatic cooling and strengthening of the lithosphere this may provide a plausible explanation for the long term persistence of the extraordinary gravity anomalies 150 mgal developed during these central australian intraplate orogenies

low p high t metamorphism and the role of heat transport by melt migration in the higo metamorphic complex kyushu japan this paper characterizes the metamorphic thermal structure of the higo metamorphic complex hmc and presents the results of a numerical simulation of a geotherm with melt migration and solidification reconstruction of the geological and metamorphic structure shows that the hmc initially had a simple thermal structure where metamorphic temperatures and pressures increased towards apparent lower structural levels subsequently this initial thermal structure has been collapsed by e w and nne ssw trending high angle faults pressure and temperature conditions using the analysis of mineral assemblages and thermobarometry define a metamorphic field p t array that may be divided into two segments the array at apparent higher structural levels has a low dp dt slope whereas that at apparent lower structural levels has a high dp dt slope this composite array cannot be explained by heat conduction in subsolidus rocks alone migmatite is exposed pervasively at apparent lower structural levels but large syn metamorphic plutons are absent at the levels exposed in the hmc transport and solidification of melt within migmatite is a potential mechanism to generate the composite array thermal modelling of a geotherm with melt migration and solidification shows that the composite thermal structure may be formed by a change of the dominant heat transfer from an advective regime to a conduction regime with decreasing depth the model also predicts that strata beneath the crossing point will consist of high grade solid metamorphic rocks and solidified melt products such as migmatite this prediction is consistent with the observation that migmatite was associated with the very high dp dt slope the melt migration model is able to generate the very high dp dt segment due to the high rate of heat transfer by advection

anatomy of a 10 km scale sheath fold mount hay ridge arunta region central australia the structural record of deep crustal flow 1 mount hay ridge arunta region central australia exposes a 10 km scale antiformal sheath fold formed in the deep crust p 0 6 0 9 gpa in proterozoic time tilting and exhumation during the devonian alice springs orogeny resulted in an oblique cross section such that the once subhorizontal sheath fold now has a moderately dipping gently folded axial plane and overturned southern limb fabric types e g s l l s were mapped in the field and finite strain data were collected from a widely distributed gabbroic granulite unit fabric and finite strain data sets exhibit remarkable correspondence and confirm the common assumption that fabric type is directly related to finite strain type for deep crustal rocks both data sets reveal a distinct pattern of oblate strain on fold limbs grading into prolate strain in the fold hinge there is no field evidence for significant strain localization and measured strain magnitudes are consistent across the fold thus we infer that different structural domains record different though coeval strain paths and that strain compatibility was maintained across domain boundaries the uniformity of foliation lineation and mesoscale fold orientations across mount hay and the consistent magnitude of maximum stretch across structural domains indicate that the sheath fold and subsidiary structures formed during a single progressive deformational event based on its geometry the inferred scale of the enclosing shear zone 8 km in thickness and regional tectonic correlations we infer that the mount hay sheath fold formed in a subhorizontal crustal scale flow zone during the paleoproterozoic strangways orogeny 1740 1690 ma the record of deformation is consistent with fold formation either on one side of a channel or through detachment flow

esr dating of glacial tills and glaciations in the urumqi river headwaters tianshan mountains china abstract electron spin resonance esr dating of the shangwangfeng the xiawangfeng and the gaowangfeng tills in the headwaters of the urumqi river was carried out using ge centers in quartz grains the shangwangfeng till is dated at 35 3 5 ka bp three dates from the lower portion of the xiawangfeng till are 171 1 17 176 18 and 184 7 18 ka bp respectively and the age of the gaowangfeng till is 459 7 46 ka bp considering the available ages i e 14 c tl and esr and the principles of geomorphology and stratigraphy the shangwangfeng till is determined to be deposited in marine isotopic stage 2 3 mis2 3 the upper part of the xiawangfeng till was formed in mis4 and the lower part was deposited in mis6 the gaowangfang till is the oldest at the head of the urumqi river corresponding to mis12 the age of the gaowangfang till also demonstrates that the tianshan mountains lay at a suitable altitude for a glacial climate at that time when the glaciers on this segment of the mountain began to develop

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

rheological controls on grenvillian intrusive suites implications for tectonic analysis abstract studies of plutonic suites in western grenville along with the recent advances in understanding magma ascent and emplacement and interactions between magma and their country rock provide insights on the use of intrusive rocks to infer the accretionary history of deeply exposed precambrian orogens in the grenvillian central metasedimentary belt of quebec terrane juxtaposition was inferred to be late based on the distribution of 1 08 ga potassic alkaline plutons this view was questioned however upon recognition that magma ascent by dyke propagation can be stalled to enable the formation of plutons upon intersecting rheologically softer units for example grenvillian marble in this context preferential ponding of magmas in marble rather than tectonics can control regional distribution of plutons the spatial association of plutons with marble and their contact relationships are at the local scale obscured by transformation of marble to skarn and by mechanical excision of marble from the paragneiss wall rock sequence in contrast plutons and dykes of an older 1 17 ga less alkaline magma association are shown by field and remote sensing studies to be evenly distributed across the various lithotectonic domains of the belt their sheet like emplacement along the belt boundary constrains grenvillian tectonic assembly in the region to be early contrasting loci type and degree of deformation of intrusive bodies illustrate that host rock rheological differences influence the final characteristics of an intrusive suite consequently the nature of the host is an important variable in inferring timing relationships and tectonic scenarios from intrusive suites in high grade terranes

mid crustal magmatic sheets in the cascades mountains washington implications for magma ascent abstract diking diapirism ascent along faults and ascent during heterogeneous ductile flow have all been championed as the most important means of magma ascent in the crust we suggest that these mechanisms are end members in a complex spectrum of ascent processes in an attempt to evaluate which combination of ascent processes formed sheet like bodies in the mid crustal 20 25 km entiat pluton washington we examined the tip regions of these sheets the sheets have length width ratios ranging from 6 to 75 with increasing ratios strongly correlated to decreasing sheet tip radii from 850 to 100 m and decreasing ratios of tip diameter sheet width from 0 66 to 0 33 thus these bodies have geometries falling between those of dikes and those associated with elliptical diapirs the sheets are not associated with faults or fracture zones extending from their tips instead sheet walls are oriented parallel to the axial planes of upright syn emplacement folds in sheets with high length width ratios magmatic foliations in sheets are folded or parallel to axial planes of host rock folds with decreasing length width ratios margin parallel foliations in both sheet and host rock are increasingly common our studies indicate that the sheets are emplaced at high angles to 1 not 3 as proposed in elastic dike models and are always associated with complex viscoelastic flow of host rock these observations rule out elastic dike and fault models and instead favor diapiric rise of magma sheets during viscoelastic behavior of host rock

tectonic record magmatic history and hydrothermal alteration in the hercynian guerande leucogranite armorican massif france abstract the guerande peraluminous leucogranite was emplaced at the end of the carboniferous in the southern part of the armorican massif at the scale of the intrusion this granite displays structural heterogeneities with a weak deformation in the southwestern part whereas the northwestern part is marked by the occurrence of s c and mylonitic extensional fabrics quartz veins and pegmatite dykes orientations as well as lineations directions in the granite and its country rocks demonstrate both e w and n s stretching therefore during its emplacement in an extensional tectonic regime the syntectonic guerande granite has probably experienced some partitioning of the deformation the southwestern part is characterized by a muscovite biotite assemblage the presence of restites and migmatitic enclaves and a low abundance of quartz veins compared to pegmatite dykes in contrast the northwestern part is characterized by a muscovite tourmaline assemblage evidence of albitization and gresenization and a larger amount of quartz veins the southwestern part is thus interpreted as the feeding zone of the intrusion whereas the northwestern part corresponds to its apical zone the granite samples display continuous compositional evolutions in the range of 69 8 75 3 wt sio 2 high initial 87 sr 86 sr ratios and low e nd t values suggest that the peraluminous guerande granite a cnk 1 1 was formed by partial melting of metasedimentary formations magmatic evolution was controlled primarily by fractional crystallization of k feldspar biotite and plagioclase an20 the samples from the apical zone show evidence of secondary muscovitization they are also characterized by a high content in incompatible elements such as cs and sn as well as low nb ta and k rb ratios the apical zone of the guerande granite underwent a pervasive hydrothermal alteration during or soon after its emplacement u th pb dating on zircon and monazite revealed that the guerande granite was emplaced 309 7 1 3 ma ago and that a late magmatic activity synchronous with hydrothermal circulation occurred at ca 303 ma these new structural petrological and geochronological data presented for the guerande leucogranite highlight the interplay between the emplacement in an extensional tectonic regime magmatic differentiation and hydrothermal alteration and provide a general background for the understanding of the processes controlling some mineralization in the western european hercynian belt

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

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

tectonometamorphic evolution of the aracena metamorphic belt sw spain resulting from ridge trench interaction during variscan plate convergence 1 the aracena metamorphic belt is a high temperature low pressure ht lp band located at the southern end of the european variscan chain it marks a suture between armorica and avalonia this belt is characterized by 1 the presence of mid ocean ridge basalt derived metabasites that were affected by an inverted ht lp metamorphism related to sw verging thrusting whose thermal peak diachronously migrated eastward 2 the occurrence of a ht lp metamorphism related to an extensional event affecting continental rocks belonging to the ossa morena zone with peak temperatures that are 150 c higher than that recorded in the oceanic metabasites and 3 the presence of near trench magmatism with high mg andesite composition these and other characteristics can be interpreted in terms of a ridge trench trench triple junction that migrated along the armorican margin during variscan oblique convergence

development of migmatites and the role of viscous segregation in high t metamorphic complexes example from the ryoke metamorphic complex mikawa plateau central japan abstract the ryoke metamorphic complex of the mikawa plateau central japan contains well preserved geological and thermal structures formed during cretaceous high temperature high t metamorphism these structures can be explained by a pervasive melt migration model however the estimated flux of melt is much higher than that expected for compaction flow of felsic melt an additional tectonic force and or melt segregation to network of veins or layers is therefore required this paper presents the results of numerical modeling of the structural evolution of immiscible viscous fluids under simple shear deformation and discusses the role of viscous segregation in enabling pervasive melt flow in hot crust the results of numerical modeling of simple shear deformation of immiscible viscous fluids reveal that low viscosity fluids segregate into lenses and layers this segregation is a type of viscous segregation that takes place to minimize the dissipation of energy in the system the rate of energy dissipation is proportional to viscosity therefore this process is expected to occur universally in immiscible viscous fluids with large viscosity contrasts and is an important process in focusing melt in partially melted rocks the structures produced by the numerical simulation are similar to those of migmatites the simulation results suggest that melts distributed randomly in partially melted rock will segregate into lenses or layers with progressive simple shear deformation during deformation enhanced segregation a melt network will form dynamically if this process occurs at small to large scales in the hot crust melts will be effectively transported upward due to buoyancy forces it is possible that pervasive migmatites at low structural levels within high t metamorphic complexes may represent fossil melt pathways formed by crustal scale viscous segregation

mechanical anisotropies and mechanisms of mafic magma ascent in the middle continental crust the sondalo magmatic system n italy

driving mechanisms for 40 km of exhumation during contraction and extension in a continental arc cascades core washington 1 in the nw north american cordillera the cascades core region of the coast plutonic complex underwent late cretaceous 96 ma to locally 73 ma sw ne contraction and crustal thickening followed by dextral transpression 73 to 55 ma then transtension 3 mm yr by local thrusting in regions undergoing crustal thickening in the central part of the core chelan block 40 km of exhumation occurred between 91 and 45 ma about half of which occurred during early contraction driven by thrusting and half during top to north arc oblique shear during reactivation of a midcrustal cretaceous thrust the dinkelman decollement the footwall of this thrust consists of the swakane biotite gneiss a cretaceous metaclastic assemblage with recorded pressures of 10 12 kbar no arc related magmatism and structures dominated by pervasive top to north shearing the hanging wall consists of the napeequa complex an oceanic assemblage with recorded pressures of 6 12 kbar voluminous arc related magmatism and complex structures indicating early top to wsw shearing younger top to north shearing and widespread folding in the napeequa top to north shearing started by 73 ma during melt present conditions at pressures 6 kbar top to north shearing in both hanging wall and footwall continued during exhumation 1 6 mm yr and cooling to greenschist facies conditions during which slip became increasingly localized eventually resulting in formation of pseudotachylite on discrete slip surfaces we suggest that arc oblique extension was driven by along arc heterogeneity in displacements erosion initially during transpression and underplating of continental sediments and later during transtension

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

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

syntectonic granite emplacement at different structural levels the closepet granite south india abstract the closepet granite in south india is a large 400 km long but only 30 km wide elongate late archaean granitic body structural levels from deep crust to upper levels crop out as evidenced by a shallowing of paleo depths from north to south all along the closepet granite this allows the study of the emplacement of the same granitic body at various crustal levels four zones have been identified a root zone where magmas are collected in active shear zones a transfer zone featuring large scale magma ascent and crystal liquid partitioning in the granitic mush a gap where the mush was filtered allowing only the liquids to rise shallow intrusions filled with this liquid the closepet granite was emplaced syntectonically field work and anisotropy of magnetic susceptibility allowed documentation of steep foliations with subhorizontal lineations both in the root and transfer zones and in the shallow intrusions remote sensing evidenced a network of shear zones bounding the closepet granite in the porphyritic root and transfer zones magmas cooled slowly thus developing strong fabrics during large scale dextral shearing ascent of residual liquids amidst the crystallizing solid framework was not recorded in the fabrics however these liquids were channelised through the gap and infilled the homogeneous shallow intrusions where rapid cooling only permitted the development of feint although wholly consistent fabrics

fault controlled pluton emplacement in the sevier fold and thrust belt of southwest montana usa abstract problems associated with syncompressional pluton emplacement center on the need to make room for magma in environments where crustal shortening not extension occurs on a regional scale new structural data from the pioneer and boulder batholiths of southwest montana usa suggest emplacement at the top of frontal thrust ramps as composite tabular bodies at crustal depths between 1 and 10 km frontal thrust facilitated pluton emplacement was accommodated by 1 a magma feeder zone created along the ramp interface 2 providing releasing steps at ramp tops that serve as initial points of emplacement and subsequent pluton growth and 3 localizing antithetic back thrusts that assist in pluton ascent a model of magma emplacement is proposed that involves these elements this model for syntectonic ramp top emplacement of plutons helps explain how space is made for plutons within fold and thrust belts

on shearing magmatism and regional deformation in neoarchean granite greenstone systems insights from the yilgarn craton abstract the structure of the neoarchean yilgarn craton is dominated by craton scale high strain zones mostly associated with highly deformed elongate granitic bodies and transposed greenstone belts these shear zones developed during widespread and prolonged magmatic activity that led to a nearly complete reworking of the felsic continental crust the spatial temporal and genetic relationships between such a voluminous and protracted event of crustal reworking and the development of the craton scale shear zone network are unclear here we combine new structural geophysical and geochemical data to investigate the relationship between crustal scale shear zones and large syntectonic plutons in the yilgarn craton of western australia we propose that archean granite greenstone systems may have commonly evolved through the interaction of three fundamental geological processes i emplacement of large scale syntectonic plutons ii activity of crustal scale shear zones iii pervasive largely syn metamorphic polyphase deformation in greenstone belts adjacent to syntectonic plutons we propose that the concept of archean regional deformation event need to be reassessed a regional event is probably pluton or batholith size and the structural metamorphic evolution of adjacent greenstone belts might have proceeded quite independently and potentially in a time transgressive way if those belts were not spatially related to the same syntectonic pluton

geophysical and structural signatures of syntectonic batholith construction the south mountain batholith meguma terrane nova scotia summary the late devonian south mountain batholith is a very large 7000 km2 composite peraluminous granitoid complex situated within the meguma terrane of the northern appalachians it is made up of two suites of granodioritic to leucogranitic plutons emplaced at approximately 380 370 ma during the acadian orogeny i e during the collision of gondwana with the eastern margin of north america a significant geophysical and geological database makes the south mountain batholith a type example of a very large syntectonic batholith emplaced within a collisional orogen gravity models reveal the plutons have flat or gently dipping floors at approximately 7 0 km depth and aspect ratios 651 they are underlain by deeper 10 km elongate northeast southwest trending roots that may indicate magma feeder zones dyke transport of granitic magma and the progressive construction of plutons by sheet injections are supported by field observations and by mapping of the anisotropy of magnetic susceptibility at the pluton scale the very narrow deformation aureole within the country rocks suggests lateral spreading of the plutons was not the main space creation mechanism during emplacement space was mostly created by vertical displacements of country rocks the data are consistent with a laccolithic model for syntectonic batholith assembly the laccolithic plutons may have been emplaced at the base of the meguma supergroup metasedimentary rocks suggesting a maximum thickness of approximately 7 0 km for the supracrustal rocks in the meguma terrane

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

numerical simulation of the microstructure and compression behavior of eckernforde bay sediments abstract transmission electron microscope tem studies conducted on sediments recovered using diver collected cores and gravity cores from eckernforde bay in the baltic sea germany reveal that the sediments consist of silt to clay sized sediment both free and bound into fecal pellets organic matter biogenic materials methane gas and microorganisms the microstructure of the upper 2 m of the sediment is complex with the clay particles arranged in a very open structure with variable inter and intra domain pore space in an effort to understand the relationship between microscopic variables e g microfabric and macroscopic geotechnical properties e g compressibility a numerical study was performed using the discrete element method the objective of this study was to develop an understanding of the micro to macro linkage of mechanical properties specifically one dimensional compression behavior of the sediment was numerically simulated and compared with experimental data compositional parameters of the sediment were obtained from specific tests conducted at the naval research laboratory and from tests conducted by other researchers participating in the project image analysis was used to quantify the microfabric parameters from tem images of the sediment the effect of organic matter was simulated by introducing inter particle contact cementation results indicate that inter particle bonding due to inter particle cementation and or the van der waals attractive force 1 contributes to the apparent overconsolidation observed in the laboratory and 2 holds the highly porous unstable microstructure together until a threshold stress level is reached change in void ratio during compression is due to inter rather than intra domain movement

mesoscale pervasive felsic magma migration alternatives to dyking abstract this paper reviews the literature on dyking as a mechanism of felsic magma extraction from a source and transport to shallower crustal levels and review the recent literature suggesting a range of alternative mechanisms of magma migration in hot crustal zones which produce mesoscale pervasive granite sheet intrusions recent papers have strongly favoured dyking as the main mechanism controlling magma migration however the initiation of dykes from a felsic magma source is fraught with difficulties even when magma is immediately available for transportation as in magma chambers within a partially molten source magma may reside in a range of structures with a wide range of shapes sizes and degrees of connectivity whereas the growth of individual dykes within a partially molten zone and the self propagation of large dykes into subsolidus crust have both been studied in some detail little attention has been given to the crucial intermediate step of the growth of a dyke network capable of producing wide crustal scale dykes the rarity of granite dyke swarms suggests that if dyking is the preferred mechanism of magma transport felsic magma sources produce only few major transporting dykes during their lifetime alternatively dyking is not an important mechanism the parameters controlling the volume of the catchment drained by one such dyke as well as other basic geometrical parameters controlling the structure of the dyke network within the source are unknown the ability of dyking to drain a partially molten source depends crucially on these variables and particularly on the horizontal permeability of the source the slow velocity of viscous felsic magmas traveling in rock pores implies that magma drained during dyking is mostly that previously extracted from the pores and resident in irregular magma bodies or dyke networks the observation that large volumes of buoyant magma are commonly present in migmatite zones and that dyking in these zones plays a secondary role suggest that dyking is inefficient and is able to extract only a fraction of the total melt available in the source in support of this conclusion recent detailed studies of exhumed hot crustal zones have revealed a range of alternative migration mechanisms characterized by mesoscale pervasive magma flow outcrop scale as opposed to porous flow pervasive migration gives rise to magma sheets preferentially emplaced parallel to high permeability zones such as foliation or bedding planes apart from local dyking three alternative mechanisms have been proposed to account for pervasive migration of magmas namely tectonic pumping magma wedging into low viscosity rocks and volatile driven intrusion because of their unfocused pervasive character these mechanisms are restricted to hot crust where magmas are not exposed to rapid freezing a model is proposed whereby heat advected with the magma pushes crustal geotherms upwards allowing pervasive magma migration to shallower depths

granite emplacement in orogenic compressional conditions the la alberca bejar granitic area spanish central system variscan iberian belt abstract the la alberca bejar granitic area comprises a set of intrusions of upper carboniferous to lower permian age located in the hinterland of the variscan belt of western spain gravity data anisotropy of magnetic susceptibility and microstructural studies carried out in the granitoid and its country rocks indicate that the granitoids of this region have tabular and inverted drop shapes and show sub horizontal magmatic fabrics and sw ne lineations the granitoid bodies were intruded episodically with single intrusive pulses recorded by deformation in the country rocks all intrusive events took place during d 3 compressive stage in the core of large d 1 anticlines and are not related to structures in the country rock a syn orogenic emplacement model for granite intrusion in such conditions is proposed the proposed model implies the activity of coeval emplacement mechanisms and room generation

interplay between deformation and magmatism during doming of the archaean shaw granitoid complex pilbara craton western australia abstract the shaw granitoid complex one of the classic granitoid domes of the archaean pilbara craton western australia evolved from a plutonic complex into a 60 km diameter steep sided dome during migmatisation of the deep crust consistent stratigraphic younging of greenstones away from the dome and underlying outward dipping domical foliations indicate that the present exposure is a crustal section the section passes downward through progressively older greenstones into a near concordant high level sheeted granite complex the ca 3470 ma north shaw suite nss to a younger sill dyke complex of leucogranite the pilga leucogranite farther below the leucogranite grades via diatexite and a network of leucogranitic dykes into the lowermost unit of migmatitic orthogneisses which have protolith ages that overlap with the nss a semi continuous solid state domal foliation extends downward from the greenstones through the nss into the migmatitic orthogneiss but is less evident in the intervening pilga leucogranite a set of nnw trending upright folds with axial planar leucogranite dykes exist in both the pilga leucogranite and underlying orthogneisses but not in the nss or greenstones field relations indicate that much of the solid state dome concordant foliation developed in a sub horizontal attitude before the leucogranite sheets were emplaced above the migmatitic orthogneisses emplacement occurred along a sub horizontal high strain zone probably at a rheological boundary between the relatively rigid nss and softer underlying migmatitic orthogneisses the sheeted leucogranite complex and underlying orthogneisses were subsequently folded with later leucogranite magmas injected along the axial surfaces of the folds these leucogranite magmas locally extended into the overlying deformed nss where they also acquired a weak domical fabric suggesting syn doming emplacement although presently estimated to be 3445 3410 ma old stratigraphic constraints suggest that the leucogranite and the dome could be as young as 3300 ma the contrasting strain patterns above and below the pilga leucogranite are not expected for horizontal tectonic models invoking thrusting core complex formation or cross folding but are similar to one observed for gravity driven vertical tectonic models thermal softening initiated deformation and then deformation localised melts at a specific rheological interface once established this interface controlled the locus of pilga leucogranite intrusion during dome evolution demonstrating the interplay between deformation and magmatism

origin of migmatites by deformation enhanced melt infiltration of orthogneiss a new model based on quantitative microstructural analysis a detailed field study reveals a gradual transition from high grade solid state banded orthogneiss via stromatic migmatite and schlieren migmatite to irregular foliation parallel bodies of nebulitic migmatite within the eastern part of the gfohl unit moldanubian domain bohemian massif the orthogneiss to nebulitic migmatite sequence is characterized by progressive destruction of well equilibrated banded microstructure by crystallization of new interstitial phases kfs pl and qtz along feldspar boundaries and by resorption of relict feldspar and biotite the grain size of all felsic phases decreases continuously whereas the population density of new phases increases the new phases preferentially nucleate along high energy like like boundaries causing the development of a regular distribution of individual phases this evolutionary trend is accompanied by a decrease in grain shape preferred orientation of all felsic phases to explain these data a new petrogenetic model is proposed for the origin of felsic migmatites by melt infiltration from an external source into banded orthogneiss during deformation in this model infiltrating melt passes pervasively along grain boundaries through the whole rock volume and changes completely its macro and microscopic appearance it is suggested that the individual migmatite types represent different degrees of equilibration between the host rock and migrating melt during exhumation the melt topology mimicked by feldspar in banded orthogneiss forms elongate pockets oriented at a high angle to the compositional banding indicating that the melt distribution was controlled by the deformation of the solid framework the microstructure exhibits features compatible with a combination of dislocation creep and grain boundary sliding deformation mechanisms the migmatite microstructures developed by granular flow accompanied by melt enhanced diffusion and or melt flow however an ams study and quartz microfabrics suggest that the amount of melt present did not exceed a critical threshold during the deformation to allow free movements of grains

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

melt redistribution during the bending of a porous partially melted layer 1 a numerical poro viscoelastic finite element flow model has been developed to simulate the redistribution of melt during the folding of a horizontal layer important mechanical properties governing the overall rheology of the buckling layer including porosity matrix viscosity melt pressure and changes in deviatoric stress are tracked simultaneously with deformation melt migrates down gradients in melt pressure to the outer hinge zone of the folds away from sites of compression to regions of local tension for a matrix with dynamic power law rheology deformation results in the formation of low viscosity strain weakening zones that coincide with zones of enhanced deviatoric stress that are potential sites for plastic deformation during fast deformation of layers with large width to height ratios the more rapidly bending material splits into domains of higher viscosity separated by lower viscosity zones porosity melt fraction distribution patterns do not appear to be controlled by the matrix rheology the numerical results and predictions of melt distribution and geometry during deformation correspond well with data from some physical analogue laboratory experiments and field observations the quantitative nature of our results should help refine more qualitative models for the rates and mechanisms of melt extraction and transport in folded crustal migmatite terranes

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

granitic magma ascent and emplacement neither diapirism nor neutral buoyancy it is probable that granitic magma ascent does not result from the intrin sic properties of the magmas within the uppermost crust neither the reduced viscosity nor the density contrast between magma and surroundings are them selves sufficient to induce either low inertia flow diapirism or fracture induced magma propagation dyking igneous diapirism is intrinsically restricted to the lower ductile crust dyking is therefore the most probable ascent mechanism for granitic magmas that reach shallow crustal levels a neutral buoyancy level in the crust at which magma ascent should stall is never observed this is demon strated by coeval emplacement of magmas with different compositions and densi ties and the negative gravity anomalies measured over many granitic plutons we suggest that deformation through strain partitioning is necessary to magma ascent pluton formation is controlled by local structures and rock types rather than by intrinsic magma properties as a result of its intermittent character deformation both local and regional induces magma pulses and this may have important consequences for the chemical homogeneity of intruded magmas ascent of granitic leucogranitic to tonalitic magma in the continental lithosphere is the most potent means of mass transfer and chemical segregation between the lower and upper crust granitic magmatism contributes to crustal recycling and to a lesser extent continental growth in rapidly accreted crust granitic rocks can occupy up to one third of the volume of the middle crust meissner 1986 together with shear zones they are the most conspicuous effects of plate convergence and collision in orogenic zones consequently they are a key element in understanding the evolution of earth s mechanical workings as most felsic magmas are emplaced in a liquid or near liquid state at temperatures over 800 a major heat source must be involved in their genesis the existence of such a heat source also has consequences for the deformation regime of the crust com monly a large temperature rise in the lithosphere will also increase the capacity of crustal material to respond to stress by deformation such a thermal anomaly will contribute to the dissipation of energy and thus buffer the temperature effects the stress regime will be imposed externally by the tectonic processes operating at the time as a reaction to a significant increase in crustal heat flow the rheological capacity of rocks to sustain stresses decreases and this activates deformation therefore most departures from a normal heat distribution in the lithosphere will trigger deformation e g formation of extensional rift zones or shear zones in convergent situations

melt extraction and accumulation from partially molten rocks current models for melt segregation and ascent are not adequate to accurately describe transport and accumulation in combination we propose that transport is discontinuous and in batches and that accumulation occurs by stepwise merging of batches a simple numerical model of jostling spheres that merge when they touch was used to represent stepwise accumulation and transport of batches by propagation of hydrofractures results of the numerical model indicate that such a system may quickly develop into a self organised critical soc state in this state the distribution of melt batch volumes can be described by a power law with an exponent m that lies between 2 3 and 1 once a self organised critical state is established the system is capable of discharging any additional melt without further change to itself deformation aids melt extraction efficiency as it increases the mobility of hydrofractures enhances accumulation and hence lowers the exponent m full connectivity of melt needs never to be reached in the system and melt transport and extraction can occur at very low melt fractions the chemical evolution of melt from source to emplacement level will be governed by the discontinuous mixing and mingling of batches each with different histories and possibly different sources if no subsequent homogenisation occurs in a magma chamber or the final emplacement structure the process can be identified by chemical heterogeneity of plutons and volcanic rocks

simulation of crustal melt segregation through cellular automata insight on steady and non steady state effects under deformation we present a numerical model of melt segregation under variable deformation regimes pure and or simple shear and strain rates the model is adapted from a cellular automaton the lagrangian description requires some initial melt content and obeys two thresholds one allows melt connection and subsequent melt motion the other rules melt escape the geometry of the flow is that of pure and or simple shear deformation is a necessary melt segregating factor with contraction being more efficient than simple shear the ratio of escaping melt to the initial melt content defines the coefficient of melt extraction it varies linearly with initial melt content thresholds and deformation which are tested separately during each run transient states of the melt extraction coefficient were characterized by exponential spikes whose amplitude depends on the initial melt content thresholds and deformation departures in melt extraction from average volume are damped when initial conditions are continuously changed reflecting a memory effect we suggest that melt extraction relates to a pinning depinning process melt extraction being discontinuous and irregular with time we suggest that detailed chemistry does not properly document the melt evolution in source rocks migmatites

strain localization and melt segregation in deforming metapelites abstract crustal rocks commonly experience partial melting in deep parts of orogenic belts present concepts predict that the melt generation is directly related to shear localization which also provides paths for the melt to migrate existing experimental studies however are insufficient to directly relate melt generation to deformation we present the results of a series of laboratory experiments on synthetic pelites to demonstrate how and when partial melt generates with respect to strain localization during non coaxial deformation at intermediate pressure 300 mpa and high temperature 750 c with a constant strain rate 3 10 4 the study reveals that shear localization and partial melting were initially independent low angle riedel shears occurred in a brittle manner when the differential stress was at its peak the generation of melt which introduced ductility in the system was associated with strong weakening of the material the amplification of melt production 22 was restricted to this strong weakening phase it decreased asymptotically with larger strain comparison with static experiments showed that 1 7 times more partial melt generates in the dynamic experiments

the role of subgrain boundaries in partial melting abstract evidence for partial melting along subgrain boundaries in quartz and plagioclase is documented for rocks from the lost creek gneiss of the llano uplift central texas the wet mountains of central colorado and the albany fraser orogen southwestern australia domains of quartz or plagioclase crystals along subgrain boundaries are preferentially involved in partial melting over unstrained domains of these minerals material along subgrain boundaries in quartz and plagioclase has the same morphology as melt pseudomorphs present along grain boundaries and is commonly laterally continuous with this former grain boundary melt indicating the material along subgrain boundaries can also be categorized as a melt pseudomorph subgrain boundaries consist of arrays of dislocations within a crystal lattice and unlike fractures would not act as conduits for melt migration instead the presence of former melt along subgrain boundaries requires that partial melting occurred in these locations because it is kinetically more favorable for melting reactions to occur there preferential melting in high strain locations may be attributed to strain energy which provides a minor energetic contribution to the reaction and leads to preferential melting in locations with weakened bonds and or the presence of small quantities of water associated with dislocations which may enhance diffusion rates or locally lower the temperature needed for partial melting

tectonic pumping of pervasive granitic melts finite element 2d models are used to study how tectonic stresses pump pervasive granitic melts within migmatites we start by assuming elliptical melt filled veins of different orientations interconnected by the assumption that they share the same pressure the melt is then redistributed in the vein array by the application of lateral compression or extension the effective permeability of vein networks is thought to be up to 13 orders of magnitude higher than the matrix permeability of 10 19 m2 this allows us to approximate pressure equilibration and neglect irreversible compaction in the matrix this simple configuration allows solution of constitutive equations for the equation of state in the viscoelastic matrix the melt in the vein array and the mechanical equilibrium along with force balance on the walls of the deforming veins a rectangular box is pressurized by a constant load applied to one side and continuously deformed by shift of another side with constant velocity new veins cannot initiate and pre existing veins cannot migrate bodily down pressure gradients however potential flaws are cut in line with the tip of every pre defined vein this means that veins not only close open and shear as they rotate but can also undergo limited propagation at rates dictated by the bulk deformation of the matrix we use constant viscosities s in the range 1017 1018 pa s deformation rates e vary between 10 10 and 10 9 s 1 and a total strain up to 4 5 isolated veins parallel and normal to 1 have melt pressures between p0 and p0 4 se the mean vein pressure differs from these extremes and is equilibrated by driving melt from shrinking veins into veins parallel to 1 which widen and lengthen the above assumptions result in an asymptotic pattern of stress distribution and the amount of melt redistributed at given strain approaching the kinematic limit with time this occurs whatever the viscosity of the matrix and the strain rate multilayered systems are modeled by pre defining a single vertical vein crossing the planar horizontal boundary between two uniform media compression parallel to the layering expels melt from the part of the vein in the more viscous layer to open and extend that part of the same vein in the less viscous layer melt moves in the opposite direction during lateral extension if sheet like bodies become sufficiently tall to become buoyant horizontal sheets resulting from lateral compression are likely to rise to higher crustal levels as diapirs while vertical sheets resulting from lateral extension are more likely to ascend as dikes

neoarchean structural evolution of the murchison domain yilgarn craton abstract the progressive strengthening of the continental lithosphere triggered a secular transition in tectonic style during neoarchean times allowing the progressive establishment of plate tectonics on our planet structural investigations in neoarchean cratons offer therefore a critical tool to constrain how such transition took place the highly mineralized yilgarn craton of western australia exposes one of the largest and best preserved portions of archean continental crust worldwide the yilgarn crust was intensely deformed during the c 2730 2650 ma neoarchean yilgarn orogeny which was accompanied by widespread granitic magmatism and was mostly unaffected by post archean pervasive tectonic events representing therefore an ideal area for the study of neoarchean tectono magmatic events here i combine new structural data with a re examination of the recent regional structural literature to offer an updated view of the structural evolution of the murchison domain one of the best exposed and mineralized portions of the yilgarn craton the result is a new fourfold scheme for the structural evolution of the domain supported by a detailed and structurally controlled geochronological dataset the preserved tectono magmatic evolution of the domain started with a pre orogenic period of dominant polydiapirism during times of prevailing tectonic quiescence this was followed by a sequence of orogenic deformation events that produced north striking large scale shear zones along which syntectonic plutons up to 100 km in size were emplaced during three major tectonic pulses characterized by bulk east west shortening between 2730 and 2660 ma the deformation events identified here at the scale of the murchison domain are likely the local expression of yilgarn wide tectono magmatic events highlighting the importance of syntectonic granitic magmatism in shaping neoarchean orogens

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

protolith heterogeneity as a factor controlling the feedback between deformation metamorphism and melting in a granulite hosted gold deposit abstract structures and alteration associated with mineralizing pre anatectic hydrothermal alteration at the challenger gold mine in south australia strongly influenced and preferentially focused subsequent metamorphism partial melting and deformation this event resulted in efficient deformation enhanced melt extraction from the host rocks immediately adjacent to mineralization leaving restitic garnet cordierite k feldspar rich mineral assemblages progressive reintegration of silicate melt into the highly restitic proximal gneiss bulk composition using mineral equilibrium modelling in the system nckfmashto indicates that hydrothermal alteration stabilized a higher proportion of hydrous minerals leading to increased fertility and thus melting in the proximal gneiss further modelling at lower temperatures provides an insight into mineral assemblages that may have been associated with mineralization temperature composition modelling provides evidence that composite gold sulphide melts were armoured by quartz and as such were immobile during peak metamorphism at challenger melt permeability of the mineralized zones was enhanced via the formation of a structural fabric folding boudinage fracturing and the formation of shear zones giving consideration to the relationships and orientations of these structures a model for preferential deformation and melt migration within the mineralized shear zone under transpression is developed where melt is essentially pumped out of the system parallel to the maximum principal elongation direction

is there a close spatial relationship between faults and plutons abstract using spatial relationships between individual plutons and faults to support genetic correlations between faulting and magmatism is meaningless since even random magmatic or tectonic processes will result in some plutons adjacent to faults our initial analyses of populations of faults and carboniferous plutons in the armorican massif france and faults and alleghanian plutons in the southern appalachians usa indicate that plutons have broad distributions with respect to faults but with a tendency for plutons to occur away from faults maxima of integrated pluton areas occur at 1 4 appalachians and 1 2 armorican of the distance between the average fault spacing in these orogens although there is a great need for statistical evaluations of relationships between populations of igneous bodies and structures in a wide variety of settings and crustal depths our initial studies suggest that faults do not preferentially channel magma during ascent or emplacement and that these are relatively unfocused processes within orogenic belts

fluid present disequilibrium melting in neoarchean arc related migmatites of daeijak island western gyeonggi massif korea the melting process of meta igneous rocks was investigated via field petrographic and geochemical analyses of the neoarchean 2 51 ga migmatite complex in daeijak island western gyeonggi massif this complex consists primarily of garnet free amphibolites and tonalitic migmatites both of which contain hornblende plagioclase and quartz as major constituents neosomes and leucosomes in the migmatite have dioritic tonalitic and tonalitic trondhjemitic compositions respectively compositions of hornblende xfe 0 39 0 42 and plagioclase an24 27 vary little between the neosomes and leucosomes the amphibolites show distinct depletions in nb ta zr and ti relative to large ion lithophile elements suggesting an arc related origin for their basaltic protolith leucosomes have lower contents of k2o mgo feo tio2 zr rb and rare earth elements ree than amphibolites and neosomes but are higher in sio2 na2o and sr contents leucosomes and neosomes have positive eu eu n 1 32 7 26 and negative 0 71 0 97 eu anomalies respectively which are attributed to the variable degree of plagioclase fractionation during the partial melting the p t condition for the migmatite formation was estimated to be 700 730 c and 4 7 5 5 kbar primarily based on the hornblende plagioclase thermobarometry and phase equilibria various lines of textural evidence such as the channel flow of melt along migmatitic layers and the segregation of melt into shear bands or boudin necks suggest a syn deformation crystallization of melt chemical disequilibrium in migmatites is documented not only by petrographic and geochemical data but also by the ree modeling between melt product and source rock disequilibrium process is most likely attributed to the rapidity of melt extraction or migration compared to chemical diffusion rate in summary the fluid present disequilibrium melting of dioritic tonalitic protoliths has produced tonalitic trondhjemitic leucosomes in a dynamic arc environment prevalent in the neoarchean crust of the korean peninsula as well as the shandong peninsula north china craton

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