a moderate melting model for the voring margin norway based on structural observations and a thermo kinematical modelling implication for the meaning of the lower crustal bodies high p wave velocities 7 1 7 8 km s lower crustal bodies lcbs imaged along volcanic margins are commonly interpreted as plume and breakup related thick mafic underplating this interpretation is partly challenged in this paper based on new seismic observations and modelling of the outer voring basin norway an exceptional strong amplitude reflection the t reflection is particularly well defined below the north gjallar ridge ngr between 7and 8 s twt the t reflection is located near the volcanic lava flows emplaced during the ne atlantic breakup 55 54 ma ago and coincides with the top of the lcb forming a mid crustal dome based on structural and temporal relationships we show that the dome clearly influences the structural development of the ngr and predates the continental breakup at least by 10 15 ma using a thermo kinematical model we tried also to investigate and quantify the relationships between the extension lcb and the magmatic production modelling suggests that significant paleocene early eocene magmatism can be produced without any temperature anomaly in the mantle if differential stretching occurs during the breakup initiation the conclusion of 2d thermo kinematical parametric analysis is that the magmatic model predicts either little extension 5 with significant melting along the outer voring basin we suggest that the continental part of the lcb could not necessarily be breakup related and so magmatic as has often been stated previously it is concluded here that the continental part of the lcb observed beneath the outer voring basin may be partly or fully attributed to inherited high pressure granulite eclogite lower crustal rocks the real amount of mafic material emplaced along the outer voring basin could be 20 40 less than thought

on the geometric relationship between deformation microstructures in zircon and the kinematic framework of the shear zone abstract we present novel microstructural analyses of zircon from a variety of strained rocks for the first time multiple plastically deformed zircon crystals were analyzed in a kinematic context of the respective host shear zones our aim was to derive how the orientation of zircon grains in a shear zone affects their deformation based on careful in situ observations for sampling we selected zircon bearing rocks that were deformed by simple shear samples covered a range of p t conditions and lithologies including various meta igneous and meta sedimentary gneisses microstructural analyses of zircon crystals in situ with scanning electron backscatter diffraction mapping show strong geometrical relationships between orientations of i the long axes of plastically deformed zircon crystals ii the crystallographic orientation of misorientation axes in plastically deformed zircon crystals and iii the foliation and lineation directions of the respective samples we assume that zircon crystals did not experience post deformation rigid body rotation and thus the true geometric link can be observed the relationships are the following a plastically deformed zircon crystals usually have long axes parallel to the mylonitic foliation plane b crystals with axes oriented at an angle 15 to the foliation plane are undeformed or fractured zircon crystals that have axes aligned parallel or normal to the stretching lineation within the foliation plane develop misorientation and rotation axes parallel to the 001 crystallographic direction zircon grains with the axis aligned at 30 60 to the lineation within the foliation plane often develop either two low miller indices misorientation axes or one high miller indices misorientation axis host phases have a significant influence on deformation mechanisms in a relatively soft matrix zircon is more likely to develop low miller indices misorientation axes than in a relatively strong matrix these relationships are independent of zircon s grain size and shape and reflect the strong geometric control of the macroscopic kinematic rotation axis on the crystal plastic deformation behavior of zircon and on the geometry of its slip systems we describe previously unknown rheological and crystallographic properties of zircon which suggest a tool for deriving an orientation of the plastically deformed zircon crystals with respect to the associated foliation and stretching lineation additionally relationships between zircon deformation microstructures and the macroscopic kinematic framework have implications for zircon geochronology if deformation events result in zircon distortion and as a consequence partial or complete resetting of the zircon isotopic system the age of deformation can be accurately dated

telescoping of three distinct magmatic suites in an orogenic setting generation of hercynian igneous rocks of the albera massif eastern pyrenees abstract the petrogenesis of the hercynian magmatic rocks of the albera massif eastern pyrenees is investigated in this work in order to establish spatial and temporal relationships between deformation episodes high temperature low pressure metamorphism and igneous activity by using fieldwork together with petrological and geochemical methods hercynian magmatic rocks comprise i calc alkaline granitoids of la jonquera composed mainly by biotite and biotite hornblende granodiorites ii the leucogranitic pluton of el castellar iii small mafic complexes composed mainly by amphibole bearing gabbros and diorites and iv stocks of anatectic leucogranites hercynian deformation structures can be classified in two phases d1 and d2 both reflecting progressive deformation first compressional then transpressional closely linked to the development of regional metamorphism spatial relationships between igneous and metamorphic rocks suggest that the emplacement of magmatic bodies was a syn d2 event and was concomitant with the metamorphic climax of the country rocks the compositional and textural variation of the rocks of the mafic complexes is interpreted to reflect fractional crystallization of hydrous mafic magma with a composition similar to that of calc alkaline basalts documenting the involvement of basaltic magmas during the generation of hercynian crustal granitoids the anatectic leucogranites crystallized from peraluminous magmas produced by partial melting of metasediments and orthogneisses similar to those found in the local migmatite zone the similar sr and nd isotope signatures of the el castellar leucogranitic pluton some anatectic leucogranites and the orthogneissess supports the hypothesis that these leucogranites were generated by partial melting of the orthogneisses based on field petrographic geochemical and isotopic lines of evidence la jonquera granodiorite can be clearly distinguished from the gabbros and from the anatectic and el castellar leucogranites the high cao content of the granodiorites precludes an origin by partial melting of local metamorphic rocks and it is suggested that the calc alkaline granitoids la jonquera were produced by fractional crystallization of a granodioritic magma generated by partial melting of metatonalites in the lower crust although the magmatic rocks were emplaced contemporaneously at about 280 my based on rb sr whole rock data they reflect the occurrence of three genetically distinct igneous suites one granodioritic one leucogranitic and one mafic the metamorphic and igneous evolution documented in the albera massif bears strong similarities with those described in the neighbouring hercynian domains from the pyrenees the hercynian regional ht lp metamorphism and magmatism in the pyrenees are two processes linked to the same deep seated thermal anomaly that was generated in a transpressive context

duration of igneous activity in the sesia magmatic system and implications for high temperature metamorphism in the ivrea verbano deep crust abstract the ivrea verbano zone and serie dei laghi of northern italy constitute a virtually complete section of the adriatic continental crust which was intruded at different levels by coeval magmas of permo carboniferous age collectively referred to as the sesia magmatic system at the deepest levels of the section the relationship between magmatic underplating and granulite facies metamorphism in the ivrea verbano zone may be studied we present new zircon u pb age data which significantly extend the record of igneous activity in this terrane and which demonstrate that igneous pulses began intruding the deep and middle crust no later than 314 ma an age corresponding to that inferred for regional granulite facies metamorphism and predating by more than 20 m y the well documented main magmatic pulse at about 292 to 282 ma a gap in age information from 316 3 ma to 276 4 ma observed in the deep crustal granulite facies paragneiss of the ivrea verbano zone overlaps a gap from circa 314 to 283 ma between igneous and reset zircons in a closely associated mafic ultramafic sill however zircon ages demonstrate intrusive activity in the middle and upper crust during this time gap we infer that prolonged maintenance of temperatures above zircon saturation in the deep crustal paragneiss could explain the gap in zircon ages in those rocks viewed in this light a gap in a geochronological record within high grade terranes might not necessarily mean an absence of events but instead may record the occurrence and duration of a thermal peak

phase equilibrium modelling of the amphibolite to granulite facies transition in metabasic rocks ivrea zone nw italy the development of thermodynamic models for tonalitic melt and the updated clinopyroxene and amphibole models now allow the use of phase equilibrium modelling to estimate p t conditions and melt production for anatectic mafic and intermediate rock types at high temperature conditions the permian mid lower crustal section of the ivrea zone preserves a metamorphic field gradient from mid amphibolite facies to granulite facies and thus records the onset of partial melting in metabasic rocks interlayered metabasic and metapelitic rocks allows the direct comparison of p t estimates and partial melting between both rock types with the same metamorphic evolution pseudosections for metabasic compositions calculated in the na2o cao k2o feo mgo al2o3 sio2 h2o tio2 o nckfmashto system are presented and compared with those of metapelitic rocks calculated with consistent endmember data and a x models the results presented in this study show that p t conditions obtained by phase equilibria modelling of both metabasic and metapelitic rocks give consistent results within uncertainties allowing integration of results obtained for both rock types in combination the calculations for both metabasic and metapelitic rocks allows an updated and more precisely constrained metamorphic field gradient for val strona di omegna to be defined the new field gradient has a slightly lower dp dt which is in better agreement with the onset of crustal thinning of the adriatic margin during the permian inferred in recent studies

characterization and p t evolution of melt bearing ultrahigh temperature granulites an example from the anapolis itaucu complex of the brasilia fold belt brazil between garnet orthopyroxene and sillimanite and coronae or sapphirine quartz ultrahigh temperature metamorphism symplectites involving corundum sapphirine spinel cordierite plagioclase sillimanite orthopyroxene biotite and ilmenite the retrograde segment of the p t path was further constrained by biotite producing reactions a composite p t path was constructed

probing the accumulation history of the voluminous toba magma the age and compositional zonation in crystals from the youngest toba tuff record the prelude to earth9s largest quaternary eruption we used allanite crystals to date and decipher this zoning and found that the crystals retain a record of at least 150 000 years of magma storage and evolution the dominant subvolcanic magma was relatively homogeneous and thermally stagnant for 110 000 years in the 35 000 years before eruption the diversity of melts increased substantially as the system grew in size before erupting 75 000 years ago

a laboratory investigation of assimilation at the top of a basaltic magma chamber the fluid dynamic processes affecting the mechanism and amount of assimilation of the roof of a magma chamber have been studied in a series of analogue laboratory experiments an aqueous solution of na 2 co 3 was first cooled and crystallized at the top of a laboratory tank to form a solid roof layer of prescribed composition the tank was then drained and refilled with warmer solution either na 2 co 3 of the same or different composition or in two experiments kno 3 the metal top in all cases was held below the eutectic temperature for the system na 2 co 3 h 2 o the experiments were continued until substantial crystallization and in some cases complete solidification were achieved starting with a dense solid roof layer of na 2 co 3 10h 2 o and ice on the na 2 co 3 side of the eutectic and a warmer eutectic na 2 co 3 solution below there was rapid convective cooling of the liquid layer accompanied by melting of the roof producing a compositional

experimental evidence for high temperature felsic melts formed during basaltic intrusion of the deep crust basaltic intrusion into the continental crust was simulated with melting experiments on layered charges of high alumina olivine tholeiite haot and metapelite at 11 kbar rapid diffusion of cao and na 2 o into the metapelite and h 2 o k 2 o sio 2 and al 2 o 3 into the basalt resulted in spectacular changes in the phase assemblage garnet megacrysts crystallized profusely across the basalt layer cumulate assemblages closely resembled rocks found in underplating localities and glass compositions graded from dacite to rhyolite even at temperatures as high as 1150 c silicic melts can be generated during interaction of basalt with metapelite at temperatures well above the h 2 o saturated granite minimum calculations based on these results and comparison with natural occurrences suggest that high temperature felsic melts with 2 o can form and segregate at rates of 1 cm yr

particle size analysis of the sediment suspended in a proglacial stream glacier de tsidjiore nouve switzerland hourly at a point samples of suspended sediment taken from the outflow stream of glacier de tsidjiore nouve switzerland over a 60 day sampling period n 1440 are shown to be dominantly composed of silt sized particles particle size sem and xrd analyses indicate a subglacial provenance for the suspended sediment temporal variations in particle size and sorting correspond poorly to fluctuations in water discharge being dominated by erratic hour to hour fluctuations and clockwise hysteresis over diurnal flow events examination of grain size and sorting dynamics over snowmelt and icemelt related ablation events during precipitation events and during glacier drainage events enables some inferences to be drawn regarding sediment source areas and supply regimes we conclude that although the bulk of the suspended sediment in the proglacial stream of glacier de tsidjiore nouve is derived directly from subglacial sources with occasional contributions from the valley train during rapid snowmelt and heavy rainfall periods a portion of the suspended load undergoes intermittent flush fall transfer through the proglacial zone which acts as a sediment source during rising flows and as a sink during periods of waning flow

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

production of hybrid granitic magma at the advancing front of basaltic underplating inferences from the sesia magmatic system south western alps italy abstract the permian sesia magmatic system of the southwestern alps displays the plumbing system beneath a permian caldera including a deep crustal gabbroic complex upper crustal granite plutons and a bimodal volcanic field dominated by rhyolitic tuff filling the caldera isotopic compositions of the deep crustal gabbro overlap those of coeval andesitic basalts whereas granites define a distinct more radiogenic cluster sri 0 708 and 0 710 respectively afc computations starting from the best mafic candidate for a starting melt show that nd and sr isotopic compositions and trace elements of andesitic basalts may be modeled by reactive bulk assimilation of 30 of partially depleted crust and 15 30 gabbro fractionation trace elements of the deep crustal gabbro cumulates require a further 60 fractionation of the andesitic basalt and loss of 40 of silica rich residual melt the composition of the granite plutons is consistent with a mixture of relatively constant proportions of residual melt delivered from the gabbro and anatectic melt chemical and field evidence leads to a conceptual model which links the production of the two granitic components to the evolution of the mafic complex during the growth of the mafic complex progressive incorporation of packages of crustal rocks resulted in a roughly steady state rate of assimilation anatectic granite originates in the hot zone of melting crust located above the advancing mafic intrusion upward segregation of anatectic melts facilitates the assimilation of the partially depleted restite by stoping at each cycle of mafic intrusion and incorporation residual and anatectic melts are produced in roughly constant proportions because the amount of anatectic melt produced at the roof is a function of volume and latent heat of crystallization of the underplated mafic melt which in turn produces proportional amounts of hybrid gabbro cumulates and residual melt such a process can explain the restricted range in isotopic compositions of most rhyolitic and granitic rocks of the permo carboniferous province of europe and elsewhere

seismic properties of rocks from an exposure of extended continental crust new laboratory measurements from the ivrea zone abstract compressional wave velocities for new samples from the ivrea verbano zone ivz and strona ceneri zone scz have been measured to 600 mpa in order to better understand the petrologic controls on velocity and to provide an expanded base for seismological models of the crust as represented by these terrains these data coupled with previous measurements show that preferred orientation of mica controls both the velocity hysteresis with pressure and seismic anisotropy of scz and ivz schists and gneisses ivz paragneisses form the basis of a linear velocity density relationship for metagraywackes of various metamorphic grade this relationship falls close to that expected for mafic rocks and overlaps velocity density data for ivz mafic gneisses one dimensional near vertical reflection models based on these data and recent structural reconstructions of the terrain show that lower crustal reflectivity can be related to high reflection coefficients associated with interlayered sequences of upper amphibolite granulite facies paragneiss mafic gneiss and ultramafic rocks transparent upper crustal levels in the model result from the overall similarity of velocities for schists gneisses and granites from the structurally and lithologically complex scz a mean lower crustal velocity of 7 1 km s is calculated for the part of the ivz composed of interlayered mafic gneiss and paragneiss and for another area dominated by a mafic layered igneous complex the velocity density analysis coupled with this observation emphasize the ambiguity inherent in interpretation of crustal composition from p wave velocity data in the absence of other constraints

regional granulite facies metamorphism in the ivrea zone is the mafic complex the smoking gun or a red herring one widely accepted paradigm for the development of continental lower crust is that regional granulite facies metamorphism is caused by intrusion of mafic magma beneath or into the crust magmatic accretion the amphibolite to granulite facies supracrustal section exposed in the ivrea zone southern alps northern italy is commonly cited as a classic example establishing this postulated genetic relationship our interpretation of the pattern of metamorphic isograds compositional trends in high grade metasedimentary rocks and textural evidence in metapelite however indicates that final emplacement of the mafic plutonic rocks the mafic complex occurred subsequent to the regional thermal maximum field and petrographic relations suggest that a spatially restricted contact melting event in crustal rocks accompanied the emplacement of the mafic complex this inference is consistent with leucosome compositions in migmatites and a low pressure high temperature metamorphic overprint recorded by mineral assemblages in wall rocks proximal to the intrusion therefore evidence of anatexis and metamorphism of crustal rocks associated unequivocally with emplacement of the mafic complex is found only within an 2 km wide contact aureole overlying the intrusion the narrow aureole associated with emplacement of the mafic complex demonstrates that in some cases emplacement of large volumes of mafic magma within the crust does not inexorably lead to regional scale granulite facies metamorphism and large ion lithophile element depletion by melt loss

influence of stretching and density contrasts on the chemical evolution of continental magmas an example from the ivrea verbano zone the southern ivrea verbano zone of the italian western alps contains a huge mafic complex that intruded high grade metamorphic rocks while they were resident in the lower crust geologic mapping and chemical variations of the igneous body were used to study the evolution of underplated crust slivers of crustal rocks septa interlayered with igneous mafic rocks are concentrated in a narrow zone deep in the complex paragneiss bearing belt and show evidence of advanced degrees of partial melting variations of rare earth element patterns and sr isotope composition of the igneous rocks across the sequence are consistent with increasing crustal contamination approaching the septa therefore the paragneiss bearing belt is considered representative of an assimilation region where in situ interaction between mantle and crust derived magmas resulted in production of hybrid melts buoyancy caused upwards migration of the hybrid melts that incorporated the last septa and were stored at higher levels feeding the upper mafic complex synmagmatic stretching of the assimilation region facilitated mixing and homogenization of melts chemical variations of granitoids extracted from the septa show that deep septa are more depleted than shallow ones this suggests that the first incorporated septa were denser than the later ones as required by the high density of the first injected mafic magmas it is inferred that density contrasts between mafic melts and crustal rocks play a crucial role for the processes of contamination of continental magmas in thick under plated crust the extraction of early felsic hybrid melts from the lower crust may be required to increase the density of the lower crust and to allow the later mafic magmas to penetrate higher crustal levels

regional metamorphism at extreme conditions implications for orogeny at convergent plate margins abstract regional metamorphism at extreme conditions refers either to alpine type metamorphism at low geothermal gradients of 30 c km extreme pressures refer to those above the polymorphic transition of quartz to coesite so that ultrahigh pressure uhp eclogite facies metamorphism occurs at mantle depths of 80 km extreme temperatures refer to those higher than 900 c at crustal depths of 80 km so that ultrahigh temperature uht granulite facies metamorphism occurs at medium to high pressures while crustal subduction at the low geothermal gradients results in blueschist eclogite facies series without arc volcanism heating of the thinned orogenic lithosphere brings about the high geothermal gradients for amphibolite granulite facies series with abundant magmatism therefore uhp metamorphic rocks result from cold lithospheric subduction to the mantle depths whereas uht metamorphic rocks are produced by hot underplating of the asthenospheric mantle at the crustal depths active continental rifting is developed on the thinned lithosphere in response to asthenospheric upwelling and this tectonism is suggested as a feasible mechanism for regional granulite facies metamorphism with the maximum temperature depending on the extent to which the mantle lithosphere is thinned prior to the rifting while lithospheric compression is associated with subduction metamorphism in accretionary and collisional orogens the thinned orogenic lithosphere undergoes extension due to the asthenospheric upwelling to result in orogen parallel rifting metamorphism and magmatism thus the rifting metamorphism provides a complement to the subduction metamorphism and its operation marks the asthenospheric heating of the orogenic lithosphere because of the partial melting and melt extraction of the lower continental crust contemporaneous granite migmatite granulite associations may serve as a petrological indicator of rifting orogeny that is superimposed on precedingly accretionary and collisional orogens the uht metamorphic rocks have occurred since the archean suggesting that the hot underplating has operated very early in the earth s history in contrast the uhp metamorphic rocks primarily occur in the phanerozoic indicating that the thermal regime of many subduction zones has changed since the neoproterozoic for the cold subduction

a systematic evaluation of the zr in rutile thermometer in ultra high temperature uht rocks the zr in rutile geothermometer is potentially a widely applicable tool to estimate peak metamorphic temperatures in rocks from diverse geological settings in order to evaluate its usefulness and reliability to record and preserve high temperatures in granulite facies rocks rutile from uht rocks was investigated to assess different mechanisms of zr re distribution following cooling from high temperature granulite facies paragneisses from the lowermost part of the ivrea zone italy incorporated as thin sheets into the extensive basaltic body of the mafic complex were selected for this study the results show that zr in rutile thermometry if properly applied is well suited to identify and study uht terranes as it preserves a record of temperatures up to 1190 c although the thermometer is susceptible to partial post peak metamorphic resetting by zr diffusion texturally homogeneous rutile grains preserve zr concentrations corresponding to temperatures of prograde rutile growth diverse rutile textures and relationships between some rutile host grains and included or adjacent zr bearing phases bear testimony to varying mechanisms of partial redistribution and resetting of zr in rutile during cooling and link zr in rutile temperatures to different steps of the metamorphic evolution rutile grains that equilibrated their zr concentrations at temperatures above 1070 c i e 1 1 wt zr could not retain all zr in the rutile structure during cooling and exsolved baddeleyite zro2 by subsequent reaction of baddeleyite exsolution lamellae with sio2 zircon needles formed before the system finally closed at 650 700 c without significant net loss of zr from the whole host rutile grain by reintegration of zircon exsolution needles peak metamorphic temperatures of up to 1190 c are derived for the studied rocks which demonstrates the suitability of this solution thermometer to record uht conditions and also confirms the extraordinary geological setting of the lowermost part of the ivrea zone

large scale folding in the upper part of the ivrea verbano zone nw italy new geological mapping has led to a new interpretation of the large scale superimposed folding in the upper part of the ivrea verbano zone italian alps the region is widely held to represent an upended section through lower continental crust of northern italy the dominant fold structure extending some 40 km along strike is the massone tight to isoclinal antiform with a hinge line strongly curved through 115 this folds pre existing large scale folds that formed during regional migmatization probably during the hercynian orogeny to form a type 2 interference geometry the region then suffered post orogenic mafic magmatic underplating and other magmatism accompanied by crustal stretching with contact metamorphism and migmatization causing the imposition of the final pattern of metamorphic isograds the ivrea verbano zone was brought into contact with the overlying metamorphic rocks of the serie dei laghi on a major shear zone sub solidus stretching continued though displacements on low angle high temperature shear zones most of the ivrea verbano zone was finally tilted to the vertical and emplaced into its present position after the mesozoic era and probably during alpine orogenesis forming the vertical limb of a crustal scale double kink

migmatites granites and orogeny flow modes of partially molten rocks and magmas associated with melt solid segregation in orogenic belts abstract this paper presents a model for the genesis of migmatites and granites during orogenic evolution based on the analysis of several phanerozoic crustal segments and a review of the physical properties of partially molten rocks and magmas this model inventories the modes of bulk flow of partially molten rocks and magmas and the mechanisms of melt solid segregation for each mode partial melting of rocks is associated with a strength decrease of two to three orders of magnitude leading to strain partitioning expressed by channel flow driven either by forces related to plate tectonics vertical channel flow or by the gravity force associated with lateral variations of crustal thicknesses horizontal channel flow although the structural characteristics of migmatites indicate that deformation plays a role in melt migration the emplacement of laccoliths of leucogranites above migmatites attests to the efficiency of the buoyancy force another decrease in apparent strength of about ten orders of magnitude is associated with the loss of continuity of the solid framework marking the transition from partially molten rocks to magmas the breakdown of the solid framework also allows for settling of the solid in suspension increasing the buoyancy of the remaining magma accordingly domes cored by diatexites former heterogeneous magmas and mantled by metatexites former partially molten rocks are interpreted as gravitational instabilities driven by the relative buoyancy of the magma and permitted by the weakness of the partially molten rocks this model provides a first order framework to elucidate the development of a crustal scale horizontal layering during the evolution of orogenic belts that are affected by partial melting in this case the middle crust is dominated by migmatites with domes cored by diatexites and mantled by metatexites that correspond to a partially molten and magmatic zone respectively the granitic dikes and sills and the associated laccoliths of leucogranites correspond to an intrusive zone overlying the partially molten zone the refractory lower crust is potentially in part formed by accumulation of solids segregated from the overlying heterogeneous magmas

permian high temperature metamorphism in the western alps nw italy during the late palaeozoic lithospheric thinning in part of the alpine realm caused high temperature low to medium pressure metamorphism and partial melting in the lower crust permian metamorphism and magmatism has extensively been recorded and dated in the central eastern and southern alps however permian metamorphic ages in the western alps so far are constrained by very few and sparsely distributed data the present study fills this gap we present u pb ages of metamorphic zircon from several adria derived continental units now situated in the western alps defining a range between 286 and 266 ma trace element thermometry yields temperatures of 580 890 c from ti in zircon and 630 850 c from zr in rutile for permian metamorphic rims these temperature estimates together with preserved mineral assemblages garnet prismatic sillimanite biotite plagioclase quartz k feldspar rutile define pervasive upper amphibolite to granulite facies conditions for permian metamorphism u pb ages from this study are similar to permian ages reported for the ivrea zone in the southern alps and austroalpine units in the central and eastern alps regional comparison across the former adriatic and european margin reveals a complex pattern of ages reported from late palaeozoic magmatic and metamorphic rocks and relics thereof two late variscan age groups 330 and 300 ma are followed seamlessly by a broad range of permian ages 300 250 ma the former are associated with late orogenic collapse in samples from this study these are weakly represented clearly dominant is the permian group which is related to crustal thinning hinting to a possible initiation of continental rifting along a passive margin

material and thermal roles of basalt in crustal magmatism case study from eastern nevada thousands of cubic kilometres of calc alkaline intermediate to silicic magma erupted in eastern nevada at 35 ma on the basis of petrologic models crustal contribution to magmatism is slightly greater than basaltic input the volume of crystals generated during differentiation is roughly twice that of erupted magma implying that intrusive complements are enriched in accumulated crystals the amount of basalt needed to drive the system thermally can be derived from the volume of crust that is heated and assimilated and is about twice the volume of basalt needed materially

formation of high δ18o fayalite bearing a type granite by high temperature melting of granulitic metasedimentary rocks southern china the genesis of a type granites has been controversial fayalite granite is a member of the most reduced a type granites commonly thought to have been primarily sourced from tholeiitic rocks in this paper we report petrography whole rock geochemistry sr nd isotope and in situ zircon hf o isotope results for a fayalite bearing a type granite suite at jiuyishan in southern china high zircon 18 o 8 0 9 8 negative zircon ehf t 6 2 to 2 3 and evolved whole rock sr nd isotopes i sr 0 7151 0 7181 end t 7 4 to 6 6 indicate the reworking of old supracrustal rocks isotopic and geochemical results particularly downward inflections of zr and ba at 70 wt sio 2 point to fractional crystallization rather than magma mixing as the controlling process for the evolution of the igneous suite integrated petrological geochemical and isotopic studies present the first convincing case that reduced a type granites can also be generated by high temperature 960 c melting of granulitic metasedimentary rocks likely related to the upwelling of the asthenosphere and or underplating and intrusion of basaltic melts we emphasize that key factors for the genesis of this unique rock type are low oxygen fugacity f o 2 low f h 2 o and high temperature

high resolution carbon isotopic records from the ordovician of south china links to climatic cooling and the great ordovician biodiversification event gobe abstract the great ordovician biodiversification event gobe represented the largest increase in diversity in the marine biosphere during the phanerozoic eon yet its causes and consequences remain poorly understood patterns of isotopic variation in high resolution 13ccarb and 13corg records from a well exposed section at honghuayuan in south china may provide important insights regarding the gobe the honghuayuan isotopic profiles which can be correlated with c isotopic records from contemporaneous sections globally reveal large perturbations to the global carbon cycle during the ordovician a 8 increase in 13corg values in the floian implies a large albeit transient increase in the burial rate of organic matter during the mid early ordovician that may have contributed to climatic cooling and played an important role in triggering the gobe a 4 increase in 13ccarb and high frequency variation in 13corg in the darriwilian to sandbian suggest a second episode of elevated organic carbon burial rates accompanied by substantial instability in the global carbon cycle during the late middle and early late ordovician this pattern may mark the onset of climate changes culminating in the end ordovician hirnantian glaciation and mass extinction event that terminated the gobe

metamorphism and anatexis in the mafic complex contact aureole ivrea zone northern italy emplacement of mantle derived magma magmatic accretion is the total thermal budget of the continental lower crust often presumed or inferred to be an important cause of regional produce regional granulite facies metamorphism and granulite facies metamorphism and crustal anatexis the juxgenerate yand heavy rare earth element hree taposition of mafic cumulates and regionally distributed granulite depleted granitoids ellis 1987 accretion of mafic facies rocks has led some to consider the ivrea zone northern italy magma and migration of partial melt will internally southern alps as an important exposure that demonstrates this stratify chemically differentiate and deplete the concausal relationship however regional ptt paths indicated by tinental lower crust in large ion lithophile elements metamorphic reaction textures and pt conditions inferred from lile magmatic accretion has been invoked to provide geothermobarometry indicate that the emplacement of mafic plutonic the heat and mass necessary for sustained magmatism in rocks mafic complex at the ivrea zone occurred during detectonic settings such as phanerozoic extensional terranes compression from ambient pressures at the regional thermal maximum lister et al 1986 gans 1987 fountain 1989 mareschal field and petrographic observations supported by pt estimates bergantz 1990 jarchow et al 1993 and magmatic indicate that regional retrograde decompression and emplacement of arcs hamilton 1981 kay kay 1981 bohlen the upper parts of the mafic complex probably accompanied lindsley 1987 hildreth moorbath 1988 extension during the late carboniferous early permian a spatially however as few exposed sections of lower continental restricted decompression melting event accompanied final emcrust show contiguous mafic intrusions and regionally placement depleting supracrustal rocks enclosed by an 2 3 km distributed granulite facies rocks estimates of the extent aureole overlying the upper mafic complex by 20 30 granite of anatexis and metamorphism accompanying magmatic component the upper mafic complex provided the thermal energy accretion have relied on numerical and analog simto reset mineral assemblages and locally overprint the regional ulations these models yield disparate results depending prograde metamorphic zonation the limited extent of the contact on whether heat transfer within the mafic intrusion aureole suggests that magmatic accretion may not inexorably cause and surrounding country rocks is primarily convective regional metamorphism and crustal anatexis campbell turner 1987 huppert sparks 1988 or conductive marsh 1989 bergantz dawes 1994 barboza bergantz 1996 a comparison with field evidence is required to discriminate between the model

isotopic evidence from the ivrea zone for a hybrid lower crust formed by magmatic underplating the large mafic complex overlying mantle peridotites in the ivrea zone of northern italy originated by underplating of mantle derived magma onto the base of continental crust and may be a unique outcrop of typical mafic lower crust sampled elsewhere only by mafic xenoliths neodymium and strontium isotope data show that the magmas assimilated large and roughly uniform proportions of crustal material the resulting hybrid magmas have the same isotopic signature as and may represent the lower crustal roots of upper crustal hercynian granitoid intrusions

the origins of granulites a metamorphic perspective although many recent reviews emphasize a uniformity in granulite pressure temperature p t conditions and paths granulites in reality preserve a spectrum of important petrogenetic features which indicate diversity in their modes of formation a thorough survey of over 90 granulite terranes or occurrences reveals that over 50 of them record p t conditions outside the 7 5 1 kbar and 800 50 c average granulite regime preferred by many authors in particular an increasing number of very high temperature 900 1000 c terranes are being recognized both on the basis of distinctive mineral assemblages and geothermobarometry petrogenetic grid and geothermobarometric approaches to the determination and interpretation of p t histories are both evaluated within the context of reaction textures to demonstrate that the large range in p t conditions is indeed real and that both near isothermal decompression itd and near isobaric cooling ibc p t paths are important amphibolite granulite transitions promoted by the passage of co 2 rich fluids as observed in southern india and sri lanka are exceptional and not representative of fluid related processes in the majority of terranes it is considered on the contrary that fluid absent conditions are typical of most granulites at or near the time of their recorded thermal maxima itd granulites are interpreted to have formed in crust thickened by collision with magmatic additions being an important extra heat source erosion alone is not however considered to be the dominant post collisional thinning process instead the itd paths are generated during more rapid thinning 1 2 mm yr exposure related to tectonic exhumation during moderate rate or waning extension ibc granulites may have formed in a variety of settings those which show anticlockwise p t histories are interpreted to have formed in and beneath areas of voluminous magmatic accretion with or without additional crustal extension ibc granulites at shallow levels very rapid 5 mm yr extensional thinning subsequent to collision it is suggested that the preservation of ibc paths rather than itd paths in many granulites is primarily related to the rate and timescale of extensional thinning of thickened crust and that hybrid itd to ibc paths should also be observed most ibc granulites and probably many itd granulites have not been exposed at the earth s surface as a result of the tectonic episodes which produced them but have resided in the middle and lower crust for long periods of time 100 2000 ma following these events the eventual exhumation of most granulite terranes only occur through their incorporation in later tectonic and magmatic events unrelated to their formation

chemical evolution of a large mafic intrusion in the lower crust ivrea verbano zone northern italy the ivrea verbano and adjacent strona ceneri zones have been described collectively as a section through the continental crust while resident in the lower crust amphibolite to granulite facies paragneiss of the ivrea verbano zone was intruded by huge volumes of mafic to intermediate plutonic rocks grouped as the mafic complex growth of the mafic complex involved hypersolidus deformation in an extensional environment isotopic and trace element variations close to the axis of this structure indicate crystallization from mantle derived melts that were extensively contaminated by crustal material previous investigations determined that the contaminant was fingerprinted by 87sr 86sr 0 71 18o 10 12 5 and a positive eu anomaly in the present study the contaminant is also shown to have been enriched in ba with respect to rb and k charnockites associated with paragneiss septa in the lower part of the mafic complex have the appropriate chemistry to be samples of the contaminating material these chemical features can be explained by melting of granulite facies paragneiss which had previously been depleted in k and rb by an earlier melting event the ba enrichment in the core of the mafic complex can be modeled by a replenishment tapping fractional crystallization rtf process operating within a small magma chamber is repeatedly replenished by mantle melts and contaminated by ba rich charnockite very high ba k in the lower part of the complex are tentatively attributed to chemical exchange between the cumulate framework and infiltrating anatectic melts from underlying paragneiss septa in contrast to the mafic complex the chemistry of coeval granites in the adjacent strona ceneri zone reflect a component derived from crustal rocks that had not been significantly depleted by a previous melting event significantly the incompatible trace element abundances in the mafic complex and strona ceneri granites are similar to model compositions for the lower and upper crust respectively

magmatic underplating extension and crustal reequilibration insights from a cross section through the ivrea zone and strona ceneri zone northern italy the thermal impact of magmatic underplating at various crustal levels is studied along a traverse through the ivrea verbano zone and strona ceneri zone in northern italy geochronological and petrologic data are compared to a two dimensional thermal kinematic model field data and numerical simulation show the strong disturbance of the temperature field in the lower and intermediate crust in relation to magmatic underplating leading to granulite to amphibolite facies metamorphism as well as reequilibration of mineral chemical and isotopic systems magmatic underplating leaves a crust with an apparently heterogeneous tectonometamorphic evolution as information on the earlier history is preserved only at upper crustal levels

water saturated and undersaturated melting of metaluminous and peraluminous crustal compositions at 10 kb evidence for the origin of silicic magmas in the taupo volcanic zone new zealand and other occurrences

nb and zr behavior in rutile during high grade metamorphism and retrogression an example from the ivrea verbano zone abstract detailed textural observations and in situ analyses emp sims and la icp ms are used to characterize trace element behavior during prograde and retrograde metamorphic reactions involving rutile the ivrea verbano zone is a classic granulite area and rocks from the strona and d ossola valleys are an example of the amphibolite to granulite facies transition although different rock types occur in the area detailed sampling and petrographic work show that rutile only occurs in granulite facies paragneisses these rocks show a rich inventory of textures that allow not only for the investigation of trace element behavior in response to prograde rutile growth but also for the effect of post peak processes on rutile chemistry nb concentrations in rutile from lower grade samples show a larger spread from 500 to 5000 ppm within one sample when compared to those from higher grades this pattern can be modeled using prograde rutile growth formed from biotite breakdown zr concentrations in rutile are characterized by an anomalously large spread and a bimodal distribution maximum zr concentrations increase according to the general metamorphic gradient known for this area temperatures from zr in rutile thermometry although feasible are considerably higher than previous calculations increasing from ca 850 to 930 c a second cluster of zr concentrations in rutile occurs at rather constant concentrations ca 1000 ppm for all localities and is interpreted to be related to intense fluid influx at high temperature and or to post peak diffusional resetting favored by slow cooling rates alteration textures characterized by a complex network of micro veins are evidence for the late fluid influx the fluid strongly affected the rutiles which is evidenced by corrosion of older rutile grains and formation of rutile veinlets

surface water freshening and high latitude river discharge in the eocene north sea abstract a shark tooth apatite 18 o record of the early palaeogene north sea reflects changes in regional hydrography by showing variable temperatures and salinities a 2 4 ma period in the early eocene was particularly influenced by substantial surface water freshening indicated by a 3 4 reduction of 18 o values the magnitude of the 18 o decrease indicates a depletion in 18 o of surface waters by 2 3 relative to eocene mean ocean water this value is lower than that of coeval lakes reconstructed from freshwater gastropod 18 o values from the paris basin suggesting that large rivers with high latitude catchment areas drained into the north sea the period of surface water freshening began close to the palaeocene eocene thermal maximum when relative sea level fall tectonic uplift and basaltic volcanism caused a temporary isolation of the north sea north atlantic and north sea surface waters became reconnected during a series of early eocene transgressions

silurian granulite facies metamorphism and coeval magmatism and crustal growth in the tongbai orogen central china abstract it is intriguing whether the formation of granulite in an orogenic belt is related to magmatism and crustal growth the tongbai orogen is one of the particular areas in the qinling tongbai dabie sulu orogen that contains both granulites and associated arc magmatic rocks and thus is an optimal place to address this issue however the formation age and genesis of the granulites and their relations to the magmatic rocks have not yet been well constrained in situ la mc icpms u pb dating and trace element and lu hf isotope analyses were undertaken on zircon grains from gneiss granulite gabbro and granodiorite samples most zircons in three granulite samples and a gneiss sample have core rim structures the cores have oval or rounded shapes oscillatory zones high th u ratios and variable u pb ages suggesting a detrital genesis the youngest group of the detrital zircons yields a weighted mean 206 pb 238 u age of 450 5 ma which is interpreted as the maximum depositional age of their protoliths a major age cluster is ca 450 490 ma corresponding to prolonged arc magmatism in the northern tongbai and qinling orogens another group of ages range from 660 to 950 ma implying that the qinling group of the tongbai orogen belongs to the south china block scb metamorphic zircons in the granulites have weak or no zoning low th u ratios and ree contents negative eu anomalies and relatively flat hree patterns which indicate that they formed contemporaneously with garnet and plagioclase and thus under granulite facies metamorphic conditions their weighted mean age of 424 4 ma is taken as the best estimated age of the granulite facies metamorphism zircons in a leucosome sample have significant negative eu anomalies and relatively flat hree patterns with a formation age of 428 4 ma indicating partial melting coeval with the granulite facies metamorphism metamorphic zircons in the gneiss show apparently negative eu anomalies and variable th u ratios and hree contents they yield a weighted mean 206 pb 238 u age of 438 4 ma which was suggested to record the timing of the prograde metamorphism zircon grains in a gabbro and a granodiorite yield u pb ages of 432 4 and 424 4 ma respectively which are interpreted as their formation ages and identical to the ages of the granulite facies metamorphism they have positive e hf t values of up to 5 1 showing clear evidence for crustal growth during their formation the appearance of the silurian granulite facies metamorphism and magmatism in the tongbai orogen provides a good paradigm of coeval granulite facies metamorphism magmatism and crustal growth in an arc continent collision orogenic belt

the effect of crystal plastic deformation on isotope and trace element distribution in zircon combined bse cl ebsd feg empa and nanosims study plastically deformed zircon grains from granulite facies ivrea verbano zone southern alps italy and amphibolite facies tauern window eastern alps austria shear zones have been investigated the main focus was on the effects of crystal plastic deformation on the distribution of trace elements and their isotopes zircon grains reveal crystal plastic deformation patterns in form of i gradual bending of the lattice high density of free dislocations ii highly deformed margins revealing a combination of low and high angle boundaries and gradual bending of the lattice and iii a low angle boundary lab network chemical and isotopic maps and profiles show that trace elements in zircon are re distributed in all plastically deformed domains changes in trace element composition along labs are sometimes revealed by brighter cl and darker bse signal labs and domains of high free dislocation density have depletion in u y yb and enrichment in ce la and nd while ti and p are either enriched depleted or remain unaffected and hf demonstrates stability y and yb are decreased in concentration across labs and have oscillating concentration in domains of high free dislocation density our observations confirm that crystal plastically deformed domains in zircon act as effective pathways for trace cations the pb isotopic system is disturbed by crystal plastic deformation microstructures as indicated by relative 207 pb 206 pb ages showing significant discordance in plastically deformed zircon domains in deformed domains a positive correlation between dislocation density and the degree of isotopic age distortion is observed fractured or porous domains and domains with high density of sub grain boundaries are enriched in common pb from the matrix and thus show significantly older relative 207 pb 206 pb ages than pristine domains in contrast pb loss occurs in domains with high free dislocation density at a distance of 5 10 m from the grain boundary caused by out diffusion of radiogenic pb this study adds to a growing database on the consequences of crystal plastic deformation for trace elements and isotopic systems in zircon we provide additional evidence that such deformation cannot be neglected and may have important implications for zircon geochronology and geochemistry

zircon and monazite petrochronologic record of prolonged amphibolite to granulite facies metamorphism in the ivrea verbano and strona ceneri zones nw italy abstract in order to improve the understanding of thermal tectonic evolution of high grade terranes we conducted a systematic study of textures ree content and u pb ages of zircon and monazite grains extracted from migmatitic metapelites across the amphibolite to granulite facies metamorphic gradient exposed in the ivrea verbano and strona ceneri zones italy this study documents the behaviour of these accessory minerals in the presence of melt the absence of relict monazite grains in the metasediments and the gradual decrease in the size of inherited zircon grains from amphibolite to granulite facies cores indicate partial to total dissolution of accessory minerals during the prograde path and partial melting the retrograde path is marked by i growth of new zircon rims r1 and r2 around inherited cores in the mesosome ii crystallisation of stubby zircon grains in the leucosome especially at granulite facies and iii crystallisation of new monazite in the mesosome stubby zircon grains have a distinctive fir tree zoning and a constant th u ratio of 0 20 together these features reflect growth in the melt conversely the new zircon grains with r1 rims have dark prismatic habits and th u ratios in the ivrea verbano zone the interquartile range iqr of u pb ages on zircon and monazite are interpreted to represent the length of zircon and monazite crystallisation in the presence of melt accordingly they provide an indication on the minimum duration for high temperature metamorphism and partial melting of the lower crust 20 ma and 30 ma in amphibolite and granulite facies respectively in amphibolite facies zircon crystallisation between 310 and 294 ma iqr is interpreted to reflect metamorphic peak condition and earlier retrograde history conversely monazite crystallisation between 297 and 271 ma iqr reflects cooling under 750 c to a temperature close to the solidus in granulite facies zircon crystallisation between 295 and 265 ma iqr is interpreted to reflect high temperature conditions which were attained after peak of metamorphism during isothermal decompression and subsequent cooling under 850 950 c the observed decrease of u pb ages in metamorphic zircon and monazite from amphibolite to granulite facies i e from the middle to the lower crust is interpreted to record slow cooling and crystallisation of the variscan orogenic root at the transition from orogenic collapse to opening of the tethys ocean

constraints on the thermal evolution of the adriatic margin during jurassic continental break up u pb dating of rutile from the ivrea verbano zone italy the ivrea verbano zone ivz northern italy exposes an attenuated section through the permian lower crust that records high temperature metamorphism under lower crustal conditions and a protracted history of extension and exhumation associated partly with the jurassic opening of the alpine tethys ocean this study presents shrimp u pb geochronology of rutile from seven granulite facies metapelites from the base of the ivz collected from locations spanning 35 km along the strike of paleozoic fabrics rutile crystallised during permian high temperature metamorphism and anatexis yet all samples give jurassic rutile u pb ages that record cooling through 650 550 c rutile age distributions are dominated by a peak at 160 ma with a subordinate peak at 175 ma both 160 and 175 ma age populations show excellent agreement between samples indicating that the two distinctive cooling stages they record were synchronous on a regional scale the 175 ma population is interpreted to record cooling in the footwall of rift related faults and shear zones for which widespread activity in the lower jurassic has been documented along the western margin of the adriatic plate the 160 ma age population postdates the activity of all known rift related structures within the adriatic margin but coincides with extensive gabbroic magmatism and exhumation of sub continental mantle to the floor of the alpine tethys west of the ivrea zone we propose that this 160 ma early post rift age population records regional cooling following episodic heating of the distal adriatic margin likely related to extreme lithospheric thinning and associated advection of the asthenosphere to shallow levels the partial preservation of the 175 ma age cluster suggests that the post rift 160 ma heating pulse was of short duration the regional consistency of the data presented here which is in contrast to many other thermochronometers in the ivz demonstrates the value of the rutile u pb technique for probing the thermal evolution of high grade metamorphic terrains in the ivz a significant decoupling between zr in rutile temperatures and u pb ages of rutile is observed with the two systems recording events 120 ma apart

exposed cross sections through the continental crust implications for crustal structure petrology and evolution abstract the continental crust is exposed in cross section at numerous sites on the earth s surface these exposures which appear to have formed by obduction along great faults during continental collision may be recognized by exposures of deep crustal rocks exhibiting asymmetric patterns of metamorphic grade and age across the faults and by distinctive bouguer anomaly patterns reflecting dipping basement structure and an anomalously deep mantle from an examination of five complexes which meet these criteria it is concluded that the most prominent layering in the crust is not compositional but metamorphic the lower crust consists of granulite facies rocks of mafic to intermediate composition while the intermediate and shallow levels consist predominantly of amphibolite facies gneisses and greenschist facies supracrustal rocks respectively post metamorphic granitic intrusions are common at intermediate to shallow levels position of discontinuities in refraction velocity where present commonly correspond to changes in composition or metamorphic grade with depth the continental crust is characterized by lateral and vertical heterogeneities of varying scale which are the apparent cause of the complex seismic reflections recorded by cocorp field observations coupled with geochemical data indicate a complex evolution of the lower crust which can include anatexis multiple deformation polymetamorphism and reworking of older crustal material the complexity of the crust is thus the result of continuous evolution by recycling and metamorphism through time in a variety of tectonic environments

high pressure high temperature seismic velocity structure of the midcrustal and lower crustal rocks of the ivrea verbano zone and serie dei laghi nw italy the ivrea verbano zone and the adjacent serie dei laghi in the inner arc of the western alps nw italy display an upended cross section through much of the thickness of the continental crust as it existed in that region in triassic jurassic time we collected a suite of oriented rock samples that represent most of the volume of the rocks of the region p and s wave velocity measurements were made in three orthogonal directions related to the mesoscopic fabric of the rocks at room temperature and up to 550 mpa confining pressure combined high temperature and high pressure measurements were made up to 700 c on a subset of the samples the lithologic units were divided into 23 different principal rock types and we present velocity data averaged in terms of these groups vertical and horizontal velocity and anisotropy sections are computed from the measured velocity data based on a restored geologic section these show that vp and vs both increase and anisotropy decreases systematically with depth of burial reflecting variations in rock type and metamorphic grade with depth vp and vs anisotropy arises mainly because of crystallographic preferred orientation or mineralogical banding with the slowest direction tending to be normal to the foliation or banding the vp velocity sections were used to compute synthetic seismic reflection profiles for the region when it lay at the bottom of the continental crust these correspond well with contemporary deep reflection profiles for regions of comparable tectonic evolution

granulite facies thermal aureoles and metastable amphibolite facies assemblages adjacent to the western fiordland orthogneiss in southwest fiordland new zealand in southwest new zealand a suite of felsic diorite intrusions known as the western fiordland orthogneiss wfo were emplaced into the mid to deep crust and partially recrystallized to high p 12 kbar granulite facies assemblages this study focuses on the southern most pluton within the wfo suite malaspina pluton between doubtful and dusky sounds new mapping shows intrusive contacts between the malaspina pluton and adjacent palaeozoic metasedimentary country rocks with a thermal aureole 200 1000 m wide adjacent to the malaspina pluton in the surrounding rocks thermobarometry on assemblages in the aureole indicates that the malaspina pluton intruded the adjacent amphibolite facies rocks while they were at depths of 10 14 kbar similar p t conditions are recorded in high p granulite facies assemblages developed locally throughout the malaspina pluton palaeozoic rocks more than 200 1000 m from the malaspina pluton retain medium p mid amphibolite facies assemblages despite having been subjected to pressures of 10 14 kbar for 5 myr these observations contradict previous interpretations of the wfo malaspina pluton as the lower plate of a metamorphic core complex everywhere separated from the metasedimentary rocks by a regional scale extensional shear zone doubtful sound shear zone slow reaction kinetics lack of available h2o lack of widespread penetrative deformation and cooling of the malaspina pluton thermal anomaly within c 3 4 myr likely prevented recrystallization of mid amphibolite facies assemblages outside the thermal aureole if not for the evidence within the thermal aureole there would be little to suggest that gneissic rocks which underlie several 100 km2 of southwest new zealand had experienced metamorphic pressures of 10 14 kbar similar high p metamorphic events may therefore be more common than presently recognized

borehole water level variations and the structure of the subglacial hydrological system of haut glacier d arolla valais switzerland late summer subglacial water pressures have been measured in a dense array of boreholes in the ablation area of haut glacier d arolla switzerland interpolated surfaces of minimum diurnal water pressure and diurnal water pressure variation suggest the presence of a subglacial channel within a more widespread distributed drainage system the channel flows along the centre of a variable pressure axis vpa some tens of metres wide that is characterized by low minimum diurnal water pressures frequently atmospheric and high diurnal water pressure variations these characteristics are transitional over a lateral distance of c 70 m to higher and more stable subglacial water pressures in the adjacent distributed system water pressure variations recorded in boreholes located close to the centre of the vpa reflect the delivery of surface derived meltwater to the glacier bed and result in a diurnally reversing transverse hydraulic gradient that drives water out from the channel into the distributed system during the afternoon and back to the channel overnight subglacial observations suggest that such flow occurs through a vertically confined sediment layer borehole turbidity records indicate that the resulting diurnal water flows are responsible for the mobilization and transport of fine debris in suspension analysis of the propagation velocity and amplitude attenuation of the diurnal pressure waves suggests that the hydraulic conductivity of the sediment layer decreases exponentially with distance from the channel falling from c 10 4 m s 1 at the channel boundary to c 10 7 ms 1 70m away these apparent hydraulic conductivities are consistent with darcian flow through clean sand and typical glacial till respectively we suggest that fine material is systematically flushed from basal sediments located adjacent to large melt season drainage channels beneath warm based glaciers this process may have important implications for patterns of glacier erosion hydrochemistry and dynamics

subglacial till a physical framework for its properties and processes the continuum theory of mixtures provides a framework around which present knowledge of subglacial till can be organized the proposed framework is provisional in two senses the constitutive description of till will require improvement and not all potentially significant processes have been included till is treated as a three component mixture of water clasts coarse grained solids and fine grained matrix solids these components obey mass balance equations and their behavior is subject to constitutive relations that describe such processes as darcian water transport fine grained sediment transport consolidation shear deformation dilatancy and comminution five state variables uniquely determine the compressibility permeability shear strength and other physical properties numerical solutions of the state evolutions equations illustrate that the properties of till are highly responsive to changes in the subglacial environment

large ion lithophile element characteristics of an amphibolite facies to granulite facies transition at gruinard bay north west scotland abstract lewisian grey gneisses from gruinard bay north west scotland retain mineralogical and geochemical evidence for scourian horn blende granulite facies metamorphism and they may be used to assess current models of elemental depletion at granulite grade their immobile major and trace element geochemistry is indistinguishable from that of lewisian amphibolite and pyroxene granulite facies counterparts the k rb th and u contents of the gruinard bay gneisses are depleted relative to amphibolite facies gneisses but generally the abundances of these elements are above those of comparable pyroxene granulites u and th have reached an advanced stage of depletion but allanite appears to be crucial in maintaining significantly higher u and th abundances at gruinard bay than in pyroxene granulites k and rb loss is less extreme and depends on the stability of the rock forming minerals k feldspar biotite and amphibole early removal of k and rb has resulted in a small rise in k rb but further preferential rb loss would have been required to generate the characteristically high k rb ratios of lewisian pyroxene granulites the residence of u and th in the accessory minerals of granulite facies gneisses which are often correlated with the residua of intracrustal partial melting renders unlikely their extreme incompatibility required by such models even if such phases are ignored high mineral melt partition coefficients for silicic melts argue against partial fusion as an efficient depletion mechanism on the other hand the advanced stage of u and th depletion reached in gruinard bay gneisses which were still partly hydrous severely restricts the role played by co2 dominated fluids and a hydrous medium is preferred

evolution of major and trace element composition during melt migration through crystalline mush implications for chemical differentiation in the crust we present the first quantitative model of heat mass and both major and trace element transport in a mush undergoing compaction that accounts for component transport and chemical reaction during melt migration and which is applicable to crustal systems the model describes the phase behavior of binary systems both eutectic and solid solution with melt and solid compositions deter mined from phase diagrams using the local temperature and bulk composition trace element concentration is also determined the results demonstrate that component transport and chemical reaction generate compositional variation in both major and trace elements that is not captured by existing geochemical models in particular we find that even for the simplest case of a homogenous insulated column that is instantaneously melted then allowed to compact component transport and reaction leads to spatial variations in major element composition that in this case produces melt that is more enriched in incompatible elements than predicted by batch melting in deep crustal hot zones dchz created by the repeated intrusion of hot mantle derived magmas buoyant melt migrating upwards accumulates in high porosity layers but has a composition corresponding to only a small fraction of batch melting because it has locally equilibrated with mush at low temperature moreover melt migration and chemical reaction in a layered protolith may lead to the rapid formation of high porosity melt layers at the interface between different rock compositions in both of these cases the melt in the high porosity layer s is less enriched in incompatible trace elements than predicted if it is assumed that melt with the same major element composition was produced by batch melting this distinctive decoupling of major and trace element fractionation may be characteristic of magmas that originate in dchz application of the model to a number of crustal systems including the ivrea verbano zone the rum layered intrusion and the holyoke flood basalt suggests that composi tional heterogeneity can be explained by buoyancy driven melt migration and compo nent transport through a reactive crystalline mush

high temperature reduced granulite facies nature of garnetites in the khabarny mafic ultramafic massif southern urals evidence from fluid and mineral analyses