geothermobarometry of high grade metapelites simultaneously operating reactions s u m m a r y experimental data on equilibria involving rock forming minerals in highgrade metapelites garnet orthopyroxene cordierite and plagioclase have been used to derive an internally consistent system of mineralogical thermometers and barometers based on component reactions and excess mixing properties of solid solutions application of these data to a granulite facies metapelite sample reveals that compositional inhomogeneity of minerals on a thin section scale is likely to be a result of several reactions opening simultaneously at the retrograde stage of metamorphic evolution the widely accepted practice of estimating pressure and temperature from separate reactions may thus lead to errors and misinterpretation of p t paths

noble gas loss may indicate groundwater flow across flow barriers in southern nevada average calculated noble gas temperatures increase from 10 to 22 c in groundwater from recharge to discharge areas in carbonate rock aquifers of southern nevada loss of noble gases from groundwater in these regional flow systems at flow barriers is the likely process that produces an increase in recharge noble gas temperatures emplacement of low permeability rock into high permeability aquifer rock and the presence of low permeability shear zones reduce aquifer thickness from thousands to tens of meters at these flow barriers which are more than 1 000 m lower than the average recharge altitude noble gases exsolve from the groundwater by inclusion in gas bubbles formed near the barriers because of greatly reduced hydrostatic pressure however re equilibration of noble gases in the groundwater with atmospheric air at the low altitude spring discharge area at the terminus of the regional flow system cannot be ruled out molecular diffusion is not an important process for removing noble gases from groundwater in the carbonate rock aquifers because concentration gradients are small

petrogenesis p t t path and tectonic significance of high pressure mafic granulites from the jiaobei terrane north china craton abstract high pressure hp mafic granulites in the jiaobei terrane are composed predominantly of garnet mafic granulites garnet hypersthene granulites and garnet amphibolites and they are found as irregular lenses or deformed dike swarms within tonalitic trondhjemitic granodioritic gneisses and granitic gneisses the hp mafic granulites contain four distinct metamorphic assemblages of which the early prograde assemblage m 1 is represented by the cores of garnets together with mineral inclusions of clinopyxene plagioclase quartz and it formed at 740 770 c and 0 90 1 00 gpa in contrast the peak assemblage m 2 consists of high ca cores in garnet high al cores in clinopyroxene and high na cores in plagioclase in the matrix which formed under p t conditions of 850 880 c and 1 45 1 65 gpa the peak metamorphism was followed by near isothermal decompression m 3 which resulted in the development of orthopyroxene clinopyxene plagioclase quartz amphibole magnetitie symplectites or coronas surrounding some garnet grains with p t conditions of 780 830 c and 0 65 0 85 gpa surrounding some garnet grains are symplectites of amphibole plagioclase quartz magnetitie which formed during a cooling retrograde stage m 4 with p t conditions of 590 650 c and 0 62 0 82 gpa an integrated study involving laser raman analysis of mineral inclusions cathodoluminescence imaging and in situ u pb dating of zircons shows that the protolith ages of the hp mafic granulites are mainly 2550 2500 ma and that the timing of the peak metamorphism of the hp mafic granulites ranges from 1900 to 1860 ma as recorded by the cores of metamorphic zircons on the other hand the medium to low pressure granulite amphibolite facies retrogression occurred mainly at 1860 1820 ma as recorded by the rims of some single zircon grains and the zircon grains that contain inclusions of garnet orthopyroxene plagioclase sphene the combination of petrography mineral compositions metamorphic reaction history thermobarometry and geochronology defines a near isothermal decompressional clockwise p t t path for the jiaobei hp mafic granulites suggesting that the jiaobei terrane underwent initial crustal thickening during 1950 1860 ma followed by relatively rapid exhumation cooling and retrogression in the period 1860 1820 ma this tectonothermal path was probably generated by subduction and collision related tectonic processes

metamorphic consequences of thrust emplacement fall mountain new hampshire metamorphic pressure temperature p t paths from the upper and lower plates of the fall mountain nappe southwest new hampshire reveal different thermal histories in the two structural levels upper plate rocks experienced early low p high t contact metamorphism to peak p t conditions of 700 750 c 3 4 5 kbar these p t conditions were followed by loading to approximately 5 6 kbar and then by nearly isobaric cooling to approximately 500 c 4 5 kbar upper plate rocks then experienced minor heating with unloading and finally cooling the lower plate rocks also experienced early low p high t contact metamorphism but only to p t conditions of 480 510 c 2 3 5 kbar nappe emplacement resulted in nearly isothermal loading to 500 c 5 6 kbar which was followed by either heating or cooling depending on the proximity of the sample to the upper plate the early pressures recorded in both the upper and lower plate rocks suggest a depth of approximately 10 km for origination of the nappe the post nappe pressures 5 6 kbar imply a total tectonic thickness of approximately 20 km or a doubling of the overburden thickness diffusion calculations suggest that the emplacement of the nappe took less than approximately 10 m y and u pb chronology on zircon suggests that it occurred between 400 and 410 ma 4o ar 39 ar cooling ages on micas indicate that the terrane was cooled to approximately 300 c by 340 ma

metamorphic interpretation of high pressure temperature metapelites with preserved growth zoning in garnet eastern grenville province canadian shield high pressure temperature metapelites that occur in close proximity to eclogitized mafic rocks in the southern part of the gagnon terrane parautochthonous belt eastern grenville province were investigated in order to constrain depths of burial and p t paths mineral assemblages and partial melting relationships in these metapelites are consistent with peak temperatures in the range between 700 and 800 c however growth zoning is apparently well preserved in garnets and only narrow rims width 100 500 m are obviously affected by diffusional retrograde resetting despite uncertainties regarding mineral assemblages and compositions of matrix minerals at early stages of garnet growth it can be shown that the observed growth zoning profiles of garnets imply increase of both pressure and temperature up to a common maximum at pressures between 1300 and 1600 mpa and that thermal relaxation did not occur during the initial stages of unloading on the other hand calculated retrograde p t conditions are consistent with steep decompression paths the inferred hair pin shaped p t path is consistent with independent evidence of rapid tectonically driven exhumation resulting in the preservation of growth zoning in garnets from such a high temperature regime

the northern margin of the grenville province in western labrador anatomy of an ancient orogenic front abstract in western labrador the northern margin of the grenville province known as the grenville front tectonic zone abuts against the superior foreland zone composed of an archean gneiss terrain and against the churchill foreland zone composed of weakly metamorphosed and deformed aphebian supracrustals in both terrains a similar tectonic style is developed in which aphebian rocks from within the grenville orogen have been thrust northwards over the adjacent foreland the grenville front is marked by mylonite phyllonite zones in the archean gneisses of the superior foreland whereas in the supracrustals it is characterized by cleavage folding grenvillian deformation involved 3 episodes of which two gave rise to northeast trending structural elements and are interpreted to be part of a single protracted event involving northerly translation of rocks from within the orogen towards the foreland a third deformation comprised large scale crossfolds and may be related to the formation of a major arcuate thrust salient spatially the northeast trending structural elements occur in the grenville front tectonic zone while the crossfolds dominate in the interior of the orogen a zone of interference structures related to the two fold trends occurs in the southern part of the grenville front tectonic zone the hudsonian front westerly limit of hudsonian deformation in the churchill province passes through the churchill foreland zone in the study area there grenvillian structures are superimposed on older hudsonian structures elsewhere in the area studied however all deformation and metamorphism of aphebian rocks is of grenvillian age metamorphic zones in pelitic rocks indicate a progressive increase in grade towards the interior of the grenville orogen the occurrence of the mineral pair kyanite k feldspar preceding the assemblage sillimanite k feldspar in the prograde sequence indicates that peak metamorphic conditions may have exceeded 8 kb and 700 c such elevated pressures during metamorphism may have been achieved by stacking of thrust slices in the grenville front tectonic zone both deformation and postorogenic uplift were diachronous in the grenville province deformation was initiated in the high grade core of the orogen and migrated northwards towards the margin whilst uplift occurred earlier in the margin than in the core

a model for garnet and plagioclase growth in pelitic schists implications for thermobarometry and p t path determinations numerical models of the progressive evolution of pelitic schists in the ncmnkfmash system with the assemblage garnet biotite chlorite staurolite plagioclase muscovite quartz h2o are presented with the goal of predicting compositional changes in garnet and plagioclase along different p t paths the numerical models support several conclusions that should prove useful for interpreting the p t paths of natural parageneses i garnet may grow along p t vectors ranging from heating with decompression to cooling with compression p t paths deduced from garnet zoning that are inconsistent with these growth vectors are self contradictory ii there is a systematic relation between garnet and plagioclase composition and growth such that for most p t paths garnet growth requires plagioclase consumption furthermore mass balance in a closed system requires that as plagioclase is consumed the remaining plagioclase becomes increasingly albitic inclusions of plagioclase in the core of garnet should be more anorthitic than those near the rim and zoned matrix plagioclase should have rims that are more albitic than the cores complex plagioclase textures may arise from the local variability of growth and precipitation kinetics iii a decrease of fe fe mg in a garnet zoning profile is a reliable indicator of increasing temperature for the assemblage modelled however there is no single reliable p monitor and inferences about p can only be made by considering plagioclase and garnet together iv consumption of garnet during the production of staurolite removes material from the outer shell of a garnet and may make recovery of peak metamorphic compositions and p t conditions impossible low peak temperatures typically recorded by staurolite bearing assemblages may reflect this phenomenon v diffusional homogenization of garnet affects the computed p t path and results in a clockwise rotation of the computed p t vector relative to the true p t path

different origins of garnet in high pressure to ultrahigh pressure metamorphic rocks abstract garnet in high pressure hp to ultrahigh pressure uhp metamorphic rocks in subduction zone commonly shows considerable zonation in major and trace elements as well as mineral inclusions which bears information on its growth mechanism via metamorphic or peritectic reactions in coexistence with relic minerals and metamorphic fluids or anatectic melts at subduction zone conditions it provides an important target to retrieve physicochemical changes in subduction zone processes including those not only in pressure and temperature but also in the durations of metamorphism and anatexis garnet from different compositions of hp to uhp metamorphic rocks may show different types of major and trace element zonation as well as mineral inclusions discrimination between the different origins of garnet provides important constraints on pressure and temperature and the evolution history for the hp to uhp metamorphic rocks magmatic garnet may occur as relics in granitic gneisses despite metamorphic modification at subduction zone conditions with spessartine increasing or flat major element profiles from inner to outer core and exceptionally higher contents of trace elements than metamorphic mantle and rim metamorphic garnet can grow at different metamorphic stages during prograde subduction and retrograde exhumation with spessartine decreasing from core to rim if the intracrystalline diffusion is not too fast the compositional profiles of metamorphic garnet in the abundances of grossular almandine and pyrope are variable depending on the composition of host rocks and co existing minerals peritectic garnet grows through peritectic reactions during partial melting of hp to uhp rocks with the composition of major elements to be controlled by anatectic p t conditions and the compositions of parental rocks and anatectic melts trace element profiles in garnet with different origins are also variable depending on the coexisting mineral assemblages the garnet forming reactions and the property of metamorphic fluids or anatectic melts mineral inclusions not only present key clues to identify the different origins of garnet but also serve as sound candidates for the temporal constraint on garnet growth

transformation of fe ti gabbro to coronite eclogite and amphibolite in the baie du nord segment manicouagan imbricate zone eastern grenville province fe ti gabbros from the baie du nord segment of the manicouagan imbricate zone metamorphosed under high p t conditions during the grenvillian orogeny have been the focus of a detailed micropetrological study textures and mineral chemistry suggest that the mineral assemblages represent progressive stages of metamorphic transformation resulting in the formation of coronas pseudomorphs after igneous phases transitional and true granoblastic eclogites the transitional and eclogitic samples also have coronas which are developed locally around igneous xenocrysts of plagioclase and olivine the deformed margins of coronitic fe ti gabbros are transformed to amphibolite and contain clinopyroxene bearing leucosomes with garnet poikiloblasts that are indicative of high p t dehydration melting interpretation of garnet zoning and thermobarometry suggest that the highest p t conditions are recorded by coronas around xenocrysts c 720 800 c at 14 17 kbar and garnet clinopyroxene cores in granoblastic assemblages c 740 820 c at 13 17 kbar in the eclogitic samples re equilibration during the early stages of exhumation at high t conditions 700 c affected all samples and is evidenced by the widespread development of pargasite bearing plagioclase collars in the coronitic and transitional metagabbros and by widespread re equilibration of the eclogites giving lower p t estimates at grain boundaries however the difference in calculated pressure conditions between coronite and eclogite samples is consistent with increasing pressure depth from the coronites 11 13 kbar to the eclogites 13 17 kbar the p t conditions recorded by these rocks define a metamorphic field gradient which suggests high heat flow through the lower crust during the grenvillian orogeny

on the interpretation of peak metamorphic temperatures in light of garnet diffusion during cooling abstract finite difference models of fe mg diffusion in garnet undergoing cooling from metamorphic peak conditions are used to infer the significance of temperatures calculated using garnet biotite fe mg exchange thermometry for rocks cooled from high grades where the garnet was initially homogeneous the calculated temperature tcalc using garnet core and matrix biotite depends on the size of the garnet the ratio of garnet to biotite in the rock vgarnet vbiotite and the cooling rate for garnets with radii of 1 mm and vgarnet vbiotite 1 tcalc is 633 700 and 777 c for cooling rates of 1 10 and 100 c ma for vgarnet vbiotite 1 and 4 and a cooling rate of 10 c ma tcalc is approximately 660 and 610 c respectively smaller and larger garnets have lower and higher tcalc respectively these results suggest that peak metamorphic temperatures may be reliably attained from rocks crystallized at conditions below tcalc of the garnet core provided that vgarnet vbiotite is sufficiently small 0 1 and that the composition of the biotite at the metamorphic peak has not been altered during cooling numerical experiments on amphibolite facies garnets with nominal peak temperatures of 550 600 c generate a well in fe fe mg near the rim during cooling maximum calculated temperatures for the assemblage garnet chlorite biotite muscovite plagioclase quartz using the fe fe mg at the bottom of the well with matrix biotite range from 23 43 c to 5 12 c below the peak metamorphic temperature for cooling rates of 1 and 100 c ma respectively maximum calculated temperatures for the assemblage garnet staurolite biotite muscovite plagioclase quartz are approximately 70 c below the peak metamorphic temperature and are not strongly dependent on cooling rate the results of this study indicate that it may be very difficult to calculate peak metamorphic temperatures using garnet biotite fe mg exchange thermometry on amphibolite facies rocks tmax 550 c because the rim composition of the garnet which is required to calculate the peak temperature is that most easily destroyed by diffusion

eclogites and polyphase p t cycling in the caledonian uppermost allochthon in troms northern norway eclogites in the tromso area northern norway are intimately associated with meta supracrustals within the uppermost allochthon of the scandinavian caledonides the tromso nappe complex the whole sequence which includes pelitic to semipelitic schists and gneisses marbles and calc silicate rocks quartzofeldspathic gneisses metabasites and ultramafites has undergone three main deformational metamorphic events d1 m1 d2 m2 and d3 m3 detailed structural microtextural and mineral chemical studies have made it possible to construct separate p t paths for these three events chemically zoned late syn to post d1 garnets with inclusions of bt pl and qtz in ky bearing metapelites indicate a prograde evolution from 636 c 12 48 kbar to c 720 c 14 15 kbar this latter result is in agreement with grt cpx geothermometry and grt cpx pl qtz geobarometry on eclogites and trondhjemitic to dioritic gneisses maximum pressures at c 675 c probably reached 17 18 kbar based on cpx pl qtz inclusions in eclogitic garnets and grt ky pl qtz and jd ab qtz in trondhjemitic gneisses post d1 pre d2 decompressional breakdown of the high p assemblages indicates a substantial drop in pressure at this stage inclusions and chemical zoning in syn to post d2 garnets from metapelites record a second episode of prograde metamorphism from 552 c 7 95 kbar passing through a maximum pressure of 10 64 kbar at 644 c with final equilibration at c 665 c 9 10 kbar the corresponding apparently co facial paragenesis grt cpx pl qtz in metabasites yields c 635 c 8 10 kbar in the metapelites post d3 grt in apparent equilibrium with bt phe and pl yield c 630 c 9 kbar the d1 m1 and d2 m2 episodes are exclusively recorded in the tromso nappe complex and must thus pre date the emplacement of this allochthonous unit on top of the underlying lyngen nappe while the d3 m3 episode is common for the two units a previously published sm nd mineral isochron grt cpx am on a partly retrograded and recrystallized ecologite of 598 107 ma represents either the timing of formation of the eclogites or the post eclogite pre d2 decompression stage while a rb sr whole rock isochron of an apparently post d1 pre d2 granite of 433 11 ma is consistent with a k ar age of post d1 pre d2 amphiboles from a retrograded eclogite of 437 16 ma which most likely record cooling below the 475 500 c isotherm after the m3 metamorphism

metamorphic p t paths from pelitic schists and greenstones from the south west tauern window eastern alps petrological data from intercalated pelitic schists and greenstones are used to construct a pressure temperature path followed by the upper schieferhulle ush series during progressive metamorphism and uplift in the south west tauern window italy pseudomorphs of al epidote fe epidote albite oligoclase chlorite after lawsonite and data on amphibole crystal chemistry indicate early metamorphism in the lawsonite albite chlorite subfacies of the blueschist facies at p 7 8 kbar geothermometry and geobarometry yield conditions of final equilibration of the matrix assemblage of 475 25 c 5 6 kbar calculations with plagioclase and phengite inclusions in garnet indicate early garnet growth at pressures of 7 5 kbar garnet zoning patterns are complex and reversals in zoning can be correlated between samples thermodynamic modelling of these zoning profiles implies garnet growth in response to four distinct phases of tectonic activity fluid inclusion data from coexisting immiscible h2o co2 nacl fluids constrain the uplift path to have passed through temperatures of 380 30 c at 1 3 0 2 kbar there is no evidence for metamorphism of ush at pressures greater than 7 5 kbar in this area of the tauern window this is in contrast to pressures of 10 kbar recorded in the lower schieferhulle only 2 3 km across strike a history of differential uplift and thinning of the intervening section during metamorphism is necessary to reconcile the p t data obtained from these adjacent tectonic units

effects of diffusional modification of garnet growth zoning on p t path calculations the effect of intragranular diffusion on chemical zoning in garnet and on p t paths calculated from that zoning was evaluated using a numerical model of multicomponent diffusion in combination with simulations of garnet growth syn and post growth diffusion of mg fe mn ca species in garnet was calculated for a model pelitic assemblage over a range of temperatures from 485 to 635 c compositions from zoned garnet as modified by diffusion hypothetical inclusions of plagioclase within garnet and matrix phases were used to reconstruct pressure temperature p t paths from isobaric and polybaric model histories p t path calculations based on numerical simulations conducted over an input isobaric heating path that reached peak temperatures between 585 and 635 c show that relaxation of garnet compositional gradients by diffusion can induce modest to appreciable curvature in the inferred paths retrieved paths also indicated somewhat smaller overall temperature changes relative to the actual temperature difference of the input path the magnitude of these distortions is shown to depend upon the heating and cooling rate and garnet crystal size as well as the actual peak temperature condition the effect of diffusion on path trajectories in simulations with thermal histories that also included cooling were comparable to heating only models that reached peak temperatures approximately 15 30 c higher compositions of garnets with radii less than 1 mm that reached actual peak temperatures of 605 c along temperature time histories characteristic of regional metamorphism experienced sufficient diffusional relaxation to introduce errors of hundreds of bars to in excess of one kilobar in path trajectories path distortions were significant at heating cooling rates less than 10 c ma but rapidly diminished for rates faster than this in polybaric simulations diffusion effects were least noticeable when the actual pressure temperature conditions changed in a clockwise sense i e convex to higher p and higher t but apprecciable modification was seen in path models that underwent counterclockwise changes in p and t reequilibration of garnet rim compositions occurred during cooling on all paths and temperature maxima obtained from garnet biotite geothermometry underestimated actual peak conditions by 40 to 70 c calculations suggest that p t path trajectories calculated from garnets of at least 1 mm size and that experienced actual thermal maxima below 585 c are not likely to be distorted by diffusional effects during regional metamorphism however p t path reconstructions based on garnet zonation with smaller grains or higher temperatures may lead to misinterpretation of crystallization history the partitioning record of peak metamorphic temperatures may be destroyed by diffusional reequilibration of garnet rim compositions during cooling seriously complicating the task of quantitatively estimating diffusion effects on path calculations

contrasting tectonic styles in the northern grenville province implications for the dynamics of orogenic fronts in western labrador tectonic elements of the parautochthonous belt of the northern grenville orogen include a regional mid crustal thin skinned metamorphic fold thrust belt structurally overlain by a thick skinned thrust sheet with a high pressure metamorphic signature in central ontario the parautochthonous belt is composed of a major reverse sense shear zone along which deep crustal rocks have been uplifted and exhumed tectonic models for both areas involve crustal shortening by delamination in the lower crust and tectonically driven exhumation up a crustal scale ramp the fold thrust belt northwest of the ramp in labrador is a result of the development of critical topography in the thrust wedge at the top of the ramp and the accretion of thrust slices to the base of the wedge during its propagation onto the foreland the fold thrust belt is absent in ontario because syntectonic erosion of the thrust wedge prevented the development of critical topography

metamorphic fractional crystallization and internal metasomatism by diffusional homogenization of zoned garnets metamorphic fractional crystallization involves fractionation of material into the interior of a crystallizing mineral such as garnet with consequent change in the effective bulk composition in assemblages with thermodynamic variance of 3 or greater the mineral assemblage compositions of coexisting phases and the zoning profile preserved in the garnet are a function of the amount of material that is fractionated homogenization of zoned garnet by intracrystalline diffusion at elevated temperatures operates on a time scale of millions to tens of millions of years depending on the temperature time path and radius of the garnet one effect of homogenization is to drive chemical reactions by a metasomatic mechanism internal metasomatism even in the absence of changes in pressure or temperature this process may affect the appearance or disappearance of minerals and may in some rocks be responsible for the first appearance of kyanite or the disappearance of staurolite in barrovian terranes

eclogitized gabbros from the eastern grenville province textures metamorphic context and implications undeformed lenses of olivine orthopyroxene gabbro belonging to the 1429 ma shabogamo intrusive suite parautochthonous belt of the eastern grenville province host the first occurrence of eclogite mineral assemblages reported from the grenville province these rocks preserve igneous textures overprinted by eclogite mineralogy and by textures related to subsequent unloading high pressure signatures include omphacite pseudomorphs after augite plagioclase replacement by garnet kyanite corundum and orthopyroxene omphacite garnet coronas between olivine and plagioclase transformations related to decompression include replacement of garnet kyanite corundum by spinel plagioclase growth of plagioclase collars between clinopyroxene and garnet and de jadeitization of omphacite clinopyroxenes with numerous plagioclase inclusions and garnets surrounded by clinopyroxene plagioclase symplectites in granoblastic mafic rocks from the same area are also evidence of decompression from high pressures these

monazite behaviour and age significance in poly metamorphic high grade terrains a case study from the western musgrave block central australia abstract integrated in situ textural chemical and electron microprobe age analysis of monazite grains in a migmatitic metapelitic gneiss from the western musgrave block central australia has identified evidence for multiple events of growth and recrystallisation during poly metamorphism in the mesoproterozoic garnet sillimanite bearing metapelite underwent partial melting and segregation to palaeosome and leucosome during metamorphism between 1330 and 1296 ma with monazite grains in leucosome recording crystallisation at 1300 ma monazite breakdown during melting is inferred to have occurred in the palaeosome during a subsequent granulite facies event at 1200 ma deformation and metamorphism of leucosome and palaeosome resulted in partial disturbance of ages and potential minor growth on 1300 ma monazite in leucosome growth of new high y hree monazite in palaeosome domains occurred during garnet breakdown in the presence of sillimanite to cordierite and spinel as a result of post peak isothermal decompression diffusive enrichment of resorbed garnet rims in y hree suggests garnet breakdown occurred slower than volume diffusion of ree monazite in both palaeosome and leucosome were subsequently partially to penetratively recrystallised during a retrogression event that is suggested to have occurred at 1150 1130 ma the intensity of recrystallisation and disturbance of ages appears linked to proximity to retrogressed garnet porphyroblasts and their occurrence in the relatively reactive or fertile local environments provided by the palaeosome mesosome volumes which caused localised changes in retrogressive fluids towards compositions more aggressive to monazite like reaction textures it is apparent that domainal equilibrium and reaction may control or at least strongly influence monazite ree and u th pb chemistry and hence ages

partial melting of metapelites in the gagnon terrane below the high pressure belt in the manicouagan area grenville province pressure temperature p t and u pb age constraints and implications metapelites of the parautochthonous gagnon terrane at the footwall of the high pressure high p belt in the manicouagan area central grenville province preserve an impressive textural record of partial melting reactions mainly in polymineralic inclusions within garnet the dominant textures were developed within the pressure temperature p t field of the continuous dehydration melting reaction biotite kyanite or sillimanite plagioclase quartz garnet k feldspar melt with sillimanite instead of kyanite in the southern part of the footwall inferred p t paths have a hair pin form in the range of 750 850 c and 1000 1500 mpa for the kyanite bearing rocks monazite crystallization ages are consistently late grenvillian either 995 or 985 ma and one sample contains monazite of both ages two of these samples also contain inherited monazite with ages of 1738 5 and 1719 30 ma indicative of an earlier metamorphic event tonalite and diorite from the same area yield archean zircon crystall

progressive metamorphism of pelitic and quartzofeldspathic rocks in the grenville province of western labrador tectonic implications of bathozone 6 assemblages aphebian metapelites and quartzofeldspathic rocks from the grenville province south of the labrador trough display progressive changes in mineral assemblages as a result of grenvillian metamorphism consistent with variation in grade from greenschist to upper amphibolite facies the following metamorphic zones have been delineated i chlorite muscovite ii chlorite muscovite biotite iii chlorite muscovite biotite garnet iv muscovite staurolite kyanite v muscovite garnet biotite kyanite vi muscovite garnet biotite kyanite granitic veins vii k feldspar kyanite granitic veins viii k feldspar sillimanite granitic veins reactions linking the lower grade metamorphic zones are interpreted to be dehydration phenomena whilst anatectic reactions occur at higher grades at lower metamorphic grades ah2o was high but it declined progressively as water entered the melt phase during higher grade anatectic reactions with the onset of vapour absent anatexis the restite assemblage became essenti

new tectonic divisions of the grenville province southeast canadian shield on the basis of geological geophysical and geochronological data the grenville province has been divided into three first order longitudinal belts the parautochthonous belt pb allochthonous polycyclic belt apb and allochthonous monocyclic belt amb these are set apart by three first order tectonic boundaries the grenville front gf allochthon boundary thrust abt and monocyclic belt boundary zone mbbz the belts are subdivided into terranes based on internal lithological character the gf separates the archean to proterozoic foreland northwest of the orogen from reworked equivalents to the southeast continuous at the scale of the orogen its main characteristic is that of a crustal scale contraction fault the pb although less clearly identified along the length of the orogen in most places represents upgraded and tectonically reworked rocks of the adjacent foreland the boundary between the pb and the apb to the southeast the abt is most clearly delineated in the eastern half of the province it is the locus of major crustal delamination along which high grade mostly middle proterozoic polycyclic terranes were tectonically transported northwest toward and onto the pb the amb comprises two separate areas underlain by the wakeham supergroup and what is currently known as the grenville supergroup respectively its basal contact the mbbz is a decollement zone of variable kinematic significance between older polycyclic rocks and tectonically overlying monocyclic rocks this first order zonation implies a tectonic polarity to the grenville province superimposed on which are second order features evident from contrasting tectonic styles and radiometric ages these characteristics are consistent with a diachronous or oblique collisional model for the grenville orogen

a comparative study of diffusion profiles in himalayan and dalradian garnets constraints on diffusion data and the relative duration of the metamorphic events cation diffusion rates at 690 30 c have been calculated by inverse modelling of observed manganese mn zonation profiles in 40 garnets from two kyanite bearing metapelite samples from the high himalayan crystalline series zanskar northwest india knowledge of the initial growth profile of mn in garnet is a pre requisite for this technique following previous workers we model mn partitioning into growing garnet in terms of a rayleigh fractionation process and demonstrate that the mngarnet whole rock partition coefficient is 60 100 three dimensional zonation profiles were obtained by successively grinding and polishing 1 cm slabs of each sample at 0 1 0 2 mm intervals and analysing the garnets at each stage thus ensuring that core sections were measured the diffusion model assumes that garnet has a spherical geometry and behaves as a closed system and simulates diffusive modification of the hypothetical mn rayleigh growth profile for each garnet the derived measure of the time integrated diffusion history for each garnet is then combined with radiometric and field relation constraints for the duration of the himalayan metamorphic event to calculate cation diffusion rates the average cation interdiffusion rate calculated for garnets in the two samples examined is 6 3 2 10 23 m2s 1 this interdiffusion rate pertains to a temperature of 690 30 c which is 0 97 t peak the peak temperature conditions experienced by the samples estimated using standard thermobarometric techniques garnet compositions are py2 17alm65 77gro6 16sp1 17 these new diffusion data are consistent with and more precise than existing high temperature 1000 c experimentally determined diffusion data although some uncertainties remain difficult to constrain qualitative comparison between diffusively modified mn growth profiles in garnets from the scottish dalradian and the himalayan garnets suggests that the duration of metamorphism affecting the dalradian garnets was 10 20 times longer than that endured by the himalayan garnets

formation of metamorphic and metamorphosed garnets in the low t uhp metagranite during continental collision in the dabie orogen abstract a combined study of major and trace elements in garnet was carried out for low t uhp metagranite in the dabie orogen the results show different fashions of element zoning in the garnet suggesting occurrence of both metamorphic and metamorphosed garnets three generations of garnet growth are distinguished on the basis of the assumption that ca contents mn contents and fe mg ratios of garnet in metagranites are a function of pressure and temperature the first generation of garnet grt i occurs in the core of a skeletal garnet showing homogeneously low x grs values and fe mg ratios but high x sps values this implies that the core garnet grew at the highest temperature but the lowest pressure representing a metamorphosed residue of magmatic garnet in protolith granite the second generation of garnet grt ii occurs in the mantles and cores of many garnet grains exhibiting increased x grs values but decreased fe mg ratios this is ascribed to a continuous increase in temperature and pressure till the peak pressure corresponding to metamorphic growth or overgrowth during the prograde subduction the third generation of garnet grt iii occurs in the rims of all garnet grains displaying decreased x grs values and fe mg ratios in response to a pressure decrease but a temperature increase till the peak temperature these rims overgrew subsequent to the peak pressure with continuous heating during the initial exhumation grt i shows steep mree hree patterns and profoundly negative eu anoalies consistent with growth from granitic melt this kind of metamorphosed garnet from protolith granite has still preserved very high contents of many trace elements such as ree rb ba sr pb th u nb and ta despite the low t uhp metamorphism grt ii and grt iii in one sample exhibit steep mree hree patterns with a continuous decrease in ree contents this suggests their growth from the almost same matrix of mineral assemblages plagioclase k feldspar muscovite during metamorphism however grt ii in the other sample displays flat to steep mree hree patterns with an increase of ree contents from core to mantle this implies that the matrix of mineral assemblages for grt ii changes from common rock forming minerals e g feldspar muscovite and biotite to ree rich minerals e g epidote allanite zircon amphibole apatite and titanite grt iii in the two samples all displays significantly lowered ree contents compared to those of grt i and grt ii this may be due to a decrease in pressure and an increase in temperature during the initial exhumation therefore the two stage growth of metamorphic garnet is evident in the low t uhp metagranite during the continental subduction zone metamorphism in addition the highest pressure occurs in the cores or mantles whereas the highest temperature occurs in the rims this suggests that the peak pressure p max did not occur contemporarily at the peak temperature t max corresponding to the hot exhumation

thermobarometry in granulites pitfalls and new approaches abstract computer models of cooling granulite facies rocks with the assemblage garnet biotite sillimanite k feldspar plagioclase quartz are presented that incorporate simultaneous reaction net transfer and exchange and multicomponent diffusion in garnet in models where cooling is accompanied by exchange reactions plus diffusion only the calculated garnet core matrix biotite temperature is always lower than the metamorphic peak by an amount that depends on the cooling rate the size of the garnet and the volumetric ratio of garnet to biotite maximum calculated core temperatures of approximately 700 750 c are to be expected from typical granulite facies garnet biotite assemblages and calculated core temperatures can easily be as low as 600 c in models where both net transfer and exchange reactions are possible calculated garnet core matrix biotite temperatures are typically higher than the metamorphic peak temperatures by more than 150 c for large garnets r 2 mm but may be lower than the metamorphic peak for small garnets r the composition of matrix plagioclase in equilibrium with garnet is very sensitive to the amount of modal plagioclase that reacts and hence calculated p t trajectories are very sensitive to the petrologic interpretation of this recrystallization history for models in which all the modal plagioclase reacts the core and rim pressures may be faithfully recovered provided that the temperature estimate for the core is reliable for models in which only a small percentage of the modal plagioclase reacts very small modal amounts of plagioclase may be in equilibrium with garnet rim which if incorrectly interpreted may lead to grossly erroneous p t path interpretations a method to test an inferred p t path model is presented that involves attempting to match the observed garnet and plagioclase zoning information with a forward model that involves simultaneous reaction and diffusion an example from western new hampshire usa is presented that closely matches the measured profile

growth and retrograde zoning in garnets from high grade metapelites implications for pressure temperature paths a previously unrecognized pattern of garnet zoning has been preserved in garnets from the upper amphibolite facies britt domain ontario grenville province some large garnets contain discrete regions of high grossular content as well as variations in the mg mg fe and mn nearly parallel to grain boundaries the latter are superimposed on and cut across regions of high grossular content such crosscutting zoning patterns have not been previously recognized in garnets the ca zoning involves changes of 4 to 10 mol grossular and is interpreted to represent growth zoning whereas the decreasing mg mg fe and increasing mn from core to rim are believed to be a diffusional retrograde effect the ca zoning indicates a pressure drop of ca 5 kbar at 700 50 c

a petrogenetic grid for pelitic schists in the system sio 2 al 2 o 3 feo mgo k 2 o h 2 o a quantitative petrogenetic grid for pelitic schists in the system kfmash that includes the phases garnet chlorite biotite chloritoid cordierite staurolite talc kyanite andalusite sillimanite and pyrophyllite with quartz h2o and muscovite or k feldspar in excess is presented the grid is based on thermodynamic data of berman et al 1985 and berman 1988 for endmember kfash and kmash equilibria and natural fe mg partitioning for the kfmash system calculation of p t slopes and the change in fe fe mg along reactions in the kfmash system were made using the gibbs method in addition the effect on the grid of mno and cao is evaluated quantitatively the resulting grid is consistent with typical buchan and barrovian parageneses at medium to high grades at low grades the grid predicts an extensive stability field for the paragenesis chloritoid biotite which arises because of the unusual facing of the reaction chloritoid biotite quartz h2o garnet chlorite muscovite which proceeds to the right with increasing t in the kfmash system however the reaction proceeds to the left with increasing t in the mnkfash system so the assemblage chloritoid biotite is restricted to bulk compositions with high fe fe mg mn typical metapelites will therefore contain garnet chlorite at low grades rather than chloritoid biotite

kyanite eclogite to amphibolite fades evolution of hydrous mafic and pelitic rocks adula nappe central alps in the southern adula nappe central alps two stages of regional metamorphism have affected mafic and pelitic rocks earlier eclogite facies with a regional zonation from glaucophane eclogites to kyanite hornblende eclogites was followed by a tertiary overprint which varied from greenschist to high grade amphibolite facies despite a common metamorphic history contrasting equilibration conditions are often recorded by high pressure mafic eclogite and adjacent predominantly lower pressure pelite assemblages this pressure contrast may be explained by different overprinting rates of the two bulk compositions during unloading the rates are controlled by a mechanism in which dehydrating metapelites provide the h2o required for simultaneous overprinting of enclosed mafic eclogites by hydration

erratum the high pressure belt in the grenville province architecture timing and exhumation we propose that the grenvillian allochthonous terranes may be grouped into high pressure hp and low pressure lp belts and examine the hp belt in detail in the western and central grenville province the hp belt is developed in paleo and mesoproterozoic rocks of the pre grenvillian laurentian margin and characterized by grenvillian eclogite and co facial hp granulite in mafic rocks pressure temperature p t estimates for eclogite facies conditions in well preserved assemblages are about 1800 mpa and 850 c in the central grenville province hp rocks formed at 1060 1040 ma and underwent a single stage of unroofing with transport into the upper crust by 1020 ma whereas farther west they underwent two stages of unroofing separated by penetrative mid crustal recrystallization before transport to the upper crust at 1020 ma unroofing processes were comparable in the two areas involving both thrusting and extensional faulting in an orogen propagating into its foreland by understacking in detail thrusting episodes preceded extension in the western grenville province whereas in the central grenville province they were coeval resulting in unroofing by tectonic extrusion in the central grenville province the footwall ramp is well preserved but any former ramp in the western grenville province was obliterated by later lower crustal extensional flow continuation of the hp belt into the eastern grenville province is not established but likely on geological grounds however the pattern of deep crustal seismic reflection in the lithoprobe eastern canadian shield onshore offshore transect ecsoot line contrasts with that father west suggesting that if present the hp rocks were exhumed by a different mechanism

mafic to felsic magmatism and crustal recycling in the obonga lake greenstone belt western superior province evidence from geochemistry nd isotopes and u pb geochronology abstract the central wabigoon subprovince of the western superior province contains mesoarchean granitoid and supracrustal rocks 3 01 2 83 ga and 2 78 2 69 ga granitoid plutons and supracrustal sequences it is a key area for understanding the relationship between greenstone belts and surrounding granitoid rocks that may have acted as basement the obonga lake greenstone belt contains two distinct assemblages 1 a ta nb th they have end values of 0 7 to 2 4 a dacitic unit in the northern assemblage has high la yb high sr y low nb y and heavy ree and is interpreted to represent a mantle modified slab melt similar to adakites in cenozoic arcs the associated enriched basalts may represent melts from the mantle wedge modified by slab melt adakitic metasomatism or they may represent an enriched oib like asthenospheric source rocks of the southern assemblage mostly show th and light ree enrichment and negative nb and ta anomalies low end values down to 0 9 can be modelled through contamination of a mafic liquid by 3 3 3 2 ga sialic crust although rocks of that age have not been observed suggested mesoarchean basement to the southern assemblage may therefore have had a complex history spanning several hundred million years the southern assemblage volcanism may have occurred in a continental arc to continental back arc system where crustal recycling played an important role

petrological and geochronological constraints on high pressure high temperature metamorphism in the snowbird tectonic zone canada the upper deck of the east athabasca mylonite triangle eamt northern saskatchewan canada contains mafic granulites that have undergone high p t metamorphism at conditions ranging from 1 3 to 1 9 gpa 890 960 c coronitic textures in these mafic granulites indicate a near isothermal decompression path to 0 9 gpa 800 c the godfrey granite occurs to the north adjacent to the upper deck high p t domain well preserved corona textures in the godfrey granite constrain igneous crystallization and early metamorphism in the intermediate pressure granulite field opx pl at 1 0 gpa 775 c followed by metamorphism in the high pressure granulite field grt cpx pl at 1 2 gpa 860 c u pb geochronology of zircon in upper deck mafic granulite yields evidence for events at both c 2 5 ga and c 1 9 ga the oldest zircon dates are interpreted to constrain a minimum age for crystallization or early metamorphism of the protolith a population of 1 9 ga zircon in one mafic granulite is interpreted to constrain the timing of high p t metamorphism titanite from the mafic granulites yields dates ranging from 1900 to 1894 ma and is interpreted to have grown along the decompression path but still above its closure temperature indicating cooling following the high p t metamorphism from c 960 650 c in 4 10 myr zircon dates from the godfrey granite indicate a minimum crystallization age of 2 61ga without any evidence for 1 9 ga overgrowths the data indicate that an early granulite facies event occurred at c 2 55 2 52 ga in the lower crust c 1 0 gpa but at 1 9 ga the upper deck underwent high p t metamorphism then decompressed to 0 9 1 0 gpa juxtaposition of the upper deck and godfrey granite would have occurred after or been related to this decompression in this model the high p t rocks are exhumed quickly following the high pressure metamorphism this type of metamorphism is typically associated with collisional orogenesis which has important implications for the snowbird tectonic zone as a fundamental boundary in the canadian shield

anticlockwise p t path of granulites from the monte castelo gabbro ordenes complex nw spain the study of mafic and aluminous granulites from the monte castelo gabbro ordenes complex nw spain reveals an anticlockwise p t path that we interpret as related to the tectonothermal activity in a magmatic arc probably an island arc the p t path was obtained after a detailed study of the textural relationships and mineral assemblage succession in the aluminous granulites and comparing these with an appropriate petrogenetic grid additional thermobarometry was also performed the granulites are highly heterogeneous with distinct compositional domains that may alternate even at thin section scale garnets are generally idiomorphic to subidiomorphic and in certain domains of the aluminous granulites they show overgrowths forming xenomorphic coronas around a more or less idiomorphic core both types of garnets show significant ca enrichment at the crystal rims which together with the other mineralogical and textural characteristics is compatible with a pressure increase with low t variation p t estimations indicate a peak of t 800 c and p 9 5 kbar attained after a significant increase in pressure that took place at high temperatures in the sillimanite field we suggest that this kind of trajectory probably anticlockwise is compatible only with a terrane heated by an intense magmatic activity after or during tectonic crustal thickening magmatic injection at the base and or into the crust which is characteristic of magmatic arcs