the composition and role of the fluid in migmatites a fluid inclusion study of the front range rocks monophase negative crystal shaped co2 inclusions occurring isolated in small clusters or in well healed intragranular fractures are common in the leucosome quartz of the 1700m y old migmatites from the east central colorado front range they are however quite rare in the mafic selvage and paleosome host rock quartz the mode of occurrence suggests that these are the earliest inclusions to form in addition to the difference in abundance of the inclusions there is a difference in co2 density distribution between migmatitic zones the temperatures of homogenization for the leucosome inclusions range and l c from 67 c to 20 c with two maxima at 21 c while those for the paleosome and selvage inclusions are 37 c to 20 c with a single maximum at 5 c these differences between the migmatitic zones which occur on the scale of a few centimeters suggest that the formation of these inclusions was related to the migmatization process the densities corresponding to the th maxima are appropriate for the p t conditions for migmatization estimated from the mineral geobarometer geothermometer these inclusions must contain nearly pure co2 as their final melting temperatures 56 5 to 57 2 c are very close to that of the triple point of co2 their composition also was confirmed by raman spectroscopic analyses

deformational history of an archean fold belt eastern point lake area slave structural province n w t archean metasedimentary rocks in the eastern point lake area of the slave structural province preserve a sequence of archean structures consisting of two generations of folds f1 and f2 with littl

population wide garnet growth zoning revealed by la icp ms mapping implications for trace element equilibration and syn kinematic deformation during crystallisation the strong partitioning of many trace elements into garnet and their slow diffusivities in both garnet and the rock matrix means that their distribution may record valuable petrogenetic information not documented by major elements in metamorphic rocks complex trace element growth zoning in garnet porphyroblasts from a garnet grade metapelite from the barrovian sequence of the sikkim himalaya is assessed using quantified la icp ms raster mapping coupled with x ray micro computed tomography the data document systematic changes in the zoning patterns from early to late nucleated crystals and also suggest that the ree y chemistry incorporated into garnet is dependent on persistent disequilibrium in the rock volume there is evidence for hree y diffusion haloes surrounding growing garnets and a heterogeneous hree y distribution in the rock matrix annuli superimposed on oscillatory zoning are not consistent with formation during some rock wide event but are dependent on the spatial disposition of the garnet annuli are interpreted to reflect an integrated history of varying growth rates and the incorporation of pre existing heterogeneities due to relatively slow matrix diffusivities conversely smooth zoning of many transition metals indicate that their distribution in garnet may be controlled by equilibrium partitioning between garnet and the rock matrix significant rotation of garnet porphyroblasts during growth is revealed due to immobility of cr over the duration of the crystallisation interval and overprinting of the heterogenous precursor cr distribution strain rate estimates derived from this zoning are on the order of 10 11 10 12 hbox s 1

amphibolites from panama anticlockwise p t paths from a pre upper cretaceous metamorphic basement in isthmian central america rocks from the metamorphic basement of the azuero and sona peninsulas panama consist of schistose amphibolites and minor amounts of metasediment in the sona peninsula strongly zoned amphiboles indicate that the amphibolites followed a progressive anticlockwsie p t path prograde from low t low p to medium t high p and are retrograded into the greenschist facies in contrast the amphibolites of the azuero peninsula are affected by a low to medium t low p metamorphism the metamorphic events of the sona amphibolites occurred prior to the intra senonian tectonic phase which affects the mesozoic formations along the pacific coast of costa rica and panama the regional significance of such a basement in isthmian central america is discussed

sequential porphyroblast growth during deformation in a low pressure metamorphic terrain orrs island harpswell neck maine poikiloblastic index minerals in pelitic rocks from the orrs island harpswell neck area of coastal maine contain inclusion textures that indicate sequential growth of progressively higher grade metamorphic minerals during development of a near vertical crenulation foliation the sequence of zones in the field is garnet staurolite staurolite andalusite staurolite sillimanite and sillimanite inclusion fabrics characteristic of different stages in crenulation cleavage development indicate that index minerals nucleated and grew sequentially biotite began to grow before deformation garnet began to grow during early stages of crenulation cleavage development staurolite grew during intermediate stages and andalusite grew relatively late when transposition of the foliation was nearly complete muscovite pseudomorphs and sillimanite were mainly post kinematic the fact that metamorphic index minerals grew sequentially in individual rocks in the same order in which they appear across the field area indicates that the high temperature part of the pressure temperature path was similar to the metamorphic field gradient metamorphism in the orrs island harpswell neck area is consistent with the magmatic heating model that has been proposed for western maine sequential development of index minerals in pelitic rocks in the orrs island harpswell neck area apparently resulted from sequential nucleation after substantial overstepping of mineral forming reactions once nucleation of an index mineral had taken place initial growth was rapid and poikiloblasts preserved inclusion trails characteristic of the prevailing stage of crenulation cleavage development because nucleation of sillimanite may have required more overstepping of the andalusite sillimanite reaction than nucleation at dehydration reactions determination of metamorphic conditions for rapidly heated rocks such as these by comparison with a petrogenetic grid is problematic garnet zoning patterns in these rocks should reflect the fact that growth of garnet interiors occurred early during metamorphism in equilibrium with a low grade assemblage only garnet rims would be expected to record the subsequent pressure temperature path

development of p t prograde and p retrograde t prograde isogradic surfaces during blueschist to eclogite regional deformation metamorphism in new caledonia as indicated by progressively developed porphyroblast microstructures the mid tertiary blueschists eclogites and eclogitic gneisses of northern new caledonia are the products of four phases of regional meta morphism and deformation d d4 omphacite lawsonite and mn rich garnet isogradic surfaces were developed during the second deformation d2 under prograde pressure and temperature conditions subsequent deformations d3 d folded these d isogradic surfaces however within the p retrograde t prograde metamor phic environment of the d phase omphacite altered to albite and chlorite as a result a late stage sub horizontal isogradic surface developed for omphacite out where this mineral is preserved as relics within syn d albite porphyroblasts other minerals that crystallized for the first time epidote or had rim additions almandine phengite during d also form nearly horizontal isogradic surfaces porphyroblastic garnet and albite contain inclusion trails which allow their microstructural development and crystallization of the matrix to be traced from d to d late syn d the temperature increased mark edly in association with an extensive exothermic decarbonation even though the rocks were in a state of pressure retrogression this caused considerable neocrystallization recrystallization and growth of matrix and porphyroblasts such that although s foliation crenulated by d and d is readily observable almost all signs of stored strain due to d and d have been removed and the deeper schists and eclogitic gneisses superficially appear to have undergone a drastic annealing recrystallization post dating deformation

thermal consequences of mantled gneiss dome emplacement abstract bodies of rock within the crust that have higher thermal conductivities than surrounding rocks serve as conduits for the diffusive flow of heat the resultant focusing of heat creates an apparent contact metamorphic aureole in the surrounding rocks without the high conductivity rocks being a heat source themselves two dimensional thermal models of mantled gneiss domes are used to consider these effects and their role in the thermal histories of metamorphic belts the complex metamorphic histories and patterns observed around gneiss domes may be the result of the emplacement of relatively high thermal conductivity rocks dome cores into lower thermal conductivity metasediments mantling strata the degree of the thermal anomaly is primarily affected by the conductivity contrast but is also dependent upon the velocity of emplacement crustal level and heat flux many of the metamorphic isograd features and pressure temperature time paths that are observed around mantled gneiss domes can be explained merely by the emplacement of high conductivity rocks into low conductivity rocks without invoking additional heat sources or significant post metamorphic deformation

continuous reactions between biotite garnet staurolite kyanite sillimanite andalusite and p t time deformation path in micaschists from the estuary of the river vilaine south brittany france microprobe analysis of the continuous chemical evolution of coexisting biotite garnet and biotite garnet staurolite has been undertaken from interbedded micaschists of the volcanodetrital group of the vilaine a thermobarometric study using pertinent mineralogical equilibria reveals a complex p t evolution continuous throughout time from high pressure medium temperature kyanite zone to medium pressure high temperature sillimanite zone then low pressure medium temperature andalusite zone the t p fh2o and xh2o variations have been calculated from coexisting biotite garnet pairs and from the equilibria paragonite in white mica quartz albite in plagioclase al silicate h2o and 3 anorthite grossular 2 alsilicate quartz the p t evolution is correlated with the continuous change in composition of minerals using p xmg and t xmg diagrams and with the evolution of assemblages this continuous p t time evolution correlated with the successive formation of s1 s2 foliations allows us to propose a p t time deformation path for the micaschists and to relate the growth of its mineral components to tectonic processes

rapid growth and strain rates inferred from synkinematic garnets penokean orogeny minnesota recent reinterpretations of the early proterozoic penokean orogeny in minnesota invoke a plate tectonic model with characteristics similar to phanerozoic collisional belts deformation during collision produced large flattening strains 89 and the development of a tectonic foliation and progressive metamorphism up to amphibolite facies as indicated by the presence of synkinematic s1 garnets biotite garnet thermometry and plagioclase garnet biotite muscovite barometry give southward increasing temperatures and pressures and indicate burial to depths of 21 25 km similar to medial and internal zones of collisional orogens worldwide gibbs method modeling on zoned synkinematic garnets indicates virtually no change in pressure p or temperature t during garnet growth growth and rotation of garnets at a point in p t space in an actively evolving deformational and thermal regime are surprising and suggest rapid garnet growth rates and high rates of strain during orogeny this is consistent with previous studies that indicate more rapid rates of tectonometamorphic processes in the precambrian relative to younger orogenic belts

metamorphism and granite genesis in the hidaka metamorphic belt hokkaido japan the main zone of the hidaka metamorphic belt is an uplifted crustal section of island arc type the crust was formed during early tertiary time as a result of collision between two arc trench systems of cretaceous age the crustal metamorphic sequence is divided into four metamorphic zones i iv in which zone iv is in the granulite facies a detailed study of the evolution of the hidaka belt based on a revised p t t analysis of the metamorphic rocks notably a newly found staurolite bearing granulite confirms a prograde isobaric heating path after a supposed event of tectonic thickening of accretionary sedimentary and oceanic crustal rocks during the peak metamorphic event c 53 ma the regional geothermal gradient attained 33 40 c km 1 and the highest p t condition obtained from the lowest part of the granulite unit is 830 c 7 kbar in this part xh2o of gt opx cd gneiss is about 0 15 and that of gt cd bt gneiss is 0 4 the p t xh2o condition of the granulite unit is well within a field where fluid present partial melting of pelitic and greywacke metamorphic rocks takes place this is in harmony with the restitic nature of the gt opx cd gneiss in the lowest part of the granulite unit the possibility that partial melting took place in the main zone is significant for the genesis of the peraluminous s type granitic rocks within it the s type granitic rocks in this zone are opx gt bt tonalite in the granulite zone gt cd bt tonalite in the amphibolite zone and cd bt mus tonalite in the bt mus gneiss zone the mineralogical and chemical nature of these strongly peraluminous tonalitic rocks permit them to be regarded as having been derived from s type granitic magma generated by crustal anatexis of pelitic metamorphic rocks in deeper crust

development of inverted metamorphic isograds in the western metamorphic belt juneau alaska an inverted metamorphic gradient is preserved in the western metamorphic belt near juneau alaska the western metamorphic belt is part of the coast plutonic metamorphic complex of western canada and southeastern alaska that developed as a result of tectonic overlap and or compressional thickening of crustal rocks during collision of the alexander and stikine terranes detailed mapping of pelitic single mineral isograds systematic changes in mineral assemblages and silicate geothermometry indicate that thermal peak metamorphic conditions increase structurally upward over a distance of about 8 km peak temperatures of metamorphism increase progressively from about 530 c for the garnet zone to about 705 c for the upper kyanite biotite zone silicate geobarometry suggests that the thermal peak metamorphism occurred under pressures of 9 11 kbar the metamorphic isograds are in general parallel to the tonalite sill that is regionally continuous along the east side of the western metamorphic belt although truncation of the isograds north of juneau indicates that the sill intrusion continued after the isograds were established our preferred interpretation of the cause of the inverted gradient is that it formed during compression of a thickened wedge of relatively wet and cool rocks in response to heat flow associated with the formation and emplacement of the tonalite sill magma garnet rim compositions and widespread growth of chlorite suggest partial re equilibration of the schists under pressures of 5 6 kbar during uplift in response to final emplacement and crystallization of the tonalite sill the combined results of this study with previous studies elsewhere in the western metamorphic belt indicate that high t high p metamorphism associated with the collision of the alexander and stikine terranes was a long lived event extending from about 98 ma to about 67 ma

the effect of fluid and deformation on zoning and inclusion patterns in poly metamorphic garnets within the bergen arcs of w norway caledonian eclogite facies assemblages t 650 c p 15 kbar have formed from grenvillian granulites t 800 900 c p 10 kbar along shear zones and fluid pathways garnets in the granulites grti pyr56 40 alm45 25gro19 14 are unzoned or display a weak ca 1 wt feo over 1000 m zoning the eclogite facies rocks contain garnets inherited from their granulite facies protoliths these relict garnets have certain areas with compositions identical to the garnets in their nearby granulite but can be crosscut by bands of a more almrich composition grtii pyr31 41alm40 47gro17 21 formed during the eclogite facies event these bands orientated preferentially parallel or perpendicular to the eclogite foliation may contain mineral filled veins or trails of eclogite facies minerals omphacite amphibole white mica kyanite quartz and dolomite steep compositional gradients up to 9 wt feo over 40 m separate the two generations of garnets indicating limited volume diffusion the bands are interpreted as fluid rich channels where element mobility must have been infinitely greater than it was for the temperature controlled volume diffusion at mineral interfaces in the granulites the re equilibration of granulite facies garnets during the eclogite facies event must therefore be a function of fracture density deformation and fluid availability the results cast doubts on modern petrological and geochronological methods that assume pure temperature controlled chemical re equilibration of garnets

peak metamorphic temperatures from cation diffusion zoning in garnet a model that relates the characteristic diffusion length and average cooling rate to peak temperature was developed for chemical diffusion in spherical geometries on the basis of geospeedometry principles and diffusion theory the model is quantitatively evaluated for cation diffusion profiles in garnet important model parameters were calibrated empirically using diffusion zoning of ca in garnet from the pikwitonei granulite domain a terrane for which the thermal history has been well characterized the results are used i to empirically test diffusion parameters for mg and fe ii and ii to develop a tool that uses the diffusion zoning of these cations in garnet to constrain peak temperature conditions for garnet bearing rocks the thermometric approach was externally tested by applying it to garnet crystals from various metamorphic terranes worldwide and comparing the results to published peak temperature estimates the results overlap within uncertainties in all cases but result that are based on fe ii and mg chemical diffusion profiles are up to three times more precise than those acquired by conventional methods the remarkable consistency of the results implies that the model is robust and provides a reliable means of estimating peak temperatures for different types of high grade metamorphic rock the tool could be of particular advantage in rocks where critical assemblages for conventional thermometry do not occur or have been replaced during retrogression

geochronologic and thermobarometric constraints on the evolution of the main central thrust central nepal himalaya the main central thrust mct juxtaposes the high grade greater himalayan crystallines over the lower grade lesser himalaya formation an apparent inverted metamorphic sequence characterizes the shear zone that underlies the thrust garnet bearing assemblages sampled along the marysandi river and darondi khola in the annapurna region of central nepal show striking differences in garnet zoning of mn ca mg and fe above and below the mct thermobarometry of mct footwall rocks yields apparent inverted temperature and pressure gradients of 18 c km 1 and 0 06 km mpa 1 respectively pressure temperature p t paths calculated for upper lesser himalaya samples that preserve prograde compositions show evidence of decompression during heating whereas garnets from the structurally lower sequences grew during an increase in both pressure and temperature in situ i e analyzed in thin section ion microprobe ages of monazites from rocks immediately beneath the greater himalayan crystallines yield ages from 18 to 22 ma whereas late miocene and pliocene monazite ages characterize rocks within the apparent inverted metamorphic sequence a lesser himalayan sample collected near the garnet isograd along the marysandi river transect contains 3 3 0 1 ma monazite ages p 0 72 gpa t 535 c this remarkably young age suggests that this portion of the mct shear zone accommodated a minimum of 30 km of slip over the last 3 ma i e a slip rate of 10 mm yr 1 and thus could account for nearly half of the convergence across the himalaya in this period the distribution of ages and p t histories reported here are consistent with a thermokinematic model in which the inverted metamorphic sequences underlying the mct formed by the transposition of right way up metamorphic sequences during late miocene pliocene shearing

garnet geochemistry of tungsten mineralized xihuashan granites in south china abstract the xihuashan complex intrusion in south china which is emplaced at ca 154 ma mainly comprises medium grained porphyritic biotite granite medium grained biotite granite and fine grained two mica granite medium grained biotite and fine grained two mica granites are important tungsten bearing granites and contain an unusual amount of garnet garnets from this intrusion are dominated by almandine and spessartine which constitute 94 to 99 of the total molecular composition of the garnet these garnets display unusual compositional zoning the cores of these garnets are rich in heavy rare earth element hree y ca and contain abundant hree and y rich mineral inclusions their rims are free of mineral inclusions and have low of these elements two types of fluorite inclusions exist in garnet y fluorite and y poor fluorite garnet exhibits specific mn zoning with a relatively mn poor core but a relatively mn rich rim thus constituting a specific spessartine inverse bell shaped profile that belongs to typical magmatic garnets all analyzed garnets have high ree content and exhibit hree enriched and lree depleted patterns with strong negative eu anomalies the incorporation of ree into garnet is in part controlled by its crystal chemistry with ree3 following a coupled substitution of the type fe2 mn2 1viii ree3 1viii si4 1iv z3 1iv the texture and compositional zoning of garnet suggest that garnet growth is largely controlled by the pressure temperature composition condition of magmatic evolution internal crystal chemical parameters and kinetics during mineral growth the garnet core grows in near equilibrium with magmatic melt under a relatively high pressure temperature p t condition by contrast the garnet rim grows rapidly by the coexisting melt fluid phase and co2 rich volatile environment under a relatively low p t condition which is virtually unfavorable for the incorporation of ree into the magmatic garnet structure garnet fingerprints the magmatic hydrothermal transition during crystallization of these granites

the importance of deformation experiments on minerals for the interpretation of metamorphic ore textures abstract for galena and partially for hematite and magnetite the experimental data are sufficient to develop constitutive relations for the various flow processes flow mechanism regimes and associated strain rates are delineated as functions of temperature stress and material properties in various types of deformation mechanism maps usually deformation mechanism maps are sections through the space strain rate normalized stress gm homologous temperature t t m structural parameters e g normalized grain size d b there is a strong need to increase the amount of experimental data on the flow behaviour of ore minerals in order to construct reliable deformation mechanism maps for additional important ore minerals and their accompanying gangue minerals deformation mechanism maps with non normalized parameters are suitable to illustrate the contrasts in the flow behaviour of ore gangue minerals during a cycle of metamorphic events

staurolite growth and metamorphic zones in the kalak nappe complex of northeastern porsangerhalvoya north norway abstract four main thrust units have been differentiated in the kalak nappe complex of northeastern porsangerhalvoya all exhibit barrovian type prograde metamorphism although the metamorphic grade increases up the nappe pile the development of planar and non kinematic fabrics is the same in relation to the deformation sequence in each of the thrust sheets early porphyroblast growth preserves a planar fabric developed during burial as a result of overthrusting this was superseded by a randomly oriented and equidimensional non kinematic fabric which developed by recrystallization associated with increased fluid concentrations due to prograde metamorphic dehydration reactions these fabric developments have been preserved in textures associated with the prograde alteration of staurolite and muscovite porphyroblasts with increased fluid concentrations the shear strength of the rock declined and thrust propagation occurred the associated shear stresses caused rotational fabrics in some garnets and aligned mineral growth

himalayan metamorphism and its tectonic implications the himalayan range exposes a spectacular assemblage of metamorphic rocks from the mid and deep crust that have fostered numerous models of how the crust responds to continental collisions recent petrogenetically based petrologic and geochronologic studies elucidate processes with unprecedented resolution and critically test models that range from continuum processes to one time events the pronounced metamorphic inversion across the main central thrust reflects continuum thrusting between ca 15 and 20 ma whereas exposure of ultrahigh pressure rocks in northwestern massifs and syntaxis granulites reflects singular early 45 ma and late 10 ma exhumation events multiple mechanisms including wedge collapse and flow of melt weakened midcrust are debated to explain pressure temperature trajectories patterns of thinning and thermal overprinting a geochronologic revolution is under way in which spatially resolved compositions and ages of accessory minerals are combined in a petrogenetically valid co

p t paths from high temperature shear zones beneath ophiolites mctamorphic rocks of the st anthony complex of north western newfoundland are best interpreted in terms of a high temperature shear zone formed between down going continental margin rocks and overriding oceanic lithosphere in a subduction zone high grade rocks immediately beneath the oceanic lithosphere peridotite display retrograde meta morphism in high strain zones whereas lower grade rocks near the base of the metamorphic complex display prograde metamorphism in high strain zones mylonite zones in meta basitcs at all levels in the complex contain the assemblage epidote hornblende albite sodic oligoclase these observations suggest that the inverted metamorphic gradient within the st anthony complex results from the fortuitous preservation of residual metamorphic assemblages from different crustal levels within an epidote amphibolite facies shear zone the degree of re equilibration is strongly dependent on the degree of strain and is best achieved in synmetamorphic mylonite zones this interpretation of the st anthony complex can be extended to other sub ophiolite metamorphic sheets which show very similar relationships it is proposed that most metamorphic sheets beneath ophiolites are high temperature shear zones the p t paths of which preserve records of burial and exhumation in subduction zones

metamorphic and structural diachroneity in the finnmarkian nappes of north norway two periods of garnet growth gt1 and gt2 have been found in the finnmarkian nappes of north norway in the kolvik nappe the lowest nappe gt1 has preserved an s2 syntectonic spiral inclusion fabric in the olderfjord nappe an earlier s1 fabric and an interkinematic inter d1 d2 fabric have been preserved in gt1 whilst only the s1 fabric has been found in gt1 in the brennsvik nappe the highest nappe in each nappe gt2 overgrew a penetrative fabric s2 wrapped around gt1 in the kolvik nappe inclusion fabrics may be continuous from gt1 into gt2 but in the higher nappes there is a distinct break gt2 may have been partially syntectonic with d3 in the brennsvik nappe chemically gt1 in the kolvik nappe and in parts of the olderfjord and brennsvik nappes has antithetic fe mn zoning in all nappes xca and xmg are weakly zoned in gt1 xmg increases outwards and is greater in the higher nappes in gt1 suggesting higher nucleation temperatures in the olderfjord and brennsvik nappes gt2 is marked by increasing xca probably due to changing garnet plagioclase equilibria although the fe mg ratio remains constant xmg is higher in gt2 than gt1 basement rocks within the nappe pile have an early pre finnmarkian growth gt1 and a later finnmarkian growth gth correlated with gt2 on the basis of chemical zoning patterns the diachroneity of gt1 is ascribed to progressively earlier compared to the structural development cessation of overstepping of garnet forming reactions before peak metamorphism in the higher nappes resulting in earlier structural events being preserved

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

a critical assessment of the tectono thermal memory of rocks and definition of tectono metamorphic units evidence from fabric and degree of metamorphic transformations

uncertainties in differential thermodynamic gibbs method p t paths a combined analytical and numerical evaluation of the uncertainties in p t paths is made for three assemblages that propagates errors in the parameters initial pressure initial temperature initial composition change of composition of monitor parameters endmember entropy and endmember volume propagated errors along an isobaric heating path t 77 c for assemblage 1 grt bt pl qtz ms chl h2o using as monitor parameters the mole fractions of almandine spessartine grossular and anorthite and ignoring uncertainties in thermodynamic properties are approximately 320 bars 1 and 8 3 c 1 if rim compositional uncertainties of 5 in major cations are assumed or 2 5 c 1 and 50 bars 1 if electron microprobe analytical uncertainties are assumed for compositions the largest source of uncertainty is from the errors in the monitor parameters and p t path uncertainties can depend critically on which monitor parameters are used if the mole fraction of annite is used as a monitor parameter in place of the anorthite content of plagioclase then propagated uncertainties are worse than 29 c 1 and 5800 bars 1 p t path uncertainties also depend on assemblage 1 precisions in assemblages 2 grt bt pl qtz ms sil h2o and 3 grt bt qtz kfs sil h2o using as monitor parameters the mole fractions of almandine spessartine grossular and anorthite are calculated to be 267 bars and 43 4 c and 372 bars and 8 2 c respectively estimates of the accuracies in p t paths that include potential errors in endmember entropy of 1 j mol k and in endmember volume of 1 cm3 mol are 324 bars and 8 5 c assemblage 1 341 bars and 48 6 c assemblage 2 and 388 bars and 9 7 c assemblage 3 use of different garnet plagioclase and muscovite activity models can change the length of a p t path by as much as 15 but does not typically change directions in p t space significantly models that incorporate changes of fluid composition shorten p t paths in assemblages 1 and 3 but do not change trajectories significantly assemblage 2 is virtually unaffected by fluid phase models for the mineral assemblages considered here and using appropriate monitor parameters propagated errors are small compared to the total path length suggesting that the differential thermodynamic approach is a precise and accurate method for determining amounts of heating or thickening during metamorphism and hence for interpreting orogenic processes

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

p t paths derived from garnet growth zoning in an extensional setting an example from the tormes gneiss dome iberian massif spain the tormes gneiss dome nw iberian massif variscan belt of spain comprises a metamorphic core complex lower unit bounded by a major extensional detachment despite metamorphic temperatures in the upper amphibolite facies 700 740 c metapelites from the highest levels of the lower unit contain garnet with preserved growth zoning these rocks were used for reconstruction of quantitative p t paths based upon interpretation of microfabrics and thermodynamic modelling of garnet zoning the results are consistent with a two stage tectonothermal evolution under high grade conditions 1 an early compressional phase of deformation that led to upper amphibolite facies barrovian type metamorphism and to p increase and t rise to approximately 9 kbar and 700 725 c 2 a subsequent major extensional phase of deformation that led to quasi isothermal decompression from 8 9 to 3 kbar at t conditions between 700 and 740 c several lines of structural textural and petrological evidence suggest that up to 15 20 km of overburden was removed from the lower unit by tectonic exhumation while these rocks were still at upper amphibolite facies conditions a final stage of quasi isobaric cooling to greenschist facies conditions is locally recorded in late low grade detachments

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

experimental study of cut marks made with rocks unmodified by human flaking and its bearing on claims of 3 4 million year old butchery evidence from dikika ethiopia abstract in order to assess further the recent claims of 3 4 ma butchery marks on two fossil bones from the site of dikika ethiopia we broadened the actualistic interpretive zooarchaeological framework by conducting butchery experiments that utilized naive butchers and rocks unmodified by human flaking to deflesh chicken and sheep long limb bones it is claimed that the purported dikika cut marks present their unexpectedly atypical morphologies because they were produced by early hominins utilizing just such rocks the composition of the cut mark sample produced in our experiments is quite dissimilar to the sample of linear bone surface modifications preserved on the dikika fossils this finding substantiates and expands our earlier conclusion that considering the morphologies and patterns of the dikika bone surface modifications and the inferred coarse grained depositional context of the fossils on which they occur the dikika bone damage was caused incidentally by the movement of the fossils on and or within their depositional substrate s and not by early hominin butchery thus contrary to initial claims the dikika evidence does not warrant a major shift in our understanding of early hominin behavioral evolution with regard to carcass foraging and meat eating

opposite p t t paths of hercynian metamorphism between the upper units of the cabo ortegal complex and their substratum northwest of the iberian massif abstract in the cabo ortegal region two metamorphic groups can be distinguished which have followed opposite p t paths in their hercynian metamorphism the upper group comprises the three uppermost units of the allochthonous complex with mafic and related lithologies of cabo ortegal the lower metamorphic group includes the two basal units of the complex and the lithological sequence outcropping in the ollo de sapo anticlinorium the lithologies of the upper group were affected by high p and medium high t eo hercynian metamorphism during a subduction episode related to the onset of the hercynian orogenic cycle as a result of such metamorphism the basic rocks were transformed into b type eclogites and also into high p mafic granulites the advance of the hercynian collision produced the progressive uplift of the subducted lithologies and finally their emplacement as large thrust sheets over the lower metamorphic group this final evolution is clearly reflected in the p t t paths of the high p lithologies which show pronounced decompressions with only minor cooling the evolution of the lower metamorphic group does not reflect high p events the characteristic regime is a barrovian type medium p metamorphism showing p t t paths with a progressive compression during heating at least partially the burial of the rock association was due to the overthrusting of a thick allochthonous pile this is represented by the upper metamorphic group

petrologic constraints on imbrication metamorphism and uplift in the sw tauern window eastern alps the pressure temperature time deformation pttd history of two tectonic units in the southwest tauern window austria italy is evaluated from p t paths of metamorphism and uplift geochronologic data and observations on fabric development petrologic data indicate burial of the lower schieferhulle lsh to depths of 35 km and the adjacent upper schieferhulle ush to only 24 28 km the units are now separated by only 2 km of section metamorphism was postkinematic in the lsh and synkinematic in the ush correlation of p t path reversals calculated from zoned garnets in the two units however indicates that juxtaposition of the lsh and ush occurred prior to metamorphism temperature time and depth time diagrams for both units show increasing cooling rates and decreasing uplift rates as the rocks approached the surface both the petrologic data and the depth time profiles for the lsh and ush point to a history of differential uplift in the early stages of decompression textural features indicate that the lsh behaved as an essentially rigid body during uplift whereas the ush experienced extensive ductile shearing the pttd data for the lsh and ush are incorporated into a model for continental collision in the eastern alps initial tectonic uplift of the lsh was accompanied by ductile thinning of the ush and overlying austroalpine units final uplift and erosion affected all of the units en masse

garnet evolution in pre variscan pelitic rocks from the lake emosson area aiguilles rouges massif western alps the chemical evolution of garnets from pelitic rocks of probable palaeozoic age corresponds to a complex metamorphic evolution of the host rocks among the almandine rich garnets alm60 80 two main types of evolution can be distinguished early mn rich garnets coexisting with kyanite may be replaced by plagioclase and then during a late stage by biotite and or sillimanite the second type of evolution corresponds to an overgrowth of mn poor late stage garnet on older mn rich garnets which corresponds to a thermal peak with sillimanite type of metamorphism this new garnet may appear either as an overgrowth with a strong discontinuity or as small new euhedral garnet or as skeletal garnet this chemical evolution of garnet corresponds to an early collisional stage of metamorphism of high pressure type with high mn values of probable ordovician age followed by uplift and a thermal peak low mn values in devonian times

the na ca amphibole albite chlorite epidote quartz geothermobarometer in the system s a f m c n h2o 2 applications to metabasic rocks in different metamorphic settings in order to illustrate different applications of the amphibole albite chlorite epidote quartz geothermobarometer pressure temperature time p t t space p t t s deformation p t t d paths have been established from literature data they are discussed as a function of the chemical equilibrium and microstructural data available in each case and compared with the conclusions already established by other methods it is clear that it is necessary to know the relative chronology of the events directions of zoning of minerals in successive microstructural positions to establish precise p t paths this enables reconstruction of complex geodynamic histories from this point of view it is necessary to analyse the maximum possible number of minerals in a few well chosen metabasic rocks showing different generations of blastesis the rocks should belong to different tectonic units to obtain the best overall picture of a metamorphic complex

pressure temperature reaction history of metapelitic rocks from the maryland piedmont on the basis of correlated garnet zoning and plagioclase inclusion composition regional structural interpretation thermobarometry and gibbs method calculations were combined to determine an approximate p t path for kyanite grade metapelitic rocks of the baltimore gneiss terrane garnet growth along the estimated p t path was modeled using the computer program diffgibbs models were evaluated by comparing calculated profiles of garnet components with measured profiles of a large garnet rich in plagioclase inclusions in a kyanite bearing sample from the area modifications in the p t path constrained by changes in garnet components along critical portions of the profile resulted in a very satisfactory match between measured and model garnet profiles in the most successful model garnet growth began at 550 c and 6500 bars in the assemblage chlorite biotite garnet muscovite plagioclase quartz with increasing temperature and with decreasing and then constant pressure at 570 c and 5724 bars staurolite was added to the assemblage and some garnet was consumed in the model garnet growth resumed after chlorite was consumed along a path having a substantial increase in pressure the region near the rim with low measured grossular content was reproduced by cessation of garnet growth as temperature increased at constant pressure to a maximum of 615 c at 6474 bars after kyanite was added to the assemblage grossular increased to the constant rim value only if temperature decreased and pressure increased cooling and decompression affected garnet zoning only very near the rim measured compositions of the rims of plagioclase inclusions in the garnet are consistent with plagioclase compositions predicted by the model this study demonstrates that complex garnet zoning can be modeled successfully even when the reaction history includes episodes of garnet consumption over a path that is consistent with geologic evidence details of garnet zoning provide surprisingly tight constraints on the p t path

pressure temperature trajectory of the sanbagawa metamorphism deduced from garnet zoning abstract most garnets in a small volume of the pelitic sanbagawa schists nucleated simultaneously grew and then ceased with their growth maintaining near equilibrium with coexisting minerals such as chlorite and biotite during the prograde metamorphism chemical trends of zoned garnets systematically shift from the pyrope poor to pyrope rich portions in a ternary mn fe mg system in order of ascending metamorphic grade a ternary mn fe mg model system suggests that the higher grade garnets started to grow at lower pressures and higher temperatures than the lower grade ones during prograde metamorphism it follows that high grade metamorphism took place at lower pressures in the garnet to oligoclase biotite zones of the sanbagawa metamorphic terrain in central shikoku

structural evolution of an a type subduction zone lofoten rombak area northern scandinavian caledonides the climactic phase of the caledonian orogeny in scandinavia involved collision of the baltic and greenland cratons in late ordovician to silurian time we infer that the collision resulted in large scale subduction of the leading edge of the baltic craton beneath an andean type greenland continental margin deep erosional levels in the lofoten rombak area norway sweden 69 n permit direct examination and evaluation of the structural evolution of a continental or a type subduction zone the early stages of collision involved en bloc underthrusting of the continental margin to depths of at least 30 km during this process deformational effects in the lower plate were restricted to its upper few hundred meters ultimately en bloc a type subduction should be limted by buoyancy effects as convergence continued the lower plate became involved in a series of imbricate thrusts with the same vergence as the intial subduction zone the temporal and spatial relationships between these basement involved thrusts are similar to those observed in foreland fold and thrust terrains field and geophysical data bearing on lower plate behavior elsewhere in the caledonldes and in other collisional orogens suggest that the structural features observed in the lofoten rombak area may be generally characteristic of a type subduction in continent continent collisional settings

contrasting p t paths in the eastern himalaya nepal inverted isograds in a paired metamorphic mountain belt petrology and phase equilibria of rocks from two profiles in eastern d f matrix assemblages are overprinted by decompression reaction textures and in zones f and g progress into the sillimanite field nepal from the lesser himalayan sequences across the main central thrust zone and into the greater himalayan sequences the two terranes were brought into juxtaposition during formation of sillimanite biotite gedrite foliation seams s3 formed at reveal a paired metamorphic mountain belt pmmb composed of two thrust bound metamorphic terranes of contrasting metamorphic conditions of 674 33 c and 5 7 1 1 kbar the contrasting average geothermal gradients and p t paths of these two metastyle at the higher structural level the greater himalayan sequences experienced high t moderate p metamorphism with an anmorphic terranes suggest they make up a pmmb the upperplate position of the greater himalayan sequences produced an ticlockwise p t path low p inclusion assemblages of quartz hercynitic spinel sillimanite have been overgrown by peak anticlockwise p t path with the high average geothermal gradient being possibly due to high radiogenic element content in this terrane metamorphic garnet cordierite sillimanite assemblages that equilibrated at 837 59 c and 6 7 1 0 kbar matrix in contrast the lower plate lesser himalayan sequences were deeply buried metamorphosed in a clockwise p t path and display minerals are overprinted by numerous metamorphic reaction textures that document isobaric cooling and re equilibrated samples preserve inverted isograds as a result of progressive ductile overthrusting of the hot greater himalayan sequences during prograde metamorphism evidence of cooling to 600 45 c at 5 7 1 1 kbar below the main central thrust the lesser himalayan sequences are a continuous though inverted barrovian sequence of high p moderate t metamorphic rocks metamorphic zones upwards from the

episodic exhumation of hp rocks inferred from structural data and p t paths from the southwestern massif central variscan belt france abstract in the southwestern massif central france hp rocks are found in lower allochthonous units i e hp units sandwiched between upper allochthonous units i e lp units and the parautochthonous unit both devoid of hp rocks hp rocks were formed at minimum conditions of 1 7 gpa 700 c in response to the northward subduction of gondwana beneath armorica during the silurian 430 390 ma their exhumation results from an 80 ma long sequence of tectonic processes effective at various scales and active specifically at each main stage of the evolution of the gondwana lithosphere hp rocks were first exhumed by upward southwestward extrusion during the early devonian 390 380 ma in relation to late stage of subduction in the northern area exhumation is achieved by rifting during the upper devonian 365 ma the most obvious event corresponds to the northwestward thrusting of lp units above hp units during early stages of continental collision 355 ma hp rocks were exhumed for a second time during orogen parallel transpression in response to late stage of continental collision 350 345 ma exhumation in internal zones is achieved by crustal scale normal faulting and erosion during the middle carboniferous 335 315 ma synorogenic extension while compression was still active in the southern external zones exhumation of the whole region is completed by regional uplift and erosional denudation during the upper carboniferous 310 290 ma postorogenic extension subduction and collision have played a key role for driving hp rocks to the surface but their efficiency and the resulting strain pattern depend on the angle of convergence upper devonian rifting and the subsequent early carboniferous crustal thickening prevents to consider exhumation of variscan hp rocks as a single and continuous process and open new questions on the related paleozoic lithosphere history

the p t evolution of the middle koli nappe complex scandinavian caledonides 68 n and its tectonic implications near 68 n the scandinavian caledonides are composed of 3 tectonic domains each of which has a different tectonostratigraphy the lower 2 domains can be related stratigraphically to scandinavia prior to caledonian deformation whereas the highest domain the middle koli nappe complex mknc represents a fore arc accretionary complex that was accreted to scandinavia during caledonian deformation subsequent to accretion the flyschoid sediments that dominate the mknc were metamorphosed to the amphibolite facies in the area covered by this study the mknc is composed of two nappes a lower langvatn nappe and an upper marko nappe each of which has a unique early metamorphic history pelitic mineral assemblages in the marko nappe constrain the peak p t to be 625 7 0 kbars whereas ultramafic mineral assemblages in the lower langvatn nappe constrain its peak temperature to be 580 c p t estimates from garnet biotite and garnet plagioclase geothermobarometry for both nappes overlap ranging from 528 c and 6 6 kbars to 620 c and 8 8 kbars with an average of 567 32 c and 8 0 0 9 kbar

plate rotation during continental collision and its relationship with the exhumation of uhp metamorphic terranes application to the norwegian caledonides lateral variation and asynchronous onset of collision during the convergence of continents can significantly affect the burial and exhumation of subducted continental crust here we use 3 d numerical models for continental collision to discuss how deep burial and exhumation of high and ultrahigh pressure metamorphic hp uhp rocks are enhanced by diachronous collision and the resulting rotation of the colliding plates rotation during collision locally favors eduction the inversion of the subduction and may explain the discontinuous distribution of ultra high pressure uhp terranes along collision zones for example the terminal scandian collision of baltica and laurentia which formed the scandinavian caledonides resulted in the exhumation of only one large hp uhp terrane the western gneiss complex wgc near the southern end of the collision zone rotation of the subducting baltica plate during collision may provide an explanation for this distribution we explore this hypothesis by comparing orthogonal and diachronous collision models and conclude that a diachronous collision can transport continental material up to 60 km deeper and heat material up to 300 c hotter than an orthogonal collision our diachronous collision model predicts that subducted continental margin material returns to the surface only in the region where collision initiated the diachronous collision model is consistent with petrological and geochonological observations from the wgc and makes predictions for the general evolution of the scandinavian caledonides we propose the collision between laurentia and baltica started at the southern end of the collisional zone and propagated northward this asymmetric geometry resulted in the counter clockwise rotation of baltica with respect to laurentia consistent with paleomagnetic data from other studies our model may have applications to other orogens with regional uhp terranes such as the dabie shan and papua new guinea cases where block rotation during exhumation has also been recorded

thrust related very low grade metamorphism in the marginal part of an orogenic wedge scandinavian caledonides the pattern and characteristics of thrust related very low grade metamorphism in the marginal part of an orogenic wedge have been investigated by combining a clay mineral crystallinity survey with detailed structural mapping of the thin skinned foreland thrust belt along the external part of the scandinavian caledonides this external part is composed of late neoproterozoic to ordovician sedimentary sequences of the autochthonous cover and lower allochthon which are overlain by the higher caledonian nappes middle and upper allochthonous units stages of the scandian phase of thrust wedge development are described which are related to the very low grade metamorphic history the initial stage involved the emplacement of cooled nappes belonging to the middle and upper allochthons with very low grade peak metamorphic conditions attained within the underlying lower allochthon cover sediments as they were progressively buried and deformed beneath the thrust wedge during this initial emplacement the isotherms are considered to have been undisturbed and dipping parallel to the wedge surface the following stages of wedge development consisted of extensive post metamorphic imbrication of the underthrusted cover sediments with a transition from basal accretion and uplift at the rear to accretion and forward propagation at the wedge s toe during accretion into the wedge the externally dipping isograd surfaces were extensively displaced from deeper levels toward higher tectonic horizons the last stage of wedge development considered here was characterized by late out of sequence thrusting with enhanced epizonal metamorphic grades developed in the vicinity of the fault zones which either resulted from further displacement of the isograds toward higher levels or from localized heating via intense fluid activity overall the pattern of metamorphic grade fabric relationships and physical calculations of heat transfer based on the geometry of the thrust wedge suggest that neither inverted temperature gradients nor shear heating were likely causes of the metamorphism in this flat lying part of the orogenic wedge the description of inverted very low grade metamorphic isograds in other marginal parts of the scandinavian caledonides which have been previously attributed to either the rapid emplacement of hot thrust nappes or the effects of dissipative shear heating are discussed in terms of variations in both the critical wedge geometry and its controlling boundary conditions

p t t deformation paths recorded by kinzigites during diapirism in the western variscan belt golfe du morbihan southern brittany france the high grade metamorphic rocks of southern brittany underwent a complex tectonic evolution under various p t conditions high p high t related to stacking of nappes during palaeozoic continentcontinent collision the east to west thrusting observed in the whole belt is strongly perturbed by vertical movements attributed to the ascent of anatectic granites in the high t area the field reconstruction of subvertical closed elliptical structures in gneisses and migmatites associated with the subhorizontal doubly radial pattern of stretching lineation in the mica schists suggests the existence of an elliptical diapiric body buried at depth beneath the present erosion level deformation is associated with a complex p t evolution partly recorded in aluminous gneisses kinzigites e g morbihanites a chronology of successive episodes of mineral growth at different compositions is established by detailed studies of the mineral microstructure relationships in x z sections using the deformation partitioning concept low and high strain zones several thermometric and barometric calibrations are applied to mineral pairs either in contact or not in contact but in equivalent microstructiiral positions with respect to the deformation history this methodology provides a continuous microstructural control of p t variations through time and leads to three p t t d paths constructed from numerous successive p t estimations path 1 is a clockwise retrograde path preserved in low strain zones which records general exhumation movements after crustal thickening paths 2 and 3 are clockwise prograde retrograde paths from high strain zones they are interpreted and discussed in the light of models of crustal anatexis and upward movement of magma diapirism deformation and p t effects induced by diapirism can be distinguished from the general deformation metamorphic history of a belt and would seem to be produced during a late stage of its history the present microstructural petrological approach to defining successive mineral equilibria in relation to progressive deformation steps provides a far more accurate evaluation of the metamorphic evolution than is possible by standard thermobarometry

the preservation potential of microstructures during static grain growth grain growth simulations using the microstructure simulation system elle have been performed in materials with a pre existing grain shape foliation as might be expected the foliation is destroyed by the end of the experiment and grain areas have increased by a factor of seven the area of material swept by the migrating grain boundaries was monitored and it was found that at every stage virtually all of the grains which survived the grain growth process contain one and only one core of unswept material remarkably these remnant unswept cores preserve a useable record of the initial grain size and the orientation of the grain shape foliation this work suggests that even for samples where no equivalent protolith can be found it may be possible to see past a grain growth episode to estimate the original grain shape and grain size of the rock and perhaps even reconstruct the grain boundary kinematics in addition the identification of unswept cores has the potential to help unravel the evolution of grain boundary chemistry in rocks during metamorphism as an example of a natural system showing these microstructures we describe a peridotite from almklovdalen norway this peridotite was infiltrated by aqueous fluids at several stages during late caledonian exhumation and retrogressive metamorphism grain boundary migration associated with the last of these infiltration events swept off abundant intragranular fluid inclusions in the original chlorite peridotite at the grain scale microstructural mapping of the fluid inclusion rich areas shows that as with the numerical simulations many of the grains retain exactly one core of unswept material examples of other natural systems discussed include dislocation density distributions and trace element zoning

porphyroblast inclusion trails the key to orogenesis detailed microstructural analysis of inclusion trails in hundreds of garnet porphyroblasts from rocks where spiral shaped inclusion trails are common indicates that spiral shaped trails did not form by rotation of the growing porphyroblasts relative to geographic coordinates they formed instead by progressive growth by porphyroblasts over several sets of near orthogonal foliations that successively overprint one another the orientations of these near orthogonal foliations are alternately near vertical and near horizontal in all porphyroblasts examined this provides very strong evidence for lack of porphyroblast rotation the deformation path recorded by these porphyroblasts indicates that the process of orogenesis involves a multiply repeated two stage cycle of 1 crustal shortening and thickening with the development of a near vertical foliation with a steep stretching lineation followed by 2 gravitational instability and collapse of this uplifted pile with the development of a near horizontal foliation gravitational spreading near coaxial vertical shortening and consequent thrusting on the orogen margins correlation of inclusion trail overprinting relationships and asymmetry in porphyroblasts with foliation overprinting relationships observed in the field allows determination of where the rocks studied lie and have moved within an orogen this information combined with information about chemical zoning in porphyroblasts provides details about the structural metamorphic p t t paths the rocks have followed the ductile deformation environment in which a porphyroblast can rotate relative to geographic coordinates during orogenesis is spatially restricted in continental crust to vertical ductile tear transcurrent faults across which there is no component of bulk shortening or transpression

controls on p t t deformation path from amphibole zonation during progressive metamorphism of basic rocks estuary of the river vilaine south brittany france edenite tremolite and edenite magnesio hornblende in equilibrium with plagioclase chlorite epidote quartz and vapour involve several types of reactions for which kd can be related to t and p thermodynamic calculation of these equilibria leads to isopleth systems given knowledge of the progressive changes of end member activities in zoned ca mg amphiboles based on microprobe analyses it is possible to construct precise pressure temperature time paths p t t paths which have been followed by metabasites during polyphase metamorphism when applied to basic rocks from the river vilaine area this method allows us to construct a p t t path that can be compared directly to the p t t path constructed from interbedded acid rocks aluminous micaschists in the same structural unit through time both basic and acid rocks underwent the same complex deformation history that can be described conveniently in the l s fabric system of flinn this allows us to construct a p t t deformation path for this structural unit these paths are interpreted in terms of an under overthrusting continental collision belt the hercynian belt and represent an illustration of the time delay caused by stacking of more than two crustal units

the petrology of retrograded eclogites of the oberpfalz forest northeastern bavaria west germany abstract in the bavarian margin of the moldanubian basement of the bohemian massif retrograded eclogitic metabasites are found within cordierite gneisses associated with garnet free serpentinised peridotites the primary eclogite assemblage gar omph qz rut ky is rarely retained typically garnet pyr 12 37 alm 38 56 gro 14 29 is prograde zoned with thin retrograde rims and a kelyphitic corona of orthopyroxene plagioclase rare or mg hastingsite plagioclase an 60 common primary omphacitic clinopyroxene jd 25 35 is preserved within garnet groundmass omphacite jd 15 25 always showing an early exsolution of plagioclase is usually replaced by a vermicular di plag an 20 symplectite itself overgrown by a diablastic hastingsite plagioclase an 40 aggregate quartz is rimmed by hypersthene with an outer margin of diopside kyanite never preserved in the metabasites is altered anisochemically to a symplectite of hercynite anorthite commonly only anorthite aggregates remain geothermobarometric studies indicate equilibration around 710 c minimum pressure 15 kb for the primary eclogite assemblage the granulite facies overprint occurred at probably slightly higher temperatures but lower pressures with later retrogression into the amphibolite fades as mostly shown by the surrounding gneisses the p t t path preserved within these metabasic rocks is that expected from a region having undergone crustal thickening by subduction and crustal imbrication their present day surface exposure underlain by crust of normal thickness would indicate the presence of a basal thrust complex that has driven under the imbricate crustal pile aiding uplift and erosion a geometry confirmed by recent geophysical studies

geothermobarometry and development of inverted metamorphism in the darjeeling sikkim region of the eastern himalayan abstract the darjeeling sikkim region provides a classic example of inverted himalayan metamorphism the different parageneses of pelitic rocks containing chlorite biotite garnet staurolite kyanite sillimanite plagioclase and k feldspar are documented by a variety of textures resulting from continuous and discontinuous reactions in the different zones microprobe data of coexisting minerals show that xmg varies in the order garnet staurolite biotite chlorite white mica is a solid solution between muscovite and phengite garnet is mostly almandine rich and shows normal growth zoning in the lower part of the main central thrust mct zone and reverse zoning in the upper part of the zone chemographical relations and inferred reactions for different zones are portrayed in afm space in the low grade zones oriented chlorites and micas and rolled garnets grew syntectonically and were succeeded by cross cutting chlorites and micas and garnet rims in the upper zones sillimanite kyanite and staurolite crystallized during a static inter kinematic phase p t contitions of metamorphism estimated through different models of geothermobarometry are estimated to have been 580 c for the garnet zone to a maximum of 770 c for the sillimanite zone the preferred values of pressure range from 5 0 kbar to 7 7 kbar models to explain the inverted metamorphism include overthrusting of a hot high himalayan slab along a c 5 km wide ductile mct zone and the syn or post metamorphic folding of isograds

allochemical retrograde metamorphism in shear zones an example in metapelites virginia usa metapelites in the altavista area southwest virginia piedmont usa underwent allochemical hydrothermal retrograde metamorphism in synmetamorphic shear zones the metapelites of the evington group were metamorphosed in a prograde sequence of chlorite staurolite and sillimanite zones garnet biotite geothermometry and phase relations support eastward increasing metamorphic grade ranging from 570 c in the staurolite zone to 650 c in the sillimanite zone at c 5 8 kbar sillimanite zone rocks later underwent progressive retrogression around shear zones which acted as fluid conduits retrograde assemblages are successively zoned around the shear zones with staurolite chloritoid and kyanite bearing assemblages the shear zones commonly contain kyanite or tourmaline veins applicable phase equilibria indicate that retrogression occurred during isobaric cooling through c 200 270 c rock compositional changes with retrogression occurred in steps sio2 was gained in the early stages of the retrogression but lost in the late stages al2o3 k2o and h2o were increasingly gained through the sequence cao was increasingly lost addition of h2o and decreasing temperatures resulted in new ferromagnesian minerals staurolite chloritoid chlorite and changes in h2o sio2 al2o3 k2o and cao contents produced muscovite and sodic plagioclase subsequent to prograde metamorphism deeply derived fluids migrated upwards along shear zones providing fluid and energy for the retrograde reactions the sheared rocks underwent fluid infiltration with fluid fluxes of 1 8 107 4 3 107 cm3 cm2 corresponding to minimum estimated fluid to rock ratios of 7 5 21 as a function of position within the shear zone fluid flow was from high to low temperature early and low to high temperature later in the retrogression

p t paths from garnet zoning a new technique for deciphering tectonic processes in crystalline terranes a procedure to calculate quantitative rock pressure temperature p t paths based on chemical zoning profiles in garnet combined with mineral chemistry from the other minerals in the assemblage has been applied to samples from diverse tectonic settings with the result that the calculated p t path is shown to be a sensitive monitor of tectonic processes terranes where metamorphic recrystallization paths are controlled by uplift and erosion show heating during decompression in the prograde p t path and cooling during decompression in the retrograde path in nappe terranes where hot rocks are emplaced over cool rocks the upper plate shows cooling during decompression whereas the lower plate shows heating during compression in nappe terranes where cool rocks are emplaced over hot the lower plate shows cooling during compression calculation of p t paths from mineral zoning should provide a powerful new tool for deciphering tectonic processes in crystalline terranes

oriented growth of sillimanite in andalusite placitas juan tabo area new mexico u s a in metapelitic schists near the sandia granite in the placitas juan tabo area new mexico nucleation sites for sillimanite are very variable but growth of sillimanite in andalusite is common either as bundles of fibrous fibrolitic sillimanite crystals or as coarser grained parallel prisms or dendritic crystals of sillimanite optical relationships indicate a coaxial relationship between the andalusite and the sillimanite but with a and b axes interchanged stacking faults formed by dissociated screw dislocations in the andalusite may have provided nucleation sites for sillimanite some large sillimanite grains appear to have formed by coarsening of fibrous sillimanite but others show no evidence of such coarsening some fibrous sillimanite occurs in folia in zones of inferred noncoaxial strain accumulation some occurs in radiating or crystallographically controlled aggregates in low strain zones between sillimanite folia these aggregates do not necessarily indicate postdeformation growth exce

bayesian noise reduction in arabia somalia and nubia arabia finite rotations since 20 ma implications for nubia somalia relative motion knowledge of nubia somalia relative motion since the early neogene is of particular importance in the earth sciences because it i impacts on inferences on african dynamic topography and ii allows us to link plate kinematics within the indian realm with those within the atlantic basin the contemporary nubia somalia motion is well known from geodetic observations precise estimates of the past 3 2 myr average motion are also available from paleo magnetic observations however little is known of the nubia somalia motion prior to 3 2 ma chiefly because the southwest indian ridge spread slowly posing a challenge to precisely identify magnetic lineations this also makes the few observations available particularly prone to noise here we reconstruct nubia somalia relative motions since 20 ma from the alternative plate circuit nubia arabia somalia we resort to trans dimensional hierarchical bayesian inference which has proved effective in reducing finite rotation noise to unravel the arabia somalia and arabia nubia motions we combine the resulting kinematics to reconstruct the nubia somalia relative motion since 20 ma we verify the validity of the approach by comparing our reconstruction with the available record for the past 3 2 myr obtained through antarctica results indicate that prior to 11 ma the total motion between nubia and somalia was faster than today furthermore it featured a significant strike slip component along the nubia somalia boundary it is only since 11 ma that nubia diverges away from somalia at slower rates comparable to the present day one kinematic changes of some 20 might have occurred in the period leading to the present day but plate motion steadiness is also warranted within the uncertainties

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

evolution and assembly of archean gneiss terranes in the godthabsfjord region southern west greenland structural metamorphic and isotopic evidence the godthabsfjord region of southern west greenland comprises several terranes that were assembled between 2750 and 2550 ma and folded during amphibolite facies metamorphism the terranes which are dominated by gneisses of different ages and show different preassembly metamorphic and structural histories are 1 the faeringehavn terrane containing the 3820 3600 ma amitsoq gneisses with granulite facies metamorphism at circa 3600 ma 2 the akia terrane containing the 3070 2940 ma nuk gneisses with granulite amphibolite facies metamorphism at circa 2980 ma 3 the tasiusarsuaq terrane containing circa 2900 ma gneisses with granulite facies metamorphism at circa 2800 ma and 4 the tre brodre terrane containing the 2800 2750 ma ikkattoq gneisses with amphibolite facies metamorphism at 2800 2750 ma metamorphic assemblages and structures formed prior to terrane assembly were variably overprinted during amphibolite facies metamorphism and heterogeneous strain associated with assembly recognition of the region as consisting of several terranes indicates that the anatomy of some archean high grade gneiss complexes may resemble that of orogens of proterozoic and phanerozoic age that formed as a consequence of plate tectonic processes

thermotectonic evolution of proterozoic and reworked archaean terranes along the nain churchill boundary in the saglek area northern labrador

structural and metamorphic evolution of the almaden de la plata core seville spain in relation to syn metamorphic shear between the ossa morena and south portuguese zones of the iberian variscan fold belt abstract the almaden de la plata core is a tectonic unit situated in the southern branch of the iberian variscan fold belt it has traditionally been regarded as an eastern extension of the aracena metamorphic belt which is located at the south southeast boundary of the ossa morena with the south portguese zone low pressure high temperature parageneses gt crd sill are found in the highest grade areas while a metamorphic gradient of medium pressure type gt sta bearing schists and gneisses is observed for the middle stages of the prograding metamorphic evolution later processes indicate an evolution to the very low grade locally retrograding previous parageneses five main tectonic events are observed d 1 oriented inclusions within cordierite and garnet and d 2 are not present in the surrounding area d 2 and d 3 have a blastomylonitic high grade metamorphism and mylonitic low grade character respectively d 4 is responsible for the present boundaries of the almaden de la plata core while d 5 corresponds to late hercynian fractures events 1 4 book place under high to low and very low metamorphic grades the metamorphic climax was reached pre to syn d 2 the metabasic rocks acebuches amphibolites show medium to high grade associations hbl plag cpx with a well developed n120 e mineral lineation these features change towards the contact with the south portuguese materials of the pulo do lobo group where metabasites are transformed into very fine grained amphibolitic schists greenschist associations with actinolite and relicts of hornblende and clinopyroxene the later are mylonitic rocks with n150 e stretching lineations and s c structures as well as sigmoidal pressure shadows indicating a southward vergence the low grade metabasites are interpreted as the bottom of a d 3 main thrust which put the ossa morena and south portuguese zones into contact these results support the hypothesis that the acebuches amphibolites of the aracena belt do not pass gradually into the pulo do lobo group but are thrust upon that unit

mineralogy petrology and p t t path of ca na amphibole assemblages saint martin des noyers formation vende france the saint martin des noyers formation is interpreted as a slice of an island arc system of lower to middle palaeozoic age located in the internal part of the variscan orogen in vendee armorican massif france metamorphosed igneous rocks range in composition from ultramafic to rhyolitic the regular increase in the feo feo mgo ratio from mafic to silicic samples results in a systematic variability in the nature and composition of the metamorphic phases in basaltic samples the occurrence of relict garnet barroisite assemblages suggests relatively high pressure conditions for the peak of metamorphism during a subsequent retrograde evolution the primary barroisitic hornblendes recrystallized to texturally complex mixtures of actinolite and hornblende despite this complication it is possible to decipher a p t t path based on amphibole chemistry the p t trajectory deduced is dominated by the effect of pressure and consistent with early underthrusting and subsequent tectonic uplift of the ancient arc of saint martin des noyers

cadomian subduction collision and variscan transpression in the badajoz cordoba shear belt southwest spain abstract the tectonothermal evolution of the badajoz cordoba shear belt southwest iberian massif spain involved the build up of a cadomian upper proterozoic lower paleozoic subduction collision orogenic wedge complex made up of five stacked tectonic slices and its deformation under transpression during hercynian late paleozoic intracontinental shearing episodes cadomian metamorphism in the high grade gneissic units of the wedge complex is characterized by peak metamorphic conditions of 720 c 8 kbar for the lowermost tectonic slice higuera de llerena mylonite gneiss and 670 c 10 kbar in the overlying gneissic slice blastomylonitic migmatic gneisses the p t conditions recorded by eclogites structurally overlying the previous gneisses enhance a scheme of inverted metamorphism since these eclogites equilibrated at 685 700 c 15 kbar during peak metamorphic conditions after the high pressure event the highest temperatures attained by the eclogites correspond to a period of nearly isothermal decompression with a drastic decrease in pressure later on the retrogressed eclogites experienced a pressure increase before they began to reequilibrate under progressively lower pandt conditions similar to those recorded by the retrogression of gneisses such a p increase in eclogites might be related to a period of underthrusting below continental crust responsible for the amalgamation of the different tectonic units of the badajoz cordoba shear belt the present results unravel that the models concerned with the geotectonic evolution of the ibero armorican arc and the respective correlations among hercynian and pre hercynian cadomian for our aims terranes across the north atlantic must be reconsidered

prograde retrograde p t t deformation path of austroalpine micaschists during variscan continental collision eastern alps a complete prograde p t path defined by 10 calculated p t fields in succession is recognized from metapelites by using geothermobarometry on garnet bearing assemblages with microstructural control overstacking of several tectonic units during an early variscan continental collision explains the complex prograde p t history isostatic uplift and deformation controlled the retrograde p t path deformation with changing character acted continuously during all stages of the evolution of the austroalpine basement complex after the intrusion of caledonian granitoids metapelites and magmatic rocks suffered a shearing deformation d1 d2 which produced sheath folds as well as the main foliation s2 spessartine rich first generation garnets situated in microlithons enclosed by s2 record the onset of shearing under increasing high pressure low temperature conditions 7 kbar 380 c geothermobarometry on second generation garnets which have been rotated during growth indicates isothermal decompression from 9 kbar to 5 kbar 500 c and subsequent recompression heating during continuing shearing this is explained by overthrusting of a tectonic unit unit 2 from ne to sw upon the micaschist unit unit 1 followed by isostatic uplift and further overstocking of a third unit unit 3 the resulting pmax of 12 kbar at 650 c and further increasing temperatures up to 680 c accompanied by decompression have been calculated using a third generation of garnets these high pressure high temperature conditions may explain the occurrence of eclogitic metabasites in adjacent regions staurolite and kyanite first appeared under decreasing pressures at the last stage of prograde p t evolution shortening deformation d3 and simultaneous growth of typical amphibolite facies minerals staurolite 2 kyanite 2 sillimanite andalusite occurred during the retrograde path a final step of variscan evolution was marked by an oppositely directed shearing d4 at t 300 c and p 3 kbar possibly indicating backthrusting or extension apart from acid intrusions no signs of a previous caledonian thermotectonic history were found in the area to the south of the defereggen antholz vals line

metamorphic evolution and thermobaric structure of the subduction related bacariza high pressure granulite formation cabo ortegal complex nw spain abstract the high pressure bacariza granulite formation comprises various lithostratigraphic units of granulite orthogneisses ultramafic mg rich mafic intermediate and common mafic granulites as well as of more exotic intercalations mineral assemblages in equilibrium in ultramafic to intermediate rocks contain garnet clinopyroxene and plagioclase with different amounts of zoisite clinozoisite kyanite quartz scapolite rutile and ilmenite depending on the granulite lithotype whereas granulite orthogneisses contain garnet phengite biotite k feldspar antiperthitic plagioclase quartz and rutile as primary phases thermobarometry of these rocks supports the existence of a high pressure metamorphism for which near peak p t conditions have been estimated at ca 790 c and 1 6 gpa the preserved fabrics and structures enable us to relate the metamorphism to coeval polyphasic deformational processes dynamic retrogression began under still high pressure granulite facies conditions 1 4 gpa and ca 740 c and is postdated by symplectitization 1 3 gpa and ca 715 c subsequent retrogression under medium pressure amphibolite facies conditions at similar temperature was either widespread and static or localized and dynamic as a result of intense deformation partitioning during uplift loading heating and subsequent decompression cooling are related to a single cycle in a subduction conduit setting this study suggests that high pressure granulite metamorphism might not be as uncommon in the high pressure metamorphic series as previously thought moreover it might constitute a diagnostic feature of convergent lithospheric settings whether or not associated with eclogite facies metamorphism in adjacent units

the metamorphic architecture of a transpressional orogen the kaoko belt namibia s 64 61 goscombe b hand m 2001b metamorphic model for the inland branch of the damara orogen a paired metamorphic mountain belt pmmb geological society of australia specialist group in tectonics and structural geology conference ulverstone

thermobarometry and p t paths from granulite facies rocks an introduction abstract p t paths from granulite terranes provide unique information about the thermal and tectonic evolution of the lower crust although methods for p t path determination are well established integration of the methods of thermobarometry reaction path analysis mineral zoning studies and diffusion studies with geochronologic analysis of temperature time histories is necessary to further interpret observed assemblages and textures granulite p t paths are characterized by either a phase of nearly isothermal decompression which is likely related to a period of tectonic denudation or nearly isobaric cooling which may be related to the thermal relaxation of the crust following either the addition of heat by magmas or rapid denudation prograde portions of the p t path are required to distinguish further between these options

metamorphic terranes isotopic provinces and implications for crustal growth of the western united states simplified maps of the western cordillera indicating distribution of metamorphic facies and times of principal recrystallization have been compiled despite rearrangement by transcurrent shuffling general trends are recognizable narrow belts of phanerozoic high p blue schist and serpentinized peridotite lie outboard from broad penecontemporaneous high t metamorphosed calcalkaline magma intruded belts bordering the pacific ocean and both are disposed approximately parallel to the present day north american margin they represent recrystallized products of mid paleozoic and younger subduction zones andean type margins and exotic island arcs in contrast dominant metamorphic mineral parageneses within the continental interior are spatially associated with chiefly mesozonal granitic plutons glaucophane schists eclogites and large tracts of ophiolitic ultramafics are absent in part due to selective overprinting of old accretionary margin lithotectonic assemblages by subsequent orogenies high t recrystallization and anatexis for example at depth within the california coast ranges high p phases are currently being destroyed farther inland a general paucity of oceanic petrotectonic assemblages is evident possibly reflecting the differential sinking of dense basaltic peridotitic units during thermal softening preserved metamorphic field gradients reflect the magnitude of subsequent uplift and erosion as well as original pt conditions late mesozoic to mid cenozoic recrystallization accompanied earlier crustal thickening ductile deformation succeeded by later extension brittle faulting mylonitization mesozoic cenozoic metamorphism involved cratonal rocks in the mojave sonoran desert but in nevada and to the north eugeoclinal sections were recrystallized metamorphic intensity decreases eastward toward the phanerozoic hinge line basement rocks in the rockies display the effects of shallow level crustal deformation but much less widespread evidence of post proterozoic plutonism and regional metamorphism initial pb nd and sr isotopic compositions and crystallization ages of igneous rocks in comparison with times of intrusion recrystallization and metamorphic facies distributions are compatible with a geologic scenario involving gradual proximal growth of the ancient continent chiefly southward from the archean wyoming craton during early and mid proterozoic time followed by late proterozoic phanerozoic westward development accretion involved the incorporation of some exotic mostly oceanic outboard terranes however continental growth in the western u s cordillera during both proterozoic and phanerozoic time periods appears to have been due chiefly to partial fusion and magma ascent above subducting paleo pacific lithospheric plates combined with metamorphic sedimentary reworking in the forearc trench back arc setting rather than resulting from the amalgamation of preexisting ancient continental fragments

contact aureoles as constraints on regional p t trajectories an example from the northern alabama piedmont usa contact aureoles of plutons emplaced into regionally metamorphosed terranes can provide indicators of physical conditions along a portion of regional metamorphic p t trajectories thereby allowing reconstruction of more complete p t loops than would be otherwise possible in the northern alabama piedmont of the southern appalachians wedowee group metapelites preserve evidence for two regional metamorphic phases overprinted by contact metamorphism adjacent to the blakes ferry trondhjemite textural evidence indicates that an early bt st grt assemblage was replaced by bt chl grt during the latter stages of regional metamorphism changes in akfm topology complex fe mg ca garnet zoning and the latestage appearance of epidote indicate that a sequence of continuous reactions bt st grt ms followed by chl ms ca grt ca pl bt fe mg grt ep occurred in response to increasing pressure and resulted in the observed changes in mineral assemblage pl ms bt grt thermobarometry indicates conditions of 580 65 c 8 5 0 8 kbar for equilibration of grt bt chl pluton emplacement subsequent to penetrative deformation caused textural annealing and mineral re equilibration by the continuous reaction bt fe mg grt ep chl ms ca grt ca pl within 50 m of the pluton conditions of 510 65 c 5 7 kbar are inferred a reconstructed p t trajectory for this area is characterized by 1 early moderate t moderate p metamorphism 2 an increase in p to approximately 8 5 kbar and 3 decompression and slight cooling prior to pluton emplacement the compressional phase of this path is interpreted to result from underthrusting of the wedowee metasediments to mid crustal levels during palaeozoic crustal thickening late stage decompression prior to intrusion records uplift of these rocks in response to movement on structurally lower thrusts

thermal modeling of extensional tectonics application to pressure temperature time histories of metamorphic rocks one and two dimensional finite difference models are used to generate theoretical pressure temperature time ptt paths for rocks uplifted from deep and intermediate crustal levels during extension via ductile stretching of the entire lithosphere pure shear and via movement of crustal blocks along rooted large scale low angle normal faults simple shear the temperature time paths of rocks uplifted by these pure shear and simple shear mechanisms have similar morphologies although rocks from pure shear settings reach their final depth at higher temperatures than do rocks uplifted by simple shear and experience a greater amount of posttectonic cooling the depth temperature paths of rocks uplifted from deep crustal levels 30 km depth via pure shear extension are often characterized by a period of nearly isothermal high dp dt uplift during the early stages of extension a modified pure shear extension model that includes enhanced heating in the lower lithosphere during extension generally produces ptt paths characterized by a more protracted period of nearly isothermal uplift in contrast simple shear uplift along low angle normal faults produces almost linear depth temperature paths with moderate dp dt and isothermal uplift is not observed for a given initial geotherm the single factor controlling the ptt paths of rocks unroofed below gently dipping normal faults that dip less than 30 is the rate at which unroofing takes place unroofing rate is defined as the rate of uplift relative to the surface and the syntectonic ptt paths are insensitive to fault dip or horizontal displacement rate current geothermometric and geobarometric techniques yield typical uncertainties in real pressure temperature and temperature time data of 1 kbar and 50 c thus it will be difficult to distinguish between these various extensional mechanisms on the basis of ptt data derived from a single sample or from a single structural level although use of data from samples collected from a broad range of structural levels may improve resolution ultimately the best prospect for distinguishing between simple shear and pure shear mechanisms through petrologic work lies in improving the precision of metamorphic pressure and temperature measurements to 0 5 kbar and 25 c

fluid inclusions as petrogenetic indicators in granulite xenoliths pali aike volcanic field chile granulite xenoliths within alkali olivine basalts of the pali aike volcanic field southern chile contain the mineral assemblage orthopyroxene clinopyroxene plagioclase olivine green spinel these granulites are thought to be accidental inclusions of the lower crust incorporated in the mantle derived basalt during its rise to the surface symplectic intergrowths of pyroxene and spinel developed between olivine and plagioclase imply that the reaction olivine plagioclase al orthopyroxene al clinopyroxene spinel 1 occurred during subsolidus cooling and recrystallization of a gabbroic protolith of the granulites

petrological and structural evolution of high grade metamorphic rocks from the floor of the alboran sea basin western mediterranean the metamorphic basement beneath the alboran sea a neogene introduction extensional basin drilled during ocean drilling program odp late orogenic extension of previously thickened conleg 161 site 976 consists of high grade pelitic schist overlying tinental lithosphere is a widely documented phenomenon migmatitic pelitic gneiss inferred assemblages in the high grade e g dewey 1988 malavieille 1993 ruppel 1995 schist evolved from garnet staurolite biotite muscovite but it is unclear whether the thermal evolution of the plagioclase assemblage 1 to biotite sillimanite k feldspar lithosphere in this situation differs significantly from that plagioclase garnet assemblage 2 to andalusite biotite associated with extension in non orogenic settings if k feldspar plagioclase assemblage 3 in the gneiss which extension is triggered by removal of part of the lower shows abundant migmatitic textures the present assemblage is biotite lithosphere e g england houseman 1989 there may andalusite sillimanite muscovite cordierite kbe a detectable rise in temperature during exhumation feldspar plagioclase garnet phase relations and thermoof rocks platt england 1994 whereas extension barometric calculations suggest that the high grade schist experienced caused by far field forces is likely to produce isothermal decompression accompanied by heating from 500 c at 10 5 kbar decompression paths followed by cooling ruppel et al through 600 30 c at 6 7 kbar assemblage 1 and thence 1988 platt et al 1999 decompression accompanied by to 650 700 c at 3 4 kbar assemblage 2 followed by cooling heating in extensional settings has been demonstrated in through 500 600 c at 2 kbar or less assemblage 3 peak the seward peninsula in alaska patrick lieberman temperature and melting occurred under low pressure conditions 1988 the cordillera darwin in the southern andes the widespread disequilibrium and overstepping of metamorphic kohn et al 1993 the tauern window in the alps reactions suggests that decompression was rapid the p t evolution selverstone et al 1984 the aegean buick holland is consistent with metamorphism in a late orogenic extensional 1989 the high himalaya inger harris 1992 and basin and suggests the existence of an external source of heat the betic cordillera van der wal vissers 1993 an probably indicating the complete removal of lithospheric mantle interesting related question is the significance of crustal beneath the extending region melting in extensional settings in principle partial melt

temperature time relationships across metamorphic zones evidence from porphyroblast matrix relationships in progressively deformed metapelites abstract in a rock pile undergoing prograde regional metamorphism the isotherms corresponding to certain metamorphic reactions shift continuously while the same rock volume develops progressive deformation fabrics as a result of increasing strain porphyroblasts in pelitic schists can provide crucial information on both these aspects of metamorphism not only do they reflect the pressure temperature evolution of that terrain but their inclusion trails are clues to the state of deformation at the time of porphyroblast growth microstructural relationships of inclusion trail patterns versus matrix foliations combined with petrological data on prograde reaction sequences can be used to reconstruct temperature time relationships in different zones within metamorphic terrains in two proterozoic areas in northern queensland the western mary kathleen fold belt and the robertson river subgroup a systematic variation of inclusion trail patterns has been found across metamorphic zones each porphyroblast species contains more advanced stages of s 2 foliation development towards its isograd compared with higher grade zones in which these porphyroblasts are present the combined microstructural petrographic observations allow to place constraints on the t t evolution of the various zones the regional distribution of inclusion trail patterns in the two study areas indicates that deformation affected high and low grade zones simultaneously

revisiting the high temperature metamorphic field gradient of the ryoke belt sw japan new constraints from the iwakuni yanai area abstract we revisit the origin of the metamorphic field gradient exposed in the ryoke belt sw japan the type locality for low pressure high temperature l p h t conditions based on combined structural petrological and geochronological data from the iwakuni yanai area w ryoke in this area metasedimentary rocks pass from schist in the north to partly migmatitic paragneiss in the south and are surrounded by numerous granitoid bodies three deformation phases are emphasized an accretionary stage or burial deformation of weak intensity d0 a vertical shortening d1 which generated a subhorizontal locally extensional foliation in the southern gneiss zone and e w upright folding d2 which reworked both the northern schist and southern gneiss zones crystallization deformation relationships indicate that in the schist zone most andalusite cordierite and mica porphyroblasts formed after d0 and before d1 syn d1 mineral assemblages define e w trending metamorphic zones with increasing grade from north to south and rare syn d2 parageneses reveal a similar pattern continuous magmatic activity lasted from ca 105 to 94 ma the oldest granitoids shimokuhara soo namera found in the western part of the belt were emplaced below schistose rocks at 105 100 ma subsequently syn to post d1 granodiorite gamano intruded concordantly with the foliation of southern high grade gneissic rocks from 100 to 94 ma a large syn d1 pluton kibe intruded the center of the belt at 98 ma whereas a younger granite iwakuni and its satellite dykes were emplaced to the northeast at 96 94 ma constraints on the timing of d1 103 99 ma and the likely diachronous d2 phase 100 94 ma suggest a deformation continuum during magma intrusion and metamorphism we conclude that the exposed metamorphic field gradient is an apparent but fortuitous alignment of p t data resulting from spatially and temporally distinct thermal events peak temperature conditions in the relatively shallow schist zone were attained during a first pre tectonic contact metamorphic event a second regional metamorphic overprint arose from the syn tectonic intrusion of granitoids in the deeper gneiss zone a younger contact metamorphic event locally affected the eastern part of the schist zone the revised tectono thermal history highlights the composite and diachronous character of the ryoke metamorphic field gradient

metamorphic response in orogens of different obliquity scale and geometry abstract investigation of material flow within transpressional orogens must involve integration of structural and metamorphic datasets to illustrate the problems in documenting flow vectors we present integrated structural metamorphic datasets from two transpressional systems the kaoko belt in namibia and the kalinjala shear zone in south australia these orogens experienced widely differing metamorphic responses to transpressional deformation integration of kinematic and metamorphic datasets from the kaoko belt indicate shallow up plunging extrusion trajectories in the orogen core and show that the maximum stretching direction pattern matches the inferred flow vectors high grade domains 800 840 c and 7 0 8 0 kb in the orogen core developed low angle upward verging maximum stretching direction trajectories whereas a low grade domain 575 600 c and 5 0 5 5 kb in the orogen core has downward verging lineation trajectories the barometric differential between these high grade and low grade domains is entirely consistent with the angle of plunge of maximum stretching directions within the high grade domains that were extruded obliquely for the amount of lateral shear estimated for the orogen core the kalinjala shear zone in south australia contrasts strongly with the kaoko belt in this example the high grade and high strain shear zone core of the orogen experienced high t high p metamorphism with low thermal gradients of 21 26 c km and steep decompressive p t paths the lower grade external domains experienced lower t lower p metamorphism with high thermal gradients of 35 37 c km sub horizontal maximum stretching directions do not match the vertical extrusional flow in the high grade core that is indicated by the metamorphic data this comparison shows that in general and on a gross scale maximum stretching directions do not necessarily correlate with the real flow vectors experienced during orogenesis in some cases maximum stretching direction recorded by deformation structures is to some degree decoupled from the vertical component of material flow consequently information pertaining to flow is often partitioned into information derived from deformation structures and information derived from the metamorphic record these two datasets must be used in concert to obtain realistic constraints on first order material flow trajectories at orogenic scales the horizontal component of flow is typically best recorded by structural fabrics maximum stretching direction and sense of shear whereas the vertical component is typically best recorded by metamorphic information such as p t paths temperature over depth ratio g and metamorphic field gradients i e t p and g across the orogen

evolution of regional metamorphism in the cape smith thrust belt northern quebec canada interaction of tectonic and thermal processes syn metamorphic re imbrication of the internal part of thrust belts can result in distinct pressure temperature time deformation p t t d pathways for different structural metamorphic domains in the early proterozoic cape smith thrust belt canada an external piggyback sequence thrusting domain is characterized by thermal peak metamorphism occurring after deformation in contrast thermal peak metamorphism in an internal domain occurred during re imbrication by out of sequence thrusting the interactions of tectonic and thermal processes have been studied using three methods i qualitative evaluation of the timing between mineral growth and deformation ii analytical p t paths from growth zoned garnet porphyroblasts and iii numerical modelling of vertical heat conduction derived p t t d pathways suggest that uplift in the external domain resulted in part from erosion and isostatic unloading in contrast paths for the internal domain indicate that the out of sequence portion of the thrust belt may have experienced faster unroofing relative to the external domain this is attributed to thickening by out of sequence thrusting and possibly to extensional faulting at now eroded higher structural levels observations on the timing of metamorphism coupled with numerical modelling suggest that the thermal peak metamorphism documented in the external domain is a consequence of the emplacement of the out of sequence thrusts stack in the internal portion of the thrust belt

the instability of rhabdophane and its unimportance in monazite paragenesis abstract the high retentivity of monazite for pb its high u and th concentrations and its emergence in staurolite zone metapelites suggests its use as a prograde thermochronometer however the monazite producing reactions in pelites are as yet incompletely understood we have investigated by experiment the suggestion that rhabdophane lreepo 4 n h 2 o should be considered as an important reactant in monazite producing reactions in pelitic compositions results of our combined hydrothermal and 1 bar heating experiments preclude rhabdophane from being the immediate precursor to neoformed monazite in lower amphibolite facies metamorphic sequences and restrict rhabdophane occurrence to the uppermost portion of the crust dehydration of rhabdophane to form monazite appears to occur in a manner that may be analogous to the dehydration equilibria of zeolite minerals dry 1 bar heating experiments indicate that la rhabdophane persists metastably to temperatures approaching 500 c xrd and visible near infrared spectroscopy examination of la rhabdophane heated isothermally in air at temperatures ranging from 200 c to 700 c indicate that structural water is largely retained up to 500 c over heating intervals of up to 24 hr structural modifications involving h 2 o appear to take place between 300 and 500 c partial decomposition of la rhabdophane to la monazite is observed at 600 c while rapid and complete reaction to la monazite occurs at 700 c experiments conducted under elevated water pressure 0 5 2 kbar at temperatures between 200 and 600 c have not encountered conditions under which rhabdophane is stable relative to la monazite h 2 o the low rhabdophane dehydration temperatures relative to the temperatures inferred for monazite formation in metapelites indicate that rhabdophane does not convert directly to monazite during pelite metamorphism rather the monazite lree component produced by rhabdophane breakdown must be incorporated in intermediate phases such as allanite and perhaps more likely lree oxides prior to monazite formation

pressure temperature paths from garnet zoning evidence for multiple episodes of thrust burial in the hinterland of the sevier orogenic belt garnets are present in two horizons of the schist of stevens spring from the basin creek area of the grouse creek mountains in northwest utah the two horizons possess different bulk compositions which resulted in garnet growth by different reactions along the same pressure temperature p t path garnet in the upper horizon grew from the breakdown of chlorite at upper greenschist facies conditions and garnet in the lower horizon grew from the breakdown of staurolite at upper amphibolite facies conditions from the upper horizon five garnets from three samples were analyzed all display growth zoning ragged morphologies associated with secondary rim consumption and yield garnet biotite geothermometry temperatures of 460 490 c from the lower horizon one garnet from each of two samples was analyzed these also display growth zoning but differ from garnet in the upper horizon in that they are dominantly idioblastic and yield garnet biotite geothermometry temperatures of 635 c garnet biotite geothermometry calculated for every point along detailed compositional traverses across the garnets revealed localized reequilibration along rims cracks and inclusions in both generations of garnet garnets from the upper horizon display prograde reequilibration and the garnets from the lower horizon display retrograde reequilibration numerical simulations of garnet growth using the gibbs method with duhem s theorem were carried out to determine p t paths the p t path defined by the modeling of five garnets from the upper horizon is an isothermal pressure increase of 1 7 kbar the p t path defined by the modeling of two garnets from the lower horizon has a steep p t trajectory d p d t 32 bars c and a total pressure change of 0 9 kbar both paths are indicative of thrust burial however the two paths cannot be reconciled as products of a single thrust episode these data are interpreted to indicate two different episodes of thrust burial during the sevier orogeny separated by 150 c of heating and partial exhumation

zoned poikiloblastic garnets p t paths and syn metamorphic uplift through 30 km of structural depth wopmay orogen canada

well preserved garnet growth zoning in granulite from the dabie mountains central china ultra high pressure eclogites and granulites both occur in the dabie mountains central china a garnet porphyroblast from felsic granulite in the dabie mountains has been analysed for compositional zoning by electron microprobe two segments of the porphyroblast have opposite compositional variations segment i from centre outward 9 mm to analytical point 18 has decreasing xsps and increasing xpyr while segment ii from analytical point 18 1 mm outward to the rim has increasing xsps and xalm and decreasing xpyr and xgrs the compositional zoning in segment i is considered as growth zoning and that in segment ii as diffusive retrograde zoning garnet growth zoning records a p t path prior to the peak granulite metamorphism the minimum p t conditions are estimated to be 1 35 gpa and 850 c for peak metamorphism based on the highest mg fe mg composition in the garnet analytical point 18 and matrix hypersthene biotite and plagioclase a symplectitic corona surrounds the porphyroblast and appears to have formed at 0 6 gpa and 700 c the well preserved growth zoning in garnet suggests a short residence time for the granulite at peak metamorphism and thus rapid tectonic uplift history the p t path is consistent with that of ultra high pressure eclogite in the area tectonic movements during a collisional event could have brought both the granulite and the eclogite to their present positions

extrusional tectonics in the core of a transpressional orogen the kaoko belt namibia the pan african kaoko belt of nw namibia provides a wellexposed example of material flow in the evolving middle and lower crust during sinistral transpressional orogenesis the kaoko belt is a composite metamorphic belt with shear zone bounded zones of contrasting metamorphic style that were metamorphosed at approximately the same time 575 550ma it is divided into a barrovian style escape zone with inclined nappes and thrusts that verge outward towards the foreland a steep high grade orogen core that experienced intense wrench style deformation and in the hinterland an older 650ma exotic coastal terrane that was reworked at moderate t low p conditions during transpressional orogenesis the kinematic lineation array defined by stretching lineations and shear sense indicators shows variation in degree and polarity of lineation obliquity to the grain of the belt lineation plunge and different polarity of vergence along the plunge with respect to the overall sinistral shear sense the gross regional lineation pattern defines an arcuate array from near orogen parallel in the orogen core to higher angle obliquity across the escape zone reflecting lateral escape towards the orogen margin two highgrade lobes within the orogen core are coincident with obliqueinclined upward flow trajectories based on the kinematic lineation array and represent oblique extrusion of lower crustal material into middle crustal levels an intervening low grade metamorphic trough within the orogen core and the coastal terrane coincides with acute oblique downward and outboard directed apparent flow trajectories these kinematic patterns are consistent with barometric constraints for peak metamorphism of 5 2kbar and 4 5kbar in the low grade trough and coastal terrane and 8 0kbar in the two high grade lobes integration of the kinematic and metamorphic datasets shows that oblique shallowly inclined extrusion trajectories occurred within the orogen core of this classic transpressional orogen and that stretching lineations approximately reflect the particle paths experienced across orogen metamorphic gradients indicate a marked contrast between the escape zone and orogen core the escape zone experienced an inverted barrovian style high p moderate t metamorphism and low thermal gradients of 20 c km the orogen core experienced moderate p high t metamorphism and high thermal gradients of 30 40 c km clockwise p t paths were experienced in all parts of the kaoko belt tight and shallow conduction dominated p t paths were experienced in all orogen core domains involving heating dominated prograde paths followed by decompression consistent with extrusional tectonics steeper and more open advection dominated p t paths were experienced in the crustal over thrust escape zone which experienced low thermal gradients