tectono metamorphic evolution of a nappe stack a case study of the swiss alps fold and thrust belts are prominent structures that occur at the front of compressional orogens to unravel the tectonic and metamorphic evolution of such complexes kinematic investigations quantitative microstructural analysis and geothermometry calcite graphite calcite dolomite were performed on carbonate mylonites from thrust faults of the helvetic nappe stack in central switzerland paleo isotherms of peak temperature conditions and cooling stages fission track of the nappe pile were reconstructed in a vertical section and linked with the microstructural and kinematic evolution mylonitic microstructures suggest that under metamorphic conditions close to peak temperature strain was highly localized within thrust faults where deformation temperatures spatially continuously increased in both directions from n to s within each nappe and from top down in the nappe stack covering a temperature range of 180 380 c due to the higher metamorphic conditions thrusting of the lowermost nappe the doldenhorn nappe was accompanied by a much more pronounced nappe internal ductile deformation of carbonaceous rock types than was the case for the overlying wildhorn and gellihorn nappes ongoing thrusting brought the doldenhorn nappe closer to the surface the associated cooling resulted in a freezing in of the paleo isotherms of peak metamorphic conditions contemporaneous shearing localized in the basal thrust initially still in the ductile deformation regime and finally as brittle faulting and cataclasis inducing ultimately an inverse metamorphic zonation with ongoing exhumation and the formation of the helvetic antiformal nappe stack a bending of large scale tectonic structures thrusts folds peak temperature isotherms and cooling isotherms occurred while this local bending can directly be attributed to active deformation underneath the section investigated up to times of 2 3 ma a more homogeneous uplift of the entire region is suggested for the very late and still active exhumation stage

shear wave birefringence a new tool for evaluating fractured reservoirs propagation characteristics of shear waves make them particularly well suited to exploration for and definition of fractured reservoirs the department of geophysics at colorado school of mines conducted a three component p and s wave seismic survey in order to determine whether shear wave polarization and splitting birefringence caused by fracture induced azimuthal anisotropy could be measured over a naturally fractured reservoir results indicate that surface seismic recordings of shear waves can be used to calculate azimuthal anisotropy and that seismic observation of azimuthal anisotropy can provide information about the location orientation and intensity of subsurface fractures data obtained from this investigation indicate that measurement of shear wave birefringence is a potentially powerful tool for predicting fracture orientation and permeability trends

coalification in the northern wildhorn nappe and adjacent units western switzerland implications for tectonic burial histories abstract vitrinite reflectance of fourteen eocene coal samples from the northern wildhorn nappe in the kander valley have been analyzed and compared to nine agewise similar coals from adjacent tectonic units the degree of coalification decreases regularly from semi anthracite in the kander valley to lignite in the plateau molasse the degree of coalification in the tertiary coals appears to be solely dependent on their tectonic history and indicates and estimated 6 km of burial by higher pennine nappes for the kander valley the rank at this locality low volatile bituminous to semi anthracite is significantly higher than in the border chain at beatenberg high volatile a bituminous the latter locality therefore was never buried deeper than about 4 km a further decrease in rank is found in the subalpine flysch and molasse high volatile b bituminous which in turn is significantly more mature than lignites from the folded plateau molasse at linden two samples of dogger coal from the pennine prealpes medianes are medium to low volatile bituminous that is intermediate between the kander valley and beatenberg their rank however was achieved before their final emplacement north of the wildhorn nappe by comparing calculated tti time temperature indices values for time depth burial curves with measured vitrinite reflectances maximum burial temperatures were estimated for the various tectonic units for the wildhorn nappe 160 180 c were obtained in the kander valley and 100 130 c in the border chain as a result of tectonic burial due to the thrusting by the pennine prealps sedimentary burial led to temperatures of 100 120 c in the subalpine molasse below the border chain prior to its thrusting

temperature and strain rate dependent microfabric evolution in monomineralic mylonite evidence from in situ deformation of norcamphor norcamphor c7h10o was subjected to plane strain simple shear in a see through deformation rig at four different strain rate and temperature conditions two transient stages in the microfabric evolution to steady state are distinguished the grain scale mechanisms associated with the microstructural and textural evolution vary with the applied temperature strain rate and strain in high temperature low strain rate experiments computer integrated polarization microscopy reveals that the texture evolution is closely related to the crystallographic rotation paths and rotation rates of individual grains high c axis rotation rates at low to intermediate shear strains are related to the development of a symmetrical c axis cross girdle by the end of the first transient stage 1 5 to 2 during the second transient stage 1 5 to 6 the cross girdle yields to an oblique c axis single girdle as c axis rotation rates decrease and the relative activity of grain boundary migration recrystallization increases steady state 8 is characterized by a stable end orientation of the sample texture and the cyclic growth rotation and consumption of individual grains within the aggregate

petrology of marble and peridotite in the seiad ultramafic complex northern california usa peridotite and infolded marble of the seiad ultramafic complex were recrystallized in the upper amphibolite facies as part of the regional progressive metamorphism of the rattlesnake creek terrane field relations including the occurrence of metarodingites and metasomatic zones between dissimilar rock types demonstrate that the metasediments and serpentinized ultramafic rocks were juxtaposed prior to regional barrovian metamorphism temperatures are estimated to have reached 760 800 c at pressures of 7 8 kbar during the peak of metamorphism four low variance parageneses have been identified within a small 3 km2 area of the complex which may reasonably be assumed to have formed under the same p and t conditions isobaric t xco2 diagrams of appropriate equilibria are presented for three different internally consistent sets of thermodynamic data despite the seemingly small numerical differences between the standard state thermodynamic properties of the data sets only one diagram allows the four observed assemblages to coexist within a reasonable temperature range all three phase diagrams require differences in fluid composition on the scale of a thin section strong evidence for effective control of pore fluid composition by local mineral reactions during metamorphism

mineralogical kar stable and sr isotope systematics of k white micas during very low grade metamorphism of limestones helvetic nappes western switzerland abstract mineralogical k ar rb sr and stable isotope analyses have been carried out on k white micas from helvetic malm limestones in order to examine their evolution during very low to low grade alpine metamorphism associated with intense ductile deformation metamorphic temperatures were estimated at 300 c from stable isotopes quartz calcite thermometry occurrence of chloritoid and epizonal illite crystallinity index k white micas consist of variable mixtures of 2m 1 phengite and muscovite as revealed by detailed x ray diffraction analyses using peak decomposition of the 060 331 spectra k ar apparent ages display a strong grain size dependence in which mainly fine grained size fractions m record alpine ages 37 15 ma however these ages provide a relative rather than an absolute chronology of the diachronous alpine metamorphic evolution of the helvetic nappes the resetting of the k ar isotopic system of k white micas to alpine metamorphic conditions reflects an apparent combination of crystallization recrystallization and radiogenic 40 ar diffusion loss the oxygen isotope compositions of micas 15 to 22 are intermediate between detrital and 18 o enriched values expected for micas neoformed within an abundant marine carbonate matrix no isotopic equilibrium has been reached between calcite and micas the variable depletion of hydrogen isotope compositions 126 to 82 is influenced by the interaction with organic matter under closed system conditions organic matter if not removed may also represent a serious source of error in k ar age determination by introducing radiogenic 40 ar contamination 87 sr 86 sr isotope ratios of micas range from 0 70879 to 0 70902 with one outlier at 0 71794 the low values reflect sr exchange with calcite occurring during crystallization recrystallization of micas under closed system conditions

vitrinite reflectance and shear induced graphitization in orogenic belts a case study from the kandersteg area helvetic alps switzerland vitrinite reflectance vr is an important indicator of incipient metamorphism and it is generally assumed that temperature and to a lesser degree time are the two main variables the role of stress in vr is controversial and will be addressed in this study in the kandersteg area north of lake oeschinen four different helvetic tectonic units are present comprising mainly limestones marls and greywackes of mesozoic and lower tertiary age vr rr rmax rmin and illite crystallinity ic were determined from a very steep section at elevations between 1700 and 2900 m a s l in general rr rmax and ic increase from tectonically higher to lower units i e from upper diagenetic 2 3 to 4 5 rr to upper anchizonal conditions 3 4 to 5 6 rmax several samples contain in addition to coaly fragments abundant semigraphitized and graphitized particles that are believed to be of authigenic origin transitional matter showsrmax values ranging from 7 to about 11 and rmin of 1 3 3 4 optical graphite showsrmax of about 15 16 and rmin of 1 6 1 8 there is good textural evidence that tectonic deformation has played a major role in the development of these strongly coalified particles the first tiny needle like crystals of graphitic material occur within narrow shear zones transecting the rock matrix the beginning of graphite formation parallels the onset of plastic deformation and dynamic recrystallization of quartz elevated vitrinite reflectance rmax and bireflectance values are found near thrust planes where simple optical observation on quartz grain morphology indicates an increase in strain intensity it appears that frictional heating associated with thin shear zones may have been responsible for the formation of authigenic graphite in the kandersteg samples

calcite graphite isotope thermometry a test for polymetamorphism in marble tudor gabbro aureole ontario canada graphitization and coarsening of organic material in carbonate bearing metasedimentary rocks is accompanied by carbon isotope exchange which is the basis of a refractory pressure independent geothermometer comparison of observed isotopic fractionations between calcite and graphite a13ccal gr with independent petrological thermometers provides the following empirical calibration over the range 400 800 c a13ccal gr 5 81 x lo6 x t k 2 61 this system has its greatest potential in marbles where calcite graphite is a common assemblage and other geothermometers are often unavailable the temperature dependency of this empirical calibration differs from theoretical calibrations reasons for this are unclear but the new empirical calibration yields temperature estimates in better agreement with independent thermometry from several terranes and is preferred for geological applications both calcite graphite isotopic thermometry and calcite dolomite solvus thermometry are applied to marble adjacent to the tudor gabbro in the grenville province of ontario canada the marble has undergone two metamorphic episodes early contact metamorphism and later regional metamorphism values of a13cca gr decrease regularly from c 8 0 in samples over 2 km from the pluton to values of 3 4 within 200m of the contact these samples appear to preserve fractionations from the early thermal aureole with the empirical geothermometer and indicate temperatures of 450 500 c away from the intrusion and 700 750 c near the gabbro this thermal profile around the gabbro is consistent with conductive heat flow models in contrast the distribution of mg between calcite and dolomite has been completely reset during later regional metamorphism and yields uniform temperatures of c 500 c even at the contact graphite textures are important for interpreting the results of the calcite graphite thermometer coarsening of graphite approaching the tudor gabbro correlates with the decrease in isotopic fractionations and provides textural evidence that graphite crystallization took place at the time of intrusion in contrast to isotopic exchange during prograde metamorphism which is facilitated by graphitization retrogressive carbon isotopic exchange appears to require recrystallization of graphite which is sluggish and easily recognized texturally resistance of the calcite graphite system to resetting permits thermometry in polymetamorphic settings to see through later events that have disturbed other systems

the structure of the helvetic nappes and its relation to the mechanical stratigraphy abstract this paper discusses structure and variations in structural style due to changes in mechanical stratigraphy of the helvetic nappes based on five cross sections the ratio n of the thickness of mechanically weak to the thickness of adjacent mechanically strong layers is of particular importance a low value of n favours imbricate thrusting and harmonic folding a high value favours detachment and disharmonic folding the transition at around n 0 5 is characterized by polyharmonic folding stratigraphic thickness variations due to local basin subsidence resulted in arcuate fold hinges and important plunges upon basin inversion lateral variations in the overall structure of the helvetic nappes is largely controlled by the initial irregular arrangements of the various small basins one more or less coherent thrust sheet the cretaceous stockwerk of the wildhorn drusberg santis nappe can be traced along the entire fold and thrust belt

experimental study of the influence of stress temperature and strain on the dynamic recrystallization of carrara marble carrara marble has been deformed experimentally at temperatures ranging between 500 and 1000 c at confining pressures of 200 and 300 mpa and up to very large strains in extension and compression in order to study the microstructural and mechanical property changes associated with dynamic recrystallization microstructural studies were made by optical microscopy on ultrathin sections and automatic grain size measurement techniques were used when the temperature is sufficiently high 600 c and the stresses are high enough for deformation twinning to occur twin boundary migration is a powerful recrystallization mechanism that does not modify the grain size at stress levels too low to activate twinning recrystallization occurs in two stages the formation of nuclei by grain boundary bulging and subgrain rotation recrystallization in the grain boundary regions followed by a second stage of grain boundary migration recrystallization to a larger grain size that eventually overprints the entire rock volume the recrystallization process requires larger prestrains at the lower temperatures the size to which migration recrystallized grains grow seems to be limited by the stress and grain size dependent twinning field boundary or by the boundary between the dislocation creep and grain size sensitive flow mechanisms within the range of experimental observations dynamically recrystallized grain size does not depend on strain rate or temperature separate empirical stress versus grain size relations are presented for rotation and migration recrystallization that may be used for palaeopiezometry no unequivocal experimental evidence of weakening resulting from recrystallization to finer grain sizes at laboratory strain rates was found however extrapolation of the experimental data to low natural strain rates suggests that weakening following recrystallization may occur in nature

differences in grain growth of calcite a field based modeling approach normal grain growth of calcite was investigated by combining grain size analysis of calcite across the contact aureole of the adamello pluton and grain growth modeling based on a thermal model of the surroundings of the pluton in an unbiased model system i e location dependent variations in temperature time path 2 3 and 1 3 of grain growth occurs during pro and retrograde metamorphism at all locations respectively in contrast to this idealized situation in the field example three groups can be distinguished which are characterized by variations in their grain size versus temperature relationships group i occurs at low temperatures and the grain size remains constant because nano scale second phase particles of organic origin inhibit grain growth in the calcite aggregates under these conditions in the presence of an aqueous fluid these second phases decay at a temperature of about 350 c enabling the onset of grain growth in calcite in the following growth period fluid enhanced group ii and slower group iii growth occurs for group ii a continuous and intense grain size increase with t is typical while the grain growth decreases with t for group iii none of the observed trends correlate with experimentally based grain growth kinetics probably due to differences between nature and experiment which have not yet been investigated e g porosity second phases therefore grain growth modeling was used to iteratively improve the correlation between measured and modeled grain sizes by optimizing activation energy q pre exponential factor k0 and grain size exponent n for n 2 q of 350 kj mol k0 of 1 7 1021 mns 1 and q of 35 kj mol k0 of 2 5 10 5 mns 1 were obtained for group ii and iii respectively with respect to future work field data based grain growth modeling might be a promising tool for investigating the influences of secondary effects like porosity and second phases on grain growth in nature and to unravel differences between nature and experiment

the concept of steady state foliation abstract a steady state foliation is thought of as a foliation with constant orientation and intensity in a rock undergoing steady state flow the problem discussed is whether such straininsensitive foliations exist in rocks and how they might be distinguished from the more familiar strain sensitive foliations steady state foliations would be useful indicators of stress or of strain rate yet misleading if used as indicators of finite strain one requirement for maintenance of steady state foliations is operation of processes that locally weaken grain shape preferred orientation or reduce grain aspect ratios or destroy lithological layering during flow these are the inverses of the processes normally discussed in connection with foliation development many such processes are known though the details of how and when they operate efficiently enough to stabilize a foliation are unknown there is however a tendency for some of them to operate more efficiently at high strains high temperatures and in flow regimes with high vorticity natural and experimental high temperature shear zones are accordingly one environment in which steadystate foliations may be encountered an experiment is described in which a thin specimen of para dichlorobenzene was given a large shearing deformation at 90 of its absolute melting point a steady state foliation was not produced but the technique is a promising one for further microstructural studies where in situ observation of material undergoing large deformations is desirable

neogene lateral extension during convergence in the central alps evidence from interrelated faulting and backfolding around the simplonpass switzerland abstract the simplon fault zone occurs at the western end of the lepontine metamorphic dome in the central alps and represents a major low angle 25 dip normal fault associated with wsw ene directed lateral extension movement began in the early miocene and continued until at least 3 ma accumulating a total vertical throw of around 10 km corresponding to 24 km displacement parallel to the fault this displacement is partitioned between a broad zone of ductile mylonites in the footwall and a more discrete cataclastic detachment fault the simplon line in the central region simplon village to zwischbergen some 3 km of the total vertical displacement is accommodated by the discrete cataclastic zone due to the higher exposed structural level the importance of the cataclastic component increases towards the northwest in the northern steep belt around the simplonpass retrograde greenschist facies mylonite formation and younger cataclastic faulting associated with lateral extension occurred contemporaneously with sse vergent large scale backfolding representing nnw sse shortening and crustal thickening the combined effect was an overall shortening across the orogen accommodated by an extension parallel to the orogen allowing lateral escape of continental material normal faulting and the resultant lateral escape was not just superficial it extended to depths of at least 15 km and probably to depths exceeding 25 km the overall geometry and kinematics of the simplon fault zone and in particular the interplay between folding and faulting is very similar to that described from the metamorphic core complexes and detachment faults of the cordillera of the southwestern usa within the central alps extension was associated with rapid exhumation of the lepontine metamorphic dome as evidenced by very rapid cooling rates determined from mineral ages in the footwall similar to the cenozoic tectonics of the himalayas and tibet orogen parallel extension in the mountain belt may have been promoted by an overall reduction in the horizontal compressive stress in this direction more limited extension of the same age and orientation is known from a large area of the european foreland greater extension in the mountain belt compared with the foreland was accommodated by major strike slip transfer fault systems and was linked to coeval overthrusting of the lowlands towards the southwest in the external southwestern alps of haute provence france coeval folding and orogen parallel extension such as at the simplonpass were developed in a dextral transpressive regime due to oblique wnw ese convergence across the alps during the neogene

granular flow in polymineralic rocks bearing sheet silicates new evidence from natural examples abstract natural deformation in carbonate mylonites bearing sheet silicates occurs via a complex interaction of granular flow and solution transfer processes and involves continuous cycles of dissolution grain boundary diffusion nucleation and growth in this way new sheet silicates a nucleate within voids formed by grain boundary sliding of calcite grains b grow and c rotate towards the shear plane as a consequence small mica grains show a wide range of orientations with respect to the shear plane but moderate to large grains are subparallel both to each other and to the shear plane increases of average grain sizes with increasing temperature of sheet silicates in mica rich layers is more pronounced than in mica poor layers in the calcitic matrix however sheet silicates can only grow via solution precipitation and mass transfer processes therefore the observed grain size variability indicates drastic differences in mass transfer behavior between the individual layers which might be related to differences in the fluid flux based on these observations a conceptual model for the microfabric evolution in sheet silicate bearing mylonites is presented

shear zones and rock history abstract shear zones that have widened with time can display margin to interior microstructural profiles unlike the real microstructural history of the interior rocks shear zones that have not widened with time can provide more reliable views of microstructural history and therefore of deformation processes even the latter zones can mislead however where there has been an annealing overprint during or following the shearing interval these sources of uncertainty need to be acknowledged more regularly and if possible eliminated by workers interpreting deformation processes from shear zone strain gradients

carbon isotopic thermometry calibrated by dolomite calcite solvus temperatures abstract the temperature dependence of carbon isotopic fractionations between calcite and graphite and between dolomite and graphite are calibrated by the calcite dolomite solvus geothermometry using marbles collected from the contact metamorphic aureole in the kasuga area central japan the carbon isotopic fractionations 13ccc gr and 13cdo gr systematically decrease with increasing metamorphic temperature the concordant relationships between the fractionations and solvus temperatures are approximately linear with t 2 over the temperature range 400 to 680 c 13ccc gr 5 6 106 t 2 k 2 4 13cdo gr 5 9 106 t 2 k 1 9 these systematic relationships between fractionation and temperature suggest that carbon isotopic equilibria between carbonates and graphite were attained in many cases the equation for the calcite graphite system has a slope steeper than bottinga s 1969 results it is however in good agreement with that of valley and o neil 1981 in the temperature range from 600 to 800 c because of the relatively high sensitivity to temperature these isotopic geothermometers are useful for determining the temperatures in moderate to high grade metamorphosed carbonate rocks

thermally induced grain growth of calcite marbles on naxos island greece the island of naxos is composed of an elliptically shaped structural and thermal dome of miocene age peak metamorphic temperatures within the central migmatite complex exceeded 700 c decreasing to about 300 c at the most distant exposures on the island equigranular calcite marbles which outcrop together with metapelites and metabasites over the whole island show a systematic pattern of increasing grain size towards the central migmatite complex with a significant discontinuity in the pattern corresponding approximately with the 500 c isotherm the microstructures and grain size distributions in the marbles are consistent with normal grain growth the variation of grain size with peak temperature attained can be explained equally well by the assumptions that a a maximum grainsize had developed particularly at higher temperatures or that b the grain size had been frozen in by a combination of cooling and coarsening both of which combine to reduce the rate of grain growth

13c 12c exchange between calcite and graphite a possible thermometer in grenville marbles the fractionation of 13c between calcite and graphite cc gr is consistently small 2 6 4 8 permil in 34 assemblages from upper amphibolite and granulite facies marbles of the grenville province in 25 samples from the adirondack mountains new york it decreases regularly with increasing metamorphic temperature the fractionations are independent of absolute 13c values of calcite 2 9 to 5 0 for t 600 800 c the adirondack data are described by cc gr 0 00748t c 8 68 this good correlation between and t suggests that carbon isotope equilibrium was attained in these high grade marbles and that the theoretical calculations of this fractionation by bottinga are approximately 2 permil too large in this temperature range because of the relatively high temperature sensitivity suggested by these results and by bottinga s calculations and the pressure independence of isotope fractionation cc gr may provide a very good thermometer for high grade marbles comparison of this field calibration for cc gr vs temperature with results from other terranes supports the utility of cc gr for geothermometry and suggests that graphite is much more sluggish to exchange than calcite that exchange between calcite and graphite occurs at temperatures as low as 300 c and that equilibrium may normally be attained only when peak metamorphic temperatures are greater than 500 600 c because 13c exchange is an unavoidable metamorphic process at temperatures above 300 c high values of 13c gr in moderate to high grade carbonate bearing rocks do not provide a sufficient criterion to infer an abiogenic origin for the graphite

a model for the formation of the iron manganese and sulphide ores of central sweden the non apatitic iron ores manganese ores and sulphide ores of central sweden which occur in a volcano sedimentary complex of early proterozoic age have a common volcanogenic origin the close association between specific ore types and metavolcanics suggests a stratigraphic control of the ores on a regional scale deposited in basins the ores formed with increasing water depth increasing ph and decreasing eh in the following order quartz banded iron ores mostly lacking sulphides non man ganiferous skarn iron ores with cu fe sulphides manganiferous skarn iron ores with zn pb sulphides and fe2 mn2 silicates eulysites the latter two ore types occur adjacent to argillites greywackes indicating a reducing environment the manganese oxide ores of the langban type represent a formation intermediate between the quartzbanded iron ores and the manganiferous skarn iron ores

a new technique to automatically quantify microstructures of fine grained carbonate mylonites two step etching combined with sem imaging and image analysis a two step etching technique for fine grained calcite mylonites using 0 37 hydrochloric and 0 1 acetic acid produces a topographic relief which reflects the grain boundary geometry with this technique calcite grain boundaries become more intensely dissolved than their grain interiors but second phase minerals like dolomite quartz feldspars apatite hematite and pyrite are not affected by the acid and therefore form topographic peaks based on digital backscatter electron images and element distribution maps acquired on a scanning electron microscope the geometry of calcite and the second phase minerals can be automatically quantified using image analysis software for research on fine grained carbonate rocks e g dolomite calcite mixtures this low cost approach is an attractive alternative to the generation of manual grain boundary maps based on photographs from ultra thin sections or orientation contrast images

the influence of nano scale second phase particles on deformation of fine grained calcite mylonites grey and white carbonate mylonites were collected along thrust planes of the helvetic alps they are characterised by very small grain sizes and non random grain shape spo and crystallographic preferred orientation cpo presumably they deformed in the field of grain size sensitive flow by recrystallisation accommodated intracrystalline deformation in combination with granular flow both mylonites show a similar mean grain size but in the grey mylonites the grain size range is larger the grain shapes are more elongate and the dynamically recrystallised calcite grains are more often twinned grey mylonites have an oblique cpo while the cpo in white mylonites is symmetric with respect to the shear plane combustion analysis and tem investigations revealed that grey mylonites contain a higher amount of highly structured kerogens with particle sizes of a few tens of nanometers which are finely dispersed at the grain boundaries during deformation of the rock nano scale particles reduced the migration velocity of grain boundaries by zener drag resulting in slower recrystallisation rates of the calcite aggregate in the grey mylonites more strain increments were accommodated by individual grains before they became refreshed by dynamic recrystallisation than in white mylonites where grain boundary migration was less hindered and recrystallisation cycles were faster consequently grey mylonites represent deformation microfabrics while white mylonites are characterised by recrystallisation microfabrics field geologists must utilise this different deformation behavior when applying the obliquity in cpo and spo of the respective mylonites as reliable shear sense indicators

carbon isotopic study on coexisting calcite and graphite in the ryoke metamorphic rocks northern kiso district central japan carbon isotope fractionation between coexisting calcite and grpahite 13ccc gr has been determined in metamorphosed limestones and calc silicate rocks from the ryoke metamorphic belt in the northern kiso district in this district the ryoke metamorphic rocks ranging from the lower greenschist facies to the upper amphibolite facies are widely distributed the fractionation of 13c 12c between calcite and graphite decreases regularly with increasing metamorphic grade and is independent of absolute 13c values of calcite this evidence suggests that carbon isotopic exchange equilibrium has been attained during metamorphism even in the greenschist facies and isotopic modification possibly caused by retrogressive metamorphism is not distinguished for t 270 650 c the fractionation is expressed by the following equation 13ccc gr 8 9 106t 2 7 1 t in k