small scale heterogeneity of phanerozoic lower crust evidence from isotopic and geochemical systematics of mid cretaceous granulite gneisses san gabriel mountains southern california an elongate belt of mid cretaceous compositionally banded gneisses and granulites is exposed in cucamonga terrane in the southeastern foothills of the san gabriel mountains of southern california banded gneisses include mafic granulites of two geochemical types type 1 rocks are similar to high al arc basalts and andesites but have higher hfse high field strength element abundances and extremely variable lile largeion lithophile element abundances while type 2 rocks are relatively low in al and similar to alkali rich mor midocean ridge or intraplate basalts intercalated with mafic granulites are paragneisses which include felsic granulites aluminous gneisses marble and calc silicate gneisses type 1 mafic granulites and calcic trondhjemitic pegmatites also oceur as cross cutting synmetamorphic dikes or small plutons small scale heterogeneity of deep continental crust is indicated by the lithologic and isotopic diversity of intercalated ortho and paragneisses exposed in cucamonga terrane geochemical and isotopic data indicate that k rb and u depletion and sm nd fractionation were associated with biotite muscovite dehydration reactions in type 1 mafic granulites and aluminous gneisses during high grade metamorphism field relations and model initial isotopic ratios imply a wide range of protolith ages ranging from early proterozoic to phanerozoic

predictive acoustical behavior of rockburst phenomena in gaoligongshan tunnel dulong river highway china abstract the excavation of the gaoligongshan tunnel encountered a complex geological condition resulting in a disastrous rockburst the rockburst during the excavation of the gaoligongshan tunnel led to casualties and to the instability of supporting structures which further damaged boring equipment therefore the forecasting of rockburst events is of great importance in this paper areas with high or low risk of rockburst are identified using the gaoligongshan tunnel stress based rockburst criterion then an acoustic emission ae method is adopted to forecast the magnitude and location of areas at high risk of rockburst based on statistical analysis of ae characteristics and rockburst events an ae based rockburst criterion is proposed as a result in the gaoligongshan tunnel the stress based and ae based rockburst criteria are helpful in determining construction schemes and selecting support measures

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

marine and continental lower permian evaporites of the prypiac trough belarus abstract sakmarian evaporites which include gypsum anhydrite halite carnallite kieserite and bischofite occur in the prypiac trough belarus within the lower and upper parts of one lithostratigraphic unit the svaboda suite the lower part of svaboda suite is up to 143 m thick and is composed of rock salt with interbeds of red claystones siltstones and potassium magnesium and magnesium sulfate salts which are correlated with the lowest potassic salts of the sakmarian kramators k suite of the dnipro donets basin these salts originated from precipitation from marine brines salts occur in a very limited area and the occurrence of sulfate facies is much wider the upper part of the svaboda suite up to 520 m thick is composed of interbedded mixed halite siliciclastic and terrigenous deposits there were two major sources of solutions flowing into the continental basin in which the subsuite was formed meteoric and continental waters and brines derived from the dissolution of devonian salts the distribution of the mixed halite siliciclastic facies was related to tectonic dislocations that controlled the depocenter and the outflow from the underlying famennian deposits of the major volume of salt into the intensively growing salt domes accordingly the tectonic framework was the major control on the evaporite basin during deposition of the upper part of the svaboda suite

carbon isotopic equilibrium between calcite and graphite in skallen marbles east antarctica evidence for the preservation of peak metamorphic temperatures abstract carbon isotope thermometry between calcite and graphite has been applied to the granulite facies marble assemblages in the skallen region of the lutzow holm bay east antarctica the carbon and oxygen isotope values of calcite from five samples which are interlayered with pelitic and charnockitic assemblages suggest within layer isotopic equilibrium textural studies of graphite crystals indicate two types one smooth highly reflecting regular shaped graphite and the other rough dull and irregular shaped graphite the 13 c values of the dull surfaced graphite are lower by about one per mil than the high reflecting graphite carbon isotopic compositions suggest that graphite with lower 13 c values is confined to the surface layer of the graphite crystal this is indicative of a later stage overgrowth during retrograde metamorphism 13 c of dull graphite was excluded in calculating the temperature of peak metamorphism temperatures of 850 c correspond to the peak metamorphic temperatures for the region this temperature estimate is consistent with the regional metamorphic conditions based on cation exchange thermometry in pelitic granulites and is interpreted to be more precise

extension in the central metasedimentary belt of the ontario grenville timing and tectonic significance a laterally extensive zone of southeast dipping marble mylonites in the central metasedimentary belt cmb of the southern ontario grenville the bancroft shear zone records southeast directed extension and a displacement of about 10 12 km calcite graphite carbon isotope thermometry indicates that the maximum temperature at which shearing occurred was ca 450 c in contrast carbon isotope fractionations 13 c cc gr within the marble protolith consistently give temperatures of 650 700 c the bancroft shear zone postdates 1060 ma and older high grade shear zones in the central gneiss belt and the cmb that are associated with northwest directed thrusting comparison of the temperature data from the retrograde marble mylonites with cooling curves for the cmb gives a late grenvillian age range of 935 1010 ma for the extensional event the geometric relation between thrusting and normal faulting in the cmb is comparable to that in parts of the himalayas and southern tibet and similarly we interpret late normal faulting in the cmb to be a result of gravitational collapse of a thrust thickened crust in contrast to the himalaya tibet region however accretion in the grenville becomes older in the direction of regional thrusting

carbonaceous material in pelitic schists of the sanbagawa metamorphic belt in central shikoku japan abstract carbonaceous material in pelitic schists of the sanbagawa metamorphic belt in central shikoku japan was separated from the host rocks and its x ray diffraction and chemical composition were studied its crystal structure and chemistry change continously with increasing metamorphic grade and approach those of well ordered graphite near the biotite isograd as graphitization is a rate process the temperature of complete graphitization differs from one metamorphic terrain to another as a function of the duration of metamorphism in an individual metamorphic terraan however the degree of graphitization is a useful indicator of relative metamorphic temperature in lower grade rocks

comparison of garnet biotite calcite graphite and calcite dolomite thermometry in the grenville orogen ontario canada the elzevir terrane of the grenville orogen in southern ontario contains metapelites and abundant graphitic marbles that were regionally metamorphosed from the upper greenschist to upper amphibolite facies comparative thermometry was undertaken with widely used calibrations for the systems garnet biotite calcite dolomite and calcite graphite temperatures that are obtained from matrix biotites paired with prograde garnet near rim analyses are usually consistent with those determined using calcite graphite thermometry however calcite graphite thermometry occasionally yields low temperatures due to lack of equilibration of anomalously light graphite application of calcite graphite and garnet biotite systems may yield temperatures up to 70 c higher than calcite dolomite in amphibolite facies rocks calcite dolomite temperatures most closely approach those from calcite graphite and garnet biotite when the samples contain a single generation of dolomite and calcite grains contain no visible dolomite exsolution lamellae however some of these samples yield temperatures considerably lower than temperatures calculated from calcite graphite and garnet biotite thermometry indicating that the calcite dolomite thermometer may have been partially reset during retrogression estimated peak metamorphic temperatures of regional metamorphism between madoc upper greenschist facies and bancroft upper amphibolite facies range from 500 to 650 c these results place the chlorite staurolite isograd at 540 c the kyanite sillimanite isograd at 590 c and the sillimanite k feldspar isograd at 650 c although each thermometer may have an absolute uncertainty of as much as 50 c the 50 to 60 c temperature differences between the isograds are probably accurate to 10 to 20 c an incomplete picture of the thermal gradients can result from the application of only one thermometer in a given area simultaneous application of several systems allows one to recognize and overcome the inherent limitations of each thermometer

garnet forming reactions in mafic granulites from enderby land antarctica implications for geothermometry and geobarometry

mineralogy and pressure temperature time path of cretaceous granulite gneisses south eastern san gabriel mountains southern california mid cretaceous granulite gneisses crop out in a narrow belt in the cucamonga region of the south eastern foothills of the san gabriel mountains southern california interlayered mafic granulites and pelitic carbonate calc silicate and quartzofeldspathic metasediments record hornblende granulite subfacies metamorphism at approximately 8 kbar and 700 800 c regional deformation and formation of banded gneisses ceased by c 108 ma although mafic intermediate magmatism and high grade metamorphism continued locally as late as c 88 ma garnet zoning in metapelitic gneisses suggests that peak metamorphism was followed locally by a period of near isobaric cooling but this interpretation requires diachronous cooling of the granulite belt which cannot be demonstrated without detailed thermo chronological data it is more likely that the entire terrane remained at granulite facies p t conditions until 88 ma followed by rapid uplift associated with juxtaposition against adjacent middle and upper crustal arc terranes uplift occurred between c 88 and 78 ma at rates of approximately 1 2 km ma 1 the geotectonic evolution of the cucamonga granulites is similar to mid cretaceous high p granulites in the sierra nevada and salinian block of central california late cretaceous uplift common to these granulites may provide an important tectonic link between dismembered mesozoic batholithic terranes in the california cordillera

reversed experiments on biotite quartz feldspar melting in the system kmash implications for crustal anatexis melting temperatures of biotite quartz and biotite quartz feldspar assemblages in the system k 2 o mgo al 2 o 3 sio 2 h 2 o have been determined in the pressure range 0 5 18 kbar the subsolidus dehydration of biotite quartz also has been investigated from 300 to 500 bars the melting equilibria studied are phlogopite quartz sanidine vapor liquid o phlogopite quartz vapor enstatite liquid k phlogopite quartz enstatite liquid k v and phlogopite quartz sanidine enstatite liquid v at 10 kbar the o reaction occurs at 700 720 c the k reaction at 780 790 c the k v reaction at 815 850 c and the v reaction at 800 850 c vapor saturated reactions were reversed isobarically in two stage experiments in which the starting assemblage was held at a temperature 10 above the temperature of previously observed melting for a duration equivalent to an initial melting experiment followed by lowering of the temperature below the temperature of initial mel

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

constraints on the application of carbon isotope thermometry in high to ultrahigh temperature metamorphic terranes nine marble horizons from the granulite facies terrane of southern india were examined in detail for stable carbon and oxygen isotopes in calcite and carbon isotopes in graphite the marbles in trivandrum block show coupled lowering of 13c and 18o values in calcite and heterogeneous single crystal 13c values 1 to 10 for graphite indicating varying carbon isotope fractionation between calcite and graphite despite the granulite facies regional metamorphic conditions the stable isotope patterns suggest alteration of 13c and 18o values in marbles by infiltration of low 13c 18o bearing fluids the extent of alteration being a direct function of the fluid rock ratio the carbon isotope zonation preserved in graphite suggests that the graphite crystals precipitated recrystallized in the presence of an externally derived co2 rich fluid and that the infiltration had occurred under high temperature and low fo2 conditions during metamorphism the onset of graphite precipitation resulted in a depletion of the carbon isotope values of the remaining fluid calcite carbon reservoir following a rayleigh type distillation process within fluid rich pockets pathways in marbles resulting in the observed zonation the results suggest that calcite graphite thermometry cannot be applied in marbles that are affected by external carbonic fluid infiltration however marble horizons in the madurai block where the effect of fluid infiltration is not detected record clear imprints of ultrahigh temperature metamorphism 800 1000 c with fractionations reaching 1000 c and would be consistent with uht metamorphism

calcite graphite isotope thermometry in amphibolite facies marble bancroft ontario this study presents calcite graphite carbon isotope fractionations for 32 samples from marble in the northern elzevir terrane of the central metasedimentary belt grenville province southern ontario canada these results are compared with temperatures calculated by calcite dolomite thermometry 15 samples garnet biotite thermometry four samples and garnet hornblende thermometry three samples cal gr values vary regularly across the area from 6 5 in the south to 4 0 in the north which corresponds to temperatures of 525 c in the south to 650 c in the north previous empirical calibration of the calcite graphite thermometer agrees very well with calcite dolomite garnet biotite and garnet hornblende thermometry whereas theoretical calibrations compare less well with the independent thermometry isograds in marble based on the reactions rutile calcite quartz titanite and tremolite calcite quartz diopside span temperatures of 525 600 c and are consistent with calculated temperature x co2 relations results of this study compare favourably with large scale regional isotherms however local variation is greater than that revealed by large scale sampling strategies it remains unclear whether the temperature cal gr relationship observed in natural materials below 650 c represents equilibrium fractionations or not but the regularity and consistency apparent in this study demonstrate its utility for thermometry in amphibolite facies marble

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

evidence for the influence of fch 4 on the crystallinity of disseminated carbon in greenschist facies rocks rhode island usa disseminated carbon has been extracted from 19 samples of arkosic rocks from the chlorite biotite and garnet zones in the narragansett basin and analysed by x ray diffraction xrd and scanning electron microscopy sem xrd of these samples yields only the 002 peak which may be as much as four times wider than the silicon internal standard peak and is always skewed to low angles 2 all 002 peaks were digitized and the four moments calculated the results suggest that width at half height w1 2 provides a poorer estimate of peak shape than coefficients of skewness and kurtosis the positive correlation of these coefficients with grain size determined by sem in the range of 0 1 100 m suggest a direct relationship between peak shape and grain size

empirical tests of carbon isotope thermometry in granulites from southern california abstract peak metamorphic carbon isotope fractionations between calcite and graphite cal gr in marbles and calc silicates from the cucamonga granulite terrane san gabriel mountains california range from 3 48 to 2 90 the data are used to test three previously published calibrations of the calcite graphite carbon isotope thermometer an empirical calibration of the calcite graphite carbon isotope thermometer gives temperatures of 700 750 c a theoretical experimental calibration of the system gives temperatures of 760 870 c an experimental calibration gives temperatures of 870 1300 c temperatures calculated using the empirical calibration are in agreement with those calculated from garnet based cation exchange thermometry when uncertainty is considered temperatures calculated using the theoretical experimental calibration overlap the upper range of cation exchange thermometry temperatures and range to 50 c higher the experimental calibration yields temperatures from 50 to 480 c higher than those from cation exchange thermometry moreover temperatures from the experimental calibration are also inconsistent with mineral and melt equilibria in the granulite phase assemblage despite the better agreement between cation exchange thermometry and the empirical calibration of the calcite graphite system temperatures calculated using the theoretical experimental calibration may be real peak metamorphic temperatures if retrograde diffusion partially reset garnet based cation exchange thermometers by c 50 c then the cation exchange temperatures are consistent with those from the theoretical empirical calibration thermometric evidence from biotite dehydration melting equilibria is consistent with either the empirical calibration if melting was fluid present or the theoretical experimental calibration if melting was fluid absent

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

origin and evolution of granulites in normal and thickened crusts rocks buried in the upper part of a doubly thickened crust during orogeny can show prograde reactions with transitional features from lower grade rocks up to granulite facies they can be exposed at the earth9s surface within this one erogenic cycle for example the granulites exposed in british columbia have concordant u pb zircon ages of 65 85 ma in contrast rocks buried in the lower part of a thickened crust will crystallize granulites that typically show only retrograde relations with younger lower grade rocks at the cessation of orogeny such rocks will undergo near isobaric cooling from the peak metamorphism and can have very long residence times at the base of a crust of normal thickness these granulites require a second orogeny to uplift and expose them at the surface as an example the archean granulites of enderby land antarctica were metamorphosed at 3070 ma at 1000 c at 8 10 kbar pressure they then followed at least a 400 c isobaric cooling path they also had a prolonged residence time of 2000 m y near the base of the crust before uplift no doubt a continuum exists between these extremes but it is proposed that many intermediate and high pressure granulites form in the lower parts of thickened crust and therefore require a two stage cycle of tectonism to be uplifted and exposed at the earth9s surface although most granulites probably form in thickened crust at active plate margins underplating of normal 30 40 km thickness crust by mantle melts during extension can also produce granulites these granulites would also undergo isobaric cooling and require a later orogeny to expose them an example is the paleozoic lachlan fold belt of eastern australia which has undergone only minor uplift since the major ultrametamorphism at about 400 ma however isotopic petrologic and geochemical differences are to be expected between these extremes of granulite formation

carbon isotope thermometry in marbles of the adirondack mountains new york carbon isotope thermometry has been applied to coexisting calcite and graphite in marbles from throughout the adirondack mountains new york eighty nine calcite graphite pairs from the amphibolite grade nw adirondacks change systematically in temperature north westwards from 680 to 640 to 670 c over a 30 km distance reflecting transitions from amphibolite facies towards granulite facies to the north west and to the south east temperature contours based on calcite graphite thermometry in the nw adirondacks parallel mineral isograds with the orthopyroxene isograd falling above 675 c and indicate that regional metamorphic temperatures were up to 75 c higher than temperatures inferred from isotherms based on cation and solvus thermometry bohlen et al 1985 fifty five calcite graphite pairs from granulite grade marbles of the central adirondacks give regional metamorphic temperatures of 670 780 c in general agreement with cation and solvus thermometry data for amphibolite and granulite grade marbles show a 12 oo range in 13ccal and 13cgr a strong correlation between carbon isotopic composition and the abundance of graphite cgr crock indicates that the large spread in isotopic compositions results largely from exchange between calcite and graphite during closed system metamorphism the trends seen in 13c vs cgr crock and 13ccal vs 13cgr could not have been preserved if significant amounts of co2 rich fluid had pervasively infiltrated the adirondacks at any time the close fit between natural data and calculated trends of 13c vs cgr crock indicates a biogenic origin for adirondack graphites even though low 13c values are not preserved in marble delamination of 17 graphite flakes perpendicular to the c axis reveals isotopic zonation with higher 13c cores these isotopic gradients are consistent with new graphite growth or recrystallization during a period of decreasing temperature and could not have been produced by exchange with calcite on cooling due to the sluggish rate of diffusion in graphite samples located 2km from anorthosite show a decrease of 0 5 0 8 oo in the outer 100 of the grains while samples at distances over 8 km show smaller core to rim decreases of c 0 2 oo correlation between the degree of zonation and distance to anorthosite suggests that the isotopic profiles reflect partial overprinting of higher temperature contact metamorphism by later granulite facies metamorphism core graphite compositions indicate contact metamorphic temperatures were 860 890 c within 1 km of the marcy anorthosite massif if samples with a significant contact metamorphic effect cal gr 3 2 oo are not included then the remaining 38 granulite facies samples define the relation 13c cal gr 3 56 106t 2 k

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

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

p t grids for silica undersaturated granulites in the systems mas n 4 and fmas n 3 tools for the derivation of p t paths of metamorphism abstract considering the minerals cordierite cd sapphirine sa hypersthene hy garnet ga spinel sp sillimanite si and corundum co in the system feo mgo al2o3 sio2 fmas the stable invariant points are co ga cd and sa constraints imposed by experimental data for the system mas indicate that under low ph2o conditions the invariant points occur at high temperature 900 c and intermediate pressure 7 10 kbar this temperature is higher than that commonly advocated for granulite facies metamorphism in granulites fe mg exchange geothermometers may yield temperatures of 100 150 c below peak metamorphic conditions and evidence for peak temperatures is best preserved by relict high temperature assemblages and by al rich cores in orthopyroxene application of the fmas grid to some well documented granulite occurrences introduces important constraints on their p t histories rocks of different bulk compositions occurring in close proximity in the field may record distinct segments of their p t paths this applies particularly to rocks with evidence for reaction in the form of coronas symplectites and zoned minerals consideration of curved reaction boundaries and xms isopleths may explain apparently contradictory results for the stability of cordierite obtained from low temperature experiments and thermochemical calculations on the one hand and hightemperature experimental data on the other

vapor absent melting at 10 kbar of a biotite and amphibole bearing tonalitic gneiss implications for the generation of a type granites vapor absent melting experiments on a biotite and amphibole bearing archean tonalitic gneiss agc150 at 10 kbar and 875 to 1050 c show that amphibole breaks down from 900 to 950 c producing garnet orthopyroxene and granitic melt biotite dehydration melting produces 10 wt melt up to 950 c via incongruent melting reactions that produce garnet orthopyroxene and titanomagnetite widespread biotite dehydration melting occurs between 950 and 975 c and produces orthopyroxene magnetite titanomagnetite and 20 wt fluorine rich melt up to 0 31 wt f minor f rich 2 7 wt biotite is present even at 1000 c our experiments show that under vapor absent conditions intrusion of hot mantle derived magmas into the lower crust is necessary to initiate widespread biotite dehydration melting in rocks with compositions like agc150 we propose that the high thermal stability of biotite in agc150 suggests that this rock is residual after a previous episode of partial dehydroxylation that left behind somewhat f enriched biotite we show that dehydration melting of such f enriched biotite produces f rich granitic liquids with compositions within the range of a type granite and leaves behind a granulitic residue consisting of orthopyroxene plagioclase quartz titanomagnetite and magnetite

late cretaceous juxtaposition of metamorphic terranes in the southeastern san gabriel mountains california new structural petrological and geochronological data from the southeastern san gabriel mountains define the amalgamation history of suspect terranes in this region four metamorphic terranes constitute the area the cucamonga san antonio san gabriel and baldy terranes and were juxtaposed in two discrete episodes during the late cretaceous the cucamonga san antonio and san gabriel terranes were juxtaposed along subparallel east west trending left lateral mylonitic fault zones during the first episode the cucamonga terrane consists of lower cretaceous granulite facies gneissic rocks these rocks were intruded in the late cretaceous by granitic rocks as the cucamonga terrane was faulted ductilely against the southern margin of the san antonio terrane mylonitized units were subjected concurrently to retrograde amphibolite facies metamorphism metasedimentary pendants in the san antonio terrane were metamorphosed under upper amphibolite facies conditions as plutonic rocks were emplaced syntectonically along its northern boundary the san antonio terrane was juxtaposed concurrently against the san gabriel terrane a terrane consisting of precambrian gneiss and precambrian and mesozoic intrusive rocks the boundary is represented by middle fork complex a ductile zone intermingling rocks of the two terranes after palinspastic restoration of local cenozoic fault displacements the left lateral mylonitic fault zones separating the terranes appear as parts of a major sinistral transform system along the southern margin of the san gabriel mountains displacement for each of the mylonitic fault zones was approximately several tens of kilometers and was part of a broad left lateral shear system this system is modeled as a lateral ramp or tear fault to a west directed synplutonic ductile thrust system underlying the san gabriel terrane crosscutting syn and post tectonic intrusives indicate that most of the displacement occurred between 88 and 78 m y ago no lithologic isotopic or structural data support previous assertions that these terranes were accreted to north america during the tertiary the late cretaceous synplutonic juxtaposition of the cucamonga san antonio and san gabriel terranes was followed by underthrusting of oceanic rocks of the baldy terrane that is pelona schist along the vincent thrust precise timing of this episode is as yet uncertain

apparent isobaric cooling paths from granulites two counterexamples from british columbia and new hampshire textural evidence for the reaction histories of pelitic granulites from british columbia and new hampshire shows that pressure temperature paths obtained from thermobarometry on these samples are an artifact of the calculation technique samples from both localities show breakdown of garnet to form cordierite a reaction that occurs during decompression additional reaction textures and fluid inclusion data also imply decompression at high temperatures for both localities however core rim thermobarometry yields nearly isobaric cooling paths similar cooling paths have been used by other authors to argue for a magmatic underplating model for regional mafic granulites we show that mineral homogenization in pelitic granulites can lead to erroneous isobaric cooling paths and hence to incorrect conclusions concerning tectonic similarities between different types of granulites

fluid heterogeneity during granulite facies metamorphism in the adirondacks stable isotope evidence the preservation of premetamorphic whole rock oxygen isotope ratios in adirondack metasediments shows that neither these rocks nor adjacent anorthosites and gneisses have been penetrated by large amounts of externally derived hot co2 h2o fluids during granulite facies metamorphism this conclusion is supported by calculations of the effect of fluid volatilization and exchange and is also independently supported by petrologic and phase equilibria considerations the data suggest that these rocks were not an open system during metamorphism that fluid rock ratios were in many instances between 0 0 and 0 1 that externally derived fluids as well as fluids derived by metamorphic volatilization rose along localized channels and were not pervasive and thus that no single generalization can be applied to metamorphic fluid conditions in the adirondacks

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

stable isotope thermometry at high temperatures determination of accurate temperatures for geological events has been the grail of stable isotope geochemistry since the seminal 1947 paper by urey in theory this should be simple calibrated mineral pairs are common analysis is rapid and there is no significant pressure correction however in spite of widespread application the promise of reliable thermometry has been elusive stable isotope temperatures in metamorphic and igneous rocks are often controversial is the fractionation between two phases a thermometer speedometer rate dependant hygrometer p h2o dependant or chimera a number of factors have contributed to this uncertainty including incomplete and sometimes conflicting calibration of isotope fractionation factors limited understanding of the kinetics of mineral exchange and the lack of microanalytical techniques this situation has improved dramatically over the past decade permitting reliable and detailed thermal histories to be inferred in a landmark paper bottinga and javoy 1975 empirically calibrated the oxygen isotope fractionation factors among ten rock forming minerals at t 500 c their compilation shows that 84 of analyzed samples contain at least three minerals meeting a concordance test for equilibrium they concluded the great majority of igneous and metamorphic rocks has conserved a state of oxygen isotope exchange equilibrium if correct this bodes well for thermometry however this conclusion was contested by deines 1977 who critically examined the existing data and concluded that less than half of the rocks analyzed to date would yield concordant 18o derived temperatures while seeming irreconcilable both conclusions are based on fact all rocks contain some degree of disequilibrium but many also preserve a thermometric record it is the purpose of this review to discuss the new strategies for reliable isotope thermometry and to explore disequilibrium processes so they may be recognized and properly interpreted the organization of this paper will be to discuss the assumptions

sedimentologic and geochemical constraints on the evolution of bristol dry lake basin california u s a two continuous cores over 400 m in length drilled in bristol dry lake a large playa in the mojave desert california u s a provide sedimentologic and isotopic information which can be integrated with surface trenches and groundwater analyses the combination of these data provide three dimensional data on the facies distribution and geochemical evolution of the basin for the last 4 ma at the surface the basin exhibits a generalized bulls eye facies distribution low gradient alluvial fans ring the playa extensive calcrete and pedogenic calcite associated with halophyte plants cement the mid to distal fan gravels and sands basinward of the distal fans in the playa margin facies an extensive gypsum zone is over 300 m wide within the gypsum zone celestite forms large decimetre size nodules which may coalesce into metre size patches basinward of the playa margin facies halite hopper crystals up to 0 5 m in diameter form in water saturated muds of the saline mud flat facies finally in the basin centre a 0 2 m thick chevron halite crust salt pan forms from evaporation of ponded ephermeral water the lateral facies distribution seen at the surface is found throughout the entire length of the cores the core drilled in the basin centre exhibits beds of alternating salt pan and saline mud flat facies whereas the core drilled in the playa margin exhibits alternating beds of playa margin sediments and saline mud flat for most of the length of the core towards the base of the playa margin core distal alluvial fan sediments are also present stable oxygen isotopes of basin centre calcite concretions from the surface and core indicate that the isotopic composition of the water precipitating the concretions has not changed substantially throughout the evolution of the basin the sulfur isotopic composition of the surface gypsum anhydrite and celestite show little variation however 34s values of anhydrite from core are isotopically lighter relative to surface gypsum and anhydrite this isotopic depletion indicates that bacterial sulfate reduction is involved in the dehydration of gypsum to anhydrite and is not caused by evolving isotopic composition of sulfate in the brine at the surface gypsum 34s values show no statistically significant trends with lateral distribution chemical analyses from shallow groundwater wells and basin centre brines from up to 150 m depth indicate that 1 the brine is dominated by na cl and ca 2 na ca mg k and cl contents increase toward the basin centre so4 increases from the alluvial fan facies to the playa margin and then sharply decreases in the basin centre and si and hco3 decrease slightly toward the basin centre this pattern is consistent with the observed evaporite mineral distribution in the basin 3 molar na cl and ca cl ratios demonstrate that the simple dissolution of previously deposited halite deposits as has been suggested for other playa basins cannot account for the proportions of these ions present in the brine 4 chemical budget calculations suggest that cl can be accounted for by atmospheric input although hydrothermal sources cannot be discounted the repititious nature of the alternating shallow brine pond halite and siliciclastics the consistency of the carbonate isotopic data from the surface and core and the agreement of water chemistry data with observed evaporite sequences indicate a relatively stable brine composition for most of the history of bristol dry lake all sedimentary structures and primary halite fabrics in the core indicate shallow water brine pond halite alternated with halite saturated siliciclastic muds in the basin centre a delicate balance between subsidence and mechanical and chemical deposition of minerals was necessary to maintain the largely ephemeral environment of deposition during the deposition of over 500 m of basin fill