paragenesis of sodic pyroxene bearing quartz schists implications for the p t history of the sanbagawa belt sodic pyroxene jadeite content x jd 0 1 0 3 occurs locally as small inclusions within albite porphyroblasts and in the matrix of hematite bearing quartz schists in the sanbagawa sambagawa metamorphic belt central shikoku japan the sodic pyroxene bearing samples are characteristically free from chlorite and their typical mineral assemblage is sodic pyroxene subcalcic or sodic amphibole phengitic mica albite quartz hematite titanite epidote spessartine rich garnet occurs in mn rich samples sodic pyroxene in epidote bearing samples tends to be poorer in acmite content average x acm 0 26 0 50 than that in the epidote free samples x acm 0 45 0 47 x jd shows no systematic relationship to metamorphic grade and is different among the three sampling regions saruta gawa asemi gawa and bessi besshi the average x jd of the saruta gawa samples 0 21 0 29 is higher than that of the asemi gawa 0 13 0 17 and bessi 0 14 0 23 the p t conditions of the asemi gawa and bessi regions are estimated at 5 5 6 5 kbar 360 c in the chlorite zone 7 8 5 kbar 440 15 c in the garnet zone and 8 9 5 kbar 520 25 c in the albite biotite zone metamorphic pressure of the saruta gawa region is systematically 1 1 5 kbar higher than that of the asemi gawa and bessi regions and materials of the saruta gawa region have been subducted to a level 3 5 km deeper than materials that underwent metamorphism at equivalent temperatures and are now exposed in the asemi gawa and bessi regions pressure slightly increases toward the north structurally high levels through the sanbagawa belt of central shikoku two types of zonal structure were observed in relatively coarse grained sodic pyroxenes in the matrix one type is characterized by increasing x jd from core to rim the other type by decreasing x jd from core to rim both types of zoned pyroxenes show an increase in x fe 2 fe2 fe2 mg from core to rim the first type of zoning was observed in a sample from the chlorite zone of lowest grade whereas the latter occurs in the garnet and albite biotite zones of higher grade the contrast in zonal structure implies that dp dt during prograde metamorphism decreased with increasing metamorphic grade and may have been negative in some samples from the higher grade zones the estimated dp dt of the prograde stage of the chlorite zone is 3 2 kbar 100 c and that of the garnet and albite biotite zones is 1 8 to 0 9 kbar 100 c the variation of dp dt at shallow and deep levels of a subduction system probably reflects the difference of heating duration and or change in thermal gradient of the subduction zone by continuous cooling of the surrounding mantle

metamorphic evolution partial melting and miocene exhumation of lower crust in the edough metamorphic core complex west mediterranean orogen eastern algeria abstract the edough massif is the easternmost crystalline massif of the maghrebides which represent the southeastern part of the west mediterranean orogen it emerges from below the greenschist facies tellian units that represent the thrusted mesozoic to eocene passive paleomargin of northern africa deposited on the thinned continental crust the top of the crystalline pile of the edough massif is characterised by metasedimentary rocks affected by medium to high pressure 12 14 kbar and medium temperature metamorphic conditions of ca 600 c the northern part of the massif as well as the overlying allochthonous amphibole peridotite unit in the north is characterised by high pressure granulite facies t 800 c and p 12 kbar nearly isothermal decompression is documented in metapelites by late to post kinematic crystallisation of sillimanite and andalusite overprinting kyanite the core of the dome comprises essentially diatexite and granite gneisses a late low pressure evolution and overprinting 2 3 kbar took place in paleozoic rocks in proximity to granite gneisses the predominant igneous mineralogy of granite gneisses suggests that they represent synmetamorphic and syn kinematic sheeted intrusions generated through a high degree melting of metasediments geochronology of the metamorphic rocks u pb on monazite sm nd on garnet wr pairs 40 ar 39 ar on amphiboles and micas yields ages bracketed between 24 and 16 ma suggesting that regional metamorphism of the edough massif took place during the late tertiary synchronous with the greenschist facies metamorphism recorded in the overlying mesozoic tellian units the edough massif represents a metamorphic core complex exhumed through tectonic denudation assisted by syn to late metamorphic low angle n dipping normal ductile shear zones an exhumation rate of 6 8 km ma and associated cooling rate of 200 300 c ma is implied we conclude that the edough massif represents part of the former thinned continental crust of africa involved in a late eocene northward subduction beneath the suboceanic domain that separated paleo europe and africa in mesozoic time the upward syn metamorphic tectonic emplacement at 40 45 km depth of slices of spinel peridotites detached from the lower mantle may be responsible for the local granulite facies occurrences their emplacement is a consequence of the long lived transtensional sinistral then dextral kinematics having taken place between the african and european plates

contact metamorphism and depth of emplacement of the manaslu granite central nepal implications for himalayan orogenesis abstract the manaslu massif central nepal provides a well exposed example of a deeply eroded pluton its contact aureole can be followed from the base to the top along medium to low grade mesozonal to anchizonal tethyan metasediments contact metamorphic mineral assemblages and thermobarometric estimations suggest that the granite was emplaced at 18 21 km for the base and 9 13 km for the roof calculated temperatures within the aureole at 550 40 c are compatible with the intrusion temperature of a leucogranitic magma microstructural evidence shows that the temperature remained high 500 c at the base of the massif during and after granite emplacement whereas towards the top of the granite deformation proceeded rapidly at lower temperature heating of the abundant calcareous rocks in the contact aureole released a local co2 rich fluid whereas a h2o and boron rich fluid seems to have pervaded the whole zone this fluid is probably exsolved from the granitic melt during its crystallization the depth of emplacement of the massif has an important implication for the reconstruction of the himalayan geodynamic evolution implying a burial of the tethyan metasediments by a major refolding of the sedimentary cover or more probably by extensive development of the north himalayan nappes towards the south for more than 70 km before granite emplacement i e before the miocene the young age of the granite and its depth of emplacement suggest a rapid tectonic denudation in the order of 1 mm a 1 probably by normal faulting north of the massif during its cooling

the evolution of water in the contact metamorphic aureole of the duluth complex northeastern minnesota contact metamorphism of the rove formation by the duluth complex northeastern minnesota resulted from the discontinuous reaction chlorite muscovite quartz cordierite biotite h 2 o and the continuous reaction muscovite biotite quartz cordierite k feldspar h 2 o bulk compositions of hornfelses and mass balance calculations indicate that metamorphism occurred isochemically except for the loss of 0 6 to 2 7 wt h 2 o calculated values of temperature and f h 2 o indicate that during progressive metamorphism h 2 o evolved from the rocks progressively diluted a co 2 rich fluid phase the relations among temperature x h 2 o and progress of the continuous reaction in an average bulk composition suggest that at t 500 c and p 1 500 bars chlorite muscovite quartz in 1 m 3 of rock reacted with at least 0 5 m 3 of a co 2 rich fluid producing a fluid with x h 2 o 0 15 the continuous reaction buffered the composition of this fluid to a value of x h 2 o 1 0 at t 600 c the relation between x h 2 o and the progress of the continuous reaction suggests that the volumetric fluid rock ratio decreased from a value of 0 55 to essentially 0 0 as temperature increased from 500 c to 600 c the study indicates that multivariate devolatilization reactions can be capable buffers of fluid composition the dynamic variables rate of temperature change rate of h 2 o evolution and rate of reaction were determined by a heat conduction model for contact metamorphism the calculated width of the aureole 180 m and temperature distribution within the aureole 500 to 620 c are consistent with observations the calculated reaction rates for the continuous reactions ranging from 1 2 10 4 to 3 10 2 moles m 3 yr are much lower than reported rates for other reactions minimum permeabilities implied by the rate of h 2 o evolution ranging from 10 12 to 5 10 15 cm 3 sec cm 3 rock are 4 to 7 orders of magnitude lower than measured permeabilities of shales these results strongly support the assumption of chemical equilibrium during metamorphism even in contact metamorphic aureoles where the rate of temperature change is great

retrograde evolution in the central vosges mountains northeastern france implications for the metamorphic history of high grade rocks during the variscan orogeny abstract metamorphism in the vosges massif is characterized by rocks formed under a wide range of pressures and temperatures high pressure rocks garnet peridotites and eclogites facies rocks are found together with low pressure rocks garnetcordierite stability field hitherto metamorphism has been interpreted within the framework of an identical p t evolution for all rock types corresponding to either two separate geotectonic events or a unique p t evolution from high pressure to low pressure conditions we studied gneisses from the vosges massif in order to constrain the p t t path they followed p t estimation indicates that gneisses equilibrated between 5 and 7 kbar during peak metamorphism time rocks later experienced cooling from temperatures above 650 c down to 550 c it is possible that the entire p t evolution of the gneisses took place within the lower part of the amphibolite facies which is inconsistent with the p t evolution of the high pressure rocks the noteworthy absence of muscovite staurolite assemblages is attributed to conditions of low h2o and low o2 activity the consequences for the geodynamics in the vosges massif are that at least two separate geotectonic events are required in order to explain the juxtaposition of high pressure rocks with low pressure amphibolite facies rocks

metamorphic evolution of the northwestern ogcheon metamorphic belt south korea the ogcheon metamorphic belt omb comprises late proterozoic to paleozoic metasedimentary and metavolcanic sequences which are intruded by mesozoic granitoid plutons to delineate the metamorphic evolution of the omb we have investigated mineral parageneses metamorphic reactions and p t conditions of pelitic and mafic schists in the chungju area northwestern part of the omb the regional metamorphic grade increases north westward from biotite to garnet zones although low pressure assemblages have developed in contact aureoles around jurassic granites garnet crystals show chemical zoning typical for prograde metamorphism with decreasing mn and increasing fe and mg from core to rim compositions of amphiboles constituting the common assemblage in the mafic schist are consistent with those of biotite and garnet zones documented in the medium pressure metabasites p t conditions of the garnet zone estimated from garnet biotite garnet chlorite and amphibole plagioclase geothermometers together with garnet plagioclase biotite muscoviterquartz geobarometers are in the range of 5 8 kbar and 520 5908c retrograde p t path based on fluid inclusion studies suggests that the exhumation of the omb has passed through the p trange of 1 3 kbar and 350 5008c following the isochore curves of the co inclusions in conjunction with structural and geochronologic data we conclude 2 that the omb has experienced a polycyclic p t evolution characterized by 1 crustal thickening during the middle paleozoic time and 2 regional retrograde metamorphism in the triassic our result further suggests that the triassic collision belt in east central china does not pass through the omb q 1998 elsevier science b v

garnetite xenoliths and mantle water interactions below the colorado plateau southwestern united states garnetite xenoliths from ultramafic diatremes in northeastern arizona provide insights into hydration and metasomatism in the mantle the garnetites contain more than 95 garnet some of which has complex compositional zonation related to growth in fractures within grains accessory minerals include rutile ilmenite chlorite clinopyroxene and zircon zircon grains in one rock were analyzed in situ to determine u pb ages and hf isotopic compositions most u pb analyses plot on or near concordia in the range 60 85ma but a few are discordant the range in 176 hf 177 hf is about 0 2818 0 2828 with grains zoned to more radiogenic hf from interiors to rims the garnetite protolith contained zircons at least 1 8ga in age and garnet and additional zircon crystallized episodically during the interval 85 60ma the garnetites are interpreted as mantle analogues of rodingites formed in metasomatic reaction zones caused by water rock interactions in proterozoic mantle during late cretaceous and cenozoic subduction of the farallon plate associated eclogite xenoliths may have been parts of these same reaction zones the rodingite hypothesis requires serpentinization in the mantle wedge 700km from the trench beginning 5 10myr before tectonism related to low angle subduction

evidence from mineral assemblages for infiltration of pelitic schists by aqueous fluids during metamorphism prograde mineral assemblages and compositions have been predicted for pelitic schist in the 10 component system na2o k2o cao mno feo mgo al2o3 sio2 co2 h2o for three cases of prograde metamorphism and fluid rock interaction 1 increasing temperature t at constant pressure p and constant pore fluid volume 1 without infiltration no infiltration case 2 increasing t at constant p accompanied by sufficient fluid infiltration that fluid composition is at all times constant large flux case and 3 increasing t at constantp accompanied by a timeintegrated fluid flux f 104 cm3 cm 2 intermediate flux case stable mineral assemblages and compositions were calculated by solving a system of non linear equations that specify mass balance and chemical equilibrium between minerals and fluid the model pelitic system includes quartz muscovite plagioclasc chlorite ankerite siderite biotite garnet staurolite andalusite kyanite sillimanite k feldspar and a coexisting binary h2o co2 fluid specifically prograde thermal metamorphism was modelled for shaw s 1956 average low grade pelite and for a moderate range of bulk rock compositions at p 3 5 and 7 kb and initial fluids with xco 2 o 0 02 0 40 the model predicts a carbonate bearing mineral assemblage for average pelite under chlorite zone conditions composed of quartz muscovite albite chlorite ankerite and siderite the mineral assemblages predicted for the noinfiltration case are unlike those typically observed in regional metamorphic terranes simulations of metamorphism for the large flux and intermediate flux cases however reproduce the sequence of mineral assemblages observed in normal barrovian regional metamorphic terranes these results suggest that regional metamorphism of pelitic schists is typically associated with infiltration of significant quantities of aqueous fluid

chloritoid bearing rocks associated with blueschists and eclogites northern new caledonia chloritoid bearing metasedimentary rocks occur in close proximity to blueschists and eclogites in the tertiary high pressure metamorphic belt of northern new caledonia the typical assemblage of chloritoid bearing rocks in the epidote zone is quartzchlorite muscovite garnet chloritoid in the omphacite zone epidote is an additional member of the chloritoid bearing assemblage paragonite is rare plagioclase was not detected and rutile and ilmenite are the fe ti oxide phases chloritoid glaucophane is not a common assemblage chloritoid bearing rocks have relatively low ca k na al ratios and the chloritoids are relatively mg rich with mg mg fe up to about 0 4 a comparison of the mineral assemblages and mineral chemistry with experimental and computed phase equilibria suggest an upper temperature limit near 560 c in the omphacite zone and a minimum temperature limit near 450 c at 10 kbar an empirical garnet chlorite fe mg exchange thermometer does not yield consistent results for the higher grade rocks suggesting ts ranging from 390 to 535 c in the omphacite zone and 420 465 c in the epidote zone the distribution coefficient kd fe mg ctd fe mg chl for chloritoid and chlorite ranges from 3 9 to 6 4 values which are lower than those 10 from lower greenschist facies rocks but are near those of upper greenschist facies and albite epidote amphibolite facies

rise and tilt of metamorphic rocks in the lower plate of a detachment fault in the funeral mountains death valley california the funeral mountains in eastern california preserve a record of early cretaceous metamorphism followed by ductile deformation uplift and low angle normal detachment faulting 40ar 39ar age spectra indicate that cooling and uplift of the lower plate began in cretaceous time uplift was accommodated by normal sense movement along a wide northwest dipping shear zone mylonitic fabrics some of which have been dated as late cretaceous deformed older high temperature metamorphic textures analyses of shear bands mica fish and porphyroclasts grain shape fabrics and folds indicate that the upper surfaces moved toward 299 12 top to the northwest relative to lower surfaces uplift continued until the near present the youngest phase being accommodated by top to the northwest movement along the detachment fault which formed subparallel to lower plate mylonitic fabrics fission track apatite data indicate that exposure of the lower plate to the surface occurred sometime after 6 ma reconstruction along the movement vector places the grapevine mountains over the funeral mountains having been displaced at least 40 km isograds and thermobarometry in pelitic schist from the lower plate indicate increasing pressures and temperatures of equilibration toward the northwest the maximum temperature and pressure was determined on a sample from monarch canyon using thermobarometry 700 c at a depth of 32 km at chloride cliff 5 km southeast of monarch canyon 4 samples yielded 575 600 c at depths of 19 27 km at indian pass 17 km southeast of monarch canyon a temperature of 490 c was determined in the southern funeral mountains about 50 km southeast of monarch canyon conodont color alteration indexes indicate temperatures of 325 425 c these data indicate that the lower plate is presently tilted strongly to the southeast from the orientation it maintained at the peak of metamorphism thermochronologic data k ar on muscovite biotite and hornblende 40ar 39ar on hornblende and fission track on apatite titanite and zircon indicate that both tilting and the transition from ductile to brittle styles of quartz deformation are confined to the interval 21 6 ma during the latter part of this interval 11 6 ma rapid uplift and movement along the detachment fault are documented the findings support current theories of detachment fault evolution in which a dipping fault surface undergoes rotation to a subhorizontal orientation while the lower plate undergoes a comparable tilt

geothermometry geobarometry and the al2sio5 triple point at mt moosilauke new hampshire the distribution of aluminum silicate minerals in pelitic schists constrains pressure and temperature conditions during the acadian orogeny to have been approximately those of the al 2 sio 5 invariant point thus providing an excellent opportunity to evaluate the internal consistency of a variety of published geothermometers and geobarometers with the al 2 sio 5 invariant point of holdaway 1971 four fluid independent equilibria yield p t estimates consistent with holdaway 1971 two fluid dependent equilibria yield internally consistent but unrealistically high temperatures our data are consistent with the conclusions that 1 only pyrope grossular mixing in the quaternary garnet system is significantly non ideal and 2 the activity coefficient for the anorthite component in plagioclase is approximately 2 at triple point p t conditions modified journal abstract

internally consistent geothermometry and h 2 o barometry in metamorphic rocks the example garnet chlorite quartz temperature and h2o activity can be determined with high precision using metamorphic mineral assemblages that define both a dehydration equilibrium and a temperature sensitive cation exchange equilibrium such determinations are obtained by applying the gibbs method and then integrating two resulting differential equations as illustrated here for the assemblage garnet chlorite quartz the first equation a geothermometer that monitors temperature based upon fe mg exchange between garnet and chlorite was calibrated using rocks at pecos baldy new mexico 0 0 05 p bars 19 02 t k 4607 ln kd 24 156 with errors of 8 c based upon analytical precision the second equation monitors differences in the activity of water between specimens 1 and 2 0 0 1 xmg chl 1 2 05 p2 p1 33 02 5 96 ln xfe chl 1 xalm 1 t2 t1 2 67 rt1ln a h2o 2 a h2o 1 5 96 t1ln xfe chl 2xalm 1 xfe chl 1xalm 2

two layer tidal circulation in a frictional rotating basin the three dimensional tidal circulation in an elongated basin of arbitrary depth is described with a coupled barotropic and baroclinic two layer model on the f plane as long as friction is not dominant near standing waves are present on the interface as well as on the surface the surface pattern is principally determined by the product of the tidal barotropic wavenumber by the basin length the interface deformation is determined by a baroclinic equivalent usually a much larger number as a result the shape of the interface is characterized by horizontally smaller features than the surface if the product of the tidal baroclinic wavenumber by the basin width is greater than one both lateral and axial modes can be excited at the interface if these modes are near resonant large internal tides can be forced directly by the co oscillating surface tide at the basin entrance the amplitude and phase of the baroclinic component are sensitive functions of the density anomaly and the interface depth as a result the phase and amplitude of the interface vary by large amounts with comparatively small changes in those parameters the model behavior is qualitatively consistent with observations in fjords and straits

late variscan strike slip tectonics between the tepla barrandian and moldanubian terranes czech bohemian massif petrostructural evidence in the variscan bohemian massif czech republic no agreement has been reached on the nature of the relationship between the tepla barrandian and the moldanubian terranes however the tepla barrandian is interpreted as a central segment separating the moldanubian and saxothuringian terranes and its position is therefore essential for any reconstruction of this part of the variscan belt our petrological and structural study in the domazlice klatovy region spans the boundary between the tepla barrandian and moldanubian terranes there is no conformity of ductile structures in both domains near their interface and their p t evolution is separate in time the development of major structures and associated metamorphism in the southwestern part of the barrandian was accompanied by syntectonic intrusions of granitoid and diorite bodies dated at about 520 ma the moldanubian migmatites are characterized by a lp ht metamorphism that possibly took place at about 320 ma the metamorphic record of the barrandian rocks shows no influence of the moldanubian partial melting the contacts are marked by semi brittle to brittle structures sinistral strike slip faulting developed in the central bohemian shear zone whereas the west bohemian shear zone acted as a semi brittle normal fault with a dextral component both shear zones may be nearly contemporaneous accommodating a bulk n s compression the central bohemian shear zone was later reactivated in a dextral regime although timing is uncertain the juxtaposition of both terranes took place probably in the waning stages of the variscan orogeny earlier than the late stephanian we argue that the late orogenic semi brittle strike slip tectonics modified considerably the face of the bohemian massif and the geodynamic models of the bohemian massif have to be reviewed with reference to this essential point

interpreting prograde growth histories of al2sio5 triple point rocks using oxygen isotope thermometry an example from the truchas mountains usa oxygen isotope compositions of kyanite andalusite prismatic sillimanite and fibrolite from the proterozoic terrane in the truchas mountains new mexico differ from one another suggesting that these minerals did not grow in equilibrium at the al2sio5 as polymorph invariant point as previously suggested instead oxygen isotope temperature estimates indicate that growth of kyanite andalusite and prismatic sillimanite occurred at c 575 615 and 640 c respectively temperature estimates reported in this paper are interpreted as those of growth for the different as polymorphs which are not necessarily the same as peak metamorphic temperatures for this terrane two distinct temperature estimates of c 580 c and c 700 c are calculated for most fibrolite samples with two samples yielding clear evidence of quartz fibrolite oxygen isotope disequilibrium these data indicate that locally and potentially regionally oxygen isotope disequilibrium between quartz and fibrolite may have resulted from rapid fibrolite nucleation pressures of mineral growth that were extrapolated from oxygen isotope thermometry results and calculated using petrological constraints suggest that kyanite and one generation of fibrolite grew during m1 at 5 kbar and that andalusite prismatic sillimanite and a second generation of fibrolite grew during m2 at 3 5 kbar m1 and m2 therefore represent two distinct metamorphic events that occurred at different crustal levels the ability of the as polymorphs to retain d 18 o values of crystallization make these minerals ideal to model prograde growth histories of mineral assemblages in metamorphic terranes and to understand more clearly the pressure temperature histories of multiple metamorphic events

regional metamorphism of the vassalboro formation south central maine usa a case study of the role of fluid in metamorphic petrogenesis progressively metamorphosed carbonate rocks of the vassalboro formation serve a natural laboratory for the investigation of fluid rock interactions during metamorphism an integrated study of their whole rock chemistry mineralogy mineral chemistry and mineral abundances has led to the following results a prograde mineral reactions involved hydrolysis as well as dehydration and decarbonation the hydrolysis reactions have resulted in an almost quantitative extraction of k and na from high grade metacarbonates b prograde mineral reactions buffered the x co 2 of coexisting metamorphic fluid during almost all of the rocks metamorphic history c the carbonate rocks were infiltrated by large volumes of h 2 o rich fluid while the buffering reactions progressed during the metamorphic event some high grade metacarbonates were infiltrated by at least 3 rock volumes of fluid combined buffering and infiltration appears to be a general metamorphic phenomenon in carbonate bearing rocks to depths of 30 km in the crust d there is an excellent positive correlation between the heat budget of individual metacarbonate rock samples and calculated fluid rock ratios convective heat transfer by metamorphic fluids is therefore likely to be an important item in the heat budget of metamorphic terrains consideration of these results leads to a model of regional metamorphism which involves infiltration of rock by large volumes of acid h 2 o rich fluid large scale acid infiltration metasomatism in this model the infiltrating fluid exerts enormous control over the mineralogical evolution of the carbonate rocks during the metamorphic event it drives mineral reactions it changes whole rock chemistry and it supplies heat

manganese enrichment in the gowganda formation of the huronian supergroup a highly oxidizing shallow marine environment after the last huronian glaciation article oxidative precipitation and authigenic enrichment of the redox sensitive element mn in sedimentary rocks can serve as a proxy for the release of high levels of o2 during the great oxidization event goe here we investigate mn abundance in sedimentary rocks of the 2 45 2 22 ga huronian supergroup canada we found authigenic mn enrichments with high mn fe ratios following the appearance of fe oxides in the firstbrook member of the gowganda formation of the huronian supergroup which was deposited immediately after the last huronian glaciation the mn bearing minerals in the firstbrook member are spessartine rich almandine and mn bearing chlorite which are likely to have been formed through post depositional diagenesis and or metamorphism using mn oxides precipitated in the ocean at the time of deposition when assuming the solution equilibrium between the atmosphere and shallow oceans oxidative mn precipitation requires that atmospheric o2 be higher than 10 2 times the present atmospheric level pal the cumulative mn amount per unit area in the firstbrook member is comparable in magnitude to that in the mn deposits in the hotazel formation of the transvaal supergroup south africa our results suggest an appearance of highly active aerobic biosphere immediately after the last huronian glaciation supporting the hypothesis that climatic recovery from the huronian glaciation accelerated the goe

construction of a composite pressure temperature path revealing the synorogenic burial and exhumation history of the sevier hinterland usa metamorphic pressure temperature p t paths derived from 16 growth zoned garnets nine from this study and seven from a previous study have been combined to construct a detailed composite path for an area in the hinterland of the cretaceous to early tertiary sevier orogenic belt in southern idaho and north west utah samples are from two proterozoic units in the footwall of the basin elba thrust the schist of mahogany peaks in the central albion mountains idaho and the schist of stevens spring in the basin creek area of the grouse creek mountains utah 40 km to the south the simulated portions of the garnets analysed in this study grew from reactions involving the breakdown of chlorite in the upper greenschist to lower amphibolite facies multiple garnets were analysed from three samples overlapping segments of p t paths from different garnets in the same sample correlate with respect to slope and garnet mn concentration the composite p t path documents three episodes of sharply increasing pressures separated by two episodes of pressure decrease all during progressively increasing temperatures the path is interpreted to represent alternating episodes of synconvergent thrusting and extensional exhumation in the hinterland of the sevier orogen burial was probably caused by the basin elba fault the only major thrust exposed in the region extensional exhumation may have occurred along the mahogany peaks or emigrant spring faults or by extensional reactivation of the basin elba fault this method of correlating partial p t paths to reveal a more complete composite path provides a powerful tool in unveiling orogenic histories in metamorphic terranes where evidence of major structures responsible for burial and exhumation is commonly obscured by later events

local and regional differences in the chemical potential of water in amphibolite facies pelitic schists mineral assemblages in different samples of amphibolite facies pelitic schists collected from two separate outcrops in the moosilauke area nh record differences in the chemical potential of water during metamorphism mineralogical petrological and field relations indicate that mineral assemblages at both outcrops equilibrated at 520 c and 3 5 4 0 kbar thermodynamic analysis of the mineral assemblages demonstrates that maximum chemical potential differences at each outcrop were of the order of 150 calories over distances of 10 20 m the differences in the chemical potential of water recorded in both bed to bed and outcrop to outcrop relations are consistent with the following conclusions 1 mineral assemblages on a specific outcrop did not equilibrate with an external reservoir of fluid of fixed composition 2 the relatively small magnitude of the chemical potential differences suggests little or no infiltration of externally derived fluid 3 these differences on the outcrop scale are probably related to initial compositional variations and the buffer capacity of the mineral assemblage and 4 the different values of the chemical potential of water exhibited by the various mineral assemblages permits an understanding of the effects of variable h2o for amphibolite facies pelitic schists

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

thermochronology of the grenville orogeny in west texas abstract the carrizo mountain group cmg comprises the westernmost exposures of the metamorphic core of the grenville orogen in texas unlike the correlative llano uplift 500 km to the east the cmg underwent one phase of medium p t metamorphism peak t was attained during d2 in all cmg exposures the peak metamorphic paleotemperatures decrease to the northwest in the overall direction of tectonic transport from staurolite to biotite grade garnet in the southeast carrizo mountains grew isothermally and probably during decompression i e part of a clockwise p t loop at ca 520 c in between biotite grade d1 and d3 events hornblende in this region cooled below closure t during d2 at 1057 6 ma within error of muscovite closure in the northwest carrizo mountains at 1059 2 ma hornblende in higher grade exposures to the southeast cooled through closure t as late as 1002 7 ma muscovite in the garnet grade and higher exposures closed to ar diffusion at 992 4 ma to 981 3 ma biotite and partially recrystallized hornblende from the northwest carrizo mountains cooled through closure in this same time period we interpret these data as signifying 1 ar loss due to peak metamorphism in hornblende southeast carrizo mountains and muscovite northwest carrizo mountains at ca 1059 ma 2 post tmax cooling through 500 c in the higher grade outcrops until ca 1000 ma and 3 cooling of all cmg exposures through 300 c due to uplift along the streeruwitz thrust and erosion from 1000 to 980 ma these ages significantly post date known tectonism in the llano uplift and extend the duration of grenville orogenesis in texas by ca 115 million years the 1000 980 ma time period is a time of thrusting in frontal regions of the grenville province of canada and the northeastern us and shearing in the natal province south africa suggesting ultimate closure of the grenville orogen and supporting recent rodinia reconstructions

a regional gradient in the composition of metamorphic fluids in pelitic schist pecos baldy new mexico two metamorphic isograds cut across graphitic schist near pecos baldy new mexico the southern isograd marks the first coexistence of staurolite with biotite whereas the northern isograd marks the first coexistence of andalusite with biotite the isograds do not record changes in temperature or pressure instead they record a regional gradient in the composition of the metamorphic fluid phase ortega quartzite which contains primary hematite lies immediately north of the graphitic schist mineral compositions within the schist change gradually toward the quartzite reflecting gradients in mu text o text 2 and mu text h text 2 text o the chemical potential gradients locally as high as 72 cal m in mu text o text 2 and 9 cal m in mu text h text 2 text o controlled the positions of the two mapped isograds the staurolite biotite isograd records where x text h text 2 text o fell below 0 80 at f text o text 2 near 10 23 bars the andalusite biotite isograd records where x text h text 2 text o fell below 0 25 at f text o text 2 near 10 22 bars dehydration and oxidation were coupled by graphite fluid equilibrium the chemical potential gradients apparently formed during metamorphism as graphite in schist reacted with hematite in quartzite local oxidation of graphite formed co2 which triggered dehydration reactions along the schistquartzite contact this process created a c o h fluid which infiltrated into overlying rocks upward infiltration local fluid rock equilibration and additional infiltration proceeded until the composition of the infiltrating fluid evolved to that in equilibrium with the infiltrated rock this point occurs very close to the staurolite biotite isograd pelitic rocks structurally above this isograd show no petrographic evidence of infiltration even though calculations indicate that volumetric fluid rock ratios may have exceeded 15 and thin rare calc silicate beds show extensive k metasomatism and quartz veining

tectonic characteristics and evolution history of the pearl river mouth basin abstract on the basis of the new understandings from tens of thousand of kilometres of geophysical material and over 60 exploration and evaluation wells providing data on the pearl river mouth basin prmb this paper analyses the special tectonic position of the prmb where three major plates i e the eurasian the pacific and the indo australian plates meet and interact the characteristics of the basin geological structure and its tectonic evolution are also discussed situated on continental and transitional crust the prmb is a mesozoic cenozoic oil and gas bearing basin developed on caledonian folding basement which was reformed during the mesozoic in the process of its formation and evolution the prmb underwent extension slight compression and re extension with the eruption of intermediate acid and basic magma meanwhile it formed double structural layers with faulting below and depression above and sediments were laid down with marine facies following continental facies hydrocarbons accumulated from continental source rocks in the marine reservoir rocks by using a fracture structure model of the uplift and depression the mechanism of the prmb and its evolution history have been interpreted

the geothermobarometric potential of tourmaline based on experimental and natural data the geothermobarometric potential of tourmaline has been assessed by investigating element exchange among tourmaline and coexisting minerals in metamorphosed pelites and graywackes and in experimental exchange between tourmaline and biotite in the natural samples a temperature dependence of tourmaline mg fe exchange with biotite staurolite garnet chlorite and muscovite and ca na exchange with plagioclase is observed equilibrium calculations for the complete mineral assemblage show that tourmaline is in compositional equilibrium with all coexisting phases which would allow for an internally consistent set of thermometers among all these phases to be defined however a prohibitively large spread is present in the k d vs t relations this is not caused by analytical effects compositional zoning or disequilibrium among the minerals the experimental results show that it is the result of inter site partitioning of elements over the y and z octahedral sites of tourmaline variations in the element distribution over these sites their relative participation in the exchange and differences in the temperature dependence of exchange with each site strongly affects the k d vs t relation observed with the slope actually changing sign depending on the elements residing at each site non ideal interactions among the elements at each site will also affect this and furthermore link every exchange to the bulk tourmaline composition and hence the element mobility in the rock the promising potential of tourmaline geothermobarometry can therefore not be fulfilled until effects of inter site partitioning and non ideal interactions are known

pressure temperature path of sanbagawa prograde metamorphism deduced from grossular zoning of garnet the zonal structure of prograde garnet in pelitic schists from the medium grade garnet zone and the higher grade albite biotite zone was examined to investigate the evolution of prograde p t paths of the sanbagawa metamorphism the garnet studied shows a bell shaped chemical zoning of the spessartine component which decreases in abundance from the core towards the rim almandine and pyrope contents and xmg mg mg fe2 increase monotonously outwards the general scheme of the zonal structure for grossular content xgrs ca fe2 mn mg ca can be summarized as 1 xgrs increases outwards inner segment and reaches a maximum at an intermediate position between the crystal core and the rim then decreases towards the outermost rim outer segment 2 the inner segment of garnet in the garnet zone samples tends to have a higher xgrs xsps values for a given xsps than those in the albite biotite zone samples 3 average xsps at the maximum xgrs position in the albite biotite zone samples ranges from 0 02 to 0 12 and is lower than that in the garnet zone samples 0 13 0 32 4 the maximum xgrs in the albite biotite zone samples 0 34 0 39 on average tends to be higher than that in the garnet zone samples 0 26 0 36 and 5 differences of xgrs between the maximum and rim in the albite biotite zone samples are between 0 10 and 0 14 and higher than those in the garnet zone samples 0 11 these facts imply that albite biotite zone materials a were recrystallized under lower dp dt conditions at an early stage of the prograde metamorphism b began their exhumation under higher p t conditions and c have been continuously heated during exhumation for a longer duration than the garnet zone materials the systematic changes of prograde p t paths can be interpreted as documenting the evolution of the sanbagawa subduction zone

tourmaline bearing rocks in the singhbhum shear zone eastern india evidence of boron infiltration during regional metamorphism frozen in reaction textures combined with mineral chemistry of tourmaline bearing metamorphic assemblages provide valuable information about fluid rock interaction during orogenesis tourmaline occurs in four distinct mineralogical associations in the singhbhum shear zone ssz of the precambrian east indian craton the tourmaline bearing rocks are associated intimately with pelitic to psammopelitic and quartzofeldspasthic rocks including meta granite the rocks were affected by two sets of folding f1 and f2 and ductile shearing associated with f1 during ca 1 6 1 8 ga tectonic activity in this belt quantitative geothermobarometry and the stability relations of the metamorphic assemblages developed in the pelitic rocks establish the presence of two episodes of metamorphism a prograde m1 event that culminated at 480 40 c 6 4 0 4 kbar and an m2 event that caused retrogression of the m1 assemblages field and petrographic observations in combination with data on chemical composition of tourmaline indicate three stages of tourmaline growth the earliest stage is represented by pretectonic pre metamorphic greenish yellow tourmaline cores in the tourmalinite from the surda area association a the textural relations demonstrate that in the second stage greenish yellow cores of oscillatory zoned tourmaline in the metagranite albite quartz biotite chlorite muscovite apatite magnetite association c very fine tourmaline grains in muscovite schist muscovite quartz association b and greenish yellow cores of tourmaline in biotite muscovite schist biotite muscovite magnetite chlorite apatite quartz association d crystallized during the f1 m1 events post tectonic greenish blue tourmaline belonging to the third stage replaces the earlier tourmaline grains along their margins and cracks compositionally tourmalines of all four associations fall in the alkali group most of the data fall in the dravite field with a few analyses straddling the boundary between schorl and dravite an abrupt compositional change was noted across the different color zones in the tourmaline grains this change is explained by one or more of the coupled substitutions al nar 1 car naal 1 and mg fe 1 where r mg fe2 mn integrating the field petrographic and phase compositional data it has been demonstrated that tourmaline growth in the second and third stages was induced by infiltration driven b metasomatism during prograde metamorphism of the rocks deep seated magma was the source of the b bearing hydrothermal fluid

analysis of continental midcrustal strain localization induced by microfracturing and reaction softening 1 low angle extensional shear zones which often characterize the brittle ductile transition of the continental crust are seen here to result from strain localization the potentially destabilizing deformation mechanism is assumed to be the progressive transformation of fractured coarse feldspar grains into white mica as observed in the east tenda shear zone alpine corsica the coupling between microfracturing and feldspar to mica reaction is coeval with strain localization that occurred in that field case at a depth close to 15 km this reaction is proposed as the main destabilizing factor responsible for the onset of localization with feldspar having a stationary dislocation creep flow stress larger than mica to test this hypothesis a rheological model is constructed based on the field observations for a mixture of three phases mica quartz and feldspar deforming at a common strain rate the phase concentrations change with time according to the feldspar to mica reaction which takes place only if feldspar grains are fractured a condition detected with the mohr coulomb criterion the tendency for the strain to localize is assessed by numerical means for the structure composed of an upper crust gliding rigidly over the lower crust which sustains an overall simple shear the onset of strain localization is defined by an increase of at least two orders of magnitude in strain rate over part of the lower crust the upper crust gliding velocity has to be increased by at least a factor of 5 for localization to occur the time lapse for this velocity change determines the depth of the shear zone 15 17 km the kinetics of the metamorphic reaction and the final amount of white mica control its width 1 4 km the time of the shear zone formation is less than half a million years

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