dating the subduction of the arabian continental margin beneath the semail ophiolite oman the presence of high pressure p eclogites along the northeast margin of the arabian continental plate beneath the semail ophiolite demonstrates that subduction of thinned continental crust to pressures of 20 kbar 80 km depth has occurred current debate centers on whether this high p metamorphism occurred prior to during or following ophiolite emplacement which is known to have started ca 95 93 ma five concordant u pb zircon ages from a garnet clinopyroxene phengite crossite eclogite from as sifah northeast oman precisely constrain the age of high p metamorphism at 79 1 0 3 ma this age demonstrates that high p metamorphism occurred during continental subduction 15 m y after formation of the ocean floor that subsequently became the semail ophiolite

two high pressure metamorphic events in ne oman evidence from 40 ar 39 ar dating and petrological data the saih hatat tectonic window in ne oman exposes basement and shelf units that structurally underlie the semail ophiolite these units were metamorphosed under high pressure conditions as evidenced by the occurrence of lawsonite schists carpholite bearing metasediments blueschists and eclogites conventional k ar and 40 ar 39 ar dating of whole rocks and white mica separates from the structurally highest region i of saih hatat and the lowest grade blueschists of the northern part of the structurally lowest saih hatat region iii yield ages of 72 80 ma white micas from the high grade blueschists and eclogites of region iii which formed at t 340 c yielded variably discordant age spectra with weighted mean plateau ages of 106 111 ma the age data combined with structural and petrological criteria suggest that many units exposed in saih hatat experienced two high pressure low temperature high p t metamorphic events the first in the early cretaceous was possibly a result of partial su

reversals in fe mg partitioning between chloritoid and staurolite chloritoid and staurolite coexist with al silicate chlorite or garnet or biotite in precambrian quartzite and schist from northern new mexico the observed fe mg reversal is not related to variable p t or minor element content including fe super 3 however it could arise from any of three factors 1 fe and mg may occur on several crystallographic sites in one or both minerals 2 some mg may not be exchangeable with fe in staurolite or 3 fe and mg may mix non ideally in one or both phases modified journal abstract

geologic section through the samail ophiolite and associated rocks along a muscat ibra transect southeastern oman mountains regional mapping at a 1 60 000 scale of a 30 km strip from the gulf of oman muscat across the oman mountains 130 km to the south provides the geologic setting for the 95 m y ibra section of the samail ophiolite where best preserved the ibra ophiolite section is an 8 km thick section of oceanic consisting of 0 5 km of pillow lavas 1 2 1 6 km of sheeted diabase dike complex 0 2 1 0 km of high level noncumulate gabbro and 3 0 5 0 km of cumulate gabbro that is underlain by tectonite peridotite 9 12 km thick the ibra section is found on the southward dipping limb of the sayah hatat antiform the tectonite peridotite represents uniformly depleted harzburgite and dunite that have been deformed by high temperature low stress asthenospheric flow discordant dunites within the tectonite peridotite appear to represent either flow crystallization products from primary picritic liquids or reaction products of these liquids with the harzburgite the structural base of the tectonite peridotite is overprinted by a high stress low temperature deformation that can be related to its oceanic detachment the layered gabbros are predominantly olivine clinopyroxene plagioclase cumulates and orthopyroxene does not occur as a cumulus phase occurrence of cumulate wehrlites and picrites at high stratigraphic levels within the layered gabbros is evidence that the gabbroic section crystallized predominantly from the bottom upward in a periodically replenished magma chamber high level gabbro represents remnants of crystallization at the roof of the magma chamber and intrudes most overlying diabase dikes both the diabase dike complex and pillow lavas are hydrothermally altered and alteration and metamorphism increase downward zeolite to epidote amphibolite facies in spite of pervasive alteration relict primary mineralogy and bulk chemistry suggest that the diabase dikes and pillow lavas are cogenetic with the underlying gabbros the present day samail thrust surface truncates ophiolite stratigraphy and puts the samail ophiolite on top of unmetamorphosed hawasina melange the last motion on this surface was probably no older than maestrichtian 70 65 m y garnet amphibolites exposed as remnants of earlier thrusting 90 m y record initial ophiolite detachment at a 14 20 km depth within the tethyan oceanic lithosphere the hawasina group which underlays the samail ophiolite is a block melange where exposed near muscat in the north and it grades into an imbricated broken formation at the southern limit of the map area the hawasina group is thrust over permian to late cretaceous shelf carbonates that represent autochthonous arabian continental shelf deposits recent post miocene collapse and dome structures such as the ibra dome have complicated ophiolite stratigraphy south of jabal dimh our geologic studies strongly indicate the samail ophiolite represents a large coherent slab of transported oceanic lithosphere formed at a late cretaceous spreading center in the tethyan sea

petrology of fe mg carpholite bearing metasediments from ne oman abstract fe mg carpholite occurs in metasediments of tectonically disrupted basement shelf and foreland basin units that structurally underlie the semail ophiolite in ne oman in the lower grade structurally higher units fe rich carpholite coexists with paragonite quartz illite kaolinite and chlorite whereas in deeper units fe mg carpholite occurs with pyrophyllite sudoite phengite and or chloritoid mineral compositions in these units indicate that chlorite is more magnesian than coexisting fe mg carpholite at low temperatures and pressures but at higher metamorphic grades xmg decreases in the order sudoite carpholite chlorite chloritoid this suggests a reversal in fe mg partitioning between fe mg carpholite and chlorite at temperatures below or close to those of the breakdown of kaolinite quartz to pyrophyllite and at xmg 0 35 phase relations and mineral equilibria indicate that the p t conditions of formation of the fe mg carpholite bearing rocks of ne oman range from 280 315 c 3 6 kbar for the structurally highest units to 325 440 c 6 9 5 kbar for the deepest units indicating a systematic down section increase in metamorphic grade textural relations in these rocks interpreted in the context of pertinent equilibria are consistent with the clockwise p t paths previously constrained for these units from petrological studies of interlayered isofacial mafic rocks

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

regional distribution of white k mica polymorphs and their phengite content in the central alps some 150 white k micas from the central alps were analysed for their polymorph and phengite content

glaucophane chloritoid bearing assemblages from ne oman petrologic significance and a petrogenetic grid for high p metapelites pelitic layers and lenses interbedded with blueschists and eclogites in saih hatat ne oman contain chloritoid and sodic amphibole bearing mineral assemblages that are useful for reconstructing the p t history of the area textural and mineral chemical relations suggest that coexisting glaucophane gln and chloritoid ctd formed at the expense of chlorite chl paragonite pg and later broke down to garnet gt pg during prograde metamorphism according to the reaction gln ctd qz gt pg h2o during retrogression gln and chl first formed at the expense of gt and pg followed by the breakdown of ctd and gt to chl the final stages of retrogression are marked by the breakdown of gln to an aggregate of chl albite ab a projection from quartz qz h2o and phengite ph on the al2o3 fe2o3 feo mgo na2o plane in the system nfmash is best suited for the representation of the phase relations in high p metapelites petrogenetic grids for the model systems nmash and nfash were calculated using program geo calc berman et al 1987 and its database berman 1988 after the retrieval of s i o and h f o for gln and ctd by mathematical programming and calculating all possible reactions among gln ctd chl jadeite jd ab gt pg talc tc pyrophyllite prl and kyanite ky the calculated petrogenetic grid for the system nfash shows that fe ctd and ferroglaucophane coexist at p 6 5 kbar and t 6 kbar this grid is consistent with the p t estimates for high p metapelites from oman new caledonia seward peninsula ile de groix sifnos and peloponnese where gln ctd bearing units are interbedded with cofacial mafic blueschists and eclogites the grid also explains the observed textural relations in the metapelites of oman and is consistent with the clockwise p t path proposed for this area but differs from the grids of guiraud et al 1990 in showing a larger stability field for gln ctd that overlaps with the stability fields of jd qz as well as ab the petrogenetic grid calculated for the system nmash shows that mg ctd gln bearing assemblages require unusually low geothermal gradients to form in metapelites it also shows that mg rich ctd tc coexist at high pressures over a wider p t range than predicted by guiraud et al 1990 this grid can therefore be applied to high p metamorphic assemblages from the eastern and western alps

petrology of lawsonite pumpellyite and sodic amphibole bearing metabasites from north east oman the assemblages chlorite pumpellyite lawsonite albite quartz chlorite lawsonite quartz epidote and chlorite epidote albite quartz occur in metabasaltic blocks and veins in a metamorphosed tectonic melange in the structurally highest unit of the autochthonous and parautochthonous section underlying the semail ophiolite in saih hatat north east oman the pre permian basement of this section contains mafic units characterized by the assemblage crossite epidote chlorite quartz albite el actinolite these assemblages indicate a down section increase in metamorphic grade from lawsonite albite facies conditions in the melange to epidote blueschist facies conditions in the basement application of empirically and experimentally based thermobarometers as well as petrogenetic grids calculated for a model basaltic system indicates that the p t conditions of metamorphism ranged from 3 to 6 kbar and 250 to 300d c for the melange and p 6 8 kbar t 310d c for the basement units textural relations interpreted in the context of petrogenetic grids indicate that these units followed clockwise p t paths of evolution the estimated p t conditions and down section increase in metamorphic grade in central western and northern saih hatat are consistent with the hypothesis relating metamorphism to the late cretaceous tectonic loading of the continental margin by an ophiolite slab 18 km in thickness these results contrast with field and petrological observations documented for blueschists and eclogites exposed along the eastern coast of saih hatat which may have formed at an earlier stage in response to an early cretaceous collisional event

obduction related planar and linear fabrics in oman under the oman ophiolites three different types of alpine synmetamorphic fabrics are seen each of them is taken as the signature of a particular stage of the obduction process the complex high grade fabric in the basal metamorphic sheet is linked to the intraoceanic detachment stage pre obduction sensu stricto the l s fabric of the muscat nappes and hatat internal border associated with low temperature high pressure metamorphism is regarded as the effect of an early obduction that is the epicontinental thrusting of the matrah peridotites the much simpler fabric of the anchizonal hawasina and autochthonous cover resulted from the gravity sliding of the semail ophiolites during the final obduction phase

uplift related retrogression history of aragonite marbles in western crete greece in the low grade high pressure 400 c 10 kbar metamorphic phyllite quartzite unit of western crete widespread occurrences of aragonite marbles have been discovered recently a sedimentary precursor is proved by relic structures bedding fossils partial or complete transformation of aragonite into calcite is ubiquitous compositional and microstructural features reflect the metamorphic history 1 the high pressure stage is documented by aragonite that is chemically characterized by incorporation of variable amounts of sr and the lack of mg the most sr rich aragonite has about 9 wt sro xsrarag 0 09 a compositional zoning observed in some aragonite crystals may be due to the prograde divariant calcite aragonite transformation in the system caco3 srco3 because the parent rocks probably were sr poor calcite limestones one can speculate that strontium has been supplied from an external source under high pressure conditions 2 during uplift calcite replacing aragonite did not equilibrate with unreplaced aragonite disequilibrium is indicated by highly variable compositions of calcite crystals that show topotactic relations to the host aragonite the calcite compositions range from that of the host aragonite sr rich and mg free to mg bearing and sr poor 3 calcite that recrystallized during retrogression is generally sr poor mean value ofxsr 0 005 mg bearing xmg 0 010 and chemically homogeneous because practically no sr remains in the calcite an interaction with a fluid phase is indicated in fine grained calcite marbles rich in solid organic matter microstructural features indicative of former aragonite may be present 4 the last stage of retrogression is documented by the appearance of radiating aragonite in veins and nodules this aragonite which shows neither deformation nor retrogression was probably formed metastably in a near surface environment

a case of obduction related high pressure low temperature metamorphism in upper crustal nappes arabian continental margin oman p t paths and kinematic interpretation in the southeastern oman mountains the saih hatat tectonic window reveals arabian continental material beneath the oceanic sensu lato nappes i e the hawasina sedimentary units and the huge sumail ophiolite the northeastern internal half of the window suffered an intense deformation associated with a high pressure low temperature hp lt metamorphism approximately dated at 80 75 ma according to the available stratigraphie data after the deformation due to the absence of collision the obduction inherited structural and metamorphic pattern was virtually unmodified a metamorphic map is established based on mineral assemblages in metapelites and metabasites low grade assemblages occur in the highest units muscat nappes quryat unit characteristic minerals are femg carpholite lawsonite associated with kaolinite or pyrophyllite and locally sudoite in the metapelites while lawsonite and blue amphiboles are found in the metabasites high grade rocks outcrop in the deepest and easternmost units as sifah antiform and are typified by an epidote glaucophane bearing eclogite assemblage intermediate units hulw show assemblages of intermediate grade chloritoid schists and glaucophane bearing metabasites higher grade assemblages such as talc chloritoid were not found inverted gradients are observed locally in the muscat nappes pyrophyllitebearing units above kaolinite quartz bearing units and these are ascribed to late metamorphic thrust displacements which are demonstrated by microstructures a complex p t evolution is fairly well documented by the mineral assemblages prograde evolution in the subophiolitic continental rocks began with a rather hot gradient of about 30 35 c km with chloritoid growth in the higher units thereafter pressure increased with the temperature remaining constant or even decreasing chloritoid replaced by kaolinite oxides and carpholite growth in synkinematic conditions the peak p t conditions are close to 8 kbar 270 c for the muscat nappes and 11 kbar 400 c for the as sifah unit the corresponding surface related gradient is low 10 15 c km during the retrograde evolution pressure and temperature generally decreased concomitantly but in some of the upper units temperature remained constant or increased slightly for some time at which point sudoite growth from carpholite took place the metamorphic pattern and mineral evolution are interpreted in the framework of the late cretaceous obduction the already cold oceanic lithosphere was thrust upon a relatively hot thinned passive margin with coeval basaltic volcanism pressure progressively increased in the passive margin due to the wedge shape of the advancing ophiolite the maximum thickness of which was about 25 km in the as sifah area in this area the overburden was also attributable to the accumulation of continental material up to 5 km thick by means of a complex set of synmetamorphic often conjugated folds and thrusts the basal peridotites of the obducted slab were probably not hotter than 300 c during its thrusting upon the continental margin the temperature of the tectonic pile below the ophiolite remained low because it was thrust above more external and still colder continental areas late thrusting in the same southerly direction controlled the retrograde evolution together with erosional and gravity driven tectonic unloading

phase relations in the greenschist blueschist amphibolite eclogite facies in the system na 2 o cao feo mgo al 2 o 3 sio 2 h 2 o ncfmash with application to metamorphic rocks from samos greece calculated phase equilibria among the minerals sodic amphibole calcic amphibole garnet chloritoid talc chlorite paragonite margarite omphacite plagioclase carpholite zoisite clinozoisite lawsonite pyrophyllite kyanite sillimanite quartz and h2o are presented for the model system na2o cao feo mgo al2o3 sio2 h2o ncfmash which is relevant for many greenschist blueschist amphibolite and eclogite facies rocks using the activity composition relationships for multicomponent amphiboles constrained by will and powell 1992 equilibria containing coexisting calcic and sodic amphiboles could be determined the blueschist greenschist transition reaction in the ncfmash system for example is defined by the univariant reaction sodic amphibole zoisite calcic amphibole chlorite paragonite plagioclase quartz h2o occurring between approximately 420 and 450 c at 9 5 to 10 kbar the calculated petrogenetic grid is a valuable tool for reconstructing the pt evolution of metabasic rocks this is shown for rocks from the island of samos greece on the basis of mineral and whole rock analyses pt pseudosections were calculated and together with the observed mineral assemblages and reaction textures are used to reconstruct pt paths for rocks from northern samos pseudomorphs after lawsonite preserved in garnet the assemblage sodic amphibole garnet paragonite chlorite zoisite quartz and the retrograde appearance of albitic plagioclase and the formation of calcic amphibole around sodic amphibole constrain a clockwise pt path that reaches its thermal maximum at some 520 c and 19 kbar the derived pt trajectory indicates cooling during exhumation of the rocks and is similar to paths for rocks from the western part of the attic cycladic crystalline complex rocks from eastern samos indicate lower pressures and are probably related to high pressure rocks from the menderes massif in western turkey

a resolution of discrepant thermodynamic properties for chamosite retrieved from experimental and empirical techniques an important deficiency in the thermodynamic data for common rock forming minerals involves chamosite the fe2 end member of chlorite solid solution the ag3 and al13 values for chamosite reported in the literature as computed from both phase equilibria data and empirical approaches exhibit huge discrepancies of up to 210 kj mol a small portion of the discrepancy 5 100 o is related to inconsistencies in the al reference state chosen for the thermodynamic retrieval calculations it is shown that the remainder can be assigned to an improper application of the van t hoffrelationship to compute standardstate thermodynamic properties 25 c i bar for chamosite from equilibrium constants derived from high temperature 575 625 c and high pressure 2 07 6 00 kbar phaseequilibria experiments we have reinterpreted these previously published experimental results by taking explicit account ofheat capacity and entropy data for chamosite predicted from additivity models following this approach we compute ag and afii for chamosite of 6495 13 4 17 and 7101 91 4 17 kj mol respectively values that are in excellent agreement with those computed from empirical techniques after inconsistencies reiated to the al reference state are taken into account in this way major discrepancies in previously reported thermodynamic data for chamosite are resolved

structure and metamorphism of blueschist eclogite facies rocks from the northeastern oman mountains the northern part of the saih hatat window oman shows high pressure metamorphic rocks derived from shelf sediments and pre premian continental basement and is atypical of sub ophiolite metamorphism elsewhere the high pressure rocks are divided into structural units originally bounded by foreland propagating thrusts formed during ophiolite obduction although now many contacts are backthrusts normal faults or extensional shear zones metamorphic breaks exist across many unit boundaries the deepest unit as sifah has eclogite facies assemblages in metabasites and metapelites which record evolution of p t conditions along a clockwise path culminating al 23 2 5kbar 540 75 c contrasting markedly with overlying units 5 10 kbar 200 500 c although separated by 40 ar 39 ar apparent ages our tectonic model relates all the high p units to a single convergent event in the late cretaceous the as sifah eclogites were exhumed in two stages i tectonic emplacement against other units at c 20 25 km depth and ii exhumation of the entire high p zone by culmination collapse after obduction

cretaceous high pressure metamorphism in ne oman and its relationship to subduction and ophiolite nappe emplacement an occurrence of glaucophane bearing schists is reported from near muscat in ne oman in rocks which underlie the upper cretaceous semail ophiolite nappe these blueschists formed by the metamorphism of quartz sandstones shales and basic igneous rocks belong to a pre permian basement in the area metamorphic micas phengites from these rocks have been dated at between 101 and 80 ma k ar ages indicating a mid to late cretaceous age for the high pressure event a syn to late f 1 assemblage of almandine glaucophane phengite in the pelites shows that the peak of metamorphism reached about 400 450 c and 4 8 kbars pressure this was followed by a greenschist facies overprint with the formation of epidote and albite porphyroblasts in the meta igneous rocks and a widespread retrogression to chlorite the high pressure metamorphism is ascribed to the effects of partial subduction of the ne oman continental margin during the initial stages of ophiolite obduction and the subsequent greenschist facies event to uplift of the area during the later stages of emplacement of the nappes

the mineralogy and metamorphic geology of low grade metasediments northern range trinidad forty one low grade metasediments from the northern range of trinidad and from the neighbouring island of tobago west indies were investigated by microscopic x ray powder diffraction guinier camera technique and electron microprobe methods the following minerals were encountered in order of decreasing abundance quartz muscovite chlorite paragonite muscovite mixed layer paragonite organic material calcite pyrophyllite dolomite albite hematite chloritoid pyrite rutile titanite rectorite stilpnomelane the chemical composition of muscovite is dependent on the mineral assemblage with si 6 1 6 2 per formula unit in two high alumina assemblages versus si 6 6 in a low alumina assemblage microprobe analyses indicate that muscovite paragonite and pyrophyllite are sometimes intergrown on a micrometre scale chlorite composition estimated from x ray data shows good correspondence with microprobe data for two al poor samples but is less satisfactory for an al rich sample observed mineral assemblages in akna are indicative of the pyrophyllite paragonite albite fades for the first time chemical data for coexisting pyrophyllite chlorite m mgo mgo feo 0 377 and chloritoid m 0 110 are given this assemblage is indicative of the low or medium pressure range of the lower greenschist facies pressure conditions during metamorphism are not well constrained but a minimum pressure of 2 kbar corresponding to 5 6 km of overburden can be estimated from the na m4 content of actinolite temperatures derived from the calcite dolomite geothermometer range between 300 and 350 c and are consistent with the presence of the assemblage pyrophyllite calcite quartz

pressure temperature time paths of regional metamorphism i heat transfer during the evolution of regions of thickened continental crust the development of regional metamorphism in areas of thickened continental crust is investigated in terms of the major controls on regional scale thermal regimes these are the total radiogenic heat supply within the thickened crust the supply of heat from the mantle the thermal conductivity of the medium and the length and time scales of erosion of the continental crust the orogenic episode is regarded as consisting of a relatively rapid phase of crustal thickening during which little temperature change occurs in individual rocks followed by a lengthier phase of erosion at the end of which the crust is at its original thickness the principal features of pressure temperature time ptt paths followed by rocks in this environment are a period of thermal relaxation during which the temperature rises towards the higher geotherm that would be supported by the thickened crust followed by a period of cooling as the rock approaches the cold land surface the temperature increase that occurs is governed by the degree of thickening of the crust its conductivity and the time that elapses before the rock is exhumed sufficiently to be affected by the proximity of the cold upper boundary for much of the parameter range considered the heating phase encompasses a considerable portion of the exhumation decompression part of the ptt path in addition to the detailed calculation of ptt paths we present an idealized model of the thickening and exhumation process which may be used to make simple calculations of the amount of heating to be expected during a given thickening and exhumation episode and of the depth at which a rock will start to cool on its ascent path an important feature of these ptt paths is that most of them lie within 50 c of the maximum temperature attained for one third or more of the total duration of their burial and uplift and for a geologically plausible range of erosion rates the rocks do not begin to cool until they have completed 20 to 40 per cent of the total uplift they experience considerable melting of the continental crust is a likely consequence of thickening of crust with an average continental geotherm a companion paper discusses these results in the context of attempts to use metamorphic petrology data to give information on tectonic processes 1984 oxford university press

reversal of fe mg partitioning between garnet and staurolite in eclogite facies metapelites from the champtoceaux nappe brittany france this paper concentrates on the petrology of eclogite facies metapelites and particularly the significance of staurolite in these rocks a natural example of staurolite bearing eclogitic micaschists from the champtoceaux nappe brittany france is first described the champtoceaux metapelites present in addition to quartz phengite and rutile two successive parageneses 1 chloritoid staurolite i garnet cores and 2 garnet rims hkyanite chloritoid detailed microprobe analyses show that garnet and chloritoid evolve towards more magnesian compositions and that staurolite is more fe rich than coexisting garnet a comparison of the studied rocks with other known occurrences of eclogitic metapelites shows that whereas staurolite is always more fe rich than garnet in high pressure eclogites the reverse is true in low to medium pressure micaschists phase relations between garnet staurolite chloritoid biotite and chlorite are analysed in the kfmash system with excess quartz phengite rutile and h20 the topology of univariant reactions is depicted for a normal and a reverse fe m g partitioning between garnet and staurolite mineral compositional changes are also predicted for varying bulk rock chemistries in the studied micaschists the zonal arrangement of garnet inclusions and the progressive compositional changes of ferromagnesian phases record part of the prograde p t path before the attainment o f peak metamorphic conditions at about 650 700 c 18 20 kb the retrograde path which records the uplift of the champtoceaux nappe occurs under decreasing temperatures