near end member magnesiochloritoid in prograde zoned pyrope dora maira massif western alps in the ultrahigh pressure metamorphic terrane of the dora maira massif western alps magnesiochloritoid up to 97 mol end member occurs along with ellenbergerite talc chlorite or kyanite as prograde inclusions in a few pyrope megablasts whereas most pyrope megablasts in this terrane are relatively homogeneous prp90 prp98 the magnesiochloritoid bearing megablasts commonly preserve a clear prograde fe mg zonation pattern from prp70 to prp98 the nearly homogeneous chloritoid inclusions also record this zonation pattern their xmg increasing from 0 70 to 0 96 from garnet core to garnet rim these zonations are shown to be prograde and to record increasing p and t conditions up to about 700 c 30 kbar the occurrence of magnesiochloritoid in some garnets simply reflects either a slightly more fe rich system general case or a more al rich and originally corundum bearing system one unzoned near end member pyrope megablast fe mg partitioning between chloritoid and zoned host garnet changes from normal to reverse i e xmgcld xmggrt from core to rim or from rim to core in other megablasts of yet similar composition in case of reverse partitioning a diffusion zone of a few tens of mu m wide exists in garnet around the inclusions toward which xmggrt decreases and reaches a normal partitioning value at the contact these features are interpreted in terms of disequilibrium and tentatively ascribed to slow diffusion in chloritoid with respect to garnet growth rate or to two competing garnet forming reactions with the metastable one which produces a more mg rich garnet being kinetically favoured the prograde inclusions of magnesiostaurolite show a consistent fe mg reversal with garnet i e xmgst xmggrt which is interpreted as an equilibrium feature at high pressure and high xmg magnesiochloritoid locally breaks down into symplectites of chlorite and an extremely low si magnesiostaurolite down to 7 0 si per formula unit which is probably metastable with respect to magnesiostaurolite corundum

stable isotope geochemistry and phase equilibria of coesite bearing whiteschists dora maira massif western alps peak metamorphic temperatures for the coesite pyrope bearing whiteschists from the dora maira massif western alps were determined with oxygen isotope thermometry the 18o smow values of the quartz after coesite 18o 8 1 to 8 6 n 6 phengite 6 2 to 6 4 n 3 kyanite 6 1 n 2 garnet 5 5 to 5 8 n 9 ellenbergerite 6 3 n 1 and rutile 3 3 to 3 6 n 3 reflect isotopic equilibrium temperature estimates based on quartz garnet rutile fractionation are 700 750 c minimum pressures are 31 32 kb based on the pressure sensitive reaction pyrope coesite kyanite enstatite in order to stabilize pyrope and coesite by the temperature sensitive dehydration reaction talc kyanite pyrope coesite h2o the a h2o must be reduced to 0 4 0 75 at 700 750 c the reduced a h2o cannot be due to dilution by co2 as pyrope is not stable at x co2 0 02 t 750 c p 30 kb in the absence of a more exotic fluid diluent e g ch4 or n2 a melt phase is required granite solidus temperatures are 680 c 30 kb at a h2o 1 0 and are calculated to be 70 c higher at a h2o 0 7 consistent with this hypothesis kyanite jadeite quartz bands may represent a relict melt phase peak p t f h2o estimates for the whiteschist are 34 2 kb 700 750 c and 0 4 0 75 the oxygen isotope fractionation between quartz 18o 11 6 and garnet 18o 8 7 in the surrounding orthognesiss is identical to that in the coesitebearing unit suggesting that the two units shared a common final metamorphic history hydrogen isotope measurements were made on primary talc and phengite d smow 27 to 32 on secondary talc and chlorite rite after pyrope d 39 to 44 and on the surrounding biotite d 64 and phengite d 44 gneiss all phases appear to be in nearequilibrium the very high d values for the primary hydrous phases is consistent with an initial oceanicderived connate fluid source the fluid source for the retrograde talc chlorite after pyrope may be fluids evolved locally during retrograde melt crystallization the similar d but dissimilar 18o values of the coesite bearing whiteschists and hosting orthogneiss suggest that the two were in hydrogen isotope equilibrium but not oxygen isotope equilibrium the unusual hydrogen and oxygen isotope compositions of the coesite bearing unit can be explained as the result of metasomatism from slab derived fluids at depth

the pyrope coesite rocks and their country rocks at parigi dora maira massif western alps detailed petrography mineral chemistry and pt path both the coarse and fine grained varieties of the partly coesite bearing pyrope quartzites their interlayered jadeite kyanite rocks and the biotite phengite gneiss country rock common to all of them were subjected to detailed petrographic and textural studies in order to determine the sequence of crystallisation of their mineral constituents which were also studied analytically by microprobe prior to pyrope and coesite growth the mg rich metapelites were talc kyanite chlorite rutile ellenbergerite schists which upon continued prograde metamorphism developed first pyrope megacrysts in silica deficient local environments at the expense of chlorite talc kyanite and subsequently the smaller pyrope crystals with coesite inclusions from reacting talc kyanite based on geobarometrically useful mineral inclusions as well as on experimentally determined phase relations a prograde pt path simplified for water activity 1 is constructed which passes through the approximate pt conditions 16 kbar and 560 c 29 kbar and 720 c and finally up to 37 kbar at about 800 c where the mg rich metapelite was a pyrope coesite rock with phengite kyanite and talc still present during the retrograde path pyrope was altered metasomatically either into phlogopite kyanite quartz or at a later stage to chlorite muscovite quartz both assemblages yield pt constraints the latter about 7 9 kbar 500 600 c the country rock gneisses have also endured high pressures of at least 15 kbar but they provide mostly constraints on the lowest portion of the uplift conditions within the greenschist facies about 5 kbar 450 c microprobe data are presented for the following minerals pyrope ellenbergerite dumortierite unusually mgti rich jadeite vermiculite formed after na phlogopite paragonite and for several generations of phengite chlorite talc phlogopite dravite and glaucophane in the high pressure rocks as well as for biotite chlorite phengites epidote garnet albite and k feldspar in the country rock gneisses an outstanding open problem identified in this study is the preservation of minerals as inclusions within kyanite and pyrope beyond their pt stability limits

coesite and pure pyrope in high grade blueschists of the western alps a first record and some consequences a pyrope quartzite originally described by vialon 1966 from the dora maira massif was resampled and reinvestigated garnet up to 25 cm in size phengite kyanite talc and rutile are in textural equilibrium in an undeformed matrix of polygonal quartz the garnet is a pyrope almandine solid solution with 90 to 98 mol mg end member it contains inclusions of coesite which has partially inverted to quartz resulting in a typical radial cracking of the host garnet around the inclusions several lines of evidence show that coesite crystallised under nearly static pressure conditions and that the whole matrix has once been coesite

microfabrics of uhp metamorphic granites in the dora maira massif western alps no evidence of deformation at great depth in the dora maira massif western alps essentially undeformed ultrahigh pressure uhp metamorphic granites brossasco granite are embedded in and locally grade into granite gneisses or augengneisses and mylonites in this study the quartz microfabrics of the undeformed granites are compared against the augengneisses and mylonites in a representative number of samples from several locations in the undeformed granites the fine grained quartz aggregates that formed from coesite upon decompression are characterized by a foam structure and random crystallographic orientation in the deformed granites the quartz microstructures and the crystallographic preferred orientation cpo indicate deformation by dislocation creep most of the deformation of the granites if not all must have happened at a late stage during exhumation after transformation of coesite to quartz at greenschist facies conditions in the middle crust the deformed granites provide no evidence of deformation during subduction at u hp metamorphic conditions and in the earlier stages of exhumation the diameter of internally undeformed slices of continental crust subducted to and exhumed from about 100 km can exceed that of the presently exposed brossasco granite i e it can be on the kilometre scale

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

deformation history of a subducted continental crust gran paradiso western alps continuing crustal shortening during exhumation eclogite facies continental basement in the western alps outcrops as tectonic windows below the oceanic units in the gran paradiso massif eclogite facies assemblages in mafic rocks display a n s stretching lineation da1 the main phase foliation formed under epidote amphibolite facies conditions da2 and defines a regional dome structure for the whole gran paradiso massif structural data including new detailed mapping microstructural analyses and metamorphic studies in the northern part of the gran paradiso cogne valley reveal the occurrence of major thrusts in this continental basement associated with the da2 deformation e w trending stretching lineations and fold axes are associated with this second and main deformation nappe stacking of the different units was achieved by top to the west shearing during exhumation of the eclogite facies basement the antiformal doming of the gran paradiso basement is thus produced by the nappe stacking during exhumation under epidote amphibolite facies

superzoned garnets in the coesite bearing brossasco isasca unit dora maira massif western alps and the origin of the whiteschists abstract rare centimeter sized superzoned garnets szgs were discovered in two coesite bearing whiteschists of the brossasco isasca unit biu southern dora maira massif dmm western alps the superzoned garnet consists of a reddish brown almandine core crowded with inclusions of staurolite chloritoid kyanite chlorite and paragonite and of a pinkish pyrope rim with sporadic inclusions of kyanite and magnesian chlorite the core rim contact is relatively sharp and marks the termination of the inclusion rich portion the core composition of the superzoned garnet is almost identical to or slightly richer in mg than that of the rim of porphyroblastic garnet in metapelites from the same unit in the rim of the superzoned garnet mg fe ratio increases abruptly towards the outermost rim whose composition is identical to that of the common pyrope in the whiteschist at the core rim boundary there is no chemical gap chloritoid and staurolite are common inclusions in the core of the superzoned garnet in the whiteschist and in the porphyroblastic garnet in the metapelite the staurolite composition si 2 00 and total r 2 grt cld chl the resulting petrogenetic grid suggests that the core of the superzoned garnet contains incompatible assemblages such as st cld chl vs cld chl ky new and literature data and results of experiments in the kfash system suggest that 1 the superzoned garnet was formed under a single prograde high pressure ultra high pressure hp uhp alpine metamorphism 2 the almandine inclusion rich core of the superzoned garnet crystallized at disequilibrium in a pelitic composition system at around 600 c and less than 16 kbar probably from a former metapelite xenolith included in a variscan granitoid and 3 the chemical environment of the host rock suddenly changed from the normal pelite to the whiteschist composition by a metasomatic process during the rim growth i e at a stage close to the uhp climax

phase relationships of ellenbergerite a new high pressure mg al ti silicate in pyrope coesite quartzite from the western alps

grain scale deformation in ultra high pressure metamorphic rocks an indicator of rapid phase transformation abstract conspicuous grain scale deformation is observed in some ultra high pressure uhp metamorphic rocks of the dora maira massif western alps although no significant strain is discernible on the mesoscopic scale in a jadeite kyanite quartz rock some of the jadeite crystals reveal 100 deformation twins indicating local differential stress levels above 0 3 gpa many kyanite crystals show marked kink or deformation bands with a slip system 100 001 in contrast the adjacent coarse grained quartz matrix grain size ca 0 2 mm which has formed from coesite during exhumation from 100 km depth reveals a foam structure the quartz grain boundary configuration is controlled by interfacial free energy the grains are optically strain free and there is no crystallographic preferred orientation preservation of this foam microstructure which indicates grain growth during low stress annealing precludes that deformation of the jadeite and kyanite crystals is a result of a late stage low temperature overprint the orientation distribution of jadeite and kyanite with and without twins or deformation bands respectively has been investigated with a combination of universal stage and ebsd techniques on the scale of a thin section there is no preferred orientation of twinned jadeite and bent kyanite crystals and undeformed crystals respectively thus the orientation of the inferred local shortening direction is random this precludes deformation driven by a homogeneous far field tectonic stress but suggests an internally controlled stress field which is highly inhomogeneous on the scale of a few grain diameters laboratory experiments show that the coesite to quartz transformation proceeds within hours after decompression from 3 0 to 2 7 gpa at 800 c the microstructures of incompletely transformed samples indicate that the quartz growing at the expense of coesite undergoes crystal plastic deformation and recrystallizes with a very fine grain size during transformation in this case the deformation of quartz is attributed to the volumetric strain v 10 inherent in the coesite quartz transformation which causes a highly inhomogeneous stress field inside the sample related to the progress of the transformation we propose that a similar process has taken place in the polyphase natural rock during exhumation with the transient stresses causing mechanical twinning of jadeite and bending or kinking of kyanite when the transformation had gone to completion grain growth obliterated the microstructures of the quartz matrix while the deformed jadeite and kyanite crystals preserved the record of inhomogeneous deformation at high temperatures the peak differential stresses locally exceeded 0 3 gpa which indicates very high strain rates and a correspondingly rapid transformation of coesite to quartz comparable to the laboratory results

a classification scheme for mantle derived garnets in kimberlite a tool for investigating the mantle and exploring for diamonds abstract a new empirical method has been devised for classification of mantle derived garnets in kimberlite simple chemical screens have been developed to distinguish between garnets from different parageneses based on mg fe ca cr ti and na values of published analyses of garnets from 2000 ultramafic xenoliths in kimberlite although crustal garnets are typically uncommon as xenocrysts in kimberlite the first step in the classification is to screen these from the mantle population using data from 600 garnet bearing crustal rocks such a screen may also prove useful in evaluating the source crust vs mantle of garnet in kimberlite exploration samples subsequent steps divide mantle garnets into eclogite peridotite and cr poor megacryst groupings and sub groups of the peridotite lherzolite harzburgite wehrlite and eclogite groups i and ii and a b c and grospydite populations important features of this classification include the fact that it is based on distinctions between groups of fundamental geological significance e g peridotite vs eclogite and it is based on a large well documented and well understood xenolith database as it utilizes oxide values and molar ratios of major and minor elements the rationale for the screens is readily understood and it is simple to use

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

geochemistry and 40ar 39ar geochronology of pseudotachylyte associated with uhp whiteschists from the dora maira massif italy abstract in situ uv laser ablation 40 ar 39 ar geochronological and geochemical data together with rock and mineral compositional data have been determined from pseudotachylyte and surrounding mylonitic gneiss associated with the uhp whiteschists of the dora maira massif italy several generations of fresh pseudotachylyte occur as irregular veins up to a few cm thick both parallel and at high angles to the foliation whole rock xrf data collected from representative lithologies of mylonitic gneiss are uniformly consistent with a mildly alkalic granitic protolith minimal compositional variation is observed between the pseudotachylyte and its surrounding mylonitic gneiss the pseudotachylyte contains newly crystallized grains of biotite and k feldspar in a matrix of glass with partially fused grains of quartz zircon apatite and titanite electron microprobe analyses of the glass show significant compositional variation that is probably strongly influenced by micrometer scale changes in mineralogy uv laser ablation icp ms traverses across the mylonitic gneiss pseudotachylyte contact are consistent with cataclastic communition of ree carriers such as epidote monazite allanite zircon and apatite before melting as an efficient mechanism of ree homogenization in the pseudotachylyte the 40 ar 39 ar data from one band of pseudotachylyte indicate formation at 20 1 0 5 ma when the mylonitic gneisses were already in a near surface position the variable effects of top to the west shear deformation within outcrops of the coesite bearing unit are reflected in localized zones of protomylonite cataclasite ultracataclasite and pseudotachylyte preservation of several generations of pseudotachylyte suggests that seismic events may have played a significant role in triggering late unroofing of the uhp rocks it is speculated that deeper crustal seismic events potentially played a role in the unroofing of the uhp rocks at earlier stages in their exhumation history

geometry and structural evolution of ultra high pressure and high pressure rocks from the dora maira massif western alps italy abstract the crystalline nappes of the dora maira massif western alps essentially made of continental material from the upper crust show petrological relics of an ultra high pressure uhp to high pressure hp cold eclogite eoalpine metamorphism they also display relics of uhp hp structures preserved in boudins and or within large uhp porphyroclasts in a retrograde greenschist facies regional deformation fabric the greenschist facies overprint has the character of a shallow dipping mylonitic foliation s m bearing a penetrative stretching lineation l m which roughly parallels the axes of coeval isoclinal folds shear sense markers indicate a w verging overthrust mechanism the uhp and hp relic structures arc of variable nature the coexistence of equant and inequant either symmetric or asymmetric fabrics indicates that the deformation at uhp hp conditions was strongly heterogeneous and partitioned this is also supported by the local preservation of hercynian magmatic fabrics the uhp and hp deformation involved at least locally rotational components although less intensive than during the latter retrograde stage the regional structural evolution is envisaged as follows i the eoalpine subducted crust was subdivided into lenticular bodies surrounded by uhp hp shear zones the main part of the exhumation processes remains unconstrained due to the sparseness and late rotation of the uhp hp structural relics conflicting models are possible depending on the interpretation of the early sense of movement normal vs reverse along the faults that limit the lens shaped units and ii the late heterogeneous regional greenschist deformation can be attributed to the eocene collapse of the alpine orogenic wedge

observations of volcanic tremor at mount st helens volcano digital recordings of ground motion during tremor episodes accompanying eruptions at mount st helens volcano in the state of washington on august 7 and october 16 18 1980 are studied the spectra of the vertical component waveforms contain at least two dominant peaks at 1 0 and 1 3 hz for all events recorded during both eruptions that were studied spectra of horizontal ground motion show peaks at 0 9 and 1 1 hz the relative amplitude of the two peaks changes between tremor episodes and during single tremor episodes and shows no consistent relation to amplitude of ground motion spectra of long period earthquakes are very similar to those of tremor events suggesting that tremor is composed of many long period earthquakes that occur over a period of time the unique waveform of tremor events observed at mount st helens must be due to a source effect since the relative amplitude of the two dominant peaks changes during tremor episodes the path effect on tremor waveforms is small since there are no peaks in the spectra of waveforms recorded during tectonic earthquakes occurring in the vicinity of mount st helens the consistency of the location of the spectral peaks for the wide range of tremor amplitudes means that there must be a physical length at the source that is constant independent of the amplitude of motion at the source variations in amplitude of motion generated during tremor events must be due to variations in the force driving the tremor amplitudes of ground motion varies between 0 11 and 4 7 m seismic moment rates during the two eruptions are calculated using the model of aki et al 1977 and found to vary between 6 1018 and 1 1020 dynes cm s 1 which are larger than values found by aki et al 1977 who studied amplitudes of shallow tremor events recorded during the october 1963 eruption of kilauea volcano in hawaii study of tremor amplitudes recorded at corvallis oregon leads to the conclusion that tremor accompanying the cataclysmic may 18 1980 eruption was at least one order of magnitude larger in amplitude than tremor during august and october

garnet omphacite phengite thermobarometry of eclogites from the coesite bearing unit of the southern dora maira massif western alps abstract using relevant geothermobarometric methods p t data were collected for the reconstruction of the metamorphic evolution of 34 eclogite samples taken from small lenses and boudins within the ultrahigh pressure uhp metamorphic coesite bearing brossasco isasca unit biu of the dora maira massif the mineral phases used clinopyroxene garnet phengite or growth zones thereof were identified as being coexistent for different stages of metamorphism on the basis of careful petrographic studies of several published geothermobarometers the garnet clinopyroxene thermometer of powell powell r 1985 regression diagnostics and robust regression in geothermometer geobarometer calibration the garnet clinopyroxene geothermometer revisited j metamorph geol 3 pp 231 243 combined with the garnet clinopyroxene phengite barometer after waters and martin waters d martin h n 1993 the garnet clinopyroxene phengite barometer terra abstr 5 pp 410 411 was chosen here because it provided the most reliable results nevertheless the scatter of p t data points for the prograde stage i peak metamorphic stage ii and retrograde stage iii development of the eclogites is still considerable among the many possible reasons for this inconsistency discussed a partial lack of equilibration of some of the eclogites during their metamorphic history should be taken into account despite the data scatter an average p t path could be estimated which includes the following coordinates for stage i 15 kbar 500 c 25 kbar 570 c 32 kbar 650 c for stage ii 36 kbar 720 c and for stage iii 24 kbar 680 c and 14 kbar 620 c this is in fair agreement with p t paths derived earlier for other rock types of the biu on the basis of other geothermobarometers

p t evolution of metasediments from the eclogite zone south central tauern window austria abstract petrologic data on the paragenesis of i kyanite zoisite marbles and ii garnet chloritoid quartz mica schists are presented with the goal of providing constraints on the pressure temperature evolution of the eclogite zone tauern window austria the peak metamorphic assemblages in the two rock types are i kyanite zoisite dolomite quartz zoisite muscovite dolomite calcite quartz and ii garnet chloritoid kyanite muscovite quartz epidote dolomite zn staurolite the estimated peak metamorphic conditions are 19 2 kbar 590 20 c secondary alteration of the kyanite zoisite marbles was accomplished in two stages the early stage resulted in the production of margarite paragonite secondary muscovite and chlorite and the later stage resulted in the formation of sudoite a di trioctahedral mg al layer silicate and kaolinite the early alteration is bracketed at conditions between 3 and 10 kbar 450 550 c and the later alteration between 200 and 350 c p 3 kbar the p t path is characterized by maximum burial to approximately 19 kbar 60 70 km at 590 c followed by nearly isothermal decompression to approximately 10 kbar 30 km and then more gradual decompression with cooling to approximately 3 kbar 10 km alteration was apparently accomplished by the influx of h 2 o rich fluids with the composition of the fluid locally buffered by the mineral assemblage

garnet chloritoid equilibria in eclogitic pelitic rocks from the sesia zone western alps their bearing on phase relations in high pressure metapelites abstract a detailed study of garnet chloritoid micaschists fom the sesia zone western alps is used to constrain phase relations in high pressure hp metapelitic rocks in addition to quartz phengite paragonite and rutile the micaschists display two distinct parageneses namely garnet chloritoid chlorite and garnet chloritoid kyanite talc has never been observed garnet and chloritoid are more magnesian when chlorite is present instead of kyanite the distinction of the two equilibria results from different bulk rock chemistries not from p t conditions or redox state estimated p t conditions for the eclogitic metamorphism are 550 600 c 15 18 kbar the presence of primary chlorite in association with garnet and chloritoid leads us to construct two possible afm topologies for the sesia metapelites the paper describes a kfmash multisystem for hp pelitic rocks which extends the grid of harte hudson 1979 towards higher pressures and adds the phase talc observed parageneses in hp metapelites are consistent with predicted phase relations critical associations are gt ctd chl and gt ctd ky at relatively low temperatures and gl chl ky and gt tc ky at relatively high temperatures

consequences of progressive eclogitization on crustal exhumation a mechanical study summary the very low water content of the granulitic unit of holsnoy in the bergen arcs norway caused its partial metastable preservation throughout caledonian burial end exhumation leading to the observed mixture of completely eclogitized and uneclogitized rocks the eclogitization of the granulite facies protolith led to a density increase 10 but also to a large rheological weakening that resulted in the localization of all deep ductile deformation in the eclogite fraction we address the consequences on subduction dynamics of the concomitant evolution of large scale density and rheology during progressive eclogitization of crust by comparing the behaviour of buried crust with various properties in the channel flow model large scale buoyancy and density are integrated in a single adimensional parameter the exhumation number which describes the capacity of the crust to exhume for a crust whose eclogitization causes a very large viscosity decrease burial resistant end member goes through a maximum in the partially eclogitized zone that is there is a given proportion of eclogite for which the crust is strongly weakened but still significantly lighter than surrounding mantle for this partially eclogitized crust with max the maximum possible downward flow is very low this zone acts as a bottleneck and all incoming crust in excess is forced backwards and starts exhuming this return flow zone is bounded downwards in the partially eclogitized zone by max while its upper limit propagates upwards at a constant rate the curve eclogite fraction controls the maximum proportion of eclogite of the crust that can be exhumed we also demonstrate that large exhumation rates reached by certain uhp units are incompatible with any stationary flow regime whatever the nature of subducted crust the additional presence of a weak layer on top of the channel favours burial for narrow channels and exhumation for large ones but does not qualitatively change the dynamics of the subduction channel this study shows that deep crustal circulation in subduction zones and exhumation from large depths are controlled by the evolution of large scale crustal properties with progressive eclogitization

a staurolite talc assemblage in tourmaline phlogopite chlorite schist from northern victoria land antarctica and its petrogenetic significance staurolite and corundum are found as inclusions in tourmaline in a talc phlogopite chlorite albite chist near mount bernstein 71 37 s 163 07 e northern victoria land antarctica these inclusions are interpreted as relics of a staurolite talc corundum chlorite assemblage that was stable during an early stage in the metamorphic cycle and subsequently armored by tourmaline probably during the middle stage pressures and temperatures during the middle stage are estimated to be 650 700 c and 5 5 6 4 kbar the transition from the early to the middle stage represents a roughly isothermal decrease in pressure of 2 3 kbar during a late retrograde stage t 300 370 c p 3 5 kbar staurolite was partly replaced by a muscovitic aggregate containing clinozoisite pumpellyite and margarite

transport of water into the lower mantle role of stishovite when subjected to lower mantle pressures and temperatures natural anhydrous basalt containing 0 2 wt h sub 2 o forms a phase assemblage in which sio sub 2 stishovite is a significant carrier of hydrogen up to 500 ppm h sub 2 o by weight as hydroxide whereas the coexisting mg fe al ca sio sub 3 perovskite appears to be not upper bound of 50 ppm wt h sub 2 o contrary to the devolatilization characteristically observed at lower pressures we find that the abundance of h sub 2 o in residual stishovite increases from approx 100 to approx 400 ppm by weight upon partially melting the high pressure mineral assemblage at 28 60 gpa we infer that the trace concentration of al within residual stishovite increases upon partial melting thereby increasing the coupled abundance of h in this crystalline phase the anhydrous component of subducted oceanic crust can thus recycle a significant amount of water into the lower mantle over the age of the earth with subducted stishovite potentially returning approx 10 sup 2 times the amount of water present in today s atmosphere

35 ma old ultrahigh pressure metamorphism and evidence for very rapid exhumation in the dora maira massif western alps abstract ion microprobe shrimp data on zircons from various rock types of an ultrahigh pressure uhp metamorphic whiteschist type pyrope quartzite lens of the dora maira massif dmm consistently show domains giving a late eocene age of 35 4 1 0 ma which is taken as the age of uhp metamorphism these domains partially replace the older oscillatory zoning pattern of the zircons formed during primary magmatic crystallization at about 275 ma zircons of a metagranitic country rock next to the uhp metamorphic lens have these primary features best preserved all zircons measured also yield intermediate ages between 275 and 35 ma with a statistical concentration between 260 and 210 ma thus the uniformity of the initial zircon population both in the lens and the country rock evidences a common protolith that is a granite intruded during the late herynian while the intermediate ages are at least partly due to incomplete resetting of the zircons during uhp metamorphism those in the 260 210 ma range may be related to rifting episodes in the permotriassic the mg rich chemistry of the whiteschist lenses is due to local metasomatic alterations of the granite perhaps by fluids derived from evaporitic sediments dating as early as the permotriassic as well the more pervasive resetting of zircon ages during uhp metamorphism in the pyrope quartzite lenses is explained by the ubiquity of fluids and or melts produced during subduction by a series of dehydration reactions that occurred only in the more hydrous mg rich protoliths and not in their drier granitic neighborhood fission track ages determined partly on the same zircon samples yielding 29 9 1 4 ma mark the time when the uhpm unit had reached about 290 c at a shallow location within the crust thus exhumation over a vertical distance of about 120 km must have occurred within about 5 6 ma indicating an average uplift rate of about 20 24 km ma and an average cooling rate of about 85 100 c ma the radiometric data obtained do not lend any support to an eo alpine cretaceous subduction event so that deep subduction and immediately following exhumation all took place during early to mid tertiary time this scenario seems to apply to large portions of the western and central alps as well calling for drastic geodynamic reinterpretations of these parts of the alps

pb sr nd isotopic behavior of deeply subducted crustal rocks from the dora maira massif western alps italy ii what is the age of the ultrahigh pressure metamorphism pb nd and sr isotope data are reported from two localities on mineral separates from mg rich metapelites and associated rocks that have been subducted to depths of at least 100 km for which metamorphic conditions are estimated at 28 33 kilobars pressure and 700 800 c and then returned to the surface initial isotope ratio data from the granitoid country rock are similar to those found in the metapelites the initial ratios indicate predominantly recycled aged granitic crustal materials for the sources of all of the samples five zircon samples 4 from pyrope megacrysts and 1 from fine grained pyrope quartzite lenses in the metapelites accurately define a chord yielding intercept ages of 304 10 and 38 0 1 4 ma in a concordia diagram zircon from the country rock also plots along the chord the zircon data together with initial nd and sr data indicate that the sedimentary sources of the rocks were derived mainly or entirely from sialic hercynian rocks ellenbergerite from pyrope megacrysts and monazite from the fine grained ground mass yield slightly younger ages of 30 34 ma apparently reflecting lower blocking temperatures than that of zircon sm nd data from a pyrope megacryst give an errorchron corresponding to an age of 38 ma in agreement with the zircon date a major question concerns the timing of the ultrahigh pressure metamorphism experimental data suggest that pyrope and quartz coesite as well as ellenbergerite formed by various metamorphic reactions if as generally assumed the ultrahigh pressure metamorphism occurred ca 100 ma ago our data require that the zircon did not experience measurable lead loss at that time but lost major amounts of lead 38 ma ago during late alpine metamorphism estimates of diffusion rates for nd in pyrope further suggest that the apparent sm nd age of 38 ma for the megacryst is not consistent with that model those problems are resolved if the ultrahigh pressure metamorphism occurred 38 40 ma ago but problems remain from ar ar dates of 100 ma on phengite an inferred 120 ma age for zircon lead loss from another study and possibly by the very rapid uplift required if the metamorphism is that young

calibration and testing of an empirical chloritoid chlorite mg fe exchange thermometer and thermodynamic data for daphnite a compilation of literature data on the fe mg composition of coexisting chlorite and chloritoid from metapelites metamorphosed at various p t conditions shows that the logarithm of the fe mg partitioning lnkd varies linearly with the inverse of temperature from about 2 4 at 300 c to about 1 3 at 600 c in contrast no trend was observed with pressure and the molar volumes of mg and fe chlorite end members suggest that the pressure dependence of lnkd is not significant therefore the chloritoid chlorite mg fe exchange reaction is a potential thermometer and has been empirically calibrated using the analyses of 112 chloritoid chlorite pairs from 28 different localities temperatures estimated using the chl cld thermometer were checked against independent estimates for 20 samples not involved in the calibration beni mzala window morocco and the results are in fair agreement with independent temperature estimates however the analytical uncertainties and errors are too large to obtain reliable temperature estimates for extremely mg rich or fe rich compositions the chl cld thermometer is unreliable at xmg cld 0 8 at 700 c and xmg cld 0 9 at 300 c using the results of the empirical calibration we calculated new thermodynamic data for daphnite implementing these data it becomes possible to estimate t and p conditions of metamorphism for the invariant chlorite chloritoid quartz aluminosilicate assemblage that is widespread in low grade metapelites these estimates appear to be relevant only in the stability field of kyanite whereas the uncertainties on the calculated pressure conditions are very large in the stability field of kaolinite and pyrophyllite

ellenbergerite a new high pressure mg al ti zr silicate with a novel structure based on face sharing octahedra ellenbergerite occurs as purple millimetre size grains associated with talc kyanite clinochlore rutile and zircon in composite inclusions within decimetre large pyrope crystals 90 98 mole percent end member in the quartzite layer of the dora maira massif western alps from which coesite has been recently reported chopin 1984 it is hexagonal a 12 255 8 c 4 932 4 a z 1 space group p63 mohs hardness 6 5 dmes 3 15 dcal 3 10 no cleavage uniaxial negative and vividly pleochroic colourless e colourless to deep lilac with colour zoning the intensely coloured variety has 1 6789 5 e 1 670 1 microprobe analysis yields sio2 39 1 p2o5 0 45 al2o3 25 1 tio2 4 0 mgo 22 2 feo 0 20 sum 99 05 wt including h2o 8 0 coulometrically the formula calculated on a o28 oh 10 basis implying 7 5 wt h2o is mg6 71 fe0 03 ti0 61 al6 00 si7 92 p0 08 o28 oh 10 the colour zoning is due to nearly complete ti zr substitution in addition ellenbergerite may contain more than 8 wt p2o5 with strictly correlated changes of si mg al and ti zr contents over 80 of which represent the sial pmg substitution

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