paleomagnetic and ams evidence for a variscan ductile clockwise rotation of the ile de groix blueschists south brittany france consequence on the late hercynian structural pattern of westernmost europe abstract the terrigenous sedimentary rocks and blueschists of the ile de groix france and surrounding sea floor display a complex metamorphic history comprising three major episodes of deformation that took place between 420 and 294 ma paleomagnetic measurements show that three components of magnetization can be isolated the low unblocking temperature component isolated below 300 c is close to the present magnetic dipole field the high component which reaches 580 c and even 680 c in some places corresponds to pre variscan magnetic directions already obtained for armorica since the groix island has always been part of the armorican microplate the corresponding magnetic inclinations can be used for dating tectonic events by comparing with the armorican apparent polar wander path apwp l1 lineations associated with sheath folds are contemporaneous with remagnetization that took place between 336 and 359 ma this is in good agreement with published radiogenic data the anisotropy of the magnetic susceptibility study shows that l1 lineations are the magnetic markers of tectonic rotation part of the dextral d2 ductile rotation of l1 around a vertical axis occurred around 310 ma paleomagnetic data also suggest that the clockwise rotation started earlier near the close of blueschist metamorphic event the clockwise rotation increases towards the southern end of groix where it trends 80 an early magnetization at 420 ma is regarded as the age of the initial emplacement of the klippe of groix restoration of the initial shape of groix island shows that the l1 fabric was broadly east west oriented before its impingement by an unevenness of the south armorican suture zone

conditions of metamorphism in an early paleozoic blueschist schist of skookum gulch northern california the late ordovician schist of skookum gulch northern california is one of only a few pre mesozoic blueschist localities in north america among these the skookum gulch occurrence is noteworthy because it contains lawsonite and has had a relatively simple metamorphic and structural history

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

phase relations of epidote blueschists phase diagrams have been computed for minerals in the system ncmash using the optimised thermodynamic data base of berman 1988 augmented by data on glaucophane and pumpellyite to illustrate the relationships between rocks containing epidote and sodic amphibole and those containing assemblages characteristic of neighbouring metamorphic facies the stoichiometries of stable bounding equilibria yield a simple list of mineral parageneses that serves to define an epidote blueschist facies and surrounding metamorphic facies the calculations suggest that when iron free the pairs zoisite glaucophane and clinozoisite glaucophane do not possess a stability field the natural fe bearing paragenesis of epidote sodic amphibole occupies a closed field in the pt diagram whose size increases dramatically as the ratio fe3 al in the minerals is increased where garnet is involved in bounding reactions such as with respect to eclogite and garnet bearing albite epidote amphibolite the epidote blueschist field decreases in size as the ratio fe2 mg is increased the classic blueschist to greenschist transition reaction is best developed in rocks with large fe3 and fe2 contents and is possibly metastable in mgal rich rocks in the latter a wedge of albite epidote actinolite barroisite amphibolite separates the epidote blueschist from the greenschits paragenesis field a first occurrence of garnet divides the epidote blueschist field into a lower temperature garnet free zone and a higher temperature garnet bearing zone although the temperatures calculated for this garnet isograd seem high the locations of all the facies boundaries are influenced by h2o activity however even minor amounts of co2 in the fluid phase x co2 0 005 render the epidote sodic amphibole paragenesis metastable so that facies boundaries cannot be shifted by varying the co2 content of the fluid phase below 600 c no mutual boundaries exist between the amphibolite facies and the epidote blueschist and eclogite facies respectively the calculated low pressure limit for the eclogite paragenesis pyralspite omphacite jd 50 quartz typical metabasite compositions is 12 to 14 kbars with h2o activity 0 9 or 1 5 to 2 kbar lower for jd 30

structure petrology and metamorphic history of the nome group blueschist terrane salmon lake area seward peninsula alaska an 80 mi 2 area adjacent to salmon lake mapped at a scale of 1 36 680 includes a section of the nome group schist that is at least 2 000 m thick it is relatively intact with relict primary lithologic layering and not melange like or highly disrupted by faults the nome group metasediments include pelitic schist graphitic quartzite pure and impure marble calcareous schist chlorite albite schist chloritic quartz albite and mafic schists metaigneous rocks include fe ti rich metabasite and granitic to tonalitic orthogneiss the protoliths are precambrian to paleozoic in age and radiometric ages indicate that the high pressure metamorphism of the nome group is late jurassic blueschist facies assemblages occur in nome group metabasites and in most metasedimentary schists the blueschists are synkinematic they formed during an episode of widely distributed penetrative deformation and mesoscopic intrafolial isoclinal folding of consistent orientation the nome group of the salmon lake area underwent a prograde monocyclic polyfacial high p t metamorphism initial metamorphism was in the lower t regime of the lawsonite glaucophane field 300 c 7 kb and prograded with increasing t and p into the epidote almandine glaucophane field through an apparent p t maximum as indicated by the assemblage glaucophane omphacite garnet 420 c 9 10 kb the later stages of the metamorphism were characterized by increased t p likely during initial stages of decompression and the return of the normal geothermal gradient to the transition between the blueschist facies and the high p albite epiclote barroisite field 450 to 525 c 7 8 kb this high p t cycle was followed by an incipient static retrogressive metamorphism at low greenschist facies conditions during the later uplift of the nome group in the late cretaceous and early tertiary

very low temperature record of the subduction process a review of worldwide lawsonite eclogites abstract lawsonite eclogites preserve a record of very low temperature conditions in subduction zones all occur at active margin settings typically characterized by accretionary complexes lithologies and as tectonic blocks within serpentinite matrix melange peak lawsonite eclogite facies mineral assemblages garnet omphacite lawsonite rutile typically occur in prograde zoned garnet porphyroblasts their matrix is commonly overprinted by higher temperature epidote bearing assemblages greenschist or amphibolite facies conditions erase former lawsonite eclogite relics various pseudomorphs after lawsonite occur particularly in some blueschist eclogite transitional facies rocks coesite bearing lawsonite eclogite xenoliths in kimberlitic pipes and lawsonite pseudomorphs in some relatively low temperature ultrahigh pressure eclogites are known using inclusion assemblages in garnet lawsonite eclogites can be classified into two types l type such as those from guatemala and british columbia contain garnet porphyroblasts that grew only within the lawsonite stability field and e type such as from the dominican republic record maximum temperature in the epidote stability field formation and preservation of lawsonite eclogites requires cold subduction to mantle depths and rapid exhumation the earliest occurrences of lawsonite eclogite facies mineral assemblages are early paleozoic in spitsbergen and the new england fold belt of australia this suggests that since the phanerozoic secular cooling of earth and subduction zone thermal structures evolved the necessary high pressure temperature conditions buoyancy of serpentinite and oblique convergence with a major strike slip component may facilitate the exhumation of lawsonite eclogites from mantle depths

rb sr and 40ar 39ar ages in blueschists from the ile de groix armorican massif france implications for closure mechanisms in isotopic systems rb sr and 40ar 39ar data mainly ranging from 345 5 to 361 5 ma and from 348 4 1 4 to 364 6 1 4 ma respectively have been obtained on phengite from the hp lt rocks from the ile de groix armorican massif france which represents one of the few occurrences of variscan blueschists petrological data show two blueschist facies units recording distinct peak p t conditions about 16 20 kbar 500 c and 12 16 kbar 450 c in the upper and lower units respectively followed by a nearly isothermal decompression in the greenschist facies rb sr and 40ar 39ar results defining two groups of ages mainly around 345 353 ma and 358 366 ma are consistent in well preserved blueschist facies rocks as well as in strongly overprinted samples from both units one sample excepted there is no evidence of inherited or excess argon and resetting of the isotopic systems oldest rb sr and 40ar 39ar ages are found in rocks with well preserved blueschist facies parageneses while younger ages are obtained on samples with a strong greenschist facies overprint this suggests that the closure of the k ar system is diachronous in samples which record the same p t evolution a closure of the k ar clock at around 450 500 c in phengite from rocks left nearly undeformed after the blueschist facies metamorphism is in agreement with petrological geochemical and structural data thus in these samples the oldest rb sr and 40ar 39ar ages around 360 370 ma represent the age of the hp metamorphism in the overprinted samples later closure of the k ar and rb sr clocks is the result of retrogression and continuing ductile deformation the small age difference between the oldest and the youngest rb sr and 40ar 39ar ages around 10 20 ma argues in favour of a relatively fast cooling

limited fluid rock interaction at marble gneiss contacts during cretaceous granulite facies metamorphism seward peninsula alaska stable isotope profiles show that flat lying marble units acted as impermeable barriers to upward fluid flow in transitional amphibolite granulite grade rocks of the kigluaik mountains seward peninsula alaska the degree of permeability is related to the composition of the marble the margin of a thick pure dolomite marble chemically reacted with underlying metasyenite ah2o 0 2 to form a 2 cm boundary layer of calcite forsterite by introduction of sio2 no fluid penetrated past this reaction front although the high temperature of metamorphism 800 c allowed transport of carbon and oxygen isotopes for an additional 2 cm by diffusion through the solid dolomite a second marble with a higher silica content underwent more decarbonation which enhanced porosity and lead to a greater extent of isotope transport 2 3 m in contact with quartzo feld spathic gneiss below an estimate of total fluid flux across the bottom of this marble layer based on the shape of the isotope profile is 1 cm3 cm2 directed down out of the marble at two other marble gneiss contacts steep isotopic gradients coincide with lithologic contacts indicating very little cross lithology fluid flow the extent of diffusional transport of isotopes in the marbles is limited and interpreted as indicating the transient presence of a pore fluid generated by thermally driven devolatilization reactions no wholesale pervasive advection of c o h fluid occurred across the thick continuous marble units near the exposed base of the kigluaik group section during the entire regional metamorphic cycle activities of pore fluid species were controlled by internal processes movement of volatiles and stable isotopes between contrasting rock types was dominantly diffusive channelized fluid pathways through the marble units developed during uplift and cooling but were not present during peak metamorphism heating of the section occurred by conduction probably from an underlying magma source and not by advection of a c o h fluid

blueschist and eclogite in mylonitic allochthons ross river and watson lake areas southeastern yukon two new occurrences of eclogite associated with mylonitic rocks are documented the first near ross river is in a quartz rich metasedimentary host that includes glaucophane schist the host forms part of a belt of high pressure rocks more than 50 km long and several kilometres wide between ross river and faro in the second occurrence north of watson lake eclogite is associated with mafic and ultramafic rocks occurring in a klippe more than 100 km2 in area that rests on unmetamorphosed triassic rocks of the north american miogeocline geothermobarometry shows that the eclogites were metamorphosed at 10 15 kb 1 kb 100 mpa between 470 and 750 c these peak conditions are comparable to those for other eclogites in the yukon and adjacent alaska the age of metamorphism and mylonitization is triassic or older the high pressure rocks occur at the leading edge of the most inboard accreted terrane along its boundary with north american miogeoclinal strata over a length of several hundred kilometres t

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

polyphase and anticlockwise p t evolution for franciscan eclogites and blueschists from jenner california usa high grade exotic blocks in the franciscan complex at jenner california show evidence for polydeformation metamorphism with eight distinct stages two parallel sets of mineral assemblages e eclogite and bs laminated blueschist representing different bulk chemistry were identified stage 1 recorded by parallel aligned inclusions s1 of crossite omphacite epidote ilmenite titanite quartz e and glaucophane actinolite epidote titanite bs in the central parts of zoned garnets represents the epidote blueschist facies the onset of a second stage stage 2 is represented by a weak crenulation of s1 and growth of garnet this stage develops a well defined s2 foliation of orientated barroisite epidote titanite e or subcalcic actinolite epidote titanite bs at c 90d to s1 with syntectonic growth of garnet defining the albite epidote amphibolite facies a third stage with aligned inclusions of glaucophane subcalcic actinolite phengite parallel to s2 in the outermost rims of large garnet grains is assigned to the transitional albite epidote amphibolite garnet bearing epidote blueschist facies the fourth stage represents the peak metamorphism and was identified by unorientated matrix minerals in the least retrograded samples in this stage the mineral assemblages garnet omphacite glaucophane phengite e and garnet winchite phengite epidote bs both represent the eclogite facies stage 5 is represented by the retrogression of eclogite facies assemblages to the epidote blueschist facies assemblages crossite glaucophane garnet omphacite epidote phengite e and glaucophane actinolite epidote phengite bs with the development of an s5 foliation subparallel to s2 stage 6 represents a crenulation of s5 with the development of a well defined s6 crenulation cleavage wrapping around relics of the eclogite facies assemblages this crenulation cleavage is further weakly crenulated during a d7 event post d7 stage 8 is recorded by the growth of lawsonite chlorite actinolite replacing garnet and by veins of lawsonite pumpellyite aragonite and phengite apatite the different yet coeval mineral parageneses observed in rock types e and bs are probably due to differences in bulk chemistry the metamorphic evolution from stage 1 to stage 8 seems to have been broadly continuous following an anticlockwise p tpath 1 epidote blueschist garnet free to 2 albite epidote amphibolite to 3 transitional epidote blueschist garnet bearing albite epidote amphibolite to 4 eclogite to 5 epidote blueschist garnet bearing to 6 7 epidote blueschist garnet free facies to 8 lawsonite pumpellyite aragonite bearing assemblages this anticlockwise p t path may have resulted from a decreasing geothermal gradient with time in the mesozoic subduction zone of california at early or pre franciscan metamorphism

calculated mineral equilibria in the greenschist blueschist eclogite facies in na 2 o feo mgo al 2 o 3 sio 2 h 2 o the new greatly expanded internally consistent dataset of holland and powell includes thermodynamic data for a wide range of mineral end members in common rock forming minerals in particular including femg 1 substitutions in glaucophane garnet chloritoid and carpholite and femg 1 and mgsial 1al 1 substitutions in talc and chlorite moreover we have the uncertainties and correlations for these data with the data we have calculated the full pressure temperature phase diagram for the system na2o feo mgo al2o3 sio2 h2o nfmash for quartz or coesite and h2o in excess in the range 300 800 c and 5 50 kbars by solving the set of non linear equations formed by the equilibrium relationships for an independent set of equations between the end members in an assemblage in nfmash the compositions of the minerals and pt can be calculated thus the changes in mgsial 1al 1 along nmash reactions and fesial 1al 1 along nfash reactions are calculated and the changes in femg 1 and mgsial 1al 1 along nfmash reactions are calculated from this information it is straightforward to generate pt diagrams for specific rock compositions mineral assemblages and mineral compositional changes in the phase diagram are discussed in relation to greenschist blueschist and eclogite facies assemblages in metapelitic rocks it is found that the correspondence between the predictions of the phase diagrams and the observations on rocks is remarkably good when semiquantitative extensions of the phase diagram to include ca mgfe 1 nasica 1al 1 fe3 al 1 and kna 1 substitutions are taken into account the agreement is essentially complete

relative motions between eurasia and north america in the bering sea region knowledge of the relative motion of the north american and eurasian plates during the late mesozoic and cenozoic provides insight into the observed timing and style of deformational events in the bering sea region periods of strong convergence from 70 to 50 ma maastrichtian to paleocene are correlated with compressional deformation between the chukotsk peninsula and northern alaska and movement along the denali fault the convergence may also be the cause of a previously proposed counterclockwise rotation of western alaska transform motion between these plates from 50 to 38 ma middle to late eocene correlates with subsidence of the bering shelf creation of a series of pull apart basins along the bering margin and cessation of calc alkaline volcanism in western alaska slight compressive convergence from 37 ma to the present may be responsible for the anticlinal deformation of basin filling sediments in the anadyr and khatyrka basins

experimentally based water budgets for dehydrating slabs and consequences for arc magma generation phase diagrams of hydrous mid ocean ridge mor basalts to 330 km depth and of hydrous peridotites to 250 km depth are compiled for conditions characteristic for subduction zones a synthesis of our experimentally determined phase relations of chlorite lawsonite epidote zoisite amphibole paragonite chloritoid talc and phengite in basalts and of phase relations from the literature of serpentine talc chlorite amphibole and phase a in ultramafics permits calculation of h2o contents in hydrous phase assemblages that occur in natural compositions this yields the information necessary to calculate water budgets for descending slabs starting from low grade blueschist conditions 10 20 km depth with h2o contents between 5 and 6 wt for hydrated oceanic crust complete dehydration is achieved between 70 and 300 km depth as a function of individual slab geotherms hydrous phases which decompose at depth below volcanic arcs are lawsonite zoisite chloritoid and talc phengite in mafic compositions and chlorite and serpentine in peridotite approximately 15 35 of the initially subducted h2o are released below volcanic arcs the contribution of amphibole dehydration to the water budget is small 5 20 and occurs at relatively shallow depth 65 90 km in any predicted thermal structure dehydration is a combination of a stepwise and a continuous process through many different reactions which occur simultaneously in the different portions of the descending slab such a dehydration characteristic is incompatible with single phase dehydration models which focus fluid flow through a unique major dehydration event in order to explain volcanic fronts as a consequence of continuously progressing dehydration water ascending from the slab will be generally available to depth of ca 150 200 km the fluid rising from the subducting lithosphere will cause partial melting in the hot portion of the mantle wedge we propose that the volcanic front simply forms above the mantle wedge isotherm where the extent of melting is sufficient to allow for the mechanical extraction of parental arc magmas thermal models show that such an isotherm ca 1300oc locates below volcanic fronts slab surface depths below such an isotherm are compatible with the observed depths of the slab surface below volcanic fronts 1998 elsevier science b v all rights reserved

regional progressive high pressure metamorphism seward peninsula alaska blueschist facies rocks on the seward peninsula constitute a structurally coherent terrane measuring at least 100 150 km radiometric age data indicate that high pressure metamorphism probably occurred in jurassic rather than in palaeozoic or precambrian time as previously suggested protolith sediments nome group are of intracontinental basin or continental margin type and of lower palaeozoic and possibly late precambrian age thus predating the high pressure metamorphism by more than 200 m y blueschist facies mineral assemblages were developed in almost all lithologies of the nome group and are best preserved in feti rich metabasites glaucophane almandine epidote and pelites glaucophane chloritoid phengite a lawsonite crossite subfacies was developed in possible nome group rocks on the east flank of the darby mountains albite epidote amphibolite facies assemblages characterize nome group rocks in the southwestern part of the peninsula metamorphism in the central zone of the terrane passed from early lawsonitic to subsequent epidote almandine glaucophane schist subfacies with the local development east of the nome river of eclogitic assemblages the high pressure metamorphic minerals were synkinematic with the development of mesoscopic scale intrafolial isoclinal folds and a flattening foliation of consistent orientation initiation of uplift probably corresponded to the growth of barroisite rims on earlier sodic and actinolitic amphiboles and partial post kinematic greenschist facies replacements record later stages of decompression ophiolites and melange are not associated with the seward peninsula blueschists the high pressure metamorphism was caused by tectonic loading of a continental plate by an allochthon of indeterminate origin the pt conditions of high pressure metamorphism were approximately 9 11 kbar 400 450 c thus falling between the pt paths of the shuksan and franciscan terranes

evidence for two burial exhumation stages during the evolution of the variscan belt as exemplified by p t t d paths of metabasites in distinct allochthonous units of the maures massif se france this paper examines the metamorphic evolution of three juxtaposed units of the maures massif france with respect to the tectonic processes related to the variscan orogeny all sampled rocks are metabasalts or metagabbros metamorphosed during palaeozoic tectonic events the inferred metamorphic evolution takes into account the relative chronology of mineral parageneses with respect to microstructures the mineral chemistry of zoned amphiboles and calculated p t t d paths derived from each unit three successive and contrasting tectono metamorphic events are clearly identified the d1 event is associated with coarse grained amphiboles of an early s1 l1 fabric that recorded prograde retrograde anticlockwise paths at high grade amphibolite facies conditions 7 8 kbar 700 750 c the d2 event is related to fine grained amphiboles of the main s2 foliation that recorded prograde retrograde clockwise paths at mp mt conditions 4 6 kbar 550 650 c the d3 event corresponds to late post s2 amphiboles crosscutting the main foliation and recording retrograde clockwise paths at lower grade conditions 4 2 kbar 500 350 c the d1 event results from silurian devonian continental subduction and subsequent thrust tectonics during an early stage of the variscan evolution before the carboniferous the d2 event is connected to the visean continental collision marked by nappe stacking burial then crustal folding and sinistral strike slip shearing exhumation the d3 event is an effect of the namurian late orogenic extension late exhumation that mostly affected the previously thickened whole central block this paper demonstrates that the whole metamorphic history of the maures massif consists of two successive stages of burial exhumation at different metamorphic conditions during the variscan mountain building similar stages of subduction uplift then collision uplift processes have also been recognised in others parts of the variscan belt and in the alpine orogen

ductile thrusting recorded by the garnet isograd from blueschist facies metapelites of the ile de groix armorican massif france

single force representation of earthquakes due to landslides or the collapse of caverns summary earthquakes due to landslides and the collapse of caverns are caused by the release of gravitational potential energy rather than elastic strain energy landslide or cavern collapse is assumed to start with dislocation motion along the lubricated interface of the slide block or fallen block that intersects the earth s free surface it can be shown that if each component of the dislocation vector is positionindependent and varies only as a function of time the resultant static displacement is spatially uniform both inside and outside the block and therefore no change occurs in static strain energy in particular if the medium is unbounded the static displacement is zero outside the block the force system equivalent to this dislocation model is a distribution of single forces equal but opposite to the dislocation acceleration multiplied by the density at each point of the volume constituting the slide block the physics behind the equivalence is discussed single force appears to be a common mechanism for indigeneous earthquakes that do not change the static strain field

two stage prograde and retrograde variscan metamorphism of glaucophane eclogites blueschists and greenschists from ile de groix brittany france on ile de groix variscan metamorphic former tholeiitic and alkaline basalts occur as glaucophane eclogites blueschists and greenschists in isolated lenses and layers within metapelites whole rock 18osmow values of the metabasites show limited variations 10 4 12 0 and no systematic differences among rock types and metamorphic grades this provides no argument for large scale blueschist to greenschist transformation driven by infiltration of externally derived fluids metamorphic mineralogical changes should have been triggered by internal fluids element variations in interlayered blue and greenschists can be attributed to magmatic fractionation assemblages with garnet clinopyroxene and glaucophane of a high pressure low temperature hp lt metamorphism m1 and naca amphiboles barroisite magnesiohornblende actinolite of a medium pressure medium temperature metamorphism m2 crystallized during deformation d1 detailed core rim zonation profiles display increasing and then decreasing aliv in glaucophane of m1 naca amphiboles of m2 mantling glaucophane and crystallized in porphyroblasts show first increasing then decreasing aliv and alvi empirically calibrated thermobarometers allowed p t path reconstructions in glaucophane eclogites of a metamorphic zone i a prograde evolution to m1 peak conditions at 400 500 c 10 12 kbar was followed by a retrograde p t path within the glaucophane stability field the subsequent m2 evolution was again prograde up to 600 c at 8 kbar and then retrograde similarly in metamorphic zones ii and iii prograde and retrograde paths of m1 and m2 at lower maximal temperatures and pressures exist the almost complete metamorphic cycle during m2 signalizes that the hp lt rocks escaped from an early erosion by a moderate second burial event and explains the long lasting slow uplift with low average cooling rates