eclogite blueschist relationships as evidenced by mineral equilibria in the high pressure metabasic rocks of sifnos cycladic islands greece

an experimental investigation of the behavior of amphibole in the system na 2 o mgo al 2 o 3 sio 2 h 2 o at high pressures the behavior of an amphibole in the system na2o mgo al2o3 sio2 h2o and its relationships with sodium micas have been examined at high pressures a sodium mica of composition intermediate between wonesite and preiswerkite is produced from low pressures to at least 28 kbar at 750 c from differing bulk compositions in water excess experiments minor amphibole has been found in water excess runs but a c2 m amphibole approaching glaucophane composition exists only in water undersaturated systems this amphibole coexists with quartz in runs on glaucophane composition from 18 kbar to 24 kbar at 700 c in this pressure range the amphibole is displaced from ideal glaucophane composition by substitution of naaaliv for si and mgm4mgm2 for nam4alm2 above 24 kbar the a site of this amphibole empties leading to the coexistence of amphibole jadeite quartz between 24 kbar and 28 kbar above 28 kbar talc forms while the modal amount of amphibole decreases tight buffering of the amphibole to ideal glaucophane composition does not occur and small amounts of naalsi 1 substitution are found in the talc this is reflected modally in the persistence of quartz with talc jadeite and amphibole amphibole and quartz continue to decrease in modal proportions with rising pressures but remain in the run products of synthesis experiments to at least 35 kbar at 750 c

tectonic stacking of hp lt metamorphic rocks in accretionary wedges and the role of shallowing slab mantle decoupling hp lt chloritoid bearing micaschists crop out widely in the central part of northern turkey and represent deep seated subduction accretionary complexes three peak metamorphic assemblages are identified in the area studied 1 garnet chloritoid glaucophane with pseudomorphs after lawsonite 2 chloritoid with pseudomorphs after glaucophane and 3 chloritoid with pseudomorphs after jadeite in addition to phengite paragonite quartz chlorite rutile and apatite the latter is interpreted as transformation of a chloritoid glaucophane assemblage to chloritoid jadeite with increasing pressure pt modeling indicates 17 and 22 25 kbars for the two peak parageneses the diversity of peak metamorphic assemblages and the pt estimates suggest that basal accretion occurred at different depths within the wedge the depth of the basal accretion is possibly controlled by the slab mantle decoupling depth stretching and thinning of the lithospheric forearc induced by the slab roll back possibly caused shallowing of the slab mantle decoupling depth which limited depth of the basal accretion from 70 80 km to 55 km within the subduction channel a slab mantle coupling depth controlled basal accretion may also explain the scarcity of eclogite and high grade blueschist facies metamorphic rocks in active intra oceanic subduction zones because the overriding plate is young and hot in intra oceanic subductions the slab and mantle are coupled at a relatively shallow depth before eclogitization of the oceanic crust this prevents accretion and exhumation of eclogite along the subduction channel

birth death and resurrection the life cycle of suprasubduction zone ophiolites 1 suprasubduction zone ssz ophiolites display a consistent sequence of events during their formation and evolution that suggests that they form in response to processes that are common to all such ophiolites this sequence includes the following 1 birth which entails the formation of the ophiolite above a nascent or reconfigured subduction zone this stage is typically characterized by the eruption of arc tholeiite lavas and the formation of layered gabbros and sheeted dike complex 2 youth during which is continued melting of refractory asthenosphere depleted during birth occurs in response to fluid flux from the subducting slab with extensional deformation of the older plutonic suite eruption of refractory lavas and the intrusion of wehrlite pyroxenite 3 maturity with the onset of semistable arc volcanism typically calc alkaline as the subduction zone matures and stabilizes and the intrusion of quartz diorite and eruption of silicic lavas and 4 death which is the sudden demise of active spreading and ophiolite related volcanism which in many cases is linked to collision with an active spreading center and the onset of shallow underthrusting of the buoyant spreading axis expressed as dikes and lavas with oceanic basalt compositions that crosscut or overlie rocks of the older suites 5 resurrection with emplacement by obduction onto a passive margin or accretionary uplift with continued subduction the early stages 1 3 may be diachronous and each stage may overlap in both time and space the existence of this consistent progression implies that ophiolite formation is not a stochastic event but is a natural consequence of the ssz tectonic setting

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

h2o transport and release in subduction zones experimental constraints on basaltic and andesitic systems phase relationships in natural andesitic and synthetic basaltic systems were experimentally investigated from 2 2 to 7 7 gpa and 550 c to 950 c in the presence of an aqueous fluid in order to determine the stability of hydrous phases in natural subducted crustal material and to constrain reactions resulting in the release of water from subduction zones to the mantle wedge water reservoirs in subducted oceanic crust at depths exceeding the amphibole stability field 70 80 km are lawsonite 11 wt h2o mg chloritoid 8 wt talc 5 wt and zoisite clinozoisite 2 wt in basaltic rocks and lawsonite zoisite clinozoisite phengite 4 wt and staurolite 2 wt in andesitic compositions the thermal stability of lawsonite at 6 0 gpa extends to 800 c and 870 c in basaltic and andesitic compositions respectively at pressures above amphibole out 2 3 2 5 gpa lawsonite reacts through continuous reactions with steep positive dp dt slopes to zoisite clinozoisite until 3 0 3 2 gpa and at higher pressures to more than 7 7 gpa to assemblages containing garnet clinopyroxene and garnet clinopyroxene kyanite in basaltic and andesitic compositions respectively on the contrary the breakdown of zoisite clinozoisite is mainly pressure sensitive phengite represents the hydrous phase with the largest stability field encountered in this study in andesite phengite is stable to more than 7 7 gpa and more than 920 c talc and staurolite contribute in minor amounts to the water balance in basaltic and andesitic rock compositions a model for water release from the subducted slab is developed combining thermal models for subduction zones with the experimentally determined phase relationships up to 1 wt and 2 wt h2o in basaltic and andesitic rocks respectively can be stored to depths beyond 200 km in cold subduction zones mainly by lawsonite and phengite dehydration rates are high until amphibole out and relatively low at greater depths the amphibole out reactions are found to release a significant amount of water in a depth interval of several kilometers however they do not represent a discrete pulse of fluid and do not completely dehydrate the descending slab fluid release at depths greater than 200 km through phengite and progressive lawsonite breakdown would hydrate the overlying mantle causing the generation of amphibole or phlogopite peridotite at higher geothermal gradients epidote zoisite contributes to fluid flux to the mantle wedge at 100 120 km depth the extensive stability field of phengite may greatly enhance the role of sediments and the small amount of potassium in mafic compositions for the fluid budget in subduction zones at increasing depth

pressure temperature and time indicators in mafic schist their application to reconstructing the polymetamorphic history of vermont mineral chemistry and overgrowth relationships in mafic schist from vermont are shown to be sensitive indicators of pressure temperature and relative time and to be good chronicles of the paleozoic history of this polymetamorphic terrane within the common assemblage amphibole chlorite epidote plagioclase quartz ti phase carbonate k mica fe 3 oxide electron microprobe analyses show that increasing metamorphic grade as defined by intercalated pelitic schist is recorded by an increase in the edenite glaucophane and tschermakite contents of amphibole in the anorthite content of plagioclase and in the substitution of al vi f 2 ti al iv for fe 2 mg mn si in biotite chlorite and muscovite with increasing pressure the glaucophane component of amphibole increases for medium pressure metamorphism the albite oligoclase gap is in the garnet zone where amphibole has between 1 2 and 1 8 formula proportion al iv and al vi fe 3 ti cr this gap is below the garnet isograd in low pressure mafic schist where al iv and al vi fe 3 ti cr in the amphibole are both less than 0 6 mineral growth periods observed are characterized by metamorphic grade and facies series and are assigned to two ordovician taconic and two devonian acadian events silurian devonian mafic schist in northeastern vermont records two periods of low pressure devonian metamorphism these events are expressed by medium pressure and low pressure metamorphism in the cambrian and ordovician rocks to the west and south in the pre silurian rocks high pressure and medium pressure metamorphism is assigned to the ordovician the observed high pressure metamorphism is confined to a 110 by 40 km area along the green mountain anticlinorium axis in north central vermont

experimental investigations of blueschist greenschist transition equilibria pressure dependence ofal2o3 contents in sodic amphiboles a new geobarometer

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

evidence for a variscan suture zone in the vendee france a petrological study of blueschist facies rocks from bois de cene in the bois de cene area blueschist facies rocks characterized by glaucophane and or chloritoid provide evidence for a suture zone in the variscan this terrain is considered to be the eastern equivalent of the ile de groix high pressure metamorphic terrain petrological study of the two characteristic types of rocks found in the area shows that the primary high pressure paragenesis was modified during a retrogression which followed substantial decompression probably at constant or decreasing temperature the simplest interpretation is that this retrogression followed tectonic emplacement within a nappe pile

thermal conductivity of fe bearing post perovskite in the earth s lowermost mantle abstract the thermal conductivity of post perovskite ppv the highest pressure polymorph of mgsio3 in the earth s mantle is one of the most important transport properties for providing better constraints on the temperature profile and dynamics at the core mantle boundary cmb incorporation of fe into ppv can affect its conductivity which has never been experimentally investigated here we determined the lattice thermal conductivities of ppv containing 3 mol and 10 mol of fe at high p t conditions of pressures up to 149 gpa and 177 gpa respectively and temperatures up to 1560 k by means of the recently developed pulsed light heating thermoreflectance technique combining continuous wave heating lasers we found that the incorporation of fe into ppv moderately reduces its lattice thermal conductivity as it increases the fe content the bulk conductivity of ppv dominant pyrolite is estimated as 1 5 times higher than that of pyrolite consisting of bridgmanite and ferropericlase in the lower mantle which agrees with the traditional view that ppv acts as a better heat conductor than bridgmanite in the earth s lowermost mantle

tectonic setting petrology and geochronology of jadeite glaucophane and chloritoid glaucophane schists from north west turkey the north west turkish blueschists represent a subducted passive continental margin sequence dominated by metaclastic rocks and marble the depositional age of the blueschist protoliths are probably palaeozoic to mesozoic while the age of the high pressure low temperature metamorphism is late cretaceous blueschists are tectonically overlain by a volcanosedimentary sequence made up of accreted oceanic crustal material that locally shows incipient blueschist metamorphism and by spinel peridotite slices the metaclastic rocks with regional jadeite and glaucophane which comprise the lower part of the blueschist unit make up an over 1000 m thick coherent sequence in the kocasu region of north west turkey rare metabasic horizons in the upper parts of the metaclastic sequence with sodic amphibole iawsonite but no garnet indicate lawsonite blueschist facies metamorphism the blueschist metaclastics in the kocasu region are practically free of calcium and ferric iron and closely approximate the nfmash system in bulk composition two low variance mineral assemblages with quartz and phengite are jadeite glaucophane chlorite paragonite and chloritoid glaucophane paragonite the metaclastics comprise up to several metres thick layers of jadeite schist with quartz phengite and nearly 100 mol jadeite phase relations in the metaclastics show that the chloritoid glaucophane assemblage even in fe2 rich compositions is stable in the jadeite stability field in the nfash system the above assemblage without the accompanying garnet has a narrow thermal stability field mineral equilibria in the metaclastics involving chloritoid glaucophane jadeite paragonite and chlorite indicate metamorphic p t conditions of 20 2 kbar and 430 30 d c yielding geothermal gradients close to 5d c km 1 one of the lowest geotherms recorded blueschists in the kocasu region which have been buried to 70 km depth are tectonically overlain by the volcanosedimentary sequence and by peridotite buried not deeper than 30 km phengites from two jadeite schists were dated by ar ar laser probe they give an age of 88 5 0 5 ma interpreted as the age of metamorphism blueschists and the overlying peridotite bodies are intruded by 48 53 ma old granodiorite bodies that were emplaced at 10 km depth this suggests that the exhumation of blueschists by underplating of cold continental crust and normal faulting at the blueschist peridotite interface occurred during the late cretaceous to palaeocene 88 53 ma

coesite bearing eclogite from the dabie mountains in central china coesite and coesite pseudomorphs are recognized in eclogite from the dabie mountains a collision zone between the sino korean and yangtze cratons in central china this third known occurrence of coesite in deep crustal rocks is within an archean gneiss terrane and is the first locality where coesite has been identified as an inclusion in both omphacite and garnet crystals at the same location coesite bearing eclogite is estimated to form at pressures of 28 kbar and temperatures from 600 to 710 c the dabie mountains eclogite underwent retrograde metamorphism to amphibolite facies after its formation the preservation of coesite in eclogite is found to be strongly controlled by the pressure temperature time path that coesite bearing eclogite followed during its uplift and retrograde metamorphism

the upper thermal stability of chlorite quartz an experimental study in the system mgo al2o3 sio2 h2o experiments up to water pressures of 21 kbar have been undertaken to bracket the reactions chlorite quartz talc kyanite h2o chlorite quartz talc cordierite h2o and talc kyanite quartz cordierite h2o by reversed runs in the system mgo al2o3 sio2 h2o mash these reaction curves intersect at an invariant point ip1 at ph2o 6 4 0 2 kbar and a temperature of 624 4 c the curve of the chlorite quartz breakdown to talc kyanite h2o at water pressures above 6 4 kbar shows a negative dp dt with the slope decreasing with rising pressure whereas the slope of the breakdown curve to talc cordierite h2o at water pressures is clearly positive the composition of the chlorite solid solution reacting with quartz has been estimated to be approximately mg4 85al1 15 al1 15si2 85o10 oh 8 over the entire pressure range investigated the composition of the talc solid solution forming by the breakdown of chlorite quartz appears to be mg2 94al0 06 al0 06si3 94o10 oh 2 at ph2o 2kbar with increasing pressure the al content of talc decreases reaching a value of about 0 06 atoms per formula unit at p h2o 21 kbar as a consequence of the new experimental data the existing phase topologies of the mash system and k2o mash system have been revised for example the invariant point ip1 and the univariant reaction curve kyanite talc h2o chlorite cordierite are stable for this reason the development of medium to high temperature metamorphic rocks compositionally approximating the mash system must be reconsidered the whiteschists from sar e sang afghanistan are treated as an example the application of the present experimental data to metamorphic rocks of more normal composition requires the examination of the influence of further components this leads to the conclusion that the introduction of fe2 into magnesian chlorite extends its stability field in the presence of quartz by 10 15 c in comparison with pure mg chlorite

formation and exhumation of blueschists and eclogites from ne oman new perspectives from rb sr and 40ar 39ar dating seven eclogite facies samples from lithologically different units which structurally underlie the semail ophiolite were dated by the 40 ar 39 ar and rb sr methods despite extensive efforts phengite dated by the 40 ar 39 ar method yielded saddle hump or irregularly shaped spectra with uninterpretable isochrons the total gas ages for the phengite ranged from 136 to 85 ma clinopyroxene phengite epidote phengite and whole rock phengite rb sr isochrons for the same samples yielded ages of 78i2 ma we therefore conclude that the eclogite facies rocks cooled through 500 u ca tc 7 8i2 ma and that the 40 ar 39 ar dates can only constrain maximum ages due to the occurrence of excess ar inhomogeneously distributed in different sites our new results lead us to conclude that high pressure metamorphism of the oman margin took place in the late cretaceous contemporaneous with ophiolite emplacement previously published structural and petrological data lead us to suggest that this metamorphism resulted from intracontinental subduction and crustal thickening along a ne dipping zone choking of this subduction zone followed by ductile thinning of a crustal mass wedged between deeply subducted continental material and overthrust shelf and slope units facilitated the exhumation of the eclogite facies rocks from depths of c 50 km to 10 15 km within c 10 ma and led to their juxtaposition against overlying lower grade rocks final exhumation of all high pressure rocks was driven primarily by erosion and assisted by normal faulting in the upper plate

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

kar and 40ar39ar study of metamorphic rocks associated with the oman ophiolite tectonic implications abstract k ar analyses on extracted minerals are reported for a variety of metamorphic rocks associated with the sumail ophiolite amphibolites lying at the sole of the ophiolite yield ages of 95 100 ma which are viewed as reflecting times of crystallization high pressure metamorphics of the saih hatat reveal complex results white micas range in age from 80 to 131 ma whereas blue amphiboles indicate ages that are systematically lower than those of coexisting white micas investigation of a few white micas by the 40 ar 39 ar step heating method yields rather intricate age spectra featuring low apparent ages in the first and the last stages of gas release and high apparent ages in between two explanations can be equally envisaged for these convex upward age spectra the first is the mixing of two generations of micas corresponding to two main metamorphisms the first one m1 is a low to medium temperature high pressure event which conceivably occurred between 130 and 114 m y ago the second m2 overprints m1 and has produced rocks typical of the greenschist facies it took place 80 m y ago and also affected the sole of the ophiolites the second is the presence of excess argon in mica mixtures with complex degassing properties thus the two metamorphic phases identified by microscopic inspection are not significantly different in age they occurred in the 70 80 ma interval moreover k ar dates on amphibole from gabbroic dikes intersecting the peridotites suggest that they are genetically linked to the mafic part of the ophiolites assuming that metamorphism is a tracer of tectonic events we view the infraophiolitic amphibolites as the result of an intraoceanic thrusting which took place near a spreading center nevertheless the uncertainty as to the age of the blueschist metamorphism precludes the possibility of indicating a timetable based on metamorphic ages for the motion shift of africa relative to eurasia during the late cretaceous a tentative model of the obduction of the sumail ophiolite is proposed assuming an age of about 130 ma for m1 we stress differences with respect to earlier reconstructions which advocated an 80 m y old blueschist metamorphism

chloritoid pyrophyllite rectorite facies rocks from brittany france electron microprobe analyses of minerals in low grade metamorphic sandy black shales of middle paleozoic age indicate that chemical equilibrium was closely approached in authigenic ferro magnesian or alkali bearing phases while aluminum silicate minerals appear to have reacted at a slower rate the k for mg fe in chlorite chloritoid pairs is 0 19 the latter phase being iron rich the pseudomorphism of detrital white micas produces kaolinite pyrophyllite and intimate mixtures of muscoviteparagonite new phases forming in other sites include chlorite chloritoid and rectorite which is an ordered paragonitebeidellite mixed layered phase these minerals were formed at temperatures below 280 c this is just above the stability of the illite smectite mixed layered minerals which have an isii ordering sequence 85 to 95 illite it was noted that ferromagnesian minerals attain chemical equilibrium on a thin section scale while the aluminous phases often seem to respond to chemical potentials operative on a submillimetric scale

implications to garnet clinopyroxene geothermometry of non ideal solid solution in jadeitic pyroxenes garnet clinopyroxene geothermometry has been applied to numerous rock types from the blueschist and eclogite terrain of the sesia zone in the western alps discrepancies in apparent temperatures of up to 600 c have been found suggesting substantial deviation from ideal behaviour of the garnet clinopyroxene equilibria in certain assemblages assemblages containing jadeitic pyroxenes rather than omphacitic or diopsidic pyroxenes yield very low kd 2 0 values and correspondingly high apparent temperatures 1 000 c the garnets coexisting with high na pyroxenes have compositions similar to those coexisting with omphacitic pyroxenes departure from ideal behaviour of these garnets is likely to be minor and is accomodated in the formulation of the geothermometer numerous examples of coexisting garnet clinopyroxene pairs indicate that there is no obvious relationship of kd with jadeite content in pyroxenes with jadeite content less than 70 but at higher values kd varies inversely with the jadeite content the dependence of kd upon the jadeite content in the high na pyroxenes is believed to be due to the preference of m2 sites for fe2 in the enstatite jadeite substitution fe2 mgna 1al 1 this substitution is usually very limited and has no demonstrable effect upon x fe cpx when the m1 fe2 mg content of the pyroxenes is high as in omphacitic and diopsidic pyroxenes however when the m1 fe2 mg content is low as in jadeitic pyroxenes the non ideality of the enstatite type exchange has a marked effect on x fe cpx and consequently on garnet clinopyroxene calculations

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

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

calculated phase relations in high pressure metapelites in the system nkfmash na2o k2o feo mgo al2o3 sio2 h2o using an internally consistent thermodynamic dataset and updated models of activity composition relation for solid solutions petrogenetic grids in the system nkfmash na2o k2o feo mgo al2o3 sio2 h2o and the subsystems nkmash and nkfash have been calculated with the software thermocalc 3 1 in the p t range 5 36 kbar and 400 810 c involving garnet chloritoid biotite carpholite talc chlorite kyanite sillimanite staurolite phengite paragonite albite glaucophane jadeite with quartz coesite and h2o in excess these grids together with calculated afm compatibility diagrams and p t pseudosections are shown to be powerful tools for delineating the phase equilibria and p t conditions of na bearing pelitic assemblages for a variety of bulk compositions from high p terranes around the world these calculated equilibria are in good agreement with petrological studies moreover contours of the calculated phengite si isopleths in p t pseudosections for different bulk compositions confirm that phengite barometry is highly dependent on mineral assemblage

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

femg partitioning between coexisting garnet and phengite at high pressure and comments on a garnet phengite geothermometer abstract the fe mg exchange reaction between coexisting garnet and phengite has been studied by reacting a natural phengite mg 67 in the presence of quartz and water at pressures of 20 35 kb and temperatures of 800 1000 c compositions of coexisting garnet and mica indicate a linear relation between both the inkd fe mg garnet fe mg phengite and temperature and inkd and pressure in the above p t range this fe mg exchange reaction between garnet and phengite is shown to be dependent on the ca content of the garnet and on the mg number of the bulk composition these two composition effects have been studied by usin phengitic mica mixes with mg numbers of 23 and 46 and by using a synthetic basaltic composition the overall results allow broad empirical calibration of separate geothermometers for pelitic and basaltic systems respectively however because of non ideality in the exchange reaction this geothermometer should not be used in any practical application outside the composition ranges studied also careful consideration of the presence of fe3 in phengite must be made if the fe3 content of the natural phengite is unknown then the temperatures obtained will be maximum temperatures only

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

pressure temperature evolution of lawsonite bearing eclogites pinchi lake british columbia blueschists and retrogressed eclogites are located along the pinchi fault zone near 54 30 n and 124 w in central british columbia the pinchi fault separates rocks of contrasting geological histories and the blueschists and eclogites occur with ultramafic rocks as fault bounded blocks the retrogressed eclogites occur as tectonic blocks now in glacial debris a few metres across blueschists occur in coherent kilometre sized tracts eclogites contain garnet omphacite rutile quartz with glaucophane lawsonite and titanite some of the lawsonite appears to be stable with omphacite and garnet tectonic blocks of eclogite from two different localities have recorded different p t histories in one tectonic block p t estimates for garnet inclusions in clinopyroxene and lawsonite c 565 c 13 1 kbar suggest that garnet and omphacite initially equilibrated outside the stability field of lawsonite a decrease in temperature as recorded in garnet rims and matrix clinopyroxene resulted in crystallization of lawsonite and other retrogressive minerals later crystallization of stilpnomelane locally pseudomorphing garnet howieite winchite and actinolite was triggered by an influx of fluid under p t conditions outside the stability field of garnet lawsonite appears to have been stable suggesting a minimum pressure of about 3 kbar in the second tectonic block clinopyroxene inclusions in garnet suggest temperatures near 350 c p 10 kbar and garnet rims equilibrated with matrix clinopyroxene suggest temperatures near 450 c at pressures above c 12 kbar

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

high pressure low temperature metamorphism in the gran paradiso basement western alps the gran paradiso basement complex of the french and italian alps is composed of metasediments termed the gneiss minuti and metabasic rocks both of which are intruded by a late hercynian granite the bonneval gneiss which crops out at the western edge of the complex is composed of highly deformed metasediments volcanics and volcaniclastic rocks eclogites now highly altered occur in the metabasic rocks kyanite and blue green amphibole are locally present in the gneiss minuti and aegirine plus riebeckite occur in the bonneval gneiss a moderately high pressure low temperature metamorphic event of probable alpine age occurred in the basement complex this metamorphic event differs from that in the overlying sesia unit and ophiolites of the schistes lustres nappe in being at lower pressures below the ab jd100 qz transition and post dating the major d2a deformation the origin of the metamorphism is discussed and interpreted as a probable consequence of the overlying nappe pile which was emplaced during the d2a event subsequent greenschist facies metamorphism in the basement complex is a consequence of thermal relaxation during uplift

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

retrograde evolution of eclogite facies rocks from ne oman evidence from fluid inclusions and petrological data eclogite facies rocks from eastern saih hatat oman metamorphosed under peak conditions of p 12 kbar and t of 550 580 c have several types of fluid inclusions in quartz clinopyroxene and garnet most of these inclusions are two phase at room t and contain an aqueous fluid belonging to the system nacl mgcl2 h2o final melting temperatures for most inclusions range from 4 to 0 c but values as low as 13 c have been recorded indicating salinities of 490 c inclusions with three phases at room t also contain a predominantly low salinity aqueous fluid with xco2 12 kbar to 4 kbar or less at t 500 550 c this conclusion is supported by the scatter in homogenization temperatures for these inclusions and their large variety of shapes and sizes which range from irregular with hair like fractures surrounded by satellite inclusions to negative crystals the replacement of ferroan dolomite ankerite and quartz by calcite talc and of rutile calcite by titanite or rutile glaucophane by ilmenite albite in these eclogites are also consistent with a stage of isothermal decompression following near isothermal decompression to depths of 10 15 km brittle deformation prevailed leading to the formation of layer parallel and crosscutting fractures that were filled by quartz calcite this stage was followed by near isobaric cooling recorded by the isochores for fluid inclusions in vein quartz and by the occurrence of dendritic networks of decrepitated and imploded fluid inclusions in these veins

high pressure metamorphism of the zermatt saas ophiolite zone switzerland within the zermatt saas ophiolite zone of the western alps a record of polystadial high pressure metamorphism is well preserved early blueschist assemblages retained as inclusions in garnets were succeeded by eclogitic assemblages which in some rocks contained lawsonite kyanite talc and chloritoid these eclogitic assemblages formed by prograde reaction from the early blueschists subsequently reaction of these eclogites with a mixed h 2 o co 2 vapour phase led to the replacement of kyanite by paragonite and the partial replacement of omphacite garnet paragenses by assemblages containing glaucophane paragonite and ankerite high pressure eo alpine metamorphism took place under conditions of 550 600 c and 17 5 20 kbar values of a h 2 o between 0 55 and 1 are high enough to accommodate equilibration with a water rich vapour phase under the highest grade conditions the presence of such a fluid phase is locally indicated by the presence of quartz rich veins containing omphacite and kyanite this vapour phase was absorbed during retrogression later rehydration is limited to areas close to albite veins tectonic contacts and bodies of metasediment the metamorphic conditions determined for the zermatt saas zone are compatible with those suggested for over and under lying units these conditions and the p t path inferred by comparing the reaction histories with a petrogenetic grid for the system na 2 o cao mgo a1 2 o 3 sio 2 h 2 o suggest that metamorphism occurred during subduction to depths of between 60 and 70 km and subsequent exhumation during the alpine orogeny

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

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

distribution of auroral precipitation at midnight during a magnetic storm on the night of november 4 1986 a very complex precipitation pattern was observed by viking in the magnetic midnight sector over scandinavia and svalbard the pass took place during a magnetic storm and during substorm recovery phase going from north to south the satellite first encountered a plasma region of bps type name derived from boundary plasma sheet and then a region of cps type derived from central plasma sheet then however a new region of bps type was traversed the quite intense most equatorward aurora corresponded to a plasma region which was not of ordinary cps type but contained sharp quasi monoenergetic peaks the high latitude midnight sector was totally dominated by eastward convection the harang discontinuity had passed northern scandinavia the first time as early as 17 to 20 mlt more than three house before the viking pass it is suggested that the particle precipitation pattern and the general shape of the aurora as observed by the viking imager can be explained in a natural way by the convection pattern the northernmost bps and cps type regions originated in the morningside convection cell while the more equatorward population of bps type had drifted in from the eveningside the interpretation is supported bymore ground based measurements by eiscat and magnetometers less

regional distribution of al silicates and metamorphic zonation in the low grade verrucano metasediments from the northern apennines italy abstract mineralogical and petrological studies of triassic verrucano metasediments of the northern apennines are reported the widespread occurrence of al silicates allows the delineation of four metamorphic zones with increasing metamorphic grade 1 kaolinite zone well perugia 2 umbria 2 kaolinite pyro phyllite zone monte argentario and part of the verrucano of the monticiano roccastrada area and monti leoni 3 pyrophyllite zone monti pisani iano monti leoni the monticiano roccastrada area and some wells in the larderello region 4 kyanite zone massa area and some wells in the larderello area the four metamorphic zones correspond to temperatures ranging from 300 c to about 450 c on the basis of the si content of muscovite and geological arguments pressures of between 3 and 5 kbar are estimated the metamorphic zones are located more or less parallel to the bent north west south east trending structural zonation of the northern apennines with the concave side towards the tyhrrenian sea during the alpine orogeny the verrucano metasediments underwent three folding phases each of which has produced an axial plane schistosity s1 s2 s3 during the first folding phase the verrucano sediments were buried increasingly deeply within the crust from east to west the climax of alpine metamorphism was attained prior to the second folding phase with crystallization of porphyroblasts of kyanite and chloritoid in a central area located between massa and larderello the inferred paleo temperature distribution pattern resembles an asymmetric thermal high defined by the kyanite zone and surrounded by the pyrophyllite zone a similar pattern is still present in the tuscan crust as indicated by a series of geothermal anomalies passing through the northern apennines

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

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

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