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

high pressure 2000 mpa kyanite and glaucophane bearing pelitic schist and eclogite from cordillera de la costa belt venezuela in the cretaceous melange of the cordillera de la costa belt introduction north central venezuela there are knockers of eclogite barroisitetwo belts of high pressure hp low temperature lt bearing eclogite and pelitic glaucophane schist these occur in a metamorphic rocks are exposed in the caribbean mounmetamorphic melange matrix that locally consists of marble sertain system of northern venezuela which is part of the pentinite amphibolite actinolite schist feldspathic schist and gneiss complex east west trending boundary zone between the graphitic schist chloritoid schist and garnet bearing mica schist caribbean and south american plates e g menendez the protoliths for these various rock types exhibit a wide age range 1966 1967 these two belts are the cordillera de la cambrian to early cretaceous recently discovered knockers of costa and villa de cura belts both of which were pelitic glaucophane schist contain mg glaucophane paragonite metamorphosed during the mid cretaceous presumably kyanite garnet talc graphite rutile quartz in a subduction zone related to the mid cretaceous the coexistence of kyanite and mg glaucophane suggests minimum leeward antilles volcanic arc e g pindell 1993 from p 2000 mpa at t 600 c eclogite knockers from the same jurassic to eocene time north and south america were outcrop contain garnet and clinopyroxene which yield 500 c for diverging and new proto caribbean sea floor was created cores 700 c for rims and p 1200 mpa the assemblage between them e g pindell 1993 therefore the leegarnet biotite phengite albite within schists of the melange matrix ward antilles arc as well as the entire caribbean mounof this locality indicates metamorphic conditions of t tain system are allochthonous and formed far to the 450 520 c at p 1800 mpa because all lithologies in this west as part of the farallon north south american plate outcrop record high p conditions this metamorphic melange formed boundary zone e g pindell 1993 before or during peak metamorphism in a mid cretaceous subduction although the two hp lt belts have similar metazone morphic ages they differ greatly in lithology and metamorphic history the cordillera de la costa belt consists of oceanic and passive continental margin rocks intermixed with paleozoic granites and granitic gneisses in this belt the eclogites and blueschists were retrograded to epidote amphiboliteand greenschist facies assemblages following an apparent p t path typical for

retrograde p t t path for the voltri massif eclogites ligurian alps italy some tectonic implications the retrograde p t trajectory of the eclogitic fe ti gabbros from the ligurian alps is constrained by the appearance of mineral parageneses post dating the na clinopyroxene garnet eclogitic assemblage and indicating the following sequence of metamorphic events 1 amphibolitic stage edenite katophorite plagioclase an33 43 oxides in symplectitic aggregates 2 glaucophanic stage a porphyroblastic glaucophanic amphibole has overgrown the symplectite winchite also occurs as thin rims around glaucophane and both amphiboles are sometimes armoured by atoll garnets 3 albite amphibolite stage barroisite katophorite albite epidote oxides chlorite overprint the glaucophanic stage minerals 4 greenschist stage represented by actinolite albite epidote oxide paragenesis the metamorphic evolution is complex and the decompression path on a p t diagram is significantly different from those defined in the literature for the voltri eclogites the main features inferred from the p t path are the following 1 the pressure climax does not match the thermal climax the maximum temperature conditions are in fact achieved during the early stage of uplift 2 a decrease in temperature suggested by the appearance of glaucophane after the amphibolitic symplectite 3 successive uplift probably accompanied by an increase in temperature the complexity of the p t path drawn for the voltri eclogites can be explained with a mechanism of successive underthrusts propagating from the innermost to the outermost sector of the ligurian alps

retrograde formation of nacl scapolite in high pressure metaevaporites from the cordilleras beticas spain a permo triassic pelite carbonate rock series with interacalated metabasitic rocks in the cordilleras beticas spain was metamorphosed during the alpine metamorphism at high pressures pmin near 18 kbar the rocks show well preserved sedimentary features of evaporites such as pseudomorphs of talc of kyanite phengitetalc biotite and of quartz after sulfate minerals and relicts of baryte anhydrite nacl and kcl indicating a salt clay mixture of illite chlorite talc and halite as the original rock the evaporitic metapelites have a whole rock composition characterized by high mg mg ca ratios 0 7 variable alkaline and sr ba contents but are mostly k2o rich 8 8 wt the f 2600 ppm cl 3600 ppm and p2o5 0 24 wt contents are also high the pelitic member of this series is a fine grained biotite rock kyanite phengite talc biotite aggregates in pseudomorphs developed in the high pressure stage albite rich plagioclase was formed when the rocks crossed the albite stability curve in the early stages of the uplift scapolite rich in nacl ca ca na mol 24 40 and poor in so4 with cl cl co3 ratios between 0 6 and 0 8 formed as porphyroblasts sometimes replacing up to 60 of the rock in a late stage of metamorphism between 10 and 5 kbar near 600 c no reaction with albite is observed and the scapolite formed from biotite by begin gathered al biotite caco 3 nacl sio 2 hfill al poor biotite scapolite mgco 3 kcl hfill mgcl 2 h 2 o hfill end gathered

eclogites and polyphase p t cycling in the caledonian uppermost allochthon in troms northern norway eclogites in the tromso area northern norway are intimately associated with meta supracrustals within the uppermost allochthon of the scandinavian caledonides the tromso nappe complex the whole sequence which includes pelitic to semipelitic schists and gneisses marbles and calc silicate rocks quartzofeldspathic gneisses metabasites and ultramafites has undergone three main deformational metamorphic events d1 m1 d2 m2 and d3 m3 detailed structural microtextural and mineral chemical studies have made it possible to construct separate p t paths for these three events chemically zoned late syn to post d1 garnets with inclusions of bt pl and qtz in ky bearing metapelites indicate a prograde evolution from 636 c 12 48 kbar to c 720 c 14 15 kbar this latter result is in agreement with grt cpx geothermometry and grt cpx pl qtz geobarometry on eclogites and trondhjemitic to dioritic gneisses maximum pressures at c 675 c probably reached 17 18 kbar based on cpx pl qtz inclusions in eclogitic garnets and grt ky pl qtz and jd ab qtz in trondhjemitic gneisses post d1 pre d2 decompressional breakdown of the high p assemblages indicates a substantial drop in pressure at this stage inclusions and chemical zoning in syn to post d2 garnets from metapelites record a second episode of prograde metamorphism from 552 c 7 95 kbar passing through a maximum pressure of 10 64 kbar at 644 c with final equilibration at c 665 c 9 10 kbar the corresponding apparently co facial paragenesis grt cpx pl qtz in metabasites yields c 635 c 8 10 kbar in the metapelites post d3 grt in apparent equilibrium with bt phe and pl yield c 630 c 9 kbar the d1 m1 and d2 m2 episodes are exclusively recorded in the tromso nappe complex and must thus pre date the emplacement of this allochthonous unit on top of the underlying lyngen nappe while the d3 m3 episode is common for the two units a previously published sm nd mineral isochron grt cpx am on a partly retrograded and recrystallized ecologite of 598 107 ma represents either the timing of formation of the eclogites or the post eclogite pre d2 decompression stage while a rb sr whole rock isochron of an apparently post d1 pre d2 granite of 433 11 ma is consistent with a k ar age of post d1 pre d2 amphiboles from a retrograded eclogite of 437 16 ma which most likely record cooling below the 475 500 c isotherm after the m3 metamorphism

the alpujarride nevado fibabride extensional shear zone betic cordillera se spain in the betic cordillera the initial thickening of the alboran domain produced an alpine high pressure low temperature hp lt metamorphism in the alpujarride and nevado filabride complexes after 19 ma this thickening process evolved to an extensional stage with uplift rates of between 1 and 2 mm year 1 the extensional stage generated structures in a heterogeneous shear regime with a top to the west sense of movement the present day alpujarride nevado filabride contact is a detachment fault generated in this shearing regime in the footwall the first structure developed in this extensional stage is a planar linear fabric characterized by constrictive strain that is axial planar to kilometric recumbent tight to isoclinal folds the planar linear fabric is folded by inclined close to open folds with axes parallel to the stretching lineation the planar linear fabric and folds are deformed by a late extensional crenulation cleavage near the detachment surface finally brittle deformation was generated within the same kinematic framework as that of the ductile structures in the hanging wall meanwhile deformation had a brittle character producing faults and joints

high pressure metamorphism in metabasites from the betic cordilleras s e spain and its evolution during the alpine orogeny the nevado filabride complex is the lowest tectonic unit of the betic zone sensu stricto ss of the betic cordilleras s e spain the upper series of this complex consists of a metamorphosed sequence intruded by basic and ultrabasic igneous rocks high pressure metamorphism in the eclogite and blueschist facies is recorded in the metabasites but this was partially obliterated by further successive metamorphic stages in the almandine amphibolite and greenschist facies

paragonite eclogites from dabie shan china re equilibration during exhumation paragonite in textural equilibrium with garnet omphacite and kyanite is found in two eclogites in the ultrahigh pressure metamorphic terrane in dabie shan china equilibrium reactions between paragonite omphacite and kyanite indicate a pressure of about 19 kbar at c 700 c however one of the paragonite eclogites also contains clear quartz pseudomorphs after coesite as inclusions in garnet suggesting minimum pressures of 27 kbar at the same temperature the disparate pressure estimates from the same rock suggest that the matrix minerals in the ultrahigh pressure eclogites have recrystallized at lower pressures and do not represent the peak ultrahigh pressure assemblages this hypothesis is tested by calibrating a garnet zoisite clinozoisite kyanite quartz coesite geobarometer and applying it to the appropriate eclogite facies rocks from ultrahigh and high pressure terranes these four minerals coexist from 10 to 60 kbar and in this wide pressure range the grossular content of garnet reflects the equilibrium pressure on the basis of the reaction zoisite clinozoisite grossular kyanite quartz coesite h o the results of the geobarometer agree well with independent pressure estimates from eclogites from other orogenic belts for the paragonite eclogites in dabie shan the geobarometer indicates pressures in the quartz stability field confirming that the former coesite bearing paragonite eclogite has re equilibrated at lower pressures on the other hand garnets from other coesite bearing but paragonite free kyanite zoisite eclogites show a very wide variation in grossular content corresponding to a pressure variation from coesite into the quartz field this wide variation partly due to a rimward decrease in grossular component in garnet is caused by partial equilibration of the mineral assemblage during the exhumation

compositions and phase relations of calcic amphiboles in epidote and clinopyroxene bearing rocks of the amphibolite and lower granulite facies central massachusetts usa calcic amphiboles coexisting with epidotegroup minerals zoisite clinozoisite epidote and or clinopyroxene plagioclase quartz garnet occur in amphibolites and calc silicate rocks that underwent amphibolite to lower granulite facies metamorphism in the acadian metamorphic high of central massachusetts usa across the region peak metamorphic conditions range from about 580 c and 6 2 kbar to 730 c and 6 3 kbar the coexistence of most ca amphiboles with fe3 rich epidote group minerals suggests the presence of fe3 in most of these amphiboles an empirical fe3 estimation for the microprobe analyses is based on two constraints the na ca content of the m4 sites of ca saturated gravimetrically analyzed hornblendes gives the relation ca m4 c 1 479 na m4 c 2 c corrected the second constraint is the stoichiometric equation ca m4 na m4 fm 15 where fm is the sum of all cations exclusive of ca na and k solving the two equations simultaneously gives 20 185 0 479 ca m4 1 479 fm starting with the uncorrected values of ca m4 u and fm m4 u u uncorrected of the all ferrous formula the normalization factor nf for calculating the corrected cations of the ferric formulas is 20 185 0 478 ca m4 u 1 479 fm u from the deficient oxygen the fe3 content which is equal to 2 23 ox can be calculated determinations of fe3 contents of four hornblende separates by mossbauer spectroscopy are in agreement with the calculated values the ca amphiboles show systematic changes in composition with increasing grade of metamorphism within the amphibolite and lower granulite facies zones increasing edenite and tschermakite substitution increasing ti content and increasing fe2 fe2 mg ratio in addition the coexisting clinopyroxenes are also characterized by an increase in fe2 fe2 mg ratio in quartz free rocks with coexisting ca amphibole and plagioclase there is an increase in the ratio x ab x ed where x ab na na ca in plagioclase and x ed na in the amphibole a site these chemical changes in mineral composition together with the disappearance of epidote at the transition to granulite facies metamorphic conditions are attributed to the continuous reaction albite epidote fe mg hornblende fe mg clinopyroxene anorthite naalsi 1 hbl h2o

partially retrograded eclogites of the munchberg massif germany records of a multi stage variscan uplift history in the bohemian massif eclogites with a wide range in bulk composition are present in the munchberg massif part of the variscan basement of the bohemian massif in north east bavaria new analyses of the primary phases garnet omphacite phengite and amphibole as well as the secondary phases clinopyroxene ii various amphiboles biotite phlogopite plagioclase margarite paragonite prehnite and pumpellyite reveal a complex uplift history new discoveries were made of samples with very jadeite rich primary omphacite as well as a secondary omphacite in a symplectite with albite various geothermobarometric techniques together with thermodynamic databases incorporating separately determined activity composition values and experimental data have clustered the minimum conditions for the primary assemblages to the p t range 650 60 c 14 3 1 kbar however jadeite in omphacite kyanite paragonite in phengite and zoisite grossular in garnet kyanite quartz relationships suggests pressures of 25 28 kbar at the same temperatures the fact that the secondary omphacite plagioclase assemblage yields pressures within a few hundred bars of the minimum pressures for the plagioclase free assemblages strongly suggests that the minimum values are serious underestimates zoning inclusion suites and breakdown reactions of primary phases in addition to new minerals formed during uplift define a polyphase metamorphic evolution which from geochronological evidence occurred solely within the variscan cycle the complex breakdown in other bohemian massif eclogites and the distinct variation in their temperatures during uplift suggest a multi stage thrusting model for the regional evolution of the eclogites such an evolution has significance with respect to incorporation of mantle slices into crustal sequences and fluid derivation from successively subducted units possibly driving the breakdown reactions

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

preservation of ice formed features in a subarctic sandy beach sequence geologic implications abstract the barrier beach shoreline near michael river labrador is ice bound for more than 6 months each year the contemporary beachface deposits record a complete yearly cycle as follows the lowermost unit d a well laminated fine to medium grained sand and the overlying unit a a graded sequence of well mixed sand and gravel near the bottom and medium grained laminated sand at the top represent foreshore accretion during summer and fall unit a is overlain by unit b a laminated fine to medium grained sand deposit containing contorted and disrupted laminae and intrabedded sand clasts features which characterize middle to upper foreshore deposits that have accreted under freezing conditions the cycle is completed by unit c a mixed gravel and sand deposit that resembles ice rafted and ice push deposits which form on cold climate beaches during spring break up of nearshore ice unit b is considered to be the freeze up deposit formed during early winter and unit c the subsequent break up deposit which accumulated during spring unit c is overlain by unit d a well laminated foreshore sand which marks the beginning of the next cycle the fact that freeze up and break up events are recorded in a yearly cycle of beachface accretion indicates that such deposits could have been preserved in the rock record if the cyclic sequence recorded in the beachface near michael river were encountered in ancient rocks it could be a good indicator of paleo climatic summer winter seasonal conditions

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

relative geobarometry from high pressure rocks of quartzofeldspathic composition from the sesia zone western alps italy in the sesia zone of the western alps italy early alpine blueschist to eclogite facies metamorphism of rocks of quartzofeldspathic composition has produced the same high pressure assemblage of quartz na pyroxene na amphibole paragonite phengite zoisite garnet magnetite sphene and fe sulphide the qfs assemblage over an area 150 km2 relative gradients in pressure and temperature over this region are reflected in the variations in mineral chemistries of the individual phases of the quartzofeldspathic assemblage through continuous reactions mineralogical discontinuities do not occur in the qfs assemblage of this region increases in the jadeite content of the pyroxenes xjd 0 48 to xjd 0 93 and in the glaucophane content of the amphiboles xgl 0 89 to xgl 0 96 occur from the southwest to the northeast of the region studied analysis of coexisting garnets and pyroxenes indicate that the compositional variation of amphiboles and pyroxenes is associated with a decrease in the grossular component of the coexisting garnet zoned pyroxenes and garnets together with the regional trends in mineral chemistries suggest that the evolution of the qfs assemblage with increasing pressure may be modelled by pressure sensitive continuous reactions in which amphibole zoisite and the more jadeitic pyroxene constitute the high pressure assemblage chemographic constraints permit the positioning in pressure temperature space of the compositional isopleths of those model continuous reactions involving these phases which meet the textural and chemical criteria observed in the natural assemblages the low dp dt slope 20 bars c of these isopleths causes the continuous reactions to be useful for geobarometric calculations at pressures above the absolute breakdown of albite to jadeite plus quartz in addition the pseudobinary loops for the other continuous reactions which are potentially useful geobarometers and involve either the naalca 1mg 1 exchange or the mgca 1 exchange are calculated comparison of mineral chemistries with the isopleths yields a relative barometric scheme for the localities studied with these barometric observations it is possible to show that the p t path which the sesia body travelled towards the final recorded state was one of increasing pressure other blueschist and eclogite occurrences from syros and sifnos which contain rocks of quartzofeldspathic composition are also examined

continuous reactions with amphibole garnet and plagioclase end member continuous and degenerate reactions are derived for the multisystem with the six components na2o cao mg fe o al2o3 sio2 h2o among the phases plagioclasess garnetss amphiboless cpx opx olivine spinel quartz and an aqueous fluid the chemography of this system is degenerate due to the co linearity 2opx ol qtz this co linearity has its implications both on reaction space and phase equilibria from a total of 28 reaction systems reaction space is derived for nine subsystems phases in parentheses are absent case a1 cpx ol cpx opx and cpx qtz case a2 spl ol spl opx and spl qtz case b ol opx ol qtz and opx qtz in the absence of either cpx or spl case a three reactions form an invariant point either cpx or spl where the co linear phases olivine opx and quartz coexist on the transformation line 2opx ol qtz changing mineral compositions force invariant points to move along the line with the different reaction curves changing their relative position according to schreinemakers rules zero contours i e the location where a phase s disappear s in reaction space correspond to singular points in phase diagrams two types are distinguished singular points of indispensable and of substitutable phases in the first case the phase disappears from the entire bundle while in the second it disappears from a single reaction in the specific case where the substitutable phases are also the co linear ones two of the three co linear phases disappear simultaneously two of the three reaction curves coincide in the system including cpx and spl case b three reactions ol opx ol qtz and opx qtz oppose three invariant points ol opx and qtz invariant points no longer move along the line 2opx ol qtz the coincidence of the zero contours of all three co linear phases in reaction space the result of the chemographic degeneracy causes the respective singular points to coincide in the phase diagrams this is the location where curves must be rearranged in a bundle to conform schreinemakers rules the reaction grs1prp2 2 ol an is fourth order degenerate and part of all nine subsystems cases a and b it can be used to relate the different phase diagrams to one another

petrogenetic grids for metacarbonate rocks pressure temperature phase diagram projection for mixed volatile systems isothermal or isobaric phase diagram sections as a function of fluid composition x f are widely used for interpreting the genetic history of metacarbonate rocks this approach has the disadvantages that 1 the influence of a key metamorphic variable either pressure p or temperature t is obscured 2 the diagrams are inappropriate for systems that are not fluid saturated these problems are avoided by constructing phase diagram projections in which the volatile composition of the system is projected onto a p t coordinate frame i e a petrogenetic grid the univariant curves of such p t projections trace the conditions of the invariant points of isothermal or isobaric phase diagram sections thereby defining the absolute stability of high variance mineral assemblages with and without a coexistent fluid phase petrogenetic grids for metacarbonate rocks are most useful for the study of regional metamorphism and for systems in which fluid composition has not been externally controlled a calculated example of a p t projection for the system cao mgo sio2 h2o co2 suggests that many assemblages e g calcite tale enstatite fluid magnesite tremolite antigorite diopside dolomite and calcite forsterite tremolite in mixed volatile systems have stability fields that make them useful as p t indicators consideration of the principles governing projection topology demonstrates that the univariant curves around a fluid present invariant point cannot be oriented independently with respect to the direction of compositional variation in the fluid phase this has the interesting predictive implication that if the direction of compositional variation along one univariant curve around an invariant point is known then the direction of compositional variation along the remaining curves can be determined solely from topologic constraints the same constraints can be applied to systems containing simple mineral solutions or melts in order to predict compositional variations

petrology structure and geotectonic implications of glaucophane bearing eclogites and related rocks from the malpica tuy mt unit galicia northwest spain abstract the malpica tuy mt unit is a narrow band of basement in galicia northwest spain composed of mica schists paragneisses and calc alkaline to peralkaline orthogneisses with minor amounts of metamorphosed basic rocks garnet omphacite glaucophane barroisite rutile phengite paragonite kyanite zoisite developed in eclogites s s and eclogitic impure sandstones garnet phengite rutile zoisite in the calc alkaline orthogeneisses and garnet phengite rutile chloritoid kyanite in the metasediments biotite breakdown in the orthogneisses and related hornfelses produced garnet with development of coronitic textures plagioclase porphyroblasts with aligned inclusions of rutile and phengite are a common feature of many paragneisses the minimum p t conditions for the equilibrium of garnet omphacite parageneses at the peak of the metamorphism were 600 c 12 2 kbar the mineral chemistry of the orthogneisses and metasediments is compatible with these conditions furthermore the structural data also suggest an in situ origin for the eclogites i e the same as the metamorphism of the enclosing rocks the crystalline area surrounding the mt unit is mainly composed of metasediments and anatectic granites the t at the peak of the metamorphism was higher than 700 c the p being of the order of 5 kbar the two domains represent different parts of the hercynian foldbelt tectonically juxtaposed the juxtaposition is the result of complex tangential tectonics three main phases of compressive deformation having been recognized a first stage of subduction of a thinned crust the materials of the mt unit and subsequent obduction development of hydrated minerals in eclogitic rocks with emplacement of these materials over an already metamorphosed crustal area is suggested the mt unit would represent a remnant of a hercynian thrust nappe

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

matrix analysis of metamorphic mineral assemblages and reactions assemblage diagrams are widely used in interpreting metamorphic mineral assemblages in simple systems they can help to identify assemblages which may represent equilibrium states to determine whether differences between assemblages reflect changes in metamorphic grade or variations in bulk composition and to characterize isograd reactions in multicomponent assemblages these questions can be approached by investigating the rank composition space range and reaction space null space of a matrix representing the compositions of the phases involved singular value decomposition svd provides an elegant way of 1 finding the rank of a matrix and detemining orthonormal bases for both the composition space and the reaction space needed to represent an assemblage or pair of assemblages and 2 finding a model matrix of specified rank which is closest in a least squares sense to an observed assemblage although closely related to least squares techniques the svd approach has the advantages that it tolerates errors in all observations and is computationally simpler and more stable than non linear least squares algorithms models of this sort can be used to interpret multicomponent mineral assemblages by straightforward generalizations of the methods used to interpret assemblage diagrams in simpler systems svd analysis of mineral assemblages described by lang and rice 1985 demonstrates the utility of the approach

eclogites and blueschists of the pam peninsula ne new caledonia a reappraisal high p rocks of the pam peninsula ne new caledonia are divided into three zones 1 an uppermost ferroglaucophane lawsonite zone of cretaceous to eocene metasediments and metavolcanics of the diahot terrane that experienced peak conditions involving p 7 9 kbar and t 400 58 c 2 albite epidote omphacite zone diahot terrane rocks that experienced blueschist facies conditions of p 12 6 1 2 kbar and t 570 36 c 3 lowermost metabasic eclogites of uncertain age that form the pouebo terrane which experienced high p conditions of p 23 9 3 0 kbar and t 600 c eclogite occurs as metre to kilometre scale pods in coarse grained hydrous mineral rich glaucophanite formed during hydration and decompression of the pouebo terrane metamorphism and deformation were consequent to 44 51 ma eocene convergence when sedimentary and ophiolitic nappes were thrust over the eclogites in a sw direction white mica ages constrain metamorphism to have ended by 37 1 ma large steps in metamorphic grade are coincident with sw dipping and ne dipping faults that separate the three zones and were formed during two stages 1 comparatively slow uplift and hydration of the pouebo terrane before it was juxtaposed with the albite epidote omphacite zone at p 14 kbar 2 comparatively rapid uplift of both the pouebo terrane and the albite epidote omphacite zone to form a domal core of eclogite flanked by significantly lower grade rocks to the sw and ne

occurrence and origins of minerals in mixed layer illite smectite rich coals of the late permian age from the changxing mine eastern yunnan china abstract minerals in coals from the changxing mine of eastern yunnan southwestern china were investigated using low temperature ashing plus x ray diffraction lta xrd optical microscopy and scanning electron microscopy with an energy dispersive x ray spectrometer sem edx the results show that minerals in the changxing coals include mixed layer illite smectite i s quartz kaolinite dolomite mg bearing siderite anatase ammonium illite k illite calcite and chamosite as well as minor amounts of hematite paragonite k feldspar and an al hydroxide mineral gibbsite the changxing coals are typically rich in mixed layer i s accounting for an average of 47 2 in their low temperature ashes mixed layer i s occurs mainly in two types 1 lentoid as thin beds and 2 fractures or cells fillings the type 1 is of terrigenous detrital origin and probably related to the weathering products of the emeishan basalt while the type 2 is of epigenetic origin and precipitated from hydrothermal fluids of a mafic magma intrusion the high k and poor na in the type 1 mixed layer i s indicate that it is in a higher evolution stage than the type 2 ammonium illite k illite and paragonite are probably products of dissolution and recrystallization of mixed layer i s while mg bearing siderite and partly chamosite are related to the fe mg al and si released from the evolution of mixed layer i s

metamorphic evolution of two different eclogites in the franciscan complex california u s a the eclogites in california u s a have been regarded as tectonic blocks which formed 150 160 ma ago and have been tectonically incorporated into younger rocks of the franciscan complex of 90 130 ma however in ward creek northern california two different types of eclogites were identified type i is a typical tectonic block and type ii is a possible in situ eclogite formed during the franciscan metamorphism most eclogites in the franciscan complex are type i eclogites type i eclogites in ward creek consist of garnet omphacite epidote glaucophane phengite rutile barroisite quartz and have coarse grained gneissic texture they have precursors of the epidote amphibolite facies and are overprinted in various degrees by blueschist facies retrograde metamorphism remnants of metamorphic rinds which consist of actinolite talc chlorite and formed during metasomatic interaction with serpentinizing peridotite occur at the margins of type i eclogite blocks type ii eclogites are medium grained and consist of garnet chloromelaniticclinopyroxene epidote glaucophane paragonite phengite rutile quartz petrochemical characters of type ii eclogites are similar to those of in situ eclogitic schists in the ward creek metabasite sequence epidote amphibolite f facies precursors metamorphic rinds and overprint of retrogressive blueschist facies metamorphism are not found in type ii eclogites garnets in type ii eclogites are medium grained unaltered and idioblastic and have higher spessartine contents sp14 25 whereas garnets in type i eclogites are coarse grained and chloritized and have lower spessartine contents sp2 7 the pt conditions of peak metamorphism for type ii eclogites are 380 10 11 5 kb type i eclogites follow a counter clockwise pt path from the epidote amphibolite through eclogite to blueschist facies whereas the presumable in situ type ii eclogites follow a clockwise pt path from the blueschist to eclogite facies

high pressure low temperature metamorphism in northern hubei province central china the qinling dabie accretionary fold belt in east central china represents the e w trending suture zone between the sino korean and yangtze cratons a portion of the accretionary complex exposed in northern hubei province contains a high pressure low temperature metamorphic sequence progressively metamorphosed from the blueschist through greenschist to epidote amphibolite eclogite facies the hongan metamorphic belt can be divided into three metamorphic zones based on progressive changes in mineral assemblages zone in the south is characterized by transitional blueschist greenschist facies zone is characterized by greenschist facies zone in the northern most portion of the belt is characterized by eclogite and epidote amphibolite facies sequences changes in amphibole compositions from south to north as well as the appearance of increasingly higher pressure mineral assemblages toward the north document differences in metamorphic p t conditions during formation of this belt preliminary p t estimates for zone i metamorphism are 5 7 kbar 350 450 estimates for zone iii eclogites are 10 22 kbar 500 50 the petrographic chemical and structural characteristics of this metamorphic belt indicate its evolutionin a northward dipping subduction zone and subsequent uplift prior to and during the final collisionbetween the sino korean and yangtze cratons

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

singular equilibria in paragonite blueschists amphibolites and eclogites chemographic constraints from high pressure mafic rocks of the introduction nevado filabride complex betic rif mountain belts se spain the relevance of singular equilibria has been largely and from worldwide high pressure terrains have been used to unravel recognized in chemical systems of interest for the earth phase relationships between amphibole garnet clinopyroxene and sciences after the early experiments of konovalov on albite in the system na2o cao feo mgo al2o3 sio2 h2o azeotropic or congruent transformations ricci 1951 ncfmash at quartz paragonite clinozoisiteand waterexhaustively discussed the fundamentals of singularities saturated conditions extensive analysis of element partitioning two types of singularities were recognized the first between amphibole and clinopyroxene suggests that the fe mg called type m and most relevant is associated with azeopartition coefficient is low at low temperature and high pressure tropic points and the second called type k is related to conditions high temperature blueschists low temperature eclogites the intersection of solvi with compositional loops as and a progressive increase is achieved by an increase in temperature occurs in the alkali feldspar system at low pressures or by a decrease in pressure or by a decrease in the sodium content morse 1970 examples and discussion of both type m of the clinopyroxene mass balance analysis of paragonite eclogites and type k singular points in igneous systems have been and amphibolites suggests that at least two singular reactions are given by morse 1980 e g see chapters 16 and 18 the possible in ncfmash clinopyroxene paragonite quartz widespread occurrence of singularities in igneous systems amphibole albite clinozoisite water garnet absent and is obviously related to the extremely large compositional amphibole clinozoisite garnet clinopyroxene paragonite range of silicate liquids the well known concept of quartz water albite absent albite absent singular equithe thermal barrier or thermal divide implies the librium is relevant for the blueschist eclogite transition whereas occurrence of singular equilibria and their importance garnet absent equilibrium may control the eclogite to amphibolite in the petrogenesis of granites e g modelled in the binary transition a complex arrangement of divariant reactions singular system kalsilite silica lindsley 1966 or basalts e g in reactions and conjugate non degenerate reactions arises in topologies the binary system forsterite silica or in the ternary system at the blueschist eclogite amphibolite transitions schreinemakers forsterite silica diopside or forsterite silica anorthite analysis in p t diagrams and mixed variable t xm xc and presnall et al 1979 has been widely discussed p xm xc diagrams suggest that complex pressure temperature in contrast the significance and implications of singular paths are not mandatory if a recurrent appearance and disappearance point nets at subsolidus conditions have been largely of similar phase assemblages is observed in paragonite bearing mafic undervalued although korzhinskii 1963 analysed azeorocks the p t location of the singular curve net is discussed in tropic transformations in subsolidus open systems only the context of nevado filabride complex mafic rocks recently have applications to metamorphic assemblages

two new geobarometers for garnet amphibolites with applications to southeastern vermont two new geobarometers for the assemblage garnet hornblende plagioclase quartz have been calibrated on the basis of the equilibrium 6 anorthite 3 tremolite 2 grossular 1 pyrope 3 tschermakite 6 quartz and its fe end member equivalent data representing 37 natural samples that equilibrated at conditions of 2 5 to 13 kbar and 500 to 800 c were fitted to the general equation rt in keq a bt p i c by using a weighted least squares method multiple correlation coefficients are high 0 98 and 0 97 lines of constant keqhave extremely shallow slopes in p t space 2 to 8 bars per degree suggesting that pressures may be deduced precisely even where temperatures are only poorly constrained typical analytical errors and temperature imprecisions propagate to very small errors in pressure about 500 bars caution is advised in applying the barometers outside the range of calibrant mineral compositions application of the calibrations to samples from southeastern vermont near the strafford chester and athens domes documents relatively high pressure metamorphism 7 to 10 kbar for these structures

metamorphic evolution and fluid composition of garnet clinopyroxene amphibolites from the tatra mountains western carpathians relics suggesting an early high pressure eclogite facies stage have been identified in the garnet and clinopyroxene bearing amphibolites of the tatra mountains in the variscan basement of the western carpathians slovakia in these rocks primary omphacite cpx i has been wholly converted to symplectites of diopside cpx ii and plagioclase apart from minor inclusions in the garnet cores amphibole and plagioclase are secondary minerals formed in the kelyphitic rims between garnet and clinopyroxene several generations of amphibole pargasite hornblende cummingtonite actinolite are evidence of a transformation down to greenschist facies conditions thermobarometric calculations from mineral inclusions in the garnet cores yield 670 700 c and 10 15 kbar recording the initial path from upper amphibolite to eclogite facies conditions the attainment of the eclogite facies stability field is inferred from the composition of a reconstructed omphacite jd36 implying a minimum peak pressure of 15 16 kbar conditions of around 650 c and 8 10 kbar record the post eclogite breakdown and partial re equilibration in the amphibolite facies region the earliest fluid inclusions contain high density nitrogen dominated water absent fluid younger are polyphase brines and two phase h2o n2 ch4 inclusions with signs of heterogeneous entrapment of coexisting gas rich and water rich immiscible phases pure nitrogen 0 5 mol ch4 is considered to have been the major component during the high pressure metamorphism in contrast the later aqueous inclusions are interpreted to represent retrogression related fluids the brines have originated by leaching and re entrapment of saline melt inclusions observed in tonalitic trondhjemitic layers of the amphibolite enclosing the eclogitic relics a similar mechanism is assumed for the origin of the h2o n2 ch4 inclusions these represent relics of the primary eclogite facies related n2 inclusions re equilibrated and re entrapped during retrogression under increasing water activity a possible source of n2 and cl rich fluids was primary amphibole which decomposed during the prograde metamorphism from the amphibolite to the eclogite facies the metamorphic pt path is generally clockwise reflecting a steep increase in pressure and temperature during burial followed by decompression and cooling the retrograded eclogites occur in boudins within an allochthonous unit of an inverted metamorphic sequence their exhumation was facilitated by tectonic transport along a ductile shear zone during the variscan orogeny

equilibration conditions of eclogite lenses from isla margarita venezuela implications for the tectonic evolution of the metasedimentary juan griego group abstract the constituent primary and secondary minerals in a suite of fifteen samples of eclogite amphibole eclogite and garnet amphibolite scattered as boudins and pods in metapelitic schists and gneisses on isla margarita venezuela have been analysed by electron microprobe to augment existing bulk rock chemical data the large sample population available allows spurious effects of stoichiometric fe3 calculation procedures and bulk rock influence to be recognised and eliminated all samples belong to one population with a relatively homogeneous character the equilibration temperatures are derived from fe 2 mg fractionation between garnet and clinopyroxene and range from 525 650 c jadeite contents of clinopyroxene and the persistence of minor stable albite constrain pressures to between 13 and 19 kbar combined with new evidence for high pressure metamorphism in the enclosing metapelites and with existing data on an eclogitic metabasic unit exposed in north eastern isla margarita la rinconada group these data show convincingly that the margarita crustal block is and has been a relatively coherent unit ever since the entire complex suffered high pressure metamorphism in late mesozoic times

reaction spaces and p t paths from amphibole eclogite to greenschist facies in the austroalpine domain oetztal complex the transition from feldspar amphibolite to eclogite is a very wide p t field that extends from some where close to 5 kbar where the garnet amphibole pair starts to appear to 10 20 kbar at albite out reaction then up to 25 30 kbar where an hydrated phase such as amphibole can be stable with pyroxene and garnet thus the assemblage garnet py amphibole tr epidote cz plagioclase ab clinopyroxene di quartz qz fluid is commonly reported in a large number of metamorphic terrains these mineral phases are complex solid solutions which adapt to variations in environmental conditions mainly by means of continuous reactions the reaction space introduced by thompson in 1982a provides a very elegant and powerful tool to approach these high variance assemblages the reactions

vapour absent melting from 10 to 20 kbar of crustal rocks that contain multiple hydrous phases implications for anatexis in the deep to very deep continental crust and active continental margins

metamorphism and phase relations in carbonate rocks from the nevado filabride complex cordilleras beticas spain application of the ttn rt cal qtz gr buffer phase relations and metamorphic conditions have been studied in metacarbonate rocks from the nevado filabride complex cordilleras beticas through forward modeling in many rock samples the assemblage titanite rutile calcite quartz graphite buffered the composition of the c o h fluid present during metamorphism over a wide range of p t conditions fluid compositions computed for this buffer are essentially binary h2o co2 mixtures this buffer also constrains the chemical potentials of tio2 cao and sio2 consequently it is possible to make a thermodynamic projection through these components to predict the stable phase relations consistent with the buffer using this method phase relations have been analyzed in a rock containing the buffer assemblage and paragonite albite phengite epidote and chlorite the equilibrium p t conditions for this assemblage are constrained by minimization of the differences between predicted and observed mineral compositions to be 560 15 c and 9 5 1 kbar conditions obtained compare well with those estimated from other studies in different lithologic units the inferred metamorphic fluid composition is h2o rich

metamorphic evolution of norwegian country rock eclogites as deduced from mineral inclusions and compositional zoning in garnets abstract detailed studies of pre early syn and post eclogite mineral associations in some west norwegian country rock eclogites have made it possible to reconstruct their metamorphic evolution pre eclogite stages are represented by various amphibolite facies associations found in the central parts of prograde zoned garnets early and syn eclogite stages are represented by various eclogitic assemblages containing low and high jd omphacites respectively the former located in the intermediate zones and the latter in the outermost rims of prograde zoned garnets and in the matrix post eclogite stages include granulite amphibolite and greenschist facies associations representing replacement phases after higher grade assemblages p t estimates indicate a prograde evolution along an extremely steep p t gradient 7 9 kbar over 100 200 c while the initial post eclogite evolution involved a nearly adiabatic decompression on the order of 10 kbar

low grade blueschist facies metamorphism of metagreywackes franciscan complex northern california metagreywackes in the eastern belt of the franciscan complex contain the assemblage qtz ab lws chl ph pmp fgl hem cal arg or compatible subassemblages blue amphibole first appears in the westernmost part of the belt and pumpellyite is absent in the eastern part the compositions of the coexisting minerals and the nature of the continuous reactions in these low grade blueschists suggest that the distribution of blue amphibole and pumpellyite in the eastern belt of the franciscan complex reflects differences of effective bulk composition rather than differences in physical conditions of metamorphism in rocks lacking pumpellyite white mica may be essential to the growth of blue amphibole but carbonate plays only a limited role the continuous reaction that limits the appearance of blue amphibole and the disappearance of coexisting pumpellyite has the general form pmp chl ab qtz hem h2o femg 1 fgl lws this reaction requires significant hydration as pressure increases in order to produce blue amphibole most of the eastern belt of the franciscan complex formed in limited ranges of temperature and pressure which are estimated to be 240 280 c 6 5 7 5 kbar pressures in the westernmost part of the area were about 1 kbar lower than in the east pressures of about 8 5 10 kbar are estimated for tectonic blocks that contain sodic clinopyroxene

amphibole evolution in variscan eclogite amphibolites from the savona crystalline massif western ligurian alps italy controls on the decompressional p t t path abstract the variscan metamorphic evolution of eclogite amphibolites from the savona crystalline massif is marked by the occurence of calcic amphiboles microtextural relations together with coupled electron microprobe and x ray investigations on amphiboles pointed out a complex decompressional evolution a prograde stage of the evolution in the eclogite facies temperature conditions estimated as ca 620 c is recorded by compositional zoning and inclusions in garnet the eclogitic event pressure conditions 12kbar produced assemblages of garnet omphacite ca amphibole zoisite quartz and rutile composition and zoning of matrix eclogitic amphiboles reveal that the subsequent evolution in the eclogite facies contemplates a late re equilibration under lower conditions of temperature ca 540 c and pressure the eclogitic amphiboles are pargasitic to edenitic hornblendes and are marked by rather high na contents at the m4 site up to 0 5 apfu the forming of coarse grained diopside plagioclase symplectites and of amphibole plagioclase coronas represent the early results of the destabilization of omphacites and garnets in the amphibolite facies pressure conditions of this stage have been estimated of ca 10 kbar while temperature conditions are poorly constrained however the increase in edenite naalsi 1 at the outermost rims of the matrix eclogitic amphiboles and the very high values of edenite and tschermack al 2 mg 1 si 1 components in the coronitic amphiboles subsilicic pargasites suggest that temperature increase accompanied the early decompression to the amphibolite facies subsequent retrogression in the low grade amphibolite facies resulted in the complete breakdown of omphacite producing finer grained symplectites this stage is also recorded by the amphibole growth magnesio hornblendes to edenitic hornblendes with negligible na amounts at the m4 site at the expense of the clinopyroxene symplectites and as outward rims around the coronitic amphiboles frequently amphibolites are completely recrystallized and do not retain any relic of high pressure assemblages temperature conditions for this late amphibolitic event have been estimated of ca 510 c while pressure conditions are inferred in the range 4 7 kbar such a complicated decompressional evolution is indicative of complex uplift tectonics after a subduction event

low temperature eclogites and eclogitic schists in mn rich metabasites in ward creek california mn and fe effects on the transition between blueschist and eclogite

zoned sodic amphibole petrologic indicator of changing pressure and temperature during tectonism in the bathurst area new brunswick canada amphibole zoned from an actinolite core through a barroisite layer to a crossite or ferroglaucophane rim coexists with epidote chlorite albite quartz phengite sphene magnetite and calcite in metabasites of the tetagouche group near bathurst new brunswick stratigraphic structural and regional metamorphic evidence suggests that these rocks were situated in the hangingwall of a se dipping subduction zone during the taconian orogeny using microprobe analyses of the coexisting minerals a dehydration reaction and a subsequent hydration reaction have been balanced in the system sio2 al2o3 tio2 fe2o3 fe mg mn o cao na2o k2o h2o so as to describe the development of the zoned amphiboles relatively large coefficients for magnetite in these reactions accord with the absence of sodic amphibole in associated magnetite free metabasites indicating that ferric iron has a significant effect on the transition from greenschist to epidote blueschist fades relatively small residuals of feo mgo and mno show that these components are only weakly partitioned between the reactant and product assemblages the procedure of holland and richardson 1979 has been used to calculate apparentp t paths that trace the growth histories of the zoned amphiboles from a low p environment to a high p environment although the apparent pressure change is implausibly large and the final temperature implausibly low it is clear that the bathurst metabasites followed a very differentp t path than those of the austrian alps despite their identical mineral assemblage