40ar 39ar and oxygen isotope studies of polymetamorphism from tinos island cycladic blueschist belt greece petrological oxygen isotope and 40ar 39ar studies were used to constrain the tertiary metamorphic evolution of the lower tectonic unit of the cyclades on tinos polyphase high pressure metamorphism reached pressures in excess of 15 kbar based on measurements of the si content in potassic white mica temperatures of 450 500 c at the thermal peak of high pressure metamorphism were estimated from critical metamorphic assemblages the validity of which is confirmed by a quartz magnetite oxygen isotope temperature of 470 c some 40ar 39ar spectra of white mica give plateau ages of 44 40 ma that are considered to represent dynamic recrystallization under peak or slightly post peak high pressure metamorphic conditions early stages in the prograde high pressure evolution may be documented by older apparent ages in the high temperature steps of some spectra eclogite to epidote blueschist facies mineralogies were partially or totally replaced by retrograde greenschist facies assemblages during exhumation oxygen isotope thermometry of four quartz magnetite pairs from greenschist samples gives temperatures of 440 470 c which cannot be distinguished from those deduced for the high pressure event the exhumation and overprint is documented by decreasing ages of 32 28 ma in some greenschists and late stage blueschist rocks and ages of 30 20 ma in the lower temperature steps of the ar release patterns of blueschist micas almost flat parts of ar ar release spectra of some greenschist micas gave ages of 23 21 ma which are assumed to represent incomplete resetting caused by a renewed prograde phase of greenschist metamorphism oxygen isotope compositions of blueschist and greenschist facies minerals show no evidence for the infiltration of a 18o enriched fluid rather the compositions indicate that fluid to rock ratios were very low the isotopic compositions being primarily controlled by those of the protolith rocks we assume that the fundamental control catalysing the transformation of blueschists into greenschists and the associated resetting of their isotopic systems was the selective infiltration of metamorphic fluid a quartz magnetite sample from a contact metamorphic skarn taken near the miocene monzogranite of tinos gave an oxygen isotope temperature of 555 c and calculated water composition of 9 1 the value of 18o obtained from this water is consistent with a primary magmatic fluid but is lower than that of fluids associated with the greenschist overprint which indicates that the latter event cannot be directly related to the monozogranite intrusion

syndeformational fluid trapping in quartz determining the pressure temperature conditions of deformation from fluid inclusions and the formation of pure co2fluid inclusions during grain boundary migration observations and microthermometric data on fluid inclusions from a terrane that underwent deformation following peak metamorphic conditions show that grain boundary migration recrystallization favours the entrapment of carbonic inclusions whereas microfracturing during brittle deformation favours the infiltration and eventual entrapment of aqueous fluids our results imply that pure co2 fluid inclusions in metamorphic rocks are likely to be the residue of deformation recrystallization process rather than representing a primary metamorphic fluid where the temperature of deformation can be deduced by other means the densities of fluid inclusions trapped during recrystallization which we call recrystallization primary fluid inclusions can be used to constrain the ambient pressure during deformation using these constraints the data imply that the post metamorphic hercynian exhumation in sardinia brought rocks at 300 c to within 3km of the surface this conclusion is similar to that described for the rapidly uplifted southern alps in new zealand

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

fluid inclusions as tectonothermobarometers relation between pressure temperature history and reequilibration morphology during crustal thickening results of experimental reequilibration studies of natural primary aqueous inclusions in quartz under conditions of hydrostatic compressive loading at 1 2 3 4 and 5 kbar and 500 c indicate that the intensity of reequilibration features is inversely proportional to inclusion volume the smallest inclusions show the first evidence of reequilibration dissolution of inclusion walls after 1 8 kbar of effective pressure for effective pressures less than 3 8 kbar decrepitation of inclusions is probably controlled by the corrosive action of water on the inclusion walls at higher effective pressures decrepitation is related to high compressive stress and is less dependent on fluid interaction with the inclusion walls the observed correlation between inclusion size intensity of reequilibration features and amount of loading combined with paragenetic and microthermometric data for the inclusions may be used to determine the pressure temperature history of the inclusions and their host rocks

direct observation of primary fluid inclusion formation observations of crystal growth in supersaturated kh 2 po 4 solutions indicate that it duplicates natural crystallization processes and formation of fluid inclusions in free liquids similar to vein formation or igneous systems continuous crystal growth by surface nucleation results in elongate fluid inclusions forndng as hollow tubes within kinked growth steps large flat fluid inclusions also form either by sealing of etch pits and other surface irregularities or along cracks propagated by crystal growth microscopic observation of real time crystal growth documents the formation of fluid inclusions along a surface or within a plane inclusions that would otherwise be identified as secondary these primary fluid inclusions resemble pseudosecondary inclusions or secondary necked down inclusions commonly observed in geologic samples furthermore these fluid inclusions and many or most in natural samples are not generated by spiral growth processes this simple analogue experiment serves as a model for crystal growth and fluid inclusion entrapment in hydrothermal and igneous systems

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

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

synthetic fluid inclusions vii re equilibration of fluid inclusions in quartz during laboratory simulated metamorphic burial and uplift p t conditions inferred from fluid inclusions in metamorphic rocks often disagree with the values predicted from mineral equilibria calculations these observations suggest that inclusions formed during early stages of regional metamorphism continue to re equilibrate during burial and subsequent uplift in response to differential pressure p t conditions accompanying burial and uplift were experimentally simulated by initially forming pure h2o inclusions in quartz at elevated temperatures and pressures and then re equilibrating the inclusions in the presence of a 20 wt nacl solution such that final confining pressures ranged from 5 kbar above to 4 kbar below the initial internal pressure of the inclusions at the temperature of re equilibration in all samples re equilibrated at confining pressures below the internal pressure some inclusions were formed that had compositions of 20 wt nacl and densities in accord with the final p t conditions additionally some inclusions were observed to contain fluids of intermediate salinities between 0 and 20 wt nacl densities of these inclusions were also consistent with formation at the re equilibration p t conditions the remainder of the fluid inclusions observed in these samples contained pure h2o and their homogenization temperatures corresponded to densities intermediate between the initial and final p t conditions in short term experiments 7 days where the initial internal overpressure exceeded 1 kbar no inclusions were found that contained the original density and none were found to have totally re equilibrated instead most h2o inclusions re equilibrated until their internal pressures were between 750 and 1500 bars above the confining pressure regardless of the initial pressure differential in a long term experiment 52 days inclusions re equilibrated at a lower confining pressure than the initial internal pressure displayed homogenization temperatures corresponding to a range in final internal pressures between 0 kbar i e total re equilibration and 1 2 kbar above the confining pressure in experiments where the confining pressure during re equilibration exceeded the initial internal pressure densities of pure h2o inclusions increased to values intermediate between the initial and final p t conditions additionally these inclusions were generally surrounded by a three dimensional halo of smaller inclusions also of intermediate density resulting in a texture similar to that previously ascribed to decrepitation from internal overpressure in extreme cases where confining pressures were 4 5 kbar above the initial pressure the parent inclusion almost completely closed leaving only the three dimensional array of small 5 m inclusions the outline of which may be several times the volume of the original inclusion groups of such inclusions closely resemble textures commonly observed in medium to high grade metamorphic rocks inclusions containing 10 and 42 wt nacl solutions trapped at 600 c and 3 kbar were re equilibrated at 600 c and 1 kbar for 5 days in dry argon to evaluate the importance of h2o diffusion as a mechanism of lowering the inclusion bulk density salinities of re equilibrated inclusions obtained from freezing point depressions and halite dissolution temperatures indicate that original compositions were preserved density changes similar to those previously described were noted in these experiments in inclusions showing no visible microfractures therefore density variations observed in inclusions in this study re equilibrated under rapid deformation conditions are considered to result from a change in the inclusion volume without significant loss of contents by diffusion or leakage

late cretaceous exhumation of the oman blueschists and eclogites a two stage extensional mechanism the continental material of the saih hatat window has been affected by a late cretaceous obduction related hp lt metamorphism below the oman ophiolite a high rate exhumation process is witnessed by the maastrichtian palaeocene onlap onto the blueschist facies rocks drastic metamorphic omissions are documented between the lowest eclogitic units and the overlying blueschist facies ones widespread late metamorphic shear structures point to a top to the nne detachment in opposition to the sense of the late cretaceous obduction the inversion of the shearing sense occurred under similar low temperature conditions in both the blueschist and eclogite facies units admitting that these hp lt metamorphic rocks formed progressively at various depth in the subducting arabian margin a two stage extensional mechanism of exhumation is suggested i early uplift of the eclogitic rocks up to the blueschists depth by ductile thinning or squeezing of a blind extensional allochthon ii exhumation of the whole hp lt metamorphic core complex by inversion of the obduction sole thrust and isostatic rebound of the lower plate

grain boundary migration of water and carbon dioxide during uplift of garnet zone alpine schist new zealand deformed quartz veins in garnet zone schist adjacent to the active alpine fault new zealand have fluid inclusions trapped along quartz grain boundaries textures suggest that the inclusions formed in their present shapes during annealing of the deformed veins many of the inclusions are empty but some contain carbon dioxide with densities that range from 0 16 to 0 80 g cm 3 no water nitrogen or methane was detected the inclusions are considerably more co2 rich than either the primary metamorphic fluid 5 co2 or fluids trapped in fracture related situations in the same or related rocks 50 co2 enrichment of co2 is inferred to have resulted from selective migration wicking of saline water from the inclusions along water wet grain boundaries after cooling induced immiscibility of a water co2 mixture inclusion volumes changed after loss of water non wetting co2 remained trapped in the inclusions until further percolation progressively removed co2 in solution this mechanism of fluid migration dominated in ductile quartz rich rocks near but below the brittle ductile transition at deeper levels hydraulic fracturing is also an important mechanism for fluid migration whereas at shallower levels advection through open fractures dominates the fluid flow regime

radial cracks around α quartz inclusions in almandine constraints on the metamorphic history of the oman mountains radiating tensional cracks around quartz inclusions in almandine have been observed in metapelite samples from the southeastern saih hatat tectonic window northeastern oman mountains these almandines show an inclusion rich glaucophane epidote and strongly deformed core with inclusions of different mineral phases the rim of the same almandines is inclusion poor and shows only quartz apatite zircon rutile and baal phosphates as inclusions quartz and apatite inclusions in the rim are single crystals often surrounded by radial cracks these radial cracks developed during uplift by the dilation of quartz 4 5 vol without a phase transformation subsequently these cracks were filled with kaolinite phengite si content 3 4 per formula unit p f u chlorite and fe oxides we calculated the appearance of radial cracks without phase transformation using the mathematical procedure of van der molen and van roermund 1 this calculation involves terms for thermal expansion isothermal compressibility and shear modulus for the example of quartz and almandine for the same p and t interval during a retrograde path published geothermobarometric estimates give pressures of between 1 0 and 2 0 gpa and temperatures of between 450 and 600 c for the peak conditions for these rocks of the saih hatat tectonic window on the basis of these p t data we calculated different retrograde p t paths in the quartz domain initiation of garnet fracturing is dependent on the p t starting conditions and the component of isothermal compression of the retrograde path the calculations yield a set of smooth monotonic curves whose exact position on the p t plane between 0 1 and 0 6 gpa and 40 and 500 c depends on the initial p t conditions and the component of isothermal compressibility of the retrograde p t paths this model can be used in general terms to estimate pressure and temperature for the following cases 1 if independent evidence such as petrological data allow the determination of the final pressure at which radial cracks appeared the initial inclusion pressure can be recalculated 2 if the initial inclusion pressure is known e g from petrological data the conditions of radial cracking can be calculated and the pair initial pressure final pressure leads to an estimate of the shape of the retrograde p t path as a function of its component of isothermal decompression in the example from the northeastern saih hatat tectonic window the late syntectonic growth of albite phengite kaolinite suggests that the final pressure for fracturing ranged between 0 4 gpa and 0 5 gpa at temperatures of 300 c these values correspond to high initial pressures of at least 2 0 gpa at a temperature of 550 c the following geodynamic model is suggested a regionally extended metamorphism led to the growth of inclusion rich garnets in the rocks from the northeastern saih hatat tectonic window at depths of about 30 km 0 1 gpa about 450 c continuing prograde metamorphism at a depth of more than 60 km with p 2 0 gpa and t 550 c affected a metapelite unit that is only exposed immediately south of as sifah village in this area clear rims of almandine grew around the older garnets and entrapped mainly quartz and apatite during uplift along a retrograde p t path with a large component of isothermal decompression radial cracks around quartz inclusions developed in the rims of almandines at a depth of about 12 km 0 4 0 5 gpa 300 c

high pressure phengite decomposition in the weissenstein eclogite munchberger gneiss massif germany the petrography mineral chemistry and petrogenesis of a sample from the weissenstein eclogite bavaria germany has been investigated the total mineral assemblage comprises garnet clinopyroxenei ii quartz amphibolei ii rutile phengite epidote allanite plagioclase biotite apatite pumpellyite titanite sphene zircon alkali feldspar and calcite textural observations combined with geothermobarometry fe mg distribution between clinopyroxene garnet and phengite garnet jadeite content of omphacite si content of phengite and an content of plagioclase provide indications of two different stages in the metamorphic evolution of the rock the main phengitequartz eclogite mineral equilibration occurred at minimum p 13 17kbar minimum t 620 50 c the retrograde symplectite stage clinopyroxeneii amphiboleii biotite plagioclase occurred at ptotal between 12 and 8 5 kbar

synchronous seafloor spreading and subduction at the paleo convergent margin of semail and arabia the semail ophiolite is the largest and best exposed ophiolite having preserved intact record of a precollision system related to neotethyan ocean closure breaking with the alternative models of ophiolite exhumation and emplacement this paper builds on integrating the joined contribution of tectonic evolution of the continental margin in the premise of the ophiolite obduction connecting the along strike segmentation of the ophiolite recorded in the lithospheric flow structure and recent high precision dating results in a 3 d model of evolution of the ophiolite and its continental margin during jurassic cretaceous time among the relevant characteristics newly accounted 1 the contamination of the ophiolite by fluids and magmas of continental origin 2 the discrimination of different types of metamorphics associated to the ophiolite and conventionally classified metamorphic soles 3 the instability of the arabian margin during the late jurassic cretaceous times 4 the new precise ages relating to overlapping of spreading and detachment and 5 the inferred vergence of neotethyan closure during late mesozoic are stressed the complex stem revealed may relate to processes associated with spreading center subduction at ridge trench transform rtt triple junction as documented along a convergent margin the semail ophiolite emplacement on the arabia continental margin would result from a temporary arabian platform directed subduction zone that consumed the southern tethyan plate during late jurassic cretaceous time

reply to comments by david m schultz on cloud resolving model simulations of multiply banded frontal clouds october a 2005 131 2617 2637

metamorphic p t paths from pelitic schists and greenstones from the south west tauern window eastern alps petrological data from intercalated pelitic schists and greenstones are used to construct a pressure temperature path followed by the upper schieferhulle ush series during progressive metamorphism and uplift in the south west tauern window italy pseudomorphs of al epidote fe epidote albite oligoclase chlorite after lawsonite and data on amphibole crystal chemistry indicate early metamorphism in the lawsonite albite chlorite subfacies of the blueschist facies at p 7 8 kbar geothermometry and geobarometry yield conditions of final equilibration of the matrix assemblage of 475 25 c 5 6 kbar calculations with plagioclase and phengite inclusions in garnet indicate early garnet growth at pressures of 7 5 kbar garnet zoning patterns are complex and reversals in zoning can be correlated between samples thermodynamic modelling of these zoning profiles implies garnet growth in response to four distinct phases of tectonic activity fluid inclusion data from coexisting immiscible h2o co2 nacl fluids constrain the uplift path to have passed through temperatures of 380 30 c at 1 3 0 2 kbar there is no evidence for metamorphism of ush at pressures greater than 7 5 kbar in this area of the tauern window this is in contrast to pressures of 10 kbar recorded in the lower schieferhulle only 2 3 km across strike a history of differential uplift and thinning of the intervening section during metamorphism is necessary to reconcile the p t data obtained from these adjacent tectonic units

cordierite gneisses of southern kerala india petrology fluid inclusions and implications for crustal uplift history the cordierite bearing gneisses occurring as elongate patches in an 8 to 10 km wide zone along the achankovil fault lineament at the northern margin of the southern kerala crustal segment represent an important lithological unit in the archaean granulite terrane of south india the textural relationships in these rocks are consistent with the following main reactions 1 garnet quartz cordierite hypersthene 2 garnet sillimanite quartz cordierite 3 hypersthene sillimanite quartz cordierite 4 sillimanite spinel cordierite corundum and 5 biotite quartz sillimanite cordierite k feldspar many of the mineral associations and reaction textures including the remarkable preservation of symplectites are indicative of partial replacement of high pressure assemblages by cordierite bearing lower pressure ones during an event of rapid decompression temperature estimates from coexisting mineral phases show 710 garnet biotite 791 garnet cordierite and 788 c garnet orthopyroxene pressure estimates from mineral assemblages range from 5 4 to 7 kb detailed fluid inclusion studies in quartz associated with cordierite show high density co2 0 80 0 95 g cm3 as the dominant fluid phase with traces of probable ch4 in the sillimanite bearing rocks the isochore for the higher density fluid inclusions defines a pressure of 5 5 kb the fracture bound co2 and co2 h2o ch4 inclusions indicate simultaneous entrapment at 400 c and 1 7 kb in the cordierite hypersthene assemblage and 340 c and 1 2 kb in the cordierite sillimanite assemblage the p t path delineated from combined solid and fluid data corresponds to the piezothermic array of the gneisses and is characterized by t convex nature indicative of rapid and virtually isothermal crustal uplift probably aided by extensional tectonics

interlayered eclogites blueschists and epidote amphibolites from ne oman a record of protolith compositional control and limited fluid infiltration abbreviations ab albite ac acmite act actinolite alm aleclogites blueschists clinopyroxene rich rocks and epidote ammandine aln allanite ap apatite aug augite bar barroisite bt biotite phibolites occur interlayered on a millimetre to decimetre scale in a cc calcite chl chlorite cpx clinopyroxene crs crossite czo clinozoisite mafic unit interbedded with calcareous and quartz mica schists in dol dolomite ep epidote fe ox fe oxides gln glaucophane gross ne oman these rocks constitute part of the continental margin grossularite gt garnet hm haematite jd jadeite kln kaolinite kspar of oman subducted during the cretaceous petrological data indicate k feldspar mgt magnetite oxychl oxychlorite pg paragonite ph that they evolved through a clockwise p t path and attained peak phengite qz quartz pyrp pyrope rt rutile spess spessartine tc talc conditions at t 560 c p 12 kbar where the mineral ttn titanite win winchite zrn zircon assemblages of the blueschists eclogites and clinopyroxene rich rocks were all stable whole rock major trace and rare earth element data indicate that the mineralogical differences between these rock types resulted from the metamorphism of interlayered basaltic flows introduction and tuffs of variable compositions possibly reflecting different degrees interlayered blueschists eclogites and or greenschists are of magmatic differentiation the interlayered epidote amphibolites common in several areas such as cazadero california are chemically similar to the blueschists and appear to have formed e g oh et al 1991 the shuksan suite washington from the eclogites and blueschists during unroofing aided by fluid state dungan et al 1983 owen 1989 the tauern infiltration oxygen isotope data for minerals and whole rocks window eastern alps e g selverstone et al 1992 and indicate low fluid rock ratios during metamorphism and unroofing several cycladic islands e g okrusch brocker 1990 the data also suggest that different layers may have interacted with this interlayering has been attributed to 1 differences fluids of different isotopic signatures throughout their history these in the p t conditions of equilibration in the different results are similar to data reported for other areas of continent layers e g reinsch 1979 ridley 1984 2 original continent collision or continental crust subduction differences in the chemical compositions of the layers before metamorphism e g dungan et al 1983 oh et al 1991 or 3 differences in the extent of retrograde overprinting as a result of variable fluid infiltration e g schliestedt matthews 1987 brocker 1990 barrientos selverstone 1993

magmatism as an essential driving force for formation of active metamorphic core complexes in eastern papua new guinea the d entrecasteaux islands in eastern papua new guinea are composed of a number of active metamorphic core complexes which have been intruded by granodiorite plutons during their formation the plutons do not appear to have been intruded by diapiric processes as previously suggested late relatively undeformed plutons form flat lying bodies which crosscut structural boundaries and are strongly discordant to core complex shear zones granodiorite magmatism and the development of the metamorphic core complexes have occurred in a linear zone which coincides with a zone of thick crust and rugged topography it is proposed that plutonism facilitated deformation in ductile extensional shear zones which resulted in tectonic exhumation of deep crustal rocks and formation of the metamorphic core complexes the source of the plutons is thought to be related to a linear zone of mantle upwelling beneath the islands related to the propagation of the woodlark seafloor spreading center into continental crust it is suggested that a localized heat source of this type which can provide heat and magmatic material to the crust is essential for the development of metamorphic core complexes

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

trace element rich brines in eclogitic veins implications for fluid composition and transport during subduction primary and pseudosecondary fluid inclusions occur in oscillatory and sector zoned omphacite in eclogitic veins from the monviso ophiolitic complex in the western alps the inclusions contain aqueous brines and daughter crystals of halite sylvite calcite dolomite albite anhydrite and or gypsum barite baddeleyite rutile sphene fe oxides pyrite and monazite this daughter mineral suite indicates high solubilites of na k ca mg fe zr ti p ba ce la th and s species and provides direct evidence for transport of high fieldstrenght large ion lithophile and light rare earth elements as dissolved species during subduction fluid inclusion heterogeneities preserved within and between adjacent grains in the veins however suggest that the scale of fluid equilibration was small a crack seal geometry in some of the veins implies that fluid release in pulses rather than steady flow controlled mineral deposition and growth in the veins from these observations we develop a model of fluid release and entrapment in which pulses of fluid are associated in time with increments of shear and tensile failure the rate of fluid release and the reduction in porosity both depend on the rate of plastic flow vein fluids may initially be derived from decreptitation of early fluid inclusions in the host eclogites small scale fluid heterogeneities implied by the fluid inclusions in the veins are best interpreted in terms of limited fluid flow and hence limited metasomatism we conclude that element recycling into the mantle wedge during subduction will depend at least as strongly on fluid transport mechanisms as on element solubilities in the fluid phase at monviso despite evidence for high trace element solubilities in saline brines the elements were not removed from the downgoing slab prior to teaching depths of 40 km

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

petrologic and age constraints on the origin of a low pressure high temperature metamorphic complex southern alaska the interrelationships between metamorphism deformation magma intrusion and 40ar 39ar geochronology were determined for a low pressure high temperature metamorphic complex which formed from an accretionary prism in the chugach mountains southern alaska compressional deformation which first produced south verging folds and associated thrusts was followed by magma intrusion and development of north verging folds synmetamorphic southward directed thrusting of metamorphosed flysch over flysch produced increased load in the footwall as documented by the distribution of mineral assemblages and by pressure temperature modeling of garnet growth the initial heating to the greenschist facies may have been accomplished by a combination of advective heating from aqueous fluids and of conductive heating from subducted young oceanic crust regionally developed amphibolite facies metamorphism followed intrusion of felsic sills the peak metamorphic conditions derived from geothermobarometry mineral assemblages and fluid inclusions ranged from 400 to 600 c at a depth of 10 km the increased heat from associated synmetamorphic concordant felsic sills raised the ambient temperatures to produce a regional distribution of andalusite and cordierite with a core zone of sillimanite bearing migmatites subsequent cooling was initially rapid 55 c ma to 350 c based on 40ar 39ar dates of 53 ma for hornblende and 50 ma for biotite and may have slowed to 11 c ma until 200 c based on an 40ar 39ar date of 35 ma for plagioclase intrusions of felsic sills and at least one pluton were along the initially north dipping foliations associated with south verging folds this suggests the source region for the melts may have been downdip in the subduction zone rather than from directly below within the accretionary prism

postmetamorphic unroofing history deduced from petrology fluid inclusions thermochronometry and thermal modeling an example from southwestern new england nonlinear unroofing rates from 10 to 2 and h 2 o rich fluid inclusions require nearly isothermal decompression followed by more isobaric cooling from these conditions comparison of this observed path to those generated by thermal models suggests that decompression was characterized by initial rapid upward en bloc velocity 1 cm yr of brief duration followed by much slower unroofing rates 0 3 mm yr published geochronologic ages in this region constrain the rapid uplift to the middle devonian and uplift slowed dramatically by the end of the devonian this differential uplift has major implications for interpretation of devonian igneous activity and sedimentation in southwestern new england

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

metamorphic petrology of a high pressure low temperature subduction complex in west central baja california mexico the western baja terrane wbt of west central baja california is an uplifted subduction complex that is divided into smaller subterranes on the basis of bounding faults and petrological differences each subterrane contains coherent early jurassic to early cretaceous sedimentary and mafic volcanic rocks not melange that have been metamorphosed under blueschist facies conditions key phases in metabasites and metaturbidites include jadeitic to acmitic clinopyroxene sodic amphibole lawsonite aragonite chlorite titanite and white mica pressure indicators include the jadeite content of clinopyroxene and the presence of aragonite temperature indicators include the presence of lawsonite the absence of greenschist facies minerals and results from vitrinite reflectance studies conditions at the peak of metamorphism were 8 kbar 225 325 c for subterrane 1 7 8 kbar 170 220 c for subterrane 2 and 5 6 kbar 175 200 c for subterrane 3 these correspond to cold geothermal gradients 6 9 km vein assemblages that include aegerine jadeite and aegerine albite aragonite lawsonite and sodic amphibole indicate uplift during continued cold conditions probably during steady state subduction

post entrapment modification of fluid inclusions due to overpressure evidence from natural samples effects of post entrapment fluid inclusion modification are examined with reference to retrogression related quartz veins from the caledonian ose thrust northern norway the inclusions occur in secondary trails and contain high density hypersaline aqueous fluids on morphological characteristics they are subdivided into type a elongate ellipsoidal and or irregular inclusions and type b more equant regular and or negative crystal form with reference to previous research on post entrapment modification of inclusions in quartz it is proposed that type a inclusions experienced little or no post entrapment modification whereas type b inclusions show features characteristic of post entrapment permanent inelastic stretching and or leakage this produces increased homogenization temperatures th associated with increased inclusion volume and lowering of density whilst maintaining constant salinity the similarity of data for degree of fill and salinity between type a and type b inclusions indicates that type b inclusions have primarily modified by stretch rather than leakage however the spread towards slightly larger volume of vapour in type b inclusions suggests that some leakage has also occurred because stretched and or partially leaked inclusions have increased th isochore projections significantly underestimate trapping pressure pt relative to unmodified inclusions therefore recognition of post entrapment inclusion modification due to overpressure is crucial to avoid misinterpretation of data but has considerable potential for constraining the detail of p t trajectories of individual rocks on this basis rocks from the ose thrust zone north norway are shown to have experienced rapid uplift on a clockwise p t t path during the final stages of caledonian scandian orogenesis

compositional re equilibration of fluid inclusions in quartz compositional modifications to salt water fluid inclusions in quartz were observed following exposure to disequilibrium conditions in a series of laboratory experiments in which samples containing inclusions of known composition were annealed at 3 kbar and 600 t 825 c in the presence of fluids having different compositions for time intervals ranging from a few days to one month changes in inclusion compositions following re equilibration were monitored using salt crystal dissolution temperatures and or ir infra red spectroscopy the behaviors of both synthetic and natural fluid inclusions were studied the synthetic samples were re equilibrated under pint pconf conditions to minimize stress in the crystal surrounding the inclusions and were subjected to both fh2ointfh2oconf and fh2ointfh2oconf after re equilibration for four days at t 600 c most inclusions displayed significant compositional changes without decrepitation salinity variations as large as 25 wt were inferred for brine inclusions exposed to fh2oint fh2oconf for 16 days at 825 c the majority of our observations are consistent with the net diffusion of water toward the reservior having the lowest h2o i e synthetic nacl h2o fluid inclusions exhibited increased tm nacl s implying lower relative h2o contents when re equilibrated in the presence of fluids having lower h2o whereas similar and in some cases the same inclusions exhibited decreased tm nacl s implying higher h2o contents after exposure to fluids having higher h2o the behavior of natural salt water fluid inclusions during re equilibration was generally consistent with corresponding observations on synthetic samples verifying that compositional changes are not restricted to synthetic inclusions our results clearly show that there was chemical communication between fluids trapped as inclusions in quartz and the external fluid reservoir additionally it is evident that although applied stress can significantly enhance the re equilibration rate strain in the crystal host around the inclusions resulting from large pressure differentials between the internal and confining fluids is not a necessary prerequisite for compositional change finally because significant compositional changes can be induced in brine inclusions in quartz during shortterm exposure to non equilibrium conditions at 600 t 825 c in the laboratory it is likely that similar changes may result at much lower temperatures during exposure of natural rocks to non equilibrium conditions over geologic time

p t evolution and fluid inclusion characteristics of retrograded eclogites munchberg gneiss complex germany kyanite eclogites occur as part of the munchberger nappe pile in ne bavaria west germany eclogites are overprinted by subsequent amphibolite facies metamorphism the preservation of primary eclogitic textures as well as symplectitic textures are indicative of rapid decompression eclogite formation is estimated to have occurred under conditions of high h2o activities at pressures between 20 and 26 kbar and temperatures ranging between 590 and 660 c as is shown by the coexistence of omphacite jd 50 kyanite zoisite and quartz minimum pressure estimates independent of the water activity range between 9 and 16 kbar at the relevant temperatures detailed studies of fluid inclusion reveal two predominant groups of aqueous brine inclusions high salinity 14 17 wt nacl equiv and low salinity 0 8 wt nacl equiv inclusions fluid compositions of both groups of inclusions yield isochores passing close to the estimated amphibolite facies pt field the compositions of these fluids are in good agreement with fluid compositions considered from mineral equilibria none of the fluid inclusions has densities appropriate for eclogite facies metamorphism but probably reflect later amphibolite facies metamorphism

the mechanism of veining and retrograde alteration of alpine eclogites the introduction of externally derived fluids into rocks of the zermatt saas zone of the swiss alps gave rise to the simultaneous formation of shear and hydraulic fractures these fractures are now filled with albite rich assemblages and surrounded by alteration halos up to c 2 m wide the alteration assemblages are zoned and an examination of reactions in p t ah2o space implies that the parageneses developed by the hydration of fluid absent eclogites a mechanical analysis of the veins after sibson 1981 shows that pfluid pload must have been at least 0 96 fluid migration into the country rocks must have been driven by excess hydraulic head either derived from the vertical extent of the veins or due to their connection to a deeper external reservoir possibly tapped along thrust surface s diffusive and capillary transport were insignificant the fluids may have been derived from underlying metasediments that were dehydrating during the quasi isothermal uplift of this part of the alps or they may have originated during the prograde mesoalpine metamorphism documented in the area

single grain argon laser probe dating of phengites from the blueschist to greenschist transition on sifnos cyclades greece measurements of 40ar 39ar age spectra on single white mica crystals of tertiary age is a new means of obtaining detailed geochronological information of alpine blueschists internal consistency of the data set as well as excellent agreement with previous conventional k ar results demonstrates that this new technique can be used with confidence to obtain information on the thermal evolution of young metamorphic belts

metamorphic amphiboles composition and coexistence amphiboles have long been key players in metamorphic petrology the concept of facies was developed for rocks of basaltic composition eskola 1920 1939 fyfe et al 1958 and within this concept amphiboles had a leading role the first appearance of amphiboles the presence of amphiboles of certain compositions or last appearance of amphiboles that are replaced by pyroxene bearing assemblages mark a number of major facies boundaries the transition from greenschist to amphibolite facies is marked in part by hornblende replacing actinolite and the granulite facies is defined by the breakdown of hornblende to orthopyroxene bearing assemblages the blueschist facies is defined by the appearance of glaucophane while the transition from blueschist facies to eclogite facies conditions is indicated by the breakdown of glaucophane to omphacitic pyroxene bearing assemblages references to amphibole names actinolite glaucophane or rocks characterized by certain amphiboles amphibolite hornblende figure prominently in six facies names turner 1981 liou et al 1985 oh et al 1988 evans 1990 peacock 1993 a large part of the literature that is devoted to metamorphic petrology of amphiboles concerns reactions which produce amphiboles from more hydrated mafic igneous rocks or lead to the destruction of amphiboles as they break down to form anhydrous or nearly anhydrous mineral assemblages consequently amphiboles can be thought of as occupying the wide intermediate portion of p t xfluid space in which metamorphic rocks are found the number of citations of the published amphibole nomenclature serves as a yardstick meterstick of the interest in amphiboles and a minimum estimate of the amount of literature devoted to this mineral two versions of the nomenclature of amphiboles have appeared 1978 and 1997 leake 1978 leake et al 1997 and they have been cited a over 2300 times in the past 28 years this works out to a minimum

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

primary fluids in low temperature eclogites evidence from two subduction complexes dominican republic and california usa eclogites occur as isolated blocks in melanges of both the samana peninsula dominican republic and the franciscan complex california usa in some of these eclogites fluid inclusions were found in omphacite and sodic calcic amphibole grains textures show that non planar populations of fluid inclusions formed during growth of clinopyroxene and amphibole in addition planar arrays of secondary fluid inclusions are found along healed cracks homogenization temperatures to liquid were used to calculate isochores for the fluid inclusions these data were compared with petrologic geothermobarometry temperature conditions of 500 700 c were estimated from garnetclinopyroxene geothermometry the jadeite contents of omphacite indicate minimum pressures of 8 11 kbar in this temperature range the p t estimates agree well with calculated isochores for primary fluid inclusions from the samana peninsula and show some overlap for both primary and secondary fluid inclusions from the franciscan complex salinities of 1 2 5 3 wt nacl equiv were estimated for both primary and secondary fluid inclusions from samana and franciscan eclogites these data suggest that low salinity aqueous fluids attended eclogite facies metamorphism and perhaps retrograde metamorphism in both subduction complexes the salinities and densities of fluid inclusions in eclogites from the samana peninsula and the franciscan complex resemble those of counterparts from garnet amphibolites of the catalina schist southern california an external source for such fluids is suggested by their homogeneous populations coupled with their low salinities geologic evidence suggests that the samana and franciscan eclogites may have been derived from a catalina like source terrane the catalina rocks are inferred to have interacted with large volumes of sediment derived fluid during subduction zone metamorphism at similar p but higher t conditions than those determined for samana and franciscan eclogite blocks these results contrast with data for fluid inclusions from eclogites of the monviso area western alps the monviso eclogites yield similar estimates for metamorphic p t to those obtained in this study but contain fluid inclusions of brine and of other saline aqueous fluids all of which are less dense than expected for incorporation at the reported eclogite facies conditions the differences between the properties of fluid inclusions from the ecologites and garnet amphibolites of the samana franciscan catalina subduction complexes and those of monviso probably reflect differences between fluid flow regimes during metamorphism

formation of hp lt rocks and their tectonic implications in the western tianshan orogen nw china geochemical and age constraints the western tianshan high pressure low temperature metamorphic belt in northwestern china formed within a paleozoic accretionary wedge on the south side of the yili central tianshan plate it comprises various lithologies including mafic metavolcanic rocks metavolcaniclastic rocks metagreywacke marble and serpentinite the metavolcanics comprise eclogite omphacitite epidosite and blueschist the trace and rare earth element characteristics of these rocks indicate morb to oib affinities suggesting a seamount like setting for the protoliths of these rocks the end value of the eclogite 8 9 indicates that the protolith was derived from a long term depleted mantle i e an n morb source supporting the trace and rare earth element characteristics of these samples sm nd isochron dating of 343 44 ma omp gln grt wr and an age of 346 3 ma grt gln were determined using an eclogite boudin within blueschist a well defined 40ar 39ar plateau age of 344 1 ma was obtained for crossite from an omphacite phengite bearing blueschist and is concordant with the sm nd isochron age for the eclogite suggesting that neither argon loss nor excess argon affected the crossite a well defined 40ar 39ar plateau age of 331 2 ma was determined for phengite from the same sample the high pressure metamorphic rocks were formed by the b type subduction of a paleozoic south tianshan ocean about 344 ma ago they were however exhumed to higher crustal levels during the collision of the tarim and yili central tianshan plates about 331 ma ago similar to an alpine type tectonic regime

synthetic fluid inclusions vi quantitative evaluation of the decrepitation behaviour of fluid inclusions in quartz at one atmosphere confining pressure the decrepitation behaviour of fluid inclusions in quartz at one atmosphere confining pressure has been evaluated using pure h2o synthetic inclusions formed by healing fractures in natural quartz three different modes of non elastic deformation referred to as stretching leakage or partial decrepitation and total decrepitation have been observed the internal pressure required to initiate non elastic deformation is inversely related to inclusion size according to the equation internal pressure kbar 4 26 d 0 423 where d is the inclusion diameter in microns regularly shaped inclusions require a higher internal pressure to initiate non elastic deformation than do irregularly shaped inclusions of similar size heating inclusions through the quartz inversion results in mechanical instability in the quartz crystal and leads to mass decrepitation of inclusions owing to structural mismatches generated by pressure gradients in the quartz around each inclusion long term heating experiments 2 years suggest that the internal pressure required to initiate non elastic deformation does not decrease significantly with time and indicates that short lived thermal fluctuations in natural systems should not alter the inclusion density and homogenization temperature inclusions that do exhibit decreased density higher homogenization temperature are however always accompanied by a change in shape from irregular to that of a negative crystal observations of this study are consistent with elasticity theory related to fracture generation and propagation around inclusions in minerals these results indicate that an inclusion will not be influenced by a neighbouring inclusion or other defect in the host phase as long as the distance between the two is 2 4 diameters of the larger of the two inclusions

garnets as open systems during regional metamorphism if a garnet fractures during metamorphism its interior will be in chemical communication with matrix phases solids and fluids and may participate in late prograde or retrograde metamorphic reactions the core of a growth zoned garnet is unlikely to be in equilibrium with matrix phases and may react locally in the vicinity of fractures or extensively resulting in dissolution and reprecipitation of the garnet interior and or production of new phases the results of petrologic techniques such as thermobarometry pressure temperature path calculations and geochronometry may be misinterpreted if sites of garnet and mineral inclusion compositional modification are not identified brittle fracturing of garnet during regional metamorphism may be facilitated by the presence of mineral inclusions the conditions at which differential expansion of an inclusion relative to its host garnet are sufficient to cause radial cracking around the inclusion vary as a function of peak pressure retrograde pressure temperature path and thermoelastic properties of the inclusion and garnet inclusion induced fracturing is favored by nearly isothermal decompression a common metamorphic pressure temperature path other fractures may form as a result of propagation of an inclusion host boundary or by mechanisms external to the garnet

redistribution of high pressure fluids during retrograde metamorphism of eclogite facies rocks voltri massif italian western alps abstract high pressure clinopyroxenes in metasomatic mafic rocks rodingites and metagabbros containing titanoclinohumite and eclogites from the voltri massif ligurian western alps retain primary two phase aqueous inclusions h 2 o nacl kcl in the metasomatized rocks the inclusions align with the c axes and the cleavage planes of diopside the eclogites display mylonitic textures primary fluid inclusions are hosted in early porphyroclasts of omphacite omphacite i and are absent in later fine grained omphacite omphacite ii that forms the mylonitic foliation omphacite i has been frequently replaced by retrograde symplectites clinopyroxene plagioclase amphibole in such cases the fluid inclusions are either absent or occur in much lower number the textural observations suggest that formation of the symplectites has been controlled by availability of the fluid inclusions inside omphacite i the symplectites have been in turn overgrown by glaucophanic and winchitic amphiboles indicating that breakdown of the omphacite i to symplectites still occurred at relatively high pressure post entrapment changes have caused a large range of densities in the analyzed inclusions although the textural evidence constrains the entrapment of fluid inclusions to high pressure conditions the highest density isochores in the eclogites indicate pressure temperature conditions 8 kbar and 550 c lower than those attained during the high pressure stage such p t estimates approach those of symplectite formation after omphacite i thus the fluid inclusions preserved in omphacite were formed by low pressure re equilibration of a fluid presumably trapped at eclogite facies conditions the inclusions kinetically triggered the retrograde breakdown of the eclogitic clinopyroxenes the compositional overlap between the analyzed inclusions and those related to the latest greenschist evolutionary stage suggests further redistribution of the high pressure fluids and close system behaviour of these rocks during their overall exhumation path

texturally early fluid inclusions in garnets evidence of the prograde metamorphic path well formed texturally early fluid inclusions in garnets from the archean pikwitonei granulite domain manitoba canada have been analyzed using microthermometric methods the mean co2 homogenization temperature to liquid for inclusions in 12 of 13 samples from the cauchon lake nelson river area is 15 2 c n 125 2 8 2 c corresponding to a co2 density of 0 82 g cm3 inclusions in the remaining sample have somewhat lower co2 homogenization temperatures mean 5 4 c n 24 the studied inclusions contain an estimated 10 to 20 vol percent h2o with minor amounts of other fluid species such as ch4 n2 and or h2s the fluid inclusions were probably trapped during early garnet growth at relatively low pressures 5 kbar if at 750 c and appear to have undergone only limited or possibly no subsequent re equilibration this interpretation is consistent with the anti clock wise p t t path heating before loading determined for the pikwitonei region by other workers for such a prograde path inclusions entrapped early at high temperatures but at relatively low pressures would experience internal underpressures during most of the subsequent prograde and retrograde phases of metamorphism the texturally early fluid inclusions in garnets from the pikwitonei region therefore cannot be used to provide direct information about the highest metamorphic temperature and pressure conditions 750 c and 7 kbar however the results obtained in this study suggest that texturally early fluid inclusions in garnets may in some cases retain evidence of the prograde metamorphic path

fluid inclusion evidence for an anticlockwise metamorphic p t path in central massachusetts acadian late silurian to early devonian metamorphism in the central maine terrane cmt in central massachusetts is characterized by an early low p high t buchan type metamorphism followed by thickening at high temperature 650d c and then by cooling to 100 200d c below peak recorded temperatures before eventual unroofing mineralogical and textural evidence for this path includes sillimanite pseudomorphs after early andalusite abundant cordierite in pelitic lithologies replacement of low p cordierite bearing assemblages by high p garnet bearing assemblages and recrystallization of mylonites associated with late shear zones to form lower t and higher p assemblages peak conditions in the highest grade rocks were 685 780d c and 5 6 kbar the cooling path passed through 550d c at about 6 5 kbar the well constrained p t path documented from geological and mineralogical evidence for the cmt offers an unusual opportunity to examine characteristics of fluid inclusions that have experienced a long lived metamorphic event spanning a broad range of p t conditions fluid inclusion data from the cmt document a range of fluid compositions co2 rich mixed co2 n2 rich n2 rich and h2o rich and densities during metamorphism densities of co2 fluid inclusions range from 0 20 to 1 03 g cm 3 medium density co2 fluid inclusions are contained in quartz inclusions within garnets in partial melt leucosomes and in quartz grains within migmatites fluid inclusions within the quartz inclusions indicate trapping conditions of 650 700d c at pressures below 5 kbar other co2 fluid inclusions from matrix quartz yield isochores which pass through 700d c and 5 2 kbar the highest density inclusions associated with rocks containing the late high p assemblages have isochores which pass below the estimated p t conditions for recrystallization of the mylonite fluid inclusion evidence suggests an early low p heating event followed by thickening at high temperature and then by nearly isobaric cooling to about 500d c with later decompression this interpretation is also consistent with previously published petrological models and supports an anticlockwise p t path for the cmt of south central massachusetts

time calibration of a pt path from the western tauern window eastern alps the problem of closure temperatures new hornblende k ar and 39ar 40ar and mica rb sr and k ar ages are used to place specific timemarks on a well constrained pressure temperature path for the late alpine metamorphism in the western tauern window after identification of excess 40ar the closure behavior of ar in hornblende is compared with that of sr and ar in phengite and biotite samples were collected in three locations whose maximum temperatures were 570 c zemmgrund 550 c pfitscher joch and 500 540 c landshuter hutte

role of extensional tectonics in exhumation of eclogites and blueschists in an oblique subduction setting northeastern venezuela eclogites and blueschists in northeastern venezuela belong to a mid to late cretaceous subduction complex fore arc terrane related to right oblique subduction of the proto caribbean plate beneath the farallon plate along the leeward antilles volcanic island arc these rocks were metamorphosed at high pressures and subsequently underwent retrogradation at decreasing pressures but at constant or slightly increasing temperatures they underwent five phases of deformation three synmetamorphic and two postmetamorphic the major principal extensions x axes of each of the three synmetamorphic deformations are subparallel to the volcanic arc the extension parallel to the arc and the associated exhumation and uplift of the subduction complex fore arc terrane may have resulted primarily from an increase of the arc parallel displacement rate of this terrane caused by an increase of obliquity of plate convergence

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

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

geochemical characterization of organic facies in the dugi otok basin the adriatic sea yugoslavia sediments in the dugi otok basin are predominantly of upper and lower cretaceous age to characterize the organic facies we used geochemical analyses of core samples from different wells and a few outcrop samples geochemical data indicate that the carbonate sequences of the lower cretaceous represent a good source rock section with a high potential for petroleum the source rocks consist both of dark grey uniform micritic limestones and highly bituminous laminated limestones they are associated with oil stains and bitumen impregnations the organic matter is predominantly of marine origin algal sapropelic with a minor input of terrestrial components the extract contents of the samples are anomalously high but contain variable amounts of hydrocarbons and low saturated aromatic hydrocarbon ratios the maturation stage is immature to marginally mature as characterized by a high proportion of resins and asphaltenes with high sulphur content an even predominance of n alkanes a high ratio of acyclic isoprenoids to n alkanes pr ph 1 and a tmax around 430 c the organic facies in the dugi otok basin varies as it is possible to find both organic rich and oxidized organic lean sediments differences in organic facies have been caused by many changes in the depositional environment with tectonic processes contributing to the heterogeneity of the organic facies

fluid inclusions in epidote minerals and fluid development in epidote bearing rocks the widespread occurrence of epidote minerals in metamorphic and igneous rocks as well as in many ore deposit types makes it a promising candidate for fluid inclusion studies apart from high to very high temperature and low to intermediate pressure conditions epidote minerals are stable over a wide range of pressure and temperature in the continental and oceanic crust e g poli and schmidt 1998 yet fluid inclusion studies on epidote minerals are surprisingly scarce even in fluid saturated environments like certain vein type deposits or hydrothermal volcanic vugs and druses for example epidote minerals are not mentioned in the subject index of roedder s 1984 outstanding summary and review of fluid inclusion studies and occurrences which lists more than sixty different host minerals for fluid inclusions nonetheless more recent studies showed fluid inclusions in epidote minerals to be the only direct witness of the physiochemical and compositional fluid evolution during certain geodynamic processes mainly found in fossil geothermal systems ore deposits and high pressure to ultra high pressure rocks the aim of this review is to outline and summarize some aspects and interpretations of geodynamic processes which are based on temperature t pressure p molar volume v and composition x data from fluid inclusions in epidote minerals as well as associated host minerals from various geological environments the review starts with a chapter on some typical mixed volatile solid fluid equilibria involving epidote minerals which are relevant for the here discussed environments this is followed by a short introduction into the basic concepts of fluid inclusion research and the role of epidote minerals the next section covers fluid inclusion studies on epidote minerals from active and fossil geothermal systems as well as low grade metamorphic rocks and constraints on the p t x properties of the fluids present in these systems this is followed by a short introduction

blueschists and eclogites a possible plate tectonic mechanism for their emplacement from the upper mantle the most significant problem of blueschists and eclogite origin is explaining the mechanism by which they were transported from great depths to the surface three aspects of this problem are discussed 1 1 prograde and retrograde trajectories in blueschist and eclogite bearing belts and the preservation conditions for such minerals as aragonite lawsonite and coesite 2 2 the mechanism by which low density crustal rocks with coesite and diamond descend to a depth of 100 150 km and the high rate of return from these depths to the surface 3 3 the origin of a tectonic mixture of rocks from different depths in melange zones using estimations of p t conditions and constructed p t t trajectories four types of blueschist belt are proposed these include two of the ernst types the alpine and franciscan and two additional types the dora maira and sanbagawa types key examples of geologically and mineralogically well studied complexes in the soviet union are discussed the kokchetav massif maksutov complex syumkeu belt borus belt the various models for the origin of different types of high pressure metamorphic belts are considered 1 1 slow uplift or uplifting of the thickened crust after cessation of subduction 2 2 more rapid tectonic pushing out 3 3 the same as in case 2 but with an additional champagne cork effect 4 4 the most complicated process with repeated deformation and metamorphism

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

denudation rates thermal evolution and preservation of blueschist terrains the erosion of low temperature high pressure blueschist facies metamorphic rocks presents a number of problems if their rate of erosion is not sufficiently rapid then the rise in temperature caused by thermal relaxation and radioactive self heating is great enough to obliterate the original low temperature mineral assemblages the problem has been investigated using a one dimensional two layer model to mimic an accretionary prism overlying oceanic lithosphere the results suggest that if no erosion takes place the maximum time that blueschist conditions may exist is about 26 m y if levels of radioactive self heating are high and approximately 100 m y if levels are very low if erosion takes place the maximum temperature reached by an element of rock is at a higher level in the earth s crust than its initial depth of burial rocks may re equilibrate at this temperature or be overprinted by incomplete reactions depending on the temperature rise and rocks involved this could result in the geotherm

a fluid inclusion study of blueschist facies lithologies from the indus suture zone ladakh india implications for the exhumation of the subduction related sapi shergol ophiolitic melange abstract the best occurrence of blueschist facies lithologies in himalaya is that of the shergol ophiolitic melange along the indus suture zone in ladakh region of north western india these lithologies are characterized by well preserved lawsonite glaucophane garnet quartz assemblages this paper presents for the first time the results of a detailed fluid inclusion study on these lithologies in order to understand the fluid p t evolution and its tectonic implications the blueschist rocks from shergol ophiolitic melange record metamorphic peak conditions at 19 kbar 470 c several types of fluid inclusions are trapped in quartz and garnet most of them being two phase at room temperature three types of fluid inclusions have been recognised basing on microtextures and fluid composition type i are primary two phase carbonic aqueous fluid inclusions v co2 l h2o type ii are two phase l h2o v h2o aqueous fluid inclusions either primary type iia or secondary type iib type iii are re equilibrated fluid inclusions in the type i primary carbonic aqueous inclusions h 2 o is strongly predominant with respect to co 2 the homogenization temperature of co 2 range from 7 to 2 c the clathrate melting temperature in such inclusions varies in between 7 1 and 8 6 c type ii two phase aqueous fluid inclusions show a wide range of salinity from 7 8 14 wt nacl eq type iia to 1 65 6 37 wt nacl eq type iib with accuracy 0 4 wt nacl eq type i and type iia primary fluid inclusions are hosted in peak minerals garnet and quartz included in garnet therefore they were likely entrapped at or near to peak p t conditions the dominantly aqueous fluid of both type i and type iia inclusions was most likely produced through metamorphic devolatilization reactions occurring in the subducting slab despite their primary nature the isochores of type i and type iia inclusions do not intersect the peak metamorphic conditions of the blueschist mineral assemblage suggesting that these inclusions stretched or re equilibrated during nearly isothermal decompression from 19 kbar to 3 kbar or less at t 290 c this conclusion is further supported by their large variability in shapes and sizes which range from irregular inclusions c arc shaped hook shape and satellite type this decompression stage was followed by nearly isobaric cooling testified by the occurrence of dendritic networks of decrepitated and imploded fluid inclusions

tectonic history of subduction zones inferred from retrograde blueschist p t paths many phanerozoic convergent plate junctions are marked by discontinuous blueschist belts reflecting relatively high pressure p prograde trajectories common blueschist paragneisses such as those of the western alps exhibit widespread overprinting by greenschist and or epidote amphibolite facies assemblages for this type of high p belt retrograde metamorphism involved fairly rapid nearly isothermal decompression some terranes underwent continued heating during early stages of pressure release uplift probably occurred as a consequence of the entrance of an island arc oceanic plateau or segment of continental crust into the subduction zone collision resulting in marked deceleration or cessation of underflow and buoyant approximately adiabatic rise of the stranded recrystallized subduction complex other high p belts such as the franciscan of western california preserve metamorphic aragonite and lack a low p overprint retrogression approximately retraced the prograde p t temperature path or for early formed high grade blocks occurred at even higher p t ratios parts of this type of metamorphic belt evidently migrated slowly back up the subduction zone in response to isostatic forces during continued plate descent and refrigeration upward motion took place as tectonically imbricated slices as laminar return flow in melange zones and perhaps partly a lateral spreading extension of the underplated accretionary prism retrograde p t trajectories of high pmore belts therefore provide important constraints on the tectonic evolution of convergent plate junctions less

low grade retrogression of a high temperature metamorphic core complex naxos cyclades greece retrogressive deformation and metamorphism are often reported from the main low angle shear zones and detachments of metamorphic core complexes but their importance is not sufficiently emphasized for the footwall interior in order to contribute to a better understanding of exhumation related retrogression processes within and at the top of metamorphic core complexes an integrated detailed microstructural textural 40 ar 39 ar geochronological and thermobarometric study on the naxos metamorphic core complex within the aegean sea is presented that provides a new perspective on low grade retrogression during exhumation through shallow ductile levels we found variable retrogressive deformation within the naxos metamorphic core complex which even pervasively affected significant portions of the migmatite grade metamorphic core and remnant high pressure areas of the metamorphic core complex where retrogression led to pervasive formation of new fabrics within greenschist facies metamorphic conditions during brittle ductile transition within a continuum of retrogression 40 ar 39 ar white mica dating allowed us to deduce three retrogressive ages at 16 52 0 39 ma within the naxos metamorphic core complex 12 6 0 28 ma moutsounas detachment shear zone on the eastern boundary of the metamorphic core complex and 10 43 0 44 ma to 8 40 0 76 ma last ductile activity along the naxos paros shear zone to the north of the metamorphic core complex a further stage of retrogression at 12 11 ma occurred along distinct low angle normal faults within the middle miocene naxos granite retrogressive microstructures low temperature calcite fabrics in marbles and chloritization in metapelites at temperatures of 350 130 c in the metamorphic core complex core resulted mainly from late stage e w shortening and folding late stage flow of hydrous fluids resulted in resetting of fabrics and enhancement of ductile deformation the middle late miocene retrogression events are also reflected by a similarly aged tectonic collapse basin in the hanging wall unit above the detachment the wide temporal range of retrogression within the naxos metamorphic core complex coincides in age with retrogressive deformation within other metamorphic core complexes of the aegean sea we interpret the long temporal range of retrogression to reflect outward southwestward retreat of the subduction and sequential activation of major detachment zones

multiphase structural evolution of a continental margin during obduction orogeny insights from the jebel akhdar dome oman mountains the structural evolution of the carbonate platform in the footwall of the semail ophiolite emplaced onto the passive continental margin of arabia helps to better understand the early stages of obduction related orogens these early stages are rarely observable in other orogens as they are mostly overprinted by later mountain building phases we present an extensive structural analysis of the jebel akhdar anticline the largest tectonic window of the oman mountains and integrate it on different scales outcrop observations can be linked to plate motion data providing an absolute timeframe for structural generations consistent with radiometric dating of veins top to s overthrusting of the semail ophiolite and hawasina nappes onto the carbonate platform during high plate convergence rates between arabia and eurasia caused rapid burial and overpressure generation and migration of hydrocarbons and bedding confined veins but no major deformation in the carbonate platform at reduced convergence rates subsequent tectonic thinning of the ophiolite took place above a top to nne crustal scale ductile shear zone deforming existing veins and forming a cleavage in clay rich layers in early campanian times ongoing extension occurred along normal to oblique slip faults forming horst graben structures and a precursor of the jebel akhdar dome campanian to maastrichtian this was followed by ne sw oriented ductile shortening and the formation of the jebel akhdar dome deforming the earlier structures thereafter exhumation was associated with low angle normal faults on the northern flank of the anticline we correlate the top to nne crustal scale shear zone with a similar structure in the saih hatat window to develop a unified model of the tectonic evolution of the oman mountains

the stacking of thrust slices in collision zones and its thermal consequences the thermal consequences of stacking up thrust slices in collision zones are investigated using simple one dimensional thermal models the metamorphic evolution geotherms and p t time paths of a given tectonic unit belonging to a crustal stacking wedge made of more than two units is governed by three effects cooling and pressure decrease associated with erosion cooling by the lowest units screen effect heating by the upper units cover effect it is shown that the efficiency of these effects are dependent on the tectonic evolution of the collision zone the metamorphic evolutions are very sensitive to the following tectonic parameters the number of thrusted units involved in thickening the time delay between each thrust and finally the mode of stacking of the different units over and understacking it appears that for a given depth of burial the temperature increase during uplift is less important in a crust thickened by three or four units than in the case of two units the screen effect during understacking is more efficient for a short time delay 10 ma between each thrust overstacking leads to higher temperatures before uplift when compared to understacking it is also shown that the thermal perturbation induced in an intermediate unit of the pile is more efficiently recorded when its thickness is rather small 10 km it is shown that for an erosion controlled uplift the shape of the p t t path depends on the position of the rocks within a given unit and on the unit position within the pile finally the general metamorphic evolutions during crustal thickening hp lt metamorphism and subsequent overprint are discussed in terms of the previously mentioned parameters in the case of the western alps it appears that the more or less efficient greenschist overprint of the units involved in thickening can be explained by different time intervals between the thrust events

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

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

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

dynamics of orogenic wedges and the uplift of high pressure metamorphic rocks subduction accretion complexes can be approximated as wedge shaped continua with a rigid buttress behind and a subducting litho spheric slab beneath thick wedges undergoing prograde metamorphism have a negligible long term yield strength and are likely to exhibit a complex nonlinear viscous rheology such a wedge will tend to deform internally until it reaches a stable configuration in which the gravitational forces generated by the wedge geometry balance the traction exerted on its underside by the subducting slab accretion of material at the wedge front will lengthen the wedge and cause it to shorten internally to regain the stable geometry this shortening will be expressed as late out of sequence thrusting backthrusting and folding conversely underplating of sediment or crustal slices will thicken the wedge which may need to extend internally to regain stability extension will cause listric normal faults that may merge downward into zones of ductile extension continued underplating at depth and compensating extension above provides a mechanism for bringing high p low t metamorphic rocks to upper levels in the rear of the wedge where they are commonly observed many major tectonic boundaries in convergent orogens such as the coast range thrust in the franciscan complex major nappe contacts in the alps and the contact between the nevado filabride and higher betic nappe complexes in the betic cordillera show abrupt increases in metamorphic grade downward across them this is consistent with their origin or reactivation as uplift related extensional structures

textural evolution of synthetic fluid inclusions in quartz during reequilibration with applications to tectonic reconstruction experimental reequilibration of synthetic 10 wt nacl h2o inclusions in natural quartz reveals that reequilibration textures show distinct differences depending upon the p t path followed by the inclusion after formation these differences combined with other geological information may be used to determine whether the sample rock followed a dominantly isothermal or isobaricp t path following entrapment the intensity and style of inclusion reequilibration features is related to the direction and magnitude of the departure of thep t path from the original isochore for the inclusion thus fluid inclusion reequilibration textures not only permit inclusionists to determine whether the rocks followed an isothermal or isobaric retrogradep t path but also the magnitude of departure of this path from one that is isochoric

fluid inclusion and mineral isotopic compositions hco in eclogitic rocks as tracers of local fluid migration during high pressure metamorphism eclogite facies metagabbros from the monviso ophiolitic complex italian western alps provide a unique opportunity to trace fluid migration processes in a portion of the oceanic crust that has undergone subduction at a depth of 40 km we have determined the omphacites 18o and the abundance d and 13c of hydrous and carbonaceous compounds present in whole rocks which are believed to trace the residual phases of what was mobilized in the original rocks during subduction prograde dehydration reactions and eclogitization of hydrothermally altered oceanic metagabbros was accompanied by approximately 90 fluid loss the remaining fluid was trapped as primary water rich fluid inclusions in omphacite megacrysts that developed at the expense of the precursor magmatic pyroxene deformation of the eclogitic rocks resulted in continuous recycling of fluid between mylonites and omphacite veins without further fluid loss from the host ductile shear zone oxygen isotopes of omphacite omp and hydrogen isotopes of water in the fluid inclusions fi analyzed in low strain rocks mylonites and undeformed deformed veins show marked variations 18oomp and dfi values ranging from 3 0 to 5 3 and from 31 to 93 respectively detailed isotopic analysis of several individual vein wallrock pairs show that the scale of isotopic equilibration is of the order of one centimeter therefore the eclogitic minerals and fluids filling the veins are concluded to be locally derived our results argue against recent models which suggest large scale mass flushing of isotopically homogeneous fluids during subduction zone metamorphism on average the h2o contents and dfi value are within the upper mantle range the carbon has been inherited from the metamorphic transformation of the original carbon present in the oceanic crust and has a mean 13c value of 24 2 1 2 two carbonaceous components can be recognized condensed carbon which represents the major carbon species and carbonate daughter crystals present in fluid inclusions 18oomp values are significantly lower than those reported for mantle minerals and are similar to pyroxene and whole rock values from hydrothermally altered oceanic crust it is suggested that the isotopic imprint of the monviso eclogitic minerals and fluids represents the signature of mid ocean ridge hydrothermal alteration and subduction zone eclogitization processes the rocks studied could be the missing link between altered oceanic gabbros and eclogitic xenoliths in addition the range in 18oomp values recorded in the different microstructural domains covers most of the 18o values reported in type a b and c eclogitic xenoliths implying that caution must be exercised when using oxygen isotopes as an indicator of the origin of eclogitic xenoliths

a new geobarometer for the assemblage biotite muscovite chlorite quartz the equilibrium thermodynamics of the reaction 4kmga1si40 o oh mg5a1 si30 o oh muscovlte chlorite muscovite biotite ka13si 0 oh 3kmg3aisi 0 0 oh 7si0 4h o quartz fluid can be formulated as a geobarometer thermo dynamic data for the mg a1 celadonite end member of muscovite is derived from the reversed mineral equilibria experiments of velde i 965 using the calculation method of powell 1978 giving afh 1298 5823 5 kj s 1298 0 3060kjk v 1298 13 8kjkbar

pressure temperature time evolution of the high pressure metamorphic complex of sifnos greece model studies of continental collision tectonics and pressure temperature time p t t predictions quantitatively demonstrate the key role of synsubduction uplift in the preservation of blueschist and eclogite facies metamorphic rocks rheological stratification of the subducting lithosphere allows development of detachment faults at the compositional boundary between the upper and lower crust material of supracrustal continental affinity is subducted to depths in excess of 50 km and then delaminated from the downgoing slab subduction continues while the detached domain is uplifted independently from the newly active hanging walls and footwalls two dimensional thermal calculations indicate that the nearby active subduction provides the cooling mechanism required to preserve blueschists and eclogites whereas the differential motions of the independently uplifted domains can explain the observed heterogeneity in p t t pathways with some domains displaying a greenschist overprint

high density co2 n2 inclusions in eclogite facies metasediments of the munchberg gneiss complex se germany the eclogite facies metasedimentary rocks in the munchberg gneiss complex t 630 30 c p 17 24 kbar locally contain co2 n2 rich fluid inclusions of extremely low molar volumes 32 cm3 mol in quartz these fluid compositions are mainly found in rocks intercalated with calcsilicate bands densities were determined from low temperature phase transitions like stable or metastable homogenization l v l partial homogenization s l v s l and the transition s l l l liquid v vapour s solid the high fluid densities are in agreement with eclogite facies pressure and temperature and subsequent amphibolite facies co2 n2 inclusions were not observed in adjacent eclogites nor in non calcareous metasediments these rock types contain predominantly h2o rich inclusions correlating with amphibolite facies conditions the variation of fluid composition with lithological differences indicates local fluid gradients and speaks against a pervasive fluid flow during eclogite facies metamorphism

exhumation of coesite bearing rocks in the dora maira massif western alps italy coesite bearing very high pressure rocks in the southern dora maira massif western alps italy emerged from a depth of 100 km during cretaceous time these rocks are now covered only by a thin fraction of the synmetamorphic overburden tectonic contacts above the coesite bearing rocks are retrograde ductile shear zones that operated with a normal sense of motion they cut out thick crustal sections from within the tectonometamorphic pile and caused the condensation of the eoalpine baric and thermal structure the result of this was the lateral dispersion of the eoalpine orogenic wedge and the facilitation of the unroofing of the very high p rocks extensional unroofing is consistent with the absence of deeply cutting cretaceous erosion and with pressure temperature paths that show that the very high p rocks cooled during decompression the very high p rocks reached relatively shallow crustal levels by ca 40 ma and the attenuated sequence overthrust a more external unit these indicate that the exhumation of the very high p rocks took place within an overall plate onvergence setting prior to the final suturing of europe and adria

the clinopyroxene plagioclase symplectite in retrograde eclogites a potential geothermobarometer the retrograde p t t path of intermediate and high temperature eclogites t 500 50 c is characterized by the transformation of omphacite into a symplectitic intergrowth of clinopyroxene and plagioclase lamellae often this symplectite shows a two stage microstructural evolution i e a coarse lamellar width symplectite s1 followed by a thinner lamellar width symplectite s2 each symplectite may also evolve independently into a final globular microstructure sg

metasomatism and partial melting in a subduction complex catalina schist southern california partial melting accompanied high pressure amphibolitization of eclogitic blocks and metasomatism of blocks and their peridotite matrix in a unit of the catalina schist a subduction zone metamorphic terrane migmatitic blocks contain leucocratic zones veins and pods with albitic plagioclase quartz muscovite mineralogically similar dikes and veins cut the ultra mafic matrix a few veins can be traced back into migmatitic blocks uniform phase proportions and igneous textures indicate that the dikes and veins crystallized from silicate melts field relations mineral and bulk compositions and comparisons with experimental data suggest that the migmatites and dikes are partial melts of variably metasomatized amphibolites element partitioning phase equilibrium and fluid inclusion data are compatible with partial melting of the amphibolites at p 8 11 kbar and t 640 750 c in the presence of a low salinity aqueous fluid although the catalina metamorphism occurred at much shallower levels than those inferred for subduction related magmatism the process envisioned is one of multistage slab derived metasomatic contamination of the mantle in the hanging wall of a subduction zone

organic microfacies and basinal tectonic control on source rock accumulation a microscopic approach with examples from an intracratonic and extensional basin abstract relationships among organic petrographic maceral assemblages and hydrocarbon source rock depositional environments distinguish broad marine terrestrial and mixed kerogen accumulation settings organic microfacies from paleozoic and mesozoic source rock intervals in the intracratonic williston basin and the extensional sverdrup basin have been studied using incident white and fluorescent light microscopy distinct anoxic organic microfacies are defined in upper ordovician bighorn group subtidal platformal kukersites and middle devonian upper elk point group carbonate lithologies that preserve a progression of depositional environments from platformal to starved basinal paleozoic carbonate platform microfacies are characterized by well preserved cyanophyceae alginite stromatolitic mats displaying upper surface textural features such as pustules pinnacles endolithic algal borings and interlaminated vertical and horizontal alginite resulting from phototactic growth potential source intervals in middle and upper triassic shelf shales from the sverdrup basin and middle devonian carbonate platform marginal mudstones in the williston basin are both characterized by high concentrations of sapropelic bituminite kerogen hosting unicellular marine alginite similar bituminite rich organic microfacies occur in middle devonian basinal settings associated with larger more abundant alginite and significant amounts of dasycladacean calcareous algae the main controls on source rock accumulation are water depth and chemistry as controlled by depositional setting and basin restriction

nitrogen bearing aqueous fluid inclusions in some eclogites from the western gneiss region of the norwegian caledonides minerals in eclogites from different localities in the western gneiss region of the norwegian caledonides age 425 ma contain a variety of fluid inclusions the earliest inclusions recognized are contained in undeformed quartz grains protected by garnet and consist of h2o n2 with x text h text 2 text o geqslant 0 8 the reconstructed p v t x properties of these fluid inclusions are compatible with peak or early retrograde metamorphic conditions matrix minerals quartz garnet apatite plagioclase contain a complex pattern of mostly truly secondary inclusions dominated by co2 and n2 the textural patterns and p v t x properties of these inclusions are incompatible with the high pressures of the eclogite forming metamorphic event but suggest that they were formed during uplift by a combination of remobilization of preexisting inclusions and influx of external fluids the fluid introduced at a late stage was dominated by co2 and did not contain n2 the present data agree with theoretical predictions of eclogite fluids from mineral equilibria and highlight the differences between granulite co2 and eclogite h2o n2 fluid regimes the provenance of the nitrogen in the eclogite fluid inclusions represents an important but unsolved question in the petrology of high pressure metamorphic rocks