coexisting sodic augite and omphacite in a sanbagawa metamorphic rock japan coexisting sodic augite and omphacite were found in a zoisite amphibolite from the iratsu epidote amphibolite mass in the sanbagawa metamorphic terrain of central shikoku japan the occurrences of the sodic augite omphacite pairs are classified into four types by texture independent composite intergrowth and exsolution types sodic augite and omphacite of the independent and composite types pair a have xna na na ca 0 15 and 0 35 respectively and were stable in the epidote amphibolite facies during the sanbagawa progressive metamorphism on the other hand xna values of sodic augite and omphacite of the intergrowth and exsolution types pair b are 0 10 and 0 44 respectively the na poor augite and na rich omphacite of the pair b were formed by re equilibration of the pair a at lower temperature the pair a of the iratsu sample suggests that a compositional gap lies between sodic augite and c2 c omphacite under epidote amphibolite facies conditions and is in marked contrast to the coexistence of sodic augite and p2 n omphacite reported from some low grade high pressure metamorphic terrains a possible phase diagram to explain the chemistry and mode of occurrence of the coexisting sodic pyroxenes is proposed

pfs mex observations of the condensing co2 south polar cap of mars abstract the condensing co2 south polar cap of mars and the mechanisms of the co2 ice accumulation have been studied through the analysis of spectra acquired by the planetary fourier spectrometer pfs during the first two years of esa s mars express mex mission this dataset spans more than half a martian year from ls 330 to ls 194 and includes the southern fall season which is found to be extremely important for the study of the residual south polar cap asymmetry the cap expands symmetrically and with constant speed during the fall season the maximum extension occurs sometime in the 80 90 ls range when the cap edges are as low as 40 latitude inside hellas and argyre basins frost can be stable at lower latitudes due to the higher pressure values causing the seasonal cap to be asymmetric within the seasonal range considered in this paper the cap edge recession rate is approximately half the rate at which the cap edge expanded the longitudinal asymmetries reduce during the cap retreat and disappear around ls 145 two different mechanisms are responsible for co2 ice accumulation during the fall season especially in the 50 70 ls range here co2 condensation in the atmosphere and thus precipitation is allowed exclusively in the western hemisphere and particularly in the longitudinal corridor of the perennial cap in the eastern hemisphere the cap consists mainly of co2 frost deposits as a consequence of direct vapor deposition the differences in the nature of the surface ice deposits are the main cause for the residual south polar cap asymmetry results from selected pfs orbits have also been compared with the results provided by the martian general circulation model gcm of the laboratoire de meteorologie dynamique lmd in paris with the aim of putting the observations in the context of the global circulation this first attempt of cross validation between pfs measurements and the lmd gcm on the one hand confirms the interpretation of the observations and on the other hand shows that the climate modeling during the southern polar night on mars is extremely sensitive to the dynamical forcing

mars orbiter camera observations of the martian south polar cap in 1999 2000 the spring summer recession of the south polar cap of mars in 1999 2000 has been investigated using the wide angle cameras of the mars orbiter camera experiment on mars global surveyor the 1999 2000 regression closely resembles the recession observed by viking orbiters in 1977 although the rates of recession are quite similar the current recession was slightly ahead of that in 1977 throughout spring the rapid development of dark terrain dubbed cryptic terrain by hugh kieffer in one sector of the south cap between ls 198 and 223 is documented the emergence and separation of the mountains of mitchel has also been recorded in detail culminating with its complete disappearance at solstice the 2000 residual cap is almost identical to that in 1977 this observation does not resolve the mystery of the very different appearance of the residual cap in 1972 the albedos in the residual cap region and in bright seasonal frost increase rapidly to high values at around solstice and then level out until the seasonal frost disappears observations of more martian years will be needed to constrain the amount of interannual variability and its relation to other phenomena such as dust storms

three dimensional prestack plane wave migration of teleseismic p to s converted phases 1 theory 1 we present the theoretical foundations for a prestack migration technique to image teleseismic p to s converted phases the method builds on teleseismic p wave deconvolution pseudostation stacking neal and pavlis 1999 and on the idea of using a plane wave decomposition for imaging as introduced by treitel et al 1982 deconvolution operators are constructed by pseudostation stacking of the array aligned to the incident p wave arrival times to produce a space variable deconvolution operator the resulting data are then muted to remove the deconvolved direct p wave pulse and pseudostation stacked over a grid of feasible slowness vectors the pseudostation stack interpolates the wave field onto a regular grid along earth s surface producing a series one per slowness vector of uniformly sampled three dimensional data cubes two space variables and time the plane wave components can be propagated downward using a form of approximate ray tracing with a three dimensional earth model this yields a series of distorted cubes topologically equivalent to the original uniformly sampled data cubes these data volumes are summed as a weighted stack with the weights derived from an integration formula for inverse scattering based on the generalized radon transform this allows an image of the subsurface to be constructed on an event by event basis beneath the array we apply this technique to data from the lodore array that was deployed in northwestern colorado the results suggest the presence of a major lithospheric scale discontinuity defined by a south dipping boundary

seismic properties of lawsonite eclogites from the southern motagua fault zone guatemala abstract we present new data on the crystal preferred orientation cpo and seismic properties of omphacite and lawsonite in extremely fresh eclogite from the southern motagua fault zone guatemala to discuss the seismic anisotropy of subducting oceanic crust the cpo of omphacite is characterized by 010 001 and it shows p wave seismic anisotropies avp of 1 4 3 2 and s wave seismic anisotropies avs of 1 4 2 7 lawsonite exhibits 001 planes parallel to the foliation and 010 axes parallel to the lineation and seismic anisotropies of 1 7 6 6 avp and 3 4 14 7 avs the seismic anisotropy of a rock mass consisting solely of omphacite and lawsonite is 1 2 4 1 avp and 1 8 6 8 avs for events that propagate more or less parallel to the maximum extension direction x the fast s wave velocity vs polarization is parallel to the z in the y z section rotated from the x z section causing trench normal seismic anisotropy for orthogonal subduction based on the high modal abundance and strong fabric of lawsonite the avs of eclogites is estimated as 11 7 in the case that lawsonite makes up 75 of the rock mass on this basis we suggest that lawsonite in both blueschist and eclogite may play important roles in the formation of complex pattern of seismic anisotropy observed in ne japan weak trench parallel anisotropy in the forearc basin domains and trench normal anisotropy in the backarc region

mapping of the water ice content within the martian surficial soil on the periphery of the retreating seasonal northern polar cap based on the tes and the omega data 1 analysis of seasonal data from the mars global surveyor thermal emission spectrometer tes shows a significant increase in thermal inertia during autumn winter and spring in the middle and high latitudes of mars at each stage of the northern seasonal polar cap s recession in spring a distinct high thermal inertia hti annulus arises around the cap s edge within this annulus we estimated and mapped the springtime water ice content in the daily thermal skin depth layer using spring and summer values of the thermal inertia in tes surface footprints the results show that the average water content in the surface soil within the hti annulus varies from 5 vol at the early stages of the seasonal polar cap retreating ls 340 360 to 1 vol at later stages ls 60 70 maximum values of water ice content within the hti annulus occur at ls 0 20 2 6 vol and ls 20 40 4 10 vol we analyzed the temporal and spatial relationship between the hti annuli and the water ice wi annuli at the edge of the northern seasonal polar cap the water ice within the wi annuli was mapped using a water ice spectral index the absorption band depth at the 1 5 m wavelength derived from the omega observatoire pour la mineralogie l eau les glaces et l activite imaging spectrometer aboard the mars express spacecraft recent omega observations show that the wi annuli formation arises only around the retreating northern seasonal polar cap never around the retreating southern seasonal cap for this reason our study is confined only to the northern hemisphere the observed relationship between the hti and wi annuli in the northern hemisphere of mars indicates a close physical interdependence between these two phenomena our results confirm that the seasonal permafrost exposed by the retreating northern polar cap within the hti annuli is actively involved today in the condensation and sublimation processes in the modern water cycle on mars the water abundance in this annual condensation sublimation cycle within the active layer may be an order of magnitude higher than all the annual water vapor abundance in the martian atmosphere our results are consistent with existence of the positive mass balance of the northern permanent polar cap in the modern climatic period

discovery of lower cretaceous synmetamorphic thrust tectonics in french lesser antilles la desirade island guadeloupe implications for caribbean geodynamics located east of guadeloupe the island of la desirade exhibits the oldest rocks of the lesser antilles arc and the eastern caribbean plate an old magmatic basement is composed of 1 late jurassic ophiolitic complex with meta basaltic pillow lavas and interbedded radiolarites 2 acid igneous complex comprising meta quartz diorite and meta rhyolitic lavas flows and 3 meta diabasic microdioritic dyke swarm complex we present and discuss the discovery of synmetamorphic thrust tectonics in this island based on detailed structural analysis we evidenced two main compressive events the first event d1 is characterized by pervasive folding associated with thrust development in the northeastern part of the island a major thrust fault the grand abaque thrust has been discovered which displaces for several kilometers the acid igneous complex onto the ophiolitic unit the second deformation event d2 is featured by a main pervasive cleavage s2 observed at the regional scale associated with upright folding f2 and by the development of two sets of conjugated steeply dipping strike slip shear zones dextral 130 n and sinistral 20 n striking respectively microstructural observations indicate that greenschist facies metamorphism is clearly contemporaneous with the development of d1 and d2 superimposed tectonic structures ar ar geochronology was performed on two bulk of adularia minerals sampled in the northeast volcanic complex within a deformed zone related to a d2 event these analyses yield well defined and concordant plateau ages at 106 2 1 7 ma and 107 2 1 8 ma thus shortening and thickening tectonics occurred during lower cretaceous albian in the available framework of caribbean geodynamics the synmetamorphic thrust tectonics we discovered in the french lesser antilles is related to the collision between an aptian albian oceanic plateau and the andean cordilleran east dipping subduction zone our results support subduction polarity reversal during lower cretaceous

segmented african lithosphere beneath the anatolian region inferred from teleseismic p wave tomography summary lithospheric deformation throughout anatolia a part of the alpine himalayan orogenic belt is controlled mainly by collision related tectonic escape of the anatolian plate and subduction roll back along the aegean subduction zone we study the deeper lithosphere and mantle structure of anatolia using teleseismic finite frequency p wave traveltime tomography we use data from several temporary and permanent seismic networks deployed in the region approximately 34 000 p wave relative traveltime residuals measured in multiple frequency bands are inverted using approximate finite frequency sensitivity kernels our tomograms reveal segmented fast seismic anomalies beneath anatolia that corresponds to the subducted portion of the african lithosphere along the cyprean and the aegean trenches we identify these anomalies as the subducted aegean and the cyprus slabs that are separated from each other by a gap as wide as 300 km beneath western anatolia this gap is occupied by slow velocity perturbations that we interpret as hot upwelling asthenosphere the eastern termination of the subducting african lithosphere is located near the transition from central anatolia to the eastern anatolian plateau or arabian eurasian collision front that is underlain by large volumes of hot underplating asthenosphere marked by slow velocity perturbations our tomograms also show fast velocity perturbations at shallow depths beneath northwestern anatolia that sharply terminates towards the south at the north anatolian fault zone nafz the associated velocity contrast across the nafz persists down to a depth of 100 150 km hence our study is the first to investigate and interpret the vertical extent of deformation along this nascent transform plate boundary overall the resolved upper mantle structure of anatolia is directly related with the geology and tectonic features observed at the surface of the anatolian plate and suggest that the segmented nature of the subducted african lithosphere plays an important role in the evolution of anatolia and distribution of its tectonic provinces

observations of the south seasonal cap of mars during recession in 2004 2006 by the omega visible near infrared imaging spectrometer on board mars express 1 the omega visible near infrared imaging spectrometer on board mars express has observed the southern seasonal cap in late 2004 and 2005 and then in the summer of 2006 these observations extended from the period of maximum extension close to the southern winter solstice to the end of the recession at ls 325 the spectral range and spectral resolution of omega make it possible to monitor the extent and effective grain size of co2 ice and h2o ice on the ground the level of contamination of co2 ice and h2o ice by dust and the column density of m sized ice grains in the atmosphere the co2 seasonal cap is very clean and clear in early southern winter contamination by h2o ice spreads eastward from the hellas basin until the southern spring equinox during southern spring and summer there is a very complex evolution in terms of effective grain size of co2 ice and contamination by dust or h2o ice h2o ice does not play a significant role close to the southern summer solstice contamination of co2 ice by h2o ice is only observed close to the end of the recession as well as the few h2o ice patches already reported by bibring et al 2004a these observations have been compared to the results of a general circulation model with good qualitative agreement on the distribution of h2o ice on the surface and in the atmosphere resolving the remaining discrepancies will improve our understanding of the water cycle on mars

mars south polar spring and summer behavior observed by tes seasonal cap evolution controlled by frost grain size thermal emission spectrometer tes observations of the recession phase of mars south polar cap are used to quantitatively map this recession in both thermal and visual appearance geographically nonuniform behavior interior to the cap is characterized by defining several small regions which exemplify the range of behavior for most of the cap while temperatures remain near the co2 frost point albedos slowly increase with the seasonal rise of the sun then drop rapidly as frost patches disappear over a period of 20 days a cryptic region remains dark and mottled throughout its cold period tes observations are compared with first order theoretical spectra of solid co2 frost with admixtures of dust and h2o the tes spectra indicate that the cryptic region has much larger grained solid co2 than the rest of the cap and that the solid co2 here may be in the form of a slab the mountains of mitchel remain cold and bright well after other areas at comparable latitude apparently as a result of unusually small size of the co2 frost grains we found little evidence for a significant presence of h2o although co2 grain size may be the major difference between these regions incorporated dust is also required to match the observations a self cleaning process carries away the smaller dust grains comparisons with viking observations indicate little difference in the seasonal cycle 12 martian years later the observed radiation balance indicates co2 sublimation budgets of up to 1250 kg m 2 regional atmospheric dust is common localized dust clouds are seen near the edge of the cap prior to the onset of a regional dust storm and interior to the cap during the storm

variations in the crustal structure beneath western turkey summary we use teleseismic receiver functions to investigate the crustal structure at two locations in westernturkey using seismic data recorded on small arrays of temporary broad band seismographs the results from these analyses are compared with receiver function results from the gdsn station anto on the anatolian plateau in central turkey the crust is 30 km thick in the region of western turkey where active normal faulting reveals present day extension in the upper crust and alkali basaltic volcanism reveals recent extension within the subcrustal lithosphere the crust is 34 km thick further east where crustal extension is still evident but less pronounced in the anatolian plateau which is not currently extending the crust is 38 km thick the level of extension estimated from these measurements of crustal thickness implies a b factor of 1 2 this value agrees with the amount of extension estimated in the upper crust from the integrated seismic strain rate b factor of 1 3 from surface faulting b factor of 1 25 and from the amount of extension in the subcrustal lithosphere estimated from the volcanism b factor 2 all indicating that the extension is approximately uniformly distributed vertically throughout the lithosphere the moho transition in this region appears to thin slightly as the degree of extension increases westwards

winter and spring evolution of northern seasonal deposits on mars from omega on mars express the omega visible near infrared imaging spectrometer on mars express has observed the retreat of the northern seasonal deposits during martian year 27 28 from the period of maximum extension close to the northern winter solstice to the end of the retreat at l s 95 we present the temporal and spatial distributions of both co 2 and h 2o ices and propose a scenario that describes the winter and spring evolution of the northern seasonal deposits during winter the co 2 rich condensates are initially transparent and could be in slab form a water ice annulus surrounds the sublimating co 2 ice extending over 6 of latitude at l s 320 decreasing to 2 at l s 350 and gradually increasing to 4 5 at l s 50 this annulus first consists of thin frost as observed by the viking lander 2 and is then overlaid by h 2o grains trapped in the co 2 rich ice layer and released during co 2 sublimation by l s 50 h 2o ice spectrally dominates most of the deposits in order to hide the still several tens of centimeters thick co 2 ice layer in central areas of the cap we propose the buildup of an optically thick top layer of h 2o ice from ice grains previously embedded in the co 2 ice and by cold trapping of water vapor from the sublimating water ice annulus the co 2 ice signature locally reappears between l s 50 and 70 what emerges from our observations is a very active surface atmosphere water cycle these data provide additional constraints to the general circulation models simulating the martian climate copyright 2011 by the american geophysical union

compact reconnaissance imaging spectrometer for mars crism south polar mapping first mars year of observations 1 we report on mapping of the south polar region of mars using data from the compact reconnaissance imaging spectrometer for mars crism instrument our observations have led to the following discoveries 1 water ice is present in the form of pole circling clouds originating from the circum hellas region beginning prior to ls 162 and diminishing markedly at ls 200 204 2 it has previously been inferred by thermal infrared observations by titus et al 2003 and co2 h2o mixture spectral models by langevin et al 2007 that surface water ice was present in the cryptic region in the final stages of sublimation the high resolution of crism has revealed regions where only water ice is present not a co2 h2o ice mixture this water ice disappears completely by ls 252 and may be the source of water vapor observed by crism in southern latitudes between ls 240 260 by m smith et al 2009 3 we have estimated surface co2 ice grain size distributions for the south pole residual cap sprc and the seasonal co2 ice cap that covers it throughout summer spring and summer our analysis suggests that grain sizes peak at ls 191 199 with an apparent grain size of 7 1 cm by the end of the summer period our analysis demonstrates minimum apparent grain sizes of 5 1 mm predominate in the sprc 4 fine grained co2 ice condenses from ls 0 40 and extends symmetrically away from the geographic pole extending beyond 80 s by ls 4 10 no evidence for unusual co2 depositional processes in the cryptic region is observed up to ls 16

thermodynamic modelling of sodic pyroxene solid solution and its application in a garnet omphacite kyanite coesite geothermobarometer for uhp metamorphic rocks a garnet omphacite kyanite coesite geothermobarometer is constructed to estimate the peak metamorphic pressure of ultrahigh pressure uhp eclogites for this we use a quaternary regular solution model for garnet and propose a new model for sodic pyroxene that assumes double site solid solution na ca m2 al fe3 fe2 mg m1si2o6 where cations in the m2 and the m1 sites mix randomly but with inter site interactions not only between nearest neighboring cations but also between next nearest neighboring cations in the new model overall excess interaction parameters are quantitatively deduced from the previous experimental data and critical temperature and the activity of the jadeite component agrees reasonably with that based on the landau theory furthermore the relations of the calculated order parameter to temperature and composition are consistent with other experimental works these facts support the usefulness of the new model application of our geothermobarometer to kyanite eclogite gave metamorphic conditions around 3 4 gpa and 800 c for both samples from the dora maira massif western alps and from rongcheng county in the su lu region of eastern china

matrix deformation mechanisms in hp lt tectonic melanges microstructural record of jadeite blueschist from the franciscan complex california abstract exhumed high pressure low temperature metamorphic melanges of tectonic origin are believed to reflect high strain accumulated in large scale interplate shear zones during subduction rigid blocks of widely varying size are embedded in a weak matrix which takes up the deformation and controls the rheology of the composite the microfabrics of a highly deformed jadeite blueschist from the franciscan complex california are investigated to help understand deformation mechanisms at depth the specimen shows a transposed foliation with dismembered fold hinges and boudinage structures several generations of open fractures have been sealed to become veins at high pressure metamorphic conditions the shape of these veins frequently restricted to specific layers indicates distributed host rock deformation during and after sealing small cracks in jadeite and lawsonite are healed with tiny quartz inclusions aligned along the former fracture surface large jadeite porphyroblasts show strain caps and strain shadows open fractures are sealed by quartz and new jadeite epitactically grown on the broken host monophase glaucophane aggregates consist of undeformed needles with a diameter between 0 1 and 2 m grown after formation of isoclinal folds only quartz microfabrics indicate some stage of crystal plastic deformation followed by annealing and grain growth aragonite in the latest vein generation shows retrogression to calcite along its rims the entire deformation happened under hp lt metamorphic conditions in the stability field of jadeite and quartz at temperatures between 300 and 450 c and pressures exceeding 1 1 4 gpa the microfabrics indicate that dissolution precipitation creep was the predominant deformation mechanism accompanied by brittle failure and vein formation at quasi lithostatic pore fluid pressure this indicates low flow strength and combined with high strain rates expected for localized deformation between the plates a very low viscosity of material in the interplate shear zone at a depth 30 45 km

titanite bearing omphacitite from the jade tract myanmar interpretation from mineral and trace element compositions abstract jadeitite is a rare rock composed predominantly of jadeite which typically occurs in association with tectonic blocks of high pressure low temperature metabasaltic rocks such as eclogite or blueschist often as a matrix in exhumed serpentinite melanges omphacitite are far less common occurring together with jadeitite such as those in the jade tract of hpakan area in myanmar the omphacitite in this locality is mostly composed of omphacite and jadeite with minor titanite ilmenite epidote and zircon the jadeite formed after omphacite shows a lower jd content than that in the neighboring white jadeitite the omphacite shows significant variation in jd content and is associated with aegirine augite both rocks show relatively linear and upslope pattern from lree to hree and a slight enrichment of ba th u zr and hf relative to chondrites in the absence of eu anomaly the titanite occurs in two groups one as discrete islands replacing ilmenite and the other as precipitation within jadeite veinlet titanite grains show convex patterns from lree to hree and depletion of sr zr and hf with no eu anomaly chemical characteristics of the titanite and ilmenite alteration around titanite suggest that the omphacitite is of secondary origin likely derived from pyroxenite through the replacement of pyroxene by jadeite based on the previous findings of jadeitization of chromitite serpentine and rodingite it is suggested that protoliths such as plagiogranite or gabbro trapped within serpentinite melange might have undergone jadeitization

fate of the subducted farallon plate inferred from eclogite xenoliths in the colorado plateau we present the first finding of the high pressure mineral coesite in lawsonite bearing eclogite xenoliths from the colorado plateau united states the presence of coesite in these xenoliths supports the hypothesis that the eclogite formed in a low temperature high pressure environment such as envisaged inside subducted oceanic lithosphere ion microprobe u pb dating of micrometer scale zircons in the eclogites yields ages ranging from 81 ma to 33 ma the two extremes in age likely indicating the age of crystallization during subduction related metamorphism and the age of recrystallization by the host magmatic event respectively these observations conclusively demonstrate that certain eclogite xenoliths from the colorado plateau originated as fragments of the subducted farallon plate which had been residing in the upper mantle since the late cretaceous this is the first conclusive evidence that any eclogite xenoliths can be directly linked to a known subducted plate

metamorphic reworking of a high pressure low temperature melange along the motagua fault guatemala a record of neocomian and maastrichtian transpressional tectonics abstract the guatemala suture zone is a major east west left lateral strike slip boundary that separates the north american and caribbean plates in guatemala the motagua fault the central active strand of the suture zone underwent two major collisional events within a system otherwise dominated by strike slip motion the first event is recorded by high pressure low temperature hp lt eclogites and related rocks that occur within serpentinites both north and south of the motagua fault lawsonite eclogites south of the fault are not significantly retrograded and give 40ar 39ar ages of 125 116 ma and sm nd mineral isochrons of 144 132 ma eclogites north of the fault give similar sm nd isochron ages 131 126 ma but otherwise differ in that they are strongly overprinted by a lower pressure assemblage and along with associated hp lt rocks give much younger 40ar 39ar ages of 88 55 ma indicating a later amphibolite facies metamorphic event we propose therefore that all serpentinite hosted eclogites along the motagua fault formed at essentially the same time in different parts of a laterally extensive lower cretaceous forearc subduction system but subsequently underwent different histories the southern assemblages were thrust southwards present coordinates immediately after hp metamorphism whereas the northern association was retrograded during a later collision that thrust it northward at ca 70 ma they were subsequently juxtaposed opposite each other by major strike slip motion this model implies that the hp rocks on opposing sides of the motagua fault evolved along a plate boundary that underwent both dip slip and strike slip motion throughout the late cretaceous as a result of oblique convergence the juxtaposition of a convergent and strike slip system means that hp lt rocks within serpentinites can be found at depth along much of the modern guatemala suture zone and its eastward extension into the northern caribbean both sets of assemblages were exhumed relatively recently by the uplift of mountain ranges on both sides of the fault caused by movement along a restraining bend recent exhumation explains the apparently lack of offset of surface outcrops along a major strike slip fault

cold subduction and the formation of lawsonite eclogite constraints from prograde evolution of eclogitized pillow lava from corsica a new discovery of lawsonite eclogite is presented from the lancone glaucophanites within the schistes lustres nappe at defile du lancone in alpine corsica the fine grained eclogitized pillow lava and inter pillow matrix are extremely fresh showing very little evidence of retrograde alteration peak assemblages in both the massive pillows and weakly foliated inter pillow matrix consist of zoned idiomorphic mg poor 0 8 wt mgo garnet omphacite lawsonite chlorite titanite a local overprint by the lower grade assemblage glaucophane albite with partial resorption of omphacite and garnet is locally observed garnet porphyroblasts in the massive pillows are mn rich and show a regular prograde growth type zoning with a mn rich core in the inter pillow matrix garnet is less manganiferous and shows a mutual variation in ca and fe with fe enrichment toward the rim some garnet from this rock type shows complex zoning patterns indicating a coalescence of several smaller crystallites matrix omphacite in both rock types is zoned with a rimward increase in xjd locally with cores of relict augite numerous inclusions of clinopyroxene lawsonite chlorite and titanite are encapsulated within garnet in both rock types and albite quartz and hornblende are also found included in garnet from the inter pillow matrix inclusions of clinopyroxene commonly have augitic cores and omphacitic rims the inter pillow matrix contains cross cutting omphacite rich veinlets with zoned omphacite si rich phengite si 3 54 apfu ferroglaucophane actinolite and hematite these veinlets are seen fracturing idiomorphic garnet apparently without any secondary effects pseudosections of matrix compositions for the massive pillows the inter pillow matrix and the cross cutting veinlets indicate similar p t conditions with maximum pressures of 1 9 2 6 gpa at temperatures of 335 420 c the inclusion suite found in garnet from the inter pillow matrix apparently formed at pressures below 0 6 0 7 gpa retrogression during initial decompression of the studied rocks is only very local late veinlets of albite glaucophane without breakdown of lawsonite indicate that the rocks remained in a cold environment during exhumation resulting in a hairpin shaped p t path

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

coexisting retrograde jadeite and omphacite in a jadeite bearing lawsonite eclogite from the motagua fault zone guatemala coexisting jadeite and omphacite were found as retrograde minerals in a jadeite bearing lawsonite eclogite from the motagua fault zone guatemala the lawsonite eclogite is characterized by the occurrence of garnet porphyroblasts up to 2 5 cm in size and the eclogite facies parageneses almandine rich garnet impure jadeite lawsonite rutile quartz garnet contains inclusions of impure jadeite phengite ferroglaucophane chlorite lawsonite rutile ilmenite and quartz textural relations and parageneses and compositions of minerals indicate that the lawsonite eclogite experienced two stages of metamorphism prograde eclogite facies stage m 1 and retrograde stage m 2 the impure jadeite jd i of the m 1 eclogite facies occurs in both the matrix and as inclusions in garnet and contains considerable amounts of augite and aegirine components jd 6 1 7 5 aug 1 6 2 4 ae 0 1 8 it is partly recrystallized to retrograde m 2 jadeite jd ii jd 7 4 8 7 aug 9 1 6 ae 0 1 1 and omphacite jd 4 2 5 0 aug 3 6 4 6 ae 7 1 6 some of these two sodic pyroxenes may have crystallized from fluids both m 2 jadeite and omphacite show textural equilibrium and are believed to have grown concurrently based on the observed compositions and the phase relations of sodic pyroxenes from carpenter 1980 the m 1 impure jadeite jd i may have had a disordered c2 c symmetry at t ca 450 c and p ca 1 8 2 4 gpa and was subsequently crystallized into jadeite jd ii plus ordered p2 n omphacite during retrogression with infiltration of fluids at t ca 300 c and p ca 0 7 gpa m 2 the extreme low t conditions during retrogression may have prevented reaction between eclogitic jadeite and adjacent minerals instead eclogitic impure jadeite plus fluid has recrystallized into the retrograde jadeite omphacite pair with a wide compositional gap

two high pressure low temperature serpentinite matrix melange belts motagua fault zone guatemala a record of aptian and maastrichtian collisions left lateral motion along the north american caribbean plate boundary has juxta posed two high pressure low temperature hp lt belts from separate cretaceous colli sions these two belts have quite different ages and different suites of high pressure as semblages yet they both contain jadeitite a relatively rare rock type this part of the plate boundary zone follows the motagua river valley in guatemala where it separates the maya block north american plate from the chortis block caribbean plate on both sides of the bounding motagua fault tectonic slices of serpentinite matrix melange host the hp lt rocks south of the fault the melange slices contain eclogite lawsonite eclogite glaucophane eclogite and blueschist blocks north of the fault the melange slices contain omphacite metabasite albitite and garnet amphibolite blocks but lack intact eclogite in addition to the dissimilar rock assemblages 40 ar 39 ar geochronology of phen gitic micas yields 77 65 ma for northern and 125 113 ma for southern blocks these data suggest that the southern belt formed during early cretaceous aptian northeastward dipping subduction of the farallon plate and collision of the chortis block with western mexico the block was then displaced southeastward along this suture in contrast the northern belt records subduction related to the maastrichtian collision of an extension of the chortis block perhaps the nicaraguan rise with the maya block

growth history of garnet in pelitic schists of the sanbagawa metamorphic terrain in central shikoku abstract during the prograde metamorphism of the sanbagawa metamorphic terrain in central shikoku garnet has grown in the pelitic schists whilst maintaining surface equilibrium with coexisting minerals such as chlorite and epidote etc in the quartz albite layer it has grown euhedrally in the micaceous layer its crystal morphology varies according to the different minerals in contact and their different orientations garnet enclosed in or sandwiched between micas is always ellipsoidal extended parallel to the basal planes of the micas however when the edge of a garnet has intruded at an acute angle into the micas euhedral garnet has grown it is concluded that the growth of garnet at the interface with micas ceased when the 110 surface of garnet came into contact with the basal plane of the micas in the tectonic c axis direction other models such as chloritization resorption pressure solution and mechanical erosion or grinding do not adequately explain the origin of the flattened garnet during retrograde metamorphism chloritization and resorption of garnets took place in the matrix resulting in the rough and irregular surface of garnet garnets included in albite porphyroblasts have been largely unaffected by the retrograde metamorphism

s p wave traveltime residuals from earthquakes and lateral inhomogeneity in the upper mantle beneath the aegean and the hellenic trench near crete

subduction of young oceanic plates a numerical study with application to aborted thermal chemical plumes we investigated numerical models of initiation and subsequent evolution of subduction of young 10 30 myr oceanic lithosphere systematic numerical experiments were carried out by varying the age of the subducting plate 10 12 5 15 17 5 20 25 and 30 myr the rate of induced convergence 2 4 and 5 cm yr and the degree of hydration 0 and 2 wt h2o of the pre existing weak oceanic fracture zone along which subduction is initiated despite the prescribed plate forcing spontaneously retreating oceanic subduction with a pronounced magmatic arc and a backarc basin was obtained in a majority of the experiments it was also found that the younger age of oceanic lithosphere results in more intense dehydration and partial melting of the slab during and after the induced subduction initiation due to the shallow dispositions of the isotherms partial melting of the subducted young crust may create thermal chemical instabilities cold plumes that ascend along the slab mantle interface until they either freeze at depth or detach from the slab and penetrate the upper plate lithosphere contributing to the nucleation and growth of a volcanic arc freezing of the plumes in the slab mantle interface is favored by subduction of very young lithosphere i e 10 myr at moderate rate 4 cm yr of convergence such aborted plumes may correspond to cretaceous partially melted morb derived slab material and associated adakitic tonalitic trondhjemitic rocks crystallized at ca 50 km depth in the slab mantle interface and exhumed in a subduction channel serpentinite melanges in eastern cuba

residual south polar cap of mars stratigraphy history and implications of recent changes abstract the residual south polar cap rspc of mars includes a group of different depositional units of co 2 ice undergoing a variety of erosional processes complete summer coverage of the rspc by 6 m pixel data of the context imager ctx on mars reconnaissance orbiter mro has allowed mapping and inventory of the units in the rspc unit maps and estimated thicknesses indicate the total volume of the rspc is currently 3 and represents less than 3 of the total mass of the current mars atmosphere scarp retreat rates in the co 2 ice derived from comparison of high resolution imaging science experiment hirise data with earlier images are comparable to those obtained for periods up to 3 mars years earlier these rates combined with sizes of depressions suggest that the oldest materials were deposited more than 125 mars years ago most current erosion is by backwasting of scarps 1 12 m in height this backwasting is initiated by a series of scarp parallel fractures in the older thicker unit these fractures form about every mars year in thinner younger materials they form less frequently some areas of the older thicker unit are lost by downwasting rather than by the scarp retreat a surprising finding from the hirise data is the scarcity of visible layering of rspc materials a result quite distinct from previous interpretations of layers in lower resolution images layers 0 1 m thick are exposed on the upper surfaces of some areas but their timescale of deposition is not known late summer albedo changes mapped by the ctx images indicate local recycling of ice although the amounts may be morphologically insignificant compact reconnaissance imaging spectrometer for mars crism data show that the primary material of all the different forms of the rspc is co 2 ice with only small admixtures of water ice and dust

hydrous carbonates on mars evidence from mariner 6 7 infrared spectrometer and ground based telescopic spectra absorption features at 2 28 and 5 4 m identified in mariner 6 7 infrared spectrometer and terrestrial telescopic spectra are consistent with the spectra of hydrous magnesium carbonates such as hydromagnesite and artinite spectral characteristics of these hydrous carbonates are different from those of the anhydrous carbonates as the former do not have the strong spectral features typically associated with anhydrous carbonates such as calcite and siderite theoretical mixing indicates that depending on the type of hydrous carbonate 10 20 wt can be incorporated into the regolith without contradicting the spectral observations or the viking x ray fluorescence chemical analysis hydrous carbonates form as weathering products of mafic minerals in the presence of h2o and co2 even in the antarctic their formation as evaporite minerals from either original magmas or hydrothermally altered rocks is consistent with the martian environment provided liquid water is or has been at least transiently present on earth formation of hydrous mg carbonates is associated with the production of amorphous iron oxides which is consistent with both the environment and the inferred surface mineralogy of mars these minerals are about 60 wt h2o co3 and oh if they are abundant everywhere at the 10 level then about 6 of the surface weight could be volatiles bound in this type of mineral although the stability of hydrous carbonates in a martian environment is uncertain there may be kinetic factors inhibiting the dehydration of these minerals which may persist metastably in the current environment although the spectroscopic evidence for anhydrous carbonates is scant the possible presence of hydrous carbonates provides an appealing mechanism for the existence of carbonates on mars

detection and monitoring of h2o and co2 ice clouds on mars we have developed an observational scheme for the detection and discrimination of mars atmospheric h2o and co2 clouds using ground based instruments in the near infrared we report the results of our cloud detection and characterization study using mars near ir images obtained during the 1990 and 1993 oppositions we focused on specific wavelengths that have the potential based on previous laboratory studies of h2o and co2 ices of yielding the greatest degree of cloud detectability and compositional discriminability we have detected and mapped absorption features at some of these wavelengths in both the northern and southern polar regions of mars compositional information on the nature of these absorption features was derived from comparisons with laboratory ice spectra and with a simplified radiative transfer model of a co2 ice cloud overlying a bright surface our results indicate that both h2o and co2 ices can be detected and distinguished in the polar hood clouds the region near 3 00 m is most useful for the detection of water ice clouds because there is a strong h2o ice absorption at this wavelength but only a weak co2 ice band the region near 3 33 m is most useful for the detection of co2 ice clouds because there is a strong relatively narrow co2 ice band at this wavelength but only broad continuum h2o ice absorption weaker features near 2 30 m could arise from co2 ice at coarse grain sizes or surface dust minerals narrow features near 2 00 m which could potentially be very diagnostic of co2 ice clouds suffer from contamination by mars atmospheric co2 absorptions and are difficult to interpret because of the rather poor knowledge of surface elevation at high latitudes these results indicate that future ground based earth orbital and spacecraft studies over a more extended span of the seasonal cycle should yield substantial information on the style and timing of volatile transport on mars as well as a more detailed understanding of the role of co2 condensation in the polar heat budget

spatial variability in the seasonal south polar cap of mars we present the first comprehensive discussion of the south seasonal polar cap spectra obtained by the mariner 7 infrared spectrometer in the short wavelength region 2 4 m in addition we correlate the infrared spectra with images acquired by the wide angle camera significant spectral variation is noted in the cap interior and regions of varying water frost abundance co2 ice frost cover and co2 ice path length can be distinguished many of these spectral variations correlate with heterogeneity noted in the camera images but certain significant infrared spectral variations are not discernible in the visible we use simple reflectance models to classify the observed spectral variations into four regions region i is at the cap edge where there is enhanced absorption beyond 3 m inferred to be caused by an increased abundance of water frost the increase in water abundance over that in the interior is on the level of a few parts per thousand or less region ii is the typical cap interior characterized by spectral features of co2 ice at grain sizes of several millimeters to centimeters these spectra also indicate the presence of water frost at the parts per thousand level a third unusual region iii is defined by three spectra in which weak co2 absorption features are as much as twice as strong as in the average cap spectra and are assumed to be caused by an increased path length in the co2 such large paths are inconsistent with the high reflectance in the visible and at 2 2 m and suggest layered structures or deposition conditions that are not accounted for in current reflectance models the final region iv is an area of thinning frost coverage or transparent ice well in the interior of the seasonal cap these spectra are a combination of co2 and ground signatures

petrology geochemistry and isotope data on a ultrahigh pressure jadeite quartzite from shuanghe dabie mountains east central china abstract in the dabie ultrahigh pressure terrane of east central china coesite bearing jadeite quartzites occur locally as intercalated layers with marble and mafic eclogite this rock assemblage is in turn enclosed within quartzofeldspathic gneisses metamorphic parageneses and kelyphitic textures reveal a multistage metamorphic evolution and complex exhumation history the primary peak metamorphic assemblage consists of jadeite garnet coesite rutile apatite minor coesite and coesite pseudomorphs occur as inclusions in jadeite and garnet three stages of retrograde assemblages are observed in the jadeite quartzites stage a is represented by the polymorphic transformation of coesite to quartz aggregates stage b is characterized by formation of coronas around jadeite porphyroblasts consisting of an inner layer of oligoclase amphibole and an outer layer of albite aegirine augite the last stage stage c involved total replacement of jadeite and most garnets by taramitic amphibole albite aegirine augite peak metamorphic p t conditions were 26 kbar at 660 c and are consistent with the estimates from the adjacent coesite bearing eclogites the jadeite quartzites display clockwise p t path that matches those of the adjacent eclogites major and trace element data suggest that the protolith of the jadeite quartzite could have been an albitized siltstone enriched in na and depleted in k and ca the highly negative present day end value 24 7 indicates a very old age for the protolith its late archean model age tdm of 2 58 ga is among the oldest so far identified for rocks from the dabie uhpm terrane concordant field relations and petrogenetic considerations suggest that all mafic politic carbonate and gneissic rocks have experienced in situ uhp metamorphism during triassic continental collision between the sino korean and yangtze cratons

high resolution reflectance spectra of mars in the 2 3 μm region evidence for the mineral scapolite spatially resolved reflectance spectra of mars in the 2 2 to 2 4 m spectral region were obtained in august 1988 using the nasa 3 m infrared telescope facility the spectra show weak absorption features due to martian atmospheric carbon monoxide and a surface mineral both co and the mineral absorptions are composed of overlapping narrow features but in many locations such as hellas chryse eden and moab the mineral absorptions are quite strong at least 3 times stronger than at the most absorbing wavelengths of co near 2 33 m therefore co complicates the analysis of the surface mineral but does not always overwhelm its signature model removal of the martian atmospheric co has been performed and the remaining absorption bands are identified as scapolite relatively strong absorptions that match bands in the spectrum of scapolite and have little or no co absorption interference are seen near 2 41 2 39 and 2 29 m absorption also occurs at the scapolite bands at 2 36 and 2 33 m but the analysis is complicated by uncertainty in the atmospheric co removal at these wavelengths weaker scapolite bands are seen at 2 44 and 2 23 m where there is virtually no atmospheric interference the scapolite bands observed on mars are due to hco3 and hso4 ions in the scapolite structure the bicarbonate and bisulfate contents appear to vary with location the scapolite in hellas is more bisulfate rich relative to that in the chryse moab eden area other locations contain little arabia syrtis major hellespontica and isidis or no scapolite e g margaritifer ausonia and erythraeum the calculated abundances are unconstrained because the amounts of hco3 and hso4 in the martian scapolites as well as their grain sizes are not known if the scapolites contain about 3 wt of each near the maximum possible the scapolite abundances probably range from about 5 wt scapolite at eden and hellas 3 5 at chryse moab and oxia palus 2 3 at arabia syrtis major and isidis to less than 2 at hellespontica syrtis minor and margaritifer assuming a relatively large grain size of 50 100 m if the characteristic grain sizes are smaller or the hco3 and hso4 contents are lower the scapolite abundances required to match the observed band depths would be higher the mineral bands are apparent in many of the mars spectra measured so it appears to be widely but not uniformly distributed the newly observed fine structure also varies greatly in both depth and spectral detail with location on mars thus there appears to be regional variations in composition the mineral phases appear to reflect local or regional geology and are not primarily contained in the homogeneous globally redistributed aeolian dust higher spectral resolution martian spectra in the 2 3 m region as well as at 3 9 m are needed to confirm the scapolite identification and to constrain its abundance

the high pressure synthesis of lawsonite in the morb h2o system lawsonite is an important water reservoir in subducting oceanic crust below the amphibole dehydration depth 70 km to determine the maximum pressure stability of lawsonite in the morb1h2o system experiments were carried out using a 1000 ton uniaxial multi anvil apparatus spi 1000 mixtures of synthetic gel12wt h2o were used for the starting materials with the average morb composition experimental p t conditions were t 5 700 900 8c and p 5 5 5 13 5 gpa run durations were 12 and 24 h lawsonite was synthesized stably up to 10 gpa and t 700 8c in the stishovite stability field and 900 8c at 8 gpa and 750 8c at 5 5 gpa in the coesite stability field with a steep positive slope for the lawsonite out reaction the lawsonite out reaction in the coesite stability field changes to have a gentle negative slope in the stishovite stability field the reaction leading to the disappearance of lawsonite is a continuous reaction due to the compositional enlargement of garnet toward the grossular end member with increasing t and p lawsonite disappears when the tie line connecting grossular rich garnet with omphacitic clinopyroxene reaches the bulk composition on the conventional ac fm ternary diagram

thermobarometry of phengite bearing eclogites in the dabie mountains of central china pressure temperature conditions for formation of the peak metamorphic mineral assemblages in phengite bearing eclogites from dabieshan have been assessed through a consideration of fe2 mg2 partitioning between garnet omphacite and garnet phengite pairs and of the reaction equilibrium celadonite pyrope grossular muscovite diopside which incorporates an evaluation of the extent of the strongly pressure dependent inverse tschermak s molecule substitution in the phengites for the latter equilibrium the calibration and recommended activity composition models indicated by waters martin 1993 have been employed and importantly yield results consistent with petrographic evidence for the stability at peak conditions of coesite in certain samples and quartz in others confirmation that in some phengite eclogite samples peak silicate mineral assemblages have equilibrated at confining pressures sufficient for the stability of coesite and in some cases even diamond rather negates previous suggestions that coesite may have been stabilized in only very localized possibly just intracrystalline domains inherent difficulties in the evaluation of peak metamorphic temperatures from fe2 mg2 partitioning between mineral phases due to uncertainties over fe3 fe2 ratios in the minerals especially omphacites and to re equilibration during extensive retrograde overprinting in some samples are also assessed and discussed our results indicate the existence in south central dabieshan of phengite eclogites with markedly different equilibration conditions within two structurally distinct tectonometamorphic terranes thus our data do not support earlier contentions that south central dabieshan comprises a structurally coherent continental crust terrane with a regional p t gradient signalling previous deepest level subduction in the north instead we recognize the central dabie ultra high pressure coesite eclogite bearing terrane to be structurally overlain by a southern dabie high pressure quartz eclogite bearing terrane at a major southerly dipping shear zone along which late orogenic extensional collapse appears to have eliminated at least 20 km of crustal section

the 1997 spring regression of the martian south polar cap mars orbiter camera observations the mars orbiter cameras moc on mars global surveyor observed the south polar cap of mars during its spring recession in 1997 the images acquired by the wide angle cameras reveal a pattern of recession that is qualitatively similar to that observed by viking in 1977 but that does differ in at least two respects the 1977 recession in the 0 to 120 longitude sector was accelerated relative to the 1997 observations after l s 240 the mountains of mitchel also detached from the main cap earlier in 1997 comparison of the moc images with mars orbiter laser altimeter data shows that the mountains of mitchel feature is controlled by local topography relatively dark low albedo regions well within the boundaries of the seasonal cap were observed to have red to violet ratios that characterize them as frost units rather than unfrosted or partially frosted ground this suggests the possibility of regions covered by co 2 frost having different grain sizes

seismic evidence for an ancient rift beneath the cumberland plateau tennessee a detailed analysis of broadband teleseismic p waveforms broadband receiver functions developed from teleseismic p waveforms recorded on the midperiod passband of regional seismic test network station rscp are inverted for vertical velocity structure beneath the cumberland plateau tennessee the detailed broadband receiver functions are obtained by stacking source equalizd horizontal components of teleseismic p waveforms the resulting receiver functions are most sensitive to the shear velocity structure near the station a time domain inversion routine utilizes the radial receiver function to determine this structure assuming a crustal model parameterized by many thin flat lying homogeneous layers lateral changes in structure are identified by examining azimuthal variations in the vertical structure the results reveal significant rapid lateral changes in the midcrustal structure beneath the station that are interpreted in relation to the origin of the east continent gravity high located northeast of rscp the results from events arriving from the northeast show a high velocity midcrustal layer not present in results from the southeast azimuth this velocity structure can be shown to support the idea that this feature is part of a keweenawan rift system another interesting feature of the derived velocity models is the indication that the crust mantle boundary beneath the cumberland plateau is a thick probably laminated transition zonemore between the depths of 40 and 55 km a result consistent with interpretations of early refraction work in the area less

pressure temperature path of sanbagawa prograde metamorphism deduced from grossular zoning of garnet the zonal structure of prograde garnet in pelitic schists from the medium grade garnet zone and the higher grade albite biotite zone was examined to investigate the evolution of prograde p t paths of the sanbagawa metamorphism the garnet studied shows a bell shaped chemical zoning of the spessartine component which decreases in abundance from the core towards the rim almandine and pyrope contents and xmg mg mg fe2 increase monotonously outwards the general scheme of the zonal structure for grossular content xgrs ca fe2 mn mg ca can be summarized as 1 xgrs increases outwards inner segment and reaches a maximum at an intermediate position between the crystal core and the rim then decreases towards the outermost rim outer segment 2 the inner segment of garnet in the garnet zone samples tends to have a higher xgrs xsps values for a given xsps than those in the albite biotite zone samples 3 average xsps at the maximum xgrs position in the albite biotite zone samples ranges from 0 02 to 0 12 and is lower than that in the garnet zone samples 0 13 0 32 4 the maximum xgrs in the albite biotite zone samples 0 34 0 39 on average tends to be higher than that in the garnet zone samples 0 26 0 36 and 5 differences of xgrs between the maximum and rim in the albite biotite zone samples are between 0 10 and 0 14 and higher than those in the garnet zone samples 0 11 these facts imply that albite biotite zone materials a were recrystallized under lower dp dt conditions at an early stage of the prograde metamorphism b began their exhumation under higher p t conditions and c have been continuously heated during exhumation for a longer duration than the garnet zone materials the systematic changes of prograde p t paths can be interpreted as documenting the evolution of the sanbagawa subduction zone

clinopyroxene a mineral telescoped through the processes of blueschist facies metamorphism textural evolution and compositional variation of clinopyroxenes in ward creek metabasites are described pyroxenes change with increasing grade from finegrained aggregates through fan shaped medium grained prisms to blocky coarse crystals characteristic features of metamorphic pyroxenes include 1 the occurrence of coexisting pyroxene pairs the compositions of which are used to delineate compositional gaps 2 the existence of large compositional variations of pyroxenes within a single specimen which record a considerable span of p and or t for crystallization and 3 the development of compositional trends in single specimens and in three metamorphic zones which are progressive in nature the first formed clinopyroxene jd20aug65ac15 in the lower lawsonite zone mimics the composition of relict igneous augite it changes continuously with increasing grade at nearly constant low xjd content towards acmite at a composition around jd20aug30ac50 the trend turns towards jadeite and intersects a solvus to form two coexisting clinopyroxenes in the middle lawsonite zone at higher grade the compositional gap becomes restricted towards the jadeite omphacite join and clinopyroxene increases in xjd toward jadeite a reversed compositional trend occurs at higher grade clinopyroxenes decrease in jadeite component at nearly constant aug ac ratio of 50 50 and finally become omphacite in the uppermost pumpellyite and epidote zones the na ca pyroxenes close to the binary join jd ac occur in the lawsonite and pumpellyite zones ranging from xjd 1 0 0 30 together with ab and qz the ubiquitous occurrence of aragonite at temperature estimates of 170 240 c by taylor coleman 1968 for these zones does not support the low temperature extrapolation of the jd ab qz curve by holland 1980 the estimated metamorphic field gradient indicates an inflection point at 7 kbar 200 c below this blueschist facies metamorphism proceeded under dominant pressure increase from 4 to 7 kbar at nearly constant temperature about 150 200 c whereas at higher grade recrystallization above the inflection point the metamorphic temperature increased from 200 to 350 c at nearly constant pressure about 7 8 kbar such an inflection point suggests the depth of underplating of either seamounts or accretionary packages in a subduction zone

coexisting chromian omphacite and diopside in tremolite schist from the chugoku mountains sw japan the effect of cr on the omphacite diopside immiscibility gap chromian clinopyroxenes with exsolution textures were found in high pressure hp tremolite schist from the osayama serpentinite melange in the chugoku mountains southwestern japan chromian omphacite jadeite jd 23 3 41 4 diopside di 46 7 60 7 kosmochlor ko 4 5 19 6 aegirine ae with up to 6 6 wt cr 2 o 3 occurs as neoblastic crystals in a foliated tremolite rich matrix or as pseudomorphs after relict chromian spinel and contains irregular shaped thin lamellae of chromian diopside jd 3 8 18 2 di 72 5 93 6 ko 1 1 13 9 ae chromian omphacite contains roughly constant jadeite plus kosmochlor components at 38 2 51 6 mol this is equivalent to the jadeite component of ordered p 2 n omphacite systematic analyses of coexisting chromian pyroxenes yield a clear immiscibility gap between omphacite and diopside the compositional gap becomes much narrower with increasing ko component addition of only 10 mol ko component narrows the omphacite diopside gap by an order of magnitude such an effect is similar to but more effective than the introduction of fe 3 on the omphacite diopside immiscibility gap chromian pyroxenes replacing relict chromian spinel are associated with other chromian silicates including phengite chlorite and pumpellyite the wide compositional gap of chromian pyroxenes and the presence of chromian pumpellyite and si rich chromian phengite indicate t p 0 8 gpa this p t estimate is consistent with parageneses of minerals in the host serpentinite the variation of cr content in chromian silicates reflects the extent of cr al substitution and may be related to a chemical heterogeneity of the cr bearing fluid

a cold early palaeozoic subduction zone in the north qilian mountains nw china petrological and u pb geochronological constraints the north qilian high pressure hp low temperature lt metamorphic belt is composed mainly of blueschists eclogites and greenschist facies rocks it formed within an early palaeozoic accretionary wedge associated with the subduction of the oceanic crust and is considered to be one of the best preserved hp lt metamorphic belts in china here we report new lawsonite bearing eclogites and eclogitic rocks enclosed within epidote blueschists in the north qilian mountains five samples contain unaltered lawsonite coexisting with omphacite and phengite as inclusions in garnet indicating eclogite facies garnet growth and lawsonite pseudomorphs were observed in garnet from an additional 11 eclogites and eclogitic rocks peak pressure conditions estimated from lawsonite omphacite phengite garnet assemblages were 2 1 2 4 gpa at temperatures of 420 510 c in or near the stability field of lawsonite eclogite and implying formation under an apparent geothermal gradient of 6 8 c km 1 consistent with metamorphism in a cold subduction zone shrimp u pb dating of zircon from two lawsonite bearing eclogitic metabasites yields ages of 489 7 ma and 477 16 ma respectively cl images and mineral inclusions in zircon grains indicate that these ages reflect an eclogite facies metamorphism an age of 502 16 ma is recorded in igneous cores of zircon grains from one lawsonite pseudomorph bearing eclogite which is in agreement with the formation age of early ordovician for some ophiolite sequences in the north qilian mountains and may be associated with a period of oceanic crust formation the petrological and chronological data demonstrate the existence of a cold early palaeozoic subduction zone in the north qilian mountains

petrology of ultrahigh pressure rocks from the southern su lu region eastern china abstract in the su lu ultrahigh p terrane eastern china many coesite bearing eclogite pods and layers within biotite gneiss occur together with interlayered metasediments now represented by garnet quartz jadeite rock and kyanite quartzite in addition to garnet omphacite rutile coesite other peak stage minerals in some eclogites include kyanite phengite epidote zoisite talc nyboite and high al titanite the garnet quartz jadeite rock and kyanite quartzite contain jadeite quartz garnet rutile zoisite apatite and quartz kyanite garnet epidote phengite rutile omphacite assemblages respectively coesite and quartz pseudomorphs after coesite occur as inclusions in garnet omphacite jadeite kyanite and epidote from both eclogites and metasediments study of major elements indicates that the protolith of the garnet quartz jadeite rock and the kyanite quartzite was supracrustal sediments most eclogites have basaltic composition some have experienced variable crustal contamination or metasomatism and others may have had a basaltic tuff or pyroclastic rock protolith the su lu ultrahigh p rocks have been subjected to multi stage recrystallization and exhibit a clockwise p t path inclusion assemblages within garnet record a pre eclogite epidote amphibolite facies metamorphic event ultrahigh p peak metamorphism took place at 700 890 c and p 28 kbar at c 210 230 ma the symplectitic assemblage plagioclase hornblende epidote biotite titanite implies amphibolite facies retrogressive metamorphism during exhumation at c 180 200 ma metasedimentary and metamafic lithologies have similar p t paths several lines of evidence indicate that the supracrustal rocks were subducted to mantle depths and experienced in situ ultrahigh p metamorphism during the triassic collision between the sino korean and yangtze cratons

late cretaceous blueschist facies metamorphism in southern thrace turkey and its geodynamic implications a blueschist facies tectonic sliver 9 km long and 1 km wide crops out within the miocene clastic rocks bounded by the strands of the north anatolian fault zone in southern thrace nw turkey two types of blueschist facies rock assemblages occur in the sliver i a serpentinite body with numerous dykes of incipient blueschist facies metadiabase ii a well foliated and thoroughly recrystallized rock assemblage consisting of blueschist marble and metachert both are partially enveloped by an upper eocene wildflysch which includes olistoliths of serpentinite metadiabase upper cretaceous and palaeogene pelagic limestone upper eocene reefal limestone radiolarian chert quartzite and minor greenschist field relations in combination with the bore core data suggest that the tectonic sliver forms a positive flower structure within the miocene clastic rocks in a transpressional strike slip setting and represents an uplifted part of the pre eocene basement the blueschists are represented by lawsonite glaucophane bearing assemblages equilibrated at 270 310 c and 0 8 gpa the metadiabase dykes in the serpentinite on the other hand are represented by pumpellyite glaucophane lawsonite assemblages that most probably equilibrated below 290 c and at 0 75 gpa one metadiabase olistolith in the upper eocene flysch sequence contains the mineral assemblage epidote pumpellyite glaucophane recording p t conditions of 290 350 c and 0 65 0 78 gpa indicative of slightly lower depths and different thermal setting timing of the blueschist facies metamorphism is constrained to c 86 ma coniacian santonian by rb sr phengite whole rock and incremental 40 ar 39 ar phengite dating on blueschists the activity of the strike slip fault post dates the blueschist facies metamorphism and exhumation and is only responsible for the present outcrop pattern and post miocene exhumation 2 km the high p t metamorphic rocks of southern thrace and the biga peninsula are located to the southeast of the circum rhodope belt and indicate late cretaceous subduction and accretion under the northern continent i e the rhodope massif enveloped by the circum rhodope belt the late cretaceous is therefore a time of continued accretionary growth of this continental domain