paragenesis of sodic pyroxene bearing quartz schists implications for the p t history of the sanbagawa belt sodic pyroxene jadeite content x jd 0 1 0 3 occurs locally as small inclusions within albite porphyroblasts and in the matrix of hematite bearing quartz schists in the sanbagawa sambagawa metamorphic belt central shikoku japan the sodic pyroxene bearing samples are characteristically free from chlorite and their typical mineral assemblage is sodic pyroxene subcalcic or sodic amphibole phengitic mica albite quartz hematite titanite epidote spessartine rich garnet occurs in mn rich samples sodic pyroxene in epidote bearing samples tends to be poorer in acmite content average x acm 0 26 0 50 than that in the epidote free samples x acm 0 45 0 47 x jd shows no systematic relationship to metamorphic grade and is different among the three sampling regions saruta gawa asemi gawa and bessi besshi the average x jd of the saruta gawa samples 0 21 0 29 is higher than that of the asemi gawa 0 13 0 17 and bessi 0 14 0 23 the p t conditions of the asemi gawa and bessi regions are estimated at 5 5 6 5 kbar 360 c in the chlorite zone 7 8 5 kbar 440 15 c in the garnet zone and 8 9 5 kbar 520 25 c in the albite biotite zone metamorphic pressure of the saruta gawa region is systematically 1 1 5 kbar higher than that of the asemi gawa and bessi regions and materials of the saruta gawa region have been subducted to a level 3 5 km deeper than materials that underwent metamorphism at equivalent temperatures and are now exposed in the asemi gawa and bessi regions pressure slightly increases toward the north structurally high levels through the sanbagawa belt of central shikoku two types of zonal structure were observed in relatively coarse grained sodic pyroxenes in the matrix one type is characterized by increasing x jd from core to rim the other type by decreasing x jd from core to rim both types of zoned pyroxenes show an increase in x fe 2 fe2 fe2 mg from core to rim the first type of zoning was observed in a sample from the chlorite zone of lowest grade whereas the latter occurs in the garnet and albite biotite zones of higher grade the contrast in zonal structure implies that dp dt during prograde metamorphism decreased with increasing metamorphic grade and may have been negative in some samples from the higher grade zones the estimated dp dt of the prograde stage of the chlorite zone is 3 2 kbar 100 c and that of the garnet and albite biotite zones is 1 8 to 0 9 kbar 100 c the variation of dp dt at shallow and deep levels of a subduction system probably reflects the difference of heating duration and or change in thermal gradient of the subduction zone by continuous cooling of the surrounding mantle

pumpellyites and coexisting minerals in different low grade metamorphic facies of liguria italy pumpellyite from four phase assemblages pumpellyite epidote prehnite chlorite pumpellyite epidote actinolite chlorite pumpellyite epidote na amphibole chlorite together with common excess phases considered to be low variance in a cao mgo feo al2o3 fe2o3 na2o sio2 h2o system have been examined in areas which underwent metamorphism in the prehnite pumpellyite pumpellyite actinolite and low temperature blueschist facies respectively the analysed mineral assemblages are compared for nearly constant basaltic chemical composition at varying metamorphic grade and for varying chemical composition basic intermediate acidic at constant metamorphic conditions low temperature blueschist facies in the studied mineral assemblages coexisting phases approached near chemical equilibrium at constant basaltic bulk rock composition the mgo content of pumpellyite increases and the xfe3 of both pumpellyite and epidote decreases with increasing metamorphic grade the fe3 being preferentially concentrated in epidote both pumpellyite and epidote compositions vary with the bulk rock composition at isofacial conditions pumpellyite becomes progressively enriched in fe and depleted in mg from basic to intermediate and acidic bulk rock compositions the compositional comparison of pumpellyites from high variance 1 3 phases assemblages in various bulk rock compositions basic intermediate acidic rocks greywackes gabbros shows that the compositional fields of both pumpellyite and epidote are wide and variable broadly overlapping the compositional effects observed at varying metamorphic grade in low variance assemblages the intrinsic stability of both fe and al rich pumpellyites extends across the complete range of the considered metamorphic conditions element partitioning between coexisting phases is the main control on the mineral composition at different p t conditions

natural deformation fabrics of plagioclase implications for slip systems and seismic anisotropy abstract the optical and tem microstructures and fabrics of twelve naturally deformed plagioclase containing rocks of compositions from an0 an80 have been studied with a view to determining the deformation mechanisms the optical indicatrix fabrics have been recorded in all samples with respect to the foliation and lineation in general and form a girdle in the foliation plane with a high concentration of normal to the foliation in some cases the orientation of 010 and 001 cleavages or albite and pericline twins has allowed the construction of pole figures in all cases 010 has been shown to be the dominant slip plane with either 001 or 100 the dominant slip direction tem shows that the dominant slip system is 010 001 with secondary 010 100 the seismic p wave anisotropy has been calculated for the first time from the lpo data of plagioclase the anisotropy coefficient is high at around 15 the minimum p velocity is normal to the lineation in the foliation plane and the maximum is sub parallel to the foliation normal the combined information on deformation kinematics and seismic anisotropy is valuable for interpretations of deep crustal seismic sections

an evaluation of criteria to deduce the sense of movement in sheared rocks some of the most useful criteria for the deduction of the sense of shear are summarized for use in areas where unequivocal field evidence is lacking apparently conflicting evidence from rotated pressure shadow regions around porphyroclasts and porphyroblasts is clarified the use of quartz crystallographic fabric asymmetry to deduce the shear sense in the bulk rock should be treated with caution and used only together with detailed microstructural observations

metamorphic evolution of the susunai metabasites in southern sakhalin russian republic the susunai complex of southeast sakhalin represents a subduction related accretionary complex of pelitic and basic rocks two stages of metamorphism are recognized 1 a local low p t event characterized by si poor calcic amphiboles 2 a regional high p t event characterized by pumpellyite actinolite epidote sodic amphibole sodic pyroxene stilpnomelane and aragonite the major mineral assemblages of the high p t susunai metabasites contain pumpellyite epidote actinolite chlorite epidote actinolite chlorite epidote na amphibole na pyroxene chlorite haematite the na amphibole is commonly magnesioriebeckite the na pyroxene is jadeite poor aegirine to aegirine augite application of empirically and experimentally based thermobarometers suggests peak conditions of t 250 300c p 4 7 6 kbar textural relationships in susunai metabasite samples and a petrogenetic grid calculated for the fe3 rich basaltic system suggest that pressure and temperature increased during prograde metamorphism

the influence of crystallographic orientation and grain boundary migration on microstructural and textural evolution in an s c mylonite abstract the analysis of the subgrain and dislocation sub structure present in grains in different crystallographic orientations within a domainal s c mylonite is reported the study reveals a range in subgrain size and development which is related to the crystal orientation of the host grain in the mylonite subgrains in grains with easy slip orientations contain large subgrains while grains in more complex slip orientations contain small subgrains the dislocation densities preserved in the mylonite are extremely variable even within individual subgrains and may be the result of a late stress pulse the analysis indicates that grain boundary migration was an important process in the microstructural evolution initiating the selective removal of some orientations and thus influencing the final crystallographic preferred orientation present in different domains the data collected is also used to assess the potential stability and preservation of microstructures during the non steady state deformation expected at the end of a deformation event

blueschist facies metamorphism shear heating and p t t paths in subduction shear zones the low temperatures recorded by blueschist facies metamorphic rocks place upper bounds on the magnitude of shear stresses in subduction zones at depths of 15 50 km in this paper steady state and transient pressure temperature time p t t paths followed by subducted material are calculated using analytical expressions presented by molnar and england 1990 supplemented by two dimensional numerical calculations in the absence of shear heating calculated steady state p t t paths are very cold intersecting 1 gpa 10 kbar at t 200 c for a wide range of subduction parameters for the case of constant shear stress in the subduction shear zone and typical thermal parameters steady state p t t paths intersect the blueschist facies for 10 60 mpa and 0 100 mpa for convergence rates of 10 and 3 cm yr 1 respectively blueschist facies metamorphism will not occur in rapid subduction zones if shear stresses exceed 60 mpa for the case of shear stress increasing linearly with depth steady state p t t paths intersect the blueschist facies when 1 9 p and 0 14 p for convergence rates of 10 and 3 cm yr 1 respectively subduction zone shear stresses may be considerably lower and possibly zero if blueschist terrains represent material metamorphosed during the early stages of subduction first 5 10 m y while the thermal structure is still evolving or if maximum temperatures in blueschist terrains are achieved during exhumation subduction zone shear stresses of several tens of mpa at 35 km depth or a few percent of lithostatic pressure are consistent with a steady state model of franciscan metamorphism a progressive underplating model for the formation of the pelona schist inverted metamorphic gradient and surface heat flow measurements in the ne japan subduction zone

water transportation from the subducting slab into the mantle transition zone using a recently developed petrogenetic grid for morb h2o we propose a new model for the transportation of water from the subducting slab into the mantle transition zone depending on the geothermal gradient two contrasting water transportation mechanisms operate at depth in a subduction zone if the geothermal gradient is low lawsonite carries h2o into mantle depths of 300 km with further subduction down to the mantle transition depth approximately 400 km lawsonite is no longer stable and thereafter h2o is once migrated upward to the mantle wedge then again carried down to the transition zone due to the induced convection at this depth hydrous phase olivine is stable and plays a role as a huge water reservoir in contrast if the geothermal gradient is high the subducted slab may melt at 700 900 c at depths shallower than 80 km to form felsic melt into which water is dissolved in this case h2o cannot be transported into the mantle below 80 km between these two endmember mechanisms two intermediate types are present in the high pressure intermediate type the hydrous phase a plays an important role to carry water into the mantle transition zone water liberated by the lawsonite consuming continuous reaction moves upward to form hydrous phase a in the hanging wall which transports water into deeper mantle this is due to a unique character of the reaction because phase a can become stable through the hydration reaction of olivine in the case of low pressure intermediate type the presence of a dry mantle wedge below 100 km acts as a barrier to prevent h2o from entering into deeper mantle 2006 published by elsevier b v on behalf of international association for gondwana research

the stability of ca na pyroxene in low grade metabasites of high pressure intermediate facies series minor metamorphic ca na pyroxene occurs as minute crystals and as thin rims around primary augites in metabasites intermediate between the prehnite pumpellyite pp and pumpellyite actinolite pa facies in the sanbagawa belt shikoku japan their compositions vary systematically with mineral paragenesis from augite to acmite with a constant jadeite component of about 20 molvo the ca na pyroxenes in the pumpellyite bearing assemblages contain a very low acmite component acaaugssjd2s which increases through ac25aug55jd2s in the alkali amphibole bearing and ac66augzsjdrs in the hematite bearing assemblages finally attaining an aegirine composition ac65aug1sjd25 in the magnetite bearing assemblages they are distinguished from relict igneous augites chemically by very low opx and tio2 contents and morphologically by fibrous crystal habit the pyroxene bearing metabasites contain assemblages of chlorite pyroxene pumpellyite epidote actinolite alkali amphibole stilpnomelane hematite and magnetite which are incompatible with the higher grade and widely developed pumpellyiteactinolite facies assemblage epidote chlorite xr 0 50 actinolite pumpellyite the reaction augite chlorite pumpellyite actinolite h2o has been suggested for such a transition the stability relations of augite chlorite pumpellyite were analyzed from comparative mineral parageneses in low grade metabasites from new caledonia new zealand and the appalachians the assemblage may have a wedge shaped p t space bounded against the high tpumpellyite actinolite facies by the above reaction and against the low t prehnite pumpellyite facies by the reaction prehnite actinolite augite chlorite with probable negative slope from the mineral parageneses in the metabasites of the sanbagawa belt and elsewhere it is concluded that the ca na pyroxenes occur within a restricted p t space between the prehnite pumpellyite and pumpellyite actinolite facies

low pressure and low temperature k bearing kosmochloric diopside from the osayama serpentinite melange sw japan kosmochloric diopside with high k content up to 0 56 wt 0 026 k atom per formula unit was discovered from the osayama serpentinite melange in the chugoku mountains sw japan k bearing clinopyroxene fills microcracks 5 150 m in wide together with uvarovite within albite vein of a tremolite rock compositions of analyzed clinopyroxene consist mainly ofkosmochlor augite 92 98 mol ko 1 9 3 8 aug 5 6 7 6 components and minor amounts of jadeite 0 6 mol aegirine 0 5 mol ca tschermak 0 3 mol and k kosmochlor 0 2 mol although the k content in clinopyroxene is also variable and heterogeneous even in a single vein clinopyroxene with higher k content occurs in ko rich part higher magnification secondary electron images confirmed that exsolution and inclusion are essentially absent in the analyzed clinopyroxenes the good negative correlation between cr na k and ca mg fe 2 indicates the cr incorporation into the octahedral site furthermore k correlates with na and cr indicating a simultaneous enrichment of k for na and cr during pyroxene growth textual relations and parageneses and compositions of minerals suggest that the k cpx precipitated together with uvarovite in brittle microcracks directly from a ca and cr rich hydrothermal fluid at approximately p 4 gpa environment permits to host relatively large k cation into the clinopyroxene structure our finding indicates that the incorporation of k into the kosmochlor diopside series solid solution with at least 0 2 cr cation p f u is possible even at low p conditions

wave coupling between the lower and middle thermosphere as viewed from timed and goce vertical coupling between the lower and middle thermosphere due to the eastward propagating diurnal tide with zonal wave number 3 de3 and the 3 5 day ultra fast kelvin wave ufkw is investigated using thermosphere ionosphere mesosphere energetics and dynamics sounding of the atmosphere using broadband emission radiometry timed saber temperatures near 100 km and gravity field and steady state ocean circulation explorer goce neutral densities and zonal winds near 260 km the analysis is performed between 45 latitude during 2011 when reliable and continuous measurements are available with geomagnetic and solar effects removed de3 and the ufkw are identified as dominant sources of day to day variability at both heights evidence is found for the vertical propagation of de3 and the ufkw from the lower to middle thermosphere over a range of time scales over 60 of the variance due to de3 and the ufkw at 260 km is traceable to variability occurring at 100 km the not exact agreement is thought to be due to the influences of wave wave interactions zonal mean winds dissipation and inherent transience that interfere with one to one mapping of structures between 100 and 260 km spectral and temporal analyses of the saber and goce data also reveal the presence of sidebands due to the modulation of de3 by the ufkw these secondary waves are responsible for up to 10 to 20 of the longitudinal and day to day variability overall vertical propagating waves together with sidebands from de3 ufkw nonlinear interactions are responsible for 60 to 80 of the total variability while geomagnetic and solar effects correlated with ap and f10 7 account for less than 20 of the variance

microstructural features of high temperature shear zones in gabbros of the northern apennine ophiolites abstract this paper presents preliminary data on the microstructural evolution of upper amphibolite facies mylonitic metagabbros from the bracco ophiolite complex northern apennines italy high temperature deformation possibly related to the initial stages of breakup and oceanization in the ligurian tethys led to the development of three different types of fabrics in the coarse grained mg gabbros the most widespread rock type in this area with progressive deformations coarse grained porphyroclastic mylonites fabric type 1 banded fabric type 2 and flinty ultramylonites fabric type 3 were formed in plagioclase and clinopyroxene the two main minerals in the studied rocks optical microstructures and scanning electron microscopy analyses indicate crystal plasticity as the main deformation mechanism a possible change from rotation recrystallization regime through grain boundary migration to diffusion assisted high temperature grain boundary sliding is suggested for the plagioclase grains in high temperature shear zones

oblique fabrics in porphyroclastic alpine type peridotites a shear sense indicator for upper mantle flow abstract a detailed structural study in alpine type peridotites of the ultramafic voltri massif in nw italy reveals the existence of oblique fabrics in porphyroclastic tectonites the dominant foliation of these tectonites is defined by stretched pyroxenes and aligned grains of spinel which formed as a result of high temperature flow in a km scale shear zone bounded by a wall rock of granular virtually undeformed spinel lherzolites the oblique fabrics in these tectonites are characterized by elongate olivine grain shapes and a marked grain boundary alignment oriented at an angle of up to 40 to the tectonite foliation in addition kink type subgrain boundaries are frequently parallel to 100 and show a preferred orientation at large angles to the tectonite foliation olivine lattice preferred orientation patterns in tectonites with this oblique grain shape fabric show bimodal distributions of 100 and 001 the oblique fabrics and bimodal orientation distributions are inferred to result from the formation of high angle kink like grain boundaries followed by deformation induced grain boundary migration at the expense of unfavourably oriented grains the sense of shear derived from the oblique fabrics is consistent with the sense of shear derived from the lattice preferred orientation patterns asymmetric pyroxene porphyroclast systems and the map scale geometry of the shear zone in which these microstructures occur it is therefore suggested that these oblique grain shape fabrics represent a reliable kinematic indicator for high temperature flow in upper mantle shear zones

parageneses and compositions of amphiboles from franciscan jadeite glaucophane type facies series metabasites at cazadero california sodic amphiboles are common in franciscan type ii and type iii metabasites from cazadero california they occur as 1 vein fillings 2 overgrowths on relict augites 3 discrete tiny crystals in the groundmass and 4 composite crystals with metamorphic ca na pyroxenes in low grade rocks they become coarse grained and show strong preferred orientation in schistose high grade rocks in the lowest grade only riebeckite to crossite appears with increasing grade sodic amphibole becomes first enriched in glaucophane component later coexists with actinolite and finally at even higher grade becomes winchite actinolite first appears in foliated blueschists of the upper pumpellyite zone it occurs 1 interlayered on a millimetre scale with glaucophane prisms and 2 as segments of composite amphibole crystals actinolite is considered to be in equilibrium with other high pressure phases on the basis of its restricted occurrence in higher grade rocks textural and compositional characteristics and fe mg distribution coefficient between actinolite and chlorite detailed analyses delineate a compositional gap for coexisting sodic and calcic amphiboles at the highest grade winchite appears at the expense of the actinolite glaucophane pair compositional characteristics of franciscan amphiboles from ward creek are compared with those of other high p t facies series the amphibole trend in terms of major components is very sensitive to the metamorphic field gradient na amphibole appears at lower grade than actinolite along the higher p t facies series e g franciscan and new caledonia whereas reverse relations occur in the lower p t facies series e g sanbagawa and new zealand available data also indicate that at low temperature conditions such as those of the blueschist and pumpellyite actinolite facies large compositional gaps exist between ca and na amphiboles and between actinolite and hornblende whereas at higher temperatures such as in the epidote amphibolite greenschist and eclogite facies the gaps become very restricted common occurrence of both sodic and calcic amphiboles and ca na pyroxene together with albite quartz in the ward creek metabasites and their compositional trends are characteristic of the jadeite glaucophane type facies series in new caledonia blueschists ca na pyroxenes are also common na amphiboles do not appear alone at low grade in metabasites instead na amphiboles coexist with ca amphiboles throughout the progressive sequence however for metabasites of the intermediate pressure facies series such as those of the sanbagawa belt japan and south island new zealand ca na pyroxene and glaucophane are not common sodic amphiboles are restricted to crossite and riebeckite in composition and clinopyroxenes to acmite and sodic augite and occur only in fe2o3 rich metabasites the glaucophane component of na amphibole systematically decreases from ward creek new caledonia through sanbagawa to new zealand this relation is consistent with estimated pressure decrease employing the geobarometer of maruyama et al 1986 similarly the decrease in tschermakite content and increase in nam4 of ca amphiboles from new zealand through sanbagawa to new caledonia is consistent with the geobarometry of brown 1977b therefore the difference in compositional trends of amphiboles can be used as a guide for p t detail within the metamorphic facies series

isotopic evidence for channeled fluid flow in low grade metamorphosed jurassic accretionary complex in the northern chichibu belt western shikoku japan the low grade metamorphosed jurassic accretionary complex of the northern chichibu belt hijikawa area western shikoku is divided into two units the hijikawa and kanogawa units that are separated by an out of sequence thrust oost the ozu kawabegawa fault the kanogawa unit south of the ozu kawabegawa fault consists mainly of sandstone shale broken formation of alternating sandstone and shale greenstone chert and pelitic melange while the hijikawa unit is characterized by a stack of subunits separated by small scale thrusts the subunits are mainly made up of basic pelitic and psammitic semischists schistose pelitic melange and chert petrological and mineralogical constraints suggest peak metamorphic conditions of 204 247 c at 1 3 kbar in the kanogawa unit and 228 289 c at 3 5 6 kbar in the hijikawa unit quartz and quartz calcite veins are widely developed in the study area especially in the hijikawa unit regional variations in stable isotopic data show that the 18oquartz and 18ocalcite values in veins tend to increase towards the ozu kawabegawa fault the 18owhole rock values are remarkably high in some subunits close to oosts within the hijikawa unit oxygen isotopic compositions from vein quartz indicate that a higher 18o fluid migrated upward from depth along the ozu kawabegawa fault within the accretionary prism during prehnite pumpellyite facies metamorphism the fluid source is inferred to be pelitic rocks at higher temperatures and greater depths within the accretionary prism

a newly discovered high pressure terrane in eastern hokkaido japan the upper jurassic nikoro greenstone complex of eastern hokkaido suffered high pressure intermediate type metamorphism characteristic minerals include lawsonite aragonite sodic pyroxene of the aegirinejadeite series winchite sodic amphibole of the glaucophane riebeckite series pumpellyite epidote and actinolite high pressure metamorphism of the nikoro greenstone complex is related to subduction of the kula plate toward the palaeo okhotsk land during cretaceous time

low grade metamorphism of the mikabu and northern chichibu belts in central shikoku sw japan implications for the areal extent of the sanbagawa low grade metamorphism in central shikoku sw japan the mikabu belt is bounded to the north by the sanbagawa belt and to the south by the northern n chichibu belt the n chichibu belt can be further subdivided into northern and southern parts there is no apparent difference in the overall geology structure or fossil and radiometric ages between the mikabu belt and the northern part of the n chichibu belt greenstones from the mikabu belt and the northern part of the n chichibu belt show evidence for similar low grade metamorphism and include the following mineral assemblages with albite chlorite in excess metamorphic aragonite sodic pyroxene quartz epidote actinolite pumpellyite glaucophane pumpellyite quartz and lawsonite not with actinolite or glaucophane these similarities suggest that the mikabu belt and the northern part of the n chichibu belt belong to the same geological unit the mb nnc complex the mineral assemblages also indicate that the mb nnc complex belongs to a different metamorphic facies from the low grade part of the sanbagawa belt that is the former represents lower temperature higher pressure conditions than the latter structural and petrological continuity between the mb nnc complex and sanbagawa belt has not yet been confirmed but both have similar radiometric ages it is therefore most likely that the mb nnc complex and sanbagawa belt belong to the same subduction complex and were metamorphosed under similar but distinct conditions these two units were juxtaposed during exhumation in contrast the southern part of the n chichibu belt is distinct in lithology and structure and includes no mineral assemblages diagnostic of the mb nnc complex and the sanbagawa belt thus the southern part of the n chichibu belt may represent a different geological unit from the mb nnc complex and sanbagawa belt

dynamic recrystallization of feldspar a mechanism for ductile shear zone formation feldspar aggregates experimentally deformed in the dislocation creep regime undergo dynamic recrystallization because recovery is difficult due to the limited climb of dislocations recrystallizing aggregates have a lower strength because of the cyclic production of small strain free grains and they develop a strong preferred orientation consistent with that observed in mylonites thus recrystallization accommodated dislocation creep may be responsible for the grain size reduction and strain softening that lead to the formation of many mylonites and ductile shear zones

evolution of oceanic gabbros from dsdp leg 82 influence of the fluid phase on metamorphic crystallizations abstract gabbro breccias were recovered from an anomalously shallow level of the ocean crust during dsdp leg 82 the rocks display evidence of metamorphic crystallization related either to localized deformation or to hydrothermal circulation of a seawater derived fluid under static conditions secondary phases consist of plagioclase amphibole and minor clinopyroxene ilmenite sphene and chlorite petrological study indicates that deformation took place at high temperature under anhydrous conditions and was followed by hydrothermal circulation the compositions of secondary minerals i e strong zonations presence of chlorine in amphiboles varying compositions of secondary plagioclase indicate that reactions of the gabbros with the fluids occurred at a low water rock ratio relations between cl na and k in amphiboles suggest penetration of at least two distinct fluids of different compositions metamorphic crystallization stopped when greenschist facies conditions were reached 350 c probably because hydrothermal circulation faded out

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

the geodynamic evolution of the alpine orogen in the cyclades aegean sea greece insights from diverse origins and modes of emplacement of ultramafic rocks

accreted oceanic materials in japan abstract the phanerozoic circum pacific orogenic belts contain numerous ocean derived materials accreted through plate converging processes japanese islands in particular display various kinds of oceanic materials of different origins including fragments of seamounts oceanic reef limestone morb like rocks and oceanic mantle and pelagic sediments the compilation of these rocks in many subduction complexes of late permian to the present led to following conclusions accretion processes work effectively only for materials primarily composing the upper portion of subducting oceanic crust i e layer 1 and layer 2 many fragments of seamount with alkali basalt 600 hot spot seamount 26 oceanic reef limestone 291 morb like basalt 200 and numerous cherts more than 1000 are recognized as ancient oceanic materials accreted to the japanese islands however gabbros and mantle materials of layer 3 and lower parts of the oceanic lithosphere scarcely occur in subduction accretion complexes except for a few examples of back arc basin or fore arc origin accretion occurs episodically in southwest japan oceanic materials were accreted intermittently in 1 a end permian 2 b middle late jurassic 3 c late cretaceous times 4 d at ca 50 ma and 5 e in miocene times while in northeast japan and hokkaido this occurred in b middle late jurassic c late cretaceous and f early cretaceous times in contrast to the general belief on accretion of younger oceanic plates the majority of japanese subduction accretion complexes were formed during the subduction of plates up to 160 ma old the accretionary events in end permian and middle late jurassic times coincide with northward collision of ancient island arcs oceanic rises or seamount chains of hot spot origin with the asian continent accretion relevant to subduction of older plates may be controlled by the collision subduction process of these topographic reliefs on an oceanic plate in addition the chronological coincidence with the continent collision amalgamation between the sino korean and siberian platforms and between the sino korean and yangtze blocks also implies collision induced voluminous supply of elastics from back arc regions and its contribution to the formation of huge accretionary complexes accreted fragments of ancient seamounts are much smaller than the average size of modern seamounts this implies that most parts of a colliding seamount are not accreted but subducted together with the underlying oceanic crust to much deeper levels with respect to the metamorphic grades for japanese subduction complexes oceanic materials have less than 1 vol in the zeolite facies 15 20 in the prehnite pumpellyite metagraywacke facies and ca 30 in the greenschist glaucophane schist facies and albite epidote amphibolite fades this relationship indicates that the major process for landward accretion of oceanic materials is not off scraping or sedimentary mixing at the trench but underplating subcretion at much deeper levels of a subduction zone

archean ocean floor metamorphism in the north pole area pilbara craton western australia abstract archean greenstones ca 3 49 3 46 ga in the north pole dome pilbara craton western australia have undergone prograde metamorphism from prehnite pumpellyite facies to greenschist amphibolite facies observations of 3000 thin sections shows that a metamorphic aureole was restricted to within a few kilometers of the contact with the 3459 ma north pole monzogranite three types of alteration were observed carbonate silicic and chlorite types the distribution of the three types of alteration suggests that secondary chemical migration of elements occurred in the whole area but that some samples were unaffected by this alteration the southern north pole area is outside of the contact metamorphic aureole and is composed of five units from north unit i to south unit v dipping south about 30 70 metamorphism of the north pole greenstones corresponds to the low pressure type which is broadly comparable to that inferred for ocean floor metamorphism prehnite pumpellyite pp transitional tr and greenschist gs facies prograde metamorphic zones occur from structural top to bottom within each of units metamorphic grade increases downward through the units and metamorphic isograds never cut unit boundaries

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