crustal velocity structure of the omineca belt southeastern canadian cordillera travel time inversion and amplitude modeling of a 350 km lithoprobe seismic refraction wide angle reflection profile determined the velocity structure of the crust and upper mantle along strike in the omineca belt of the canadian cordillera the upper crust to 12 18 km depth has velocities from 5 6 to 6 2 km s 1 and two shear zones the monashee decollement and gwillim creek shear zone are imaged by the wide angle reflections and velocity trends minor velocity differences on either side of the monashee decollement may be related to separate rock origins prominent reflections define the boundaries of a low velocity midcrustal layer from 10 15 km to 20 25 km depth with velocities less than 6 1 km s 1 the low velocities of the midcrust associated with high electrical conductivities and high heat flow may be considered as support for the hypothesis of fluids in the cordilleran crust though other possibilities such as the effect of high temperatures on rock velocities are possible in the lower crust velocities range from 6 4 6 5 km s 1 at the top of the lower crust to 6 6 6 8 km s 1 at its base the moho is very clearly defined by the refraction wide angle reflection data and has a gentle southerly dip crustal thicknesses are 35 37 km a thin crust mantle transition zone of 1 2 km thickness in which velocities vary between 7 6 and 7 7 km s 1 is consistent with coincident reflection data upper mantle velocities range from 7 9 to 8 1 km s 1 with indications from the data of upper mantle layering in comparison with neighboring regions the omineca belt has an anomalously thin crust low crustal velocities and a low velocity upper mantle similar only to the basin and range province the velocity structure may partly mirror the temperature profile which has overprinted the geological signature of the region as measured by the seismic refraction method the characteristics of a thin crust and lithosphere along with low velocities from midcrust to mantle suggests that both the basin and range and the southern canadian cordillera are currently being heated from a source within the mantle

graphitization of carbonaceous matter during metamorphism with references to carbonate and pelitic rocks of contact and regional metamorphisms japan this study is an attempt to correlate the graphitization process of carbonaceous matter during metamorphism with metamorphic grade graphitization can be parameterized using crystal structure and chemical and isotopic compositions the extent of graphitization could be characterized mainly by temperature duration of metamorphism and rock composition we compared the graphitization trends for two metamorphic terrains a contact aureole of the kasuga area and a regional metamorphic terrain of high temperature low pressure type of the ryoke metamorphic terrain in northern kiso area central japan and for two different lithologies carbonate and pelite using x ray diffractogram dta tg analysis and chemical and stable isotope analyses during contact metamorphism graphitization and carbon isotopic exchange reactions proceeded simultaneously in pelitic and carbonate rocks the decreases in basal spacing d 002 of the carbonaceous matter in carbonate rocks is greatly accelerated at temperatures higher than about 400 c furthermore carbon isotopic ratios of graphite in carbonate rocks also change to 13c enriched values implying exchange with carbonates the beginning of this enrichment of 13c in the carbonaceous matter coincides with an abrupt increase of the graphitization processes carbon isotopic shifting up to 5 in pelites could be observed as metamorphic temperature increased probably by about 400 c carbonaceous matter in pelitic rocks is sometimes a mixture of poorly crystallized organic matter and well crystallized graphite detritus dta tg analysis is an effective tool for the distinction of detrital graphitic material two sources for the original carbon isotopic composition of carbonaceous matter in pelites in the kasuga contact aureole can be distinguished about 28 and 24 regardless of the presence of detrital graphite and were mainly controlled by depositional environment of the sediments graphitization in limestones and pelitic rocks in regional metamorphism proceeds further than in a contact aureole in the low temperature range the differences in extent of graphitization between the two metamorphic regions is large however at temperatures higher than 600 c the extent of graphitization in both regions is indistinguishable the degree of graphitization is different in limestones and pelitic rocks from the ryoke metamorphic terrain we demonstrate that the graphitization involves a progressive re construction process of the crystal structure the sequence of the first appearance of crystal inter planar spacing correlates with the metamorphic grade and indicates the crystal growth of three dimensional structured graphite

temperature and strain gradients through lesser himalayan rocks and across the main central thrust south central bhutan implications for transport parallel stretching and inverted metamorphism in order to understand mass and heat transfer processes that operated during himalayan orogenesis we collected temperature finite and incremental strain and kinematic vorticity data through a 5 km thickness of lesser and greater himalayan rocks in southern bhutan this transect crosses two major shear zones the main central thrust mct and shumar thrust st raman spectroscopy on carbonaceous material and garnet biotite thermometry are integrated with deformation temperatures from quartz petrofabrics these data define inverted field gradients that correspond in structural position with the mct and st which are separated by sections in which temperatures remain essentially constant temperatures increase from 400 500 c to 700 750 c between 675 m below and 200 m above the mct this defines a 269 44 c km inverted gradient interpreted to have formed via high magnitude 100 250 km shearing on a discrete mct zone delineated by the limits of inverted metamorphism temperatures increase from 300 400 c to 400 530 c across the st which is attributed to differences in maximum burial depth of hanging wall and footwall rocks strain and vorticity data indicate that lesser and greater himalayan rocks were deformed by layer normal flattening transport parallel lengthening and foliation normal shortening increase from 38 71 and 36 49 respectively between 2 3 1 0 km below the mct the mct acted as a stretching fault with translation on the order of 100 s of km accompanied by transport parallel stretching of footwall and hanging wall rocks on the order of 10 s of km this demonstrates that stretching accommodated between major shear zones can make a significant contribution to cumulative mass transfer

unpairing metamorphic belts p t paths and a tectonic model for the ryoke belt southwest japan southwest japan is divided into outer and inner zones by the median tectonic line mtl a major transcurrent fault the outer zone is composed of the sambagawa high pressure intermediate or high p t type metamorphism chichibu and shimanto belts in the inner zone the ryoke belt andalusite sillimanite or low p t type metamorphism was developed mainly within a jurassic accretionary complex this spatial relationship between high p t type and low p t type metamorphic belts led miyashiro to the idea that metamorphic belts were developed as paired systems textural relationships and petrogenetically significant mineral assemblages in pelites from the ryoke belt imply peak p t conditions of 5 kbar and up to 850 c in migmatitic garnet cordierite rocks from the highest grade metamorphic zone it is likely that the thermal anomaly responsible for metamorphism of the ryoke belt was related to a segment of the farallon izanagi ridge as it subducted under the eastern margin of the asian continent during the cretaceous the sequence of mineral assemblages developed in pelites implies a metamorphic field gradient with shallow dp dt slope inferred to have been generated by a nested set of hairpin like clockwise p t paths these p t paths are characterized by limited prograde thickening minor decompression at peak t and near isobaric cooling features that may be typical of p t paths in low p t type metamorphic belts caused by ridge subduction a ridge subduction model for the ryoke belt implies that juxtaposition of the high p t metamorphic rocks of the sambagawa belt against it was a result of terrane amalgamation belt parallel ductile stretching recorded as syn metamorphic predominantly constrictional strain in both ryoke and sambagawa belt rocks and substantial sinistral displacement on the mtl are consistent with left lateral oblique convergence diachroneity in fast cooling of the ryoke belt is implied by extant thermochronological data and is inferred to relate to progressive sw to ne docking of the sambagawa belt thus an alternative interpretation of paired metamorphic belts in japan is that they represent laterally contemporaneous terranes rather than outboard and inboard components of a trench arc paired system amalgamation of laterally contemporaneous terranes during large translations of forearcs along continental margins may explain other examples of paired metamorphic belts in the geological record

pressure dependence of graphitization implications for rapid recrystallization of carbonaceous material in a subduction zone we report the results of kinetic experiments of graphitization at various pressures 0 5 8 0 gpa and durations 1 s to 24 h at 1200 c the natural carbonaceous material in sedimentary rocks from the shimanto accretionary complex and the hidaka metamorphic belt japan underwent systematic changes in crystallinity and morphology with increasing pressure to assess the pressure dependence of graphitization we adopted three approaches to formulating the graphitization kinetics using a power law rate model a johnson mehl avrami kolmogorov model and a superposition method activation volumes of 21 7 3 0 to 45 7 4 5 cm3 mol 1 and 0 7 0 2 to 16 8 1 8 cm3 mol 1 were obtained for pressures from 0 5 to 2 0 gpa and 2 0 to 8 0 gpa respectively such large negative activation volumes might arise from structural modification and compression in the primary carbonaceous material we applied the experimental data to the arrhenius type equation of graphitization extrapolated to geological p t t conditions our model predicts that carbonaceous material undergoing metamorphism for 10 myr at pressures of 0 5 3 0 gpa will begin to crystallize at around 350 420 c and transform fully to ordered graphite at around 450 600 c depending on the peak pressure thus natural graphitization might proceed much more rapidly than previously estimated owing to the large negative activation volumes for the reaction rate this indicates that subducted carbonaceous materials will completely convert to fully ordered graphite by rapid recrystallization and metamorphic devolatilization before reaching sub arc depths 100 km

raman geothermometry of carbonaceous material in the basal ediacaran doushantuo cap dolostone the thermal history of extremely negative δ13c signatures in the aftermath of the terminal cryogenian snowball earth glaciation abstract extremely negative 13 c values as low as 48 of blocky calcite cements filling cavities sheet cracks and fractures in the basal ediacaran cap dolostone of the doushantuo formation in south china have been considered as evidence for methane release possibly associated with gas hydrate destabilization in the aftermath of the terminal cryogenian snowball earth glaciation alternatively these extremely negative 13 c values may have resulted from oxidation of thermogenic methane derived from localized hydrothermal activity long after the deposition of the doushantuo cap dolostone the latter interpretation was based on carbonate clumped isotope thermometry suggesting that the precipitation of extremely 13 c depleted calcite occurred at temperatures up to 476 c mean 378 c n 4 whereas carbonate fabrics with moderately negative 13 c values generally 10 were formed at much lower temperatures mean 112 c n 4 to assess these competing hypotheses we systematically investigated the petrography carbonate carbon and oxygen isotopes and carbonaceous material cm raman spectroscopy of the doushantuo cap carbonate raman geothermometry suggests that cm associated with different microfabrics and 13 c signatures including sedimentary fabrics with moderately negative 13 c values e g peloids stromatolite laminae microbial mat fragments diagenetic fabrics with moderately negative 13 c values e g some void filling or crack filling cements and diagenetic fabrics with extremely negative 13 c values e g void filling or crack filling blocky and bladed calcites experienced temperatures generally 13 c signatures and paleotemperatures whereas the origins of these extremely negative 13 c values remain unresolved the raman geothermometric data challenge the previous interpretation that they were derived from thermogenic methane related to localized hydrothermal activity at temperatures up to almost 500 c that only affected the thermal history of the calcite with extremely negative 13 c values instead the new data suggest that the analyzed microfabrics experienced the same peak temperatures

extending the applicability of the raman carbonaceous material geothermometer using data from contact metamorphic rocks the degree of graphitization of carbonaceous material cm has been widely used as an indicator of metamorphic grade previous work has demonstrated that peak metamorphic temperature t o f regional metamorphic rocks can be estimated by an area ratio r2 of peaks recognized in raman spectra of cm the applicability of this method to low pressure 50 that adequately reflects the range of sample heterogeneity using this procedure with 532 nm laser and adapting our new calibration the errors of the raman cm geothermometer for contact metamorphic rocks decrease to 30 c

raman spectra of carbonaceous material in metasediments a new geothermometer metasedimentary rocks generally contain carbonaceous material cm deriving from the evolution of organic matter originally present in the host sedimentary rock during metamorphic processes this organic matter is progressively transformed into graphite s s and the degree of organisation of cm is known as a reliable indicator of metamorphic grade in this study the degree of organisation of cm was systematically characterised by raman microspectroscopy across several mesozoic and cenozoic reference metamorphic belts this degree of organisation including within sample heterogeneity was quantified by the relative area of the defect band r2 ratio the results from the schistes lustres western alps and sanbagawa japan cross sections show that 1 even through simple visual inspection changes in the cm raman spectrum appear sensitive to variations of metamorphic grade 2 there is an excellent agreement between the r2 values calculated for the two sections when considering samples with an equivalent metamorphic grade and 3 the evolution of the r2 ratio with metamorphic grade is controlled by temperature t along the tinos cross section greece which is characterised by a strong gradient of greenschist facies overprint on eclogite facies rocks the r2 ratio is nearly constant consequently the degree of organisation of cm is not affected by the retrogression and records peak metamorphic conditions more generally analysis of 54 samples representative of high temperature low pressure to high pressure low temperature metamorphic gradients shows that there is a linear correlation between the r2 ratio and the peak temperature t c 445 r2 641 whatever the metamorphic gradient and probably the organic precursor the raman spectrum of cm can therefore be used as a geothermometer of the maximum temperature conditions reached during regional metamorphism temperature can be estimated to 50 c in the range 330 650 c a few technical indications are given for optimal application

characterization of organic matter and its implications for pyrite hosted refractory gold mineralization along the south purulia shear zone eastern india abstract refractory gold bearing sulphide mineralization is spatially associated with the carbonaceous matter rich meta sedimentary rocks of the proterozoic south purulia shear zone spsz the carbon rich layers occurring as black continuous laminae are the metamorphosed derivatives of carbonaceous matter which were originally present in the sedimentary rocks and bear the characteristics of initial sedimentary environment of the refractory gold bearing formations of spsz the carbonaceous laminae aligned along the bedding foliation planes got remobilized into the sulfide bearing quartz carbonate veins during the metamorphic and deformational events the typical disordered bands as well as graphitic band in the raman spectra indicate that the carbonaceous matter is partially graphitized from the precursor carbon rich layers the light and homogeneous carbon isotopic values 13c 26 7 to 28 2 of the graphitized carbon suggests its origin due to the evolution from an organic precursor the 13c values fall within the range typical for organic carbon deposited during the paleo to meso proterozoic period the organic matter is considered to have played an essential role in buffering the oxygen fugacity of the initial sedimentary basin the dissolved sulfate ions in seawater were reduced resulting in sulfidation and adsorption of au in sedimentary pyrites to form refractory gold later during the deformational stages the hydrothermal fluids interacted with the graphitized matter which provided suitable redox conditions for the precipitation of auriferous sulfides such as pyrite pyrrhotite chalcopyrite and arsenopyrite within the quartz carbonate veins

geothermometry geobarometry and the al2sio5 triple point at mt moosilauke new hampshire the distribution of aluminum silicate minerals in pelitic schists constrains pressure and temperature conditions during the acadian orogeny to have been approximately those of the al 2 sio 5 invariant point thus providing an excellent opportunity to evaluate the internal consistency of a variety of published geothermometers and geobarometers with the al 2 sio 5 invariant point of holdaway 1971 four fluid independent equilibria yield p t estimates consistent with holdaway 1971 two fluid dependent equilibria yield internally consistent but unrealistically high temperatures our data are consistent with the conclusions that 1 only pyrope grossular mixing in the quaternary garnet system is significantly non ideal and 2 the activity coefficient for the anorthite component in plagioclase is approximately 2 at triple point p t conditions modified journal abstract

structure and origin of the gold mineralization in the nacimiento block the los burros deposits central california abstract the los burros gold deposit is located within the franciscan complex of the nacimiento block california although this deposit was mined during the late 19th and early 20th centuries its origin and its relationship with the regional metamorphism and deformation are unclear in order to better understand the mode of formation of the deposit and its possible link with a post subduction thermal anomaly we performed a detailed geological and structural mapping within the los burros district coupled with a u th he thermochronological study on apatite and zircon fluid inclusion and stable isotope o and c analyses of the ore deposit provide constraints on the temperature and potential source of the fluid s responsible for the los burros gold formation our results demonstrate that the los burros au deposit formed from a metamorphic aqueous co2 ch4 fluid at temperatures of 300 c results from raman spectroscopy of carbonaceous materials confirm the presence of a thermal anomaly in the range 260 320 c near the los burros mining district as previously inferred by vitrinite reflectance techniques our structural and preliminary thermochronological analyses show that the los burros au deposit consists of a series of meter wide quartz calcite sulfide en echelon veins formed during a late transpressive event related to san andreas fault activity and the subduction of a slab window during the past 30 ma our new data suggest that the thermal anomaly recorded in the vicinity of the los burros mining district is not associated with a magmatic derived hydrothermal system such as described within the epithermal mclaughlin deposit in northern california but rather with a late transpressive deformation event that uplifted deeply buried metamorphic rocks possibly connected to a local metamorphic overprint

two dimensional velocity structure beneath northern eurasia derived from the super long range seismic profile quartz abstract a two dimensional velocity structure for the mantle down to a depth of 700 km derived from super long range nuclear explosion seismic data recorded along the quartz profile in northern eurasia is presented the profile extends from the russian platform across western siberia to the altai mountains a detailed structural image of the lithosphere and asthenosphere shows strong lateral variations along the profile the subcrustal lithosphere to a depth of 100 km is characterized by lateral variations of the p wave velocity between 7 7 and 8 7 km sec the seismic data are consistent with the ural mountains having a crustal root another important finding in the data is an additional discontinuity in the mantle transition zone at 530 km this discontinuity could represent one or more phase transitions known to occur at about this depth at present the nuclear seismic records are believed to represent a key data set to prove the existence of this much debated upper mantle discontinuity

graphitization in a high pressure low temperature metamorphic gradient a raman microspectroscopy and hrtem study the graphitization of carbonaceous material cm in a high pressure metamorphic gradient is characterized along a cross section in the schistes lustres formation western alps along this 25 km cross section both the cm precursor and the host rock lithology are homogeneous and the prograde evolution of the pressure temperature metamorphic conditions from the lower blueschist facies 13 kbar 330 c to the eclogite facies 20 kbar 500 c is tightly constrained by literature data raman microspectroscopy shows that at the micrometre scale this process is progressive and continuous with increasing metamorphic grade and that the structure of cm is very sensitive to temperature variations at the nanometre scale hrtem the cm is composed of a mixture of a microporous phase and an onion ring like phase both known as non graphitizing under the effect of temperature at ambient pressure the hp lt graphitization produces structurally and microtexturally heterogeneous cm with increasing metamorphic grade the graphitization of the two types of cm proceeds up to the triperiodic graphite stage because of microtextural and structural changes that are specific to each type of cm the microporous material is progressively transformed into graphite through a macroporous transitional stage in this case graphitization mainly occurs on the pore walls as a result of pore growth in the case of concentric onion ring like material graphitization occurs in the regions with the largest radius of curvature i e on the outer part of the ring in comparison with 1 bar experiments pressure seems to induce microtextural changes which allows the subsequent structural modifications of the starting material

mantle heterogeneities and their significance results from lithoprobe seismic reflection and refraction wide angle reflection studiesthis article is one of a series of papers published in this special issue on the theme lithoprobe parameters processes and the evolution of a continent lithoprobe contribution 1486 within lithoprobe s 10 transects data from more than 20 000 km of multichannel seismic mcs reflection profiling and 12 refraction wide angle reflection r war surveys were acquired while the

phase relations of pumpellyite actinolite facies metabasites in the sanbagawa metamorphic belt in central shikoku japan abstract sanbagawa metabasites metamorphosed at conditions near the upper limit of the pumpellytic actinolite facies were examined in terms of phase equilibria in the five component system al2o3 fe2o3 cao mgo feo the fe3 content of epidote measured as x ep fe fe fe al of epidote in the assemblage epidote chlorite actinolite pumpellyite decreases gradually towards the higher grade pumpellyite free areas the progressive change in xfeep can be detected within one metabasite bed 200 meters thick near the upper limit of the pumpellyite actinolite facies the mg fe2 substitution as expressed by variation of fe fe mg in chlorite 0 40 0 55 has little effect on the fe3 al ratios of epidote and pumpellyite in the above mentioned assemblage the lowet xfeep in the pumpellite bearing assemblage is 0 15 and hence the upper limit of the pumpellyite actinolite facies is defined by the appearance of an epidote chlorite actinolite assemblage with x ep fc c 15

crust and upper mantle velocity structure of the intermontane belt southern canadian cordillera as part of the lithoprobe southern cordillera transect seismic refraction data were recorded along a 330 km long strike profile in the intermontane belt an iterative combination of two dimensional traveltime inversion and amplitude forward modelling was used to interpret crust and upper mantle p wave velocity structure this region is characterized by i a thin near surface layer with large variations in velocity between 2 8 and 5 4 km s and low velocity regions that correlate well with surface expressions of tertiary sedimentary and volcanic rocks ii an upper and middle crust with low average velocity gradient possibly a weak low velocity zone and lateral velocity variations between 6 0 and 6 4 km s iii a distinctive lower crust characterized by significantly higher average velocities relative to midcrustal values beginning at 23 km depth approximately 8 km thick with average velocities of 6 5 and 6 7 km s at top and base iv a depth to moho as defined by wide angle reflections that aver

application of raman microspectrometry in the study of structural changes in precambrian kerogens during regional metamorphism abstract a series of kerogens isolated from upper proterozoic shales and schists from a regionally metamorphosed complex barrandian region czechoslovakia were examined by raman microspectrometry changes in the spectra indicate evolution in the chemical structure of the kerogen in the direction of the metamorphic gradient

the association of poorly ordered graphite coke and bitumens in greenschist facies rocks of the ponikla group lugicum czech republic the result of graphitization of various types of carbonaceous matter greenschist facies rocks of the ponikla group ordovician silurian czech republic contain several types of carbonaceous matter that differ in their morphology texture reflectance and raman characteristics the first type consists of large up to 3 mm irregularly bound particles of low reflectance romin 0 9 romax 5 6 the area ratio of the 1585 cm 1 to 1350 cm 1 raman peaks 1 08 1 17 indicates an intermediate degree of graphitization the formation of this type of highly porous particle displaying a texture reminiscent of regular or needle coke is attributed to the thermal alteration of the amorphous structureless kerogen of the precursor sediments the second type consists of lamellar particles up to 30 m thick which can be associated with the latter or can occur independently in white mica rich laminae this type is characterized by high bireflectance romin 0 6 romax 11 9 and by lower ratios 0 70 0 82 of the raman peak areas these particles are interpreted as the product of solid state diffusion controlled graphitization of a chemically homogeneous organic material e g of graptolite periderms the third type consists of isometric up to 2 mm large commonly fractured grains and fragments which mainly occur in quartz rich laminae in reflected light the texture is either homogeneous or consists of various types of anisotropic mozaics the raman peak area ratios 0 75 1 14 indicate a highly variable degree of structural ordering these particles are considered as the remains of metamorphosed bitumens accumulated in the sandy laminae of the original sediments the fourth type consists of small particles of carbonaceous matter maximum length 25 m thickness 1 2 m which occur adjacent to crystal faces of white micas this type is probably the product of epitaxial growth of graphite from the gaseous phase the results of this work indicate that the differences in the degree of graphitization of the carbonaceous matter in low grade metamorphosed rocks can be mainly related to the initial nature of the sedimentary organic matter and to its premetamorphic history

carbon isotopic thermometry calibrated by dolomite calcite solvus temperatures abstract the temperature dependence of carbon isotopic fractionations between calcite and graphite and between dolomite and graphite are calibrated by the calcite dolomite solvus geothermometry using marbles collected from the contact metamorphic aureole in the kasuga area central japan the carbon isotopic fractionations 13ccc gr and 13cdo gr systematically decrease with increasing metamorphic temperature the concordant relationships between the fractionations and solvus temperatures are approximately linear with t 2 over the temperature range 400 to 680 c 13ccc gr 5 6 106 t 2 k 2 4 13cdo gr 5 9 106 t 2 k 1 9 these systematic relationships between fractionation and temperature suggest that carbon isotopic equilibria between carbonates and graphite were attained in many cases the equation for the calcite graphite system has a slope steeper than bottinga s 1969 results it is however in good agreement with that of valley and o neil 1981 in the temperature range from 600 to 800 c because of the relatively high sensitivity to temperature these isotopic geothermometers are useful for determining the temperatures in moderate to high grade metamorphosed carbonate rocks

raman spectrum of carbonaceous material a possible metamorphic grade indicator for low grade metamorphic rocks raman spectral analyses of carbonaceous material cm extracted from pelitic samples along two sections traversing the metamorphic belt of taiwan were carried out in the present study the results show similar spectral variations of cm with metamorphic grade as those documented in the literature however continuous sampling from zeolite facies through prehnite pumpellyite facies to greenschist facies metamorphic rocks in the present study does reveal some interesting features on the raman spectra of cm that were not noted before both the raman d disordered o ordered peak area i e integrated intensity ratio and the d o peak width i e full width at half maximum fwhm ratio of the cm decrease with progressive metamorphism but the most prominent change in the d o peak area ratio occurs in samples of lower greenschist facies metamorphic grade while the most significant decrease in the d o peak width ratio occurs in samples near the boundary of prehnite pumpellyite facies and greenschist facies this phenomenon is interpreted as a result of the decoupling of the changing rates of in plane crystallite size and degree of defects of cm with progressive metamorphism it is postulated that the raman spectrum of cm can serve as a metamorphic grade indicator to distinguish samples of prehnite pumpellyite facies metamorphic grade from those of greenschist facies metamorphic grade

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

denudation history of the high t p ryoke metamorphic belt southwest japan constraints from chime monazite ages of gneisses and granitoids chime chemical th u total pb isochron method monazite ages were determined for gneisses and granitoids from the eastern and western parts of the ryoke belt separated by about 500 km the monazite ages for the gneisses are concentrated between 102 and 98 ma and are interpreted as the time of monazite formation under lower amphibolite facies conditions the peak metamorphism seems to be contemporaneous with the emplacement of the geologically oldest plutons that are dated at c 95 ma in both the eastern and western parts in the eastern part plutonism continued from c 95 ma to c 68 ma at intervals of 2 10 ma whereas in the western part it ceased at c 85 ma the chime monazite ages agree well with the relative age of granitoids derived from intrusive relationships of granitoids in both parts these lines of evidence are incompatible with a current view that the plutonometamorphism in the ryoke belt becomes younger towards the east the chime monazite ages coupled with available data on the depth at which the ryoke metamorphism took place and the emplacement of individual plutons show that the western part was eroded more rapidly about 1 5 mm year 1 than the eastern part about 0 8 mm year 1 over the time span from 91 to 85 ma the denudation rates agree well with those in active orogenic belts like the alps and himalayas

graphite thermometry in a low pressure contact aureole halifax nova scotia abstract intrusion of the late devonian south mountain batholith southern nova scotia produced a low pressure contact metamorphic aureole in its metasedimentary host rocks the effects of contact metamorphism are particularly well developed in pelitic rocks of the halifax group on the eastern margin of the batholith contact metamorphic isograds and mineral assemblages suggest low pressure metamorphism with p t conditions at the contact estimated at 2 5 3 0 kbar and ca 650 c in this study raman spectroscopy of carbonaceous material rscm was used to obtain temperatures from graphite which is common throughout the contact aureole temperature estimates range from ca 360 c just outside the cordierite in isograd to ca 640 c in the sillimanite k feldspar zone near the contact the latter consistent with the temperature estimated from the corresponding silicate mineral assemblage three different rscm calibrations produced very similar results except at the high temperature end of the observed range a thermal profile constructed from the rscm data was used to constrain a 2d numerical model for post intrusion conductive cooling of the batholith along its eastern margin comparison of rscm vs model thermal profiles suggest that observed differences between the thermal structure of the inner and outer aureole were controlled by the subsurface geometry of the pluton contact the model predicts that peak temperatures in country rocks within 1 km of the contact were reached within 50 ka of intrusion but that the outer part of the aureole took 250 500 ka to reach peak temperatures the results confirm the utility of rscm thermometry for acquiring temperature data over a range of metamorphic grades

thermal metamorphism in the lesser himalaya of nepal determined from raman spectroscopy of carbonaceous material the determination of metamorphic conditions is critical to the understanding of the formation of mountain belts however all collisional mountain belts contain large volumes of accreted sediments generally lacking metamorphic index minerals and are therefore not amenable to conventional petrologic investigations by contrast these units are often rich in carbonaceous material making it possible to determine thermal metamorphism through raman spectroscopy of carbonaceous material rscm method a technique that has been recently calibrated beyssac et al j metamorph geol 20 2002 859 871 the lesser himalaya lh is one of these problematic cases with a very poor mineralogy but a key structural position within the himalayan system that makes lh considered as diagnostic of the overall thermal behaviour of the orogen this work demonstrates the performance of the rscm technique and shows that this technique might thus be used to detect inter sample variations as small as 10 15 c but absolute temperatures can only be determined to 50 c due to the uncertainty on the calibration this study reveals that the lh has undergone a large scale thermal metamorphism with temperature decreasing progressively from about 540 c at the top to less than 330 c within the deepest exhumed structural levels

upper mantle structure in the southeastern canadian cordillera a seismic broadside wide angle experiment in canada s southeastern cordillera has revealed an anomalous and unexpected seismic arrival analysis of the data shows that the arrival is unlikely to be a crustal or shallow mantle phase or multiple or converted waves the kinematic characteristics of this arrival dictate that it is a reflection from within or from the base of a low velocity zone 7 8 8 0 km s 1 situated at a depth of 59 to 62 km that we identify as the top of the asthenosphere this leads to the conclusion that the base of the lithosphere is at a depth of 50 2 km in the region the interpreted shallow asthenosphere bolsters earlier conclusions of a heat source at depth in the southeastern canadian cordillera the interpretation also shows that stratification in the upper mantle is resolvable by current wide angle seismic experiments

crustal velocity structure of the omineca and intermontane belts southeastern canadian cordillera we provide information on the physical properties across two important morphogeological regions of the southeastern canadian cordillera the omineca and intermontane belts and tie together two previously interpreted wide angle seismic lines which have separately probed each zone along strike the data analyzed here line 7 of lithoprobe s 1990 southern cordillera refraction experiment in southern british columbia is along a 410 km profile from the fraser river to radium hot springs near the rocky mountain trench iterative forward and inverse modeling of the seismic travel times constrained by seismic amplitude characteristics and earlier cross line interpretations defines a thin crust with relatively low velocities the upper crust has an average thickness of 15 3 km and an average p wave velocity of less than 6 1 km s 1 with velocities at the bottom reaching 6 2 6 3 km s 1 it is further characterized by velocity gradients which decrease markedly with depth and by low velocity trends which correlate with the traces of crustal scale shear zones a high velocity region at the west end of the profile is coincident with a major uplift structure the midcrust has an average thickness of 9 2 km with velocities below 6 1 km s 1 except for the extreme west end of the profile this defines a midcrustal low velocity zone lvz in the omineca belt and partly in the intermontane belt the lower crust from 24 km to 34 km depth has velocities averaging 6 6 6 7 km s 1 higher velocities in the deep crust may mark the westward extent of north american cratonic rocks at the okanagan valley the upper mantle velocity has an average 8 0 km 5 increasing to 8 2 km s 1 at the maximum depth of ray penetration at 48 km the model presented here compliments well the earlier interpretations of vertical incidence seismic reflection profiles the velocity structure of the crust especially the lvz closely correlates with the regional isotherms while there seems to be no well defined velocity boundary between the intermontane and omineca belts there are distinct velocity differences between the intermontane and neighboring coast plutonic complex to the west the low velocity characteristics of the omineca and much of the intermontane belts may be a consequence of temperature control of the physical parameters of the crust the lithosphere in the southeastern canadian cordillera is clearly anomalous with respect to much of northwest north america and in terms of crustal thickness and velocity structure it is similar only to the basin and range province of the united states

kinetics and duration of metamorphic mineral growth in a subduction complex zircon and phengite in the nagasaki metamorphic complex western kyushu japan we applied interface controlled kinetics including interface controlled ostwald ripening and non hydrostatic dissolution precipitation to metamorphic zircon and phengite growth in a high pressure high p metamorphic complex this kinetic modeling yields the growth duration of metamorphic zircon based on the assumption that dissolution and precipitation of zircon is much slower than that of phengite the model was applied to zircon and phengite growth in the nishisonogi unit of the high p nagasaki metamorphic complex western kyushu japan given that detrital zircons that are tens of microns in size remain after metamorphism our model assumption is justified our results show that the duration of metamorphic zircon growth in an individual rock ranges from 10 to 20 myr in general the duration of metamorphic zircon growth in the whole nishisonogi unit is ca 30 myr this prolonged duration implies that high p metamorphism is sustained by continuous subduction of hydrated oceanic crust and overlying trench fill sediments the continuous subduction and accretion of these materials may supply the metamorphic fluid into the high p metamorphic domain in the deeper part of the accretionary prism thereby contributing to interface controlled kinetics in the metamorphic complex

evaluating raman spectra of carbonaceous matter by automated iterative curve fitting abstract raman spectroscopy of carbonaceous material rscm estimates the maximum metamorphic temperature of coal graphite and cm enclosed in metapelites however the evaluation of cm raman spectra is often biased by subjectivity which reduces the overall comparability of rscm results to decrease subjectivity an automated evaluation software is presented it is based on the iterative and simultaneous modeling of signal and baseline functions two new rscm parameters are introduced termed raman area ratio rar and scaled total area sta these parameters are tested on a sample suite of organic rich shales and slates of the teuschnitz syncline thuringia germany of well constrained very low to low metamorphic grade defined by illite crystallinty and vitrinite reflectance a correlation of both rscm parameters with vitrinite reflectance verifies that the rscm parameters reflect the metamorphic conditions this correlation prevails at different excitation wavelengths 488 nm 633 nm 785 nm used for the raman measurements

thermal structure of a fossil subduction wedge in the western alps new peak metamorphic temperatures are obtained by raman spectroscopy of carbonaceous material to document the thermal structure of the central western alps with high sampling resolution we show that peak metamorphic t gradually increases eastward from 330 to 350 c ultra dauphinois to subbrianconnais units 350 to more than 400 c brianconnais domain including the zone houillere where metamorphic index minerals are rare and from 350 to more than 500 c liguro piemontese domain combined with other constraints on the metamorphic evolution this dataset reveals a good preservation of the overall thermal structure of the fossil subduction wedge with no particular thermal overprint during collision however local confrontation with p t estimates and radiometric ages reveals more subtle variations within tectonic units and across the main contacts that are linked to the past activity of the major thrusts and extensional shear zones during subduction and exhumation

iodine 129 cfc 11 transit times for denmark strait overflow water in the labrador and irminger seas 1 iodine 129 discharged from nuclear fuel reprocessing plants in france and the united kingdom is transported into the nordic seas on timescales of 3 5 years tracer 129i is subsequently injected into intermediate waters that overflow the sills between greenland iceland and scotland and ventilate the north atlantic deep waters nadw during the early 1990s discharges of 129i increased by 600 resulting in a large well resolved tracer front whose passage through the nordic seas is presently being observed by a time series of 129i measurements on the woce ar7w section in the labrador sea the highest 129i levels were measured below depths of 3000 m in denmark strait overflow water dsow these levels increased by about 300 between 1997 and 2001 to values 40 107 atoms l indicating that the leading edge of the tracer front from the early 1990s was being observed entering the nadw using a simple mixing advection model 129i and cfc 11 results were used to calculate transit times of 0 4 2 6 years for the flow of dsow from formation regions in the nordic seas to the labrador sea measurements on samples collected in the irminger sea in 2001 gave transit times of 0 3 years for core dsow immediately south of denmark strait and about 2 years for locations off the southern tip of greenland an increase in dsow transit times observed in the labrador sea between 1999 and 2001 probably reflects a more general weakening in the subpolar gyre of the north atlantic during the 1990s