partial melting during tectonic exhumation of a granulite terrane an example from the larsemann hills east antarctica anatectic migmatites in medium to low pressure granulite facies metasediments exposed in the larsemann hills east antarctica contain leucosomes with abundant quartz and plagioclase and minor interstitial k feldspar and assemblages of garnet cordierite spinel ilmenite sillimanite qualitative modelling in the system k2o feo mgo al2o3 sio2 h2o tio2 o2 in conjunction with various p t calculations indicate that the high grade retrograde evolution of the terrane was dominated by decompression from peak conditions of c 7 kbar at c 800 c to 4 5 kbar at c 750 c extensive partial melting during decompression involved the replacement of biotite by the assemblage cordierite garnet spinel within the leucosomes these leucosomes represent the site of partial melt generation the cordierite garnet spinel ilmenite assemblage representing the solid products and excess reactants from the melting reaction the extraction and accumulation of this decompression generated melt led to the formation of syntectonic pegmatites and extensive granitic plutons leucosome development and terrane decompression proceeded during crustal transpression synchronous with upper crustal extension during a progressive early palaeozoic collisional event subsequent retrograde evolution was characterized by cooling as indicated by the growth of biotite replacing spinel and garnet thin mantles of cordierite replacing spinel and quartz within metapelites and garnet replacing orthopyroxene and hornblende within metabasites p t calculations on late mylonites indicate lower grade conditions of formation of c 3 5 kbar at c 650 c consistent with the development of late cooling textures

revised empirical garnet biotite muscovite plagioclase geobarometer in metapelites the garnet biotite muscovite plagioclase gbmp barometer was empirically revised for p t conditions of 1 14 kbar and 450 840 c using 263 metapelitic rock samples from all over the world this barometer is based on activity models for garnet biotite and plagioclase identical to those of the well calibrated garnet biotite thermometer and the garnet aluminosilicate plagioclase quartz gasp barometer the gbmp barometer is less temperature dependent than the gasp barometer and can be applied to either al2sio5 absent or al2sio5 bearing metapelites the total error of the gbmp barometer is estimated to be about 1 2 kbar on considering input temperature error and analytical errors of chemical compositions of the phases involved the random error of the gbmp barometer is evenly distributed with respect to pressure temperature and mineral composition simultaneous application of the gbmp barometer and the garnet biotite thermometer identifies the correct stability field for al2sio5 bearing metapelites application of the gbmp barometer to metapelitic rocks within the same geological terranes or thermal contact aureoles yielded similar pressures within error a spreadsheet for implementing the proposed gbmp geobarometer is supplied on the journal s website

garnet biotite geothermometry revised new margules parameters and a natural specimen data set from maine the garnet biotite geothermometer has been recalibrated using recently obtained margules parameters for iron magnesium calcium garnet mn interactions in garnet and al interactions in biotite as well as the fe oxidation state of both minerals fe mg and dwal margules parameters for biotite have been retrieved by combining experimental results on 6 al free and 6 al bearing biotite using statistical methods margules parameters per mole of biotite are bt w 5 40 719 2 30t j mole mgfe

empirical garnet biotite plagioclase quartz gbpq geobarometry in medium to high grade metapelites on the basis of the net transfer reactions among garnet biotite plagioclase and quartz for both mg and fe end member models the garnet biotite plagioclase quartz gbpq geobarometer was empirically calibrated under physical conditions of p 1 0 11 4 kbar and t 515 878 c based on the input garnet biotite temperatures and garnet aluminosilicate plagioclase quartz gasp pressures of 224 natural aluminosilicate bearing metapelitic samples collated from the literature the calibrations are internally consistent with the asymmetric quaternary solid solution model of garnet the symmetric quaternary solid solution model of biotite and the al avoidance ternary solid solution model of plagioclase in calibrating the garnet biotite geothermometer and the gasp geobarometer the resulting two gbpq barometer formulae reproduce the input gasp pressures well within 1 0 kbar mostly within 0 5 kbar for both aluminosilicate bearing and aluminosilicate absent metapelites the two gbpq barometry formulae yielded identical pressures whether the sample was included or not included in calibrating the gbpq barometry the random error of the gbpq barometry may be expected as 1 2 kbar the dp dt slopes of these two gbpq formulae are close to that of the gasp barometer in p t space applications of the gbpq barometry of aluminosilicate absent metapelites to the rocks within a thermal contact aureole or rocks within a limited geographical area without post metamorphic structural discontinuity show no obvious pressure change it may be concluded that the two gbpq barometry formulae derived in this study may be used as practical tools for metamorphic pelites under the conditions of 515 878 c and 1 0 11 4 kbar in the composition range of xgros 3 in garnet xan 17 in plagioclase and x bio 3 in biotite application of the gbpq barometer beyond the calibration range should be undertaken with caution

ferric ferrous ratios h 2 o contents and d h ratios of phlogopite and biotite from lavas of different tectonic regimes complete chemical analyses including ferric and ferrous iron h2o contents and dd values for 16 phlogopite and biotite and 2 hornblende separates are presented samples were obtained from volcanic rocks from four localities 1 phlogopite phenocrysts from minette lavas from the western mexico continental arc 2 biotite and hornblende phenocrysts from andesite lavas from mono basin california 3 phlogopite and biotite from clinopyroxenite nodules entrained in potas sic lavas from the east african rift uganda and 4 phlogopite phenocrysts from a wyomingite lava in the leucite hills wyoming the fe 2 o 3 contents in the mi cas range from 0 8 to 10 5 wt corresponding to 0 09 to 1 15 fe 31 per formula unit pfu water contents vary from 1 6 to 3 0 wt corresponding to 1 58 to 3 04 oh pfu significantly less than would be expected for a site fully occupied by hydroxyl cation and anion based normalization procedures provide accurate mineral for mulae with respect to most cations and anions but are unable to generate accurate estimates of fe 31 yfe t a nd overestimate oh at the expense of o on the hydroxyl site these inaccuracies are present despite acceptable adjust ed totals and stoichiometric calculated site occupancies the phlogopite and biotite phenocrysts in arc related lavas from western mexico and eastern california have the highest fe 31 yfe t ratios 56 87 reflecting high magmatic oxygen fugacities dnno 51 2t o 15 in contrast to those from uganda 25 40 and the leucite hills 23 there is no correlation between the oh content and the fe 31 yfe t ratio in the micas values of k mgyfe 21 d 2s errors were calculated for three phlogo pite olivine pairs 0 12 0 12 0 26 0 14 0 09 0 12 two biotite hornblende pairs 0 73 0 08 and 1 22 0 10 and a single phlogopite augite pair 1 15 0 12 values of k fyoh d for two biotite and hornblende pairs could not be determined without significant error because of the extremely low f contents 0 2 wt of the four phases the dd values obtained in this study encompass a large range 2137 to 243 the phlogo pite and biotite separates from uganda have dd values of 270 to 249 which overlap those believed to represent primary mantle there is a larger range in dd values 2137 to 243 for phlogopite phenocrysts from west ern mexico minette lavas although their range in d 18 o values 5 2 6 2 is consistent with normal mantle it is unlikely therefore that the variable dd values reflect heterogeneity in the mantle source region of the minette magmas nor can the extremely low dd values reflect degassing of h2 or h2o since almost 100 loss of dis solved water in the magma is required an unrealistic scenario given the stability of the hydrous phenocrysts the very low dd values of the mascota minette phlogo pites require that the hydrogen be introduced from an external source e g meteoric water whatever the pro cess responsible for the observed hydrogen isotope com position it had no effect on the d 18 o value fo2 ah2o or bulk composition of the host magmas

geochemistry of dendritic anorthosites and associated pegmatites in the skaergaard intrusion east greenland evidence for metasomatism by a chlorine rich fluid abstract in the upper zone uza of the skaergaard intrusion dendritic anorthosites and pegmatites are found in a zone of deformed gabbro along the flank of a trough banded structure apatite biotite some having over 2 wt c1 and amphibole have higher chlorine contents than those found in the surrounding rocks and indicate the presence of a high temperature aqueous fluid having a high hcl hf fugacity ratio the fluid must have been derived locally by exsolution of magmatic volatiles because hydrous minerals in nearby hydrothermal veins have low chlorine contents and the structures are far from the margin of the intrusion apatite and quartz in the pegmatites have abundant fluid inclusions containing cubic halide crystals giving evidence for a separate aqueous fluid coexisting with an evolved residual melt the pegmatites are also characterized by mafic and felsic clots with compositions similar to immiscible silicate liquids dendritic anorthosites have equal or higher abundances of zr hf ta and rare earth elements than the surrounding gabbros yet they contain little pyroxene or fe ti oxides the unusual trace element enrichment patterns and the associated rare assemblage of zirconolite zircon baddeleyite and allanite suggest that the chlorine rich fluid was enriched in trace elements derived from the residual melt and the reacted mafic minerals deformation of the magma at a crystallinity below the critical melt fraction resulted in localized exsolution and channeling of a magmatic aqueous fluid and an evolved residual melt dissolution of pyroxene olivine and fe ti oxides allowed for further deformation and expulsion of the mafic melt leaving a plagioclase rich residue these structures are the product of enhanced reaction locally of an aqueous fluid which may have moved upward as a nearly planar reactive front throughout the lower crystallization zone in addition to the transport of trace elements migration of a high temperature fluid through the zone of crystallization could aid in the compaction of igneous cumulates and the development of layering this study presents the first evidence for a high temperature chlorine rich aqueous fluid in the layered series of the skaergaard intrusion and illustrates the important effect of such fluids on the physical and chemical evolution of layered intrusions

mineral chemistry of ti rich biotite from pegmatite and metapelitic granulites of the kerala khondalite belt southeast india petrology and further insight into titanium substitutions precise chemical composition including fe 3 and h of biotite from a pegmatite dike and its host granulite from the kerala khondalite belt of se india has been determined using a multi technique approach involving emp sims mossbauer and c h n elemental analysis biotite in these rocks formed at t 800 850 c and p 5 1 kbar the full analyses were normalized on the basis of o 12 x y z oh x cl y f z biotite in the pegmatite is ti f and cl rich 0 33 0 46 and 0 16 apfu respectively h 2 o poor oh 0 86 pfu has x mg 0 49 and fe 3 fe tot 3 the low octahedral vacancies 0 06 pfu and the high oxygen content in the hydroxyl site oh f cl 1 49 pfu confirm the role of the ti oxy substitution as a major exchange vector in these high t biotites in the host granulite fine grained biotite is fe 3 free has low cl 0 03 apfu and more variable composition with ti f and x mg in the ranges 0 26 0 36 0 52 0 67 and 0 67 0 77 respectively the number of octahedral vacancies is relatively large 0 10 0 18 pfu and the sum of volatiles oh f cl varies from 1 71 to 2 06 pfu systematic variations of x mg are a function of the microstructural position and are in agreement with retrograde exchange reactions biotite included in or in contact with garnet has the maximum values whereas crystals in the matrix have the minima titanium has systematic negative correlations with f x mg and oh f cl whereas al and octahedral vacancies are virtually constant these trends indicate that the ti vacancy along with substitutions involving al cannot explain the observed short scale variations conversely the ti oxy exchange appears to be active resulting from combination of two vectors the more conventional hydroxylation ti 4 2o 2 fe mg 2 2oh and the fluorination ti 4 2o 2 fe mg 2 2f the systematic retrograde redistribution involves not only fe and mg as commonly observed but also ti f and h in a way such to eliminate the primary ti oxy component of biotite

efficiency of eigenfunctions for discriminant analysis of subaerial non tidal beach profiles abstract a one year set of subaerial beach profiles from the southern mediterranean beaches of israel measured at fortnightly intervals was analysed by applying eigenfunctions and computations of scour and fill the eigenfunction associated with the first eigenvalue is shown not to be efficient for beach categorization however the second eigenfunction effectively classified three subenvironments of the exposed beaches these constitute the upper foreshore the stable segment and the upper backshore a model of subaerial beach types based on the second eigenfunction is confirmed by scour and fill indices and by morphological characteristics

petrogenesis of the contact metamorphic rocks beneath the stillwater complex montana the stillwater gabbroic complex was emplaced at about 2700 ma in a metasedimentary section consisting of pelitic schist iron formation and quartzite contact metamorphism resulted in the development of a 500 1000 m hypersthene hornfels zone at the contact and a 500 1500 m cordierite cummingtonite hornfels zone farther from the contact weakly foliated biotite schist occurs beyond the hornfels pelitic rocks are poor in potassium generally lack muscovite and typically contain the assemblage garnet staurolite gedrite chlorite cordierite biotite plagioclase ilmenite quartz the diagnostic hornfels assemblage is cordierite cummingtonite biotite plagioclase ilmenite quartz cummingtonite is replaced by hypersthene near the contact the assemblages in the biotite schist formed during a low pressure regional metamorphism temperature ranged from 450 625 c in biotite schist to 650 800 c in hypersthene hornfels pressure estimates range from 100 250 mpa in biotite schist to 50 300 mpa in hypersthene hornfels the alpha h 2 o in the hypersthene zone is estimated to have been 0 27 orthoamphibole in the biotite schist has compositions that fall on a well defined linear trend from fe 3 5 mg 3 5 si 8 o 22 oh 2 to na 0 7 fe 3 5 mg 1 4 al 2 0 al 2 5 si 5 5 o 22 oh 2 there is no evidence for any immiscibility between anthophyllite and gedrite at the pressure of metamorphism orthoamphibole was replaced by clinoamphibole with increasing grade the petrogenetic grid calculated from estimates of thermodynamic data for the observed compositions of minerals indicates that most reactions in biotite schist occur in the temperature range 525 550 c at 200 mpa the formation of the assemblage cordierite cummingtonite occurs at 600 c and hypersthene is stable above 700 c the net reaction during formation of cordierite cummingtonite hornfels from biotite schist indicates little loss of h 2 o and a small increase in solid volume the pressure and temperature of metamorphism are consistent with subvolcanic emplacement of the complex at a depth of 6 7 km in metasedimentary country rock following a period of regional metamorphism

calibration of the garnet biotite al2 sio5 quartz geobarometer for metapelites a garnet biotite al2sio5 quartz gbaq geobarometer was empirically calibrated using more than 700 natural metapelites with a broad compositional range of garnet and biotite under p t conditions of 450 950 c and 1 17 kbar in the calibration activity models of garnet and biotite identical to those in the garnet biotite geothermometer of holdaway 2000 were used therefore the gbaq geobarometer and the garnet biotite geothermometer can be simultaneously applied to iteratively estimate metamorphic p t conditions successful application of the gbaq geobarometer to natural metapelites certify its validity most importantly when plagioclase is absent or cao in garnet and or plagioclase are deficient this geobarometer may prove useful for estimating metamorphic pressures the random error of the present gbaq geobarometer is inferred to be around 1 8 kbar an electronic spreadsheet is available as table s4 to apply the gbaq geobarometer in combination with the garnet biotite geothermometer this article is protected by copyright all rights reserved

empirical garnet muscovite plagioclase quartz geobarometry in medium to high grade metapelites abstract based on the net transfer reactions among garnet muscovite plagioclase and quartz for both mg and fe end member models the garnet muscovite plagioclase quartz gmpq geobarometry was empirically calibrated under the physical conditions of p 1 0 11 4 kbar and t 505 745 c for 128 natural metapelitic rock samples collected from the literature the input temperatures and pressures were simultaneously determined by the garnet biotite thermometer and the garnet aluminosilicate plagioclase quartz gasp barometer the gmpq calibrations adopted the same asymmetric quaternary solid solution model of garnet and the same al avoidance asymmetric ternary model of plagioclase as the calibrations of the garnet biotite geothermometer and the gasp geobarometer a symmetric fe mg al vi ternary solid solution model of muscovite was adopted and the margules parameters of muscovite were obtained through regression the mg and fe model reactions along with the assumption of whether the ferric iron content in muscovite is 0 or 50 resulted in four gmpq barometry formulae the gmpq barometry formulae reproduce the input gasp pressures well within 1 0 kbar mostly within 0 5 kbar for both aluminosilicate bearing and aluminosilicate absent samples the gmpq barometry formulae yield identical pressures for every sample whether the sample was included or not in calibrating the barometers for each of the mg or fe model reaction the formulae gave identical pressures within 40 bars the random error of the gmpq barometry may be expected as 1 4 kbar the d p d t slopes of these gmpq formulae are close to that of the gasp barometer in the p t space applications of the gmpq barometry to aluminosilicate absent metapelites within a limited geographic area without postmetamorphic structural discontinuity generally show no pressure difference it may be concluded that the gmpq barometry formulae derived in this work may be used as practical tools for metamorphic pelites under the conditions of 505 745 c and 1 11 4 kbar in the composition range of x gros 3 in garnet and x an 17 in plagioclase

yangzhumingite and phlogopite from the kvaloya lamproite north norway structure composition and origin abstract this study reports a petrographic and crystal chemical analysis of three types of micas yangzhumingite light brown phlogopite and dark brown phlogopite found in a lamproitic dyke at the kvaloya island north norway the study was carried out integrating different analytical techniques electron microprobe single crystal x ray diffraction inductively coupled plasma mass spectrometry mossbauer and micro fourier transform infrared spectroscopy kvaloya yangzhumingite second occurrence in nature was characterized for the first time in detail via single crystal x ray diffraction the different mica types are distinguishable on the basis of the vi fe and mg versus si content yangzhumingite composition is intermediate between those of kmg 2 75 si 3 5 al 0 5 o 10 f 2 and kmg 2 50 si 4 o 10 f 2 synthetic compounds reported in the literature light and possibly dark brown phlogopite is a mg rich fluorotetraferriphlogopite the latter having a greater fe content the infrared spectra of yangzhumingite and the light brown phlogopite show the occurrence of oh absorption bands respectively at 3586 cm 1 which correlates well with the measured f content 3707 cm 1 and 3686 cm 1 assigned mainly to 3mg 2 k oh phlogopitic environment structural analyses performed only on yangzhumingite and light brown phlogopite show that both samples are 1 m polytypes with the expected space group c 2 m crystal chemical details are compatible with the following major substitution mechanisms 2 xii k vi 2 xii vi r 2 where r 2 mg fe oh f for yangzhumingite and 2 vi r 2 vi ti 4 vi ti vacancy oh f for light brown phlogopite all three types of micas formed at relatively constant low pressure but over a large temperature range in equilibrium with a grain boundary fluid that underwent significant changes in composition during reaction progress light brown phlogopite cores and dark brown phlogopite rims formed during crystallization from the lamproitic magma while yangzhumingite formed as a result of reactions between the already formed phlogopite and the highly reactive fluid that was derived from the volatile rich lamproite magma

hydrothermal metal supplies enhance the benthic methane filter in oceans an example from the okinawa trough abstract anaerobic oxidation of methane aom is an important process that regulates methane budget in the global carbon cycle sulfate is traditionally regarded as the most important electron acceptor for aom however recent studies reveal that reactive metal reduction driven aom metal aom may also present on natural environments partitioning the methane c sink from anaerobic oxidation between sulfate reduction driven aom sulfate aom and metal aom is thus becoming a scientific challenge to be addressed this study aimed to show that metal aom has a potential to complement sulfate aom especially when methane seeps met sediments enriched in iron and manganese oxides supplied by hydrothermal plumes samples of authigenic carbonate were collected from the cold seeps on the western slope of the okinawa trough ot a semi enclosed back arc basin with cold seeps and hydrothermal activities coexist spatially petrologic and mineralogical observations provided faithful evidence of a close genetic relationship between goethite and authigenic carbonates likely indicating the presence of metal aom although some contributions of organoclastic fe reduction cannot be ruled out the most possible place of metal aom was between the lower part of the sulfate methane transition zone smtz and the immediate upper part of the methanogenic zone the characteristics of 13ccarb 53 7 to 3 3 vienna peedee belemnite vpdb also point to our carbonate samples as the products of aom these authigenic carbonates was further identified as three possible types of carbonate of different sources sulfate aom metal aom and hydrothermal carbonates depending on the correlations among concentrations of carbonate associated sulfate cas 13ccarb and 18ocarb based on these findings we tentatively suggested that the reduction of reactive metals carried by hydrothermal plumes might drive the methane oxidation in the adjacent cold seep areas resulting in the precipitation of unique carbonates and enhancement the efficiency of the benthic methane filter this study reported the coupling or interaction between the two extreme submarine environments by metal aom for the first time which might help in understanding and improving the global carbon and metal cycles now and in the past

a mossbauer study of pseudotachylytes redox conditions during seismogenic faulting the redox conditions during frictional melting provide information on the physical and chemical conditions during seismic slip in the crust here we examine frictional melts from five localities by analyzing host rocks and corresponding pseudotachylytes using mossbauer spectroscopy the faults examined are located at south mountain arizona fort foster maine two localities long ridge fault north carolina and the homestake shear zone colorado the main iron bearing phases in the pseudotachylytes are phyllosilicates biotite muscovite and clays and iron oxides magnetite and hematite and minor pyrite the ferrous ferric ratios of the phyllosilicates in the host rocks are the same as those in the pseudotachylytes except for the hematite bearing pseudotachylyte from the long ridge fault which is more oxidized the magnetites in the host rocks and the corresponding pseudotachylytes have different ferric and ferrous iron distributions which is attributed to different cation chemistry rather than redox conditions with the exception of the south mountain locality the ferric ferrous ratios of the micas are interpreted to record the primary redox state of the pseudotachylyte melt as the calculated oxygen fugacities are consistent with magnetite and hematite equilibria pyrite bearing pseudotachylytes plot 0 1 log10 units above the fayalite magnetite quartz fmq buffer magnetite bearing pseudotachylytes plot 2 4 log10 units above the fmq buffer and hematite bearing pseudotachylytes plot 3 5 log10 units above the hematite magnetite hm buffer hematite bearing pseudotachylytes together with previous oxygen isotope data are inferred to represent melting in the presence of externally derived pressurized water other localities are inferred to represent melting under rock buffered closed system conditions if the localities studied are representative of seismogenic faulting the calculated oxygen fugacities indicate that in the system c o h s h2o and co2 should be the dominant fluid species this is the first detailed study of the redox state of pseudotachylytes

crystal chemistry and petrologic significance of fe3 rich phlogopite from the tapira carbonatite complex brazil this contribution deals with the crystal chemistry of phlogopite and fe3 rich phlogopite from the tapira alkaline carbonatite complex brazil to assess the petrological significance and genetic conditions of these rocks the tapira complex consists of a layered intrusion composed mainly of ultramafic rocks dunite wehrlite clinopyroxenite bebedourite garnet magnetitite perovskite magnetitite and glimmerite with subordinate carbonatite the wide range of textural optical and crystal chemical characteristics of ph logopite is related to the variation of 102 ah20 and ac02as well as magma bulk chemical composition during fractional crystallization phlogopite from alkaline silicate rocks ranging from dunite to bebedourite is characterized by fairly constant al content moderate 4 fe3 substitution and variable amounts of ti the 4 fe3 substitution accompanied by crystals showing reverse pleochroism increases during fractional crystallization these features correspond to crystallization at low pressure and high 102 and ah20 in the presence of moderate saturation in ti bearing phases al203 in the magma or both phlogopite from silicate carbonatite rocks classified as ferriphlogopite on the basis of strong reverse pleochroism related to 4 fe3 tetrahedral substitution also presents low to very low ai fe2 and ti contents these features suggest very high 102 h20 and c02 conditions in the presence of strong saturation in ti bearing phases as well as very low al203 content in the liquid the crystal structure refinements of tapira phlogopite show that fe3 substitutes for si in tetrahedral sites fe distribution is completely disordered so the resulting space group is c21m the octahedral site composition is similar to that of phlogopite the octahedral sites being preferentially occupied by mg the presence of fe3 in the tetrahedral sheet enlarges the whole structure this enlargement is reflected by an increase in cell edge lengths and a decrease in b angle values the increase in distortion of the tetrahedral ring a angle up to 11 is necessary for the tetrahedral and octahedral sheets to fit together

devonian sedimentation in the xiqingshan mountains implications for paleogeographic reconstructions of the sw qinling orogen abstract the qinling orogen between the north china and yangtze plates comprises various accreted and collisional terranes recording the evolution of the proto tethyan ocean and the formation of east asia knowledge of the provenance and tectonic setting of a thick succession of e w striking devonian sedimentary rocks sandwiched tectonically between the shangdan and mianlue ophiolitic suture zones in this belt is essential to understanding the evolution of the qinling orogen and the assembly of the north china and yangtze plates during the late paleozoic systematic studies of the devonian sedimentary facies in the xiqingshan mountains indicate the northern margin of the south qinling belt was uplifted prior to the early devonian lower devonian sandstones are dominated by feldspathic litharenite q 12 31 f 18 37 l 40 65 indicative of a continental arc derivation detrital zircon u pb data demonstrate that the lower and middle devonian sandstones were derived respectively from ca 770 ma and ca 400 ma felsic arc related magmatic sources devonian sedimentation in the xiqingshan mountains is interpreted to have occurred in a foreland basin between the yangtze plate and the north qinling continental arc

single crystal x ray studies of trioctahedral micas coexisting with dioctahedral micas in metamorphic sequences from western maine a crystal chemical study of thirteen biotite twelve of 1 m polytype and one of 2 m 1 polytype and four muscovite samples was made the biotite coexists with the muscovite samples are from metamorphic terranes and from granitic and granodioritic bodies occurring in three areas of western maine the metamorphic mineral zones identified by mineral compatibilities are in order of increasing metamorphic grade the lower sillimanite zone lsz the upper sillimanite zone usz and the k feldspar sillimanite zone k sz the muscovite compositions cluster near ideal muscovite and display a small celadonite substitution and a small but variable paragonite substitution the biotite composition has values of vimg2 vi mg2 fe2 ranging from 0 26 to 0 54 and significant octahedral al content 0 48 vial 0 72 apfu in metamorphic biotite samples 0 51 vial 0 67 in those from granites in trioctahedral micas from western maine and especially in those with graphite there are a greater number of interlayer vacancies than in common micas interlayer vacancies have an increase in interlayer cation basal oxygen atom distances and a decrease in tetrahedral flattening angle thus suggesting a reduced interlayer charge with a few exceptions tetrahedral rotation angle is related to crystallization temperature in particular decreases with a temperature increase and is also related to octahedral chemical substitutions results tentatively suggest for micas from metamorphic environments a direct influence of genetic parameters t and f o2 on mica crystal structure and not just chemical composition

influences of reaction history and chemical diffusion on p t calculations for staurolite schists from the littleton formation northwestern new hampshire geothermometry from staurolite grade pelitic assemblages may fail to reflect peak temperatures of metamorphism because of 1 prograde gamet consuming reactions and 2 diffusional reequilibration during cooling in response to retrograde reactions gamet dissolution during staurolite formation leads to sharp compositional gradients within a few micrometers of garnet rims these gradients may then be modified by diffusion on cooling and lead to anomalous temperature estimates calculated from thermo barometers involving garnet numerical simulations of mineral growth and diffusion were used to examine these effects for a prograde metamorphic sequence from the salmon hole brook syncline northwestern new hampshire in higher grade assemblages mineral textures indicate that staurolite grew by the consumption of gamet and gamet biotite temperature estimates fall in the range 515 t 30 c interbedded layers containing garnet and biotite but not staurolite yield temperature estimates of 565 t 25 c in the eastern portion of the syncline rocks containing kyanite staurolite garnet biotite assemblages indicate temperatures of 550 t 25 c hypothetical growth and reaction histories that adhere to the modal mineralogical and known geochronological constraints were developed using a thermodynamically rigorous treatment of mineral modes and compositions incorporating calculations for multicomponent diffusion in garnet these models demonstrate that temperature estimates from staurolite schists may underestimate actual thermal maxima by more than 50 c gamet growth accompanying the reaction staurolite kyanite biotite garnet can lead to higher absolute temperature estimates but these may still be at least 50 c lower than actual peak temperatures simulations also demonstrate that fe enrichment in biotite accompanying retrograde production of chlorite may lead to calculated temperatures that are 10 50 c higher than would occur where only diffusional exchange operated on cooling evidence from additional metamorphic terranes suggests that this reaction model has wide applicability

evidence for neoarchean ni cu bearing mafic intrusions along a major lithospheric structure a case study from the south rae craton canada abstract deformed and metamorphosed mafic to ultramafic intrusive rocks are common along and adjacent to the snowbird tectonic zone a major paleoproterozoic structure separating the rae and hearne cratons on the rae side of this major lithospheric structure we investigated a series of mafic bodies that intrude high grade paragneiss and are associated with ni cu pge mineralization by linking field observations geochronology and metamorphic petrology our results indicate archean 2 63 ga crystallization ages for these mafic intrusions over a minimum along strike extent of 500 km and a later high temperature 820 c and 7 5 kbar paleoproterozoic metamorphic overprint at t

equilibria within the mineral assemblage quartz muscovite biotite garnet plagioclase and implications for the mixing properties of octahedrally coordinated cations in muscovite and biotite interaction parameters derived using empirical calibration methods indicate strong non ideality in the mixing of octahedrally coordinated cations in muscovite and biotite the data set used for calibration comprises mineral compositions from 49 samples containing quartz muscovite biotite garnet plagioclase and al2sio5 kyanite or sillimanite pressures and temperatures in the data set were determined through the simultaneous application of geothermometry based on the garnet biotite femg1 exchange equilibrium and geobarometry based on the anorthite breakdown equilibrium two equilibria yielded simple expressions from which binary interaction parameters for octahedrally coordinated cations in biotite could be directly determined a four component fe2 mg al ti regular symmetric mixing model was assumed for biotite one equilibrium yielded a simple expression from which an interaction parameter for the mixing of the mgal celadonite component in muscovite could be directly determined two sets of calculations were performed utilizing different calibrations of the garnet biotite geothermometer and the anothite breakdown geobarometer and different garnet activity models both placed samples within or near the stability field of the al2sio5 phase present in each sample and both yielded similar values for the interaction parameters within narrow uncertainties indicating that the values are insensitive to differences in the underlying methods using the derived interaction parameters activity models were formulated for the annite phlogopite eastonite and siderophyllite components of biotite and for the mgal celadonite component of muscovite these were utilized for the empirical calibration of 45 fluid independent equilibria involving unique combinations of phase components from the mineral assemblage garnet plagioclase biotite muscovite quartz forty three of the equilibria may be applied as geobarometers to equilibrium assemblages of quartz muscovite biotite garnet plagioclase when care is taken to insure that applications are restricted to valid compositional ranges for these the calibrations yielded multiple correlation coefficients ranging from 0 953 to 0 998 and standard deviations of the residuals ranging from 597 to 118 bars