mechanical interactions between rigid particles in a deforming ductile matrix analogue experiments in simple shear flow abstract the mechanical interaction between two or more particles is investigated in newtonian simple shear flow the experimental models allow observation of both the particle rotation and the deformation pattern around the particles the finite and instantaneous strain patterns around rigid particles are strongly heterogeneous and asymmetric with high finite strain zones aligned to the direction of maximum finite stretch these correspond to local flow with low vorticity number heterogeneous strain patterns around rigid particles spread over a distance of 1 2 times the particle length and this distance increases with increasing bulk strain where rigid particles are more concentrated these patterns coalesce and the overall pattern is then strongly controlled by the particles and cannot be simply related to the external boundary conditions in rocks the characteristic asymmetric strain pattern around rigid isolated particles is a reliable shear criterion which becomes unreliable with high concentrations of rigid particles particle rotation is significantly disturbed when the distance between adjacent particles of equal size is shorter than their length that is only in very concentrated suspensions of rigid particles in a composite shape fabric the development of the sub fabric corresponding to the smaller minerals will be more disturbed

strain softening along the mct zone from the sikkim himalaya relative roles of quartz and micas abstract in the darjeeling sikkim himalaya two distinct faults form the main central thrust mct the structurally higher mct1 and the lower mct2 each has accommodated translation greater than 100 km the lower mct2 places greater himalayan amphibolite grade paro lingtse gneiss over lesser himalayan greenschist grade daling metapelites the mct2 is folded by the underlying lesser himalayan duplex and is exposed at different structural positions of the fold at pelling the mct2 zone is exposed as a 373 m thick nw dipping fault zone that exposes 19 m of hanging wall mylonitized lingtse gneiss the lingtse protolith shows evidence of amphibolite grade plastic deformation features in quartz and feldspar within the hanging wall mylonite zone hwmz quartz and feldspar have undergone grain size reduction by different deformation mechanisms and feldspars are sericitized suggesting the presence of fluids during deformation we estimate a temperature of 300 c within the fault zone during fluid assisted retrogression and deformation reaction softening of feldspars produced a large proportion of intrinsically weak matrix this in combination with development of a strong foliation defined by parallel mica grains resulted in strain softening along the mct2 zone and concentrated the deformation along a thin zone or zones

strain rates from snowball garnet spiral inclusion trails in garnet porphyroblasts are likely to have formed due to simultaneous growth and rotation of the crystals during syn metamorphic deformation thus they contain information on the strain rate of the rock strain rates may be interpreted from such inclusion trails if two functions are known 1 the relationship between rotation rate and shear strain rate 2 the growth rate of the crystal we have investigated details of both functions using a garnetiferous mica schist from the eastern european alps as an example the rotation rate of garnet porphyroblasts was determined using finite element modelling of the geometrical arrangement of the crystals in the rock the growth rate of the porphyroblasts was determined by using the major and trace element distributions in garnet crystals thermodynamic pseudosections and information on the grain size distribution for the largest porphyroblast size fraction size l 12 mm we constrain a growth interval between 540 and 590 c during the prograde evolution of the rock assuming a reasonable heating rate and using the angular geometry of the spiral inclusion trails we are able to suggest that the mean strain rate during crystal growth was of the order of 6 6 10 14 s 1 these estimates are consistent with independent estimates for the strain rates during the evolution of this part of the alpine orogen

dynamic recrystallization mechanisms and their transition in the daling thrust dt zone darjeeling sikkim himalaya abstract the daling thrust dt delineates a zone of intense shear localization in the lesser himalayan sequence lhs of the darjeeling sikkim himalaya from microstructural studies of deformed quartzite samples we show a transition in the dynamic recrystallization mechanism with increasing distance from the dt dominated by grain boundary bulging blg recrystallization closest to the dt and progressively replaced by sub grain rotation sgr recrystallization away from the thrust the transition is marked by a characteristic variation in the fractal dimension d of grain boundaries estimated from the area perimeter method for the blg regime d 1 046 which decreases significantly to a value as low as 1 025 for the sgr regime using the available thermal data for blg and sgr recrystallization we infer increasing deformation temperatures away from the dt in the hanging wall based on the quartz piezometer our estimates reveal strong variations in the flow stress 59 00 mpa to 16 00 mpa over a distance of 1 2 km from the dt deformation mechanism maps constructed for different temperatures indicate that the strain rates 10 12 s 1 to 10 14 s 1 comply with the geologically possible range finally we present a mechanical model to provide a possible explanation for the cause of stress intensification along the dt

population wide garnet growth zoning revealed by la icp ms mapping implications for trace element equilibration and syn kinematic deformation during crystallisation the strong partitioning of many trace elements into garnet and their slow diffusivities in both garnet and the rock matrix means that their distribution may record valuable petrogenetic information not documented by major elements in metamorphic rocks complex trace element growth zoning in garnet porphyroblasts from a garnet grade metapelite from the barrovian sequence of the sikkim himalaya is assessed using quantified la icp ms raster mapping coupled with x ray micro computed tomography the data document systematic changes in the zoning patterns from early to late nucleated crystals and also suggest that the ree y chemistry incorporated into garnet is dependent on persistent disequilibrium in the rock volume there is evidence for hree y diffusion haloes surrounding growing garnets and a heterogeneous hree y distribution in the rock matrix annuli superimposed on oscillatory zoning are not consistent with formation during some rock wide event but are dependent on the spatial disposition of the garnet annuli are interpreted to reflect an integrated history of varying growth rates and the incorporation of pre existing heterogeneities due to relatively slow matrix diffusivities conversely smooth zoning of many transition metals indicate that their distribution in garnet may be controlled by equilibrium partitioning between garnet and the rock matrix significant rotation of garnet porphyroblasts during growth is revealed due to immobility of cr over the duration of the crystallisation interval and overprinting of the heterogenous precursor cr distribution strain rate estimates derived from this zoning are on the order of 10 11 10 12 hbox s 1

residence of ree y th and u in granites and crustal protoliths implications for the chemistry of crustal melts a systematic study with laser ablation icp ms scanning electron microscopy and electron microprobe revealed that 70 95 wt of ree except eu y th and u in granite rocks and crustal protoliths reside within reeythu rich accessories whose nature composition and associations change with the rock aluminosity the accessory assemblage of peraluminous granites migmatites and high grade rocks is composed of monazite xenotime in low ca varieties apatite zircon thorthosilicate uraninite and betafite pyrochlore metaluminous granites have allanite sphene apatite zircon monazite and thorthosilicaie peralkaline granites have aeschinite fergusonite samarskite bastnaesite fluocerite allanite sphene zircon monazite xenotime and th orthosilicate granulite grade garnets are enriched in nd and sm by no less than one order of magnitude with respect to amphibolite grade garnets granulitegrade feldspars are also enriched in lree with respect to amphibolite grade feldspars accessories cause non henrian behaviour of ree y th and u during melt solid partitioning because elevated fractions of monazite xenotime and zircon in common migmatites are included within major minerals their behaviour during anatexis is controlled by that of their host settling curves calculated for a convecting magma show that accessories are too small to settle appreciably being separated from the melt as inclusions within larger minerals biotite has the greatest tendency to include accessories thereby indirectly controlling the geochemistry of ree y th and u we conclude that ree y th and u are unsuitable for petrogenetical modelling of granitoids through equilibrium based trace element fractionation equations

major and trace element zoning in garnets from calcareous pelites in the nw shelburne falls quadrangle massachusetts garnet growth histories in retrograded rocks

monazite xenotime garnet equilibrium in metapelites and a new monazite garnet thermometer prograde suites of pelitic rocks were examined with electron microprobe introduction and laser ablation inductively coupled plasma mass spectrometry to monazite ce la th po4 plays an important role in determine the systematics of element partitioning between coexisting studies of igneous and metamorphic petrogenesis foremonazite xenotime and garnet monazite grains that grew in most monazite is used to date specific events in a equilibrium with xenotime are enriched in y and dy compared with petrogenetic sequence e g parrish 1990 harrison et al monazite that grew in xenotime absent assemblages y and heavy 1997 hawkins bowring 1999 foster et al 2000 rare earth element contents of monazite coexisting with xenotime monazite may contain a large percentage of the sample increase with rising temperature monazite xenotime y gd and rare earth element ree budget and thus exert an y dy partitioning is systematic within a metamorphic grade and important influence on the evolution of melt composition increases slightly with increasing metamorphic grade suggesting wark miller 1993 bea 1996 the thermobarometric that monazite xenotime pairs approached partitioning equilibrium potential of monazite coexisting with xenotime has been garnet and monazite in both xenotime bearing and xenotime absent recognized leading to calibration of monazite geoassemblages show a strong r 0 94 systematic relationship thermometers gratz heinrich 1997 1998 heinrich between inverse temperature and ln keq for the net transfer equiet al 1997 andrehs heinrich 1998 in addition the librium yag oh ap 25 4 qtz 5 4 grs 5 4 an recognition that nearly all lead in monazite is radiogenic 3ypo4 mnz 1 2h2o suggesting that garnet and monazite parrish 1990 montel et al 1994 has led to development crystallized in compositional equilibrium the following temand application of electron microprobe monazite dating perature keq relationship for the equilibrium above has been derived techniques suzuki adachi 1991 montel et al 1996 williams et al 1999 t c 1 45p bars 447772 32052 567 40 rln keq 273 15 full realization of monazite as a thermochronometer requires detailed understanding of the specific reactions responsible for its formation previous studies of monazite with a precision of some 30 c for temperature estimates our petrogenesis have focused on monazite compositional observations suggest that major and accessory phases interact in a zonation zhu o nions 1999a 1999b monazite coupled fashion during metamorphism and also approach a state growth kinetics ayers et al 1999 behavior of monazite of compositional equilibrium as reactions proceed during hydrothermal alteration poitrasson et al 1996 crowley ghent 1999 and melting events watt 1995 or to a limited extent the influence of major phase mineral assemblage in monazite reactivity zhu

rapid growth and strain rates inferred from synkinematic garnets penokean orogeny minnesota recent reinterpretations of the early proterozoic penokean orogeny in minnesota invoke a plate tectonic model with characteristics similar to phanerozoic collisional belts deformation during collision produced large flattening strains 89 and the development of a tectonic foliation and progressive metamorphism up to amphibolite facies as indicated by the presence of synkinematic s1 garnets biotite garnet thermometry and plagioclase garnet biotite muscovite barometry give southward increasing temperatures and pressures and indicate burial to depths of 21 25 km similar to medial and internal zones of collisional orogens worldwide gibbs method modeling on zoned synkinematic garnets indicates virtually no change in pressure p or temperature t during garnet growth growth and rotation of garnets at a point in p t space in an actively evolving deformational and thermal regime are surprising and suggest rapid garnet growth rates and high rates of strain during orogeny this is consistent with previous studies that indicate more rapid rates of tectonometamorphic processes in the precambrian relative to younger orogenic belts

studies of late prehistoric and modern opal phytoliths from coastal sand dunes and machair in northwest britain abstract this paper investigates the possibilities of identifying the origins of and classifying modern and prehistoric sand samples from coastal dune environments based on a study of their phytolith content a simple conservative classification of phytolith morphotypes has been developed for specimens from this type of sedimentary environment this classification has facilitated the recognition of anthropogenic and non anthropogenic depositional levels in ancient sand dunes on the basis of their contained phytolith suites areas of ancient human occupation and activity are shown to be characterized by phytolith concentrations which are larger by orders of magnitude than those that occur naturally in these coastal dune systems correspondence analysis has been used to investigate the extent to which some specific aspects of past human activity can be distinguished specifically the introduction of cut peat plant and animal waste and possible differences in the character of grazing or pasture in the dune systems

geochronologic and thermobarometric constraints on the evolution of the main central thrust central nepal himalaya the main central thrust mct juxtaposes the high grade greater himalayan crystallines over the lower grade lesser himalaya formation an apparent inverted metamorphic sequence characterizes the shear zone that underlies the thrust garnet bearing assemblages sampled along the marysandi river and darondi khola in the annapurna region of central nepal show striking differences in garnet zoning of mn ca mg and fe above and below the mct thermobarometry of mct footwall rocks yields apparent inverted temperature and pressure gradients of 18 c km 1 and 0 06 km mpa 1 respectively pressure temperature p t paths calculated for upper lesser himalaya samples that preserve prograde compositions show evidence of decompression during heating whereas garnets from the structurally lower sequences grew during an increase in both pressure and temperature in situ i e analyzed in thin section ion microprobe ages of monazites from rocks immediately beneath the greater himalayan crystallines yield ages from 18 to 22 ma whereas late miocene and pliocene monazite ages characterize rocks within the apparent inverted metamorphic sequence a lesser himalayan sample collected near the garnet isograd along the marysandi river transect contains 3 3 0 1 ma monazite ages p 0 72 gpa t 535 c this remarkably young age suggests that this portion of the mct shear zone accommodated a minimum of 30 km of slip over the last 3 ma i e a slip rate of 10 mm yr 1 and thus could account for nearly half of the convergence across the himalaya in this period the distribution of ages and p t histories reported here are consistent with a thermokinematic model in which the inverted metamorphic sequences underlying the mct formed by the transposition of right way up metamorphic sequences during late miocene pliocene shearing

sub millimeter heterogeneity of yttrium and chromium during growth of semi pelitic garnet x ray maps of yttrium and chromium in two garnets from a semipelitic schist from central nepal reveal parallel spiraling bands of high and low concentration 10 30 m wide this zoning mimics similar concentration bands defined by foliation in the matrix outside the garnets the garnets apparently incorporated the matrix zoning intact during growth at conditions up to lower amphibolite facies these observations and interpretations indicate that yand cr concentrations remained heterogeneous at the scale of tens of micrometers during breakdown of matrix minerals and growth of garnet this conclusion extends results of previous work which showed that y and cr do not homogenize at the centimeter to millimeter scale during metamorphism to amphibolite facies the width of the concentration bands in the garnets places a constraint on the maximum effective intracrystalline diffusion coefficient for both y and cr in garnet of about 10 21 cm 2 s for peak metamorphic conditions of about 9 kbar at 5708c different concentrations of yand cr at different positions around the rim of each garnet indicate disequilibrium for these elements between garnet and matrix phases the presence of an internal foliation defined by the yand cr concentration bands rules out pre kinematic garnet growth most useful in conjunction with other indicators zoning of these elements in garnet is thus a tool for deciphering the relative timing of deformation and garnet growth

trace element distributions in silicates during prograde metamorphic reactions implications for monazite formation to assess the petrogenetic relationship between monazite and major silicates during prograde metamorphism ree were measured across coexisting zoned silicates in garnet through kyanite grade pelitic schists from the great smoky mountains western blue ridge terrane southern appalachians to establish ree concentrations and distributions before and after the monazite in isograd and to identify the role major silicates play in the formation of monazite results indicate significant scavenging of light rare earth elements lree from silicates during the monazite in isograd reaction however the absolute concentration of lree hosted in the silicates was insufficient to produce monazite in the quantity observed in these schists monazite must have formed mainly from either the dissolution of allanite or some other source of concentrated lree possibly adsorbed onto grain boundaries even though direct evidence for allanite is lacking in a majority of the samples laser ablation icp ms analyses and theoretical thermodynamic calculations show that monazite may have formed as a result of contributions from both allanite and major silicates allanite breakdown initially formed monazite and monazite production drew lree liberated from allanite major silicates and possibly from crystal boundaries in many rocks the reaction was further promoted by the staurolite in reaction allowing for rapid isogradic monazite growth

rates of li diffusion in garnet coupled transport of li and y rees numerical simulation of stranded diffusion profiles in partially resorbed garnet crystals from the aureole of the makhavinekh lake pluton labrador canada yields quantitative rates of intracrystalline diffusion for li in garnet diffusion coefficients for li at 700 900 c and 0 53 gpa are 0 5 1 5 log10 units lower than those for divalent cations in the same samples and match those for y and yb this correspondence likely stems from coupled substitution of li and y or ree ions in the garnet structure the requirement of local electroneutrality limits li mobility to be no faster than that of y rees because of this coupling li zoning in garnet is retained to comparatively high temperatures making garnet a valuable monitor of the behavior of li in deep crustal systems

strain and strain rate in a synkinematic trondhjemitic dike evidence for melt induced strain softening during shearing bohemian massif czech republic abstract emplacement related deformation of a cambrian trondhjemitic dike located at the western border of the tepla barrandian unit central european variscides is studied in detail the trondhjemitic melt was emplaced at t 750 c into an active low temperature ca 350 c east northeast trending dextral transcurrent shear zone thermal modelling indicates that the rheologically critical melt fraction and the solidus of the dike were achieved after 1 and 3 days of cooling respectively the major part of the shearing occurred within 8 days after the melt emplaced the zonation of the dike in strain magnitude mineralogy and geochemistry can be explained by the laterally varying rheology from margin to centre during cooling and shearing the deformation was volume constant and the strain data plot in the apparent constrictional field the unusually high value of the calculated longitudinal strain rate 2 8 10 6 s 1 equivalent to a displacement rate of 2 cm h 1 is probably related to the intruding melt that lubricated the shear zone and thus enhanced its displacement velocity consequently melts that intrude as dikes into active transcurrent shear zones may significantly weaken the strength of the middle and upper continental crust

the significance of intergranular diffusion to the mechanisms and kinetics of porphyroblast crystallization the spatial disposition compositional zoning profiles and size distributions of garnet crystals in 11 specimens of pelitic schist from the picuris range of new mexico usa demonstrate that the kinetics of intergranular diffusion controlled the nucleation and growth mechanisms of porphyroblasts in these rocks an ordered disposition of garnet centers and a significant correlation between crystal radius and near neighbor distances manifest suppressed nucleation of new crystals in diffusionally depleted zones surrounding pre existing crystals compositional zoning profiles require diffusionally controlled growth the rate of which increases exponentially as temperature increases with time an acceleration factor for growth rate can be estimated from a comparison of compositional profiles for crystals of different sizes in each specimen crystal size distributions are interpreted as the result of nucleation rates that accelerate exponentially with increasing temperature early in the crystallization process but decline in the later stages because of suppression effects in the vicinity of earlier formed nuclei simulations of porphyroblast crystallization based upon thermally accelerated diffusionally influenced nucleation kinetics and diffusionally controlled growth kinetics quantitatively replicate textural relations in the rocks the simulations employ only two variable parameters which are evaluated by fitting of crystal size distributions both have physical significance the first is an acceleration factor for nucleation with a magnitude reflecting the prograde increase during the nucleation interval of the chemical affinity for the reaction in undepleted regions of the rock the second is a measure of the relative sizes of the porphyroblast and the diffusionally depleted zone surrounding it crystal size distributions for the picuris range garnets correspond very closely to those in the literature from a variety of other localities for garnet and other minerals the same kinetic model accounts quantitatively for crystal size distributions of porphyroblastic garnet phlogopite sphene and pyroxene in rocks from both regional and contact metamorphic occurrences these commonalities indicate that intergranular diffusion may be the dominant kinetic factor in the crystallization of porphyroblasts in a wide variety of metamorphic environments

oscillatory and sector zoned garnets record cyclic rapid thrusting in central nepal 1 garnets with oscillatory zoning repeated compositional reversals and sector zoning crystallographically controlled compositional differences were discovered in rocks from near the main central thrust in the langtang region central nepal this zoning results from crystal growth kinetic effects related to extraordinarily rapid possibly cyclic thrusting if cyclic thrusting occurred then timescales of thrust rate inhomogeneities are 25 kyr implying that satellite determination of modern fault movement rates may not accurately reflect slip rates or strain partitioning in orogens on timescales exceeding thousands of years sector zoning in ca but not y mn or mg implies maximum intracrystalline diffusion rates for ca of 10 21 cm2 s at 550 c but faster rates for other elements

late miocene movement within the himalayan main central thrust shear zone sikkim north east india in the sikkim region of north east india the main central thrust mct juxtaposes high grade gneisses of the greater himalayan crystallines over lower grade slates phyllites and schists of the lesser himalaya formation inverted metamorphism characterizes rocks that immediately underlie the thrust and the large scale south tibet detachment system stds bounds the northern side of the greater himalayan crystallines in situ th pb monazite ages indicate that the mct shear zone in the sikkim region was active at c 22 14 15 and 12 10 ma whereas zircon and monazite ages from a slightly deformed horizon of a high himalayan leucogranite within the stds suggest normal slip activity at c 17 and 14 15 ma although average monazite ages decrease towards structurally lower levels of the mct shear zone individual results do not follow a progressive younging pattern lesser himalaya sample kbp1062a records monazite crystallization from 11 5 0 2 to 12 2 0 1 ma and peak conditions of 610 25 c and 7 5 0 5 kbar whereas in the mct shear zone rock chg14103 monazite crystallized from 13 8 0 5 to 11 9 0 3 ma at lower grade conditions of 525 25 c and 6 1 kbar the p t t results indicate that the shear zone experienced a complicated slip history and have implications for the understanding of mid crustal extrusion and the role of out of sequence thrusts in convergent plate tectonic settings

formation of monazite via prograde metamorphic reactions among common silicates implications for age determinations three lines of evidence from schists of the great smoky mountains nc indicate that isogradic monazite growth occurred at the staurolite in isograd at 600 c 1 monazite is virtually absent below the staurolite in isograd but is ubiquitous several hundred grains per thin section in staurolite and kyanite grade rocks 2 many monazite grains are spatially associated with biotite coronas around garnets formed via the reaction garnet chlorite muscovite biotite plagioclase staurolite h2o 3 garnets contain high y annuli that result from prograde dissolution of garnet via the staurolite in reaction followed by regrowth and rare monazite inclusions occur immediately outside the annulus and in the matrix but not in the garnet core larger monazite grains also exhibit quasi continuous th zoning with high th cores and low th rims consistent with monazite growth via a single reaction and fractional crystallization during prograde growth common silicates may host sufficient p and lrees that reactions among them can produce observable lree phosphate specifically phosphorus contents of garnet and plagioclase are hundreds of parts per million and dissolution of garnet and recrystallization of plagioclase could form thousands of phosphate grains several micrometers in diameter per thin section lrees may be more limiting but sheet silicates and plagioclase can contain tens to 100 ppm lree so recrystallization of these silicates to lower lree contents could produce hundreds of grains of monazite per thin section monazite ages determined via electron and ion microprobes are 400 ma directly linking prograde barrovian metamorphism of the western blue ridge with the acadian orogeny in contrast to previous interpretations that metamorphism was taconian 450 ma interpretation of ages from metamorphic monazite grains will require prior chemical characterization and identification of relevant monazite forming reactions including reactions previously viewed as involving solely common silicates

the rotation rate of cylindrical objects during simple shear the rotational behaviour of circular objects is modelled using a two dimensional finite element simulation we present results for linear and non linear viscous rheologies and explore parameter space for the rheology contrast between object and matrix and for object size relative to the shear zone width l for high our results confirm analytical descriptions for l 0 in that the rotation rate of the object is half of the bulk shear strain rate however we show that for 0 1 l 0 9 the rotation rate as a function of l can have a minimum a maximum or it can decrease or increase depending on the stress exponent and in fact for some rheologies the rotation rate may decrease to 0 3 of the shear strain rate at intermediate l 0 5 20 area of porphyroblast in thin section these counter intuitive results have important consequences for the interpretation of the rotational behaviour of minerals in deforming rocks for example in rocks where garnet constitutes a volumetrically significant proportion of the bulk rock the effective shear zone boundaries may be given by neighbouring crystals corresponding to intermediate l as described here thus the interpretation of the rotation rate of crystals in such rocks can only be performed if the distance to the effective system boundaries is known

deformation partitioning and porphyroblast rotation in meta morphic rocks a radical reinterpretation abstract most porphyroblasts never rotate during ductile deformation provided they do not intern ally deform during subsequent events with the exception of relatively uncommon but spectacular examples of spiralling garnets instead the sur rounding foliation rotates and reactivates due to partitioning of the deformation around the porphyroblast consequently porphyroblasts commonly preserve the orientation of early foliations and stretching lineations within strain shadows or inclusion trails even where these structures have been rotated or obliterated in the matrix due to subsequent deformation these relationships can be readily used to help develop an understanding of the processes of foliation development and they demonstrate the prominent role of reactivation of old foliations during subse quent deformation they can also be used to determine the deformation history as porphyro blasts only rotate when the deformation cannot partition and involves progressive shearing with no combined bulk shortening component

chromium and manganese zoning in pelitic garnet and kyanite spiral overprint and oscillatory zoning patterns and the role of growth rate x ray composition maps and quantitative analyses for mn ca and cr have been made for six pelitic and calc pelitic garnet crystals and al fe and cr analyses maps have been made for two kyanite crystals from lower and mid upper amphibolite facies rocks from the grenville province of western labrador using an electron microprobe analyser and a laser ablation icp ms garnet with spiral snowball internal fabrics si has spiral zoning in major elements implying that growth was concentrated in discrete regions of the crystal at any one time spiral zoning cr zoning is parallel to si in low amphibolite facies garnet with both straight and spiral internal fabrics indicating that the garnet overprinted a fabric defined by cr rich mica chlorite epidote and cr poor quartz plagioclase layers during growth overprint zoning and that cr was effectively immobile in contrast in mid upper amphibolite facies garnet porphyroblasts lacking si cr zoning is concentric implying that cr diffusion occurred cr zoning in kyanite porphyroblasts appears superficially similar to oscillatory zoning with up to three or four annuli of cr enrichment and or depletion present in a single grain however the variable width continuity cr concentration and local bifurcation of individual annuli suggest that an origin by overprint zoning may be more likely the results of this study explain previously observed nonsystematic cr zoning in garnet and irregular partitioning of cr between coexisting metamorphic mineral pairs in addition this study points to the important role of crystal growth rate in determining the presence or absence of inclusions and the type of zoning exhibited by both major and trace elements during fast growth inclusions are preferentially incorporated into the growing porphyroblast and slow diffusing elements such as cr are effectively immobile whereas during slow growth inclusions are not generally included in the porphyroblast and cr zoning is concentric

diffusion limited ree uptake by eclogite garnets and its consequences for lu hf and sm nd geochronology garnets from the zermatt saas fee eclogites contain narrow central peaks for lu yb tm er and at least one additional small peak towards the rim the ree sm eu gd tb dy are depleted in the cores but show one prominent peak close to the rim these patterns cannot be modeled using rayleigh fractionation accompanied by mineral breakdown reactions instead the patterns are well explained using a transient matrix diffusion model where ree uptake is limited by diffusion in the matrix surrounding the porphyroblast observed profiles are well matched if a roughly linear radius growth rate is used the secondary peaks in the garnet profiles are interpreted to reflect thermally activated diffusion due to temperature increase during prograde metamorphism the model predicts anomalously low 176lu 177hf and 147sm 144nd ratios in garnets where growth rates are fast compared to diffusion of the ree and these results have important implications for lu hf and sm nd geochronology using garnet

trace element mapping by la icp ms assessing geochemical mobility in garnet a persistent problem in the study of garnet geochemistry is that the consideration of major elements alone excludes a wealth of information preserved by trace elements particularly the rare earth elements rees this is despite the fact that trace elements are generally less vulnerable to diffusive resetting and are sensitive to a broader spectrum of geochemical interactions involving the entire mineral assemblage including the growth and or dissolution of accessory minerals we outline a technique for the routine acquisition of high resolution 2d trace element maps by la icp ms and introduce an extension of the software package xmaptools for rapid processing of la icp ms data to visualise and interpret compositional zoning patterns these methods form the basis for investigating the mechanisms controlling geochemical mobility in garnet which are argued to be largely dependent on the interplay between element fractionation mineral reactions and partitioning and the length scales of intergranular transport samples from the peaked hill shear zone reynolds range central australia exhibit contrasting trace element distributions that can be linked to a detailed sequence of growth and dissolution events trace element mapping is thus employed to place garnet evolution in a specific paragenetic context and derive absolute age information by integration with existing u pb monazite and sm nd garnet geochronology ultimately the remarkable preservation of original growth zoning and its subtle modification by subsequent re equilibration is used to see through multiple superimposed events thereby revealing a previously obscure petrological and temporal record of metamorphism metasomatism and deformation

origins of yttrium and rare earth element distributions in metamorphic garnet highly variable distributions of yttrium and rare earth elements y rees are documented in pelitic garnet from the picuris mountains new mexico and from passo del sole switzerland and in mafic garnet from the franciscan complex california the wide variety of these y ree zoning patterns and those described previously in other occurrences imply diverse origins linked to differing degrees of mobility of these elements through the intergranular medium during garnet growth in the picuris mountains large early nucleating crystals have radial profiles of y ree dominated by central peaks and annular maxima in patterns that vary systematically with atomic number superimposed on these features are narrow spikes in hrees and mrees located progressively rimward with decreasing atomic number in contrast profiles in small late nucleating crystals contain only broad central maxima for all y rees in garnet from passo del sole y ree zoning varies radically from sample to sample in some rocks crystals of all sizes display only central peaks for all y rees in others profiles exhibit irregular fluctuations in y ree contents that match up with small scale patchy zoning in y and ca x ray maps in the franciscan complex y ree in garnet cores fluctuate unsystematically but mantles and rims display concentric oscillatory zoning for both major elements and y rees our interpretation of the complexity of y ree distributions in metamorphic garnet centres on the concept that these distributions vary primarily in response to the length scales over which these elements can equilibrate during garnet growth very short length scales of equilibration due to low intergranular mobility produce overprint zoning characterized by small scale irregularities higher but still restricted mobility yields diffusion controlled uptake characterized by patterns of central peaks and annular maxima that vary with atomic number and are strongly influenced by t t paths during garnet growth still greater mobility permits progressively greater potentially rock wide equilibration with major and accessory phase assemblages leading to mineralogical controls an unchanging mineral assemblage during garnet growth produces bell shaped profiles resembling those produced by rayleigh fractionation whereas changes in major and or accessory phase assemblages produce profiles with distinct annuli and sharp discontinuities in concentration the very high mobility associated with influxes of y ree bearing fluids can cause these element distributions to be dominated by factors external to the rock yielding profiles characterized by abrupt shifts or oscillations that are not correlated to changes in mineral assemblages

yttrium zoning in metamorphic garnets abstract yttrium zoning has been measured in garnets from the straits schist connecticut usa using the synchrotron x ray fluorescence sxrf microprobe sxrf compositional maps for mn and fe show that the major elements are concentrically zoned about the gannet core in the classic bell shaped profiles considered indicative of growth zoning yttrium however displays oscillatory zoning which varies three orders of magnitude in abundance from about 3 to 4000 ppm and is correlated with textural zoning in garnet the major element abundances by contrast display no visible inflection related to textural zoning at this time it is not possible to unequivocally identify the mechanism which has produced this oscillatory zoning most likely this reflects either the breakdown of y enriched phases changes in the rate of garnet growth with time or changes in the effective y concentration of matrix phases due to breakdown and reprecipitation of garnet as the equilibrium reactions involving gannet change this study demonstrates that the sxrf microprobe provides excellent spatially resolved compositional maps for trace elements in porphyroblasts something other analytical techniques have been unable to do it also suggests that the diffusion rates for y in garnet are probably significantly slower than those for major cations such as mn fe mg and ca as a result trace element mapping potentially provides a clearer record of the reaction history of the rock provides details of how chemical zoning in garnet is produced and allows evaluation of assumptions about chemical equilibrium between minerals

three dimensional patterns of garnet nucleation and growth three dimensional compositional zoning patterns in garnet zone garnets from harpswell neck maine united states suggest complex patterns of nucleation and growth compositional zoning is often asymmetric and core compositions do not always coincide with the geometric center of the garnet in detail garnet zoning patterns show multiple isolated high mno areas that we interpret as sites of garnet nucleation if it is assumed that garnet growth is close to equilibrium and the mn zoning is not significantly modified by diffusion then the mno contours or isosurfaces can serve as a proxy for patterns of garnet nucleation and growth computer generated three dimensional animations of these patchwork mno zoning patterns reveal that many garnets contain multiple nuclei that grew and coalesced to form a single crystal new nuclei often appeared next to preexisting porphyroblasts and became incorporated into the growing crystal contrary to the widely held idea that garnets grow radially outward from a single nucleus we propose that many of the garnets examined in this study grew by the coalescence of multiple nuclei

mineral leucosome trace element partitioning in a peraluminous migmatite a laser ablation icp ms study abstract low pressure peraluminous migmatites from the pena negra complex central spain stayed partially molten for 5 10 ma until the latest hercynian deformations allowed the segregation of melt as discordant leucosome veins due to long residence within its source the melt had time enough to equilibrate with residual phases other than accessories included within major refractory minerals we estimated crystal melt partition coefficients as the concentration ratios between leucosome samples appearing to be pure melts and mesosome minerals which were analyzed for trace elements with a laser probe coupled to an icp mass spectrometer our data reveal that when biotite is stable li rb cs tl sc v cr ni nb and ta become strongly compatible the role of biotite in fractionating y th u and the ree is insignificant cordierite strongly fractionates li and be and also has some effect on the hree and u garnet produces extreme fractionation of sc y and the hree the ree partition coefficients for ferromagnesian silicates increase with the atomic number this effect being progressively more important through biotite cordierite and garnet k feldspar strongly fractionates ba and pb but plays a secondary role to biotite for rb and cs k feldspar fractionation does not change the lree hree ratio but plagioclase fractionation produces a significant decrease in the lree hree ratio both feldspars greatly fractionate sr and eu monazite fractionation produces a dramatic depletion in ree th and u as well as a decrease in the lree hree and th u ratios apatite also fractionates the ree although in contrast with monazite it increases the lree hree ratio and does not affect the th u ratio zircon fractionation like apatite and garnet produces a strong depletion in the hree and a concomitant increase in the lree hree ratio in contrast with monazite zircon fractionation causes the th u ratio to increase

a new kinematic evolutionary model for the growth of a duplex an example from the rangit duplex sikkim himalaya india abstract the lesser himalayan duplex lhd is a prominent structure through much of the lesser himalayan fold thrust belt in the darjeeling sikkim himalaya a component of the lhd is exposed in the rangit window as the rangit duplex rd the rd consists of ten horses of the upper lesser himalayan sequence gondwana buxa upper daling the duplex varies from hinterland dipping in the north through an antiformal stack in the middle to foreland dipping in the south the ramgarh thrust rt is the roof thrust and based on a balanced cross section the main himalayan sole thrust is the floor thrust at a depth of 10 km and with a dip of 3 5 n retrodeformation suggests that the rd initiated as a foreland dipping duplex with the early ramgarh thrust as the roof thrust and the rt as the floor thrust the rt became the roof thrust during continued duplexing by a combination of footwall imbrication and concurrent rt reactivation this kinematic history best explains the large translation of the overlying mct sheets the restoration suggests that rd shortening is 125 km and the original gondwana basin extended 142 km northward of its present northernmost exposures within the window

effective bulk composition changes due to cooling a model predicting complexities in retrograde reaction textures diffusive processes are a strong function of temperature thus during cooling of rocks mineral grains may develop zoning profiles as successively larger parts of the grain close to the diffusive exchange with the rock one of the consequences of this process is that during cooling successively larger parts of zoned minerals depending on grain size are effectively removed from the reacting part of the rock volume thus the effective bulk composition of metamorphic rocks changes during cooling and the rate of its change will be a function of grain size because the sequence of metamorphic reactions seen by a given rock is a strong function of its bulk composition this process may have the consequence that two rocks of identical overall bulk composition but of different grain size may experience a different sequence of reactions qualitatively identical peak paragenesis may therefore react to form qualitatively different retrograde reaction textures the model is applied to examples in the pelitic system there garnet is usually the slowest diffusing phase developing zoning profiles during cooling and the effective removal of garnet from the reacting rock volume will cause changes of the effective bulk composition it is shown that during cooling of pelitic rocks from amphibolite facies conditions typical aluminous peak parageneses of garnet muscovite kyanite biotite may react to form either staurolite chlorite or muscovite or different combinations thereof depending on grain size during cooling from the granulite facies aluminous peak parageneses of garnet cordierite sillimanite may form biotite either on the expense of cordierite or garnet also depending on grain size the two examples are illustrated with a series of reaction textures reported for amphibolite and granulite terrains in the literature

a comparison of three methods for the extraction of phytoliths from sediments abstract this paper compares three methods commonly used to extract fossil phytoliths from sediments a basic procedure using heavy liquid flotation and oxidation is compared with two other procedures across a range of sediment types commonly encountered in archaeological studies the three procedures are 1 a heavy liquid flotation method hlf 2 a burning method pow and 3 another heavy liquid flotation method hlfpol similar to hlf but adapted to allow the extraction of pollen and spores as well as phytoliths within a single process comparisons of the resulting output using these three techniques for phytolith extraction show that different methods can produce different results and therefore basic techniques should be modified according to the characteristics of the sediments for which they are used while all the techniques showed similarities in assemblage results there were problems associated with disaggregation and effective separation of light and heavy fractions in particular with the pow procedure the evidence suggests that morphotype selection occurred both within the physical sorting process and in the process of inverting slides to shed excess residue in both cases it is difficult to suggest a solution to the problem the results show clearly that the advantages gained by using the pow procedure are largely outweighed by the problems encountered with its use and because of possible size shape selection it is not recommended for general extraction procedures the heavy liquid flotation procedures on the other hand are shown to produce more concentrated residues with higher levels of clarity and less potential than the pow procedure for sample bias the use of a non toxic heavy liquid sodium polytungstate now allows the process to be used in relative safety it is recommended that analysts use heavy liquid flotation procedures with chemical treatments specific to sediment requirements

porphyroblast inclusion trails the key to orogenesis detailed microstructural analysis of inclusion trails in hundreds of garnet porphyroblasts from rocks where spiral shaped inclusion trails are common indicates that spiral shaped trails did not form by rotation of the growing porphyroblasts relative to geographic coordinates they formed instead by progressive growth by porphyroblasts over several sets of near orthogonal foliations that successively overprint one another the orientations of these near orthogonal foliations are alternately near vertical and near horizontal in all porphyroblasts examined this provides very strong evidence for lack of porphyroblast rotation the deformation path recorded by these porphyroblasts indicates that the process of orogenesis involves a multiply repeated two stage cycle of 1 crustal shortening and thickening with the development of a near vertical foliation with a steep stretching lineation followed by 2 gravitational instability and collapse of this uplifted pile with the development of a near horizontal foliation gravitational spreading near coaxial vertical shortening and consequent thrusting on the orogen margins correlation of inclusion trail overprinting relationships and asymmetry in porphyroblasts with foliation overprinting relationships observed in the field allows determination of where the rocks studied lie and have moved within an orogen this information combined with information about chemical zoning in porphyroblasts provides details about the structural metamorphic p t t paths the rocks have followed the ductile deformation environment in which a porphyroblast can rotate relative to geographic coordinates during orogenesis is spatially restricted in continental crust to vertical ductile tear transcurrent faults across which there is no component of bulk shortening or transpression

monazite xenotime miscibility gap thermometry i an empirical calibration rare earth element ree and yttrium concentrations of coexisting monazite and xenotime were determined from a suite of seven metapelites from the variscan fold belt in ne bavaria germany the metapelites include a continuous prograde mainly low p 3 5 kbar metamorphic profile from greenschist c 400 c to lower granulite facies conditions c 700 c the lree la sm are incorporated preferentially in monoclinic monazite reo9 polyhedron whereas the hree plus y are concentrated in tetragonal xenotime reo8 polyhedron the major element concentrations of both phases in all rocks are very similar and do not depend on metamorphic grade monazite consists mainly of la ce and nd la0 20 0 23 ce0 41 0 45 nd0 15 0 18 po4 all other elements are below 6 mol likewise xenotime consists mainly of ypo4 with some dy and gd solid solutions y0 76 0 80 dy0 05 0 07 gd0 04 0 06 in contrast the minor hree concentrations in monazite increase strongly with increasing metamorphic grade y dy and gd increase by a factor of 3 5 from greenschist to granulite facies rocks monazite crystals often show zonation with cores low in hree and rims high in hree that is interpreted as growth zonation attained during prograde metamorphism similarly sm and nd in xenotimes increase by a factor of 3 4 with increasing metamorphic grade prograde zonation in single crystals of xenotime was not observed the xhree y in monazite and xlree in xenotime of the seven rocks define two limbs along the strongly asymmetric miscibility gap from c 400 c to 700 c the empirical calibration of the monazite miscibility gap limb coexisting with xenotime is appropriate for geothermometry due to its contents of u and th monazite has often been used for u pb age determination the combination of our empirical thermometer on prograde zoned monazite along with possible age determination of zoned single crystals may provide information about prograde branches of temperature time paths

the nature and evolution of the main central thrust structural and geochronological constraints from the sikkim himalaya ne india the main central thrust mct is a prominent continental scale fault within the himalaya its definition has been the topic of some debate in the literature after a brief consideration of the state of discussion to clarify the definition we use in this work we report features from the field to the microstructural scale of a particularly well exposed section in sikkim ne india the nature of the protoliths as well as the overlying and underlying rocks is characterized in terms of e nd the dates of motion on the fault are constrained using u pb geochronology of zircon and monazite from pegmatitic dikes that cross cut the deformation fabric it is found that the mechanism of deformation recorded in the fault zone rocks is different compared to that found in the overlying greater himalayan gh or the underlying lesser himalayan lh rocks the gh and lh have different protolith characteristics as well combined with existing data on p t history dates of metamorphism and cooling and exhumation rates of the gh and the lh our measurements show that major motion on this fault occurred before 20 ma at 450 700 c but after peak metamorphism of rocks 750 800 c in this zone isolated events occurred in this zone as late as 11 ma possibly in the brittle domain this underscores the pulsed nature of movement over an extended period on such major faults and the related difficulties in dating fault movement determination of the rates of movement and designating a fault plane as in or out of sequence within a propagating deformation front

trace element zoning in a metamorphic garnet trace element zoning has been measured in an amphibolite garnet from the tauern window austria by using an ion microprobe humps in the zoning profiles of na sc v y and the heavy rare earth elements mark a period of open system behavior these humps correspond to a part of the major element zoning profile that is interpreted as a p t reversal the source of the mass excesses of these elements remains ambiguous they were derived either on a thin section scale by the breakdown of trace element enriched refractory minerals or externally from unusual trace element enriched fluids p t paths determined from garnet zoning may require modification if open system behavior is important during garnet growth

deformation fields around a fault embedded in a non linear ductile medium summary the geological record of deformation is often characterized by a combination of discontinuous deformation in which strain is concentrated in faults and continuous deformation in which strain is distributed through the material where slip occurs on a fault that terminates the surrounding material is deformed in the lower crust and in cases where large strains occur over long geological time scales it is appropriate to model the deformation using a viscous probably non linear viscous rheology we describe a method for practical finite element solution of this problem using a dynamically self consistent formulation for stress and displacement on a fault of arbitrary geometry the accuracy of the method is tested by comparison with an analytical solution for the linear rheology we describe here the instantaneous deformation fields around a mode ii fault under both plane strain and plane stress conditions and a range of rheological exponents n where strain rate is proportional to deviatoric stress to the nth power the distributions of stress and strain rate around the fault tip are controlled primarily by the rheological exponent n a localized zone of high strain rate projects beyond the end of the fault if n is about 3 or greater and the degree of localization of deformation increases with the value of n the zone of high shear strain rate can be defined in practical terms by considering 1 the region in which the creep velocity differs by more than 20 per cent from the velocity on the nearby external boundary and 2 the region in which the maximum shear strain rate is greater than about twice the externally imposed shear strain rate for n 1 the volumes so defined differ considerably but for large values of n the two definitions both describe the same narrow zone of deformation beyond the end of the fault evaluation of the navier coulomb criterion for brittle failure of the medium surrounding the fault tip shows first that brittle failure is much more likely on the extensional side of the fault than the compressional side it also shows that the volume of material subject to brittle failure decreases rapidly with increasing n because of the relatively weaker stress singularity we analyse previously published displacement versus distance data for faults terminating in sedimentary rocks at 0 1 to 100 m length scales under different tectonic conditions in order to determine the rheological exponent n these analyses result in n values between approximately 0 85 and 5 for the different faults with error bounds on n typically pl 1 the variation in n values may result from differences in pressure temperature and fluid conditions at the time of faulting more importantly the analysis demonstrates a new method for the determination of the effective rheological exponent under in situ geological conditions

geothermobarometry and development of inverted metamorphism in the darjeeling sikkim region of the eastern himalayan abstract the darjeeling sikkim region provides a classic example of inverted himalayan metamorphism the different parageneses of pelitic rocks containing chlorite biotite garnet staurolite kyanite sillimanite plagioclase and k feldspar are documented by a variety of textures resulting from continuous and discontinuous reactions in the different zones microprobe data of coexisting minerals show that xmg varies in the order garnet staurolite biotite chlorite white mica is a solid solution between muscovite and phengite garnet is mostly almandine rich and shows normal growth zoning in the lower part of the main central thrust mct zone and reverse zoning in the upper part of the zone chemographical relations and inferred reactions for different zones are portrayed in afm space in the low grade zones oriented chlorites and micas and rolled garnets grew syntectonically and were succeeded by cross cutting chlorites and micas and garnet rims in the upper zones sillimanite kyanite and staurolite crystallized during a static inter kinematic phase p t contitions of metamorphism estimated through different models of geothermobarometry are estimated to have been 580 c for the garnet zone to a maximum of 770 c for the sillimanite zone the preferred values of pressure range from 5 0 kbar to 7 7 kbar models to explain the inverted metamorphism include overthrusting of a hot high himalayan slab along a c 5 km wide ductile mct zone and the syn or post metamorphic folding of isograds

scales of disequilibrium and rates of equilibration during metamorphism mounting evidence suggests that partial disequilibrium meaning disequilibrium for some elements but not for others may be a common but rarely detected phenomenon during metamorphic mineral growth even in ordinary prograde reactions that progress to completion detailed examination of compositional variations in garnet crystals over a range of metamorphic grade suggests distinctly different scales of equilibration for common elements with strong temperature dependence under lower greenschist facies conditions fe and mg may equilibrate at hand sample scale whereas mn and ca may not equilibrate even at millimeter scale although mn may achieve hand sample scale equilibration under upper greenschist facies conditions ca and many trivalent cations e g rees may not do so until temperatures exceed those of the middle amphibolite facies even in the lower granulite facies some elements e g y yb show indications of disequilibrium at sub centimeter scales during garnet growth analysis and numerical modeling of undisputed disequilibrium textures demonstrate that the most common impediment to equilibration during metamorphism is the sluggishness of intergranular diffusion the same mechanism known to govern porphyroblast crystallization in many metamorphic environments despite this importance to petrology few quantitative determinations exist of inter granular diffusion rates under metamorphic conditions using numerical simulations of coupled intergranular and intracrystalline diffusion processes in coronal textures around partially resorbed garnet crystals from the llano uplift u s a a very precise and relatively accurate estimate is obtained for the rate of intergranular diffusion of al in fluid undersaturated systems this result and an earlier estimate for fluid saturated systems provide bracketing values for al diffusivity during metamorphism

p t paths from garnet zoning a new technique for deciphering tectonic processes in crystalline terranes a procedure to calculate quantitative rock pressure temperature p t paths based on chemical zoning profiles in garnet combined with mineral chemistry from the other minerals in the assemblage has been applied to samples from diverse tectonic settings with the result that the calculated p t path is shown to be a sensitive monitor of tectonic processes terranes where metamorphic recrystallization paths are controlled by uplift and erosion show heating during decompression in the prograde p t path and cooling during decompression in the retrograde path in nappe terranes where hot rocks are emplaced over cool rocks the upper plate shows cooling during decompression whereas the lower plate shows heating during compression in nappe terranes where cool rocks are emplaced over hot the lower plate shows cooling during compression calculation of p t paths from mineral zoning should provide a powerful new tool for deciphering tectonic processes in crystalline terranes

disequilibrium for ca during growth of pelitic garnet compositional zoning in hundreds of almandine rich garnets in amphibolite by facies micaceous quartzites from the picuris range north central new mexico usa indicates that although mn mg and fe achieve chemical equilibrium at hand sample scale during garnet growth ca does not instead ca concentrations at the surface of growing garnets appear to depend strongly on kinetic factors that govern the local chemical environment yielding disequilibrium for ca at scales larger than the region immediately surrounding an individual porphyroblast detailed zoning profiles were obtained for 371 garnet crystals in a small volume of a single sample of garnetiferous quartzite and core analyses were made of 97 additional crystals each analysis was made on a section that passed precisely through the morphological centre of the crystal located by means of 3 d imagery from computed x ray tomography the data reveal strong correlations between crystal size and concentrations of mn mg and fe but not ca in garnet cores a relationship between crystal size and isolation rigorous cross correlations among concentrations of mn mg and fe but not ca and systematic variations in ca concentrations as a function of crystal size and core composition that are anomalous in comparison to the behaviour of the other divalent cations we interpret these observations as the result of thermally accelerated diffusion controlled garnet growth in circumstances that promoted rapid intergranular diffusion and thus rock wide equilibration of mn mg and fe but that prevented equilibration at similar scale for ca because of its more sluggish intergranular diffusion the anomalous behaviour of ca is made evident in these garnets by the presence of sharp spikes in ca concentration which are demonstrably not a consequence of any simultaneous rock wide event such as a change in pressure temperature or some other intensive parameter instead ca concentrations probably reflect the local extent of reaction in the immediate vicinity of each porphyroblast to the degree that such kinetic factors introduce departures from chemical equilibrium for ca thermobarometric estimates that involve grossular contents of pelitic garnet will be in error

inclusion trail patterns in porphyroblasts from the foothills terrane california a record of orogenesis or local strain heterogeneity a major problem with the current use of porphyroblast matrix microstructural relationships to infer orogenic histories such as multiple orthogonal orogenic events is that other evidence for these events is typically lacking for example a comparison of regional relationships and local structures formed in and adjacent to porphyroblasts present in contact aureoles in the foothills terrane sierra nevada california shows that 1 except in shear zones contact aureoles and local zones along lithological contacts the foothills terrane has a single regional cleavage although locally formed by multiple processes 2 the regional cleavage and locally developed porphyroblast inclusion trails have variable orientations and neither dataset supports the formation of dominantly subhorizontal and subvertical cleavages in this orogen 3 structural and metamorphic heterogeneities occur at all scales and can markedly affect inclusion trail patterns in porphyroblasts 4 complex porphyroblast growth features and internal inclusion trail patterns can form in porphyroblasts that grow during short time intervals in contact aureoles indicating that local complexity in porphyroblasts does not imply regional complexity because of these conclusions multiple datasets rather than data acquired only from porphyroblasts should be considered when attempting to understand the evolution of orogens furthermore using microstructural information preserved only in porphyroblasts to infer orogenic processes and plate motions is generally unjustified

orogen transverse tectonic window in the eastern himalayan fold belt a superposed buckling model abstract the eastern lesser himalayan fold thrust belt is punctuated by a row of orogen transverse domal tectonic windows to evaluate their origin a variety of thrust stack models have been proposed assuming that the crustal shortening occurred dominantly by brittle deformations however the rangit window rw in the darjeeling sikkim himalaya dsh shows unequivocal structural imprints of ductile deformations of multiple episodes based on new structural maps coupled with outcrop scale field observations we recognize at least four major episodes of folding in the litho tectonic units of dsh the last episode has produced regionally orogen transverse upright folds f 4 the interference of which with the third generation f 3 orogen parallel folds has shaped the large scale structural patterns in dsh we propose a new genetic model for the rw invoking the mechanics of superposed buckling in the mechanically stratified litho tectonic systems we substantiate this superposed buckling model with results obtained from analogue experiments the model explains contrasting f 3 f 4 interferences in the lesser himalayan sequence lhs the lower order terrain scale folds have undergone superposed buckling in mode 1 producing large scale domes and basins whereas the rw occurs as a relatively higher order dome nested in the first order tista dome the gondwana and the proterozoic rocks within the rw underwent superposed buckling in modes 3 and 4 leading to type 2 fold interferences as evident from their structural patterns

timing duration and inversion of prograde barrovian metamorphism constrained by high resolution lu hf garnet dating a case study from the sikkim himalaya ne india abstract we investigated lu hf isotopic systematics in garnets from gradually higher grade metamorphic rocks from the first appearance of garnet at c 500 c to biotite dehydration melting at c 800 c in the sikkim himalaya india exceptionally precise lu hf ages obtained for the barrovian metasedimentary sequence in the lesser himalaya lh correspond to the time of early garnet formation on a prograde path and show remarkable correlation with increasing metamorphic grade and decreasing structural depth the youngest age is reported for the garnet zone 10 6 0 2 ma and then the ages become progressively older in the staurolite 12 8 0 3 ma kyanite 13 7 0 2 ma and sillimanite 14 6 0 1 ma zones the oldest age of 16 8 0 1 ma was recorded at the top of the sequence in the zone marking the onset of muscovite dehydration melting directly below the main central thrust mct these ages provide a tight constraint on the timing and duration of the barrovian sequence formation which lasted about 6 ma the age pattern is clearly inverted with respect to structural depth but shows normal correlation with the metamorphic grade i e earlier garnet growth in higher grade rocks the timing of the peak metamorphic conditions estimated for the garnet and kyanite zones suggests that thermal climax occurred in all zones within a relatively short time span about 10 13 ma ago the metamorphic inversion in the sikkim himalaya must therefore have occurred after the metamorphic peak was attained in all grades lu hf garnet ages in the overthrust higher himalaya hh continue to be older but do not show a clear progression there is about 6 ma jump to 22 6 0 1 ma within the mct zone and even older ages of 26 28 ma were obtained for migmatites from the lower part of the hh the lu hf system in garnets from the hh records the time of melting at or near the thermal peak rather than dates initial garnet growth as in the lh the age pattern obtained in this study provides precise time constraints on the continental collision models which must take into account early and prolonged anatexis in the hh progressive delay in initial garnet growth in different barrow zones in the lh nearly contemporaneous metamorphic peak in all grades and post peak inversion of the coherent lh block during exhumation

rates of fe mg mn and ca diffusion in garnet extraction of the thermal and chemical histories preserved by modified compositional zoning in garnet is compromised by large uncertainties in experimentally determined diffusion coefficients for principal divalent cations some of this uncertainty derives from long down temperature extrapolations but much of it arises from an inability to account accurately for large variations in diffusivity as a function of garnet composition numerical simulation of stranded diffusion profiles in partially resorbed natural garnets constrains diffusivities at low temperature and reveals a systematic dependence of diffusion rates on host garnet composition a comprehensive model incorporating a simple compositional dependence allows accurate prediction of diffusion rates for fe mg mn and ca for nearly all garnet compositions across the full range of geologically relevant temperatures pressures and oxygen fugacities

crustal reworking and hydration insights from element zoning and oxygen isotopes of garnet in high pressure rocks sesia zone western alps subduction zones represent one of the most critical settings for fluid recycling as a consequence of dehydration of the subducting lithosphere a better understanding of fluid flows within and out of the subducting slab is fundamental to unravel the role of fluids during burial in this study major and trace element geochemistry combined with oxygen isotopes were used to investigate metasediments and eclogites from the sesia zone in order to reconstruct the effect of internal and external fluid pulses in a subducted continental margin garnet shows a variety of textures requiring dissolution precipitation processes in presence of fluids in polycyclic metasediments garnet preserves a partly resorbed core related to pre alpine high temperature low pressure metamorphism and one or multiple rim generations associated with alpine subduction metamorphism in eclogites garnet chemical zoning indicates monocyclic growth with no shift in oxygen isotopes from core to rim in metasediments pre alpine garnet relics show 18o values up to 5 3 higher than the alpine rims while no significant variation is observed among different alpine garnet generations within each sample this suggests that an extensive re equilibration with an externally derived fluid of distinct lower 18o occurred before or in correspondence to the first alpine garnet growth while subsequent influxes of fluid had 18o close to equilibrium the observed shift in garnet 18o is attributed to a possible combination of 1 interaction with sea water derived fluids during pre alpine crustal extension and 2 fluids from dehydration reactions occurring during subduction of previously hydrated rocks such as the serpentinised lithospheric mantle or hydrated portions of the basement

strain rates at high temporal resolution from curved inclusion trails in garnet passo del sole central swiss alps quantitative strain rates at outcrop scale are very difficult to obtain but they may be estimated from crystals with curved inclusion trails by calculating rotation rates from growth rates and corresponding deflections of the internal foliation garnet in a quartzose pelite at passo del sole in the central swiss alps is extraordinarily valuable for calculation of strain rates during alpine orogenesis because the unusual zoning patterns clearly define the kinetics of its nucleation and growth complex concentric zoning patterns can be correlated from one crystal to another in a hand sample based on compositional and microstructural similarities the ubiquity of these features demonstrates that all garnet crystals nucleated at nearly the same time compositional bands whose radial widths are proportional to crystal size provide evidence for growth governed by the kinetics of intergranular diffusion of locally sourced nutrients together these constraints increase the reliability of estimates of rates of garnet growth and the strain rate calculations that depend on them to obtain growth rates p t conditions during garnet crystallization were modelled in a series of pseudosections and compositional evolution was connected to rates of garnet growth by means of an independently determined heating rate these growth rates combined with measured amounts of curvature of inclusion trails indicate that the time averaged strain rate at passo del sole during alpine metamorphism was on the order of 10 14 s 1 strain rates calculated using rotational v non rotational models are similar in magnitude the constraints on crystallization kinetics also allow direct calculation of strain rates during individual stages of garnet growth revealing short term increases to values on the order of 10 13 s 1 these higher strain rates are correlated with the growth of concentric high ca or high mn zones in garnet which implies that strain softening associated with the transient passage of fluids is responsible for acceleration of deformation during these intervals

trace element fractionation during high grade metamorphism and crustal melting constraints from ion microprobe data of metapelitic migmatitic and igneous garnets and implications for sm nd garnet chronology abstract rare earth element ree and other trace element y sr ti cr v na abundances in garnet from a garnet bearing metapelite a pelitic migmatite a syn tectonic granite and a post tectonic leucogranite were measured by secondary ion mass spectrometry sims in order to identify the effective variables on the trace element distribution between garnet and the host rock garnet from the garnet bearing metapelite the pelitic migmatite and the syn tectonic granite is zoned with respect to ree the cores are enriched by a factor of 2 3 relative to the rims for the garnets from the garnet bearing metapelite equilibrium distribution following a simple rayleigh fractionation is responsible for the decreasing concentrations in ree from core to rim garnet from the pelitic migmatite shows a more complex trace element pattern following distinct enrichment and depletion patterns for ti v cr and ree from core to rim these features suggest disequilibrium between garnet and the associated melt in which the enrichment of trace elements probably correspond to a period of open system behaviour in these rocks at a time when the garnet originally nucleated in the metamorphic environment was incorporated into the melt the garnet from the syn tectonic granite shows stepwise decreasing concentrations in ree from core to rim a ree rich core can be distinguished from a broad ree depleted rim notably from core to rim an inflection of the yb er and yb dy ratios is visible whereas the decrease of hree abundance in the core region of the garnet from the syn tectonic granite may arise from equilibrium partitioning during garnet growth the inflection can be interpreted as a result of partial melting garnet cores with high yb er and yb dy 1 nucleated in the metamorphic environment without the presence of a melt whereas the rims with lower yb er and yb dy

diffusional homogenization of light ree in garnet from the day nui con voi massif in n vietnam implications for sm nd geochronology and timing of metamorphism in the red river shear zone abstract high grade migmatitic and mylonitic gneisses from the day nui con voi massif in northern vietnam record temperatures of 760 810 c at pressures of 6 10 kbars high temperature conditions have resulted in the development of major element diffusional profiles in garnet laser ablation icp ms analyses of trace elements indicate that ree and hf closely followed rayleigh like fractionation trends but underwent significant post crystallization modification light ree and to a lesser extent hree rim to rim zonation profiles show progressive flattening with the decreasing garnet size nd and sm are completely homogenized in the crystals smaller than 1 5 mm while lu always preserves variable degrees of core to rim concentration gradient the observed ree patterns are interpreted as resulting from the combination of protracted garnet growth of progressively smaller crystals and intracrystalline diffusion this had profound influence on sm nd geochronology and resulted in isochron ages ranging from 50 to 32 ma the youngest age was obtained for a sample where all garnet crystals are smaller than 2 mm in which light ree profiles are completely or nearly completely homogenized thus only the youngest age represents geologically meaningful event and 31 7 0 9 ma age is interpreted as the best estimate of the resetting episode due to high temperature diffusional homogenization of light ree during early oligocene metamorphism older sm nd ages reflect mixed analyses of variably reset individual garnet crystals lu hf isotopic analyses of bulk garnet fractions despite yielding high parent daughter isotopic ratios appeared very scattered and did not allow defining isochron ages instead apparent ages defined by whole rock and individual garnet fractions range from c 80 to 160 ma very old apparent ages are interpreted as being the consequence of intracrystalline lu diffusion and preservation of the original hf distribution which leads to lower 176 lu 177 hf ratios and thus steeper older isochrons back diffusion of lu during commonly observed resorption played a subordinate role in modifying isotope systematics

yttrium zoning in garnet coupling of major and accessory phases during metamorphic reactions the concentration of yttrium in pelitic garnets as a function of metamorphic grade has been examined in relation to the distribution of xenotime ypo 4 in samples from new england and british columbia samples with xenotime present only as inclusions in garnet generally possess high y cores and concentrations that drop off discontinuously along zoning shoulders of variable width to low y outboard regions samples with matrix xenotime are restricted to the garnet zone y concentration of these garnets generally decreases smoothly from core to rim xenotime may also be present in reaction zones around garnet in xenotime bearing samples y g r t is strongly temperature dependent and ranges from 5000 ppm in the garnet zone to 150 ppm in the sillimanite zone measured yttrium zoning profiles in xenotime absent samples are reproduced with both rayleigh fractionation and diffusion models but p t histories of the samples examined favor the rayleigh model with garnet volume bulk rock yttrium and mode of y hree accessory phases controlling the profile shape high yttrium annuli in staurolite zone samples may form by garnet overgrowth of proximal matrix enriched in yttrium due to garnet consumption during discontinuous staurolite forming reactions an increase in y g r t and hree g r t in garnet from anatectic samples is related to dissolution of phosphates in vapor absent peraluminous melt with partitioning of highly compatible y and hree into garnet grown during anatexis textural analysis reveals that phosphates are absent from regions of garnet grown in equilibrium with melt a main result of this study is identification of an intimate coupling between major pelite phases and accessory phases during reaction progress this coupling is of great advantage in that it may be used to 1 calibrate sensitive geothermometers and geobarometers 2 identify particular regions of garnet grown in different garnet producing reactions over a range of grades and 3 reveal portions of pelite reaction history invisible to major elements

the duration of prograde garnet crystallization in the uhp eclogites at lago di cignana italy abstract the distinct core to rim zonation of different rees in garnet in metamorphic rocks specifically sm relative to lu suggests that sm nd and lu hf isochron ages will record different times along a prograde garnet growth history therefore ree zonations in garnet must be measured in order to correctly interpret the isochron ages in terms of the garnet growth interval which could span several m y new ree profiles garnet crystal size distributions and garnet growth modeling combined with previously published sm nd and lu hf geochronology on a uhp eclogite of the zermatt saas fee zsf ophiolite lago di cignana italy demonstrate that prograde garnet growth of this sample occurred over a 30 to 40 m y interval relative to peak metamorphism at 38 to 40 ma garnet growth is estimated to have begun at 11 to 14 kbar pressure at 70 to 80 ma although such a protracted garnet growth interval is surprising this is supported by plate tectonic reconstructions which suggest that subduction of the liguro piemont ocean occurred through slow and oblique convergence these results demonstrate that ree zonations in garnet coupled to crystal size distributions provide a powerful means for understanding prograde metamorphic paths when combined with sm nd and lu hf geochronology

identification of growth mechanisms in metamorphic garnet by high resolution trace element mapping with la icp tofms garnet is one of the most robust and ubiquitous minerals that record element zoning during crustal metamorphism in addition to major elements zoning in trace elements can provide a wealth of information to document the changing conditions of garnet growth and modification however mapping trace elements at low concentrations over large areas and with high resolution has remained a major challenge we present a comprehensive investigation of the te distribution in garnet from three alpine samples that underwent a complex evolution at different metamorphic conditions the te distribution in garnet grains is mapped in 2d in thin section with a novel approach using laser ablation inductively coupled plasma time of flight mass spectrometry la icp tofms to achieve a lateral resolution of 5 m and limits of detection for the heavy rare earth elements ree down to 0 2 g g comparison with major element zoning measured by electron probe microanalysis and trace elements measured by conventional la icpms spot analysis testifies to the accuracy of the measurements garnet in an amphibolite facies metapelite from campolungo central alps that recorded metamorphism to 600 c preserves y ree trace element zoning that closely matches that of ca in this sample there is no notable diffusive modification for trace elements y ree zoning is dominated by rayleigh fractionation in the core and by the sporadic breakdown of accessory phases producing annuli in the rim of the garnet a granulite facies garnet from malenco eastern central alps formed during subsolidus heating followed by peritectic melting reactions up to temperatures of 800 850 c major and trace element zoning are decoupled indicating diffusional resetting of major elements whereas trace elements still largely document the growth history enrichment of trace elements in the garnet mantle may be related to the consumption of biotite v cr and the dissolution of zircon zr and monazite y ree in the melt diffusion of y hree at the core mantle boundary occurred over a length scale of 200 m garnet in an eclogite from the sesia zone western alps p 2 gpa t 600 c displays pronounced fluid related veinlets visible in feo mgo and mno which cross cut the primary growth zoning surprisingly complex y ree and cr zoning is not affected by the veinlets indicating that they did not form by a crack seal mechanism but are rather related to a selective replacement process the trace element maps provide a detailed insight into the growth and modification of garnet and thus allow assessment of equilibrium versus disequilibrium processes and assist in determination of p t conditions garnet dating diffusion modelling as well as documenting fluid induced modifications

tectonics and structural zonation of southern tibet china the yarlung zangbo suture zone is composed of several thrust sheets with distinct stratigraphies states of deformation and metamorphic grades successive deformations have been produced by persistent north south shortening since obduction of the ophiolites by late cretaceous palaeocene times a possible decoupling of the crust may be associated with the intracontinental thrust system which has formed in the subducted indian plate this article emphasizes the role of shear deformations in the building of a collision belt

strain localization and porphyroclast rotation it has been debated for decades whether rigid inclusions such as porphyroclasts and porphyroblasts do or do not rotate in a softer matrix during deformation experiments and numerical simulations with viscous matrix rheologies show ongoing rotation of circular inclusions whereas using mohr coulomb plasticity results in nonrotation because the rocks in which inclusions are found normally undergo deformation by dislocation creep we applied a full field crystal plasticity approach to investigate the rotation behavior of rigid circular inclusions we show that the inclusion9s rotation strongly depends on the anisotropy of the matrix minerals strongly anisotropic minerals will develop shear bands that reduce the rotation of inclusions inhibition of rotation can only occur after a significant amount of strain our results may help to explain why geologic rigid objects often show evidence for rotation but not necessarily in accordance with the viscous theory that is usually applied to these systems

porphyroblast rotation eppur si muove in a number of recent papers the theory has been postulated that porphyroblasts as a rule do not rotate with respect to geographical coordinates and can be used to determine the original orientation of older foliations complex inclusion patterns in spiral garnets have even been used to advocate a new model of orogenesis involving several alternating phases of horizontal shortening and extension critical assessment of the assumptions and data used to support the theory of irrotational porphyroblasts reveals numerous flaws millipede structures used as proof for flow partitioning can also form by other flow geometries evidence quoted to support irrotational behaviour of porphyroblasts is unsound porphyrobl asts do occur in sets with a preferred orientation of the internal foliation trace but these cannot be shown to represent original orientations microstructures which resemble truncation planes in spiral garnets are used as evidence that these structures developed by several phases of deformation and as proof for periodic extension and horizontal shortening in orogenesis they can however also be explained by intermittent growth of a rotating porphyroblast during a single phase of deformation finally porphyroblast sets in which orientation is a function of aspect ratio indicate that porphyroblast rotation with respect to kinematic axes does occur in at least some situations

dolomitization and dolomite neomorphism in the back reef facies of the bonneterre and davis formations cambrian southeastern missouri the back reef facies of the bonneterre and davis formations consists of dolomitized cryptalgalaminates and partially dolomitized peloid mudstones these subfacies were deposited on tidal flats and in shallow lagoons that existed in the st francois mountains of southeast missouri during late cambrian time the back reef facies is bounded on the seaward side by dolomitized algal bioherms and grainstone banks which host the southeast missouri mississippi valley type mvt lead zinc copper ore deposits cryptalgalaminates are replaced by both fine crystalline planar dolomite and coarse crystalline nonplanar dolomite peloid mudstones are partially replaced but mainly by coarse crystalline nonplanar dolomite planar dolomite replacing cryptalgalaminates shows gradational transitions into coarse crystalline nonplanar dolomite the transitions are accompanied by progressive increase in average crystal size and an increased skewness toward the coarse size fraction planar dolomite is enriched in 18 o relative to stratigraphically equivalent marine limestones coarse crystalline nonplanar dolomite which replaces both cryptalgalaminates and peloid mudstones has delta 13 c and delta 18 o values transitional between those of planar dolomite and late dolomite cements associated with mvt mineralization fluid inclusion analysis of nonplanar dolomite and associated dolomite cements indicates precipitation at elevated temperatures 60 to 114 degrees c by saline basinal fluids these data indicate that the back reef facies was subjected to a complex diagenetic history involving both early and late diagenetic dolomitization and dolomite neomorphism fine crystalline planar dolomite was formed during early diagenetic dolomitization of the cryptalgalaminate subfacies by a fluid approaching seawater composition coarse crystalline nonplanar dolomite was formed as a result of neomorphism of pro existing planar dolomite and dolomitization of peloid mudstones after burial accompanying mvt mineralization in southeast missouri

styles of post subduction collisional orogeny influence of convergence velocity crustal rheology and radiogenic heat production abstract a number of models of continental collision already exist but the role of foregoing oceanic plate subduction or variable coupling between plates still remains to be explored in addition heat generation by radioactive decay may be quite variable for example our geochemical data from low to high grade metamorphic rocks of the lesser and higher himalayan sequences reveal measured heat production values on the order 4 5 w m 3 which is considerably higher than normal inferred upper crustal values 3 therefore we performed 2d numerical experiments of oceanic continental subduction followed by continental collision for varying continental lower crustal strength convergence velocity and radiogenic heating rate oceanic subduction produces thinning and upwarping of the overriding lithospheric wedge such as is observed in andean type collisional settings cascadia and central andes after collision two main orogenic styles develop depending on the values of key parameters either a slow convergence rates weak shear heating and consequently strong rheological coupling between plates and or a weak lower crust generate a two sided thick and narrow symmetrical collisional zone or b high convergence rates strong shear heating and therefore weak rheological coupling between plates and a strong lower crust produce a one sided thin and broad asymmetrical collisional zone in this case a vertical lithospheric body divides two upper crustal wedges that develop distinct deformation patterns in both cases enhanced radiogenic heating favors the formation of midcrustal partially melted channels that propagate toward the surface and the pro foreland partial melting triggers lateral extrusion of metamorphic rocks from different depths 15 23 kbar 700 c on top of coherent lp lt units the geometry of metamorphic isograds inverted shapes of p t paths at different grades and exhumation rates are similar to those observed in parts of the himalayan chain indicating that the models capture the essence of real systems the model results demonstrate that internal heat production rate is a crucial parameter in determining the timing of slab breakoff polarity reversal and exhumation of the inverted metamorphic sequence with respect to the timing of partial melting late stages of some numerical experiments showed interesting geodynamical evolution of the collisional zone with the appearance of polarity reversal roll over of both plates and roll back of the subducting slab irrespective of these details hundreds of km of continental lithosphere are recycled into the mantle in all models supporting the idea that continental crustal consumption is indeed a common and considerable process during collision this affects inferences about extents of convergence and or shortening during orogenesis moreover crustal recycling into the deep mantle during collisional orogeny needs to be considered in models of crustal growth and evolution

a simple preparation technique for the study of opal phytoliths from archaeological and quaternary sediments abstract this note describes a simple and inexpensive method for the extraction and counting of opal phytoliths from archaeological and quaternary sediments

models of garnet differential geochronology abstract rayleigh distillation models are developed to describe theoretical growth zoning of lu hf rb sr sm and nd in typical garnet crystals from metapelites and metabasites effects of diffusion limited transport within the matrix and intracrystalline diffusion are also considered qualitatively theoretical zoning profiles show strong depletions of lu in garnet rims compared to cores but virtually invariant hf rb sr sm and nd profiles generally consistent with natural profiles for lu and hf and previously published models theoretical isochron diagrams for lu hf exhibit distinctive arcuate distributions and high mswds consistent both with himalayan data and with expectations that garnet growth durations exceed chronologic resolution by as much as an order of magnitude predicted isochron diagrams for sm nd and rb sr exhibit vertical arrays for garnet and high mswds that are generally lower than for lu hf in metapelites inherent chronologic resolution for bulk separates is best for lu hf in metapelites and rb sr but analytical considerations favor rb sr or sm nd for chronologic zoning studies diffusion limited transport in the rock matrix strongly influences zoning profiles but does not change the main trends on isochron diagrams intracrystalline diffusion will initially rotate lu hf isochrons to steeper slopes giving older apparent ages the natural himalayan data indicate growth of garnet in one rock from the greater himalayan sequence at 34 ma consistent with previously measured monazite ages from the same rock data from another himalayan rock suggest polymetamorphism that includes a paleozoic component

high resolution la icp ms mapping of deep sea polymetallic micronodules and its implications on element mobility abstract enrichments of rey rare earth yttrium and other trace metals co and ni in deep sea ferromanganese fe mn micronodules have received increasing attention in both deep sea research and mineral exploration due to the presence of multiple easily crushed and poorly crystallized phases in micronodules the genesis of micronodules and their adsorption of various trace elements are poorly understood to address this gap we examined the spatial distributions of elements in cross sections of micronodules from the western tropical north pacific ocean using high resolution hr la icp ms raster mapping coupled with laser raman and x ray photoelectron spectroscopy xps the ferromanganese micronodules we studied are dominated by fe and mn oxides with minor carbonate minerals such as siderite rhodochrosite and calcite la icp ms maps show that these micronodules consist of a mn rich core and a fe rich rim the fe enriched rim is enriched in as and surrounds a mg mn cu co and ni concreted core laser raman maps show that the micronodule core contains more birnessite an important scavenger of trace metals in deep sea sediments than the rim the birnessite filled core of these micronodules does not have elevated rey indeed birnessite line channels may feed metal rich fluid containing rey to adjacent minerals including well crystallized bio apatite and zeolite as high ce and y levels are spatially correlated with these minerals the observed element profiles and xps observations showing the coexistence of multiple oxidation states of mn 2 3 and 4 fe 2 and 3 and ce 3 4 demonstrate that the fe mn phases of these micronodules are of a diagenetic origin and that they are sites of redox driven metal enrichment in deep sea sediment

isotopic dating of strain fringe increments duration and rates of deformation in shear zones the time scales over which deformation in the earth9s crust remains localized in shear zones are poorly known as are the associated strain rates we have determined the longevity and rates of deformation using rubidium strontium rb sr microsampling dating of increments of fibrous strain fringes from a pyrenean shear zone the fibers grew quasi continuously through a protracted deformation history between 87 and 50 million years ago over a period comparable to that of an orogeny during a short interval between 66 and 62 million years a rise in strain rate from 1 1 10 15 to 7 7 10 15 seconds 1 occurred this acceleration correlates with an abrupt change in fiber growth direction and a stress field inversion from gravitational collapse to renewed horizontal crustal shortening

trace element zoning in garnet from the kwoiek area british columbia disequilibrium partitioning during garnet growth trace element zoning in garnets from two contact metamorphosed rocks from the kwoiek area british columbia hollister 1969a was measured with an ion microprobe zoning profiles have three distinct parts with chemical breaks defined by co variation of major and trace elements important features of the trace element zoning profiles are 1 roughly bell shaped zoning profiles for y and the hrees 2 an abrupt increase in ti at a point midway through each garnet with inflections in the zoning profiles of other elements li na cr v y zr and the hree and 3 irregular cr and v profiles unlike mn zoning the zoning profiles of most other trace elements cannot be easily modeled using simple rayleigh fractionation models ti activity in the two samples is buffered by phase relations with ilmenite garnets from a continuously heated contact metamorphic environment should display continuous ti zoning profiles if equilibrium was maintained and provided the ti buffering assemblage did not change during garnet growth the irregular ti profiles suggest disequilibrium behavior several elements cr v may indicate breakdown of a phase enriched in trace elements during metamorphism the source for the mass excess of these elements is probably the refractory cores of ilmenite grains either differing matrix transport rates of trace lements or interface kinetic controlled segregation could explain the unusual trace element behavior at the element inflection point the preferred explanation involves segregation of elements at the interface of the garnet that were trapped during episodes of rapid garnet growth

porphyroblast rotation and strain localization debate settled the topic of whether or not porphyroblasts rotate relative to a fixed external reference frame during deformation has engendered much debate mainly owing to the fact that pre deformational orientations of porphyroblasts and their inclusion trails can rarely be reconstructed in 20 years of

porphyroblast rotation during crenulation cleavage development an example from the aureole of the mooselookmeguntic pluton maine usa in the low pressure high temperature metamorphic rocks of western maine usa staurolite porphyroblasts grew at c 400 ma very late during the regional orogenesis these porphyroblasts which preserve straight inclusion trails with small thin section scale variation in pitch were subsequently involved in the strain and metamorphic aureole of the c 370 ma mooselookmeguntic pluton the aureole shows a progressive fabric intensity gradient from effectively zero emplacement related deformation at the outer edge of the aureole 2900 m map distance from the pluton margin to the development of a pervasive emplacement related foliation adjacent to the pluton the development of this pervasive foliation spanned all stages of crenulation cleavage development which are preserved at different distances from the pluton the spread of inclusion trail pitches in the staurolite porphyroblasts as measured in two dimensional 2 d thin sections increases nonlinearly from 16 to 75 with increasing strain in the aureole these data provide clear evidence for rotation of the staurolite porphyroblasts relative to one another and to the developing crenulation cleavage the data spread is qualitatively modelled for both pure and simple shear and both solutions match the data reasonably well the spread of inclusion trail orientations 40 75 in the moderately to highly strained rocks is similar to the spread reported in several previous studies we consider it likely that the sample scale spread in these previous studies is also the result of porphyroblast rotation relative to one another however the average inclusion trail orientation for a single sample may in at least some instances reflect the original orientation of the overgrown foliation

particle motion and the theory of charcoal analysis source area transport deposition and sampling principles from particle motion physics were applied to recurring problems of the interpretation of stratigraphic charcoal data 1 fires within catchments of lakes often produce no record in fossil charcoal curves and 2 periods characterized by no local fire e g 20th century fire suppression often display as much charcoal as times when local fire was frequent quantitative theory on source area transport deposition and sampling of charcoal shows the relationship between particle sizes counted by alternative methods of charcoal analysis pollen slides for particles 5 80 m in diameter petrographic thin sections for particles 50 10 000 m diameter and charcoal diagrams the relationship between diameter and critical and deposition velocities results in fundamental aerodynamic differences between the sizes of particles quantified by the two methods charcoal recorded on pollen slides is of a size that is difficult to lift but once entrained it remains in suspension thin section charcoal is lifted at relatively low wind velocities but it is not suspended at normal surface wind speeds thermal buoyancy during fire lofts charcoal above the forest canopy depending on particle size and wind speed pollen slide charcoal sizes are underrepresented near the fire because they remain in suspension and are preferentially exported from the burn area thin section charcoal is convected to lower heights on average and is deposited nearby following fire thin section charcoal is redistributed locally by wind and thus may enter lakes because of cohesive forces and aerodynamics more pollen slide charcoal remains on the ground and less enters lakes source areas for pollen slide charcoal are subcontinental to global and diagrams of pollen slide charcoal are biased toward nonlocal charcoal they can be used to interpret importance of fire for broad spatial and temporal scales thin section charcoal represents the catchment fire regime simulation models that generate charcoal during fire mix sediments and then sample at specified intervals indicate that 1 in the absence of sediment mixing the pollen slide method should consistently resolve individual fires that occur with an expectation of 30 50 yr 2 unless samples are continuous neither method will produce useful estimates of fire frequency and 3 even a modest amount of sediment mixing will obscure the signal

behaviour of rigid objects during deformation and metamorphism a test using schists from the bolton syncline connecticut usa the behaviour of spherical versus highly ellipsoidal rigid objects in folded rocks relative to one another or the earth s surface is of particular significance for metamorphic and structural geologists two common porphyroblastic minerals garnet and staurolite approximate spherical and highly ellipsoidal shapes respectively the motion of both phases is analysed using the axes of inflexion or intersection of one or more foliations preserved as inclusion trails within them we call these axes fias for foliation inflexion intersection axes for staurolite this motion can also be compared with the distribution of the long axes of the crystals schists from the regionally shallowly plunging bolton syncline commonly contain garnet and staurolite porphyroblasts whose fias have been measured in the same sample garnet porphyroblasts pre date this fold as they have inclusion trails truncated by all matrix foliations that trend parallel to the strike of the axial plane however they have remarkably consistent fia trends from limb to limb the fias trend 175 and lie 25 nnw from the 020 strike of the axial trace of the bolton syncline the plunge of these fias was determined for six samples and all lie within 30 of the horizontal eleven of these samples also contain staurolite porphyroblasts which grew before during and after formation of the bolton syncline as they contain inclusion trails continuous with matrix foliations that strike parallel to the axial trace of this fold the staurolite fias have an average trend of 035 15 ne from the 020 strike of the axial plane of this fold the total amount of inclusion trail curvature in staurolite porphyroblasts about the axis of relative rotation between staurolite and the matrix i e the fia is greater than the angular spread of garnet fias although staurolite porphyroblasts have ellipsoidal shapes their long axes exhibit no tendency to be preferentially aligned with respect to the main matrix foliation or to the trend of their fia this indicates that the axis of relative rotation between porphyroblast and matrix the fia was not parallel to the long axis of the crystals it also suggests that the porphyroblasts were not preferentially rotated towards a single stretch direction during progressive deformation five overprinting crenulation cleavages are preserved in the matrix of rocks from the bolton syncline and many of these result from deformation events that post date development of this fold staurolite porphyroblast growth occurred during the development of all of these deformations most of which produced foliations staurolite has overgrown and preserved as helicitic inclusions crenulated and crenulation cleavages i e some inclusion trail curvature pre dates porphyroblast growth the deformations accompanying staurolite growth involved reversals in shear sense and changing kinematic reference frames these relationships cannot all be explained by current models of rotation of either or both the garnet and staurolite porphyroblasts in contrast we suggest that the relationships are consistent with models of deformation paths that involve non rotation of porphyroblasts relative to some external reference frame further we suggest there is no difference in the behaviour of spherical or ellipsoidal rigid objects during ductile deformation and that neither garnet nor staurolite have rotated in schists from the bolton syncline during the multiple deformation events that include and post date the development of this fold