new insights into the geologic evolution of the grenvillian trenton prong inlier central appalachian piedmont usa new geochemical and 40ar 39ar hornblende and biotite data from the grenvillian trenton prong inlier provide the first constraints for the identification of lithotectonic units their tectonic setti

prolonged acadian orogenesis revelations from foliation intersection axis fia controlled monazite dating of foliations in porphyroblasts and matrix numerous phases of garnet growth are revealed by detailed studies of foliation inflection intersection axes preserved in porphyroblasts fias in acadian metamorphic rocks in the chester dome region of vermont u s a a regionally consistent succession of four different fia tends in garnet porphyroblasts has been dated by analyzing monazite inclusions with an electron microprobe these monazite inclusions which lie within the various foliations that define the fias provide absolute ages for multiple periods of deformation and episodic phases of garnet growth the monazite inclusions reveal a progression in foliation ages from 431 2 to 349 3 million years within porphyroblasts and from 366 3 to 327 5 million years in pervasive matrix foliations three samples of schist reveal ages ranging through 75 to 70 million years from the cores of porphyroblasts through the medians to the rims and then into the matrix ages determined from monazite grains within garnet porphyroblasts link directly to periods of multiple deformation and episodic garnet growth defined by the fias the four fia sets began forming prior to 424 3 405 6 386 6 and 366 4 million years ago respectively thus multiple stages of garnet growth occurred throughout acadian deformation and metamorphism in vermont and orogenesis was far more prolonged than previously thought invariably garnet growth occurred early in the accompanying deformation event dating of monazite inclusions without careful separation of phases of garnet growth by fia studies will lead to a spread of ages that will confuse rather than elucidate the metamorphic and structural history analysis of monazite grains in the matrix alone will likely only present ages for the youngest events these ages can be amalgamated from grain to grain to yield apparently precise ages but such ages reveal nothing about the deformation and metamorphic processes operating during orogenesis or the overall continuity of the deformation and metamorphism that accompanies plate motion fia and microstructural studies of inclusion trail successions allow numerous phases of garnet growth to be distinguished and then dated using monazite that has dissolved and regrown many times as multiple successive foliations developed this in turn allows very long and involved metamorphic histories to be fully integrated with similarly complex structural histories

one dimensional thermal modelling of acadian metamorphism in southern vermont usa one dimensional thermal 1dt modelling of an acadian devonian tectonothermal regime in southern vermont usa used measured metamorphic pressures and temperatures and estimated metamorphic cooling ages based on published thermobarometric and geochronological studies to constrain thermal and tectonic input parameters the area modelled lies within the vermont sequence of the acadian orogen and includes i a western domain containing garnet grade pre silurian metasedimentary and metavolcanic rocks from the eastern flank of an acadian composite dome structure rayponda sadawga dome and ii an eastern domain containing similar but staurolite or kyanite grade rocks from the western flank of a second dome structure athens dome approximately 10 km farther east using reasonable input parameters based on regional geological petrological and geochronological constraints the thermal modelling produced plausible p t paths and temperature time t t and pressure time p t curves information extracted from p t t modelling includes values of maximum temperature and pressure on the p t paths pressure at maximum temperature predicted ar closure ages for hornblende muscovite and k feldspar and integrated exhumation and cooling rates for segments of the cooling history the results from thermal modelling are consistent with independently obtained pressure temperature and ar cooling age data on regional metamorphism in southern vermont modelling results provide some important bounding limits on the physical conditions during regional metamorphism and indicate that the pressure contemporaneous with the attainment of peak temperature was probably as much as 2 5 kbar lower than the actual maximum pressure experienced by rocks along various particle paths in addition differences in peak metamorphic grade garnet grade versus staurolite grade or kyanite grade and peak temperature for rocks initially loaded to similar crustal depths differences in calculated exhumation rates and differences in 40ar 39ar closure ages are likely to have been consequences of variations in the duration of isobaric heating or crustal residence periods and tectonic unroofing rates modelling results are consistent with a regional structural model that suggests west to east younging of specific acadian deformational events and therefore diachroneity of attainment of peak metamorphic conditions and subsequent 40ar 39ar closure during cooling modelling is consistent with the proposition that regional variations in timing and peak conditions of metamorphism are the result of the variable depths to which rocks were loaded by an eastward thickening thrust nappe pile rooted to the east new hampshire sequence as well as by diachronous structural processes within the lower plate rocks of the vermont sequence

enriched seismic imaging by using multiple scattering abstract the migration of full wavefields primaries multiples by using the concept of secondary sources at each subsurface grid point is summarized these secondary sources are two way and contain the grid point reflection as well as the grid point transmission properties they assist the primary sources in illumination of the subsurface full wavefield migration is implemented as a closed loop process in which the estimated reflectivity properties are used to generate all higher order scattering multiples and transmission effects such that the modeled full wavefield response matches the observed measurements in addition velocities can be updated simultaneously theoretical considerations and examples lead to the principal conclusion that multiples should be used not removed

p t d histories from quartz a case study of the application of the titaniq thermobarometer to progressive fabric development in metapelites 1 this study investigates the ability of quartz to record segments of the pressure temperature deformation p t d path in poly deformed metamorphic terranes and the associated application of the ti in quartz thermobarometer titaniq metapelites from the strafford dome eastern vermont were selected for this study because they record progressive fabric development during prograde metamorphism and have well constrained p t d histories from previous work results of this investigation are in agreement with previous studies and demonstrate that quartz in these samples records additional distinct intervals of the p t d path six preserved quartz equilibration events include 1 early prograde growth during burial 2 kyanite in quartz producing reactions 3 prenappe emplacement equilibration during isobaric heating 4 precipitation of quartz in quartz feldspar domains during crenulation cleavage development due to solution transfer 5 quartz producing chlorite out reactions during heating postnappe emplacement and 6 retrograde quartz overgrowths from an influx of siliceous fluids modeling quartz volume allows for the identification of quartz producing reactions in p t space the same may be done for ti phases to better constrain activities during reequilibration events an integration of cathodoluminescence imaging microstructural and petrographic investigation isochemical forward stability modeling and thermobarometry allows identification and interpretation of zoning patterns in quartz to unravel histories that would otherwise not be obtainable

tectonometamorphic evolution of the southern quebec appalachians 40ar 39ar evidence for middle ordovician crustal thickening and silurian early devonian exhumation of the internal humber zone published age data from the humber zone of the southern quebec appalachians indicate that deformation and metamorphism occurred during middle to late ordovician time this paper presents the results of a regional scale geochronological study of 70 amphibole and mica single grains by the 40 ar 39 ar laser step heating method which brings forward a more complete middle ordovician early devonian tectonometamorphic evolution of the quebec appalachian humber zone biotites have plateau ages varying between 420 and 411 ma amphiboles are often compositionally rimmed and or contain biotite inclusions their age spectra are generally disturbed but may suggest the occurrence of two distinct events silurian 429 424 ma and middle ordovician 462 460 ma two main groups of muscovite ages have been obtained middle ordovician 469 461 ma and silurian early devonian 431 411 ma but middle to late devonian 387 376 ma ages are also locally present middle ordovician ages are interpreted to record crustal thickening by nappe emplacement during regional taconian metamorphism silurian early devonian ages are attributed to consequences of backthrusting and normal faulting a statistical analysis of silurian early devonian muscovite age spectra reveals an irregular decrease in weighted apparent age maxima across the internal humber zone this age decreasing trend is interpreted to result from the temporal ca 431 411 ma and possibly spatial northwest to southeast progression of deformation recrystallization and fluid circulation during tectonic exhumation of the internal humber zone in southern quebec tectonic models for the northern appalachian internal humber zone should take into account the strong body of evidence for silurian early devonian hinterland directed and extensional deformation as documented in southern quebec

effects of diffusional modification of garnet growth zoning on p t path calculations the effect of intragranular diffusion on chemical zoning in garnet and on p t paths calculated from that zoning was evaluated using a numerical model of multicomponent diffusion in combination with simulations of garnet growth syn and post growth diffusion of mg fe mn ca species in garnet was calculated for a model pelitic assemblage over a range of temperatures from 485 to 635 c compositions from zoned garnet as modified by diffusion hypothetical inclusions of plagioclase within garnet and matrix phases were used to reconstruct pressure temperature p t paths from isobaric and polybaric model histories p t path calculations based on numerical simulations conducted over an input isobaric heating path that reached peak temperatures between 585 and 635 c show that relaxation of garnet compositional gradients by diffusion can induce modest to appreciable curvature in the inferred paths retrieved paths also indicated somewhat smaller overall temperature changes relative to the actual temperature difference of the input path the magnitude of these distortions is shown to depend upon the heating and cooling rate and garnet crystal size as well as the actual peak temperature condition the effect of diffusion on path trajectories in simulations with thermal histories that also included cooling were comparable to heating only models that reached peak temperatures approximately 15 30 c higher compositions of garnets with radii less than 1 mm that reached actual peak temperatures of 605 c along temperature time histories characteristic of regional metamorphism experienced sufficient diffusional relaxation to introduce errors of hundreds of bars to in excess of one kilobar in path trajectories path distortions were significant at heating cooling rates less than 10 c ma but rapidly diminished for rates faster than this in polybaric simulations diffusion effects were least noticeable when the actual pressure temperature conditions changed in a clockwise sense i e convex to higher p and higher t but apprecciable modification was seen in path models that underwent counterclockwise changes in p and t reequilibration of garnet rim compositions occurred during cooling on all paths and temperature maxima obtained from garnet biotite geothermometry underestimated actual peak conditions by 40 to 70 c calculations suggest that p t path trajectories calculated from garnets of at least 1 mm size and that experienced actual thermal maxima below 585 c are not likely to be distorted by diffusional effects during regional metamorphism however p t path reconstructions based on garnet zonation with smaller grains or higher temperatures may lead to misinterpretation of crystallization history the partitioning record of peak metamorphic temperatures may be destroyed by diffusional reequilibration of garnet rim compositions during cooling seriously complicating the task of quantitatively estimating diffusion effects on path calculations

metamorphic thermal and tectonic evolution of central new england mountain skitchewaug and big staurolite nappes reactivation of a new detailed tectonic model is presented for the acadian orogenic this fabric during thrusting is recorded in some rocks of the big belt of central new england vermont and new hampshire that staurolite nappe by rotated garnets that grew during near isothermal accounts for a wide range of petrological and structural observations loading only the sillimanite isograd crosses the fall mountain three belts are considered the eastern vermont merrimack and skitchewaug nappe boundary metamorphic breaks across the skitintervening bronson hill belts specific observations in eastern chewaug big staurolite nappe boundary at the base of the big vermont that are accounted for in the model include the following staurolite nappe and at the margin of the keene and alstead domes p t paths are clockwise with maximum pressures near the athens require post metamorphic thrusting when p t conditions were in chester and strafford domes of 8 11 kbar but with maximum the greenschist facies these observations can be explained by a pressures decreasing to 3 5 kbar at the boundary with the bronson relatively simple model involving in sequence thrusting from east to hill belt differential exhumation of the vermont domes relative to west commencing in central new hampshire at 400 410 ma the rocks in easternmost vermont is required by the recorded differences preservation of the low grade belt along the vermont new hampin maximum pressure 5 6 kbar 15 20 km and the present day shire border requires that crustal thickening in vermont was not geographical separation 7 10 km specific observations in new caused by emplacement of new hampshire nappes onto eastern hampshire that are explained include the following p t paths in vermont and that the nappes of western new hampshire had time the merrimack belt are counter clockwise with maximum pressures to cool before final juxtaposition against the low grade belt cooling of 4 5 kbar and are related to high regional heat flow and heat ages constrain this final juxtaposition to have occurred in the transfer by early acadian plutons p t paths in the bronson hill carboniferous suggesting that the acadian was a prolonged event belt are intimately associated with structural position an early spanning as much as 100 myr contact metamorphism is evidenced in the skitchewaug and fall mountain nappes near contacts with the early acadian bethlehem gneiss 400 410 ma peak metamorphic temperature rises upwards in the nappe sequence an inverted metamorphic sequence

a paleozoic pangaea paleozoic paleogeographies should be consistent with all available reliable data however comparison of three different devonian paleogeographies that are based largely or wholly on the data of remanent magnetism show them to be inconsistent in many regards when these three paleogeographies are provided with possible ocean surface current circulation patterns and have added to them lithofacies and biogeographic data they also are shown to be inconsistent with such data a pangaeic reconstruction positioned in the southern hemisphere permits the lithofacies and biogeographical data to be reconciled in a plausible manner

patterns of mineral occurrence in metamorphic rocks patterns in the occurrence of minerals in metamorphic rocks suggest additional opportunities for investigating chemical and physical processes during metamorphism three such patterns are reviewed first trace minerals in metamorphic rocks commonly occur with regular distributions indicating their participation in prograde reactions that can be mapped as isograds examples include the distribution of allanite and monazite in pelitic rocks and of zircon and baddeleyite in siliceous dolomites recognition of these isograds points to the potential for developing a chronology of specific chemical reactions during metamorphism and for defining the p t conditions of those reactions second the mineralogical products of retrograde metamorphism in many cases occur in distinctive associations that are consistent with partial mineral fluid equilibrium examples include the distribution of retrograde calcite quartz and tremolite in siliceous limestones and of retrograde tremolite dolomite brucite and serpentine in siliceous dolomites from contact aureoles among other things application of partial equilibrium to retrograde metamorphic rocks leads to constraints on the amount and direction of fluid flow in contact aureoles as they cool third pseudomorphs are typically absent from prograde metamorphic rocks but are common in retrograde metamorphic rocks the distribution may be explained by the effect of force of crystallization the pattern of occurrence of pseudomorphs thus suggests novel phenomena during metamorphism that develop from an interplay between chemical and mechanical processes

a laser microprobe mass spectrometric study of ar kr k cl and br in an unconformity garnet associated fluid inclusions staurolite and micas from vermont u s a abstract garnet prophyroblasts in paragonite schist of the pinney hollow fm on the west limb of the athens mantled gneiss dome in vermont exhibit apparent angular unconformities cores of garnets contain inclusions of chloritoid rutile and ilmenite that define a foliation which is truncated at a high angle by the foliation in the rims electron microprobe traverses across a garnet indicate that the core rim boundary is the beginning of a steep compositional gradient in ca mg mn and fe that characterizes the rim following electron microprobe study a sample was irradiated in a nuclear pile and then studied by laser microprobe noble gas mass spectrometry ar isotopes were measured in garnet and in adjacent micas and staurolite and ar plus kr isotopes were measured in fluid inclusions fi in quartz in garnet and in fi trapped 40 ar 36 ar varies between 300 and 2000 with no simple correlations indicated between trapped ar isotope ratios and k cl and 84kr or petrographic features similar trapped ar occurs in the core and rim of garnet and in fi in quartz most fi indicate 40 ar e cl 10 3 and cl 36 ar 6 atom ratios br abundances in fi are usually too low to be measured indicating br cl 3 atom ratio 84 kr 36 ar in fi varies between 0 02 and 0 08 gases extracted from laser craters in white micas in schist outside the garnet indicate mean 40 ar 39 ar ages of 362 5 ma and in biotites 332 7 ma with k bearing inclusions in the garnet rim yielding ages close to 360 ma in the garnet core interpretation of 40 ar 39 ar ages is ambiguous due to the combination of low concentrations of in situ radiogenic 40ar and comparatively large amounts of trapped 40are poorly correlated with other parameters measured

implications of low temperature cooling history on a transect across the colorado plateau basin and range boundary west central arizona fission track ages of apatite and zircon from metamorphic plutonic and sedimentary rocks along a 80 km transect across the colorado plateau basin and range boundary in west central arizona show differences in the low temperature cooling histories between the provinces the transect extends from cypress mountain in the colorado plateau transition zone to the eastern buckskin mountains in the basin and range along the northeast margin of the basin and range province metamorphic rocks exposed in the footwall of a major detachment fault system yield zircon and apatite fission track ages of 16 10 ma these ages are similar to k ar fusion ages of biotite and age minima of k feldspar on 40ar 39ar age spectra and collectively indicate rapid cooling one k feldspar age spectrum has an age maximum of about 22 8 ma an age minimum of 11 8 ma and a spectrum whose shape is suggestive of reheating possibly in middle miocene time the heating event was probably related to hydrothermal activity during emplacement of cu and mn deposits in and above the detachment fault zone effects of this heating are only locally detected in rocks above the detachment fault in the poachie range fission track ages of apatite and zircon are 60 50 and 80 70 ma respectively the disparity between the apatite and zircon ages indicates that the rocks cooled slowly in late cretaceous and early tertiary time probably due to gradual uplift and erosion total uplift and denudation in the area of the poachie range since cretaceous time is 6 km or more to the northeast of the range fission track ages of apatite and zircon increase and diverge indicating that apparent uplift decreased in that direction the apatite ages from the poachie range are concordant with early tertiary hornblende ages determined in other studies of lower plate rocks near the southwest end of the transect the ages represent cooling of crystalline rocks after cretaceous regional metamorphism and magmatism near bagdad 20 km northeast of the poachie range 2 km or less of erosion has occurred since intrusion of high level plutons and dikes and caldera formation in late cretaceous time remnants of an erosion surface that developed in middle tertiary time are preserved in the transition zone volcanic and sedimentary rocks at least as old as early miocene were deposited on the erosion surface and filled valleys cut into it dissection of these deposits began about 8 ma we interpret these data combined with those from other studies to indicate that in cretaceous time southward thrusting and later extensive magmatism in the middle crust led to thickening and heating of the crust the cretaceous igneous rocks at bagdad are high level manifestations of this magmatism uplift and slow cooling occurred in late cretaceous and early tertiary time in late oligocene and early miocene time during northeast southwest extension middle crustal rocks moved southwest put from beneath the southwest margin of the transition zone tectonic denudation rapidly exposed the crust that had been brought up from a depth of 10 km or more and rapidly cooled in the eastern buckskin and harcuvar mountains middle miocene reheating occurred locally in the lower plate along the detachment and in nearby parts of the upper plate

obtaining equilibrium oxygen isotope fractionations from rocks theory and examples a generalized approach for retrieving equilibrium isotope fractionations from natural rocks is proposed in which models of prograde reaction histories and retrograde diffusional exchange are used to identify coexisting minerals with similar isotope closure temperatures examples using literature data and new analyses from 32 natural amphibolite facies schists demonstrate both the feasibility and limitations of obtaining equilibrium oxygen isotope fractionations from minerals in natural rocks by screening samples according to the theoretical models natural data are shown to have highly consistent mineral fractionations 2 reproducibilities of 0 16 to 0 54 that within uncertainty reproduce experimental determinations among the minerals quartz biotite muscovite and calcic amphibole this correspondence indicates that the proposed theoretically based selection criteria improve the likelihood of measuring equilibrium fractionations the new data further corroborate the expected progressive enrichment of 18o in the orthosilicates with increasing al si relative to fe mg ky grt 1 05 st grt 0 6 and st cld 0 3 at 525 575 c in contrast typical samples that fail to satisfy screening criteria exhibit fractionations involving quartz biotite and amphibole that are strongly disequilibrium because of exchange during cooling theoretical screening of samples prior to isotope analysis allows robust independent assessment of theoretical and experimental determinations of equilibrium isotope fractionations

laser 40ar 39ar evaluation of slow cooling and episodic loss of 40ar from a sample of polymetamorphic muscovite volume diffusion models predict that crystals with large diffusion dimensions can record a wide range of thermal conditions in the earth s crust direct measurements of the zoning of radiogenic argon 40 in single muscovite porphyroblasts from a complex terrain in the vermont appalachians record multiple crustal events that span 150 million years the crystal radius was the effective dimension for argon diffusion approximately 1000 micrometers late deformation features inside the crystals locally decreased the diffusion dimension and promoted loss of argon 40 zoning patterns of radiogenic isotopes as observed in this study are an increasingly important diagnostic tool for studying the thermal record of tectonic processes

metamorphic p t paths from calcic pelitic schists from the strafford dome vermont usa metamorphism of the gile mountain formation and waits river formation in the strafford dome and townshend brownington syncline in east central vermont records two nappe style events d1 and d2 followed by doming d1 formed a muscovite biotite ilmenite schistosity subparallel to compositional layering so and was followed by heating to garnet grade the temperature and pressure at the end of d1 are estimated to be c 450d c and 6 8 kbar d2 variably crenulated and folded s1 during a nearly isothermal pressure increase of 1 2 kbar calculated from compositions of garnet which have inclusions trails with progressive crenulation and rotation of the s1 fabric similar p t paths are computed for most of the area suggesting that the later schistosity developed during emplacement of a regional nappe 3 6 km thick there is a general lack of d3 dome stage microstructures near the strafford willoughby arch staurolite and kyanite overgrew s2 in pelites and plagioclase with increasing xan overgrew s2 in calcic pelites reflecting post d2 heating to a maximum of 550 600d c metamorphic pressures at the end of d2 are fairly constant on the west side of the dome indicating minor dome stage uplift in contrast pressures at the thermal peak of metamorphism decrease by more than 4 kbar east of the dome the observed pattern of isotherms and isobars is mainly the result of post metamorphic differential uplift and unroofing finally a minor retrograde metamorphism produced the assemblage albite epidote k feldspar muscovite chlorite with grade increasing east toward the connecticut river

simple models of co2 release from metacarbonates with implications for interpretation of directions and magnitudes of fluid flow in the deep crust simple one dimensional models of coupled advection hydrodynamic surroundings can account for basic t fluid composition reaction dispersion reaction are used to investigate processes of co2 release progress relationships observed in much of the acadian orogen of from metacarbonate beds during deep crustal 8 kbar acadian new england the results provide an alternative to up t flow prograde metamorphism in new england usa two broad models scenarios that account for these relationships by large pervasive in which reaction progress is controlled by gradients in h2o co2 horizontal fluid fluxes up regional t gradients fluid composition between diverent rock types are presented in the first divusional exchange of volatiles across lithologic contacts is significant co2 generated during prograde temperature t rise is transported away from metacarbonate layers to surrounding 1 metapelitic layers which generate h2o by dehydration and or 2 key words metamorphism fluid flow flow direction carbon dioxide

regional scale mid jurassic to late cretaceous unroofing from the adirondack mountains through central new england based on apatite fission track and u th he thermochronology abstract apatite fission track aft ages of 178 to 70 ma for 108 samples from the adirondack mountains and eastern new york state vermont western massachusetts and connecticut and western new hampshire indicate that widespread unroofing occurred throughout the region during the middle jurassic to late cretaceous additionally u th he ages of 167 to 85 ma were determined on nine of those samples and support cooling rates of 1 8 2 2 c m yr from 100 to 65 c during the middle jurassic to late cretaceous assuming a geothermal gradient of 25 c km unroofing rates for the region were estimated to be on average 0 07 0 08 km m yr the aft age discontinuities between the high peaks region and the fault dissected southeastern adirondack mountains and along the ammonoosuc fault in the connecticut river valley between vermont and new hampshire suggest differential unroofing that may have been accommodated by fault reactivation syn post late cretaceous time after the youngest aft ages 82 and 70 ma resp

geochronologic studies in central new england i evidence for pre acadian metamorphism in eastern vermont sup 40 ar sup 39 ar geochronology on hornblendes from central new england reveals a grenvillian cooling age 1026 plus minus 12 ma along the eastern margin of the green mountain massif acadian ages in east central vermont 350 397 ma and along the bronson hill anticlinorium in new hampshire and massachusetts 407 280 ma and late ordovician to silurian ages 440 451 ma along a belt in eastern vermont and central massachusetts these ages indicate a minimal acadian overprint in eastern vermont the highest degree of acadian overprint being located along the axis of the domes in the connecticut valley synclinorium these data suggest that the formation of the domes occurred in the acadian moreover the late ordovician to silurian cooling ages in eastern vermont suggest that much of the metamorphic mineralogy and fabric observed in the connecticut valley synclinorium may be pre silurian in age

p t t paths and differential alleghanian loading and uplift of the bronson hill terrane south central new england late paleozoic u pb ages of sphene and 4 0 ar 3 9 ar cooling ages of amphibole and muscovite from rocks of the bronson hill terrane in connecticut and central massachusetts reflect a late paleozoic alleghanian overprinton acadian metamorphic rocks prograde alleghanian sphenes crystallized during the late pennsylvanian and eliminate the possibility that amphibole ages reflect delayed permian cooling from devonian acadian metamorphism fourteen new amphibole ages from connecticut form a north to south trend of decreasing age from 294 to 245 ma while in massachusetts four new amphibole ages together with three others from the literature produce a random carboniferous pattern seven new muscovite ages support existing data indicating uniform cooling throughout the bronson hill terrane through 350 c in the early triassic the rate of permian cooling defined by amphibole muscovite pairs increases from 4 c my in northern connecticut to 50 c my near long island sound hinged loading and hinged but delayed exhumation in the southern part of the bronson hill terrane with the hinge in central connecticut explain these ages and cooling rates as well as a southerly increasing metamorphic field gradient one dimensional thermal modeling indicates that loading of bronson hill rocks must have begun by the late mississippian the time of peak alleghanian metamorphic temperature decreases southward from early permian in northern connecticut to late permian to the south these results demonstrate that the metamorphic effects of the alleghanian orogeny are not restricted to the avalon terrane of southeastern new england on the contrary the alleghanian orogeny reset 4 0 ar 3 9 ar mineral ages recrystallized minerals partially melted felsic rocks and transposed fabrics at least as far west as the bronson hill terrane in south central new england

contrasting p t t paths thermochronologic evidence for a late paleozoic final assembly of the avalon composite terrane in the new england appalachians strongly contrasting pressure temperature time paths for the avalon composite terrane and the structurally overlying putnam nashoba zone in eastern new england obtained from thermochronologic and thermobarometric data are best explained by a late paleozoic underthrusting of cover rocks by the avalon composite terrane we present new ar and u pb thermochronologic data that show that in the southern hope valley zone permian 280 ma anatectic metamorphic conditions of 700 c and 6 kbar were quenched by relatively rapid cooling 12 c m y and exhumation 0 5 km m y for 40 m y in contrast peak metamorphic conditions in the putnam nashoba zone predate silurian intrusions and slower cooling 3 5 c m y began at about 400 ma one dimensional thermal modeling suggests that these two belts were not in thermal equilibrium during the permian metamorphism of the avalon composite terrane because of the absence of high grade alleghanian metamorphism in rocks overlying the avalon terrane we conclude that high grade alleghanian metamorphism in the avalon terrane occurred east of rocks now overlying it and that significant motion between avalon and this cover occurred after peak alleghanian metamorphism similarly contrasting metamorphic histories between avalon inliers willimantic window massabesic complex gneiss pelham dome and their cover rocks reveals the regional significance of this boundary the core rocks all show permian cooling but the cover rocks show post acadian cooling ages decreasing from east to west to the pelham area where hornblende ages in avalon and cover differ by only 35 rather than 80 m y model calculations show that thermal equilibrium between instantaneously thrusted blocks of rocks is generally obtained in tens of millions of years consequently underthrusting of avalon is constrained to be middle mississippian or younger because the leading edge of the underthrusting block would have been heated the longest and would have most closely approached thermal equilibrium with its cover core rocks of the pelham dome must have been relatively close to this leading edge thus carboniferous to permian underplating from a generally eastward direction best explains these thermochronologic relationships

reaction softening by dissolution precipitation creep in a retrograde greenschist facies ductile shear zone new hampshire usa we describe strain localization by a mixed process of reaction and microstructural softening in a lower greenschist facies ductile fault zone that transposes and replaces middle to upper amphibolite facies fabrics and mineral assemblages in the host littleton schist near claremont new hampshire here na poor muscovite and chlorite progressively replace first staurolite then garnet and finally biotite porphyroblasts as the core of the fault zone in approached across the transect higher grade fabric forming na rich muscovite is also progressively replaced by fabric forming na poor muscovite the mineralogy of the new phyllonitic fault rock produced is dominated by na poor muscovite and chlorite together with late albite porphyroblasts the replacement of the amphibolite facies porphyroblasts by muscovite and chlorite is pseudomorphic in some samples and shows that the chemical metastability of the porphyroblasts is sufficient to drive replacement in contrast element mapping shows that fabric forming na rich muscovite is selectively replaced at high strain microstructural sites indicating that strain energy played an important role in activating the dissolution of the compositionally metastable muscovite the replacement of strong high grade porphyroblasts by weaker na poor muscovite and chlorite constitutes reaction softening the crystallization of parallel and contiguous mica in the retrograde foliation at the expense of the earlier and locally crenulated na rich muscovite defined foliation destroys not only the metastable high grade mineralogy but also its stronger geometry this process constitutes both reaction and microstructural softening the deformation mechanism here was thus one of dissolution precipitation creep activated at considerably lower stresses than might be predicted in quartzo feldspathic rocks at the same lower greenschist facies conditions this article is protected by copyright all rights reserved

late ottawan extension in the eastern adirondack highlands evidence from structural studies and zircon and monazite geochronology syn to postorogenic collapse is increasingly being recognized as a fundamental process in most orogens although the mechanism of collapse at mid to lower crustal levels is not well understood the adirondack highlands of the grenville province in northern new york are an exhumed granulite terrane and represent an ideal setting in which to investigate mid to lower crustal orogenic processes although the highlands experienced high grade metamorphism and crustal thickening during the ottawan orogeny few extensional shear zones have been recognized in this region we document a new shear zone in the eastern adirondack highlands that accommodated postorogenic extensional collapse during the late ottawan the shear zone accommodated extension via noncoaxial top to the southeast shear although we cannot rule out a significant coaxial vertical thinning component some earlier thrust motion may also have occurred in the zone u pb sensitive high resolution ion microprobe zircon and in situ electron probe u th pb monazite ages demonstrate that extension occurred from ca 1050 to 1025 ma following peak metamorphism during the ottawan orogeny our results suggest that late ottawan extension in the adirondack highlands was more widespread than previously recognized and was likely driven by gravitational collapse extension along this eastern adirondack highlands shear zone may connect with the tawachiche shear zone to the north in quebec indicating that tectonic exhumation during the ottawan occurred on a system of interconnected shear zones top to the northeast extension in the eastern adirondack highlands was synchronous with top to the northwest extension on the carthage colton shear zone in the western adirondack highlands suggesting that the highlands granulite terrane may have been partially exhumed as a large scale symmetrical gneiss dome

oxygen isotope geochemistry of the amphiboles isotope effects of cation substitutions in minerals abstract the occurrence of coexisting amphiboles in rocks and the likelihood of concurrent isotope closure allows equilibrium oxygen isotope fractionations among the amphiboles to be recovered from natural samples oxygen isotope analyses of mineral separates using laser fluorination show that coexisting amphiboles increasingly partition 18o in the order hornblende gedrite 0 9 previously published data for hornblende actinolite glaucophane and garnet show that act hbl 0 2 gln grt 1 and hbl grt 0 thus glaucophane strongly partitions 18o relative to the calcic amphiboles the fractionation between two amphiboles of arbitrary composition can be predicted from the known fractionations for mica endmembers pyroxene endmembers and exchange components such as caal nasi 1 naal camg 1 camg 1 mgfe 1 femn 1 kna 1 kal si 1 and fe3 al 1 applications of the exchange component method reproduce measured amphibole fractionations to within 0 1 to 0 2 whereas other predictive methods cause misfit for typical metamorphic hornblende of 0 5 at 575 c although the isotope effects of cation exchanges may be small at high t they magnify dramatically for minerals formed in surficial diagenetic and low t metamorphic environments different composition clays are predicted to have equilibrium 18o differences of 2 9 if the isotope fractionation can be determined for one mineral endmember then calibrated exchanges allow accurate prediction of the isotope fractionations for intermediate compositions of most ortho ring chain and sheet silicates

exhumation and tilting of the western metamorphic belt of the coast orogen in southern southeastern alaska the uplift history of the western metamorphic belt in southern southeastern alaska can be traced using emplacement depths and cooling ages of mid cretaceous plutons which intruded the metamorphic belt during the period 101 ma to 89 ma during the period 101 ma to 94 ma the rocks of the metamorphic belt now at the surface were buried to a depth of approximately 30 km pressure data indicate that by 89 ma the western portion of the metamorphic belt was at a shallower crustal level 23 km within the study area while the eastern side remained at a depth of approximately 30 km cooling curves constructed from minerals in mid cretaceous plutonic rocks indicate that the western side of the metamorphic belt cooled earlier than the eastern side an observation consistent with the inferred uplift history southwestward transport of metamorphic belt rocks within the hanging walls of thrust and high angle faults may have provided one mechanism for uplift during mid to late cretaceous time the eastern portion of the western metamorphic belt cooled rapidly at 55 ma probably in response to a period of rapid uplift the coast shear zone an early tertiary high angle fault with west side up displacement bounds the east side of the metamorphic belt we suggest tertiary uplift occurred by tilting of the metamorphic belt due to movement along this fault

the massabesic gneiss complex new hampshire a study of a portion of the avalon terrane geochemical data from the 625 ma massabesic gneiss complex of southern new hampshire show strong affinities for other avalonian rocks of southern new england and suggest continental rifting in the late proterozoic migmatized paragneiss the dominant rock type in the complex has major and trace element compositions that are compatible with graywackes from continental arcs the para gneiss also has strong lithologic metamorphic and isotopic similarities to the rocks of the hope valley zone of connecticut avalon suggesting a possible hope valley massabesic correlation at 625 ma the paragneiss end values are similar to avalonian crust in other locations of the orogen two types of amphibolite are present in minor amounts in the paragneiss of the massabesic gneiss complex the first type is a paramphibolite and consists of calc silicate layers in the massabesic paragneiss the second type is metaigneous major and trace element abundances reveal that the protoliths of the orthoamphibolites range from continental rift alkaline basalts and tholeiites to n type morbs orthoam phibolite end 625 ma values range from 2 4 to 4 as expected of rift related magmas derived from partial melting of a depleted mantle source and have the same values as iapetus ocean floor rocks of similar age orthoamphibolite major and trace element geochemical characteristics overlap those of the middlesex fells amphibolites of the esmond dedham zone of eastern massachusetts avalon which range from alkaline to transitional basalts erupted in a continental rift setting the compositions of orthoam phibolites define a potential magmatic continuum produced by batch partial melting of the mantle initiated during continental rifting and proceeded to ocean basin forma tion the inferred continuity of mafic magmatism from the esmond dedham mid dlesex fells formation to the massabesic gneiss complex and hope valley zone suggests that these zones are not distinct lithotectonic zones but are parts of a single landmass massachusetts avalon esmond dedham represents the continental section of avalon where the alkaline to transitional magmas of the early rifting stages are preserved according to our tectonic reconstruction the massabesic gneiss complex is the oceanward continental margin represented by volcanoclastic sediments with the morbs representing the initiation of ocean basin development the leading edge of this landmass of which the massabesic gneiss complex is the only observable remnant collided with laurentia during the acadian orogeny the inboard thicker more continental trailing edge that is platform avalon esmond dedham collided later during the alleghanian orogeny

partitioning of deformation along an orogen and its effects on porphyroblast growth during orogenesis an identical succession of foliation inflection intersection axis trends in porphyroblasts fias is present in three separate areas affected by acadian deformation and metamorphism along 130 kms of the appalachians from east central vermont to north central massachusetts the fias trend successively nw se sw ne w e nnw sse and ssw nne and appear to have formed during the same succession of deformations which have been dated in se vermont as follows not yet dated pre 425 ma 425 to 404 ma 404 to 385 ma and 385 to 360 ma respectively the two areas that are furthest apart in east central vermont and north central massachusetts have strikingly similar total distributions of fias in garnet porphyroblasts that show a somewhat inverse relationship to those observed in the chester athens dome area of se vermont that separates them that is the number of samples containing the first formed fia set is greatest for the two regions lying furthest apart and decreases in younger sets whereas the number of samples containing the first fia set is the least in the central region and increases for more recently developed sets partitioning of deformation into zones of progressive shearing and shortening has been demonstrated to control the sites of development of differentiated crenulation cleavage versus porphyroblast growth and sites of porphyroblast growth on one limb of a regional fold versus the other we argue that partitioning of the deformation around competent feldspathic gneiss bodies at an orogen scale produced the variation in the number of samples containing the various fia sets between the central and outer areas described above porphyroblasts most commonly grew during sw ne directed shortening to the north and south of the chester athens dome region of the appalachians but only rarely grew in the latter region this suggests that deformation at this time partitioned around an outlier of competent basement feldspathic gneiss that underlies the chester athens dome region and this protected the rocks above and in the strain shadow to the nw and se from the effects of the great bulk of the deformation that occurred over the period of time during which nw se trending fias formed we attribute the increase in the number of samples containing progressively younger fias in the chester athens dome area during subsequent periods of nw se n s wsw ene and wnw ese directed shortening to the rise in temperature and pressure recorded in the rocks above the chester athens dome with peak conditions occurring during the development of the nnw sse oriented fia set between 404 and 385 ma as the temperature and pressure increased the gneiss below would have become less competent allowing the deformation associated with the succession of fia sets to partition more pervasively through the dome core this provided more sites for porphyroblast nucleation and growth and hence the number of samples recording these fia sets increased the concomitant decrease in the number of samples in which porphyroblasts grew in the rock within the areas to the north and south with successive fias possibly resulted from a progressive increase in competency of these rocks as the number of porphyroblasts increased

diachronous supracrustal extension in an intraplate setting and the origin of the connecticut valley gaspe and merrimack troughs northern appalachians in the appalachians of mainland canada and new england silurian early devonian rocks are preserved in the connecticut valley gaspe and merrimack troughs and rest unconformably or in fault contact with older rocks belonging to laurentia and to gander avalon respectively the silurian early devonian rocks consist of marine clastic deposits with subordinate carbonates lava flows and terrestrial deposits the origin of these sedimentary basins is still poorly understood metamorphic ages and structures in the laurentian margin major unconformities and syn sedimentary normal faulting in both troughs argue for significant crustal extension during deposition the connecticut valley gaspe and merrimack troughs are separated by inliers of pre ordovician to ordovician rocks which we interpret as silurian basement highs that would have been buried in devonian times to form a composite sedimentary basin volcanic rocks are widely distributed in time and space in both basins and are mostly subalkaline within plate tholeiites which is consistent with a tectonic setting involving crustal extension rather than with subduction granitic plutons are abundant in the merrimack trough and attest to high temperatures at mid crustal levels it is suggested that crustal extension was responsible for the formation of both basins and that heating of the lower crust in the merrimack trough during the silurian was the result of synorogenic collapse likely triggered by delamination at the laurentia medial new england boundary delamination of the subducted slab and the upwelling of the asthenosphere would have caused 1 isostatic uplift and formation of basement highs 2 magmatism in the lower crust and regional scale contact metamorphism in the upper crust and 3 collapse of metamorphic terranes and the formation of subsiding sedimentary basins

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