accelerated hydration of the earth s deep crust induced by stress perturbations the metamorphic cycle associated with the formation of mountain belts produces a lower crust containing little or no free fluid1 2 the introduction of external fluids to dry and impermeable volumes of the earth s crust is thus a prerequisite for the retrogressive metamorphism later observed in such regimes such metamorphism can cause significant changes in the crust s physical properties including its density rheology and elastic properties3 4 on a large scale the introduction of fluids requires the presence of high permeability channels such as faults or fractures which are the result of external tectonic stresses but extensive interaction between externally derived fluids and the fractured rock requires efficient mass transport away from the initial fractures into the rock itself and this transport often occurs over distances much longer than expected from grain boundary diffusion here we present both field observations and a simple network model that demonstrate how the transport of fluids into initially dry rock can be accelerated by perturbations in the local stress field caused by reactions with fluids we also show that the morphology of reaction fronts separating dry from wet rocks depends on the anisotropy of the external stress field

h2o transport and release in subduction zones experimental constraints on basaltic and andesitic systems phase relationships in natural andesitic and synthetic basaltic systems were experimentally investigated from 2 2 to 7 7 gpa and 550 c to 950 c in the presence of an aqueous fluid in order to determine the stability of hydrous phases in natural subducted crustal material and to constrain reactions resulting in the release of water from subduction zones to the mantle wedge water reservoirs in subducted oceanic crust at depths exceeding the amphibole stability field 70 80 km are lawsonite 11 wt h2o mg chloritoid 8 wt talc 5 wt and zoisite clinozoisite 2 wt in basaltic rocks and lawsonite zoisite clinozoisite phengite 4 wt and staurolite 2 wt in andesitic compositions the thermal stability of lawsonite at 6 0 gpa extends to 800 c and 870 c in basaltic and andesitic compositions respectively at pressures above amphibole out 2 3 2 5 gpa lawsonite reacts through continuous reactions with steep positive dp dt slopes to zoisite clinozoisite until 3 0 3 2 gpa and at higher pressures to more than 7 7 gpa to assemblages containing garnet clinopyroxene and garnet clinopyroxene kyanite in basaltic and andesitic compositions respectively on the contrary the breakdown of zoisite clinozoisite is mainly pressure sensitive phengite represents the hydrous phase with the largest stability field encountered in this study in andesite phengite is stable to more than 7 7 gpa and more than 920 c talc and staurolite contribute in minor amounts to the water balance in basaltic and andesitic rock compositions a model for water release from the subducted slab is developed combining thermal models for subduction zones with the experimentally determined phase relationships up to 1 wt and 2 wt h2o in basaltic and andesitic rocks respectively can be stored to depths beyond 200 km in cold subduction zones mainly by lawsonite and phengite dehydration rates are high until amphibole out and relatively low at greater depths the amphibole out reactions are found to release a significant amount of water in a depth interval of several kilometers however they do not represent a discrete pulse of fluid and do not completely dehydrate the descending slab fluid release at depths greater than 200 km through phengite and progressive lawsonite breakdown would hydrate the overlying mantle causing the generation of amphibole or phlogopite peridotite at higher geothermal gradients epidote zoisite contributes to fluid flux to the mantle wedge at 100 120 km depth the extensive stability field of phengite may greatly enhance the role of sediments and the small amount of potassium in mafic compositions for the fluid budget in subduction zones at increasing depth

40ar 39ar ages in mantle xenolith phlogopites determining the ages of multiple lithospheric mantle events and diatreme ascent rates in southern africa and malaita solomon islands kimberlites are extraordinary natural phenomena ascending through the earth s lithosphere entraining xenoliths to erupt at the surface within hours to days of their inception deep within the lithospheric mantle with the realization that some ar ar phlogopite grain core ages may be indicative of geological events we have undertaken high spatial resolution ar ar dating of phlogopites in xenoliths and megacrysts from kimberley monastery and letseng in southern africa and malaita in the solomon islands to est whether other mantle phlogopite cores may yield meaningful ages modelling of ar diffusive loss profiles from phlogopite grain boundaries to cores provides information on both the eruption age and the duration of outgassing within the kimberlite magma and hence yields estimates on diatreme ascent rates the ascent durations are very similar for all of the southern african pipes studied yielding durations of 0 9 6 9 days assuming an average kimberlite magma temperature of 1000 c these can be compared to estimates from phlogopite xenoliths from siberian diamond bearing kimberlites yielding ascent durations of 2 15 hours assuming the same magma temperature

deep crustal embrittlement and fluid flow during granulite metamorphism in the limpopo belt south africa deep crustal rocks generally deform in a ductile manner and have a very low permeability however in the southern marginal zone of the limpopo belt solutions fluids have moved through high grade rocks and reacted with these on a regional scale along a major shear zone strike slip environments there have more widespread alteration than thrust fault environments under high grade metamorphic conditions brittle failure and hydraulic fracture processes in particular have occurred in these deep crustal rocks the latter phenomenon provides a mechanism for developing permeability and plumbing systems for the migration of fluids and or liquids through the deep crust modeling of the physical conditions of brittle failure under these unusual conditions would suggest that a mobile phase of low viscosity and water activity participated in the fracture process

contrasting assemblages and petrogenetic evolution of corona and noncorona gabbros in the grenville province of western labrador contrasting coronas around olivines pyroxenes and ilmenites occur in metagabbros in two adjacent terranes in the grenville province of western labrador in the parautochthonous gagnon terrane coronas around olivines display evidence of evolution from double coronas of orthopyroxene and fluorine rich pargasitic amphibole through triple coronas of orthopyroxene garnet clinopyroxene pargasitic amphibole to double coronas of orthopyroxene garnet clinopyroxene parentheses enclose phase occurring as inclusions in previous phase in the allochthonous lac joseph terrane coronas around olivines consist of orthopyroxene clinopyroxene spinel or orthopyroxene pargasitic amphibole spinel brackets enclose phases occurring in symplectic intergrowths possible reactions that explain the development of these assemblages are investigated and it is concluded that the system was closed at the level of the corona the silica required for the olivine plagioclase reaction in gagnon terrane was gene

coexisting clinopyroxene spinel and amphibole spinel symplectites in metatroctolites from the buck creek ultramafic body north carolina blue ridge buck creek metatroctolites preserve evidence of three stages in the geologic history of the region a relict igneous stage a high pressure granulite facies metamorphic stage and a hydrous alteration stage during high pressure granulite facies metamorphism adjacent olivine and plagioclase reacted to produce complex coronitic textures relict olivine is surrounded by radial columnar orthopyroxene and plagioclase is rimmed by fine grained symplectite composed of separate grains of clinopyroxene spinel tschermakitic amphibole spinel symplectite and minor clinopyroxene sapphirine symplectite examination of textural relationships in back scattered electron images and x ray element maps of the symplectites demonstrates that amphibole spinel and clinopyroxene spinel symplectites grew simultaneously at high temperature to replace plagioclase in these rocks clinopyroxene sapphirine symplectite grew in the latest stages of plagioclase replacement in some samples a successful model for the main corona forming reaction in sample bc7 1 00 pl 1 72 ol 0 17 h 2 o 0 59 di 0 17 amp 0 68 spl 0 79 opx results only if amphibole is included the model predicts relative volumes of product clinopyroxene amphibole and spinel that are generally consistent with the proportions in imaged symplectites proportions of clinopyroxene and amphibole hosted symplectite appear to have been governed by local mass balance of na and al in plagioclase and or availability of h 2 o the most successful model for formation of the inner clinopyroxene sapphirine corona 1 00 pl 1 47 ol 0 17 h 2 o 0 59 di 0 17 amp 0 30 spr 10 ox 0 57 opx also includes amphibole which is present in association with clinopyroxene sapphirine symplectite sufficient h 2 o for formation of symplectic amphibole must have been present in the intergranular fluid of relict igneous troctolites or have infiltrated during metamorphism at high p t conditions

petrology of the chunky gal mountain mafic ultramafic complex north carolina the buck creek ultramafic body situated within a belt of alpine type ultramafites in the north carolina blue ridge is completely enveloped by the chunky gal mountain amphibolites the mafic and ultramafic units were in contact prior to the earliest metamorphic and deformational events the complex is at least partly bounded by a major thrust fault and appears to have been emplaced after the first stage of deformation the amphibolites are divided into two units types i and ii on the basis of texture mineralogy trace element geochemistry and geothermometry however these are indistinguishable in terms of major element compositions type i amphibolites were metamorphosed under prograde granulite facies conditions 725 25 c whereas type ii amphibolites were metamorphosed under prograde amphibolite facies conditions 520 20 c pressure was 4 to 6 kbar over the entire complex a second retrograde event was of middle green schist facies the amphibolites appear to have been derived from a cumulate gabbroic protolith and probable igneous differentiation trends are preserved despite the high grades of metamorphism this complex may represent either the basal section of an ophiolite sequence with calc alkaline affinity or a mafic pluton into which an ultramafic diapir intruded within a rifted continental environment

tectonics of the southern and central appalachian internides the appalachian orogen was constructed along the ancient precambrian continental margin of eastern north america by a series of compressional events that began in the ordovician and episodically spanned much of the paleozoic era the processes of accretion of suspect and exotic terranes together with terrane collision and ultimately completion of the wilson cycle by continent continent collision resulted in construction of the appalachian orogen figure 1 these paleogeographic elements are rep resented by the various lithotectonic units visible today in the southern and central appalachians figure 2 these subdivisions represent the late precambrian rifted margin and early paleozoic platform that were formed directly upon north american grenvillian basement cumberland allegheny plateau valley and ridge and western blue ridge the offshore north american slope rise assemblage hamburg klippe southern appalachian eastern blue ridge and inner piedmont and higher thrust sheets of the maryland northern virginia piedmont and the exotic carolina or avalon and wilmington terranes the carolina ter rane contains the alleghanian high grade metamorphic core these elements form the basis for this analysis and review of the present state of knowledge of the tectonic history of the southern and central appalachian internides the appalachian orogen has been the subject of study by geologists at least since the early nineteenth century early fundamental concepts such as the geosynclinal theory of mountain building dana 1873 hall 1883

matrix analysis of metamorphic mineral assemblages and reactions assemblage diagrams are widely used in interpreting metamorphic mineral assemblages in simple systems they can help to identify assemblages which may represent equilibrium states to determine whether differences between assemblages reflect changes in metamorphic grade or variations in bulk composition and to characterize isograd reactions in multicomponent assemblages these questions can be approached by investigating the rank composition space range and reaction space null space of a matrix representing the compositions of the phases involved singular value decomposition svd provides an elegant way of 1 finding the rank of a matrix and detemining orthonormal bases for both the composition space and the reaction space needed to represent an assemblage or pair of assemblages and 2 finding a model matrix of specified rank which is closest in a least squares sense to an observed assemblage although closely related to least squares techniques the svd approach has the advantages that it tolerates errors in all observations and is computationally simpler and more stable than non linear least squares algorithms models of this sort can be used to interpret multicomponent mineral assemblages by straightforward generalizations of the methods used to interpret assemblage diagrams in simpler systems svd analysis of mineral assemblages described by lang and rice 1985 demonstrates the utility of the approach

a tem study of disequilibrium plagioclase breakdown at high pressure the role of infiltrating fluid high pressure metamorphism 600 c 20 kbar of the allalin gabbro western alps resulted in the breakdown of plagioclase an63 to fine grained zoisite jadeite kyanite and quartz in rare cases this reaction failed to reach completion the resulting textures of partial reaction have been studied by transmission and analytical electron microscopy in localised regions of a plagioclase crystal where the extent of reaction is 10 only zoisite developed and the orientation relationship left 100 right text zo left 101 right text p1 1 and left 012 right text zo left 010 right text p1 1 is frequently present in regions where 10 50 of plagioclase has transformed the reaction plagioclase h2o zoisite kyanite quartz nasica 1al 1 pl has occurred the systematic orientation relationship between plagioclase and zoisite is absent at 50 transformation complete breakdown of plagioclase occurred in localized micron scale domains by the reaction plagioclase h2o zoisite jadeite kyanite quartz and the reaction products are variably orientated with respect to each other incomplete reaction together with the concentration of reaction products around cracks in original plagioclase grains suggests that extent of reaction was controlled primarily by the availability of h2o the textural observations are interpreted in terms of two possible disequilibrium reaction models 1 plagioclase persists metastably with its original igneous composition to a pressure 17 kbar at 600 c reaction to the equilibrium assemblage then develops adjacent to cracks in response to the presence of aqueous fluid at intracrystalline sites only partial reaction occurs because jadeite and sometimes kyanite and quartz fail to nucleate for kinetic reasons 2 localized regions of a plagioclase crystal partially equilibrate at several stages during the increase of pressure 9 17 kbar at 600 c possibly due to discrete episodes of fluid infiltration in both these models the extent of reaction may be limited by nasi caal interdiffusion in plagioclase

chromite symplectites in mg suite troctolite 76535 as evidence for infiltration metasomatism of a lunar layered intrusion abstract despite the very low chromium concentrations in its cumulus olivine 140 ppm lunar troctolite 76535 contains large amounts of cr sporadically but highly concentrated in symplectite assemblages consisting of mg al chromite and two pyroxenes previously proposed symplectite formation mechanisms include crystallization of trapped interstitial melt diffusion of cr from cumulus olivine and or remobilization of cumulus chromite grains these mechanisms would imply that the highly cr depleted nature of mg suite parental magmas and their source materials inferred from cumulus olivine may be illusory we have conducted a detailed petrologic and textural study of symplectites as well as chromite veins intercumulus assemblages olivine hosted melt inclusions and clinopyroxene troilite veins in 76535 with the goals of constraining the origin of the symplectites and the degree of cr depletion in mg suite magmas relative to other lunar basalts orthopyroxene and clinopyroxene in melt inclusions are depleted in cr relative to their symplectite counterparts averaging 900 and 1200 ppm vs 7400 and 8100 ppm cr 2 o 3 respectively olivine in contact with symplectite assemblages may exhibit a diffusion profile of cr going into olivine whereas olivine boundaries away from symplectites show no diffusion profile there is also a distinct lack of primary chromite as inclusions in cumulus phases and melt inclusions multiple textural observations melt inclusion chemistry and modeling of chromite olivine equilibrium rule out previously proposed symplectite formation mechanisms and strongly suggest that chromite was not a primary crystallization product of the 76535 parental magma accordingly the post cumulus addition of cr and fe is required to produce the symplectites after considering multiple models the addition of cr and fe to 76535 via infiltration metasomatism by an exogenous chromite saturated melt is the model most consistent with multiple textural and geochemical observations failure of models that call upon cr diffusion out of olivine grains imply that the observed cr depleted nature of olivine observed in many mg suite lithologies is a primary feature of the cr depleted nature of the mg suite parental magmas and their source materials this substantial depletion of cr in the magma relative to mare basalt magmas still requires a satisfactory explanation in order to be consistent with mg suite petrogenetic models and currently accepted bulk moon compositions additionally if the intimate interaction of migrating melts with early lunar crustal lithologies was a widespread phenomenon after lmo solidification it provides another mechanism by which to reset or delay closure of radiogenic isotopic systems and explain the mg suite ferroan anorthosite age overlap

stratigraphy and tectonics of the virginia north carolina blue ridge evolution of a late proterozoic early paleozoic hinge zone late proterozoic 690 570 ma through ordovician rocks of the virginia north carolina blue ridge and valley and ridge record the evolution of an eastward facing passive margin and its subsequent deformation during continent arc collision taconic orogeny comparison of blue ridge stratigraphy with modern rifts and passive margins suggests that late proterozoic rifting involved substantial crustal attenuation in the eastern blue ridge the axial zone of the blue ridge from central virginia southward is occupied by a major fault system hayesville fries rock fish valley fault contrasts between rocks deposited east and west of this fault suggest that it is a reactivated hinge zone separating highly attenuated continental crust to the east from continental crust of more normal thickness to the west during rifting normal faulting and rapid subsidence of attenuating crust in the eastern blue ridge produced rift basins in which deep water clastic and volcanic rocks accumulated lynchburg group ashe formation landward of the hinge zone in the western blue ridge mainly terrestrial to shallow water clastic and volcanic rocks were deposited in a series of subaerial rift basins swift run catoctin grandfather mountain mount rogers formations the hinge zone not only controlled depositional patterns throughout the rift and passive margin history but it was also reactivated during the taconic collision when attenuated crust and its cover were thrust westward over the shelf edge turbidite dominated rift sequences of the eastern blue ridge are unlike typical rift deposits which are characterized by predominantly subaerial and shallow water sedimentation deep water rifts may be common on attenuated crust as in the bay of biscay however they are poorly known from the stratigraphic record because of their susceptibility to destruction during orogeny

granulites at winding stair gap north carolina the thermal axis of paleozoic metamorphism in the southern appalachians granulite facies rocks in the thermal axis of paleozoic taconic metamorphism are exposed in southwestern north carolina in a 370 m long road cut at winding stair gap lithologic units are composed of metasedimentary and metaigneous rocks some of which are now migmatites structural analysis of the exposure indicates that deformation occurred before during and after the peak of metamorphism interpretation of the predeformation stratigraphy suggests five of the six lithologic units may correlate with intervals in the late precambrian tallulah falls formation or the upper part of the late precambrian or possibly early paleozoic coweeta group the sixth unit an orthopyroxenite may be related to presumed ophiolites elsewhere in the hayesville thrust sheet prograde metamorphic conditions of t 750 775 c p 6 5 7 0 kbar were determined from mineral stability fields and cation exchange geothermometry geobarometry partitioning of water into anatectic melts of quartzofeldspathic gneiss and garnet sillimanite schist units resulted in ph2o ptotal during the prograde event an isobaric cooling path from the metamorphic peak to 600 c may be related to expulsion of partial melts that may be represented by pegmatites and trondhjemite dikes at higher structural levels limited retrograde effects were likely facilitated by water released from crystallization of the remaining in situ melts at t 700 c

newly discovered eclogite in the southern appalachian orogen northwestern north carolina abstract recent detailed geologic mapping in the blue ridge belt of northwestern north carolina has revealed the first reported eclogite sensu stricto in the southern appalachian orogen the eclogite occurs at the base of the ashe metamorphic suite within a shear zone that separates the spruce pine thrust sheet from the underlying grenville age basement rocks this contact juxtaposes rocks of apparent oceanic affinities predominantly metapelite metabasite and meta ultramafic rocks with rocks of the ancient north american craton the primary metamorphic assemblage in the eclogite is garnet omphacite quartz rutile hornblende and plagioclase occur only as retrograde amphibolite facies phases in the eclogite pressure temperature estimates for the primary eclogite assemblage are consistent with minimum pressures of 13 17 kbar at 625 790 c while the retrograde amphibolite facies assemblage suggests conditions of 8 5 12 kbar and 650 730 c ultramafic blocks also occur within the shear zone containing the eclogite the presence of high pressure metamorphic rocks associated with ultramafic rocks and pelitic schists within a shear zone separating continental rocks from oceanic rocks is strong evidence for an ancient subduction event and indicates that at least part of the ashe metamorphic suite is a subduction related accretionary melange

climate induced changes in holocene calcareous tufa formations bohemian karst czech republic abstract about 70 localities where holocene calcareous tufa is formed recently and or was formed in the past are known in the bohemian karst a small karst area located sw of prague all known tufa accumulations display a very similar pattern of lithological and biostratigraphic evolution reflecting climatic changes erosion events and biota succession a 17 m thick tufa accumulation at svat jan pod skalou was selected and thoroughly studied as a typical well developed representative of local tufa deposits tufa formation started at about 9500 bp on a flat fluvial gravel terrace of late glacial early holocene age deposition of lithologically uniform pure hard porous tufa continued until about 6500 bp from that time a more unstable climate with several dry periods and erosion events produced a lithologically varied complex of loose tufa alternating with embedded soils and scree layers termination of the tufa deposition occurred about 2500 bp and was followed by partial erosion connected with relocation of the spring below the tufa body holocene climatic changes were recorded in lithology molluscan assemblages and oscillations of oxygen and carbon stable isotope ratios in carbonate the observed patterns are in good agreement with the evolution of calcareous tufa deposits throughout central europe

the wayah granulite facies metamorphic core southwestern north carolina high grade culmination of taconic metamorphism in the southern blue ridge the wayah granulite facies metamorphic core in the eastern blue ridge southwestern north carolina contains the metamafic assemblage hornblende orthopyroxene clinopyroxene which defines the hornblende lower granulite facies the metamorphic progression from the kyanite almandine zone into the wayah granulite core is continuous across the hayesville fault a major premetamorphic peak tectonostratigraphic boundary this metamorphic transition includes a distinct muscovite absent alkali feldspar second sillimanite zone retrogression is limited this entire range in metamorphic grade apparently was produced during a single prograde paleozoic likely taconic metamorphic event the hayesville fault crosses the study area mostly within the first sillimanite zone northwest of the wayah granulite core both supracrustal sequences juxtaposed along this fault consist predominantly of metasedimentary rocks probably deposited during the late proterozoic although paragneiss pre dominates the southeastern terrane is distinguished by the presence of metamorphosed felsic mafic and ultramafic igneous rocks no evidence supports the presence of grenville basement within the study area the second of four deformational events synchronous with the metamorphic peak created northeast southwest to east west trending upright isoclinal folds which dominate the outcrop pattern macroscopic f 2 folds deformed the hayesville fault application of mineralogic thermometry and barometry yields conditions which range from 585 c 5 5 kb at the kyanite sillimanite isograd to 842 c 9 8 kb in the hornblende granulite facies macroscopic distributions of migmatite and perthite published experimental data and barometric calculations suggest that t 669 c p 6 52 kb and x h 2 o 0 62 at the second sillimanite isograd results indicate that the wayah granulite facies core constitutes the high grade culmination of paleozoic metamorphism in the southern appalachian blue ridge

retrograde metamorphism of eclogite in the southern appalachian mountains u s a a case involving seamount subduction this work supports a growing body of evidence that the ashe metamorphic suite ams of the eastern blue ridge province in north carolina has an ensimatic origin and is part of a subduction related accretionary melange marking the taconic suture between the north american craton and the inner piedmont in a palinspastic reconstruction the thrust fault at the base of the ams appears to have intercepted the greatest depths i e highest p metamorphic rocks beneath parts of the ams now exposed adjacent to the grandfather mountain window the greatest volume of mafic rock is found in these same areas we suggest that the nascent subduction related basal thrust fault was deflected downward by an obstacle in the form of an isolated mafic volcanic edifice on the oceanic crust a sea mount pelitic and mafic rocks dominate the ams north of the grandfather mountain window retrograded eclogite occurs in the amphibolite near the base of the ams textures and mineralogy indicate that an original eclogite assemblage was subjected to the following sequence of parageneses a eclogite i facies omphacite garnet quartz b eclogite ii facies omphacite garnet epidote quartz c symplectic diopside plagioclase garnet epidote quartz d amphibolite facies diopside plagioclase garnet epidote hornblende quartz e amphibolite facies plagioclase garnet epidote hornblende quartz p t conditions estimated from geothermobarometry applied to relevant mineral compositions are c 720 c and c 16 kbar for b eclogite ii facies c 655 c and c 8 5 kbar for e amphibolite facies

cryptic crustal events elucidated through zone imaging and ion microprobe studies of zircon southern appalachian blue ridge north carolina georgia compositional zoning reveals multistage growth histories and resorption events in zircon from a high grade terrane in the eastern blue ridge of north carolina and georgia these zoning patterns were used to guide high resolution ion microprobe dating that places important constraints on the evolution of the southern appalachian crust zircons from granulite facies metapelite have unzoned rims that yield concordant u pb ages of 495 14 ma we interpret this as the time of rim growth which occurred during peak metamorphism early in the protracted orogenic history of the region detrital cores characterized by truncated euhedral zoning are of grenville age 1 04 1 26 ga zircons from the whiteside and rabun plutons have well defined rounded inherited cores and euhedral oscillatory zoned magmatic rims rims of rabun zircons record magmatic crystallization at 374 4 ma whereas whiteside rims yield a 466 10 ma crystallization age cores from both plutons include 1 1 1 3 ga and 2 6 2 7 ga ages these data indicate that there was no single voluminous episode of plutonism in this area that similar material underpinned the region at least from 370 to 470 ma and that previously unrecognized archean basement or archean basement derived sedimentary rock was present in the southern appalachians results of this study verify the value of combining zoning and ion microprobe studies using conventional u pb methods or ion microprobe dating without knowledge of zoning would have made interpreting the events recorded in these zircons and the ages that they yield difficult or impossible

the origin of olivine plagioclase coronas in metagabbros from the adirondack mountains new york olivine plagioclase coronas in metagabbros from the adirondack mountains new york usa are spatially well organized reaction textures consisting most commonly of sequential layers of orthopyroxene clinopyroxene plagioclase and garnet the textures are characteristic of diffusion controlled reaction kinetics although similar coronas have been interpreted by previous workers in terms of an isochemical steady state diffusion model petrographical relations and material balance calculations establish that coronas in the adirondack metagabbros cannot be treated as isochemical and do not form in a single stage steady state process instead they evolve through time in a complex open system reaction in this study the isochemical diffusion model is modified to account for elemental fluxes across the outer boundaries of the coronal reaction band thereby approximating the open system behaviour of the coronas the sequence and relative proportions of product minerals calculated by the open system steady state model correspond closely to those observed in coronas of the adirondacks over a wide range of values for the relative diffusivities of chemical components involved in the reaction regardless of the particular method used to determine material balance in the reaction texture despite this correspondence petrographical evidence for successive replacement of coronal product layers reveals that the adirondack coronas evolved through one or more transient states rather than forming in a single stage steady state process there is no evidence that the successive replacement of coronal product layers resulted from changes in pressure or temperature but there is petrographical evidence that these changes resulted from modification of the composition of reactant plagioclase as the corona forming reaction proceeded this is confirmed by the fact that the evolution of the coronas over time can be replicated with the open system diffusion model by simulating the effect of the gradual exhaustion of plagioclase as a source of the ca and si components required for reaction these simulations suggest that successive stages in the evolution of the coronas are characterized by these product sequences i orthopyroxene clinopyroxene plagioclase garnet ii orthopyroxene clinopyroxene garnet and iii orthopyroxene garnet all of these stages and the transitions between them are observed petrographically coronas in adirondack metagabbros appear therefore to have originated in a complex open system diffusion controlled reaction in which the product assemblages changed as the reaction progressed