retrograde fluids in granulites stable isotope evidence of fluid migration widespread retrograde alteration assemblages document the migration of mixed h sub 2 o co sub 2 fluids into granulite facies rocks in the adirondack mountains fluid migration is manifest by 1 veins and patchy intergrowths of chlorite plus minus sericite plus minus calcite 2 small veins of calcite many only identifiable by cathodoluminescence and 3 high density co sub 2 rich or mixed h sub 2 o co sub 2 fluid inclusions the distinct and varied textural occurrences of the alteration minerals indicate that fluid rock ratios were low and variable on a local scale stable isotope ratios of c o and s have been determined in retrograde minerals from samples of the marcy anorthosite massif and adjacent granitic gneisses charnockites retrograde calcite in the anorthosite has a relatively small range in both delta sup 18 o sub smow and delta sup 13 c sub pdb 8 6 to 14 9 and minus 4 1 to 0 4 respectively probably indicating that the hydrothermal fluids that precipitated the calcite had exchanged with a variety of crustal lithologies including marbles and orthogneisses and that calcite was precipitated over a relatively narrow temperature interval values of delta sup 34 s sub cdt that range from 2 8 to 8 3 within the anorthosite can also be interpreted to reflect exchange between orthogneisses and metasediments the recognition of retrograde fluid migrationmore is particularly significant in granulite facies terranes because the controversy surrounding the origin of granulites arises in part from differing interpretations of fluid inclusion data specifically the timing of entrapment of high density co sub 2 rich inclusions results indicate that retrograde fluid migration which in some samples may leave only cryptic petrographic evidence is a process capable of producing high density co sub 2 rich fluid inclusions less

geochemistry and origin of albite gneisses northeastern adirondack mountains new york albite gneisses containing up to 8 7 percent na2o and as little as 0 1 k2o comprise a significant part of the proterozoic lyon mountain gneiss in the ausable forks quadrangle of the northeastern adirondacks new york state two distinct types of albite gneisses are present one is a trondhjemitic leucogneiss lag consisting principally of albite ab95 ab98 and quartz with minor magnetite and locally minor amounts of amphibole or acmiterich pyroxene lag probably originated by metamorphism of a rhyolitie or rhyodacitic ash flow tuff with a type geochemical affinities following post depositional analcitization in a saline or saline alkaline environment the other type is a mafic albite gneiss mag containing albite and pyroxene along with 0 45 percent quartz minor amphibole and titanite mag locally displays pinstripe banding and contains albite ab98 megacrysts up to 5 cm across its precursor may have been a sediment composed of diagenetic analcite or albite dolomite and quartz both types of albite gneiss are interlayered with granitic gneisses lmg of variable composition derived from less altered tuffs a potassium rich up to 9 7 k2o microcline gneiss facies may have had a protolith rich in diagenetic k feldspar we propose that the albite gneisses and associated granitic gneisses are the granulite facies metamorphic equivalent of a bimodal dominantly felsic volcanic suite with minor intercalated sediments probably including evaporites the volcanics were erupted in an anorogenic setting such as an incipient or failed intracontinental rift deposition took place in a closed basin playa lake environment where diagenetic alteration resulted in redistribution of the alkalis and strong oxidation

geochemistry of shale and sedimentary pyrite as a proxy for gold fertility in the selwyn basin area yukon selwyn basin area strata contain sedimentary pyrite with au above background levels when analyzed by laser ablation inductively coupled mass spectrometry hyland group rocks contain framboidal pyrite contents of 670 ppb au 1223 ppm as and 5 3 ppm te the mean of all types of sedimentary pyrite in the hyland group is 391 ppb au 1489 ppm as and 3 8 ppm te these levels are similar to sedimentary pyrite in host lithologies from major orogenic gold districts in new zealand and australia comparison of whole rock and pyrite data show that rocks deposited in continental slope settings with significant terrigenous input contain pyrite that is consistently enriched in au as te co and cu although data are limited whole rock samples of stratigraphic units containing au rich pyrite also contain high au indicating that most of the au is within sedimentary pyrite based on geologic characteristics and comparison of pyrite chemistry data with whole rock chemistry selwyn basin area strata have the necessary ingredients to form orogenic gold deposits au enriched source rocks metamorphic conditions permissive of forming a metamorphic ore fluid and abundant structural preparation for channeling fluids and depositing ore

oxygen isotope evidence for shallow emplacement of adirondack anorthosite oxygen isotopic analysis of wollastonites from the willsboro mine adirondack mountains new york reveals a 400 ft wide zone of 18o depletion at anorthosite contacts values of 18o vary more sharply with distance and are lower to 1 3 than any yet reported for a granulite fades terrain exchange with circulating hot meteoric water best explains these results and implies that the anorthosite was emplaced at relatively shallow depths 10 km in marked contrast to the depth of granulite fades metamorphism 23 km these 18o depletions offer the first strong evidence for shallow emplacement of anorthosite within the grenville province and suggest that regional metamorphism was a later and tectonically distinct event

fluid inclusions in adirondack granulites implications for the retrograde p t path investigation of fluid inclusions in granitic and calc silicate gneisses from the adirondack mountains new york has revealed the presence of various types including 1 co2 rich 2 mixed h20 c02 salt and 3 aqueous inclusions with no visible co2 many if not all of these inclusions were trapped or modified after the peak of granulite facies metamorphism as shown by textural relations or by the lack of agreement between the composition of the fluids found in some inclusions and the composition of the peak metamorphic fluid as estimated from mineral equilibria many fluid inclusions record conditions attained during retrograde cooling and uplift with minimum pressures and temper atures of 2 to 3 kbar and 200 to 300 c the tempera tures and pressures derived from the investigation of these inclusions constrain the retrograde p t path and the results indicate that a period of cooling with little or no decompression

determination of oxygen diffusion rates in magnetite from natural isotopic variations the oxygen isotope compositions of magnetite grains hosted in a calcite marble from the bancroft terrane of the ontario grenville province vary systematically with grain size the 18 o values of magnetite and corresponding closure temperatures t c based on calcite magnetite range from 18 o 2 6 t c 505 c for a grain radius of 0 075 mm to 18 o 5 5 t c 660 c for a grain radius of 1 15 mm the 18 o of the calcite is constant within a scale of 100 m at a value of 12 3 the observed isotopic variations can be fit to the diffusion model of dodson by the method of least squares r 0 98 to yield an activation energy q 211 20 kj mole and a pre exponential factor d o 4 3 3 3 1 9 x 10 7 cm 2 s for a cooling rate of 4 c m y the activation energy estimate is independent of the assumed cooling rate but the calculated pre exponential factor varies as follows d o cm 2 s d t d t x 1 08 x 10 7 note d t d t is in c m y the activation energy is identical to an experimental determination by giletti and hess but the pre exponential factor is 100 times lower the difference is attributed to the water rich conditions in the experiments and the absence of fluid in the slowly cooled marbles investigated in this study the strong dependence of diffusion rate on water presence or f h 2 o may be used as a sensor for water rich fluids during cooling in natural systems

ion microprobe analysis of oxygen isotope ratios in granulite facies magnetites diffusive exchange as a guide to cooling history ion microprobe analysis of magnetites from the adirondack mountains ny yields oxygen isotope ratios with spatial resolution of 2 8 m and precision in the range of 1 1 sigma these analyses represent 11 orders of magnitude reduction in sample size compared to conventional analyses on this material and they are the first report of routinely reproducible precision in the 1 per mil range for analysis of 18o at this scale high precision micro analyses of this sort will permit wide ranging new applications in stable isotope geochemistry the analyzed magnetites form nearly spherical grains in a calcite matrix with diopside and monticellite textures are characteristic of granulite facies marbles and show no evidence for retrograde recrystallization of magnetite magnetites are near to fe3o4 in composition and optically and chemically homogeneous a combination of ion probe plus conventional brf5 analysis shows that individual grains are homogeneous with 18o 8 9 1 smow from the core to near the rim of 0 1 1 2 mm diameter grains depth profiling into crystal growth faces of magnetites shows that rims are 9 depleted in 18o these low 18o values increase in smooth gradients across the outer 10 m of magnetite rims in contact with calcite these are the sharpest intracrystalline gradients measured to date in geological materials this discovery is confirmed by bulk analysis of 150 350 m diameter magnetites which average 1 2 lower in 18o than coarse magnetites due to low 18o rims conventional analysis of coexisting calcite yields 18o 18 19 suggesting that bulk 18o cc mt 9 3 and yielding an apparent equilibration temperature of 525 c over 200 c below the temperature of regional metamorphism consideration of experimental diffusion data and grain size distribution for magnetite and calcite suggests two contrasting cooling histories the data for oxygen in calcite under hydrothermal conditions at high p h2o indicates that diffusion is faster in magnetite and modelling of the low 18o rims on magnetite would suggest that the adirondacks experienced slow cooling after grenville metamorphism followed by a brief period of rapid cooling possibly related to uplift conversely the data for calcite at low p h2o show slower oxygen diffusion than in magnetite modelling based on these data is consistent with geochronology that shows slow cooling through the blocking temperature of both minerals suggesting that the low 18o rims form by exchange with late low temperature fluids similar to those that infiltrated the rock to serpentinize monticellite and which infiltrated adjacent anorthosite to form late calcite veinlets in either case the ion microprobe results indicate that two distinct events are recorded in the post metamorphic exchange history of these magnetites recognition of these events is only possible through microanalysis and has important implications for geothermometry

preservation of premetamorphic oxygen isotope ratios in granitic orthogneiss from the adirondack mountains new york usa the adirondack mountains new york expose a diverse group of proterozoic igneous rocks that were metamorphosed to granulite facies conditions during the ottawan phase of the grenville orogeny oxygen isotope data for seventy whole rock samples of gabbroic to granitic meta igneous rocks primarily from the charnockite suites from the tupper and saranac sheets in the central adirondacks demonstrate a correlation between 18 o wr and major element composition within continuous mappable meta plutonic units no such relationship is seen among nonconsanguineous granitoids variations in mineral 18 o values and large differences in 18 o between rocks with nonconsanguineous protoliths but similar bulk composition demonstrate that these rocks were not infiltrated by or isotopically equilibrated through a pervasive metamorphic fluid values of 18 o for mineral separates preserve generally high temperature fractionations indicative of dominantly closed system retrograde exchange values of 18 o wr in adirondack orthogneisses were not significantly shifted during granulite facies metamorphism and were dominantly controlled by processes active during premetamorphic magmatism values of 18 o wr may thus serve as a petrogenetic indicator allow discrimination of rock units and serve as a source constraint for meta igneous rocks fayalite meta granites with low 18 o values can be discriminated from surrounding granitoids having high 18 o values a record of assimilation in the evolution of differentiated granitic units is preserved the preservation of primary igneous 18 o values in granulite facies orthogneiss imposes constraints on the synmetamorphic and postmetamorphic fluid history of the adirondack highlands oxygen isotopic compositions at the hand sample scale have been preserved through granulite facies metamorphism fluid absence or low fluid rock ratios on a regional scale are indicated

oxygen isotope variations in granulite grade iron formations constraints on oxygen diffusion and retrograde isotopic exchange the oxygen isotope ratios of various minerals were measured in a granulite grade iron formation in the wind river range wyoming estimates of temperature and pressure for the terrane using well calibrated geothermometers and geobarometers are 730 50 c and 5 5 0 5 kbar the mineral constraints on fluid compositions in the iron formation during retrogression require either very co2 rich fluids or no fluid at all in the iron formation isotopic temperature estimates from quartz magnetite fractionations are controlled by the proximity to the enclosing granitic gneiss and range from 500 c qz mt 10 0 within 2 3 meters of the orthogneiss contact to 600 c qz mt 8 0 farther from the contact temperature estimates from other isotopic thermometers are in good agreement with those derived from the quartz magnetite fractionations

diffusion effects on oxygen isotope temperatures of slowly cooled igneous and metamorphic rocks recently obtained data on oxygen diffusion in feldspars quartz and hornblende permit the prediction of the apparent18o16o temperatures that would be measured in a rock that consisted only of those three minerals and cooled slowly from high temperature the computed temperatures would be based on the differences in the18o16o ratios between coexisting pairs of minerals the present calculation takes into account the diffusion rates for oxygen as a function of temperature the cooling rate of the rock the mineral grain sizes and the mode of the rock for mineral grains 1 mm in radius and a cooling rate of 10 c m y the minimum difference in apparent temperature between quartz feldspar and feldspar hornblende pairs will be 115 c despite the assumption of a normal uneventful slow cooling history to room temperature further the apparent quartz hornblende temperature will range over 30 c 590 620 c depending on the mode of the rock for a cooling rate of 1000 c m y the apparent difference in temperature can be as much as 400 c consequently consistency in temperatures obtained by oxygen isotope analysis should not be expected in most high grade metamorphic rocks or igneous rocks which are cooled slowly departures from the pattern of temperatures obtained in this model would imply a very rapid quench from high temperature or a complex history for the rock for some minerals including hornblende the relation between temperature and the equilibrium fractionation of oxygen isotopes between coexisting phases has been derived from observed relations in natural specimens the choice of the specimens used for such calibrations needs to be re evaluated in light of these findings this may result in a change in the equilibrium equation constants an example from the literature the san jose tonalite baja california mexico was modelled and yields 18o concentrations in the minerals that correspond closely with the measured values this suggests that the model used is appropriate that the rock has had a simple thermal history and that it cooled at 100 200 c m y over the temperature range 800 500 c the set of paleotemperatures obtained for a rock will in general yield neither the mineral closure temperatures nor the formation or crystallization temperatures on the other hand the cooling rate of the rock may be derived from the data this in turn may have important tectonic implications with regard to denudation and uplift rates

sr and nd isotope geochronology geologic history and origin of the adirondack anorthosite abstract we have analyzed samples from the adirondack marcy massif for rb sr and sm nd isotopes in an attempt to determine directly the primary crystallization age of a proterozoic massif type anorthosite rock suite the oldest age obtained 1288 36ma is from a 4 point sm nd isochron defined by igneous textured whole rock and mineral separate data from a local layered sequence gradational from oxiderich pyroxenite to leuconorite this age is older than silver s 1969 1113 ma zircon age of associated charnockites but is within the window of permissible anorthosite ages based on previous geochronology and field relationships as such 1288 ma may represent the time of crystallization of the massif for the most part however both sm nd and rb sr isotopic systems did not survive granulite facies metamorphism internal isochrons based on whole rocks and minerals yield ages between 995 and 919 ma these isotopic data suggest that the granulite fades metamorphism experienced by the massif was a prograde event that occurred a minimum of 100 ma and as much as 350 ma after crystallization of the massif the relatively large range in rb abundance and in calculated initial 87 sr 86 sr 0 7039 0 7050 and 143 nd 144 nd ratios among anorthosite suite rocks particularly those at or near the contacts of the marcy massif is explicable by variable contamination with crustal materials and or fluids derived from surrounding acidic metaplutonic rocks paragneisses and marbles despite uncertainies caused by crustal contamination and metamorphic resetting of primary ages marcy samples have epsilon nd values between 0 44 and 5 08 implying a source for the massif with long term depletion in light rare earth elements a probable source material would be depleted mantle

metamorphic fluids in the deep crust evidence from the adirondacks unusual mineral assemblages of tremolitic hornblende diopside enstatite quartz have been examined to estimate the temperature t and water fugacity at the time of the granulite fades metamorphism in the adirondacks the pyroxenes are closely represented by the system cao mgo sio2 allowing application of experimental data1 with only small corrections for reduced values of activities tremolite equilibria2 buffered log 3 0 0 1 equivalent to 0 9 kbar of water pressure at 7 kbar 710 c nearby marbles contain wollastonite calcite quartz and buffered 1 1 kbar consistent with 5 9 kbar inferred local gradients in fluid compositions suggest that deep crustal fluid conditions are complex and heterogeneous indicating that fluid movements whether of mantle3 or deep crustal origin are channelized rather than pervasive thus some granulite facies rocks were wholly or partly closed systems with respect to externally derived fluids permitting localized buffering of fluid compositions recognition of restricted fluid mixing limits theories that call for massive amounts of pervasive fluid flow to facilitate melting cause metasomatism or to stabilize granulite facies mineralogy

age of xenolith bearing olivine metagabbro eastern adirondack mountains new york a u pb zircon age of 1143 6 pm 7 ma has been obtained on a xenolith bearing olivine metagabbro in the eastern adirondack highlands thus fixing a minimum age for metamorphism represented by truncated foliation in the xenolith and the igneous crystallization age of the rock the igneous age predates commonly accepted metamorphic ages of 1000 1100 ma for the adirondacks and documents that more than one high grade metamorphic event affected the region

post granulite facies fluid infiltration in the adirondack mountains granulite facies lithologies from the adirondack mountains of new york contain alteration assemblages composed dominantly of calcite chlorite sericite these assemblages document fluid infiltration at middle to upper crustal levels cathodoluminescence of samples from the marcy anorthosite massif indicates that the late fluid infiltration is more widespread than initially indicated by transmitted light petrography alone samples that appear unaltered in transmitted light show extensive anastomosing veins of calcite 0 05 mm wide along grain boundaries in crosscutting fractures and along mineral cleavages the presence of the retrograde calcite documents paleopermeability in crystalline rocks and is related to the formation of high density co sub 2 rich fluid inclusions recognition of this process has important implications for studies of granulite genesis and the geophysical properties of the crust

a new look at stable isotope thermometry interdiffusion between coexisting minerals affects all rocks and causes resetting and discordance of stable isotope geothermometers that is commonly observed in slowly cooled igneous and metamorphic rocks the fast grain boundary fgb model describes the stable isotope fractionations and intracrystalline zonation which result from closed system interdiffusion eiler et al 1991 1992 this model assumes that grain boundary diffusion is much faster than volume diffusion and it accounts for exchange among all minerals in a rock previous models of closure temperature violate mass balance restrictions and will be inaccurate in most rocks modeling results are described for amphibolites and hornblende granites and gneisses biotite granites schists and gneisses pelitic and semi pelitic rocks garnet peridotites anorthosites gabbros pyroxenites and related rocks and calc silicate rocks examples of mineral pairs and specific rock types that allow accurate stable isotope thermometry include plagioclase pyroxene in pyroxene bearing anorthosites and garnet quartz in garnetiferous quartzites in contrast the same mineral pairs in related rocks such as pyroxenites and pelitic schists will exhibit reset apparent temperatures closed system processes are capable of producing a variety of patterns of stable isotope resetting discordance mineral zonation and fractionation reversals examples include large reversals of quartz feldspar fractionations in micaceous rocks and oscillatory zonation in feldspar from some quartz rich rocks these results permit reinterpretation of many studies of stable isotope thermometry speedometry and retrograde alteration history fgb modeling of mineral zonation provides an important new guide to applying recently developed micro analytical tools to slowly cooled rocks application of the fgb model to quartzo feldspathic gneisses from the adirondack mountains new york demonstrates the usefulness of diffusion modeling in discriminating closed system diffusion controlled retrogression from open system retrogression and illustrates the possible importance of incorporating the effect of water activity on mineral diffusivity

intercrystalline stable isotope diffusion a fast grain boundary model we formulated a numerical model for stable isotope interdiffusion which predicts the temperatures recorded between two or more minerals and the intragranular distribution of stable isotopes in each mineral as functions of mineral grain sizes and shapes diffusivities modes equilibrium isotopic fractionations and the cooling rate of a rock one of the principal assumptions of the model is that grain boundaries are regions of rapid transport of stable isotopes this fast grain boundary fgb model describes interdiffusion between any number of mineral grains assuming that local equilibrium and mass balance restrictions apply on the grain boundaries throughout the volume modeled the model can be used for a rock containing any number of minerals and number of grain sizes of each mineral several grain shapes and any thermal history or domain size desired previous models describing stable isotope interdiffusion upon cooling have been based on dodson s equation or an equivalent numerical analogue the closure temperature of dodson is the average bulk temperature recorded between a mineral and an infinite reservoir by using dodson s equation these models have treated the closure temperature as an innate characteristic of a given mineral independent of the amounts and diffusion rates of other minerals such models do not accurately describe the mass balance of many stable isotope interdiffusion problems existing models for cation interdiffusion could be applied to stable isotopes with some modifications but only describe exchange between two minerals under specific conditions the results of fgb calculations differ considerably from the predictions of dodson s equation in many rock types of interest actual calculations using the fgb model indicate that closure temperature and diffusion profiles are as strongly functions of modal abundance and relative differences in diffusion coefficient as they are functions of grain size and cooling rate closure temperatures recorded between two minerals which exchanged stable isotopes by diffusion are a function of modal abundance and differences in diffusion coefficient and may differ from that predicted by dodson s equation by hundreds of degrees c either or both of two minerals may preserve detectable zonation which may in some instances be larger in the faster diffusing mineral rocks containing three or more minerals can record a large span of fractionations resulting from closed system processes alone the results of fgb diffusion modeling indicate that the effects of diffusive exchange must be evaluated before interpreting mineral fractionations concordant or discordant recorded within any rock in which diffusion could have acted over observable scales the predictions of this model are applicable to thermometry evaluation of open or closed system retrogression and determination of cooling rates or diffusion coefficients

nature and timing of anatexis in the eastern and southern adirondack highlands metapelitic rocks in the eastern and southern adirondack highlands contain two feldspar quartz leuco somes that pre date most deformational fabric as well as granulite facies recrystallization whole rock xrf analyses of these leucosomes yield cipw norms which correspond closely with experimental melts in the system ab or qt an h 2 o thus indicating that the leucosomes are anatectic in origin derived p t values for anatexis indicate high temperatures and low pressures suggestive of widespread contact metamorphism the heat source is believed to have been provided by anorthositic gabbroic magmas postulated to have been anorogenically emplaced prior to the grenville orogeny

oxygen isotope fractionation in quartz albite anorthite and calcite laboratory measurements of equilibrium oxygen isotope fractionation in quartz albite anorthite and calcite have been carried out by anhydrous exchange between silicates and calcite at temperatures of 600 c and above exchange in these systems is as rapid as exchange between silicates and water fractionation factors can be summarized in the following equations qc 0 38 106t 2 abc 0 57 106t 2 anc 1 59 106t 2 from which the silicate pair fractionations are readily obtained these results are compared with published theoretical estimates as well as data derived from hydrothermal experiments some significant differences are found in particular it is difficult to reconcile all of the hydrothermal data either with theoretical calculations or with the present experimental data the new experiments provide an internally consistent set of fractionation factors suitable for isotopic thermometry and for test of disequilibrium in natural systems

age and regional relationships of granitoid rocks of the adirondack highlands approximately 55 of the adirondack highlands is underlain by granitic to tonalitic rocks that can be subdivided into five groups based on age composition and field occurrence 1 early calcalkaline tonalites ca 1300 ma and associated granitic rocks ca 1250 ma of the southern and eastern highlands 2 older anorogenic plutonic rocks 1145 1160 ma that occur throughout the highlands 3 younger anorogenic plutonic rocks ca 1130 ma that envelop anorthosite massifs in the central highlands 4 younger granitic rocks 1095 1105 ma of the northwest highlands and 5 late leucogranitic rocks 1060 1080 ma that occur throughout the northern highlands the data presented here are in good agreement with zircon ages from the green mountains of vermont as well as from southern ontario and quebec and suggest similar geologic histories for these regions these results document a protracted and complicated evolution for this portion of the grenville province

denudation outpaced by crustal thickening in the eastern tianshan the modern high topography of the tianshan resulted from the reactivation of a paleozoic orogenic belt by the india asia collision today the range exhibits tectonically active forelands and intermontane basins based on quantitative morphotectonic observations and age constraints derived from cosmogenic 10be dating single grain post infrared infrared stimulated luminescence p ir irsl dating and modeling of fault scarp degradation we quantify the deformation in the nalati and bayanbulak intermontane basins in the central eastern tianshan our results indicate that at least 1 4 mm yr of horizontal crustal shortening is accommodated within these two basins this shortening represents over 15 of the 8 5 0 5 mm yr total shortening rate across the entire range at this longitude this shortening rate implies that the eastern central tianshan is thickening at a mean rate of 1 4 mm yr a rate that is significantly higher than the average denudation rate of 0 14 mm yr derived from our cosmogenic analysis this discrepancy suggests that the tianshan range has not yet reached a steady state topography and remains in a transient state of topographic growth most likely due to limited denudation rates driven by the arid climate of central asia

juvenile middle proterozoic crust in the adirondack highlands grenville province northeastern north america nd isotope data indicate that minimal amounts of significantly older crust have contributed to the genesis of the oldest ca 1 3 13 5 ga plutons in the adirondack highlands these are magmatic arc tonalites with positive initial epsilon sub nd values and sm nd depleted mantle model ages t sub dm that are within 70 m y of the time of their crystallization granitoids of the anorthosite mangerite charnockite granite suite dated at 1 156 1 134 ma as well as the 1 100 1 050 ma plutons associated with the ottawan phase of the grenvillian orogenic cycle also have positive initial epsilon sub nd values and t sub dm ages similar to the tonalites derivation of both groups of granitoids by crustal melting of the magmatic arc is consistent with the available isotopic and geochemical data juvenile late middle proterozoic crust that formed during or just prior to the grenville cycle appears to dominate the southwestern grenville province as well as the grenville inliers to the south in contrast most of the contiguous grenville province in canada comprises largely reworked older crust

isotopic constraints on emplacement age of anorthositic rocks of the marcy masiff adirondack mts new york the igneous emplacement age of anorthositic rocks comprising the marcy massif have been constrained by fixing minimum and maximum ages for their intrusion minimum ages have been fixed by a u pb dating of cores of air abraded zircons from the anorthosite 1113 ma b u pb dating of baddeleyite within the anorthosite 1087 ma c u pb dating of baddeleyite from an olivine metagabbro crosscutting marcy massif anorthosite 1109 ma d u pb zircon dating of jotunitic rocks crosscutting the marcy anorthosite massif 1143 pm 31 ma maximum emplacement ages are based upon a sheets of anorthositic rock crosscutting mangeritic charnockitic gneiss dated by u pb zircon at 1125 pm 10 ma 1134 pm 4 ma and 1135 pm 43 ma and b anorthosite related pyroxenite dikes crosscutting mangerite that contains xenoliths of identical pyroxenite these results constrain emplacement of the marcy anorthosite to 1113 1138 ma with the most probable age in the interval 1125 1135 ma this time of in