late archean quetico accretionary complex superior province canada the 1200 km long quetico subprovince consists of monotonous metagraywacke with derived migmatite and granite in thrust and or transcurrent fault contact with the adjacent wabigoon and wawa metavolcanic subprovinces imbrication of sedimentary wedges derived from volcanic arcs to the north and south produced a 10 100 km wide stratigraphically north facing pile thermal relaxation in the 20 m y following accretion of the southern arc resulted in melting of the accretionary pile to produce peraluminous granite and associated low pressure metamorphism

rare earth element mobility during prograde granulite facies metamorphism significance of fluorine mafic gneisses occur as lenses or thin layers in spatial association with tonalitic leucosomes in a granulite zone of the quetico subprovince of the superior province ontario canada and exhibit concentric zoning with a biotite rich margin orthopyroxene rich outer zone clinopyroxene rich central zone and occasionally patches of relict amphibolites within the clinopyroxene rich zone the granulites biotite orthopyroxene and clinopyroxene rich zones in the mafic gneisses are characterized by significant amounts of rare earth element ree bearing fluorapatite 1 10 vol and other ree rich minerals allanite monazite and zircon fluorapatite shows an increase in modal abundance from the biotite and orthopyroxene rich zones to the clinopyroxene rich zone but is rare in the relict amphibolites textural evidence and element partitioning indicate that fluorapatite and other ree rich minerals was part of the peak metamorphic assemblages whole rock geochemical analyses confirm that the granulites in the mafic gneisses contain anomalously high contents of ree and high field strength elements hfse whereas the relict amphibolites are geochemically typical of tholeiitic basalts mass balance calculations reveal that ree and hfse were introduced into the mafic gneisses during the prograde granulite facies metamorphism pointing to ree mobility under granulite facies metamorphic conditions the presence of high f contents in the ree rich minerals and their associated minerals e g biotite and hornblende suggests that ree and hfse may have been transported as fluoride complexes during the granulite facies metamorphism this conclusion is supported by previously published results of hydrothermal experiments on the partitioning of ree between fluorapatite and f rich fluids at 700 c and 2 kbar

infiltration driven dehydration and anatexis in granulite facies metagabbro grenville province ontario canada dark hornblende garnet rich quartz absent metagabbro boudins from the seguin subdomain ontario grenville province are transected by anastomosing light coloured veins rich in orthopyroxene clinopyroxene plagioclase and sometimes quartz the veins vary in texture from fine grained diffuse veins and patches that overprint the metagabbro to coarse tonalitic leucosomes with sharp borders the diffuse veins and patches are suggestive of channellized subsolidus dehydration of the metagabbro while the tonalitic leucosomes are suggestive of local internally derived anatexis all vein types grade smoothly into each other with the tonalitic leucosomes being the latest relative to the host metagabbro the veins have higher si na ba sr lower fe mg ca ti and similar al the coarser veins are enriched in k plagioclase becomes steadily enriched in na in the transition from host metagabbro an47 to the veins an35 and in the coarsest veins it is antiperthitic differences in composition of the other minerals between host metagabbro and vein are minor pressure temperature estimates are scattered but indicate a minimum temperature during vein formation of 700 c at about 8 kbar mass balance constraints indicate that the veins formed from the metagabbro in an open system the transecting veins are interpreted to represent pathways of si na ba sr k al enriched low ah2o fluids that metasomatized the host metagabbro to form the anhydrous veins an initial period of localized solid state dehydration of the metagabbro represented by the diffuse veins was followed by a transition to localized anatexis represented by the tonalitic leucosomes the change to anatexis may have been due to the addition of k to the infiltrating fluid the source and delivery mechanism of the fluids is unknown

a regional perspective of the quetico metasedimentary belt superior province canada alternating greenstone granite and metasedimentary gneiss belts are a first order tectonic feature of the southern superior province the tectonic development of the quetico metasedimentary belt is reviewed with regard to depositional structural and metamorphic plutonic history over its 1200 km length the belt consists of marginal metasedimentary schists of turbiditic origin and interior metasedimentary migmatite and peraluminous leucogranite polyphase deformation has resulted in a steep easterly striking foliation and regional gently east plunging stretching lineation metamorphic grade varies in a low p facies series from greenschist at the belt margins to upper amphibolite and local granulite in the central migmatite intrusive granite zone mineral assemblages in the central zone yield estimates of metamorphic pressure that increase systematically eastward over 800 km from about 250 mpa 2 5 kbar near the canada united states border to 600 mpa 6 kbar in granulites adjacent to the kapuskasi

phosphorus and the rare earth elements in felsic magmas an assessment of the role of apatite abstract the solubility of fluorapatite in 17 silica rich melts in the system na 2 o k 2 o al 2 o 3 sio 2 with and without cao or caf 2 was determined at 1 kbar water pressure and 750 900 c apatite saturation occurs at levels of dissolved p 2 o 5 ranging between 0 04 0 02 and 0 28 0 13 wt with only 4 values outside the 0 09 0 20 wt range the results demonstrate not only that apatite is a common liquidus phase in felsic melts but also that under most circumstances it remains in the residue during episodes of partial fusion of the crust given a solubility limit of 0 14 wt dissolved p 2 o 5 the mean of the experimental values a source containing as little as 0 05 p 2 o 5 must be 35 melted before apatite is lost from the residue and no longer buffers the melt p 2 o 5 concentration at the saturation value higher abundances of p 2 o 5 in the source postpone the loss of residual apatite to still higher degrees of melting and if the source p 2 o 5 content exceeds 0 14 wt apatite must be residual for all degrees of melting increasing in abundance as melting proceeds the generally secondary influence of apatite on the rare earth element ree patterns of melt and residue is most apparent when garnet and or amphibole is minor or lacking in the residue fractional crystallization of intermediate e g andesitic magmas toward felsic compositions invariably results in saturation in apatite and some consequent depletion of ree in the melt

co 2 melts and granulite metamorphism the discovery that granulites contain co2 rich fluid inclusions while amphibolites in the same terrane contain h2o rich or mixed co2 h2o inclusions1 has led to a debate over the processes that produce granulite metamorphism one theory2 4 maintains that granulites form as the result of a massive influx of co2 from lower crustal or mantle sources such an influx could be responsible for the marked depletion many granulite terranes show in large ion lithophile elements2 5 others however maintain that granulite facies metamorphism is a necessary complement of crustal melt ing6 9 recently lamb and valley10 11 have argued that granulite metamorphism may be a product of any one of three processes co2 streaming partial melting or recrystallization of originally dry rocks here we suggest that these processes need not be independent and that they all may be related to the passage of melts through the crust

mass transport mechanisms in deforming rocks recognition using microstructural and microchemical criteria we establish microstructural and microchemical criteria that can be used to distinguish the extent to which mass transport during deformation occurred by diffusion or fluid infiltration as well as the extent to which it was pervasive or channelized continuous or episodic diffusive mechanisms are indicated by a small scale of mass transfer and symmetrical patterns of redistribution of chemical components infiltration typically leads to asymmetric patterns of chemical variation because of smearing out of chemical anomalies in the direction of fluid movement and can lead to unpredictable directions of chemical change because of bypassing of layers by permeable pathways and nonpervasive flow examples of the use of the criteria include the following 1 patterns of compositional variation in olivine across the layering in lherzolitic mylonites 2 detection of microveins containing more calcic plagioclase even where they have been completely recrystallized int he matrix of a granitic mylonite and 3 complex and variable lengthwise zoning patterns in adjacent plagioclase porphyroblasts in a lithologically complex mylonite indicating transient nonpervasive fluid infiltration combined microstructural and microchemical studies coupled with thermodynamic calculations are essential if the sites of mineral dissolution and growth together with the direction of chemical change and the likely sources ofmore metasomatic components are to be defined less

the timescale and mechanism of granulite formation at kurunegala sri lanka incipient charnockite formation at kurunegala in sri lanka is characterized by the growth of orthopyroxene at the expense of amphibole and biotite in an originally homogeneous gneiss mineral equilibria in the charnockite assemblage record pressure temperature p t conditions of 738 60 c and 6 9 1 2 kbar at 17 0 1 2 log fo2 and ah2o 0 18 0 16 wholerock trace element and isotopic measurements show that charnockite formation was accompanied by a systematic depletion of sm rb pb u sr nd with a fractionation of rb sr sm nd and th u ratios and crystallization of the charnockite assemblage at 535 5 ma major element fe mg ca and sm nd equilibration between minerals occurred at 524 9 ma whereas pb and rb sr underwent continued exchange to 501 5 ma and 486 1 ma respectively trace element data for both amphibolite and charnockite minerals show that depletion on a whole rock scale can be accounted for either by changes in mineral modes or trace element abundances within the immediate area of dehydration the fractionation of sm nd on a whole rock scale is controlled by the breakdown of amphibole without the growth of a major new host phase for sm in the charnockite rubidium and sr are dependent on the relative behaviour of biotite plagioclase and alkali feldspar modelling of dehydration melting involving the breakdown of amphibole biotite and alkali feldspar reproduces the observed sm nd and rb sr fractionation and indicates the loss of small melt fractions on a cm scale from the charnockite these observations suggest that partial melting is the most plausible means of effecting both the dehydration and depletion that accompanies charnockite formation

isotopic evidence for involvement of c0 2 bearing magmas in granulite formation it has been suggested1 2 that co2 bearing magmas play an important part in the formation of granulites these magmas crystallize directly into granulites and also expel fluids which promote the development of granulite facies mineral assemblages in adjacent country rocks direct evidence for carbonic fluids remains elusive however as granulite facies aureoles adjacent to charnockitic intrusives1 may result either from the influx of carbonic fluids derived from the intrusives or simply from extraction of water into a vapour absent melt here we report results of a detailed study of carbon isotope compositions of graphite associated with a charnockite dyke from the ponmudi quarry of south india higher graphite abundances and an anomalously heavy carbon isotopic composition 13cgr 8 1 0 at the dyke margins indicate that externally derived co2 rich fluids were transported into the ponmudi paragneisses by the intrusive and precipitated nearly quantitatively as graphite on encountering reducing country rocks the occurrence of large orthopyroxene crystals along the dyke margins suggests that these fluids also served to dehydrate the country rocks this dyke may thus be representative of the widespread transport of carbonic fluids in this terrain by felsic magmas

granulite facies metamorphism in the quetico subprovince north of manitouwadge ontario a zone of granulites defined by an orthopyroxene in isograd and extending more than 100 km in length and about 10 km in width occurs near the southern margin of the quetico subprovince north of manitouwadge ontario mineral assemblages in metasedimentary rocks and associated migmatites consist of quartz plagioclase garnet orthopyroxene biotite cordierite sillimanite k feldspar hercynite magnetite ilmenite and other accessory phases minor mafic gneisses and calc silicate pods or lenses are also present from equilibrium phase relations and thermobarometry the granulites experienced a thermal peak event 4 6 kbar 1 bar 100 kpa 680 770 c a h2o of 0 15 0 25 and fo2 of 1 2 log units above the fmq buffer in association with d2 deformation followed by a retrogression 550 660 c and 3 4 kbar and a later hydrothermal alteration 1 2 kbar and 200 400 c the distribution and calculated peak metamorphic conditions of the granulite zone in the quetico subprovince are similar to those of

deep crustal oxygen isotope variations the wawa kapuskasing crustal transect ontario we have studied the oxygen isotopic composition of rocks from a 100 km transect through the central superior province of ontario representing progressively the shallower terrains of the kapuskasing structural zone ksz the wawa gneiss terrane wgt and the michipicoten greenstone belt mgb these rocks range in age from 2 76 to 2 60 ga and correspond to a section through approximately 20 km of crustal thickness equivalent lithologic types have similar range of 18o values at each crustal level tonalitic to granodioritic rocks 6 4 to 9 5 dioritic and anorthositic rocks 5 5 to 7 6 mafic gneisses group 1 majority 5 7 to 7 1 group 2 8 1 to 9 5 18o values exhibit a remarkable correlation with sio2 values similar to that observed in unaltered plutonic rocks of equivalent composition paragneisses have significantly higher 18o values 9 3 to 12 2 low grade metavolcanic and metasedimentary rocks of the mgb are 18o enriched compared to their high grade equivalents in the ksz and wgt 7 4 to 13 3 for mafic to felsic metavolcanic rocks 11 4 to 14 7 for clastic metasediments coexisting minerals exhibit 18o fractionation consistent with equilibrium but corresponding to uniform isotopic temperatures about 553 to 584 c across the entire transect lower than the inferred metamorphic temperatures in the highest grade ksz terrane the lack of distinctive isotopic differences between equivalent rock types in the ksz wgt and mgb suggests that there is no significant gradient in 18o with depth in the crust the majority of mafic gneisses group 1 appear to have been emplaced either as subaerial extrusives intrusive sills or less likely as submarine extrusives that were hydrothermally altered at high temperatures the less abundant group 2 mafic rocks have the 18o values typical of greenstones that were altered at low temperature by seawater and isotopically resemble low grade rocks in the michipicoten and abitibi belts in general no major changes in whole rock isotopic composition appear to have occurred during granulite facies metamorphism implying limited flux of water or co2 the continuous linear gradient in 18o versus sio2 in the high grade rocks cannot be due to differentiation of a mafic source magma a model involving an association between mantle derived mafic magma and 18o enriched crustal materials is more consistent with the oxygen isotopic and the ree data

thermo tectonic evolution of an archean accretionary complex u pb geochronological constraints on granulites from the quetico subprovince ontario canada abstract u pb geochronological data have been obtained for a granulite zone in the quetico subprovince an archean accretionary complex of the superior province north of manitouwadge ontario canada this granulite zone represents the culmination of a low pressure high temperature regional metamorphism in the quetico subprovince and an apparent continuation of a northward increase in metamorphic grade in the wawa subprovince and across the quetico wawa boundary a u pb zircon age of 2666 1 ma from a concordant magnetite bearing granitic pegmatite in the quetico granulite zone is consistent with the inferred timing of regional metamorphism in amphibolite facies zones 2671 2665 ma and indicates that the low pressure high temperature metamorphism in the quetico subprovince commenced during the main compressional deformation titanite in a calc silicate rock from low leucosome fraction layered migmatites that might represent incipient melting gave a 207pb 206pb age of 2658 1 ma the timing of the granulite facies metamorphism is constrained by u pb zircon ages of 2650 2 and 2651 3 ma from mafic granulite and garnet bearing tonalitic leucosome respectively that are similar to reported u pb ages for mafic granulites in the upper levels of the kapuskasing structural zone the present u pb data suggest a diachronous development of peak metamorphic conditions of the regional metamorphism in the quetico and wawa subprovinces this diachronous development and the zonal distribution of the regional metamorphism are probably related to oblique collision at a convergent plate margin and differential rates of erosional uplift away from the quetico wawa boundary

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

melt related metasomatism in mafic granulites of the quetico subprovince ontario constraints from o sr nd isotopic and fluid inclusion data mafic granulites in the archean quetico subprovince north of manitouwadge ontario occur as isolated lenses or discontinuous layers in spatial association with tonalitic leucosomes in metasedimentary rocks and exhibit concentric zoning from a biotite rich margin to an orthopyroxene rich outer zone and a clinopyroxene rich central zone with internal orthopyroxene bearing leucosomes and rarely patches of relict amphibolites within the clinopyroxene rich zone microstructural and microchemical evidence suggests that the mafic granulites formed from amphibolites by combined infiltration diffusion processes in the presence of a p f bearing silicate melt p 2o5 melt 0 24 0 28 wt and ac o 2 rich hypersaline fluid the whole rock and mineral 18 o values of the mafic granulites 8 9 v smow indicate oxygen isotope equilibration between amphibolites 6 6 6 9 and associated tonalitic leucosomes 9 5 10 at 700 800 c strontium and nd isotope data and u pb zircon ages confirm isotopic homogenization at the leucosome amphibolite boundaries during the peak granulite facies metamorphism at about 2650 ma texturally early co2 rich fluid inclusions in quartz and garnet yield p t conditions similar to those of the peak granulite facies metamorphism hypersaline fluid inclusions occur in textural coexistence with the early co2 rich inclusions but are invariably low in homogenization temperatures 178 234 c this study shows that silicate melts not only provide a conduit for co2 rich fluids but also interact directly with country rocks for the formation of granulites also the o sr nd isotope data show that the documented mobility of rare earth elements in the quetico granulite zone is localized in scale and related to anatexis of local metasedimentary rocks during the granulite facies metamorphism resume les granulites mafiques dans la sous province de quetico archeenne au nord de manitouwadge en ontario apparaissent sous forme de lentilles isolees ou de couches discontinues associees spatialement a des leucosomes tonalitiques dans les roches metasedimentaires elles exhibent un zonage concentrique evoluant d une marge riche en biotite a une zone externe riche en orthopyroxene et a une zone centrale riche en clinopyroxene incluant des leucosomes internes contenant un orthopyroxene occasionnellement sont rencontrees des plages d amphibolites relictuelles dans la zone riche en clinopyroxene les analyses microstructurales et microchimiques suggerent que les granulites mafiques derivent d amphibolites transformees par les processus combines d infiltration diffusion en presence d un magma silicate contenant p et f p2o5 magma 0 24 0 28 par poids et d un fluide riche en co2 hypersalin

on the origin of co2 rich fluid inclusions in migmatites nearly pure co2 fluid inclusions are abundant in migmatites although h2o rich fluids are predicted from the phase equilibria processes which may play a role in this observation include 1 the effects of decompression on melt 2 generation of a co2 bearing volatile phase by the reaction graphite quartz biotite plagioclase melt orthopyroxene co2 rich vapour 3 selective leakage of h2o from co2 h2o inclusions when the pressure in the inclusion exceeds the confining pressure during decompression and 4 enrichment of grain boundary vapour in co2 by subsolidus retrograde hydration reactions

quantification of lower crustal synmetamorphic fluid fluxes in the kapuskasing structural zone based on oxygen isotope profiles across two paragneiss mafic gneiss contacts detailed oxygen isotope profiles were measured across two very similar amphibolite grade paragneiss mafic gneiss contacts located 2 km apart within the kapuskasing structural zone the first profile is asymmetrical about die contact 18 o values for whole rocks and minerals are homogeneous in the paragneiss whole rock 18 o 10 4 to 11 4 smoothly decrease by about 2 over 2 5 m moving into the mafic gneiss and are homogeneous in the remainder of the mafic gneiss whole rock 18 o 7 0 to 7 4 the second profile is flat with homogeneous whole rock and mineral 18 o values in bom lithologies and mafic gneiss values whole rock 18 o 8 7 to 9 8 that are higher by about 3 relative to typical mafic gneiss values mineral mineral isotopic fractioations are fairly constant in all samples and are typical of high grade metamorphic rocks despite their differences both profiles can be explained by the advection of a mixed h 2 o co 2 fluid in equilibrium with the paragneiss across the contact into the mafic gneiss coupled with diffusion of oxygen in the fluid modeling constrains the net fluid flux required to form me first profile to be 1 3 m 3 m 2 and for me second profile to be 10 m 3 m 2 thus the net fluid flux during metamorphism varied probably by at least a factor of five over 2 km these fluxes could have been generated locally within the kapuskasing structural zone during metamorphism either by metamorphic devolatilization reactions or by the release of volatiles from crystallizing magmas

anatexis of interlayered amphibolite and pelite at 10 kbar effect of diffusion of major components on phase relations and melt fraction we have conducted h2o undersaturated melting experiments on charges consisting of a layer of a sillimanite bearing metapelite and a layer of garnet bearing amphibolite with the goal of studying how layered metamorphic rocks interact during anatexis experiments with the layered charges were performed at 10 kbar 1 gpa at 900 c 334 h 925 c 341 h and 950 c 50 161 332 h dehydration melting of the amphibolite alone and of the pelite alone were studied at the same p t conditions when run alone at 900 c the pelite and the amphibolite yielded ca 40 vol and 15 vol melt respectively corresponding melt fractions at 950 c were ca 50 vol and 30 vol when run side by side melt abundance in the pelite is fairly uniform and ca 20 vol higher than in the solo runs at all temperatures studied in contrast complex zoning patterns parallel to the interface develop in the amphibolite layer far from the interface the melt fraction is 5 10 vol lower than in the amphibolite solo runs closer to the interface the melt fraction in the amphibolite layers are comparable to or somewhat higher than in the amphibolite solo runs and both clinopyroxene and plagioclase react out and are replaced by orthopyroxene and quartz melt fraction closest to the interface reaches 65 70 vol and the only crystalline phase present is garnet with hollow cores the increase in melt fraction in the pelite layers of the layered runs is caused by transfer of na from the amphibolite to the pelite which forces biotite to be eliminated at lower temperature along the biotite garnet reaction boundary k is transferred from the pelite to the amphibolite and the lower melt fraction in the distal end of the amphibolite layers is caused by faster diffusion of na compared to k which results in a net depletion of alkalies ca is transferred from the amphibolite to the pelite and the resulting changes in cao concentrations affect the stabilities of plagioclase pyroxenes garnet and quartz and cause the observed zoning in the amphibolite layer ca entering the pelite layer induces crystallization of large plagioclase neoblasts radiating from the interface into the pelite layer these results show that diffusion of components between contiguous lithologies undergoing anatexis can cause profound changes in phase relations melt fractions and restite compositions

methane in fluid inclusions from granulites a product of hydrogen diffusion many fluid inclusion studies of granulite grade rocks reveal the presence of co2 rich inclusions that appear to have been trapped near the peak of metamorphism final melting temperatures of co2 tm co2 reported for these inclusions are often below the co2 triple point of 56 6 c and some are below 60 c this freezing point lowering is usually attributed to the presence of a second volatile component such as ch4 and the presence of ch4 has been confirmed in some cases by raman or mass spectroscopy a co2 melting temperature near 56 6 c is commonly offered as evidence that the inclusions contain nearly pure co2 however significant amounts of ch4 may be present but cause seemingly insignificant freezing point depressions c o h fluid speciation calculations for conditions representative of granulite facies metamorphism indicate that ch4 may comprise a significant portion of peak metamorphic fluids when graphite is present but it is never a significant species in co2 rich fluids in most cases the amount of ch4 should be virtually undetectable with microthermometric or spectroscopic techniques only in aqueous fluid inclusions can co2 and ch4 both be significant species post trapping speciation changes to fluid inclusions at constant mass cannot account for the reported compositions unless graphite precipitates in the inclusions thus the observed compositions require post trapping compositional changes due to loss or gain of components we have modeled variations in the fugacities of molecular fluid species in inclusion and matrix fluids during uplift from 6 kb and 800 c assuming hypothetical uplift pressure temperature paths which are concave or convex toward the temperature axis t concave and t convex respectively our results suggest that for rocks buffered at o2 within one log unit of the fayalite magnetite quartz equilibrium fmq 1 most uplift paths result in external h2 overpressures of bars to tens of bars at temperatures 400 c the highest overpressures are generated during t concave uplift compositional changes resulting from equilibration of such gradients via diffusive addition of hydrogen to peak metamorphic fluid inclusions and concomitant reduction of co2 by conversion to ch4 and h2o are consistent with the compositions of fluid inclusions reported from granulite terranes previous workers have postulated that co2 rich fluid inclusions in granulites could originate from post trapping diffusive loss of h2o from h2o co2 inclusions in response to an h2o gradient between the inclusion and matrix fluids the results of the present study suggest that for fluids buffered by fmq 1 this is possible only if 1 1 uplift is t convex and the matrix fluid composition remains nearly constant or 2 2 the matrix fluid evolves toward relatively h2o poor compositions the latter could occur if influx of co2 rich fluids occurs during uplift

archean and proterozoic mineralization and tectonics at the renco mine northern marginal zone limpopo belt zimbabwe gold mineralization at the renco mine occurs within the northern marginal zone of the limpopo belt along east northeast trending structures that formed in the late arehean 2 6 ga pb pb step leach age on syntectonic garnet under granulite facies the mineralization is hosted by a massive enderbite which intruded at 2 57 ga u pb zircon age north northeast trending pyroxenite dikes were intruded around 2 4 ga pb pb step leach age of garnet and pyroxene probably associated with the intrusion of the great dyke and satellites the northern marginal zone had been exhumed to crustal levels corresponding to greenschist facies by this time the major gold bearing pyrrhotite chalcopyrite or bismuth mineralization occurred under greenschist facies conditions 400 degrees c this occurred at approximately 2 0 ga as deduced from cooling of biotite associated with mineralization 1 88 ga rb sr age this ca 2 0 ga age of the mineralization implies a link with the important early proterozoic tectono thermal event in the limpopo belt during which shearing occurred under greenschist facies conditions in the triangle shear zone the reneo deposit is not as formerly suggested a high grade mineralization although there is some evidence for a precursory high grade phase of mineralization in the late archean its genesis is quite distinct from most of the greenstone associated deposits of the zimbabwe craton which are known or inferred to be late archean in age

incongruent dissolution of ree and sr rich apatite in peraluminous granitic liquids differential apatite monazite and xenotime solubilities during anatexis the relative solubilities of monazite mnz xenotime xno and apatite ap ree and sr rich and ree and sr poor have been studied in peraluminous granitic liquids in month long experiments at 750 c and 200 mpa ph20 in contrast with the high solubility of the apatite 0 7 wtojop205 in strongly peraluminous liquids monazite and xenotime have much lower solubilities 0 05 wt jop205 in mildly to strongly peraluminous compositions p205 in the liquid is 0 03 0 04 wt joat xenotime saturation and 0 02 0 05 wt jo at monazite saturation in keeping with the low p in the glasses re203 contents are below emp detection thresholds 50 08 wt jo for all conditions of experiments saturation of liquid at equilibrium and local saturation around apatite apatite dissolves incongruently crystallizing ree rich monazite on its surface 1 4 m iong grains resulting in similar low ree contents in liquids monazite precipitation occurs along the margins of dissolving apatite crystals even though the bulk liquid is not monazite saturated the abundance of monazite microcrystals increases with the ree content of the apatite and the degree of apatite dissolution the reaction relationship ap liqi mnz liq2 stemming from differences in relative solubilities greater than an order of magnitude between apatite and monazite results in the dissolution of much smaller amounts of ree into peraluminous liquids than expected by simple evaluation of apatite ree contents the amount of ree contributed from apatite directly to peraluminous granitic liquid is related to the amount of apatite dissolved by simple mass balance only if the total ree content of the apatite is sufficiently low that monazite saturation in liquid 50 100 ppm re203 at 750 c is avoided during dissolution of sr rich apatite the sr partitions into the liquid and at 750 c and 200 mpa ph20 the diffusion coefficient of sr in liquid is 2 x 10 10 cm2js r2 0 659 the reaction relationships described above may have application to some textural features observed in natural igneous rocks for example clusters of monazite microcrystals might be indicators of dissolved apatite monazite morphology can be used to distinguish the source of its ree and the general petrological process rock anatexis or magma crystallization under which the monazite formed in addition monazite microcrystals could serve as nucleation sites for other minerals which might explain their common inclusion in biotite and amphibole within granitoids

open system dehydration of amphibolite morton pass wyoming elemental and nd and sr isotopic effects we have undertaken a geochemical and sm nd and rb sr isotopic study of banded metabasites in the contact aureole of the 1 43 ga laramie anorthosite complex wyoming in order to determine the isotopic effects of open system high temperature metamorphism samples were collected from two locations along strike one that experienced peak temperatures of 720 c and one that was subjected to temperatures up to 810 c at the higher temperature location metabasites adjacent to marble have undergone extreme dehydration presumably due to fluxing of co 2 rich fluids these hornblende poor rocks also show textural evidence of high temperature shearing element mobility is most evident within this zone of dehydration variations in chemical composition are homogenized and elements including ti fe k na rb sr y sm and nd have been removed metabasites at greater distances from the marble also provide evidence of open system behavior albeit less dramatic rb k and sr contents are low throughout the hi

residence and redistribution of ree y zr th and u during granulite facies metamorphism behaviour of accessory and major phases in peraluminous granulites of central spain accessory minerals are thought to play a key role in controlling the behaviour of certain trace elements such as ree y zr th and u during crustal melting processes under high grade metamorphic conditions although this is probably the case at middle crustal levels when a comparison is made with granulite facies lower crustal levels differences are seen in trace element behaviour between accessory minerals and some major phases such a comparison can be made in central spain where two granulite facies terranes have equilibrated under slightly different metamorphic conditions and where lower crustal xenoliths are also found differences in texture and chemical composition between accessory phases found in leucosomes and leucogranites and those of melanosomes and protholiths indicate that most of the accessory minerals in melt rich migmatites are newly crystallized this implies that an important redistribution of trace elements occurs during the early stages of granulite facies metamorphism in addition the textural position of the accessory minerals with respect to the major phases is crucial in the redistribution of trace elements when melting proceeds via biotite dehydration melting reactions in granulitic xenoliths from lower crustal levels the situation seems to be different as major minerals show high concentration of certain trace elements the distribution of which is thus controlled by reactions involving final consumption of al ti phlogopite a marked redistribution of hree y zr between garnet and xenotime where present and zircon but also of lree between feldspars k feldspar and plagioclase and monazite is suggested

the radiogenic isotope characteristics of dikes and sills associated with the mesoproterozoic midcontinent rift near thunder bay ontario canada abstract new radiogenic isotope data are presented for intrusive rocks of the mesoproterozoic midcontinent rift from the north shore of lake superior these mafic rocks are amongst the older portions of the rift sequence and offer insights into the early development and evolution of the rift two sill suites are recognized within the study area an earlier spatially restricted ultramafic unit termed the riverdale sill and the predominant logan sills as well as three dike sets the north east trending pigeon river swarm the north west trending cloud river dikes and the mt mollie dike in addition an enigmatic extrusive unit termed the devon basalts is also present the logan sills are characterized by 87sr 86sri values of 0 7048 0 7119 with 143nd 144nd values from 0 512012 to 0 512187 whereas the riverdale sill is characterized by 87sr 86sri values ranging from 0 7037 to 0 7078 and 143nd 144nd of 0 511938 0 511984 both the cloud river and pigeon river dikes show a range of isotope data with the cloud river dikes having 87sr 86sri values ranging from 0 7042 to 0 7064 and 143nd 144nd values from 0 512222 to 0 512379 whereas the pigeon river dikes have 87sr 86sri values from 0 7041 to 0 7243 and 143nd 144nd from 0 511640 to 0 512383 the mt mollie dike is characterized by 87sr 86sri of 0 7041 0 7074 and 143nd 144nd values range from 0 512014 to 0 512371 the majority of samples are characterized by negative ndt which are interpreted to be the result of contamination of a plume related mantle source by older crustal material the lack of correlation with other indices of fractionation e g sio2 for the majority of samples implies interaction with subcontinental lithospheric mantle either through assimilation or melting of the mantle rather than contamination during emplacement however a small number of samples appear to have assimilated material during emplacement resulting in strongly negative ndt values the radiogenic isotope data are consistent with a suite of rocks that have undergone complex contamination histories requiring variable crustal residence times and assimilation of material both at depth in large magma chambers and during emplacement

partitioning of f between h2o and co2 fluids and topaz rhyolite melt implications for mineralizing magmatic hydrothermal fluids in f rich granitic systems fluid melt distribution coefficients for f have been determined in experiments conducted with peraluminous topaz rhyolite melts and fluids consisting of h2o and h2o co2 at pressures of 0 5 to 5 kbar temperatures of 775 1000 c and concentrations of f in the melt ranging from 0 5 to 6 9 wt the major element f and cl concentrations of the starting material and run product glasses were determined by electron microprobe and the concentration of f in the fluid was calculated by mass balance the h2o concentrations of some run product glasses were determined by ion microprobe sims the solubility of melt in the fluid phase increases with increasing f in the system the solubility of h2o in the melt is independent of the f concentration of the system with up to 6 3 wt f in the melt no evidence of immiscible silica and fluoriderich liquids was detected in the hydrous but water undersaturated starting material glasses 8 5 wt f in melt or in the water saturated run product glasses f concentrates in topaz rhyolite melts relative to coexisting fluids at most conditions studied however df wt f in fluid wt f in melt increases strongly with increasing f in the system maximum values of df in this study are significantly larger than those previously reported in the literature linear extrapolation of the data suggests that df is greater than one for water saturated peraluminous granitic melts containing 8 wt f at 800 c and 2 kbar df increases as temperature and as h2o h2o co2 of the fluid increase for topaz rhyolite melts containing 1 wt f and with h2o rich fluids df is independent of changes in pressure from 2 to 5 kbar at 800 c for melts containing 1 wt f and in equilibrium with co2 bearing fluids the concentrations of f in fluid increases with increasing pressure f and lithophile element enriched granites may evolve to compositions containing extreme concentrations of f during the final stages of crystallization if f in the melt exceeds 8 wt df is greater than one and the associated magmatic hydrothermal fluid contains 4 molal f such f enriched fluids may be important in the mass transport of ore constituents i e f mo w sn li be rb cs u th nb ta and b from the magma

oxygen diffusion rates in quartz exchanged with co2 oxygen self diffusion rates were determined in quartz samples exchanged with18o enriched co2 between 745 and 900 c and various pressures and the diffusion profiles were measured using an ion microprobe the activation energy q and preexponential factor d0 at p co2 p tot 100 bar for diffusion parallel to the c axis are 159 13 kj g atom and 2 10 0 75 0 55 10 8 cm2 s this rate is approximately 100 times slower than that obtained from hydrothermal experiments and 100 times faster than a previous 1 bar quartz o2 exchange experiment the oxygen diffusion rate measured at 0 6 bar 888 c and at 900 c in vacuum is in agreement with the previous 1 bar exchange experiments with18o2 the effect of higher co2 pressures is small at 900 c the diffusion rate exchanged with co2 is 2 35 10 15 cm2 s at 100 bar 2 24 10 15 cm2 s at 3 45 kbar and 8 13 10 15 cm2 s at 7 2 kbar there is probably a diffusing species other than oxygen that enhances the oxygen diffusion rate in these quartz co2 systems relative to that occurring at very low pressures or in a vacuum the effect of this diffusing species however is not as strong as that associated with h2o preserved oxygen isotope fractionations between coexisting minerals in a slowly cooled high grade metamorphic terrane will vary depending upon whether a water rich phase was present or not closure temperatures will be approximately 100 c higher in rocks where no water rich phase was present during cooling the measured fractionations between coexisting minerals in metamorphic rocks may potentially be used as a sensor of water presence during retrogression

diffusion controlled growth of pyroxene bearing margins on amphibolite bands in the granulite facies of rogaland southwestern norway implications for granulite formation in the rogaland granulites of southern norway thin anhydrous pyroxene bearing margins 5 10 mm are observed mainly in migmatitic banded gneisses at the contact between hornblende rich metabasites and charnockites according to field data the development of these margins post dates any deformation petrographic data show that they are zoned three different types have been recognized 1 metabasite plagioclase orthopyroxene plagioclase clinopyroxene gneiss 2 metabasite plagioclase orthopyroxene clinopyroxene plagioclase clinopyroxene gneiss 3 metabasite plagioclase orthopyroxene gneiss

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

phosphorus in s type magmas the p2o5 content of feldspars from peraluminous granites pegmatites and rhyolites

radiogenic isotope characteristics of the mesoproterozoic intrusive rocks of the nipigon embayment northwestern ontario the ca 1110 1120 ma diabase sills of the nipigon embayment represent the oldest phase of the midcontinent rift recognized to date the sills can be subdivided into five geochemically distinct suites nipigon mcintyre inspiration shillabeer and jackfish sm nd rb sr and pb pb isotope data for the sills can be used to evaluate the source regions and emplacement histories of these sills the endt values for all the sill suites are consis tently negative 0 5 to 6 6 but show coherent variations both within and between suites the negative endt values can be interpreted as the result of contamination of a plume related mantle source by older crustal material the initial strontium sri values 0 7032 0 7068 for the sills indicate that at least two distinct contaminants are required 1 a source with strongly negative endt and lower sri likely archean metasediment or granite and 2 one with elevated sri likely sedimentary rocks of the proterozoic sibley group the pb data for the nipigon and inspiration sills form distinct linear arrays consistent with variable contamination of a source comparable to that of other igneous suites of the midcontinent rift the radiogenic isotopes preserve a complex contamination history best interpreted as the result of 5 contamination by a variety of archean and proterozoic sources the ultramafic jackfish sill shows the least ev idence of contamination and is interpreted to have been emplaced along a crustal scale fault with little interaction with crustal material in contrast the other sill suites have undergone complex contamination histories requiring variable crustal residence times and assimilation of material both at depth in large magma chambers and during emplacement

fluid heterogeneity during granulite facies metamorphism in the adirondacks stable isotope evidence the preservation of premetamorphic whole rock oxygen isotope ratios in adirondack metasediments shows that neither these rocks nor adjacent anorthosites and gneisses have been penetrated by large amounts of externally derived hot co2 h2o fluids during granulite facies metamorphism this conclusion is supported by calculations of the effect of fluid volatilization and exchange and is also independently supported by petrologic and phase equilibria considerations the data suggest that these rocks were not an open system during metamorphism that fluid rock ratios were in many instances between 0 0 and 0 1 that externally derived fluids as well as fluids derived by metamorphic volatilization rose along localized channels and were not pervasive and thus that no single generalization can be applied to metamorphic fluid conditions in the adirondacks

timing and mechanisms of carbon isotope exchange in granulite facies calc silicate boudins rauer group east antarctica mineralogically zoned decimeter diameter calc silicate boudins enclosed within paragneisses from little italy island rauer group east antarctica underwent granulite facies metamorphism p 5 700 900 mpa t 5 840 6 40 8c followed by near isothermal decompression to 200 400 mpa during decompression several mineral reactions occurred in the presence of a pore fluid 0 4 the calc silicate boudins show a general xco2 increase in calcite d 13 c values from core as low as 217 to rim 2 9t o 2 10 by contrast d 18 o values show little variation across the boudins the d 13 c profiles are similar to those predicted to result from diffusion within a sphere that is surrounded by a homogeneous reservoir diffusion of carbon isotopes probably occurred synchronous with the post peak metamorphic mineral reactions at that time centimeter scale diffusion would have been facilitated by the presence of the fluid while isotopic exchange between the minerals and the fluid would have been promoted by recrystallization for metamorphic porosities of 10 25 2 10 23 the d 13 c profiles could have formed in a few thousand to hundreds of thousands of years small millimeter to centimeter scale variations in d 13 c values that may have been initially present within the boudins would have been homogenized on much shorter timescales than those required to form the profiles the calculated timescales may reflect the time over which metamorphic recrystallization occurred and a reactionenhanced porosity was present graphite formed locally in the paragneisses only at margins of the boudins probably reflects the local escape of co 2 bearing fluids from the boudins into relatively low rocks fo2