fluid flow and mass transport at the valentine wollastonite deposit adirondack mountains new york state the valentine wollastonite skarn in the north west adirondack mountains new york is a seven million ton deposit which resulted from channellized infiltration of hzo rich silica bearing fluids the wollastonite formed by reaction of these fluids with non siliceous calcite marble the skarn formed at the contact of the syenitic diana complex and was subsequently overprinted by grenville age granulite facies metamorphism and retrograde hydrothermal alteration during uplift calcite marbles adjacent to the deposit have generally high 6 o values c 21 0 typical of grenville marbles which have not exchanged extensively with externally derived fluids carbon isotopic fractiona tions between coexisting calcite and graphite in the marbles indicate equilibration at 675 c consistent with the conditions of regional metamorphism oxygen isotopic ratios from wollastonite skarn are lower than in the marbles and show a 14 variation 1 to 13 some isotopic heterogeneity is preserved from skarn formation and some represents localized exchange with low4 0 retrograde fluids detailed millimetre to centimetre scale isotopic profiles taken across skarn marble contacts reveal steep 8i8o gradients in the skarn with values increasing towards the marble the gradients reflect isotopic evolution of the fluid as it reacted with high 6 o calcite to form wollastonite calcite in the marble preserves high 6 o values to within 5 mm of the skarn contact the preservation of high 6 o values in marbles at skarn contacts and the disequilibrium fractionation between wollastonite skarn and calcite marble across these contacts indicate that the marbles were not infiltrated with significant quantities of the fluid thus the marbles were relatively impermeable during both the skarn formation and retrograde alteration skarn formation may have been episodic and fluid flow was either chaotic or dominantly parallel to lithological contacts although these steep isotope gradients resemble fluid infiltration fronts they actually represent the sides of the major flow system because chromatographic infiltration models of mass transport require the assumption of pervasive fluid flow through a permeable rock such models are not applicable to this hydrothermal system and by extension to many other metamorphic systems where low permeability rocks restrict fluid migration pathways minimum time integrated fluid fluxes have been calculated at the valentine deposit using oxygen isotopic mass balance reaction progress of fluid buffering reactions and silica mass balance all three approaches show that large volumes of fluid were necessary to produce the skarn but silica mass balance calculations yield the largest minimum flux and are hence the most realistic

origin and metamorphic history of an early cretaceous polybaric granulite terrain fiordland southwest new zealand regionally extensive two pyroxene granulites in fiordland southwest new zealand are products of metamorphism of a suite of anhydrous magmas which crystallized two pyroxenes the granulite protolith igneous charnockitic rock synkinematically intruded metasediment and other orthogneiss in an early cretaceous subduction related magmatic arc and during cooling experienced deformation induced recrystallization to form granoblastic gneiss the granulites occur side by side with coeval rocks of amphibolite facies mineral zoning and textural relationships in both granulites and amphibolite facies rocks provide evidence of two distinct periods of crystallization 1 an early high temperature comparatively low pressure event accompanying magmatic intrusion andalusite sillimanite facies series recorded locally in the country rock followed by 2 high pressure metamorphism under conditions of 650 700 c at 12 13 kbar garnet granulite locally overprinted earlier formed two pyroxene granulite during the latter event the pressure increase 6 kbar between the two events is attributed to crustal thickening by overthrusting and is equivalent to unloading of a 20 km thick slab over rocks already buried at mid crustal depths both events occurred over a 20 m y interval between the time of magmatic emplacement of the granulite protolith and uplift controlled final cooling of the terrain the phanerozoic granulites in fiordland share some petrologic similarities with precambrian granulite terrains suggesting that at least some aspects of the former may serve as a useful model for development of the latter

evidence for multiple metamorphic events in the adirondack mountains n y field evidence consisting of 1 rotated foliated xenoliths 2 country rock foliation truncated by isoclinally folded igneous intrusions bearing granulite facies assemblages document one or more early dynamothermal event s of regional scale and high grade early metamorphism resulted in pronounced linear and planar fabric throughout the adirondacks and preceded the emplacement of the anorthosite mangerite charnockite granite alaskite amca suite which contains xenoliths of the metamorphosed rocks olivine metagabbros believed to be approximately contemporaneous with the amca suite also crosscut and contain xenoliths of strongly foliated metasediments these intrusive rocks caused contact metamorphism in the metasediments which locally exhibit both anatectite and restite assemblages subsequently this already complex framework underwent three phases of folding including an early recumbent isoclinical event and was metamorphosed to granulite facies p t conditions the age of the early metamor

lu hf and pbsl geochronology of apatites from proterozoic terranes a first look at lu hf isotopic closure in metamorphic apatite abstract the mineral apatite is characterized by elevated and highly variable lu hf ratios that in some cases allow for single crystal dating by the lu hf isotopic system apatites from the adirondack lowlands and otter lake area in the grenville province and from the black hills south dakota yield lu hf ages that are consistently older than their respective pb step leaching ages isotopic closure for the lu hf system therefore occurs before u pb system closure in this mineral in the adirondack lowlands where h2o activity was low lu hf systematics of cm sized apatite crystals remained undisturbed during upper amphibolite facies metamorphism 700 to 675 c at 1170 1130 ma the relatively old lu hf ages of 1270 and 1230 ma observed for these apatites correlate with decreasing crystal size in contrast apatite from the fluid rich otter lake area and black hills yields unrealistically low apparent lu hf closure temperatures implying that in these apatites fluids facilitated late exchange the lu hf ages for the metamorphic apatites were thus controlled either by the prevailing temperature and grain size or by fluid activity

sapphirine bearing quartzite from the eastern ghats granulite terrain vizianagaram india sapphirine occurs in a quartzite in the eastern ghats granulite province near vizianagaram india sapphirine is developed at the contact of quartzite and spinel magnetite cordierite bands and occurs as small grains throughout the quartzite bulk chemical analyses of sapphirine bearing rock suggest that the protolith was a quartzite with aluminous and ferruginous cements the presence of monazite and rare earth element ree analyses also indicate a sedimentary origin textural relations indicate that spinel sapphirine hypersthene and sillimanite formed in an early metamorphic event at very high temperature and elevated pressure low activity of water and high oxygen fugacities accompanied reactions forming these minerals later metamorphic reactions accompanied by higher activity of water produced cordierite and biotite mineralogic geothermometry also supports a multi stage metamorphic history experimental evidence for temperatures of 850 c or more required to stabilize sapphirine and quartz indicat

contamination of the marcy anorthosite massif adirondack mountains ny petrologic and isotopic evidence oxygen isotope analyses of 101 samples from the marcy anorthosite massif 61 from this study 40 from taylor 1969 indicate that two major and distinct processes of crustal contamination have affected the massif ninety percent of the 93 samples with over 65 plagioclase are enriched in 18o by 2 6 relative to normal anorthosites or gabbros the average 18o for 83 enriched samples is 9 5 depletions in 18o occur in 8 of the samples which have values ranging from 3 0 to 5 8 only 2 of the samples fall within the normal magmatic range for anorthosites low 18o values of 3 0 to 5 8 in the anorthosite occur only near contacts and a gradient in 18o occurs near the contact within the border zone of the massif low 18o values in both the anorthosite and adjacent wollastonite skarns with 18o down to 1 3 were probably caused by isotopic exchange with heated meteoric water when the anorthosite intruded at shallow levels prior to grenvilleage 1 1 by granulite facies metamorphism the 18o enrichment was ascribed to exchange between anorthosite and 18o rich marble by means of a pervasive h2o co2 fluid during the regional metamorphism by taylor 1969 however a number of lines of evidence argue against this hypothesis 1 the preservation of premetamorphic low 18o values in anorthosite from the border zone as well as preserved gradients in 18o from a number of localities 2 mass balance calculations of the amount of marble necessary to produce the 18o enrichment 3 metamorphic phase equilibria which buffer p text co text 2 and p text h text 2 text o to low values and 4 recent oxygen isotope analyses show homogeneity which indicates that magmatic oxygen isotope compositions have been preserved we evaluated the importance of magmatic assimilation of country rock at the present level of intrusion as an alternative cause of the 18oenrichment samples from 2 distinct lobes of the massif were analyzed the ne lobe where xenoliths of metasedimentary country rock are common and the nw lobe where xenoliths are scarce and the country rock is dominantly granitic the mean values of 18o for these two lobes are 9 6 in the ne and 9 3 in the nw thus magmatic assimilation at the present level of exposure probably had only localized and relatively minor effects on the oxygen isotope composition of the massif this conclusion is supported by rb sr data variations in rb content and 87sr 86sr i show that such crustal contamination is localized generally occurring only in samples near the border zone all of the available results suggest that the 18o enrichment is a magmatic feature acquired prior to intrusion at the present level of exposure

retrograde deformation within the carthage colton zone as recorded by fluid inclusions and feldspar compositions tectonic implications for the southern grenville province the carthage colton zone ccz located in the northwestern portion of the proterozoic adirondack terrane forms the boundary between the adirondack lowlands and highlands and is characterized by textures indicative of ductile deformation samples from three rock types have been collected from this zone 1 pyroxene bearing syenite gneisses of the diana igneous complex 2 paragneiss samples from near the northwestern boundary of the diana complex 3 a quartz rich metasediment feldspar geothermometry performed on the diana metasyenites shows that porphyroclasts are relict igneous grains that formed att 900 c while dynamic recrystallization occurred at temperatures as low as 470 to 550 c as shown by the compositions of feldspar neoblasts all samples examined in this study from the ccz contain a suite of co2 rich fluid inclusions that are distinctive both texturally and in their microthermometric behavior th 27 7 to 7 1 c as compared to co2 rich adirondack fluid inclusions that do not lie within this zone th 45 9 to 31 0 c the results from fluid inclusion microthermometry and feldspar geothermometry restrict the conditions of dynamic recrystallization to temperatures and pressures of 400 to 550 c and 3 to 5 kbar the retrograde pressure temperature path must pass through these conditions similar fluid inclusion results have been obtained from the parry sound shear zone pssz which is a grenvillian shear zone that is located in southern ontario lamb and moecher 1992 however the inferred retrogradep t paths for these two areas the ccz and the pssz are different and this difference may be a result of late deformation along shear zones that are located between the two areas

young high temperature granulites from the base of the crust in central mexico characterization of the lower continental crust is essential to understanding processes of crustal formation and evolution many workers have concluded that the lower crust is composed of granulite facies rocks see for example refs 1 2 but many exposed granulite facies terranes seem to have mid crustal rather than lower crustal origins3 5 here we present new data from direct samples of the lower continental crust in the form of metapelitic rock fragments brought to the surface by quaternary volcanism in central mexico these xenoliths contain exsolved ternary feldspars that record extremely high minimum metamorphic temperatures of 950 1 100 c the highest yet known to be preserved in deep seated metamorphic rocks the metamorphism seems to be the result of underplating by basaltic magma and to be regional in extent our barometry on these samples demonstrates that granulite facies metamorphic rocks are present in the lower crust and provides important constraints on tectonic models and interpretations of geophysical data chemical and text lira i evidence suggest that the xenoliths were not able to cool to below 850 900 c before being entrained in their host magmas indicating that the granulite facies metamorphism occurred no more than 30 myr ago

resetting of oxybarometers and oxygen isotope ratios in granulite facies orthogneisses during cooling and shearing adirondack mountains new york the petrography petrology and oxygenisotope geochemistry of granulite facies granitic and syenitic orthogneisses of the diana and stark complexes adirondack mountains new york show that the extent and nature of resetting of isotopic and mineralogic systems is highly variable there is a strong correlation between retrogression and shearing and the rocks may be divided texturally into 1 unsheared lithologies that preserve little retrogressed pyroxene or hornblendebearing peak metamorphic mineralogies and 2 sheared rocks that underwent retrogression marked by the growth of late biotite in centimetre to metre wide shear zones after the peak of metamorphism oxygen fugacities in the unsheared lithologies were estimated for reintegrated mineral compositions from magnetiteilmenite mt ilm and ferrosilite magnetic quartz fs mt qtz equilibria mt ilm yields logfo2mt ilm values of 15 9 to 17 6 0 6 to 1 3 log units below the fayalite magnetite quartz buffer fmq and temperatures of 670 745 c that agree with those from other geothermometry and phase equilibria studies these data suggest that aside from oxyexsolution of ilmenite from magnetite the fe ti system underwent only minor resetting during cooling and the fe ti oxides yield good estimates of peak metamorphic temperatures and fo2 in unsheared ilmenite magnetite orthopyroxene quartz assemblages values of logfo2mt ilm are lower than logfo2fs mt qtz by an average of 0 6 when the orthopyroxene activity model of sack and ghiorso is used minor resetting of the fe ti oxides analytical errors and errors in the placement of end member reactions probably account for this relatively small difference in fo2 values whole rock 18o values of unsheared diana and stark lithologies range from 4 0 to 10 3 reflecting pre regional metamorphic oxygen isotope ratios peak metamorphic minerals preserve high temperature oxygen isotope fractionations and in many samples the effective diffusion of oxygen in minerals ceased at higher temperatures than predicted from wet experimental diffusion data these data suggest that the rocks did not contain an aqueous fluid phase during cooling the combination of petrologic isotopic and textural data also permits a detailed study of shearing and retrogression ilmenites in the sheared lithologies underwent greater degrees of hematite loss than in the unsheared rocks resulting in logfo2mt ilm values as low as 24 1 3 1 log units below fmq and mt ilm temperatures that are up to 175 c below regional estimates sheared rocks also have higher 18o values up to 13 3 during shearing 18o values of biotite k feldspar and magnetite reset readily while the degree of isotopic resetting of quartz correlates with the intensity for recrystallization

granulite facies amphibole and biotite equilibria and calculated peak metamorphic water activities samples located near the oregon dome anorthosite massif in the south central adirondack mountains new york contain the fluid buffering mineral assemblages amphibole clinopyroxene orthopyroxene quartz or biotite quartz orthopyroxene k feldspar these rocks were metamorphosed under granulite facies conditions t 725 750 c p 7 5 kbar during the grenville orogeny the mg rich nature of amphiboles micas and pyroxenes allow accurate calculation of water activities because corrections for the effects of solid solution are relatively small the activity of water was low during the peak of granulite facies metamorphism with h2o 0 15 0 14 wollastonite occurrences indicate that the co2 was low 0 3 in nearby rocks demonstrating that large quantities of co2 did not infiltrate in a pervasive manner the combination of low h2o with low co2 is consistent with the hypothesis that magmatic processes were dominant generating dry fluid absent conditions

a delta plain sheet sand in the fraser river delta british columbia canada abstract borehole data in the fraser river delta demonstrate that a nearly continuous sand unit generally 8 to 20 m thick underlies the surficial silts and sands of most of the delta plain the sand unit consists of one or more sharp based fining upward sequences and is interpreted to be a complex of distributary channel deposits distributary channel migration and the resulting accumulation of channel sands has occurred primarily in a tidal flat environment due to the interaction of tidal and fluvial processes and the high proportion of sand in the sediment load this sand unit thus provides a model for sand rich river and tide dominated delta plains jetties now maintain the position of the main channel of the fraser river where it crosses the tidal flats and prevent channel migration and further storage of sand in the lower delta plain as a result more of the available sand is being transported directly to the river mouth where failures transport the sand downslope consequently these structures contribute to instability at the river mouth

origin and age of adirondack anorthosites re evaluated with nd isotopes we report the sm nd systematics in seven whole rock anorthosites in plagioclase and garnet mineral separates from one anorthosite and in five mangerite norite charnockite rocks from the snowy mountain dome in the south central adirondacks the plagioclase two garnet separates and the whole rock anorthosite define a four point internal isochron of age 1 098 7 2 m y the six whole rock anorthosites and the anorthositic gabbro in conjunction with the above internal isochron define a similar age of 1 095 7 2 m y and an initial 143nd 144nd ratio of 0 51135 1 2 this age of 1 100 m y is considered to be the time of crystallization of the anorthositic rocks the initial nd isotope ratio of the combined whole rock and mineral isochron indicates an nd value of 2 5 which suggests a light rare earth element depleted source for the anorthositic parent magma in the upper mantle the trace element and isotopic data also support the view that gabbroic anorthosite is the parent magma for the anorthosites five whole rock samples of norite mangerite charnockite found in close association with the anorthosites do not define a precise isochron the initial nd values of these rocks at 1 100 m y show a range from 2 7 to 5 1 which demonstrates that the anorthositic rocks and the mangerite charnockite are not cogenetic the isotopic data further demonstrate that the mangerite charnockite rocks are mantle derived meta igneous rocks and not of sedimentary derivation

the carthage colton mylonite zone adirondack mountains new york the site of a cryptic suture in the grenville orogen u pb ages were determined on metamorphic sphenes and monazites from the late proterozoic adirondack highlands and lowlands in the vicinity of the carthage colton mylonite zone monazites were extracted from metapelites and sphenes were separated from marbles calc silicate gneisses and granite gneisses in order to determine the timing and the duration of metamorphism as well as the cooling histories for rocks on either side of the mylonite zone monazite ages from the lowlands range from 1171 1137 ma sphene ages in the lowlands range from 1156 1103 ma those from the highlands immediately to the east of the mylonite zone range from 1050 982 ma the ages indicate that the last high grade metamorphism in the highlands is ca 100 m y younger than in the lowlands and that both terranes had separate cooling histories at least until ca 1000 ma sphenes from within the carthage colton mylonite zone yield ages of about 1098 ma which are distinct from sphene ages on either side of the shear zone the mineral

the metamorphism and migmatization of the xanthe echinos metamorphic complex central rhodope greece the xanthe echinos metamorphic complex in the central hellenic rhodope records early tertiary convergent tectonics on the eurasian continental margin of neotethys it consists principally of layered gneiss and migmatite with minor amphibolite and calc silicate rocks three metamorphic events are recognised a high pressure eclogite event cretaceous medium pressure barrovian type metamorphism eocene and later low pressure greenschist facies metamorphism petrographic composition and electron microprobe analysis of minerals in forty four samples of gneiss and migmatite shows that the eocene barrovian event is characterized by replacement of clinopyroxene by pargasitic hornblende in clinopyroxene garnet and clinopyroxene edenitic hornblende assemblages the compositional variation and textures of the amphiboles suggest initial growth of edenitic hornblende zoned to hornblende and development of syn to late tectonic pargasitic hornblende under higher temperature conditions associated with migmatization the textures zoning and paragenetic relationships of sodic calcic amphiboles are consistent with uplift accompanied by synmetamorphic deformation partial melting in the migmatites involved plagioclase k feldspar quartz the solidification products of the produced melt are present in the form of quartz monzonitic leucosome layers and is also represented in syntectonic plutons metamorphic pressures of 7 5 8 5 kbar att 600 c were estimated from garnet clinopyroxene pairs in gneisses adjacent to the migmatites amphibole plagioclase thermometry of migmatite samples yielded temperatures of 728 746 c at p 8 5 kbar the activity of h2o was probably less than 1 0 in the partially melted rocks the geothermobarometric results indicate an increase of the geothermal gradient during uplift probably related to intrusion of basic magma into the crust and or changes in geothermal gradient associated with crustal thickening

adirondack highlands northwest lowlands boundary a multiply folded intrusive contact with fold associated mylonitization the boundary between the adirondack highlands and the northwest lowlands near harrisville new york is a multiply folded intrusive contact between metamorphosed sedimentary rocks of the lowlands and metamorphosed intrusive rocks of the diana complex in the highlands this is shown by detailed geologic mapping and analysis of stratigraphy and minor structural features regionally developed protomylonitic foliation in the diana complex is an axial plane foliation formed during isoclinal folding of dry quartz syenite at granulite facies metamorphic grade lack of major post intrusive fault displacement along the diana complex northwest lowlands boundary is demonstrated by correlation of xenoliths in the diana complex with immediately adjacent metamorphosed sedimentary rock units of the northwest lowlands and by continuity of major folds across the boundary a stratigraphic succession consisting of seven rock units has been recognized in northwest lowlands rocks of the harrisville area rocks in these units include from oldest to youngest 1 garnet sillimanite cordierite gneiss 2 rusty pelitic gneiss with biotite gneiss pyroxene amphibolite and quartzite 3 graphitic calcite marble 4 siliceous marble 5 rusty calc silicate rocks 6 diopside gneiss and 7 calc silicate granulite four phases of folding are evident in these stratigraphic units but only the last three phases affect intrusive quartz syenite gneisses of the diana complex first phase isoclinal folds in bedding have a pervasive axial plane foliation which is the regional foliation associated regional lineations are parallel to first phase fold axes foliation and lineation formed during the first phase of folding were deformed by second phase isoclinal folds which have a weak to strong axial plane foliation or slip cleavage quartz syenite of the diana complex intruded folded metamorphosed sedimentary rocks early during second phase folding and was itself isoclinally folded somewhat later during the second phase isoclinal folding of quartz syenite gneiss is associated with development of protomylonitic to mylonitic axial plane foliation which is most intense in the northwest border zone of the diana complex due to lithic contrast with rocks northwest of the border third phase folds are isoclinal to open and are locally characterized by axial plane slip cleavage or fracture cleavage the axial surfaces of folds associated with the first three phases of deformation have a general northeasterly trend and northwesterly dip but their axes have varied plunges two dimensional heart and anchor and hook patterns are a product of interference of second and third phase folds fourth phase deformation resulted in northwest trending open folds with locally developed axial plane slip or fracture cleavage interference of third and fourth phase folds is manifest in dome and basin patterns that are elongate to the northeast and overturned to the southeast similar to dome and basin patterns throughout the northwest lowlands late stage ductile shearing produced very fine grained thin mylonite and ultramylonite in shear zones that transect the preexisting protomylonite foliation in quartz syenite gneiss later brittle deformation resulted in closely spaced fractures quartz and epidote veins and local minor faults however no evidence for major faulting has been observed along this part of the adirondack highlands northwest lowlands boundary

the miocene pliocene macusani volcanics se peru i mineralogy and magmatic evolution of a two mica aluminosilicate bearing ignimbrite suite

migmatites from grenville quebec metamorphic p t x conditions in transitional amphibolite granulite facies rocks abstract migmatitic rocks near grenville quebec preserve features indicative of reactions at the onset of granulite facies metamorphism in this area metapelites and metacarbonates of the classic grenville series are spatially associated with granitic gneiss and metabasite and flank a paleozoic fe rich syenite stock near this intrusion the metapelite is diatexitic and nearly devoid of biotite indicating the involvement of biotite during melting in the contact aureole of this intrusion outside of the contact aureole metapelites and associated rocks contain biotite and are metatexitic these features suggest two episodes of migmatization the earlier predating the syenite the later synchronous with this intrusion hornblende rich metabasites near the syenite contain a two part neosome consisting of coarse grained leucosome veins and patches that are enclosed by fine grained pyroxene rich envelopes migmatization is attributed to dehydration melting in the presence of co2 rich fluids possibly derived from nearby carbonate rocks prior to and or during emplacement of the syenite the occurrence of isolated mafic clots in the mesosome and rarity of melanosome seams on leucosomes suggest that some melts were mobile on an outcrop scale these observations suggest that the leucosomes formed by the segregation of melts which coupled with co2 flux dehydrated the wallrock along narrow margins forming the pyroxene rich neosomes back reaction with residual fluids led to the local scapolitization of plagioclase and the concomitant formation of coronal garnet on pyroxene in neosomes thermobarometry of corona structures within the contact aureole generates diffusional mg fe blocking temperatures 550 c at 5 5 kbar extrapolated up temperature p sensitive equilibria for the coronas yield similar pressures 8 9 kbar as texturally equilibrated assemblages for which high temperatures 750 50 c xco2 0 90 0 95 were determined for rocks sampled inside and outside of the contact aureole this suggests that the grenville migmatites had not been substantially decompressed by the time that the syenite was emplaced

steep oxygen isotope gradients at marble metagranite contacts in the northwest adirondack mountains new york usa products of fluid hosted diffusion steep gradients in oxygen isotope ratios are recorded from two contacts between grenville marbles calcite 18o 21 24 and metamorphosed granitic intrusives whole rock 18o 11 12 in the northwest adirondacks oxygen and carbon isotope fractionations between coexisting minerals in the marbles and the metagranites are consistent with isotopic equilibration at peak metamorphic temperatures the 18o profiles are sigmoidal with inflection points at the contacts and are developed over 5 6 m quantitative fluid infiltration diffusion modeling suggests that the isotopic gradients are best explained as the result of fluid hosted diffusion for porosities of 10 3 to 10 6 the profiles could have formed in 103 107 years via an interconnected grain boundary fluid the granites were probably emplaced at shallow crustal levels prior to the regional grenville metamorphism and isotopic exchange is most likely to have occurred at that time contact metamorphism at these localities involved devolatilisation of the marbles and the time estimates may reflect the periods over which fluid producing reactions took place and an enhanced porosity was maintained in the marbles there is a progressive increase in the percentage of diopside and k feldspar at the expense of phlogopite and tremolite towards the metagranites these mineralogical changes probably occurred due to the temperature increase associated with granite intrusion and diffusion of h2o from the granite the two localities lie on either side of the regional metamorphic amphibolite to granulite facies transition the preservation of isotopic profiles formed during contact metamorphism suggests that there was little pervasive fluid flow at this crustal level during the regional metamorphism or at any time thereafter steep isotopic gradients are present at orthogneiss marble contacts elsewhere in the adirondacks suggesting that this conclusion may be generally applicable to this terrain

the role of synmetamorphic igneous rocks in the metamorphism and partial melting of metasediments northwest adirondacks field and petrologic studies along the adirondack lowlands highlands boundary near harrisville ny indicate that heat from the synmetamorphic intrusion of the diana syenite complex intrusion temperature of 1 050 c played a major role in the local metamorphic thermal regime and was responsible for extensive partial melting of adjacent metasedimentary units major paragneiss of engel and engel metamorphic temperatures inferred from two feldspar and spinel quartz assemblages decrease from 850 950 c along the diana metasediment contact to 650 700 c 2 3 km away from the contact metamorphic pressures are 7 0 5 kb as determined from coexisting plagioclase garnet sillimanite quartz kyanite sillimanite and garnet rutile ilmenite sillimanite quartz grail in the paragneiss migmatites consisting of quartz microcline perthite sodic plagioclase leucosomes are generally concordant with the melanosome consisting of biotite sillimanite garnet spinel plagioclase corundum cordierite qualitatively the amount of partial melt and occurrences of corundum bearing assemblages decrease away from the diana contact activity of h2o inferred from coexisting biotite sillimanite quartz garnet k feldspar ranges from 0 01 to 0 17 and is five to ten times lower in corundum bearing rocks

thermometry and barometry of some amphibolite granulite facies rocks from the otter lake area southern quebec grenville rocks from a 2500 km2 area centered on otter lake quebec some 75 km northwest of ottawa are in the uppermost amphibolite to lower granulite facies orthopyroxene occurs occasionally in both metabasic and charnockitic rocks the temperature of metapmorphism was approximately 675 c based upon oxide feldspar and garnet clinopyroxene thermometry little thermal gradient could be detected across the area carbonate thermometry using reintegrated calcite compositions yielded lower temperatures of 600 c maximum while garnet biotite and other kd thermometers yielded scattered and for the most part unreasonable results metamorphic pressure calculated from the reaction anorthite grossular sillimanite quartz was 5 0 0 5 kbar 500 50 mpa similar calculations based upon the reactions garnet quartz anorthite orthopyroxene and garnet quartz anorthite clinopyroxene yielded pressures of 5 5 7 0 kbar 550 700 mpa pressure calculations based upon assemblages of cordierite g

thermal gradients in the sri lankan granulite terrane a garnet biotite thermometric approach thermal zoning of the highland complex sri lanka has been delineated using the fe2 mg distribution coefficient between garnet and biotite from garnet biotite gneiss samples collected with wide geographical distribution in order to minimize the potential for retrograde fe mg exchange and maximize the potential for retaining peak equilibrium kd garnet biotite and temperature garnet and biotite included within feldspar and quartz without other mineral inclusions have been selected the calculated results indicate four distinct temperature contours with kd values varying from 1 84 to 6 38 and temperature varying from 996 to 591 c from the present results it is possible to divide the highland complex into two major metamorphic zones a high temperature area in the central region and a low temperature area in the south western and north eastern region in conjunction with the metamorphic pressure variations estimated from the granulites of the highland complex in previous studies it is shown that the high and low temperature areas are complemented by a high pressure region towards the eastern side and a low pressure region towards the western side of this complex this thermal dome is interpreted to be an artifact of the different crustal levels exhumed following pan african metamorphism

texturally early fluid inclusions in garnets evidence of the prograde metamorphic path well formed texturally early fluid inclusions in garnets from the archean pikwitonei granulite domain manitoba canada have been analyzed using microthermometric methods the mean co2 homogenization temperature to liquid for inclusions in 12 of 13 samples from the cauchon lake nelson river area is 15 2 c n 125 2 8 2 c corresponding to a co2 density of 0 82 g cm3 inclusions in the remaining sample have somewhat lower co2 homogenization temperatures mean 5 4 c n 24 the studied inclusions contain an estimated 10 to 20 vol percent h2o with minor amounts of other fluid species such as ch4 n2 and or h2s the fluid inclusions were probably trapped during early garnet growth at relatively low pressures 5 kbar if at 750 c and appear to have undergone only limited or possibly no subsequent re equilibration this interpretation is consistent with the anti clock wise p t t path heating before loading determined for the pikwitonei region by other workers for such a prograde path inclusions entrapped early at high temperatures but at relatively low pressures would experience internal underpressures during most of the subsequent prograde and retrograde phases of metamorphism the texturally early fluid inclusions in garnets from the pikwitonei region therefore cannot be used to provide direct information about the highest metamorphic temperature and pressure conditions 750 c and 7 kbar however the results obtained in this study suggest that texturally early fluid inclusions in garnets may in some cases retain evidence of the prograde metamorphic path

the origins of granulites a metamorphic perspective although many recent reviews emphasize a uniformity in granulite pressure temperature p t conditions and paths granulites in reality preserve a spectrum of important petrogenetic features which indicate diversity in their modes of formation a thorough survey of over 90 granulite terranes or occurrences reveals that over 50 of them record p t conditions outside the 7 5 1 kbar and 800 50 c average granulite regime preferred by many authors in particular an increasing number of very high temperature 900 1000 c terranes are being recognized both on the basis of distinctive mineral assemblages and geothermobarometry petrogenetic grid and geothermobarometric approaches to the determination and interpretation of p t histories are both evaluated within the context of reaction textures to demonstrate that the large range in p t conditions is indeed real and that both near isothermal decompression itd and near isobaric cooling ibc p t paths are important amphibolite granulite transitions promoted by the passage of co 2 rich fluids as observed in southern india and sri lanka are exceptional and not representative of fluid related processes in the majority of terranes it is considered on the contrary that fluid absent conditions are typical of most granulites at or near the time of their recorded thermal maxima itd granulites are interpreted to have formed in crust thickened by collision with magmatic additions being an important extra heat source erosion alone is not however considered to be the dominant post collisional thinning process instead the itd paths are generated during more rapid thinning 1 2 mm yr exposure related to tectonic exhumation during moderate rate or waning extension ibc granulites may have formed in a variety of settings those which show anticlockwise p t histories are interpreted to have formed in and beneath areas of voluminous magmatic accretion with or without additional crustal extension ibc granulites at shallow levels very rapid 5 mm yr extensional thinning subsequent to collision it is suggested that the preservation of ibc paths rather than itd paths in many granulites is primarily related to the rate and timescale of extensional thinning of thickened crust and that hybrid itd to ibc paths should also be observed most ibc granulites and probably many itd granulites have not been exposed at the earth s surface as a result of the tectonic episodes which produced them but have resided in the middle and lower crust for long periods of time 100 2000 ma following these events the eventual exhumation of most granulite terranes only occur through their incorporation in later tectonic and magmatic events unrelated to their formation

contribution of deviatoric stresses to metamorphic p t paths an example appropriate to low p high t metamorphism p t paths for a simple situation appropriate to many low p high t lpht terranes in which metamorphism and deformation are localized by advection of heat in magmas has been modelled assuming a medium with a power law rheology with an inverse exponential dependence of stress and temperature and capable of sustaining deviatoric stress in the order of 100 mpa at 400d c and strain rates of up to 10 13s 1 numerical simulations and analytical approximations for p t histories appropriate for simple convergent deformation histories show that the destruction of the deviatoric stress field around large intrusions may result in significant decompression near the metamorphic temperature peak moreover for a specified strain rate and temperature evolution p t paths may vary from clockwise to anticlockwise merely as a function of vertical distance from the heat source inasmuch as mounting independent evidence suggests that the crust can support deviatoric stresses of up to about 100 mpa at temperatures of 400 500d c and that the shear strength of the crust is strongly temperature dependent in the range 400 800d c these results suggest that caution should be taken in the tectonic interpretation of p t paths involving decompression of the order of 100 mpa or less in lpht terranes the results illustrate a plausible mechanism for the close spatial association of both clockwise and anticlockwise p t paths documented in some lpht terranes

raman spectroscopic characterization of h2o in co2 rich fluid inclusions in granulite facies metamorphic rocks abstract water h2o has been identified during raman analyses at 150 200 c of fluid inclusions from granulites that were previously thought to contain pure co2 at room temperature h2o in a co2 rich fluid inclusion forms an optically unresolvable liquid wetting the inclusion walls during heating the h2o phase evaporates into the co2 phase to produce a homogeneous h2o co2 fluid throughout the inclusion volume and the h2o raman peak at 3640 cm 1 is clearly resolvable at elevated temperatures to test and confirm the method synthetic h2o co2 fi of known composition 87 5 mol and 95 mol co2 were also analyzed previously studied granulite samples from the adirondack mountains usa and from the kerala khondalite belt sittampundi complex and sevitturangampatti india were tested for the presence of h2o in co2 rich fluid inclusions traces of h2o were found in co2 rich fluid inclusions in quartz crystals from the adirondacks and from the kerala khondalite belt h2o was not detected in co2 rich secondary fluid inclusions in garnet from the sittampundi complex and from sevitturangampatti rather hydrogarnet was detected along the walls of inclusions in garnet as evidenced by a peak at 3661 cm 1 the hydrogarnet is interpreted to be a step daughter phase that formed by reaction of the h2o in the fluid inclusion with the garnet host during retrogression these observations confirm the presence of an h2o bearing co2 rich fluid at some time during granulite petrogenesis but do not resolve the question of whether these co2 rich fluids were trapped at peak metamorphic conditions or during retrogression or whether the fluid inclusions have reequilibrated following entrapment

post metamorphic co2 rich fluid inclusions in granulites in granulite facies samples from the adirondack mountains ny estimates of peak metamorphic co2 fugacities based on mineral equilibria are not consistent with estimates based on data for high density co2 rich fluid inclusions of the 21 adirondack samples investigated for this study all contain co2 rich inclusions inclusions occur in quartz apatite and garnet they range in size from 3 to 50 m and are without visible h2o in a few of the inclusions freezing point determinations and preliminary laser raman spectroscopy show the presence of small amounts 3 of other fluids n2 and h2s co2 liquid vapor homogenization temperatures are between 46 and 31 c corresponding to densities between 1 14 and 0 5 gm cc some of these densities are consistent with peak metamorphic entrapment 1 06 to 1 1 gm cc

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

on the causes of 18 o depletion and 18 o 16 o homogenization during regional metamorphism the east humboldt range core complex nevada previous stable isotope studies at lizzies basin revealed that metasedimentary rocks are 18o depleted relative to protolith values particularly in the lower parts of the section lower zone where the rocks are also isotopically homogeneous on a scale of hundreds of meters quartz 18o 9 0 to 9 6 per mil in contrast metasedimentary rocks at higher levels at lizzies basin upper zone are less 18o depleted and more heterogeneous in 18o in order to understand more fully the isotopic evolution of this terrane a series of detailed meter scale traverses across various metamorphic and igneous lithologies were completed at lizzies basin and at the structurally higher angel lake locality traverses in the lizzies basin lower zone and in the lower parts of angel lake angel lake lower sequence across various silicate lithologies including abundant granitoids reveal similar degrees of homogeneity although the average 18o values are higher at angel lake in contrast traverses which include substantial thicknesses of marble and calc silicate gneiss and very little granitoid have more heterogeneous quartz 18o values 11 9 to 13 4 per mil and also have a higher average 18o 12 9 per mil than observed elsewhere the scale of 18o 16o homogeneity in quartz observed at lizzies basin and angel lake meters to hundreds of meters requires fluid mediated isotope exchange which accompanied tertiary metamorphism there is a correlation between the degree of 18o depletion in metasedimentary rocks 18o 16o homogenization between lithologies and the proportion of granitoids leucogranites in particular within any part of the section and a corresponding anticor relation with the proportion of marble this points to a causal relationship whereby the leucogranites as well as the tertiary hornblende diorite and biotite monzogranite acted as both a relatively low 18o reservoir and a source of fluids to enhance exchange while the marbles hindered isotope deplction and homogenization by acting as relatively high 18o reservoirs and impermeable layers material balance calculations help delineate the plausible mechanisms of exchange between granitoids and metasediments single pass infiltration of magmatic fluids from the granitoids is not capable of reproducing all of the observations fluidmediated exchange by convective recirculation of magmatic fluids on a scale of meters is the mechanism which explains all of the observations the generalized model for the isotopic evolution of the east humboldt range core complex provides and excellent opportunity to establish the main causes and controlling factors of 18o depletion and 18o 16o homogenization during regional metamorphism

origin of potassic c type adakite magmas experimental and field constraints the postcollisional mesozoic c type or high k adakitic intrusions k adakites of china lack either temporal or spatial association with subduction and have k2o na2o around unity otherwise their geochemistry is very similar to ordinary adakites new experimental data geological observations and theoretical considerations lead us to suggest that these k adakites were high t 1050 and probably 1075 c rather hydrous magmas 6 wt h2o produced by fluid absent partial melting of metatonalites meta andesites or possibly potassic metabasalts at pressures exceeding 2 gpa their peculiar chemistry is a consequence of both the elevated k2o na2o in the protolith and the very high pressure of partial melting the most likely tectonic setting is one of extreme crustal thickening followed by delamination of the eclogitic keel and partial melting of this continental crustal material at mantle depths with high mantle heat flow during orogenic collapse

constraints on melting and magma production in the crust major intrusions of granitic rocks are found in several tectonic settings and in all cases crustal melts may contribute to the volumes of granitic magma high grade metamorphism and partial melting of the crust take place predominantly under fluid absent conditions we present a model for calculating the amounts of melt that may be formed by fluid absent breakdown of micas and amphiboles in common crustal rock types pelitic quartzofeldspathic intermediate and mafic melt proportions depend mainly on the kind of source rock the pressure at which melting takes place the temperature and the hydrous mineral h2o content of the source as a consequence of the pressure dependence of water solubility in silicate melts any given source rock will produce more melt by a given fluid absent reaction at lower pressure at a given pressure higher temperature reactions can produce more melt from a given source rock based on a survey of the compositions of common rock types we show that the amounts of melt can vary from 50vol thus crustal rocks vary widely in their fertility as magma sources depending on the types and amounts of hydrous phases they contain in general muscovite breakdown in pelites will yield only small quantities of melt and lead to migmatite formation biotite breakdown in pelites occurs at higher temperature and because most high grade pelites below granulite grade are biotite rich can yield up to about 50 vol of granitoid melt rocks of intermediate composition and hornblende rich mafic rocks are potentially highly fertile magma sources also provided that the high temperatures necessary for biotite and hornblende breakdown are realized pyroxene rich mafic rocks and quartzofeldspathic rocks are much less potentially fertile data suggest that mechanisms exist for the efficient segregation of melt and restite in systems with 30and probably 20vol melt the pressure temperature history of a region can greatly influence crustal source fertility through its control over the occurrence of subsolidus dehydration and melting equilibria

thermobarometry and p t t paths the granulite to eclogite transition in lower crustal xenoliths from eastern australia lower crustal suite xenoliths in basaltic and kimberlitic magmas are dominated by mafic granulites and may also include eclogites and garnet pyroxenites pressures of up to 25 kbar obtained from such xenoliths are well in excess of an upper value of c 12 kbar for exposed granulite terranes palaeogeotherms constructed from xenoliths for the lower crust beneath the phanerozoic fold belts of eastern australia sea and beneath the eastern margin of the australian craton emac indicate two distinct thermal regimes the two geotherms have similar form with the emac curve displaced c 150 c to lower temperatures reaction microstructures show the partial re equilibration of primary igneous assemblages to granulite and eclogite assemblages and are interpreted to reflect the cooling from magmatic temperatures variations in mineral compositions and zoning are used to constrain further the history of several emac xenoliths to near isobaric trajectories detailed graphical models are constructed to predict compositional changes for isobaric p t paths at 7 14 21 kbar to transform an sea type geotherm to a cratonic geotherm the models show that for the assemblage grt cpx ky plag qtz the changes associated with falling temperature in xgr xjd increase and xan decrease will be greater at higher pressures these results indicate that discernible zoning is more likely to be preserved in the higher pressure xenoliths the zoning recorded in clinopyroxene from mafic granulite xenoliths over the pressure range c 12 22 kbar suggests isobaric cooling of a large crustal thickness 30 35 km an isobaric cooling path is consistent with magma accretion models for the transition of a crust mantle boundary from an sea type geotherm to a cratonic geotherm the coexistence of granulite and eclogite over the depth range 35 75 km beneath the emac indicates that the granulite to eclogite transition in the lower crust is controlled by p t conditions bulk chemistry and kinetic factors at shallower crustal levels typified by exposed granulite terranes isobaric cooling may not result in the transition to eclogite

carbon isotope thermometry in marbles of the adirondack mountains new york carbon isotope thermometry has been applied to coexisting calcite and graphite in marbles from throughout the adirondack mountains new york eighty nine calcite graphite pairs from the amphibolite grade nw adirondacks change systematically in temperature north westwards from 680 to 640 to 670 c over a 30 km distance reflecting transitions from amphibolite facies towards granulite facies to the north west and to the south east temperature contours based on calcite graphite thermometry in the nw adirondacks parallel mineral isograds with the orthopyroxene isograd falling above 675 c and indicate that regional metamorphic temperatures were up to 75 c higher than temperatures inferred from isotherms based on cation and solvus thermometry bohlen et al 1985 fifty five calcite graphite pairs from granulite grade marbles of the central adirondacks give regional metamorphic temperatures of 670 780 c in general agreement with cation and solvus thermometry data for amphibolite and granulite grade marbles show a 12 oo range in 13ccal and 13cgr a strong correlation between carbon isotopic composition and the abundance of graphite cgr crock indicates that the large spread in isotopic compositions results largely from exchange between calcite and graphite during closed system metamorphism the trends seen in 13c vs cgr crock and 13ccal vs 13cgr could not have been preserved if significant amounts of co2 rich fluid had pervasively infiltrated the adirondacks at any time the close fit between natural data and calculated trends of 13c vs cgr crock indicates a biogenic origin for adirondack graphites even though low 13c values are not preserved in marble delamination of 17 graphite flakes perpendicular to the c axis reveals isotopic zonation with higher 13c cores these isotopic gradients are consistent with new graphite growth or recrystallization during a period of decreasing temperature and could not have been produced by exchange with calcite on cooling due to the sluggish rate of diffusion in graphite samples located 2km from anorthosite show a decrease of 0 5 0 8 oo in the outer 100 of the grains while samples at distances over 8 km show smaller core to rim decreases of c 0 2 oo correlation between the degree of zonation and distance to anorthosite suggests that the isotopic profiles reflect partial overprinting of higher temperature contact metamorphism by later granulite facies metamorphism core graphite compositions indicate contact metamorphic temperatures were 860 890 c within 1 km of the marcy anorthosite massif if samples with a significant contact metamorphic effect cal gr 3 2 oo are not included then the remaining 38 granulite facies samples define the relation 13c cal gr 3 56 106t 2 k

liquefaction features on the fraser delta evidence for prehistoric earthquakes sand dykes and sills and vented sand boils on the fraser river delta and adjacent serpentine river floodplain in southwestern british columbia record one or more major liquefaction events the source of the dykes is a shallow subsurface saturated sand unit that was deposited in foreslope and distributary channel environments during holocene progradation of the fraser delta the dykes cut steeply through a crust of delta topset muds and locally flatten out as sills in overlying peaty sediments at two sites evidence was found for venting of sand onto a subaerial or intertidal surface liquefaction and upward movement of sand and water locally deformed the intruded sediments causing some subsidence and uplift of the delta surface all observed liquefaction features are younger than ca 3500 bp and at least some are younger than 2400 bp it is not clear however whether they formed during one or several separate events a review of possible causes suggests that the liquefaction features probably are seis

very high temperature moderate pressure metamorphism in the new russia gneiss complex northeastern adirondack highlands metamorphic aureole to the marcy anorthosite the new russia gneiss complex in the northeastern adirondack highlands of new york includes meta anorthosite gneiss and anatectic gneiss derived from metagabbro mangerite and charnockite metamor

beginning of melting and composition of first melts in the system qz ab or h 2 o co 2 solidus temperatures have been determined for minimum melt compositions in the system qz sio2 ab naalsi3o8 or kalsi3o8 at p fluid 2 5 and 10 kbar and at various water activities the dry solidus was investigated in a dry argon environment water activities ah2o between 0 0 and 1 0 were obtained by using h2o co2 mixtures the or ab or ratio of first melts increases considerably with decreasing water activity at 10 kbar it is 0 28 in the water saturated system and 0 56 at water activity 0 1 the qz content does not change with changing water activities the ab content of minimum melts formed at high pressures and low ah2o may remain almost constant in ascending magmas that are cooling and crystallizing qz content increases at the expense of the or component solidus temperatures decrease considerably when ah2o increases slightly from zero at 10 kbar the temperature difference between dry melting and the solidus for ah2o 0 1 is 120 c the influence of pure co2 on the solidus is very limited in the investigated p t range the solidus is approximatively 760 c at ah2o 0 5 between 2 and 10 kbar and approximatively 830 c at ah2o 0 3 this means that melting of quartz feldspar assemblages may induce dehydration reactions at p t conditions of the granulite facies

pressure temperature time paths and a tectonic model for the evolution of granulites petrologic studies and application of well calibrated mineralogic thermometers and barometers reveal several important features common to many granulite terrains 1 peak metamorphic pressure and temperature conditions cluster around values of 7 5 pm 1 kbar and 800 pm 50 c implying average geothermal gradients of 30 35 c km 2 in comparison with upper amphibolite facies rocks especially in paired amphibolite granulite terrains it is evident that granulites are distinguished from amphibolites by the former s higher metamorphic temperatures but not higher pressure 3 initial cooling of granulite terrains from peak conditions is nearly isobaric for garnet closure temperatures of 600 pm 50 c the dp dt retrograde paths inferred from compositionally zoned garnet rims are 2 8 b c in most granulites 4 primary coarse sillimanite is found in most granulites and retrograde kyanite is known in a few these data suggest that granulites form as a result of anomalous thermal gradients

thermal buffering effects at the solidus implications for the equilibration of partially melted metamorphic rocks abstract a simple thermal model is used to investigate to what extent buffering effects of latent heat of fusion may prolong the thermal evolution of a one dimensional statically cooling partially or completely melted heat source for a melting model appropriate for crustal rocks the time of cooling of rocks through the solidus may be about tripled by this process depending on the melting model cooling may halt completely near the solidus for time spans comparable to the thermal time constant of the heat source in order to test the influence of this buffering effect on the equilibration of partially melted metamorphic rocks these thermal model results are coupled with a simple diffusion model that relates the degree of equilibration of a simple assemblage to temperature and cooling rate it is shown that for a broad range of initial heat source temperatures freezing of selected mineral equilibria may occur in a comparably narrow temperature range at and around the solidus the calculations may have some relevance to low p high t metamorphic terranes in as much as they may provide an explanation for the widely observed equilibration and partial re equilibration of rocks near the crystallisation temperatures of partial melts

partial melting and the formation of granulite facies assemblages in namaqualand south africa abstract dehydration melting reactions in which water from a hydrous phase enters the melt leaving an anhydrous solid assemblage are the dominant mechanism of partial melting of high grade rocks in the absence of externally derived vapour equilibria involving melt and solid phases are effective buffers of ah2 o the element partitioning observed in natural rocks suggests that dehydration melting occurs over a temperature interval during which for most cases ah2o is driven to lower values the mass balance of dehydration melting in typical biotite gneiss and metapelite shows that the proportion of melt in the product assemblage at t 850 c is relatively small 10 20 and probably insufficient to mobilize a partially melted rock body granulite facies metapelite biotite gneiss and metabasic gneiss in namaqualand contain coarse grained discordant unfoliated anhydrous segregations surrounded by a finer grained foliated matrix that commonly includes hydrous minerals the segregations have modes consistent with the hypothesis that they are the solid and liquid products of the dehydration melting reactions bt sil qtz pi grt crd kfs l metapelite bt qtz pl opx kfs l biotite gneiss and hbl qtz opx cpx pl l metabasic gneiss the size shape distribution and modes of segregations suggest only limited migration and extraction of melt growth of anhydrous poikiloblasts in matrix regions development of anhydrous haloes around segregations and formation of dehydrated margins on metabasic layers enclosed in migmatitic metapelites all imply local gradients in water activity also they suggest that individual segregations and bodies of partially melted rock acted as sinks for soluble volatiles the preservation of anhydrous assemblages and the restricted distribution of late hydrous minerals suggest that retrograde reaction between hydrous melt and solids did not occur and that h2o in the melt was released as vapour on crystallization this model combined with the natural observations suggests that it is possible to form granulite facies assemblages without participation of external fluid and without major extraction of silicate melt

early history of the carthage colton shear zone grenville province northwest adirondacks new york u s a abstract the adirondack mountains expose two distinct tectonic elements of the proterozoic grenville province of northeastern north america the adirondack lowlands and highlands the lowlands are located along the eastern edge of the metasedimentary belt and the highlands form the western portion of the granulite terrane the two are separated by the carthage colton shear zone ccsz u pb titanite and 40ar 39ar hornblende ages in the lowlands are ca 100 m yr older than in the highlands across the ccsz while both the lowlands and the highlands record a history of metamorphism during the elzevirian orogeny ca 1150 ma only the highlands record evidence of a major phase of grenvillian thermotectonic activity ottawan orogeny at ca 1090 1030 ma proposed tectonic models require that the lowlands were either laterally separated from the highlands or were at structurally higher levels during this metamorphism new u pb and 40ar 39ar ages from titanite and hornblende bearing mylonites in the dana hil

metamorphism of reduced granulites in low co 2 vapour free environment pervasive flooding of co2 has been proposed as the cause of granulite facies metamorphism that is capable of producing many distinctive characteristics of the deep continental crust reduced water activity orthopyroxene bearing assemblages depletion of large ion lithophile lil elements and dehydration1 6 the calculations presented here of c o h fluid composition for conditions of granulite facies metamorphism show that oxygen fugacity f o2 estimates from many terranes are sufficiently low that the addition of co2 rich fluid causes graphite to precipitate for values of pressure p and temperature t common to granulites and with f o2 slightly below the quartz fayalite magnetite buffer qfm the addition of co2 sufficient to grow 10 vol orthopyroxene requires the precipitation of 1 5 vol graphite as 0 1 vol graphite is readily recognizable but is not reported in most low f o2 granulites these rocks have not been flooded by co2 and low f h2o is probably due to extraction of a magma or recrystallization of an already dry rock7 8 granulite terranes may thus result from a combination of these three processes and the dominance of any one cannot now be demonstrated on a regional basis

migmatization and secondary granitic magmas effects of emplacement and crystallization of primary granitoids in southern bohemia austria the sauwald and muhl zones of the prebatholithic moldanubian middle crust in northern austria contain metapelites and metaluminous to weakly peraluminous metagreywackes respectively both zones were affected by low pressure high temperature metamorphism and anatexis the metapelites of the sauwald zone became in situ diatexites probably by fluid absent reactions involving the breakdown of muscovite and the partial breakdown of biotite the biotite plagioclase quartz gneisses of the muhl zone experienced only slight melting following this event and while the mid crust was still hot additional heat was locally advected into the muhl zone by the intrusion of the weinsberg granite this brought about fluid present partial melting of the biotite plagioclasequartz gneisses producing relatively large volumes of metaluminous to weakly peraluminous i type schlieren granite this cool wet restite rich magma remained close to its site of generation thus infracrustal i type granitoids may be formed anywhere in the crust and not always at high t under special circumstances the heat and fluids from granitic magmas can spawn secondary granites also relatively low initial 87sr 86sr values of around 0 707 in i type rocks do not necessarily indicate either lower crustal magma sources or mixing with mantle derived magma the weinsberg granite magma came from the lower crust p probably 700 mpa where widespread fluid absent breakdown of biotite plagioclase quartz assemblages occurred the necessary high heat flow was probably provided by newly underplated mafic magmas however these seem not to have mixed or mingled with the crustally derived weinsberg magmas deep equivalents of the muhl zone metagreywackes may have formed the weinsberg protolith fluid absent experiments show that the melting temperature probably exceeded 850 c and that a garnet bearing orthopyroxene rich residue should be present in the lower crust fluid present experiments demonstrate that the availability of free h2o can radically alter the characteristics of the partial melts from apparent s type mineralogy with fluid absent melting to i type mineralogy with wet melting

a corundum quartz assemblage from the eastern ghats granulite belt india evidence for high p t metamorphism corundum quartz bearing assemblages occur in small lenses in granulite facies metapelites in rayagada north central part of the eastern ghats granulite belt india corundum porphyroblasts and quartz coexist with porphyroblastic almandine rich garnet hercynite spinel ilmenite and magnetite corundum and quartz are separated by sillimanite or a composite corona consisting of sillimanite and garnet whereas corundum shows sharp grain boundaries with spinel ilmenite and magnetite porphyroblastic corundum contains prismatic sillimanite inclusions in which irregularly shaped quartz is enclosed two distinct reactions are inferred from the textural features corundum quartz sillimanite and spinel quartz garnet sillimanite from the petrographical features we infer that corundum quartz garnet spinel was the peak metamorphic assemblage although large uncertainties exist regarding the positions of the respective reactions in p t space from several published experimental results and theoretical calculations a peak metamorphic condition of 12 kbar and 1100 c is estimated as the lower stability limit of the corundum quartz assemblage decompression from the peak p t condition to c 9 kbar 950 c is inferred

corundum quartz and mg staurolite bearing granulite from the limpopo belt southern africa implications for a p t path abstract a new occurrence of the rare corundum quartz assemblage and magnesian staurolite has been found in a gedrite garnet rock from the central zone of the neoarchean limpopo belt in zimbabwe poikiloblastic garnet in the sample contains numerous inclusions of corundum quartz sillimanite magnesian staurolite sapphirine orthopyroxene and sapphirine sillimanite assemblages as well as monophase inclusions corundum often containing subhedral to rounded quartz occurs as subhedral to euhedral inclusions in the garnet quartz and corundum occur in direct grain contact with no evidence of a reaction texture the textures and fe mg ratios of staurolite inclusions and the host garnet suggest a prograde dehydration reaction of st grt crn qtz h2o to give the corundum quartz assemblage peak conditions of 890 930 c at 9 10 kbar are obtained from orthopyroxene sapphirine and garnet staurolite assemblages a clockwise p t path is inferred with peak conditions being followed by retrograde conditions of 4 6 kbar and 500 570 c the presence of unusually magnesian staurolite mg fe mg 0 47 0 53 and corundum garnet assemblages provides evidence for early high pressure metamorphism in the central zone possibly close to eclogite facies the prograde high pressure event followed by high to ultrahigh temperature metamorphism and rapid uplifting of the limpopo belt could have occurred as a result of neoarchean collisional orogeny involving the zimbabwe and kaapvaal cratons

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

on the formation of granulites the tectonic settings for the formation and evolution of regional granulite terranes and the lowermost continental crust can be deduced from pressure temperature time p t time paths and constrained by petrological and geophysical considerations p t conditions deduced for regional granulites require transient average geothermal gradients of greater than 35 c km 1 implying minimum heat flow in excess of 100 mwm 2 such high heat flow is probably caused by magmatic heating tectonic settings wherein such conditions are found include convergent plate margins continental rifts hot spots and at the margins of large deep seated batholiths however particular p t time paths do not allow specific tectonic settings to be distinguished at this time under different conditions both clockwise cw pmax attained before tmax and anticlockwise acw pmax attained slightly after tmax paths are possible in the same tectonic setting both cw and acw end member paths can yield nearly isobaric cooling ibc paths such cooling paths are clearly not an artefact of thermobarometry but can be constrained by solid solid and devolatilization equilibria and geophysical modelling in terms of understanding the evolution of the deep crust a potentially significant group of regional granulite terranes are those that show evidence for acw ibc paths such paths are the likely result of i episodic igneous activity resulting in intrusions within all levels of the crust ii thickening of the crust by magmatic underplating iii slow uplift as a result of the formation of a deep garnet rich crustal root and iv excavation resulting from a later tectonic event unrelated to that resulting in the formation of the granulites the later event might be triggered by the delamination of the garnet rich lowermost crust

p t conditions mechanism and timing of the localized melting of metapelites from the petronella shear zone and relationships with granite intrusions in the southern marginal zone of the limpopo belt south africa

ultra high temperature metamorphism of metapelitic granulites from kondapalle eastern ghats belt implications for the indo antarctic correlation a suite of quartz and corundum bearing metapelitic granulites intruded by layered gabbronorite pyroxenite anorthosite at kondapalle eastern ghats belt preserves a multitude of reaction textures involving oxide and silicate minerals that attest to several prograde and retrograde reactions in the quartz bearing associations the reactions are a biotite sillimanite quartz garnet liquid b garnet sillimanite spinel magnetite quartz c fe2tio4 o2 ferrian ilmenite magnetite d reversal of reaction b e fe2o3 rich ilmenite plagioclase quartz fe2o3 poor ilmenite garnet o2 reactions in the corundum bearing associations are f spinel biotite sillimanite garnet liquid g biotite sillimanite garnet ti rich spinel corundum liquid h biotite sillimanite garnet corundum liquid i fe2tio4 feal2o4 o2 ferrian ilmenite fe3o4 al2o3 in ilmenite j garnet corundum spinel sillimanite to examine the paragenetic evolution of the metapelitic granulites a petrogenetic grid for the kfmash system at high temperatures and pressures involving quartz and corundum was constructed the sequence of inferred reactions documents an anticlockwise heating cooling path reintegrated compositions of spinel with 10 mol fe2tio4 and feldspars indicate ultra high temperature uht of metamorphism 1000 c comparable with the liquidus temperature of the enclosing magmatic rocks crystallization pressures inferred for the magmatic rocks and the pressure constraints imposed by the petrogenetic grid on the metapelite assemblages indicate that the emplacement of the igneous suite and the accompanying uht metamorphism occurred in the lower crust 8 kbar reported u pb cooling ages of monazite and allanite from a late pegmatite suggest the uht metamorphism to be older than 1600 ma the deduced p t history and the absence of grenvillian high grade metamorphism in the study area provide important constraints on the configuration of east gondwana in particular on the continuation of the napier rayner terrane boundary into the eastern ghats belt