exhumation of eclogites insights from depth time path analysis abstract literature data on geochronology and pressure temperature estimates have been used to assess the depth time paths for high pressure units in the alps and the variscan belt these depth time paths consistently show that exhumation rates decrease through time the first stages of the exhumation are rapid 1 mm yr 1 and last less than a few million to a few tens of million years in contrast the final 10 20 km ascent to the surface is at least one order of magnitude slower for the deepest and most important part of the exhumation trajectory the chronology of the retrogressive metamorphism excludes gravitational collapse as the major driving force material balance and structural arguments also exclude isostatically assisted erosion processes prism dynamics and tectonically assisted erosion are the most likely mechanisms accounting for the kinetics of exhumation isostasy assisted erosion and shallow level extension are nevertheless required to eventually bring the high pressure rocks to exposure

determining the significance of high grade shear zones by using temperature time paths with examples from the grenville orogen ductile shear zones preserve essential information on processes that are active in orogenic roots but the significance of these zones is often difficult to interpret structural petrologic and geochronologic data from shear zones yield elements of the history that are not necessarily synchronous however by combining these data with temperature time t t paths insights are obtained into the nature of shear zones the relation between bounding blocks and orogenic evolution of the deep crust this procedure is illustrated with two examples from the mid proterozoic grenville orogen t t paths from 1160 to 900 ma are based on u pb dating of metamorphic minerals including garnet closure temperature t c 800 c monazite t c 725 c sphene t c 600 c and rutile t c 400 c and 40 ar 39 ar hornblende ages t c 480 c comparison of t t paths from adjacent blocks allows predictions about the significance kinematics and timing of displacement of shear zones in the grenville orogen t t paths can distinguish between major terrane boundaries e g the carthage colton shear zone and within terrane shear zones e g the bancroft shear zone thus these data can also be used to identify individual tectonic terranes in the deep crust this integrated approach to analysis of shear zones provides constraints needed to determine the nature and rate of deep orogenic processes in areas that are complicated by high metamorphic grades

a seismic based cross section of the grenville orogen in southern ontario and western quebec a schematic crustal cross section is presented for the southwestern grenville province based on reprocessed lithoprobe near vertical incidence seismic reflection data and compiled seismic refraction wide angle velocity models interpreted with geological constraints the schematic crustal architecture of the southwest grenville province from southeast to northwest comprises allochthonous crustal elements frontenac adirondack belt and composite arc belt that were assembled prior to ca 1160 ma and then deformed and transported northwest over reworked rocks of pre grenvillian laurentia and the laurentian margin primarily between 1120 and 980 ma reworked pre grenvillian laurentia and laurentian margin rocks are interpreted to extend at least 350 km southeast of the grenville front beneath all of the composite arc belt three major structural boundary zones the grenville front and adjacent grenville front tectonic zone the central metasedimentary belt boundary thrust zone and the elzevir frontenac bou

from granulites to eclogites in the sesia zone italian western alps a record of the opening and closure of the piedmont ocean the sesia zone italian western alps offers one of the best preserved examples of pre alpine basement reactivated under eclogite facies conditions during the alpine orogenesis a detailed mineralogical study of eclogitized acid and basic granulites and related amphibolites is presented in these rare weak to undeformed rocks microstructural investigations allow three main metamorphic stages to be distinguished a a medium to low p granulite stage giving rise to the development of orthopyroxene garnet plagioclase brown amphibole ilmenite biotite in basic granulites and garnet k feldspar plagioclase cordierite sillimanite biotite ilmenite in acid granulites b a post granulite re equilibration associated with the development of shear zones producing discrete amphibolitization of the basic granulites and widespread development of biotite sillimanite cordierite spinel in the acid rocks c an eo alpine eclogite stage giving rise to the crystallization of high p and low t assemblages in an effort to quantify this evolution independent well calibrated thermobarometers were applied to basic and acid rocks for the granulite event p t estimates are 7 9 kbar and 700 800 c and for subsequent retrograde evolution p t was 4 5 kbar and 600 c for the eo alpine eclogite metamorphism pressure and temperature conditions were 14 16 kbar and 550 c the inferred p t path is consistent with an uplift of continental crust produced by crustal thinning prior to the subduction of the continental rocks in the light of the available geochronological constraints we propose to relate the pre alpine granulite and post granulite retrograde evolution to the permo jurassic extensional regime the complex granulite eclogite transition is thus regarded as a record of the opening and of the closure of the piedmont ocean

reply to discussion by p j barnett on generation of heavy mineral glacial indicator dispersal trains from a diabase sill nipigon region northwestern ontario larson and mooers 1753 barnett 2006 poses a number of questions regarding some of the field observations and assumptions we used in our interpretation of a heavy mineral dispersal train derived from an isolated eastern outlier of diabase of the nipigon sills in northwestern ontario larson and mooers 2005b he also brings attention to some details of the regional glacial geologic setting we did not address in our analysis and interpretation of the dispersal train and highlights new field data barnett and dyer 2005 relevant to interpretation of past glacial processes acting in the broader nipigon region we thank barnett for his comments and feel they serve in part to highlight the variety of problems regarding glacial erosion and entrainment and transport to which we believe our conceptual model of glacial indicator dispersal larson and mooers 2004 is able to contribute barnett 2006 brings attention to the fact that tills exposed at surface in the beardmore geraldton region contain a wide range 1 up to 23 in carbonate content thorleifson and kristjansson 1993 high carbonate contents in these tills are a clear signal of long distance transport of carbonate bearing debris from the hudson bay lowlands the occurrence of high carbonate tills in patches of thick streamlined drift has been interpreted by hicock 1988 and hicock and others 1989 to mean that all surface tills in this region were deposited at the base of a fastflowing ice stream however the ice stream model does not explain the widely disparate values in till carbonate occurring over distances of only a few kilometres we believe a different interpretation explains the same data larson and mooers 2005a tills sampled in our limited study area contained virtually none of the paleozoic carbonate and proterozoic greywacke clasts characteristic of high carbonate tills found in the region we believe this observation is inconsistent with an origin of the till forming material by longdistance transport of debris from the hudson bay lowlands by an ice stream and rather that it reflects local erosion and entrainment and short distance transport however the origin of the tills we sampled in our study area is not relevant to the interpretations we presented determining whether the till in our study area was generated by erosion and entrainment into the englacial basal debris layer of the ice sheet by erosion and entrainment in a subglacial deforming debris layer or by another process was beyond the scope of our study we deliberately made no explicit assertion regarding the processes by which diabase was eroded and entrained and transported we did however attempt to document the rates at which the erosion and entrainment process worked the only explicit assumptions we incorporated into our interpretations were of the debris mass per unit bed area m and of the velocity at which the basal debris layer was transported u these values were used to estimate the erosivity e recorded in the till sheet and indirectly the absolute erosion and entrainment rate e of diabase into the basal debris layer examination of the equations presented in larson and mooers 2004 and the modified equations presented in larson and mooers 2005b shows that increasing the estimated m results in increased estimates of the values of e and e increasing u likewise results in increased estimates of e however estimates of the erosion length scale and the relative rates of erosion of the diabase and greenstone are unaffected by varying these quantities we feel that the m and u estimates that we incorporated into our interpretation of the heavy mineral dispersal data represent reasonable values for an ice sheet and that the resulting e estimates are likewise reasonable it is important to note that the absolute values of erosion and entrainment we have estimated for diabase and greenstone lithologies lie within the range documented for modern glacial systems cf hallet et al 1996 barnett 2006 questions our conclusion that the diabase sill in our study area was significantly more resistant to glacial erosion and entrainment than the archean greenstones into which it was intruded notwithstanding the higher susceptibility of gabbro diabase to physical and chemical

deformation history of a subducted continental crust gran paradiso western alps continuing crustal shortening during exhumation eclogite facies continental basement in the western alps outcrops as tectonic windows below the oceanic units in the gran paradiso massif eclogite facies assemblages in mafic rocks display a n s stretching lineation da1 the main phase foliation formed under epidote amphibolite facies conditions da2 and defines a regional dome structure for the whole gran paradiso massif structural data including new detailed mapping microstructural analyses and metamorphic studies in the northern part of the gran paradiso cogne valley reveal the occurrence of major thrusts in this continental basement associated with the da2 deformation e w trending stretching lineations and fold axes are associated with this second and main deformation nappe stacking of the different units was achieved by top to the west shearing during exhumation of the eclogite facies basement the antiformal doming of the gran paradiso basement is thus produced by the nappe stacking during exhumation under epidote amphibolite facies

evidence for protracted prograde metamorphism followed by rapid exhumation of the zermatt saas fee ophiolite major and trace element zoning in garnet in combination with rb sr sm nd and lu hf geochronology provide evidence for a protracted garnet growth history for the zermatt saas fee zsf ophiolite western alps four new lu hf ages from pfulwe c 52 46 ma and one from chamois c 52 ma are very similar to a previously published lu hf age from lago di cignana overall the similarity of geochronological and garnet zoning patterns suggests that these three localities had a similar prograde tectonic history commensurate with their similar structural position near the top of the zsf samples from the lower part of the zsf at saas fee and st jacques however produced much younger lu hf ages c 41 38 ma neither differences in whole rock geochemistry which might produce distinct garnet growth histories nor rare earth element zoning in garnet can account for the age differences in the two suites this suggests a much later prograde history for the lower part of the zsf supporting the idea that it was subducted diachronously such a model is consistent with changes in subduction vectors based on plate tectonic reconstructions where early oblique subduction which produced long prograde garnet growth changed to more orthogonal subduction which corresponds to shorter prograde garnet growth six new rb sr phengite ages range from c 42 to 39 ma and in combination with previously published rb sr ages constrain the timing of the transition from eclogite to upper greenschist facies p t conditions the proximity of the zsf in the saas fee region to the underlying continental monte rosa unit and the similarity of peak metamorphic ages suggest these two units were linked for part of their tectonic history this in turn indicates that the monte rosa may have been partly responsible for rapid exhumation of the zsf unit

formation burial and exhumation of basement nappes at crustal scale a geometric model based on the western swiss italian alps abstract information from geological and reflection seismic data from the western swiss italian alps and from numerical models is used to build a geometrical model that can explain some of the major tectonic metamorphic features of alpine type basement nappes the model gives a geometrical and mechanical explanation for the initiation burial and subsequent uplift and partial exhumation of basement nappes at a crustal scale three main tectonic stages during convergence are distinguished and each correlated with the formation of specific nappe structures the first two stages are single vergent nw and correspond to the subduction of continental margin crust and the formation and uplift of high pressure rocks simple shear flow and superimposed wedge shaped pure shear flow is proposed for the creation and intrusion of high pressure nappes of the monte rosa type the third stage is characterized by a doubly vergent style with both pro and retro movements the former created nw vergent nappes as seen in the external alpine massifs and the latter caused backfolding and thrusting typical of the internal alps this third stage corresponds to the neoalpine movements in the western swiss italian alps and is accompanied by a generalized uplift mountain building and molasse sedimentation

u pb geochronology of the mazinaw terrane an imbricate segment of the central metasedimentary belt grenville province ontario the mazinaw terrane in the central metasedimentary belt of the grenville province comprises volcanic sedimentary and plutonic rocks that were intensely folded and faulted and metamorphosed to as high as upper amphibolite facies u pb geochronology establishes an early period of magmatism and sedimentation at about 1280 1240 ma probably in a marginal basin setting and a multistage rhetamorphic evolution in the period between 1100 and 980 ma which was probably related to crustal thickening by imbrication during compression and wedging of the terrane some of the earliest magmatism formed calc alkalic volcanic rocks of kashwakamak formation at 1276 2 ma an associated sedimentary assemblage was intruded by the helena trondhjemite stock at 1267 5 ma a younger succession formed between 1250 and 1240 ma includes the bimodal volcanic mazinaw lake formation alaskitic granites of the norway lake and abinger plutons and a granodioritic gneiss phase of the cross lake pluton these units were covered

generation of a heavy mineral glacial indicator dispersal train from a diabase sill nipigon region northwestern ontario the heavy clinopyroxene mineral pigeonite forms a glacial indicator dispersal train originating from diabase intrusions in the nipigon region of northwestern ontario analysis and interpretation of the pigeonite dispersal pattern adjacent to the up ice portion of the diabase provides a number of insights into the nature of glacial erosion of bedrock and the generation of heavy mineral dispersal trains bedrock erosion and entrainment rates at the time of pigeonite dispersal train formation were high 3 14 mm a 1 suggesting that bedrock erosion was rapid yet spatially and temporally restricted contrasting erosion rates between the diabase and surrounding greenstone lithologies suggests that modern shield topography is not an assemblage of equilibrium bedforms with respect to the ice sheet this agrees with hypothesized low total erosion of shield bedrock during the pleistocene pigeonite grain size coarsens over the diabase source indicating that most of the pigeonite was quarried from outcrops as coar

prograde high to ultrahigh pressure metamorphism and exhumation of oceanic sediments at lago di cignana zermatt saas zone western alps pelagic metasediments and morb type metabasalts of the former tethyan oceanic crust at cignana valtournanche italy experienced uhp metamorphism and subsequent exhumation during the early to late tertiary maximum pt conditions attained during uhp metamorphism were 600 630 c 2 7 2 9 gpa which resulted in the formation of coesite glaucophane eclogites in the basaltic layer and of garnet dolomite aragonite lawsonite coesite phengite bearing calc schists and garnet phengite coesite schists with variable amounts of epidote talc dolomite na pyroxene and na amphibole in the overlying metasediments during subduction the rocks followed a prograde hp uhp path which in correspondance with the jurassic age of the tethyan crust reflects the thermal influence of relatively old and cold lithosphere and of low to moderate shear heating inflections on the prograde metamorphic path may correspond to thermal effects that arise from a decrease in shear heating due to brittle plastic transition in the quartz aragonite dominated rocks induced convection in the asthenospheric mantle wedge and or heat consumption by endothermic reactions over a restricted pt segment during subduction after detachment from the downgoing slab some 50 70 ma before present the cignana crustal slice was first exhumed to ca 60 km and concomitantly cooled to ca 550 c tracing back the uhp hp prograde path displaced by 50 80 c to higher temperatures exhumation at this stage is likely to have occurred in the benioff zone while the subduction of cool lithosphere was going on subsequently the rocks were near isothermally exhumed to ca 30 km followed by concomitant decompression and cooling to surface conditions at 500 c 1 gpa during this last stage the uhpm slice arrived at its present tectonic position with respect to the overlying greenschist facies combin zone in contrast to the well preserved hp uhpm record of the coesite glaucophane eclogites the hp uhp assemblages of the metasediments have been largely obliterated during exhumation relics from which the metamorphic evolution of the rocks during prograde hp metamorphism and the uhp stage can be retrieved are restricted to rigid low diffusion minerals like garnet dolomite tourmaline and apatite

comment on subglacial erosion and englacial sediment transport modeled for north american ice sheets by d h d hildes g k c clarke g e flowers s j marshall

geochronology of the belmont lake metavolcanic complex and implications for crustal development in the central metasedimentary belt grenville province ontario u pb analyses of zircon and titanite were carried out on eight rocks from the belmont lake metavolcanic complex a volcano sedimentary sequence in the central metasedimentary belt of the grenville province of southeastern ontario the ages of concordant supracrustal rocks within the complex do not accord with stratigraphic position the youngest volcanic age is from a rhyolite near the base of the sequence the oldest age is from a dacite in the middle this is overlain by a rhyolite in age a rhyolite at the top of the sequence appears to contain zircon inherited from a source about 1870 ma old the complex is therefore interpreted as comprising a lithotectonic sequence composed of structurally interleaved segments of contrasting age tectonic emplacement of these segments was most likely along previously unrecognized thrust faults the supracrustal rocks were subjected to at least two major deformational events as well as a late metamorphism the earliest event was probably associated with thrusting t

geochronology and geochemistry of precambrian gneisses metabasites and pegmatite from the tobacco root mountains northwestern wyoming craton montanathis article is one of a series of papers published in this special issue on the theme of geochronology in honour of tom krogh t e krogh deceased april 2008 the middle mountain metamorphic domain of the montana metasedimentary terrane northwestern wyoming craton within the northwestern tobacco root mountains mainly comprises migmatized tonalitic gneiss interlayered with amphibolitic hornblende gneiss both of which are cut by metamorphosed mafic rocks together these gneisses are defined as middle mountain gneiss archean tonalitic gneiss from west of and amphibolitic gneiss from east of the bismark fault give from chemically and air abraded zircon grains u pb id tims ages of 3325 5 1 7 and 3340 ma respectively these results reflect primary magmatic ages and show that the middle mountain gneiss extends into the northern area of the central fault block between the bismark and mammoth faults older crustal processes in the tonalitic gneiss are evidenced by inherited grains the oldest of which is 3460 ma a metabasite hosted in tonalitic gneiss in the bismark fault selvage zone yields a zircon age of 2468 ma which is interpreted as the time of

excess 40ar free phengite in ultrahigh pressure metamorphic rocks from the lago di cignana area western alps abstract ar ar analyses of phengites and paragonites from the ultrahigh pressure metamorphic rocks zoisite clinozoisite schist garnet phengite schist and piemontite schist in the lago di cignana area western alps were carried out with a laser probe step heating method using single crystals and a spot dating method on thin sections eight phengite and two paragonite crystals give the plateau ages of 37 42 ma with 96 100 of 39 ar released each rock type also contains mica crystals showing discordant age spectra with age fractions 20 35 ma significantly younger than the plateau ages phengite inclusions in garnet give ages of 43 2 1 1 ma and 44 4 1 5 ma which are significantly older than the spot age 36 4 1 4 ma from the matrix phengites and the plateau ages from the step heating analyses inclusion ages 43 and 44 ma are consistent with a zircon shrimp age 44 1 ma in this area these results suggest that the oceanic materials that underwent a simple subduction related uhpm form excess 40 ar free phengite and that the peak metamorphism is ca 44 ma or little older we suggest that matrix phengites experienced a retrogression reaction changing their chemistry contemporaneously with deformation related to the exhumation of rocks releasing significant radiogenic 40 ar from the crystals this has lead to the apparent ages of the matrix phengites that are significantly younger than the inclusion age

high pressure eo alpine parageneses developed in magnesian metagabbros gruppo di voltri western liguria italy aluminous magnesian metagabbroic layers and pods occur in the beigua serpentinite of western liguria where they are associated with more voluminous fe ti rich lithologies both types were subjected to localized early rodingitization all rocks show the effects of a pervasive essentially isochemical high pressure early alpine metamorphism for which a sequence of mineral assemblages is partially preserved eclogite glaucophanic eclogite na al actinolitic amphibolite greenschist the nature of the eo alpine high pressure recrystallization and the origin of the protoliths are the subjects of this paper

distinguishing between seafloor alteration and fluid flow during subduction using stable isotope geochemistry examples from tethyan ophiolites in the western alps large amounts of fluid bound up in the hydrated upper layers of the ocean crust are consumed at convergent margins and released in subduction zones through devolatilization the liberated fluids may play an integral role in subduction zone processes including the generation of arc magmas however exhumed subduction zone rocks often record little evidence of large scale fluid flow especially at deeper levels within the subduction zone basaltic pillows from the high pressure corsican and zermatt saas ophiolites show a range of 18o values that overall reflect seafloor alteration prior to subduction however comparison between the 18o values of the cores and rims of the pillows suggests that the 18o values of the pillow rims at least have been modified during subduction and high pressure metamorphism pillows that have not undergone high pressure metamorphism generally have rims with higher 18o values than their cores whereas the converse is the case in pillows that have undergone high pressure metamorphism this reversal in the core to rim oxygen isotope relationship between unmetamorphosed and metamorphosed pillows is strong evidence for fluid rock interaction occurring during subduction and high pressure metamorphism however the preservation of different 18o values in the cores and rims of individual pillows and within and between different pillows suggests that fluid flow within the subduction zone was strongly channelled resetting of the 18o values in the pillow rims was probably due to fluid hosted diffusion that occurred over relatively short time scales 1 myr

monazite u th pb geochronology of the central metasedimentary belt boundary zone cmbbz grenville province ontario canada the central metasedimentary belt boundary zone cmbbz is a crustal scale shear zone that juxtaposes the central gneiss belt and the central metasedimentary belt of the grenville province geochronological work on the timing of deformation and metamorphism in the cmbbz is ambiguous and the questions that motivate our study are how many episodes of shear zone activity did the cmbbz experience and what is the tectonic significance of each episode we present electron microprobe data from monazite the u th pb chemical method to directly date deformation and metamorphism recorded in five garnet biotite gneiss samples collected from three localities of the cmbbz of ontario west guilford fishtail lake and killaloe all three localities yield youngest monazite dates ca 1045 ma most of the monazite domains that yield these dates are high y rims in comparison with this common late ottawan history the earlier history of the three cmbbz localities is less clearly shared the west guilford samples have m

exhumation of ultrahigh pressure metamorphic oceanic crust from lago di cignana piemontese zone western alps the structural record in metabasites the metamorphic and deformational history of coesite bearing ultrahigh pressure metamorphic oceanic crust from lago di cignana valtoumanche western alps as recorded along the decompression and cooling path has been resolved to derive constraints on the physical state of the crust during exhumation the following history is proposed eclogites derived from pillow basalts were deformed by dislocation creep of pyroxene at a depth of more than 80 km in the stability field of coesite the deformation was progressively localised into shear bands over the first 40 km of subsequent exhumation the rocks remained essentially unmodified any deformation must have been localised beyond the limits of the present outcrop retrogression first in a closed system started without concomitant deformation at a depth of about 40 km and temperatures of 550 to 500 c subsequently the system size increased fluid infiltration focussed along tensile fractures is documented by the formation of veins vein shapes and orientations indicate high pore fluid pressures and low differential stress after cooling to about 350 450 c at depths of 12 to 18 km localised ductile deformation transformed the retrogressed eclogites into greenschists for that stage the microstructure of the quartz veins indicates deformation by dislocation creep at falling temperatures and increasing differential stress final cooling to below ca 300 c took place at depths of 7 to 12 km as indicated by the density of fluid inclusions later deformation in the brittle field cannot be correlated with the p t path

diffusion limited ree uptake by eclogite garnets and its consequences for lu hf and sm nd geochronology garnets from the zermatt saas fee eclogites contain narrow central peaks for lu yb tm er and at least one additional small peak towards the rim the ree sm eu gd tb dy are depleted in the cores but show one prominent peak close to the rim these patterns cannot be modeled using rayleigh fractionation accompanied by mineral breakdown reactions instead the patterns are well explained using a transient matrix diffusion model where ree uptake is limited by diffusion in the matrix surrounding the porphyroblast observed profiles are well matched if a roughly linear radius growth rate is used the secondary peaks in the garnet profiles are interpreted to reflect thermally activated diffusion due to temperature increase during prograde metamorphism the model predicts anomalously low 176lu 177hf and 147sm 144nd ratios in garnets where growth rates are fast compared to diffusion of the ree and these results have important implications for lu hf and sm nd geochronology using garnet

extension in the central metasedimentary belt of the ontario grenville timing and tectonic significance a laterally extensive zone of southeast dipping marble mylonites in the central metasedimentary belt cmb of the southern ontario grenville the bancroft shear zone records southeast directed extension and a displacement of about 10 12 km calcite graphite carbon isotope thermometry indicates that the maximum temperature at which shearing occurred was ca 450 c in contrast carbon isotope fractionations 13 c cc gr within the marble protolith consistently give temperatures of 650 700 c the bancroft shear zone postdates 1060 ma and older high grade shear zones in the central gneiss belt and the cmb that are associated with northwest directed thrusting comparison of the temperature data from the retrograde marble mylonites with cooling curves for the cmb gives a late grenvillian age range of 935 1010 ma for the extensional event the geometric relation between thrusting and normal faulting in the cmb is comparable to that in parts of the himalayas and southern tibet and similarly we interpret late normal faulting in the cmb to be a result of gravitational collapse of a thrust thickened crust in contrast to the himalaya tibet region however accretion in the grenville becomes older in the direction of regional thrusting

effect of cooling on the water content and mechanical behavior of metamorphosed rocks calculations of the decline in f h2o in equilibrium with typical metamorphic assemblages during cooling indicate that there will be a large drop in pore water pressure within 100 of the metamorphic maximum the loss of pore fluid and the generation of large effective pressures across grain boundaries will inhibit rock deformation by those mechanisms that depend on the presence of an aqueous fluid phase in many metamorphic belts extensive deformation occurs below the metamorphic maximum during heating that is while fluid is being generated whereas rocks do not deform in the same ductile manner on cooling even while temperature is high this may reflect changing mechanical properties of the rocks rather than merely changes in regional stresses mobile belts could be made rigid in this way in about 20 m y

source tracing of detrital serpentinite in the oligocene molasse deposits from the western alps barreme basin implications for relief formation in the internal zone we present the first contribution of tracing the source area of ophiolitic detritus in the alpine molasse deposits by raman spectroscopy the lower oligocene molasse deposits preserved in the barreme basin in the sw foreland of the western alpine arc are known for the sudden arrival of the first exotic detritus coming from the internal alpine zones among them the pebbles of serpentinized peridotites have so far not been studied we show that they only consist of antigorite serpentinite implying that they originate from erosion of high temperature blueschists in contrast the upper oligocene lower miocene molasse shows mixed clasts of serpentine including antigorite and lizardite without any evidence of chrysotile this suggests that they were derived from a less metamorphosed unit such as the low temperature blueschist unit taking into account the sediment transport direction in the basin and the varied metamorphic characteristics of the other ocean derived detritus we constrain the lithologic nature of the source zones and the location of the relief zones identified as the internal alps se of the pelvoux external crystalline massif available structural data and in situ thermochronological data allow the reconstruction of the oligocene to early miocene collisional geometry of the palaeogene subduction wedge this phase corresponds to two major phases of uplift evolving from a single relief zone located above the ivrea body during early oligocene times and persisting up to early miocene times then during late oligocene early miocene times a second relief zone developed above the brianconnais zone at that time the internal western alps acquired its double vergency

the mechanism of veining and retrograde alteration of alpine eclogites the introduction of externally derived fluids into rocks of the zermatt saas zone of the swiss alps gave rise to the simultaneous formation of shear and hydraulic fractures these fractures are now filled with albite rich assemblages and surrounded by alteration halos up to c 2 m wide the alteration assemblages are zoned and an examination of reactions in p t ah2o space implies that the parageneses developed by the hydration of fluid absent eclogites a mechanical analysis of the veins after sibson 1981 shows that pfluid pload must have been at least 0 96 fluid migration into the country rocks must have been driven by excess hydraulic head either derived from the vertical extent of the veins or due to their connection to a deeper external reservoir possibly tapped along thrust surface s diffusive and capillary transport were insignificant the fluids may have been derived from underlying metasediments that were dehydrating during the quasi isothermal uplift of this part of the alps or they may have originated during the prograde mesoalpine metamorphism documented in the area

high pressure metamorphism in mafic schist from northern vermont the only confirmed occurrence of glaucophane and omphacite in the appalachian mountains of new england is in mafic schist from tillotson peak 44 48 72 33 north central vermont mineral assemblages observed all with epidote sphene magnetite phengitic muscovite carbonate calcite or dolomite apatite sulfide are 1 actinolite glaucophane chlorite garnet quartz 2 actinolite glaucophane chlorite albite garnet 3 actinolite glaucophane omphacite garnet 4 glaucophane chlorite garnet quartz 5 glaucophane omphacite quartz 6 albite glaucophane chlorite quartz and 7 albite paragonite chlorite quartz electron microprobe investigations show that plagioclase chlorite sphene white mica carbonate and magnetite grains are relatively homogeneous however amphibole pyroxene epidote and garnet grains are zoned indicating incomplete equilibration and changing physical conditions during metamorphism glaucophane in assemblage 6 has the general formula rim composition na 1 9 m4 ca 0 1 mg 1 8fe 1 2 2 al 1 0 vi fe 0 3 3 si 7 9 al 0 1 iv o 22 oh 2 glaucophane in assemblages 1 to 5 is somewhat poorer in the glaucophane endmember and richer in actinolite ca rich amphibole grains are zoned extensively with barroisite cores and actinolite rims omphacite grains in assemblages 3 and 5 have the general formula rim composition na 0 5 ca 0 5 mg 0 4 fe 0 1 2 al 0 4 vi fe 0 1 3 si 2o 6 in spite of the significant zoning the partitioning of elements among the minerals at their grain margins suggests that all seven assemblages above may coexist at the same physical conditions of metamorphism in mafic rocks with a relatively small range in bulk composition correlation of mineral compositions and assemblages with mineral stability data suggests high pressure facies series metamorphism at about 9 2 kb and 450 100 c with p h2o p total it is proposed that the metamorphism occurred during the ordovician

geochemical and stable isotope resetting in shear zones from taschalp constraints on fluid flow during exhumation in the western alps fluid flow at greenschist facies conditions during exhumation of the western alps occurred in several penecontemporaneous systems including shear zones at lithological contacts deformed contacts between serpentinite bodies and metabasalts albite veins within metabasalts and calcite quartz veins within calcareous schists fluid flow in shear zones that juxtapose metasediments and ophiolitic rocks within the piemonte unit reset o and h isotope ratios 18o values are buffered by the wall rocks however calculated fluid 2h values are similar within all the shear zones suggesting that they formed an interconnected network the similarity of 2h values of the sheared rocks and those of unsheared calcareous schists suggests that the fluids were derived from or had equilibrated with the schists that envelop the ophiolite rocks time integrated fluid fluxes at the sheared contacts estimated from changes in si in metabasalts were up to 105 m3 m 2 with the fluid flowing up temperature driven either by topography or seismic pumping individual shear zones were active for c 2 3 myr implying average fluid fluxes of up to 10 9 m3 m 2 s 1 rocks in shear zones within the ophiolite away from contacts with the metasediments show much less marked isotopic and geochemical changes implying that fluid volumes decreased into the ophiolite unit consistent with the source of fluids being the metasediments fluids were generated by dehydration reactions that were intersected during exhumation and while many rocks show the affects of fluid rock interaction large scale fluid flow between major units was not common

the high pressure stability of hydrous phases in orogenic belts an experimental approach on eclogite forming processes abstract metamorphic evolution of orogenic belts is characterized by reactions involving hydrous phases stability relations of hydrous phases depend on h 2 o activity availability during metamorphism however hydrous phases do occur in synthetic and natural systems also at water undersaturated conditions h 2 o undersaturated experiments in the cao al 2 o 3 sio 2 h 2 o model system were performed in order to predict the role of hydrous phases in h 2 o undersaturated compositions at constant pressure and temperature these experiments yielded different assemblages of hydrous anhydrous phases depending on the experimental bulk h 2 o content in natural eclogites of mafic to intermediate bulk composition common hydrous phases include amphibole lawsonite zoisite mg chloritoid paragonite and talc complex continuous reactions delimit the stability fields of the different assemblages involving these hydrous phases such continuous reactions can be used as powerful petrogenetic tools to derive p t conditions of natural eclogites derivation of h 2 o undersaturated phase relationships for natural and synthetic systems show that hydrous phases also occur in h 2 o undersaturated bulk compositions at eclogite facies conditions comparison of experimentally determined phase relationships in mafic rocks with pressure temperature evolution of orogenic terrains demonstrates that at subsolidus conditions a trend toward h 2 o saturation is necessarily maintained during prograde p t paths in orogenic belts as far as dehydration reactions occur any prograde p t path necessarily leads to a maximized occurrence of hydrous phases and thus a maximized h 2 o preservation is achieved during metamorphism

white mica k ar geochronology of hp uhp units in the lago di cignana area western alps italy tectonic implications for exhumation abstract high pressure and ultra high pressure hp uhp blueschist and eclogite facies metabasaltic and metasedimentary rocks occur in four different tectonic units near lago di cignana western alps we have determined k ar ages for white micas matrix phengite and paragonite from the lago di cignana uhp unit lcu 39 41 ma the lower and upper units of the zermatt saas meta ophiolite lu and uu 37 38 ma and 38 41 ma respectively and the combin unit cu 36 40 ma these k ar ages overlap with single grain ar ar plateau ages 36 42 ma previously determined for phengites from lcu metasediments matrix white micas have been severely deformed during exhumation and their chemistries differ from those of micas included in garnet although individual mica grains in the matrix could have experienced different degrees of deformation which have reset their k ar systems bulk white mica separates provide the average age of all the individual grains in the separate the similarity of ages determined for white micas from the lcu lu uu and cu units regardless of rock type and mineral species suggests that these four units were metamorphosed together as part of a single metamorphic sequence in the piemonte liguria paleosubduction zone and were subsequently exhumed together however present day structural relationship among those units and the limited occurrence of uhp minerals in lcu suggests that the exhumation of lcu was more rapid than that for lu uu and cu the age gaps between the youngest value of white mica k ar ages in each unit and the inferred timing of the metamorphic peak u pb age 44 ma is 5 7 6 and 8 myr for lcu lu uu and cu respectively these intervals are considerably shorter than that determined for the sanbagawa hp metamorphic belt of southwest japan 31 myr the short interval observed for the lago di cignana units that we have studied is consistent with the model of rapid exhumation of the uhp bearing metamorphic domain suggesting the exhumation rate is higher than 18 mm y in the early stage of exhumation from the deepest level ca 120 km to the lower crust ca 30 km

provenance of jurassic tethyan sediments in the hp uhp zermatt saas ophiolite western alps rubidium sr and sm nd isotope data and rare earth element ree concentrations of the metasedimentary rocks within the zermatt saas zs ophiolite complex of the western alps are used to investigate element mobility and to determine the provenance of the metasediments in order to place constraints on the precollisional paleogeography of the piemont ligurian portion of the neo tethys ocean present day 8 7 sr 8 6 sr variations for the zs metasediments scatter about an early tertiary alpine metamorphic age whereas local nappes that have been interpreted to reflect african apulian and european basements scatter about a variscan like age this suggests 8 7 sr 8 6 sr isotope systematics were nearly completely homogenized for most of the zs metasediments during early tertiary metamorphism probably because they were relatively wet prior to metamorphism in contrast to the sr isotope data ree data and nd isotope compositions of the zs metasediments overlap those of average upper continental crust average shale and the local nappes and nd model ages of the zs metasediments overlap with those of variscan age rocks these relations suggest that the rees of the zs metasediments were not disturbed during high to ultra high pressure alpine metamorphism based on ree data nd isotope compositions and mixing models the zs metasediments comprise two groups that require distinct source terranes one group group i seems to be mixing of an old crustal component such as the paragneissic basement nappe samples with metasediments similar to the second group group ii the source material for group ii is dominated by homogenization of the variscan like orthogneissic basement nappe samples the provenance of group i samples is interpreted to be local where source nappes must have been proximal to the piemont ligurian basin prior to alpine convergence the similarity in dispersion of nd isotope compositions of the metasediments and likely source terranes suggests that the metasediments reflect deposition in small isolated basins early in the formation of the piemont ligurian ocean

1 1 ga k rich alkaline plutonism in the sw grenville province u pb zircon and baddeleyite dating of six syenitic stocks establishes that the ultrapotassic potassic alkaline and shoshonitic magmatism with island arc affinities in the central metasedimentary belt cmb of the southwestern grenville province canada took place between 1089 and 1076 ma along a 400 km long northeast trending plutonic belt these ages indicate that ultrapotassic rocks with arc affinities are not unique to the phanerozoic west to east emplacement ages along a northern and southern cross section of this belt range from 1083 2 ma kensington through 1081 2 ma lac rouge to 1076 1 3 ma loranger in the north and from 1089 3 4 ma loon lake and 1088 2 ma calabogie to 1076 2 ma westport in the south although closely spaced in time in detail these ages suggest a slight younging of this magmatic activity to the southeast integration of the geochronological data with the spatial extent and potassic character of the plutons shows that the k rich alkaline suite is distinct from the nepheline syenite belt of the bancroft terrane and from the syenite monzonite suite of the frontenac terrane of the cmb and it is considered to be a magmatic episode unique to the elzevir terrane and its gatineau segment the timing and the postmetamorphic emplacement of these plutons indicate that the regional greenschist to granulite facies metamorphism of the country rock precise age unknown is older than 1089 ma throughout the entire elzevir terrane the potassic magmatism is interpreted as the initiation of the 1090 1050 ma ottawan orogeny in the elzevir terrane thus the regional metamorphism in this terrane previously assigned to the ottawan orogeny is an earlier event the contemporaneous emplacement of this postmetamorphic plutonic belt with keweenawan volcanism is at variance with current tectonic models which consider the keweenawan rift to be formed at the same time as regional metamorphism in the cmb

stable isotope geochemistry of alpine ophiolites a window to ocean floor hydrothermal alteration and constraints on fluid rock interaction during high pressure metamorphism the subduction of hydrated oceanic lithosphere potentially transports large volumes of water into the upper mantle however despite its potential importance fluid rock interaction during high pressure metamorphism is relatively poorly understood the stable isotope and major element geochemistry of pennine ophiolite rocks from italy and switzerland that were metamorphosed at high pressures are similar to that of unmetamorphosed ophiolites suggesting that they interacted with little pervasive fluid during high pressure metamorphism cover sediments also have oxygen isotope ratios within the expected range of their protoliths in the rocks that escaped late greenschist facies retrogression different styles of sub ocean floor alteration may be identified using oxygen isotopes petrology and major or trace element geochemistry within the basalts zones that have undergone high and low temperature sub ocean floor alteration as well as relatively unaltered rocks can be distinguished serpentinites have 18o and 2h values that suggest that they were formed by hydration on or below the ocean floor the development of high pressure metamorphic mineralogies in metagabbros occurred preferentially in zones that underwent sub ocean floor alteration and which contained hydrated fine grained reactive assemblages given that the transformation of blueschist facies metabasic rocks to eclogite facies assemblages involves the breakdown of hydrous minerals e g lawsonite zoisite and glaucophane and will thus liberate considerable volumes of fluids metamorphic fluid flow must have been strongly channelled high pressure quartz calcite omphacite glaucophane titanoclinohumite veins that cut the ophiolite rocks represent one possible channel however stable isotope and major element data suggest that they were not formed from large volumes of exotic fluids fluids were more likely channelled along faults and shear zones that were active during high pressure metamorphism such strong fluid channelling may cause fluids to migrate toward the accretionary wedge especially along the slab mantle interface which is probably a major shear zone this may preclude all but a small fraction of the fluids entering the mantle wedge to flux melting additionally because fluids probably interact with relatively small volumes of rock in the channels they cannot scavenge elements from the subducting slab efficiently

dating of ultramafic rocks from the western alps ophiolites discloses late cretaceous subduction ages in the zermatt saas zone the zermatt saas zone was part of the middle to late jurassic tethyan lithosphere that underwent oceanic metamorphism during mesozoic time and subduction during eocene time hp to uhp metamorphism in upper valtournanche serpentinite metarodingite and eclogite record a dominant s2 foliation that developed under 2 5 0 3 gpa and 600 20 c during alpine subduction serpentinites contain clinopyroxene and rare zircon porphyroclasts clinopyroxene porphyroclasts show fringes within s2 with similar compositions to that of grains defining s2 zircon cores show zoning typical of magmatic growth and thin fringes parallel to the s2 foliation these features indicate crystallization of clinopyroxene and zircon fringes during hp syn d2 metamorphism related to the alpine subduction the u pb zircon dates for cores and fringes reveal crystallization at 165 3 2 ma and 65 5 5 6 ma respectively the middle jurassic dates are in agreement with the known ages for the oceanic accretion of the tethyan lithosphere the late cretaceaous paleocene dates suggest that the zermatt saas zone experienced high pressure to ultra high pressure hp uhp metamorphism at c 16 ma earlier than previously reported this result is in agreement with the evidence that in the western alps the continental sesia lanzo zone reached the subduction climax at least from 70 ma and was exhumed during ongoing oceanic subduction our results are further evidence that the zermatt saas ophiolites diachronically recorded heterogeneous hp uhp metamorphism

horizontal contraction or horizontal extension heterogeneous late eocene and early oligocene general shearing during blueschist and greenschist facies metamorphism at the pennine austroalpine boundary zone in the western alps mylonitic structures related to two orogenic events are described from the upper and lower contacts of the combin zone and the immediately overlying upper austroalpine dent blanche nappe mont mary klippe and the directly underlying lower austroalpine etirol levaz slice the first event late eocene in age commenced during blueschist facies p t conditions but pre dated the peak of subsequent greenschist facies overprint the second event early oligocene in age took place during retrograde greenschist facies conditions most sense of shear indicators associated with the retrograde mylonites indicate top se shearing but subordinate top nw displacing shear sense indicators have also been mapped mylonitic top se shearing appears to be restricted to the combin zone and its upper and lower contacts within the dent blanche nappe and mont mary klippe and at the base of the etirol levaz slice structures were observed which developed during blueschist greenschist facies conditions and are in conjunction with the p t t history of these rocks inferred to be older associated kinematic data indicate a top nw shear sense comparable blueschist greenschist facies shear sense indicators have not been observed in the combin zone nonetheless the foliation in the combin zone shows a progressive evolution from blueschist facies to greenschist facies to retrograde greenschist facies conditions this indicates that the combin zone and the immediately over and underlying austroalpine units shared a common tectono metamorphic evolution since the late eocene finite strain data reveal oblate strain fabrics which are thought to result from a true flattening strain geometry flow path modelling reveals a general non coaxial deformation regime and corroborates significant departures from a simple shear deformation in the study area mylonitic top se shearing in the combin zone is attributed to early oligocene backfolding and backthrusting of the mischabel phase temperature time curves suggest slight reheating in the monte rosa nappe underneath and cooling in the dent blanche nappe above the combin zone hence confirming a thrust interpretation for this event the top nw displacing structures are thought to result from late eocene emplacement of the dent blanche nappe and the combin zone onto the middle pennine barrhorn series along the combin fault as related structures initiated during mildly blueschist facies conditions in the dent blanche nappe and the underlying combin zone and both were emplaced together onto the greenschist facial barrhorn series it is concluded that the structures developed as the nappes moved upward relative to the earth s surface thus the combin fault is regarded as a thrust the geometry of this structure indicates that the combin fault is an out of sequence thrust that locally cut down section hence top nw out of sequence thrusting caused local thinning of the metamorphic structural section in association with horizontal shortening out of sequence thrusts cutting down section and back thrusts offer the possibility of explaining the pronounced break in the grade of metamorphism across the combin fault i e the contact between the eclogite facial zermatt saas zone and the overlying lower grade combin zone by contractional deformation

magmatic and tectonic emplacement of the pukaskwa batholith superior province ontario canadathis article is one of a series of papers published in this special issue on the theme of geochronology in honour of tom krogh the neoarchean pukaskwa batholith consists of pre syn and post tectonic phases emplaced over an interval of 50 million years pre tectonic phases are broadly synvolcanic and have a high al tonalite trondhjemite granodiorite ttg affinity interpreted to reflect derivation by partial melting of basaltic crust at lower crustal or upper mantle depths minor syn tectonic phases slightly post date volcanism and have geochemical characteristics suggesting some involvement or interaction with an ultramafic mantle source component magmatic emplacement of pre and syn tectonic phases occurred in the midcrust at paleopressures of 550 600 mpa and these components of the batholith are thought to be representative of the midcrust underlying greenstone belts during their development subsequent to emplacement of the syntectonic phases and likely at approximately 2680 ma the pukaskwa batholith was uplifted as a structural dome relative to flanking greenstone belts synchronously with ongoing regional sinistral t

geothermometry from 40ar39ar dating experiments abstract we have selected a variety of minerals from three basic intrusions within the high grade metamorphic terrane near haliburton ontario for k ar dating by application of 40 ar 39 ar incremental heating experiments we conclude that the discordant k ar mineral dates can best be explained by a slow cooling history from the stepwise heating experiments we calculated isotopic closure temperatures using dodson s theory for slow cooling and from these constructed a cooling curve this curve was then compared to independent constraints to test the overall validity of our procedures and assumptions the results are geologically reasonable insofar as 1 a the maximum calculated temperatures from hornblendes of 700 c are consistent with independent approximate metamorphic grid arguments 2 b the ar closure temperatures for biotites 350 400 c are consistent with independent evidence and 3 c minerals with older k ar dates have higher calculated closure temperatures the resultant geological cooling curve implies that a maximum temperature of 700 c was reached before 1000 ma ago and thereafter the temperature decreased via changes in uplift and cooling rates to below 200 c by 700 ma ago we have discovered that plutonic plagioclases are very sensitive indicators comparable to k felds pars of ar loss and therefore can usefully extend the range of application of the 40 ar 39 ar stepwise heating method we also describe experiments in which we have tested the effects on the ar systematics of acid leaching of plagioclases

p t evolution of glaucophane omphacite bearing hp lt rocks in the western tianshan orogen nw china new evidence for alpine type tectonics the late palaeozoic western tianshan high pressure low temperature belt extends for about 200 km along the south central tianshan suture zone and is composed mainly of blueschist eclogite and epidote amphibolite greenschist facies rocks p t conditions of mafic garnet omphacite and garnet omphacite blueschist which are interlayered with eclogite were investigated in order to establish an exhumation path for these high pressure rocks maximum pressure conditions are represented by the assemblage garnet omphacite paragonite phengite glaucophane quartz rutile estimated maximum pressures range between 18 and 21 kbar at temperatures between 490 and 570 c decompression caused the destabilization of omphacite garnet and glaucophane to albite ca amphibole and chlorite the post eclogite facies metamorphic conditions between 9 and 14 kbar at 480 570 c suggest an almost isothermal decompression from eclogite to epidote amphibolite facies conditions prograde growth zoning and mineral inclusions in garnet as well as post eclogite facies conditions are evidence for a clockwise p t path analysis of phase diagrams constrains the p t path to more or less isothermal cooling which is well corroborated by the results of geothermobarometry and mineral textures this implies that the high pressure rocks from the western tianshan orogen formed in a tectonic regime similar to alpine type tectonics this contradicts previous models which favour franciscan type tectonics for the southern tianshan high pressure rocks

fabrics produced mimetically during static metamorphism in retrogressed eclogites from the zermatt saas zone western italian alps abstract lattice preferred orientations lpos are commonly interpreted to form by dislocation creep consequently they are used to infer deformation at the metamorphic grade at which the minerals were stable especially if those minerals show a shape fabric here we show that lpos can occur through mimicry of a pre existing lpo so they formed statically not during deformation omphacite and glaucophane lpos occur in eclogite facies rocks from the zermatt saas unit of the northwest italian alps barroisite grew during greenschist facies retrogression and has an lpo controlled significantly by the eclogite facies omphacite and glaucophane lpos rather than directly by deformation using spatially resolved lattice orientation data from the three key minerals collected using electron backscatter diffraction we deploy a new technique of interphase misorientation distribution analysis to prove this barroisite lpo develops by mimicry of omphacite via a particular lattice orientation relationship and by direct topotactic and epitactic replacement of glaucophane lpo in turn influenced anisotropic grain growth resulting in a barroisite grain shape fabric thus regional retrogression during exhumation of the zermatt saas high pressure rocks was in large part static rather than dynamic as previously interpreted in general the possibility of mimetic fabrics forming during metamorphic reactions must be borne in mind when interpreting direct structural observations and seismic anisotropy data in terms of deformation in both crust and mantle

proterozoic subduction and terrane amalgamation in the southwestern grenville province canada evidence from ultrapotassic to shoshonitic plutonism a late grenvillian 1089 1076 ma subduction regime followed by terrane amalgamation is postulated as the paleoenvironment of a 400 km long belt of potassium rich alkaline and shoshonitic plutons in the central metasedimentary belt of the southwestern grenville province canada emplacement of the plutons postdates the regional metamorphism in the country rock but predates major shear zones that form a structural boundary for the plutonic belt the extent timing and magmatic affinities of the suite delineate the gatineau domain within the current mont laurier terrane of quebec and are compelling evidence for the allochthonous nature of the elzevir terrane and its extension eastward to rideau lake in ontario and northward into the gatineau domain the belt trends northeast the emplacement ages are younger to the southeast and the magmatic affinities are those of island arc ultrapotassic to shoshonitic rocks this is interpreted to reflect the existence of a southeast dipping northeast trending subduction zone beneath the combined elzevir gatineau terrane between 1089 and 1076 ma subsequent to subduction amalgamation of the elzevir terrane to the other terranes formed the central metasedimentary belt which then accreted and collided with the allochthonous polycyclic belt these events provide evidence that the ottawan orogeny commenced at ca 1090 ma in the central metasedimentary belt a modern analogue of this plutonism and its tectonic setting may have been the magmatism and arc continent collision and subduction setting of the sunda are indonesia

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

geologic transect across the grenville orogen of ontario and new york revised cross sections of the western grenville province incorporate new geologic results and reprocessed seismic reflection data the geology is presented in terms of three tectonic elements 1 pre grenvillian laurentia and its margin with ca 1740 and 1450 ma continental arc plutons and associated supracrustal rocks 2 composite arc belt of allochthonous 1300 1250 ma volcanic arcs and sedimentary rocks and 3 frontenac adirondack belt characterized by supracrustal and granitoid rocks and anorthosites of uncertain affinity that may represent a distinctive part of the composite arc belt or an offshore micro continent rocks of the composite arc and frontenac adirondack belts were amalgamated with each other by ca 1160 ma were then thrust over laurentia during ca 1080 1035 ma and ca 1010 980 ma phases of convergence and were dissected and exhumed by 1040 ma normal faults penetrative deformation was restricted to that part of the pre grenvillian laurentian margin that lies to the southeast of the grenville front and parts of the accreted composite arc and frontenac adirondack belts the laurentian rocks in the grenville province are bounded to the northwest and southeast by southeast dipping ductile thrust and or normal shear zones the composite arc and frontenac adirondack belts to the southeast are bounded by ductile and brittle ductile thrust and or normal faults that separate domains with contrasting cooling histories despite a long pre grenvillian tectonic and plutonic history the present crustal architecture and much of the seismic reflectivity were acquired during 1080 980 ma phases of compression and extension

u mo and ree mineralization in late tectonic granitic pegmatites southwestern grenville province canada abstract late tectonic granitic pegmatites occur throughout most of the southwestern grenville province of ontario and quebec these vary from mineralogically simple 3 4 major phases to complex 4 major phases pegmatites the latter are variably enriched in u th mo and rare earth elements ree and zr and nb the mineralogical and whole rock geochemical data obtained from simple pegmatites suggest crystallization from low temperature h 2 o undersaturated partial melts generated in amphibolite to granulite grade felsic paragneisses and orthogneisses that have undergone an earlier stage of melting these magmas coalesced and fractionated during ascent to mid crustal levels the mineralogical complexity and increase in abundance of ferromagnesian silicates in individual pegmatites arises from interaction with the host rocks hybridization skarnification previous research has shown that there is an empirical relationship between ferromagnesian silicate content and u th and ree enrichment although this phenomena has not been investigated petrogenetically interaction with the host rocks hybridization produced compositional changes in the pegmatites through bimetasomatic two way exchange processes that modified normal fractional crystallization processes high concentrations of ca fe mg and possibly ti depending on the host rock are interpreted to indicate bimetasomatic exchange resulting in a mineralogical modification of the pegmatitic magma for instance the introduction of ca into the pegmatite magma may lead to local saturation of titanite and or allanite high concentrations of u th ree and nb in these titanites suggests that elemental distribution coefficients are affected within the pegmatite melts by the appearance of hybrid titanite the role of a fluid phase in the hybridization skarnification process is two fold 1 the fluid phase will enhance bimetasomatic diffusive exchange of constituents between the host rocks e g ca fe mg ti and the pegmatite si al k na fe u th ree nb zr h 2 o f cl over that of normal melt solid diffusivities and 2 the fluid may be enriched in certain constituents that become saturated during metasomatic interaction between the pegmatite and host rocks

comparison of the high pressure schist belts of new caledonia and sanbagawa japan the high pressure schist terranes of new caledonia and sanbagawa were developed along the oceanic sides of sialic forelands by tectonic burial metamorphism the parent rocks were chemically similar as volcanic sedimentary trough or trench sequences and metamorphic temperatures in both belts were 250 to 600 c from phase equilibria curves total pressures were higher for new caledonia 6 15 kb than for sanbagawa 5 11 kb and the estimated thermal gradients were 7 10 c km and 15 c km respectively pt paths identify the higher pressure in new caledonia p differences 2 kb at 300 c and 4 kb at 550 c with consequent contrast in progressive regional metamorphic zonation for pelites in the two areas lawsonite epidote omphacite new caledonia and chlorite garnet biotite sanbagawa in new caledonia the na amphibole is dominantly glaucophane and na pyroxenes associated with quartz are jadeite jd95 100 and omphacite in sanbagawa the amphibole is crossite or riebeckite and the pyroxene is omphacite jd50 for both areas garnet rims show increase in pyrope content with advancing grade but sanbagawa garnets are richer in almandine progressive assemblages within the two belts can be equated by such reactions as new caledonia sanbagawa glaucophane paragonite h2o albite chlorite quartz glaucophane epidote h2o albite chlorite actinolite and the lower pressure japanese associations appear as retrogressive phases in the new caledonia epidote and omphacite zones

sequence and style of major post nappe structures simplon pennine alps abstract in the upper pennine nappe complex of the simplon pennine alps switzerland and italy at least three phases of major post nappe folding in places associated with thrusting can be distinguished these are superimposed on an earlier formed partly chaotic complex of tectonic units including the bernhard and monte rosa continental flakes and the zermatt saas and antrona ophiolite complexes the earliest post nappe folds were essentially isoclinal throughout the whole region and were accompanied by a strong schistosity which is the main foliation in most areas later two successive phases of back folding led to the present overall structure both phases typically show rapid variations in style from open folds lacking axial planar schistosity to very tight structures with complete foliation transposition this has been demonstrated by systematically mapping the major axial traces over the whole region successively removing the major structures in reverse order shows that the ophiolite complexes were originally part of a continuous unit marking an important suture between the bernhard and monte rosa nappes

chlorine isotope constraints on fluid rock interactions during subduction and exhumation of the zermatt saas ophiolite 1 chlorine isotope compositions of high pressure 2 3 gpa serpentinite rodingite and hydrothermally altered oceanic crust aoc differ significantly from high and ultrahigh pressure 3 2 gpa metasedimentary rocks in the aosta region italy texturally early serpentinites rodingites and aoc have bulk 37cl values indistinguishable from those of modern seafloor analogues 37cl 1 0 to 1 0 in contrast serpentinites and aoc samples that recrystallized during exhumation have low 37cl values 2 7 to 0 5 37cl depletion correlates with progressive changes in bulk chemistry hp uhp metasediments have low 37cl values median 2 5 that differ statistically from modern marine sediments median 0 6 cl in metasedimentary rocks is concentrated in texturally early minerals indicating modification of seafloor compositions early in the subduction history the data constrain fluid sources during both subduction and exhumation related phases of fluid rock interaction 1 marine sediments at the top of the downgoing plate likely interacted with isotopically light pore fluids from the accretionary wedge in the early stages of subduction 2 no pervasive interaction with externally derived fluid occurred during subsequent subduction to the maximum depths of burial 3 localized mixing between serpentinites and fluids released by previously isotopically modified metasediments occurred during exhumation in the subduction channel most samples however preserved protolith signatures during subduction to near arc depths

zoned high pressure assemblages in pillow lavas of the zermatt saas ophiolite zone switzerland pillow lavas of the zermatt saas ophiolite zone of the western swiss alps in places consist of zoned high pressure parageneses these assemblages which developed subsequent to the metamorphic peak are of lawsonite eclogite in the cores of pillows and dolomite bearing blueschist or glaucophane eclogite in the rims the matrix between the pillows comprises dolomite glaucophane rich material an analysis of reactions in the system na2o cao feo mgo al2o3 sio2 h2o co2 ncfmas h2o co2 indicates that one possible model is that these assemblages formed cofacially due to systematic intra pillow variations in the chemical potentials of h2o and co2 pillow rims equilibrated at higher values of co2 and lower values of h2o than pillow cores textural evidence suggests that infiltration has controlled the distribution of co2 generated by the breakdown of dolomite bearing assemblages during the return of the rocks to the surface variation in h2o content was caused by ocean floor hydrothermal alteration in these rocks therefore blueschist assemblages have been stabilised by higher values of co2 and eclogites by higher values of h2o if fluids were present as suggested by various lines of evidence then blueschist pillow rims equilibrated at higher co2 co2 h2o than eclogitic pillow cores

the age and origin of grenville province uraniferous granites and pegmatites uranium mineralization in pegmatites and granites occurs widely in the grenville province but is especially important near bancroft ontario and north of mont laurier and johan beetz in quebec most investigators have concluded that these rocks were produced by anatexis of a uranium enriched sedimentary protolith we have used rb sr isotopic methods to test this and other hypotheses and have reexamined the petrologic and structural features that led to these hypotheses rb sr whole rock isochron ages from all three areas range from 937 to 980 ma some 150 ma younger than the age of the peak of grenville metamorphism as inferred from numerous ages from syntectonic intrusive rocks and high grade metasediments the ages of the pegmatites are similar to or just slightly greater than k ar ages from this region the initial 87sr 86sr ratios range from 0 7028 to 0 7054 plus one poorly defined value of 0 709 excluding this last one these values are only slightly greater than mantle ratios and calculations show

sharbot lake terrane and its relationships to frontenac terrane central metasedimentary belt grenville province new insights from u pb geochronology the sharbot lake terrane of the central metasedimentary belt in ontario comprises marbles minor tholeiitic basalts and siliciclastic rocks metamorphosed at low to moderate degrees and is intruded by several suites of mafic to felsic plutons gabbroic and associated monzogranitic phases of the lavant complex in northwestern parts of the terrane yield ages of 1224 2 ma for zircon and monazite and 1221 1211 ma for titanite the wolf grove structure a gneissic domain in the northeastern part of the area was migmatized and intruded by granites at 1168 ma prior to its tectonic juxtaposition on amphibolites and marbles of the sharbot lake terrane correlative events recorded by gneiss quartzite and marble from carleton place farther east stress the strong affinity of the wolf grove structure with rocks of the frontenac terrane the 1156 2 ma posttectonic maberly stock intrudes rocks near the boundary zone between the frontenac and sharbot lake terranes suggesting that the two terranes were linked by

u pb geochronology of the raglan gabbro belt central metasedimentary belt ontario implications for an ensialic marginal basin in the grenville orogen the raglan gabbro belt of the ontario grenville orogen is coincident with the top of the central metasedimentary belt boundary thrust zone a major mid crustal shear zone separating the central gneiss belt in the footwall from the central metasedimentary belt in the hanging wall it has been suggested that the gabbros making up the belt are coeval that they formed in a marginal basin within the central metasedimentary belt and that they formed a horizon of theologically stiff material that controlled the localization of the top of the boundary thrust zone during its initiation as the marginal basin closed at ca 1190 ma u pb zircon dating of plutons within the raglan gabbro belt was undertaken to test the coeval nature of intrusions in the belt magmatic crystallization ages for three of the gabbros fall in the range 1246 1227 ma and a fourth yields a minimum age of ca 1175 ma the results are permissive of a common origin for the gabbros and allow that the raglan gabbro belt may have been related to

a thermal tectonic model for high pressure metamorphism and deformation in the sesia zone western alps the sesia zone western alps is a slice of austroalpine continental crust that was once metamorphosed at t 500 560 c and p 14 16 kbar at 130 100 ma b p pervasive blueschist eclogite facies assemblages record this event and a later greenschist facies overprint is only well developed in the nw part of the zone on the basis of thermal calculations it is difficult to explain the preservation of high pressure assemblages in a simple model of underthrusting and subsequent uplift erosion an alternative model is proposed in which the sesia zone was emplaced by underthrusting of the austroalpine continental margin prior to subduction of oceanic lithosphere of the piemonte basin subduction of relatively cool oceanic lithosphere below the underthrust sesia zone then continued for a period of time e g 20 ma thus suppressing thermal recovery of this latter unit exhumation of sesia zone rocks began during this subduction episode by uplift on the insubric line erosion and by underplating of oceanic

focused fluid flow during subduction oxygen isotope data from high pressure ophiolites of the western alps abstract during subduction hydrothermally altered oceanic crust undergoes extensive dehydration the water released migrates upwards and may escape along the slab mantle interface or continue rising to hydrate the mantle overlying the subducting slab oxygen isotope ratios from eclogitic ophiolites of the western alps correlate closely with lithology samples with little altered basalt as a protolith have 18 o values of 5 1 6 0 material representing low temperature alteration zone rocks have 18 o values of 5 9 6 6 and rocks orginating in high temperature alteration zones have 18 o values of 4 6 5 3 these isotopic values closely approach those of the ocean crust and unmetamorphosed ophiolites these data suggest that the alpine rocks have not experienced significant fluid flow during metamorphism implying that the escape of fluid from subducted ocean crust is highly focused probably along structurally controlled pathways to the top of the slab the fluid may then continue largely along the slab mantle interface although a portion of it rises further to hydrate the overlying mantle

high pressure metamorphism of the zermatt saas ophiolite zone switzerland within the zermatt saas ophiolite zone of the western alps a record of polystadial high pressure metamorphism is well preserved early blueschist assemblages retained as inclusions in garnets were succeeded by eclogitic assemblages which in some rocks contained lawsonite kyanite talc and chloritoid these eclogitic assemblages formed by prograde reaction from the early blueschists subsequently reaction of these eclogites with a mixed h 2 o co 2 vapour phase led to the replacement of kyanite by paragonite and the partial replacement of omphacite garnet paragenses by assemblages containing glaucophane paragonite and ankerite high pressure eo alpine metamorphism took place under conditions of 550 600 c and 17 5 20 kbar values of a h 2 o between 0 55 and 1 are high enough to accommodate equilibration with a water rich vapour phase under the highest grade conditions the presence of such a fluid phase is locally indicated by the presence of quartz rich veins containing omphacite and kyanite this vapour phase was absorbed during retrogression later rehydration is limited to areas close to albite veins tectonic contacts and bodies of metasediment the metamorphic conditions determined for the zermatt saas zone are compatible with those suggested for over and under lying units these conditions and the p t path inferred by comparing the reaction histories with a petrogenetic grid for the system na 2 o cao mgo a1 2 o 3 sio 2 h 2 o suggest that metamorphism occurred during subduction to depths of between 60 and 70 km and subsequent exhumation during the alpine orogeny

kinematic reworking and exhumation within the convergent alpine orogen abstract kinematic data from the internal zones of the western alps indicate both top to se and top to nw shearing during synkinematic greenschist facies recrystallisation rb sr data from white micas from different kinematic domains record a range of ages that does not represent closure through a single thermal event but reflects the variable timing of synkinematic mica recrystallisation at temperatures between 300 and 450 c the data indicate an initial phase of accretion and foreland directed thrusting at ca 60 ma followed by almost complete reworking of thrust related deformation by se directed shearing this deformation is localised within oceanic units of the combin zone and the base of the overlying austroalpine basement and forms a regional scale shear zone that can be traced for almost 50 km perpendicular to strike the timing of deformation in this shear zone spans 9 ma from 45 to 36 ma the se directed shear leads to local structures that cut upwards in the transport direction with respect to tectonic stratigraphy and such structures have been interpreted in the past as backthrusts in response to ongoing alpine convergence however on a regional scale the top to se deformation is related to crustal extension not shortening and is coincident with exhumation of eclogites in its footwall during this extension phase deformation within the shear zone migrated both spatially and temporally giving rise to domains of older shear zone fabrics intercalated with zones of localised reworking top nw kinematics preserved within the combin zone show a range of ages the oldest 48 ma may reflect the final stages of emplacement of austroalpine units above piemonte oceanic rocks prior to the onset of extension however much of the top to nw deformation took place over the period of extension and may reflect either continuing or episodic convergence or tectonic thinning of the shear zone 40ar 39ar data from the region are complicated due to the widespread occurrence of excess 40ar in eclogite facies micas and partial ar loss during alpine heating reliable ages from both eclogite and greenschist facies micas indicate cooling ages in different tectonic units of between 32 and 40 ma these ages are slightly younger than rb sr deformation ages and suggest that cooling below ca 350 c occurred after juxtaposition of the units by se directed extensional deformation our data indicate a complex kinematic history involving both crustal shortening and extension within the internal zones of the alpine orogen to constrain the palaeogeographic and geodynamic evolution of the alps requires that these data be integrated with data from the more external zones of the orogen complexity such as that described is unlikely to be restricted to the western alps and spatially and temporally variable kinematic data are probably the norm in convergent orogens recognising such features is fundamental to the correct tectonic interpretation of both modern and ancient orogens

rapid exhumation of the zermatt saas ophiolite deduced from high precision smnd and rbsr geochronology sm nd isotope data from garnets in ultrahigh pressure coesite bearing eclogite from the western alps were obtained to determine the age of peak metamorphism and exhumation rates of deeply buried oceanic crust in the zermatt saas ophiolite complex we report an exceptionally well constrained sm nd isochron age of 40 62 6 ma from the lago di cignana eclogite difficulties in dating alpine eclogites using this method were overcome by using an hf acid leaching procedure on garnet to remove mineral inclusions the zermatt saas rocks reached greenschist facies conditions around 38 2m a on the basis of rb sr whole rock phengite isochrons these data combined with existing estimates of the pressure and temperature conditions of the eclogite give an estimate for the initial exhumation rate of 10 to 26 km m y depending on the inferred pressure for the recrystallization of phengite followed by slow exhumation of 0 3 km m y from 34 to 14 ma the initial exhumation rates at lago di cignana are among the highest yet determined for high pressure terranes and are well constrained because the age has been determined directly on the high pressure assemblage the rapid exhumation rates presented here place critical constraints on collisional tectonic models 1999 elsevier science b v all rights reserved

the timing of and nature of greenschist facies deformation and metamorphism in the upper pennine alps the upper pennine nappes in the western alps are cut by structures that locally place european basement rocks over those of the piemonte zone which represent the sutured remains of alpine ocean crust in the taschalp area of switzerland the mischabel backfold and associated se verging shearing carries gneisses of the grand saint bernard nappe over eclogitic material of the zermatt saas zone greenschist facies assemblages are developed locally in the foot wall and hanging wall of the mischabel structure as well as in the sheared rocks of the thrust zone rb sr 40ar 39ar and u pb studies of white micas amphiboles and sphenes show that greenschist assemblages developed at circa 44 ma in the zermatt saas zone of the footwall and at circa 40 ma in the thrust zone rb sr data from the literature document muscovite and biotite ages in strongly foliated grand saint bernard gneisses of the hanging wall of circa 36 and 32 ma respectively these are thought to indicate that deformation on the mischabel structure continued for at least 5 m y rapid cooling inferred for the zermatt saas ophiolites in the mid eocene may document either the tectonic emplacement of the ophiolite over the underlying monte rosa massif or their extensional unroofing it is not possible to determine with certainty whether the mischabel structure is extensional or contractional on a crustal scale the age of cooling and deformation reported here show that it is not possible that the mischabel structure was responsible for the exhumation of the high pressure assemblages in the zermatt saas and monte rosa rocks

carbonate till as a soft bed for pleistocene ice streams on the canadian shield north of lake superior silty carbonate till derived from erosion of paleozoic carbonate and proterozoic rocks within and adjacent to hudson bay covers extensive areas of the canadian shield north of lake superior it is hypothesized that this carbonate till could have acted as low resistance substrata for overriding ice streams by deforming and or supporting high subglacial water pressures contrary to assumptions presented in some current models for ice flow within the laurentide ice sheet it need not be assumed that shield terrain in these areas acted as a rigid bed generating large basal shear stresses and inhibiting ice flow indeed erratic dispersal patterns long distance glacial transport and splayed patterns of ice flow indicators in areas of thick till cover may be better explained by rapid ice flow events or ice streams enhanced by the thickness distribution impermeability and susceptibility to deformation of fine carbonate till

subducted and obducted ophiolites of the central alps paleotectonic implications deduced by their distribution and metamorphic overprint abstract the scenario of the closure of the tethyan realm in the central alps as reconstructed from the distribution and intensity of high pressure overprint can be interpreted as follows in the western part of the alps consecutive subduction and collision occurred in two well separated oceanic domains first in the piemontais domain zermatt saas zone and then in the valais domain versoyen further to the east where the two oceanic domains had merged ino a single ocean closure of the piemontais ocean led to obduction of oceanic crust onto the northern continental margin the ophiolites so produced were later involved in a greenschist facies stage which is related to the second collision and definitive closure of the alpine realm the change in tectono metamorphic evolution is spatially related to the eastern extremity of the brianconnais rise

petrological constraints on the eclogite and blueschistfacies metamorphism and p t t paths in the western alps phase relationships in the model mafic system and geothermobarometry allow discrimination of four main groups of high p rocks in the nappes of the western alps very high p eclogite facies including kyanite eclogites and coesite pyrope assemblages eclogite facies paragonite zoisite eclogites high t blueschist facies glaucophane garnet lawsonite assemblages and low t blueschist facies glaucophane lawsonite pumpellyite assemblages the blueschist facies eclogite facies transition is promoted chiefly by increasing t low bulk xmg and relatively low h2o the variety of assemblages and the heterogeneous approach to equilibrium observed in the alpine rocks are not only constrained by the intersection of the reaction surfaces in p t x space but also by the effect and timing of the processes which control kinetics i e pervasive deformation and fluid infiltration the faster rate of dehydration reactions relative to hydration reactions along with the fact that different bulk compositions crossed the reaction curves at different temperatures and times all may have induced h2o gradients and contributed to the heterogeneous distribution of deformation through a process of reaction enhanced ductility also mass transfer may have been an effective process in determining the type of high p assemblage in particular rock volumes as regards the p t t paths only the post climax histories are recorded well in the alpine nappes post eclogitic exhumation paths at decreasing temperatures characterize structurally higher nappes which were first subducted during the early alpine cretaceous event in contrast more or less isothermal decompression paths characterize structurally deeper nappes formed by westward propagation of the underthrust surfaces during the early alpine event and the subsequent meso alpine palaeogene collision between the european and african plates in the western alps prevalent eclogite facies conditions were attained during the metamorphic climax of the early alpine subduction while blueschist facies recrystallization characterizes the early alpine exhumation of the eclogitized units and the subsequent intracontinental underthrusts linked to the meso alpine continental collision

dehydration induced fracturing of eclogite facies peridotites implications for the mechanical behaviour of subducting oceanic lithosphere structural analysis of eclogite facies peridotites from the voltri massif in northwest italy shows that deformation in the upper 60 km of the alpine serpentinite peridotite dominated subduction zone was strongly localized in antigorite rich serpentinite mylonites deformation in the peridotite wall rock was dominated by the development of sub planar shear fracture systems and veins with olivine titanian clinohumite bearing assemblages fracturing of the peridotite wall rock was closely associated with synkinematic antigorite breakdown in serpentinite mylonites during peak subduction hp lt metamorphism and is inferred to have occurred as a result of hydraulic fracturing the deformational structures observed allow to place some constraints on the mechanical behaviour of the erro tobbio peridotites during subduction it is inferred that fracturing of the peridotite may have occurred at transient fluid pressures in excess of 0 9 times the lithostatic pressure the shear stresses along the subducting plate contact were controlled by the rheology of serpentinite mylonites which suggests that maximum values of about 60 mpa were reached at 30 km depth while below 40 km depth they decreased to less than 5 mpa these shear stresses are distinctly lower than those inferred for basalt dominated subduction zones

pillow lavas as protoliths for eclogites evidence from a late precambrian cambrian continental margin seve nappes scandinavian caledonides pillow lavas containing an eclogitic mineral assemblage locally with glaucophane crossite are described from the tsakkok lens within the seve nappes north central scandinavian caledonides critical primary relationships to the host metasedimentary rocks which are dominated by marble and quartz garnet phengite schist are preserved it is argued that these clastic rocks and the ongoing igneous activity in the tsakkok lens are late precambrian to cambrian in age and that these rocks were deposited emplaced along the outermost part of a continental margin related to the continent baltica

harvey cardiff domain and its relationship to the composite arc belt grenville province insights from u pb geochronology and geochemistrythis article is one of a series of papers published in this special issue on the theme of geochronology in honour of tom krogh combined mapping u pb isotope dilution thermal ionization mass spectrometry id tims geochronology and geochemical studies in harvey cardiff domain in the western composite arc belt reveal the presence of a long lived magmatic metamorphic history between 1290 and 1030 ma following intrusion of gneissic tonalite of the anstruther and burleigh gneiss complexes at ca 1290 ma diorite of the salmon burn intrusive complex was emplaced at 1242 1 1 1 ma a potassium feldspar megacrystic monzogranite in the salerno creek deformation zone that forms the boundary between harvey cardiff domain and bancroft terrane was emplaced at 1211 3 1 5 ma similar to the 1220 1 6 ma old junction pluton and a previously reported age of ca 1229 ma from another methuen suite granite in the domain all three ages are 20 to 30 million years younger than methuen suite ages elsewhere in the composite arc belt 1250 1240 ma deformation along the salerno creek deformation zone is constrained between 1211 ma the age of th

serpentinites of the zermatt saas ophiolite complex and their texture evolution the zermatt saas serpentinite complex is an integral member of the penninic ophiolites of the central alps and represents the mantle part of the oceanic lithosphere of the tethys metamorphic textures of the serpentinite preserve the complex mineralogical evolution from primary abyssal peridotite through ocean floor hydration subduction related high pressure overprint meso alpine greenschist facies metamorphism and late stage hydrothermal alteration the early ocean floor hydration of the spinel harzburgites is still visible in relic pseudomorphic bastite and locally preserved mesh textures the primary serpentine minerals were completely replaced by antigorite the stable assemblage in subduction related mylonitic serpentinites is antigorite olivine magnetite diopside the mid tertiary greenschist facies overprint is characterized by minor antigorite recrystallization textural and mineral composition data of this study prove that the hydrated mineral assemblages remained stable during high pressure metamorphism of up to 2 5 gpa and 650 c the zermatt saas serpentinites thus provide a well documented example for the lack of dehydration of a mantle fragment during subduction to 75 km depth

sm nd isotopic evidence on the age of eclogitization in the zermatt saas ophiolite rocks within the zermatt saas ophiolite of the western alps have undergone eclogite facies metamorphism during subduction prior to the alpine collision the metamorphic history of these rocks is well defined with eclogitic assemblages being followed by the limited growth of blueschist assemblages of glaucophane and paragonite subsequent greenschist alteration occurs adjacent to faults veins and metasediments away from such sources of water retrogression is very limited sm nd isotopic analyses of an essentially unretrogressed eclogitic metabasalt suggest that eclogite facies metamorphism occurred at 52 18 ma the large uncertainty is due to the presence of very small amounts of nd rich epidote present as inclusions within garnet as the closure temperature of garnet to sm nd is thought to be 600dc resetting due to post high pressure diffusion is thought to be insignificant given the fine grained protolith to the sample analysed and its extensive deformation under eclogite facies conditions incomplete homogenization of pre metamorphic isotopic variations is also considered unlikely to be responsible for the young age a tertiary age of eclogitization means that models of early alpine evolution based on the cessation of high pressure metamorphism in the cretaceous need to be revised

the eclogitized pillows of the betic ophiolitic association relics of the tethys ocean floor incorporated in the alpine chain after subduction in the volcanic sequence of the betic ophiolitic association boa volcanic structures and textures are preserved in spite of being metamorphosed to the edogite facies the original quenched glassy margins of the pillows and lava flows are still recognizable by the darker colour fine grained textures and scarcity of phenocrysts the boa eclogitized pillows have chemical compositions very similar to the basalts enriched in lil elements erupted nowadays at the mid oceanic ridges magmatism which generated the boa most likely began under continental rift conditions at the triassic jurassic boundary and continued under ocean floor extensional conditions during the lower and middle jurassic in age and petrological characteristics this magmatism is equivalent to that of the western tethys ophiolites during the late cretaceous due to the collision of african and iberian plates the boa ophiolites were subducted and underwent a metamorphism in the eclogite fades whose climax in the lugros outcrop can be estimated at 650 700 c and about 20 kb

the formation of eclogite facies metatroctolites and a general petrogenetic grid in na2o cao feo mgo al2o3 sio2 h2o ncfmash eclogite facies metatroctolites from a variety of western alps localities voltri monviso lanzo allalin zermat saas etc that preserve textural evidence of their original form as bimineralic olivine plagioclase rocks are considered in terms of calculated mineral equilibria in the system na2o cao feo mgo al2o3 sio2 h2o ncfmash pseudosections based on a new petrogenetic grid for ncfmash presented here are used to unravel the metamorphic history of the metatroctolites considering the rocks to consist of different composition microdomains corresponding to the original olivine and plagioclase grains on the basis that the preservation of the mineral assemblage in each microdomain will tend to be from where on a rock s p t path the metamorphic fluid phase is used up via rehydration reactions p t pseudosections contoured for water content and p t path mh2o amount of water pseudosections are used to examine fluid behaviour in each microdomain we show that the different microdomains are likely to preserve their mineral assemblages from different places on the p t path for the olivine microdomain the diagnostic mineral assemblage is chloritoid talc garnet omphacite the preservation of this assemblage in the light of the closed system p t path mh2o relationships implies that the microdomain loses its metamorphic fluid as it starts to decompress and in the absence of subsequent hydration the high pressure mineral assemblage is then preserved in the plagioclase microdomain the diagnostic assemblage is epidote or zoisite kyanite quartz suggesting a lower pressure of about 2 gpa than for the olivine microdomain in the light of p t path mh2o relationships development of this assemblage implies breakdown of lawsonite across the lawsonite breakdown reaction regardless of the maximum pressure reached it is likely that the plagioclase microdomain was mainly fluid absent prior to lawsonite breakdown only becoming fluid present across the reaction then immediately becoming fluid absent again

u pb geochronology of the grenville orogen of ontario and new york constraints on ancient crustal tectonics based on lithological structural and geophysi cal characteristics the proterozoic grenville orogen of southern ontario and new york has been divided into domains that are separated from each other by ductile shear zones in order to constrain the timing of meta morphism u pb ages were determined on metamorphic and igneous sphenes from marbles calc silicate gneisses amphibolites granitoids skarns and pegmatites in addi tion u pb ages were obtained for monazites from meta pelites and for a rutile from an amphibolite these mineral ages constrain the timing of mineral growth the duration of metamorphism and the cooling history of the different domains that make up the southern part of the exposed grenville orogen based on the ages from metamorphic minerals regional and contact metamorphism occurred in the following intervals central granulite terrane