rutile u pb age depth profiling a continuous record of lithospheric thermal evolution abstract understanding of the thermal and geophysical evolution of the lower continental crust is limited by the resolution of conventional thermochronology intracrystalline daughter nuclide distribution profiles preserve a rich and underutilized record of thermal history using laser ablation inductively coupled plasma mass spectrometry we outline here a method to simultaneously acquire 206 pb 238 u age and trace element profiles from u bearing accessory phases inversion of 206 pb 238 u age depth profiles yields thermal history information from an extended temperature range compared to inversion of age versus grain size relationships thermally activated volume diffusion of pb and zr in rutile is sensitive to the thermal evolution of the mid to lower lithosphere we document the ability of laser ablation depth profiling to simultaneously resolve 206 pb 238 u age and zr diffusion profiles in the outer 35 m of lower crustal rutile euhedra from the ivrea zone southern alps with 1 2 m depth resolution inversion of the age profiles reveals a continuous cooling history characterized by initially rapid cooling from 600 c at 180 ma followed by a period of slower cooling from 525 c to 450 c combined with the topology of zr diffusion profiles these data indicate that the ivrea zone underwent a brief thermal pulse in the early jurassic plausibly associated with hyperextension of the adriatic margin inversion of near surface 206 pb 238 u age distributions can be employed to resolve otherwise inaccessible thermal history information from the lower lithosphere

ultra high temperature metamorphism of mafic granulites from south altyn orogen west china a result from the rapid exhumation of deeply subducted continental crust

significant zr isotope variations in single zircon grains recording magma evolution history zircons widely occur in magmatic rocks and often display internal zonation finely recording the magmatic history here we presented in situ high precision 2sd

collisional snowbird tectonic zone resurrected growth of laurentia during the 1 9 ga accretionary phase of the hudsonian orogeny the 2800 km long snowbird tectonic zone is one of the most controversial tectonic features of the canadian shield metamorphic and in situ geochronologic data reported here reveal that a 1 9 ga medium to high pressure belt extends along most of this tectonic zone in contrast to recent interpretations a collisional origin is indicated by the length of this metamorphic belt tectonic thickening documented in parts of it geological contrasts across it and subduction type microdiamonds within it this collisional event marks a pre 1 865 ga phase of the hudsonian orogeny involving microcontinent accretion that was fundamental to the growth of laurentia

upb la mc icp ms dating of rutile new reference materials and applications to sedimentary provenance in response to the general lack of sufficiently abundant and high quality rutile usingle bondpb reference materials for in situ geochronology we have characterised two new potential rutile 1 8 ga reference materials sugluk 4 and pca s207 from granulite facies belts of the canadian shield namely the northern cape smith belt of quebec and the snowbird tectonic zone sasatchewan characterisation includes id tims and la icp ms usingle bondpb dating imaging and trace element analysis we compare these materials with existing rutiles used already r19 and r10 luvizotto et al 2009 and zack et al 2011 and show that the measured usingle bondpb compositions i e including any common pb of our rutiles are considerably more homogeneous this makes possible a usingle bondpb normalisation procedure not reliant upon a common pb correction that results in a significant decrease in the uncertainty contribution from the common pb correction and better reproducibility of reference materials and unknowns for provenance analysis and other applications the reproducibility is 2 4 2rsd for 206pb 238u and 207pb 206pb only slightly greater than long term data for zircon reference materials we show in a rutile provenance study from young orogens bhutan himalaya and canadian cordillera that the sensitivity of our analytical set up allows dating of 90 of rutiles in a sediment using a 50 m laser ablation spotsize within samples containing rutile as young as 10 20 ma and obviates the requirement for u concentration pre screening thus reducing or eliminating rutile selection bias unsuccessful analyses are due to poor quality rutiles with predominant common pb 207pb signal below detection or u content below 1 2 ppm we have used the 207pb method using the tera wasserburg diagram to correct for substantial common pb in very young and or very low u rutiles rather than developing an on line correction since rutile ages reflect mainly the time of cooling rutile is a sensitive recorder of metamorphic thermochronological information and therefore is an excellent complement to detrital zircon u207pb data the contrast between zircon and rutile signatures in himalayan samples with rutile as young as 10 ma is shown to be very dramatic most zircons from the same sample are 480 ma with only a few grains or metamorphic rims reflecting miocene metamorphism as such rutile provides complementary information about the thermal events within the source regions of the grains rutile usingle bondpb dating is an underexploited provenance method with wide applicability to sedimentary provenance studies

striding athabasca mylonite zone complex archean deep crustal deformation in the east athabasca mylonite triangle northern saskatchewan the geophysically defined snowbird tectonic zone is manifested in northernmost saskatchewan as a deep crustal multistage mylonitic structure the east athabasca mylonite triangle the triangle located at the northeastern apex of a stiff crustal scale lozenge is composed of mid archean annealed mylonites and late archean ribbon mylonites formed during two granulite facies events 850 1000 c 1 0 gpa the flow pattern in the mylonites is geometrically and kinematically complex and corresponds to that expected adjacent to the apex of a stiff elliptical volume subjected to subhorizontal regional extension parallel to its principal axis the late archean mylonites are divided into an upper structural deck entirely occupied by a dip slip shear zone and an underlying lower deck the latter is divided into two upright conjugate strike slip shear zones separated by a low strain septum which deformed by progressive coaxial flow the flow pattern in the mid archean mylonites is compatible with that of

the oldest uhp eclogites of the world age of uhp metamorphism nature of protoliths and tectonic implications coesite bearing eclogitic rocks from the pan african belt in northern mali were dated by multichronological methods rb sr sm nd and ar ar rb sr and sm nd isotope analyses on whole rock and mineral separates of an omphacite kyanite micaschist and a mafic eclogite yielded concordant ages of about 620 ma this is interpreted as the time of the eclogitisation which represents the oldest ultrahigh pressure uhp metamorphic event so far recorded in the continental crust the nearly identical rb sr and sm nd ages suggest very rapid exhumation as observed in many phanerozoic uhp rocks 40ar 39ar dating of phengites from two uhp micaschists gave much older ages of 1045 and 760 ma phengite from a quartzite collected from the same site yielded a plateau date of 623 3 ma which is in agreement with the rb sr and sm nd dates the two 40ar 39ar ages older than 625 ma testify once more the recurrent problem of excess ar in ultrahigh pressure phengites the occurrence of the mali uhp eclogites indicates that by the late precambrian the earth has cooled enough to sustain the formation and preservation of deeply subducted uhp metamorphic rocks the new ages of ca 620 ma also constrain the amalgamation in nw gondwana to have occurred in the latest proterozoic but not in the middle cambrian this is supported by several recent chronological data for the southern segment of the major suture zone in the dahomeyide orogenic belt

stabilization and reactivation of cratonic lithosphere from the lower crustal record in the western canadian shield new u pb geochronology for an extensive exposure of high pressure granulites in the east lake athabasca region of the western canadian shield is consistent with a history characterized by 2 55 ga stabilization of cratonic lithosphere 650 million years of lower crustal residence and cratonic stability and 1 9 ga reactivation of the craton during lithospheric attenuation and asthenospheric upwelling high precision single grain and fragment zircon data define distinctive discordia arrays between 2 55 and 1 9 ga u pb ion microprobe spot analyses yield a similar range of u pb dates with no obvious correlation between date and cathodoluminescence zonation we attribute the complex u pb zircon systematics to growth of the primary populations during a 2 55 ga high pressure granulite facies event 1 3 gpa 850 c recorded by the dominant mineral assemblage of the mafic granulite gneisses with subsequent zircon recrystallization and minor secondary zircon growth during a second high pressure granulite facies event 1 0 gpa 800 c at 1 9 ga the occurrence of two discrete granulite facies metamorphic events in the lower crust separated by an interval of 650 million years that included isobaric cooling for at least some of this time suggests that the rocks resided at lower crustal depths until 1 9 ga we infer that this phase of lower crustal residence and little tectonic activity is coincident with an extended period of cratonic stability detailed structural and thermochronological datasets indicate that multistage unroofing of the lower crustal rocks occurred in the following 200 million years extended lower crustal residence would logically be the history inferred for lower crust in most cratonic regions but the unusual aspect of the history in the east lake athabasca region is the subsequent lithospheric reactivation that initiated transport of the lower crust to the surface we suggest that a weakened strength profile related to the 1 9 ga heating left the lithosphere susceptible to far field tectonic stresses from bounding orogens that drove the lower crustal exhumation an ultimate return to cratonic stability is responsible for the preservation of this extensive lower crustal exposure since 1 7 ga

timescales and significance of high pressure high temperature metamorphism and mafic dike anatexis snowbird tectonic zone canada new geochronological isotopic and geochemical data for a spectacular swarm of deep crustal migmatitic mafic dikes offer important insight into processes operative during 1 9 ga high pressure high temperature metamorphism along the snowbird tectonic zone in northern saskatchewan high precision u pb zircon dates reveal anatexis of chipman mafic dikes at 1 896 2 0 3 ma during syntectonic and synmetamorphic intrusion at conditions of 1 0 1 2 gpa 750 c u pb zircon dates of 1 894 1 891 ma for cross cutting pegmatites place a lower bound on major metamorphism and deformation at the currently exposed crustal levels the persistence of elevated temperatures for 14 m y following peak conditions is implied by younger u pb titanite dates and by sm nd whole rock isotopic data that suggest the derivation of the pegmatites by melting of a mafic source limited melting of the host felsic gneiss at 1 9 ga despite high temperature is consistent with evidence for their previous dehydration by granulite facies metamorphism in the archean spatial heterogeneity in patterns of mafic dike and tonalitic gneiss anatexis can be attributed to lateral peak temperature and compositional variability we correlate 1 896 ma chipman mafic dike emplacement and metamorphism with substantial 1 9 ga mafic magmatism over a minimum along strike extent of 1 200 km of the snowbird tectonic zone this suggests a significant continent wide period of asthenospheric upwelling that induced incipient continental rifting extension was subsequently terminated by hinterland contraction associated with trans hudson accretion and orogenesis little activity in the lower crust for ca 650 m y prior to proterozoic metamorphism and mafic magmatism implies an extended interval of cratonic stability that was disrupted at 1 9 ga this episode of destabilization contrasts with the record of long term stability in most preserved cratons and is important for understanding the lithospheric characteristics and tectonic circumstances that control the destruction or survival of continents

stratigraphy and sedimentology of the paleoproterozoic thluicho lake group southwestern rae province canada alluvial basin development in the hinterland of the taltson orogen abstract the thluicho lake group is a middle paleoproterozoic succession of greenschist facies continental clastic rocks that was deposited on variably mylonitized crystalline basement rocks in the southwestern rae province of the canadian shield near the end of 2 02 1 92 ga thelon taltson orogenesis two informal formations are proposed for this fining upward succession the powder lake formation comprises predominantly coarse grained rocks including conglomerate gulo lake member pebbly sandstone camel lake member and sandstone wellington lake member whereas the overlying camsell portage formation consists of sandstone to siltstone falls member rhythmic sandstone and argillite waterloo lake member and argillite slate island member the two informal formations represent two principal facies associations a lower conglomerate sandstone facies association representing an alluvial fan to distal braided plain that underwent episodic flash flooding events and an upper sandstone siltstone muds

thermobarometric constraints on burial and exhumation of 2 billion year old eclogites and their metapelitic hosts abstract one of the first appearances of eclogite facies mineral assemblages in the geological record occurs in the c 2000 ma palaeoproterozoic usagaran belt in central tanzania where the extended margin of the tanzanian craton is interpreted to have been subducted mafic rocks are interpreted to have contained the mineral assemblage garnet omphacite rutile quartz amphibole this high pressure assemblage has been overprinted by a secondary mineral assemblage containing clinopyroxene plagioclase hornblende ilmenite orthopyroxene mineral equilibria forward modelling indicates that the eclogite facies assemblages reached minimum peak pressure temperature p t conditions of 17 kbar and 700 c inclusions in garnet document a prograde p t history consistent with burial through upper amphibolite facies conditions and possible partial melting petrological and compositional evidence from garnet suggests that following peak metamorphism the eclogite facies rocks were heated while stalled at approximate peak pressures temperature estimates derived from zr concentrations in interpreted texturally retrograde rutile support a near isothermal post peak p t evolution for the eclogite facies rocks an evolution that terminates at retrograde p t conditions of approximately 7 6 8 2 kbar and 680 790 c the relict eclogite domains form part of a larger assemblage with enclosing migmatitic metapelitic lithologies the isimani suite the metapelitic gneisses contain garnet kyanite biotite staurolite hornblende plagioclase muscovite rutile quartz and preserve minimal evidence of a high pressure history conceivably due to post peak mineralogical recrystallisation p t modelling inclusion assemblages and compositional zonation patterns in porphyroblastic garnet suggests the metapelitic gneisses similarly to the relict eclogites experienced burial to minimum peak pressures of approximately 16 5 17 kbar compositional zoning patterns in eclogitic garnet suggest the isimani system was buried reached peak metamorphic conditions and was subsequently exhumed within a timeframe of up to 20 myr a tectonic regime involving crustal thickening and subduction followed by extensional exhumation of the entire isimani suite is our preferred model for the development of the c 2000 ma usagaran belt

modest movements spectacular fabrics in an intracontinental deep crustal strike slip fault striding athabasca mylonite zone nw canadian shield geometry and strain partitioning within lower crustal intraplate strike slip shear zones can be extremely complex compared with analogous structural levels of interplate strike slip shear zones sited at plate margins striding athabasca mylonite zone canadian shield is a spectacular ca 500 km long granulite facies continental intraplate shear zone the shear zone is composed of middle archean granulite facies annealed mylonites ca 3 13 ga and late archean ca 2 62 2 60 ga granulite facies ribbon mylonite belts which thread a sinuous course along a chain of crustal scale lozenges cored by relatively stiff rocks of mafic to intermediate composition to the northeast the mylonites form a n s trending 5 10 km thick dextral strike slip belt to the southwest this bifurcates into a pair of conjugate strike slip shear zones overlain by a contemporaneous dip slip shear zone striding athabasca mylonite zone was kinematically inefficient as a strike slip fault and cannot have accommodated large wallrock displacements nevertheless spectacular granulite facies ribbon mylonites were formed throughout the shear zone reflecting the very high temperatures ca 850 1000 c high recrystallization rate strain rate ratios and the transpressive nature of the deformation wk 1 possibly accommodated by significant volume loss by magma migration

precambrian tectonics and the physicochemical evolution of the continental crust i the gabbro eclogite transition revisited abstract we report the first occurrence of retrograde eclogite shear zones cutting proterozoic granulites from australia the musgrave range granulites were metamorphosed at about 12 kbar pressure and 850 900 c the granulites then cooled isobarically and developed a variety of lower temperature mineral reaction coronas typical of garnet granulites these rocks are cut by a major shear zone the davenport shear in which lower temperature fine grained eclogite developed this demonstrates that mafic rocks in the deep crust can cool down into the thermodynamic stability field of eclogite without undergoing transformation to eclogite we agree with harte et al 1981 that rocks lacking a fluid phase in stable tectonic regions are unlikely to maintain local equilibrium at ambient geotherm conditions until depths well within the upper mantle are reached it is proposed that many isobaric high pressure granulites could have cooled down into the eclogite field before their later excavation to the surface and did not transform to an eclogite mineralogy the inability of the lower crust to transform into eclogite by retrograde reaction has important implications for the seismic petrologic and geochemical character of the lower crust the continental moho cannot be caused by the gabbro eclogite transition preservation of relatively low density high temperature granulite and gabbroic mineral assemblages in the lower continental crust prohibits major recycling of lower crust back into the mantle once it is removed from active plate boundaries however in island arcs with magmatically thickened crust mafic rocks at a depth of 60 to 70 km will be in the eclogite stability field at high enough temperatures 800 900 c for eclogite to rapidly form and sink back into the mantle this could result in a net accreted crust of broadly andesitic composition in contrast later growth of continental crust by basalt underplating in stabilised shield regions will not result in subsequent eclogite formation and lower crustal recycling back into the mantle will not occur thus estimates of the average composition of the continental crust which are based on a mass balance of plate margin basaltic input from the mantle will not be adequate

geochronological constraints on the legs lake shear zone with implications for regional exhumation of lower continental crust western churchill province canadian shield the legs lake shear zone marks the southeastern boundary of an extensive region 20 000 km2 of high pressure 0 8 1 5 gpa granulite facies rocks in the western churchill province canada the shear zone is one of the largest exhumation related structures in the canadian shield and coincides with the central segment of the 2 800 km long snowbird tectonic zone the movement history of this shear zone is critical for the development of models for the exhumation history of the high pressure region we used electron microprobe u th pb dating of monazite with supplemental id tims u pb geochronology to place constraints on the timing of shear zone activity combining these and other data we suggest that regional exhumation occurred during at least three distinct phases over an 150 million year period the first phase involved high temperature decompression from 1 0 to 0 8 0 7 gpa shortly following 1 9 ga peak metamorphism possibly under an extensional regime the second phase involved rock uplift and decompression of the hanging wall to 0 5 0 4 gpa during east vergent thrusting across the legs lake shear zone at ca 1 85 ga this phase was likely driven by early collision related convergence in the trans hudson orogen the final phase of regional exhumation involving the removal of 15 20 km of overburden from both footwall and hanging wall likely occurred after 1 78 ga and may have been related to regional extensional faulting

formation and exhumation of ultra high pressure rocks during continental collision role of detachment in the subduction channel 1 uhp rocks commonly form and exhume during the transition from oceanic subduction to continental collision their exhumation in subduction channels depends on the balance between down channel shear traction and up channel buoyancy thermal mechanical upper mantle scale numerical models are used to investigate how variations in material properties of the subducting continental margin affect this balance changes in shear traction leading to crustal decoupling detachment are investigated by varying the onset of strain weakening thermal parameters and convergence velocity variations in buoyancy force are investigated by modifying subducted material density and volume the model results are interpreted in terms of the exhumation number e which expresses the role of the pressure gradient channel thickness effective viscosity and subduction velocity peak metamorphic conditions exhumation velocity and timing of exhumation are temporally and spatially variable and are sensitive to the evolution of e the models reproduce natural ptt constraints and indicate that neither slab breakoff nor surface erosion is required for uhp exhumation

monazite u pb and th pb geochronology by ion microprobe with an application to in situ dating of an archean metasedimentary rock abstract monazite grains from nine different precambrian rock samples and having concordant isotope dilution u pb and th pb ages were used to develop and evaluate techniques for u pb th pb and pb pb isotopic dating using an ion microprobe operating at high mass resolution r 5500 1 an unidentified isobar at 203 960 amu complicated the determination of 204 pb in the monazites with higher th but it appeared to have been eliminated when using a slight energy offset the 207 pb 206 pb ratios of individual spots were determined in 10 min analyses to 0 3 0 5 1 of the true values using a 20 m wide o 2 primary beam whereas the uncertainty in the mean of several spots on a single generation of monazite was typically 0 1 0 4 2 instrumental bias in the measured pb uo and pb tho values of individual analyses was monitored using linear relationships between 206 pb uo vs uo 2 uo and 208 pb tho vs uo 2 uo the bias appears to vary somewhat with monazite th content requiring the use of compositionally matched standards individual spot measurements of the 206 pb 238 u and 208 pb 232 th ratios in unknown monazites of low to moderate th can be determined to 2 1 the dating technique was applied to in situ analysis of small monazite inclusions hosted in an amphibolite grade metapelite of archean origin both the contextual setting and the morphology of the monazite grains were found to be important factors in interpreting the timing of growth of the host metamorphic mineral e g garnet staurolite plagioclase euhedral monazite grains armoured in low cao garnet and matrix plagioclase yielded an age of 2548 17 ma interpreted as dating an early episode of low pressure metamorphism partially resorbed monazite within a high cao garnet rim yielded ages of ca 2500 ma but are interpreted as inherited from the first event rather than syn genetic with respect to the growth of this garnet an age of 1755 30 ma was obtained for anhedral monazite associated with late chlorite filled fractures apparently reflecting post tectonic retrogression

the cora lake shear zone athabasca granulite terrane an intraplate response to far field orogenic processes during the amalgamation of laurentia the cora lake shear zone clsz is a 4 6 km wide localized high strain zone that bisects the polydeformed athabasca granulite terrane northern saskatchewan it also coincides with the geophysical trace of the snowbird tectonic zone the clsz represents a major lithotectonic and thermobarometric discontinuity within the exposure of 20 000 km2 of high pressure granulites most rocks have a strong mineral lineation plunging moderately to the southwest the northwestern subdomain hangingwall is characterized by ca 2 6 ga plutonic rocks that contain an early subhorizontal gneissic layering ca 2 57 ga that was overprinted by large amplitude folds and a partitioned but pervasive axial planar dextral shear fabric at ca 1 9 ga thermobarometry suggests metamorphic conditions of 0 9 gpa and 750 c during both of the phases of tectonism the footwall is predominantly underlain by the ca 3 3 3 0 ga chipman tonalite layers of intercalated mafic and felsic granulite and the widespread 1 9 ga chipman

geochronological constraints on metamorphism magmatism and exhumation of deep crustal rocks of the kramanituar complex with implications for the paleoproterozoic evolution of the archean western churchill province canada u pb and sm nd geochronology establishes an important paleoproterozoic 1 9 ga history for the kramanituar complex located in the interior of the archean western churchill province and provides further insight into the snowbird tectonic zone a crustal scale 2000 km feature whose role during precambrian time remains controversial the kramanituar complex is a window of deep crustal rocks dominated by a granulite facies metagabbroic suite of 1902 ma age with minor supracrustal rocks and charnockite these yield peak equilibrium conditions of 12 15 kbar and 850 900 c which are bracketed between 1910 ma the age of prograde metamorphic monazite in sillimanite and kyanite bearing paragneiss and 1901 ma the cooling ages of titanite and rutile in gabbroic and paragneissic rocks although mineral assemblages reflecting peak metamorphic conditions are widespread some rocks record near isothermal decompression to 8 kbar and 800 c the timing of uplift and exhumation is tightly bracketed between magmatic crystallization of gabbroic anorthosite under granulite facies conditions at 1902 ma and widespread rutile cooling ages of 1901 ma time averaged cooling rates of 100 c ma are estimated for gabbroic anorthosite and leucogranite rocks surrounding the kramanituar complex are mainly amphibolite facies archean plutonic and supracrustal rocks those to the north were at deep crustal conditions coeval with the complex whereas those to the south appear to preserve a record of older mainly neoarchean tectonometamorphic assemblages and fabrics the southern boundary of the complex is inferred to be a normal fault that accommodated tectonic unroofing of the kramanituar complex at 1 9 ga this geochronological data set highlights the protracted history of a segment of the geophysically defined snowbird tectonic zone a structure along which latest significant magmatism and tectonometamorphism has been interpreted as either 2600 or 1800 ma penetrative 1900 ma activity in the east trending chesterfield segment as documented at the kramanituar complex suggests that the complex may have represented a favorably oriented segment of a crustal scale archean fault that was reactivated during paleoproterozoic time possibly in response to collision between the slave and churchill cratons thelon orogen at 1 97 1 9 ga

deep subduction and rapid exhumation role of crustal strength and strain weakening in continental subduction and ultrahigh pressure rock exhumation 1 the exhumation of crustal ultra high pressure uhp material depends on temporal and spatial variations in its detachment within the subduction channel this dependence is investigated using numerical models with variable initial crustal strengths representing a range of initial crustal compositions and parameterized strain weakening representing a range of processes that reduce effective crustal viscosity during deformation competition between down channel shear traction favoring subduction and up channel buoyancy favoring exhumation is expressed as the exhumation number e which can vary with time and position along the channel exhumed lower strength crust which resists subduction owing to weak down channel traction records peak conditions 38 kbar given sufficient strain weakening exhumation proceeds at 60 km ma 1 indicating that buoyancy e 1 drives exhumation in these models in all models exhuming uhp material forms a deforming ductile plume with a range of possible structural relationships predicted between exhumed uhp and hp materials

the significance of u pb zircon dates in lower crustal xenoliths from the southwestern margin of the kaapvaal craton southern africa the combination of high precision id tims u pb geochronology with cathodoluminescence cl imaging and spatially resolved microsampling of zircon provides a high fidelity record of the temporal evolution of samples of the middle and lower crust a suite of xenoliths exhumed in cretaceous kimberlites at the southwestern margin of the kaapvaal craton including eclogite granulite amphibolite and greenschist facies plutonic rocks are the subject of these detailed zircon studies cl imaging demonstrates the complexity of chemical and growth structure in single zircons and highlights the importance of sampling on an intragrain scale to extract end member ages for zircon growth events for these diverse metamorphic rocks linking the zircon chronology to geologic events and processes requires an understanding of the kinetics and thermodynamics of zircon stability during metamorphism using an empirical approach we attempt to relate zircon crystallization dissolution and regrowth to metamorphic conditions through a detailed analysis of zircon chemical zonation patterns mineral inclusions and petrography these lines of evidence indicate the crystallization of zircon under p t conditions ranging from eclogite through greenschist facies we discuss four examples of this approach 1 at the lowest grade of metamorphism examined primary igneous zircon 1900 ma in a syenite xenolith from the klipfontein 08 kimberlite underwent two episodes 1015 and 964 ma of probable fluid mediated resorption and reprecipitation in the greenschist facies 2 an upper amphibolite facies xenolith from the markt kimberlite contains two distinct populations of blocky and fir tree sector zoned zircon which are suggested to date prograde hornblende breakdown reactions 1107 ma and melt infiltration under retrograde metamorphic conditions 1092 ma respectively 3 at granulite facies metamorphic zircon crystallized at 1114 ma during prograde reactions involving hornblende breakdown in a granulite xenolith also from the markt kimberlite later 960 ma zircon resorption followed by minor secondary crystallization occurred in both the amphibolite and granulite xenoliths during retrograde greenschist facies reactions possibly enhanced by fluids 4 in a much older episode of eclogite facies metamorphism recorded in a xenolith from the lovedale kimberlite zircon crystallized with the peak metamorphic mineral assemblage or possibly early on the cooling path in the late archean 2630 ma each of these examples provides unique information regarding the evolution of the kaapvaal craton while the links to metamorphic reactions and p t conditions clarify the interpretation of zircon dates obtained from these samples of the middle to lower crust

thermobarometry and geochronology of the uvauk complex a polymetamorphic neoarchean and paleoproterozoic segment of the snowbird tectonic zone nunavut canada the uvauk complex is an ultramylonite bounded granulite facies suite of anorthosite gabbro that forms part of the chesterfield inlet segment of the snowbird tectonic zone following initial anorthosite gabbro magmatism at ca 2 71 ga and a cryptic 2 62 2 60 ga event marked by zircon and monazite growth the uvauk complex experienced two high grade tectonometamorphic events at 2 56 2 50 and 1 91 1 90 ga similar to the 2 56 2 50 ga development of other shear zones in the region the upper amphibolite facies to granulite facies moderately high pressure 8 4 11 0 kbar and 705 760 c 1 kbar 100 mpa m1 event is interpreted to have involved the structural emplacement of ca 2 71 ga uvauk complex rocks on ca 2 68 ga tonalitic rocks to the south granulite facies high pressure 11 2 14 7 kbar and 695 865 c m2 metamorphism gabbroic magmatism and mylonite development within the complex at ca 1 9 ga culminated with 3 5 kbar decompression at high temperature clockwise pressure temperature time p t t

reos and upb geochronological constraints on the eclogite tonalite connection in the archean man shield west africa abstract mantle derived eclogite xenoliths and tonalite trondhjemite granodiorites ttg that occur in the man shield west africa sample different levels of archean lithosphere chemical and oxygen isotope systematics indicate that low mgo eclogites from the koidu kimberlite are ancient remnants of subducted oceanic crust that may have been involved in regional archean crust formation re os whole rock isotopic data for these eclogites scatter about a line with slope corresponding to an archean age of 3 44 0 76 ga 2 with re os model ages ranging from 1 4 to 3 9 ga this wide range of model ages overlaps with the age range for crust formation and metamorphism in the man shield in situ u pb ages of zircons from crustal rocks have been measured by laser ablation icp ms a tonalitic gneiss has discordant zircons with rare old cores 3 6 ga and an upper concordia intercept at 2890 9 ma 2 zircons from a mafic lower crustal granulite xenolith are concordant at 2686 32 ma our results together with previously published ages for man shield rocks indicate an early archean crust formation event followed by major crustal growth at 2 9 3 0 ga and a last major metamorphic event at 2 7 ga these data show that the eclogites and the continental crust of the west african craton overlap in time of formation but only at the very broad age uncertainty provided by the eclogite re os results they are permissive of archean crustal growth by melting of the protoliths of the materials now sampled as the koidu eclogite xenoliths if so this suggests that archean crustal growth in the man shield occurred in a convergent margin setting

eclogites with a short lived granulite facies overprint in the moldanubian zone czech republic petrology geochemistry and diffusion modelling of garnet zoning the petrology and geochemistry of a newly discovered suite of high pressure garnet clinopyroxene bearing rocks from the monotonous series of the moldanubian zone of the bohemian massif southwest czech republic have been investigated three types common eclogites quartz kyanite clino zoisite eclogites and garnet hornblende clinopyroxenites are distinguished by petrography and geochemistry all underwent a significant degree of partial breakdown under granulite and amphibolite facies conditions during exhumation important features include the growth of orthopyroxene in breakdown domains after garnet and omphacite and anorthite spinel corundum exceedingly peraluminous sapphirine replacing kyanite garnet zoning and inclusion patterns support a prograde evolution from low pressures for at least some of the samples the post eclogite stage granulite facies overprint indicates that high temperatures prevailed during exhumation but preservation of zoning in some garnets and the results of diffusion modelling suggest that this overprint took place over a very short time scale the geochemical and petrological results allow characteristic differences to be recognized between these eclogites and metabasites found in other tectonic units of south bohemia and consequently the assigning of all high pressure rocks to a single now disrupted tectonic unit is a gross simplification that seriously misrepresents the tectono metamorphic history of the region

partially retrograded eclogites of the munchberg massif germany records of a multi stage variscan uplift history in the bohemian massif eclogites with a wide range in bulk composition are present in the munchberg massif part of the variscan basement of the bohemian massif in north east bavaria new analyses of the primary phases garnet omphacite phengite and amphibole as well as the secondary phases clinopyroxene ii various amphiboles biotite phlogopite plagioclase margarite paragonite prehnite and pumpellyite reveal a complex uplift history new discoveries were made of samples with very jadeite rich primary omphacite as well as a secondary omphacite in a symplectite with albite various geothermobarometric techniques together with thermodynamic databases incorporating separately determined activity composition values and experimental data have clustered the minimum conditions for the primary assemblages to the p t range 650 60 c 14 3 1 kbar however jadeite in omphacite kyanite paragonite in phengite and zoisite grossular in garnet kyanite quartz relationships suggests pressures of 25 28 kbar at the same temperatures the fact that the secondary omphacite plagioclase assemblage yields pressures within a few hundred bars of the minimum pressures for the plagioclase free assemblages strongly suggests that the minimum values are serious underestimates zoning inclusion suites and breakdown reactions of primary phases in addition to new minerals formed during uplift define a polyphase metamorphic evolution which from geochronological evidence occurred solely within the variscan cycle the complex breakdown in other bohemian massif eclogites and the distinct variation in their temperatures during uplift suggest a multi stage thrusting model for the regional evolution of the eclogites such an evolution has significance with respect to incorporation of mantle slices into crustal sequences and fluid derivation from successively subducted units possibly driving the breakdown reactions

eclogitized gabbros from the eastern grenville province textures metamorphic context and implications undeformed lenses of olivine orthopyroxene gabbro belonging to the 1429 ma shabogamo intrusive suite parautochthonous belt of the eastern grenville province host the first occurrence of eclogite mineral assemblages reported from the grenville province these rocks preserve igneous textures overprinted by eclogite mineralogy and by textures related to subsequent unloading high pressure signatures include omphacite pseudomorphs after augite plagioclase replacement by garnet kyanite corundum and orthopyroxene omphacite garnet coronas between olivine and plagioclase transformations related to decompression include replacement of garnet kyanite corundum by spinel plagioclase growth of plagioclase collars between clinopyroxene and garnet and de jadeitization of omphacite clinopyroxenes with numerous plagioclase inclusions and garnets surrounded by clinopyroxene plagioclase symplectites in granoblastic mafic rocks from the same area are also evidence of decompression from high pressures these

neoarchean arc magmatism and subsequent collisional orogenesis along the eastern rae domain western churchill province implications for the early growth of laurentia abstract the athabasca granulite terrane on the eastern margin of the rae domain of the western churchill province contains a well exposed record of neoarchean magmatism deformation and metamorphism circa 2 63 2 60 ga plutonic rocks range from 58 01 to 73 71 sio2 and vary from ferroan to magnesian they form shallow linear trends on modified alkali lime index diagram plotting largely in the calc alkaline field on tectonic discrimination diagrams all samples plot in the volcanic arc field samples are lree enriched and contain relatively steep negative slopes and flat depleted hree profiles when normalized to chondrites the entire neoarchean plutonic complex independent of lithology contains strong enrichments in the liles and pb and depletions in hfses monazite u th total pb results from a migmatitic paragneiss suggest that partial melting occurred at ca 2 57 ga within the chipman subdomain monazite results from the adjacent northwestern subdomain suggest that deformation occurred at ca 2 57 ga in sims and icp ms zircon u pb geochronology corroborates these conclusions although the region is dominated by paleoproterozoic localized shear zones internal to each subdomain is evidence of a widespread tectonothermal event during the neoarchean that involved arc plutonism crustal thickening lower crustal flow at pressures exceeding 1 0 gpa and extensive partial melting of the lower continental crust we interpret this sequence of magmatic deformational and anatectic processes to reflect the transition from a continental arc to a collisional tectonic setting the recognized series of events presented herein is likely correlative to the 2 60 2 50 ga macquoid orogen which has been recognized further to the northeast and likely parallels the eastern margin of the rae domain of the western churchill province these data support an important component of modern style plate tectonics during continental magmatism and accretion in the neoarchean

variable p t paths and hp uhp metamorphism in a precambrian terrane gridino russia petrological evidence and geodynamic implications abstract the gridino complex represents one of the oldest eclogite facies terranes on earth it consists of blocks boudins and lenses of eclogites pyroxenites and epidosites as well as deformed eclogitized dikes within biotite amphibole gneisses detailed petrological studies of the pyroxenites and different types of eclogites reveal considerable diversity in metamorphic pressure p temperature t conditions from 1 3 gpa at 660 c to 3 0 gpa at 660 c and fluid regimes wet vs dry experienced by these rocks dike related rocks escaped prograde metamorphism and reached higher pressures than the lenses and blocks that experienced considerable prograde metamorphic reworking the variability in p t conditions and the shapes of p t paths are in agreement with the results of thermomechanical modeling and data from u hp metamorphic rocks exhumed during continent continent collision in the phanerozoic the t p ratio estimated for an eclogitized dike from eclogitovii island of the gridino complex corresponds to the gradients of

contractional uplift of deep crustal rocks along the legs lake shear zone western churchill province canadian shield the legs lake shear zone juxtaposes high pressure 1 0 gpa granulite and eclogite facies rocks with low pressure 0 5 gpa amphibolite to granulite facies rocks and thus may represent an important crustal scale exhumation related structure in the western canadian shield field mapping and structural and petrologic analysis document the deformation and metamorphic history of rocks within and adjacent to the shear zone at least two important phases of deformation are recorded 1 early oblique thrusting d2 that placed high grade rocks over lower grade rocks and 2 more discrete and lower grade brittle ductile normal faulting d3 that may represent the later part of the exhumation history the northwest dipping shear zone consists of 5 8 km of mylonite in map view which bounds the southeast margin of the east athabasca mylonite triangle high pressure granulite facies metamorphism 750 850 c 1 0 1 2 gpa occurred in the east athabasca mylonite triangle at ca 1900 ma prior to d2 juxtaposit

petrophysical constraints on magnetic anomalies associated with metamorphic reactions in northern saskatchewan canada1 the western chipman domain of the east athabasca mylonite triangle in northern saskatchewan canada displays a large positive aeromagnetic anomaly that is the result of retrograde magnetite produc

reconstruction of a large deep crustal terrane implications for the snowbird tectonic zone and early growth of laurentia the 2800 km long snowbird tectonic zone is a well recognized but still enigmatic feature in the western canadian shield it has been interpreted as a paleoproterozoic continental suture or an archean strike slip fault system but here we suggest that the distinctive geometry of the central snowbird tectonic zone is primarily due to the interaction of crosscutting paleoproterozoic intracontinental thrust and strike slip shear zones having a length of hundreds of kilometers first a major zone of thrust sense shearing coeval with early continent continent collision between the superior and western churchill provinces accommodated uplift of a large exposure of granulite facies lower continental crust younger strike slip shear zones perhaps analogous to asian fault systems behind the himalayan orogen offset the thrust zone thus the current geometry and distribution of deep crustal rocks in this region represent a relatively late stage in the tectonic evolution of the western churchill province rather than an accretionary one earlier structures oriented at a high angle to the snowbird tectonic zone may record the fundamental accretionary history in this part of laurentia

multistage exhumation and juxtaposition of lower continental crust in the western canadian shield linking high resolution u pb and 40ar 39ar thermochronometry with pressure temperature deformation paths 1 new u pb titanite apatite rutile and 40ar 39ar hornblende muscovite biotite data are linked with existing pressure temperature deformation paths to impose detailed temporal constraints on the juxtaposition and unroofing of domains in an extensive 20 000 km2 region of exhumed lower crust in the east lake athabasca region of the western canadian shield in the 200 m y between circa 1 9 ga high pressure granulite facies metamorphism and circa 1 7 ga unconformable deposition of athabasca basin sediments on the exhumed rocks our analysis reveals at least three distinct phases of unroofing at different rates specifically we distinguish 1 an early phase of extensional unroofing in the chipman domain at rates of 1 5 2 0 km m y from 1 0 gpa to 0 8 gpa associated with mafic magmatism and metamorphism 2 an episode of regional contractional uplift and associated unroofing along the legs lake shear zone at circa 1850 ma from 0 7 0 8 gpa to 0 4 0 5 gpa and 3 a final period of extensional unroofing at rates of 0 2 0 3 km m y from 0 4 to 0 5 gpa that culminated in transport of current exposures to near surface conditions the cooling patterns and retrograde assemblages are consistent with pauses between exhumational phases the apparent convergence of disparate higher temperature histories in several deep crustal domains at circa 1 89 1 88 ga implies their juxtaposition at 0 7 0 8 gpa conditions regional east directed thrusting of the deep crustal domains as a coherent unit at 1 85 ga juxtaposed the granulites with middle crustal hearne domain rocks at 0 4 0 5 gpa conditions the detailed exhumational history allows correlation with changing regional tectonic regimes associated with the amalgamation of laurentia the temporal and spatial heterogeneity of exhumation patterns in the east lake athabasca region may be a common feature of the unroofing histories of lower crustal rocks

eclogite facies relics and a multistage breakdown in metabasites of the ktb pilot hole ne bavaria implications for the variscan tectonometamorphic evolution of the nw bohemian massif complex reaction textures in coronitic metagabbros and retrograded eclogites of the ktb pilot and an adjacent drilling provide evidence for a multistage metamorphic history in the variscan basement of the nw bohemian massif the eclogites show complete metamorphic recrystallization leaving no textural or mineral relics of their igneous precursors in contrast textural relics of the igneous protolith are still preserved in the metagabbros where the metamorphic overprint under high pressure conditions achieved only partial replacement of the initial assemblage plagioclase augite amphibole olivine or orthopyroxene ilmenite to form the eclogite facies assemblage garnet omphacite kyanite zoisite quartz rutile the garnets in the metagabbros occur in the typical necklace fashion at the borders between the original plagioclase and mafic phase domains in the same rocks omphacite formed by a topotactic reaction mechanism replacing igneous augite as well as in smaller grains at the margins of the texturally igneous clinopyroxene where it occurs without fixed orientation with respect to the relict phase both eclogites and metagabbros show a partial breakdown under high pressure granulite transitional to high pressure amphibolite facies conditions during which omphacite broke down to vermicular symplectites of diopside plagioclase a later pervasive medium pressure metamorphism under amphibolite facies conditions led to the development of assemblages dominated by hornblende plagioclase titanite phases prevailing in the overwhelming majority of the surrounding metabasites subsequent vein associated retrogression produced minerals typical of the greenschist to zeolite facies all metamorphic stages may be represented in a single thin section but although the overall reaction sequence is apparent the obvious disequilibrium in the rocks makes the use of conventional geothermobarometry difficult however calculations made by assuming an approach to domainal equilibrium show that both the eclogite facies and early breakdown occurred above 10 kb as the metamorphic unit hosting these particular metabasites is generally characterized by pressures below 10 kb these results have important implications for understanding the tectonometamorphic evolution of the region the relationship between the studied rocks and other units in the nw bohemian massif exhibiting a multistage metamorphic evolution is discussed and possible tectonic models evaluated

revealing hidden structures the application of cathodoluminescence and back scattered electron imaging to dating zircons from lower crustal xenoliths abstract the use of cathodoluminescence cl and or back scattered electron bse imaging techniques with in situ ion probe analyses of zircons can help unravel complex crustal histories of metamorphic rocks that otherwise might remain elusive using these techniques we have imaged zircons from three lower crustal xenolith suites that have previously been dated by shrimp sensitive high resolution ion microprobe in all three cases the zircons are featureless in transmitted light but cl and bse reveal internal structures that correlate with distinct growth events generally cl and bse images reveal similar structures with cl showing finer detail neither imaging technique is capable of delineating all growth features in every sample and the best results are obtained using a combination of the two techniques igneous cores in zircons commonly emit a different color cl emission in many cases zoning in the cores is truncated indicating that the zircons either spent time in the supracrustal environment after their initial crystallization and prior to the granulite facies event s or that part of the core zircon was resorbed during the subsequent metamorphic event metamorphic rims when present are commonly 10 to 30 m thick and are nearly always unzoned and featureless igneous cores and metamorphic overgrowths commonly have distinctive cl emission spectra and trace element concentrations however the cl spectra can only be used to qualify chemical differences as a linear relationship has not been shown to exist between cl intensity and trace element concentration in natural zircons in many cases the hf y p u and the heavy rare earth elements hrees concentrations can be correlated to igneous and metamorphic growth using a combination of cl and bse imaging techniques and in situ trace element analyses with either the electron microprobe or pixe particle induced x ray emission

archean crust as revealed in the kapuskasing uplift superior province canada in the central superior province of the canadian shield a 120 km wide transition from the low grade michipicoten greenstone belt to the high grade kapuskasing structural zone represents an oblique section through some 20 km of archean crust uplifted along a northwest dipping thrust fault the restored vertical section through the upper and middle crust consists of three megalayers with undulating boundaries 1 0 to 25 km a high grade heterogeneous gneissic assemblage in part older than the upper supracrustal succession

zirconium in rutile speedometry new constraints on lower crustal cooling rates and residence temperatures abstract the incorporation of zirconium into the mineral rutile tio2 has been both empirically and experimentally calibrated as a measure of rutile crystallization temperatures watson et al 2006 this temperature sensitive system has been employed as a geothermometer with applications to a number of different geologic settings and rock types experimentally measured kinetics for zr diffusion in rutile cherniak et al 2007 indicate that zr can be lost to temperature dependent diffusion warranting further investigation of the geologic significance of calculated temperatures coupling diffusion kinetics with numerical solutions to the diffusion equation provides a means to forward model the time and temperature dependency of the system modeled results indicate a strong dependency of zr concentration in rutile on both 1 initial cooling rate following high temperature metamorphism crystallization and 2 temperature and duration of long term geologic residence zr concentrations measured in rutile from lower crustal xenoliths that resided at 25 45 km depths for 2000 my reveal zr concentrations in the approximate grain center that are consistent with temperatures measured by independent thermometers forward models for zr diffusion show that preserving a zr record of these initial temperatures in the center of a rutile crystal with a 50 m radius requires rapid cooling 300 c ma from magmatic metamorphic temperatures followed by a long term residence 2000 my at temperatures

structural petrological and u pb shrimp geochronological study of the western beaverlodge domain implications for crustal architecture multi stage orogenesis and the extent of the taltson orogen in the sw rae craton canadian shield abstract the beaverlodge domain is a polydeformed precambrian terrane in the southwestern rae craton whose rocks variably record the effects of archean tectonism and four major paleoproterozoic tectonic events the 2 5 2 3 ga arrowsmith orogeny the 1 99 1 93 ga taltson orogeny the ca 1 90 ga snowbird orogeny and the ca 1 87 1 80 ga hudsonian orogeny structural metamorphic analysis and in situ u pb shrimp zircon and monazite geochronology reported herein demonstrate that this domain consists of two main structural levels variably exposed due to complex interference folding a lower level comprising mainly high grade archean ca 3 0 and 2 7 2 6 ga metaplutonic and metasedimentary gneisses and a higher level comprising mainly ca 2 33 to 1 2 that led to development of the composite ese trending s 0 s 1 s 2 transposition foliation followed by more open folding d 3 the absence of taltson age plutonic rocks except for low volume crustal melts suggests that this cycle was driven by crustal thickening consistent with a clockwise p t t path derived for one sample the high degree of strain along the contact between archean rocks and the murmac bay group along with local structural discordance indicates some degree of translation of cover rocks over basement during this stage pelitic rocks at higher structural levels low to medium grade zone underwent prograde metamorphism to middle amphibolite facies whereas rocks at deeper structural levels high grade zone attained upper amphibolite facies conditions monazite data bracket prograde metamorphism and related garnet growth between 1939 6 ma and 1934 5 ma the second stage dated most precisely at 1906 7 ma involved refolding of d 1 to d 3 structures about ne trending axes f 4 and coeval dextral shearing and is correlated with snowbird orogenesis rocks of the low to medium grade zone were held at roughly the same crustal level whereas rocks in the high grade zone were uplifted via dextral oblique displacement along domain bounding shear zones instigating decompression garnet to cordierite reactions this study demonstrates that after

paleoproterozoic amphibolite facies retrogression and exhumation of archean metapelitic granulites in the southern marginal zone of the limpopo belt south africa abstract granulite facies terrains that have undergone extensive retrogression provide insights into the recycling of volatiles in the crust during orogenesis the timing of retrogression is key to understanding the geodynamic circumstances of such events this study has investigated amphibolite facies retrogression of melt depleted granulite facies metapelites from the south marginal zone smz of the limpopo belt the granulites formed via incongruent partial melting reactions that consumed biotite and produced garnet and rutile as prominent peritectic products at 2 7 ga under conditions of 11 kbar and 875 c peak metamorphic garnet is replaced by cordierite orthopyroxene symplectites with cordierite subsequently partially replaced by fine grained intergrowths of amphibolite facies assemblages including rutile zr in rutile thermometry yielded a temperature range of 731 846 c for the granulite facies rutiles and 629 690 c for rutile in the retrograde assemblage all rutile generations yielded a youngest u pb age of 2 1 ga some granulite facies rutiles record older dates 2 55 2 25 ga which are non systematic with regard to crystal size and represent non reset relicts of metamorphic recrystallization during a potentially complex history of amphibolite facies metamorphism between 2 7 and 2 2 ga the youngest 2 1 ga rutile u pb age represents the time of final exhumation related cooling of the rocks following a discrete paleoproterozoic amphibolite facies retrograde event coexisting orthoamphiboles in the peak metamorphic assemblage indicate a maximum peak temperature of 625 c the lack of age size correlation and intragrain age zoning in rutile suggest fast cooling following the amphibolite facies retrograde event which is inferred to be not much older u pb dating of zircon from the same rocks confirms that the metapelites underwent granulite facies metamorphism at 2695 35 ma thus the metapelites of the smz record both an archean granulite facies event and a polymetamorphic paleoproterozoic amphibolite facies history that culminated in a mid amphibolite facies peak inducing partial retrogression of the granulites the paleoproterozoic amphibolite facies event is tentatively correlated with mantle melting documented between 2 3 and 2 1 ga to produce mafic and intermediate volcanic rocks in the pretoria group on the kaapvaal craton

using calculated chemical potential relationships to account for replacement of kyanite by symplectite in high pressure granulites calculated mineral equilibria are used to account for the formation of sapphirine plagioclase spinel plagioclase and corundum plagioclase symplectites replacing kyanite in quartz plagioclase garnet kyanite granulite facies gneisses from the southern domain of the athabasca granulite terrane a segment of the snowbird tectonic zone in northern saskatchewan canada metamorphic conditions of 14 kbar and 800 c are established for the high pressure garnet kyanite assemblage using constraints from p t pseudosections and zr in rutile thermometry replacement of kyanite by symplectites reflects the reaction of kyanite with the matrix following near isothermal decompression to 10 kbar the chemical potential gradients developed between the kyanite and the matrix led to diffusion that attempted to flatten the gradients kyanite persisting as a stable phase while it is consumed by symplectite from its edge in this local equilibrium model the mineral and mineral compositional spatial relationships are shown to correspond to paths in na2o cao k2o feo mgo in the model chemical system na2o cao k2o feo mgo al2o3 sio2 nckfmas with sio2 and al2o3 taken to be completely immobile the values of na2o and cao are constrained by fixing p t conditions and choosing appropriate na2o and cao values that correspond to the observed plagioclase compositions feo mgo diagrams show the corresponding spatial relationships with kyanite and the symplectite phases these results demonstrate that the replacement of kyanite by sapphirine plagioclase and spinel plagioclase appears to be metastable with respect to replacement by corundum plagioclase replacement by corundum plagioclase does also occur apparently overprinting pre existing symplectite and also kyanite ignoring corundum the resulting diagrams account for the spatial relationships and compositions observed in the spinel plagioclase and sapphirine plagioclase symplectites they are predicted to occur over both a wide range of p t conditions 6 11 kbar 650 850 c and plagioclase compositions xan 0 5 0 9 the wide range of p t conditions that may result in identical spatial and compositional relationships suggests that such reaction textures may be of limited use in accurately quantifying the p t conditions of retrograde metamorphism

timing of high pressure metamorphism in the yukon tanana terrane canadian cordillera constraints from u pb zircon dating of eclogite from the teslin tectonic zone zircon from eclogite near last peak in the teslin tectonic zone yielded a u pb isotopic age of 269 2 ma 2 the first precise age for such a rock in the yukon tanana terrane of the canadian cordillera both the morphology and geochemistry of the eclogitic zircons indicate a metamorphic origin and the u pb age therefore constrains the timing of peak high pressure metamorphism in this rock the u pb age demonstrates for the first time that an early permian high pressure metamorphic event occurred in rocks now making up the teslin tectonic zone and possibly elsewhere in the yukon tanana terrane this u pb age provides a new geochronologic pin in the evolution of the yukon tanana terrane prior to its mesozoic accretion to the north american continental margin and combined with recent 40ar 39ar muscovite data indicates that high pressure metamorphism at this time was a relatively short lived event

contrasts in sillimanite deformation in felsic tectonites from anhydrous granulite and hydrous amphibolite facies shear zones western canadian shield abstract the deformation behavior of crustal materials in variably hydrated metamorphic environments can significantly influence the rheological and seismic properties of continental crust optical observations and electron backscatter diffraction ebsd analyses are used to characterize sillimanite deformation behavior in felsic tectonites from two deformation settings in the athabasca granulite terrane western canadian shield under estimated conditions of 0 8 1 0 gpa 725 850 c in the cora lake shear zone the data suggest that sillimanite deformed by dislocation creep with slip in the 001 direction accompanied by subgrain rotation recrystallization where sillimanite locally remained undeformed strain was concentrated in surrounding weaker phases under hydrated conditions of 0 4 0 6 gpa 550 650 c in the grease river shear zone textures and cathodoluminescence imaging point to dissolution precipitation creep as the major deformation mechanism for sillimanite resulting in synkinematic growth of foliation parallel euhedral sillimanite in a preferred orientation with 001 parallel to the lineation the results suggest that temperature fluid content and modal mineralogy of the surrounding phases may all have significant influence on sillimanite deformation but that preferential alignment of sillimanite 001 parallel to the lineation persists regardless of contrasts in the conditions or mechanisms of deformation

geochronological constraints on the evolution of high pressure felsic granulites from an integrated electron microprobe and id tims geochemical study abstract a combined geochronological geochemical and nd isotopic study of felsic high pressure granulites from the snowbird tectonic zone northern saskatchewan canada has been carried out through the application of integrated electron microprobe and isotope dilution thermal ionization mass spectrometry id tims techniques the terrane investigated is a 400 km 2 domain of garnet kyanite k feldspar bearing quartzofeldspathic gneisses monazite in these granulites preserves a complex growth history from 2 6 to 1 9 ga with well armored high y and th grains included in garnet yielding the oldest u pb dates at 2 62 to 2 59 ga in contrast matrix grains and inclusions in garnet rims that are not well armored are depleted in y and th and display more complicated u pb systematics with multiple age domains ranging from 2 5 to 2 0 ga 1 9 ga monazite occurs exclusively as matrix grains zircon is typically younger 2 58 to 2 55 ga than the oldest monazite sm nd isotope analysis of single monazite grains and whole rock samples indicate that inclusions of archean monazite in garnet are similar in isotopic composition to the whole rock signature with a limited range of slightly negative initial nd in contrast grains that contain a paleoproterozoic component show more positive initial nd most simply interpreted as reflecting derivation from a source involving consumption of garnet and general depletion of hree s our preferred interpretation is that the oldest monazite dates record igneous crystallization of the protolith the ca 2 55 ga dates in zircon and monazite record an extensive melting event during which garnet and ternary feldspar formed very high pressure 1 5 gpa metamorphism during the paleoproterozoic at 1 9 ga produced kyanite from garnet breakdown and resulted in limited growth of new monazite and zircon in the case of monazite this is likely due to the armoring and sequestration of early formed monazite such that it could not participate in metamorphic reactions during the high pressure event as well as the depletion of the ree s due to melt loss following the early melting event

multiphase inclusions in peritectic garnet from granulites of the athabasca granulite terrane canada evidence of carbon recycling during neoarchean crustal melting abstract in the last decade the study of fluid and melt inclusions in partially melted rocks has become a key tool to acquire unprecedented information about crustal anatectic processes in this study we report the results of the microstructural and microchemical investigation on multiphase inclusions trapped within peritectic garnet of a neoarchean felsic granulite from the upper deck domain of the athabasca granulite terrane canada inclusions have been studied by sem eds fib sem serial sectioning and raman microspectroscopy and classified in terms of size and hosted phases type i multiphase inclusions are small 15 m primary in origin and do not show evidence of decrepitation their multiphase assemblage is made of ferroan magnesite quartz and graphite in association with minor amounts of corundum pyrophyllite and zn spinel calcite dolomite and zinc bearing sulphide may also be present the coexistence of quartz and corundum in these inclusions is interpreted as the product of metastable growth within pores of extremely small size the fluid phase of type i inclusions always present in amounts 40 vol is co2 rich 96 5 mol with traces of n2 3 3 mol and ch4 0 2 mol these carbon rich type i inclusions coexist in the same cluster with primary melt inclusions nanogranitoids type ii these are large up to 50 m and composed of k feldspar quartz and plagioclase with minor amounts of graphite biotite and aluminosilicate because nanogranitoids are droplets of anatectic silicate melt formed and trapped during incongruent melting of crustal rocks the coexistence of type i multiphase inclusions proves the presence of a carbon rich fluid during the neoarchean anatexis 800 950 c 0 6 1 4 gpa of this portion of continental crust in a likely situation of melt fluid immiscibility according to phase equilibria modelling the uncommon multiphase assemblage within type i inclusions is here interpreted as the result of a post entrapment carbonation reaction between an original co2 bearing fluid and the garnet host during rock cooling from uht conditions

magnetotelluric investigations of the lithosphere beneath the central rae craton mainland nunavut canada new magnetotelluric soundings at 64 locations throughout the central rae craton on mainland nunavut constrain 2 d resistivity models of the crust and lithospheric mantle beneath three regional transects responses determined from colocated broadband and long period magnetotelluric recording instruments enabled resistivity imaging to depths of 300 km strike analysis and distortion decomposition on all data reveal a regional trend of 45 53 but locally the geoelectric strike angle varies laterally and with depth the 2 d models reveal a resistive upper crust to depths of 15 35 km that is underlain by a conductive layer that appears to be discontinuous at or near major mapped geological boundaries surface projections of the conductive layer coincide with areas of high grade archean metasedimentary rocks tectonic burial of these rocks and thickening of the crust occurred during the paleoproterozoic arrowsmith 2 3 ga and trans hudson orogenies 1 85 ga overall the uppermost mantle of the rae craton shows resistivity values that range from 3000 m in the northeast beneath baffin island and the melville peninsula to 10 000 m beneath the central rae craton to 50 000 m in the south near the hearne domain near vertical zones of reduced resistivity are identified within the uppermost mantle lithosphere that may be related to areas affected by mantle melt or metasomatism associated with emplacement of hudsonian granites a regional decrease in resistivities to values of 500 m at depths of 180 220 km increasing to 300 km near the southern margin of the rae craton is interpreted as the lithosphere asthenosphere boundary

an extended episode of early mesoproterozoic metamorphic fluid flow in the reynolds range central australia the products of metamorphic fluid flow are preserved in zones within the marbles and metamorphosed semipelites of the upper calcsilicate unit in the granulite portion of the late palaeoproterozoic reynolds range group northern arunta block central australia the zones of retrogression characterized by minerals such as wollastonite grossular and clinohumite local resetting of oxygen isotopic compositions and local major element metasomatism were channelways for water rich fluids derived from granulite facies metapelites u th pb isotopic ages measured by the shrimp ion microprobe on zircon and monazite from a granulite facies semipelite an early semiconcordant aluminous quartz rich fluid flow segregation and a late discordant quartz rich segregation record some of the extended thermal history of the area zircon cores from the semipelite show its likely protolith to be an igneous rock 1812 11 ma old itself derived from a source containing zircon as old as 2 2 ga low th u overgrowths on the zircon grew during granulite facies metamorphism at 1594 6 ma monazite cooled to its blocking temperature at 1576 8 ma zircon cores from the semiconcordant segregation are dominantly 2 3 ga old indicating that the source of the fluids was not the particular metamorphosed semipelite studied two generations of low th u overgrowths on the zircon give indistinguishable ages for the older and younger of 1589 8 and 1582 8 ma respectively the monazite age is the same 1576 12 ma zircon from the late discordant segregation gave 1568 4 ma fluid flow occurred for at least 18 3 ma and ended 26 3 ma after the peak of metamorphism suggesting a very slow cooling rate of 3 c ma 1 the last regional high grade metamorphism in the reynolds range occurred at 1 6 ga not 1 78 ga as previously thought the high grade event at 1 78 ga is a separate event that affected only the basement to the reynolds range group

eclogites of the snowbird tectonic zone petrological and u pb geochronological evidence for paleoproterozoic high pressure metamorphism in the western canadian shield eclogite occurs within the southern domain of the east athabasca mylonite triangle in northern saskatchewan situated at the boundary between the archean rae and hearne provinces of the western canadian shield the east athabasca mylonite triangle is a fundamental exposure of the 3 000 km long snowbird tectonic zone the eclogite occurs in association with a variety of lower crustal high pressure granulites that record a complex metamorphic history from 2 6 to 1 9 ga temperatures of the eclogite facies metamorphism are constrained by garnet clinopyroxene exchange thermometry at 920 1 000 c minimum pressure conditions are recorded by the jadeite quartz albite geobarometer at 1 8 2 0 gpa a near isothermal decompression path to granulite facies conditions is inferred from retrograde reaction textures involving the formation of granulite facies assemblages such as orthopyroxene plagioclase and pargasite plagioclase u pb idtims zircon geochronology of the eclogite yields a weighted mean 207pb 206pb date of 1 904 0 0 3 ma which we interpret as the time of peak eclogite facies metamorphism shrimp in situ analyses of metamorphic zircons included within omphacitic clinopyroxene support this interpretation with a weighted mean 207pb 206pb date of 1 905 19 ma inclusion suites of high pressure phases and the petrographic setting of zircon are a direct link between zircon growth and eclogite facies metamorphism zircon from one eclogite sample has older cores that are 2 54 ga which is a minimum age for the emplacement or earliest metamorphism of the gabbroic protolith u pb rutile data indicate slow cooling at 1 c ma below 500 c from 1 88 to 1 85 ga the formation and exhumation of the eclogites at ca 1 9 ga has important implications for the tectonic significance of the snowbird tectonic zone during the paleoproterozoic the eclogites described here are consistent with transport of continental crust to mantle depths during the paleoproterozoic followed by rapid buoyancy driven exhumation to normal lower crustal depths

the effect of hydrous mineral content on competitive strain localization mechanisms in felsic granulites abstract this study investigates the grain scale mechanisms that facilitated strain localization in the lower crust within the east athabasca mylonite triangle of northern saskatchewan canada there two distinct lithotectonic units the upper deck and northwestern domain are juxtaposed across a km scale shear zone that has been previously interpreted to record thrust and or normal sense displacement our study reports new quartz c axis opening angle ti in biotite and ti in quartz geothermometry and amphibole plagioclase geothermobarometry within syn kinematic phases indicating that both units were deformed and metamorphosed at upper amphibolite to granulite facies 725 c and 0 8 gpa while sharing similar conditions the dominant mechanisms of deformation across the two units are strongly linked to modal proportion of syn kinematic hydrated mineral phases detailed microstructural and crystallographic preferred orientation cpo analyses reveals that specimens poor in hydrated minerals 10 pressure solution and re precipitation processes enhanced phase mixing and promoted grain boundary sliding resulting in weakly developed or absent quartz cpo these results indicate a strong mineralogical and fluid control on deformation and therefore which strain softening process es may dominate in lower crustal shear zones

a new calibration of seismic velocities anisotropy fabrics and elastic moduli of amphibole rich rocks 1 a large portion of the middle to lower crust beneath the continents and oceanic island arcs consists of amphibolites dominated by hornblende and plagioclase we have measured p and s wave velocities vp and vs and anisotropy of 17 amphibole rich rock samples containing 34 80 vol amphibole at hydrostatic pressures p up to 650 mpa combined petrophysical and geochemical analyses provide a new calibration for mean density average major element contents mean vp p and vs p coefficients intrinsic vp and vs anisotropy poisson s ratios the logarithmic ratio rs p and elastic moduli of amphibole rich rocks the vp values decrease with increasing sio2 and na2o k2o contents but increase with increasing mgo and cao contents the maximum 0 38 0 40 km s and minimum s wave birefringence values occur generally in the propagation direction parallel to y and normal to foliation respectively amphibole plays a critical role in the formation of seismic anisotropy whereas the presence of plagioclase quartz pyroxene and garnet diminishes the anisotropy induced by amphibole crystallographic preferred orientations cpos the cpo variations cause different anisotropy patterns illustrated in the flinn diagram of vp x vp y vp y vp z plots the results make it possible to distinguish in terms of seismic properties the amphibolites from other categories of lithology such as granite granodiorite diorite gabbro diabase felsic gneiss mafic gneiss eclogite and peridotite within the earth s crust hence amphibole aligned by dislocation creep anisotropic growth or rigid body rotation is the most important contributor to the seismic anisotropy of the deep crust beneath the continents and oceanic island arcs which contains rather little phyllosilicates such as mica or chlorite

striding athabasca mylonite zone implications for the archean and early proterozoic tectonics of the western canadian shield study of the northern saskatchewan district of mackenzie segment of the snowbird tectonic zone suggests that fragments of relatively stiff mid archean crust possibly arc related have controlled the localization shape and complex kinematics of the multistage striding athabasca mylonite zone during the archean as well as the geometry of the early proterozoic rifted margin of the western churchill continent by the late archean the striding athabasca mylonite zone was located in the interior of the western churchill continent well removed from the contemporaneous plate margins except for the alberta segment the snowbird tectonic zone was not the site of an early proterozoic plate margin we suggest that the geometry of the archean early proterozoic boundary in the western canadian shield represents a jagged continental margin composed of a pair of reentrants defined by rifted and transform segments these segments were inherited from early proterozoic breakup and controlled by the archean structure

magnetism of the lower crust observations from the chipman domain athabasca granulite terrain northern canada abstract magnetic properties of lower crustal rocks produce anomalies seen in satellite aeromagnetic and ground studies and are assumed to be responsible for observed long wave length anomalies lwa of 20 nt the soon to be launched swarm satellites will provide extensive data on the magnetization of the lower to middle crust in anticipation of this event we are investigating magnetic properties in a superbly exposed section of lower crust in northern saskatchewan the athabasca granulite terrain agt is a complex region of felsic and mafic lower crustal rocks part of the snowbird tectonic zone stretching ne sw across the canadian shield the agt is composed of a sequence of rocks identified as lower crustal in origin by their high pressure 1 0 gpa and high temperature 800 c metamorphism dated at 2 6 ga and 1 9 ga with uplift and exhumation at 1 85 1 80 ga the agt is characterized by low negative aeromagnetic anomalies with distinct large positive anomalies in the southern and central regions the chipman domain on the east side consists of tonalites mafic granulites and granite intruded by the chipman dike swarm at 1 9 ga where anomalies cut across mapped lithologic boundaries susceptibility measurements from both field and lab readings range over several orders of magnitude from 1 10 5 to 3 10 1 si with higher values related to both mafic granulite and some tonalite samples remanence values also show considerable variability from 0 1 ma m to 90 a m with the weakest magnetization found in the chipman dikes and the fehr granite forty samples out of 89 have koenigsberger ratios greater than 1 but low initial remanence limits its influence on anomalies hysteresis and low temperature measurements identify magnetite as the predominant iron oxide this section of lower crustal rocks has paramagnetic granites and dikes with ferromagnetic mafic granulites and bimodal tonalites defined by geographic location

isotopic evidence for neoarchean continuity across the snowbird tectonic zone western churchill province canada abstract the snowbird tectonic zone stz the ca 2800 km long boundary between rae and hearne domains of the western canadian shield remains of uncertain tectonic significance but is currently interpreted as a ca 1 9 ga suture the eastern rae domain is well studied and underlain by a 20 000 km2 region of high pressure granulites containing ca 2 60 ga arc rocks and granulite to amphibolite grade structures which formed at ca 2 57 ga d1 m1 and 1 90 ga d2 m2 the 1 85 ga legs lake shear zone juxtaposed the high p granulites of the eastern rae domain structurally above lower amphibolite to greenschist grade supracrustal gneisses of the hearne domain zircon u pb and hf isotopic analyses were acquired from igneous and metasedimentary rocks across the stz and rae hearne boundary to assess the validity of the suture model with specific focus given to the 5 km wide legs lake shear zone results indicate that both 2 60 2 55 ga magmatism and deformation continue unimpeded across the stz and rae hearne boundary furthermore hf isotopic data from igneous zircon from 2 60 ga plutonic rocks across the stz display little variation in ehf indicating that no major crustal break corresponds to the rae hearne boundary or any of the shear zones that parallel the stz on the ground older plutonic rocks show a greater variability in ehf which may indicate a disparate origin grade intensity and timing of paleoproterozoic features vary along the transect and display little continuity across the rae hearne domain boundary in conjunction with existing field and isotopic data we interpret that the rae and hearne domains were together by 2 60 ga and that the stz was host to repeated paleoproterozoic reactivation driven by the latter stages of taltson thelon orogenesis final indentation of the slave craton into the rae domain and accretionary processes on the southern periphery of the hearne domain from 1 93 to 1 84 ga furthermore assessing the viability of major shear zones within high grade terranes as potential sutures is complicated by the lateral and vertical heterogeneity of the crust juxtaposed by them

formation of paleoproterozoic eclogitic mantle slave province canada insights from in situ hf and u pb isotopic analyses of mantle zircons abstract in situ hf isotope analyses and u pb dates were obtained by laser ablation mc icp ms for a zircon bearing mantle eclogite xenolith from the diamondiferous jericho kimberlite located within the archean slave province nunavut canada the u pb zircon results yield a wide range of ages 2 0 to 0 8 ga indicating a complex geological history of importance one zircon yields a u pb upper intercept date of 1989 67 ma providing a new minimum age constraint for zircon crystallization and eclogite formation in contrast hf isotope systematics for the same zircons display an intriguing uniformity and corresponding hf depleted mantle model ages range between 2 1 0 1 and 2 3 0 1 ga the youngest hf model age is within error to the oldest u pb date the jericho eclogites have previously been interpreted as representing remnants of metamorphosed oceanic crust and their formation related to paleoproterozoic subduction regimes along the western margin of the archean slave craton during the wopmay orogeny hf isotope compositions and u pb results for the jericho zircons reported here are in good agreement with a paleoproterozoic subduction model suggesting that generation of oceanic crust and eclogite formation occurred between 2 0 and 2 1 ga the slightly older hf depleted mantle model ages 2 1 to 2 3 ga may be reconciled with this model by invoking mixing between crustal derived hf from sediments and more radiogenic hf associated with the oceanic crust during the 2 ga subduction event this results in intermediate hf isotope compositions for the jericho zircons that yield fictitiously older hf model ages

a revised geodynamic model for the lapland kola orogen abstract the paleoproterozoic lapland kola orogen in fennoscandia has been studied for decades and several plate tectonic models have been proposed including one sided subduction zone either towards sw or nw or two opposite verging subduction zones before the collision based on new structural and isotope data from finland and recently published data from russia we propose a revised tectonic model for the paleoproterozoic lapland kola orogen the main components are foreland in the ne followed by cryptic suture inari arc retro arc basin and retro arc foreland in the sw the latter three constitute the inari orocline subduction towards present sw and subsequent arc magmatism inari arc started at ca 1 98 ga followed by voluminous sedimentation in the deepening retro arc basin underplating of a mid ocean ridge caused flat subduction and magmatic flare at 1 92 ga over a broad distance in the retro arc basin rapid heating led to melting of the retro arc basin sediments and voluminous amounts of granulite facies diatexites formed during collision d1 at 1915 1910 ma large thrust nappes formed on the foreland deformation in the retro arc basin is seen as recumbent folding and shearing of diatexites in the lower parts of the basin and thrusting of metatextite diatexite packages in the upper parts a post collisional stage is seen as 1904 ma appinites and decompression derived granites at 1 90 1 89 ga renewed shortening d2 due to far field effects in sw at 1 88 1 87 ga led to thick skin shortening of the archean middle crust large scale crustal duplexing of already cooled granulites towards the retro arc foreland and inclined upright folding of granulites in the opposite direction towards the inari arc a switch in the stress field from ne sw to nw se led to orogen parallel contraction and buckling started along a dextral strike slip fault zone to form the inari orocline buckling is seen in the bending of pre orocline fabrics and formation of syn orocline fabrics radial conical folds d3 radial fractures a strike slip fault zone and thrusting at the hinge zone the end result is a mega scale parallel multi layer fold composed of the inari arc retro arc basin and possibly also the heated retro arc foreland

magmatism and early variscan continental subduction in the northern gondwana margin recorded in zircons from the basal units of galicia nw spain in situ uranium lead dating la sf icpms and sims and lu hf isotope analyses la mc icp ms of zircon from eclogite facies rocks from the basal units of the variscan belt in galicia constrain their magmatic and metamorphic evolution and give some clues about the nature and origin of the involved basement the samples studied are two felsic gneisses two eclogites and one eclogitic gneiss of intermediate composition metatonalite oscillatory zoned zircon cores from the felsic samples gave a main clustering of u pb ages at 493 2 and 494 2 ma and some older ages that represent inherited cores zircon grains from the intermediate and one of the mafi c rocks show no inherited cores and yielded ages of 494 3 and 498 6 ma respectively interpreted as time of protolith crystallization variably developed homogeneous zircon rims in one felsic gneiss yielded an age of 372 3 ma and very tiny zircons of one eclogite gave 350 2 ma both of which we interpret as metamorphic ages the new age data demonstrate that the calc alkaline magmatic suite described in the basal unit is ca 20 ma older than the alkaline to peralkaline plutonic suite of the same unit dated at 472 2 ma rodriguez et al 2007 and thus probably represents a distinct geologic event overgrowth rims are interpreted as metamorphic on the basis of their lu hf and th u ratios the 372 3 age is considered as dating the high pressure high p metamorphism and is essentially in agreement with previous ar ar and rb sr data this high p metamorphism marks the initial early variscan subduction of the gondwana margin the inherited zircon ages and hf isotopic composition of zircons point to a considerable input of crustal material with west african craton provenance to the felsic magma

metamorphic pt path and zircon u pb dating of archean eclogite association in gridino complex belomorian province russia abstract the eclogites from the gridino eclogite bearing melange complex in the belomorian province fennoscandian shield may represent one of the oldest terrain in the world the belomorian province is a superposition of archean and palaeoproterozoic collisional orogens which experienced repeated high grade metamorphic reworking and intense polyphase tectonic deformation in the early precambrian main purpose of this study is to determine the metamorphic age s and evolutionary pt path of eclogite as boudin like body within the gridino tectonic melange representative rock materials of well preserved plagioclase free eclogite and associated retrogressed eclogitic rock were sampled from stolbikha island in the gridino area for our research the eclogite sample mainly comprises omphacite garnet amphibole rare quartz and some accessory minerals rutile magnetite whereas the retrogressed eclogite sample is made of diopside na rich plagioclase symplectites amphibole quartz and accessory minerals thermodynamic modeling points to a clockwise metamorphic pt path with peak conditions of p 18 5 kbar t 710 c and a warm subduction is inferred thus la icp ms zircon u pb dating yields diverse metamorphic episodes from the neoarchean to the palaeoproterozoic while metamorphic zircon formed at eclogite facies condition is identified by studying the trace element distribution and partitioning in and between zircon and co existing eclogite facies minerals omphacite garnet consequently neoarchean eclogite facies metamorphic event of ca 2 70 ga can be distinguished for the gridino eclogite complex and a regional scale thermal event may have occurred in the period of 2 70 2 61 ga

the high pressure stability of zoisite and phase relationships of zoisite bearing assemblages the fluid absent reaction 12 zoisite 3 lawsonite 7 grossular 8 kyanite 1 coesite was experimentally reversed in the model system cao al2o3 sio2 h2o cash using a multi anvil apparatus the upper pressure stability limit for zoisite was found to extend to 5 0 gpa at 700 c and to 6 6 gpa at 950 c additional experiments both in the h2o sio2 saturated and in the h2o al2o3 saturated portions of cash provide further constraints on high pressure phase relationships of lawsonite zoisite grossular kyanite coesite and an aqueous fluid consistency of the present experiments with the h2o saturated breakdown of lawsonite is demonstrated by thermodynamic analysis using linear programming techniques two sets of data consistent with databases of berman 1988 and holland and powell 1990 were retrieved combining experimental phase relationships calorimetric constraints and recently measured elastic properties of solid phases the best fits result in g f 1 298 zoisite 6 499 400 j and s 1 298 zoisite 302 j k and g f 1 298 lawsonite 4 514 600 j and s 1 298 lawsonite 220 j k for the dataset of holland and powell and g f 1 298 zoisite 6 492 120 j and s 1 298 zoisite 304 j k and g f 1 298 lawsonite 4 513 000 j and s 1 298 lawsonite 218 j k for the dataset of berman examples of the usage of zoisite as a geohygrometer and as a geobarometer in rocks metamorphosed at eclogite facies conditions are worked profiting from the thermodynamic properties retrieved here

syntectonic generation and segregation of tonalitic melts from amphibolite dikes in the lower crust striding athabasca mylonite zone northern saskatchewan vapor absent melting in a swarm of amphibolite dikes in the lower crust has produced segregations of tonalitic and trondhjemitic composition in a variety of textures and structures that dramatically illustrate the mechanisms of melt segregation during deformation the 3 2 ga chipman dikes intrude the mylonitized chipman tonalite within the striding athabasca mylonite zone of northern saskatchewan dike emplacement spans a major sinistral transpressive ductile deformation and granulite facies metamorphic event older dikes are intensely folded and sheared younger dikes are undeformed major and trace element analyses indicate that the dikes are tholeiitic basalts similar in composition to midocean ridge basalt thermobarometry finds conditions of 750 850 c 1 0 gpa the youngest most pristine dikes contain hornblende and plagioclase with minor clinopyroxene and garnet older migmatitic dikes have tonalitic to trondhjemitic segregations spatially associated with garnet crystals where small the segregations occupy tails or strain shadows on every garnet crystal where garnets and segregations are large leucosomes form an interconnected network that extends into the host tonalite tonalitic pools probably of dike origin collect in boudin necks and fold hinges the chipman dikes are interpreted to have been emplaced solidified and partially melted during ductile shearing in the lowermost crust perhaps near the base of an archean island arc they appear to offer an exceptional view of magma genesis where underplated mantle derived mafic magmas provide not only heat but also a component of material to new felsic magmas deformation is essential to the process allowing access of mafic magma into the lower crust and facilitating the segregation of the new felsic melts from their source rocks

two neoarchean supercontinents revisited the case for a rae family of cratons abstract a variety of early paleoproterozoic supercontinents or supercratons have been proposed including kenorland superia and vaalbara improved geologic knowledge of the churchill province canadian shield suggests that the classical kenorland configuration with linked rae and hearne cratons is invalid and represents a younger ca 1 9 ga nuna configuration we review the early paleoproterozoic tectonic history of the rae craton and propose that the ca 2 55 2 50 ga macquoid and 2 50 2 28 ga arrowsmith orogens represent critical piercing points for supercontinent reconstruction utilizing the stratdb and dateview databases we have identified nearly two dozen regions worldwide that show effects of these orogenic events including the rae gawler mawson peninsular india west africa amazonia north china and sask crustal blocks we propose that these cratonic blocks form a distinct rae family lineage utilizing new time space plots for the rae family and components of the superia and vaalbara supercratons we demonstrate that the rae family lacks the distinctive early paleoproterozoic depositional and glaciogenic record characteristic of superia and vaalbara but has an important contemporaneous record of juvenile crust formation and contractional orogenesis we propose the collective history of the rae family is best reconciled if it represents the now dispersed blocks of a third earliest paleoproterozoic supercraton that we term nunavutia in this model the magmatic and tectonometamorphic record of these cratons includes an early 2 55 2 45 ga period of interior collisional orogenesis in which the supercraton amalgamated and a later phase of long lived peripheral orogenesis 2 45 2 28 ga prior to its partial break up at ca 2 05 ga forming within an otherwise globally quiescent period the history of the rae family is important for assessment of constrasting models of early paleoproterozoic geodynamic activity

high pressure granulites formation recovery of peak conditions and implications for tectonics high pressure granulites are characterised by the key associations garnet clinopyroxene plagioclase quartz in basic rocks and kyanite k feldspar metapelites and felsic rocks and are typically orthopyroxene free in both basic and felsic bulk compositions in regional metamorphic areas two essential varieties exist a high to ultrahigh temperature group and a group representing overprinted eclogites the high to ultrahigh temperature type formerly contained high temperature ternary feldspar now mesoperthite coexisting with kyanite is associated with garnet peridotites and formed at conditions above 900 c and 1 5 gpa clinopyroxene in subordinate basic rocks is al rich and textural evidence points to a high pressure high temperature melting history the second variety contains symplectite like or poikilitic clinopyroxene plagioclase intergrowths indicating former plagioclase free i e eclogite facies assemblages this type of rock formed at conditions straddling the high pressure amphibolite high pressure granulite field at around 700 850 c 1 0 1 4 gpa importantly in the majority of high pressure granulites orthopyroxene is secondary and is a product of reactions at pressures lower than the peak recorded pressure in contrast to low and medium pressure granulites which form at conditions attainable in the mid to lower levels of normal continental crust high pressure granulites of nonxenolith origin mostly represent rocks formed as a result of short lived tectonic events that led to crustal thickening or subduction of the crust into the mantle short times at high temperature conditions are reflected in the preservation of prograde zoning in garnet and pyroxene high pressure granulites of both regional types although rare are known from both old and young metamorphic terranes e g c 45 ma namche barwa e himalaya 400 340 ma european variscides 1 8 ga hengshan china 1 9 ga snowbird saskatchewan and 2 5 ga jianping china this spread of ages supports proposals suggesting that thermal and tectonic processes in the lithosphere have not changed significantly since at least the end of the archean

multiple burial exhumation episodes revealed by accessory phases in high pressure granulite facies rocks rae craton nunavut canada the big lake shear zone blsz is a 60 km long deep crustal structure adjacent to the northern segment of the snowbird tectonic zone stz nunavut canada in a region characterized by intensely sheared high pressure granulite facies rocks the units exposed here provide an exceptional record of the lower crust that preserves crucial evidence bearing on interpretation of superimposed neoarchean and paleoproterozoic tectono metamorphic events deformation along the blsz postdates 2650 ma the age of a mylonitized monzogranite and predates 2190 ma the age of cross cutting macquoid mafic dykes metamorphic assemblages and p t modeling suggest crustal thickening in the neoarchean with peak conditions of 800 c and 14 15 kbar at ca 2530 ma mylonitic fabrics developed at ca 2505 ma t 800 c p 10 c ma and partial exhumation post ca 2505 ma followed by re burial at 1900 ma rather than slower isobaric cooling at depth these results indicate that the long term residence models proposed by several authors for domains in the central stz are not applicable to the blsz region and are more compatible with a ca 1 9 ga collisional setting

evidence for neoarchean ni cu bearing mafic intrusions along a major lithospheric structure a case study from the south rae craton canada abstract deformed and metamorphosed mafic to ultramafic intrusive rocks are common along and adjacent to the snowbird tectonic zone a major paleoproterozoic structure separating the rae and hearne cratons on the rae side of this major lithospheric structure we investigated a series of mafic bodies that intrude high grade paragneiss and are associated with ni cu pge mineralization by linking field observations geochronology and metamorphic petrology our results indicate archean 2 63 ga crystallization ages for these mafic intrusions over a minimum along strike extent of 500 km and a later high temperature 820 c and 7 5 kbar paleoproterozoic metamorphic overprint at t

age and provenance of the paleoproterozoic thluicho lake group based on detrital zircon u pb shrimp geochronology new insights into the protracted tectonic evolution of the southwestern rae province canadian shield abstract the middle paleoproterozoic thluicho lake group comprises coarse grained continental clastic rocks deposited in fault controlled alluvial basin s in the southwestern rae province of the canadian shield the group nonconformably overlies crystalline basement that records late taltson high grade metamorphism ca 1 93 ga and is folded and metamorphosed to greenschist facies providing an important window of insight into tectonic activity in this region this paper reports 100 detrital zircon ages for two samples from the lower and upper parts of the thuicho lake group four analyses of one of the youngest detrital zircons from the stratigraphically higher member define a weighted mean 207pb 206pb age of 1922 6 ma establishing a maximum depositional age for the group this result accords with regional geological relationships indicating that the group is intermediate in age between development of two major tectonic fronts an older ca 1 93 ga east southeast trending front that formed at the height of thelon taltson orogeny and a younger ca 1 91 1 90 ga northeast striking front expressed by the black bay fault and related northeast striking structures affiliated with the snowbird tectonic zone the younger age limit of the group may therefore be older 1 91 1 90 ga than the previously reported minimum age constrained by cross cutting dykes dated at ca 1 82 ga in terms of isotopic provenance the bulk of analyses of the combined dataset lie within two principal age ranges 2750 2500 ma and 2475 2250 ma the older of these is dominated by ca 2600 ma zircons whereas the younger contains two statistically distinct subgroups of 2410 and 2310 ma this signature is consistent with derivation almost exclusively from immediately surrounding basement rocks of the nolan taltson zemlak ena and beaverlodge domains abundant 2475 2250 ma zircons provide indirect evidence for the 2 45 2 3 ga arrowsmith orogeny despite proximity to the taltson magmatic zone 1990 1960 ma zircons are nearly completely absent suggesting that thluicho lake basins formed in the hinterland of the orogen possibly separated from the foreland taltson arc by a continental divide alternatively taltson arc plutons may not have been unroofed at the time of deposition of thluicho lake strata the data presented herein support the previously proposed broad correlation between the thuicho lake and nonacho groups however there are apparent differences in detail between respective basin settings and controls suggesting that this correlation requires further investigation

u pb zircon geochronology of unusual eclogite facies rocks from syros and tinos cyclades greece low temperature eclogite facies rocks from syros and tinos cyclades greece include meta igneous blocks of unclear origin meta olistostrome or tectonic melange with very high trace element concentrations e g zr up to 4850 ppm y up to 475 ppm the unusual geochemistry is considered to be the result of metasomatic alteration in a subduction zone environment locally metasomatic exchange with an ultramafic matrix further enhanced compositional anomalies a concordant u pb zircon age of 78 1 ma recorded for an omphacitite from syros is interpreted as the time of pre eocene high pressure metamorphism in the cyclades zircon dates of 61 and 63 ma for a jadeitite from tinos possibly indicate an additional discrete event deformation these results are a first geochronological indication that high pressure metamorphism in the cyclades commenced signif icantly earlier than indicated by previous white mica chronology which provided ages between c 50 and 40 ma

coronitic metagabbro and eclogite from the grenville province of western quebec interpretation of u pb geochronology and metamorphism we present new u pb and metamorphic data on high pressure coronitic metagabbros from three distinct structural settings in the parautochthonous belt of the grenville province in western quebec intrusive ages are i for metagabbro close to the grenville front correlative with the sudbury dykes defined in ontario ii for an eclogitized lens at the base of the highest structural level sl4 a new age for mafic magmatism in the western grenville and iii for metagabbro from sl4 interpreted as correlative with metagabbros from the algonquin and shawanaga domains in ontario metamorphism in all cases is grenvillian with the best constrained age of 1069 3 ma for the metagabbro of sl4 metamorphic grade increases from the grenville front to the south the mafic rocks preserve relict igneous textures overprinted by garnet clinopyroxene that developed as coronas and or pseudomorphs after igneous phases the highest grade metagabbros contain omphacite and some lack primary plagioclase therefore b

petrological and geochronological constraints on high pressure high temperature metamorphism in the snowbird tectonic zone canada the upper deck of the east athabasca mylonite triangle eamt northern saskatchewan canada contains mafic granulites that have undergone high p t metamorphism at conditions ranging from 1 3 to 1 9 gpa 890 960 c coronitic textures in these mafic granulites indicate a near isothermal decompression path to 0 9 gpa 800 c the godfrey granite occurs to the north adjacent to the upper deck high p t domain well preserved corona textures in the godfrey granite constrain igneous crystallization and early metamorphism in the intermediate pressure granulite field opx pl at 1 0 gpa 775 c followed by metamorphism in the high pressure granulite field grt cpx pl at 1 2 gpa 860 c u pb geochronology of zircon in upper deck mafic granulite yields evidence for events at both c 2 5 ga and c 1 9 ga the oldest zircon dates are interpreted to constrain a minimum age for crystallization or early metamorphism of the protolith a population of 1 9 ga zircon in one mafic granulite is interpreted to constrain the timing of high p t metamorphism titanite from the mafic granulites yields dates ranging from 1900 to 1894 ma and is interpreted to have grown along the decompression path but still above its closure temperature indicating cooling following the high p t metamorphism from c 960 650 c in 4 10 myr zircon dates from the godfrey granite indicate a minimum crystallization age of 2 61ga without any evidence for 1 9 ga overgrowths the data indicate that an early granulite facies event occurred at c 2 55 2 52 ga in the lower crust c 1 0 gpa but at 1 9 ga the upper deck underwent high p t metamorphism then decompressed to 0 9 1 0 gpa juxtaposition of the upper deck and godfrey granite would have occurred after or been related to this decompression in this model the high p t rocks are exhumed quickly following the high pressure metamorphism this type of metamorphism is typically associated with collisional orogenesis which has important implications for the snowbird tectonic zone as a fundamental boundary in the canadian shield

a decomposition of the atlantic meridional overturning circulation into physical components using its sensitivity to vertical diffusivity abstract the sensitivity of the atlantic ocean meridional overturning circulation to the vertical diffusion coefficient in the global coupled atmosphere ocean sea ice model climber 3 is investigated an important feature of the three dimensional ocean model is its low diffusive tracer advection scheme the strength mmax of the atlantic overturning is decomposed into three components 1 the flow ms exported southward at 30 s 2 the large scale upward flow that balances vertical diffusion in the atlantic and 3 a wind dependent upwelling flux wbound along the atlantic boundaries that is not due to vertical diffusion the export of water at 30 s varies only weakly with but is strongly correlated with the strength of the overflow over the greenland scotland ridge the location of deep convection is found to be mixing dependent such that a shift from the nordic seas to the irminger sea is detected for high values of the ratio r ms mmax gives a measure of the interhemispheric overturning efficienc

microstructural tectonometamorphic processes and the development of gneissic layering a mechanism for metamorphic segregation the mary granite in the east athabasca mylonite triangle northern saskatchewan provides an example and a model for the development of non migmatitic gneissic texture gneissic compositional layering developed through the simultaneous evolution of three microdomains corresponding to original plagioclase orthopyroxene and matrix in the igneous rocks plagioclase phenocrysts were progressively deformed and recrystallized first into core and mantle structures and ultimately into plagioclase rich layers or ribbons garnet preferentially developed in the outer portions of recrystallized mantles and with further deformation produced garnet rich sub layers within the plagioclase rich gneissic domains orthopyroxene was replaced by clinopyroxene and garnet and hornblende if sufficient water was present which were in turn drawn into layers with new garnet growth along the boundaries the igneous matrix evolved through a number of transient fabric stages involving s c fabrics s c c fabrics and ultramylonitic domains in addition quartz veins were emplaced and subsequently deformed into quartz rich gneissic layers moderate to highly strained samples display extreme mineralogical compositional segregation yet most domains can be directly related to the original igneous precursors the mary granite was emplaced at approximately 900 c and 1 0 gpa and was metamorphosed at approximately 750 c and 1 0 gpa the igneous rocks crystallized in the medium pressure granulite field opx pl but were metamorphosed on cooling into the high pressure grt cpx pl granulite field the compositional segregation resulted from a dynamic mutually reinforcing interaction between deformation metamorphic and igneous processes in the deep crust the production of gneissic texture by processes such as these may be the inevitable result of isobaric cooling of igneous rocks within a tectonically active deep crust