dehydration melting of amphibolite at 10 kbar the effects of temperature and time we have simulated the dehydration melting of a natural low k calcic amphibolite 67 4 hornblende 32 5 anorthite in piston cylinder experiments at 10 kbar and 750 1000 c for 1 9 days the solidus temperature is lower than 750 c garnet appears at 850 c the overall reaction is hb p l cpx al hb ca hb ga opx three stages of reaction are 1 melting dominated by the growth of clinopyroxene and garnet with little change in composition of liquid or garnet 2 a reversal of this reaction between 875 c and 900 c with decreases in the amounts of liquid and garnet and 3 a large increase in liquid along with the loss of hornblende and decrease of plagioclase while clinopyroxene and garnet increase garnet is enriched in pyrope and zoned from fe cores to mg edges range 3 mol pyrope liquid composition is enriched first in an to 950 c and then in ab the liquids are more calcic and aluminous than natural tonalites which is attributed to the plagioclase composition an90 the formation of peraluminous liquid from the metaluminous amphibolite is caused by anorthite not h2o saturated conditions the results are consistent with an amphibolite phase diagram with relatively high solidus temperatures in the garnet absent field 900 1000 c but with a solidus backbend at 7 9 kbar coincident with the garnet in boundary hornblende breakdown due to garnet formation in a closed system must make h2o available for h2o undersaturated melting right down to the h2o saturated solidus below 700 c which defines a large low temperature pt area where hydrous granitoid melts can be generated with residual garnet and hornblende

ttgs in the making natural evidence from inyoni shear zone barberton south africa despite the consensus that ttgs the main constituents of the archaean continental crust worldwide originated by partial melting of garnet bearing amphibolites natural evidence is scarce a large variety of archaean amphibole rich rocks including migmatitic amphibolites and hornblende rich cumulates was exhumed as a tectonic melange in the inyoni shear zone isz near barberton south africa likely at the time of emplacement of the 3 2 ga nelshoogte badplaas plutons this unique collection provides the opportunity to investigate partial melting of garnet amphibolites as well as the differentiation processes occurring in the ttg magmas en route to the surface the isz migmatitic amphibolites are characterized by quartz plagioclase leucosomes in equilibrium with garnet amphibole titanite epidote garnet compositions are characterized by high almandine and grossular contents actually the leucosomes and neosomes likely lost part of their melt component and mesosomes may have been also modified towards restite rich compositions due to melt segregation restite bearing rocks are very iron rich according to their high contents in garnet besides the mg rich compositions of the hornblende cumulates and their ree distribution patterns resemble those of the nearby tt tonalite trondhjemite plutons thermobarometric estimates from the isz migmatitic rocks cluster in the range 720 800 c and 1 1 1 2 gpa for the melting reaction and the derived geothermal gradient is ca 17 22 c km these conditions are consistent with either water present melting or more likely water absent epidote dehydration melting such a geothermal gradient is therefore assigned to the genesis of large volumes of medium pressure ttg magmas that will likely form at greater depth unmelted garnet bearing amphibolites correspond to slightly lower conditions the magmatic cumulates crystallized at ca 0 6 gpa the cumulate rocks evidence that the ttg parental magmas en route to the surface experienced fractional crystallization of a large amount of amphibole before crystallizing as tonalite trondhjemite plutons at slightly shallower depths ca 0 5 gpa

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

226ra 230th excess generated in the lower crust implications for magma transport and storage time scales excesses of 226 ra in arc magmas have been interpreted as resulting from flux melting of the mantle above subducting slabs followed by fast ascent rates of magma from slab to surface up to 1000 m yr however we demonstrate that incongruent melting of the lower crust could either maintain or augment mantle derived 226 ra excesses and so reduce inferred vertical transport rates we developed an incongruent continuous melting model and both the incongruent melting reaction and ingrowth effects contribute to the 226 ra excess in particular we found that dehydration melting of amphibolite can produce modeled 226 ra excesses greater than 300 mixtures of such amphibolite dehydration melts with mantle melts will likely retain a 238 u excess subducted slab signature this amphibolite dehydration melting process will also produce elevated ratios of light rare earth elements to heavy rare earth elements similar to those observed in several arc settings which may distinguish these magmas from those with 226 ra excesses produced by slab dewatering alone

granite genesis and mafic felsic magma interaction in the lower crust field observations from an exposed section of deep continental crust the athabasca granulite terrane agt saskatchewan canada provide a view of granite genesis and a mechanism for deep seated contamination of felsic and mafic magmas the 1 9 ga chipman mafic dike swarm was emplaced into the chipman tonalite ca 3 3 ga and the megacrystic fehr granite ca 2 6 ga at a crustal depth of 40 km the fehr granite shows evidence for extensive partial melting and generation of granitic leucosome mafic dikes and granitic leucosome display magma mingling and mixing textures similar to those widely described from shallow crustal exposures the agt provides a view of a dynamic heterogeneous and locally fertile deep crust mantle derived mafic magma promotes extensive partial melting of fertile granitoids which in turn filter and entrap later mafic dikes and sills the result is almost inevitable mingling and hybridization i e contamination of mafic and felsic end members this interaction of magmas in the deep crustal environment may account for the isotopic and compositional signatures of igneous rocks at shallower crustal levels that typically record contamination of crustal melts by mantle material and vice versa

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

petrology and u pb geochronology of mafic high pressure metamorphic coronites from the tshenukutish domain eastern grenville province abstract a suite of mafic and ultramafic rocks from the tshenukutish domain eastern grenville province have been the subject of a detailed petrographic and geochemical study combined with u pb geochronology the tshenukutish domain is the highest tectonic level in the recently discovered eclogite bearing manicouagan imbricate zone the mafic and ultramafic rocks are characteristic of two of the lithotectonic segments in the domain the lac espadon segment and the baie du nord segment in the lac espadon segment cumulate textures and geochemical characteristics suggest that the units represent a layered mafic and ultramafic complex and an unrelated apatite rich fe ti mafic pegmatite nelsonite body in contrast the samples from the baie du nord segment represent fairly homogeneous fe ti gabbros and anorthosites the samples studied show abundant petrographic evidence for high pressure metamorphism for instance the widespread development of garnet clinopyroxene and kyanite bearing metamorphic assemblages estimates of the conditions of high pressure metamorphism in previous studies and melting textures in deformed and hydrated gabbro margins are in the range 750 900 c and 1600 1800 mpa the gabbroic end members from both segments suggest within plate tholeiite petrotectonic signatures ree patterns of gabbros and cumulates in both segments suggest fractionation from a tholeiitic source cumulate textures fractionation and inherited zircons suggest high crustal level emplacement of the magmatic protoliths in the tshenukutish domain in the lac espadon segment u pb ages of igneous zircon and baddeleyite give an age range of 1643 1629 ma for the emplacement of the layered complex the nelsonite in the les gives a poorly constrained igneous zircon age of 1596 126 99 ma in the baie du nord segment baddeleyite gives an age of 1170 5 ma for emplacement of the fe ti gabbros this is the youngest intrusion in the td and thus gives a maximum limit for the onset of grenvillian metamorphism ages of high p metamorphism are further constrained by zircon formed during partial melting of deformed and hydrated fe ti gabbro margins 1046 3 ma overgrowth of igneous zircon 1042 22 28 ma in the nelsonite saccharoidal zircon replacing baddeleyite 1030 10 7 ma and 1030 12 ma and new growth of equant zircon 1012 12 ma from the undeformed interior of the gabbros these ages indicate that the high p metamorphism of the tshenukutish domain was late grenvillian ottawan retrogression to amphibolite facies is constrained by the ages of titanite 1006 997 ma in deformed gabbro margins rutile ages range between 960 and 929 ma and are interpreted as cooling ages the difference between the ages of titanite and rutile suggest slow cooling of the tshenukutish domain after initial high pressure and temperature metamorphism and exhumation

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

displacement features associated with fault zones a comparison between observed examples and experimental models abstract fault zones commonly consist of discontinuous surfaces or are composed of different kinds of en echelon fractures tension cracks r or p type fractures which give the brittle shear zone a width translation along these faults and related features can occur by sliding on the elementary planes and or by solution deposition processes with opening of transtension zones and possible dilatation of the shear zone in order to understand how fault planes develop and to investigate the mechanical conditions corresponding to natural configurations displacement along an analogue fault model was examined under conditions of direct shear the experimental fracture patterns were then compared with the natural features the structural elements of certain faults created by testing and showing transpression and transtension zones were found to be similar to natural domino structures bounded by elementary shears and could be compared with computed situations it can be inferred that stresses are reoriented inside the shear zone the angle between elementary fractures depends on their order of development transpression and transtension zones occur systematically and the shear zone undergoes dilatancy under low normal stresses

transformation of fe ti gabbro to coronite eclogite and amphibolite in the baie du nord segment manicouagan imbricate zone eastern grenville province fe ti gabbros from the baie du nord segment of the manicouagan imbricate zone metamorphosed under high p t conditions during the grenvillian orogeny have been the focus of a detailed micropetrological study textures and mineral chemistry suggest that the mineral assemblages represent progressive stages of metamorphic transformation resulting in the formation of coronas pseudomorphs after igneous phases transitional and true granoblastic eclogites the transitional and eclogitic samples also have coronas which are developed locally around igneous xenocrysts of plagioclase and olivine the deformed margins of coronitic fe ti gabbros are transformed to amphibolite and contain clinopyroxene bearing leucosomes with garnet poikiloblasts that are indicative of high p t dehydration melting interpretation of garnet zoning and thermobarometry suggest that the highest p t conditions are recorded by coronas around xenocrysts c 720 800 c at 14 17 kbar and garnet clinopyroxene cores in granoblastic assemblages c 740 820 c at 13 17 kbar in the eclogitic samples re equilibration during the early stages of exhumation at high t conditions 700 c affected all samples and is evidenced by the widespread development of pargasite bearing plagioclase collars in the coronitic and transitional metagabbros and by widespread re equilibration of the eclogites giving lower p t estimates at grain boundaries however the difference in calculated pressure conditions between coronite and eclogite samples is consistent with increasing pressure depth from the coronites 11 13 kbar to the eclogites 13 17 kbar the p t conditions recorded by these rocks define a metamorphic field gradient which suggests high heat flow through the lower crust during the grenvillian orogeny

archean and early proterozoic events along the snowbird tectonic zone in northern saskatchewan canada abstract the snowbird tectonic zone is a major boundary structure in the canadian shield separating rocks of the rae and hearne lithostructural domains field investigations along portions of the snowbird tectonic zone exposed in northern saskatchewan have revealed a major granulite facies mylonite belt that records deformation and metamorphism at several intervals in the archean and early proterozoic early assemblages are dominated by granulite grade gneisses that were deformed in a major left lateral strike slip shear zone at ca 3200 ma the major episode of deformation along this portion of the snowbird tectonic zone occurred at ca 2600 ma again at granulite facies in a dominantly right lateral strike slip shear zone little evidence of proterozoic deformation is recorded within this region of the snowbird tectonic zone itself but neighboring rocks in the rae province to the west preserve granulite grade right lateral fabrics dated at ca 1850 ma these constraints suggest that the snowbird tectonic zone was a focus of major regional deformation throughout the middle and late archean but that regional deformation was partitioned outward to the surrounding rae and hearne province rocks during the early proterozoic these data suggest that the snowbird tectonic zone was not a major continental suture zone in the early proterozoic

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

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

porphyroclast systems as kinematic indicators porphyroclasts of relatively strong minerals in mylonites commonly have an internal monoclinic shape symmetry defined by tails of dynamically recrystallized material the geometry of a porphyroclast and its tails called a porhyroclast system can serve as a valuable indicator of the sense of vorticity porphyroclast systems have been divided into and types on the basis of the geometry of the tails types have wedge shaped recrystallized tails whose median lines lie on opposite sides of a reference plane parallel to the tails and containing the symmetry axis for the system types are further subdivided into a a types in which the porphyroclast is isolated in a relatively homogeneous matrix and b types in which the porphyroclast system is associated with a shear band foliation in the matrix types typically have narrow recrystallized tails whose median lines cross the reference plane adjacent to the porphyroclast consequently embayments of matrix material occur adjacent to the porphyroclasts and the tails display characteristic bends a porphyroclast system in a mylonite develops when the relatively weak dynamically recrystallized grain aggregate in the porphyroclast mantle changes its shape due to non coaxial flow in the adjacent matrix this behaviour has been modelled in shear box experiments passive marker lines around rigid cylinders embedded in silicone putty were subjected to simple shear the experiments were modified to simulate a change in recrystallization rate r with respect to rate of deformation by decreasing the diameter of the rigid cylinder during deformation at variable rates the ratio r appears to be one of the most important factors in determining which porphyroclast system will develop at high r values flow of recrystallized material away from the porphyroclast is continuously appended by the production of new grains and wedge shaped a type tails develop at low r values relatively few new grains are added to the tails which become thinned and deflected by drag due to the spinning motion of the porphyroclast in addition most porphyroclast systems at low shear strains are of a type or lack monoclinic symmetry whereas types are only developed at high shear strain values complex porphyroclast systems characterized by two generations of tails are observed in many of the natural and model shear zones studied and may form due to fluctuating r conditions that allow isolated a and type porphyroclast systems to be used as sense of vorticity indicators are the systems should have a monoclinic shape symmetry matrix grain size should be small with respect to porphyroclast size matrix fabric should be homogeneous deformation history should be simple and observations should be made on sections normal to the inferred bulk vorticity vector for the mylonite

hydrogeology of the coventry region uk an urbanized multilayer dual porosity aquifer system abstract the coventry groundwater investigation has researched the sources and movement of chlorinated hydrocarbon solvents in the groundwater system underlying the coventry region the hydrogeological scene is set in this paper as a basis for the other aspects of the investigation which are reported in the other papers in this volume the deep intercalcated sandstones and mudstones of the permo carboniferous which underly the study area behave as a multilayer aquifer although individual aquifer units are not regionally extensive the sandstone units are fractured and behave as dual porosity aquifers with rapid tracer movement in fractures and diffusion into the low permeability but porous matrix the water balance of the groundwater system has changed from natural conditions because of man s influence although direct recharge has been reduced by the impermeable surfaces of roads and towns substantial extra recharge arises in the urban areas from leaking water mains

melt segregation in the continental crust before melt will segregate from its restite and source there must be both a driving force and space into which the melt can move the continental crust is anisotropic and in most cases is subject to deviatoric stresses during melting under these conditions melt segregation begins when an interconnected network of melt forms and the source becomes permeable melt is driven by pressure gradients from its restite toward dilatant sites set up in the heterogeneously deforming source the source is continually drained of melt and so need never contain more melt than that required to maintain permeability 26 where the solid matrix breaks down and diapirism can occur

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

granite emplacement by crustal boudinage example of the calmayo and el hongo plutons cordoba argentina this study deals with the structure and emplacement of the calmayo and el hongo trondhjemite plutons famatinian belt of cargentina it provides structural data from the granites and the country rocks and a study of the magnetic fabric in the plutons new u pb geochronological data yield intrusion ages of 512 1 3 4 ma and 500 6 4 5 ma for the calmayo and el hongo plutons respectively the el hongo massif and the southern part of the calmayo trondhjemite preserve magmatic structures whereas the northern domain of calmayo shows the imprint of solid state deformation the main foliation in the country rocks outlines a boudin like pattern at the map scale and the granites are located along boudin necks suggesting that the emplacement of these tron dhjemite plutons was linked to large scale boudinage of the country rocks

the origin of granite the role and source of water in the evolution of granitic magmas temperature and water content are the two most important parameters in the formation of granitic magmas evidence from volcanic and plutonic lithologies suggests that water contents of 2 to 4 wt are present in most silicic magmas calculations based on the stability of biotite yield water fugacities within the melt phase from about 500 to 2 000 bars although these calculations determine the log f h 2 o and the effective uncertainty of 0 5 log units yields a large absolute uncertainty comparison with crystallization experiments demonstrates that less than 2 water would require significant percentages of crystallization at temperatures above 900 c with liquidus temperatures about 1000 c water contents greatly in excess of 4 wt would mean that the magma would become vapor saturated at high pressures and would tend to crystallize during ascent to a fine grained granite before reaching shallow depths the main sources of water for magma generation are the dehydration of hydrous silicates within the crust and volatiles transported into the crust from subducted oceanic crust and upper mantle in the form of hydrous basalts and andesites dehydration reactions of muscovite biotite and horn blende are of particular significance anatectic granites may be partially classified in terms of the probable dehydration reaction responsible for their generation melts generated from muscovite dehydration are relatively cool peraluminous in composition high in k na ratio and generally high in initial 87 sr 86 sr and delta 18 o biotite generated melts tend to be higher in temperatures peraluminous to meta aluminous in composition moderately high in k na ratio and relatively high in strontium and oxygen ratios both types may contain metasedimentary enclaves granitoids generated by hornblende dehydration would be much higher in initial temperature peralka line to meta aluminous in composition and lower in k na ratio and in general would have lower strontium and oxygen ratios volatiles deposited in hydrothermally altered oceanic crust and upper mantle will be released during subduction in the form of hydrous basalts and andesites these melts are important energy transfer mechanisms to drive anatexis within the crust if these melts encounter a silicic magma chamber when intruding the crust they will be trapped below the lower density granitic melt while partially quenching against the cooler melt they may release volatiles and heat which will aid in the melting process granites generated in this way will vary in their geochemical properties depending on the relative importance of crustal versus mantle magma systems in general they will have moderate to high initial temperatures be meta aluminous in composition be variable but somewhat low in k na ratio and have lower initial strontium and delta 18 o ratios than will melts derived completely from the crust the fact that most terrestrial spreading centers are subaqueous thus may aid in the formation of a thick granitic continental crust

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

dehydration melting and water saturated melting of basaltic and andesitic greenstones and amphibolites at 1 3 and 6 9 kb

hornblende dehydration melting in mafic rocks and the link between massif type charnockite and associated granulites eastern ghats granulite belt india a massif type intrusive charnockite body in the eastern ghats granulite belt india is associated with hornblende bearing mafic granulite two pyroxene granulite and enderbitic granulite the charnockite is characterised by pervasive gneissic foliation s1 this is axial planar to the folded layers of hornblende bearing mafic granulite f1 folds indicating that the granulite protoliths were present before the development of s1 two pyroxene granulite and enderbitic granulite occur as lenticular patches disposed along the foliation and hence could be syngenetic to s1 the tonalitic to granodioritic metaluminous to weakly peraluminous compositions and relatively high sr rb of the charnockite are consistent with its derivation by partial melting of a mafic protolith strong y depletion lack of sr depletion and strongly fractionated ree patterns with high la yb n ratio but relatively lower hree gd lu fractionation with marked positive eu anomalies suggest major residual hornblende as well as garnet but not plagioclase consistent with the hornblende dehydration melting in the source rocks such a residual mineralogy is broadly similar to those of some of the hornblende bearing mafic granulite inclusions which have compositional features indicative of a restitic nature quantitative modelling supports an origin for the charnockite melts by partial melting of a hornblende rich mafic granulite source although source heterogeneity is very likely given the rather variable trace element contents of the charnockite the whole rock and mineral compositions of the two pyroxene granulites and enderbitic granulites are consistent with them representing peritectic phase segregations of hornblende dehydration melting a clockwise p t path implies that melting could have occurred in thickened continental crust undergoing decompression

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

the role of strain localization in the segregation and ascent of anatectic melts namaqualand south africa granulite facies gneisses of the late proterozoic okiep copper district of the namaqua province in south africa preserve evidence of a range of anatectic melt features that reflect the initial stages of segregation and ascent of crustally derived magmas during high grade metamorphism these melt bodies include both in situ and sharply transgressive subvertical pipe like bodies that show vertical dimensions of several hundreds of metres and horizontal dimensions of tens to hundreds of metres migmatite bodies are spatially closely linked with narrow upright zones characterized by intense high strain fabrics locally referred to as steep structures they display a progressive textural development from diktyonitic textures in in situ bodies via schollen and raft textures to largely homogeneous intrusive granites that have migrated vertically for distances of several hundreds of metres or more the intimate association of the migmatite bodies with the steep structures reflects strain induced melt segregation facilitated by increased permeabilities in these ductile deformation zones and migration of melts into sites of strain incompatibility at the intersections between the regional subhorizontal gneissosity and the superimposed high strain zones subsequent melt migration was focused along the network of subvertical structural anisotropies provided by the steep structures melt migration was controlled by a combination of buoyancy shear enhanced melt compaction during ongoing deformation melt compaction due to the settling of wall rock fragments from higher stratigraphiclevels and subordinate brittle fracturing the unusual geometry of the steep structures and the intensely heterogeneous nature of the strain and the absence of similar strain features and voluminous melt bodies elsewhere in the granulite facies terrane suggest a positive feed back mechanism between melt generation and strain localization in steep structures the structural development of the migmatite bodies illustrates that the efficiency of melt segregation and migration in a mid crustal segment is dependent not only on the fertility of its lithologies but also on its deformational style

ductile thrusting at mid crustal level southwestern grenville province the northwestern boundary zone of the central metasedimentary belt grenville province in the haliburton area ontario is a stack of alternating tonalitic and syenitic crystalline thrust sheets transported toward the northwest on out of sequence upper amphibolite facies ductile thrust zones during the grenvillian orogeny at 1060 ma approximately 100 ma after the initiation of thrusting in the underlying central gneiss belt kinematics of the deformation are complex predominant northwestward thrusting was at least partly coeval with subordinate northeastward thrusting late synmetamorphic extensional shears cut both thrusts and thrust sheets minor late thrusting on discrete ductile shear zones postdates the extensional structures belts of mechanically weak pelite appear to have localised the thrust sheets highly mobile marble behaved as a relatively low viscosity fluid during transport able to intrude and erode more competent wall rock

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

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

zoning patterns in orthopyroxene and garnet in granulites implications for geothermometry compositional maps of orthopyroxene and garnet of contrasting grain size and in contact with different minerals were made from two paragneiss granulites from the minto terrane of northern quebec the compositional maps provide clear evidence of late exchange of fe fe mg after ca in garnet and al in orthopyroxene had been quenched in the extent of late fe mg exchange was controlled by neighbouring minerals with negligible fe mg gradients against plagioclase and quartz and substantial gradients against exchangeable fe mg minerals cores of grains in contact with exchangeable fe mg neighbours are progressively more reset in fe fe mg as grain size decreases whereas cores of even small grains surrounded by only plagioclase and quartz are not significantly different in fe fe mg than cores of the largest grains gradients of ca in garnet and of al in orthopyroxene in grains of uniform fe fe mg preserve a high temperature retrograde history during which intergranular exchange effected compositional uniformity of mineral rims and intragranular fe mg diffusion in garnet and orthopyroxene was rapid enough to homogenize fe fe mg the transition from efficient intergranular exchange at relatively high temperatures to local fe mg exchange at lower temperatures may have been controlled by loss of an intergranular exchange medium in the rock possibly an internally generated dehydration melt phase implications for geothermometry of granulites include the following numerical values are particular to this study 1 core compositions of garnet and orthopyroxene grains in contact with exchangeable neighbours may be reset in fe fe mg relative to the most refractory compositions by an amount equivalent to 120d c 2 fe mg exchange thermometry using even the most refractory fe fe mg compositions may not record peak granulite conditions possibly recording instead the temperature at which an intergranular exchange medium was lost from the rock and 3 temperature sensitive net transfer equilibria involving al solubility in orthopyroxene yield temperatures up to 150d c higher than maximum fe mg exchange temperatures even in grains with flat fe fe mg compositional profiles making them a better means of estimating peak granulite temperatures than fe mg exchange thermometry

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

high grade metamorphism in the chapleau foleyet area ontario archean rocks superior province kapuskasing structural zone in migmatitic mafic gneiss paragneiss dioritic and tonalitic rocks a lower grade garnet clinopyroxene plagioclase gt cpx pl zone and patchy higher grade orthopyroxene zones are distinguished based on mineral melt equilibria minimum conditions for the gt cpx pl zone are 750 degrees c 6 kbar a h 2 o 0 5 0 7 and for the orthopyroxene zone 770 degrees c 6 kbar a h 2 o 800 degrees c pressure values are 5 4 8 4 kbar and define a nne trending area of relatively high p in the kapuskasing zone supporting the interpretation of a tilted crustal section modified journal abstract

some remarks on high temperature low pressure metamorphism in convergent orogens many high temperature low pressure high t low p metamorphic terranes show evidence for peak mineral growth during crustal thickening strain increments at pressures near the maximum attained during the heating cooling cycle such terranes are not readily explained as the conductive response to crustal thickening since the resulting moho temperatures would greatly exceed the crustal liquidus and because heating due to conductive equilibration on length scales appropriate to lithospheric scale strains must greatly outlast the deformation consequently high t low p metamorphism may be generated during crustal thickening only when significant heat is advected within the crust as for example may occur during the segregation of granitic melts we show that without the addition of asthenospheric melts and at strain rates appropriate to continental deformation the conditions required for significant lower crustal melting during deformation are only likely to be attained if heat flow into the lower crust during crustal thickening is increased substantially for example by removing the mantle part of the lithosphere a simple parameterization of lithospheric deformation involving the vertical strain on the scale of the crust c and the lithosphere 1 respectively allows the potential energy of the evolving orogen to be readily evaluated using this parameterization we show that an important isostatic consequence of the deformation geometries capable of generating such high t low p metamorphism during crustal thickening with c1 is an imposed upper limit to crustal thicknesses which is much lower than for homogeneous deformations fc f1 for the same initial lithospheric configuration

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

melt enhanced deformation a major tectonic process convergent tectonics between continental crustal blocks result in deep burial and anatexis of supracrustal rocks anatectic and or mantle derived melts combine to form a melt weakened zone in the thickened lower crust the accumulation of melt eventually leads to crustal failure along melt lubricated shear zones rapid 1 mm yr vertical component movements of large crustal blocks result we refer to these crustal displacements as tectonic surges the melt lubricated shears are characterized by the close association of sheared country rocks with foliated or massive igneous sills and plutons rocks that formed at different crustal levels are juxtaposed across these shear zones one result of surges with large lateral component of movement is metamorphic inversion with high pressure and high temperature metamorphic rocks structurally over lower pressure and temperature assemblages large and rapid vertical surges may displace crust containing abundant melt and may result in high t p including granulite facies metamorphism and preservation of metamorphic textures caused by rapid decompression

garnet growth during amphibolite anatexis implications of a garnetiferous restite formation of a garnet clinopyroxenite restite may have important physicochemical consequences for deep crustal and mantle processes we have experimentally simulated the dehydration melting of amphibolite and report the modal and compositional evoltuion of the mafic granulitic mineral assemblage the restite with an emphasis on garnet a natural low k calcic amphibolite was powdered and run in sealed gold capsules in a piston cylinder apparatus at 10 kbar 750 1000 c 1 to 9 day runs and fo 2 ni nio garnet first appears in 8 day runs at 825 25 c above 850 c clinopyroxene and poikilitic garnet comprise most of the restite garnet peaks at 950 c at 32 vol of the material within the capsule or 41 vol of the restite assemblage and disappears between 975 1000 c there is no cross over of fe mg partitioning between coexisting liquids and garnets liquids remain mg poorer than garnets throughout the melting interval but do increase in mg with increasing temperature thus the hotter liquids may be less likely to react with mantle wedge peridotite the modes are consistent with ree patterns of archean tonalitic trondhjemitic rocks generated by the partial melting of garnet bearing mafic crust a garnet clinopyroxenite with 41 vol garnet has a density of 3 5g cm 3 which may induce lower crustal delamination in hot nonthickened amphibolitic crust this crust is seismically indistinguishable from lithosphere or mantle because garnet growth traps water bearing hornblende inclusions the restite may contain up to 0 3 wt water thus delivery of poikilitic garnet to the mantle possibly by delamination or by subduction may be an important mechanism for adding water to the mantle

introduction to special section mechanisms and consequences of melt segregation from crustal protoliths much of petrological effort has been concentrated on attempts to discern define and interpret regularly recurring patterns of igneous and metamorphic petrogenesis i do not wish to deny the existence of discernable order in petrological phenomena but every geological event is unique if we concentrate too heavily on discernment of order and pattern may we perhaps overlook or underrate the unique quality of each igneous or metamorphic episode possibly uniqueness may have as great significance as conformity to pattern when we attempt to fit the phenomena of petrology into the broad framework of geology francis j turner presidential address to the mineralogical society of america november 11 1969

syntectonic granite emplacement at different structural levels the closepet granite south india abstract the closepet granite in south india is a large 400 km long but only 30 km wide elongate late archaean granitic body structural levels from deep crust to upper levels crop out as evidenced by a shallowing of paleo depths from north to south all along the closepet granite this allows the study of the emplacement of the same granitic body at various crustal levels four zones have been identified a root zone where magmas are collected in active shear zones a transfer zone featuring large scale magma ascent and crystal liquid partitioning in the granitic mush a gap where the mush was filtered allowing only the liquids to rise shallow intrusions filled with this liquid the closepet granite was emplaced syntectonically field work and anisotropy of magnetic susceptibility allowed documentation of steep foliations with subhorizontal lineations both in the root and transfer zones and in the shallow intrusions remote sensing evidenced a network of shear zones bounding the closepet granite in the porphyritic root and transfer zones magmas cooled slowly thus developing strong fabrics during large scale dextral shearing ascent of residual liquids amidst the crystallizing solid framework was not recorded in the fabrics however these liquids were channelised through the gap and infilled the homogeneous shallow intrusions where rapid cooling only permitted the development of feint although wholly consistent fabrics

thermobarometry in granulites pitfalls and new approaches abstract computer models of cooling granulite facies rocks with the assemblage garnet biotite sillimanite k feldspar plagioclase quartz are presented that incorporate simultaneous reaction net transfer and exchange and multicomponent diffusion in garnet in models where cooling is accompanied by exchange reactions plus diffusion only the calculated garnet core matrix biotite temperature is always lower than the metamorphic peak by an amount that depends on the cooling rate the size of the garnet and the volumetric ratio of garnet to biotite maximum calculated core temperatures of approximately 700 750 c are to be expected from typical granulite facies garnet biotite assemblages and calculated core temperatures can easily be as low as 600 c in models where both net transfer and exchange reactions are possible calculated garnet core matrix biotite temperatures are typically higher than the metamorphic peak temperatures by more than 150 c for large garnets r 2 mm but may be lower than the metamorphic peak for small garnets r the composition of matrix plagioclase in equilibrium with garnet is very sensitive to the amount of modal plagioclase that reacts and hence calculated p t trajectories are very sensitive to the petrologic interpretation of this recrystallization history for models in which all the modal plagioclase reacts the core and rim pressures may be faithfully recovered provided that the temperature estimate for the core is reliable for models in which only a small percentage of the modal plagioclase reacts very small modal amounts of plagioclase may be in equilibrium with garnet rim which if incorrectly interpreted may lead to grossly erroneous p t path interpretations a method to test an inferred p t path model is presented that involves attempting to match the observed garnet and plagioclase zoning information with a forward model that involves simultaneous reaction and diffusion an example from western new hampshire usa is presented that closely matches the measured profile

evolution of structurally contrasting anatectic migmatites in the 3 kbar ballachulish aureole scotland anatectic migmatites of contrasting structural style are found adjacent to the contacts of the ballachulish igneous complex argyllshire scotland on the east flank evidence for migmatization is largely restricted to the local development of millimetre centimetre scale kfs qtz rich leucocratic segregations which accompany fragmentation of brittle hornfels layers and ductile deformation of mm cm scale semipelitic layers large volumes of semipelitic rock rich in feldspar and quartz on the east flank show no migmatitic features and bedding is usually preserved undisturbed right up to the contact on the west flank in contrast similar semipelitic rocks show widespread migmatitic features and disruption of layering is substantial and widespread over a 400 m wide zone within the west flank migmatites 1 100 cm scale rigid bedding fragments schollen may be suspended and disoriented in a semipelitic matrix that underwent ductile deformation the p t conditions on both flanks are in the same range 3 kbar and 650 700 c the contrast in gross structural style is believed to result from differences in the volumes of melt produced and differences in the proportion of rock in which the critical melt fraction of the rocks was exceeded on the east flank only on a mm cm scale was enough melt locally accumulated to cause disruption of some layers and segregation of melt on the west flank melting proceeded substantially in a broad tract of semipelitic rocks resulting in larger scale contrasts in rheology that led to the present chaotic structures in this zone because migmatization occurred at a pressure too low for muscovite dehydration melting and at temperatures too low for substantial biotite dehydration melting the different amounts of melting on the east and west flanks most probably resulted from the introduction of differing amounts of externally derived water on the east flank and throughout most of the aureole the absence of melting even in quartzofeldspathic protoliths indicates that there was no substantial movement of fluid towards or away from the igneous complex during migmatization the contrasting situation on the west flank may have resulted from devolatilization of underlying quartz diorite magma 690 710 c which released heat and fluids into the overlying quartz and feldspar rich semipelites solidus temperature 650 680 c

internal differentiation of the archean continental crust fluid controlled partial melting of granulites and ttg amphibolite associations in central finland fault bound blocks of granulite and enderbite occur within upper amphibolite facies migmatitic tonalitic trondhjemitic granodioritic ttg gneisses of the iisalmi block of central finland these units record reworking and partial melting of different levels of the archean crust during a major tectonothermal event at 2 6 2 7 ga anhydrous mineral assemblages and tonalitic melts in the granulites formed as a result of hydrous phase breakdown melting reactions involving amphibole at peak metamorphic conditions of 8 11 kbar and 750 900 c a nominally fluid absent melting regime in the granulites is supported by the presence of carbonic fluid inclusions the geochemical signature of light rare earth element lree depleted mafic granulites can be modelled by 10 30 wt partial melting of an amphibolite source rock leaving a garnet bearing residue the degree of melting in intermediate granulites is inferred to be less than 10 wt and was restricted by the availability of quartz pressure temperature estimates for the ttg gneisses are significantly lower than for the granulites at 660 770 c and 5 6 kbar based on the p t conditions melting of the ttg gneisses is inferred to have occurred at the wet solidus in the presence of an h 2 o rich fluid a hydrous mineralogy abundant aqueous fluid inclusions and the absence of carbonic inclusions in the gneisses are in accordance with a water fluxed melting regime low ree contents and strong positive eu anomalies in most leucosomes irrespective of the host rock composition suggest that the leucosomes are not melt compositions but represent plagioclase quartz assemblages that crystallized early from felsic melts furthermore similar plagioclase compositions in leucosomes and adjacent mesosomes are not a migmatite paradox as both record equilibration with the same melt phase percolating along grain boundaries

partial melting of two amphibolites contrasting experimental results under fluid absent conditions a large portion of the lower continental crust may be amphibolitic in composition and without a free fluid phase as a consequence h2o undersaturated or fluid absent melting of amphibolites may be responsible for the formation of some granites and migmatites produced during major orogenic events in an attempt to determine the systematics of melting under fluid absent conditions a series of piston cylinder experiments was conducted on two natural amphibolites one a meta alkali basalt aba with a total water content of wt contained in hornblende and the other a meta island arc tholeiite iat which has 1 1 3 wt water contained in hornblende cummingtonite and biotite the experimentally determined melting ranges of the two amphibolites showed that the solidus temperatures and sta temperature interval over which amphibole was stable were controlled by the amphibolites different bulk compositions and their resulting metamorphic assemblages the volume of melt produced by melting of the two amphibolites were compared with estimated amounts based on burnham s 1979 water melt solubility model and the fluid absent melting model presented by clemens and vielzeuf 1987 the observed melt volumes were greater than estimated as the water content of melt largely detemines the volume of melt produced independent measurements of the water content of the glass formed during partial melting in the aba were made by thermogravimetric analyses the water content of the aba glass is 2 wt which is less than the assumed melt water content water content of the melt used in previous modeling of fluidabsent anatexis in mafic lithologies as a consequence more melt can be expected during fluid absent partial melting of mafic lower crust as is observed in the experiments a modification of the clemens and vielzeuf 1987 fluid absent melting model for mafic compositions has been made using the experimental data available on melting in basaltic systems and is presented here for pressures of 5 8 and 10 kbar tectonic scenarios in which the crust is thickened i e by collision then undergoes extension or where a previously thinned crust is later rethickened provide enough heat so that amphibolite melting under fluid absent conditions can become importan and hence responsible for some melts produced during post collisional magmatism the results may also have applications to melting in hydrated oceanic crust in subduction zones and in island arc terains

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

generation of tonalitic and dioritic magmas by coupled partial melting of gabbroic and metasedimentary rocks within the deep crust of the famatinian magmatic arc argentina the source regions of dioritic and tonalitic magmas have been identified in a deep crustal section of the famatinian arc sierras pampeanas of western argentina the source zones of intermediate igneous rocks are located at the transition between a gabbro dominated mafic unit and a tonalite dominated intermediate unit in the upper levels of the mafic unit mafic magmas intruded into metasedimentary wall rocks crystallized mainly as amphibole gabbronorite and caused the partial melting of the surrounding metasediments in turn the leucogranitic melts sourced from the metasedimentary rocks intruded into the newly crystallized but still hot mafic layers and catalysed the process of partial melting of the gabbroic plutonic rocks the gabbroic rocks became mafic migmatites comprising amphibole rich pyroxene bearing mesosomes and leucotonalitic veins significantly most of the mafic migmatites have isotopic compositions sr sr t 50 7063 and end t 1 4 0 94 to 2 24 similar to those of the gabbroic rocks and distinct from those of their complementary leucotonalitic veins sr sr t 1 4 0 7075 0 7126 and end t 5 2 65 providing evidence for the idea that melting of the mafic rocks was triggered by the intrusion of leucogranitic anatectic melts sr sr t 1 4 0 715 and end t 1 4 6 21 mass balance calculations show that the model reaction plagioclase amphibole leucogranitic melt leucotonalitic melt clinopyroxene orthopyroxene can better explain the partial melting of the gabbroic rocks based on field observations we argue that the coalescence of leucotonalitic veins in the mafic migmatites led to breakdown of the solid matrix to form melt dominated leucotonalitic pools however the leucotonalitic veins that crystallized before leaving behind the mafic migmatitic rock are chemically elemental and isotopic more evolved than the dioritic and tonalitic rocks we envisage that once detached from their source region the leucotonalitic magmas were able to react commingle and mix with entrained fragments of both mafic and metasedimentary rocks this process gave rise to melts that became tonalitic and dioritic magmas this study concludes that the generation of intermediate magmas is a multistage process with three critical steps 1 influx and emplacement of hydrous mafic magmas into a deep crust containing metasedimentary country rocks 2 physically and chemically coupled melting of mafic and metasedimentary rocks leading to the formation of a leucotonalitic vein and dyke system that coalesces to form leucotonalitic or tonalitic magma bodies 3 retrogression of the leucotonalitic magmas by partially assimilating entrained fragments of their mafic and metasedimentary precursors the dimensions of the

partial melting in amphibolites in a deep section of the sveconorwegian orogen sw sweden garnet amphibolite metataxites at the steningekusten nature reserve in southwestern sweden contain tonalitic patches and veins whole rock chemistry suggests that the protoliths were mafic igneous rocks with alkaline affinities orthopyroxene megacrysts are present in leucosome in parts of these garnet amphibolites but absent in others orthopyroxene megacrysts were formed by vapor absent melting initiated by incongruent melting of biotite followed by the breakdown of hornblende the net reaction was bt hbl pi qtz opx melt cpx gt melting occurred at pressures of approximately 1 gpa and temperatures which probably exceeded 800 degrees c pyroxenes are surrounded by hornblende quartz symplectites and hornblende in these coronas has distinctly lower concentrations of na k and ti than that in adjacent mesosorne the hornblende rims formed upon cooling and reaction with crystallizing melt this created a barrier to further reaction thus preserving the orthopyroxene megacrysts garnet amphibolite metatexites lacking pyroxene megacrysts have features characteristic of vapor present melting including lack of peritectic phases predicted by vapor absent melting reactions larger amounts of leucosome 14 versus 7 and less distinct melanosomes the variation in these migmatites reflects open system behavior either on a regional scale with the migration of aqueous fluids into the amphibolites or on a local scale with the migration of melt within the amphibolites zircons from all units have cl dark core domains that are dated at 1415 1390 ma the core zones are cut and overgrown by cl dark and cl bright rims that are dated at 975 965 ma the zircon rims are thin in the mesosome but are thicker in the leucosome suggesting that they formed during migmatization new growth of zircon associated with migmatization at ca 970 ma corresponds to the timing of crustal scale partial melting in the deep regions of the sveconorwegian orogen synchronous with east west extension and the intrusion of mafic dykes if partial melting was driven by an influx of aqueous fluids they were probably derived from a relatively cool source region which would indicate tectonic juxtaposition of hotter and cooler terranes c 2015 elsevier b v all rights reserved less

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

re examining pluton emplacement processes abstract previous pluton emplacement studies have attempted to explain how space is made during pluton emplacement in fact the only means of making space during emplacement of mantle derived magmas in the crust are 1 lowering the moho or 2 outward displacement of the earth s surface other pluton emplacement mechanisms are material transfer processes mtps that do not increase the volume of the crust one frequently cited mtp for emplacement of concentrically zoned and ballooning plutons is ductile flow of wall rocks around the plutons the expected structures and total strains near such plutons are dependent on the three dimensional pluton shape the width of the deforming aureole whether the pluton is a piercing or non piercing diapir and in the latter case the number of body radii the pluton travels we have re examined a few such plutons and calculated the amount of material transfer caused by ductile flow in no case is more than 40 and in most cases only 15 35 of the required material transfer accommodated by wall rock flow the contact aureoles around these plutons are too narrow and or strains too low indicating that the plutons are predominantly discordant bodies with narrow 0 1 0 4 pluton radii concordant aureoles a comparison of strains in natural contact aureoles and those made in mechanical models of diapirs indicates that natural strains are at least an order of magnitude less than model strains we suggest that the similarities between models of diapiric ascent of spherical bodies and concentrically zoned plutons are superficial because the models are too simplistic instead we argue that the ascent and emplacement of magmas require multiple near field and far field mtps these mtps will show gradients with depth distance from the magma and with time rates of near field mtps must be rapid whereas far field rates are probably nearer to long term average rates of orogenic processes

jurassic cordilleran dike swarm shear zones implications for the nevadan orogeny and north american plate motion a cogenetic and coeval tonalitic and mafic dike swarm has been identified within a southern fragment the owens mountain area of the western foothills terrane california the dikes were mylonitized and transposed rotated into subparallel orientation during emplacement from 155 to 148 ma u pb zircon data which coincides in time with the nevadan orogeny steeply southeast plunging fold axes and s fold geometries indicate a sinistral sense of shear possibly with some dip slip motion as well this shear zone may be the southern and possibly deeper extension of the bear mountains fault zone this and other late jurassic cordilleran dike swarms record a complex pattern of sinistral sense transtension transpression that developed at the apparent polar wander j2 cusp 150 ma and during subsequent rapid northwestward acceleration of north america the late jurassic nevadan orogeny is a manifestation of these dramatic changes in magnitude and direction of north american motion

pluton emplacement along an active ductile thrust zone piute mountains southeastern california interaction between deformational and solidification processes plutons that are emplaced synchronously with regional deformation offer the opportunity to understand the partitioning of deformation and metamorphism in time and space during orogeny the 85 ma east piute pluton of the eastern mojave desert is an unusually clear example of a syntectonic pluton it was emplaced during thrusting in the southern cordillera combined petrologic and structutal studies lead to empirical models for the dynamic interaction of magmatic and deformational processes during granitoid emplacement and final crystallization trace and major element and isotopic data suggest that closed system differentiation of a single magma batch was responsible for compositional variation granodiorite to leucocratic granite correlation between degree of chemical evolution of compositional units and intrusive sequence suggests that discrete melt segregation events punctuated fractional crystallization modeling of trace element data suggests segregation of monzogranite melts after 50 70 crystallization of the magma and segregation of leucocratic granite after about 80 crystallization deformation promoted segregation of melt fractions at discrete times during crystallization during final ascent and after the magma had crystallized sufficiently 50 70 to attain shear strength monzogranite liquid was incompletely segregated along the active thrust zone at the southern margin of the pluton at essentially the present crustal level and after about 80 crystallization the remaining leucocratic granite melts were more effectively segregated and migrated into thrust related extensional openings including tension gashes undeformed pegmatite and aplite dike arrays indicate that final crystallization coincided with final increments of shear zone movement solid state deformational fabrics and brittle synthetic thrusts that offset the pluton margin record sharp temperature decline during and after crystallization because plutons crystallize rapidly they can be used to estimate strain rates during orogeny thermal modeling suggests that the transition from rheological liquid to solid in the east piute pluton occurred during an interval of about 10 4 yr as final increments of ductile deformation coincided with final crystallization of evolved compositional units total strain recorded by fabric in the pluton apparently accumulated during this interval shear strains estimated from the s c fabric in the granodiorite unit and rotation of pegmatite filled vein arrays are on the order of 1 thus strain rates in the shear zone were on the order of 10 12 sec during crystallization of the pluton this corresponds to displacements of tens of centimeters per year these high strain rates suggest punctuated melt enhanced movement episodes during contractional orogeny

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

experimental deformation of partially melted granitic aggregates abstract the effects of varying amounts of partial melt on the deformation of granitic aggregates have been tested experimentally at conditions 900 c 1500 mpa 10 4 to 10 6 s where melt free samples deform by dislocation creep with microstructures approximately equivalent to those of upper greenschist facies experiments were performed on samples of various grain sizes including an aplite 150 m and sintered aggregates of quartz albitemicrocline 10 50 and 2 10 m water was added to the samples to obtain various amounts of melt 1 15 in the aplite 1 5 in the sintered aggregates optical and tem observations of the melt distribution in hydrostatically annealed samples show that the melt in the sintered aggregates is homogeneously distributed along an interconnected network of triple junction channels while the melt in the aplites is inhomogeneously distributed the effect of partial melt on deformation depends an melt amount and distribution grain size and strain rate for samples deformed with 1 melt all grain sizes exhibit microstructures indicative of dislocation creep for samples deformed with 3 5 melt the 150 m and 10 50 m grain size samples also exhibit dislocation creep microstructures but the 2 10 m grain size samples exhibit abundant tem scale evidence of dissolution precipitation and little evidence of dislocation activity suggesting a switch in deformation mechanism to predominantly melt enhanced diffusion creep at natural strain rates melt enhanced diffusion creep would predominate at larger grain sizes although probably not for most coarse grained granites the effects of melt percentage and strain rate have been studied for the 150 m aplites for samples with 5 and 10 melt deformation at 10 6 s squeezes excess melt out of the central compressed region allowing predominantly dislocation creep conversely deformation at 10 5 s produces considerable cataclasis presumably because the excess melt cannot flow laterally fast enough and a high pore fluid pressure results for samples with 15 melt deformation at both strain rates produces cataclasis presumably because the inhomogeneous melt distribution resulted in regions of decoupled grains which would produce high stress concentrations at point contacts at natural strain rates there should be little or no cataclasis if an equilibrium melt texture exists and if the melt can flow as fast as the imposed strain rate however if the melt is confined and cannot migrate a high pore fluid pressure should promote brittle deformation

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

rheological evolution of a pseudotachylyte bearing deep crustal shear zone in the western canadian shield abstract crustal scale continental strike slip fault systems are typically thought to be thermally segregated into an upper seismogenic expression and a lower zone of ductile deformation likely extending into the lower crust the ability of the lower crust to sustain stress is fundamental to understanding the mechanical behavior of the lithosphere yet significant debate exists regarding the strength of the ductile crust the cora lake shear zone in the western churchill province of the canadian shield is a 4 7 km wide zone of granulite to upper amphibolite facies mylonite which hosts synkinematic pseudotachylyte suggesting a locally strong lower crust kilometer scale mapping suggests that the shear zone localized at a major lithological and terrane boundary with the pseudotachylyte network centered within the ca 1 km wide more intensely deformed finer grained ultramylonitic core in this contribution we use optical petrography and electron backscatter diffraction to describe microstructure across a transect of the cora lake shear zone by combining these observations with grain size piezometry experimental flow laws and regional pressure temperature paths we conclude that stress amplification from a relatively common type of km scale lithological heterogeneity and cooling history is the most reasonable explanation for the deep crustal pseudotachylyte