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

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

mafic magma intraplating anatexis and hybridization in arc crust bindal batholith norway distinct with nd and o in the migmatites from 7 6 to 9 6 the dioritic velfjord plutons 448 ma were emplaced into and from 10 9 to 13 5 and in the contact granites regional migmatitic metapelitic and metacarbonate rocks at midfrom 5 2 to 7 5 and from 9 6 to 12 3 respectively crustal levels corresponding to pressures of 700 mpa exthus the contact granites could have a source that is isotopically humation to 400 mpa began while the migmatites were in a distinct from but mineralogically similar to the diatexites or they partly molten state with increasing proximity to the plutons could result from mixing of magma similar to the high k leucosomes regional stromatic migmatites change to diatexite and diatexitic with dioritic magmas mass balance calculations are consistent dikes are common within 500 m of the contacts we interpret these with the latter interpretation with proportions of granitic to dioritic relationships to indicate that heat from the plutons resulted in contact magmas from 7 1 to 7 3 emplacement and solidification of the migmatization in a zone up to 1 km wide typical residual dioritic plutons provided zones of structural anisotropy along which mineralogy in the diatexites is plagioclase quartz biotite high k leucosome magmas and contact granite magmas collected garnet sillimanite k feldspar consistent with biotite these magmas were injected by additional dioritic magma and dehydration melting podand dike like leucosomes consist of two further hybridized because the solidi of the plutons were several types earlier high k granitic ones with mineral assemblages hundred degrees higher than that of the granitic magmas the pluton identical to the migmatites and later low k tonalitic ones in which walls acted as long lived hot rigid surfaces along which magmas sillimanite is sparse and garnet absent the high k leucosome collected and migrated magmas can be explained by biotite dehydration melting at 700 mpa within the aureole mafic magmas were locally injected into and hybridized with the diatexites and the high k leucosome

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

ages and petrogenesis of jurassic and cretaceous intrusive rocks in the matsu islands implications for lower crust modification beneath southeastern china abstract major and trace element whole rock sr nd and hf isotope zircon u pb age and hf o isotope data are reported for the intrusive rocks from the matsu islands in the coastal area of southeastern se china in order to study the ages sources and petrogenesis of these rocks and evolution of the lower crust the rocks include gneissic granite massive granite brecciated granite and diabase secondary ion mass spectrometer sims zircon u pb dating reveals that the rocks in the matsu islands were emplaced at 160 ma 130 ma and 94 ma the jurassic granites 160 ma have high sio2 74 1 74 5 wt and k2o na2o 8 32 8 33 wt contents and high rb sr ratios of 0 6 1 2 and la yb cn ratios of 12 6 19 4 their relatively high initial 87sr 86sr ratios 0 7074 0 7101 variable and negative end t values 9 2 to 5 4 and variable zircon ehf t 17 0 to 5 2 and 18o 4 7 8 1 values indicate they were mainly derived from an ancient lower crustal source but with involvement of high ehf t and low 18o materials the early cretaceous diabase 130 ma has sio2 content of 56 5 wt relatively high mgo concentration low initial 87sr 86sr ratio and negative end t value similar to geochemical features of other cretaceous mafic rocks in the coastal area of se china zircons from the diabase have high ehf t values 5 5 to 0 2 and relatively low 18o values of 4 2 5 0 these characteristics indicate that the parental magma of the diabase was generated by partial melting of enriched lithospheric mantle which have been metasomatised by altered oceanic crust derived low 18o fluids for the cretaceous granitoids 130 ma and 94 ma they have relatively low sio2 68 0 71 3 wt and k2o na2o 5 30 7 55 wt contents and low rb sr ratios and la yb cn ratios of 5 8 7 1 they have low initial 87sr 86sr ratios 0 7071 0 7082 homogeneous end t 4 3 to 4 5 and relatively high zircon ehf t values 3 7 to 1 2 and low 18o values 4 6 5 9 their isotopic compositions are similar to those of the diabases in this study as well as other cretaceous mafic rocks in the coastal area of se china suggesting that the sources of the cretaceous granitoids might be the newly formed lower crust related to the underplated mafic rocks whole rock geochemical sr nd and zircon hf o isotopic compositions indicate that the jurassic granitoids are most likely generated by partial melting of relatively ancient basement rocks whereas the cretaceous granitoids were generated by partial melting of relatively young lower crustal rocks with addition of mantle derived magma this distinction implies that the pre existing ancient lower crust beneath the coastal area of se china has been modified by large scale mafic magma underplating therefore underplating of mantle derived mafic magma would result in modification of the pre existing ancient lower crust and formation of the relatively juvenile lower crust

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

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

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

automated determination of auroral breakup during the substorm expansion phase using all sky imager data this paper describes a novel method for quantitatively and routinely identifying auroral breakup following substorm onset using the time history of events and macroscale interactions during substorms all sky imagers substorm onset is characterized by a brightening of the aurora that is followed by auroral poleward expansion and auroral breakup this breakup can be identified by a sharp increase in the auroral intensity i t and the time derivative of auroral intensity i t utilizing both i t and i t we have developed an algorithm for identifying the time interval and spatial location of auroral breakup during the substorm expansion phase based solely on quantifiable characteristics of the optical auroral emissions we compare the time interval determined by the algorithm to independently identified auroral onset times from three previously published studies in each case the time interval determined by the algorithm is within error of the onset independently identified by the prior studies we further show the utility of the algorithm by comparing the breakup intervals determined using the automated algorithm to an independent list of substorm onset times we demonstrate that 50 of the breakup intervals characterized by the algorithm are within the uncertainty of the times identified in the list the quantitative description and routine identification of an interval of auroral brightening during the substorm expansion phase provides a foundation for unbiased statistical analysis of the aurora and to probe the physics of the auroral substorm and identify the processes leading to auroral substorm onset

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

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

u pb zircon dating geochemical and sr nd hf isotopic compositions of early indosinian intrusive rocks in west qinling central china petrogenesis and tectonic implications the qinling dabie sulu orogenic belt is the junction between the north and south china blocks which resulted from the final amalgamation of china continents during the indosinian indosinian granitoids are widespread in the qinling orogen and their geneses can thus constrain the evolution of china continent we carried out a combined u pb zircon dating and geochemical study for the shuangpengxi granodiorite pluton and the xiekeng diorite granodiorite pluton in the middle part of the west qinling orogen u pb zircon dating shows that the magma crystallization ages of 242 3 ma for the shuangpengxi pluton and 244 242 ma for the xiekeng pluton geochemical and sr nd hf isotopic compositions reveal that the magma of the shuangpengxi granodiorite was derived from partial melting of crustal materials the xiekeng diorites can be divided into high al diorite and high mg diorite both of them resulted from partial melting of enriched lithospheric mantle but their mantle source had been modified by previous slab derived melt the high al diorite was formed by fractional crystallization of olivine pyroxene and or preferential accumulation of plagioclase and the high mg diorite was formed by fractional crystallization of olivine and or preferential accumulation of pyroxene the xiekeng granodioritic porphyry was formed by mixing of crust derived and mantle derived melts we propose that the early indosinian magmatism resulted from break off of subducted oceanic slab after collision the slab break off model can well explain the linear distribution of the early indosinian plutons and rapid crustal uplift during the middle triassic in the west qinling

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

interaction between two contrasting magmas in the albtal pluton schwarzwald sw germany textural and mineral chemical evidence the magmatic evolution of the variscan albtal pluton schwarzwald sw germany is explored using detailed textural observations and the chemical composition of plagioclase and biotite in both granite and its mafic magmatic enclaves mmes mmes probably formed in a two step process first mafic magma intruded a granitic magma chamber and created a boundary layer which received thermal and compositional input from the mafic magma this is indicated by corroded granitic quartz crystals and by large granitic plagioclase xenocrysts which contain zones of higher anorthite and partly crystallized from a melt of higher sr content texturally different plagioclase types e g zoned and inclusion rich types correspond to different degrees of overprint most likely caused by a thermal and compositional gradient in the boundary layer the intrusion of a second mafic magma batch into the boundary layer is recorded by a thin an50 zone along plagioclase rims that crystallized from a melt enriched in sr most probably the second mafic intrusion caused disruption of the boundary layer dispersal of the hybrid magma in the granite magma and formation of the enclaves rapid thermal quenching of the mmes in the granite magma is manifested by an30 overgrowths on large plagioclase grains that contain needle apatites our results demonstrate the importance of microtextural investigations for the reconstruction of possible mixing end members in the formation of granites

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

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

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

crust mantle interaction during syn collisional magmatism evidence from the oamikaub diorite and neikhoes metagabbro damara orogen namibia abstract the early syntectonic 563 7 6 1 ma old oamikaub diorite damara orogen namibia consists of metaluminous magnesian calc alkalic to calcic diorites granodiorites and granites associated gabbro diorites and gabbros belong to the neikhoes metagabbro linear major and trace element variations imply that the rock suite evolved through fractional crystallization processes involving amphibole biotite fe ti oxides zircon and apatite initial sr 87sr 86sr 0 7058 0 7123 and nd e nd 2 1 to 18 8 isotopic compositions are highly variable and negatively correlated indicating that assimilation of crustal components occurred unradiogenic initial 206pb 204pb 16 23 17 23 and 207pb 204pb ratios 15 50 15 57 suggest derivation from or interaction with ancient crust with low u pb two gabbro diorites have mgo ni and cr abundances that are compatible with derivation of these rocks from upper mantle lithologies their initial e nd values 2 1 and 7 4 and 87sr 86sr ratios 0 7058 and 0 7076 imply derivation from an aged metasomatized lithospheric mantle other mafic samples have mgo abundances and compatible element concentrations that exceed the values commonly accepted for primary mafic melts implying some accumulation of clinopyroxene and amphibole the granodiorites form a homogenous group in which the isotope data initial e nd 12 4 to 14 1 initial 87sr 86sr 0 7083 0 7096 imply a lower crustal source the granites are also magnesian and calc alkaline but two of them are strongly peraluminous their isotope data initial e nd 13 2 to 18 8 initial 87sr 86sr 0 7099 to 0 7123 imply derivation from more ancient sources alternatively these samples gained their isotope systematics through extensive afc processes from parental granodiorites a common subduction zone environment as suggested from negative nb ta anomalies in multi element diagrams seems unlikely for all samples because of a lack of isotopically depleted signatures the data from the oamikaub diorite and other mafic complexes are better explained by a flat subduction model involving mainly continental mantle lithosphere and crust with limited if any melting of asthenospheric mantle

fluid present disequilibrium melting in neoarchean arc related migmatites of daeijak island western gyeonggi massif korea the melting process of meta igneous rocks was investigated via field petrographic and geochemical analyses of the neoarchean 2 51 ga migmatite complex in daeijak island western gyeonggi massif this complex consists primarily of garnet free amphibolites and tonalitic migmatites both of which contain hornblende plagioclase and quartz as major constituents neosomes and leucosomes in the migmatite have dioritic tonalitic and tonalitic trondhjemitic compositions respectively compositions of hornblende xfe 0 39 0 42 and plagioclase an24 27 vary little between the neosomes and leucosomes the amphibolites show distinct depletions in nb ta zr and ti relative to large ion lithophile elements suggesting an arc related origin for their basaltic protolith leucosomes have lower contents of k2o mgo feo tio2 zr rb and rare earth elements ree than amphibolites and neosomes but are higher in sio2 na2o and sr contents leucosomes and neosomes have positive eu eu n 1 32 7 26 and negative 0 71 0 97 eu anomalies respectively which are attributed to the variable degree of plagioclase fractionation during the partial melting the p t condition for the migmatite formation was estimated to be 700 730 c and 4 7 5 5 kbar primarily based on the hornblende plagioclase thermobarometry and phase equilibria various lines of textural evidence such as the channel flow of melt along migmatitic layers and the segregation of melt into shear bands or boudin necks suggest a syn deformation crystallization of melt chemical disequilibrium in migmatites is documented not only by petrographic and geochemical data but also by the ree modeling between melt product and source rock disequilibrium process is most likely attributed to the rapidity of melt extraction or migration compared to chemical diffusion rate in summary the fluid present disequilibrium melting of dioritic tonalitic protoliths has produced tonalitic trondhjemitic leucosomes in a dynamic arc environment prevalent in the neoarchean crust of the korean peninsula as well as the shandong peninsula north china craton