identifying granite sources by shrimp u pb zircon geochronology an application to the lachlan foldbelt the potential genetic link between granites and their host sediments can be assessed using zircon age inheritance patterns in the lachlan fold belt southeastern australia granites and associated high grade metasedimentary rocks intrude low grade ordovician country rock this relationship is well exposed in the tallangatta region northeast victoria part of the wagga omeo metamorphic complex in this region granites two i types and two s types have intruded during the mid late silurian between approximately 410 430 ma based on the ages of magmatic zircons the age spectra for inherited zircons from the granites have been compared with those of detrital zircons from the enclosing low and high grade metasediments in broad terms both for detrital zircons in all four sediments and for inherited zircons in three of the four granites the dominant ages are early paleozoic and late precambrian with sporadic older precambrian ages extending up to 3 5 ga the ages of the youngest detrital zircons from the low grade lockhart and talgarno terranes limit the time of sedimentation to ca 466 ma or younger the youngest detrital zircons from two samples of the high grade gundowring terrane are 473 ma making these sediments ordovician or younger not cambrian as originally suggested however the individual age spectra for the four selected metasediments are not well matched when closely examined the age spectra of the inherited zircons in the granites also do not adequately match those in any of the metasediments thus the metasediments might not be representative of the actual source rocks of the granites while the exact source of the granites cannot be identified from the analysed samples the existence of a large population of ca 495 ma inherited zircon grains in the s type granites requires that the granite source contains a significant proportion of cambrian or younger material this does not preclude the existence of a precambrian basement to the lachlan fold belt but indicates that at the level of s type magma generation a cambrian and or younger protolith is required

evidence for a widespread mafic cover sequence and its implications for continental growth in the northeastern superior province abstract archean greenstone belts of the northeastern superior province nesp were emplaced over a 160 ma period 2 88 2 72 ga spanning a major episode of crustal reworking in which early tonalite trondhjemite plutonism evolved to dominant granite granodiorite and pyroxene bearing felsic plutonism the numerous greenstone belts that crop up across the craton contain lavas belonging to three mafic volcanic suites 1 mg tholeiites that have chemical compositions typical of many archean basalts with 4 10 wt mgo 9 15 wt fe2o3t 0 4 1 2 wt tio2 and la sm ratios between 1 and 3 2 fe tholeiites that have similar mgo contents and la sm ratios but markedly higher fe2o3t 11 20 wt tio2 1 0 2 6 wt and gd yb ratios that may reflect derivation from a distinct garnet bearing mantle source 3 light rare earth element lree enriched rocks that have the chemical characteristics typical of calc alkaline mafic magmas with higher sio2 al2o3 and la sm ratios but lower fe2o3t and tio2 than the tholeiitic suites at a given mgo content the presence of unconformities within single greenstone belts numerous inheritance ages in volcanic rocks replicating those of older volcanic plutonic assemblages and the recognition of widespread geographically separate yet coeval 2 78 ga extrusive assemblages containing the distinctive fe tholeiites do not support a plate tectonic model for a large portion of the nesp the correlations in composition and ages of greenstone belts are more consistent with a model in which they represent the remnants of an extensive autochthonous mafic cover sequence nd isotopic evidence for the presence of evolved crust in some syn volcanic felsic plutons and volcaniclastic rocks imply that this mafic cover sequence was erupted onto an older granitoid crust the amalgamation of two isotopically distinct terranes at ca 2 76 2 74 ga created a proto cratonic mass that consisted of a composite tonalite trondhjemite greenstone crust underplating by mafic mantle melts and widespread insulation after 2 75 ga increased temperatures sufficiently to produce extensive melting of this crust to generate pyroxene bearing felsic rocks and granodiorite to granite partial melts the isotopically enriched character of mantle derived magmas after 2 75 ga may reflect more extensive contamination by a felsic crust affected by widespread partial melting furthermore a decrease of nb th ratios in the lree enriched mafic rocks younger than 2 75 ga reflects a change in the nature of the crustal contaminant from tonalite trondhjemite prior to 2 75 ga to granite granodiorite afterwards

multistage zircon and titanite growth and inheritance in an archean gneiss complex winnipeg river subprovince ontario abstract the paper presents results from a u pb isotopic investigation of zircon and titanite from three phases of the archean sioux lookout gneiss complex at the margin of the gneissic plutonic winnipeg river subprovince wrs of the superior province the data reveal an intricate data pattern caused by inheritance and multistage crystallization and record the timing of two major orogenic events followed by a protracted post metamorphic evolution a zircon age of 2889 3 ma for a foliated pink granite correlates with that of similar units elsewhere in the superior province and reflects one of the first major episodes of crustal assembly in the pre kenoran evolution of the region tonalite and diorite from the same site yield zircon and titanite ages of 2704 2 and 2705 5 ma which correspond to the timing of major magmatism related to kenoran collision of the wrs with the adjacent wabigoon volcanic domains tonalite and diorite both contain abundant xenocrystic zircon moreover the late kinematic diorite also includes xenocrystic titanite which has a minimum age of 2730 ma and coexists with younger ca 2705 ma titanite and zircon two generations of titanite are also present in both the tonalite and the granite the earliest one reflects the 2704 ma plutonic metamorphic event whereas the second one indicates post metamorphic crystallization at 2650 and 2627 ma caused by superimposed hydrothermal activity and retrogression the data support the concept that the titanite can remain a closed system to pb diffusion at elevated temperatures as long as the crystals escape magmatic or metamorphic recrystallization conversely they also show that titanite can easily be produced by low temperature processes hence generalized interpretation of titanite ages in terms of regional cooling can be inappropriate

provenance of meso to neoproterozoic cover sediments at the ming tombs beijing north china craton an integrated study of u pb dating and hf isotopic measurement of detrital zircons and whole rock geochemistry abstract the north china craton ncc was subjected to an extensional regime after the luliang movement at 1 8 ga and then was covered by an extensive meso to neoproterozoic sedimentary succession namely the changcheng jixian and qingbaikou groups in ascending order we report age spectra for detrital zircons and monazites hf isotopic systematics of detrital zircons and whole rock chemical and nd isotopic compositions for sediments from the succession in the ming tombs area beijing one of the typical meso to neoproterozoic areas in the ncc detrital zircons of six sedimentary samples have two distinct age peaks at 2 52 ga and 1 85 ga there are some detrital zircons at 2 4 2 0 ga but none at 2 3 ga and only a few 2 7 ga the detrital zircon age spectra change with time sediments in the lower succession changcheng group and in the upper successions jixian and qinbaikou groups are dominated by significant detrital zircon populations of late neoarchean and late paleoproterozoic ages respectively the 2 5 ga detrital zircons of the changcheng group have ehf 2 5 ga values and tdm hf model ages mainly ranging from 2 to 7 and 2 8 to 2 7 ga respectively detrital monazites of a sample from the jixian group exhibit a major age peak between 1 95 and 1 80 ga with some data between 2 0 and 1 95 ga the sedimentary rocks of the changcheng group are characterized by high k2o contents mostly 7 09 15 20 and insignificant eu anomalies eu eu 0 71 1 16 they have tdm nd model ages ranging from 2 70 to 2 43 ga being older than the tdm nd ages 2 11 and 1 99 ga of sedimentary samples from the qingbaikou group based on a comparison with ages for the early precambrian 1 8 ga basement of the ncc it can be concluded that 1 the sediments of the meso to neoproterozoic cover were undoubtedly derived from the ncc itself or once neighboring terranes 2 variations in the detrital zircon age spectra from the lower to the upper successions reflect provenance evolution in that the lower crustal late paleoproterozoic rocks were exposed at the surface after the upper crustal late neoarchean rocks had already been eroded

crustal evolution of the western minto block northern superior province canada abstract geochemical and nd isotope data are presented for suites of meso to late archean 2 7 2 9 ga granitoid rocks from the western minto block of the northeast canadian superior province the samples provide coverage of the inukjuak and tikkerutuk domains and include biotite amphibole and or orthopyroxene bearing tonalites granodiorites and granites from tonalite trondhjemite granodiorite ttg and monzodiorite suites both suites are characterized by strong light rare earth element ree and large ion lithophile lil element enrichment variably negative to positive eu anomalies and moderate heavy ree depletion high la yb and sr y ratios with correspondingly low yb and y concentrations in both suites likely reflect parental melts that left residual garnet correlations between eu anomalies and fractionation indices suggest that compositional variations in both suites reflect plagioclase accumulation in tonalite samples and plagioclase and fe ti oxide fractionation among granitic samples the nd isotope compositions range from nd values of 2 to 10 the largest range found among granitoids of the minto block and indicate the presence of a crustal contaminant of ca 3 8 ga in age there is a decrease in nd values from east to west across the studied region and a measurable decrease across tikkerutuk and inukjuak domain boundary this suggests increasing addition of an older crustal component in the inukjuak domain possibly as a result of recycling of early archean crust from the nearby ca 3 8 ga nuvvuagittuq sequence

archaean cratonization and deformation in the northern superior province canada an evaluation of plate tectonic versus vertical tectonic models abstract the archaean minto block northeastern superior province is dominated by tonalite trondhjemite enderbite pyroxene tonalite granodiorite and granite with subordinate mafic rocks and supracrustal belts the plutons have been interpreted as the batholithic roots of andean type plate margins and intra oceanic arcs existing horizontal tectonic models propose that penetrative recrystallization and transposition of older fabrics during terrane assembly at 2 77 and 2 69 ga produced a n nw tectonic grain in the douglas harbour domain northeastern minto block tonalite and trondhjemite dominate the faribault thury complex 2 87 2 73 ga and enderbite constitutes 50 100 km scale ovoid massifs troie and qimussinguat complexes 2 74 2 73 ga magmatic muscovite and epidote in tonalite trondhjemite have corroded edges against quartz plagioclase suggesting resorption during ascent of crystal charged magma foliation maps and air photo interpretation show the common development of 2 10 km scale ovoid structures throughout the douglas harbour domain outcrop and thin section scale structures imply that many plutons experienced a phase of syn magmatic deformation typically followed by high temperature sub magmatic overprints thermobarometric data for plutons indicate near solidus recrystallization at 4 6 kbar pressures the common preservation of syn magmatic fabrics in plutons of different ages seems incompatible with the origin of these fabrics through superimposed regional orogenesis the broad uniformity of intrusion ages and lithologies throughout the minto block and the rarity of shallowly dipping planar fabrics also seem inconsistent with accretion of disparate older terranes each of which should preserve distinct histories a possible alternative explanation for these features is provided by vertical tectonic models whereby buoyant felsic magmas ascended as crystal slurries while dense supracrustal rocks and solidified felsic intrusions emplaced into them subsided as cold fingers 10 20 km scale instabilities shear between upwelling and downwelling limbs would have concentrated in the weak intrusions generating steeply plunging syn magmatic fabrics and producing ductile overprints in solidified rocks

multiple zircon growth within early archaean tonalitic gneiss from the ancient gneiss complex swaziland abstract u pb age determinations by ion microprobe reveal multiple episodes of zircon growth and recrystallization within a single sample of tonalitic gneiss from the ancient gneiss complex the oldest episode at3644 4ma 2 produced the dominant type of zircon characteristically purplish brown and massive in texture this probably constitutes unaltered zircon precipitated from the original magma recrystallization accompanied and obscured by early pb loss took place within the oldest grains at3504 6ma and3433 8ma whole new grains grew at these times also we interpret the post 3644 ma growth as due to episodic deformational and metamorphic events that transformed the tonalite pluton into foliated banded gneiss in addition many grains are visibly overgrown by two layers of younger zircon of different colour and texture dated at2986 20ma and2867 30ma euhedral finely zoned whole grains having the 2986 ma age are present also evidently contributed by very thin felsic veins associated with the nearby lochiel granite the age of3644 4ma combined with precise zircon u pb dating of volcanics from the onverwacht group reported elsewhere demonstrates that at least part of the ancient gneiss complex is older than the barberton greenstone belt

2 7 ga crustal growth in the north china craton evidence from zircon u pb ages and hf isotopes of the sushui complex in the zhongtiao terrane abstractthe major precambrian crustal growth in the north china craton ncc has been thought to have occurred at 2 5 ga here we present secondary ion mass spectroscopy sims and laser ablation inductively coupled mass spectrometry la icp ms zircon u pb ages and hf isotope compositions from dioritic and tonalite trondhjemite granodiorite ttg gneisses of the neoarchean paleoproterozoic metamorphic terrane in the central ncc zircons from the trondhjemitic samples yield emplacement ages at and ma those from the dioritic gneiss yield an age of ma the ttg gneisses are characterized by a na rich and k poor nature strongly fractionated rare earth element patterns high sr 418 318 and sr y 78 49 low y 5 4 6 4 and ybn 3 0 3 6 and low mgo 0 89 1 10 mg 47 42 cr 39 20 and ni 11 8 nb ta 16 12 which suggests that magmas were derived from the partial melting of a flatly subducted oceanic slab the dioritic gneiss shows higher mgo 3 17 mg 50 cr 44

late paleozoic to triassic evolution of the gondwana margin evidence from chilean frontal cordilleran batholiths 28 s to 31 s upper paleozoic to triassic chilean granitoids in the andean frontal cordillera between 28 s and 31 s record crustal and mantle conditions at the gondwana margin during the final assembly and initial breakup of the pangea supercontinent this period overlaps the end of paleozoic terrane accretion and precedes andean subduction integration of new trace element and isotopic data with other information on the granitoids and the regional geology leads to a tectonic model that has implications for other parts of the gondwana margin in the model the carboniferous to early permian is a period of oblique convergence associated elqui complex granitoids are diverse those in the guanta and montosa units are predominantly related to subduction processes whereas those in the cochiguas and el volcan units are dominated by melting of the subduction complex and older crust progressive oblique collision of the last pre pangea terrane equis along the margin resulted in crustal thickening associated with shortening deformation of foreland basinal sedimentary rocks and uplift of the elqui complex subsequent gravitational collapse of the inactive slab and lithospheric delamination resulted in the production of large amounts of basalt which intruded and melted the crust producing the post collisional ingaguas complex the los carricitos granitoids formed in thickened crust whereas the chollay el colorado and el leon units formed in thinner crust the ingaguas complex is part of the choiyoi granite rhyolite province whose formation similar to that of other gondwana silicic provinces was probably accentuated by anomalously hot upper mantle associated with the pangea supercontinent

crustal magnetization and long wavelength aeromagnetic anomalies of the minto block quebec measured susceptibilities of 3500 samples from a 500 500 km area of the minto block superior province quebec have an average value of 0 03 si equivalent to induced magnetizations of 1 5 a m charnockitic lithologies igneous orthopyroxene bearing granodiorite granite diorite that constitute large parts of the minto region yield the highest susceptibilities the variations in susceptibility show a good correlation with the variations in the observed magnetic field including those at long wavelengths 100 km suggesting that charnockitic and related granitoid rocks could be responsible for the long wavelength anomalies modeling of a magnetic profile perpendicular to the strike of mapped lithotectonic subprovinces in the minto block supports this contention charnockitic lithologies are not usually considered a possible sources of long wavelength anomalies mafic rock types are generally favored the dominant minto lithologies are more felsic but are more efficient in the production of magnetite from the available iron

new pieces to the archaean terrane jigsaw puzzle in the nuuk region southern west greenland steps in transforming a simple insight into a complex regional tectonothermal model in the south of the nuuk region of west greenland our 1980s mapping recognized four archaean gneiss terranes f ringehavn tre brodre tasiusarsuaq and akia terranes with different protolith ages and separate early tectonothermal histories later in the archaean these were juxtaposed and then experienced the same 2700 2500 ma tectonothermal events here we abandon extrapolation of only these four terranes across the whole region and distinguish two new terranes in the ne the northernmost isukasia terrane previously regarded as the northernmost exposure of the f ringehavn terrane consists of palaeoarchaean rocks 3600 ma tectonically bounded on its south by the 3075 2960 ma kapisilik terrane these were juxtaposed and metamorphosed together by 2950 ma the previously recognized f ringehavn terrane to the sw is another separate entity of palaeoarchaean rocks that was juxtaposed with adjacent terranes only after c 2800 ma hence in an increasingly complex regional model there were several mid to neoarchaean terrane assembly events with superimposed orogenies from c 2950 ma until after 2700 ma although the f ringehavn and isukasia terranes were incorporated into the later archaean terrane collage at different times they might be fragments from a larger palaeoarchaean complex rifted apart from c 3500 ma onwards

geochronological constraints on the development of the cross lake greenstone belt northwest superior province manitoba recent 1 20 000 mapping combined with 11 age determinations from metavolcanic metaplutonic and metasedimentary rocks in the cross lake greenstone belt provide a framework for interpreting the stratigraphic structural intrusive and metamorphic evolution of the belt these data in combination with u pb ages from adjacent granitoid domains are used to model a cratonizing event in the northwest superior province the cross lake greenstone belt is made up of three distinct unconformable groups of supracrustal rocks i the pipestone lake group 2760 ma a thick sequence of pillowed and massive tholeiitic basalt flows and related high level gabbros inter preted as a back arc spreading sequence ii the gunpoint group 2730 ma a fining upward sequence of clastic sediments interbedded with rhyodacitic fragmental volcanic rocks and iii the cross lake group 2710 ma a fining upward fluvial marine clastic sedimentary sequence with shoshonitic volcanic rocks near the top deposited in a restricte

metamorphic zircon trace elements and neoarchean metamorphism in the ca 3 75 ga nuvvuagittuq supracrustal belt quebec canada abstract the nuvvuagittuq supracrustal belt nsb in northwestern quebec ranks as one of the oldest granitoid gneiss complexes thus far discovered emplacement ages for intrusive trondhjemitic dikes that transect amphibolites and quartz magnetite schists at the porpoise cove outcrops of the nsb cluster at 3 75 ga as with all other pre 3 7 ga terranes thus far documented the nsb has been metamorphosed and multiply deformed in a combined approach to explore the thermal history of the nsb conventional garnet biotite and plagioclase hornblende geothermometry was coupled with zircon u th pb depth profiles ree partitioning and ti in zircon thermometry to show that neoarchean metamorphism in the belt reached upper amphibolite facies conditions 640 c this metamorphism resulted in some cases in thick zircon overgrowths on older cores and likely corresponds to the amalgamation of terranes in the northeast superior province and the initiation of widespread igneous activity in the vicinity of the nsb an earlier resolvable metamorphic episode from a zircon depth profile recorded at 3 62 ga probably corresponds to the intrusion of the granitoids which envelope the nsb

constraints on early archean crustal extraction and tholeiitic komatiitic volcanism in greenstone belts of the northern superior province numerous small remnants of archean greenstone belts in the northern superior province ca 2875 2710 ma have chemical characteristics similar to those of the larger greenstone belts of the southern superior province and preserve direct evidence of crustal conditions prior to the major volcanic events of the late archean wawa abitibi subprovinces ca 2760 2700 ma three of the best preserved belts are engulfed in tonalite intrusions of the faribault thury complex ftc and exhibit common chemical characteristics which may imply a similar origin the dominant tholeiitic basalts typically have mgo contents 7 wt tio2 15 low cao al2o3 1 2 and chondritic gd yb ratios similar to 2 7 ga al undepleted komatiites the trace element ratios of komatiitic rocks are indistinguishable from those of the associated tholeiites suggesting either a derivation from similar mantle sources or a comagmatic relationship nb thpm 0 8 1 1 la cepm 0 9 1 3 nb ce 0 7 0 9 y hopm 1 and th lapm 0 7 1 1 numerical modelling of trace and major elements during low pressure crystal fractionation reproduces the spectrum of both inferred liquid and cumulate compositions and is consistent with a comagmatic origin between the komatiites and tholeiites the relatively low nb th ratios of these mid archean volcanic rocks relative to both modern day basalts and late archean basalts may indicate that they were derived from a mantle source that had not lost its crustal components nor seen significant recycled oceanic crust high nb th the extraction of continental crust from this archean mantle source might then postdate the ftc volcanism and may be associated with the generation of the voluminous tonalites that engulf the belts 445

giant granulite terranes of northeastern superior province the ashuanipi complex and minto block the ashuanipi complex and minto block of the superior province are large regions that have been classified as high grade gneiss terranes on the basis of the presence of orthopyroxene bearing units like the granite greenstone and metasedimentary subprovinces of the southern superior province the two terranes consist predominantly of intrusive rocks but are distinguished by their primary magmatic orthopyroxene both high grade and gneiss are misnomers because granulite facies gneisses are only sparingly present and the regions consist dominantly of massive unmetamorphosed plutonic rock the ashuanipi complex consists of a deformed metamorphosed package of metasedimentary rocks and primitive early tonalite cut by widespread orthopyroxene garnet granodiorite diatexite as well as plutons of tonalite granite and syenite based on its lithological and chronological similarity and on strike position the complex appears to be the continuation of metasedimentary subprovinces such as the quetico its evolution involved deposition of immature greywacke in an accretionary prism early arc tonalitic magmatism and deformation followed by widespread intracrustal magmatism in the period 2700 2670 ma both metamorphic and igneous rocks record equilibration under granulite facies conditions 700 835 c 0 35 0 65 gpa a 0 3 and indicate exposure levels of 20 km the minto block at the latitude of leaf river consists of several north northwest trending domains of similar scale and diversity to the east trending subprovinces of the southern superior province the central goudalie domain is dominantly amphibolite facies tonalitic rocks including some with ages 3 ga with small belts of volcanic and sedimentary origin lake minto domain contains poorly preserved supracrustal remnants in a plutonic complex comprising hornblende granodiorite clinopyroxene orthopyroxene granodiorite orthopyroxene biotite diatexite and granite the hornblende granodiorite suite constitutes most of the utsalik and tikkerutuk domains and is thought to represent continental arc magmatism on the basis of their distinct aeromagnetic and lithological character two additional domains are evident north of the leaf river area the inukjuak domain in the west and the douglas harbour domain in the east the northerly grain of the minto block appears to have been established in situ with respect to the easterly belts of the southern superior province i e no large scale block rotation during the same interval of time 3 0 2 7 ga modification of the tectonic framework for the superior province is required to explain minto arc magmatism in the interval 2730 2690 ma ago a continental magmatic arc built the berens river and bienville subprovinces and minto block on the southern and eastern edges respectively of a northern protocratonic foundation in the same period primitive volcanic arcs and accretionary prisms developed outboard on oceanic crust and were accreted to form a southern tectonic regime

how feasible was subduction in the archean as the asthenospheric mantle rises at oceanic spreading centres it undergoes partial melting producing oceanic crust and depleted mantle both of which have lower intrinsic density than the asthenospheric mantle from which they were derived with a warmer asthenosphere in the archean these effects are enhanced leading to the possibility that subduction was no longer feasible i investigate the density of the oceanic crust and underlying mantle for a mantle with temperatures 200 c higher than today using models of the chemistry of melting and the mineralogy of the ensuing rocks for the melting model used crustal thicknesses are 21 km and the depth to which the mantle is partially melted is 114 km compared with 7 and 54 km for a comparable model of modern earth two thermal evolution models for archean oceanic lithosphere are examined one assumes twice the heat flow into the base of the plates which severely restricts the depths to which the plates can cool with age a second assumes the plates c

minto block superior province missing link in deciphering assembly of the craton at 2 7 ga plate tectonic models of the superior province are rooted in the granite greenstone and metasedimentary belts of the southern part of the craton north striking domains of the minto block in the northeastern part of the province evolved at similar times in 3 1 2 8 2 725 and 2 69 ga events requiring expansion of models of late archean assembly to accommodate minto geology western and eastern protocratons 3 1 2 8 ga rifted at 2 79 ga to produce an ocean basin that was mostly consumed by subduction at 2 725 ga the leaf river plutonic suite of cale alkalic hornblende biotite orthopyroxene clinopyroxene granodiorite represents magmatic arcs built on the protocratons whereas the intervening goudalie domain containing fault bounded fragments of rifted continental crust rift volcanics primitive oceanic crust 2724 ma island arc rocks and a 2718 ma back arc assemblage marks the suture terminal collision at 2 7 ga led to thickening and crustally derived granitoid magmatism the southern superior province also experienced vigorous activity between 2 725 and 2 69 ga as island arcs oceanic plateaus continental fragments and accretionary prisms were amalgamated progressively from north to south in a regime of dextral transpression then stitched by granites a northern proto superior craton had continental magmatic arcs built on its eastern and southern flanks in response to west northwest directed subduction orthogonal convergence in the east produced wide plutonic arcs in contrast to terrane accretion tectonics in the more oblique regime along the southern margin

u pb and hf isotope records in detrital and magmatic zircon from eastern and western dharwar craton southern india evidence for coeval archaean crustal evolution abstract u pb and lu hf isotopic records in detrital and magmatic zircons from the eastern and the western dharwar craton edc and wdc suggest presence of 3 4 ga crustal components in both the blocks magmatic zircon from the western block has ages ranging between 3 1 and 3 2 ga while those from the eastern block show a bi modal age distribution with records of 3 0 3 2 ga protolith components as well as 2 7 and 2 5 ga metamorphic overgrowths detrital and magmatic zircons from granitoid conglomerate clasts of the western dharwar block show initial hf values consistent with models for the early archaean mantle suggesting their juvenile character lower hf values in younger as well as detrital zircons from eastern dharwar block reflect importance of reworking of 3 2 ga crust the combined u pb and hf isotope study strengthens suggestions for contemporaneous crust formation processes in both the western and eastern parts of the dharwar craton

crustal growth through successive arc magmatism reconnaissance u pb shrimp data from the northeastern superior province canada abstract north of the la grande subprovince in northern quebec the superior province is dominated by plutonic rocks of the bienville subprovince and minto block which have been further subdivided into domains on the basis of reconnaissance geological mapping and geochronology and projected along strike using aeromagnetic anomalies to test the validity of the extrapolations u pb ages were obtained through shrimp analyses of zircon from 18 small archival samples selected from a suite of 2000 specimens collected during a helicopter reconnaissance survey conducted in the 1960s the results reveal a history of successive arc magmatism between 3 07 and 2 70 ga followed by metamorphism emplacement of crustally derived granites 2 702 2 685 ga and late hydrothermal events ca 2 65 ga in the northeastern minto block the douglas harbour domain previously extrapolated from dated localities to the northeast is substantiated through an age of 2 87 ga the older tonalitic crust is punctured by 2 734 2 725 ga granodioritic plutons of the leaf river suite that locally carry 3 01 ga inherited zircon the northern utsalik domain is made up of pyroxene and hornblende bearing leaf river plutons with consistent 2 725 2 723 ga ages and local inheritance of 2 82 and 2 77 ga however the southern extension of utsalik domain contains pyroxene bearing granodiorites both older 2 762 ga and younger 2 692 ga than the leaf river suite a tonalitic rock from the southern goudalie domain with an age of 2 833 ga supports a linkage to 2 84 2 83 ga units of the qalluviartuuq domain along strike to the north tikkerutuk domain represents a north trending 50 km wide calc alkaline magmatic arc with ages between 2 71 and 2 70 ga that extends southward into the bienville subprovince on the basis of aeromagnetic anomalies and geochronology zircon inheritance ages indicate that the tikkerutuk arc was built upon antecedent arcs of 2 84 2 77 and 2 725 ga the inukjuak domain in the west consists mainly of granite dated in one locality at 2 72 ga

differential response of u pb systems in coexisting accessory minerals winnipeg river subprovince canadian shield implications for archean crustal growth and stabilization the u pb isotopic systems of zircon monazite titanite and some apatite and the pb isotopic composition of k feldspar have been investigated in three areas of the winnipeg river subprovince wrs of the superior province canada in order to define the timing of magmatic and metamorphic processes in this archean gneissic granitoid terrain

pre 3750 ma supracrustal rocks from the nuvvuagittuq supracrustal belt northern quebec geochemistry and u pb ion microprobe zircon geochronology guided by high resolution mapping 1 50 scale was used to define a minimum age of ca 3750 ma for supracrustal rocks from the nuvvuagittuq supracrustal belt nsb in the northern superior province quebec canada mineralogy and geochemistry of critical field relationships preserved at the porpoise cove locality describe a supracrustal succession of mafic amphibolites and ultramafic rocks finely banded quartz magnetite units with intermixed coarse grained ferruginous quartz pyroxene rocks and quartz biotite schists that superficially resemble polymict metaconglomerates with large up to 10 cm deformed polycrystalline quartz and mafic clasts all units in the mapped outcrop have sharp lithological contacts narrow dominantly trondhjemitic orthogneiss sheets locally preserve intrusive contact relationships to the supracrustals the total extent of the supracrustal enclave is 8k m 2 it is strongly deformed and the full deformation history appears to be shared by all units with later modifications from leucogranitoid intrusions the quartz biotite schists record complex zircon growth at 3500 and 2800 ma interpreted to reflect metamorphic events zircons separated from orthogneisses in the enclave including one sheet that transects a banded quartz pyroxene magnetite unit yield magmatic 207 pb 206 pb ages of ca 3750 ma these ages are slightly younger than earlier provisional reports for an nsb orthogneiss from porpoise cove the nuvvuagittuq supracrustals are the oldest rocks thus far reported for the minto block they overlap in age with the ca 3 70 3 83 ga isua supracrustal belt and akilia association rocks in west greenland and they represent an important new area for exploration of the early earth

metamorphic evolution of granulites in the minto block northern quebec extraction of peak p t conditions taking account of late fe mg exchange in the central minto block of northern quebec the lake minto and goudalie domains are dominated respectively by orthopyroxene bearing plutonic suites granite granodiorite and diatexite and a tonalitic gneiss complex both of which contain scattered remnant paragneisses two main granulite grade mineral assemblages are observed in the paragneiss garnet grt orthopyroxene opx plagioclase quartz gopq and garnet grt cordierite crd sillimanite plagioclase quartz gcspq these show distinct lithological associations with the gcspq assemblages occurring exclusively within the diatexite in the lake minto domain petrogenetic grid considerations demonstrate that the gopq rocks are higher grade than the gcspq rocks maximum temperatures for gopq rocks obtained from equilibria based on al solubility in orthopyroxene in equilibrium with garnet range from 950 to 1000d c significantly higher than garnet orthopyroxene fe mg exchange temperatures of 700 50d c the latter probably representing a closure temperature below peak conditions the al temperatures were corrected for late cation exchange by adjusting the fe fe mg ratios in garnet and orthopyroxene to achieve internal consistency between the gopq thermometers and barometers grt crd thermometry records temperatures of 750 50d c peak p t conditions range from 5 6 kbar and 750 800d c in the goudalie and eastern lake minto domains to 7 10 kbar and 950 1000d c in the western and central lake minto domain this variability contrasts with the uniform crustal pressures of 5 1 kbar recorded by the gcspq assemblages in the diatexites and the hornblende granodiorites c 4 5 kbar across the same area the gopq rocks are inferred to record earlier p t conditions that prevailed before the formation of gcspq assemblages and the intrusion of the granodiorites partial p t paths in gopq rocks from both domains based on net transfer equilibria corrected for fe mg resetting document cooling of 100 250d c from thermal peak conditions concomitant with a modest pressure decrease of 2 3 kbar although textures diagnostic of isobaric cooling are not developed the paths are consistent with a tectonic model in which granulite metamorphism and crustal thickening in the minto block were consequences of magmatic underplating the progression from higher p t conditions recorded by gopq assemblages to lower p t conditions recorded by gcspq assemblages is attributed to variable amounts of synmagmatic uplift and cooling in a single continuous thermal event in the minto crust associated with protracted crustal magmatism in the goudalie and eastern lake minto domains where gopq and gcspq rocks and hbl granodiorites have similar p t conditions of equilibration the crust may not have been thickened as much as further west where gopq p t conditions are significantly higher than those of the hornblende granodiorites and the gcspq rocks

s type granites in the western superior province a marker of archean collision zones detailed field observations indicate that neoarchean s type granites were emplaced along and or proximal to some terrane tectonic boundary zones in the western superior province southeastern m

quantifying the timing and rate of crustal evolution global compilation of radiometrically dated detrital zircon grains abstractmultiple models steady state episodic and early growth followed by crustal reworking have been postulated to explain the evolution of earth s continental crust an independent assessment of these models is now possible as a result of the massive numbers of detrital zircon grains that have been dated individually over recent years using u pb and hf isotopes a compilation of 200 000 published zircon dates reveals a global age distribution u pb with six prominent statistically significant peaks 3 2 3 0 2 7 2 5 2 0 1 7 1 2 1 0 0 7 0 5 and 0 3 0 1 ga in most cases these peaks are recorded on all seven continents most of the peaks are detectable also in modern sediments a comparison of grains with both u pb ages and hf isotope model ages available for 5100 detrital zircons suggests that the age frequency distribution of detrital grains reflects predominantly episodic crustal recycling rather than crustal growth the zircon age distributions and associated data provide consistent ev

paleo to mesoarchean basement recycling and terrane definition in the northeastern superior province quebec canada abstract the northeastern superior province nesp is a 500 km 700 km segment of plutonic archean crust generated by 1 2 ga of tectonomagmatic activity the nesp exhibits the classical plutonic series of archean terranes low k tonalite trondhjemite granodiorite ttg and high k granodiorite granite monzogranite ggm with unusually large volumes of px ttgs mostly enderbites and subordinate charnockites volcanic rocks constitute t 0 45 to 5 03 mafic to felsic volcanic rocks and mafic to ultramafic plutonic rocks t 0 40 to 6 90 these radiogenic signatures as well as inherited zircon cores record recycling of mesoarchean 3 8 2 9 ga crust either through direct melting or by coupled assimilation fractional crystallization processes in comparison the riviere arnaud terrane is underlain by younger t values 2 34 to 0 67 indicating either a juvenile character or recycling of fairly juvenile late mesoarchean early neoarchean crust overall geochemical and isotopic data of the late neoarchean

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

internal structures of zircons from archaean granites from the darling range batholith implications for zircon stability and the interpretation of zircon u pb ages internal structures in zircons from granitoids from the late archaean darling range batholith show secondary features revealed by hf etching which record reconstitution of the zircons and modification of the distribution of trace elements during post crystallisation cooling of the granitoid zircons from the granites commonly contain unzoned to weakly zoned cores surrounded by rims showing oscillatory zoning which has been modified by recrystallisation the most striking feature is the development of high trace element concentration areas found in zircons from a number of granites these structures range from enhanced trace element concentrations in primary zones to a single accumulation of most trace elements in one band about half way between the outer edge and the centre of the zircon in any zircon the extent of the concentration of trace elements towards the formation of a single trace element band appears to be inversely related to the fading and broadening of primary oscillatory zones in the outer rim this suggests that the trace element bands formed by migration of trace elements from the outer primary zones to new concentration sites on an inner set of primary zones this explanation is supported by the formation of multiple curved trace element bands that transgress primary zoning and the determination of younger shrimp ages on depleted zircon outer rims compared to remnant primary oscillatory zoned areas of the zircon and unzoned centres also observed in some granite zircons is a finely convoluted zoning which overprints oscillatory zoning in parts of a zoned zircon and in rare cases occurs throughout the zircon this structure is explained in terms of secondary migration and reconcentration of trace elements in curved bands all structures can be transgressed by generally rounded lobes and patches of low u weakly nebulously zoned zircon this is interpreted as a late stage interaction between the zircon and fluids formed during cooling and crystallisation of the granitoid resulting in recrystallisation of affected parts of the zircon with accompanying loss of trace elements from the zircon

a reply to how many arcs can dance on the head of a plume by jean bedard precambrian research 2012 abstract bedard s 2013 comment on wyman 2013 itself an outgrowth of a 5ias workshop debate has clarified several points concerning the catalytic delamination driven model of bedard 2006 many of the references to archean plate tectonic models however are expressed in terms of an improbable uniformitarian viewpoint that could not be applied to the modern earth whereas differences between archean and phanerozoic plateau basalts have been attributed to a less depleted mantle source any similar arguments from advocates of archean plate tectonics are characterized as special pleading bedard 2013 does not address key issues raised by wyman 2013 and when the paper does attempt to resolve apparent dilemmas related to craton development it undermines the new refinements made to the non plate tectonic scenario that bedard favors

solar cycle and diurnal dependence of auroral structures in order to facilitate usage of optical data in space climate studies we have developed an automated algorithm to quantify the complexity of auroral structures as they appear in ground based all sky images the image analysis is based on a computationally determined arciness value which describes how arc like the auroral structures in the image are with this new automatic method we have analyzed the type of aurora in about 1 million images of green aurora 557 7nm captured at five camera stations in finnish and swedish lapland in 1996 2007 we found that highly arc like structures can be observed in any time sector and their portion of the auroral structures varies much less than the fraction of more complex forms the diurnal distribution of arciness is in agreement with an earlier study with high arc occurrence rate in the evening hours and steadily decreasing toward the late morning hours the evolution of less arc like auroral structures is more dependent on the level of geomagnetic activity and solar cycle than the occurrence of arcs the median arciness is higher during the years close to the solar minimum than during the rest of the solar cycle unlike earlier proposed the occurrence rate of both arcs and more complex auroral structures increases toward the solar maximum and decreases toward the solar minimum the cyclic behavior of auroral structures seen in our data is much more systematic and clear than previously reported visual studies suggest the continuous arciness index describing the complexity of auroral structures can improve our understanding on auroral morphology beyond the few commonly accepted structure classes such as arcs patches and omega bands arciness can further be used to study the relationship of auroral structures at different complexity levels and magnetospheric dynamics

neoarchean crustal evolution in western shandong province of the north china craton the role of 2 7 2 6 ga magmatism abstract the neoarchean era is an important period for the formation of continental crust in the early earth the north china craton ncc records magmatism during 2 8 2 7 ga and 2 6 2 5 ga with the latter more extensive however the 2 7 ga rock assemblages are significantly different from those of the 2 5 ga rocks based on detailed field investigation we present results of zircon geochronology by using shrimp and la icp ms techniques on the shanggang pluton in the central belt of the western shandong province wsp the shanggang pluton is composed of 2 71 2 69 ga strongly deformed tonalite on the eastern and northern margins with the central part occupied by younger 2 67 2 65 ga and 2 63 2 61 ga trondhjemite most zircon grains with apparent 207pb 206pb ages of 2 67 2 66 ga in our samples have low th u ratios 0 05 0 20 resulted from low to moderate th contents of 10 130 ppm and relatively higher u contents of 100 3000 ppm consistent with those of anatectic zircons some of grains carry 2 7 ga inherited cores our study reveals that the crustal re working event at 2 67 2 66 ga might have been more widespread than currently known although the 2 67 2 66 ga zircons have positive ehf t values close to that of coeval depleted mantle wan et al 2014 together with data from previous studies it is notable that more trondhjemite and even some granite were generated during 2 67 2 60 ga relative to the older ttg suites 2 72 2 69 ga the younger ones 2 67 2 60 ga are dominated by trondhjemite the compositional variations of ttg suites through the early to middle neoarchean suggest that the 2 67 2 60 ga magmatic and anatectic events should be related to the compositional differentiation of the archean continental crust of the wsp

timing and kinematics of crustal movement in the northern superior superterrane insights from the gull rapids area of the split lake block manitoba abstract the northern superior superterrane in the northwestern superior province is characterized by an ancestry back to ca 3 8 ga and contains one of the largest and best preserved high grade terranes on earth the pikwitonei granulite domain and its northeast extension the split lake block this high grade terrane occurs in the superior boundary zone a complex transitional zone that bounds the northwestern margin of the archean superior province and separates it from the paleoproterozoic crust of the trans hudson orogen detailed structural and geochronological data from the exceptionally well exposed gull rapids area at the eastern margin of the split lake block shows evidence for both neoarchean kenoran and paleoproterozoic trans hudson deformation and metamorphism more importantly it indicates that the uplift of the split lake block was achieved by shear zone movement in the neoarchean not in the palaeoproterozoic synchronous with peak metamorphism and dyke emplacement at ca 2686 ma this indicates that uplift of the high grade domains in the northern superior superterrane was neither related to its collision with the north caribou superterrane at ca 2 72 ga nor to the ca 1 8 ga trans hudson orogeny instead this uplift might have occurred during a major period of superior wide active tectonism toward the end of the neoarchean kenoran orogenesis

juvenile magmatism and crustal recycling at the end of the neoarchean in western shandong province north china craton evidence from shrimp zircon dating western shangdong province experienced major crustal growth as a result of 2 75 to 2 50 ga tectonothermal events different from other archean areas in the north china craton besides early neoarchean tonalite trondhjemite granodiorite ttg and supracrustal assemblages there are large volumes of late neoarchean gabbroic dioritic and granitic rocks in western shandong shrimp zircon dating of thirty one samples of different rock types from late neoarchean rocks yielded a narrow age range from 2560 to 2490 ma based on these data and previously published results the following conclusions can be drawn 1 the archean basement can be divided into three belts a late neoarchean 2525 2490 ma crustally derived granite belt in the northeast an early neoarchean 2 75 2 60 ga rock belt in the center and a late neoarchean 2550 2500 ma belt of juvenile rocks in the southwest 2 the tectonic regime in western shandong province probably changed from compressional to extensional at around 2525 ma between 2530 and 2520 ma 3 crustal recycling with addition from the mantle occurred at the end of the neoarchean 4 the late archean magmatic rocks were probably formed in an arc environment

ferropicrite driven reworking of the ungava craton and the genesis of neoarchean pyroxene granitoids article history voluminous pyroxene bearing intermediate to felsic plutons were emplaced during a 20 50 million year long spatially extensive neoarchean igneous event that culminated in the cratonization of north america s 500 km wide ungava craton the crystallization ages of pyroxene bearing plutons coincide with the emplacement of numerous ca 2 72 2 70 ga fe rich ultramafic mafic intrusions of the qullinaaraaluk suite q suite that are scattered across the disparate domains of the ungava craton a high proportion of relatively sodic pyroxene bearing granitoids with intermediate silica contents fall in a compositional gap between the q suite and pyroxene free granitoids suggesting that the pyroxene granitoids may be formed by the simultaneous fractional crystallization and assimilation of older tonalitic and trondhjemitic tt crust by the q suite magmas we estimate that pyroxene granitoids containing 65 wt sio2 may reflect 40 50 wt contamination of mantle derived picritic magma by trondhjemitic melts of the pre 2 74 ga ttg crust the craton wide occurrence of the q suite intrusions and pyroxene granitoids suggests that underplating by ferropicritic magmas played a key role in the cratonization of the ungava craton at the end of archean a major contribution of mantle derived magmas to the petrogenesis of the ca 2 74 2 70 ga pyroxene granitoids is consistent with the proposed global generation of voluminous juvenile continental crust ca 2 7 ga

neoarchean metagabbro and charnockite in the yinshan block western north china craton petrogenesis and tectonic implications abstract properly calibrating the magmatic record of mantle and crustal origin in archean granite greenstone terranes is crucial for understanding the petrogenetic and geodynamic processes that generated early continental landmasses this geochronological and geochemical study documents a neoarchean bimodal metaplutonic suite of metagabbro and charnockite from the yinshan block north china craton the meta gabbroic rocks show an sio 2 range from 45 9 to 51 7 and high mgo content from 6 7 to 16 4 with enrichment in ba and light rare earth elements la n yb n 3 11 5 99 and depletion in high field strength elements together with their enriched whole rock nd nd t 0 23 to 1 49 and zircon hf hf t 0 1 6 9 isotopic signatures relative to the archean depleted mantle in the craton these rocks are supposed to originate from the second stage high temperature partial melting of refractory depleted mantle that experienced prior basaltic magma extraction and subsequent metasomatism by subduction related fluids the associated charnockites range in sio 2 from 61 3 to 69 6 and exhibit a magnesian calc alkalic and metaluminous character with variable sr y and la n yb n ratios these elemental features plus their evolved isotopic compositions nd t 0 93 1 85 zircon hf t 0 3 to 2 5 are consistent with partial melting of newly underplated mafic lower crustal protolith in combination with widespread occurrence of metasomatized lithospheric mantle derived magmas e g high mg basalts sanukitoid suites and juvenile crust extracted potassic granites in the ncc during late neoarchean time such a mafic and felsic magma association not only attests to the establishment of a craton scale subduction related metasomatized sub continental lithospheric mantle but also encapsulates a scenario of coupled lithospheric mantle crust formation at 2 7 ga and juvenile crustal reworking at 2 5 ga within a modern style convergent continental margin possibly featuring episodic slab break off events

contrasted tectonic styles for the paleoproterozoic evolution of the north china craton evidence for a 2 1 ga thermal and tectonic event in the fuping massif structural analysis along with 40ar 39ar and u pb datings in the fuping massif provide new insight into the evolution of the eastern part of the trans north china belt north china craton from 2 7 ga to 1 8 ga d1 is responsible for the development of a dome and basin structure coeval with crustal melting giving rise to migmatite and nanying gneissic granites at 2 1 ga this dome and basin architecture resulted from the interference between a n s compression of a weak ductile crust and gravity driven vertical flow in a high thermal regime the next events involved flat lying ductile thrusting d2 and normal faulting d3 dated at around 1880 ma and 1830 ma respectively the d2 and d3 events belong to the trans north china orogeny that results in the final amalgamation of the north china craton the d1 deformation is considered as evidence for an earlier orogen developed around 2 1 ga prior to the trans north china orogeny the change in the deformation style between the 2 1 ga and 1 8 ga could be viewed as a consequence of the cooling of the continental crust in the north china craton

secular variation in greenstone sequence development emphasizing superior province canada abstract evidence from the superior province of the canadian shield suggests the existence of several distinct lithostratigraphic associations in greenstone belts 1 quartz arenite and carbonate bearing sequences 2 mafic ultramafic volcanic sequences 3 mafic to felsic volcanic cycles and 4 timiskaming type sequences of fluviatile sediments and calc alkaline to alkaline volcanics recently recognized quartz arenite bearing sequences in the sachigo and wabigoon subprovinces are from base to top quartz arenite stromatolite bearing carbonate oxide facies iron formation and komatiite or tholeiite primary structures in the quartz arenites indicate shallow water deposition and represent mature sedimentation granitoid provenance of these sediments is postulated based upon petrography of coarse arenites and clasts of associated conglomerates the few dated superior province quartz arenite bearing sequences are 2 85 ga the shallow water origin of the primary structures and biota and the areal extent of the quartz arenite and carbonate sequences invite comparison with platform sediments at passive margins platform sequences unconformably overlie 3 ga granitoid rocks and older mafic volcanics where original lower contacts are preserved in two localities mafic volcanic sequences contain sheet flows iron formations and deep water argillites based on physical volcanology and sedimentology these sequences are compared to modern deep water mafic plain and hawaiian volcanism they generally underlie mafic to felsic volcanic cycles and overlie platform sequences mafic sequences are 2740 2700 ma in age mafic to felsic volcanic cycles consist of deep water tholeiitic flows overlain by shallow water to subaerial intermediate to felsic pyroclastics and flows the facies style of volcanism petrography and geochemistry of the mafic to felsic cycles most closely resemble the characteristics of crust in continental magmatic arc systems this type of lithostratigraphic association varies in age from 2775 to 2700 ma in the superior province with minor exceptions timiskaming type sequences consist of fluviatile conglomerate immature arenites and wackes with subaqueous to subaerial volcanics of variable chemistry longitudinal and lateral variation within fault bounded basins is extreme and the sequences have generally been affected by only late stages of deformation the combination of rock types facies distribution the fault bounded nature of the repositories and the locus of sedimentation along zones of transcurrent displacement suggest timiskaming sequences are best compared with modern pull apart basins superior province timiskaming sequences are within the superior province platform sequences are the earliest postorogenic lithostratigraphic unit in greenstone belts followed by mafic plain sequences arc volcanics and pull apart basins we suggest that platform sequences based upon limited evidence are preceded by an orogeny and that pull apart basins represent a subsequent orogeny the lithostratigraphic units above have distinct metallogenic associations 1 platform sequences based upon compilation of several shields may contain detrital au and u and oxide facies iron formations associated felsic volcanics contain no recognized volcanogenic cu zn deposits in the superior province the following metallogenic associations are noted 2 mafic plain sequences contain komatiite flow related ni deposits cu ni and platinum group deposits are associated with flows and coeval intrusions 3 arc volcanic sequences contain volcanogenic cu zn sulphide deposits and scarce deposits of porphyry type cu mo and 4 timiskaming sequences and related plutonism are related to lode au

relating crustal magnetization and satellite altitude magnetic anomalies in the ungava peninsula northern quebec canada abstract a prominent satellite altitude magnetic anomaly 6 nt located over the ungava peninsula northern quebec is coincident with a region of intense aeromagnetic anomalies and high susceptibilities of exposed rocks inversion of a combined pogo magsat anomaly data set shows that the susceptibility levels required to satisfy the satellite altitude anomaly range from 0 03 to 0 04 si assuming a 40 km thick crust these values are well within the range of measured surface susceptibilities in the minto region and the anomaly can be explained if the measured values are representative of the average upper crustal magnetization therefore the ungava peninsula is one of the few areas on earth where a direct relation between surface rock properties and satellite magnetic anomalies can be made

allochthonous 2 78 ga oceanic plateau slivers in a 2 72 ga continental arc sequence vizien greenstone belt northeastern superior province canada embedded within the vast granitoid terrane of the minto block of northeastern superior province are late archean greenstone belts of the goudalie domain that preserve a long lived record of continent ocean interaction the vizien greenstone belt is one such belt and it contains four fault bounded structural panels the 2786 ma mafic ultramafic sequence is an allochthonous package of pillowed basaltic andesite komatiite and volcaniclastic rocks cut by peridotite and gabbro sills the mafic rocks are lree depleted tholeiites which have primitive mantle prim normalized abundances of th la nb and a range of nd values from 0 1 to 1 7 the 2722 ma lac serindac bimodal subaerial tholeiitic volcanic sequence contains andesite locally with tonalite xenoliths basalt gabbro sills lenses of quartz rich sedimentary rocks and a thick upper rhyolite sequence the lac serindac tholeiites are lree enriched have prim normalized th la nb high zr to 300 ppm and ti contents and low nd values from 0 8 in basalt to 1 4 in rhyolite the 2718 ma basement cover sequence comprises 2 94 ga tonalitic gneiss unconformably overlain by clastic sediments and a thin upper sequence of 2700 ma gabbro siliceous high mg basalt shmb and andesite the shmb are characterised by lree depletion and nd values of 2 6 whereas the andesite is lree enriched and has nd values of 0 3 the 2786 ma mafic ultramafic sequence is interpreted as a sliver of plume related oceanic plateau crust the 2724 lac lintelle sequence represents a continental arc formed on the eastern protocraton the 2722 ma lac serindac volcanic sequence represents late continental rift deposits the various 2 8 2 7 ga supracrustal sequences were accreted deformed and metamorphosed to mid amphibolite facies during late stage assembly of the minto block between 2 718 and 2 693 ga

evolution of the darling range batholith yilgarn craton western australia a shrimp zircon study ma and slow cooling indicated by marginal recrystallization and primary and secondary internal structures observed in zircons from continued pb loss from the zircons until 2628 2616 ma granites from the late archaean darling range batholith record stages in the evolutionary history of the granites and provide a basis for a shrimp u pb study on the timing of granite evolution many granite zircons contain cores a few low u cores retain concordant to slightly discordant u pb ages of 2 8 ga some show discordant intermediate ages but most have nearly concordant

structural constraints on the tectonic evolution of a late archean greenstone belt in the northeastern superior province northern quebec canada abstract a detailed structural analysis shows that the vizien greenstone belt of northeastern superior province records five generations g1 to g5 of ductile deformation as well as brittle faulting g1 is indicated by the local presence of a pre g2 foliation and is interpreted to be related to thrusting g2 is responsible for the main penetrative foliation s2 axial planar to tight to isoclinal f2 folds and subparallel to major boundaries between lithotectonic panels it is concentrated in a thrust with sense of shear opposite to that interpreted for g1 f3 and f4 folds are open to tight with nnw sse and e w trending axial surfaces respectively they warp the s2 foliation and major panel boundaries and dominate the map pattern g5 is associated with dextral transcurrent movement along a nnw sse trending shear zone through understanding of its geometry and deformation history the vizien belt has been unfolded into its much simpler pre g3 geometry this greatly helps in the investigation of relationships between the different panels proposed terranes their tectonic settings and the kinematics of panel amalgamation the structural study in concert with geological and geochemical work leads to a tectonic model for the evolution of the greenstone belt which involves subduction of oceanic lithosphere and continental arc volcanism at 2724 ma intra arc rifting at 2722 ma and collision between 2718 and 2693 ma with associated obduction of oceanic plateau crust formation of melange and deposition in a possible foreland basin the g1 and g2 deformations are interpreted to be related to synthetic thrusting and back thrusting respectively during the collision

suspended sediment transport in an estuarine tidal channel within san francisco bay california abstract a recently developed instrumentation system has been used to monitor simultaneously flow conditions and suspended sediment distribution in the bottom boundary layer of a tidal channel within san francisco bay california measurements were made every 15 min over six successive flood and ebb tidal cycles they included mean velocity profiles from four electromagnetic current meters within 1 m of the seabed mean suspended sediment concentration profiles from seven miniature nephelometers placed within 1 m of the seabed near bottom pressure fluctuations vertical temperature gradient and bottom photographs additionally suspended sediment was sampled from four levels within 1 m of the seabed three times during the tidal cycle the instrument system was retrieved during each slack water period to exchange suspended sediment sample bags while the instrument was deployed std nephelometer measurements were made thoughout the water column and water samples were collected each 1 h and bottom sediment was sampled at the deployment site size distributions of the suspended sediment samples estimates of particle settling velocity s friction velocity u and reference concentration c a at z 20 cm were used in the suspended sediment distribution equations to evaluate their ability to predict the observed suspended sediment profiles three suspended sediment particle conditions were evaluated 1 individual particle sizes in the 4 11 62 5 0 5 m size range with the reference concentration c a at z 20 cm c 2 individual particle sizes in the 4 6 size range flocs representing the 7 11 size range with the reference concentration c a at z 20 cm c f and 3 individual particle sizes in the 4 6 size range flocs representing the 7 11 size range with the reference concentration predicted as a function of the bed sediment size distribution and the square of the excess shear stress an analysis was also carried out on the sensitivity of the suspended sediment distribution equation to deviations in the primary variables s u and c a in addition computations of mass flux were made in order to show vertical variations in mass flux for varying flow conditions

age structure and orogenic significance of the berens river composite batholiths western superior province the berens river area of northwestern ontario is underlain mainly by archean felsic plutonic rocks which enclose minor supracrustal and gneissic enclaves and merge with the greenstone belt rich uchi subprovince to the south u pb geochronology using zircon and monazite shows that the batholiths evolved mainly between 2750 and 2690 ma by sequential and essentially continuous intrusive activity into an older substratum composed of 3000 2800 ma volcanic and tonalitic crust there is a broad but not strict compositional transition from early biotite tonalite and hornblende tonalite progressing with time towards a greater abundance of hornblende granodiorite to granite and finally to late biotite granite rare peraluminous granites and sanukitoid dioritic monzodioritic to granitic plutons the tonalite suites were predominantly synvolcanic the late granitic intrusions postdated volcanism but were largely synchronous with the main compressional events that caused widespread sedimentation deformation

2 7ga juvenile crust formation in the north china craton taishan xintai area western shandong province further evidence of an understated event from u pb dating and hf isotopic composition of zircon abstract the evolution of the north china craton ncc is well known for a marked 2 55 2 50 ga tectonothermal event however supracrustal and intrusive rocks of 2 75 2 70 ga are in fact widely distributed in the western shandong province the most important area of archaean basement in the eastern part of the ncc this paper reports shrimp u pb dating and la icpms hf isotopic composition of zircons from 2 75 2 70 ga supracrustal and trondhjemite tonalite granodiorite ttg rocks in that area three fine grained hornblende biotite gneiss samples known locally as leptynite with meta volcanic or volcanosedimentary rock protoliths and five ttg samples have shrimp zircon u pb ages varying from 2 75 to 2 70 ga and 2 74 to 2 71 ga respectively zircons from most of the samples have high positive hf t values 4 7 to 10 0 and t dm hf ages 2 85 2 60 ga similar to their zircon u pb ages this indicates that the rocks represent largely juvenile crustal additions derived from depleted mantle only a short time before however some granitoids show hf t zircon values of 13 6 to 5 1 and t dm hf of 3 51 2 80 ga therefore the strong 2 75 2 70 ga tectonothermal event in the western shandong province involved not only juvenile addition to the continental crust but also intracrustal recycling of older components combined with craton wide data it is shown that the ncc is similar to many other cratons around the world where tectonothermal events of 2 7 ga are well developed however the main difference is that in the ncc superimposed 2 5 ga tectonothermal events were much stronger than in most other cratons

meso and neoarchean evolution of the island lake greenstone belt and the northwestern superior province evidence from lithogeochemistry nd isotope data and u pb zircon geochronology abstract the island lake greenstone belt is located on the northern margin of the north caribou terrane a large reworked mesoarchean crustal block in the northwestern superior province the belt experienced a long and complex geological history manifested by an older mesoarchean 2 897 2 852 ga and a younger neoarchean 2 744 ga volcanic assemblages comprising chronologically and geochemically distinct mafic and felsic volcanic rocks and associated plutonic rocks the mafic volcanic rocks in these assemblages are subdivided into two groups with distinct geochemical signatures 1 enriched in th and lree and with pronounced negative nb ta and ti anomalies and 2 with broadly flat ree patterns without apparent nb ta and ti anomalies the geochemical characteristics of these mafic volcanic rocks are interpreted to have formed in magmatic arc settings with a protracted history consistent with nd isotope data that require variable inputs of an older ca 3 0 ga crustal source i e the north caribou terrane for their formation crustal contamination appears to decrease with decreasing age in the volcanic assemblages and associated plutonic rocks together these data suggest that the meso and neoarchean volcanic assemblages are parts of a primary volcanic stratigraphic sequence built on the same ca 3 0 ga basement and have autochthonous relationships with each other u pb zircon id tims geochronology constrained the ages of two episodes of deformation d 1 and d 2 in the belt two dykes crosscutting an older foliation place a minimum age of ca 2 723 ga on the d 1 deformation and two syn kinematic dykes dated movement along d 2 transpressional shear zones at ca 2 70 ga geological geochemical and geochronological data indicate that the tectonic evolution in the island lake greenstone belt and in the northwestern superior province took place in three main stages the first two stages involved the generation of meso and neoarchean volcanic assemblages and contemporaneous plutonic rocks which is interpreted to be due to southward dipping subduction under the north caribou terrane this plate tectonic model is in contrast with well documented northward dipping subduction south of the north caribou terrane the third stage involved movement along late transpressional shear zones at ca 2 70 ga at the end of this geodynamic process the hudson bay terrane was terminally docked to the north caribou terrane along the north kenyon fault

deformation coupling between the archean pukaskwa intrusive complex and the hemlo shear zone superior province canada abstract archean greenstone belts typically form narrow sheared basins separating bulbous tonalo trondjhemo granodioritic ttg intrusive complexes the role played by gravity in the development of such dome and keel structures constitutes a key question in archean tectonics the pukaskwa intrusive complex pic hemlo greenstone belt system stands as a remarkable example of the dome and keel architecture that commonly occurs in archean terrains abundant strain markers in the greenstone belt and in the hemlo shear zone hsz attest of late sinistral strike slip kinematics d2 whereas in general the quartzofeldspathic coarse grained rocks of the pukaskwa intrusive complex bear little macroscopically visible kinematic indicators most likely due to pervasive recrystallization the pic consists dominantly of a heterogeneous assemblage of ttg plutonic rocks and gneisses which overall are less dense than the greenstone rocks the study of anisotropy of magnetic susceptibility ams based on 120 stations and 1947 specimens from the pic reveals east west trending prolate and plano linear fabrics across the northern margin of the complex i e along the hsz since geotherms were higher in the archean than in the present the effective viscosity of the ttg units would have been sufficiently low to allow their diapiric ascent through denser greenstone rocks here we propose an alternative model where thrust tectonics is responsible for the early structuration of the pic later transpressive tectonics causes strain localization along internal strike slip shear zones and along lithological boundaries

geochemical evolution of the minto block a 2 7 ga continental magmatic arc built on the superior proto craton abstract a 400 km long by 100 km wide geological transect across the minto block of northeastern superior province reveals five plutonic suites and minor amphibolite to granulite grade supracrustal remnants the oldest regionally significant plutonic rocks include 3 0 0 1 ga foliated to gneissic tonalite characterized by high na k ratios high al contents light ree enrichment and initial gend 1 8 the leaf river plutonic suite is the most widespread with crystallization ages of about 2725 5 ma across the transect the suite comprises massive to foliated pyroxene and hornblende bearing granodiorite tonalite granite diorite and minor gabbro pyroxenite and syn plutonic mafic dykes the rocks are i type calc alkaline have variable na k ratios moderate to high ba and sr contents and ree patterns with variable light ree enrichment depletion of heavy rees with increasing silica and neutral negative or positive eu anomalies initial nd values for the suite range from 0 5 to 1 3 mean 0 4 and initial 87 sr 86 sr ratios isr are 0 7020 the diatexite plutonic suite 2713 ma occurs within a 150 km wide zone and consists of orthopyroxene garnet granodiorite containing 25 50 inclusions of granulite grade mafic supracrustal and tonalitic inclusions the variable major and trace element chemistry of the suite with generally high ni and cr contents reflects heterogeneous source materials and the presence of non liquid components initial nd values are from 0 1 to 1 7 monzogranite and orthopyroxene granite dykes plugs and plutonic masses both 2690 ma comprise the last important plutonic suites both biotite monzogranite and orthopyroxene granite are light ree enriched but the former have negative eu anomalies and the latter positive initial nd values range from 0 7 to 3 2 for monzogranite isr 0 7016 to 0 7067 and 0 7 to 0 2 for orthopyroxene granite isr 0 701 to 0 702 the initial nd and isr values of the 2 7 ga plutonic suites are lower and higher respectively than estimatesf of the contemporaneous depleted mantle suggesting significant and wide scale reworking of older tight ree enriched high rb sr lithosphere early tonalite orthogneiss from goudalie domain may be derived by partial melting of mafic crust at high pressures the lithological and geochemical diversity of the 2725 ma leaf river plutonic suite indicate a range of parental magma compositions and petrogenetic processes but fractional crystallization of mantle derived basaltic melts which had assimilated small amounts of significantly older lower crust may have been the dominant mechanism crustal inclusion charged granodioritic diatexite 2713 ma was probably derived by partial melting of heterogeneous lower crustal materials under relatively dry conditions late biotite monzogranite was generated by partial melting of older sialic crust and orthopyroxene granite by deeper melting or fractional crystallization of more juvenile source materials with garnet as a residual mineral we infer from the presence of coeval mafic dykes in all plutonic suites that heat for melting was ultimately derived from intrusion of basaltic magmas at depth the 2 7 ga magmatism in the minto block occurred during a period of active production and accretion of juvenile oceanic terranes in the southern superior province the northwest directed subduction regime inferred from the southern superior province arc terranes may also have been responsible for the magmatism that acted on 3 ga lithosphere in the northern superior province melting within the mantle wedge beneath the proto cratonic lithosphere and in the lower crust occurred within a tectonic setting similar to that of modem continental magmatic arcs