evolution of the south central segment of the archean abitibi belt quebec part iii plutonic and metamorphic evolution and geotectonic model textural criteria permit distinction between the pre kenoran and kenoran phases of plutonism and metamorphism the pre kenoran plutons and pre kenoran metamorphic phases are directly related to the volcanic evolution synvolcanic tonalite trondhjemite plutons and swarms of mafic and felsic dykes core central volcanic complexes the volcanic rocks underwent three types of pre kenoran metamorphism namely a pervasive alteration a thermal contact metamorphism that affected narrow aureoles around synvolcanic plutons and a high intensity hydrothermal alteration that affected cross cutting pipes in central volcanic complexes synkinematic kenoran metamorphism resulted in the growth of minerals chlorite actinolite etc parallel to schistosities synkinematic metamorphic grade ranges form the pumpellyite prehnite facies to the amphibolite facies late to post kinematic metamorphic phases resulted in the growth of minerals across schistosities syn to post kinematic plutons are not voluminous in the part o

radiocarbon dates and the late quaternary palaeogeography of the province of the eastern cape south africa abstract a total of 193 dates are listed from the eastern cape middle stone age hunter gatherers existed in the drakensberg prior to the rigours of cold climatic conditions after ca 22 000 bp these uplands were reoccupied under more favourable climatic conditions after ca 12 600 bp but were apparently abandoned between ca 6000 bp and 3000 bp hunter gatherer occupation throughout the holocene is indicated at lower altitudes with in migration of pastoralists ca 1800 bp in the fish river area and with iron age farmers entering coastal districts and adjacent river valleys from ca 1400 bp sand dunes accumulated in the holocene adjacent to the indian ocean flood plain development in the early holocene was succeeded by incision of rivers in the later holocene flood plain deposition began again in the southern drakensberg ca 1000 bp palynological studies evidence marked climatic oscillations around the late glacial holocene boundary with apparent stability at high altitude subsequent to 2700 bp

experimental calibration of the aluminum in hornblende geobarometer with application to long valley caldera california volcanic rocks a new geobarometer based on the al content of igneous hornblendes in equilibrium with melt fluid biotite quartz sanidine plagioclase sphene and magnetite or ilmenite has been calibrated experimentally the calibration was performed by equilibrating the required phase assemblage over the pressure range 2 8 kbar at 740 780c and then analyzing euhedral hornblendes in equilibrium with glass melt experiments were performed on natural samples of both volcanic and plutonic rocks earlier empirical calibrations of this geobarometer relied on analyzing natural hornblendes from plutons with the required phase assemblage and inferring pressure from nearby metamorphic country rocks the experimental calibration differs from the empirical calibrations especially above 5 kbar and shows that the al content of hornblendes in equilibrium with the required phase assemblage is greater for a given total pressure than previously thought the geobarometers uncertainty is dramatically reduced the derived equation is p plus minus 0 5 kbar 3 minus 3 46 plus minus 0 24 4 23 plus minus 0 13 al sup t the geobarometer is applied to post bishop tuff volcanic rocks from long valley caldera california and reveals that most rhyodacites in this complex erupted from depths of about 6 km these eruptions occurred over 500 000 yr suggesting that the rhyodacitic magma reservoir beneath longmore valley had reached a steady p depth t state less

origin of late archean granite geochemical evidence from the vermilion granitic complex of northern minnesota the 2 700 ma vermilion granitic complex of northern minnesota is a granite migmatite terrane composed of supracrustal metasedimentary rocks mafic rocks tonalitic and granodioritic plutonic rocks and granite the metasedimentary rocks are predominantly graywacke which has been regionally metamorphosed to garnet sillimanite muscovite bearing biotite schist and has locally undergone anatexis the mafic rocks form early phases within the complex and are of two types 1 basaltic amphibolite and 2 monzodiorite and essexite rich in large ion lithophile elements lile the members of the early plutonic suite form small bodies that intrude the metasedimentary rocks and mafic rocks producing an early migmatite the granite is of two distinct varieties 1 white garnet muscovite biotite leucogranite s type chappell and white 1974 and 2 grayish pink biotite magnetite lac la croix granite i type the leucogranite occurs in the early migmatite and in paragneissic portions of the complex whereas the lac la croix granite is a late stage intrusive phase that invades the early migmatite and metasediment producing a late migmatite and forms a batholith this study focuses specifically on the origin of granite in the vermilion granitic complex chemical mass balance calculations suggest that the s type two mica leucogranite had a metagraywacke source and that the i type lac la croix granite formed via partial fusion of calc alkaline tonalitic material which may have been similar to rocks of the early plutonic suite this model is satisfactory for petrogenesis of similar late archean post kinematic granites throughout the canadian shield

archean lamprophyre dikes of the superior province canada distribution petrology and geochemical characteristics shoshonitic lamprophyre dikes of late archean age are a volumetrically minor yet widespread component of magmatism in the superior province and preferentially occur along major structures that delineate subprovince boundaries lamprophyre swarms are most prominently developed in fault graben where they are associated with alkali gabbro and trachytes and molasse sediments mineralogically the dikes are characterized by zoned phlogopite or hornblende phenocrysts restriction of feldspar to the groundmass globular segregations of k feldspar and calcite panidiomorphic texture and the sporadic presence of crustal xenoliths compositionally fresh dikes have contents of sio2 47 51 wt tio2 0 55 0 60 and p2o5 0 36 1 58 and k2o na2o ratios 1 5 2 3 and mg numbers 78 80 comparable to phanerozoic shoshonitic lamprophyres unaltered dikes are characterized by extreme enrichments of the lithophile elements k rb ba cs th u and light rare earth elements lree and enhanced abundances of cr 650 920 ppm co 37 42 ni 264 355 and sc 22 28 mid ocean ridge basalt normalized plots display 1 consistent enrichments of k rb and ba relative to sr and lree 2 variable enrichments of rb and ba relative to k and 3 troughs at ta nb and ti compared with fresh rocks altered samples have systematically lower k rb ba and sr contents but similar ree distributions and profiles from th yb and sc ni on normalized diagrams suites of post archean ultramafic lamprophyres in the superior province are enriched in large ion lithophile elements lile lree and compatible elements relative to their archean counterparts and lack troughs at ta nb and ti emplacement of the lamprophyres was diachronous from north 2705 ma to south 2674 ma probably being related to localized extension during the successive accretion of individual subprovinces archean lamprophyre magmas were derived from a depleted mantle source variably enriched in lile and lree by sediments and altered oceanic crust with small variable degrees of crustal contamination whereas the post archean lamprophyres lacked a plate margin setting

depletion of light rare earth elements in felsic magmas contrary to simple generalizations about their behavior light rare earth elements lree do not act as incompatible elements in very felsic magmas in fact lree concentrations typically decrease often drastically during differentiation of such magmas the simplest explanation for this depletion involves the separation of minute easily overlooked quantities of an extremely lree rich accessory mineral either monazite or allanite our data indicate that felsic liquids with 50 ppm lree may be saturated in either of these accessories and that the concentration required for saturation decreases in increasingly felsic liquids this accounts for incompatible behavior of lree even at high concentrations in mafic magmas in contrast to compatible behavior at low concentrations in felsic magmas partitioning of lree into solid rather than liquid has important implications for trace element and nd isotope modeling of crustal anatexis as well as for magma differentiation

magmatic epidote and its petrologic significance epidote is a major magmatic mineral in tonalite and granodiorite in a belt coextensive with the mesozoic accreted terranes between northern california and southeastern alaska textural and chemical evidence indicates that epidote crystallized as a relatively late but magmatic mineral that formed through reaction with hornblende in the presence of a melt phase the observed relations concur with experimental data on crystallization of epidote from synthetic granodiorite at 8 kbar total pressure plutonic rocks bearing magmatic epidote must have formed under moderately high pressures corresponding to lower crustal depths under fairly oxidizing conditions

plutonic rock types series discrimination of various granitoid series and related rocks abstract the streckeisen qap diagram discriminates four characteristic type series among the large variety of granitoids associated in intrusions three of them integrate varied compositions from basic to acid terms 1 the calcalkaline series with its low k or trondhiemite medium k or granodioritic high k or monzonitic variants 2 the alkaline series with its characteristic syenitic members 3 the tholeiitic series deprived of k feldspars the fourth 4 corresponds to mobilizates associated with migmatites of varied composition but always quartz rich the generalization in the qapf system allowing the plotting of undersaturated alkaline series provides an excellent visual display of the major associations and trends for the plutonic rocks

crust enriched mantle derived tonalites in the early proterozoic penokean orogen of wisconsin the early proterozoic 1 82 to 1 89 ga penokean orogenic province represents the initial phase of a major period of crustal growth of the north american proterozoic craton in central wisconsin the orogeny is dominated by the intrusion of variably deformed and metamorphosed tonalite trondhjemite suites in part hosted by archean gneisses and migmatites the magmatism is calc alkaline and similar in composition to modern high k orogenic andesites moderate abundances of sr 600 750 ppm and ree in the most primitive tonalites are consistent with derivation from an eclogitic altered tholeiite rather than a crustal archean amphibolitic tholeiite or enriched mantle source existing nd sr isotopic data for other tonalites of the wisconsin penokean orogen also indicate a mantle origin but with some admixture of archean crustal component this study shows that an archean component most likely was incorporated during emplacement the tonalite to trondhjemite differentiation of these magmatic suites is not

upb zircon and rutile chronology of archean greenstone formation and gold mineralization in the val d or region quebec abstract u pb geochronology provides an absolute time framework for the evolution of the sigma gold deposit and surrounding rocks at val d or southern abitibi subprovince the bourlamaque batholith the largest pluton in the area gives a 2699 8 1 0 ma u pb zircon age this pluton cuts the val d or formation which hosts the mineralization a u pb zircon age of 2704 9 1 1 ma on a felsic volcanic rock the colombiere rhyolite 13 km east of the mine dates that formation the gold bearing quartz vein system at sigma is hosted by andesites and two generations of porphyry intrusions all metamorphosed to the greenschist facies the oldest porphyry porphyritic diorite shows the same deformation as the volcanic rocks and has a 2703 7 2 5 ma u pb zircon age the porphyritic diorite and volcanic rocks are cut by feldspar porphyry dykes which post date regional folding and have a 2694 0 2 2 ma u pb zircon age regional greenschist metamorphism has been dated directly with a u pb date of 2684 7 ma on rutile in the colombiere rhyolite the mineralization and hydrothermal alteration in the mine are superimposed on the metamorphic minerals hydrothermal rutile from an alteration halo around the veins in andesite has a 2599 9 ma u pb age textural evidence clearly indicates that the wall rock alteration and vein filling are contemporaneous and hence the vein system and gold mineralization appear to have developed at least 80 m y after the formation and metamorphism of host greenstones

petrology of amphibolite facies mafic and ultramafic rocks from the catalina schist southern california metasomatism and migmatization in a subduction zone metamorphic setting the catalina schist of southern california is a subduction zone metamorphic terrane it consists of three tectonic units of amphibolite high p greenschist and blueschist facies rocks that are structurally juxtaposed across faults forming an apparent inverted metamorphic gradient migmatitic and non migmatitic metabasite blocks surrounded by a meta ultramafic matrix comprise the upper part of the catalina amphibolite unit fluid rock interaction at high p high t conditions caused partial melting of migmatitic blocks metasomatic exchange between metabasite blocks and ultramafic rocks infiltration of silica into ultramafic rocks and loss of an albitic component from nonmigmatitic clinopyroxene bearing metabasite blocks partial melting took place at an estimated p 8 11 kbar and t 640 750 c at high h2o activity the melting reaction probably involved plagioclase quartz trondhjemitic melts were produced and are preserved as leucocratic regions in migmatitic blocks and as pegmatitic dikes that cut ultramafic rocks the metasomatic and melting processes reflected in these rocks could be analogous to those proposed for fluid and melt transfer of components from a subducting slab to the mantle wedge aqueous fluids rather than melts seem to have accomplished the bulk of mass transfer within the mafic and ultramafic complex

i type granitoids as indicators of a late paleozoic convergent ocean continent margin along the southern flank of the central european variscan orogen a belt of carboniferous to permian i type granitoid bodies is in the basement thrust sheets of the alps which correspond to the southern flank of the central european variscan orogen these predominantly granodioritic plutons contrast with a contemporaneous belt of s type granites which extends to the north of the alps in the moldanubian section of the variscan orogen the observed regional duality of variscan plutonism resembles the plutons along active circum pacific continental margins in that there are outer i type plutons and inner s type granitoid belts therefore central europe could have undergone a cordilleran type orogenic event in late paleozoic time involving northwestward subdcution of the paleo tethys ocean

a review of the superior province of the canadian shield a product of archean accretion abstract the superior province consists of northern and southern high grade gneiss subprovinces and a central region of alternating granite greenstone and metasedimentary belts subprovinces are commonly fault bounded and display contrasting lithological assemblages metamorphic and structural styles geophysical characteristics and ages they are comparable to phanerozoic suspect terranes the middle and late archean rocks of the superior province are of mantle and juvenile crustal origin with little evidence of inheritance of components older than ca 3 1 ga high grade gneiss subprovinces are mainly deeper level equivalents of the supracrustal rich belts but one minnesota river valley is part of an older ca 3 6 ga terrane that was juxtaposed late in the tectonic history granite greenstone terranes are characterized by low grade volcano sedimentary sequences tholeiitic komatiitic submarine lava plain tholeiitic calc alkalic submarine to subaerial central complexes early platform type quartz arenite stromatolitic carbonate late timiskaming type shoshonitic alkalic volcanic fluvial sediment that range in age from ca 3 0 ga to 2 7 ga the volcanics represent oceanic island arc and continental arc volcanism the quartz arenite carbonate sequences deposition on relatively stable early formed ca 3 0 ga crust and the timiskaming type sequences deposition in pull apart basins developed along strike slip faults late archean ca 2 75 2 70 ga greenstone sequences occur throughout the superior province middle archean ca 3 0 2 8 ga sequences are common in northern and central superior province rare or absent in the south volcanism was accompanied by mafic to trondhjemitic plutonism the metasedimentary belts consist of variably metamorphosed volcanogenic turbidites with detrital zircons ranging in age from ca 3 0 ga to 2 7 ga the metasediments were deposited after major volcanism and probably represent accretionary prisms late archean orogenesis accompanied and followed by calc alkalic plutonism youngs southward from about 2 73 2 72 ga to 2 70 2 68 ga it involved large scale north south compression and dextral transpression and displays thin to thick skinned transitions the contemporaneity of events along the lengths of the belts and their southward younging coupled with the evidence of major compression are consistent with subduction driven oblique accretion of oceanic and continental volcanic arcs accretionary sedimentary wedges older microcontinental fragments etc in a convergent margin setting analogous to parts of the pacific rim differences eg abundance of komatiites and tonalitic plutonic suites are attributable to a hotter archean mantle

geochemistry and age of timiskaming alkali volcanics and the otto syenite stock abitibi ontario tephrites and trachytes of the timiskaming volcanics from the kirkland lake area ontario and syenites and a granite from the nearby otto stock are characterized by extreme enrichment of incompatible elements coupled with relative depletion of nb ti and to a lesser extent zr and y the volcanic rocks have a whole rock sm nd isochron age of 2740 117 ma 2 error and minerals separated from the otto stock a sm nd age of 2544 50 ma conventional and ion probe u pb analyses of zircons from the otto stock yielded an upper intercept age of 2700 19 ma whereas the more concordant ion probe analyses had a mean 207 pb 206 pb age of 2671 8 ma 2 the latter is interpreted as the age of emplacement of both the volcanics and the pluton and the sm nd mineral isochron age is thought to reflect a period of later disturbance probably during regional metamorphism a high initial e nd of 2 5 1 5 for kirkland lake volcanics indicates long term isotopic depletion of their source this value is the same as that for volcanic rocks throughout the abitibi belt and indicates that any chemically enriched material in the source cannot have been much older than the volcanics themselves an environment remote from older continents is inferred

tectonic framework of the himalaya karakoram and tibet and problems of their evolution the himalaya the karakoram and tibet were assembled by the successive accretion to asia of continental and arc terranes during the mesozoic and early tertiary the jinsha and banggong sutures in tibet join continental terranes separated from gondwana ophiolites were obducted onto the shelf of southern tibet in the jurassic before the formation of the banggong suture the kohistan ladakh terrane contains an island arc that was accreted in the late cretaceous on the shyok suture and consequently evolved into an andean type batholith further east this trans himalayan batholith developed on the southern active margin of tibet without the prior development of an island arc ophiolites were obducted onto the shelf of india in the late cretaceous to lower palaeocene before the closing of tethys and the formation of the indus yarlung zangbo suture at about 50 ma post collisional northward indentation of india at ca 5 cm a 1 since the eocene has redeformed this accreted terrane collage palaeomagnetic evidence suggests this indentation has given rise to some 2000 km of intracontinental shortening expressions of this shortening are the uplift of mid crustal gneisses in the karakoram on a late tertiary breakback thrust folding of palaeogene redbeds in tibet south directed thrust imbrication of the foreland and shelf of the indian plate north directed back thrusts along the indus suture zone post miocene spreading and uplift of thickened tibet giving rise to n s extensional faults and strike slip faults which allowed eastward escape of tibetan fault blocks

archean volcanoes in southwestern abitibi belt ontario and quebec form composition and development southwestern abitibi volcanic belt is composed of numerous deformed volcanoes that were originally circular to subcircular in outline 100 200 km in diameter and 10 16 km in stratigraphic thicknes

nd and sr isotope systematics for the kamiskotia montcalm area implications for the formation of late archean crust in the western abitibi subprovince canada geochemistry and nd isotopic compositions are used to characterize mantle and crustal sources and to provide constraints on petrogenetic models for tholeiitic calc alkalic and lamprophyric suites in the kamiskotia montcalm area the kamiskotia gabbroic complex kgc and cogenetic bimodal volcanic rocks have end t 2 2 to 2 6 consistent with a direct derivation from a long term light rare earth element lree depleted mantle the montcalm gabbroic complex has decreasing end t upsection from 2 8 to 1 0 consistent with contamination by long term lree enriched with respect to the long term lree depleted abitibi mantle crust during fractionation two calc alkalic lamprophyre samples characterized by large ion lithophile element lile and lree enrichment and high mgo ni and cr contents have end t of 2 5 and 2 8 indicating a derivation from a depleted mantle source that had undergone recent trace element enrichment a different lamprophyre suite is extremely lile and lree enriched and h

growth and differentiation of the continental crust declining radiogenic heat production since the archaean has resulted in a secular evolution from a regime of numerous fast moving small thin torsionally weak plates to the present regime of larger thicker torsionally stronger plates moving at an average rate of less than one sixth of the archaean rate this has been accompanied by episodic changes in geological effects by 2500 ma b p about 85 of the present crustal mass had grown by the addition and amalgamation of mafic and calc alkaline rocks in oceanic arcs at an average rate of 11 17 pg a during the early proterozoic the first large cratons were stabilized and locally thickened and differentiated the proterozoic was an era of little continental growth falling average sea level and intracontinental deformation by 700 ma b p cratons had become much more stable marginal accretionary terrians had begun to develop with an average phanerozoic growth rate of 1 64 pg a and blueschists and ophiolites sensu stricto witness the advent of the extant plate tectonic regime

genesis of the kinabalu sabah granitoid at a subduction collision junction the kinabalu batholith is a late neogene granitoid in northwestern sabah east malaysia apparently marking the locus at which subducted south china basin lithosphere interacted with roots of the northern sabah collision suture the exposed batholith comprises a relatively small core of biotite quartz monzodiorite bqm grading out to dominant hornblende quartz monzonite hqm both lithologies contain mafic igneous and metasedimentary inclusions and are cut by late stage aplite dikes major element data indicate the bqm k2o na2o ratios ranging 0 72 1 03 represents a low k type while hqm k2o na2o ratios ranging 1 35 5 58 is a distinct high k type magmaphile element distributions support this distinction hqm showing higher k rb lree and lower ta and nb contents than bqm indicating the more extensive interaction of hqm with sial least squares mass balance models suggest that hqm evolved through the combined effects of fractional crystallization and crustal assimilation while bqm was dominated by fractional crystallization however similar plagioclase zoning patterns in both lithologies suggest they are comagmatic rather than generated by melting of separate sources it is concluded that low k type melts formed by subduction induced remelting of underplated lower crust underwent high pressure sialic contamination with development of high k character these provided access to later formed low k melts which were less contaminated and consolidated to form the pluton core the unusual zonation from inner low k to outer high k type compositions may indicate that the cessation of subduction prevented upward migration of the melting anomaly and thermal maturation of the pluton

archaean orogenic ultrapotassic magmatism an example from the southern abitibi greenstone belt the clericy pluton in the southern abitibi greenstone belt of quebec is a syenitic intrusion cross cut by nordmarkitic dykes it is probably one of the oldest known ultrapotassic intrusions about 2 7 ga it crystallized in a crustal environment under low ph20 2 5 kbar and low fo2 conditions most trace elements behaved as compatible elements suggesting that sphene and apatite were involved together with clinopyroxene during low pressure crystal fractionation the large range of initial isotopic ratios calculated for 2 68 ga nd 1 9 to 2 3 as well as the trace element distribution suggest that the syenitic pluton is the product of differentiation from a parental magma with a composition similar to that of timiskaming type archaean shoshonites contaminated by a crustal component on the other hand the nordmarkitic dykes are probably the result of crustal anatexis of metasedimentary rocks and some addition of the syenitic magma the clericy peralkaline syenite differs chemically from typical intraplate syenites in being saturated with respect to quartz and displaying lower ti and hfse contents it is related to an ultrapotassic volcano plutonic event which characterized the last 20 ma of igneous activity in the abitibi greenstone belt it may represent an increased rate of convergence of small heterogeneous plates during a subduction driven accretion tectonic event which possibly marked the culmination of the kenoran orogeny in the superior province of the canadian shield

petrogenetic interpretation of granitoid rock series using multicationic parameters abstract granitoid rock compositions from a range of tectonic environments are plotted on a multicationic diagram devised by de la roche and his coworkers this shows that there is a systematic change through an orogenic cycle which leads progressively to the ultimate development of alkaline magmas possible source materials and mechanisms of magma generation are considered from analysis of mineral compositional vectors these suggest that most granitoid series result from a two stage process first fractional crystallisation of clinopyroxene olivine and calcic plagioclase from a basic source with tholeiitic affinities produces a magma of intermediate composition this magma then undergoes periodic mixing with a felsic magma followed by in situ fractionation to generate individual intrusions within granitoid series

geochemistry of archean alkalic volcanic rocks from the crystal lake area east of kirkland lake ontario canada abstract twelve samples of timiskaming volcanics from the crystal lake area were analyzed for major and trace elements ta th zr hf v cr sc rare earth elements ree y etc by x ray fluorescence and instrumental neutron activation analysis techniques the samples which include mafic trachytes trachytes and leucite trachytes represent several batches of archean alkalic magmas differentiated at shallow depths the cr sc cr systematics and ree zr th and zr y characteristics show however that their primary magmas generated at great depths in the enriched mantle where garnet remained in the refractory residue of partial melt the chemical similarity between the timiskaming samples and modern alkalic rocks in mature volcanic arcs strongly implies that the timiskaming volcanism represents the last stage of a late archean island arc volcanism present in the southwestern abitibi belt superior province the occurrence of timiskaming type alkalic rocks is not rare in archean greenstone belts thereby suggesting that a tectonic setting similar to modern volcanic arc system might have been common in late archean time

geobarometry differential block movements and crustal structure of the southwestern abitibi greenstone belt canada the southern volcanic zone of the archean abitibi greenstone belt comprises several fault bounded tectonic blocks with distinct volcanic sedimentary sequences pressure temperature time p t t histories granite compositions and metallogenic provinces pressure distributions have been mapped out from an extensive new database derived from al in hornblende and other geobarometers in the composite crustal structure of the southern volcanic zone determined from the pressure time data the lacorne block and pontiac subprovince represent deeper crustal levels overlain by rocks exposed at the surface in the round lake or lac abitibi type blocks which are in turn capped by those exposed at the surface in a blake river rouyn type block exposure of the deeper crustal levels and juxtaposition of these distinct tectonic blocks are interpreted to have occurred by differential block uplift caused by strike slip faulting which was related to oblique collisional tectonics during late tectonic development of the superior province in late archean time

geodynamic implications of devonian silicic arc magmatism in the sierra nevada and klamath mountains california most proposed geodynamic models integrate the northern sierra nevada and eastern klamath mountains in a single island arc system new petrological and geochemical data show that the upper devonian sierra buttes formation of the northern sierra nevada displays important differences from the lower devonian balaklala formation of the eastern klamath mountains the cale alkalic affinity of the sierra buttes rhyolites light rare earth element ree enriched low e nd suggests that they were erupted in a continent based island arc in contrast the lower devonian balaklala low k tholeiitic rhyolites light ree depleted high e nd are considered to be part of an intraoceanic island arc sequence moreover the paleozoic tectogenesis of these two island arc sequences is different the lower devonian balaklala rhyolites were affected by the late devonian early carboniferous orogeny whereas the sierra buttes formation is on basement already affected by a pre late devonian deformation therefore at least after carboniferous time and before late triassic time the eastern klamath and northeastern sierra nevada island arcs cannot be considered lateral counterparts

isotopic and trace element arguments for the lower crustal origin of hercynian granitoids and pre hercynian orthogneisses massif central france abstract the cadomian hercynian basement of the massif central france contains both pre metamorphic orthogneisses 600 410 ma and post metamorphic undeformed granitoids 360 275 ma major trace element and sr nd isotopic analyses of these intrusions indicate slight differences in source material for orthogneisses and two types of underformed granitoid a granodiorite monzogranites and leucogranites orthogneisses have moderately enriched ree patterns la yb 5 3 13 3 variable hree contents ybn 4 2 25 and strong negative eu anomalies eu eu 0 23 0 36 their tnd values are low 5 8 to 1 7 indicating a strong crustal component in the source but tsr values are also generally low 25 to 100 and overlap the field of age corrected isotopic values for lower crustal granulite xenoliths from the massif central the source of the orthogneisses is therefore considered to be dominated by a lower crustal meta igneous component with additional input from a lower crustal metasedimentary source granodiorites monzogranites tend to show strong lree enrichment la yb 11 26 with small eu anomalies eu eu 0 54 0 75 and ybn values between 6 and 8 5 these granitoids form the least evolved end members of a series trending to highly evolved leucogranites with similar la yb ratios 11 36 but with generally larger eu anomalies eu eu 0 24 0 62 and lower ybn values 3 8 initial isotopic ratios of the leucogranites tnd 7 9 to 4 5 and tsr usually 100 mostly fall within the field of age corrected lower crustal metasedimentary granulite xenoliths from the region and it is proposed that these leucogranites were formed by partial melting of lower crustal metasediments granodiorites and monzogranites have tnd between 6 5 and 3 4 and tsr values

the tectonic evolution of the abitibi greenstone belt of canada based on structural geochemical sedimentological and geochronological studies we have formulated a model for the evolution of the late archaean abitibi greenstone belt of the superior province of canada the southern volcanic zone svz of the belt is dominated by komatiitic to tholeiitic volcanic plateaux and large bimodal mafic felsic volcanic centres these volcanic rocks were erupted between approximately 2710 ma and 2700 ma in a series of rift basins formed as a result of wrench fault tectonics the svz superimposes an older volcanic terrane which is characterized in the northern volcanic zone nvz of the abitibi belt and is approximately 2720 ma or older the nvz comprises basaltic to andesitic and dacitic subaqueous massive volcanics which are cored by comagmatic sill complexes and layered mafic anorthositic plutonic complexes these volcanics are overlain by felsic pyroclastic rocks that were comagmatic with the emplacement of tonalitic plutons at 2717 2 ma the tectonic model envisages the svz to have formed in a series of rift basins which dissected an earlier formed volcanic arc the nvz analogous rift environments have been postulated for the hokuroko basin of japan the taupo volcanic zone of new zealand and the sumatra and nicaragua arcs the difference between rift related submergent volcanism in the svz and emergent volcanism in the nvz resulted in the contrasting metallogenic styles the former being characterized by syngenetic massive sulphide deposits whilst the latter was dominated by epigenetic porphyry type cu au deposits

geochemistry of fine grained clastic sediments in the archean abitibi greenstone belt canada implications for provenance and tectonic setting abstract archean sandstone and shale samples from turbiditic tf and alluvial facies af in the southern volcanic zone of the abitibi greenstone belt canada have been analyzed for major and trace elements the sandstones from tf are mainly greywackes whereas those from af are lithic arenite sio 2 ranges from 51 to 68 geochemically the clastic sediments cluster into mafic element mep and low mafic element lmep populations the mep sediments which include turbiditic facies sediment samples from the larder lake group in the kirkland lake area are characterized by elevated abundances of mgo 5 ni 200 ppm cr 400 ppm sc 22 ppm and feo 7 5 abundances lower contents of incompatible elements such as th u ta hf zr and total rees n ratios are 4 5 with eu eu 1 the lmep sediments which include samples from the beatty dome and three nations lake formations in timmins the kewagama group noranda area and timiskaming group in the kirkland lake area however show reverse abundance distributions the geochemical gaps shown by the major and trace elements define two populations of sediments suggesting contrasting provenances for them the mep sediments require a dominantly mafic and ultramafic 90 source with 10 felsic contribution whereas the lmep sediments require a mixture of felsic 75 and mafic 25 rocks this compositional distribution of the sediments is closely related to their tectonic environments the mep was formed in basins proximal to or within an active oceanic rift consequently having ultramafic and mafic rocks as the major source for the lmep calc alkaline volcanic sequences formed in an island arc like setting were the major source these two contrasting volcanic plutonic terranes were tectonically juxtaposed during late archean collisional tectonics

petrology and tectonic significance of gabbros tonalites shoshonites and anorthosites in a late paleozoic arc root complex in the wrangellia terrane southern alaska plutonic rocks intrusive into the late paleozoic tetelna formation of southern alaska are the underpinnings of the late paleozoic skolai arc of the wrangellia terrane there are four groups of intrusive rocks within the skolai arc 1 gabbro diorite plutons that contain gabbroic to anorthositic cumulates along with a differentiated series of gabbros and diorites of basaltic to andesitic composition 2 silicic intrusions including tonalite granodiorite and granite 3 monzonitic to syenitic plutonic rocks of the ahtell complex and related dikes and sills 4 fault bounded bytownite anorthosite of uncertain age and association these anorthosites may be related to post skolai nikolai greenstone magmatism the silicic rocks yield discordant u pb zircon ages of 290 320 ma early to late pennsylvanian relative age relations suggest that the oldest intrusive rocks are the gabbro diorite plutons the youngest are the monzonitic rocks and that the silicic rocks span this range the gabbro diorite pluto

a proposed model for the formation of reversely zoned plutons based on a study of the lacorne complex superior province quebec the lacorne complex is a calc alkaline mass of archean age located in the abitibi region of quebec the complex can be subdivided into three zones the inner zone varies in composition from hornblende biotite gabbro breccia to monzonite with all rocks containing 5 modal quartz the outer zone rocks contain more than 5 modal quartz and range in composition from biotite hornblende quartz diorite to granodiorite the third zone consisting of muscovite biotite garnet leucomonzogranite is exposed only in the northwestern portion of the complex the transition between each zone is relatively abrupt but continuous based on 400 modal estimates of stained slabs and 107 whole rock and trace element analyses it is demonstrated that both the inner zone and the outer zone rocks are reversely zoned mafic cores are surrounded by more felsic borders various methods that could explain the observed geochemical trends are considered the data indicate that crystal liquid equilibria were maintained during cry

petrology of the archean pontiac and kewagama sediments and implications for the stratigraphy of the southern abitibi belt we present an interpretation of the petrology of two archean sedimentary groups in the southeastern portion of the abitibi greenstone belt 1 the pontiac sediments which represent an assemblage of quartz rich sandstones and 2 the kewagama sediments which are quartz poor and rich in volcanic fragments the pontiac sediments display a north south decrease in bed thickness sand shale ratio and grain size suggesting derivation from a northern source relative to their present position petrographic and rare earth element ree analysis indicates that three provenances contributed to the pontiac 1 a mature craton 2 light heavy ree fractionated igneous rocks probably of tonalite trondhjemite composition and 3 ultramafic mafic volcanics in contrast to the pontiac sediments the kewagama sediments were derived from acid to mafic volcanic rocks ratio of 3 1 with minor contributions from plutonic rocks and from graded sedimentary rocks possibly the pontiac the ree data indicate a source frac

evidence for contrasting compositional spectra in comagmatic intrusive and extrusive rocks of the late archean blake river group abitibi quebec the flavrian pluton is a sill shaped intrusion in the blake river group brg volcanic rocks in the noranda region of the abitibi greenstone belt the pluton is dominated by trondhjemites and tonalites with minor peripheral quartz gabbro and hybrid phases the brg volcanic rocks consist of a bimodal suite of basalt andesite and rhyolite the flavrian trondhjemites are geochemically identical to the rhyolitic lavas of the brg sio2 72 la sm 3 4 la yb 3 6 zr y 3 9 y nb 3 1 and the flavrian gabbroic and dioritic rocks are identical to the brg basalts and andesites sio2 58 la sm 3 0 la yb 5 5 zr y 4 2 y nb 3 3 however the tonalitic rocks of the flavrian pluton have no extrusive equivalents in the brg the different compositional spectra of the extrusive and intrusive rocks are interpreted as being a result of a transition in magma chamber evolution from a zoned open system that was active during the evolution of the volcanic rocks to closed system plutonic crystallization

icp ms a powerful tool for high precision trace element analysis in earth sciences evidence from analysis of selected u s g s reference samples abstract inductively coupled plasma mass spectrometry icp ms is a powerful analytical technique with considerable application to geochemistry in this paper we demonstrate the power of icp ms as a tool in petrogenetic studies data for 28 elements in seven u s g s basalt and andesite reference samples agv 1 bcr 1 bhvo 1 bir 1 dnc 1 w 1 and w 2 are presented accuracy and precision standard deviation for 26 of these elements is either excellent a detailed description of the icp ms technique we have developed is given a simple hf hno 3 dissolution in screwtop teflon bombs was used for sample preparation icp ms instrumental sensitivity was measured by external solutions with surrogate calibration for 4 elements matrix correction was made by use of standard addition naturally occurring internal standards rb y ce and pb were used to correct for instrumental drift between sample and spiked sample measurements we report data for the elements li cs rb sr ba zr hf nb ta y sc u th pb and all 14 of the ree in addition the procedure acquired data for be mo w tl and bi many elements in this array are crucial for the evaluation of petrogenetic models and source tracing herein lies the real power of icp ms one technique one dissolution small sample size good limits of detection good to excellent accuracy and good precision for many geochemically important elements

chemical composition of archean pontiac metasediments southwestern abitibi belt superior province three mica schists from the pontiac group have a la yb ratio of 19 a la content of 32 ppm and a th content of 6 ppm these values are higher than those of the average archean upper crust la y

controls on source and depth of emplacement of granitic magma large granitic batholiths emplaced in high grade metamorphic rocks at mid crustal levels were relatively hydrous compared with major batholiths emplaced at very shallow levels those emplaced at mid crustal levels were probably generated by partial melting of crustal rocks at only slightly deeper levels those emplaced at shallow levels are associated with penecontemporaneous volcanic ejecta and were probably generated at deep crustal or possibly mantle depths

ree geochemistry and isotopic data of archean silicic volcanics and granitoids from the pilbara block western australia implications for the early crustal evolution eighteen silicic volcanic rocks of the warrawoona group and ten associated plutonic rocks from the pilbara block western australia have been chosen for geochemical and isotopic studies silicic volcanics of the unsb upper member of north star basalt are dated at 3 56 3 57 by both the rb sr and the sm nd methods the respective 1 initial isotopic composition values are 0 7005 5 sr and 0 50810 39 nd this age is consistent with the stratigraphic interpretation that the talgatalga subgroup in which the north star basalt occupies the lowermost position is overlain by the duffer formation whose age was earlier established at 3 45 by the zircon u pb method the new rb sr data on six silicic lava samples from the duffer formation yield an isochron of 3 23 0 28 2v though imprecise this age agrees with the zircon age within error limits rb sr ages of 2 3 2 4 obtained for the panorama rocks and the wyman formation do not correspond to their initial eruption ages chemical arguments suggest that these ages represent the time of metasomatism associated with the widespread thermal event in this region about 2 3 2 4 ago geochemically most of these analyzed rocks volcanic and plutonic are of tonalite trondhjemitegranodiorite ttg composition a typical feature found in many other archean terrains they generally show fractionated ree patterns except the panorama formation rocks furthermore the wyman formation rhyolites and the post tectonic adamellites show significant negative eu anomalies suggesting a similar mode of magma generation and a probable genetic link theoretical considerations suggest that most of these ttg rocks could have been generated by partial melting of amphibolitic or basaltic sources followed by fractional crystallization although the archean granitic gneisses often possess mantle like isr values the trace element data indicate that they could not have been derived by direct melting of upper mantle materials the immediate tectonic implication is that in any archean terrain the formation of na rich continental crust of ttg suite must be preceded by the presence of basaltic crust the occurrence of this basaltic crust is a matter of controversy such crust might have been totally destroyed by repeated melting processes or its remnants are now represented by some of the mafic ultramafic enclaves within the tonalite trondhjemite batholiths

a multi disciplinary seriation of early homo and paranthropus bearing palaeocaves in southern africa abstract fossils of early homo and paranthropus have been recovered from several sites in southern africa unfortunately their precise age has historically been difficult to assess hampering the reconstruction of their relationships to each other and to fossils from eastern africa multi dating strategies combining biochronological archaeological palaeomagnetic electron spin resonance esr and uranium series techniques are now clarifying their age the following sequencing of sites is suggested swartkrans member 1 2 0 ma gondolin 1 8 ma kromdraai 1 8 1 7 ma sterkfontein m5a 1 8 1 4 ma swartkrans m2 1 7 1 1 ma sterkfontein m5b 1 4 1 1 ma sterkfontein m5c 1 3 0 8 ma swartkrans m3 1 0 0 6 ma the position of coopers d and drimolen is difficult to access because they only have faunal age ranges 1 9 1 6 ma esr suggests mixing is a potential major problem in multi generational sites the oldest southern african representatives of early homo and paranthropus occur around 2 1 1 9 ma in swartkrans member 1 and are recorded almost continuously in the palaeocave deposits until around 1 0 0 6 ma in swartkrans member 3 currently these data suggest that paranthropus and homo first occur significantly later in the southern african record than the eastern african record moreover paranthropus persists much later in southern africa than in eastern africa

ridge collision at convergent margins implications for archean and post archean crustal growth abstract growth of continental crust today occurs dominantly at convergent plate margins through magmatic addition and tectonic accretion lateral and vertical related to 1 1 steady state subduction 2 2 non steady state episodic processes ridge forearc collision is one episodic process that can cause significant highly diachronous thermal and tectonic effects when a buoyant bathymetrically high and thermally active oceanic spreading ridge approaches collides with and potentially subducts below a continental margin near field effects within 100 km of collision site in the forearc may include anomalous heat anomalous magmatism ophiolite obduction forearc deformation and tectonic erosion of the base of the forearc far field effects generally postdate collision and may include changes in the thermal structure of the continent changes in the composition of arc magmas a reduction or hiatus in arc magmatism uplift and or subsidence of the continental margin marginal basin formation underplating of subducted slabs and intraplate deformation effects of ridge collision can vary depending on the triple junction geometry the chile margin triple junction cmtj where the actively spreading chile ridge is colliding with the south american forearc is the best modern example of ridge collision near field effects of pliocene recent ridge collisions near the cmtj include 1 1 emplacement of the pliocene taitao ophiolite less than 15 km from the trench 2 2 intrusion of pliocene granitic plutons and 3 3 forearc deformation formation of a pull apart basin on the continental shelf and late tertiary recent movement on a major fault along the arc forearc boundary far field effects include reduction in subduction related seismicity and volcanism chemical changes in arc magmatism and possibly uplift and or subsidence ridge collison features in southern chile are similar to features of archean greenstone belts suggesting that some greenstone sequences may have formed in a ridge collision setting and that ridge collision may have been an important process in archean crustal growth these features include granitic rocks closely associated with ophiolitic rocks formed near a continental margin mafic ultramafic section overlain by basaltic to andesitic volcanic rocks of mixed calc alkaline and morb character intercalated with volcaniclastic sedimentary rocks differences between archean and phanerozoic tectonic processes mainly related to higher heat production in the past probably have affected the nature and importance of ridge collision in archean crustal growth during the archean greater ridge length and smaller plates probably resulted in more ridge collisions and related processes contributing to a higher continental growth rate in particular underplating of subducted slabs and direct slab melting may have been important processes in archean continental growth but are of relatively minor importance today

archean metavolcanics from the rouyn noranda district abitibi greenstone belt quebec 2 mobility of trace elements and petrogenetic constraints qualitative evaluation of altered mafic pillows from the rouyn noranda district of the abitibi metavolcanic belt reveals that for low grade metamorphism the high field strength elements hfs zr y ti and the rare earth elements ree are immobile however where alteration is extreme the ree elements are leached and redeposited by carbonate rich and k rich metasomatic fluids whilst zr y and zr ti remain unchanged rubidium ba and to a lesser extent sr are mobile at all levels of alteration however provided certain filters are applied the original geochemical values of the lavas for the alkaline elements the hfs elements and the ree can be defined the geochemical variations thus identified for the volcanic series of the rouyn noranda district are ascribed to a combination of contamination and fractional crystallization of a primitive tholeiitic magma

mantle derivation of archean amphibole bearing granitoid and associated mafic rocks evidence from the southern superior province canada amphibole bearing late archean 2 73 2 68 ga granitoids of the southern superior province are examined to constrain processes of crustal development the investigated plutons which range from tonalite and diorite to monzodiorite monzonite and syenite share textural mineralogical and geochemical attributes suggesting a common origin as juvenile magmas despite variation in modal mineralogy the plutons are geochemically characterized by normative quartz high al2o3 15 wt na rich fractionation trends mol na2o k2o 2 low to moderate rb generally 150 ppm are comagmatic with the evolved granitoids and indicate that the suites are mantle derived isotopic studies of archean monzodioritic rocks have shown lree enrichment and initial 143nd 144nd ratios indicating derivation from mantle sources enriched in large ion lithophile elements lile shortly before melting mineral assemblages record lower ph2o with increased alkali contents of the suites this evidence in conjunction with experimental studies suggests that increased alkali contents may reflect decreased ph2o during mantle melting these features indicate that 2 73 ga tonalitic rocks are derived from more hydrous mantle sources than 2 68 ga syenitic rocks and that the spectrum of late archean juvenile granitoid rocks is broader than previously recognized comparison with phanerozoic and recent plutonic suites suggests that these archean suites are subduction related

archean geodynamics and the abitibi pontiac collision implications for advection of fluids at transpressive collisional boundaries and the origin of giant quartz vein systems abstract giant quartz vein systems of all ages from the archean to cenozoic are characterised by similarities of geodynamic setting local structure paragenesis metal budget p t t paths and chemical fluid dynamic and isotopic properties that collectively imply a singular hydrothermal process such quartz vein provinces form in regional brittle ductile shear zones that define terrane boundaries including closure of back arc basins and arc continent or continent continent collisions examples include the svz of the archean abitibi belt the norseman wiluna belt yilgarn block kolar schist belt india foothills metamorphic belt california and coast ranges megalineament in the cordillera and the cenozoic monte rosa district insubric line evidence for an archean collisional environment comes from the abitibi and pontiac subprovince tectonic boundary which hosts giant quartz vein systems in this region the amphibolite facies lacorne tectonic block in the archean abitibi greenstone belt is anomalous with respect to the prevalent low grade supracrustal sequences in neighbouring blocks the lacorne block has mature clastic sediments with a zircon provenance age spectrum from 3040 2691 ma and two granitic magma series a late syntectonic monzodiorite monzonite granodiorite syenite series formed over 2685 2670 ma similar to phanerozoic volcanic arc granites and post tectonic garnet muscovite granites emplaced at 2650 2630 ma which compositionally resemble phanerozic collisional s type granites the pontiac subprovince to the south of the abitibi greenstone belt shares all the above features with the lacorne block and collided with and was locally thrust under the abitibi belt with differential uplift of the lacorne block following collision to generate a tectonic window following collision of allochthonous terranes the conjuction of large volumes of subcreted oceanic crust and sediments between accreted terranes displaced isotherms rise and metamorphic dehydration are all necessary conditions for forming giant quartz vein systems metamorphic fluids are expelled along the terrane boundary structures at deep levels and focussed into second and higher order splays at mid crustal levels where quartz and au precipitation occurs hydrothermal fluids that formed the giant quartz veins are remarkably uniform in h o c and sr isotopic compositions albeit with small provincial variations the veins and gold precipitated at 270 360 c from fluids with low salinity and moderate co2 at 2 3 kbar in the brittle ductile transition

origin and early growth rate of continents abstract data suggest that early continents formed over mantle sinks and grew rapidly between 2 7 and 3 0 ga ago nd isotopic results indicate that archean tonalites which are largely responsible for early continental growth are derived chiefly from ancient depleted mantle sources trace element and oxygen isotope data however indicate enriched sources for such tonalites these constraints can be explained if the mantle is fractionated into depleted dmr and enriched emr reservoirs during early degassing of the earth 4 4 ga blocks of emr are recycled back into the mantle and the emr dmr mixtures serve as sources for archean basalts these basalts are enriched in incompatible elements by deep sea alteration metasomatism in sinks or by other mantle processes and then undergo wet partial melting to produce tonalite magmas enrichment must occur not more than 150 ma prior to tonalite production basalt plateaus accumulate at sinks and as cooling continues geotherms at sinks pass into the eclogite stability field dense eclogite roots form on the basalt plateaus which pulls them into the mantle between 2 7 and 3 0 ga ago resulting in rapid tonalite production and thus in rapid continental growth

the himalayas a modern analogue for archaean crustal evolution abstract present crustal evolution models fail to account for the generation of the large volumes of continental crust in the required time intervals if the oceanic crust was appreciably thicker in the archaean as geothermal models would indicate then oceanic crustal collision on the scale of the present day himalayan continental collision zone may have been a frequent occurrence in the archaean resulting in extensive partial melting of the hydrous underthrust oceanic crust to produce voluminous tonalite melts leaving a depleted stabilised basic residuum

geochemistry and origin of archean felsic metavolcanic rocks central noranda area quebec canada felsic magma petrogenesis was studied by analyzing 24 stratigraphically controlled archean andesite to rhyolite lava flows of both tholeiitic and calc alkalic affinity from the upper noranda subgroup quebec using instrumental neutron activation and x ray fluorescence techniques the lavas have moderate values of la yb n 0 9 3 8 and low values of 100 th zr 1 according to calculations following batch partial melting and rayleigh fractional crystallization models both the calc alkalic and tholeiitic felsic volcanic rocks are probably products of shallow level fractional crystallization of mafic parental magmas formed respectively by lower 7 for calc alkalic and higher 14 for tholeiitic degrees of partial melting of a primitive mantle source contribution to the magma genesis from plausible crustal materials was negligible a back arc type diapirism is geochemically suggested for the tectonic model of origin of noranda felsic magmas in conformity with geological observations felsic volca

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

single zircon age constraints on the tectonic juxtaposition of the archean abitibi greenstone belt and pontiac subprovince quebec canada abstract zircons from metasediments and granitoids in the high grade lacorne block within the low grade archean abitibi greenstone belt have been dated by the single zircon pb evaporation technique yielding 207pb 206pb minimum ages detrital zircons in the mature clastic metasediments of the lacorne block display a range of ages from 2691 8 ma to 3042 6 ma the younger zircon ages 2691 8 2695 4 2715 16 and 2719 4 ma thus impose an upper limit for deposition and indicate that the high grade lacorne block is not basement to the abitibi supracrustal sequence 2747 2680 ma existence of abundant 69 older detrital zircons 2750 ma suggests in turn that the abitibi supracrustal rocks are not the source of the lacorne sediments two generations of granitoids occur in the lacorne block an early monzodiorite monzonite granodiorite syenite series mmgs 2671 4 to 2675 24 ma and a younger s type garnet muscovite granite series gmg 2630 20 to 2655 8 ma this contrasts with granitoid magmatism in the abitibi greenstone belt which ended at 2675 ma the pontiac subprovince to the south of the abitibi greenstone belt shares all of the above features of the lacorne block including detrital zircon ages as well as the composition and timing of granitoid magmatism this is interpreted to indicate that the lacorne block was originally part of the same tectonic terrane as the pontiac subprovince after development of the mmgs magmatism 21670 2680 ma the pontiac subprovince locally underthrust the abitibi greenstone belt and crustal thickening promoted partial melting of underthrust pontiac metasediments to form the 2644 13 ma s type granites later differential uplift exposed the lacorne block as a high grade tectonic window in the abitibi greenstone belt

uranium lead isotopic ages of plagiogranites from the troodos ophiolite cyprus and their tectonic significance zircon populations from two plagiogranites in the plutonic section of the troodos ophiolite yield u pb dates between 90 3 0 7 and 92 4 0 7 ma zircons from one of the two samples yield u pb dates of 90 3 0 7 and 92 1 0 7 ma this difference is greater than our analytical error and the zircons are therefore considered to be slightly discordant two fractions from another sample are internally and externally concordant and have an average date of 91 6 1 4 ma this date is adopted as the age of plagiogranite crystallization the plutonic section of the troodos ophiolite is complex in that several generations of intrusion can be identified the plagiogranites occur at the top of the intrusive section and represent fractionated residua of magma chambers related to the lower pillow lavas they are possibly slightly older than small ultramafic layered complexes that represent the final intrusive episode within the troodos ophiolite there is good agreement between our results and the ages of radiolaria which occur in umbers intercalated with the upper pillow lavas of troodos blome and irwin 1985 this shows that the 75 to 85 ma k ar dates on the lavas and sheeted dikes desmet et al 1978 delaloye and desmet 1979 delaloye et al 1980 staudigel et al 1986 represent age resetting unrelated to ophiolite formation the troodos u pb zircon ages are slightly lower than u pb zircon ages from the samail ophiolite in oman which vary between 93 5 and 97 9 ma and average 95 ma tilton et al 1981 the small difference in age between the troodos and the samail ophiolites and the geochemical similarities of basalts from both complexes to island arc volcanic rocks imply that a 3000 km axis of rift related island arcs formed between 91 and 98 ma emplacement of the ophiolites followed during the early maastrichtian

u pb zircon geochronology in the southwestern abitibi greenstone belt superior province the abitibi belt is the largest continuous greenstone belt in the superior province of the canadian shield it comprises several composite komatiitic tholeiitic calc alkalic and sedimentary sequences that are folded transected by major faults and intruded by various generations of plutonic rocks precise u pb geochronology has been carried out in the belt for the past decade to solve chronostratigraphic and metallogenetic problems this paper presents new zircon ages and reassesses previously published ones now refined by the addition of abraded and concordant zircon analyses volcanic and subvolcanic units of the timmins area yield the following ages 2727 1 5 ma for a tuff of the upper deloro formation 2703 1 5 ma for a tuff of the upper tisdale formation and 2698 4 ma for the krist fragmental assigned to the top of the tisdale formation the age of a dunite intrusive into the upper deloro formation is revised at 2707 3 ma whereas the aquarius diorite east of timmins yields a poorly define

mantle derived archaean monozodiorites and trachyandesites the 2 680 2 750 myr old1 5 intrusive granite greenstone terranes6 8 in the rainy lake region ontario and in and adjacent to the vermilion district minnesota of the superior province in north america include a wide variety of volcanic and intrusive rocks with initial pb sr and nd isotope ratios close to those for the mantle4 5 9 12 the isotopic constraints require the production of large volumes of silica oversaturated magmas from silica undersaturated mantle within 100 200 myr we present here geochemical data on monzodiorites and trachyandesites from the rainy lake area which are strongly enriched in large ion lithophile elements lile we conclude that they are derived by direct melting of the mantle at depths of 50km in either anhydrous or hydrous conditions their rare earth element ree abundances and initial 143nd 144nd ratios suggest that their mantle sources were enriched in lile shortly before melting the monzodiorites trachyandesites and their granodioritic derivatives may comprise up to 20 of the exposed igneous rocks in the rainy lake region and vermilion district if these rocks are as abundant in other archaean terranes a significant part of the early continental crust could have formed by direct melting of lile enriched mantle

40ar 39ar age constraints on the thermal history of the archean abitibi greenstone belt and the pontiac subprovince implications for terrane collision differential uplift and overprinting of gold deposits 40ar 39ar mineral age spectra of granitic and metamorphic rocks in conjunction with existing conventional zircon geochronology indicate that at least two major late archean thermal events affected tectonic blocks of the abitibi southern volcanic zone svz and the juxtaposed pontiac subprovince the earlier thermal activity 2690 2670 ma was accompanied by the intrusion of voluminous syntectonic plutons and caused low pressure greenschist facies metamorphism in the svz and intermediate pressure metamorphism in the pontiac subprovince the second thermal event 2660 2630 ma was coeval with the emplacement of syncollisional s type garnet muscovite granites in the pontiac subprovince and the higher grade lacorne block of the abitibi svz and reset the k ar systems in preexisting rocks magmatic amphibole from the syntectonic round lake batholith 2695 ma u pb zircon age of the abitibi svz has a slightly disturbed ar release spectrum with an upper plateau age of 2669 6 ma signifying that the low gra

timing of magmatism and deformation in the kamiskotia kidd creek area western abitibi subprovince canada abstract the kamiskotia kidd creek area comprises well preserved granitoid greenstone belt lithologies of the western most abitibi subprovince similar in composition and structural style to other late archean granitoid greenstone terranes e g kalgoorlie norseman western australia belingwe zimbabwe slave province canada it contains the kidd creek volcanogenic massive sulfide deposit and lesser deposits of the kamiskotia district and lode gold deposits associated with the destor porcupine fault zone near the prolific porcupine gold district the magmatic and tectonic history for the area is provided through detailed structural mapping and high precision u pb geochronology field observations support the following sequence of magmatic and structural events in the kamiskotia area 1 formation of older mafic and intermediate volcanic rocks followed by the emplacement of the kamiskotia gabbroic complex kgc and cogenetic massive sulfide bearing bimodal volcanic rocks kv also accompanied by coeval intrusion of granitoid a into the crystallizing kgc 2 regional bulk rotation about a sub horizontal axis of this stratigraphy to form a ne facing monocline 3 emplacement of three large granitoid intrusions imparting their respective contact strain aureoles on the stratigraphy 4 regional sub horizontal bulk north south shortening that formed a non penetrative east west flattening fabric with an element of 5 late dextral transpression that formed high strain zones related to western destor porcupine fault zone dpfz within less competent volcanic units and 6 emplacement of alkalic dikes associated with the late stages of transpressive deformation u pb ages help substantiate this chronology and aid in the correlation of kamiskotia lithologies across the southern abitibi subprovince zircon ages for the kgc and a kamiskotia rhyolite are 2707 2 ma and 2705 2 ma respectively these ages are coeval with the reid rhyolite with a zircon age of 2706 2 ma but are significantly younger than the kidd creek rhyolite and the prosser rhyolite with zircon ages of 2717 2 ma data refined by the addition of an abraded concordant analysis and 2716 6 3 ma respectively two of the granitoid intrusions have zircon ages of 2696 1 5 and 2694 4 ma the older granitoid intrusion has a titanite population with a 207 pb 206 pb minimum age of 2692 5 ma which may represent a protracted cooling history or a metamorphic event after the setting of the zircon u pb system two zircon fractions from the younger granitoid intrusion exhibit inheritance with 207 pb 206 pb minimum ages up to 2926 ma indicating the presence of significantly older crust in this region timing of north south compression and late transpression may be constrained by a u pb garnet titanite age on a late to post tectonic alkalic dike at 2687 3 ma

u pb systemstics in single zircons from the pontiac sediments abitibi greenstone belt twenty six detrital zircon grains from the late archaean pontiac sediments of the abitibi greenstone belt were dated individually by the u pb method in order to identify possible elements of an ancient pre 2 7 ga shield the results show that at least two components were present at the source areas of these sediments 1 materials of about 2700 2725 ma which were most likely derived from contemporaneous acid igneous rocks and 2 significantly older materials parts of which are at least 2940 ma old which were either part of an uplifted sedimentary basin or a sialic basement complex these results can be used to suggest a close link between igneous activity and basin subsidence and that the development of the abitibi volcanic belt was closely associated with the presence of some older sialic blocks

evolution of the south central part of the archean abitibi belt quebec part i stratigraphy and paleogeographic model the region comprises parts of the southern zone of the abitibi greenstone belt and of the bellecombe gneiss belt and the belleterre angliers greenstone belt the stratigraphic sequence of the abitibi belt consists of 1 a lower ultramafic division 2 a middle tholeiitic division and 3 an upper diverse division these divisions lap onto an older volcanic sequence to the north and west in the bellecombe belt sediments overlie the lower ultramafic division these sediments coarsen toward the abitibi belt and toward the stratigraphic top the volcanic rocks of the belleterre angliers belt overlie the sediments of the bellecombe belt five paleogeographic phases have been recognized during phase i a deep marine ultramafic lava plain covered the south part of the abitibi belt and the bellecombe belt faulting took place at the duparquet destor break during phase ii tholeiitic basalts formed a deep marine lava plain in the west of the abitibi belt whereas flows erupted mainly from central volcanic c

generation of archean tonalites thermal evolution models of young subducting oceanic plates under archean conditions suggest that tholeiitic basalts in the subducting wet crust melted at shallow depths 50 to 80 km and produced archean tonalites the latent heat of fusion consumed by the crust kept the overlying peridotitic mantle wedge below the melting temperature of peridotite this prevented both fusion in the wedge and production of cale alkaline island are volcanic rocks similar to those observed in modern subduction zones

petrogenesis of mantle derived lile enriched archean monzodiorites and trachyandesites sanukitoids in southwestern superior province reply in southwestern superior province diorite monzodiorite and trachyandesite sanukitoids occurring within syn to post tectonic intrusive complexes and within greenstone belts have the following chemical characteristics 55 60 wt sio2 mgo 6 wt mg 0 60 ni and cr both 100 ppm na2o k2o 6 wt sr and ba both 600 1800 ppm and rare earth element ree patterns that are strongly light rare earth element lree enriched cen 80 250 ybn 4 10 and show no eu anomalies sanukitoids and their granodioritic derivatives constitute at least 5 of the exposed crust in the study area the sanukitoids cannot be derived by melting fractionation or crustal contamination of basalts or lamprophyres that are coeval with the sanukitoids crustal contamination of komatiites fails to explain the high large ion lithophile element lile contents of the sanukitoids rather we suggest that the sanukitoids were derived by hydrous melting of lile enriched mantle peridotite at pressures between 10 and

the geochemical characteristics of granitoids in contrasting arcs and comments on magma sources geochemical variations in modern subduction related igneous suites with respect to arc maturity in time and space are illustrated using data for both volcanic suites basalt andesite and dacite and intrusive granitoid suites diorite tonalite granodiorite and granite from circum pacific arcs using trace element data we suggest that two groups of processes control the natural variation in the parental magmas of both suites a subduction zone enrichment of lithospheric mantle locally coupled with crustal assimilation allied with fractional crystallization afc in zones of thickened crust all of which yield magmas with enhanced concentrations of the large ion lithophile lil elements k rb th u lrees etc b with increasing distance from the active trench contributions from within plate sub continental lithosphere producing mantle derived magmas with enhanced levels of high field strength hfs elements among which nb ta hf and y are particularly distinctive thus even for the evolved granitoids of intrusive arc series ratios of hfs lil elements not significantly affected by crystal fractionation such as ta nb k rb la may throw some light on the origin of mafic intermediate precursor magmas in terms of these elements we suggest the following groupings for interpreting the tectonic associations of granitoid suites 1 primitive calcic arc granitoids with low lil and hfs element abundances 2 normal calc alkaline continental arc granitoids with enhanced lil element abundances and low hfs lil ratios 3 mature alkali calcic arc granitoids with high levels of lil and hfs elements and higher hfs lil ratios 4 back arc anorogenic alkaline granitoids with the highest levels of hfs elements

quantitative trace element modelling of the crystallization history of the kinojevis and blake river groups abitibi greenstone belt ontario the archean tholeiitic kinojevis suite is characterized by an iron enrichment trend and abundant fe ti oxides in its evolved basalts andesites and rhyolites the rare earth element ree patterns of the suite remain flat from the basalts through to the rhyolites with the development of small negative eu anomalies quantitative modelling of the trace elements from little altered samples is consistent with the mineralogy suggesting that the suite was produced through fractional crystallization of olivine pyroxene plagioclase and fe ti oxides the evolved rhyolites are interpreted as having developed by greater than 90 fractional crystallization in a high level magma chamber the calc alkaline blake river group conformably overlies the kinojevis rocks and is characterized by enrichment in alkalis and silica the ree patterns are light rare earth element lree enriched and the felsic rocks have prominent negative eu anomalies geochemical modelling shows that the suite could have developed either th

an open system fractional crystallization model for very early continental crust formation abstract a model is discussed which seeks to explain the origin of archaean tonalitic trondhjemitic granodioritic ttg crust by an open system fractional crystallization mechanism following the assumption that in the pre and early archaean the melt which formed above sites of large scale mantle upwelling was too voluminous and too hot to solidify an almost global near surface partially molten zone could form the magma in this layer would converge above mantle sinks and undergo fractional crystallization recharged fractionating reservoirs would form at sites of convergence the major element composition of the magma in such reservoirs would evolve towards one resembling that of ttg rocks provided that amphibole was a significant fractionating phase from such reservoirs their incompatible as well as compatible trace element abundances would also become similar to those of ttg terrains it is proposed that such terrains resulted from the consolidation of these recharged fractionating reservoirs it is shown that the reservoirs would remain indistinguishable from the primitive mantle in terms of radiogenic isotopes so that model ages cannot be greater than the age of their consolidation consideration of the main aspects of greenstone belts and archaean high grade terrains indicates no contradictions with the model the intrusion of late archaean adamellites and granites is interpreted as an indirect effect of pressure induced breakdown of amphibole in the cumulate underlying the ttg crust which in turn resulted from crustal shortening cratonization is seen as the result of consolidation not only of the crust but also of the underlying cumulate as subcontinental lithosphere

trace element geochemistry of archean volcanic rocks and crystal growth in southwestern abitibi belt canada trace element geochemistry on 176 controlled samples in three adjoining abitibi 2 7 b y volcanic piles confirms the fundamental concept of archean volcanic cycles composed of lower tholeiitic and upper calc alkalic parts the volcanic piles each as much as 16 km thick are either unicyclic or multicyclic in the latter case the cycles display secular geochemical trends all the volcanic rocks studied are indicated to have come from mantle sources a fundamental feature of abitibi magma genesis is the simultaneous presence of both depleted and undepleted mantle sources abitibi tholeiitic basalts closely resemble modern mid ocean ridge basalts abitibi calc alkalic andesites and alkalic rocks are very similar to modern oceanic island arc andesites and to some modern volcanic arc high k rocks respectively the principal constraints to geodynamic processes are interpreted in terms of pulsating migrating mantle diapirism a type of hot spot tectonics mechanism involving a layered mantle responsible for the early tholeiitic depleted mantle source and later calc alkalic undepleted mantle source parts respectively of the volcanic cycles