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

petrology of the archean pontiac and kewagama sediments and implications for the stratigraphy of the southern abitibi belt discussion lajoie and ludden presented data on the petrography traceelement chemistry and bedding characteristics of the pontiac and kewagama sedimentary units from which they proposed a paleogeographic environment of sedimentation and discussed the relative position of these units in the stratigraphy most of the sedimentological and chemical data concord with our paleogeographical model dirnroth et al 1982 and as such they are a valuable addition to our paper thus textural parameters sedimentary structures and bedding characteristics suggest that pontiac sediments are turbidites derived from the subaerial erosion of a northern high relief and tectonically unstable source area and that they accumulated in a relatively deep water environment below wave base lajoie and ludden p 1310 trace element compositions of the two sedimentary units indicate sources that are partly volcanicplutonic in nature and fractionated in lightlheavy rare earth elements ree relative to the adjacent blake river group lajoie and ludden p 13 11 h e s e chemical data are consiitent with the ree contents of three mica schists of the pontiac group published by ujike 1984 the petrography of the kewagama sediments also suggests a derivation from a nearby subaerial source of high relief and mostly volcanic composition lajoie and ludden p 131 1 unfortunately lajoie and ludden s interpretation of the paleogeographic model is weakened by a misrepresentation of the pontiac metasediments as being metaquartzite with framework consisting of more than 90 quartz this statement is based on an evaluation of the detrital mode of the framework composition of pontiac metasediments as presented in the qm p l triangle of dickinson and suczek 1982 lajoie and ludden fig 3 however it is inappropriate to evaluate the framework composition of strongly deformed and metamorphosed greywackes in the q p l triangle as lajoie and ludden have done ingersoll and suczec 1984 p 985 warn

late archean quetico accretionary complex superior province canada the 1200 km long quetico subprovince consists of monotonous metagraywacke with derived migmatite and granite in thrust and or transcurrent fault contact with the adjacent wabigoon and wawa metavolcanic subprovinces imbrication of sedimentary wedges derived from volcanic arcs to the north and south produced a 10 100 km wide stratigraphically north facing pile thermal relaxation in the 20 m y following accretion of the southern arc resulted in melting of the accretionary pile to produce peraluminous granite and associated low pressure metamorphism

provenance of upper precambrian lower cambrian shales in the central iberian zone spain evidence from a chemical and isotopic study geochemical and isotopic sr nd o data are presented for upper precambrian and lower cambrian shale sequences from the central iberian zone spain in order to define geochemical discriminants between them and to assess the nature of the source regions supplying these basins elemental data especially th sc ratios coupled with endt values of 2 5 suggest that the upper precambrian rocks contain a greater proportion of a much younger more mafic component than the lower cambrian sequence which has high th sc and endt of 7 3 coherent trends between elemental and o isotope data imply a homogeneous recycled source as the dominant provenance for the upper precambrian shales whereas a lack of such coherence for the lower cambrian sediments suggests a more heterogeneous provenance the sedimentological geochemical and isotopic data are consistent with the upper precambrian succession being derived ultimately from a polycyclic mixture of young pre cadomian mafic sources and older 2 ga crust in contrast the lower cambrian sequence was mainly derived from old more felsic crust which would have contributed increasingly greater proportions of material as the younger sources were progressively removed by erosion

nd sr and o isotopic constraints on the petrogenesis of mafic intrusions in the proterozoic trans hudson orogen of central canada sm nd isotopic data from five mafic intrusions in the la ronge lynn lake domain indicate crystallization between 1 88 and 1 79 ga the data from a single intrusion in the glennie lake domain suggest a slightly younger age of about 1 71 ga these results indicate a cogenetic origin of the mafic intrusions with the associated volcanic and plutonic rocks initial end values vary from 4 5 to 0 6 and can be modeled by mixing proterozoic mantle derived magmas with 17 archaean crust or subduction of 3 sediment similar to pelites from the wollaston domain the initial end value of 0 for a late proterozoic mackenzie dike is compatible with an origin from mantle sources affected by hudsonian subduction processes rb sr data indicate variable and generally high initial 87sr 86sr isotopic ratios which reflect in part isotopic redistribution during metamorphism 18o values of coexisting clinopyroxene orthopyroxene and plagioclase indicate isotopic disequilibrium plagioclase samples show variable degrees o

a regional perspective of the quetico metasedimentary belt superior province canada alternating greenstone granite and metasedimentary gneiss belts are a first order tectonic feature of the southern superior province the tectonic development of the quetico metasedimentary belt is reviewed with regard to depositional structural and metamorphic plutonic history over its 1200 km length the belt consists of marginal metasedimentary schists of turbiditic origin and interior metasedimentary migmatite and peraluminous leucogranite polyphase deformation has resulted in a steep easterly striking foliation and regional gently east plunging stretching lineation metamorphic grade varies in a low p facies series from greenschist at the belt margins to upper amphibolite and local granulite in the central migmatite intrusive granite zone mineral assemblages in the central zone yield estimates of metamorphic pressure that increase systematically eastward over 800 km from about 250 mpa 2 5 kbar near the canada united states border to 600 mpa 6 kbar in granulites adjacent to the kapuskasi

rare earth element and mineralogic changes in holocene soil and stream sediment a case study in the wet mountains colorado u s a size fractions of soil and stream sediment developed on plutonic metamorphic or mixed plutonic metamorphic sources in the wet mountains colorado have been analyzed for ree rare earth elements and mineralogy the clay sized fractions 0 5 m and 0 5 2 m or 2 m have ree contents that are similar to more enriched than the source eu sm ratios that are similar to the source and la lu ratios that are higher than their corresponding source the 0 5 m fractions have lower ree contents than the corresponding 0 5 2 m fraction in every stream sediment for example unweathered san isabel batholith near the sampled stream sediment contains ree 511 712 ppm and eusmratio 0 17 0 19 and lalu cn 9 8 10 3 the corresponding 0 5 and 0 5 2 m fractions of sediment formed from san isabel batholith are enriched in the ree ree 1135 and 2225 ppm respec are slightly lower in eu sm ratios 0 16 and 0 14 respec and are higher in la lu ratios 12 2 and 16 3 respec than the unweathered rocks the silts have lree light ree contents and eu sm ratios very similar to the source due to similar mineralogy but silts have hree heavy ree contents higher than the source due to high zircon for example silt sized fractions in stream sediment draining the san isabel batholith contain ree 808 ppm eusmratio 0 18 and lalu cn 7 4 in contrast the sands contain lower ree contents similar to higher eu sm ratios and lower la lu ratios than the corresponding source due to depletion in heavy minerals concentrated in the ree and containing lower eu sm ratios relative to quartz and feldspar depleted in the ree and containing higher eu sm ratios for example the sand fraction from stream sediment from the san isabel batholith has ree 206 ppm eusmratio 0 17 and lalu cn 5 6 small variations of accessory minerals concentrated in the ree like allanite or sphene may cause drastic variation in ree content in the sand fraction clays and silts derived only from the plutonic rocks have higher ree and similar to lower eu sm ratios than corresponding fractions derived only from migmatites e g ree of silt fractions from migmatites 224 ppm and from plutons 375 808 ppm eu sm ratios from migmatites 0 17 and from plutons 0 11 0 18 the ree content and the eu sm ratio of clays and silts derived from migmatites are much more similar to the average of mcps mid continent platform sediment than corresponding fractions derived from the plutons mcps has ree 192 ppm eusm 0 19 lalu 8 0 the eusm ratio of sands derived from plutons and migmatite are lower to mostly higher than the mcps the ree content of sands from plutons are not necessarily higher than the mcps due to variation in heavy minerals like allanite or sphene concentrated in the ree sediments derived from mixed sources have ree contents in fine fractions that may not necessarily reflect the ree content of the nearest source for example 0 5 2 m and silt fractions developed on the san isabel batholith with a migmatite upstream have the high ree content of the batholith ree in 0 5 2 m fraction 1100 ppm silt 573 ppm while the corresponding 0 5 m fraction has the very low ree content of the migmatites ree 110 ppm these results suggest fine grained material near the source may sample a much larger area than coarser material near the source thus analysis of fine grained sediment associated with coarse grained material near the source may give a much different impression of the source than would determination of the source using classic petrographic examination of the sand or gravels

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

u pb ages and tectonic significance of late archean alkalic magmatism and nonmarine sedimentation timiskaming group southern abitibi belt ontario the paper presents u pb ages for zircons of the calc alkalic to alkalic igneous suite and associated alluvial fluvial sedimentary rocks of the timiskaming group in the late archean abitibi greensto

composition of modern deep sea sands from arc related basins summary petrographic and chemical examination of modern deep sea sands shows a clear distinction between active and passive plate tectonic settings among active settings sands from forearc basins of island arcs and basins from strike slip continental margins can be distinguished but those from other arc related settings overlap considerably in composition in particular sands from continental margin subduction zones and those from the backarc of island arcs appear to be indistinguishable chemically arc related sands are very close in composition to ancient greywacke sandstones for instance sodium almost always exceeds potassium thus it is not necessary for sodium to be added to sands diagenetically to make greywackes as has been suggested the influence of tectonics on the composition of sandstones has long been a central feature of sandstone petrology the geosynclinal model of global tectonics stimulated much thought on this subject probably the best known ideas are those of krynine e g 1942 subsequently such research declined but the plate model for global tectonics has produced a new wave of interest e g crook 1974 schwab 1975 potter 1978 dickinson suczek 1979 much of this work has relied on data from ancient sediments where diagenetic changes and difficulty in de ciphering the tectonic setting limit the useful ness of the conclusions much more work is needed on recent sediments to establish a base line against which ancient sediments can be compared we have previously reported valloni maynard 1981 the results of our work on the petrography of a set of recent deep sea sands from piston cores in this paper we shall com pare this petrographic information with bulk chemistry and attempt to relate these to tecto nics in arc related settings

spatial patterns of glaciers in response to spatial patterns in regional climate abstract glaciers are direct recorders of climate history and have come to be regarded as emblematic of climate change they respond to variations in both accumulation and ablation which can have separate atmospheric controls leading to some ambiguity in interpreting the causes of glacier changes both climate change and climate variability have characteristic spatial patterns and time scales the focus of this study is the regional scale response of glaciers to natural patterns of climate variability using the pacific northwest of north america as the setting the authors employ a simple linear glacier model to study how the combination of patterns of melt season temperature and patterns of annual accumulation produce patterns of glacier length variations regional scale spatial correlations in glacier length variations reflect three factors the spatial correlations in precipitation and melt season temperature the geometry of a glacier and how it determines the relative importance of temperature and

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

sedimentary rocks and crustal evolution tectonic setting and secular trends a significant change in the composition of turbidites deposited at active tectonic settings is seen at the archean post archean transition in comparison to post archean active margin turbidites archean greenstone turbidites exhibit more uniform th sc ratios but with a similar average greater hree depletion gd n yb n 2 less eu depletion eu eu mostly 0 85 and an absence of low th u ratios th u 3 these data indicate that differing mantle sources and or conditions of mantle melting existed during the archean and that intracrustally differentiated rocks older cratons or young differentiated arcs were relatively unimportant as provenance components for archean turbidites in contrast some sedimentary rocks preserved in archean platformal sequences of high grade terranes display significant negative eu anomalies similar to the majority of post archean shales indicating that the general process of cratonization including intracrustal differentiation has taken place since at least 3

geochemistry and tectonic setting of lewisian clastic metasediments from the early proterozoic loch maree group of gairloch nw scotland abstract the early proterozoic 2 0 ga loch maree group lmg of the lewisian complex consists of a supracrustal sequence of metasedimentary schists and associated tholeiitic metavolcanic amphibolites within the archaean basement the metasediments comprise three main mapped units the flowerdale schists kerrysdale schists and charlestown schists together with numerous narrower bands the dominant clastic metasediments within the schist units semipelites and psammites are chemically distinguished as a series of greywackes and lithic sandstones with variable lil and transition element abundances fractionated ree patterns and overall compositions similar to paas three chemical groups are recognized whose range of composition largely reflects the mixing between two main source components lewisian gneissose basement with an intermediate acid continental crust composition and local rift related amphibolites with a primitive tholeiitic composition which also incorporates components not preserved within the lmg the geochemistry of the clastic metasediments confirms a previous tectonic model for the lmg as an intracontinental extensional rift receiving clastic detritus from the adjacent uplifted basement as well as from internal volcanogenic sources

internal structure of the cadillac tectonic zone southeast of val d or abitibi greenstone belt quebec the kirkland lake larder lake cadillac break is a major fault zone of the abitibi greenstone belt well known for its spatially associated gold camps a detailed structural analysis of this fault zone in the val d or area has shown the presence of a 200 750 m wide zone of high strain defined here as the cadillac tectonic zone ctz which includes the narrower schist zone generally regarded as the cadillac break the boundaries of the ctz which coincide with major lithologic contacts strike approximately east west and dip 80 to the north the internal structure of the ctz is best analysed in terms of two increments of the same progressive deformation the d1 strain increment is characterized by an east west striking subvertical s1 foliation at low angle in strike to the bound aries of the ctz and containing subvertical elongation lineation primary lithological contacts have been transposed into s1 and folded by f1a folds f1b intrafolial folds of s1 ranging from noncylindrical subhorizontal f

nd isotopic evidence for early to late archean 3 4 2 7 ga crustal growth in the western superior province ontario canada abstract new sm nd analyses of archean metavolcanic plutonic gneissic and metasedimentary rocks from sachigo and berens river subprovinces north caribou terrane and winnipeg river and wabigoon subprovinces in conjunction with previously published data are used to trace the crustal evolution of the western superior province wsp from ca 3 4 to 2 7 ga the extensive wsp geochronological database which contains 3 17 2 55 ga plutonic rocks allows the calculation of precise e t nd values and the identification of the main periods of new crustal addition at 3 4 0 1 3 02 2 92 and 2 76 2 69 ga mass balance calculations indicate that the western superior province represents a collage of archean crusts comprising 7 44 and 48 wt of crust created at 3 4 3 0 and 2 7 ga respectively although these calculations depend on the isotopic signatures of the mantle and those of the average crust at different times several important observations can be made 1 the importance of pre 2 7 ga terranes recycled in the 2 7 ga orogen 2 the existence of time intervals of 150 300 ma dominated by intra crustal processes resulting in a large range of crystallization ages u pb zircon ages that do not reflect new additions from the mantle 3 much of the new crustal additions at 2 7 ga were contaminated by crustal material subducted into the source region ttg suites or were contaminated during intrusion into the crust sanukitoids

chemical and mineralogical comparison of the pennsylvanian fountain formation colorado u s a an uplifted continental block to sedimentary rocks from other tectonic environments the mineralogic and chemical composition of composites of cobble and pebble sized rock clasts clay stones siltstones fine sandstones and size fractions of two sandstones of the fountain formation near beulah colorado were determined the sandstones are arkosic and most plot in the basement uplift or transitional continental fields of qfl diagrams the mineralogic composition of the sandstones is similar to that of the composite of rock clasts at a given location al mg fe ti mn sc th ree and cs are concentrated in the clay minerals or heavy minerals and are diluted by quartz or feldspar consequently they are inversely correlated with si cr hf k rb ba and sr are poorly correlated with si cr is concentrated in clay and in heavy minerals in the silts k rb ba and sr are correlated with k feldspar mica or clay mineral abundances sedimentary rocks from the fountain formed from an uplifted continental block the element concentrations plot in the fields of active or passive continental margins in many tectonic discrimination diagrams probably as a result of derivation from granitoid sources of similar composition the ree patterns eu anomaly size k and rb concentrations of the fine sandstones are mostly similar to those of the corresponding composite of rock clasts at a given location suggesting that these parameters might be the most reliable in distinguishing sources the most reliable discrimination diagram for uplifted tectonic blocks like the fountain may be a plot of k2o vs eu anomaly size since both of these parameters were very similar in concentration in both the sedimentary rocks and presumed source of the fountain most sedimentary rocks in this study have larger negative eu anomalies than those from the nasc north american shale composite or mcps midcontinent platform sediment this suggests a more evolved source for the fountain than that for average platform sediment

subprovince accretion tectonics in the south central superior province development of tectonic subprovinces as shear bounded granite greenstone and sediment dominated terranes during the late archaean is reviewed and interpreted from relationships between portions of

archean wrench fault tectonics and structural evolution of the blake river group abitibi belt quebec structural analysis of the blake river group volcanic rocks in the rouyn noranda region shows the formations to be distributed largely in z shapes resulting from the interference of two early fold systems oriented west northwest east southeast and east west respectively the first of these two fold systems is probably related to the shortening associated with left lateral movement along the two major fractures in the region namely the porcupine destor and larder lake cadillac faults the second system appears to be the result of north south compression perpendicular to the two major fractures the two major faults the first system of early folding the normal and reverse faults and the minor dextral and sinistral strike slip faults that have been observed in the blake river group rocks can all be integrated into one tectonic system that of wrench fault tectonics the orientations of the principal structures recognized in the abitibi belt major shear zones folding and faulting suggest that the d

deep structure of an archaean greenstone terrane more than half of the continental crust was formed before about 2 500 million years ago1 yet there is still considerable uncertainty regarding the structure and evolution of the ancient archaean cratons2 7 modern analogues proposed for the setting of archaean greenstone belts include aborted intra cratonic rifts continental margins and various oceanic settings2 7 alternatively archaean tectonics may have been quite different from the plate tectonics we see today2 4 6 the abitibi belt in canada is the largest archaean greenstone ternane3 and has been a touchstone for many models because its surface geology is relatively well known8 10 it contains significant gold and base metal deposits10 and it is less deformed and has suffered less erosion than most archaean ter ranes here we present deep seismic reflection data from part of the abitibi belt which reveal extensive subhorizontal and gently dipping reflection zones truncated by steeply inclined discontinuities which project to major fault zones at the surface our preliminary model of the abitibi belt comprises a mostly juvenile magmatic crust similar to that exposed in the nearby kapuskasing uplift11 12 dissected and imbricated by deep penetrating discontinuities associated with thrust and strike slipe faulting in view of the similarity of greenstone belts worldwide2 7 13 our seismic sections provide new constraints on the general processes that have affected archaean metavolcanic terranes

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

compositions and sources of metasediments in the upper dharwar supergroup south india texturally mature quartz rich sandstones texturally mature mafic rich sandstones and texturally immature graywackes are abundant in the late archean chitradurga group of the dharwar craton in south india when compared to graywackes of paleozoic age from northeastern north america the chitradurga graywackes have slightly higher average concentrations of na and ferromagnesian elements indicating the existence of a more tonalitic and mafic rich provenance during the late archean in karnataka the regional tonalitic gneiss peninsular gneiss is the predominant source of these sediments although mafic ultramafic volcanic debris from unidentified sources is also important neither intermediate nor felsic volcanics nor an extensive metasedimentary terrane were prominent in the provenance although the composition of the craton was dominantly sialic the presence of abundant ferromagnesian debris in some of the texturally mature sandstones that probably formed in shallow water intracratonic or platform

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

the influence of source rock type chemical weathering and sorting on the geochemistry of clastic sediments from the quetico metasedimentary belt superior province canada abstract the geochemistry of clastic sediments is controlled by at least four factors source rock composition weathering of the source rocks sorting of the weathered detritus and post depositional alteration during diagenesis and metamorphism in order to evaluate the relative importance of these factors major trace and rare earth element contents were determined for distal and proximal clastic turbidites from the archaean quetico metasedimentary belt the degree of metamorphism of the samples ranges from middle greenschist to upper amphibolite facies simple mixing of archaean mafic and acidic rocks of similar composition to those found in the adjacent granite greenstone terrains will account for the tio2 feot mgo cr sc y and hree contents of the clastic turbidites the metasediments are enriched in lree relative to the mafic acidic rock mixing line indicating that the acidic source was enriched in the lree compared to archaean terrains from other parts of the world some quetico turbidites have an additional fe rich source component possibly a chemical sediment incorporated at the time of turbidite slumping cao na2o k2o rb and sr form trends on bivariant plots that are oblique to the mafic acidic rock mixing line and hence cannot be accounted for by simple mixing of the source rocks chemical weathering of feldspars prior to deposition can account for cao na2o k2o rb and sr trends since cao na2o and sr are lost to the weathering solutions and k2o and rb are retained on the product clays the higher na 2 o cao k 2 o na 2 o rb k 2 o and sr cao ratios but lower cao content of the distal samples suggest that they were derived from a bulk sediment that had experienced a greater degree of chemical weathering than the proximal sediments sorting is the factor that most strongly influences the composition of individual samples within the proximal sample group the coarse grained metagraywackes with their higher quartz feldspar and heavy mineral contents are enriched in sio2 cao na2o sr lree zr and hf relative to the fine grained clay rich metapelites metapelites are enriched in tio2 al2o3 feot mgo k2o cr zn sc y hree cs and rb as a result of their higher clay contents thus the use of clastic sediment compositions in determining average crustal compositions e g for the ree can only give satisfactory results when all grain size fractions are sampled in their correct relative proportions the chemical differences observed between the high and low grade samples can be accounted for by source rock type chemical weathering of the source rocks or sorting post depositional element mobility does not appear to be an important factor in determining the composition of the quetico metasediments

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

deep crustal oxygen isotope variations the wawa kapuskasing crustal transect ontario we have studied the oxygen isotopic composition of rocks from a 100 km transect through the central superior province of ontario representing progressively the shallower terrains of the kapuskasing structural zone ksz the wawa gneiss terrane wgt and the michipicoten greenstone belt mgb these rocks range in age from 2 76 to 2 60 ga and correspond to a section through approximately 20 km of crustal thickness equivalent lithologic types have similar range of 18o values at each crustal level tonalitic to granodioritic rocks 6 4 to 9 5 dioritic and anorthositic rocks 5 5 to 7 6 mafic gneisses group 1 majority 5 7 to 7 1 group 2 8 1 to 9 5 18o values exhibit a remarkable correlation with sio2 values similar to that observed in unaltered plutonic rocks of equivalent composition paragneisses have significantly higher 18o values 9 3 to 12 2 low grade metavolcanic and metasedimentary rocks of the mgb are 18o enriched compared to their high grade equivalents in the ksz and wgt 7 4 to 13 3 for mafic to felsic metavolcanic rocks 11 4 to 14 7 for clastic metasediments coexisting minerals exhibit 18o fractionation consistent with equilibrium but corresponding to uniform isotopic temperatures about 553 to 584 c across the entire transect lower than the inferred metamorphic temperatures in the highest grade ksz terrane the lack of distinctive isotopic differences between equivalent rock types in the ksz wgt and mgb suggests that there is no significant gradient in 18o with depth in the crust the majority of mafic gneisses group 1 appear to have been emplaced either as subaerial extrusives intrusive sills or less likely as submarine extrusives that were hydrothermally altered at high temperatures the less abundant group 2 mafic rocks have the 18o values typical of greenstones that were altered at low temperature by seawater and isotopically resemble low grade rocks in the michipicoten and abitibi belts in general no major changes in whole rock isotopic composition appear to have occurred during granulite facies metamorphism implying limited flux of water or co2 the continuous linear gradient in 18o versus sio2 in the high grade rocks cannot be due to differentiation of a mafic source magma a model involving an association between mantle derived mafic magma and 18o enriched crustal materials is more consistent with the oxygen isotopic and the ree data

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

the pontiac problem quebec ontario in the light of gravity data a belt of archean quartzose metasedimentary gneisses with minor mafic volcanic rocks the pontiac group lies south of the blake river and older archean mafic volcanic rocks of the abitibi greenstone belt and is separated from them by the larder lake cadillac break to the west of the pontiac group on strike is the archean larder lake group of turbidite conglomerate argillite limestone and iron formation with abundant mafic flows and intrusions these strata also lie south of the larder lake cadillac break and south of the blake river and older archean mafic volcanic rocks the western contact between the pontiac and larder lake groups is covered by a narrow north south strip of proterozoic cobalt sedimentary rocks on the basis of gravity work that compares the bouguer gravity anomaly gradient across the cadillac break with that across the west margin of the pontiac group it is proposed that the larder lake and pontiac groups are separated by a north south fault and that the pontiac group repr

another look at rare earth elements in shales linear correlation and mass balance considerations indicate that clays are more important than zircon or other heavy minerals in hosting both light and heavy ree in cratonic shales considering the uniformity of the eu anomaly and ree distributions in cratonic shales it seems likely that detritus is thoroughly mixed during weathering and sediment transport and that the ree patterns in shales reflect the average ree pattern of the sources regardless of the ratio of clays to heavy minerals in the shales shales suspended river loads and near shore marine sediments all have broadly similar ree patterns and reported hree depletions in the latter two groups may reflect incomplete dissolution of heavy minerals during sample preparation ree distributions in shales and suspended river loads determined by inna are similar to those in average upper continent crust and ree distributions in shale averages nasc and paas seem to be reasonably representative of the average composition of post archean upper continental crust

u pb geochronology of archean metasedimentary rocks in the pontiac and abitibi subprovinces quebec constraints on timing provenance and regional tectonics abstract u pb isotopic ages of single detrital zircons have been obtained from metasandstones in the northern part of the pontiac subprovince and from the cadillac kewagama and lac caste groups within the southern abitibi subprovince youngest detrital zircon ages from these sequences are 2685 2686 2687 and 2693 ma respectively the lac fourniere pluton which intrudes pontiac group metasedimentary rocks is dated at 2682 1 ma a similar age was previously measured from a pluton that cuts the kewagama group these data bracket deposition of greywacke in the pontiac and abitibi subprovinces to 2685 3 ma in contrast the youngest zircon found in a volcaniclastic unit interbedded with timiskaming group metawacke is 2673 2 ma while the age of a pluton that intrudes the timiskaming is 2672 1 ma thus the timiskaming group is distinctly younger than the other metasedimentary rocks the normalized probability distributions of detrital zircon ages from the pontiac and the abitibi metasedimentary rocks as well as of dated igneous rocks from the southern abitibi subprovince all show that their major zircon populations formed in the age range 2685 2710 ma the distributions for the abitibi metaturbidites and dated igneous rocks are very similar suggesting that metasedimentary rocks in the abitibi belt were mostly derived from the surrounding granite greenstone terrain the largest peak of the pontiac distribution is a few million years younger than that from the abitibi metasedimentary rocks but there is a correlation between major peaks and both groups show similar age ranges for older pre abitibi components 2750 2850 and 3020 3030 ma deposition of the ca 2685 ma sediments was virtually syn deformational based on previous field geochemical and isotopic studies as well as the detrital zircon age data the pontiac group may represent a foreland basin that may have developed from an accretionary prism the source of much of the sediment may have been an uplifted extinct continental arc to the north the arc was mostly eroded into the sediment basin and perhaps tectonically overridden by a southward advancing nappe complex comprised of its back arc basin now partly preserved in the southern volcanic zone of the abitibi subprovince metaturbidites exposed between metavolcanic assemblages within the greenstone belt may represent overturned unconformities at the structural base of the greenstones these were brought up by high angle faults which also localized deposition of non marine timiskaming type sediments it is tentatively suggested that pontiac type metasedimentary rocks extend most of the way beneath the abitibi subprovince the oldest part of the basin emerging as the quetico group in the western superior province

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

geochemical oxygen and neodymium isotope compositions of metasediments from the abitibi greenstone belt and pontiac subprovince canada evidence for ancient crust and archean terrane juxtaposition abstract the abitibi greenstone belt agb and pontiac subprovince ps in the southwestern superior province are adjacent greenstone plutonic and metasedimentary dominated terranes respectively separated by a major fault zone metasediments from these two contrasting terranes are compared in terms of major and trace element and o and nd isotope compositions and detrital zircon ages the following two compositional populations of metasediments are present in the low grade abitibi southern volcanic zone 1 1 a mafic element enriched population mep characterized by flat depleted ree patterns enhanced mg cr co ni and sc low incompatible element contents and minor or absent normalized negative troughs at nb ta and ti and 2 2 a low mafic element population lmep featuring lree enriched patterns enhanced rb cs ba th and u contents and pronounced normalized negative troughs at nb ta and ti these geochemical features are interpreted to indicate that the mep sediments were derived from an ultramafic and mafic dominated oceanic provenance whereas the lmep sediments represent mixtures of mafic and felsic arc source rocks the ps metasediments are essentially indistinguishable from abitibi lmep on the basis of majorelement and transition metal abundances suggesting comparable types of source rocks and degrees of maturity but are distinct in terms of some trace elements and o isotope compositions the pontiac metasediments are depleted in 18o and enriched in cs ba pb th u nb ta hf zr and total ree and also have higher ratios of rb k cs rb ba rb ta nb th la and ba la relative to the abitibi lmep two subtypes of ree patterns have been identified in ps metasediments both have moderately fractionated distributions with la yb n 3 9 17 and 7 8 13 6 respectively but the first has minor eu anomalies eu eu 0 89 1 16 whereas the second features pronounced negative anomalies eu eu 0 52 0 81 coupled with high concentrations of incompatible elements such as k rb th u nb and ta the first subtype is interpreted to be derived from provenances of mixed mafic and felsic volcanic rocks whereas the eu depleted type has geochemical features that are typical of post archean sediments or archean k rich granites and volcanic equivalents abitibi metasediments have nd 2 7 ga values of 0 71 to 1 83 corresponding to model tchur 2 82 2 55 ga with the majority around 2 65 ga close to the depositional age of 2 7 ga in contrast ps metasediments are characterized by nd 2 7 ga 0 95 to 3 51 corresponding to t dhur 2 7 to 3 1 ga consistent with a detrital zircon age spectrum of 2 69 ga to 3 1 ga pontiac subprovince and abitibi metasediments have contrasting provenances suggesting their separate initial geodynamic settings and later tectonic juxtaposition major and trace elements o and nd isotopic compositions and detrital zircon ages collectively suggest that some of the ps metasediments were derived mainly from juvenile rocks whereas others require various degrees of contribution from older 3 0 3 5 ga evolved crust mixed with juvenile 2 7 ga rocks older components were derived either directly from erosion of an ancient continental arc or from contemporaneous volcanic rocks which may contain recycled older components if the ps represents in part an accretionary prism to the abitibi southern volcanic zone then oblique subduction may have been involved which allowed the pontiac subprovince to juxtapose or sample exotic source rocks

stratigraphic and isotopic link between the northern stikine terrane and an ancient continental margin assemblage canadian cordillera geologic and isotopic data strongly imply a late triassic depositional link between a juvenile volcanic arc northern stikine terrane and an outboard ancient continental margin assemblage nisling terrane in the canadian cordillera two sandstone samples and a schist clast from a conglomerate layer at the base of the upper triassic stuhini group northern stikine terrane have nd depleted mantle model ages of 1400 1430 and 1600 ma respectively other stuhini group rocks have model ages of 390 660 and 690 ma three samples of nisling terrane schist and gneiss yield nd model ages of 910 1770 and 2450 ma and highly radiogenic 87 sr 86 sr ratios these isotopic data corroborate stratigraphic evidence that detritus at the base of northern stikine was shed from the nisling terrane and strengthen interpretations that these terranes became linked by late triassic time thus upper triassic strata of the northern stikine terrane may have accumulated on top of or adjacent to an exotic continental fragment a rifted fragment of the north american margin or the in situ north american margin

geochemistry of archean shales from the witwatersrand supergroup south africa source area weathering and provenance with a few exceptions shales from the archean witwatersrand supergroup 2800 ma in south africa are depleted in na ca lile ree and hfse compared to phanerozoic shales cr co and ni are enriched in all witwatersrand shales and fe and mg are high in shales from the west rand groups wrg and lower central rand group crg shales from the crg and uppermost wrg are enriched in na al lile ree hfse and transition metals relative to shales from the lower wrg chondrite normalized ree patterns for all witwatersrand shales are enriched in light ree and exhibit small to moderate negative eu anomalies a positive correlation of ree and al2o3 contents in the shales suggests that ree are contained principally in clay minerals relative to shales from the crg shales from the wrg exhibit depletions of na ca and sr a feature probably reflecting intense chemical weathering of their source rocks cia indices in witwatersrand shales are variable chiefly 70 98 even within the same shale unit such variations reflect chiefly variable climatic zones or rates of tectonic uplift in source areas with perhaps some contribution from provenance and element remobilization during metamorphism compared to present day upper continental crust all but the orange grove roodepoort and k8 shales appear to have been derived from continental sources depleted in lile ree and hfse and enriched in transition metals computer mixing models based on six relatively immobile elements th hf yb la sc co and four source rocks indicate that the relative proportions of granite basalt and komatiite increased with time in sediment source areas at the expense of tonalite the contributions of basalt and komatiite appear to reach a maximum during deposition of the booysens shale and granite during deposition of the k8 shales and possibly during deposition of the orange grove shales

evolution of the south central segment of the archean abitibi belt quebec part ii tectonic evolution and geomechanical model in this paper we describe the relations between the paleogeographic and tectonic evolution of the southwestern part of the archean abitibi and bellecombe belts volcanism in the abitibi belt created a very thick anisotropic plate composed of competent volcanic rocks and broken by the duparquet destor break the depocenters of the upper division of diverse volcanic rocks subsided about 10 km relative to their surroundings and some central volcanic complexes within this division were consolidated by synvolcanic plutons and their thermal metamorphic aureole the cadillac break a normal fault separated the abitibi and bellecombe belts the latter consisted of comparatively incompetent sedimentary rocks on top of a basement composed of ultramafic mafic flows north south compression of the volcanic terrain during the kenoran orogeny produced a set of flexure folds f1 that curve around the consolidated cores of central volcanic complexes generally in an easterly direction synclinoria nucleated at the dee

sandstone petrographic evidence and the chugach prince william terrane boundary in southern alaska the contact between the upper cretaceous valdez group and the paleocene and eocene orca group has been inferred to be the boundary between the chugach and the prince william tectonostratigraphic terranes sandstone petrographic data from the prince william sound area show no compositional discontinuity across this contact these data are best explained by considering the valdez and orca groups to be part of a single terrane a thick flysch sequence derived primarily from a progressively unroofing magmatic arc with increasing input from subduction complex sources through time

the archean proterozoic transition evidence from the geochemistry of metasedimentary rocks of guyana and montana abstract chemical and isotopic compositions of ancient sedimentary rocks have in some cases provided evidence of first order changes yin the composition and extent of continental crust from the archean to the proterozoic we reversed the typical sampling pattern comparing metapelites and carbonates from proterozoic greenstone belts with those from an archean continental margin sequence and found the reverse of the typical age composition relations metapelites and volcaniclastic metagreywackes from the early proterozoic greenstone belts of northern guyana resemble those of many archean greenstone belts in ree trace and major element compositions mean ratios la sm 5 0 la th 5 1 eu eu 0 80 th u 3 4 median k 2 o na 2 o 0 8 schists from an archean continental marginal sequence in montana have compositions like many post archean metapelites derived from continental sources mean ratios la sm 6 8 la th 2 7 eu eu 0 67 th u 5 2 median k 2 o na 2 o 1 7 other archean continental basin sediments also resemble their post archean counterparts implying that upper continental crusts of these eras may not have differed significantly in these respects however temporal change may have occurred in upper crustal contents of ni and cr many archean clastic metasedimentary rocks of both continental and greenstone belt provenance have contents of these compatible elements higher than in their post archean equivalents dolomitic limestone from a greenstone belt of northern guyana has a very low 87 sr 86 sr ratio of 0 7007 such low values may reflect the local influence of mantle derived volcanic rocks during carbonate deposition diagenesis or metamorphism and they may not be reliable indicators of the evolution of sr in seawater archean dolomitic marbles from the continental marginal sequence of montana have 87 sr 86 sr ratios of 0 7065 and higher significantly more radiogenic than many accepted values for carbonates of archean greenstone belts

constraints on the geodynamic evolution of the southern superior province u pb la icp ms analysis of detrital zircon in successor basins of the archean abitibi and pontiac subprovinces of ontario and quebec canada abstract to investigate the tectonic development of the southern superior province new u pb laser ablation inductively coupled mass spectrometry analysis of detrital zircon was conducted on graywacke samples from successor basins of the abitibi and pontiac subprovinces the three successor basin successions investigated are the 2690 2685 ma porcupine assemblage and the 2679 2669 ma timiskaming assemblage of the abitibi subprovince and the 2682 ma sedimentary rocks of the pontiac subprovince predominately neoarchean zircon grains 80 95 of the total zircon population reflect local sources in the abitibi and pontiac subprovinces the successor basins were deposited within 100 ma of the majority of the detrital zircon grains primary crystallization ages which is consistent with patterns observed at modern convergent margin settings porcupine assemblage samples contain 5 mesoarchean zircon while pontiac subprovince and timiskaming assemblage samples contain 18 and 13 respectively no local mesoarchean sources exist in the abitibi or pontiac subprovinces suggesting input from an adjacent hinterland the higher proportion of mesoarchean zircon in the timiskaming assemblage relative to the porcupine assemblage suggests that detritus from the hinterland was more prevalent during the later stages of collision probably as a result of progressive uplift and denudation of the hinterland the high proportion of mesoarchean zircon in the pontiac subprovince sedimentary rocks may reflect an additional mesoarchean source comparison to previously published geochronology data for the superior province indicates that the hinterland was to the northwest and is in part composed of the winnipeg river marmion and opatica subprovinces patterns of contractional deformation successor basin formation and felsic plutonism progressively young to the south in the southern superior province and are interpreted in terms of accretionary processes detrital transport across subprovince or terrane boundaries progressive hinterland emergence and foreland directed propagation of the deformational front recorded by successor basins is similar to that observed in modern style collisional or accretionary orogens

the evolution of the southern abitibi greenstone belt in light of precise u pb geochronology this paper summarizes u pb zircon ages for the archean southern abitibi greenstone belt and integrates them together with zircon and baddeleyite data for three new samples into a general framework for the evolution of the region new ages are reported for the ghost range gabbroic complex 2713 super 7 sub 5 ma and for a pyroclastic volcanic rock of the gauthier group 2700 or 3 ma a previously determined age of 2701 super 3 sub 2 ma for the felsic volcanic skead group is confirmed by magmatic zircons from a second sample that also contains a xenocrystic component or 2720 ma indicating that the skead volcano was built on top of an older volcanic sequence the compiled ages show that the southern abitibi greenstone belt developed between 2750 and 2670 ma the earliest ages date remnants of volcanic complexes that occur at the periphery of the belt major preorogenic magmatism was confined mainly to the period 2720 to 2700 ma in general each episode was characterized by the essentially coeval formation of komatiitic tholeiitic and calc alkalic igneous rocks taking into consideration the known geologic geochemical and isotopic data this association is consistent with evolution in paired active are and back are systems older crust was absent from the region but it may have interacted with these arcs and rift systems farther to the west the extensional regime was followed by a period of compression that caused folding and thrusting emplacement of calc alkalic plutons and deposition of turbidites between 2700 and 2688 ma the magmatism became predominantly alkalic during the main timiskaming period between 2681 and 2676 ma this event was accompanied by the deposition of alluvial fluvial sequences and was followed by renewed compression that caused polyphase folding and thrusting probably between approximately 2676 and 2670 ma lamprophyre dikes were associated with these final stages of deformation postorogenic activity includes faulting and episodes of hydrothermal activity locally associated with gold mineralization ages in the range 2630 to 2580 ma for hydrothermal minerals e g titanite and rutile camflo suggest a correlation of these events with lower crustal magmatism and metamorphism as recorded for example in the adjacent kapuskasing zone

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

18o16o and 2h1h changes during progressive hydration of a zechstein anhydrite formation abstract the stable oxygen and hydrogen isotopic composition of crystallization water in gypsum has been measured in the zechstein billingham main anhydrite in north east england the samples represent a vertical drillcore section through the formation at about 40 metres depth and have suffered alteration by groundwater from above and below this has resulted in a central zone of relict anhydrite surrounded by hydrated zones in which alteration developed symmetrically the object of the study was to obtain evidence about the nature of the gypsification process the groundwater flux involved the geochemistry and the possible timing minor element contents na k mg sr cl of both anhydrite and bulk gypsum were similar anomalously high concentrations of cl mg na and k in the hydration transition zone correlate with the presence of trace halide and sulphate minerals and suggest localised leaching of these elements during the hydration process however sr is markedly uniform indicating immediate reprecipitation as srso4 during the hydration process the 18 o 16 o and 2 h 1 h data for crystallization water form a very marked trend with depth with a characteristic slope for 2 h versus 18 o of about 5 sofer 1978 which corresponds to the calculated rayleigh type isotopic fractionation experienced by water when reacting with anhydrite to form gypsum in a closed system taken together the stable isotopic geochemical and petrographie data indicate that the hydration reaction involved a very restricted flux of water just sufficient for the hydration the penetration of water probably occurred through micro fractures accompanying late tertiary to quarternary uplift and denudation the process of gypsification self seals the micro fracture network and thereby controls influx in an environment of limited groundwater circulation

geochronology of the lumby lake greenstone belt a 3 ga complex within the wabigoon subprovince northwest ontario the lumby lake greenstone belt lies within the central predominantly metaplutonic terrane of the western wabigoon subprovince it is conformable with gneiss of the marmion lake tonalite batholith u pb analyses of zircons from two felsic tuffs and a quartz porphyry within the greenstone belt give a scattered lead loss pattern indicative of early disturbance the ages of the oldest concordant fractions agree however and define an age of 2999 1 m y for emplacement of these felsic units zircons from a tonalite gneiss within the marmion lake batholith are also somewhat disturbed the oldest abraded fraction is concordant and has an age of 3003 5 m y indicating that the gneiss is probably close in age to the supracrustal rocks u pb analyses of rutile from one of the felsic units give an age of 2690 5 m y and record the effect of late archean metamorphism three titanite fractions from the tonalite gneiss agree within error and have an age of 2809 1 m y the age and consistency of the titanite fractions indicate that titanite in this rock was formed as a result of a metamorphic event and was never subsequently disturbed this age is therefore a minimum estimate for high grade metamorphism of the gneiss the presence of a pre 2809 ma fabric in a gneiss of regional extent suggests that an early metamorphic deformational event affected this area these data constitute the first firm evidence for extensive 3 0 ga crust in the southern superior province and give the first evidence of a regional deformational event which predates cratonization of the superior province

nd model age mapping of the southeast margin of the archean foreland in the grenville province of ontario nd model ages are used to map the southeast margin of superior southern province basement within the grenville province of ontario this boundary located 60 km southeast of the grenville front is tentatively identified as a collisional suture between an archean foreland to the northwest and a proterozoic island are of penokean age to the southeast

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

comparison of isotopic and petrographic provenance indicators in sediments from tertiary continental basins of new mexico abstract isotopic variations of nd in sedimentary rocks provide a new tool for provenance determination we have compared the isotopic technique with classic petrographic indicators of provenance in sediments from two tertiary basins of the rio grande rift north central new mexico the siliciclastic sediments in these basins were derived from varying proportions of precambrian basement and oligocene volcanic rocks which are isotopically and petrographically distinct in the analyzed sediments a good correspondence exists between petrographic and isotopic provenance indicators the data also suggest that the isotopic tracer is more sensitive to minor detrital input from a second source than is the framework petrology the similar sm nd ratios for all analyzed sediments and the identical isot pic values of interlayered mudstone and sandstone indicate that no significant fractionation of sm and nd occurs during the formation of the sediments thus the nd model ages of the sediments preserve the crust formation age of the source the sr isotopic and concentration data show trends generally similar to the nd data but they do not indicate the sedimentary provenance as clearly this ambiguity is a consequence of strong rb sr source heterogeneities and rb sr fractionation during the sedimentary cycle

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

geochemistry and petrology of recycled orogen derived sediments a case study from upper precambrian siliciclastic rocks of the central iberian zone iberian massif spain abstract two areas of upper precambrian siliciclastic rocks separated from each other by 150 km were selected in the central iberian zone ciz for a petrological and geochemical study the petrology of the coarser grained rocks indicates the absence or scarcity of igneous clasts the presence of extrabasinal and intrabasinal clasts and homogeneous features of sandstones ranging from very fine to fine grained the modal data of these rocks plot close to the qt l and qm lt sides of qtfl and qmflt triangular diagrams suggesting provenance from a recycled orogen the geochemical data on the shales and sandstones from the two areas studied are very uniform both groups of rock show similar ranges of relevant major element ratios such as al 2 o 3 tio 2 17 46 21 51 17 46 21 34 for shale and sandstone respectively al 2 o 3 feo t 2 40 3 79 2 18 3 33 and feo t feo t mgo 0 71 0 77 0 68 0 76 together with parallel average rare earth element ree patterns and similar values for some key trace element ratios la th th yb th u zr hf sm nd and cr sc other trace element ratios la sc th sc cr th and cr zr are uniform in the shales group and also in the sandstones group although minor differences become apparent when both groups are compared these differences are probably related to the quartz dilution effect affecting the relative abundance of phyllosilicates in these rocks and indirectly the sc and cr contents given their covariations with al2o3 the zr yb zr y and zr th ratios of the shales are uniform the sandstones show higher and more variable values and there is no correlation between zr abundance and any of these ratios in the shales or sandstones together with the parallel ree patterns shown by both groups of rock these results suggest the absence of heavy mineral fractionation during the transport of detrital material and or the relative importance of other minerals probably phyllosilicates in the control of yb y and th abundances apparently all the groups of rocks show a remarkable geochemical homogeneity despite the distance between the sampling areas thus the petrological and chemical data are consistent and point to a very extensive homogeneous and stable source region as the most plausible contributor to the upper precambrian siliciclastic rocks in the ciz the possible relationship between this zone and n gondwana is also discussed

the age and provenance of metasedimentary rocks in the quetico subprovince ontario from single zircon analyses implications for archean sedimentation and tectonics in the superior province abstract u pb isotopic analyses have been performed on single detrital zircon grains from greywacke samples collected along the northern boundary of the quetico subprovince in northwest ontario secondary lead loss was virtually eliminated by choosing crack free grains and removing the outer surface using air abrasion thus the 207 pb 206 pb age can be considered a good approximation to the age of crystallization of each grain zircons in the quetico metasediments show a range of ages from 2698 3 ma to 3009 4 ma deposition and metamorphism of the metasediments is bracketed within a 10 5 ma time span between crystallization of the youngest detrital zircon grain and a late tectonic intrusion the blalock pluton which cuts the sediments and has a zircon age of 2688 4 ma the most easterly metasediment sample contains zircons with ages which cluster around 3007 6 ma which is within error of the age of a batholith dated in the adjacent marmion lake area of the wabigoon subprovince this along with the fact that most detrital zircons were only slightly abraded by sedimentary transport suggests that the source of many zircons may have been proximal from the north there is also a significant cluster of young zircon ages in the range 2698 2709 ma surprisingly only a few ages correspond to the time of most intense arc type felsic volcanism 2710 2745 ma in the main greenstone belt of the western wabigoon subprovince sedimentary deposition occurred at a time of extensive deformation in the adjacent wabigoon subprovince the pattern of detrital grain ages indicates that this involved uplift of a complex older sialic landmass petrographic evidence for the presence of substantial felsic volcanic detritus as well as the fact that many of the zircons only slightly pre date deposition indicates that much of the sediment was probably derived from erosion of active felsic volcanic centers these data combined with regional geochronological and field studies favour deposition in a basin formed by late orogenic collisional events

samarium neodymium elemental and isotopic systematics in sedimentary rocks abstract trace element sm nd th sc and nd isotopic data for sedimentary rocks are examined in order to evaluate 1 1 possible secular trends in the composition of the sedimentary mass and upper continental crust 2 2 the role of tectonic setting in controlling the chemical and nd isotopic composition of the sedimentary mass when plotted against either sedimentation age or average provenance age determined mainly from nd model ages sm nd in sedimentary rocks decreases abruptly from about 0 21 to 0 19 at the archean proterozoic transition consistent with a change towards a more felsic upper crustal composition the change is only somewhat less pronounced for turbidite sediments than for those of continental affinity a trend toward increasing sm nd approaching or exceeding archean values is observed for phanerozoic turbidites this latter change is interpreted as indicating undifferentiated young crust as a component in the sediment which has been lost from the older post archean record due to sedimentary and igneous intracrustal recycling processes when plotted against sedimentation age th sc ratios exhibit a trend directly comparable to that of sm nd in this case th sc increases abruptly at the archean proterozoic boundary from about 0 5 to 1 0 and then decreases to about 0 7 during the phanerozoic nd isotopic composition of modern sediments is affected by tectonic association with those deposited at active margin settings tending to have higher nd reflecting younger provenance and higher and more scattered sm nd reflecting less homogenized and differentiated provenance it is likely that these distinctions can be traced back for about 1 2 ga but with the current data base are not readily resolved in older sedimentary rocks the degree to which the preserved sedimentary mass is open to new additions from the mantle either through replacement during sedimentary recycling or sediment mass growth is highly variable but averages about 15 25 during the phanerozoic

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

geochemistry and provenance of sediments from the pongola supergroup south africa evidence for a 3 0 ga old continental craton abstract the 3 0 ga pongola supergroup of south africa is one of the oldest well preserved supracrustal successions the sequence is divided into the nsuze chiefly volcanics quartzites and pelites and the overlying mozaan groups chiefly quartzites and pelites relative to north american shale composite nasc most pongola pelites have similar la yb and eu eu ratios and large ion lithophile elements lile high field strength elements hfse ree v and sc contents ni 30 300 ppm and cr 100 800 ppm contents are greater than those of nasc while cr ni ratios are generally higher than shales and pelites of all ages these trends may reflect residual cr enrichment during intense chemical weathering of komatiitic source rocks local populations of nsuze pelites based on major and trace element data may result from source heterogeneities and or contemporaneous volcanic input into the sedimentary basin pongola quartzite compositions and detrital modes are similar to those of sandstones from phanerozoic passive margin and basement uplift tectonic settings ree lile and hfse contents in pelites suggest that pongola source rocks were remarkably similar in composition to average phanerozoic upper continental crust trace element geographic and isotopic constraints favor 3 2 to 3 0 ga hood granites as a major source for pongola detritus moderate contributions from mafic sources are also required whereas relatively minor contributions from tonalite and komatiite are allowed these results indicate the existence of a chemically evolved felsic craton in southern africa by 3 0 ga this craton appears to have been more evolved than sources for either older moodies 3 3 ga or most of the younger witwatersrand sediments 2 8 2 7 ga pelites from the kaapvaal craton record at least two mafic to felsic source area compositional cycles in pelites deposited from the mid to late archean additional cycles may be masked by a depositional or erosional hiatus between the moodies and pongola sequences these sedimentary patterns suggest that crustal development in southern africa was episodic in nature during the early precambrian

rare earth element patterns in archean high grade metasediments and their tectonic significance abstract metasediments from two contrasting types of archean high grade terrains are interpreted as being derived from distinct tectonic settings the kapuskasing structural zone canada represents the deep roots of a typical greenstone belt whereas the limpopo province southern africa and western gneiss terrain australia mainly consist of shelf sediments deposited on a granitic basement and then buried to the depths required for granulite fades metamorphism upper amphibolite to granulite fades paragneisses from the kapuskasing structural zone have ree patterns similar to those of greenstone belt sediments except where partial melting has occurred forming restites with eu enrichment and melts with eu depletion except in this latter instance metamorphism has not affected the primary ree patterns ree patterns in archean upper amphibolite to granulite facies metasediments from the central limpopo province and western gneiss terrain show wide differences ranging from patterns which resemble those in post archean terrigenous sediments to typical archean sedimentary rock patterns the diversity in ree patterns for these shallow shelf metasediments is interpreted as resulting from derivation from local provenances samples with post archean patterns displaying eu depletion are interpreted as being derived from k rich granitic plutons which were portions of small stable early archean terrains precursors of the widespread late archean proterozoic episode of major cratonic development

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

rare earth element geochemistry of australian paleozoic graywackes and mudrocks provenance and tectonic control abstract in the paleozoic graywackes of eastern australia there is an increase in the total abundance of rare earth elements ree a light to heavy ree ratio and a decrease in the chondrite normalized eu anomaly with the increase in sio2 al2o3 and k2o na2o ratios this is due to the change in the dominant source rocks from andesite to dacite to granite gneiss and sedimentary rocks similarly an increase in total ree and a decrease in eu anomaly is also seen in the associated mudrocks with the increase in total clay content this correlation in paleozoic sedimentary rocks suggests that the ree characteristics of terrigenous sedimentary rocks reflect the depositional tectonic setting of the basin the oceanic island arc type graywackes are characterised by their lower total ree abundance only slight enrichment of lree over hree and the absence of a negative eu anomaly on chondrite normalised plots the continental island arc type graywackes are discriminated by their higher ree abundance and la yb ratio the presence of a small negative eu anomaly on the chondrite normalised plots and the significant enrichment of eu compared to paas post archean average australian shale the sedimentary rocks deposited on andean type active continental margins passive margins platform and cratonic basins are all characterised by the high enrichment of lree over hree and the presence of a pronounced negative eu anomaly on chondrite normalised plots and can not be discriminated from each other by ree patterns alone

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

nd sr isotopic provenance analysis of upper cretaceous great valley fore arc sandstones the provenance of fore arc sandstones of the upper cretaceous great valley group was investigated by using measurements of sm nd and rb sr isotope ratios and major and trace element concentrations the objective was to elucidate the erosional history of the adjacent sierra nevada arc and its relation to magmatism and fore arc sedimentation relatively detailed provenance information is obtainable because the currently exposed plutonic rocks of the arc and by inference the cretaceous volcanic rocks have large geographic gradients in isotopic composition mainly from west to east across the arc the relation of nd to sr and to major and trace elements in great valley sandstones is the same as in the plutonic rocks of the sierra nevada this observation confirms that the arc was the primary source of sediment and that the chemistry of the detritus was not significantly disturbed by sedimentary processes with decreasing nd there is an increase in k 2 o rb pb ba la and u and mgo ni and cr decrease the nd of sandstones decreases with decreasing depositional age from 0 7 to 5 0 in 97 to 73 ma strata the nd and age of the sandstones closely match those of the plutonic rocks of the sierra nevada this observation indicates that the location of the mean sandstone source and the active volcanic front were nearly coincident in time and space thus the isotopic compositions of the sandstones directly record the temporal and geographic variations in arc magmatism the volcanic roof of the sierra nevada arc was evidently chemically and isotopically equivalent to its plutonic roots prearc metasedimentary rocks did not significantly contribute to the isotopic or geochemical composition of the fore arc sandstones

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

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

contrasting types of late archean plutonic rocks in northwestern ontario implications for crustal evolution in the superior province abstract the late archean aulneau and lount lake batholiths have contrasting geochemical characteristics that are interpreted to be a consequence of derivation from different sources the aulneau batholith which was intruded into low grade metavolcanic rocks of the wabigoon subprovince is characterized by high na2o and low k2o rb and y together with enriched light rare earth elements lree near chondritic heavy rare earth elements hree negligible eu anomalies and mantle type initial sr and nd isotopic compositions these geochemical characteristics are consistent with derivation from the partial melting of metamorphosed tholeiitic basalt in contrast the lount lake batholith which was intruded into older 2 83 3 17 ga tonalitic plutonic rocks of the winnipeg river subprovince has relatively high k2o rb y and ree low na2o and prominent eu anomalies the initial sr and nd isotopic compositions of the lount lake batholith are consistent with a crustal source at least 200 400 ma older than the crystallization age of the batholith these geochemical characteristics indicate that the lount lake batholith originated from the partial melting of deeper level equivalents of the tonalitic metaplutonic rocks 2 83 3 17 ga which they intrude these results suggest that reworked 3 0 ga crust may be extensive in the winnipeg river subprovince and that the geochemical and isotopic signature of late archean plutonic rocks may be a useful reconnaissance tool in the identification of areas of extensive older crust the results can be interpreted within a uniformitarian model diachronous ensimatic magmatic arcs represented by the winnipeg river and western wabigoon subprovinces developed independently collision of these arcs at 2707 8 ma marks the cessation of subduction and related magmatic activity and the initiation of high grade metamorphism and melting in tectonically thickened portions of the crust

smnd isotopic geochemistry of sediments from taiwan and implications for the tectonic evolution of southeast china abstract an isotopic and geochemical study on the miocene to late paleozoic sediments and metasediments of taiwan has been conducted in order to decipher the crustal evolution of se china with the exception of a few late paleozoic sediments most samples from taiwan have a narrow range in sr and nd isotopic ratios and sm nd crustal residence ages t dm assuming derivation from a depleted mantle source the isotopic characteristics are similar to those of suspended material from major rivers around the world in particular they closely resemble those reported for the yangtze river sediment and the pre cretaceous granitoids of se china no obvious trend in t dm or isotopic ratios has been found with respect to the depositional ages indicating no pronounced secular variation however a few late paleozoic samples might have been influenced by fresh mantle inputs to their provenance proterozoic t dm of the sediments combined with proterozoic pb components in zircons from taiwan s granitoids suggest that important crustal growth in se china took place in the middle proterozoic the isotopic and geochemical characteristics of the sediments suggest that their sources were primarily recycled pre existing continental crust ree contents of taiwan sediments measured in this study are similar to the suspended load in major rivers paas post archean average australia shale nasc north american shale composite and the continental shelf slope sediments from the east china sea off the mouth of the yangtze they seem to represent products derived from typical continental crust lower hree abundances in taiwan sediments suspended load in major rivers and surface sediments from the e china sea than those of paas suggest that the source of hree in paas is likely to be slightly different from that for these recent terrigenous sediments an alternative for the explanation of this hree discrepancy is that a certain proportion of hree in these recent sediments were left in the residue partially dissolution of heavy mineral in analysis procedures particularly zircon even in a teflon bomb might cause this hree discrepancy late yanshanian granitoids now widely exposed on the se china coast have higher nd and lower sr isotopic ratios than those for taiwan s sediments so these granitic intrusions were not a major source for the sediments indicating that they were not exposed to erosion until the miocene or later