fluid compositions and p t conditions of vein type uranium mineralization in the beaverlodge uranium district northern saskatchewan canada the beaverlodge district in northern saskatchewan is known for vein type uranium mineralization most of the uranium deposits are spatially related to major structures and hosted by ca 3 2 1 9 ga granitic rocks and albitite derived from them and by ca 2 33 ga murmac bay group amphibolite all of which are unconformably overlain locally by deformed but unmetamorphosed redbeds of the ca 1 82 ga martin group and by the flat lying ca 1 75 1 5 ga athabasca group the uranium mineralization is mainly hosted in fault rocks breccias and carbonate quartz albite veins referred to as breccia style and vein style mineralization respectively with the latter being the focus of this study most of the mineralized veins occur in the basement rocks although some crosscut the martin group this study examines the field petrographic fluid inclusion and c o isotope characteristics of mineralized and non mineralized veins from 19 deposits occurrences as well as from the martin group with an aim to better understand the mineralizing environment and processes the coexistence of liquid dominated l v vapour dominated v l and vapour only v fluid inclusions within individual fluid inclusion assemblages fias in the veins suggests fluid immiscibility and heterogeneous trapping the l v inclusions with the lowest homogenization temperatures th within individual fias are interpreted to represent homogeneous trapping of the liquid phase which yield th values from 78 to 330 c mainly 100 to 250 c and salinities from 0 2 to 30 8 wt nacl equivalent mass spectrometric analysis of bulk fluid inclusions shows that the volatiles are dominated by h2o average 97 2 mol with minor amounts of co2 ch4 h2 o2 n2 ar and he fluid pressures were estimated to be 200 bars based on the inference of fluid immiscibility fluid temperatures of 100 to 250 c and low concentrations of non aqueous volatiles 3 mol the 18ovpdb and 13cvpdb of carbonate minerals associated with mineralization range from 20 5 to 8 9 and 10 1 to 0 9 respectively the 18ovsmow values of the parent fluids calculated using the th values range from 9 6 to 17 0 with the majority from 0 to 5 0 o isotopes of paired equilibrium quartz and calcite analyzed by secondary ion mass spectrometry sims yield temperatures from 161 to 248 c which are consistent with the fluid inclusion data the new fluid inclusion and stable isotope data are inconsistent with a metamorphic or magmatic hydrothermal model as proposed in some previous studies for breccia style and vein style mineralization but rather support a model in which the vein type uranium mineralization took place at relatively low temperature 100 to 250 c and shallow 2 km conditions with fluid pressure fluctuating between hydrostatic and sub hydrostatic regimes possibly related to episodic faulting the mineralizing fluids were mainly sourced from the martin lake basin and uraninite was precipitated as a result of mixing between this basin derived fluid and fluids carrying reducing agents fe2 ch4 derived from the basement although fluid rock reactions and fluid immiscibility may have also played a role

diagenetic origin for quartz pebble conglomerates the occurrence of quartz pebble conglomerates qpc in the rock record increases backward through time from the tertiary through the precambrian the positive correlation between qpc abundance and age is valid both for numbers of reported qpc and for qpc as a percentage of all conglomerate and at both the era and the period level qpc are usually interpreted as being due to intense chemical weathering protracted transport or sediment recycling but none of these can account for the age distribution of qpc which is the opposite of the global mass age distribution for sedimentary rocks precambrian and tertiary conglomerates with similar sources and sedimentology have vastly different clast populations nonquartzose clasts being much more abundant in the younger rocks comparison of the petrology of qpc and polymict conglomerates shows that qpc have consistently higher proportions of diagenetic secondary matrix and pressure solved grain contacts we conclude that diagenetic factors play an important role in qpc formation by preferentially destroying less durable clasts

comparisons between tethyan anorthosite bearing ophiolites and archean anorthosite bearing layered intrusions implications for archean geodynamic processes

archean greenstone belts are not oceanic crust archean greenstone belts with a preponderance of mafic volcanic rocks often preserved in tectonically complex sequences are obvious candidates in the search for remnants of archean ocean crust we review the tectonic setting and stratigraphy of a number of greenstone sequences previously interpreted as archean ophiolites and conclude on the basis of basal unconformities presence of xenocryst zircons geochemical and isotopic evidence for crustal contamination intrusive relationships with older basement and their internal stratigraphy that none of these examples is derived from archean oceanic crust thermal tectonic and isostatic constraints imply that archean oceanic basins did exist and were covered with several kilometers of water we consider it possible indeed probable that relict archean oceanic crust is preserved in granite greenstone terrains or other archean tectonic settings but that an unequivocal archean ophiolite has not yet been recognized

neoarchean magmatism in the southeastern amazonian craton brazil petrography geochemistry and tectonic significance of basalts from the carajas basin abstract the neoarchean carajas basin is situated in the northern part of the carajas mineral province the main rock assemblages of this basin consist of volcano sedimentary sequences composed predominantly of mafic volcanic rocks and banded iron formations the grao para group ggp is the dominant sequence in the basin the basalts of grao para group parauapebas formation are investigated using petrography geochemistry and geochronology to decipher the nature and tectonic setting of the magmatism these volcanic rocks occur in extensive succession of massive or amygdaloidal lava flows at least 370 m in thickness the basalts are grayish green amygdaloidal porphyritic aphanitic or fine grained and hypocrystalline the primary mineral assemblages consist predominantly of clinopyroxene and plagioclase on the other hand the secondary mineral assemblages consist s mainly of chlorite albite fe epidote quartz and calcite and are interpreted as product of submarine hydrothermal alteration and or sub greenschist facies metamorphism geochemical investigations on volcanic rocks of the parauapebas formation show 51 12 55 26 wt sio2 0 69 0 92 wt tio2 7 02 12 35 wt feo and mgo ranging from 4 38 to 7 38 wt moreover the rocks are sub alkaline plot in the transitional and calc alkaline fields and show either arc like trace element patterns or similar to those of the upper continental crust these features include lile and lree enrichment and nb and ti depletion shrimp zircon u pb dating constrains the crystallization ages at 2749 6 5 and 2745 5 ma early neoarchean regarding volcanic mafic rocks our data suggest that the basaltic rocks were derived from the subcontinental lithospheric mantle affected by upper continental crustal components although this volcanism could be originated by the opening of a back arc continental basin a rift setting is more plausible on the basis of regional geology thus the carajas basin was likely formed in an extensional regime related to a rift setting at ca 2 75 ga we propose that this rift basin is related to post orogenic setting in the passage from the mesoarchean to the neoarchean the rifting process could be associated to a slab breakoff and subsequent asthenosphere upwelling that provided heat for partial melting of the lithospheric mantle

petrology age and tectonic setting of the white rock formation meguma terrane nova scotia evidence for silurian continental rifting the white rock formation in the yarmouth area of the meguma terrane of southern nova scotia consists mainly of mafic tuffaceous rocks with less abundant mafic flows epiclastic and clastic sedimentary rocks and minor intermediate and felsic crystal tuff it is divided into seven map units that appear to young from west to east inconsistent with a previously assumed synclinal structure the white rock formation is flanked on both northwest and southeast by mainly the cambrian to lower ordovician halifax formation the western contact is interpreted to be a sheared disconformity whereas the eastern contact appears to be a major brittle fault and shear zone that juxtaposes different crustal levels the granitic brenton pluton forms a faulted lens within the eastern shear zone a felsic tuff from the upper part of the white rock formation yielded a u pb zircon age of 438 3 2 ma identical within error to published ages for the brenton pluton and felsic volcanic rocks near the base of the white rock formati

intracrustal complexity in the united states midcontinent preliminary results from cocorp surveys in northeastern kansas unusually clear indications of complex structure in the mid to lower crust is revealed by seismic reflection surveys in northeastern kansas this complexity contrasts markedly with the layer cake simplicity of both the overlying sedimentary cover and most previous crustal models for the central united states seismic sections collected by cocorp consortium for continental reflection profiling as part of a major east west traverse across the neniaha ridge and midcontinent geophysical anomaly indicate that below a thin relatively flat layered paleozoic sedimentary section the deep crust is characterized by numerous dipping and arcuate reflections and diffractions in many places layered and crosscutting these reflections suggest convoluted three dimensional folded faulted and intruded structures specific identification of these deep features may be possible if future surveys can trace them to accessible depths the basement above these reflection complexes contains significantly fewer reflections consistent with but not necessarily diagnostic of the granitic terrane that dominates basement drill hole samples in the region among the events at these shallower basement depths are several east dipping reflections some of which may be major faults travel times corresponding to expected moho depths about 36 km are characterized less by specific reflections than by an apparent decrease in the density and number of reflections while evidence of crustal heterogeneity is common among deep reflection studies the kansas seismic results outlined in this brief report stand out as being unusually clear representations of such

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

tectono stratigraphy of late archaean greenstone terranes in the southern eastern goldfields western australia abstract late archaean greenstone terranes in the southern part of the eastern goldfields are regionally extensive faultbounded entities defined on the basis of small but distinct differences in stratigraphy and structure the greenstones comprise volcano sedimentary successions that were all deposited at the same time c 2720 2675 ma on sialic crust hence the terranes are interpreted to represent contemporaneous probably adjacent basins these basins were affected by the same history of granite plutonism and regional deformation c 2675 2620 ma early subhorizontal deformation is poorly understood but includes at least locally significant thrust stacking d1 with a south to north movement direction this was followed by regional east west extension affecting all supracrustal successions above a province wide detachment now lying at depths between 4 and 7 km this detachment may have evolved from the greenstone basement contact but only after extensive reworking clastic basins developed along or above major extensional faults but no volcanism is associated with this extension subsequent east west d2 shortening resulted in regional scale thrusting folding and associated late stage transcurrent fault movements the d2 event is contemporaneous with a major thermal event represented by voluminous granite emplacement c 2660 ma and associated peak regional metamorphism in the greenstone succession predominantly monzo granite derived from older sialic crust was emplaced throughout the period of basin formation and deformation early synvolcanic granite c 2719 2710 ma is a minor component late synvolcanic granite c 2685 2675 ma is widespread and syn d2 granite c 2660 ma is abundant there are a few small late tectonic intrusions c 2640 ma to 2600 ma the lateral lithostratigraphic persistence of the contemporaneous basins the syn d2 low pressure regional metamorphism and the province wide monzo granite magmatism suggest that the greenstone basins possibly developed in a tectonic setting similar to that of failed rifts above aborted spreading centres the basins closed in response to far field compressional stresses

review of archean supracrustal assemblages of the southern abitibi greenstone belt in ontario canada products of microplate interaction within a large scale plate tectonic setting abstract previously formulated plate convergent models account for many aspects of the large scale regularity and parallelism between different subprovinces of the archean superior province such models do not however appear to adequately explain the within subprovince evolution of 2 75 2 68 ga supracrustal assemblages of the late archean southern abitibi greenstone belt principal supracrustal assemblage classes include l assemblages dominated by tholeiitic metabasalt and metakomatiite with varying amounts of tholeiitic intermediate to felsic metavolcanic rocks these assemblages probably formed in extensional settings the ratio of basalt to komatiite and the ratio of basalt komatiite to intermediate felsic metavolcanic rocks may be a function of mantle temperature and crustal thickness at the site of extension respectively 2 assemblages dominated by intermediate to felsic fragmental and or effusive rocks are generally of calcalkalic affinity and likely formed in island arc settings 3 assemblages containing a substantial component of iron formation may represent older supracrustal assemblages that form structural and or stratigraphic basement to some of the younger supracrustal assemblages 4 assemblages consisting almost wholly of turbidite facies metasedimentary rocks appear to postdate volcanic activity related to the previous assemblages deposition of these sediments broadly coincides with the onset of regional deformation these assemblages may represent distal deposits related to an emerging collisional orogen 5 assemblages consisting of alluvial fluvial metasedimentary rocks with or without spatially associated alkalic metavolcanic rocks are associated with crustal scale shear zones and are the youngest assemblages within the greenstone belt alluvial fluvial assemblages likely represent the final emergence of the consolidating orogen geochronological data and assemblage distribution suggest that the supracrustal assemblages of the southern abitibi greenstone belt may represent the amalgamation of products of diverse and intricate microplate tectonic settings repeated in time and space prior to the impingement of the large scale plate tectonic regime that dictated the final large scale configuration of the archean superior province

taltson basement gneissic rocks upb and nd isotopic constraints on the basement to the paleoproterozoic taltson magmatic zone northeastern alberta the taltson magmatic zone is a 300 km long northerly trending belt of archean to paleoproterozoic granitic basement gneiss amphibolite supracrustal gneissic rocks and voluminous paleoproterozoi

field and geochemical characteristics of mesoarchean to neoarchean volcanic rocks in the storo greenstone belt sw greenland evidence for accretion of intra oceanic volcanic arcs abstract the storo greenstone belt southern west greenland consists of thrust imbricated slices of mesoarchean 3060 ma and neoarchean ca 2800 ma mafic to ultramafic volcanic rocks volcaniclastic sediments and gabbro anorthosite associations the belt underwent polyphase metamorphism at upper amphibolite facies conditions between 2650 and 2600 ma the contacts between the mesoarchean and neoarchean volcanic rocks and surrounding eoarchean to neoarchean tonalite trondhjemite granodiorite ttg gneisses are tectonic and typically bounded by high grade mylonites regardless of age the volcanic rocks are dominated by mafic amphibolites with a tholeiitic basalt composition near flat to slightly enriched light rare earth element lree patterns la smcn 0 91 1 48 relatively flat to slightly depleted heavy ree hree gd ybcn 1 0 1 28 and pronounced negative nb ta anomalies nb nb 0 34 0 73 on chondrite and primitive mantle normalized diagrams these geochemical characteristics are consistent with subduction zone geochemical signatures and partial melting of a shallow

stratigraphy of the steep rock group northwest ontario a major archaean unconformity and archaean stromatolites the archaean steep rock group of northwest ontario situated in the wabigoon subprovince of the superior province canada comprises five formations wagita formation clastics mosher carbonate jolliffe ore zone dismal ashrock and witch bay formation metavolcanics reinvestigation of the geology of the group has shown that the basal clastics of the wagita formation 0 150 m unconformably overlie the marmion complex a massive tonalite tonalite gneiss terrane 3 ga old overlying the basal elastics is the mosher carbonate 0 500 m containing diverse stromatolite morphologies extensive zones of carbonate breccia occur adjacent to fault zones and mafic dykes stratigraphically above the mosher carbonate is the jolliffe ore zone 100 400 m which is divided into a lower manganiferous paint rock member and an upper goethite member within the jolliffe ore zone thin layers of buckshot ore occur these are horizons of haematitic pisolites and fragments set in a lighter ferruginous matrix of kao

a historical approach to continental flood basalt volcanism insights into pre volcanic rifting sedimentation and early alkaline magmatism abstract continental flood basalts are widely thought to be produced from mantle plume heads however plume theories do not observe any role for lithospheric rifting before flood basalt events and consider irrelevant the fact that most if not all continental flood basalts have erupted through deep rifts containing thick sedimentary sequences at best plume theories invoke selective capture of such deep rifts or lithospheric thinspots by rising mantle plume heads however the fact that cfbs of the world erupted through deep ancient rift zones and alternative dynamical considerations of flood basalt genesis directly lead towards a new historical approach to flood basalt emplacement this approach takes cognizance of the basic unity of geological history and processes satisfactorily explains pre volcanic rifting sedimentation mantle metasomatism and early pre tholeiite enriched alkaline magmatism for tens of millions of years both incubating and impacting plume heads ought to lead to pre volcanic lithospheric doming which is usually not observed in flood basalts continental or oceanic flood basalt events instead seem to be derived by convective partial melting during sudden lithospheric pull apart splitting along pre existing lithospheric discontinuities such as deep rifts or fracture zones

remnants of an archean oceanic plateau belingwe greenstone belt zimbabwe stratigraphic and structural data from the archean zimbabwe craton suggest that a major detachment surface exists within the belingwe greenstone belt the surface separates ultramafic and mafic magmatic rocks of the upper greenstones in the hanging wall from an ancient gneiss complex older volcanic sedimentary rocks and a shallow water sedimentary sequence in the footwall rocks dated at ca 2 7 ga above the detachment surface form the proposed mberengwa allochthon the regionally extensive upper greenstone succession represents tectonically emplaced allochthonous sheets not indigenous magmas erupted within autochthonous continental rifts magmatic rocks of the mberengwa allochthon resemble oceanic plateaus preserved in younger mountain belts comparison of the zimbabwe craton with the proterozoic birrimian terranes of west africa leads us to suggest that precambrian continental growth may have been characterized by intense structural imbrication related to the difficulty of subduction of buoyant oceanic lithosphere

growth and demise of an archean carbonate platform steep rock lake ontario canada the steep rock group of northwest ontario s wabigoon subprovince is one of the world s thickest archean carbonate platform successions it was deposited unconformably over a 3001 2928 ma gneissic t

enriched archean lithospheric mantle beneath western churchill province tapped during paleoproterozoic orogenesis ultrapotassic rocks of the christopher island formation baker lake basin were emplaced across an enormous area 240 000 km 2 minimum of the western churchill province ca 1 83 ga these rocks extend across the snowbird zone a geophysical feature postulated by others to represent a paleoproterozoic suture that welded the rae and hearne domains minette dikes and flows of the rae and hearne domains display identical nd 1830 values and incompatible element patterns and thus appear to have originated from a common lithospheric mantle source christopher island nd model ages cluster at 2 8 ga and nd data from one archean lamprophyre and three 2 45 to ca 2 2 ga mafic suites suggest that enriched lithospheric mantle sources beneath both the rae and hearne domains existed well before ca 1 83 ga inconsistent with paleoproterozoic suturing along the snowbird zone in contrast to commonly invoked models that envisage melting of local enriched domains christopher island ultrapotassic rocks appear to have originated from an extensive reservoir we suggest that such a reservoir was created during an archean metasomatic event perhaps owing to flat subduction and that it remained in nearly complete isolation until tapped during paleoproterozoic extension related to squeezing of western churchill crust between flanking wopmay and trans hudson orogens

faster spreading or greater ridge length in the archean dissipation of more heat from a hotter early earth by the plate tectonics mechanism would require subduction of on average younger lithosphere in principle this could be achieved equally well by faster spreading by having greater ridge length or some combination thereof present day evidence however suggests that younger oceanic lithosphere subducts more slowly than older if this applied in the archean then the greater ridge length alternative prevailed a quantitative expression is derived which indicates that oceanic heat loss is proportional to the cube root of ridge length this relationship means that if archean heat flow was 3 times that of the present 27 times as much ridge would have been required which would suggest that the archean earth was covered by many small plates moving slowly

chronology of devonian to early carboniferous rifting and igneous activity in southern magdalen basin based on u pb zircon dating fifteen u pb zircon radiometric age determinations have been made on igneous rocks of middle devonian to early carboniferous age from the southern margin of the magdalen basin in cape breton island and northern mainland nova scotia volcanic rocks interbed with early rift basin sedimentary rocks with some palynological biostratigraphy dated intrusive rocks cut these sedimentary units our biostratigraphically constrained ages are in close agreement with the current devonian time scale combined with previously published data the age determinations show that igneous activity occurred in four pulses middle devonian 390 385 ma early late devonian 375 370 ma latest devonian to early tournaisian 365 354 ma and late tournaisian to early visean ca 339 ma middle devonian 385 389 ma volcanic rocks are confined to the guysborough group the fisset brook formation basalt and minor rhyolite in the type area and elsewhere in cape breton island and northern mainland nova scotia is late devonian c

why was flood volcanism on submerged continental platforms so common in the precambrian abstract many archean and proterozoic greenstone belts contain volcanic rocks that erupted subaqueously but interacted with continental crust on their way to the surface this points to emplacement in a tectonic setting that was rare to absent throughout the phanerozoic the eruption of flood basalts onto submerged continental platforms to explain the situation requires that sea levels were high during periods of flood volcanism perhaps because the ocean basins were partially filled by extensive active and high standing oceanic ridges or by oceanic plateaus the common preservation of crust contaminated submarine basalts even in the oldest supracrustal sequences may indicate that continental crust was voluminous even at an early stage in earth history

continental extensional setting for the archean belingwe greenstone belt zimbabwe the tectonic setting of the archean ngezi group greenstones in the belingwe greenstone belt zimbabwe is controversial both autochthonous continental and oceanic settings have been proposed the sedimentary facies of the basal manjeri formation have been logged in surface sections and in drill cores deposition occurred under fluviatile and shallow marine conditions followed by alluvial fans and fan deltas detrital mineralogy rare earth element patterns and sm nd depleted mantle model ages between 2 9 and 3 7 ga are consistent with derivation from very local sources the facies and geochemical association imply sedimentation in an extensional continental setting

the age and origin of the north shore plutons in the rae province goldfields area saskatchewan the north shore plutons are peraluminous granitoids emplaced within the murmac bay group supracrustals exposed on the north shore of lake athabasca saskatchewan 207pb 206pb ages obtained using the single zircon pb evaporation technique indicate emplacement at 1952 18 ma coeval with taltson magmatism that occurred during the 1 9 2 0 ga thelon orogeny which influenced the rae province the granitoids inherited zircons from archean source terranes dated at approximately 2 5 and 3 0 ga detrital zircons from the younger source terrane were also identified in the murmac bay group thus constraining the maximum age of the group to the paleoproterozoic rare earth element and incompatible element data and rb sr and sm nd isotope systematics from the north shore plutons and adjacent host rocks indicate that the plutons are crustal melts generated from melting of murmac bay group rocks and archean crust at deeper levels granitic gneisses described as older granites represent mixtures of variable amounts of

u pb geochronologic constraints on the crustal evolution of the long range inlier newfoundland the long range mountains of newfoundland expose the northeastern most basement inlier of the appalachian orogen u pb results for two samples of basement gneiss reveal crustal formation ages of 1466 10 ma western brook pond charnockite and 1530 8 ma cat arm road gneiss indicating an affinity with pinwarian magmatism 1510 1450 ma in labrador a third sample of basement gneiss from the cat arm road dam site yielded a minimum formation age of 1631 ma these basement gneisses were intruded by granite plutons during two periods of grenvillian magmatism group i at 1032 1022 ma and group ii at 993 985 ma group i intrusions include the 1032 0 1 5 ma lomond river granite and the 1022 0 2 0 ma lake michel igneous suite group ii intrusions include the 999 4 ma potato hill charnockite 993 7 ma horse chops granite 984 9 1 6 ma cloud river granite and may include the apsy granite the metamorphic history of the long range inlier is complex and three discrete proterozoic metamorphic events can

is the lomagundi event present on the rae craton a case study from the murmac bay group shortly after the initial rise of atmospheric oxygen in the paleoproterozoic era a major perturbation occurred in the global carbon cycle which is manifested as a long lived positive carbon isotope excursion recorded in 2 22 2 06 ga carbonate rocks known as the lomagundi event beyond its significance for evolving seawater composition this geochemical event can be used as an indirect age marker in paleoproterozoic sedimentary successions documenting further occurrences of this event in other paleoproterozoic carbonate rocks confirms that the event was global and reflects ambient seawater composition this event however has only been documented in two successions in canada despite the ubiquity of paleoproterozoic aged rocks on the canadian shield our study focuses on metacarbonate rocks from the paleoproterozoic aged murmac bay group on the southwestern margin of the rae craton in northern saskatchewan measured 13c values up to 7 8 fall within the range of the lomagundi event but most value

initiation of 2 45 2 1 ga intracratonic basin sedimentation of the hurwitz group keewatin hinterland northwest territories canada continental to shallow marine rocks of the hurwitz group exposed as outliers across the hearne province in northern canada were deposited between 2 45 and 2 11 ga on a 3 3 2 6 ga granite greenstone basement initiation of the hurwitz basin was characterized by low relief and rates of subsidence that never progressed beyond the intensity of regional sagging during stage 1 polymictic conglomerate lower noomut formation was deposited in palaeovalleys cut into archaean basement and subarkose to quartz arenite quartz pebble conglomerate upper noomut formation was deposited on a low relief sand plain abundant first cycle quartz rich sandstones are attributed to intense humid climate weathering in low relief source areas and at sites of temporary storage as well as to repeated fluvial and eolian reworking interbedding of pyritic palaeoplacers and continental redbeds denotes fluctuating levels of atmospheric oxygen disappearance of pyritic palaeoplacers upsection signifies ultimate changeover to an atmosphere with sustained free oxygen stage 2 padlei formation was marked by a climatic shift toward cold glacial conditions as indicated by dropstones greywacke till pellets and possible ikaite pseudomorphs in lacustrine rhythmites stage 3 kinga formation represents a return to wet warm conditions with fluvial eolian deposition of subarkose and quartz arenite on a low relief sand plain maguse member and deposition of supermature quartz arenite in a vast 100 000 km2 minimum shallow water lake or series of lakes whiterock member local chert beds at the top of the kinga formation hawk hill member formed by spring discharge due to regional gravity driven convective groundwater flow an intracratonic setting is implied by 1 a symmetric basin centred fill 2 absence of features indicating a shelf slope break 3 absence of rift rocks 4 a prolonged slow subsidence history 5 likely correlation with other continental to shallow marine deposits scattered across the keewatin hinterland and 6 positioning 1000 2000 km away from closest coeval passive margin sequences we speculate that hurwitz basin formed due to lithospheric stretching related to a large scale mantle upwelling during the protracted breakup of an earliest palaeoproterozoic supercontinent kenorland

new depositional age constraints for the murmac bay group of the southern rae craton canada abstract the murmac bay group is one of several early supracrustal successions that were deposited on the southern rae craton churchill province western canadian shield and subsequently affected by paleoproterozoic tectono metamorphism where best understood the succession comprises basal quartzite and or psammite with intercalated dolostone patchy iron formation mafic volcanic flows and overlying psammopelite pelite previous attempts to date the murmac bay group have resulted in both archean and paleoproterozoic estimates re interpretation of critical field relationships together with acquisition of new geochronological data have greatly improved estimates for its age of deposition which in turn yield new constraints on the tectonic evolution of the region circa 2 33 2 32 ga detrital zircon from three of four dated samples including a rare basal conglomerate establishes a maximum age for the initiation of deposition a 2322 5 ma quartz feldspar porphyry dyke intruding mafic volcanic rocks near the base of the succession indicates that deposition of the murmac bay group was initiated ca 2 33 ga coeval with the earliest members of a 2 33 2 29 ga granitic suite these post collisional granites were emplaced following a 2 37 ga thermal culmination attributed to the arrowsmith orogeny deposition of the murmac bay group continued until at least 2 17 ga the youngest detrital zircon age analyzed from the upper part of the succession deposition was over by ca 1 93 ga when the murmac bay group underwent regional metamorphism and was intruded by the donaldson lake pluton for which a revised ca 1 93 ga crystallization age has been obtained according to our new age constraints the upper murmac bay group may be temporally correlative with the waugh lake group exposed about 100 km to the west northwest at the eastern margin of the taltson magmatic zone and with rocks of the rutledge river basin located 150 km to the northwest a stronger case can be made for lithostratigraphic correlation of the murmac bay group with the lower three of four assemblages in the amer and ketyet river groups 750 km to the northeast the abundance of ca 2 17 ga detrital zircon in the murmac bay group together with the presence of 2 2 2 07 ga detritus in the amer and ketyet river groups rutledge river basin and in younger sedimentary successions in the region are thought to record an early accretionary event on the western margin of the rae craton

construction of an evolutionary deglaciation model for the irish midlands based on the integration of morphostratigraphic and geophysical data analyses alternative established models for the deglaciation of the midlands of ireland are tested against an interpretation of a suite of deglacial sediments covering an area of 600 km2 in the east central midland area interpretation of the sediments is based on geomorphological mapping lithostratigraphic characterization of exposures and geotechnical data supported by electrical resistivity tomography ert and ground penetrating radar gpr gpr depicted small scale sedimentological and deformational structures within low conductivity soft sediments such as cross bedding planar bedding channel like features and faulting planes and revealed the internal architecture of eskers glaciodeltas subaqueous fans and raised bogs ert data permitted the detection of depth to bedrock and the lithological characterization of unconsolidated sediments the ten sites presented were surveyed by traditional mapping methods and or geophysical techniques this allowed the construction of a local model of the deglaciation of the area which recognized five main stages an ice sheet covering most of ireland withdrew as a single body as far as the midlands at this stage two main directions of ice retreat are identified from the spatial distribution of meltwater overflow channels esker and morainic ridges and ice marginal glaciolacustrine deposits a pattern of deglacial sedimentation into an expanding ice marginal glacial lake is depicted the glacial lake was dammed to the west by two ice dome fronts one decaying to the north west and another to the south west and by the shannon basin watershed to the east glacial lake outlets identified along the watershed and the altitude of the topset foreset interface zone depicted in glaciodeltaic deposits allowed the identification of three lake water levels the highest level is at 87 89 m ordnance datum od the second lake level at 84 m od and the third at 78 m od copyright 2012 john wiley sons ltd

stratigraphy sedimentology and physical volcanology of the henik group central ennadai rankin greenstone belt northwest territories canada late archean paleogeography of the hearne province and tectonic implications abstract extensive areas of the hearne and rae provinces are underlain by archean metavolcanic and metasedimentary rocks in the rae province supracrustal sequences with prominent continental to shallow water quartz arenites have been interpreted by others as rift and passive margin deposits in the hearne province to the southeast the ennadai rankin greenstone belt is exposed in an area ca 700 200 km forming the second largest greenstone belt in canada it comprises mainly greenschist facies grade mafic and felsic volcanic siliciclastic chemogenic and volcaniclastic rocks stratigraphic sedimentologic and physical volcanologic data based on 1 50 000 scale mapping in the central part of the belt indicate deposition in a subaqueous mafic volcanic plateau slope basin system that was well removed from a continental influence water depth was below storm wave base as indicated by preservation of delicate lamination in pelitic and iron formation units absence of features suggestive of subaerial exposure oscillatory flow or tidal currents and lack of interfingering rocks that could represent laterally adjacent shelf coastal or continental environments reconnaissance level regional geologic and geochronologic data indicate an overall pattern of supracrustal deposition in the hearne province consisting of a large mafic volcanic plain isolated felsic volcanic centres with lenses and belts of deep water sedimentary rocks at least two speculative yet testable tectonic models are consistent with the inferred paleogeography for both the rae province is considered to be the western continental hinterland to the ensimatic hearne province the rae and hearne provinces may record late archean continental extension sea floor spreading oceanic volcanic arc growth diachronous back arc spreading and collapse and arc continent collision with thrusting of oceanic rocks on to attenuated rae crust alternatively the hearne province may represent northwestward accretion of multiple volcanic arc trench pairs

early ordovician continental break up in variscan europe ndsr isotope and trace element evidence from bimodal igneous associations of the southern massif central france abstract the high grade marvejols group and the low grade albigeois cevennes sedimentary sequence contain bimodal igneous rocks of early ordovician age which are representative of a widespread thermal event in the european variscides comparison of their nd sr isotope and trace element characteristics provides additional evidence for their origin in an ensialic extensional setting as an alternative to the back arc model proposed by previous authors we propose that these associations record a continental break up episode unrelated to contemporaneous subduction in this model the widespread early paleozoic bimodal magmatism marks the birth of a mid european oceanic arm we propose that the break up was controlled by both transtensional processes and mantle plume activity high grade associations such as the marvejols group and low grade bimodal associations such as the albigeois cevennes might be regarded as representing opposite rifted passive margins the northern marvejols type margin was involved in a late ordovician silurian subduction like process with hp ht metamorphism in contrast the southern albigeois cevennes margin remained largely unaffected possibly as part of the overriding plate of the subduction zone pseudo calc alkaline signatures unrelated to subduction processes may occur in magmatic rocks associated with continental break up episodes in this case negative nb anomalies are produced by the addition of crustal components enriched in th and lree relative to nb this inferred alternative origin of nb anomalies has important bearing on the paleogeodynamic settings based on geochemical data

the archean murmac bay group evidence for a giant archean rift in the rae province canada abstract the lower amphibolite to granulite facies archean murmac bay group of the southwestern rae province canada formed during continental extension rifting of an underlying granitoid basement complex that includes the 3060 40 ma lodge bay granite and the 2999 7 ma cornwall bay granite the contact between the murmac bay group and the basement complex is an unconformity that is locally marked by polymictic conglomerate overlying the conglomerate is basal quartzite of the murmac bay group that contains a laterally extensive 10 km intraformational oligomictic quartzite pebble conglomerate that may have formed by uplift during rifting distinctive medium to coarse grained gabbro dykes and sills that cut both the basement complex and the basal quartzite are interpreted as feeders to stratigraphically overlying tholeiitic basalt flows that have geochemical signatures consistent with formation in a continental rift setting the quartzite basalt contact exhibits rare stratigraphic interlayering and is a horizon along which oxide and silicate facies iron formation dolostone and pelite locally occur ultramafic rocks with komatiitic compositions also intrude these lower stratigraphic units the upper murmac bay group is dominated by pelitic to psammitic rocks that may represent deepening of the sedimentary basin during continued extension ancient approximately 3 62 and 3 7 ga grains dominate the detrital zircon population of the murmac bay group sediments but deposition occurred after approximately 2 77 ga the youngest detrital zircon in our study the age of the murmac bay group is thus constrained as younger than approximately 2 77 ga and older than the crosscutting 2631 12 ma dead man and donaldson lake granites the gross lithological and stratigraphic make up of the murmac bay group closely resembles that of the prince albert woodburn lake and mary river groups of the northern rae province all three of which have been interpreted as approximately 2 7 ga continental rift successions we propose that the murmac bay group is correlative with these successions if correct the total length of this neoarchean rift is at least 2000 km extending from baffin island to northwestern saskatchewan canada the supracrustal rocks of the rift record the break up of a large paleo to early neoarchean continent nunavutia which experienced magmatic events at approximately 3 0 3 62 and 3 7 ga

nd isotopic evolution of the taltson magmatic zone northwest territories canada insights into early proterozoic accretion along the western margin of the churchill province the taltson magmatic zone is a north trending belt of early proterozoic granitoids exposed over an area exceeding 18 000 km 2 in the northwest territories of canada three petrologically and geochronologically discrete intrusive suites comprise the taltson zone these are the 1986 ma deskenatlata suite the 1955 to 1925 ma slave suite and the 1935 ma konth suite the deskenatlata suite is composed of biotite hornblende diorite to granite and displays a more mafic geochemical character relative to the other two intrusive suites initial epsilon nd values for the deskenatlata suite range from 2 7 to 3 6 the slave and konth suites are compositionally heterogeneous to homogeneous leucocratic granitoids that contain numerous xenoliths of dominantly pelitic high grade paragneiss and display a peraluminous granite mineralogy and geochemistry initial epsilon nd values for the konth suite cluster within 4 7 and 5 5 and are completely overlapped by the 4 1 to 6 0 range of the pelitic xe

north mountain basalt from digby nova scotia models for a fissure eruption from stratigraphy and petrochemistry the early mesozoic quartz normative north mountain basalts in southwestern nova scotia digby area form three units a coarse massive lower flow 190 m bearing minor lenses of mafic pegmatite a middle unit of thin amygdaloidal basaltic flows 50 m and an upper flow unit of massive phenocryst rich basalt 160 m the two thick units show phenocrysts of orthopyroxene bronzite and or pigeonite augite and zoned plagioclase in a granular matrix of augite pigeonite and plagioclase variation diagrams and chondrite normalized rare earth element patterns relate all chemical diversity between and within flows to removal and or accumulation of plagioclase and pyroxene phenocrysts 1 1 high k rb and ba appear related to assimilation of continental crust constancy of fractionation independent element ratios and variations in phenocryst content vertically and along the 200 km strike of the basalts suggest 1 crystal settling and accumulation together with assimilation and mixing in a lower

precambrian crustal evolution seismic constraints from the canadian shield whether or not plate tectonic processes operated on a younger hotter earth remains ambiguous seismic data from new networks in the hudson bay region of the canadian shield where the precambrian geological record spans more than 2 billion years offer fresh scope to address this problem using receiver function analyses we show that the crust of the rae domain which exhibits ages of paleo to neoarchean 3 9 2 7 ga is likely felsic to intermediate in composition average vp vs 1 73 and seismically transparent with a sharp moho there is little evidence for modern style plate tectonics and based on the simplicity and spatial extent of the felsic crust models favouring vertical tectonic processes such as crustal delamination or plume activity appear better suited to the results data from the hearne domain which exhibits widespread 2 7 ga granite and greenstone geology show a more complex crust with higher vp vs ratios consistent with a greater mafic component the trans hudson orogen tho proposed to be a himalayan scale mountain belt during the paleoproterozoic is thought to have formed during the 1 8 ga collision of the superior and churchill plates results from the quebec baffin island segment of the tho appear to map out the first order shape of the underthrusting superior plate with elevated vp vs ratios likely representing the rifted margin of the superior craton consistently thicker crust is observed beneath central and southern baffin island 43 km coincident with widespread high grade metamorphic surface geology these features can be explained by crustal thickening due to stacking of accreted terranes during continent continent collision analogous to the present day tibetan plateau followed by erosion when reviewed in light of age and compositional constraints from the geological record our seismic observations point towards secular crustal evolution from non plate tectonic during the paleo to mesoarchean evolving towards fully developed modern style plate tectonics during the paleoproterozoic

accretion of the archean slave province continental rift models have long been applied to the archean slave province of northwestern canada a reassessment of these models shows them to be incompatible with observed geological relations and suggests that contractional tectonic models may be more appropriate than extensional ones regions composed of different rock suites e g orthogneisses vs mafic volcanics are separated by high strain zones recording large displacements it is proposed that the high strain zones separate four distinct terranes that have been juxtaposed during collisional orogenesis from west to east these include the anton terrane interpreted as an archean microcontinent the sleepy dragon terrane possibly an exhumed more eastern part of the anton terrane the contwoyto terrane a westward verging fold and thrust belt containing tectonic slivers of greenstone volcanics and the hackett river volcanic terrane interpreted as an archean island arc the contwoyto and hackett river terranes represent a paired accretionary prism and island arc system that formed above an east dipping subduction zone these collided with the anton microcontinent producing a basement nappe expressed as the sleepy dragon terrane during the main accretion event within the slave province the whole tectonic assemblage was intruded by late kinematic to postkinematic granitoids

an archean quartz arenite andesite association in the eastern baltic shield russia implications for assemblage types and shield history abstract shallow water sedimentary units are generally considered scarce in archean greenstone belts we describe an unusual quartz arenite subaerial andesite association within the archean hisovaara greenstone belt a fragment of the parandovo tikshozero belt within the karelian craton of the baltic shield the hisovaara greenstone belt consists of several lithotectonic assemblages 1 a komatiite tholeiite assemblage 2803 ma based on ages of cross cutting dikes 2 an andesite quartz arenite assemblage cut by similar dikes 3 an assemblage of coarse volcaniclastic rocks and 4 an upper mafic assemblage of tholeiitic basalts with minor pyroxene komatiite volcanic rocks the andesite quartz arenite assemblage 100 to ca 750 m thick has basal amygdaloidal fragmental andesites overlain by massive andesites then amygdaloidal and plagioclase phyric andesites unconformably overlying the andesite is a unit of quartz rich sandstones 6 40 m thick dominated by quartz arenite extending several km along strike at the north end the quartz arenite succession consists of basal andesite overlain by quartz arenite exhibiting hummocky cross stratification followed by aluminous coarse metasediments and sulfidic argillite and an unconformably overlying tholeiite komatiite unit at the south end the succession is basal andesite regolith cross bedded quartz arenite weathered andesites a second quartz arenite argillite and then subaerial rhyolite ree and hfse geochemistry has been obtained on the rocks of the andesite quartz arenite assemblage the quartz arenites contain low abundances and chondrite normalized patterns vary from relatively fractionated to flat with most of the variation related to grain size with pebbly units having higher abundance and more fractionated patterns combined major and trace element geochemistry indicates that a sodic felsic source with some admixture of mafic material will explain the geochemistry of the quartz arenites the andesites display moderately fractionated spidergrams with negative anomalies at ti ta and nb typical of arc volcanism the andesite quartz arenite assemblage represents accumulation of shallow water quartz rich sediments in a setting typical of the later stages of arc volcanism in which the volcanic edifice is subaerial at the southern end of the assemblage however at the north end our evidence is interpreted as indicating subaerial andesitic volcanism subsidence to a shallow marine basin which then deepens and rifts therefore the hisovaara andesite quartz arenite assemblage provides a linkage in archean greenstones between assemblages representing continental volcanism and a platform to rift setting the presence of an erosional interval in 2 8 ga greenstones suggests possible pre 2 7 ga orogeny in the baltic shield the pre 2 7 ga quartzrich sedimentation is similar in age to platformal assemblages in the pre 2 7 ga north caribou terrane of the superior province canada

formation of an archaean continent about 30 of the earth is covered by continents but only about 10 small kernels of these continentsae known as archaean cratonsae are continental fragments formed before 2 5 gyr ago the kaapvaal craton of south africa which formed and stabilized between 3 7 and 2 7 gyr ago is one of the oldest reasonably sized examples of these continental fragments it consists of a mosaic of subdomains that have been welded together by processes similar to those of modern day plate tectonics the earliest subdomains may have owed their origin to the onset of efficient recycling from the earth s hydrosphere into the mantle

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

tectonic history of the barberton greenstone belt south africa 490 million years of archean crustal evolution rocks of the 3570 3080 ma barberton greenstone belt record a geological history of at least 490 million years m y and were affected by regional thermal perturbations for over 1500 m y small tectonic wedges of 3570 3511 ma schistose tonalite are the oldest rocks yet recognized but little is known about their tectonothermal history the earliest well recognized tectonothermal events 3490 3450 ma represent mid ocean ridge like processes these were followed by two periods of arc related and trench related processes separated by 160 m y the first 3445 3416 ma recording an intraoceanic suprasubduction like environment and the second 3260 3225 ma intra arc and interarc like processes culminating in arc amalgamation accretion like convergent processes dominated between 3230 ma and 3080 ma overlapping with the assembly of the kaapvaal craton at 3080 ma the barberton greenstone belt experienced a sudden shift from dominantly convergent to dominantly transtensional tectonism and was mineralized with mesothermal gold along a regional shear system soon afterward transtension became distributed across the entire kaapvaal craton heralding the onset of formation of the witwatersrand basin by 3070 ma

ensialic origin for the ngezi group belingwe greenstone belt zimbabwe volcanic rocks of the ngezi group in the belingwe greenstone belt have been interpreted as continental basalts extruded onto or oceanic crust obducted onto a succession of stable shelf sedimentary rocks that nonconformably overlie continental basement the volcanic succession locally contains quartzose sandstones and the stratigraphy is unlike ocean crust primary volcanic and sedimentary structures indicate a shallow subaqueous to subaerial origin all structural features are compatible with later refolding of an autochthonous successon there is no structural manifestation of obduction these criteria refute an oceanic setting for the volcanic rocks rather they are consistent with an ensialic origin for the ngezi group

geology lithogeochemistry and u pb geochronology of the aberdeen lake area nunavut new insights into the neoarchean tectonic evolution of the central rae domain abstract the aberdeen lake supracrustal belt is one of several neoarchean greenstone belts preserved within the rae domain of the western churchill province we divide the belt into 1 a lower sequence of ca 2750 ma komatiite ttg tonalite trondhjemite granodiorite intrusions and ttg contaminated fe tholeiitic mafic gneiss 2 a middle sequence of ca

age and evolution of the lower crust beneath the western churchill province u pb zircon geochronology of kimberlite hosted granulite xenoliths nunavut canada abstract zircon from granulite facies xenoliths can preserve unique geodynamic records of deep and mostly inaccessible levels of the continental crust within archean cratons we present the first such records from beneath two archean crustal blocks that were affected by multiple episodes of regional scale proterozoic metamorphism and are located within the western churchill province nunavut canada zircon u pb geochronology and trace element analysis were carried out on samples that originated in the deep crust at the time of kimberlite eruption as confirmed with raman spectroscopy zircon was recovered from four metabasites grt cpx plg from the rankin inlet area of the chesterfield block and five metabasites grt cpx plg hbl opx and three metatonalite anorthosites plg cpx plg hbl opx from the repulse bay area of the rae craton sem cl imaging reveals several types of growth zoning diagnostic of igneous and metamorphic zircon crystallization the latter are related to local high temperature recrystallization and micro fracturing and are spatially associated with zones of apparent pb loss and or intracrystalline pb variability evidence for the formation of paleoarchean mesoarchean to neoarchean crust is preserved in oscillatory zoned igneous cores having 207pb 206pb ages of ca 3 5 ga and ca 3 0 2 6 ga two periods of paleoproterozoic metamorphism and zircon growth followed with the oldest episode at ca 2 0 1 9 ga in the chesterfield block metamorphic zircon rims from all twelve xenoliths have an age range of ca 1 75 to 1 70 ga the trace element compositions of the metamorphic rims differ from those of older igneous and metamorphic cores with the rims characterized by lower hree contents and low lu gd n suggesting that garnet crystallization occurred syn to pre rim growth at ca 1 75 ga the archean and paleoproterozoic ages correspond broadly to the upper crustal histories of the western churchill province however the prevalence of ca 1 75 1 70 ga metamorphic rims indicates regional scale metamorphic reworking of the lower crust across the region during a period of intracratonic extension and associated magmatism regional magmatic underplating and or remobilization of the lithosphere at ca 1 75 1 70 ga to an extent not widely appreciated represents a terminal stage in western churchill cratonization including stabilization of reworked archean crust relative to underlying diamondiferous mantle

sedimentology of quartz pebble conglomerates and quartzites of the archean bababudan group dharwar craton south india evidence for early crustal stability the 3200 to 3000 ma bababudan group locally as thick as 1800 m consists of a variety of rocks indicating deposition on a stable platform u and au bearing pyritiferous quartz pebble conglomerate a few meters thick overlies a regolith at the basal unconformity with a gneissic basement the bulk of the column consists of mafic volcanics including subaerial flows lahars and sills ultramafic and felsic rocks iron formation and pelites are minor lithologies in most of the sequence but iron formation forms a major unit at the top of the group quartzite units intercalated with the volcanics are generally less than 40 m thick and make up 25 to 50 of the column they are mineralogically mature and texturally submature recrystallization generally obscures original textures but units that were originally clayey quartz arenites have survived total recrystallization rounded zircons dominate the nonopaque detrital heavy mineral suite verifying a history of extensive abrasion paleocurrent patterns bas

the central slave basement complex part i its structural topology and autochthonous cover new field and geochronological data are used to define the distribution of mesoarchean basement rocks in the south central slave province this distribution reflects a single contiguous basement terrane that we propose to call the central slave basement complex it shows a structural topology that is internally consistent and compatible with known regional folding and faulting events a sample of a proposed basement gneiss below the courageous lake greenstone belt central slave province has been dated by u pb methods and yields an age of 3325 8 ma consistent with the new basement distribution this sample also contains 2723 3 ma metamorphic zircon and ca 2680 ma titanite the central slave basement complex is overlain by a thin discontinuous but distinctive cover sequence that includes minor volcanic rocks clastic sedimentary rocks and banded iron formation all previously known and some new occurrences of this distinctive cover sequence occur in the immediate stratigraphic hanging wall of the