origin and tectonometamorphic history of the repulse bay block melville peninsula nunavut exotic terrane or deeper level of the rae craton the rae craton on melville peninsula nunavut comprises several lithotectonic domains including a structurally and lithologically distinct yet poorly known crustal terrane the repulse bay block rbb this study presents new lithological and petrographic observations combined with u pb zircon data to better understand the archean and paleoproterozoic crustal evolution of the rbb the new data demonstrate that the central eastern rbb consists of the following i upper amphibolite to granulite facies ca 2 73 2 71 ga intermediate granitoid gneisses and gabbroic sheets ii ca 2 69 ga two pyroxene charnockite to enderbite intrusions and iii thin slivers of both archean and paleoproterozoic supracrustal rocks inherited zircon also attests to the presence of a mesoarchean to paleoarchean substrate a semi pelitic gneiss from one of the paleoproterozoic supracrustal panels was deposited sometime after ca 1 89 ga and shows a similar detrital zircon age profile to a 1 92 ga semi pelitic gneiss fr

the origin of ferrous zoning in allende chondrule olivines abstract olivine crystals in allende type 1 chondrules have ferrous rims fa15 44 a few microns thick at grain surfaces and along crosscutting cracks this ferrous olivine contains as much as 2 atomic pet al and cr and 0 2 pct ti along cracks profiles of fe al cr and ti are consistent with difrusional emplacement these elements moved from an external source through the cracks as a vapor or by grain boundary diffusion then into the olivine by lattice diffusion the emplacement did not occur during an epoch of planetary metamorphism that partly equilibrated matrix and chondrule olivines because e g neither matrix nor primary magnesian chondrule olivine contains even 1 10 this much al cr and ti we argue that the source of these elements was the nebular gas and emplacement occurred prior to accretion the ferrous zones at olivine grain boundaries do not display simple diffusion profiles rather they appear to be a composite of two concentric rims the inner rims have compositions similar to those of the ferrous zones along cracks and contain the highest concentrations of al cr and ti the outer rims up to 20 m thick where grain boundaries are at the surfaces of chondrules are of uniformly high fa content and relatively low al cr and ti concentration these outer rims appear to be additional material that condensed onto the chondrules after the diffusional emplacement event discussed earlier matrix olivine grains which are identical to these rims in composition condensed simultaneously

collisional snowbird tectonic zone resurrected growth of laurentia during the 1 9 ga accretionary phase of the hudsonian orogeny the 2800 km long snowbird tectonic zone is one of the most controversial tectonic features of the canadian shield metamorphic and in situ geochronologic data reported here reveal that a 1 9 ga medium to high pressure belt extends along most of this tectonic zone in contrast to recent interpretations a collisional origin is indicated by the length of this metamorphic belt tectonic thickening documented in parts of it geological contrasts across it and subduction type microdiamonds within it this collisional event marks a pre 1 865 ga phase of the hudsonian orogeny involving microcontinent accretion that was fundamental to the growth of laurentia

absolute transports of mass and temperature for the north atlantic current subpolar front system the flow of subtropical waters carried into the northern north atlantic ocean by the north atlantic current subpolar front system nac spf is an important component of the meridional overturning circulation these waters become colder and denser as they flow through the subpolar region both by mixing with the colder subpolar waters and by atmospheric cooling the relative roles of these two processes remain to be quantified and the mechanisms driving lateral mixing need to be better understood to address those questions a new methodology is developed to estimate the mean absolute transports of mass and heat for the top 1000 dbar in the region of the nac spf for the time period 1993 2000 the transports are obtained by combining historical hydrography with isopycnal rafos float data from the area the mean absolute transport potential field shows an nac spf pipe defined by two bounding transport potential contours this pipe transports 10 0 6 3 5 sv sv 106 m3 s 21 top 1000 dbar from the subtropics into the eastern subpolar north atlantic in contrast to earlier studies the northward flowing nac follows a distinct meandering path with no evidence of permanent branches peeling off the current before reaching the northwest corner as the current enters the northwest corner it loses its tight structure and maybe splits into two or more branches which together constitute the eastward flow along the spf the eastward flow between the northwest corner and the mid atlantic ridge is not as tightly defined because of the meandering and or eddy shedding of the branches constituing the spf as the flow approaches the mid atlantic ridge it converges to cross above the charlie gibbs and faraday fracture zones the mean absolute temperature transport top 1000 dbar by the 10 sv pipe was estimated across 10 transects crossing the nac spf because the mean mass flux is constant in the pipe variations in the mean temperature transports result from lateral exchange and mixing across the pipe s side walls and from air sea fluxes across the surface of the pipe the nac spf current loses 0 18 6 0 05 pw on its transit through the region most of the loss occuring upstream of the northwest corner the heat loss is 10 times the corresponding heat lost to the atmosphere we conclude that cross frontal exchange induced by the steep meanders of the northward flowing nac is the main mechanism by which heat is lost along the current in the region between the tail of the grand banks and the mid atlantic ridge

mineralogy and petrology of chondrules and inclusions in the mokoia cv3 chondrite all objects 100 m in apparent diameter in five polished thin sections of the mokoia cv3 chondrite were studied and classified number and volume percentages and mean apparent size of each type of chondrule and inclusion were determined three major types of olivine chondrules were observed igneous chondrules recrystallized chondrules and chondrules that appear to be accretional aggregates coarse grained cai s have igneous textures and mineral parageneses while fine grained cai s are aggregates containing varying proportions of al rich concentric objects ca rich chaotic material and inclusion matrix chondrules and refractory inclusions in mokoia and allende are broadly similar in texture and mineral chemistry but mokoia refractory inclusions contain phyllosilicates rather than feldspathoids and melilite rich cai s are more abundant in allende we think that most cai s formed during the metamorphism partial melting and incomplete distillation of primitive dust aggregates when they were heated in the solar nebula in the process ca rich melt appears to have been physically separated from al rich residues producing the observed fractionation of ca from al into distinct constituents of cai s some cai s may be aggregates of devitrified amorphous metastable condensates inclusion matrix may have condensed from silicate rich vapors produced during distillation mokoia inclusion matrix contains phyllosilicates that are probably primitive nebular material

phyllosilicates in the mokoia cv carbonaceous chondrite evidence for aqueous alteration in an oxidizing environment abstract most cv chondrites contain little if any phyllosilicate mineralization a petrographic and transmission electron microscopy study of the mokoia cv carbonaceous chondrite shows that the matrix chondrules aggregates and inclusions all contain considerable amounts of phyllosilicates those in the matrix are mostly fe bearing saponite while those in chondrules aggregates and inclusions are both febearing saponite and na rich phlogopite serpentine occurs as minor coherent intergrowths within the phlogopite the saponite resulted from aqueous alteration of fe rich olivine and the phlogopite resulted from alteration of ca rich clinopyroxene anorthite spinel and ca al si rich glass iron rich olivines in the mokoia matrix contain extremely thin planar zones parallel to 100 with diffuse streaks along a in their diffraction patterns such planar zones are not known from terrestrial olivines saponite forms preferentially along these zones with its 001 planes roughly parallel to olivine 100 the planar zones probably consist of precipitates of fe oxide or fe hydroxide that were produced during oxidation of olivine while it cooled from a high temperature by formation of the fe rich precipitates olivine became fe deficient and thus was structurally disturbed during subsequent hydration saponite formed by preferentially replacing these regions in olivine simultaneously part of the fe was mobilized and precipitated as magnetite along the olivine peripheries the mineralogy and occurrence of phyllosilicates in mokoia differ from those in the ci and cm chondrites the differences suggest that aqueous alteration of the three meteorite groups probably occurred under a variety of conditions

temporal variability of the subarctic front near the charlie gibbs fracture zone we document temporal variability of the northernmost branch of the north atlantic current known as the north subarctic front nsaf for the period 1976 1985 the front s location and temperature salinity t s parameters were determined from 120 repeat transects along three standard sections occupied by russian ships 1 ocean weather station ows c 52 75 n 35 5 w to st john s newfoundland 2 along 35 w from 52 n to 36 5 n and 3 ows c to cape st vincent portugal the nsaf exhibits significant interannual variability of its location shifting 200 km to 300 km in a few months along the ows c st john s section the front reached its southernmost location in 1976 1977 then experienced two 250 to 300 km northward shifts in 1978 1979 and 1980 1981 all of the three shifts peaked in wintertime these events nearly coincide with events of the opposite sign along the ows c cape st vincent section the northernmost location in 1976 1977 and two 200 km southward shifts in 1978 1979 and 1980 1981 the nsaf s behavior is thus coherent near the charlie gibbs fracture zone at the 500 km scale frontal time series associated with the moving front of t and s at the 200 m level for the cold side of the nsaf reveal two events an s200 minimum in the winter of 1976 1977 which might be a manifestation of the great salinity anomaly of the 1970s and a sharp drop of s200 in early 1984 which might signal the arrival of another large salinity anomaly the great salinity anomaly of the 1980s

directly measured mid depth circulation in the northeastern north atlantic ocean the circulation of water masses in the northeastern north atlantic ocean has a strong influence on global climate owing to the northward transport of warm subtropical water to high latitudes but the ocean circulation at depths below the reach of satellite observations is difficult to measure and only recently have comprehensive direct observations of whole ocean basins been possible here we present quantitative maps of the absolute velocities at two levels in the northeastern north atlantic as obtained from acoustically tracked floats we find that most of the mean flow transported northward by the gulf stream system at the thermocline level about 600 m depth remains within the subpolar region and only relatively little enters the rockall trough or the nordic seas contrary to previous work our data indicate that warm saline water from the mediterranean sea reaches the high latitudes through a combination of narrow slope currents and mixing processes at both depths under investigation currents cross the mid atlantic ridge preferentially over deep gaps in the ridge demonstrating that sea floor topography can constrain even upper ocean circulation patterns

u pb zircon geochronology and depositional history of the montresor group rae province nunavut canada paleoproterozoic metasedimentary successions of the northwestern canadian shield provide records of tectonic events but the definition of depositional ages has proved elusive although previously

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

fayalite rich rims veins and halos around and in forsteritic olivines in cais and chondrules in carbonaceous chondrites types compositional profiles and constraints of their formation abstract fayalite rich rims veins and halos around and in forsteritic olivines are a wide spread phenomenon in chondrules ca al rich inclusions cais and single grains in carbonaceous chondrites the presence of fayalite rod like crystals and laths in rims veins in wall of pores and as fluffy network bridging neighbouring olivines pyroxenes feldspars etc is strongly suggestive that the fayalitic olivine was formed by condensation presumably from the solar nebula gas the formation of the fayalitic olivine was probably caused by an increase in the h 2 o h 2 ratio to a ratio between 0 1 1 subsequent to condensation of forsterite at that stage feni inclusions in olivine were also oxidized and fayalitic halos around the metal were then formed by fe diffusion along with addition of sio 2 from the solar gas or loss of mgo to the solar gas the fa rich olivine rims and veins display a narrow compositional variation between fa 34 and fa 46 subsequent to condensation of fa rich olivine and oxidation of feni metal fe diffused in forsterite this diffusion was probably enhanced due to the presence of point defects in olivine or the formation of a nonstoichiometric phase analogous to laihunite enriched in al 2 o 3 and cr 2 o 3 however the presence of al 2 o 3 and cr 2 o 3 rich discrete domains cannot be excluded cooling rates calculated by modelling of the diffusion profiles are indicative of rapid cooling subsequent to the condensation of fayalitic olivines we obtain cooling rates ranging from 2000 c day and 10 c day at an initial temperature of 1200 c and 900 c respectively

mineralogy and petrology of amoeboid olivine inclusions in co3 chondrites relationship to parent body aqueous alteration petrographic and mineralogic studies of amoeboid olivine inclusions aois in co3 carbonaceous chondrites reveal that they are sensitive indicators of parent body aqueous and thermal alteration as the petrologic subtype increases from 3 0 to 3 8 forsteritic olivine fa0 1 is systematically converted into ferroan olivine fa60 75 we infer that the fe si and o entered the assemblage along grain boundaries forming ferroan olivine that filled fractures and voids as temperatures increased fe 2 from the new olivine exchanged with mg 2 from the original aoi to form diffusive haloes around low feo cores cations of mn 2 ca 2 and cr 3 were also mobilized the systematic changes in aoi textures and olivine compositional distributions can be used to refine the classification of co3 chondrites into subtypes in subtype 3 0 olivine occurs as small forsterite grains fa0 1 free of ferroan olivine in petrologic subtype 3 2 narrow veins of feo rich olivine have formed at forsterite grain boundaries with increasing alteration these veins thicken to form zones of ferroan olivine at the outside aoi margin and within the aoi interior by subtype 3 7 there is a fairly broad olivine compositional distribution in the range fa63 70 and by subtype 3 8 no forsterite remains and the high fa peak has narrowed fa64 67 even at this stage there is incomplete equilibration in the chondrite as a whole e g data for coarse olivine grains in isna co3 8 chondrules and lithic clasts show a peak at fa39 we infer that the mineral changes in aoi identified in the low petrologic types required aqueous or hydrothermal fluids whereas those in subtypes 3 3 largely reflect diffusive exchange within and between mineral grains without the aid of fluids

matrices of carbonaceous chondrite meteorites the morphology classification and chemistry of the matrices of carbonaceous chondrite cc meteorites is reviewed based on recent research results the various kinds of ccs are examined in terms of their matrix mineralogy alteration processes in ccs are discussed

early paleoproterozoic supracrustal assemblages of the rae domain nunavut canada intracratonic basin development during supercontinent break up and assembly abstract the basal paleoproterozoic supracrustal succession of the churchill province in northern canada comprises sedimentary basins overlying the archean rae and hearne domains the rae domain supracrustal succession e g amer ketyet river chantrey and montresor groups comprises four lithotectonic assemblages which include in ascending stratigraphic position 1 shallow marine fluvial quartzarenite with mainly neoarchean detrital zircons of local provenance 2 shallow marine quartzarenite with mainly local provenance that locally includes carbonates mafic volcanic flows 3 abrupt but conformable transition to basinal dark shales and overlying platformal carbonates that pass upward into siltstones and sandstones that record gradual emergence of the platform sandstones within this assemblage display a substantially different detrital zircon provenance signature dominated by early paleoproterozoic zircons with grains as young as 1 95 ga 4 unconformably to conformably overlying immature marine sandstones including turbidites with a significant component of 2 0 1 9 ga detritus probably derived from a combination of local and distal sources the thickness and relative compositional maturity of the basal lithotectonic assemblage 1 records a protracted period of enhanced weathering on a slowly subsiding but stable supercontinent e g kenorland this was followed by mafic volcanism and marine transgression consistent with partial rifting of the rae domain and crustal extension attending supercontinent break up lithotectonic assemblage 2 strata of lithotectonic assemblage 3 record basin flooding and deepening followed by platformal sedimentation along the margins of extended continental blocks assemblage 4 is interpreted to represent deposition in foreland basins during early amalgamation of laurentia part of nuna at ca 1 9 ga the stratigraphy and detrital zircon geochronology of the hurwitz group which overlies the hearne domain is broadly similar to the supracrustal sequence of the rae domain but differs through the presence of basal diamictites and lack of 2 5 ga detritus a major local source in the rae our data suggest that the rae and hearne domains were assembled together by max 1 91 ga i e max age of deposition of the uppermost hurwitz and amer groups the paleoproterozoic stratigraphic assemblage framework for the churchill province compares well with that of other cratons derived from break up of kenorland and ultimately amalgamated in nuna examples from laurentia include supracrustal rocks that overlie the southern margin of the superior province southern province e g huron supergroup animikie group marquette range supergroup whitewater group and mistassini otish group the paleoproterozoic assemblages of the southern and churchill provinces exhibit first order stratigraphic similarities that indicate a shared history of stable continental sedimentation followed by rifting basin opening and basin closure

the amer belt remnant of an aphebian foreland fold and thrust belt aphebian supracrustal sequences occur as outliers throughout the northwestern portion of the churchill structural province of the canadian shield in the amer lake area medium to high grade poly

trans hudsonian far field deformation effects in the rae foreland an integrated geological 3d magnetic model abstract the intracratonic rae cover sequence deposited ca 2 2 1 9 ga forms a useful marker for unravelling tectonic events that affected the archean rae province at ca 2 0 1 9 and 1 85 ga polyphase deformation is recognized within the rae cover rocks including the 70 10 km montresor belt and attributed to distal effects of the ca 1 85 ga trans hudson orogeny in this contribution we explore the 3d geometry and structural history of the montresor belt previously considered to be a simple syncline lying unconformably on archean basement new geological geophysical and geochronological results define a more complex history in which lower montresor units were thrust imbricated with basement gneisses and metamorphosed to the amphibolite facies mid to upper greenschist facies upper montresor units exposed in an open synform are superficially less deformed however using high resolution aeromagnetic data and distinct magnetic marker units considered proxies for bedding we constructed a set of forward models to explore the three dimensional geometry of the belt the re analysis outlines a set of pre synform structures defined by low angle truncations of the magnetic markers geometric relationships indicate the presence of at least three faults at low angles to bedding interpreted as d1 piggy back thrusts and bracketed by available geochronology between 1 924 and 1 87 ga d1 strain in the upper montresor strata is significantly less intense than that further south in rae cover rocks consistent with a more distal foreland setting during the trans hudson orogeny the montresor belt preserves a record of the trans hudson tectonic style at relatively shallow crustal levels as a result of its foreland setting and structural history including a syn orogenic extensional detachment event

sedimentological and geochemical basin analysis of the paleoproterozoic penrhyn and piling groups of arctic canada abstract paleoproterozoic metasedimentary successions on the western churchill province record a history of regional tectonic events spanning the breakup and amalgamation of supercontinents metasedimentary successions in the northeastern part of the trans hudson orogeny including the penrhyn and piling groups of melville peninsula and baffin island respectively have been understudied with respect to their basin evolution records we present new sedimentological geochemical and geochronological data for the penrhyn and piling groups that record deposition on the rae craton new detrital zircon u pb age data indicate that deposition of the penrhyn group started after 1897 ma and post dates deposition of most of the piling group the new maximum age constraints and stratigraphy of the penrhyn group do not support previous lithostratigraphic correlation with the piling group deposition of the lower piling group likely occurred in an epicontinental basin setting the occurrence of detritus from the meta incognita microcontinent in the upper piling group bravo lake and longstaff bluff formations is interpreted to signal the onset of foreland basin deposition the details of sedimentation in the piling foreland basin provide an additional constraint to discerning the polarity of subduction between the rae craton and meta incognita microcontinent that closed the piling basin we conclude that the piling foreland system shows characteristics of a pro foreland basin implying that the rae craton was the lower plate in the collision with the meta incognita microcontinent finally with new stratigraphic and detrital zircon geochronological data for the penrhyn and piling groups we provide a comparison with other sedimentary successions on the rae craton whose histories were influenced by similar regional tectonic events

structural petrological and u pb shrimp geochronological study of the western beaverlodge domain implications for crustal architecture multi stage orogenesis and the extent of the taltson orogen in the sw rae craton canadian shield abstract the beaverlodge domain is a polydeformed precambrian terrane in the southwestern rae craton whose rocks variably record the effects of archean tectonism and four major paleoproterozoic tectonic events the 2 5 2 3 ga arrowsmith orogeny the 1 99 1 93 ga taltson orogeny the ca 1 90 ga snowbird orogeny and the ca 1 87 1 80 ga hudsonian orogeny structural metamorphic analysis and in situ u pb shrimp zircon and monazite geochronology reported herein demonstrate that this domain consists of two main structural levels variably exposed due to complex interference folding a lower level comprising mainly high grade archean ca 3 0 and 2 7 2 6 ga metaplutonic and metasedimentary gneisses and a higher level comprising mainly ca 2 33 to 1 2 that led to development of the composite ese trending s 0 s 1 s 2 transposition foliation followed by more open folding d 3 the absence of taltson age plutonic rocks except for low volume crustal melts suggests that this cycle was driven by crustal thickening consistent with a clockwise p t t path derived for one sample the high degree of strain along the contact between archean rocks and the murmac bay group along with local structural discordance indicates some degree of translation of cover rocks over basement during this stage pelitic rocks at higher structural levels low to medium grade zone underwent prograde metamorphism to middle amphibolite facies whereas rocks at deeper structural levels high grade zone attained upper amphibolite facies conditions monazite data bracket prograde metamorphism and related garnet growth between 1939 6 ma and 1934 5 ma the second stage dated most precisely at 1906 7 ma involved refolding of d 1 to d 3 structures about ne trending axes f 4 and coeval dextral shearing and is correlated with snowbird orogenesis rocks of the low to medium grade zone were held at roughly the same crustal level whereas rocks in the high grade zone were uplifted via dextral oblique displacement along domain bounding shear zones instigating decompression garnet to cordierite reactions this study demonstrates that after

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

new insights into the formation of fayalitic olivine from allende dark inclusions abstract although considerable progress has been made in unraveling the origin s of fayalitic olivines in dark inclusions dis many questions remain still unresolved and or controversial we combine a chemical and petrographic study of the allende dark inclusion 4884 2b amnh new york and atem studies of a fragment of the dark inclusion allende af nhm vienna and discuss an alternative way in which fayalitic olivines could have formed allende dark inclusion 4884 2b contains a few aggregates with variable proportions of transparent and feathery olivine two such objects aggregates a and b are the focus of this study as they preserve glasses that can help in deciphering the nature of the processes involved during olivine growth and subsequent olivine transformation the petrographic and chemical characteristics of aggregates a and b indicate that the forsteritic stack olivines may be pseudomorphs of clear olivine crystals the atem studies in all af suggest that fayalitic olivines may be the result of secondary processes e g metasomatic exchange reactions operating in the solar nebula transformation may have occurred through the mediation of a dry gas phase involving nonvolatile major elements such as mg and fe e g dohmen et al 1998 this mechanism could reveal olivine growth patterns e g stacked platelets due to a rapid growth regime and may have contributed to the development of their fibrous aspect while preserving the shape i e volume of the crystals this highly selective process did completely or partially transform ferromagnesian minerals but affected the fine grained mesostasis only slightly

neoarchean continental crust formation and paleoproterozoic deformation of the central rae craton committee bay belt nunavut integrated mapping structural analysis and u pb geochronology of the committee bay area nunavut establish a record of neoarchean crustal growth followed by penetrative paleoproterozoic deformation supracrustal rocks include a lower ca 2 73 ga mafic ultramafic volcanic dominated sequence a middle economically significant 2 71 ga intermediate volcanic bearing sequence with intercalated sulphidized gold bearing iron formation and an upper 2 69 ga clastic komatiite quartzite sequence following an 80 million year hiatus this succession was intruded by voluminous ca 2 61 2 57 ga granodiorite tonalite granite diorite which do not appear to have thermally or tectonically affected the supracrustal belt instead three generations of structures record polyphase paleoproterozoic deformation of the region d1 structures are consistent with a doubly vergent structural fan developed at ca 2 35 ga in response to the arrowsmith orogeny that affected the western rae margin penetrative d2 structures do

fe ni metal in primitive chondrites indicators of classification and metamorphic conditions for ordinary and co chondrites we report the results of our petrological and mineralogical study of fe ni metal in type 3 ordinary and co chondrites and the ungrouped carbonaceous chondrite acfer 094 fe ni metal in ordinary and co chondrites occurs in chondrule interiors on chondrule surfaces and as isolated grains in the matrix isolated ni rich metal in chondrites of petrologic type lower than type 3 10 is enriched in co relative to the kamacite in chondrules however ni rich metal in type 3 15 3 9 chondrites always contains less co than does kamacite fe ni metal grains in chondrules in semarkona typically show plessitic intergrowths consisting of submicrometer kamacite and ni rich regions metal in other type 3 chondrites is composed of fine to coarse grained aggregates of kamacite and ni rich metal resulting from metamorphism in the parent body we found that the number density of ni rich grains in metal number of ni rich grains per unit area of metal in chondrules systematically decreases with increasing petrologic type thus fe ni metal is a highly sensitive recorder of metamorphism in ordinary and carbonaceous chondrites and can be used to distinguish petrologic type and identify the least thermally metamorphosed chondrites among the known ordinary and co chondrites semarkona is the most primitive the range of metamorphic temperatures were similar for type 3 ordinary and co chondrites despite them having different parent bodies most fe ni metal in acfer 094 is martensite and it preserves primary features the degree of metamorphism is lower in acfer 094 a true type 3 00 chondrite than in semarkona which should be reclassified as type 3 01

high temperature condensation of iron rich olivine in the solar nebula abstract olivine is the most abundant mineral in many chondritic meteorites it is also the first major mg silicate to condense from a solar gas upon cooling the composition of olivine provides important information concerning its origin results of calculations are presented for the fe mn and ni contents of olivine condensing from a gas with solar composition but variable oxygen fugacity a basic computational method is described that takes into account the depletion of the gas in mg and si during condensation the first olivine that condenses from a solar gas has less than 10 2 mol feo independent of the oxygen fugacity of the otherwise solar gas at lower temperatures when most of the mg is condensed more feo can be incorporated in forsterite by condensation if the oxygen fugacity is sufficiently high the feo mno ratio of the first condensing olivine is above 1000 decreasing with decreasing temperature and increasing oxygen fugacity to values of around 100 which are actually observed in fayalitic olivine rims in allende the ni content of condensed olivine is below 10 7 mol even at high oxygen fugacities indicating that condensation of nio in olivine is a very unlikely process any ni in olivine in the ppm range is therefore indicative of metal olivine equilibration under increasingly oxidizing conditions h 2 o h 2 ratios of 0 01 0 1 chromite is an early phase to condense forming before forsterite isolated olivine grains in allende and olivine on the outside of chondrules have feo rich rims texture composition and morphology of these rims indicate that they formed by condensation from a gas our calculations suggest that the gas was enriched by a factor of 100 in oxygen relative to solar composition gas and that mg was almost completely condensed when rim formation occurred the sharp boundary between forsteritic and fayalitic olivine suggests an increase in oxygen fugacity and rapid formation of fayalitic olivine by condensation cr enrichment at the rim interface with the underlying forsterite probably reflects stability of chromite under oxidizing conditions conventional models of feo bearing silicate formation require oxidation of metal silicate assemblages at temperatures as low as 400 k it is shown here that diffusion at this temperature is so slow that the time needed for formation of the observed feo bearing phases would exceed the lifetime of the solar nebula other oxygen fugacity indicators reflect similar conditions for example the hercynite component of spinel in fine grained al rich inclusions indicates equilibration with a gas of similar composition as that responsible for condensation of fayalitic olivine suggesting formation of feo rich spinel by gas solid reactions mechanisms for producing oxidizing conditions in the solar nebula are briefly discussed

isopycnal rafos floats as roving hydrographers in the north atlantic current region the properties measured by isopycnal floats can be compiled to provide hydrographic information about the sampled surfaces the height and temperature of two specific volume anomaly surfaces reported by 84 rafos floats deployed in the north atlantic current nac region are compared here with the climatological hydrographic database the mean pathway of the nac and other prominent features of the region are similar in float derived and climatological surfaces the most striking difference between floats and climatology an extended trough in the nac path at 44 n in the float derived surface reflects the sampling time scales the baroclinic transport of the nac estimated from rafos floats using the observed climatological relationship between the potential energy anomaly and the height of the specific volume anomaly surfaces was 42 sv at 42 n and 24 sv at 47 n in reasonable agreement with climatological estimates

oscillatory zoned fayalitic olivine from the lewis cliff 90500 cm2 meteorite abstract a grain of fayalitic olivine with oscillatory fe rich and fe poor zoning occurs in the antarctic lewis cliff 90500 cm meteorite suggesting reaction via diffusion this grain is continuously enclosed by a fine grained rim ranging from 20 to 100 m in thickness phyllosilicates and other hydrous phases occur at three broken edges of the grain as well as within its surrounding rim suggesting in situ alteration via aqueous processes this olivine grain plus its rim provides evidence for both anhydrous reaction that predated the accretion of the meteorite parent body and hydrous alteration that presumably occurred on the parent body

the formation and alteration of the renazzo like carbonaceous chondrites iii toward understanding the genesis of ferromagnesian chondrules to better understand the formation conditions of ferromagnesian chondrules from the renazzo like carbonaceous cr chondrites a systematic study of 210 chondrules from 15 cr chondrites was conducted the texture and composition of silicate and opaque minerals from each observed feo rich type ii chondrule and a representative number of feo poor type i chondrules were studied to build a substantial and self consistent data set the average abundances and standard deviations of cr2o3 in feo rich olivine phenocrysts are consistent with previous work that the cr chondrites are among the least thermally altered samples from the early solar system type ii chondrules from the cr chondrites formed under highly variable conditions e g precursor composition redox conditions cooling rate with each chondrule recording a distinct igneous history the opaque minerals within type ii chondrules are consistent with formation during chondrule melting and cooling starting as s and ni rich liquids at 988 1350 c then cooling to form monosulfide solid solution mss that crystallized around olivine pyroxene phenocrysts during cooling fe ni metal crystallized from the s and ni rich liquid and upon further cooling mss decomposed into pentlandite and pyrrhotite with pentlandite exsolving from mss at 400 600 c the composition texture and inferred formation temperature of pentlandite within chondrules studied here is inconsistent with formation via aqueous alteration however some opaque minerals fe ni metal versus magnetite and panethite present in type ii chondrules are a proxy for the degree of whole rock aqueous alteration the texture and composition of sulfide bearing opaque minerals in graves nunataks 06100 and grosvenor mountains 03116 suggest that they are the most thermally altered cr chondrites

cathodoluminescence microscopy and spectroscopy of forsterite from kaba meteorite an application to the study of hydrothermal alteration of parent body highly forsteritic olivine fo 99 2 99 7 in the kaba meteorite emits bright cathodoluminescence cl cl spectra of red luminescent forsterite grains have two broad emission bands at approximately 630 nm impurity center of divalent mn ions in the red region and above 700 nm trivalent cr ions in the red ir region the cores of the grains show cl blue luminescence giving a characteristic broad band emission at 400 nm also associated with minor red emissions related to mn and cr ions cl color variation of kaba forsterite is attributed to structural defects electron probe microanalyzer epma analysis shows concentrations of ca al and ti in the center of the forsterite grain the migration of diffusible ions of mn cr and fe to the rim of the kaba meteoritic forsterite was controlled by the hydrothermal alteration at relatively low temperature estimated at about 250 c while ca and al ions might still lie in the core a very unusual phase of feo wustite was also observed which may be a terrestrial alteration product of feni metal

3d geophysical inversions of the north east amer belt and their relationship to the geologic structure the amer lake area is located within the churchill structural province in the kivalliq region of nunavut approximately 160 km north west of baker lake two distinct geophysical geological entities are structurally intercalated an archean mixed granitoid gneiss metasedimentary metavolcanic basement and the unconformably overlying paleoproterozoic amer group metasediments from east of amer lake stretching toward the south west these two entities form the amer fold and thrust belt at the north east end of this belt high resolution aeromagnetic data define a distinctive oval shape that has been interpreted as a south west trending doubly plunging synform the outcrop within the interior of this structure is sparse resulting in limited structural data and speculative geological interpretations with multiple geometries possible the high resolution aeromagnetic data compiled through an industry government consortium and newly acquired detailed gravity profiles were modelled to provide constraints on the geometry of this synform we document a geophysical geological feedback process whereby the available geological and geophysical data were used to derive constraints on inversion models for the synform starting with available limited litho structural data the presence of a double plunging synform was directly inferred from the aeromagnetic data segments of the aeromagnetic data have 2d morphology and so can be modelled using a simple parametric 2d dipping slab inversion approach models of profiles extracted from the aeromagnetic data were used to provide preliminary dip and magnetic susceptibility constraints for the three lakes mudstone with iron formation and the five mile lake basalt landsat imagery outlined the spatial limits of the stratigraphically underlying non magnetic ayagaq quartzite incorporating these outputs as bounds in the input reference model for a ubc gif 3d magnetic inversion helped to accentuate the geological structure in the output mesh an enhanced inversion that incorporates both geological and geophysical constraints the validity of the resulting inversion model was tested by computing 2d forward models of the gravity profile data the inversion model generated by this study emphasizes the importance of integrating information from as many knowledge sources as one can find more trust can be placed on forward and inversion models where there is agreement among all data sets and a coherency of structural style

pathways of cross frontal exchange in the north atlantic current the north atlantic current nac forms part of the boundary between the subtropical and subpolar gyres in the north atlantic ocean the current has topographically controlled stationary meanders that appear to grow and decay a region east of the current in the newfoundland basin contains water of mixed subpolar subtropical properties suggesting that there is exchange across the nac this study considers data from isopycnal rafos floats launched in the nac region from 1993 to 1995 we use the rafos data to define the frontal zone as a pressure range where the jet is most likely to be found this definition requires a latitudinal dependence as the nac shoals to the north floats shallower and deeper than this range are defined to be on the subpolar and subtropical side respectively these definitions are used to estimate mixing that occurs between the current and its surroundings and to estimate the relative quantity of exchange of water parcels between the two gyres only small quantities of mass exchange from one gyre to the other are found but there is a distinct asymmetry leading to a mean flux from the subpolar to subtropical sides we also find that floats spend significant time in the frontal region and are frequently exchanged between fast and slow moving waters particularly at the meander extrema diffusion while in the jet leads to eddy cross frontal exchange which is important for the exchange of properties across the nac

a circumpolar gravest empirical mode for the southern ocean hydrography historical hydrographic data from the southern ocean are projected into a baroclinic stream function space generating a three dimensional gravest empirical mode gem parameterized by pressure geopotential height and longitude the low dimensional gem projection is intrinsically steady vertically coherent and equivalent barotropic the gem fields capture more than 97 of the total density and temperature variance in the antarctic circumpolar current region

fayalite in the kaba and mokoia carbonaceous chondrites abstract the olivines in the kaba and mokoia cv3 carbonaceous chondrites range from almost pure forsterite fo99 6 to almost pure fayalite fa99 9 individual grains of fayalite of such high purity have not previously been reported from meteorites they can reach 100 m in diameter and occur in the matrix chondrules forsterite enstatite aggregates and rims around ca al rich inclusions chondrules and aggregates the fayalite is three to nine times richer in mn than in bulk ci chondrites many grains are associated with magnetite troilite and pentlandite an assemblage that suggests relatively oxidizing conditions we propose that the fayalite formed through reaction of sio g released by decomposition of enstatite with magnetite and sulfide the lower thermal limit for fayalite formation is determined by the above reaction whereas the upper thermal limit is controlled by magnetite stability thermodynamic calculations indicate that fayalite coexisting with magnetite and sulfides could have formed in a nebular environment that had a h 2 o h 2 ratio substantially greater than the canonical solar nebula coexisting olivines having endmember compositions indicate that they experienced little or no thermal metamorphism

dark inclusions in the mokoia cv3 chondrite evidence for aqueous alteration and subsequent thermal and shock metamorphism mokoia is a cv3 chondrite that contains abundant phyllosilicate mineralization we present a detailed petrographic and scanning electron microscopic study of 24 dark inclusions dis that we found in mokoia the overall texture and constituent minerals of the dis resemble those in the host meteorite fe bearing saponite and na rich phlogopite the same phyllosilicates as in the host meteorite occur in the dis which strongly suggests that the dis have a similar alteration history to the host meteorite however the dis show several distinct differences from the host meteorite olivine grains in the di matrices are more homogeneous in fe fe mg ratio than those in the host meteorite matrix phyllosilicates in the dis are less abundant than in the host meteorite and they have been dehydrated to various extents these characteristics suggest that the dis have experienced higher degree of thermal metamorphism than the host meteorite in addition the matrices in the dis are more compacted than those in the host meteorite most olivine grains in the dis show undulatory extinction in transmitted crossed polarized light and some show planar fractures while such olivine grains are rare in the host meteorite two of the dis contain si mg fe and o rich melt veins these characteristics indicate that most dis have been shocked to shock stage s3 s4 while the host meteorite is shock stage s1 virtually unshocked thermal metamorphism of the dis was likely caused by shock heating these results are consistent with the contention previously proposed for the dis in cv3 chondrites i e the dis have experienced aqueous alteration and subsequent dehydration on the cv parent body we suggest that thermal and shock metamorphism occurred locally to various extents after pervasive aqueous alteration in the mokoia parent body

fayalitic halos around inclusions in forsterites from carbonaceous chondrites abstract spindle shaped fayalitic halos occur in forsterites of the allende kaba mokoia vigarano and ningqiang carbonaceous chondrites the halos have 1 ellipsoidal or less commonly rounded shapes 2 long axes parallel to each other within given host crystals 3 round bleb like inclusions of ni rich metal and sulfide in their centers and 4 compositional gradients that are most fe rich near the inclusions the fe and mg compositional profiles are well approximated by the error function which in combination with halo shapes indicates a diffusional origin the high ni contents relative to kamacite of the metal inclusions suggest fe loss we propose that the halos formed by oxidation of the fe ni metal in the inclusions and fe 2 diffusion into adjoining forsterite the halo dimensions suggest they formed between 1000 and 1250 k carbonaceous chondrites are characterized by among other features minerals and some organic molecules that are only stable at relatively low temperatures probably less than 500 k and certainly lower than 800 k the conditions required to form the fayalitic halos by diffusion are inconsistent with such low temperatures this combination leads us to conclude that the halos formed prior to their incorporation in the parent body ies of their host meteorites possibilities are that the halos formed in the nebula or within a prior parent body from which material was dispersed and reaggregated

aqueous alteration in the kaba cv3 carbonaceous chondrite the kaba cv3 carbonaceous chondrite has undergone pervasive aqueous alteration that produced fe bearing saponite in chondrules and matrix saponite in chondrules is coarse grained and shows a crystallographic orientation relationship such that c of saponite parallels a of enstatite matrix saponite exhibits a variety of textures including coarse and fine grained clusters isolated packets and sparse lamellar replacements of fe rich olivine the coarse grained clusters were probably derived from altered chondrules that were fragmented during regolith gardening the fine grained clusters and isolated packets formed from material released during the partial dissolution of matrix silicates much saponite is associated with submicron fe ni sulfides suggesting that conditions were not oxidizing during aqueous alteration we believe that the alteration occurred after accretion and on the kaba parent body because saponite is present throughout the sample by analogy to terrestrial occurrences the saponite in kaba probably formed at low temperatures 100 c the distribution of saponite in kaba is determined by local bulk compositions enstatite in chondrules has been altered more extensively than the fine grained olivine in matrix the alteration products in kaba resemble those in other aqueously altered carbonaceous chondrites such as the mokoia cv3 chondrite and in certain ci chondrites there are however significant differences kaba lacks the abundant high al phyllosilicates in chondrules and in cais reported from mokoia also lacking in kaba is the serpentine and ferrihydrite found in orgueil alteration products in kaba matrix are dissimilar from those in co3 chondrites even though their prealteration mineralogies were similar an observation that probably reflects different temperatures during alteration

historical position of the north atlantic current the path of the north atlantic current nac within the newfoundland basin is determined from the historical hydrographic database 1910 1988 horizontal probability distributions for dynamical and property front locations on specific volume anomaly surfaces are derived from an analysis of all available hydrographic stations in the basin these probability distributions indicate where the fronts were most likely to have been observed in the past they are observed to exhibit a pattern of stationary meanders with troughs located over the southcast newfoundland rise over the newfoundland seamounts and to the east of the flemish cap the width of the nac s envelope varies from 300 km over the southeast newfoundland rise temporal subsamples of the historical data indicate that the path and meander patterns are persistent features for all seasons and for the four decades between 1950 and 1990

hydrous and anhydrous alterations of chondrules in kaba and mokoia cv chondrites chondrules in the bali like cv chondrite kaba and the allende like portion of the mokoia breccia have been studied to explore the relationship between hydrous alteration to form phyllosilicates and anhydrous alteration resulting in secondary olivine zonation replacement of enstatite by ferroan olivine and formation of feldspathoids nepheline and sodalite all kaba chondrules experienced extensive hydrous alteration whereas anhydrous alteration was minor and resulted only in the olivine zonation on the other hand all of the mokoia chondrules experienced both extensive anhydrous and hydrous alteration bronzite rims formed between relic enstatite grains and phyllosilicates in both kaba and mokoia during the hydrous alteration petrographic observations indicate that phyllosilicates in mokoia postdate formation of the secondary ferroan olivine and feldspathoids we conclude that anhydrous alteration in kaba and mokoia predated hydrous alteration and took place before accretion of chondrules into the cv parent asteroid

oxygen silicon and mn cr isotopes of fayalite in the kaba oxidized cv3 chondrite constraints for its formation history the iron rich olivine end member fayalite occurs in the matrix chondrules ca al rich inclusions cais silicate aggregates and dark inclusions in the kaba and mokoia oxidized cv3 chondrites in most occurrences fayalite is associated with magnetite and troilite to help constrain the origin of the fayalite fa98 100 we measured oxygen and silicon isotopic compositions and mn cr systematics in fayalite from two petrographic settings of the kaba meteorite one setting consists of big fayalite laths embedded in the matrix and radiating from a core of fine grained magnetite and sulfide while the other setting consists of small fayalite magnetite sulfide assemblages within or at the surface of type i barred or porphyritic olivine chondrules oxygen in the big fayalite laths and small chondrule fayalites falls on the terrestrial fractionation line and is distinct from that in chondrule forsterites which are enriched in 16o 17o 4 oxygen in the big fayalite laths may be isotopically heavier than that in chondrule fayalites silicon isotopes suggest that forsterite is 1 amu heavier than adjacent fayalite within kaba chondrules however we were unable to confirm large silicon isotopic differences among fayalites reported previously the mn cr data for big kaba fayalites give an initial 53mn 55mn ratio of 2 07 0 17 10 6 consistent with literature results on mokoia chondrule fayalites the combined data suggest that fayalites in both petrographic settings formed at about the same time 9 7 ma after the formation of cais our data indicate that those fayalite magnetite troilite assemblages replacing metal inside and around chondrules formed by aqueous alteration on the meteorite parent body the formation site and mechanism for the big fayalite laths is less clear but the petrographic setting indicates that they did not form in situ none of the models that have been suggested for formation of these fayalites is entirely satisfactory

the subpolar mode water of the north atlantic ocean abstract the warm waters of the subtropical and subpolar basins of the north atlantic have tight regional temperature salinity relationships and are conventionally called the regional central waters a volumetric census of the temperature salinity characteristics of the north atlantic by wright and worthington 1970 shows that waters characterized by certain segments of the t s relationships have large volumes compared to those of other segments volumetric mode waters such mode waters appear as layers with increased vertical separation between isopycnals pycnostads the present study reports on the existence of pycnostads in the central and eastern north atlantic these subpolar mode waters are formed by deep winter convection in the subpolar north atlantic and participate in the upper water circulation of the northern north atlantic the seasonal outcropping of the pycnostads occurs within and adjacent to the north atlantic current the irminger current the east and west greenland currents and

two neoarchean supercontinents revisited the case for a rae family of cratons abstract a variety of early paleoproterozoic supercontinents or supercratons have been proposed including kenorland superia and vaalbara improved geologic knowledge of the churchill province canadian shield suggests that the classical kenorland configuration with linked rae and hearne cratons is invalid and represents a younger ca 1 9 ga nuna configuration we review the early paleoproterozoic tectonic history of the rae craton and propose that the ca 2 55 2 50 ga macquoid and 2 50 2 28 ga arrowsmith orogens represent critical piercing points for supercontinent reconstruction utilizing the stratdb and dateview databases we have identified nearly two dozen regions worldwide that show effects of these orogenic events including the rae gawler mawson peninsular india west africa amazonia north china and sask crustal blocks we propose that these cratonic blocks form a distinct rae family lineage utilizing new time space plots for the rae family and components of the superia and vaalbara supercratons we demonstrate that the rae family lacks the distinctive early paleoproterozoic depositional and glaciogenic record characteristic of superia and vaalbara but has an important contemporaneous record of juvenile crust formation and contractional orogenesis we propose the collective history of the rae family is best reconciled if it represents the now dispersed blocks of a third earliest paleoproterozoic supercraton that we term nunavutia in this model the magmatic and tectonometamorphic record of these cratons includes an early 2 55 2 45 ga period of interior collisional orogenesis in which the supercraton amalgamated and a later phase of long lived peripheral orogenesis 2 45 2 28 ga prior to its partial break up at ca 2 05 ga forming within an otherwise globally quiescent period the history of the rae family is important for assessment of constrasting models of early paleoproterozoic geodynamic activity

mineral equilibrium in fractionated nebular systems we investigated the equilibrium mineral assemblages in chemically fractionated nebular systems using a computer routine that finds the set of minerals and gases which minimizes the gibbs free energy of a system with stipulated elemental abundances diagrams are presented showing the equilibrium mineralogy as a function of temperature 400 2300 k for unfractionated solar material and five fractionated systems the fractionated systems were defined by mixing in various proportions the following four volatility components that solar material can be divided into refractory dust carbonaceous matter ices and h2 gas dust enrichment is seen to increase temperatures of condensation evaporation and the fe2 content of mafic minerals and to permit existence of stable melt phases enrichment of dust and organic matter produces mineral assemblages that are similar in many ways to those of enstatite chondrites but with mafic minerals that are far more reduced fe2 poor than those in primitive enstatite chondrites enrichment of dust organics and ices leads to highly ferrous mineralogies even at the highest temperatures but does not predict the stability of hydrous phases above 450 k

the arrowsmith orogeny geochronological and thermobarometric constraints on its extent and tectonic setting in the rae craton with implications for pre nuna supercontinent reconstruction abstract geochronological and thermobarometric data for two widely spaced and previously unstudied areas of the rae craton canada provide important temporal and spatial links between previously investigated areas in the northern and southern rae craton that had been interpreted to have been reworked during the ca 2 5 2 3 ga arrowsmith orogeny on southern boothia peninsula upper amphibolite to granulite facies metamorphism dated by in situ shrimp u pb analysis of monazite occurred at ca 2 64 2 60 2 54 2 50 2 49 2 44 and 2 37 2 34 ga with retrograde monazite growth at 2 27 ga shrimp u pb ages of zircon in two metasedimentary rocks provide a further indication of metamorphism at ca 2 54 and 2 50 ga which also corresponds to the age determined for a k feldspar augen granite textural relationships indicate that the regional foliation formed between ca 2 54 ga and 2 50 2 49 ga peak metamorphic conditions of 700 c and 5 6 kbar are interpreted to have been reached during the 2 37 2 34 ga event in the mccann mcarthur lake region of the southwestern rae craton upper amphibolite to granulite facies metamorphism is dated by in situ shrimp monazite ages of ca 2 53 2 47 and 2 37 2 28 ga textural relationships indicate deformation initiated by ca 2 37 ga and terminated by ca 2 30 ga metamorphic conditions of 5 kbar and 700 735 c were quenched at ca 2 3 ga following peak conditions of 7 kbar and 800 c the evidence for regional deformation at 2 54 2 49 ga on southern boothia peninsula supports an andean type accretionary margin setting for the arrowsmith orogeny a collisional event may have terminated high grade metamorphism in the northern region by ca 2 34 ga while metamorphism associated with continental arc magmatism continued in the south until at least ca 2 28 ga these combined data demonstrate the continuity and broad extent of the arrowsmith orogen along the western flank of the rae craton extending 1500 km from northern saskatchewan to northern baffin island the broad extent of this globally rare orogenic event provides an important piercing point for supercontinent reconstructions which are discussed in a companion paper the inferred convergent tectonic activity between ca 2 54 and 2 28 ga is interpreted to have occurred on the flank of a growing supercontinent a setting that could account for an apparent slowdown but not shutdown in global tectonics

age and provenance of the paleoproterozoic thluicho lake group based on detrital zircon u pb shrimp geochronology new insights into the protracted tectonic evolution of the southwestern rae province canadian shield abstract the middle paleoproterozoic thluicho lake group comprises coarse grained continental clastic rocks deposited in fault controlled alluvial basin s in the southwestern rae province of the canadian shield the group nonconformably overlies crystalline basement that records late taltson high grade metamorphism ca 1 93 ga and is folded and metamorphosed to greenschist facies providing an important window of insight into tectonic activity in this region this paper reports 100 detrital zircon ages for two samples from the lower and upper parts of the thuicho lake group four analyses of one of the youngest detrital zircons from the stratigraphically higher member define a weighted mean 207pb 206pb age of 1922 6 ma establishing a maximum depositional age for the group this result accords with regional geological relationships indicating that the group is intermediate in age between development of two major tectonic fronts an older ca 1 93 ga east southeast trending front that formed at the height of thelon taltson orogeny and a younger ca 1 91 1 90 ga northeast striking front expressed by the black bay fault and related northeast striking structures affiliated with the snowbird tectonic zone the younger age limit of the group may therefore be older 1 91 1 90 ga than the previously reported minimum age constrained by cross cutting dykes dated at ca 1 82 ga in terms of isotopic provenance the bulk of analyses of the combined dataset lie within two principal age ranges 2750 2500 ma and 2475 2250 ma the older of these is dominated by ca 2600 ma zircons whereas the younger contains two statistically distinct subgroups of 2410 and 2310 ma this signature is consistent with derivation almost exclusively from immediately surrounding basement rocks of the nolan taltson zemlak ena and beaverlodge domains abundant 2475 2250 ma zircons provide indirect evidence for the 2 45 2 3 ga arrowsmith orogeny despite proximity to the taltson magmatic zone 1990 1960 ma zircons are nearly completely absent suggesting that thluicho lake basins formed in the hinterland of the orogen possibly separated from the foreland taltson arc by a continental divide alternatively taltson arc plutons may not have been unroofed at the time of deposition of thluicho lake strata the data presented herein support the previously proposed broad correlation between the thuicho lake and nonacho groups however there are apparent differences in detail between respective basin settings and controls suggesting that this correlation requires further investigation

neoarchean high potassium granites of the boothia mainland area rae domain churchill province u pb zircon and sm nd whole rock isotopic constraintsthis article is one of a series of papers published in this special issue on the theme of geochronology in honour of tom krogh the boothia mainland region of the north central rae domain is underlain by remnants of a neoarchean volcano sedimentary sequence dismembered by two regionally extensive neoarchean high potassium granitoid suites with rare occurrences of a structurally interleaved paleoproterozoic sedimentary cover sequence the granitoids and their gneissic equivalents are dominated by variably deformed and metamorphosed i type metaluminous polyphase commonly porphyritic to augen biotite hornblende monzogranite and subordinate granodiorite with rare tonalite new geochronological results the first for this area demonstrate that the widespread neoarchean granitoid plutonism is dominantly 2 61 2 59 ga with a less prominent 2 66 ga plutonic event the age of zircon recrystallization suggests that ca 2 60 ga archean metamorphism and fabric development s1 affected the 2 66 ga plutons prior to or contemporaneously with intrusion of the voluminous ca 2 6 ga suite end t for the ca 2 61 2 59 ga suite range from

geophysical models of the montresor metasedimentary belt and its environs central nunavut canada recent identification of hydrothermally altered rocks and breccia in the underexplored montresor belt of paleoproterozoic metasedimentary rocks suggests the possible presence of undiscovered mineralization this study examines potential field data from the region with the goal of identifying subsurface features that could be associated with or serve as vectors to mineralization subsurface alteration zones faulting and or igneous intrusions gravity data were used to model regional and local geological features using known geology and physical properties from the study area and environs as constraints and documents dense intrusive bodies underlying the paleoproterozoic sequences maps of transformed apparent magnetic susceptibility values outline corridors of weak magnetization that correspond to observed zones of non magnetic breccia and epidote hematite quartz alteration imputing the apparent susceptibility and rock property information into a magnetic forward model defines the geometry of this al