sm nd evidence for a major 1 5 ga crust forming event in the central grenville province the majority of gray gneisses from an 80 000 km 2 area in the central quebec grenville province define a sm nd isochron that yields an age of 1 53 0 07 ga major element trace element and initial nd isotopic data all point to a subduction related orogenic source for this terrane it is attributed to a major crustal extraction event that formed a mid proterozoic island arc the age of the terrane agrees with published data from the eastern and western grenville province in newfoundland and ontario respectively we suggest that 1 5 ga are fragments were accreted along much of the length of the laurentian foreland within 100 m y of their formation in an event marked by widespread granitoid plutonism

contrasting zoning profiles in high grade garnets evidence for the allochthonous nature of a grenville province terrane in the central part of the grenville province proterozoic metasediments of the reservoir cabonga terrane arc underlain by gently dipping archean migmatites a major mylonite zone located within the migmatites is the base of a large thrust sheet made up of the reservoir cabonga terrane and a migmatitic sole garnets from meta aluminous assemblages in proterozoic metasediments mylonites and migmatites display contrasting types of ca zoning patterns garnets with ca rich cores are found along the western edge of the reservoir cabonga terrane whereas garnets with flat ca profiles are found in the remaining proterozoic metapelites in the mylonites and in the archean migmatites considering samples in which an infinite reservoir of ca ions is warranted by abundant plagioclase in the matrix pressures well in excess of 1200 mpa at temperatures between 700 and 800 c are indicated for a metamorphic event recorded in the cores of garnets from the western edge of the reservoir cabonga terrane the inner rim of garnet records pressures of around 1000 mpa for the same temperature range in homogeneous garnets a single p t set around 800 mpa and 750 c is recorded in the migmatites some of the mylonites initiate their recrystallization under relatively high pressures which is continued during decompression and record the lowest p t values these metamorphic signatures suggest that the proterozoic metapelites were already metamorphosed under relatively high pressure when emplaced on to archean migmatites and that the entire area was subsequently subjected to metamorphism at a lower pressure

osl chronology of sand deposits and climate change of last 18 ka in gurbantunggut desert northwest china the development of the gurbantunggut desert is mainly controlled by the westerly wind and at present has little influence from indian and southeast asian monsoons a combined study using optically stimulated luminescence osl dating ground penetrating radar gpr surveys and climatic proxies analysis was carried out in the southern part of the desert the chronology extends back to 18 ka and is constructed based on 16 osl ages from boreholes in the linear dune body and the inter dune area the chronology suggests that sand deposition in the last 18 ka experienced two rapid accumulation phases at 11 and 2 5 ka ago which were also evident from the gpr surveys these periods relate to increased aridity in the region five climate phases are identified from the last 18 ka based on the osl chronology and climatic proxy analysis with grain size and magnetic susceptibility measurements the deglacial period 18 3 10 4 ka is characterized by climate instability and possible glacial melting events the holocene optimum peaked 8 5 ka and terminated 3 6 ka ago when the regional climate became arid osl samples from the dune body cluster around 2 5 ka which indicates rapid advance extension of dune bodies at this time copyright 2011 john wiley sons ltd

nd isotope mapping of the lac dumoine thrust sheet implications for large scale crustal structure in the sw grenville province terra nova 24 363 372 2012 abstract over 100 new nd model ages are presented for orthogneisses from the lac dumoine region western quebec interpreted as a thrust sheet of the grenvillian allochthonous polycyclic belt the thrust sheet has an isotope signature consistent with other terranes of the allochthonous belt with tdm ages 1 8 ga in the main allochthon but ages of 1 8 2 0 ga in an underlying tectonic unit itself thrust over parautochthonous archean crust however the new data also reveal a half window of palaeoproterozoic crust that divides the allochthon into two lobes showing them to be relatively thin skinned nappes originating from the baskatong crustal scale ramp when combined with new nd isotope mapping for algonquin park ontario these results suggest that the main ramp of the allochthon boundary thrust is located further to the se than previously thought with a strong sw ne linear trend that coincides with dipping reflectors imaged on lithoprobe seismic line 32 south of algonquin park

crustal formation in the grenville province nd isotope evidence ninety eight new nd isotope analyses are presented for gneissic rocks from the grenville province when combined with over 250 published nd analyses and some unpublished analyses these data can be used to establish a first order crustal formation age map for most of the grenville parautochthon and allochthonous polycyclic belt in particular the geographic extents of a juvenile labradorian arc labradoria and a juvenile 1 5 ga arc quebecia are defined each with an area probably in excess of 100 000 km2 in addition pre 1 75 ga arcs are identified in ontario barilia and labrador makkovikia this work largely fills the last major gap in the continent wide crustal formation age map of the precambrian shield of north america

northeast extension of proterozoic terranes of mid continental north america granite and rhyolite ranging in age from 1 8 1 34 m y are known to underlie much of mid continental north america west of the phanerozoic michigan basin u pb zircon ages of similar rocks exposed adjacent to the grenville province in ontario suggest that the mid continental terranes extend northeast beneath and beyond the michigan basin prevalence of similar u pb zircon ages obtained from metaplutonic rocks southeast of the grenville front and affected by the 1 2 1 0 ga grenvillian orogeny prompts the interpretation that deformed equivalents of the mid continental proterozoic rocks form the bulk of the crust in the central gneiss belt of the grenville province east of lake huron farther east pre 1 34 ga terranes appear to be tectonically occluded by 1 28 1 22 ga volcanic sedimentary and plutonic rocks of the central metasedimentary belt themselves deformed metamorphosed and intruded by younger grenvillian plutons

the location of the allochthon boundary thrust and the archean proterozoic suture in the mattawa area of the grenville province nd isotope evidence abstract over 100 nd isotope analyses are used to map the crustal formation ages of grenville orthogneisses in a 10 000 km 2 area near mattawa on the ontario quebec border lithotectonic terranes with mapped outcrops of eclogite or coronitic metagabbro have nd model ages less than 1 8 ga and are interpreted as fragments of the allochthonous polycyclic belt however large areas of orthogneiss to the north of mattawa have model ages of 1 9 2 9 ga interpreted as archean crust that was extensively plutonised in paleo and mesoproterozoic magmatic events based on this evidence a revised location is proposed for the allochthon boundary thrust abt which passes to the south of the bonfield batholith and mattawa but then trends to the north and is correlated with the lac watson shear zone some allochthonous klippen are identified northwest of the main trace of the abt representing the dissected remains of what was originally a large allochthonous thrust sheet within the parautochthon a major step in depleted mantle model ages is observed in the vicinity of the mattawa fault separating archean and paleoproterozoic crustal blocks this boundary is interpreted as an ancient crustal suture reactivated by faulting associated with the ottawa bonnechere graben it is concluded that nd isotope analysis is a useful technique for detailed mapping of major tectonic boundaries in highly metamorphosed orogenic belts such as the grenville province

crustal evolution along a seismic section across the grenville province western quebec results of deep seismic reflection survey along a 375 km long transect of the grenville province in western quebec are combined with a review of geological observations and published isotopic ages the seismic profile offers a remarkably clear image of the crust mantle boundary and a good definition of the various crustal blocks crust about 44 km thick beneath the grenville front zone thins abruptly to ca 36 km southeastward perhaps the result of extension on southeast dipping surfaces extending to the moho other zones of relatively thin crust although less pronounced occur where proterozoic crust overlies archean crust and beneath the morin anorthosite complex the thickest crust is found at the extreme southeast of the transect east of the morin anorthosite from northwest to southeast three main crustal subdivisions are 1 deformed archean rocks with southeast dipping reflectors in the grenville front zone 2 an archean parautochthon with northwest dipping reflectors extending to the lower

monazite as a metamorphic chronometer south of the grenville front western quebec monazite was utilized as a chronometer to examine the effects of high grade metamorphism across the parautochthonous belt and allochthon boundary thrust of the grenville province in western quebec this study in addition to previous geo chronological studies indicates an archean component in the gneisses which is consistent with the presence of more than one set of peak metamorphic conditions single grain monazite analyses from metasedimentary gneisses from four locations within the parautochthonous belt yielded grenvillian u pb dates of 1000 5 to 1006 2 ma the location farthest to the northwest 45 km southeast of the grenville front included monazite with a distinct archean signature southeast of this point an archean signature was not detected in the monazite at 70 km southeast of the grenville front monazite yielded two discrete ages of 1005 2 and 1020 3 ma xenotime from one location indicated that the closure temperature of this mineral may be equivalent to that of monazite 725 25

erratum metamorphic constraints on the evolution of the gneisses from the parautochthonous and autochthonous polycyclic belts grenville province western quebec the tectono metamorphic history of polycyclic grey gneisses located in the central grenville province of western quebec has been constrained along a transect perpendicular to the length of the gr

tectono stratigraphic evolution of the early proterozoic wisconsin magmatic terranes of the penokean orogen the early proterozoic penokean orogen developed along the southern margin of the archean superior craton the orogen consists of a northern deformed continental margin prism overlying an archean basement and a southern assemblage of oceanic arcs the wisconsin magmatic terranes the south dipping niagara fault suture zone separates the south facing continental margin from the accreted arc terranes the suture zone contains a dismembered ophiolite the wisconsin magmatic terranes consist of two terranes that are distinguished on the basis of lithology and structure the northern pembine wausau terrane contains a major succession of tholeiitic and calc alkaline volcanic rocks deposited in the interval 1860 1889 ma and a more restricted succession of calc alkaline volcanic rocks deposited about 1835 1845 ma granitoid rocks ranging in age from about 1870 to 1760 ma intrude the volcanic rocks the older succession was generated as island arcs and or closed back arc basins above the south dipping subducti

new tectonic divisions of the grenville province southeast canadian shield on the basis of geological geophysical and geochronological data the grenville province has been divided into three first order longitudinal belts the parautochthonous belt pb allochthonous polycyclic belt apb and allochthonous monocyclic belt amb these are set apart by three first order tectonic boundaries the grenville front gf allochthon boundary thrust abt and monocyclic belt boundary zone mbbz the belts are subdivided into terranes based on internal lithological character the gf separates the archean to proterozoic foreland northwest of the orogen from reworked equivalents to the southeast continuous at the scale of the orogen its main characteristic is that of a crustal scale contraction fault the pb although less clearly identified along the length of the orogen in most places represents upgraded and tectonically reworked rocks of the adjacent foreland the boundary between the pb and the apb to the southeast the abt is most clearly delineated in the eastern half of the province it is the locus of major crustal delamination along which high grade mostly middle proterozoic polycyclic terranes were tectonically transported northwest toward and onto the pb the amb comprises two separate areas underlain by the wakeham supergroup and what is currently known as the grenville supergroup respectively its basal contact the mbbz is a decollement zone of variable kinematic significance between older polycyclic rocks and tectonically overlying monocyclic rocks this first order zonation implies a tectonic polarity to the grenville province superimposed on which are second order features evident from contrasting tectonic styles and radiometric ages these characteristics are consistent with a diachronous or oblique collisional model for the grenville orogen

age of the cabonga nepheline syenite grenville province western quebec this study presents isotope dilution thermal ionization mass spectrometry id tims u pb data for megacrystic zircon from the cabonga nepheline syenite complex of the reservoir cabonga terrane grenville province this terrane is an allochthonous slab metamorphosed under granulite grade conditions 1 18 1 14 ga and transported onto migmatites of the grenvillian parautochthon at about 1 02 ga the very low uranium and lead concentrations of zircon from the cabonga nepheline syenite cns did not permit determination of a statistically significant crystallization age using laser ablation microbeam techniques consequently extensive microsampling 15 zircon chips guided by x ray and electronic imaging followed by id tims analyses were carried out on a single megacrystic zircon a regression of 13 out of 15 u pb isotopic analyses results in a crystallization age of 117 1 3m a for the cns criteria based on zircon morphologies zoning patterns varying th u ratios 0 3 0 9 and a highly fractionated zr hf ratio 68 suggest an igneous derivation for the cns the cabonga alkaline rocks intruded under high grade metamorphic conditions pre ceded the onset of the widespread and ubiquitous 1 15 0 01 ga anorthosite mangerite charnockite granite magmatism in the southern part of the grenville province resume cette etude presente des donnees de geochronologie u pb par dilution isotopique spectrometrie de masse par ionisation thermique id tims sur des zircons a megacristaux provenant du complexe de syenite a nepheline de cabonga du terrane du reservoir cabonga province de grenville ce terrane est une dalle allochtone qui a ete meta morphisee au facies des granulites vers 1 18 1 14 ga et transporte par dessus des migmatites du parautochtone gren villien vers 1 02 ga les tres faibles concentrations d uranium et de plomb dans le zircon de la syenite a nepheline de cabonga n ont pas permis de determiner un ge de cristallisation statistiquement significatif au moyen de techniques d ablation par laser a microfaisceaux de nombreux micro echantillonnages 15 eclats de zircon guides par imagerie aux rayons x et electronique et suivis d analyses par dilution isotopique et spectrometrie de masse par ionisation thermique id tims ont ete effectues sur un seul megacristal de zircon une regression de 13 des 15 analyses u pb isotopiques a donne un ge de cristallisation de 117 1 3m apour la syenite a nepheline de cabonga selon des criteres bases sur la morphologie des zircons les patrons de zonalite les rapports th u variables 0 3 a 0 9 et un rapport zr hf hautement fractionne 68 la syenite a nepheline de cabonga proviendrait de roches ignees les roches alcalines de cabonga mises en place sous des conditions de metamorphisme eleve precedent le debut du magmatisme 1 15 0 01 ga anorthosite mangerite charnokite granite generalise et tres etendu dans la partie sud de la province de grenville

crustal structure of the grenville front and adjacent terranes under the auspices of the great lakes international multidisciplinary program on crustal evolution approximately 320 km of deep seismic reflection data were collected in lake huron along a profile that extends east from the manitoulin terrane across the grenville front to the interior of the grenville orogen the manitoulin terrane is characterized by a series of gently east dipping reflections at about 20 km depth that separate a highly reflective lower crustal layer from a markedly less reflective upper layer imaged by strong reflections at the western end of a spectacular band of moderately east dipping reflections the grenville front clearly truncates manitoulin terrane structures to the west these data are interpreted in terms of a speculative two stage model involving 1 creation of a major decollement during northward collision of an allochthonous terrane with the southern superior cratonic margin 1 83 1 89 ga penokean orogeny and 2 northwest directed stacking of microterranes at the southeastern margin of the craton followed by crust penetrating ductile imbrication under high pressure high temperature conditions leading to the ramping of deeply buried rocks to the near surface 1 0 1 3 ga grenvillian orogeny

quantitative palaeoclimate reconstruction as an inverse problem a bayesian inference of late holocene climate on the eastern tibetan plateau from a peat cellulose δ18o record there has been a sustained need to quantify past climate changes from proxy records to better understand the driving mechanisms and thus to improve the prediction of the future transfer function is an intuitive and frequently used method in this regard however this method is unable to provide interpretive and predictive results from proxy records because physical processes are not included the inverse proxy modelling method opened up a new avenue for quantitative palaeoclimate reconstructions through the integration of proxy records with deterministic models it is fundamentally different from the statistical approach representing a conceptual advance in quantitative palaeoclimatology here we demonstrate the potential of this method by placing a mechanistic model and a 6000 year long peat cellulose 18o record obtained from the high cold and monsoonal eastern tibetan plateau c 3500 m a s l in a bayesian paradigm in this worked example the marginal posterior probability distributions of palaeo

the southern limit of archean crust and significance of rocks with paleoproterozoic model ages nd model age mapping in the grenville province of western quebec abstract detailed nd model age mapping in the grenville province of western quebec has identified biotite gneisses with crustal formation age intervals of 2 5 2 9 ga and 1 8 2 0 ga although the presence of archean crust in the grenville parautochthonous belt has been known for some time the results of nd model age mapping combined with geochemical and aeromagnetic data indicate that the archean parautochthon extends more than 20 km to the south of the renzy shear belt this belt which was previously defined as the southern limit of archean crust corresponds to an isolated allochthon with nd model ages ranging from 2 0 to 1 4 ga and does not demarcate the allochthon boundary thrust rocks with model ages of 1 8 2 0 ga exposed to the south of the archean crust geochemically possess an arc affinity and are interpreted as a paleoproterozoic arc a model age boundary between archean crust and this arc has been defined by this study the age boundary probably represents a suture along which the southern margin of laurentia and the arc converged

nd isotope mapping of allochthonous grenvillian klippen evidence for widespread ramp flat thrust geometry in the sw grenville province abstract over sixty new nd isotope analyses are used to constrain the extent of several small grenvillian lithotectonic domains that form erosional outliers of the grenvillian allochthonous polycyclic belt the identification of these tectonic klippen when combined with previous mapping of large nappes shows that the allochthonous polycyclic belt forms a mid upper crustal sub horizontal thrust sheet that was transported to the nw over the archean proterozoic parautochthon this thrust sheet was corrugated by folding and dissected by erosion leaving the klippen as synformal remnants nd isotope mapping of these remnants when coupled with seismic reflection data from the lithoprobe programme suggests that these thrust sheets were exhumed on relatively steep crustal scale ramps before transportation to the nw on sub horizontal shear zones this type of thrust geometry previously observed in the central eastern grenville province was termed ramp flat geometry by rivers et al 2012 re examination of lithoprobe seismic data for the georgian bay transect shows that reflectors under the parry sound klippe have a synformal character in the mid crust suggesting that this is also part of an undulose sub horizontal thrust sheet exhumed on a steep crustal scale ramp rather than the previous model of a very long ramp with low inclination hence we argue that the georgian bay section has a geometry resembling the ramp flat trajectory of the allochthon boundary thrust previously discerned elsewhere in the grenville province

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

nd isotopes and the origin of 1 9 1 7 ga penokean continental crust of the lake superior region nd isotopic data on 26 samples demonstrate the origin of the early proterozoic crust of the penokean orogen the three major components are 1 the marquette range supergroup a predominantly miogeoclinal continental margin sequence deposited before 1 85 ga on archean basement 2 ca 1 88 ga felsic metavolcanic rocks of the wisconsin magmatic terrane to the south of evolved island arc affinity and 3 1 87 1 76 ga granitoids that intrude the metavolcanic rocks initial nd values for the metavolcanic rocks of the magmatic terrane range between 0 0 and 2 4 and t dm ages between 1 9 and 2 2 b y the volcanic rocks primarily represent new crustal material that had only a limited archean input probably through mixing of subducted sediments into the magma source area nd t values for sedimentary rocks in the lower part of the marquette range supergroup indicate an archean source most likely the 2 7 ga superior province to the north on the other hand the sedimentary rocks in the upper part of the marquette range supergroup graywackes of the upper michigamme formation have initial nd values between 0 8 and 1 5 indicative of an early proterozoic source these graywackes probably are foredeep deposits derived from the volcanic rocks of the wisconsin magmatic terrane to the south thus the time of deposition of the upper michigamme formation dates the final convergence of the wisconsin magmatic terrane with the continental margin the granitoids of the wisconsin magmatic terrane have a wide range of nd t values from 4 5 to 4 0 they represent mixtures of variable amounts of new crustal material and recycled archean detritus the more negative nd t values occur in samples close to the northern boundary of the wisconsin magmatic terrane which is marked by a mylonitic shear zone the niagara fault zone we suggest that the miogeoclinal lower marquette range supergroup rocks having an archean nd signature became involved in magma genesis close to the collisional boundary whereas increasingly lesser amounts of this older material were available for mixing farther away from the suture the 1 9 1 7 ga penokean events involved major growth of new crust from the mantle

a neodymium isotope study of plutons near the grenville front in ontario canada abstract sm nd analysis of meta plutonic rocks near the grenville front in ontario allows their division into two groups granitoid plutons with model ages of 2 ga are intruded into a crustal section formed largely of archean or re worked archean basement plutons with model ages of plutons south of the proposed suture line can be explained by anatexis of penokean age crust alone while plutons north of this line contain mixtures of archean and proterozoic nd these mixed signatures suggest that the nw grenville province may be analogous to northern wisconsin and michigan in this region a very early proterozoic miogeocline supplied by the superior province is overlain by a formation of mixed superior penokean provenance which heralded the collision of a penokean arc with the archean craton despite very strong sm nd evidence for the presence of old crustal components in the plutons u pb analysis did not reveal the presence of inherited zircon hence the absence of such zircons should not be taken in isolation as evidence against crustal re working

nd isotope mapping of a cryptic continental suture grenville province of ontario abstract nd isotope data are presented for 70 archean and paleoproterozoic grey gneisses from the grenville parautochthonous belt in ontario these data constrain an age boundary which separates partially reworked archean basement to the northwest from a paleoproterozoic arc terrane to the southeast this boundary is interpreted as a crustal scale continental suture but its exposure is cryptic due to metamorphic recrystallisation during the grenville orogeny mesoproterozoic stitching plutons injected across the boundary suggest that it has a minimum age of 1 5 ga and is probably a paleoproterozoic structure archean basement northwest of the suture was intensely plutonised by the 1 75 1 7 ga killarnean ensialic arc juvenile paleoproterozoic basement southeast of the suture has a maximum crustal formation age of less than 2 0 ga it is probably an accreted penokean terrane which was reworked by the same killarnean ensialic arc

the geochemistry of mafic gneisses from the renzy terrane western grenville province quebec implications for the geodynamic setting of the early mesoproterozoic laurentian margin abstract this study presents new geochemical data on mafic gneisses from the renzy terrane rt located in the grenvillian parautochthonous belt of western quebec this region is of particular interest since it contains one of the rare ni cu co massive sulfide deposits ever mined in the grenville province rt mafic gneisses have been divided into 3 groups based mainly on major element compositions trace element profiles normalized to the primitive mantle and incompatible trace element ratios the behaviour of high field strength elements hfse ta zr hf ti th and large ion lithophile elements lile ba rb are particularly useful for discriminating rt mafic gneisses major and trace element compositions of group 1 mafic ultramafic gneisses are very similar to those of the rt ultramafic sheets the mafic gneisses are therefore interpreted as melagabbroic sills cogenetic with these sheets the trace element composition of group 1 mafic gneisses is typical of magmas formed from a previously metasomatized depleted mantle source these rocks have high th ta ratios and show strong to moderate negative anomalies in ti zr and hf on multi element plots normalized to the primitive mantle mafic gneisses of group 2 show geochemical features transitional between extensional and compressional settings these rocks generally have low concentrations of hfse and high concentrations of lile typical features of mafic rocks formed in a compressional environment however some of them are fe ti rich with minor or no negative anomalies in hfse and low to moderate concentrations of lile on multi element diagrams normalized to primitive mantle features more typical of an extensional environment mafic gneisses of group 3 have a geochemical signature typical of extensional environments or of ocean island basalt oib these gneisses have no negative anomalies in hfse and have low concentrations of lile rt mafic gneisses are similar in composition to mafic granulites in the bondy gneiss complex in the central metasedimentary belt and to shawanaga domain amphibolites from the central gneiss belt two paleoarc back arc environments in the grenville province based on the contrasting geochemical signatures found in the rt mafic gneisses and the similarities with known arc back arc settings in the grenville province we propose that the rt was formed by arc back arc magmatic activity most likely associated with a period of arc magmatism along the proterozoic laurentian margin oib like mafic gneisses found in the rt are similar to those from the kipawa region formed by intraplate continental magmatism in the parautochthonous belt of the grenville province

geochronology of proterozoic metamorphism in the deformed southern province northern lake huron region canada abstract metamorphic geochronology and thermochronology of the deformed southern province north of lake huron in ontario canada reveals a complex and protracted tectonothermal history for the proterozoic southern laurentide margin that is distinct from that of the lake superior region to the west while penokean interval deformation is well constrained in the lake superior area a well defined geon 18 geochronometric signature is lacking in the huronian vicinity the oldest metamorphic ages revealed by in situ monazite u th total pb electron microprobe analyses are ca 1800 ma with a strong geon 17 yavapai interval and local geon 14 15 magmatic influences furthermore conventional 40 ar 39 ar incremental step heating of hornblende indicates paleoproterozoic heating through 500 c occurred only adjacent to geon 17 intrusions regional analysis of mica cooling ages suggests that mesoproterozoic thermal overprinting of primary geon 17 magmatism and metamorphism is limited to south of the murray fault the penokean orogeny long considered the dominant paleoproterozoic event in the great lakes region is now recognized as only the first of several accretionary events that impinged on the southern laurentide margin however the penokean orogen is restricted to the 2100 ma embayment of the superior province rifted margin moreover subsequent tectonothermal pulses yavapai mazatzal and ca 1450 ma recorded in the rocks of the southern province in ontario mimic that documented across much of the central and western united states based on the distinct lack of penokean interval ages and the strong geon 17 and geon 14 total pb and 40 ar 39 ar geochronometric signature the tectonothermal history of southern province more closely resembles that of the southwest united states than the lake superior region

basement tracing using mid proterozoic anorthosites straddling a palaeoterrane boundary ontario canada abstract the st charles and mercer anorthosites intrude mesoproterozoic gneisses and granitoids in the northwestern grenville province of ontario the two intrusions are hosted by the west bay batholith but lie on opposite sides of a neodymium model age break as identified by dickin can j earth sci 37 2000 165 the st charles and mercer anorthosites give imprecise but essentially identical ages of ca 1245 ma assuming grenvillian lower intercepts the high errors are largely consequences of very shallow discordia while there are some differences in the extent of deformation such that metamorphic facies vary from greenschist to amphibolite in both but with more pervasive gneissosity in the mercer anorthosite the age dates and setting suggest that the two intrusions have experienced essentially identical geological histories the petrography and geochemistry of the two intrusions is dominated by the effects of plagioclase accumulation modified by varying amounts of intersitial pyroxene and trapped liquid the samples show chondrite normalised ree profiles with modest lree enrichment and pronounced positive eu anomalies and high field strength element hfse depletion nb zr hf the mercer anorthosite displays higher trace element abundances on average smaller europium anomalies and more lree enrichment with la yb n from 6 to 9 for the mercer and around 2 4 for st charles consistent with differences in cumulate liquid proportions the sm nd isotopic system shows a similar distinction whereby the mercer samples give very consistent signatures with e nd 1240 values of 2 1 to 2 6 while those for st charles are 3 9 to 6 5 the isotopic distributions are interpreted in terms of assimilation of two distinct lower crustal rock types during ascent through the crust the st charles anorthosite ascended through relatively old radiogenic archaean basement which is comparatively incompatible element poor and the mercer anorthosite ascended through basement comprising a mixture of archaean and proterozoic material with more incompatible enriched and more isotopically juvenile signatures this interpretation suggests that the apparent suture between archaean foreland and an accreted palaeoproterozoic arc represents a significant deep crustal lineament in accord with previous interpretation this study supports the efficacy of the application of anorthositic rocks as sensitive samplers of lower crust attributable to the fact that anorthosites are mineralogically and geochemically relatively simple systems being largely monomineralic and that the amount of crustal assimilation is small and therefore does not overwhelm the host signature

sw grenville province canada the case against post 1 4 ga accretionary tectonics abstract seven accretionary sutures formed between 1 16 and 1 03 ga have been identified by different authors in the ontario quebec adirondack oqa segment of the mesoproterozoic grenville orogen in canada with one exception the inferred accretionary terrane boundaries lie within or at the margins of the central metasedimentary belt cmb located between the central gneiss belt and the adirondack highlands central granulite terrane however geological geochronological and petrological data suggest that the grenville orogen on both sides of the proposed terrane boundaries sutures preserves a common 1 4 1 03 ga tectonomagmatic history inconsistent with its origin as a post 1 4 ga collage of exotic tectonic blocks features which straddle the proposed 1 16 1 03 ga sutures from the central gneiss belt via the adirondack highlands to the mauricie area include 1 mesoproterozoic continental crust 1 5 1 4 ga forming the host and or basement to younger magmatic and supracrustal suites 2 a 1 35 1 3 ga continental arc remnants of which occur from the cmb boundary zone cmbbz in ontario to the appalachians in the united states built on the 1 5 1 4 ga continental crust 3 intrusions of 1 17 1 13 ga age in the central gneiss belt mafic suite and the adirondack highlands and their quebec extension amcg suite i e anorthosite massifs and related granitoids 4 relics of 1 18 1 14 ga sedimentary basins in the northwestern cmb and the mauricie area we propose that an alternative model can adequately account for the observed geology of this part of the grenville orogen wherein the rocks of the oqa segment were part of an andean type margin between 1 4 and 1 2 ga at 1 35 1 3 ga a continental magmatic arc was built upon the southeastern margin of laurentia represented by the 1 5 1 4 ga mesoproterozoic continental crust the arc split at 1 3 ga forming an ensialic back arc basin relics of which now occur in the northwestern part of the cmb and the back arc basin was flanked to the southeast by an active 1 28 1 25 ga arc collision between the laurentian margin and another continent amazonia occurred at 1 2 ga resulting in closure of the back arc basin and initiation of thrusting along the cmbbz post collisional lithospheric shortening led to convective removal of thickened subcontinental lithosphere upper mantle melting and extension of the overlying crust resulting in widespread magmatic activity at 1 17 1 13 ga including emplacement of the amcg massifs crustal extension generated sedimentary basins now represented by the st boniface sediments in the mauricie area 1 18 to between 1 15 and 1 09 ga and the penecontemporaneous flinton group in the northwestern cmb renewed post collisional granulite facies shortening commenced at 1 12 ga manifested as nappes in the central gneiss belt and thrusting in the mauricie area continued post collisional shortening at 1 08 1 05 ga was more localised resulting in reactivation of thrusting in the cmbbz and initiation of the kinematically compatible tawachiche shear zone along the eastern border of the quebec extension of the adirondack highlands the characteristics of the oqa segment of the grenville orogen can all be accounted for in the context of 1 a 1 4 1 2 ga southeast facing andean type margin to a laurentian upper plate associated with northwest dipping subduction 2 continental collision at 1 2 ga and 3 subsequent continued post collisional shortening without invoking accretion of exotic terranes between 1 4 and 1 0 ga

regional cross section of the southern province adjacent to lake huron ontario implications for the tectonic significance of the murray fault zone the southern province in the lake huron area of ontario and the penokean fold belt of minnesota wisconsin and michigan have the earmarks of a collisional orogen the huronian supergroup a southerly facing passive margin sequence was deposited during early aphebian early proterozoic crustal stretching and ocean basin formation along the southern margin of the superior province archean craton it accumulated as four unconformity bounded northerly tapering onlapping clastic wedges megacycles the thickness and facies variations reflect syndepositional down to the basin south normal faulting that controlled the accumulation and preservation of the lower three huronian megacycles these are overlapped northward by the youngest megacycle an extensive sheet of clastic sediments recording post stretching regional subsidence of the cratonic margin due to cooling and thermal contraction soft sediment folds in the rocks of the youngest megaycycle to the extreme south probably indicate southward slu

ductile nappe extrusion in constrictive strain at the origin of transverse segments of the allochthon boundary thrust in the manicouagan imbricate zone central grenville province quebec abstract separating the parautochthonous and allochthonous belts the allochthon boundary thrust abt is a major lithotectonic boundary that generally runs parallel with the length of the grenville province but with some transverse segments in this contribution we investigate one of these segments the manicouagan imbricate zone miz by adding new field based data supplemented by new u pb deformation ages to a compilation of previous research conducted on both sides of the abt in the region our results reveal striking differences between the footwall and hanging wall of the abt in terms of strain and structures and concludes that the transverse orientation of the abt is not related to folding an updated strain analysis allows us to determine that deformation of the miz was dominated by a constrictive type of strain we build on a previously proposed conceptual model to document ductile extrusion over basement ramps driven by tectonic forcing and lateral density contrast in the upper crust at ca 990 ma the insertion of the hot ductile nappe into partially molten metasediments of the surrounding parautochthonous belt was accommodated by constrictive strain within the former and flattening and transverse folding in the latter

precise u pb ages for grenvillian and pre grenvillian thrusting of proterozoic and archean metamorphic assemblages in the grenville front tectonic zone canada while the position of the grenville front is placed at a structural feature associated with the onset of high grade rocks when approached from the north the time of first metamorphism in these rocks is in general pre grenvillian and highly variable at killarney amphibolite to granulite grade rocks were metamorphosed lineated and foliated to their present front parallel configuration mainly by 1453 6 ma north of north bay amphibolite grade migmatites have been subjected to a pre 2650 ma metamorphism granulite grade gneisses south of the grenville front near val d or record a 2640 ma metamorphic event south of chibougamau mafic enclaves in tonalite south of the front were converted to migmatitic amphibolite at about 2680 to 2690 ma in central labrador a 1650 ma foliated granite is present south of the grenville front and at least some of the banding is this old at the atlantic coast granulite facies metamorphism in front parallel gneisses predates a 1790 1662 ma deformation in contrast each of these areas shares a near common age of second metamorphism that is variable in degree and accompanies nw directed thrusting or crustal loading near killarney on georgian bay titanite records an episode of 977 2 ma partial resetting the degree of which increases with distance into the grenville province here grenvillian deformation where present is manifest as feldspar porphyroclastic fabric with strong se plunging lineations near north bay archean pegmatites in the grenville front zone were similarly deformed at 988 10 ma and strongly rodded ductile amphibolite underwent late melting to produce postlineation tonalite in which new zircon tips grew at 987 2 ma a 991 4 ma cooling age for titanite in this melt is analytically identical to the zircon growth age hence rapid cooling occurred south of chibougamau new zircons grew in high pressure coronite in response to tectonic loading at 995 6 ma and in central labrador melt pods that developed in 1650 granites in response to thrusting record a zircon new growth age of 995 2 ma titanite in the granitic host rock has an analytically identical age of 992 4 ma again indicating rapid cooling this second event is recorded by 28 titanite and zircon analyses for samples up to 225 km apart that record a 1651 5 ma to 988 5 ma two age mixing line grenville resetting at about 1037 19 ma in the grenville front zone on the labrador coast is indicated but major thrusting is pre 1662 ma u pb isotopic results indicate that the grenville front zone is characterized along its length by partially reset u pb systems high pressure assemblages local melting and rapid cooling crustal loading and shearing with lead loss related to mechanically induced recrystallization and local heat that was rapidly terminated by rebound and cooling are required