neoproterozoic sedimentation and tectonics of the laurentian midcontinent detrital zircon provenance of the jacobsville sandstone lake superior basin usa and canada

the labradorian orogeny in the grenville province eastern labrador canada two minor intrusions within the hawke river terrane in the grenville province eastern labrador have upper intercept u pb ages as follows i for a megacrystic intermediate dyke and ii 1622 3 ma for a pegmatite dyke the upper intercepts date time of emplacement and the lower intercepts 508 and 320 ma respectively record the timing of early paleozoic events in the region a third sample a sillimanite bearing pelitic gneiss from the paradise metasedimentary gneiss belt in the hawke river terrane yielded a range of ages between 1647 and 1627 ma from seven single zircon analyses the zircons in the metasedimentary gneiss are interpreted as detrital because of their variation in morphology the range of ages obtained and their extreme variation in u content a detrital origin implies that the sedimentary protolith must have been deposited after 1627 ma and is therefore not the same age as morphologically similar metasedimentary gneiss that occurs as enclaves within pre 1670 ma migmatized quartz d

lithoprobe line 55 integration of out of plane seismic results with surface structure metamorphism and geochronology and the tectonic evolution of the eastern grenville province lithoprobe line 55 in the grenville province of eastern quebec provides unusually good control on the three dimensional 3 d geometry and structural relationships among the major lithological units there archean basement underlies the exposed proterozoic rocks along the entire seismic line and there is a lateral ramp in this basement immediately behind a lobate stack of thrust slices of high pressure metamorphic rocks comprising the manicouagan imbricate zone miz integration of the 3 d geometry with p t and geochronological data allows derivation of a tectonic model for the region the miz was buried to depths 60 km at 1050 ma preservation of its high pressure assemblages and the absence of metamorphism at 990 ma which is characteristic of lower pressure metamorphic rocks that tectonically overlie them indicates the miz rocks were rapidly unroofed early in the tectonic history there were two discrete pulses of crustal thickening during the grenvillian orogeny in this region the first inv

late labradorian metamorphism and anorthosite granitoid intrusion cape caribou river allochthon grenville province labrador evidence from u pb geochronology in the cape caribou river allochthon ccra metaigneous and gneissic units occur as a shallowly plunging synform in the hanging wall of the grand lake thrust system glts a grenvillian structure that forms the boundary between the mealy mountains and groswater bay terranes the layered rocks of the ccra are cut by a stockwork of monzonite dykes related to the dome mountain suite and by metadiabase amphibolite dykes that probably form part of the ca 1380 ma mealy swarm the mafic dykes appear to postdate much of the development of subhorizontal metamorphic layering within the lower parts of the ccra the uppermost least metamorphosed units of the ccra the north west river anorthosite metagabbro and the dome mountain monzonite suite have been dated at 1625 6 and 1626 2 ma respectively an amphibolite unit that concordantly underlies the anorthosite metagabbro and is intruded discordantly by monzonite dykes has given metamorphic ages of 1660 3 and 1631 2 ma granitoid gneisses that form the

the michael gabbro and other mesoproterozoic lithospheric probes in southern and central labrador the michael gabbro 1426 ma and the shabogamo gabbro 1459 ma represent two iarge diabase intrusive episodes that affected the northwestern margin of the northeastern grenville province both have sustained variable effects of subsequent grenvillian metamorphism other broadly contemporaneous magmatic activity in the region includes the formation of elssnian anorthosite granitoid complexes 1 46 1 29 ga that intruded churchill province and nain province rocks to the north the harp dykes 1 27 ga and mealy dykes 1 25 ga petrologic and geochemical data show that the michael gabbro and shabogamo gabbro are similar with the latter displaying more cumulate rock characteristics and the former having compositions closer to those of rapidly cooled magma both have compositions comparable to those of other continental diabases and to some continental flood basalts sr and nd initial isotopic compositions of michael gabbro 1426 ma 4 7 to 6 0 1 1426 ma 8 7832 0 7w and shabogamo gabbro i459 ma 4 0 to 5 5 1 1459 ma 8 7020 0 7060 are alike and overlap suggesting similar sources and processes of development comparisons with other nearly contemporary mafic suites in central and southern labrador show that only the mealy dykes have a distinctly more radiogenic isotopic signature paradoxically other rnafic suites that intrude older basement rocks north of the grenville front have less enriched nd signatures than do those that intrude younger basement rocks south of the front the argument is made that the subcontinental lithospheric mantle and not crustal contamination played the most influential role in evoiution of the magmas

roots of the labradorian orogen in the grenville province in southeast labrador evidence from marine deep seismic reflection data marine deep seismic reflection data obtained as part of the eastern canadian shield onshore offshore transect ecsoot lithoprobe project from offshore southeast labrador are evaluated using geodynamic and gravity models and a tectonic interpretation is developed the geodynamic model explains the seismic reflection data in terms of doubly vergent tectonism resulting from southward directed underthrusting of mantle and lowermost crust middle and upper crustal seismic reflectors are considered to be proshears and retroshears and correlated through potential field data with major mylonite zones at terrane boundaries mapped onshore as there is little evidence of subsequent severe structural modification apart from strike parallel grenvillian dextral transpression which does not alter the crustal geometry along the line of the seismic transect the seismic reflectors are considered to represent labradorian structures inferred from onshore geochronological data to be mostly related to a 1665 ma labradorian accretionary event the southward directed underthrusting of the mantle and lowermost crust is consistent with existing models for southward labradorian subduction coast parallel gravity and magnetic anomalies are interpreted as an expression of a neoproterozoic to early phanerozoic extensional basin the existence of which had little effect on the underlying crystalline basement other than to downfault it relative to that exposed onshore

micromorphology of iceberg scour in clays glacial lake agassiz manitoba canada abstract icebergs plough through unconsolidated lake sea sediments gouging out kilometre long grooves 100s of metres wide and tens of metres deep although the surface morphology of iceberg scours is well documented little is known about what scours look like in stratigraphic section particularly where surface characteristics are absent e g through decay or burial this investigation establishes a definitive suite of diagnostic criteria for identifying iceberg scours in clays by macroscopically and microscopically 2d thin sections examining sediment deformation below iceberg scours in former glacial lake agassiz manitoba canada structures unequivocally attributed to iceberg scour in this investigation include sub horizontal microfabrics folds sheath folds augen shapes normal faults reverse thrust faults discrete shears partially destroyed clasts intraclasts type iii multiple domains water escape structures flow structures unistrial plasmic fabric dropstones realigned bedding and specific structural sequences this research will be particularly valuable in palaeoenvironmental reconstruction and in predicting glacial dynamics in addition it may eventually aid structural engineering on polar shelves which could be of great value to oil and gas companies

labradorian and grenvillian crustal evolution of the goose bay region labrador new u pb geochronological constraints major initial crust formation in the northeastern grenville province of labrador occurred during the labradorian orogeny 1710 1620 ma prior to re deformation in the grenvillian orogeny between 10

the lithoprobe abitibi grenville transect two billion years of crust formation and recycling in the precambrian shield of canada we summarize the results of lithoprobe studies in the neoarchean southeastern superior province and the mesoproterozoic grenville province in the southeastern precambrian shield of canada through two composite cross sections based on seismic reflection data which define dramatically different styles of crust formation and tectonic accretion in the neoarchean and mesoproterozoic in the neoarchean the structures at the surface are steep with discontinuous and flatter structures at depth much of the crust appears to be juvenile and the predominant process of crustal growth is inferred to have been subduction accretion of primitive crust in a prograding arc system in the mesoproterozoic surface structures are shallow and the seismic character of the crust is continuous over the entire cross section archean parautochthonous rocks and reworked archean crust comprise a very significant proportion of the preserved crust in the mesoproterozoic and provided the backstop to the grenvillian orogeny result

new insights on the 1 7 1 0 ga crustal evolution of the central grenville province from the manicouagan baie comeau transect abstract integrated field relationships and u pb geochronology of rocks exposed along the southern part of the manicouagan reservoir central grenville province quebec provide insights into protolith formation and thermal events that range in age from ca 1 7 to 0 9 ga and share common features with the rest of the grenville province and beyond from presently lower to higher structural levels main findings include the recognition of a ca 1 70 1 75 ga granitic magmatism and metamorphism in the southern part of the parautochthonous gagnon terrane correlative in age with late stages of magmatism in the makkovik orogen to the northeast b a ca 1 69 gabbro anorthosite suite island domain which is inferred to represent the southwesternmost exposure of labradorian crust in the central grenville province c a 1 4 ga volcaniclastic sequence and associated plutonic rocks canyon domain which based on lithological association and age is inferred to represent the nne extension of the montauban island arc and d a ca 1 2 ga bimodal felsic mafic volcanic to intermediate volcaniclastic sequence banded complex inferred to have originated in an intra continental rift system contemporaneously with the composite arc belt southwestern grenville province and with widespread extensional magmatism elsewhere in the canadian shield in addition 1 3 ga granitoid plutons are documented to intrude labradorian units at the nw boundary of canyon domain the significance of these plutons is ambiguous as they may represent either stitching plutons marking the accretion of the 1 4 ga canyon domain to the laurentian margin or the first stage in the development of the ca 1 2 ga continental rift system the hinterland units island domain canyon domain and banded complex were metamorphosed under medium pressure granulite facies conditions during the culmination of the grenvillian orogeny between 1 08 and 1 04 ga this metamorphism was accompanied by ca 1 07 ga a type granitic magmatism in canyon domain in addition locally abundant ca 0 98 ga late tectonic felsic pegmatite and ultrapotassic dykes occur in canyon domain this event is coeval with titanite ages documented in a wide range of units the data on the grenvillian evolution suggest mantle derived magmatism and associated metamorphism in the hinterland during both the culmination and the waning stages of the grenvillian orogeny

paleomagnetism of pre grenvillian mafic intrusions from the grenville province southeast labrador paleomagnetic results are reported for middle proterozoic layered intrusions and dykes from within the grenville province of coastal southeastern labrador rock units studied include the little gra

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

lithoprobe reflection studies of archean and proterozoic crust in canada abstract lithoprobe canada s national collaborative earth science research project established to develop a comprehensive understanding of the evolution of the north american continent is a multidisciplinary program spearheaded by seismic reflection studies five recently recorded seismic lines discussed in this paper are located in precambrian regions the mesoproterozoic grenville province the paleoproterozoic trans hudson orogen tho the paleoproterozoic archean basement of alberta and the archean superior province data acquired across the grenville orogen in eastern quebec show strong reflectivity throughout the crust upper crustal reflections can be correlated with exposed structural elements including extensional shear zones and packages of deformed high pressure rocks eclogites in a marine survey across the grenville orogen off southeastern labrador seismic images show variably dipping reflections and a structural high associated with a major gravity anomaly data acquired across central alberta show crustal scale thrust stacking and imbrication of the archean hearne craton to the east across the trans hudson orogen images of similar collisional features are observed geochronologic constraints indicate contemporaneity of tectonic activity between the two regions at 1 8 ga suggesting that collisional tectonic activity was coeval over a broad crustal region ca 1000 km across strike in the superior province seismic data across a collision zone involving the northern abitibi greenstone belt and the arc related opatica plutonic belt show spectacular crustal reflectivity and dipping reflections that extend 8 s 30 km into the mantle the latter feature is interpreted as representing a relict 2 69 ga old suture associated with subduction providing the first direct evidence that plate tectonics was active in the late archean these five examples supported by other lithoprobe results refute a number of generalizations about crustal reflectivity that have been made in the past and illustrate how reflection studies combined with other geoscience studies can lead to a better understanding of precambrian tectonics reflectivity persists throughout the crust there is no general separation into a poorly reflective upper crust and a reflective lower crust crustal reflectivity in archean and proterozoic regions is as pervasive as that in areas of more recent tectonism the precambrian reflection moho is generally well defined but shows a range of characteristics relative ages of reflectors can be discerned and tectonic significance can be attached to characteristic features of the crustal reflectivity

a revised model for the crustal structure of the sw grenville province ontario canada the grenville province forms the exhumed remnants of a 1 1 ga collisional orogeny that telescoped an older continental margin terranes with distinct crustal formation ages can be mapped using nd isotopes revealing a ramp flat thrust structure the ramp is identified by the presence of retrogressed eclogites and its trajectory is refined using nd model ages the main allochthon is locally overlain by the parry sound klippe but is also underlain by a tectonic duplex northwest directed nappes represent remnants of a corrugated thrust sheet but a ring shaped remnant was also preserved where the thrust sheet was down buckled under the dense rocks of parry sound domain

lateral ramps as an aid to the unroofing of deep crustal rocks seismic evidence from the grenville province we suggest that lateral ramps in the basement to orogenic wedges may be important agents in the rapid unroofing of deeply buried continental crust our suggestion is based on a study of the distribution of high pressure metamorphosed continental rocks in the mesoproterozoic grenvillian orogen in eastern quebec these rocks were buried to depths of more than 50 km through crustal thickening during a continental collision and were then rapidly unroofed they are currently exposed in a 100 km 60 km lobate embayment north of the manicouagan reservoir using a combination of surface structural data and seismic reflection data from lithoprobe line 55 we reconstruct the three dimensional form of the high pressure rocks our reconstruct ion methods are based on demonstrable correlations between the attitudes of gneissic layering at the surface and the attitudes of reflectors on the seismic profile they make extensive use of the fact that much of the seismic reflectivity on line 55 comes from outside the vertical plane of the seismic line because of the sinuous nature of the line and its obliquity to tectonic strike the validity of the reconstructions is demonstrated by the generation of a synthetic seismogram from the interpreted geometry that is very similar to the observed seismogram our reconstruction shows that a steep lateral ramp extends to the base of the crust immediately behind the broad lobe of high pressure rocks at the surface the lateral ramp probably provided a favorable pathway for the extrusion of deep seated rocks from the deep crust and their transport onto the foreland

a group of rifting events in the terminal paleoproterozoic in the north china craton abstract rifting events of the terminal paleoproterozoic are well preserved in the north china craton they include an aulacogen and cratonic marginal rift basins mafic dyke swarms two anorthosite suites and rapakivi granites all of these events took place after metamorphism deformation and magmatism of the late paleoproterozoic between 2 0 ga and 1 85 ga or 1 80 ga the rifting events belong to an anorogenic event group reflecting breakup of a late paleoproterozoic united north china craton an intracratonic rift basin in the center of the north china craton is named the yanshan taihangshan aulacogen a stratigraphic thickness of 9193 m was deposited within the basin including three groups and twelve formations with time range ca 1 8 ga to 0 85 ga the oldest changcheng group of the sequence was deposited in a graben and is mainly composed of alluvial basal conglomerates terrestrial clastic rocks and shallow marine sandstone and carbonates with super high potassium basalt trachyte the marginal basins occur around the northern western and southern margins of the north china craton respectively located in langshan baiyun obo helanshan mountain and the adjacent regions between henan shanxi and shaanxi provinces the stratigraphic sequences of the basins are generally made up of clastic rocks in the lower parts and carbonates in the upper part with a thick volcanic sequence with u pb age of 1840 14 ma exposed in the southern marginal regions based on stratigraphic sequences isotopic data and volcanic features the initial rifts took place at about 1 85 ga to 1 80 ga mafic dyke swarms are largely developed in central parts of the north china craton where they intrude archean granulite and high amphibolite rocks a new single zircon u pb isotopic age of 1769 2 5 ma has been reported from a mafic dyke in east henshan mountain northwestern shanxi province an anorthosite suite agms is exposed along two deep fault belts in northern hebei province isochron ages of 1683 193 ma by rb sr and 1735 239 ma by sm nd were measured from an anorthosite body the rapakivi granites crop out along the miyun qiangzilu xinglong fault in an e w direction and are characterized by megacrysts of alkali feldspars mantled by plagioclase thus showing many of the petrographic features of classic rapakivi granites worldwide a typical rapakivi body preserved in shachang of miyun county beijing has a u pb age of 1716 31 ma super high potassium volcanics located in the upper changcheng group have been dated at 1683 67 ma and 1625 6 ma the time range for all magmatic events is between 1 72 ga and 1 62 ga

u pb constraints for the plutonic and tectonometamorphic evolution of lake melville terrane labrador and implications for basement reworking in the northeastern grenville province abstract along the northeastern margin of the grenville province in canada crust of labradorian age 1 71 1 62 ga was variably reworked during northward present day coordinates propagation of the 1 20 1 00 ga grenvillian orogen to provide constraints on the nature of grenvillian reworking we describe previously unreported field relationships and present ten new u pb ages from a transect across lake melville terrane lmt which forms the leading edge of the allochthonous polycyclic belt in the study area exhumation of granulites on the northern margin of lmt along the rigolet thrust zone occurred during the interval 1617 1610 ma and was therefore a late labradorian feature approximately 120 m y after the labradorian orogeny during the interval 1489 1474 ma dioritic to granitic plutons inferred to be related to pinwarian 1 51 1 47 ga magmatism were emplaced in northern lmt this together with a revised u pb baddeleyite and zircon age of 1472 27 21 ma for the michael gabbro suggests that pinwarian ensialic arc magmatism was coeval with the emplacement of the michael and shabogamo gabbro dyke swarms the data also indicate that pinwarian magmatism was not limited to the pinware terrane but occurred throughout the entire grenville province in eastern labrador implying that the region was already accreted to the laurentian margin prior to the grenvillian orogeny the principal phase of grenvillian deformation and uppermost amphibolite facies metamorphism in lmt occurred between 1 08 and 1 05 ga and appears to have been focused between the rigolet shear zone and the anorthosite dominated mealy mountains terrane grenvillian amphibolite facies reactivation of the rigolet shear zone was limited to its upper structural levels only with the central and lower levels preserving labradorian fabrics and granulite facies assemblages high grade grenvillian deformation within lmt was dominated by tight to isoclinal folding and belt parallel transposition minor thrusting followed by localized strike slip shearing at the northeastern and southwestern margins

postrift history of the eastern central atlantic passive margin insights from the saharan region of south morocco the passive margin of south morocco is a low elevated passive margin it constitutes one of the oldest margins of the atlantic ocean with an early jurassic breakup and little geological data are available concerning its postrift reactivation so far we investigated the postrift thermal history of the onshore part of the margin with low temperature thermochronology on apatite crystals fission track and u th sm he ages we obtained are significantly younger than the breakup 190 ma fission track ages range from 107 8 to 175 16 ma with mean track lengths from 10 7 0 3 to 12 5 0 2 m u th sm he ages range from 14 1 to 185 15 ma using inverse modeling of low temperature thermochronological data we demonstrate that the south moroccan continental margin underwent a complex postrift history with at least two burial and exhumation phases the first exhumation event occurred during late jurassic early cretaceous and we attribute this to mantle dynamics rather than to intrinsic rifting related processes such as flexural rebound the second event from late cretaceous to early paleogene might record the onset of africa europe convergence we show a remarkably common behavior of the whole moroccan passive margin during its early postrift evolution the present day differences result from a segmentation of the margin domains due to the africa europe convergence finally we propose that varying retained strengths during rifting and also the specific crustal lithospheric geometry of stretching explain the difference between the topographical expressions on the continental african margin compared to its american counterpart

the chief lake complex revisited and the problem of correlation across the grenville front south of sudbury ontario abstract south of sudbury the boundary between the southern and grenville provinces the grenville front is placed along the faulted southeastern edge the ca 1 46 ga chief lake complex clc a southeast dipping thrust sense mylonite zone lies in the immediate hanging wall of the grenville front boundary fault gfbf separating the clc in the southern province from metasedimentary granitoid anorthositic and amphibolitic gneiss in the grenville province to the southeast the gfbf also coincides with an abrupt break in the continuity of southeast trending diabase dykes of the 1 24 ga sudbury swarm the northern clc is a composite intrusion whose units include and are separated by large rafts of the early paleoproterozoic huronian supergroup in which relict stratigraphy outlines engulfed folds mafic rocks along the northwest contact of the clc previously interpreted as metasomatized nipissing gabbro 2 22 ga are in fact a phase of the chief lake intrusive suite commingled with granite the northwest contact coincides in part with a steep southeast side down pre intrusion fault that was later reactivated with reverse sense of displacement the chief lake granitoid rocks become increasingly foliated southeast toward the grenville front but two features suggest that at least some of the deformation of the chief lake intrusive suite was syn emplacement 1 the granitoid component of commingled rocks may be strongly foliated in places where the mafic component is undeformed and 2 included huronian metasedimentary rocks in general lack the constrictional fabric of their enveloping plutonic rocks sudbury dykes cut across the fabric of the clc but the facts that they are themselves buckled locally metamorphosed to greenschist facies and are offset by faults marked by ultramylonite and cataclasite attest to tectonism of grenvillian age in the footwall for a limited distance northwest of the gfbf granitoid rocks dated at 1 74 and 1 47 ga southwest of the study area and archean tonalitic rocks and 2 47 ga anorthositic gabbro to the northeast occur on both sides of the grenville front in the study area units of paragneiss in the hanging wall of the gfbf cannot be assigned to specific huronian formations in the southern province footwall however although not proven it is reasonable to correlate other hanging wall rocks namely mylonitic gabbroic anorthosite amphibolite and granitic gneiss to the 2 47 ga east bull lake intrusive suite nipissing gabbro and chief lake granite respectively in the southern province whereas sudbury dykes do not intrude the mylonitic rocks immediately above the gfbf they do reappear to the southeast as dyke segments and pods within mylonite bounded panels where they cut earlier isoclinal folds and exhibit a progressive increase in metamorphic grade across a gradient shallower than that in the enclosing gneissic rocks they document that much of the tectonic history recorded by their host rocks is pre grenvillian even so there is no evidence that the grenville front was ever the locus of a suture

depositional and tectonic setting of the paleoproterozoic lower aillik group makkovik province canada evolution of a passive margin foredeep sequence based on petrochemistry and u pb tims and lam icp ms geochronology abstract the paleoproterozoic lower aillik group is a deformed metasedimentary metavolcanic succession located in the makkovik province of labrador eastern canada the group is situated near the boundary between reworked archaean gneiss of the nain north atlantic craton and juvenile paleoproterozoic crust that was both tectonically accreted and formed on or adjacent to this craton during the ca 1 9 1 78 ma makkovikian orogeny the lower aillik group is structurally underlain by archaean gneiss and structurally overlain by ca 1860 1807 ma bimodal dominantly felsic volcanic and volcaniclastic rocks of the upper aillik group we present geochemical data from metavolcanic rocks and u pb geochronological data from several units of the lower aillik group in order to address the depositional and tectonic history of this group u pb data were obtained using both thermal ionization mass spectrometry tims and laser ablation microprobe inductively coupled plasma mass spectrometry lam icp ms two quartzite units near the structural base of the lower aillik group contain detrital zircons only of archaean age and are interpreted to have been deposited on the nain craton during post 2235 ma rifting and initiation of a passive continental margin overlying mafic metavolcanic rocks contain thin horizons of intermediate tuff one of which is dated at 2178 4 ma this relatively old age and an inferred stratigraphic relationship with underlying sedimentary units suggest that the volcanic rocks represent transitional oceanic crust consistent with their geochemical similarity to tholeiitic rifted margin sequences of mesozoic age in eastern north america a package of interlayered psammitic and semipelitic metasedimentary rocks that appears to stratigraphically overlie the mafic volcanic unit is dominated by paleoproterozoic detrital zircons but also contains archaean grains this package was deposited after 2013 ma the age of the youngest concordant zircon the u pb data imply a minimum 165 m y time gap between mafic volcanism and sedimentation and are consistent with deposition of the psammite semipelite unit in an evolving foredeep that heralded the approach of a paleoproterozoic arc terrane accretion of this terrane to the nain cratonic margin at ca 1 9 ga initiated the makkovikian orogeny although the lower aillik group is highly deformed and may contain internal tectonic boundaries or be incomplete the u pb and geochemical data allow quantitative assessment of a prolonged rift drift basin closure cycle that characterized the early paleoproterozoic evolution of the southern nain cratonic margin

proterozoic southward accretion and grenvillian orogenesis in the interior grenville province in eastern labrador evidence from u pb geochronological investigations abstract this study is a fascinating example from the eastern grenville province of unraveling the triple challenges inherent in the geochronological investigation of any multi orogenic region namely inheritance emplacement and subsequent metamorphism the existence of hitherto unknown pre labradorian crust within allochthonous grenvillian terranes is demonstrated formerly unrecognized spatial relations between labradorian and pinwarian crust are outlined and a new perception of mid grenvillian magmatic and metamorphic events is presented the pre labradorian crust has an age range of 1800 1770 ma and consists of calc alkaline plutonic rocks intruding earlier supracrustal assemblages containing archean and early paleoproterozoic inherited zircons evidence is provided from dated enclaves and inheritance in younger rocks that similar aged pre labradorian crust once existed farther south the geochronologically defined geographic extent of early labradorian crust having a 1680 1655 ma age range is expanded this crust is intruded by the mid labradorian mealy mountains intrusive suite for which new results confirm its 1650 1630 ma time of emplacement pinwarian orthogneiss dated between 1513 and 1496 ma and previously unknown in the region is now recognized to be common sufficient data exist to define for the first time a boundary between labradorian and pinwarian orthogneiss cryptic in the field establishing the presence of pinwarian aged crust to the southwest lacking labradorian inheritance except close to the boundary a belt of layered mafic anorthositic and monzogranitic intrusions is situated close to the boundary current data favour a pinwarian age for emplacement of these rocks mid grenvillian plutonism is demonstrated by 1043 ma ages from two k feldspar megacrystic granodiorite bodies they are situated on either flank of a crustal scale northeast trending northeast closing fold the fold which is 40 km wide and at least 160 km long and exposes pinwarian orthogneiss in its core post dates emplacement of the 1043 ma intrusions the fold probably formed at the peak of grenvillian tectonism in the region between 1030 and 1015 ma and certainly before intrusion of weakly deformed granitoid rocks at 992 ma final activity included the emplacement of late to post grenvillian granitoid plutons between 964 and 951 ma a pluton having a 951 ma age is the youngest known in the eastern grenville province cooling and stabilization was completed by ca 940 ma broader implications of the results in conjunction with existing data are that i the northeasternmost part of the eastern grenville province was only peripherally affected by grenvillian orogenesis ii an east northeast offshore extrapolation of the grenville front as a major tectonic feature does not exist iii high grade metamorphic mineral assemblages in the groswater bay and hawke river terranes are pre grenvillian rather than an eastward expression of a grenvillian high pressure belt and iv the north shore of the eastern st lawrence estuary represents across orogen exposure rather than orogen parallel as previously supposed

proterozoic orogens of the northeastern canadian shield new information from the lithoprobe ecsoot crustal reflection seismic survey as part of the eastern canadian shield onshore offshore transect ecsoot lithoprobe acquired 1250 km of deep seismic reflection data along the coast of labrador and across ungava bay to image evidence of proterozoic crustal accretion to the archean nuclei of the nain and superior provinces of the canadian shield the relatively pristine archean crust of the nain province has low reflectivity and generally lacks systematic reflector orientations reworking of archean crust on the margins of the makkovik province has little effect on this weak signature in contrast the archean crust in the eastern churchill rae province appears to have been overprinted by a strongly developed whole crustal easterly dipping reflection fabric interpreted to result from proterozoic collision of the nain and superior provinces in the paired new quebec and torngat orogens juvenile proterozoic crust in the makkovik and grenville provinces also shows strong whole crustal dipping reflection fabrics interpretable as outw

metamorphic conditions and 40ar39ar geochronologic contrasts across the grenvile front zone coastal labrador canada the grenville front zone on the eastern labrador coast transects lower and middle proterozoic metamorphic and plutonic rocks and embodies elements of a protracted metamorphic history that involved two periods of pre grenvillian metamorphism onto which grenvillian effects were superimposed the earliest event minimum age 1 8 ga was a granulite facies metamorphism ca 650 750 c and minimum pressure between 4 0 and 6 0 kbar that occurred prior to emplacement of plutons of the benedict mountains intrusive suite bmis and produced the white bear islands granulite complex these rocks were subsequently affected by a progressive amphibolite facies retrogression from about 700 to 550 c at 5 kbar this event was synchronous with the intrusion and or deformation of the bmis which yields rb sr isochron crystallization ages of 1 7 1 8 ga hornblende within deformed units of the bmis north of the grenville front record 40ar39ar plateau ages of 1 66 1 64 ga interpreted to date cooling through temperatures needed for intracrystalline retention of 40ar following the pre grenvillian metamorphism following the intrusion of the michael gabbro ca 1 4 ga grenvillian orogenesis was superimposed on the area during the grenvillian orogeny the area was sliced into three thrust bound blocks which contain contrasting metamorphic mineral assemblages ranging from greenschist facies in the north to lower amphibolite facies maximum 560 630 c moderate pressures in the south collectively these represent a thrust induced telescoped metamorphic sequence precise 40ar39ar dating of individual grenvillian tectonothermal events in this area is precluded by the widespread incorporation of extraneous excess 40ar components within various mineral phases

paleomagnetism and geochemistry of carboniferous sandwich bay dykes from coastal labrador paleomagnetic petrographic and geochemical data are used to characterize a suite of dykes in the sandwich bay area south of cartwright labrador these dykes are carboniferous in age and their separate identity is indicated by the following i distinct paleomagnetic signatures ii homogeneous geochemistry iii cross cutting relations with other dated rock units in the area and iv a whole rock k ar date of 327 13 ma the dikes have a soft present earth9s field component superposed on a stable magnetization at d 325 5 i 26 9 n 14 k 91 6 95 4 6 with the corresponding paleopole at 16 2 n 157 6 e dp dm 2 8 5 2 the stable magnetization is shown to be primary from baked contact tests no kiaman overprint is present the paleopole determined for the dykes suggests emplacement of the dykes at equatorial latitudes during the carboniferous times

upb geochronology of labradorian and later events in the grenville province eastern labrador u pb dating of selected rock units in the hawke river and groswater bay terranes of the grenville province in eastern labrador has refined the timing of metamorphism and magmatism during the labradorian orogeny 1710 1620 ma in the northern part of the hawke river terrane a major migmatization event is confirmed between 1671 4 3 ma for a tonalitic gneiss and a newly determined age of 1662 3 ma for a granite dyke that cross cuts the migmatized fabric of its host this event can be correlated with similar coeval activities in the lake melville terrane to the south and labradorian thrusting at the grenville front in the paradise metasedimentary gneiss belt high grade metamorphism has been dated in two localities in pillowed mafic volcanic flows associated with the metasedimentary gneisses which produced new zircon growth at 1645 ma in a garnet amphibolite assemblage and in pegmatitic melt pods hosted in high grade pelitic gneiss at 1638 11 3 ma the time of this metamorphic event places a minimum depositional age on the supracrustal assemblage in the paradise metasedimentary gneiss belt that conflicts with a previously reported age for sedimentation at 1627 ma the 1645 ma metamorphic event was coeval with emplacement of a layered mafic intrusion 1644 2 ma in the groswater bay terrane a large granite body between the groswater bay terrane and the hawke river terrane 1645 7 5 ma which is a minimum age for juxtaposition of the two terranes but inferred from other data to be 1665 ma and leucogabbro from the white bear arm complex 1640 ma which is adjacent to the paradise metasedimentary gneiss belt the faulted contact between the white bear arm complex and the paradise metasedimentary gneiss belt is intruded by the 100 km long paradise arm pluton at 1639 2 ma indicating contact between the two domains at this time pegmatitic phases in the white bear arm complex indicate later magmatism at 1629 3 2 ma current geological models argue for accretion of terranes never widely separated at 1665 ma followed by crustal thickening and uplift between 1645 and 1639 ma deformation in the hawke river terrane during the pinwarian event ca 1500 1470 ma is suggested by complex data for monazite and baddeleyite from the white bear arm complex and from titanite data for rocks elsewhere in the hawke river terrane grenvillian metamorphic effects are present in the region but do not appear to have had a major impact

wide angle seismic imaging of pristine archean crust in the nain province labrador a detailed refraction wide angle reflection seismic experiment was carried out in northern labrador to determine the velocity structure of relatively unaltered archean crust in the nain province six 3 component land seismometers were used to record an airgun source along a profile parallel to the coast forward modeling of traveltimes and amplitudes yields a p and s wave velocity model that shows two crustal blocks separated by a fault magnetic data suggest but do not prove that the fault is the offshore continuation of the handy fault a southwards thickening of the lower crust across the fault indicates that a transcurrent component might have been associated with the faulting the total crustal thickness is 33 km to the north and 38 km to the south of the fault the presence of pms reflections imply a sharp transition at the moho upper crustal velocities of 5 8 6 3 km s and poisson s ratios of 0 20 and 0 24 north and south of the fault respectively are consistent with a gneissic composition

dating ductile deformation using u pb geochronology examples from the gilbert river belt grenville province labrador canada the gilbert river belt in the grenville province in southeastern labrador is a distinctive west northwest trending zone of locally intense deformation and voluminous granitoid plutonism up to 30 km in width in an attempt to directly quantify the timing of deformation in ductile shear zones within the belt rocks interpreted as having been intruded synchronously with ongoing deformation were sampled for u pb isotopic analysis three of these samples are 2 m wide granitic veins that have sharp intrusive contacts that truncate ductile deformation fabrics but are themselves deformed at metamorphic conditions similar to their host rocks and are therefore interpreted as having intruded after the initiation of deformation and fabric development but prior to cessation of this deformation the first vein is syntectonic with respect to amphibolite facies deformation and yielded a zircon age of the second vein intruded synchronously with the development of a zone of amphibolite facies straight gneisses wh

crustal evolution in eastern labrador constraints from precise u pb ages abstract new u pb ages for rocks of eastern labrador complement previous u pb dates for the grenville and makkovik provinces and substantiate the following conclusions eastern labrador consists of two distinct crustal blocks that are different in age as well as magmatic metamorphic and structural overprint the northern block includes archaean to lower proterozoic basement rocks 1900 ma and two distinct episodes of rhyolitic volcanism at 1860 and 1807 ma high grade metamorphism deformation and anatexis affected this block between 1860 and 1760 ma ago makkovikian orogeny the southern block comprises the trans labrador batholith and the various terranes of the grenville province in contrast to the northern region the southern block shows no proof for the existence of continental crust older than 1710 ma this suggests that large amounts of juvenile material were added between 1710 and 1630 ma ago as dated by a large series of precise rock ages at least towards its end this crust forming event was associated with high grade metamorphism deformation and anatexis labradorian orogeny the 1710 1630 ma period of activity was followed by three distinct phases of anorogenic plutonism causing the emplacement of pegmatites 1500 ma a widespread suite of cotonitic metagabbros michael gabbro 1426 ma and granitic syenitic bodies 1296 ma during the grenvillian orogeny the region achieved its present structural configuration including the emplacement of granitoid plutons the structurally highest regions the mealy mountains and the hawke river terranes were thrust over the lake melville terrane and part of the groswater bay terrane and they were only marginally affected by grenvillian metamorphism the more deeply buried lake melville and groswater bay terranes however were significantly modified during tectonothermal events at 1030 ma and 980 960 ma respectively