bluuebunny/crossref_metadata_2025 huggingface dataset:
40Ar/39Ar dating of ribbon-textured veins and wall-rock material from Meguma lode gold deposits, Nova Scotia: implications for timing and duration of vein formation in slate-belt hosted vein gold deposits
Authors: Kontak Daniel J , Horne Richard J , Sandeman Hamish , Archibald Douglas , Lee Jim KW
Published: April 2011

Results of 15 40Ar/39Ar age spectra for whole-rock argillite samples collected from within and adjacent to veins from eight Meguma gold districts in the Meguma Terrane of southern Nova Scotia are presented. The samples give excellent plateau ages (i.e., concordance of plateaux, correlation, and integrated ages) that range from ca. 379 to ca. 403 Ma, and there is no apparent correlation of age with bulk rock composition (K2O, Al2O3) or sampling localities. In addition, apparent ages for samples from a district are similar within analytical error, except for one deposit, and there is no difference for samples from within ribbon-textured veins versus samples collected outside of the gold districts (i.e., in areas free of quartz veining). The results of this work compare well to previous whole-rock 40Ar/39Ar dating of Meguma Group samples and we concur with previous workers that the range in dates (i.e., 380-410 Ma) reflects diachronous cooling of the area through the intracrystalline retention temperature for argon in mica (i.e., approx. 300-350°C). However, with respect to the vein samples, there is a marked difference between the 40Ar/39Ar ages of vein-hosted whole-rock samples and hydrothermal minerals (amphibole, muscovite, biotite) from the same deposits previously dated, which indicates that whole-rock samples have retained their metamorphic ages and have not been reset by the later hydrothermal event responsible for vein formation, despite being incorporated within the high-temperature fluids (approx. 400-450°C). This discrepancy in ages indicates that the wall rocks and veins were in thermal disequilibrium, as the vein temperatures were well above that required to cause diffusion of argon out of mica phases within the whole-rock samples, and implies therefore that the fluids must have been derived from depth. The results have the following important implications for models of vein formation: (i) vein formation was rapid and is consistent with models of hydrofracturing due to fluid overpressure, and (ii) the vein-forming fluids were derived from depth and cannot have been produced by a lateral secretion processes whereby fluids and gold are derived from the Meguma Group.
First report of protoceratopsians (Neoceratopsia) from the Late Cretaceous Judith River Group, Alberta, Canada
Authors: Ryan Michael J , Currie Philip J
Published: April 2011

Protoceratopsians are best known in North America from associated skeletal material of Montanoceratops from the early Maastrichtian of Montana and Campanian of Alberta and Leptoceratops from the late Maastrichtian of Alberta and Wyoming. We report here the first occurrence of protoceratopsian elements from the middle Campanian (Dinosaur Park Formation) of Alberta. The specimens consist of a fragmentary right dentary and an almost complete left dentary which can be referred to Leptoceratops sp. Recent examination of Albertan microvertebrate material has identified cf. protoceratopsians teeth from the latest Santonian (Milk River Formation), extending the record of Albertan protoceratopsians back almost 20 million years. The rarity of these small ornithischians in the fossil record of Alberta may have been due to ecological exclusion from the wet, coastal environments that were preferred by the larger, more abundant ceratopsids.
Holocene delevelling of Devon Island, Arctic Canada: implications for ice sheet geometry and crustal response
Authors: Dyke Arthur S
Published: April 2011

The raised beaches and deltas of Devon Island contain an abundance of dateable materials. A large set of radiocarbon dates (228), 154 of which are new, are used to construct relative sea level curves and isobase maps for the island. The best materials for this purpose are driftwood logs (61 dates) and bowhead whale bones (74 dates) from raised beaches and mollusc shells from marine-limit deltas (20 dates) or from altitudes close to marine limit (14 dates). During the last glacial maximum, the island is thought to have lain beneath the southeastern flank of the Innuitian Ice Sheet. The relative sea level history is congruent with that inferred ice configuration. The island spans half the ice sheet width. Relative sea level curves are of simple exponential form, except near the glacial limit where an early Holocene emergence proceeded to a middle Holocene lowstand below present sea level, which was followed by submergence attending the passage of the crustal forebulge. The response times of relative sea level curves and of crustal uplift decrease from the uplift centre toward the limit of loading, but the change appears strongest near the limit. The Innuitian uplift is separated from the Laurentide uplift to the south by a strong isobase embayment over Lancaster Sound. Hence, ice load irregularities with wavelengths of about 100 km were large enough to leave an isostatic thumbprint in this region of the continent. The apparent absence of a similar embayment over Jones Sound probably indicates a greater Late Wisconsinan ice load there, or a thicker crust than in Lancaster Sound.
Age and origin of the Jan Lake Complex: a glimpse at the buried Archean craton of the Trans-Hudson Orogen
Authors: Ashton K E , Heaman L M , Lewry J F , Hartlaub R P , Shi R
Published: April 2011

The largely buried Sask Craton is a continental fragment or microcontinent that collided with and underthrust the Paleoproterozoic Flin Flon - Glennie protocontinent along the Pelican Décollement Zone at 1840-1830 Ma. Rocks of the Sask Craton are exposed in three tectonic windows. Those of the Pelican Window have been named the Jan Lake Complex and comprise [Formula: see text] 2960 Ma arc-derived leucocratic orthogneisses, migmatitic paragneisses, and a tholeiitic, within-plate igneous suite comprising 2488 Ma dioritic to gabbroic rocks and 2450 Ma enderbitic and charnockitic rocks including the Sahli Granite. Ages of ca. 2450 Ma are also common from the other two tectonic windows to the Sask Craton, suggesting that emplacement of the igneous suite was widespread and perhaps part of the coeval Matachewan Igneous Event. The absence of rocks in the [Formula: see text] 2960 and ca. 2450 Ma age ranges on both the adjacent Superior and Rae-Hearne cratonic margins makes it improbable that the Sask Craton was derived by simple fragmentation without large-scale tectonic transport. The overthrust Paleoproterozoic rocks represent a high-grade northwestern extension of the Flin Flon volcanic belt and include 1856 Ma leucotonalite and 1843 Ma quartz monzodiorite. An 1830 Ma suite of homogeneous, calc-alkaline enderbitic rocks, which intrude Burntwood semipelitic migmatites throughout the Kisseynew Domain, has also been emplaced in mylonites of the Pelican Décollement Zone. Zircon and monazite ages in the 1812-1803 Ma range record a high-grade metamorphic event that resulted from collision between the Sask Craton and Flin Flon - Glennie Complex.
Metamorphism of the MacLean Lake and Central Metavolcanic belts, La Ronge domain, Trans-Hudson Orogen: pressure-temperature variations and tectonic implications
Authors: Yang Haiming , Kyser Kurt , Ansdell Kevin
Published: April 2011

Metamorphic assemblages differ between the metasedimentary MacLean Lake belt and the adjacent Central Metavolcanic belt in the La Ronge domain, Trans-Hudson Orogen. The former consists of meta-arkoses, psammitic gneisses, metaconglomerates, and calc-silicate gneisses of upper amphibolite facies (600-740°C, 440-660 MPa) with local migmatization, whereas the latter is comprised mainly of metavolcanic and plutonic rocks, with minor metasedimentary schists of greenschist to lower amphibolite facies (480-630°C, 520-560 MPa). Petrographic evidence indicates that peak metamorphic conditions were reached towards the end of D1 deformation during which the Central Metavolcanic belt was thrust onto the MacLean Lake belt along the McLennan Lake tectonic zone, which separates the two belts. Peak metamorphic assemblages did not undergo retrograde alteration during D2 deformation, indicating that high temperature was maintained during D2 deformation. Differences in pressure (P) and temperature (T) between the northeastern and southwestern parts of the Central Metavolcanic belt may have resulted from tilting along strike after peak metamorphism. Peak temperatures increase gradually from the Central Metavolcanic belt to MacLean Lake belt across the McLennan Lake tectonic zone. Peak pressures in the two belts are similar, implying that the Central Metavolcanic belt thrust sheet was probably thin. The P-T data for the MacLean Lake belt indicate a relatively high thermal gradient (40-50°C/km), similar to that in the metasedimentary Kisseynew domain in the orogen.
Eocene Challis-Kamloops volcanism in central British Columbia: an example from the Buck Creek basin
Authors: Dostal J , Robichaud D A , Church B N , Reynolds P H
Published: April 2011

Eocene volcanic rocks of the Buck Creek basin in central British Columbia are part of the Challis-Kamloops volcanic belt extending from the United States across British Columbia to central Yukon. The volcanic rocks include two units, the Buck Creek Formation, composed of high-K calc-alkaline rocks with predominant andesitic composition, and the overlying Swans Lake unit made up of intraplate tholeiitic basalts. Whole rock 40Ar/39Ar data for both units show that they were emplaced at 50 Ma. They have similar mantle-normalized trace element patterns characterized by a large-ion lithophile element enrichment and Nb-Ta depletion, similar chondrite-normalized rare earth element patterns with (La/Yb)n ~4-14 and heavy rare earth element fractionation, and overlapping epsilonNd values (2.4-3.1) and initial Sr-isotope ratios ( ~ 0.704). These features suggest derivation of these two units from a similar mantle source, probably garnet-bearing subcontinental lithosphere. The differences between tholeiitic and calc-alkaline suites can be due, in part, to differences in the depth of fractional crystallization and the crystallizing mineral assemblage. Fractional crystallization of the calc-alkaline magmas began at a greater (mid-crustal) depth and included fractionation of Fe-Ti oxides. The volcanic rocks are probably related to subduction of the Farallon plate under the North American continent in a regime characterized by transcurrent movements and strike-slip faulting.
Geochemical and Nd-Pb isotopic systematics of late Archean granitoids, southwestern Slave Province, Canada: constraints for granitoid origin and crustal isotopic structure
Authors: Yamashita Katsuyuki , Creaser Robert A , Stemler James U , Zimaro Tony W
Published: April 2011

New geochemical and Nd-Pb isotopic data for ~ 2.62-2.59 Ga granitoids from the southwest Slave Province are used to determine the source(s) of granitoid magmas, to evaluate the role of pre-2.8 Ga basement during this magmatism, and to refine the existing Nd-Pb isotopic structure of the western Slave Province. The Pb isotopic data require crust older than ~3.2 Ga as a granitoid protolith, whereas the Nd isotopic data require input from juvenile crustal material. This discrepancy is explained if the granitoid protoliths are mixtures of ancient basement and ~2.7 Ga juvenile crust in varying proportions. Specifically, granitoids from the southwestern Slave Province require 10-30% basement, whereas granitoids from other parts of the western Slave Province require >50%. Incorporation of basement as a protolith may be achieved indirectly, by assimilation of basement during juvenile ~2.7 Ga magmatism, or directly during ~2.62-2.59 Ga magmatism. The granitoid isotopic data suggest that indirect basement input was important on a regional scale, but direct input may have also taken place in some areas of the western Slave Province, particularly along the ~111°W "isotopic boundary" zone previously recognized. The geochemical characteristics of these granitoids are compatible with an origin by partial melting of dominantly amphibolite and metasedimentary rocks to produce the ~2.61 Ga and ~2.59 Ga magmatism, respectively; partial melting occurred in response to regional crustal thickening at this time.
U-Pb ages and tectono-magmatic evolution of Middle Ordovician volcanic rocks of the Wild Bight Group, Newfoundland Appalachians
Authors: MacLachlan Kate , Dunning Greg
Published: April 2011

The Wild Bight Group and spatially associated South Lake Igneous Complex form one of the Ordovician oceanic terranes of the central mobile belt of the Newfoundland Appalachians. An integrated study of these rocks, involving detailed mapping, geochemistry, Sm-Nd isotopic analyses and U-Pb geochronology, has shown that there are two temporally and genetically distinct volcanic sequences within the Wild Bight Group. The younger sequence comprises a lower volcanic succession associated with coarse volcaniclastic rocks and an upper volcanic succession interbedded with argillite, chert, and minor greywacke. The lower volcanic succession has calc-alkaline affinties, and isotopic evidence for minor crustal contamination. It is interpreted to represent a volcanic arc formed in proximity to the Gondwanan margin, above an east-dipping subduction zone. The age of this volcanic sequence is confined to 472 ± 3 Ma by felsic tuffs which occur stratigraphically above and below it. The upper volcanic unit has predominantly enriched tholeiitic to alkaline geochemical characteristics with isotopic signatures indicative of little or no crustal contamination, and is interpreted to represent arc rifting. The age of this sequence was determined indirectly by dating two geochemically related gabbro sills (472+2-9 Ma and 471 ± 4 Ma). This work shows that despite different lithologies and stratigraphic and structural relationships between Early and Middle Ordovician sequences in the northern and southern Exploits Subzone, they have undergone essentially the same tectono-magmatic events. The age constraints on the magmatic events in the Wild Bight Group provide evidence for the timing of "obduction" of Early Ordovician oceanic sequences and the reversal of subduction polarity along the Gondwanan margin, suggested by previous workers.
Age structure and orogenic significance of the Berens River composite batholiths, western Superior Province
Authors: Corfu Fernando , Stone Denver
Published: April 2011

The Berens River area of northwestern Ontario is underlain mainly by Archean felsic plutonic rocks, which enclose minor supracrustal and gneissic enclaves and merge with the greenstone-belt-rich Uchi Subprovince to the south. U-Pb geochronology using zircon and monazite shows that the batholiths evolved mainly between 2750 and 2690 Ma by sequential and essentially continuous intrusive activity into an older substratum composed of 3000-2800 Ma volcanic and tonalitic crust. There is a broad, but not strict, compositional transition from early biotite tonalite and hornblende tonalite, progressing with time towards a greater abundance of hornblende granodiorite to granite, and finally to late biotite granite, rare peraluminous granites, and sanukitoid (dioritic, monzodioritic to granitic) plutons. The tonalite suites were predominantly synvolcanic. The late granitic intrusions postdated volcanism, but were largely synchronous with the main compressional events that caused widespread sedimentation, deformation, and metamorphism in other parts of the region. The age patterns and compositional features of the batholiths and the spatial and temporal links between their evolution and those of the supracrustal sequences in the greenstone belts of the region are consistent with mechanisms of magma generation and emplacement at converging plate margins.
Paleoclimatic changes during the Late Pleistocene - Holocene in Laguna Babícora, near the Chihuahuan Desert, México
Authors: Ortega-Ramírez Jose R , Valiente-Banuet Alfonso , Urrutia-Fucugauchi Jaime , Mortera-Gutiérrez Carlos A , Alvarado-Valdez Guillermo
Published: April 2011

The paleoclimatic variability of northern Mexico since the Late Wisconsinan is determined from sedimentological, geochemical, and stratigraphic studies of the lacustrine sequence of Laguna Babícora (29.4°N, 107.7°W; elevation 2100 m asl). Chronological control is based on 13 radiocarbon dates (6 from this study and 7 previously reported) from four stratigraphic profiles that cover a range from 16 342 to 2800 BP. Two major periods of increased humidity are recognized by lake-level variation during the Late Wisconsinan and early Holocene. The Late Wisconsinan humid period may be related to jet stream migration to southerly latitudes and the influence of the mobile polar high. This period has been identified in the sedimentary record of El Diablo profile with a radiocarbon date of 16 342 ± 201 BP. The early Holocene humid period is recorded in El Diablo (10 976 ± 115 BP) and Cano profiles (9614 ± 130 BP). It is interpreted as a result of the Younger Dryas climatic anomaly characterized by a temperature decrease and cold paleoclimatic conditions similar to those for the Late Wisconsinan. The early Holocene was followed by an overall gradual drying that reached a maximum around 6000 BP, during the middle Holocene. For the late Holocene evidence of widespread erosion, sedimentation changes, flooded surfaces, and paleosoils indicates that Laguna Babícora was subjected to short-term climatic changes.
Sequential palynological changes across the composite Cretaceous-Tertiary (K-T) boundary claystone and contiguous strata, western Canada and Montana, U.S.A.
Authors: Sweet A R , Braman D R , Lerbekmo J F
Published: April 2011

The boundary claystone in the western Canada Basin is composed of three intimately associated layers with a regional distribution: a basal brownish-grey hackly claystone, a middle brownish-black satiny claystone, and an upper brown laminated shale. These layers appear to represent a graded succession in terms of grain size and depositional rate. An abrupt reduction in the relative abundance of gymnospermous pollen, representing canopy vegetation, immediately precedes the hackly layer. The hackly claystone is accepted as being originally formed of microtektites from a ballistically transported ejecta blanket deposited within minutes or hours of the bolide's impact and associated with a heat pulse. The variable miospore content of the hackly layer is considered to be from local, reproductively active understorey vegetation. The shocked-quartz-rich satiny claystone is interpreted as being formed by fine debris deposited over days, weeks, or months. Its homogeneous texture suggests a short depositional event, but must have involved enough time for a residual Cyathidites-Ulmoideipites survival flora to release miospores. Further, the presence of this flora requires the continuation of light levels required for photosynthesis and temperatures generally above freezing. A settling time of several years for very fine debris and aerosols would fit with the apparent seasonal interlayering of the organic tissue in the laminated shale. The abruptly overlying Laevigatosporites-Kurtzipites recovery flora initiates the succession leading to the return of a canopied swamp, and its presence may signal the clearing of the atmosphere of all impact-generated debris, except for postulated elevated CO2 abundances.
Detrital zircon geochronology of Neoproterozoic to Permian miogeoclinal strata in British Columbia and Alberta
Authors: Gehrels George E , Ross Gerald M
Published: April 2011

U-Pb ages have been determined on 250 detrital zircon grains from Neoproterozoic through Permian miogeoclinal strata in British Columbia and Alberta. Most of the grains in these strata are >1.75 Ga and are interpreted to have been derived from nearby basement provinces (although most grains were probably cycled though one or more sedimentary units prior to final deposition). Important exceptions are Ordovician sandstones that contain grains derived from the Peace River arch, and upper Paleozoic strata with detrital zircons derived from the Franklinian orogen, Salmon River arch (northwestern U.S.A.), and (or) Grenville orogen. These provenance changes resulted in average detrital zircon ages that become progressively younger with time, and may also be reflected by previously reported shifts in the Nd isotopic signature of miogeoclinal strata. In addition to the grains that have identifiable sources, grains of ~1030, ~1053, 1750-1774, and 2344-2464 Ma are common in our samples, but igneous rocks of these ages have not been recognized in the western Canadian Shield. We speculate that unrecognized plutons of these ages may be present beneath strata of the western Canada sedimentary basin. Collectively, our data provide a record of the ages of detrital zircons that accumulated along the Canadian Cordilleran margin during much of Paleozoic time. Comparisons between this reference and the ages of detrital zircons in strata of potentially displaced outboard terranes may help reconstruct the paleogeography and accretionary history of the Cordilleran orogen.
Archean neptunian fissures and early history of the Destor-Porcupine fault zone, Timmins, Ontario, Canada
Authors: Beavon Roy V
Published: April 2011

Neptunian dikes and fissures are intimately associated with a minor Archean sedimentary basin near Timmins, Ontario, in the southwestern part of Abitibi Subprovince of the Canadian Shield. These structures are associated with the late Archean Timiskaming unconformity, and were formed by clastic sedimentation in fissures opened by the reactivation and dilation of basement faults along a major crustal shear. A "pull-apart" origin is indicated for the sedimentary basin by published township maps and the underground geology of the Dome gold mine. The neptunian dikes and fissures are discussed in relation to previous stratigraphic and tectonic interpretations of the Timmins area.
Geochronology and geochemistry of the Pola de Allande granitoids (northern Spain): their bearing on the Cadomian-Avalonian evolution of northwest Iberia
Authors: Fernández-Suárez Javier , Gutiérrez-Alonso Gabriel , Jenner George A , Jackson Simon E
Published: April 2011

The Pola de Allande pre-Variscan tonalite-granodiorite plutons are located in the Narcea Antiform, at the boundary zone between the Cantabrian and west Asturian Leonese zones of the Iberian Variscan belt. These granitoids were intruded into a Neoproterozoic siliciclastic sedimentary sequence with subordinate volcanic intercalations and were subsequently overprinted by Variscan thrust-related shear deformation. U-Pb laser ablation inductively coupled plasma - mass spectrometry (ICP-MS) dating of zircons from two plutons yielded concordant ages of intrusion of 605 ± 10 and 580 ± 15 Ma. To the authors' knowledge, this is the first reported U-Pb Cadomian-Avalonian age for igneous rocks in this section of the Iberian Variscan belt. These intrusions are coeval with the main episode of dominantly calc-alkaline magmatic activity related to Cadomian-Avalonian subduction. Major and trace element composition of the granitoids is characteristic of I-type high-K calc-alkaline granitoids generated in continental arc settings, and are comparable to those of coeval granitoids in other areas of the Cadomian-Avalonian belt. Sr and Nd isotopic signatures indicate that the genesis of the Pola de Allande granitoids involved either mixing of mantle melts of Cadomian extraction with an older enriched crust (Eburnean-Icarthian, i.e., ca. 2 Ga old crust) or melting of a mafic infracrustal protolith with a Grenville age (ca. 1.2 Ga) mantle extraction. The Neoproterozoic sediments, hosting the Pola de Allande granitoids and present in large areas of northwest Iberia, may represent the back-arc basin of the subduction complex in which the Avalon composite terrane constituted the main magmatic arc.
The gravity field over the Ungava Bay region from satellite altimetry and new land-based data: implications for the geology of the area
Authors: Telmat Hamid , Mareschal Jean-Claude , Gariépy Clément
Published: April 2011

Gravity data were obtained along two transects on the southern coast of Ungava Bay, which provide continuous gravity coverage between Leaf Bay and George River. The transects and the derived gravity profiles extend from the Superior craton to the Rae Province across the New Quebec Orogen (NQO). Interpretation of the transect along the southwestern coast of Ungava Bay suggests crustal thickening beneath the NQO and crustal thinning beneath the Kuujjuaq Terrane, east of the NQO. Two alternative interpretations are proposed for the transect along the southeastern coast of the bay. The first model shows crustal thickening beneath the George River Shear Zone (GRSZ) and two shallow bodies correlated with the northern extensions of the GRSZ and the De Pas batholith. The second model shows constant crustal thickness and bodies more deeply rooted than in the first model. The gravity models are consistent with the easterly dipping reflections imaged along a Lithoprobe seismic line crossing Ungava Bay and suggest westward thrusting of the Rae Province over the NQO. Because no gravity data have been collected in Ungava Bay, satellite altimetry data have been used as a means to fill the gap in data collected at sea. The satellite-derived gravity data and standard Bouguer gravity data were combined in a composite map for the Ungava Bay region. The new land-based gravity measurements were used to verify and calibrate the satellite data and to ensure that offshore gravity anomalies merge with those determined by the land surveys in a reasonable fashion. Three parallel east-west gravity profiles were extracted: across Ungava Bay (59.9°N), on the southern shore of the bay (58.5°N), and onshore ~200 km south of Ungava Bay (57.1°N). The gravity signature of some major structures, such as the GRSZ, can be identified on each profile.
Teleseismic studies of the lithosphere below the Abitibi-Grenville Lithoprobe transect
Authors: Rondenay Stéphane , Bostock Michael G , Hearn Thomas M , White Donald J , Wu Hua , Sénéchal Guy , Ji Shaocheng , Mareschal Marianne
Published: April 2011

In the past decade, the Abitibi-Grenville Lithoprobe transect has been the site of numerous geological and geophysical surveys oriented towards understanding the lithospheric evolution of the southeastern Superior and adjoining Grenville provinces. Among the different geophysical methods that have been employed, earthquake seismology provides the widest range of information on the deep structures of the upper mantle. This paper presents a review of studies, both complete and ongoing, involving teleseismic datasets that were collected in 1994 and 1996 along the transect. A complete shear-wave splitting analysis has been performed on the 1994 dataset as part of a comparative study on electrical and seismic anisotropies. Results suggest a correlation between the two anisotropies (supported by xenolith data) and favour a lithospheric origin for the seismic anisotropy. The two anisotropies are believed to represent the fossilized remnants of Archean strain fields in the lithospheric roots of the Canadian Shield. Preliminary splitting results for the 1996 experiment suggest that the S-wave azimuthal anisotropy may be depth dependent and laterally varying. Ongoing receiver function analysis and traveltime inversion studies provide velocity models of the crust and upper mantle beneath the study area. Preliminary receiver function results reveal the presence of an S-velocity increase at ~90-100 km depth which appears to be laterally continuous over 200 km. Traveltime inversion models indicate the presence of an elongate, low-velocity anomaly beneath the southern portion of the 1996 array which strikes obliquely to major geological structures at the surface (e.g., Grenville Front). Preliminary interpretation relates this anomaly to the same process (e.g., fixed mantle plume, continental rifting) responsible for the emplacement of the Monteregian Hills igneous province.
Geochemical characteristics of the Eden Lake Complex: evidence for anorogenic magmatism in the Trans-Hudson Orogen
Authors: Halden Norman M , Fryer Brian J
Published: April 2011

The Eden Lake Complex intrudes granitic rocks located between the Lynn Lake and Leaf Rapids tectonic domains in the Reindeer Zone of the Trans-Hudson orogenic terrane in northern Manitoba. It consists of a number of magmatic phases. Of particular interest are aegirine-bearing monzonites and late rare earth element (REE)- and fluorine-enriched pegmatites. The aegirine monzonite ranges in SiO2 content from 51 to 76 wt.%, showing a continuous fractionation trend from low to high silica. The CaO, Fe2O3T, and MgO contents are low (about 1-6 wt.%), TiO2 contents range from 0.8 to about 0.1 wt.%, and total alkalis range from about 11 to 13 wt.%. The early evolution of the complex was influenced by pyroxene fractionation; during the later stages, K-feldspar fractionation had an increasing effect on the bulk composition of the magma. Accumulation of K-feldspar has obscured the tectonic signature of the complex, but elevated REE, high field strength element, and fluorine contents are consistent with an A-type granitoid association. The complex is associated with REE-bearing pegmatites. Total REE contents in the monzonites range from 1000 ppm in low-silica phases to 20 ppm in high-silica phases, with La/LuCN values ranging from 80 to 25. REE patterns reflect the fractionation of K-feldspar, pyroxene, and minor amounts of titanite, apatite, and zircon. Y/Nb and Yb/Ta values (relatively constant at ~1 and 1.5, respectively) are consistent with derivation from a depleted- mantle source enriched in REE; moreover, positive εNd values of 0.3-1.7 (with an assumed age of 1750 Ma) are consistent with derivation from an initially light rare earth element (LREE)-depleted source. Elevated Σ REE contents and LREE-enriched characteristics of the complex suggest the source region was altered prior to generation of the magma(s).
Submarine fan deposystems and tectonics of a Late Cretaceous forearc basin along an accretionary convergent plate boundary, MacColl Ridge Formation, Wrangell Mountains, Alaska
Authors: Trop Jeffrey M , Ridgway Kenneth D , Sweet Arthur R , Layer Paul W
Published: April 2011

Analysis of Upper Cretaceous sedimentary and volcanic strata in the Wrangell Mountains of south-central Alaska provides an opportunity to study the tectonics, depositional systems, and provenance of a forearc basin that developed along an accretionary convergent plate boundary. New data from the 1150 m thick MacColl Ridge Formation indicate that deposition occurred during the Campanian on a coarse-grained submarine fan that was derived from an uplifted allochthonous terrane exposed in the hanging wall of a fault system that separated the forearc basin from the subduction complex. New age controls include palynoflora indicative of a late middle to late Campanian age, and compatible radiometric age determinations of volcanic vitric-crystal tuffs near the top of the formation which have 40Ar/39Ar isochron ages of 79.4 ± 0.7 and 77.9 ± 2.1 Ma. Sedimentological and paleontological data show that sedimentation occurred on the inner portions of a sand- and gravel-rich submarine fan system. Evidence for this interpretation includes dominance of channelized sediment gravity flow deposits, particularly turbidites and debris flows; microflora indicative of open-marine conditions; unidirectional paleocurrent indicators; and syndepositional slump features. The pyroclastic eruptions that formed the vitric-crystal tuffs of the MacColl Ridge Formation are interpreted as products of the Late Cretaceous Kluane magmatic arc that bordered the forearc basin to the north. Sandstone and conglomerate compositional data combined with northward-directed paleocurrent indicators suggest that detritus was derived mainly from igneous rocks of the allochthonous Wrangellia terrane located in the hanging wall of the Border Ranges fault system along the southern margin of the basin. From a regional perspective, deposition of the MacColl Ridge Formation was coeval with the early part of Campanian-Maastrichtian synorogenic sedimentation and contractile deformation documented throughout the northwestern Cordillera.
A new specimen of Plioplatecarpus (Mosasauridae) from the lower Maastrichtian of Alberta: comments on allometry, functional morphology, and paleoecology
Authors: Holmes Robert , Caldwell Michael W , Cumbaa Stephen L
Published: April 2011

An incomplete specimen of the plioplatecarpine mosasaur Plioplatecarpus from the lower Maastrichtian locality of Scabby Butte (Saint Mary River Formation) closely resembles the slightly older Plioplatecarpus primaevus from Saskatchewan (Bearpaw Formation). However, quantitative and qualitative differences are observed: the vertebrae of the Scabby Butte specimen are more gracile and, although of similar shape and proportions, are relatively smaller. Since these differences cannot be readily explained by allometry or sexual dimorphism, the hypothesis that this specimen represents a new species is provisionally accepted. However, in the absence of diagnostic cranial features, a new species is not named. The morphology of the well-preserved forelimb indicates that the animal cannot be reconstructed as a subaqueous flyer, but probably used the forelimbs for paddling. The associated matrix indicates preservation in overbank deposits in a deltaic system exhibiting no evidence of open marine environments. This suggests that this mosasaur was able to exploit estuarine and freshwater environments.
Age and isotopic composition of late Archean leucogranites: implications for continental collision in the western Superior Province
Authors: Larbi Y , Stevenson R , Breaks F , Machado N , Gariépy C
Published: April 2011

U-Pb and Sm-Nd isotopic data are presented for leucogranites and pegmatites from the boundary zones of the English River, Winnipeg River, and Wabigoon subprovinces in the late Archean Superior Province of northwestern Ontario. The Ghost Lake batholith and the Separation Rapids pluton are posttectonic, rare-metal-bearing, S-type leucogranites that were generated during the final stages of the amalgamation of the Superior Province. U-Pb dating of monazites yields ages of 2650 ± 3 Ma for a pegmatite from the Dryden area and 2646 ± 2 Ma for the Separation Rapids pluton. Sm-Nd data from these granitoids are compared with similar data from late Archean intrusions and adjacent rocks from the same regions. Values of εNd range from 0 to +2 for pretectonic tonalites, from -2 to +2 for both the Ghost Lake batholith and the Separation Rapids pluton, from +1 to +3.5 for metavolcanic rocks, and from -0.5 to -1.5 for metasedimentary rocks. There is an overall trend of decreasing εNd values from pretectonic tonalites to the latest leucocratic pegmatites. This reflects the origin of more and more granitoids as a result of anatexis as the crust grew and thickened through accretion. The ranges of εNd values found among leucogranites from the two regions overlap with the isotopic values of the basalts and sediments. This suggests that the leucogranites were generated by similar processes involving both a crustal and a juvenile mantle component. Furthermore, the ages from this study and from pegmatites (2652-2643 Ma) in the Bird River greenstone belt provide the best estimate of the age of rare metal mineralization in the western Superior Province. Similar ages (2651-2639 Ma) for rare-metal-bearing leucogranites in southern Abitibi subprovince suggest a specific period of emplacement over a wide area.
Field, geochemical, and isotopic evidence for magma mixing and assimilation and fractional crystallization processes in the Quottoon Igneous Complex, northwestern British Columbia and southeastern Alaska
Authors: Thomas J B , Sinha A K
Published: April 2011

The quartz dioritic Quottoon Igneous Complex (QIC) is a major Paleogene (65-56 Ma) magmatic body in northwestern British Columbia and southeastern Alaska that was emplaced along the Coast shear zone. The QIC contains two different igneous suites that provide information about source regions and magmatic processes. Heterogeneous suite I rocks (e.g., along Steamer Passage) have a pervasive solid-state fabric, abundant mafic enclaves and late-stage dikes, metasedimentary screens, and variable color indices (25-50). The homogeneous suite II rocks (e.g., along Quottoon Inlet) have a weak fabric developed in the magmatic state (aligned feldspars, melt-filled shears) and more uniform color indices (24-34) than in suite I. Suite I rocks have Sr concentrations <750 ppm, average LaN/YbN = 10.4, and initial 87Sr/86Sr ratios that range from 0.70513 to 0.70717. The suite II rocks have Sr concentrations >750 ppm, average LaN/YbN = 23, and initial 87Sr/86Sr ratios that range from 0.70617 to 0.70686. This study suggests that the parental QIC magma (initial 87Sr/86Sr approximately 0.706) can be derived by partial melting of an amphibolitic source reservoir at lower crustal conditions. Geochemical data (Rb, Sr, Ba, and LaN/YbN) and initial 87Sr/86Sr ratios preclude linkages between the two suites by fractional crystallization or assimilation and fractional crystallization processes. The suite I rocks are interpreted to be the result of magma mixing between the QIC parental magma and a mantle-derived magma. The suite II rocks are a result of assimilation and fractional crystallization processes.
Field relations, U-Pb geochronology, and Sm-Nd isotope geochemistry of the Point Lake greenstone belt and adjacent gneisses, central Slave craton, N.W.T., Canada
Authors: Northrup C J , Isachsen C , Bowring S A
Published: April 2011

Data from the Point Lake area, central Slave craton, suggest an intimate tectonic and paleogeographic association between volcano-sedimentary supracrustal rocks and adjacent gneisses. Granite plutons and orthogneisses yield U-Pb zircon crystallization ages ranging from ca. 3230 to 2818 Ma. Numerous mafic dykes cut the gneisses, and two have been dated by U-Pb zircon geochronometry at 2673 ± 3 and 2690 ± 3 Ma, ages similar to those of volcanic rocks in the Point Lake greenstone belt. Although high-strain zones form the greenstone-gneiss in most places, a structural repetition of granite about 4 km east of Keskarrah Bay is cut by numerous mafic dykes and apparently overlain depositionally(?) by pillow basalt. Mafic volcanic and plutonic rocks from Point Lake have initial (2.7 Ga) εNd values ranging from about +2.2 to -6.3, significantly lower than the depleted mantle at that time. The Nd data suggest either derivation from a more isotopically evolved reservoir, or assimilation of crust similar to the granite gneiss at Point Lake. We infer from the presence of mafic dykes of appropriate age in the basement and the low initial εNd values of some pillow basalts that the volcanic sequence developed on the older granitic crust. The supracrustal rocks may have been deposited in a back-arc basin floored at least in part by attenuated continental material. Closure of the basin, bulk east-west shortening, and sinistral oblique or strike-slip faulting then obscured the original relations between the volcanic and gneissic rocks.
A near-ridge origin for seamounts at the southern terminus of the Pratt-Welker Seamount Chain, northeast Pacific Ocean
Authors: Cousens Brian , Dostal Jarda , Hamilton T S
Published: April 2011

Three seamounts close to the south end of the Pratt-Welker Seamount Chain, Gulf of Alaska, have been sampled to test whether or not mantle plume-related volcanism extends south of Bowie Seamount. Lavas recovered from Oshawa, Drifters, and Graham seamounts are weathered, Mn-encrusted pillow lavas and sheet-flow fragments, commonly with glassy rims. The glasses and holocrystalline rocks are tholeiitic basalts, with light rare earth element depleted to flat primitive mantle normalized incompatible element patterns and radiogenic isotope compositions within the ranges of mid-ocean ridge and near-ridge seamount basalts from the Explorer and northern Juan de Fuca ridges. Chemically, the seamount lavas strongly resemble older, "shield-phase" tholeiitic rocks dredged from the flanks of southern Pratt-Welker seamounts, but are distinct from the younger alkaline intraplate lavas that cap Pratt-Welker edifices. The weathered, encrusted basalts were most likely erupted in a near-ridge environment, adjacent to Explorer Ridge, between 11 and 14 Ma. No evidence of plume-related activity is found in this area. Compared with northeast Pacific mid-ocean ridge and alkaline intraplate basalts, Graham seamount lavas have anomalously high 206Pb/204Pb, which does not appear to be a function of sea-floor alteration, magma contamination, or mixing between previously identified mantle components. All near-ridge seamounts in the northeast Pacific exhibit isotopic heterogeneity that does not correlate with major or trace element composition, suggesting that the mantle sources of all near-ridge seamounts have been variably depleted by prior, but recent melting events.
Tectonic and regional metamorphic implications of the discovery of Middle Ordovician conodonts in cover rocks east of the Green Mountain massif, Vermont
Authors: Ratcliffe Nicholas M , Harris Anita G , Walsh Gregory J
Published: April 2011

Middle Ordovician (late Arenigian - early Caradocian) conodonts were recovered from a dolostone lens in carbonaceous schist 30 m below the base of the Pinney Hollow Formation in the Eastern Cover sequence near West Bridgewater, Vermont. These are the first reported fossils from the metamorphic cover sequence rocks east of the Green Mountain, Berkshire, and Housatonic massifs of western New England. The conodonts are recrystallized, coated with graphitic matter, thermally altered to a color alteration index (CAI) of at least 5, and tectonically deformed. The faunule is nearly monospecific, consisting of abundant Periodon aculeatus Hadding? and rare Protopanderodus. The preponderance of Periodon and the absence of warm, shallow-water species characteristic of the North American Midcontinent Conodont Province suggest a slope or basin depositional setting. The conodont-bearing carbonaceous schist is traceable 3 km southeast to the Plymouth area, where it had been designated the uppermost member of the Plymouth Formation, previously regarded as Early Cambrian in age. The age and structural position of the carbonaceous schist above dolostones of the Plymouth Formation but below the Pinney Hollow Formation (upper Proterozoic and Lower Cambrian?) suggest that this unit may be correlative or time transgressive with the Ira Formation, which underlies the Taconic allochthons in the Vermont Valley. Such a correlation supports the concept of placing the western limit of the root zone of the Taconic allochthons beneath the Pinney Hollow Formation. An approximate absolute age assignment for the conodont-bearing rock is between 470 and 454 Ma. This suggests that dynamothermal metamorphism during the Taconian orogeny on the east flank of the Green Mountains was younger than early Caradocian, which is in accord with the middle Caradocian age of the Ira Formation west of the Green Mountain massif.
Sedimentology, stratigraphy, and geomorphology of glacial Lake Calgary at Cochrane, Alberta, Canada
Authors: Fisher Timothy G
Published: April 2011

Stratigraphic and sedimentologic field data in the Cochrane, Alberta, area demonstrate that glaciolacustrine sediment comprising the Calgary Formation underlies glaciofluvial and fluvial sediment of the Bighill Creek Formation, previously dated at 11.4 ka BP. A continuous, conformable contact between sediments of glacial Lake Calgary and underlying till indicates that the lake was coeval with initial deglaciation of the area. The lake formed during retreat of the Cordilleran ice up the Bow Valley that was once previously coalescent with the Laurentide Ice Sheet. Rhythmic, graded, and convoluted glaciolacustrine sediments record continuous and high rates of sedimentation in this reach of glacial Lake Calgary, further implying that the lake formed early in deglacial time.
Timing of plutonism, deformation, and metamorphism in the Yellowknife Domain, Slave Province, Canada
Authors: Davis W J , Bleeker Wouter
Published: April 2011

U-Pb zircon, titanite, and monazite ages for plutonic rocks are used to constrain the timing of post-2660 Ma deformational and metamorphic events in the Yellowknife Domain, southern Slave Province. Diorite and tonalite plutons of the Defeat Suite yield ages of 2628 ± 3 and 2624 ± 3 Ma, older than previous, less precise estimates for this suite of ca. 2618-2620 Ma. The Defeat Suite plutons crosscut and postdate upright F1 folds in turbidites of the ca. 2680-2660 Ma Burwash Formation. D1 deformation is therefore bracketed between 2660 and 2630 Ma. The D1 event significantly predates deposition of polymict conglomerates of the Jackson Lake Formation. A second folding event (F2), represented by northwest- to north-trending steeply plunging folds of bedding and D1 structures, affected the area after intrusion of the Hidden Lake pluton at 2606 ± 2 Ma, and synchronously with intrusion of the 2596 ± 2 Ma Sparrow Lake pluton, a member of the two-mica granite Prosperous suite. The time of initiation and the duration of D2 deformation are not well defined. It may have affected units as young as the 2586 ± 2 Ma Morose Granite. Metamorphic isograds are spatially related to Defeat, Prosperous, or even younger plutonic complexes, and therefore formed at different times over a 30 Ma period in different parts of the Yellowknife Domain. The ca. 2620-2630 Ma Defeat-type plutonism started in the Yellowknife Domain 10-15 Ma earlier than similar intrusive events in the central and northern Slave Province and highlights diachronous development of plutonism in the period from 2630 to 2610 Ma. In contrast, the ages of younger granites in the Yellowknife Domain (Prosperous, Morose) are within the range of intrusive ages documented for similar plutonic suites elsewhere in the Slave Province.
Siliceous microfossils and mineral magnetic characteristics in a sediment core from Lake Manitoba, Canada: a remnant of glacial Lake Agassiz
Authors: Risberg Jan , Sandgren Per , Teller James T , Last William M
Published: April 2011

A 14.2 m long core was recovered from the southern Lake Manitoba basin. The sediment, consisting mainly of silty clay, was studied for siliceous microfossil content and mineral magnetics; 14 new accelerator mass spectrometry (AMS) radiocarbon dates provide chronological control of the paleoenvironmental history of the basin. The basal 5 m contains ice-rafted clasts and is largely barren of siliceous microfossils; these sediments were deposited when the lake was part of glacial Lake Agassiz. Sediments immediately overlying the barren part of the sequence contain AMS dates of 7700-7400 BP and reflect a dramatic change in conditions in the basin. Diatom abundances rise abruptly. Magnetic characteristics change substantially. The presence of freshwater taxa such as Stephanodiscus niagarae, together with brackish water diatoms, indicate that shallow, turbid, high-nutrient conditions with variable salinity occurred during the early part of the middle Holocene. Although climatic conditions throughout the northern Great Plains are known to have become drier and warmer during the mid-Holocene, there is a distinct change in diatom taxa in the Lake Manitoba sequence toward less saline conditions at this time. The presence of the riverine diatom Aulacoseira granulata in this interval supports previous conclusions that these freshwater conditions resulted from the northward diversion of the Assiniboine River into the basin. Following this, diatoms indicate an abrupt increase in salinity to >1500 mg·L-1 total dissolved solids between 4000 and 2600 BP, reflecting the diversion of the fresh waters of the Assiniboine River away from Lake Manitoba. Increasingly cooler and wetter conditions during the late Holocene, combined with differential isostatic rebound, caused a freshening of the lake during the late Holocene.