bluuebunny/crossref_metadata_2025 huggingface dataset:
Isolation of early Paleohelikian remanence in Grenville anorthosites of the French River area, Ontario
Authors: Stupavsky M. , Symons D. T. A.
Published: April 2011

Specimens were collected at 19 sites from Precambrian anorthosites in the French River region of the Grenville Province. AF and thermal step demagnetization analyses indicate the presence of four components: A, B, C, and D. After bulk AF and thermal demagnetization at 550, 650, and 670 °C, conventional two-tiered statistical analysis reveals only the presence of the B component for the collection. Point density contour plots isolate objectively all four components. Vector subtraction shows that the A component survives up to ~40 ± 10 mT or 300 ± 100 °C and resides in magnetite. Its pole position of 45°W, 26°N, δp = 9°, δm = 10° is consistent with it being a Grenvillian orogenic metamorphic overprint acquired 975 Ma ago at the same time as the K–Ar dates were reset. The B component survives up to 100 mT and resides both in magnetite up to 585° C and in hematite up to 670 °C. Its pole position of 26°W, 13°S, δp = 1°, δm = 2° is ~30° off the pre-1000 Ma portions of the APW path, but falls directly on the 1725 Ma portion. This fits the known whole-rock Rb–Sr and zircon U–Pb ages of 1700 ± 100 Ma found for these rocks, which date the Hudsonian Orogeny. The C component is found only at 650 °C or above. Its pole of 57°W, 27°S, δp = 4°, δm = 7° is 60° away from 1000–1400 Ma portion of the APW path but falls directly on the 1800 Ma path. The D component is isolated only after the 670 °C treatment. Its pole of 1°W, 6°N, δp = 4°, δm = 7° falls on the 1250 Ma segment of the APW path. It is attributed to alteration associated with the Sudbury olivine diabase dikes of this age, which are adjacent to some sites. Thus the Grenville Orogeny is essentially only a thermal event in the anorthosites, and the B and C components were acquired during cooling after the more intense Hudsonian Orogeny. Also population level screening provides a more efficient and objective way of isolating multicomponents than conventional tiered statistics after bulk AF or thermal cleaning of a collection.
Emplacement and differentiation of an Archean subvolcanic metapyroxenite–metagabbro sill in the Favourable Lake area, northwestern Ontario
Authors: Raudsepp M. , Ayres L. D.
Published: April 2011

A sequence of five fractionated, subvolcanic, pyroxenite–gabbro sills of tholeiitic basalt initial composition was intruded into the Archean Favourable Lake metavolcanic–metasedimentary sequence. Individual sills are up to 2.7 km long, range in thickness from 17 to 240 m, and are confined to a relatively thin lacustrine metachert and fine clastic unit. The sills were emplaced sequentially upward into water-rich, unlithified country rocks, with succeeding sills being emplaced prior to final consolidation of underlying sills. Sills are either in contact or separated by thin country-rock septa, but later sills did not intrude earlier sills.Greenschist facies metamorphism and pre-metamorphism alteration have destroyed primary mineralogy and have modified chemical variations, but primary textures are well preserved by pseudomorphs. Each sill comprises a lower ultramafic zone of clinopyroxene–olivine and clinopyroxene–orthopyroxene cumulates and an upper mafic zone of clinopyroxene–plagioclase – iron–titanium oxide and plagioclase – iron–titanium oxide cumulates. The upper part of the mafic zone is largely cumulus plagioclase that either accumulated in situ or floated upwards. The lower part of the ultramafic zone is relatively constant in composition and crystallization appears to have been largely in a closed system with only local magma flowage. In the mafic zone, crystallization was apparently in a more open system with surges of magma related to volcanic processes being derived either from elsewhere in the sills or externally. The most fractionated component is granophyric gabbro that is locally concentrated into large intrusive lenses.
A giant pterosaur (Reptilia: Archosauria) from the Judith River (Oldman) Formation of Alberta
Authors: Currie Philip J. , Russell Dale A.
Published: April 2011

A specimen discovered in the Judith River Formation, Dinosaur Provincial Park, documents the presence of a giant pterosaur in Alberta during Campanian times. The element lacks both ends, but is probably a femur. The size suggests that the wingspan of the animal from which it was derived would have been about 13 m. A diagnostic vertebra from a smaller specimen demonstrates that the giant pterosaur is probably Quetzalcoatlus.
Howe lake: a hydrodynamic blowout structure
Authors: Christiansen E. A. , Gendzwill D. J. , Meneley W. A.
Published: April 2011

Howe Lake (formerly called "Goudie Lake") is an unusual feature of southeast Saskatchewan. Almost perfectly circular (diameter 295 m), it is much deeper (26 m) than almost any other prairie lake. Initially noted in 1952, numerous investigations culminated in deep drilling and geophysical programs in the 1969–1976 period. The lake is the surface expression of a deep, funnel-shaped depression filled with coarse colluvium at the bottom and finer material near the top. A central pipe extends to a depth of at least 138 m. Radiocarbon dates and the history of deglaciation give evidence that the structure was formed between 12 000 and 12 500 years BP, shortly after the last ice sheet melted in the area. The structure was probably formed by a hydrodynamic blowout of water from the Mannville Group 400 m below surface. Extreme overpressure of water in the Mannville Group was probably induced by recharge from meltwater of the continental glacier then standing a few kilometres to the north of Howe Lake. The blowout was initiated at Howe Lake by fractures in the subsurface, related to a salt solution-collapse structure. Howe Lake probably served as a valve controlling the artesian pressure in the Mannville Group over a large area, until the active ice margin had retreated northeastward exposing lower elevation discharge areas. Hydrodynamic blowouts provide another process for the origin of breccia-filled pipes.
The late Quaternary geology of the Chalk River region, Ontario and Quebec
Authors: Catto N. R. , Patterson R. J. , Gorman W. A.
Published: April 2011

Glacial ice covered the Chalk River area through most of the Wisconsin Stage. About 11 300 years ago, an ice retreat was followed immediately by a short incursion of Champlain Sea waters, which deposited at least 2.5 m of clay, silt, and sand. A local readvance, probably associated with the St. Narcisse event, deposited till on the marine sediments. Following the final retreat of the ice from the area, lacustrine and aeolian deposition occurred locally for a short time.About 10 500 years ago, the North Bay drainage route opened, greatly increasing the discharge of the Ottawa River. A faint terrace at a present elevation of 209 m probably formed at this time. Changes in the drainage routes of proglacial lakes and in the rate of ice retreat caused a general decrease in discharge rates, and resulted in the formation of pronounced terraces, now at 180, 160, and 129 m, and fainter terraces at 170, 141, and 137 m. By about 5000 years BP, the North Bay outlet closed, and the river fell to approximately 111 m, its present elevation at Chalk River.During the whole period of terrace formation, alluvial sands were being deposited and, as river levels fell, exposed sands were reworked by the wind until anchored by vegetation. Charcoal horizons within the aeolian sequences indicate that forest fires occasionally destroyed the vegetation cover, re-initiating aeolian activity. Locally, active dunes are present near Chalk River, but most of the area has been stabilized by vegetation.
The Ordovician–Silurian boundary, northern Canadian Cordillera: graptolite and conodont correlation
Authors: Lenz A. C. , McCracken A. D.
Published: April 2011

The Upper Ordovician graptolite facies of the northern Canadian Cordillera is divided into two biostratigraphic units, the Dicellograptus ornatus Zone and the Pacificograptus pacificus Zone. Interbedded limestone yielded conodont Fauna 12. The uppermost Ordovician equivalents to the graptolite Climacograptus extraordinarius Zone and conodont Fauna 13 are absent, signifying a widespread stratigraphic hiatus probably attributable to the effects of glacially induced regression.The lowest Silurian graptolite fauna, of the Glyptograptus persculptus Zone, is tentatively identified in some sections, although it is absent in others. Conodonts from the zone, although present, are not diagnostic.Thicknesses of the lowest Silurian G. persculptus Zone sediments are anomalously high in comparison with average thicknesses of the zone elsewhere in the world.
The Devonian goniatite genera Pinacites and Foordites from Alaska
Authors: House M. R. , Blodgett R. B.
Published: April 2011

The Devonian goniatite genera Pinacites and Foordites are described from west-central Alaska. This is the first time that Pinacites has been found in North America. The faunas are compared with those of the Eifelian Stage of Europe and in particular with the lower part of the Chotéč Limestone of Czechoslovakia.
Paleomagnetism of the Little Dal lavas, Mackenzie Mountains, Northwest Territories, Canada
Authors: Morris W. A. , Aitken J. D.
Published: April 2011

The Proterozoic stratigraphic column of the Mackenzie Mountains is dominated by two main successions, the platformal "Mackenzie Mountains Supergroup" beneath, and the Windermere-equivalent Rapitan Group and younger strata, of rift-depression and slope origin, above. The former succession is at least partly older than 770 Ma, the latter younger than 770 Ma. These two main successions are locally separated by an unconformity-bounded succession, the "copper cycle." An important question is whether the copper cycle is more closely related in time and in origin to the older or the younger main succession. Determination of the paleomagnetism of the basaltic lavas locally preserved at the top of the Little Dal Group (top of the Mackenzie Mountains Supergroup) and comparison of their remanence directions with those published for other rocks bearing on the question were thought to be one way of shedding light on this question. Accordingly, paleomagnetic investigation of 10 sites in the Keele River area and six sites in the Thundercloud Range area was undertaken to obtain the remanence direction related to the initial extrusion of the lavas. Coherent directional groupings were only obtained from Little Dal lavas in the Keele River area. Of the three magnetizations found, LD-L (D = 304°, I = 20°, α95 = 7°) is assumed to represent the magnetization acquired on crystallization of the lavas. If this assumption is correct, the significant difference from the direction LD-A (D = 265°, I = 26°, α95 = 4°) reported elsewhere for strata low in the Little Dal Group suggests either that the lavas significantly postdate the group or that significant movement of the North American plate occurred during accumulation of the 2 km or so of platformal strata between the lower Little Dal beds and the lavas. The new results presented here also admit the conclusion that the Little Dal lavas do not represent the same igneous events as diabase intrusions dated at about 770 Ma that cut the lower Little Dal.
A possible annelid from the Trenton Limestone (Ordovician) of Quebec, with a review of fossil oligochaetes and other annulate worms
Authors: Morris S. Conway , Pickerill R. K. , Harland T. L.
Published: April 2011

A single specimen of fossil worm from the Trenton Limestone, recovered from a quarry northeast of Quebec City, may represent a new form that cannot be directly compared with any other known fossil or recent worm. The specimen is incomplete and shows evidence of decay, but the remarkable preservation of the surviving portion shows well defined transverse lines, interpreted as segmentation, and traces of internal organs. There is, however, no evidence for either setae or parapodia. In certain respects the anatomy of this worm is reminiscent of the annelids, and in particular the oligochaetes, but any similarity may be superficial. Other fossil worms that have been interpreted as oligochaetes are reviewed briefly, and a redescription of the worm Protoscolex covingtonensis Ulrich from the Eden Group (Ordovician) of Kentucky is given.
Lag of the Fraser glacial maximum in the Pacific Northwest: pollen and macrofossil evidence from western Fraser Lowland, British Columbia
Authors: Hicock Stephen R. , Hebda Richard J. , Armstrong John E.
Published: April 2011

Pollen and macrofossil evidence from two sites in northwestern Fraser Lowland reveals that Abies lasiocarpa – Picea cf. engelmannii forest and parkland grew there about 18 000 years ago under cold humid continental conditions. Taxus brevifolia was also a significant constituent of this forest. This plant assemblage resembles the ESSF Biogeoclimatic Zone of subalpine elevations in the northern interior of British Columbia (900–2250 m). Climate was probably cold with low to moderate rainfall and characterized by long, cold, wet winters and very short, probably dry, frost-free summers. Mean annual temperature was depressed about 8 °C and the tree line was probably 1200–1500 m lower than today.Fraser Lowland was probably removed from Pacific oceanic influence because the land–sea interface was located on the continental shelf to the west of Vancouver Island and Washington about the time of the last global glacial maximum, global depression of sea level, and Quadra Sand aggradation in the Pacific Northwest. Lowland glaciation was probably delayed because of insufficient precipitation in the drier macroclimate and the precipitation shadow created behind mountains on Vancouver Island and Olympic Peninsula. We speculate that, as Laurentide ice decayed, there was a northward shift of zonal weather patterns over the eastern Pacific, bringing very wet winters to the Fraser Lowland and providing moisture for rapid, extensive, Vashon glaciation, which culminated about 14 500 BP, lagging at least 3000 years behind the Laurentide glacial maximum.
Discrete single-domain and pseudo-single-domain titanomagnetite particles in silicic glass of an ash-flow tuff
Authors: Geissman John Wm. , Newberry Nancy G. , Peacor Donald R.
Published: April 2011

Single-domain and pseudo-single-domain titanium-poor magnetite grains are present in silicic vitrophyre glass of an ash-flow tuff from western Nevada. Conventional transmission electron microscopy and analytical electron microscopy corroborate inferences, made solely from rock magnetic and remanence data, that the thermoremanent magnetization in the vitrophyre is carried by such grains. The magnetite grains vary in size, shape, and distribution throughout the glass. They probably crystal lized during violent eruption and welding of the ash flow, at temperatures well in excess of 600 °C.
A mid-Holocene record of vegetative change in central Alberta
Authors: Vance Robert E. , Emerson Don , Habgood Thelma
Published: April 2011

Sediments from three lakes in central Alberta, Smallboy Lake (53°35′N, 114°8′W), E.I. Pond (53°38′N, 112°51′W), and Hastings Lake (53°25′N, 112°53′W), have been analyzed for their pollen content, charcoal remains, and (in two lakes) pyrite spherule concentration. The earliest record (radiometrically dated at 7400 years BP) indicates the existence of mixed-wood parkland vegetation. By 5000 years BP the regional vegetation had a considerably more open structure than now and was subject to frequent fires, presumably a response to the warm, dry Hypsithermal climate of this time. The termination of the Hypsithermal Interval (4000 years bp) is recorded in all three lakes by a marked increase in precipitation. The onset of a cooler, moister climatic regime stimulated forest closure and reduced regional fire activity, although the local vegetation of each of the three study sites responded in a unique way to the changing climate. By 3000 years BP the vegetation resembled the modern vegetation. Little change is recorded in the sediments from 3000 years BP to the present.
Correlation of two Helikian peralkaline granite – volcanic centres in central Labrador
Authors: Hill J. D. , Thomas A.
Published: April 2011

Recent mapping in central Labrador has resulted in the recognition and correlation of two Neohelikian peralkaline silicic igneous centres. The Flowers River igneous suite is circular in shape, covers an area of approximately 1720 km2, and consists of undeformed comenditic granite in contact with extrusive equivalent porphyry, felsite, tuff, and breccia. The Letitia Lake volcanic complex has been deformed by the Grenvillian Orogeny into an elliptical structure that covers approximately 450 km2. The complex consists of comenditic granite and syenite of the Arc Lake intrusive suite and related porphyry, rhyolite, tuff, and volcanogenically derived sediments of the Letitia Lake Group. Undersaturated aenigmatite–nepheline gneisses and syenites of the Red Wine alkaline complex are associated in space and time with the peralkaline silicic rocks of the Letitia Lake complex. The two centres are separated by 175 km and are an integral part of a Neohelikian period of uplift and continental rifting that involved formation of plateau basalts, terrestrial sediments, diabase dikes, and peralkaline magmatic centres in a belt extending from south Greenland to Lake Superior.
Sedimentary environments and postglacial history of the Fraser Delta and lower Fraser Valley, British Columbia
Authors: Clague John J. , Luternauer John L. , Hebda Richard J.
Published: April 2011

The Fraser River delta, which is about 1000 km2 in area above low tide level, has been built into the Strait of Georgia in southwestern British Columbia during the Holocene. Present-day sedimentary environments, including foreslope, tidal flat, river channel, floodplain, and bog, also existed earlier during the delta's development. Borehole data reveal a succession of sedimentary environments related to Holocene progradation of the delta south and west of New Westminster. At each site, marine basin and distal foreslope sediments are overlain by proximal foreslope materials, which in turn are overlain by coarser intertidal platform and channel deposits capped by floodplain and bog sediments.Initial growth of the Fraser Delta was preceded both by deglaciation of the region and by the rapid westward extension of the Fraser River floodplain down a partially submerged, glacially scoured trough east of New Westminster. Irregularities on the trough floor were covered by fluvial, deltaic, marine, and lacustrine sediments as the floodplain extended westward. About 10 000 years ago, the Fraser River began to empty directly into the Strait of Georgia through a gap in the Pleistocene uplands at New Westminster. A delta was constructed south and west from this site as the sea dropped below its present level relative to the land. Deltaic progradation continued after sea level stabilized at about −12 m elevation after 8000 years BP. A marine transgression between 7000–7500 and 5000–5500 years ago inundated parts of the Fraser proto-delta and temporarily inhibited its seaward advance. This transgression ended with the sea perhaps 1 or 2 m below its present position, whereupon a large area of the delta became emergent and large bogs began to form. During the remainder of the Holocene, the Fraser Delta grew westward, but apparently not southward, under a regime of relatively stable sea levels.
The greenschist facies of an Archean assemblage near Wawa, Ontario
Authors: Studemeister Paul A.
Published: April 2011

A stock of trondhjemite intrudes an Archean succession metamorphosed to the greenschist facies at Gutcher Lake, 30 km north of Wawa in Ontario. The stock is 4 km2 in plan and is partly enveloped by an aureole of epidote–amphibole hornfels up to 1 km wide. Within this aureole chlorite partly replaces biotite; chlorite, calcite, and quartz mantle hornblende and epidote; zoned amphibole has a rim of actinolite; and albite varies from 0 to 8% An. The stock has chlorite pseudomorphous after biotite, and feldspar is mottled by white mica and has a clear rim of albite. Fractures filled with quartz, calcite, ankerite, white mica, chlorite, pyrite, and native gold cross-cut the stock and its aureole. Wall rocks to these veins were modified by hydrothermal alteration with addition of Si, Fe, K, H2O + CO2, S, and Rb, leaching of Na, and a shift in Fe2+/Fet from ~0.66 to ~0.90. Primary inclusions in the vein quartz have a solution with a CO2 gas bubble that homogenizes into the liquid at around 300 °C.Initial contact metamorphism of volcanic rocks at T = 450–550 °C and P < 200 MPa (2 kbar) formed an aureole of epidote–hornblende hornfels near the stock. Subsequent regional metamorphism during the Archean at T = 325–450 °C and P = 200–300 MPa (2–3 kbar) retrograded the stock and its contact aureole to a lower greenschist assemblage. The retrogression involved hydration and CO2 fixation in hornfels and trondhjemite by a hot reducing fluid of low salinity. This metamorphic fluid precipitated native gold with quartz and pyrite along fractures in response to cooling and chemical reaction with wall rocks.
Geomagnetic variation studies in the southern Appalachians: preliminary results
Authors: Mareschal Jean-Claude , Musser James , Bailey Richard C.
Published: April 2011

Magnetic field variations have been recorded at 12 stations located on a traverse across the Blue Ridge and the Piedmont provinces of the southern Appalachian system in Georgia. The records were Fourier analyzed, and the transfer functions, relating the anomalous vertical magnetic field variations to the horizontal field, were obtained for different periods ranging between 16 and 4000 s. The transfer functions for the southeasternmost stations seem to be affected by the coast effect and possibly by the Coastal Plain sediment. In the northwestern part of the survey, the direction of the transfer functions was away from the coast. This suggests the presence of a strong conductor in the lower crust or the upper mantle beneath the Blue Ridge and extending farther toward the northwest.
A survey of the Jurassic System of China
Authors: Yi-gang Wang , Dong-li Sun
Published: April 2011

Terrestrial Jurassic rocks are widespread in China, whereas marine Jurassic rocks are confined to Xizang (Tibet), South Qinghai, South Xinjiang, and, in limited parts of the sequence, to West Yunnan, Guangdong, South Hunan, and East Heilongjiang. Geographically, occurrences of Jurassic deposits may be divided into three regions and 13 districts: (I) the Xizang–Qinghai region, with the South Tethyan Himalayan, North Tethyan Himalayan, Lhasa, Xanza–Shiquanhe, Bangon–Dengqen, Karakorum–Tanggula, and Qamdo districts; (II) the Coastal Volcanic region of East China, with the Guangdong–Hunan, Fujian–Zhejiang, Heilongjiang, and Northeast China districts; and (III) the Stable Giant Basin region of West China, with the Sichuan–Yunnan and Northwest China districts. These regions and districts are described briefly, and preliminary biostratigraphic sequences and palaeogeography are presented. The upper and lower boundaries of the Jurassic System in China are briefly discussed.The Jurassic seas of China evidently were extensions of the Mediterranean (Tethyan) Sea and of the Pacific Ocean. The main transgressions occurred during Sinemurian, Bathonian–Callovian, and Tithonian – Early Cretaceous times. All three transgressions of the Mediterranean Sea were important, whereas only the earlier two transgressions from the Pacific Ocean were important.The relief and climate of China during the Jurassic Period are also discussed briefly. After Triassic tectonism, the ancient mountains, such as the Altai, Tian Shan, Kunlun, Altun, Hengduan, Qilian, Qinling, Dabei, North China Plateau, Huaibei Plateau, and Yunnan–Guizhou Plateau, originated. Latitudinal and longitudinal tectonic belts were important in the geological development of China and significantly influenced biogeography, palaeogeography, and climate.
Revisions to the stratigraphy of the Upper Ordovician Collingwood beds of Ontario—a potential oil shale
Authors: Russell D. J. , Telford P. G.
Published: April 2011

Preliminary results from a drilling programme aimed at evaluating the oil shale potential of the Whitby Formation (Upper Ordovician) of Ontario showed that the Craigleith Member of this unit possessed the greatest promise. However, some preliminary lithostratigraphic observations contradicted those of earlier workers, prompting a detailed analysis of the lithostratigraphy, log response, and organic geochemistry of this unit of organic-rich interbedded shale and limestone, also known informally as the "Collingwood shales." Previously grouped with overlying non-calcareous shales, these strata are found to be in gradational contact with underlying limestones of the Lindsay Formation. This, together with a sharp decrease in carbonate and organic carbon contents at their top, leads to their redefinition as the Collingwood Member of the Lindsay Formation. Analysis of subsurface data shows that the unit now termed the "Collingwood Formation" in well records does not contain any of the Collingwood strata as originally defined, and that the potential oil shales of the newly defined Collingwood Member have a significantly restricted distribution. The non-calcareous blue-grey shales previously referred to the upper part of the Whitby Formation are redefined as the Blue Mountain Formation, which is dominated by the brown and grey slightly organic and calcareous Rouge River Member only in the area east of Toronto.
The Triassic and Jurassic paleogeography and evolution of the Qinghai–Xizang (Tibet) Plateau
Authors: Wang Yi-Gang , Sun Dong-Li
Published: April 2011

The Kunlun, Qilian, and Qinling mountains already existed in embryonic form as coastal ranges in Triassic times. Marine conditions predominated to the south. The southern coastline is demarcated by the northern margin of the Tertiary Siwalik sediments. The paleo-landmass of Longmenshan–Sichuan–Yunnan separated this sea from that of southwest China. By Jurassic times, the coastline of the Eurasian continent within what is now China had already moved southwards to a line along the Kunlun and Hengduan mountains.Mainly on the basis of differences between continental and oceanic crust, several different seas can be distinguished: the Himalayan, Lhasa, Qiangtang, Hengduan, and Triangle seas, together with the Qaidam Peninsula and Xining Bay. Triassic and Jurassic transgressions and regressions of these seas are discussed in detail.During Carboniferous and Permian times the Lhasa and Himalayan seas were joined together with the main Gondwana Plate to the south. In Triassic times, when the India–Pakistan Subcontinent split from Gondwanaland, the Lhasa Sea was a small microplate, which separated from the Himalayan Sea along a deep fracture that follows the line of the Yarlung Zangbo (River). It drifted northeastwards relatively rapidly. The Triangle Sea was a similar microplate that drifted northwards. In Late Triassic times the Triangle Microplate collided with Eurasia along the margin of the Qaidam–Qinling fault belt. The Bangong Co – Nu Jiang deep fracture is regarded as the boundary between the northern and southern margins of the eastern Tethys, on the basis of Carboniferous to Jurassic paleobiogeographical provinces within the region studied.During Triassic and Early and Middle Jurassic times, an oceanic basin separated the Lhasa Sea from the Qiangtang and Hengduan seas. In the Late Jurassic Epoch, the Lhasa Microplate converged with the Qingtang Microplate, which had become attached to Eurasia. The former was subducted northwards. As a result, the boundary between the southern and northern margins of East Tethys was shifted southwards during Cretaceous times to the line of the Yarlung Zangbo (River) deep fracture. At the end of the Cretaceous and on into Eocene times, the Himalayan Sea closed with the collision of the India–Pakistan Subcontinent and the Lhasa Plate. Folding and faulting followed and the paleo-oceanic history of the Qinghai–Xizang Plateau came to an end.
Geochemistry of Precambrian basaltic rocks from the Central African Republic (Equatorial Africa)
Authors: Dostal J. , Dupuy C. , Poidevin J. L.
Published: April 2011

The two Archaean greenstone belts (Bandas and Bogoin) in the Central African Republic (Equatorial Africa) are 250 and 150 km long. The metavolcanic rocks in the belts are predominantly komatiitic and tholeiitic basalts. Komatiites include both Al-depleted and Al-undepleted types. The komatiites and light-REE-depleted tholeiites were probably derived from a similar upper mantle source. However, the tholeiitic basalts enriched in light REE from the upper volcanic strata of the Bandas belt were generated from a different source. The dolerites from Proterozoic dyke swarms and sills differ from the basalts mainly in their abundances and ratios of several incompatible elements such as K, Rb, Th, and light REE. They were derived from a distinct, incompatible-element-enriched upper mantle source.The average background gold levels in the Bandas belt and dolerite dyke swarms are comparable to those in equivalent rocks from North America. The exception is the Bogoin greenstone belt, which has anomalously high gold abundances.
Archaean stromatolites from the Steep Rock Group, northwestern Ontario, Canada
Authors: Wilks M. E. , Nisbet E. G.
Published: April 2011

Reinvestigation of the late Archaean stromatolites of the Steep Rock Group has shown that a wide variety of forms is present, including domed and tabular bioherms and biostromes. Both columnar and noncolumnar structures are present. Branching is common in some columnar forms. Facing directions in the stromatolites are consistent with other field evidence showing that the base of the Steep Rock Group is an unconformity.
An 40Ar/39Ar geochronological and electron microprobe investigation of an Archean pyroxenite and its bearing on ancient atmospheric compositions
Authors: Hanes J. A. , York D. , Hall C. M.
Published: April 2011

A detailed 40Ar/39Ar step-heating geochronological study of pyroxene mineral separates from the Munro–Warden ultramafic sill in the Archean Abitibi Greenstone Belt of the Canadian Superior Province yields a precise primary age of 2703 ± 11 Ma (1σ). This is in excellent agreement with previous U–Pb zircon dates of 2700–2712 Ma for the enclosing felsic volcanics and a Sm–Nd age of 2622 ± 60 Ma for the sill. Electron microprobe analyses, coupled with the argon evolution patterns, demonstrate that deuteric amphibole is responsible for this primary age, whereas the volumetrically dominant diopside shows anomalously young ages. An isotope correlation plot reveals two components of contaminating argon with differing 40Ar/36Ar ratios in distinctly different sites. One component, trapped during the deuteric formation of amphibole, has an 40Ar/36Ar ratio that is significantly less (258 ± 3) than the present-day atmospheric value of 295.5. This may be an indication of the atmospheric ratio 2.7 Ga ago. The other component has a high 40Ar/36Ar ratio and was trapped during a later thermal event. The data demonstrate the potential of the 40Ar/39Arep-heating method for not only obtaining primary ages from low-potassium Archean ultramafic rocks, but also providing information on ancient atmospheric compositions.
Postglacial relative sea-level change, Port au Port area, west Newfoundland
Authors: Brookes I. A. , Scott D. B. , McAndrews J. H.
Published: April 2011

We first report pollen and foraminifera analyses and radiocarbon dates from two cores taken from salt-marsh deposits bordering Port au Port Bay, southwestern Newfoundland. Results show that relative sea level (RSL) stood at 2.8 m below present higher high-water level (HHWL) at 2770 ± 300 years BP and at −1.8 m at 2365 ± 175 years BP at the core sites. They permit calculation of a rate of late Holocene RSL change from western Newfoundland. We then report other available dates bearing on the earlier RSL record of this area.A date of 5800 ± 200 years BP fixes the age of minimum RSL in Port au Port Bay at 11–14 m below present. A date of 9350 ± 120 years BP from St. George's provides a minimum age for the passage of sea level below present there. A date of 12 600 ± 140 years BP from Stephenville fixes a sea level at 29 m above present, whereas one of 13 600 ± 110 years BP from Abrahams Cove dates the marine limit at 44 m. These geographically restricted data closely constrain a curve of postglacial RSL change in the Port au Port Bay – northern St. George's Bay area. The form of the curve supports a recent model predicting sea-level response to wastage of a limited late Wisconsinan ice load in the wider region.
Age relations, chemistry, and petrogenesis of mafic alkaline dikes from the Monteregian Hills and younger White Mountain igneous provinces
Authors: Eby G. Nelson
Published: April 2011

The mafic alkaline dikes of the Monteregian Hills and younger White Mountain igneous provinces can be divided into three groups: (1) K2O-rich alnoites; (2) moderately to strongly undersaturated monchiquites, camptonites, and basanites; and (3) slightly undersaturated to critically saturated camptonites and alkali olivine basalts. The dikes were emplaced between 139 and 107 Ma, with the bulk of the activity occurring in three discrete intervals: 139–129, 121–117, and 110–107 Ma. The first two intervals correspond to the times of emplacement of the main Monteregian intrusions. There is no apparent geographic pattern to the ages.Chemical evolution of the group 2 and group 3 magmas was largely controlled by the removal of olivine, clinopyroxene, and Fe–Ti-rich oxides. The group 2 dikes are generally enriched in REE and have higher La/Yb ratios (18–28) than the group 3 dikes (La/Yb = 9–23). For the majority of the samples Zr/Hf ratios (30–43) and Rb/Ba × 102 ratios (4.8–11.6) fall in the range of primary basalts, but some samples have higher ratios, indicating crustal contamination.Trace-element models indicate that the group 2 and group 3 magmas originated by variable degrees of melting of a metasomatized spinel lherzolite whereas the group 1 magmas most likely originated in a carbonated garnet lherzolite mantle. The thermal energy for the melting may have been provided by a mantle plume.
The stratigraphy and sedimentology of the Fundy Group (Triassic) of the St. Martins area, New Brunswick
Authors: Nadon G. C. , Middleton G. V.
Published: April 2011

Triassic sediments exposed along the south shore of New Brunswick, near St. Martins, were deposited primarily by a series of alluvial fans prograding eastward from the western margin of the Fundy half graben and are interbedded with fluvial conglomerates deposited by an axially flowing river system. The sediments are divisible into three formations, which are described here formally for the first time. These are, from base to top, the Honeycomb Point, Quaco, and Echo Cove formations. The basal Honeycomb Point Formation consists of approximately 990 m of red beds exposed in several fault blocks. The western outcrops consist of coarse breccias of the proximal fan, the Browns Beach Member, which fines eastward into sheetflood deposits of the mid- to distal fan, the McCumber Point Member. Both units contain eolian sands that indicate paleowind direction from the northeast. These are overlain by fluvial conglomerates of the Quaco Formation, 190–300 m thick, which were deposited by a large, braided river system that flowed north along the axis of the graben. Renewed growth of the alluvial fans led to the deposition of the Echo Cove Formation, 850–1300 m thick, on the conglomerates. The coarsest breccias are found in the westernmost exposures, the Stony Brook Member, and along the coast the formation is divided into the basal redbed unit, the Berry Beach Member, which grades into green beds of the Fownes Head Member, which in turn is overlain by the red beds of the Melvin Beach Member. Pollen recovered from the upper portion of the Fownes Head Member places the unit in the mid-Carnian (and possibly Ladinian).
The Anvil plutonic suite, Faro, Yukon Territory
Authors: Pigage Lee C. , Anderson Robert G.
Published: April 2011

The Anvil plutonic suite consists of three phases: a peraluminous muscovite–biotite granite (Mount Mye phase) and two metaluminous to peraluminous hornblende–biotite granodiorite and minor granite intrusions (Orchay and Marjorie phases). The suite is massive or foliated, equigranular or seriate, and contains alkali-feldspar megacrysts. The Marjorie phase is characteristically porphyritic.Geochemical trends are irregular for the suite and for individual phases. High-K2O, low-CaO, and low-MgO compositions typify the silicic, calc-alkaline suite. Hornblende-bearing phases contain less SiO2, K2O, and Rb and more cafemic oxides, TiO2, Sr, Ba, and Y than the Mount Mye phase and are compositionally similar to coeval South Fork volcanics.Isochrons from some of the Orchay phase whole-rock samples (t = 99 ± 2.5 Ma; 87Sr/86Sri = 0.7161 ± 0.0001) and from whole rocks and minerals of the Mount Mye phase (t = 100 ± 2 Ma; 87Sr/86Sri = 0.7405 ± 0.0001) indicate they are coeval but not comagmatic, accounting for the lithologic, petrographic, and geochemical distinctions. Similar K–Ar isotopic ages (81–102 Ma) suggest rapid cooling and therefore high-level emplacement. Together, the isotopic ages provide a minimum (youngest) age for the main deformation of the surrounding metasediments and a maximum (oldest) age for movement along the Tintina Fault.A petrographically and geochemically distinct sample from the Orchay phase yielded a Rb–Sr isochron age of 61 ± 1.5 Ma and an initial 87Sr/86Sr ratio of 0.7090 ± 0.0001, implying intrusive activity in the Paleocene.Field relations, lithology, petrography, geochronometry, and geochemistry suggest that the Orchay and Marjorie phases are plutonic equivalents of the South Fork volcanics. Similarities in plutonic style characterize the extensive mid-Cretaceous igneous event in southeast Yukon.
The Nares Strait gravity anomaly and its implications for crustal structure
Authors: Jackson H. R. , Koppen L.
Published: April 2011

A negative free-air gravity anomaly is associated with Nares Strait, the waterway that separates Ellesmere Island and Greenland. Two east–west gravity profiles that cross Ellesmere Island and Nares Strait were collected. A low with values in the range of −100 to −120 mGal (−1000 to −1200 μm/s2) was observed, and two-dimensional crustal models were created to identify the cause of the anomaly. The gravity anomaly cannot be attributed wholly to the bathymetry of the strait or to the sedimentary rocks underlying the strait. Crustal models that reproduce the anomaly have a M discontinuity that slopes under Nares Strait towards Ellesmere Island so that the crust beneath Ellesmere Island is thickened. The anomaly is similar to those associated with ancient and modern suture zones, regions of collided continental crust. Plate reconstructions suggest Nares Strait is a collisional boundary between the North American Plate (Ellesmere Island) and the Greenland Plate. The gravity anomaly supports this interpretation of Nares Strait.
Reconnaissance paleomagnetic study of igneous rocks from the eastern sector of the Labrador Trough
Authors: K.-Seguin Maurice , Clark Thomas
Published: April 2011

A paleomagnetic study has been carried out on 75 samples of metagabbro and metaperidotite from 14 sites in the Labrador Trough. The sites were distributed over a distance of 500 km and are situated in sills intruded into formation near the top of the trough stratigraphic column. Four components (A, B, C, and D) were obtained after alternating field (AF) and thermal demagnetization. The mean directions of magnetization are A. 143°, +33°. α95 = 14.4°. k = 15.4, 8 sites: B, 69°, +51°, α95 = 10.3d, k = 22.9, 10 sites: C. 278°. +15°, α95 = 7.7°, k = 28, 14 sites; and D, 200°, +16°. α95 = 13.3°, k = 16.2, 9 sites. All components are secondary. The fold test is negative, and consequently no primary magnetization has been preserved in the isolated components. Component C may have erased all others in zones of highest metamorphism. The paleopoles obtained for components D, A, and C are, respectively, 270°E. 23°S (δP = 10°. δm = 18°); 327°E, 9°S (δp = 11°. δm = 20°); and I9°E, 11°S (antipole) (δp = 10°. δm = 17°). The paleopole positions obtained for these three components are compared with me existing late Aphebian and early Paleohelikian apparent polar wander (APW) path for North America and fall in the 1750–1850 Ma interval.
Geochemistry and origin of volcanic rocks from Tuzo Wilson and Bowie seamounts, northeast Pacific Ocean
Authors: Cousens Brian L. , Chase R. L. , Schilling J.-G.
Published: April 2011

The origin of the Tuzo Wilson Seamounts, 50 km south of the Queen Charlotte Islands, has been ascribed by various workers to either the Pratt–Welker mantle plume, which has formed the Pratt–Welker seamount chain, or the formation of a new segment of the Explorer–Juan de Fuca spreading ridge system. Abundances of major and trace elements in dredged alkali basalts from Tuzo Wilson and Bowie seamounts (360 km northwest of Tuzo Wilson Seamounts) are typical of alkaline volcanism on ocean islands associated with mantle plumes, but 87Sr/86Sr ratios (0.70252–0.70258) fall within the range of mid-ocean ridge basalts (MORB) from the Explorer and Juan de Fuca ridges. Geochronological and chemical data from the Pratt–Welker, Bowie, and Tuzo Wilson seamounts suggest that the Tuzo Wilson Seamounts are in an early stage of development as a result of activity of the Pratt–Welker mantle plume but that contributions from both a depleted and an undepleted mantle source are necessary to reconcile trace-element and Sr isotope values. Modelling of rare-earth behaviour during partial melting indicates that neither the Tuzo Wilson nor Bowie basalts could be generated from a mantle source similar to that of the Explorer or Juan de Fuca MORB, unless recent metasomatism has enriched the seamounts' source region in incompatible elements.
The petrology, geochemistry, and economic potential of the Musquodoboit batholith, Nova Scotia
Authors: MacDonald M. A. , Clarke D. B.
Published: April 2011

The Musquodoboit batholith of southwestern Nova Scotia is a massive, post-tectonic granitoid intrusion that was emplaced into the regionally deformed and metamorphosed Meguma Group metasedimentary rocks. The batholith is composed primarily of medium- to coarse-grained monzogranites into which two small (≈1 km2) porphyries and numerous dykes have been injected. All rocks contain quartz, alkali feldspar, plagioclase, muscovite, and biotite (with the exception of some leucocratic dykes). Cordierite is a major constituent in most monzogranitic rocks and also occurs in some leucocratic dykes. Andalusite and garnet 0are also present as accessory phases in some rocks.Major-element chemical analyses indicate that all rock types in the Musquodoboit batholith are peraluminous. Compositions resemble those of the eastern part South Mountain batholith; however, slightly higher concentrations of Al2O3 and P2O5 distinguish the Musquodoboit batholith from the central part of the South Mountain batholith. Major- and trace-element data indicate that magmatic differentiation has operated; however, the decrease in Σ 8 REE's, Th/U, and K/Rb from monzogranite to dyke rocks suggests that stripping by hydrothermal fluids has also occurred.Various field, petrographic, and geochemical data yield equivocal estimates of the economic potential of the Musquodoboit batholith.
A late Precambrian rift-related igneous suite in western Newfoundland
Authors: Williams Harold , Gillespie R. T. , Breemen Otto Van
Published: April 2011

A granite that yields a U–Pb zircon age of 602 ± 10 Ma is associated with mafic and silicic volcanic rocks and metamorphic equivalents near Deer Lake in western Newfoundland. The granitic rocks are named the Round Pond granite, and the combined granite–volcanic suite is assigned to the Hughes Lake complex. All of the rocks are contained in the Hughes Lake structural slice that occurs above other allochthonous rocks and the autochthonous Cambrian–Ordovician carbonate sequence of western Newfoundland.The Round Pond granite is cut by metadiabase dykes. Mafic volcanic rocks, interpreted as coeval with the dykes, occur along the southeast side of the granite. A thick sequence of arkosic metagreywackes and psammitic to pelitic schists of the Mount Musgrave Group occurs stratigraphically above the mafic volcanic rocks. Regional correlations imply that the Mount Musgrave Group is of late Precambrian – Early Cambrian age, thus setting an upper stratigraphic limit to the age of the Hughes Lake complex.Perthitic and granophyric textures and the chemistry of the Round Pond granite are typical of anorogenic high-level hypersolvus intrusions. Nearby pink silicic volcanic rocks are probably consanguineous with the granite and together with the mafic volcanics form a bimodal suite.Bimodal volcanic suites and related mafic dykes and granitic intrusions imply rift tectonic settings. Occurrences along the west flank of the Appalachian Orogen are equated with the initiation of an ancient continental margin and the opening of an Iapetus Ocean. The 602 ± 10 Ma age of the Round Pond granite dates the rifting in western Newfoundland. Older isotopic ages on similar rocks in the southern Appalachians of the United States suggest a diachronous Precambrian rifting and Iapetus opening that propagated northward, much like the Mesozoic opening of the North Atlantic Ocean.
Sedimentation rates in Baffin Island fiord cores from comparative radiocarbon dates
Authors: Andrews J. T. , Jull A. J. T. , Donahue D. J. , Short S. K. , Osterman L. E.
Published: April 2011

Five accelerator (TAMS system, University of Arizona) radiocarbon dates on small (30–50 mg) paired bivalves or shell fragments have been obtained on two piston cores. These dates are compared with conventional and accelerator 14C dates on the total organic fraction (< 2 μm size fraction) from similar levels in the two cores. The shell dates are always younger by several thousands of years. We argue that the shell dates are the more reliable; for example, we demonstrate that pre-Quaternary pollen is abundant in the lowermost 6 m in core HU77-159 from Frobisher Bay. Since the occurrence of shells is rare in piston cores, we have developed an empirical relation that enables us to change dates on organic matter to a more probable "true" age. If this correction is applied to dates on the total organic fraction that we obtained on an additional five cores from fiord and shelf sites, a consistent pattern of sedimentation rate variations is shown. This indicates that sedimentation rates were highest between ca. 9000 and 6500 years BP, a time when fiord glaciers in Baffin Island were undergoing substantial retreat.