PERIDOTITIC INTRUSIONS NEAR QUETICO AND SHEBANDOWAN, NORTHWESTERN ONTARIO: A CONTRIBUTION TO THE PETROLOGY AND GEOCHEMISTRY OF ULTRAMAFIC ROCKS

1964 ◽  
Vol 1 (1) ◽  
pp. 63-98 ◽  
Author(s):  
D. H. Watkinson ◽  
T. N. Irvine
Keyword(s):  
Lake Superior ◽  
Olivine Basalt ◽  
Chemical Change ◽  
Ultramafic Rocks ◽  
Tholeiitic Magma ◽  
Mafic Intrusions ◽  
Northwestern Ontario ◽  
Serpentinized Peridotite ◽  
Small Stock ◽  
Petrology And Geochemistry

A study has been made of several dozen small, stock-like ultramafic to mafic igneous bodies that occur in two clusters, 50 and 75 miles west of the head of Lake Superior. One cluster, the Quetico intrusions, is dominantly hornblende peridotite, hornblendite, and feldspathic hornblendite; the other, the Shebandowan intrusions, comprises serpentinized peridotite and subordinate gabbro. Both intrude eugeosynclinal deposits and are followed by extensive granitic, and minor syenitic intrusions, and both have associated Ni–Cu sulphides containing minor Pt and Pd.The Quetico intrusions are shown to have formed by fractional crystallization of tholeiitic olivine basalt liquid, possibly carrying suspended olivine and clinopyroxene when emplaced. Crystallization was at a water partial pressure of at least 2500 bars and, thus, a depth of 5 or more miles. The Shebandowan intrusions resemble alpine-type peridotite bodies structurally but, chemically and petrographically, are more like the differentiates of stratiform mafic intrusions. Their peridotite has formed from tholeiitic magma by early fractionation of olivine; the gabbro probably represents residual magmatic liquid. Serpentinization, which is postmagmatic, apparently has not caused extensive chemical change beyond addition of H2O, O2, and some CO2, and local redistribution of lime.The rocks in the two groups of intrusions differ appreciably in Cu and Ni, and a correlation between the Cu/Ni ratios of the rocks and their respective ores is illustrated. This correlation, and the sulphur isotopic composition, both indicate derivation of the sulphides from the intrusions.

Sources of placer platinum in Yukon: provenance study from detrital minerals

2008 ◽  
Vol 45 (8) ◽  
pp. 879-896 ◽  
Author(s):  
Yana Fedortchouk ◽  
William LeBarge
Keyword(s):  
Unknown Origin ◽  
Heavy Mineral ◽  
Source Rocks ◽  
Ultramafic Rocks ◽  
Chromian Spinel ◽  
Mafic Intrusions ◽  
Platinum Group Minerals ◽  
Mineral Sample ◽  
Back Arc ◽  
Ocean Ridge

Source rocks for the platinum group minerals (PGM), historically reported in a number of Yukon placers, remain either unknown or poorly understood. A study of heavy-mineral samples from five creeks draining bedrock in west and central Yukon was undertaken to confirm the presence of placer platinum, to determine which mafic–ultramafic rock is the source of PGM in Kluane area, southern Yukon, and to explain platinum occurrences in Canadian and Florence creeks, central Yukon, where no known mafic–ultramafic rocks are present. Diverse composition of chromian spinel and clinopyroxenes from three creeks in the Kluane area indicate several sources of ultramafic rocks, including fragments of Alpine-type peridotites formed in back-arc basin and mid-ocean-ridge settings, and a source rock for zoned zinc-rich chromites of unknown origin. The Kluane ultramafic sills are the most likely source of PGM in this area. The heavy-mineral sample from Canadian Creek returned one PGM grain, no chromite, and abundant ilmenite and titanomagnetite. A group of chromium-rich magnesian ilmenites (∼4 wt.% MgO) closely match the composition of ilmenites from continental mafic intrusions produced during continental rift magmatism. This supports the continental rifting event recently proposed for this part of Yukon and indicates the economic potential of the Canadian Creek platinum occurrence. Composition of spinel from Florence Creek sample indicates an Alaskan-type intrusion as the source of PGM.

The Mid-Atlantic Ridge near 45 °N. VI. Remanent intensity, susceptibility, and iron content of dredged samples

1970 ◽  
Vol 7 (2) ◽  
pp. 226-238 ◽  
Author(s):  
E. Irving ◽  
W. A. Robertson ◽  
F. Aumento
Keyword(s):  
Iron Content ◽  
Rock Type ◽  
Chemical Change ◽  
Sampling Bias ◽  
Axial Zone ◽  
Mean Intensity ◽  
Basaltic Rocks ◽  
Active Volcanism ◽  
Serpentinized Peridotite ◽  
Mid Atlantic Ridge

Results from 27 dredge hauls (75 samples) spaced from 150 km west to 70 km east of the Median Valley of the Mid-Atlantic Ridge at about 45 °N are reported. Basalt is the most common rock type. The basalts have a mean remanent intensity of 92 × 10−4 and a mean susceptibility of 0.9 × 10−4 cgs cm−3. The remanence varies with distance from the axis, samples from the Median Valley (mean 574 × 10−4) being ten times more magnetic than samples at a greater distance. Most of this decrease of intensity occurs within a few kilometers (less than 6 km) of the central axis and within the zone of active volcanism. It is suggested that this dramatic drop in intensity is caused by viscous decay enhanced by thermal cycling or by chemical change in the narrow volcanic axial zone.Certain other properties of the basalts vary with distance; the blocking temperatures and stability (versus a.f. demagnetization) increase, and the ratio FeO/Fe2O3 decreases with distance. These changes are most marked at the inner slopes of the Crestal Mountains not within the narrow axial zone, and it is possible that they reflect sampling bias, the samples from the Median Valley being from flow margins, whereas the collections from the flanks contain material from the centers of flows.Non-basaltic rocks include serpentinized peridotite, greenstone, gabbro, and diabase. Serpentinized peridotite samples are strongly magnetic and have a mean intensity of 23 × 10−4 cgs cm−3. Greenstones, gabbros, and diabases are weakly magnetized, with mean intensities of about 10−4. Moreover, basalt showing partial alteration to greenstone has intermediate intensities showing that such a metamorphic process effectively demagnetizes a rock. This result is more consistent with the idea that Layer 3 is composed predominantly of gabbro and metamorphosed basalt rather than of serpentinized peridotite. The remanence and susceptibilities of 18 "erratic" samples, which are thought from other evidence to have been deposited by Pleistocene icebergs, have a wide and irregular spread.

Perovskite, loparite and Ba-Fe hollandite from the Schryburt Lake carbonatite complex, northwestern Ontario, Canada

1994 ◽  
Vol 58 (390) ◽  
pp. 49-57 ◽  
Author(s):  
R. Garth Platt
Keyword(s):  
Fluid Phase ◽  
Ultramafic Rocks ◽  
Subsequent Reaction ◽  
Carbonatite Complex ◽  
Silicate System ◽  
Northwestern Ontario ◽  
Intimate Association ◽  
Free Mica ◽  
Host Rocks ◽  
Light Rare Earth Elements

AbstractWithin a suite of felsic-free, mica-rich alkaline ultramafic rocks of the Schryburt Lake carbonatite complex of northwestern Ontario, loparite and Ba-Fe hollandite occur in intimate association with perovskite. The host rocks have variable modal proportions of Mg-olivine, phlogopite, magnetite, ilmenite, apatite and carbonate (generally calcite) with minor Mg-salite. Thus, they correspond to ultramafic lamprophyres (i.e. aillikites), in the sense of Rock (1990) or the lamprophyric facies of the melilitite clan, in the sense of Mitchell (1993).Perovskite is the principal titanate phase, forming both euhedral and anhedral grains, the latter showing evidence of marginal resorption. It exhibits complex zonal patterns due principally to variations in the light rare earth elements, Na and Nb. In the nomenclature suggested, they may be termed perovskite and cerian perovskite. Loparite forms as small euhedral overgrowths on corroded perovskite cores. Chemically they are essentially solid solutions of loparite, lueshite and perovskite. Consequently, they may be termed calcian-loparite, calcian niobian loparite, niobian calcian loparite, loparite and niobian loparite. Titanates of the hollandite group are rare accessory minerals whose composition closely approach that of the septatitanate BaFe2+Ti7O16.The complex zoning of the perovskite grains has been attributed to the periodic introduction of carbonatite-derived fluids enriched in REE, Na and Nb into the silicate system during perovskite crystallization. Subsequent reaction of the early perovskite with F-bearing fluids leads to a localized environment enriched in Ti, Na, Nb and REE derived from both the fluid phase and the unstable perovskite. Loparite subsequently crystallizes from these micro-chemical environments.

Rb–Sr geochronology of the Coldwell Complex, northwestern Ontario, Canada

1982 ◽  
Vol 19 (9) ◽  
pp. 1796-1801 ◽  
Author(s):  
R. Garth Platt ◽  
Roger H. Mitchell
Keyword(s):  
Upper Mantle ◽  
Lake Superior ◽  
Parental Magma ◽  
Initial Ratio ◽  
North Shore ◽  
The North ◽  
Northwestern Ontario ◽  
Alkaline Intrusion ◽  
Intrusive Rocks ◽  
Igneous Activity

The Coldwell Complex of Northwestern Ontario is North America's largest structurally and petrologically complex alkaline intrusion. Situated on the north shore of Lake Superior, it consists of at least three intrusive centres and is cross-cut by a diverse suite of coeval–cogenetic dikes. The main intrusive rocks range from gabbros to ferroaugite syenites, nepheline syenites, and quartz syenites. The dikes are predominantly lamprophyric. A seventeen point whole rock Rb–Sr isochron (MSWD 2.22) gives an age of 1044.5 ± 6.2 Ma (2σ) and an initial ratio of 0.70354 ± 0.00016 (2σ). The age is late Neohelikian and is younger than the bulk of igneous activity (Keweenawan activity) prevalent in the Lake Superior Basin during the Neohelikian. The low initial ratio indicates an upper mantle origin for the parental magma of the complex.

Ultramafic intrusions of the Abitibi area, Ontario

1969 ◽  
Vol 6 (2) ◽  
pp. 281-303 ◽  
Author(s):  
N. D. MacRae
Keyword(s):  
General Structure ◽  
Basaltic Magma ◽  
Tholeiitic Basalt ◽  
Ultramafic Rocks ◽  
The South ◽  
South Side ◽  
Original Order ◽  
Cyclic Unit ◽  
Petrology And Geochemistry ◽  
Group 2

A study has been made of the petrology and geochemistry of a 30-mile (48.3 km) segment of a belt of ultramafic-gabbroic igneous bodies extending past the south side of Lake Abitibi, Ontario. In general, the bodies are sill-like and are differentiated into major layers of peridotite, clinopyroxenite, and gabbro, the layers generally being in that stratigraphic order. In detail, the intrusions fall into four groups: (1) complex sills in which there is a cyclic repetition of layers; (2) simple differentiated sills showing only one sequence of the above rock layers; (3) bodies composed only of peridotite and dunite; and (4) bodies composed wholly of gabbroic rocks.One of the group (2) bodies has a chilled margin equivalent in composition to a tholeiitic basalt. The general structure of the intrusions and their petrographic and chemical features indicate that they are differentiated from basaltic magma by gravity-controlled fractionation. However, it appears that while solidifying, some of the intrusions were open to periodic addition or subtraction of magma. Thus, in the intrusions showing cyclic repetition of layers, it is apparent that the magma was altered prior to the formation of each cyclic unit such that the original order of mineral crystallization was repeated. For other intrusions, it can be inferred that large amounts of partly crystallized liquid were expelled such that each of these intrusions is now largely or wholly represented by ultramafic rocks. The bodies composed wholly of gabbro may be derived from the expelled magma.

Deglaciation chronology and revegetation in northwestern Ontario

1985 ◽  
Vol 22 (6) ◽  
pp. 850-871 ◽  
Author(s):  
Svante Björck
Keyword(s):  
Black Spruce ◽  
Lake Superior ◽  
The South ◽  
Migration Routes ◽  
White Pine ◽  
Cool Period ◽  
Radiocarbon Dates ◽  
Northwestern Ontario ◽  
History Of ◽  
Ice Retreat

Along a 420 km transect in northwestern Ontario, Canada, sediments from four lakes were analyzed with respect to lithology, pollen, and macrofossils. Radiocarbon dates show that the region was deglaciated between ca. 11 500 and 8000 years BP, and periods of both rapid ice retreat and readvance influenced the history of Glacial Lake Agassiz. In the south the ice sheet was succeeded by a lengthy interval of park–tundra with stands of spruce, ash, and elm. The ash and elm seem to have disappeared during a suggested cool period (11 100–10 200 years BP). Farther north the park–tundra phase lasted not more than 50–100 years after ca. 10 200 years BP before boreal trees dominated. The climatic change around 10 200 years BP permitted the very rapid migration of spruce, larch, birch, and jack or red pine into northwestern Ontario from northern Minnesota. The migration routes for Pinus strobus (white pine), Alnus rugosa, and A. crispa were divided, however: one from the south (south of Lake Superior) and one from the east-southeast (north of Lake Superior). White pine reached its maximum distribution 6500–6000 years BP, when the limit was probably 150–200 km north of today's. The composition of the boreal forest during the altithermal was only slightly changed, but the influx of presumed prairie pollen reached a peak ca. 8000–7000 years BP. Since then Picea mariana (black spruce) gradually became the dominating tree species.

40Ar/39Ar Spectrum Ages on Logan Intrusions, a Lower Keweenawan Flow, and Mafic Dikes in Northeastern Minnesota—Northwestern Ontario

1975 ◽  
Vol 12 (5) ◽  
pp. 821-835 ◽  
Author(s):  
Gilbert N. Hanson
Keyword(s):  
Lake Superior ◽  
Igneous Rocks ◽  
Basalt Flow ◽  
Mafic Rocks ◽  
Stepwise Heating ◽  
Northwestern Ontario ◽  
Igneous Activity ◽  
Higher Temperature

The 40Ar/39Ar method using stepwise heating has been applied to 11 Precambrian mafic rocks in Minnesota and Ontario to test the technique on samples varying from extremely altered to essentially unaltered and to date some of the Lower and Middle Keweenawan igneous rocks. Four essentially unaltered samples give plateau ages which appear to be reliable and may date the time of intrusion. One strongly metamorphosed sample gives a plateau age which may date the time of metamorphism. The altered mafic rocks give patterns which are inconsistent, difficult to interpret, and have higher ages for the higher temperature fractions. Essentially unaltered samples may also give inconsistent patterns. For the Keweenawan igneous activity along the northwest shore of Lake Superior near the Minnesota–Ontario border and a Logan sill near Lake Nipigon, the plateau ages suggest that three Lower Keweenawan Logan intrusions occurred 1150 to 1170 m. y. ago, although it is possible that the sills near the Minnesota–Ontario border are older; that the Keweenawan igneous activity ended about 1135 m.y. ago, based on one late dike analysis; and that a basal Keweenawan basalt flow was metamorphosed and cooled enough to retain argon by 1095 m.y. ago.

Early Historic Populations In Northwestern Ontario: Archaeological And Ethnohistorical Interpretations

1975 ◽  
Vol 40 (1) ◽  
pp. 54-63 ◽  
Author(s):  
Charles A. Bishop ◽  
M. Estellie Smith
Keyword(s):  
Seventeenth Century ◽  
Social Organization ◽  
Lake Superior ◽  
Cultural Ecology ◽  
Northern Ontario ◽  
Ethnic Boundaries ◽  
Northwestern Ontario ◽  
International Border ◽  
North And South

There seems to be little doubt that major population shifts occurred in Northern Ontario and Manitoba following the Iroquois wars of the seventeenth century and the advent of the fur trade. These movements have tended to blur pre-contact ethnic boundaries. Although recent archaeological reports have provided much needed data, differing interpretations of that data have only further obscured the issue. For instance, while some scholars have argued that Blackduck Focus sites in western Northern Ontario and northern Minnesota were occupied by Algonkian-speakers (either Ojibwa or Creej, others have interpreted the materials as having an Assiniboin authorship. In support of the latter view, we analyze both the archaeological and early ethnohistorical data. From these sources we conclude that, at contact, the Ojibwa extended no further west than Michipicoten Bay, while the Cree occupied most of Northern Ontario except for a strip of about 50 mi north and south of the present International border west of Lake Superior which was Assiniboin territory. Proper determination of ethnic boundaries is of theoretical importance to studies of cultural ecology and social organization.

Morphological and genetic analyses identify a new record of a glacial relict: pygmy whitefish (Prosopium coulterii) from northwestern Ontario

2014 ◽  
Vol 92 (3) ◽  
pp. 267-271 ◽  
Author(s):  
P.J. Blanchfield ◽  
E.B. Taylor ◽  
D.A. Watkinson
Keyword(s):  
North American ◽  
New Record ◽  
Lake Superior ◽  
Missouri River ◽  
River System ◽  
Relict Species ◽  
Genetic Analyses ◽  
Northwestern Ontario ◽  
New Finding ◽  
Glacial Relict

The pygmy whitefish (Prosopium coulterii (Eigenmann and Eigenmann, 1892)) is a glacial relict species with a disjunct North American distribution that, apart from its most easterly known location in Lake Superior, is predominantly found in northern and western regions of Canada. Here we report on a new finding of pygmy whitefish from Winnange Lake in northwestern Ontario that extends the range of this species ∼320 km from its most easterly distribution in Lake Superior and almost 1500 km east of the closest previously known western localities. Genetic analyses confirmed that the fish from Winnange Lake were most closely related to the lineage that includes fish from Lake Superior and likely also originated via postglacial dispersal from a refugium in the upper Mississippi – Missouri river system.

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