Petrology and Sedimentation of the Archean Quetico Metasediments in the Atikokan-Mine Centre Area, Northwestern Ontario
1988-12
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Petrology and Sedimentation of the Archean Quetico Metasediments in the Atikokan-Mine Centre Area, Northwestern Ontario
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1988-12
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Abstract
The Quetico Subprovince in the Atikokan-Mine Centre region of Northwestern Ontario comprises an Archean gneiss-granite superbelt between the Wabigoon and the Wawa-Shebandowan Subprovinces to the north and south, respectively. A one-to two-kilometer wide marginal zone of low-grade metasedimentary rocks parallels the Quetico fault along the northern portion of the Quetico Subprovince in the study area. These metasediments are dominated by the resedimented facies association of greywacke, pebbly sandstone, mudstone and conglomerate. The metagreywackes are regularly bedded and graded and display sedimentary features typical of turbidite deposition. The stratigraphic nomenclature in the Rainy Lake area which was established at the beginning of the century by A.C. Lawson can still beapplied, although the stratigraphic relationships have been reversed. Poulsen and others (1980) have shown unequivocally that the Coutchiching Series (of which the Quetico Metasediments may be correlative) is not the oldest unit in the area because it overlies the Keewatin metavolcanic unit. However, the relationships between the Quetico metasediments, the Seine Series, and the Steep Rock Series still need to be resolved and and zircon geochronology should help in correlating these units. All rocks in the Quetico Subprovince have undergone at least greenschist facies metamorphism. The metasediments have been deformed, progressively metamorphosed southward and migmatized toward a central axis so that the original textures and most of the sedimentary features have been obliterated. The core of the Quetico Subprovince is made up of metasedimentary migmatite remnants, amphibolite, and granitic intrusives, many of anatectic origin. Bedding stikes east and dips are nearly vertical. Two episodes of folding have been recognized in the eastern and central portions of the study area. Three sets of folds have been recognized along the western margin. An initial folding episode produced recumbent folds with an associated early shistosity (S1). This early schistosity was modified by a second generation of tight to isoclinal folds which produced a penetrative and almost perfectly planar schistosity (S2). Lineations found in the bedding planes trace the attitudes of fold axes and plunge moderately to steeply to the east and west. Along the western margin of the study area, kinks and small chevron folds are related to the folding of the S2 schistosity around steeply dipping S3 axial fold surfaces. In the eastern half of the study area, relatively thick, graded metagreywacke beds are prominent; thirty kilometers to the west along Highway 11, turbidite beds become thinner and more silt-and mud-rich. In the western half of the study area, interlaminated siltstone and mudstone packages are more common, as are Bouma B and DE sequences. Areal variations in the relative proportions of lithologic types and in the character of Bouma sequences all indicate a general east to west facies transition from proximal near Atikokan (mid-fan), to distal (lower-fan) near Mine Centre. Modal analyses of 41 metagreywacke and pebbly sandstone samples reveal that reworked rhyolite-dacite volcanic rocks and coeval tonalitic intrusives were major sediment sources. Mafic volcanics, chert, slate, minor potassic granite and possibly basement were additional sources and point to a mixed provenance as confirmed by QmFLt plots. QFL plots indicate an orogenic source for much of the detritus but this inference is suspect. A dissected magmatic arc is the logical choice to explain a mixed provenance. The sediments may have been derived by the rapid erosion of unconsolidated material on volcanic edifices and associated coeval tonalitic plutons. Transportation of detritus to unstable basin margins was likely accomplished by high velocity streams. The temporary elastic piles were likely jarred loose and transported downslope into back-arc or fore-arc environments. Heavy mineral analysis reveals the vast majority of the heavy minerals present to be metamorphic species such as biotite, apatite, actinolite and epidote. A zircon varietal study reveals dominantly pink to purple (hyacinth) and brown (malacon) zircons. Most zircons appear as short stubby simple prisms with pyramidal ends. Length to width ratios are commonly equal to or less than 2. Unzoned varieties are more common than those that are zoned or contain a dark core. U-Pb geochronology has provided an absolute time framework for deposition of the Quetico Metasediments. Zircons recovered from the Blalock pluton define a 2688 +4/-3 Ma minimum age for sediment deposition. The youngest detrital zircons from the Quetico Metasediments are in the range of 2700 +/-3 Ma; the oldest zircons are approximately 3004 Ma. These two dates roughly bracket the age of sediment deposition although it is likely that most sedimentation took place in the range of 2700 to 2750 Ma because this is the time of extensive volcanism and plutonism in this part of the Superior Province. Preliminary U-Pb analyses from adjacent greenstone and batholithic terranes have yielded age dates that correlate with those determined from detrital zircons recovered from the Quetico Metasediments. These units must be regarded as possible sediment.
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A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by Frank Pezzutto in partial fulfillment of the requirements for the degree of Master of Science, December 1988. Plate 1 referenced in the thesis is also attached to this record.
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Pezzutto, Frank. (1988). Petrology and Sedimentation of the Archean Quetico Metasediments in the Atikokan-Mine Centre Area, Northwestern Ontario. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/220194.
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