Geology and Geochronology of the Wollaston Lake Fold Belt in Northwestern Manitoba
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Abstract:
Recent studies in the northeastern extension of the Wollaston fold belt in Manitoba suggest that Aphebian metasedimentary rocks of the southern Churchill province comprise a geosynclinal facies that is conformably overlain by a platform facies. A possible period of emergence and embryonic Hudsonian tectonism separates this lower sequence from a blanket of continental deposits, which is probably of large regional extent and includes part of the Hurwitz Group in the District of Keewatin.Rubidium–strontium total-rock isochron ages have been obtained for five rock units in the Kasmere Lake area of Manitoba. The ages reveal that the metasedimentary rocks of the Wollaston fold belt are bounded to the east by an Archean complex comprising a differentiated hypersthene-bearing rock series having a minimum age of 2745 ± 124 Ma ( 87 Rb = 1.39 × 10 −11 yr −1 ) and a quartz monzonite unit having a minimum age of 2636 ± 163 Ma.Hudsonian igneous rocks are mostly syn- to post-kinematic. The age of 1855 ± 62 Ma for a quartz monzonite of batholithic dimension coincides with the ages of similar syntectonic Hudsonian intrusions in the Northwest Territories. The age of 1800 ± 60 Ma obtained from a pelitic gneiss unit is a minimum age for the Hudsonian metamorphism and dates the end of the Hudsonian orogeny in this part of the Churchill province. An age of 1941 ± 25 Ma for a granitic gneiss along the western margin of the Wollaston fold belt indicates early or pre-Hudsonian magmatic activity.Keywords:
Quartz monzonite
Orogeny
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<p>Tables containing sample details and isotope ratios used to calculate the U-Pb and Ar/Ar dates. Extra sample pictures, cross sections showing sample locations, and age spectrum plots and isochron diagrams of the samples analyzed for Ar/Ar geochronology.</p>
Geochronology
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Thirteen samples from two Kangâmiut dykes give a Rb–Sr whole-rock isochron age of 1950 ± 60 Ma. This age is interpreted as the age of intrusion of the dykes. The post-dyke Nagssugtoqidian orogeny took place between ca. 2000 and ca. 1800 Ma and can be correlated with the Hudsonian in Canada and the Laxfordian in Scotland.
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Abstract A Rb-Sr whole rock isochron for the Finspång augen gneiss, south central Sweden, yields an age of 1834 ± 21 Ma which is compared with intrusive ages for similar rocks in the neighbourhood. The augen gneiss is interpreted as an outer ‘skin’ surrounding younger granite intrusions.
Radiometric dating
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The Lac Croche complex is a deformed basin-shaped body of igneous origin, situated northeast of the Morin anorthosite pluton. Rb–Sr isotopic studies of the rock-types comprising the complex yield the following results: (a) The mangeritic augen gneiss, leuconoritic gneiss, monzonite, and monzonorite crystallized approximately 1124 m.y. ago with an initial 87 Sr/ 86 Sr ratio of 0.7042, compatible with these rocks being derivatives from a common source; (b) The K-feldspar – quartz gneiss crystallized approximately 1143 m.y. ago with an 87 Sr/ 86 Sr ratio of 0.7025; and (c) the granite crystallized approximately 961 m.y. ago with an 87 Sr/ 86 Sr ratio of 0.7084. Because the mangeritic augen gneiss and leuconoritic gneiss were deformed with the complex and the monzonite and monzonorite crystallized after this deformation, and because these rock types define an isochron, we conclude that the deformation of the Lac Croche complex took place approximately 1124 m.y. ago. If this deformation was a result of the emplacement of the Morin pluton, then this latter body also had to be emplaced at this time. These events constitute the main penetrative deformation in the area and represent the Grenville orogenic event.
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<p>Tables containing sample details and isotope ratios used to calculate the U-Pb and Ar/Ar dates. Extra sample pictures, cross sections showing sample locations, and age spectrum plots and isochron diagrams of the samples analyzed for Ar/Ar geochronology.</p>
Geochronology
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Muscovite
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Basal Gneiss Region, west Norway gives an Rb-Sr whole-rock isochron age of 1775 ±57 m.y. A foliated granodiorite in more homogeneous gneisses (Fetvatn gneiss) gives an age of 960 ±lO m.y. Similar ages from other areas indicate that most of the Basal Gneiss Region original ed during the Svecofennian and Sveconorwegian orogenies. The role of the Caledonian orogeny in the evolution of the Basal Gneiss Region remains unsettled. The 960 m.y. intrusion possesses a pronounced schistosity indicating recrys talliza tion in a stress field during the Caledonian orogeny or perhaps a late stage of the Sveconorwegian orogeny. The diminished role of the Caledonian orogeny in the formation of portions of the North Atlantic Caledonian System should modify theories for the evolution of this orogen.
Orogeny
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