Date of Award


Degree Type


Degree Name

Doctor of Philosophy


The Phinney-Dash Lakes (PDL) complex of the western Wabigoon Subprovince, northwestern Ontario is predominantly extrusive rhyodacite with minor rhyolite and clastic and chemical sediments that overlie basalt. The basalt is intruded at its base by a tonalite batholith which is massive near the PDL complex but is gneissic and migmatitic westward, toward the centre of the batholith. The basalt is also more deformed and amphibolitic near the gneissic tonalite; the batholith contact here is intruded by late, massive to lineated, amphibolite xenolith-rich quartz diorite to diorite. The PDL complex and adjacent rocks were emplaced near the north margin of an axis of gneissic diapirs and late intrusions.;Trace element partitioning calculations suggest that rhyodacite magma of the PDL complex is a partial melt of amphibole and plagioclase in basalt under relatively dry conditions, leaving a residue of plagioclase and pyroxene. The tonalite may also be a partial melt of basalt but by comparison with the rhyodacite, the conditions were wetter and only the plagioclase component was melted, leaving an amphibole and pyroxene-rich residue. The quartz diorite to diorite is interpreted to be a hybrid rock derived from its host basalt by addition of feldspathic fluid, rich in incompatible elements and Sr. This fluid may have had its origin in the mantle or by dehydration or decarbonation of large volumes of basalt metamorphosed to granulite.;It is inferred from the regional geology, supported by published zircon U-Pb geochronology that the crust in the PDL area before the emplacement of the gneissic diapirs was relatively stable for a period of at least 40 Ma and possibly for almost 300 Ma. The tonalite batholith, intruded at approximately 2724 Ma B.P. was not a cause of major crustal disturbance. the central part of this batholith was remobilized as gneissic diapirs by heat that came either from degassing of mantle or from entrapment of basalt liquid low in the crust. The zircon U-Pb age of approximately 2711 Ma B.P. of conformable felsic tuff near the top of the volcanic sequence containing the PDL complex and the zircon U-Pb age of 2699 Ma B.P. of the late quartz diorite to diorite bracket the period of deformation of the supracrustal rocks in the PDL area.



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