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ABSTRACT: Geology of the Grenville Basement of Southwestern Ontario and Documentation of Widespread Post-Grenville Alteration

R. M. Easton, T. R. Carter

Examination of cuttings and drill core from approximately 500 drill holes that intersect basement in southwestern Ontario, in conjunction with interpretation of aeromagnetic and gravity data for the region, has enabled us to identify several major lithotectonic domains within the basement of southwestern Ontario. These domains are of the same scale and form as domains exposed in the Central Gneiss Belt (CGB) of the Grenville Province to the northeast. Granite is the rock type most commonly found in the holes, with syenite, monzonite, and tonalite also being abundant; gabbroic rocks are uncommon. The most distinctive domain lithologically is the Cambridge domain, which contains a belt of marbles and associated metaclastic rocks, and which may be analogous to the Parry Sound Domain of t e Central Gneiss Belt. The Fishog domain is an extension of the highly magnetic, granite-tonalite-monzonite rocks exposed in the CGB west of Minden and is distinct geophysically.

The position of the Grenville Front has been redefined: It runs roughly north-south along the St. Clair and Detroit Rivers, approximately 30-50 km east of where many published maps show its position. In addition, the location of the Central Metasedimentary Belt Boundary Zone has been better defined and is in agreement with previous interpretations of its position.

We have also been able to document the extent of weathering and alteration of the basement rocks associated with the Precambrian-Paleozoic unconformity. Weathering along outcrop exposures of the unconformity has been studied by the Ontario Geological Survey in the Madoc area, and the presence of regoliths and karsting that developed after 900 Ma but prior to Middle Ordovician carbonate deposition has been well documented. Similar features are present throughout southwestern Ontario and extend, on average, from 2 m to 5 m below the Precambrian-Paleozoic contact. In addition, alteration has also occurred along the unconformity in southwestern Ontario, with authigenic potassium feldspar and sausseritization of plagioclase being common products of this alteration. Anyone studying the Prec mbrian basement of southwestern Ontario must sample beneath this weathered and altered zone to obtain fresh material.

This is a first attempt to map the Grenvillian basement in southwestern Ontario. Geochronology, additional holes in areas of limited drilling, and geochemical studies of the pre-Ordovician weathering at the top of the Precambrian basement will lead to further refinement of this map.

AAPG Search and Discovery Article #90998 AAPG Eastern Section Meeting, London, Ontario, Canada, September 10-12, 1990