TECTONIC SIGNIFICANCE OF ALASKITE-GRANITE-GRANODIORITE BODIES IN THE COMPOSITE ARC BELT OF SOUTHEAST ONTARIO: STRUCTURAL EVIDENCE FROM THE BLACK DONALD AND MAZINAW DOMAINS

Tabular and crescent-shaped bodies of strained, 1250-1240 Ma, alaskite-granite-granodiorite (AGG), also known as Methuen Suite rocks, are widely distributed within the domain aggregate of the Grenvillian Composite Arc Belt. Such wide distribution suggests that the AGG bodies were emplaced after or at an advanced stage of arc amalgamation. Some geologists believe that AGG bodies – together with 1245-1224 Ma gabbroic plutons – stitched together the ‘fragments’ of arc assemblages making up the tectonic collage of the belt. However, most bodies lie within the structural domains of the belt rather than straddle the domain boundaries.

The internal structure and lithologic pattern of most AGG bodies remain to be delineated in the field. We have mapped the lithology and structure of southwestern portions of the Addington Granite and Schooner-Norcan Lakes body (SNLB), situated, respectively, in the Mazinaw and Black Donald domains. Strained alaskite outcrops at the shores of Stoco Lake and elsewhere in the southwestern Addington Granite, but occupies <20% of its surface area. Within and adjacent to the well-exposed southwestern portion of the SNLB, 25 cm – 25 m thick sheets of unstrained alaskite have sharp contacts and are parallel to (i) the main foliation of their folded and highly foliated host rocks and (ii) the boundaries of 15-75 cm thick, concordant mini-sheets (transposed dikes?) of severely deformed, potassic pegmatite. Evidently, the alaskite sheets postdate the ductile deformation within and adjacent to the SNLB, if not throughout the Black Donald domain. In the absence of U-Pb geochronologic data, this relationship may be explained by several tectonic scenarios. We favour a scenario in which the AGG magmatism outlasted, at least locally, the processes of arc amalgamation and associated ductile deformation.