Petrography and Provenance of an Archean Conglomerate Manitou Lake Northwestern Ontario

Abstract

<p> The Archean "Loose Pebble Bay" conglomerate, Manitou Lake, northwestern Ontario, contains a variety of clast types, not all of which can be readily ascribed to local lithologies. This study was undertaken to determine the modal clast composition of the conglomerate, and investigate the origin of the clasts. The conglomerate lies near the top of a stratigraphic sequence which includes mafic and felsic volcanic rocks, iron formations, conglomerates, sandstones and argillites. The conglomerate unit itself consists of interbedded conglomerate and sandstone, and probably represents a channel-fill deposit of an ancient submarine fan.</p> <p> Modal percentages of clast types were obtained using a line-intercept method. They indicate that most of the debris in the conglomerate can be reasonably attributed to uplift and erosion of the lateral equivalents of the underlying stratigraphy, except for the granitoid clasts, which have no known origin within the area.</p> <p> Petrographic examinations of the clasts indicate that field identifications must be confirmed with thin section investigations.</p> <p> The modal composition of granitoid clasts was determined utilizing both thin sections and stained slabs. Most of the granitoid clasts are granodiorite, or lie just within the granite field, adjacent to the granodiorite field. Textural studies of the granitoid clasts suggest that gneissic and allotriomorphic-granular textured rocks may have formed by deformation or partial recrystallization along grain boundaries of previously hypidiomorphic-granular rocks. Textures generally indicate intrusive origin and slow cooling, although two granophyric samples may have solidified at relatively shallower depths than the other granitoid rocks.</p> <p> The textures and compositions of most of the granitoid clasts suggest that they were derived from one intrusive body. Intrusion of such a body into the volcanic-sedimentary belt, followed by uplift and erosion, would account for the presence in the "Loose Pebble Bay" conglomerate of granitic clasts and clasts similar to the underlying rocks. However, no evidence of such an intrusion has been found in the Manitou Lake area, and two of the granitoid clasts are noticeably different in composition from the others. The possibility of a pre-existing sialic basement cannot be ruled out.</p>