The Barnum Lake Pluton, Thunder Bay, Ontario

Abstract

North of Thunder Bay. Ontario, rocks of the Shebandowan–Wawa and Quelico Bell form the site for a number of plutons. These intrusions are distributed in an east–northeast trending line over a distance of 45 km. The Barnum Lake Pluton occupies a central position in this zone and has been emplaced into pelitic to semipditic metasedimentary schists and gneisses.The major rock type is a quartz monzonite in which composite megacrysts of feldspar occur abundantly. Composition of the matrix varies from quartz dioritic to tonalitic. Mafic-rich inclusions confined to a narrow zone paralleling the contact are of dioritic to monzodioritic composition.Major element and mineralogical variations in the rocks are primarily caused by magmatic processes. No evidence of partial melting has been found in the enclosing rocks.The internal structure of the pluton is the result of primary flow. A high degree of conformity characterizes the foliated border phase, contact, and the attitude of the country rocks.Development of contact metamorphic textures in the regionally metamorphosed metasedimentary sequence suggests a late syntectonic emplacement for the pluton.