Behavior of an Archean Granitic Batholith

Abstract

The Bamaji–Blackstone granite has 'invaded' volcanic rocks of the Lake St. Joseph area, northwestern Ontario. The resulting deformation, close to the granite, is substantially pure shear in three dimensions. This is indicated by flattened but not noticeably unidirectionally extended pillows, and by chocolate-tablet boudinage. Both features imply extension in all directions parallel to the granite margin and marked compression on horizontal axes normal to the granite margin. The flattening (compression) is also suggested by buckled granitic dikes enclosed in the flattened pillows. The discrepancy between the strains in pillows and dikes is explained as due to the late arrival of the dikes. Flattening persists away from the granite, but is increasingly accompanied by marked extension on sub-vertical axes, a deformation pattern akin to die drawing.The flattening mode is ascribed largely to the diapiric uprise of the granite, much modified by inflation of the granite due to the arrival of granitic magma late in the 'invasion' process. The granitic dikes presumably represent some magma that escaped.