A petrofabric study of the Falcon Lake stock, Eastern Manitoba

Abstract

An investigation has been carried out to determine the degree of preferred orientation shown by both plagioclase and quartz crystals in rocks from the eastern half of the Falcon Lake Stock. For the plagioclase crystals, the orientation of the (O1O) composition planes of albite twinning was determined. These (O1O) planes, parallel to the largest faces of the lath-shaped crystals, were found to show a strongly preferred orientation. This planar parallelism has been related to alignment of the crystals in primary flow layers within the stock. The flow layers in the quartz monzonite core were found to be steeply inclined, possibly as a result of re-orientation of a pre-existing plagioclase "mesh" by upward streaming of an acidic differentiate; the mechanism suggested Brownell (1941) to explain the formation of the core. In the outer parts of the stock, there appears to be an arch of flow layers. A close relationship has been found to exist between the orientation of the quartz c-axes and the inferred attitudes of the flow layers. In general the majority of the quartz axes are consistently contained within the plane of the flow layers and most of the axes are also horizontal. It is suggested that the quartz grains assumed their preferred orientation as they crystallized, in response to a prolongation of the forces which originally propelled the magma upwards. It would appear that their orientation was also influenced by the pre-existing framework of plagioclase crystals which had already been aligned within parallel flow layers.