Flowage Differentiation in the Ultramafic Sills of the Cape Smith - Wakeham Bay Fold Belt, Ungava, P.Q.

Abstract

<p> intrusive Ultramafic sill of the Cape Smith-Wakeham Bay Fold Belt, located in the Ungava Peninsula P.Q., was studied. Measurements of olivine crystals were made on enlarged images of thin sections to determine the average olivine grain size and volume across the intrusion. </p> <p> The Bravo ultramafic sills exhibit a peculiar petrographic and chemical zoning, the rocks becoming increasingly rich in olivine as one moves toward the centre of the intrusion. Rock Compositions range from gabbro at the margin to olivine rich peridotite near the centre. The olivine grain size distribution exhibits n similar zoning, the grain size increasing toward the centre. However, the maximum value is skewed somewhat to the south along the edge of the central olivine rich zone, coinciding with the maximum value of Ns (Nickel in sulfides). </p> <p> The zoning is a consequence of flowage differentiation. Along the margins of the intrusion grain dispersive pressure (the pressure due to mechanical interaction between phenocrysts is dominant and grains are forced toward the centre. Within the centre of the intrusion, where the increased crystal concentration results in plug flow, the force of gravity is dominant and the largest grains make their way to the base of the plug. </p> <p> The Bravo Ultramafic Sills are pre-tectonic in origin being intruded into a group of eugeosynclinal strata in a sub-horizontal attitude. </p>