The origin of laminated and massive anorthosite, Sept Iles layered intrusion, Québec, Canada

Abstract

Well-laminated plagioclase-rich rocks (‘anorthosite’) occur as layers in mineralogically similar but massive rocks in the Sept Iles intrusion. Several phases of crystallization of poikilitic clinopyroxene have preserved the intermediate stages in the production of these rocks. Plagioclase nucleated and initially grew in random orientations in the stationary part of the boundary layer. There, in the absence of other events, crystallization produced the massive anorthosite. The laminated anorthosite may have been produced by episodic increases in the velocity of the magma adjacent to the boundarylayer induced by magmatic density-currents. Such currents would have caused the zone of simple shear of the boundary-layer to migrate outwards into the previously stationary part. This simple shear would then have rotated the existing plagioclase crystals, as well as crystals that nucleated at that time, towards the shear-plane. Some rocks show evidence of several periods of enhanced flow-velocities. The production of the lamination may have weakened the crystal-mush and enabled slumping to occur, producing folds and contorted layers. Finally, compaction liberated large quantities of intercumulus magma of which some was trapped as granophyric segregations and some escaped by overturing and churning the crystal-pile. Earlier-formed granophyric segregations and laminated layers were disrupted during this process to produce a range of more-complex rocks including igneous breccias.