Emplacement of the Nain anorthosite: diapiric versus conduit ascent

Estimation of settling velocities of large orthopyroxene megacrysts, found within anorthosite intrusions, are calculated and compared with ascent rate s achieved by diapirism and conduit propagation. Calculations suggest that diapirism is far too slow to be an appropriate ascent mechanism for anortho sitic crystal mush and favour conduit emplacement. The intrusions of the Na in Plutonic Suite (NPS) are located along the Abloviak shear zone, which ma rks the boundary between the Nain and Churchill provinces, and within the z one of juxtaposition of the Saglek and Hopedale blocks of the Nain Province . These crustal weaknesses have probably controlled the emplacement and dis tribution of the intrusions. Contact relations between intrusions of anorth osite and their gneissic host rock provide evidence for two emplacement sty les within the NPS, the first typified by strongly deformed and recrystalli zed rocks, and the second by an outer border zone of mafic rocks. It is pro posed that these differences in intrusive style are due to differences in d uctility contrast between the magma and its surrounding host rocks, such th at those intrusions emplaced into the thermally softened shear zone have de formed margins, whereas those intruded into the cooler Archaean crust have undeformed margins.