Ultramafic intrusions in the Lewis Hills Massif, Bay of Islands Ophiolite Complex, Newfoundland: Implications for igneous processes at oceanic fracture zones

Abstract

The Coastal Complex of western Newfoundland has been interpreted as oceanic crust and upper mantle which formed in an oceanic fracture zone. Within the Coastal Complex, a suite of peridotite bodies and associated basaltic dikes occur. The least-deformed peridotites in these bodies have cumulate textures and have apparently been separated from any gabbroic cumulates which formed in association with them. Intrusive relationships with surrounding metamorphic and cumulate rocks are well preserved and suggest that the peridotite bodies are crystal mush intrusions derived from mobilized ultramafic cumulates which were originally deposited within relatively small isolated magma chambers. These processes are believed to be characteristic of ridge-transform intersections or “leaky” transform regions. Two petrologically distinct types of peridotite have been identified. One type has lithologies and mineral chemistry similar to those of many oceanic plutonic rocks, and these lithologies may be interpreted as differentiates produced in the evolution of typical Mid-Ocean Ridge Basalts (MORBs). The second type of peridotite, although lithologically similar to the first, has relatively high CaO/Al 2 O 3 ratios and relatively low TiO 2 and Na 2 O. This second type of peridotite cannot be simply related to the genesis of typical MORBs. Chilled margins and fine-grained dikes along the margins of some of these peridotite bodies are high-magnesium basalts.