Geochemical and geological evidence bearing on the origin of the Bay of Islands and Coastal Complex ophiolites of western Newfoundland

Abstract

Suggestions that ophiolitic rocks of the Bay of Islands Complex (BOIC) and Coastal Complex (CC) of southwestern Newfoundland were formed at a seafloor spreading center are supported by: 1. (1) whole-rock trace and major element data from diabases and basalts, 2. (2) spinel mineral chemistry from ultramafic rocks, 3. (3) the internal geology of the ophiolite terranes, and 4. (4) regional geologic and tectonic constraints in the western Newfoundland Appalachians. Diabase and basalt samples from the BOIC and the northern CC massifs are tholeiitic and geochemically indistinguishable from present-day mid-ocean ridge basalts (MORBs). Many diabases from the Lewis Hills massif (the southernmost part of the CC) are also identical to MORBs, but there is also an unusual suite of diabases from the Lewis Hills that is unlike any other suite yet reported from a variety of tectonic settings including island arcs, ocean islands or spreading centers. This suite is interpreted to have been produced by unusual magma generation processes in the deep structural levels of an oceanic ridge-ridge transform fault. Approximately 90% of spinels from ultramafic rocks in the BOIC and CC have Cr/(Cr + Al) ratios that are less that 0.60, comparable to those values reported from ultramafic rocks of the present-day ocean basins. These geochemical constraints, in combination with previously-described geological relationships within the ophiolite terranes and regional tectonic relationships in the western Newfoundland Appalachians, suggest that the BOIC and CC ophiolites formed within a main ocean basin (i.e., the early Paleozoic Iapetus Ocean) along a seafloor spreading center and ridge-ridge transform fault, respectively. There is little evidence to suggest formation in either an island arc or back-arc basin tectonic setting. As such, the BOIC and CC ophiolites are probably well suited for detailed petrologic, structural and geophysical studies to determine the processes involved in the generation and evolution of contemporary oceanic crust and upper mantle.