Ultramafic intrusions in the Lewis Hills massif, Bay of Islands ophiolite complex, Newfoundland: implications for igneous processes at oceanic fracture zones.

Three distinct types of ophiolite ultramafic rocks all occur at the same structural level. They include residual upper mantle harzburgite tectonites, variably deformed dunite, wehrlite, clinopyroxenite cumulates and the intrusive bodies described here. The 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 may be a differentiate produced in the evolution of typical MORB; the other, containing higher CaO/Al ₂ O ₃ ratio and lower TiO ₂ and Na ₂ O, cannot be related simply to the genesis of typical MORB. Such peridotites are identified only in the Coastal complex, probably restricted to a fracture-zone region. If incorporated into rising serpentinite diapirs and/or deformation and serpentinization related to transform faulting, the significance of such peridotites to the regional geology of the sea-floor ophiolite complex becomes difficult to determine. Thus, peridotites sampled in fracture zones need not be representative of the oceanic mantle remote from fracture zones and should be used with caution in petrological modelling of the oceanic lithosphere. (Preceding abstract) -L.C.H.