The petrology and petrogenesis of the Bay of Islands Ophiolite Suite, Western Newfoundland

Malpas, John Graham (1976) The petrology and petrogenesis of the Bay of Islands Ophiolite Suite, Western Newfoundland. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The Bay of Islands Complex, situated on the west coast of Newfoundland, forms the upper slice assemblage of a number of allochthonous slices that were emplaced on platformal carbonate rocks of the ancient North American continental margin during the lower Middle Ordovician. The Complex consists of a complete ophiolite sequence disposed in four main massifs and believed to represent obducted portions of oceanic lithosphere. During early stages of displacement from the oceanic environment, a basal dynamothermal aureole was developed and remains part of the Complex. Temperatures of metamorphism in this aureole suggest that the ophiolitic rocks were just formed and still hot when displaced. This, together with other evidence, suggests their production in a marginal basin or in the environs of a ridge, subduction zone, transform triple point. -- Petrological, mineralogical and chemical data show that the Complex may be divided into a lower series of ultramafic tectonites representing mantle material, and a higher series of cumulate and extrusive rocks capped by clastic sediments which may be correlated with oceanic crust. -- Examination of the tectonite series reveals a lower spinel lherzolite member overlain by harzburgites. Both rock types are cut by numerous deformed and undeformed olivine-pyroxene veins. These veins represent early crystallisation products from a picritic tholeiite magma derived at 18 kb and 1100°C by ~ 23% partial melting of a spinel lherzolite to leave a harzburgite residuum. The remainder of the magma crystallised as different cumulate and extrusive rocks under crustal conditions. -- The petrogenetic model derived from the mineralogy and chemistry implies a rising diapiric body beneath an accreting centre and allows for the production of tholeiitic, transitional and mildly alkaline basalts from a similar parent. The nature of the basalt erupted depends upon rate of upwelling and rate of crysallisation within the diapir. Prolonged fractionation at pressures of 8-18 kb may explain the derivation of off ridge-axis alkaline series. Furthermore, the model places limits on the depths of melting within the rising diapir and suggests that crystallisation takes place at depths of less than approximately 60 km. -- Comparison of the Bay of Islands Complex with oceanic lithosphere and other ophiolite suites suggests it is possibly the ideal type example with which to study oceanic crust/mantle relationships.

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