Models of the oceanic lithosphere derived from constraints imposed by the geology of ophiolite complexes require a magma chamber to produce the cumulate and other plutonic portions of the oceanic crust and upper mantle. Evidence from the Bay of Islands ophiolite complex indicates that layered plutonic rocks have not formed solely due to differential crystal settling to the horizontal floor of a magma chamber, but have formed largely by heterogeneous nucleation and in situ crystallization along gently inclined as well as steeply inclined bounding surfaces of a large magma reservoir. Coherent, well defined and systematically oriented, large-scale, arcuate patterns described by mesoscopic igneous layering have been identified in the plutonic sections of two ophiolite massifs in the Bay of Islands complex. These layering patterns seem to outline the shape of a large, continuously evolving, steady-state magma chamber that extended along a ridge axis and ended at a transform fault intersection.
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