Major element variations in basalts and diabases from the North Arm Mountain Massif, Bay of Islands ophiolite: Implications for magma chamber processes at mid-ocean ridges

Abstract

The major element compositions of aphyric diabases from the North Arm Mountain Massif range from primitive basalts to highly evolved ferrobasalts. From modeling of these compositional variations, it is clear that the diabases have undergone highly variable (~ 10 to ~ 90%) increments of crystallization at low pressures. The dominant minerals inferred to have crystallized from the liquids that formed these diabases are olivine, plagioclase, and clinopyroxene, the same minerals that dominate the underlying cumulate gabbroic and ultramafic rocks. The projection of these compositions onto pseudo-liquidus phase diagrams suggests that the primary magma(s) from which this North Arm suite was produced were generated by melting at 10–25 kbar (or greater) pressure. There is no indication in the major element variations of these diabases of the operation of high-pressure crystal factionation processes, which is not surprising because low-pressure fractionation processes will effectively mask evidence for earlier high-pressure processes. It is suggested that the tapping of intercumulus liquid from crystal mush areas and the tapping of discrete, fractionated magmas within the roof zone of magma chambers is a method of producing a wide variety of magma compositions from a single magma chamber in which the majority of the chamber may be actively convecting and therefore remain relatively uniform in composition.