Formation of chromitites by assimilation of crustal pyroxenites and gabbros into peridotitic intrusions: North Arm Mountain massif, Bay of Islands ophiolite, Newfoundland, Canada

Abstract

The lowermost crust of the North Arm Mountain massif, Bay of Islands ophiolite, is dominated by peridotitic intrusions that assimilated and reacted with host gabbros, pyroxenites and peridotites. Cr‐spinel is a common accessory and locally forms chromitite layers, schlieren, or rims. Thermobarometric calculations and fractionation models suggest that the Cr‐spinels formed at oxidation states between QFM and QFM+1.5. Postcumulus and subsolidus diffusional exchanges with silicate minerals increased the Fe/Mg ratio of Cr‐spinels at North Arm but cannot explain the diversity of Cr‐spinel Cr/Al. Nor can closed or open‐system fractional crystallization of a single parental magma in cotectic proportions generate the required range of Cr/Al in Cr‐spinel. Either a variety of primary magmas was present or the Cr/Al ratio of the melt was perturbed by assimilation of host rocks, with assimilation inducing crystallization of spinel in greater than cotectic proportions. Outcrop‐scale structures and phase relationships suggest that high‐Cr/Al chromitites that are associated with contacts between intrusive peridotites and host pyroxenites are by‐products of the incongruent dissolution of pyroxenes into a Cr‐spinel‐saturated magma. Aluminous chromitites occur higher in the section, in association with hybrid feldspathic peridotites and gabbros that formed by disaggregation of feldspathic cumulates into peridotitic intrusions. These rocks commonly have disseminated anhedral Cr‐spinels and may also contain thin chromitite schlieren, both with low Cr/Al spinels. The disseminated Cr‐spinels fill dissolution pits in feldspar xenocrysts, implying that they formed by incongruent dissolution of feldspar. The H2O needed to stabilize the amphibole included in the low‐Cr/Al Cr‐spinels was probably derived from the reactant gabbros, many of which were previously affected by subseafloor alteration. Locally, chromitites and disseminated Cr‐spinels have higher concentrations of Ti, V, Fe2+, and Fe3+, which we attribute to assimilation of gabbros rich in Fe‐Ti oxides. The composition of Cr‐spinels in chromitites at North Arm depends more on the nature of the minerals being assimilated than on the composition of the magma.