Cumulate Recycling and Crustal Evolution in the Bay of Islands Ophiolite

Abstract

The lower crustal section of the North Arm Mountain Massif of the Bay of Islands Ophiolite is composed of interlayered gabbroic and ultramafic cumulates. Despite pervasive ridge-related deformation, key contact relationships are preserved in low-strain domains or late-kinematic intrusions. Angular and crenulated gabbroic enclaves in ultramafic rocks and ultramafic dikes in gabbroic rocks imply that many of the ultramafic rocks intruded into older gabbroic hosts: olivine gabbro, gabbro, gabbro-norite, norite, and Fe-oxide-amphibole gabbros. The ultramafic intrusions are composed of dunite, wehrlite, harzburgite, lherzolite, and olivine websterite. Clino- to ortho-pyroxenite reaction rims line many peridotitic/gabbroic contacts. The voluminous pyroxenites of the lower crust are petrologically identical to in situ pyroxenite reaction rims and are thought to represent partially digested gabbros. Fine-grained harzburgitic sills crosscut the foliation of wehrlitic host rocks, with orthopyroxenite reaction rims at many of the contacts. Medium- to coarse-grained harzburgitic cumulates are important locally in the lower crust. Where pyroxenite reaction rims are detached or absent, the interaction between the hot, intruding, primitive magma and host gabbroic cumulates led to the formation of hybrid olivine gabbro, troctolite, and feldspathic peridotite. Feldspars in these hybrids occur as embayed grains and clots (anorthositic or gabbroic), commonly with discontinuous aluminous Cr-spinel rims. Aluminous chromitite schlieren are commonly found within hybrid feldspathic peridotite bodies. High-Cr spinel (Cr# = Cr/Cr + Al >0.5) layers are associated with lower crustal and upper mantle orthopyroxenite schlieren. The coronitic textures and spatial associations suggest that chromitites in ophiolites are by-products of the assimilation of gabbro (Cr# <0.5) and pyroxenite (Cr# >0.5) by invading primitive magmas.