Magmatic and fluid processes in the Upper Mantle: a study of the Bay of Islands ophiolite complex, Newfoundland

Abstract

An extremely heterogeneous section of upper mantle exposed over 2 km² in the Springers Hill area of the Lewis Hills Massif, Bay of Islands Ophiolite Complex, consists of two distinct associations: low-Al peridotites and pyroxenites (LALPP), and high-Al peridotites, pyroxenites and gabbros (HALPPG). These represent the processes of magma and fluid generation, migration and evolution in the mantle wedge of a subduction zone. -- The LALPP have Cr-spinel of Cr# = 47-90, Mg# = 23-65, and fo-med in the order of harzburgite-orthopyroxenite (and associated dunite and chromititel-clinopyroxenite (and associated dunite, chromitite and websterite). Harzburgite is a refractory residue from partial melting and complete removal of low-Ti tholeiitic magma. This harzburgite has straight, positively sloping chondrite-normalized REE patterns and Pd/lr < 1. Dykes of orthopyroxenite have U-shaped chondrite-normalized PEE patterns and very high Pd/lr ratios, and formed from hydrous. As-saturated magma of boninitic affinity, which previously had fractionated significant quantities of Cr-spinel and relatively minor olivine. By mechanical mixing or magmatic impregnation, a component of orthopyroxenite was added to some harzburgites; these harzburgites have U-shaped chondrite-normalized REE patterns and Pd/lr >1. Dykes of cllnopyroxenite formed from LREE-depleted, high Pd/lr, S-saturated, low-Ti tholeiitic magma. The LALPP formed at P ≤ 7-8 kbar in upwelling asthenosphere (T=1300 ± lOCC) below a zone of crustal accretion in a supra-subduction zone environment. -- The HALPPG contain spinel of approximately Cr# = 30, Mg# = 65, and comprise dunite, wehrlite, olivine cllnopyroxenite, gabbro and amphibole peridotite. The HALPPG formed in the lithosphere (T = 900 ± 100°C) at P = 5-9 kbar by combined assimilation and crystal fractionation, which involved fluid-bearing magma and LALPP. The replacement of LALPP by HALPPG involved dissolution of ortho- and clinopyroxene, precipitation of clinopyroxene, plagioclase, Ca-amphibole and phlogopite, and recrystallization of olivine and spinel. In minerals and whole-rock samples of HALPPG, the Mg# is largely controlled by the LALPP protolith, whereas concentrations of Ti, Al, Ca, Na, K and REE are controlled by the magma or fluid. -- The magma parental to the HALPPG was fluid-bearing, LREE-depleted, Sr-rich and SiO₂ -undersaturated, and formed by partial melting of amphibolitized oceanic crust during intra-oceanic thrusting and obduction of the Bay of Islands Ophiolite Complex. The fluid component of this magma became LREE- and Eu- enriched and SiO₂ -saturated by fractionation of Ca-amphibole and reaction with LALPP. As such, this suggests a link between fluids within metamorphic soles of ophiolites, metasomatized mantle peridotites and pyroxenites, and forearc magmatic and fluid processes leading to the formation of wehrlitic bodies and high-MgO, SiO₂ -saturated magmas or fluids of boninitic affinity.