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

Edwards, Stephen John (1991) Magmatic and fluid processes in the Upper Mantle: a study of the Bay of Islands ophiolite complex, Newfoundland. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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    Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission.
    (Original Version)

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.

Item Type: Thesis (Doctoral (PhD))
URI: http://research.library.mun.ca/id/eprint/6815
Item ID: 6815
Additional Information: Volume 1: Text. Volume 2: Tables and figures. The two volumes have been assembled sequentially into one digital item. -- Bibliography: v. 1, leaves [159]-198.
Department(s): Science, Faculty of > Earth Sciences
Date: 1991
Date Type: Submission
Geographic Location: Canada--Newfoundland and Labrador--Islands, Bay of, Region; Canada--Newfoundland and Labrador--Springers Hill Region; Earth--Mantle
Library of Congress Subject Heading: Ophiolites--Newfoundland and Labrador; Geology--Newfoundland and Labrador--Islands, Bay of, Region; Magmatism--Newfoundland and Labrador--Islands, Bay of, Region; Bay of Islands Ophiolite Complex (N.L.)

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