Abstract

Three ophiolites are currently recognized in the southern Quebec Appalachians, from south to north: the Mont-Orford ophiolite, dominated by tholeiitic to transitional–alkaline basalt and diabase, and the Thetford Mines and Asbestos ophiolites, which are made up of boninites and tholeiitic basalts. This contribution documents the origin, geochemistry, and geodynamic evolution of the Lac-Brompton ophiolite, which was previously interpreted as a diapiric serpentinite mélange and genetically attributed to the Mont-Orford ophiolite. Our study of its igneous rocks rather indicates that the Lac-Brompton ophiolite is an exhumed segment of oceanic lithosphere represented by mantle rocks of harzburgitic composition and a discontinuous crust made up of boninitic volcanic rocks and intrusions of gabbro, pyroxenite, and granitoids that, respectively, overlie and crosscut the harzburgite. The ophiolitic mantle and crustal rocks and underlying infra-ophiolitic tectonic sole are overlain by sedimentary rocks belonging to the Saint-Daniel Mélange. The internal stratigraphy of the ophiolite and relationships with the Saint-Daniel Mélange suggest that the mantle rocks were exhumed to seafloor level prior to boninitic magmatism, and that the mantle and crustal rocks were deeply eroded during or shortly after obduction. The intra-oceanic evolution of the Lac-Brompton ophiolite is believed to have been comparable to the one of ultramafic core complexes exposed along slow-spreading ridge segments. The stratigraphic and geochemical characteristics of the Lac-Brompton ophiolite suggest that it forms, with the Asbestos and Thetford Mines ophiolites, a composite slab of forearc seafloor.

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