Abstract
The Early to Middle Ordovician Québec Appalachian ophiolites contain three geochemically distinct groups of extrusive and intrusive rocks. The predominant group consists of Ni- and Cr-rich olivine-clinopyroxene phyric lavas and dikes grading in composition from basalt and andesite to rhyodacite. These rocks are geochemically similar to arc-related boinites. They are closely associated, at the base of the extrusive sequence, with aphyric lavas and dikes which have geochemical affinities with volcanic arc basalts. The third group is restricted to a single locality, Mount Orford, at the southwestern end of the ophiolite belt. It consists of alkaline diabases occurring in interdigitation with arc basaltic diabases to form a 1000-m-thick sheeted sill complex. The Thetford Mines ophiolite is interpreted to be typical of ocean crust created by fore-arc rifting in a subduction zone environment. The Mount Orford ophiolite, with its alkaline rocks, is considered to be remnant of the back-arc section of the same island-arc system.
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