Geochemistry of Mount Orford ophiolite complex, Northern Appalachians, Canada

The Orford ophiolite complex is part of an ultramafic-mafic belt occurring within the Québec Appalachians. These allochtonous bodies were emplaced during the Early Ordovician. The Orford complex comprises (from bottom to top): ultramafic cumulate rocks (dunite to pyroxenite), gabbros, sheeted sills (diabase) and volcanic rocks. Representative rocks (10 ultramafic cumulate rocks, 5 gabbros, 8 diabases, 8 volcanic rocks) from all rock units were analysed for major elements and Ba, Cr, Ni, Rb, Sr, Y, Zn, Zr and REE. Rare-earth element abundances in cumulate dunites are ∼0.04−0.08× chondrite. Pyroxene-bearing cumulates are all strongly LREE-depleted and have HREE abundances ranging from 0.6× to 2.5× chondrite. Some gabbros analysed have up to 40−45× chondrite HREE. The diabases of the sheeted sill complex and the volcanic rocks have MORB-like REE patterns. Geochemical modelling of the REE strongly suggests that ultramafic cumulates, gabbros, sheeted sills and volcanic rocks are coeval and were derived from a LREE-depleted, MORB-type lherzolitic mantle source. An arc environment is favoured as a tectonic setting for the Mount Orford ophiolite complex.