Late Silurian-Early Devonian slab break-off beneath the Canadian Appalachians: Insights from the Nashwaak Granite, west-central New Brunswick, Canada

Abstract

Late Silurian-Early Devonian granites are widespread in the Miramichi Highlands of New Brunswick, but their petrogenesis is controversial and hinders an understanding of the geological evolution of the Canadian Appalachians. Here we present a detailed geochemical and geochronological study of the Nashwaak Granite, and compare it with coeval granites in this region to indicate a Late Silurian-Early Devonian slab break-off regime in the Canadian Appalachians. The Nashwaak Granite consists of two-mica granite, biotite granite, and biotite granite dyke swarms. These rocks intruded at ca. 420 Ma and their compositions show a highly siliceous, calc-alkaline, and peraluminous characteristic. They show similar REE distribution patterns, similar depletion of Ba, Sr, Nb, P, and Ti, and similar Nd isotopes as contemporaneous granites inboard of the Salinic suture zone (Bamford Brook Fault). Together, these data suggest that Late Silurian-Early Devonian granites in the Miramichi Highlands may be derived from partial melting of the Neoproterozoic to lower Cambrian Ganderian basement resembling the Brookville terrane. Enriched subcontinental lithospheric mantle may have also been involved in the magma system (where coeval gabbro occurs within the Granite), but a contribution from asthenospheric mantle is absent. These linearly-distributed and post-collisional granites are most likely formed by Salinic slab break-off, with corresponding uplift and extension recorded by 422-419 Ma bimodal volcanic rocks, unconformities in sedimentary rocks, and extensional deformation (D3) in the Brunswick subduction complex of northern New Brunswick.