Geochemical evolution of earliest Carboniferous continental tholeiitic basalts along a crustal-scale shear zone, southwestern Maritimes basin, eastern Canada

Abstract

Continental tholeiites were extruded locally within the extensional wrench Maritimes basin from about 375 to 330 Ma (Late Devonian–Early Carboniferous). Basalts of the Early Carboniferous Diamond Brook Formation are up to 1.5 km thick and overlie a succession of predominantly felsic volcanic rocks reaching several kilometres in thickness. The volcanic rocks and co-genetic plutons occur along a crustal-scale strike–slip fault that facilitated the transport of relatively primitive magma, resulting in basalts with high Mg number, Ni and Cr, ε Nd>+4, and relatively low TiO 2, V, Y and Zr. Fractional crystallization of plagioclase and lesser clinopyroxene, in the absence of iron-oxide fractionation, resulted in repetitive extrusion cycles hundreds of metres in thickness that show an upward increase in plagioclase phenocrysts or megacrysts and in TiO 2 and other high field-strength elements (HFSEs). Rare andesites are interpreted as products of partial melting of underplated gabbro. The relatively primitive basalts resemble low-Ti basalts found in other continental flood basalt provinces and have trace-element and isotopic characteristics that have been ascribed in the literature either to partial melting of, or contamination of asthenospheric melts by, subcontinental lithospheric mantle. In the Maritimes basin, similar continental tholeiites were extruded intermittently over a 45-Ma period. High FeO t (∼12% normalised to 8% MgO) suggests high potential temperature in the source mantle, presumably within a mantle plume. The youngest eruption products geochemically and isotopically resemble HIMU-OIB. Plume activity may have gradually migrated eastward from late Devonian to Permian, from the Maritimes basin through Scotland to Norway and north Germany.