Geochemistry of pre-Taconian mafic volcanism in the Humber Zone of the northern Appalachians, Québec, Canada

Abstract

There are two types of pre-Taconian mafic volcanics in the Humber Zone of the northern Appalachians: (1) relatively small scattered volumes of transitional to alkaline basalts and hypabyssal intrusives; and (2) localized, though more voluminous, subalkaline tholeiitic volcanics. They formed during a Late Proterozoic to Early Paleozoic period of continental lithospheric extension which led to the break-up of Laurentia and to the opening of the Iapetus ocean, and were subsequently deformed and metamorphosed during the Ordovician Taconian and Late Silurian to Middle Devonian Acadian Orogenies. Subalkaline tholeiitic basalts have major-element and REE abundances similar to that of modern N-MORB. However, they are further characterized by variable [ La Sm ] N (0.7–1.7) and ϵ Nd values (+2.8 to +7.0, at 540 Ma), and Ti Zr , Ta La , Th Ta and P Ce ratios forming more-or-less developed Ti, Ta and P troughs on mantle-normalized diagrams. Primitive subalkaline tholeiitic basaltic melts were most likely derived from a depleted asthenospheric source, while heterogeneity in the ϵ Nd , La Sm , Ti Zr , Ta La , Th Ta and P Ce values is attributed to variable degrees of assimilation and fractional crystallization of both lower- and upper-crustal material during ascent and eruption. The transitional to alkaline basalts are enriched in moderately to highly incompatible trace elements (LREE, Ti, P, Hf, Ta, Th), but unlike tholeiites, they do not show evidence of crustal assimilation. Transitional basalts have less fractionaed REE ( 3<-[ La Yb ] N ≤9 ) and lower ϵ Nd values (+3.3 to +4.9) than alkaline basalts ( 12≤ La Yb ] N ≤20 ; ϵ Nd = +5.9 to +6.4). The enriched character of the transitional to alkaline basalts is either explained by extraction from a heterogeneous enriched asthenospheric mantle source, or a metasomatized lithospheric mantle source previously enriched by OIB veins. Although the available geochemical data cannot discriminate between contributions from the lithospheric and asthenospheric mantles, relatively high and positive ϵ Nd values indicate that the source of transitional and alkaline basalts had not suffered long-term enrichment, and that contribution from the Late Archean and Early to Middle Proterozoic mantle lithosphere is unlikely.