Elsevier

Chemical Geology

Volume 119, Issues 1–4, 5 January 1995, Pages 55-77
Chemical Geology

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

https://doi.org/10.1016/0009-2541(94)00104-GGet rights and content

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 [LaSm]N (0.7–1.7) and ϵNd values (+2.8 to +7.0, at 540 Ma), and TiZr, TaLa, ThTa and PCe 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 ϵNd, LaSm, TiZr, TaLa, ThTa and PCe 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<-[LaYb]N≤9) and lower ϵNd values (+3.3 to +4.9) than alkaline basalts (12≤LaYb]N≤20; ϵNd = +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 ϵNd 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.

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    Present address: CRL-Géoconseil, 16 rue Hébert, BUR. 2, Québec, Qué. G1R 3T7, Canada.

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