Geochemistry of mafic magmas from the Ungava orogen, Québec, Canada, and implications for mantle reservoir compositions at 2.0 Ga

Abstract

The Ungava orogen of northern Québec is one of the best preserved Proterozoic mobile belts of the world, recording > 200 Ma of plate divergence and convergence. Voluminous magmatism associated with rifting of the Superior Province basement ≈2.04 Ga resulted in the development of a volcanic rift margin sequence and an ocean basin. Four distinct mafic magma suites were erupted: (1) continental basalts (Eskimo Formation, western and central Povungnituk Group) with moderate to high Zr/Nb and negative Nb anomalies which have interacted with the continental crust (ϵ Nd(2.0 Ga)) from −7.4); (2) mafic lavas from the Flaherty Formation, eastern Povungnituk Group and some Watts Group lavas associated with passive margin rifting, having slightly enriched isotopic signatures ( ϵ Nd(2.0 Ga) = +2.7 to +4.4) compared to the contemporaneous depleted mantle, high (Nb/Y) n and low Zr/Nb ratios (≈4.4 and ≈8.9, respectively); (3) a highly alkaline OIB-like suite ( ϵ Nd(2.0 Ga) = +2.3 to +3.2, (Nb/Y) n> 12) within the Povungnituk Group composed of nephelinites, basanites and phonolites; and (4) depleted Mg-rich basalts and komatiitic basalts ( ϵ Nd(2.0 Ga) ≈ + 4.5 to + 5.5) with trace-element characteristics of N-MORB, but with higher Fe and lower Al than primitive MORB (Chukotat Group, Ottawa Islands and some Watts Group samples). The ocean basin into which these lavas were erupted was subsequently destroyed during subduction between ≈1.90 and ≈1.83 Ga, resulting in the development a magmatic arc (Narsajuaq terrane and Parent Group). The Ungava magmas provide a unique window into the mantle at 2.0 Ga. The chemical and isotopic similarity of these Proterozoic magmas to modern-day magmas provides strong evidence that the interplay between depleted mantle, OIB mantle and sub-continental mantle during the Proterozoic was comparable to that of the modern Earth.