Ferropicrite-driven reworking of the Ungava craton and the genesis of Neoarchean pyroxene-granitoids

Abstract

Voluminous, pyroxene-bearing, intermediate to felsic plutons were emplaced during a 20–50 million year long, spatially extensive Neoarchean igneous event that culminated in the cratonization of North Americaʼs ∼500 km-wide Ungava craton. The crystallization ages of pyroxene-bearing plutons coincide with the emplacement of numerous ca. 2.72–2.70 Ga, Fe-rich, ultramafic/mafic intrusions of the Qullinaaraaluk suite (Q-suite) that are scattered across the disparate domains of the Ungava craton. A high proportion of relatively sodic pyroxene-bearing granitoids with intermediate silica contents fall in a compositional gap between the Q-suite and pyroxene-free granitoids, suggesting that the pyroxene-granitoids may be formed by the simultaneous fractional crystallization and assimilation of older tonalitic and trondhjemitic (TT) crust by the Q-suite magmas. We estimate that pyroxene-granitoids containing ∼65 wt.% SiO2 may reflect ∼40–50 wt.% contamination of mantle-derived picritic magma by trondhjemitic melts of the pre-2.74 Ga TTG crust. The craton-wide occurrence of the Q-suite intrusions and pyroxene-granitoids suggests that underplating by ferropicritic magmas played a key role in the cratonization of the Ungava craton at the end of Archean. A major contribution of mantle-derived magmas to the petrogenesis of the ca. 2.74–2.70 Ga pyroxene-granitoids is consistent with the proposed global generation of voluminous juvenile continental crust ca. 2.7 Ga.