Dual sources of ensimatic magmas, Hearne domain, Western Churchill Province, Nunavut, Canada: Neorchean “infant arc” processes?

Abstract

In the Henik segment of the Central Hearne supracrustal belt, Neoarchean supracrustal rocks accumulated in deep-water depositional systems that were well removed from a continental influence. Repeated in time and space, these systems consisted of mafic lava plain, slope, and apron subenvironments, with local emergent to near-emergent felsic volcanic centres. Volcanic and plutonic rocks are subalkaline, and range in composition from basalt to rhyolite. Consistent with field relationships, the volcanic rocks are juvenile and, with predominantly positive values that are independent of compositional variation, display little evidence of contamination by continental crust. However, despite primary interlayering on scales of 10s of metres, mafic and felsic rocks display markedly different rare earth element (REE) characteristics. The mafic volcanic rocks are MORB-like and display a tholeiitic magma evolution trend, flat REE patterns, and positive values, suggesting that they were derived from partial melting of depleted mantle. In marked contrast, the felsic rocks display steeply sloping REE patterns with significantly lower heavy REE abundances, negative Nb anomalies, and positive Zr and Hf anomalies suggesting derivation from partial melting of slightly older basaltic crust. This duality of magma sources supports a generalized model for the Central Hearne supracrustal belt that draws analogies from Eocene intraoceanic “infant arc” processes described from the southwestern Pacific Ocean, rather than from those normally associated with fully developed Phanerozoic subduction.