Abstract

Detailed mapping of a portion of the North Spirit Lake greenstone belt reveals a basal basaltic volcanic unit (ca. 3 Ga) with a komatiitic component and strong major and trace element geochemical affinity with typical mid-ocean ridge basalts (e.g., N-MORB). The overlying transitional unit includes basaltic tuffs, which have close geochemical affinity to the underlying basalts, and to N-MORB. Interbedded with and overlying the tuffs is a thin, laterally continuous sulfide-rich unit with chert interbeds, which is in turn overlain by interbedded carbonate, chert, and iron-rich sedimentary rocks. The chemical sedimentary rocks of the transitional unit display rare-earth element (REE) patterns and positive Eu anomalies consistent with derivation from fluids associated with submarine hydrothermal vent fields. The sulfide unit is interpreted to represent proximal hydrothermal vent deposits derived from black smoker activity near a spreading center. The overlying carbonate and chert mounds and beds are interpreted as hydrothermal deposits formed at lower temperatures in a more distal setting relative to the spreading center. The carbonate deposits may have formed in an environment analogous to that of the Lost City hydrothermal field (Kelley et al., 2001), where abundant chimneys, dendrites, and beds of carbonate are produced ~15 km from the mid-Atlantic rift axis. The basal volcanic unit consequently is interpreted as oceanfloor basalts formed at an Archean spreading center, with the basaltic tuffs representing the last volcanic emanations, and with the chemical sediments representing hydrothermal-vents deposits both proximal and distal to the spreading center. The capping siliciclastic unit likely is part of an accretionary wedge, possibly representing ocean closure after a protracted seafloor spreading episode. The sedimentary rocks were derived, in part, from a weathered tonalite-trondhjemite-granodiorite source. The supracrustal rocks of the North Spirit Lake greenstone belt provide an unusually clear window into some tectonic activity on the primitive Earth, including evidence for the existence of ocean-floor basalts, proximal and distal hydrothermal activity, and possibly a protracted cycle of ocean opening and destruction.

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