The physical volcanology and petrogenesis of Archean komatiitic lavas and volcaniclastic rocks in the Selkirk Bay area, Prince Albert greenstone belt, Melville Peninsula, Nunavut, Canada

The 2.96 Ga Prince Albert greenstone belt (PAGB) of the Melville Peninsula (Nunavut, Canada) contains multiple units of differentiated and undifferentiated komatiitic flows and laterally extensive units of komatiitic tuff These units are serpentinized, chloritized, amphibolitized, and less commonly, carbonate altered The tuffs retain element abundances typical of Al-undepleted komatiites, which are identical to those of associated komatiitic flows, indicating that the tuffs and flows are comagmatic. Excellent textural preservation of the flows is present at a few outcrops, which display well developed upper spinifex and lower cumulate zones typical of differentiated komatiite flows. However, the tuffs do not preserve primary microscale textures and the tuffs are uniformly fine-grained and largely massive to planar- laminated. This, coupled with their extensive nature (up to 10 km laterally; up to 2m thick vertically), lack of contacts with komatiitic flows, and their komatiitic mineralogical and chemical composition support a pyroclastic origin, deposition via eruption fed fallout and mass flow, and subaqueous deposition during intervals characterized by lower rates of effusive volcanism. The low volatile content of the komatiitic magma, indicated by lack of vesicles in associated komatiitic flows and the lack of scoriaceous lapilli in the fine-grained tuff suggest an origin through phreatomagmatic pyroclastic eruptions. The Selkirk Bay komatiitic lavas and tuffs have liquid compositions up to 30 wt% MgO, consistent with their derivation from a high magnesium komatiitic parental magma. There is evidence for up to 10% crustal contamination, which is also supported by slight enrichments in the LREE and Th relative to Nb. Major and minor elemental trends are indicative of olivine being the dominant liquidus mineral during the early stages of crystallization.;Komatiitic flows of the PAGB were erupted onto massive to pillowed basalt where they formed thick channelized, massive, undifferentiated flows flanked by spinifex-textured sheet/lobate flows. Synvolcanic structural basins and down faulting are thought to be responsible for flow channelization and also controlled the deposition of local, framework-supported, ultramafic mass flow deposits and localized deposits of felsic volcaniclastic rocks. The komatiitic tuffs occur at two stratigraphic intervals indicating that they are a product of at least two periods of pyroclastic eruptions. The uppermost strata document a change to a more mafic-dominated lava sequence interlayered with both difièrentiated and undifferentiated komatiitic flows, which are overlain by thick deposits of felsic volcaniclastic rocks.