Abstract:

The 2.96 Ga Prince Albert greenstone belt (PAGB) o f the Melville Peninsula (Nunavut, Canada) contains multiple units o f differentiated and undifferentiated komatiitic flows and laterally extensive units ofkomatiitic tuff These units are serpentinized, chloritized, amphibolitized, and less commonly, carbonate altered. The tuffs retain element abundances typical o f Al-undepieted komatiites, which are identical to those o f associated komatiitic flows, indicating that the tuffs and flows are comagmatic. Excellent textural preservation o f 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 planarlaminated. This, coupled with their extensive nature (up to 10 km laterally; up to 2m thick vertically), lack of gradational contacts with komatiitic flows, and their komatiitic mineralogical and chemical composition support a pyroclastic origin, deposition via eruption-fedfallout and mass flow, and subaqueous deposition during intervals characterized by lower rates of effusive volcanism. The low volatile content o f the komatiitic magma, indicated by lack o f vesicles in associated komatiitic flows and the lack o f 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 fo r up to 10% crustal contamination, which is also supported by slight enrichments in the TREE 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.<br/>Komatiitic flows o f 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 fo r 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 ofpyroclastic eruptions. The uppermost strata document a change to a more mafic-dominated lava sequence interlayered with both differentiated and undifferentiated komatiitic flows, which are overlain by thick deposits o f felsic volcaniclastic rocks.