Amphibolites of the Shawanaga domain, Central Gneiss Belt, Grenville Province, Ontario: tectonic setting and implications for relations between the Central Gneiss Belt and Midcontinental USA

The Lighthouse gneiss association is an amphibolite-dominated assemblage situated in the allochthonous Shawanaga domain of the Central Gneiss Belt (CGB), Grenville Province. It consists of two units in which the amphibolites differ in chemical composition and associated lithologies. The upper unit consists of: (i) thick intervals of amphibolite interpreted as mafic flows and which form about half of the unit; (ii) a varied assemblage of thinly interlayered and laminated amphibolite and metasediment interpreted to have originated as mafic tuffs interlayered with sediment with a quartzofeldspathic source; and (iii) thick horizons of semipelite or regularly interlayered psammites and pelites interpreted to be metamorphosed greywacke-shale turbidites shed from a quartzofeldspathic source. The lower unit has a much higher proportion of thick amphibolite interpreted as mafic flows and the metasediments are semipelitic. Geochemically, both upper and lower amphibolites are olivine-normative tholeiites but have distinct trace element characteristics. The mantle-normalized trace element pattern of amphibolite in the upper unit lacks a subduction imprint and suggests an asthenospheric source, but that of the lower amphibolite has a subduction imprint (or an indication of eruption through attenuated continental crust). A back-arc environment is most consistent with the chemistry of the amphibolites and the nature of the associated metasediments. The recognition of the rocks of the Lighthouse gneiss association as juvenile material and their juxtaposition with another juvenile member of the Shawanaga domain, the metarhyolite-rich Sand Bay gneiss association, leads to a refinement of current models for the tectonic evolution of the CGB before the ≈1200–1000 Ma Grenville orogenic cycle. We also suggest that all elements of the Proterozoic geology of Midcontinental USA are present, although jumbled by Grenvillian tectonism, in the CGB. These elements include units interpreted as oceanward-younging juvenile arcs that are overlain by a granite–rhyolite layer and pierced by coeval plutons.