Neoarchaean volcanic rocks, Central Hearne supracrustal belt, Western Churchill Province, Canada: geochemical and isotopic evidence supporting intra-oceanic, supra-subduction zone extension

The Kaminak segment of the Central Hearne supracrustal belt (CHSB) Western Churchill Province, Canada, comprises a diverse sequence of Neoarchaean volcanic and less abundant metasedimentary rocks that were emplaced as two assemblages between 2695–2711 Ma and 2681–2686 Ma, respectively. These are intruded by uncommon pre-tectonic diorites and tonalites (ca. 2691 Ma), voluminous syn-tectonic tonalites and granodiorites (ca. 2679–2686 Ma) and, rare post–tectonic potassic monzogranite and syenite (ca. 2659–2666 Ma). Metasedimentary rocks include turbidites, epiclastic tuffs, minor iron formation (oxide) and rare volcanic conglomerates. Volcanic rocks of assemblage I include abundant pillowed and massive basalts to andesites with less common silicic lavas, tuffs and volcaniclastic debris flow deposits. Assemblage II contains voluminous silicic tuff and volcaniclastic debris flow deposits but fewer basaltic to andesitic flows. The critical diagnostic feature of the CHSB is the stratigraphic intercalation of compositionally diverse basaltic, andesitic and felsic volcanic rocks throughout both assemblages. Mapping, U–Pb geochronology and lithogeochemistry suggest that an initial MORB-like basaltic plain containing widespread intercalations of dacite to rhyolite was replaced at ca. 2688 Ma by a relatively short-lived, dacite to rhyolite dominated magmatic environment characterized by localized felsic volcanic centres and a bloom of 2686–2679 Ma tonalitic to granodioritic plutons. Basaltic to andesitic rocks are dominated by iron-rich tholeiites, although the proportion of calc-alkaline rocks increases with silica content. Felsic volcanic rocks all exhibit calc-alkaline affinities. The wide range in chemistry of the basaltic to andesitic rocks of both volcanic assemblages implies diverse mantle sources capable of generating voluminous MORB-, with less common ARC-, NEB(OIB)- and rare BABB-like rocks. Similarly, the variable composition of the felsic volcanic rocks indicates both anatexis of eclogitic to garnetiferous mafic crust and also extensive fractionation of mafic precursors in crustal magma chambers. Two geochemically distinct, arc-like mafic suites were generated through contamination of primary mantle-derived magmas by juvenile, ca. 2700 Ma silicic crust either in their mantle source or through assimilation upon ascent. ϵNdt isotopic data are comparable to contemporaneous depleted mantle with only local evidence for incorporation of older, ⋙2700 Ma crust. SB may have formed via tectono-magmatic processes comparable to those of the Eocene, infant-arcs of the SW Pacific, whereby the formation of a thick sequence of coeval, intercalated, compositionally diverse mantle- and crustal-derived rocks, are generated in an extensional supra-subduction setting. The cessation of supra-subduction zone extension at ca. 2688 Ma, was followed by the short-lived development of felsic volcanic edifices (incipient arc), the extrusion of mafic to felsic magmas and the concomitant intrusion of voluminous syn-kinematic tonalitic plutons. This accompanied a major change in the tectono-magmatic setting accompanying and presumably following the termination of extensional, supra-subduction zone processes.