Evidence for contrasting compositional spectra in comagmatic intrusive and extrusive rocks of the late Archean Blake River Group, Abitibi, Quebec

Abstract

The Flavrian pluton is a sill-shaped intrusion in the Blake River Group (BRG) volcanic rocks in the Noranda region of the Abitibi greenstone belt. The pluton is dominated by trondhjemites and tonalites, with minor peripheral quartz gabbro and hybrid phases. The BRG volcanic rocks consist of a bimodal suite of basalt–andesite and rhyolite. The Flavrian trondhjemites are geochemically identical to the rhyolitic lavas of the BRG (SiO 2 ≥ 72%, La/Sm = 3.4, La/Yb = 3.6, Zr/Y = 3.9, Y/Nb = 3.1), and the Flavrian gabbroic and dioritic rocks are identical to the BRG basalts and andesites (SiO2 < 58%, La/Sm = 3.0, La/Yb = 5.5, Zr/Y = 4.2, Y/Nb = 3.3). However, the tonalitic rocks of the Flavrian pluton have no extrusive equivalents in the BRG. The different compositional spectra of the extrusive and intrusive rocks are interpreted as being a result of a transition in magma-chamber evolution from a zoned open system that was active during the evolution of the volcanic rocks to closed-system plutonic crystallization. The latter destroyed the compositional bimodality of the magma chamber and resulted in the evolution of intermediate compositions (tonalites) generated by both fractional crystallization and magma mixing.