Upper mantle processes beneath the 2.7 Ga Abitibi belt, Canada: a trace element perspective

Abstract

Abstract Mantle processes associated with volcanism in the Abitibi belt remain poorly understood despite numerous detailed studies of its volcano-sedimentary successions. This paper employs log plots of Nb/Y and Zr/Y to distinguish the sources of well-defined Abitibi volcanic suites and to clarify processes in the upper mantle during greenstone belt development. Mafic rocks of the Southern Volcanic Zone in the Abitibi belt define two main trends on log plots of Nb/Y versus Zr/Y. Trends for the Kidd–Munro Assemblage and Malartic Block correspond to mixtures of strongly deleted plume asthenosphere and adakitic melts. The shared trace element systematics of these volcanic successions confirm they are parts of a single authochthonous supracrustal assemblage that extended across the southern Abitibi belt. The younger Kamiskotia Assemblage and Blake River Group correspond to crustal extension episodes within an Abitibi island arc. Trace element plots establish that they were derived from mantle plume-related asthenosphere and not the late Archean MORB-source upper mantle associated with island arc assemblages elsewhere in the southern Superior Province. The data suggests widespread lateral flow of mantle plume asthenosphere occurred in the vicinity of the Abitibi arc. Mantle mixing with adakitic melts continued between ∼2716 and ∼2698 Ma. The data are not consistent with wet komatiite models that generate ultramafic volcanic rocks from typical upper mantle following hydration of a mantle wedge. Chemical mixing trends for mafic magma sources indicate melt hybridization of the Abitibi mantle wedge. The trends require a slab melt origin for adakitic magmas rather than internal differentiation of underplated mafic crust. Comparisons with other 2.7 Ga greenstone belts suggest that mantle plume interaction with Archean subduction zones, as proposed for the Abitibi belt, can be distinguished in the NbZrY systematics of associated mafic volcanic rocks.