Geochemistry of Mesoarchean felsic to ultramafic volcanic rocks of the Lac Guyer area, La Grande Subprovince (Canada): Evidence for plume-related magmatism in a rift setting

Abstract

The Lac Guyer area of the La Grande Subprovince (Superior Province) hosts Mesoarchean east-trending volcano-sedimentary sequences subdivided into 3 stratigraphic units (Mintisch and Rouget Formations and Guyer Group), which were emplaced between 2847 and 2806 Ma on a gneissic-tonalitic basement. These approximately 35 to 175 km-long sequences comprise felsic to ultramafic volcanic rocks that have been metamorphosed to the amphibolite and granulite facies and record mainly two Archean episodes of ductile deformation.The Mesoarchean tholeiitic basalt, komatiitic basalt, and komatiite appear to be genetically related based on petrologic modeling, their close spatial association, and their major and trace element signatures. They are characterized by the following: depletion to enrichment in LREE relative to HREE; Zr/Ypm, Ti/Vpm, and La/Smpm ratios higher than or close to 1; Ti/Zrpm and Nb/Thpm ratios lower than or close to 1; common negative anomalies in Nb-Ta ± Ti; and local positive to negative anomalies in Hf-Zr. Komatiitic basalt and komatiite are mostly Al-undepleted and rarely Al-depleted. According to current komatiite melting models, this suggests that these rocks formed by a high degree of partial melting of a garnet-bearing lherzolite source that consumed any residual garnet and progressively depleted the primitive mantle source in incompatible elements. Basalt and komatiitic basalt compositions were likely produced by assimilation and fractional crystallization (AFC) processes from komatiitic magmas (∼21–23 wt% MgO), as suggested by AFC modeling and their geochemical characteristics, whereas komatiite compositions were controlled by partial melting processes and crustal contamination.The Mesoarchean calc-alkaline rhyolite, dacite, and andesite are enriched in LREE relative to HREE, enriched in Th relative to LREE, and have negative anomalies in Nb-Ta-Ti, negative to positive anomalies in Hf-Zr, as well as high Zr/Y and Ti/V ratios and low Ti/Zr ratios. Such geochemical signatures could be explained either by concurrent assimilation and fractional crystallization of the basaltic magmas in the lower crust or by magma mixing between basalt and lower crustal anatectic magmas.The spatial and temporal distribution of Mesoarchean volcanic rocks within the Lac Guyer area and their major and trace element characteristics suggest that they were emplaced into a continental rift associated with a mantle plume. The geochemical similarities between these rocks and the rift-related volcanic rocks of the younger Yasinski Group (2751–2732 Ma), and the presence of other Mesoarchean volcanic sequences with rift affinity indicate that the La Grande Subprovince was affected by two main plume-related rifting events during the Archean (ca. 2.88–2.81 and 2.75–2.71 Ga), each produced by a single or multiple magmatic plumes.