An archean oceanic felsic dyke swarm in a nascent arc: The Hunter Mine Group, Abitibi Greenstone Belt, Canada

The 2730-Ma-old Hunter Mine Group (HMG), a dominantly felsic subaqueous volcanic sequence, was formed during early are construction in the Abitibi greenstone belt (Quebec, Canada). The western part of the HMG contains a felsic dyke swarm up to 1.5 km wide and traceable up-section for 2.5 km. Five distinct generations were identified: (1) aphanitic to feldspar-phyric dykes; (2) quartz-feldspar-phyric dykes with < 5% quartz phenocrysts; (3) quartz-feldspar-phyric dykes with 10-25% quartz phenocrysts; (4) dacitic feldspar-phyric dykes; and (5) mafic dykes. The felsic dykes collectively constitute more than 90% of the dyke swarm. Geochemically, they resemble modem calc-alkaline dacites and rhyolites. Their mantle-normalized incompatible trace-element patterns display a moderate enrichment of Th and light REE relative to HFSE and heavy REE as well as negative Nb, Ta, Eu and Ti anomalies. Most of the major- and trace-element abundance variations in these rocks can be explained by crystallization of feldspars. Geochemical data including depleted mantle-like epsilon(Nd) values suggest that an older sialic substrate was not involved in their genesis. We infer that the felsic rocks were generated by melting of mafic oceanic crust. The swarm was emplaced during nascent oceanic island-are development and was related to rifting of the are. The conformably overlying MORE-like basalts and basaltic komatiites of the Stoughton-Roquemaure Group used the same conduits and further indicate splitting of the are. HMG and associated parts of the Abitibi greenstone belts bear a strong resemblance to modem rifted intraoceanic arcs of the western Pacific.