Abstract

Felsic magma petrogenesis was studied by analyzing 24 stratigraphically controlled Archean andesite-to-rhyolite lava flows of both tholeiitic and calc-alkalic affinity from the upper Noranda Subgroup, Quebec, using instrumental neutron activation and X-ray fluorescence techniques. The lavas have moderate values of [La/Yb]N (0.9–3.8) and low values of 100 × Th/Zr (~1). According to calculations following batch partial melting and Rayleigh fractional crystallization models, both the calc-alkalic and tholeiitic felsic volcanic rocks are probably products of shallow-level fractional crystallization of mafic parental magmas formed respectively by lower (~7 % for calc-alkalic) and higher (~14% for tholeiitic) degrees of partial melting of a primitive mantle source.Contribution to the magma genesis from plausible crustal materials was negligible. A back-arc-type diapirism is geochemically suggested for the tectonic model of origin of Noranda felsic magmas, in conformity with geological observations. Felsic volcanic rocks with compositions analogous to the studied samples exist in several other Archean terrains of the Canadian Shield, suggesting thereby that the late Archean sialic crust was at least in part produced by volcanic rocks ultimately derived from the primitive mantle.

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