Geochemistry of Archean meta-igneous rocks, Lake Despair area, Wabigoon Subprovince, northwestern Ontario
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abstract
The Lake Despair area, northwestern Ontario, is underlain by a series of igneous and meta-igneous rocks including mafic and felsic metavolcanic rocks, gneisses (Footprint gneiss), and plutonic granitoids (Jackfish Lake Plutonic Complex; Northwest Bay Complex). The metavolcanic sequence consists of a bimodal suite of interlayered metabasalts of tholeiitic character and metadacites with metamorphic grade ranging from greenschist to amphibolite facies. Trace element studies of the metabasalt, while indicating an oceanic provenance, do not uniquely distinguish between possible oceanic tectonic settings. The metadacites, the Footprint gneiss, and the Northwest Bay Complex are chemically similar to the high Al tonalitic rocks common in other Archean terrains. The Footprint gneiss, which forms part of the gneissic core of the Rainy Lake batholith, was derived from an igneous parent, and was quasi-isochemically metamorphosed.The youngest major intrusion, the Na-alkalic Jackfish Plutonic Complex, is composed of hornblende diorite and monzodiorite with minor leucodiorite, quartz monzodiorite, granodiorite, and Na-syenite. The diorite and monzodiorite crystallized from an intermediate magma formed by partial melting of a garnet and (or) amphibole-rich basaltic parent. Remaining residual liquids, enriched in alkalis and silica by amphibole fractionation, crystallized to form the more leucocratic rock types. Perthitic microcline megacrysts, characteristic of the quartz monzodiorite and granodiorite, formed near the end of magmatic processes. The Na-syenite was emplaced as a separate pulse of magma or possibly was produced by the interaction of alkalis and the upper portions of the solidified rim of the magma chamber.
