Abstract

Variolitic lavas from Archean tholeiitic series north and south of Rouyn–Noranda (Abitibi Metavolcanic belt, Canada) contain large, sharply defined, spheroidal to subspheroidal felsic varioles (up to 5 cm in diameter) set in a ferruginous matrix of more mafic composition. Quench texture and flow differentiation studies indicate that the variolites were produced by rapid cooling of a two-liquid magma, and that these liquids were in contact and chemically discrete prior to extrusion. Physical mixing models do not adequately account for these contiguous magmas, yet a liquid immiscible model demonstrably satisfies almost all variolite field, microscopic, microprobe, and chemical data. We conclude Archean variolites are formed by immiscible splitting of a magma of tholeiitic composition.

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