Archean variolites—quenched immiscible liquids

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.