Large-scale rhyolite peperites (Jurassic, southern Chile)

Development of a Jurassic volcano-tectonic rift basin in the southern Andes created a setting in which thick, rhyolitic volcaniclastic sequences accumulated in submarine environments and were penetrated by hypabyssal intrusions during or shortly after deposition. In the Ultima Esperanza District of southern Chile, extensive masses of peperite were produced when rhyolite magma underwent quenching, disruption, and commingling with wet, unconsolidated sediments during intrusion at shallow levels beneath the sea floor. The peperite forms discordant intrusive masses with volumes of up to several cubic kilometers, in which large, widely spaced, coherent rhyolite feeder pods are surrounded by, and grade into closely packed and dispersed peperite. Closely packed peperite consists of tightly fitting clasts separated by sediment-filled fractures. In dispersed peperite, the sediment forms a matrix surrounding large masses of fractured rhyolite and smaller more widely separated rhyolite clasts; evidence of in situ quench fragmentation is well preserved on both outcrop and thin-section scales. Thin sections show that clast margins and, in some cases, entire small clasts underwent cooling-contraction granulation, releasing shards of quenched rhyolite and fragments of phenocrysts into the adjacent sediment.Interaction between magma and wet sediment was non-explosive and involved fluidization of the host sediments, creating space for the intruding magma and causing pervasive injection of highly mobile sediment along thermal contraction cracks in quench-fragmented rhyolite. The ability of the magma to undergo complex intermixing with large volumes of sediment, with widespread preservation of in situ fragmentation textures, is interpreted to reflect a relatively low magma viscosity, presumably caused by retention of volatiles in the magma at the ambient pressures involved.Beds of redeposited peperite within the rift-basin fill indicate that some of the intrusive peperite masses reached the sea floor, undergoing slumping and mass flow. The peperites were thus an important local source of coarse-grained debris during the evolution of the basin.