A genetic model of pillowed dike and peperite formation interpreted from Pleistocene and Archean volcanogenic sequences

Abstract

Pillowed dikes and associated peperite and volcaniclastic sediments in modern volcanogenic settings in Iceland are used to develop a conceptual framework of the formation of peperite in the context of relative shear strength of magma and host sediment. This model is then applied to an Archean age volcanic system in the Vermilion District of Minnesota, USA, where well-developed peperite is preserved in association with a pillowed dike intruding saturated volcaniclastic sediment. Pillowed dike formation requires a high water/rock ratio and interaction of the magma and wet sediment that can be described as a fuel-coolant system, where magma serves as fuel and the wet sediment or vapor as coolant. Peperite is then formed essentially in-situ by disintegration of magma intruding and mingling with unconsolidated or poorly consolidated, wet sediments. The breakup of parent magma into peperite can happen explosively by steam explosions in the sediment or non-explosively by quench fragmentation. The degree of mixing of the igneous and sedimentary components then depends on the relative viscosity or shear strength of the magma and wet sediment. We define this ratio as the system fluidity parameter(S_f ), where S_f= τ_m/τ_s, the ratio of the shear strength of the magma (τ_m) and sediment (τ_s), respectively. Sf essentially varies from 0 (low system fluidity; case 1) to positive values, up to or even greater than one (high system fluidity; case 2). The conceptual model is bounded by two limiting cases. First, if host sediment has a large shear strength relative to the magma, Sf is small and mixing will be kept to a minimum. In an explosive setting, mixing and peperite formation will occur near the pillowed dike, but large-scale mixing and sediment deformation will be minimal. In non-explosive systems, it is likely that peperite will be confined to a narrow zone near dikes. In either case, dikes appear as well defined tabular or pipe-like bodies with discrete boundaries. In contrast, in cases where sediment strength is similar to magma rheology, Sf approaches unity and magma emplacement will cause extensive deformation of host sediment. Whether explosive or non-explosive, deformation and mixing will occur throughout the magma/sediment system. Magma quenched at conduit margins will be incorporated into host sediment and can be mixed well beyond conduits. Dike margins will be diffuse, and migration and branching of conduits will lead to complex geometries. Pillowed dike and the associated peperite system in the Archean Vermilion district exhibit similarities to Pleistocene peperite deposits in Iceland by both having diffuse, pipe-like magmatic bodies with peperitic margins. Magma/sediment deformation and mixing is extensive, exhibited by deformed magmatic pillows over 10 m away from dike margins. In contrast to Iceland, Archean deposits of northern Minnesota lack preservation of deformed sediment likely due to weathering and deformation.