Mechanics of Flow Differentiation in Ultramafic and Mafic Sills

Abstract

Experimental model studies support the view that the mechanical process of magmatic flow differentiation may be a quantitatively important factor in causing a fractionation of rock types, mineralogical zoning, and chemical differentiation in nature. Scale models were constructed using homogeneous mixtures of solids in suspension and partly differentiated solids in solid-fluid mixtures in order to study the process of magmatic flow differentiation in ultramafic and mafic sills with thicknesses of 5-2,000 feet. Structures geometrically similar to some of the sills of north Skye and Soay, Scotland; the Leopard rock sills of Laborador trough; and the Palisades, New Jersey, were produced in quasi-scale models. Study of these models yields a more satisfactory explanation for many of the observed features than do other hypotheses. Conditions of magma flow in the presence of a discontinuity or magma floor and formation of sill by the intrusion of fluid between two parallel surfaces were simulated. In models, agglomeration of crystals and shear rotation appeared to produce structures geometrically similar to olivine nodules and mottled plagioclase. From experimentation, the volume concentration of intratelluric crystals in the crystal-melt mush during flow and the velocity-pressure relationship of the flowing melt have been found to be critical in determining the nature of differentiation producing chemical and crystal fractionation in nature. The wall effect and fluid slippage along the walls due to the continuity of flow imposed by the presence of a discontinuity or magma floor are considered significant in producing crystal-free or non-porphyritic margins and inward migration of intratelluric crystals to form central plug of phenocrysts.