Primary Banding in Basic Plutonic Rocks

Abstract

The nature and suggested modes of origin of the mineralogically unlike bands so frequently observed in bodies of basic plutonic rock are reviewed. A mechanism for the production of some of these bands is suggested, involving simultaneous crystallization and settling of two or more minerals in a magma lighter than any of them. By a process of rhythmic differential settling, rhythmically alternating bands of differing composition may be produced. The same process may account for anorthosite bodies without necessitating the assumption of the crystallization of plagioclase from a liquid heavier than itself, provided only that the plagioclase be lighter than the mean specific gravity of liquid and heavy minerals together. Experimental evidence is adduced in support of the suggested mechanism.