THE ORIGIN OF BASIC-ULTRABASIC SILLS WITH S-, D-, AND I-SHAPED COMPOSITIONAL PROFILES BY IN SITU CRYSTALLIZATION OF A SINGLE INPUT OF PHENOCRYST-POOR PARENTAL MAGMA

Abstract

An attempt is made to develop an in situ crystallization model based on the concept of Soret fractionation to explain the origin of commonly observed S-, D-, and I-shaped compositional profiles in sills formed from a single pulse of phenocryst-poor parental magma. The model envisages that the various composi- tional profiles observed in sills can be interpreted in terms of different combinations of four principal units—Basal Zone and Layered Series forming the floor sequence, and Top Zone and Upper Border Series constituting the roof sequence. The Basal and Top Zones represent mirror images of the Layered and Upper Border Series, respectively, and therefore are referred to as basal and top reversals. It is proposed that the formation of basal and top reversals takes place through the non-equilibrium Soret differentiation of liquid boundary layers which form as a consequence of the temperature gradient imposed by the cold country rock. In contrast, the Layered and Upper Border Series originate predominantly through the crystal-liquid boundary layers developing in equilibrium conditions. The model permits the production of S-, D-, and I-shaped compositional profiles from the same magma composition. All that is necessary to produce a specific shape of compositional profile is an appro- priate temperature gradient imposed by the cold country rock on the liquid boundary layers of a parental magma of a given composition.