The Zoned Clinopyroxenes of the Shiant Isles Sill, Scotland

Abstract

The zoned clinopyroxenes in samples from a near-vertical section through the main Shiant Isles sill have been studied using the electron microprobe. In terms of Ca: Mg: Fe the over-all fractionation trend is much greater than had been believed from bulk pyroxene analyses and is of a distinctive calcic augite-hedenbergite type. In this respect it differs from both the augite-ferroaugite and salite-aegirine trends generally regarded as typical of tholeiitic and alkaline basic magmas respectively, and it is proposed that this trend represents a third major type typical of mildly alkaline basic magmas. The Al, Ti, and Na contents of the pyroxenes are dependent on the degree of fractionation as represented by Mg/(Mg+Fe) and it is suggested that the Al and Ti contents are controlled by the Ti content of the liquid and the crystallization of titaniferous magnetite whereas the entry of Na into the pyroxene as aegirine is dependent on the activity of Fe3+ in the liquid. To provide a basis for discussion of the crystallization of the zoned clinopyroxenes the existing petrogenetic hypotheses for the Shiant Isles sill are reviewed and a new model proposed for its emplacement and differentiation. It is concluded that the differences between the pyroxenes of the mildly alkaline Shiant Isles sill and those of more strongly alkaline basic magmas result from fundamental chemical differences in the magma. The lower Ca contents of the early pyroxenes are due to the slightly higher Si activity of the magma and the subsequent trend towards hedenbergite rather than aegirine is due to crystallization under conditions of relatively lower oxygen fugacity, probably as a result of lower initial H2O content of the magma.