The Zoned Clinopyroxenes of the Shiant Isles Sill, Scotland

The zoned clinopyroxenes in samples from a near-vertical sectionthrough the main Shiant Isles sill have been studied using theelectron microprobe. In terms of Ca: Mg: Fe the over-all fractionationtrend is much greater than had been believed from bulk pyroxeneanalyses and is of a distinctive calcic augite-hedenbergitetype. In this respect it differs from both the augite-ferroaugiteand salite-aegirine trends generally regarded as typical oftholeiitic and alkaline basic magmas respectively, and it isproposed that this trend represents a third major type typicalof mildly alkaline basic magmas. The Al, Ti, and Na contentsof the pyroxenes are dependent on the degree of fractionationas represented by Mg/(Mg+Fe) and it is suggested that the Aland Ti contents are controlled by the Ti content of the liquidand the crystallization of titaniferous magnetite whereas theentry of Na into the pyroxene as aegirine is dependent on theactivity of Fe³⁺ in the liquid. To provide a basis for discussion of the crystallization ofthe zoned clinopyroxenes the existing petrogenetic hypothesesfor the Shiant Isles sill are reviewed and a new model proposedfor its emplacement and differentiation. It is concluded thatthe differences between the pyroxenes of the mildly alkalineShiant Isles sill and those of more strongly alkaline basicmagmas result from fundamental chemical differences in the magma.The lower Ca contents of the early pyroxenes are due to theslightly higher Si activity of the magma and the subsequenttrend towards hedenbergite rather than aegirine is due to crystallizationunder conditions of relatively lower oxygen fugacity, probablyas a result of lower initial H₂O content of the magma.