Petrology and geochemistry of the Loch Ba ring-dyke, Mull (N.W. Scotland): an example of the extreme differentiation of tholeiitic magmas

Abstract

The Loch Ba ring-dyke in the Tertiary igneous central complex of Mull, N.W. Scotland is composed predominantly of a banded rhyolitic welded tuff. The rhyolite contains numerous inclusions of dark aphanitic rock. The textural relationships between the different rocks indicate rapid, violent and intimate mixing during emplacement of the dyke. The dark glassy component varies continuously from basaltic andesite to andesite, dacite and rhyolite. These glasses are enriched in FeO and depleted in MgO at a given SiO₂ content in comparison to other tholeiitic highly differentiated volcanic rocks. The rhyolite contains an average of 4% phenocrysts and is associated with the mineral assemblage plagioclase (An₃₂ to An₂₁)-sanidine(Or_50–60)-hedenbergite-fayalite-magnetite-ilmenite-apatite-zircon. Mineral aggregates involving either plagioclase-hedenbergite-ilmenite or plagioclase-fayalite-magnetite are common, but aggregates containing fayalite and hedenbergite together are scarce. The dark glassy components are either phenocryst free or contain less than 0.2% phenocrysts. The main phenocrysts associated with the dark glasses are plagioclase (An₆₅-An₃₀), high calcium clinopyroxene ranging continuously from augite to pure hedenbergite, pigeonite, magnetite, ilmenite and rare apatite. Zoning in minerals is generally weak or absent. The plagioclase feldspar, high calcium clinopyroxenes and pigeonites have similar compositional ranges to the minerals observed in the Middle and Upper Zones of the Skaergaard Intrusion. The mineral compositions are systematically related to SiO₂ content and Mg number of the glasses. The data demonstrate that mineral compositions and assemblages similar to the Skaergaard form from silica-rich andesitic to rhyolitic liquids. The various mafic glasses are interpreted to have been derived from a zoned magma chamber underlying an upper layer of rhyolitic magma. Differentiation is attributed to fractional crystallization of the observed mineral assemblages causing SiO₂ enrichment and FeO depletion. However, glasses with less than 57% SiO₂ have unusual compositions with very low MgO and P₂O₅ as well as variable Al₂O₃ and TiO₂. Their peculiarities could be explained by andesitic magmas assimilating cumulate mineral aggregates precipitated from more differentiated dacite and rhyolite magmas. The bulk compositions of these cumulates have high FeO, low SiO₂ and negligible MgO and P₂O₅. It is suggested that the high density of the mineral aggregates containing fayalite-hedenbergite-magnetite and ilmenite caused them to settle through the zoned chamber to be assimilated by high temperature, less differentiated magmas.