Abstract

Most of the ring complexes in the British Tertiary Igneous Province include at least one net-veined or mixed-magma ring-dyke. Net-veined ring-dykes consist of a granophyre host rock containing mafic pillows, with areas of granophyric net-veined mafic rock. Mixed-magma ring-dykes contain mafic micropillows dispersed in a silicic host rock. The ‘mafic’ component covers a wide compositional range with basaltic-andesite to andesite abundant. These magmas represent both fractionates of basaltic magma and hybrids of silicic, intermediate and mafic magma. Analogous compositional diversity in extrusive rocks of Hekla and Askja, Iceland, indicate that the Hebridean composite ring-dykes originate from compositionally zoned magma chambers. Density relationships suggest that zoning in chambers of tholeiitic systems can only result for magmas more evolved than ferrobasalt. Zonation can result from crystallization on the sidewalls and roof of the chamber (producing a boundary layer flow of less dense fluid), wall melting and magma replenishment. Hybrid mafic pillows originate by mixing within the chamber, especially when lower-density olivine tholeiite is emplaced beneath ferrobasalt magma. Subsidence of a crustal block into an underlying compositionally and thermally zoned chamber results in commingling of silicic, intermediate and mafic magmas within both the reservoir and the ring-dyke, producing pillows and net-veining. Mixing is incomplete due to the contrasted viscosities and solidus temperatures of the magmas. Mixing and net-veining is also enhanced by volatile exsolution and explosive emplacement of magma at sub-volcanic levels, with mixed magma eruptions sometimes occurring at the surface.

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