The genetic significance of almandine-pyrope phenocrysts in the calc-alkaline Borrowdale Volcanic Group, Northern England

A suite of garnet-bearing andesites and dacites from the Ordovician of N. W. England is described and major- and trace-element analyses of the garnet phenocrysts are presented. The garnets are of almandine-pyrope composition, with minor amounts of spessartine and grossular, and often show marked reversed zoning; the crystal becoming progressively enriched in pyrope towards the margin. Garnets from the dacites are consistently richer in almandine and spessartine than are those from the andesites.
From a consideration of the chemistry of the garnet phenocrysts and host rocks, especially La and Y abundances, it is shown that garnet could not have been removed from the magma in quantities sufficient to affect the liquid composition. Consequently the magma must have evolved by some process other than crystal fractionation. It is proposed that the magma was generated by the partial melting of oceanic crust along an ancient Benioff zone, stored at depth (possibly at the crust/mantle interface) long enough for garnet to nucleate, and then transferred rapidly to the surface. Isobaric crystallisation of the garnet phenocrysts at depth could explain the reversed zoning observed.