Composite lava flows of Ordovician age in the English Lake District

Abstract

Three composite andesitic lava flows, within the Ordovician Borrowdale Volcanic Group, are described. They are 35-250 m thick and in their simplest form consist of a massive, flow-jointed andesite lower part and a flow-laminated to blocky basaltic andesite upper part with a zone of interlayering between them. One flow has a very thin, discontinuous, basal unit of xenolithic, flow-laminated basaltic andesite. The interlayered zone comprises layers and lenses of andesite, basaltic andesite and mixed rocks consisting of closely packed inclusions of pale coloured andesite in a chlorite-rich matrix. The rocks have undergone several stages of alteration: (a) deuteric alteration, including silicification associated with autobrecciation of the upper component and inter-magma reactions in the interlayered zone; (b) soda-metasomatism, preferentially of basaltic andesite; (c) metamorphism, attributed to the Lake District granite batholith, giving an actinolite-sericite assemblage; (d) regional metamorphism to give a chlorite-epidote-actinolite-sphene assemblage with rare prehnite and pumpellyite; (e) calcite veining. These alterations have modified the bulk chemistry of the rocks, but the concentrations of Al, P, Ti, V, Cr, Co, Ni, Ga, Y, Zr, Nb, La, Ce and Th are largely unaffected on the scale of a 2 kg sample. Using these elements the composition of the lavas compares with high-K orogenic andesites emplaced through continental crust. Each of the flows shows distinct chemical characteristics which may be explained by variations of phenocryst content or the assimilation of cognate xenoliths. Variations within flows can be explained by magma mixing and fractional crystallization of plagioclase, minor pyroxene, ilmenite and zircon. A mechanism for eruption is proposed in which a volatile-rich basic magma intruded de-gassed cooler, less dense andesite magma in a high level sub-volcanic chamber. The basic magma formed the lower layer in the chamber. On cooling and crystallization of plagioclase near the interface the volatile content of the residual basic liquid rose and the density decreased allowing the liquids to form an emulsion-like layer at the interface. This separated and rose into the andesite magma and the process was repeated to form an interlayered zone at the junction. Before mixing was complete the magma was erupted as a series of composite flows.