Major Element Chemical Variation in the Eocene Lavas of the Isle of Skye, Scotland

Abstract

The Lower Eocene lavas of northern Skye are preserved over an area of approximately 1500 km2 in a shallow faulted oval basin. Seventy-four new major element chemical analyses have been made of specimens showing minimal post-consolidation alteration. These demonstrate that the early volcanics vary from hypersthene-to nepheline-normative basalts; the former containing less TiO2 and P2O5 but more K2O than the latter. The compositions of these basalts straddle the low-pressure thermal divide near the critical plane of silica under-saturation, the normative ‘join’ OI–Pl–Cpx; implying that their variation was caused by high-pressure, upper mantle processes. The unusual incompatible element pattern of the lavas suggests that a K-rich mantle phase, tentatively taken to be phlogopite, was involved in their genesis. At subvolcanic pressures the spectrum of basaltic magmas was split by the join olivine-plagioclase-augite, producing two divergent trends; from alkali basalts, via relatively Si- and K-poor and Fe- and Ti-rich hawaiites and mugearites to benmoreite, and from hypersthene-normative basalts, via relatively Si- and K-rich and Fe- and Ti-poor intermediates to trachyte. One-atmosphere melting experiments on 21 lavas are used to supplement the chemical investigation of these low-pressure trends. Two flows of aphyric, low-alkali tholeiitic basalt, with compositions quite distinct from all other Skye lavas, have been discovered interleaved among the alkali-rich rocks near the top of the volcanic pile, at present exposure level. The almost patternless chemical variation of all the lavas with time suggests that magma chambers, as conventionally envisaged, did not exist beneath the Skye volcanic field, but rather that fissure eruptions were fed from a sponge-like plexus of conduits and small reservoirs, within which magma, affected to varying extents by upper mantle processes, remained for varying periods, pursuing low-pressure fractionation trends. The chemistry of the main Skye basalts and the low-alkali tholeiites, plus available data on other post-lava Skye basic igneous rocks, such as the Cuillin Layered Intrusion, other gabbros in the central complexes, and late-stage alkali dolerite dykes, are all combined in an attempt to ascribe the variation of the basic magmas which approached or reached the surface of Skye to the growth, culmination, and waning stages of a thermal event in the upper mantle beneath that area.