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

The Lower Eocene lavas of northern Skye are preserved over anarea of approximately 1500 km² in a shallow faulted oval basin.Seventy-four new major element chemical analyses have been madeof specimens showing minimal post-consolidation alteration.These demonstrate that the early volcanics vary from hypersthene-tonepheline-normative basalts; the former containing less TiO₂and P₂O₅ but more K₂O than the latter. The compositions of thesebasalts straddle the low-pressure thermal divide near the criticalplane of silica under-saturation, the normative ‘join’OI–Pl–Cpx; implying that their variation was causedby high-pressure, upper mantle processes. The unusual incompatibleelement 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 wassplit by the join olivine-plagioclase-augite, producing twodivergent trends; from alkali basalts, via relatively Si- andK-poor and Fe- and Ti-rich hawaiites and mugearites to benmoreite,and from hypersthene-normative basalts, via relatively Si- andK-rich and Fe- and Ti-poor intermediates to trachyte. One-atmospheremelting experiments on 21 lavas are used to supplement the chemicalinvestigation of these low-pressure trends. Two flows of aphyric, low-alkali tholeiitic basalt, with compositionsquite distinct from all other Skye lavas, have been discoveredinterleaved among the alkali-rich rocks near the top of thevolcanic pile, at present exposure level. The almost patternlesschemical variation of all the lavas with time suggests thatmagma chambers, as conventionally envisaged, did not exist beneaththe Skye volcanic field, but rather that fissure eruptions werefed from a sponge-like plexus of conduits and small reservoirs,within which magma, affected to varying extents by upper mantleprocesses, remained for varying periods, pursuing low-pressurefractionation trends. The chemistry of the main Skye basaltsand the low-alkali tholeiites, plus available data on otherpost-lava Skye basic igneous rocks, such as the Cuillin LayeredIntrusion, other gabbros in the central complexes, and late-stagealkali dolerite dykes, are all combined in an attempt to ascribethe variation of the basic magmas which approached or reachedthe surface of Skye to the growth, culmination, and waning stagesof a thermal event in the upper mantle beneath that area.     

Low Alkali, High Calcium Olivine Tholeiite Lavas from the Isle of Skye, Scotland

The recognition in Skye of olivine tholeiite lava flows, withlow alkali (1?7–1?9 per cent Na₂O,0?04–0?14 percent K₂O) and high calcium (12?7 per cent CaO), is reported.An account of their mineralogy and petrology, seven major elementanalyses, some trace element data, and the results of one atmospheremelting experiments are presented. These lavas, quite distinctfrom the plateau lavas in Skye, are postulated as representingthe early stages of a central cone volcano in S.W. Skye andthought to be closely related to the parental magmas of theCuillins layered basic intrusion complex.     

Low Alkali, High Calcium Olivine Tholeiite Lavas from the Isle of Skye, Scotland

The recognition in Skye of olivine tholeiite lava flows, withlow alkali (1·7–1·9 per cent Na₂O, 0·04–0·14per cent K₂O) and high calcium (12·7 per cent CaO), isreported. An account of their mineralogy and petrology, sevenmajor element analyses, some trace element data, and the resultsof one atmosphere melting experiments are presented. These lavas,quite distinct from the plateau lavas in Skye, are postulatedas representing the early stages of a central cone volcano inS.W. Skye and thought to be closely related to the parentalmagmas of the Cuillins layered basic intrusion complex. ^* Present address: Department of Earth & Space Sciences, S.U.N.Y. at Stony Brook, N.Y. 11794, U.S.A.