Inclusions and Megacrysts from Undersaturated Tuffs and Basanites, East Fife, Scotland

Abstract

This paper deals with inclusions, megacrysts, and nodules from a group of Stephanian and Permian vents and associated intrusives on the Fife (Scotland) coast near Elie. The petrography and chemistry of inclusions of spinel Iherzolite, wehrlite, and clinopyroxenite are described. The Elie Ness vent contains coarse-grained plutonic nodules (Elie type nodules) and megacrysts of pyrope, sub-calcic augite, kaersutite, and anorthoclase. Elie type nodules are divisible into five groups: (1) kaersutite-olivine-pyroxenite, (2) type 1 +oligoclase, (3) biotite-pyroxenite, (4) sodic amphibole-biotite-albite, (5) biotite-albite. Experimental studies show that sub-calcic augite and pyrope phenocrysts could have coprecipitated from an alkali basalt magma at P > 25 kb, T = 1300–1450 °C. It is proposed that the primary alkali basalt liquid was formed by partial melting of a vapour-free, mica-bearing garnet Iherzolite mantle at a depth of c. 100 km, with subsequent pyrope-augite phenocryst crystallization at not less than 70 km depth. Geochemical studies of clinopyroxenes from the Elie type nodules indicate crystallization within the lower crust. It is proposed that types 1 and 2 nodules are cumulates from the alkaline basaltic liquid, intercumulus kaersutite representing compositions of liquids intermediate on the Fife basalt trend. Type 3 nodules may represent basaltic liquids at the basic end of the Fife trend, wholly crystallized at pressure from 10–15 kb. Experimental data on stability of anorthoclase in its host basanite show it to be present in the basanite melting interval only at P < 9 kb (dry). It is thought that crystallization of anorthoclase may be associated with formation of types 4 and 5 nodules, possibly from a trapped pocket of evolved alkaline liquid at upper crustal levels. The Elie Ness eruption must have been rapid enough to strip the accumulated pyroxenites from the lower crust and carry unresorbed garnet megacrysts from depths of over 70 km. Spinel-lherzolite inclusions are found only in late stage basic sheets whose intrusion may be unrelated to the initial violent tuff eruptions.