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

This paper deals with inclusions, megacrysts, and nodules froma group of Stephanian and Permian vents and associated intrusiveson the Fife (Scotland) coast near Elie. The petrography andchemistry of inclusions of spinel Iherzolite, wehrlite, andclinopyroxenite are described. The Elie Ness vent contains coarse-grainedplutonic nodules (Elie type nodules) and megacrysts of pyrope,sub-calcic augite, kaersutite, and anorthoclase. Elie type nodulesare 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 pyropephenocrysts could have coprecipitated from an alkali basaltmagma at P > 25 kb, T = 1300–1450 ?C. It is proposedthat the primary alkali basalt liquid was formed by partialmelting of a vapour-free, mica-bearing garnet Iherzolite mantleat a depth of c. 100 km, with subsequent pyrope-augite phenocrystcrystallization at not less than 70 km depth. Geochemical studiesof clinopyroxenes from the Elie type nodules indicate crystallizationwithin the lower crust. It is proposed that types 1 and 2 nodulesare cumulates from the alkaline basaltic liquid, intercumuluskaersutite representing compositions of liquids intermediateon the Fife basalt trend. Type 3 nodules may represent basalticliquids at the basic end of the Fife trend, wholly crystallizedat pressure from 10–15 kb. Experimental data on stabilityof anorthoclase in its host basanite show it to be present inthe basanite melting interval only at P < 9 kb (dry). Itis thought that crystallization of anorthoclase may be associatedwith formation of types 4 and 5 nodules, possibly from a trappedpocket of evolved alkaline liquid at upper crustal levels. TheElie Ness eruption must have been rapid enough to strip theaccumulated pyroxenites from the lower crust and carry unresorbedgarnet megacrysts from depths of over 70 km. Spinel-lherzoliteinclusions are found only in late stage basic sheets whose intrusionmay be unrelated to the initial violent tuff eruptions.