The mineral chemistry and petrology of Tertiary pitchstones from Scotland

Sub-silicic to silicic pitchstones are widespread throughout the British Te rtiary Igneous Province (BTIP), with examples being found at all the major igneous centres. Both highly porphyritic and almost completely aphyric vari eties occur, and take the form of sills, dykes and lava flows. Here we pres ent previously unreported mineral chemistry data on phenocryst and microcry stallite populations from a number of pitchstones from throughout the BTIP. Phenocryst assemblages are completely anhydrous, comprising mixtures of pl agioclase, sanidine, fayalite, orthopyroxene, pigeonite, ferroaugite, ferro hedenbergite and quartz. Microcrystallite assemblages are also diverse, con sisting of sanidine, ferrohedenbergite, fayalite and, occasionally, almost pure end-member ferrosilite, as well as hydrous phases such as ferrohornble nde and biotite. Textural and mineral chemistry observations support interp retations derived from whole-rock and residual glass major element analyses , together with whole-rock trace element and the available Sr-Nd-Pb isotope data, that the Tertiary pitchstones of Scotland are either the products of intimate mixing between a range of basaltic magmas with hydrous crustal me lts, or were formed by the crustal contamination of basaltic magmas.