Regional Variation in the Composition of British Caledonian Granites

Abstract

Trend surface analysis has been used to measure the regional variation in composition of the Caledonian granites of the British Isles. The variations in <tex-math>$Na_{2}O/K_{2}O$</tex-math>, and in normative albite, orthoclase, and quartz, are illustrated by contour maps. The trend surfaces for each of these constituents show a marked parallelism to Caledonian structures in the region. The surfaces for <tex-math>$Na_{2}O/K_{2}O$</tex-math>, albite, and orthoclase show some indication of bilateral symmetry, such that a belt of high normative albite-low normative orthoclase compositions corresponds broadly to the central highly metamorphosed zone of the orogenic belt. Comparison of the magma compositions with relevant experimentally investigated systems suggests that the Caledonian granitic magmas originated under water pressures that were greatest in the central part of the orogenic belt.