The petrology of the metamorphosed syenite in Glen Dessarry, Inverness-shire

Abstract

The Glen Dessarry syenite is composed of two types of syenite which were intruded before or during a phase of almandine–amphibolite facies regional metamorphism. The chemistry. of the syenites, taken together with their field relations and mineralogy, suggests that they are derived from a parent magma crystallizing principally aegerine-augite, biotite and magnetite. The syenites themselves crystallized as alkali-feldspar, pyroxene, biotite, magnetite, spheric, apatite (calcite, allanite, zircon) rocks. During metamorphism and deformation, the feldspar, by this time mesoperthite, unmixed into granular orthoclase and sodic plagioclase. A fluid phase entering the igneous rocks gave rise to disequilibrium pyroxene-hornblende bearing assemblages and equilibrium pyroxene-free assemblages (the criterion of equilibrium being regularity of distribution of Mn and Mg between co-existing phases); the same fluid effected an over-all reduction of the original rocks. On the assumption that the fluid contained only water and hydrogen, its composition is calculated to be at least as rich in hydrogen as 20mol per cent. Probably during a later metamorphic episode, small amounts of epidote and quartz formed in the syenites at the margins of the complex.