Metamorphic Reactions in Some Scottish Basic Intrusions

Abstract

Three main mineralogical alterations were studied in two syn-orogenic basic igneous suites, the 'Younger' basic complex and the Glen Scaddle intrusion. They were examined using optical microscopy, electron probe microanalysis, scanning and transmission electron microscopy and x-ray diffraction. Olivine and plagioclase reacted to form coronas comprising a layer sequence olivine-orthopyroxene - amphibole - (amphibole + spinel symplectite) - plagioclase, separated by a sharp boundary. In some specimens, discontinuous layer of amphibole + anorthite symplectite is developed. Quantitative model reactions are written based on the evidence of restricted movements of Al and Si in plagioclase when made over to symplectites. This is shown by the inheritance of Al/Si ratios of plagioclase in both symplectites. Fe, Mg and O released from olivine ———»orthopyroxene transformation moved into the amphibole + spinel symplectite, causing the boundary to move towards olivine. Igneous clinopyroxene changed to metamorphic compositions of higher Ca while reacting to actinolite by exchanging Ca, Mg and Fe with the environment. At the initial stage, when metamorphic pyroxene produced predominated over actinolite, the reaction consumed Ca and released Fe, Mg. Ca was released when more actinolite was produced. Ca may partly come from the simultaneous reaction actinolite + plagioclase ___, hornblende + quartz. Orthopyroxene is pseudomorphed by cummingtonite which simultaneously reacts with plagioclase forming hornblende rims. Possible model reactions for each system are proposed. They suggest variable degrees of open - system behaviour. Although Fe and Mg partition approaches metastable equilibrium between actinolite and hornblende in the Glen Scaddle area, in which hornblende closely approaches equilibrium with plagioclase, the abruptness of the actinolite core/hornblende rim transition is interpreted as due to sluggishness of Al diffusion. Thus this boundary may be inherited from the original plagioclase/pyroxene interface and is not an equilibrium feature. Primary hornblende of high Ti content is equilibrated at lower temperature by exsolving oriented ilmenite and rutile in approximately a closed system. The orientations of ilmenite are controlled by semi-coherent relations of oxygen atom packing to the hornblende.