The nature and occurrence of discrete zoning in plagioclase from recently erupted andesitic volcanic rocks, Montserrat

Abstract

Recently erupted rocks from Montserrat, Lesser Antilles, were found to be coarsely porphyritic including plagioclase, magnesio–hornblende, orthopyroxene, titano–magnetite, and rare quartz phenocrysts. Zonations within plagioclase show three distinct periods within the evolution of the andesitic magmas. Initial steady state oscillatory overgrowths (An 49–54) represent an early period of growth within a confined andesitic magma chamber. Interaction with an intruding more mafic hotter magma resulted in thick zones of dissolution features that cut through these early growth layers. The dissolution zones and crystal fragments are overgrown by thin calcic (An 86) plagioclase rims. A plagioclase growth rate of 10 −7cm/s suggests that the calcic plagioclase rims were formed in approximately 10 h, consistent with the idea that mineral growth may increase magma viscosity inducing conduit plugging and shallow pressurization resulting in cycles of dome inflation, eruption, and explosive activity. Thermodynamic modeling of heating and mixing of the andesitic magma with a basaltic liquid produces plagioclase of composition An 79, not the observed composition of the late plagioclase (An 86). We conclude that the final plagioclase grew under non-equilibrium conditions as a result of the mixing event.