Phase equilibria in quartzitic garnet-kyanite-chloritoid micaschist from the Eclogite Zone, Tauern Window, Eastern Alps

Abstract

In a quartzitic garnet-kyanite-chloritoid micaschist garnet is present as compact, partly euhedral grains and as web-like types with numerous inclusions of quartz. Compact types overgrown by web garnets or alternating layers of both types are also common. Continuous zoning is observed with increase of pyrope and decrease of grossular and spessartine component towards garnet rim. A first stage of garnet growth together with chloritoid is recognized, followed by a garnet rim stage including kyanite. Calculated P-T pseudosections in the KNCFMMnASTH system suggest a prograde path with garnet growth starting in the assemblage chloritoid-chlorite-lawsonite-glaucophane (muscovite-quartz-rutile-H2O excess phases). Later breakdown of chloritoid +chlorite is followed by the reaction chloritoid+quartz = garnet+kyanite+H2O. The calculations suggest that glaucophane was present along a major portion of the prograde P-T path. Conditions of ~20.5 kbar, 540 °C are suggested for growth of garnet core and ~19–21 kbar, 580–590 °C for garnet rim coexisting with the assemblage garnet-kyanite-chloritoid-paragonite. During exhumation, cooling and influx of H2O took place. As a result high-XMg chloritoid was formed in the matrix and lower-XMg chloritoid in garnet inclusions. In a later event at lower P-T conditions, minor amounts of low-Si phengite, biotite, chlorite and clinozoisite replaced high-Si phengite, kyanite, garnet and chloritoid.