Chemical zoning in the large phenocrysts of the Godóvár tuff,Börzsöny Mts., Hungary

Large phenocrysts of clinopyroxene, amphibole in the Godóvár tuff show normal zoning with mg-values decreasing from the center towards the border zones. Clinopyroxenes exhibit a primitive Cr-diopsidic (Mg₄₈Fe₇Ca₄₅) core which with a sudden change in the composition is surrounded by an augitic rim (Mg₄₄Fe₁₄Ca₄₂). No such abrupt change can be observed in amphibole crystals which are typical pargasites and only the outermost zones may pass into the magnesio-hastingsite field. It is argued that the Cr-diopside may have separated from a basaltic liquid at depths greater than about 18 km, while the other pyroxene zones with the pargasite and plagioclase crystals accumulated in a storage chamber at a depth of about 12–15 km.