Der petrogenetische Werdegang der Klinopyroxene in den tertiären Vulkaniten der Hocheifel

Abstract

The course of crystallization of basalt forming clinopyroxenes in the olivine-basalt-hawaiite-mugearite-trachyte-and basanitoid (= basanite)-series from the Hocheifel area (Western Germany) has been presented by the author in two earlier papers (Hucken-holz, 1965 a, 1965 b). The present paper deals with the evolution of clinopyroxenes from the ankaramites in the same area. The ankaramites are richer in olivines and clinopyroxenes than the normal olivine basalts and may be a product of accumulation of olivines and clinopyroxenes in an olivine basalt magma. The first clinopyroxene formed in the ankaramites is a greenish chromian salite Ca47.1 Mg40.9 Fe12.0 which is preserved in the cores of the phenocrysts. Strongly zoned brown titansalite Ca45 Mg42Fe13 surrounds the chromian salite (HF 5) or appears in independent microphenocrysts (HF53). The groundmass clinopyroxene is titansalite Ca45Mg37Fe18 with a small optic axial angle. Phenocrysts of nickel-rich olivine Fe12–14 and chrome spinell were formed together with chromium clinopyroxene. The same minerals together with orthopyroxenes have been observed in the olivine basalts, hawaiites and basanitoids (= basanites). This paragenesis indicates high pressure and high temperature in the alkali basalts of the Hocheifel during an initial stage of magmatic evolution. With the decrease of pressure, mineral reaction occurred with the alkali basalt melt. The high pressure clinopyroxenes of the ankaramites and basanitoids (= basanites) were changed to a larger extent than the clinopyroxenes of the olivine basalts and hawaiites or were completely destroyed (HF 53) because they are in contact with the hot magma for a longer period of time. This favours the reaction between the solids and the melt, and the high pressure garnet and/or enstatite components in the clinopyroxenes are substituted by Ca-Tschermak’s and titanaugite molecules.