Der petrogenetische Werdegang der Klinopyroxene in den tertiären Vulkaniten der Hocheifel II. Die Klinopyroxene der Basanitoide

Abstract

The course of crystallization of basalt forming clinopyroxenes in the olivine basalt-hawaiite-mugearite-trachyte series from the Hocheifel area (Western Germany) has been presented by the author in, an earlier paper (Huckenholz 1964, part I). The present paper deals with the evolution of clinopyroxenes from the basanitoid rocks in the same area. The basanitoids are richer in clinopyroxenes than the normal olivine basalts and may be a product of accumulation of clinopyroxenes in an olivine basalt magma.

Mineral separation is very troublesome in fine-grained volcanic rocks but special procedures (as outlined in part I) allow chemical, optical and X-ray analyses of three generations of clinopyroxenes and of other basalt forming minerals. The first clinopyroxenes formed in the basanitoids is a greenish chromian salite Ca₄₅Mg₄₃Fe₁₂ which is preserved in the cores of the phenocrysts. Strongly zoned brown titansalite Ca₄₅Mg₄₁Fe₁₄ surrounds the chromian salite or appears in independent microphenocrysts. The groundmass clinopyroxene is a sodian titansalite Ca₄₆Mg₃₇Fe₁₇ with a small optic axial angle. In contrast to the olivine basalt-trachyte series the clinopyroxenes of the basanitoids are richer in titanium and aluminum.

Phenocrysts of nickel-rich olivine Fe_14–16, relics of orthopyroxene Fs₁₄ and chrome spinell were formed together with the chromian clinopyroxene. The same minerals have been observed in the olivine basalts and hawaiites. 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 reactions occurred with the alkali basalt melt. The high pressure clinopyroxenes of the basanitoids were changed to a larger extent than the clinopyroxenes of the olivine basalts and hawaiites because they are in contact with the 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 elinopyroxenes are substituted by Ca-Tschermak's and titanaugite molecule.