Mineral textures of olivine minette and their significance for crystallization history of parental magma; an example from the Moldanubian Zone (the Bohemian Massif)

Abstract

One of the best-preserved dykes of olivine minette among the lamprophyre dyke swarm in the Moldanubian Zone of the Bohemian Massif (Czech Republic) was investigated. The minette, exposed at Horní Kožlí Village (near Prachatice town), has porphyric texture with phenocrysts of olivine, clinopyroxene, orthopyroxene and biotite in a fine-grained matrix consisting of K-feldspar, biotite, clinopyroxene and minor plagioclase and quartz. Accessory minerals are apatite, Cr-rich spinel and iron sulphides. Olivine is mostly replaced by talc and rimmed by two zones (coronas) – a talc-rich inner zone and a biotite-rich outer zone. Rarely, larger grains of quartz with a corona of clinopyroxene are present. The clinopyroxene grows mostly perpendicular to the quartz rim and radially penetrates the quartz crystal. Three stages of mineral crystallization were distinguished. The first stage with apatite, olivine, biotite, spinel, orthopyroxene and part of the clinopyroxene occurred in the mantle position. During the second stage, felsic phases (K-feldspar, plagioclase, quartz) in the matrix were crystallized. The enrichment of the residual melt by silica and Na occurred as the result of both fractionation and contamination during magma ascent through the granulite facies crust during post-collision orogeny in the Bohemian Massif. Minerals related to the third stage were formed during filling of the vesicles (quartz with reaction rims of clinopyroxene) and subsequent alteration (talc after olivine). The origin of quartz with clinopyroxene reaction rims (‘quartz ocelli’) is explained by filling of cavities formed by the escape of volatiles.