Mineralogy and petrogenesis of a Ba–Ti–Zr-rich peralkaline dyke from Šebkovice (Czech Republic): Recognition of the most lamproitic Variscan intrusion (original) (raw)

Abstract

A peralkaline, ultrapotassic dyke found at Šebkovice (Třebíč district, western Moravia) is a mineralogically extreme member of a dyke swarm occurring along the southeastern border of the Moldanubian Region of the Bohemian Massif. The dyke shows a simple zoning, with a very fine-grained marginal zone grading into a medium-grained central zone. It has a primary mineral assemblage of microcline and potassic amphiboles, with accessory apatite and altered phlogopite. The microcline exhibits an unusual red luminescence colour and pronounced substitution of Fe 3+ for Al, with measured contents of Fe 2 O 3 up to 8.5 wt.% (0.31 apfu Fe 3+). Amphiboles have very high K (up to 0.99 apfu) and Si contents; their compositions follow an alkaline fractionation trend from potassic-richterite to potassic-magnesio-arfvedsonite, characterized by an increase of Na/K and a decrease of Ca, Mg, Fe 2+ and Ti via heterovalent substitutions [B] Ca + [C] (Mg,Fe 2+) → [B] Na + [C] Fe 3+ and Ti +Mg → 2Fe 3+. The most evolved apatite is significantly enriched in SrO (up to 9.7 wt.%; 0.49 apfu Sr). The core of the dyke and late veinlets contain unique late-to post-magmatic Ba-Ti-Zr-bearing mineral assemblages of baotite, henrymeyerite, titanite, rutile, benitoite and bazirite. Anhedral baotite fills interstices distributed inhomogeneously in the dyke centre; it is locally replaced by a Ba-bearing titanite + henrymeyerite + rutile + quartz assemblage. Henrymeyerite (the second record in a lamproite) shows variable Fe/Ti ratios and represents a solid solution of the hepta-and hexatitanate components. Euhedral crystals of benitoite and bazirite are enclosed in the late-stage quartz-titanite-apatite veinlets in the finegrained margin of the intrusion. In terms of a mineralogical-genetic classification, the Šebkovice dyke can be considered as a new high-silica (~57 wt.% SiO 2) variety of lamproite (variety Šebkovice), and represents a unique expression of post-collisional potassic magmatism on the southeastern border of the Bohemian Massif. The peralkaline dykes from this area show mineralogical and geochemical features similar to those of silicarich orogenic lamproites emplaced at destructive plate margins. In terms of the modern classification of lamproites, the Šebkovice dyke is the first lamproite recognised in the Variscan orogenic belt.

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