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

摘要

A peralkaline, ultrapotassic dyke found at Sebkovice (Treble district, western Moravia) is a mineralogically extreme member of a dyke swarm occurring along the south-eastern 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 Fe2O3 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 Sebkovice dyke can be considered as a new high-silica (～57 wt.% SiO2) variety of lamproite (variety Sebkovice), and represents a unique expression of post-collisional potassic magmatism on the south-eastern border of the Bohemian Massif. The peralkaline dykes from this area show mineralogical and geochemical features similar to those of silica-rich orogenic lamproites emplaced at destructive plate margins. In terms of the modern classification of lamproites, the Sebkovice dyke is the first lamproite recognised in the Variscan orogenic belt.