Datable remnants of uraninite microcrystals of Proterozoic age in Variscan high-grade metamorphic graphite schists from the Southern Bohemian Massif, Austria

Abstract

Graphite-bearing metamorphic rocks in the Southern Bohemian Massif regularly contain tiny (1–10 µm) uraninite crystals. Spatially highly resolved, electron-beam-based, U-Th-total Pb dating reveals an unexpectedly great geochronological potential for these micrometre-sized uraninite crystals. Most of them gave relatively precise geochronological information for the Variscan high-grade metamorphism in this area (around 340 Ma), but older remnant uraninite domains were also recognized and dated at an Ediacaran and an even older Tonian age. The most spectacular result of our study is the discovery of a 5 µm uraninite grain with a well-defined threefold age zoning: Ediacaran and Tonian inner domains are overgrown by a Variscan uraninite shell and survived Variscan high-grade metamorphic conditions transitional to granulite facies without suffering substantial lead loss. High radiogenic Pb contents of around 10 wt% in the core of this grain are stabilized in the form of PbS nanodomains.