Age and formation conditions of anatase from the Bavarian Pfahl giant quartz vein, Germany

The ~150 km long Bavarian Pfahl is a major inactive shear zone cutting through Late Palaeozoic basement of the Moldanubian zone of the Variscan orogen in central Europe. Due to the absence of conventional mineral geochronometers, it long presented a challenge for radiometric dating. Here, we present high spatial resolution U-Pb analyses of rare anatase crystals by large-geometry secondary ionization mass spectrometry yielding a 209 ± 28 Ma (2σ uncertainty) concordia intercept age. This age is more precise than previous Rb-Sr age determinations on sericite- and fluid inclusion-bearing quartz. The youngest Variscan granite intrusions around the Pfahl zone are thus c. 100 Ma older than the Bavarian Pfahl quartz lode, ruling out magmatic fluid sources. Common 207Pb/206Pb = 0.8619 ± 0.0039 derived from linear regression of anatase analyses overlaps with values for galena mineralization within and adjacent to the Pfahl zone, pointing towards hydrothermal leaching of common basement sources. Oxygen isotopic equilibrium thermometry for quartz and TiO2 phases suggests anatase formation at 162 ± 4 °C, overlapping previously estimated quartz precipitation temperatures, albeit at their lower range. A Late Triassic extensional geodynamic environment is inferred for the emplacement of the Bavarian Pfahl, which significantly postdates late-orogenic deformation. With the caveat that only few radiometric age determinations are available, the new age for the Bavarian Pfahl falls in between reported ages for other giant quartz veins in the Variscan basement of Germany.