Dating multiply overprinted Sn-mineralized granites—examples from the Erzgebirge, Germany

摘要

Granites and primary tin mineralization in the Erzgebirge were dated using (1) conventional U–Pb dating of uraninite inclusions in mica, (2) Rb–Sr dating of inclusions in quartz that represent highly evolved melts, (3) Re–Os dating of magmatic–hydrothermal molybdenite, and (4) chemical Th–U–Pb dating of uraninite. Conventional isotope dilution and thermal ion mass spectrometry and chemical Th–U–Pb dating of uraninite in granites from the Ehrenfriedersdorf mining district provide ages of 323.9 ± 3.5 Ma (2σ; Greifenstein granite) and 320.6 ± 1.9 and 319.7 ± 3.4 Ma (2σ, both Sauberg mine), in agreement with U–Pb apatite ages of 323.9 ± 2.9 and 317.3 ± 1.6 Ms (2σ, both Sauberg mine). Rb–Sr analysis of melt inclusions from Zinnwald gives highly radiogenic Sr isotopic compositions that, with an assumed initial Sr isotopic composition, permit calculation of precise ages from single inclusions. The scatter of the data indicates that some quartz-hosted melt inclusions have been affected by partial loss of fluid exsolved from the melt inclusion. Re–Os dating of two molybdenite samples from Altenberg provides ages of 323.9 ± 2.5 and 317.9 ± 2.4 Ma (2σ). Together with age data from the literature, our new ages demonstrate that primary tin mineralization and the emplacement of the large Sn-specialized granites in the Erzgebirge fall in a narrow range between 318 and 323 Ma. Primary Sn mineralization occurred within a short interval during post-collisional collapse of the Variscan orogen and was essentially synchronous over the entire Erzgebirge. In contrast to earlier claims, no systematic age difference between granites of the eastern and western Erzgebirge was established. Furthermore, our data do not support a large age range for Late-Variscan granites of the Erzgebirge (330–290 Ma), as has been previously suggested.