Chemical composition of radioactive accessory minerals: implications for the evolution, alteration, age, and uranium fertility of the Fichtelgebirge granites (NE Bavaria, Germany)

摘要

The late Variscan granites of the Fichtelgebirge form an older, Namurian intrusive complex (OIC-p and OIC-e) and a younger, post-Westphalian intrusive complex (YIC-1 and YIC-2). Both complexes have distinct radioactive accessory-mineral assemblages and compositions. The OIC-p biotite monzogranites contain monazite-(Ce) and minor thorite, but apparently lack magmatic xenotime-(Y) and uraninite. The more evolved OlC-e two-mica granites bear monazite-(Ce) occasionally rich in Th (up to 21 wt percent ThO_2) and U (8 wt percent UO_2), xenotime-(Y) of moderate U content (< 3.3 wt percent UO_2), and uraninite poor in Th and the REE. The most fractionated YIC Li-mica granites (YIC-2) may contain cheralite extremely high in Th (40.5 wt percent ThO_2) and U (8.6 wt percent UO_2), xenotime-(Y) rich in U (6.3 wt percent UO_2), and also a Th-REE-poor uraninite. Dissolution-reprecipitation processes account for the formation of huttonite-rich domains in monazite-(Ce), uraninite in U-depleted areas in zircon, and Mn-Fe-rich phosphorus minerals in fluorapatite. The accommodation of U in Th-poor uraninite made the OIC-e and YIC granites highly susceptible to U leaching. However, currently known U mineralization spatially associated with these granites is economically negligible. Electron-microprobe analyses yield a mean U-Th-total Pb monazite+xenotime age of 321 +- 8 Ma (2 sigma) for the OIC, which is further supported by the average age of uraninite in one of the studied OIC-e granites (323 +- 6 Ma). These results are in harmony with previous age data and imply a penecontemporaneous emplacement of the OIC-p and OlC-e granites, between about 326 and 321 Ma. The YIC-1 and YIC-2 granites display statistically indistinguishable mean ages of 294 +-6 Ma and 296 +- 6 Ma. These age data imply formation of the YIC granites during one intrusion period and question recent models postulating their generation during two discrete magmatic episodes separated by approx 15 Ma. The mean monazite+xenotime age of 296 +- 4 Ma obtained for the emplacement of the Younger Intrusive Complex, together with the age of uraninite in one of the YIC-1 granite samples (294 +- 3 Ma), suggest intrusion of this granite assemblage at the Carboniferous/Permian boundary. The YIC postdates compositionally similar granites in the neighbouring Erzgebirge-Vogtland Zone by 20-25 Ma, which emplaced penecontemporaneously with the OIC.