Monazite to the rescue: U–Th–Pb dating of the intrusive history of the composite Karkonosze pluton, Bohemian Massif

The large (~700 km2) composite Karkonosze (Krkonoše) pluton in the West Sudetes, on the border between Poland and the Czech Republic, consists mainly of porphyritic and equigranular granitoids, but contains a range of lithologies from lamprophyre to leucogranite. The absolute age and duration of the plutonism have proved difficult to determine. Previous age measurements by Rb–Sr, Ar–Ar and U–Pb range from ~330 to 290 Ma, with more recent results converging to ~320–300 Ma. Dating of zircon and monazite from samples of various major and minor lithologies by SIMS U–Th–Pb, several from the geochemical study of Słaby and Martin (2008), has narrowed the possible age range further. U–Pb ages measured on eight of ten zircon and monazite samples are in the range ~314–311 Ma. Zircon ages measured on the two major types of porphyritic granitoid are 313 ± 3 and 311 ± 4 Ma, and monazite ages are 312 ± 2, 313 ± 3 and 311 ± 3 Ma. Monazite from one hybrid granitoid has an age of 314 ± 3 Ma, and zircon from another an age of 314 ± 4 Ma. The results indicate that the monazite U–Pb age of an equigranular granite, at 318 ± 6 Ma, is consistent with geological evidence suggesting it predates the porphyritic granitoids.