Coeval late-Variscan emplacement of granitic rocks: an example from the Regensburg Forest, NE Bavaria

摘要

Detailed field mapping in the Regensburg Forest, West Bohemian Massif, has enabled us to trace the distribution of a group of minor intrusions (granodiorites, granite porphyry dykes, two-mica granites). In order to unravel their genetic relation and to constrain their age, geochemical and geochronological studies were carried out.~(207)Pb/~(206)Pb evaporation analyses of zircons gave ages of 324 + - 2 Ma for a granodiorite (Stallwang granodiorite), and 323+ -2 Ma for a granite porphyry dyke. U-Th-Pb dating of monazite from a two-mica granite body gave concordant ages of 323 + - 4 Ma (average~(207)Pb/~(235)U-age), 322 + - 5 Ma (average ~(206)Pb/~(238)U-age) and 323 + 5 Ma (average~(208)Pb/~(232)Th-age). All these ages define the crystallization times and therefore reflect the simultaneous emplacement of the granitoids. The age determination was largely complicated by the predominance of inherited zircon cores. Combining zircon geochronology with geochemical and Sr-Nd data, it becomes evident that the granitoids were derived from Palaeozoic crustal protoliths which, in turn, contain components from Proterozoic (or older) rocks. A source-rock evaluation based on major element oxides allows discriminating between different protoliths. Lithological compositions similar to presently exposed country rock gneisses (mainly greywacke transformed into more or less homogeneous diatexite) are a potential source for the two-mica granites. The geochemistry of the granite porphyry dykes and the granodiorite is consistent with the experimental results of melting mafic pelitic and amphibolitic protoliths, respectively.