Geochemie a zirkonove U-Pb stari derflickeho granodioritu z dyjskeho masivu

Two studied granodiorites from the Derflice village (east of the town of Znojmo, southern Moravia) in the NE-part of the Dyje- (Thaya-) Massif (Fig. 1) are homogeneous fine-grained rocks composed of quartz (35 wt%), altered plagioclase (35 wt%), potassium feldspar (25 wt%) and chlorite (5 wt%) with accessory amounts of muscovite, apatite, hematitized ilmenite, and zircon. Appropriate petrological name of these rocks are chloritized biotite granodiorites. Chemically and mineralogically (Table 1, Fig. 2), the Derflice granitoids resemble granodiorites of the Tetčice suite from the western branch of the Brno Massif.
Representative oscillatory zoning (Fig. 3) revealed in cathodoluminescence (CL) images of zircon grains in all studied samples is shown in Fig. 1, and U-Pb concordia diagrams for zircons from the Thaya (Dyje) Massif are in Fig. 4. Dating of samples DF-B and DF-C yielded concordia ages of 603±3 Ma and 603±5 Ma (2σ), respectively (for geochronological data of individual zircon grains see Table 2), and are interpreted as the Upper Proterozoic intrusive age of the granodiorite. These ages are close to the age of zircons from granodiorite of the nearby Western Granitoid Complex of the Brno Massif (603±3 Ma, 2σ; Soejono et al. 2016, in review). Comparable ages supporting the idea that the granitoids of the Thaya Massif are assumed to be a south-western continuation of the Western Granitoid Complex of the Brno Massif. The presence of granitic rocks in the Thaya (Dyje) Massif with the Upper Proterozoic (Ediacaran) crystallization ages provides further evidence for a widespread Cadomian magmatic activity along the northern margin of Gondwana.

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