Three generations of granitoids emplaced over a 300 My time span in the Strzelin Massif, Fore-Sudetic Block, SW Poland: Mutual relationships and implications for secular crustal evolution

Abstract

In the Sudetes, northeastern Bohemian Massif, three groups of (meta-)granitoids occur: 1) Cadomian orthogneisses, with a protolith c. 600–540 Ma old, 2) Early Ordovician (~500 Ma) orthogneisses, and finally 3) Variscan (~300 Ma) granitoids. The three groups exist together in the Karkonosze–Izera Massif in the West Sudetes and in the Strzelin Massif in the East Sudetes. In the Strzelin Massif, the ~600 Ma old granitoids belong to the Strzelin Complex ascribed to the Brunovistulian microcontinent, whereas the ~500 Ma orthogneisses are part of the allochtonous Stachów Complex, correlated with the Saxo–Thuringian Zone. During the Variscan Orogeny, the Stachów Complex was thrust onto the Strzelin Complex. Subsequently, both complexes were intruded by Variscan granitoids. Trace-element data support the crustal affinity of the three generations of granitoids but they do not provide evidence for their mutual relationships. Neodymium isotope data suggest that, while the 600 Ma orthogneisses were produced from ancient, purely crustal source materials, the genesis of the Ordovician metagranites involved protoliths that were isotopically less mature. Finally, Variscan granitoids required a significant contribution of juvenile (mantle-derived) components. The general increase of initial εNd values with decreasing age is reflected by the younging average crustal residence ages of the source materials to each particular group of granitoids in the Strzelin Massif. Although they are allochtonous in the massif, the 500 Ma gneisses fit very well to the scheme: the younger granites, the less negative values of the εNdi. The zircon inherited-core ages suggest that the protoliths of the 600 Ma and 500 Ma gneisses originated from different sources derived from Avalonia- and Armorica-type terranes, respectively.