Niemcza diorites and moznodiorites (Sudetes, SW Poland): a record of changing geotectonic setting at ca. 340 Ma

Granites sensu lato in the Sudetes intruded in several episodes during the Variscan orogeny recording different stages of crust and mantle evolution. Correlating precise ages with geochemistry of the Variscan granites provides information on the evolution of these sources within the Variscan orogen. The Variscan intrusive rocks from the Niemcza Zone (Bohemian Massif, Sudetes, SW Poland) include undeformed dioritic to syenitic rocks and magmatically foliated granodiorites. In this study we analysed low SiO2 (48–53 wt.%) monzodioritic rocks from Przedborowa and Koźmice. The monzodiorites contain late-magmatic zircons with ages of 341.8 ± 1.9 Ma for Przedborowa and 335.6 ± 2.3 Ma for Koźmice, interpreted as emplacement ages of the dioritic magmas. Older Przedborowa rocks are lower in K, Mg, Rb and Ni than the Koźmice rocks and similar compositional trend is also observed in the Central Bohemian Plutonic Complex. The implication is that the mantle underlying the Niemcza Zone became more enriched from ca. 342 to ca. 336 Ma, probably following the collision of the Saxothuringian and Moldanubian/Lugian domains. The magmatism related to the collision occurred ca. 12 Ma later than that in the Central Bohemian Plutonic Complex, but was accompanied by a similar change in magma chemistry from high-K (Przedborowa) to shoshonitic (Koźmice, Kośmin enclaves) and probably to ultrapotassic (Wilków Wielki)