Polyphase Permo-Carboniferous magmatism adjacent to the Intra-Sudetic Fault: constraints from U–Pb SHRIMP zircon study of felsic subvolcanic intrusions in the Intra-Sudetic Basin, SW Poland

The SHRIMP U–Pb dating of zircons from felsic subvolcanic rocks in the Carboniferous formations of the Intra-Sudetic Basin in SW Poland verifies previous views on the timing of Late Palaeozoic magmatic activity in this area and constrains the links between regional tectonics and intra-basinal volcanism in the mid-European Variscides. Two main stages of magmatism are identified: the Late Carboniferous late orogenic stage, and the Early Permian post-orogenic stage. The Carboniferous late orogenic subvolcanic to volcanic activity was contemporaneous with the formation of the Karkonosze granite pluton to the west and comprised episodic emplacement of predominantly felsic laccoliths and sills within the basin fill during the Westphalian–Stephanian in two sub-stages at ca. 313–310 Ma and 306–305 Ma. Local folding of the Carboniferous succession peaked at ca. 303–300 Ma adjacent to the Intra-Sudetic Fault, in relation to late-stage sinistral movements along this regional wrench fault. The emplacement of minor subvolcanic intrusions at ca. 293 Ma completed the activity in the northern Intra-Sudetic Basin; this Permian post-orogenic volcanism, however, developed fully in areas further south. The predominance of subvolcanic intrusions over extrusions in Carboniferous can be linked to the trapping of rising magmas within the several kilometres thick sedimentary basin fill. Zircon xenocrysts in the Carboniferous subvolcanic rocks indicate that older crustal lithologies, possibly metamagmatic and metasedimentary rocks of dominantly Neoproterozoic–Palaeozoic ages, were involved in petrogenesis as magma sources and/or contaminants. The crustal influence on magma formation was stronger during the earlier magmatic events.