A Carboniferous/Permian, calc-alkaline, I-type granodiorite from the Małopolska Block, Southern Poland: implications from geochemical and U-Pb zircon age data

Abstract

A granodiorite from borehole WB-102A in the Dolina Będkowska, the Małopolska Block (MB), Southern Poland, yielded a mean U-Pb zircon age of 300 ą3 Ma with SHRIMP II. No inherited older component was detected. Geochemically, it is a K-rich, I-type, calc-alkaline granodiorite with supra-subduction characteristics (negative Nb and Ti anomalies). Silicic igneous rocks are abundant at the MB margin along the Kraków-Lubliniec Fault Zone (KLFZ) across which it adjoins to the Upper Silesian Block (USB) where such rocks are scarce. Both blocks belong to the Variscan foreland. Granitic rocks cannot, however, generate at foreland settings. Thus, the hypothesis is put forward that the parent melt for the silicic rocks was derived from the thickened lower crust of the Variscan orogenic belt owing to extensional decompression melting, and transported away towards pull-apart openings developed along the crustal-scale fault zone (KLFZ) that underwent a complex strike-slip history around the Carboniferous/Permian boundary.