Ambiguous geological position of Carboniferous rhyodacites in the Intra-Sudetic Basin (SW Poland) clarified by SHRIMP zircon ages

Abstract

Rhyodacite sheets (the Sady Górne Rhyodacites) in the lowermost part of the Permo-Carboniferous Intra-Sudetic Basin molasse fill have been mapped as intrusives but, later on, based on ambiguous field and petrographic evidence, reinterpreted as lower Carboniferous lavas and tuffs; if so, they would mark the earliest episode of late-orogenic volcanism in the Intra-Sudetic Basin and in the whole Sudetes region in SW Poland. However, re-examination of field relationships and new observations are consistent with an intrusive emplacement of the rhyodacites as conformable to semiconformable, simple to composite sheets. SHRIMP zircon study indicates that the rhyodacites contain rare inherited zircons of ca. 560 Ma, and ca. 470 Ma (or slightly older), and a main population of zircons with an average concordia age of 306.1 ±2.8 Ma. This latter age documents the emplacement of the rhyodacites during a mid/late late Carboniferous (Westphalian) stage of volcanism in the Intra-Sudetic Basin in the Central European Variscides. This post-orogenic volcanism was possibly initiated several million years later than previously assumed, and could have comprised a few pulses over a relatively prolonged time span of millions of years