From subduction to collision: Genesis of the Variscan granitic rocks from the Tatric Superunit (Western Carpathians, Slovakia)

Abstract

Granitic rocks from the core mountains of the Tatric Superunit (Western Carpathians, Slovakia) were dated by means of the sensitive high-resolution ion microprobe (SHRIMP) zircon U–Th–Pb method. The dated granitic rocks yielded a broad interval of the Concordia ages from 365 ± 5 Ma to 332 ± 3 Ma and largely invalidated the former hypothesis of a duality/antagonism in emplacement ages of the Variscan Sand I-type granites (Mississippian vs. Pennsylvanian) in the Central Western Carpathians (CWC). Generally, the obtained magmatic ages cluster in two separate intervals reflecting different stages of orogeny. The older, Famennian–Tournaisian event (365–350 Ma) was related to subduction of the Rheic Ocean, whereas the younger, mostly Visean event (348–332 Ma) was associated with collisional melting. The Th/U ratios of analysed zircons are compatible mainly with their magmatic origin (Th/U > 0.2), while the lower ones (Th/U < 0.1) in some zircons can indicate competition for Th with monazite and allanite, commonly present in the analysed granitic rocks. The new dating confirmed common zircon inheritance in Western Carpathian granites with inherited zircon cores showing the Neo-Archean to Paleo-Proterozoic (2800–1690 Ma) and Ediacaran to Late Ordovician (623–448 Ma) ages. The lack of any significant differences in magmatic/emplacement ages and the common “dirty” or hybrid character of both Iand S-type granites in the CWC indicate that the melted, often mixed sources and/or subsequent modification by hybridization and assimilation processes were mostly responsible for their general “alphabetic” designation (I-/S-type).