Tectonothermal history of the basement rocks within the NW Dinarides: new40 Ar/39 Ar ages and synthesis

Abstract

Abstract Very low-grade and low-grade metamorphosed basement rocks from distinct inliers of the Africa-derived northwestern Dinarides (Medvednica Mts and Paleozoic Sana-Una Unit, respectively) have been studied with the multigrain step-heating 40 Ar/ 39 Ar technique in order to compare and reveal their tectonothermal history. 40 Ar/ 39 Ar ages from detrital white mica of the very low-grade basement rocks of the Paleozoic Sana-Una Unit gave a Variscan age of ~335 Ma. The new age is in agreement with 40 Ar/ 39 Ar ages from the very low-grade basement exposed at Petrova and Trgovska Gora of the NW Dinarides. Within low-grade metamorphic basement rocks from the Medvednica Mts, we found no Variscan ages. White mica from phyllitic basement rocks of the Medvednica Mts gives predominantly early Alpine ages ranging between 135 and 122 Ma and younger Alpine ages of ~80 Ma. The early Alpine ages of 135 and 122 Ma are interpreted as the date to the onset of ductile nappe stacking predating the formation of Gosau-type collapse basins. The late early Alpine event of ~80 Ma can be traced in the entire Cretaceous-aged orogen of the Circum- Pannonian Region and is synchronous with subsidence of the Gosau-type basins and opening and closure of the neighbouring Sava-Vardar Zone.