Tectonothermal Evolution of the Apuseni Mountains, Romania: Resolution of Variscan versus Alpine Events with 40Ar/39Ar Ages

Abstract

Polyphase Alpine transcurrent faulting and thrusting in the Apuseni Mountains in Romania resulted in the juxtaposition of contrasting structural units that include both metamorphic assemblages and Permian‐Mesozoic cover. Resolution of the chronology of Alpine events and distinction from pre‐Alpine evolution has been possible by systematic 40Ar/39Ar mineral dating of medium‐ and low‐grade assemblages. Hornblende and muscovite within nonretrogressed northern sectors of the Someş gneissic assemblage record rapid postmetamorphic cooling between 317 and 300 Ma, following a regionally penetrative “Late” Variscan tectonothermal overprint. The Codru amphibolite‐granodiorite assemblage records markedly older 40Ar/39Ar mineral ages (400–365 Ma hornblende; 340–335 Ma muscovite) that date delayed postmetamorphic cooling following “Early” Variscan tectonothermal activity. Although variably overprinted during Mesozoic tectonism, “Late” Variscan tectonothermal activity (320–300 Ma) is also evident from analysis of muscovite concentrates from mylonitic granite and schist within low‐grade assemblages in the regional Highiş‐Biharia shear zone (HBSZ). Shallow‐dipping phyllonites throughout western sectors of the HBSZ record effects of Aptian to Albian (114–100 Ma) north‐vergent thrusting and concomitant low‐grade metamorphism. A penetratively sheared Triassic phyllonite in the footwall yielded a similar whole‐rock plateau, age of ca. 117 Ma. Southern sectors of the Baia de Arieş carbonate‐lense gneissic assemblage record both Jurassic (186–156 Ma) and Early‐Middle Cretaceous (124–111 Ma) ages in different structural units. The 40Ar/39Ar results suggest that the Apuseni Mountains experienced a complex tectonothermal evolution that included “Early” and “Late” Variscan events and polyphase Alpine tectonism within major shear zones developed in interior sectors of the Carpathian arc.