Tectonothermal evolution of the Apuseni Mountains, Romania: Resolution of Variscan versus alpine events with 40Ar/39Ar ages

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 Somes 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 Highis-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 Aries 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 are.