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

Polyphase Alpine transcurrent faulting and thrusting in the Apuseni Mountai ns in Romania resulted in the juxtaposition of contrasting structural units that include both metamorphic assemblages and Permian-Mesozoic cover. Reso lution of the chronology of Alpine events and distinction from pre-Alpine e volution has been possible by systematic Ar-40/Ar-39 mineral dating of medi um- and low-grade assemblages. Hornblende and muscovite within nonretrogres sed northern sectors of the Somes gneissic assemblage record rapid postmeta morphic cooling between 317 and 300 Ma, following a regionally penetrative "Late" Variscan tectonothermal overprint. The Codru amphibolite-granodiorit e assemblage records markedly older 40Ar/39Ar mineral ages (400-365 Ma horn blende; 340-335 Ma muscovite) that date delayed postmetamorphic cooling fol lowing "Early" Variscan tectonothermal activity. Although variably overprin ted during Mesozoic tectonism, "Late" Variscan tectonothermal activity (320 -300 Ma) is also evident from analysis of muscovite concentrates from mylon itic 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-v ergent thrusting and concomitant low-grade metamorphism. A penetratively sh eared Triassic phyllonite in the footwall yielded a similar whole-rock plat eau, age of ca. 117 Ma. Southern sectors of the Baia de Aries carbonate-len se gneissic assemblage record both Jurassic (186-156 Ma) and Early-Middle C retaceous (124-111 Ma) ages in different structural units. The 40Ar/39Ar re sults suggest that the Apuseni Mountains experienced a complex tectonotherm al evolution that included "Early" and "Late" Variscan events and polyphase Alpine tectonism within major shear zones developed in interior sectors of the Carpathian are.