Combined garnet and zircon geochronology of the ultra-high temperature metamorphism: Constraints on the rise of the Orlica-Śnieżnik Dome, NE Bohemian Massif, SW Poland

Abstract

Garnet and zircon geochronology combined with trace element partitioning and petrological studies provide tight constraints on evolution of the UHT-(U)HP terrain of the Orlica-Śnieżnik Dome (OSD) in the NE Bohemian massif. Lu-Hf dating of peritectic garnet from two mesocratic granulites constrained the time of its initial growth at 346.9±1.2 and 348.3±2.0Ma recording peak 2.5GPa pressure and 950°C temperature. In situ, U-Pb SHRIMP dating of zircon from the same granulite gave a younger age of 341.9±3.4Ma. Ti-in-zircon thermometry indicates crystallization at 810–860°C pointing to zircon formation on the retrograde path. Lu partitioning between garnet rim and zircon suggest equilibrium growth and thus U-Pb zircon age constrain the terminal phase of garnet crystallization which lasted about 6Ma.All Sm-Nd garnet ages obtained for mesocratic and mafic granulites are identical and consistently younger than the corresponding Lu-Hf dates. They are interpreted as reflecting cooling of granulites through the Sm-Nd closure temperature at about 337Ma.The estimated PTt path documents the ca. 10Ma evolution cycle of the OSD characterized by two distinct periods: (1) 347 - >342Ma period corresponds to nearly isothermal decompression resulting from crustal scale folding and vertical extrusion of granulites, and (2) at >342–337Ma which corresponds to a fast, nearly isobaric cooling.