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±2.0 Ma recording peak 2.5 GPa pressure and 950 °C temperature. In situ, U-Pb SHRIMP dating of zircon from the same granulite gave a younger age of 341.9±3.4 Ma. Tiin-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 6 Ma. 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 337 Ma. The estimated PTt path documents the ca. 10 Ma evolution cycle of the OSD characterized by two distinct periods: (1) 347->342 Ma period corresponds to nearly isothermal decompression resulting from crustal scale folding and vertical extrusion of granulites, and (2) at >342-337 Ma which corresponds to a fast, nearly isobaric cooling.

Integrating X‐ray mapping and microtomography of garnet with thermobarometry to define the P–T evolution of the (near) UHP Międzygórze eclogite, Sudetes, SW Poland

“…The initial P-T estimates obtained for the Międzygórze eclogites suggested peak metamorphic conditions in the coesite stability field at pressures >27 kbar and temperatures of~700-800°C (Bröcker & Klemd, 1996). Comparable estimates were published from other parts of the area for granulites (22-27 kbar and 875-950°C (Ferrero, Wunder, Walczak, O'Brien, & Ziemann, 2015;Kryza et al, 1996) or even 30-35 kbar and 850-920°C (Walczak, Anczkiewicz, Szczepański, Rubatto, & Košler, 2017) and eclogites intercalated in the granulites (21-28 kbar and 800-1,000°C; Klemd & Bröcker, 1999). Bakun-Czubarow (1991b, 1992 described the presence of quartz pseudomorphs after coesite in the granulite-eclogite association from the Stary Gierałtów locality (Figure 1).…”

“…Geochronological studies from the nearby granulites and surrounding rocks establish a time frame for the exhumation process (Anczkiewicz et al., ; Budzyń, Jastrzębski, Kozub‐Budzyń, & Konečný, ; Klemd & Bröcker, ; Lange et al., ; Walczak et al., ). Exhumation commenced at 349–347 Ma (Budzyń et al., ; Walczak et al., ) and after c . 6 Ma (342 Ma) reached the stage of zircon crystallization and recrystallization on the retrograde path (Turniak et al., ; Walczak et al., ).…”

“…Lu–Hf dating of garnet from mesocratic granulites that records peak P–T conditions of 25 kbar and 950°C constrained the time of initial growth as c . 348–347 Ma (Walczak et al., ). In situ, U – Pb SHRIMP dating of zircon from the same granulite gave a younger age of c .…”

“…The SHRIMP U-Pb zircon dating of (U)HP granulites and eclogites documented the importance of a Carboniferous metamorphic event comprising the late-stage eclogite facies metamorphism and lower pressure HT overprint at c. 340 Ma (Bröcker et al, 2009(Bröcker et al, , 2010. Lu-Hf dating of garnet from mesocratic granulites that records peak P-T conditions of 25 kbar and 950°C constrained the time of initial growth as c. 348-347 Ma (Walczak et al, 2017). In situ, U-Pb SHRIMP dating of zircon from the same granulite gave a younger age of c. 342 Ma.…”

Meso–Cenozoic cooling and exhumation history of the Orlica‐Śnieżnik Dome (Sudetes, NE Bohemian Massif, Central Europe): Insights from apatite fission‐track thermochronometry

“…Reicherter et al, ). The Orlica‐Śnieżnik Dome (OSD) represents a mantled gneiss dome formed during Early Carboniferous crustal‐scale folding (Chopin et al, ; Walczak et al, ). Widespread mid‐Triassic to mid‐Cretaceous erosion in this area erased most of the Permo‐Mesozoic sedimentary record (see Migoń & Lidmar‐Bergström ).…”