Exhumation and metamorphism of an ultrahigh-grade terrane: geochronometric investigations of the Sudete Mountains (Bohemia), Poland and Czech Republic

Abstract

The Sudete Mountains, NE Bohemian Massif (Czech Republic and Poland), preserve abundant eclogitic and granulitic centimetre- to decimetre-scale boudins enveloped in a predominantly migmatitic matrix. Published geochronometry and thermobarometry from the UHP and UHT rocks broadly constrain the crystallization and initial stage 1 exhumation history for these units; however, the timing of stage 2 metamorphism and associated unroofing is less well constrained. New in situ ion microprobe Th–Pb monazite results, together with complementary U–Pb zircon and electron microprobe analyser total-Pb monazite results, on 11 amphibolite-facies gneissic to migmatitic samples, place important temporal constraints on the second stage of UHP and UHT metamorphism–exhumation. The Orlica–Snieznik Dome records UHP metamorphism occurring at 375 Ma and subsequent exhumation to midcrustal levels in supra-Barrovian conditions at c . 345–330 Ma. In contrast, the western Gory Sowie Block preserves evidence of HP-granulite conditions at c . 400 Ma, and exhumation to mid-crustal levels at 380–370 Ma, revealing a c . 30 million years difference in exhumation events between the neighbouring terranes. The eastern Gory Sowie Block preserves ages similar to the Orlica–Sneiznik Dome, suggesting that different preserved metamorphic–cooling histories are juxtaposed across the Sudetic Marginal fault. The bounding Niemcza shear zone yields preliminary Th–Pb dates that range from 380 ± 8 Ma to 283 ± 2 Ma, preserving a protracted metamorphic record that spans the exhumation history of the region. The distinct collapsed geochronologies of both terranes probably reflect rapid vertical transport of low-viscosity crust under supra-Barrovian conditions near the mid-crustal high-strength lid during oblique (transpressional) convergence.