A section of the orogenic middle crust (Orlica-Snie_ znik Dome, Polish/Czech Central Sudetes) was examined to constrain the duration and significance of deformation (D) and intertectonic (I) phases. In the studied metasedimentary synform, three deformation events produced an initial subhorizontal foliation S1 (D 1 ), a subsequent subvertical foliation S2 (D 2 ) and a late subhorizontal axial planar cleavage S3 (D 3 ). The synform was intruded by pre-, syn-and post-D 2 granitoid sheets. Crystallization-deformation relationships in mica schist samples document I 1-2 garnet-staurolite growth, syn-D 2 staurolite breakdown to garnet-biotite-sillimanite/andalusite, I 2-3 cordierite blastesis and late-D 3 chlorite growth. Garnet porphyroblasts show a linear Mn-Ca decrease from the core to the inner rim, a zone of alternating Ca-Y-and P-rich annuli in the inner rim, and a Ca-poor outer rim. The Ca-Y-rich annuli probably reflect the occurrence of the allanite-to-monazite transition at conditions of the staurolite isograd, whereas the Ca-poor outer rim is ascribed to staurolite demise. The reconstructed P-T path, obtained by modelling the stability of parageneses and garnet zoning, documents near-isobaric heating from~4 kbar/485°C to~4.75 kbar/575°C during I 1-2 . This was followed by a progression to 4-5 kbar/580-625°C and a subsequent pressure decrease to 3-4 kbar during D 2 . Pressure decrease below 3 kbar is ascribed to I 2-3 , whereas cooling below~500°C occurred during D 3 . In the dated mica schist sample, garnet rims show strong Lu enrichment, oscillatory Lu zoning and a slight Ca increase. These features are also related to allanite breakdown coeval with staurolite appearance. As Lu-rich garnet rims dominate the Lu-Hf budget, the 344 AE 3 Ma isochron age is ascribed to garnet crystallization at staurolite grade, near the end of I 1-2 . For the dated sample of amphibolebiotite granitoid sheet, a Pb-Pb single zircon evaporation age of 353 AE 1 Ma is related to the onset of plutonic activity. The results suggest a possible Devonian age for D 1 , and a Carboniferous burialexhumation cycle in mid-crustal rocks that is broadly coeval with the exhumation of neighbouring HP rocks during D 2 . In the light of published ages, a succession of telescoping stages with time spans decreasing from c. 10 to 2-3 Ma is proposed. The initially long period of tectonic quiescence (I 1-2 phase, c. 10 Ma) inferred in the middle crust contrasts with contemporaneous deformation at deeper levels and points to decoupled P-T-D histories within the orogenic wedge. An elevated gradient of 30°C km À1 and assumed high heating rates of c. 20°C Ma À1 are explained by the protracted intrusion of granitoid sheets, with or without deformation, whereas fast vertical movements (2-3 Ma, D 2 phase) in the crust require the activity of deformation phases.

In the Variscan Bohemian Massif, orthogneiss complexes of different sizes, derived from early Palaeozoic granitoids, are accompanied by metavolcano-sedimentary successions. They are common in the Sudetes, SW Poland, and belong mainly to the Saxothuringian Terrane. In order to better characterize the evolution of the latter, new U-Pb and O isotopic zircon analyses, combined with zircon typology and new U-Th-Pb isotopic monazite analyses, were performed. The present data show that the S-type granitic precursors of the Doboszowice orthogneisses (Fore-Sudetic Block) and the Śnieżnik gneisses (Sudetes) were formed at ca. 495 Ma from differently evolved magmas. Protoliths of the Doboszowice orthogneisses developed entirely in the crust, whereas the precursors to the Śnieżnik gneisses received some mantle input. Metasediments that accompany the Doboszowice orthogneisses reveal zircon spectra, which point to a late Cambrian-Early Ordovician sedimentary age. These spectra, in addition to predominant grains with Ediacaran ages, contain up to 10% of zircons dated at ca. 1.0 Ga. Therefore, it is suggested that the parent basin was supplied with detritus coming from areas, located in the Libyan-Nubian part of North Africa. Using also earlier published data, such a provenance is assigned to the units that now occur in the eastern part of the Fore-Sudetic Block. Late Cambrian-Early Ordovician granite intrusions and the concurrent accumulation of sediments originated in an extensional setting of the peri-Gondwana rifted continental margin or a back-arc setting. The Pb/U and Pb/Th monazite data constrain the ca. 346-341-Ma peak of the Variscan regional metamorphism in the eastern Saxothuringian Terrane and the prolonged juxtaposition of now adjacent tectonic units at least until 330 Ma.

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