Int J Earth Sci (Geol Rundsch) (2004) 93:361–383 DOI 10.1007/s00531-004-0386-7 ORIGINAL PAPER U. A. Glasmacher · I. Matenaar · W. Bauer · V. N. Puchkov Diagenesis and incipient metamorphism in the western fold-and-thrust belt, SW Urals, Russia Received: 2 October 2002 / Accepted: 17 January 2004 / Published online: 1 May 2004  Springer-Verlag 2004 Abstract In the western fold-and-thrust belt of the southern Urals, the Kübler and rkai indices determined on shales, slates and phyllites record an increase from lower late diagenetic to epizonal grade from west to east. The metamorphic grade varies strongly within the dif- ferent tectonic segments, which are separated by major thrusts. The increase of diagenetic and incipient meta- morphic grade from the footwall to the hanging wall of all major Upper Palaeozoic thrusts indicates a pre-Permo/ Triassic origin. West of the Avzyan thrust zone, the di- agenetic to incipient metamorphic grade is related to the Palaeozoic basin development and reached the final grades in Late Carboniferous to Early Permian times. East of the first Avzyan thrust in the Yamantau anticli- norium, the diagenetic to lower greenschist metamorphic grade is possibly of Neoproterozoic origin and might be related to the development of the Neoproterozoic basin at the eastern margin of the East European Craton. The eastern part of the Yamantau anticlinorium was exhumed below 200 C in the Late Carboniferous or Early Per- mian. The diagenetic grade of the autochthonous Palaeo- zoic sedimentary units increases toward the stack of Palaeozoic nappes and might partly be caused by the deformational process due to the emplacement of the Palaeozoic nappes. Within the Timirovo thrust sheet, the decrease of metamorphic grade with stratigraphic age developed prior to the emplacement of the nappes. The upper anchizonal metamorphic grade of the Upper De- vonian slates of the Zilair nappe results from the defor- mation process related to the Lower Carboniferous nappe emplacement. Keywords Diagenesis · Incipient metamorphism · Kübler and rkai indices · Southern Urals · URSEIS’95 Introduction The Uralide orogen forms a linear N–S-trending Palaeo- zoic fold-and-thrust belt that extends for more than 2,000 km from the Arctic Ocean to near the Caspian Sea (Fig. 1). It resulted from the Palaeozoic two-stage colli- sion of the East European Craton (EEC) with the Sibirian/ Kazakhstanian continents and intervening accretion of island arc and microcontinental fragments (Hamilton 1970; Zonenshain et al. 1984, 1990; Puchkov 1988, 1993, 1997). The Main Uralian fault (MUF) is the major suture zone between the East European Craton to the west and the oceanic, island arc and continental terranes of Asia to the east (Zonenshain et al. 1984, 1990; Echtler and Hetzel 1997). In comparison to other Palaeozoic orogens (e.g. the Variscides, Caledonides etc.), the Uralides show a general bilateral symmetry with a western fold-and-thrust belt, a central magmatic arc zone (Magnitogorsk zone) and an eastern thrust belt with deep reaching crustal shear zones (Berzin et al. 1996). However, unlike other Palaeo- zoic orogens that underwent post-orogenic collapse and extension, the Uralide fold belt reveals an intact, well- preserved orogen with a deep crustal root within a stable continental interior (Echtler et al. 1996; Steer et al. 1998). The crustal root is interpreted as partially eclogitized lower crust (Döring and Götze 1999; Leech 2001). As the orogen is isostatically compensated and appears as though it has never departed far from this state, it represents an orogen that has escaped delamination of the lower crust and post-orogenic extensional collapse (Giese 2000; Leech 2001). Thus, the Uralides are an exceptional ex- U. A. Glasmacher ( ) ) Forschungsstelle Archäometrie der Heidelberger Akademie der Wissenschaften, Max-Planck-Institut für Kernphysik, Postfach 103980, Heidelberg, Germany e-mail: ua.glasmacher@mpi-hd.mpg.de Tel.: +49-6221-516321 Fax: +49-6221-516633 I. Matenaar · W. Bauer Geologisches Institut, RWTH Aachen, Wüllnerstrasse 2, 52056 Aachen, Germany V. N. Puchkov Ufimian Geoscience Center, Russian Academy of Sciences, K. Marx Str., 450000 Ufa, Russia