Abstract The Kasimovian Stage is the lower stage of the Upper Pennsylvanian Subsystem, in which a series of considerable biotic and abiotic events happened and changed the Earth. The Variscan orogeny and the Late Paleozoic Glaciation are two major events that caused geographical isolation of marine faunas and difficulties for a global correlation of biostratigraphy. Regional timescales of the Kasimovian across major continents are reviewed here. A global correlation of the Kasimovian is tentatively established based on a detailed review of major fossil groups such as conodonts, fusulines and some macrofossils. The index taxon for the base of the Kasimovian Stage has not been selected. The conodont species Swadelina subexcelsa , Idiognathodus heckeli , I. turbatus and I. sagittalis have good potential. Among them, I . heckeli is considered the best marker for the base of the Kasimovian Stage because it can mark a bioevent in a wide geographical range, and more importantly, it has a clear taxonomic definition within a phylogenetic lineage. The fusuline Montiparus might be regarded as an auxiliary marker to define the base of the Kasimovian based on its wider distribution. Other proxies, i.e. isotopic dating and strontium, carbon and oxygen isotopic stratigraphy throughout the Kasimovian, are also reviewed. The Global Boundary Stratotype Section and Point (GSSP) candidates for the Kasimovian Stage include the Naqing section, South China, the Usolka section, South Urals and the Afanasievo section, Moscow Basin. The Naqing section is regarded as the most appropriate GSSP candidate in terms of its complete sedimentary succession, well-recorded conodont lineages and well-studied bio-, chemo- and cyclo-stratigraphy.

ABSTRACT The Maksyutov complex is a mid- to late-Paleozoic high- to ultrahigh-pressure (HP-UHP) eclogite-bearing subduction zone terrane in the south Ural Mountains. Previous reports of radial fractures emanating from quartz inclusions in garnet, omphacite, and glaucophane, cuboid graphite pseudomorphs after matrix diamond, and microdiamond aggregates preserved in garnet identified by Raman spectroscopy indicate that parts of the complex were subjected to physical conditions of ~600 °C and >2.8 GPa for coesite-bearing rocks, and >3.2 GPa for diamond-bearing rocks. Peak UHP eclogite-facies metamorphism took place at ca. 385 Ma, and rocks were exhumed through retrograde blueschist-facies conditions by ca. 360 Ma. Bulk analyses of 18 rocks reflect the presence of mid-oceanic-ridge basalt (MORB), oceanic-island basalt (OIB), and island-arc tholeiite (IAT) basaltic and andesitic series plus their metasomatized equivalents. To more fully constrain the petrotectonic evolution of the complex, we computed isochemical phase equilibria models for representative metabasites in the system Na 2 O-CaO-K 2 O-FeO-MgO-Al 2 O 3 -SiO 2 -H 2 O-TiO 2 based on our new bulk-rock X-ray fluorescence (XRF) data. Both conventional Fe-Mg exchange thermometry and phase equilibrium modeling result in higher peak equilibrium temperatures than were previously reported for the complex. Pseudosection analysis provides minimum P-T conditions of 650–675 °C and 2.4–2.6 GPa for peak assemblages of the least retrogressed Maksyutov eclogites, whereas Fe-Mg exchange thermometry yields temperatures of 750 ± 25 °C for a pressure of 2.5 GPa. We interpret our new P-T data to reflect a thermal maximum reached by the eclogites on their initial decompression-exhumation stage, that defines a metamorphic field gradient; the relict coesite and microdiamond aggregates previously reported testify to pressure maxima that define an earlier prograde subduction zone gradient. The eclogitic Maksyutov complex marks underflow of the paleo-Asian oceanic plate and does not represent subduction of the Siberian cratonal margin.