Zircon provenance analysis of the local extensional basins of the Sudetian orogen in the East Ural zone

Introduction. Two structural zones are traditionally distinguished in the Eastern Urals. They are the Magnitogorsk zone and the East Ural zone, which are divided by a narrow suture. The Early Sudetian (Visean) orogenic phase is marked by a structural unconformity in the base of Upper Visean terrigenous-carbonate sequence both in the suture zone and the East Ural zone. According to drilling data granitic-pebble-bearing conglomerates are present at the base of this sequence. The Sudetian rearrangement is implied in the Magnitogorsk zone by the end of rifting and the initiation of carbonate deposition.

The Pre-Sudetian basement of the East Ural zone is comprised of the Lower Paleozoic deformed metamorphic rocks (gneisses, schists and carbonaceous quartzites), clastic deposits of the Ordovician (wackes and meta-arkoses) and the Lower Carboniferous (greywackes), as well as the volcanic rocks of the Tournasian-Early Visean and the granitoids of the Neplyuevka complex, dated 355-340 Ma. The Pre-Sudetian basement of the Magnitogorsk zone is represented by igneous complexes of the Devonian – Early Carboniferous age.

This research aims to determine the source areas and migration paths of the sediments for local basins associated with the Sudetian orogenic phase. The basins at the conjunction of the two megazones could derive clastic material from both.

Materials and methods. The specimens were collected from quarries near the Novinka village where sandstones of the terrigenous-carbonate succession are exposed. The sandstones are commonly cross-bedded, medium to coarse-grained, have sub-arkose, arkose, greywacke or, sometimes, quartz-arenitic composition. 210 zircon grains were extracted from two samples. 99 zircon grains characterized by discordancy no more than 5% have been chosen for the evaluation of age distribution parameters.

Results and discussion. The dating results appeared to be unexpected. Firstly, no single analysis yielded a Devonian isotope age, and only a single grain yielded the Tournaisian isotope age. Secondly, the vast majority of the zircon grains appeared to have the Cambrian and Ordovician isotope ages, with the main peak corresponding to the beginning of the Ordovician (480 Ma) and secondary ones corresponding to the beginning of the Late Ordovician (450-460 Ma), the Middle Cambrian (510-520 Ma) and the Early Cambrian (530-540 Ma).

So, the Magnitogorsk zone could not house the zircone source area for the local basin associated with the Sudetian orogenic phase. The clastic material could only be derived from the East Ural zone. However, the study area does not contain any known igneous complexes with suitable ages. The local source areas of detrital zircons are, in fact, associated with the scarps of metamorphic complexes of the East Ural zone, which host the zircons with the isotope ages of 478±5 Ma and 529±6 Ma.

Financial support. The research has been funded by RFBR and CNF as a part of the research project № 19-55-26009. The U-Pb dating of the zircon is executed as a part of the research project № АААА-А18-118053090045-8 of State task of IGG UB RAS. Centre of collective usage ‘Geoportal’,  Lomonosov Moscow State University (MSU), provided access to remote sensing data.