LATE ORDOVICIAN EXPLOSIVE VOLCANISM IN SIBERIA

In Siberia, several individual K-bentonite beds have been identified in the Upper Sandbian – Katian cool-water carbonateStages of the Late Ordovician. During the Cambrian, Ordovician and Silurian the Siberian Platform, which constitutes the core of the Siberian paleocontinent, was located in the low latitude tropical area migrating slowly from the southern hemisphere in the Cambrian and Lower Ordovician to the northern hemisphere in the Upper Ordovician and Silurian. The Lower Ordovician and the lower part of the Middle Ordovician series of the basin are represented by succession of warm-water tropical-type carbonates. The Upper Ordovician series by contrast is represented by succession of cool-water carbonates dominated by bioclastic wackestone and packstone beds intercalated with fine-grained terrigenous sediments. All K-bentonite beds have been found within the Upper Ordovician cool-water carbonate succession. Their appearance points to the intensive explosive volcanism on or near the western margin of the Siberian craton in Late Ordovician time, which is not consistent with most paleogeographic reconstructions for this time. X-ray diffraction analysis of the clay fractions of six samples shows they are dominated by illite/smectite mixed layer clay with Reichweite values ranging from 1 to 3. Some samples show the presence of chlorite in addition to the illite/smectite mixed layers. Raman spectroscopy fully supports the XRD data and shows, in addition, the presence of minor amounts of calcite and organic matter. SEM images reveal the presence of volcanogenic mineral. And based on major and trace element XRF analyses these beds are equivalent to the K-bentonite beds from the Upper Ordovician Mangazea and Dolbor formations of thesouthwestern part of the Tungus basin in Siberia. They appear to be derived from the alteration of volcanic ash falls and their appearance points to the intensive explosive volcanism on or near the western (in present day orientation) margin of the Siberian craton in Late Ordovician time. The timing of volcanism in the Ordovician of Siberia is surprisingly close to the period of volcanic activity of the Taconic arc near the eastern margin of Laurentia. It looks like both arcs were activated by the same plate tectonic reorganization.