CONSTRAINING THE AGES OF LATE DEVONIAN EXTINCTION EVENTS IN THE CENTRAL ASIAN OROGENIC BELT (CAOB): U-PB ZIRCON AGES AND IGNEOUS PETROLOGY

Two of the top six most devastating extinction events in Earth's history occurred during the Late Devonian. Studies of the Kellwasser Anoxia Event at the Frasnian-Famennian (F-F) boundary and the Hangenberg Anoxia Event at the Devonian-Carboniferous (D-C) boundary have mostly been conducted on the continental margins of Europe and North America, but evidence of these extinction events have also been identified in sediments located in an understudied area: the Central Asian Orogenic Belt (CAOB).

Late Devonian clastic sediments and lava flows from the CAOB have been collected from the Zhulumute Formation (Xinjiang Province, China) and the Samnuuruul Formation (Khovd Province, Mongolia). The Zhulumute Formation contains volcaniclastic sandstones and conglomerates with detrital zircons that grade upwards into siltstones and limestones. The Samnuuruul Formation contains conglomerate beds with interbedded limestones that grade into sandstone, siltstone, and tuffite layers, with interbedded pillow basalts and lava flows found in some locations. Trace element concentrations suggest both units have an island arc volcanic signature, consistent with Late Devonian tectonic models for the region, and with the observed stratigraphy. U/Pb geochronology of zircons in basalts and volcaniclastic sediments (using a thermal ionization mass spectrometer (TIMS)) is being used to constrain the ages of the Kellwasser and Hangenberg ocean anoxia events in the CAOB. Initial results from the Zhulumute Formation tentatively suggest a Middle Ordovician age of 452 ± 2 Ma, consistent with tectonic models of Marianas-type island arc development around the Hongguleleng Ocean. Initial results from the Samnuuruul Formation suggest a Late Devonian age of 376 ± 2 Ma, consistent with initial conodont biostratigraphy. These results will be the first radiometric ages for Late Devonian sediments in the CAOB thought to contain both the F-F boundary and the D-C boundary in an open oceanic setting.