LATE DEVONIAN ROCKS FROM A NEW SITE: THE BAYANKHOSHUU RUINS OF SOUTHWESTERN MONGOLIA

The Late Devonian (383-359 Ma) was characterized by prolonged climate instability and contained several mass extinctions. Our understanding of the Late Devonian extinction event is compromised with paleographic sample bias as nearly all study locations are in North America and Europe (which were paleogeographically adjacent, as Pangea was forming at this time) and in South China. For this reason, we sampled a new section in a paleogeographically unique location: the Bayankhoshuu Ruins section of southwestern Mongolia, which likely represents an isolated volcanic island arc environment. The site spans two adjacent terranes, the Gurvansayhan Terrane and the Mandalovoo Terrane, and samples were collected from the Botuulkhudag Formation’s Takhuul and Minjin Members. Optical microscopy, X-ray diffraction, cathodoluminescence, and scanning electron microscopy with energy dispersive X-ray microanalysis thus far indicates that Bayankhoshuu Ruins are characterized by a mixture of sedimentary rocks, volcanic rocks, and volcaniclastic rocks that have experienced low-grade metamorphism, up through prehnite-pumpellyite facies. The metamorphism and mineral assemblages are consistent with the tectonic setting but preclude our ability to detect the primary mineralogy and stable isotope signatures associated with any extinction event horizons. Although the new data collected for this site is not appropriate for determining the cause of these Late Devonian extinction events, it has produced a revised stratigraphic and metamorphic framework for this understudied region.