Whole-rock 40Ar/39Ar geochronology, geochemistry, and stratigraphy of intraplate Cenozoic volcanic rocks, central Mongolia

A new comprehensive suite of 235 whole-rock 40Ar/39Ar ages ranges from Holocene to ca. 30 Ma for Cenozoic volcanic rocks in central Mongolia. Their total preserved volume is similar to 1540 km(3), but prior to erosion, the volume may have been as high as 2900 km(3). The 235 new stratigraphically referenced ages, combined with surface mapping, suggest that volcanic output began to increase in the early Miocene and peaked in the middle Miocene, with a gradual decrease in volume through the Holocene. Trace-element inversion modeling suggests that the volcanic rocks were derived by low degrees of partial melting of a garnet lherzolite source at a depth greater than 62 km. The low total volumes and the lack of an apparent age-progressive hotspot track suggest that the volcanism cannot be attributed to a deep mantle plume beneath central Mongolia. The long-term gradual increase and subsequent decrease in volcanic output may also rule out delamination as the cause of the volcanism. Instead, we favor a small-scale mantle upwelling to explain the cause of the intraplate volcanism in the region. This upwelling could have been the result of one or more different processes, including edge-driven convection, redirection of asthenospheric flow by the thick lithosphere of the Siberian craton to the north, shear-driven upwelling, delamination, drips, or upwelling related to stagnant slabs in the mantle transition zone.