Toward an Improved Understanding of Intraplate Uplift and Volcanism: Geochronology and Geochemistry of Intraplate Volcanic Rocks and Lower-Crustal Xenoliths

Abstract The origin of diffuse intraplate volcanism and 4000 m high topography of the Hangay Mountains in central Mongolia is enigmatic, as it is not explained by or predicted by traditional plate tectonic models. Chapter One presents new whole-rock 40Ar/39Ar ages that range from Holocene to ~30 Ma for the Cenozoic volcanic rocks in central Mongolia. The total volume of the Cenozoic volcanic rocks is approximately 1540 km3, but prior to erosion, the volume may have been as high as 2900 km3. Volcanism began to increase gradually in the early Miocene and peaked in the middle Miocene, with a gradual decrease in volume through the Holocene. The low total volumes and the lack of an age progressive hot-spot track rule out the presence of a mantle plume. The long-term gradual increase and subsequent decrease in volcanic output may also rule out delamination. Chapter Two presents the results of a geochemical study to understand the source of the volcanic rocks. Isotopic compositions of the volcanic rocks fall between prevalent mantle (PREMA) and enriched mantle (EM1) and are similar to other Cenozoic basalts from east Asia, though markedly different from the depleted MORB mantle (DMM) isotopic signature of the lithospheric mantle in Mongolia, suggesting the volcanic rocks must be derived from a sub-lithospheric source. A mantle upwelling beneath the region is likely, though the specific cause can not be identified by geochemistry and geochronology alone. In Chapter Three, two-pyroxene granulite lower crustal xenoliths brought to the surface by the Cenozoic volcanism were studied using U-Pb geochronology and geochemistry. U-Pb geochronology indicates the lower crust in central Mongolia began to form by at latest the Permo-Triassic, likely during the formation of the Central Asian Orogenic Belt (CAOB), which is consistent with arc-like geochemical signatures. Recent seismology studies indicate that the high elevations of the Hangay region could be isostatically supported by a thick crust. My new data suggests the lower crust, and by inference the high topography of the Hangay, formed in the Late Paleozoic to Early Mesozoic during the formation of the CAOB, which is significantly older than any other previous estimates.