Volcanic stratigraphy, 40Ar/39Ar Geochronology and Geochemistry of Hangay Dome Volcanic Rocks, Central Mongolia

We report new ⁴⁰Ar/³⁹Ar ages for basalts from the Hangay region ranging from ~1 Ma to ~23 Ma. Combined with geomorphic and stratigraphic observations and an existing body of ages that range from 33 Ma to 5 Ka, these data show that multiple episodes of high-frequency eruptions of laterally extensive flows occurred between ~23 and ~6 Ma. A later stage of valley-filling eruptions occurred between ~4 Ma and ~1 Ma. Flows across this range of ages occur in a number of locations within the Hangay, suggesting that the region has been the site of volcanism for over 30 Ma, with no discernible age progression. In the headwaters of the Orkhon River, a valley-filling sequence (~3-4 Ma) has been inset into a more extensive sequence of plateau-like basalts (~21-23 Ma). This relationship suggests that a significant amount of erosion occurred during the Miocene to Early Pliocene. This erosion could have been due to uplift of the Hangay dome, but the presence of significant paleotopography below the older sequence makes it possible that elevations were already relatively high, and that post-eruption erosion reflected regrading by a regional drainage network that had been clogged by basalt flows. Further field and paleoelevation studies will be required to resolve this ambiguity and also take into account any climatic forcing. ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios for the basaltic rocks from the region range from 0.7039-0.7050 and 0.5120-0.5127 respectively. A plot of Sr vs Nd ratios shows limited correlation with a broad positive trend as opposed to the negative trend of the mantle Sr-Nd array. The lowest ⁸⁷Sr/⁸⁶Sr values are found in basalts with ɛ_Nd between -3 and -4. Defining whether these isotopic signatures reflect crustal contamination of magmas derived from the asthenosphere or primary magma generation from the continental lithospheric mantle will be the goal of additional isotopic studies. The lack of any age-progressive hotspot track and unified flood-basalt province in central Mongolia over the 33 m.y. duration of volcanism suggests that uplift and volcanism in the region was not likely to have been caused by a mantle plume as previously suggested. Delamination of the lower lithosphere also seems unlikely given the significant duration of magmatic activity and the fact that some of the volcanism predates the inferred uplift of the Hangay Dome. Asthenospheric upwelling or a small-scale edge-driven convection cell caused by the thick Siberian cratonic keel to the north of the Hangay could explain the region's uplift and long-lived volcanism.