Geochemistry and geochronology of Hangay Dome volcanic rocks: exploring the source of high topography and volcanism in an intracontinental setting

The Hangay dome in central Mongolia is an anomalous uplifted continental interior that is partially covered by diffuse Cenozoic basaltic rocks. Here we present new data on the geochemistry, stratigraphy, geomorphology and ⁴⁰Ar/³⁹Ar ages of the basaltic rocks to help elucidate the cause of the uplift and 33 Ma of volcanism in the region. ¹⁸⁷Os/¹⁸⁸Os ratios for the basaltic rocks range from 0.1363-0.3440. The higher values implicate crustal contamination, but the less radiogenic values limit the amount of contamination to the point where the Sr, Nd and Hf isotopic composition of the lavas are little affected, allowing them to be used as reliable tracers of the initial melt source. 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. These values are higher and lower, respectively, than Sr and Nd isotopic composition of the majority of spinel peridotite xenoliths contained in recent Hangay lavas, implicating a sub-lithospheric source for the magmas. The basalts have isotopic compositions approaching the EM-1 enriched mantle end member, similar to a number of other sites of young east Asian magmatism. An EM-1 type mantle source may have been generated regionally across East Asia by incorporation of pelagic sediments into the upper mantle during the protracted history of terrane accretion and subduction associated with the formation of the Central Asian orogenic system. New stratigraphically correlated ⁴⁰Ar/³⁹Ar ages for basalts from the Hangay region show that multiple episodes of laterally extensive flows occurred between 28.30×0.19 and 4.11×0.11 Ma. This first phase of volcanism was the most voluminous and long-lived. A later stage of valley-filling eruptions occurred between 3.28×0.50 Ma and 5 Ka. Flows across this range of ages occur in a number of locations within the Hangay, with no discernable age progression, indicating that the region has been the site of volcanism for over 30 Ma. Flow stratigraphy clearly shows the presence of substantial paleotopography that has been completely blanketed by flows, and a complex history of incision spanning the duration of volcanism. A plume source for the volcanism unlikely, given the basalt geochemistry, the lack of a large flood basalt province, and the lack of a discernable hot spot track. Large-scale slab-like delamination of the lower lithosphere also seems unlikely given the significant duration and diffuse nature of the magmatic activity. Geodynamic processes consistent with the high topography in the Hangay, the long duration of volcanism, and the sub-lithosphereic EM-1 isotopic signature include shear driven asthenospheric upwelling, thermal blanketing of the asthenosphere by thick crust, several small scale lithospheric drips or a small-scale edge-driven convection cell in the upper mantle.