Unravelling intraplate Cenozoic magmatism in Mongolia: Reflections from the present-day mantle or a legacy from the past?

Abstract

<p>In Mongolia, East Asia, intraplate magmatism has occurred intermittently from the late Cretaceous to present day. During the Mesozoic, basaltic volcanism was widespread across much of the southern and eastern parts of Mongolia. In contrast, the Cenozoic magmatism mainly extends in central parts of the country in a band trending north to south. This magmatism occurs in small, diffusely dispersed and relatively small volume (<30km<sup>2</sup>) plateaus (Barry et al., 2003). An exception to this is the Dariganga plateau in the southeast of Mongolia with >200 volcanic cones, covering an area of >10,000km<sup>2</sup>. Windley et al., (2010) and Sheldrick et al., (2018), proposed that the Mesozoic magmatism was caused by widespread but patchy removal of lithospheric mantle from beneath parts of Mongolia, NE China and Russia. Although several models have tried to explain the Cenozoic magmatism in Mongolia, there is no clear evidence for the cause of the volcanic activity. Isotopic studies on volcanic rocks from the Hangai Dome in central Mongolia revealed an asthenospheric origin for the melts (Barry et al., 2003). Could the Cenozoic volcanism be the result of melts that originated during a Mesozoic event? These melts could have been trapped in the lithospheric mantle since the Mesozoic and variably remobilised more recently. Or is there a mechanism causing melting during the Cenozoic, which can give insights into present-day conditions in the underlying mantle? Here, we examine the possibilities of (a) a direct link between the late Mesozoic and Cenozoic volcanic events in Mongolia leading to a multistage modification of the melt composition and (b) mechanism(s) in the asthenosphere/lithosphere causing present-day melting. In order to assess these possibilities we will compare the melt sources of the magmatism, focusing on three contrasting regions. (1) In central Mongolia, Cenozoic basalts occur on the flanks of the uplifted Hangai Dome, which is thought to have been uplifted in the Mesozoic (McDannell et al., 2018) but did not experience any volcanism at the time. (2) The Gobi Altai, which experienced both Mesozoic and Cenozoic magmatism, separated by ~40-50 Ma gap, but did not undergo any Mesozoic uplift. And finally, (3) the Dariganga plateau, which has experienced extensive volcanism during the late Cenozoic but not during the Mesozoic and in contrast to Hangai, underwent Mesozoic basin development rather than uplift. We will compare and contrast mantle sources of these regions to determine whether Mesozoic events have influenced the composition of the Cenozoic magmatism. Additionally, study of tomographic images from the upper mantle below central Mongolia will help us identify possible mechanisms that could have contributed towards the present-day melting of the upper mantle.</p>