Investigating the long term evolution of intraplate basalt magmatism in Mongolia

Cenozoic basalts are widespread across central Mongolia and hold critical clues to deciphering the mantle dynamics at work beneath central Asia. The basalts occur so far away from plate boundaries that they raise important questions about their origin and relationship to local tectonics and deeper mantle processes. The purpose of the proposed research is to investigate the chronostratigraphy and geochemistry of Cenozoic basalt lavas to better establish the temporal evolution of regional magmatism, and the implications for mantle dynamics and its correlation with the surface geology.

Detailed studies on the Cenozoic basalts of Mongolia have concentrated on the most recent volcanism around the topographically elevated Hangai Dome in Central Mongolia (e.g. Barry et al., 2003, Hunt et al., 2012), where seismic evidence suggests a slightly warm upwelling supports the dome. This phase of magmatism has been tracked back to ~30 Ma in the Gobi, to the south of Hangai (Barry et al., 2001, 2003). Basalts with much older dates (>95 and 30 Ma; Yarmolyuk et al., 2015) are exposed between Hangai and the Gobi, but dating is based on K-Ar technique. K-Ar dating is notoriously unreliable and the exact timing of these basalts and their geochemistry hold an important key to understanding what role mantle dynamics has played in the long-term evolution of the province.

Building on a wealth of expertise between the University of Leicester and the Open University (OU), the student will investigate these southern, central Mongolian basalts. The occurrence of these basalts apparently extends the longevity of the enigmatic intra-plate volcanism and would have considerable implications for understanding any kind of long-lived thermal anomaly beneath the region. The student will establish a well-constrained volcanic stratigraphy based on robust Ar-Ar chronology, and examine geochemical affinities with wholerock trace element and isotope datasets.

Methodology

Fieldwork

The student will examine and sample basalt exposures across SW Hangai and the Gobi. The fieldwork will be in collaboration with Prof. Bat-Ulzii of the School of Geology and Mining at the Mongolian University - Science and Technology Institute, Ulaanbaatar, Mongolia, with whom we have a long-standing collaborative relationship. Prof. Bat-Ulzii is an experienced field geologist with first-hand knowledge of geochemical and Ar dating analyses (e.g.Bat-Ulzii et al., 2013).

Analyses

The student will undertake Ar-Ar sample preparation and analyses at the OU to determine magmatic chronology. Trace, rare earth element and isotope analyses will interrogate the magmatic rocks for source characteristics.

Modelling

Using the geochemical data, the student will undertake melt generation modelling to better constrain the magmatic event(s) and the changes of source characteristics over time.

Year 1

The project will start in January 2018 to maximise the 3-year studentship, with fieldwork in the early summer of 2018. Prior to fieldwork the student will use satellite imagery to assess the best locations to collect samples from and will gather information on dated samples from existing literature (K-Ar dates in Russian journals). Following fieldwork the student will learn sample preparation for Ar-Ar analyses, XRF wholerock and ICPMS trace and REE analyses. They will attend a national conference for networking, and present preliminary findings.

Year 2

Sample preparation and analyses will continue, with the emphasis on the student receiving full training in running their own analyses and learning to process data. The student will also contribute to the preparation of a NERC facility proposal requesting radiogenic isotope analyses. Present findings at a national conference and prepare paper(s) for submission.

Year 3

The student will complete isotope analyses and conduct detailed modelling of results to determine melt history and source characteristics for thesis and publication submission