The litospheric mantle beneath Central Mongolia: costraints from spinel-bearing peridotite xenoliths and high-pressure experiments

This study investigates the poorly understood lithospheric mantle of the Central Asian Orogenic Belt, focusing on spinel-bearing mantle xenoliths collected from volcanic structures in the Mandakh-Mandal-Gobi region of Central Mongolia. These xenoliths, primarily lherzolites with minor harzburgites and one pyroxenite, are hosted in alkaline lavas aged between 82.5 ± 2 Ma and 51 ± 2 Ma. The xenolith textures range from porphyroblastic to protogranular, with melt-crystal reaction features varying by sampling area. Major elements are relatively homogeneous within xenoliths, but Rare Earth Elements (REE) and other incompatible element patterns show significant heterogeneity, particularly in xenoliths from the last magmatic event, which reflect metasomatism and basalt extraction of an older and probably deeper Sub-Continental Lithospheric Mantle. Given the challenges of accurately estimating formation depths for spinel peridotite xenoliths, we conducted high-pressure experiments at the Bayerisches Geoinstitut to simulate mantle conditions beneath the study area and refine thermobarometric methods. Experiments were performed with a piston cylinder press in a pressure-temperature (P-T) window of 5–25 kbar and 1000–1200°C, using synthetic oxides and powdered mineral compositions. The results helped to better evaluate the current geothermobarometric methods and to construct phase stability fields in pseudosections, revealing that samples from the latest magmatic event show higher equilibrium P-T conditions, consistent with observed mineral chemistry. The combined natural and experimental data, provide insight into the geodynamic evolution of the region. Initial magmatism at the Mandal-Gobi volcano sampled a relatively homogeneous, depleted mantle, while subsequent events associated with graben formation at Log Uul introduced limited melt interaction. In contrast, the Rigo dyke samples reflect a deeper, different mantle portion enriched in light REE and depleted in heavy REE, indicative of significant prior melting events and metasomatism. This evolving sequence suggests mantle upwelling and sampling during lithospheric extension, contributing to a complex, heterogeneous mantle beneath Central Mongolia.