Evidence for fluid and melt generation in response to an asthenospheric upwelling beneath the Hangai Dome, Mongolia

Abstract

The Hangai Dome, Mongolia, is an unusual high-elevation, intra-continental plateau characterized by dispersed, low-volume, intraplate volcanism. Its subsurface structure and its origin remains unexplained, due in part to a lack of highresolution geophysical data. Magnetotelluric data along a ~610 km profile crossing the Hangai Dome were used to generate electrical resistivity models of the crust and upper mantle. The crust is found to be unexpectedly heterogeneous. The upper crust is highly resistive but contains several features interpreted as ancient fluid pathways and fault zones, including the South Hangai fault system and ophiolite belt that is revealed to be a major crustal boundary. South of the Hangai Dome a clear transition in crustal properties is observed which reflects the rheological differences across accreted terranes. The lower crust contains discrete zones of low-resistivity material that indicate the presence of fluids and a weakened lower crust. The upper mantle contains a large low-resistivity zone that is consistent with the presence of partial melt within an asthenospheric upwelling, believed to be driving intraplate volcanism and supporting uplift.

Microstructures, Water Contents, and Seismic Properties of the Mantle Lithosphere Beneath the Northern Limit of the Hangay Dome, Mongolia

“…High melt contents in the sublithospheric mantle beneath the Hangay Dome are also not required by joint modeling of seismic velocities and topography (Fullea et al, ). The origin of the magneto‐telluric anomalies reported by Comeau et al () remains thus unexplained.…”

“…The deep lithosphere and the asthenosphere beneath the Hangay Dome are characterized by high electrical conductivity, which was interpreted by Comeau et al (2018) as most likely due to the presence of~6% melt (see Yoshino et al, 2006, for experimental calibration). Nevertheless, to produce such high partial melting degrees at >70-km depth, abnormal mantle temperatures and/or compositions, not consistent with the present data, are required.…”

“…Geochemistry, Geophysics, Geosystems of seismic velocities and topography (Fullea et al, 2012). The origin of the magneto-telluric anomalies reported by Comeau et al (2018) remains thus unexplained.…”

“…In addition to seismic investigations, magnetotelluric (MT) data were acquired along a 610‐km long, NNE to SSW profile cross cutting the Hangay Dome, imaging a large low‐resistivity zone (bulk resistivity 30–60 Ωm) beneath the dome at depths of 70–140 km (Comeau et al, ). Based on calculations by Gardés et al (), for a temperature of 1200 °C, unrealistic water contents over 350 ppm H 2 O by weight (ppm H 2 O wt.)…”

“…in olivine would be necessary to explain such low electrical resistivity. Therefore, Comeau et al () interpret the MT data as indicative of partial melting (~6% melt) at 70‐ to 120‐km depth, possibly resulting from an asthenospheric upwelling, which would control the volcanism and support the regional uplift.…”