Geophysical imaging of a continental intraplate basaltic volcanic system from source to surface

Abstract

Quaternary–Neogene aged, alkaline basalt flows and clusters of volcanic cones are found in Central Mongolia, which is in the continental interior thousands of kilometres from active tectonic margins [1]. Using a high-resolution, multi-scale, magnetotelluric dataset, we generate both 3-D and 2-D electrical resistivity models of the lithosphere and uppermost mantle beneath this region [2, 3]. We focus on two volcanic fields that are separated by about 100 km. By examining the geophysical models and considering the available evidence we propose the following: i) narrow, vertical, low-resistivity anomalies located in the upper–middle crust beneath the surface expressions of volcanism represent the remnant signatures of ancient, ephemeral magma pathways (or collection of pathways) and record the location of magma ascent; ii) widely distributed low-resistivity zones in the lower crust can be explained by very small amounts of saline fluids in a thermally perturbed region, and are hard to reconcile with magma storage; and iii) a local low-resistivity zone in the lithospheric mantle and a broad, doming low-resistivity feature in the uppermost mantle are interpreted to represent a metasomatized lithospheric mantle and a mantle upwelling and thermal anomaly explained by low-percent partial melt, which is inferred to be the source for intraplate volcanism. Thus the geophysical images reveal magma generation and transport from a mantle source to the surface beneath a continental intraplate basaltic volcanic system. Furthermore, they are consistent with geochemical and petrological evidence from erupted lavas [1] that point to a single common mantle source region for both volcanic zones and a volatile-enriched, metasomatized sub-continental lithospheric mantle, as well as limited crustal contamination and rapid magma ascent.