Detailed studies have been made on a number of alkalic volcanic fields in the Gobi Desert, Mongolia. These volcanics, described in Russian literature as Cenozoic, are found in close proximity to large-scale east-west trending strike-slip faults. The area studied is of particular interest because it is located close to the Hangay Dome, the topographic expression of a proposed mantle plume (Windley and Allen, 1993).
The Gobi Desert is cut by a prominent array of Cenozoic left-lateral strike slip faults, which are accommodating an eastward displacement of crust relative to the stable Siberian craton, to the north (Cunningham et. al., 1996). These appear to be associated with parallel structures seen in the Tien Shan Mountains and Tibet, which have been interpreted as the accommodation of the indentation of India, by escape tectonics (Molnar and Tapponnier, 1975).
Within a region of 6000 km2, three volcanic fields have been documented, each of c. 100 km2. Two of these fields comprise basaltic plateaus, built of at least five, 8-12 m thick, flows. The third area consists of fissure-erupted alkalic basalts with associated dykes found parallel to the major strike-slip faults, linking volcanic centres. These dykes show evidence of magma mixing, and a vesicular texture which indicates near-surface emplacement.
Eighty samples were collected from the three volcanic fields during the 1995 field season. A variety of mantle and crustal xenoliths, < 20 cm in size, were found in concentrations from both plateau and fissure settings. The basalts displayed variable vesicularity and impressive columnar jointing. Trachytic textures in an olivine-rich groundmass were seen particularly in the fissure eruptives. Geochemical data will allow us to evaluate the roles of lithosphere and (plume?) asthenosphere in the formation of the basalts. We would predict that the long time span of the volcanic activity (as much as 50 my) may have facilitated substantial mobilisation and incorporation of lithospheric mantle by the underlying thermal anomaly.
Cunningham, W.D. et al., Earth Planet. Sci. Lett . (1996, in the press).
Molnar, P. & Tapponnier, P., Science 189, 419-426 (1975).
Windley, B. & Allen, M., Geology 21, 295-298 (1993).