Cenozoic volcanism in Mongolia is part of a diffuse volcanic province in central Asia including northern China and Siberia, south of the Baikal Rift.  This province is predominantly basaltic and commonly alkaline including trachybasalts, basanites, tephrites, and hawaiites.  Most of the volcanic fields are low volume centers with cinder cones and lava sequences tens of meters thick.  In a belt across central Mongolia, this province can be divided into three areas: the Khovsgol Nuur area in the north, the Khangay Nuruu in central Mongolia, and the Gobi Altai in the south.  The origin of this diffuse volcanic province is equivocal.  The province traverses areas of diverse Cenozoic tectonism including transpression in the Gobi Altai and rifting in Khangay Nuruu and Khovsgol Nuur area.  This suggests that magmatism is driven by sublithospheric processes.  A mantle plume has been invoked to explain the province.  Barry et al. (2002) provide several lines of evidence against a plume and develop a model for magmatism driven by a shallow thermal anomaly heating metasomatized lithosphere.

Figure 2. Volcanic rocks of the Khangay Mountains region (after Whitford-Stark, 1987)

Cenozoic volcanic rocks are found on both the northern and southern flanks of the Khangay Nuruu (Fig. 2).  On the north side of the range are four Miocene to Recent volcanic fields which are generally confined to river valleys and half grabens.  The largest and most extensive of these volcanic fields lies in the Tariat depression. Some Tariat lavas have abundant crustal and mantle xenoliths.  The Shavryn Tsaram site has produced diamond-bearing xenoliths.

On the southern flank of the Khangay Nuruu is a dissected series of plateau basalts that within an Oligocene to Miocene sedimentary sequence.  The Oligocene lavas are interbedded with red-beds of the Shand-Gol Suite and have K-Ar ages of between 31 and 24 Ma.  Miocene lavas make up the upper stratigraphy of the plateau of the southern Khangay.  These lavas have K-Ar ages of 20 to 17 Ma.  The south Khangay basalts are of diverse composition including trachybasalts, limburgites, analcime “andesite basalts”, and sanidine- and biotite-bearing basalts, clearly reflecting extremely potassic compositions.  These Tertiary lavas will be the focus of the petrology portion of this project.