Tectonic implications of garnet-bearing mantle xenoliths exhumed by Quaternary magmatism in the Hangay dome, central Mongolia

Garnet-bearing mantle xenoliths have been recovered from Quaternary alkali basalts, both within and peripheral to the Hangay dome of central Mongolia. Microfabric analysis and thermobaromery, combining empirical thermobarometers and the self-consistent dataset of THERMOCALC, indicate that garnet websterites from the Shavaryn-Tsaram volcanic centre at the dome core were formed in the spinel-lherzolite upper mantle at pressures of 17-18 kbars and temperatures of 1,070-1,090A degrees C, whereas garnet lherzolites were derived from greater depths (18-20 kbars). Garnet lherzolites from the Baga Togo Uul vents near the dome edge were formed at 18-22 kbars under significantly cooler conditions (960-1,000A degrees C). These xenoliths reveal reaction coronas of (1) orthopyroxene, clinopyroxene, plagioclase and spinel mantling garnets; (2) spongy rims of olivine replacing orthopyroxene and (3) low-Na, low-Al clinopyroxene replacing primary clinopyroxene. Trace-element abundances indicate that clinopyroxene from these coronas is in chemical equilibrium with the host magma. The thermobarometric and textural data suggest that lherzolite xenoliths from both sites were derived from depths of 60-70 km and entrained in magma at 1,200-1,300A degrees C. The average rate of ascent, as determined by olivine zoning, lies in the range 0.2-0.3 m s(-1). The contrast in thermal profiles of the upper mantle between the two sites is consistent with a mantle plume beneath the Hangay dome with elevated thermal conditions beneath the core of the dome being comparable to estimates of the Pleistocene geotherm beneath the Baikal rift.