LOWER CRYSTAL XENOLITHS FROM MONGOLIA AND THEIR BEARING ON THE NATUREOF THE DEEP DUST BENEATH CENTRAL-ASIA

Rare lower crustal xenoliths found in Cenozoic alkali basalts from the Tariat region in central Mongolia and the Dariganga Plateau in south- eastern Mongolia are the only direct samples of lower crustal material known so far from central and eastern Asia. They are two-pyroxene gra nulites, including some garnet granulites, as well as scarce amphiboli te-facies rocks. The xenoliths are broadly basaltic to andesitic in bu lk chemical composition. Their igneous protoliths appear to represent underplated fractionated liquids and cumulates from such liquids. The xenoliths yield equilibration temperatures of 840 +/- 30 degrees C (We lls, 1977) and, for Tariat garnet granulites only, pressures of 14 +/- 1.5 kbar. For central Mongolia, these estimates indicate unusually gr eat depths of origin which, however, are in line with some geophysical models for that area. Extensive to complete kelyphitisation has affec ted the garnets where originally present in the Tariat suite; neverthe less, the kelyphite has largerly preserved the major element and REE c ompositions of the original garnet. Mineral and whole-rock Sm-Nd data obtained for three samples from Tariat and Dariganga indicate, within large errors, low or zero ages. These may either indicate that the roc ks are young (Cenozoic) or that ambient temperatures in the lower crus t were high enough to permit continuous isotopic equilibration on a mi neral-to-mineral scale.