DISTRIBUTION AND EVOLUTION OF ZIRCONIUM MINERALIZATION IN PERALKALINE GRANITES AND ASSOCIATED PEGMATITES OF THE KHAN BOGD COMPLEX, SOUTHERN MONGOLIA

摘要

The western part of the Khan Bogd complex, in the south Gobi desert, southern Mongolia, comprises (in order of emplacement): microcline-phyric granite, peralkaline aegirine-arfvedsonite granite (main intrusive phase) and aplite-pegmatite veins confined predominantly to the apical parts of the intrusion. These rocks are interpreted to represent products of extreme fractional crystallization involving alkali feldspar, quartz and, to a lesser extent, ferromagnesian silicates and, in the final stages of magma evolution, release of a silica-saturated orfhomagmatic fluid. Geochemically, this evolutionary path involved an increase in peralkalinity index (from 1.0 in the porphyritic granite to 1.3 in the main phase to 2.5 in the pegmatite) and whole-rock Zr content (864 to 1130 to 16900 ppm,respectively). In the porphyritic granite, zircon with alow Hf content (Zr/Hf in the range 55-75) is the principal Zr phase, whereas alkali-Ca zirconosilicate minerals are characteristically absent.