Two types of magma sources of rare-metal alkali granites

Abstract

The geological setting, age, and magma sources of rare-metal ores at the Khalzanburgetdei deposit in western Mongolia and the Khan Bogd occurrence in southern Mongolia are considered. The Khalzanburgetdei deposit, 392 Ma in age, is situated in the Lake Zone of the Early Caledonides of the Central Asian Foldbelt at a branch of a triple junction of grabens. The Khan Bogd occurrence, about 290 Ma in age, is located in the South Mongolian Hercynian Zone and related to a continental rift conjugated in space and time with Hercynian island-arc systems. To estimate features of rare-metal magma sources, the Nd and O isotopic compositions were used, as well as ratios of incompatible elements (Nb-U, Nb-Zr, La-Yb, Th-Ta, and Ce-Pb). Since the granitic magma was commonly saturated with these elements, their ratios may be used for estimation of magma source compositions, trends of magma fractionation, and accumulation of rare-metal minerals. A mixture of an OIB source and crustal material (Caledonian ophiolitic and island-arc complexes) served as a source of rare-metal granitic rocks of the Khalzanburgetdei deposit, while the Khan Bogd occurrence was related to a source of subduction-related basic rocks, probably, with participation of the depleted and enriched mantle and continental crust.