Age, Composition, and Formation Mechanisms of the Bural Alkaline Granitoid Complex, Eastern Sayan Rare-Metal Province, with Reference to the Problem of Rare-Metal Magma Generation

The Bural alkaline granitoid complex is located in the southwestern part of the Eastern Sayan rare-metal province and makes up a compact field of dike and stock-shaped bodies of alkaline syenites, granites, and leucogranites with subordinate gabbromonzonites. The age of the rocks is estimated at 301 ± 4 Ma, which is close to the age of other alkaline granitoids in the region, including the Ulug-Tanzek and Ulan-Tologoy rare-metal deposits. The Bural complex comprises two groups of alkali-salic rocks, differing in contents of trace elements, primarily REE. The first group includes rocks whose REE contents exceeding 185 ppm. They compose a differentiated series enriched of REE, Nb, Ta, Zr, Th, and U in the final silicic (alkaline-granite) derivatives, formed as a result of fractionation of mafic melts. The other group includes leucogranites and syenites “depleted” in REE. The leucogranites correspond to eutectic melts and have features typical of rare-metal magmas of the metaluminous series: strongly reduced REE contents (<35 ppm) at relatively high concentrations of Rb, Cs, Ta, and Nb. The associated “depleted” syenites have higher contents of incompatible elements and resulted from interaction between leucogranite melts and melts of the differentiated series. A model proposed to explain the origin of the Bural complex assigns a leading role in the development of magmatic system to differentiated subalkaline mafic melts. On the one hand, they triggered the anatectic melting of the crust leading to the formation of metaluminous rocks, and on the other, they differentiated to alkali-salic melts enriched in rare elements.