THE KHALDZAN-BUREGTEI MASSIF OF PERALKALINE RARE-METAL IGNEOUS ROCKS: STRUCTURE, GEOCHRONOLOGY, AND GEODYNAMIC SETTING IN THE CALEDONIDES OF WESTERN MONGOLIA

Abstract

The composition, age, and geological setting is studied for the Khaldzan-Buregtei group of massifs of peralkaline rare-metal granitoids. The massifs are confined to a large fault zone separating the Late Caledonian structures of the Mongolian Altai and Early Caledonian structures of the Ozernaya Zone. The massifs are hosted by the rocks of ophiolite complex and granitoids various in age and composition. The geochronological (U-Pb and Ar-Ar) studies demonstrated the multistage magmatic history of the region. The ophiolites in the basement of the Caledonides were formed within 570-520 Ma, and the island-arc complexes situated near the alkaline massifs originated at 546 Ma ago (Ar-Ar method). The collision in the region was accomplished by emplacement of tonalites and granodiorites (495 Ma, U-Pb method). Granites of normal alkalinity were emplaced at later stages at about 465 +/- 11, 441 +/- 2, and 355.4 +/- 1.4 Ma (U-Pb method). The alkali granitoid massifs have multiphase structures and include rare-metal ores. The Khaldzan-Buregtei massif was formed in the following sequence: (1) nordmarkites with coeval dolerites, (2) peralkaline granites with coeval dolerites, (3) ekerites, fine-grained peralkaline granites and pegmatites, (4) pantellerites, (5) rare-metal peralkaline granites, (6) potassium alkali basic rocks and, conventionally, leucocratic syenites and (7) miarolitic rare-metal peralkaline granites. The minerals of Nb, Zr, REEs, and, particularly, Y, U, Th, and Be are the most valuable ore minerals in the massif. The U-Pb ages were determined for the following rocks of the massif. 392.2 +/- 2.3 Ma for peralkaline granites of the second phase, 390.8 +/- 1.2 Ma for syenites of the sixth phase, and 395.0 +/- 1.3 Ma for quartz-zircon-orthite-epidote ore. The Ar-Ar method gave consistent results or some older ages. All these data constrain the age of the massif within 391-395 Ma. The Khaldzan-Buregtei group of alkali rock massifs belongs to the Early-Middle Paleozoic Altai-Sayan intraplate region of the Central Asian Caledonides. The distribution of the massifs and an associated wide dike swarm is controlled by an extension zone, the southern branch of the triple junction in this region.