卷 24, 编号 5 (2016)

Data on the composition, inner structure, and magma sources of giant batholith in the Central Asian Orogenic Belt are analyzed with reference to the Khangai batholith. The Khangai batholith was emplaced in the Late Permian–Early Triassic (270–240 Ma) and is the largest accumulations (>150000 km²) of granite plutons in central Mongolia. The plutons are dominated by granites of normal alkalinity and contain subalkaline granites and more rare alkaline granites. The batholith is hosted in the Khangai zonal magmatic area, which consists of the batholith itself and surrounding rift zones. The zones are made up of bimodal basalt–trachyte–comendite (pantellerite) or basalt-dominated (alkaline basalt) volcanic associations, whose intrusive rocks are dominated by syenite and granite, granosyenite, and leucogranite. Both the batholith and the rift zones were produced within the time span of 270–240 Ma. Although the rocks composing the batholith and its rift surroundings are different, they are related through a broad spectrum of transitional varieties, which suggests that that the mantle and crustal melts could interact at various scale when the magmatic area was produced. A model is suggested to explain how the geological structure of the magmatic area and the composition of the magmatic associations that make up its various zones were controlled by the interaction between a mantle plume and the lithospheric folded area. The mantle melts emplaced into the lower crust are thought to not only have been heat sources and thus induced melting but also have predetermined the variable geochemical and isotopic characteristics of the granitoids. In the marginal portions of the zonal area, the activity of the mantle plume triggered rifting associated with bimodal and alkaline granite magmatism. The formation of giant batholiths was typical of the evolution of the active continental margin of the Siberian paleocontinent in the Late Paleozoic and Early Mesozoic: the Khangai, Angara–Vitim, and Khentei batholiths were formed in this area within a relatively brief time span between 300 and 190Ma. The batholiths share certain features: they consist of granitoids of a broad compositional range, from tonalite and plagiogranite to granosyenite and rare-metal granites; and the batholiths were produced in relation to rifting processes that also formed rift magmatic zones in the surroundings of the batholiths. The large-scale and unusual batholith-forming processes are thought to have occurred when the active continental margin of the Late Paleozoic Siberian continent overlapped a number of hotspots in the Paleo- Asian Ocean. This resulted in the origin of a giant anorogenic magmatic province, which included batholiths, flood-basalt areas in Tarim and Junggar, and the Central Asian Rift System. The batholiths are structural elements of the latter and components of the zonal magmatic areas.

The Ulaan Tolgoi massif of rare-metal (Ta, Nb, and Zr) granites was formed at approximately 300Ma in the Eastern Sayan zone of rare-metal alkaline magmatism. The massif consists of alkaline salic rocks of various composition (listed in chronologic order of their emplacement): alkaline syenite → alkaline syenite pegmatite → pantellerite → alkaline granite, including ore-bearing alkaline granite, whose Ta and Nb concentrations reach significant values. The evolution of the massif ended with the emplacement of trachybasaltic andesite. The rocks of the massif show systematic enrichment in incompatible elements in the final differentiation products of the alkaline salic magmas. The differentiation processes during the early evolution of the massif occurred in an open system, with influx of melts that contained various proportions of incompatible elements. The magma system was closed during the origin of the ore-bearing granites. Rare-metal granitoids in the Eastern Sayan zone were produced by magmas formed by interaction between mantle melts (which formed the mafic dikes) with crustal material. The mantle melts likely affected the lower parts of the crust and either induced its melting, with later mixing the anatectic and mantle magmas, or assimilated crustal material and generated melts with crustal–mantle characteristics. The origin of the Eastern Sayan zone of rare-metal alkaline magmatism was related to rifting, which was triggered by interaction between the Tarim and Barguzin mantle plumes. The Eastern Sayan zone was formed in the marginal part of the Barguzin magmatic province, and rare-metal magmas in it were likely generated in relation with the activity of the Barguzin plume.

The paper presents detailed geochemical data on the rocks of the Zashikhinsky Massif and mineralogical–geochemical characteristics of the ores of the eponymous deposit. The rare-metal granites are divided into three facies varieties on the basis of the degree of differentiation and ore potential: early facies represented by microcline–albite granites with arfvedsonite, middle facies represented by leucocratic albite–microcline granites, and late (most ore-bearing) facies represented by quartz–albite granites grading into albitites. Microprobe data were obtained on major minerals accumulating trace elements in the rocks and ores. All facies of the rare-metal granites, including the rocks of the fluorite–rare-metal vein, define single compositional trends in the plots of paired correlations of rock-forming and trace elements. In addition, they also show similar REE patterns and spidergrams. The latter, however, differ in the depth of anomalies of some elements. Obtained geological, petrographic, and geochemical data suggest a magmatic genesis of the rocks of different composition and their derivation from a single magma during its differentiation. On the basis of all characteristics, the Zashikhinskoe deposit is estimated as one of the largest tantalum rare-metal deposits of alkaline-granite type in Russia.