Volume 26, Nº 4 (2018)

The Khangai batholith is one of the largest groups of granitoid plutons produced in Central Asia in the Late Permian–Early Triassic, at 270–240 Ma. The batholith occurs in the Khangai collage of Precambrian terranes, which include Early Precambrian crustal blocks (Dzabkhan and Tarbagatai) and Early to Late Neoproterozoic structures of the Songino block in their surroundings. The axial zone of this collage is overprinted by a basin filled with Devonian volcanic–siliceous rocks and Early to Middle Carboniferous terrigenous rocks. The isotopic parameters (Nd and Pb) of granitoids in the Khangai batholith indicate that the melts were derived from compositionally contrasting crustal sources and a single mantle one. The massifs hosted in the Precambrian blocks were produced with the involvement of lower crustal material, with various ages of the origin of the crust and its differentiation into upper and lower ones. The crust of the Tarbagatai and Dzabkhan blocks was produced in the Early Archean and was differentiated at the Archean–Proterozoic boundary. The crust of the Songino block was formed in the Paleoproterozoic and differentiated in the Early Neoproterozoic. According to the Pb and Nd isotopic parameters of granitoids in the Khangai Basin, the regional continental crust was close to the juvenile one, i.e., the continental crust of the Khangai Basin had still not been differentiated by the time when the Khangai batholith was produced. A single mantle source was involved in the origin of the melts of granitoids of the Khangai batholith in various tectonic blocks. The evolution of the Pb isotopic composition of this sources is consistent with the Stacey–Kramers model at µ = 9.5. This source can be identified with the enriched mantle, which has a higher U/Pb ratio than the depleted mantle and lower ε_Nd(T) of 0 to +2.

An Early Cretaceous (120 ± 5 Ma) trachyrhyolite lava sheet in the Nyalga basin, Central Mongolia, includes a domain (∼0.5 km²) of unusual fluorite-enriched rocks with anomalously high concentrations of CaO (1.2–25.7 wt %) and F (0.6–15 wt %). The textures and structures of the rocks suggest that they were produced by two immiscible melts: fluoride–calcium (F–Ca) and trachyrhyolitic. Data on mineral-hosted inclusions and SEM EDS studies of the matrixes of the rocks indicate that a F–Ca melt occurred in the trachyrhyolitic magmas during its various evolutionary episodes, starting from the growth of minerals in a magmatic chamber and ending with eruptions on the surface. Elevated fluorine concentrations (up to 1.5–2 wt %) in local domains of the trachyrhyolitic melt may have resulted in the onset of its liquid immiscibility and the exsolution of a F–Ca liquid phase. This was associated with the redistribution of trace elements: REE, Y, Sr, and P were preferably concentrated in the F–Ca melt, while Zr, Hf, Ta, and Nb were mostly redistributed into the immiscible silicate liquid. The F–Ca melt contained oxygen and aqueous fluid and remained mobile until vitrification of the trachyrhyolitic magma. The oxygen-enriched F–Ca phase was transformed into fluorite at 570–780°? and a high oxygen fugacity Δlog fO₂ (0.9–1.7) relative to the NNO buffer. Ferrian ilmenite, monazite-group As-bearing minerals, and cerianite crystallized under oxidizing conditions, and the titanomagnetite was replaced by hematite. The Ca- and F-enriched rocks were affected by low-density (0.05–0.1 g/cm³) aqueous fluid, which was released from the crystallizing trachyrhyolitic melt, and this led to the partial removal of REE from the F–Ca phase. The chondrite-normalized REE and Y patterns of the fluidmodified rocks show positive Y anomalies and W-shaped minima from Gd to Ho. A composition of the F–Ca phase close to the original one is conserved in mineral-hosted inclusions and in relict isolations in the rocks matrix. It is so far unclear why fluorite did not crystallize from the F–Ca melt contained in the trachyrhyolitic magma. Conceivably, this was favored by high-temperature oxidizing conditions under which the melt accommodated oxygen and aqueous fluid. The possible origin of mobile oxygen-bearing fluorite–calcic melt at subsolidus temperature should be taken into account when magmatic rocks and ores are studied. Fluorite and accompanying ore mineralization might have been formed in certain instances not by hydrothermal–metasomatic processes but during the fluid–magmatic stage as a result of the transformation of F–Ca melt enriched in REE, Y, and other trace elements.

In a number of industries (ferrous and nonferrous metallurgy, glass-making and silicate-producing technologies), interaction between refractory materials with melts results in sequences of reaction zonation (reaction columns) that show all principal features of diffusion-controlled metasomatic zoning. However, in contrast to the latter, reaction melt is generated together with crystalline phases in the rear zones of the columns. This melt is neither mechanically displaced melt that affects the refractory materials, nor produced by melting. The process generating this melt is most adequately defined as replacement by melt. The principal characteristics of the zoning are discussed below with reference to the corrosion of chromite–periclase refractory materials with melted slag in nickel-producing metallurgy. Similarities between the relations observed under different conditions and in different systems and the evolutionary dynamics of the process, specifics of melt generation and changes in its composition in the zones are demonstrated below with the use of data on other technologies and their experimental modeling. The mechanism of melt replacement is applicable to describing natural reaction processes of magma interaction with host rocks (magmatic replacement), with the following unobvious implications. (1) It is reasonable to expect that the minerals of the rocks should host melt inclusions. (2) It is reasonable to expect that certain minerals should be found in two distinct populations: (i) those in equilibrium with melt in the reaction column and (ii) those crystallizing from the cooling melt. (3) Two or more zones of the column can consist of the same minerals, but their proportions should be different. (4) Plastic deformations in the rear zones of the column (magmatic replacement) should be associated with brittle ones in the pristine host rocks and frontal (metasomatic) zones. (5) In contrast to the rocks of metasomatic columns, the material of magmatic-replacement zones can flow through fractures cutting across the host metasomatic rocks and thereby intersect the outer metasomatic zones.