G. S. Ripp

The phosphate and sulfate-phosphate minerals in the sillimanite-bearing rocks of the Kyakhta deposit are considered. The mineral assemblages of the high-Al rocks were formed during prograde and retrograde stages of metamorphism. The first stage is characterized by the formation of sillimanite, corundum, muscovite, quartz, rutile, titanohematite, magnetite, feldspar, biotite, lazulite, and wagnerite. The muscovite composition showed that sillimanite paragenesis was formed at temperatures above 510–600°C. According to oxygen isotope thermometry, the minimum metamorphic temperature for quartz and titanohematite is 690°C. Andalusite, diaspore, quartz, pyrophyllite, muscovite, and a wide range of phosphates and sulfate-phosphates crystallized during the retrograde stage. The decrease in temperature and increase in the water content led to the following sequence of mineral formation: Mg-Fe-Al-Ca-REE-rich phosphates (lazulite, scorzalite, augelite, apatite, and monazite) → Ca-Sr sulfate-phosphates (woodhouseite and svanbergite) → sulfate (barite) → Sr-Ca-Ba aluminophosphates (goyazite, crandallite, and gorceixite). The chemical compositions of phosphates and sulfate-phosphates minerals and their formation conditions are discussed.

The paper reports Ar-Ar, Rb-Sr, and U-Pb (SHRIMP II) geochronologic data on carbonatites in Transbaikalia, related metasomatically altered rocks, and comagmatic silicate alkaline rocks. Metamorphic processes at two carbonatite occurrences were dated at 550–559 Ma (U-Pb and Rb-Sr methods). Geochronologic data make it possible to distinguish two major epochs when carbonatite were formed: Late Mesozoic in southwestern Transbaikalia and Late Riphean-Vendian in northern Transbaikalia. Small carbonatite occurrences are also known in the Vitim and Baikal alkaline provinces, which were formed in the Middle-Late Paleozoic. The Late Mesozoic carbonatite-forming epoch is definitely correlated with the development of the Western Transbaikalia rift structure and the Late Riphean-Vendian epoch, with the breakup of Laurasia in the Late Riphean.

The composition and nature of high-Cr minerals in lithic clasts from the carbonatites of the Veseloe occurrence, northern Transbaikalia, were considered. In order to determine their source, the Cr-bearing phases were compared with chromite, magnetite, and rutile from ultrabasic rocks, mantle xenoliths, and eclogites. It was suggested that the xenoclasts studied were formed at great depths, whereas the carbonatites were directly derived from the mantle rather than formed by the crustal differentiation of a silicate-carbonate melt.

This paper presents the results of a study of the Vesely carbonatite occurrence in a new carbonatite-bearing area of northern Transbaikalia. The REE patterns, oxygen and carbon isotope compositions of carbonates and magnetites, and strontium isotope composition of dolomite allow us to classify the rocks as carbonatites. This study focused on minerals from xenoliths that were identified as mantle-derived on the basis of structure and mineral composition. The compositions of Cr-bearing chlorite, phengite, magnetite, rutile, ilmenite, and titanite are reported. Aggregates of closely intergrown magnetite and rutile occurring in the xenoliths resemble exsolution structures. A possible origin of these aggregates is discussed.