RELICTS OF FLUORIDE-CALCIUM (FLUORITIC) AND SALT MELTS IN ROCKS OF THE ARY-BULAK MASSIF (EAST TRANSBAIKALIA)

Abstract

The relevance of the study of ongonites, which are subvolcanic analogs of rare-metal Li-F granites, is caused by obtaining new data on petrology and geochemistry of F and rare elements enriched with felsitic igneous rocks. Investigation of the processes associated with the separation of the initial homogeneous silicate melt into immiscible liquid phases with a contrasting composition is one of the important tasks of magmatic petrology. The aim of the research is to study the phenomena of fluoride-silicate liquid immiscibility during the evolution of ongonite magma based on the data of studies of melt and fluid inclusions in minerals. Objects: rocks of the Ary-Bulak massif, East Transbaikalia, Russia. Methods: thermometric study of melt and fluid inclusions, analysis of inclusions using laser ablation system (LA-ICP-MS), cathodoluminescent study of mineral zoning, determination of the composition of minerals, phases and silicate glasses by energy dispersive spectrometry (SEM EDS). Results. The inclusions of silicate melts, fluoride melts and saline brine-melts, as well as fluids coexisting with them in quartz and topaz were studied. Homogenization temperatures of inclusions: 350–400 °С for FI, 370–570 °С for saline brines-melts, 700–730 °С for melt inclusions up to 30 μm in size (no homogenization is observed in larger melt inclusions). The formation of the rocks of the massif involved P-Q type fluids – steam solutions, NaF-containing with an admixture of chlorides. Melt inclusion contains crystallization products of ongonite melt (quartz, sanidine, albite, fluorite, mica, villiomite, W-ixiolite). Melt inclusion glasses have a per aluminous composition, high contents of F (2–10 wt. %) and water (10–15 wt. %), some of them contain Cs2O (up to 5,6 wt. %) and As2O5 (14,4 wt. %). The authors have found out the inclusions of fluoride melts and brines-melts: with fluoride-calcium (F-Ca) phase or fluorite (prevailing), with aluminofluorides and phases with different ratios of chlorides Na, K, Cs, Mn, Fe, Ca, fluorides K, Al , Na. F-Ca inclusions are characterized by high concentrations of Y and LREE. The results of studying the inclusions indicate a fluoride-silicate liquid immiscibility in ongonite magma. It was found that, together with the ongonite melt, there were fluoride melts, brine-melts and water-salt fluids. The data obtained demonstrate the leading role of the F-Ca (fluorite) melt in the concentration of REE and Y in ongonite magma.

Keywords

• fluoride-silicate liquid immiscibility
• fluid and melt inclusions
• fluoride-calcium (f-ca) melt
• ongonite
• ary-bulak massif