Fluoride-calcium (F-Ca) melt in rhyolitic magma: Evidence from fluorite-rich rhyolites of the Nyalga Basin, Central Mongolia

Abstract

Unusual varieties of Early Cretaceous (120 ± 3 Ma) rhyolites in the Nyalga basin (Central Mongolia) bear exceptionally high percentages of fluorite reaching 30–36 wt%. The origin of fluorite in the rhyolites does not fit the existing models of magmatic crystallization in haplogranitic melts or subsolidus postmagmatic and hydrothermal fluoritization. We suggest that the fluorite-enriched rhyolites formed after eruption of magma that contained a mixture (emulsion) of immiscible rhyolitic and F-Ca melts. The F-Ca melt separated from the rhyolitic magma as a result of fluoride-silicate liquid immiscibility when the concentration of F in the Ca-bearing subaluminous–peraluminous rhyolitic melt locally increased to 1–2 wt%. Trace and minor elements, mainly REE, Y, Sr, P, Zr, Hf, Ta and Nb partitioned between the two immiscible melts. The F-Ca melt existed in the course of rhyolitic magma evolution from the crystallization of phenocrysts in the magma source till eruption and quenching on the Earth's surface. The effective viscosity of the magma (mixed rhyolitic and F-Ca melts with fluid bubbles) may have been of the same order of magnitude as that of viscous liquid during the formation of the rhyolite matrix at 750–650°С. The quenching of the F-Ca melt formed an F-Ca phase consisting of submicrometer fluorite particles, while crystallization of rhyolitic glass produced quartz-sanidine symplectites. According to SEM EDS and LA-ICP-MS data, the F-Ca phase typically shows large ranges of O, Y, La, Ce, Nd, Sr, Sc, P, Si (wt%) and trace element (ppm) concentrations varying for orders of magnitude. The variations result from recrystallization of the F-Ca phase under the effect of a low-density (0.05–0.1 g/cm3) aqueous fluid released from the degassing rhyolitic melt. Upon interaction with the fluid, micrometer fluorite particles of the F-Ca phase liberated from impurities and transformed into larger crystalline segregations. The process occurred within the 570–780°С temperature range at a high oxygen fugacity of ΔlgfO2 Ni-NiO (0.9–1.7). The oxidized environment maintained crystallization of ferrian ilmenite, monazite-group As-bearing minerals, and cerianite, as well as replacement of titanomagnetite by hematite. The O2− → F− substitution in the fluorite structure led to the formation of O-vacancy centers responsible for luminescence of the F-Ca phase in the rhyolite matrix upon laser excitation. The compositions close to the primary F-Ca phase have preserved in relict globules existing as inclusions in the matrix and in enclosed minerals. The F-Ca melt could exist in a metastable liquid state during the eruption of rhyolitic magma at an oxygen fugacity remaining high till the subsolidus temperature of the rhyolitic melt. The fluoride-silicate liquid immiscibility with participation of an oxygenated F-Ca melt may be a common feature of F-rich silicate magmas at high oxygen fugacity. Fluorite and related mineralization in many igneous rocks and magmatic ores possibly resulted from transformation of the F-Ca melt enriched in REE, Y and other minor elements.