The paper reports the results of melting experiments on the fluorite-enriched rhyolites of the Nyalga Basin (Central Mongolia). The fluoride–calcium (F–Ca) melt was obtained in a wide range of P-T parameters (1250–750°С, 5.5–1 kbar). Fluoride–silicate liquid immiscibility was observed at F > 2.5 wt % and CaO > 5.3 wt % in the initial system. An increase of temperature and pressure was accompanied by a significant increase in the concentrations of REE, Y, Sr, P, Th, U, Nb, Co, Cu, Sn, Sb, and Mo in the F–Ca melt. The peculiarity of the D_REE distribution coefficients between F–Ca and silicate melts can lead to the formation of M-type tetrad effects for the first, third, and fourth tetrads in the chondrite-normalized REE patterns of silicate melt. The F–Ca melt has existed up to subsolidus temperatures of the rhyolitic melt. None of the models of magmatic crystallization of fluorite in haplogranitic melts or subsolidus postmagmatic and hydrothermal fluoritization can explain the origin of fluorite-enriched rhyolites. It is suggested that these rocks were formed from magma containing the emulsion of rhyolitic and F–Ca melts. The fluoride–silicate liquid immiscibility resulted in the redistribution of trace elements (REE, Y, Sr, P, Zr, Hf, Ta, Nb, Sc, Li, Be, and Rb) between melts. The formation of rock matrix was accompanied by degassing of the rhyolitic melt. An effective viscosity of the magma (melts emulsion with fluid bubbles) is comparable with the viscosity of a liquid. The F–Ca melt was quenched into F–Ca phase consisting of submicron fluorite particles, while solidification of the rhyolitic melt and silicate glass devitrification resulted in the formation of quartz–sanidine symplectites. The F–Ca phase has the elevated contents of O, Sr, LREE, Y, Si, sometimes Sc, P, and Al. The isomorphous substitution of O^2– → F^– in the fluorite structure led to the formation of aggregates of oxygen-vacancy centers, which are responsible for the laser-induced luminescence of the F–Ca phase in the rhyolite matrix. The wide variations of the REE, Y, Sr, Th, Nb, Ta, Zr, and Hf contents in the F–Ca phase are related to its recrystallization under the effect of fluid, which was released during degassing of rhyolitic melt. The submicron-sized fluorite particles in the F–Ca phase during the interaction with fluid were gradually liberated from impurities (besides Sr) and transformed into larger crystalline segregations. It is suggested that the oxygenated F–Ca melt exists in a metastable supercooled state under oxidizing conditions during eruption of rhyolitic magma. This is inconsistent with previously obtained experimental data on crystallization of fluorite from CaF₂- and H₂O-saturated haplogranitic melts at < 950°С and 1–2 kbar. Using rhyolites as example, it is shown that fluorite and associated ore mineralization (monazite-group minerals, cerianite) were derived from F–Ca melt with elevated REE and Y contents. In many igneous rocks and magmatogenic ores, fluorite could be a product of transformation of the F–Ca melt.

中文翻译：

---

实验数据和流纹岩中氟化物-钙熔体组成的演变

本文报道了尼亚加盆地（中蒙古）富含萤石的流纹岩的熔融实验结果。氟化物-钙（F-Ca）熔体是在多种PT参数（1250-750°С，5.5-1 kbar）下获得的。在初始系统中，F> 2.5 wt％且CaO> 5.3 wt％观察到氟化物与硅酸盐液体的不溶混性。温度和压力的增加伴随着F-Ca熔体中REE，Y，Sr，P，Th，U，Nb，Co，Cu，Sn，Sb和Mo浓度的显着增加。D _REE的独特性F–Ca和硅酸盐熔体之间的分布系数会导致在硅酸盐球粒陨石归一化REE模式中，第一，第三和第四四分体形成M型四分体效应。F-Ca熔体一直存在到流纹熔体的亚固相线温度以下。萤石质熔岩中的萤石岩浆结晶或亚固相后岩浆和水热氟化的岩浆结晶模型都不能解释富含萤石的流纹岩的起源。这表明这些岩石是由岩浆形成的，岩浆中含有流纹岩和F-Ca熔体的乳状液。氟硅酸盐液体的不混溶性导致熔体之间痕量元素（REE，Y，Sr，P，Zr，Hf，Ta，Nb，Sc，Li，Be和Rb）的重新分布。岩石基质的形成伴随着流纹质熔体的脱气。岩浆的有效粘度（具有气泡的熔融乳液）与液体的粘度相当。F-Ca熔体被淬灭到由亚微米萤石颗粒组成的F-Ca相，而流变熔体的凝固和硅酸盐玻璃的失透导致形成了石英-山梨共沸物。F-Ca相的O，Sr，LREE，Y，Si，Sc，P和Al含量较高。O的同构取代 P和Al。O的同构取代 P和Al。O的同构取代^{2 –} →F ^–萤石结构中的碳原子导致氧空位中心聚集体的形成，这是流纹岩基质中F-Ca相的激光诱导发光的原因。F-Ca相中REE，Y，Sr，Th，Nb，Ta，Zr和Hf含量的广泛变化与其在流质熔体脱气期间释放的流体作用下的重结晶有关。与流体相互作用时，F-Ca相中的亚微米级萤石颗粒逐渐从杂质（Sr除外）中释放出来，并转变为较大的晶体偏析。建议在流纹岩浆喷发过程中，在氧化条件下，氧化的F-Ca熔体以亚稳态过冷状态存在。这与先前获得的有关从CaF中萤石结晶的实验数据不一致<950°С和1-2 kbar时_2-和H ₂ O饱和的触木质熔体。以流纹岩为例，结果表明，萤石和相关的矿石矿化（独居石类矿物，陶粒）是从稀土元素和Y含量较高的F-Ca熔体中衍生出来的。在许多火成岩和成岩矿石中，萤石可能是F–Ca熔体转变的产物。