I. S. Peretyazhko

Immiscibility of calcium fluoride and aluminosilicate melts in ongonite from the Ary-Bulak intrusion, Eastern Transbaikal region

The addition of fluorides of Na, K, Ca, and other elements into silicate melts lowers the degree of their homogeneity owing to the formation of sybotaxic groups enriched in fluorine and cations-modifiers [1]. Many fluoride–silicate systems are characterized by a microheteregoneous structure in the hyperliquidus region related to immiscibility [2]. Under certain conditions, the evolution of natural magmatic systems gives rise to the formation of fluoride melts. Such examples are not numerous and mostly pertain to the effects of immiscibility in sodic fluoride melts (melts– brines) associated with peralkaline granitic magmas. The existence of calcium fluoride melts is confirmed by inclusions of fluoritic glass in mantle xenoliths captured by alkali basalt in New Zealand [3]. As was suggested previously, the residual glasses in ongonite from the Ary-Bulak intrusion were formed as a result of quenching of a calcium alumino-silicofluoride melt consisting of Ca, F, Al, and Si with an admixture of Na and K [4]. In this communication, we present new data on the active participation of calcium fluoride melts in crystallization of this rock.

Borocookeite, a new member of the chlorite group from the Malkhan gem tourmaline deposit, Central Transbaikalia, Russia

Abstract Borocookeite, ideally Li1+3xAl4-x(BSi3)O10(OH,F)8 (where “x” varies in the range 0.00-0.33 apfu), in which [4]Al is replaced by B relative to cookeite, occurs as a late-stage pocket mineral in the Sosedka and Mokhovaya pegmatite veins, Malkhan gem tourmaline deposit, Chikoy district, Chita oblast, Russia. Borocookeite proper, as well as boron-rich cookeite, is light grey with a pinkish or yellow hue and occurs as a dense, massive crypto-flaky aggregate or thin crusts and snow-like coatings on crystals of quartz, tourmaline, and feldspars from miarolitic cavities. Fragments of elbaite, danburite, and albite are included in the borocookeite mass. In some pockets, the coating is composed of borocookeite and boron-rich muscovite (or boromuscovite) which are not distinguishable visually. Chemical analysis yields (wt%): SiO2 34.19, TiO2 0.02, Al2O3 41.77, FeO 0.06, MnO 0.07, MgO 0.04, CaO 0.08, Na2O 0.01, K2O < 0.01, Li2O 4.65, Rb2O 0.004, Cs2O 0.005, B2O3 4.06, BeO 0.05, H2O+ 14.17, H2O- 0.11, F 1.22, -O = F 0.51, total 100.00. The empirical formula calculated on the basis of 28 positive charges is: Li1.61Al3.80(Al0.44B0.60Be0.01Si2.95)Σ4.00O10[F0.33(OH)7.81]Σ8.14. The unitcell parameters, calculated from X-ray powder diffraction data, are a = 5.110(4), b = 8.856(3), c = 14.080(6) Å, b = 96.93°(4) these values are smaller than those for cookeite. Dcalc = 2.69(1) g/cm3. The mineral has a Mohs hardness of 3, light pinkish-grey streak, greasy luster, perfect (001) cleavage, and no parting or fracture. Optical properties (for white light): α = 1.574, β = 1.580, γ = 1.591 (all ± 0.002), 2Vcalc = 72°, dispersion not determined. The optical sign and the angle of the optical axis were not measured because of the small size and strong curvature of the mineral flakes. Borocookeite, as well as other boron-rich phyllosilicate minerals, crystallized from evolved residual solutions in miarolitic cavities at temperatures not less 265-240 °C. The ratio of activities of K, Li, B, F, and H2O in the mineral-forming fluids of isolated evolving pockets determined whether borocookeite or boromuscovite formed separately or together.

First 40Ar/39Ar age determinations on the Malkhan granite-pegmatite system: Geodynamic implications

The Malkhan granite-pegmatite system located in Central Transbaikalia, in the southwestern portion of the Malkhan-Yablonovy structure-formational zone of the Caledonian folding comprises two granite massifs (Bolsherechensk and Oreshny) and a miarolitic pegmatite field of the same name, which adjoins the Chikoi deep-seated fault and Lower Cretaceous Chikoi rift depression in the north. The first 40Ar/39Ar data were obtained on porphyritic biotite granites of the Oreshny massif and on K-feldspar, muscovite, and lepidolite from the Oktyabrskaya pegmatite vein. According to these data, the age of the granitepegmatite system is 123.8–127.6 Ma, which is consistent with the age of Lower Cretaceous rocks from the Chikoi depression. The intimate spatial relationship and isochronism between the Chikoi depression and the Malkhan granite-pegmatite system are strongly suggestive of a rift regime that affected its evolution, thus highlighting the need to regard the evolution of this system as being intimately related to depression development. Such a model can easily be realized within the framework of the concept of a metamorphic core complex, which was used to explain the nature of Transbaikal-type rift depressions and conjugate granite-gneiss swells.

A First Finding of Anomalously Cs-Rich Aluminosilicate Melts in Ongonite: Evidence from Melt Inclusion Study

The chemical properties of cesium allow accumulation of this element in the late silicic derivatives of igneous complexes, in particular, in rare-metal granites, pegmatites, and related metasomatic rocks. However, the Cs content can be high enough only in pegmatites to form its own mineral (pollucite), which occasionally occurs in considerable amounts. In other rocks, Cs concentrates largely in micas and feldspars. The possible maximal level of Cs accumulation in the melt remains poorly studied. Important new information has been obtained from the study of volcanic glass and melt inclusions (MI) in minerals. The rhyolitic glass concentrates 220‐870 ppm Cs, occasionally up to 3770 ppm [1]. To date, the highest Cs 2 O content (1.2‐5.5 wt %) has been detected in MIs captured by quartz from miarolitic pegmatite of the Malkhan field of the central Transbaikal region [2, 3] and the southwestern Pamirs (Leskhozovskaya and Vezdarinskaya veins). These inclusions contain products of crystallization of the late pegmatitic melt [3] and the inferred high-temperature meltlike gels [4]. While studying ongonite of the AryBulak Massif, we detected MIs filled with a residual glass that contains up to 17 wt % Cs. This is reliable evidence in favor of the existence of natural melts extremely enriched in Cs. In this communication, we describe these unusual inclusions and discuss Cs distribution in ongonites. The Ary-Bulak Massif is a dome-shaped stock exposed over ~0.8 km 2 among the Devonian volcanosedimentary rocks [5]. The massif is composed largely of porphyritic ongonite. A zone of aphyric rock 50‐100 m wide occurs only near the southwestern contact zone. Beyond this zone, an ongonite variety with anomalously high contents of CaO (3.3‐21.8 wt %) and F (2.7‐16 wt %) has been found near the same locality. The high CaO (7.8‐18 wt %) and F (7.1‐15.5 wt %) contents are inherent to aphyric rocks as well. Prosopite CaAl 2 F 4 (OH) 4 has been identified in this rock for the first time as an abundant mineral (6‐26 wt %). It was established that interstices between minerals of the groundmass of the Ca- and F-rich rocks are filled with submicrometric intergrowths of “fluoritic” and “K-feldspathic” phases. The “fluoritic” phase is a partly devitrified calcium fluoride melt with the following admixtures (wt %): O (3‐12), Al (0.5‐3.3), Si (0.2‐1.5), Sr (0.3‐ 0.5, occasionally up to 1.0‐1.3), Na (up to 0.5), and S (up to 0.3). The “K-feldspathic” phase is commonly close in composition to sanidine (rims around tabular albite crystals) but characterized by local enrichment in Ca (1.5‐4.0 wt %). We provided evidence for the joint crystallization of immiscible aluminosilicate and calcium fluoride melts in the presence of HF-bearing aqueous fluid during formation of Ca- and F-rich rocks [6, 7]. The Cs content in rocks from the Ary-Bylak Massif

Unique Clinkers and Paralavas from a New Nyalga Combustion Metamorphic Complex in Central Mongolia: Mineralogy, Geochemistry, and Genesis

The paper presents mineralogical and geochemical data on clinkers and paralavas and on conditions under which they were formed at the Nyalga combustion metamorphic complex, which was recently discovered in Central Mongolia. Mineral and phase assemblages of the CM rocks do not have analogues in the world. The clinkers contain pyrogenically modified mudstone relics, acid silicate glass, partly molten quartz and feldspar grains, and newly formed indialite microlites (phenocrysts) with a ferroindialite marginal zone. In the paralava melts, spinel microlites with broadly varying Fe concentrations and anorthite–bytownite were the first to crystallize, and were followed by phenocrysts of Al-clinopyroxene ± melilite and Mg–Fe olivine. The next minerals to crystallize were Ca-fayalite, kirschsteinite, pyrrhotite, minerals of the rhönite–kuratite series, K–Ba feldspars (celsian, hyalophane, and Ba-orthoclase, Fe3+-hercynite ± (native iron, wüstite, Al-magnetite, and fresnoite), nepheline ± (kalsilite), and later calcite, siderite, barite, celestine, and gypsum. The paralavas contain rare minerals of the rhönite–kuratite series, a new end-member of the rhönite subgroup Ca4Fe82+Fe43+O4 [Si8Al4O36], a tobermorite-like mineral Ca5Si5(Al,Fe)(OH)O16 · 5H2O, and high- Ba F-rich mica (K,Ba)(Mg,Fe)3(Al,Si)4O10F2. The paralavas host quenched relics of microemulsions of immiscible residual silicate melts with broadly varying Si, Al, Fe, Ca, K, Ba, and Sr concentrations, sulfide and calcitic melts, and water-rich silicate–iron ± (Mn) fluid media. The clinkers were formed less than 2 Ma ago in various parts of the Choir–Nyalga basin by melting Early Cretaceous mudstones with bulk composition varies from dacitic to andesitic. The pyrogenic transformations of the mudstones were nearly isochemical, except only for volatile components. The CM melt rocks of basaltic andesitic composition were formed via melting carbonate–silicate sediments at temperatures above 1450°C. The Ca- and Fe-enriched and silicaundersaturated paralavas crystallized near the surface at temperatures higher than 900–1100°C and oxygen fugacity

Processes of the formation of mugearitic and benmoreitic magmas on Nemrut Volcano (East Turkey)

f_{O_2 }

Inclusions of unusual late magmatic melts in quartz from the Oktyabr'skaya pegmatite vein, malkhan field (central Transbaikal Region)

fO2 between the IW and QFM buffers. In local melting domains of the carbonate–silicate sedimentary rocks and in isolations of the residual melts among the paralava matrix the fluid pressure was higher than the atmospheric one. The bulk composition, mineral and phase assemblages of CM rocks of the Nyalga complex are very diverse (dacitic, andesitic, basaltic andesitic, basaltic, and silica-undersaturated mafic) because the melts crystallized under unequilibrated conditions and were derived by the complete or partial melting of clayey and carbonate–silicate sediments during natural coal fires.

The Malkhan granite-pegmatite system

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.