Description: Possible topologies of miscibility gaps in arsenian (Cu,Ag) 10 (Fe,Zn) 2 (Sb,As) 4 S 13 fahlores are examined. These topologies are based on a thermodynamic model for fahlores whose calibration has been verified for (Cu,Ag) 10 (Fe,Zn) 2 Sb 4 S 13 fahlores, and conform with experimental constraints on the incompatibility between As and Ag in (Cu,Ag) 10 (Fe,Zn) 2 (Sb,As) 4 S 13 fahlores, and with experimental and natural constraints on the incompatibility between As and Zn and the nonideality of the As for Sb substitution in Cu 10 (Fe,Zn) 2 (Sb,As) 4 S 13 fahlores. It is inferred that miscibility gaps in (Cu,Ag) 10 (Fe,Zn) 2 As 4 S 13 fahlores have critical temperatures several °C below those established for their Sb counterparts (170 to 185°C). Depending on the structural role of Ag in arsenian fahlores, critical temperatures for (Cu,Ag) 10 (Fe,Zn) 2 (Sb,As) 4 S 13 fahlores may vary from comparable to those inferred for (Cu,Ag) 10 (Fe,Zn) 2 As 4 S 13 fahlores, if the As for Sb substitution stabilizes Ag in tetrahedral metal sites, to temperatures approaching 370°C, if the As for Sb substitution results in an increase in the site preference of Ag for trigonal-planar metal sites. The latter topology is more likely based on comparison of calculated miscibility gaps with compositions of fahlores from nature exhibiting the greatest departure from the Cu 10 (Fe,Zn) 2 (Sb,As) 4 S 13 and (Cu,Ag) 10 (Fe,Zn) 2 Sb 4 S 13 planes of the (Cu,Ag) 10 (Fe,Zn) 2 (Sb,As) 4 S 13 fahlore cube.

Description: The system NaAlSi 3 O 8 (albite, Ab)-H 2 O offers a simple and tractable model to study the thermodynamics of the volatile constituent H 2 O in felsic magmas. Although it has been studied in this context for nearly 100 years, developing a comprehensive model that adequately describes the activity of H 2 O ( \({a_{{H_2}O}}\) ) in hydrous albite liquids and vapors has proven challenging. There are several problems. First, \({a_{{H_2}O}}\) in hydrous liquids relies on melting experiments in the presence of mixed fluids with reduced H 2 O activity (H 2 O-CO 2 and H 2 O-NaCl), but models of \({a_{{H_2}O}}\) in these coexisting fluids have lacked sufficient accuracy. Second, the role of the solubility of albite in H 2 O has been assumed to be negligible; however, it is important to take solubility into account at pressure ( P ) above 0.5 GPa because it becomes sufficiently high that H 2 O activity at the wet solidus is significantly less than 1. Third, the dry melting temperatures and wet solidus temperatures are inconsistent between the datasets. We address these issues by combining previous experimental work on T – \({X_{{H_2}O}}\) liquidus relations at 0.5–1.5 GPa with accurate activity formulations for H 2 O in mixed fluids (Aranovich and Newton, 1996, 1999). This yields isobaric T – \({a_{{H_2}O}}\) sections at 0.5, 0.7, 1.0 and 1.5 GPa. Data at each isobar were fit to cubic equations, which were used to derive the following equation for liquidus T as a function of \({a_{{H_2}O}}\) and P : $$T\left( {{a_{{H_2}O}},P} \right) = {m_0} + {m_1}{a_{{H_2}O}} + {m_2}a_{{H_2}O}^2 + {{m_3}a_{{H_2}O}^3}^\circ C$$ where T is °C, m 0 = 1119.6 + 112.3 P , m 1 =–856.5–578.9P, m 2 = 1004.1 + 952.9 P , and m 3 =–477.1–618.0 P . The equation is valid at 0.5 〈 P 〈 1.5 GPa and T solidus 〈 T 〈 T dry melting . The nonzero solubility of albite in pure H 2 O is incorporated into the model to give the correct liquidus H 2 O activity when truncating the model equation in the limiting case where T → T solidus at a given pressure. This model equation reproduces both the liquidus-H 2 O contents and activities from the solubility measurements of Makhluf et al. (2016) in the binary system Ab -H 2 O at 1.0 GPa. The model equation also accurately reproduces the liquidus H 2 O activities from Eggler and Kadik (1979) and Bohlen et al. (1982) when the Aranovich and Newton (1999) activity formulation for CO 2 –H 2 O mixed fluids is applied to their datasets.

Description: Isotopic dates newly obtained for the northwestern portion of the Angara–Vitim batholith are consistent with preexisting data on the duration of the Late Paleozoic magmatic cycle: 55–60 Ma (from 325 to 280 Ma). These data also indicate that alkaline mafic magmatism in western Transbaikalia began simultaneously with the transition from crustal granite-forming processes to the derivation of granites of a mixed mantle–crustal nature, with gradual enrichment of the juvenile component in the source of the magmas. Analysis of the currently discussed geodynamic models of Late Paleozoic magmatism shows that a key role in all models of extensive granite-forming processes in the region is assigned to mafic mantle magmas, which can be generated in various geotectonic environments: subduction, delamination, decompression, and a mantle plume. The plume model is most consistent with the intraplate character of the Angara–Vitim batholith. The derivation of the vast volume of granitic material (approximately 1 million km 3 ) should have required a comparable volume of mafic magma that should have been pooled in the middle crust of the Baikal fold area. However, the density structure of the region does not provide evidence of significant volumes of mafic rocks. This suggests that the mechanism of plume–lithospheric interaction that should have induced extensive crustal melting and the origin of vast granite areas was more complicated than simply conductive melting of crustal protoliths in contact with mafic intrusions.

Description: An important role of the early Neoproterozoic juvenile crustal growth in the formation of the Khangai group of Precambrian terranes in the Central Asian Orogenic Belt was demonstrated by the example of the Holbo Nur Zone of the Songin Block. Magmatic complexes of this zone correspond to different settings of the Early Neoproterozoic ocean: oceanic islands, mid-ocean ridges, intraoceanic island arcs, and turbidite basins. Obtained data on volcanic rocks and associated granitoids constrain a timing of the island-arc magmatic complexes, at least within the interval of 888–859 Ma. The comparison of structures of the Songino and Tarbagatai blocks of the Khangai group of terranes showed that they share many common features in their geology and evolution and may be united into the single Songino–Tarbagatai terrane. This terrane was formed owing to the Early Neoproterozoic (~800 Ma) accretion of the ocean island, spreading, island-arc, and turbidite complexes of the oceanic plate to a stable continental massif represented by the Early Neoproterozoic Ider Complex of the Tarbagatai Block. The involvement of the Dzabkhan terrane into a Khangai collage of terranes is constrained between the formation of the volcanic rocks of the Dzabkhan Formation (~770–755 Ma), which are unknown in the Songino–Tarbagatai terrane, and the Tsagaan-Olom carbonate cover (~630 Ma), overlying both the Dzabkhan and Songino–Tarbagatai terranes. It was proposed that the formation of the Precambrian terranes of the Central Asian Orogenic Belt began from the Early Neoproterozoic accretion to the Rodinia supercontinent. The fragmentation of the latter above a mantle superplume at the end of the Early Neoproterozoic spanned also the newly formed fold area. This led to the formation of terranes, which included both fragments of the Paleoproterozoic craton and Early Neoproterozoic structures. Subsequent amalgamation of these Precambrian crustal fragments into composite terranes possibly occurred at the end of the early Baikalian tectonic phase.

Description: The paper discusses petrological effects related to interaction between rocks and concentrated aqueous salt fluids (brines) at lower crustal metamorphism. These effects arise mainly from the low H 2 O activity typical of brines, while preserving and even increasing transport properties relative to pure H 2 O or H 2 O–nonpolar gas fluids. The paper presents thermodynamic properties of the halogen-bearing end members of the biotite solid solution based on experimental data, and examples illustrating how they can be employed to calculate the activities (concentrations) of alkali halides in the fluid. Action of brines significantly changes conventional views on the solubility of several minerals and on the distribution of elements (including trace elements) between minerals, melts, and fluids. The specific role of brines is also in bringing to interaction zones not only water but also alkali metals and Ca, which results in numerous metasomatic net-transfer reactions involving mafic minerals and/or exchange reactions with feldspars that produce new mineral assemblages with lower melting temperature, i.e., cause granitization of rocks as defined by D.S. Korzhinskii. Brines also exert fine “tuning” of metasomatic and melting processes: even at equal pressure, temperature, and water activity values metasomatism may or may not trigger melting depending on the Na/K/Ca ratio in the fluid phase.

Description: The paper presents newly obtained original data on the morphology, internal structure (as seen in cathodoluminescence images, CL), and composition of more than 400 zircon grains separated from gabbroids and plagiogranites (OPG) sampled at the axial zone of the Mid-Atlantic Ridge (MAR). The zircons were analyzed for REE by LA-ICP-MS and for Hf, U, Th, Y, and P by EPMA. Magmatic zircon in the gabbroids crystallized from differentiating magmatic melt in a number of episodes, as follows from systematic rimward increase in the Hf concentration, and also often from the simultaneous increase in the (U + Th) and (Y + P) concentrations. These tendencies are also discernible (although much less clearly) in zircons from the OPG. Zircon in the OPG is depleted in REE compared to the least modified zircons in the gabbro, which suggests that the OPG were derived via partial melting of gabbro in the presence of seawater-derived concentrated aqueous salt fluid. Another reason for the REE depletion might be simultaneous crystallization of zircon and apatite. The CL-dark sectors, which are found in practically all of the magmatic zircon grains, have Y/P (a.p.f.u.) ≫ 1 which most likely resulted from OH accommodation in the zircon structure, a fact suggesting that the OPG parental melt contained water. High-temperature hydrothermal processes induced partial to complete recrystallization of zircon (via dissolution-reprecepitation), a process that was associated with ductile and brittle deformations of the zircon-hosting rocks. The morphology of the hydrothermal zircons varies depending on pH and silica activity in the fluid from weakly corroded subhedral crystals with typical vermicular microtopography of the crystal faces to completely modified grains of colloform structure. Geochemically, the earlier hydrothermal transformations of the zircons resulted in their enrichment in La and other LREE, except only Ce, whose concentration, conversely, decreases compared to that of the unmodified magmatic zircons. The hydrothermal zircon displays a reduced Ce anomaly and its most altered domains typically host minute inclusions of xenotime, U and Th oxides and silicates, and occasionally also baddeleyite, which suggests that the hydrothermal fluid was reduced and highly alkaline. These features were acquired by the seawater-derived fluid when it circulated within the axial MAR zone area due to phase separation in the H 2 O–NaCl system and particularly as a result of fluid interaction with the abyssal peridotites of oceanic core complexes. Our data demonstrate that zircon is a sensitive indicator of tectonic and physicochemical processes in the oceanic crust.

Description: This paper reports the results of the first comprehensive petrological study of mafic enclaves widespread in the products of recent (2006–2012) eruptions of Bezymianny Volcano, Kamchatka. Four types of mafic enclaves were distinguished on the basis of the composition and morphology of minerals, P–T conditions of formation of mineral assemblages, and structural and textural characteristics of the rocks. Disequilibrium assemblages of mafic enclaves indicate a complex structure of the magmatic plumbing system of the volcano, including a shallow chamber with andesite–basaltic andesite magmas and a deep reservoir filled in part with plagioclase–hornblende cumulates and fed by basic magmas with mantle harzburgite xenoliths. The mafic enclaves were formed at different levels of the magmatic plumbing system of the volcano and correspond to different degrees of mixing of interacting magmas. The most abundant enclaves were formed during magma ascent from the deep reservoir (960–1040°C, 5–9 kbar) into the shallow andesitic chamber (940–980°C). Enclaves of plagioclase–hornblende cumulates from the basic magmas feeding the deep reservoir ( T 〉 1090°C and P 〉 9 kbar) are much less common.

Description: The Tuva–Mongolian terrane of the Central Asian Orogenic Belt is a composite structure with a Vendian–Cambrian terrigenous–carbonate cover. The Sangilen block in the southern part of the belt is a smaller composite structure, in which tectono–stratigraphic complexes of different age that were produced under various conditions were amalgamated in the course of Early Paleozoic tectonic cycle. The P–T parameters of the Early Paleozoic metamorphism in the western part of the Sangilen block corresponded to the amphibolite facies. The gneisses of the Erzin Complex contain relict granulite-facies mineral assemblages. The granulites are dominated by metasediments typical of deep-water basins on passive continental margins. The only exception is granulites of the Lower Erzin tectonic nappe of the Chinchlig thrust system: these rocks are metatholeiites, tonalites, and trondhjemites, whose REE patterns are similar to those of MORB. The composition of these granulites and their high Sm/Nd ratios indicate that the rocks were derived from juvenile crust that had been formed in an environment of a mature island arc or backarc basin. It is reasonable to believe that these rocks are fragments of the Late Riphean basement of the Sangilen block. The average 206 Pb/ 238 U zircon age of the garnet–hypersthene granulites is 494 ± 11 Ma. With regard for the zircon age of the postmetamorphic granitoids, the granulite-facies metamorphism occurred within the age range of 505–495 Ma. The peak metamorphic temperature reached 910–950°C, and the pressure was 3–4 kbar, which corresponds to ultrahigh-temperature/low-pressure (UHT–LP) metamorphism. The garnet–hypersthene orthogranulites were formed at a temperature that decreased to ~850°C and pressure that increased to ~5.5‒7 kbar. It can be hypothesized that the earlier UHT–LP granulites were produced at an elevated heat flux and were later (in the course of continuing collision) overlain by a relatively cold tectonic slab, and this leads to a certain temperature decrease and pressure increase. This relatively cold slab could consist of fragments of the Vendian elevated-pressure metamorphic belt whose development terminated at the Vendian–Cambrian boundary before the onset of the Early Paleozoic regional metamorphism.

Description: Titanium contents of quartz have been analyzed in samples of granulites from various metamorphic complexes of eastern Siberia (Sutam, Chogar, and Sharyzhalgai) that contain mineral assemblages conventionally regarded as indicative of “ultrahigh-temperature” metamorphism. The related TitaniQ temperature estimates (Wark and Watson, 2006) are consistent with those of other mineralogical geothermometers and are commonly much lower than “ultrahigh-temperature”.

Description: Experimental investigations in the system rare-metal granite–Na 2 O–SiO 2 –H 2 O with the addition of aqueous solutions containing Rb, Cs, Sn, W, Mo, and Zn at 600°C and 1.5 kbar showed that the typical elements of rare-metal granites (Li, Rb, Cs, Be, Nb, and Ta) are preferentially concentrated in hydrosilicate liquids coexisting with aqueous fluid. The same behavior is characteristic of Zn and Sn, the minerals of which are usually formed under hydrothermal conditions. In contrast, Mo and W are weakly extracted by hydrosilicate liquids and almost equally distributed between them and aqueous fluids. Liquids similar to those described in this paper are formed during the final stages of magmatic crystallization in granite and granitepegmatite systems. The formation of hydrosilicate liquids in late magmatic and postmagmatic processes will be an important factor controlling the redistribution of metal components between residual magmatic melts, minerals, and aqueous fluids and, consequently, the mobility of these components in fluid-saturated magmatic systems enriched in rare metals.