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: New isotopic-geochemical data are reported on the Late Cretaceous–Paleocene ultrapotassic volcanic rocks of the alkaline–ultrabasic complex of the Valagin Ridge, Eastern Kamchatka. The high Mg, low Ca and Al contents at high K/Na ratios in these rocks make them similar to the Mediterranean-type lamproites and ultrapotassic rocks. The low contents of high-field strength (HFSE) and heavy rare-earth (HREE) elements relative to the MORB composition, and the low Sr and high Nd isotopic ratios indicate the formation of their primary melts from a depleted mantle source. The enrichment of the ultrapotassic rocks in the large-ion lithophile elements (LILE) can be explained by the fluid influx in melts during melting of subsided oceanic crust.

Description: The geochemical and zircon geochronological (U-Pb, SHRIMP-II) study of Mesoarchean gabbros of the South Vygozersky and Kamennoozersky greenstone structures of Central Karelia made it possible to distinguish four gabbro types: (1) Fe–Ti gabbro, 2869 ± 12 Ma, (2) gabbro compositionally close to tholeiitic basalts, 2857 ± 7 Ma, (3) leucogbabbro, 2840 ± 5 Ma; and (4) melanogabbro, 2818 ± 14 Ma. From the early to late gabbros, the rocks are depleted in Ti, Fe, V, Y, Zr, Nb, Hf, REE and enriched in Mg, Ca, Cr, Ni. According to the systematics (Condie, 2005), the Nb/Y, Zr/Y, Zr/Nb ratios in the studied Late Archean gabbros are close to those of primitive mantle, while the gabbros in composition are similar to those of plumederived ocean-plateau basalts. Their magma sources were derived from different mantle reservoirs. The leucogabbro and melanogabbro with similar εNd = +4 were derived from a depleted mantle source (DM). The gabbro close in composition to tholeiitic basalts and having the elevated positive ε Nd (+4.9) was derived from a strongly depleted mantle source. Insignificant admixture of crustal material or lithospheric mantle is inferred in a source of the Fe–Ti gabbro (with lowest ε Nd = +2.1).

Description: Fragments of heterogeneous cosmonegic substance (nickelphosphide Ni 3 P and ZnAl 2 ) were found using high resolution analytical electron microscopic techniques, for the first time in samples from a large meteorite crater: the Zhamanshin astrobleme in Kazakstan. Inasmuch as such fragments cannot simultaneously occur in meteorite of any one type, we suggest that the impactor of the Zhamanshin crater was of comet nature.

Description: The paper presents detailed isotope-geochronological, geological, and petrologic–mineralogical data on lavas of one of the greatest Quaternary magmatic area in the Greater Caucasus, the Kazbek neovolcanic center, including polygenetic Kazbek stratovolcano and a number of subordinate volcanic cones in its vicinities. The research was conducted based on a representative collection of more than 150 geological samples that characterize most of the volcanic cones and lava flows of different age, some of which were known previously, and other were discovered by the authors. The high-precision K–Ar data obtained on these materials make it possible to reproduce the evolutionary history of youngest magmatism at the Kazbek center and evaluate the total duration of this evolution at ~450 ka. The magmatic activity was subdivided into four phases (at 460–380, 310–200, 130–90, and 〈50 ka) with long-lasting interludes in between. Because the latest eruptions occurred in the Kazbek vicinity in the Holocene, this volcano is regarded as potentially active. The volcanic rocks of the Kazbek center make up a continuous compositional succession of basaltic (trachy)andesite–(trachy)andesite–dacite and mostly belong to the calc–alkaline series. The principal petrographic characteristics of the rocks and the composition of their phenocryst minerals are determined, mineral assemblages of these minerals are distinguished in the lavas of different type, and the temperature of the magmatic melts is evaluated. A principally important role in the petrogenesis of the Kazbek youngest magmas is proved to have been played by fractional crystallization and replenishment of mafic melts in the magmatic chambers beneath the volcano, which resulted in their mixing and mingling with the residual dacite melt and the origin of high-temperature hybrid andesite lavas. The comprehensive geological studies, involving interpretation of high-resolution satellite images, allowed the authors to compile the first detailed (1: 25 000) volcanologic map of the Kazbek center and a geochronologic chart supplemented with a stratigraphic column, which illustrate the origin sequence of the volcanic vents and their lava flows, geological relations between them, as seen in reference geological sections, and variations in the composition of the magmatic products with time.

Description: Cretaceous to Eocene plutonic and volcanic rocks of the Sabzevar zone have an adakite characteristic with high Sr/Y ratio, depleted HFSE and enriched LILE features. Most of the Sabzevar adakites are high silica adakites with low Ni, Cr and Co contents. LREE/HREE ratio is high, while K 2 O content is low to intermediate. Adakites in the Sabzevar zone are exposed in two areas, which are named southern and northern adakites here. The combination of Sr, Nd and Pb isotopic data with major and trace elements indicates that the adakitic rocks are formed by partial melting of the Sabzevar oceanic slab. Nb/Ta content of the samples indicates that the adakitic magmas were generated at different depth in the subduction system. Dy/Yb ratios of adakitic samples indicate positive, negative and roughly flat patterns for different samples, suggesting garnet and amphibole as residual phases during slab-derived adakitic magma formation. Sabzevar adakites emplaced during late to post-kinematic events. Sabzevar oceanic basin demised during a northward subduction by central Iranian micro-continents (CIM) and Eurasia plate convergence.

Description: A classification suggested for alkaline ultramafic rocks of the Ary-Mastakh and Staraya Rechka fields, Northern Anabar Shield, is based on the modal mineralogical composition of the rocks and the chemical compositions of their rock-forming and accessory minerals. Within the framework of this classification, the rocks are indentified as orangeite and alkaline ultramafic lamprophyres: aillikite and damtjernite. To estimate how much contamination with the host rocks has modified their composition when the diatremes were formed, the pyroclastic rocks were studied that abound in xenogenic material (which is rich in SiO 2 , Al 2 O 3 , K 2 O, Rb, Pb, and occasionally also Ba) at relatively low (La/Yb) PM , (La/Sm) PM , and not as much also (Sm/Zr) PM and (La/Nb) PM ratios. The isotopic composition of the rocks suggests that the very first melt portions were of asthenospheric nature. The distribution of trace elements and REE indicates that one of the leading factors that controlled the diversity of the mineralogical composition of the rocks and the broad variations in their isotopic–geochemical and geochemical characteristics was asthenosphere–lithosphere interaction when the melts of the alkaline ultramafic rocks were derived. The melting processes involved metasomatic vein-hosted assemblages of carbonate and potassic hydrous composition (of the MARID type). The alkaline ultramafic rocks whose geochemistry reflects the contributions of enriched vein assemblages to the lithospheric source material, occur in the northern Anabar Shield closer to the boundary between the Khapchan and Daldyn terranes. The evolution of the aillikite melts during their ascent through the lithospheric mantle could give rise to damtjernite generation and was associated with the separation of a C–H–O fluid phase. Our data allowed us to distinguish the evolutionary episodes of the magma-generating zone during the origin of the Triassic alkaline ultramafic rocks in the northern Anabar Shield.

Description: The Vorochistoozersky, Nizhnepopovsky, and Severo-Pezhostrovsky gabbro-anorthosite massifs have been studied in the central part of the Belomorian Province, Fennoscandian Shield. The similarity of geological setting and rock composition of these massifs suggests their affiliation to a single complex. The age of the gabbro-anorthosites was determined by U-Pb (SHRIMP II) zircon dating of gabbro-pegmatites from the Vorochistoozersky massif at 2505 ± 8 Ma. The studied massifs were overprinted by the high-pressure amphibolite facies metamorphism. Relicts of magmatic layering and primary magmatic assemblages preserved in the largest bodies. The massifs consist mainly of leucocratic gabbros but also contain rocks of the layered series varying in composition from olivinite to anorthosite. The presence of troctolites in the layered series indicates the stability of the olivine–plagioclase liquidus assemblage and, respectively, shallow depths of melt crystallization. Despite the composition differences between gabbro-anorthosites of the Belomorian and peridotite–gabbronorite intrusions Kola provinces, these simultaneously formed massifs presumably mark a single great igneous event. It also includes the gabbronorite dikes in the Vodlozero terrane of the Karelian province, the Mistassini swarm in the Superior province, and the Kaminak swarm in the Hearne Craton, Canadian Shield. The large igneous province of age ~2500 Ma reflects the oldest stage of within-plate magmatism after a consolidation of the Neoarchean crust of the Kenorland Supercontinent (Superia supercraton).

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.