Description: Reactions and partial melting of peraluminous rocks in the presence of H 2 O-CO 2 –salt fluids under parameters of granulite-facies metamorphism were modeled in experiments on interaction between orthopyroxene–cordierite–biotite–plagioclase–quartz metapelite with H 2 O, H 2 O-CO 2 , H 2 O-CO 2 -NaCl, and H 2 O-CO 2 -KCl fluids at 600 MPa and 850°C. Rock melting in the presence of H 2 O and equimolar H 2 O-CO 2 fluids generates peraluminous (A/CNK 1 〉 1.1) melts whose composition corresponds to magnesian calcic or calc–alkaline S-type granitoids. The melts are associated with peritectic phases: magnesian spinel and orthopyroxene containing up to 9 wt % Al 2 O 3 . In the presence of H 2 O-CO 2 -NaCl fluid, cordierite and orthopyroxene are replaced by the association of K-Na biotite, Na-bearing gedrite, spinel, and albite. The Na 2 O concentrations in the biotite and gedrite are functions of the NaCl concentrations in the starting fluid. Fluids of the composition H 2 O-CO 2 -KCl induce cordierite replacement by biotite with corundum and spinel and by these phases in association with potassium feldspar at X KCl = 0.02 in the fluid. When replaced by these phases, cordierite is excluded from the melting reactions, and the overall melting of the metapelite is controlled by peritectic reactions of biotite and orthopyroxene with plagioclase and quartz. These reactions produce such minerals atypical of metapelites as Ca-Na amphibole and clinopyroxene. The compositions of melts derived in the presence of salt-bearing fluids are shifted toward the region with A/CNK 〈 1.1, as is typical of so-called peraluminous granites of type I. An increase in the concentrations of salts in the fluids leads to depletion of the melts in Al 2 O 3 and CaO and enrichment in alkalis. These relations suggest that the protoliths of I-type peraluminous granites might have been metapelites that were melted when interacting with H 2 O-CO 2 -salt fluids. The compositions of the melts can evolve from those with A/CNK 〉 1.1 (typical of S-type granites) toward those with A/CNK = 1.0–1.1 in response to an increase in the concentrations of alkali salts in the fluids within a few mole percent. Our experiments demonstrate that the origin of new mineral assemblages in metapelite in equilibrium with H 2 O-CO 2 -salt fluids is controlled by the activities of alkaline components, while the H 2 O and CO 2 activities play subordinate roles. This conclusion is consistent with the results obtained by simulating metapelite mineral assemblages by Gibbs free energy minimization (using the PERPE_X software), as shown in log( \({a_{{H_2}O}}\) )–log( \({a_{N{a_2}O}}\) ) and log( \({a_{{H_2}O}}\) )–log( \({a_{{K_2}O}}\) ) diagrams.

Description: We studied the petrography, mineralogy, and geochemistry of the Paleoproterozoic (2.06 Ga) granites of the Katugin massif (Stanovoy suture zone), which hosts the combined rare-metal Katugin deposit. Three groups of granites were distinguished: (1) biotite ( Bt ) and biotite–riebeckite ( Bt–Rbk ) granites of the western block of the massif; (2) biotite–arfvedsonite ( Bt–Arf ) granites of the eastern block; and (3) arfvedsonite ( Arf ), aegirine–arfvedsonite ( Aeg–Arf ), and aegirine ( Aeg ) granites of the eastern block. The Bt and Bt–Rbk granites of the first group are mainly metaluminous and peraluminous rocks with rather high CaO contents and the minimum F contents among the granites described here. It was suggested that the granites of this group could be derived from a source dominated by crustal rocks with a small addition of mantle materials. These granites probably crystallized from a metaluminous–peraluminous melt with elevated CaO and moderate F contents. Melts of such compositions are least favorable for the crystallization of ore minerals. The Bt–Arf granites of the second group are mainly peralkaline and show high contents of CaO and Y and low contents of Na 2 O and F. A mixed mantle–crust source was proposed for the Bt–Arf granites. The initial melt of the Bt–Arf granites could have a peralkaline composition with elevated CaO content and moderate to high F content. The Arf , Aeg–Arf , and Aeg granites of the third group are enriched in ore mineral and were classified as peralkaline granites with very low CaO contents, elevated Na2O and F contents, and usually very high contents of Zr, Hf, Nb, and Ta. Based on the geochemical and isotopic data, it was supposed that the source of the granites of the third group could be derivatives of basaltic magmas produced in an OIB-type source with a minor addition of crustal material to the magma generation zone. It was suggested that the primary melt of this granite group could be a peralkaline CaO-poor and F-rich silicic melt, which is most favorable for the crystallization of ore minerals. Based on the analysis of the geochemical characteristics of the three granite groups and their relationships within the Katugin massif, a qualitative model of its formation was proposed. According to this model, the Bt and Bt–Rbk granites of the western block crystallized first, followed by the Bt–Arf granites of the eastern block and, eventually, the Arf , Aeg–Arf , and Aeg granites enriched in ore minerals.

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: Gibbs energy minimization is the means by which the stable state of a system can be computed as a function of pressure, temperature and chemical composition from thermodynamic data. In this context, state implies knowledge of the identity, amount, and composition of the various phases of matter in heterogeneous systems. For seismic phenomena, which occur on time-scales that are short compared to the timescales of intra-phase equilibration, the Gibbs energy functions of the individual phases are equations of state that can be used to recover seismic wave speeds. Thermodynamic properties relevant to modelling of slower geodynamic processes are recovered by numeric differentiation of the Gibbs energy function of the system obtained by minimization. Gibbs energy minimization algorithms are categorized by whether they solve the non-linear optimization problem directly or solve a linearized formulation. The former express the objective function, the total Gibbs energy of the system, indirectly in terms of the partial molar Gibbs energies of phase species rather than directly in terms of the Gibbs energies of the possible phases. The indirect formulation of the objective function has the consequence that although these algorithms are capable of attaining high precision they have no generic means of treating phase separation and expertise is required to avoid local minima. In contrast, the solution of the fully linearized problem is completely robust, but offers limited resolution. Algorithms that iteratively refine linearized solutions offer a compromise between robustness and precision that is well suited to the demands of geophysical modeling.