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Vol 31, No 4 (2023)
Articles
High Fractionated Granites of the Raumid Massif (S. Pamir): O-Isotope and Geochemical Study
Abstract
The processes of crystallization differentiation, retrograde isotopic exchange, and autometamorphism are considered using the “natural laboratory” – Eocene granites of the Raumid massif, which has eight intrusion phases. The work is based on a comprehensive (oxygen isotope, petrographic, geochemical) study of representative samples of each of the intrusion phases of the massif. Isotopic and geochemical studies of rock-forming minerals (Qz, Pl, Kfs, Bt), as well as their altered varieties, have been carried out. The geochemical features of the Raumid’s granites are correspond to both A-type granites and highly fractionated I-type granites. We show that the rocks of the Raumid massif are not the geochemical analog of the Qiangtang Eocene granitoids of the Central Tibet and the Vanj complex, as previously assumed (Chapman et al., 2018). The differentiation conditions for acidic melts that formed the Raumid pluton (T = 750–800°C, P = 4.5–7.8 kbar with predominant Pl crystallization) were estimated. The intrusion of melts into the hypabyssal zone proceeded at least in two stages: early (γ1–γ3) and late (γ4–γ8), although it is possible that the rocks of the γ7 and γ8 phases belonged to a separate stage. The closing temperature of the oxygen isotopic system of quartz (Тq) varies from 420 to 610°C, which indicates a wide range of cooling rates for these rocks. The effect of the multiphase nature of melt intrusion on the Тq and apparent cooling rates is considered. The study of altered and unaltered minerals makes it possible to establish that autometamorphism partially overlapped with the retrograde oxygen isotope exchange in the cooling rock. The modelling of the δ18О values of feldspars during autometamorphism in a system closed with respect to fluid show that the Pl saussuritization and Kfs kaolinitization proceeded at a limited fluid/mineral ratio (W/M = 0.3–0.05), which could be provided by aqueous fluid separated during the Raumid pluton cooling.
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Age and Isotope-Geochemical Characteristics of Ta, Nb, W, Sn Mineralization Associated with Rare-Metal Granites (Khangilay Ore District, Eastern Transbaikalia)
Abstract
The age relations between the formation of the parent massif and the time of crystallization of the associated ore mineralization were established on the basis of isotope-geochronological study of the massifs-deposits of the Khangily ore district with various metallogenic specialization in Eastern Transbaikalia. In the Orlovka massif of Li-F granites, the crystallization time of columbite-tantalite (145 ± 1 Ma) and cassiterite (144.2 ± 0.3 Ma) (U-Pb, ID-TIMS) is almost identical to the crystallization time of zircon (140.6 ± 2.9 Ма (U-Pb, SHRIMP) and 145 ± 1 Ma (U-Pb, CA-ID-TIMS)) – an age marker of the formation of massifs. This fact testifies to the magmatogenic nature of rare-metal mineralization. In the Spokojnoye massif – the “standard type” of rare-metal peraluminous granites – a time gap with an interval of 0.6–3.8 Ma was revealed between the time of formation of the massif (141.3 ± 1.8 Ма, U-Pb, SHRIMP, 146.9 ± 0.7 Ма, Rb-Sr isotopic system) and crystallization of wolframite (141.8 ± 0.6, Rb-Sr isotopic system and 140.1 ± 1.4 Ма, Sm-Nd isotopic system). Perhaps this interval corresponds to the time of formation of the hydrothermal system, with which tungsten mineralization is genetically related.
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Experimental Investigation of the Interaction of Amphibole with Highly Salted H2O-NaCl-KCl Fluid at 750°C, 700 MPa: Application to Alkaline Metasomatism of Amphibole Rocks
Abstract
The article presents experimental data on the interaction of amphibole with NaCl-H2O and (K, Na)Cl-H2O solutions at varying salt content. When interacting with the H2O-NaCl fluid, amphibole remains the predominant mineral in all experiments. In addition to it, Na-phlogopite, plagioclase, and nepheline/sodalite are formed. At \({{a}_{{{{{\text{H}}}_{{\text{2}}}}{\text{O}}}}}\) > 0.6, amphibole melts. When amphibole interacts with the H2O-NaCl-KCl fluid at \({{X}_{{{{{\text{H}}}_{{\text{2}}}}{\text{O}}}}}\) < 0.40 and ХKCl/(ХKCl + ХNaCl) in the fluid, defined as ХNaCl = 0.506 – 0.84ХKCl, the amphibole is replaced by an association of nepheline with acid plagioclase, sodalite, and biotite. At ХKCl/(ХKCl + ХNaCl) > 0.3, nepheline, sodalite, and plagioclase become unstable, K-feldspar appears, and biotite, clinopyroxene, and amphibole remain. At ХKCl/(ХKCl + ХNaCl) > 0.5, the association Cpx + Bt + Kfs + Grt (grossular–andradite) is stable. Thus, the grossular–andradite garnet is an indicator of high potassium activity in fluids, while nepheline is an indicator of high sodium activity. Na → K isomorphism is observed in amphibole and biotite, and Ca → Na isomorphism in clinopyroxene, but in general these minerals (unlike nepheline and garnet) remain stable in the range of wide K/Na variations in the fluid. Clinopyroxene in experiments corresponds to Ca-Fe-Mg compositions with varying, sometimes high aluminium content, amphiboles belong to the pargasite-hastingsite composition. With an increase in \({{a}_{{{{{\text{H}}}_{{\text{2}}}}{\text{O}}}}}\) (\({{X}_{{{{{\text{H}}}_{{\text{2}}}}{\text{O}}}}}\) > 0.57), i.e. a decrease in the gross salinity of the fluid, melts appear, the composition of which varies from trachyte to phonolitic. As ХKCl/(ХKCl + ХNaCl) increases in fluids, the alumina content of melts decreases. An increase in the total salinity of the fluid leads to an increase in the content of potassium in the melt and a decrease in the content of chlorine in it. Experiments have shown that the interaction of amphibole with fluids containing high concentrations of NaCl and KCl leads to the formation of mineral associations typical of alkaline metasomatism of amphibole rocks and concomitant HCl enrichment of the fluid phase. The replacement of fluids with a high saline component by highly acid ones leads to the leaching of Ca, Mg, Fe from metamorphic rocks, their transport and redeposition. Thus, in some cases, a significant removal of FeO, MgO, CaO is a consequence of the interaction of host rocks with water-salt solutions.
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Thermodynamic Model of the Fluid System H2O–CO2–NaCl–CaCl2 at P-T Parameters of the Middle and Lower Crust
Abstract
Based on the earlier obtained equations of state for the ternary systems H2O–CO2–CaCl2 and H2O–CO2–NaCl, an equation of state for the four-component fluid system H2O–CO2–NaCl–CaCl2 is derived in terms of the Gibbs excess free energy. A corresponding numerical thermodynamic model is build. The main part of the numerical parameters of the model coincides with the corresponding parameters of the ternary systems. The NaCl–CaCl2 interaction parameter was obtained from the experimental liquidus of the salt mixture. Similar to the thermodynamic models for H2O–CO2–CaCl2 and H2O–CO2–NaCl, the range of applicability of the model is pressure 1–20 kbar and temperature from 500°C to 1400°C. The model makes it possible to predict the physicochemical properties of the fluid involved in most processes of deep petrogenesis: the phase state of the system (homogeneous or multiphase fluid, presence or absence of solid salts), chemical activities of the components, densities of the fluid phases, and concentrations of the components in the coexisting phases. The model was used for a detailed study of the phase state and activity of water on the H2O–CO2–salt sections when changing the ratio \( {{{{x}_{{{\text{NaCl}}}}}} \mathord{\left/ {\vphantom {{{{x}_{{{\text{NaCl}}}}}} {\left( {{{x}_{{{\text{NaCl}}}}} + {{x}_{{{\text{CaC}}{{{\text{l}}}_{{\text{2}}}}}}}} \right)}}} \right.} {\left( {{{x}_{{{\text{NaCl}}}}} + {{x}_{{{\text{CaC}}{{{\text{l}}}_{{\text{2}}}}}}}} \right)}} \) from 1 to 0. Changes in the composition and density of coexisting fluid phases at a constant activity of water and changes in the total composition of the system are studied. A set of phase diagrams on sections H2O–NaCl–CaCl2 for different mole fractions of CO2 is obtained. Pressure dependencies of the maximal activity of water in the field of coexisting unmixable fluid phases are obtained for several salt compositions of the system. Due to removal of restrictions resulting from a smaller number of components in ternary systems, the thermodynamic behavior of systems with a mixed composition of the salt is significantly differ from the behavior of those with a single salt component.
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Petrogeochemical Characteristics of Late Devonian Metagabbro-Dolerites of the Kara Astrobleme (Nenets Autonomous District, Russia)
Abstract
The article presents new results of detailed petrographic and petrogeochemical studies of metagabbro-dole-rites from the center of the Kara Astrobleme. The data obtained made it possible to narrow the petrographic diversity of the rocks to two varieties: metagabbro-dolerites and quartz metagabbro-dolerites, formed during the differentiation of the original magmatic melt. The geochemical features of the rocks indicate the heterogeneity of the magma-generating source. Metagabbro-dolerites and quartz metagabbro-dolerites are characterized by the presence of planar elements of low and middle stages of impact metamorphism.
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Moissanite in Rocks of the Bobruisk Ledge of the Belarusian Crystalline Massif of the East European Craton
Abstract
A large range of minerals, native, intermetallic, amorphous compounds containing K, Na, Fe, Mn, Ca, Ba, Sr, Cu, Pb, Co, Ni, Sn, Zn, Al, Ce, Nd, La, Pr, Sm, Y, Yb, Nb, Hf, W, Mo, Zr, Cr, V, Ag, Ti, Si, has been found in the Bobruisk buried ledge of the Belarusian crystalline massif in the western part of the East European craton. As, P, Bi, O, H, F, Cl, S, Se, C, B, N F. Among them there is moissanite. It forms xenomorphic and subidiomorphic separations up to 1.5 mm in size and is represented by a hexagonal polytype modification 6H. Inclusions inherent in meteorites were found in one of its grains – sinoite (Si2N2O), xifengite (Si5Fe3) and awaruite (Ni3Fe). Moissanite, the native, intermetallic, amorphous compounds associated with it, are developed in the form of scattered, poor, multi-grained inclusions in the rocks of three structural-material complexes of the centuries. This indicates the superimposed nature of mineralization. The whole set of crystalline and amorphous secretions observed with moissanite is proposed to be designated as Bobruiskites. Mineral formation, with a high degree of probability, is due to the impact interaction of the meteoroid with rocks of the East European craton.
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