Vol 31, No 4 (2023)

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 (Т_q) 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 Т_q 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.

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 Х_KCl/(Х_KCl + Х_NaCl) in the fluid, defined as Х_NaCl = 0.506 – 0.84Х_KCl, the amphibole is replaced by an association of nepheline with acid plagioclase, sodalite, and biotite. At Х_KCl/(Х_KCl + Х_NaCl) > 0.3, nepheline, sodalite, and plagioclase become unstable, K-feldspar appears, and biotite, clinopyroxene, and amphibole remain. At Х_KCl/(Х_KCl + Х_NaCl) > 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 Х_KCl/(Х_KCl + Х_NaCl) 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.

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.