Association of Fe–Ti oxides in subalkaline dolerites of the Rai-Iz massif (Polar Urals)

Abstract

The relevance of the research. Subalkaline dolerites, Fe–Ti oxides of which were studied in this work, occur within the largest deposits of chromium ores of the Rai-Iz massif. The study of the material composition and age of these rocks was carried out by the authors for the first time. The purpose of the research is to reveal the patterns of changes in the chemical composition of the main accessory minerals of rocks – titanomagnetite and ilmenite, and on the basis of the obtained data to evaluate the conditions for the formation of subalkaline dolerites of the massif. Research methodology. The chemical compositions of minerals were determined using microprobe analysis on a CAMECA SX 100 and scanning electron microscopy on a Jeol JSM-6390LV setup with an INCA Energy 450 X-Max 80 energy dispersive attachment. Raman spectra were obtained on a LabRam HR 800 Evolution spectrometer equipped with an Olimpus BX-FM microscope. The studies were carried out at the “Geoanalyst” сenter for the collective use of scientific equipment of the IGG, UB RAS. Results. The typomorphism of the chemical composition of titanomagnetite and ilmenite has been established for petrographic varieties of dolerites allocated on the massif. The T–fO2 parameters of dolerite formation were estimated and compared with metaultramafites. It is shown that titanomagnetite in amphibole dolerites undergoes maghemitization. The highest temperatures (1076–1126 °C) are recorded in pyroxene dolerites. The oxygen fugacity in these rocks is FMQ +0.6–+1 units. Hyalodolerite from the chilled margin of the pyroxene dolerite dike gives a temperature of 811–818 °C and an oxygen fugacity of 1.8 units above FMQ. The compositions of the associated ilmenite and titanomagnetite from amphibole dolerites correspond to fO2 = –2–0 units relative to FMQ and temperature 580–720 °C. The trend of change in T–fO2 parameters shows that the injection of the mafic melt occurred under conditions of metamorphism of ore-bearing ultramafic rocks. Conclusions. The formation of dolerite veins occurred under conditions of ultramafic rock metamorphism. The increased fugacity of oxygen at a relatively low temperature led to the maghemitization of titanomagnetite, rather than to the redistribution of components in the magnetite-ilmenite paragenesis.