Ultramafites of Ilmeny-Vishnevogorsky complex (South Urals): Ti-in-zircon geothermometry

The article presents data on determining the crystallization temperature of zircons (78 crystals) from ultramafites of the Ilmeny-Vishnevogorsky complex and the Karabash massif by Ti-in-zircon geothermometer. The content of RE and REE in zircon is determined by Secondary-Ion Mass Spectrometry using the ion microprobe CAMEKA IMS-4F. A variety of heterogeneous polychronous zircons from ultramafic rocks include varieties with certain specific mineralogical-geochemical and age characteristics. The Early Proterozoic magmatic zircons PR1 (1770-1740 Ma) are characterized by the concentration of titanium from 16.7 to 29.9 ppm, low contents SREEср = 482 ppm, LREEср = 25.4 ppm and impurity elements Caav = 33 ppm, Baav = 1.4 ppm and Sr £ 0.5 ppm. High Ce and minimum Eu are distinctly manifested. Th/Uср = 0.65. The Cambrian magmatic zircons V-Є1 (580-520 Ma) contain 7.9-12.9 ppm titanium, higher concentrations SREEav = 623 ppm, LREEav = 30 ppm and impurity elements Caav = 17 ppm, Th/Uav = 0.86. The Early Silurian magmatic zircons O3-S (450-420 Ma) are characterized by titanium concentration from 7.4 to 19.6 ppm, high SREEav = 1142 ppm, low LREE/HREE = 0.03 and Th/Uav = 0.42. Transition and hydrothermal type of zircons (274-1793 Ma) have wide variations of titanium concentrations from 2.5 to 1160 ppm, that are mainly characterized by high concentrations in zircon LREE, Ca, Ba and Sr. For a real assessment of the crystallization temperature of zircon, and, therefore ultramafites zircons of magmatic type can be used. The crystallization temperature of magmatic zircon types (the formation-transformation of ultramafites) amount to Early Proterozoic PR1 - (822.5 ± 37)°C; Cambrian V-Є1 - (742 ± 17)°C and Early Silurian O3-S - (768 ± 27)°C.