Comparative in-situ U–Th–Pb geochronology and trace element composition of baddeleyite and low-U zircon from carbonatites of the Palaeozoic Kovdor alkaline–ultramafic complex, Kola Peninsula, Russia
التفاصيل البيبلوغرافية
العنوان:
Comparative in-situ U–Th–Pb geochronology and trace element composition of baddeleyite and low-U zircon from carbonatites of the Palaeozoic Kovdor alkaline–ultramafic complex, Kola Peninsula, Russia
The most widely used geochronometer for dating geological events is the U–Pb isotope system used on zircon crystals. However, in silica-undersaturated ultramafic and alkaline rocks, baddeleyite (ZrO2) is the predominant zirconium mineral. We present the results of 65 U–Th–Pb SIMS (SHRIMP-II) analyses of baddeleyite grains from carbonatite and phoscorite rocks of the Paleozoic Kovdor alkaline–ultramafic complex, Kola Peninsula, Russia. There are no significant differences in the obtained ages, either for baddeleyite from different host rocks or morphotypes of baddeleyite crystals, and dates vary little between different analytical sessions. The batch calculations of baddeleyite data show a concordant age of 379.1 ± 3.7 Ma, and a weighted mean 206Pb/238U age of 376.5 ± 4.3 Ma. Accessory zircons from the same host rocks are characterized by extremely low uranium content (mainly