Calcic Garnets as a Source of Information on the Age of Ultramafic Alkaline Intrusions in the Kola Magmatic Province

Abstract

The paper presents data of U–Pb (ID-TIMS) geochronologic studies of schorlomite–morimotoite–andradite garnet from the Vuoriyarvi, Sallanlatva, and Salmagorsky massifs in the Kola alkaline province. The garnet is relatively rich in U (5–9 ppm) and poor in common Pb (Pbc/Pbt = 0.2–0.7), contains U as an isomorphous component, and does not host inclusions of any U-bearing minerals. The U–Pb ages of garnet from the various rocks of the massifs (373 ± 2 to 377 ± 1 Ma) are mutually consistent and correspond to the main pulse of alkaline ultramafic magmatism in the Kola province (360–380 Ma). The geochronologic data indicate that the U–Pb system of schorlomite–morimotoite–andradite garnet is stable and can be utilized as a reliable geochronometer for alkaline ultramafic rocks.