DURATION OF FORMATION OF MAGMATIC SYSTEM OF POLYPHASE PALEOZOIC ALKALINE COMPLEXES OF THE CENTRAL KOLA: U-Pb, Rb-Sr, Ar-Ar DATA

Abstract

The estimation of the duration of multiphase alkaline complex formation is a very complicate problem for the following reasons: first, the effect of younger magmatic phases provokes disturbance of isotopic systems in rocks formed at older stages; second, the recrystallization of accessory mineral phases—the main REE concentrators in alkaline rocks—limits feasibility of Sm–Nd and Rb–Sr isochron timing; third, undersaturation of alkaline magmas in silica promotes the replacement of liquidus zircon with other Zr-bearing phases; and fourth, zircons in some Paleozoic alkaline rocks are characterized by extremely low uranium contents, and this sharply narrows application of the U–Pb method.