Abstract:
Data are presented on the Kurga polyphase intrusion, which is located within the Kola alkaline province in the northeastern Baltic Shield. The rock assemblages composing the intrusion are demonstrated to be unique to the province. In addition to ultramafics, the intrusion contains widespread rocks of the larvikite–laurdalite series, which are absent from other alkaline complexes of the area. Neither foidites nor carbonatites, which are inherent members of all alkaline–ultramafic series, were found in the massif. A distinctive feature of the rocks is their relatively low alkalinity: the massif is dominated by miaskitic hypersthene-normative syenites of the larvikite–laurdalite series. The Rb–Sr isochron age of the intrusion (404 10 Ma) suggests that its origin was probably related to the initial stage of the Paleozoic tectono-magmatic reactivation of the area and that subalkaline and alkaline magmatic rocks were produced in the area during not only Middle but also Early Devonian time. The U–Pb zircon age of the pyroxenite and larvikite (387 7 Ma) dates the crystallization of the accessory zircon during the postmagmatic stage of massif crystallization at a lower alkalinity and an elevated silica activity. The initial stage of mantle magmatism in northeastern Fennoscandia (the Kurga intrusion) is thought to have proceeded no less than 10–20 m.y. earlier than the main stage of the alkaline and carbonatite magmatism, and was related to the final tectonic stage in the North Atlantic belt of the Caledonides.