THE KOLA ALKALINE PROVINCE OF THE CIS AND FINLAND - PRECISE RB-SR AGES DEFINE 380-360 MA AGE RANGE FOR ALL MAGMATISM

Twenty-one new Rb-Sr mineral and biotite-whole rock isochrons are pres ented for intrusive rock complexes of the Palaeozoic Kola Alkaline Pro vince in the CIS and Finland. Samples from the ultrabasic-alkaline and nepheline-syenite complexes Khibina, Lovozero, Afrikanda, Turiy Mys, Ozernaya Varaka, Sokli and Iivaara all yield ages in the very narrow r ange from 361.7 to 377.3 Ma, i.e. late Middle Devonian to Upper Devoni an. These data represent crystallization rather than intrusion ages, b ut are nevertheless argued as marking very closely the time of intrusi on. There is no indication of bimodal age distribution. The emplacemen t of the alkaline magmas is related temporally to the subsidence of th e Kontozero Graben of the Central Kola Peninsula. Sr initial ratios of the agpaitic nepheline-syenites of the giant Khibina and Lovozero int rusions scatter between 0.703 and 0.704. Because of high Rb/Sr ratios of the rocks, these isotope ratios exclude long-term pre-enrichment of alkalis in the sources of the magmas prior to segregation and intrusi on, and thus episodic enrichment during earlier rifting events in the northern Baltic Shield. The magma formation may be related to wrench m ovements in the surroundings of the major Trollfjord-Komagelv Fault, w hich forms part of the Late Caledonian Arctic- North Atlantic Megashea r System.