The age of the Sokli carbonatite, Finland, and some relationships of the North Atlantic alkaline igneous province

Abstract

Nine dates from the Sokli area are presented, including four not previously published. Dates of the carbonatite range from 334 to 392 my. with one markedly discordant date of 247 my. Fenites give dates ranging from 354 ± 19 my, close to the carbonatite contact, to 488 ± 18 and 1 220 ± 40 my. at approximately one kilometre from the contact. Unfenitized basement granite and syenite are dated at 1 760 + 30 and 1 740 + 35 my. A summary is presented of the spatial and temporal relationships of the carbonatites and alkaline complexes of the North Atlantic alkaline igneous province as defined by Doig (1970), for which there are now available some 50 isotopic dates. These fall into three main groups which correspond closely in time with the peaks of the Caledonian — Appalachian, Grenville, and Hudsonian — Svecokarelian orogenic episodes. All three groups are represented in Canada and Europe, and two of them in Greenland. It is pointed out that in addition to the carbonatites, most of the world's described alkaline complexes which are layered occur within the province. The correspondence between the ages of emplacement of the igneous intrusions and the orogenic episodes is ascribed to deep tensional faulting in the forelands caused by down buckling of crustal plates along the orogenically active zones. The production of carbonatite magma and layered alkaline igneous complexes in this relatively narrow zone, and over a time span of 2 000 my. during which there was presumably differential motion between crustal plates and mantle, suggests that the lithosphere probably played an important role in the production of these exotic igneous rocks.