U-PB GEOCHRONOLOGY OF LAYERED MAFIC INTRUSIONS IN THE EASTERN BALTIC SHIELD - IMPLICATIONS FOR THE TIMING AND DURATION OF PALEOPROTEROZOIC CONTINENTAL RIFTING

Paleoproterozoic layered mafic-ultramafic intrusions are widespread in the eastern Baltic (Fennoscandian) Shield and mark the initial stages of continental rifting and outpouring of voluminous flood basalts. U- Pb zircon/baddeleyite ages have been determined for seven layered mafi c intrusions and a subvolcanic felsic unit associated with two Paleopr oterozoic volcanic-sedimentary belts: the Pechenga-Imandra-Varzuga bel t in the Kola Peninsula, and the Sumi-Sariola belt in Karelia. The lay ered intrusions in the Sumi-Sariola belt show an age progression from 2449.0 +/- 1.1 Ma for the Burakovka complex, located at the southeaste rn edge of the belt, to 2442.1 +/- 1.4 Ma and 2441.3 +/- 1.2 Ma for th e Lukkulaisvaara and Tsipringa intrusions of the Olanga complex in nor thern Karelia (northeastern part of the belt). These age results indic ate a total duration of mafic magmatism along the ca. 800 km belt in K arelia to be less than 10 m.y.. The Pechenga-Imandra-Varzuga belt, Kol a Peninsula, contains two distinct age groups of layered mafic intrusi ons and related rocks. The younger of these overlaps with the ages obt ained for the Olanga intrusions and includes the 2441.0 +/- 1.6 Ma Ima ndra lopolith and a spatially associated felsic subvolcanic unit dated at 2442.2 +/- 1.7 Ma. The older group of mafic layered intrusions in the Kola Peninsula includes the 2501.5 +/- 1.7 Ma Pana tundra intrusio n, the 2504.4 +/- 1.5 Ma Monche pluton and the 2505.1 +/- 1.6 Ma Gener al'skaya mountain intrusion. The temporal and spatial distribution of the 2.50-2.44 Ga rift-related igneous activity in the Baltic Shield is consistent both with the model of starting mantle plume, and with the model of passive rifting controlled by pre-existing lithospheric weak nesses.