Lamprophyres in the Early Proterozoic volcanic complex of the Pechenga structure, Kola peninsula

The upper portion of the Early Proterozoic sequence of the Pechenga structu re was detected to contain a series of subvolcanic lamprophyre bodies. Thes e are sills of meso- and melanocratic holocrystalline massive biotite (kers antite) and amphibole (odinite) rocks and explosion breccias of camptonitic composition. The U-Pb isotopic age of the micaceous lamprophyres (kersanti tes) is 1711 +/- 12 Ma and corresponds to the Rb-Sr age of lamproites from the Por' ya Bay area(1710 +/- 10 Ma). The lamprophyres affiliate with the s ubvolcanic rock association of the final evolutionary stages of the Pecheng a structure. This association also includes trachydacite, Na-trachydacite, high-Ti Na-trachyte, and ultrasilicic ferrorhyolite. An analogous triple ro ck assemblage (lamprophyre-trachyte-ultrasilicic ferrorhyolite) is also typ ical of Early Proterozoic structures in eastern Finland, a fact that led us to suggest that rock associations of this type are specific of the orogeni c and postorogenic stages in the evolution of Early Proterozoic foldbelts i n the Baltic Shield.