PALEOMAGNETISM OF PRECAMBRIAN BASIC INTRUSION AND DYKES OF NORTHERN KARELIA, EASTERN FENNOSCANDIAN SHIELD

Palaeomagnetic results for mafic and ultramafic intrusions and their c ountry rocks in Russian Karelia are presented. The oldest rock type is pyroxene-hornblende gabbro, regarded as late Archaean or Palaeoproter ozoic in age. Later, at the early stage of formation of a volcanic-plu tonic association, including layered intrusions and metavolcanites, ga bbro-norite dykes were emplaced. Sm-Nd isotopic analyses of this assoc iation yield mineral and whole-rock ages of 2.47-2.35 Ga with epsilon( Nd) = -0.4 to 1.2 for gabbro-norite dykes, Based on field relations, t he youngest rock types are pyroxene-plagioclase porphyry dykes and gab bro-dolerite dykes. Three distinct magnetization components were isola ted by alternating field and thermal demagnetization. The most common remanence component B, with a mean direction of D = 30 degrees, I = 58 degrees, k = 52, alpha(95) = 10 degrees (N = 5 geological units), is an overprint interpreted to have been acquired at the last stage of th e Svecofennian orogeny (1.90-1.70 Ga). Component A, with a mean direct ion of D = 350 degrees, I = 49 degrees, k = 69, alpha(95) = 9 degrees (N = 5 geological units), is also an overprint, probably acquired at t he onset of the Svecofennian orogeny. Both components are found in all rocks studied. Component C, with a mean direction of D = 126 degrees, I = 73 degrees, k = 167, alpha(95) = 10 degrees (N = 3 geological uni ts), is isolated as a primary remanence in the pyroxene-homblende gabb ro intrusion and gabbro-norite dykes. The results are compared with ot her contemporaneous palaeomagnetic poles from Finland and Russian Kare lia.