Palaeomagnetism of Precambrian basic intrusion and dykes of Northern Karelia, eastern Fennoscandian Shield

Abstract

Palaeomagnetic results for mafic and ultramafic intrusions and their country rocks in Russian Karelia are presented. The oldest rock type is pyroxene-hornblende gabbro, regarded as late Archaean or Palaeoproterozoic in age. Later, at the early stage of formation of a volcanic-plutonic association, including layered intrusions and metavolcanites, gabbro-norite dykes were emplaced. Sm-Nd isotopic analyses of this association yield mineral and whole-rock ages of 2.47-2.35 Ga with ∈ Nd = −0.4 to 1.2 for gabbro-norite dykes. Based on field relations, the youngest rock types are pyroxene-plagioclase porphyry dykes and gabbro-dolerite dykes. Three distinct magnetization components were isolated by alternating field and thermal demagnetization. The most common remanence component B, with a mean direction of D = 30°, I = 58°, k = 52, α 95 = 10° ( N = 5 geological units), is an overprint interpreted to have been acquired at the last stage of the Svecofennian orogeny (1.90-1.70 Ga). Component A, with a mean direction of D = 350°, I = 49°, k = 69, α 95 = 9° ( N = 5 geological units), is also an overprint, probably acquired at the onset of the Svecofennian orogeny. Both components are found in all rocks studied. Component C, with a mean direction of D = 126°, I = 73°, k = 167, α 95 = 10° ( N = 3 geological units), is isolated as a primary remanence in the pyroxene-hornblende gabbro intrusion and gabbro-norite dykes. The results are compared with other contemporaneous palaeomagnetic poles from Finland and Russian Karelia.