Mineral Ages from the Finnish Precambrian

Abstract

The number of isotopically dated zircons from the basement complex of the Karelian belt has been doubled to include six samples. The ages are all discordant and are compatible with loss of lead by a continuous diffusion mechanism. Two possible mechanisms are that diffusion took place at a constant value of the parameter,$$D/a^{2}$$, or under conditions such that $$D/a^{2}$$ increased linearly in proportion to the amount of radiation damage. The lead age data are also compatible with an episodic loss of lead about 500 m.y. ago, but there is no evidence that episodic loss should have occurred at that time. The regional metamorphism which affected the rocks 1,700-1,900 m.y. ago, as recorded by the mica age values, did not seriously alter the uranium-lead ratios in the zircons of the pre-Karelian basement complex; however, zircon from the gneiss domes to the west may have experienced some lead loss as a result of the regional metamorphism. Four detrital zircon samples from Svecofennian schists near Tampere have discordant isotopic ages suggesting an age of 2,300 m.y. Since igneous rocks of this age are as yet unknown in Finland, the value is provisional. Zircon from a granodiorite intruding the schists has a nearly concordant age of 1,900 m.y. Comparison of the zircon ages from the schists and granodiorite shows that the 1,900 m.y. age must represent the time of crystallization of zircon in the granodiorite, not a time of resetting as a result of intense metamorphism. These data strengthen the conclusions of earlier studies that plutonic rocks associated with both the Karelian and Svecofennian orogenic belts crystallized at about the same time. Data bearing on the retention of lead in zircon are presented. Zircon in a lens of schist 40 X 150 m. enclosed in the 1,900 m.y. granodiorite mass has not had its age record erased in spite of the fact that the zircons in the schist must have been heated to the temperature of the intrusive mass. Other data in the literature establish that loss of lead from zircon may take place under less severe conditions. The uranium and thorium content of the samples appears to be one factor controlling lead loss, samples having high radioactivity losing lead more readily. This effect is probably caused by radiation damage to the crystal structure or by inhomoge-neous distribution of radioactivity.