Archean—Paleoproterozoic boundary at the Karelian craton: First ion-microprobe U—Pb (SHRIMP II) data on zircons from mafic volcanics

Abstract

In spite of the long history of study of Karelian geol� ogy, the question concerning position of the Archean– Proterozoic boundary remains open thus far. According to the traditional scheme developed by Krats [1], this boundary was placed at the base of the Sumian Super� horizon. The lower part of the latter is made up of two sedimentary–volcanogenic sequences, each of which (from the bottom upward) consists of highly mature sedimentary rocks (quartzites, arkoses) and basaltic andesites. The Sumian unit is terminated by a rhyodac� itic volcanic sequence. The U–Pb age of the felsic vol� canics determined by the conventional zircon tech�