Bulletin of the Geological Society of Finland (Jun 1986)

A revision of the Proterozoic-Archean boundary of the Northern Bothnian schist belt with a discussion of the geochemistry of related basic metavolcanics

  • Y. Kähkonen,
  • E. Mattila,
  • J. Nuutilainen

DOI
https://doi.org/10.17741/bgsf/58.1.008
Journal volume & issue
Vol. 58, no. 1
pp. 109 – 129

Abstract

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Geophysical analysis suggests the need for a revision of the eastern border of the early Proterozoic Northern Bothnian (or Kiiminki) schist belt. Recent geological field work has revealed places where the discordance and hiatus between Proterozoic and Archean formations are seen or have been constructed. The geochemistry of the basic rocks (mainly metavolcanics) on both sides of the discordance is compared on the basis of XRF-analyses for major and some trace (Zr, Cr, Ni, V, Cu, Zn and Sr) elements from 80 samples and neutron activation analysis for rare earth elements (REE) from 26 samples. The rocks are divided into groups (the Archean volcanigenic amphibolites of Puutturi and Käkiperä, the Archean metagabbros of Kalliomaa, the Proterozoic metadiabases of Puutturi and Käkiperä, and the Proterozoic metabasalts of the Kiiminki schist belt), all of which have individual geochemical characteristics. The basic rocks tend to have tholeiitic affinities (as revealed by Ti/Zr and V/Zr diagrams) but the Archean amphibolites of Puutturi are calc-alkaline. The latter group and the metagabbros of Kalliomaa are higher in Cr than are the other groups (at low Zr) and the Proterozoic metadiabases are lower in Cr than are the metabasalts of the schist belt (at low Zr). Chondrite-normalized REE patterns vary from consistent LREE enrichments in the Puutturi amphibolites to LREE depletions in the MORB-like pillowed metabasalts of the schist belt. The metadiabases are enriched in LREE but have convex-up patterns. The amphibolites of Käkiperä and the metagabbros of Kalliomaa display both depletions and enrichments in LREE. Fractional crystallization has caused some of the variation within the groups but cannot be the cause of all the features observed. It is also evident that the Proterozoic metadiabases and the metabasalts of the schist belt are not comagmatic.

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