V. R. Vetrin, O. M. Turkina, and O. Nordgulen
15 kbar (type A, garnet-amphibolite restite)
and 8 kbar
(type B, plagioclase-amphibole restite). The conditions under which these rocks originated
appear to be similar to those recognised for the Garsjo plagiogneiss, which is
located in
the Svanvik-Lotta segment of the Kola-Norwegian block, and probably belongs to the
upper
part of the Bjo rnevatn greenstone belt. Protoliths of certain Garsjo gneisses
(type C) were
generated by partial melting of a metabasic source rock enriched in light rare earth
and other
incompatible elements (e.g. Sr), and corresponding in composition to TH2 basalt.
In
contrast to the Garsjo gneiss, the tonalite-trondhjemite rocks of the KSDB were
derived
from relatively poor-differentiated metabasic rocks which are characterised by lower
La/Yb
ratios and lower Sr content, and are compatible to TH1 tholeiite. On the Kola Peninsula,
homologues of the tonalite-trondhjemite rocks of the KSDB are likely to be found
within the
Olenegorsk greenstone belt in the central part of the Kola Peninsula.
It is concluded that the formation of tonalite-trondhjemite rocks has considerable
influence on the composition of the lower crust of the region.