RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 3, NO. 3, PAGES 163–196, doi:10.2205/2001ES000060, 2001

Surface analogues of "grey gneiss'' among the Archaean rocks in the Kola Superdeep Borehole (experience from petrologic-geochemical modelling of lower crust composition and conditions of formation of tonalite-trondhjemite rocks)

O. M. Turkina
United Institute of Geology, Geophysics and Mineralogy of the Siberian Branch of RAS, Novosibirsk, Russia

O. Nordgulen
Geological Survey of Norway, Trondheim, Norway

Abstract

[1]  Two types of plagiogneiss have been distinguished among Archaean rocks of the KSDB. Their protoliths formed at P ≥ 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 Garsj\o\ plagiogneiss, which is located in the Svanvik-Lotta segment of the Kola-Norwegian block, and probably belongs to the upper part of the Bj\o rnevatn greenstone belt. Protoliths of certain Garsj\o\ 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 Garsj\o\ 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.

Received 1 August 2001; published 20 August 2001.

Keywords: Archaean rocks, Kola Superdeep Borehole, types of plagiogneiss.

Version of this paper in Russian