Two Main Types of the Early Precambrian Lower Crust

E. V. Sharkov Institute of Ore Deposits Geology, Petrology, Mineralogy and Geochemistry (IGEM),

Russian Academy of Sciences, 109017 Moscow, Russia

The Early Precambrian lower crustal rocks from the basement of Russian platform (the Baltic Shield) and Siberian platform (the Southern Prisayn'e) were studied. On the Baltic Shield they were found as deep-seated xenoliths of garnet granulites and eclogites in the Elovy Island diatrem (the Kola Peninsula) within the Belomorian domain - eastern continuation of the Archaean Karelian granite-greenstone terrane which was strongly reworked in the Paleoproterozoic (c. 2.45 Ga ago). Their chemical composition have intermediate position between tholeiitic and island-arc basalts. The rocks were formed under conditions P = 12-19 kbar and T = 750-930°C. Their model Sm-Nd age varies from 2.9 to 2.4 Ga (Aftalion et al., 1991).

In the Southern Prisayn'e (Eastern Siberia,150 km to NW from Lake Baikal) lower crustal formations are represented by tectonic block (Arbansky massif) within the Late Archaean Onotsky granite-greenstone terrane. The massif formed by
garnet granulites and eclogites with interlayers of garnet-kyanite schists, concordant thick (about 100 m) bodies of Gr-bearing enderbite-gneisses, and lenticular bodies of spinel peridotites and pyroxenites (Kempton et al., 1995). In its turn, Onotsky terrane is a large tectonic block among the Paleoproterozoic (c. 2.45 Ga) (Sharkov et al., 1995) Sharyzhalgay granulite
belt. Tectonic emplacement of the Arbansky massif took place c. 2 Ga ago. These rocks, probably, were primary a volcanic-sedimentary sequence, and enderbite-gneisses with relics of magmatic textures were intrusive bodies as well as ultramafics. In differ from the Baltic rocks, chemical composition of the garnet granulites is unusual for igneous rocks, and it suggests their restite origin. The rocks were formed under different P-T conditions: garnet-bearing ones at high pressures (eclogite at P = 29-34 kbar, Gr granulites at P = 15-22 kbar, T = 840-960°C). It, probably, means that the lower crust here was formed by tectonic slices of rocks from different depths which were intruded by ultramafics and "soldered" together before c. 2 Ga ago, when they were thrusted out as an uniform body.

Studied complexes represent two main types of the lower crustal rocks which were formed during the Archaean and the Early Proterozoic. The origin of the Kola type was linked with underplating beneath granite-greenstone terranes, whereas the origin of the Prisaynian one was probably related to zones of "drawing" beneath granulite belts.


Aftalion, M. et al., J. Geology 99, 851-862 (1991).

Kempton P. et al., Lithos (1995, in press).

Sharkov, E.V. et al., In 6th Intern.Kimberlite Conf., Extended Abstracts, 506-508 (Novosibirsk, 1995).