The simulation of the genesis of the komatiite-tholeiitic series in the Vedlozero-Segozero greenstone belt (Upper archean V-S.g.b. lies in Central Karelia, Russia, it extends approximately north-south from Lake Segozero to Lake Vedlozero for about 300 km and comprises the Hautavaara, Koikary, Palaselga, Semch, Sovdozero and some other structures. The komatiite-tholeiitic series occur in the lower parts of the stratigraphic sequences, where they make up 800 to 2800 m thick strata-bound units and have an age of 2900-3100 Ma.) has led the author to conclude that peridotite komatiites are formed at 45-55% degrees of melting of mantle lherzolite at a pressure of 30-35 Kbar (Svetov, 1995).
The fact that all the komatiites belong to the Yilgarn
[Al-undepleted type with the ratios CaO/Al2O3<1, (0.25-1); Al2O3/TiO2=13-22, Ti/Y=200-290, Ti/Zr=60-100 close to average mantle ratios] type alone and at the same time fall into different REE topological groups [1. Komatiites showing flat chondritic REE distribution -(La/Sm)n=0.8-1.1, (Gd/Yb)n=0.9-1.2, (Ce/Yb)n=0.9-1.1 Palaselga structure,
2. Slightly LREE depleted komatiites, (La/Sm)n=0.4-0.7, (Gd/Yb)n=0.89-1.4,(Ce/Yb)n=0.6-0.8 Koykary structure, Siivikkovaara strukture Kuhmo g.b., 3. Komatiites showing slight LREE enrichment and flat HREE distribution, (La/Sm)n=1.5, (Gd/Yb)n=1.1-1.3, (Ce/Yb)n=0.9-1 Sovdozero structure, 4. Komatiites LREE enrichment, Ilomantsi g.b.] is due to their genesis and the geochemical characteristics of the mantle sources.
The petrogenesis of C. Karelia komatiites indicates that they were generated in a single mantle reservoir which considerably differ in some chemical characteristics such as minor and REE content from the mantle sources of peridotite komatiites in East Finland, North Karelia and the Kola Peninsula.
Gruau, G. & Tourpin, S., Contrib. Mineral. Petrol. 112, 66-82 (1992).
Nurmi, P.A. & Sorjonen-Ward, P., Geol.surv. Fin. Spec. pap. 17, 147-184 (1993).
Svetov, S.A. Abst. of MAEGS 9, 115-116 (1995).
Vrevsky, A. B., Abst. of MAEGS 9, 126 (1995).
Table 1: The mantle sources compositions of peridotite komatiites.
Ilomantsi Kuhmo Koykary Palaselga Sovdozero
Finland Finland C.Karelia C.Karelia C.Karelia
AGE (Ma) 2750* 2750±20a 2935 3000 3000
Cr ~3000 ~2000 ~3000 ~3350 ª3000
Ni ~2000 ~1000 ~2000 ~2000 ~2000
Co 77 - 80 100 70
Zr 3.2 22 9~10 9~10 9~10
Sr 30 7 9 4 12
Y 2.4 >4 3~4 3~4 3~4
Nb 5 - 0.5 0.5~0.7 0.5~0.7
Ce 3.1 ~1.1 0.65 1.45 1.3
Nd 1.3 1.3 0.72 0.98 0.82
Sm 0.5 0.5 0.29 0.27 0.3
Eu 0.15 0.15 0.11 0.18 0.11
Yb - 0.49 0.38 0.32 0.35
*Nd +1.3b +3.0a - +1.7c -
*O18 - +4a - +7.1c -
a Gruau and Tourpin (1992); b Nurmi and Sorjonen-Ward (1993); c Vrevsky (1995).