Научная деятельность

Geochemistry and Petrogenesis of the upper Archean Komatiite-Tholetitic Series in the Vedlozero-Segozero Greenstone Belt, Central Karelia

The Upper Archean Vedlozero-Segozero greenstone belt lies in Central Karelia.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 discussed in the present paper 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. At the present time, the rocks of the komatiite-tholeiitic series are metamorphosed to low epidote-amphibolite grade.Therefore, all structural and textural rock varieties such as spinifex and cumulate structures are preserved. It has been found by studying the effect of metamorphism on rock geochemistry that in the samples taken the elements are not redistributed and evolutionary trends are not distorted by the graphic interpretation of compositions. The geochemical study of the rocks that constitute the series suggests that in the Vedlozero-Segozero greenstone belt the komatiites represent an Al-undepleted (Yilgarn) type with the rations CaO/Al2O3<1, ( 0.25-1); Al2O3/TiO2=13-22, Ti/Y=200-290, Ti/Zr=60-100 close to average mantle rations. In the structures, tholeiites are geochemically similar to komatiites. This is apparent in similar element rations and the presence of common genetic trends ranging from peridotite komatiites to tholeiites.
The REE topology of Vedlozero-Segozero komatiites provides the basis for subdividing them into several types:
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.
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.
The fact that all the komatiites belong to the Yilgarn type alone and at the same time fall into different REE topological groups is due to their genesis and the geochemical characteristics of the mantle sources of their melts. It has been shown by calculating the temperatures at which komatiitic lava is generated and ejected that lava ejection temperatures did not exceed 1600 degrees (1540-1560 - Koykary structure, 1560-1600 -Palaselga structure, 1580-1600-Sovdozero structure) and its generation temperature in the source was 1800 degrees. The simulation of the genesis of the above series has led the author to conclude that peridotite komatiites are formed at 40-55% degrees of melting of mantle lherzolite at a pressure of 35-40 KBar. The petrogenetic characteristics of comagmatic tholeiites are most correctly correlated with a model of their formation during the fractional differentiation of deep-seated melts compositionally corresponding to peridotite and pyroxenite komatiites. The petrogenesis of 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.

Последние изменения: 16 июля 2006