Petrography of komatiitic metavolcanites

from Koikary and Palaselga structures, Vedlozero-Segozero greenstone belt (Central Karelia, Russia)

Microstructures are observed as relics in the case of Archean komatiites and are well preserved in less strongly metamorphosed Proterozoic strata. In Archean komatiites metamorphic alteration most commonly does not exceed an epidote-amphibolite facies.

As a result, primary minerals are replaced. This is what we see, for example, in the Vedlozero-Segozero greenstone belt. Present mineral paragenesis is represented by actinolite, tremolite, hlorite, serpentinite, talc and magnetite.

In the Vetreny Belt, Proterozoic (2.39 Ga) volcanics retain a primary mineral assemblage which consists of olivine ( 80-90 Fo ), clinopyroxene and chrome-spinel.

Photomicrograph of spinifex-texture in upper zone (A2) in the lava flow (relict of Ol spinifex)

Photomicrograph of relict random oriented pyroxene spinifex-texture (The rock is composed of actinolite and minor chlorite)

Photomicrograph of spinifex-texture relict in lava flow

Photomicrograph of komatiitic tuff


In the Koikary - Semch zone and in the Palaselga and Sovdozero structures, regional metamorphism is related to the main (Reboly) phase of folding. There, the rocks are metamorphosed to greenschist to amphibolite grade (andalusite - sillimanite pressure type)

When studying komatiite - tholeiite series, attention is given primarily to their geochemical characteristics. However, there arises the question whether the chemical compositions of magmatic rocks agree with those of their metamorphosed varieties.

Correlation of the geochemical compositions of unequally altered rocks has led the author to conclude for all the structures studied that :

1. Metamorphic processes did not result in the marked redistribution of petrogenic, minor and rare elements, as shown by analysis of some samples. Neither do they distort evolutionary trends.
2. CaO, Na2O and K2O are observed to be redistributed only in strongly carbonatized samples.Their exclusion from access reduces a probable error.
3. The geochemical characteristics of metamorphosed rocks in komatiite - tholeiite series agree with relevant primary values estimated for volcanics.
4. Recalculation of chemical analyses for an anhydrous residue diminishes the error which arises from the high water content of secondary minerals.

1. Introduction
2. Geology
3. Layered flows
4. Structures
5. Petrography
6. Geochemistry
7. Mantle sources
8. References
9. Researcher (Sergei Svetov)

Последние изменения: 20 октября 2006