Petrology and geochemistry of metamorphosed komatiites and basalts from the Tungurcha greenstone belt, Aldan Shield

Abstract

Supracrustal rocks of the Tungurcha greenstone belt in the western Aldan Shield occur as enclaves of actinolite schist and amphibolite within ∼ 3.2 Ga old tonalitic gneisses. Geological and chemical data indicate the supracrustal rocks were komatiites and tholeiites. The high-magnesium rocks have been subdivided into three groups based on Al Ti and Gd Yb ratios. One group has approximately chondritic Al Ti and Gd Yb and corresponds to Al-undepleted komatiite of the type found in Munro Township (Nesbitt et al., 1979). A second group has lower Al Ti and higher solGd Yb , and is similar to Al-depleted komatiite of the type found in the Barberton greenstone belt. In the third group Al Ti is even lower and Gd Yb higher than in the other two groups, but Ca Al is similar to that of the second group. This third type of komatiite shows strong enrichment of incompatible elements, and its low Al Ti is attributed to Ti enrichment, not Al-depletion. The SmNd age of komatiites is 3232±199 Ma ( ϵ Nd=0.7±0.2) and that of the basalts 2873±88 Ma ( ϵ Nd=+2.1±0.3). Significant scatter is apparent even amongst samples from a single flow, and it is believed that the scatter results not from differences in initial isotopic compositions but from disturbance of the SmNd system during later metamorphism. In view of the large errors and the difference between the two results, little significance can be placed on either the age or the initial isotopic compositions.