Olivine-bearing rocks of the Lapland granulite belt (Baltic Shield)

Abstract

Olivine-bearing varieties of garnet–clinopyroxene crystalline schists of the Lapland granulite belt have been studied in detail for the first time. Two types of olivine (iron mole fractions of 27 and 38%) are distinguished. Olivine with lower Fe content occurs as inclusions in clino- and orthopyroxene and in terms of СаО and Cr contents is close to magmatic minerals. Olivine with high Fe content presumably suffered highand moderate-temperature metamorphism. The olivine-bearing rocks contain several grains of omphacite with 30–37 mol % jadeite and garnet with 44–50 mol % pyrope, which can be regarded as relict assemblages of the early stage of eclogitization of a magmatic protolith. The presence of symplectites indicates their retrograde transformation during decompression. The protoliths of the studied rocks could be olivine gabbronorites and pyroxenites. It was found that the rocks contain high-alumina minerals: corundum, spinel, and sapphirine. In addition, Al2O3 content in some amphibole grains is as high as 19 wt %. This indicates that the ascent of the deep-seated rocks was accompanied by interaction with Al-rich fluid. The positive Eu anomaly in the olivine-bearing rocks and some of their minerals is indicative of the reducing character of fluid. Activation of fluid reworking leading to the formation and transformation of the olivine-bearing rocks, transfer of alumina and its precipitation at different depths are related to the processes at the base of the Paleoproterozoic rift system of Karelides.