Zoned accessory chromites from Archean komatiites of Eastern Fennoscandia, Karelian Craton (Russia): metamorphic fingerprints

Svetov S. A., Chazhengina S. Y., Singh V. K., Rybnikova Z. P., Mishra S.

Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2019. V. 148. N 4. P. 80-94

https://doi.org/10.30695/zrmo/2019.1484.06

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Language: English

Abstract

SEM and LA-ICP-MS analyses were used for the study of accessory chromite in Archean (3.0— 2.8 Ga) komatiites from the most representative greenstone belts of the Karelian Craton. Zoned chromite, subjected to varying degrees of metamorphic alteration, was identified in the komatiites. Two types of zoned chromite, differing in the composition of their cores, were recognized. Al-rich zoned chromite of type I shows the low extent of metamorphic alteration and has cores with Cr# 0.70—0.86, Mg# 0.03—0.12, and Fe# 0.06—0.23. It is enriched in Zn, Co, and Mn but strongly depleted in Ga, Ni, and V. The study reveals that the primary magmatic distribution of minor and trace elements in the cores was strongly redisturbed during prograde metamorphism of greenschist to low amphibolite facies. Zoned chromite with the low Al₂O₃ content of type II has cores with higher Fe# (0.42—0.49), Cr# (0.88—1), and lower Mg# (0—0.04) as compared to cores of Al-rich chromite, that corresponds to the more intensive metamorphic alteration. Since the primary composition of studied zoned chromite was significantly obliterated during metamorphism, established patterns can be used as a guide for the reconstruction of metamorphic processes.

Key words: zoned chromite, minor and trace elements, komatiites, Archean, Karelian Craton