Formation of Atoll Garnets in the Banded Iron Formation of Maru Schist Belt
Aliyu Abdu IBRAHIM

Abstract
Petrographic and microprobe analyses from the banded iron formations (BIF) of the Maru Schist Belt shows that the BIF contain occurrences of atoll-like garnets. The formation of the atoll garnet is discussed using textural, chemical and Backscatter Electron image (BSE) characteristics. The garnets are of almandine (Alm - 45) composition in the outer rims, with increased amount of spessartine (Spst - 40) in the central rims, and a low amount of grossularite and pyrope. The garnet replaced by chlorite (chamosite), with magnetite inclusions are contained in silicate facies BIF. Textural evidence reveals incipient garnet replacement by chamosite along the Fe - rich rims. Based on the microprobe analysis and BSE data it is supposed that atoll-like forms of the garnets in the BIF developed by replacement of the preexisting garnets mainly by chamosite, hematite, and ilmenite under varying temperature pressure conditions (from high grade - garnet zone, to low grade - chlorite zone) typical of polymetamorphic regions.

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