SPESSARTINE-GROSSULAR GARNETS IN METAVOLCANICS FROM THE CENTRAL BUNDELKHAND GREENSTONE COMPLEX OF THE BUNDELKHAND CRATON, INDIAN SHIELD: PARAGENESES, ZONING, STRINGERS, AND INCLUSIONS

Abstract

Archean metavolcanics from the Mauranipur Structure of the Central Bundelkhand greenstone complex have been subjected to three stages of metamorphism and metasomatism: 1) amphibolite-facies metamorphism (~2.7 Ga), 2) retrograde metamorphism and associated hydrothermal metasomatism (~2,68 Ga), including propylitization, and 3) prehnite-pumpellyite-facies metamorphism presumably associated with the formation of hydrothermal quartz veins (~1,8 Ga). Garnet is stable at all multiple metamorphic stages, varying in composition from almandine-spessartine to grossular and being associated with amphibole (from pargasite and tschermakite to grunerite-cummingtonite and actinolite in composition), clinopyroxene, chlorite, zoisite-clinozoisite, K-feldspar, etc. At the final stages of blastesis it is equiponderous with calcite, albite, prehnite and pumpellyite. Complex, contrasting chemical zoning with well-defined grain boundaries and zonal variations in the mineral composition of inclusions is formed in some of the garnet grains. The number of zones (three) is comparable with the number of epigenetic stages. As the BSE-images of the garnet are contrasting (because of the inverse relationship of FeO and CaO concentrations), the zoning pattern was examined and stringers, “interseasonal anomalies” and inclusions of grossular garnet in spessartine-almandine were revealed. Garnet grains are not always a closed system; recrystallization may occur in them.

Keywords

• архей
• метаморфизм
• метасоматоз
• парагенезисы
• гранат
• зональность граната
• стрингеры
• бунделкхандский кратон
• индийский щит