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.
Highlights
Archean metavolcanics from the Mauranipur Structure of the Central Bundelkhand greenstone complex have undergone three stages of metamorphism and metasomatism: 1) amphibolite-facies metamorphism (~2.7 Ga), 2) retrograde metamorphism and associated 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, clinopyroxene, chlorite, zoisite-clinozoisite, K-feldspar, etc
The central Bundelkhand Archean greenstone complex, Bundelkhand Craton, Central India: geology, composition, and geochronology of supracrustal rocks // Int. Geol
Summary
Бунделкхандский кратон состоит главным образом из неоархейских микроклиновых гранитов [Ramakrishnan, Vaidyanadhan, 2010; Singh et al, 2019]. 1 – аллювиальные отложения; 2 – кварцевые жилы (рифы) (1,9–1,8 млрд лет); 3–7 – зеленокаменный комплекс: 3 – дайка кислых пород (2,56 млрд лет), 4 – полосчатые кварциты (BIF), 5 – кислые метавулканиты (2,81 млрд лет), 6 – метабазальты (местами с реликтами шаровой текстуры), 7 – метаультрамафиты и высокомагнезиальные основные породы; 8 – метасоматические породы (около 2,7 млрд лет); 9 – гранитоиды (3,55–2,50 млрд лет); – точки отбора образцов и их номер; – предполагаемые разломы (а) и надвиги (б); 12–14 – элементы залегания: – полосчатости, – сланцеватости и линейности, – опрокинутое залегание. В борту Центрально-Бунделкхандского террейна, в линзе высокомагнезиальных корундсодержащих флогопит-хлоритовых сланцев были установлены метаморфические преобразования в условиях эклогитовой фации (>18 кбар), возраст которых оценивается в 2780 ± 64 млн лет [Saha et al, 2011]. Для метавулканитов зеленокаменных комплексов кратона по данным [Singh, 2012] предполагался метаморфизм низких ступеней, времени проявления 3,2–2,6 млрд лет, в РТ-условиях 480–550 °С и 4–5 кбар
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