Magmatic enclaves in the Archaean Bundelkhand Granitoid (India) formed by hydrated mantle-lower crust interactions

Abstract

Unravelling the role of crust-mantle processes in the evolution of large silicic-intermediate magmatic systems is a major contemporary research frontier. Magmatic enclaves (MEs), common in granitoid, may provide important information and bring additional constraints in this endeavour. Critical to this is whether the MEs represent earlier rock (xenoliths) or cumulates (and restites) or contemporaneous magma(s) to host granitoid integral to the same tectono-magmatic system. Using new field, petrography and microstructural evidence, and detailed bulk geochemical compositions (major and trace element compositions including REE), we recognize for the first time MEs of felsic-intermediate lower crustal litholgies, e.g. magmatic charnockites (730°C–855°C), TTG (∼817°C) and felsic granulites (∼616°C), and mafic MEs (enriched basalts/picro-basalt: 1170°C–1217°C) in the ∼2.7–2.3 Ga Bundelkhand granitoid (India). We show that MEs represent coeval magmas that experienced variable extents of crustal-mantle melt interactions, yet coexisted in the same reservoir as that of the host granitoid and incorporated into the mush at deeper crustal levels (∼5–10 kbar) in the NeoArchaean-Palaeoproterozic tectono-magmatic system. Our study highlights transcrustal Bundelkhand granitoid not only exposes lower crustal lithologies, including TTG, but the role of hydrated OIB-like mantle was critical to Archaean mid-to lower-crustal anatexis in fluid-present conditions. The fluid-present lithospheric melting may explain the production of sodic crust, including TTG in the ancient Earth, irrespective of geodynamic setting.