Origin of magnetite phenocrysts in mafic magmatic enclaves of the Bamuni pluton, Mikir Massif, Northeast India

Abstract

Magnetite (Fe₃O₄) has been widely documented across igneous and metamorphic scenarios; yet the mechanisms governing its genesis in mafic magmatic enclaves (MMEs) and mixed rocks are inadequately constrained. The current work presents a distinctive effort to elucidate the underlying physicochemical conditions governing magnetite formation in MMEs triggered by magma mixing. Mafic magmatic enclaves are common structures in plutonic environments, and field investigations in the Bamuni Pluton of Mikir Massif, Northeast India, demonstrate mafic-felsic magma interaction, as evidenced by a widespread occurrence of MMEs throughout the pluton. Distinct euhedral magnetite phenocrysts occur within a biotite-rich matrix in the inner part of double-layered MMEs. P-T conditions estimated using biotite compositions show a pressure and temperature range of 3.93–5.89 kbar and 749–767 °C, respectively. These biotite compositions were also used as a parameter to estimate oxygen fugacity (fO₂), and subsequent calculations revealed that their crystallization took place in an oxidizing environment. The calculated logfO₂ values of biotite crystallization range between −9.9 and −9.5. Mafic-felsic magma interaction is evidenced by the presence of different disequilibrium textures, such as quartz ocelli, titanite ocelli, biotite clots, and acicular apatite. Apart from forming disequilibrium textures, magma mixing also facilitates substantial mass transfer leading to the destabilization of existing minerals and formation of new minerals. Subsequent to mafic magma intrusion and formation of MMEs in the studied pluton, precursor clinopyroxene possibly destabilized to amphibole. Further interaction between both the mafic and felsic magmas led to the diffusion of K₂O from the felsic to mafic domain. An increase in K₂O concentration in the mafic domain thereby caused the breakdown of amphibole to biotite, indicating an advanced hybridization stage. Liberation of FeO during the transformation of amphibole to biotite subsequently contributed to the formation of magnetite in the MMEs under oxidizing conditions.