Magnetite-manganoan ilmenite transformation and its implications for magma mixing and migmatization in the Assam-Meghalaya Gneissic Complex, Northeast India

The migmatites of Chandrapur area near Guwahati, which forms a part of the Assam-Meghalaya Gneissic Complex (AMGC) in Northeast India, have preserved magnetite ocelli. Petrographic observations have revealed that magnetite crystals occurring in the ocellar rock are surrounded by partial to complete thin rims of biotite. Moreover, mineral chemical analyses, backscattered electron images, and quantitative elemental mapping have revealed that magnetite crystals occurring in the ocellar rock have been extensively replaced by manganoan ilmenite at the periphery. From field observations, textural, and mineral chemical analyses, it has been inferred that invasion of hot, volatile-bearing pegmatitic magmas into thermally rejuvenated basement gneisses of the AMGC during the late Pan-African tectonothermal episode (ca. 530-450 Ma) induced very restricted partial melting in the gneissic rocks producing neosomes. The newly formed neosomes were then infiltrated by the nearby pegmatitic melts leading to mixing between the two melts. The mixing event facilitated transformation of magnetite to manganoan ilmenite owing to diffusion of elements like Mn and Ti from the neosome to the pegmatitic domains. Moreover, formation of ilmenite released surplus Fe hosted in the magnetite that combined with in situ K and Al, and diffusing Mg from the neosome to form biotite crystals around magnetite. From this study, it can be concluded that magnetite-manganoan ilmenite transformation may be considered as one of the petrogenetic indicators to decipher magma mixing events.