Fractal analysis and geochemical characterization of mafic magmatic enclaves in the Kathalguri Pluton, Mikir Massif (Northeast India): implications for Pan-African bimodal magmatism

The similar to 515 Ma Kathalguri Pluton of Mikir Massif, Northeast India, presents the first evidence of Pan-African bimodal magmatism from the easternmost part of the Indian shield. It is characterized by a number of outcrop-scale features like mafic flows and mafic magmatic enclaves (MME) within granitic host rocks, which suggests that mafic magma intruded into the felsic magma chamber during its evolution. Hybrid intermediate rocks are also encountered in the granitoid body indicating proper mixing between the mafic and felsic magmas. The mafic flows and MME are observed at the lower portion of the pluton, while the hybrid intermediate rocks are distributed in the upper part. Such field observations suggest that the felsic magma chamber was vertically zoned when mafic magma intruded into it. Textural features associated with magma mixing like resorbed crystals, boxy-cellular morphology, and oscillatory zoning in plagioclase, including orthoclase-microcline transformation are preserved in the hybrid rocks. Geochemical signatures confirm that magma mixing has played an important role in the evolution of the Kathalguri Pluton. Our results also suggest that differential mobility of elements was responsible for variable rates of homogenization in the magma mixed system. From fractal analysis, it has been inferred that MME from the Kathalguri Pluton underwent different degrees of interaction with the felsic magma. From lighter to darker MME (based on color index), the complexity of the interface morphologies increases with increasing D-box (fractal dimension) and log V-R (viscosity ratio) values, which indicates decreasing rate of homogenization between the mafic and felsic end-members. Moreover, viscosity calculations also suggest that the more viscous smaller MME are more evolved than the relatively less viscous larger MME. The mafic rocks preserved in the host pluton show higher LILE/HFSE and LREE/HREE ratios, and dominantly plot in the field of 'within-plate basalts' in various tectonic discrimination diagrams. From the results presented in this work, we infer that the Kathalguri Pluton developed in a post-collisional, within plate extension setting through melting of mid- to lower-crustal rocks by mantle-derived mafic magmas. The mafic magmas during the Pan-African episode were generated in a continental rift setting, which was probably activated by asthenospheric upwelling.

Automated summary

The Kathalguri Pluton in Northeast India shows evidence of Pan-African bimodal magmatism and the important role of magma mixing in its evolution.