Fractal analysis and formation of biotite clots within double-layered mafic magmatic enclaves in the Bamuni Pluton, Mikir Massif, Northeast India

Abstract

Biotite clot is a rare magma mixing texture and its occurrence may provide valuable information to understand magma mixing processes. Back-veining, a common yet underappreciated phenomenon is emphasized here, which was liable for the formation of biotite clots within double-layered mafic magmatic enclaves (MME) in the Bamuni Pluton, Mikir Massif, Northeast India. From field observations, the existence of two distinct portions in this pluton, viz. lower portion comprising coarse-grained granite and upper portion consisting of fine-grained variety, depicts that the pluton represents a fossilized vertically zoned granitic magma chamber. Moreover, many MME of varying sizes are distributed throughout the pluton. The presence of quartz ocelli, sieve-textured plagioclase, acicular apatite, and biotite clots within MME at the upper portion of the pluton suggests mixing between mafic and felsic magmas. Biotite clots are distributed within outer layer of double-layered MME at the upper part of this pluton. Composition of biotite found in clot-bearing MME, composition of biotite found in granitic rocks surrounding the MME, and composition of biotite found in granitic rocks away from the MME, all plot in the field of siderophyllite. Compositions of plagioclase from the three zones plot in the fields of oligoclase and albite. Moreover, feldspars from the granitic rocks surrounding the MME plot in fields of anorthoclase and sanidine. Furthermore, fractal dimension (Dbox) and viscosity ratio (log VR) calculations reveal that the outer layers of the double-layered MME with lower Dbox and log VR values correspond to higher degree of hybridization between the MME and felsic host in comparison to the inner layers of the MME. Viscosity model also reveals that the viscosity of outer layer plot in between the inner layer of the MME and host granitic rocks. From our results, we infer that biotite clots within double-layered MME were formed due to the back-veining of felsic melt during the interaction between hotter MME and granitic host.