Origin of graphic texture by magma mixing-induced undercooling in the Borjuri diorite pluton, Mikir Massif, Northeast India

Abstract

Graphic intergrowths from the Borjuri diorite pluton in the Mikir Massif of Northeast India were investigated to reflect on the role of magma mixing in their formation. The work presented discusses how magma mixing triggers an abrupt change in liquidus temperature (undercooling), driving the formation of graphic intergrowths. Previously, fractal dimensions were utilised in experimental work to understand the involvement of undercooling in graphic texture formation. The present study follows the earlier documented experimental work and puts forward a new approach, as it employs fractal and lacunarity calculations in natural systems (i.e., natural diorite from the Mikir Massif). The fractal and lacunarity calculations presented in this work demonstrate the complexity of graphic intergrowths, in order to qualitatively assess the relative degree of undercooling. Undercooling, resulting from the rise in liquidus temperature, was achieved due to loss of volatiles or devolatilization during magma mixing. Observed disequilibrium textures such as rapakivi, anti-rapakivi, skeletal amphiboles, acicular apatite, biotite clots, dendritic and zoned plagioclase, provides a reliable explanation for an origin linked to magma mixing, further providing a different perspective for the growth of such graphic intergrowths. Signatures of magma mixing have been reported from numerous Pan-African felsic plutons in the Assam-Meghalaya Gneissic Complex. Considering the regional viewpoint and the textural signatures observed in the diorites, magma mixing could be the only plausible explanation for the formation of the diorites.