Submagmatic Fabric in the 2.6 Ga Bundelkhand Granitoid, India:Evidence from Microstructure

Abstract

Primary foliations in igneous rocks are the key to understanding processes within the magma chamber, cooling history, paleostress regime and strain during emplacement of granitoid plutons. In the 2.6 Ga Bundelkhand granitoid rocks, weak to moderately strong regional fabric is the result of submagmatic grainsupported flow during syn-kinematic emplacement of the magma. Microstructural and outcrop-scale evidences provide an excellent record for interpreting the significance of this fabric development vis-à-vis rheological state during fabric formation. Preferentially oriented tabular feldspar phenocrysts in the granitoid show deformation such as reformed shape, marginal fragmentation and recrystallization, whereas the finer interstitial grains of quartz remain undeformed or mildly deformed. On the basis of the characteristic foliated structure of the Bundelkhand granitoid and the specific contrasting deformation characteristics of feldspar and quartz grains, we propose that the fabric is submagmatic in origin and perhaps formed during syn-emplacement deformation environment of the granitoid.

Grain-scale anatomy of the Bundelkhand granite: Implications for the interplay of magmatic to sub-magmatic deformation mechanisms

“…Instead of considering the red and grey granitoids are of separate entities, Sensarma et al (2018) suggested that the original grey-coloured granitoid transformed to red-coloured granitoid due to mantle-derived high temperature fluid-rock metasomatic alteration. The conclusion of Sensarma et al (2018) is further supported by the observation that both grey-and red-coloured granitoids show similar modal mineralogical compositions (Sarkar et al 2017) and contemporaneous crystallisation age (Kaur et al 2016). We, therefore, use the common term granitoid to refer to both red-and grey-coloured granitoid rocks that exhibit an undoubtedly porphyritic texture under the microscope (figure 1c).…”

Reddening of ~2.5 Ga granitoid by high‐temperature fluid linked to mafic dyke swarm in the Bundelkhand Craton, north central India

“…To determine the relative proportions of dominant mineralogy including different types of feldspar and to ascertain interrelation between the red colouration and primary modal abundance of feldspar types in these granitoids, modal analyses of both grey and red granitoids were done (see fig. 2 in Sarkar et al, ). It is important to note that both grey and red granitoids fall in the monzogranite field, except for limited samples, which plot as granodiorite and syeno‐granite and thus could not be petrographically discriminated.…”

“…Large euhedral to subhedral feldspar crystals and relatively smaller grained quartz and mafic ferromagnesian minerals (in variable proportions <10%) constitute the granitoids. The inequidimensional and elongated tabular feldspar crystals and ferromagnesian minerals are preferentially arranged imparting a weak submagmatic fabric (Sarkar et al, ), which gives a crude apparent gneissic appearance to the rocks. Under the microscope, green‐coloured microveins are abundantly and intricately present particularly along grain boundaries, fractures, and cleavage planes in the feldspars in the red granitoids.…”

“…Length of pen is 13 cm [Colour figure can be viewed at wileyonlinelibrary.com] red granitoids were done (see fig. 2 in Sarkar et al, 2017). It is important to note that both grey and red granitoids fall in the monzogranite field, except for limited samples, which plot as granodiorite and syeno-granite and thus could not be petrographically discriminated.…”

“…For example, near Datia the red granitoid is intruded by the NW‐SE trending dolerite dyke (see fig. 1 in Sarkar, Matin, & Sensarma, ). In the north central part near Jhansi, the reddening of granitoid is vivid and inescapable in the vicinity of a dolerite dyke (see Figure a).…”