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

Abstract

The origin of reddening of granitoid is controversial. Red granitoids, occurring spatially with grey granitoids with a thin transitional zone is a volumetrically significant litho‐type in the Bundelkhand Craton, north central India. Using detailed petrography and microstructure study, and phase compositions and elemental X‐ray maps, we demonstrate for the first time that pervasive infiltration of Fe‐Mg‐Na‐K‐rich fluid caused re‐equilibration of ~2.5 Ga grey monzogranite during both brittle and ductile deformation in the craton. The reddening of granitoid is attributed to replacement and precipitation of tiny Fe‐rich particles in porous feldspars by the high temperature Fe‐Mg‐Na‐K‐rich fluid (~950°C), plausibly from crust–mantle depths. The Fe‐Mg‐Na‐K‐rich fluid with high to moderately high FeO (≤27 wt%), MgO (≤19 wt%), Al2O3 (≤20 wt%), Na2O (<1.59 wt%), K2O (<2.74 wt%), low CaO, (≤0.28 wt%), and FeO/MgO (<1) occurs as discrete interconnected network of green colour veins in the rock. We infer that the veins filled with green‐coloured material along deep shear planes/mylonitic fabric in granitoids and grain boundaries and fractures of felsic minerals (feldspar and quartz) acted as highly permeable network conducive for pervasive fluid activity in the area that influence the normative mineralogy of granitoids yielding pyroxene in the norm. Replacement of original K‐feldspar by plagioclase (albitization), and again by K‐feldspar (K‐feldspathization) by deep fluid took place during which Al2O3 was broadly conserved, with no significant gain or loss in alkalies, a marginal loss in CaO and, but a huge gain in FeO and MgO. The Fe‐Mg‐Na‐K‐rich fluid (with normative olivine + pyroxene) is responsible for near isochemical subsolidus alteration and replacement–precipitation process of feldspars in granitoids. We suggest that the Fe‐Mg‐Na‐K‐rich fluid is of crust–mantle derivation and could intrinsically be linked to Paleoproterozoic dolerite dyke swarm activity in the craton.

The origin and localization of “vermiculite” along the intra‐terrane shear zones in the Bundelkhand Craton, India: Mechanism and implication

“…Before the current investigation, a general term "green-coloured vein material" was assigned to the vein infills (e.g., Sensarma et al, 2018); however, the XRD analysis and microscale observations carried out in the present study reveal that these green-coloured vein materials consist of vermiculite + epidote + quartz (Figure 6a). The most significant peaks of XRD analyses at d values 14.209 Å (41.96%), 7.097 Å (100%), Boettcher, 1966;Davies & Bisset, 1948;Johnson, 1964).…”

Superplastic deformation inside the knife-sharp shear bands in mid-crustal granites

“…NW-SE and NE-SW are nearly coeval conjugate sets, and N-S trending deformation was the youngest among the events (Rao et al, 2005;Bhattacharya and Singh, 2013;Singh and Bhattacharya, 2017). The BGGC carries the imprints of 0.5-0.6 Ga old Pan-African event, responsible for the knife-sharp juvenile shear bands in the craton (Kaur et al, 2016;Sensarma et al, 2018). This event is preceded by the post ~2.5 Ga event of grey-granite to pink-granite transformation in the presence of fluids, which induced reactions in nominally anhydrous minerals like feldspars and increased the porosity of the granites (Putnis, 2021).…”

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

“…The boundary between red and grey granitoid is gradational where the redcoloured granitoid fades gradually into greycoloured granitoid with progressive decrease in red colouration. 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).…”

“…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).…”