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.