Fluid evolution in the Bundelkhand granodiorite, North Central India: Implications to hydrothermal activities in the Bundelkhand Craton

The Bundelkhand granodiorite (BG) constitutes the bulk of the granitoid complex in the Bundelkhand Craton and preserves the imprints of its evolution from magmatic to protracted hydrothermal stage as deduced from petrography. Hornblende thermobarometry and plagioclase – hornblende thermometry furnish pressure temperature ranges of 1.48 – 5.97 kbar and 523.6℃ – 724.2C whereas Ti-content in biotite furnishes a temperature range of 489℃ to 669℃. A very broad temperature history is retrieved from biotite-apatite pairs which vary from 824℃ – 394℃. Magmatic epidote preserves its primary texture and furnishes temperature as high as 573 °C while the hydrothermally altered grains furnish temperatures down to 160 °C. Chlorite thermometry furnishes a temperature range of ~217℃ to 340℃ by using the semi-empirical formulations. A near similar range of temperatures (95 ℃ to 400 ℃) have been obtained from fluid inclusion microthermometry. The residual and late-stage fluid in BG is dominantly a low saline aqueous fluid (~0 to 5 wt% NaCl eq) with a broader salinity value of ~0 to 21.06 wt% NaCl eq. with restricted occurrences of H2O-CO2/CO2 fluid. The aqueous-carbonic inclusions represent a high-temperature fluid > 330℃ corresponding to a pressure of 2.6kb which is correlated to the lower pressure value from hornblende barometry. Laser Raman microspectrometry reveals many silicate, oxide, and carbonate daughter / captive phases in the aqueous-carbonic inclusions. Thus, the Bundelkhand granodiorite preserves imprints of fluid activity at high temperatures corresponding to the initial stage of exsolution of CO2-charged fluid, to lower temperatures as a result of cooling, mixing with meteoric fluid that caused variations in salinity and prolonged internal evolution of the late-stage fluid. The fluid characteristics in BG domain is strikingly similar to that in the giant quartz reefs (Rout et al., 2022) even to the extent of identical solid phase bearing inclusions. Simulation of crystallization evolution of hydrous BG magma from liquidus temperature (~800 °C) and pressure of 3.5 kilo bars to 400 °C and 1500 bars was attempted. The results broadly reproduced the sequence of crystallization of primary minerals of observed chemistry and indicated high water content of the residual melt after 60% crystallization that quite explains the late-stage fluid activity. A broad contemporaneity of crystallization of quartz in BG (both matrix and veins) and formation of giant quartz reef from the same late-stage fluid can thus be surmised.