Chapter 31 - Paleoarchean Crustal Evolution of the Bundelkhand Craton, North Central India

Abstract

Paleoarchean tectonic evolution of tonalite–trondhjemite–granodiorite (TTG) gneisses from the western part of the Bundelkhand tectonic zone (BTZ) has been studied by integrated structural analyses, petrological observations, and thermodynamic modeling and U–Pb SHRIMP zircon ages. U–Pb SHRIMP zircon ages reveal that the TTGs crystallized at ∼3.4Ga and inherited zircons from a preexisting ∼3.5Ga crust. TTGs demonstrate fractionated rare-earth element patterns with negative Nb, Ti, and positive Pb anomalies. Three phases of folding have been observed in the TTGs. First, two phases leading to coaxial deformation and generation of hook-shaped folds with N–S trending axial planar foliation defined by oriented hornblende and biotite against parallel leucocratic layers constituted of plagioclase and quartz. Melt present textures, and deformation microstructures from the N–S foliation plane indicate that both phases of folding occurred during crystallization of the magma. P-T estimates by conventional hornblende-plagioclase thermometers and barometers and P-T pseudosection analyses suggest that assemblages in the N–S foliation domains equilibrated at ∼6.5–8.5kbar, 630–720°C, with possible near isobaric cooling to 500°C. Third stage of foliation led to formation of open folds with E–W closures and sparsely developed axial planar foliation, which is intruded by ∼2.5Ga potassic granite. E–W trending brittle-ductile shear planes (with sinistral shear sense) are observed in the TTGs, and also the intruded potassic granite. Within a protomylonite TTG, brittle deformation of plagioclase and ductile recrystallization of quartz along with chloritization, albitization are observed, indicating low temperature (∼300°C) shearing at ∼3–4kbar pressure. We correlate Pan African ages (640–600Ma) obtained from lower intercept the U–Pb SHRIMP zircon data from the TTGs as the age of shearing. The TTGs thus have been exhumed at a depth of ∼10–15kbar along the BTZ possibly during the Pan-African time.