Neodymium Isotope Constraints on the Origin of TTGs and High-K Granitoids in the Bundelkhand Craton, Central India: Implications for Archaean Crustal Evolution

Abstract

The Bundelkhand craton in central India consists mainly of abundant high-K granitoids formed at the Archaean-Proterozoic boundary and several enclosed rafts of TTGs (tonalite-trondhjemite-granodiorites) up to 3.5 Ga. Therefore, the Bundelkhand craton is a key locality for studies on Archaean crustal growth and the emergence of multisource granitoid batholiths that stabilised a supercontinent at 2.5 Ga. Based on their geochemical characteristics, the high-K granitoids are divided into low silica–high Mg (sanukitoids and hybrids) and high silica–low Mg (anatectic) groups. We aim to provide new insights into the role of juvenile versus crustal sources in the evolution of the TTG, sanukitoid, hybrid, and anatectic granitoids of the Bundelkhand craton by comparing their key geochemical signatures with new Nd isotope evidence on crustal contributions and residence times. The ages and geochemical signatures as well as εNd(⁠t ⁠) values and Nd model ages of TTGs point towards partial melting of a juvenile or short-lived mafic crust at different depths. Paleoarchaean TTGs show short crustal residence times and contributions from the newly formed crust, whereas Neoarchaean TTGs have long crustal residence times and contributions from the Paleoarchaean crust. This may reflect the transition from melting in a primitive oceanic plateau (3.4-3.2 Ga) in plume settings, resulting in a Paleoarchaean protocontinent, to 2.7 Ga subduction and island arc accretion along the protocontinent. The 2.5 Ga high-K granitoids formed at convergent subduction settings by partial melting of the mantle wedge and preexisting crust. Sanukitoids and hybrid granitoids originated in the mantle, the latter showing stronger crustal contributions, whereas abundant anatectic granitoids were products of pure crustal melting. Our Nd data and geochemical signatures support a change from early mafic sources to strong crust-mantle interactions towards the A-P boundary, probably reflecting the onset of supercontinent cycles.