Priyadarshi Chowdhury

Abstract

Earth’s earliest continental crust, archived in “cratons”, is composed of granitoids and greenstone belts. The granitoids are dominantly represented by a sodium-rich, tonalite-trondhjemite-granodiorite (TTG) suite of rocks that formed via partial melting of hydrated metabasalts at pressures mostly > 1 GPa. However, all cratons show a change from TTG magmatism to more potassium-rich granite magmatism (K₂O/Na₂O > 0.8) during the late Archean. These K-granites form a minor component of the Archean continental crust but represent its thermal and physical maturation and stability. While a lot of work has been done on understanding the petrogenesis of TTGs, the origin of K-granite remains less understood. It remains debated how Na-rich TTGs (K₂O/Na₂O < 0.6) or mafic rocks can produce highly K-rich melts, and what melting reactions drive them. Ample number of experimental works, simulating the anatexis of TTG and mafic sources under different physical conditions, have provided diverging outcomes. Here, we use thermodynamic modelling and geochemical modelling to reveal the melting reactions and pressure-temperature (P-T) conditions of Archean K-granite formation. The modelled melt compositions are then compared with those of natural K-granites to validate the results. We model the role of melt-loss and degree of melting in the generation of K-granites. The results show that the melting of TTG sources produces potassic melt compositions at low melt-fractions (15-20 vol. %) and at 800-900°C and < 1.2 GPa. The formation of these melt-fractions is controlled by the dehydration-melting of biotite. Trace element composition of these K-rich modelled melts are consistent with that of the natural K-rich granitoids. Therefore, we infer that intra-crustal melting of the older TTG dominated crust can produce significant K-granite magma that leaves a granulitic residue in the lower crust leading to a more differentiated and thick crust.

Acknowledgment: This talk is based on an ongoing work in collaboration with Dr. Subham Mukherjee (The Univ. of Delhi, India).