Evidence for Protracted Intracrustal Reworking of Palaeoarchaean Crust in the Pilbara Craton (Mount Edgar Dome, Western Australia)

Abstract

~3.5-2.8 Ga granitoids from the Pilbara Craton in Western Australia are one of the most ancient and best-preserved records of early processes of continental crust generation. A number of recent studies have focused on the nature of the mantle source from which Pilbara granitoids derived, yet no consensus has been reached on whether the mantle was chondritic or depleted in the Eo/Palaeoarchaean. Here we present integrated whole-rock (major and trace elements) and zircon (U-Pb and Lu-Hf isotopes) data for 10 granitoids sampled across the Mount Edgar Dome, which recorded four main magmatic events between 3.47 and 3.23 Ga. Whole-rock major and trace element analyses suggest that the samples belong to two distinct petrogenetic groups. The first group is akin to the tonalite-trondhjemite-granodiorite (TTG) suite, representing highly fractionated magmas initially formed by partial melting of a basaltic crust. The second group, here classified as granites, is best interpreted by the remelting of a basaltic crust and the addition of more evolved material, and it is striking that TTG-like and granitic magmas occurred coevally in time and space. Overall, both groups were formed through intense intracrustal differentiation processes that lead to the loss of significant geochemical information about their original sources. High-precision Lu-Hf analyses in zircon allow to obtain such information and to trace back the isotopic composition of the Palaeoarchaean mantle. A clear change from superchondritic to subchondritic Hf isotope compositions is observed between 3.47 and 3.23 Ga. The superchondritic Hf isotope composition of the 3.47 Ga old granitoids substantiates derivation from a depleted mantle source that separated from the chondritic mantle prior to 3.8 Ga. The presence of ca. 3.5 Ga old inherited zircons in younger magmas suggests that crustal remelting processes were involved in their generation. We propose that all granitoids investigated in this study had their crustal sources originated from a single mantle–crust differentiation event that occurred at 3.50 Ga. This event resulted in the differentiation, from the same original mantle, of two distinct crustal reservoirs, i.e., a mafic reservoir with a 176Lu/177Hf ratio of 0.023, and a reservoir of intermediate/felsic composition ( 176Lu/ 177Hf=0.013). 3.32-3.31 Ga-old granitoids were produced by remelting of the mafic reservoir, whereas 3.43 and 3.23 Ga granitoids derived from the intermediate/felsic reservoir. Overall, our data suggest that protracted intracrustal remelting processes and differentiation have played a key role in the formation, evolution, and maturation of the building blocks of continents during the Palaeoarchaean.