Eoarchean apatite uncovers felsic foundations of the Pilbara Craton

Abstract

The fragmented record of early Earth is largely preserved in Archean granite-greenstone terranes, yet there is evidence that these ancient continental nuclei may themselves have been built on preexisting crustal substrates. The East Pilbara Terrane (EPT), Western Australia, is the archetypal granite-greenstone terrane, with exposed crust dating back to ca. 3.5 Ga. Radiogenic Hf, Nd, and Pb isotopic studies indicate that the EPT was built on older crustal foundations. However, apart from rare Eoarchean enclaves, xenocrystic and detrital zircons, the age and composition of these crustal foundations remains elusive. Here, we present detrital apatite geochronology and trace-element geochemistry from Paleo-Neoarchean sedimentary successions of the EPT. Apatite U-Pb dates largely record reset ages associated with Meso- and Neoarchean thermal events. The Lu-Hf system in the same crystals retains primary crystallization ages up to ca. 3.84 Ga—the oldest detrital apatite grains discovered to date. Sr-Y systematics indicate that >70% of Eoarchean detrital apatite grains were sourced from tonalite-trondhjemite-granodiorite−type felsic rocks, and the relative proportion of felsic to mafic sources did not change significantly throughout the Archean. We conclude that the EPT was built on Eoarchean crustal foundations that were comparable in composition to the presently exposed Paleo-Mesoarchean granite-greenstone crust, including an important component of felsic rocks.