The Magmatic and Metamorphic Record of the 3.5-3.3 Ga Mt. Edgar Complex, Pilbara Craton – Told by Coupled Hf and Nd Isotopes and Garnet Geochronology

Abstract:

Understanding the geochemical evolution of Earth’s mantle and crustal reservoirs remains a longstanding question in the geosciences. In this study we focus on the granite-greenstone dome-and-keel terranes of the Pilbara Craton—specifically in the Mt. Edgar Granitic Complex (MTE)—to better understand how the earliest continents grew and stabilized. Interpretation of Hf and Nd isotope data, which provides an important window into these processes, can be ambiguous in Earth’s oldest rocks due to post-crystallization trace-element mobility. Here, we apply a “full-arsenal” analytical approach to these rocks by coupling: U-Pb ages and Hf isotopes in zircon; U-Pb ages and Nd isotopes in monazite, titanite, and apatite; Lu-Hf and Sm-Nd garnet geochronology; and whole rock Lu-Hf and Sm-Nd isotopes. U-Pb ages of zircons in MTE granitic rocks range between 3.49-3.32 Ga and have broadly chondritic initial Hf isotope compositions (εHf= ~0 ± 1). U-Pb ages of monazite, titanite, and apatite also range between 3.49-3.32 Ga, in agreement with the zircon ages of these rocks; these also have chondritic initial Nd isotope compositions (εNd= ~0 ± 1). Whole rock Hf and Nd compositions (εHf_(i) = ~0 ± 1 and εNd_(i)= ~0 ± 1) agree well with the isotope compositions of their accessory mineral assemblages. Garnet Lu-Hf and titanite U-Pb geochronology of metabasaltic rocks in the Warrawoona Group yield ages of 3.33-3.30 Ga, which we interpret as the timing of metamorphism, and have a corresponding initial Hf isotope composition of εHf = +0.2. The systematic agreement between Hf and Nd isotopes and U-Pb accessory mineral ages demonstrates these systems have remained closed since the Paleoarchean – the Hf and Nd isotope systems are coupled in these 3.5-3.3 Ga rocks. Together, Hf and Nd isotopes suggest derivation of magmas from a chondritic source (at ~3.5-3.3 Ga), and that this crust represents juvenile additions from the mantle without significant input from older evolved crust. Overlapping igneous and metamorphic ages suggest the time between crustal production and deformation was short.