The Eoarchean Muzidian gneiss complex: Long-lived Hadean crustal components in the building of Archean continents

Abstract

The nature of Earth's first crust and the processes that formed it are poorly constrained due to limited exposures of >3.7 billion-year-old (Ga) rocks. Here we report the discovery of a new Eoarchean terrane named the Muzidian gneiss complex in the Yangtze craton of central China where rocks as old as 3.81 Ga are preserved. In this study, we characterized six samples (including five TTGs and an amphibolite) through zircon U-Pb age, Hf isotope and bulk-rock 146,147Sm-142,143Nd isotope compositions. The subchondritic zircon initial Hf isotope compositions and the negative μ142Nd values for the 3.81 – 3.65 Ga samples reveal that these rocks were most likely reworked from >4.3 Ga mafic crust. The 2.5 – 2.4 Ga rocks in the same complex are the youngest felsic rocks on Earth with deficits in 142Nd, highlighting the long-lived role of Hadean crustal components in the building of a stable continent. The 147Sm-143Nd isotopic systematics of these rocks are disturbed. The currently available data for global Eoarchean rocks suggest two distinct lineages for > 3.6 Ga Eoarchean crustal blocks, one produced by reworking of Hadean mafic crust with the isotopic signature of coupled negative εHf(t) and μ142Nd, and the other by melting of incompatible-element-depleted mantle sources residual to Hadean crust formation characterized by positive μ142Nd and near chondritic εHf(t). The Eoarchean samples of the Muzidian gneiss complex in the present study, along with >3.6 Ga rocks in the Acasta, Napier and Nuvvuagittuq regions imply a Hadean crustal source distinct from the products expected for magma ocean crystallization.