Diversity in early crustal evolution: 4100 Ma zircons in the Cathaysia Block of southern China

Abstract

Zircons are crucial to understanding the first 500Myr of crustal evolution of Earth. Very few zircons of this age (>4050Ma) have been found other than from a ∼300km diameter domain of the Yilgarn Craton, Western Australia. Here we report SIMS U-Pb and O isotope ratios and trace element analyses for two ∼4100Ma detrital zircons from a Paleozoic quartzite at the Longquan area of the Cathaysia Block. One zircon (207 Pb/ 206 Pb age of 4127 ± 4Ma) shows normal oscillatory zonation and constant oxygen isotope ratios (δ18O = 5.8 to 6.0‰). The other zircon grain has a ∼4100Ma magmatic core surrounded by a ∼4070Ma metamorphic mantle. The magmatic core has elevated δ18O (7.2 ± 0.2‰), high titanium concentration (53 ± 3.4ppm) and a positive cerium anomaly, yielding anomalously high calculated oxygen fugacity (FMQ + 5) and a high crystallization temperature (910°C). These results are unique among Hadean zircons and suggest a granitoid source generated from dry remelting of partly oxidizing supracrustal sediments altered by surface waters. The ∼4100Ma dry melting and subsequent ∼4070Ma metamorphism provide new evidence for the diversity of the Earth's earliest crust. © 2014 Macmillan Publishers Limited. All rights reserved.