Evolution of the Neoarchean lower crust beneath Western Liaoning of the North China Craton: Evidence from U-Pb-Hf-O isotopes in zircon

Abstract

A comprehensive analysis of U-Pb geochronology and Hf-O isotopes of zircons from granulite xenoliths hosted in the Cretaceous basalt and zircon xenocrysts in the Jurassic andesitic volcanic rocks of Western Liaoning within the North China Craton (NCC) provides an opportunity to understand the evolution of the lower crust in the region. Zircons from these granulites yield 207Pb/206Pb ages of 1678–2646 Ma and consistent upper intercept ages of 2514–2544 Ma. In contrast, zircon xenocrysts yield two periods of 207Pb/206Pb ages of Neoarchean (2500–2591 Ma) and Paleoproterozoic (1733–2499 Ma), which produce consistent upper intercept ages of 2543–2549 Ma and lower intercept ages of 1761–1789 Ma, respectively. The Neoarchean zircons from the granulites and xenocrysts generally have similar positive εHf(t) values resulting in Hf depleted mantle model ages (TDM) of 2.5–2.9 Ga with a mode of 2.7 Ga and variable δ18O (5.05‰ to 8.81‰) values. However, the δ18O values of zircons from the xenocrysts are a little bit higher than those of zircons from granulites (peak of 8.00‰ for the xenocrysts and 6.00‰ for the xenoliths). Some Paleoproterozoic zircon xenocrysts have negative εHf(t) values (−12.8 to −9.39) and similar 176Hf/177Hf ratios and δ18O values to those of the Neoarchean zircons, whereas the other Paleoproterozoic zircons show chondritic εHf(t) (−4.34 to 1.16) and higher δ18O (7.70‰ to 10.19‰) values. All these data demonstrate that the primordial lower crust beneath Western Liaoning originally formed in the Neoarchean, perhaps around 2.6–2.7 Ga. The ubiquitous presence of ca. 2.5 Ga magmatic zircons illustrates that an important melting event indeed happened in the lower crust beneath Western Liaoning, similar to the thermal event throughout the entire NCC. In addition, the Neoarchean lower crust underwent another important thermal event during the Paleoproterozoic, which was considered to be associated with the final amalgamation of the Western and Eastern blocks of the NCC. Thus, we can conclude that there indeed existed an Archean lower crust beneath the Western Liaoning until to the Late Mesozoic, and this Archean lower crust underwent a similar multiple-stage (Neoarchean and Paleoproterozoic) evolutionary history as those throughout the NCC.