New evidence for Neoarchean (ca. 2.7 Ga) crustal growth in the North China Craton

Abstract

The North China Craton (NCC) is characterized by major ca. 2.5 Ga tectonothermal events, which is markedly different from most other typical cratons worldwide. Whereas, ca 2.7 Ga rocks are sporadically exposed in the whole craton, and only have been identified in several localities. In this contribution, we report new discovery of ca. 2.7 Ga rocks from the western Huixian in the NCC. Zircon U–Pb dating reveals their protoliths were emplaced between 2714 and 2704 Ma. The rocks are tonalitic gneisses in composition, and present similar geochemical characteristics to the Archean TTG suites and Phanerozoic high-SiO2 adakites worldwide. Zircons from the three tonalitic gneisses yield variable positive εHf(t) values of 2.3–6.7, 4.2–8.7, and 3.3–6.3, with Hf model average ages of ca. 2848 Ma, ca. 2838 Ma, and ca. 2840 Ma, respectively. The measured zircon δ18O values range from 5.5‰ to 6.2‰, 5.1 to 6.1‰, and 5.2 to 6.2‰, with weighted mean values of 5.7‰, 5.7‰, and 5.8‰, respectively, all of them overlap with the mantle range within two sigma uncertainty limits. The Hf-O isotope compositions in zircons from the tonalitic gneisses indicate that their protoliths originated in a juvenile crust-forming event at ca. 2.7 Ga. Available data suggest ca. 2.7 Ga is a major period of crustal growth in the NCC. Geochemical signatures of the ca. 2.7 Ga tonalitic gneiss reflect that their protoliths formed by partial melting of hydrated mafic rocks with variable proportions of the amphibole and garnet residues to be left behind in the residuum but absence of plagioclase and rutile in their source. The rocks are classified as HP-TTG, their protolith formed under amphibolite-facies conditions. It is noted that the ca. 2.7 Ga tonalitic gneisses in Huixian display low MgO (<2 wt%), Ni (16.6–23.5 ppm), and Cr (14.6–28.8 ppm) as well as uniform positive εHf(t) values and mantle-like zircon δ18O values, indicating that the protolith magma was likely produced by partial melting of a thickened lower oceanic crust.