Deciphering the origin of early continental crust: insights from newly identified ~3.8 Ga granitoids in the Anshan area, North China Craton

Abstract

Listen

Translate article

ABSTRACT Numerous questions regarding the formation and evolution of Earth’s early continental crust remain unresolved. Ancient rocks are essential to understanding these early geological processes. This study provides new geochronology, geochemistry, and zircon Hf-O isotope data from recently discovered Eoarchean granitoids in the Anshan area of the North China Craton (NCC). Zircon U-Pb dating indicates that these granitoids formed at ca. 3.8 Ga and include both trondhjemitic and monzogranitic gneisses. The trondhjemitic gneisses are characterized by high SiO2 and Na2O contents, low magnesium number (Mg#), low Sr/Y and (La/Yb)N ratios, and slightly negative Eu anomalies. These rocks exhibit enrichment in light rare earth elements (LREE) with flat heavy rare earth element (HREE) patterns. Their zircon εHf(t) values range from −3.34 to + 1.09, with two-stage model ages (TDM 2) between 4.43 and 3.99 Ga. The magmatic zircons show δ18O values of + 5.33‰ – +6.98‰. These geochemical and Hf-O isotope features suggest that the trondhjemitic gneisses, classified as low-pressure type, likely formed through the partial melting of Hadean to early Eoarchean mafic proto-crust under upper-amphibolite facies conditions. In comparison, the monzogranitic gneisses display higher SiO2 and K2O levels, lower Sr/Y, (La/Yb)N, and (Gd/Yb)N ratios, and more pronounced negative Eu anomalies. These monzogranitic gneisses are enriched in Rb, Th, and U and depleted in Ba, Sr, Nb, P and Ti. Such geochemical traits classify them as highly differentiated granites. Zircon Hf isotope data reveal radiogenic values, with TDM 2 ages between 4.23 and 4.01 Ga and εHf(t) values ranging from −1.60 to + 0.85. These evidences suggest that the monzogranitic gneisses were derived from the partial melting of ancient felsic source and underwent fractional crystallization during the late stages of magma evolution. The diversity of the ca. 3.8 Ga granitoids indicates the presence of a highly evolved continental crust in the NCC during the early Eoarchean. Hf isotope data show that the earliest crust grew primarily through the partial melting of juvenile crust derived from a depleted mantle, accompanied by certain reworking or recycling of pre-existing continental material. The ca. 3.8 Ga trondhjemitic and monzogranitic gneisses likely formed in an extension environment, such as intraplate rifting setting. Continental crust evolution in the NCC during early Eoarchean was primarily driven by magma underplating associated with asthenospheric mantle upwelling, potentially linked to mantle plume activity.