Decoding the onset of ca. 3.8 Ga continental nuclei in Anshan, North China: A review integrated with 1:10,000 geological mapping, zircon U-Pb dating, and Si-O-Nd-Hf-W isotopes

The Archean Anshan Complex (AAC) in northeastern China is one of the best-preserved ca. 3.8 Ga outcrops worldwide, which are key to understanding the evolution of the early Earth. In this study, we carried out 1:10,000 geological mapping and integrated this with published whole-rock geochemical, zircon U-Pb age, and O-Nd-Hf-W isotopic data, along with new zircon U-Pb and Si-O isotopic data. We used these data to establish a geochronological framework for the AAC and investigate its origins and tectonic setting, and differentiation of the early Earth. Our new mapping shows that the AAC can be divided into several rock associations based on the zircon U-Pb ages and lithologies: (1) Eoarchean (ca. 3.8 Ga) trondhjemitic gneiss and meta-quartz diorite; (2) Paleoarchean (ca. 3.3 Ga) gneiss and migmatite; (3) Mesoarchean (ca. 3.1 Ga) granitic gneiss; (4) Mesoarchean (ca. 3.0 Ga) monzogranite; (5) Neoarchean (ca. 2.5 Ga) K-rich granite; and (6) Archean supracrustal rock. The ca. 3.8 Ga rocks are exclusively exposed as enclaves within the ca. 3.3 Ga gneiss and migmatite unit at Dongshan, Guodishan, Shengousi, and Hujiamiao. Some ca. 3.8 Ga components at Baijiafen and Dongshan cannot be visually identified due to strong deformation and migmatization. The ca. 3.8 Ga Anshan trondhjemitic gneiss and meta-quartz diorite have coupled heavy Si-O isotopic compositions, indicative of the involvement of supracrustal silicified materials in their sources. (La/Yb)N ratios vary widely in the different ca. 3.8 Ga rock types (i.e., 9-53 for the trondhjemitic gneisses and 3-8 for the meta-quartz diorites), indicating they formed at different pressures. We suggest the ca. 3.8 Ga trondhjemitic gneisses were derived by partial melting of a subducted slab, and the 3.8 Ga meta-quartz diorites might have been derived by partial melting of a mantle wedge that had been metasomatized by fluids released from the subducted slab. Highly variable epsilon Nd(t) (-4.5 to +10.1) and epsilon Hf(t) (-3.8 to +6.1) values of the ca. 3.8 Ga rocks indicate that their protoliths involved at least two end-member components that were derived from enriched and ultra-depleted mantle reservoirs. Finally, the 182W excess (mu 182W = +8.3 to +12.9) of the ca. 3.8 Ga rocks might be due to either early mantle differentiation or a lack of late accreted materials. However, 142Nd isotopes (mu 142Nd = +1.8 to +9.2) of these ca. 3.8 Ga rocks do reveal a differentiation event in the early Earth that produced two end-member reservoirs with positive and negative mu 142Nd anomalies. We conclude that plate tectonics likely had an important role in the onset of continental nuclei in North China at ca. 3.8 Ga, and the precursors of these oldest rocks can be used to decipher multiple mantle-crust differentiation events during 4.5-3.8 Ga.

Automated summary

The Archean Anshan Complex in northeastern China, with its well-preserved ca. 3.8 Ga outcrops, provides important insights into the early evolution of the Earth. Through detailed geological mapping and integration of various isotopic and geochemical data, this study established a geochronological framework for the complex, shedding light on its origins, tectonic setting, and differentiation processes in the early Earth.