Crustal growth and reworking of the eastern North China Craton: Constraints from the age and geochemistry of the Neoarchean Taishan TTG gneisses

The Taishan Complex is located in the central part of the eastern North China Craton (NCC), and preserves extensive Neoarchean TTG gneisses and potassic granitoids. Their formation age, geochemical features and petrogenesis can provide important insights into understanding the crustal growth and reworking events involving in the evolution of the craton. Previous studies were mainly focused on geochronology of these Neoarchean rock assemblages, but lacked study on their petrogenesis and related tectonic regime. In this study, we carried out lithological, geochronological and geochemical analyses for the representative TTG gneisses of the Taishan Complex. U-Pb zircon age dating results reveal that the Taishan TTG gneisses were mainly emplaced at 2718-2548 Ma. Lu-Hf isotopic data show uniform positive zircon epsilon Hf(t) values ( + 2.2 to + 6.6), with Hf model ages of 2949-2665 Ma, indicating that the TTG gneisses were mostly derived from juvenile crustal materials extracted from the depleted mantle during the Mesoarchean to Neoarchean. The delta O-18 values of zircons vary from + 4.6 parts per thousand to + 6.7 parts per thousand, indicating that the source region was not significantly affected by the supracrustal materials. Geochemical data show that these TTG gneisses have high Na2O (3.81-4.83 wt%) and ratio of Zr/Sm (24-80), but show low Mg-# (40-56), content of Sr (292-508 ppm), and ratios of Sr/Y (35.6-66.6) and Nb/Ta (10.1-15.9). These TTG gneisses were probably derived from partial melting of juvenile crustal materials under a relatively low-pressure environment, which were equilibrated with a certain amount of amphibole and plagioclase and minor garnet in the residue. It indicates that a large-scale magmatic underplating could be a most possible tectonic regime operating in this region during the Neoarchean. Our new results, combined with the available data from the adjacent areas, reveal a significant crustal growth event in the eastern NCC at ca. 2.94-2.70 Ga, which was followed by the pervasive crustal reworking event at ca. 2.70-2.55 Ga and regional metamorphism at ca. 2.55-2.45 Ga.