Crustal thickening and continental formation in the Neoarchean: Geochemical records by granitoids from the Taihua Complex in the North China Craton

Abstract

The Earth’s lithosphere changed dramatically in many aspects during the Neoarchean era. In this period, the crust became more felsic and shows significant diversification in the composition of granitoids. Although this change has been noticed for many years, the mechanism that drove the change remains unclear. Here we report a geochemical study of Neoarchean granitoids from the Taihua Complex in the North China Craton. Zircon U-Pb dating reveals two episodes of granitoid magmatisms at ca. 2.65 Ga and ca. 2.55–2.50 Ga. Sodic TTGs at ca. 2.65 Ga are characterized by low K2O/Na2O ratios of < 0.6, high Sr/Y and (La/Yb)N ratios, positive Eu anomalies, positive zircon ɛHf(t) values of 0.9 to 5.2, and zircon δ18O values of 6.3–6.5‰, suggesting that they were derived from partial melting of the thickened oceanic crust. In contrast, K-rich granitoids at ca. 2.65 Ga have higher K2O/Na2O ratios, low Sr/Y and (La/Yb)N ratios, negative Eu anomalies, mostly positive zircon ɛHf(t) values of −0.4 to 8.0, and high zircon δ18O values of 7.0–8.4‰. The K-rich granitoids at 2.55–2.50 Ga show similarly high Sr/Y and (La/Yb)N ratios to the sodic TTGs at ca. 2.65 Ga. All these geochemical evidences suggest that these Neoarchean K-rich granitoids were derived from partial melting of the low-T seawater-hydrothermally altered oceanic basalt at different depths. Therefore, the sodic TTG and K-rich granitoids are linked to each other through both similarity and difference in their source compositions rather than magmatic processes such as partial melting and crystal fractionation. On the other hand, high-K granites at ca. 2.55–2.50 Ga are highly siliceous, have very high K2O/Na2O ratios of 4.0–7.9, and contain many relict zircons with U-Pb ages of ca. 3.49–2.68 Ga, indicating their formation by reworking of the pre-existing continental crust. Using a filtered global geochemical database, we find that the Neoarchean is the most important period for the growth of both sodic TTGs and K-rich granitoids. The Sr/Y and (La/Yb)N ratios of these granitoids also reach the maximum during this period, suggesting global-scale thickening of the oceanic crust caused by warm plate convergence. Once the orogenic lithospheric mantle was foundered for thinning, the thickened oceanic crust experienced partial melting at different depths. This gave rise to the different types of granitoid for growth of the continental crust along convergent plate boundaries. Therefore, partial melting of the thickened crust at different depths is a viable mechanism for the compositional transition of continental crust in the Neoarchean.