Tectonic switch from a lithospheric rift to an active continental margin in the Paleoproterozoic: Evidence from low δ18O granites from the Trans-North China Orogen in the North China Craton

Abstract

Tectonic switch from rift zones to subduction zones is common along convergent plate boundaries. While this process is susceptible to retrieving from Phanerozoic rock records, the difficulty has been encountered for Precambrian rock records because of the relative lack of characteristic geological signatures. Nevertheless, such a difficulty can be overcome by finding of specific geochemical signatures in ancient rock records. This paper reports our finding of low δ18O zircons from the Trans-North China Orogen (TNCO) in the North China Craton (NCC), where the tectonic switch would occur during the early to middle Paleoproterozoic in association with the amalgamation of supercontinent Columbia. A combined study of zircon U-Pb ages and Hf-O isotopes as well as whole-rock major-trace elements and Nd isotopes were carried out for magmatic rocks from the Taiyue complex in the southern part of the TNCO. Zircon U-Pb dating yields two episodes of magmatism at ca. 2.34–2.31 Ga and ca. 2.2–2.1 Ga. These magmatic rocks are dominated by the ca. 2.18–2.16 Ga granites, 2.18–2.17 Ga diorites and 2.17–2.11 Ga mafic–ultramafic cumulates that intruded the 2.34–2.30 Ga granites and diorites. The 2.2–2.1 Ga diorites, mafic-ultramafic cumulates, and the regional mafic dykes/intrusions exhibit continuously varying major element compositions, arc-like trace element patterns, and consistent zircon Hf and whole-rock Nd isotope compositions, indicating their derivation from the same suite of continental arc magmas. Such primitive arc magmas would evolve through fractionation and accumulation of pyroxenes and plagioclase into dioritic magmas. The two episodes of granites are similar in major and trace element compositions, generally belonging to alkali-calcic or calc-alkalic A-type granitoids. Although both groups show a small difference in their zircon Hf isotope compositions, they exhibit a big difference in their zircon O isotope compositions. The 2.31 Ga granites show variably low zircon δ18O values of 3.4–5.5‰, mostly lower than normal mantle zircon values. The 2.18 Ga granites also exhibit variable zircon δ18O values from 3.6 to 6.0‰, but mostly mantle-like values. It is inferred that the 2.31 Ga granites would acquire their low δ18O signatures from partial melting of the high-T seawater-hydrothermally altered Archean crust in an early Paleoproterozoic continental rift. This rift would extend for at least 300 km along the TNCO. The 2.18 Ga granites are closely associated with the 2.2–2.1 Ga diorites and mafic–ultramafic cumulates. They would be most likely to form through differentiation of the 2.2–2.1 Ga continental arc magma, with their low 18O signatures being contaminated by the low δ18O 2.3 Ga granites. The low 18O signatures in the 2.3 Ga and 2.18 Ga granites indicate that the southern part of the TNCO would have probably evolved from a lithospheric rift to an active continental margin during ca. 2.3–2.1 Ga.