Geochronology and geochemistry of the Xiaoqinling Taihua Complex in the southern Trans-North China Orogen: Implications for magmatism during the early Paleoproterozoic global tectono-magmatic shutdown

Abstract

The Taihua Complex in the Xiaoqinling area of the southern Trans-North China Orogen (TNCO) records extensive magmatism during the early Paleoproterozoic. This contrasts with the tectono-magmatic quiescence recorded globally at this time. Based on zircon U−Pb geochronological and whole-rock geochemical data, the 2.36–2.30 Ga granitic gneisses in the Taihua Complex can be subdivided into three groups. Group-1 gneisses (2.36–2.33 Ga), mainly in the Dumutai area, are high-K2O granitoid rocks, which record the earliest magmatism. These rocks have fractionated rare earth elements (REE) patterns, marked negative Eu−Sr anomalies, and variable whole-rock εNd(t) (−1.67 to +2.41) and εHf(t) values (−0.89 to +3.41), and were likely derived by partial melting of a mixture of pre-existing tonalite–trondhjemite–granodiorite (TTG) and K-rich rocks with residual plagioclase in the source at high-temperature and low-pressure conditions. Group-2 gneisses (2.33–2.31 Ga), mainly in the Jialu and Xitongyu areas, are granitoid rocks that formed after the Group-1 gneisses. These rocks have less fractionated REE patterns, low La/Yb and Sr/Y ratios, and slightly positive to negative whole-rock εNd(t) (−3.91 to +2.00) and εHf(t) values (−7.99 to +0.87), which resulted from high-degree partial melting of ancient crust in an extensional setting. Group-3 gneisses (~2.30 Ga) are TTG rocks from the Bayuan area, which represent the youngest magmatism. These rocks have high La/Yb and Sr/Y ratios, high MgO, Cr, and Ni contents, and negative whole-rock εNd(t) (−1.71 to −1.67) and εHf(t) values (−0.57 to −0.10), which indicate derivation by partial melting of delaminated lower crust and interaction with mantle materials. Samples from all groups have variable zircon εHf(t7/6) (−6.38 to +7.70) and δ18O values (3.97‰ − 6.79‰), indicative of a heterogeneous magmatic source. Most zircons have slightly positive to negative εHf(t7/6) values, indicating significant reworking of ancient crustal materials and limited crustal growth during this period. The age, geochemistry, and petrogenesis of the studied granitoid rocks record orogenic collapse and a transition from post-collisional extension to crustal delamination in the early Paleoproterozoic. This followed late Archean subduction–collision–accretion processes in the southern NCC.