Zircon U-Pb age and Lu-Hf isotope constraints on Precambrian evolution of continental crust in the Songshan area, the south-central North China Craton

摘要

A combined whole-rock geochemical and zircon U-Pb and Lu-Hf isotope study on two Precambrian lithotectonic units in the Songshan area, south-central North China Craton (NCC), was performed to constrain the tectonic evolution of the Trans-North China Orogen (TNCO) during collision between the Eastern and Western Blocks of the NCC. The metamorphic unit at Shipaihe consists of dioritic gneiss with minor amphibolitic enclave, whereas the magmatic unit at Shicheng is composed of granite. Zircon U-Pb dating demonstrate that the dioritic gneiss and enclave amphibolite were originally emplaced contemporaneously at ca. 2.5Ga. The magmatic zircons exhibit e{open}_(Hf)(t) values of 2.2-7.8 and Hf model ages of 2.52-2.73Ga, suggesting that the dioritic gneiss was produced by reworking of the juvenile crust. The magmatic zircons of amphibolite have e{open}_(Hf)(t) values of 0.7-8.1 and Hf model ages of 2.51-2.79Ga. The granite primarily gave zircon U-Pb ages of ca. 1.78Ga, with inherited zircons in ca. 1.87Ga. Zircon Lu-Hf isotopic analyses yield negative e{open}_(Hf)(t) values of -16.7 to -1.8 and Hf model ages of 2.55-3.47Ga, indicating that the granite was mainly derived from reworking of ancient Archean crust. The Neoarchean dioritic gneiss and amphibolite show enrichment of LREE and LILE but depletion of HREE and HFSE, suggesting their derivation from anatexis of juvenile arc-type crust and enriched lithospheric mantle, respectively. The dioritic gneiss also has highly fractionated REE patterns and negligible negative Eu anomalies, implying anatexis at high pressures, where garnet and possibly amphibole as residual phases. The Paleoproterozoic granite has high K_2O+Na_2O and Zr, high ratios of total FeO/MgO and Ga/Al, and low contents of CaO, Al_2O_3, Ba, Sr and Eu/Eu* values, consistent with the characteristics of A-type granites. The negative Eu, Ba and Sr anomalies suggest that plagioclase acted as a residual phase during partial melting at relatively low pressures. Taken together, the Songshan metamorphic complex would originally derive from the reworking of juvenile arc-type crust and coeval partial melting of juvenile, enriched lithospheric mantle. Thus, an active continental margin was present in the late Neoarchean in south-central NCC, generating arc-type magmatism between the Eastern and Western Blocks. During the collisional orogeny, the arc-type crust in southern NCC underwent intensive metamorphism that followed by post-collisional extension at ca. 1.78Ga, and the reworking of the old continent in the southern TNCO yield Shicheng A-type granite.