Petrogenesis and geodynamics of Late Archean magmatism in eastern Hebei, eastern North China Craton: Geochronological, geochemical and Nd–Hf isotopic evidence

Abstract

The eastern part of the North China Craton contains large volumes of Late Archean hornblenditic, tonalitic, dioritic, granodioritic and granitic rocks. LA-ICPMS zircon U–Pb dating of three plutons from eastern Hebei, eastern North China Craton, record emplacement ages of 2526–2515 Ma and metamorphic ages of 2500–2490 Ma for the diorite, granodiorite, and monzogranite and 2440 Ma for the K-feldspar granite. Hornblendites have high MgO (8.6–10.6 wt.%), Cr (674–1126 ppm), Ni (125–159 ppm) and V (224–315 ppm) at low (basic) silica contents (SiO 2 = 45.7–49.0 wt.%). They are enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs) and depleted in high field strength elements (HFSEs), with positive ɛ Nd( t) values (+1.2 to +2.0), indicating they were derived from an enriched mantle source. The monzogranites have high Sr and Ba contents and Sr/Y and La/Yb ratios, low Y and HREEs abundances, and positive ɛ Nd( t) and zircon ɛ Hf( t) values with Nd and Hf model ages less than 3.0 Ga, reflecting that they were derived from partial melting of a juvenile lower crust at high pressure. However, the K-feldspar granites, with low Sr and Ba concentrations and high HREE and Y contents, are considered to have originated from partial melting of juvenile crustal materials at shallow crustal levels. The diorites and granodiorites have variable major and trace element concentrations. Magmatic zircons from these rocks have heterogeneous ɛ Hf( t) values but record uniform homogeneous U–Pb ages, indicating they resulted from mixing of mafic and felsic magmas, coupled with fractional crystallization. Because of the widespread distribution of Late Archean magmatism and evidence for magma mixing, possibly related to mafic magma underplating, it appears likely that this event in the eastern North China Craton may relfect mantle plume activity, that resulted in significant growth of continental crust during the Late Archean.