Two types of Neoarchean basalts from Qingyuan greenstone belt, North China Craton: Petrogenesis and tectonic implications

Geochronological, geochemical and isotopic studies were carried out on the basalts from the Qingyuan greenstone belt to understand the petrogenesis of the basalts and the mantle source characteristics, providing insights into Neoarchean geodynamic evolution of the North China Craton. The Qingyuan greenstone belt is analogous to the typical Barberton-type greenstone belt in lithological units. U-Pb isotopic dating using the LA-ICP-MS method on zircons from the basalts and the associated andesite reveals that they erupted at 2510-2545 Ma and were subjected to subsequent 2486-2491 Ma metamorphism. The basalts are high-Fe tholeiitic, showing coherently positive εNd(t) and εHf(t) values. The geochemical and isotopic studies strongly imply that they experienced significant fractional crystallization and minor/no crustal contamination during magma evolution. On the basis of chondrite normalized REE patterns, the basalts can be further divided into two types, viz. the enriched and the depleted. The enriched basalts are abundant in LREE and REEs contents, depleted in HFSEs (Nb, Zr, Ti), endowed with arc-like geochemical peculiarities. Modeling reveals that the parent magma of the enriched basalts has been derived from <10% partial melting of spinel-garnet lherzolites. The depleted basalts are characterized by somewhat left-inclined to slightly right-inclined REE patterns and relatively low REEs contents, interpreted to have been formed when the rising plume head mixed with the upper mantle. We suggest the plume head contribution of >70% to the depleted basalts. The basalts from the Neoarchean Qingyuan greenstone belt witness the episode of the vertical growth of continental crust at ca. 2.5 Ga. Combined with other geological considerations, a large Archean mantle plume model is favored to account for the generation of the late Neoarchean basalts from Qingyuan greenstone belt in the Eastern Block, North China Craton.