Geochronology, geochemistry and Sr–Nd–Pb–Hf isotopes of the Paleoproterozoic mafic dykes from the Wulashan area, North China Craton: Petrogenesis and geodynamic implications

Abstract

Zircon LA-MC-ICP-MS U–Pb geochronology, major and trace element geochemistry, and Sr–Nd–Pb–Hf isotope analysis were performed on the Paleoproterozoic mafic dykes from the Wulashan area to investigate their petrogenesis and tectonic implications. The mafic dykes are diabase porphyries with a dominant mineral assemblage of plagioclase and clinopyroxene. Zircon U–Pb isotopic data yield weighted mean 207Pb/206Pb ages of 1769±12 and 1770±5Ma for the NEE–EW trending and the N–S trending mafic dykes, respectively. Geochemically, these rocks belong to medium-K or high-K calc-alkaline series with K2O+Na2O contents of 3.39 to 5.28wt.%. They show low MgO contents (3.92–4.63wt.%) and Mg# values (32–37) and high contents of TiO2 (2.38–3.23wt.%) and FeOt (13.60–15.29wt.%). The dykes exhibit right-declined REE patterns, with (La/Yb)N of 4.63–9.35 and LREE/HREE of 5.02–7.92, and are characterized by enrichment in Rb, Ba, K, La, and Ce but depletion in Nb, Ta, Th, U, and Sr. Particularly, the NEE–EW trending samples display pronounced positive Ti anomalies, whereas the N–S trending samples exhibit negative P anomalies. All of the studied mafic dykes display similar Nd–Pb–Hf isotopic compositions with εNd(t) ranging from −3.41 to −1.49, (206Pb/204Pb)i of 15.0010 to 16.6626, (207Pb/204Pb)i of 15.2192 to 15.3468, (208Pb/204Pb)i of 35.0147 to 37.2389, and zircon εHf(t) values varying from −2.8 to 3.8. Geochemical modeling reveals that the NEE–EW and N–S trending mafic dykes were derived from 8% to 10% and 2%–4% partial melting of a common mantle source with ∼2% garnet. During emplacement, the parental mafic magma of the NEE–EW trending dykes experienced fractionation of clinopyroxene and accumulation of Ti–Fe oxides, whereas the N–S trending ones experienced fractionation of olivine and apatite. The selective enrichment of LILEs and LREEs, together with negative εNd(t) values and EMI-like Pb isotopic compositions, suggests that the dykes were derived from the partial melting of subcontinental lithospheric mantle metasomatized by slab-derived fluids. Furthermore, the marked depletion in Th and U and decoupled Nd–Hf isotopes indicate the incorporation of continental crustal components into the mantle prior to partial melting. Based on the geochemical features and previous regional geological data, we propose an extensional tectonic setting for the formation of the mafic dykes from the Wulashan area, possibly related to the post-orogenic setting following the collision between the Yinshan and Ordos Blocks.