Origin and tectonic implications of Early Cretaceous Siziwangqi volcanic rocks from the North China Craton

Abstract

We present new whole rock major and trace element and, Sr–Nd–Hf isotope data for Mesozoic basalts and andesites from the Siziwangqi area, northwestern China, in order to constrain their petrogenesis and regional tectonic implications. The basalts belong to the shoshonitic series and can be subdivided into olivine (Ol-) and plagioclase (Pl-)basalts. The Ol-basalts have moderate MgO contents (5.1–5.5 wt%) and Mg# values (48.7–50.3), and trace element patterns similar to typical ocean island basalts with positive Nb–Ta and negative Pb anomalies. We interpret the Ol-basalts as mantle-derived rocks. The Pl-basalts and andesites are more differentiated than the Ol-basalts, and have low MgO contents (3.9–4.5 and 0.55–3.4 wt%, respectively) and Mg# values (40.3–43.3 and 26.9–47.4; respectively). Trace element (e.g., low Nb/U and Ce/Pb ratios) and Sr–Nd–Hf isotope data for the Pl-basalts and andesites indicate an origin by fractionation and mixing between the mantle-derived magma parental to the Ol-basalts and a crust-derived silicic magma. Crustal melting was induced by the underplating of the mafic parental magmas of the Ol-basalts.Geochemical features of the Ol-basalts, including high SiO2, Nb, Ta, Na2O/K2O, and Fe/Mn and low Rb/Sr and Dy/Yb, are best explained by the vein-plus-wall rock melting of a metasomatized mantle source consisting of amphibole–clinopyroxene-rich veins and surrounding peridotites. The Ol-basalts have enriched Sr–Nd isotopic compositions ((87Sr/86Sr)i = 0.70585–0.70597; εNd(t) = −2.8 to −2.6) and decoupled Hf–Nd isotopes with positive ∆εHf(t) (+4.5 to +5.0) that are consistent with recent enrichment of the lithosphere by subduction components from the Paleo-Asian oceanic slab. We propose that the Siziwangqi magmatism was related to intra-continental extension of the North China Craton.