Magnesium and zinc isotope evidence for recycled sediments and oceanic crust in the mantle sources of continental basalts from eastern China

Abstract

To investigate the origin of compositional variations in intraplate alkaline and tholeiitic basalts, this study reports a combined study of metal stable (i.e., Mg and Zn) and radiogenic (i.e., Sr and Nd) isotopes of 50 Cenozoic continental basalts from the South China Block, eastern China. These basalts are classified into high- and low-CaO basanites, trachybasalts, and tholeiites, and have variable Mg–Zn–Sr–Nd isotopic compositions (δ26Mg = −0.29 to −0.46‰, δ66Zn = 0.27 to 0.50‰, 87Sr/86Sr = 0.703233 to 0.704175, and εNd = 3.8 to 8.0), reflecting mantle heterogeneity. In detail, the high-CaO basanites and some trachybasalts have high Ca/Al and δ66Zn, low Hf/Hf* and δ26Mg, and depleted SrNd isotopic compositions, consistent with partial melts of carbonated mantle formed by reaction of the mantle with slab-derived carbonate melts. The low-CaO basanites, some trachybasalts, and tholeiites have low Ca/Al and δ26Mg, high Hf/Hf* and δ66Zn, and depleted SrNd isotopic compositions. These characteristics are related to recycled carbonated eclogites, rather than carbonates, which is consistent with the correlations between δ26Mg, δ66Zn, and Zn/Fe values. In addition, previously published data for the late Mesozoic to Cenozoic basalts from eastern China reveal another group of basalts with high Ba/Th ratios and enriched SrNd isotopic compositions, indicative of significant contributions from siliceous sediments. Our study demonstrates that the compositional variations of the late Mesozoic to Cenozoic alkaline and tholeiitic basalts from eastern China reflect variable proportions of oceanic slab components in their mantle sources.