Geochemistry of high‐Nb basalt‐andesite in the Erguna Massif (NE China) and implications for the early Cretaceous back‐arc extension

This paper presents new zircon laser ablation-inductively coupled plasma-mass spectrometry U-Pb age and whole-rock geochemical data for the high-Nb basalts-andesites (HNBs) from the Meiletu Formation in NE China. In comparison with common arc basalts and Nb-enriched basalts, the Meiletu HNBs (140 Ma) show high TiO2 (1.7-3.2 wt.%), P2O5 (0.7-1.7 wt.%), Nb (14.6-34.0 ppm), and Zr (314-600 ppm) contents, and high (Nb/Th)(PM) (0.26-0.91), (Nb/La)(PM) (0.22-0.36), Nb/U (8.2-24.8) ratios. They show strongly light rare earth element (REE) enriched chondrite-normalized REE patterns with unusually high total REE contents [(TREE)cn = 978-1802]. Unlike adakite-associated HNBs derived from a mantle wedge metasomatized by adakites, the Meiletu HNBs were derived from low degree (<2%) partial melting of the garnet peridotite at considerable depth (>80 km). The Meiletu HNBs were generated in an intracontinental back-arc extension setting. Combined with previous geological observations, it is argued that Early Cretaceous tectonics of the Erguna Massif is dominated by the Mongol-Okhotsk oceanic slab-rollback and back-arc extension.