Geochemical characteristics and intraplate extension of Late Devonian bimodal volcanic rocks in Chifeng area of Inner Mongolia

The volcanic rocks of the Chaotugou Formation in Chifeng area of Inner Mongolia consist of meta-basalts and metarhyolites and exhibit typical "double peaks", with the SiO₂ content being 46.24%-56.6% and 77.59%-85.75% respectively. Zircon LA-ICP-MS U-Pb dating of rhyolite yielded U-Pb ages of(359±1) Ma-(360±1) Ma, which represent the time of crystallization of the volcanic rocks. The volcanic rocks were formed in late Devonian Famennian age. Geochemically, meta-basalts (or basaltic andesites) are characterized by enriched LREE, depleted HREE, slightly negative Eu, Nb, Ta, Sr and Ti anomalies and high abundances of LILE and HFSE, with the (La/Yb)_N ratios from 2.4 to 8.76, and the ⁸⁷Sr/⁸⁶Sr initial values from 0.705076 to 0.707770; in addition, they have relatively high positive ε_Nd(t)values (+0.51 to +4.90) and relatively young T_DM (964-1090 Ma) ages. The mafic rocks might have resulted from the mixture of depleted mantle with crustal source. Meta-rhyolites are generally characterized by rich silicon, alkali and poor magnesium. They are characterized by enriched LREE, depleted HREE, noticeable negative Eu (δEu 0.12-0.43), Nb, Ta, Sr, P and Ti anomalies, relatively high U, Th and rather low Ba, Sr, Ti and P; the (La/Yb)_N ratios are from 7.07 to 11.07, ⁸⁷Sr/⁸⁶Sr initial values are high (0.713757-0.739647), negative ε_Nd(t) values are low (-1.75 to +2.54), and T_DM ages are relatively young (846-1156 Ma); besides, most zircons have the positive ε_Hf(t) values (-2.3 to +6.9) and relatively young T_DM ages (707-1074 Ma). The felsic rocks show an A-type affinity and might have come from a juvenile crustal source. According to their geochemistry and previous regional geological studies, the late Devonian volcanic rocks are basalts and rhyolites, suggesting a typical bimodal volcanic rock association and implying an extensional setting.