Termination of the Hegenshan Orogen in the Xing'an–Mongolian Orogenic Belt, North China: Geochemical and zircon U–Pb geochronological constraints from Early Permian mafic dykes.

Based mainly on field geological observation, we identified the Dong Ujimqin mafic dykes in Inner Mongolia, north of Hegenshan ophiolite accretionary belt. The dykes are mainly dolerite, and their ages and source were determined using geochronological, geochemical, zircon Hf, and whole‐rock Sr–Nd isotopic data. Photomicrographs reveal ophitic texture and amygdale structures of the dolerite dykes. Laser ablation multiple collector inductively coupled plasma mass spectrometry U–Pb analysis of zircons yielded ages of 277.9 ± 1.8 and 275.9 ± 1.2 Ma for two of these mafic dykes, placing them within the Early Permian. The dykes are classified as alkaline and tholeiitic series according to their major elements and characterized by high Na2O (2.62–4.06 wt.%), high TiO2 (1.63–2.71 wt.%), and low K2O (0.74–2.06 wt.%). The mafic dykes have similar geochemical characteristics that exhibit large‐ion lithophile element enrichment relative to high‐field‐strength elements, depletion in Ta and Nb, and an asthenosphere‐like Sr–Nd–Hf isotopic signature. They have high Sm/Nd (0.131–0.141) and LaN/YbN (3.75–9.17), near‐chondritic heavy rare earth elements ([Gd/Yb]N = 1.21–2.39), and low Rb/Sr (0.03–0.08) ratios with negligible Eu anomalies (Eu/Eu* = 0.85–1.00). The dykes contain relatively low amounts of radiogenic Sr, as shown by (87Sr/86Sr)i values (0.7035–0.7042), and have positive values of εNd(t; 4.31–6.96) and εHf (t; 6.0–14.5) and negative values of εSr(t; −0.28 to −10.31), suggesting that the mafic dykes have been derived from partial melts of the depleted lithospheric mantle contaminated by crust in the source region and formed in post‐orogenic settings. All the above indicate an extensional event, which was related to the post‐subduction of the Paleo‐Asian Ocean, at ~277–275 Ma, and significant vertical continental growth of the crustal continuum occurred in the continental interior during post‐collisional extension. [ABSTRACT FROM AUTHOR]