Xierqu is a skarn Fe–Cu deposit associated with porphyritic diorite, in the Kalatag district of eastern Tianshan, China. The diorite yields a sensitive high-resolution ion microprobe U–Pb zircon date of 391 ± 3 Ma interpreted as its age of emplacement. Major element geochemistry shows that the diorite is a calc-alkaline assaying 16.7–19 wt.% of Al2O3, with Mg# values of 56.8–67.1, and Al2O3/(CaO + Na2O + K2O) (A/CNK) values of 0.55–0.94. Trace element characteristics of the diorite show that it is enriched in Ba, K, and Sr and depleted in Nb, Ta, and Ti, with a positive Eu (1.18–1.44) anomaly and high Sr/Y and La/Yb ratios. Sr–Nd isotopic analyses yielded positive εNd(t) values ranging from 7.1 to 9.8 with low initial 87Sr/86Sr ratios of 0.703806 to 0.703856. These geochemical and isotopic characteristics indicate that the diorite is adakite-like emplaced in an island-arc setting and resulted from partial melting of subducted oceanic crust in a mantle wedge during the Devonian.