In the Chinese Altai, on the northern side of the Erqis fault, the ∼10-m-wide Qiemuerqieke gabbro is composed almost entirely of hornblende and plagioclase. Its relative crystallization sequence is olivine, hornblende, plagioclase, and it shows a narrow compositional range in SiO2 (48.7–50.2 wt%), MgO (6.33–8.54 wt%), FeO (5.27–6.46 wt%), Na2O (3.06–3.71 wt%), and K2O (0.26–0.37 wt%). These contents result in a high Mg# value (68–72) and a low K2O/Na2O ratio of ∼0.1. It has (87Sr/86Sr)i ratios of 0.70339–0.70350, εNd(t) values of 4.8–6.0, and zircon εHf(t) of 11.4–15.8; these values demonstrate a mantle-derived source, and a whole-rock δ18O of ∼6.7‰ suggests a mantle wedge origin. The presence of magmatic hornblende suggests a relatively high water fugacity, and the crystallization temperature (715–826 °C) calculated using Ti-in-zircon thermometry is considerably lower than that of a normal mafic melt but consistent with an origin from a water-bearing magma. The gabbro has a secondary ion mass spectrometry zircon U-Pb age of 276.0 ± 2.1 Ma, which is coeval with the 275 Ma mantle plume in the northern Tarim craton, but the Qiemuerqieke petrological and geochemical data do not indicate an abnormally high mantle temperature or a deep mantle signature, which would commonly characterize a mantle plume source. Our results integrated with published data support a model of juvenile crustal growth by a subduction-related process.