In the Chinese Altai, on the northern side of the Erqis fault, the similar to 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 similar to 0.1. It has (Sr-87/Sr-86)(i) ratios of 0.70339-0.70350, epsilon(Nd(t)) values of 4.8-6.0, and zircon epsilon(Hf(t)) of 11.4-15.8; these values demonstrate a mantle-derived source, and a whole-rock delta O-18 of similar to 6.7 parts per thousand suggests a mantle wedge origin. The presence of magmatic hornblende suggests a relatively high water fugacity, and the crystallization temperature (715-826 degrees 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.