Permian hornblende gabbros in the Chinese Altai from a subduction-related hydrous parent magma, not from the Tarim mantle plume

Abstract

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 SiO 2 (48.7-50.2 wt%), MgO (6.33-8.54 wt%), FeO (5.27-6.46 wt%), Na 2 O (3.06-3.71 wt%), and K 2 O (0.26-0.37 wt%). These contents result in a high Mg# value (68-72) and a low K 2 O/Na 2 O ratio of ~0.1. It has ( 87 Sr/ 86 Sr) i ratios of 0.70339-0.70350, ε Nd(t) values of 4.8-6.0, and zircon ε Hf(t) of 11.4-15.8; these values demonstrate a mantle-derived source, and a whole-rock δ 18 O 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. © 2013 Geological Society of America.