Petrogenesis of the Carboniferous Intrusive Rock in the Xiaobaishitou District of East Tianshan, Northwest China: Magma Evolution and Tectonic Significance

The Xiaobaishitou gabbro-diorite pluton comprises a medium-grained gabbro-diorite suite and a fine-grained diorite suite, which intrude the Kawabulag Group in the East Tianshan Orogen of the Central Asian Orogenic Belt (CAOB). A combination of mineral chemistry, zircon U-Pb age, whole-rock geochemistry, Sr-Nd isotopes, and in situ zircon Hf isotopes for newly found gabbro-diorite from the Xiaobaishitou district in the Central Tianshan Terrane (CTT) is presented to investigate the petrogenesis and tectonic or even crustal evolution of the East Tianshan Orogen. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U-Pb analyses indicate that the gabbro-diorite was formed at 324.7 +/- 2.4 Ma. The isolated clinopyroxene formed under higher P-T-integral O-2 melt conditions (10.7-14.6 kbar; 1199-1269 degrees C; high integral O-2) than those for the hornblende, plagioclase, and zircon (557-687 degrees C; moderate integral O-2) in the gabbro-diorite, which reveals a multilevel, magmatic storage system. The gabbro-diorite is characterized by fractioned REE patterns, enriched LILEs (e.g., Ba and Pb), negative anomalies of HFSEs (e.g., Nb and Ta), and low La/Yb and Sr/Y ratios, which are typically indicative of crustal contamination and accounted for by subduction-related fluids. The rock also characterized by typical features of high compatible elements (MgO = 3.14-11.65 wt%, Cr = 1-157 ppm, Ni = 6-830 ppm), high Mg-# (47 -74), positive e(Hf)(t) values (+5.1 to +10.3) and e(Nd)(t) values (+2.3 to +4.4). These features suggest that the Xiaobaishitou gabbro-diorite was most likely derived from metasomatic mantle and contaminated minor crustal components. Taking into account the spatial and temporal distribution of the Carboniferous magmatic rocks in the CTT, we suggest the formation of the Xiaobaishitou gabbro-diorite was attributed to southward subduction of the Kangguer Ocean.