Age and petrogenesis of late Paleozoic granites from the northernmost Alxa region, northwest China, and implications for the tectonic evolution of the region

The Wudenghan, Huhetaoergai and Zhuxiaobuguhe plutons, northern Alxa region, in the southern Central Asia Orogenic Belt are dated by U-Pb zircon to 383 +/- 3, 356 +/- 3 and 286 +/- 2 Ma, respectively. The late Devonian Wudenghan monzogranite, a highly fractionated I-type granite with epsilon (Nd)(t) (-0.2 to -0.1) and very low (Sr-87/Sr-86) (t) (0.704719-0.706113), is from mantle-derived magmas and shows volcanic arc characteristics. The early Carboniferous Huhetaoergai granodiorite with medium-K calc-alkaline peraluminous characteristics represents a volcanic arc granite generated from partial melting of lower continental crust combined with mantle-derived input. The early Permian Zhuxiaobuguhe pluton, an unfractionated calc-alkaline granodiorite with moderately low epsilon (Nd)(t) (-2.0 to -1.1) and low (Sr-87/Sr-86) (t) (0.708370-0.708462), was likely derived from partial melting of the mafic lower crust of a paleo-volcanic arc and represents a post-collisional granite. Our revised tectonic evolution of the region includes (1) northward subduction of the oceanic crust represented by the Engger Us Ophiolitic Belt and formation of the late Devonian Wudenghan monzogranite, (2) northward subduction of the ocean between the Huhetaoergai and Zhusileng tectonic zones and the formation of the Huhetaoergai volcanic arc granite during the early Carboniferous and (3) the emplacement of the Zhuxiaobuguhe pluton in the early Permian during post-collisional extension