Petrogenesis and Tectonic Implications of the Cenozoic Alkaline Complex in Kuzigan, Taxkorgan, West Kunlun

Kuzigan alkaline complex is one of the major parts of the Taxkorgan alkaline complexterrane. It is mainly composed of aegirine-augite syenite, quartz aegirine-augite syenite, and alkali granite. There is great difference among the three rocks in the content of SiO₂, 50.26%-54.11%, 59.74%-60.43% and 69.59%-72.13% for aegirine-augite syenite, quartz aegirine-augite syenite and alkali granite, respectively. Aegirine-augite syenite and quartz aegirine-augite syenite has similar Rittman index, namely 7.83-10.97 and 8.51-9.05, distinctly differing from 2.49-3.71 of alkali granite. These three rocks are strongly enriched in light rare earth elements (LREE) as well as some large ion lithophile element (LILE), such as Rb, Ba, Sr, and depleted in High Field Strength Element (HFSE) such as Nb, Ta and Ti. Abnormal high Sr, Ba and ∑ REE content may imply the mixing of enriched mantle materials. LA-ICP-MS in-situ zircon U-Pb dating obtain the ages of 10.9±0.1 Ma and 11.9±0.4Ma for the quartz aegirine-augite syenite and alkali granite respectively, representing the crystallization ages of the two rocks. The εHf(t) values of the quartz aegirine-augite syenite and alkali granite are of -9.78-3.24 and -13.54-5.78, showing the crust-mantle mixing in magma source area. According to the comprehensive analysis and combined with regional geological data, we preliminarily hypothesized that the Kuzigan alkaline complex is a product of the crust-mantle mixing, which is caused by the asthenosphere flowing northward due to the delamination of the Tibetan plateau at ~25Ma, and the asthenosphere upwelling as a result of the obstruction of the root of the Tarim craton lithosphere. Therefore, the formation of the complex is a fernwirkung of the magmation after Tibetan plateau large-scale delamination, and it signifies that during ~11 Ma the study area was in extensional tectonic setting.