Devonian arc-related granitoids in the Northwestern Chinese Tianshan, Central Asian Orogenic Belt: implications for the bending of the Kazakhstan Orocline

The Kazakhstan collage system, occurring as a tightly curved configuration with oroclinal structures, holds significant information to uncover the orogenic process and continental geodynamics of the CAOB. Palaeomagnetic data and regional geological correlation document that Kazakhstan orocline was not formed until the Middle Devonian, broadly corresponding to the time of Junggar-Ocean subduction. To exert temporal constraints on the subduction and understand associated magmatism, we conducted zircon U-Pb dating and Hf isotopic analysing as well as whole-rock geochemical and Nd-Sr study for the Devonian magmatic rocks in the Northwestern Chinese Tianshan. Zircon U-Pb dating results showed two phases of magmatism of 390.6 ± 1.8 Ma and 371.2 ± 2.4 Ma, respectively. These rocks have monzogranite compositions, with high SiO₂ (75.9–77.9 wt.% and 74.1 wt.%), medium-high K₂O (2.06–3.85 wt.% and 3.81–4.82 wt.%) concentrations, showing calc-alkaline and peraluminous characteristics (A/CNK = 1.02–1.03 and 1.03–1.09). They also display evident depletions in Ba, Nb, Ta, P, and Ti relative to LREEs, Rb, and Th, showing striking features of subduction-related magma. The variations of zircon ε_Hf(t) values (−0.60 to +5.53 and +2.12 to +8.39 for two samples) may suggest an extensive interaction between old crustal rocks and mantle-derived juvenile components. Therefore, the Middle-Late Devonian magmatism can be correlated with Devonian Volcanic Belt in Kazakhstan, which constituted a magmatic belt generated by the subduction of Junggar-Ocean plate. The steady-state distributions and long-term geochemical variations of Early Devonian-Early Carboniferous arc-related magmatism in the Northwestern Chinese Tianshan allow us to conclude the lack of retreat of magmatic belts in the southern limb of the Kazakhstan Orocline. Accordingly, rapid retreat in the west and north limbs of orocline vs. normal subduction in the Northwestern Chinese Tianshan, supports the asymmetric retreat model interpreting the first stage of bending of Kazkhstan collage system.

以弧形构造（oroclinal structures）紧密弯曲为特征的哈萨克斯坦拼贴构造体系（Kazakhstan collage system），对于揭示中亚造山带（Central Asian Orogenic Belt，CAOB）的造山作用与大陆动力学机制具有关键科学意义。古地磁数据与区域地质对比研究显示，哈萨克斯坦弧形造山带（Kazakhstan orocline）直至中泥盆世才得以形成，其形成时间大致对应准噶尔洋（Junggar-Ocean）的俯冲阶段。为约束该俯冲作用的时间进程并厘清相关岩浆作用机制，我们针对中国天山北西段的泥盆纪岩浆岩开展了锆石U-Pb定年（zircon U-Pb dating）、Hf同位素分析（Hf isotopic analysing）以及全岩地球化学与Nd-Sr同位素研究。锆石U-Pb定年结果揭示两期岩浆活动，分别为390.6±1.8 Ma与371.2±2.4 Ma。这些岩石具有二长花岗岩（monzogranite）的成分特征，SiO₂含量较高（75.9–77.9 wt.% 与74.1 wt.%），K₂O含量处于中高水平（2.06–3.85 wt.% 与3.81–4.82 wt.%），表现出钙碱性（calc-alkaline）与过铝质（peraluminous）的地球化学特征（A/CNK值分别为1.02–1.03与1.03–1.09）。相较于轻稀土元素（LREEs）、铷（Rb）与钍（Th），这些岩石显著亏损钡（Ba）、铌（Nb）、钽（Ta）、磷（P）与钛（Ti），具备俯冲带相关岩浆的典型地球化学特征。两个样品的锆石ε_Hf(t)值变化范围分别为−0.60至+5.53与+2.12至+8.39，这指示古老地壳物质与幔源新生组分之间存在广泛的物质相互作用。因此，中晚泥盆世的岩浆活动可与哈萨克斯坦境内的泥盆纪火山岩带相对比，该火山岩带属于准噶尔洋板块俯冲作用形成的岩浆带。中国天山北西段早泥盆世至早石炭世弧相关岩浆作用（arc-related magmatism）的稳态分布与长期地球化学演化特征表明，哈萨克斯坦弧形造山带南翼不存在岩浆带后撤（retreat）现象。据此，造山带西翼与北翼的快速后撤，结合中国天山北西段的正常俯冲作用，支持了解释哈萨克斯坦拼贴构造体系第一期弯曲的不对称后撤模型。