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

Abstract

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 εHf(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.