Petrogenesis and tectonic setting of the Chaheilingashun pluton, North Alxa Block, NW China: constraints from whole-rock geochemistry, zircon U–Pb ages, and Hf isotope compositions

Abstract

To better constrain the petrogenesis and tectonic setting of the Chaheilingashun pluton in the southern Ya-Gan Fault, North Alxa Block, NW China, this study presents new zircon U–Pb ages, whole-rock geochemistry, and Hf isotopes of diorites from the Chaheilingashun pluton. The Chaheilingashun pluton is composed of medium- to coarse-grained monzonitic diorite, with moderate to high SiO2 contents (58.22–62.77 wt.%), moderate total alkali contents (Na2O + K2O, 5.72–7.66 wt.%), and high Al2O3 contents (14.26–17.69 wt.%), which indicate that the diorite exhibits calc-alkaline to alkaline and metaluminous to peraluminous series. The correlations of SiO2 with oxides and the right-dipping oblique trends of rare earth elements (REEs) demonstrate that the magma experienced fractional crystallization. In particularly, the correlations of Rb with Ba and Sr, and the weak negative Eu anomalies indicate that the plagioclase has undergone fractional crystallization. In addition, this paper has shown that both fractional crystallization and partial melting also contributed to the magmatic process through the Eu anomalies and La/Sm-La diagram. The positive eHf(t) values (2.2–4.6) and low Mg#, Cr, and Ni indicate that diorite was generated from the partial melting of juvenile mafic crust. Together with previous results, the diorites are inferred to have formed in an post-collisional extensional environment. The zircon U–Pb age of diorite from the Chaheilingashun pluton is dated at 289.3 Ma, suggesting that the final closure time of the Paleo-Asian Ocean was might before 289.3 ± 2.3 Ma.