The Timing of the Paleo-Asian Oceanic Closure: Geochemical Constraints from the Jigede Gabbro in the Alxa Block

Abstract

This paper investigates the Jigede gabbros from the Shalazhashan tectonic belt, Alxa Block. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U-Pb dating reveals that they were emplaced in the Middle Permian (ca. 262 Ma). All the gabbro samples collected from the intrusion exhibit low contents of TiO₂ (0.24–0.37 wt %) and \({\text{F}}{{{\text{e}}}_{{\text{2}}}}{\text{O}}_{{\text{3}}}^{{\text{T}}}\) (4.87–5.41 wt %), but high levels of MgO (11.15–12.15 wt %), CaO (11.45–12.75 wt %), Al₂O₃ (14.18–17.08 wt %) and high Mg# (0.81–0.83). Relative to HREEs and LILEs, the gabbros are enriched in LREEs and depleted in Th, Nb, Ce, Zr, Hf, and Ti, with positive Eu, Sr and U anomalies. In contrast to MORB, the Jigede intrusion has higher initial ⁸⁷Sr/⁸⁶Sr values of 0.7046 to 0.7054 and lower ε_Nd(T) of + 1.8 to + 4.8. Elemental and isotopic data suggest that the intrusion was likely to have been produced by partial melting of a shallow lithospheric mantle source modified by slab-derived fluids, with subsequent cumulation of plagioclase. Thus, the Jigede intrusion was emplaced in a back-arc setting, in response to the Paleo-Asian oceanic subduction. In addition, the final closure of the Paleo-Asian Ocean did not occur prior to the Middle Permian.