Back-arc magmatism of the Proto-Tethys Ocean in the West Kunlun Orogenic Belt: evidence from the Ordovician Nanpingxueshan Nb-enriched gabbros

ABSTRACT

The West Kunlun Orogenic Belt (WKOB) in the northwestern Tibetan Plateau records key information about the tectonic evolution of the Proto-Tethys Ocean. To further constrain the Early Palaeozoic tectonic evolution of the WKOB, we report petrography, geochronology, whole-rock geochemistry, and Sr-Nd-Hf-O isotope data of the newly identified Ordovician Nanpingxueshan gabbros in the southern WKOB. Laser ablation-inductively coupled plasma-mass spectroscopy (LA-MC-ICP-MS) U-Pb zircon dating reveals that the gabbroic intrusion was emplaced during 476–471 Ma. Geochemically, the gabbroic rocks exhibit a tholeiitic signature and can be subdivided into two groups, i.e. normal arc-like gabbros (AGs) and Nb-enriched gabbros (NEGs). The AGs have TiO₂ (1.20–1.64%), P₂O₅ (0.18–0.25%), and Nb (4.60–7.64 ppm) contents, fractionated REE patterns ((La/Yb)_N = 2.64–4.11), and Nb-Ta depletion (Nb/La = 0.37–0.50). Compared with AGs, the NEGs have higher TiO₂ (1.74–2.68%), P₂O₅ (0.34–0.55%), and Nb (10.79–14.08 ppm) contents and Nb/La ratios of 0.45–0.53, which are similar to typical Nb-enriched arc basalts. Isotopically, the AGs have depleted whole-rock ε_Nd(t) values of 3.22 to 4.04 and negative to positive zircon ε_Hf(t) values of −1.8 to 7.8, while the NEGs exhibit slightly lower whole-rock ε_Nd(t) values of 0.69 to 3.18, zircon ε_Hf(t) values of −7.8 to 4.6, and mostly δ¹⁸O values greater than those of normal mantle zircon. Integrated analyses reveal that the AGs and NEGs can be attributed to a similar mantle source metasomatized by different proportions of subducted sediment-derived melt during the subduction process. Our new data, combined with the results of previous studies, suggest that the southward subduction of the Proto-Tethys Ocean under the WKOB initiated at ca. 530 Ma. The long-term southward subduction led to the formation of the massive Early Palaeozoic accretionary complex in the South Kunlun Terrane, while trench retreat led to migration of the arc magmatism northward from the Mazar – Tianshuihai Terrane to the South Kunlun Terrane, as well as back-arc extension in the Mazar – Tianshuihai Terrane.

KEYWORDS:

• Nb-enriched gabbros
• back-arc extension
• West Kunlun Orogenic Belt
• Proto-Tethys Ocean

Acknowledgments

We sincerely thank Dr Jian Zhang and Jianzhen Geng for the LA-MC-ICP-MS zircon dating and in situ Lu-Hf isotope analyses and Dr Wengang Liu for the whole-rock Sr-Nd isotope analyses.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Supplementary material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/00206814.2024.2355624.

Additional information

Funding

The work was supported by the the National Nature Science Foundation of China [42262007]; the Tianchi Doctoral Programme in the Xinjiang Uygur Autonomous Region [TCBS202002]; the Open Project of the Xinjiang Uygur Autonomous Key Laboratory [2021D04007]; the Key Science Project of the Xinjiang Uygur Autonomous Region [2019B00011-1, 2020A03005-1].