The constraint of petrogenesis of Middle Ordovician magmatic rocks on the closure timing of the Proto-Tethys Ocean in the eastern Kudi area, Western Kunlun Mountains

Although previous researchers have conducted numerous studies on the early Paleozoic tectonic evolution of the Proto-Tethys in the Western Kunlun Mountains, there are still different opinions in understanding the closure timing of the Proto-Tethys Ocean. In this article, we have conducted geological, petrological, geochemical, and isotopic geochronological studies of the Middle Ordovician volcanic rocks and monzogranite of the intrusive accretion complex in the Mailun area of the eastern Kudi. The results show that rhyolite samples have SiO2 contents ranging from 75.83% to 76.24%, K2O contents ranging from 4.68% to 5.35%, Na2O contents ranging from 3.09% to 4.22%, and A/CNK values ranging from 1.00 to 1.05, indicating that the rhyolite is a weak peraluminous rock and belongs to the high potassium alkaline series. The rhyolite samples have∑REE contents rangingfrom 161.95×10–6 to 238.53×10–6, (La/Yb)N values ranging from 10.11 to 10.97, (Gd/Yb)N values varying from 1.26 to 1.33, and δEu values ranging from 0.24 to 0.26. The monzogranite samples have SiO2 contents ranging from 69.39% to 74.81%, K2O contents ranging from 4.01% to 5.41%, Na2O contents ranging from 3.62% to 3.99%, and A/CNK values ranging from 1.01 to 1.20, indicating that they belong to the weak to to strong peraluminous rocks and/or belong to the high potassium alkaline - shoshonitic series. The monzogranite samples have ∑REE contents ranging from 233.08×10–6 to 754.92×10–6, (La/Yb)N values ranging from 12.04 to 34.84, (Gd/Yb)N values ranging from 1.60 to 5.26, and δEu values varying from 0.54 to 0.7. The rhyolite in the Middle Ordovician volcanic strata in the Eastern Kudi area has the LA-ICP-MS zircon U–Pb age of (473.4±4.8) Ma. It is believed that the rhyolite was formed in the early Middle Ordovician Epoch in the active continental marginal environment based on its zircon age and geochenical characteristics. The monzogranite has the LA-ICP-MS zircon U–Pb age of (459.3±1.1) Ma, indicating that it was formed in the late Middle Ordovician Epoch. It belongs to the strong peraluminous S-type granite with its magma sourced from the metamorphosed sedimentary rock. The monzogranite was formed at the beginning of the post-collision stage, indicating that the closure of the Proto-Tethys Ocean in the Eastern Kudi area was in the late Middle Ordovician Epoch. Results of previous researches and this study have shown that the Proto-Tethys Ocean in the Western Kunlun Mountains is characterized by many islands. The development of Kudi Ophiolite and Kangxiwa-Subashi Ophiolite indicates that the Proto-Tethys Oceanic plate was subducted sequentially from north to south.