The evolution of Changning-Menglian Ocean in the Qingping area of Shuangjiang County, western Yunnan: Constraints from age and geochemistry of plagioclase amphibolite

A large number of data about ages of Early Paleozoic relict oceanic crust and corresponding arc magma records of the Changning-Menglian suture zone in recent years have been obtained.However, there are relatively few records related to the Late Paleozoic oceanic evolution and lack of high-precision chronological data, which severely constrains the understanding of the Changning-Menglian oceanic evolution.LA-ICP-MS zircon U-Pb dating of two plagioclase amphibolite samples in the Qingping region of Shuangjiang County yields the weighted average ages of 442±3 Ma, 410±6 Ma and 365±6 Ma respectively, of which 410±6 Ma is the inheritance age, and the rest represents the magmatic crystallization age.Geochemical characteristics show that the content of SiO₂ is 47.28%~48.18% (sample D6164, t=365 Ma), averaging 47.90%, and the ratio of K₂O/Na₂O is less than 1, rich in sodium and poor in potassium.It is characterized by LREE enrichment, HREE depletion, enrichment of large ion lithophile element Ba, no obvious loss of high field strength elements of Nb and Ta, weak negative anomalies of Ti and Zr, indicating the overall characteristics of E-MORB.Sr-Nd-Pb isotopic compositions, (⁸⁷Sr/⁸⁶Sr)_t=0.705288~0.705750, (¹⁴³Nd/¹⁴⁴Nd)_t=0.512373~0.512424, ε_Nd(t)=4.01~5.00, averaging 4.72, indicate that the magma source of amphibolite was mainly derived from the enriched mantle (EMII) with DUPAL anomaly.The comprehensive analysis shows that the ocean basin was developed in the period of 442~365 Ma in the Qingping area, and the Changning Menglian suture zone once experienced the evolution of Proto-Paleo Tethys Ocean.