| Abstract:The Qianmai ophiolitic mélange in the Lancang, southwestern Yunnan, is the southern extension of the Wanhe ophiolitic mélange in Mengku region, which record the process of proto-Tethys Ocean from expansion to subduction. In this paper, zircon U-Pb chronology, Lu-Hf isotope and whole rock geochemistry are studied on the plagiogranite in the ophiolitic mélange. The plagiogranite is characterized by relatively high SiO2, Na2O, Al2O3 and low in K2O, MgO contents. Its geochemical data shows that granite is enriched in LREE with negative Eu anomaly. The primitive mantle-normalized spider-grams are characterized by Rb, Th, U, K enrichments and high in Y, Yb contents, especially for a negatives Sr, Nb-Ta-Ti anomaly, which is consistent with the characteristics of island arc magma. Zircons from plagiogranite are typical magmatic zircon crystals with negative Eu and positive Ce anomalies. Their ratios of Th/U are 0.29-0.85 and 0.46-0.62, which is typical for magmatic zircons. LA-ICPMS zircon U-Pb dating shows that the weighted mean 206Pb/238U ages are 489±3 Ma and 464±4 Ma, indicating the related to Early Paleozoic magma activity. All zircons have native εHf（t）values(-10.10～-1.92), and the model ages (1.60～2.07 Ga) are older than the formation age. In conclusion, the plagiogranite should be derived by partial melting of amphibolite phase in the migration shear zone of oceanic crust (residual mineral phases is:Pl+Hb+Opx+Ilm(without Grt)) in the condition of low temperature and low pressure. Besides, combining with regional geological data, the Proto-Tethys opened before the late Cambrian. |
