Opening of the Longmu Co–Shuanghu–Lancangjiang ocean:constraints from plagiogranites

This paper reports new zircon U-Pb ages,and Hf isotope and whole-rock major and trace element data for Cambrian plagiogranites from the Tuobeiling ophiolite in central Qiangtang,Tibetan Plateau.Zircon SIMS and LAICP-MS U-Pb dating of the plagiogranites yield weighted mean ages of 504.8±4.2 and 491.6±1.5 Ma,respectively.The zircons from plagiogranites exhibit positive eHf(t)values(ranging from 11.46 to 15.16),indicating that the plagiogranites are derived from depleted mantle.These plagiogranites are characterized by high SiO2and Na2O,low K2O,low REE contents,and flat REE distribution patterns.These rocks have geochemical compositions typical of oceanic plagiogranite and,considered along with their petrography and field relationships,are interpreted to have derived from anatexis of hydrated amphibolites by ductile shearing during transports of the oceanic crust.The formation age of such type of plagiogranite is slightly younger than that of the associated section of oceanic crust.Thus the new results from these plagiogranites suggest that the Longmu Co–Shuanghu–Lancangjiang ocean had probably opened before the Middle Cambrian.

This paper reports new zircon U-Pb ages, and Hf isotope and whole-rock major and trace element data for Cambrian plagiogranites from the Tuobeiling ophiolite in central Qiangtang, Tibetan Plateau. Zircon SIMS and LA- ICP-MS U-Pb dating of the plagiogranites yield weighted mean ages of 504.8 ± 4.2 and 491.6± 1.5 Ma, respectively. The zircons from plagiogranites exhibit positive εHf（t） values （ranging from 11.46 to 15.16）, indicating that the plagiogranites are derived from depleted mantle. These plagiogranites are characterized by high SiO2 and Na2O, low K2O, low REE contents, and flat REE distribution patterns. These rocks have geochemical compositions typical of oceanic plagiogranite and, considered along with their petrography and field relationships, are interpreted to have derived from anatexis of hydrated amphibolites by ductile shearing during transports of the oceanic crust. The formation age of such type of plagiogranite is slightly younger than that of the associated section of oceanic crust. Thus the new results from these plagiogranites suggest that the Longmu Co-Shuanghu-Lancangjiang ocean had probably opened before the Middle Cambrian.