Geochemistry and LA-ICP-MS Zircon U-Pb Dating of the Guandiping Amphibolite in the Mian-Lue Structure Zone

Located in Mianxian County and with a typical heap crystal structure, the Guandipin ophiolite is an important part of the Mian-lue ophiolite structure zone. In this study, rock geochemistry and LA-ICP-MS zircon U-Pb dating are applied to analyze the intrusive environment and period of the amphibolite. The result shows that the amphibolite belongs to Calc-alkaline series with typically high content of MgO and CaO but low K₂O. The TiO₂ content is similar to that of active continental margin and island arc volcanic rocks. Besides, it is rich in LILE such as Ba, Th, U and LREE, while low in HFSE of Nb and Ta. Moreover, the fractionation of light and heavy REE is not obvious. These features indicate that it is probably the product of the northward subduction of the Mian-lüe oceanic basin under island arc environment. Its geochemistry characteristics are similar to the Indosinian granites, suggesting that they may derive from the same source, and are contaminated by the continental crust. LA-ICP-MS zircon U-Pb dating result shows that the diagenetic age of the amphibolite is 222.5±2.1 Ma, which indicates that it was formed in the Late Triassic Epoch. This age is a bit earlier than the granites in the study area, implying that the ancient Mian-lüe was formed in the Late Triassic Epoch. This age is a bit earlier than the granites in the study area, implying that the ancient Mian-lüe oceanic crust continued northward subduction until the forming of island arc volcanic rocks in Late Triassic or later, and then entered the collision orogenic stage.