Material composition and oceanic basin evolution process of the Litang mélange in the middle part of the Ganzi-Litang ophiolite mélange belt

The Litang mélange, in which conserves complete mélange system is developed, including remnant of ophiolite, oceanic island blocks, intra-oceanic arc blocks, flysch formation, remnant of rift, high-ultrahigh pressure belt, is located in the middle part of the Ganzi-Litang ophiolite mélange belt around Xinlong-Litang county town, and is an ideal object for the restoring and inversing of the evolution of the Ganzi-Litang ocean basin. On the basis of previous studies and combined with recent research results, the mineral composition, tectonic environment and formation age of Litang melange are described in detail, which further constrain the space-time, nature and evolution of Ganzi-Litang ocean basin. The results of LA-ICP-MS zircon U-Pb dating show that the age of ophiolite is (346±17)Ma, (286.2±5.1)Ma, (219.5±2.2)Ma, (216.1±2.3)Ma, the age of oceanic island is (271±10)Ma, (245.1±1.5)Ma, (211.8±1.8)Ma, and the latest age of detrital zircon is (196±3)Ma in the siltstone interlayer of Ruihuanshan Formation in Jurassic. Combined with a large number of fossil identification results, the analysis shows that the age of the Litang mélange probably ranges from the Middle Devonian to the Early Cretaceous, indicating that it records the continuous evolution of Ganzi-Litang ocean basin from the Middle Devonian to early Cretaceous. With all the achievement mentioned above, the formation and evolution mode of the Ganzi-Litang ocean basin from the Late Paleozoic to the Mesozoic is approximately established.