Identification of the Late Triassic oceanic island rock assemblages in the Ganzi-Litang ophiolite mélange belt and its constraints on the tectonic evolution of the Ganzi-Litang oceanic basin

The Ganzi-Litang ophiolite mélange belt is located in the eastern Tethys domain. It is an important part of the Sanjiang (Nujiang River, Lancangjiang River and Jinshajiang River) archipelagic arc-basin system in Southwest China, with a complete trench-arc-basin system. The petrological, geochemical and zircon U-Pb dating analyses of the oceanic island-type "basalt + carbonate" rock assemblage in the Xiaba Formation from the Lazaga Mountain in the Litang area are carried out to provide new evidence for the reconstruction of the Late Triassic ocean-continent configuration of the Ganzi-Litang Ocean. Geochemical analyses show that the SiO₂ content of the basalts is 42.16%~48.32%, and the TiO₂ content is 2.81%~3.75%. The total rare earth elemnt (ΣREE) ranges from 164.51×10^-6 to 414.40×10^-6, and the light rare earth elements are more enriched than the heavy rare earth elements (La/Yb)_N=9.35~34.31, and are obviously enriched in lithophile elements such as Rb, Ba, Th, U, K, and high field strength elements such as Nb, Ta, Zr, Ti, etc. The REE distribution curve and trace element spider patterns of the basalts are similar to typical oceanic island basalt (OIB). The zircon U-Pb dating indicates that oceanic-island basalts were formed at 211 Ma. These data further indicate that an oceanic island environment existed in the Ganzi-Litang Ocean during the Late Triassic, and also provide new evidence for understanding that the Ganzi-Litang Ocean was in the subduction stage during the Late Triassic.