Citation: | YAN Songtao, WU Qingsong, ZHU Lidong, LI Hu, DAI Xuejian, QIN Meng. 2023. 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. Geological Bulletin of China, 42(10): 1684-1695. doi: 10.12097/j.issn.1671-2552.2023.10.006 |
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.
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Tectonic location map of the study area(a) and geological sketch map of the Litang area, Sichuan(b)
The microphotographs of the ocean island rock association
The CL images(a) and U-Pb concordia plots(b) of zircons for basalt from the Xiaba Formation
The Nb/Y-Zr/TiO2 diagram of basalts from the Xiaba Formation
Chondrite-normalized REE patterns(a) and primitive mantle-normalized trace element spider diagrams(b) of basalts from the Xiaba Formation
Plots of Nb/Yb-Ta/Yb(a) and Nb/Yb-Th/Yb(b) for basalts from the Xiaba Formation
Tectonic discrimination diagrams of basalts from the Xiaba Formation
The cross bedding and microtexture of the lithic quartz sandstone from the Xiaba Formation
Synthesis column maps of the Solomom Malait seamount(a), Karamay oceanic island(b) and the Xiaba Formation oceanic island(c)