Triassic oceanic–continental transition of the Ganzi–Litang Ocean basin: Evidence from high-Mg andesites and Nb-enriched basalts in the Litang area, Eastern Tibet

Abstract

Petrological evidence of the oceanic–continental transition of intra-oceanic arcs enables the identification and reconstruction of the transformation of ocean basins into continents. In this study, we present new elemental and Sr–Nd isotopic compositions and zircon U–Pb ages for Triassic andesites and basalts in the Litang area, Eastern Tibet. Andesites in Cunge and basalts in Juewu contain zircon grains with U–Pb ages of 246 ± 2 and 248 ± 2 Ma, respectively, indicating a Middle Triassic origin. Andesites in Cunge have high SiO2 contents (53.1–56.5 wt%), MgO contents (5.15–7.13 wt%), and Mg# values (54–61), similar to Sanukite-type high-Mg andesites. Basalts in Juewu have high TiO2 (1.43–1.80 wt%) and Nb (12.5–20.2 ppm) contents, and high Nb/U and (Nb/La)N values, typical of Nb-enriched basalts. The andesites and basalts were both characterized by low (87Sr/86Sr)i values (0.7033–0.7044) and positive εNd(t) values (+4.2–+4.8). Therefore, interactions between melt from the subducting slab and mantle wedge peridotite formed andesites in Cunge, whereas the partial melting of mantle peridotite metasomatized by an adakitic melt (produced by the melted slab) formed basalts in Juewu. The findings of this study indicate that the formation of the andesites and basalts in the Litang area were related to intra-oceanic subduction magmatism, which was the product of the westward subduction of the Ganzi–Litang Ocean basin in the Middle Triassic.