Early Middle Triassic malic dikes from the Baoshan subterrane, western Yunnan: implications for the tectonic evolution of the Palaeo-Tethys in Southeast Asia

摘要

The time of final closure of the Palaeo-Tethys and the Sibumasu-Indochina collision in Southeast Asia represents a major unresolved geologic problem. Here, we present zircon chronology, whole-rock elemental, Sr-Nd, and zircon Hf isotopic geochemistry for newly discovered mafic dikes from the northern segment of the Sibumasu terrane, to provide constraints on this issue. Zircon U-Pb data indicate that the dikes were emplaced at 240 ± 3 Ma. These are the earliest Mesozoic magmatic rocks reported so far in the Sibumasu terrane, the late Palaeozoic passive margin of the Palaeo-Tethys. They are subalkaline tholeiites, showing geochemical characteristics similar to those of enriched mid-ocean ridge basalts (E-MORBs). They have ~(87)Sr/~(86)Sr(t) ratios of 0.703161-0.703826, εNd(t) of+4.8 to +7.5, and zircon εHf(t) of+9.2 to +13.3, implying strong mantle depletion. They were derived by partial melting of asthenospheric mantle and underwent subsequent frac tional crystallization and lithospheric assimilation. The geologic-petrologic evidence suggests that the mafic dikes were generated in a collisional setting, when suturing of the Baoshan and Simao subterranes (the two subterranes are part of the Sibumasu and Indochina terranes, respectively) occurred. These early Middle Triassic mafic dikes provide an upper limit for Sibumasu-Indochina collision. In conjunction with previous work, we conclude that the final closure of the Palaeo-Tethys and collision of the Sibumasu and Indochina terranes took place during the late Permian to Early Triassic.