Age and origin of tuffites from the Hekou Formation in the Napo basin, Southwest Guangxi and its tectonic implications

The Napo basin, Southwest Guangxi, locates in the Babu-Song Hien tectonic belt and shows widespread Early-Middle Triassic magmatic and massive marine clastic rocks, which plays a key role in studying the evolution of the Paleo-Tethys orogeny along the southern margin of South China block. In this paper, we have carefully analyzed the petrology, geochemistry and zircon U-Pb-Hf isotope for two sets of tuffites in the middle-upper part of the Middle Triassic Hekou Formation, Napo basin. LA-ICP-MS zircon U-Pb dating results of the two tuffite samples yield the ages at 240. 6 +/- 1. 0Ma and 236. 3 +/- 0. 8Ma, respectively, indicating that the clastic rocks of Hekou Formation should be deposited during the Middle Triassic Ladinian stage, some of which may extend to the early Camian of Late Triassic. Whole-rock geochemical results indicate that the tuffites are subalkaline dacitic, enriched in light rare earth elements (LREEs), large ion lithophile elements (LILEs), Rb, Th and U, depleted in high field strength elements (HFSEs), and have significant negative anomalies of Nb, Ta and Ti. The tuffite zircons have lower epsilon(Hf )(t) values (-18. 6 similar to -6. 3) and older crustal model ages (t(DM)(C) =1664 similar to 2434Ma). Combined with trace elements, we suggest these zircons may have been formed in subductionrelated setting. In summary, the tuffite samples from the Hekou Formation in Napo basin display the geochemical characteristics of island-arc volcanic rock, which are inferred to be derived from the Middle-Late Triassic acidic volcanic rocks in Pingxiang area, Southwest Guangxi. Given the existing geological survey results, it is concluded that the Babu-Cao Bang Paleo-Tethys branch ocean along the southern margin of the South China block should subduct northward during the Late Permian-early Late Triassic, and subsequently closed with scissor-style from west to east. The collision between the South China block and North Vietnam terrane should occur in the middle to late stage of Late Triassic.