The tectonic setting of the Early-Middle Triassic volcanic-sedimentary succession in Funing-Napo area, the south margin of Nanpanjiang Basin, South China

The Late Paleozoic to Early Mesozoic tectonic evolution process along the Sino-Vietnam border, particularly the subduction polarity of the Late Paleozoic ocean basin, is still controversial.Massive Early—Middle Triassic magmatic rocks and thick Triassic siliciclastic system occurred in the south margin of Nanpanjiang Basin provide important magmatic and sedimentary evidences for settling dispute.The Funing-Napo Triassic volcanic rocks, as the largest magmatic outcrop, are the ideal object for studying the tectonic evolution along the Sino-Vietnam border.The systematic profile survey and geological mapping indicates that the volcanic-sedimentary succession that consists of lower basaltic andesites overlain by carbonate conglomerates, pebbly sandstones and calcareous sandstones, which is similar to the volcanic-sedimentary sequence in island arc setting.Zircon SHRIMP U-Pb dating results of basaltic andesites are 247±1 Ma and 246±3 Ma, which are consist with field occurrence that basaltic andesite is overlain by Middle Triassic clastic rocsk.Combined with previous results, it is determined that the volcanic rocks were formed at 247~242 Ma.The geochemical analysis shows that basaltic andesites are enriched in LILEs (Rb, Th and U) and LREE, with a remarkable negative Nb-Ta and Ti anomalies.Both the geochemical characteristics of basaltic andesites and the volcanic-sedimentary sequence suggest that the Early—Middle Triassic volcanic-sedimentary succession was formed in a subduction-related arc setting.The spatial and temporal distribution characteristics of Early-Middle Triassic volcanic arc and ophiolitic mélange, implies that a north-dipping subduction occurred along Sino-Vietnam border.