Mid-Neoproterozoic magmatic rocks from the southeastern part of the Nanhua Basin constrain the tectonic setting of the basin. Data from rhyolitic tuffs of the Louqian Formation that overlie the Cathaysia Block in southeastern South China indicate accumulation of the unit at ca. 770 Ma, and provide an upper age limit on collisional assembly of the Yangtze and Cathaysia blocks. The zircons within the tuffs display largely positive ε_Hf(t) (−3.66 to +13.94) and juvenile T_DM (Hf) values (1.74–0.90 Ga), which indicate involvement of Neoproterozoic to late Paleoproterozoic crustal materials in their generation. Geochemically, the tuffs are peraluminous and characterized by both A-type and arc affinities with high SiO₂, Ga, accentuated negative Eu anomalies, strong depletion in high field strength elements and high zircon saturation temperatures (T_Zr), which are comparable to contemporaneous (800–750 Ma) felsic rocks from the northwestern of the Nanhua Basin. These characteristics suggest the Nanhua Basin lay inboard of a convergent plate margin arc. This arc is represented by 860–730 Ma magmatism along the western and northern margin of the South China Craton (Panxi-Hannan Belt), with the Nanhua Basin occupying an upper plate, back arc setting.