The early Paleozoic Kwangsian event, the first compressive orogeny after the Rodinia assemblage in the South China Block (SCB), is characterized by the angular unconformity between the Devonian and pre-Devonian sequences and widespread granitoids and small-volume mafic rocks. However, the mechanism triggering this event is highly debated and the associated mantle source is poorly known. Our recent investigations identified the occurrence of early Silurian high-Mg basalt–andesite–dacite in the Dashuang area along the northwestern Yunkai Domain in the SCB. This study presents a set of new zircon U–Pb geochronological, whole-rock geochemical and Sr–Nd isotopic data for these volcanic rocks. They yield weighted mean 206Pb/238U ages of c. 430–443 Ma, similar to the age-peaks of early–middle Paleozoic magmatism and metamorphism in the SCB. These volcanic rocks have high MgO, Cr and Ni contents and Mg-numbers. They also exhibit arc-like geochemical affinities with crustal-like Sr–Nd isotopic compositions, similar to the Neoproterozoic high-Mg basalt–andesite in the eastern SCB. Such signatures, combined with other geological observations, suggest that the Dashuang high-Mg rocks were derived from an ancient (probably early Neoproterozoic) mantle wedge source. The early Paleozoic intra-continental reactivation in response to late-orogenic extension triggered the melting of this palaeosubduction-modified mantle, which led to the formation of these volcanic rocks.