Garnet-bearing silicic volcanic rocks are rare in fossil orogens and usually record a transient stage from regional compression to extension. This study reports newly identified 839 ± 3 Ma garnet-bearing dacitic volcanic rocks associated with the Fuchuan ophiolite complex (FOC) in the eastern Jiangnan Orogen (JO), Southeast China. The presence of these unusual rocks provides new constraints on the late Neoproterozoic tectonic evolution of the orogen.

The garnet-bearing dacitic volcanic rocks of the garnets are weakly peraluminous and exhibit trace element and Nd isotopic signatures similar to those of post-orogenic, strongly peraluminous granites in the eastern JO, indicating a similar crustal source. The garnets are almandine-rich (76–79 wt%) and characterized by low CaO (<2.5 wt%), MnO (<2.6 wt%) and TiO2 (<0.1 wt%), consistent with garnets in peraluminous S-type volcanic rocks globally. Integrated petrological, geochemical and zircon Hf isotopic evidence indicates that the primary magma originated from partial melting of a heterogeneous lower-crustal source, comprising both juvenile basaltic and ancient pelitic components. High zircon saturation temperatures (>900°C) further imply the heating of coeval underplating mantle-derived mafic magma, analogous to the mechanism forming ‘hot granites’.

Integrating our findings with regional geology, we propose that the garnet-bearing dacitic volcanic rocks associated with the FOC formed in an ensialic back-arc basin along the southeastern margin of the Yangtze Craton. The occurrence of the garnet-bearing magmatism records the onset of back-arc extension, likely following the ∼880–860 Ma arc–continent collision and subsequent subduction polarity reversal.