An anomalous rollback process of Mesozoic flat-slab subduction in South China

The Mesozoic magmatic rocks within the South China Block exhibit a widespread distribution, extending up to 1300 km inland. In order to better understand the Mesozoic tectonic evolution within the interior South China Block, we present new geochronological and geochemical investigations on the Danguanzhang granitic pluton in the region where multiple episodes of Mesozoic magmatism were initiated in South China. The Danguanzhang pluton is primarily composed of Guizhumao granites (168 ± 1 Ma) as its main body and Gaojian granites (246 ± 1 Ma) as the eastern part. Both the Guizhumao and Gaojian granites are highly differentiated I-type granite based on their high SiO2 contents (>72 wt%), metaluminous-weakly peraluminous (A/CNK = 0.99–1.12), fractionated rare earth element (REE) patterns ([La/Yb]N = 1.1–13.3), significantly negative Eu anomalies (δEu = 0.02–0.14), and a negative correlation between P2O5 and SiO2. They were derived from the partial melting of Paleoproterozoic metaigneous rocks and show negative whole-rock ɛNd(t) values (−9.12 to −9.38 and − 9.61 to −9.8, respectively) and zircon ɛHf(t) values (−7.0 to −12.8 and − 6.8 to −11.8, respectively). The Guizhumao granites exhibit a close affinity with the widely distributed early Yanshanian (165–150 Ma) I-type granites within the interior South China Block, marking the onset of the early Yanshanian magmatism in South China. Conversely, the Gaojian granites, coeval with the Indiosinian magmatism, are associated with inland subduction. A compilation of previously published geochronology data reveals distinct episodes of Mesozoic magmatism within the interior South China Block (250–200 Ma, 195–180 Ma, and 168–150 Ma), which contrast with the typical flat-slab subduction scenario that typically exhibits two episodes of magmatism corresponding to flat subduction and rollback. Therefore, the interior South China Block might have undergone complex deep dynamic processes in the Mesozoic, involving prolonged flat-slab subduction and unique slab rollback caused by slab break-off and foundering in the central part of the subducted slab.