The Late Mesozoic geological framework of South China is characterized by voluminous volcanic rocks and their intrusive counterparts. However, the petrogenesis and tectonic implications of the Late Mesozoic magmatism in South China are poorly constrained. In this study, we report new petrographic, geochemical, zircon U-Pb geochronological, whole-rock Sr-Nd isotopic and zircon Lu-Hf isotopic data for the Yihe biotite monzogranite in the Heyuan area, southeastern South China Block, with the aim of determining its age and petrogenesis, important for understanding the late Mesozoic tectonic evolution of the South China Block. Petrographic features suggest suggest that Yihe pluton mainly consists of biotite monzogranite. Zircon LA-ICP-MS dating results show that biotite monzogranites formed during the Late Jurassic (161.1~157.0Ma). The Yihe granite is metaluminous to weakly peraluminous and characterized by high SiO2 (75.33％~77.25％), K2O(4.47％~5.60％), but low content of Al2O3(11.47％~12.61％), depletion in P, Ti, Ba, Sr, and enrichment in Rb, U, Th, with pronounced negative Eu anomalies (δEu=0.13~0.28). Zircon saturation temperatures are relatively high (748~794°C), together with the trace-element systematics, indicate a highly fractionated I-type affinity. The Yihe biotite monzogranite has relatively high initial 87Sr/86Sr ratios (0.711789 to 0.716214), low εNd(t) values (-8.25 to -8.21), and zircon εHf(t) values (-11.3 to -7.0), with Nd and Hf two-stage model ages (TDM2) of 1613~1616 Ma and 1648~1923 Ma, respectively. These features suggest that the Yihe magma was generated mainly by partial melting of ancient crustal materials, with mantle-derived contributions more likely expressed as limited heat input rather than large-scale material mixing, and subsequently underwent fractional crystallization during ascent and emplacement. Combined with regional geological background, we propose that the Yihe biotite monzogranite formed in a Late Jurassic extensional regime that was probably related to slab break-off after Paleo-Pacific flat-slab subduction and the associated local delamination/derooting of the subcontinental lithosphere beneath South China.