This study presents new zircon U?Pb ages and whole-rock geochemical data for the Fanyindong (FYD) and Jincheng (JC) granitoid intrusions in coastal Zhejiang and Fujian Provinces, respectively, southeast China, to investigate their petrogenesis and provide new constraints on the regional tectonic evolution and crustal growth. Zircon U?Pb ages indicate that the FYD and JC intrusions were emplaced at c. 170 Ma and 193 Ma, respectively. Both granitoids belong to the high-K calc-alkaline series, and display arc-like trace element patterns, with enrichment of large ion lithophile elements and Nb-Ta depletion. The FYD granitoids show adakitic geochemical characteristics, with high radiogenic Sr isotope ratios, and the most enriched bulk-rock Nd and zircon Hf isotopic compositions (crustal Nd?Hf model ages = ca. 2.35?2.75 Ga) ever reported in the Mesozoic granitoids within southeast China. Geochemical characteristics suggest that the FYD granitoids were generated by partial melting of mostly Paleoproterozoic basement rocks in a thickened lower crust. However, whole-rock Sr?Nd and zircon Hf isotopes of the JC granites show depleted source signatures. Geochemical signatures suggest the JC granites originated from the partial melting of a mixed source, consisting of the pre-existing, ancient, lower crust and juvenile, mantle-derived, basaltic rocks. The data from this study, in conjunction with previously published data, indicates that there exists a NE-trending, Early?Middle Jurassic, arc-related, magmatic rock belt along the southeast coast of China, and the Paleo-Pacific Plate subduction most likely started at c. 200 Ma. The early stage subduction of the Paleo-Pacific plate and the induced reactivation and extension of pre-existing E?W striking faults in the Nanling region of southeastern China gave rise to the generation of the JC granites. Ongoing subduction of the Paleo-Pacific plate resulted in a thickening of the continental margin, and subsequent crustal anatexis of basement materials produced the FYD granitoids.