Has the Southern Tan-Lu Fault Zone in eastern China been thickened?

Early Cretaceous adakitic rocks are widespread along the Tan-Lu Fault and the Dabie Orogen in eastern China. Conventional models attribute their origin to partial melting of thickened lower crust, associated with lithospheric thinning in the North China Craton. Our study presents comprehensive geochemistry for low-Mg adakitic rocks (LMAs) from the Southern Tan-Lu Fault Zone (STLFZ), and compares them with those from the Dabie Orogen. The STLFZ LMAs share similar Sr-Nd-Hf isotopic signatures with the Dabie LMAs, suggesting a shared source derived from the Paleoproterozoic basement of the northern Yangtze Block. However, the STLFZ LMAs are geochemically distinct, with lower La/Yb, Sr/CaO, and Nb/Ta ratios, but higher Nb/La ratios, reflecting differences in residual mineral assemblages and melting conditions. Phase equilibrium modeling reveals that the STLFZ LMAs likely formed through fluid-fluxed melting under garnet amphibolite facies, contrasting with the eclogite-facies dehydration melting for the Dabie LMAs. This implies that the STLFZ was not significantly thickened at the time of melting. Furthermore, the STLFZ LMAs were emplaced at ca. 130−115 Ma, later than the Dabie LMAs (ca. 143−130 Ma). These spatial, temporal, and genetic differences probably reflect the reactivation of the Tan-Lu Fault (ca. 146−130 Ma), which triggered dehydration melting, crustal foundering, and mantle metasomatism in the Dabie Orogen. Subsequent underplating of hydrous magmas beneath the STLFZ then facilitated fluid-fluxed melting under normal crustal thickness conditions. These findings provide new insights into the heterogeneous mechanisms of adakitic magmatism and the role of major tectonic structures in controlling crustal evolution.