Abstract: The paper studies the petrography, geochronology, whole-rock geochemistry, and Sr-Nd isotope of the intermediate-basic magmatic rocks in Nandu and Tanrong areas of southeastern Guangxi. The results show that the main rock types of Nandu and Tanrong intermediate-basic magmatic rocks are monzonite and aegirine-augite syenite, with the zircon U-Pb ages of 159.8 Ma and 156.8 Ma, respectively, indicating both formed during the Late Jurassic. The Nandu monzonite is more basic, while the Tanrong syenite is relatively intermediate. In the SiO₂-K₂O diagram, all samples plot within the shoshonitic series. The LREEs are significantly enriched relative to HREEs, exhibiting an overall right- dipping pattern. The Nandu samples show obvious negative Ba anomalies, while Tanrong samples distinct negative Sr and Ti anomalies. The isotope characteristics reveal that the I_Sr values of Nandu samples range from 0.70554 to 0.70959, with the εNd(t) values from -10.58 to -0.75, mostly falling within the enriched mantle and close to the Bulk Silicate Earth(BSE) area. The I_Sr values of Tanrong samples are 0.69033-0.69615, with the εNd(t) value of 0.36, mainly plotting between depleted mantle and enriched mantle fields. In summary, the study area was in a back-arc extensional tectonic setting associated with the westward subduction of Paleo-Pacific Plate during the Late Mesozoic. The Nandu monzonite samples were likely formed by partial melting of enriched mantle material, with the mantle-derived magma contaminated by crustal material during ascent, accompanied by a certain degree of plagioclase fractional crystallization, while the Tanrong aegirine-augite syenite was probably generated by partial melting of newly formed lower crust, induced by heat under the influence of partial melting of enriched mantle.