Many lamprophyre dykes occurring in north Guangxi province are probably the source rock for proximal diamond placer deposit, but their formation age has long been a pending geological issue. This study shows that phlogopites (phenocryst and matrix) in lamproite in the Rongshui area have a 40 Ar- 39 Ar cooling age of 203~204 Ma, indicating that they formed in late Triassic. Analytical results show that the dykes are characterized by SiO 2 content of 55. 68%~56. 85%, high K (K 2O/Na 2O=1. 87~3. 06) and high Mg# (71. 90~73. 20), enrichment of LREE and relative depletion of HREE with distinct differentiation between LREE and HREE, slightly Eu anomaly, enrichment of large ion lithophile elements (LILE) and depletion of high field strength elements (HFSE), geochemically suggesting that the dykes sourced from the enriched lithospheric mantle metasomatized by subduction- related fluids. This study shows that the Mesozoic lithospheric mantle under the southwestern margin of the Yangtze Carton is the an enriched mantle which experienced metasomatization of subduction fluid and thinning event due to lithospheric extension in the late Triassic, and its formation was probably related to the NW- trending subduction of the paleo- Pacific plate.