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　40Ar-39Ar　cooling　age　of　203~204　Ma,　indicating　that　they　formed　in　late　Triassic.　Analytical　results　show　that　the　dykes　are　characterized　by　SiO2　content　of　55.68%~56.85%,　high　K　(K2O/Na2O=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.　©　2019,　Science　Press.　All　right　reserved.