Due　to　the　lack　of　high-precision　geochronological　and　systematic　structural　analyses,　the　tectonic　evolution　and　paleogeography　of　the　Sabah　region　in　North　Borneo　are　not　well　understood.　In　this　study,　we　conducted　zircon　U-Pb　dating,　zircon　Hf　isotope,　and　whole-rock　elemental-isotope　geochemical　analyses　of　monzonites　and　gabbros　in　the　eastern　part　of　Sabah,　North　Borneo.　The　dating　results　showed　that　the　monzonites　intruded　at　Middle　Triassic　(239.8–234.8　Ma),　and　the　gabbro　intruded　at　Late　Triassic　(217.2±2.5　Ma),　respectively.　Geochemically,　the　monzonites　have　typical　adakite　characteristics,　such　as　high　Mg#　(56–59),　high　Al2O3,　high　Sr/Y　and　La/Yb　ratios,　low　Y　and　Yb　contents,　depletion　of　high-field-strength　elements　such　as　Nb,　Ta,　Ti,　etc.,　and　depleted　Nd-Hf　isotopic　compositions　(whole-rock　ΕNd(t)=6.5–7.2,　zircon　ΕHf(t)=11.0–16.5).　The　gabbros　consist　of　both　N-MORB　and　E-MORB　types　and　are　characterized　by　the　depletion　of　HFSEs,　significant　negative　Nb-Ta　anomalies,　and　depleted　Nd　isotopic　compositions　(ΕNd(t)=6.7–8.7).　Comprehensive　analyses　indicated　that　the　monzonites　derived　from　interaction　between　the　adakitic　melt　resulted　from　partial　melting　of　the　subducting　oceanic　crustal　sheet　and　the　overlying　mantle　peridotite　and　experienced　crystallization　of　olivine　and　pyroxene　during　magma　ascending;　whereas　the　N-MORB-type　gabbro　was　derived　from　partial　melting　of　depleted　lithospheric　mantle　metasomatized　by　fluids　released　from　subducted　oceanic　crust　and　experienced　fractional　crystallization　of　olivine　and　pyroxene;　E-MORB-type　gabbros　resulted　from　partial　melting　of　oceanic　seamounts　or　oceanic　islands,　the　melt　was　mixed　with　the　N-MORB-type　gabbro　parent　magma　during　its　ascending　and　experienced　fractional　crystallization　of　olivine　and　pyroxene.　Considering　the　rock　associations　(e.g.,　high-magnesium　andesites　and　adakites,　and　E-MORB　gabbro)　in　the　region,　we　suggest　that　the　Triassic　magmatism　in　eastern　Sabah　belongs　to　continental　island-arc　magmatism.　Further　comparative　analysis　revealed　that　Triassic　continental　arc　magmatism　occurs　along　the　southeast　coasts　of　China　and　Taiwan　Island,　southeast　Vietnam,　the　islands　of　the　Philippines,　the　Nansha　Islands,　and　Borneo,　suggesting　that　a　giant　active　continental　margin　of　Southeast　Asia　was　formed　under　westward　subduction　of　the　Paleo-Pacific　Plate,　with　the　entire　Borneo　being　part　of　this　active　continental　margin　island　arc.　©　2025　Science　Press.　All　rights　reserved.