<p>In order to reveal the genesis and tectonic background of the mafic rocks in the South Altyn area, the diagenetic age and geochemical characteristics of diabase gabbro in the Saitama area were discussed by using LA-ICP-MS zircon dating and geochemical analysis. The diabase gabbro in the Saitama area of the South Altyn region is ²⁰⁶Pb/²³⁸U and has an age of 423&#xa0;Ma. The SiO₂ content of diabase gabbro ranged from 48.28 to 54.53%, and the average value was 50.76%. The variation range of Mg# value was 25.80 ~ 57.97, and the average value was 45.17. The content of K₂O + Na₂O varied between 2.46 and 7.63%, and the average value was 4.71%, which belonged to the alkaline series. It enriches large ionic lithophilic elements (LILE) and light rare earth elements such as Rb, Ba, Th, U, K, La, and Ce, and loses high field strength elements (HFSE) such as Nb, Ta, Ti and heavy rare earth elements. The study of lithogenesis shows that the mantle source magma of diabase gabbro in the Saitama area has the addition of crustal materials, and the mode of action should be the mixing of the source area, and the addition of crustal materials may be related to the dismantling and subsidence of the thickened lithosphere after the deep subduction of the South Altyn Basin in the middle and late Ordovician. The occurrence of mafic–ultramafic rocks around 423&#xa0;Ma in the Saitama area may imply the transition of the tectonic system from extrusion to extension.</p>