The newly discovered Hongshanliang Formation represents the oldest stratigraphic unit within the Baogutu tectonic belt along the northwestern margin of the Junggar basin, with the age of Late Devonian. It is dominated by red siliceous rock, tuffaceous siliceous siltstone, basalt, and andesitic basalt. Previous geochemical and tectonic studies of volcanic rocks in this region were limited to the Carboniferous- Permian period. However, the basalts from the Hongshanliang Formation provide important information about the pre- Carboniferous magmatism. Geochemical analysis reveals two distinct basalt types: tholeiitic and alkaline. Tholeiitic basalts generally are characterized by low TiO2 (0. 30%~1. 16%), Al2O3 (12. 80%~18. 46%) contents, high TFe2O3 (8. 97%~13. 57%) and Mg# (42. 23~64. 37), and display a right- tilted rare earth element (REE) pattern with depletions in Nb, Ta, and Ti. These geochemical features are consistent with N- MORB and IAT affinities, suggesting fluid activity induced by subduction in a back- arc basin setting. This tectonic environment facilitated a 5%~10% partial melting of spinel lherzolite in a depleted mantle source. Conversely, alkaline basalts display elevated TiO2 (1. 74%~1. 88%), Al2O3 (15. 92%~18. 07%) contents, low TFe2O3 (6. 39%~7. 62%) and Mg# (41. 84~47. 13), lack distinct negative Nb and Ta anomalies. Their geochemical signature closely resembles OIBs, indicating a 5%~10% partial melting of garnet + spinel lherzolite. This suggests an origin possibly related to a seamount/ocean island setting associated with a mantle plume. Integrating these findings with previous research, this study considers that back- arc basin and mantle plume magmatism developed simultaneously during the Devonian period in the northwestern margin of the Junggar basin.