Abstract: The Guongmangco ophiolitic mélange is well developed in the middle part of Shiquanhe-Yongzhu-Jiali ophiolitic mélange belt, and its formational environment can provide important evidence for investigating the tectonics of the Shiquanhe-Yongzhu-Jiali ophiolitic mélange belt. In this paper, the geochemistry of siliceous rocks was studied to judge the formational environment of the Guomangco ophiolitic mélange. Siliceous rocks occur as intercalated beds in basalt and have abundant Late Triassic-Early Cretaceous radiolarite. The siliceous rocks have SiO2 content of 71.38%~77.67%, high Al2O3 content of 8.62%~11.51%, MnO/TiO2 ratios of 0.28~0.35, (Ce/Ce?)SN ratios of 0.92~0.94 and (La/Ce)SN ratios of 1.13~1.17, implying the influence of the input of terrigenous components. However, they have higher V, Ni and Cu content and V/Y ratios than the siliceous rocks in epicontinental settings, similar to things of siliceous rocks in ocean mid-ridge and basin. Guomangco siliceous rocks were probably formed in an ocean basin relatively far away from the continent and mainly affected by the input of terrigenous components. In combination with the geochemistry of metamorphic peridotite, mafic dike and basalt, the authors hold that the Guomangco ophiolitic mélange was probably formed in a back-arc basin close to the continental margin.