Metamorphic rocks of the Mesoproterozoic Aksu Group are developed in Aksu area, southwestern Tianshan Mountains. Based on detailed field and petrographic observation as well as geochemical investigation, the authors hold that the protoliths of the gneiss and the hornblende schist are felsic volcanic rocks and mafic rocks respectively, whereas the protoliths of mica schist are graywacke. Chemical components of the gneiss indicate that SiO2 and Al2O3 content of the samples varies from 72.36% to 74.6% and from 12.28% to 13.55%, respectively. TiO2, Fe2O3, FeO, MnO, MgO and CaO content is obviously low: Fe2O3=0.44%～1.2%, FeO=0.68%～1.74%，MgO=0.23%～1.17%，and CaO=0.41%～1.31%. Chemical components of the hornblende schist suggest that TiO2, TFe2O3, MgO and CaO content is obviously high: TiO2=2.27%，TFe2O3=15.79%，MgO=5.77%，and CaO=7.75%, but SiO2 and Al2O3 content of the sample is only 49.88% and 12.9% respectively. Compared with PASS, mafic rocks are rich in SiO2 and Na2O (varying from 61.51%～75.53% and 1.33%～4.38%, 69.16% and 2.91% on average respectively), but poor in Al2O3, TFe2O3, MgO and CaO. The petrological and chemical characteristics of the gneiss and hornblende schist suggest that the rocks are of calc-alkaline series, with σ from 1.8 to 2.3, and the chrondrite-normalized REE patterns are all characterized gnificant enrichment of light rare elements, with no significant fractionation of heavy rare earth elements. In the by si MORB normalized multi-element variation diagram, multi-element variations show relative concentration of K, Rb, Th, evident depletion of Ti, Nd, Ta, and clear negative P anomalies. The chrondrite-normalized and PASS-normalized REE patterns of the mica schist are similar to patterns of active continental margin and continental arc greywacke. All the gneiss samples are plotted in the field defined by volcanic arc granites in geochemical-tectonic discrimination diagrams，which suggest that they were formed in a continental volcanic arc closely related to the subduction zone. This conclusion is also supported by the geochemical-tectonic discrimination diagrams of the simultaneously-deposited metamorphic sedimentary rocks. In both La-Th-Sc and Th-Sc-Zr/10 discrimination diagrams, almost all of the samples are plotted in the continental arc. Chemical characteristics of the mica schist suggest that it was formed in a sedimentary environment related to continental island arcs, such as back arc basin. Combined with regional background, all these characteristics suggest that metamorphic rocks in the Aksu Group were formed in a continental volcanic arc-back arc basin and might have been related to a global supercontinent Columbia event.