Abstract: Owing to the unique sedimentary characteristics, the Youjiang Basin has attracted considerable attention from numerous geologists. The Middle Triassic of the Youjiang Basin was filled with a set of thick flysch sediments, which are predominantly fine-grained sandstones, siltstones, mudstones and transition rocks. However, there are still controversies about the provenance of the clastic rocks. In this paper, detailed petrological and geochemical analyses of the Middle Triassic clastic rocks in Ceheng have been carried out. Results show that the clastic rocks are featured by low compositional maturity and textural maturity, which reflects a proximal sedimentation. The content of SiO2 (average=58.49%) is low with a wide range of variation, which indicates that the quartz contents are relatively low. The high content of Al2O3 (average=12.46%) indicates the high contents of clay minerals and feldspars. The average ratio of Al2O3/SiO2 is 0.22, which is consistent with the graywacke (0.20~0.23), indicating that the clastic rocks are low in maturity and also have proximal sedimentary characteristics. The high content of CaO (average 7.83%) indicates more carbonates. The trace elements are characterized by the enrichment of Rb, Th, La, Nd, Hf and Pb,
but relatively depleted in P, Ti, Sr, Nb and Ta. Rare earth elements are featured by large variation range of ΣREE, enrichment of light rare earth elements and Eu depletion. Analysis of major, trace and rare earth elements reveals that the source rocks of clastic rocks are mainly felsic rocks, with a little mixture of intermediate rocks and quartzose sedimentary rocks. The tectonic setting of the source area is dominated by continental island arc, and partially passive continental margins and active continental margins. The chemical index of alteration (CIA) is 53.93~74.80 (average=65.12). The CIA and Th/U ratios indicate that the source area of clastic rocks mainly experienced moderate chemical weathering.

Key words: Youjiang Basin, Ceheng, Middle Triassic, provenance, tectonic setting