Depositional Chemistry of Cherts of the Late Paleozoic in Napo Rift Basin, Guangxi and Its Implication for the Tectonic Evolution

The Napo rift basin on the southern margin of the Youjiang basin was dominated by deep-water sediments including cherts, mudstones and marine basalts during the Late Paleozoic. Major and rare earth elements of the Upper Devonian Liujiang Formation and Lower-Middle Permian Sidazhai Formation cherts in Napo are analyzed in this study. The SiO₂ contents of samples range from 88.55% to 99.03%, and the PAAS components are less than 20%, which indicate few terrigenous clastic components in the Upper Paleozoic cherts. The high Al/(Al+Fe+Mn) ratios (0.45 to 0.94) and no positive Eu anomalies (0.51 to 0.95) indicate non-hydrothermal origins. Considering the effect of diagenetic SiO₂ dilution, their ∑REE+Y values are between twice and five times those of PAAS-like compositions, indicating that they were deposited in a basin far away from the terrigenous input environment. The Upper Devonian Liujiang Formation of Yanxin and Yutang have moderately negative Ce anomalies (0.37 to 0.72, 0.58 to 0.89, respectively) and higher Y/Ho ratios (39.05 to 83.74, 34.33 to 36.70, respectively), indicating these cherts were deposited in the open-rift basin far away from the terrigenous input. The Lower-Middle Permian Sidazhai Formation cherts of Yutang show similar geochemistry characteristics of the mature ocean basin cherts, with obviously negative Ce anomalies (Ce/Ce^*=0.12 to 0.33). On the basis of our studies about cherts, we conclude that the depositional chemistry of the Upper Paleozoic cherts records the evolution process of the Youjiang basin from the rift basin during the Late Devonian into an open-ocean basin during the Early-Middle Permian.