Abstract:Deepwater sedimentary siliceous rocks and limestones, widely developed in Luodian, South Guizhou during MiddleUpper Devonian time，belong to a special formation type of siliceous rocks, because their formation type is distinctly different from siliceous rock developed within the island arctype or accretionary environments. The petrological, mineralogical, and geochemical study was carried out for 42 samples collected from the MiddleUpper Devonian siliceous sediments at Mengjiang and Guoshizhai in Luodian county, South Guizhou. The results reveal that the middle Devonian siliceous sedimentary rocks are high in Al2O3（0.62%~22.1%）, TiO2 (0.01%~0.84%)，Fe2O3 (0.41%~10.5%) and ∑REE content (7.87×10-6~319×10-6），and Ce/Ce*（0.51~0.96）, and relatively low in Y/Ho（26.6~37.0，ave. 308） and （La/Ce）N（1.06~1.99，ave. 1.28）; while the Mengjiang siliceous sediments of Upper Devonian are low in Al2O3（2.42%~10.6%）, TiO2（0.06%~0.43%）, Fe2O3（0.62%~2.65%）, ∑REE（28.7×10-6~163×10-6）, Ce/Ce*（0.45~0.82）, and relatively high in Y/Ho（28.2~36.7，ave. 34.2）and （La/Ce）N （1.34~2.34，ave. 1.82）. Except the effect of SiO2 dilution, the siliceous sediments in the study area have greatly variable REE contents and indicate multiple sources in origin, with large part from seawater from Middle to Upper Devonian and minor from terrigenous clastics. The discrimination diagrams of Al2O3/TiO2Al（Al+Fe+Mn）, AlFeMn and （∑REE+Y）SiO2/Al2O3, and the major and rare earth element geochemical characteristics of the MiddleUpper Devonian siliceous sediments at Guoshizhai and Mengjiang in Luodian, indicate that these siliceous sediments formed in a normal marine sedimentary environment, with the depositional environment changing from restricted margin rift basin to an openocean rift basin during Middle to Upper Devonian. This study is of important value for further understanding the evolution for the ShuichengZiyunNandan rift basin in the Late Paleozoic and Youjiang basin during Middle to Upper Devonian.
