CHEN Wu, QIN Ying, LUO Xiang-jian, ZHANG Jia-wei, LU Shu-fan, FU Hong-bin, PAN Ying-juan^*

(Guizhou Geological Survey, Guiyang 550018, China)

Abstract: There is a set of ferruginous and aluminous rocks enriched with rare earth elements in the bottom of Upper Permian Xuanwei Formation in the Hujiagou area in Hezhang County of the northwestern Guizhou. Through analysis of major and trace elements of various rockson in the Hujiagou profile and research of their sedimentary geochemical characteristics, the sedimentary paleoenvironment of ferruginous and aluminous rock series in the bottom part of the Xuanwei Formation in the study area has been revealed. The research indicates that samples from the upper part of the ferruginous rock segment and the lower part of the aluminous rock segment have Sr/Ba values ranging from 0.60 to 0.97 and V/Zr values ranging from 0.63 to 2.23, indicating a brackish-marine environment. These reflect that those rocks of ferruginous and aluminous rock segments were formed in a terrestrial sedimentary region which might have been influenced by seawater at that time. Those samples have relatively high values of Al₂O₃/MgO and Mg/Ca, indicating a warm and humid depositional environment. Moreover, clastic rocks in the middle and lower parts of the Xuanwei Formation have higher values of Al₂O₃/MgO and Mg/Ca than the ferruginous and aluminous rocks, suggesting that clastic rocks of the Xuanwei Formation were formed in an even more humid depositional environment. All samples have U/Th values ranging from 0.18 to 0.41 and Ce_anom values ranging from -0.28 to 0, indicating that they were formed in oxidized to weakly oxidized environments. Especially, samples of the aluminous rock segment have relatively high,U/Th values but relatively low Ce_anom values, suggesting that the aluminous rocks were formed in a relative weakly oxidized environment, reflecting that they were formed in a relatively deep sedimentary water body. It is beileved that the REE-enriched aluminous rock was formed by continental marginal offshore lacustrine sediments whose sedimentation was partially influenced by seawater. This unique depositional environment might be conducive to the REE enrichment and preservation.

    Keywords: geochemical characteristics; ferruginous and aluminous rock series; sedimentary geochemical indicators; depositional environment;     the northwestern Guizhou