Geochemistry of Proterozoic Wufoshan Group sedimentary rocks in the Songshan area, Henan Province: Implications for provenance and tectonic setting

The Archean-Paleoproterozoic metamorphic crystalline basement and subsequent sedimentary cover well outcrop in the Songshan area along the southern margin of the North China Craton (NCC). The Wufoshan Group, covering on the Dengfeng Group and Songshan Group with angular unconformity, is the earliest sedimentary stratum extensively distributed in Songshan area. Sandstones have higher SiO2 contents (70.18% similar to 95.21%) than mudstones (20.79% similar to 52.27%), with relatively lower Fe2O3, MgO, CaO and K2O contents, indicating more basic constitutes and potassium-rich minerals in mudstones. All sedimentary samples show that the light rare earth elements (LREE) are enriched relative to the heavy REE with moderately negative Eu anomalies. Mudstones have higher total REE (Sigma REE = 131 x 10(-6) similar to 169 x 10(-6)) than sandstones (Sigma REE = 7.30 x 10(-6) similar to 158 x 10(-6)), implying the absorption of REE by clay minerals abundant in fine sediments. The lower index of compositional variability (ICV = 0.89 similar to 3.59) of sandstones indicate they have higher grade of maturity and formed in a relatively stable tectonic setting, whereas the higher ICY values (4.04 similar to 17.50) and lower SiO2/Al2O3 (3.90 similar to 5.58) and La/V ratios (0.45 similar to 0.65) of mudstones indicate that they have relatively lower grade of maturity and formed in an active sedimentary basin. The chemical index of alteration (CIA = 7.52 similar to 66.1) of the Wufoshan Group samples display that they have experienced low grade of chemical weathering and alteration. The petrologic and geochemical signatures show that the sedimentary rocks of the Wufoshan Group sourced from post-Archean medium-acid components with minor basic rocks. Sandstones in the middle and lower part of the Wufoshan Group formed in a passive continental margin, the same as contemporary sedimentary sequences in the southern margin of the NCC, implying that the tectonic setting shifted to a stable sedimentary stage after the eruption of Xiong'er Group volcanic rocks. However, mudstones in the upper part formed in a sedimentary basin related to a continental arc. More crustal activities and distinct changes in sedimentary environments from Mesoproterozoic to Neoproterozoic indicate the transition from passive to active continental margin, a possible effect of the early tectonic events of the Qinling Orogenic Belt.