Geochronology and geochemistry of the Paleoproterozoic Fangniushan supracrustal strata in the Xiaoshan area, southern North China Craton: Implications for tectonic evolution

The Fangniushan supracrustal strata located in Xiaoshan area is one of the best-preserved Paleoproterozoic lowgrade metasedimentary units exposed in the southern North China Craton, which is mainly composed of quartzite and feldspar quartzite with minor pebbly quartzite and conglomerates. It unconformably contacts with the underlying Taihua Complex and the overlying Paleoproterozoic Xiong'er Group. Ages of the youngest detrital zircons from the quartzites combined with the formation age of the overlying Xiong'er Group bracket the depositional time of the Fangniushan supracrustal strata at ca. 1.87-1.80 Ga. Low CIA ( < 70), ICV (0.48-1.46) values and high SiO2/Al2O3 (4.1-75.1) ratios for the Fangniushan metasedimentary rocks are suggestive of chemically mature source that experienced weak to moderate chemical weathering in their source areas, which is also supported by the A-CN-K signatures. Detrital zircon U-Pb ages of the Fangniushan metasedimentary rocks yield three populations of similar to 2.00 to 2.24 Ga, similar to 2.25 to 2.40 Ga and similar to 2.50 to 2.88 Ga. REE patterns combined with trace elements characteristics involving La, Th, Zr, Hf, Co and Sc reveal that the Fangniushan metasedimentary rocks dominantly received detritus from felsic sources in the Xiaoshan and adjacent Xiaoqinling, Xiong'ershan and Zhongtiaoshan areas. Taking all these geochronological and geochemical signatures into account, the Fangniushan supracrustal strata was most likely deposited in a retro-arc foreland setting. In combination with previous studies on other Paleoproterozoic metavolcanic-sedimentary sequences, we prefer that the southern NCC might have experienced tectonic regime change from rift to subduction during Paleoproterozoic.