Abstract:

Recently, a well-preserved carbonate-derived Mesoproterozoic ancient weathering crust is identified at the boundary (~1400 Ma) between the Tieling Formation and the Xiamaling Formation at the Zishikou section, Laishui County, Hebei Province. Based on the field observation and  analyses of petrology and mineralogy, it can be referred that this ancient weathering crust is mainly composed of limonitebearing ferric mudstone and illite claystone. Compared with element contents of the Upper Continental Crust, Al₂O₃,TiO₂,Fe₂O₃, K₂O in the ancient weathering crust have obvious and different enrichments, SiO₂ is leached slightly and CaO, MgO, Na₂O are leached seriously, trace elements, including Sc, Cu, Ga, Zr, Hf, Nb, Ta, In, Th, Li, V, Zn, Rb, Cs, W, Pb, U, Mo, Bi, are unchanged or moderately enriched, but Cr, Sr, Cd are leached, REE are enriched and fractionated. Ce shows positive anomaly and Eu shows negative anomaly. All  leaching and enrichment features, including that of U/Th, Fe³⁺/Fe²⁺, abnormality of Ce, variations of CIA and BA indexes, indicate that this ancient weathering crust should be formed under the oxidizing, moderate to strong degree of weathering conditions. Consequently, all analyses suggest that the north part of the North China Craton should be a warm and humid tropical to subtropical climate environment at the transitional time between the Calymmian Period (1600~1400 Ma) and the Ectasian Period (1400~1200 Ma) of the Mesoproterozoic Era.

Key words: ancient weathering crustlpaleoclimate, Mesoproterozoic Era, North China Craton