华北克拉通北部14亿年前古风化壳的地球化学特征及古气候意义

最近在河北涞水紫石口剖面的中元古界铁岭组-下马岭组界线(~1400 Ma)附近，发现了一套保存完好的碳酸盐岩古风化壳。野外观测和岩石矿物学研究显示，该古风化壳的岩性以含褐铁矿的铁质泥岩和伊利石粘土岩为主。相对上地壳平均化学组分来说，古风化壳的主量元素氧化物中Al₂O₃、TiO₂、Fe₂O₃和K₂O富集，SiO₂轻微亏损，CaO、MgO和Na₂O强烈亏损；微量元素中Sc、Cu、Ga、Zr、Hf、Nb、Ta、In、Th、Li、V、Zn、Rb、Cs、W、Pb、U、Mo和Bi等化学性质稳定或易吸附于粘土中的元素含量接近上地壳平均化学组分值或呈不同程度富集，易溶元素Cr、Sr和Cd相对亏损；稀土元素全部富集，轻稀土元素与重稀土元素分异程度大，Ce正异常，Eu负异常。U/Th、Fe³⁺/Fe²⁺、Ce的正异常、化学蚀变指数与风化淋滤指数等多种指标都指示该古风化壳是在强氧化的中等到强烈的风化淋滤程度条件下形成。上述研究表明，在中元古代盖层纪(Calymmian Period，1600~1400 Ma)与延展纪(Ectasian Period，1400~1200 Ma)之交，剖面所在的华北克拉通北部应属于温暖湿润的热带-亚热带气候。

Geochemistry and Implication to Paleoclimate of the~1.4 Ga Ancient Weathering Crust in the North of the North China Craton

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.