华北房山景儿峪组顶部古风化壳常量元素地球化学特征及其古气候意义
Major Element Compositions and Paleoclimatic Implications of Paleo-Regolith on Top Jingeryu Formation in Fangshan, North China

风化壳的元素地球化学组成可以反映风化壳的发育类型及其气候特征，地史时期形成的古风化壳的元素地球化学组成可以揭示古风化壳的类型及其古气候特征.华北地区新元古代晚期(景儿峪组)至寒武纪(府君山组)期间的地层记录缺乏连续性，广泛发育一套典型的古风化壳，该古风化壳的发育过程可能记录了晚新元古代至早寒武纪期间的古气候特点.针对华北房山地区景儿峪组顶部保存的古风化壳中常量元素的组成、元素迁移富集特征和风化系数等分布规律开展了较系统分析，结果表明：(1)SiO2、Al2O3、TFe2O3、CaO是景儿峪组顶部古风化壳的主要组分，Al2O3、TFe2O3、TiO2、K2O在风化壳的中上部相对富集，SiO2则轻微亏损，CaO、Na2O、MgO、P2O5被迁移淋失；(2)硅铝系数、硅铝铁系数、化学蚀变指数(CIA)、残积系数、风化淋溶系数(BA)等地球化学指标的垂向变化特征指示该古风化壳形成于温暖湿润条件下中等强度的化学风化作用，其风化过程可能经历了由较弱至较强再逐渐减弱的演变过程；(3)与现代发育于湖南、贵州、云南等地碳酸盐岩类基岩之上的风化壳元素地球化学特征对比，发现景儿峪组顶部古风化壳的Si淋失度和Fe、Al富集度均较低.综合研究区古风化壳的常量元素地球化学特征，同时结合新元古代晚期至寒武纪的华北板块古纬度迁移特征，认为房山地区景儿峪组顶部发育的古风化壳形成于温暖湿润的亚热带-热带气候，为脱硅富铝化程度较低的硅铝粘土型风化壳.
The geochemical composition of regolith can reflect its development processes and the background climatic characteristics of weathering processes, and geochemical compositions of the paleo-regolith formed in earth history can reveal the type of paleo-regolith and record paleoclimatic changes. The record between the Late Neoproterozoic (the Jingeryu Formation) and the Cambrian (the Fujunshan Formation) in North China is a stratigraphic hiatus due to the tectonic uplift and erosion. A typical paleo-regolith widely occurs and is supposed to have recorded paleoclimate changes during the Late Neoproterozoic to Early Cambrian. According to the compositions of major elements, enrichment in elemental migrations and coefficient of weathering distribution of paleo-regolith on the topmost Jingeryu Formation in the Fangshan area, North China, this study suggests that: (1) The major element compositions of the paleo-regolith on the topmost Jingeryu Formation are dominated by the elements of SiO2, Al2O3, TFe2O3, CaO. And the elements of Al2O3, TFe2O3, TiO2, K2O are relatively concentrated in the middletoupper parts of the regolith while the SiO2 is slightly lost. (2) The vertical variations of geochemical proxies such as SiO2/Al2O3, SiO2/(Al2O3+TFe2O3), chemical index of alteration (CIA), residual coefficient and BA value indicate that the paleoregolith should have been formed under a warm and humid condition with moderate chemical weathering. The chemical weathering might have gone through an evolution from weak to strong and again to weak in degree. (3) Compared to the element geochemical characteristics of modern regolith on the topmost carbonate settings from Hunan, Guizhou and Yunnan, it is inferred that the paleoregolith on the topmost Jingeryu Formation has relatively lower deficiency in Si and enrichment in Fe and Al. Consequently, by comprehensive analysis of the geochemical characteristics of paleo-regolith on the topmost Jingeryu