Measuring and adjusting the weathering and hydraulic sorting effects for rigorous provenance analysis of sedimentary rocks: a case study from the Jurassic Ashikita Group, south‐west Japan
Measuring and adjusting the weathering and hydraulic sorting effects for rigorous provenance analysis of sedimentary rocks: a case study from the Jurassic Ashikita Group, south‐west Japan
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DOI：
10.1111/j.1365-3091.2008.00963.x
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发表时间：
2008-12
期刊：
影响因子：
3.5
通讯作者：
T. Ohta
中科院分区：
文献类型：
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作者：
T. Ohta
本研究考察了分布于日本西南部的侏罗纪Ashikita群的物源，该群由Idenohana组、Kyodomari组和Sakamoto组组成。在物源分析中使用了两张地球化学图，充分考虑了风化作用引起的成分变化(MFW图)和水力分选过程(SiO_2/Al_2O_3-Na2O/K_2O图)。MFW图描绘了沉积岩的风化趋势，并通过将风化趋势追溯到未风化的区域来估计原始烃源岩的成分。Idenohana组和Kyodomari组的风化趋势向后延伸到中间和长英质火成岩领域。相反，坂本组的沉积物不符合线性风化趋势，表明源岩不能近似为火成岩。在SiO_2/Al_2O_3-Na_2O/K_2O图上，沉积物按组成趋势组织，其范围反映了水力分选效应引起的组成变化。在这张图上，可以通过读取趋势的倾角来区分来自火成岩和再循环沉积源区的沉积物。利用这一原理，Idenohana组和Kyodomari组的源岩被解释为火成岩，Sakamoto组的源岩被解释为循环沉积岩。因此，这些图解同时通过量化和调整风化和分选效应来估计源岩成分，揭示了Ashikita群物源的系统转变。Idenohana组和Kyodomari组主要来自火成岩物源，在地层顺序上从中间成分转变为长英质成分。然而，Sakamoto组的沉积物主要来自一个再循环的沉积物源。
The present study examines the provenance of the Jurassic Ashikita Group distributed in south‐west Japan, which is composed of the Idenohana, Kyodomari and Sakamoto Formations. Two geochemical diagrams for provenance analysis were utilized, which incorporate full consideration of compositional modifications resulting from weathering (MFW diagram) and hydraulic sorting processes (SiO2/Al2O3–Na2O/K2O diagram). The MFW diagram delineates weathering trends of sedimentary rocks and allows estimation of the original source rock composition by tracing the weathering trends backwards to an unweathered domain. Weathering trends of the Idenohana and Kyodomari Formations extend backward to the domain of intermediate and felsic igneous rocks. In contrast, sediments of the Sakamoto Formation do not fit into a linear weathering trend, indicating that the source rock cannot be approximated to igneous rocks. On the SiO2/Al2O3–Na2O/K2O diagram, sediments are organized into compositional trends, in which the range reflects compositional variations induced by the hydraulic sorting effect. On this diagram, sediments derived from the igneous and recycled sedimentary provenances can be distinguished by reading the inclination of the trend. By utilizing this principle, source rocks of the Idenohana and Kyodomari Formations are interpreted as igneous rocks and those of the Sakamoto Formation are interpreted as recycled sedimentary rocks. Therefore, these diagrams concurrently estimate the source rock composition through quantifying and adjusting the weathering and sorting effects, and reveal a systematic transition in the provenance of the Ashikita Group. The Idenohana and Kyodomari Formations were supplied chiefly from an igneous provenance, which shifted from intermediate to felsic compositions in stratigraphic order. Whereas, sediments of the Sakamoto Formation were sourced primarily from a recycled sedimentary provenance.