4:00 PM - 4:15 PM
[SGL18-07] Geochemical characteristics of the Japan Arc based on the logarithmic distribution of chemical data for each rock facies
Keywords:Japan arc, geochemical characteristics, lognormal distribution, geological distribution
The structure and formation process of the continental crust has long attracted attention from both chemical and physical perspectives. In addition, the chemical composition of the continental crust reflects the formed process of the crust at that time, and in addition to dating, the spatiotemporal distribution of petrological and geochemical data is examined, and compositional variation and their geochemical origins are investigated. The growth of the continental crust since the Proterozoic has occurred in "fluctuation zones" formed on continental margins, and the Japan Arc is an example of such a fluctuation zone, and its geology, rocks, and their chemical composition are could be the key to understanding how island arcs, and ultimately continents, grow. The Japan Arc has been located in the convergence margins of the continents (South China and North China massifs) since at least the Paleozoic, and is a field where the geological structure development has been studied for a long time, and research to date has clarified this. The growth process of the Japan Arc is attracting attention when considering the growth of the continental crust, especially during the Phanerozoic. In addition, since many chemical and physical data on rocks that make up the foundation of the Japan Arc have been announced, Japan Arc is a suitable field for understanding crust formation. Many studies have been conducted on the “geological composition ratio” and “average chemical composition” as a result of the geological development of the Japan Arc. Estimating the “geological classification” and the corresponding “average chemical composition of each component'” remains assignment. This report uses "Seamless Geological Map" by GSJ and "DODAI Database" for geological constituents and chemical data, and examines the chemical characteristics and regional characteristics of the Japanese archipelago from statistical values and large-scale data analysis.
In previous reports (JpGU 2023, MGI29-P05, etc.), we have reported on lithofacies classification, geographical distribution of chemical data, and chemical composition distribution for each lithology. In this process, issues such as the density of chemical data classified into each rock facies and problems in statistical processing of chemical data were taken up. In particular, the chemical composition of rocks may show extremely high concentrations of certain elements due to the concentration of incompatible elements during the magma differentiation process. Therefore, it is necessary to consider the influence of this “existence of highly concentrated data”. For many elements, the data distribution peaks around 0 concentration and shows a monotonically decreasing distribution toward higher concentrations, whereas various statistical values are based on a “normal distribution”. There are problems such as questions about the positioning of numerical values. We addressed these problems by taking the “logarithm” of the concentration. Use the logarithm, many of the elements whose distribution monotonically decreases from 0 in antilogarithm show a data distribution similar to a normal distribution, and it has become possible to perform statistical processing on extremely high concentration data all at once. In this report, we compare the characteristics of the average chemical composition of the Japan Arc using logarithms with previous research and average crustal composition data from around the world, and discuss the characteristics of the data and its position in the history of geological structure development of NE and SW Japan Arcs.
We thank Mr. Yoshiharu Nishioka (AIST) for his cooperation in calculating the area ratio of each rock type based on the Seamless Geological Map.
In previous reports (JpGU 2023, MGI29-P05, etc.), we have reported on lithofacies classification, geographical distribution of chemical data, and chemical composition distribution for each lithology. In this process, issues such as the density of chemical data classified into each rock facies and problems in statistical processing of chemical data were taken up. In particular, the chemical composition of rocks may show extremely high concentrations of certain elements due to the concentration of incompatible elements during the magma differentiation process. Therefore, it is necessary to consider the influence of this “existence of highly concentrated data”. For many elements, the data distribution peaks around 0 concentration and shows a monotonically decreasing distribution toward higher concentrations, whereas various statistical values are based on a “normal distribution”. There are problems such as questions about the positioning of numerical values. We addressed these problems by taking the “logarithm” of the concentration. Use the logarithm, many of the elements whose distribution monotonically decreases from 0 in antilogarithm show a data distribution similar to a normal distribution, and it has become possible to perform statistical processing on extremely high concentration data all at once. In this report, we compare the characteristics of the average chemical composition of the Japan Arc using logarithms with previous research and average crustal composition data from around the world, and discuss the characteristics of the data and its position in the history of geological structure development of NE and SW Japan Arcs.
We thank Mr. Yoshiharu Nishioka (AIST) for his cooperation in calculating the area ratio of each rock type based on the Seamless Geological Map.