INTERPRETING PROVENANCE CHANGES THROUGH TIME IN A MIOCENE MARINE BASIN, BOSO PENINSULA, JAPAN

The provenance of sediments in active tectonic settings may be determined from paleogeographic reconstructions and from major and trace element geochemistry. This study focuses on Miocene age mudrocks, sandstones and tuffs from the Boso Peninsula, Japan. The provenance of sediments in the basin changed over time due to the uplift of the Mineoka Mountain Range, as changing plate movements resulted in subduction and then obduction of the Mineoka ophiolite. This is an ideal location to interpret the influence of provenance on sediment geochemistry. Samples were collected from the Amatsu Formation, a shallow marine sequence of mudrocks and minor sandstones, punctuated by volcanic ash and tuff deposits. We determined their mineralogy and major and trace element geochemistry. Discriminant function plots for major elements in the mudrocks show a gradual change in provenance from intermediate to mafic over time. Trace element plots (e.g. Th, Sc and REE) mostly show the same provenance pattern. However, in some samples the major and trace element signals disagree. Some of the sediments include a sand component, where the major elements are dominated by the sand grains and the trace elements reflect the mud matrix. These two components have different provenance signatures (sand is intermediate, mud is mafic). In a few mudrock samples, there appear to be traces of volcanic ash, and this material dominates the trace element signature over the background mudrock (ash is mafic, mud can be intermediate or mafic). This corroborates with the geochemistry of the tuff layers which is mafic, suggesting that there were also minor or more distant pyroclastic eruptions with the same composition. Overall, the trend in provenance from intermediate to mafic agrees with the paleogeography of the Boso Peninsula determined from plate tectonic reconstructions. Initially, there was an input of detritus with intermediate provenance from the Honshu Arc to the north, then the provenance gradually became more mafic with the introduction of a southern source, the Mineoka Ophiolite complex. Infrequent pyroclastic eruptions of mafic composition interrupt this overall trend, contributing to the geochemical signature.