PETROTECTONICS OF ADAKITES: IMPLICATIONS FOR TECTONIC RECONSTRUCTION OF THE EASTERN HOKKAIDO, JAPAN

“Adakite” was initially defined as andesitic, dacitic to rhyolitic rocks characterised by high Sr/Y and strongly depleted Y and heavy rare earth elements (HREEs) concentrations named after Adak Island, Alaska, at 1990 by Defant and Drummond. The petrogenesis of adakite was originally proposed as partial melting of young subducted lithosphere because their geochemical features such as high Sr/Y and depleted HREEs were simply thought to be brought by the existence of residual garnet and/or amphibole of eclogite and/or amphibolite suits in the subducted oceanic lithosphere. The occurrence of adakite, therefore, seems to have an important implication for the tectonic setting. Since the beginning of 2000s, dozens of studies have reported adakite or “adakitic rock” which are not putatively derived from slab melting but possibly from partial melting of lower crust or high pressure fractional crystallisation even they have similar geochemical characteristics such as high Sr/Y ratio and strongly depleted HREEs. The multiple origins of adakites and adakitic rocks are supported by reconsideration of the previous tectonic implication.

In this study, principal component analysis (PCA) and k-means cluster analysis (KCA) of previously published adakite database from GEOROC is applied to distinguish adakites which have different origins. We find that several trace elements are able to classify adakites and adakitic rocks into some groups which have different origins properly.

The Late Cretaceous to Paleogene sedimentary rocks in the eastern Hokkaido, Japan, is interpreted as a forearc basin deposit of the Paleo-Kuril Arc. Igneous rock associations composed of andesite and alkaline basalt have been observed in the sedimentary facies. Our chemical analysis shows the andesitic samples are classified as adakite. Here, we show the origin of igneous rock associations of the eastern Hokkaido by applying the statistical analysis with the geochemical database which is mentioned above, and some tectonic implications for there.