Keywords:Kamuikotan high-P/T type metamorrphic rocks, amphibolite and blueschist tectonic blocks in serpentinite mélange, counterclockwise P-T paths, subduction initiation
It has been recently noted that serpentinite mélange which contains tectonic blocks of high-P/T type metamorphic rocks such as blueschist and eclogite and their counterclockwise P-T paths could indicate the subduction initiation (e.g. Willner et al., 2004; Escuder-Viruete et al., 2011; Agard et al., 2016; Bhowmik and Ao, 2016; Hunziker et al., 2017). We investigated tectonics in amphibolite and blueschist tectonic blocks in mélange, which are distributed at the highest structural level of the high-P/T type Kamuikotan metamorphic rocks in Hokkaido, northern Japan. We reconstructed their individual pressure-temperature (P-T) paths based on petrography, mineral chemistry and phase equilibria modeling. As a result, tectonic blocks in this study area are divided into six rock types: garnet-epidote amphibolite, epidote amphibolite, amphibolite, plagioclase-poor amphibolite, blueschist and quartz-rich blueschist. From the reconstruction of P-T paths, it is found that most of the tectonic blocks experienced blueschist facies metamorphism probably at the maximum depth, while the initial paths vary from a counterclockwise P-T path shown by different kinds of amphibolite to a prograde P-T path shown by blueschist and quartz-rich blueschist. We have concluded that these P-T paths, on the whole, could have reflected the cooling history of subduction channel from the beginning to steady state of subduction. The geothermal gradient could have been changed from ~25 °C/km to ~10 °C/km within c. 25 myr estimated by previously reported radiometric ages. These tectonic blocks could have started to subduct at a different timing to show the different P-T paths, and juxtaposed against each other at a certain depth in the subduction channel. Since these tectonic blocks show little deformation structures formed during exhumation stage, they might have behaved as a rigid body in serpentinite (i.e. mélange matrix) after they were incorporated in it during subduction stage.