Significance of serpentinite-hosted exhumation channels in a palaeo-subduction complex, Nishisonogi unit, Nagasaki Metamorphic Complex: P–T trajectories of mélange blocks and coherent schists

Abstract

Serpentinite mélanges occur as thin horizons up to 350 m thick within coherent schists of the Nishisonogi unit of the Nagasaki Metamorphic Complex located in western Kyushu, which represent serpentinite-hosted exhumation channels within a Cretaceous subduction complex. This study gives the petrography of coherent schists and various tectonic block types embedded in serpentinite or chlorite-actinolite schist of the mélange. The P–T conditions recorded in these rocks were compared, to clarify a possible exhumation process and style of the Nishisonogi unit. In the mélange, two types of tectonic blocks were recognized based on peak temperature conditions: a lower temperature type (400–590°C) and a higher temperature type (780–830°C), the latter of which shows temperatures higher than those of the coherent schists (370–450°C). The garnet glaucophanite in the coherent schists shows a prograde pressure increment from 0.9 to 2.3 GPa. The serpentinite mélange comprises tectonic blocks with peak temperatures higher than those of the coherent schists, indicating that blocks from the deep structural levels of the lower-plate underwent tectonic mixing into the mélange during exhumation. Ultrahigh-pressure (UHP) conditions have not been recorded either in the coherent schists or in the serpentinite mélanges, although microdiamonds have been reported from the Yukinoura mélange located at the western margin of the Nishisonogi unit (Nishiyama et al., Scientific Reports, 2020, 10, 11645). A possible correlation between the Nishisonogi unit and other HP metamorphic rocks or belts (Renge rocks, Suo Belt and Sanbagawa Belt) was examined to conclude that the Nishisonogi unit is a unique paleo-subduction complex in terms of the coexistence of HP subduction complex with intercalated serpentinite mélanges and a UHP mélange derived from the slab-mantle interface.