Proto-Japan and tectonic erosion: Evidence from zircon geochronology of blueschist and serpentinite

Abstract

Blueschist facies rocks in the Shikoku island of southwest Japan were extruded from depth as blocks in the Kurosegawa serpentinite mélange, where they occur in association with other tectonic blocks of tonalite-trondhjemite-granodiorite(TTG)rocks and calc-alkaline volcanics. Here we report magmatic zircons in the blueschist that yield 206Pb/238U mean ages in the range 505 ± 3 to 503 ± 3 Ma. These zircons crystallized at a mid-ocean ridge within basaltic rocks, and migrated to the continental margin of paleo-Asia. Subduction of the oceanic lithosphere into the mantle at a depth of ~40 km resulted in metamorphism under jadeite-glaucophane facies conditions at 250 Ma. The blueschist facies rocks were then exhumed to the surface along a splay fault connected with the forearc region through tectonic extrusion. Zircons separated from serpentinite yielded a 206Pb/238U mean age of 152 ± 3 Ma, marking the timing of serpentine protrusion to the surface, consistent with geologic constraints. The serpentinites also contain minor ca. 500 Ma zircons, similar to those in the blueschist. The predominance of detrital ca. 150 Ma zircons in the serpentinite suggests extensive tectonic mixing of Jurassic trench turbidites with the mantle wedge. The Kurosegawa serpentinite mélange belt extends along the strike of the orogenic belt for more than 800 km, parallel to the active trench. We propose that this belt marks the location of extensive tectonic erosion and provides an excellent case study for the destruction of continental crust along a convergent plate boundary.