Subhorizontal tectonic framework of the Horoman peridotite complex and enveloping crustal rocks, south‐central Hokkaido, Japan

Abstract

Geological observations in the Horoman area, south-central Hokkaido, show that the Horoman peridotite complex of the Hidaka metamorphic belt is a tectonic slice about 1200 m thick. The peridotite slab is intercalated into a gently east-dipping structure. The underlying unit is a Cretaceous-Paleogene accretionary complex. Riedel shear planes in the sedimentary layers of the accretionary complex near the structural bottom of the peridotite slab indicate top-to-the-west (thrust) displacement. The overlying unit is composed of felsic-pelitic gneisses and mafic-felsic intrusive rocks (the Hidaka metamorphic rocks). The boundary surface between the peridotite complex and metamorphic rocks forms a domal structure. Microstructures of sheared metamorphic rocks near the structural top of the peridotite slab indicate top-to-the-east (normal) displacement. The results combined with previous studies suggest that the Horoman peridotite complex was emplaced onto the Asian margin (Northeast Japan) during the collision between the Asian margin and the Hidaka crustal block.

The influence of melt infiltration on the Li and Mg isotopic composition of the Horoman Peridotite Massif

“…122 La) enrichment in the region is most likely to be melt. At the end of Miocene (~ 23 Ma), the 123 peridotites were metasomatised by fluid and formed phlogopite veins in the mantle wedge 124 above the Hidaka subduction zone during emplacement based on Rb-Sr dating on samples 125 with phlogopites (Yoshikawa et al, 1993;Yamamoto et al, 2010). During uplift and 126 emplacement the peridotite was transformed from garnet-to spinel-to plagioclase-facies 127 peridotites (Takazawa et al, 1996a;Ozawa, 2004).…”

“…Horoman massif (Yoshikawa et al, 1993;Yamamoto et al, 2010). In this case the minimum 501 cooling time from melt percolation to olivine closure is ~0.3 Ma.…”

Reply to Comment on ‘Metamorphic olivine after dehydration embrittlement in Serpentinite: Case study from the Shiraga Serpentinite mass in the Sanbagawa high P/T metamorphic belt, central Shikoku, Japan’ by Wallis and Endo, https://doi.org/10.1111/iar.12328

“…The presence of the convergent structure is originally shown in the schematic cross section (Figure 2b) of Shiota's pioneering work (Shiota, ). The convergent structure of the serpentinite (ultramafic) mass due to imbrication is rather common in metamorphic belts like the Horoman peridotite in the Hidaka belt, Hokkaido, Japan (Yamamoto et al, ). Therefore, reconstructing the convergent structure (imbricated structure) through a geological map and in cross section remains important.…”