Ultramafic rocks in the Kurosegawa tectonic zone, Southwest Japan.

Abstract

The Kurosegawa tectonic zone extends from Kyushu to the western Kii Peninsula and con tains ultramafic rocks with two distinct textural and chemical types of olivine and clinopyroxene. The ultramafic bodies were mainly harzburgites or lherzolites which originally equilibrated at around amphibolite facies conditions and subsequently re-equilibrated near 400•Ž. Ultramafic rocks in the Chichibu belt, from the eastern Kii Peninsula to the Kanto Mountains, have the same textural and chemical characteristics as those in the Kurosegawa tectonic zone ; accordingly , they are considered to be an eastern extension of the zone. Petrology of the ultramafic rocks and the distribution of lithologic types within the Kurose gawa tectonic zone indicate that the ultramafic rocks and the associated granitic and high-grade metamorphic rocks were originally situated towards the western side of the zone, and were tectonically sliced and crushed by left-lateral movement during the Early Cretaceous. The more mobile serpentinized ultramafic rocks were transported laterally far to the east along the strike slip systems, but the other rigid materials remained within the western area.

Petrogenesis and geotectonics of the Mikame ultramafic body, western Shikoku, Japan

“…Several brief petrological descriptions of the Mikame ultramafic rocks were given by Ishimoto (1973), Suzuki (1977), and Yokoyama (1987). They reported that the Mikame ultramafic rocks are predominantly dunite and wehrlite, with minor amounts of harzburgite, lherzolite, websterite, and orthopyroxenite.…”

“…The isotherm curves are from Obata et al (1974). For comparison, the plots of the serpentinites from the Mariana forearc (Murata et al 2009) and the Kurosegawa belt (Yokoyama, 1987) are also shown. The symbols are the same as those in Figure 4.…”

“…However, Masuda (1977) found granitic rocks equivalent to a member of the Kurosegawa belt in southern margins of this area, and interpreted that, together with the granitic rocks, the Mikame ultramafic rocks belong to the Kurosegawa belt. Yokoyama (1987) also performed a petrological study of the Mikame ultramafic rocks, and concluded that it is a member of the Kurosegawa belt, although he noted the petrological differences between the Mikame ultramafic body and other members of the Kurosegawa belt. Isozaki and Itaya (1990) also recognized the Mikame body as a member of the Kurosegawa belt based on the lithological assemblage in this area.…”

Antigorite crystallographic preferred orientations in serpentinites from Japan

“…Serpentinite A: olivine-bearing serpentinite mylonite from the Toba area Lenticular serpentinite bodies intrude a Jurassic accretionary complex and pre-Cretaceous metamorphic rocks in the Toba area, southwest Japan (Fig. 1a) (Saka et al, 1988;Yokoyama, 1987). Occasionally, the serpentinite body contains blocks of schists that have undergone highpressure metamorphism (Saka et al, 1988).…”

“…Veins consist mainly of olivine with lesser diopside, magnetite, and antigorite. Yokoyama (1987) estimated the temperature of vein formation as 400°C, based on the Mg-Fe equilibrium in olivine-clinopyroxene assemblages.…”