Nikoro Gr

Tokoro Belt

Synonym: 仁頃, にこる

The Nikoro Group is characterized by abundant occurrences of volcanic and volcanogenic clastic rocks associated with chert and lmestone.

The volcanic rocks consist mainly of basaltic pillow and massive lava, with minor amounts of dolerite and felsic rocks. Minor and small bodies of troctolite, gabbro, wehrlite and tonalite also occur (Bamba 1981, 1984; Niida 1981).

Clastic rocks are dominantly green-coloured sandstone and conglomerate with abundant voicaniclastic materials. and have also been described as sandy or epiclastic hyaloclastites (Bamba 1984; Sakakibara et al. 1986).

These sandstones are polymictic consisting dominantly of basaltic volcanic clasts occasionally associated with rock fragments of andesite, granitoid rocks and crystalline schists.

See “age” section.

Chert occurs overlying basaltic rocks and has been dated as Late Jurassic-Early Cretaceous in age using radiolarian fossils (Kiminami et al. 1983; Okada et al. 1989; Iwata et al. 1990). Early Cretaceous (Aptian) planktonic foraminifers were reported from pelagic limestone conformably overlying Late Jurassic chert (Okada et at. 1989).Precise ages from the clastic rocks have not been successfully obtained, but they are inferred to be Late Cretaceous based on radiolarian evidence (Sakakibara et al. 1993).

The Nikoro Group was once considered to represent islandarc rocks (Komatsu 1981; Kiminami & Kontani 1983a).
Although, based on whole-rock chemistry, Bamba (1984) identified both tholeiitic and alkaline basalts. He considered the former to be of abyssal basalt origin, comprising an ophiolitic assemblage togetber with gabbro and troctolite.
Alkali basalts are considered as accreted fragments of subducted seamounts (Niida 1981; Sakakibara et al. 1986).

The majority of the Nikoro Group underwent zeolite to prehnite pumpellyite facies metamorphism. However, in the northeastern parts of the belt, high-pressure-low-temperature (HP-LT) metamorphic minerals represented by aragonite, Lawsonite, sodic pyroxene and sodic amphibole occur in basaltic rocks (Sakakibara 1986a, b, 1991). These parts of the Tokoro Belt therefore comprise a HP-LT metamorphic subduction complex.
In these rocks, epidote and lawsonite co-occur with pumpellyite and sodic pyroxene and not with sodic amphibole, showing slightly lower pressure than typical blueschists in which epidote or lawsonite + sodic amphibole assemblages are common. Metamorphic conditions of 200-300°C at 0.5-0.6 GPa have been estimated by Sakakibara (1991).