Tokoro terrane 

(subduction zone - oceanic crust) 

(Hokkaido Island, Japan)

Consists chiefly of two major units. (1) A lower unit that consists of ultramafic cumulate rocks, diabase, 
pillow basalt, trachyte, alkali rhyolite, basaltic hyaloclastite and other volcaniclastic rocks, chert, and 
limestone (Nikoro Group). Jurassic Stromatopora and Upper Cretaceous foraminiferas occur in limestone, 
and Upper Jurassic to Lower Cretaceous radiolarians occur in chert. The mafic to ultramafic igneous rocks 
are derived from alkali basalt magmas, and are metamorphosed at the zeolite, prehnite-pumpellyite, sodic 
pyroxene-chlorite, and pumpellyite-actinolite facies (high-pressure intermediate facies type). And (2) an 
unconformably overlying upper unit that consists of Upper Cretaceous to Paleogene sedimentary rocks 
(Saroma Group). REFERENCES: Kiminami and Kontani, 1983; Research Group of Tokoro Belt, 1984; 
Iwata and Tajika, 1986; Kontani and others, 1986; Sakakibara and others, 1986; Tajika, 1988; Kato and 
others, 1990.