West Oshima Plutons Fm
Period:
Cretaceous
Age Interval:
Province:
Hokkaido
Type Locality and Naming
West Oshima Belt
Synonym:
Lithology and Thickness
Cretaceous plutonic rocks intruded into the Jurassic accretionary complexes are surmarized by Tsuchiya et al. (1986) and Tsuchiya (2010). According to them, the plutonic rocks are dominated by calc-alkaline tonalite to granodiorite batholiths both of ilmenite and magnetite series. Alkaline (shoshonitic) series rocks of gabbro, ultramafic cumulate and monzo-gabbro/ diorite are also locally present. In addition to these intrusive, Early Cretaceous rhyolitic volcaniclastic rocks including welded tuff also occur in the Okushiii Island off SW Hokkaido.
Lithology Pattern:
Volcanics
Relationships and Distribution
Lower contact
West Oshima accretionary Gr (unconformable) in the West Oshima belt.
Upper contact
No data.
Regional extent
West Oshima Belt. Plutonic rocks in the Oshima Belt form part of the Cretaceous granite belt exposed in the NE Honshu Arc.
GeoJSON
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Fossils
Age
Radiometric ages (mainly by K-Ar method) from Oshima Belt granites range from 130 to 65 Ma with a cluster at 115-100 Ma.
Age Span:
Beginning stage:
Barremian
Fraction up in beginning stage:
0
Beginning date (Ma):
126.50
Ending stage:
Cenomanian
Fraction up in the ending stage:
0.762
Ending date (Ma):
95.47
Depositional setting
Depositional pattern:
Additional Information
Although some radiometric ages of the Oshima Belt granites are coeval with those in the Northern and Southern Kitakami belts, adakite granites known in the latter are lacking in the Oshima Belt.
Compiler:
Hayato Ueda. 2g Hokkaido. in Moreno, T., WALLIS, S., Kojima, T. & Gibbons, W. (Eds) 2016. The Geology of Japan. Geological Society. London. 201-220.