Iwashimizu Complex Fm
Period:
Cretaceous, Jurassic, Triassic
Age Interval:
Province:
Hokkaido
Type Locality and Naming
Kamuikotan Belt
Synonym:
Lithology and Thickness
The second group (Types I and If by Sakakibara & Ota 1994) consists dominantly of metabasites, and chert associated with pelitic and psammitic rocks, and occurs mainly as coherent masses (Iwashimizu Complex by Kawamura et al. 1998; Ueda 2005). These rocks underwent blueschist facies and lower-grade (lawsonite-albite facies) metamorphism characterized by common occurrences of aragonite, lawsonite, sodic amphiboles and jadeitic pyroxene, as well as epidote and pumpellyite. Lower grade parts of them are very weakly deformed, occasionally preserving original sedimentary and volcanic structures.
Lithology Pattern:
Gneiss
Relationships and Distribution
Lower contact
The first Kamuikotan Complex Fm (conformable?)
Upper contact
The third Kamuikotan Complex Fm (conformable?)
Regional extent
In the cores of anticlines within the Yezo forearc basin.
GeoJSON
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Fossils
In the least recrystallized parts of the Iwashimizu Complex, chert yields Late Triassic-earliest Cretaceous radiolarians, and mudstone contains middle Early Cretaceous radiolarians (Hori & Sakakibara 1994). MidCretaceous (Albian-Cenomanian) radiolarians were also reported from another aragonite-bearing pelitic unit (Ganpiyama Complex: Kawamura et al. 2001) isolated within serpentinites.
Age
Age Span:
Beginning stage:
Capitanian
Fraction up in beginning stage:
0
Beginning date (Ma):
264.34
Ending stage:
Aptian
Fraction up in the ending stage:
0
Ending date (Ma):
121.40
Depositional setting
Depositional pattern:
Additional Information
Metabasites of the Twashimizu Complex show chemical characteristics dominantly of oceanic-island basalts (Nakano & Komatser 1979; Kimura et al. 1994; Skakibara et al. 1999).
These fossil ages and the basalt geochemistry of the Iwashimizu Complex Fm are the same as those of the Naizawa Complex in the Idonnappu Zone, and Watanabe et al. (1994) and Iwasaki et al. (1995) correspondingly proposed that the former was the deeply subducted equivalent of the latter.
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.