Poroshiri Ophiolite Fm
Period:
Cretaceous
Age Interval:
Province:
Hokkaido
Type Locality and Naming
Sorachi-Yezo Belt (Poroshiri Ophiolite)
Synonym: 幌尻
Lithology and Thickness
The ophiolite consists mainly of amphibole schists, metagabbro and metacumulates of the amphibolite facies, with peridotite and metasediments (represented by viridine-muscovite-quartz schist: Suzuki et at. 1965).
These rocks are locally intruded by dykes, which also underwent metamorphism (Miyashita & Niida 1981), and the original ophiolite sequence has been rearranged by thrusts and folds (Miyashita 1983) with synmetamorphic deformation occurring during dextral transpression (Jolivet & Miyashita 1985; Arita et at. 1986; Arai & Miyashita 1994).
The Hidaka metamorphic rocks to the east are partly involved in folds and thrusts within the ophiolite (Arai & Miyashita 1994; Arai et at. 1995).
Lithology Pattern:
Metavolcanics
Relationships and Distribution
Lower contact
No data.
Upper contact
No data.
Regional extent
On the cast (structurally lower part) of the Idonnappu Zone, a north-south-trending zone exposes the Poroshiri Ophiolite (Miyashita & Yoshida 1988; Miyashita et al. 1994) and is bounded by a thrust fault (Hidaka Western Boundary Thrust or WBT).
GeoJSON
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Fossils
Age
The geochemistry of basaltic rocks and the mineralogy of meta-cumulate and peridotite all suggest that the ophiolite, as well as the later dykes, originated from normal oceanic crust consisting solely of mid-oceanic ridge magmatic rocks (Miyashita & Niida 1981; Miyashita & Yoshida 1988: Miyashita et al. 1994, 2007).Kizaki (2000) reported a zircon U-Pb age of 96.7 + 2.6 Ma from a quartz amphibolite presumably of plagiogranite origin, suggesting that the Poroshiri Ophiolite represents mid-Cretaceous oceanic crust. Because this age is contemporaneous with pelagic sediments in the Late Cretaceous unit of the Idonnappu Zone, Ueda (2006) considered that the Poroshiri Ophiolite is a relic of the subducted oceanic slab on which the Idonnappu Zone accretionary complex was formed.
Age Span:
Beginning stage:
Cenomanian
Fraction up in beginning stage:
0
Beginning date (Ma):
100.50
Ending stage:
Turonian
Fraction up in the ending stage:
0
Ending date (Ma):
93.90
Depositional setting
Metamorphic grade generally increases from west to east (Miyashita 1981, 1983; Osanai et al. 1986; Miyashita et al. 1994; Tanaka et al. 2012). Metabasites along the wester margins (WBT) of the ophiolite show greenschist facies mineralogy (zone A) and these grade into the amphibolite facies (zone C) via the epidote amphibolite facies (zone B), across several-hundred-metre intervals showing apparent geothermal gradients up to c. 200°C km-1. Central and eastern parts (zone Co of the ophiolite show relatively homogeneous metamorphic temperatures, except for the easternmost parts (zone D) which grade through upper amphibolite to granulite facies. Metamorphic amphiboles commonly show prograde compositional zoning. This is attributed to early ocean-floor metamorphism followed by heating from the east as the ophiolite was juxtaposed with hot granulites of the Hidaka Metamorphic Belt (Osanai et a. 1986). A white mica K-Ar age of 17.1 + 0.4 Ma (Shibata et al., 1984) suggests Miocene exhumation in conjunction with the Hidaka Metamorphic Belt.
Depositional pattern:
Additional Information
It underwent greenschist to amphibolite (or locally granulite) facies metamorphism and was once regarded as a western part (the Wester Zone) of the Hidaka Metamorphic Belt (Osanaiet al. 1986; Komatsu et at. 1989). However, its oceanic rock character strongly contrasts with the continental crustal section typifying the main zone of the Hidaka Metamorphic Belt. The Poroshiri Ophiolite is here included in the Sorachi-Yezo Belt, taking into account its genetic relation with the Idonnappu Zone accretionary complexes (Komatsu et al. 1992; Ueda 2006).
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.