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Geochronology of Sanbagawa schists, SW Japani ;" t L Tetsumaru ITAYAl VJ,L; :?;4:ff

Research Institute of Natural Sciences, Okayama University of Sciencex; )I; :: iz 4F rF t>F ' 4 :Southwest Japan, a segment of the circum‑Pacific orogenic chain, consists of several subduction‑accretion complexes and high‑pressure metamorphic belts of Phanerozoic age. The Sanbagawa high‑pressure schist belt belongs to the Jurassic subduction complexes. The belt extends about 800km along

the Pacific Ocean side of southwestern portion of the Japan arc. The Sanbagawa metamorphic sequence

has long been studied extensively on its general geology, petrology and structural geology.
Geochronology of the belt has also been well documented since Banno and Miller (1965), in particular,
aiter the systematic K‑Ar and Ar‑Ar phengite analyses by Monie et al. (1987), Itaya and Takasugi ( 1 988),

Takasu and Dallmeyer ( 1 990).

The timing of peak metamorphism has been estimated from the Rb‑Sr whole‑rock isochron ages
(1 16d: lOMa; Minamishin et al., 1979) ofthe high‑pressure schists in central Shikoku area. Okamoto etal. (1999) gave SHRIMP U‑Pb zircon ages ( I 1 2‑ I 1 8Ma) from quartz eclogite, which are consistent withthe Rb‑Sr whole rock age. These suggest that the Sanbagawa metamorphic rocks exhumed afier the ageof I I OMa that should be older than the K‑Ar and Ar‑Ar ages of K‑bearing minerals such as phengites,biotites and amphiboles. Most of K‑Ar and Ar‑Ar mineral ages are younger than the age but some rocks

have given the ages siginificantly older than 1 10Ma, e.g.. K‑Ar biotite age (128Ma) from omphacite
bearing amphibolite and K‑Ar phengite ages ( 1 23 and 1 36Ma) from quartz eclogite both of which occur
in the lratsu mafic ‑ ultramafic body. These are likely due to excess argon trapped in the minerals during

the metamorphism. The Sanbagawa schist clasts in Eocene Kuma conglomerate have also extremely
older ages for garnet amphibolites (Takasu and Dallmeyer, 1 992; Itaya, unpubl. data). Takasu and andDallmeyer ( 1 992) considered that the older age rocks exhumed as the Kuma nappe predating theSanbagawa main schists. This is unlikely because the Ar‑Ar age spectra of amphibole are very irregularand have no plateau region and the total gas ages are also so heterogeneous among the grains from thesame sample (Itaya, unpubl. data). This type of amphiboles probably formed in heterogenous excess
argon environment during the metamorphism though the phengite associated with amphiboles has the age
consistent with that ofthe main Sanbagawa schists.

Itaya and Takasugi (1988) cawied out the conventional K‑Ar .study of phengites from theSanbagawa schists in central Shikoku and gave ages that ranged from 88Ma to 64Ma. Monie et al.

(1987) and Takasu & Dallmeyer (1990) have reported 40Ar/39Ar ages of phengite from the same
metamorphic sequence, which are the same as the conventional K‑Ar ages of Itaya and Takasugi (1988).
This indicates that the conventional K‑Ar phengite analyses are applicable to revealing the timing andkinetics of exhumation of the whole Sanbagawa metamorphic sequence as followed by Itaya and Fuj ino(1 999) and Miyashita and Itaya (in review).

‑ 101 ‑Phengites in the pelitic and psammitic schists generally occur forming aggregates consisting offine‑grained phengite crystals and are extremely fine‑grained in domains close to relatively rigid garnet

and albite porphyrob]asts, indicating that deformation‑induced grain‑size reduction had taken place inphengite during the ductile deformation accompahying the exhumation of host schists. The matrix
phengites in schists are chemically heterogeneous on the thin‑section scale. The matrix phengites

having low Si values are likely to have been formed during retrograde metamorphism in extremely
restricted equilibrium domains.

The two or three different types of schists from the same outcrop, which have a similar grain sizeof phengite, have similar K‑Ar ages, suggesting that the closure temperature does not depend on
chemistry. However, the hematite‑rich quartzose schist with strong grain‑size reduction of both
phengite and quartz has a significantly younger K‑Ar phengite age than the pelitic and quartzose schists

in the same outcrop that do not show grain‑size reduction. We suggest that, the exhumation tectonics ofthe schists, which have experienced strong ductile deformation at temperatures less than ca. 3500C,
played an important role resulting in the observed variation in age.

Although K‑Ar phengite ages have discordant relation among the different types of schists from thesame out crop, there are two contrasting age‑temperature‑structural relations for the Sanbagawa
metaorphic sequence. Systematic K‑Ar phengite analyses show a younging polarity from Southern Unit
(82Ma) to Northern unit (58Ma) in the Kanto Mountains. The age range is similar to that in the centralShikoku. The older schists occur in the higher metamorphic grade zone in the central Shikoku while in
the lower‑grade zone in the Kanto Mountains. The thermal structures in the central Shikoku are inverted,
so that the highest‑grade zone occurs in the upper or middle parts of the apparent stratigraphic succession.

In contrast, the Kanto Mountains have a normal thermal structure: the higher‑grade zone is in the lower
part of the apparent stratigraphic succession. The different tectonic features in exhumation offer the two

contusting age ‑ temperature ‑ structure relations at the western side of Sanbagawa belt in the central

Shikoku and the eastem end of Sanbagawa belt in the Kanto Mountains that are 800km distant from eachoth er.

‑ 102 ‑