Department of Earth and Planetary Sciences, University of Tokyo
The Sambagawa metamorphic belt is bordered by Izumi Cretaceous forearc sediments to the north and to the south by the Chichibu accretionary complex of the Jurassic age. The boundary between Izumi group and the Sambagawa belt is the transcurrent fault named as Median Tectonic Line. The southern boundary is the thrust dipping northward.
Metamorphic rocks were highly deformed plastically and in the brittle manner (Toriumi 1990, Wallis, 1993, Toriumi and Hara, 1995, Toriumi and Yamaguchi, 2000). Quantitative strain magnitude distribution in the Sambagawa metamorphic belt was proposed by means of shape change of radiolarian fossils filled by polycrystalline quartz in metachert. The strain geometry was also determined: the strain magnitude increases with increasing metamorphic temperature, the strain geometry changes from pizza pie to pencil shape with increasing strain magnitude, and the maximum elongation orientation is parallel to the mineral lineation trending EW direction.
The deformation path of the Sambagawa metamorphism in the subduction orogen is still unclear, but the total strain magnitude and strain geometry can serve our plastic flow scheme in the process of subduction and exhumation. Garnet growth took place prograde process, that is the subduction process, and thus the deformation during garnet growth should indicate the subduction deformation.
The Sambagawa metamorphism occurred in the age of 120-60Ma determined by K-Ar and Ar-Ar dating. Judging from the amphibole dating about 100-90 Ma and muscovite about 80-70 Ma, the cooling process should take place in the period from 90 to 70 Ma, that is the retrograde metamorphism(Itaya and Takasugi, 1983, Takasu and Dallmayer,1990). The prograde metamorphism may be the time of 120 to 90 Ma. Ryoke metamorphism also occurred during 90 - 60 Ma judging from the K-Ar and Rb-Sr ages of micas, and thus it seems that both metamorphism took place coeval.
Sambagawa metamorphism is a high pressure intermediate type regional metamorphism from blueschist facies to eclogite facies conditions. In order to obtain the detailed P-T-D paths we conducted the differential thermodynamic method to garnet zoning in pelitic schists and amphibole zoning in basic schists. The systems are garnet - muscovite - paragonite - epidote - chlorite - albite - quartz - fluid and amphibole - epidote - chlorite - albite - quartz - fluid. The former is tirvariant system and the latter is four variant system. As garnet has three independent chemical variable and amphibole dose five independent chemical components, pressure and temperature paths can be inferred exactly without chemistries of epidote, chlorite and mica in the garnet and amphibole systems.
The prograde paths were obtained by garnet inversion as follows; pressure increases from 0.4 to 1.0 Gpa with increasing temperature from 520 to 600C (Enami, 1998, Inui, 1999, Inui and Toriumi, 2001, Okamoto and Toriumi, 2001). On the other hand, the retrograde paths were deduced to be the hair-pin and anticlockwise paths by amphibole zoning patterns. The paths suggested by many authors until now that were clockwise paths were very different from those deduced by the present studies. The clockwise paths are similar to the general trends of temperature and pressure conditions of metamorphic zones, but this is inconsistent with progressive zoning of amphiboles from hornblende core to winchite and actinolite rim. Anticlockwise paths strongly indicate that the early stage of prograde metamorphism is characterized by low pressure and high temperature metamorphism which is very similar to the Ryoke metamorphism. Garnet zoning patterns do not conflict with these paths judging from theoretical first appearance conditions of garnet at given bulk compositions.
Iwamori (1999) proposed the detailed thermal evolution models of the subduction related metamorphic pressure and temperature paths and concluded that 10kb and 600C indicates too high temperature conditions to be available in the cold subduction system. Such high temperature conditions require the very young plate subduction because of high heat flux below the metamorphic orogen. In this case, the model predicts that the subduction of orogen results nearly adiabatic pressure increase as the metamorphic P - T path, and this is very consistent with the prograde paths obtained here. Further, retrograde paths showing anticlockwise path can be easily interpreted by corner flow of the orogen which is cooled from wedge mantle because of temperature conditions in the wedge mantle relative to the very young slab.
Important fact is that the retrograde paths are characterized by the formation of retrograde amphiboles, chlorite and epidote from hornblende assemblage and it shows clearly the hydration reactions. Thus, the retrograde reactions need a large amount of water influx to the metamorphic orogen, instead of prograde reactions producing garnet from chlorite need a large outflux of water from the orogen.
Wintsch, Bynes, Toriumi (1999) obtained the P-T-D paths from the pull apart graines of amphiboles which are composed of hornblende core, pulled apart wintschite, and actinolite rim. The timing of the ductile deformation illustrated by pulled apart texture is just after formation of wintschite and during the formation of actinolite. Therefore, the large amounts of plastic deformation took place at the period of exhumation with hydration reaction.
The strain geometry change with increasing metamorphic temperature is from the oblate shape in the low grade zone to prolate shape in the higher grade zone (Toriumi and Noda, 1986, Toriumi and Masui, 1986). Considering the large amounts of strain indicating tangential stretching at the retrograde metamorphism (that is exhumation stage), the prograde metamorphic deformation should be followed by extense plastic deformation trending EW direction forming the prolate shape strain ellipsoid. Toriumi (1985) proposed combined deformation matrix model in the 3D corner flow model by oblique subduction that the prograde metamorphic deformation has been resulted from drag from the slab and retrograde deformation has been derived from the drag of the wedge mantle. This produces the along axis vorticities and it has been suggested by Wintsch et al. (1999).