Isotopic ages of siliceous shale from Hida-Kanayama, central Japan.

Abstract

Whole-rock samples of siliceous shale, assigned to lower Tithonian by radiolarian fossils, from Hida Kanayama give a Rb-Sr isochron age of 128±3Ma.K-Ar ages for these samples range from 125 to 133 Ma except two much older ages, which are perhaps caused by the presence of detrital material. On the basis of the concordance between Rb-Sr and K-Ar ages and the lack of significant thermal event, the age of 128Ma is interpreted as indicating the time when the whole-rock became closed with respect to Rb-Sr system, thus there should be a period of about 20Ma between the deposition and chemical diagenesis.The initial 8°Sr/86Sr ratio for the shale is determined to be 0.7155 ± 0.0004 and much higher than the contemporaneous marine e'Sr/86Sr ratio. This fact suggests the existence of older rocks in the provenance.

Ridge-trench interactions and high- T -low- P metamorphism, with particular reference to the Cretaceous evolution of the Japanese Islands

“…4) continental affinity (the Kitakami allochthonous terrane; see Saito & Hashimoto 1982), into which was emplaced a linear belt of Early Cretaceous plutons, the Kitakami batholith (part of the Kitakami magmatic arc), the full extent of which has been delineated using aeromagnetic mapping (Finn 1994;Finn et al 1994). Magmatic activity in the Kitakami arc ceased by c. 110Ma (Shibata 1968), but proabably shifted westward as evidenced by Late-Cretaceous plutons in Sikhote Alin (Fig. 4).…”

“…Radiolarians suggest a pre-Early Cretaceous stratigraphic age for the protolith accretionary complex sediments (Wakita 1987). Very low-grade protolith rocks have yielded a Rb-Sr isochron age of 1284-14Ma and wholerock K-Ar ages of c. 133-125Ma (Shibata & Mizutani 1980), and K-Ar mica ages of c. 134-122Ma (Takami et al 1993;Takami & Itaya 1996), interpreted to record the age of low-grade subduction-related metamorphism. The age of the superimposed Ryoke belt metamorphism is suggested to have been c. 100Ma, in both the west and east of the Ryoke belt, based on recently published Th-U-total Pb ages on monazite from samples of high-grade parts of the belt (Suzuki et al 1994(Suzuki et al , 1996Suzuki & Adachi 1998), interpreted to date peak metamorphism.…”

Deformation process and kinematics of mélange in the Early Cretaceous accretionary complex of the Mino‐Tamba Belt, eastern southwest Japan

“…The terminal age of both the siliceous and terrigenous mudstone is Tithonian‐Berriasian [ Wakita , 1988a]. The siliceous mudstone at Okukanayama in the eastern part of the area (Figure 2) give whole rock ages of 133–125 Ma with both Rb‐Sr and K‐Ar methods [ Shibata and Mizutani , 1980]. The Early Cretaceous isotopic age has been interpreted as representing the peak of regional metamorphism during the accretion process [ Takami and Itaya , 1996; Takami , 2000].…”

Isotopic ages of Jurassic siliceous shale and Triassic bedded chert in Unuma, Central Japan.

“…In the siliceous shale in set (a) of Hida-Kanayama (SHIBATA and MIZUTANI, 1980), the K-Ar whole-rock ages of four samples among six are identical with the Rb-Sr whole-rock isochron age; this implies that both K-Ar and Rb-Sr systems of the siliceous shale became closed at the same time. There are, however, two samples having much older K-Ar whole-rock ages.…”

“…We reported isotopic ages of Jurassic si liceous shale in Hida-Kanayama, Central Japan (SHIBATA and MIZUTANI, 1980). The result shows that the whole-rock system yields a Rb-Sr isochron age of 128 ± 3 Ma, whereas the deposition age assigned by radiolarian fossils to be early Tithonian is geochronologi cally about 145 Ma, if we adopt a time scale summarized by ARMSTRONG and M_CDOWALL (1974).…”