Ken Shibata

Initial 87Sr/86Sr ratios of plutonic rocks from Japan

Initial 87Sr/86Sr rations were determined for more than 80 plutonic rocks in Japan. The 87Sr/86Sr ratios of gabbroic and granitic rocks show no significant difference in plutonic terranes where both rocks occur closely associated, implying a genetic relationship between them (e.g., Green Tuff belt) or reequilibration at deep level (e.g., Ryoke belt). Wherever granitic rocks occur independently from gabbroic rocks, the granites have higher ratios than the gabbros.Initial 87Sr/86Sr ratios of the granitic rocks are low (<0.706) in Northeast Japan but high (<0.706) in Southwest Japan, the boundary being the Tanakura Tectonic Line. Within Southwest Japan, the ratios are low along the Japan Sea side of the southernmost area. This regional variation is generally correlated with thickness of the continental crust as deduced from the Bouguer anomaly.Initial 87Sr/86Sr ratios of the granitic rocks vary from 0.7037 to 0.7124. The low group (<0.706) is considered to consist of essentially mantle-derived magmas contaminated by crustal material in lesser but varying degree, because of its geological setting and initial 87Sr/86Sr values. The high group may have been formed by contamination of a deep-seated magmas by crustal material or by generation of the main part of the magmas within the continental crust. The ratios of individual belts reflect their own history depending upon age and Rb/Sr ratio of the crustal material.Initial 87Sr/86Sr ratios of granitic rocks are generally low for the magnetite-series but high for the ilmenite-series. Thus, a negative correlation is observed between initial ratios and δ34S for most Cretaceous-Paleogene granites. However, Neogene ilmenite-series granites are low in both initial 87Sr/86Sr and δ34S indicating interaction of the granitic magma with young sedimentary rocks enriched in 32S.

Along-arc lateral variation of Rb−Sr and K−Ar ages of Cretaceous granitic rocks in Southwest Japan

Cretaceous granitic rocks were emplaced over a distance of 700 km along arc in Southwest Japan. Rb−Sr and K−Ar ages of a major group of these granitic rocks, with ilmenite series ore mineralogy, were examined. Rb−Sr whole rock ages of 92.8±4.0 Ma and Rb−Sr and K−Ar biotite ages of 80–88 Ma were obtained on one group of these granitic rocks from Kamo-Sera area of central Hiroshima Prefecture. The K−Ar ages of various minerals, combined with the Rb−Sr whole-rock age, give a smooth cooling curve, which suggests a 5 to 10 Ma time-lag between intrusion and cooling at 300° C for the Cretaceous granitic rocks. The Rb−Sr whole-rock and Rb−Sr/K−Ar biotite ages of these granitic rocks become younger eastward along the Southwest Japan arc, and the time-lag between the two systems remains constant at 5 to 10 Ma over the entire area. The along-arc age variation does not support the genetical relationship of the Cretaceous granitoids with steady-state subduction. The Cretaceous granitic province at the eastern margin of Eurasian continent was, at least partly, formed by an episodic event such as ridge subduction.

RbSr whole-rock ages of Precambrian metamorphic rocks in the Kamiaso conglomerate from central Japan

Abstract Whole-rock samples of metamorphic and granitic cobbles and boulders from the Kamiaso conglomerate in central Japan yield well-defined Rb Sr isochron ages of 1985 ± 25 my and 1820 ± 40 my. These ages are the oldest yet obtained for rocks in the Japanese Islands, and provide key evidence for the middle Precambrian metamorphic and igneous events in the provenance of these rocks. The age of 1985 my defined by six samples of quartzo-feldspathic gneiss may be the time of emplacement of the original granitic rocks. The 1820 my age indicates the time of extensive regional metamorphism and igneous activity. Precambrian episodes in the provenance of the Kamiaso conglomerate are summarized as follows: (1) 2000 my — formation of granitic rocks, (2) 1800–1600 my — high grade metamorphism accompanied by igneous activity, (3) 1200–1000 my — some significant thermal event. Judging from rock types and geochronological data, it can be said that metamorphic rocks in the Kamiaso conglomerate are remarkably similar to those of the Matenrei and Nangnim systems in North Korea. The Precambrian complex from which the metamorphic and granitic rocks were derived, was exposed to the north not far from the present site of the Kamiaso conglomerate in the late Paleozoic time, and it was probably a part of the large Precambrian continent in East Asia.