Sr isotopic ratios and Rb, Sr and REE abundances are determined for granitic clasts from the middle Permian Kozaki Formation in the Kurosegawa Belt, Kumamoto, Southwest Japan, in order to elucidate their source and formation. Garnet-biotite granodiorite clasts, uniquely found in the Kozaki Formation, show M-type REE tetrad patterns and wide variation of Rb/Sr ratios, which enable their petrogenesis and age determination. The REE patterns indicate shallow formation of the garnet-bearing granodiorite. The Rb-Sr isotopic data yield an age of 251 ± 46 Ma with an initial ratio of 0.7055 ± 0.0006. The Rb-Sr age is close to the sedimentation age of the Kozaki Formation defined by paleontological data and K-Ar ages of the corresponding granitic clasts in the Permian conglomerate (“Usuginu conglomerate”) of the Kitakami area, Northeast Japan. The initial Sr isotopic ratio of 0.7055 is slightly higher than the ratios around 0.7040 reported for the Permian granitic rocks from the Akaishi Range and the eastern Kyushu and the corresponding Permian granitic clasts in the Kitakami area. The initial ratio for the garnet-biotite granodiorite clasts is also slightly higher than the ratios (0.7037 and 0.7032) at 345 and 400 Ma reported for Yatsushiro granite of the Kurosegawa Belt (Nohda, 1973; Yanagi, 1975). These Sr isotopic data suggest that the garnet-biotite granodiorite clasts correspond to rocks differentiated from the Kurosegawa granitic rocks. The Sr initial ratio of 0.7055 at 251 Ma for the garnet-bearing granodiorite clasts is slightly enriched but still primitive, and the Sr initial ratio clearly indicates that the clasts were not derived from typical-continental crust. The Rb-Sr isotopic data and our preliminary Sm-Nd isotopic data for the garnet-biotite granodiorite clasts yield ε_Sr (251 Ma) = +18 and ε_Nd (251 Ma) = +6, suggesting an intra-oceanic arc provenance for the garnet-biotite granodiorite body from which the clasts in the Kozaki Formation had been derived.

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Sulfur and carbon isotope ratios have been determined on carbonaceous shales of the 2.7 Ga Jeerinah Formation, Hamersley Group, Australia. The analyses were performed using the Nd-YAG laser microprobe method. The δ¹³C_(PDB) values of organic matter range from -38.3‰ to -35.1‰ with an average of -37.0‰ (44 analyses). These carbon isotope compositions support the previously proposed hypothesis that methanogens and methanotrophs were involved in the carbon cycles in the 2.7 Ga Hamersley ocean. Sulfur isotope compositions are ranging from +0.4‰ to +10.2‰ with an average of +4.6‰ (90 analyses). A notable feature is variable δ³⁴S values within a micro-scale area; approximately 6 to 7‰ variations of δ³⁴S values were detected within 5 × 5 mm areas. Such δ³⁴S variations clearly indicate that pyrites in the Jeerinah shales were formed as a result of the sulfate reduction. This suggests that the 2.7 Ga Hamersley ocean contained appreciable amount of dissolved sulfate, opposing to the previously popular H₂S-rich ocean model. Because of no evidence for the hydrothermal alterration on the examined samples, pyrites in the 2.7 Ga Jeerinah shales were most likely formed by the biological sulfate reduction in sediments.

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We have measured U concentrations, and elemental ratio of ²³⁸U/²⁰⁶Pb, and isotopic compositions of Pb in thirteen zircons extracted from a volcaniclastic rock of the Kamiaso conglomerate in the Mino terrane and sixty three analyses in fifty zircons from the Amo gneiss, a metamorphic rock originated from an intermediate volcanic rock in the Hida terrane of central Japan by means of an ion microprobe. Zircons from the Kamiaso were first analyzed in Australian National University (ANU) and then measured again by Hiroshima University (HU) Sensitive High Resolution Ion Micro Probe. Both radiometric ages of ²³⁸U/²⁰⁶Pb^∗ and ²⁰⁷Pb^∗/²⁰⁶Pb^∗ obtained by the ANU instrument agree well with those by HU except for one sample. There are six geochronological episodes in the Kamiaso sample; about 2550 Ma, 2000 Ma, 1300 Ma, 920 Ma, 250 and 220 Ma. U-Pb zircon geochronology of the Amo sample indicates that there are six concordant zircon episodes and three concordia intercept episodes; about 3420 Ma, 2560 Ma, 1840 Ma, 1130 Ma, 580 Ma, 400 Ma, 360 Ma, 285 Ma, and 250 Ma. The 3420 Ma is the oldest age so far reported in Japan. The 2560 Ma event may have a relation to the period of the evolution of the Sino-Korean craton between 2.7-2.5 Ga. The third age peak of about 1840 Ma suggests that the zircons were derived from the Ryongnam massif in South Korea with the similar formation age. The younger ages of 1130 Ma, 580 Ma, 400 Ma, 360 Ma and 285 Ma may be minor events recorded in the Hida metamorphic belt. The latest age of 250 Ma may be the most important metamorphic event in Hida terrane. Rare earth element abundance patterns of the Amo zircons show progressive increase with atomic number together with positive Cc anomaly, which is a typical zircon signature. Small negative anomaly of Eu is found in zircons with the age of 2560 Ma and 250 Ma while zircons with intermediate ages show large anomaly. There is no correlation between Ce and Eu anomaly, suggesting that both anomalies may not have been simply controlled by the oxidation state during zircon crystallization.

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A geochemical study was carried out on Lower Cretaceous sedimentary rocks of the Wakino Subgroup, Kenseki Formation, and Sasayama Group, distributed in the Inner Zone of Southwest Japan. The chemical characteristics of the Lower Cretaceous sediments indicate that these rocks are immature first-order sediments derived from igneous and/or meta-igneous rocks of predominantly felsic composition. The sediments from the Kenseki Formation and the Sasayama Group, however, show high Cr and Ni abundances, suggesting a significant contribution of detritus from ultramafic rocks. Weathering at the source areas was moderate. The high Th/U ratios (mostly > 3.8), negative Eu anomalies (Eu/Eu^∗ between 0.67 and 0.93) and Th/Sc ratios (mostly between 0.5 and 1) of the Lower Cretaceous sediments suggest their derivation dominantly from an old upper crust with minor amounts of young arc-derived detritus. The major, trace and rare earth element compositions imply that deposition took place in an active continental margin environment. The small amounts of young arc-derived material in the sediments support the inference by other workers that arc magmatism was not so prominent in Southwest Japan during the early Cretaceous.

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