ALBERT

Notes: Abstract Rb–Sr and Sm–Nd isochron ages were determined for whole rocks and mineral separates of hornblende-gabbros and related metadiabases and quartz-diorite from Shodoshima, Awashima and Kajishima islands in the Ryoke plutono-metamorphic belt of the Setouchi area, Southwest Japan. The Rb–Sr and Sm–Nd whole-rock-mineral isochron ages for six samples range from 75 to 110 Ma and 200–220 Ma, respectively. The former ages are comparable with the Rb–Sr whole-rock isochron ages reported from neighboring Ryoke granitic rocks and are thus due to thermal metamorphism caused by the granitic intrusions. On the contrary, the older ages suggest the time of formation of the gabbroic and related rocks. The initial 87Sr/86Sr and 143Nd/144Nd ratios of the gabbroic rocks (0.7070–0.7078 and 0.51217–0.51231 at 210 Ma, respectively) are comparable with those of neighboring late Cretaceous granites and lower crustal granulite xenoliths from Cenozoic andesites in this region. Because the gabbroic rocks are considered to be fragments of the lower crustal materials interlayered in the granulitic lower crust, their isotopic signature has been inherited from an enriched mantle source or, less likely, acquired through interaction with the lower crustal materials. The Sr and Nd isotopic and petrologic evidence leads to a plausible conclusion that the gabbroic rocks have formed as cumulates from hydrous mafic magmas of light rare earth element-rich (Sm/Nd 〈 0.233) and enriched isotopic (ɛSr 〉 0 and ɛNd 〈 0) signature, which possibly generated around 220–200 Ma by partial melting of an upper mantle. We further conclude that they are fragments of refractory material from the lower crust caught up as xenoblocks by granitic magmas, the latter having been generated by partial melting of granulitic lower crustal material around 100 Ma.

Notes: Abstract The Ryoke Belt in the Ikoma Mountains, Nara Prefecture, Japan, is composed mainly of various granitic, intermediate and gabbroic rocks. Igneous activity in this area is divided into two periods, early–middle Jurassic and late Cretaceous, based on isotopic dating. The intermediate plutonic rocks in the Fukihata area are composed of two rock types: Kyuanji quartz diorite and Fukihata tonalite. Rb–Sr whole-rock isochron ages have been determined for both plutonic rocks. Their ages and initial 87Sr/86Sr ratios are as follows: the Kyuanji quartz diorite has an age of 161.0 ± 17.9 Ma with an initial 87Sr/86Sr ratio of 0.70727 ± 0.00007, while the Fukihata tonalite has an age of 121.4 ± 24.6 Ma with an initial 87Sr/86Sr ratio of 0.70753 ± 0.00020. Our chronological results indicate that the Kyuanji quartz diorite belongs to the Jurassic mafic rocks, such as the Ikoma gabbroic mass, while the Fukihata tonalite belongs to the early Cretaceous granitic rocks. Both these intermediate plutonic rocks have different chemical characteristics and were derived from different magmas.

Notes: Abstract Rb–Sr and K–Ar chronological studies were carried out on granitic and metamorphic rocks in the Ina, Awaji Island and eastern Sanuki districts, Southwest Japan to investigate the timing of intrusion of the granitoids in the Ryoke belt. Intrusions of ‘younger’ Ryoke granitic magmas took place in the Ina district between 120 Ma and 70 Ma, and cooling began immediately after the emplacement of the youngest granitic bodies. Igneous activity in Awaji Island was initiated at 100 Ma and continued to 75 Ma. Along-arc variations of Rb–Sr whole-rock isochron ages suggest that magmatism began everywhere in the Ryoke and San-yo belts at almost the same time (ca 120 Ma). The last magmatism took place in the eastern part of both belts. Rb–Sr and K–Ar mineral ages for the granitoids young eastwards. The age data suggest that the Ryoke belt was uplifted just after the termination of igneous activity. Initial Sr and Nd isotopic ratios for the Ryoke granitoids indicate that most were derived from magmas produced in the lower crust and/or upper mantle with uniform Sr and Nd isotopic compositions. Several granitoids, however, exhibit evidence of assimilation of Ryoke metamorphic rocks or older Precambrian crustal rocks beneath the Ryoke belt.

Notes: Abstract The Ryoke Belt is one of the important terranes in the South-west Japan Arc (SJA). It consists mainly of late Cretaceous granitoid rocks, meta-sedimentary rocks (Jurassic accretionary complexes) and mafic rocks (gabbros, metadiabases; late Permian–early Jurassic). Initial ɛSr (+ 25– + 59) and ɛNd (− 2.1–−5.9) values of the metadiabases cannot be explained by crustal contamination but reflect the values of the source material. These values coincide with those of island arc basalt (IAB), active continental margin basalt (ACMB) and continental flood basalt (CFB). Spiderdiagrams and trace element chemistries of the metadiabases have CFB-signature, rather than those of either IAB or ACMB. The Sr–Nd isotope data, trace element and rare earth element chemistries of the metadiabases indicate that they result from partial melting of continental-type lithospheric mantle. Mafic granulite xenoliths in middle Miocene volcanic rocks distributed throughout the Ryoke Belt were probably derived from relatively deep crust. Their geochemical and Sr–Nd isotopic characteristics are similar to the metadiabases. This suggests that rocks, equivalent geochemically to the metadiabases, must be widely distributed at relatively deep crustal levels beneath a part of the Ryoke Belt. The geochemical and isotopic features of the metadiabases and mafic granulites from the Ryoke Belt are quite different from those of mafic rocks from other terranes in the SJA. These results imply that the Ryoke mafic rocks (metadiabase, mafic granulite) were not transported from other terranes by crustal movement but formed in situ. Sr–Nd isotopic features of late Cretaceous granitoid rocks occurring in the western part of the Japanese Islands are coincident with those of the Ryoke mafic rocks. Such an isotopic relation between these two rocks suggests that a continental-type lithosphere is widely represented beneath the western part of the Japanese Islands.

Notes: Abstract Granitoids are widely distributed in the Ryoke belt and have been divided into four main igneous stages based on their field setting. In this paper, we present Rb–Sr isochron ages for the younger Ryoke granitoids (second stage to fourth stage) in the Kinki district. The Yagyu granite (second stage) gave a Rb–Sr whole-rock isochron age of 74.6 ± 10.9 Ma with an initial 87Sr/86Sr ratio of 0.70938 ± 0.00016, and a Rb–Sr mineral isochron age of 71.8 ± 0.1 Ma. The Narukawa granite (second stage) yielded a Rb–Sr mineral isochron age of 79.5 ± 0.4 Ma. A Rb–Sr whole-rock isochron age of 78.3 ± 3.0 Ma with an initial 87Sr/86Sr ratio of 0.70764 ± 0.00014 was obtained for the Takijiri adamellite (third stage). The Katsuragi quartzdiorite (fourth stage) gave a Rb–Sr whole-rock isochron age of 85.1 ± 18.3 Ma (initial 87Sr/86Sr ratio of 0.70728 ± 0.00006), and mineral isochron ages of 76.9 ± 0.5 Ma and 74.8 ± 0.5 Ma. The Minamikawachi granite (fourth stage) gave a Rb–Sr whole-rock isochron age of 72.8 ± 2.0 Ma with an initial 87Sr/86Sr ratio of 0.70891 ± 0.00021. These age data indicate that the igneous activity in younger Ryoke granitoids of Kinki district occurred between 80 and 70 Ma, except for the Katsuragi quartz diorite. The isotopic data on the various igneous stages in Kinki district correspond with the relative timing from field observations. Based on the initial 87Sr/86Sr ratios, the granitoids of the Ryoke belt in Kinki district are spatially divided into two groups. One is granitoids with initial 87Sr/86Sr ratio of 0.707–0.708, distributed in the southern part of the Ryoke belt. The other is granitoids with initial 87Sr/86Sr ratio of 0.708–0.710 distributed in the northern part of the Ryoke belt. The initial 87Sr/86Sr ratios of granitoids increase with decreasing (becoming younger) Rb–Sr whole-rock isochron ages.

Notes: Abstract Miyanohara tonalite occurs in the middle part of the Higo metamorphic belt in the central Kyushu, Southwest Japan. This tonalite intrudes into early Permian Ryuhozan metamorphic rocks in the south and is intruded by Cretaceous Shiraishino granodiorite in the north. The Miyanohara tonalite yielded three mineral ages: (i) 110–100 Ma for Sm–Nd and Rb–Sr internal isochrons and for K–Ar hornblende; (ii) 183 Ma for Sm–Nd internal isochron; and (iii) 211 Ma for Sm–Nd internal isochron. The ages of 110–100 Ma may indicate cooling age due to the thermal effect of the Shiraishino granodiorite intrusion. The ages of 183 Ma and 211 Ma are consistent with timing of intrusion of the Miyanohara tonalite based on geologic constraints. The hornblende in the sample which gave 183 Ma shows discontinuous zoning under microscope, whereas the one which gave 211 Ma does not show zonal structure. These mineralogical features suggest that the 183 Ma sample has suffered severely from later tectonothermal effect compared with the 211 Ma sample. Therefore, the age of 211 Ma is regarded as near crystallization age for the Miyanohara tonalite. The magmatic process, geochronology and initial Sr and Nd isotope ratios for the Miyanohara tonalite are similar to those of early Jurassic granites from the Outer Plutonic Zone of the Hida belt that constitutes a marginal part of east Asia before the opening of the Japan Sea. Intrusion of the Miyanohara tonalite is considered to have taken place in the active continental margin during the late Triassic.

Notes: Abstract Whole-rock chemical and Sr and Nd isotope data are presented for gabbroic and dioritic rocks from a Cretaceous-Paleogene granitic terrain in Southwest Japan. Age data indicate that they were emplaced in the late Cretaceous during the early stages of a voluminous intermediate-felsic magmatic episode in Southwest Japan. Although these gabbroic and dioritic rocks have similar major and trace element chemistry, they show regional variations in terms of initial Sr and Nd isotope ratios. Samples from the South Zone have high initial 87Sr/86Sr (0.7063–0.7076) and low initial Nd isotope ratios (ɛNd, −2.5 to −5.3); whereas those from the North Zone have lower initial 87Sr/86Sr (usually less than 0.7060) and higher Nd isotope ratios (ɛNd, −0.8 to + 3.3). Regional variations in Sr and Nd isotope ratios are similar to those observed in granitic rocks, although gabbroic and dioritic rocks tend to have slightly lower Sr and higher Nd isotope ratios than granitic rocks in the respective zones. Limited variations in Sr and Nd isotope ratios among samples from individual zones may be attributed partly to a combination of upper crustal contamination and heterogeneity of the magma source. Contamination of magmas by upper crustal material cannot, however, explain the observed Sr and Nd isotope variations between samples from the North and South Zones. Between-zone variations would reflect geochemical difference in magma sources. The gabbroic and dioritic rocks are enriched in large ion lithophile elements (LILE) and depleted in high field strength elements (HFSE), showing similar normal-type mid-ocean ridge basalt (N-MORB) normalized patterns to arc magmas. Geochronological and isotopic data may suggest that some gabbroic and dioritic rocks are genetically related to high magnesian andesite. Alternatively, mantle-derived mafic or intermediate rocks which were underplated beneath the crust may be also plausible sources for gabbroic and dioritic rocks. The magma sources (the mantle wedge and lower crust) were isotopically more enriched beneath the South Zone than the North Zone during the Cretaceous-Paleogene. Sr and Nd isotope ratios of the lower crustal source of the granitic rocks was isotopically affected by mantle-derived magmas, resulting in similar initial Sr and Nd isotope ratios for gabbroic, dioritic and granitic rocks in each zone.

Notes: Late Miocene (7–9 Ma) basaltic rocks from the Monbetsu-Kamishihoro graben in northeast Hokkaido have chemical affinities to certain back-arc basin basalts (referred to herein as Hokkaido BABB). Pb-, Nd- and Sr-isotopic compositions of the Hokkaido BABB and arc-type volcanic rocks (11–13 Ma and 4–4.5 Ma) from the nearby region indicate mixing between the depleted mantle and an EM II-like enriched component (e.g. subducted pelagic sediment) in the magma generation. At a given 87Sr/86Sr, Hokkaido BABB have slightly lower 143Nd/144Nd and slightly less radiogenic 206Pb/204Pb compared with associated arc-type lavas, but both these suites are difficult to distinguish solely on the basis of isotopic compositions. These isotopic data indicate that while generation of the Hokkaido BABB involves smaller amounts of the EM II-like enriched component than do associated arc lavas, Hokkaido BABB are isotopically distinct from basalts produced at normal back-arc basin spreading centers. Instead, northeast Hokkaido BABB are more similar to basalts erupted during the initial rifting stage of back-arc basins. The Monbetsu-Kamishihoro graben may have developed in association with extension that formed the Kurile Basin, suggesting that opening of the basin continued until late Miocene (7–9 Ma).