Rb-Sr and Sm-Nd isotopic studies of mafic igneous rocks from the Ryoke plutono-metamorphic belt in the Setouchi area, Southwest Japan: Implications for the genesis and thermal history

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 (ε(Sr)>0 and ε(Nd)<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.