Sr-Nd isotopic systematics and geochemistry of intermediate plutonic rocksfrom Ikoma Mountains, Southwest Japan: evidence for a sequence of Mesozoicmagmatic activity in the Ryoke Belt

The Ryoke Belt in the Ikoma Mountains, Nara Prefecture, Japan, is composed mainly of various granitic, intermediate and gabbroic rocks. Igneous activi ty in this area is divided into two periods, early-middle Jurassic and late Cretaceous, leased on isotopic dating. The intermediate plutonic rocks in the Fukihata area are composed of two rock types: Kyuanji quartz diorite an d Fukihata tonalite. Rb-Sr whole-rock isochron ages have been determined fo r both plutonic rocks. Their ages and initial Sr-87/Sr-86 ratios are as fol lows: the Kyuanji quartz diorite has an age of 161.0+/-17.9 Ma with an init ial Sr-87/Sr-86 ratio of 0.70727+/-0.00007, while the Fukihata tonalite has an age of 121.4+/-24.6 Ma with an initial Sr-87/Sr-86 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.