Sr-Nd isotopic systematics and geochemistry of intermediate plutonic rocks from Ikoma Mountains, Southwest Japan: evidence for a sequence of Mesozoic magmatic 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 activity 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 and Fukihata tonalite. Rb-Sr whole-rock isochron ages have been determined for both plutonic rocks. Their ages and initial Sr-87/Sr-86 ratios are as follows: the Kyuanji quartz diorite has an age of 161.0+/-17.9 Ma with an initial 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.