ABSTRACT

Petrographic, petrochemical and Sr-Nd isotopic data are presented for granitoids and microdioritic enclaves from two Cretaceous stocks (Togouchi granodiorite and Tateiwayama granite porphyry) from the Togouchi-Yoshiwa district, northwest Hiroshima prefecture, SW Japan. The data are used to examine the genetic relationships between the microdioritic enclaves and their granitoid hosts. The granodiorite, granite porphyry and the microdioritic enclaves are all calc-alkaline in nature, and belong to the I-type ilmenite series. The Togouchi granodiorite has a Rb-Sr whole rock isochron age of 85.6 ± 4.7 Ma with an initial Sr isotope ratio (SrI) of 0.70634 ± 0.00012 (2σ). The Tateiwayama granite porphyry has a slightly younger Rb-Sr whole isochron age (77.4 ± 3.1 Ma) but similar SrI of 0.70653 ± 0.00015, suggesting that both stocks may have been derived from the same source. Despite diverse whole rock chemistry, the microdioritic enclaves in the respective intrusives have quite similar initial Sr and Nd isotope ratios, suggesting that they formed by fractional crystallization of a single magma, and also that the source of the enclaves in both intrusives had similar geochemical characteristics. In both stocks, however, the enclaves have distinctly lower initial Sr isotope ratios than their respective host rocks, indicating that they were derived from a different source than their hosts. In view of the geochemical and Sr-Nd isotope data, we infer that the enclave magmas were derived from a similar LILE- and LREE-enriched source to that of the Cretaceous basalts and gabbroic-dioritic rocks that are sporadically distributed in SW Japan. Such mafic to intermediate magmas were probably derived from the upper mantle, and transferred both heat and material to the lower crust, thus producing granitic magmas by partial melting. Successive mafic magmas or their differentiates could then have been injected into the granitic magma chamber, trapped and quenched, resulting in the formation of microdioritic enclaves in both granitic stocks.