ABSTRACT Investigation of gabbroic and dioritic rocks is important for evaluating the geochemical state of the mantle wedge. To reveal the temporal evolution of the mantle wedge from the Early Cretaceous to the Paleogene, we compared gabbroic and dioritic rocks that intruded into the Yamizo Group. We assessed the crystallization age and source material of the Iwafune diorite, located between gabbroic and dioritic rocks of Northeast (NE) and Southwest (SW) Japan, using geological, geochemical, and zircon U–Pb data. The Iwafune diorite consists of a main‐diorite and a gabbroic diorite, associated with granite. The gabbroic diorite (54.75–54.90 wt.% SiO 2 ; SrI = 0.70925–0.70939) is more primitive than the main‐diorite (56.69–58.61 wt.% SiO 2 ; SrI = 0.70996–0.71020). The whole‐rock composition of the Iwafune diorite shows island‐arc basalt affinity, characterized by enrichment in Pb and light rare earth elements, and depletion in Nb, Ta, Ti, and heavy rare earth elements. Zircon U–Pb ages of the main‐diorite, gabbroic diorite, and granite are 64.9 ± 1.2, 65.6 ± 1.2, and 64.7 ± 2.1 Ma, respectively. Geochemical data and mass‐balance modeling show that the main‐diorite was derived from the gabbroic diorite through assimilation–fractional crystallization process. Trace element characteristics and isotopic constraints indicate that the Iwafune diorite originated from partial melting of a metasomatized mantle peridotite enriched in Sr content and having a high 87 Sr/ 86 Sr ratio than those of depleted mantle. The crystallization age and SrI value of the Iwafune diorite indicate that it belongs to the SW Japan suite. The temporal rise in SrI values from the gabbroic and dioritic rocks of the NE to SW Japan is not attributable to crustal mixing but likely reflects changes in the 87 Sr/ 86 Sr ratio of the mantle peridotite itself.
Yamazaki et al. (Thu,) studied this question.