Zircon U–Pb ages of the Cretaceous gabbroic and granitic rocks from the Kajishima, northwest Shikoku, southwest Japan

Abstract

We carried out zircon U–Pb dating of the Cretaceous gabbroic and granitic rocks exposed in the Kajishima to determine their magmatic ages. The plutonic rocks in the Kajishima are classified into seven lithologies: Hbl–bearing olivine norite, Type–1 Hbl–bearing troctolite, Type–2 Hbl–bearing troctolite, Type–1 pyroxene hornblende gabbronorite, and Type–2 pyroxene hornblende gabbronorite, massive granodiorite, and deformed granodiorite. The new zircon U–Pb ages are: 90.77 ± 0.99 Ma for the Type–2 Hbl–bearing troctolite, 92.35 ± 0.74 Ma for the Type–1 pyroxene hornblende gabbronorite, 91.33 ± 1.03 Ma for the massive granodiorite, and 84.11 ± 1.12 Ma for the deformed granodiorite. The new zircon U–Pb ages suggest multiple episodes of magmatic intrusion in Kajishima during ~ 92 and 84 Ma. Although two gabbroic rocks and a massive granodiorite show similar zircon U–Pb ages, subsequent granitic magmatism is unraveled in Kajishima, which is contrary to previous geological studies that postulated an evolution of all plutonic rocks from a single gabbroic parent magma.