Zircon trace‐element compositions in Miocene granitoids in Japan: Discrimination diagrams for zircons in M‐, I‐, S‐, and A‐type granites

Abstract

AbstractDetrital zircons demonstrate high resistance to alteration and, as such retain information about their formation ages and parental magmas for a long period of time. Geochemical researchers have proposed a wide variety of discrimination diagrams applicable to detrital zircons. In our research, we focused on the conventional classification scheme for granites (Mantle, M; Igneous, I; Sedimentary, S; and Alkaline, A types) and sought to characterize zircon trace‐element compositions that are sensitive to differences among these granite types. To accomplish this, we examined trace‐element compositions of zircons extracted from granitoids in the Ohmine granitic rocks and the Ashizuri plutonic complex in southwestern Japan. The zircons showed systematic differences in Nb/P, Ta/P, Ce/P, Ce/Nd, Y/P, Th/U, and Sc/Yb ratios and the Eu anomaly. Zircons in A‐type granite are rich in Nb, Ta, Ce, and Y, and their signatures clearly reflect those elements in their parental bodies. Sc/Yb ratios of zircons in A‐type granites are <0.1, which is similar to those of ocean‐island‐type zircons. Despite their high abundance at the whole‐rock level, zircons in S‐type granite are characterized by low Nb/P, Ta/P, and Th/U ratios. This is attributable to the depletion of Nb, Ta, and Th in the magma by ilmenite and monazite prior to zircon crystallization. In general, S‐type granitic magmas exhibit reducing environments, which decrease the proportions of Ce4+ and Eu3+. These effects lead to a low Ce/Nd ratio and a large negative Eu anomaly in S‐type zircons. On the basis of these findings, we recommend the combined use of Nb/P–Ce/P or Ta/P–Ce/P crossplots and of Sc/Yb ratios to discriminate zircons in M‐, I‐, S‐, and A‐type granites. Although the crossplots are created using data from Miocene granitoids in Japan, the discrimination diagrams are based on the general features of each type of granite.