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

Abstract

Detrital 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.