5:15 PM - 6:45 PM
[MIS23-P08] Host rocks of Hadean zircons; based on comparison with the zircons in Japanese Cenozoic granitoids
Keywords:Hadean zircons, Cenozoic granitoids, Trace elements
Constraining the composition and tectonic affiliation of Hadean crusts is one of the crucial issues in Earth science. The record of detrital zircon grains dates back to the Hadean, therefore it can offer key insights into the tectono-magmatic systems on the Hadean Earth. Various geochemical approaches have been used to obtain information of the host rocks of Hadean detrital zircons. The zircon REE + Y versus P diagram suggests that Hadean Jack Hills zircons were predominantly sourced from igneous (I-type) magmas because of the low P concentrations (Burnham and Berry, 2017). However, there are overlaps of both types at low P region of the discrimination diagram. To overcome this problem, Chen et al. (2023) developed high-dimensional machine learning approaches, and they proposed that approximately 45% of the Hadean Jack Hills zircons belonged to sedimentary (S-type) melts. Within the alphabetical classification of granite, mantle type (M-type) and Alkaline type (A-type) are also included. After creating the discrimination diagram to separate these 4 types, we revisit the host rocks of Hadean zircons.
Cenozoic granitoids are widely distributed throughout Japan islands and have been extensively studied by Japanese geologists. Thus, we postulate that the Cenozoic granitoids in Japan are the most suitable for testing zircon classification. We have examined trace-element compositions of zircons extracted from Japanese Cenozoic granitoids, and we recommend the combined use of Nb/P-Ce/P or Ta/P-Ce/P crossplots to discriminate zircons in M-, I-, S-, and A-type granites (Sawaki et al., 2022). Zircons in A-type granite are rich in Nb, Ta, and Ce, whereas zircons in S-type granite are poor in these elements. Zircons in M-type granite are rich in Ce but poor in Nb and Ta. The compositional range of zircons in I-type granite is situated at the center of the other 3 types. In addition, zircons in S-type granite are characterized by low Ce/Nd ratios. In general, S-type granitic magmas exhibit reducing environments, which decrease the proportions of Ce4+. These effects lead to a low Ce/Nd ratio.
Based on our discrimination diagram, most Hadean zircons plotted within the I-type field, and none of Hadean zircons plotted within the S-type field. On the other hand, some Hadean zircons plotted on the transitional region between A- and I-type field. Among Cenozoic Japanese granitoids, the zircons in the granite exposed at the Mt. Shibisan have similar compositions to these Hadean zircons. Whole-rock chemical composition of the Shibisan granodiorite shows affinities to A-type granite. Therefore, alkaline magmatism also contributed to form the Hadean crust.
Based on weak positive Ce anomaly seen in the Hadean zircons, it has been considered that the Hadean continental crust was built from melts of reduced deep-mantle origin (Yang et al., 2014). However, this Ce anomaly is based on the deviation of Ce relative to adjacent La and Pr. Because the generally low La concentration in zircon has a significant impact on the Ce anomaly, we used the Ce/Nd ratio without La to evaluate the degree of Ce enrichment. The range of Ce/Nd ratios of Hadean zircons is same as that of M- and I-type granite and is higher than that of S-type granite. This fact suggests that the redox state of Hadean magma was at the same level as modern oxidative granitic magma.
Cenozoic granitoids are widely distributed throughout Japan islands and have been extensively studied by Japanese geologists. Thus, we postulate that the Cenozoic granitoids in Japan are the most suitable for testing zircon classification. We have examined trace-element compositions of zircons extracted from Japanese Cenozoic granitoids, and we recommend the combined use of Nb/P-Ce/P or Ta/P-Ce/P crossplots to discriminate zircons in M-, I-, S-, and A-type granites (Sawaki et al., 2022). Zircons in A-type granite are rich in Nb, Ta, and Ce, whereas zircons in S-type granite are poor in these elements. Zircons in M-type granite are rich in Ce but poor in Nb and Ta. The compositional range of zircons in I-type granite is situated at the center of the other 3 types. In addition, zircons in S-type granite are characterized by low Ce/Nd ratios. In general, S-type granitic magmas exhibit reducing environments, which decrease the proportions of Ce4+. These effects lead to a low Ce/Nd ratio.
Based on our discrimination diagram, most Hadean zircons plotted within the I-type field, and none of Hadean zircons plotted within the S-type field. On the other hand, some Hadean zircons plotted on the transitional region between A- and I-type field. Among Cenozoic Japanese granitoids, the zircons in the granite exposed at the Mt. Shibisan have similar compositions to these Hadean zircons. Whole-rock chemical composition of the Shibisan granodiorite shows affinities to A-type granite. Therefore, alkaline magmatism also contributed to form the Hadean crust.
Based on weak positive Ce anomaly seen in the Hadean zircons, it has been considered that the Hadean continental crust was built from melts of reduced deep-mantle origin (Yang et al., 2014). However, this Ce anomaly is based on the deviation of Ce relative to adjacent La and Pr. Because the generally low La concentration in zircon has a significant impact on the Ce anomaly, we used the Ce/Nd ratio without La to evaluate the degree of Ce enrichment. The range of Ce/Nd ratios of Hadean zircons is same as that of M- and I-type granite and is higher than that of S-type granite. This fact suggests that the redox state of Hadean magma was at the same level as modern oxidative granitic magma.