Presentation Information
[R6P-05]Geochemical characteristics of the Cretaceous mantle wedge in Southwest Japan: Implications for magmatic cycles
*Masaaki OWADA1, Shogo kodama2, Mariko Nagashima1, Atsushi Kamei3 (1. Yamaguchi University, 2. Taiheiyo Consultant, West Japan Technology Center, 3. Shimane University)
Keywords:
Active continental margin,Magmatic flare-ups and lulls,Non-stationary plate motion,Mantle wedge
The subducting oceanic plate along the East Asian continent changed its subduction angle, rate, and direction during the Cretaceous to Paleogene periods. Magmatic activity during this period is believed to be influenced by plate dynamics, resulting in phenomena such as magmatic flare-ups and lulls. Although many researchers have investigated the shift of the volcanic front to reveal this plate motion, it is equally important to understand the underlying mantle dynamics, including its physicochemical properties.
In this paper, we address the geochemical changes in the mantle wedge based on a geochemical study including Nd isotope compositions and the geochronology of mantle-derived mafic rocks. The target rocks in this study are gabbros and diorites from the Kyoto to northern Kyushu areas ranging from 90 to 107 Ma, which belong to the San’yo and Ryoke belts, as well as rocks from the San’in belt dating to the late Cretaceous (75–65 Ma).
The compiled dataset for these rocks shows that initial epsilon Nd values (εNdi) progressively decreased from +4 to -5 between 107 and 90 Ma. Furthermore, Th/La ratios exhibit a negative correlation with εNdi. These geochemical trends and their temporal variations suggest that subducted sediments increasingly interacted with the mantle wedge, producing progressively enriched mantle sources from 107 to 90 Ma. In contrast, the Late Cretaceous mafic rocks show εNdi values progressively increasing from -4 to +2 between 75 and 65 Ma, reflecting the contribution of a depleted material, such as the upwelling asthenosphere.
Overall, this temporal variation in the mantle wedge indicates a dynamic (non-static) subduction system during the Cretaceous, which triggered these magmatic flare-ups and lulls. Investigating these temporal chemical changes provides crucial insights into the mechanisms driving such magmatic cycles in active continental margins.
In this paper, we address the geochemical changes in the mantle wedge based on a geochemical study including Nd isotope compositions and the geochronology of mantle-derived mafic rocks. The target rocks in this study are gabbros and diorites from the Kyoto to northern Kyushu areas ranging from 90 to 107 Ma, which belong to the San’yo and Ryoke belts, as well as rocks from the San’in belt dating to the late Cretaceous (75–65 Ma).
The compiled dataset for these rocks shows that initial epsilon Nd values (εNdi) progressively decreased from +4 to -5 between 107 and 90 Ma. Furthermore, Th/La ratios exhibit a negative correlation with εNdi. These geochemical trends and their temporal variations suggest that subducted sediments increasingly interacted with the mantle wedge, producing progressively enriched mantle sources from 107 to 90 Ma. In contrast, the Late Cretaceous mafic rocks show εNdi values progressively increasing from -4 to +2 between 75 and 65 Ma, reflecting the contribution of a depleted material, such as the upwelling asthenosphere.
Overall, this temporal variation in the mantle wedge indicates a dynamic (non-static) subduction system during the Cretaceous, which triggered these magmatic flare-ups and lulls. Investigating these temporal chemical changes provides crucial insights into the mechanisms driving such magmatic cycles in active continental margins.