[SCG55-P19] Genesis of the low-Sr Granitoid in the northern Kyushu
Keywords:North Kyushu, Granitoids
The Hirao granodiorite and related granite dike are exposed on the east side of KTT. The Hirao granodiorite occurs as a stock with 16 km N-S and 4 km E-W, and has Sr contents of 213-291 ppm. In this study, the authors discussed the magmatic processes of the Hirao granodiorite and petrogenesis of the low-Sr granitoids by comparing the Ushikiri-yama granodiorite (high-Sr granitoid, details reported by Eshima et al., 2019).
The Hirao granodiorite and the granite dike have similar mineral assemblage, consisting mainly of plagioclase, quartz, K-feldspar, biotite and hornblende. In addition, the granodiorite and the granite dike geochemically resemble each other; for instance, their REE patterns and Sr-Nd isotopic values. The result of mass balance calculation reveals that the granite dike magma can be formed by subtraction of plagioclase and hornblende from the Hirao granodiorite magma.
The different Sr contents between the high- and low-Sr granitoids in this region can be explained by the following magmatic processes; magma mixing, assimilation of host rock, degree of fractional crystallization or partial melting, and composition of source materials. The magma mixing and assimilation of the host rocks were ruled out in terms of field occurrence. The geochemical study including trace elements and Sr-Nd isotopic compositions revealed that the Hirao granodiorite and the Ushikiri-yama granodiorite were shared with similarity source materials. Moreover, the magmatic processes of the both granodiorite were similar regarding the degree of fractionation and partial melting. Considering geochemical characteristics combined with melting experiments, the Hirao granodiorite representative of the low-Sr granitoid magma can be produced by relatively low-pressure melting rather than the high-Sr granitoid; thereby, leaving plagioclase as the residual phase at the source region.