| Abstract |
The Takidani granitoid rocks (TG) is reported to be the youngest exposed pluton on the Earth (1.9-2.4 Ma), and forms part of a volcano-plutonic complex system with the Pliocene Hotaka Andesites. In this study, petrographic and petrochemical characteristics of the TG, enclaves and Hotaka Andesites are described, together with preliminary pressure estimations for the TG. Overall, TG has a reverse zonal structure, based on the modal abundance of constituent minerals and chemical composition of surface samples. The inner zone of TG is composed of hornblende rich biotite granodiorite (Bi-Hb-Gdt) with whole rock SiO2 ranging from 66 to 70 wt%, whilst granite and biotite rich hornblende granodiorite (Hb-Bi-Gdt) occur in the outer zone. In the eastern and western parts of the pluton, an apparent exception to the reverse zonal structure is evident. At the edge of the eastern zone, porphyritic granodioritite (P-Gdt) occurs that has a SiO2 content of 68-71 wt% with porphyritic granite (PGrt: 70-73 wt%) closer to the core of the intrusion. In hornblende rich biotite granite (Bi-Hb-Grt) located in the western part of the TG, SiO2 content increases from 68 wt% to 71 wt% approaching the inner zone. Thus, in the eastern and western parts of the pluton, a normally zoned chemical structure is formed. Accordingly, the genesis and emplacement process of the Takidani pluton has been characterized by multiple magma processes i.e. with extended in situ crystallization of reversely stratified magma in the marginal zone. A reverse zonal chemical structure of TG might be major evidence for an underlying stratified magma chamber. |