[S-CG45] Petrology, Mineralogy & Resource Geology

[SCG45-P04] A field geological study of the granite emplacement history: Example of the Kinpusan pluton, Kanto Mountains, central Japan.

*Daichi Murakami¹, Ken Yamaoka², Hiroshi Mori¹ (1.Shinshu University, 2.National Institute of Advanced Industrial Science and Technology )

Keywords:pluton emplacement, granite, Kofu granitic complex, contact metamorphism, zircon, Miocene

The geometry and intrusion time interval of the incremental magma unit when pluton construction are expected to control significantly the thermal structure and its evolution over time of the pluton inside. However, there are only a few examples of constraints of such incremental history based on field observations despite the presence of a large number of plutons on the Earth. In this study, we present results of detailed field studies on the Kinpusan granitic pluton area, central Japan combined with a preliminary investigation of the thermal structure analysis of the contact metamorphic aureole and the zircon U-Pb dating conducted to elucidate three-dimensional variations in lithology and thermal history during pluton emplacement. The spatial distribution of felsic magmatic enclaves in the granite suggests that the pluton was incrementally emplaced by under-accretion of sheet-like magma. The coarsening of grain size toward the bottom of the pluton and the increase of occurrences of aplite dyke and miarolitic cavities indicate that the pluton bottom remained in a high-temperature condition that can sustain crystal mush for a longer period than the roof part. On the other hand, outcrop-scale observations of fine-grained chilled margin facies at the upper boundary of the incremental magma sill indicate that little magma chamber growth occurred across the sill-like magma unit. The thermal structure of the contact metamorphic aureole at the sidewall of the pluton is constrained by the Raman carbonaceous material geothermometry and shows a slight lower temperature trend than the typical felsic pluton. Zircon U-Pb dating applied to three samples taken from different magma unit and show a downward youngering trend of about 0.1 million years, although larger error range. These observations support that the Kinpusan pluton is a relatively rare example that preserving a number of internal contacts during growth of the pluton.