[S-CG48] Petrology, Mineralogy & Resource Geology

[SCG48-04] Magma evolution and growth process of the Ushikiri-yama granodiorite body a member of the Cretaceous Northern Kyushu batholith: Implication for the crustal growth of the initial Cretaceous plutonic activity in northern Kyushu

*Keisuke Eshima¹, Masaaki Owada¹ (1.Yamaguchi University)

Keywords:Ushikiri-yama granodiorite body, Cretaceous Northern Kyushu batholith, Initial plutonic activity, Growth process, Magma flow pattern, East Asian active continental margin

Comprehension of how to form igneous bodies and the magma flow patterns are crucial in petrology, volcanology, and economic geology. In volcanology and economic geology, research on magma flow patterns and the three-dimensional (3D) structure of sheet-like intrusive bodies has led to important discoveries, including the identification of magma sources and the formation of deposits containing Platinum-Group Elements (e.g., Barnes et al., 2016; Magee et al., 2016a, b 2019; Martin et al., 2019; Galland et al., 2019; Stephens et al., 2021). On the other hand, the concept of sheet-like intrusions has become prevalent in the study of emplacement shapes, particularly in understanding the space problem of plutonic rocks. This is currently being explored through field observation, geophysical analysis, analog experiments, and numerical modeling of the emplacement process of plutonic rocks and the formation of the Earth's crust (e.g., Kavanagh et al., 2006; Stevenson et al., 2007, 2008; Menand, 2008; Miller et al., 2011; Magee et al., 2018; Gill et al., 2022). According to Magee et al. (2013), the formation of magma flow patterns, the coalescence of magma, and the emergence of discrete sheet-like intrusive rocks are all related to the rheology of magma. The magmatic processes that occur prior to solidification also play a significant role in the emplacement of plutonic bodies, in contrast to volcanic rocks. Additionally, it is widely accepted that large-scale plutonic bodies (e.g., batholiths) are formed through the coalescence of small-scale plutonic bodies (Annen, 2011). Furthermore, many petrological models propose that magmatic systems are open to the heat and mass accretion. In summary, a thorough examination of the emplacement and growth processes of small-scale plutonic bodies is crucial for understanding the formation of batholiths and the processes that lead to the maturation of the Earth's crust for the various tectonic setting.
The ascending and emplacement processes of granitoid magma combined with magma genesis are essential issues in various scales of intrusive rocks. This study aims to reconstruct and reevaluate the formation of the Ushikiri-yama granodiorite body, a small-scale plutonic body in the Northern Kyushu batholith. The Ushikiri-yama granodiorite occurs as a laccolith shape divided into the south and north bodies. Both bodies are emplaced along the low-angle rigidity boundary of the host rocks between the crystalline limestone and pelitic metamorphic rocks. The magmatic foliation and lineation reveal that the granodiorite magma extends to the south and north directions from the central part of the Ushikiri-yama granodiorite to form the laccolith. The zircon U–Pb dating of south and north bodies give ages of 112.1 ± 0.8 Ma and 110.8 ± 0.6 Ma, respectively; thereby, the granodiorite magmas were active at different pulses. Moreover, these ages correspond to the initial stage of the Northern Kyushu batholith. The Ushikiri-yama granodiorite magma ascended from the source region through the same conduit at least two pulses and emplaced at the shallow crustal level utilizing the low-angle rigidity boundary to form the small-scale laccolith as a pioneering magmatic activity of the Northern Kyushu batholith. As the northern part of Southwest Japan and the southern part of Korean peninsula during the Cretaceous has been thought to be situated within the extensional setting due to the slab roll-back, the results of this study provide new insights into the complicated batholiths forming processes and crustal evolution under the extensional regime.