Origin of the Sandstone Dikes Intruding Into the Miocene Shidara Group, Southwest Japan Based on Sandstone Composition and Detrital Zircon U–Pb Ages

ABSTRACT Geological conditions and fluid migration records that lead to the formation of sandstone injectites, including sandstone dikes, are attracting attention from various perspectives, including resource development. It is crucial to know the direction of intrusion from the parent sand body and the type of dike to understand the formation mechanism of sandstone dikes, while the location of the parent sand body is often unknown. Although the parent sand body of sandstone dikes intruding the Miocene Hokusetsu Subgroup of the Shidara Group in Southwest Japan is also unknown, this study has identified its parent sand body by comparing the sandstone composition and detrital zircon U–Pb age spectra of the sandstone dikes with those of each formation of the Hokusetsu Subgroup. The sandstone of the sandstone dikes are feldspathic sandstones similar to those of the lower part of the Hokusetsu Subgroup. Furthermore, the detrital zircon U–Pb ages of the sandstone dikes, which are characterized by a major cluster at about 95 Ma and minor clusters at 73–72 Ma, constrain its parent sand body among the lower Hokusetsu Subgroup. It resembles the zircon age spectrum of the lower Umedaira Sandstone Member, the uppermost horizon of the lower part of the Hokusetsu Subgroup. This means that the sandstone dikes originated from the lower part of the Umedaira Sandstone Member. This fact suggests that comparing the sandstone properties, such as sandstone composition and detrital zircon U–Pb age spectra of sandstone dikes with those of the host rock, is an effective method for identifying the parent sand body.