Lithologic variation and plutonic history of the Late Cretaceous granitoids in the Rokko Mountains, southwest Japan

Abstract

Late Cretaceous granitic rocks are widely distributed in the Rokko Mountains. They are divided into Nunobiki Granodiorite, Yahatadani Granite, Dobashi Quartz Diorite and Rokko Granite, according to their field occurrence and petrographical characteristics. The Nunobiki Granodiorite occurs at the southern foot of the Rokko Mountains. This granodiorite shows a weak gneissic fabric, trending east-west and dipping to the north. This rock is subdivided into porphyritic and equigranular facies. The Yahatadani Granite occurs as a small pluton in the southeastern Rokko Mountains. The Nunobiki Granodiorite and the Yahatadani Granite belong to the Younger Ryoke granitoids. The Dobashi Quartz Diorite occurs as aggregations of small xenoblocks within the Rokko Granite. The Rokko Granite is the most extensively developed pluton in the Rokko Mountains. This rock is subdivided into medium- to coarse-grained facies, fine-grained facies and micrographic facies according to the grain-size differences and petrographical characteristics. The Rokko Granite is a vertically zoned pluton. The fine-grained facies occupies the higher part, the medium-grained facies the middle part, and the coarse-grained facies the lower part in altitude of the Rokko Mountains. The upper part of the Rokko Granite is rich in SiO₂ and K₂O, whereas the lower part of the Rokko Granite is rich in TiO₂, MgO, and CaO.