西南日本内帯の後期中生代岩脈群の地質学的・岩石学的研究

Geological and Petrological Studies of Late Mesozoic Dyke Swarms in the Inner Zone of Southwest Japan

YOKOYAMA Shunji

廣島大學地學研究報告

Geological report of the Hiroshima University

Late Mesozoic igneous activity in the inner zone of Southwest Japan occurs in three modes, eruption of volcanic rocks, emplacement of plutonic rocks and intrusion of dykes as swarms. This paper presents the results of detail studies of geology and petrology of dykes in Shodoshima and the regional correlation of dykes in Southwest Japan.
Dykes can be divided into five distiguishable generations with reference to intrusive age. These five dyke generations correspond to five cycles of rhythmic magmatism from basic to acid.
The dyke-activity of five generations, especially, are closely related to Late Cretaceous plutonic activity in space and time. Dykes of the earlier three generations intruded in the closing stage of plutonic activity in the Ryoke and Sanyo belts, namely showing synplutonism. Though dykes of the later two generations took place after the plutonic activity ceased, dykes belonging to these generations are also placed in the Ryoke and Sanyo belts.
The detail studies of time relationships between intrusion of dykes and cooling history of plutonic rocks have been performed in Shodoshima. Dykes of the first and second generations occur in the Tanoura Gabbroic Complex and the Yoshino Granodiorite belonging to the synkinematic granites which are characterized by development of E-W-trending gneissosity (approximately.correlative to the Younger Ryoke Granites). These dykes took place before the formation of gneissosity. Complete cooling-consolidation of the Tanoura Gabbroic Complex was nearly contemporaneous with the formation of gneissosity. However plutonic activity of the Yoshino Grandiorite took place continuously until the latest stage of dyke-activity of the third generation, and was nearly contemporaneous with that of the Shodoshima Adamellite belonging to the post-kinematic granites in the Sanyo belt which free from any apparent sign of the regional deformation. Thus dykes of the third generation run across the gneissoity of the Yoshino Granodiorite, and some of them show typical synpulutonism in the Yoshino Granodiorite and the Shodoshima Adamellite.

jpn

廣島大學理學部地學教室

[ISSN] 0073-2303

[NCID] AN00213676