山陰中部における後期白亜紀乃至古第三紀の火成活動

その他

Hitherto described is the successive igneous activity, which continued from late Mesozoic to Paleogene, in the central San-in region.
ESKOLA (1932) stated that the formation of a granitic magma is intimately related to an orogenic movement, and that a granitic magma is formed when a granitic material having the lowest melting temperature is squeezed out of a deeper part of a geosyncline. The present writer considers that the parental "San-in granitic magma" in the central San-in began to intrude in connection with some orogenic movevement, perhaps the Akiyoshi orogenic movement.
This enormous "acidic granitic magma" ascended up to near the ground surface, mostly in late Mesozoic, and effused out, due to deroofing of the roof rock, or partly consolidated in the vents, thus forming the great quantities of late Mesozoic acidic vocanic rocks. Although these acidic volcanic rocks are intruded by granitic plutonic rocks, it can be said that the volcanic rocks and the plutonic rocks were originally the same material constituting the enormous acidic "San-in granitic magma".
PERRlN et ROUBAULT (1949), RAMBERG (1944), and others have long stressed the role of metasomatism in the genesis of granite, and they have minimized the part played by magma. In central San-in, the plutonic rocks are associated with greatly predominant acidic volcanic rocks intimately. Therefore, the granitic plutonic rocks are regarded as magmatic origin. Their general composition is granitic to granodioritic, and is closer to the lowest crystallizing point shown in the experiments of Ab-Or-Qz-H_2O system by TUTTLE et BOWEN (1958). Consequently, it will be easier to explain that largely liquid granitic and granodioritic magmas, formed by almost complete fusion, of deep-seated rocks dr squeezed upward from deep zones of partial fusion, have repeatedly effused out or invaded the upper crust on a large scale.
The batholith is composite, having been formed by the successive emplacement of at least four major intrusives. Accordineg to the K-A dating by KAWANO et UEDA (1966), these batholithic activities may have been a short-continued process.
The results of K-A dating by KAWANO et UEDA (1966) indicate that the crystallization and consolidation of these plutonic rocks extend over the Paleogene period. This results satisfactorily explain the field evidences.