Intrusion Mechanism of a Granite Batholith

The Yagyu batholith, which is placed in the northern margin of the Ryoke belt of the Central Kinki Province, Japan, is one of granite batholiths which intruded after the regional metamorphism and strong regional-tectonism in the Ryoke belt ceased. The intrusion mechanism of the southern half of Yagyu batholith has been analysed in this paper. It is a zoned pluton, whose mantle and core consist mainly of quartz diorite — granodiorite facies and of adamellite facies respectively, and forms a tongue-like shape jutting toward the south. The gneissosity develops concordantly with the shape of the batholith and the intensity tends to increase from its core toward its margin. The structural trends of wall rocks are generally parallel or subparallel to the batholith boundary, showing that the southern half of Yagyu batholith had forcefully intruded forming gneissosity and strongly deforming its wall rocks.
Microtextures of constituent minerals (quartz and plagioclase) of the Yagyu batholith have been analysed. Quartz is recognized as two textural units, quartz grains and quartz pools: quartz grains as individual crystals distinguished from each other by high-angle boundaries, and quartz pools as individual domains occupied only or almost only by quartz grains and bounded mainly by other minerals than quartz. It has been clarified that quartz pools were commonly initially older single quartz grains as one of magmatic crystallization-induced microtextures and strongly elongated during the forceful intrusion of the batholith, and that quartz grains as constituents of quartz pools are secondary grains produced by dynamic recrystallization of older quartz grains related to the deformation which occurred after the emplacement of the batholith had been almost completely performed. Plagioclase also is recognized as two textural units, plagioclase grains and plagioclase pools: plagioclase grains as individual crystals distinguished from each other by distinct boundaries, and plagioclase pools as individual domains occupied only or almost only by plagioclase grains, showing euhedral outlines and single zonal structures of chemical composition. It has been pointed out that plagioclase grains (pl-grains) as constituents of plagioclase pools tend to increase in number but to decrease in size with increase of the intensity of development of gneissosity, showing that the formation of pl-grains is ascribed to the deformation related to the formation of gneissosity during the forceful intrusion of the batholith. The formation mechanism of pl-grains has been also briefly considered. And it has been concluded that, when the intrusion of the southern half of Yagyu batholith began, the lithofacies fractionation had already ceased and its mantle had already been almost completely consolidated.