塙深成岩体の分化とジルコンの形態との関係

抄録

The Hanawa pluton reported by Tanaka and Ochiai (1988) is devided into main bodies and small fine-grained quartz diorite-tonalite bodies. K-Ar ages using hornblende by Shibata and Utsumi (1983) are 117 Ma for fine-grained quartz diorite-tonalite bodies and 99 Ma for main bodies which intrude the former.
The main bodies are devided into granodiorite, foliated granodiorite-tonalite and medium to fine-grained tonalite-granodiorite facies which are probably formed by succesive intrusion of magma differentiated at deeper level. Furthermore the granodiorite is subdivided into mylonitelike, large sphene bearing, porphyritic and fine-grained sub-facies, which are seem to be differentiated at shallower level.
Each facies contains zircons showing characteristic morphology; granodiorite facies (110 dominant type), foliated granodiorite-tonalite facies (100 dominant type), medium to fine-grained tonalite-granodiorite facies (110/100 intermediate type), fine-grained quartz diorite-tonalite facies (E/F dispersed type).
The morphology of zircons in each rock facies is different in response to petrochemical characters. Prism index of zircon from main bodies decreases systematically with increasing SiO₂ contents of the rocks and with decreasing FeO, CaO, and MgO contents. Therefore the prism index of zircon is related to differentiation of granite magma. While prism index of zircon in fine-grained quartz diorite-tonalite of older stage is completely different from the above trend of the main bodies.
On the other hand U, Th and HREE contents in the zircons of the Hanawa pluton show a wide range from several ten to several hundreds ppm. Elongation index of zircon is clearly related to concentration of these elements substituting Zr.
In conclusion, morphological variations of zircons in granitic rocks reflect evolution of magma. Therefore the study of morphology of zircon is useful for understanding of petrogenesis of granite magma.