著者

山田 悟資 角替 敏昭

出版者

島根大学

雑誌

島根大学教育学部紀要. 自然科学 (ISSN:05869943)

巻号頁・発行日

vol.32, pp.15-27, 1998-12-01

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The Zimbabwe Craton, which is a typical Archean granite-greenstone terrane, contains two different types of granitoids ; older gneiss (3500~3000 Ma) and younger granite (2900~2600 Ma). This study focused on morphological characteristics of zircons in the older gneiss and the younger granite to understand mineralogical and petrological differences between the two granitoids.13; All samples examined in this study have two types of zircon. One is an elongated, fine-grained, and euhedral zircon (Type-A). The younger granite contains many Type-A zircons without any resorption texture, because the granitoid did not suffer later metamorphism and/or partial melting. The other type is a coarse-grained zircon with overgrowth texture around older core (Type-B). This type is abundant in the older gneiss. For example, sample ZC10A, which is a typical older gneiss, contains many Type-B zircons. We considered that the core of the zircons was formed during crystallization stage of the protolith of the older gneiss. The rock was then metamorphosed and/or partly molten and formed overgrowth texture. Presence of both euhedral and anhedral zircons in one specimen may imply that euhedral zircons were included in earlier-stage minerals(ex.biotite) before the last stage of metamorphism and/or melting.13; The younger granite contains Type-B zircons smilar to the older gneiss, although their amomt is little. We thus regard the origin of the younger granite as a partial melting of the older granite. Some zircons of the older gneiss have thus survived during later thermal event and now present in the younger granite.