Cretaceous plutonic rocks distributed in Yuga and Seongsan.myeon are of quartz monzodiorite,granodiorite and monzogranite(granite B) in rock types. The modal composition changes systematically and each plutons are divided into the two rock types convenienly, ie. intermediate and acidic rock types:The intermediate rock is composed of quartz monzodiorite mainly and granodiorite, whereas the acidic rock is composed of monzogranite mainly and quartz monzonite. Zirocons are prominently included in plagioclase of the rock forming minerals in the intermediate rock, whereas in the acidic rock, in hornblende and biotite of the rock forming minerals prominently, The amounts of the included zircons are related to the relative amounts of plagioclase or hornblende and biotite in the two rock types. In the ratio of the included zircons of the main rock forming minerals, those of plagioclase are narrowly constant, whereas those of hornblende and biotite are broadly change in the two rock types and the latters in the acidic rock are higher than those in the intermediate rock. The size of zircon grains gradationally decrease from the intermediate rock parts to the acidic rock. From the frequency distribution of lenghth and breadth, those of the intermediate rock are bimodal or polymodal, and those of the acidic rock are unimodal shape. From these fact, it is suggested that the two rock types are comagmatic origin, but zircon concentrates of each rock type may have been crystallized under some different magmatic environment in the physico-chemical condition.

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