Journal of the Faculty of Science, Hokkaido University. Series 4, Geology and mineralogy;Vol.XVIII, Nos.1-2


Petrology of the Kesengawa Granodiorite, Kitakami Mountains, Northeast Japan

Hoe, Soeng Gi

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The Kesengawa granodiorite mass, a representative Cretaceous granitoid intrusive of the Kitakami mountain region, is divided into a basic and an acidic rock group. The basic rock group (melanocratic fine-grained porphyritic rocks) consists of microdiorite (facies 1), porphyritic microdiorite (facies 2) and quartz diorite (facies 3), and the acidic group (leucocratic coarse-grained equigranular rocks) consists of granidiorite-I (facies 4) and granodiorite-II (facies 5). A large number of measurements of the optical properties of plagioclases presented in each rock facies has been carried out. In general, the author could always discriminate four domains for the anorthite content of the plagioclase in each rock facies. These domains are tentatively called here as A, B, C and D. The plots of optically measured anorthite contents on the standard line gradually shift from the basic towards the acidic side in accordance with the change of rock facies from (1) to (5). Such tendency is summarized as follows: A, (B) (facies 1) → C, (B, D) (facies 2) ⇔ B, (A) (facies 3) → C, (B, D) (facies 4) → C, D (facies 5)-in the core part of plagioclases. C, (D) (facies 1) → C (facies 2) ⇔ C (facies 3) → D, (C) (facies 4) → D, (C) (facies 5)-in the mantle part of plagioclases. A vast number of dykes (plagioclase porphyrite, microdiorite and diabase) intruded into Palaeozoic and Mesozoic sedimentary strata prior to the emplacement of the Kesengawa granodiorite mass. The anorthite contents of plagioclases in these dykes fall into the same domain of facies (1). The author concludes that the generation process of the Kesengawa granodiorite mass can be interpreted as follows: 1) Subsequent to the generation of the dykes, melanocratic fine-grained porphyritic rock facies such as (1), (2) and (3) were formed along a fracture zone. 2) These basic facies were partially assimilated by the acidic rock facies, especially granodiorite-I (facies 4) which was generated after the former basic rocks. 3) Finally, granodiorite-II (facies 5) intruded into the formerly generated rocks.