香川県鷲(わし)ノ山の新生代角閃石-斜方輝石安山岩中の含斜方輝石捕獲岩類

概要

論文の詳細を見る
40 xenoliths and several xenocrysts were collected from Washino-yama, Kagawa Prefecture. They were carried up by the Cenozoic hornblende-orthopyroxene andesite which was ejected on the late Cretaceous biotite adamellite of the Sanuki Plain. This report concerns mainly with the petrography of the xenoliths from Washino-yama. The xenoliths from Washino-yama are commonly small in size. They are grouped into 10 rock types after the mineral assemblages ;i.e., Type 1 to Type 8, diorite-porphyry and microdiorite. The mineral assemblages of them vary to one another, but the main mineral assemblage is represented by plagioclase + orthopyroxene, except for Types 7 and 8 (Tables 1 and 2). 9 rock-types have orthopyroxene as an essential mineral and, further, 4 rock-types among them are associated with clinopyroxene. The orthopyroxene are ferrohypersthene to hypersthene. The orthopyroxene with high Al_2O_3 contents more than 4% is found in Types 1 and 3 xenoliths, and such orthopyroxene usually shows strong pleochroism. And further, the Al_2O_3 contents of the orthopyroxene vary conspicuously grain by grain even in one rock fragment (Fig. 12), but those of clinopyroxene seem to be nearly constant in one rock fragment. Hornblende is found in 4 rock-types, two of which are accompanied by two-pyroxenes. The hornblende is pargasitic or ferro-pargasitic in composition according to the terminology of LEAKE (1978, Fig. 11). Hornblende of xenoliths is very alike that of the enclosing andesite in chemistry as we1l as in optic characters. Garnet is found in 4 rock-types, and it shows no zoning pattern in any case. The pyrope molecule in garnet attains 38% in Type 3, but in others it is less than 15%. Plagioclase shows granular texture or often porphyritic texture in some xenoliths. Only plagioclase of Type 8 is exceptional because it occurs as a cicular and radial crystals. The structural state of plagioclase is the high temperature type in Type 5, the low temperature type in Types 1 and 7, and the intermediate state in Types 2 and 6 (Fig. 7). Several rock-types of xenoliths are chemically analysed. Judging from the chemical and mineral ogical properties of the xenoliths, some of them are igneous rocks but the others are metamorphic rocks which would be derived from some igneous rocks. Thus, the xenoliths from Washino-yama can be classifiedin to 4-groups by their origin and petrographical features, as follows: Types 1, 2, 3 and 6……Granulite or granulite facies rocks, Types 7 and 8……Amphibolite facies rocks, Diorite-porphyry and microdiorite……Igneous rocks, not metamorphosed, Types 4 and 5……Igneous rocks, may not be metamorphosed. The granulite facies rocks, along with the amphibolite facies rocks, would be a part of the lower crust which is now overlain by granites of the Chugoku batholith. Type 5 and perhaps Type 4 xenoliths might be derived from synorogenic plutons, but both of the diorite-porphyry and microdiorite might be derived from postorogenic intrusives.
日本地質学会の論文

香川県鷲(わし)ノ山の新生代角閃石-斜方輝石安山岩中の含斜方輝石捕獲岩類

スポンサーリンク

概要

著者

関連論文

スポンサーリンク