飛騨帯における火成活動の変遷の化学的特徴

概要

論文の詳細を見る
Chemical compositions of magmatic products published or unpublished in the Hida belt, are examined interms of SiO_2-(Na_20+K_20), CaO-alkali lime index, SiO_2-FeO・/MgO and TiO_2-FeO・/MgO diagrams after the check of chemical migration, using the K_20/Na_20-(Na_20+K_20) diagrams. Concluding remarks are as follow: 1) Five stages of igneous activity are recognized from modes of occurrence and radiometric ages. Amphibolite concordant with surrounding rocks, which are estimated to have been formed as basaltic lavas or sills at geosynclinal stage, are mildly alkaline rocks showing probable Coombs trend in MIYASHIRO'S sense, as far as the SiO_2-(Na_20+K_20) diagram and CIPW norm classification are concerned. But they belong to oceanic tholeiites in the CaO-alkali lime index diagram. This discrepancy may suggest strong chemical migration through submarine alteration and regional metamorphism at least more than twice. 2) Inishi plutonic rock and grey granite were formed in the second stage, at the thermal peak of regional metamorphism in the deeper part of the metamorphic belt. Those are plotted mainly in the TH and CA field in the SiO_2-(Na_20+K_20) diagrams, and their majority are calc-alkalic in MIYASHIRO'S classification. 3) The Mozumi-Kamioka metabasite mass, cross-cutting the distribution trend of gneiss, is the products of the third stage of magmatism. It belongs also to mildly alkaline rocks showing Straddle-type trend and varies from basalt to trachyte in composition. Both the first stage amphibolite and the third stage metabasite belong to mildly alkaline rocks, but the chemical difference is very clear, i. e., the former is richer in TiO_2 and poorer in Al_20_3 at given FeO・/MgO ratio. Small amounts of possible tholeiite are similar in composition to tholeiite in matured-type island arcs or active continental margins. 4) The fourth stage igneous activity is the Funatsu-stage plutonism in the early Mesozoic, whose products are divided into earlier Shimonomoto-type and later Funatsu type rocks. Both belong to mainly CA and TH series rocks. The THseries rocks in the former are richer in TiO_2 than the latter. TiO_2 contents of both types are comparable with those of matured-type island arcs or active continental margins. 5) The last stage Cretaceous granites are not accompanied dy basic to intermediate members. Most of them are calc-alkalic. These are the most acidic members among all igneous rocks in the Hida belt, typical ones characterizing matured-type island arcs. 6) It is clear as for the magma composition that the younger in age the rocks are, the richer in SiO_2 they are, except the second stage plutonic rocks. 7) The ratio of CA series rocks in each magmatic stage, increases towards Cretaceous in age, except the second stage plutonic rocks. Above mentioned facts may suggest that the acidic crust in the Hida belt became thicker from geosynclinal stage to post orogenic Cretaceous age, though remains a possibility that the oceanic crust existed only at geosynclinal stage.
1979-09-20

飛騨帯における火成活動の変遷の化学的特徴

スポンサーリンク

概要

著者

関連論文

スポンサーリンク