北上山地，ＩＶ帯の深成岩類を貫くアルカリに富む岩脈類

Abstract

Peculiar dyke rocks rich in alkalies and other incompatible elements intrude into the Himekami and Hinomiko plutons of Zone IV in the Kitakami Mountains. Representative rock types are as follows: 1) Shoshonite with the mineral assemblage biotite-orthopyroxene-olivine-clinopyroxene-plagioclase-alkali feldspar; and 2) High-K andesite with biotite-hornblende-plagioclase-alkali feldspar-quartz. The assemblage of the first rock types is similar to those of monzogabbro, which characterizes Zone IV plutons. Phenocrysts of olivine and orthopyroxene are surrounded by small crystals of cummingtonite and pale colored biotite. Composition of olivine is Fo74-70, while phenocryst and groundmass clinopyroxene are Wo40En45Fs15 and Wo43En35 Fs22 respectively, and that of phenocryst orthopyroxene has Wo3En72Fs25 core and Wo1En55Fs44 rim. Compositions of phenocryst plagioclase, groundmass plagioclase and alkali-feldspar are An60-41 An43-21 and Or91-31 respectively. The second rock type is rather coarase and porphyritic having phenocrysts of plagioclase and hornblende, and groundmass biotite, hornblende, plagioclase, alkali feldspar, quartz, apatite and Fe-Ti oxides. Hornblende is distinctly zoned and rich in (Na+K) and Al, different from that of the Cretaceous plutons in the Kitakami Mountains. Major element chemistry of these rocks indicates that both rock types are characteristically rich in K2O and poor in TiO2. They show MORB-normalized patterns poor in Nb and rich in Rb and Ba, which are characteristics of island arc or orogenic shoshonite rock series. The occurrence of these rocks provides important information as to the genesis of plutons and tectonic environments of the Kitakami Mountains during Early Cretaceous age.