Amphiboles of a Metagabbro--Amphibolite Sequence, Hidaka Metamorphic Belt, Hokkaido

The western part of the Hidaka Metamorphic Belt, Hokkaido, consists of primary pyroxene gabbro and lesser amounts of olivine gabbro that have been dynamically metamorphosed to metagabbro—gabbroic amphibolite-amphibolite-epidote amphibolite during uplift and shearing about 23 m.y. ago. Textures and the presence of relic and recrystallized amphibole and plagioclase in the same rock indicate incomplete reaction and non attainment of equilibrium during recrystallization. EPMA and bulk analyses of 165 amphiboles indicate a continuous overall compositional range from actinolite to dark green hornblende (with 100 mg/(Mg+Fe2++Fe3+Mn) ratios varying from 89.5 to 32.0) marked by increasing Al, Fe, Ti, and Na. A compositional gap is usually present between relic and recrystallized amphiboles in any one rock which becomes more prominent with increasing shearing. In addition to host rock chemical control, amphibole composition is largely dependent on the An content of coexisting plagioclase. Locally epidote and sphene exert a strong influence on both amphibole and plagioclase compositions. Amphibole Ti and Mn contents decrease with shearing and Fe enrichment of the host rocks largely as a result of the incoming of rutile, sphene, and Fe-Ti oxides. Analysis of host rock oxidation ratio and amphibole compositions indicates that the rocks essentially behaved as closed systems to oxygen during metamorphism. Al1V-AlIV, AlIV-Fe3+, and AlIV-(Na, K)A are the main substitutions in the amphiboles. Within any one rock the recrystallized amphiboles are enriched in Al, Fe, Ti, and Na relative to the relice amphiboles. Increasing metamorphism results in a progressive change of amphiboles (recrystallized) to more Fe and Si (rather than Al) rich compositions reflecting the trend towards greenschist where Fe-actinolite (+Mg chlorite) would be stable. Differentiation of the amphiboles is within the limits of Si⇌Al replacement and the compositional limits of the early stage reaction rim and replacement amphiboles in the host olivine and pyroxene metagabbros.