Magmatic zoisite and epidote in tonalite of the Ryoke belt, central Japan

Abstract

Magmatic epidote and zoisite commonly occur in Cretaceous tonalite of the Hazu area in the Ryoke belt, central Japan. The tonalite is mainly composed of amphibole, biotite, plagioclase, quartz, and epidote/zoisite with minor ilmenite, magnetite, pyrite, zircon, and apatite. Small amounts of K-feldspar occur as an interstitial phase between other felsic phases or perthitic patches in plagioclase. Epidote occurs as inclusions in plagioclase, as interstitial phase in the matrix, and as secondary phase in chlorite pseudomorphs after biotite, and in saussuritized plagioclase. The XFe [= Fe3+/(Al + Fe3+)] value of the secondary epidote ranges from 0.27 to 0.39. Epidote inclusions in plagioclase and interstitial grains contain less Fe3+ (XFe = 0.08–0.29), Fe3+-poor epidote with XFe 0.20 and their host plagioclase. Epidote grains with XFe > 0.20 in plagioclase and the matrix are a magmatic phase that crystallized directly from the tonalite magma. The Fe3+-poor epidote (XFe < 0.20) and zoisite were probably formed by a local reaction between the trapped melt and its host plagioclase, and these are considered not to have been in equilibrium with the tonalite magma. Compositions of amphibole-plagioclase assemblages allowed for temperature estimates in the range of 730–770 °C and minimum pressures of 0.47–0.57 GPa for the epidote/zoisite-bearing tonalites of the Hazu area. Epidote/zoisite-free tonalites occur in other areas of the Ryoke belt. There may be several tonalite bodies that record different intrusion processes and solidification depths in the Ryoke belt.