The mineral assemblages of sulfides in the Ryoke metamorphic rocks of the Narai Komagane area, northern Kiso-Ina district, were studied by using an ore microscope and an electron microprobe analyzer. Most of the original rocks are pelitic and psammitic rocks and the metamorphic grade ranges from the greenschist facies to the upper amphibolite facies. Principal sulfides occurring in the rocks are pyrrhotite (Po), pyrite (Py) and chalcopyrite (Cpy). Pyrrhotite occurs most frequently and commonly contains a small, lenticular crystal of chalcopyr-ite. The mineral assemblages of sulfides are Po+Py+Cpy (22), Po+Cpy (18), Po+Py (11), Po (9), Py (5), Py+Cpy (4) and Cpy (1) (numbers in parentheses represent the frequency of occurrence). The coexistence of pyrrhotite and chalcopyrite is noteworthy considering the grade of metamorphism.
Judging from the experimental results of the Cu-Fe-S system, the assemblages of sulfides observed do not represent those formed at peak metamorphic stage, but re-equilibrated ones at lower temperatures. The equilibrium assemblage of sulfide minerals at peak metamorphic stage is considered to be pyrrhotite+ intermediate solid solution (iss)±pyrite. The presence of primary iss in these rocks, however, seems unlikely except for some specimens. Textural evidence of pyrrhotite-chalcopyrite composite grains (Po (Cpy)) and its widespread occurrence leads to an interpretation that copper was incorporated into pyrrhotite solid solution and that iss did not appear in most of the rocks. Thus, the following processes are assumed for the formation of pyrrhotite-chalcopyrite composite grains:
1. Pyrrhotite solid solution rich in Cu crystallized under peak metamorphic conditions.
2. Exsolution of iss from pyrrhotite took place during cooling.
3. Iss changed into chalcopyrite at temperatures near 325°C.
In contrast to silicate minerals, sulfide minerals have been re-equilibrated at low temperatures ( ?? 325°C) during cooling of the Ryoke metamorphic rocks.

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K-Ar ages were determined for Miocene Ryozen basalts from east of Fukushima City, Northeast Japan. The Ryozen basalts mainly comprise primitive basalt lava flows of the island arc tholeiite series with subordinate amounts of pyroclastic flows. Ten basalt specimens gave almost identical K-Ar ages of 12.4-16.0 Ma, corresponding to Middle Miocene age. If we combine the data of previous studies with the present ones, we can conclude that the Ryozen district have two periods of activities in Miocene. A weak andesitic volcanism took place about 22 Ma, whereas very intence basaltic activity occurred about 13-16 Ma. The volcanic front of 13-16 Ma was estimated to lie 30-50km east of the Quaternary volcanic front.

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Samples of metabasaltic dikes and lavas from early Proterozoic rock units in the southcentral Lake Superior region were analyzed for major and selected trace elements in order to compare their chemical composition with those of rocks from modern, known tectonic environments. The rocks exhibit a relatively wide compositional range in which the least evolved samples show a chemical composition similar to that of T-type MORB.
Based on geochemical characteristics and tectonic discrimination diagrams, I conclude that the rock suites were emplaced in an extensional regime. The geochemical patterns of these rocks are similar to those of British Tertiary Volcanic Province and continental tholeiitic basalts from Rdendeka. which ernnted in more recent rift environments.

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