Rh-Sr, Sm-Nd and K-Ar systematics of metamorphosed pillowed basalts and associated Besshi-type deposits in the Sanbagawa Belt, Japan

Samples from metamorphosed pillowed basalts and related Besshi-type deposit s occurring in the Sanbagawa belt of the Shikoku Island, southwest Japan, h ave been analyzed for Sr-87/Sr-86, Nd-143/Nd-144 and K-40/Ar-40. This is to investigate the tectonic settings in which the original submarine volcanis m and associated Besshi-type mineralization occurred, as well as the age of metamorphism. Eight whole-rock samples of the pillow lavas metamorphosed i n pumpellyite-actinolite facies conditions yield a Rb-Sr isochron age of 10 7 +/- 15 Ma with an initial ratio of 0.70401 +/- 0.00006, while they do not define a Sm-Nd isochron. We interpret the results as the metamorphic age, an interpretation consistent with the previously reported Rb-Sr whole-rock age for the Sanbagawa pelitic schists. The overall ranges of the initial ep silon values at T = 107 Ma are: epsilon Nd (T)= +7.8 to +4.3; epsilon Sr(T) = +2.2 to -7.0, suggesting that the most likely source for the pillowed bas alts is depleted oceanic mantle, a conclusion supported by the previous Pb isotope studies. The K-Ar ages determined for twelve mineral separates from the Besshi-type deposits range from about 60 to 112 Ma, with a mean age of about 80 Ma, in agreement with the previous K-Ar and Ar-Ar data for the Sa nbagawa pelitic and basic schists. The youngest age, 60 Ma, was obtained fo r sericite from the Hinooku deposit metamorphosed in pumpellyite- actinolit e facies conditions, while the oldest one for hornblende from the spotted a mphibolite in the immediate vicinity of the Shiragayama deposit metamorphos ed in albite-biotite grade. The oldest age, 112 Ma, is interpreted to date the peak metamorphism, consistent with the Rb-Sr data, though a possibility of excess hr cannot always be ruled out. In view of the closure temperatur es of muscovite (350 degrees C) in the biotite zone, it is suggested that o ur K-Ar age data (<about 80 Ma) represent the age of the retrograde metamor phism or subsequent uplift. Datable microfossils found in the Sanbagawa bel t of Shikoku suggest that the submarine basaltic volcanism and related Bess hi-type mineralization occurred in an oceanic basin away from the trench re gion in Late Triassic (conodont) to Late Jurassic (radiolarian) times.