The use of apatite as a record of initial 87Sr / 86Sr ratios and indicator of magma processes in the Inagawa pluton, Ryoke belt, Japan

Sr isotopic analysis of apatite in the extensive Inagawa pluton of southwestern Japan reveals the spatial distribution of initial 87Sr / 86Sr (SrI) within this body with an unprecedented resolution. While the SrI values in older units show the largest variation (0.7093–0.7107), the younger ones have relatively homogeneous SrI values around 0.7096. The older units also contain numerous mafic microgranular enclaves, which are absent in the younger units. Combining these results with bulk rock analyses and field observations suggests the wide variation in petrology and bulk chemistry in the Inagawa pluton is not solely due to fractional crystallization as previously thought. Mixing between mafic and felsic melts has also played a significant role. The lack of enclaves and homogeneous SrI values in the younger units suggest that mixing was complete by this time.