Age of formation for the Ishikawa composite mass, Abukuma mountains, inferred from Nd and Sr isotopic systematics

Sm-Nd and Rb-Sr isotopic compositions and REE abundances were measured on the Ishikawa composite mass, which is one of the older granitic rocks in the Abukuma mountains. Three mineral and two whole-rock samples define a Sm-Nd isochron yielding an age of 111±42 Ma with an initial 143Nd/144Nd ratio of 0.51251±3. Eight whole-rock samples give a Rb-Sr isochron age of 106±16Ma with an initial 87Sr/86Sr ratio of 0.70518±15. These ages are interpreted to represent the time of intrusion, supporting the Jurassic age of the Gosaisho metamorphic rocks confirmed by the fossil evidence. The Nd model ages of the Jshikawa mass relative to CHUR are as young as intrusion ages, whereas those relative to DM are 600-800Ma. On the basis of the REE patterns and eNd-eSr relationship, the Ishikawa mass is thought to be derived from the source material similar to CHUR. It is therefore suggested that the intrusion of the Ishikawa mass occurred subsequently to the differentiation of the source magma from the mantle.