北部北上山地の階上深成岩体のRb-Sr年代
1992 年 87 巻 5 号 p. 187-196
詳細
1992 年 87 巻 5 号 p. 187-196
The Hashigami plutonic mass, northern Kitakami, Iwate prefecture, Japan, has been dated by Rb-Sr method. The plutonic mass consists of three successive intrusives of tonalitic to granodioritic composition. The Rb and Sr abundances of the rocks from the first intrusive are within limited ranges, and thus their Sr isotopic compositions do not change much. Although the rocks from the second intrusive do not vary much in Sr and Rb concentrations, they yielded an isochron of 135±25 Ma with a low initial 87Sr/86Sr ratio (=0.70355±0.00006). The 87Rb/86Sr ratios of the last intrusive rocks are low and similar to each other. Their 87Sr/86Sr ratios are also low and do not change much. None of the rocks from the first and the last intrusives is on the isochron. All intrusives might have originated from the common parental material, but crustal assimilation during upwelling possibly disturbed their Rb-Sr systematics.
