ABSTRACT

Sm-Nd, Rb-Sr and K-Ar geochronological and isotopic analyses were made on the Hayachine-Miyamori arc ophiolite in the Kitakami Mountains, Northeast Japan. The Sm-Nd isochron for four whole-rock samples gives an age of 510 ± 70 Ma and an initial ¹⁴³Nd/¹⁴⁴Nd ratio of 0.51229 ± 0.00010 (ε_Nd = +6.4 ± 2.0). The initial ε_Nd for individual rocks ranges from +6.3 to +7.0, and is lower than that of the MORB. The age of 510 Ma is interpreted to represent the crystallization age of the Hayachine-Miyamori ophiolite. K-Ar ages of eight hornblendes range from 244 to 473 Ma, within which ages younger than about 400 Ma are thought to be affected by Cretaceous granitic intrusions. The K-Ar hornblende ages may indicate the times of tectonic emplacement to shallower levels. The initial ⁸⁷Sr/⁸⁶Sr ratios of eight whole rock samples from the Hayachine-Miyamori complexes show a wide variation: ranging from –4.0 to +31.9 as ε_Sr. The nearly constant ε_Nd and variable ε_Sr suggest various extent of seawater alteration for these samples. Amphibolites whose protolith may be extrusive or shallow intrusive rocks give higher ε_Sr than plutonic rocks, which is consistent with circulation of sea water during the metamorphism. However, coarse-grained gabbro intruded into host peridotites still have higher ε_Sr than the MORB. The lower ε_Nd and higher ε_Sr for these gabbroic rocks, which occur in the host peridotites with arc-related geochemical characteristics, suggest that these isotopic signatures were recorded within an arc upper mantle. This mantle signature may be explained by an involvement of radiogenic fluid or melt derived from subducting slab, which had been altered by sea water at a mid-ocean ridge.