Ordovician arc ophiolite, the Hayachine and Miyamori complexes, Kitakami Mountains, Northeast Japan isotopic ages and geochemistry

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 l43Nd/]44Nd ratio of 0*5122±0.00010 (eNd= ±6.4 ±2.0). The initial eNd for individual rocks ranges from ±6.3 to ±7.0, and is lower than that of the MORB The age of 510Ma 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 horn-blende ages may indicate the times of tectonic emplacement to shallower levels, The initial87 Sr/86Sr ratios of eight whole rock samples from the Hayachine-Miyamori complexes show a wide variation: ranging from —4.0 to ±31,9 as e. The nearly constant ENd and variable E& sug-gest various extent of seawater alteration for these samples. Amphibolites whose protolith may be ex-trusive or shallow intrusive rocks give higher eSrthan plutonic rocks, which is consistent with circulation of sea water during the metamorphism. However, coarse-grained gabbro intruded into host peridotites still have higher eSr than the MORE. The lower eNd and higher eSrfor 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. © 1992, GEOCHEMICAL SOCIETY OF JAPAN. All rights reserved.