Trends of Sr and Nd isotopes through time near the Japan Sea in northeastern Japan

Abstract

Nd and Sr isotopic analyses were made on selected volcanic rocks of known stratigraphic age from a limited area in the “Green Tuff” region, northeastern Japan, and in adjacent areas. Two ultramafic inclusions from Ichinomegata were also analyzed. The volcanic rocks range in age from 27 to 6 Ma, and vary in ε_(Nd) from −0.9 to 5.7 and in ε_(Sr) from 14.1 to −16.3. The isotopic data plot generally along the region of the “mantle array”. When plotted against time, the isotopic ratios are found to be a regular function of geologic age. The results indicate that: (1) the volcanic rocks in the area are part of a “related” sequence or evolutionary pattern over a time period of 27 Ma; (2) the “evolutionary” pattern goes from an initially enriched source to that of a very depleted source. The shift in isotopic composition appears to reflect the ongoing lining of magma conduits or the decreasing extraction of lower crustal contributions with increasing infiltration of oceanic magma types in an environment where the crust is thinning. Based on the geotectonic development of this area due to the back-arc spreading of the Japan Sea, we identify two different stages: the pre-opening stage (> 15 Ma) and the post-opening stage (< 15 Ma). The volcanic rocks of the pre-opening stage are characterized by an enriched isotopic signature, and those of the post-opening era by a depleted signature. The host rocks of the Kuroko ore deposit lie within this evolutionary trend and contain a substantial amount of continentally derived materials processed through an oceanic environment. In general, we infer that the degree of continental crust involvement is high during the pre-opening stages of a back-arc basin when it is associated with a rifted continental mass.