Abstract
Mid-Eocene granites in Sakhalin reveal a new period of juvenile crustal growth in Cenozoic Northeast Asia after a magma gap from 56 to 45 Ma. In South Sakhalin, this study presents new whole-rock geochemical and Sr-Nd isotopic data, together with zircon U-Pb ages and Hf isotope ratios, of granitic rocks and associated dikes from the Okhotsk and Aniva plutons. The two plutons emplaced in the Mid-Eocene show contrasting geochemical characteristics. The Okhotsk granites (44 to 42 Ma) have transitional I- and A-type granite features, and contains rare inherited zircons. The Aniva granites (40 Ma), essentially biotite-cordierite bearing, possess typical S-type features. This suite has abundant inherited zircons dated from 48 to 2446 Ma. Whole-rock Sr-Nd and zircon Hf isotopic data indicate that the genesis of the two plutons involved partial melting of mixed sources with dominant juvenile crustal components and magma mixing between underplated mantle-derived magmas and crustal partial melts. The Okhotsk granites have εNd (T) values of +3.1 to +3.7, initial 87Sr/86Sr ratios of 0.7047 to 0.7048, and zircon εHf (T) values of +11 to +16. The Aniva granites have εNd (T) values of +0.5 to +0.9 and initial 87Sr/86Sr ratios of 0.7052 to 0.7055, and zircon εHf (T) values of +4 to +12. According to Sr-Nd and Nd-Hf isotopic mixing calculations, the proportion of the juvenile component (=mantle-derived materials) is estimated to be more than 70%, and that of the upper crust component is less than 30% in the sources of the Okhotsk and Aniva plutons. We further argue that, based on Nd and Hf isotopic constraints, the Okhotsk and Aniva granites are correlated to the Mid-Eocene granites in the western and eastern parts, respectively, of the Hidaka terrane in Hokkaido. From the Sikhote-Alin area to Sakhalin and Hokkaido, the increase in juvenile sources and the decrease in recycled crustal materials are exhibited in the Nd isotopic signatures of the granitoids from early Cretaceous to Miocene. The granitic magmatism in South Sakhalin and Hokkaido may be the most prominent example of juvenile crustal growth among these areas.
