Geochronology and Sr-Nd-Pb-Hf-O isotope geochemistry of Miocene intrusive rocks from Tsushima Islands, Japan: Constraints on petrogenesis and tectonic setting

Abstract

We present geochemical and Sr-Nd-Pb-Hf-O isotopic data and zircon U-Pb ages for Miocene intrusive rocks (granitoids, mafic microgranular enclaves (MMEs), and diabases) from Tsushima Islands between Kyushu (Japan) and southeastern Korea. A granite yielded an age of 16.23 ± 0.16 Ma. The granitoids are mostly I-type, but are transitional between metaluminous and peraluminous compositions. All samples exhibit enrichment in LREE and LILE (e.g., K, Rb, and Pb), and depletion in HFSE (e.g., Nb, Ta, and Ti), implying formation in a subduction-related tectonic setting. However, on tectonic discrimination diagrams, data for all samples plot between the orogenic and anorogenic fields, indicating that Tsushima Islands were in a transitional tectonic regime from subduction to continental extension at ca. 16 Ma. The δ18Oclinopyroxene values of a diabase range from 5.65‰ to 5.84‰. The Sr-Nd-Pb isotopic compositions of the diabases (87Sr/86Sr = 0.70423–0.70481; εNd = −0.8 to +3.6; 206Pb/204Pb = 18.27–18.40) are within the range of Miocene Eoil basalts from the Cenozoic Yangnam Basin in southeastern Korea, indicating the mantle source was a metasomatized mantle wedge. The granitoids are characterized by more enriched isotopic compositions than the diabases (87Sr/86Sr = 0.70611–0.70756; εNd = −6.0 to −3.8; 206Pb/204Pb = 18.43–18.55), and the MMEs have an intermediate isotopic composition. δ18Oquartz values of the granitoids vary from 8.68‰ to 9.25‰, and zircons from a granite have εHf(t) values of −1.7 ± 1.2. The MMEs may have formed by partial melting of amphibolitic lower crust, whereas the granitoids possibly had a hybrid source consisting of amphibolite and graywacke.