Generation of Cenozoic granitoids in Hokkaido (Japan): Constraints from zircon geochronology, Sr-Nd-Hf isotopic and geochemical analyses, and implications for crustal growth

Abstract

The island of Hokkaido is a young accretionary terrane, basically built with a Jurassic accretionary complex and Cretaceous arc in the west (= NE Japan arc terrane), a Cretaceous-Paleogene forearc basin and accretionary complex with the Hidaka metamorphic belt in the center, and a Cenozoic island arc with Cretaceous basement in the east (= Chishima or Kuril arc terrane). Though volumetrically small, Paleogene and Neogene granitoids are widespread in central Hokkaido (Hidaka Belt). Granitoids are the most representative component of the continental crust, so in this work we aimed to study the mode of generation and source characteristics of these granitoids in order to assess the crustal composition of Hokkaido and examine the general problem of continental growth. New zircon geochronology on nine granitic and one gabbroic rocks from the Hidaka Belt reveals three distinct magmatic episodes, two in the Eocene at 45–46 Ma (3 granites), and 37.0 ± 0.5 Ma (1 granite), and one in the Miocene at 18 to 19 Ma (5 granites and 1 gabbro). The Miocene episode represents the most important granitic emplacement in Hokkaido. The early Eocene zircon ages of 45 to 46 Ma are identified for the first time for granitoids that occur in the northern part of the Hidaka Belt. The zircon age of 37 Ma for a granite from Shirataki is rather rare in Hokkaido, but similar ages had been reported for a tonalite and a granite from the Hidaka metamorphic belt. Geochemically, all granites are slightly peraluminous but not S-type, and they possess volcanic arc granitoid characteristics. Their REE distribution patterns are typically “granitic,” showing fractionated patterns with LREE enrichment and distinct negative Eu anomaly. The whole-rock isotopic signatures [I_Sr = 0.7044 to 0.7061; ε_Nd(t) = +1.0 to +4.7; T_DM-1 = 400-1000 Ma] reveal their largely juvenile characteristics. This is corroborated by the zircon Hf isotopic compositions [ε_Hf(t) = +8 to +19]. The Eocene granites were most probably generated by melting of subducted accretionary complex in a prolonged period from 46 to 37 Ma in supra-subduction zone; whereas the Miocene granites were also generated by melting of accretionary complex in a back-arc rifting setting. In both cases, the involved accretionary complex was probably dominated by the mantle-derived lithological component with little Paleozoic or older crustal material. Hokkaido provides an excellent example of juvenile crust addition to the continental crust.