Whole-rock major, trace and Sr-Nd isotope composition of the Cretaceous granitoids in the Takanawa Peninsula of western Ryoke Belt, southwest Japan

Abstract:

The Ryoke-Sanyo Belt in southwest Japan consists of metamorphic rocks and granitoids of the Cretaceous I-type and ilmenite series. It has been suggested that a transect from the San-yo to Ryoke Belt represents a crustal cross section of the Cretaceous Asian continental margin down to mid-crustal depths, and the Ryoke Belt represents the deep crust (e.g., Nakajima, 1994, Lithos). Therefore, understanding the petrological processes of granitoid formation in the Ryoke Belt would contribute to understanding deep crustal processes along the continental margin. In this study, we present new whole-rock major, trace and isotope data of the various types of granitoid in Takanawa Peninsula of western Ryoke Belt.

The granitoids in the Takanawa Peninsula intrudes into the Ryoke metamorphic rocks comprising mainly of clastic metasedimentary rocks of Jurassic accretionary complex. Various types of granitoids including quartzdiorites, tonalites, granodiorites and granites are observed in this area. Recently, Shimooka et al. (2019, JMPS) reported zircon U-Pb ages (ca. 97 Ma to 89 Ma) from six granitoid samples.

Based on the whole-rock geochemical data obtained in this study, eleven plutons are identified in the granitoids. Each pluton shows distinctive trends in SiO₂ vs. trace element diagrams suggesting that the granitoids formed from multiple magmas. The Sr-Nd isotopic compositions of the granitoids, however, show relatively narrow ranges (εSrI: +35.2 - +50.0 and εNdI: -6.3 - -3.3) suggestive of similar sources for the eleven granitoid plutons. Mafic rocks exposed around the Takanawa Peninsula show comparable enriched isotope composition to the granitoids (e.g., Kodama et al., 2019, JMPS) implying that the mafic rocks represent source materials of the granitoids. The granitoids in the study area are probably derived by anatexis of mafic lower crust, formed through underplating of mafic magmas from isotopically enriched mantle source, in a Cretaceous Asian continental margin.