Crustal evolution in the Gyeongsang Arc, southeastern Korea: Geochronological, geochemical and Sr-Nd-Hf isotopic constraints from granitoid rocks

Abstract

A loosely assembled patchwork of Cretaceous to Paleogene granitoid plutons comprise the platform of the Gyeongsang Arc in southeastern Korea and represent a high-flux pulse of magmatism in Northeast Asia. The present study uses new and published zircon U-Pb ages, whole-rock geochemical and Sr-Nd isotopic data, and zircon Hf isotopic data of the plutons to decipher the crustal evolution in the Gyeongsang Arc and compare it with adjacent accretionary terranes. The ion microprobe zircon U-Pb ages range from 91 to 27 Ma, revealing the selective occurrence of Paleogene plutons in the eastern coastal area, to the east of the NNE-trending Yangsan fault system. Paleoproterozoic to Paleogene xenocrystic cores are occasionally observed in the zircon grains. The plutons are composed of dominantly magnesian, high- to medium-K calc-alkaline granites-granodiorites exhibiting typical geochemical characteristics of subduction zone magma, and less abundant ferroan alkaline granites. Their trace element patterns require residual amphibole and feldspar, but not garnet in the source. The whole-rock Sr-Nd and zircon Hf isotopic compositions of the granitoids (initial ⁸⁷Sr/⁸⁶Sr = 0.7043 to 0.7069; ε_Nd(t) = –4.8 to +2.9; ε_Hf(t) = –5.0 to +19.7) are distinctly more primitive than those of inland granitoids distributed outside of the Gyeongsang backarc basin. An important role of relatively young, most likely Paleozoic juvenile crust in the formation of the Late Cretaceous granitoids is suggested by the time-ε_Hf trend of high-ε_Hf zircons that converges toward data points of the Late Permian Yeongdeok adakite composing the arc basement. The asthenospheric mantle input is highlighted by significantly high (>+17) ε_Hf(t) values of some zircons from the early Eocene alkaline plutons. Subsequent reworking of the rejuvenated crust yielded granitoid plutons possessing slightly but recognizably higher ε_Nd(t) and ε_Hf(t) values than the older plutons. These isotopic features demonstrate that the Cretaceous-Paleogene calc-alkaline granitoids in the Gyeongsang Arc are not “juvenile” (sensu stricto) despite their mostly positive epsilon Nd and Hf values, but are basically a product of crustal reworking. The Sr-Nd-Hf isotopic compositions of Late Cretaceous granitoids in the Gyeongsang Arc are comparatively more primitive than those in adjacent accretionary terranes such as Southwest Japan and Fujian province in southeastern China, reflecting differences in the formation age of the basement on which the arc system was built.