Emplacement ages and petrogenesis of the Sunchang and Namwon granitoids, South Korea

Abstract

We present new sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon ages for two adjacent plutons (Sunchang and Namwon) along the Middle Jurassic Honam Shear Zone, South Korea, and report their major- and trace-element and Sr-Nd-Pb isotopic compositions to constrain the emplacement ages and petrogenesis of the plutons. The Sunchang foliated granites yield U-Pb ages of 186–178 Ma, suggesting long-lasting magmatic activity. The emplacement age of the Namwon pluton (192–180 Ma) indicates that it was broadly coeval with or slightly older than the Sunchang pluton. Some zircon grains contain ancient (Neoarchean to Paleo-Proterozoic) inherited cores. The REE patterns of the granitoids are roughly U-shaped with no strong negative Eu anomalies and show arc-like enrichments in LILEs and depletion in Nb or Ta, similar to I-type granites. The Sunchang granites yield initial 87Sr/86Sr ratios of 0.71543–0.71675, εNd(t) values of −29.0 to −26.0, and 206Pb/204Pb ratios of 18.44–18.90, and the Namwon granitoids yield initial 87Sr/86Sr ratios of 0.71229–0.71652, εNd(t) values of −27.5 to −20.9, and 206Pb/204Pb ratios of 17.93–18.23. The granitoids have adakite-like elevated Sr/Y (25–78) and La/Yb (15–66) ratios but are potassic, which is inconsistent with slab-derived melts. Furthermore, they have relatively low MgO at a given SiO2 content compared with high-SiO2 adakites and low Mg# (30–48) and Ni (<5 ppm), Co (<8 ppm) and Cr (<52 ppm) contents. They resemble C-type adakites, which form by the partial melting of thickened lower continental crust with residual garnet and amphibole. Underplating mafic magma from the mantle wedge or shear heating may have been the source of heat for the crustal melting. The close spatiotemporal association of these C-type adakitic rocks suggests a genetic link between them. The theory that the massive to weakly foliated Namwon granitoids were emplaced post-tectonically needs to be revised, as the Namwon and Sunchang granitoids formed during the same magmatic episode in the regional-scale shear zone.