Origin of adakite-like plutons in southern Korea

Abstract

We present Sr–Nd–Pb isotopic compositions for two adakite-like intrusions in southern Korea (Jindong and Bongnae), including major and trace element concentrations, and the sensitive high-resolution ion microprobe (SHRIMP) zircon U–Pb age. Our aim is to constrain the origin and tectonomagmatic processes that gave rise to the plutons. A SHRIMP U–Pb zircon age of 88.7±0.7Ma was obtained from a Jindong granodiorite sample. The Jindong plutons belong to the medium-K calc-alkaline series, and have an overall enrichment in large ion lithophile elements (LILE), such as K, Rb, Ba, and Pb, and a relative depletion in high field strength elements, such as Nb, Ta, and Ti, compared with the neighboring elements in the primitive mantle-normalized multi-trace element variation diagram. It was found that (87Sr/86Sr)i=0.70475–0.70596, (143Nd/144Nd)i=0.512547–0.512604, [(εNd)i=+0.4 to 2.2], and (206Pb/204Pb)i=18.19–18.37, which fall within the field of the arc-type Cretaceous to Tertiary Bulguksa intrusive rocks in the Sr–Nd and Pb–Pb isotopic correlation diagrams. The elevated Sr/Y (17–40), but low La/Yb (3.7–8.8) ratios of the Jindong plutons, together with their coherent geochemical trends and U-shaped rare earth element (REE) patterns, indicate that they were not formed by slab melting, but by amphibole-dominated fractional crystallization of the Bulguksa-like arc magma. A SHRIMP U–Pb zircon age of 226.5±2.5Ma was obtained from a Bongnae tonalite sample. The Bongnae plutons belong to the shoshonitic series. Their trace element patterns resemble the Jindong samples, but are characterized by significantly negative U anomalies in mafic rocks. They also have elevated MgO, Ni, Co, Cr, Rb, Ba, Sr, and low Al2O3 and Na2O contents at a given SiO2 concentration compared with the Jindong plutons, and are typified by highly radiogenic Sr and unradiogenic Nd and Pb isotopic compositions: [(87Sr/86Sr)i=0.71096–0.71290, (143Nd/144Nd)i=0.511641–0.511681, (εNd)i=−13.0 to −13.8, and (206Pb/204Pb)i=17.53–17.45]. The Bongnae plutons have adakite-like elevated Sr/Y (36–56) and La/Yb (12.9–42.7) ratios, but are highly potassic compared with slab melts. The petrogenesis of the C-type Bongnae adakites is consistent with formation by the partial melting of two components (a lower crust and metasomatized lithospheric mantle) in a post-collisional tectonic setting. Asthenospheric upwelling induced by slab breakoff after continental collision between the north and south China blocks may be a heat source for melting of the continental lithosphere. The Paleo-Tethys subduction before continental collision in this area or lower crustal delamination and subsequent low fractions of melting may have induced metasomatic overprinting in LILE in the overlying lithospheric mantle.