Geochronology, and geochemical and Nd-Sr isotopic characteristics, of Triassic plutonic rocks in the Gyeonggi Massif, South Korea: Constraints on Triassic post-collisional magmatism

Abstract

Sensitive high-resolution ion microprobe (SHRIMP) zircon U-Pb ages, and whole-rock chemical and isotopic (Sr-Nd) compositions, have been measured on some Middle Triassic plutonic rocks associated with post-collisional magmatism in the Gyeonggi Massif (Hongseong, Namyang and Yangpyeong areas), South Korea. Plutonic rocks from the southwest (Hongseong) and central (Yangpyeong) Gyeonggi Massif and central Okcheon Belt (Cheongsan) have ages of ca. 227-226, 231 and 225 Ma, respectively. The plutonic rocks from Hongseong are dominated by granite and quartz syenite, from Namyang by granite, and from Yangpyeong by quartz monzonite to quartz syenite and gabbro. Three rocks from the Gyeonggi Massif have the depletions in Ta-Nb, P and Ti, and enrichment in large ion lithophile elements (LILE) characteristic of plutonic rocks that are subduction-related or derived from an arc-related crustal source. The Hongseong quartz syenite and Yangpyeong quartz monzonite to quartz syenite have features of shoshonitic and high Ba-Sr granitoids, with LREE enrichment and no significant Eu anomaly. The plutonic rocks from the Gyeonggi Massif have high initial 87Sr/86Sr ratios (0.7083-0.7144) and strongly negative εNd(t) (- 20.3 to - 10.4), indicating derivation of the magmas from sources in the middle to upper crust. These high-K calc-alkaline and shoshonitic plutonic rocks formed during periods of relaxation that separated the culminations of post-orogenic uplift/collapse in the transitions from orogenic to anorogenic events after the Early-Middle Triassic continent-continent collision (249-229 Ma) between the North and South China blocks in South Korea. The plutonic rocks of the Gyeonggi Massif support therefore a possible eastern extension of the Early-Middle Triassic Dabie-Sulu collision belt in China.