The Hongcheon area in the central Gyeonggi massif is a unique carbonatite locality in South Korea. The age and petrogenesis of this uncommon rock type and associated rare earth element (REE) mineralization still remain uncertain. The NNE trending, 20-50 m wide and similar to 2 km long Fe-REE ore bodies are hosted within a swarm of carbonatite dykes intruding Precambrian basement gneisses. The intrusive nature of the dykes, fenite alteration halos, exsolution intergrowths of constituent minerals and stable isotope data in the literature collectively attest to the ore formation by crystallization of carbonatite magma. The"carbonatites are composed primarily of dolomite, ankerite, siderite, magnetite, monazite, apatite, strontianite and pyrite with subordinate quartz, barite, columbite, fergusonite and calcite. The principal carrier phase of REEs is monazite. The REE contents of monazite vary narrowly (TREO = 66.1-69.4 wt.%) irrespective of the textural occurrence. Although the monazite shows a sample-to-sample variation in La/Nd ratio, the textural varieties from each rock sample are similar with respect to this ratio. Thorium contents in monazite are consistently low (average = ca. 2500 ppm) with unusually high (average = ca. 2200) Th/U ratios. Sensitive high-resolution ion microprobe (SHRIMP) dating of monazite yielded a weighted mean Pb-208/Th-232 age of 232.9 +/- 1.6 Ma, which agrees with a weighted mean Pb-206/U-238 age of 2272 +/- 83 Ma within uncertainties. This age, coupled with comparable intrusion ages documented for kimber-lites and monzonite-syenite-gabbro-mangerite suite from central Korea, demonstrates the occurrence of mantle derived alkaline igneous activities and associated REE mineralization following the North and South China collision. The intrusion of the Hongcheon carbonatite and potassic or ultrapotassic suite in central Korea may have resulted from the post-collisional detachment of the subducted slab and consequent upwelling of hot asthenosphere and melting of the overriding lithospheric mantle. Initial Nd-Sr isotopic ranges of the Hongcheon carbonatite (epsilon Nd = ca. 26, Sr-87/Sr-86 = 0.703-0.706) and previous trace element data deny a petrogenetic linkage with the coeval silicate magmas. The metasomatism in the lithospheric mantle source of the Hongcheon carbonatite must have occurred in the distant past (>1.7 Ga) to generate significantly negative epsilon Nd values. (C) 2016 Elsevier B.V. All rights reserved.
