Geochemical and mineralogical characteristics of the Yonghwa phoscorite-carbonatite complex, South Korea, and genetic implications

Abstract

The Yonghwa phoscorite-carbonatite complex occurs as an isolated individual body with an inclined pipe shape within the Precambrian Gyeonggi Massif, South Korea. The phoscorite consists mainly of olivine, apatite, magnetite, carbonates, amphibole, and phlogopite, and can be subdivided into two types, olivine-rich and apatite-rich. The carbonatite is composed of calcite, Mg-rich dolomite, Fe-rich dolomite, magnetite, apatite, and/or siderite. Intensive fenitization occurred along the boundary between the complex and the wall rocks of leucocratic banded gneiss and garnet-bearing metabasite. The paragenetic sequences of the phoscorite-carbonatite complex demonstrate that the early crystallization of silicate minerals was followed by the crystallization of carbonates as the carbonatitic melt cooled. Magnetite occurs within the complex, and the carbonatites have Fe contents that are higher than typical ferrocarbonatites, due to the high magnetite contents. The rare earth elements (REEs) in the phoscorites and carbonatites are weakly fractionated and show enrichments of LREEs and Nb relative to HREEs. Furthermore, the apatites reflect the fractionated trends of LREEs relative to HREEs. Phoscorite apatites are enriched in Sr and show substitutions between Ca and Sr. Mica chemistry reflects the evolutionary trend of Fe2+ and Mg2+ in the phoscorite-carbonatite melt without Al substitution. Micas exhibit high values of Mg# in the phoscorite-carbonatite complex, but lower values in fenites. Via thermodynamic analysis, the early stability fields of magnetite-pyrrhotite-graphite-carbonate assemblages indicate that the Yonghwa phoscorite and carbonatite crystallized under conditions of 600 degrees C, 2 kbar, and X-CO2 = 0.2. Afterward, melts underwent an evolution to the late stability fields of magnetite-pyrite-pyrrhotite-ilmenite assemblages. The delta C-13 and 8180 isotopic compositions of carbonates in the Yonghwa phoscorite-carbonatite complex are -8.2 parts per thousand to -3.4 parts per thousand and 6.6 to 11.0 parts per thousand., respectively, and together with the sulfur isotope compositions of the sulfides (delta S-34 values of about 0.2 parts per thousand to 2.2 parts per thousand) indicate a primary mantle source of the magmas. Phlogopites from the fenites yielded K-Ar ages of 193.4 +/- 4.9 and 195.0 +/- 5.1 Ma, which demarcate the timing of the cooling of the phoscorite-carbonatite intrusion, and indicate that the phoscorite-carbonatite may be related to a post-collisional magmatic regime. The discovery of this complex marks the first known occurrence in Korea, of Fe and Nb-REE mineralization related to phoscorite-carbonatite igneous activity. (C) 2016 Elsevier B.V. All rights reserved.