Timing of deposition of the Yuli Group distributed in the Northeastern Yeongnam Massif constrained by zircon and monazite U-Pb ages

Abstract

In order to determine the timing of deposition of the Yuli group distributed in the northeast of the Yeongnam Massif, U-Pb zircon and monazite age determination for the metasedimentary rocks from the Goseonri Formation and the Gakhwasa Formation of the Yuli Group was conducted using LA-MC-ICP-MS. The detrital zircons from the Goseonri Formation show well-developed roundness. In the cathodoluminescence images, most zircon grains show well developed oscillatory zoning inside. In some zircon grains, metamorphic rims are observed outside, and their thickness is not constant. The Th / U ratio of the outer rim is less than 0.1, confirming that this part is made of metamorphic origin. Roundness was also developed in these zircons with metamorphic rims, indicating that detrital zircons were deposited after metamorphism. In the data with U-Pb discordancy less than 10%, the 207Pb/206Pb of oscillatory zones is concentrated between 2600 Ma and 2100 Ma. In data below 10% discordancy, the 207Pb/206Pb ages of the metamorphic rim are concentrated at 1878±5.2 Ma. Therefore, the provenance of the sediments that make up the Goseonri Formation is interpreted to include rocks metamorphosed at about 1870 Ma, probably the surrounding northeast Yeongnam massif metamorphic rocks. Later sediments from these sources were deposited. The detrital zircons from the Gakhwasa Formation are similar to the Goseonri Formation, with interal zircon ages less than 10 % discordancy ranging 2000 Ma to 2500 Ma. The 207Pb/206Pb age of the outer metamorphic rim of the Gakhwasa Formaton is calculated as 1,849±14 Ma, indicating deposition after this time. The metamorphic age obtained from the Gakhwasa Formation is more suitable to limit the oldest deposition time of the Yuli Group because the error is smaller than that of Goseongri Formation. The 207Pb/206Pb age of the pegmatite intruding the Yuli Group was measured at 1838±5 Ma, which constrain the youngest limit of the Yuli Group deposition. Therefore, the deposition of the Yuli Group can be limited to periods between 1838±5 Ma and 1878±5.2 Ma. In summary, the Yuli Group’s sedimentation period was later than the main metamorphic phase of the northeast Yeongnam Massif, but ealier than the igneous event that led to pegmatite intrusion. Metamorphic mineral assemblages, including muscovite, andalusite, and sillimanite, have been identified from metasedimentary rocks from the Gakhwasa Formation. These metamorphic minerals indicate that metasedimentary rocks have experienced metamorphism between 501℃ and 770℃ of temperature and less than 3.75 kb of pressure. Metamorphic mineral assemblages of the Gakhwasa Formation are similar to those of the neighboring Dueumri Formation and can be interpreted as overlapping effects of contact metamorphism caused by the Chunyang granite.