ZIRCON U-PB GEOCHRONOLOGY AND HF ISOTOPE GEOCHEMISTRY OF PRECAMBRIAN METAMORPHIC ROCKS FROM NORTHEASTERN YEONGNAM MASSIF, KOREA

The Korea peninsula comprises three Archean-Proterozoic blocks (Nangrim, Gyeonggi, and Yeongnam Massifs). This study presents the results of U-Pb and Hf isotopic analyses of zircons from Paleoproterozoic metamorphic rocks in the north-eastern Yeongnam Massif. Detrital zircons from the metasedimentary rocks yield a predominant age population of ca. 2.5 Ga. They frequently have textureless rims that were overgrown at ca. 2.03–1.85 Ga, indicating post-depositional overprints associated with the intrusion of neighboring granitoids and metamorphism. The (meta)granitoids are divided into three groups of banded or augen biotite gneisses (group I), massive cordierite or two mica granitic gneisses (group II), and a garnet-bearing leucogranite (group III). The emplacement ages of group I and II metagranitoids are constrained to be ca. 1.98 Ga. The upper intercept discordia ages of ca. 1.86 Ga indicated by the metamorphic overgrowth rims of zircons from group I and II metagranitoids agree with the emplacement age of group III leucogranite (1867 ± 6 Ma). The lower intercept ages of the zircons indicate repeated Pb loss events in the Neoproterozoic to Paleozoic. Most zircons have negative ε_Hf values corresponding to two-stage Hf model ages (T_2DM) from 3.4 to 2.7 Ga. Zircons from group I metagranitoids display a narrow T_2DM range (2.74 ± 0.09 Ga). The protoliths of group I metagranitoids are considered to be I-type granites that was derived by infracrustal melting at depth. In contrast, the scattered Hf model ages of zircons from group II metagranitoids are suggestive of crystallization from heterogeneous S-type magmas derived from the partial melting of supracrustal rocks. The Neoarchean Era (ca. 2.75–2.62 Ga) marks the most important stage of crustal formation in the north-eastern Yeongnam Massif. The Paleoproterozoic (ca. 2.50–1.98 Ga) magmas from which the zircons crystallized were principally a product of crustal reworking.