PRECAMBRIAN CRUSTAL EVOLUTION OF THE NORTHEASTERN YEONGNAM MASSIF, KOREA REVEALED BY ZIRCON U-PB GEOCHRONOLOGY AND HF ISOTOPE GEOCHEMISTRY

The northeastern Yeongnam Massif mainly comprises Paleoproterozoic metasedimentary rocks and (meta)granitoids. The (meta)granitoids are divided into three lithologic 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). Ion microprobe zircon U-Pb data from group I and II metagranitoids consistently constrained the timing of magmatic emplacement at ca. 1.98 Ga, which is about 100 Ma older than magmatic ages reported for the Jirisan complex in the middle to southwestern part of the massif. In the age pattern of inherited zircons from the metagranitoids and detrital zircons from the metasedimentary rocks, an age population of ca. 2.5 Ga is predominant with subordinate clusters at ca. 2.7, 2.3, and 2.1 Ga. Metamorphic overgrowth rims of the zircons yielded coeval U-Pb ages 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. These age constraints do not allow an indisputable correlation of the northeastern Yeongnam Massif with Paleoproterozoic terranes in eastern China. LA-MC-ICP-MS data showed that most zircons had negative epsilon Hf values corresponding to two-stage Hf model ages from 3.4 to 2.7 Ga, demonstrating Neoarchean to Paleoproterozoic reworking of the Paleo- to Neoarchean crust. 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 northeastern Yeongnam Massif. The Paleoproterozoic magmas from which the zircons crystallized were principally a product of crustal reworking.