The Silurian-Devonian magmatism recorded in detrital zircons from the Andong area, northeastern Yeongnam Massif, Korea

摘要

We report in situ U-Pb ages and Hf isotopic compositions of detrital zircons in three metasedimentary gneisses from the Andong area, northeastern Yeongnam Massif, Korea. Zircons from two xenoliths entrained in ca. 250 Ma tonalitic-granodioritic metagranitoids respectively show predominant Silurian clusters slightly overlapping each other within uncertainties (428 +/- 7 and 419 +/- 10 Ma), together with subordinate Archean-Proterozoic peaks. A younger Devonian population (372 +/- 5 Ma) is yielded by the youngest and most dominant age component in a biotite gneiss of the gneiss-marbleamphibolite complex occurring to the southeast of the metagranitoids. The magmatic origin of these zircons is corroborated by their oscillatory cathodoluminescence zoning and relatively high Th/U ratios (0.6 +/- 0.4). Therefore, the depositional timing of the protolith of the gneisses is constrained to be Late Paleozoic. Scattered metamorphic rim dates of the detrital zircons (ca. 260-220 Ma) reflect post-depositional thermal overprints resulting from recurrent arc-related magmatism. Highly variable initial epsilon Hf values of the Silurian zircons (+6 to -18) indicate contributions from both primitive melts and pre-existing crustal materials, which are typical for continental arc magmas. The Silurian-Devonian magmatism recorded in the detrital zircons is possibly linked with the Pb loss event(s) inferred from the zircon data of the Paleoproterozoic basement rocks of northeastern Yeongnam Massif and the overlying Cambrian sediments.