APPLICATION OF ALLANITE, MONAZITE AND ZIRCON GEOCHRONOLOGY TO A GRANITE GNEISS COBBLE FROM METADIAMICTITE OF THE OGCHEON METAMORPHIC BELT, KOREA

The U–Th–Pb ages of allanite, monazite and zircon from a granitic gneiss cobble were investigated to constrain the timing of deposition and peak metamorphism of the Hwanggangri Formation, Ogcheon metamorphic belt, Korea. This formation consists primarily of greenschist-facies metadiamictite which contains various clasts, ranging in size from granule to boulder. The clasts comprise 3–30 % of the rock by volume, and consist of mostly quartzite, in addition to minor granite, granitic gneiss, shale, limestone and dolomite. These clastic rocks are enclosed in a pelitic to calcareous phyllite matrix. The mineral assemblage of the pelitic matrix is characterized by the coexistence of biotite, chlorite and muscovite, together with epidote, plagioclase and quartz. The U–Th–Pb isotopic compositions of the accessory minerals were measured using a SHRIMP II ion microprobe. Most of the zircon crystals are 100–300 mm in length, gray and translucent, but their CL images mostly show concentric, oscillatory zonation which is characteristic of an igneous origin. Forty-two spot analyses from 25 of the most transparent zircon crystals defined a Pb loss array from ~1.88 Ga to ~400 Ma on a Tera-Wasserburg diagram. The crystallization age of the zircon was determined from 14 spot analyses that were less than 1% discordant. Their weighted mean ²⁰⁷Pb/²⁰⁶Pb age was 1863 ± 12 Ma (tσ). Fifty-three spots were analyzed from 20 allanite crystals. These crystals, 100–300 mm in length, do not show a distinct zoning pattern in BSE images, and contain a range of inclusions including monazite, thorite and zircon. The weighted mean ²⁰⁸Pb/²³²Th age of 49 spot analyses from 18 crystals is 413 ± 8 Ma (tσ), and that of 4 spots from 2 crystals is 249 ± 14 Ma (tσ). Both sets of data show high Th/U ratios (500–4000), but different LaSiO₂⁺/Th⁺ ratios (>4 and ~1, respectively). The Silurian age is reproduced in a monazite crystal ~200 mm in diameter, and its weighted mean ²⁰⁸Pb/²³²Th age from four spot analyses is 416 ± 13 Ma (tσ). The Th/U ratios in the analyses are also high (~1500), suggesting a paragenetic affinity with the Silurian allanite. All the above data suggest the regional metamorphism of the metadiamictite at ~250 Ma. However, its deposition age, either pre- or post-Silurian, relies upon the metamorphic condition of the coexisting allanite and monazite.