SHRIMP allanite U-Th-Pb dating of bimodal Triassic metamorphism of Neoarchean tonalitic gneisses, Daeijak Island, central Korea

The microstructures, compositions and U-Th-Pb ages of allanite from tonalitic gneisses in a Neoarchean migmatite complex, Daeijak Island, central Korea, have been investigated. Allanite crystals up to ∼300 μm in diameter occur with accessory apatite, ilmenite, magnetite and zircon primarily in the major foliation defined by aggregates of biotite and hornblende. The allanite is commonly rimmed by clinozoisite, and has a range of oscillatory to patchy compositional zoning. Its total LREE + Th content ranges from 0.58 to 0.83 atoms per 12.5 oxygens. The ²⁰⁸Pb/²³²Th isotopic ages of allanite from two tonalitic gneiss samples measured using the SHRIMP II ion microprobe show the same two age clusters, 229 ± 2 and 215 ± 4 Ma in sample DE28, and 227 ± 7 and 213 ± 4 Ma in sample DE43. The allanite ²⁰⁶Pb/²³⁸U ages from sample DE28 are similarly clustered, but those from sample DE43 are consistently younger, ∼185 Ma. These results, indicating a bimodal Triassic metamorphic overprint, are in contrast to the Neoarchean age (∼2.51 Ga) of thick zircon overgrowths in sample DE28. Allanite in both samples has retained its ²⁰⁸Pb/²³²Th crystallization age(s) through an event that caused major Pb loss from its U-Pb system, recording the later metamorphic history of a Neoarchean migmatite terrane.