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

Abstract

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 2 0 8 Pb/ 2 3 2Th 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 2 0 6 Pb/ 2 3 8 U 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 2 0 8 Pb/ 2 3 2Th 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.

Recurrent rare earth element mineralization in the northwestern Okcheon Metamorphic Belt, Korea: SHRIMP U–Th–Pb geochronology, Nd isotope geochemistry, and tectonic implications

“…Intensity of the O 2 − primary ion beam was maintained between 3.0 and 7.0 nA, and the spot size of the primary beam was approximately 25 μm diameter on the target surface. For allanite, the retardation lens was used to suppress scattered ions, and 232 Th was additionally measured for 232 ThO/ 232 Th-208 Pb/ 232 Th calibration (Gregory et al, 2007;Kim et al, 2009a). Th/U ratios for allanite were estimated using a fractionation factor derived from the measured 232 ThO/ 238 UO versus 208 Pb/ 206 Pb of the standard (Williams et al, 1996).…”

“…Although the Th-Pb isotopic system alone cannot provide a consistency test on the calculated ages, it has some advantages over the U-Pb isotopic system arising from better counting statistics and smaller proportion of common 208 Pb. Moreover, the use of Pb/Th ages avoids the problem of excess 206 Pb from the decay of initial 230 Th (Schaerer, 1984), and the Th-Pb isotopic system of allanite appears to be more resistant to later metamorphism than its U-Pb isotopic system (Kim et al, 2009a). Gregory et al (2007) showed that allanite grains with sufficiently high REE + Th contents (N0.5 cations per formula unit), like our samples (Table 1), yielded SHRIMP Pb/Th ages comparable to their reference ID-TIMS ages, without a matrix-induced effect.…”

Lithology and Geology of Deokjeok Island, Western Gyeonggi Massif, Central Korea

“…한편, 경기육괴 북서쪽 연변부에서는 한반도에서 가 장 오래된 암석이 대이작도에서 발견된 이후 이 일대 에 대한 연구가 비교적 활발히 이루어지고 있다 (Cho et al, 2008). 최근까지 연구가 이루어진 경기육괴 서 부 지역의 도서는 대이작도 (Cho et al, 2008;Kim et al, 2009;Lee and Cho, 2013), 영흥-선재-대부 도 (Na et al, 2012)가 있다. 대이작도에는 후기 시생 대(~2.51 Ga)에 관입한 토날라이트질 혼성암이 산출 하며, 이들은 트리아스기(~230 Ma)에 변성작용을 경 험하였다 (Cho et al, 2008;Kim et al, 2009).…”

“…최근까지 연구가 이루어진 경기육괴 서 부 지역의 도서는 대이작도 (Cho et al, 2008;Kim et al, 2009;Lee and Cho, 2013), 영흥-선재-대부 도 (Na et al, 2012)가 있다. 대이작도에는 후기 시생 대(~2.51 Ga)에 관입한 토날라이트질 혼성암이 산출 하며, 이들은 트리아스기(~230 Ma)에 변성작용을 경 험하였다 (Cho et al, 2008;Kim et al, 2009). 영 흥-선재-대부도에는 태안층과 대비되는 변성퇴적암류 들이 넓게 분포함이 밝혀졌으며 (Na et al, 2012), Vol.…”

In-situ analyses of zircon and other minerals: Contributions to the Asian geology and tectonics

“…This study demonstrates that a detailed investigation is required to understand the temporal relationship between accessory mineral growth and its microstructure such as corona or symplectite. Kim et al (2009) report the SHRIMP U-Th-Pb dating of allanite from tonalitic gneisses in Daeijak Island, central Korea. The U-Pb and Th-Pb isotopic systems in allanite provide the timing for bimodal metamorphic events in the range of ca.…”