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논문 기본 정보
- 자료유형
- 학술저널
- 저자정보
- 발행연도
- 2009.1
- 수록면
- 305 - 315 (11page)
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초록· 키워드
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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.
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