IN-SITU ELECTRON MICROPROBE TH-U-TOTAL PB DATING OF PRECAMBRIAN DETRITAL ZIRCON IN LATE PALEOZOIC BLUESCHIST, SW JAPAN

Precise electron microprobe analysis of the Th, U and Pb concentration is applicable for age determination of Th- and U-bearing minerals such as monazite and zircon. Recently, we succeeded in the precise analysis of Th, U and Pb in zircon by a JEOL JXA-8900R EPMA, using an accelerating voltage of 15 kV, a probe current of 500nA and the CIT-ZAF method for matrix corrections. The precise analytical method was applied to in-situ age determination of detrital zircon in mafic meta-sediment (blueschist) from the Renge metamorphic belt in Chugoku Mountains, SW Japan. The blueschist consists mainly of glaucophane, lawsonite, pumpellyite and quartz with minor amount of albite, titanite, and chlorite. The metapelites (quartz-phengite schists) intercalated with the blueschist yield ca. 320 Ma of phengite K-Ar ages (Tsujimori and Itaya, 1999). The detrital zircon occurs as aggregate of fragments of various sizes (maximum 0.2 mm). Each fragment is chemically heterogeneous and has sometimes a remarkable zonal structure, suggesting that the detrital fragment has been a part of the poly-metamorphosed zircon crystal. More than 180 spots (5 µm) on detrital zircon were analysed after the textual observation using the BSE, CL and X-ray images. The PbO varies from 0.016-0.049 wt.% and *UO2 (UO₂ plus the equivalent ThO₂) from 0.095 to 0.361 wt.%. The apparent ages and *UO₂ are calculated using the CHIME age calculation program by Kato et al. (1999). The apparent ages for individual spots are subdivided into three groups; namely 721-879 Ma, 962-1272 Ma, and 1257-1671 Ma. No zircon overgrowth during the Late Paleozoic blueschist facies metamorphism is recognized. In particular, individual points of the youngest subgrain are arrayed linearly from the origin on the PbO versus *UO₂ diagram, defining an isochron age of 799 +/- 26 Ma (MSDW=0.10). Principal source of such multiple zoned zircons are expected in the high-grade metamorphic rocks in the Precambrian basements in Eastern Asia that records polymetamorphic evolution. The presence of Precambrian detrital zircon in the Late Paleozoic Renge blueschist implies that some reworked Precambrian zircon was supplied to Phanerozoic accretionary complexes in present Japanese Island.