| 蓟县长城系大红峪组火山岩的单颗粒锆石U-Pb法准确定年 |
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| A PRECISE U-Pb SINGLE ZIRCON AGE DETERMINA- TION FOR THE VOLCANICS OF DAHONGYU FORMATION,CHANGCHENG SYSTEM IN JIXIAN |
| U-Pb single zircon age determination with 205Pb/235U mixed spike could give precise and reliable U-Pb ages simply and guickly, particularly it could be a fine dating method to be used in China. In this paper we report a precise and reliable U-Pb single zircon age for volcanics of the Dahongyu Formation, Changcheng System in Jixian by this method, and discuss its important significance for confirming the geological time scale of the Changcheng Period.A key to U-Pb single zircon age determination with 205Pb/235U mixed spike is getting very low total blanks of U and Pb. The analytic work we report here was undertaken fay Li Huimin in the laboratory of the Australian National University and the Australian Bureau of Mineral Resources. Pb blanks of reagents measured during this study are 2-5 pg/ml for HCI and HF, 1-3 pg/ml H2O, 10-20 pg/ml for HNO3. The average total blanks are 10-20 pg for Pb and 10 pg for U.Zircons were dissolved and U and Pb were separated and determinined following the technique outlined by T. Krogh (1973) with something modified. Single zircon was dissolved in a PFA vial of 0.35 ml volume with 0.1 ml HF for 10-15 hours (205±5℃).A 205Pb/235U mixed spike was used. U and Pb extraction procedures were standard ion-exchange column method with a small column of 50 μ1 volume and HCI, but U and Pb were taken out off column together with water. U and Pb were loaded onto outgassed single Re filaments in a silica gel-phosphoric acid mixture and analysed on a Finnigan MAT 261 mass spectrometer. Pb isotope data were recorded in filament temperature range of 1200-1300℃ and U isotope data were recorded in filament temperature range of 1350-1450℃.A Precise age of 1625.3±6.2Ma (2σ) and another age of about 2400 Ma for zircons separated from the Dahongyu Formation volcanics in Jixian were obtained by this method. The precise 1625.3 Ma age is considered the strat- igraphic age of the Dahongyu Formation volcanics, and 2400 Ma age can be interpreted as a xenocryst age. These ages confirm complex origin of zircons separated from the Dahongyu Formation volcanics, at least confirm that the magma was contaminated by early Proterozoic strata during extrusion. By U-Pb single zircon age determination, not only the magmatic age of volcanics, but also older xenocryst age was obtained. This is important for studying volcanic the rocks origin and evolution of the relevantPrecise and accurate age determination for the Dahongyu Formation volcanics supplies a reliable standard age for dis-cussing the geological time range of the Dahongyu Formation. It was based on U-Pb single zircon age for the Dahongyu Formation volcanics that the time range of this formation is referred between 1650 Ma and 1600 Ma. |
| LU Song-nian,LI Hui-min.1991.A PRECISE U-Pb SINGLE ZIRCON AGE DETERMINA- TION FOR THE VOLCANICS OF DAHONGYU FORMATION,CHANGCHENG SYSTEM IN JIXIAN[J].Acta Geoscientica Sinica,12(1):137-146. |
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