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Approaching the Age Problem for Some Metamorphosed Precambrian Basement Rocks and Phanerozoic Granitic Bodies in the Wuyishan Area:The Application of EMP Monazite Age Dating

CHEN Cheng-hong   

  1. Department of Geological Sciences, Taiwan University, Taipei 10617
  • Received:2008-03-20 Revised:2008-03-20 Online:2008-03-20 Published:2008-03-20

Abstract: We use EMP monazite, along with LA-ICPMS U-Pb zircon, dating methods to assert on 5 metamorphic rocks from Wuyisahn Precambrian basement and 5 adjacent Phanerozoic granitic rocks. Results reveal that monazites were largely grown in the Caledonian stage and probably overprinted by a younger age at the Hercynian. Among them, 533.0±44.4 Ma and 451.0±17.7 Ma (no age group from zircon but all >800 Ma) are obtained for a migmatite of Wanquan Group, 422.0±13.7 Ma and 360.0±14.9 Ma (436.4±4.7 Ma from zircon) for a migmatitic leucosome from Badu Group, and 443.3±11.3 Ma (no age group from zircon but concentrated at 450-350 Ma) for a mica quartz schist from Tienjinping Formation. No monazite was found in gneissic rocks from Mayuan Group and granulite from Wanquan Group, but an age of 859.1±9.2 Ma is obtained from zircon.For Phanerozoic granites, 162.3±4.4 Ma is obtained for Hetien body and 215.3±4.5 Ma for Fuchen body, and 418.7±16.4 Ma and 360.6±15.3 Ma for Weipu, 441.8±12.3 Ma and 350±45 Ma for Ninghua and 429.3±15.6 Ma for Zengfengding bodies.Accordingly, we conclude that (1) Wuyishan basement rocks have been strongly influenced by the Caledonian orogeny leading to not only the granitic intrusion but also the strong modification on these rocks; (2) Hercynian event is only reflected by the high-U monazite but not the zircon ages, indicating that the influence of this orogenesis may be limited if it really existed; (3) the fact that the Archean zircons are mostly of inherited origin seems not support the existence of such an Archean basement in Wuyishan; (4) the provenance for Paleoproterozoic monazites that are abundant in W Taiwan sediments has not yet been proven to be correlated to the Wuyishan area as suggested earlier.