Hf Isotopic Compositions of Zircons from 2.7 Ga Metasedimentary Rocks and BiotitePlagioclase Gneiss in the Mengjiatun Formation Complex, Western Shandong Province
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    Abstract:

    LuHf isotopic compositions of zircons separated from the garnet quartzite(M08), staurolitebearing garnetbiotitequartz schist(D242Y2) of the Mengjiatun formation complex and biotite plagioclase gneiss(M03) of the TTG gneisses at Mengjiatun village, Xintai City, western Shandong Province, have been analyzed through the LaMCICPMS method. All the rocks mentioned above were formed 2.7Ga ago. Among them, the magmatic cores of detrital origin zircons of the garnet quartzite give the 176Lu/177Hf and 176Hf/177Hf ranges 0.001730~0.002546 and 0.281249~0.281360, respectively, and the zircon rims of the sample have the 176Lu/177Hf and 176Hf/177Hf ranges 0.000123~0.002070 and 0.281241~0.281318, respectively. The magmatic cores of detrital origin zircons from the staurolitebearing garnetbiotitequartz schist yield the 176Lu/177Hf and 176Hf/177Hf values 0.001334~0.002169 and 0.281226~0.281324, respectively, and the metamorphic rims of the zircons from the schist 0.000445~0.001375, 0.281227~0.281309, respectively. While the ratios of magmatic zircons from the biotite plagioclase gneiss are 0.000315~0.000847, 0.281186~0.281265, respectively. εHf(t) values of the magmatic (detrital)zircons from garnet quartzite and staurolitebearing garnetbiotitequartz schist of the Mengjiatun formation complex and the orthogneiss are 3.75~7.26, 2.31~7.26 and 3.21~6.27, respectively. In addition, the TDM1 ages of the zircons of all the samples are close to the SHRIMP zircon UPb ages, suggesting they were derived from partial melting of the juvenile crustal materials. Combining with the regional geology information, the authors deduce that the 2.7Ga magmatism is an important magmatic tectonothermal event in western Shandong Province area, even in the North China Craton. Many zircons in the metasedimentary rocks show the tendency that the metamorphic overgrowing rims have lower 176Lu/177Hf but higher 176Hf/177Hf values than that of the magmatic cores, demonstrating the substantial influence of the metamorphic garnet on the LuHf isotopic system of zircons.

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Du Lilin, Yang Chonghui, Zhuang Yuxun,-.2010. Hf Isotopic Compositions of Zircons from 2.7 Ga Metasedimentary Rocks and BiotitePlagioclase Gneiss in the Mengjiatun Formation Complex, Western Shandong Province[J]. Acta Geologica Sinica,84(7):991-1001

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History
  • Received:October 22,2009
  • Revised:April 15,2010
  • Adopted:April 02,2010
  • Online: July 26,2010
  • Published: