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EXCESS ARGON IN METAMORPHIC AMPHIBOLE BY TECTONIC SHEARING: EVIDENCE FROM Sm-Nd, Rb-Sr AND 40Ar/39Ar DATING ON PLAGIOCLASE AMPHIBOLITES OF THE DANFENG GROUP, NORTH QINLING AREA

Sun Weidong1, Li Shuguang1 2, Chen Wen3, Sun Yong4, Zhang Guowei4,   

  1. 1. Department of Earth and Space Sciences, University of Science and Technology of China, Hefei,230026; 2. Advanced Research Center on Earth Science and Astronomy, Third World Academy of Science, Hefei,230026; 3. Institute of Geology, Chinese Academy of Geological Sciences, Beijing, 100037; 4. Department Geology, Northwest University, Xi’an, 710069
  • Received:1996-12-20 Revised:1996-12-20 Online:1996-12-20 Published:1996-12-20

Abstract: The evolution history of the famous E-W-striking Qinling-Dabie orogenic belt which separates the North China Block and Yangtse Block, has been a matter of concern for many years. There are still many disagreements on the formation time of this orogenic belt. The continental subduciton of the Yangtse Block occurred in Triassic time, as shown by the ages of ultrahigh-pressure metamorphic rocks from the Dabie Mountains, while the calc-alkaline magmaticsm terminated in Devonian in North Qinling as shown by U-Pb analyses of zircons from I-type granites which is consistent with the Sm-Nd mineral isochron age of mafic pluton in Heihe, North Qinling belt. One important way to solve this problem is to figure out the uplifting history of the North Qinling Mountains. Since the closure temperature for argon in amphibole has been accurately determined, its 40Ar/39Ar method is widely used in studying the uplifting history of orogenic metamorphic rocks. It is very important to know how and in what kind of condition the excess argon occurred in metamorphic amphibole because that excess argon in amphibole has often been reported. Two metamporphic amphibolte samples of the Danfeng Group in north Qinling area were determined using Sm-Nd, Rb-Sr and 40Ar/39Ar methods. Sm-Nd, Rb-Sr mineral isochron ages of sample QSP-10 and QH-3 are consistent with each other. They are also consistent with other dating results on metamorphic time of the Danfeng Group. In fact 400 Maagrees well as a collision time in Shangdan fault. The 40Ar/39Ar spectrum age (488Ma) of amphibole in QSP-10 sampled from a sheared zone in Puyu is much higher than its Sm-Nd and Rb-Sr mineral isochron ages (414Ma, 410Ma). While the 40Ar/39Ar age of amphibole from QH-3, which was not sheared, is similar to its SM-Nd and Rb-Sr mineral isochron ages. In general, amphibole is an ideal mineral for K-Ar or 40Ar/39Ar dating because of good retention properties for radiogenic argon. The result of their study suggests that amphibole from sheared zone is different from others. One possible reason for this is that shearing leads to fluid activity and hence mobiled argon is easy tobe transferred to metamorphic amphibole from country rocks by fluid. It could be concluded that shearing might play an important role on the formation of excess argon in metamorphic amphibole from tectonic zone. To avoid sampling in shear zone is essential to reduce the possibility of excess argon and to improve the reliability of results from studies on orogenic isochronology with 40Ar/39Ar methods.